Optimization of p-Type Cu2O Nanocube Photocatalysts Based on Electronic Effects

The size effect in semiconductor photocatalysis has been widely investigated but still remains elusive. Herein, employing p-type Cu2O nanocubes as the heterogeneous photocatalysts, we propose a feasible size optimization strategy to enhance the photocatalytic performance of semiconductors. With the size of Cu2O increasing from 2.5 nm (exciton Bohr radius) to 5 nm (twice the exciton Bohr radius), the corresponding calculated band gap of Cu2O decreases from 3.39 to 2.41 eV, indicating that controlling the size to above twice the exciton Bohr radius is vital for retaining the visible-light response of Cu2O. Based on the theoretical calculations and experimental measurements of the charge carrier dynamics, we found that the synthesized 30 nm Cu2O nanocubes have an electron diffusion length of 191 nm, while 229 nm Cu2O nanocubes show an electron diffusion length of 45 nm. An electron diffusion length larger than the semiconductor particle size lowers the electron–hole recombination, resulting in a visible-light CO generation rate 23.4 times higher for the smaller Cu2O nanocubes than that for the larger ones. These results verify that confining Cu2O size to within the minority carrier diffusion length and above twice the exciton Bohr radius is a promising way to enhance Cu2O photocatalytic activity.