Band engineering of delafossite CuB1-xFexO2 (B = Al, Cr, Sc) solid solutions as photocathodes: Visible-light response and performance enhancement

The optimization of wide-bandgap semiconductor photoelectrodes for efficient visible-light response and enhanced photoelectrochemical activity is a major challenge in the field of hydrogen production from water splittin. To overcome this challenge, the integration of narrow-bandgap semiconductor photoelectrodes to form solid solutions has emerged as a promising strategy. Delafossite ABO2 compounds offer an ideal platform for this approach due to their family of materials with multiple compatible members and their ability to readily form solid solutions. In this study, we focus on the formation of continuous solid solutions by combining a narrow-bandgap semiconductor CuFeO2 photocathode with three typical wide-bandgap semiconductor CuBO2 (B=Al, Cr, Sc) photocathodes. The synthesis of these solid solutions was achieved using solid-state reaction and hydrothermal methods. Characterization of the optical properties confirms that CuB1-xFexO2 (0 < x < 1) solid solutions exhibit strong light absorption within the visible light region. Photoelectrochemical tests performed under visible light irradiation demonstrate excellent photoelectrochemical activity. Notably, the photocurrent densities of CuAl0.7Fe0.3O2, CuCr0.7Fe0.3O2, and CuSc0.8Fe0.2O2 photocathodes are approximately 7.29, 11.00, and 7.32 times higher, respectively, than those of the corresponding pure CuAlO2, CuCrO2, and CuScO2 photocathodes. Additionally, these solid-solution photocathodes exhibit significantly improved monochromatic incident photon-to-electron conversion efficiency. Attributed to the integration of Fe components, the bandgap and conduction band edge of CuBO2 (B=Al, Cr, Sc) materials are effectively decreased. The optimal photoactivity of CuB1-xFexO2 solid solutions under visible light irradiation is attained when the visible light response, reduction potential, and kinetic behavior of photogenerated carriers reach equilibrium. This finding underscores the importance of balancing these key factors to achieve superior performance in solid solution photocathodes. Based on our results, we summarize several universal principles for constructing high-performance solid solution photocathodes.