The effect of anodization conditions on the morphology of porous tungsten oxide layers formed in aqueous solution

Abstract

Nanostructured WO3 layers were obtained by one-step anodic oxidation of tungsten in an aqueous solution containing fluoride ions. A detailed investigation of the growth of anodic films on metallic substrate was focused on the study of the influence of anodization conditions, such as anodization potential, duration of the process, electrolyte composition, applied temperature, and electrolyte agitation on morphological features of as-received materials. Such kind of comprehensive studies has been performed for the first time. It was found that the optimal conditions allowing synthesis of nanostructures with well-defined pores and the thickest possible oxide layer are: anodization potential of 50 V, anodization time of 4 h, the electrolyte containing 1 M ammonium sulfate and 75 mM ammonium fluoride, temperature of 20 °C, and agitation of electrolyte of 250 rpm. A post-treatment procedure was developed to effectively remove the precipitates formed on the surface of anodic layers during anodization. The obtained materials were also examined as photoanodes in photoelectrochemical (PEC) water splitting experiments. It was confirmed, that PEC performance of nanostructured WO3 photoanodes can be significantly improved (higher photocurrents by 24%) after ultrasonic treatment in a 20 wt% HF solution for 10 s and ethanol for 5 s.