The structure of SnO2 nanoparticles (avg. 5 nm diameter) with a few layers of water on the surface has been elucidated by atomic pair distribution function (PDF) methods using in situ neutron total scattering data and molecular dynamics (MD) simulations. The analysis of PDF data with the hydroxylated MD hydration model demonstrates that surface hydration layers stabilize the SnO2 nanoparticles and inhibit nanoparticle growth. Upon heating under vacuum (dehydration) to 250 ºC, nanoparticles start to grow with rapid increases of crystallinity before completion of dehydration. The minimum concentration of hydroxyl groups required to stabilize the nanoparticles corresponds to ~0.7 monolayer coverage. Low activation energy of 44(8) kJ/mole for particle growth is consistent with high resolution transmission electron microscopy observations in which SnO2 nanoparticles are observed to grow via an orientated attachment process. Observation of larger a lattice dimension for nanoparticles as compared with microsized SnO2 bulk crystallites suggest that particle-size-dependent structural modifications are related to the presence of sorbed water species, which further governs the structural stability and energy of SnO2 nanocrystals. Thus, a careful evaluation of surface water contents and structural configuration of sorbed water species is critical for interpreting many macroscopic thermochemistry properties of hydrated SnO2 nanoparticles.
Citation: Journal of the American Chemical Society
Pub Type: Journals