ZnO for Photoelectrochemical Hydrogen Generation

Abstract

The rise in the Earth’s surface temperature on an annual basis has stimulated scientific and engineering interest in developing and implementing alternative energy sources. Besides cost, the main requirements for alternative energy sources are renewability and environmental friendliness. A prominent representative that allows the production of “green” energy is the conversion of solar photons into a practical energy source. Among the existing approaches in solar energy conversion, the process of photoelectrochemical (PEC) hydrogen extraction from water, which mimics natural photosynthesis, is promising. However, direct decomposition of water by sunlight is practically impossible since water is transparent to light waves longer than 190 nm. Therefore, applying a photoelectrochemical process using semiconductor materials and organic compounds is necessary. Semiconductor materials possessing appropriately positioned valence and conduction bands are vital constituents of photoelectrodes. Certain materials exhibit semiconductor characteristics that facilitate the reduction-oxidation (RedOx) reaction of water (H₂O) under specific circumstances. ZnO holds a unique position in the field of photocatalysis due to its outstanding characteristics, including remarkable electron mobility, high thermal conductivity, transparency, and more. This article offers an overview of studies exploring ZnO’s role as a photocatalyst in the generation of hydrogen from water.

1. Introduction

The deterioration of the ecology and climate of the planet, as well as the greenhouse effects caused by the excessive consumption of hydrocarbon energy carriers, motivate the scientific community to create alternative ways for energy production. One of the most promising prospects of alternative energy is the utilization and conversion of the Sun’s electromagnetic waves. Solar energy hits the Earth’s surface in a large amount, about 32 × 10²⁴ J per year. The conversion of light into the necessary form of energy is of great scientific and applied importance for humankind’s industrial and economic activities. The effective conversion of only 0.001% of the light energy coming from the Sun will satisfy the energy needs of modern society [1]. Two main methods of transforming solar energy into electricity or heat, are solar panels that generate electricity and solar collectors that concentrate thermal energy. The main problem from these methods is the storage and transportation of the resulting power. Therefore, the issue of creating alternative energy technologies is open [2]. One of the promising ways of solar energy utilization is the conversion of photons into chemical energy in the form of hydrogen gas [3]. Hence, there is a large literature on PEC, both books [4,5] and review articles [6,7], as well as individual articles.

Today, fossil fuels, especially natural gas, are sources of about 95% of the hydrogen consumed in the world. However, fossil resources are not feasible for hydrogen resources because replenishment takes geological times, and much worse, the additional carbon dioxide emissions from hydrogen production by fossil fuels contributes to global warming. Thus, such hydrogen cannot be considered a truly environmentally friendly fuel. Hence, the optimal approach for hydrogen generation involves utilizing alternative energy sources such as hydroelectric power, wind power, low-energy nuclear reactions, and solar-driven water-splitting methods. Among these alternatives, solar-driven technology appears to be the most promising because it imposes fewer constraints compared to wind and hydroelectric power.

Solar-driven hydrogen production encompasses three main methods: (1) thermochemical water-splitting, (2) photobiological water-splitting, and (3) photocatalytic or photoelectrochemical (PEC) water-splitting. Among these, PEC water-splitting stands out as the most promising technique for producing pure hydrogen. In contrast to thermochemical and photobiological approaches, PEC reactions offer several advantages, including (1) reasonable efficiency in converting solar energy to hydrogen energy, (2) cost-effectiveness, (3) the capacity to generate hydrogen and oxygen separately during the reaction, and (4) versatile reactor systems suitable for both widespread market applications and domestic use [8]. It is known that ZnO occupies an important place in photocatalysis and has attracted much attention as a photoanode for photoelectrochemical water splitting due to its unique properties such as high electron mobility, excellent optical properties, high thermal conductivity, wide availability, and low toxicity [9,10]. This article discusses the use of ZnO as a photocatalyst for semiconductor photoanodes [11] for the production of hydrogen from water. With this review, the authors would like to review the potential and status of photocatalytic zinc oxide in the reaction of water splitting under the action of light.

2. Principles of PEC Hydrogen Production

The mechanism of energy transformations in natural photosynthesis is a model for photon conversion into chemical energy by water-splitting. PEC hydrogen evolution involves the participation of photocatalytic systems. Their activation occurs when they are under irradiation with specific wavelengths of light. The fundamentally, the photons provide energy, the electrochemistry leads to charge separation, and the catalysts, improve reaction rates.

The primary materials of photoelectrodes in PEC systems are semiconductor materials, and organic molecules. However, the efficiency of organic molecules as photocatalysts for extracting hydrogen from water is relatively low compared to semiconductor materials [12]. In this regard, it is essential to develop methods for manufacturing active layers of PEC systems based on semiconductors [13].

The water-splitting reaction refers to uphill reactions, the energy required to break the water molecule H₂O into H₂ and ½O₂ in standard conditions is ∆G = 237.2 kJ/mol by Equation (1) below. According to the Nernst equation, the ∆E° in ΔG is equivalent to 1.23 V per electron transfer. Therefore, to perform the RedOx reaction of water, the semiconductor material must absorb a photon with an energy of at least 1.23 eV, corresponding to light wavelengths shorter than 1000 nm. When irradiated photoactive electrodes act as catalysts, it provides the necessary energy to the system to overcome the energy barrier. The absorbance of light quanta by the active material triggers the RedOx reaction of water [14] by Equation (2) below. The RedOx reaction begins after an absorbed photon generates an electron-hole pair. In detail, the absorbed light energy pumps the conduction band of a semiconductor with electrons from the valence band. Similarly, light energy pumps the normally empty LUMO level with electrons from the normally filled HOMO level of the organic molecules [15]. The electrons in the conduction band move to the photocatalysis/electrolyte interface and start the water-splitting reduction half-reaction. Positive carriers in the valence band migrate to the centers of the oxidative half-reaction, triggering the mechanisms of oxygen evolution [16].

$$2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}_2+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}$$

(1)

Oxidation energy 1.23 V versus SHE (standard hydrogen electrode).

$$2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2$$

(2)

The reduction energy is 0 V versus SHE.

The water reduction reaction must involve two electron-hole pairs (two photons) (2 × 1.23 eV, or 2.46 eV); the oxidative reaction of water requires four electron-hole pairs (four photons).

Both photocatalyst powder dispersed in water (photochemical system) and the electrode of PEC cells produce solar-driven hydrogen production [17]. If the water-splitting reaction proceeds on powder surfaces, the resulting separation of released H₂ and O₂ are complex due to the lack of sufficient spatial distancing of the centers of RedOx half-reactions. In exploiting photoelectrodes, the centers of half-reactions have spatial distance (the minimum distance between the (photo)cathode and (photo)anodes is relatively larger than the size of regions saturated with the released gas), which provides favorable conditions for the separation of the released gases. This fact, is an advantage of using PEC electrodes. Therefore, it is promising for producing hydrogen fuel.

The quantum efficiency of the PEC cell quantifies how effectively light energy is transformed into chemical energy through the process of water splitting. Achieving a sufficiently high hydrogen production efficiency is possible if the PEC reactor meets the following requirements [18]:

(1)

Large amount of absorbed photons.

(2)

Reducing the level of electron-hole recombination.

(3)

Fast charge carriers transportation to the centers of RedOx reactions.

(4)

Reducing number of charge carriers trapping zones.

(5)

Increase in the surface area of the RedOx centers.

Optimizing the active layer’s transverse-longitudinal dimensions and roughness adjusts the light-harvesting efficiency by semiconductor materials. Thus, photoactive layers based on nanorods capture more photons than nanosized films (Figure 1a,b) [19]. The schematic representation in Figure 1b,c shows multiple photons reflection back and forth insight nanoarray resulting in an increase of photon harvesting.

Furthermore, the character of the developed surface affects the capability of the material to absorb incident light, and can reduce the number of photons harvested by a unit surface area, reducing the the open circuit voltage. The size and crystallinity of a catalyst primarily determine the rate of separation and migration of photogenerated charge carriers (Figure 1c). Greater crystallinity results in fewer defects, enhancing the likelihood of charge carriers moving towards RedOx reaction centers. Defects serve as traps and recombination sites for photogenerated electrons and holes, causing a decline in photocatalytic activity. Reducing particle size reduces the distances photogenerated electrons and holes need to travel to reach reaction centers, thereby reducing the chances of recombination [20,21]. The density of donors also effects on electron–hole separation efficiency. A high donor density level increases the width of the depletion region, which leads to a decrease in the electron-hole separation efficiency (Figure 1d–f) [19].

2.1. Requirements for Semiconductors for PEC Reactions

The exploitation of only wide-gap semiconductor materials as a photoelectrode element is burdened with insufficient sensitivity of the material to the visible part of the electromagnetic spectrum. That is because, wide-gap semiconductors react only to the UV part of the spectrum, which is no more than 4% of all sunlight falling on the Earth. The extension of photoelectrode sensitization into the visible region of solar radiation, which is about 46% of photons incident on the Earth, is the critical task for improvement of PEC processes.

The ways of increasing light harvesting by photoelectrodes, include zone engineering, creating multi-junction systems, and considering size effects. Connecting two or more semiconductors in a hierarchical wide-gap-to-narrow-gap manner increases the number of captured photons from solar radiation (Figure 2). In addition, the band transition hierarchy allows improvements in charge carrier transport and increases material stability. Figure 2 shows, a diagram of electron transport between a wide-gap semiconductor and narrow-gap materials with and without nanoplasmonics. Combined PEC devices, known as tandem cells, show the best efficiency at the moment [22]. Only photoactive semiconductors possessing a band gap wide enough for water-splitting (at least 1.23 eV), with a conduction band potential more negative than the hydrogen reduction potential E°(H⁺/H₂), and a valence band potential more positive than the water oxidation potential E°(O₂/H₂O) are considered suitable [23]. Additionally, the photocatalysts must exhibit resistance to both photo-induced and corrosion processes, as these are crucial requirements for the efficiency of PEC systems. Furthermore, the effectiveness of PEC processes is significantly influenced by the rate at which charge carriers transition to the semiconductor/liquid interface, which has a direct impact on the material’s stability. Introducing plasmonic nanoparticles between the photactive layers of semiconductors can improve charge transfer and photocatalysis stability.

For rapid interphase transitions of charge carriers, sacrificial donors are employed. For instance, ethylenediaminetetraacetic acid, triethanolamine, or IO³⁻/I⁻ RedOx mediators are oxidized faster than H₂O. IO³⁻/I⁻ RedOx mediators are often used in the cells which combine photoanode and photocathode, known as the Z-scheme, introduced in 1979 by Bard. The Z-scheme closely mimics the natural photosynthesis process found in green plants, where photosystems I and II capture photons at wavelengths of 700 and 680 nm, respectively, leading to the oxidation of H₂O and the production of O₂. The quantum yield, in this case, is close to unity. Thus, two different photocatalysts form one PEC system using the corresponding RedOx shuttle intermediary of the Z-scheme. In such cells, visible light is redistributed more efficiently than in conventional one-step systems because of reducing activation energy for a single photocatalyst [24]. Despite a significant increase in efficiency, Z-schemes are still of no interest from a practical point of view since the production of sacrificial donors or mediators leads to an increase in costs, and hence, an increase in the price of the produced H₂ [25]. Using some chemical industry wastes as sacrificial donors in the PEC process is also not practical since the supply of sacrificial donors in large-scale hydrogen production must be uninterrupted.

When a semiconductor comes into contact with a different liquid, gas, or metal that possesses distinct Fermi levels or RedOx potentials, it causes the redistribution of electrical charges at the interface. This redistribution aims to balance the chemical potential between the semiconductor and the second phase and triggers the creation of a double layer [26].

Immersing photoelectrodes in the electrolyte causes balancing of the electrolyte’s Fermi level and the photoelectrodes’ Fermi level. Such a process occurs when charges are transferred from a system with a more negative Fermi level to a more positive one. Charge transfer forms a Helmholtz double layer [27].

The balance in the electrolyte-photoelectrode system is attained when the Fermi level of the RedOx photoelectrode matches the potential of the electrolyte. Under these conditions, in an undoped semiconductor material, the Fermi level resides in the central region between the conduction band and the valence band. For n-type semiconductors, the Fermi level is situated close to the conduction band, while in p-type semiconductors, it is positioned near the valence band. Thus, when an n-type photoelectrode is immersed in a solution, electrons move from the semiconductor to the electrolyte. Conversely, in the case of a p-type semiconductor, its Fermi level is lower than that of the electrolyte, causing electrons to flow from the electrolyte to the semiconductor. At equilibrium, a region with a positive charge, referred to as the depletion layer, forms. Consequently, the edges of the valence and conduction bands bend upwards, creating a potential barrier that impedes further electron transfer from the semiconductor to the electrolyte across the electrolyte/semiconductor interface. Such band bending is shown in the bottom of Figure 1.

2.2. PEC Cell Photoanode Materials

Research activity in the field of PEC cells using photoelectrodes dates back to the 1970s, after the publication of the work of scientists from Fujishima and Honda [28], in which the authors used a photoanode made of a layer of titanium dioxide, a wide-gap semiconductor. Since then, many materials have been developed that exhibit photocatalytic activity. According to the properties of electronic configurations, such starting materials are divided into four groups:

(1)

metal oxides d⁰, such as Zr⁴⁺, Nb⁵⁺, Ta⁵⁺, W⁶⁺, Mo⁶⁺, and Ti⁴⁺

(2)

metal oxides d¹⁰ (Zn²⁺, Ga³⁺, Ge⁴⁺, Sn⁴⁺, Sb⁵⁺, and In³⁺);

(3)

metal oxides f⁰ [29], such as Ce⁴⁺;

(4)

non-oxide photocatalysts (Ta₃N₅, CdS) [30].

Photocatalysts that rely on transition metal cations featuring vacant d-orbitals are described by an electron configuration of d⁰, whereas those based on regular metal cations with fully occupied d-orbitals are characterized by an electron configuration of d¹⁰. Transition metal cations that possess partially filled d-electrons (i.e., electron configuration: 0 < n < 10) within photocatalyst materials contribute to an inefficient photoresponse. On the other hand, metal oxides with electron configurations of d⁰ or d¹⁰, such as Mo⁶⁺ and Zn²⁺, are anticipated to exhibit active photocatalytic properties in the context of water separation [31].

Metal oxides such as TiO₂, ZnO, Fe₂O₃, CdS, etc., are widely used as the base (first layer in the path of the light wavefront) anode material for PEC cells [32]. However, the selection of semiconductor photoelectrodes combining the properties of visible light absorbents with high chemical stability and efficient charge separation is in the active area of research. No material known today can combine these properties to the desired degree. Metal oxides are known for good chemical stability. However, they frequently exhibit various drawbacks, including relatively wide band gaps, limited optical absorption capabilities, modest catalytic performance, and low charge carrier mobility, coupled with a brief exciton lifetime [33]. Using photocatalysts based on tantalates, such as K³Ta₃Si₂O₁₃, Sr₂Ta₂O₇, A⁰ Ta₂O_6, and others active in water decomposition, even without using co-catalysts [34], is also limited due to the large values of the band gap of these materials.

When designing photoelectrodes, the following points are essential parameters:

(1)

choosing appropriate materials to serve as photocatalysts with favorable band levels and high absorption coefficients in their bands;

(2)

creating extremely crystalline layers of photocatalysts covering the entire electrode, with the correct concentrations of dominant carriers;

(3)

stablishing an ohmic contact (to minimize the Schottky barrier) between the semiconductor and any co-catalysts, if they are employed;

(4)

development of high-activity co-catalysts for surface electrocatalysis;

(5)

Efficient location and maximization of the concentration of reductive/oxidative co-catalysts on photocatalyst surfaces [35].

Exploitation of nanotechnologies and nanomaterials proved to be a practical approach to address several of the above issues. A wide range of physical and chemical methods is roughly divided into top-down and bottom-up approaches to produce metal oxide nanoparticle. Physical techniques like abrasion or ball milling are considered top-down methods. Nanopowders produced through these top-down processes typically show a broad range of particle sizes, making it challenging to control their size, shape, and morphology. Moreover, potential structural and surface impurities can have a substantial impact on the surface chemistry and, consequently, the catalytic characteristics of nanomaterials. Regardless of their production method, nanoparticles can be categorized based on their geometric shapes into three groups: zero-dimensional, one-dimensional, and two-dimensional nanostructures. The useful properties of nanomaterials depend on their dimensionality, along with the details of their composition and structure. Structural aspects include defects, as well as the basic lattice structures of the nanomaterials.

Broadening the reactivity of wide band-gap photocatalysts to encompass visible light can be achieved through the creation of solid solutions between semiconductors possessing wide and narrow band gaps but sharing a similar lattice structure. Solid solutions involving two or more semiconductors result in structures where the lattice positions are influenced by the components within the solid solution. The band gap of materials can be tuned by altering the composition of the solid solution. Examples of semiconductor blends include GaN-ZnO, ZnSCdS, ZnS-AgInS₂, and CdS-CdSe [36]. The blending of semiconductors (i.e., forming composites) is a recognized approach for designing photocatalysts responsive to visible light [37]. The semiconductor blending approach involves combining a high-band-gap semiconductor material with a low-band-gap semiconductor possessing a more negatively positioned conduction band [38]. Consequently, electrons from the low-band-gap semiconductor can be incorporated into the high-band-gap semiconductor, enhancing the light absorption capability of the hybrid photocatalyst. Another benefit of employing composite semiconductor photocatalysts is the potential to minimize carrier recombination by facilitating interparticle electron transfer [39]. In these photocatalyst compositions, semiconductor particles are in electronic contact without being mixed at the molecular level.

To successfully combine semiconductors, the following criteria must be satisfied:

(1)

the conductivity level of the narrow-gap semiconductor being investigated should have a more negative potential than that of the wide-gap semiconductor;

(2)

the position of the conduction level in the wide-gap semiconductor must be more negative than the recovery potential;

(3)

electron injection should occur rapidly and efficiently;

This approach is exemplified by composites like CdS-TiO₂, CdS-ZnO [36], and a BiVO₄ materials, which are small-sized photocatalysts capable of generating oxygen from a silver nitrate solution under visible light [40]. Heterogeneous photocatalysis involves a complex series of reactions, and its oxidation process is not yet fully understood. In the traditional heterogeneous photocatalytic process, the response occurs in the adsorbed phase, and the overall process can be broken down into four steps:

(1)

transport of reactants in the liquid phase to the catalyst’s surface;

(2)

adsorption of reactants on the photocatalyst’s surface, activated by photon energy during this stage;

(3)

photocatalytic reactions taking place on the catalyst’s surface;

(4)

desorption of RedOx reaction products from the photocatalyst’s surface [41].

Introduction of doping elements into the semiconductor electrode material leads to a change in the spectral dependence of the system photocurrent; for example, alloying titanium dioxide arrays with chromium or vanadium, or alloying ZnO layers with aluminum, increases the photocurrent passing through the electrochemical cell [42]. A series of oxynitride materials for photocatalytic hydrogen evolution has also been developed. The oxynitrides (Ga_1−xZn_x) (N_1−xOx) and (Zn_1+xGe_x) (N₂O_x) were found to function as stable photocatalysts for the reduction and oxidation of water when irradiated with visible light. In this case, solid solutions with the electronic configuration d¹⁰ achieve a general separation of water under visible light without noticeable deterioration in the material’s chemical resistance. Some d⁰-type oxynitrides apply to the two-stage water separation system, which can collect a wide range of visible photons [43]. Bulk Nb/Ti layers made by anodizing Ti-Nb alloys can exhibit enhanced PEC water-splitting properties. A noticeable effect is achieved at a relatively low concentration of Nb, about 0.1 at.%. Compared to other doping approaches (such as N or C doping), this effect is stable over time (i.e., no significant photodegradation observed). Because of the vast possibilities for controlling morphology and geometry in the synthesis of layers of TiO₂ nanotubes, further optimization of this material demonstrates the importance of carrying out work on using TiO₂ to obtain optimal results in PEC water separation [44]. In [45], nanocrystalline mesoporous TiO₂ was produced through a hydrothermal process employing titanium butoxide as the initial substance. The photocatalytic performance of the resulting TiO₂, treated at the suitable calcination temperature, exhibited a notably superior level compared to commercially available TiO₂. Furthermore, the deposition of the Ru molecular catalyst on the surface with a nanostructured TiO₂ film shows relatively good catalytic efficiency initiated by visible light [46].

As discussed above, the design of PEC systems with the simultaneous use of a photoanode and a photocathode can significantly reduce the activation energy of the water-splitting reaction. A striking example is systems consisting of a photoanode based on ZnO, TiO₂, and a photocathode based on Cu₂O. To employ Cu₂O photocathodes in the photoelectrochemical (PEC) splitting of water, it is essential to stabilize them against photocorrosion when exposed to aqueous electrolytes. One effective approach involves applying ultrathin layers of wide-band-gap semiconductor oxides onto cuprous oxide using atomic layer deposition. These ultrathin films serve a dual purpose: (1) they facilitate the extraction of charges by creating a p-n junction, (2) they shield the active material from the aqueous electrolyte, thus preventing photocorrosion and promoting hydrogen production [47]. Additionally, photogenerated electrons migrating to the conduction band function as reducing agents for converting H⁺ to H², while holes that migrate to the semiconductor’s surface contribute to the decomposition of H₂O to O₂ and H⁺ (3)–(6) [48]:

$$\mathrm{ZnO}\stackrel{\mathrm{h}\mathsf{\nu }}{\to } \mathrm{ZnO}\left({\mathrm{e}}_{\mathrm{cb}}^{-}+{ \mathrm{h}}_{\mathrm{vb}}^{+}\right)$$

(3)

$$2{\mathrm{e}}_{\mathrm{cb}}^{-}+2{\mathrm{H}}^{+}\to {\mathrm{H}}^{·}+{\mathrm{H}}^{·}={\mathrm{H}}_2$$

(4)

$${\mathrm{h}}_{\mathrm{vb}}^{+}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{O}}^{+}\to {\mathrm{OH}}^{·}+{\mathrm{H}}^{+}$$

(5)

$${\mathrm{OH}}^{·}+{\mathrm{OH}}^{·}\to {\mathrm{H}}_2\mathrm{O}+\frac{1}{2}{\mathrm{O}}_2$$

(6)

The quantum yield of the cell measures the quality of the photocatalytic system. The quantum yield (Q_out) of H₂ and O₂ as a percentage is calculated by the Formulas (7) and (8):

$${\mathrm{Q}}_{\mathrm{out}}=\left(\frac{\left(2{\ast \mathrm{number} \mathrm{of} \mathrm{molecules} \mathrm{H}}_2\right)}{\mathrm{number} \mathrm{h}\mathsf{\nu }}\times 100\right);$$

(7)

$${\mathrm{Q}}_{\mathrm{out}}=\left(\frac{\left(2{\ast \mathrm{number} \mathrm{of} \mathrm{molecules} \mathrm{O}}_2\right)}{\mathrm{number} \mathrm{h}\mathsf{\nu }}\times 100\right);$$

(8)

Assessing a PEC or photochemical cell by directly measuring its quantum yield requires precise measurements of both the gas volume produced and the number of incident photons on the photocatalyst. However, replicating these conditions accurately in natural settings can be challenging. Consequently, it is more practical to evaluate a PEC or photochemical system by calculating its photocatalytic efficiency, denoted as η, using the following Formula (9):

$$\mathsf{\eta }=\left(\left(1.229\mathrm{V}-{\mathrm{V}}_{\mathrm{bias}}\right)\ast \mathrm{J}\right)/\mathrm{P}$$

(9)

The standard potential needed for the water molecule splitting is 1.229 V. V_bias represents the external power source’s bias voltage, while J denotes the current density flowing through the PEC cell (A/m²). Lastly, P signifies the power of the electromagnetic radiation emitted by the light source (W/m²) [49].

2.3. Photoactive ZnO

Over the past few decades, there has been a significant global emphasis on researching wide-bandgap semiconductors, with particular attention directed towards ZnO. This heightened interest is primarily due to ZnO’s outstanding electrical properties, including its high electron mobility, excellent thermal conductivity, transparency, and its wide and direct transition between the valence and conduction bands (3.37 eV). Moreover, ZnO boasts a high exciton binding energy, which makes it a promising material for optical devices leveraging exciton effects [50]. Its versatility extends its applicability to a wide range of fields, including optoelectronics, transparent electronics, lasers, sensors, and even the cosmetic industry [51]. A striking example of the versatility of ZnO is the use of bulk and thin-film ZnO coatings in ZnO varistors, referred to by a variety of names, including nonlinear resistors, variable resistors, surge suppressors, network filters, and voltage limiters [52], as well as gas sensors, e.g., for monitoring the level of ammonia gas [53]. ZnO nanoparticles are applied in the textile industry, to cotton and woolen fabrics for sun protection effects, thermal and mechanical resistance [54], antibacterial coatings [55], and photocatalysts [56].

ZnO preferentially crystallizes in the stable hexagonal wurtzite structure, and it is worth noting that it can also crystallize with a zinc blende lattice [57]. ZnO possesses lattice parameters of a = 3.296 nm and c = 0.520 nm, along with a density of 5.60 g/cm³. The electronegativity values for O²⁻ and Zn²⁺ are 3.44 and 1.65, respectively, leading to a strong ionic bond between the zinc and oxygen ions. The wurtzite structure is relatively easy to understand. It involves each oxygen ion being surrounded tetrahedrally by four zinc ions, and vice versa. This crystal formation is characterized by stacking these ions on top of each other along the C axis. The tetrahedral arrangement of O²⁻ and Zn²⁺ in ZnO results in a non-central structure comprising two interpenetrating hexagonally closed-packed sublattices of zinc and oxygen. These sublattices are shifted in relation to each other by 0.375 along the hexagonal axis. This particular arrangement in the ZnO crystal is responsible for its piezoelectric properties.

Wurtzite ZnO has four common crystal faces, which include the polar Zn-terminated (0001) and O-terminated (000$\overline{1}$) faces, oriented along the c-axis. It also includes the non-polar (11$\overline{2}$0) (A axis) and (10$\overline{1}$0) faces, both of which have an equal number of Zn and O atoms. Among these, the polar and (1010) surfaces are stable, while the (11$\overline{2}$0) surface is less stable [58].

Among many semiconductor materials that can efficiently convert solar energy into hydrogen in a PEC cell, ZnO remains a widely used material due to several critical factors for photocatalysis: suitable band potential, low electrical resistance, lack of toxicity, low cost, and resource intensity [59,60,61]. Hence, the utilization of ZnO as a photoelectrode in large-scale PEC water splitting systems, pertinent to global energy needs, is strongly favored.

Comprehensive investigations into ZnO as a semiconductor photoanode for sunlight-driven water decomposition have highlighted both its potential and limitations. The primary drawbacks of ZnO encompass the rapid recombination of photo-generated electron-hole pairs, limited ability to absorb visible light [62], and relatively low stability when exposed to an aqueous solution [63]. These drawbacks inherently impose constraints on ZnO’s effectiveness in solar energy conversion. In recent times, substantial research endeavors have been directed towards addressing these limitations through surface modifications of ZnO. These modifications include methods like carbon doping and combining it with carbon-based substances. Consequently, the effective combination of porous ZnO nanostructures and carbon hybridization has the potential to yield a photoanode with enhanced hydrogen production efficiency and prolonged operational durability in a watery environment [64].

Despite the existing disadvantages, ZnO has a handy feature since it can be grown at relatively low temperatures (below 773 K), which allows production of structures of various shapes and sizes, such as granular, needle-like, and flower forms, nanorods, nanowires, nanosheets, nanoring plates, rings, tubes, etc. Electrochemical synthesis of ZnO layers is a simple and versatile method for deposition arrays with highly controlled morphology (Figure 3).

Electrochemical deposition enables forming of ZnO films with 1D, 2D, and 3D structures. The electrogenerated base is the primary reaction leading to the formation of oxides [65]. Generally, aqueous electrolytes containing dissolved zinc ions are the primary solution for the electrodeposition of ZnO arrays. One of the most used sources of zinc ions is Zn(NO₃)₂. Adding KCl to the electrolyte leads to the deposition of ZnO structures with a pronounced hexagonal texture [65,66,67,68,69,70,71]. The combination of electrochemical synthesis with other methods has gained wide popularity. For example, hydrothermal treatment of an electrodeposited ZnO improves the luminescent properties of the semiconducting array [72]. Improving the photocatalytic properties of PEC cell electrodes based on electrodeposited ZnO layers is also achieved by decorating ZnO structures with TiO₂ particles [73]. When decorating (creating heterojunctions), it is necessary to pay particular attention to the quality of heterojunctions and adherence to the hierarchy of the built-in layer architecture [74]. Decorating ZnO arrays with narrow-gap semiconductors, such as CdS, CuO, Cu₂O, Fe₂O₃, etc., allows the expansion of light absorption to the visible part of the solar spectrum to increase the efficiency of photocatalytic activity [75] (Figure 2).

2.4. Modification of ZnO for the Water-Splitting Reaction

Metal sulfides have the potential to be employed in photocatalytic processes triggered by visible light. Typically, the valence band of these substances comprises sulfur’s 3p orbitals, resulting in more negatively positioned values and a narrower band gap when compared to metal oxides [17]. Various techniques are available for producing narrow-gap semiconductor CdS in different forms, including nanoparticles, nanospheres, and nanorods [76], and nanosheets [77], including the production of CdS nanocrystals. They can be achieved by cheap and efficient biological [78] and wet chemical methods [79]. CdS is a semiconductor capable of absorbing visible light and is, therefore, often studied as an electrode for PEC cells and a photocatalyst for water splitting. However, the practicality of CdS as a photocatalyst is capped due to anodic degradation called photocorrosion [80]. Corrosion is one of the limiting aspects of the usage of CdS in devices for converting light energy [81]. RedOx reactions can prevent destructive decomposition under suitable RedOx potential and high speed. Moreover, rapid charge transfer at the interfaces between semiconductors, and between the semiconductor and water contributes to the photoanode’s prolonged stability. Of particular interest are methods for producing composite materials using cadmium sulfide. The developing various architectures for incorporating CdS particles into composites will improve the material’s resistance to photocorrosion. It will also play a favorable role in increasing the efficiency of PEC reactions [82,83,84,85,86,87]. Different composite structures were obtained by coupling ZnO and CdS. For example, Figure 4 shows several attempts of ZnO decoration with CdS particles from nanodots (Figure 4a) and core-shell structures (Figure 4b,c), as well as a sandwich-like architecture (Figure 4d).

CdS possesses a greater electron affinity compared to ZnO. In accordance with the Anderson model, this results in the formation of a second-type heterojunction between CdS and ZnO, a schematic representation of which is in Figure 5a. Therefore, when photoinduced electron-hole pairs are created in CdS, electrons from the CdS conduction band efficiently move to the ZnO conduction band through ballistic diffusion. The transfer of a single electron takes only 18 picoseconds, which is shorter than the exciton lifetime in CdS [88]. Figure 5b,c show micrographs of the ZnO/CdS nanorod obtained by hydrothermal/spin coating. An increase in the deposited CdS layers on ZnO nanorods leads to a thicker coating, which reduces the efficiency of the photocatalytic cell. It is indirectly confirmed by measurements of the photocurrent in Figure 5d.

Zone engineering for a mixture of semiconductors makes it possible to obtain hydrogen-generating photoactive materials operating in the visible spectrum of electromagnetic radiation. Thus, solid-state mixing of cadmium sulfide with wide-gap ZnS produces semiconductors with a controlled band gap. Xing et al. [89] showed that the Cd_0.62Zn_0.16S solid solution is the optimal material with good photocatalytic hydrogen production efficiency. Wang et al. reported that 30 h of ZnO/CdS photocatalyst stability in the process of hydrogen evolution from an electrolyte with S²⁻ and SO₃²⁻ as sacrificial reagents [90]. Hailing Lin et al. in their work [91] demonstrated that ZnO/ZnS photoanode demonstrates much higher photosensitivity than pure ZnO (Figure 6). Multi-component ZnS-ZnO-CuS-CdS heterostructures with multiple electronic transitions also have excellent photocatalytic activity. It is reported in [92] that effective charge separation in the ZnS-ZnO-CuS-CdS heterostructure helps to reduce the recombination of charge carriers, thereby improving the photocatalytic properties of the material. Even nanometer adjustment of the thickness of the ZnO surface coating with a narrow-gap CdS controls the efficiency of the PEC decomposition of water [93], which can reach a hydrogen generation rate of 22.12 mmol/g×h [94]. An additional improvement in the transport properties of charge carriers is feasible by introducing plasmonic particles between the ZnO and CdS.

Harmonic oscillations of electrons in the conduction band of plasmonic nanoparticles resonate with external electric field oscillations, increasing the photosensitivity of semiconductor materials. Thus, an increase in external electromagnetic radiation leads to intense oscillations of the electron cloud and the formation of an electron deficit in one region of the plasmonic structure, and the supersaturation of the negative charge in another region. The charge redistribution creates an electric field inside and outside metal nanoparticles, which is directed opposite to the electric field of light. When the electron density shifts, a Coulomb restoring force arises, which starts a series of oscillations in the nanoparticle. These coherent fluctuations in charge density and electric field are known as localized surface plasmons, and they can persist for approximately ten femtoseconds. After about 100 femtoseconds, electron-electron scattering occurs, resulting in the generation of hot electrons. These hot electrons then interact with phonons over the course of about one picosecond, ultimately leading to their injection into the semiconductor’s conduction band. The frequency at which conduction electrons oscillate within a metal nanoparticle with minimal power dissipation is referred to as the plasmon resonance frequency.

Surface plasmon resonance refers to the collective resonance of free electrons on the surface of a metal. The frequency and intensity of plasmonic oscillations depend on factors like the size and shape of the metal and the dielectric constant of the surrounding environment. Noble metal plasmonic nanoparticles, such as gold (Au) and silver (Ag), exhibit excellent resistance to degradation and display high absorption coefficients across a wide spectrum, spanning ultraviolet, visible, and near-infrared radiation [95]. Using noble metals as materials with pronounced plasmon resonance properties is widespread in planar waveguides, photonic crystals [96], and biosensor devices [97].

Nanoparticles with surface plasmon resonance properties have also found their application in photocatalysis. Thus, the deposition of nanoparticles of noble metals on the surface of photoactive layers of electrodes in PEC systems broadens the spectral sensitivity of photocatalysts. Deposition of plasmonic nanoparticles such as Ag [98], Au [99], or others [100] is an effective way to improve ZnO water-splitting ability. Exploitation of metal nanoparticles on ZnO arrays is not limited by plasmonic properties. Moakhar [101] improved the absorption of visible and PEC indicators of water splitting with light by using samples based on ZnO decorated with Au–Pd bimetallic particles due to a combination of plasmonic and catalytic properties. Zhang explains the increase in the efficiency of water decomposition after the decoration of a ZnO array with Ag nanoparticles by claiming an improvement in electronic mobility, which favors photocorrosion [102]. Au incorporated on the surface of the ZnO increases the light-harvesting efficiency because of surface plasmon resonance, suppresses the recombination of photogenerated electron-hole pairs, and improves the ability to capture light [102,103].

An increase in the current density when the photoactive material is illuminated is another evidence of the improvement in the properties of PEC. Ma et al. showed that after the deposition of Au nanoparticles on ZnO nanorods, the photocurrent increased by a factor of 3.08 [104]. The introduction of Au nanoparticles between ZnO and CdS increases the hydrogen generation rate because of the electron transfer mediator function of Au nanoparticles [105]. Besides Au nanoparticles, Ag nanoparticles have good plasmonic and catalytic properties. The silver nanoparticle deposition significantly increases ZnO’s photocatalytic properties [106]. Typical approaches to synthesizing Ag nanoparticles include colloidal chemistry, nanolithography, and photothermal reduction of nanoparticles at the deposition site. Although simple, these approaches involve several complex steps and often introduce unwanted chemicals that compromise the plasmon resonance effect and complicate charge transfer. As a result, it is interesting to develop simple but effective methods for obtaining plasmonic nanoparticles and composites with the participation, for example, of core-shell structures [107]. Thus, Ag/TiO₂ composites are produced on a large scale using a microwave hydrothermal strategy followed by a photochemical reduction process under xenon lamp irradiation [108]. ZnO/Ag composites are widely used [56,109].

The binding energies of silver (Ag), zinc (Zn), and oxygen (O) within the Ag/ZnO structures differ significantly when compared to their metallic forms. This discrepancy arises from the interaction between Ag and ZnO. The self-organization of nanoparticles can produce such composite materials by thermal evaporation of thin layers of Ag, followed by low-temperature annealing [110], the solvothermal method [111], or electrospinning followed by heat treatment to remove the polymer and convert silver nitrate into silver nanoparticles [112]. Another no less exciting but relatively simple and effective method for preparing composites is the method of chemical deposition of plasmonic Ag particles on 1D ZnO. The Ag interlayers surface morphology can be managed by adjusting the deposition parameters during Ag layer formation. Simultaneously, the configuration of the ZnO/Ag interface profoundly influences the surface plasmon resonance process. Research has demonstrated that the overall enhancement of plasmon resonance in the ZnO/Ag system hinges on two critical factors: the size of the Ag particles and their distribution density. Larger Ag particles result in a greater scattering cross-section when exposed to light, while a higher particle density leads to more effective radiating antennas [113].

In addition to the photocatalytic properties of silver nanoparticles, the ZnO structures deposited on the surface enhances the luminescent properties of the latter. A tenfold increase in luminescence is reported after the deposition of Ag particles on the surface of 1D ZnO [114]. Most recently. Adam et al. reported about Ag/Ag₂WO₄ as promising candidate for optimal use of sunlight, which provides a higher photocurrent of 3 mA cm⁻² measured at 1.23 V (vs. Ag/AgCl) for ZnO/Ag/Ag₂WO₄, which is 3 times the photocurrent achieved by ZnO nanorods [115]. Mustafa, with co-authors, demonstrated the exploitation of nanoplasmonic Ag to prepare a photoanode of ZnO/Ag₂WO₄/AgBr, which exhibits a photocurrent density five times greater than bare ZnO nanorod-based photoanode (Figure 7) [116].

The incorporation of co-catalysts onto the surface of the photoactive ZnO also plays a significant role in increasing the photocurrent density of a PEC cell or enhancing the rate of the RedOx reaction within a photochemical system, as exemplified by ZnO/Ti₃C₂TX [99]. It is achieved through multiple mechanisms:

(1)

co-catalysts can effectively lower the activation energy or induce an overvoltage, thereby facilitating the release of hydrogen and oxygen on the semiconductor surface;

(2)

co-catalysts aid in the separation of electron-hole pairs at the interface between the co-catalyst and the semiconductor;

(3)

co-catalysts serve to mitigate the issue of photocorrosion in the semiconductor material [117].

Table 1. ZnO-based catalysts, morphology, photocurrent density, electrolyte, and hydrogen evolution.

(ZnO)Catalysts/Co-Catalysts	Morphology	Photocurrent Density (IPCE) without Co-Catalysts	Photocurrent Density (IPCE) with Co-Catalysts	Bias	Electrolyte	Hydrogen Evolution Reaction	Ref.
(ZnO)/Ti3C2TX	nanorods/flask	0.83 mA cm−2	1.2 mA/cm2	1.23 VRHE	1 M potassium borate (pH 9.3)		[99]
(ZnO)/Ni-MOF	film	(6.4%)	(11.0%)	0.5 V	Na2SO4		[118]
GaN/(ZnO)/CoPi	(1) nanorods (2) nanocomb		(1) 4.81 A/cm2 (~22%) (2) 4.46 mA/cm2 (19.1%)	1.23 VRHE	0.5 M NaOH		[119]
Ni(OH)2/ZIF-8/(ZnO)/NF	nanorods/branches	0.92 mA/cm2	1.95 mA/cm2 (40.05%)	1.23 VRHE	0.1 M KOH		[120]
(ZnO)/MnO2	nanorods	0.49 mA/cm2	0.95 mA/cm2	1.2 VAg/AgCl	0.5-M Na2SO3		[121]
(ZnO)/Au/g-C3N4/Pt	3D urchin-like		0.3 mA/cm2	0 VRHE	0.2 Na2SO4	6.75 μmol/h·cm2	[122]
GaN:(ZnO)/Rh2−yCryO3	nanorods				distilled water containing 10 vol% methyl alcohol	53.44 μmoL·g−1·h−1	[123]
ZnS-ZnO	composite				0.03 M NaClO4 (methanol-water solution)	247 µmol H2 h−1·g−1	[124]

provides a comprehensive overview of ZnO-based photocatalytic materials utilized in various water-splitting reactions across diverse electrolytes and with varying applied bias conditions.

3. Conclusions

Photoelectrochemical production of hydrogen is relevant in the energy industry, since the use of hydrocarbon energy sources leads to crisis such as global warming, a decrease in fresh water levels and deterioration in the quality of air masses. Among the wide range of applications, the UV-responsive wide bandgap semiconductor ZnO has applications in photocatalysis and photoelectrochemical processes. The benefits of ZnO are very promising in photocatalytic water splitting and PEC water splitting for hydrogen production. In many respects, ZnO is comparable to TiO₂, which is widely used in photocatalysis. At the same time, methods for the synthesis of ZnO, such as electrochemical deposition, hydrothermal synthesis, and vapor deposition, have advantages in terms of simplicity and in terms of energy costs. On the other hand, ZnO has disadvantages related to photocorrosion and solubility in electrolytes. Therefore, it is critical to create nanocomposites with coatings for fast electron transfer while increasing the service life of ZnO coatings, such as cascade systems with a specified hierarchy of architectures. The creation of heterojunctions of a wide-gap semiconductor with a narrow-gap semiconductor expands the sensitivity of the photoelectrode to visible light, which increases the number of photon captures and their conversion into excitons. Examples of such cascade structures are ZnO/CdS, ZnO/CdSe, ZnO/Fe₂O₃ and others composites. Maintaining the hierarchy of sequential deposition of semiconductors in a composite, taking into account the position of the levels of conduction bands and band gaps, is an important aspect of the formation of a photocatalyst. Control of geometric parameters, such as the thickness of semiconductor films, is also an important aspect, since the carrier path length is limited by the characteristics of the material, which affects the efficiency of charge separation and the rate of photocorrosion of structures. Controlling the surface morphology of nanocomposites is also important because it allows one to vary the level of photon absorption by the material. The search for and development of cocatalysts for ZnO is ongoing, which could lead to potential practical applications of this material in hydrogen energy.