Ultrathin ALD TiO2 shells for enhanced photoelectrochemical solar fuel generation

ZnO/TiO2 core/shell heterojunction arrays were fabricated via the atomic layer deposition (ALD) of various ultrathin TiO2 layers on hydrothermally-synthesized hexagonal ZnO micro/nanorods. Current-voltage (I-V) characteristics of the photoanodes made using 10–75 cycles of TiO2 showed significant suppression in the dark current. Specifically, the sample made of 50 ALD cycles of TiO2 exhibited a 45% decrease in the dark current compared to the bare ZnO electrodes. Further increase in number of deposition cycles resulted in stability deterioration. The decay time constant measurements revealed electron lifetimes in the core/shell electrodes that are longer than those in pristine ZnO electrodes. Finally, Mott−Schottky analysis and the positron annihilation measurements confirmed the decrease in the donor density and surface defects for the core/shell electrodes. The results attained in this study represent a step towards a facile method to stabilize ZnO photoanodes from photodeterioration under the harsh operating conditions of water splitting