Nanoporous TiO2 and WO3 Films by Anodization of Titanium and Tungsten Substrates: Influence of Process Variables on Morphology and Photoelectrochemical Response

Abstract

The photoelectrochemical response of nanoporous films, obtained by anodization of Ti and W substrates in a variety of corrosive media and at preselected voltages in the range from 10 to 60 V, was studied. The as-deposited films were subjected to thermal anneal and characterized by scanning electron microscopy and X-ray diffraction. Along with the anodization media developed by previous authors, the effect of poly(ethylene glycol) (PEG 400) or d-mannitol as a modifier to the NH₄F electrolyte and glycerol addition to the oxalic acid electrolyte was studied for TiO₂ and WO₃, respectively. In general, intermediate anodization voltages and film growth times yielded excellent-quality photoelectrochemical response for both TiO₂ and WO₃ as assessed by linear-sweep photovoltammetry and photoaction spectra. The photooxidation of water and formate species was used as reaction probes to assess the photoresponse quality of the nanoporous oxide semiconductor films. In the presence of formate as an electron donor, the incident photon to electron conversion efficiency (IPCE) ranged from ∼130% to ∼200% for both TiO₂ and WO₃ depending on the film preparation protocol. The best photoactive films were obtained from poly(ethylene glycol) (PEG 400) containing NH₄F for TiO₂ and from aqueous NaF for WO₃.