Hole trap, charge transfer and photoelectrochemical water oxidation in thickness-controlled TiO2 anatase thin films

Abstract

Despite the symbolism of TiO2 in photoelectrochemistry, its fundamental nanoscale properties in oxidation reaction have often reported inconsistent results. We demonstrated the simple model of TiO2 thin films (TFs) for photoelectrochemical (PEC) oxidation reaction (OR) of water, which is constituted of Indium Tin Oxide (ITO) and TiO2 TFs, by depositing it directly onto ITO coated glass substrates through Atomic Layer Deposition (ALD). Using ALD with a sub-nanometer thickness control, 9 to 90 nm in thickness (200 to 2000 in no. of ALD cycles) of TiO2 TFs were fabricated and annealed for crystallization into anatase phase. Anatase TiO2 TF photoanodes showed an optimum PEC OR performance at the thickness of 45 nm (corresponding to around 1000 ALD cycles) in the electrolyte solution of 0.1 M NaOH (pH = 12.9) under UV light illumination of 365 nm in wavelength. Different thicknesses of TiO2 TFs were characterized by electrochemical and optical measurements. Increasing bulk traps in TiO2 TFs up to the critical thickness improved charge transfer and PEC OR by increasing holes' lifetime. However, it was found to be detrimental when it became dominant along with the increase of TFs' thickness. This work provides insight in designing highly efficient TiO2 photoelectrodes with the precisely controlled thickness that can have a significant effect on charge transfer kinetics and thus PEC performances.