Deposition, Topographic and Spectroscopic Studies of Metal Nanoclusters on Photocatalytic Surfaces

Author: Jesse Daughtry

Daughtry, Jesse, 2023 Deposition, Topographic and Spectroscopic Studies of Metal Nanoclusters on Photocatalytic Surfaces, Flinders University, College of Science and Engineering

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Photocatalysis, the catalysis of chemical reactions using energy from solar photons, offers promising pathways to the production of fuel, chemical storage of solar energy and industrial catalysis. However photocatalytic materials such as TiO2 require improvements to their properties and performance. The deposition of co-catalysts such as chemically-synthesised, atomically-precise nanoclusters, which have been shown to improve photocatalytic performance due to their size-specific properties and quantised energy levels, have been proposed as a pathway to improving performance and tuning the electronic structure of photocatalytic systems. To facilitate and improve the design of such systems, it is imperative to develop new methods to better characterise the physical and electronic surface interactions between metal nanoclusters and their photocatalytic supports. Ultra-high vacuum based spectroscopic techniques, offer extreme surface sensitivity, and are uniquely positioned for investigations into the surface interactions of nanoscale objects and their supporting surfaces.

Obtaining electronic information from nanocluster adorned photocatalytic surfaces has been a challenge without powerful, specialised equipment and delicate sample preparation techniques. These factors have meant that isolating the effects of deposited nanoclusters with respect to surface conditions has been difficult. The approach to improving investigations in such systems was three-fold. Firstly, an improved method of synthesising photocatalytic TiO2 substrates for surface sensitive spectroscopies was required. Secondly, a deposition method which would allow for chemically-synthesised metal clusters to be deposited directly onto sample surfaces in-situ without causing the loss of surface treatments, or introducing unwanted species. Finally, the use of newly developed techniques was compared against legacy methods and used to investigate and isolate the electronic structure of nanoclusters, photocatalyst surfaces and the interactions between them using vacuum spectroscopy.

To build a complete picture of these methods and systems, a combination of scanning probe microscopy, electron microscopy, surface-sensitive spectroscopies and other methods were required. Topographical characterisation of surfaces and any deposited nanoclusters was achieved through a combination of atomic force microscopy, scanning electron microscopy and transmission electron microscopy, while material phase analysis was performed using x-ray diffraction methods. Elemental compositions and chemical states were investigated using x-ray photoelectron spectroscopy, which in turn informed the analysis of meta-stable ion electron spectroscopy and ultraviolet photon spectroscopy measurements which probe the electronic structure and density of states of these systems.

The development of a new method of nanoparticulate, amorphous TiO2 tin film was achieved. These films were characterised and found to not only reduce undesirable chemical species, but also enabled simple surface modification through sample heating introducing Ti3+ defects and transition to anatase and rutile crystallinity. A method for in-situ deposition of solvated chemically synthesised metal nanoclusters was developed, allowing for the controlled deposition of disperse sub-monolayer nanocluster films onto target samples under vacuum conditions. These photocatalyst and pulsed nanocluster deposition (PNCD) innovations were combined in a comparison of legacy deposition methods to PNCD samples with PNCD showing improvements in surface treatment retention, and improvements to XPS and MIES analysis of electronic DOS. Finally, the concept of an experimental system allowing the deposition of multiple chemically synthesised nanoclusters into vacuum was explored, showing inter-cluster and cluster-surface impacts on electronic DOS.

The developments presented in this thesis outline a complete system for the deposition and spectroscopic study of chemically synthesised metal nanoclusters onto photocatalytic TiO2 surfaces. The results achieved offer pathways for the preparation, investigation, and analysis of the electronic structure and surface coverage for these photocatalytic systems, as well as pointing to the possibility for some techniques to find more general utility.

Keywords: Physics, Surface Science, Photocatalysis, Titanium Dioxide, Nanocluster, TiO2, Spectroscopy, Gold Nanocluster, Ruthenium Nanocluster, MIES, UPS, XPS, AFM, SEM, Vacuum Deposition, Mica, PNCD, Pulsed Nanocluster Deposition, RF Sputter Deposition,

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2023
School: College of Science and Engineering
Supervisor: Professor Gunther Andersson