Photochemistry of hematite photoanodes under zero applied bias

Abstract Surface photovoltage spectroscopy (SPS) was used to observe photochemical charge separation and oxidation reactions on Fe 2 O 3 nanorod arrays under zero applied bias. Nanorod films were grown from FeCl 3 under hydrothermal conditions followed by calcination at 550 °C. A negative photovoltage of up −130 mV is observed under 2.0–4.5 eV (0.1 mW cm −2 ) illumination, confirming 2.0 eV as the effective bandgap of the material, and electrons as majority carriers. SPS in the presence of air, nitrogen, water, oxygen, and under vacuum suggest that the photovoltage is associated with the oxidation of surface water and with reversible surface hole trapping on the 1 min time scale and de-trapping on the 1 h time scale. O 2 promotes water oxidation by increasing the concentration of surface holes. Sacrificial donors KI, H 2 O 2 or potassium hydroxide increase the voltage to −240 and −400 mV, due to improved hole transfer. Cobalt oxide and Co-Pi cocatalysts quench the voltage, which is tentatively attributed to the removal of surface states and enhanced e/h recombination. An energy diagram is used to relate the experimental photovoltage to the built-in potentials at the respective interfaces.