Towards the photophysical nature of the triphenylphosphine-stabilized nonagold cluster and its catalytic applications
| dc.contributor.advisor | Metha, Gregory F. | |
| dc.contributor.advisor | Kee, Tak W. | |
| dc.contributor.author | Madridejos, Jenica Marie Lorica | |
| dc.contributor.school | School of Physics, Chemistry and Earth Sciences | en |
| dc.date.issued | 2022 | |
| dc.description.abstract | Atomically-precise gold clusters are well-defined structures with an exact number of atoms and possess distinct electronic and optical properties that are strongly influenced by the ligands. A major interest in the field of gold nanochemistry stems from the different potential applications of these systems. For example, photophysical properties of gold clusters such as efficient phosphorescence and ultrafast charge transfer could be useful for sensing, imaging and photonics. In heterogeneous catalysis, phosphine-ligated Au clusters offer greater potential due to the more labile Au−P bond and structural nonrigidity than the thiolate and alkynated counterparts. This dissertation tackles the research gap regarding the electronic structure and photophysical properties of gold nanoclusters, specifically the fluxional nonagold cluster [Au9(PPh3)8]3+ with two known structural forms, by employing computational methodologies and spectroscopic techniques. Firstly, a review focused on computational chemistry and quantum chemical simulation studies of phosphine-stabilized gold clusters highlights the need for a more computationally feasible method for these nanoclusters. Next, the development and implementation of the density functional tight-binding parameters describing gold-phosphine clusters are tested against density functional theory and experimental data such as geometric structure and infrared spectra. This tight-binding method for nanoscale gold-phosphine clusters simplifies the geometry optimization and frequency calculation that would often require heavy computational resources for systems with hundreds of atoms. In particular, the parameters are used to predict the geometric and electronic structures and far-infrared spectrum of the large metalloid-like Au108S24(PPh3)16 cluster. Next, time-dependent density functional theory formalism is reviewed and computational parameters and methods for excited state calculations such as appropriate density functionals, scalar relativistic effects, and spin orbit coupling corrections are benchmarked for better simulations of the absorption spectra of the two different isomers of [Au9(PPh3)8]3+. The structural isomerism of the [Au9(PPh3)8]3+ cluster impedes the correct analyses of the cluster’s optical properties. The photophysical properties of the two isomers of this cluster are considered by utilizing steady-state and transient absorption spectroscopy, alongside calculations of their optical properties using spin orbitcorrected time-dependent density functional theory. In solution, the nonagold cluster exists as a crown-like structure with a C4 core symmetry, while the experimental crystal structure exhibits a butterfly-like D2h core symmetry. The two isomers have different electronic structure, and therefore display distinct optical properties. The peaks of the experimental steady-state absorption spectra of [Au9(PPh3)8]3+ in dichloromethane and methanol solutions are analyzed and assigned to that of simulated spectra for the crown isomer. The transient absorption results show fast decay constants of 2 and 45 ps, which are assigned to fast intersystem crossing and non-radiative decay of the crown isomer. The effect of small counteranions on the stability of butterfly isomer is studied using steady-state absorption spectroscopy and demonstrates the difference in charge density distribution of the two isomers. Computational determination of energy barrier between the two structural isomers via transition state calculations accounts for fast structural isomerization of the nonagold cluster. The presence of the two isomers in gas-phase can be explained by the low energy barrier and the peaks of the gas-phase electronic absorption spectrum of [Au9(PPh3)8]3+ are rationalized by the combined simulated spectra of the two isomers. Finally, the catalytic potential of the [Au9(PPh3)8]3+ cluster deposited on titania (TiO2) is tested for the low-temperature conversion of methane and water. The photocatalyzed reaction produced mainly hydrogen and carbon dioxide, thus resembling low-temperature steam methane reforming and water-gas shift reaction, plus low amounts of ethane and other short-chained hydrocarbons. The complications of the photocatalytic methane transformation arise from agglomeration of the Au clusters which limits repeatability and possible scale-up. Potential future studies based on this dissertation are also considered and discussed. Further improvements in the quantum chemical simulations of Au nanoclusters using density functional theory and its tight-binding counterpart are recommended to ensure accuracy of the ground-state calculations and to provide better simulations of absorption spectra. Likewise, the results of the photocatalysis require further investigation on any potential role for the nonagold cluster in methane transformation and possible surface modification of the photocatalysts. | en |
| dc.description.dissertation | Thesis (Ph.D.) -- University of Adelaide, School of Physics, Chemistry and Earth Sciences, 2022 | en |
| dc.identifier.uri | https://hdl.handle.net/2440/138201 | |
| dc.language.iso | en | en |
| dc.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 | en |
| dc.subject | gold clusters | en |
| dc.subject | density functional theory | en |
| dc.subject | excited states | en |
| dc.subject | optical properties | en |
| dc.title | Towards the photophysical nature of the triphenylphosphine-stabilized nonagold cluster and its catalytic applications | en |
| dc.type | Thesis | en |
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