Iron cation vacancies in Pt(iv)-doped hematite

Abstract

Platinum-doping of hematite (α-Fe₂O₃) is a popular method to increase the performance of hematite in photoelectrochemical applications. The precise mode of Pt incorporation is however unclear, as it can occur as Pt⁰, Pt²⁺ or Pt⁴⁺, either on the surface, as dispersed inclusions, or as part of the hematite crystal lattice. These different Pt-doping varieties can have major effects on the hematite performance. Here, we employ a high-pressure synthesis method assisted by silicate liquid flux to grow Pt-doped hematite crystals large enough for elemental analysis by wavelength dispersive spectroscopy (WDS). We find that the total cations are lower than the expected 2 atoms per formula unit, and together with Fe, they are inversely correlated with Pt contents. Linear regressions in compositional space reveal that the slopes are consistent with 4Fe³⁺ = 3Pt⁴⁺ + V_Fe as the charge-balanced substitution mechanism. Therefore, Pt⁴⁺-doping of hematite at high oxygen fugacities, which does not allow Fe²⁺ to form, will lead to removal of Fe and formation of cation vacancies. Our hematite also contains significant Al³⁺, Ti⁴⁺ and Mg²⁺, raising the possibility of fine tuning the hematite properties by co-doping with other elements. Photoelectrochemical performance of cation vacancy bearing hematite is experimentally understudied and is a potentially promising future field of study.