STRAIN AND STRUCTURAL DISORDER IN NANOPARTICLES: EFFECTS ON STABILITY AND REACTIVITY

Lattice strain and complex structural disorder in nanosized particles can substantially modify a material’s physicochemical and electronic properties. Such modifications remain only partly understood and are important for tailoring functional nanomaterials and evaluating the role of nanoparticles in environmental systems. Recent work on nanosized samples of sphalerite (ZnS), ferrihydrites of different sizes and compositions, and uraninite (UO₂) provide interesting examples of compounds that exhibit varying degrees of size-dependent strain and structural disorder. Restructuring of the near-surface regions is often considered as the dominant factor driving structural disorder and can cause lattice expansions or contractions that propagate through the entire particle [1]. Internal strain also varies in response to changes in the surface environment (e.g., binding of water, adsorbed species), and this effect can be large enough to drive a bulk structural transformation [2]. Compositional variations are another factor that may be coupled with size-dependent effects. For example, aging of ferrihydrite under certain conditions drives physicochemical changes that include particle growth and the filling of structural vacancies [3]. The resulting relatively ordered form of ferrihydrite has less lattice strain and overall structural disorder, and increased thermodynamic stability relative to its disordered precursor. In contrast, nanobiogenic uraninite exhibits surface restructuring but with no detectable lattice strain [4]. The lack of interior strain appears related to nanobiogenic uraninite exhibiting thermodynamic and kinetic properties consistent with recent findings that oxides with higher cation oxidation states, and actinide oxides in particular have relatively low surface energies [5] and shows how the effects of structural disorder are currently difficult to generalize.

[1] Gilbert et al. (2004) Science 305: 651

[2] Zhang et al. (2003) Nature 424: 1025

[3] Michel et al. (2010) Proc. Natl. Acad. Sci. USA 107: 2787

[4] Schofield et al. (2008) Environ. Sci. Technol. 42: 7898

[5] Navrotsky et al. (2010) Thermodynamic issues in nanoscale actinide oxides. In: Goldschmidt, June 13-18, Knoxville, TN