Novels materials based on 2-dimensional arrays of metal nanoparticles and microparticles

Abstract

The purpose of this thesis is to study the mechanism of interfacial self-assembly and develop multi-functional 2-dimensional films based on metal nanoparticles and microparticles via self-assembly combined with other subsequent simple post-processing steps.

The work is based on a general and novel method recently developed by the group in QUB that allows charged colloidal nanoparticles to self-assemble into a monolayer at liquid-liquid interface with the assistance of “promotor” molecules such as TBA+ or TPB- which give a charge-screening effect. In the current work, the first investigation was centred on CTAB which was found to promote nanoparticle self-assembly in a way that appeared similar to previously investigated promoters. However, in this thesis more detailed studies showed that its actual role in the self-assembly process was more complex than expected and was concentration dependent. It was found that as well as acting as a promoter at low concentration, CTAB can also work as a modifier when it is present at higher concentration, chemically bonding to the surface of particles and altering the charge, which also induces particle localisation at the interface. This is the first dual function promoter/modifier to be discovered although it is possible that many other compounds may be able to give similar effects.

The next part of the thesis focuses on extending the established self-assembly method to create new functional 2D films of photoactive nanoparticles. Self-assembled TiO2 nanoparticle films on polymer supports were the first films of this general type to be used for dye degradation. However, these films have limitations associated with charge recombination which may be reduced by adding noble metals. Here the possibility of enhancing the photocatalytic activity was investigated with Au and Pt nanoparticles which were sputtered or photodeposited onto the film to construct metal/TiO2 composite photocatalytic materials. Detailed studies of the uniformity, stability and photocatalytic activity of the metal/TiO2 composite films obtained by using both these methods (sputtering or photodeposition) were carried out to allow the methods to be compared. It was found that the sputtering approach for loading metal particles on semiconductor films is superior to the photodeposition method in that it is simple, inexpensive and reproducible. Most importantly, the photodeposition significantly improves the performance of the initial TiO2, so that they have much higher efficiency than standard commercial materials while at the same time being very flexible and dramatically lighter than existing systems.

The interfacial self-assembly approach was then extended from nanoparticles to micro-particles where Cu microparticles were chosen as the building blocks because of their wide use and low cost. It was found that these microparticles can also self-assemble at liquid-liquid interfaces even though they are much larger and heavier than the nanoparticles which have previously been studied. The surface wettability of Cu particles was altered chemically to optimise the localisation process which is determined principally by minimising interfacial surface tension. It was found that particles made hydrophilic by acidic treatment led to formation of spherical assemblies while those made more hydrophobic by thiol modification gave 2D films which could then be further modified by galvanic deposition of fractal metallic Ag or Au structures. These had the different levels of roughness required to form Cassie-Baxter superhydrophobic materials after modification with polyfluorothiols to lower the surface energy. In this thesis, the films were used as substrates for SERS detection following droplet evaporation from the surface but due to their flexibility they also have numerous possible applications not available with traditional rigid superhydrophobic materials. For example, the film can also be attached onto various substrates, including paper and acetate sheet, by using the polymer layer as a thermosetting adhesive. These can in turn be patterned using a standard laser printer to apply ink which has a different hydrophobicity from the superhydrophobic coating. Proof-of-principle experiments show that the final printed film-on-substrate composites have considerable potential to be applied in droplet-based microfluidic systems.

Thesis embargoed until 31 July 2027.

Date of Award	Jul 2022
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Steven Bell (Supervisor) & Panagiotis Manesiotis (Supervisor)