Platinum-doped α-Fe₂O₃ for enhanced water splitting efficiency: A DFT+ U study

Hematite (α-Fe₂O₃) is commonly considered for converting solar energy into hydrogen fuel through water splitting. Recent experiments performed in 2013 reached a maximum efficiency in Fe₂O₃ photoelectrochemical cells while using platinum-doped Fe₂O₃. In order to understand how platinum increases efficiency, we use the density functional theory + U (DFT+U) method to model the bulk and the (0001) surface of platinum-doped Fe₂O₃. We also give a unique ligand field theory combined with Bader charge analysis to explain changes resulting from symmetry breaking by the dopant. First, we find that, although platinum has a lower oxidation state than usual n-type dopants, platinum donates electrons. We find a theoretical ideal doping range of 0.64-2.96 atom % for enhanced electron conductivity, which is within the optimal range obtained by previous experiments. Second, we find that the energy gap decreases upon doping, improving solar energy absorption. Third, in agreement with previous experiments, we calculate an unfavorable increase in overpotential for oxidizing water upon platinum doping. Since platinum has both good and bad effects, we recommend bypassing this duality by platinum doping with a gradient-based strategy: high doping in the bulk for enhanced conductivity and low doping at the surface to not interfere with catalysis. We anticipate that experimentally testing our proposed strategy will advance the development of better electrodes for photoelectrochemistry.