Effects of catalyst material and atomic layer deposited TiO2 oxide thickness on the water oxidation performance of metal–insulator–silicon anodes

Abstract

We report on the effects on water oxidation performance of varying (1) the nanoscale TiO₂ thickness and (2) the catalyst material in catalyst/TiO₂/SiO₂/Si anodes. Uniform films of atomic layer deposited TiO₂ are prepared in the thickness range ∼1–12 nm on degenerately-doped p⁺-Si, yielding water oxidation overpotentials at 1 mA cm⁻² of 300 mV to 600 mV in aqueous solution (pH 0 to 14). Electron/hole transport through Schottky tunnel junction structures of varying TiO₂ thickness was studied using the reversible redox couple ferri/ferrocyanide. The dependence of the water oxidation overpotential on ALD-TiO₂ thickness, with all other anode design features unchanged, exhibits a linear trend corresponding to ∼21 mV of added overpotential at 1 mA cm⁻² per nanometer of TiO₂ for TiO₂ thicknesses greater than ∼2 nm. For thinner TiO₂ layers, an approximately thickness-independent overpotential is observed. The linear behavior for anodes with thicker TiO₂ layers is consistent with the predicted effect of bulk TiO₂-limited electronic conduction on the voltage required to sustain the current density across the TiO₂/SiO₂ insulator stack. Eight different oxygen evolution catalysts of thickness 1–3 nm are studied. For the anodes investigated, 3 nm of Ir or Ru gave the best water oxidation performance, but both thinner layers and other catalysts can be quite effective, suggesting the potential for reduced materials cost. Lastly, a flat band voltage analysis of solid state thin film capacitors was done for five different gate metals on n-Si to probe junction energetics directly relevant to a photoanode. The results are consistent with a Schottky junction in which the Fermi level at the semiconductor surface is unpinned.