Controlling the electronic and photoexcited properties of cuprous oxide (Cu 2 O) through slight modifications of the synthesis method can impact a wide range of emerging technologies. Herein, we consider copper vacancies in Cu 2 O as a prototype of a ptype oxide semiconductor for studying the impact of crystal and electronic structure on carbon dioxide photoreduction. Oriented films of copper vacancy modulated Cu 2 O consisting of nano twin structures are electrodeposited by changing the potential in an aqueous alkaline copper(II)-lactate solution. The copper vacancies introduce tail states inside the band gap, improving the hole concentration and facilitating the charge separation and transfer in the Cu 2 O photocathode. This study gives an in-depth view of how a cation-deficient structure regulates and promotes photoelectrochemical activity toward CO 2 reduction.