Electronic Properties of Pure and Fe-Doped β-Ni(OH)2: New Insights Using Density Functional Theory with a Cluster Approach

Abstract

NiOx has recently emerged as a robust catalyst with high catalytic activity for water oxidation reaction. Despite extensive studies, the origin of the high oxygen evolution reaction activity upon Fe doping is not fully solved, even for one of its simplest phases, β-Ni(OH)2. We present here density functional theory calculations using for the first time a cluster approach to revisit the electronic structure of pure and Fe-doped β-Ni(OH)2. First, our findings agree with a recent hypothesis that the band gap of the pure case reduces upon Fe doping. Second, in agreement with earlier calculations, we find that the highest occupied state consists of O and Ni states in pure and Fe-doped β-Ni(OH)2. However, the lowest unoccupied orbitals are Ni and O for pure β-Ni(OH)2 and mainly Fe for Fe-doped β-Ni(OH)2. We argue that the two different states for the highest occupied state and for the lowest unoccupied state of Fe-doped β-Ni(OH)2 may lead to low electron-hole recombination. Third, the delocalized nature of the band edge states that may be associated with high mobility is not damaged after Fe doping. These electronic effects could be some of the reasons experiments show that doping β-Ni(OH)2 with Fe enhances overall efficiency.