IIIT Hyderabad Publications |
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Gold Nanoclusters and Nanoparticles: Enhanced Catalytic and Raman Scattering PropertiesAuthor: Shweta Bhardwaj Date: 2017-02-07 Report no: IIIT/TH/2017/12 Advisor:Tapan Kumar Sau AbstractGold nanostructures especially gold nanoparticles and nanoclusters (NCs) have emerged as very prominent metal nanosystems because of their several unique properties. We present here our studies of a physical and a chemical property that are modified enormously by such gold nanostructures, in addition to the synthesis and characterizations of the gold NCs and nanoparticles. The physical and the chemical properties were „surface-enhanced Raman scattering (SERS)’ and the catalytic activities respectively.Currently it is of paramount importance to develop new synthesis strategies that will give us opportunity to observe a system evolving from its few-atom, ultra-small, clusters to nanoparticle size. We have been successful to develop a new one-pot aqueous synthesis protocol where gold nanoclusters starting with size < 1 nm evolved with time to almost ~7.5 nm size nanoparticles. The temporal evolution of nanosystem was studied by various methods like UVVis spectroscopy, electrochemical, MALDI-mass spectrometry, TEM, etc. These systems can be used to study the particle size effects on the catalytic reactions.Raman scattering provides a wealth of information about the chemical structure and dynamics of molecules and materials. However, the limitations of low Raman cross sections of the scatterers can be overcome by the surface-enhanced Raman scattering (SERS) on the roughened metal electrodes and/or colloidal aggregations. Researchers had attempted a variety of substrates for the signal enhancement. Since the magnitude of Raman enhancement depends on a number of factors such as the size, shape, orientation, extent of aggregation, etc. of the nanostructures, it is, therefore, practically very difficult to produce nanostructures that give reproducible and stable SERS. Nanoclusters that have precise compositions, well-defined structures and coordination can be viable SERS substrates. Here we explored the enhanced Raman scattering properties of different sized gold nanoclusters. The gold clusters were found to produce significant static chemical enhancements in Raman signals for small molecules. Various factors such as molecule–cluster interaction energies, charge transfer from the molecule to the metal cluster and total change in the polarizability of the system were calculated and their roles and contributions in the Raman enhancement mechanism were explored. Ligand to cluster charge transfer, HOMO-LUMO gaps of the complexes and the orientation of ligand molecules with respect to gold clusters played significant roles in the chemical enhancement mechanism.We investigated the nanoparticle surface and morphology effects on SERS. We demonstrated that the nature of the surface structure and morphology of the gold nanoparticles strongly influence the Raman signal of a Raman probe. Drastic drop in Raman signal due to the disappearance of sharp edges and tips of the particles directly proved that such sharp structures can enhance the Raman signal to a large extent which is popularly known as “lightening rod effect”. In addition to the individual size and geometry effects, assembled nanosystems can produce extremely large Raman signal enhancements. This effect has been employed for the detection of low concentration analytes placed among assembled nanoparticles. We used sulfur containing amino acid cysteine for linear and controlled assembly of gold nanoparticles of different shapes. Raman enhancement factor was maximal for multi-spiked gold nanoparticles followed by gold nanorod and nanospheres. Finally, we studied the enhanced chemical effects in terms of catalytic activities of gold nanoclusters and nanoparticles. We compared the catalytic effects of the gold nanosystem sizes/concentrations on the reduction reaction of o–nitro aniline to 1,2–benzenediamine. Rate constant value decreased as the size of gold nanocluster increased from <1 nm to ~7.5 nm. Smaller gold nanosystems acted as better catalysts for the reduction reaction compared to the larger ones. This demonstrates that free-standing gold nanoclusters can act as better catalysts than the nanoparticles. Full thesis: pdf Centre for Computational Natural Sciences and Bioinformatics |
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