ChemInform Abstract: Photoelectrochemical and Impedance Characteristics of Specular Hematite. Part 1. Photoelectrochemical, Parallel Conductance, and Trap Rate Studies.

ChemInform Abstract

Natural specular hematite samples (Quebec, Canada) containing 0.6 wt.% Ti without preheating in air or without removal of the surface layer by grinding and polishing do not give any photocurrent, most probably because of the high carrier density in the surface layer as a result of oxygen loss. Substantial photocurrents as photoanodes in a PEC cell are developed by these samples when heated in air to 900 °C for 1 h or more after polishing to remove the surface layer. Current-Voltage and impedance measurements and photocurrent spectroscopy show inherent bulk states at 1.5 eV optical (∼ 0.5-0.7 eV below the conduction band edge) to be responsible for trapping of the majority carriers and for electron-hole recombination and surface states. On addition of pyrogallol, a strong reducing agent and hence an electron donor, the trapping and detrapping effects due to surface states are completely eliminated and the photocurrent greatly enhanced. This behavior is due to the filling of surface states through electron donation from the chemisorbed pyrogallol and the subsequent transfer of these electrons to the photogenerated holes in the valence band.