X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD) have been used to characterize both local and long range structure in a series of nano-scaled metal oxide systems. Their unit cell lattice parameters have been observed to contract systematically with crystallite size and composition relative to that observed for the macro-scaled crystallite structure. In rocksalt NiO, the contraction in a0 is approximately 0.2% for 25 nm nano-crystallites but proportionally smaller in solid-solution ZnxNi1-xO and CuxNi1-xO (0≤x≤0.3), in which larger guest cations (Zn2+ and Cu2+) substitute for the smaller Ni2+. While both Zn2+ and Cu2+ form homogeneous solid solutions in NiO, with random guest substitution, the copper cation undergoes a small Jahn-Teller distortion in the solid solution while no difference in Zn2+-O2- and Ni2+-O2- coordination and bond lengths are observed for ZnxNi1-xO. Both nano-scaled PdO and CuxPd1-xO (0≤x≤0.75) show contractions in a0 and c0 unit cell parameters, with cell contractions increasing with decreased particle size. Reports of lattice differences between macroscaled and nanoscaled oxide crystallites are controversial. However, to date many describe a lattice expansion for nanoparticles with decrease in crystallite size. For example, the contraction is not intrinsic to the rocksalt structure since NiO exhibits a contraction whereas both forms of ZnO (rocksalt and wurtzite) show a slight expansion. Instead the ionic v.s. covalent nature of the oxide lattice must be considered in determining the effect of nano-scaled changes in local and long range crystal structure.

Local and long range structure in metal oxide nanoparticles

249th National Meeting (2015)

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