Modelling the properties of gold nanoproperties
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The physical and chemical properties of gold nanoparticles were investigated using first-principles density-functional theory in the local density approximation. The low-energy structures of gold nanoparticles containing from 3 to 38 atoms were determined using a combination of semi-empirical potential and DFT calculations. At the DFT-LSDA level, planar structures persist up to Au6 and lie close in energy to the 3D ground-state structures for 7 ≤ n ≤ 10 atoms. Ordered structures are predicted as being stable for n ≤ 9. Beyond this size, disordered structures dominate with the notable exception of the tetrahedral Au20. The present DFT calculations predict numerous structural isomers being energetically equivalent or lying close in energy to the ground-state structures. The HOMO-LUMO energy gap is found to decrease as the cluster size increases, which indicates a transition towards a metallic behaviour at larger sizes. The thermal behaviour of gold clusters containing 7, 13 and 20 atoms was investigated using isothermal density-functional molecular-dynamics simulations. At each size, the global minimum energy structure and a low lying isomer are used as the starting structures. In most cases, the clusters exhibit high-temperature melting without showing any sharp transition from a solid-like to a liquid-like phase, but rather pass through a region of transformation between structural isomers. The starting structure used in the simulation is shown to have a considerable effect upon the subsequent thermal behaviour. The Au20 ground-state structure contrasts with the other clusters and remains tetrahedral up to its melting at about 1200 K. The adsorption of Au2o on a regular and defective (100) MgO surface was investigated using a combination of DFT MID simulations, single-point calculations and structure optimisations. Au20 is found to bind quite weakly to the regular (100) MgO surface, favouring Au-O interactions. The Mulliken population analysis used in conjunction with charge density maps reveals that the resulting bond mainly arises from an electrostatic polarisation of the cluster by the substrate. When adsorbed on a surface F-centre defect. Au20 binds to the oxide surface with a significantly enhanced strength. The analysis of the resulting interaction indicates that, in addition to being polarised, Au20 is charged by the defective surface. Based upon the results of the present investigation, different reaction mechanisms for the oxidation of CO by Au20 supported on MgO are proposed.
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