A computational study on CO adsorption onto SnO2 small grains

The focus of this study is on the adsorption properties of nanocrystalline materials and SnO2 has been chosen as the appropriate example. Nanocrystalline SnO2, in fact, has many practical applications for gas sensors but is virtually absent from the current physico-chemical literature on nanomaterials. The purpose of this study is a deeper understanding of its gas-absorbing properties in order to better exploit its gas-sensor applications. Therefore model structures consisting on small SnO2 grains have been considered and the adsorbed system is generated by depositing a CO molecule above a tin, or oxygen, atom on the grain surface. The calculations illustrate the structural properties of grains, their binding and adsorption energies and their conductance and are based on semi-empirical Hartree–Fock and scattering theories. It has been found that the molecule is stably bonded to the grain without penetration or intermixing and adsorption is not dissociative. These are also properties of adsorption on the surfaces of bulk samples. However the analysis of the adsorbed system indicates that stable adsorption derives from the molecule being integrated into the grain structure. Furthermore adsorption depends on the grain shape, on the adsorption site and on the orientation of the molecule. These dependencies do not exist in the bulk, though the values of the adsorption energy may be similar in the two cases. In agreement with known properties of structures of finite size, the conductance changes with the grain structure. A relevant result is that the dependence of this quantity on the grain size and shape is the same as the one of the binding and of the adsorption energies.