Synergetic effect of Sn addition and oxygendeficient atmosphere to fabricate active hematite photoelectrodes for light- induced water splitting

This work describes the design of a microwave-assisted method using hydrothermal conditions to fabricate pure and Sn-doped hematite photoelectrodes with varied synthesis time and additional thermal treatment under air and N-2 atmosphere. The hematite photoelectrode formed under N-2 atmosphere, with Sn deposited on its surface-which is represented by material synthesized at 4 h-exhibits the highest performance. Hence, Sn addition followed by high temperature annealing conducted in an oxygen-deficient atmosphere seems to create oxygen vacancies, and to prevent the segregation of dopant to form the SnO2 phase at the hematite crystal surface, reducing its energy and suppressing the grain growth. The increased donor number density provided by the oxygen vacancies (confirmed by x-ray photoelectron data), and a possible reduction in the grain boundary energy or hematite crystal interface might favor charge separation, and increase the electron transfer through the hematite into the back contact (FTO substrate). In consequence, the light-induced water oxidation reaction efficiency of Snhematite photoelectrodes was significantly increased in comparison with pure ones, even though the vertical rod morphology was not preserved. This finding provides a novel insight into intentional Sn addition, revealing that dopant segregation at the hematite crystal surface (or at the grain boundaries) could-by increasing the electron mobility-be the more relevant factor in developing active hematite photoelectrodes than the control of columnar morphology.