Fig. 1. Schematic diagram of photocatalytic water splitting over a semiconductor photocatalyst loaded with reduction and oxidation cocatalysts.

Fig. 2. Brief schematic diagram of the entire content of this review.

Fig. 3. Schematic diagram of reductive photodeposition (a) and oxidative photodeposition (b).

Fig. 4. Categories of the photodeposited earth-abundant cocatalysts in photocatalytic water splitting.

Fig. 5. Roles of the photodeposited earth-abundant cocatalysts in photocatalytic water splitting.

Fig. 6. Influencing factors in the photodeposition of earth-abundant cocatalysts.

Fig. 7. Modification strategies toward photodeposition of earth-abundant cocatalysts for photocatalytic water splitting.

Fig. 8. Design considerations on photodeposition of earth-abundant cocatalysts for photocatalytic water splitting.

Fig. 9. (a) High-magnification TEM image of the Ni-TiO2 composite. (b) Comparison of the photocatalytic H2 evolution activity over Ni-TiO2 with different addition amounts of Ni(NO3)2. (c) Schematic illustration of Ni photodegradation and photocatalytic H2 production over Ni-TiO2. Reproduced with permission from Ref. [97]. Copyright 2013, The Royal Society of Chemistry.

Fig. 10. TEM image (a), HRTEM image (b), and SEM-EDX elemental mapping images (c) of C, N, and Ni of the Ni/g-C3N4 composite. (d) Comparison of the photocatalytic H2 evolution activity of Ni/g-C3N4 with different Ni loading amounts. (e) Photocatalytic H2 evolution of Ni/g-C3N4 irradiated by natural and stimulated sunlight. (f) Photocatalytic H2 evolution of Ni/g-C3N4 in the recycle experiment. (g) UV-Vis diffuse reflectance spectra of g-C3N4 and Ni/g-C3N4 and the H2 evolution rate of Ni/g-C3N4 along with different excitation light wavelengths. (h) Schematic diagram of photocatalytic H2 evolution over Ni/g-C3N4. Reproduced with permission from Ref. [50]. Copyright 2016, The Royal Society of Chemistry.

Fig. 11. (a) Schematic illustration of the photochemical synthesis of Ni1/CdS. XANES spectra (b) and the corresponding Fourier transform curves (c) of NiS, Ni1/CdS, and Ni(OH)2 reference at Ni K-edge. (d) Photocatalytic H2 evolution activity of Ni1/CdS, Ni(OH)2 NPs/CdS, and CdS. (e) Long-term photocatalytic H2 evolution activity of Ni1/CdS. (f) Gibbs free energy for H* adsorption on Ni1/CdS, Ni(OH)2 NPs/CdS and CdS. (g) Charge density distributions of conduction band edges of Ni1/CdS, Ni(OH)2 NPs/CdS and CdS. (h) Differential charge density maps of Ni1/CdS and Ni(OH)2 NPs/CdS. Reproduced with permission from Ref. [82]. Copyright 2020, Elsevier.

Fig. 12. HRTEM image (a) and selected area electron diffraction patterns (b) of ZnxCd1-xS/MoS2 junction. (c) Comparison of photocatalytic H2 evolution of ZnxCd1-xS/MoS2 with different atom ratios of Zn/Cd. (d) The loading contents of MoS2. Reproduced with permission from Ref. [52]. Copyright 2013, The Royal Society of Chemistry.

Fig. 13. (a) Schematic illustration of synthesizing MoS2 deposited g-C3N4 using photodeposition and physical mixing treatment. (b) Photocatalytic H2 evolution activities over MoS2 with different contents of deposited g-C3N4 composites using photodeposition. Single-particle photoluminescence images of pure g-C3N4 (c) and MoS2 deposited g-C3N4 composites using photodeposition (d) and physical mixing (e) treatment. Time-resolved diffuse reflectance spectra of pure g-C3N4 (f) and MoS2 deposited g-C3N4 composites (g) using photodeposition and heat mixing (h) treatment. Reproduced with permission from Ref. [24]. Copyright 2018, John Wiley and Sons.

Fig. 14. (a) Schematic diagram of photochemical preparation of NiS loaded g-C3N4 hybrid. TEM (b) and HRTEM (c) images of NiS loaded g-C3N4. (d) Comparison of the photocatalytic H2 evolution activity in the control experiment. (e) Photocatalytic H2 evolution in the recycle experiment. (f) UV-Vis diffuse reflectance spectra of g-C3N4 and NiS loaded g-C3N4 and the H2 evolution rate of NiS loaded g-C3N4 along with different excitation light wavelengths. (g) Schematic diagram of photocatalytic H2 evolution mechanism over NiS loaded g-C3N4. Reproduced with permission from Ref. [73]. Copyright 2018, Elsevier.

Fig. 15. (a) Schematic illustration for the photodeposition route of CoxP loaded CdS. (b) SEM elemental mapping of Cd, S, Co, and P of CoxP loaded CdS. (c) Photocatalytic H2 evolution activity over CoxP loaded CdS. (d) UV-Vis diffuse reflectance spectra of CdS and CoxP loaded CdS and the H2 evolution rate of CoxP loaded CdS along with different excitation light wavelengths. (e) Schematic illustration for photocatalytic H2 evolution mechanism of CoxP loaded CdS. (a-e) Reproduced with permission from Ref. [56]. Copyright 2016, Elsevier. (f) TEM elemental mapping of C, N, Ni, and P of NixP/g-C3N4 composite. (g) Schematic illustration for photocatalytic H2 evolution mechanism of NixP/g-C3N4 composite. (f,g) Reproduced with permission from Ref. [55]. Copyright 2017, American Chemical Society.

Fig. 16. (a) Schematic diagram of the TEOA-mediated photodeposition formation of Ni-P loaded g-C3N4. TEM images (b,c) and HRTEM image (d) of Ni-P loaded g-C3N4. (e) Photocatalytic H2 evolution in the control experiment. (f) Schematic illustration of photocatalytic H2 evolution mechanism over Ni-P loaded g-C3N4. Reproduced with permission from Ref. [87]. Copyright 2020, Springer.

Fig. 17. (a) HRTEM image of Co-Pi loaded CdS. (b) Comparison of the photocatalytic hydrogen production rates of CdS loaded with different amounts of Co-Pi and 1 wt% Pt in lactic acid aqueous solution under visible light irradiation. (c) Schematic illustration for photocatalytic H2 evolution mechanism of Co-Pi loaded CdS. (a-c) Reproduced with permission from Ref. [140]. Copyright 2016, Elsevier. (d) Comparison of the photocatalytic H2-production activities for TiO2 and Co3O4 composite samples. Reproduced with permission from Ref. [141]. Copyright 2016, Elsevier.

Fig. 18. (a) Schematic diagram for the preparation of CdS@MoS2@Co-Pi. TEM images of pure CdS (b) and CdS@MoS2@Co-Pi (c). (d) EDX elemental mapping of Cd, Mo, S, Co, and P in CdS@MoS2@Co-Pi. (e) Photocatalytic H2 evolution activity over different cocatalysts. (f) Schematic diagram for the photocatalytic H2 evolution mechanism of CdS@MoS2@Co-Pi. (g) Recycling test of photocatalytic H2 evolution over CdS@MoS2@Co-Pi. Reproduced with permission from Ref. [94]. Copyright 2019, American Chemical Society.

Fig. 19. Optimized slab structures for {100} (a), {010} (b), and {001} (c) slabs of Bi2MoO6. Calculated densities of states for {100} (d), {010} (e), and {001} (f) slabs of Bi2MoO6. (g) Schematic diagram of charge-separation process within Bi2MoO6. (h) SEM image of CoOx-Bi2MoO6 using photodeposition. (i) Photocatalytic oxygen evolution activity over pure Bi2MoO6 and CoOx-Bi2MoO6 using photodeposition and impregnation. Reproduced with permission from Ref. [15]. Copyright 2018, American Chemical Society.

Fig. 20. (a) High-resolution Co 2p XPS spectra of TaON/LVCoOx and TaON/HVCoOx. (b) Schematic illustration of the crystal structure of TaON/LVCoOx; (c) Normalized Co K-edge XANES spectra of TaON/LVCoOx and control samples, indicating the dominant Co2+ species in the bulk. TEM images of LVCoOx (d) and HVCoOx (e) (the inset is the particle size distribution of CoOx). (f) Photocatalytic O2 evolution activity over different photocatalysts. (g) Absorption spectrum and wavelength-dependent AQE of photocatalytic O2 production for TaON/LVCoOx. (h) Schematic diagram for the photocatalytic O2 evolution mechanism over CoOx loaded TaON. Reproduced with permission from Ref. [86]. Copyright 2021, The Royal Society of Chemistry.

Fig. 21. (a,b) SEM images of Co-Pi loaded BiVO4. (c) Photocatalytic O2 evolution over CoPi loaded BiVO4 with different loadings of Co-Pi. (d) Photocatalytic O2 evolution over BiVO4 modified with different cocatalysts. Reproduced with permission from Ref. [61]. Copyright 2012, American Chemical Society.

Fig. 22. SEM images of BiVO4 (a) and CoOx(OH)y/BiVO4 (b). (c) Photocatalytic O2 evolution over bare BiVO4 (a, blue), NiOx(OH)y/BiVO4 (b, magenta), and CoOx(OH)y/BiVO4 (c, red). SEM images of BiVO4 loaded by NiOx(OH)y with contents of 10 wt% (d), 1 wt% (e), and 0.1 wt% (f). (g) Schematic diagram for the energy band alignment for CoOx(OH)y/BiVO4 and NiOx(OH)y/BiVO4 along the (010) and (110) directions. (h) Schematic diagrams for oxygen evolution over CoOx(OH)y/BiVO4 and NiOx(OH)y/BiVO4 using NaIO3 as sacrificial agent. Reproduced with permission from Ref. [63]. Copyright 2020, American Chemical Society.

Fig. 23. (a) Dark field STEM image and histograms of metal particle size distribution of CoOx loaded MIL-125(Ti)-NH2. (b) Photocatalytic H2 and O2 evolution over CoOx loaded MIL-125(Ti)-NH2. (a,b) Reproduced with permission from Ref. [64]. Copyright 2019, Elsevier. (c) Photocatalytic H2 and O2 evolution over different SrTiO3(Al) based photocatalysts. Reproduced with permission from Ref. [160]. Copyright 2020, The Royal Society of Chemistry. (d) Photocatalytic H2 and O2 evolution over mpg-CNx-CoPi. Reproduced with permission from Ref. [151]. Copyright 2013, The Royal Society of Chemistry. (e) Photocatalytic overall water splitting over MnOx/CdS/Ti3+-SrTiO3 in the repeated experiment. Reproduced with permission from Ref. [65]. Copyright 2021, Elsevier. (f) Photocatalytic H2 and O2 evolution over Ba5Ta4O15 loaded by Cr2O3 with different deposition amounts. Reproduced with permission from Ref. [66]. Copyright 2016, John Wiley and Sons. (g) Schematic illustration for the deposition mechanisms of reductively photodeposited CrO42- and oxidatively photodeposited Cr2+. Reproduced with permission from Ref. [159]. Copyright 2017, John Wiley and Sons.

Fig. 24. TEM images of NiOx loaded GaN:ZnO (a) and CrOx-NiOx loaded GaN:ZnO (b). (c) Time courses of visible-light-driven overall water splitting on CrOx-NiOx loaded GaN:ZnO (circles) NiOx loaded GaN:ZnO (diamonds). Solid and open symbols indicate H2 and O2, respectively. (a-c) Reproduced with permission from Ref. [162]. Copyright 2010, John Wiley and Sons. (d) H2 and O2 evolution over CrOx-CuO loaded GaN:ZnO under ultraviolet-visible light irradiation. Reproduced with permission from Ref. [163]. Copyright 2011, The Royal Society of Chemistry.

Fig. 25. Selected-area electron diffraction pattern obtained from Rh/Cr2O3/CoOOH-loaded SrTiO3:Al (a) and corresponding transmission electron microscopy image of a particle (b). (c) Particle morphology and crystal orientation. Simulations of photocarrier distributions in SrTiO3:Al particles: Mapping of conduction-band energy, Ec (d); density of electrons (e-), n (e); density of holes (h+), p (f); energy band diagram (g); and electron and hole densities (h) as functions of position (x′, y′) with work function difference ΔWel = 0.2 eV. (i) Effect of ΔWel on electron-to-hole-density ratio at the {100} and {110} facets. (j) Ultraviolet-visible diffuse reflectance spectrum of bare SrTiO3:Al (black solid line) and wavelength dependence of external quantum efficiency (EQE) during water splitting on Rh/Cr2O3/CoOOH-loaded SrTiO3:Al (red symbols). (k) Recycle performance of Rh/Cr2O3/CoOOH-loaded SrTiO3:Al in photocatalytic water splitting. Reproduced with permission from Ref. [167]. Copyright 2020, Nature.

Fig. 26. (a) Schematic diagram for photodeposition of Pt and MnOx on CdS/g-C3N4 heterojunction. (b) Photocatalytic overall water splitting over CdS/g-C3N4 co-loaded by Pt and MnOx. (a,b) Reproduced with permission from Ref. [63]. Copyright 2020, American Chemical Society. (c) Schematic diagram for photocatalytic overall water splitting mechanism of Pt-ZnIn2S4/rGO/Co3O4-BiVO4 composite. (d) Photocatalytic overall water splitting over Pt-Znln2S4/GO/Co3O4-BiVO4. (c,d) Reproduced with permission from Ref. [89]. Copyright 2021, Springer.