Beneficial surface passivation of hydrothermally grown TiO2 nanowires for solar water oxidation

Abstract

Rutile TiO2 nanowires (TONWs) with a length of 2.0μm were synthesized using a facile hydrothermal method in a strong acid solution. To investigate the effect of surface passivation of TONW arrays, a TiO2 layer with a thickness varying from 5 to 20nm on TONW arrays was applied by atomic layer deposition (ALD). No distinct morphological modification was observed in all prepared TONW arrays in the environment where the diameter of the TONW arrays was systematically increased from 10 to 40nm. In this study, Mott-Schottky analysis revealed that 10nm TiO2-coated TONW (denoted as TiO2(10nm)/TONW) arrays showed the highest electronic conductivity, followed by the 5nm, 20nm, and 0nm TiO2/TONW arrays. The photoelectrochemical (PEC) performance was assessed in 0.1M KOH, which revealed that TiO2(10nm)/TONW arrays displayed a photocurrent density (3.92mA/cm2 at 0.5VNHE) higher than that (2.72mA/cm2) of TONW arrays. This may be ascribed to the surface passivation of trap or defect sites by the thin TiO2 surface coating, leading to the increased electron densities and improving the PEC performance. For a more definitive examination, photovoltage decay measurement was performed to calculate the decay lifetime, which is closely correlated to the electron-hole recombination reaction. In this study, TiO2(10nm)/TONW arrays exhibited a decay lifetime (0.7s) shorter than that (1.1s) of TONW arrays, proving the suppressed charge recombination in the thin TiO2/TONW arrays.