Fabrication of superior α-Fe₂O₃ nanorod photoanodes through ex-situ Sn-doping for solar water splitting

Doping transition metals into 1-D nanostructures is of crucial importance for their application in photovoltaics and photoelectrochemical (PEC) systems; performance enhancements arise from both dopant incorporation and the 1-D nanostructures. Both in-situ and ex-situ doping methods have been demonstrated for 1-D hematite (α-Fe₂O₃) nanostructures, with tin (Sn) as the dopant, for photoelectrochemical water oxidation. In-situ Sn-doped hematite photoanodes adopted a morphology consisting of nanocorals with the (104) plane as the preferred direction of crystal growth. As an alternative solution, ex-situ doping not only preserves the vertically-aligned nanorod morphology but also sustains the preferred orientation of the (110) axis, which is favorable for high conductivity in pristine hematite photoanodes. In-situ Sn-doping was carried out by the same method: Sn precursors were added and dissolved in ethanol during the hydrothermal synthesis. Ex-situ doping was carried out in two stages (during pre-deposition and during high temperature sintering). During pre-deposition, a defined amount of the Sn precursor was introduced near the surface region of the 1-D nanostructure, and the Sn content was controlled by changing the concentration of the precursor solution. In subsequent high temperature sintering (800 °C), the dopant atoms diffused into the hematite lattice to attain the desired doping profile. We found that ex-situ Sn-doping resulted in a 60% increase in the photocurrent while in-situ Sn-doping yielded an increase of only 20% in the photocurrent, as compared with pristine hematite photoanodes, at 1.4 V vs. RHE. The improvement in the photocurrent was caused by a combination of Sn dopants in the hematite, which act as electron donors by increasing the donor density, and better surface charge transfer kinetics, thereby enhancing the overall device performance.