Epitaxial growth of WO₃ nanoneedles achieved using a facile flame surface treatment process engineering of hole transport and water oxidation reactivity

Charge carrier dynamics and light harvesting ability are most important for the performance of a photoanode in photoelectrochemical (PEC) systems. In this work, through a facile flame surface treatment process in a reducing atmosphere, oriented WO₃ nanoneedles are grown on pre-formed vertically aligned nanohelices. Nanohelices have excellent light harvesting abilities on their own; however, the addition of nanoneedles to the top of nanohelices increases the light harvesting abilities even further. More importantly, the reducing atmosphere for the post-treatment process enhances the metallic properties of WO₃, changes the band position to facilitate hole transport, and modifies the flat band potential, all of which contribute to an improved performance in terms of photocurrent density and onset. The as-fabricated WO₃ nanohelices/nanoneedles with a metallic interface have also been used for heterojunction photoanode fabrication for water oxidation through two- and four-electron pathways for H₂O₂ and O₂ production, respectively.