Fig. 1. (Color online) PEC water splitting in (a) the n-type semiconductor-based PEC system, (b) p-type semiconductor-based PEC system, and (c) tandem system^[6].

Fig. 2. (Color online) Main processes of PEC water splitting for n-type semiconductors.

Fig. 3. (Color online) (a) ZnO model of the hexagonal wurtzite structure, (b) schematic illustrations of atoms and charges distribution in the unit cell of Wurlitzer-structure ZnO, where F and P represent the applied stress and the induced electric dipole moment, respectively^[11].

Fig. 4. (Color online) Energy potentials of ZnO and redox potentials for PEC water splitting at pH = 7, relative to NHE (normal hydrogen electrode).

Fig. 5. (Color online) Carrier transport mechanism of the ZnO photoanode.

Fig. 6. (Color online) Schematic illustration of the preparation processes of CS ZnO/TiO₂ and BN ZnO/TiO₂^[24].

Fig. 7. (Color online) Effect of element doping on band structure^[27].

Fig. 8. (Color online) (a) Schematic diagram of N gradient doped ZnO nanorods and stepped band structure to promote carrier separation^[15]. (b) Morphological benefits of Y doping and schematic of increased electron mobility from trap filling^[30].

Fig. 9. (Color online) Schematic diagrams of the forms of (a) type-II junction, (b) p–n junction, (c) Z-scheme system, and (d) hot-electron injection^[3].

Fig. 10. (Color online) Schematic illustration of the proposed mechanism for the charge transfer (a) in ZnWO₄/ZnO photoanode^[31], (b) between ZnO and MoS_x co-catalyst^[32], (c) for the system of ZnO/CdS/PbS ONTs^[19], and (d) ZnO–Au–SnO₂^[34].

Fig. 11. (Color online) (a) Bilateral CdS–ZnO–ZnO–CdSe nanowire array photoanode structure and corresponding energy level diagram^[37]. (b) Synthetic route diagram and (c) schematic of the potential energy diagram of the ZnO/ZnFe₂O₄/PbS nanorod arrays electrode^[38].

Fig. 12. (Color online) (a) The main mechanism of Au/3D ZnO nanowire photoelectrode^[40]. (b) Schematic diagram of bending the sample to bending radius R under light^[41].

Fig. 13. (Color online) Schematic band alignment of charge transport and recombination models in (a) ZnO and (b) FVO/ZnO photoanodes^[48].

Fig. 14. (Color online) Piezo-phototronic effect on the photoelectrocatalytic process (photoanode). Illustration of the photoelectrocatalytic process (a) without strain, (b) under tensile strain, and (c) under compressive strain^[11].

Fig. 15. (Color online) Schematic illustration of the enhanced catalytic performance induced by piezotronic effect and unique asymmetric nanostructure under light irradiation and ultrasonic actuation (ϕ_SB, Schottky barrier; E_CB and E_VB, the CB and VB of ZnO, respectively; E_f, the Fermi level of the Asy–Au–ZnO composite structure)^[53].