Functional Blocking Layer of Twisted Tungsten Oxide Nanorod Grown by Electrochemical Anodization for Photoelectrochemical Water Splitting

Abstract

Using electrochemical anodization of tungsten foil in a fluorinated-based electrolyte, the twisted WO3 nanorods (T-WO3 NRs) were grown, and the film properties were controlled by post-annealing conditions. The anodized WO3 material was composed of two distinct layers; the T-WO3 NRs which functions as an active layer for the PEC reaction and another compact WO3 (C-WO3) barrier which acts as a blocking layer for the charge transport. From more resistance of thicker C-WO3 layer, to minimize the formation of the C-WO3 layer under constant applied voltage, we performed rapid-thermal annealing under air ambient as a post-thermal treatment. As the post-annealing time increased, the thickness of T-WO3 NRs layer remained at approximately 680 nm, but the C-WO3 thickness gradually increased from about 91 nm at 1 min to 633 nm at 120 min. Unexpectedly, the anodic WO3 film having a compact layer thickness of about 475 nm exhibited the highest PEC performance, corresponding to a photocurrent density of 0.75 mA cm−2 at 1.23 V vs reversible hydrogen electrode, subsequently followed by anodic WO3 films with less thick compact layers. This suggests that the C-WO3 layer promotes the charge separation/transfer events by formation of cascading band alignments between the T-WO3 NR and C-WO3 layers.