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.