New benchmark for water photooxidation by nanostructured alpha-Fe2O3 films

Thin films of silicon-doped Fe2O3 were deposited by APCVD (atmospheric pressure chemical vapor deposition) from Fe(CO)5 and TEOS (tetraethoxysilane) on SnO2-coated glass at 415 degrees C. HRSEM reveals a highly developed dendritic nanostructure of 500 nm thickness having a feature size of only 10-20 nm at the surface. Real surface area determination by dye adsorption yields a roughness factor of 21. XRD shows the films to be pure hematite with strong preferential orientation of the [110] axis vertical to the substrate, induced by silicon doping. Under illumination in 1 M NaOH, water is oxidized at the Fe2O3 electrode with higher efficiency (IPCE = 42% at 370 nm and 2.2 mA/cm2 in AM 1.5 G sunlight of 1000 W/m2 at 1.23 VRHE) than at the best reported single crystalline Fe2O3 electrodes. This unprecedented efficiency is in part attributed to the dendritic nanostructure which minimizes the distance photogenerated holes have to diffuse to reach the Fe2O3/electrolyte interface while still allowing efficient light absorption. Part of the gain in efficiency is obtained by depositing a thin insulating SiO2 interfacial layer between the SnO2 substrate and the Fe2O3 film and a catalytic cobalt monolayer on the Fe2O3 surface. A mechanistic model for water photooxidation is presented, involving stepwise accumulation of four holes by two vicinal iron or cobalt surface sites.