Crystalline-face-dependent photoelectrochemical properties of single crystalline CuGaSe2 photocathodes for hydrogen evolution under sunlight radiation

Photoelectrochemical (PEC) water splitting by using a electrode based on a single crystalline compound provide a good opportunity for studying on fundamental aspects of net semiconductor bulk properties related to PEC performance. In this study, single crystalline CuGaSe2 wafers exposed (321), (312), and (112) facets were used as sources of photocathodes for investigating PEC properties for water reduction into hydrogen under sunlight radiation related to their surface properties. A CuGaSe2 single crystalline ingot grown by using the zone melting method was cut along with different directions to obtain wafers with different crystal orientations. The electrical and electron-energy properties of wafers after annealing in an Se atmosphere indicated the formation of a Cu-deficient component on their surfaces when the wafers oriented (321) and (312) directions were used, whereas formation of a component was not observed on the wafer oriented in the (112) direction. Since the Cu-deficient component acted as a hole blocking layer to suppress the recombination of photoexcited carries at the heterojunction, photocathodes based on wafers having (321) and (312) orientations showed enhanced PEC performance for water reduction compared to the PEC performance of a photocathode based on a wafer having the (112) orientation. Highly active photocathodes for water reduction based on polycrystalline thin films usually have preferential (112) orientation, whereas those of densely packed powder forms would not be probable for controlling crystalline orientations. Therefore, present results would be a guide to achieve further improvements of PEC water reduction using such conventional systems.