The effects of 4d and 5d transition metals (Nb, Mo, Ta, W) on photosensitization property of the rutile titanium oxide were investigated by a series of experiments and a molecular orbital method using a cluster model of the rutile.
The cluster model used in this study is MTi10O22, in which the central Ti atom in the cluster is replaced by each transition element, M. The electronic structure of the rutile is simulated by the DV-Xα method using the cluster. Photoelectrode characteristics of the rutile titanium oxide doped with each transition element are evaluated by a series of experiments. The doped specimens are prepared by oxidation of Ti-5 mol%M alloys. The results obtained from the experiments and the calculation are as follows; It is confirmed by a conventional X-ray diffraction method that we can make the pure rutile titanium-oxide as well as the oxides doped with the transition elements can be made by means of the oxidation of single phase of Ti-M alloys. It is suggested by the calculation that the donor levels of the additional transition elements appear in the energy gap of the rutile and that the donor levels of the additional transition elements lower in order of Nb, Mo, Ta, W, and their levels are 0.17eV, 0.58eV, 0.79eV, 0.89eV, respectively, below the conduction band of the rutile. The photocurrents density in the anodic polarization measurements starts to increases at about 0V, and it saturates at about 1200mV vs. SCE. The saturation photocurrent-density of the (Ti, M)O2 specimens depends on the additional element, M, and it lowers in order of M: Nb, Ta, Mo, W. It is found that the increase of photocurrent density at 620, 730nm in the (Ti, Ta)O2 and (Ti, W)O2 rutile specimens is caused by the existence of the donor levels of the additional transition elements.