Core–shell porous ZnO p–n homojunction nanorod arrays for achieving superior water splitting efficiency via piezocatalytic and piezo-phototronic effects

Abstract

Photoelectrochemical (PEC) water splitting converts solar energy into chemical energy using semiconductor photocatalysts to split water into hydrogen and oxygen under light illumination and external bias, representing a promising green hydrogen method. In this study, ZnO nanorod (NR) arrays were selected as active materials due to the optical surface area, suitable band structure, and intrinsic piezoelectric properties, enabling piezoelectric modulation of the energy bands. To overcome the rapid carrier recombination that limits the PEC performance of ZnO, annealing treatment and Cu doping were combined with piezocatalytic and piezo-phototronic effects to enhance carrier transfer efficiency. The annealing treatment enhances crystallinity and induces pore formation, therefore enhancing piezoelectricity. Meanwhile, the independent Cu doping process enables Cu atoms to diffuse from all the top and side surfaces into the NRs to a depth of approximately 40 nm, forming a Cu-doped ZnO (CZO) shell structure, leading to a ZnO/CZO core–shell p–n homojunction. The optimized annealed porous ZnO and CZO/ZnO photoelectrodes exhibited photocurrent densities of 0.29 mA cm⁻² and 0.49 mA cm⁻², corresponding to 1.6-fold and 2.7-fold improvements over pristine ZnO, respectively. The CZO/ZnO photoelectrode achieved an IPCE of 33.9%, approximately threefold higher than that of pristine ZnO. Additionally, the piezoelectric properties of ZnO were leveraged to induce band bending and facilitate charge separation through the piezo-phototronic effect. Notably, under upward bending (strain, ε = −0.15%), the CZO/ZnO photoelectrode achieved a photocurrent density of 0.71 mA cm⁻², which is 4 times that of pristine ZnO, highlighting the synergistic effects of doping, p–n junction and piezoelectric modulation in enhancing PEC efficiency.