Synergetic effect of Sn addition and oxygen-deficient atmosphere to design active hematite photoelectrodes for light-induced water splitting

This work describes a microwave-assisted hydrothermal conditions method to design pure and Sn-hematite photoelectrodes at different synthesis time with additional thermal treatment under air and N2 atmosphere. The hematite photoelectrode designed under N2 atmosphere and Sn deposited on its surface, which is represented by material synthesized at 4 hours exhibit the highest performance. Hence, the Sn-addition followed by high annealing temperature conducted at oxygen-deficient atmosphere seems to create of oxygen vacancies and have prevented the dopant segregation to form SnO2 phase at the hematite crystal reducing its energy and suppressing the grain growth. The increased number of donor 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 the charge separation and increase the electron transfer through the hematite into the back contact (FTO substrate). As consequence, the light-induced water oxidation reaction efficiency of Sn-hematite photoelectrode was significantly increased in comparison with pure ones, even though the vertical rod morphology was not preserved. This finding provides a novel insight on the intentional Sn-addition revealing that the dopant segregation at the hematite crystal surface (or at the grain boundaries) could be the more relevant factor for developing active hematite photoelectrodes by increasing the electron mobility than the columnar morphology control.