Bilateral gradient defect engineering integrated atomic in-layer homojunctions for efficient photoelectrochemical water splitting

Severe recombination of photo-generated carriers restricts the application of photoelectrochemical water split-ting. Designing carrier spatial separation paths at the atomic level is an innovative but challenging strategy to address this problem. Herein, we first propose a gradient defect model confined in atomic layers. Bilateral gradient Cd doping terminated at the outer surfaces of five-atomic-layer BiOI nanosheets was achieved by a linear pressurized gas-assisted (LPGA) technique with self-adapting oxygen vacancies generated unexpectedly. Different doping concentrations between layers trigger the rearrangement of energy levels, resulting in distributed atomic homojunctions with "pyramid-like " band alignment to synergistic modulate carrier separa-tion. Thus, the gradient Cd doping BiOI displays a charge separation efficiency of 78.1%, which is 12.0 and 7.9 times higher than those for pristine BiOI and uniform doping BiOI. Furthermore, the record photocurrent density of 4.68 mA cm(-2) at 0 VRHE is the highest absolute value of BiOI photocathodes reported to date and outperforms most bismuth-based photocathodes. Our work provides a new method to modulate photo-generated carrier separation at the atomic level and deepens the understanding of homojunction semiconductors.