Luminescent nanoclusters of silver and gold

Abstract

Gold and silver clusters with sizes up to ∼100 atoms have unique properties, such as luminescence, that are not found in bulk materials or in larger nanoparticles. Clusters can be prepared with atomic monodispersity rather than a size distribution, as is common for larger nanoparticles. This allows for correlation between size and properties on an atomic level. This thesis describes the synthesis and characterisation of gold and silver clusters in aqueous solutions. The clusters are capped with lipoic acid, a bidentate ligand which binds to Ag or Au with two sulfur atoms and provides an efficient barrier against aggregation. The silver clusters are luminescent and atomically monodisperse, with 29 Ag atoms and 12 ligands. Over time, the clusters degrade and lose their luminescence, although this process is reversible, which is possibly unique for such Ag clusters. Further investigation into the synthesis procedure of Ag29 found that reduction of silver ions in the presence of lipoic acid initially results in the formation of larger Ag clusters with around 100 atoms. These are then etched by excess ligands to give Ag29. The luminescence efficiency and stability of Ag29 can be enhanced by doping with gold. A singly doped cluster is formed, Au1Ag28. With X-ray spectroscopy, we find that the Au atom preferentially occupies the central position in the cluster. Doping of Ag29 with more than a few percent of Au does not lead to the formation of stable clusters. Pure Au clusters with lipoic acid can be prepared, but these are polydisperse. However, the average size and optical properties of the clusters can be tuned by varying the NaOH concentration during synthesis. A lower NaOH concentration corresponds to a larger average cluster size. The synthesis proceeds in two steps. We find that in the first step of the synthesis, the Au precursor HAuCl4 reacts with lipoic acid resulting in the formation of large, unstable Au nanoparticles. This reaction is inhibited at high NaOH concentrations, where it must compete with hydrolysis of HAuCl4. The resulting AuCl4-xOHx- species (x =3–4) may be less reactive towards LA. The second step in the synthesis of Au clusters involves the reduction of the synthesis intermediates to form clusters. Many of the conclusions presented in this thesis are based on X-ray spectroscopy studies. This technique was used to demonstrate that the synthesis of Ag29 clusters proceeds via larger particles, that the Au atom in Au1Ag28 is preferentially located in the centre of the cluster, and that Au cluster samples with high NaOH concentration contain AuCl4-xOHx- species. A great advantage of X-ray spectroscopy is that it is element-selective. It enables us to study for instance only the Au dopant in bimetallic clusters, or all Ag species during the synthesis of Ag29. Solids, solutions, mixtures and disordered species can all be measured, and no purification is necessary to remove excess salts or ligands as these are invisible in X-ray spectroscopy.