Cathodic shift of onset potential for water oxidation of WO3 photoanode by Zr+ ions implantation

Abstract

Tungsten trioxide is one of the most widely studied semiconductors for photoelectrochemical water splitting. However, its onset potential is too positive. In a photoelectrochemical system, a low onset potential and a high photocurrent for a photoanode are important for enhancing the efficiency of water splitting. It is an effective way to adjust the onset potential by changing the conduction and valence band level. Doping is a powerful way to alter the positions of the energy levels of semiconductors to improve their photoelectrochemical performance. In this paper, we present a method of ion implantation to alter the energy levels by implanting Zr+ ions into WO3. Cathodic shifts of the photocurrent onset potential for water oxidation are achieved. The systematic studies show that ion implantation followed by thermal annealing treatment can form substitutional Zr4+ in WO3. The upward shifts of the conduction band and valence band lead to the cathodic shifts of the onset potential. Two combined factors lead to the upward shift of the conduction band. One is strain induced after doping in the lattices. Another is due to the higher energy level of the Zr 4d orbital than the W 5d orbital. Meanwhile, the oxygen vacancy introduced during the ion implantation can cause an upward shift the valence band maximum. The results indicate that the upward shifts of the conduction band minimum and valence band maximum are good for the photoelectrochemical water splitting. It also shows that an ion implantation technique combined with thermal annealing could be an effective way to enhance the performance of the photoanode for water splitting.