Author: Hassan Al Qahtani
Al Qahtani, Hassan, 2016 Characterisation of Au9 –Nanoclusters Deposited on Titania Surfaces Using Spectroscopic and Microscopic Techniques, Flinders University, School of Chemical and Physical Sciences
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In this work chemically made clusters stabilised by organic ligands, Au9 (PPh3)8(NO3)3 (Abbreviated Au9), deposited on titania surfaces are investigated by using various surface sciences techniques to study gold clusters and the effects of titania surfaces. Supported size specific metallic nano-clusters have emerged as superior catalysts in a few cases but until now fabrication and activation of catalysts using chemically synthesised atomically precise gold (Au) clusters supported on titania has not been well understood. Titanium oxide (TiO2) is used widely as support for metal based catalysts compared with other ‘inert’ supports (e.g. SiO2, BN). It was demonstrated in the case of ideal flat TiO2 surfaces that defects significantly affect adsorption energy, cluster shape, and electronic structure of Au nanoclusters supported on them and influence their unique catalytic properties. Au clusters strongly bind to reduced titania surfaces due to defects, such as oxygen vacancies. Defects can be created by pre-treatment of titania surfaces before Au cluster deposition, such as sputtering with Ar+, acidic pre-treatment and annealing in UHV. Chemical reactivity of metal clusters is determined by three main factors: (a) size of clusters, (b) geometric arrangement of atoms forming clusters and (c) electronic structure. Also, metal oxide supports play an important role in changing the catalytic activity of nanoclusters. In this study, various surface techniques were used to investigate the above factors using various substrates and various pre-treatment methods. Chemical compositions of clusters deposited on titania surface and removal of ligands after heat treatment were monitored using XPS. The heating process caused the Au clusters to partially agglomerate with the remaining isolated precise Au clusters. Aberration corrected HAADF-STEM with low electron-beam acceleration voltage wasused to resolve the detailed structure of ultra small protected Au9 clusters deposited on titania nanosheet at atomic resolution for the first time. Such microscopes are capable of directly determining the geometric structure of Au9 clusters deposited on substrates. We compared experimental Au9 structures using STEM with DFT calculations of clusters of nine Au atoms in the gas phase. While substrate influence on Au9 structure is missing in DFT calculations, it is possible to classify structures of Au9 clusters deposited on titania as found experimentally using STEM. AFM and STM observations show nanoclusters are highly distributed and their average size was measured. STM showed an individual Au9 deposited on titania nanosheet. In addition, Metastable Induced Electron Spectroscopy (MIES) was applied to quantitatively determine valance electronic structure of supported Au clusters.
Keywords: Gold nanoclusters-DFT calculation-STM-XPS-AFM-STEM-Titania support
Subject: Chemistry thesis
Thesis type: Doctor of Philosophy
School: School of Chemical and Physical Sciences
Supervisor: Prof.Gunther Andersson