Controlling crystallographic orientation in h-WO3 films to maximize photoelectrochemical water splitting efficiency

Abstract

Hexagonal tungsten trioxide (h-WO₃) is a theoretically promising photoanode material due to its unique crystal structure and enhanced charge transport properties, which results from the W–O–W bond angle approaching 180°. However, its practical applicability in photoelectrochemical water splitting has been limited owing to improper crystallographic orientation. In this study, we adopted a precise precursor-control strategy within a hydrothermal synthesis framework to enable selective facet orientation in h-WO₃ films. By leveraging NH₄⁺ ion adsorption, we directed growth predominantly along the [110] axis while modulating the exposure of the (200) and (001) facets. The WO₃-110 photoanode achieved a superior photocurrent density of 1.86 mA cm⁻² at 1.23 V vs. RHE, surpassing WO₃-200 and WO₃-001 by factors of 1.6 and 3.3, respectively. The (110) facet demonstrated a photo-induced charge separation efficiency of 58.8%, highlighting its role in efficient charge separation. This efficiency was further evidenced by the WO₃-110 photoanode's notable performance in preliminary PEC oxidation tests of glucose and glycerol, yielding photocurrent densities of 2.71 mA cm⁻² and 2.93 mA cm⁻², respectively. These findings not only validate the theoretical promise of h-WO₃ but also reveal how crystal facet engineering can unlock its potential, advancing the field of facet engineering in photoelectrocatalysis.