Revealing the relationship between photoelectrochemical performance and interface hole trapping in CuBi(2)O(4) heterojunction photoelectrodes

p-Type CuBi(2)O(4) 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 CuBi(2)O(4) 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, BiVO(4), and Ga(2)O(3)) were tested between CuBi(2)O(4) and conformal TiO(2) as the protection layer. RuO(x) was used as the co-catalyst for hydrogen evolution. Factors that limit the photoelectrochemical performance of the CuBi(2)O(4)/TiO(2)/RuO(x), CuBi(2)O(4)/CdS/TiO(2)/RuO(x), CuBi(2)O(4)/BiVO(4)/TiO(2)/RuO(x) and CuBi(2)O(4)/Ga(2)O(3)/TiO(2)/RuO(x) heterojunction photoelectrodes were revealed by comparing photocurrents, band offsets, and directed charge transfer measured by modulated surface photovoltage spectroscopy. For CuBi(2)O(4)/Ga(2)O(3)/TiO(2)/RuO(x) photoelectrodes, barriers for charge transfer strongly limited the performance. In CuBi(2)O(4)/CdS/TiO(2)/RuO(x), 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 CuBi(2)O(4)/CdS interface. Hole trapping limited the performance moderately in CuBi(2)O(4)/BiVO(4)/TiO(2)/RuO(x) and strongly in CuBi(2)O(4)/TiO(2)/RuO(x) photoelectrodes. For the first time, our results show that hole trapping is a key factor that must be addressed to optimize the performance of CuBi(2)O(4)-based heterojunction photoelectrodes.