Bismuth(III) oxide, which is pale yellow, functions as an amphoteric semiconductor, although it is relatively unstable upon illumination with UV light. The energy level positioning, corrosion limits and photoactivity of samples of n-type bismuth oxide have been determined by electrochemical techniques. It was found that this material is able to oxidise water to molecular oxygen upon illumination with UV light.
Bismuth oxide forms a wide range of well-defined solid solutions with many other oxides and, in particular, niobium oxide can be used to form several distinct structures, each based upon defective fluorite. Each of these structures will function as an n-type semiconductor, retaining the basic properties of undoped bismuth oxide. However, the exact composition of the solid solution determines the band-gap energy of the semiconductor. Low concentrations of niobium in the lattice induce a substantial lowering in the band-gap energy and give rise to orange coloured materials that can collect a higher fraction of the solar spectrum. Photoelectrochemical studies showed that the presence of niobium caused the valence band to move to lower potentials but had little effect upon the energy of the conduction band. The photochemical properties of these solid solutions were studied using the photooxidation of alcohols as a test system. The results are discussed in terms of the structure of the oxide.