Ambipolar Behavior of Hematite Surface Induced by Water Adsorption Revealed under Environmental Conditions

In this work, we determine the electronic structure and charge carrier dynamics of α-Fe₂O₃(0001) ultrathin film deposited on Pt(111) as a function of water pressure by combined near ambient pressure-time-resolved photoelectron spectroscopy (NAP-TR-PES) measurements and DFT calculations. Under ultrahigh vacuum (UHV) α-Fe₂O₃ exhibits the expected typical <i>n</i>-type semiconductor behavior with a surface photovoltage (SPV) shift of 31 meV. Surprisingly, when exposed to water a completely different comportment appears. At a partial pressure of water of <i>P</i>_H₂O = 0.02 hPa, a much smaller SPV (7 meV) appears that is shifted in the opposite direction, which is thus characteristic of a <i>p</i>-type semiconductor. Finally, at <i>P</i>_H₂O > 0.1 hPa, SPV is canceled, which is consistent with a layer of metallic nature. DFT calculations explain these experimental findings well as originating from a modification of the surface structure and electronic properties of the layer consecutive to water adsorption. It is shown that the Fermi level position in the band gap of hematite depends on the quantity of adsorbed water, giving rise to the ambipolar behavior of the surface, which is able to transport both negative and positive charges when exposed to the appropriate water pressure.