Synthesis, Structural Characterisation and Self Assembly of Nanoparticles

Abstract

This thesis is concerned with the synthesis, structural characterisation and self assembly of various nanocrystalline materials. These materials include gold, lead sulfide and lead selenide with substantial focus given to the noble metal palladium. The aim of this research was to obtain size and shape control over nanoparticles formed from solution phase synthesis for various applications. This was realised with chemical techniques using organic surfactants as growth controlling agents. The morphology, composition, internal crystal structure and applicable properties of the as synthesised nanoparticles were fully investigated to give a complete characterisation. Characterisation was carried out using a number of techniques including Super and High Resolution Transmission Electron Microscopy (SHREM, HREM), Synchrotron Powder X-Ray Diffraction (XRD), Selected Area Electron Diffraction (SAED) and Energy Dispersive X-Ray Spectroscopy (EDS).

The first chapter in this thesis focuses on the synthesis and self assembly of monodisperse gold nanoparticles into nanoparticle superlattices (NPSLs), an exciting new type of material. The nanoparticles were prepared using a well known chemical method at room temperature. They were then arranged into NPSLs by a simple evaporation technique. Intermediate structures to the SLs were isolated which gave an insight into their formation. This showed that the NPs first self assembled into an energetically unfavourable bilayer before forming the most thermodynamically preferred three dimensional structure. This behaviour was due to the presence of organic capping ligands.

The second chapter is concerned with the synthesis and characterisation of lead chalcogenide nanoparticles (lead sulfide and lead selenide). These are semiconductor materials which can provide a photocurrent when illuminated with infra-red radiation which makes them ideal candidates for solar cell technology. The nanoparticles were synthesised using a bench top solvothermal method. By varying the nature of the surfactant system, the precursor and the reaction time and temperature a wide range of nanoparticles with different sizes and shapes were prepared. A type of lead sulfide nanoparticles was then chosen for capping ligand exchange experiments. The new method developed here provides a facile route to water soluble lead chalcogenide nanoparticles and a means to more easily extract a photocurrent when used in solar cell applications.

The remainder of this thesis is focussed on the synthesis and structural characterisation of palladium nanoparticles. Palladium is a very important catalytic metal therefore control over its size and shape on the nanoscale is of primary concern. In the third chapter of this thesis various types of palladium nanoparticles were produced using solution phase techniques in a pressure reaction vessel. By varying the nature of the surfactant system, the precursor and the reaction pressure, temperature and time the size and shape of the resulting nanoparticles could be controlled. These included spherical and worm-like nanoparticles as well as novel pod-like and highly branched palladium nanostructures. These complex shapes were the first evidence of this kind of morphology for palladium and provide a new and exciting material for catalytic applications.

The final chapter in this thesis features a full structural characterisation and growth mechanism for the novel, complex palladium nanostructures along with an investigation into their catalytic and hydrogen absorption properties. The structural characterisation of a palladium tripod provides the first direct evidence of complex growth from a symmetrical nanoparticle core possessing the face centred cubic crystal structure. The growth of the highly branched palladium nanostructures is then tracked in real time. It is shown that the growth involves the formation of nuclei followed by tripod intermediates and finally highly branched nanostructures. By varying the nature of the surfactant system the kinetics of the reaction and hence the morphology of the resulting nanostructures can be controlled. A full growth mechanism is therefore proposed. The catalytic activity of the highly branched palladium nanostructures towards a simple organic transformation reaction is investigated. Finally, the hydrogen absorption and desorption properties of the highly branched nanostructures is explored. The results presented here regarding palladium nanoparticles are applicable to other industrially important noble metals such as gold, silver and platinum.

A final conclusion chapter is then presented along with ideas for future research.