Photoelectrochemical properties of anodic oxide film on niobium

Photoelectrochemical properties of niobium oxide film formed by anodic oxidation in a saturated boric acid solution have been studied. The oxide film is considered as an n-type semiconductor from the polarization curves measured in 0.5N sulfuric acid solution. Anodic photocurrent is observed when the oxide film is illuminated by an ultraviolet light while it is anodically polarized. From the spectrum of the photocurrent of the film, the band gap energy of the oxide is determined to be about 3.3 eV. When the specimen is polarized at a cathodic potential before the photocurrent measurements, the film shows an additional photoresponse for the light with longer wavelength than that corresponds to the band gap energy. This can be explained by the formation of new electron levels in the band gap by cathodic hydrogen charging.