Environment-dependent shape and properties of gold nanoparticles: a first principles study

Abstract

In the present Thesis a computational study that links extensive quantummechanicalcalculations, based on density functional theory (DFT), to Wul constructionsin order to predict the environment-dependent equilibrium shape andproperties of large gold nanoparticles is presented.We firstly present the methodology used to obtain the equilibrium shape, includingthe main aspects of the density functional theory and the Wul constructionmethod. We continue by presenting the code developed to construct any clean orhaving a molecule adsorbed at a given site Au(hkl) surface. Then, we study the adsorptionof CO and the dissociative adsorption of dimethyl disulde (CH3S-SCH3)on every possible Au(hkl) with h; k; l 3 plus the kinked Au(421) and we discusstrends on adsorption energies, bond lengths and bond angles as the surfacestructure changes.We calculate the surface energy per unit area for clean surfaces and use it togetherwith the Wul construction method to predict the equilibrium shape ofclean gold nanoparticles with diameters up to several tenths of a nanometer, inaccessibleby direct atomistic simulations. We also present the codes developed toconstruct atomistic models of nanoparticles using the Wul construction methodand analyse the geometrical features of them. We point out a surprising agreementbetween Wul construction, experiments, and atomistic simulations at smallsizes. Au nanoparticles smaller than 16.3 nm in diameter have truncated octahedralshape, exposing only (111) and (100) faces. Larger nanoparticles also exposehigher-index faces, mostly (332).Then we use the surface energies per unit area and the results for the adsorptionenergy to obtain the interface energy per unit area between Au and CO or Auand thiolates (CH3S-) at low pressure and temperature. These results are thenused to obtain the equilibrium shape of CO- or thiolate-covered Au nanoparticles.In agreement with experimental data, Au nanoparticles in CO are found to bemore spherical and more reactive compared to Au nanoparticles in noninteractingenvironments. Gold nanoparticles change their shape upon adsorption of thiolatestowards shapes of higher sphericity and higher concentration of step-edge atoms.Finally, we check the stability of these nanoparticles through Molecular Dynamicssimulations and we present a theoretical method providing shape-dependentspectroscopic properties of gold nanoparticles.