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Sn-doped hematite nanostructures for photoelectrochemical water splitting.

We report on the synthesis and characterization of Sn-doped hematite nanowires and nanocorals as well as their implementation as photoanodes for photoelectrochemical water splitting. The hematite nanowires were prepared on a fluorine-doped tin oxide (FTO) substrate by a hydrothermal method, followed by high temperature sintering in air to incorporate Sn, diffused from the FTO substrate, as a dopant. Sn-doped hematite nanocorals were prepared by the same method, by adding tin(IV) chloride as the Sn precursor. X-ray photoelectron spectroscopy analysis confirms Sn(4+) substitution at Fe(3+) sites in hematite, and Sn-dopant levels increase with sintering temperature. Sn dopant serves as an electron donor and increases the carrier density of hematite nanostructures. The hematite nanowires sintered at 800 °C yielded a pronounced photocurrent density of 1.24 mA/cm(2) at 1.23 V vs RHE, which is the highest value observed for hematite nanowires. In comparison to nanowires, Sn-doped hematite nanocorals exhibit smaller feature sizes and increased surface areas. Significantly, they showed a remarkable photocurrent density of 1.86 mA/cm(2) at 1.23 V vs RHE, which is approximately 1.5 times higher than that of the nanowires. Ultrafast spectroscopy studies revealed that there is significant electron-hole recombination within the first few picoseconds, while Sn doping and the change of surface morphology have no major effect on the ultrafast dynamics of the charge carriers on the picosecond time scales. The enhanced photoactivity in Sn-doped hematite nanostructures should be due to the improved electrical conductivity and increased surface area.