Conditions for magnetic and electronic properties of ultrathin Ni–Fe hydroxide nanosheets as catalysts: a DFT+U study

Abstract

The effects of cation ordering and surface compensating anions on the magnetic structure and catalytic properties of unilamellar Ni–Fe hydroxide nanosheets are studied by using the density functional theory (DFT) plus U method. Fe-segregation in the nanosheets yields magnetic domains with different spin alignments, while the surface compensating anions affect the local moments and valence states of the Fe atoms. The two conditions do not radically change the super-exchange nature of interactions between the paramagnetic metal centers, but facilitate the formation of various magnetic superlattices in the nanosheets. The calculated free energy change of the intermediates shows that the most stable magnetic structure of Ni–Fe hydroxide nanosheets exhibits superior catalytic activity towards oxygen reduction/evolution reactions, which is indicative of magnetic catalyst. This is due to the cycle transition between Fe2+ and Fe3+ ions in the reactions, which determines the sequence of cleavage of the O–H bond and the release of the OH group, controlling the rate-limiting steps of the reaction. The relationship of magnetism and catalytic activity of Ni–Fe hydroxide nanosheets is established by the valence state change of the Fe ions, which will be helpful to open the way for the design of hydroxide/layered double hydroxides (LDHs)-based magnetic catalysts.