Effect of nickel monolayer deposition on the structural and electronic properties of the low miller indices of (bcc) iron: A DFT study

Metal clusters of both iron (Fe) and nickel (Ni) have been found in nature as active electro-catalytic sites, for example in the enzyme carbon mono-oxide dehydrogenase found in autotrophic organisms. Thus, surface modification of iron with nickel could improve the surface work function to enhance catalytic applications. The effects of surface modifications of iron by nickel on the structural and electronic properties have been studied using spin-polarised density functional theory calculations within the generalised gradient approximation. The thermodynamically preferred sites for Ni adsorption on the Fe (100), (110) and (111) surfaces have been studied at varying monolayer coverages (including 0.25 ML and 1 ML). The work function of the bare Fe surfaces is found to be of the order (100) ∼ (111) < (110) i.e. 3.80 eV ∼ 3.84 eV < 4.76 eV, which is consistent with earlier studies. The adsorption energies show that monolayer Ni deposition is thermodynamically favoured on the (100) and (111) surfaces, but not on the (110) surface. Expansion of the first interlayer spacing (d12) of all three Fe surfaces is observed upon Ni deposition with the extent of expansion decreasing in the order (111) > (110) > (100), i.e. 6.78% > 5.76% > 1.99%. The extent of relaxation is magnified on the stepped (111) surface (by 1.09% to 30.88%), where the Ni coordination number is highest at 7 compared to 5 on the (100) facet and 4 on the (110) facet. The Ni deposition changes the work functions of the various surfaces due to charge reordering illustrated by charge density plots, where the work function is reduced only on the (110) surface by 0.04 eV, 0.16 eV and 0.17 eV at 1 ML, 0.5 ML and 0.25 ML respectively, with a concomitant increase in the surface dipole (polarity). This result implies enhanced electron activity and electrochemical reactivity on the most stable and therefore frequently occurring Ni-doped (110) facet compared to the clean (110) facet, which has implications for the development of improved Fe electro-catalysts (for example for CO2 activation and reduction). These findings improve our understanding of the role of surface topology and stability on the extent of Ni interactions with Fe surfaces and the extent to which the Fe surface structures and properties are altered by the Ni deposition.