Back Electron Transfer at TiO2 Nanotube Photoanodes in the Presence of a H2O2 Hole Scavenger.

Abstract

Adding charge scavengers, which usually are more unstable than water, is an effective method to quantify the quantum efficiency loss of photoelectrode during the charge separation, transfer, and injection processes of the water splitting reaction. Here, we detected, on TiO2 nanotube photoanodes after using hydrogen peroxide (H2O2) as a hole scavenger, a nearly 40% saturated photocurrent decrease in alkaline electrolyte and a negligible saturated photocurrent difference in acid electrolyte. We found that the photoelectrons were trapped in the surface states of TiO2 with nearly the same storage capacity of electrons in a wide range of pH values from 1.0 to 13.6. However, kinetics of a back reaction, H2O2 reduction by the photoelectrons trapped in surface states, is about 10 times higher for that in alkaline electrolyte than in acid electrolyte. As a result, the pH-dependent kinetic difference in H2O2 reduction induced the negative effects on the saturated photocurrent. Our results offer a new insight into understanding the effects of back electron transfer on electrochemical behaviors of surface states and charge scavengers.