Fabrication of superior alpha-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 (alpha-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 Sri content was controlled by changing the concentration of the precursor solution. In subsequent high temperature sintering (800 degrees 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. (C) 2015 Elsevier B.V. All rights reserved.