A new strategy of using forward gradient self-doping to improve the charge separation efficiency in metal oxide photoelectrodes is proposed. Gradient self-doped CuBiO 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 CuBiO 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 CuBiO, which can facilitate charge separation. Compared to homogeneous CuBiO photocathodes, CuBiO photocathodes with a forward gradient show highly improved charge separation efficiency and enhanced photoelectrochemical performance for reduction reactions, while CuBiO photocathodes with a reverse gradient show significantly reduced charge separation efficiency and photoelectrochemical performance. The CuBiO photocathodes with a forward gradient produce record AM 1.5 photocurrent densities for CuBiO up to -2.5 mA/cm at 0.6 V vs RHE with HO 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 CuBiO are maintained. Lastly, forward gradient self-doped CuBiO photocathodes are protected with a CdS/TiO heterojunction and coated with Pt as an electrocatalyst. These photocathodes demonstrate photocurrent densities on the order of -1.0 mA/cm at 0.0 V vs RHE and evolve hydrogen with a faradaic efficiency of ∼91%.

Cupric oxide (CuO) is a promising material for large-scale, economic solar energy conversion due to the abundance of copper, suitable band gap, and ease of fabrication. For application as a photocathode for water splitting, the main challenge is prevention of the inherent photocorrosion in aqueous media. Photoelectrochemical measurements of bare CuO thin films prepared by oxidation of electroplated Cu indicated that the vast majority of the photocurrent in 1 M phosphate buffer solution (pH 7) comes from photocorrosion of the CuO into metallic Cu, with a faradaic efficiency for hydrogen evolution of ∼0.01%. We found that deposition of an n-type CdS buffer layer underneath a protective TiO 2 layer yielded a stable and efficient photoelectrode, with the champion electrode giving 1.68 mA cm −2 at 0 V RHE and an onset potential of ca. 0.45 V RHE . We attribute a favorable band alignment of CuO/CdS for the record photovoltage obtained with this material and a high conformality of the TiO 2 layer on the sulfide surface for the high stability of hydrogen-producing photocurrents (faradaic efficiency ∼100%).

A highly efficient Sb2Se3photocathode is resistant to photocorrosion in strongly acidic media without protective overlayers.

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