Cocatalysts for Photocatalytic Overall Water Splitting: A Mini Review

Abstract

Photocatalyst overall water splitting is usually restricted by low carrier separation efficiency and a slow surface reaction rate. Cocatalysts provide a satisfactory solution to significantly improve photocatalytic performance. In this review, some recent advances in cocatalysts for photocatalytic overall water splitting are gathered and divided into groups. Firstly, the loading method of the cocatalyst is introduced. Then, the role of the cocatalyst applied for the photocatalytic overall water splitting process is further discussed. Finally, the key challenges and possible research directions of photocatalytic overall water splitting are proposed. This review is expected to promote research on the design of efficient cocatalysts in photocatalytic systems for overall water splitting.

1. Introduction

With the rapid development of society, greenhouse gases and pollutants released by the massive combustion of traditional fossil fuels have caused global warming and environmental pollution. Therefore, it is urgent to find renewable energy to substitute fossil energy for carbon-neutral development. Solar energy is safe, convenient, and low-cost [1,2]. In the development and utilization of solar energy, photocatalysis has attracted extensive attention [3,4,5,6,7,8]. Photocatalysis utilizes light energy as an energy source to catalyze various reactions through catalysts to satisfy different requirements. At present, the more common applications of photocatalysis include photocatalytic water splitting [4,5], photocatalytic CO₂ reduction [7,9], photocatalytic organic synthesis [10], photocatalytic degradation [11,12], etc. Hydrogen, a clean and non-polluting energy with high energy density, is available for storing and transporting. More importantly, water is the only combustion product. Since Honda and Fujishima first reported the phenomenon of photoelectrochemical water splitting on TiO₂ electrodes in 1972 [13], the splitting of water into hydrogen using semiconductor photocatalysts has emerged as a promising strategy to produce hydrogen fuel using sunlight and water [3,6,10,14,15,16,17]. Due to the distinctive merits of no additives and bias-free, photocatalytic overall water splitting (POWS) into hydrogen and oxygen is considered the most promising method [4,5,18,19].

In general, POWS involves three main steps (Figure 1): (1) A photon with energy greater than the bandgap energy of the photocatalytic semiconductor is absorbed by the semiconductor, and excites an electron from the valence band to the conduction band, leaving a hole in the valence band. Such electrons and holes are referred to as photogenerated electron-hole pairs or photogenerated carriers. Since photo-generation of electrons/holes relies on excitation of the semiconductor with light irradiation, photocatalysts with a wide range of light responses and good light absorption coefficient should be developed, which could be realized by tailing modification of photocatalysts via doping or solid-solution construction [20]. (2) Photogenerated carriers separate and migrate towards the surface redox-active sites. During the migration process of carriers to the surface of the photocatalyst, some electrons and holes will recombine, and only the electrons and holes that successfully reach the redox-active sites can participate in the redox reaction. As such, accelerated charge separation should be realized via building hetero/homo-junctions [15,21]. (3) Photogenerated carriers reaching the redox-active sites are consumed in the process of photocatalytic hydrogen and oxygen production. Due to the high activation energy of water splitting, not all the photogenerated carriers reaching the photocatalyst surface could participate in the water splitting reaction timely, which, consequently, results in surface recombination, finally greatly limiting the efficiency of POWS [22,23]. Therefore, various strategies, including loading cocatalysts, have been employed to boost the surface redox reaction.

For facilitating the surface redox reaction, various methods of loading cocatalysts on the photocatalyst have been developed, such as impregnation [24,25], photodeposition [26,27], and so on [28,29,30,31]. The cocatalysts supported by different loading methods exhibit variations, including loading sites, loading amounts, and particle sizes, which play a significant role in photocatalytic performance. Therefore, summarizing the various loading methods of cocatalysts and the influence on photocatalyst efficiency is highly desired for developing novel photocatalytic systems.

Appropriate cocatalysts loaded on semiconductor photocatalysts can act as reaction sites and catalyze the progress of the reaction, promoting charge separation and migration driven by the interfacial junction formed between the cocatalyst and the semiconductor photocatalyst [23,32]. Noble metals with large work function (Table 1) and suitable adsorption-desorption energy, such as Pt [33,34], Pd [35,36], Rh [27,37], Au [38,39], Ag [31,40,41], etc., have been widely applied on photocatalysts to improve photocatalytic performance. To reduce costs, many non-noble metal and non-metal cocatalysts have also been developed to replace noble metals for hydrogen production, such as Ni [42,43], Cu [44,45], Co [46], C [47,48], etc.

Oxygen production is a 4-electron process, which is considered as the rate-limiting step in the POWS reaction. Introducing an appropriate oxygen-generating cocatalyst can reduce the activation energy barrier of the oxygen-generating reaction and simultaneously accelerate the desorption of the generated oxygen, thereby achieving a higher oxygen generation rate. Common noble metal oxygen-generating cocatalysts include IrO₂ [49] and RuO₂ [50,51,52], while non-noble metal cocatalysts include MnO_x [53,54], NiO_x [55], CoO_x [25,56], NiCo₂O₄ [29], etc.

For POWS, the reverse hydrogen-oxygen recombination reaction is an important factor hindering photocatalytic activity. In this regard, researchers have developed various cocatalysts to isolate the contact between oxygen and hydrogen production sites, such as CrO_x [27,57], NiO_x [58,59], etc. Another way to prevent the reverse reaction of hydrogen-oxygen recombination is by depositing hydrogen-generating and oxygen-generating cocatalysts into the different sites of the photocatalyst and successfully separating the hydrogen-generating and oxygen-generating sites to suppress the reverse reaction [26,49,60,61]. Another problem with POWS is the oxidation of photocatalysts and cocatalysts by the holes or produced oxygen, also known as photocorrosion. For this reason, researchers have developed a series of cocatalysts to inhibit photocorrosion, primarily a protective shell that keeps out oxygen [62,63,64,65].

In this review, the preparation methods of several common cocatalysts are introduced first and the advantages of various methods are then comparatively discussed to facilitate the subsequent design and preparation of cocatalysts. Focusing first on the different roles of cocatalysts, the cocatalysts for POWS are then categorized and the roles of various cocatalysts in the process of POWS are summarized. Hopefully, this paper will serve as a link for the development of promoters for POWS.

2. Cocatalysts with Various Loading Methods

Various methods of loading cocatalysts on the photocatalyst have been developed. The loading methods can be mainly divided into two categories. One is cocatalysts that are directly grown on the surface of the photocatalyst using precursors, referred to here as the direct addition-type method. The other is loading the as-synthesized cocatalyst on the surface of the photocatalyst, referred to as the composite-type method. Direct addition-type methods, such as solvothermal treatment, precipitation, immersion, photo-deposition, etc., could ensure sufficient and tight interfacial contact between cocatalyst and photocatalyst semiconductors, thereby facilitating charge separation and transfer. Compound-type methods involve synthesizing the desired cocatalyst and then loading on the photocatalyst by impregnation, grinding, physical mixing, and other methods. For Compound-type synthesis, the microstructure of the cocatalyst can be precisely controlled and the synthesis conditions are not limited by the photocatalyst semiconductor.

2.1. Direct Addition-Type Cocatalysts

Direct addition-type cocatalysts refer to those cocatalysts directly grown on the surface of photocatalyst with corresponding precursors, which contributes to the formation of a tight interface between the cocatalyst and the photocatalyst. For this reason, the cocatalyst loaded by direct addition-type methods can effectively reduce the resistance at the interface. Moreover, the cocatalyst loading site, particle size, and distribution are easily controlled via this loading method, and special structures such as core–shell structures can be formed through layer-by-layer loading. As reported by Liu et al. in 2020, the prepared C quantum dots were added to the raw materials and then used a hydrothermal method to prepare a C-loaded CoP photocatalyst. The POWS efficiency was 4 times higher than that of the physical mixture of C quantum dots and CoP [48]. The research shows that the product obtained from an in situ growth method have relatively tight chemical bonds, thus leading to better performance.

2.1.1. Impregnation

The impregnation-calcination method is one of the most common methods applied to the cocatalysts deposition on photocatalyst semiconductors (Figure 2a) [23,66]. Photocatalysts were first immersed into the precursor solution of the cocatalyst. During the following slow evaporation process of the solvent, the supersaturated cocatalyst precursor continued to precipitate and evenly disperse on the surface of the photocatalyst. The metal compounds attached to the photocatalyst surface were pyrolyzed into metal or metal oxide nanoparticles at different temperatures/atmospheres. Afterward, a photocatalyst with a uniformly supported cocatalyst was obtained. Due to the interactions within high-temperature treatment, the cocatalyst prepared by the impregnation-calcination method was tightly combined with the photocatalyst.

Domen’s group first reported that a mixed oxide cocatalyst of rhodium and chromium was supported on (Ga_1−xZn_x)(N_1−xO_x) by the impregnation method [67]. As the sources of rhodium and chromium, Na₃RhCl₆·2H₂O and Cr(NO₃)₃·9H₂O were loaded on the photocatalyst by immersion method. After calcination, RhCrO_x particles with a diameter of 10–20 nm and good crystallization were formed, which were uniformly distributed on the surface of (Ga_1−xZn_x)(N_1−xO_x). The apparent quantum yield (AQY) of the samples at POWS achieved was 2.5% at 420–440 nm, which is about an order of magnitude higher than previously reported. The impregnation method can also be compliant with molecular cluster cocatalysts. For example, Seo et al. loaded the synthesized [MO₃S₄]⁴⁺ molecular clusters on the NaTaO₃ surface via the impregnation method and obtained molecular cluster cocatalysts with a diameter of about 1 nm [68]. The hydrogen production rate of POWS is 28 times higher than that of pure NaTaO₃.

In addition to oxide cocatalyst, the immersion calcination method can also obtain phosphide cocatalyst by changing the calcination atmosphere. Xue et al. successfully loaded Co and Ni precursor on PCN by the impregnation method and then employed NaH₂PO₂·H₂O as P source to obtain Co_xNi_yP cocatalyst through a high-temperature phosphorization process [69]. The formation of the P⁺-P^δ-Co/Ni^δ+ chemical bridge between the cocatalyst and the photocatalyst has been demonstrated to significantly enhance charge transfer and separation, as well as H-H bond formation and H₂ desorption. Then, the H₂ production rate reached 239.3 μmol h⁻¹ g⁻¹ for POWS.

2.1.2. Photodeposition

Photochemical deposition has been widely applied to the load of metals and their oxides on photocatalyst semiconductors as cocatalysts (Figure 2b) [70,71,72]. A typical photodeposition process can be described as follows: The photocatalyst is excited by light to generate photogenerated electrons and holes, which migrate to the reduction and oxidation active sites on the surface of the photocatalyst, respectively. The photogenerated electrons migrated to the reduction active site will preferentially react with metal cations in solution to reduce metal cations into metal elements. Likewise, during photodeposition of metal oxide cocatalysts, metal cations are oxidized to metal oxides by photogenerated holes that migrate to the oxidation active sites. Therefore, the photodeposition method can selectively load metal cocatalysts on the active sites to shorten the photogenerated carrier migration distance and improve the carrier separation efficiency, and the particle size can be adjusted by means of metal precursor solution concentration, deposition time, different sacrificial agents, etc., which is difficult to achieve by other methods.

Domen’s group reported that the excellent POWS performance obtained by photodepositing RhCrO_x on SrTiO₃ surface was about 2 times as that obtained from impregnation method [72]. Subsequent studies by this group showed that the photodeposited rhodium and chromium oxides exhibited a core–shell structure with metallic Rh core and Rh(III)-Cr(III) mixed oxide as core and shell, respectively. During the experiment, the authors found that the concentration of K₂CrO₄ solution during the photo-deposition process could directly influence the valence state of deposited Cr. The POWS performance of the core–shell structure supported by photodeposition method is 5.8 times than that of the mixed oxide of Rh(III) and Cr(III) [71].

After the metal is deposited on the photocatalyst surface by using the photodeposition method, the metal compound can also be obtained by further processing. In our previous work, Co was deposited on the surface of Al: STO by the photodeposition method and then phosphatized at a high temperature to obtain CoP [70]. The photodeposition process makes the contact between the cocatalyst and the photocatalyst more intimate than that prepared by the impregnation method. Thus, the activity of CoP loaded on SrTiO₃ by the photodeposition method was nearly 1.3 times higher than that by the impregnation method.

2.1.3. Electrostatic Adsorption and Low-Temperature Heat Treatment

Electrostatic adsorption and low-temperature heat treatment is also a common method for loading cocatalysts. In particular, this method is commonly used when it is desired to support single-atom cocatalysts. Single-atom metal loading not only maximizes the utilization of metal, but also promotes the separation and transfer of charge carriers. Xiao et al. embedded sodium copper chlorophyllin salt into the melamine-based supramolecular precursor and then the pyrolysis was controlled to form a single-atom Cu-loaded C₃N₄ [73]. The visible-light photocatalytic hydrogen production rate was 30 times higher than that of bulk C₃N₄. Melamine was hydrolyzed and self-assembled in the presence of phosphoric acid, while chlorophyll sodium copper salt was inserted into melamine due to large layer spacing. During thermal decomposition, Cu was anchored to a N atom via a Cu-N_x bond with N in C₃N₄ planes. As an excellent cocatalyst for hydrogen production, single-atom Pt can be loaded on the surface of g-C₃N₄ [74], CdS [75], and TiO₂ [76] through electrostatic adsorption and low-temperature heat treatment, respectively. The corresponding photocatalytic performance was 8.6 times, 7.69 times and 591 times higher than that by nanoparticles loading, respectively. It is worth noting that the reports of single-atom cocatalysts mainly focus on photocatalytic hydrogen production and there are no relevant reports on single-atom cocatalysts to achieve POWS.

2.1.4. Others

In addition to the above-mentioned methods of loading cocatalysts, there are other direct addition-type methods such as hydrothermal treatment, controlled ink-jet printing, and so on. MoS₂-RuO₂ modified CdS with MoS₂ at the ends and RuO₂ on the sidewalls of CdS nanowires were prepared by Qiu et al. [77]. In the process of hydrothermal treatment, the amino group on the surface of CdS concentrates the precursor of MoS₂ on the two ends of the CdS nanowire. When RuO₂ is further loaded, the interaction between Ru and amino group loads RuO₂ on the sidewalls of the nanowire. Thereby, the photocatalyst realized the separation of hydrogen and oxygen production sites. Consequently, the POWS efficiency is 10 times higher than that with unseparated sites. In 2021, A. Aguirre-Astrain et al. decorated the photocatalyst with semiconductor oxides using the controlled ink-jet printing method [28]. Firstly, the metal precursor solution was printed on the surface of the BaV₂O₆ film and then the metal oxide-modified BaV₂O₆ photocatalyst was obtained after high-temperature calcination. Ultimately, the POWS activity of CuO/CuO₂-modified BaV₂O₆ was 30 times higher than that of pure BaV₂O₆.

2.2. The Composite Type of Cocatalysts

Although cocatalyst directly synthesized on the photocatalyst has the above advantages, it is necessary to ensure that the photocatalyst does not change in the process of synthesis. Therefore, the synthesis conditions of direct addition-type methods are restricted by the inherent quality of the photocatalyst. However, synthesizing the cocatalyst first and then loading it provides another choice to prepare the cocatalyst, which allows us to synthesize the desired cocatalyst without restriction.

2.2.1. Grinding

A non-noble metal Mn₂Co₂C@C/Mn₂N_0.86 composite cocatalyst composed of Mn₂N_0.86 and N-doped graphitized carbon-coated Mn₂Co₂C nanoparticles were fabricated by Zhou et al. and loaded on SrTiO₃ and g-C₃N₄ by a mechanical grinding method [78]. During the illumination process, Mn₂N_0.86 was hydrolyzed and formed a MnOOH and Mn₂Co₂C dual cocatalyst through an in situ growing method. Thereinto, Mn₂Co₂C is used for hydrogen production and MnOOH is used for oxygen production; finally, POWS was achieved and the corresponding AQY reached 2.54% and 1.45% at 400 and 420 nm, respectively. The FeNi inter-metallic compound nanoparticles coated with N-doped graphitized carbon prepared by pyrolysis were also loaded on the g-C₃N₄ surface by grinding [79]. The secondary charge transfer in FeNi alloys can promote carrier separation and accelerate hydrogen desorption, which is 283 times as high as that of primary g-C₃N₄. As a result, a high AQY up to 24.78% (400 nm) was obtained.

2.2.2. Calcination

Garcia Esparza et al. used mesoporous g-C₃N₄ as a template and carbon source to synthesize tungsten carbide nanocrystals with different structures and compositions using tungsten precursors at different temperatures. The cocatalyst was loaded onto sodium-doped SrTiO₃ by the immersion method and POWS was achieved [80]. Recently, a new hybrid cocatalyst CoO_x-Mo₂N was prepared by a hydrothermal method and loaded by calcining the mixture of CoO_x, Mo₂N and Ge₃N₄ in N₂ flow [81]. Among them, Mo₂N is used for hydrogen production and CoO_x is used for oxygen production, and the combination of them realizes POWS. The activity is 1.5 times higher than that by Ge₃N₄ loaded with the spatial separated dual cocatalysts CoO_x and Mo₂N.

3. Application of Cocatalysts for POWS

In photocatalytic water splitting, cocatalysts play various roles in enhancing the photocatalytic performance of semiconductor photocatalysts (Figure 3). First, the cocatalyst can trap photogenerated carriers to enhance the carrier transfer and separation efficiency. Secondly, the cocatalyst can provide reaction sites, enhance the adsorption and desorption of surface species, and reduce the overpotential of the surface reaction, thus increasing the H₂ and O₂ generation rate. For POWS, some cocatalysts also have unique functions, such as inhibiting the reverse reaction of H₂ and O₂, or inhibiting the photocorrosion of the photocatalyst and the cocatalyst. In this section, the roles of various cocatalysts in POWS will be discussed for designing an efficient POWS system.

3.1. Promote the Water Splitting Reaction

3.1.1. Hydrogen-Production Cocatalysts

The band gap of a photocatalyst realizing POWS should not be less than 1.23 eV [82], while the potential of conduction band bottom should be more negative than H⁺/H₂ (0 V vs. NHE at pH = 0) and the potential of valence band top should be more positive than O₂/H₂O (1.23 V vs. NHE at pH = 0). Beyond that, the oxidation and reduction reaction on the surface of the photocatalyst requires a certain overpotential. Even with the appropriate potential to split water, many semiconductor photocatalysts cannot produce hydrogen/oxygen without cocatalysts, which is mainly restrained by the severe carrier recombination and slow surface reaction rate. Appropriate cocatalysts loaded on semiconductor photocatalysts can act as reaction sites and catalyze the progress of the reaction, thus promoting charge separation and accelerating the reaction rate. Many materials have been developed as cocatalysts used for hydrogen production, among which NiO_x and RhCrO_x are widely used for POWS.

As early as 1980, Domen et al. achieved POWS by anchoring the NiO cocatalyst on SrTiO₃ [83]. Through a series of comparison experiments, it was found that the NiO cocatalyst on the surface of SrTiO₃ was composed of metallic Ni particles and a layer of NiO, in which hydrogen was generated on Ni and oxygen was generated on NiO [42]. Moreover, in-depth studies on the state of Ni-NiO cocatalysts in photocatalytic reactions were carried out by XPS, TEM, etc. The results show that the Ni(OH)₂ on the surface will be oxidized to NiOOH when illuminated, and when the illumination is stopped, the NiOOH will undergo a disproportionation reaction to form embedded Ni(OH)₂ and Ni particles [58].

RhCrO_x as a cocatalyst for POWS was first reported by Maeda et al. in 2006. The RhCrO_x was supported on (Ga_1−xZn_x)(N_1−xO_x) surface through the impregnation-calcination method [67]. Subsequently, they developed a photo-deposition method to prepare Rh-CrO_x core–shell structure hydrogen production cocatalysts [57]. Since then, RhCrO_x and Rh-CrO_x core–shell structures have been widely used in various photocatalysts to achieve POWS [26,30,37,50,72,84,85,86,87,88,89,90]. The role of each component in the RhCrO_x cocatalyst was explored through electron spin resonance spectral and the results show that RhO_x can accelerate the hydrogen production reaction process and promote the production of superoxide radical and singlet oxygen in competition with the hydrogen production reaction, while CrO_x inhibits the production of these two species [90]. Although CrO_x is not able to accelerate the hydrogen production reaction, the co-action with RhO_x enhances the hydrogen production performance (Figure 4a). The Al-doped SrTiO₃ loaded with RhCrO_x and MoO_y prepared by the impregnation-calcination method realized POWS and an AQY of 69% was achieved at 365 nm [91].

A Pd@Pt core–shell structure cocatalyst was successfully prepared. Additionally, it was further deposited on TiO₂ nanosheets to improve the efficiency of Pt cocatalysts on POWS by reducing the load amount of Pt, and finally obtained an enhanced performance of 7.2 times when compared with Pt-TiO₂ [92]_. Pd@Pt cocatalyst has improved the capabilities of charge trapping and accelerated charge separation through interfacial charge polarization, while optimizing lattice structure and enhancing the adsorption capacity of H₂O, thereby enhancing the photocatalytic performance and simultaneously reducing the load amount of Pt (Figure 4b).

Other materials such as W_xC [80] and RuO_x [49] have also been developed for hydrogen production cocatalysts for POWS catalysts.

3.1.2. Oxygen-Production Cocatalysts

Oxygen production reaction is generally considered as the rate-limiting step due to its 4-electron process. Accordingly, increasing the oxygen production reaction rate can effectively improve photocatalytic efficiency [3,14,93,94]. Appropriate oxygen-generating cocatalyst loading can effectively reduce the activation energy and increase the oxygen-generating reaction rate [32,95,96,97,98,99].

As a common cocatalyst for oxygen generation, RuO₂ has been widely used in POWS [51,52,100,101,102]. For the first time, β-Ge₃N₄ supported by RuO₂ realized POWS for non-oxide photocatalysts (Figure 5a) [51]. Besides RuO₂, Yuan et al. used the photo-deposition method to deposit MnO_x on the Z-scheme photocatalyst composed of γ-MnS and Cu₇S₄ to achieve POWS [53]. Oxygen generation cocatalyst MnO_x and hydrogen production cocatalyst PtS were selectively deposited on a ZnIn₂S₄/WO₃ Z-scheme photocatalyst with an AQY of 0.5% at 420 nm [103].

Cobalt oxide phosphate is a common electrocatalyst for water oxidation, which was introduced into the photocatalytic system by Li et al. in 2013 [104]. The cobalt-oxide-phosphate cocatalyst was loaded on mpg-CN_x using the direct photo-deposition method and impregnation-photo-assisted oxidative conversion method, which was very efficient for the photocatalytic water oxidation reaction with electron scavenger. With hole scavenger, the cobalt-oxide-phosphate will be reduced to a cobalt-oxo/hydroxy-phosphate compound, an HER electrocatalyst. In the POWS process, part of the cobalt-oxide-phosphate is converted into cobalt-oxo/hydroxy-phosphate compound, thus achieving POWS. Other Co species, such as CoOx, Co(OH)₂, etc. were also used as efficient oxygen-evolution cocatalysts in the POWS system [105,106].

Figure 5

(a) SEM image of RuO₂-loaded β-Ge₃N₄. Reproduced with permission from Ref. [51]. Copyright 2005, American Chemistry Society. (b) OER rate with AgNO₃ of fabricated photocatalysts. (c) Wavelength-dependent AQY (orange dots) of Pt·Ni(OH)₂/C₃N₄. (b,c) Reproduced with permission from Ref. [107]. Copyright 2019, Elsevier.

[Figure omitted. See PDF]

NiP was obtained by the in situ photo-oxidation Ni(OH)₂ method and, subsequently, selectively photo-deposited Pt to prepare Pt clusters decorated with Ni(OH)₂ nanoparticles as an oxygen-generating cocatalyst [107]. The close contact between nanosized Pt and Ni(OH)₂ produces a strong Pt^σ+-O^σ− interaction, which is beneficial to the adsorption of H₂O, and, at the same time, greatly reduces the activation energy of the H-O-H bond and promotes oxygen production (Figure 5b), and the AQY for POWS can reach 4.2% at 420 nm (Figure 5c).

3.2. Suppressed Backward Reactions

3.2.1. Separate Hydrogen and Oxygen Production Sites

In 2012, Townsend et al. supported NiO_x on the surface of SrTiO₃ by the impregnation-calcination method and achieved excellent POWS performance [42]. It has been shown that NiO_x is composed of Ni metal particles and NiO particles, and neither Ni or NiO can achieve POWS. NiO can be generated by photodeposition after loading Ni. The surface photovoltaics prove that Ni acts as a hydrogen production site. Comparative experiments of deposited Pt confirm that NiO is not a hydrogen-producing site. Therefore, it is concluded that Ni is a hydrogen-generating cocatalyst, and NiO is an oxygen-generating cocatalyst. The Ni and NiO are supported at different sites to prevent the reverse reaction, thereby achieving excellent POWS performance. Subsequently, the photo-deposition method was used to selectively deposit MnO_x and Pt on the (110) surface and the (010) surface of BiVO₄, respectively, which was acted as an oxygen-generating and hydrogen-generating cocatalyst. Consequently, an AQY of 5.5% at 420 nm at POWS was realized [53]. The oxygen-production cocatalyst reduced graphene oxide and hydrogen-production cocatalyst Ag nanoparticles were loaded on different parts of the surface of SnS₂ nanosheets, respectively (Figure 6) [31]. This structure realizes the separation of hydrogen production and oxygen production sites, improves the carrier separation efficiency, and suppresses the reverse reaction. Thus, POWS was obtained under visible light irradiation.

3.2.2. Oxygen Barrier

In the core–shell structure cocatalyst, the shell can effectively prevent the contact between H₂ generated on the core and O₂ in the environment, thus inhibiting the occurrence of reverse reaction (Figure 7). Maeda et al. utilized a photo-deposition method to prepare Rh-Cr₂O₃ core–shell cocatalysts (Figure 8a) [57]. The study showed that Rh core was the hydrogen-producing site and Cr₂O₃ shell isolated the contact between the hydrogen-producing site and O₂, preventing the reverse reaction. On this basis, a Cr₂O₃ shell covered on Pt by Qi et al. also played a significant role in preventing the reverse reaction [108].

Ni-NiO core–shell structure was reported to be loaded on Ta₂O₅, achieving POWS [59]. It is shown that Ni has excellent catalytic performance for hydrogen production, but at the same time, Ni also acts as the reverse reaction site of H₂ and O₂, which limits the further improvement of the POWS performance. The researchers found through experiments that H₂ generated on the Ni surface can penetrate the NiO shell, while H₂O and O₂ cannot. This isolates the generation sites of H₂ and O₂, inhibits the reverse reaction, and enhances the POWS performance (Figure 8b).

3.2.3. Other Means of Inhibiting Recombination

In 2018, Wang et al. found in the POWS experiment of Pt-loaded TiO₂ that halogen atoms can be adsorbed on the surface of the Pt cocatalyst, which decreases the adsorption and activation degree of hydrogen and oxygen molecules on Pt, thereby suppressing the reverse reaction (Figure 8c) [109,110].

3.3. Restrain Photocorrosion

A large number of holes and generated O₂ during the POWS process will cause severe photocorrosion of photocatalyst semiconductors, which limits the commercial application of photocatalysts [27,50,63,111].

3.3.1. Restrain the Photocorrosion of Photocatalyst

Thaminimulla et al. found that depositing a small amount of Cr oxide on the surface of Ni-deposited K₂La₂Ti₃O₁₀ by the impregnation method could improve the POWS activity and stability of the photocatalyst [43]. Recently, an Al₂O₃ shell was loaded on the surface of CdS as a cocatalyst and isolated CdS from generated O₂, which effectively inhibited the photocorrosion of CdS and achieved an AQY of 0.11% at 430 nm [112]. Inductively coupled plasma emission spectrometer (ICP) analysis showed that before loading Al₂O₃, the post-reaction solution contained a large amount of Cd²⁺, indicating that there was a severe photocorrosion phenomenon in the reaction. Nevertheless, after loading Al₂O₃, the change of Cd²⁺ content in the solution before and after the reaction was almost negligible, so the Al₂O₃ shell on CdS effectively inhibits the photocorrosion of CdS (Figure 9a,b). Similarly, the Ni₂P shell loaded on the surface of CdS as a cocatalyst also achieved a similar effect and achieved an AQY of 3.89% at 430 nm for POWS (Figure 9c,d) [62]. Recently, uniform CdS nanorods protected by ultrathin NiOOH achieved POWS and exhibited good photostability over 25 h [113]. Another example is removing oxygen-production sites from a photocatalyst to a cocatalyst. CoO_x was loaded as a cocatalyst providing oxygen-production sites, which could extract holes from a GaN/InGaN photocatalyst. As a consequence, impressive stability of >580 h was achieved for POWS [114,115].

3.3.2. Restrain the Photocorrosion of Cocatalyst

Zhang et al. found that the Ni@NiO core–shell structure cocatalyst deposited on the surface of TiO₂ can only generate hydrogen but not oxygen in the POWS reaction [64]. The research shows that the Ni@NiO core–shell structure cocatalyst deposited on the surface of TiO₂ will gradually be corroded and dissolved during the hydrogen production process, leaving the NiO shell, so POWS cannot be achieved (Figure 10a,b). The generation of H₂ is a chemical reaction in the Ni corrosion process. Therefore, inhibiting the photocorrosion of the cocatalyst is also one of the factors to consider to improve the performance of POWS.

In 2015, Busser et al. loaded Cu on Ga₂O₃ to replace the noble metal Rh, and simultaneously used CrO_x and MoO_x to suppress the photoreduction of CuO, which effectively improved the stability of the CuO cocatalyst [45]. On this basis, another layer of CrO_x was deposited on the surface of the Ni/NiO cocatalyst and CrO_x significantly inhibited the dissolution of Ni [116]. In 2019, with the quantum efficiency of Al-SrTiO₃ loaded with RhCrO_x as high as 50%, Lyu et al. further loaded CoOOH and TiO₂, which effectively inhibited the dissolution of Cr and improved the stability of the photocatalyst, maintaining POWS activity for over 1000 h [65].

4. Conclusions and Perspectives

The low surface redox reaction rate is one of the vital influencing factors for poor POWS efficiency. Loading cocatalysts is a modification strategy to boost the surface redox reaction. Therefore, numerous cocatalysts have been developed and applied for POWS. This review summarizes the recent progress made in the field of cocatalysts in enhancing POWS performance in regard to loading methods and functions (Table 2). Although highly active and stable cocatalysts as well as various loading means have been developed, numerous challenges to achieving efficient and stable POWS still remain. Thus, significant efforts should be devoted to the following aspects:

• (1). Explore suitable low-cost cocatalysts for hydrogen and oxygen production. Currently, the most efficient cocatalysts for hydrogen production and oxygen production are still noble metals and their oxides, while non-noble metal cocatalysts still need to improve their performance and stability. Therefore, it is urgent to develop efficient and stable non-noble-metal cocatalysts to reduce the costs of the catalytic system. Many transition metal-based cocatalysts have been developed as an alternative to noble metal cocatalysts, which is conducive to reducing the preparation cost of photocatalytic systems effectively. Although the efficiency of non-noble metal-based cocatalysts still needs to be improved, non-noble metal-based cocatalysts are widely used. For example, non-noble metals and their phosphides can be used as cocatalysts for hydrogen production, and their oxides can be used as cocatalysts for oxygen production, the inhibition of reverse reactions, and the prevention of photocorrosion, etc. Transition metal-based cocatalysts that could promise a comparable performance as noble metal are yet to be developed.

• (2). Develop a single-atom cocatalyst. It is widely acknowledged that single-atom metal cocatalysts could maximize the utilization of metal and reduce the costs of a catalyst system. Moreover, single-atom photocatalysts with unique unsaturated coordination environments also exhibit excellent performance in various reactions. However, although single-atom cocatalysts have been widely used in the field of photocatalysis, it is of great significance to develop single-atom cocatalysts for POWS.

• (3). Develop new methods for loading cocatalysts. The activity of cocatalyst-modified photocatalysts is highly dependent on the methods of cocatalyst preparation and loading, which determine the properties of the cocatalyst and the interface between the cocatalyst and the base photocatalyst. However, the currently developed methods cannot be simply adopted on different photocatalysts and different precursors of cocatalysts. Therefore, it is necessary to develop new general/universal strategies for cocatalyst loading, especially those low-cost methods, for instance, simply blending non-noble-metal cocatalysts with photocatalysts in the reaction solution, which is suitable for large-scale commercial applications.

• (4). Comprehensively configure different types of cocatalysts to improve the performance of the catalytic system. For POWS, accelerating the formation of hydrogen and oxygen, inhibiting backward reactions, and reducing photocorrosion are significant factors that need to be considered. It is difficult for a single cocatalyst to achieve all the above functions, so how to rationally configure cocatalysts with different functions is worth further study.

Table 2

The most promising cocatalyst for POWS catalyst with H₂ or O₂ production.

Loading Method	Photocatalyst	H2-Production Cocatalyst	O2-Production Cocatalyst	Light Source	Activity (μmol h−1 g−1)	AQY (%)	Stability at Least (h)	Ref.
Impregnation	SrTiO3	NiO		(Hg)	0.098 (H2)0.049 (O2)		100	[83]
	(Ga1−xZnx)(N1−xOx)	Rh2-yCryO3		λ > 200 nm (Xe)	1760 (H2)880 (O2)			[72]
	SrTiO3	Ni@NiOx		AM 1.5G	18 (H2)9 (O2)			[58]
	STO:Al	Graphene-RhCrOx		(Xe)	6375 (H2)3083 (O2)	0.67 (360 nm)	16	[89]
	(Ga1−xZnx)(N1−xOx)	Rh2-yCryO3		367 nm (LED)		0.16 (367 nm)	20	[87]
	STO-Al	RhCrOx		(Xe)	3288 (H2)166.24 (O2)		12	[88]
	ZrO2/TaON	Cr2O3/RuOx	IrO2/RuOx	λ > 300 nm (Xe)			10	[49]
	mpg-CNx		CoPi	λ > 400 nm (Xe)	13.6 (H2)6.6 (O2)		15	[104]
	ß-Ge3N4		RuO2	(Hg)	933 (H2)	9 (300 nm)	25	[51]
Impregnation-calcination	Al:STO	MoOy-RhCrOx		λ > 300 nm (Xe)		69 (365 nm)	16	[91]
	(Ga1−xZnx)(N1−xOx)	RhCrOx		λ > 400 nm (Hg)	3080 (H2)1540 (O2)	51 (430 nm)	35	[67]
Hydrothermal -impregnation	TiO2	Pd@Pt		(Xe)	601.7 (H2)			[92]
Photodeposition	(Ga1−xZnx)(N1−xOx)	Rh-CrOx		λ > 400 nm (Hg)	1153 (H2)593 (O2)			[57]
	SrTiO3	NiOx		λ > 250 nm (Xe)	28 (H2)14 (O2)			[42]
	Ta2O5	Rh-CrOx		λ > 250 nm (Hg)	204 (H2)91 (O2)			[37]
	Ga2O3	Rh		λ > 250 nm (Hg)	249 (H2)126 (O2)			[37]
	GaN:ZnO	Rh2-yCryO3		λ > 400 nm (Xe)	17.5 (H2)7 (O2)		2160	[50]
	Y2Ti2O5S2	Rh/Cr2O3	IrO2	λ > 420 nm (Xe)		5.3 ± 0.3 (H2)2.3 ± 0.1 (O2) (450 nm)	20	[26]
	(Zn1+xGe)(N2Ox)	Rh2-yCryO3		λ > 200 nm (Xe)	53.3 (H2)25.7 (O2)			[72]
	SrTiO3	Rh2-yCryO3		λ > 200 nm (Xe)	3533.3 (H2)1788.7 (O2)			[72]
	Ca2Nb2O7	Rh2-yCryO3		λ > 200 nm (Xe)	500 (H2)216.7 (O2)			[72]
	ß-Ga2O3	Rh2-yCryO3		λ > 200 nm (Xe)	4156.7 (H2)1950 (O2)			[72]
	H-Bi0.5Y0.5VO4	Rh@Rh2O3		λ > 300 nm (Xe)	516 (H2)258.5 (O2)		16	[85]
	GaN:ZnO	Mn3O4 + Rh/Cr2O3		λ > 420 nm (Xe)			12	[30]
	InGaN/GaN	Rh/Cr2O3		λ > 300 nm (Xe)	37.94 (H2)20.78 (O2)(μmol h−1)	1.86 (400 nm)	20	[86]
	γ-MnS/Cu7S4		MnOx	(Xe)	718 (H2)	18.8 (420 nm)	20	[53]
	ZnIn2S4/WO3	PtS	MnOx	(Xe)	14.85 (H2)5.6 (O2)	0.5 (420 nm)	24	[103]
Photodeposition-adsorption	Y2Ti2SO5	Cr2O3-Rh	IrO2	λ > 420 nm (Xe)		0.36 (420 nm) 0.23 (500 nm) 0.05 (600 nm)0.007 (STH)	22	[84]
Photodeposition-impregnation	CNN/BDCNN	Pt	Co(OH)2	λ > 300 nm (Xe)	62.9 (H2)31.25 (O2)	11.76 (420 nm) 1.16 (STH)	24	[106]
hydrothermal -photodeposition	C3N4	Pt	Ni(OH)2	(Xe)	1330 (H2)632 (O2)	4.2 (420 m)	18	[107]

Author Contributions

Conceptualization, X.G. and S.Z.; formal analysis, L.T. and X.G.; investigation, L.T.; resources, L.T.; writing—original draft preparation, L.T.; writing—review and editing, A.D. and J.Q.; funding acquisition, X.G. and S.Z. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Schematic descriptions for photocatalytic overall water splitting.

Figure 2. Schematic illustrations of (a) impregnation and (b) photodeposition method.

Figure 3. Various roles of cocatalyst for POWS.

Figure 4. (a) Diagram of the mechanism by which RhCrOx promotes photocatalytic performance: RhOx could facilitate both the desired hydrogen evolution reaction (HER) and undesired reactions of superoxide radicals and singlet oxygen formation. On the contrary, CrOx inhibits formation of these two ROS although it could not promote the POWS reaction. Reproduced with permission from Ref. [90]. Copyright 2022, American Chemistry Society. (b) Photocatalytic H2 evolution from water with the samples as catalysts under the same irradiation conditions. Reproduced with permission from Ref. [92]. Copyright 2022, Wiley.

Figure 6. Morphology and microstructure analysis of rGO/SnS2/Ag. (a) SEM image and (b) TEM image, (c) HRTEM lattice fringes. (d) rGo diffraction patterns, (e) rGO/SnS2 diffraction patterns, (f) rGO/SnS2/Ag diffraction patterns. Reproduced with permission from Ref. [31]. Copyright 2022, Elsevier.

Figure 7. Core–shell structured cocatalyst.

Figure 8. (a) HR-TEM images of Rh-loaded (Ga1−xZnx)(N1−xOx) after photodeposition of the Cr shell. Reproduced with permission from Ref. [57]. Copyright 2006, Wiley. (b) H2 production rate vs. reaction time of samples oxidized at different temperatures (different oxide shell thickness). Reproduced with permission from Ref. [59]. Copyright 2015, Elsevier. (c) H2 and O2 recombination rates for Pt-TiO2, F-Pt-TiO2, Cl-Pt-TiO2, Br-Pt-TiO2 and I-Pt-TiO2. Reproduced with permission from Ref. [109]. Copyright 2018, Elsevier.

Figure 9. (a) The cadmium ion concentration changes in the solution of CdS NPs and Pt/CdS@Al2O3 composite with light irradiation time (b) Cycling runs for the photocatalytic hydrogen evolution activity under visible light over various samples: (a) CdS NPs, (b) Pt/CdS, (c) Pt/CdS@Al2O3 with artificial gill and (d) Pt/CdS@Al2O3 without artificial gill. (a,b) Reproduced with permission from Ref. [112]. Copyright 2018, Elsevier. (c) HRTEM images of 10Ni2P@CdS and 1Pt@CdS. (d) The concentration of Cd2+ for each run in the reaction solution of CdS and 10Ni2P@CdS under visible light irradiation. (c,d) Reproduced with permission from Ref. [62]. Copyright 2018, Elsevier.

Figure 10. (a) Partial void/shell structure areas of deactivated materials show cleavages in between TiO2 and Ni metal. (b) Void/shell structures after 50% deactivation. Reproduced with permission from Ref. [64]. (a,b) Copyright 2015, American Chemistry Society.

Table 1

The values of work function (Φm) of different noble metals and non-noble metals (eV).

Noble Metals	Φm	Non-Noble Metal	Φm
Pt	5.65	Fe	4.50
Pd	5.12	Co	5.00
Rh	4.98	Ni	5.15
Au	5.12	Cu	4.65
Ag	4.26

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