Photocatalytic Oxygen Evolution from Water Splitting

Abstract

Photocatalytic water splitting has attracted a lot of attention in recent years, and O₂ evolution is the decisive step owing to the complex four-electrons reaction process. Though many studies have been conducted, it is necessary to systematically summarize and introduce the research on photocatalytic O₂ evolution, and thus a systematic review is needed. First, the corresponding principles about O₂ evolution and some urgently encountered issues based on the fundamentals of photocatalytic water splitting are introduced. Then, several types of classical water oxidation photocatalysts, including TiO₂, BiVO₄, WO₃, α-Fe₂O₃, and some newly developed ones, such as Sillén–Aurivillius perovskites, porphyrins, metal–organic frameworks, etc., are highlighted in detail, in terms of their crystal structures, synthetic approaches, and morphologies. Third, diverse strategies for O₂ evolution activity improvement via enhancing photoabsorption and charge separation are presented, including the cocatalysts loading, heterojunction construction, doping and vacancy formation, and other strategies. Finally, the key challenges and future prospects with regard to photocatalytic O₂ evolution are proposed. The purpose of this review is to provide a timely summary and guideline for the future research works for O₂ evolution.

Introduction

Over the last few decades, the problems of energy shortage and environmental pollution have become the major issues that need to be solved for the sustainable development of modern society and economics.^[1] Since TiO₂ photoelectrode was found to be able to decompose H₂O into H₂ and O₂ under UV irradiation in 1972, semiconductor-based photocatalytic technologies have attracted extensive attentions for environmental purification and energy conversion.^[2] A large variety of semiconductor photocatalysts have been developed, such as TiO₂, ZnO, g-C₃N₄, sulfides, bismuth-based compounds, black phosphorus, etc.,^[3–9] which show various applications in the areas of degradation of domestic sewage, industrial dyes and pesticides,^[10,11] water splitting for H₂, O₂, and H₂O₂,^[12–14] carbon dioxide conversion for generating CO, CH₄, HCHO, CH₃OH, etc.,^[15,16] nitrogen fixation,^[17] removal of harmful indoor gases,^[18,19] solar cell, etc.^[20] The photocatalytic process generally includes three stages: i) the generation of the electron–hole pairs in the bulk phase of photocatalysts under light irradiation (hv ≥ E_g); ii) the migration of electrons and holes from bulk to the surface of photocatalysts, and some of them will be recombined during this process; and iii) the residual electrons and holes on the surface of photocatalysts will react with the adsorbed reactants for reduction and oxidation reactions.^[21]

Photocatalytic overall water splitting (OWS) is a thermodynamically uphill process (ΔH_ϕ = 285.5 KJ mol⁻¹), which cannot occur spontaneously. It is composed of two half reactions, namely, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), both of which are the processes of Gibbs free with. It can be realized through the solar light irradiation over photocatalysts, which has become one of the most promising ways to produce clean energy due to the advantages of cleanness, low cost and high physicochemical stability, the equations are as follows^[22] [Image Omitted. See PDF][Image Omitted. See PDF]

The photocatalytic water splitting reactions are usually carried out in the presence of sacrificial agents to accelerate the reduction/oxidation process in the form of the rapid consumption of photogenerated holes/electrons. The generally used sacrificial agents are methanol, lactic acid, triethanolamine, and Na₂S–Na₂SO₃ for H₂ evolution, and silver nitrate, ferric chloride, and sodium iodate for O₂ evolution.^[6,23–25] Compared to H₂ evolution, the O₂ evolution is a four-electrons transferring process with high reaction barrier, which is the rate-determining step in OWS reaction.^[22] Considering the extra kinetic overpotential requirement in the actual reaction situation due to the characteristic of multielectron/proton processes of water oxidation, the bandgaps of the used photocatalysts should be generally no smaller than 1.6 eV to conquer the defect of sluggish kinetics, which can be ascribed to the multistep reactions of the existent active intermediates during the O₂ evolution process. The reaction process is distinct under the acidic or alkaline condition, and the equations are as follows^[26] [Image Omitted. See PDF][Image Omitted. See PDF]

According to the three basic stages of photocatalytic reaction, the efficiency of photocatalytic O₂ evolution is also mainly determined by light absorption, photogenerated charge separation, and surface catalytic reaction, which can be expressed as follows^[26] [Image Omitted. See PDF]

In order to evaluate the photocatalytic activity and physicochemical stability of O₂ producing photocatalyst, the most common method is to test the turnover frequency (TOF), turnover number (TON), and apparent quantum efficiency (AQE) of the reaction system, and the corresponding formulas are in the following^[27,28] [Image Omitted. See PDF][Image Omitted. See PDF][Image Omitted. See PDF]

In particular, the AQE is the ratio of the moles of generated O₂ per unit reaction time to the incident photons absorption number at a certain monochromatic wavelength, whereas the conversion efficiencies for O₂ evolution photocatalysts are distinct based on their different kinds of bandgaps (direct and indirect). Compared with indirect photocatalysts, the direct semiconductor materials have little change in the momentums during the migration process of photogenerated charge carriers after the absorption of incident photons, leading to the faster water oxidation rates than those of indirect ones.^[26–28]

Previously, the published reviews that are related to the photocatalytic O₂ evolution from water splitting mainly focused on some specific class of materials, such as BiVO₄, metal–organic frameworks (MOFs), cobalt complexes, etc.,^[29–31] or limited to the theoretical calculations, defects design from crystal structures, etc.^[32,33] So far, there is a lack of a systematic overview of the literatures on the photocatalytic O₂ evolution. Herein, we provide a review for the advances on the researches of photocatalytic O₂ evolution by starting with the fundamentals of photocatalysis and photocatalytic O₂ evolution, and then introduce various types of water oxidation photocatalysts based on different crystal structures, compositions, and morphologies. Then, we focus on the diverse strategies that improve the performance of photocatalytic O₂ evolution. Finally, the challenges and future prospects in this field are proposed. The summary contents are shown in Figure 1.

Figure 1. Schematic diagram of general contents in this review.

Photocatalysts for Oxygen Evolution

As O₂ evolution reaction is the rate-determining step during the process of photocatalytic OWS (schematic diagram shown in Figure 2), there are some critical requirements for these high efficiency water oxidation photocatalysts, including: i) the valence band (VB) potentials of the photocatalysts should be higher than those of the O₂/H₂O (1.23 eV); ii) to make more use of solar energy, bandgaps of the photocatalysts should be in the range of 1.23–3 eV; iii) the separation of the photogenerated electron–hole pairs within the photocatalysts should be fast under illumination, and then the corresponding redox reactions can occur in time on the surface of photocatalysts.^[34,35] In addition, the water oxidation active sites should be sufficient on the surface of the photocatalysts in order to restrain the rapid recombination of the photogenerated electrons and holes on the surface. Nowadays, the heterogeneous photocatalysts are still the most commonly researched photocatalysts.

Figure 2. Schematic diagram for photocatalytic water splitting.

TiO₂

Titanium dioxide (TiO₂) can be mainly divided into four types: TiO₂(B), brookite, anatase, and rutile, all of whose crystal structures are composed of TiO₆ octahedra (Figure 3a–d). As the bandgap of TiO₂ is 3.2 eV, the photogenerated charge carriers can only be excited by UV light with a wavelength of less than ≈390 nm.^[36] The most common form of TiO₂(B) is the layered titanate, and the brookite TiO₂ belonging to rhombic system has a low physicochemical stability. As anatase TiO₂ has more oxygen defects inside the crystal and greater band bending to capture the electrons and promote charge separation than the rutile,^[37] the photocatalytic activity of the anatase TiO₂ is usually better than that of the rutile phase.^[38] Commercial TiO₂ (P25) is composed of both anatase and rutile, which has a higher photocatalytic activity than the monocomponent one, due to the formation of energy barrier at the interface that improves the separation efficiency of the photogenerated charge carriers.^[39,40]

Figure 3. Crystal structures of a) TiO2(B), b) brookite, c) anatase, and d) rutile phases of TiO2. Reproduced with permission.[36] Copyright 2014, American Chemical Society. e) TEM image of P25 nanoparticles. Reproduced with permission.[41] Copyright 2015, Elsevier. f) HRTEM image of rutile TiO2 nanoparticles. Reproduced with permission.[43] Copyright 2015, Springer Nature. g) SEM image of TiO2 nanowires. Reproduced with permission.[44] Copyright 2008, Elsevier. h) XRD patterns of solid and mesoporous TiO2 nanorods. i,j) SEM images of solid and mesoporous TiO2 nanorods. Reproduced with permission.[45] Copyright 2013, American Chemical Society. k) SEM image, and l) HRTEM image and SAED pattern (inset) of black TiO2 nanotubes. Reproduced with permission.[46] Copyright 2014, Royal Society of Chemistry. m) TEM image of anatase TiO2 nanosheets. Reproduced with permission.[47] Copyright 2016, Elsevier. n) Synthesis schematic of mesoporous TiO2 flakes. o) XRD pattern of mesoporous TiO2 flakes. p,q) SEM images of rose petal and mesoporous TiO2 flakes. r) HRTEM image and SAED pattern (inset) of mesoporous TiO2 flakes. Reproduced with permission.[48] Copyright 2015, Royal Society of Chemistry.

The commonly used routes for synthesis of anatase and rutile TiO₂ are hydrothermal and calcination methods. In general, the corresponding precursors were first synthesized with some titanium-containing substances and organic solvents, and then the hydrothermal/solvothermal or calcination processes at different temperatures were utilized to form the anatase, rutile or mixed-phase TiO₂,^[41–43] which were confirmed by transmission electron microscopy (TEM) (Figure 3e,f). For instance, with increasing the temperature, the crystalline phase gradually transformed from the TiO₂(B) to anatase, and finally some of them were transformed into rutile, showing the 1D nanowires morphology with photocatalytic activity higher than that of the P25 nanoparticles due to the large specific surface area (Figure 3g).^[44] In addition to the conventional preparation methods, researchers have also tried to combine the traditional routes with some additional conditions to synthesize TiO₂ with novel microstructures. For example, the porous rutile TiO₂ nanorods were obtained with the synergistic silica as template under the hydrothermal environment, indicating that the crystal growth can be guided under external interference (Figure 3h–j). This 3D porous structure has a larger specific surface area and more exposed active crystal planes than the counterparts with traditional morphology, thus making TiO₂ more catalytically active.^[45] 1D black TiO₂ nanotubes can be obtained through the annealing followed by aluminum reduction treatment, which can produce abundant oxygen vacancies (OVs) on the surface (Figure 3k,l).^[46] Besides, different dimensions of TiO₂ can be obtained by the addition of diverse acids under the hydrothermal process, such as the 1D nanorods (by adding hydrochloric acid), 2D nanosheets (by adding hydrofluoric acid) (Figure 3m), etc.^[47] Similarly, the 2D anatase TiO₂ flakes with microporous arrays were prepared after preliminary heat treatment and calcination with the rose petals as a natural mesoporous template, rendering TiO₂ more efficient light absorption and reactive sites, which is known as the petal effect (Figure 3n–r).^[48]

Compared with anatase TiO₂, the rutile TiO₂ has a larger potential in the water oxidation due to the physicochemical characteristics.^[49] In order to find out the relationship between O₂ generation capability and physicochemical properties of rutile TiO₂, a series of TiO₂ were synthesized by calcination at temperature gradients. It was found that the crystallinity of the samples got better as the calcination temperatures increased (Figure 4g), as confirmed by the scanning electron microscopy (SEM) images that the samples changed from the original amorphous form to the regular grains (Figure 4a–f). The TiO₂ sample calcined at 1073 K (R-1073) showed the highest O₂ evolution rate (44.1 µmol h⁻¹ in the FeCl₃ solution; Figure 4i). The authors speculated that the water oxidation performance of the samples is related to not only the sizes and specific surface areas, but also the OVs. To verify this conjecture, the R-1073 sample was calcined in the hydrogen (H₂) atmosphere. There were no significant changes in the crystallinity after treatment, whereas the absorption intensity in the visible region of the H₂-treated R-1073 was slightly higher than before, which may be due to the increase of the OVs concentration in the sample (Figure 4h). Regardless of NaIO₃ or FeCl₃ served as the sacrificial agent, the O₂ evolution performance of R-1073 after calcination under H₂ was greatly improved in comparison with the noncalcined one.^[50]

Figure 4. a–f) SEM images of samples calcined at varieties of temperatures. g) XRD patterns of samples calcined at varieties of temperatures. h) DRS of H2-treated/untreated R-1073. i) Time courses of oxidation evolution for untreated R-1073. Reproduced with permission.[50] Copyright 2014, American Chemical Society.

BiVO₄

In recent years, the n-type semiconductor bismuth vanadate (BiVO₄) has attracted wide attention for photocatalytic water oxidation due to the moderate bandgaps (2.3–2.5 eV), suitable VB position, high photochemical stability, and low cost.^[51]

The typical crystalline phase of BiVO₄ is monoclinic scheelite, of which the crystal structure is composed of the distorted VO₄ tetrahedron and BiO₈ dodecahedron, as shown in Figure 5a–c.^[52] Density functional theory (DFT) calculation results demonstrated that BiVO₄ is a direct bandgap semiconductor with a bandgap of ≈2.2 eV (Figure 5d,f). It has a higher hole transferring rate than other water oxidation semiconductors, such as In₂O₃,^[52] as verified by the density of states (DOS) (Figure 5e).^[53] Fascinatingly, the large polaron can be formed under the polaronic state from holes within BiVO₄, which contributes to the stability of the monoclinic scheelite BiVO₄ (Figure 5g,h).^[54]

Figure 5. a) The crystal structure, and b) corresponding vertical view and c) side view of BiVO4. d) The band structure and f) corresponding brillouin zone of BiVO4. Reproduced with permission.[52] Copyright 2011, Royal Society of Chemistry. e) The total and projected density of states of BiVO4. Reproduced with permission.[53] Copyright 2015, Royal Society of Chemistry. g) The bond lengths of BiO bonds of BiO8 dodecahedra from h) the density of states of nonpolaronic hole states for BiVO4. Reproduced with permission.[54] Copyright 2013, American Physical Society.

BiVO₄ generally has a decahedral shape with smooth surfaces and sharp edges when prepared by hydrothermal reaction with NH₄VO₃ and Bi(NO₃)₃ as the raw materials, and the diameters of these as-prepared BiVO₄ nanoparticles varied considerably depending on the different synthetic conditions (Figure 6a,b).^[55,56] The lattice spacing of 0.308 nm was attributed to the {121} facets of BiVO₄ (Figure 6c,d).^[56] Introduction of a certain amount of chelating agent to the hydrothermal environment usually produced BiVO₄ with the pure monoclinic phase and a starlike shape (Figure 6e,f).^[57] Furthermore, the crystalline phases and photoabsorption of BiVO₄ also varied as the molar ratios of Bi(NO₃)₃ and NH₄VO₃ changed (Figure 6g,h), leading to modulated photocatalytic properties of BiVO₄.^[58]

Figure 6. a) SEM image and corresponding diagram (inset) of BiVO4. Reproduced with permission.[55] Copyright 2018, Royal Society of Chemistry. b) TEM image and size distributions (inset) of BiVO4 nanoparticles. c) HRTEM image and d) lattice spacing of BiVO4 nanoparticles. Reproduced with permission.[56] Copyright 2015, Wiley. e) XRD pattern and f) TEM image of starlike BiVO4. Reproduced with permission.[57] Copyright 2009, American Chemical Society. g) XRD patterns and h) DRS of BiVO4 with Bi/V molar ratios of 1/1, 2/1, 4/1, and 6/1. Reproduced with permission.[58] Copyright 2015, Royal Society of Chemistry. i) SEM images of single BiVO4 crystals, and BiVO4 deposited by j) Au, k) Pt, l) MnOx, m) Au/MnOx, and n) Pt/MnOx; the scale bar is 500 nm. Reproduced with permission.[61] Copyright 2013, Springer Nature. o) SEM image and p) corresponding diagram of 30-faceted BiVO4. q) Time courses of oxidation evolution for decahedron, truncated decahedron, and 30-faceted BiVO4. Reproduced with permission.[62] Copyright 2017, Wiley.

Monoclinic BiVO₄ shows a large potential for water oxidation.^[59,60] The active sites of photocatalytic reduction and oxidation reactions of the BiVO₄ decahedron are closely related to the exposing crystal facets, as revealed by photodeposition of metal/metal oxides.^[53] As shown in Figure 6i–l, the BiVO₄ crystals presented a typical regular decahedron structure, and the front and side facets were {010} and {110} facets, respectively. Under illumination, Au and Pt particles were selectively deposited on the {010} facets, while MnO_x was more prone to be deposited on the {110} facets, indicating that the reduction and oxidation active sites were located on the {010} and {110} facets, respectively. The following deposition of Au/MnO_x and Pt/MnO_x was also consistent with the above conclusions (Figure 6m,n).^[61] As the exposing facets determine the photocatalytic activity, BiVO₄ with multiple exposing facets was studied. The BiVO₄ crystals with 30 facets were obtained by a decahedral-etching engineering under the participation of gold nanoparticles with the controllable concentrations (Figure 6o,p). In addition to the {010} and {110} facets, some uncommon ones, such as {121}, {321}, and {132} facets had also been exposed, offering more possibilities for the photocatalytic activity enhancement of BiVO₄. The BiVO₄ with 30 facets showed a higher photocatalytic O₂ evolution rate, 3–5 times that of decahedron BiVO₄. The AQE reached 18.3% at 430 nm (Figure 6q).^[62] Besides exposing facets, the effect of morphology on the photocatalytic activity of BiVO₄ is another hot topic. By adjusting the pH value of reaction solution, BiVO₄ with the distinct morphologies were obtained. Under the acidic condition, BiVO₄ showed a regular decahedron structure, whereas leaf-like products were obtained in the alkaline media. Owing to the different crystal sizes and bandgaps, these BiVO₄ crystals synthesized at different pH showed different photocatalytic O₂ evolution performance, and the BiVO₄ decahedron exhibited the highest catalytic activity.^[63]

WO₃

Tungsten trioxide (WO₃) has been widely used for photo(electro)catalytic water oxidation on account of its suitable bandgap (2.5–2.8 eV), positive VB, and high physicochemical stability.^[64,65]

The crystal structure of monoclinic WO₃ is composed of the distorted WO₆ octahedrons (Figure 7a), in which the marked A site is a good position for doping based on the perovskite-like structure of WO₃.^[66] The top of VB maximum is mainly dominated by the O 2p states (Figure 7e), which can be divided into three parts according to the energy size: low energy region (−7.5 to −6 eV), medium energy region (−6 to −2 eV), and high energy region (−2 to 0 eV) (Figure 7b–d), while the bottom of conduction band (CB) minimum is mainly occupied by the W 5d states (Figure 7e).^[67] It can also be inferred from the DOS of WO₃ that doping or defects with metal or nonmetal atoms is one of the effective ways to improve its photocatalytic performance.

Figure 7. a) The monoclinic structure of WO3; blue and red spheres represent W and O atoms, respectively. Reproduced with permission.[66] Copyright 2008, American Physical Society. b–d) The electron charge density of WO3 from the upper VB. e) The density of states of WO3, the zero of energy, and the CB minimum are indicated by the vertical solid line and vertical dashed line, respectively. Reproduced with permission.[67] Copyright 2012, American Chemical Society.

Morphology regulation is the mostly involved among the researches of WO₃ photocatalysts. 1D WO₃ nanorod arrays on fluorine-doped tin oxide (FTO) were obtained by the hydrothermal-calcination method at different temperatures. When the calcination temperature increased to 450 °C, the phase of as-synthesized 1D WO₃ nanorod arrays gradually transformed from orthorhombic to monoclinic phase (Figure 8a), as demonstrated by the emergence of (002), (020), and (200) peaks. After calcination at 500 °C, the monoclinic WO₃ nanorod arrays with the exposed {202} facets were uniformly covered on the FTO substrate (Figure 8b).^[68] Similar to this convenient method, the 1D WO₃ nanowire arrays can be also successfully prepared on the FTO substrate under a relatively high temperature (≈550 °C; Figure 8c,d).^[69] Compared to 1D WO₃, 3D WO₃ can promote the separation of photogenerated carriers more effectively. Using the wire mesh as a template, the 3D WO₃ nanosheets with a thickness of 200 nm were uniformly grown on this scaffold (Figure 8e–g). The exposed {002} facets indicated the successful synthesis of monoclinic phase (Figure 8h).^[70] In addition to nanosheets, the monoclinic WO₃ can be deposited on FTO substrate in the form of nanofilms, enriching the 3D WO₃ materials (Figure 8i,j).^[71] Due to the structure characteristic of WO₃ composed of WO₆ perovskite units, OVs are easier to be introduced into its lattice to obtain the defective 2D nanosheets through calcination in air, which improved the photo(electro)catalytic performance (Figure 8k–p).^[72]

Figure 8. a) XRD patterns of WO3 nanorod arrays with heat treatment of different temperatures. b) SEM image of WO3 nanorod arrays with calcination treatment of 500 °C. Reproduced with permission.[68] Copyright 2016, Elsevier. c) SEM image and d) TEM image of WO3 nanowires. Reproduced with permission.[69] Copyright 2014, Royal Society of Chemistry. e–g) SEM images of wire mesh and corresponding WO3 nanosheet arrays grown on it. h) TEM and HRTEM images (inset) of WO3 nanosheet arrays. Reproduced with permission.[70] Copyright 2014, Wiley. i,j) SEM images of WO3 nanofilms. Reproduced with permission.[71] Copyright 2014, Royal Society of Chemistry. k) TEM image, l) STEM image, and m) HAADF-STEM image of WO3 nanosheets with nondefect treatment. n) STEM image and o) HAADF-STEM image of WO3 nanosheets with defect treatment. p) Lattice diagram with OVs of as-synthesized WO3 nanosheets with defect treatment. Reproduced with permission.[72] Copyright 2016, American Chemical Society. q) Schematic diagram of the synthesis of H0.23WO3 crystal and monoclinic WO3 nanosheets from tungsten boride precursor. r) XRD patterns of tungsten boride precursor and WO3 crystals series. s,t) SEM images of H0.23WO3 crystals, u) quasi-cubic-like WO3 crystals, and v–x) monoclinic WO3 nanosheets. y) Crystal models of (002), (020), and (200) facets of monoclinic WO3. Reproduced with permission.[73] Copyright 2012, Royal Society of Chemistry.

The exposure ratios of the crystal facets have different effects on the photocatalytic O₂ evolution activity of WO₃. The monoclinic WO₃ cubes and WO₃ nanosheets with different exposing facets were obtained by hydrothermal/calcination treatment of the precursors (Figure 8q). The exposed facets of WO₃ cubes were mainly {002}, {020}, and {200} facets with exposing ratio of 1:1:1, while that of WO₃ nanosheets were mainly {002} facets (Figure 8r–y). The normalized O₂ evolution rate of the WO₃ cubes reached 5.9 µmol h⁻¹, which was more than eight times that of the WO₃ nanosheets (0.7 µmol h⁻¹). In contrast, the WO₃ nanosheets showed high activity for photoreduction of CO₂ to CH₄ (0.34 µmol h⁻¹ g⁻¹), indicating that the exposing facets have large impacts on the photocatalytic activity of monoclinic WO₃.^[73] Besides the facets, the particle size is another factor to influence the photocatalytic activity of WO₃. For instance, the O₂ evolution performance of as-synthesized WO₃ nanodots, nanoplates, and microcrystals was distinct from each other in the case of similar photoabsorption, among which the WO₃ nanodots had the highest O₂ production, implying the direct relationship between the particle sizes and photocatalytic activity of WO₃.^[74]

α-Fe₂O₃

According to the different elemental ratios and crystal structures, iron oxides can be divided into ferrous oxide (FeO), iron trioxide (Fe₂O₃), and ferroferric oxide (Fe₃O₄), among which the hematite Fe₂O₃ (α-Fe₂O₃) has a good performance in the photocatalytic field due to its narrow bandgap (1.9–2.2 eV), superior stability, and abundant natural resources compared with other forms of iron oxides.^[75,76]

From the perspective of crystal structure (Figure 9a,b), there are six equivalent crystalline directions in (001) plane in hematite, which are perpendicular to c-axis, based on the Cornell and Schwertmann theory. As the closely packed plane, the crystal growth rates of α-Fe₂O₃ along [100] direction and other five equivalent crystalline directions in (001) plane would be more slowly than that of [001] direction, making the crystal of α-Fe₂O₃ multifaceted.^[77,78] As seen from the band structure and DOS of α-Fe₂O₃ (Figure 9c,d), the CB minimum is mainly occupied by the Fe 3d orbitals, whereas the O 2p orbitals constitute the VB maximum. According to the electronic structure of α-Fe₂O₃, doping with IIIA elements (e.g., Al, Ga, In) in the lattice of α-Fe₂O₃ can promote the photocatalytic activity of α-Fe₂O₃ and have little effect on its bandgap and band edge energy at the same time, which can be regarded as a good strategy.^[79]

Figure 9. The crystal model of α-Fe2O3 viewed from a) [001] direction and b) [110] direction. Reproduced with permission.[77,78] Copyright 2010, Wiley and 2006, American Chemical Society. c) The band structure and d) partial density of states of α-Fe2O3. Reproduced with permission.[79] Copyright 2010, American Chemical Society.

As the O₂ evolution performance of α-Fe₂O₃ can be greatly influenced by the morphology and size, a large number of investigations on the microstructure regulation of α-Fe₂O₃ were conducted by using different synthetic techniques under various conditions. The 0D α-Fe₂O₃ quantum dots were obtained by the facile microwave-assisted reverse micelle route (Figure 10a,b). It was shown that the narrow size distribution of as-synthesized quantum dots was 2–5 nm, which endowed α-Fe₂O₃ with higher photocatalytic activity (Figure 10c).^[80] While 1D α-Fe₂O₃ nanorods (Figure 10d–f), nanotubes (Figure 10g) and nanofibers (Figure 10h–j) can be obtained by high-temperature calcination, deposition, combustion, and electrospinning.^[81–85] Compared to 0D or 1D α-Fe₂O₃, the most widely used strategy for synthesizing 2D and 3D α-Fe₂O₃ is the hydrothermal or solvothermal route. 2D hexagonal α-Fe₂O₃ nanosheets with a thickness of 15 nm were prepared with a facile solvothermal approach, with the lattice spacing of 0.25 nm for the exposed (110) plane (Figure 11a–c). When they are combined with g-C₃N₄ nanosheets to form a Z-scheme system, the OWS reaction with H₂ and O₂ evolution rate of 38.2 and 19.1 µmol h⁻¹ g⁻¹, respectively, were achieved.^[86] The truncated nano-octahedra structure of nanoparticles were obtained by hydrothermal method (Figure 11d,e), and the average size of these particles was about 800 nm. The uneven edges of these particles increased the exposure of crystal facets and specific surface areas, which were beneficial to the photocatalytic reactions.^[87] The 3D α-Fe₂O₃ are generally constructed by 0D or 2D α-Fe₂O₃. For instance, the solid/hollow α-Fe₂O₃ nanospheres were always assembled by the nanoparticles or nanosheets through hydrothermal process, leading to the unique hierarchical structure that allowed higher specific surface area and more catalytic active sites compared to nanoparticles or nanosheets (Figure 11f,g).^[88,89] With potassium ferricyanide as the precursor, α-Fe₂O₃ dendrites were synthesized by hydrothermal treatment at 180 °C for 12 h with trunk length of 6–7 µm and branches of 2–2.5 µm (Figure 11h). Under hydrothermal reaction at 180 °C for 12 h, porous α-Fe₂O₃ nanocubes with average edge length of 100 nm and various sizes of pores on their surface were fabricated. The hollow structure made α-Fe₂O₃ much easier to access reactants for accelerating photocatalytic reactions (Figure 11i,j).^[90]

Figure 10. a) TEM image, b) HRTEM image and FFT pattern (inset), and c) size distributions of α-Fe2O3 quantum dots. Reproduced with permission.[80] Copyright 2016, Royal Society of Chemistry. d) TEM image of α-Fe2O3 nanorods. Reproduced with permission.[81] Copyright 2010, American Chemical Society. e) SEM image and f) AFM image of α-Fe2O3 nanorods arranged on silicon substrates. Reproduced with permission.[82] Copyright 2009, Elsevier. g) SEM image and partial enlarged section (inset) of α-Fe2O3 nanotubes. Reproduced with permission.[83] Copyright 2010, Elsevier. h) Schematic diagram of electrostatic spinning. i) SEM image of α-Fe2O3 nanofibers. Reproduced with permission.[84] Copyright 2012, Royal Society of Chemistry. j) SEM image of α-Fe2O3 nanofibers. Reproduced with permission.[85] Copyright 2011, Springer Nature.

Figure 11. a) SEM image, and b,c) TEM and HRTEM images of α-Fe2O3 nanosheets. Reproduced with permission.[86] Copyright 2017, Wiley. d) XRD pattern and e) SEM image (structural representation inset) of α-Fe2O3 octahedral nanoparticles. Reproduced with permission.[87] Copyright 2012, Royal Society of Chemistry. f) SEM image of α-Fe2O3 spheres. Reproduced with permission.[88] Copyright 2012, Royal Society of Chemistry. g) SEM image of α-Fe2O3 hollow spheres. Reproduced with permission.[89] Copyright 2013, Royal Society of Chemistry. h) SEM image of α-Fe2O3. i) SEM image and j) dark-field TEM image of porous α-Fe2O3 nanocubes. Reproduced with permission.[90] Copyright 2013, Springer Nature.

The transition of dimensions of α-Fe₂O₃ can occur during the synthetic process of α-Fe₂O₃. As illustrated in Figure 12a, the directional assembly from 1D iron oxide hydroxide chloride nanorods to 3D porous α-Fe₂O₃ nanocages was realized by introducing metal ions (Ni²⁺) and surfactant (PVP) as the precursors. The α-Fe₂O₃ hollow nanocages were composed of nanorods with rough surfaces. The hematite phase was confirmed by the exposed crystal facets (Figure 12b–d). To study the transformation mechanism during the preparation process, the intermediate products synthesized at different times were observed. As shown in Figure 12e–g, the products gradually changed from nanorods to microspheres and then to hollow nanocages with increasing the reaction time, which may provide a new insight into the morphology regulation of α-Fe₂O₃.^[91] The water oxidation performance of α-Fe₂O₃ was also explored in terms of the morphology. The O₂ evolution rate of the α-Fe₂O₃ hollow nanospheres assembled from ultrathin nanosheets reached 70 µmol h⁻¹ g⁻¹, which was much higher than that of α-Fe₂O₃ nanorods (32 µmol h⁻¹ g⁻¹) and commercial Fe₂O₃ nanoparticles (14 µmol h⁻¹ g⁻¹) (Figure 12h,i), emphasizing the importance of the morphology regulation for the photocatalytic O₂ evolution performance of α-Fe₂O₃.^[92] similarly, the as-synthesized single crystalline nanospheres also demonstrated higher O₂ evolution rate than bulk crystals and irregular particles under either solar or visible light irradiation. Moreover, the O₂ evolution rates of these catalysts were found to be inversely proportional to the logarithm of the sizes (Figure 12j).^[93]

Figure 12. a) Schematic diagram of transition of 1D iron hydroxide nanorods to 3D α-Fe2O3 hollow nanocages. b,c) SEM images of α-Fe2O3 hollow nanocages. d) HRTEM image and SAED pattern (inset) of α-Fe2O3 hollow nanocages. SEM images of hydrothermally synthesized α-Fe2O3 hollow nanocages with different reaction times of e) 1 h, f) 3 h, and g) 5 h in 200 °C. Reproduced with permission.[91] Copyright 2012, Royal Society of Chemistry. h) TEM image of α-Fe2O3 hollow nanospheres. i) The O2 evolution rates of Fe2O3 nanoparticles, α-Fe2O3 nanorods, and α-Fe2O3 hollow nanospheres. Reproduced with permission.[92] Copyright 2013, Royal Society of Chemistry. j) Relationship between particle size and oxidation evolution rate for bulk crystals, irregular particles, and single crystalline nanospheres of α-Fe2O3. Reproduced with permission.[93] Copyright 2011, Royal Society of Chemistry.

Sillén–Aurivillius Perovskites

Sillén et al. found a series of compounds that were made up of the [M₂O₂]²⁺ metal oxide layers (M = Ca, Sr, Ba, Cd, Li, Na, Sb, Bi) separated by the halide layers and proposed the formula of X_n (n = 1, 2, 3) to represent the number of halide layers that separated the metal oxide layers, such as BiPbO₂X (X = Cl, Br, I) and BiOX (X = Cl, Br, I) are the X₁ and X₂ type of structure, respectively. Later, Aurivillius discovered in 1949 that some layered bismuth oxide compounds consist of the alternately stacked [Bi₂O₂]²⁺ units and perovskite layers, like Bi₂MoO₆, Bi₄Ti₃O₁₂, Bi₃TiNbO₉, etc., the general formula can be described as (Bi₂O₂)²⁺(A_n−1B_nO_3n+1)²⁻, where A and B represent the cations. In recent years, it has been discovered that some compounds possess both of the above two structural characteristics, and they are called Sillén–Aurivillius perovskites; the crystal structures can be expressed as [(Bi₂O₂)₂X]²⁺(A_n−1B_nO_3n+1)²⁻, where A and B represent the cations and X represents the halide anions. This type of material has recently been found to demonstrate excellent performance of photocatalytic O₂ evolution from water splitting.^[94–98]

In 2016, Ryu Abe's group first reported the photocatalytic activity for O₂ production over the Sillén–Aurivillius structured Bi₄NbO₈Cl under visible light, which has a layered crystal structure of the alternately stacked [NbO₄]⁶⁻ perovskite units and [(Bi₂O₂)₂Cl]⁶⁻ blocks. Different from some simple oxyhalides, such as BiOCl and BiOBr, the VB maximum of Bi₄NbO₈Cl is mainly occupied by the highly dispersive O 2p orbitals and is not prone to self-oxidation due to the stable oxygen anions, so it has great potential for the catalytic water oxidation (Figure 13a,b).^[99] In recent years, Abe's group has gained abundant experiences on the synthesis of Sillén–Aurivillius structured photocatalysts. Bi₄NbO₈Cl was traditionally synthesized by solid-state reaction (SSR), which led to the morphology of irregular chunks of the as-prepared samples (Figure 13c). Based on SSR experience, a certain proportion of alkali metal chlorides (CsCl and NaCl) were be added into the halogen precursors to make up for the halogen volatilization and to provide a solid solution environment (called as flux method). As a result, the Bi₄NbO₈Cl nanosheets with well-defined (00l) planes were obtained when calcination temperature reached above the melting points of CsCl and NaCl like 650 °C (Figure 13d,e,h), 700 °C (Figure 13f), or 800 °C (Figure 13g).^[100] When FeCl₃ serves as the sacrificial agent, the photocatalytic O₂ evolution performance of Bi₄NbO₈Cl is the best (Figure 13l).^[99] Importantly, the O₂ evolution rates of Bi₄NbO₈Cl synthesized by the flux method were much higher than those by SSR regardless of with or without the addition of cocatalysts (e.g., RuO₂, Pt) (Figure 13m).^[100] Inspiringly, the OWS reaction was realized when Bi₄NbO₈Cl was coupled with the H₂ evolution photocatalyst such as Rh-doped SrTiO₃ to construct the Z-scheme systems, and the system with Bi₄NbO₈Cl synthesized by flux method exhibited a higher photocatalytic activity than that prepared by SSR.

Figure 13. a) Crystal structure of Bi4NbO8Cl. b) Total and partial density of states of Bi4NbO8Cl. Reproduced with permission.[99] Copyright 2016, American Chemical Society. SEM images of Bi4NbO8Cl synthesized through c) solid-state reaction and d,f,g) flux method. e) TEM image and h) SAED pattern of Bi4NbO8Cl synthesized through flux method. Reproduced with permission.[100] Copyright 2019, American Chemical Society. SEM images of Bi4NbO8Cl synthesized through i) two-step polymerized complex method, j) solid-state reaction, and k) conventional single-step solid-state reaction. Reproduced with permission.[101] Copyright 2018, Royal Society of Chemistry. l) Time courses of oxidation evolution for Bi4NbO8Cl with different sacrifices. Reproduced with permission.[99] Copyright 2016, American Chemical Society. m) The O2 evolution rates of pristine Bi4NbO8Cl, RuO2-loaded Bi4NbO8Cl, and Pt-loaded Bi4NbO8Cl synthesized through flux method and solid-state reaction. Reproduced with permission.[100] Copyright 2019, American Chemical Society. n) Time courses of oxidation evolution for Bi4NbO8Cl synthesized through two-step polymerized complex method, solid-state reaction, and conventional single-step solid-state reaction. Reproduced with permission.[101] Copyright 2018, Royal Society of Chemistry. SEM images of o) Bi4TaO8Cl, p) Bi4TaO8Br, q) flux-treated Bi4TaO8Br, and r) flux-treated Bi4TaO8Cl. Total and partial density of states of s) Bi4TaO8Cl and t) Bi4TaO8Br. u) The O2 evolution rates of Bi4TaO8X and RuO2-loaded Bi4TaO8X. Reproduced with permission.[102] Copyright 2017, Wiley. v) The O2 evolution production of Bi4TaO8X (X = Cl, Br) and MoO3–Bi4TaO8X with Fe(NO3)3 and Ag(NO3)3 as the sacrificial agent, respectively. Reproduced with permission.[103] Copyright 2018, Wiley. w) Band structures of Bi4TaO8X (X = Cl, Br). x) Time courses of overall water splitting evolution for Bi4TaO8Br combined with Ru(0.1 wt%)/Rh-doped SrTiO3 with Fe3+/Fe2+ as redox cycle mediator. y) Schematic diagram of overall water splitting evolution for Bi4TaO8Br-based Z-scheme system in Fe3+/Fe2+ redox cycle mediator. Reproduced with permission.[102] Copyright 2017, Wiley.

The different morphologies of Bi₄NbO₈Cl were obtained by the two-step polymerized complex method (2PC), solid-state reaction (2SSR), and conventional single-step solid-state reaction (1SSR) at the calcination temperature of 973 K (Figure 13i–k), and Bi₄NbO₈Cl synthesized by 2PC route had much smaller size and larger specific surface area than the samples synthesized by 2SSR and 1SSR, implying that the morphology of Bi₄NbO₈Cl was associated with the synthetic routes. The O₂ evolution production of Bi₄NbO₈Cl prepared by 2PC was also higher than those of samples synthesized by 2SSR and 1SSR routes (Figure 13n). Naturally, the OWS activity of the 2PC synthesized Bi₄NbO₈Cl was also higher than that of 1SSR prepared sample when combined with Rh-doped SrTiO₃ with the existence of Fe³⁺/Fe²⁺ redox mediator, highlighting the status of O₂ evolution reaction that as the decisive step for the overall water splitting.^[101]

Besides Bi₄NbO₈Cl, other Sillén–Aurivillius perovskites with similar crystal structures were also synthesized, including Bi₄TaO₈Cl and Bi₄TaO₈Br. The DOS plots in Figure 13s,t stated that the O 2p orbitals were powerfully hybridized with Bi 6s orbitals, which is a common characteristic of the electronic structures of most bismuth-based materials. Similar to Bi₄NbO₈Cl, the morphologies of Bi₄TaO₈X (X = Cl, Br) were transformed from the original bulk particles to the regular nanosheets with the thickness of ≈100 nm when the flux method was adopted with NaCl and KCl as the flux agents (Figure 13o–r). The O₂ evolution rates of Bi₄TaO₈X treated with fluxes were higher than that without fluxes. The O₂ production activity of all the Bi₄TaO₈X was promoted after loading the cocatalyst, and flux-treated Bi₄TaO₈X showed larger improvement (Figure 13u).^[102] The interfacial modulation is considered to be an effective way to enhance the photocatalytic performance of photocatalysts. The O₂ evolution performances of Bi₄TaO₈X were substantially improved after being decorated with MoO₃ particles by the impregnation route (Figure 13v). After the introduction of MoO₃, the photogenerated electrons would migrate from the VB of Bi₄TaO₈X to the CB of MoO₃ due to their distinct Fermi levels, thus forming a built-in electric field to effectively separate the charge carriers. Taking into account that the bandgap positions of both of Bi₄TaO₈Cl and Bi₄TaO₈Br meet the thermodynamic requirements of photocatalytic water splitting reaction (Figure 13w), when Bi₄TaO₈Br was coupled with Rh-doped SrTiO₃ to form the Z-scheme system, the yield ratio of H₂ to O₂ was close to 2:1 and it also showed a high stability, which provides an new reference for OWS (Figure 13x,y).^[103]

To make up for the volatilization effect of halogen species, excess halogen precursors were added in the SSR process of Bi₄MO₈X (M = Nb, Ta; X = Cl, Br) to obtain the samples denoted as ex-Bi₄MO₈X (M = Nb, Ta; X = Cl, Br).^[100] As can be seen from Figure 14a–d, both Bi₄MO₈X and ex-Bi₄MO₈X were pure phase, and the addition of excess halogen precursors did not affect the crystal structure of Bi₄MO₈X. When calcination temperature increased to 1073 K, some small-sized particles appeared in the as-synthesized Bi₄MO₈X, which could be ascribed to the halogen species volatilization. Note that this phenomenon was effectively alleviated for ex-Bi₄MO₈X, indicating that excess halogen species compensated for the volatilization of halogen species (Figure 14e–l). With the increase of calcination temperature, the O₂ evolution rate of Bi₄MO₈X first increased and then decreased, reaching the maximum in 1073 K (Figure 14m), indicating that excessive anionic defects emerged at high temperature was not conducive to the O₂ evolution activity of Bi₄MO₈X. Obviously, the O₂ evolution rate of ex-Bi₄MO₈X was higher than Bi₄MO₈X at each calcination temperature, which was attributed to the less anionic defects in ex-Bi₄MO₈X than in Bi₄MO₈X.^[104]

Figure 14. a–d) XRD patterns of Bi4MO8X and ex-Bi4MO8X synthesized with varieties of calcination temperatures. e–l) SEM images of Bi4MO8X and ex-Bi4MO8X calcined at 1073 K. m) The O2 evolution rates of Bi4MO8X and ex-Bi4MO8X synthesized with varieties of calcination temperatures. Reproduced with permission.[104] Copyright 2018, Royal Society of Chemistry. Crystal models of n) single-layered perovskite oxyhalides Bi4MO8Cl and o) double-layered perovskites A4A′M2O11Cl. p) Schematic diagram of revised-lone-pair theory with interaction between O 2p orbitals and Bi/nearby anion/cation 6s and 6p orbitals. q) Bandgaps of double-layered perovskites A4A′M2O11Cl. Reproduced with permission.[105] Copyright 2019, American Chemical Society.

Lately, a new series of double-layered Sillén–Aurivillius perovskites (denoted as A₄A′M₂O₁₁Cl, A, A′ = Bi, Pb, Ba, and Sr; M = Ta, Nb, and Ti) were synthesized via the polymerized complex (PC) route by Abe's group (Figure 14n,o). The VB positions of the as-synthesized novel double-layered perovskites were generally more negative than those of some metallic oxides due to the interaction between O 2p orbitals and Bi/nearby anion/cation 6s and 6p orbitals (Figure 14p,q). Compared to the single-layered perovskites Bi₄MO₈X, the band levels of A₄A′M₂O₁₁Cl can be neatly tuned due to the distinct valences of cations in A₄A′M₂O₁₁Cl (A, A′, M) and their various combinations. The O₂ evolution activities of A₄A′M₂O₁₁Cl largely depended on the ratios of Cl/Bi on the surfaces, which were consistent with Bi₄MO₈X, and the best O₂ evolution performance was obtained by Ba₂Bi₃Nb₂O₁₁Cl (17.6 µmol h⁻¹). Besides, the Z-scheme system consisting of Ba₂Bi₃Nb₂O₁₁Cl and Rh-doped SrTiO₃ with Fe³⁺/Fe²⁺ revealed an appreciable OWS AQE of 0.7% at 420 nm, comparable to the oxygen evolution activity of bare Bi₄TaO₈Cl.^[105]

Other Photocatalysts for Oxygen Evolution

H₂WO₄ belongs to the orthorhombic phase and its crystal structure consists of WO₅(H₂O) octahedral units, which has a wide absorption region for visible light. It was synthesized by the dehydration of H₄WO₅, generating products composed of aggregated plate-like particles with the sizes of 50–500 nm (Figure 15a). The O₂ evolution performance of H₂WO₄ was the best when using Fe(NO₃)₃ as the sacrificial agent, whereas Fe₂(SO₄)₃ was not conducive to the O₂ evolution production, which was attributed to the fact that it was more easy for Fe³⁺ to form the cation complex with SO₄²⁻ (Figure 15b). The O₂ evolution rate of H₂WO₄ was directly proportional to its specific surface area, demonstrating that the larger specific surface area favors the reduction of Fe³⁺ (Figure 15c). Similar to the above perovskites, H₂WO₄ can also achieve OWS reaction when combined with Rh-doped SrTiO₃, and the evolution activity of H₂ and O₂ was closely related to the solution environment, e.g., the gas yield in the Fe(ClO₄)₃ solution was more higher than that in distilled water (Figure 15v).^[106]

Figure 15. a) SEM image of H2WO4. b) Time courses of oxidation evolution for H2WO4 with different sacrifices. c) Specific surface areas and oxidation evolution rates of a series of H2WO4 samples. Reproduced with permission.[106] Copyright 2017, Royal Society of Chemistry. SEM images of d) KCa2Nb3O10 and e) H+/KCa2Nb3O10, and f) SEM image and g) TEM image of ex-Ca2Nb3O10/K+. h) Time courses of oxidation evolution for KCa2Nb3O10, ex-Ca2Nb3O10/K+, and RuO2-loaded samples of them. i) Time courses of oxidation evolution for ex-Ca2Nb3O10/K+ loaded with varieties of cocatalysts. Reproduced with permission.[107] Copyright 2015, Royal Society of Chemistry. j–q) Digital pictures and r,s) DRS of M3ReO8. t) Total and partial density of states of Y3ReO8. u) Amount of oxidation evolution for M3ReO8 in 10 h. Reproduced with permission.[108] Copyright 2017, Royal Society of Chemistry. v) Time courses of overall water splitting evolution for H2WO4 combined with Ru(0.7 wt%)/Rh-doped SrTiO3 in i) Fe(ClO4)3 aqueous solution and ii) distilled water. Reproduced with permission.[106] Copyright 2017, Royal Society of Chemistry. Time courses of overall water splitting evolution for w) PtO(0.5 wt%)/KCa2Nb3O10 or x) PtO(0.5 wt%)/ex-Ca2Nb3O10/K+ combined with Pt(0.5 wt%)/Rh-doped SrTiO3 in KI aqueous solution. y) Time courses of overall water splitting evolution for ex-Ca2Nb3O10/K+/H+ combined with Ru(0.7 wt%)/Rh-doped SrTiO3 in Fe(ClO4)2 aqueous solution. Reproduced with permission.[107] Copyright 2015, Royal Society of Chemistry.

As one kind of cation-exchangeable layered metal oxides, KCa₂Nb₃O₁₀ can be easily intercalated by small molecules, which is beneficial for water oxidation. After exfoliating the H⁺/KCa₂Nb₃O₁₀ (K⁺ of KCa₂Nb₃O₁₀ was replaced by H⁺) into nanosheets (denoted as ex-Ca₂Nb₃O₁₀/K⁺) by continuous stirring, the specific surface area was greatly improved compared to that of KCa₂Nb₃O₁₀ (Figure 15d–g). With NaIO₃ as sacrificial agent, the O₂ evolution performance of the exfoliated samples was higher than that of KCa₂Nb₃O₁₀ with or without the cocatalyst RuO₂ (Figure 15h), owing to the more exposed active sites of ex-Ca₂Nb₃O₁₀/K⁺. Some other cocatalysts, such as IrO₂, can also promote the O₂ evolution of ex-Ca₂Nb₃O₁₀/K⁺, and PtO was the most effective one (Figure 15i). The constructed Z-scheme system of ex-Ca₂Nb₃O₁₀/K⁺–SrTiO₃ exhibited a more efficient OWS reaction with higher H₂ and O₂ yields than that of KCa₂Nb₃O₁₀–SrTiO₃ in the KI aqueous solution, owing to the lower O₂ evolution activity and selectivity for KCa₂Nb₃O₁₀ (Figure 15w,x). Moreover, when RuO₂ and Fe(ClO₄)₂ were selected as the cocatalyst and reaction solution for this Z-scheme system, respectively, both H₂ and O₂ productions were greatly improved (Figure 15x,y).^[107]

The rare-earth rhenates M₃ReO₈ (M = Y, La, Nd, Sm, Eu, Gd, Dy, and Yb) were obtained by the SSR route (Figure 15j–q). Compared to the corresponding oxides Re₂O₇, M₃ReO₈ had more intense photoabsorption in the visible light region (Figure 15r,s). Taking Y₃ReO₈ as the representative, the DOS revealed that the CB minimum of Y₃ReO₈ was mainly occupied by Re 5d and O 2p orbitals, while the O 2p orbitals nearly took up the VB maximum of Y₃ReO₈ (Figure 15t). Based on the previous findings, the photogenerated carries were easier to be recombined in the band, which was occupied by the R 4f orbitals of RVO₄ compounds (R = Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb), thus Gd₃ReO₈ and Y₃ReO₈ exhibited the higher O₂ production than other M₃ReO₈ samples (Figure 15u).^[108]

In the homogenous system, the onset potentials of some ruthenium-based compounds (Figure 16a) were lower than those of Ru^3+/2+ of some normally used photosensitizers, such as polypyridyl ruthenium compounds [Ru(bpy)₃]²⁺ (R₁ = COOEt, R₂ = H) (Figure 16b), so the water oxidation reactions can be driven by [Ru(bpy)₃]²⁺. In a three-components photocatalytic reaction system composed of the photocatalyst, photosensitizer, and sacrificial agent (Na₂S₂O₈) (Figure 16c), the No.1 compound in Figure 16a demonstrated the best O₂ evolution performance (TOF = 20 min⁻¹) with the quantum efficiency of 17.1% at 473 nm (Figure 16d),^[109] and the reaction equations are as follows [Image Omitted. See PDF][Image Omitted. See PDF]

Figure 16. Chemical formulas of a) ruthenium-based photocatalysts and b) polypyridyl ruthenium photosensitizer [Ru(bpy)3]2+. c) Schematic diagram of three-component photocatalytic reaction system. d) Time courses of oxidation evolution and TON for No.1 to No.4 ruthenium-based photocatalysts from (a). Reproduced with permission.[109] Copyright 2013, Elsevier. e) Chemical formulas of cobalt-based porphyrins photocatalysts. f) Time courses of oxidation evolution for three kinds of cobalt-based porphyrins from (e). Time courses of g) oxidation evolution and h) TOF for COTPPS in different pH values. i) The O2 evolution rates as a function of concentrations of COTPPS. Reproduced with permission.[110] Copyright 2013, Royal Society of Chemistry.

Since some ruthenium-based compounds have the good water oxidation performance under the assistance of the photosensitizer, some researchers then focused on the specific cobalt-based porphyrins in the same three-components reaction system (Figure 16e). The as-synthesized cobalt(II) tetrakis(p-sulfonatophenyl)porphyrin (COTPPS) showed the highest O₂ evolution production with the largest TOF of 0.17 s⁻¹, when the pH of the reaction solution was 11.0 and [Ru^II(bpy)₃](NO₃)₂ and Na₂S₂O₈ served as the photosensitizer and sacrificial agent, respectively (Figure 16f–h). Additionally, the O₂ evolution rate exhibited a linear relationship with the concentration of COTPPS (Figure 16i). The corresponding reaction equations are as follows^[110] [Image Omitted. See PDF][Image Omitted. See PDF][Image Omitted. See PDF]

As a kind of organic–inorganic hybrid material, MOFs are generally constructed by the self-assembly of organic ligands and metal ions/clusters through coordination bonds, featuring the advantages of low density, high porosity, and large specific surface area.^[111,112] In recent years, people have stepped up the researches of water oxidation of MOFs. Cu₃PO₄(C₂N₃H₂)₂OH has a monoclinic crystal structure, in which the copper-polyhedral layers and copper-triazole layers were alternately stacked along [001] direction to form a 3D structure (Figure 17a,b). The more positive VB potential than O₂/H₂O endowed Cu₃PO₄(C₂N₃H₂)₂OH with the potential of superior water oxidation (Figure 17c). As seen from Figure 17d, the O₂ evolution performance of Cu₃PO₄(C₂N₃H₂)₂OH reached ≈4.2 mL after 1 h photoirradiation with the addition of sulfite as sacrificial agent.^[113]

Figure 17. a) XRD pattern of Cu3PO4(C2N3H2)2OH. b) Crystal structure of Cu3PO4(C2N3H2)2OH along [001] direction. c) Schematic diagram of O2 evolution of Cu3PO4(C2N3H2)2OH. d) Time courses of oxidation evolution for Cu3PO4(C2N3H2)2OH with/without sacrifices. Reproduced with permission.[113] Copyright 2014, Elsevier. e) XRD pattern of Bi-mna. f) Time courses of oxidation evolution for Bi-mna. g) Electron localization function plots and h) total and partial density of states of Bi-mna. i) Time-resolved fluorescence decay spectra of Bi-mna and H2mna. Reproduced with permission.[114] Copyright 2014, Wiley.

As introduced above, the bismuth-based photocatalysts have become a series of potential materials for water oxidation due to the abundant resources and environment-friendly feature. Bi³⁺ ions easily coordinate with the organic ligands, so the Bi-based MOFs are ideal candidates worth to be explored. For instance, a orthorhombic bismuth-based metal–organic framework (denoted as Bi-mna) was obtained by solvothermal method with Bi(NO₃)₃·5H₂O and 2-mercaptonicotinic acid as the precursors (Figure 17e). The O₂ evolution rate reached 216 µL h⁻¹ with AgNO₃ as the sacrificial agent (Figure 17f). The calculated result of the electron localization function in Figure 17g indicated that the Bi atoms allowed the increase of delocalization of electrons in the covalent bond, which facilitates the migration of charge carriers. The Fukui functions showed that the photogenerated electrons were transferred from S atoms to pyridine rings with Bi atoms as the bridge, and this conclusion was also confirmed by the DOS of Bi-mna (Figure 17h). Furthermore, the measured time-resolved fluorescence decay spectra revealed that the average lifetime of Bi-mna (1.1 ns) was much longer than H₂mna (100 ps), indicating that Bi-mna was able to more efficiently separate photogenerated charge carriers (Figure 17i).^[114] The O₂ evolution production performance of non-strategic photocatalysts under distinct reaction conditions is shown in Table 1.

Table 1 The O₂ evolution production performance of photocatalysts under different conditions

Photocatalyst	Sacrificial agent	Amount of sacrificial agent	Light source	Incident light [nm]	Oxygen production rate	AQE	Refs.
TiO2	FeCl3	10 × 10−3 m	300 W Xe	≥350	44.1 µmol h−1	–	[50]
BiVO4	NaIO3	20 × 10−3 m	300 W Xe	≥420	57 µmol h−1	18.3% at 430 nm	[62]
BiVO4	AgNO3	50 × 10−3 m	Sunlight	–	82.8 µmol h−1 g−1	–	[63]
BiVO4	AgNO3	50 × 10−3 m	300 W Xe	≥420	310 µmol h−1	–	[115]
BiVO4	H3PMo12O40	10 × 10−3 m	300 W Xe	≥400	1.03 µmol h−1	–	[116]
BiVO4	K2S2O8	11 mg	Sunlight	–	≈4.32 µmol h−1	–	[117]
BiVO4	AgNO3	50 × 10−3 m	300 W Xe	≥420	≈60 µmol h−1	–	[118]
BiVO4	AgNO3	10 × 10−3 m	300 W Xe	≥400	533 µmol h−1 g−1	–	[119]
WO3	AgNO3	850 mg	300 W Xe	≥400	5.9 µmol h−1	–	[73]
WO3	NaIO4	10 × 10−3 m	300 W Xe	≥400	31.6 µmol h−1	–	[74]
WO3	FeCl3	8 × 10−3 m	300 W Xe	≥400	≈10 µmol h−1	2.8% at 420 nm	[120]
WO3	KIO3	2.5 × 10−3 m	300 W Tu	≥420	1256.3 µmol h−1	–	[121]
Fe2O3	Na2S2O8	60 mg	300 W Xe	≥420	70 µmol h−1 g−1	–	[92]
Fe2O3	AgNO3	20 × 10−3 m	300 W Xe	≥400	1071 µmol h−1 g−1	0.61% at 375 nm	[93]
Bi4NbO8Cl	FeCl3	5 × 10−3 m	300 W Xe	≥400	≈4.2 µmol h−1	0.4% at 420 nm	[99]
Bi4NbO8Cl	FeCl3	5 × 10−3 m	300 W Xe	≥400	≈2.55 µmol h−1	–	[122]
Bi4TaO8Cl	FeCl3	5 × 10−3 m	300 W Xe	≥400	≈16 µmol h−1	0.9% at 420 nm	[101]
Bi4TaO8Br	AgNO3	5 × 10−3 m	300 W Xe	≥420	≈37.5 µmol h−1	22.3% at 420 nm	[102]
Bi4TaO8Cl	AgNO3	5 × 10−3 m	300 W Xe	≥400	≈68 µmol h−1	2.3% at 420 nm	[104]
Ba2Bi3Nb2O11Cl	FeCl3	5 × 10−3 m	300 W Xe	≥400	17.6 µmol h−1	0.7% at 420 nm	[105]
PbBiO2Cl	Fe(NO3)3	5 × 10−3 m	300 W Xe	≥400	≈2.28 µmol h−1	0.9% at 400 nm	[123]
Bi6NbWO14Cl	AgNO3	5 × 10−3 m	300 W Xe	≥400	≈5.08 µmol h−1	1.4% at 420 nm	[124]
H2WO4	Fe(NO3)3	5 × 10−3 m	300 W Xe	≥400	≈6.83 µmol h−1	–	[106]
Gd3ReO8	AgNO3	10 × 10−3 m	300 W Xe	≥400	≈0.45 µmol h−1	–	[108]
[Ru(bda)L2]a)	Na2S2O8	10 × 10−3 m	300 W Xe	≥400	≈50 µmol h−1	17.1% at 473 nm	[109]
COTPPS	Na2S2O8	5 × 10−3 m	300 W Xe	≥400	≈24.4 µmol h−1	–	[110]
Cu3PO4(C2N3H2)2OH	S2O32−	0.01 × 10−3 m	200 W Tu	≥400	4 mL h−1	0.13% at 420 nm	[113]
Bi-mnab)	AgNO3	100 mg	300 W Xe	≥420	216 µL h−1	–	[114]
CoFPS	Na2S2O8	1 × 10−3 m	300 W Xe	≥400	0.05 µmol s−1	–	[125]

a)H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid; L = N-cyclic aromatic ligands b)mna = 2-mercaptonicotinic acid. Strategies for Enhancing Photocatalytic Oxygen Evolution

The challenges of photocatalytic O₂ evolution with the participation of sacrificial agents are mainly from the inferior kinetics of the four-electron migration of water oxidation and intrinsic drawbacks of photocatalysts, such as the low utilization of visible light, easy recombination of photogenerated carriers, and self-oxidation poisoning from photogenerated holes.^[126,127] To overcome the disadvantages originating from the above processes, the researchers have put forward various strategies, including cocatalysts loading, heterojunction fabrication, doping and defects formation, and others strategies, such as formation of special microstructure, surface modification, and solid solution construction.

Cocatalysts Loading

In the photocatalytic water splitting process, oxidation reaction requires more rigorous thermodynamic and kinetic conditions. As a small amount of cocatalysts can provide the active sites and trap the photogenerated charge carriers, they can reduce the activation energy and enhance the photocatalytic performance.^[128] Therefore, the strategy of loading suitable cocatalyst on the surface of semiconductor is necessary. For the O₂ evolution reaction, the cocatalysts can be divided into the noble metals (e.g., Ag, Ru, Rh, Ir), noble metal oxides (e.g., RuO₂, RhO₂, IrO₂), and transition metal oxides (TMOs) (e.g., MnO₂, Co₃O₄).^[129,130]

Pt is the mostly used noble metal to decrease the overpotential and to promote the charge separation by collecting electrons. For example, when the loading amount of Pt was 0.5 wt% at 823 K by the impregnation method, the uniformly dispersed Pt species were observed clearly on the surface of WO₃ (Figure 18a), and the as-synthesized PtO_x/WO₃ exhibited the highest O₂ evolution activity (Figure 18b,c). The secondary loading of metal oxides (CoO_x, MnO_x, RuO₂, IrO₂) further boosted the O₂ evolution performance of PtO_x/WO₃, and the most effective one was RuO₂ with a O₂ evolution rate of 41 µmol h⁻¹ (Figure 18d).^[131] Au nanoparticles with diameters of ≈2 and ≈11 nm were in situ loaded on the surface of α-Fe₂O₃ particles by photodeposition (Figure 18e–g). The α-Fe₂O₃ loaded with larger size Au species showed higher O₂ evolution production, due to the stronger surface plasmon resonance (SPR) effect (Figure 18h).^[132] As a typical nitrogen oxide, TaON has attracted widespread attention in water oxidation by virtue of the suitable band position and intense visible light absorption.^[133,134] To explore the influence of cocatalyst on the O₂ evolution performance of TaON, Ru-loaded TaON was synthesized by the impregnation method. It was found that the O₂ evolution production of TaON reached the maximum when the calcination temperature and the loading amount of Ru was 623 K and 0.5 wt%, respectively (Figure 18i,j). SEM images revealed that the Ru precursor was mostly transformed into RuO₂ with grain size of ≈30 nm when the calcination temperature was 623 K (Figure 18k), while the agglomeration of RuO₂ appeared upon higher calcination temperature (Figure 18l), which led to the decreased O₂ evolution activity of TaON. When Pt/ZrO₂/TaON and RuO₂/TaON were separately used as the reduction and oxidation catalysts to form a Z-scheme system, the yield of H₂ and O₂ was close to 2:1, achieving the OWS in NaI aqueous solution.^[135] On the basis of aforementioned research work, the O₂ evolution activity of TaON loaded with other metal cocatalysts (Rh, Co, Ir) by the impregnation route was also explored. As shown in Figure 18m–o, the most effective cocatalyst for the O₂ evolution of TaON was Rh species, and the rate reached ≈43 µmol h⁻¹ when the calcination temperature was 500 °C. In addition, the O₂ evolution rate of TaON loaded with two cocatalysts was higher than those of solely loaded samples, demonstrating the superiority of dual-loading of cocatalysts (Figure 18p). Besides, the Z-scheme system consisting of Rh/Ru/TaON and Pt/ZrO₂/TaON showed higher photocatalytic efficiency than that of solely loaded TaON.^[136]

Figure 18. a) TEM image of WO3 loaded with 0.5 wt% Pt. b) Time courses of oxidation evolution for WO3 as a function of loading amount of Pt. c) Time courses of oxidation evolution for WO3 as a function of calcination temperatures. d) The O2 evolution rates of pristine Pt–WO3 and Pt–WO3 loaded with various kinds of second oxidative cocatalysts. Reproduced with permission.[131] Copyright 2012, Royal Society of Chemistry. e) TEM image of pristine α-Fe2O3 nanoparticles. TEM images of f) 2 nm (size distributions inset) and g) 11 nm Au-loaded α-Fe2O3. h) Time courses of oxidation evolution for pristine α-Fe2O3 and Au-loaded α-Fe2O3. Reproduced with permission.[132] Copyright 2012, Royal Society of Chemistry. i) Time courses of oxidation evolution for TaON loaded with 0.5 wt% Ru at varieties of calcination temperatures. j) Time courses of oxidation evolution for TaON with various loading amounts of Ru. SEM images of TaON calcined at k) 623 K and l) 723 K, respectively. Reproduced with permission.[135] Copyright 2011, American Chemical Society. m) The O2 evolution rates of TaON loaded with Rh, Co, and Ir as a function of calcination temperatures. TEM images of TaON n) loaded with 1 wt% Rh and o) pure sample. p) Time courses of oxidation evolution for TaON loaded/coloaded with Rh, Ru, and Co at varieties of calcination temperatures. Reproduced with permission.[136] Copyright 2019, Royal Society of Chemistry.

Considering the rare resources and high cost of noble metals, some common transition metals have been employed as cocatalysts to replace the noble metals in recent years. As shown in Figure 19a–c, Co₃O₄ nanoparticles with diameters of 3, 10, and 40 nm were fabricated, in which the 3 nm Co₃O₄ presented a good colloidal stability in solution (Figure 19d). Though high O₂ evolution activity was obtained, it decreased for all the Co₃O₄ after a period of time due to the flocculation effect. Fortunately, this phenomena can be alleviated by using porous SBA-15 as the a carrier (Figure 19e).^[137] The loading of TMOs is a good strategy to improve the photocatalytic activity of TiO₂.^[138,139] When TMOs were loaded on the surface of the TiO₂ nanosheets through solvothermal method (Figure 19f–h). It can be seen from the scanning transmission electron microscopy (STEM) images that there were irregular clusters (MnO_x) on the surface of the TiO₂ nanosheets compared to pristine TiO₂ nanosheets (Figure 19i,j). The very similar phenomenon was also observed for other cocatalysts. Interestingly, the O₂ evolution rate of TiO₂ nanosheets loaded with CoO_x (47 µmol h⁻¹) was higher than that of Ru/Ir-loaded counterparts, reflecting the superiority of TMO cocatalysts (Figure 19k,l).^[140]

Figure 19. a–c) TEM images and d) surface areas of Co3O4 nanoparticles (digital pictures of Co3O4 nanoparticles with diameter of 3 nm in inset). e) Time courses of oxidation evolution for a series of Co3O4 samples. Reproduced with permission.[137] Copyright 2013, American Chemical Society. f) XRD patterns and g,h) SEM images of pristine TiO2 nanosheets and MnOx-loaded TiO2 nanosheets. STEM images of i) pristine TiO2 nanosheets and j) MnOx-loaded TiO2 nanosheets. k) The O2 evolution rates and l) schematic diagram of reaction mechanism for pristine TiO2 nanosheets and various kinds of cocatalyst-loaded TiO2 nanosheets. Reproduced with permission.[140] Copyright 2013, American Chemical Society. m) Schematic diagram of crystal facets selection of reduction/oxidation cocatalysts by photodeposition. SEM images of BiVO4 loaded with n) CoOx, o) Pt, p) MnOx, q) Au, and r) Pt/MnOx on {110}/{010} facets of BiVO4. s) The O2 evolution rates of MnOx/Co3O4-loaded BiVO4 deposited by varieties of reduction cocatalysts. Reproduced with permission.[142] Copyright 2014, Royal Society of Chemistry.

Since the photogenerated electrons and holes have the feature of selective migration based on different crystal facets of some specific semiconductors (e.g., BiVO₄),^[141] it is significant to explore the distributions of different types of cocatalysts on the crystal facets. Under photoirradiation, the Pt and CoO_x were respectively accumulated on {010} and {110} facets of BiVO₄ (Figure 19m–o), which was distinct from the random deposition of cocatalysts resulted by the impregnation method. The selective photodeposition feature of BiVO₄ was also confirmed by other single deposition (MnOx, Au) (Figure 19p,q) and dual deposition (Pt and MnO_x) (Figure 19r). Notably, the O₂ evolution performance of BiVO₄ loaded with MnO_x or Co₃O₄ was improved by deposition of Pt, Au, and Ag (Figure 19s), attributing to the synergetic effect of the reductive and oxidative cocatalysts.^[142]

As one of the most abundant metals on earth, Fe species as cocatalysts have a great potential in the substitution of the traditional noble metal oxides. Due to the poor conductivity of Fe(oxy)hydroxides, 2D ultrathin FeOOH nanosheets with a thickness of ≈1.8 nm were prepared by a bottom-up strategy, which had abundant OVs compared to the bulk FeOOH and FeOOH nanoparticles (Figure 20a–c). The FeOOH nanosheets can be easily loaded on BiVO₄ by electrostatic interaction, leading to the O₂ evolution rate of 67.2 µmol h⁻¹ with 0.5% loading amount of FeOOH nanosheets (Figure 20d).^[143]

Figure 20. a) TEM image and b) AFM image of ultrathin FeOOH nanosheets. c) ESR spectra of FeOOH with distinct morphologies. d) Time courses of oxidation evolution for pristine BiVO4 and various kinds of BiVO4/FeOOH materials. Reproduced with permission.[143] Copyright 2019, Royal Society of Chemistry. e) Schematic diagram of reaction mechanism and f) the O2 evolution production of BiVO4 loaded with cobalt-based compounds. g) Kelvin probe force microscopy and h) surface photovoltage of pristine BiVO4 and cobalt-based compound-loaded samples. Reproduced with permission.[144] Copyright 2016, Elsevier. i) TEM image of MIL-101. j) TEM image and EDX mapping image (inset) of Co element of CoPOM encapsulated in MIL-101. k) Time courses of oxidation evolution for pristine CoPOM, CoPOM/MIL-101, and recycled CoPOM/MIL-101. Reproduced with permission.[145] Copyright 2015, Elsevier. l) SEM image of WO3 loaded with B2O3. HRTEM images of WO3 loaded with m) B2O3 and n) B2O3−xNx. o) Time courses of oxidation evolution for B2O3-loaded WO3 and B2O3−xNx-loaded WO3. Reproduced with permission.[146] Copyright 2012, Royal Society of Chemistry. p,q) TEM images of WO3 nanowires and WO3/graphene nanowires. r) Time courses of oxidation evolution for WO3 nanowires and WO3/graphene nanowires. Reproduced with permission.[147] Copyright 2014, Hindawi Publishing Corporation. SEM images of s) graphene and t) WO3/graphene. u) Time courses of oxidation evolution for graphene, WO3, WO3/graphene, and physical mixed WO3/graphene. Reproduced with permission.[148] Copyright 2012, Royal Society of Chemistry. v) TEM image of α-Fe2O3/rGO. w) Time courses of oxidation evolution for pristine α-Fe2O3 and α-Fe2O3/rGO. x) Schematic diagram of reaction mechanism of α-Fe2O3/rGO. Reproduced with permission.[149] Copyright 2013, American Chemical Society.

In addition to the TMOs as cocatalysts, transition metal complexes also play an important role in promoting the water oxidation activity of photocatalysts. BiVO₄ loaded with three kinds of cobalt-based complexes (Co₄O₄(O₂CMe)₄L₄, L = py, Mepy, and CNpy, denoted as 1–3, respectively; Figure 20e) were prepared by heating and refluxing routes, among which the BiVO₄ loaded with No. 3 cobalt-based molecular catalyst exhibited an excellent photocatalytic activity with a high TOF of 2 s⁻¹ (Figure 20f). Kelvin probe force microscopy (KPFM) and surface photovoltage (SPV) measurements revealed that the ΔCPD signal (reflecting the migration extent of photogenerated holes to the surface of photocatalysts) and SPV amplitude were enhanced for BiVO₄/Co₄O₄(O₂CMe)₄L₄ (Figure 20g,h), indicating that the separation efficiency of photogenerated carriers of BiVO₄ was improved by loading of cobalt-based complexes.^[144]

Due to the stable structure, the MOF materials can also be used as the supports of photocatalysts to promote the O₂ evolution. To improve the stability and photocatalytic activity of the polyoxometalates [Co(H₂O)₂(PW₉O₃₄)₂]¹⁰⁻ (denoted as CoPOM), the MOF MIL-101 was applied as the support to encapsulate CoPOM inside the cavity. TEM and energy dispersive X-ray (EDX) mapping images (Figure 20i,j) showed that the CoPOM nanoparticles were evenly distributed in the channel of MIL-101 by ion exchange approach. After being immobilized inside the cavity of MIL-101, the O₂ evolution production of the composite was higher than that of pristine CoPOM, and the slight decrease in O₂ evolution activity after one circle reaction was ascribed to the leakage of CoPOM within MIL-101 (Figure 20k).^[145]

Compared with traditional metal-based cocatalysts, the nonmetallic oxide cocatalysts can be modified by the heteroatoms, such as nitrogen doping. Through calcination under ammonia atmosphere, WO₃ loaded with B₂O_3−xN_x nanoclusters was synthesized. TEM images (Figure 20m,n) showed obvious nanocluster layers of both WO₃/B₂O₃ and WO₃/B₂O_3−xN_x. The O₂ production amount of WO₃/B₂O_3−xN_x was higher than WO₃/B₂O₃ (Figure 20o), demonstrating positive effect of B₂O_3−xN_x in promoting the O₂ evolution of WO₃ than B₂O₃. It provided an insight for the exploration of nonmetallic cocatalysts.^[146] Another widely used 2D cocatalyst is graphene, which can act as the carrier of photocatalysts to promote the separation of photogenerated carriers owing to the excellent conductivity. In Figure 20p–r, the WO₃/graphene nanowires with different mass ratios of graphene that were prepared by a facile hydrothermal route displayed higher O₂ evolution performance than the WO₃ nanowires, because graphene promoted the separation efficiency of charge carriers by accepting the photogenerated electrons from WO₃.^[147] Due to the existence of the low resistance conduction path, the WO₃/graphene with the particle diameter of ≈12 nm exhibited higher O₂ evolution production than bare WO₃ and physically mixed WO₃ and graphene (Figure 20s–u).^[148] Similarly, the photocatalytic activity of α-Fe₂O₃ was also elevated by coupling with reduced graphene oxide (rGO) as the electron transfer platform (Figure 20v–x).^[149]

Heterojunction Construction

Two crucial factors that hinder the photocatalytic performance of semiconductor photocatalysts are the insufficient photoabsorption and the rapid recombination of photogenerated electron and holes during their migration from the bulk to the surface of photocatalysts. By combining two or more photocatalysts with the suitable band structures to form the II-type or Z scheme heterojunction, the weak photoabsorption of wide-bandgap semiconductors can be compensated by narrow-bandgap semiconductor. Moreover, the photogenerated electrons and holes can be effectively separated between different components on the basis of their band structures, collectively resulting in enhanced photocatalytic O₂ evolution activity.

II-type Heterojunction

II-type heterojunction is formed by two or more semiconductors with staggered band structures. The energy potential difference can propel the photogenerated electrons and holes to migrate to the semiconductors with more positive CB and more negative VB, respectively. II-type heterojunction is a p–n junction if it consists of a p-type semiconductor and an n-type semiconductor. The as-formed built-in electric field in the p–n junction will further boost the directional transfer of photogenerated carriers of the photocatalysts.^[150–152] The schematic diagrams of II-type heterojunction and p–n heterojunction are shown in Figure 21.

Figure 21. Schematic diagram of II-type heterojunction and p–n heterojunction.

Through a facile in situ crystallization, the CoAl-layered double hydroxide (CoAl-LDH) was grown on the surface of TiO₂ hollow nanospheres (TiO₂@CoAl-LDH) to construct TiO₂@CoAl-LDH hollow nanospheres with average size of ≈250 nm, which achieved the utilization of the full sunlight spectrum (Figure 22a–c). Under the irradiation of simulated sunlight, the O₂ evolution rate of TiO₂@CoAl-LDH (2.34 mmol h⁻¹ g⁻¹) was much higher than that of the pristine TiO₂ hollow nanospheres (0.27 mmol h⁻¹ g⁻¹) (Figure 22d). The DOS calculations revealed that the coupling of TiO₂ and CoAl-LDH made the CB potential of TiO₂ decreased and bandgap of CoAl-LDH narrowed at the same time, thus the photogenerated electrons can migrate from the CB of CoAl-LDH to that CB of TiO₂, while holes from VB of TiO₂ were injected into that of CoAl-LDH, leading to the effective separation of charge carriers (Figure 22e).^[153] The FeTiO₃–TiO₂ porous hollow architectures with the interconnected nanosheets anchored on the surface were synthesized by a two-step solvothermal method followed by the calcination treatment (Figure 22g,h). X-ray absorption near-edge spectra (XANES) of Ti K-edge revealed that the FeTiO₃–TiO₂ and pristine component had the similar local symmetry (Figure 22i). When the atomic ratio of Ti to Fe was 0.75, the O₂ evolution rate of FeTiO₃–TiO₂ hollow sphere reached the maximum, ≈2 times that of pristine FeTiO₃ (Figure 22j). The improved O₂ evolution performance was attributed to the formation of the II-type heterojunction between FeTiO₃ and TiO₂, which promoted the separation of photogenerated charge carriers (Figure 22l).^[154] Interestingly, inspired by the architecture of butterfly wings in nature, 3D WO₃/BiVO₄ heterojunction was fabricated by a facile one-step sol–gel route with Paris papilio as the biological template (Figure 22m). As shown in Figure 22n,o, both the as-prepared WO₃ and WO₃/BiVO₄ photocatalysts retained the quasi-honeycomb morphology after the calcination treatment. The O₂ yield of WO₃/BiVO₄ was 950 µmol after 5 h irradiation, which was 7.6-fold higher than that of pristine BiVO₄ (Figure 22p). The synergetic effects of the porous quasi-honeycomb structure and the formed II-type heterojunction between WO₃ and BiVO₄ that boosted the visible light absorption and photogenerated charge separation, thereby contributing to the increased photocatalytic O₂ evolution production (Figure 22q).^[155]

Figure 22. a) Schematic diagram of the synthesis and O2 evolution of TiO2@CoAl-LDH hollow nanospheres. b) SEM image and c) TEM image of TiO2@CoAl-LDH hollow nanospheres. d) Time courses of oxidation evolution along with three cycling tests for TiO2, CoAl-LDH, and TiO2@CoAl-LDH hollow nanospheres. e) Schematic diagram of reaction mechanism of TiO2@CoAl-LDH hollow nanospheres. Reproduced with permission.[153] Copyright 2015, Wiley. f) TEM image of FeTiO3. g,h) TEM and HRTEM image of FeTiO3–TiO2 hollow spheres. i) The X-ray absorption near-edge spectra of Ti K-edge of TiO2, FeTiO3, and FeTiO3–TiO2. j) The O2 evolution rates of pristine TiO2 and FeTiO3–TiO2 hollow spheres with mass ratios of 1:0.25, 1:0.5, 1:0.75, and 1:1. k) UV–vis spectrum and wavelength-dependent O2 evolution rates of FeTiO3–TiO2 hollow spheres. l) Schematic diagram of reaction mechanism of FeTiO3–TiO2 hollow spheres. Reproduced with permission.[154] Copyright 2015, Royal Society of Chemistry. m) Schematic diagram of synthesis of WO3/BiVO4. SEM images of n) pristine WO3 and o) WO3/BiVO4. p) Time courses of oxidation evolution for WO3, BiVO4, and WO3/BiVO4. q) Schematic diagram of reaction mechanism of WO3/BiVO4. Reproduced with permission.[155] Copyright 2017, Royal Society of Chemistry.

Through a facile impregnation route followed by calcination treatment, the B-doped g-C₃N₄ was deposited on the dual-phases BiVO₄ (monoclinic and tetragonal phases, denoted as BVOMT) to form the p–n heterojunction (Figure 23a). TEM and high-resolution TEM (HRTEM) images demonstrated that the surface of g-C₃N₄ nanosheets was decorated with BiVO₄ cashew nut (Figure 23b,c). The BVOMT showed higher O₂ evolution rate (452.8 µmol h⁻¹ g⁻¹) than the monoclinic-phase BiVO₄ (321 µmol h⁻¹ g⁻¹) and tetragonal-phase BiVO₄ (256 µmol h⁻¹ g⁻¹) (Figure 23d,e). Furthermore, when BVOMT was combined with B-doped g-C₃N₄, the as-synthesized BVCN-50 with the strongest visible light absorption exhibited the highest O₂ evolution rate (1027.2 µmol h⁻¹ g⁻¹), which was attributed to the formation of p–n heterojunction (Figure 23f).^[156]

Figure 23. a) Schematic diagram of synthesis of BiVO4-B-doped g-C3N4. b) TEM and c) HRTEM image of BiVO4-B-doped g-C3N4. d) DRS of varieties of samples. e) The O2 evolution rates of varieties of samples. f) Schematic diagram of reaction mechanism of BiVO4-B-doped g-C3N4. Reproduced with permission.[156] Copyright 2019, American Chemical Society.

Z-Scheme Heterojunction

Although the II-type/p–n heterojunction can effectively improve the separation efficiency of the photogenerated charge carriers, the photogenerated carriers are all transferred to lower energy levels, which reduces the redox driving force of photocatalysts.^[157,158] As a result, the concept of Z-scheme has been proposed by recombination of the photogenerated holes of PS I and the electrons of PS II (Figure 24). The advantage of the Z-scheme heterojunction is that it can spontaneously promote the separation of photogenerated carriers and maintain the strong redox abilities of photocatalysts, which has attracted enormous attentions in photocatalytic water splitting in recent years.^[159]

Figure 24. Schematic diagram of Z-scheme heterojunction without (left) and with (right) mediators.

The photodeposition experiment revealed that the {010} and {110} facets of BiVO₄ were the reduction and oxidation facets, respectively (Figure 25b), while the endpoint and surface of ZnO nanorods were the accumulation regions of electrons and holes, respectively. To probe the charge separation and migration behavior between ZnO and BiVO₄, the 1D/3D ZnO/BiVO₄ was assembled by hydrothermal route followed by calcination treatment (Figure 25a). SEM images and electron paramagnetic resonance (EPR) spectra confirmed that the OV-rich ZnO nanorods were vertically anchored on the surface of BiVO₄ (Figure 25c–e). Under visible light illumination, the O₂ evolution rate of ZnO/BiVO₄ (68 µmol h⁻¹) was much higher than that of the pristine BiVO₄ (Figure 25f). It was demonstrated that the photogenerated electrons and holes were separately gathered on {010} and {110} facets of BiVO₄ driven by the internal electric field. Partial holes on {110} facets were recombined with the electrons that enriched on the endpoints of ZnO nanorods to form a Z-scheme heterojunction, which led to efficient charge separation.^[160] Besides, the mixed-phase BiVO₄ (monoclinic and tetragonal phases) were applied as the bridge to construct BiVO₄/g-C₃N₄ Z-scheme junction for accelerating the charge separation and water oxidation (Figure 25g).^[161] To realize the tight coupling between α-Fe₂O₃ and g-C₃N₄, an SSR route was carried out to synthesize the 2D/2D α-Fe₂O₃/C–g-C₃N₄ (C–g-C₃N₄ indicates g-C₃N₄ with amorphous carbon). The weaker peak in XRD patterns of α-Fe₂O₃/C-C₃N₄ attributed to the exfoliation and partial carbonization of g-C₃N₄ after calcination (Figure 25h), as confirmed by the obvious amorphous carbon around the edges of g-C₃N₄ (Figure 25i). Owing to the formed Z-scheme junction between α-Fe₂O₃ and C–g-C₃N₄, the O₂ evolution production rate of α-Fe₂O₃/C–g-C₃N₄ (22.3 µmol h⁻¹) was 30-fold higher than that of the pristine g-C₃N₄ (0.7 µmol h⁻¹) (Figure 25j,k).^[162]

Figure 25. a) Schematic diagram of synthesis of ZnO/BiVO4. b) SEM images of BiVO4 and ZnO photodeposited with Ag and Co3O4. SEM images of c) BiVO4 and d) ZnO/BiVO4. e) ESR spectra of varieties of samples. f) The O2 evolution rates of varieties of samples. Reproduced with permission.[160] Copyright 2018, Elsevier. g) Schematic diagram of water oxidation of mixed-phase BiVO4/g-C3N4, BVO-T, and BVO-M are represented the monoclinic scheelite and tetragonal zircon phase, respectively. Reproduced with permission.[161] Copyright 2019, Royal Society of Chemistry. h) XRD patterns of α-Fe2O3/C3N4-r and α-Fe2O3/C–g-C3N4. i) TEM image of α-Fe2O3/C-C3N4. j) The O2 evolution production of varieties of samples. k) Schematic diagram of reaction mechanism of α-Fe2O3/C-C3N4. Reproduced with permission.[162] Copyright 2018, American Chemical Society.

As a layered perovskite, Bi₂MoO₆ has a great potential in the field of photocatalysis due to the layered crystal structure and suitable bandgap. The heterojunction of Bi₂MoO₆ hybridized with the good electron transporters, such as g-C₃N₄, has shown excellent photocatalytic performances in terms of the degradation of organic pollutants, water splitting for H₂ evolution, etc.^[163–166] To further improve the activity or stability, it is significant to introduce appropriate electron transfer media, like Au, Rh, and Ru. Recently, the Bi₂MoO₆/Ru/g-C₃N₄ catalyst was fabricated by the solvothermal method combined with the reduction of precursor (Figure 26a,d). DRS indicated that the incorporated Ru endowed the material with the absorption in the entire visible light region (Figure 26e). The calculation results of charge density difference and band structure manifested the formation of chemical bonds between Ru and O atoms of Bi₂MoO₆ (Figure 26g–j). Under visible light irradiation, Bi₂MoO₆/Ru/g-C₃N₄ exhibited the highest O₂ evolution rate of 328.34 µmol h⁻¹ g⁻¹, which was ≈3 and 25 times that of Bi₂MoO₆ and g-C₃N₄, respectively (Figure 26f). It was attributed to that the metallic Ru as the electron transfer media promoted the recombination of photogenerated electrons from Bi₂MoO₆ and holes from g-C₃N₄, allowing Bi₂MoO₆ to retain the strong oxidizing capability (Figure 26k).^[167]

Figure 26. a) Schematic diagram of synthesis of Bi2MoO6/Ru/g-C3N4. SEM images of b) g-C3N4, c) Bi2MoO6, and d) Bi2MoO6/Ru/g-C3N4. e) DRS of varieties of samples. f) The O2 evolution rates of varieties of samples. Charge density difference of g) Ru/g-C3N4 and h) Ru/Bi2MoO6. Band structures of i) g-C3N4 and Ru/g-C3N4, and j) Bi2MoO6 and Ru/Bi2MoO6. k) Schematic diagram of reaction mechanism of Bi2MoO6/Ru/g-C3N4. Reproduced with permission.[167] Copyright 2019, American Chemical Society.

Doping and Vacancy Formation

Doping or vacancy creation can break the periodicity of crystal atomic arrangement and induce the lattice distortion, which lead to the formation of impurity states or defect states in the forbidden band, thus extending the light response range.^[168] Besides, they can also promote the separation of photogenerated charge carriers, enhancing the photocatalytic O₂ evolution performance.

Doping

Since 1982, it has been found that the incorporation of a certain amount of transition metals, such as Cr and Ru, into TiO₂ allows it to absorb more visible light. After that, metallic doping was employed to improve the photocatalytic activity of semiconductors.^[169] At the beginning of this century, nonmetallic elements doping (e.g., B, N) has gradually become the mainstream, as they bring more flexible tunability in the improvement of photocatalytic activity.^[170,171] Nowadays, a generally accepted viewpoint is that the effect of metallic/nonmetallic doping for photocatalysts can be affected by the types of element, doping methods, concentration of dopant, and doping position.

Based on the mature research works of elemental doping in TiO₂, the concept of gradient doping with nonmetallic heteroatoms in TiO₂ to improve the electronic structure has been proposed. As shown in Figure 27a,b, the anatase TiO₂ microspheres doped with boron were synthesized by hydrothermal method and heat treatment with TiB₂ as the precursor of titanium and boron. The X-ray photoelectron spectroscopy (XPS) spectra with argon ion sputtering revealed that the binding energy of B 1s in the TiO₂ microspheres changed from 187.9 to 192.2 eV, indicating the transition of substitutional boron (B^δ−) to interstitial boron (B^σ+) during the heat treatment process (Figure 27c). It was found that the O₂ evolution of TiO₂ microspheres after thermal treatment was 4.5 times that of previous one (Figure 27d), which was attributed to the diffusion of boron from the core to the edges of microspheres, making the VB of TiO₂ shifted to a more positive energy level with stronger oxidation ability (Figure 27e).^[172] Since the effect of doping can be affected by the types of element, it is interesting to investigate the influence of the valence state of doped elements in photocatalysts. As shown in Figure 27f, the O₂ evolution rates of TiO₂ doped with W⁶⁺, Ta⁵⁺, or Nb⁵⁺ were higher than that of the pristine TiO₂, while doping of Zr⁴⁺, Sn⁴⁺, or Ge⁴⁺ had little effect on the photocatalytic activity of TiO₂. In contrast, the O₂ evolution performance of TiO₂ was reduced by doping In³⁺, Ga³⁺, or Al³⁺. According to the classical Kröger–Vink theory, when the metallic cations with a higher valence than Ti⁴⁺ were doped into the TiO₂ lattice (denoted as donor doping), the electron will increase in concentration and be captured into the Ti⁴⁺ lattice, tuning the latter into Ti³⁺ species. Then, the Fermi energy level was shifted upward and a built-in electric field was formed, promoting the separation of photogenerated charge carriers (Figure 27g).^[173]

Figure 27. a) Schematic diagram of crystal structures of TiB2 and anatase TiO2. b) SEM image of TiO2 microspheres. c) Argon ion sputtering dependent XPS spectra of B 1s of TiO2 microspheres. d) Time courses of oxidation evolution for TiO2 microspheres i) before and ii) after heat treatment. e) Schematic diagram of boron distribution and electronic structure inside TiO2 microspheres before and after heat treatment. Reproduced with permission.[172] Copyright 2012, Wiley. f) The O2 evolution rates of pristine rutile TiO2 and TiO2 doped with various kinds of metallic cations. g) Schematic diagram of reaction mechanism of TiO2 doped with metallic cations. Reproduced with permission.[173] Copyright 2018, Elsevier. h) Digital picture, and i) optical microscopy image and model of butterfly wings. j,k) TEM and HRTEM image of BiVO4 doped with 1.5 wt% C. l) DRS and m) time courses of oxidation evolution for pristine BiVO4 and BiVO4 doped with various amounts of C. Reproduced with permission.[174] Copyright 2013, Royal Society of Chemistry. SEM images and enlarged views (inset) of n) WO3 and o) Cs–WO3. p) Time courses of oxidation evolution, and q) the O2 evolution rates and energy barrier of WO3 and Cs–WO3. r) Open circuit voltage decay of WO3 and Cs–WO3. s) Schematic diagram of reaction mechanism of WO3 and Cs–WO3. Reproduced with permission.[175] Copyright 2019, American Chemical Society.

The butterfly wings in nature are composed of uniformly arranged structures, in which the morphology of porous honeycomb endows them with the strong absorbance for the external sunlight (Figure 27h,i). Inspired by this architecture, the C-doped BiVO₄ was synthesized by the sol–gel route followed by subsequent thermal treatment with butterfly wings as the sacrificial template. As shown in Figure 27j,k, the original porous honeycomb structure was basically retained for the as-synthesized material and the lattice spacing of 0.308 nm was assigned to the {121} facets of BiVO₄. DRS revealed that the incorporation of C allowed BiVO₄ stronger absorption intensity in visible light region (Figure 27l). When the temperature of thermal treatment and doping amount of C was 400 °C and 1.5 wt%, respectively, the sample showed the highest O₂ evolution activity (800 µmol L⁻¹; Figure 27m), which was ascribed to the synergistic effect of the unique structure and C doping, resulting in enhanced absorption of visible light and separation of the carriers.^[174]

In order to investigate the main factor that hinder the photocatalytic water oxidation, WO₃ treated with cesium (denoted as Cs–WO₃) was synthesized by the impregnation approach. After the introduction of cesium, an amorphous layer appeared on the surface of Cs–WO₃ with slightly changed size (Figure 27n,o). Compared to pristine WO₃, the O₂ evolution rate of Cs–WO₃ increased by three times (51.3 µmol h⁻¹) (Figure 27p). Contrary to previous view that the lower energy barrier was beneficial to the photocatalytic reaction, Cs–WO₃ with a higher energy barrier had a longer photogenerated carrier lifetime than that of WO₃ as revealed by the open circuit voltage decay curves (Figure 27q,r). Based on these advances, Li's group proposed that the main bottleneck for O₂ evolution is to enrich the long-lived photogenerated holes that were involved in the oxidation reaction on the surface of photocatalysts (Figure 27s),^[175] as widely supported by other works.^[176,177]

The band position of TiO₂ can be adjusted through the nitridation, and its water oxidation performance can be improved by the donor doping,^[173,178] thus the synergistic effect of metal and nonmetal codoping on the water oxidation activity of rutile TiO₂ was investigated. Kazuhiko's group reported that the rutile TiO₂ nanorods codoped with Ta/N species (denoted as TiO₂:Ta/N) were synthesized by the microwave-assisted hydrothermal route followed by the subsequent calcination treatment in the ammonia atmosphere (Figure 28a–c). The doping sites of Ta⁵⁺ within the crystal of TiO₂ were mainly along the [101] and [110] directions (Figure 28d,e). DRS and schematic diagram of band positions in Figure 28f,h revealed that the increased absorbance ability of visible light for TiO₂:Ta/N was ascribed to the doping of N species, while the doping of single Ta species enlarged the bandgap of TiO₂. The transient absorption spectra indicated that the as-synthesized TiO₂:Ta/N showed a much lower concentration of deeply trapped charge carriers compared with N-doped TiO₂, implying a higher photocatalytic activity of TiO₂:Ta/N (Figure 28g). Based on above results, it can be proclaimed that the role of Ta doping is to improve the separation efficiency of photogenerated carriers of TiO₂, while the incorporation of N species endows TiO₂ with stronger visible light absorbance. Additionally, the O₂ evolution rate rose with increasing the doping amount of Ta species and the temperature of microwave-assisted treatment, indicating that the photocatalytic activity of rutile TiO₂ was affected by both the doping level and the crystallinity of the catalysts (Figure 28i,j). Importantly, the photocatalytic activity of OWS was closely related to the types of redox cycle mediator and the kinds of O₂ evolution photocatalyst. It was revealed that the TiO₂–SrTiO₃ Z-scheme system exhibited a higher catalytic activity with Fe³⁺/Fe²⁺ as the redox cycle mediator than that with IO₃⁻/I⁻ because the Fe²⁺ electron donor has a stronger capability in hindering the backward reaction of SrTiO₃ (Figure 28k,l). Furthermore, the TiO₂:Ta/N showed the highest OWS performance (Figure 28m), and a solar-to-hydrogen conversion efficiency (STH) of 0.021% was obtained with Fe³⁺/Fe²⁺ as the redox cycle mediator, and a higher STH of 0.039% was achieved with TiO₂:Ta/N (IrO₂ as the cocatalyst) as the O₂ evolution component in the Rh-doped SrTiO₃-based Z-scheme system.^[179,180]

Figure 28. TEM images of a) pristine rutile TiO2, and b) TiO2 doped with Ta and c) codoped with Ta/N. Reproduced with permission.[179] Copyright 2017, Royal Society of Chemistry. HAADF-STEM images, enlarged views, and crystal structures of TiO2 codoped with Ta/N in d) [121] and e) [111] directions. Reproduced with permission.[180] Copyright 2019, Royal Society of Chemistry. f) DRS of pristine TiO2 and varieties of samples. g) Transient absorption spectra of TiO2 doped with N and codoped with Ta/N. h) Schematic diagram of band positions of TiO2 doped with N, Ta, and codoped with Ta/N. Reproduced with permission.[179] Copyright 2017, Royal Society of Chemistry. The O2 evolution rates of TiO2 codoped with Ta/N as a function of i) doping amounts of Ta species and j) temperatures. Reproduced with permission.[180] Copyright 2019, Royal Society of Chemistry. Time courses of overall water splitting evolution for RuO2/Ta–N codoped TiO2 combined with RuO2/Rh-doped SrTiO3 in k) NaIO3 aqueous solution and l) FeCl3 aqueous solution. m) Time courses of overall water splitting evolution for varieties of O2 evolution photocatalysts combined with RuO2/Rh-doped SrTiO3 in FeCl3 aqueous solution. Reproduced with permission.[179] Copyright 2017, Royal Society of Chemistry.

Vacancy Formation

In addition to doping, another strategy for improving the O₂ evolution performance based on defect modulation is the vacancy creation. Vacancy is one of the intrinsic defects, which can introduce the defect energy level into the forbidden band of semiconductors, thereby enhancing the light absorbance ability and charge separation.^[181–185]

OVs are one of the most common vacancies, and the construction of surface OVs is more beneficial to the separation of photogenerated carriers than the OVs in the bulk.^[186] In recent years, 2D monocrystalline nanosheets are the ideal candidates to create OVs. For instance, the OVs were in situ generated on the surface of BiOCl single-crystalline nanosheets with the exposed {010}/{001} facets by the hydrothermal method followed by UV-light irradiation. The water molecules adsorbed over OVs on the {010} facets in the dissociated manner were more easily to be oxidized than that on {001} facets in the molecular manner, thus leading to an enhanced O₂ production activity.^[187] Yan et al. fabricated WO₃ single-crystalline nanosheets with OVs introduced by the exfoliation and the following calcination treatment (Figure 29a). Compared to pristine WO₃ nanosheets, the morphology was hardly changed after the calcination procedure, while HRTEM images illustrated that there were amorphous layers with a thickness of ≈1 nm on the edge of WO₃ nanosheets via calcination in both vacuum and H₂ atmospheres (Figure 29b–d). The slight shift of (020) diffraction peak to a higher 2θ angle and the localized surface plasmon resonance (LSPR) peaks in the infrared region of the DRS spectra also proved the successful creation of OVs on the outside surface of WO₃ nanosheets (Figure 29e,f). The free carrier density of WO₃ nanosheets calcined in vacuum and H₂ atmospheres was calculated to be 2.5 × 10²¹ and 2.0 × 10²¹ by the following Drude formula, respectively [Image Omitted. See PDF]where ω_p, N_e, e, ε₀, and m_e denote the bulk plasma frequency, charge carrier density, elementary charge, permittivity of free space, and effective mass of an electron, respectively. Under simulated solar light, WO₃ nanosheets calcinated in H₂ atmosphere demonstrated the highest O₂ evolution rate of 1593 µmol h⁻¹ g⁻¹, ≈2.6-fold higher than that of the pristine WO₃ nanosheets (606 µmol h⁻¹ g⁻¹), which was attributed to the LSPR effect induced by the OVs that promoted the utilization of the solar energy (Figure 29g).^[188]

Figure 29. a) Schematic diagram of synthesis of WO3 nanosheets with OVs. SEM images and HRTEM images (inset) of b) pristine WO3 nanosheets, c) WO3 nanosheets calcined in H2 atmosphere, and d) vacuum. e) Enlarged view of XRD patterns of pristine WO3 nanosheets, and WO3 nanosheets calcined in H2 atmosphere and vacuum. f) DRS of pristine WO3 nanosheets, and WO3 nanosheets calcined in H2 atmosphere and vacuum. g) The O2 evolution rates of pristine WO3 nanosheets, and WO3 nanosheets calcined in H2 atmosphere and vacuum with varieties of irradiation conditions. Reproduced with permission.[188] Copyright 2015, Wiley.

Other Strategies

Apart from the above strategies for improving the O₂ evolution performances of photocatalysts, others methods including the formation of special microstructure, surface modification, and solid solution construction, can also effectively promote the O₂ evolution production. Fabrication of photocatalysts with special morphology often brings advantages, such as more favorable visible light absorption, faster migration of photogenerated charge carriers and larger specific surface area.^[189–191] Since BiVO₄ has the good morphological plasticity, BiVO₄ with the unique conical shape was obtained by a simple two-phase approach. The morphology of BiVO₄ underwent a series of transformations during the synthetic process (Figure 30a–g), and the most regular cone structure was observed after 40 min reaction. Besides, the dominantly exposed facets of BiVO₄ changed as the reaction time increased (Figure 30h,i). Under visible light irradiation, the O₂ evolution rate of as-synthesized conical BiVO₄ was three times that of BiVO₄ nanosheets, which reflected the influence of formation of special microstructure on the photocatalytic performance of BiVO₄ (Figure 30j).^[192]

Figure 30. SEM images of BiVO4 with reaction time of a) 10 min, b) 20 min, c) 30 min, d) 40 min, e) 60 min, and f) 120 min. g) EDS mapping images of BiVO4 with reaction time of 40 min. h) Enlarged view of XRD patterns of BiVO4 with varieties of reaction times. i) Intensity ratios of (040)/(110) plane as a function of dissolution time of Bi(NO3)3·5H2O. j) Time courses of oxidation evolution for conical BiVO4 and BiVO4 nanosheets. Reproduced with permission.[192] Copyright 2020, Royal Society of Chemistry. k) Digital picture, l) DRS, and m) local XRD patterns of pristine WO3 and WO3 with thermal treatment in varieties of temperatures under the H2 atmosphere. SEM images and HRTEM images of n,o) pristine WO3 and p,q) WO3 with thermal treatment in 200°C under the H2 atmosphere. Reproduced with permission.[193] Copyright 2013, Elsevier.

Li's group has investigated the influence of the hydrogenation treatment with a gradient of temperature for the O₂ evolution rate of WO₃ nanoparticles. The distinct thermal treatment temperatures led to the different colors of the samples (Figure 30k). Compared to pristine WO₃, the hydrogenated WO₃ showed stronger absorbance in visible light region (Figure 30l) and the obvious shift of the (002), (020), and (200) diffraction peaks to a lower 2θ angle due to the H₂ intercalation effect within the crystal of WO₃ (Figure 30m), whereas there was little impact on the morphology of WO₃ (Figure 30n,p). The WO₃ treated by H₂ at 200°C exhibited the highest O₂ evolution rate (75.3 µmol h⁻¹), ≈2.3-fold higher than the pristine WO₃ (32.6 µmol h⁻¹), benefiting from the formation of the H_xWO₃ layer around the edge of WO₃ (Figure 30o,q).^[193]

The hydrophilicity or hydrophobicity of the surface of photocatalysts also plays a key role in the interfacial contact between the photocatalyst and the loaded cocatalyst. In this regard, Li's group engineered the surface of Ta₃N₅ with MgO nanolayer as the modifier. TEM images showed that there was a layer of MgO deposited on the edge of Ta₃N₅ by calcination and subsequent nitridation treatment (Figure 31a,b); it then quickly changed into Mg(OH)₂ after contacting with the water molecules outside. The lattice spacing of 0.213 and 0.244 nm indicated the existence of both CoO and Co₃O₄ nanoparticles as the cocatalyst (denoted as CoO_x) that were closely loaded on the surface of Ta₃N₅ (Figure 31c,d). When the amount of MgO was 2%, the contact angle between Ta₃N₅ and CoO_x reached the saturated value and the catalyst exhibited the highest O₂ evolution rate of 1.2 mmol h⁻¹ (Figure 31e,f). Ta₃N₅ modified by MgO displayed a better water oxidation performance than pristine Ta₃N₅ (Figure 31g), which was attributed to the formation of the MgO layer on the surface of Ta₃N₅ that improved the deposition of CoO_x, leading to the enhancement of the separation efficiency of photogenerated charge carriers.^[194]

Figure 31. a,c) TEM and b,d) HRTEM images of a,b) Ta3N5–MgO and c,d) Ta3N5–MgO loaded with CoOx. e) Contact angles tests of i) Ta3N5–MgO and ii) Ta3N5–MgO loaded with CoOx as a function of the contents of magnesium. f)The O2 evolution rates of Ta3N5–MgO loaded with CoOx as a function of the contents of magnesium. g) Time courses of O2 evolution production for i) Ta3N5–MgO loaded with CoOx and ii) Ta3N5 loaded with CoOx. Reproduced with permission.[194] Copyright 2015, Wiley. h) SEM image of Sr0.96Ba0.04TiO3 nanoparticles. i,j) TEM image, size distributions of Au (inset), and HRTEM image of Sr0.96Ba0.04TiO3 deposited with Au. k) XRD patterns of SrTiO3, BaTiO3, and Sr0.96Ba0.04TiO3. l) Time courses of O2 evolution production for Au/SrTiO3, Au/BaTiO3, and Au/Sr0.96Ba0.04TiO3. m) The O2 evolution rates of Sr0.96Ba0.04TiO3 as a function of doping amounts of barium. Reproduced with permission.[196] Copyright 2017, Elsevier.

Besides, construction of solid solutions was considered to be an effective way for controllable adjustment of bandgap and energy levels.^[195] Ye's group incorporated a certain proportion of barium into SrTiO₃ by the hydrothermal approach to construct the Sr_0.96Ba_0.04TiO₃ solid solution. The diameter of the as-synthesized solid solution nanoparticles was ≈100 nm (Figure 31h) with the crystal structure kept the same to the original SrTiO₃ and BaTiO₃ (Figure 31k). The size distributions of Au nanoparticles as cocatalyst deposited on the surface of the solid solution was between 1 and 4 nm (Figure 31i,j). Under visible light irradiation, Sr_0.96Ba_0.04TiO₃ showed the highest O₂ evolution rate (6.7 µmol h⁻¹) when 3% Ba was doped, ≈4.79 and 1.97-fold higher than that of pure SrTiO₃ and BaTiO₃, respectively. The enhanced O₂ production activity was mainly attributed to the optimized energy level of SrTiO₃ by the Ba doping (Figure 31l,m).^[196] The O₂ evolution production performance of strategic photocatalysts under distinct reaction conditions is shown in Table 2.

Table 2 Strategies for enhancing photocatalytic O₂ evolution of semiconductor photocatalysts

Photocatalyst	Sacrificial agent	Amount of sacrificial agent	Light source	Incident light [nm]	Oxygen production rate	AQE	Refs.
CoOx–TiO2	NaIO3	20 × 10−3 m	150 W UV	–	47 µmol h−1	15.5% at 365 nm	[140]
CoOx–TiO2	AgNO3	100 × 10−3 m	UV light	–	34.42 µmol h−1	–	[197]
Ir–TiO2	KIO3	1.5 × 10−3 m	400 W UV	–	≈140 µmol g−1	–	[198]
Carbon dots/TiO2	AgNO3	15 mg	114 W UV	–	3598 µmol h−1 g−1	–	[199]
TiO2@CoAl-LDH	AgNO3	10 mg	300 W Xe	–	2.34 mmol h−1 g−1	–	[153]
FeTiO3–TiO2	AgNO3	10 × 10−3 m	300 W Xe	–	71 µmol h−1	9.1% at 420 nm	[154]
TiO2/SrTiO3	AgNO3	–	300 W Xe	–	103 µmol h−1	35.4% at 365 nm	[200]
g-C3N4/a-TiO2/r-TiO2	AgNO3	10 × 10−3 m	Sunlight	–	198.8 µmol h−1	–	[201]
WO3–TiO2/Nb2O5	Fe2(SO4)3	–	250 W Hg	–	151.8 µmol L−1 h−1	–	[202]
Ag3PO4/CeO2/TiO2	–	–	300 W Xe	≤400	23.13 µmol h−1	–	[203]
B3+–TiO2	AgNO3	16.7 × 10−3 m	–	–	≈50 µmol h−1	–	[172]
Ta5+–TiO2	AgNO3	0.45 × 10−3 m	UV light	–	≈21 µmol h−1	–	[173]
Ta/N–TiO2	NaIO3	100 × 10−6 m	360 W Xe	≥420	≈15 µmol h−1	0.5% at 420 nm	[179]
Ta/N–TiO2	FeCl3	1 × 10−3 m	300 W Xe	≥400	≈13 µmol h−1	–	[180]
S–TiO2	Fe(OH)3	–	500 W Xe	≥350	≈2.9 µmol h−1	–	[204]
Ta/N–TiO2	AgNO3	10 × 10−3 m	300 W Xe	≥400	≈5.4 µmol h−1	–	[205]
Pt–Co3O4/BiVO4	NaIO3	20 × 10−3 m	300 W Xe	≥420	≈160 µmol h−1	–	[142]
Co4O4(O2CMe)4CNpy4–BiVO4	NaIO3	10 × 10−3 m	300 W Xe	≥420	≈51 µmol h−1	4.5% at 420 nm	[144]
Pt/MnOx/BiVO4	NaIO3	20 × 10−3 m	300 W Xe	≥420	≈660 µmol h−1 g−1	–	[61]
FeOOH/BiVO4	AgNO3	850 mg	300 W Xe	≥400	67.2 µmol h−1	2.4% at 420 nm	[143]
Au/CoOx/BiVO4	K3[Fe(CN)6]	10 × 10−3 m	300 W Xe	≥420	32 µmol h−1	10.3% at 420 nm	[206]
BiVO4/MCM-41	AgNO3	–	300 W Xe	–	172.8 µmol h−1	–	[207]
BiVO4/B–g-C3N4	AgNO3	5 × 10−3 m	150 W Xe	≥420	1027.2 µmol h−1 g−1	–	[156]
BiVO4/ZnO	AgNO3	50 × 10−3 m	300 W Xe	≥420	68 µmol h−1	5.0% at 450 nm	[160]
BiVO4@Cu2O	AgNO3	50 × 10−3 m	300 W Xe	≥420	1500 µmol h−1 g−1	–	[208]
BiVO4/g-C3N4	AgNO3	50 × 10−3 m	300 W Xe	–	600 µmol h−1 g−1	2.36%	[161]
CeVO4/rGO/BiVO4	AgNO3	1000 mg	–	–	85.68 µmol L−1 g−1	–	[209]
C–BiVO4	AgNO3	4.5 × 10−3 m	300 W Xe	≥420	800 µmol L−1	–	[174]
Mo–BiVO4	AgNO3	50 × 10−3 m	Sunlight	–	942 µmol h−1 g−1	–	[210]
Conical BiVO4	AgNO3	25 × 10−3 m	300 W Xe	≥400	230 µmol h−1 g−1	–	[192]
Pt/RuO2/WO3	NaIO3	10 × 10−3 m	300 W Xe	≥420	41 µmol h−1	14.4% at 420 nm	[131]
B2O3−xNx@WO3	AgNO3	850 mg	–	≥400	74.5 µmol h−1	–	[146]
WO3/graphene	Fe2(SO4)3	10 × 10−3 m	150 W Xe	≥400	40 µmol L−1 h−1	–	[147]
WO3@graphene	Fe2(SO4)3	0.16 × 10−3 m	300 W Xe	–	43.1 µmol L−1 h−1	–	[148]
Ru–WO3	NaIO3	5 × 10−3 m	300 W Xe	≥300	65 µmol h−1	–	[211]
WO3/rGO	AgNO3	30 × 10−3 m	300 W Xe	–	580 µmol h−1 g−1	–	[212]
WO3/BiVO4	–	–	300 W Xe	≥420	20 µmol h−1 mg−1	–	[155]
WO3/TiO2	–	–	300 W Xe	–	178 µmol h−1 g−1	1.8% at 400 nm	[213]
WO3/Ag3PO4	AgNO3	510 mg	300 W Xe	≥420	≈425 µmol h−1 g−1	–	[214]
WO2–WO3	AgNO3	10 × 10−3 m	500 W Xe	≥400	265 mmol h−1 g−1	–	[215]
Cs–WO3	AgNO3	–	300 W Xe	≥420	51.3 µmol h−1	3.7%	[175]
WO3−x–V/HT	NaIO3	10 × 10−3 m	Sunlight	–	1593 µmol h−1 g−1	9.3% at 420 nm	[188]
S–WO3	–	–	250 W Xe	≥500	76.7 µmol L−1 h−1 g−1	–	[216]
HxWO3–WO3	AgNO3	10 × 10−3 m	300 W Xe	≥420	75.3 µmol h−1	–	[193]
Au–α-Fe2O3	Na2S2O8	5.33 × 10−3 m	300 W Xe	≥420	10.81 µmol h−1	–	[132]
α-Fe2O3/rGO	AgNO3	68 mg	300 W Xe	–	752 µmol h−1 g−1	–	[149]
Fe2O3/C–C3N4	AgNO3	10 × 10−3 m	–	≥420	22.3 µmol h−1	–	[162]
Pt–Bi4NbO8Cl	FeCl3	2 × 10−3 m	300 W Xe	≥400	≈96 µmol h−1	2.5% at 420 nm	[100]
MoO3–Bi4TaO8Cl	AgNO3	5 × 10−3 m	300 W Xe	≥420	≈28 µmol h−1	25% at 420 nm	[103]
Fe/Ru–Bi4TaO8Cl	FeCl3	4 × 10−3 m	300 W Xe	≥400	26 µmol h−1	1.6% at 420 nm	[217]
Ru–TaON	NaIO3	1 × 10−3 m	300 W Xe	≥420	13.0 µmol h−1	–	[135]
Rh/Ru–TaON	NaIO3	1 × 10−3 m	300 W Xe	≥400	≈46.67 µmol h−1	6.9% at 420 nm	[136]
Co3O4@SBAa)	Na2SO4/Na2S2O8	975 mg/357 mg	300 W Xe	≥450	≈135 µmol h−1	–	[137]
CoPOM@MIL-101b)	Na2S2O8	0.375 × 10−3 m	300 W Xe	≥420	≈68.38 µmol h−1	–	[145]
Ru/Co–TaON	NaIO3	1 × 10−3 m	300 W Xe	≥400	≈12.5 µmol h−1	–	[218]
Ir/TiO2/Ta3N5	NaIO3	1 × 10−3 m	300 W Xe	≥420	24.1 µmol h−1	–	[219]
Rh–KCa2Nb3O10	NaIO3	5 × 10−3 m	300 W Xe	≥300	≈3.4 µmol h−1	–	[107]
Bi2MoO6/Ru/g-C3N4	NaIO3	396 mg	300 W Xe	≥420	328.34 µmol h−1 g−1	–	[167]
N–KCa2Nb3O10	AgNO3	10 × 10−3 m	300 W Xe	≥400	≈1.92 µmol h−1	–	[220]
CoOx/MgO–Ta3N5	AgNO3	1500 mg	300 W Xe	≥420	1.2 mmol h−1	11.3% at 500 nm	[194]
Au/Sr0.96Ba0.04TiO3	AgNO3	5 × 10−3 m	300 W Xe	≥400	6.7 µmol h−1	–	[196]
Fe-TAMLc)	Na2S2O8	8.3 × 10−3 m	Sunlight	–	0.79 µmol h−1	–	[221]

a)SBA = Mesoporous silica b)COPOM = [Co₄(H₂O)₂(PW₉O₃₄)₂]¹⁰⁻ c)Fe-TAML = Biuret modified tetra-amidomacrocyclic ligands. Conclusions, Challenges, and Perspectives

Under the social background of advancing the sustainable development of energy resources, the photocatalytic water splitting has been gradually become the focus due to the features of abundant resources, environmental-friendly, etc. As the rate-determining step of the water splitting reaction, water oxidation is the key bottleneck that restricts the efficiency during this process. This review summarizes the latest research progresses of photocatalytic water oxidation. The content includes the introduction of several classical water oxidation photocatalysts (e.g., TiO₂, BiVO₄, WO₃), featuring the crystalline structures, synthesis approaches, and morphologies. On this basis of the critical issues that hinder the photocatalytic activity of photocatalysts, such as the low utilization of visible light and fast recombination of photogenerated charge carriers, the corresponding effective solutions, including the cocatalyst loading, heterojunction construction, doping and vacancy formation, and other strategies, are summarized.

In the last ten years, although a series of oxygen evolution photocatalysts have been developed, there is still a long way to go before the practical industrial applications. The photocatalysts for oxygen evolution still suffer from low efficiency or poor physicochemical stability, and especially most of them require the presence of sacrificial agents and cocatalysts, which also undoubtedly increase the economic costs to the industrial applications. In the future, more efforts are in need:

• i)The development of efficient water oxidation photocatalysts is still the present research focus, and the Sillén–Aurivillius perovskites will show significant potential for photocatalytic O₂ evolution. First, the VB of these perovskites such as Bi₄NbO₈Cl is mainly occupied by the O 2p orbitals. It makes them not be easily corroded by the photogenerated holes, thus demonstrating high photochemical stability. Second, the synthesis methods of this kind of materials are diverse, including 1SSR, 2SSR, 2PC, and flux, which allows the preparation process to be flexible and easy to optimize the photocatalytic performance. Additionally, the Sillén–Aurivillius perovskites have diverse compositions, which enables the adjustable light absorption or bandgap and photocatalytic O₂ evolution activity. Therefore, the rational design strategies based on crystal structure and band structure are expected to yield high-performance perovskites for O₂ evolution in the future.
• ii)At present, the strategies for O₂ evolution performance enhancement are mainly achieved by improving the light absorption ability and charge separation efficiency, whereas the researches on the surface catalytic reaction are rarely involved. Actually, the reactive sites of photocatalysts are closely related to the adsorption of reactants and the reaction activation energy. For example, the construction of the surface defects can obviously enrich the active sites of photocatalysts. Therefore, the exploration of the catalytic active sites for O₂ evolution is expected to be one of the research priorities in the future.
• iii)Most of the reported works mainly focused on promoting the separation efficiency of photogenerated charge carriers to achieve the purpose of improving the O₂ evolution performance. However, less attention has been paid to the physicochemical stability of photocatalysts during the photocatalytic reaction process, whereas the actual situation is that the physicochemical stability of most of as-synthesized materials is often affected by many factors, such as the synthetic routes and conditions. In addition, some O₂ evolution photocatalysts are prone to be self-poisoned by the photogenerated holes, which results in the inactivation after photoreactions. Thus, developing effective tactics for improving the chemical and physicochemical stability of O₂ evolution photocatalysts are necessary.

Besides, the spatial separation for the occurrence of reduction and oxidation reactions should be considered, which can effectively inhibit the inverse reaction that usually occurs on the surface of photocatalyst. For instance, the hydrogen farm strategy proposed by Li's group is a promising direction,^[222] and in which we believe more breakthroughs will be achieved.

Acknowledgements

This work was jointly supported by the National Natural Science Foundations of China (Nos. 51972288, 51672258 and 51772279) and the Fundamental Research Funds for the Central Universities (2652018287).

Conflict of Interest

The authors declare no conflict of interest.