Beneficial CuO Phase Segregation in the Ternary p-Type Oxide Photocathode CuBi₂O₄

Assessing the intrinsic material performance of emerging copper-based ternary oxide photocathode candidate materials such as CuBi2O4 (CBO) has been challenging due to the formation of phase-segregated domains in films with stoichiometric nonideality. However, we find films with CuO phase segregation demonstrate improved photoelectrochemical (PEC) performance, the origin of which inspired this deeper investigation. Uniform and compact CBO thin films with Bi:Cu ratios of 2.10, 1.97, 1.78, and 1.38 were grown by spin-coating. Although CuO was detected by Raman and X-ray diffraction in the 1.38 film only, high resolution energy-dispersive X-ray spectroscopy mapping revealed the presence of 10–20 nm CuO particles at the CBO/FTO interface in the 1.38, 1.78, and 1.97 samples. The greater number of CuO particles in the 1.38 sample resulted in a 25% enhancement in incident photon-to-current efficiency performance but could not be attributed to CuO-related light absorption. X-ray photoelectron spectroscopy characterization of the type-II band alignment was used to confirm that the particles behave as hole-selective contacts. The presence of nanoparticulate heterojunctions improves carrier collection of low diffusion length holes, enhancing the performance of the heterojunction beyond that of a fully planar derivative.