Gradient Self-Doped CuBi₂O₄ with Highly Improved Charge Separation Efficiency

A new strategy of using forward gradient self-doping to improve the charge separation efficiency in metal oxide photoelectrodes is proposed. Gradient self-doped CuBi₂O₄ photocathodes are prepared with forward and reverse gradients in copper vacancies using a two-step, diffusion-assisted spray pyrolysis process. Decreasing the Cu/Bi ratio of the CuBi₂O₄ photocathodes introduces Cu vacancies that increase the carrier (hole) concentration and lowers the Fermi level, as evidenced by a shift in the flat band toward more positive potentials. Thus, a gradient in Cu vacancies leads to an internal electric field within CuBi₂O₄, which can facilitate charge separation. Compared to homogeneous CuBi₂O₄ photocathodes, CuBi₂O₄ photocathodes with a forward gradient show highly improved charge separation efficiency and enhanced photoelectrochemical performance for reduction reactions, while CuBi₂O₄ photocathodes with a reverse gradient show significantly reduced charge separation efficiency and photoelectrochemical performance. The CuBi₂O₄ photocathodes with a forward gradient produce record AM 1.5 photocurrent densities for CuBi₂O₄ up to -2.5 mA/cm² at 0.6 V vs RHE with H₂O₂ as an electron scavenger, and they show a charge separation efficiency of 34% for 550 nm light. The gradient self-doping accomplishes this without the introduction of external dopants, and therefore the tetragonal crystal structure and carrier mobility of CuBi₂O₄ are maintained. Lastly, forward gradient self-doped CuBi₂O₄ photocathodes are protected with a CdS/TiO₂ heterojunction and coated with Pt as an electrocatalyst. These photocathodes demonstrate photocurrent densities on the order of -1.0 mA/cm² at 0.0 V vs RHE and evolve hydrogen with a faradaic efficiency of ∼91%.