Effects of catalyst material and atomic layer deposited TiO_2 oxide thickness on the water oxidation performance of metal-insulator-silicon anodes

摘要

We report on the effects on water oxidation performance of varying (1) the nanoscale TiO_2 thickness and (2) the catalyst material in catalyst/TiO2_2/SiO_2/Si anodes. Uniform films of atomic layer deposited TiO_2 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 TiO_2 thickness was studied using the reversible redox couple ferri/ferrocyanide. The dependence of the water oxidation overpotential on ALD-TiO_2 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 TiO_2 for TiO_2 thicknesses greater than ~2 nm. For thinner TiO_2 layers, an approximately thickness-independent overpotential is observed. The linear behavior for anodes with thicker TiO_2 layers is consistent with the predicted effect of bulk TiO_2-limited electronic conduction on the voltage required to sustain the current density across the TiO_2/SiO_2 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.