In-situ Synchrotron Radiation Technique Study Of Surface Ligand Modification Mechanism Of Gold Nanocrystals

Gold nano-materials have attracted increasing research attention due to their unique structures, properties and applications in various fields, such as catalyst, biomedical, sensor and luminescent materials et al. However, to achieve the perfect application of these materials, there are still many unresolved problems of great importance blocking the rapid development of this field. Some of these problems are listed as:how to achieve the controllable synthesis of nanoclusters; the initial growth mechanism of metal nano-materials. Thereinto, the functionalization of material has been the most concerned problem. According to the structure of gold nano-materials, the exsisting functionalization methods can be divided into two categories:tuning of metallic core and tuning of surfactant. In this dissertation, we conducted thorough researches on the surfactant behavior of gold nano-materials (including nanaoparticles and nanoclusters) with a combination of UV-vis and X-ray absorption fine structure (XAFS) spectroscopy and Matrix-assisted laser desorption/ionization time of flight mass spectrometry techniques. Besides, we also synthesized CoSe2two-dimensional (2D) ultrathin nanosheets by exfoliating the CoSe2-based inorganic-organic lamellar nanohybrids. This graphene-like2D nanosheets embodied perfect oxygen revolution reaction (OER) catalytic property. By means of XAFS and positron annihilation measurement (PAS), together with first-principle calculations, the reason of property modification was investigated. This dissertation include:1. Adsorption kinetic process of thiol ligands on gold nanocrystalsGold nanoparticles (NPs) capped by thimbleful triphenylphosphine (PPha) with a narrow size distribution of0.3nm were prepared by a reduction in ethanol and then excess dodecanethiol was injected into the solution. TEM images indicated that sizes of NPs almost remain constant during the ligand adsorption process and in-situ X-ray absorption fine structure (XAFS) revealed that the ligand adsorption process could be divided into two stages, that is, a slow stage on vertex and edge sites followed by a fast stage on facet sites. The second stage could be described by the Langmuir kinetics equation with an adsorption rate constant of0.0132min-1. In-depth analysis of XAFS spectra indicated a elongation of Au-S and surface Au-Au bond by0.04A and0.06A, respectively which was a certain result of successive ligand adsorption on different geometric position. A clear picture describing the thiol adsorption process on Au NPs is presented experimentally for the first time and quantitative revolutions of Au NPs in the adsorption process were also revealed.2. In-situ study of ligand desorption process of Au nanoclusters～1.0nm Au nanoclusters was prepared by liquid chemical method and then a process of ethanol to hexane solvent-exchange was monitored by in-situ XAFS and UV-vis techniques. We found Au nanoclusters kept their size and size distribution unchanged in this process. In-depth analysis reveals that solvent-exchange leads to quick desorption of thiol ligand from the surface of nanoclusters which changes their electronic structures and makes them unstable. As a result, the remained Au cores undergo a slower atomic structural rearrangement, producing a face-centered-cubic structured clusters from a low-symmetry icosahedron structure. A facile solvent-exchange method of tuning the atomic and electronic structure of Au nanoclusters was reported for the first time.3. Mechanism of ligand exchange reaction on the surface of Au nanoclustersA whole process of ligand exchange between Au25(SCH2CH2Ph)18and PhSeH was monitored by a combination of in-situ UV-vis absorption and XAFS spectra as well as in-situ MALDI-TOF mass spectrometry. In this process, in addition to a traditional exchange pattern of one incoming ligands with one original protecting ligands, a brand new pattern of fracture-ligand exchange-rebinding-reorganization was detected. Specifically, at the beginning, six Au2(SCH2CH2Ph)3of Au25cluster entirely or partially separated from Au13core in the ligand exchange process; and then the separated fragments would exchange with solvated PhSeH; nextly the resulting fragments would rebind to the Au13core and a new Au25(SePh)18was formed after an structure reorganization. This brand new ligand exchange mechanism will give a lot of insights into further functionalization of Au nanoclusters.4. Vacancy-rich ultrathin CoSe2nanosheets:fabrication and performance studyUltrathin CoSe2nanosheets with single-unit thickness (1.5nm) were fabricated by exfoliating their nanohybrid in ethanol solution. Our developed CoSe2ultrathin nanosheets with atomic thickness can effectively catalyze the oxygen evolution reaction (OER) with low overpotential (0.32V), which is superior to that of its bulk counterparts and most reported Co-based electrocatalysts. The positron annihilation spectrometry and XAFS spectra provide clear evidence that a large number of VCo vacancies formed in the ultrathin nanosheets and these vacancies may be formed in the fabrication process where the ultrasonic treatment enables the DETA to drag the Co atoms detached from the lattice. First-principles calculations proved that Vco vacancies could serve as active sites to effectively adsorb H2O molecules, resulting in the significantly improved OER catalytic performance. In summary, we not only demonstrated the potential of a notable, affordable, and earth-abundant water oxidation electrocatalyst based on ultrathin CoSe2nanosheets but also opened up a promising avenue into the exploration of excellent active and durable catalysts that can replace noble metals for oxygen electrocatalysis.