Fig. 1. Scheme showing overall structure of the present review article.

Fig. 2. Mechanism of photocatalytic water splitting over the surface of a semiconductor photocatalyst [27].

Fig. 3. The electronic band structure and thermodynamics of photocatalytic water splitting over the surface of a semiconductor photocatalyst [30].

Fig. 4. WBG Systems I) with suitable band structure for water splitting and II) not with suitable inherent band structure for water splitting.

Fig. 5. Volcano plot of WBG systems.

Fig. 6. Synthesis of various WBG semiconductor based systems.

Fig. 7. Schematic representation of design strategies of efficient HER electro and photocatalyst.

Fig. 8. Mechanism of electrocatalytic HER from acidic and alkaline medium.

Fig. 9. Free energy diagram of HER: (a) Volmer-Heyrovsky; (b) Volmer-Tafel route.

Fig. 10. Mechanism of direct (a) and indirect (b) mechanism of HER.

Fig. 11. Mott-Schottky plots vs. Ag/AgCl electrode (a), Normal hydrogen electrode (b), VB-CB positions (c) of TiO2, SnO2 and g-C3N4 [198].

Fig. 12. (a) EIS of bare SPCE and ND-GCSs@SPCE electrodes (a larger presentation of bare SPCE data is shown in the inset). (b) HER stability test of the ND-GCSs@SPCE electrocatalyst before and after 1000 cycles. Reproduced with permission from Ref. [230].

Fig. 13. HER activity of different electrodes. Linear sweep voltammograms of NHC, HC, graphite and Ti electrodes in 0.5 mol L?1 H2SO4 (scan rate 5 mV s?1). Reproduced with permission from Ref. [231].

Fig. 14. Tauc plot of CeO2 (A), Ce/Al (3:1) (B), Ce/Al (1:1) (C), and Ce/Al (1:3) (D). Reprinted with permission from Ref. [5]. Copyright 2020, Elsevier.

Fig. 15. (a) 3D representations of Cu2O particles. (i) Cubic; (ii) octahedron; (iii) truncated Octahedron; (iv) truncated octahedron PVP-Cu2O; (v) star. Morphologies of Cu2O. (b) Cubic. (c) Octahedron. (d) Truncated Octahedron. (e) Truncated octahedron PVP-Cu2O. (f) Star. (g) XRD patterns Cu2O particles of different geometry. Reprinted with permission from Ref. [253]. Copyright 2023, MDPI.

Fig. 16. Straddling heterojunction (a), staggered heterojunction (b), general representation (c) of Broken-up heterojunction. (d) Broken-up heterojunction in VO/AC/GCO composite. Reprinted with permission from [7]. Copyright 2021, Elsevier. Reprinted with permission from [273]. Copyright 2021, Elsevier. Reprinted with permission from [275]. Copyright 2018, Wiley.

Fig. 17. Charge transfer at Z-scheme heterojunction (a), S-scheme heterojunction (b), p-n junction (c), and Schottky junction (d). Reproduced with permission from [276]. Copyright 2017, Elsevier. Reproduced with permission from [280]. Copyright 2023, ACS. Reproduced with permission from [282]. Copyright 2010, ACS. Reproduced with permission from [283]. Copyright 2013, Elsevier.

Fig. 18. Charge transfer route in z-scheme and s-scheme system.

Fig. 19. Charge transfer route in p-n junction (a) and Schottky junction (b).

Fig. 20. Charge transfer process through dual S-scheme (a), dual z-scheme (b) and combined S-scheme-p-n junction (c). Reprinted with permission from [285], Copyright 2017, ACS. Reprinted with permission from [286]. Copyright 2018, Elsevier. Reprinted with permission from [287]. Copyright 2013, Springer.

Fig. 21. Flower like surface morphology obtained by SEM imaging. (a) ZnO. (b,c) Optimized ZnO-ZnS composite. Reprinted with permission from Ref. [20]. Copyright 2023, Elsevier.

Fig. 22. Mechanism of hydrogen production by photoelectrocatalysis.

Fig. 23. SAC Diatomic catalysis.