Revealing the relationship between photoelectrochemical performance and interface hole trapping in CuBi₂O₄...

p-Type CuBi₂O₄ is considered a promising metal oxide semiconductor for large-scale, economic solar water splitting due to the optimal band structure and low-cost fabrication. The main challenge in utilizing CuBi₂O₄ as a photoelectrode for water splitting, is that it must be protected from photo-corrosion in aqueous solutions, an inherent problem for Cu-based metal oxide photoelectrodes. In this work, several buffer layers (CdS, BiVO₄, and Ga₂O₃) were tested between CuBi₂O₄ and conformal TiO₂ as the protection layer. RuO was used as the co-catalyst for hydrogen evolution. Factors that limit the photoelectrochemical performance of the CuBi₂O₄/TiO₂/RuO , CuBi₂O₄/CdS/TiO₂/RuO , CuBi₂O₄/BiVO₄/TiO₂/RuO and CuBi₂O₄/Ga₂O₃/TiO₂/RuO heterojunction photoelectrodes were revealed by comparing photocurrents, band offsets, and directed charge transfer measured by modulated surface photovoltage spectroscopy. For CuBi₂O₄/Ga₂O₃/TiO₂/RuO photoelectrodes, barriers for charge transfer strongly limited the performance. In CuBi₂O₄/CdS/TiO₂/RuO , the absence of hole traps resulted in a relatively high photocurrent density and faradaic efficiency for hydrogen evolution despite the presence of pronounced deep defect states at the CuBi₂O₄/CdS interface. Hole trapping limited the performance moderately in CuBi₂O₄/BiVO₄/TiO₂/RuO and strongly in CuBi₂O₄/TiO₂/RuO photoelectrodes. For the first time, our results show that hole trapping is a key factor that must be addressed to optimize the performance of CuBi₂O₄-based heterojunction photoelectrodes.