Gradient\nSelf-Doped CuBi₂O₄ with\nHighly Improved Charge Separation Efficiency

Abstract

A new\nstrategy of using forward gradient self-doping to improve\nthe charge separation efficiency in metal oxide photoelectrodes is\nproposed. Gradient self-doped CuBi₂O₄ photocathodes\nare prepared with forward and reverse gradients in copper vacancies\nusing a two-step, diffusion-assisted spray pyrolysis process. Decreasing\nthe Cu/Bi ratio of the CuBi₂O₄ photocathodes\nintroduces Cu vacancies that increase the carrier (hole) concentration\nand lowers the Fermi level, as evidenced by a shift in the flat band\ntoward more positive potentials. Thus, a gradient in Cu vacancies\nleads to an internal electric field within CuBi₂O₄, which can facilitate charge separation. Compared to homogeneous\nCuBi₂O₄ photocathodes, CuBi₂O₄ photocathodes with a forward gradient show highly improved\ncharge separation efficiency and enhanced photoelectrochemical performance\nfor reduction reactions, while CuBi₂O₄ photocathodes\nwith a reverse gradient show significantly reduced charge separation\nefficiency and photoelectrochemical performance. The CuBi₂O₄ photocathodes with a forward gradient produce record\nAM 1.5 photocurrent densities for CuBi₂O₄ up\nto −2.5 mA/cm² at 0.6 V vs RHE with H₂O₂ as an electron scavenger, and they show a charge separation\nefficiency of 34% for 550 nm light. The gradient self-doping accomplishes\nthis without the introduction of external dopants, and therefore the\ntetragonal crystal structure and carrier mobility of CuBi₂O₄ are maintained. Lastly, forward gradient self-doped\nCuBi₂O₄ photocathodes are protected with a CdS/TiO₂ heterojunction and coated with Pt as an electrocatalyst.\nThese photocathodes demonstrate photocurrent densities on the order\nof −1.0 mA/cm² at 0.0 V vs RHE and evolve hydrogen\nwith a faradaic efficiency of ∼91%.