Photoreduction of Hydrogen Cations on TiO2 and Its Impact on Surface Band Bending and the Charge Carrier Recombination Rate: A Photoluminescence Study under High Vacuum Conditions

The interaction between hydrogen species and TiO2 surfaces is important because of its relevance to hydrogen production in the photocatalytic splitting of water on TiO2. In this study, the effect of the photocatalytic reduction of hydrogen cations, a half photoelectrochemical reaction run under high vacuum conditions, on the surface band bending of TiO2 was investigated by photoluminescence (PL) spectroscopy. Exposure of TiO2 to molecular hydrogen had no effect on the PL emission from TiO2, but the PL intensity increased under UV irradiation when TiO2 was exposed to hydrogen cations produced by ionization of molecular hydrogen by a cold cathode pressure gauge. The PL intensity increase, signaling a decrease in the upward band bending and increased electron-hole radiative recombination rate, is caused by charge transfer during photoreduction of hydrogen cations. Residual photoexcited holes left in TiO2 due to the transfer of photoexcited electrons to hydrogen cations tend to accumulate at the TiO2 surface, balancing the originally trapped electrons at the TiO2 surface and thereby lowering the original upward band bending. These unusual observations point out that charge transfer during a photoelectrochemical reaction at a semiconductor interface alters the surface band bending and photoexcited electron-hole recombination rate in ways that are likely to impact the efficiency of photocatalytic devices.