Low-temperature activation of hematite nanowires for photoelectrochemical water oxidation

Hematite (α-Fe₂O₃) nanostructures have been extensively studied as photoanode materials for photoelectrochemical (PEC) water oxidation. However, the photoactivity of pristine hematite nanostructures is fairly low and typically requires thermal activation at temperature of 650 °C or above. Here, we report a new method for enhancing the photocurrent of hematite nanowires at a substantially lower temperature of 350 °C by means of a two-step annealing process (activation process). Hydrothermally grown β-FeOOH nanowires were first annealed in a pure N₂ environment at 350 °C to form magnetite, followed by partial oxidation in air to convert magnetite to hematite. During this process, Fe²⁺ sites (oxygen vacancies) were intentionally created to increase the donor density and therefore the electrical conductivity of hematite. The oxygen-deficient hematite nanowire photoanode created at low temperature (350 °C) show considerably enhanced photoactivity compared to pristine hematite sample that prepared by thermal annealing of β-FeOOH nanowires at 550 °C in air. Moreover, this low-temperature annealing method can be coupled with an element doping method to further increase the photoactivity of hematite nanowire. Sn-doped hematite nanowires prepared by the same low-temperature annealing method show at least three fold enhanced photocurrent compared to the undoped sample. Significantly, the highest temperature in the entire annealing process was 350 °C, which is the lowest activation temperature ever reported for hematite nanowire photoanodes. Cool customer: A low-temperature (350 °C) annealing process for improving the performance of hematite nanowires for photoelectrochemical water oxidation is reported. Oxygen vacancies are created in the hematite structure to increase the donor density and the electrical conductivity. The low-temperature activated hematite photoanode shows substantially enhanced photoactivity compared to the hematite sample prepared at 550 °C in air.