The effects of adjusting pulse anodization parameters on the surface morphology and properties of a WO3 photoanode for photoelectrochemical water splitting

An ordered porous nanostructure provides a large reaction interface with an unusually high number of active sites, meaning that such a nanostructure is especially applicable to photoelectrochemical (PEC) water splitting. Therefore, we prepared WO3 films on W foil by pulsed anodization using square-pulse on/off voltage followed by calcination, and scrutinized the effects of reaction parameters-particularly the duty ratio, frequency, and F- ion content-on the surface morphology and PEC behavior of the films. The WO3 films produced with a pulsed voltage of 50 V, a duty ratio of 20%, and a pulse frequency of 200 Hz in an electrolyte of 0.06 M NH4F showed an ordered and porous morphology. WO3 films prepared under optimized conditions yielded a water splitting photocurrent density of 1.33 mA cm(-2) at a bias potential of 1.2 V when exposed to AM 1.5 G 1-sun illumination in 0.5 M Na2SO4 electrolyte. The high PEC activity of the ordered porous WO3 films can be attributed to their ordered porous nanostructure, which results in a much larger surface area than in compact or disordered porous structures. Moreover, the ordered porous WO3 films also exhibited excellent stability and a high incident-photon-to-charge conversion efficiency (IPCE) of 57.8% at 350 nm and a bias potential of 1.2 V. This research demonstrates that the pulsed anodization technique allows the controlled fabrication of porous WO3 nanostructures for application in PEC water oxidation.