We use theoretical simulations to study how oxidative and humid environments affect the chemical composition, shape and structure of ceria nanoparticles. Based on our calculations, we predict that small stoichiometric ceria nanoparticles will have a very limited stability range when exposed to these environments. Instead, we find that reduced ceria nanoparticles are stabilized without changing their inherent shape through the adsorption of oxygen molecules in the form of superoxo species and water in the form of hydroxo species. Based on our results, we propose a redox-cycle for meta-stable ceria nanoparticles without the formation of explicit oxygen vacancies, which is important for understanding the low-temperature oxygen chemistry of ceria at the nanoscale.
The surface structures of ZnO surfaces and ZnO nanoparticles, with and without water, were studied with a reactive
force field (FF) within the ReaxFF framework, and molecular dynamics (MD) simulations. The force field parameters
were fitted to a training set of data points (energies, geometries, charges) derived from quantum-mechanical B3LYP
calculations. The ReaxFF model predicts structures and reactions paths at a fraction of the computational cost of the
quantum-mechanical calculations. Our simulations give the following results for the (10-10) surface. (i) The alternating
H-bond pattern of Meyer et al. for one monolayer coverage is reproduced and maintained at higher temperatures. (ii)
Coverages beyond one water monolayer enhances ZnO hydroxylation at the expense of ZnO hydration. (iii) This is
achieved through an entirely new H-bond pattern mediated via the water molecules in the second layer above the ZnO
surface. (iv) During a desorption process, the desorption rate slows significantly when two monolayers remain.
Simulations of nanoparticles in water suggest that these conclusions are relevant also in the nano case.