Charge Distribution in Nanostructured TiO₂ Photoanode Determined by Quantitative Analysis of the Band Edge Unpinning

Abstract

Conduction band (CB) edge position, trap state distribution and electron density are considered to be the most important properties of the mesoporous semiconductor photoanodes for detailed understanding of the various charge transfer processes associated with them. The electroactive surface states on these high surface area electrodes cause potential drop across the Helmholtz layer resulting in the unpinning of band edges. Herein, we present a spectroelectrochemical method that determines the magnitude of the CB unpinning and its effect on the overall electron distribution. The position of CB edge is determined from the band gap widening, also known as Burstein-Moss shift, observed under negative bias. Additionally, the spatial location of the trap states is also estimated. Trapped electrons are found to be distributed on the surface as well as in the bulk and with increasing negative potential bulk traps dominate the overall trap density.