Density functional studies of the electronic structure and catalytic properties of small bare and ligand-protected gold clusters

Abstract

In this thesis, bare and thiolate-protected gold nanoclusters and thiolated polymeric gold, silver and copper complexes are computationally studied by using density functional codes GPAW (grid projector augmented wave) and CPMD (Car-Parrinello molecular dynamics and metadynamics). Since 2007, breakthroughs in experimental structural determination of Au25(SR)−1 18 and Au102(SR)44 clusters (SR = thiolate) have revised the understanding of bonding motifs at the gold-sulfur nano-interface. The structure of both clusters can be written via a "Divide and Protect" structural motif as AuNcore[Aux(SR)x+1]y where a considerable number of Au atoms of the cluster are outside of the metal core, covalently bound with thiolates in the protective ligand layer. The Au25 cluster can be written as Au13[Au2(SR)3]−1 6 featuring an approximately icosahedral Au13 core and six oligomeric Au2(SR)3 units or "semi-rings". The comparative analysis of thiolated Cu, Ag, and Au complexes, related to the "semirings", shows that the metal-thiolate bond is sterically flexible, i.e., ring-like, helixlike or catenane-like complexes are energetically competitive for homoleptic complexes (MeSM)x, where x ≤ 12 (Me is methyl and M = Cu, Ag, Au). Among these complexes, the gold-thiolate bond is dominantly covalent with a slight electron charge transfer to sulfur and the copper-thiolate bond is most ionic among the three. The stability of the Au25 nanocluster can be understood from electron-shell-closing arguments. In its anionic form the cluster has 8 Au(6s) electrons in the metal core, composing a shell closing of P-type and rendering the cluster as a closed shell "superatom". Chemical modifications of the Au25 cluster was investigated by (i) replacing one Au atom by a Pd atom in the metal core or in the ligand shell, or by (ii) changing the nature of a simple methylthiolate ligand to more electronegative by chlorination. Both of these studies were motivated by experimental work and are helpful in interpreting the structural and electrochemical behaviour of doped clusters. The Au25 cluster, either in its all-gold or doped form, is a robust building block that could be used for designing cluster-based nanomaterials with tunable electronic, optical or magnetic properties. Finally, catalytic properties of (i) small, bare and neutral gold clusters (Au2 and Au4) for H2O2 formation and (ii) activated, partially thiolate-protected Au25 clusters for CO oxidation were studied. Reaction pathways for competing channels in H2O2 (hydrogen peroxide or water formation) were studied by room-temperature molecular dynamics and metadynamics. It was concluded that small gold clusters are fluxional (their structure can change during the reactions) and the size of the gold cluster can affect the probability of a given reaction channel. Activation of the Au25 cluster by partial removal of the protective layer changes the electron count in the metal core and renders the cluster an electropositive species that can bind molecular oxygen and produce CO2 with considerably low (below 0.7 eV) activation barriers.