The reduction of hematite nanostructures was achieved by the pyrolysis of NH3BH3 (AB) solution, which introduced oxygen vacancies in hematite and significantly improved the photocurrent for solar water oxidation. Moreover, by covering the sample with a crucible in the annealing process, a significant cathodic shift of the onset potential (up to 70 mV) was observed. Data suggested that a higher inside pressure was introduced by the cover, which led to a further reduction of hematite in the near surface region. The depth-reduction resulted in a cathodic shift compared to the sample treated in the surface region. The cathodic shift was also confirmed at various concentrations of AB and annealing temperatures. Our results suggest that the treating depth could be a key role for the low onset potential of hematite for solar water oxidation.
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