3 years ago

Gradient Self-Doped CuBi2O4 with Highly Improved Charge Separation Efficiency

Gradient Self-Doped CuBi2O4 with Highly Improved Charge Separation Efficiency
Fatwa F. Abdi, Wilman Septina, Sean P. Berglund, S. David Tilley, Dennis Friedrich, Fuxian Wang, Roel van de Krol, Peter Bogdanoff, Abdelkrim Chemseddine
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%.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b07847

DOI: 10.1021/jacs.7b07847

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