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.