Bifunctional TiO2 underlayer for α-Fe2O3 nanorod based photoelectrochemical cells: enhanced interface and Ti4+ doping

Abstract

A thin, compact TiO₂ underlayer for hematite-based photoelectrochemical cells was prepared by simple spin coating and showed a dramatic increase in device performance and photocurrent density. The introduction of TiO₂ underlayers induced a noticeable change in the nanostructure. In contrast to the conventional strategies based on underlayers, the compact TiO₂ underlayers can act as both a charge recombination barrier and also as a source for titanium dopants. One could simply take advantage of fortuitous doping of Sn from FTO into hematite lattice during the activation step, and is converted into intentional doping of Ti⁴⁺ from the TiO₂ underlayer into the hematite lattice. Ti⁴⁺ doping in hematite lattice is highly probable during the sintering of FTO/TiO₂/α-Fe₂O₃ photoanodes at 800 °C, which has been confirmed by XPS measurements. Based on electrochemical studies, it is evident that the TiO₂ underlayer effectively suppresses charge recombination at the FTO/α-Fe₂O₃ interface and provides possible Ti⁴⁺ doping apart from Sn diffusion from FTO substrates when sintered at high temperature (800 °C). In contrast, only charge recombination was suppressed at lower sintering temperature (550 °C). This is the first report on the elemental doping of Ti⁴⁺ from the TiO₂ underlayer when sintered at high temperature.