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Type: Theses
Title: The characterisation, photocatalytic performance, and theoretical investigation of small gold clusters supported on titanium dioxide nanoparticles
Author: Alvino, Jason Frankie
Issue Date: 2015
School/Discipline: School of Physical Sciences
Abstract: Renewable photocatalytic systems that can use solar radiation to produce chemical fuels are appealing technologies that have garnered much interest over the last few decades. The development of novel gold-based heterogeneous photocatalysts has the potential to drive this new form of green chemistry. In this thesis, ligand-protected, atomically precise gold clusters supported on titanium dioxide nanoparticles are characterised by various surface science techniques and their photocatalytic potential for water photolysis is investigated. Given the difficultly associated with acquiring experimental data in the incredibly small size regime of clusters, complementary density functional theory calculations are also performed to match with experimental data and to help elucidate the interactions occurring between gold atoms and the titanium dioxide surface. The characterisation of these gold clusters begins with the far infra-red absorption spectra of isolated Au₆(Ph₂P(CH₂)₃PPh₂)₄(NO₃)₂, Au₈(PPh₃)₈(NO₃), Au₉(PPh₃)₈(NO₃)₃, Au₁₁(PPh₃)₈Cl₃, Pd(PPh₃)Au₆(PPh₃)₆(NO₃)₂, and Pt(H)(PPh₃)(AuPPh₃)₇(NO₃)₂ recorded using synchrotron light. These experiments reveal a series of unique peaks between 50 and 475 cm⁻¹ that are assigned to specific vibrational modes by comparison with density functional theory calculations. The distinct peaks for each cluster can be assigned to the calculated cluster core vibrations: 80.4 and 84.1 cm⁻¹ for Au₆; 165.1 and 166.4 cm⁻¹ for Au₈; 170.1 and 185.2 cm⁻¹ for Au₉; 173.7 and 182.2 cm⁻¹ for Au₁₁; 158.9, 195.2, and 206.7 cm⁻¹ for Au₆Pd; and 156.3, 171.8, and 173.5 cm⁻¹ for Au₇Pt. There is also a strong absorption for all clusters near 420 cm⁻¹ that are assigned to P-Ph₃ vibrations. Characterisation is continued with the use of Synchrotron X-ray and TEM techniques to investigate the full effect of treatments that are undertaken to remove the organic protective ligands after the gold clusters are supported upon titanium dioxide. These experiments show an increase in particle size and loss of ligands with successively harsher post-treatments. Acidic pre-treatment and the form of the titanium dioxide support are shown to have a strong impact on the severity of agglomeration and ligand loss. These results are also supported by X-ray absorption near edge structure and extended X-ray absorption near edge structure analysis of complementary X-ray absorption experiments. The photocatalytic potential for water photolysis by these catalysts is also investigated using a new experimental apparatus designed and built during the project. It consists of a heterogeneous gas-phase reactor operating near ambient conditions that can deliver constant and controlled samples of gas to a residual gas mass spectrometer for analysis of the gas composition over time. Using this apparatus, Au₈(PPh₃)₈(NO₃)₂, Au₉(PPh₃)₈(NO₃)₃, and Au₁₀₁[P(C₆H₅)₃]₂₁Cl₅ supported on anatase or P25 TiO₂ nanoparticles are investigated for their activity towards water photolysis and their peak H₂ production rates are quantified. Anatase-supported samples are found to increase in activity with successively harsher post-treatment conditions, suggesting that increasing the size of the gold particles is beneficial for photocatalytic activity. In contrast, samples prepared on acid-washed P25 or acid-washed anatase show evidence of an ideal gold particle size and ligand coverage after relatively gentle post-treatment conditions. Finally, density functional theory investigations are undertaken of Au₁, Au₂, Au₃, and Au₄ clusters bound to the stoichiometric and oxygen-deficient titanium dioxide anatase(101) surface using an atomic-centred basis set approach. Numerous isomers are found within 0.5 eV of the lowest energy structure for Au₂, Au₃, and Au₄. The structural parameters, binding energies, infrared spectra, charge transfer, and density of states for each isomer are described in detail. Key findings include: increased binding energy as the number of Au atoms are increased; strong Au-Au stretching and cluster breathing modes that shift considerably between isomers; charge transfer from the Au clusters to the titanium dioxide surface; and the introduction of numerous occupied gold states at the valence band edge that extend into the band gap. This work provides the foundation for future studies that will use these anatase models to investigate the properties of small Au clusters with photocatalytically relevant molecules and their reaction pathways.
Advisor: Metha, Gregory Francis
Kee, Tak W.
Huang, David
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 2015.
Keywords: photocatalysis
gold cluster
ligated clusters
DFT
TiO₂
Nanoparticle
water-splitting
Provenance: This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legals
DOI: 10.4225/55/58dc7333731ef
Appears in Collections:Research Theses

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