Defect-Cluster-Boosted Solar Photoelectrochemical Water Splitting by n-Cu O Thin Films Prepared Through Anisotropic Crystal Growth.

Anisotropic growth of Cu O crystals deposited on an indium-doped tin oxide-coated glass substrate through facile electrodeposition and low-temperature calcination results in favorable solar photoelectrochemical water splitting. XRD, TEM, and SEM reveal that appreciable oxygen vacancies are populated in the Cu O crystals with a highly branched dendritic thin film morphology, which are further substituted by Cu atoms to form Cu antisite defects exclusively along the [111] direction. The post-thermal treatment presumably accelerates such migration of the lattice imperfections, favoring the exposure of the catalytically active (111) facets. The Cu O thin film derived in this way shows n-type conduction with a donor concentration in the order of 10  cm and a flat-band potential of -1.19 V vs. Ag/AgCl, which is also confirmed by Mott-Schottky analysis. The material is employed as a photoanode and delivers a photocurrent density of 2.2 mA cm at a potential of 0.3 V vs. Ag/AgCl, surpassing reported values more than twofold. Such superiority mostly originates from the synergism of the selective facet exposure within the Cu O crystals, which have decent crystallinity, as shown by Raman and photoluminescence spectroscopy, and a favorable bandgap of 2.1 eV, as confirmed by UV/Vis spectroscopy. The n-type Cu O thin film reported herein holds excellent promise for solar-related applications.