**Towards Green, Enhanced Photocatalysts for Hydrogen Evolution**

Editors

**Andrea Speltini Antonella Profumo**

MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin

*Editors* Andrea Speltini Department of Chemistry University of Pavia Pavia Italy

Antonella Profumo Department of Chemistry University of Pavia Pavia Italy

*Editorial Office* MDPI St. Alban-Anlage 66 4052 Basel, Switzerland

This is a reprint of articles from the Special Issue published online in the open access journal *Catalysts* (ISSN 2073-4344) (available at: www.mdpi.com/journal/catalysts/special issues/photocatalysts hydro evolution).

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## **Contents**


### **About the Editors**

#### **Andrea Speltini**

Andrea Speltini is Assistant Professor at the University of Pavia. After his Ph.D. research, which focused on the application of carbon nanotubes in separation science, in particular as solid-phase extraction sorbents and chromatographic stationary phases, as a post-doc researcher he has been involved in the development of analytical methods for the determination of contaminants and pharmaceuticals in complex matrices. A consistent part of the research has been conducted in the field of environmental chemistry, in particular to explore alternative photocatalytic systems for hydrogen gas evolution from water, in the presence of sacrificial (waste)biomasses. He is currently working on carbon materials for the sample treatment of environmental, food and biological matrices, and on the application of novel composite catalysts based on TiO2, graphitic carbon nitride, carbon dots and perovskite for H<sup>2</sup> photogeneration under solar light.

#### **Antonella Profumo**

Antonella Profumo is Full Professor of Analytical Chemistry and Director of the Department of Chemistry of the University of Pavia. Her main research activities, in collaboration with her research group of young researchers, focus on the development and application of selected analytical procedures for preconcentration on carbon-based solid phases in different matrices (environmental, food, and biological samples); the use and application of analytical procedures in the field of forensic pedology; and the employment of novel catalysts for Hydrogen generation by sacrificial water splitting in the presence of renewable biomasses. She runs scientific collaborations with public and private foundations that refer to the Department for analytical guidance, in particular in the sector of natural spring and drinking waters, and in the environmental field as far as concerns emissions of inorganic and organic pollutants from industrial plants and the recovery of material from waste in a circular economy perspective.

## **Preface to "Towards Green, Enhanced Photocatalysts for Hydrogen Evolution"**

Nowadays, more and more people are becoming aware that environmental protection is a priority. The constant growth of energy demand combined with the adverse effects on the global environment caused by use of fossil fuels is increasingly requiring clean and renewable energy sources. The most desired goal is producing energy in new ways, possibly exploiting green sources, in the framework of a *circular economy*. To this aim, *photocatalysis* represents a smart, energy-saving, and very promising technology that relies on the use of an abundant energy source as solar light. In the recent years, *hydrogen photocatalytic production* by water splitting and aqueous-phase reforming of different organics has been under deep study by many research groups because H<sup>2</sup> gas is a highly desired energy vector that produces water by combustion, thus perfectly matching the requirements of green chemistry. Although most of the available data derive from lab-scale studies, it seems believable that, in the near future, H<sup>2</sup> could be obtained on a large scale by making use of inexhaustible, green sources such wastewater and biomass derivatives with the aid of another inexhaustible resource as sunlight. However, this necessarily requires that *efficient*, *recyclable* and *safe* photocatalysts are designed. The authors of this work are involved in environmental studies including the development of alternative H2-producing photocatalytic systems, so the aim of this book is to gather some of the recent studies in the context of *hydrogen photocatalytic production*. This topical cluster could be interesting for researchers working in the fields of materials science, applied photocatalysis and environmental sciences.

> **Andrea Speltini, Antonella Profumo** *Editors*

#### *Editorial* **Towards Green, Enhanced Photocatalysts for Hydrogen Evolution**

**Antonella Profumo 1,\* and Andrea Speltini 2,\***


The constant growth of energy demand joined with the adverse effects on the global environment induced by use of fossil fuels is increasingly requiring new routes to obtain clean and renewable energy sources. Hydrogen photocatalytic production by water splitting or aqueous-phase reforming of different organics has been under deep investigation for some years; H<sup>2</sup> gas is the most desirable energy vector because the only product by combustion is water. H<sup>2</sup> could be obtained at the large scale in the near future by exploiting inexhaustible, green sources such as sunlight, wastewater and biomass derivatives, if efficient, recyclable and safe photocatalysts will be designed.

Besides a review article, this Special Issue gathers research on the preparation, characterization and application of new organic/inorganic composites endowed with photo(electro) catalytic properties. These materials were tested under either UV–visible or, even more conveniently, under visible light for H<sup>2</sup> evolution in "sacrificial water splitting" or "photoreforming" systems.

A new 3D porous carbon nitride catalyst has been proposed by Qiu et al. [1]. The material was synthesized by a simple bottom-up procedure entailing self-assembly of a melamine–cyanuric acid complex followed by ethanol and glycerol intercalation prior to calcination. This route has the merit to yield a 3D hierarchical pancake-like highly porous carbon nitride with enhanced light-harvesting capacity, expanded band gap, prolonged charge carriers' lifetimes, and higher surface area and reduction ability towards hydrogen ions to produce gas-phase H2, compared to the bulk material. Under visible-light radiation and platinum as a co-catalyst, the hydrogen evolution rate (HER) from triethanolamine aqueous solution was 430 µmol gcat <sup>−</sup><sup>1</sup> <sup>h</sup> −1 , 9-fold larger than that afforded by non-porous carbon nitride. The semiconductor showed photochemical stability. Indeed, it was successfully reused for three additional photoreactions. The behavior of the novel catalyst was studied in water containing a fine chemical, as a proof-of-concept sacrificial agent. Three other research studies focused on H<sup>2</sup> evolution from water in the presence of more sustainable electron donors, such as saccharides and alcohols [2–4], testing new catalysts as well.

Carbon nitride–perovskite composites, which presently denote a cutting-edge research field, were investigated as new photoactive micro-sized materials for H<sup>2</sup> evolution from glucose aqueous solution as a representative sacrificial biomass [2]. In particular, the synergism between the newer lead-free perovskite and carbon nitride, due to improved charge carrier separation derived from the positive band-alignment between the two semiconductors, has been exploited to set up a sacrificial water splitting system working under simulated solar light. The H<sup>2</sup> production was optimized by a design of experiments, achieving an HER higher than 900 µmol gcat <sup>−</sup><sup>1</sup> <sup>h</sup> −1 , 12-fold larger compared to pure carbon nitride. The best-performing composite also provided H<sup>2</sup> with no metal co-catalyst, and in the presence of untreated starch, selected as an abundant and low-cost biopolymer, and therefore exploiting the sacrificial role of a raw polysaccharide. Despite the lower surface area, the perovskite–carbon nitride composite results were attractive compared to nanometric P25 TiO2, relative to H<sup>2</sup> evolution.

**Citation:** Profumo, A.; Speltini, A. Towards Green, Enhanced Photocatalysts for Hydrogen Evolution. *Catalysts* **2021**, *11*, 93. https://doi.org/10.3390/catal11010093

Received: 8 January 2021 Accepted: 8 January 2021 Published: 12 January 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

A representative biomass-derived substrate such as ethanol has been selected in the research of Luo et al. [3] to demonstrate the feasibility of a more sustainable method of obtaining H2. Indeed, the contemporary production of bioethanol has reached several tens of million tons per year, and the possibility of ethanol photoreforming under mild conditions is of great relevance. This paper by Luo shows that coupling selective decoration with Au nanoparticles of TiO<sup>2</sup> nanorods and the TiO2/Cu2O p–n junction produces H<sup>2</sup> along with acetaldehyde. The latter is stoichiometrically formed because the C–C cleavage of ethanol does not occur, resulting in no release of greenhouse gases such as carbon dioxide. Under simulated solar light, the new photocatalytic system Au@TiO2/Cu2O affords an HER higher than 8500 µmol gcat <sup>−</sup><sup>1</sup> <sup>h</sup> <sup>−</sup><sup>1</sup> over the composite Au@Cu2O/TiO<sup>2</sup> and Au@TiO2, and it maintains unchanged performance for at least five consecutive catalytic runs.

In the report by Adamopolous and co-workers [4], a nano-sized TiO2/WO<sup>3</sup> bilayer catalyst was employed as a photoanode in a photoelectrochemical cell to produce H<sup>2</sup> from aqueous ethanol solution. The system proposed, which involved photoelectrocatalytic alcohol reforming, takes advantage of the high oxidative power and visible light absorption of WO3, used in the photoanode, stabilization of charge carriers by electron-transfer from TiO<sup>2</sup> to WO3, and passivation of tungstate surface states, which reduces the number of charge recombination sites involved by the titania layer. The latter constituent also scatters back to the bottom WO<sup>3</sup> layer part of the incident light resulting in a higher photocurrent production, proportional to the applied voltage, thus in greater H<sup>2</sup> formation at the cathode of the cell.

This Special Issue also includes a review article focused on the application of 2D materials and composites as potential photocatalysts for water splitting [5]. With more than 200 references covering the last two decades, but with particular attention to the papers published in recent years, after providing the reader with some introductory sections summarizing the fundamentals of water splitting photocatalysis, the application of various materials is presented and discussed through comprehensive tables reporting key information about each photocatalytic system (e.g., catalyst band gap, light source, type and amount of co-catalyst, sacrificial agent used, and HER). The paper covers selected studies on graphitic carbon nitride and graphene-based photocatalysts, metal phosphides, metal organic frameworks and derivatives, together with those on the more recent phosphorene. The review emphasizes the progress in modern nanomaterial applications, for instance by metal nanoparticles doping, surface functionalization, synthesis-controlled morphology, which are essential to achieve the most desired properties, i.e., low charge recombination, high light harvesting capability, good electron conductivity, fast kinetics, and large surface area. From the survey by Saleem and co-authors, it emerged that the use of 2D materials, their combinations and derivatives, are now at the basis of further advancements in photocatalytic water splitting.

We take the chance to thank the authors and their co-authors for the contributed publications to this Special Issue.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


## **Photoelectrocatalytic Hydrogen Production Using a TiO2**/**WO<sup>3</sup> Bilayer Photocatalyst in the Presence of Ethanol as a Fuel**

**Panagiotis Marios Adamopoulos, Ioannis Papagiannis, Dimitrios Raptis and Panagiotis Lianos \***

Department of Chemical Engineering, University of Patras, 26500 Patras, Greece; cmng3238@upnet.gr (P.M.A.); ion.papg@gmail.com (I.P.); dgraptis86@yahoo.gr (D.R.)

**\*** Correspondence: lianos@upatras.gr; Tel.: +30-2610-997513

Received: 13 September 2019; Accepted: 19 November 2019; Published: 21 November 2019 -

**Abstract:** Photoelectrocatalytic hydrogen production was studied by using a photoelectrochemical cell where the photoanode was made by depositing on FTO electrodes either a nanoparticulate WO<sup>3</sup> film alone or a bilayer film made of nanoparticulate WO<sup>3</sup> at the bottom covered with a nanoparticulate TiO<sup>2</sup> film on the top. Both the electric current and the hydrogen produced by the photoelectrocatalysis cell substantially increased by adding the top titania layer. The presence of this layer did not affect the current-voltage characteristics of the cell (besides the increase of the current density). This was an indication that the flow of electrons in the combined semiconductor photoanode was through the WO<sup>3</sup> layer. The increase of the current was mainly attributed to the passivation of the surface recombination sites on WO<sup>3</sup> contributing to the limitation of charge recombination mechanisms. In addition, the top titania layer may have contributed to photon absorption by back scattering of light and thus by enhancement of light absorption by WO3. Relatively high charge densities were recorded, owing both to the improvement of the photoanode by the combined photocatalyst and to the presence of ethanol as the sacrificial agent (fuel), which affected the recorded current by "current doubling" phenomena. Hydrogen was produced under electric bias using a simple cathode electrode made of carbon paper carrying carbon black as the electrocatalyst. This electrode gave a Faradaic efficiency of 58% for hydrogen production.

**Keywords:** WO3; TiO2; hydrogen production; photoelectrocatalysis

#### **1. Introduction**

Photoelectrocatalytic hydrogen production is one of the most popular research subjects because it promises an effective route for converting solar energy and storing it as chemical energy in the form of hydrogen. The popularity of hydrogen stems from the fact that it has the highest gravimetric heat of combustion (~286 kJ/mole), while its combustion leads to the production of water. Hydrogen is mainly produced by reforming of fossil fuels. It can also be produced by electrolysis using, for example, renewable electricity. Theoretically, 1.23 V are necessary to split water by electrolysis; however, in reality, much higher voltages and expensive electrocatalysts are necessary. In this sense, water splitting by photoelectrocatalysis [1] is a very promising approach since it necessitates much lower electric biases. However, oxidation of water is a four-electron process:

$$\rm 2H\_2O \xrightarrow{} O\_2 + 4H^+ + 4e^- \tag{1}$$

Therefore, it is necessary to simultaneously extract four units of charge in order to oxidize water and produce molecular oxygen. This is not easy and necessitates oxygen evolution co-catalysts. On the

contrary, it is easier to oxidize an organic substance. For example, in the case of ethanol, oxidation leads to the formation of acetaldehyde, which is a two electron process, therefore easier to realize, either directly [2–4]:

$$\text{C}\_2\text{H}\_6\text{O} \rightarrow \text{CH}\_3\text{CHO} + 2\text{H}^+ + 2\text{e}^- \tag{2}$$

or by the intermediate of hydroxyl radicals:

$$\text{OH}^- \rightarrow \text{^\bullet OH} + \text{e}^- \text{ and } \text{C}\_2\text{H}\_6\text{O} + 2\text{^\bullet OH} \rightarrow \text{CH}\_3\text{CHO} + 2\text{H}\_2\text{O} \tag{3}$$

For most organic substances, oxidation and subsequent mineralization proceed by steps, as in the above case of ethanol, which usually are two electron processes, therefore easier to oxidize than water. In terms of oxidation potential, oxidation of ethanol to acetaldehyde corresponds to −0.21 V [5], i.e., it is much lower than the above mentioned potential of 1.23 V necessary to oxidize water. Molecular hydrogen production by reduction of protons or water is also a two electron process [3,4]:

$$\text{2H}^+ + \text{2e}^- \rightarrow \text{H}\_2 \text{ (acidic environment)}\tag{4}$$

or

$$2\text{H}\_2\text{O} + 2\text{e}^- \rightarrow 2\text{OH}^- + \text{H}\_2 \text{ (alkaline environment)}\tag{5}$$

Therefore, photoelectrocatalytic hydrogen production by using an organic substance as a fuel is even more of a promising approach, which has been adopted in the present work.

The standard configuration of a photoelectrocatalytic cell for hydrogen production involves a photoanode electrode carrying a semiconductor photocatalyst and a cathode electrode where reductive reactions are carried out, producing hydrogen. Both electrodes are immersed in an electrolyte, which increases the internal cell conductivity and provides the ionic species necessary to allow device functioning in a cyclic manner [3,4]. Figure 1 shows a simplified design of a photoelectrocatalysis cell. When photons are absorbed by the photocatalyst, electron-hole (e−-h+) pairs are generated. Holes are consumed by oxidation reactions, such as (2) or (3), while electrons move through an external circuit and are consumed by reduction reactions at the cathode. The overall result of oxidation and reduction can be represented, for example, by combining (2) or (3) with (4) or (5), respectively [2]:

$$\text{C}\_2\text{H}\_6\text{O} \xrightarrow{\text{e}} \text{CH}\_3\text{CHO} + \text{H}\_2\tag{6}$$

which describes the consumption of ethanol, from now on referred to as the "fuel", to produce hydrogen by means of a photoelectrocatalytic process. Acetaldehyde may be further oxidized until complete mineralization and further hydrogen production. The complete mineralization of ethanol may be expressed by the following overall reaction [3,4]:

$$\rm C\_2H\_6O + 3H\_2O \to 2CO\_2 + 6H\_2 \tag{7}$$

which is the same as ethanol reforming [6] and can be rightfully called "photoelectrocatalytic alcohol reforming".

Photoelectrocatalysis necessitates then the presence of a photocatalyst. There exists a rich literature on the choice of photocatalysts [4,7,8]. Researchers searching for a new photocatalyst have developed resourceful arguments in their favor; however, it is a matter of fact that very few among them simultaneously possess more than one quality that distinguishes each one of them from the others. Nanostructured titania is the best such example, but WO<sup>3</sup> is also a distinguished choice as well [9–11]. Titania is an n-type, low cost, non-toxic semiconductor, easy to process, and having a strong adhesive quality on various types of electrodes. It possesses a strong oxidative potential (about 3 V in the case of anatase) and a conduction band conveniently located with respect to the hydrogen production potential (about −0.2V vs. NHE) [12]. Titania also possesses relatively large charge carrier mobility. For example, its hole diffusion length is of the order of 10<sup>4</sup> nm, much larger than that of WO<sup>3</sup> (~150 nm) and

–

hematite (2–4 nm) [13]. These properties would make titania an ideal photocatalyst; however, it still suffers from substantial electron-hole recombination, while its light absorption range is limited only to the UV. Nanoparticulate tungsten oxide is also a very popular choice, and thanks to its substantial range of visible light absorption, it has been studied for several decades [11,14–17] as an alternative to the UV absorbing titania. Indeed, WO<sup>3</sup> has a bandgap ranging between 2.5 and 2.8 eV, and it may thus absorb light up to 500 nm, which accounts for 12% of the solar radiation on the surface of the Earth [17]. WO<sup>3</sup> is an n-type indirect semiconductor. It is easy to synthesize and deposit on electrodes; it has a moderate hole-diffusion length (~150 nm [17], as already said); it is resistant against photocorrosion; and it is stable at relatively low pH values. For this reason, WO<sup>3</sup> has been studied as a photoanode material for photoelectrochemical water splitting applications [11,14–20]. Its valence band is located approximately at +2.8 V vs. NHE [12]; therefore, it also possesses high oxidative power, and it is well placed for water and organics' oxidation. Its conduction band is located at positive potentials (approximately +0.2 to +0.3V vs. NHE, cf. [12]); therefore, it is located substantially lower than that of titania. If the two semiconductors are then brought into contact, it is expected that photogenerated electrons may be transferred from TiO<sup>2</sup> to WO3, thus achieving charge separation and limiting e <sup>−</sup>-h + recombination. However, it is understood that since the photon absorption spectral range for titania is limited, its role as an electron source for WO<sup>3</sup> is of limited importance. There is another role that titania may play, which is of utmost importance, and this is the passivation of surface states of WO<sup>3</sup> that a top cover of titania can offer. This role will be investigated in the present case. − <sup>−</sup>→ <sup>−</sup>→ <sup>−</sup> −

→ <sup>−</sup>

<sup>−</sup>→• <sup>−</sup> • →

**Figure 1.** Schematic representation of the reactor employed in the present work, showing the working electrode (photoanode), counter electrode (cathode electrode carrying an electrocatalyst), reference electrode, and connections with a potentiostat, which provides the necessary bias. Hydrogen production is monitored in a de-aerated environment obtained by Ar flow.

→ In addition to the surface states' passivation effect, there is one more reason to enhance WO3's performance. As will be discussed below, contrary to the transparent WO<sup>3</sup> layer, the top titania layer is a light scattering film, which scatters back to WO<sup>3</sup> at least part of the incident light. This situation supports conditions for enhancement of light absorption by the WO<sup>3</sup> layer, and this, of course, results in higher photocurrent production.

Unbiased flow of electrons from the photoanode to the cathode electrode would necessitate a substantial potential difference between the two electrodes. The potential of the photoanode is roughly defined by the conduction band of the photocatalyst, while that of the cathode electrode by the redox potential of the reduction reaction. In the case of hydrogen production, the potential in a strongly acidic environment is around 0 V vs. NHE. The conduction band of titania lies at about −0.2 V vs. NHE, as already said. This difference of 0.2 V is too small, and it is eliminated by losses. It is even worse in

the case of WO3, which has a conduction band slightly more positive than the hydrogen production potential. Therefore, no unbiased flow of electrons is expected for a WO<sup>3</sup> photoanode and is hardly expected in the case of a TiO<sup>2</sup> photoanode. In other words, when the above two photocatalysts are used, production of hydrogen may be obtained only by applying a bias. This is the reason that in the present work, hydrogen was produced under electric bias, as schematically illustrated in Figure 1.

By taking into account the above knowledge, in the present work, we are studying photoanodes made by combining nanoparticulate WO<sup>3</sup> with nanoparticulate TiO2, in a bilayer structure comprising a bottom film made of WO<sup>3</sup> and a top film made of TiO2, in order to investigate the merits of such a combination. Measurements have focused on photoelectrocatalytic hydrogen production by using ethanol as the model fuel [19] with the understanding that other organic materials, either biomass byproducts or water soluble pollutants, may apply as well [20].

#### **2. Results and Discussion**

In the present work, as already said, the photoanodes were made either by deposition of a WO<sup>3</sup> nanoparticulate film alone on an FTO electrode or by adding a layer of nanoparticulate titania on the top. As seen in Figure 2A, WO<sup>3</sup> nanoparticles were polydisperse with the size ranging between 20 and 50 nm. The BET specific surface area was 26 m2g −1 , and X-ray diffractograms (not shown) revealed the formation of monoclinic WO<sup>3</sup> crystallites of size around 25 nm as calculated by using Scherrer's formula. The quantity of WO<sup>3</sup> in the film was approximately 4.5 mg cm−<sup>2</sup> . When the titania layer was added on the top, a complete coverage of the lower WO<sup>3</sup> film was obtained by exposing only the titania film. The SEM image of the film in Figure 2B then showed a typical titania nanostructure. The quantity of titania was approximately 4 mg cm−<sup>2</sup> . The latter maintained the characteristics previously described in other publications [21,22]. The thickness of the combined semiconductor film was approximately determined by its scanning electron microscope profile, and it was below 1 µm. − − −

**Figure 2.** FE-SEM images of a WO<sup>3</sup> /FTO (**A**) and a TiO<sup>2</sup> /WO<sup>3</sup> /FTO (**B**) film.

The spectroscopic characteristics of the WO<sup>3</sup> film before and after titania deposition are shown in Figure 3A. WO<sup>3</sup> film alone absorbed light up to about 480 nm, i.e., within a spectral range typical of this semiconductor. When titania was added on the top, the reflectance-absorption spectrum presented a different aspect depending on which side of the film was exposed to light. When the light passed through the transparent FTO electrode (cf. Figure 3B), the spectrum (Curve 2) showed similar characteristics as those of WO<sup>3</sup> alone (Curve 1). Since the photoanode operates by this mode (Figure 3B), it was seen that the spectroscopic characteristics of WO<sup>3</sup> prevailed. When absorption was recorded by front reflection, i.e., by means of the top titania layer, the corresponding curve (Curve 3) had a typical aspect of a nanocrystalline titania film. This is an additional indication of complete coverage of the WO<sup>3</sup> layer by (the light scattering) titania.

**Figure 3.** (**A**) Reflection-absorption spectra of the WO<sup>3</sup> (1) and the TiO<sup>2</sup> /WO<sup>3</sup> (2,3) photoanode. Curve 3 was obtained by front side reflection and Curve 2 by reflection through the supporting FTO glass. Curve 3 is typical of the nanoparticulate titania absorption spectrum. (**B**) Graphical representation of the direction of photons in the presently used setup.

The presence of TiO<sup>2</sup> on the top of WO<sup>3</sup> was expected to encourage electron-hole separation, thanks to the difference between the levels of the conduction band of these semiconductors, which may reach more than 0.4 V, with the titania conduction band being the more negative of the two (cf. the band levels in [23]). However, as already said, the role of titania as a sensitizer of WO<sup>3</sup> was expected to be limited since its photon absorption range was limited. On the contrary, deposition of titania on the top of the WO<sup>3</sup> film may result in the passivation of surface defects of the latter, and this would limit charge recombination sites. Passivation of surface defects has been observed and recorded in TiO2/WO<sup>3</sup> and other semiconductor combinations [24–30]. In addition, titania films are more stable than WO<sup>3</sup> films; therefore, the top titania layer provided protection against aggressive electrolytes. Finally, the top titania layer provided a light scattering film, in contrast to the transparent WO<sup>3</sup> film, which may send light back to WO3, thus enhancing its capacity for photon harvesting. This is especially convenient in the present case where light was absorbed through the supporting FTO layer. This configuration is illustrated by the design of Figure 3B. Light scattering by the titania layer was due to the agglomeration of titania nanoparticles into large clusters (cf. Figure 2B).

The beneficial effects that the bilayer photocatalyst film had on the activity of the photoelectrocatalytic cell were immediately seen by recording current-voltage curves, as seen in Figure 4. More than a 30% increase of current was recorded when the titania layer was added on the top, compared with a photoanode made of WO<sup>3</sup> alone. The curves of Figure 4 were recorded under the same conditions, differing only by the composition of the photocatalyst film. The curves were recorded in light chopping mode demonstrating that the voltage range of pure photocurrent extended between about 0.2 and 1.8 volts vs. Ag/AgCl. The small anodic peak below this range was due to capacitance current deriving from the adsorption of cations from the electrolyte into the pores of the mesoporous film [21]. The increase of the dark current above 1.8 V vs. Ag/AgCl was due to electrolysis. The electrolyte was 0.5 M NaClO<sup>4</sup> with 5% v/v ethanol added in the anode compartment. In accordance with previous publications [19,20], in the absence of ethanol (not shown; cf. the data in [19]), lower currents were obtained in both photoanode cases. This was expected as previously explained [19,20]. Ethanol acted as a hole scavenger, which further limited charge recombination, thus resulting in higher currents. Moreover, transient species formed by ethanol oxidation induced current increase by the "current doubling" phenomenon [31]. Indeed, the maximum current density expected for a photoanode with light absorption threshold at 480 nm (cf. Figure 3A) was no more than 5 mA cm−<sup>2</sup> [4]. However, the current density presently increased under bias well beyond this limit,

−

and this was rationalized only by the current doubling effect. The addition then of the titania layer and the presence of the fuel (ethanol) resulted in the production of substantial photocurrent.

**Figure 4.** Current density-voltage curves for a photoelectrocatalytic cell made of two different photoanodes. Recording was done in light chopping mode demonstrating the voltage range of the pure photocurrent.

− − − − The increase of the current when titania was added on the top of WO<sup>3</sup> was not due to titania acting as the photocatalyst by itself. When a photoanode was made by using titania alone, by taking care to deposit the same quantity of photocatalyst (4 mg cm−<sup>2</sup> ), under the same conditions of the cell configuration and the electrolyte and fuel concentration, the behavior of the current-voltage curve presented a completely different aspect, as seen in Figure 5. The maximum current density was only 0.42 mA cm−<sup>2</sup> , while in the case of Figure 4, the current increase was more than 3 mA cm−<sup>2</sup> . Most importantly, open circuit voltage (the point where the current became zero) was <−1 V vs. Ag/AgCl, while in Figure 4, the open circuit voltage was approximately located at 0.0 V vs. Ag/AgCl. The difference in the onset potential between a WO<sup>3</sup> and a TiO<sup>2</sup> photoanode roughly reflected the difference in the electrochemical potential of each semiconductor, and it was systematically observed. The fact that the addition of titania on the top of WO<sup>3</sup> preserved the characteristics of the latter simply means that the bottom lying WO<sup>3</sup> layer defined the level at which electrons left the anode electrode. It was furthermore observed that in the case of titania alone, the photocurrent soon reached saturation by the voltage increase, while in the case of WO<sup>3</sup> and TiO2/WO3, the current continuously increased with bias voltage. In other words, the addition of the titania layer on the top of WO<sup>3</sup> resulted only in a current increase without affecting the original appearance of the pure WO<sup>3</sup> current-voltage curve. Obviously, photogenerated electrons in the titania layer were transferred to the electrode only by means of the underlying WO<sup>3</sup> layer, and titania did not act as a photocatalyst by itself. Because of the continuous increase of current with increasing voltage and since hydrogen production was proportional to the current flowing through the external circuit of the cell, it was expected that the hydrogen production rate should strongly depend on the applied electric bias.

**Figure 5.** Current density-voltage curves for a photoelectrocatalytic cell made of a (pure) titania photoanode. Recording was done in light chopping mode demonstrating the voltage range of the pure photocurrent. The anodic rise at negative voltages is due to the capacitance current deriving from the adsorption of Na cations from the electrolyte solution [21].

At this point, a question arises about the fate of the photogenerated holes. An approximate positioning of band levels for TiO<sup>2</sup> and WO<sup>3</sup> nanoparticulate semiconductors and of the oxidation potentials of the three possible hole scavengers in the present system is presented by the diagram in Figure 6 (cf. also the band levels in [23]). Titania nanoparticles used for the present application were a mixture of anatase and rutile. For this reason, the valence band level could not be clear cut and was expected to lie between 3.0 and 2.8 V vs. NHE. A similar situation existed for WO<sup>3</sup> due to the fact that its bandgap may range between 2.5 and 2.8 eV, as already said. This created an uncertainty concerning the direction of the hole transfer. In Figure 6, we favored the transfer of holes to titania, which, being on the top, facilitated the interaction with hole scavengers in the solution. In view of structural imperfections in both semiconductors, it was hard to make a strong assertion on this matter. In any case, the level of 3.0 V vs. NHE was positive enough to produce •OH radicals. However, vast losses, again due to structural imperfections, may make transfer of holes to OHions less probable than the most obvious hole scavenging by ethanol or water. Oxidation of ethanol, as already said, is easier than water therefore, ethanol remained the most probable hole consumer.

**Figure 6.** Approximate band levels for nanoparticulate TiO<sup>2</sup> and WO<sup>3</sup> and oxidation potentials of the possible hole scavengers in the present system.

The hydrogen production rate by employing the two types of photoanode was indeed monitored, and the results are presented in Figure 7. In Figure 7, we tried to demonstrate the strong variation of the hydrogen production rate as a function of the applied bias. No hydrogen production was observed without a forward bias. Three different electric biases were applied, chosen within the above photocurrent range. Thus, each curve was divided into three parts, one for each applied bias, as marked on the graph. Each part demonstrated a period of a rising rate followed by a period of a practically constant rate. A fast increase of the hydrogen production rate took place after changing the applied bias, and it corresponded to the filling of tubes and detection device with the additional hydrogen produced. The variation of the hydrogen production rate with applied bias was a clear demonstration that hydrogen produced by reduction reactions at the cathode depended on the current flowing through the external circuit of the cell. Indeed, by comparing Curve #2 of Figure 4 with Curve #2 of Figure 7, it is found that the hydrogen produced was roughly proportional to the current. In addition, the combined TiO2/WO<sup>3</sup> photoanode produced a higher quantity of hydrogen than the WO<sup>3</sup> photoanode, in accordance with the higher current flow. The maximum hydrogen production rate recorded under the present conditions, with 1.6 V bias vs. Ag/AgCl, was 4.4 µmol.min-1. The corresponding current was 24.3 mA. The Faradaic efficiency for hydrogen production under the present conditions could be calculated by the following equivalent [32]: 1 <sup>µ</sup>mole/min H<sup>2</sup> corresponds to (10−<sup>6</sup> moles) <sup>×</sup> (6.022 <sup>×</sup> <sup>10</sup><sup>23</sup> molecules/mole) <sup>×</sup> (2 electrons/H<sup>2</sup> molecule) <sup>×</sup> (1.6 <sup>×</sup> <sup>10</sup>−<sup>19</sup> Coulomb/electron)/60 s, which is equal to 3.21 mA. Consequently, 4.4 µmole.min−<sup>1</sup> correspond to 14.1 mA. Since the average real current was 24.3 mA, then the Faradaic efficiency was 58% (14.1/24.3 = 0.58). The Faradaic efficiency depended on the device configuration and reactor parameters, as well as the efficiency of the cathode electrode and the quality of the electrocatalyst. The presently obtained value was relatively low, owed mainly to the use of an inexpensive electrocatalyst (carbon paper loaded with carbon black), but also to the fact that the reactor parameters were not optimized. Comparable Faradaic efficiencies were also obtained in a previous work [32]. It must be stressed at this point that the quality of the photoanode and the strength of the applied bias mainly dictated the flowing current, while the functionality of the cathode electrode mainly affected the Faradaic efficiency. It relied then mainly on the counter electrode to improve Faradaic efficiency, and this will be a matter of our future projects. − − −

**Figure 7.** Variation of the photoelectrocatalytic hydrogen production rate by applying various electric biases. Biases are marked on the graph, and they are expressed in volts vs. Ag/AgCl electrode. The two curves correspond to the two photoanodes.

The data of Figure 7 corresponded to relatively short-time measurements. When the experiment was run for a longer period of time, a slow current drop was observed, which was accompanied by an analogous drop of the hydrogen production rate (cf. the data in our previous work [32]). The proportionality of current with the hydrogen production rate preserved the value of the Faradaic efficiency, in accordance with the above discussion. It was then again concluded that the photoanode and its immediate environment, as well as the applied bias dictated the current production, while the capacities of the counter electrode dictated the Faradaic efficiency for hydrogen production.

#### **3. Materials and Methods**

#### *3.1. Materials*

Reagents were obtained from Sigma Aldrich and were used as received. Millipore water was used in all experiments. SnO2:F transparent conductive electrodes (FTO, resistance 8 ohm/square) were purchased from Pilkington (Pilkington, Toledo, OH, USA) and carbon paper from SGL Technologies GmbH (Meitingen, Germany, thickness 190 µm), and carbon black was a donation from Cabot Corporation (Vulcan XC72, Billerica, MA, USA).

#### *3.2. Construction of WO<sup>3</sup> Photoanodes*

A quantity of 0.4 g of high purity tungsten powder (99.99%) with average particle sizes up to 10 microns reacted with 3 mL of 30% aqueous hydrogen peroxide [33] under sonication for 2–3 h until a transparent colorless solution was obtained. The excess of H2O<sup>2</sup> was catalytically decomposed using a Pt foil [34,35] by keeping the mixture at 10 ◦C with a thermostated water bath. Then, the solution was mixed with 3 mL of ethanol and 0.3 g of the surfactant Triton X-100, and the mixture was deposited on previously cleaned FTO slides cut into the appropriate dimensions. FTO slides were cleaned first with soap and then by sonication in acetone, ethanol, and water. Deposition was made by spin-coating at 2000 rpm for 20 s. After depositing 3 subsequent layers, the obtained film was calcined for 10 min at 500 ◦C (heating rate of 20 ◦C/min). The procedure was repeated at least six times. This rather tedious procedure was necessary to make a compact film without voids. The active area of the film was 1 cm × 1 cm in the case when it was used for IV measurements. Larger films of 3.5 cm × 5 cm were employed for hydrogen production.

#### *3.3. Preparation of the TiO<sup>2</sup> Paste and Deposition on the WO<sup>3</sup> Film*

Deposition of titania on the top of the WO<sup>3</sup> film was achieved by using a paste made of commercial titania nanoparticles. The paste was prepared by modifying a previously published procedure [36]. Three grams of Degussa P25 powder were placed in a small mortar with 0.5 mL of acetic acid and carefully ground for 3 min. Then, 2.5 mL of triple-distilled water were added in 5 equal doses while grinding the mixture. Grinding continued by then gradually adding 10 times ethanol by 1 mL and subsequently 6 times by 1.25 mL. At the end of this procedure, a big amount of ethanol, almost 30 mL, was added, which helped to detach titania from the walls of the mortar. The mixture was then sonicated several times, for 2 s each time, until a homogeneous dispersion was obtained. Consequently, we added 10 g of 2-(4-methyl-1-cyclohex-3-enyl)propan-2-ol (terpineol) and repeated ultrasonication again until a homogeneous dispersion was obtained. Finally, we added an amount of ethyl cellulose (5% w/w) dissolved in EtOH (10% w/v) and again repeated ultrasonication to obtain a homogeneous mixture. The whole procedure lasted 50–60 min, and when it ended, the solution was ready for use. This paste can be condensed in a rotary evaporator; however, in the present work, it was used as is, in order to apply it by spin-coating. Indeed, titania was applied on either a WO<sup>3</sup> covered electrode or a plain FTO glass by spin-coating at 3000 rpm for 30 s.

#### *3.4. Construction of the Counter Electrode*

The counter electrodes that were used in the present work were made of carbon cloth on which a carbon black paste was deposited by the following procedure: 0.3 g of carbon black was mixed with 8 mL of distilled water by vigorous mixing in a mixer (more than 4000 r.p.m.) until it became a viscous paste. This paste was further mixed with 0.1 mL polytetrafluorethylene (Teflon 60% wt. dispersion in water) and then applied on a carbon paper cut into the appropriate dimensions. The paste was first dried at 80 ◦C, and then, it was sintered at 340 ◦C. The procedure was repeated so as to deposit 0.5 mg of carbon black per cm<sup>2</sup> . The active area of the electrode was 17.5 cm<sup>2</sup> (5 cm × 3.5 cm).

#### *3.5. Apparatus and Measurements*

The apparatus used for photoelectrochemical measurements and hydrogen production is schematically presented by Figure 1. It was made of two Pyrex cylinders separated by an ion transfer membrane (ROBU, Germany, porosity SGQ 5, diameter 25 mm, thickness 2 mm). One cylinder accommodated the photoanode and the reference electrode (Ag/AgCl) and the other the counter electrode. The counter electrode compartment was sealed using appropriate fittings, which allowed de-aeration of the solution by Ar flow, collection of the produced H2, and counter electrode electric connection. The photoanode compartment was exposed to the ambient environment. The active area of the counter electrode was 3.5 × 5 cm<sup>2</sup> , while the area of the photoanode was 1 × 1 cm<sup>2</sup> in the case of current-voltage measurements, but increased to 3.5 × 5 cm<sup>2</sup> in the case of hydrogen production. Each compartment was filled with 200 mL of 0.5 M NaClO4. However, in the anode compartment, we also added 5% v/v ethanol. Illumination was made in all cases using a Xe lamp providing an intensity of 100 mW·cm−<sup>2</sup> at the position of the photoanode. Hydrogen was detected online by using Ar as the carrier gas and an SRI 8610C gas chromatograph. Calibration of the chromatograph signal was accomplished by comparison with a standard of 0.25% H<sup>2</sup> in Ar. Current-voltage curves and application of electric bias were accomplished with an Autolab potentiostat PGSTAT128N (Metrohm AG, Herisau, Switzerland)

#### **4. Conclusions**

This work demonstrated the improvement of the photoelectrocatalytic behavior and the increase of the hydrogen production rate by adding a titania film on the top of a WO<sup>3</sup> photoanode. The electrochemical characteristics of the photoanode were not modified by TiO<sup>2</sup> film addition, but the obtained photocurrent largely increased, while the quantity of hydrogen proportionally increased as well. This effect was mainly ascribed to the passivation of WO<sup>3</sup> surface states, thus decreasing the number of charge recombination sites. This result is in accordance with the observations described in [24]. Increased photocurrent (and hydrogen) production may additionally be ascribed to the TiO<sup>2</sup> light scattering effect.

**Author Contributions:** P.M.A.: investigation. I.P.: investigation. D.R.: investigation. P.L.: conceptualization.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Article*

## **Enhanced Hydrogen Production from Ethanol Photoreforming by Site-Specific Deposition of Au on Cu2O**/**TiO<sup>2</sup> p-n Junction**

**Lan Luo 1,**† **, Tingting Zhang 1,**† **, Xin Zhang <sup>1</sup> , Rongping Yun <sup>1</sup> , Yanjun Lin <sup>1</sup> , Bing Zhang <sup>2</sup> and Xu Xiang 1,\***


Received: 15 April 2020; Accepted: 8 May 2020; Published: 13 May 2020

**Abstract:** Hydrogen production by photoreforming of biomass-derived ethanol is a renewable way of obtaining clean fuel. We developed a site-specific deposition strategy to construct supported Au catalysts by rationally constructing Ti3<sup>+</sup> defects inTiO<sup>2</sup> nanorods and Cu2O-TiO<sup>2</sup> p-n junction across the interface of two components. The Au nanoparticles (~2.5 nm) were selectively anchored onto either TiO<sup>2</sup> nanorods (Au@TiO2/Cu2O) or Cu2O nanocubes (Au@Cu2O/TiO2) or both TiO<sup>2</sup> and Cu2O (Au@TiO2/Cu2O@Au) with the same Au loading. The electronic structure of supported Au species was changed by forming Au@TiO<sup>2</sup> interface due to the adjacent Ti3<sup>+</sup> defects and the associated oxygen vacancies while unchanged in Au@Cu2O/TiO<sup>2</sup> catalyst. The p-n junction of TiO2/Cu2O promoted charge separation and transfer across the junction. During ethanol photoreforming, Au@TiO2/Cu2O catalyst possessing both the Au@TiO<sup>2</sup> interface and the p-n junction showed the highest H<sup>2</sup> production rate of 8548 µmol gcat <sup>−</sup><sup>1</sup> h <sup>−</sup><sup>1</sup> under simulated solar light, apparently superior to both Au@TiO<sup>2</sup> and Au@Cu2O/TiO<sup>2</sup> catalyst. The acetaldehyde was produced in liquid phase at an almost stoichiometric rate, and C−C cleavage of ethanol molecules to form CH<sup>4</sup> or CO<sup>2</sup> was greatly inhibited. Extensive spectroscopic results support the claim that Au adjacent to surface Ti3<sup>+</sup> defects could be active sites for H<sup>2</sup> production and p-n junction of TiO2/Cu2O facilitates photo-generated charge transfer and further dehydrogenation of ethanol to acetaldehyde during the photoreforming.

**Keywords:** ethanol reforming; hydrogen production; Au catalyst; Ti3<sup>+</sup> defect; site-specific deposition

#### **1. Introduction**

Hydrogen is extensively used in various industrial processes, e.g., in the petrochemical industry, metallurgy, fine chemical engineering, etc. [1]. Hydrogen, as a clean and renewable fuel, has aroused tremendous attention from both academic and industrial perspectives in the past decades because its energy-extraction process produces only water as a byproduct and emits no greenhouse gases, e.g., CO<sup>2</sup> or any pollutants [2,3]. At present, industrial production of hydrogen depends predominantly on steam reforming of CH4, an energy-consuming process accompanied by CO<sup>2</sup> emissions. Therefore, it is highly desirable to develop efficient, economical and energy-neutral processes for sustainable H<sup>2</sup> production.

Ethanol could be produced in a sustainable way from huge-amount, low-grade biomass, e.g., lignocelluloses and agriculture waste besides from the conventional petrochemical route [4]. In addition, over several tens of million tons biomass-derived ethanol (bio-ethanol) is produced per year. Therefore, ethanol reforming provides an alternative and promising means of H<sup>2</sup> production from abundant and low-value biomass resources [5,6]. Photoreforming could be one of the most promising potential reforming ways because H<sup>2</sup> production is activated on catalysts by sunlight and is achieved conveniently at room temperature and ambient pressure [7–10]. Another advantage of ethanol photoreforming is that hydrogen in the gas phase and acetaldehyde in the liquid phase can be formed at a stoichiometric rate without CO<sup>2</sup> emission. Acetaldehyde can be used directly in many situations, or can be further converted into other chemicals. Exploiting high-efficiency catalysts is of great importance for achieving high activity in ethanol photoreforming under mild conditions.

Metal-supported TiO<sup>2</sup> photocatalysts have been extensively studied in alcohol photoreforming. Au-supported TiO<sup>2</sup> catalysts have been tuned with respect to metal particle size, annealing conditions in different gas atmospheres, and the phase structure of TiO<sup>2</sup> (anatase and rutile) [11]. The AuPd alloy was further studied for adjusting the electronic structure of metal component, resulting in enhanced activity in this reaction [12]. The films of metal/TiO<sup>2</sup> supported on glass were tested under UV light irradiation of ethanol/water mixtures, revealing that Pt was slightly more active than Au under those conditions [13]. It is recognized that the longitudinal SPR mode of Au is the main channel for transferring the hot electrons from Au toTiO<sup>2</sup> [14]. The selective deposition of TiO<sup>2</sup> or other electron acceptors at the tip of Au nanorods (NRs) or at the edge of Au nanodisks (NDs) results in highly active plasmonic photocatalysts [15]. Yang and co-workers studied the gold nanodisks and TiO<sup>2</sup> nanophases sandwich between zeolite nanosheets for hydrogen production by plasmonic photocatalytic reforming of methanol [16]. It has been reported that anatase TiO<sup>2</sup> is more active than the rutile one, and the higher surface availability of the former for Au is beneficial to H<sup>2</sup> production from ethanol under UV light irradiation. However, the significant amounts of other gaseous products (mostly CO, CO<sup>2</sup> and CH4) were also released, probably by further photo-induced decomposition of ethanol [17].

It is highly desirable to explore alternative strategies of catalyst design for enhancing hydrogen production from bio-ethanol photoreforming and simultaneously inhibiting C–C cleavage of ethanol. Herein, we propose a strategy for constructing both p-n junctions between two kinds of semiconducting oxides and Au@TiO<sup>2</sup> interface by site-specific Au deposition for enhancing hydrogen production from ethanol reforming. The results indicate that the interaction of Au and the adjacent Ti3<sup>+</sup> defects on Au@TiO2/Cu2O catalyst affects the electronic structure of Au. In addition, the p-n junction between Cu2O and TiO<sup>2</sup> facilitates the charge separation and transfer across the interface. The simultaneous construction of the p-n junction and the metal/oxide interface on the supported catalyst results in a record-level H<sup>2</sup> production rate of 8548 µmol gcat <sup>−</sup><sup>1</sup> h <sup>−</sup><sup>1</sup> under simulated solar light and excellent recycling stability. Accompanied by hydrogen production, acetaldehyde at a stoichiometric rate is solely produced in the liquid phase, indicative of efficient inhibition of C–C cleavage during ethanol reforming.

#### **2. Results and Discussion**

The procedure for catalyst preparation is shown in Scheme 1. The experimental details are stated in Section 3 Materials and Methods.

High-resolution transmission-electron microscope (HRTEM) photographs are shown in Figure 1. Cu2O nanocubes with an average size of ~40 nm were grown on TiO<sup>2</sup> nanorods with a lateral size of 80~120 nm. The close contact between them can be clearly observed and the lattice d-spacing of 0.243 nm and 0.31 nm is assigned to the plane (103) of anatase phase TiO<sup>2</sup> [18] and (110) of cubic phase Cu2O [19], respectively (Figure 1a). The junction can be formed between two oxides having intimate contact [20]. The Au nanoparticles are site-specific deposited on either TiO<sup>2</sup> or Cu2O or both, showing the same average size of 2.5 nm and high dispersion (Figure 1b–d). The Au loading is determined to be 0.85 wt% in the three catalysts Au@Cu2O/TiO2, Au@TiO2/Cu2O andAu@TiO2/Cu2O@Au by ICP analyses. The lattice distance of 0.235 nm corresponds to the plane (111) of cubic phase Au [21]. The Au nanoparticles display a well-defined spherical morphology and high crystallinity (Figure 1b–d). The size and loading of Au nanoparticles change a little, whatever Au is deposited on any support. EDS elemental mappings show the homogeneous distribution of Ti and O in the nanorods and Cu and O in the nanocubes (Figure 1)

**Scheme 1.** The procedure for catalyst preparation by site-specific deposition of Au on the support.

–

**Figure 1.** HRTEM images and EDS mapping of (**a**) Cu2O/TiO<sup>2</sup> (**b**) Au@Cu2O/TiO<sup>2</sup> , (**c**) Au@TiO<sup>2</sup> /Cu2O, (**d**) Au@TiO<sup>2</sup> /Cu2O@Au, with histogram of Au NPs size distribution of each sample (100 particles are counted).

X-ray diffraction patterns of the samples indicate that TiO<sup>2</sup> exist in the form of anatase phase (JCPDS No. 21-1272). The second phase in the samples with p-n junction can be indexed to cubic Cu2O (Figure S1). The diffraction related to Au phase is not observed in the Au-supported samples because of the low loading of Au (~0.85 wt%) and/or high dispersion of small-sized Au particles (~2.5 nm) on the surface of the support. X-ray photoelectron spectra (XPS) were used to analyze the chemical

**O**

**O<sup>C</sup>**

**V**

**O<sup>L</sup>**

**(c)**

**(C)**

**Intensity (a.u.)**

**(b)**

**(a)**

**90 89 88 87 86 85 84 83 82**

**Binding energy (eV)**

**Au 4f Au**

**0**

**535 534 533 532 531 530 529 528**

**Binding energy (eV)**

**(a)**

**(b)**

**(c) Intensity (a.u.)**

**(d)**

**O 1s**

**(B)**

**466 464 462 460 458 456**

**Ti 4+ Ti 3+**

**Binding energy (eV)**

**(a)**

**(b)**

**(c)**

**(d)**

**Ti 2p (A)**

**Intensity (a.u.)**

valence and electronic structure of each element. Two peaks were fitted at binding energy (B.E.) of 457.9 and 458.3 eV in the Ti 2p3/<sup>2</sup> region of Cu2O/TiO<sup>2</sup> and Au-supported catalysts, corresponding to Ti3<sup>+</sup> and Ti4<sup>+</sup> species, respectively (Figure 2A) [22–24]. The B.E. values hardly shift among these samples. However, the peak area ratio of Ti3<sup>+</sup> to Ti4<sup>+</sup> is lower when Au is deposited on TiO<sup>2</sup> i.e., Au@TiO2/Cu2O and Au@TiO2/Cu2O@Au (0.96 and 1.03), compared to that of Au-free TiO2/Cu2O and Au deposition on Cu2O i.e., Au@Cu2O/TiO<sup>2</sup> (1.17 and 1.11) (Table 1). This indicates that the site-specific deposition of Au differs at defect-rich TiO<sup>2</sup> and Cu2O, leading to a variation to Ti3<sup>+</sup> content when Au is deposited to TiO2. In our previous studies, the ratio of Ti3+:Ti4<sup>+</sup> increased as the amount of NaBH<sup>4</sup> increased. When the ratio reaches around 1, even when increasing the amount of NaBH4, the ratio will not increase any more, which suggests that it is approaching saturation of Ti3+:Ti4<sup>+</sup> ratio [25].

**Figure 2.** XPS spectra of (**A**) Ti 2p, (**B**) O 1s: (**a**) Cu2O/TiO<sup>2</sup> , (**b**) Au@TiO<sup>2</sup> /Cu2O, (**c**) Au@TiO<sup>2</sup> /Cu2O@Au, (**d**) Au@Cu2O/TiO<sup>2</sup> , and (**C**) Au 4f core level spectra: (**a**) Au@TiO<sup>2</sup> /Cu2O, (**b**) Au@TiO<sup>2</sup> /Cu2O@Au, (**c**) Au@Cu2O/TiO<sup>2</sup> .


**Table 1.** Ti 2p XPS analyses of the samples.

<sup>1</sup> The value refers to the ratio of the respective integral peak area.

The O 1s spectra were deconvoluted to three peaks at 530.0 eV, 532.0 eV and 533.5 eV (Table 2), respectively, which are assigned to lattice oxygen (OL), adsorbed oxygen adjacent to surface oxygen vacancy (OV) and surface chemisorbed or dissociated oxygen species (OC) (Figure 2B) [25]. The B.E. values of each oxygen species shift little among different samples. Nevertheless, the OV/O<sup>L</sup> ratio associated with the peak area is decreased when Au is deposited on Au@TiO2/Cu2O and Au@TiO2/Cu2O@Au (0.71 and 0.74) compared with that of TiO2/Cu2O (0.86). The OV/O<sup>L</sup> ratio in Au@Cu2O/TiO<sup>2</sup> (0.81) is close to that of TiO2/Cu2O (0.86). This tendency is consistent with that of the Ti3+/Ti4<sup>+</sup> ratio. The amount of oxygen vacancy (OV) decreases with the decreasing Ti3<sup>+</sup> content because the Ti3<sup>+</sup> defects are closely associated with O<sup>V</sup> in defect-rich TiO<sup>2</sup> support [26].


**Table 2.** O 1s XPS analyses of the samples.

<sup>1</sup> The value refers to the ratio of the respective integral peak area.

Au 4f spectra of each Au-supported catalyst were shown and compared in Figure 2C. The main Au 4f peaks are located at 87.8 eV (4f <sup>5</sup>/2) and 84.0 eV (4f <sup>7</sup>/2). The fitted peak appears at B.E. of 84.0 eV in the Au 4f7/<sup>2</sup> region of Au@Cu2O/TiO2, which is assigned to Au<sup>0</sup> species [27]. In contrast, when Au is deposited on both TiO<sup>2</sup> and Cu2O, the B.E. shifts to a lower value of 83.7 eV, with a negative shift of 0.3 eV (Table 3). The negative shift of B.E. hints at the increased electron density of Au<sup>0</sup> species on Au@TiO2/Cu2O@Au compared to that on Au@Cu2O/TiO2. The B.E. continually shifts to a lower value of 83.4 eV when Au is only deposited on TiO2. The more negative shift (−0.6 eV) indicates higher electron density of Au because of strong metal-support interaction (SMSI) between Au nanoparticles and defect-rich TiO<sup>2</sup> in Au@TiO2/Cu2O [28,29]. The negatively charged Au adjacent to Ti3<sup>+</sup> sites associated with oxygen vacancies could be active sites for ethanol reforming [25]. The higher electron density of Au on Au@TiO2/Cu2O facilitates proton reduction to hydrogen during ethanol photoreforming.

**Table 3.** Au 4f XPS analyses of the samples.


<sup>1</sup> The value refers to the shift compared to that of Au@Cu2O/TiO2.

XPS spectra were carried out to determine the chemical states of Cu species in the catalysts. Figure 3A shows the Cu 2p core level spectra. The binding energies at 932.1 eV and 951.9 eV, respectively, in Cu 2p3/<sup>2</sup> and Cu 2p1/<sup>2</sup> region were assigned to Cu<sup>+</sup> species [30]. The binding energies hardly shift among these catalysts. There is no shake-up satellite peak in the Cu 2p spectra. The absence of the satellite peak excludes the existence of Cu2<sup>+</sup> species [31,32]. However, the Cu<sup>+</sup> and Cu<sup>0</sup> species cannot be distinguished in Cu XPS spectra because the binding energies assigned to Cu<sup>+</sup> and Cu<sup>0</sup> are very close to each other. The difference in binding energies is merely around 0.1–0.2 eV [33,34]. Consequently, Cu LMM Auger spectra were analyzed to distinguish between Cu<sup>0</sup> and Cu<sup>+</sup> species (Figure 3B). The spectra were fitted to two peaks at 917.0 eV and 919.0 eV, respectively, which is assigned to Cu<sup>+</sup> and Cu<sup>0</sup> species [35]. The Cu<sup>+</sup> is the dominating species on the basis of Auger spectra. The coexistence of Cu<sup>+</sup> and Cu<sup>0</sup> species in Cu2O nanocubes or nanoparticles is also observed in the previous literatures [36]. There is no shift in the kinetic energies regardless of Au deposition on either Cu2O or TiO2. This is an indication that the chemical states of Cu2O changes little in these catalysts.

,

**Figure 3.** (**A**) Cu 2p core level spectra: (**a**) Cu2O/TiO<sup>2</sup> , (**b**) Au@TiO<sup>2</sup> /Cu2O, (**c**) Au@Cu2O/TiO<sup>2</sup> , (**d**) Au@TiO<sup>2</sup> /Cu2O@Au, and (**B**) Cu LMM Auger spectra: (**a**) Cu2O/TiO<sup>2</sup> , (**b**) Au@TiO<sup>2</sup> /Cu2O, (**c**) Au@Cu2O/TiO<sup>2</sup> , (**d**) Au@TiO<sup>2</sup> /Cu2O@Au.

The electron paramagnetic resonance (EPR) spectra are shown in Figure 4A. An intense EPR signal appears at a g value of 2.003 for Cu2O/TiO2. This signal is usually assigned to surface O-species, formed via the interaction of dioxygen and Ti3<sup>+</sup> defects in TiO<sup>2</sup> [24,37–39]. The EPR signal appears at the same g value for Au@Cu2O/TiO2, Au@TiO2/Cu2O@Au and Au@TiO2/Cu2O. The intensity of the signal in Au-supported catalysts is lower than that in un-supported Cu2O/TiO2. The intensity of O-related EPR signal (g = 2.003) could be associated with the Ti3+/Ti4<sup>+</sup> ratio in TiO2. The lower signal intensity in Au-supported catalysts indicates the smaller Ti3+/Ti4<sup>+</sup> ratio. The Au@TiO2/Cu2O has the smallest Ti3+/Ti4<sup>+</sup> ratio of 0.96 among these samples, showing the lowest signal intensity of g value. This is consistent with the results presented in XPS Ti 2p analyses (Table 1).

The digital photos of the catalysts display that all are in black color (Figure 4B). The UV-Vis absorption spectra were compared in Figure 4C. Compared to the spectrum of TiO2, the Cu2O/ TiO<sup>2</sup> junction shows stronger absorption in the visible light region besides in the UV regime. The Au-supported catalysts possess more intensive absorption throughout the visible light wavelength. The absorption band (~560 nm) associated with Au surface plasmon resonance (SPR) is not clearly observed within the UV-vis absorption spectra of Au-supported catalysts because of the intense visible absorption of Cu2O/TiO<sup>2</sup> itself [40]. The fluorescence emission spectra of TiO<sup>2</sup> and Cu2O/TiO<sup>2</sup> showed a broad band peaked at 423 nm under the excitation of 350 nm (Figure 4D). The latter has lower emission intensity, indicating that the carrier recombination is inhibited due to the existence of Cu2O/TiO<sup>2</sup> junction. After Au is deposited on either TiO<sup>2</sup> or Cu2O, the emission intensity of supported catalysts further decreases. It indicates that forming the interface of Au and semiconducting oxides facilitates the separation of photogenerated electrons and photogenerated holes by charge transfer across the metal-oxide interface [22,41].

**Figure 4.** The EPR spectra (**A**) and digital photos (**B**) of Cu2O/TiO<sup>2</sup> , Au@Cu2O/TiO<sup>2</sup> , Au@TiO<sup>2</sup> / Cu2O@Au, and Au@TiO<sup>2</sup> /Cu2O; UV-Vis absorption spectra (**C**) and Fluorescence emission spectra (**D**) of TiO<sup>2</sup> , Cu2O/TiO<sup>2</sup> , Au@Cu2O/TiO<sup>2</sup> , Au@TiO<sup>2</sup> /Cu2O@Au, and Au@TiO<sup>2</sup> /Cu2O. The excitation wavelength is 350 nm.

− The photocurrent tests were carried out to evaluate the charge separation within the catalysts. The photocurrent−time traces of photoelectrodes were obtained in the photoelectrochemical cell at a bias of 1.23 V vs. RHE under chopped AM 1.5G light illumination (Figure 5). The Au loading on Cu2O/TiO<sup>2</sup> enhances the photocurrent compared to Cu2O/TiO<sup>2</sup> itself, which indicates the improved charge separation across the metal/oxide interface. The Au@TiO<sup>2</sup> electrode shows a higher photocurrent than Au@Cu2O/TiO2. The interface of Au and TiO<sup>2</sup> is more favorable to charge separation than that of Au and Cu2O due to the lower conduction band potential of TiO<sup>2</sup> than that of Cu2O [42]. In addition, the existence of Ti3<sup>+</sup> defects associated with adjacent oxygen vacancies intensifies the interaction of Au and TiO<sup>2</sup> [25]. The Au@TiO2/Cu2O electrode exhibits the highest photocurrent among all electrodes, 2.5 times higher than Au@TiO2. This hints at the contribution of TiO2/Cu2O p-n junction combined with the interface of Au and TiO<sup>2</sup> to charge separation and transfer. The photocurrent of Au@TiO2/Cu2O is also higher than Au@TiO2/Cu2O/Au. This is an indication that the site-specific deposition of Au on TiO<sup>2</sup> leads to charge separation more efficiently under light illumination than the random deposition of Au on both TiO<sup>2</sup> and Cu2O.

− **Figure 5.** Photocurrent-time traces of photoelectrodes at a bias of 1.23 V vs. RHE under chopped AM 1.5G illumination (100 mW·cm−<sup>2</sup> ), back illumination. Solution: 0.1 M phosphate buffer (pH 7).

μ <sup>−</sup> <sup>−</sup> μ <sup>−</sup> <sup>−</sup> μ <sup>−</sup> <sup>−</sup> μ <sup>−</sup> <sup>−</sup> μ <sup>−</sup> <sup>−</sup> μ <sup>−</sup> <sup>−</sup> The photoreforming of ethanol was achieved using pure ethanol in an Ar atmosphere (1.4 bar) under simulated solar irradiation. The H<sup>2</sup> production rate is contrasted on each catalyst (Figure 6A). The rate of H<sup>2</sup> production reaches 6932 µmol gcat <sup>−</sup><sup>1</sup> h <sup>−</sup><sup>1</sup> on Au@Cu2O/TiO2, 8.35 times higher than that on Cu2O/TiO<sup>2</sup> (830 µmol gcat <sup>−</sup><sup>1</sup> h −1 , Table 4). It is apparent that proton reduction is greatly accelerated on the Au-supported catalyst for H<sup>2</sup> production. Additionaly, the charge separation is improved by loading Au on the semiconductor, evidenced by photocurrent measurements (Figure 5). The Au@TiO<sup>2</sup> catalyst exhibits higher activity (7143 µmol gcat <sup>−</sup><sup>1</sup> h −1 ) towards H<sup>2</sup> production than Au@Cu2O/TiO2. This suggests that constructing the interface of Au and TiO<sup>2</sup> is more favorable than that of Au and Cu2O, which could direct the structural design of catalysts for ethanol photoreforming. The rate of H<sup>2</sup> production on Au@TiO2/Cu2O (8548 µmol gcat <sup>−</sup><sup>1</sup> h −1 ) is apparently higher than that on either Au@TiO<sup>2</sup> (7143 µmol gcat <sup>−</sup><sup>1</sup> h −1 ) or Au@TiO2/Cu2O@Au (7348 µmol gcat <sup>−</sup><sup>1</sup> h −1 ). Table S4 compares the H<sup>2</sup> yield rates reported for TiO2-supported noble-metal catalysts with the result in this work. The p-n junction between TiO<sup>2</sup> and Cu2O contributes the enhanced H<sup>2</sup> production. However, the selective deposition of Au on defect-rich TiO<sup>2</sup> is essential for higher activity because the Au-Ov-Ti3<sup>+</sup> sites are more active in ethanol photoreforming [25]. The quantity of Au-Ov-Ti3<sup>+</sup> sites is relatively less in randomly deposited Au@TiO2/Cu2O@Au than in site-specifically deposited Au@TiO2/Cu2O. As a consequence, the latter shows higher activity than the former. In addition, the lower H<sup>2</sup> production rate of Au@Cu2O/TiO2may be due to the alloying of Au-Cu, leading to a smaller number of active sites on gold according to the previous report [43].


**Table 4.** H<sup>2</sup> production via photoreforming of ethanol on the catalysts under simulated solar light.

<sup>1</sup> Suspension of the photocatalyst (1 g/L) in ethanol under stirring were irradiated with simulated solar light (100 mW cm−<sup>2</sup> ) at Ar atmosphere (1.4 bar) at 25 ◦C for 6 h.

−

μ <sup>−</sup> <sup>−</sup>

**0**

**5**

**10**

**15**

**Current density (μA/mg)**

**20**

**25**

**30**

**Au@TiO<sup>2</sup>**

**Au@TiO<sup>2</sup>**

**Au@TiO<sup>2</sup> Au@Cu<sup>2</sup>**

**Cu<sup>2</sup> O/TiO<sup>2</sup>**

**/Cu<sup>2</sup> O**

**O/TiO<sup>2</sup>**

**/Cu2O@Au**

μ <sup>−</sup> <sup>−</sup>

**0 50 100 150 200**

**Time (s)**

μ <sup>−</sup> <sup>−</sup>

μ <sup>−</sup> <sup>−</sup>

μ <sup>−</sup> <sup>−</sup> μ <sup>−</sup> <sup>−</sup>

**Figure 6.** (**A**) H<sup>2</sup> and acetaldehyde production rate via the photoreforming of ethanol under simulated solar light (AM 1.5G). (**B**) The cycle tests of Au@TiO<sup>2</sup> /Cu2O for photocatalytic hydrogen evolution. Conditions: suspension of the photocatalyst (1 g/L) in ethanol under stirring were irradiated with simulated solar light at Ar atmosphere (1.4 bar) for 6 h.

The products except H<sup>2</sup> in the gas phase, e.g., CH4, CO, CO2, are below 0.3% on all catalysts (Table 4), which clearly indicates the efficient inhibition of C–C cleavage during the photoreforming. Also, high-purity H<sup>2</sup> can be produced on these catalysts. For instance, the purity of H<sup>2</sup> is beyond 99.9% on Au@TiO2/Cu2O catalyst. Acetaldehyde is the only detectable product in the liquid phase, evidenced by GC-MS analysis (Figure S2). The high selectivity of acetaldehyde is due to the absence of O<sup>2</sup> activation to peroxide intermediates and hydroxyl species on the surface of Au NPs under Ar ambient [43,44]. In addition, acetaldehyde is produced in almost stoichiometric yield with H2, which is consistent with the reforming pathway of ethanol via a redox process [11]. The photo-generated holes in the valence band of semiconductor oxidize ethanol to acetaldehyde and the electrons on the active sites adjacent to Au reduce protons to H<sup>2</sup> during the process.

The optimal Au@TiO2/Cu2O catalyst was recycled five times and the H<sup>2</sup> yield was hardly decreased (Figure 6B). The structural stability of catalysts was further evidenced by HRTEM and Ti 2p, O 1s, Au 4f XPS and Cu LMM Auger analyses. The size of Au particles remains 2.5 nm, the same as that before the reactions (Figure S3). In addition, the Au loading after reaction is not changed by ICP measurement (0.84 wt%). The Ti3+/Ti4<sup>+</sup> ratio and OV/O<sup>L</sup> ratio in the catalyst have no change before and after reactions (Figure S4, Tables S1 and S2). The peak in the Au 4f region appears at 83.5 eV and has little shift compared to that before reactions (Figure S4, Table S3). Additionally, the peaks in Cu 2p XPS and Cu LMM Auger spectra keep unchanged after photoreforming (Figure S5). These findings verify that the catalyst is structurally stable and keeps activity after repeated use.

The in situ FTIR spectra of ethanol adsorption on the catalysts were first collected in the dark after adsorption of ethanol for 40 min, and then recorded every hour during photoreforming. The spectra of the three supported catalysts showed intensive absorption bands at 1000~1200 cm−<sup>1</sup> , 1200~1500 cm−<sup>1</sup> , 2700~3100 cm−<sup>1</sup> , and 3600~3800 cm−<sup>1</sup> , which are respectively assigned to the vibrations of ν(C–O), δ(CHx), ν(CHx), and ν(O–H) of ethanol (Figure 7) [45].

**Figure 7.** In situ FTIR spectra of ethanol adsorption on the catalysts during the photoreforming: (**A**) Au@Cu2O/TiO<sup>2</sup> , (**B**) Au@TiO<sup>2</sup> /Cu2O, (**C**) Au@TiO<sup>2</sup> /Cu2O@Au.

− The band at 1755 cm−<sup>1</sup> gradually increases with the extension of irradiation time, which corresponds to the vibration related with acetaldehyde molecule [46]. This is consistent with the increased acetaldehyde production in liquid phase with the reaction. The band shifts little

−

among the three catalysts. The band at around 2350 cm−<sup>1</sup> , assigned to the vibration of CO<sup>2</sup> [47], does not appear in the FTIR spectra. This verifies that the C–C cleavage of ethanol is greatly inhibited on the three Au-supported catalysts during the photoreforming. Consequently, the dehydrogenation of ethanol to acetaldehyde is the dominating step, accompanied with high-purity H<sup>2</sup> production on these catalysts.

The energy levels of semiconductors play a critical role in the activity and selectivity of ethanol photoreforming. In a catalyst with a p-n junction, the charge separation and transfer are strongly dependent on the relationships of energy levels [48]. The flat band potentials (Efb) of the TiO<sup>2</sup> nanorods and Cu2O nanocubes were determined by electrochemical Mott-Schottky measurements at varied frequencies and room temperature. The Mott-Schottky plots of TiO<sup>2</sup> exhibit positive slopes, indicative of their characteristic of n-type semiconductor (Figure 8A). The Efb values are estimated by extrapolating the linear portion of the plots measured at varied frequencies to the intercept of *x*-axis. It is generally considered that the bottom of the conduction band (CB) in n-type semiconductors is approximately equal to its flat band potential. [49,50] Consequently, the conduction band of TiO<sup>2</sup> nanorod is −0.63 V vs. Ag/AgCl. This potential was converted to −0.02 V vs. RHE according to the following formula [51]. − −

$$\rm{E\_{RHE}} = \rm{E\_{Ag/AgCl}} + 0.197 + 0.059pH\_{\prime} \tag{1}$$

The bandgap of TiO<sup>2</sup> nanorod was estimated to be 2.76 eV using the Kubelka−Munk equation, i.e., F(R) = (1 − R)<sup>2</sup> /2R, where R is the reflectance (Figure S6). This value is smaller than that reported for bulk anatase TiO<sup>2</sup> of ~3.2 eV [52]. It is documented that theTi3<sup>+</sup> defect-rich anatase TiO<sup>2</sup> exhibits narrower bandgap owing to the involvement of defect energy levels [53]. The valence band TiO<sup>2</sup> nanorod is calculated to be 2.74 eV from the bandgap and the conduction band level. − −

The Mott-Schottky plots of Cu2O show negative slope, an indication of p-type semiconductor (Figure 8B). The flat band potential of 0.65 V vs. Ag/AgCl was converted to 1.26 V vs. RHE according to Formula (1). It is usually considered that the bottom of valence band (VB) in p-type semiconductors was 0.30 V more positive than the flat band potential [54,55]. The valence band level of Cu2O is 1.56 V vs. RHE. Based on the bandgap of Cu2O (2.2 eV) [56,57], the conduction band (CB) level of Cu2O is estimated to be −0.64 V. −

**Figure 8.** Mott-Schoottky plots of (**A**) TiO<sup>2</sup> nanorods and (**B**) Cu2O nanocubes measured at varied frequencies and room temperature. Solution: 0.1 M phosphate buffer (pH 7).

Based on the findings above, we proposed possible charge transfer pathways on the Au@TiO2/Cu2O catalyst during ethanol photoreforming. The p-n junction is formed when p-type Cu2O and n-typeTiO<sup>2</sup> come into contact with each other (Scheme 2). The photo-generated holes are transferred from the valence band of TiO<sup>2</sup> to that of Cu2O owing to the more positive valence band level of TiO2. The electrons generated on Cu2O under visible light irradiation are transferred to the conduction band of TiO2. In addition, the hot electrons of Au under visible light irradiation are transferred to the conduction band of TiO<sup>2</sup> due to the SPR effect of Au. The strong interaction between Au and defect-rich TiO<sup>2</sup> enhances the hot electron transfer from Au to the interface of Au/TiO2. As a consequence, the interface of Au and TiO<sup>2</sup> is rich in the electrons, leading to high electron density on the Au-Ov-Ti3<sup>+</sup> interfacial sites. The proton reduction is promoted on the electron-rich interfacial sites, resulting in higher H<sup>2</sup> production rate than the Au@Cu2O/TiO<sup>2</sup> catalyst without the interfacial sites. Consequently, the dual effects of Au-Ov-Ti3<sup>+</sup> active sites and p-n junction endow Au@TiO2/Cu2O catalyst with higher activity towards ethanol photoreforming by the site-specific deposition of Au on defect-rich TiO<sup>2</sup> nanorods.

**Scheme 2.** Diagram of energy levels and charge transfer on Au@TiO<sup>2</sup> /Cu2O catalyst.

#### **3. Materials and Methods**

#### *3.1. Materials*

C2H5OH (anhydrous ethanol), CH3OH (methanol), CuSO<sup>4</sup> (anhydrous copper sulfate), PVP (polyvinyl pyrrolidone), TBOT (tetrabutyltitanate), HNO<sup>3</sup> (nitric acid), NaBH<sup>4</sup> (sodium borohydride), AA (ascorbic acid), HAuCl4·3H2O, EDTA (Ethylenediamine) were obtained from Shanghai Aladdin Biochemical Technology Co. Ltd. (Beijing, China). H<sup>2</sup> (99.9%), N<sup>2</sup> (99.9%) and Ar (99.9%) were supplied by Beijing Haipu Gas Co. Ltd. (Beijing, China). All reagents were of analytical grade and used as received. Deionized water was used throughout the experiments.

#### *3.2. Synthesis of Defect-Rich TiO<sup>2</sup> Nanorods*

The defect-rich TiO<sup>2</sup> nanorods were synthesized by a NaBH<sup>4</sup> reduction method according to a previous literature [22]. In a typical procedure, the mixture (TiO2/NaBH<sup>4</sup> = 2:1 molar ratio) was continuously stirred for 30 min and treated at 350 ◦C for 1 h in a flow of N2. The sample was washed with diluted HCl solution and water thoroughly. The sample was named as TiO2.

#### *3.3. Synthesis of Cu2O Nanocubes*

The Cu2O nanocubes were prepared by a chemical precipitation method, modified from a literature's one [58]. In a typical run, 155 mg of CuSO<sup>4</sup> and 400 mg of PVP were dissolved in 160 mL of deionized water to obtain a light blue solution. The pH was adjusted to 10 by dropwise addition of NaOH (1 M). An amount of 176 mg of ascorbic acid (AA) was dispersed in the solution to reduce Cu2<sup>+</sup> to Cu2O. The suspension was stirred for 20 min. Subsequently, the suspension was centrifuged and washed thoroughly with deionized water and ethanol to remove PVP. The product was collected after the color of solution changed from light blue to brownish yellow. The solid product was finally dried under vacuum at 60 ◦C.

#### *3.4. Synthesis of Cu2O*/*TiO<sup>2</sup>*

2 g of TiO<sup>2</sup> nanorod sample was added into the CuSO<sup>4</sup> and PVP solution, and the pH of the solution was adjusted to 10 with an aqueous solution of NaOH (1 M). Subsequently, AA was added to the solution and stirred for 20 min. The solid product was collected by centrifugation and washed with ethanol and water. The product was finally dried under vacuum at 60 ◦C.

#### *3.5. Synthesis of Au@TiO<sup>2</sup> and Au@TiO2*/*Cu2O*

2 g of TiO<sup>2</sup> nanorod sample was dispersed in 200 mL of H2O containing 2 mL of HAuCl<sup>4</sup> solution (0.051 M). The pH of suspension was adjusted to 10 with an aqueous solution of NaOH (1 M). 10 mL of methanol was added and stirred for 3 h. The Au@TiO<sup>2</sup> sample was obtained by a photo-reduction method under UV irradiation. After the photo-reduction, the precipitation was centrifuged, washed with water thoroughly to neutral and dried in vacuum at 60 ◦C. The Au@TiO<sup>2</sup> product was suspended in 160 mL of aqueous solution containing 155 mg of CuSO<sup>4</sup> and 400 mg of PVP. The pH was adjusted to 10 with an aqueous solution of NaOH (1 M). 176 mg of AA was added and stirred for 20 min. The solid product was collected by centrifugation and washed with by ethanol and water and finally dried under vacuum at 60 ◦C. The product was denoted as Au@TiO2/Cu2O.

#### *3.6. Synthesis of Au@Cu2O*/*TiO<sup>2</sup>*

The as-synthesized Cu2O and PVP was suspended in 100 mL of H2O. 2 mL of HAuCl<sup>4</sup> solution (0.051 M) was added and the pH of suspension was adjusted to 10 with an aqueous solution of NaOH (1 M). 10 mL of methanol was added and stirred for 3 h. The Au@Cu2O sample was obtained by a photo-reduction method under visible light irradiation. The as-synthesized TiO<sup>2</sup> nanorod was suspended in the solution and stirred for 1 h. The solid product was collected by centrifugation and washed with by ethanol and water and finally dried under vacuum at 60 ◦C. The product was denoted as Au@Cu2O/TiO2.

#### *3.7. Synthesis of Au@TiO2*/*Cu2O@Au*

2 g Cu2O/TiO<sup>2</sup> sample was suspended in 200 mL of H2O containing 2 mL of HAuCl<sup>4</sup> solution (0.051 M). The pH of suspension was adjusted to 10 with an aqueous solution of NaOH (1 M). 10 mL of methanol was added and stirred for 3 h. The Au@TiO2/Cu2O@Au sample was obtained by a photo-reduction method under UV-visible light irradiation. The solid product was collected by centrifugation and washed with ethanol and water and finally dried under vacuum at 60 ◦C.

#### *3.8. Characterization*

Powder X-ray diffraction (XRD) patterns of the samples were obtained by a Shimadzu XRD-6000 diffractormeter using graphite-filtered CuKα radiation (40 kV, 30 mA, λ = 0.15418 nm) in a 2theta range of 10−70◦ .

Transmission electron microscopy (TEM) measurements were carried out on a JEOL JEM-3010 high-resolution transmission electron microscope.

Elemental analysis was performed on a Shimadzu ICPS-7500 inductively coupled plasma atomic emission spectrometer (ICP-AES).

The XPS spectra were recorded on a Thermo VG ESCALAB MK II X-ray photoelectron spectrometer at a pressure of 2 × 10−<sup>9</sup> Pa using Al Kα X-ray as the excitation source (1486.6 eV). The positions of all binding energies were calibrated using theC1s line at 284.6 eV.

UV-visible diffuse reflectance spectra were performed on a Shimadzu UV-3000 spectrometer equipped with an integrating sphere attachment with BaSO<sup>4</sup> (10 mg) as reference.

*Catalysts* **2020**, *10*, 539

Photoluminescence (PL) emission spectra were recorded by a Hitachi F-7000 spectrofluorometer using laser excitation at a wavelength of 350 nm.

Electron paramagnetic resonance (EPR) measurements were performed on a Bruker E500 spectrometer with a 9.53 GHz X-band. The sample mass was 50 mg. The spectra were recorded in the magnet field range of 318–328 mT.

The liquid phase products were determined by GC-MS measurements using a Thermo Fisher ISQ Trace1300 and the sample volume was 1 µL.

Electrochemical impedance spectroscopy (EIS) was performed on an electrochemical workstation at an open-circuit voltage of 0.3 V vs. RHE under illumination with 10 mV amplitude of perturbation and a frequency between 1.0 kHz, 1.5 kHz and 2.0 kHz. Mott-Schottky plots were measured at room temperature in the dark.

In situ Fourier Transform Infrared Spectroscopy (FTIR) was conducted in an in situ reaction cell on a Bruker Tensor II spectrometer installed with MCT narrow-band detector. The sample was pretreated in a flow of high-purity N<sup>2</sup> at 100 ◦C for 1 h. After an initial scan as the background spectrum, ethanol was induced into the cell through a flow of N<sup>2</sup> for 30 min. After flowing N<sup>2</sup> to remove the residual ethanol vapor, the FTIR spectra were collected in the range of 4000~950 cm−<sup>1</sup> at room temperature.

#### *3.9. Photoreforming of Ethanol*

In a typical run, 25 mg of catalyst was added to 25 mL of ethanol and ultrasonically dispersed for 15 min. The suspension was transferred into a high-pressure stainless steel reactor (volume: 50 mL) equipped with a sapphire crystal window. A flow of Ar was purged into the reactor for 30 min and the reactor was evacuated. The Ar was re-purged and the pressure in the reactor was maintained at 1.4 bar. The suspension was irradiated by a 300 W Xenon lamp equipped with an AM 1.5G filter (100 mW·cm−<sup>2</sup> ) under magnetic stirring for 6 h. After the reaction, the catalyst was removed from the solution by filtration. The gaseous products were detected by an online gas chromatograph (GC-2014C, Shimadzu, Japan) equipped with a high-sensitivity thermal conductivity detector (TCD) and Ar was used as the carrier gas. The liquid products were analyzed by a gas chromatograph (GC-2014C, Shimadzu) equipped with a flame ionization detector (FID). The reaction rate of H<sup>2</sup> followed the formula below:

$$\text{Reaction rate of production} = \frac{\text{n}\_{\text{production}}(\text{\mu mol})}{\text{m}\_{\text{cat}}(\text{g}) \times \text{time} \,(\text{h})} \tag{2}$$

#### *3.10. Recycled Use*

The catalyst was separated from the solution after the reaction, washed with deionized water, and finally dried under vacuum at 60 ◦C for 6 h. The dried catalyst was reused in a next catalysis run under the same reaction conditions.

#### **4. Conclusions**

The TiO2/Cu2O-supported Au catalysts were delicately constructed by the site-specific deposition of Au on either defect-rich TiO<sup>2</sup> nanorods or Cu2O nanocubes. The selective anchoring of Au nanoparticles on TiO<sup>2</sup> nanorods combined with the p-n junction of TiO2/Cu2O leads to the highest activity towards ethanol photoreforming. The H<sup>2</sup> production rate reaches a record level of 8548 µmol gcat <sup>−</sup><sup>1</sup> h <sup>−</sup><sup>1</sup> under simulated solar light. The acetaldehyde in liquid phase was generated at an almost stoichiometric rate, which indicates the effective inhibition of C−C cleavage of ethanol to CH<sup>4</sup> or CO2. Extensive spectroscopic studies verified that Au species adjacent to Ti3<sup>+</sup> defects and the associated oxygen vacancies on TiO<sup>2</sup> nanorods activate the proton reduction to H2. The p-n junction between TiO<sup>2</sup> and Cu2O facilitates charge separation and transfer owing to the matching of energy levels, which accelerates photo-generated hole transfer and the dehydrogenation of ethanol to acetaldehyde on the junction. This site-specific deposition strategy could be applicable in the enhancement in

other biomass hydrogen production by rationally designing the delicate structure of catalysts and maximizing the catalytic capability.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/10/5/539/s1, Figure S1: XRD patterns; Figure S2: GC-MS spectra; Figure S3: HRTEM photographs; Figure S4: Ti 2p, O 1s and Au 4f XPS spectra; Figure S5: Cu 2p XPS and Cu LMM Auger spectra; Figure S6: Bandgap estimation; Table S1: Ti 2p XPS analyses; Table S2: O 1s XPS analyses; Table S3: Au 4f XPS analyses; Table S4: Comparisons of photocatalysts for ethanol photoreforming.

**Author Contributions:** Conceptualization, X.X.; methodology, T.Z. and X.Z.; formal analysis, L.L., T.Z., X.X.; investigation, X.Z., L.L.; data curation, L.L., T.Z., R.Y.; writing—original draft preparation, L.L., T.Z., X.X.; writing—review and editing, T.Z., R.Y., Y.L., B.Z., X.X.; supervision, X.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (grant 21978021, 21521005), the National Key R&D Program of China (grant 2017YFA0206804), and the Fundamental Research Funds for the Central Universities.

**Conflicts of Interest:** The authors declare no competing financial interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Article*

## **A 3D Hierarchical Pancake-Like Porous Carbon Nitride for Highly Enhanced Visible-Light Photocatalytic H2 Evolution**

#### **Xiaobin Qiu, Lingfang Qiu \*, Mengfan Ma, Yingying Hou and Shuwang Duo \***

Jiangxi Key Laboratory of urfacSe Engineering, Jiangxi Science and Technology Normal University, Nanchang 330013, China; xbluyin@126.com (X.Q.); MF1783975@126.com (M.M.); hyy18170401838@163.com (Y.H.)

**\*** Correspondence: qlf1108@163.com (L.Q.); swduo@imr.ac.cn (S.D.)

Received: 26 November 2019; Accepted: 2 January 2020; Published: 4 January 2020

**Abstract:** Polymeric carbon nitride is a fascinating visible-light-response metal-free semiconductor photocatalyst in recent decades. Nevertheless, the photocatalytic H<sup>2</sup> efficiency is unsatisfactory due to the insufficient visible-light harvesting capacity and low quantum yields caused by the bulky structure seriously limited its applications. To overcome these defects, in this research, a 3D hierarchical pancake-like porous carbon nitride (PPCN) was successfully fabricated by a facile bottom-up method. The as-prepared photocatalyst exhibit enlarged surface area, enriched reactive sites, improved charge carrier transformation and separation efficiency, and expanded bandgap with a more negative conduction band towardan enhanced reduction ability. All these features synergistically enhanced the photocatalytic H<sup>2</sup> evolution efficiency of 3% Pt@PPCN (430 <sup>µ</sup>mol g−<sup>1</sup> <sup>h</sup> −1 ) under the visible light illumination (λ ≥ 420 nm), which was nine-fold higher than that of 3% Pt@bulk <sup>C</sup>3N<sup>4</sup> (BCN) (45 <sup>µ</sup>mol g−<sup>1</sup> <sup>h</sup> −1 ). The improved structure and enhanced photoelectric properties were systematically investigated by different characterization techniques. This research may provide an insightful synthesis strategy for polymeric carbon nitride with excellent light-harvesting capacity and enhanced separation of charges toward remarkable photocatalytic H<sup>2</sup> for water splitting.

**Keywords:** pancake-like porous carbon nitride; bottom-up method; water splitting; visible light photocatalyst

#### **1. Introduction**

With fossil fuel reserves dwindling every day, energy shortage and environmental pollution issues have become increasingly prominent. Developing clean and renewable energy sources is urgent to meet the sustained growth in energy demand [1–6]. Since the discovery of the Honda–Fujishima effect in 1972 [7], photocatalytic hydrogen evolution reaction (HER) through water splitting has gained considerable attention due to its enormous potential to convert renewable solar energy to clean sustainable energy [8–12]. Researchers have devoted a significant amount of effort to investigate more effective photocatalytic systems for hydrogen evolution via photocatalytic water splitting [13–16]. Among multitudinous proposed semiconductor photocatalysts, polymeric carbon nitride (CN) has been considered promising for visible-light driven hydrogen evolution due to its tunable bandgap, low cost, earth-abundant nature, nontoxicity, facile preparation, being environmentally benign, and its remarkable physical and chemical stability [17–22]. Unfortunately, the photocatalytic performance of bulky CN was severely limited, resulting from the small specific surface area, large recombination rates of photo-excited electron-hole pairs, weak light-harvesting capacity, and inadequate catalytic active sites [23–28]. To address these shortcomings, many strategies have been developed, such as heteroatom

doping [29,30], electronic structure modulation [31,32], nano-structuring [33,34], dye sensitization [35–37], and heterojunction construction [38–41]. Among them, nano-structuring is considered one of the most efficient and accessible strategies, as the enlarged specific surface area simultaneously abundant and quality active sites are not only favorable to the transfer of photo-excited charge carriers but also shortened the diffusion length for both ions and electrons [42,43]. On the other hand, the nano-structuring designed materials commonly exhibit the quantum confinement effect [24] with an enlarged electronic bandgap would be favorable to an enhanced redox ability toward photocatalytic reaction, promoting the quality and quantity of photo-induced charge carriers to some extent.

The nanostructural CN can be achieved by many methods. template-assistance strategies, top-down methods, and hydrothermal approaches have been regarded as representative and impelling strategies to fabricate multidimensional CN [44,45]. Nevertheless, these methods have many unsatisfactory deficiencies, such as a shortage of template species, large time consumption, and being environmentally unfriendly, resulting from the introduction of hazardous reagents, challenging the removal of the byproducts [46,47]. In contrast, the highly efficient photocatalysis materials benefited from the improved structure and accelerated the separation efficiency of photo-induced charge carriers, which is relatively convenient to obtain by a facile bottom-up method [48,49].

Porous carbon nitride as a photocatalyst was introduced by Kang et al. Nevertheless, the material showed undesirable photocatalytic performance with a poor quantum efficiency of ~0.5%. Rahman et al. improved it to some extent, which reached a benchmark quantum efficiency of ~6% [50,51]. The synthesized photocatalyst exhibited similar structure compared to that synthesized by Rahmanetal. While the PPCN presented a higher quantum efficiency of ~19%.In this work, we report a low-cost and straightforward bottom-up method for the production of hierarchical pancake-like porous carbon nitride, the typical synthesis procedure is presented in Scheme 1. Typically, the cyanuric acid–melamine complex, which is suitable for controlling molecular self-assembly [52], was firstly synthesized. Then, ethanol and glycerol intercalated into the framework of the precursor. Next, a thermal-induced exfoliation and polycondensation process was performed to obtain the resultant products. The as-synthesized PPCN exhibited unique hierarchical pancake-like porous morphology structure and remarkable photoelectric properties. As expected, by virtue of an enlarged specific surface area up to 79.663 m2g −1 , enhancing the light-harvesting capacity, and enriching the reactive sites, the hierarchical porous architecture revealed a highly efficient photocatalytic H<sup>2</sup> evolution efficiency for water splitting under visible light irradiation (λ ≥ 420 nm), which is about nine-fold higher than that of the bulk carbon nitride (BCN). Various characterization approaches were performed to investigate the typical structure formation mechanism and the photoelectric properties of PPCN for the remarkably enhanced hydrogen evolution efficiency. − λ ≥

**Scheme 1.** Schematic illustration of hierarchical pancake-like porous C3N<sup>4</sup> formation.

#### **2. Results and Discussion**

#### *2.1. Morphology and Structure Characterization*

The morphologies of BCN and PPCN were investigated via SEM and TEM. It can be seen that BCN appears as a large solid block material (Figure 1a) with seriously agglomerated structure. The light absorption, charge transport, and mass immigration were severely restrained owing to this bulky structure. In contrast, a 3D hierarchical pancake-like porous carbon nitride (Figure 1b–d) exhibited the opposite morphology with an enlarged dimension and abundant pores, which is greatly favorable for the dispersion and diffraction of the light toward the enhanced light absorption capacity. Meanwhile, TEM images (Figure 1e–g) displayed the mesoporous and lattice distance (Figure 1h) of 0.348 nm.

**Figure 1.** (**a**) SEM image of BCN, (**b**–**d**) SEM images of PPCN, (**e**–**g**) TEM images, and (**h**) lattice distance pattern of PPCN.

The structure can be exfoliated into hierarchical porous architecture and generated abundant pores using a suitable hybrid of alcohol and glycerol as the imbedding reagents which supporting abundant gas at the polycondensation process, leading to a hierarchical pancake-like porous architectural carbon nitride formation. N<sup>2</sup> adsorption/desorption isotherms (Figure 2a) was performed to prove this unique structure. The calculated BET specific surface area of PPCN is 79.663 m2g −1 , which is seven-fold

− −

higher than that BCN (10.643 m2g −1 ). Meanwhile, the peak intensity centered at 37.2 nm in BJH pore distribution curves of PPCN sample (Figure 2b) also exhibited a larger value than BCN implying an abundant nanoporosity.

**Figure 2.** (**a**) N<sup>2</sup> adsorption-desorption isotherm curves and (**b**) BJH pore size distribution curves of BCN and PPCN.

− − − − The XRD patterns of PPCN is shown in Figure 3a. Compared to the BCN, PPCN presented a broader and weaker diffraction peak located at 27.2◦ (002), which is originated from the stacking reflection of interlayer toward a reduced dimension structure [53]. Meanwhile, the peak at 12.9◦ , ascribed to the repeats in the plane of tri-s-triazine unit could hardly been observed owing to the abundant pores in the interface. Fouriertransform infrared (FT-IR) spectroscopy (Figure 3b) demonstrates that the PPCN possessing a similar chemical structure with BCN. The 810 cm−<sup>1</sup> peaks are characteristic signal of triazine unit, and the peaks ranges from 1200 to 1600 cm−<sup>1</sup> is corresponding to the C-N heterocycles. In addition, the broad band peaks located at 3000–3600 cm−<sup>1</sup> were contributed to-NH stretching vibrations [54]. It is notably that the terminal-NH absorption peak at 3000–3600 cm−<sup>1</sup> of PPCN vibrate more strongly compared to that of BCN, which can be attributed to the difference in the crystal structural between BCN and PPCN.

X-ray photoelectron spectroscopy (XPS) was further performed to investigate the composition and chemical states of BCN and PPCN. As can be seen in Figure 3c, the presence peaks of C, N, and O are found both in the PPCN and BCN. It should be noted that the higher intensity of O 1s signal of PPCN is easy for the absorption of water and CO<sup>2</sup> due to the hierarchical porous architecture structure. The corresponding high-resolution spectra of C 1s, N 1s, O1s are shown in Figure 3d–f, respectively. The C 1s XPS spectrum revealed three peaks, which can be ascribed to the adventitious carbon species (284.4 eV) [55], C–NH<sup>X</sup> species on the edges of aromatic rings (286.03 eV) [56], and the sp<sup>2</sup> -hybridized C atom in N==C(–N)<sup>2</sup> (288.0 eV) [57], which is consistent with that of BCN. The intensity of the sp 2 -bonded carbon of PPCN was increased compared to the BCN may be attributed to the loss of lattice nitrogen. The N 1s XPS signals can be deconvolved into three peaks at398.46, 399.75, and 400.93 eV, which originated from the sp<sup>2</sup> -hybridized N attached to C–N=C groups, bridging N in N–(C)<sup>3</sup> groups, and the amine functional groups (C–N–H), respectively [58,59]. Additionally, the O 1s spectra can be divided into two peaks located at 532.68 and 530.71 eV, which belongs to the adsorbed water and CO<sup>2</sup> [60].

**Figure 3.** (**a**) XRD and (**b**) FT-IR spectra of BCN and PPCN, Survey and high-resolution XPS spectra of BCN and PPCN, (**c**) survey, (**d**) C 1s, (**e**) N 1s, and (**f**) O 1s of BCN and PPCN.

#### *2.2. Photoelectric Property*

UV–VIS diffuse reflectance (DRS) was used to assess the optical absorption properties of BCN and PPCN. Figure 4a indicates that BCN has an absorption edge at 460 nm. Compared with BCN, the PPCN edge shows apparent blue shift, indicating the increase in band gap of semiconductors caused by the quantum confinement effect. The bandgaps for PPCN and BCN were 2.97 and 2.80 eV according to the Kubelka–Munk function [61] (Figure 4b). Consequently, the enlarged bandgap of PPCN would endow the photo-induced electrons and holes with relatively stronger redox abilities toward photocatalytic reactions. The energy band structure of materials can be determined by the conduction band or valence band together with the band gap. In this article, we use the valence band to estimate the conduction band position [62]. The valence band (VB) in Figure 4c of BCN and PPCN was obtained by the XPS valence band spectra. From the band gap date in Figure 4b, the valence band potentials of PPCN and BCN were 1.98 and 2.30 eV vs. NHE, respectively. Finally, the energy band structure was depicted in Figure 4d. Obviously, the more negative conduction band levels of PPCN would contribute to a more powerful e– reduction ability owing to the more negative CB.

**Figure 4.** (**a**) UV–VIS DRS spectra and (**b**) Kubelka-Munk transformed function of BCN and PPCN, (**c**) valence band XPS spectra, (**d**) energy band structure of BCN and PPCN.

The photoluminescence tests were further performed with a 325 nm excitation wavelength to assess the separation efficiency of photo-induced electron-hole pairs. As shown in Figure 5a, PPCN presented a lower emission intensity compared with BCN, which indicated of reduced recombination of photo-induced electron-hole pairs in PPCN. It is apparent from this image that the absorption peak of PPCN shifted to a lower value resulted from the quantum size confinement, which is consistent with the DRS.

τ τ The time-resolved PL decay spectra were also performed to evaluate the photo-physical characteristics of photogenerated charge carriers of BCN and PPCN. As shown in Figure 5b, the PPCN exhibited a slower exponential decay and fluorescence than that of BCN. τ<sup>1</sup> is originated from the free excitons recombination in semiconductor and τ<sup>2</sup> is ascribed to the non-illuminated recombination of charge carriers in the surface defect states [63]. The short-lived and long-lived lifetime of charge carriers in PPCN (1.743 and 7.595 ns) are both higher than that BCN (1.211 and 5.733 ns). Mostly, the calculated average lifetime in PPCN was 5.089 ns which substantially exceeds 3.696 ns in BCN. The prolonged carriers lifetime and larger percentage of long lifetime charge carriers in PPCN are undoubtedly favorable for promoting the probability of charge carries to involve in photocatalytic reactions. The photocurrent response can also prove this similar conclusion [64]. As depicted in Figure 5c, the PPCN exhibited increased photocurrent intensity than BCN during six light on–off recycles, demonstrating a more efficient separation and transfer of photo-induced electrons. Furthermore, PPCN exhibited a smaller charge transfer resistance (Figure 5d) than that of BCN. Therefore, the hierarchical pancake-like porous architectural structure of PPCN is benefit to the highly efficient separation and migration of photo-induced electron-hole pairs.

**Figure 5.** (**a**) Fluorescence emission spectra and (**b**) time-resolved fluorescence excited by incident light of 365 nm of BCN and PPCN, (**c**) photocurrent, and (**d**) EIS spectra of BCN and PPCN.

#### *2.3. Photocatalytic Performance*

λ ≥ − − − − Though photocatalyst based on carbon nitride without precious Pt is a trend in recent years [65–67], platinum plays a remarkable role in photocatalytic hydrogen production due to the low overpotential and the optimal Gibbs free energy for proton [68,69]. Figure 6a presented the HER of PPCN with 3%Pt loading under visible light (λ ≥420 nm) using 15 vol% TEOA as electron donor. As shown in Figure 6a, 3%Pt@PPCN had an HER of 430 µmol g−<sup>1</sup> h −1 , which was nine-fold higher than that of 3%Pt@BCN (45 µmol g−<sup>1</sup> h −1 ). The apparent quantum yield (AQE) for the Pt@PPCN reached as high as 19%at 420 nm (see the calculation in Supplementary Materials). Recycling production H<sup>2</sup> evolution test of 3%Pt@PPCN was further performed to assess the reusability of photocatalyst. As can be seen in Figure 6b, the PPCN maintained an excellent HER stability under visible light irradiation, demonstrating the excellent structure stability of PPCN.

Overall, a rational photocatalytic HER mechanism is illustrated in Scheme 2 based on the above results and discussions. Under visible light illumination, the electrons in the valence band (VB) of PPCN are activated to the conduction band (CB) and then are trapped by the Pt for proton reduction. The holes in the VB (+1.98 eV) of PPCN oxidized the sacrificial regent TEOA and converted H2O into oxidation products (such as H2O2). The CB level of PPCN (−0.99 eV) is more negative than that of BCN (0.50 eV), which greatly boosted the reduction capacity of the PPCN. As a consequence, benefited from the 3D hierarchical pancake-like porous architecture, the utilization capacity of light absorption of PPCB would be forcefully enhanced by the scattering and diffraction in the pores and vertical plane, which significantly effective to boost the light-harvesting. More than that, the enlarged specific surface

area offered more reactive sites and reduced the recombination rate of photogenerated h+–e–pairs, boosting a highly efficient photocatalytic H<sup>2</sup> evolution reaction.

λ ≥ **Figure 6.** (**a**) H<sup>2</sup> production of the BCN and PPCN with 3%Pt loading, (**b**) cycling H<sup>2</sup> production of 3 wt % Pt@PPCN, Reaction conditions: (**a**,**b**) purified water photocatalytic H<sup>2</sup> production with TEOA as sacrificial under visible light (λ ≥ 420 nm), every 6 h as a period by recovering the catalyst and started with fresh purified water.

**Scheme 2.** Proposed photocatalysis charge transfer mechanism of Hierarchical pancake-like porous C3N<sup>4</sup> in TEOA solution.

#### **3. Experimental**

#### *3.1. Reagents*

Melamine was purchased from Tianjin Damao Chemical Regent Co., Ltd. (Tianjin, China). Phosphorous acid, glycerol, and ethanol were all supplied by Xilong Scientific Co., Ltd (Jiangsu, China). All reagents used in this study were at least of analytic grade without further purification. DI water was used in the whole experiment.

#### *3.2. Synthesis of Bulk C3N<sup>4</sup>*

In the typical procedure, 10.0 g melamine powders were placed into a crucible sealed with a cover followed by calcinating at 550 ◦C for 4 h in a silica oxide tube furnace with a ramping rate of 2.5 ◦C/min under N<sup>2</sup> flow (99.999%, 50 mL/min). The as-obtained solid blocks were named bulk C3N4.

#### *3.3. Synthesis of Precursor*

A total of 1.0 g melamine and 1.2 g phosphorous acid were added to 100 mL deionized water in a magnetic stirrer kept vigorous stirring for 1h at 80 ◦C. Next, the solution was transferred into Teflon-lined stainless-steel autoclave at 180 ◦C for 10 h. The mixture was centrifuged, washed with deionized water for several times to obliterate the phosphorus species. Finally, the precursors were obtained after drying at 60 ◦C.

#### *3.4. Preparation of Hierarchical Pancake-Like Porous Carbon Nitride*

Typically, 0.6 g precursor was refluxed with a mixed aqueous solution of 5 mLglycerol and 15 mL ethanol for 3 h in the temperature 90 ◦C. Then, the powders were washed by ethanol three times and dried at 60 ◦C. After that, the powders were transferred into a crucible tightly coated with a single layer aluminum foil paper. Finally, the crucible was placed into a muffler and heated to 500 ◦C for 6 h with a ramping rate of 2 ◦C/min to obtain the PPCN.

#### *3.5. Characterization*

X-ray diffraction (XRD, Shimadzu XRD-6100, Kyoto, Japan) was performed to identify the crystal phase and structural variation of the samples with a Cu–Ka radiation at 40 kV and 30 mA. Scanning electron macroscopy (SEM, Zeiss Sigma, Jena, Germany) and transmission electron microscope (TEM, JEM-2010, JEOL, Tokyo, Janpan) images was recorded to study the morphologies of the samples. Fourier transform infrared (FT-IR) spectra (PerkinElmer Spectrum, Waltham, USA) were recorded to examine the surface functional groups of samples. X-ray photoelectron spectroscopy (XPS) was conducted on thermo ESCALAB 250XI (Axis Ultra DLD Kratos AXIS SUPRA; phi-5000 versaprobe). Nitrogen-adsorption-desorption isotherms (Quantachrome autosorb-IQ2, Quantachrome Instruments, Florida, USA) were performed to measure the specific surface area and pore volume of the samples in a Brunauer-Emmett-Teller (BET) method. UV–VIS diffusion reflectance spectra (DRS) of the samples were recorded on a PerkinElmer Lambda with BaSO<sup>4</sup> as a reference. Photoluminescence (PL) spectra were recorded with an excitation wavelength of 325 nm. The time-resolved fluorescence decay spectra were acquired on a Hamamatsu compact fluorescence lifetime spectrometer (C11367, Quantaurus-Tau, Hamamatsu, Japan) with 365 nm excitation wavelength and 470 nm emission wavelength.

#### *3.6. Electrochemical Analysis*

The photocurrent and electrochemical impedance spectroscopy (EIS) were investigated with a CHI660C electrochemical workstation equipped with standard three-electrodes in 0.5 M Na2SO<sup>4</sup> aqueous solution. Among the electrodes, Photocatalyst was deposited on the Fluorine doped Tin Oxide (FTO) electrode and served as the photoanode. Pt wire as the counter electrode and saturated calomel electrode (SCE) as the reference electrode. The preparation approach of photoanode was presented as follows: 50 mg catalyst was dispersed in 100 µL ethanol to obtain a slurry. Next, the slurry was carefully coated onto a 7 mm × 14 mm FTO conductive glass and dried in room temperature. A 500 W Xe lamp quipped with a 420 nm cut-off filter was used as the light irradiation source. The EIS spectrum was recorded using the PPCN as the working electrode at a potential of 0.24 V versus SCE with the frequency ranging from 1 MHz to 0.01 Hz.

#### *3.7. Photocatalytic Hydrogen Evolution*

Photocatalytic hydrogen production performance was performed via a sealed reactor and a recirculated cooling water system. Typically, 25 mg of photocatalyst powder was dissolved in 80 mL aqueous solution (68 mL 3 wt % H2PtCl<sup>6</sup> solution and 12 mL triethanolamine). After that, the reactor was degassed and illuminated 2 h under a full arc light 120 mW·cm−<sup>2</sup> (λ > 300 nm) with a 420 nm wavelength cut off. Finally, the generated hydrogen was monitored by GC.

#### **4. Conclusions**

In summary, a 3D hierarchical pancake-like porous carbon nitride with remarkable photocatalytic H<sup>2</sup> production efficiency was synthesized by a facile bottom-up strategy. 3 wt % @Pt/PPCN showed 430 µmol g−<sup>1</sup> h −1 for photocatalytic H<sup>2</sup> production under visible light (λ ≥ 420 nm), which was nine times higher than that of 3 wt % @Pt/BCN (45 µmol g−<sup>1</sup> h −1 ). The enlarged specific surface area, prolonged charge carriers lifetime, boosted light-harvesting and utilization capacity, and improved charge separation and transfer efficiency are all contributed to the optimized photocatalytic capacity. In addition, this research provides a simple and environmental-friendly approach to design highly-efficient polymeric carbon nitride catalysts for potential applications in solar energy-driven photocatalytic water splitting.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/10/1/77/s1.

**Author Contributions:** Conceptualization, X.Q., L.Q. and S.D.; validation, X.Q., L.Q. and S.D.; formal analysis, X.Q.; investigation, X.Q.; resources, X.Q.; data curation, X.Q., M.M. and Y.H.; writing—original draft preparation, X.Q.; writing—review and editing, X.Q.; visualization, X.Q.; supervision, L.Q. and S.D.; project administration, L.Q. and S.D.; funding acquisition, L.Q. and S.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors gratefully acknowledge the support from the financial support of National Natural Science Foundation of China (no.41763020), the Natural Science Foundation of Jiangxi Province (No.20171BAB206008), the Foundation of Jiangxi Educational Commission (no. GJJ180596, GJJ180603), and the Doctoral Scientific Research Starting Foundation of Jiangxi Science and Technology Normal University (no. 2017BSQD003).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Article*

## **Carbon Nitride-Perovskite Composites: Evaluation and Optimization of Photocatalytic Hydrogen Evolution in Saccharides Aqueous Solution**

**Andrea Speltini 1,\* , Lidia Romani <sup>2</sup> , Daniele Dondi <sup>2</sup> , Lorenzo Malavasi <sup>2</sup> and Antonella Profumo 2,\***


Received: 29 September 2020; Accepted: 29 October 2020; Published: 30 October 2020

**Abstract:** The application of hybrid photocatalysts made of carbon nitride and lead-free perovskites, namely DMASnBr3/g-C3N<sup>4</sup> and PEA2SnBr4/g-C3N4, for the H<sup>2</sup> evolution from saccharides aqueous solution is described. The novel composites were tested and compared in terms of hydrogen evolution rate (HER) under simulated solar light, using Pt as a reference co-catalyst, and glucose as a representative sacrificial biomass. The conditions were optimized to maximize H<sup>2</sup> generation by a design of experiments involving catalyst amount, glucose concentration and Pt loading. For both materials, such parameters affected significantly H<sup>2</sup> photogeneration, with the best performance observed using 0.5 g L−<sup>1</sup> catalyst, 0.2 M glucose and 0.5 wt% Pt. Under optimized conditions, DMASnBr3/g-C3N<sup>4</sup> showed a 5-fold higher HER compared to PEA2SnBr4/g-C3N4, i.e., 925 µmoles g−<sup>1</sup> h <sup>−</sup><sup>1</sup> and 190 µmoles g−<sup>1</sup> h −1 , respectively (RSD ≤ 11%, *n* = 4). The former composite, which affords an HER 15-fold higher in aqueous glucose than in neat water, provided <sup>H</sup><sup>2</sup> also with no metal co-catalyst (around 140 <sup>µ</sup>moles g−<sup>1</sup> <sup>h</sup> −1 ), and it was reusable for at least three photoreactions. Encouraging results were also collected by explorative tests on raw starch solution (around 150 µmoles g−<sup>1</sup> h −1 ).

**Keywords:** hydrogen; catalyst; photocatalysis; biomass; solar light; perovskite; carbon nitride; design of experiments

#### **1. Introduction**

The search for new photocatalytic systems working under solar light for hydrogen production is increasingly triggering the interest of the scientific community. In the framework of novel photocatalysts, graphitic carbon nitride (g-C3N4) has emerged in the last decade as one of the most promising material to run H<sup>2</sup> photoproduction from water under visible light, due to cost-effective and easy synthesis, chemical stability, narrow band-gap and band potentials suitable to perform relevant redox reactions in aqueous solution [1,2]. Its intrinsic limitations, namely relatively high charge carriers recombination, low surface area and restricted light harvesting in the visible region, have been in part compensated by several strategies. For instance, metal and non-metal doping, structural and morphological modifications, dye-sensitization, and combination with co-catalysts of different nature (e.g., carbon nanotubes, carbon dots, bimetallic deposition) have proved to be rewarding in terms of enhanced photocatalytic activity towards H<sup>2</sup> generation from water also in the presence of sacrificial agents [2,3]. Among these, mainly fine chemicals have been used, as recently reviewed by Nasir et al. [2], while just a few studies were undertaken in aqueous biomass solutions or directly in wastewaters [4–6].

In recent times, the application of metal halide perovskites (MHPs) in the photocatalysis field has aroused great interest in view of the relatively narrow bandgaps compared to traditional metal oxide photocatalysts [7,8]. MHPs possess exclusive optical properties, viz. consistent light absorption in the visible region, tunable band-gap and extended carriers lifetimes, conveniently exploited in photovoltaics and optoelectronics [9].

Recently, coupling MHPs with g-C3N<sup>4</sup> has become a cutting-edge research field [2,10,11], although limited work has been done so far concerning photocatalysis [2]. Recently, lead-free MHPs showing an exceptional stability in water were synthesized and characterized [12–14], thus opening a new avenue for the preparation and application of innovative photocatalysts. In this context, our group advantageously coupled dimethylammonium and phenylethylammonium-based perovskites—DMASnBr<sup>3</sup> and PEA2SnBr4, respectively—to g-C3N4, providing new micro-sized composites with excellent photocatalytic properties towards H<sup>2</sup> production from triethanolamine (TEOA) aqueous solution; in particular, up to a 20-fold increase of hydrogen evolution rate (HER) was achieved compared to g-C3N<sup>4</sup> alone, due to a synergistic effect between the two constituents in the composite [12,13], essentially due to improved charge carrier separation. Such an effect results from the positive band-alignment between the two semiconductors, namely the MHP and g-C3N4, and the perovskite is selected based on its bandgap in order to exploit this synergistic effect. It was also observed in an explorative test that such catalytic systems are capable of generating H<sup>2</sup> from glucose solution under UV-visible light, with HERs from 30 to 100 times higher relative to pristine g-C3N<sup>4</sup> [12,13].

In this study, the two best performing materials we previously identified, namely 5% PEA2SnBr4/g-C3N<sup>4</sup> and 33% DMASnBr3/g-C3N4, were systematically tested, under simulated solar light, in aqueous glucose through a design of experiments (DoE) with the aim to maximize HER and compare the performance of each photocatalyst under the best conditions. The most performant composite was further investigated for its photo-chemical stability by recycling tests, compared to the well-known Evonik Aeroxide® P25 TiO2, used without any metal co-catalyst, and in the presence of soluble starch as a low-cost and abundant polysaccharide.

#### **2. Results and Discussion**

Based on the preliminary findings obtained with carbon nitride-perovskite composites in TEOA solution [12,13], the two best performing materials, 33% DMASnBr3/g-C3N<sup>4</sup> and 5% PEA2SnBr4/g-C3N4, are investigated here for a systematic study of H<sup>2</sup> photoproduction from glucose aqueous solution. Glucose was selected as a biomass-derived sacrificial agent because in the wastewaters from food industry sugars are present at considerable amounts [4,5], and Pt was used as the reference metal co-catalyst because of its excellent properties for water reduction due to the large work function, resulting in a strong Schottky barrier effect [15,16].

#### *2.1. Comparison between DMASnBr3*/*g-C3N<sup>4</sup> and PEA2SnBr4*/*g-C3N<sup>4</sup> after Chemometric Optimization*

With the aim of investigating the behavior of the two catalysts and maximizing H<sup>2</sup> evolution, a chemometric approach was chosen to easily individuate the effects of the key operational parameters of heterogeneous photocatalysis, namely the relative amounts of catalyst, glucose and metal [17–20]. Accordingly, a full 2<sup>3</sup> experimental design was setup considering glucose concentration (x1), catalyst amount (x2) and Pt loading (x3). As reported in Table 1, the experimental domain was comprised of two levels (−1 and +1) of each variable.


**Table 1.** Experimental domain for the 2<sup>3</sup> factorial design.

HERs obtained under the different conditions (mean values of duplicate tests), according to the experimental matrix, are reported in Table 2.

**Table 2.** Mean hydrogen evolution rates (HERs) obtained in the conditions of the experimental plan for both composite photocatalysts.


The values were used as the experimental response (y), and they were modeled (Matlab® software) relative to each variable (x<sup>i</sup> ), in line with the following equation:

$$\mathbf{y} = \mathbf{b}\_0 + \mathbf{b}\_1 \mathbf{x}\_1 + \mathbf{b}\_2 \mathbf{x}\_2 + \mathbf{b}\_3 \mathbf{x}\_3 + \mathbf{b}\_{12} \mathbf{x}\_1 \mathbf{x}\_2 + \mathbf{b}\_{13} \mathbf{x}\_1 \mathbf{x}\_3 + \mathbf{b}\_{23} \mathbf{x}\_2 \mathbf{x}\_3 \tag{1}$$

Figure 1 shows the plot of the coefficients (b<sup>i</sup> ) of the model, while the response surfaces are gathered in Figure 2.

**Figure 1.** Plots of the coefficients of the model for (**a**) DMASnBr<sup>3</sup> /g-C3N<sup>4</sup> and (**b**) PEA2SnBr<sup>4</sup> /g-C3N<sup>4</sup> . Stars indicate the significance of the coefficients (\* *p* < 0.05, \*\* *p* < 0.01, \*\*\* *p* < 0.001), while error bars indicate the confidence intervals (*p* = 0.05).

− − −

 − **Figure 2.** Response surfaces obtained by Matlab® elaboration for (**a**) DMASnBr<sup>3</sup> /g-C3N<sup>4</sup> and (**b**) PEA2SnBr<sup>4</sup> /g-C3N<sup>4</sup> (for level codes −1 +1 see Table 1); the number close to each curve represents the HER.

The models elaborated on the experimental results are:

$$\text{HER} = 390 + 99\mathbf{x}\_1 - 251\mathbf{x}\_2 - 64\mathbf{x}\_3 - 70\mathbf{x}\_1\mathbf{x}\_2 - 6\mathbf{x}\_1\mathbf{x}\_3 + 60\mathbf{x}\_2\mathbf{x}\_3\tag{2}$$

$$\text{HER} = 101 + 15\mathbf{x}\_1 - 43\mathbf{x}\_2 - 41\mathbf{x}\_3 + 14\mathbf{x}\_1\mathbf{x}\_2 - 12\mathbf{x}\_1\mathbf{x}\_3 + 4\mathbf{x}\_2\mathbf{x}\_3\tag{3}$$

for DMASnBr3/g-C3N<sup>4</sup> and PEA2SnBr4/g-C3N4, respectively.

As apparent from the significance of the coefficients (Figure 1), the picture is similar for both composites, as the three variables affect significantly the HER (*p* < 0.001), although glucose concentration to a minor extent for PEA2SnBr4/g-C3N<sup>4</sup> (*p* < 0.01); in particular, the photoreaction is favored by high glucose concentration and low levels of catalyst in the suspension and metal photodeposited on the catalyst surface. In addition, some interactions are statistically relevant, in particular x1–x<sup>2</sup> and x2–x<sup>3</sup> for DMASnBr3/g-C3N4. In line with the response surfaces (see Figure 2), HER increases by keeping the catalyst amount at the low level but with the highest concentration of sacrificial agent, whereas both catalyst and co-catalyst at the lowest level enhance H<sup>2</sup> evolution. For PEA2SnBr4/g-C3N4, x1–x<sup>2</sup> and x1–x<sup>3</sup> are significative (*p* < 0.01 and *p* < 0.05, respectively); accordingly, the response surfaces show that also in this case, HER is maximized with low Pt loading and catalyst amount, and the high level of sacrificial biomass is useful, especially when working with 0.5 wt% Pt.

These findings can be explained considering that at low sacrificial substrate concentration, the mass transfer of glucose from water to the catalyst surface is the bottleneck of photocatalytic reaction, whilst in concentrated solutions, interfacial reactions govern the process, due to the saturation of glucose on the catalyst [17]. Furthermore, excessive Pt loading reduces the catalytic surface available for light absorption; additionally, a high concentration of suspended powder causes scattering of the incident radiation, thus decreasing the overall process [15,18].

Based on these outcomes, the selected conditions were 0.2 M glucose, 0.5 g L−<sup>1</sup> catalyst, and 0.5 wt% Pt. These are advantageous, involving the use of small amounts of both catalyst and metal co-catalyst, and suggesting that large sample dilution could be avoided when using food industry wastewaters, where the total sugar content is up to tens of grams per liter [5].

The reproducibility was good, with relative standard deviations (RSD) ≤11% and ≤15% observed on four independent experiments for DMASnBr3/g-C3N<sup>4</sup> and PEA2SnBr4/g-C3N4, correspondingly.

For the best performing composite, DMASnBr3/g-C3N4, the optimal conditions were further confirmed by validation of the model, working at the test point (x<sup>1</sup> = 0; x<sup>2</sup> = 0; x<sup>3</sup> = 0). Being the mean HER (407 ± 69 µmoles g−<sup>1</sup> h −1 , *p* = 0.05, *n* = 4) not significantly different from the value predicted by Equation (1), viz. 390 µmoles g−<sup>1</sup> h −1 , it is demonstrated that the model is effective and it can be applied to the whole experimental domain.

The chemometric study provided key information. First, it enabled pointing out the most convenient conditions for H<sup>2</sup> evolution, which resulted in a three-fold increase in HER relative to the first pilot work [13]; at the same time, the results collected clearly highlight the superior photocatalytic activity of DMASnBr3/g-C3N4.

Given the very small difference between the MHPs' band gaps—2.85 and 2.74 eV for DMASnBr<sup>3</sup> and PEA2SnBr4, respectively [12,13]—and the higher surface area of PEA2SnBr4/g-C3N<sup>4</sup> (5.5 m<sup>2</sup> g −1 vs. 3.4 m<sup>2</sup> g <sup>−</sup><sup>1</sup> of DMASnBr3/g-C3N4), it could be speculated that the better performance of the first catalyst derives from the different band alignments of the MHPs' valence band (VB) and conduction band (CB) edges with those of carbon nitride (gathered in Figure 3). This results in a different electron transfer and charge carriers stabilization mechanism.

The synergistic effect between the two constituents of the DMASnBr3-based photocatalyst, which works as a Z-scheme heterojuction, has been explained in terms of favorable band alignment and prolonged charge carrier lifetimes [13]. In the case of PEA2SnBr4, this is also true, but it is known that 2D perovskites have higher excitonic binding energies that may slightly reduce the charge carrier dynamics, thus affording a lower photocatalytic efficiency [21].

Another reason rationally stands in the better dispersibility of the DMA composite in aqueous phase due to its lower hydrophobicity. Because of the low surface area of the composites, good mechanical stirring is important; on the other hand, the fast sedimentation of the powders can be an advantage to recover them from water after photocatalysis.

−

−

≤ ≤

−

**Figure 3.** Scheme of the redox potentials (pH 7 vs NHE) of relevant reactions in relation to the band edges positions of as-prepared g-C3N<sup>4</sup> and metal halide perovskites (MHPs).

#### *2.2. Further Investigation of DMASnBr3*/*g-C3N<sup>4</sup>*

− −

− −

The photocatalytic behavior of the composite containing the DMASnBr<sup>3</sup> perovskite was further studied. Table 3 summarizes the HERs collected by key tests aimed at assessing the role of the metal co-catalyst, of the glucose biomass and to evaluate the contribute of "direct water splitting", i.e., H<sup>2</sup> evolved from water in the absence of the sacrificial donor [22].

**Table 3.** Key experiments for evaluating the photocatalytic system based on 33% DMASnBr<sup>3</sup> /g-C3N<sup>4</sup> (simulated solar light).


<sup>1</sup> RSDs ≤ 11% (*n* = 3); n.q.; not quantifiable (<0.008 µmoles h−<sup>1</sup> ).

As is apparent, the model biomass has a major role in sustaining H<sup>2</sup> evolution, which is 15-fold higher relative to the HER observed in neat water (the contribution of "direct water splitting" is <7%). In addition, the metal co-catalyst largely rules the photoreaction to give gas-phase H2, both in the presence of the sacrificial agent and in pure water. Notice that appreciable HER was gained also with no metal, underlining the potentiality of such photoactive material. This evidence corroborates a photoreaction mechanism typical of the photocatalytic systems for H<sup>2</sup> production from water [1,3,23], wherein:

(1) The catalyst absorbs the radiation generating the characteristic charge separation (holes and electrons);

(2) The sacrificial organic material serves as scavenger of the oxidizing species and, by undergoing gradual oxidation (photoreforming), supplies electrons;

(3) The metal works as an electron collector and is the active site for hydrogen ions reduction while hindering charge carriers' recombination and backward reactions [3].

Regarding the DMASnBr3/g-C3N<sup>4</sup> composite, valence band holes—not hydroxyl radicals in the solution—are generated in g-C3N<sup>4</sup> and these trigger biomass oxidation to boost H<sup>2</sup> formation from water, while avoiding a complete aqueous phase reforming of the organic substrate until mineralization [1], which instead occurs in titanium dioxide photocatalysis, especially with Pd co-catalyst [16,22,24]. The negligible H<sup>2</sup> evolution (<0.008 <sup>µ</sup>moles h−<sup>1</sup> ) observed by irradiation of neat water and 0.2 M glucose, as the control tests (Table 3), further substantiated the proposed mechanism.

The synergistic effect between carbon nitride and perovskite described above and assessed in foregoing work [13] was here confirmed in glucose solution under the best conditions. Indeed, HER was 74 and 40 µmoles g−<sup>1</sup> h −1 for g-C3N<sup>4</sup> and DMASnBr<sup>3</sup> individually tested, respectively, against 925 µmoles g−<sup>1</sup> h <sup>−</sup><sup>1</sup> of the hybrid catalyst (see Table 3).

The values of apparent quantum yield (AQY), calculated as the percent ratio H<sup>2</sup> moles/incident photons moles [5] and turn over number (TON), calculated as the ratio H<sup>2</sup> moles/Pt moles [16], for DMASnBr3/g-C3N<sup>4</sup> in the optimized conditions are reported in Table 4.

**Table 4.** Apparent quantum yield (AQY) and turn over number (TON) values for DMASnBr<sup>3</sup> /g-C3N<sup>4</sup> (0.5 g L−<sup>1</sup> catalyst, 0.5 wt% Pt). −


These results are very interesting and highlight the efficiency of the composite catalyst that, despite the very low surface area (<4 m<sup>2</sup> g −1 , 18-times lower than that of the commercial nanometric P25 TiO<sup>2</sup> [25]), promoted HER equal to about one third of the latter (2906 <sup>µ</sup>moles g−<sup>1</sup> <sup>h</sup> −1 ), under the same conditions. To point out the role of surface area and to provide comparable data, we tested a lower amount of P25 TiO<sup>2</sup> (0.028 g L−<sup>1</sup> ) to have approximately the same catalytic surface area of 0.5 g L−<sup>1</sup> of the composite. HER was 377 µmoles g−<sup>1</sup> h −1 , 2.5-times lower than that afforded by employing the composite. − − − − − − −

Additional trials were devoted to investigate the stability of the catalyst upon subsequent irradiations. As is shown in Figure 4, no loss of efficiency was noticed using the recycled powder in a new sample solution, and only a 25% decrease in H<sup>2</sup> evolution in a third treatment.

− **Figure 4.** Reusability of DMASnBr<sup>3</sup> /g-C3N<sup>4</sup> (0.5 g L−<sup>1</sup> , 0.5 wt% Pt, 0.2 M aqueous glucose), RSD < 11% (*n* = 3).

The X-ray diffraction (XRD) profiles overlaid in Figure 5 attest the preservation of the pristine structure in the composite employed after the three sequential photoreactions (18 h total irradiation time) in aqueous glucose.

**Figure 5.** XRD patterns collected on (**a**) DMASnBr<sup>3</sup> (green), g-C3N<sup>4</sup> (black) and as-prepared DMASnBr<sup>3</sup> /g-C3N<sup>4</sup> (blue); (**b**) the composite after three irradiations (azure) compared to the as-prepared one (blue) and reference metallic tin (red).

Although in the XRD pattern of the as-synthesized catalyst the fingerprints of the perovskite are not visible, a broad signal centered on the principal peak of the carbon nitride that stabilizes an amorphous structure of the perovskite is present, as previously discussed [13]. Superimposable patterns were observed on both fresh and recycled materials (see Figure 5b); moreover, no peaks related to metallic Sn were found after catalysis (instead observed working in TEOA solution, data not shown). These findings support both the integrity of the hybrid material and the maintenance of the perovskite in the photocatalyst. Thus, this can be utilized for at least three consecutive irradiations, as a proof of photo-chemical stability that strengthens the applicability of DMASnBr3/g-C3N<sup>4</sup> for a potential practical use.

Such new outcomes prompted us to test the composite in the presence of a polysaccharide sacrificial substance, and starch was chosen as the representative bio-polymer because of its low cost, large availability and attractiveness as bio-oxygenated for H<sup>2</sup> photogeneration [5,24].

− Hydrogen gas was effectively produced from starch solution (see Table 4), at a concentration of 4.5 g L−<sup>1</sup> (the maximum solubility [5]), although at lower amount compared to glucose. This is justified considering that that the rate of hydrogen evolution from water decreases with increasing molecular weights and structure complexity of carbohydrates [15–17,24]. Additionally, since the VB holes are the oxidizing species of the present catalytic system, an intimate contact between the sacrificial substrate and the catalyst surface is certainly required. This could be more favored with monosaccharides than polysaccharides with a branched skeleton such as starch, which realistically has slower mass transfer kinetics from solution to the composite surface sites [16,24].

Such new findings highlight that DMASnBr3/g-C3N<sup>4</sup> is able to carry forward H<sup>2</sup> photoproduction exploiting the sacrificial role of a raw polysaccharide such as soluble starch, which thus can be employed directly without any pre-treatment, e.g., enzymatic hydrolysis or microwave-assisted hydroxylation [26,27].

#### **3. Materials and Methods**

−

Preparation and characterization of the two new composites 5% PEA2SnBr4/g-C3N<sup>4</sup> and 33% DMASnBr3/g-C3N<sup>4</sup> was reported by these authors in very recent works [12,13].

 − Surface area measurements were carried out by the Brunauer, Emmett and Teller (B.E.T.) single point method using a Flowsorb II 2300 (Micromeritics, Norcross, GA, USA) apparatus. The sample was accurately weighed and degassed at 80 ◦C for 1.5 h, under a continuous stream of a N2-He (30:70) mixture, and then it was put in liquid nitrogen for gas adsorption. Evonik Aeroxide® P25 TiO<sup>2</sup> (10–50 nm, 60.8 m<sup>2</sup> g −1 [25]) was supplied by Evonik Industries AG (Hanau, Germany). H<sup>2</sup> evolution experiments were conducted in distilled water containing 0.025-0.2 M glucose (99.9%, Carlo Erba

−

Reagents) or 4.5 g L−<sup>1</sup> soluble starch (ACS reagent, Carlo Erba Reagents), irradiated in Pyrex glass containers [28]. After addition of the catalyst (0.028, 0.5, 2 g L−<sup>1</sup> ), the suspension was deoxygenated by Ar bubbling (20 min) and irradiated for 6 h, under magnetic stirring. When using Pt as the co-catalyst, chloroplatinic acid (H2PtCl6, 38% Pt basis) from Sigma-Aldrich (Milan, Italy) was employed as the metal source. Since Pt is in situ photodeposited on the catalyst surface, after Ar bubbling, a small volume from an H2PtCl<sup>6</sup> aqueous solution (0.15, 3 or 15 g L−<sup>1</sup> ) was added using a 10–100 µL micropipette to the catalyst suspension, directly in the photoreactor. This was closed with sleeve stopper septa and was irradiated, as described in the following, achieving simultaneous Pt deposition and H<sup>2</sup> production [23,28–30]. Irradiation was done under simulated solar light using a Solar Box 1500e (CO.FO.ME.GRA S.r.l., Milan, Italy) set at a power factor of 500 W m-2, and equipped with UV outdoor filter made of IR-treated soda lime glass. The apparent photon flux, measured as previously described [5], was 1.53 × 10-7 photons moles s−<sup>1</sup> . The headspace H<sup>2</sup> was quantified by gas chromatography coupled with thermal conductivity detection (GC-TCD) [28]. The results obtained relative to H<sup>2</sup> evolution are shown in the paper as micromoles of gas per gram of catalyst per hour (µmoles g−<sup>1</sup> h −1 , HER). Reproducibility was appraised on the RSD% from four independent photoproduction runs. For catalyst recycling tests, the catalyst was recovered by filtration (0.45 µm nylon membrane), washed with plenty of distilled water, left drying at room temperature for several weeks in the dark, and finally re-used under the optimized conditions. Room temperature Cu-radiation XRD spectra were acquired by a Bruker D8 diffractometer (Billerica, MA, USA).

#### **4. Conclusions**

Lab-scale tests proved the superior photocatalytic activity of DMASnBr3/g-C3N<sup>4</sup> against PEA2SnBr4/g-C3N<sup>4</sup> for H<sup>2</sup> evolution from glucose solution, under simulated solar light, using Pt as a co-catalyst. The systematic optimization of the experimental conditions by a DoE provided HER higher than 900 µmoles g−<sup>1</sup> h <sup>−</sup><sup>1</sup> using the former composite. The results herein collected highlight the importance to work under selected conditions to maximize the H<sup>2</sup> yield while reducing the use of catalyst and co-catalyst. The comparison with P25 TiO<sup>2</sup> strengthened the catalytic efficiency of DMASnBr3/g-C3N4, while avoiding the handling of nano-sized powders. Notably, a rewarding H<sup>2</sup> formation (around 140 <sup>µ</sup>moles g−<sup>1</sup> <sup>h</sup> −1 ) was gained, also avoiding any metal deposition. DMASnBr3/g-C3N<sup>4</sup> showed good photochemical stability to be utilized for two consecutive runs without any loss of efficiency and with a decrease of 25% in the third cycle, as verified by H<sup>2</sup> evolution measurement and XRD analysis. The system turned out to work also in raw starch solution as sustainable, model sacrificial biomass, providing ca. 150 µmoles g−<sup>1</sup> h −1 . These outcomes corroborate the potential application of this new class of photocatalysts for clean energy retrieval under sustainable conditions and deserve further investigation to move the photoproduction experiments on sugar-rich wastewaters under natural solar light.

**Author Contributions:** Conceptualization, A.S., L.M. and A.P.; validation, A.S. and A.P.; formal analysis, A.S.; investigation, A.S. and L.R.; data curation, A.S.; writing—original draft preparation, A.S.; writing—review and editing, L.M., D.D. and A.P.; visualization, A.S. and L.R.; supervision, A.S. and A.P.; funding acquisition, A.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The Authors acknowledge Giuseppina Sandri (Department of Drug Sciences, University of Pavia) for surface area measurements.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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*Review*

## **Two-Dimensional Materials and Composites as Potential Water Splitting Photocatalysts: A Review**

#### **Zubia Saleem, Erum Pervaiz \*, M. Usman Yousaf and M. Bilal Khan Niazi**

Department of Chemical Engineering, School of Chemical & Materials Engineering (SCME), National University of Sciences & Technology (NUST), Sector H-12 Islamabad, Islamabad 44000, Pakistan; zubia.saleem92@gmail.com (Z.S.); usman.yousaf93@gmail.com (M.U.Y.); m.b.k.niazi@scme.nust.edu.pk (M.B.K.N.)

**\*** Correspondence: erum.pervaiz@scme.nust.edu.pk; Tel.: +92-51-9085-5113

Received: 28 November 2019; Accepted: 18 January 2020; Published: 24 April 2020

**Abstract:** Hydrogen production via water dissociation under exposure to sunlight has emanated as an environmentally friendly, highly productive and expedient process to overcome the energy production and consumption gap, while evading the challenges of fossil fuel depletion and ecological contamination. Various classes of materials are being explored as viable photocatalysts to achieve this purpose, among which, the two-dimensional materials have emerged as prominent candidates, having the intrinsic advantages of visible light sensitivity; structural and chemical tuneability; extensively exposed surface area; and flexibility to form composites and heterostructures. In an abridged manner, the common types of 2D photocatalysts, their position as potential contenders in photocatalytic processes, their derivatives and their modifications are described herein, as it all applies to achieving the coveted chemical and physical properties by fine-tuning the synthesis techniques, precursor ingredients and nano-structural alterations.

**Keywords:** water splitting; photocatalyst; graphene oxide; phosphorene; graphitic carbon nitride; MOFs

#### **1. Introduction**

Today, energy is one of the utmost critical environmental, economic and political issues due to the contributing factors of continuously increasing global population, escalating urbanization and growing energy consumption. These dynamics are compelling researchers and front-runners in political, social, environmental and industrial fields towards a shift from conventional energy production methods towards more innovative, sustainable, efficient, safe and environmentally benign technologies and materials. The total energy consumption of the entire globe is 18.5 TW per year currently, which is expected to rise to 40.8 TW by 2050 [1]. Approximately 85% of total energy usage comes from fossil fuel. It is estimated that there are sufficient fossil fuel resources to meet global energy demands for several centuries [2]. However, irreversible climate change due to generation of greenhouse gases, depletion of fossil fuel reserves, geopolitical conflicts and other environmental concerns are continuous threats for secure and sustainable energy supply. Therefore, the world needs to devise advanced, sustainable methods and sources competitive to fossil fuels in terms of efficiency and economics while ensuring minimal environmental damage [3]. The sun is an everlasting source of inexhaustible energy with which to produce hydrogen, which proves to be potential candidate to meet secure energy demands due to environmental friendliness, high energy value and the possibility of onsite production [4]. The amount of solar energy reaching earth is around 9.5 × 10<sup>16</sup> J/s; that is 10<sup>4</sup> times the total usage by entire human population, endorsing its candidature for potential sustainable energy sources [5]. The major research concern for harnessing solar energy is the production of solar fuel. Conventional technologies of solar energy production suffer from many downsides, such as the limited

storage capacity for the energy, the intermittent availability in certain areas and the economic feasibility of the process. The steam reforming process is traditionally used for hydrogen production from fossil fuels, which is not considered as sustainable in the long haul due to massive carbon dioxide emission and depleting resource trends. Catalytic steam reforming using fossil fuels is virtually the only industrial process responsible for producing the entire hydrogen supply of the world today, with electrolysis contributing only a meager 4% via water electrolysis. There is a growing interest in generating hydrogen from sustainable and renewable resources using environmentally friendly methods to deliver inexpensive and clean energy [6]. Photocatalytic water splitting activity is one of the key research areas for onsite hydrogen production and storage [7–16]. It is one of the emerging process for harnessing solar energy into fuel in the form of hydrogen without any use of fossil fuels or emission of carbon dioxide. There is a growing interest in the field of semiconductor photocatalysis [17] which could be used for water dissociation into constituent oxygen and hydrogen gases using solar energy, which is abundantly available all over the world. Solar energy conversion in the form of hydrogen via photocatalytic water splitting has become an attractive subject [18].

The prerequisites to an energy efficient, cost effective and operationally feasible photocatalyst are the ability to effectively harvest and utilize photons in the visible light region to generate electron-hole pairs; lower band gap energies; unimpeded mobility of charge carriers; a low recombination rate of electron-hole pairs; and a greater surface area, resulting in a higher number of exposed active sites. The earliest discoveries fulfilling these requirements comprised granular semi-conducting materials and their derivatives, such as ZnO and TiO<sup>2</sup> [19]. But these conventional photocatalysts have several shortcomings, such as:


Further research and development in this regard led to the conclusion that the nano-materials are better photocatalysts because of their high surface area property [24,25]. Among nanomaterials, 2D materials are promising photocatalyst alternatives due to their sheet-like structures; electrical, thermal and mechanical properties; the operational advantages of being easily handleable; having the ability to form integrated composites with highly photoactive co-catalysts; and having higher surface to volume ratios. The bi-layer and mono-layer 2D nanocrystals and nanosheets provide the basis for highly exposed active sites and faster charge transfer schemes [26], while the doped precious and photo-active metals and their related compounds offer the added compensation of higher light absorption capacity and lower band gap energy [27,28].

#### *1.1. Photocatalysis: Theoretical Digest*

Hydrogen production by photocatalytic water splitting has proven to be a superlative solution for resource and economics related problems in novel hydrogen production methods. The process is centered on a semiconducting catalyst material and sunlight to convert the light energy to chemical energy, also termed "artificial photosynthesis." The photons of light having higher energy than the energy gap of semiconducting material, when irradiated on the photocatalyst material, results in photo generated charge carriers usually termed the electron hole pair. This generation of electron-hole pairs in the conduction band and valence band acts as the foundation for redox reactions, as indicated below [29,30]:

Overall:

$$2H\_2O \text{ (l) } \xrightarrow{Photon\ energy > 1.23 \text{ eV}} O\_2(g) + 2H\_2\text{ (g)}$$

*Catalysts* **2020**, *10*, 464

(݈) ܱଶܪ2

$$E^o = 1.23\,\text{V}.$$

= 1.23 ܸ.

(݃) ଶܪ2 + (݃)

௧ ௬ வଵ.ଶଷ ሱۛۛۛۛۛۛۛۛۛۛۛۛۛۛۛۛۛሮ ܱ<sup>ଶ</sup>

ܧ

The oxidation and reduction half reactions at anode and cathode, respectively, under acidic (low pH) solutions proceed as: 2ܪଶܱ (݈) + 4ℎା → ܱ<sup>ଶ</sup> ܧ௫ௗ௧ = −1.23 ܸ

Oxidation:

$$2H\_2O \text{ (}l\text{)} + 4h^+ \rightarrow \text{ O}\_2(g) + 4H^+(aq)$$

$$E^\circ\_{oxidation} = -1.23 \text{ V}.$$

Reduction:

$$2H^{+} + 2e^{-} \rightarrow O\_{2}(g) + 2H\_{2} \text{ (g)}.$$

$$E\_{reduction}^{o} = \text{0 } V.$$

Under basic (high pH) conditions, the chemical reactions are represented as: Oxidation: → (ݍܽ)ିܪ2ܱ ଵ ଶ ܱ<sup>ଶ</sup> 2݁ି + (݈) ܱଶܪ + (݃)

$$2OH^-(aq)\rightarrow \frac{1}{2}\text{ O}\_2(g) + H\_2\text{O}\ (l) + 2e^-\ .$$

Reduction:

$$2H\_2O \text{ (}l\text{)} + 2e^- \rightarrow H\_2 \text{ (}g\text{)} + 2OH^-.$$

The holes generated in the valence band resultantly are responsible for oxidation of adsorbed species (water in this case) releasing oxygen, and the agitated electrons from the conduction band are the reducing agents to produce hydrogen from in-contact water molecules as shown in Figure 1.

**Figure 1.** Schematic illustration of basic processes in photocatalytic water splitting. (Reprinted with permission from ACS Catal. 2018, 8, 3, 2253–2276, Publication Date: 30 January 2018. Copyright (2018) American Chemical Society [23]).

The production of charge carriers, such as electron and hole ones, is a fundamental step for all photocatalytic processes; other crucial factors include the width of material band gap and the energy levels of both conduction and valence bands of the material. For a feasible reaction to occur, the conduction band of the electron acceptor species should be more positive than the conductive band of the semiconductor photocatalyst, whereas the valence band of the electron donor type must be more negative than the semiconductor's valence band, and the minimum band gap must be greater than 1.23 eV [31]. As a result of the interface electron transfer process, the redox reaction takes place and the photolysis of water molecules occur.

#### *1.2. Limitations of Photocatalytic Water Splitting*

1. To catalyze the splitting of water at the interface of electrolyte and electrode, the charge carriers are required to be transferred to catalyst surface once the electron-hole pairs are created. The quick recombination of photo-generated electron-hole pairs (Figure 2) releasing heat or photon energy before they can catalyze the redox reactions is a major challenge in this step, requiring a high degree of crystallinity [32].


**Figure 2.** Schematic of charge carrier dynamics in a semiconductor: one electron transported to the surface; two holes transported to the surface; three surface recombinations; four bulk recombinations; five electron transfers to respective acceptor molecules; and six hole trappings by a donor molecule. (Reprinted with permission from [34]. Copyright (2019), Elsevier).

#### **2. Two Dimensional Materials as Photocatalysts for Water Splitting**

A good photocatalyst must possess the characteristics of photo-activity, thermal stability, photo-stability, chemical inertness, low cost, non-toxicity and post-reaction recoverability [35]. To tackle the environmental pollution factor, metal-free catalysts are always the first priority for synthesize with

maximum efficiency. The first and second generation of catalysts include combinations of inorganic materials [36] and various oxides and sulfides of metals, but the main problem of these materials is the toxic and corrosive nature [33,37]. Among various other photocatalyst materials available, 2D materials are promising candidates for the water splitting applications, because of merits, such as:


The fundamental strategies to synthesizing 2D nanosheets comprise top-down and bottom-up methods, which vary on the basis of starting precursors and treatment methods also shown in Figure 3.

**Figure 3.** Schematic illustration of top-down and bottom-up strategies toward the synthesis of 2D nanosheets. (Reprinted with permission from [41]. Copyright (2015), Elsevier).

Till now, various 2D materials have been discovered by researchers, such as graphene, graphitic carbon nitride, black phosphorus/phosphorene and hexagonal boron nitride. All these materials have proven potential for water splitting activity by photocatalysis [42]. 2D materials show unusual structural, electronic and optical properties [43]. However, some limitations have been observed in terms of reaction completion due to the recombination rate of the electrons and holes which carry out reactions. In the search for alternatives to metal-based systems, metal-free elementals and compound photocatalysts have recently been developed. Red-P, alpha-sulfur and boron are notable examples reported in the literature [44]. However, these elemental photocatalysts exhibit low photocatalytic activity. There are various techniques that researchers employ to enhance activity majorly, including doping, sensitization, tuning morphology and making heterostructures with other materials [45]. The search for a non-toxic, inexpensive, corrosion resistant photocatalyst with a focus on the design and efficiency of photocatalyst, while assessing the activity for economical and scalable hydrogen generation, is ongoing. Table 1 summarizes the discussed literature in a comprehensive manner. The main focus of this paper is to go over 2D materials, their modifications and the different techniques that have been employed to enhance their photocatalytic water splitting performances. Said materials include, but are not limited to, graphitic carbon nitride, graphene, black phosphorus, metal phosphides and metal organic frameworks. Figure 4 presents the bandgap energies of all potential 2-D materials. 


**Table 1.** A summary of recent studies on ultra-thin 2D photocatalysts. (Adapted with permission from [46], copyright (2017); Wiley and [47], copyright (2019); Nanotechnology, NCBI and [48], copyright (2019), Springer) (Unless otherwise specified).


**Table 1.** *Cont.*

**Figure 4.** Bandgap energies of several layered materials used for nanoelectronics. (Reprinted with permission from [41]. Copyright (2015), Elsevier).

#### *2.1. Graphitic Carbon Nitride and Derivatives*

Graphitic carbon nitride is metal-free polymeric catalyst comprised of repeating triazine units. It is proving to be a highly efficient catalyst for photocatalytic activity with band gap of 2.56–2.70 eV. Due to highly efficient performance with an environmentally friendly nature, it is one of the most highly esteemed materials for water splitting. Due to some of its limitations, research is underway to make it more feasible and economical with improved activity. Different techniques, such as morphological alteration by changing temperature, modifying g-C3N<sup>4</sup> with some metal and doping, for making heterostructures, have been investigated [79–81]. These modifications are targeted toward achieving a highly efficient catalyst by lowering the recombination rate of electrons and holes, improving value and improving environmental friendliness as much as possible [82]. 2D nanomaterials are being fabricated by facile and reliable methods: top-down ones include thermal exfoliation, chemical exfoliation, mechanical cleavage and selective etching; the bottom up approaches include chemical vapor deposition and wet synthesis methods. The most employed technique for the fabrication of 2D g-C3N<sup>4</sup> is thermal treatment [83]. This facile method is easy to scale up for industrial level synthesis. However, in order to improve the photocatalytic performance, different techniques are employed and are under investigation to get maximum efficiency of a given material. Different temperature controlled synthesis techniques have been investigated to alter the morphology of the material [84]. Quan Gu investigated the morphological evolution of graphitic carbon nitride nanostructures and also evaluated their photocatalytic activities under visible light. The materials were synthesized at different temperatures, and showed different morphologies, optical properties, energy band structures and photocatalytic activity for water splitting. By changing the temperature, different structures, such as nanosheets, nanotubes and nanoparticles, were observed. BET surface area increased dramatically from 4.5 m<sup>2</sup> /g of bulk g-C3N<sup>4</sup> to 210.1 m<sup>2</sup> /g of modified sample obtained at 540 ◦C. The exfoliated structures showed better activity for water splitting; i.e., 297 and 120 µmol/g·h for HER and OER respectively [85]. Zhi -An Lan modified g-C3N<sup>4</sup> with bromine to investigate the change in properties. It was observed to get enhanced optical, conductive and photocatalytic properties of g-C3N4. The optimal catalyst CNU-Br 0.1 demonstrated more than two times higher H<sup>2</sup> evolution activity than pure CNU [86]. Junqing Yan fabricated g-C3N4/TiO<sup>2</sup> heterojunction by the hydrothermal method to get good production and separation of charge carriers. The fabricated TiO2/g-C3N<sup>4</sup> samples are active

in the hydrogen evolution reaction of water splitting with the best performance of 700 µmol/g·h and better photo-stability [87,88]. Recently, Yuhao Yang proposed a 2D/2D heterojunction strategy in order to compensate for low efficiency of charge separation in ultra-thin g-C3N<sup>4</sup> 2D sheets. The high surface area interface of g-C3N<sup>4</sup> nanosheets and 2D TiO2, coupled with favorable band structure, reliability of operation and stability of TiO<sup>2</sup> nanosheets resulted in high H<sup>2</sup> evolution rates and a robust, recyclable photocatalyst [89]. Guigang Zhang conducted some experimental studies of various redox cocatalysts, reporting g-C3N<sup>4</sup> modified with Pt, PtOx, and CoOx. The results proved that cocatalysts, especially Pt/g-C3N4, show potential for water splitting activity without the use of a sacrificial agent [88]. Yaping Zeng fabricated NiTiO3/g-C3N<sup>4</sup> heterostructured material by the sol gel method. It was observed that NiTiO<sup>3</sup> had a suitable band gap and unique photo response in visible light range. It showed three times the activity of NiTiO3/g-C3N<sup>4</sup> hybrid catalysts, as compared to pure g-C3N<sup>4</sup> and 3 wt%, the catalysts presented optimal activity. This is ascribed to the strong light absorption in the visible region and the prolonged recombination rate of electrons and holes [90].

Xiaojie She aimed at the template free synthesis of porous ultrathin nonmetal doped g-C3N<sup>4</sup> nanosheets for increased H<sup>2</sup> production. The formation of an ultrathin structure and the introduction of oxygen proved favorable for the enhancement of the photocatalytic performance. The average H<sup>2</sup> evolution rate of came out at ≈ 3786 µmol/g·h; that is five times that of bulk g-C3N<sup>4</sup> photocatalyst material. This increased activity was endorsed due to more adsorption and active sites, the enhanced redox ability and improved electron transportability due to the introduction of the electrophilic groups (C-O, C=O and COOH) [91]. Mohammad W. Kadia et al., in their work, claimed to increase the efficiency of water splitting using visible light, and also increased the charge separation using mesoporous g-C3N<sup>4</sup> which was coated over by WO3. They further claim that by coating a semiconductor surface with a metal-oxide, it can capture the surface electrons, and hence can increase the charge separation. The parameters affected the WO3/g-C3N<sup>4</sup> in charge separation ability, mesoporous structure gap between valence and conduction band high surface area [92]. Recently, a step-heterojunction scheme was demonstrated by Junwei, Fu. et al. [69], whereby 2D/2D WO3/g-C3N<sup>4</sup> photocatalyst was developed by ultrasonic exfoliation of WO<sup>3</sup> and dual-step thermal etching of g-C3N<sup>4</sup> in bulk forms. The resulting material showed 1.7 times higher H<sup>2</sup> production activity by barricading the charge recombination. This advancement is owed to the fact that the electronic transfer on interface of both materials from g-C3N<sup>4</sup> to WO<sup>3</sup> balanced the higher fermi level of g-C3N<sup>4</sup> and the lower fermi level of WO3. This mechanism creates a continuous electron transfer trend and simultaneously prevents the useful charge carriers (electrons from CB of g-C3N<sup>4</sup> and holes from VB of WO3) from recombining. KNbO<sup>3</sup> has been researched extensively in terms of photocatalytic activity for water splitting, but it has low quantum efficiency, showing water splitting activity only under UV light irradiation. It has been used to enhance g-C3N<sup>4</sup> activity by making heterojunction. Dongbo, Xu. et al., in their research, presented the synthesis of g-C3N4/KNbO<sup>3</sup> heterojunction composites by the hydrothermal method and used them for water splitting and hydrogen production using sunlight. The g-C3N<sup>4</sup> and KNbO<sup>3</sup> had better performances for water splitting by more than two and 1.8 times that of their pure forms respectively [93].

Seza, A. et al. presented microwave assisted synthesis of g-C3N4/SnO<sup>2</sup> by simple pyrolysis of urea. It is the first ever synthesis of g-C3N<sup>4</sup> using microwave assistance, wherein urea is converted to g-C3N<sup>4</sup> nanorod, and then the uniform distribution of SnO<sup>2</sup> over the surface makes it a promising material for visible light photoactivity to be used for water splitting [94]. Jingran Xiao, et al., prepared g-C3N<sup>4</sup> on ZnO wires, and then incorporated nano clusters of Pt over the surface. Both the g-C3N<sup>4</sup> and ZnO act as photo anodes while Pt carries the charges generated; i.e., electrons generated. This showed great improvement in the photoactivity in sun light; the efficiency increased by 9.5 times that of ZnO pure and 4.5 times that of g-C3N4/ZnO. The recombination of electron holes further reduced by the newly formed anode with Pt over surface. Composites of ZnO and g-C3N<sup>4</sup> were prepared and nano-clusteres were decorated over the surface, which eventually increased the performance [95]. Feng Guoa et al., formed another heterojunction of g-C3N<sup>4</sup> with CoO; studies showed that this metal oxide decoration over the surface of graphite carbon nitrite increased the photo activities of the newly formed photocatalyst. The formation of this hetero-conjunction catalyst was done through the "solvothermal" process. Among all of the combinations, 30 wt% CoO and g-C3N<sup>4</sup> had H<sup>2</sup> evolution rates of 50.2 µmol/g·h and 27.8 µmol/g·h for O<sup>2</sup> evolution [96]. Kelin He et al. fabricated a nano-heterojunction co-catalyst system based on the doping of hexagonal Ni3C nanoparticles produced by thermolysis at low temperature on the base g-C3N<sup>4</sup> nanosheets via simple grinding method. The catalyst comprising of over 15 wt% Ni3C content showed the hydrogen production rate of 1518 µmol/g·h when irradiated with visible light and an apparent quantum yield 116 fold greater than pure g-C3N4; i.e., 0.4% at 420 nm successfully showing better results than noble metal 0.5 wt% Pt/g-C3N<sup>4</sup> sample. This relatively low cost, good performance, non-noble metal addition of co-catalyst enables a decrease in hydrogen production over-potential, improvement in oxidation kinetics and effective charge carrier separation [97]. Another Non-noble metal alternative with superior hydrogen evolution rate of 169 µmol/g·h has been reported by Lu Chen et al. The technique centered on the combination of Ni3N nanoparticles and g-C3N<sup>4</sup> nanosheets via a hydrothermal method and subsequent annealing step which ensures stabilization of Ni3N structures on g-C3N<sup>4</sup> and maintains the hydrogen productivity. The notable advantages of this material include the lower cost, higher throughput than 3 wt% Pt/g-C3N<sup>4</sup> material, multiple cycles operation, better carrier separation and visible light operation. These properties are attributed to the optimal loading and distribution of Ni3N active co-catalyst medium and better stability of Ni3N phase with a minimal conversion to less active Ni or Ni(OH)<sup>2</sup> states [98]. Based on the single layer fabrication and promising scalable strategy of single layer SnS<sup>2</sup> nanosheets of 3-atom thickness exhibiting above 38% water splitting efficacy under visible light by Yongfu Sun et al. Shao-hua Chen et al. further investigated the prospects of coupling SnS<sup>2</sup> nanosheets with g-C3N<sup>4</sup> to overcome the performance limitations of afore-mentioned research. The results were conformed with the intended improvements of better charge carrier separation and band alignment owing to the weak Vander Walls heterojunction between SnS<sup>2</sup> and g-C3N<sup>4</sup> [99]. Another approach using environmentally benign materials and methods was introduced by Rongchen Shen et al., in which the hybridization of Co2P nanoparticles formed from phosphorization technique of cobalt metal organic framework and g-C3N<sup>4</sup> nanosheets. The resulting g-C3N4/Co2P/K2HPO<sup>4</sup> material formed by simple grinding method exhibited a 556 times superior H<sup>2</sup> production rate compared to simple g-C3N4, i.e., 556 µmol/g·h, owing to the lower hydrogen production over potentials, better light absorbance, diminished charge carrier combination tendency and provision of an alternate reaction pathway by proton-reduction [100]. A recent approach to produce cobalt phosphide (CoP)/g-C3N<sup>4</sup> cocatalysts has been presented by Xiao-Jun Sun et al., involving dual-step calcination of ZIF-67. The calcination was carried out to formulate CoP nanoparticles, which were found to boost the hydrogen release rate to as high as 201.5 µmol/g·h with a meager NPs loading of 1.42 wt%. ZIF-67 was formulated by the traditional method and g-C3N<sup>4</sup> by melamine thermal condensation; they were subsequently combined, ground and calcined at a high temperature. The H<sup>2</sup> release rate exhibited a directly proportional relation with increasing ZIF-67 concentration until 5 wt% (1.42% CoP), indicating deterrence in ability to absorb light. Moreover, the lower water contact angle of the hybrid material than those of the individual components conforms to the assumption of better water absorption ability and reduction of adsorbed proton specie. These factors, along with the notably lower charge transport resistance indicated by photocurrent experiments, contribute to the exemplary alliance and compatibility of CoP and g-C3N<sup>4</sup> as photocatalysts [101]. Another methodology to incorporate graphite carbon nitride as a root material was verified by garnishing carbon based quantum dots on g-C3N<sup>4</sup> nano-tubes by thermal co-polymerization method. Yang Wang et al. reported the resultant hydrogen release rate as high as 3538.3 µmol/g·h due to reduced charge recombination and greater light capturing capabilities, demonstrating 10.94% quantum yield. The co-condensation of these components encourages tube arrangement and micro-sized composite structure to stimulate transfer of photoelectrons. Introduction of C-QDs notably shifted the pure g-C3N<sup>4</sup> nano-sheet structure towards nano-tube structure, because of the tough amide bonds created among C-QDs and urea precursor. Furthermore, the band gap of untreated base material was shifted from 2.70 to 2.60 eV due to the structural change and multi-reflective containment of incident light radiations in the nano-tubes [102]. Nan Wang et al., in their research, developed MnO<sup>2</sup> with a co-modified nanocarbon tube with a g-C3N<sup>4</sup> ternary composite. Different properties were studied, and the compound was characterized by different conventional techniques. It was observed that this ternary compound shows more active photocatalysis in contrast with carbon nanotubes (CNTs)/C3N4. Here, CNTs capture the electrons generated when light falls over the surface of this catalyst, which resultantly increases the rate of water splitting; hence, more hydrogen is formed. Hydrogen production is increased with rate of 4067 µmol/g·h [103]. Figure 5 shows the composite g-C3N4/GO with MoS2 as potential photogenerated carrier separator.

. **Figure 5.** Schematics of MoS2/g-C3N<sup>4</sup> /GO composite and charge carrier separation. (Reprinted with permission from [104]. Copyright (2017), American Chemical Society).

#### *2.2. Graphene-Based Photocatalysts for Water Splitting*

Another metal-free photocatalyst for photocatalytic water splitting activity is graphene. It has gotten immense attention recently. In 2004, demonstration of the "sticky-tape peeling" of atomically thin sheets of carbon atoms from a mass of graphite brought a revolution in 2D materials research. This thinnest-ever material is known as "graphene" [105]. It is characterized by excellent chemical, mechanical and electrical properties [106]. For example, graphene is 100 times harder than steel; superior to copper in terms of electrical and thermal conductivity; and is flexible and transparent. Proposed future applications include uses ranging from computer chips and flexible displays to batteries and fuel cells [107].

Graphene is proving to be a potential candidate for the production of hydrogen, due its low cost and environmentally friendly nature, and shows unique charge carrier mobility properties, large specific

μ

π

surface area (single layer: 2650 m<sup>2</sup> /g) [108], high transparency, structural flexibility, chemical stability and unique optoelectronic properties [109]. Graphene has been established as a viable and efficient material choice for improved catalytic operation with various other semi-conducting materials [29] due to its extended two-dimensional sp<sup>2</sup> hybrid carbon network framework structure. It offers excellent electron transporting and accepting characteristics, due to which it behaves as a multi-dimensional passageway for electrons, and hence, helps to effectively separate the photo-generated charge carriers. However, the possibility and efficacy of graphene to transport electron holes is yet to be explored and studied. Moreover, a great number of established studies have been concentrated on combinations of a single type of nanoparticle and graphene film, for use as photocatalysts to improve hydrogen gas production (Figure 5); but the incorporation of two or more components as co-catalysts on graphene for greater selectivity and improved performance has not been achieved. Graphene can be produced via top down or bottom up approach (Figure 6) and synthesis methodology effects the intrinsic property of the graphene. The atomically thin and flexible graphene layers can not only provide a support for dispersing metallic or oxide nanoparticles and provide a highly conductive matrix, but also can induce easier electron transfer from the conduction band of semiconductors to graphene due to large energy level offset formed at the interface, leading to an efficient charge separation.

**Figure 6.** Top-down and bottom-up synthesis schematics of graphene. (Reprinted from [110], Copyright (2017), MDPI).

The different structures of graphene have been explored to enhance the photocatalytic performance of graphene. Graphene can be only used with a combination of other materials due to its lack of a natural band gap. Yibo Yan et al. prepared functionalized graphene quantum dots to improve performance [111]. Since the development of photocatalytic technology, TiO<sup>2</sup> has been a photocatalyst due to its high efficiency, low cost and good stability. TiO2/graphene composites are currently being considered as one of the promising candidates for photocatalytic applications. The study by Angel Pérez del Pino et al. is focused on developing an inexpensive, metal-free, eco-friendly and benign photo-active catalyst while eliminating unsafe metals and reagents with superior efficiency and practicability. Laser-exposure leads to the development of defects in lattice structure, crinkles and holes, nitrogen integration, reduction of GO recognized by a drop-in oxygen atoms attached to GO and a rise in carbon content. It was further ascertained that the proliferation in N-content in GO structure results in better charge segregation, hydrogen liberation and photo-catalytic efficacy. Thus, it was logically

concluded that the favorable factors for improving performance are nitrogen content assimilated in graphene and a degree of reduction, because redox reactions are favorably promoted by carbon adjacent to pyridinic-N. A hydrogen gas production rate of up to 89.29 µmol/L was recorded [112]. Shixiong Min et al. prepared TiO2–rGO–CoO<sup>x</sup> advanced photo-catalyst via two step hydrolysis-thermal process to separately integrate TiO<sup>2</sup> and CoO<sup>x</sup> species on rGO sheets. Despite the direct non-participation of rGO as a catalyst in redox processes in water splitting, its incorporation results in noteworthy increases in gas production rates and catalytic activity due to its quality of greater electron and hole kinetics, translating to lower charge combination and transportation of electrons from TiO2, and hole transport to CoO<sup>x</sup> [113]. Cheng, P., and Yang, Z. et al., improved the performance of typical photocatalyst TiO<sup>2</sup> by combining it with graphene, which has unique properties, such as flexible structure, huge specific surface area, high transparency and electron mobility, making it a worthy contender to be combined with TiO<sup>2</sup> to improve its photoactivity. The experimental results specified the solvothermal reduction of graphene oxide to graphene sheets, thereby augmenting the light absorption ability of P25–GR nanocomposites and also their charge separation efficiency. Graphene played the role of photogenerated electron acceptor due to its 2D π-conjugation structure and helped as an effective transporter in the separation of electron–hole pairs. The hybrid displayed superior activity towards the evolution of hydrogen from methanol solution under the illumination caused by Xe-lamp when compared to bare P25 due to improved ability for light absorption and a lower electron–hole pair recombination rate (Figure 7). The examination of photocatalytic activity of P25–GR nanocomposites based on graphene content affirmed a mass ration of 0.5 wt% GR to P25 as the optimum, with further addition of graphene leading to a decreased photocatalytic activity [39]. Li et al. worked to enhance photo-activity of traditional TiO2-rGO promoter and introduce an innovative method to fabricate these materials which is environmentally friendly, single-step, intense chemical-free and upgrades the gas production capability of the system by dropping band gap energy. The absence of surface Ti3<sup>+</sup> species and its presence in the bulk of the sample results in better performance with respect to H<sup>2</sup> liberation, creation of an alternate e-conduit, modifying the reaction from UV to the visible spectrum and absorbing charge transporters. Materialization of titanium–carbon bonds leads to reducing the band gap of titanium oxide, expanding the photosensitivity range, declining the charge conduction resistance and facilitating conduction between rGO and TiO<sup>2</sup> (Figure 8). Hydrogen productivity grew with increasing rGO content and laser exposure duration. Peak H<sup>2</sup> formation rate recorded was 16000 µmol/g·h and solar transformation efficiency was 14.3%, which were the subsequent outcomes of strongly coupled TiO<sup>2</sup> and rGO components [91]. μ

**Figure 7.** TEM image (**a**) and HRTEM image (**b**) of P25–10%GR [39]. (Reprinted from Int. J. Hydrogen Energy, Volume 37, Cheng P., et al., TiO2–graphene nanocomposites for photocatalytic hydrogen production from splitting water, Pages 2224–2230. Copyright (2012), with permission from Elsevier).

μ

**Figure 8.** Mechanisms of UV and visible light activation of TiO<sup>2</sup> with graphene. (Reprinted from [110], Copyright (2017), MDPI).

Sankeerthana Bellamkonda et al. studied unique photocatalytic composites of graphene and carbon nanotube amalgamations of semiconductor materials. The linking of reduced TiO2-x facets with carbon matrix exhibited the extraordinary photoactivity for the dissociation of water to produce hydrogen at room temperature. These nanohybrid composites produced hydrogen at 29,000 µmol/g·h, with 14.6% solar energy conversion efficiency, to hydrogen. The augmented photoactivity of this nanohybrid composite is ascribed to O<sup>2</sup> vacancies on the surface, the presence of Ti3<sup>+</sup> sites in the TiO<sup>2</sup> lattice and the interfacial interaction between TiO<sup>2</sup> and graphene [114]. Multi-layers of graphene result in increased light absorption, and the formation of 111 oriented gold nanoplatelets on multilayer graphene was analyzed by Diego Mateo et al. The orientation of incorporated Au NPs was detected using XRD and EDX. The hydrogen and oxygen production rates noted were 12,000 and 9000 µmol/g·h respectively. When compared with previous literature using similar materials, it was concluded that the enhanced rates were a result of using specialized one-step synthesis process along with the highly oriented attachment of Au particles on graphene material, which affected the strength of interactions, ability of charge separation and the photo-catalytic performance [115]. Mohamed, Mokhtar et al. found that energy conversion efficiency is also prohibited in most of these metal oxides due to their high band gap energies (Eg). To mitigate such problems, spinel families, AFe2O<sup>4</sup> (A = Co, Cu, Zn) have been examined as photocatalysts for H<sup>2</sup> production; particularly, manganese ferrite (MnFe2O4) hybridized with graphene/graphene oxide to decrease E<sup>g</sup> and particles size, and to enhance both electronic conductivity and surface area. Enhanced light absorption ability of these hybrids induced more charge carriers, thereby increasing their apparent quantum efficiency to work as photocatalysts for water splitting. A nanospindle-shaped GO1:Mn1 hybrid absorbed more visible light for the stated purpose of generating high current density values exceeding traditional semiconductors, which improved its performance as a photocatalyst [116]. Katsuya Iwashina et al. studied the photo-catalytic activity for water splitting using the rGO-TiO<sup>2</sup> composite (oxygen-producing promoter) as an electron-transfer facilitator and sulfide-metal materials as hydrogen-producing promoters. Sulfide metal materials in unaided form are unfitting for direct use for water-splitting promoters due to issues relating to photo-corrosion. However, when combined with semiconducting materials for hydrogen production and using rGO-TiO<sup>2</sup> functionalized material for oxygen liberation, efficient gas production was observed. This scheme is also feasible for use in solar-aided reactions. Loading of noble metal co-catalysts with sulfide metals affected the activity owing to the particle sizes and

extents of distribution. Moreover, the incorporation of platinum via adsorption showed the highest photo-catalytic character [117]. Zhenhua Pan et al. made RGO overcome the charge transfer challenge between loaded catalyst specks on a conductive sheet by introducing reduced graphene oxide as an arbitrating redox binder. The degree of reduction is an established factor in determining the equilibrium between hydrophilicity and conductive ability necessary for a photo-active material, which was ascertained in this case by XPS and Raman spectroscopy. The water dissociation rate was enhanced 3.5 fold by the incorporation of rGO in catalytic sheets, when compared to those that were devoid of rGO under identical circumstances, due to its inherent property of assisting in charge transmission. The role of Au as mechanical support and protection against self-oxidation was reiterated by results. Moreover, rGO helped in robust fixation of other active particulates on the Au sub-layer. Increasing its content to 1.25% (optimum) led to gradual growth in effectiveness, but a further rise in GO content caused the undesired excessive coverage of active species. Furthermore, the samples with superior dispersion of particulates presented an extra boost in efficiencies by as much as 1.7 times [118]. Graphene oxide (GO) based hybrid materials could be employed in several applications, including hydrogen generation. Carbon-based 2D structures processed by graphene oxide and its reduced form have controllable, layered structures; huge surface areas; and tunable optical, electronic and magnetic properties. Certain orthodox photocatalytic materials, such as Pt, TiO2, ZnS and CdS, could be dispersed onto these GO/rGO matrices, resulting in efficient hybrids for less energy intensive H<sup>2</sup> production via photocatalytic water dissociation reaction. Doping of metal ion, particularly Co2<sup>+</sup> and Al3+, considerably enhanced the production of hydrogen by rGO, indicating that efficient photocatalyst could be synthesized using simple hybrids of rGO and transition metal ions [119]. A novel combination of materials was studied by Xiujun Fan, et al. Nano-crystals of three transition metal carbides were enclosed in thin graphene nano-ribbons via hot filament chemical vapor deposition technique to obtain highly pure Fe, Co and Ni carbides supported on the peaks of graphite thorn-like upright nano-ribbon structure. The stimulating hierarchical structure produced inspired a comprehensive and relative study of the sizes, shapes and other dispersion factors of carbides on 2D graphene vertical conical arrays. The GNRs not only act as a supporting substrate to enhance stability but also prevent the aggregation of carbides on the surface, holding them well into place. The porous channels also expedite the electron transport to the core, causing a distinct decline in conduction resistance. The suitability of these materials for ORR has been indicated by ample reduction of oxygen at low Tafel slopes of 39, 41 and 45 mV/dec for Fe, Co and Ni carbide GNRs, respectively—comparable to 32 mV/dec value for Pt/C electrode. The HER current densities produced by M3C-GNRs (M = Fe, Co, Ni) were 166.6, 79.6 and 116.4 mA/cm<sup>2</sup> at 200 mV over-potential. These operative statistics can be ascribed to the coupled carbide-graphene effects of high roughness factors, large active area exposure and superior conducting ability and porosity of graphene [120].

#### *2.3. Phosphorene*

Phosphorene is a two dimensional (2D) material with a band gap of 0.3 to 2 eV from bulk phosphorus (Figure 9) to single layer phosphorene. The band gap of phosphorene is tunable depending on applied strain. The properties of phosphorene change with change in stress, number of layers and applied electric field [121,122]. It consists of singled-layer black phosphorus, is similar to graphene, is included in the family of monolayer-flatland materials and arranged in a hexagonal puckered lattice [50,123].

**Figure 9.** The layered and anisotropic crystal structure of elemental black phosphorus. (Reprinted with permission from [41]. Copyright (2015), Elsevier).

The advantage of phosphorene over graphene is that it shows a band gap. Black phosphorous was discovered around 100 years ago, but the discovery of phosphorene is just over four years old. Therefore, very little experimental research of phosphorene has been reported [124]. A monolayer of black phosphorous (BP) was synthesized using the sticky-tape technique used earlier for graphene; the material was titled phosphorene. Phosphorene has the promising properties of niche 2D materials. It shows the quantum confinement effect in a direction perpendicular to the 2D plane, which induces distinct electronic and optical properties; natural passivation of the surface without any dangling bond; no lattice mismatch concerns for assembling vertical heterostructures with other 2D materials; a high specific surface area; and a strong interaction with light. These properties are considered extremely necessary for photocatalytic applications. Additionally, phosphorene has a tunable band gap and shows anisotropic properties that make it suitable for a variety of applications [125]. Black phosphorous is the most stable allotrope of phosphorous. It has layered in-plane strong bonds along with weak van der Waals interlayer interactions. This layered structure provides the opportunity for fabrication of very thin layers of phosphorous by exfoliation from a bulk size. The techniques which have been reported for the fabrication of phosphorene so far are based on micromechanical cleavage using sticky-tape and solvent exfoliation methods. Single layer phosphorene has also been fabricated by combining mechanical exfoliation with plasma thinning technique. This sticky-tape micro mechanical exfoliation can yield phosphorene of high crystal quality, but with very low product density. In order to avoid chemical degradation, black phosphorus must be exfoliated using anhydrous and oxygen-free solvents such as acetone, chloroform, isopropyl alcohol or dimethyl form-amide. The technique of exfoliation with sonication results in low cost and a high yield of 2D phosphorene. It also seems feasible as a process for development in large scale production. However, it becomes practical only if there is a good matching of the surface tension between the layer crystal and the solvent, and if the solvent can stabilize the nanosheet to evade restacking and aggregation. Phosphorene shows a highly anisotropic structure in nature. The monolayer structure of black phosphorus is a 2D material; however, its excitonic properties closely resemble quasi-1D material, such as carbon nano-tubes. Similar to bulk black phosphorus, phosphorene shows quantum confinement perpendicular to the 2D plane. The band gap of phosphorene can be modulated appropriately for photon absorption in the ultraviolet to the near-infrared region of the solar spectrum. Apart from theoretical studies, there is actually very little published work that proves the water splitting capability of phosphorene, conclusively, at the laboratory scale [126]. However, some work has been done for phosphorene hybrids with various oxides, graphene and few sulfides. The hybrid of black phosphorous and TiO<sup>2</sup> has been reported as a photocatalyst in the quest for improved activity as shown in Figure 10. It was demonstrated that when bare BP was irradiated by visible light, it increased its degradation after various runs. However, when TiO<sup>2</sup> was loaded to form TiO2/BP hybrid, the degradation was reduced by 90%. The reason for the improved stability can be described as due to the substitution of Ti atoms into the BP matrix to make BP

resistant to moisture or oxygen. This study suggests that alteration of phosphorene with semiconductor materials might be a solution to increase the stability of phosphorene to ambient degradation and phosphorene can better be used as a photocatalyst [127]. In addition to BP/TiO2, BP/SnO2, BP/WO<sup>3</sup> and other oxides also need to be investigated, as no research has been published yet. Yongqing Cai et al. reported that graphene could be suitable as a supporting layer for encapsulating phosphorene to improve interaction and provide collaborative effects. This hybrid combination needs in depth study to find conclusive findings [128]. The eminent ability of black phosphorus nano-sheets to employ the solar energy efficiently (approximately 75%) in photocatalytic operation and the superior charge separation capacity of cobalt phosphide (CoP) can effectively be integrated in a single material, as supposed by Bin Tian, et al. [129]. The synthesis procedure consists of a straightforward solvo-thermal step involving white phosphorus and a Co-source (Co (NO3)2·6H2O) instead of the relatively tedious, expensive and complicated procedure of bulk BP synthesis and later exfoliation to nano-sheets. The special effects of CoP addition on BP resulted in manifold increase in H<sup>2</sup> release rate, 30 before and 735 µmol/g·h after CoP addition. MoS2/phosphorene and WS2/phosphorene hybrids demonstrate type II heterostructures. Type II is more expedient for electron-hole separation than type I and type III. The band gap of MoS2/phosphorene varies between 0.59 to 1.06 eV, whereas it varies between 0.90 to 1.44 eV for the WS2/phosphorene hybrid [130]. It seems that MoS2/phosphorene cannot be used as a water splitting photocatalyst because its band gap is much below the thermodynamics band gap requisite of 1.23 eV for a photocatalyst [131]. Very little work has been done for phosphorene/sulfide hybrids. Other potential 2D sulfides/phosphorene hybrids, such as CdS/phosphorene, CdSe/phosphorene and WS2/phosphorene, are yet to be explored. However, sulfide compounds are easily degradable in water, as they get irradiated by light. Hence, along with band gap congruency, stability is also major drawback with sulfides photocatalyst materials [132].

**Figure 10.** TiO<sup>2</sup> substitution on BP [127]. (Reprinted with permission from Lee, H.U., et al., Sci. Rep., 2015. 5: p. 8691).

#### *2.4. Metal Phosphides*

Phosphides of many transition metals are emerging as very popular photocatalysts due to their ability to adsorb hydrogen atoms and release H<sup>2</sup> gas, easily proving themselves to be very practical HER (hydrogen evolution reaction) catalysts, as confirmed by Shi and Zhang [133]. Furthermore, their tuneability to perform as dual function catalysts has been investigated via co-catalysis mechanism

to enhance both HER and OER processes by incorporation of metal hydroxo/oxo groups on material surfaces. Nickel, cobalt and iron phosphides and their binary and ternary derivatives have been tried and tested by many researchers for photocatalytic applications and have showed promising outcomes. The P-content in the metal catalyst material has been suggested as a major deciding factor in determining the HER activity of material, as it is found to be responsible for the catalyst's interaction and affinity with H atoms. This trend has been reported by Yuan Pan et al. [134] and Sengeni Anantharaj, et al. [135], who confirmed the direct relation between HER catalytic activity and P content; i.e., an increase in the P ratio results in a corresponding increase in activity as well as in release rate. The OER's (oxygen evolution reaction) activity depends principally on the optimal bond energy values of the interaction between catalytic material and reaction intermediates (oxide, peroxide and hydro peroxide). Nickel phosphide was investigated as a dual functional photocatalyst by Marc Ledendecker, et al. [136]. It showed high activities for both HER and OER, attaining 10 mA/cm<sup>2</sup> under 1.7 V. Ni5P<sup>4</sup> nanostructure was grown directly on Ni-sheets by an extremely straightforward process involving the heating of elemental phosphorus and Ni-foil sheet under inert conditions. The material showed good performance for the especially imperative OER kinetics, which is normally a challenging task in photocatalysis. This feature can be the consequence of formation of highly active NiOOH intermediate deposition on the surface which contributes to reduce the operation over-potential, the shift in electronic structure, superior current density and stable operation in a 20-h test conditions in acidic environment [136]. An alternate variation of nickel phosphide, Ni2P as a potential electrode catalyst was suggested by Lucas A. Stern, et al. [137], wherein the active specie was found to promote both the HER and OER kinetics presenting an economically feasible and efficient single-material substitute catalyst. The Ni2P nanoparticles and nanowires were fabricated through simple and scalable heating of the precursor ingredients, both of which showed comparable OER activities. The detailed structural studies showed the shell/core structure of NiOx/Ni2P which contributes prominently in the process as the Ni2P core offers an electronic passageway to the NiO<sup>x</sup> shell, thereby providing better diffusive characteristics and improved activity as compared to equivalent sized NiO<sup>x</sup> nanoparticles. The reported values of current density 10 mA/cm<sup>2</sup> achieved with overpotential were as insignificant as 290 mV for OER and 10 mA/cm<sup>2</sup> , with 136 mV overpotential for HER in acidic medium driving the developed catalyst material towards the mainstream photocatalytic water splitting systems as a promising and practical candidate material for both anodic and cathodic reactions [137]. Apart from nickel, other transition metal phosphides have also been investigated for catalysis to release hydrogen and oxygen from water by using incident light source. Cobalt phosphides emerged as a stimulating opportunity in this regard, and several reviewers and researchers have explored the utility of different forms and combinations of Co phosphides. Yang Y, et al. [138] formulated cobalt centric thin sheets via an intricate multistep process involving plasma cleaning of glass plate substrate, sputter-coating conduction nano-layers of chromium (10 nm) and gold (40 nm), Co-layer deposition by aqueous plating solution of CoSO4, formation of porous thin Co-oxide layer by anodic treatment of Co-layer under static voltage and then finally chemical vapor deposition of phosphorus for conversion to Co-phosphide while conserving the regular porous film (500 nm thickness). The ratio of film active area to the electrode surface area (denoted by roughness factor) was calculated as a parameter to endorse the high activity of material without the necessity of external additives. Hence, the proposition was confirmed by an observed increase in RF from six (compact-before anodic treatment) to 95 (highly porous). The operational tests showed steady operation and an exceptional HER activity of 30 mA/cm<sup>2</sup> at overpotentials of 175 mV (in acidic condition) and 430 mV (under basic conditions) which is even higher than most noble metal based contemporary catalysts, such as CoP nanoparticles (20 mA/cm<sup>2</sup> ), Co/CNT (10 mA/cm<sup>2</sup> ), MoP (10 mA/cm<sup>2</sup> ), Ni2P (20 mA/cm<sup>2</sup> ) and C3N<sup>4</sup> (10 mA/cm<sup>2</sup> ). Similarly, notable results were observed in terms of OER activity as well; i.e., current density of 30 mA/cm<sup>2</sup> at 330 mV overpotential which is again higher than most up-to-date studied catalysts, such as CoP nanoparticles (25 mA/cm<sup>2</sup> ), Au/Co3O<sup>4</sup> (25 mA/cm<sup>2</sup> ) and most ternary selections as well [138]. A valuable comparative analysis of different raw materials for CoP fabrication and their effectiveness in

terms of HER efficiencies as scrutinized by Afriyanti Sumboja, et al. [139] to provide an outlook for large-scale and commercially applicable source of CoPs manufacture. Salts such as cobalt acetate and cobalt acetylacetonate were phosphodized by sodium hypophosphite in a single-step. The CoP formed from the former ingredient exhibited better HER promotion (overpotentials: 160 mV in acidic, 175 mV in basic media) due to greater phosphide fraction (92.5%) and greater surface area (ECSA: 10–15 cm<sup>2</sup> ). While the latter being a readily oxidizable salt produces a greater fraction of metallic Co rather than phosphide (63.5% CoP), lower surface area (ECSA: 7–12.5 cm<sup>2</sup> ) and greater overpotential (169 mV in acidic, 188 mV in basic media) [139]. The application of Co-phosphide nanosheets as HER promoter in 1M KOH media has been studied by Liang Su, et al. [140] wherein the novel approach of effects of surface refurbishment of CoP nanosheets are exposed. The CoP nanosheets were conventionally synthesized by phosphidation of Cobalt oxides and then engineered electrochemically to promote an irrevocable and stable transformation into Co(OH)x/CoP hybrid configuration that displayed excellent HER catalysis producing 10 mA/cm<sup>2</sup> with 100 mV overpotential as compared to the untreated CoP nanosheet overpotential of 180 mV. The dispersion of hydroxyl group on the nanosheet surface as an outcome of 10 hours' exposure to 20 mA/cm<sup>2</sup> exhibit a coarser surface, greater turnover frequency of 0.234 per second and reliable operation even after 5000 cycles. These improvements in performance by the surface modification are attributed to the arrangement of Co(OH)<sup>x</sup> complexes which are efficient promoters of the breakdown of water into molecular components. As this stage is the rate-determining step of HER mechanism, this surface makeover of CoP nano-sheets proves to be highly favorable for the process and the copiously released H-atoms interact with adjoining CoP molecules to release hydrogen gas readily. The merger of both Co(OH)x and CoP specie is most beneficial for the process as compared to single-component systems as established by the comparative assessment of Co(OH)x/CoP (100 mV overpotential), untreated CoP (180 mV) and C3N<sup>4</sup> (300 mV) samples [140]. CoP nano-rods supported on Nickel foam as reported by Yun-Pie Zhu, et al. [141] is a novel strategy to fabricate dual-function HER and OER promoters with high efficacy, high porosity, enhanced diffusive transport and surface area (148 m<sup>2</sup> /g) while maintaining high conductivity. The synthesis procedure revolves around potentio-deposition of nanorods on Ni-foam while the CoP deposited is produced in situ, facilitated by direct electron passageways provided by Ni-foam. Current densities of 10,100 and 800 mA/cm<sup>2</sup> at 54, 121 and 235 mV overpotentials for HER and 100 mA/cm<sup>2</sup> for overall cell voltage of 1.62 V were accomplished which are even more efficient than Pt and IrO<sup>2</sup> system [141]. Ternary variants, such as Ni-Co-P have also been developed successfully to integrate the desired properties of component materials. Yingjie Li, et al. [142] cultivated Ni-Co-P nano-sheet arrangements on Ni foam by two stage hydrothermal and phosphorization scheme. The synthesized material demonstrated extraordinary performance for both HER and OER catalysis even when compared with the traditional commercially used noble metal (Pt/C, Ir/C) electrodes and the separately tested Ni2P and Co2P nano-sheet arrays produced by the same method. The overpotential values for Ni-Co-P ternary electrodes for HER and OER were 133 and 308 mV respectively necessary to operate at 50 mA/cm<sup>2</sup> and the cell voltage required was as minimal as 1.77 V. The remarkable activity and stability of material can be credited to marginal electron resistance, normalized hydrogen binding forces, material's inherent high activity and a distinctive super-aerophobic nano-framework [142].

A similar arrangement was prepared by Liang H, et al. [143] with plasma-facilitated growth of Ni-Co-P on Ni foam, concluding in a menial over-potential of 32 mV for HER and 280 mV for OER with 10 mA/cm<sup>2</sup> current density and 1.58 V cell potential. Bi-functional high porosity Ni-Co-P nano-sheet catalyst supported on carbon-fiber paper was developed by Rui Wu, et al. [144] by electrodeposition resulting in identical sheet arrangements with subsequent annealing to generate spongy Ni0.1Co0.9P structure. The annealing temperature, Ni: Co ratio of 1:9 and the precursors were deciding factors for optimal synthesis of active material. The detailed characterization results indicated that resulting material operated at high turn-over frequency (0.24 per sec for H2, 0.12 per sec for O2), reliable stability (20 h operation, 1000 cycles), better electron transmission ability and interactive sites contributed by Ni fraction and high efficiency. Cell voltage required for operation at 10 mA/cm<sup>2</sup> was a competitive

value of 1.89 V and the over-potential was 125mV as compared to 174 and 196 mV for CoP and Ni2P electrodes [144]. An ample amount of laudable work in the ternary Ni-Co-P catalyst and its composites category can be mentioned such as NiCoP/Carbon cloth composite as dual function catalyst reported by Cheng Du, et al. [145], with overpotentials as petite as 44 and 62 mV for HER and OER respectively at 10 mA/cm<sup>2</sup> current density and 1.77V cell potential for 100 mA/cm<sup>2</sup> output paves the way for practical non-noble metal electrode materials. NiCoP/r-GO composite for stimulating both HER and OER synthesized by Jiayuan Li, et al. [146] presented 10 mA/cm<sup>2</sup> current density with cell voltage 1.59V with stable operation for more than 75 hrs. The coupling of Co-doping in Ni2P and their combination with rGO resulted in a boost in electron transport, catalytic activity and surface arrangement of active spots. An elegant and novel heterostructure involving NiCoP@Cu3P nano-sheets was cultivated on Cu foam by Xingxing Ma, et al. [147] as a favorable candidate for both HER and OER catalysis showcasing 54 mV and 309 mV over-potentials for HER and OER at 10 mA/cm<sup>2</sup> . Similarly an intricate system of NiCoP nano-sheets grown on N-doped Carbon-coated Ni foam as formulated by Miaomiao Tong, et al. [148] through hydrothermal and phosphorization route required 31.8 mV and 308.2 mV for HER and OER to accomplish 10 mA/cm<sup>2</sup> current density. The durable performance and activity experiments showed comparable outcomes to the commercial Pt/C and Ru2O catalysts.

#### *2.5. Metal Organic Frameworks and Derivatives*

Metal organic framework or MOF is the peculiar category of crystalline synthetic materials that has claimed a prominent position in the league of modern multi-functional materials due to the distinctive properties of high porosity, adaptable pore structures, larger surface area and amendable elemental compositions. MOFs comprise of a network assembly of metal centers linked to organic bridging ligands which result in a variety of geometries, compositions and pore spacing dependent upon the synthesis procedure, conditions and initiators. The unique metal-centric interlinked geometric morphology allows MOF applications in various fields including, but not limited to, gas adsorption and separation, catalysis, drug delivery systems, optics and sensors [149–157] and the possibilities and opportunities of its efficient utilization and improvements are continuously being analyzed by researchers every day [158,159]. Numerous MOF-based catalyst materials have been synthesized, characterized and upgraded for photocatalytic water splitting reactions in recent times [160,161] with a focus to develop novel and feasible bi-functional promoters to make the concept of commercial solar hydrogen production from water a practical reality [162]. The standard practices to convert MOFs into practical photocatalysts comprise of inclusion of light-sensitive building blocks into the MOF backbone for instance aminoterephthalates [163,164] and porphyrins [165], Doping with any active metal or noble metal in order to enhance catalytic capacities [166], inorganic nano-particles encapsulated in MOFs [167], assembling ternary composites and introduction of electron donor or trapping materials to prevent charge recombination challenges [168]. A primary perspective of efforts in this regard will be presented herewith. The long-standing fact of high activity of noble metals in water splitting systems photocatalytically was incorporated into MOF-based materials by Meng Lan, et al. [169] to generate 3 types of Pt-tempered heterojunctions; i.e., Pt-ZnS-CoS, Pt-Zn3P2-CoP and Pt-ZnO-Co3O4. The synthesis procedure involved the ternary steps of Zn-Co ZIF preparation by previously reported methods [170–173], the sulfidation, phosphorization or oxidation of base ZnCo ZIF for each sample separately and the subsequent doping of Pt nano-particles. The twin-metal ZIF backbone facilitates the light absorption ability, increases the active area contact and favors charge carrier transportation and separation, while the dispersed Pt particles capture the electrons and motivate the hydrogen production. The resulting hydrogen release rates by above listed hetero-junctions are 8210, 9150 and 7800 µmol/g·h respectively. In the structural analysis, it was noted that the polyhedral MOF shape and porosity was retained after Pt-doping as well and the operational durability was noted to be unaffected even after 5 cycles. Cheng Wang, et al. [174] reported the fabrication of two kinds of Zr-carboxylate MOFs by intricate methods and their subsequent center-doping with Pt-nanoparticles whereby the produced Pt@MOF material displayed the ability to catalyze HER, recyclable nature by

simple centrifugation and the turnover number of 7000. Another example of incorporating Pt into MOF units for enhanced activity has been reported by Dengke Wang, et al. [175] wherein, the Pt nano-particles were disseminated uniformly via a novel photo-reduction method in MIL-100(Fe) MOF which has formerly been established as a water stable and photo-excitable material [176]. The introduction of optimal fraction of Pt (0.8%) in MOF structure verified the hydrogen release increase from 5.9 to 109 µmol/g·h (18.5 times of pristine MOF). This prime value is observed due to the collaborating mechanism of electrons generation by photo-active MOF structure and the immediate electron seizing by central Pt also acting as the prime H<sup>2</sup> production site. This coordinated functioning results in enhanced charge separation without compromising on the exposed excitation area of for light. Qin Liu, et al. [177] used a permeable Ni-foam as a substrate to utilize its conductive capacity, high exposed surface region and ease of electronic diffusion through the channels towards the active dispersed Ni-MOF arrays which allowed the current capacity to reach 100 mA/cm<sup>2</sup> at 320 mV over-potential and turnover frequency of 0.25 mol/s O2. The in-situ hydrothermally grown Ni-MOF nano-sheet arrays on Ni-foam was analyzed thoroughly and compared to commercial Ru2O/NF OER catalyst and simple NF (Nickel foam). The results displayed the trends of lower Tafel slope 123 mV/dec than NF (193 mV/dec) but higher than Ru2O/NF (72 mV/dec) due to the different OER mechanisms followed by these materials. Faradaic efficiency measured for oxygen release by contrasting the theoretical and practically released oxygen was nearly 100% with a stable operation for 20 h test time endorsing its practicality as a successful OER promoter. In 2016, ultra-thin nano-sheets of bi-metallic NiCo MOF were prepared with a purpose to promote oxygen discharge from water by Shenlong Zhao, et al. [178]. Utilization of nano-sized ultra-thin MOF sheets introduces the merits of lower mass transport resistance, better electronic pathways for conduction, superior exposure to the remote active sites and un-bound highly active free metal sites which act in combination with the intrinsic effectiveness of Ni and Co base metals in OER catalysis. The 2D NiCO, ultra-thin MOF nano-sheets supported on Co-sponge exhibited reassuring performance with over-potential 189 mV at 10 mA/cm<sup>2</sup> , continuous and stable operating time of 200 h and Faradaic efficiency of 99.3%. These parameters verified the hypothesized collective benefits of incorporating two-metals in MOF network and the working decision to use ultra-thin MOF structure on a porous support. A similar study using a different combination of metals, i.e., Ni-Fe ultrathin (≈3.5 nm) MOF nano-sheets, was conducted by jingjing duan, et al. [179]. This Ni-Fe combo also showed superior analytical parameters due to the identical structural and elemental choices as previously listed work. The over-potential observed in this case was 240 mV for 10 mA/cm<sup>2</sup> with noteworthy turnover frequency of 3.8 per sec and stable efficient operation for 20000 s. A three-component engineered MOF structure was developed by Rui Lin, et al. [180] that combined established photo-active ingredients to make a strong partnership of UiO-66 MOF, CdS and reduced graphene oxide. Inspired to resolve the electron-hole recombination challenge in CdS photocatalysts, combining CdS/rGO composite with UiO-66 MOF not only resolved this issue but added the benefits of its high contact area, considerable thermal stability [181] uniform pore channels and proven photo-active nature of UiO-66 for hydrogen production [182]. Consequently, the UiO-66/CdS/1%-rGO combination showed the best charge separation capacity as compared to other proportions of rGO and the hydrogen release 13.8 times superior to pristine CdS. Another variation of ternary MOF photocatalyst is based on MoS2/UiO-66/CdS fusion structure [183] which delivered 32,500 µmol/g·h H<sup>2</sup> release rate. A CdS/MOF combination was developed to promote H<sup>2</sup> evolution reaction by Jiao He, et al. [184] with the unusual idea of supporting CdS active material on photocatalytically inert MIL-101 MOF defining its sole purpose of providing greater exposed active area. MIL-101 has Cr centric structure with benefits of excellent air, water and heat stability. The supported CdS@MOF formed a crystalline cubic shape and the highest H<sup>2</sup> release rate was noted for 10 wt% proportion on CdS reaching upto 75,500µmol/g.h. Another specimen using MIL-101 was prepared by amine sensitization along with Pt co-catalyst doping in 2014 to obtain greater hydrogen efficiency and stability [185]. Furthermore, Co-oxide restrained in MIL-101 was prepared by intricate method involving double solvo-thermal technique to create a robust material (TOF: 0.012/sec per Co-atom) with a rich oxygen

productivity (88%) [186]. The inventive strategy to produce a photo-sensitive MOF combination was explored to directly synthesize the MOF using dye-resembling organic azobenzene tetra carboxylic acid as an organic ligand and the metal (Gd) salt as the central backbone element. Xiaojun Sun, et al. [187] used a solvo-thermal method to synthesize yellow colored Gd-MOF crystals which were later also metal-laden with Ag (1–2%) as a second catalyst. The experimental comparison showed 154.2 µmol/g·h H<sup>2</sup> release from pristine Gd-MOF and 212 µmol/g·h from Ag (1.5%)/Gd-MOF under UV–Vis exposure due to the introduction of electron-trapping noble metal nature. Ag proved to be a suitable and feasible option as a noble metal dopant due to its cost effectiveness, lower electron transport resistance and decreased charge re-combination by capturing excited electron species released from photoactive MOF [188]. An alternate approach to integrate the qualities of superior photo-sensitivity, high HER activity and larger exposed surface area is to prepare syndicate of materials serving these purposes as formulated by Jiao He, et al. [189] wherein, a light sensitive Cr MOF UiO-66 was loaded with Pt nano-particles for HER activity and rhodamine B (RhB) dye for photo-activation. The impact of this fusion is clearly evident from H<sup>2</sup> evolution rates whereby pure UiO-66 displayed no perceptible H<sup>2</sup> release, Pt/UiO-66 showed 3.9 µmol/g·h and adsorbed RhB/Pt/UiO-66 showed 116 µmol/g·h which is 30 times greater than un-treated Pt/UiO-66. A different photo-active MOF variant MIL-125-NH<sup>2</sup> for H<sup>2</sup> production from water can be used as the base material with a MoS integration, as discovered by Tu N. Nguyen, et al. [190], to produce H<sup>2</sup> evolution rate as high as 2094 and 1454 µmol/g·h with quantum productivity 11% and 5.8% using two different kinds of MoS clusters; i.e., Mo3S<sup>13</sup> and IT-MoS respectively. MoS<sup>2</sup> is an inherently photo-active compound [191] and the sources of light responsive behavior are the vacancy defects and sulfur-edges only [192,193] while the basal faces do not contribute in catalysis and the size of particles is inversely related to the HER activity [194] Robust interaction between the supporting MOF and MoS result in higher exposed active area, lower electron conduction resistance and high stability. In an innovative approach to replicate the active nodular-structure of [NiFe] hydrogenase complex [195], a two-dimensional layered photo-responsive MOF [Ni2(PymS)4]<sup>n</sup> was fabricated via cautiously supervised multi-step hydrothermal process [196]. The resultant material showed robust water and air stability, bi-atomic Ni webbed morphology with 4 PymS<sup>−</sup> linkers and turnover frequency of 10.6 per hour and hydrogen release upto 6017 µmol/g·h using TEA as electron source, flourescien as photo-activator and white LED or visible light as source. The pH of solution, nature of sensitizer, nature of electron source and amount and particle size of catalyst were observed to be the deciding factors to determine the practical competence of the catalyst. A dual function catalyst with the aim of promoting both HER and OER components of the water dissociation process has many advantages and has been a center of research since decades now. A MOF-based route to achieve this goal was proposed by Yang An, et al. [169]. Pt and CoPi components were dispersed onto the innately photo-active MIL-125 (Ti) MOF [131] to boost hydrogen and oxygen discharge and suppress charge recombination tendency by the deposited components. The experimental H<sup>2</sup> and O<sup>2</sup> liberation rates under UV–Vis exposure were 42.33 and 21.33 µL/h. Separate influence of trace materials on the half-reactions were confirmed by appraising the MIL-125 (Ti) with CoPi@MIL-125 (Ti) and Pt@MIL-125 (Ti), which verified that the OER activity of CoPi-activated sample amplified to 307 µL/h from 143 µL/h, whereas in case of HER, introduction of Pt on MOF increased the activity 25 fold. Furthermore, the order of Pt and CoPi doping is a significant factor as a higher activity was noted when Pt was loaded first. Recently, an aluminum-centric MOF devised from 2-aminoterephthalic acid (ATA) was recognized as an active oxygen evolution promoter and modification of this material with Ni assimilation resulted in a dual purpose (HER and OER) overall water dissociation catalyst [197]. The half reaction analysis validated the coordinating nature of both Al and Ni components as OER for Al-ATA-Ni MOF was 5166.7 µmol/g·h contrasted with 550 µmol/g·h and HER for Al-ATA-Ni MOF was 1200 µmol/g·h and the material without Ni was found inactive towards hydrogen production. A bipyridine-centered MOF-253 was modified by confining Pt complex into the network with the intention to promote HER and simultaneously act as a photo-exciter [198]. The base MOF-253 possesses the anticipated merits of high exposed area and promising stability [199], which is evident in the

resulting compound and the addition of Pt component translates to five times increase in photo-activity. Another Ti-centric MOF was amended by incorporating Ru-complex established on the similar strategy to promote HER in visible light presence [200]. Despite being a relatively new concept and material, 2D MOFs have found various application in electrocatalysis, photocatalysis and heterogeneous catalysis in general [201]. The focus on optimization of manufacturing procedure, product stability, surface coating and immobilization techniques and scalability can bring this novel category of materials into limelight for future catalytic processes due to the inherent benefits of exposed active sites, turnover frequencies, effective coordinated properties of metal and organic ligands and easy handling capability.

#### **3. Conclusions and Future Outlook**

A tremendous amount of research and development has been carried out to improve, study and implement various strategies in photocatalytic water splitting using different categories of materials, composites, heterojunctions and immobilizing schemes, as summarized in Table 2 [202,203]; however, the use of 2D materials, their combinations and derivatives is a developing concept in photocatalytic water splitting research, which poses itself as an excellent opportunity for advancement and further exploration of innovative materials. With tunable and unique architectures, distinctive features and desirable structural intricacies being the specialties of 2D materials, it seems imperative to delve into this arena for progressive photocatalysis systems. Keeping these positive outlooks and prospects in view, this review highlights the evolution of modern nanomaterials, 2D photocatalysts and MOF derivatives to attain the coveted properties of low charge recombination, high photo absorbance activity, efficient electron conductivity, fast kinetics and large surface area for water splitting to produce hydrogen. It has been concluded that production of hydrogen can be enhanced using various techniques, including doping with noble metal particles, surface functionalization, nano-heterostructures, synthesis-controlled morphology and surface decoration of nano-particles in 2D cages. The most common backbone materials noted in this regard are the graphitic carbon nitride, and graphene, which serve as supporting pillars and high area foundations in case of composite materials and hetero-structural modifications. Meanwhile, phosphorene has emerged as a relatively novel material and limited research has been done to explore the possibilities of practical application of this material. Metal phosphides and metal organic frameworks have been extensively incorporated in 2D heterostructures and as supporting cocatalysts to improve performance and functional enhancement of base materials and assure performance improvement with further research and experimentation. With the acknowledgement of the fundamentally essential properties displayed by the 2D base materials and their variants, as discussed in this article, it is safe to conclude that photocatalytic research and the commercial application of this promising technology has found 2D materials and their hybrids to be promising and stimulating candidates. However, despite the progress in research, the challenges associated with 2D materials present an opportunity to explore improvements in existing 2D materials and the development of new alternatives with novel electronic and structural features, such as electrochemically reduced thin films (e.g., α-Fe2O3) [28], layered oxyhalides (FeOBr, Bi4VO8Cl, etc.), thiophosphates (CoPS3), metal chalcogenides, 2D MOFs and metal-free semiconducting materials [46]. Moreover, the setbacks associated with cost effective, bulk production of these ultrathin photocatalysts limits the scalability for prospective commercial operation which needs to be addressed and resolved. The odds of bulk scale synthesis from nano-layered and non-layered materials to manufacture 2D thin sheets must be improved to make thickness-controlled and efficient 2D photocatalysts. Debottlenecking of issues related to stability in water splitting reaction conditions, such as agglomeration, corrosion and oxidation, must be prioritized to explore surface deformation control and reduction of surface energies. Fast charge carrier re-combination could be mitigated by sacrificial agents, but better practicality demands investigation of alternative solutions, such as 2D/2D, 2D/1D and 2D/0D heterojunctions to introduce interfacial charge separation. Modern systems and industries thrive on the basis of technology; thus, optimization and simulation tools must be introduced to foster research and development in photocatalysis and eliminate the gap between theoretical and real-time reaction systems.

**Table 2.** Summary of photocatalytic water splitting systems discussed in this article involving g-C3N4, graphene, phosphine, metal phosphides and metal organic frameworks MOFs.



**Table 2.** *Cont.*

**Author Contributions:** All authors have contributed to the research work. Z.S. and M.U.Y. are masters students and both have written this paper under the supervision of E.P. She gave the main idea to the students about writing a comprehensive review article related to very emerging field of water splitting and guided students about how to write. M.B.K.N. have helped us a lot in grammatical purview and proof reading. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research is funded by Higher Education Commission (HEC) Pakistan, financial grant no 2017/HEC/NRPU/10482 and APC was funded by National University of Sciences & Technology (NUST), Islamabad Pakistan.

**Acknowledgments:** Author Erum Pervaiz would like to acknowledge NUST Pakistan and the HEC project (2017/NRPU-10482) for financial support.

**Conflicts of Interest:** All authors have mutually agreed upon this submission and there is no conflict of interest that needs to be declared.

#### **References**


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