Photoelecrochemical investigation of film formation at titanium and niobium anodes in acid fluoride media

摘要

The properties of the surface oxide films during electrodissolution of titanium and niobium anodes in acid fluoride media are investigated by photoelectrocemical and impedance techniques.In slow voltammetries a linear dependence of photocurrent signal on applied potential is observd,consistent with a constant film formation ratio;an estimate of the latter is given on the basis of capacity data.Phenomena of depassivation observed at the Nb anode during voltammetric and potential step experiemtns are characterised and attriubted to instability of the oxide film.The photocurrent spectra of both surface oxides change in shape under polarisation,showing a limited decrease of the photoresponse for penetrating wavelengths close to the bandgap and a marked decay for the shorter,strongly absorbed wavelengths.The phenomenon may be attriubted to formation of a duplex oxide film,with a compact photoactive layer in contact with the metal substrate and an outer layer a tthe fim|solution interface,rich in defects and possibly porous/hydrated,which acts as an optical filter.