Atomic structure relaxation in nanocrystalline NiO studied by exafs spectroscopy: Role of nickel vacancies

Nanocrystalline NiO samples have been studied using the Ni K-edge extended x-ray absorption fine structure (EXAFS) spectroscopy and recently developed modeling technique, combining classical molecular dynamics with ab initio multiple-scattering EXAFS calculations (MD-EXAFS). Conventional analysis of the EXAFS signals from the first two coordination shells of nickel revealed that (i) the second shell average distance R(Ni-Ni₂) expands in nanocrystalline NiO compared to microcrystalline NiO, in agreement with overall unit cell volume expansion observed by x-ray diffraction; (ii) on the contrary, the first shell average distance R(Ni-O₁) in nanocrystalline NiO shrinks compared to microcrystalline NiO; (iii) the thermal contribution into the mean-square relative displacement σ2 is close in both microcrystalline and nanocrystalline NiO and can be described by the Debye model; (iv) the static disorder is additionally present in nanocrystalline NiO in both the first Ni-O₁ and second Ni-Ni₂ shells due to nanocrystal structure relaxation. Within the MD-EXAFS method, the force-field potential models have been developed for nanosized NiO using as a criterion the agreement between the experimental and theoretical EXAFS spectra. The best solutions have been obtained for the 3D cubic-shaped nanoparticle models with nonzero Ni vacancy concentration C_vac: C_vac0.4-1.2% for NiO nanoparticles having the cube size of L3.6-4.2 nm and C_vac1.6-2.0% for NiO thin film composed of cubic nanograins with a size of L1.3-2.1 nm. Thus our results show that the Ni vacancies in nanosized NiO play important role in its atomic structure relaxation along with the size reduction effect.