Improving the photoelectrochemical performance of spin-coated WO₃/BiVO₄/ZnO photoanodes by maximizing charge transfer using an optimized ZnO decoration layer

The photoelectrochemical (PEC) method has potential for harvesting solar energy to eventually resolve the fossil fuel crisis owing to its green and renewable characteristics and inherently low conversion efficiency. Herein, we introduce a controlled ZnO decoration layer on a WO₃/BiVO₄ type II heterojunction array using a facile spin-coating method to enhance the PEC performance of photoelectrodes. The ZnO decoration layer plays a crucial role in attaining superior charge transfer, reduced charge recombination, and stable photoactivity in the composite WO₃/BiVO₄/ZnO systems. An optimally decorated ZnO layer with oxygen vacancies in the WO₃/BiVO₄/ZnO photoanode allows extended light absorbance in the UV and visible wavelength regions with a narrow band gap and passivates the underlying WO₃/BiVO₄ type II heterojunction surfaces by covering the defect sites. Thus, the optimized WO₃/BiVO₄/ZnO-3 photoanode exhibits the highest photocurrent density (190 μA cm⁻² at 1.23 V_RHE) with the lowest onset potential (0.41 V_RHE), reproducible photo-reaction under chopped light, the smallest charge transfer resistance in the Nyquist plot (1755 Ω), the highest quantum efficiencies for ABPE, IPCE, and APCE (0.036% at 0.92 V_RHE, 6% at 380 nm at 1.23 V_RHE and 11% at 380 nm, respectively), the highest reliability (30% decay after 1500 s), and high charge injection (63%) and charge separation (7.3%) efficiencies. These findings demonstrate that the component optimization in WO₃/BiVO₄/ZnO-x composite photoelectrodes is crucial in achieving high PEC performance by improving charge separation and reducing charge recombination.