Spatially Engineered WO₃ Nanofibers on BiVO₄: A Route to High-Efficiency Photoelectrochemical Water Splitting

This study employs aerosol-assisted chemical vapor deposition (AACVD) to fabricate WO₃/BiVO₄ heterojunction photoanodes with an inverted architecture (WO₃ atop BiVO₄). The unique permeable nanofiber morphology of WO₃ provides a solution to enhance water oxidation performance. By correlating precursor volume (10-40 mL) and spatial position within the deposition chamber (inlet/mid/outlet) with film properties, we demonstrate that a midreactor position yields "grass-like" WO₃ nanofibers (diameter: 100-230 nm, length: 3.5-3.98 μm), enabling dual functionality: (i) > 50% light transmittance to the underlying BiVO₄ absorber, and (ii) electrolyte penetration into the heterointerface between WO₃ and BiVO₄. In contrast, rod-like WO₃ produced near the inlet causes severe light scattering, reducing the incident photon-to-current efficiency (IPCE) by six times above wavelengths of 350 nm. Optimized samples, produced with a deposition volume of 30 mL to deposit WO₃ atop of BiVO₄ positioned in the middle of the deposition chamber (i.e., WO₃-30/BiVO₄-mid), achieve a photocurrent density of 0.82 mA·cm^-2 at 1.23 V_RHE under 1 sun irradiance, which is 121% higher than single-layer BiVO₄ (0.37 mA·cm^-2) and exceeds some conventional WO₃-under/BiVO₄ heterojunctions in which WO₃ is underneath BiVO₄. Transient absorption spectroscopy confirms prolonged carrier lifetimes in our unique heterostructure through improved charge-carrier separation. This work challenges current traditional heterojunction design rules for the WO₃/BiVO₄ system by showcasing how permeable WO₃ nanostructures atop BiVO₄ photoanodes can improve light harvesting and facilitate charge-carrier separation to significantly improve activity.