Next Article in Journal
On a Response Surface Analysis: Hydrodeoxygenation of Phenol over a CoMoS-Based Active Phase
Next Article in Special Issue
Modified BaMnO3-Based Catalysts for Gasoline Particle Filters (GPF): A Preliminary Study
Previous Article in Journal
Asymmetric Synthesis of Both Enantiomers of Dimethyl 2-Methylsuccinate by the Ene-Reductase-Catalyzed Reduction at High Substrate Concentration
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Review

g-C3N4-Based Direct Z-Scheme Photocatalysts for Environmental Applications

by
Javier Fernández-Catalá
1,2,*,
Rossella Greco
2,
Miriam Navlani-García
1,
Wei Cao
2,
Ángel Berenguer-Murcia
1 and
Diego Cazorla-Amorós
1
1
Inorganic Chemistry Department, Materials Science Institute, University of Alicante, Ap. 99, 03080 Alicante, Spain
2
Nano and Molecular Systems Research Unit, University of Oulu, 90014 Oulu, Finland
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(10), 1137; https://doi.org/10.3390/catal12101137
Submission received: 29 August 2022 / Revised: 14 September 2022 / Accepted: 22 September 2022 / Published: 28 September 2022

Abstract

:
Photocatalysis represents a promising technology that might alleviate the current environmental crisis. One of the most representative photocatalysts is graphitic carbon nitride (g-C3N4) due to its stability, cost-effectiveness, facile synthesis procedure, and absorption properties in visible light. Nevertheless, pristine g-C3N4 still exhibits low photoactivity due to the rapid recombination of photo-induced electron-hole (e-h+) pairs. To solve this drawback, Z-scheme photocatalysts based on g-C3N4 are superior alternatives since these systems present the same band configuration but follow a different charge carrier recombination mechanism. To contextualize the topic, the main drawbacks of using g-C3N4 as a photocatalyst in environmental applications are mentioned in this review. Then, the basic concepts of the Z-scheme and the synthesis and characterization of the Z-scheme based on g-C3N4 are addressed to obtain novel systems with suitable photocatalytic activity in environmental applications (pollutant abatement, H2 production, and CO2 reduction). Focusing on the applications of the Z-scheme based on g-C3N4, the most representative examples of these systems are referred to, analyzed, and commented on in the main text. To conclude this review, an outlook of the future challenges and prospects of g-C3N4-based Z-scheme photocatalysts is addressed.

1. Introduction

Environmental protection and energy generation are two of the great challenges that mankind is presently facing, since they have a large negative impact on our society, affecting, among others, air pollution, climate change, water pollution, thermal pollution, and solid waste disposal [1,2,3]. Such problems have been caused by rapid industrialization, uncontrolled environmental pollution, and current energy scenario based on fossil fuels [4]. Aiming to solve these issues, current policymakers are passing regulations to reach full environmental sustainability and avoid a prolonged energy crisis and environmental deterioration. One interesting approach to solve this issue is to replace the use of carbon-based energy with solar energy [5,6]. Indeed, solar energy is a green, inexhaustible, and clean resource [6,7]. Another essential approach to achieve the sustainability of our planet is the degradation of pollutants to less toxic products and the reduction of CO2 to obtain value-added products, which can be achieved under the irradiation of solar light [8,9].
Photocatalysis represents a promising technology that might alleviate the current environmental crisis [10]. Semiconductor photocatalysts may generate H2 by water splitting, degrade organic pollutants, reduce CO2 into high value-added products, etc. using solar energy. Moreover, heterogeneous photocatalysis can be performed under very mild conditions (room temperature and atmospheric pressure) [11,12]. Although photocatalytic technology presents relevant characteristics, such as being clean, safe, and renewable, it is still far from commercial implementation, especially in solar-to-fuel conversion [13,14]. Titanium dioxide (TiO2) has been widely studied in the field of solar energy conversion after the pioneering work reported by Fujishima and Honda in 1972 based on photoelectrochemical water splitting on a TiO2 electrode [15]. Furthermore, this semiconductor has received great interest from the scientific community for its use in pollutant abatement due to its advantages, e.g., low price, high durability, and abundance [16]. To understand the relevance of photocatalysis, it is important to briefly describe the reaction mechanism [17]. Initially, a photon with energy equal to or greater than the energy band gap (Eg) of the semiconductor is absorbed by the semiconductor. Then, a photoexcited electron (e) is promoted from the valence band (VB) to the conduction band (CB). This effect leads to the formation of a hole (h+) in the VB and, consequentially, to the generation of an electron-hole pair (e-h+). The produced e and h+ can recombine on the surface or within the photocatalyst rapidly and thus decrease the photocatalytic activity or they can also migrate to the surface of the semiconductor and initiate the redox reaction(s) with the species adsorbed on the surface of the photocatalyst. The generated holes can induce oxidation processes, which occurs in organic molecules abatement, and the electrons can promote reduction processes such as hydrogen evolution in water splitting and CO2 reduction. To develop efficient photocatalysts, light absorption is an essential step in the manufacture of the material [18]. Pristine TiO2 is active only under UV light with a wavelength below 387 nm due to its wide bandgap (3.2 eV). This fact is the main drawback of TiO2 since solar energy is mainly concentrated (between 95 and 97%) in the IR and visible light regions [16]. Consequently, the exploration of visible-light-responsive photocatalysts with high efficiency is a highly interesting topic for the scientific community [19,20].
Graphitic carbon nitride (g-C3N4) is a very promising material due to its potential application in photocatalytic pollutant degradation, photocatalytic H2 production, and CO2 reduction. Its bandgap of 2.7 eV makes it a good candidate as a visible-light-responsive photocatalyst. In addition, g-C3N4 presents a unique electronic band structure, low cost, and easy preparation, allowing it to be a possible alternative to TiO2 in solar energy applications [21,22]. However, pristine g-C3N4 still exhibits low photoactivity in several applications such as water splitting and CO2 reduction due to the rapid recombination of photo-induced e-h+ pairs [23]. In recent years, several strategies have been developed by the scientific community to improve the photocatalytic efficiency of g-C3N4, i.e., morphological control, doping elements, deposition of noble metals, and construction of heterojunctions [24,25].
The latest advancements on g-C3N4-related photocatalytic systems are based on g-C3N4-based heterostructures due to the effective separation of photogenerated e-h+ pairs [26,27]. Traditional heterojunctions are categorized into type-I, type-II, and type-III, as shown in Figure 1. A different pathway of the traditional type-II is the Z-scheme family (Z-schemes and S-schemes) [28,29], as shown in Figure 1 [30,31]. Since the present review is focused on direct Z-scheme heterojunctions, only type II heterojunctions will be described, as shown in Figure 1b. Type II heterojunction formation using g-C3N4 is a noteworthy method used to increase the photocatalytic efficiency by suppressing the recombination of charge carriers [24]. However, type II heterostructures limit the oxidation and reduction efficiency of the heterojunction material from the parent materials [27]. To solve this drawback, the synthesis and study of Z-scheme photocatalysts may be a good compromise since Z-scheme-based photocatalysts present the same band configuration but follow a different charge carrier recombination mechanism. Thus, in the Z-scheme, the strongest reduction and oxidation potentials of each semiconductor are preserved similarly to what happens in a natural process such as photosynthesis [32,33].
There are two types of Z-scheme semiconductor heterojunctions [34,35]. The first one is the indirect Z-scheme (redox-mediated and all solid state), where the transport route of photogenerated charge carriers is not achieved directly but by the addition of an electron mediator. The second type is the direct Z-scheme, which consists of two semiconductors in close contact, eliminating the demand for an electron mediator. These novel direct Z-scheme photocatalysts have attracted the attention of the scientific community, increasing the use of these photocatalysts. Indeed, the absence of mediators eliminates the backward reaction and light-shielding effects [36,37]. The layered structure of g-C3N4 is a suitable building block for Z-scheme photocatalyst construction due to the possible high surface area, provided that a layered material is obtained, and its ability to perform photocatalytic reduction reactions (water splitting and CO2 reduction), making it possible to couple g-C3N4 with a wide range of oxidation-type photocatalysts to fabricate Z-scheme photocatalysts [21,38,39]. This fact has boosted the study of Z-scheme photocatalysts based on g-C3N4 for their use in environmental applications, as evidenced by the increase in the number of scientific publications on this topic during the last decade (Figure 2). Therefore, it becomes timely to carry out a bibliographic review in which the most relevant works of direct Z-scheme photocatalysts based on g-C3N4 and their environmental applications are compiled and summarized.
In this review, we will briefly describe pristine g-C3N4, although it still exhibits low photoactivity in several applications such as water splitting and CO2 reduction due to the rapid recombination of photo-induced e-h+ pair. Then, a new alternative to improve the photocatalytic activity of g-C3N4 based on the design of novel Z-schemes will be widely described. Although Z-schemes are applied in relevant photocatalytic reactions, such as environmental applications [34,40], energetic applications [40], transformation of organic compounds to added-value products [41,42,43], and photocatalytic biorefineries [44], among others, this review will cover most relevant works on g-C3N4-based Z-scheme photocatalysts and their use in environmental applications such as pollutant abatement and CO2 reduction due to the high impact in our society, as was mentioned previously. In addition, energy aspects, e.g., water splitting, are also briefly reviewed.

2. Pristine g-C3N4

Graphitic carbon nitride is a stable polymeric and layered semiconductor formed by the polymerization of abundant nitrogen-containing precursors, such as melamine or urea [45,46]. The first report about g-C3N4 was written in the 1830s by Berzelius and Liebig [47]. Since then, many research studies have been carried out to investigate the structure of this material and to develop different synthetic routes [45]. The structure of g-C3N4 is based on triazine or heptazine units polymerized in a layered structure (Figure 3a) [48,49]. In the last decades, g-C3N4 has been presented as a promising material for its use in several applications such as sensors, energy storage, and photovoltaic cells, among others, since it presents facile, low-cost, and environmentally friendly preparation methods with promising stability and good physicochemical properties [45,50]. Focusing on the application of this review, g-C3N4 is a good candidate as a photocatalyst because it is a visible-light-active photocatalyst with a bandgap of ~2.7 eV (~460 nm) [51,52]. In contrast to other semiconductors, g-C3N4 does not contain metal ions, which may be leached into the ecosystem and cause unfavorable impacts therein. Instead, it is composed of Earth-abundant carbon and nitrogen elements and can be easily synthesized using nitrogen-rich organic precursors by various methods. The obtained g-C3N4 is endowed with desirable electronic structures and unique morphologies, and high thermal stability up to 600 °C in air [46].
Nowadays, there are several innovative and relevant synthesis methods of g-C3N4, that may be classified as solvothermal, chemical vapor deposition (CVD), plasma sputtering reaction deposition, and polycondensation [21,53]. Among them, thermal polycondensation is the most attractive method due to its simplicity and low cost. Nitrogen-rich small molecules can polymerize into g-C3N4 following a calcination process at 450–650 °C (Figure 3b) [54]. The choice of the precursor exhibits effects on the electronic band structures and the textural properties of the obtained pristine bulk g-C3N4 [55]. In this sense, Wang et al. demonstrated in 2009 that g-C3N4 was a promising visible-light photocatalyst for H2 evolution. The as-prepared g-C3N4 achieved steady H2 production from water containing triethanolamine as a sacrificial electron donor on light illumination (λ > 420 nm) even in the absence of noble metal co-catalysts such as Pt (4 μmol/h), opening a new approach for the search of materials for H2 production [56]. After this pioneering work, the scientific community has carried out a lot of work to synthesize g-C3N4 by thermal treatment of nitrogen-rich precursors such as urea, thiourea, melamine, cyanamide, and dicyandiamide, among others, to obtain g-C3N4 in different morphologies such as nanosheets or nanotubes [57,58]. However, pristine g-C3N4 presents several drawbacks for its use in photocatalysis, including a low specific surface area, insufficient visible light utilization, and, the most relevant, rapid recombination of photogenerated charge carriers [23].

3. Direct Z-Scheme Photocatalysts Based on g-C3N4

3.1. Direct Z-Scheme Photocatalysts

To compensate for the drawbacks of pristine g-C3N4 for its use in photocatalysis, as described in Section 2, a novel alternative is the design of heterojunctions, defined as the interface between two different semiconductors, which can result in suitable band alignments [59,60]. The design of type II heterojunctions using g-C3N4 is an interesting method to increase the photocatalytic efficiency by suppressing the recombination possibilities of the generated e-h+ pairs. In type-II heterojunction photocatalysts, the CB and the VB levels of semiconductor 1 are higher than the corresponding levels of semiconductor 2. Electron and hole pairs are separately generated in 1 and 2 subjected to light irradiation. The photogenerated electrons will transfer to semiconductor 2 while the photogenerated holes will migrate to semiconductor 1. Consequently, type II heterostructures diminish the oxidation and reduction efficiency of the isolated materials, which is the main drawback of this heterostructure (Figure 4c) [21].
Inspired by natural photosynthesis (Figure 4a), the artificial Z-scheme heterojunction design is a great alternative for the development and manufacture of novel photocatalysts, with outstanding results in numerous applications [61,62]. The Z-scheme photocatalytic system concept was proposed by Bard in 1979 for the first time [63]. Similar to what happens in photosynthesis, under light excitation, the electrons generated in a semiconductor with low reduction potential are recombined with the holes with low oxidation potential, i.e., those with the highest potential. This phenomenon generates electrons and holes isolated in the Z-scheme system with maximum redox abilities, making it the greatest advantage of these materials (Figure 4b) [34]. The traditional Z-scheme introduced by Bard [63] needs a shuttle redox mediator (electron acceptor/donor pair) to form a liquid-phase Z-scheme. Thus, Z-scheme photocatalysts suffer from the limitations of redox mediator reversibility and their specific applications, e.g., CO2 reduction, can only be applied in the liquid phase [34]. The second generation of Z-scheme (all solid-state Z-scheme photocatalysts) was discovered in 2006 by Tada et al. [64]. These are composed of two different semiconductors with a solid-phase electron mediator as a noble metal nanoparticle (NP) or carbon material (graphene and carbon nanotubes) [65]. To solve the inconvenience of using electron mediation, Yu et al. [66] constructed a direct Z-scheme photocatalyst by combining g-C3N4 and TiO2 in 2013. The interfacial contact between the semiconductors facilitates the direct electron transfer without the help of an electron mediator. This novel direct Z-scheme system presented the advantage of significantly reducing the construction cost [37].
In fact, g-C3N4 has received significant attention due to its potential use as a reductant semiconductor in direct Z-scheme photocatalysts given its superior chemical and physical features [26,33]. In addition, the great advantage of its use as part of Z-scheme photocatalysts is that the VB and CB positions are located at approximately +1.6 and −1.1 eV, respectively [28]. This redox potential converts g-C3N4 into an interesting reduction semiconductor for its use in several applications, such as H2 production and CO2 reduction (Figure 5) [67]. Nevertheless, the formation of the g-C3N4/semiconductor interface is a challenge for a Z-scheme heterostructure [33,68]. Therefore, it is necessary to design and modify novel routes of synthesis to tackle the drawbacks of these systems.

3.2. Synthesis and Characterization of Z-Scheme Photocatalysts Based on g-C3N4

Generally, the synthesis of Z-scheme photocatalysts is crucial to obtain systems with high efficiency in photocatalysis. In the literature, there are several methods to synthesize catalysts or composites such as deposition, solid-state synthesis, and hydrothermal synthesis, among others [22,37,69]. In the synthesis of Z-scheme photocatalysts, two semiconductors are combined to optimize the oxidation and reduction potential by the recombination of e-h+ pairs, which makes intimate contact between both semiconductors crucial [63]. To increase the activity of photocatalysts based on g-C3N4, there are several methods [58,64], such as varying the texture of g-C3N4 by means of template synthesis or deposition of metal cocatalysts. However, this section of the manuscript will focus on the synthesis of the direct Z-scheme without any mediator.
The most relevant synthetic methodologies used by the scientific community to design direct Z-scheme systems based on g-C3N4 are (Figure 6):
Solid-state synthesis. This methodology is broadly used in the synthesis of pristine g-C3N4 materials [47,70] and it is most employed to synthesize Z-scheme photocatalysts based on g-C3N4 [71]. This methodology is based on the calcination of one or a mixture of precursors in air or an inert gas atmosphere at high temperatures [72]. There are some crucial experimental parameters in the synthesis of materials using solid-state methodologies such as the heating rate, calcination temperature, and calcination time to control the crystallinity, morphology, surface properties, and phase structure of the composite [73,74]. This methodology is very interesting for the design of Z-scheme materials, as reported by W. Yu et al. [75], who synthesized g-C3N4 in the presence of pre-synthesized WO3 to obtain a direct Z-scheme g-C3N4/WO3 photocatalyst. Another noticeable work that uses this methodology was reported by L. Lu et al. [74]. In this work, the authors studied the effects of the calcination temperature on the photocatalytic activity of direct Z-scheme TiO2/g-C3N4. Although this methodology is widely used in the synthesis of the g-C3N4-based direct Z-scheme, the high temperatures used and the low control of the composition are the main drawbacks.
Deposition precipitation method. This methodology is commonly used for the synthesis of photocatalysts when one of the precursors is cationic and the other is anionic. A uniform precipitated composite is formed [76,77]. The deposition precipitation technique is based on the formation of a precipitate on the surface of another component by slow addition or in situ growth of a substance, following the addition of a precipitating agent at a low temperature [78,79]. Although this methodology is not widely used in direct Z-schemes, it is an interesting methodology due to the easy fabrication of these systems [80].
Impregnation. Impregnation is another popular method for the synthesis of catalysts [81,82] and it has been used for the fabrication of Z-scheme photocatalysts [83,84]. In this approach, a solid precursor or material is in contact with a solution containing the precursor to be deposited on the solid surface. There are two methods of impregnation: (1) the wet impregnation method, in which the solid precursor is introduced with an excess volume of the second precursor solution, and (2) incipient wetness impregnation, in which the volume of the second precursor solution used is equal or less than the pore volume of the solid [82]. Due to the advantages of this methodology, Feng et al. [85] reported a composite synthesized using a simple impregnation-heating method in which MoO3 nanoparticles were in situ supported on g-C3N4 sheets. This Z-scheme photocatalyst had photocatalytic activity in CO2 reduction to fuels under simulated sunlight radiation. Zhou et al. [86] reported a simple impregnation method to synthesize Z-scheme g-C3N4 decorated with TiO2 nanotubes with improved visible-light photocatalytic activity in pollutant abatement. Another example is the work of Jin et al. [84], where the authors reported the use of a one-step impregnation method to prepare direct Z-scheme LaCoO3/g-C3N4 photocatalysts.
Hydrothermal synthesis. In the 21st century, hydrothermal technology is one of the preferred methods for the synthesis of materials in various interdisciplinary fields such as advanced materials technology, nanotechnology, biotechnology, etc. due to the ease of processing particles with high purity, high crystallinity, controlled stoichiometry, and controlled chemical and physical characteristics, and to the environmental friendliness [87,88]. Hydrothermal processing is defined as a heterogeneous reaction performed under high temperatures and pressure in the presence of an aqueous solvent to dissolve and recrystallize substances that are relatively insoluble under ambient conditions [89]. It became one of the mostly used synthetic methodologies for the synthesis of g-C3N4-based Z-scheme photocatalysts. Jo et al. reported the synthesis of Z-scheme g-C3N4/TiO2 photocatalysts for isoniazid degradation. Moreover, they studied the effect of the TiO2 morphology in the synthesis of the Z-scheme photocatalysts and their catalytic activity [90]. Di et al. [91] reported the synthesis of a direct Z-scheme based on g-C3N4, synthesizing a C3N4/SnS2 photocatalyst with an in situ hydrothermal method at 140 °C, and the photocatalysts had a superior visible-light CO2 reduction performance. Recently, Lu et al. [92] reported a 2D/2D g-C3N4/BiVO4 Z-scheme heterojunction using the hydrothermal methodology with remarkable photocatalytic activity enhancement of CO2 conversion promoted by efficient interfacial charge transfer. In this sense, Wu et al. [93] synthesized 2D g-C3N4-supported nanoflower-like NaBiO3 using a facile hydrothermal synthesis.
Photo-deposition methodology. Photo-deposition is a common technique for loading a cocatalyst (such as Pt NPs) onto a photocatalyst via photoreduction [94]. Photo-deposition is the phenomenon through which a cocatalyst is deposited on the surface of a semiconductor, upon illumination of a solution containing the cocatalyst precursor and the support [95]. In the last years, the scientific community has developed a great interest in the photo-deposition method to obtain Z-schemes [17,95]. One representative example is the work reported by Jiang et al. [96], where two routes for constructing the Fe2O3/g-C3N4 direct Z-scheme through photo-deposition were demonstrated.
The characterization of direct Z-scheme photocatalysts is crucial to identify Z-scheme heterojunctions because type II heterojunctions and Z-scheme photocatalysts have similar structures [37]. The main difference between both heterojunctions is the charge carrier mechanism, as described in Section 3.1. In the last years, researchers have studied and developed several experimental and theoretical simulation methods to characterize these novel heterostructures (Figure 6) [33,37]. The most interesting and widely used methodology to characterize and understand the mechanism of Z-scheme systems is the radical species trapping methodology [97,98]. Indeed, this methodology can be applicable because Z-scheme semiconductor 1 with a high oxidizing capacity can produce OH while semiconductor 2 with a sufficient reduction potential is capable of generating O2•− species. However, type-II heterojunction photocatalysts with a low redox (reduction or oxidation) ability can only generate one type of radicals (either O2•− or OH). To elucidate this effect, the radical scavenging methodology is applied. In radical scavenging experiments, a chemical agent is introduced in the photocatalytic medium system to quench a radical, consequently decreasing the activity of the studied reaction [99,100]. Common scavengers used in the literature are tert-butyl alcohol (TBA) and isopropanol (IPA) for OH, and N2 gas and p-benzoquinone (BQ) for O2•− while ammonium oxalate (AO), triethanolamine (TEA), and disodium EDTA are used for holes (h+) [100]. Other characterization techniques used to study Z-scheme systems are photoluminescence (PL) spectroscopy and electron paramagnetic resonance (EPR) spectroscopy [101,102,103]. During the last years, there has been an increase in the application of other spectroscopic characterization techniques such as ultraviolet photoelectron spectroscopy (UPS), transient absorption spectroscopy (TAS), surface photovoltage spectroscopy (SPS), and in situ irradiated X-ray photoelectron spectroscopy (ISI-XPS) to verify the Z-scheme charge transfer mechanism [26,104]. Another methodology used to verify Z-schemes in heterojunctions is photocatalytic reduction testing because the photogenerated electrons with adequate reduction potential in a semiconductor can be used to produce some selective products that a semiconductor with a lower reduction potential is unable to generate [33,105]. To experimentally verify the proposed system, this methodology usually needs the aid of computational calculations based on density functional theory [106,107,108]. In the last years, this methodology has been used to calculate the Fermi level and to interpret the charge transfer mechanism.
Finally, it is relevant to indicate that there are several methodologies used to synthesize direct Z-scheme photocatalysts, as described in this section. However, it is essential to choose and develop easy and sustainable methodologies focusing on the intimate contact between both semiconductors to obtain direct Z-scheme photocatalysts. It is also necessary to use and develop powerful characterization tools to investigate the charge-transfer mechanism to elucidate whether the heterojunctions synthesized and studied are Z-scheme photocatalyst or another heterostructure, such as type II.

4. Environmental Applications of Direct Z-Schemes Based on g-C3N4

As mentioned in previous sections, the use of semiconductors capable of utilizing sunlight has received great interest in environmental remediation and in contributing to the solution of the energy crisis [10,109]. g-C3N4 is a semiconductor with optimal properties (absorption in visible light) for its use in this field, although it has several drawbacks [54,110], as shown in Section 2. Therefore, an alternative for g-C3N4 to be a key material in the current scenario is the use of the direct Z-scheme based on g-C3N4 [33,67]. Hence, g-C3N4-based direct Z-schemes have been widely studied by scientists for their application in photocatalytic pollutant remediation [111,112,113], photocatalytic H2 production [114,115], and photocatalytic CO2 reduction [85,116,117]. In this section, the most representative direct Z-schemes based on g-C3N4 for their use in environmental applications focusing on pollutants remediation, H2 production, and CO2 photoreduction will be presented. Additionally, in the main text, the most relevant aspects of the developed direct Z-schemes based on g-C3N4 for these three relevant environmental applications are described.

4.1. Pollutant Remediation

During the last decades, photocatalytic pollutant remediation has been intensively studied, leading to the synthesis of a plethora of photocatalysts active in the degradation of different pollutants. Many reports were also focused on the study of the so-called advanced oxidation processes (AOPs), which are fundamental for pollutant remediation. Indeed, AOPs include by definition the formation of highly reactive oxygen species (ROS), e.g., O2•−, OH, and more recently SO42•− radicals, which can generate cascade processes during pollutant degradation. Indeed, these radicals have the capability of rapidly reacting with most common pollutants. This will generate radicals on the organic skeleton of the molecule and bring about defragmentation, which might lead to a complete mineralization of the contaminant in the form of CO2 [118]. The use of heterogeneous photocatalysts can positively influence the formation of these species. As shown in Equation (1), the promotion of electrons from the VB to the CB origins holes, which might react with water or SO2O82− for the generation of OH (see Equations (2) and (3)) or SO42•− radicals (Equation (4)), respectively, which participate in the subsequent radical reaction pathway for pollutant degradation [119,120]. On the other hand, the electrons accumulated at the CB might react with O2 to generate O2•− radicals (Equation (5)), fundamental for the following cascade reactions [121]:
S C + h ν e C B + h V B +  
h V B + + O H s u r f a c e O H
E0 = 2.8 V (V vs. NHE, pH 7, 25 °C, 1 atm)
h V B + + H 2 O a b s o r b e d O H + H +
h V B + + S 2 O 8   a d s o r b e d 2 S O 4
E0 = 2.6 V (V vs. NHE, pH 7, 25 °C, 1 atm)
e C B + O 2   a b s o r b e d O 2
E0 = −0.33 V (V vs. NHE, pH 7, 25 °C, 1 atm)
As mentioned in the text, the use of heterojunctions plays a key role in overcoming the main drawbacks of photocatalysts. Thus, the oxidation processes involved in the formation of ROS might be more favored by the composite than its isolated components [122]. The coupling of g-C3N4 with different semi-conductors gives birth to an extended family of direct Z-scheme heterojunctions, active in AOPs (Figure 7) and, subsequently, applied in pollutant remediation [67].
In the following pages, we will analyze some of the most important examples of pollutant photodegradation by direct Z-schemes based on g-C3N4. Mainly, we will focus on some of the most representative pollutants used in photodegradation studies and some of the most illustrative results are summarized in Table 1.
Among the different pollutants, rhodamine B (RhB) is one of the most widely studied in recent reports related to this topic. Indeed, this water-soluble molecule is used as a red dye in the textile industry, thus representing one of the most concentrated wastes in water. Being classified as carcinogenic, RhB must be removed from the residual water, and this can be achieved by the use of photocatalysis [123]. Metal-free g-C3N4-based direct Z-scheme photocatalysts have been extensively applied in the degradation of RhB with highly remarkable results. M. Tang et al. [124] developed a ternary system composed of AgI/Ag3PO4/g-C3N4 with high activity in the degradation of RhB and a neonicotinoid pesticide due to the low recombination rate assured by the presence of the three compounds, as shown in Figure 8a. Indeed, the use of isolated components or even binary systems containing these materials did not lead to the same high yield in degradation. This finding was also demonstrated by Y. He et al. [116] using SnO2-x/g-C3N4 composite in RhB degradation. Figure 8b depicts another advantage of this heterostructure, i.e., an increase in the surface area, which is known to be one of the main drawbacks of the application of g-C3N4 in catalysis [116]. Another noteworthy example is the 2D/2D SnS2/g-C3N4 heterostructure developed by H. Che et al. [125], where the efficiency of the charge carrier separation is enhanced by the high interfacial contact area generated from the contact between the 2D materials. In addition, the photocatalytic activity of this catalyst is boosted because both OH and O2•− radicals might be generated, as presented in Figure 8c [126]. Considering lower dimensionality systems, 0D materials have found applicability in the generation of novel composites. For example, Y. Fu et al. [111] developed the MoS2 QD/g-C3N4 heterostructure and demonstrated that the interaction between the 0D QD and 2D moieties can stabilize the QDs. Photocurrent measurements (Figure 8d) highlight that in the composites, the interfacial charge transfer is faster and the recombination of photoexcited charges is inhibited, thus boosting the photocatalytic activity in RhB degradation.
Tetracycline (TC) is a common antibiotic but with many side effects to live bodies. Its wide usage in husbandry (cattle, swine, poultry, and fishery) leads to an increase in its concentration in water and poses a threat to the natural balance. Among other degradation techniques, photocatalysis has found application in the degradation of this molecule [127,128,129], and g-C3N4-based direct Z-scheme photocatalysts have been extensively applied for this purpose. One recent report developed by J. Kang et al. [130] shows how the construction of the heterostructure g-C3N4/Na-BiVO4 can boost the photodegradation of TC, promoted in this case by the formation of peroxymonosulfate radical. Indeed, the calculated CB and VB presented in Figure 9a demonstrate that the use of g-C3N4 can overcome one of the main drawbacks of the promising photocatalyst BiVO4, i.e., the low CB potential. Similarly, peroxymonosulfate-mediated TC photodegradation was demonstrated by C. Jin et al. [131] to be successful when using the 2D/2D Co3O4/g-C3N4 heterostructure. In this case, the two-dimensionality allows a better separation and transfer of the charges, as demonstrated by the photoluminescence measurements in Figure 9b. In addition, S. Wang [132] et al. showed that the use of α-Fe2O3/g-C3N4 in TC degradation could improve the photocatalytic activity of the isolated components due to the formation of interfacial -NH/-NH2 groups on the g-C3N4 surface. The synergy between the two components is determined by the fact that the defect-rich g-C3N4 improves the absorption of visible light on the material and α-Fe2O3 diminishes the recombination rate.
RhB and TC represent two of the most common pollutants that have been studied in photodegradation processes, but, certainly, according to the latest publications, there are other dangerous pollutants, which are present worldwide in high concentrations in water [133]. Consequently, photocatalysts based on g-C3N4 have also been used for the degradation of different pharmaceutical products or persistent organic pollutants (POPs), e.g., fluoroquinolone antibiotics, benzodiazepines, or parabens. Y. Zhou et al. [134] successfully used a 3D/2D MOF-derived CoCeOx/g-C3N4 Z-scheme heterojunction in the photocatalytic degradation of carbamazepine. The presence of the MOF-derived CoCeOx increased the surface area, overcoming one of the main drawbacks of g-C3N4. Additionally, J. Huang et al. [135] showed that the combination of g-C3N4 and TiO2 originated an active photocatalyst for the visible-light elimination of enrofloxacin in water, as indicated in Figure 9c. In this case, the presence of g-C3N4 favors the absorbance of TiO2 under visible light, giving more applicability to the well-known photocatalyst TiO2. Furthermore, J. Meng et al. [136] synthesized the WO3/g-C3N4 heterojunction for the degradation of methylparaben. In this case, the heterostructure avoids the fast recombination typical in both materials and the presence of g-C3N4 enhances the formation of O2•− radicals, which are involved in the degradation process (Figure 9d).
In summary, direct Z-schemes are a great alternative to improve the catalytic activity of bare photocatalysts in pollutant abatement, obtaining total conversion in a few hours. However, it is necessary to test the photocatalysts under more realistic conditions since most of the catalytic tests were performed with isolated pollutants in pure water.
Figure 7. ROS formation catalyzed by a heterostructure. Reproduced from [137]. Copyright 2022, Elsevier.
Figure 7. ROS formation catalyzed by a heterostructure. Reproduced from [137]. Copyright 2022, Elsevier.
Catalysts 12 01137 g007
Figure 8. (a) Photocatalytic mechanism in the presence of the AgI/Ag3PO4/g-C3N4 heterostructure. Reproduced from [124]. Copyright 2020, Elsevier. (b) TG profiles of g-C3N4, its composite with SnO2-x (SC), and their physical mixture (SC-PM). Reproduced from [116]. Copyright 2015, Elsevier. (c) Proposed photocatalytic mechanism for the degradation of RhB by SnS2/g-C3N4. Reproduced from [126]. Copyright 2019, Elsevier. (d) Transient photocurrent response of g-C3N4 and MoS2 QD/g-C3N4 composites. Reproduced from [111]. Copyright 2017, Elsevier.
Figure 8. (a) Photocatalytic mechanism in the presence of the AgI/Ag3PO4/g-C3N4 heterostructure. Reproduced from [124]. Copyright 2020, Elsevier. (b) TG profiles of g-C3N4, its composite with SnO2-x (SC), and their physical mixture (SC-PM). Reproduced from [116]. Copyright 2015, Elsevier. (c) Proposed photocatalytic mechanism for the degradation of RhB by SnS2/g-C3N4. Reproduced from [126]. Copyright 2019, Elsevier. (d) Transient photocurrent response of g-C3N4 and MoS2 QD/g-C3N4 composites. Reproduced from [111]. Copyright 2017, Elsevier.
Catalysts 12 01137 g008
Figure 9. (a) UV–vis DRS spectra (i) and Mott–Schottky plots (ii) of catalysts, XPS valence spectra (iii) of g-C3N4, BiVO4, and Na-BiVO4. Reproduced from [130]. Copyright 2021, Elsevier. (b) Photoluminescence spectra: (i) g-C3N4, (ii) 1%Co3O4/g-C3N4, (iii) 3%Co3O4/g-C3N4, (iv) 5%Co3O4/g-C3N4, (v) 10%Co3O4/g-C3N4, and (vi) 15%Co3O4/g-C3N4. Reproduced from [131]. Copyright 2020, Elsevier. (c) Proposed visible-light photocatalytic enrofloxacin degradation mechanism by g-C3N4/TiO2. Reproduced from [135]. Copyright 2020, Elsevier. (d) Charge carrier transfer mechanism in the presence of WO3/g-C3N4 direct Z-scheme photocatalyst. Reproduced from [136]. Copyright 2021, Elsevier.
Figure 9. (a) UV–vis DRS spectra (i) and Mott–Schottky plots (ii) of catalysts, XPS valence spectra (iii) of g-C3N4, BiVO4, and Na-BiVO4. Reproduced from [130]. Copyright 2021, Elsevier. (b) Photoluminescence spectra: (i) g-C3N4, (ii) 1%Co3O4/g-C3N4, (iii) 3%Co3O4/g-C3N4, (iv) 5%Co3O4/g-C3N4, (v) 10%Co3O4/g-C3N4, and (vi) 15%Co3O4/g-C3N4. Reproduced from [131]. Copyright 2020, Elsevier. (c) Proposed visible-light photocatalytic enrofloxacin degradation mechanism by g-C3N4/TiO2. Reproduced from [135]. Copyright 2020, Elsevier. (d) Charge carrier transfer mechanism in the presence of WO3/g-C3N4 direct Z-scheme photocatalyst. Reproduced from [136]. Copyright 2021, Elsevier.
Catalysts 12 01137 g009
Table 1. g-C3N4-based direct Z-scheme photocatalysts for pollutant remediation.
Table 1. g-C3N4-based direct Z-scheme photocatalysts for pollutant remediation.
Z-Scheme
Photocatalyst
FabricationIrradiation SourcePollutant/Photodegradation
Performance/Reaction Time (min)
Reference
SnO2−x/g-C3N4solid state350W Xenon lamp
(420 nm < λ < 800 nm)
RhB/100%/60 min[116]
MoS2 QD/g-C3N4microemulsion method350W Xenon lamp
(λ > 420 nm)
RhB/100%/9 min[111]
Bi3O4Cl/g-C3N4solid phase calcination method250W Xenon lamp
(λ > 420 nm)
RhB/98.3%/90 min[125]
β-Bi2O3/g-C3N4sonication350W Xenon lamp
(λ > 420 nm)
RhB/98%/80 min[138]
g-C3N4/ZnOthermal
atomic layer deposition
300W Xenon lampCephalexin/92.7%/60 min[139]
SnS2/g-C3N4solvothermal method300W Xenon lamp
(λ > 400 nm)
RhB/94.8%/60 min[126]
Fe3O4-OQs/Bi2O4/g-C3N4hydrothermal method250W Xenon lamp
(λ > 420 nm)
RhB/92.7%/160 min[140]
g-C3N4/TiO2solvent evaporation method500 W Xenon lampRhB/100%/20 min[141]
α-Fe2O3/g-C3N4calcination100W LED lamp
(λ = 420 nm)
Tetracycline/95.0%/60 min[142]
ZnO/g-C3N4hydrothermal methodVisible lightAtrazine/90%/180 min[143]
Co3O4/g-C3N4solid state 300W Xenon lamp
(λ > 420 nm)
Tetracycline/90.0%/60 min[131]
AgI/Ag3PO4/g-C3N4in situ ion exchange method300W Xenon lamp
(λ > 420 nm)
Nitenpyram/100%/4 min[124]
g-C3N4/anatase TiO2calcination350W Xenon lamp
(λ > 420 nm)
Enrofloxacin/98.5%/60 min[135]
g-C3N4/Na-BiVO4hydrothermal method300W Xenon lamp
(λ > 420 nm)
Tetracycline/98.2%/40 min[130]
WO3/g-C3N4hydrothermal method300W Xenon lamp
(λ > 400 nm)
MPB/98.2%/60 min[136]
ZnO/g-C3N4hydrothermal methodSolar lightRh B/98%/100 min[144]
α-Fe2O3/g-C3N4sonication500 W Xenon lampTetracycline/97.1%/80 min[132]
CoCeOx/g-C3N4solid state300W Xenon lamp
(λ > 420 nm)
Carbamazepine/90.1%/60 min[134]

4.2. H2 Production

One of the greatest current challenges of our society from an environmental and energy point of view is the production and use of clean fuel from renewable energies [2,145]. During the last decades, H2 has become a highly relevant alternative to fossil fuels because it has clean combustion and high energy density [145]. Therefore, it is essential to develop technologies to allow the production of H2 without using fossil fuels [146]. This subsection is placed here, as the water splitting uses similar starting radicals to pollutant abatement in aqueous ambience. The generation of H2 by water splitting using sunlight has received great interest from the scientific community and in society [147,148]. The water splitting reaction that generates H2 and O2 (Equation (6)) is a non-spontaneous endergonic reaction (ΔG = +237 KJmol−1), which means that energy is needed for the reaction to occur [148]:
H 2 O 1 2 O 2 + H 2   Δ G = 237   KJmol 1
One alternative to the generation of H2 from water splitting is the use of photocatalysis, which would require the use of sunlight and a semiconductor (photocatalysis). For the reaction to occur, the CB potential of the semiconductor should be more negative than the reduction potential of H2O (0 V vs. NHE) for H2 generation, and the VB potential of the semiconductor should be more positive than the oxidation potential of H2O (1.23 V vs. NHE) for O2 generation [148,149]. An interesting semiconductor for this reaction is g-C3N4 since the VB and CB positions are located at approximately +1.6 and −1.1 eV, respectively, with a band gap of ~2.7 eV, making the reaction of water splitting possible using sunlight [56,150]. However, the g-C3N4 photocatalyst presents low activity in water splitting [23], as mentioned in Section 2. To solve this low activity of g-C3N4 in water splitting, the scientific community has focused on studying heterojunctions based on g-C3N4 [26,60]. A heterojunction that has aroused the interest of the scientific community is the direct Z-scheme because the latter is present in the systems responsible for the water splitting reaction in photosynthesis due to the decrease in the e-h+ pair recombination rate [38,98]. This makes the direct Z-schemes promising systems, which may lead to adequate quantum efficiency values for their commercialization [38]. In this subsection, several direct Z-scheme photocatalysts based on g-C3N4 in different conditions used in water splitting are summarized in Table 2. In addition, a detailed discussion of the most relevant Z-scheme photocatalysts from the point of view of the catalytic activity and reaction mechanism will be described in the main text.
R. Ye et al. [151] described a CoTiO3/g-C3N4 novel Z-scheme heterostructure photocatalyst for H2 evolution (858 µmol g−1 h−1) under visible light with effectively intimate interfaces linked through the Co–O–N or Ti–O–N bonds, as shown in Figure 10a. The authors showed that this system presents a direct Z-scheme mechanism by PL, PEC, and ESR analyses. The work described by R. Ye et al. focusing on Z-scheme CoTiO3/g-C3N4 was useful for further development of novel heterojunction photocatalytic systems based on perovskite, polymer, and composite photocatalysts for photocatalytic H2 production. X. She et al. [152] showed a novel strategy for the synthesis of an α-Fe2O3/2D g-C3N4 Z-scheme hybrid structure containing ultrathin g-C3N4 platelets. In the synthesis methodology used in this study, α-Fe2O3 promotes the formation of g-C3N4 nanosheets. Moreover, the authors claimed that the Fe2O3/2D g-C3N4 direct Z-scheme photocatalyst exhibits unprecedented photocatalytic activity toward H2 evolution (31,400 µmol g−1 h−1), with a remarkably high quantum efficiency of 44.35% at a wavelength of 420 nm, as shown in Figure 10b. The enhanced performance of H2 evolution in α-Fe2O3/2D g-C3N4 was attributed to the 2D structure of g-C3N4, allowing an efficient transfer of the photoexcited electron to the reactant. Another factor relevant to improving the photocatalytic activity of this system is the Z-scheme structure due to the low electron-hole recombination in both α-Fe2O3 and 2D g-C3N4 and the close interface contact between α-Fe2O3 and 2D g-C3N4, enabling fast electron transfer from the conduction band of α-Fe2O3 to the valence band of 2D g-C3N4. This work shows that the dimensionality (2D) and the porous hierarchy of the systems present a great effect on the photocatalytic activity of Z-schemes. Moreover, Fe2O3/2D g-C3N4 direct Z-scheme photocatalysts exhibit great values in H2 production, indicating the effectivity and interest of these artificial Z-schemes for their use in this application. Considering the effect of the 2D materials, Y. Yang, et al. [153] designed and constructed a direct Z-scheme van der Waals heterojunction composed of ultrathin WO3·H2O and g-C3N4 nanosheets using simple hydrothermal synthesis to achieve efficient water splitting without the addition of any sacrificial agents (482 µmol g−1 h−1). This WO3·H2O/g-C3N4 system could efficiently transport the electrons, decreasing the e-h+ recombination rates, as shown in Figure 10c, which led to a considerable improvement in the photocatalytic performance. Another relevant parameter in the study of Z-schemes is the presence of interfacial defects since they might be beneficial to the recombination of interfacial photogenerated e/h+ and could serve as Z-scheme pathways for charge transfer. L. Kong et al. [154] fabricated a direct Z-scheme g-C3N4/Ti3+-doped TiO2 to show this effect. The authors observed that the Ti3+ defects were introduced under CB of TiO2, which improved the interfacial charge transfer channels for Z-scheme g-C3N4/Ti3+-doped TiO2 composite and improved the visible light absorption. These properties were studied using a radical species trapping methodology, UV-Vis spectroscopy, and EPR analysis. Due to the enhanced properties, this system has high activity (1938 μmol∙h−1∙g−1) under simulated solar light and good stability in photocatalytic H2 evolution. S. Qin et al. [155] showed a promising way to construct Z-scheme heterostructures based on metal-organic cages (MOCs) and semiconductors for their use in photocatalytic H2 generation. The authors synthesized an MOC, integrating four organic photosensitized ligands M4− and two Pd2+ catalytic centers. Then, the MOC was successfully immobilized by hydrogen bonds to obtain a robust heterogeneous direct Z-scheme g-C3N4/MOC-Q1 (Figure 10e). This innovative system presents high H2 evolution activity (4495 μmol g–1 h–1). In this manuscript, it was described that Z-scheme systems improved the photocatalytic H2 production rate, but, on the other hand, the photoconversion efficiency is still far from the requirement for practical applications.
One novel methodology based on direct Z-scheme system photocatalysts is the design of dual-Z-scheme systems that ensure the efficient transmission of photogenerated carriers and further optimize the structure of the Z-scheme system. Y. Yang et al. [114] successfully constructed an efficient dual-Z-scheme heterojunction, CdS/W18O49/g-C3N4 (CWOCN), by introducing CdS nanoparticles in the W18O49/g-C3N4 (WOCN) composite. This dual-Z-scheme system exhibits extraordinary photocatalytic activity and stability. The TRPL measurement results demonstrated that the formation of a dual-Z-scheme system facilitates the transmission of photogenerated electrons and prolonged the electrons’ lifetime, thus boosting the photocatalytic activity. It led to a prominent photocatalytic H2 evolution rate (11,658 μmol h−1 g−1). This study offers guidance for exploiting and designing efficient and practical photocatalysts.
To conclude this section, there are several methodologies based on direct Z-scheme system photocatalysts used to obtain photocatalysts that are active in H2 production. However, this technology is far from commercialization due to the low reproducibility of the materials and low quantum efficiency.
Figure 10. (a) Schematic illustration of the mechanisms for charge carrier separation of CoTiO3/g-C3N4 direct Z-schemes and the molecular interaction between CoTiO3 and g-C3N4 through the formation of chemical bonds. Reproduced from [151]. Copyright 2016, ACS Publications. (b) Catalytic activity (Turnover frequency) of ML g-C3N4, α-Fe2O3/ML g-C3N4, 2D g-C3N4, and α-Fe2O3/2D g-C3N4 for water splitting. Reproduced from [152]. Copyright 2017, Willey. (c) PL spectra of g-C3N4, WO3·H2O, and WO3·H2O/g-C3N4 and visible-light-driven photocatalytic H2 and O2 generation for WO3·H2O/g-C3N4. Reproduced from [153]. Copyright 2018, Willey. (d) Schematic diagram of the mechanism for the separation and transfer of photoinduced charge carrier in CN-T-H composite. Reproduced from [154]. Copyright 2018, Elsevier. (e) Schematic illustration of a heterostructure of g-C3N4/MOC-Q1. Reproduced from [155]. Copyright 2021, ACS Publications. (f) The mechanism of dual-Z-scheme heterojunction (CWOCN) for H2 production. Reproduced from [114]. Copyright 2021, Willey.
Figure 10. (a) Schematic illustration of the mechanisms for charge carrier separation of CoTiO3/g-C3N4 direct Z-schemes and the molecular interaction between CoTiO3 and g-C3N4 through the formation of chemical bonds. Reproduced from [151]. Copyright 2016, ACS Publications. (b) Catalytic activity (Turnover frequency) of ML g-C3N4, α-Fe2O3/ML g-C3N4, 2D g-C3N4, and α-Fe2O3/2D g-C3N4 for water splitting. Reproduced from [152]. Copyright 2017, Willey. (c) PL spectra of g-C3N4, WO3·H2O, and WO3·H2O/g-C3N4 and visible-light-driven photocatalytic H2 and O2 generation for WO3·H2O/g-C3N4. Reproduced from [153]. Copyright 2018, Willey. (d) Schematic diagram of the mechanism for the separation and transfer of photoinduced charge carrier in CN-T-H composite. Reproduced from [154]. Copyright 2018, Elsevier. (e) Schematic illustration of a heterostructure of g-C3N4/MOC-Q1. Reproduced from [155]. Copyright 2021, ACS Publications. (f) The mechanism of dual-Z-scheme heterojunction (CWOCN) for H2 production. Reproduced from [114]. Copyright 2021, Willey.
Catalysts 12 01137 g010
Table 2. Summary of direct Z-scheme photocatalysts based on g-C3N4 for water splitting.
Table 2. Summary of direct Z-scheme photocatalysts based on g-C3N4 for water splitting.
Z-Scheme
Photocatalyst
Fabrication MethodologyIrradiation SourceH2 Production Activity (µmol g−1 h1) and AQEReference
CoTiO3/g-C3N4Solid-StateXenon lamp
(300 W, λ ≥ 420 nm)
858
AQE: 38.4% (365 nm)
[151]
g-C3N4/ZnODepositionXenon lamp
(300 W, λ ≥ 420 nm)
322[156]
g-C3N4/PSiPolycondensation reactionXenon lamp
(300 W, λ ≥ 400 nm)
870[157]
2D α-Fe2O3/g-C3N4Solid-StateXenon lamp
(300 W, λ ≥ 420 nm)
31400
AQE: 44.35% (420 nm)
[152]
WO3.H2O/g-C3N4Hydrothermal methodXenon lamp
(300 W, λ > 400 nm)
482
AQE: 6.2% (420 nm)
[153]
g-C3N4/Ti3+-TiO2Solid-StateXenon lamp
(300 W, λ > 400 nm)
1938[154]
Nb2O5/g-C3N4HydrothermalXenon lamp
(1000 W, 1.5G)
110,000[115]
Bi2O2CO3/g-C3N4Heat treatment methodXenon lamp
(300 W, λ ≥ 400 nm)
965
AQE: 7.14% (420 nm)
[158]
2D/2D g-C3N4/Sn3O4Calcined in N2Xenon lamp
(300 W, λ ≥ 400 nm)
1960[159]
g-C3N4/MOC-Q1Deposition Xenon lamp
(300 W, λ ≥ 420 nm)
4495
AQE: 0.50% (425 nm)
[155]
CdS/W18O49/g-C3N4 (CWOCN)Chemical bath depositionXenon lamp
(300 W, λ ≥ 420 nm)
11,658
AQE: 26.73% (420 nm)
[116]
Cu2O/g-C3N4Solid-StateXenon lamp
(300 W, λ ≥ 420 nm)
266.3
AQE: 13.40% (420 nm)
[160]

4.3. CO2 Photoreduction

The great use of fossil fuels in the last decades has led to significant CO2 emissions into the atmosphere, resulting in global warming responsible for climate change, which represents one of the greatest challenges of the current society [161,162]. An innovative alternative to reduce the emissions of CO2 into the atmosphere would be to capture CO2 [163] and transform it into high value-added products using sunlight, mimicking natural photosynthesis [164]. However, CO2 is a stable covalent molecule due to the C=O bond, resulting in high dissociation energy (531 kJ mol−1) [165]. CO2 photoreduction is an alternative method to break the C=O bond using solar energy. Nevertheless, this process is a multielectron transfer process, which may result in a great variety of products with different carbon oxidation states, including CH4, CH3OH, HCHO, HCOOH, and CO [166] (see Equations (7)–(13)):
C O 2 + 2 e C O 2 ·
E0 = −1.90 V (V vs. NHE, pH 7, 25 °C, 1 atm)
C O 2 + 2 H + + 2 e   H C O O H
E0 = −0.61 V (V vs. NHE, pH 7, 25 °C, 1 atm)
C O 2 + 2 H + + 2 e   C O + H 2 O
E0 = −0.53 V (V vs. NHE, pH 7, 25 °C, 1 atm)
C O 2 + 4 H + + 4 e   H C H O + H 2 O
E0 = −0.48 V (V vs. NHE, pH 7, 25 °C, 1 atm)
C O 2 + 6 H + + 6 e   C H 3 O H + H 2 O
E0 = −0.38 V (V vs. NHE, pH 7, 25 °C, 1 atm)
C O 2 + 8 H + + 8 e   C H 4 + 2 H 2 O
E0 = −0.24 V (V vs. NHE, pH 7, 25 °C, 1 atm)
2 H 2 O + 2 e 2 O H + H 2  
E0 = −0.41 V (V vs. NHE, pH 7, 25 °C, 1 atm)
Another challenge in the photocatalytic reduction of CO2 is that the reduction of H2O to H2 (see Equation (1)) has a similar reduction potential to the reduction of CO2. This fact means that in the aqueous ambience, there is competition between both reactions [167]. To overcome the challenges presented above in the reduction of CO2, the scientific community has focused on designing selective photocatalysts for the reduction of CO2 to various high value-added products [168]. Among the different photocatalysts developed, g-C3N4 should be highlighted. G. Dong et al. reported that pristine g-C3N4 without any cocatalyst presented selectivity in the photocatalytic reduction of CO2 into CO under visible-light irradiation in the presence of water vapor [169]. However, the efficiency of this photocatalyst was low due to the high e-h+ pair recombination rate [47]. As mentioned throughout this review, an alternative used to reduce the e-h+ recombination ratio, which is one of the challenges of g-C3N4, is the design and synthesis of heterojunctions [26]. Within the use of heterojunctions for the photoreduction of CO2, artificial direct Z-scheme photocatalysts have received great interest from the scientific community since these systems mimic the electronic processes that occur in photosynthesis to reduce CO2 into hydrocarbons [33,34]. Therefore, the study of these artificial systems for the reduction of CO2 has grown noticeably in the last years, being a key approach to understanding the photocatalytic mechanism and to design efficient systems capable of being marketed [39,170,171]. In this subsection, several direct Z-scheme photocatalysts based on g-C3N4 used in CO2 photoreduction are summarized in Table 3. In addition, a detailed discussion of the most relevant photocatalyst Z-scheme from a synthesis, catalytic activity, and reaction mechanism perspective will be described in the main text.
W. Yu et al. [172] described the coupling effect of ZnO for the improved photoactivity of g-C3N4 and the authors also proposed a direct Z-scheme mechanism to explain the better performances of the g-C3N4/ZnO binary composite photocatalytic system. The g-C3N4/ZnO photocatalytic system exhibited enhanced photocatalytic activity for CO2 reduction compared with pure g-C3N4, with selectivity towards CH3OH. This enhancement of photocatalytic CO2 reduction activity is attributed to the highly efficient ZnO-to-g-C3N4 electron transfer occurring at the intimate contact interface between the g-C3N4 and ZnO phases. This work provided new insights into the rational construction of a g-C3N4-based photocatalytic system and the design of a direct Z-scheme system without an electron mediator for the photocatalytic CO2 reduction reaction. To enhance the photocatalytic activity of the materials, Z. Jiang et al. [170] described that the Z-scheme hybrid material should present several characteristics to accelerate the reduction process, including a 3D hierarchical structure and preferably basic sites, which promote CO2 adsorption. In addition, these systems should present an efficient separation of the e-h+ pairs and an enhancement in the reduction character of electrons in the conduction band of g-C3N4. In this work, a hierarchical direct Z-scheme hybrid was developed by combining 3D urchin-like α-Fe2O3 and g-C3N4 photocatalysts (Figure 11a) for the photocatalytic reduction of CO2 without the use of any sacrificial agent or cocatalyst. The hierarchical system improved the photocatalytic reduction of CO2 to form CO (27.2 µmol g−1 h−1), and the system was stable for four cycles (Figure 11a). N.T. Thanh Truc et al. [173] synthesized a Nb-TiO2/g-C3N4 direct Z-scheme system, in which the photo-excited e in the CB of the Nb-TiO2 combines with the photo-excited h+ in the VB of the g-C3N4, preserving the existence of e in the CB of the g-C3N4 and h+ in the VB of Nb-TiO2. Thus, the established Nb-TiO2/g-C3N4 direct Z-scheme system produced a huge amount of available e/h+ pairs for the reduction of CO2 into various valuable fuels. Moreover, the authors observed that the photo-current response of the prepared Nb-TiO2/g-C3N4 (~57.55 µA/cm2) was much higher than that of g-C3N4 (~3.92 µA/cm2) and Nb-TiO2 (~12.41 µA/cm2) under the full visible light spectrum (Figure 11b). This result showed that the photo-excited e in the CB of the Nb-TiO2 tends to combine with photo-excited h+ in the VB of the g-C3N4, preserving the existence of e in the CB of the g-C3N4 and h+ in the VB of the Nb-TiO2, indicating the power of this characterization technique to address the Z-scheme mechanism. M. Lu et al. [92] synthesized 2D/2D ultrathin nanosheets of g-C3N4/BiVO4 Z-scheme heterojunction via thermal-polymerization and a subsequent hydrothermal method. The 2D/2D g-C3N4/BiVO4 ultrathin nanosheets exhibited a great photocatalytic evolution rate for CO (145 µmol g−1 h−1) and CH4 (133 µmol g−1 h−1) (Figure 11c). The study presented by Lu et. al. evidenced the relevance of using 2D materials. Indeed, the direct Z-scheme heterojunction and face-to-face interfacial contact between 2D g-C3N4 and 2D BiVO4 accelerated the charge transfer and separation, improving the catalytic activity of these systems in CO2 reduction. B. Tahir et al. [117] designed 3D/2D WO3/g-C3N4 microspheres with an effective interfacial contact using a facile single-step hydrothermal method. The direct growth of WO3 microspheres with g-C3N4 enables good interaction among both semiconductors, enabling proficient charge carrier separation (Figure 11d). The synergistic effect with the larger interfacial contact area and proficient charge carrier separation of this system improves the CO and CH4 production over WO3/g-C3N4 (145 and 133 μmol g−1 h−1, respectively), which are 2- and 4-fold higher than the use of pristine g-C3N4. This work reveals that effective interfacial contact between both semiconductors present on the Z-scheme photocatalyst is a key factor to promoting the photocatalytic CO2 conversion to solar fuels under visible light irradiation. J. Zhao et al. [174] showed the development of a novel direct Z-scheme Bi19S27Br3/g-C3N4 composite using the ionic liquid-assisted solvent-thermal method. The Bi19S27Br3/g-C3N4 composites showed enhanced CO2 photoreduction activity for the production of CO (12.87 μmol g−1 h−1). The authors observed in this work that the C-S bond boosts the transfer of photogenerated charge between Bi19S27Br3 and g-C3N4, as shown in Figure 11e. Therefore, the construction of a chemical bond-bridged direct Z-scheme is a promising strategy for precisely tailoring the photogenerated charge separation direction of the photocatalyst.
In conclusion, although the synthesis of Z-schemes has been successfully achieved and they are active in CO2 photoreduction, nowadays, the photocatalytic efficiencies of Z-schemes are still too low for their commercialization.
Table 3. Summary of direct Z-scheme photocatalysts based on g-C3N4 for CO2 photoreduction.
Table 3. Summary of direct Z-scheme photocatalysts based on g-C3N4 for CO2 photoreduction.
Z-Scheme
Photocatalyst
FabricationIrradiation SourceProducts/Production
(µmol g−1 h−1)/AQE
Reference
ZnO/g-C3N4Solid-state300 W simulated solar Xe arc lampCH3OH: 0.6[172]
SnO2-x/g-C3N4Solid-state500 W Xe lampCO: ~19
CH3OH: ~4
CH4: ~2
[116]
g-C3N4/SnS2Hydrothermal method300 W Xenon lamp
(λ ≥ 420 nm)
CH3OH: 2.24
CO: 0.64
[91]
MoO3/g-C3N4impregnation method350 W Xenon lamp
(800 nm > λ > 420 nm)
CO: ~18
CH3OH: ~7
CH4: ~1
[85]
α-Fe2O3/g-C3N4Impregnation–hydrothermal
method
Xenon lamp
0.21 Wcm−2
CO: 27.2
AQE: 0.963% (420 nm)
[170]
AgCl/g-C3N4Deposition-precipitation method11 W fluorescent lampCH4: ~2
CH3COOH: ~0.75
HCOOH: ~0.31
AQE: 0.211% (475 nm)
[175]
Cu2V2O7/g-C3N4Calcination methodology20 W white bulbs
(700 nm > λ > 400 nm)
CH4: 305
CO: 166
O2: 706
[176]
g-C3N4/FeWO4Sonochemical method300 W Xenon lamp
(100 mW/cm2)
CO: 6
AQE: ~0.3% (420 nm)
[83]
(Nb)TiO2/g-C3N4Calcination methodologyTwo 30 W white bulbsCH4: 562
CO:420
HCOOH:698
[173]
2D/2D g-C3N4/BiVO4Hydrothermal method300 W Xenon lamp
(λ ≥ 420 nm)
CO: ~5.2
CH4: ~4.6
[92]
NiMoO4/g-C3N4Calcined methodology30 W LED,
(700 nm > λ > 400 nm)
CH4: 635
CO: 432
O2: 1853
HCOOH: 647
[177]
α-Fe2O3/g-C3N4Hydrothermal method300 W xenon lampCO: 17.8
AQE: 0.31% (420 nm)
[178]
3D/2D WO3/g-C3N4Hydrothermal method300 W Xenon lamp
(100 mW cm−2)
CO: 145
CH4: 133
[117]
La2Ti2O7/g-C3N4Ultrasonic-deposition methodFour blue LED
(4 × 3 W, 420 nm)
CH3OH: ~4
CO: ~2.5
AQE: 3.61% (420 nm)
[179]
Bi2S3/g-C3N4Hydrothermal method300 W xenon lampCO: 6.84
CH4: 1.57
H2: 1.38
AQE: 2.31% (420 nm)
[180]
ZnO/ZnWO4/g-C3N4Calcination methodXenon lamp
(300 W, 0.95 mW/cm2)
CH4: 6.2
CH3OH: 3.8
CH3CH2OH: 2.1
CO: 1.3
[181]
NiTiO3/g-C3N4Ultrasonic-calcination method300 W Xenon lamp
(λ ≥ 420 nm)
CH3OH: 13.74[182]
Bi19S27Br3/g-C3N4Physical mixture
(Strong grinding)
300 W xenon lampCO: 12.87[174]

5. Conclusions and Outlook

In conclusion, g-C3N4-based direct Z-scheme photocatalysts are innovative alternatives for overcoming the main drawbacks of the parent material, i.e., high e-h+ recombination rate and inadequate redox potential. In this review, a comprehensive overview of the recent research on g-C3N4-based Z-scheme photocatalysts is described, focusing on the utilization of these systems in three environmental applications, such as pollutant abatement, H2 evolution, and CO2 reduction. There is undoubtedly a substantial amount of ground to cover by the scientific community since there are few reports where the synthesis and catalytic activity of novel g-C3N4-based direct Z-scheme photocatalysts have been studied. Especially, several synthetic methodologies (hydrothermal, solid-state, impregnation, etc.), characterization techniques to study the properties of Z-schemes (EPR, PL …), and the application of these materials in environmental reactions have been investigated, focusing on the effect of the catalysts and their catalytic activity to obtain efficient solar photocatalysts. Nevertheless, the photocatalytic efficiencies are still low, and many catalytic mechanisms remain unclear. These characteristics represent the main drawbacks hindering the commercialization of these systems. The challenges for new researchers in the design and applicability of g-C3N4-based direct Z-scheme photocatalysts might be in the synthetic methodology of this photocatalyst, focusing on the interaction between g-C3N4 and another SC, since the intimate contact between both systems has a strong influence on the photocatalytic activity.
The effect of the dimensionality and surface chemistry of g-C3N4 and the hetero- counterpart present in the Z-scheme photocatalyst is a paradigm in heterostructure synthesis, and thus, in the improvement of the photocatalytic activity. Although the transfer directions of photogenerated charge carriers in Z-scheme systems are studied by EPR, photoluminescence spectroscopy, reactive species scavenging experiments, and theoretical calculations, it is necessary to develop more powerful characterization tools to investigate the charge-transfer mechanism. Subsequently, it is also mandatory to understand the electron photoexcitation, trapping, and migration at the interface through a series of operando characterization techniques and computational studies.
Regarding the applicability of Z-schemes in environmental reactions, it is essential to develop new techniques to elucidate the mechanisms of catalytic reactions in all the stages of these catalytic processes (adsorption, reaction, and desorption), considering that the monitoring of the reaction is sometimes complex due to the formation of by-products. This scenario opens the door for the design and engineering of new Z-schemes based on g-C3N4, and the development of characterization techniques and studies of new mechanisms to obtain active photocatalysts in environmental applications under visible light.

Author Contributions

Conceptualization, J.F.-C., R.G., M.N.-G., Á.B.-M. and D.C.-A.; writing—original draft preparation, J.F.-C. and R.G.; writing—review and editing R.G., M.N.-G., W.C., Á.B.-M. and D.C.-A.; supervision, M.N.-G., W.C., Á.B.-M. and D.C.-A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by European Union-Next Generation EU, MINECO, and University of Alicante: MARSALAS21-09, Generalitat Valenciana: CDEIGENT/2018/027, University of Alicante: GRE20-19-A. PID2021-123079OB-I00 project funded by MCIN/AEI/10.13039/501100011033 and “ERDF A way of making Europe”, European Union’s Horizon 2020 research and innovation programme: Grant Agreement 101002219 and Generalitat Valenciana: Proyecto Prometeo CIPROM/2021/70.

Data Availability Statement

Not applicable.

Acknowledgments

J.F.-C. thanks European Union-Next Generation EU, MINECO, and University of Alicante for a postdoctoral researcher grant (MARSALAS21-09). M.N.-G. would like to thank the Plan GenT project from Generalitat Valenciana (CDEIGENT/2018/027), and the Vicerrectorado de Investigación y Transferencia de Conocimiento de la Universidad de Alicante (GRE20-19-A) for the financial support. PID2021-123079OB-I00 project funded by MCIN/AEI/10.13039/501100011033 and “ERDF A way of making Europe”, European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement 101002219), and Generalitat Valenciana (Proyecto Prometeo CIPROM/2021/70) are also acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Toman, M.A.; Morgenstern, R.D.; Anderson, J. The Economics of “When” Flexibility in the Design of Greenhouse Gas Abatement Policies. Annu. Rev. Environ. Resour. 1999, 24, 431–460. [Google Scholar]
  2. Vijayavenkataraman, S.; Iniyan, S.; Goic, R. A Review of Climate Change, Mitigation and Adaptation. Renew. Sustain. Energy Rev. 2012, 16, 878–897. [Google Scholar] [CrossRef]
  3. Manisalidis, I.; Stavropoulou, E.; Stavropoulos, A.; Bezirtzoglou, E. Environmental and Health Impacts of Air Pollution: A Review. Front. Public Health 2020, 8, 14. [Google Scholar] [CrossRef]
  4. Paltsev, S. Energy Scenarios: The Value and Limits of Scenario Analysis. Wiley Interdiscip. Rev. Energy Environ. 2017, 6, 1–19. [Google Scholar] [CrossRef]
  5. Solangi, K.H.; Islam, M.R.; Saidur, R.; Rahim, N.A.; Fayaz, H. A Review on Global Solar Energy Policy. Renew. Sustain. Energy Rev. 2011, 15, 2149–2163. [Google Scholar] [CrossRef]
  6. Kannan, N.; Vakeesan, D. Solar Energy for Future World—A Review. Renew. Sustain. Energy Rev. 2016, 62, 1092–1105. [Google Scholar] [CrossRef]
  7. Kabir, E.; Kumar, P.; Kumar, S.; Adelodun, A.A.; Kim, K.H. Solar Energy: Potential and Future Prospects. Renew. Sustain. Energy Rev. 2018, 82, 894–900. [Google Scholar] [CrossRef]
  8. Alharbi, N.S.; Hu, B.; Hayat, T.; Rabah, S.O.; Alsaedi, A.; Zhuang, L.; Wang, X. Efficient Elimination of Environmental Pollutants through Sorption-Reduction and Photocatalytic Degradation Using Nanomaterials. Front. Chem. Sci. Eng. 2020, 14, 1124–1135. [Google Scholar] [CrossRef]
  9. Saravanan, A.; Senthil Kumar, P.; Vo, D.V.N.; Jeevanantham, S.; Bhuvaneswari, V.; Anantha Narayanan, V.; Yaashikaa, P.R.; Swetha, S.; Reshma, B. A Comprehensive Review on Different Approaches for CO2 Utilization and Conversion Pathways. Chem. Eng. Sci. 2021, 236, 116515. [Google Scholar] [CrossRef]
  10. Melchionna, M.; Fornasiero, P. Updates on the Roadmap for Photocatalysis. ACS Catal. 2020, 10, 5493–5501. [Google Scholar] [CrossRef]
  11. Long, Z.; Li, Q.; Wei, T.; Zhang, G.; Ren, Z. Historical Development and Prospects of Photocatalysts for Pollutant Removal in Water. J. Hazard. Mater. 2020, 395, 122599. [Google Scholar] [CrossRef]
  12. Maeda, K.; Domen, K. Photocatalytic Water Splitting: Recent Progress and Future Challenges. J. Phys. Chem. Lett. 2010, 1, 2655–2661. [Google Scholar] [CrossRef]
  13. Nishiyama, H.; Yamada, T.; Nakabayashi, M.; Maehara, Y.; Yamaguchi, M.; Kuromiya, Y.; Nagatsuma, Y.; Tokudome, H.; Akiyama, S.; Watanabe, T.; et al. Photocatalytic Solar Hydrogen Production from Water on a 100-M2 Scale. Nature 2021, 598, 304–307. [Google Scholar] [CrossRef] [PubMed]
  14. Tuller, H.L. Solar to Fuels Conversion Technologies: A Perspective. Mater. Renew. Sustain. Energy 2017, 6, 3. [Google Scholar] [CrossRef] [PubMed]
  15. Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37–38. [Google Scholar] [CrossRef] [PubMed]
  16. Nakata, K.; Fujishima, A. TiO2 Photocatalysis: Design and Applications. J. Photochem. Photobiol. C Photochem. Rev. 2012, 13, 169–189. [Google Scholar] [CrossRef]
  17. Wenderich, K.; Mul, G. Methods, Mechanism, and Applications of Photodeposition in Photocatalysis: A Review. Chem. Rev. 2016, 116, 14587–14619. [Google Scholar] [CrossRef]
  18. Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D.W. Understanding TiO2 Photocatalysis Mechanisms and Materials. Chem. Rev. 2014, 114, 9919–9986. [Google Scholar]
  19. He, R.; Cao, S.; Zhou, P.; Yu, J. Recent Advances in Visible Light Bi-Based Photocatalysts. Chin. J. Catal. 2014, 35, 989–1007. [Google Scholar] [CrossRef]
  20. Han, G.; Sun, Y. Visible-Light-Driven Organic Transformations on Semiconductors. Mater. Today Phys. 2021, 16, 100297. [Google Scholar] [CrossRef]
  21. Wen, J.; Xie, J.; Chen, X.; Li, X. A Review on G-C3N4-Based Photocatalysts. Appl. Surf. Sci. 2017, 391, 72–123. [Google Scholar] [CrossRef]
  22. Hayat, A.; Al-Sehemi, A.G.; El-Nasser, K.S.; Taha, T.A.; Al-Ghamdi, A.A.; Shah Syed, J.A.; Amin, M.A.; Ali, T.; Bashir, T.; Palamanit, A.; et al. Graphitic Carbon Nitride (g–C3N4)–Based Semiconductor as a Beneficial Candidate in Photocatalysis Diversity. Int. J. Hydrogen Energy 2022, 47, 5142–5191. [Google Scholar] [CrossRef]
  23. Gan, X.; Lei, D.; Wong, K.Y. Two-Dimensional Layered Nanomaterials for Visible-Light-Driven Photocatalytic Water Splitting. Mater. Today Energy 2018, 10, 352–367. [Google Scholar] [CrossRef]
  24. Ismael, M. A Review on Graphitic Carbon Nitride (g-C3N4) Based Nanocomposites: Synthesis, Categories, and Their Application in Photocatalysis. J. Alloys Compd. 2020, 846, 156446. [Google Scholar] [CrossRef]
  25. Huang, D.; Li, Z.; Zeng, G.; Zhou, C.; Xue, W.; Gong, X.; Yan, X.; Chen, S.; Wang, W.; Cheng, M. Megamerger in Photocatalytic Field: 2D g-C3N4 Nanosheets Serve as Support of 0D Nanomaterials for Improving Photocatalytic Performance. Appl. Catal. B Environ. 2019, 240, 153–173. [Google Scholar] [CrossRef]
  26. Fu, J.; Yu, J.; Jiang, C.; Cheng, B. G-C3N4-Based Heterostructured Photocatalysts. Adv. Energy Mater. 2018, 8, 1701503. [Google Scholar] [CrossRef]
  27. Li, Y.; Zhou, M.; Cheng, B.; Shao, Y. Recent Advances in G-C3N4-Based Heterojunction Photocatalysts. J. Mater. Sci. Technol. 2020, 56, 1–17. [Google Scholar] [CrossRef]
  28. Liao, G.; Li, C.; Li, X.; Fang, B. Emerging Polymeric Carbon Nitride Z-Scheme Systems for Photocatalysis. Cell Rep. Phys. Sci. 2021, 2, 100355. [Google Scholar] [CrossRef]
  29. Belousov, A.S.; Fukina, D.G.; Koryagin, A.V. Metal–organic framework-basedheterojunction photocatalysts for organicpollutant degradation: Design, construction, and performances. J. Chem. Technol. Biotechnol. 2022, 97, 2675–2693. [Google Scholar] [CrossRef]
  30. Zhao, Y.; Linghu, X.; Shu, Y.; Zhang, J.; Chen, Z.; Wu, Y.; Shan, D.; Wang, B. Classification and Catalytic Mechanisms of Heterojunction Photocatalysts and the Application of Titanium Dioxide (TiO2)-Based Heterojunctions in Environmental Remediation. J. Environ. Chem. Eng. 2022, 10, 108077. [Google Scholar] [CrossRef]
  31. Low, J.; Yu, J.; Jaroniec, M.; Wageh, S.; Al-Ghamdi, A.A. Heterojunction Photocatalysts. Adv. Mater. 2017, 29, 1601694. [Google Scholar] [CrossRef] [PubMed]
  32. Liao, G.; Li, C.; Liu, S.Y.; Fang, B.; Yang, H. Emerging Frontiers of Z-Scheme Photocatalytic Systems. Trends Chem. 2022, 4, 111–127. [Google Scholar] [CrossRef]
  33. Ghosh, U.; Pal, A. Graphitic Carbon Nitride Based Z Scheme Photocatalysts: Design Considerations, Synthesis, Characterization and Applications. J. Ind. Eng. Chem. 2019, 79, 383–408. [Google Scholar] [CrossRef]
  34. Zhang, W.; Mohamed, A.R.; Ong, W.J. Z-Scheme Photocatalytic Systems for Carbon Dioxide Reduction: Where Are We Now? Angew. Chem.-Int. Ed. 2020, 59, 22894–22915. [Google Scholar] [CrossRef] [PubMed]
  35. Yuan, Y.; Guo, R.T.; Hong, L.F.; Ji, X.Y.; Lin, Z.D.; Li, Z.S.; Pan, W.G. A Review of Metal Oxide-Based Z-Scheme Heterojunction Photocatalysts: Actualities and Developments. Mater. Today Energy 2021, 21, 100829. [Google Scholar] [CrossRef]
  36. Low, J.; Jiang, C.; Cheng, B.; Wageh, S.; Al-Ghamdi, A.A.; Yu, J. A Review of Direct Z-Scheme Photocatalysts. Small Methods 2017, 1, 1700080. [Google Scholar] [CrossRef]
  37. Li, X.; Garlisi, C.; Guan, Q.; Anwer, S.; Al-Ali, K.; Palmisano, G.; Zheng, L. A Review of Material Aspects in Developing Direct Z-Scheme Photocatalysts. Mater. Today 2021, 47, 75–107. [Google Scholar] [CrossRef]
  38. Zhang, H.; Tian, W.; Zhang, J.; Duan, X.; Liu, S.; Sun, H.; Wang, S. Carbon Nitride-Based Z-Scheme Photocatalysts for Non-Sacrificial Overall Water Splitting. Mater. Today Energy 2022, 23, 100915. [Google Scholar] [CrossRef]
  39. Lin, J.; Tian, W.; Zhang, H.; Duan, X.; Sun, H.; Wang, S. Graphitic Carbon Nitride-Based Z-Scheme Structure for Photocatalytic CO2 Reduction. Energy Fuels 2021, 35, 7–24. [Google Scholar] [CrossRef]
  40. Kumar, Y.; Kumar, R.; Raizada, P.; Aslam, A.; Van Le, Q.; Singh, P.; Nguyen, V.-H. Novel Z-Scheme ZnIn2S4-based photocatalysts for solar-driven environmental and energy applications: Progress and perspectives. J. Mater. Sci. Technol. 2021, 87, 234–257. [Google Scholar] [CrossRef]
  41. Li, Y.; Pan, C.; Wang, G.; Leng, Y.; Jiang, P.; Dong, Y.; Zhu, Y. Improving the photocatalytic activity of benzyl alcohol oxidation by Z-scheme SnS/g-C3N4. New J. Chem. 2021, 45, 6611–6617. [Google Scholar] [CrossRef]
  42. Ma, X.; Huo, X.; Hao, K.; Song, L.; Yu, Q.; Liu, T.; Wang, Z. Visible Light Driven VO2/g-C3N4 Z-Scheme Composite Photocatalysts for Selective Oxidation of DL-1-Phenylethyl Alcohol under Vis-LEDs Irradiation and Aerobic Oxidation. Chem. Select 2021, 6, 2101–2211. [Google Scholar]
  43. Belousov, A.; Suleimanov, E.V. Application of metal–organic frameworks as an alternative to metal oxide-based photocatalysts for the production of industrially important organic chemicals. Green Chem. 2021, 23, 6172–6204. [Google Scholar] [CrossRef]
  44. Ma, J.; Yang, X.; Yao, S.; Guo, Y.; Sun, S. Photocatalytic Biorefinery to Lactic Acid: A Carbon Nitride Framework with O Atoms Replacing the Graphitic N Linkers Shows Fast Migration/Separation of Charge. ChemCatChem 2022, 14, e2022000. [Google Scholar] [CrossRef]
  45. Rono, N.; Kibet, J.K.; Martincigh, B.S.; Nyamori, V.O. A Review of the Current Status of Graphitic Carbon Nitride. Crit. Rev. Solid State Mater. Sci. 2021, 46, 189–217. [Google Scholar] [CrossRef]
  46. Inagaki, M.; Tsumura, T.; Kinumoto, T.; Toyoda, M. Graphitic Carbon Nitrides (g-C3N4) with Comparative Discussion to Carbon Materials. Carbon 2019, 141, 580–607. [Google Scholar] [CrossRef]
  47. Ong, W.J.; Tan, L.L.; Ng, Y.H.; Yong, S.T.; Chai, S.P. Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer to Achieving Sustainability? Chem. Rev. 2016, 116, 7159–7329. [Google Scholar] [CrossRef]
  48. Wang, J.; Wang, S. A Critical Review on Graphitic Carbon Nitride (g-C3N4)-Based Materials: Preparation, Modification and Environmental Application. Coord. Chem. Rev. 2022, 453, 214338. [Google Scholar] [CrossRef]
  49. Kroke, E.; Schwarz, M. Novel Group 14 Nitrides. Coord. Chem. Rev. 2004, 248, 493–532. [Google Scholar] [CrossRef]
  50. Alaghmandfard, A.; Ghandi, K. A Comprehensive Review of Graphitic Carbon Nitride (g-C3N4)–Metal Oxide-Based Nanocomposites: Potential for Photocatalysis and Sensing. Nanomaterials 2022, 12, 294. [Google Scholar] [CrossRef]
  51. Liu, N.; Li, T.; Zhao, Z.; Liu, J.; Luo, X.; Yuan, X.; Luo, K.; Luo, K.; He, J.; Yu, D.; et al. From Triazine to Heptazine: Origin of Graphitic Carbon Nitride as a Photocatalyst. ACS Omega 2020, 5, 12557–12567. [Google Scholar] [CrossRef] [PubMed]
  52. Wang, X.; Blechert, S.; Antonietti, M. Polymeric Graphitic Carbon Nitride for Heterogeneous Photocatalysis. ACS Catal. 2012, 2, 1596–1606. [Google Scholar] [CrossRef]
  53. Ismael, M.; Wu, Y. A Mini-Review on the Synthesis and Structural Modification of g-C3N4-Based Materials, and Their Applications in Solar Energy Conversion and Environmental Remediation. Sustain. Energy Fuels 2019, 3, 2907–2925. [Google Scholar] [CrossRef]
  54. Papailias, I.; Giannakopoulou, T.; Todorova, N.; Demotikali, D.; Vaimakis, T.; Trapalis, C. Effect of Processing Temperature on Structure and Photocatalytic Properties of G-C3N4. Appl. Surf. Sci. 2015, 358, 278–286. [Google Scholar] [CrossRef]
  55. Nguyen, T.K.A.; Pham, T.T.; Nguyen-Phu, H.; Shin, E.W. The Effect of Graphitic Carbon Nitride Precursors on the Photocatalytic Dye Degradation of Water-Dispersible Graphitic Carbon Nitride Photocatalysts. Appl. Surf. Sci. 2021, 537, 148027. [Google Scholar] [CrossRef]
  56. Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J.M.; Domen, K.; Antonietti, M. A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009, 8, 76–80. [Google Scholar] [CrossRef]
  57. Aslam, I.; Hassan Farooq, M.; Ghani, U.; Rizwan, M.; Nabi, G.; Shahzad, W.; Boddula, R. Synthesis of Novel G-C3N4 Microrods: A Metal-Free Visible-Light-Driven Photocatalyst. Mater. Sci. Energy Technol. 2019, 2, 401–407. [Google Scholar] [CrossRef]
  58. Wu, X.; Liu, C.; Li, X.; Zhang, X.; Wang, C.; Liu, Y. Effect of Morphology on the Photocatalytic Activity of G-C3N4 Photocatalysts under Visible-Light Irradiation. Mater. Sci. Semicond. Process. 2015, 32, 76–81. [Google Scholar] [CrossRef]
  59. Ren, Y.; Zeng, D.; Ong, W.J. Interfacial Engineering of Graphitic Carbon Nitride (g-C3N4)-Based Metal Sulfide Heterojunction Photocatalysts for Energy Conversion: A Review. Chin. J. Catal. 2019, 40, 289–319. [Google Scholar] [CrossRef]
  60. Zhu, B.; Cheng, B.; Fan, J.; Ho, W.; Yu, J. G-C3N4-Based 2D/2D Composite Heterojunction Photocatalyst. Small Struct. 2021, 2, 2100086. [Google Scholar] [CrossRef]
  61. Wang, Y.; Suzuki, H.; Xie, J.; Tomita, O.; Martin, D.J.; Higashi, M.; Kong, D.; Abe, R.; Tang, J. Mimicking Natural Photosynthesis: Solar to Renewable H2 Fuel Synthesis by Z-Scheme Water Splitting Systems. Chem. Rev. 2018, 118, 5201–5241. [Google Scholar] [CrossRef] [PubMed]
  62. Huang, D.; Chen, S.; Zeng, G.; Gong, X.; Zhou, C.; Cheng, M.; Xue, W.; Yan, X.; Li, J. Artificial Z-Scheme Photocatalytic System: What Have Been Done and Where to Go? Coord. Chem. Rev. 2019, 385, 44–80. [Google Scholar] [CrossRef]
  63. Bard, A.J. Photoelectrochemistry and Heterogeneous Photo-Catalysis at Semiconductors. J. Photochem. 1979, 10, 59–75. [Google Scholar] [CrossRef]
  64. Tada, H.; Mitsui, T.; Kiyonaga, T.; Akita, T.; Tanaka, K. All-Solid-State Z-Scheme in CdS-Au-TiO2 Three-Component Nanojunction System. Nat. Mater. 2006, 5, 782–786. [Google Scholar] [CrossRef] [PubMed]
  65. Sharma, S.; Dutta, V.; Raizada, P.; Khan, A.A.P.; Van Le, Q.; Thakur, V.K.; Biswas, J.K.; Selvasembian, R.; Singh, P. Controllable Functionalization of G-C3N4 Mediated All-Solid-State (ASS) Z-Scheme Photocatalysts towards Sustainable Energy and Environmental Applications. Environ. Technol. Innov. 2021, 24, 101972. [Google Scholar] [CrossRef]
  66. Yu, J.; Wang, S.; Low, J.; Xiao, W. Enhanced Photocatalytic Performance of Direct Z-Scheme g-C3N4-TiO2 Photocatalysts for the Decomposition of Formaldehyde in Air. Phys. Chem. Chem. Phys. 2013, 15, 16883–16890. [Google Scholar] [CrossRef] [PubMed]
  67. Lin, J.; Tian, W.; Zhang, H.; Duan, X.; Sun, H.; Wang, H.; Fang, Y.; Huang, Y.; Wang, S. Carbon Nitride-Based Z-Scheme Heterojunctions for Solar-Driven Advanced Oxidation Processes. J. Hazard. Mater. 2022, 434, 128866. [Google Scholar] [CrossRef]
  68. Zhao, D.; Guan, X.; Shen, S. Design of Polymeric Carbon Nitride-Based Heterojunctions for Photocatalytic Water Splitting: A Review. Environ. Chem. Lett. 2022, 7, 1–19. [Google Scholar] [CrossRef]
  69. Xu, Q.; Zhang, L.; Yu, J.; Wageh, S.; Al-Ghamdi, A.A.; Jaroniec, M. Direct Z-Scheme Photocatalysts: Principles, Synthesis, and Applications. Mater. Today 2018, 21, 1042–1063. [Google Scholar] [CrossRef]
  70. Zhang, Y.; Liu, J.; Wu, G.; Chen, W. Porous Graphitic Carbon Nitride Synthesized via Direct Polymerization of Urea for Efficient Sunlight-Driven Photocatalytic Hydrogen Production. Nanoscale 2012, 4, 5300–5303. [Google Scholar] [CrossRef]
  71. Kumar, S.; Karthikeyan, S.; Lee, A.F. G-C3N4-Based Nanomaterials for Visible Light-Driven Photocatalysis. Catalysts 2018, 8, 74. [Google Scholar] [CrossRef]
  72. Wiley, J.B.; Kaner, R.B. Rapid Solid-State Precursor Synthesis of Materials. Science 1992, 255, 1093–1097. [Google Scholar] [CrossRef] [PubMed]
  73. Stein, A.; Keller, S.W.; Mallouk, T.E. Turning down the Heat: Design and Mechanism in Solid-State Synthesis. Science 1993, 259, 1558–1564. [Google Scholar] [CrossRef] [PubMed]
  74. Lu, L.; Wang, G.; Zou, M.; Wang, J.; Li, J. Effects of Calcining Temperature on Formation of Hierarchical TiO2/g-C 3N4 Hybrids as an Effective Z-Scheme Heterojunction Photocatalyst. Appl. Surf. Sci. 2018, 441, 1012–1023. [Google Scholar] [CrossRef]
  75. Yu, W.; Chen, J.; Shang, T.; Chen, L.; Gu, L.; Peng, T. Direct Z-Scheme g-C3N4/WO3 Photocatalyst with Atomically Defined Junction for H2 Production. Appl. Catal. B Environ. 2017, 219, 693–704. [Google Scholar] [CrossRef]
  76. Chrouda, A.; Mahmoud Ali Ahmed, S.; Babiker Elamin, M. Preparation of Nanocatalysts Using Deposition Precipitation with Urea: Mechanism, Advantages and Results. ChemBioEng Rev. 2022, 9, 248–264. [Google Scholar] [CrossRef]
  77. Wen, X.J.; Shen, C.H.; Fei, Z.H.; Fang, D.; Liu, Z.T.; Dai, J.T.; Niu, C.G. Recent Developments on AgI Based Heterojunction Photocatalytic Systems in Photocatalytic Application. Chem. Eng. J. 2020, 383, 123083. [Google Scholar] [CrossRef]
  78. Gupta, B.; Melvin, A.A.; Matthews, T.; Dash, S.; Tyagi, A.K. TiO2 Modification by Gold (Au) for Photocatalytic Hydrogen (H2) Production. Renew. Sustain. Energy Rev. 2016, 58, 1366–1375. [Google Scholar] [CrossRef]
  79. Olivares, F.; Peón, F.; Henríquez, R.; del Río, R.S. Strategies for Area-Selective Deposition of Metal Nanoparticles on Carbon Nanotubes and Their Applications: A Review. J. Mater. Sci. 2022, 57, 2362–2387. [Google Scholar] [CrossRef]
  80. Guo, W.; Fan, K.; Zhang, J.; Xu, C. 2D/2D Z-Scheme Bi2WO6/Porous-g-C3N4 with Synergy of Adsorption and Visible-Light-Driven Photodegradation. Appl. Surf. Sci. 2018, 447, 125–134. [Google Scholar] [CrossRef]
  81. Hutchings, G.J.; Kiely, C.J. Strategies for the Synthesis of Supported Gold Palladium Nanoparticles with Controlled Morphology and Composition. Acc. Chem. Res. 2013, 46, 1759–1772. [Google Scholar] [CrossRef] [PubMed]
  82. Campanati, M.; Fornasari, G.; Vaccari, A. Fundamentals in the Preparation of Heterogeneous Catalysts. Catal. Today 2003, 77, 299–314. [Google Scholar] [CrossRef]
  83. Bhosale, R.; Jain, S.; Vinod, C.P.; Kumar, S.; Ogale, S. Direct Z-Scheme g-C3N4/FeWO4 Nanocomposite for Enhanced and Selective Photocatalytic CO2 Reduction under Visible Light. ACS Appl. Mater. Interfaces 2019, 11, 6174–6183. [Google Scholar] [CrossRef]
  84. Jin, Z.; Hu, R.; Wang, H.; Hu, J.; Ren, T. One-Step Impregnation Method to Prepare Direct Z-Scheme LaCoO3/g-C3N4 Heterojunction Photocatalysts for Phenol Degradation under Visible Light. Appl. Surf. Sci. 2019, 491, 432–442. [Google Scholar] [CrossRef]
  85. Feng, Z.; Zeng, L.; Chen, Y.; Ma, Y.; Zhao, C.; Jin, R.; Lu, Y.; Wu, Y.; He, Y. In Situ Preparation of Z-Scheme MoO3/g-C3N4 Composite with High Performance in Photocatalytic CO2 Reduction and RhB Degradation. J. Mater. Res. 2017, 32, 3660–3668. [Google Scholar] [CrossRef]
  86. Zhou, D.; Yu, B.; Chen, Q.; Shi, H.; Zhang, Y.; Li, D.; Yang, X.; Zhao, W.; Liu, C.; Wei, G.; et al. Improved Visible Light Photocatalytic Activity on Z-Scheme g-C3N4 Decorated TiO2 Nanotube Arrays by a Simple Impregnation Method. Mater. Res. Bull. 2020, 124, 110757. [Google Scholar] [CrossRef]
  87. Darr, J.A.; Zhang, J.; Makwana, N.M.; Weng, X. Continuous Hydrothermal Synthesis of Inorganic Nanoparticles: Applications and Future Directions. Chem. Rev. 2017, 117, 11125–11238. [Google Scholar] [CrossRef]
  88. Kaya, C.; He, J.Y.; Gu, X.; Butler, E.G. Nanostructured Ceramic Powders by Hydrothermal Synthesis and Their Applications. Microporous Mesoporous Mater. 2002, 54, 37–49. [Google Scholar] [CrossRef]
  89. Byrappa, K.; Adschiri, T. Hydrothermal Technology for Nanotechnology. Prog. Cryst. Growth Charact. Mater. 2007, 53, 117–166. [Google Scholar] [CrossRef]
  90. Jo, W.K.; Natarajan, T.S. Influence of TiO2 Morphology on the Photocatalytic Efficiency of Direct Z-Scheme g-C3N4/TiO2 Photocatalysts for Isoniazid Degradation. Chem. Eng. J. 2015, 281, 549–565. [Google Scholar] [CrossRef]
  91. Di, T.; Zhu, B.; Cheng, B.; Yu, J.; Xu, J. A Direct Z-Scheme g-C3N4/SnS2 Photocatalyst with Superior Visible-Light CO2 Reduction Performance. J. Catal. 2017, 352, 532–541. [Google Scholar] [CrossRef]
  92. Lu, M.; Li, Q.; Zhang, C.; Fan, X.; Li, L.; Dong, Y.; Chen, G.; Shi, H. Remarkable Photocatalytic Activity Enhancement of CO2 Conversion over 2D/2D g-C3N4/BiVO4 Z-Scheme Heterojunction Promoted by Efficient Interfacial Charge Transfer. Carbon 2020, 160, 342–352. [Google Scholar] [CrossRef]
  93. Wu, Y.; Zhao, X.; Huang, S.; Li, Y.; Zhang, X.; Zeng, G.; Niu, L.; Ling, Y.; Zhang, Y. Facile Construction of 2D G-C3N4 Supported Nanoflower-like NaBiO3 with Direct Z-Scheme Heterojunctions and Insight into Its Photocatalytic Degradation of Tetracycline. J. Hazard. Mater. 2021, 414, 125547. [Google Scholar] [CrossRef]
  94. Lakshmanareddy, N.; Navakoteswara Rao, V.; Cheralathan, K.K.; Subramaniam, E.P.; Shankar, M.V. Pt/TiO2 Nanotube Photocatalyst–Effect of Synthesis Methods on Valance State of Pt and Its Influence on Hydrogen Production and Dye Degradation. J. Colloid Interface Sci. 2019, 538, 83–98. [Google Scholar] [CrossRef]
  95. Magdziarz, A.; Colmenares, J.C. In Situ Coupling of Ultrasound to Electro-and Photo-Deposition Methods for Materials Synthesis. Molecules 2017, 22, 216. [Google Scholar] [CrossRef] [PubMed]
  96. Jiang, W.; Qu, D.; An, L.; Gao, X.; Wen, Y.; Wang, X.; Sun, Z. Deliberate Construction of Direct Z -Scheme Photocatalysts through Photodeposition. J. Mater. Chem. A 2019, 7, 18348–18356. [Google Scholar] [CrossRef]
  97. Sari, F.N.I.; Yen, D.T.K.; Ting, J.M. Enhanced Photocatalytic Performance of TiO2 through a Novel Direct Dual Z-Scheme Design. Appl. Surf. Sci. 2020, 533, 147506. [Google Scholar] [CrossRef]
  98. Rhimi, B.; Wang, C.; Bahnemann, D.W. Latest Progress in G-C3N4 Based Heterojunctions for Hydrogen Production via Photocatalytic Water Splitting: A Mini Review. J. Phys. Energy 2020, 2, 042003. [Google Scholar] [CrossRef]
  99. Pourshirband, N.; Nezamzadeh-Ejhieh, A. An Efficient Z-Scheme CdS/g-C3N4 Nano Catalyst in Methyl Orange Photodegradation: Focus on the Scavenging Agent and Mechanism. J. Mol. Liq. 2021, 335, 116543. [Google Scholar] [CrossRef]
  100. Chiu, Y.H.; Chang, T.F.M.; Chen, C.Y.; Sone, M.; Hsu, Y.J. Mechanistic Insights into Photodegradation of Organic Dyes Using Heterostructure Photocatalysts. Catalysts 2019, 9, 430. [Google Scholar] [CrossRef]
  101. Bai, Y.; Wang, P.Q.; Liu, J.Y.; Liu, X.J. Enhanced Photocatalytic Performance of Direct Z-Scheme BiOCl-g-C3N4 Photocatalysts. RSC Adv. 2014, 4, 19456–19461. [Google Scholar] [CrossRef]
  102. Jiang, W.; Zong, X.; An, L.; Hua, S.; Miao, X.; Luan, S.; Wen, Y.; Tao, F.F.; Sun, Z. Consciously Constructing Heterojunction or Direct Z-Scheme Photocatalysts by Regulating Electron Flow Direction. ACS Catal. 2018, 8, 2209–2217. [Google Scholar] [CrossRef]
  103. Zhang, J.; Hu, Y.; Jiang, X.; Chen, S.; Meng, S.; Fu, X. Design of a Direct Z-Scheme Photocatalyst: Preparation and Characterization of Bi2O3/g-C3N4 with High Visible Light Activity. J. Hazard. Mater. 2014, 280, 713–722. [Google Scholar] [CrossRef] [PubMed]
  104. Low, J.; Dai, B.; Tong, T.; Jiang, C.; Yu, J. In Situ Irradiated X-Ray Photoelectron Spectroscopy Investigation on a Direct Z-Scheme TiO2/CdS Composite Film Photocatalyst. Adv. Mater. 2019, 31, 1802981. [Google Scholar] [CrossRef]
  105. Han, Q.; Li, L.; Gao, W.; Shen, Y.; Wang, L.; Zhang, Y.; Wang, X.; Shen, Q.; Xiong, Y.; Zhou, Y.; et al. Elegant Construction of ZnIn2S4/BiVO4 Hierarchical Heterostructures as Direct Z-Scheme Photocatalysts for Efficient CO2 Photoreduction. ACS Appl. Mater. Interfaces 2021, 13, 15092–15100. [Google Scholar] [CrossRef]
  106. Zhou, Z.; Niu, X.; Zhang, Y.; Wang, J. Janus MoSSe/WSeTe Heterostructures: A Direct Z-Scheme Photocatalyst for Hydrogen Evolution. J. Mater. Chem. A 2019, 7, 21835–21842. [Google Scholar] [CrossRef]
  107. Abdul Nasir, J.; Munir, A.; Ahmad, N.; Haq, T.U.; Khan, Z.; Rehman, Z. Photocatalytic Z-Scheme Overall Water Splitting: Recent Advances in Theory and Experiments. Adv. Mater. 2021, 33, 105195. [Google Scholar] [CrossRef]
  108. Lu, Q.; Eid, K.; Li, W.; Abdullah, A.M.; Xu, G.; Varma, R.S. Engineering Graphitic Carbon Nitride (g-C3N4) for Catalytic Reduction of CO2 to Fuels and Chemicals: Strategy and Mechanism. Green Chem. 2021, 23, 5394–5428. [Google Scholar] [CrossRef]
  109. Saravanan, A.; Kumar, P.S.; Vo, D.V.N.; Yaashikaa, P.R.; Karishma, S.; Jeevanantham, S.; Gayathri, B.; Bharathi, V.D. Photocatalysis for Removal of Environmental Pollutants and Fuel Production: A Review. Environ. Chem. Lett. 2021, 19, 441–463. [Google Scholar] [CrossRef]
  110. Yu, X.; Ng, S.F.; Putri, L.K.; Tan, L.L.; Mohamed, A.R.; Ong, W.J. Point-Defect Engineering: Leveraging Imperfections in Graphitic Carbon Nitride (g-C3N4) Photocatalysts toward Artificial Photosynthesis. Small 2021, 17, 2006851. [Google Scholar] [CrossRef]
  111. Fu, Y.; Li, Z.; Liu, Q.; Yang, X.; Tang, H. Construction of Carbon Nitride and MoS2 Quantum Dot 2D/0D Hybrid Photocatalyst: Direct Z-Scheme Mechanism for Improved Photocatalytic Activity. Chin. J. Catal. 2017, 38, 2160–2170. [Google Scholar] [CrossRef]
  112. Jing, L.; Xu, Y.; Liu, J.; Zhou, M.; Xu, H.; Xie, M.; Li, H.; Xie, J. Direct Z-Scheme Red Carbon Nitride/Rod-like Lanthanum Vanadate Composites with Enhanced Photodegradation of Antibiotic Contaminants. Appl. Catal. B Environ. 2020, 277, 119245. [Google Scholar] [CrossRef]
  113. Zhou, D.; Chen, Z.; Yang, Q.; Shen, C.; Tang, G.; Zhao, S.; Zhang, J.; Chen, D.; Wei, Q.; Dong, X. Facile Construction of G-C3N4 Nanosheets/TiO2 Nanotube Arrays as Z-Scheme Photocatalyst with Enhanced Visible-Light Performance. ChemCatChem 2016, 8, 3064–3073. [Google Scholar] [CrossRef]
  114. Yang, Y.; Chen, J.; Liu, C.; Sun, Z.; Qiu, M.; Yan, G.; Gao, F. Dual-Z-Scheme Heterojunction for Facilitating Spacial Charge Transport Toward Ultra-Efficient Photocatalytic H2 Production. Sol. RRL 2021, 5, 2100241. [Google Scholar] [CrossRef]
  115. Idrees, F.; Dillert, R.; Bahnemann, D.; Butt, F.K.; Tahir, M. In-Situ Synthesis of Nb2O5/g-C3N4 Heterostructures as Highly Efficient Photocatalysts for Molecular H2 Evolution under Solar Illumination. Catalysts 2019, 9, 169. [Google Scholar] [CrossRef]
  116. He, Y.; Zhang, L.; Fan, M.; Wang, X.; Walbridge, M.L.; Nong, Q.; Wu, Y.; Zhao, L. Z-Scheme SnO2-x/g-C3N4 Composite as an Efficient Photocatalyst for Dye Degradation and Photocatalytic CO2 Reduction. Sol. Energy Mater. Sol. Cells 2015, 137, 175–184. [Google Scholar] [CrossRef]
  117. Tahir, B.; Tahir, M.; Mohd Nawawi, M.G. Highly STable 3D/2D WO3/g-C3N4 Z-Scheme Heterojunction for Stimulating Photocatalytic CO2 Reduction by H2O/H2 to CO and CH4 under Visible Light. J. CO2 Util. 2020, 41, 101270. [Google Scholar] [CrossRef]
  118. Deng, Y.; Zhao, R. Advanced Oxidation Processes (AOPs) in Wastewater Treatment. Curr. Pollut. Reports 2015, 1, 167–176. [Google Scholar] [CrossRef]
  119. Zhang, Y.; Zhou, J.; Chen, X.; Wang, L.; Cai, W. Coupling of Heterogeneous Advanced Oxidation Processes and Photocatalysis in Efficient Degradation of Tetracycline Hydrochloride by Fe-Based MOFs: Synergistic Effect and Degradation Pathway. Chem. Eng. J. 2019, 369, 745–757. [Google Scholar] [CrossRef]
  120. Garza-Campos, B.; Brillas, E.; Hernández-Ramírez, A.; El-Ghenymy, A.; Guzmán-Mar, J.L.; Ruiz-Ruiz, E.J. Salicylic Acid Degradation by Advanced Oxidation Processes. Coupling of Solar Photoelectro-Fenton and Solar Heterogeneous Photocatalysis. J. Hazard. Mater. 2016, 319, 34–42. [Google Scholar] [CrossRef]
  121. Zhang, Z.; Zhang, M.; Deng, J.; Deng, K.; Zhang, B.; Lv, K.; Sun, J.; Chen, L. Potocatalytic Oxidative Degradation of Organic Pollutant with Molecular Oxygen Activated by a Novel Biomimetic Catalyst ZnPz(Dtn-COOH)4. Appl. Catal. B Environ. 2013, 132–133, 90–97. [Google Scholar] [CrossRef]
  122. Xie, L.; Du, T.; Wang, J.; Ma, Y.; Ni, Y.; Liu, Z.; Zhang, L.; Yang, C.; Wang, J. Recent Advances on Heterojunction-Based Photocatalysts for the Degradation of Persistent Organic Pollutants. Chem. Eng. J. 2021, 426, 130617. [Google Scholar] [CrossRef]
  123. Al-Buriahi, A.K.; Al-Gheethi, A.A.; Senthil Kumar, P.; Radin Mohamed, R.M.S.; Yusof, H.; Alshalif, A.F.; Khalifa, N.A. Elimination of Rhodamine B from Textile Wastewater Using Nanoparticle Photocatalysts: A Review for Sustainable Approaches. Chemosphere 2022, 287, 132162. [Google Scholar] [CrossRef] [PubMed]
  124. Tang, M.; Ao, Y.; Wang, C.; Wang, P. Facile Synthesis of Dual Z-Scheme g-C3N4/Ag3PO4/AgI Composite Photocatalysts with Enhanced Performance for the Degradation of a Typical Neonicotinoid Pesticide. Appl. Catal. B Environ. 2020, 268, 118395. [Google Scholar] [CrossRef]
  125. Che, H.; Che, G.; Dong, H.; Hu, W.; Hu, H.; Liu, C.; Li, C. Fabrication of Z-Scheme Bi3O4Cl/g-C3N4 2D/2D Heterojunctions with Enhanced Interfacial Charge Separation and Photocatalytic Degradation Various Organic Pollutants Activity. Appl. Surf. Sci. 2018, 455, 705–716. [Google Scholar] [CrossRef]
  126. Song, Y.; Gu, J.; Xia, K.; Yi, J.; Chen, H.; She, X.; Chen, Z.; Ding, C.; Li, H.; Xu, H. Construction of 2D SnS2/g-C3N4 Z-Scheme Composite with Superior Visible-Light Photocatalytic Performance. Appl. Surf. Sci. 2019, 467–468, 56–64. [Google Scholar] [CrossRef]
  127. López-Peñalver, J.J.; Sánchez-Polo, M.; Gómez-Pacheco, C.V.; Rivera-Utrilla, J. Photodegradation of Tetracyclines in Aqueous Solution by Using UV and UV/H2O2 Oxidation Processes. J. Chem. Technol. Biotechnol. 2010, 85, 1325–1333. [Google Scholar] [CrossRef]
  128. Chen, Y.; Hu, C.; Qu, J.; Yang, M. Photodegradation of Tetracycline and Formation of Reactive Oxygen Species in Aqueous Tetracycline Solution under Simulated Sunlight Irradiation. J. Photochem. Photobiol. A Chem. 2008, 197, 81–87. [Google Scholar] [CrossRef]
  129. De Godos, I.; Muñoz, R.; Guieysse, B. Tetracycline Removal during Wastewater Treatment in High-Rate Algal Ponds. J. Hazard. Mater. 2012, 229–230, 446–449. [Google Scholar] [CrossRef]
  130. Kang, J.; Tang, Y.; Wang, M.; Jin, C.; Liu, J.; Li, S.; Li, Z.; Zhu, J. The Enhanced Peroxymonosulfate-Assisted Photocatalytic Degradation of Tetracycline under Visible Light by g-C3N4/Na-BiVO4 heterojunction Catalyst and Its Mechanism. J. Environ. Chem. Eng. 2021, 9, 105524. [Google Scholar] [CrossRef]
  131. Jin, C.; Wang, M.; Li, Z.; Kang, J.; Zhao, Y.; Han, J.; Wu, Z. Two Dimensional Co3O4/g-C3N4 Z-Scheme Heterojunction: Mechanism Insight into Enhanced Peroxymonosulfate-Mediated Visible Light Photocatalytic Performance. Chem. Eng. J. 2020, 398, 125569. [Google Scholar] [CrossRef]
  132. Wang, S.; Teng, Z.; Xu, Y.; Yuan, M.; Zhong, Y.; Liu, S.; Wang, C.; Wang, G.; Ohno, T. Defect as the Essential Factor in Engineering Carbon-Nitride-Based Visible-Light-Driven Z-Scheme Photocatalyst. Appl. Catal. B Environ. 2020, 260, 118145. [Google Scholar] [CrossRef]
  133. Aravind kumar, J.; Krithiga, T.; Sathish, S.; Renita, A.A.; Prabu, D.; Lokesh, S.; Geetha, R.; Namasivayam, S.K.R.; Sillanpaa, M. Persistent Organic Pollutants in Water Resources: Fate, Occurrence, Characterization and Risk Analysis. Sci. Total Environ. 2022, 831, 154808. [Google Scholar] [CrossRef] [PubMed]
  134. Zhou, Y.; Zhou, L.; Ni, C.; He, E.; Yu, L.; Li, X. 3D/2D MOF-Derived CoCeOx/g-C3N4 Z-Scheme Heterojunction for Visible Light Photocatalysis: Hydrogen Production and Degradation of Carbamazepine. J. Alloys Compd. 2022, 890, 161786. [Google Scholar] [CrossRef]
  135. Huang, J.; Li, D.; Li, R.; Chen, P.; Zhang, Q.; Liu, H.; Lv, W.; Liu, G.; Feng, Y. One-Step Synthesis of Phosphorus/Oxygen Co-Doped g-C3N4/Anatase TiO2 Z-Scheme Photocatalyst for Significantly Enhanced Visible-Light Photocatalysis Degradation of Enrofloxacin. J. Hazard. Mater. 2020, 386, 121634. [Google Scholar] [CrossRef]
  136. Meng, J.; Wang, X.; Liu, Y.; Ren, M.; Zhang, X.; Ding, X.; Guo, Y.; Yang, Y. Acid-Induced Molecule Self-Assembly Synthesis of Z-Scheme WO3/g-C3N4 Heterojunctions for Robust Photocatalysis against Phenolic Pollutants. Chem. Eng. J. 2021, 403, 126354. [Google Scholar] [CrossRef]
  137. Liu, N.; Lu, N.; Yu, H.T.; Chen, S.; Quan, X. Enhanced Degradation of Organic Water Pollutants by Photocatalytic In-Situ Activation of Sulfate Based on Z-Scheme g-C3N4/BiPO4. Chem. Eng. J. 2022, 428, 132116. [Google Scholar] [CrossRef]
  138. Zhang, L.; Wang, G.; Xiong, Z.; Tang, H.; Jiang, C. Fabrication of Flower-like Direct Z-Scheme β-Bi2O3/g-C3N4 Photocatalyst with Enhanced Visible Light Photoactivity for Rhodamine B Degradation. Appl. Surf. Sci. 2018, 436, 162–171. [Google Scholar] [CrossRef]
  139. Li, N.; Tian, Y.; Zhao, J.; Zhang, J.; Zuo, W.; Kong, L.; Cui, H. Z-Scheme 2D/3D g-C3N4@ZnO with Enhanced Photocatalytic Activity for Cephalexin Oxidation under Solar Light. Chem. Eng. J. 2018, 352, 412–422. [Google Scholar] [CrossRef]
  140. Qin, Y.; Li, H.; Lu, J.; Ma, C.; Liu, X.; Meng, M.; Yan, Y. Fabrication of Magnetic Quantum Dots Modified Z-Scheme Bi2O4/g-C3N4 Photocatalysts with Superior Hydroxyl Radical Productivity for the Degradation of Rhodamine B. Appl. Surf. Sci. 2019, 493, 458–469. [Google Scholar] [CrossRef]
  141. Zhang, X.; Li, L.; Zeng, Y.; Liu, F.; Yuan, J.; Li, X.; Yu, Y.; Zhu, X.; Xiong, Z.; Yu, H.; et al. TiO2/Graphitic Carbon Nitride Nanosheets for the Photocatalytic Degradation of Rhodamine B under Simulated Sunlight. ACS Appl. Nano Mater. 2019, 2, 7255–7265. [Google Scholar] [CrossRef]
  142. Guo, T.; Wang, K.; Zhang, G.; Wu, X. A Novel α-Fe2O3@g-C3N4 Catalyst: Synthesis Derived from Fe-Based MOF and Its Superior Photo-Fenton Performance. Appl. Surf. Sci. 2019, 469, 331–339. [Google Scholar] [CrossRef]
  143. Truc, N.T.T.; Duc, D.S.; Van Thuan, D.; Tahtamouni, T.A.; Pham, T.D.; Hanh, N.T.; Tran, D.T.; Nguyen, M.V.; Dang, N.M.; Le Chi, N.T.P.; et al. The Advanced Photocatalytic Degradation of Atrazine by Direct Z-Scheme Cu Doped ZnO/g-C3N4. Appl. Surf. Sci. 2019, 489, 875–882. [Google Scholar] [CrossRef]
  144. Gayathri, K.; Teja, Y.N.; Prakash, R.M.; Hossain, M.S.; Alsalme, A.; Sundaravadivel, E.; Sakar, M. In Situ-Grown ZnO Particles on g-C3N4 Layers: A Direct Z-Scheme-Driven Photocatalyst for the Degradation of Dye and Pharmaceutical Pollutants under Solar Irradiation. J. Mater. Sci. Mater. Electron. 2022, 33, 9774–9784. [Google Scholar] [CrossRef]
  145. Jain, I.P. Hydrogen the Fuel for 21st Century. Int. J. Hydrogen Energy 2009, 34, 7368–7378. [Google Scholar] [CrossRef]
  146. Nazir, H.; Louis, C.; Jose, S.; Prakash, J.; Muthuswamy, N.; Buan, M.E.M.; Flox, C.; Chavan, S.; Shi, X.; Kauranen, P.; et al. Is the H2 Economy Realizable in the Foreseeable Future? Part I: H2 Production Methods. Int. J. Hydrogen Energy 2020, 45, 13777–13788. [Google Scholar] [CrossRef]
  147. Jafari, T.; Moharreri, E.; Amin, A.S.; Miao, R.; Song, W.; Suib, S.L. Photocatalytic Water Splitting-The Untamed Dream: A Review of Recent Advances. Molecules 2016, 21, 900. [Google Scholar] [CrossRef]
  148. Fajrina, N.; Tahir, M. A Critical Review in Strategies to Improve Photocatalytic Water Splitting towards Hydrogen Production. Int. J. Hydrogen Energy 2019, 44, 540–577. [Google Scholar] [CrossRef]
  149. Wang, Q.; Domen, K. Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies. Chem. Rev. 2020, 120, 919–985. [Google Scholar] [CrossRef]
  150. Chen, X.; Shi, R.; Chen, Q.; Zhang, Z.; Jiang, W.; Zhu, Y.; Zhang, T. Three-Dimensional Porous g-C3N4 for Highly Efficient Photocatalytic Overall Water Splitting. Nano Energy 2019, 59, 644–650. [Google Scholar] [CrossRef]
  151. Ye, R.; Fang, H.; Zheng, Y.Z.; Li, N.; Wang, Y.; Tao, X. Fabrication of CoTiO3/g-C3N4 Hybrid Photocatalysts with Enhanced H2 Evolution: Z-Scheme Photocatalytic Mechanism Insight. ACS Appl. Mater. Interfaces 2016, 8, 13879–13889. [Google Scholar] [CrossRef] [PubMed]
  152. She, X.; Wu, J.; Xu, H.; Zhong, J.; Wang, Y.; Song, Y.; Nie, K.; Liu, Y.; Yang, Y.; Rodrigues, M.T.F.; et al. High Efficiency Photocatalytic Water Splitting Using 2D A-Fe2O3/g-C3N4 Z-Scheme Catalysts. Adv. Energy Mater. 2017, 7, 1700025. [Google Scholar] [CrossRef]
  153. Yang, Y.; Qiu, M.; Li, L.; Pi, Y.; Yan, G.; Yang, L. A Direct Z-Scheme Van Der Waals Heterojunction (WO3·H2O/g-C3N4) for High Efficient Overall Water Splitting under Visible-Light. Sol. RRL 2018, 2, 1800148. [Google Scholar] [CrossRef]
  154. Kong, L.; Zhang, X.; Wang, C.; Xu, J.; Du, X.; Li, L. Ti3+ Defect Mediated G-C3N4 /TiO2 Z-Scheme System for Enhanced Photocatalytic Redox Performance. Appl. Surf. Sci. 2018, 448, 288–296. [Google Scholar] [CrossRef]
  155. Qin, S.; Lei, Y.; Guo, J.; Huang, J.F.; Hou, C.P.; Liu, J.M. Constructing Heterogeneous Direct Z-Scheme Photocatalysts Based on Metal-Organic Cages and Graphitic-C3N4 for High-Efficiency Photocatalytic Water Splitting. ACS Appl. Mater. Interfaces 2021, 13, 25960–25971. [Google Scholar] [CrossRef]
  156. Wang, J.; Xia, Y.; Zhao, H.; Wang, G.; Xiang, L.; Xu, J.; Komarneni, S. Oxygen Defects-Mediated Z-Scheme Charge Separation in g-C3N4/ZnO Photocatalysts for Enhanced Visible-Light Degradation of 4-Chlorophenol and Hydrogen Evolution. Appl. Catal. B Environ. 2017, 206, 406–416. [Google Scholar] [CrossRef]
  157. Shi, Y.; Chen, J.; Mao, Z.; Fahlman, B.D.; Wang, D. Construction of Z-Scheme Heterostructure with Enhanced Photocatalytic H2 Evolution for g-C3N4 Nanosheets via Loading Porous Silicon. J. Catal. 2017, 356, 22–31. [Google Scholar] [CrossRef]
  158. Yang, C.; Xue, Z.; Qin, J.; Sawangphruk, M.; Rajendran, S.; Zhang, X.; Liu, R. Visible Light-Driven Photocatalytic H2 Generation and Mechanism Insights into Bi2O2CO3 /G-C3N4 Z-Scheme Photocatalyst. J. Phys. Chem. C 2019, 123, 4795–4804. [Google Scholar] [CrossRef]
  159. Cui, Y.; Wang, H.; Yang, C.; Li, M.; Zhao, Y.; Chen, F. Post-Activation of in Situ B–F Codoped g-C3N4 for Enhanced Photocatalytic H2 Evolution. Appl. Surf. Sci. 2018, 441, 621–630. [Google Scholar] [CrossRef]
  160. Xu, B.; Wang, B.; Zhang, H.; Yang, P. Z-Scheme Cu2O Nanoparticle/Graphite Carbon Nitride Nanosheet Heterojunctions for Photocatalytic Hydrogen Evolution. ACS Appl. Nano Mater. 2022, 5, 8475–8483. [Google Scholar] [CrossRef]
  161. Olabi, A.G.; Abdelkareem, M.A. Renewable Energy and Climate Change. Renew. Sustain. Energy Rev. 2022, 158, 112111. [Google Scholar] [CrossRef]
  162. Kuc, T.; Rozanski, K.; Zimnoch, M.; Necki, J.M.; Korus, A. Anthropogenic Emissions of CO2 and CH4 in an Urban Environment. Appl. Energy 2003, 75, 193–203. [Google Scholar] [CrossRef]
  163. Sullivan, I.; Goryachev, A.; Digdaya, I.A.; Li, X.; Atwater, H.A.; Vermaas, D.A.; Xiang, C. Coupling Electrochemical CO2 Conversion with CO2 Capture. Nat. Catal. 2021, 4, 952–958. [Google Scholar] [CrossRef]
  164. Lewis, N.S. Developing a Scalable Artificial Photosynthesis Technology through Nanomaterials by Design. Nat. Nanotechnol. 2016, 11, 1010–1019. [Google Scholar] [CrossRef] [PubMed]
  165. Greenwood, N.N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Elsevier Butterworth-Heinemann: Oxford, UK, 2005. [Google Scholar]
  166. Khalil, M.; Gunlazuardi, J.; Ivandini, T.A.; Umar, A. Photocatalytic Conversion of CO2 Using Earth-Abundant Catalysts: A Review on Mechanism and Catalytic Performance. Renew. Sustain. Energy Rev. 2019, 113, 109246. [Google Scholar] [CrossRef]
  167. Chang, X.; Wang, T.; Gong, J. CO2 Photo-Reduction: Insights into CO2 Activation and Reaction on Surfaces of Photocatalysts. Energy Environ. Sci. 2016, 9, 2177–2196. [Google Scholar] [CrossRef]
  168. Collado, L.; Reynal, A.; Coronado, J.M.; Serrano, D.P.; Durrant, J.R.; De la Peña O’Shea, V.A. Effect of Au Surface Plasmon Nanoparticles on the Selective CO2 Photoreduction to CH4. Appl. Catal. B Environ. 2015, 178, 177–185. [Google Scholar] [CrossRef]
  169. Dong, G.; Zhang, L. Porous Structure Dependent Photoreactivity of Graphitic Carbon Nitride under Visible Light. J. Mater. Chem. 2012, 22, 1160–1166. [Google Scholar] [CrossRef]
  170. Jiang, Z.; Wan, W.; Li, H.; Yuan, S.; Zhao, H.; Wong, P.K. A Hierarchical Z-Scheme α-Fe2O3/g-C3N4 Hybrid for Enhanced Photocatalytic CO2 Reduction. Adv. Mater. 2018, 30, 1706108. [Google Scholar] [CrossRef]
  171. Liu, T.; Hao, L.; Bai, L.; Liu, J.; Zhang, Y.; Tian, N.; Huang, H. Z-Scheme Junction Bi2O2(NO3)(OH)/g-C3N4 for Promoting CO2 Photoreduction. Chem. Eng. J. 2022, 429, 132268. [Google Scholar] [CrossRef]
  172. Yu, W.; Xu, D.; Peng, T. Enhanced Photocatalytic Activity of G-C3N4 for Selective CO2 Reduction to CH3OH via Facile Coupling of ZnO: A Direct Z-Scheme Mechanism. J. Mater. Chem. A 2015, 3, 19936–19947. [Google Scholar] [CrossRef]
  173. Thanh Truc, N.T.; Giang Bach, L.; Thi Hanh, N.; Pham, T.D.; Thi Phuong Le Chi, N.; Tran, D.T.; Nguyen, M.V.; Nguyen, V.N. The Superior Photocatalytic Activity of Nb Doped TiO2/g-C3N4 Direct Z-Scheme System for Efficient Conversion of CO2 into Valuable Fuels. J. Colloid Interface Sci. 2019, 540, 1–8. [Google Scholar] [CrossRef] [PubMed]
  174. Zhao, J.; Ji, M.; Chen, H.; Weng, Y.X.; Zhong, J.; Li, Y.; Wang, S.; Chen, Z.; Xia, J.; Li, H. Interfacial Chemical Bond Modulated Bi19S27Br3/g-C3N4 Z-Scheme Heterojunction for Enhanced Photocatalytic CO2 Conversion. Appl. Catal. B Environ. 2022, 307, 121162. [Google Scholar] [CrossRef]
  175. Murugesan, P.; Narayanan, S.; Manickam, M.; Murugesan, P.K.; Subbiah, R. A Direct Z-Scheme Plasmonic AgCl@g-C3N4 Heterojunction Photocatalyst with Superior Visible Light CO2 Reduction in Aqueous Medium. Appl. Surf. Sci. 2018, 450, 516–526. [Google Scholar] [CrossRef]
  176. Thanh Truc, N.T.; Hanh, N.T.; Nguyen, M.V.; Le Chi, N.T.P.; Van Noi, N.; Tran, D.T.; Ha, M.N.; Trung, D.Q.; Pham, T.D. Novel Direct Z-Scheme Cu2V2O7/g-C3N4 for Visible Light Photocatalytic Conversion of CO2 into Valuable Fuels. Appl. Surf. Sci. 2018, 457, 968–974. [Google Scholar] [CrossRef]
  177. Thanh Truc, N.T.; Pham, T.D.; Nguyen, M.V.; Van Thuan, D.; Trung, D.Q.; Thao, P.; Trang, H.T.; Nguyen, V.N.; Tran, D.T.; Minh, D.N.; et al. Advanced NiMoO4/g-C3N4 Z-Scheme Heterojunction Photocatalyst for Efficient Conversion of CO2 to Valuable Products. J. Alloys Compd. 2020, 842, 155860. [Google Scholar] [CrossRef]
  178. Shen, Y.; Han, Q.; Hu, J.; Gao, W.; Wang, L.; Yang, L.; Gao, C.; Shen, Q.; Wu, C.; Wang, X.; et al. Artificial Trees for Artificial Photosynthesis: Construction of Dendrite-Structured α-Fe2O3/g-C3N4Z-Scheme System for Efficient CO2 Reduction into Solar Fuels. ACS Appl. Energy Mater. 2020, 3, 6561–6572. [Google Scholar] [CrossRef]
  179. Wang, K.; Jiang, L.; Wu, X.; Zhang, G. Vacancy Mediated Z-Scheme Charge Transfer in a 2D/2D La2Ti2O7/g-C3N4 nanojunction as a Bifunctional Photocatalyst for Solar-to-Energy Conversion. J. Mater. Chem. A 2020, 8, 13241–13247. [Google Scholar] [CrossRef]
  180. Guo, R.T.; Liu, X.Y.; Qin, H.; Wang, Z.Y.; Shi, X.; Pan, W.G.; Fu, Z.G.; Tang, J.Y.; Jia, P.Y.; Miao, Y.F.; et al. Photocatalytic Reduction of CO2 into CO over Nanostructure Bi2S3 Quantum Dots/g-C3N4 Composites with Z-Scheme Mechanism. Appl. Surf. Sci. 2020, 500, 144059. [Google Scholar] [CrossRef]
  181. Zhu, L.; Li, H.; Xu, Q.; Xiong, D.; Xia, P. High-Efficient Separation of Photoinduced Carriers on Double Z-Scheme Heterojunction for Superior Photocatalytic CO2 Reduction. J. Colloid Interface Sci. 2020, 564, 303–312. [Google Scholar] [CrossRef]
  182. Guo, H.; Wan, S.; Wang, Y.; Ma, W.; Zhong, Q.; Ding, J. Enhanced Photocatalytic CO2 Reduction over Direct Z-Scheme NiTiO3/g-C3N4 Nanocomposite Promoted by Efficient Interfacial Charge Transfer. Chem. Eng. J. 2021, 412, 128646. [Google Scholar] [CrossRef]
Figure 1. Different types of heterostructures: (a) type-I, (b) type-II, and (c) type-III heterojunctions. Reproduced from [30]. Copyright 2022, Elsevier.
Figure 1. Different types of heterostructures: (a) type-I, (b) type-II, and (c) type-III heterojunctions. Reproduced from [30]. Copyright 2022, Elsevier.
Catalysts 12 01137 g001
Figure 2. (a) Number of publications per year on g-C3N4-based Z-scheme photocatalysts (source: Scopus; date: 17 July 2022; keywords: “g-C3N4”; “Z-scheme”; “photocatalysis”). (b) Number of publications per year on individual keywords: g-C3N4-based (black-line) Z-scheme (red-line) and photocatalysis (blue-line) (source: Scopus; date: 8 September 2022).
Figure 2. (a) Number of publications per year on g-C3N4-based Z-scheme photocatalysts (source: Scopus; date: 17 July 2022; keywords: “g-C3N4”; “Z-scheme”; “photocatalysis”). (b) Number of publications per year on individual keywords: g-C3N4-based (black-line) Z-scheme (red-line) and photocatalysis (blue-line) (source: Scopus; date: 8 September 2022).
Catalysts 12 01137 g002
Figure 3. (a) Structure of graphitic carbon nitride g-C3N4. Reproduced from [46]. Copyright 2019, Elsevier. (b) Schematic diagram of g-C3N4 synthesis by thermal polymerization from various precursors (thiourea, melamine, cyanamides, dicyanamide, and urea). Reproduced from [47]. Copyright 2016, ACS Publications.
Figure 3. (a) Structure of graphitic carbon nitride g-C3N4. Reproduced from [46]. Copyright 2019, Elsevier. (b) Schematic diagram of g-C3N4 synthesis by thermal polymerization from various precursors (thiourea, melamine, cyanamides, dicyanamide, and urea). Reproduced from [47]. Copyright 2016, ACS Publications.
Catalysts 12 01137 g003
Figure 4. (a) Scheme of the charge separation and transformation process mechanism of natural photosynthesis. Adapted from [33]. Copyright 2019, Elsevier. (b) Z-scheme schematic diagram and (c) type II schematic diagram. Adapted from [28].
Figure 4. (a) Scheme of the charge separation and transformation process mechanism of natural photosynthesis. Adapted from [33]. Copyright 2019, Elsevier. (b) Z-scheme schematic diagram and (c) type II schematic diagram. Adapted from [28].
Catalysts 12 01137 g004
Figure 5. Band structure of g-C3N4 and some oxidation semiconductors. Adapted from [37]. Copyright 2021, Elsevier.
Figure 5. Band structure of g-C3N4 and some oxidation semiconductors. Adapted from [37]. Copyright 2021, Elsevier.
Catalysts 12 01137 g005
Figure 6. Scheme of representative synthesis and characterization methodologies of g-C3N4-based direct Z-schemes.
Figure 6. Scheme of representative synthesis and characterization methodologies of g-C3N4-based direct Z-schemes.
Catalysts 12 01137 g006
Figure 11. (a) TEM images of α-Fe2O3/g-C3N4 (A) and high magnitude TEM images of α-Fe2O3/g-C3N4 (B). Time courses of photocatalytic CO evolutions (A), average CO production rates of g-C3N4, α-Fe2O3 (B), and α-Fe2O3/g-C3N4 hybrid and recycling test of α-Fe2O3/g-C3N4 (C). Reproduced from [170]. Copyright 2018, Wiley. (b) The photo-current response of the Nb-TiO2, g-C3N4, and Nb-TiO2/g-C3N4 under different excitation light conditions at an applied potential of 0 V (vs. NHE). Reproduced from [173]. Copyright 2019, Elsevier. (c) Schematic diagram of the photocatalytic process over face-to-face 2D/2D g-C3N4/BiVO4 and proposed mechanism schematics of direct Z-scheme g-C3N4/BiVO4 heterojunctions for photocatalytic CO2 reduction. Reproduced from [92]. Copyright 2020, Elsevier. (d) Schematic diagram of the photocatalytic process over direct Z-scheme WO3/g-C3N4 and stability analysis of WO3/g-C3N4 for photocatalytic CO2 reduction to CO and CH4. Reproduced from [117]. Copyright 2020, Elsevier. (e) Schematic illustration of the preparation and CO2 photoreduction process of the Bi19S27Br3/g-C3N4 composite. Reproduced from [174]. Copyright 2022, Elsevier.
Figure 11. (a) TEM images of α-Fe2O3/g-C3N4 (A) and high magnitude TEM images of α-Fe2O3/g-C3N4 (B). Time courses of photocatalytic CO evolutions (A), average CO production rates of g-C3N4, α-Fe2O3 (B), and α-Fe2O3/g-C3N4 hybrid and recycling test of α-Fe2O3/g-C3N4 (C). Reproduced from [170]. Copyright 2018, Wiley. (b) The photo-current response of the Nb-TiO2, g-C3N4, and Nb-TiO2/g-C3N4 under different excitation light conditions at an applied potential of 0 V (vs. NHE). Reproduced from [173]. Copyright 2019, Elsevier. (c) Schematic diagram of the photocatalytic process over face-to-face 2D/2D g-C3N4/BiVO4 and proposed mechanism schematics of direct Z-scheme g-C3N4/BiVO4 heterojunctions for photocatalytic CO2 reduction. Reproduced from [92]. Copyright 2020, Elsevier. (d) Schematic diagram of the photocatalytic process over direct Z-scheme WO3/g-C3N4 and stability analysis of WO3/g-C3N4 for photocatalytic CO2 reduction to CO and CH4. Reproduced from [117]. Copyright 2020, Elsevier. (e) Schematic illustration of the preparation and CO2 photoreduction process of the Bi19S27Br3/g-C3N4 composite. Reproduced from [174]. Copyright 2022, Elsevier.
Catalysts 12 01137 g011
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Fernández-Catalá, J.; Greco, R.; Navlani-García, M.; Cao, W.; Berenguer-Murcia, Á.; Cazorla-Amorós, D. g-C3N4-Based Direct Z-Scheme Photocatalysts for Environmental Applications. Catalysts 2022, 12, 1137. https://doi.org/10.3390/catal12101137

AMA Style

Fernández-Catalá J, Greco R, Navlani-García M, Cao W, Berenguer-Murcia Á, Cazorla-Amorós D. g-C3N4-Based Direct Z-Scheme Photocatalysts for Environmental Applications. Catalysts. 2022; 12(10):1137. https://doi.org/10.3390/catal12101137

Chicago/Turabian Style

Fernández-Catalá, Javier, Rossella Greco, Miriam Navlani-García, Wei Cao, Ángel Berenguer-Murcia, and Diego Cazorla-Amorós. 2022. "g-C3N4-Based Direct Z-Scheme Photocatalysts for Environmental Applications" Catalysts 12, no. 10: 1137. https://doi.org/10.3390/catal12101137

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop