The Controllable Synthesis And Growth Mechanism Of Gold-diphosphine Nanoclusters

Small metal clusters composed of a few to tens of core atoms have size-specific unusual physical and chemical properties that differ from those of larger nanocrystals, which are of fundamental importance as new functional materials. Among them a great deal of research has gone into ligand-coordinated gold nanoclusters because of their potential applications in fields such as catalysis, optical and electrical properties, imaging and biology. However there are still some problems to be solved in gold nanoclusters’field. Some of the problems include:how to control synthesis of gold nanoclusters with high yield and accurate number of gold atoms, futher research on growth mechanism of gold nanoclusters, optimize the system of ligand exchange reaction, reveal the relationship between structure and property of gold nanoclusters and so on. Diphosphine ligands containing alkyl chain, with the general formula of P(Ph)2-(CH2)M-P(Ph)2 or LM, where the two P atoms are separated by an alkyl chain with a number M of CH2 spacers, there may be some differtent characteristics between LM-and other ligands-protected gold nanoclusters. In this dissertation, with a combination of X-ray absorption fine structure (XAFS), UV-vis, mass spectrometry (including electrospray ionization mass spectrometry ESI-MS and matrix assisted laser desorption ionization time of flight mass spectrometry MALDI-TOF-MS) and nuclear magnetic resonance (NMR) techniques we conducted thorough researches on kinetic control synthesis, in-situ reaction mechanism and thiol etching of diphosphine ligands LM-protected gold nanoclusters. This dissertation includes:1. Control synthesis of gold-diphosphine nanoclustersUV-vis and MALDI-TOF-MS were combined to study the reaction kinetics of L5-protected gold nanoclusters synthesized from Au2L5Cl2 precursor by changing solvent (dichloromethane, ethanol absolute or acetone), reductant (borane-tert-butylamine TBAB or sodium borohydride NaBH4) and reaction temperature (room temperature or 0℃). It revealed that the reaction speed was different under different kinetic control. Reaction speed is reduced by reduction of temperature, but it nearly made no difference to the product distribution in the ethanol /NaBH4 reaction system. To some extent solvent and reductant were crucial to product distribution, strong reductant NaBH4 led to polydisperse product while weak reductant TBAB was prone to produce monodisperse product as long as appropriate solvents such as ethanol or dichloromethane were chose. And ethanol (or dichloromethane)/TBAB reaction system persisted more than one hour, which provided conditions for real-time monitoring reaction growth kinetic process.2. Study on growth mechanism of gold-diphosphine nanoclustersThe formation process of 1,5-bis(diphenylphosphino) pentane (L5)-protected Au nanocluster through the reduction of precursor Au2L5Cl2 by borane-tert-butylamine (TBAB) is traced by a combination of time-dependent x-ray/UV-vis absorption spectroscopies and mass spectrometry. The whole reaction persisted about one hour and the colour of the reaction solutions slowly changed from ligh yellow, yellow, orange, orange red and finally to brownish red. It is found that Au4L52Cl was formed in the initial 10min followed by the formation of Au5L52 and Au7-Au11 could be observed subsequently. At 30 and 60 min, the main products were Au8L53Cl and Au11L54Cl2, respectively. We can conclude that the cluster formation is achieved in a nucleating/growth manner including two distinct reaction steps. The initial generation of dual-core basic unit Au4L52Cl is a critical step, which allows for a subsequent size-growth process via incorporation of the existing Au monomer (Au(I)-Cl) or Au2L5 to form Au5L52, Au8L53Cl, Au10L54Cl and finally main Au11L54Cl2. This work advances one step further toward understanding the mechanism of formation and growth of diphosphine ligands-protected Au NCs and may be helpful to the design and controllable synthesis of nanoclusters.3. Study on thiol etching of gold-diphosphine nanoclusters1,5-bis(diphenylphosphino) pentane (L5)-protected Au11 nanocluster was firstly synthesized in ethanol absolute solution, then the Au11 nanocluster was dissolved in chloroform solution and dodecanethiol was added, the whole reaction proceeded for 48h under 55℃.The thiol-etching reaction in chloroform for L5-protected Au11 nanoclusters is presented, and synchrotron radiation X-ray absorption fine structure, UV-vis absorption and mass spectra are combined to identify the reaction products. It is revealed that a gold(I)-thiolate complex Au2L5(SR) is produced, quite different from the case of thiol-etching PPh3-protected Au clusters where formation of Au25 or Au11 clusters is achieved. This work deepens understanding of etching reactions of Au clusters, different ligands have different characteristic which may have specific influence on thiol-etching reaction and lead to different products. It’s significant for thiol-etching reaction and ligands effect.