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

Thesize effect in semiconductor photocatalysis has been widelyinvestigated but still remains elusive. Herein, employing p-type Cu2O nanocubes as the heterogeneous photocatalysts, we proposea feasible size optimization strategy to enhance the photocatalyticperformance of semiconductors. With the size of Cu2O increasingfrom 2.5 nm (exciton Bohr radius) to 5 nm (twice the exciton Bohrradius), the corresponding calculated band gap of Cu(2)Odecreases from 3.39 to 2.41 eV, indicating that controlling the sizeto above twice the exciton Bohr radius is vital for retaining thevisible-light response of Cu2O. Based on the theoreticalcalculations and experimental measurements of the charge carrier dynamics,we found that the synthesized 30 nm Cu2O nanocubes havean electron diffusion length of 191 nm, while 229 nm Cu2O nanocubes show an electron diffusion length of 45 nm. An electrondiffusion length larger than the semiconductor particle size lowersthe electron-hole recombination, resulting in a visible-lightCO generation rate 23.4 times higher for the smaller Cu2O nanocubes than that for the larger ones. These results verify thatconfining Cu2O size to within the minority carrier diffusionlength and above twice the exciton Bohr radius is a promising wayto enhance Cu2O photocatalytic activity.