Functionalization of lamellar molybdenum disulphide nanocomposite with gold nanoparticles

This work explores the functionalization of an organic-inorganic MoS2 lamellar compound, prepared by a Chemical Liquid Deposition Method (CLD), that has an interlamellar distance of ~5.2 nm, using clusters of gold nanoparticles. The gold nanoparticles have a mean diameter of 1.2 nm, a stability of ~85 days, and a zeta potential measured to be ζ = -6.8 mV (solid). The nanoparticles are localized in the hydrophilic zones, defined by the presence of amine groups of the surfactant between the lamella of MoS2. SEM, TEM, EDAX and electron diffraction provide conclusive evidence of the interlamellar insertion of the gold nanoparticles in the MoS2.

Silica bonded stationary phases: Innovative functionalization methods and novel chromatographic separations based on molecular recognition

The research work included in this thesis examines the synthesis, characterization and chromatographic evaluation of novel bonded silica stationary phases. Innovative methods of preparation of silica hydride intermediates and octadecylsilica using a “green chemistry” approach eliminate the use of toxic organic solvents and exploit the solvating power and enhanced diffusivity of supercritical carbon dioxide to produce phases with a surface coverage of bonded ligands which is comparable to, or exceeds, that achieved using traditional organic solvent-based methods. A new stationary phase is also discussed which displays chromatographic selectivity based on molecular recognition. Chapter 1 introduces the chemistry of silica stationary phases, the retention mechanisms and theories on which reversed-phase liquid chromatography and hydrophilic interaction chromatograpy are based, the art and science of achieving a well packed liquid chromatography column, the properties of supercritical carbon dioxide and molecular recognition chemistry. Chapter 2 compares the properties of silica hydride materials prepared using supercritical carbon dioxide as the reaction medium with those synthesized in an organic solvent. A higher coverage of hydride groups on the silica surface is seen when a monofunctional silane is reacted in supercritical carbon dioxide while trifunctional silanes result in a phase which exhibits different properties depending on the reaction medium used. The differing chromatographic behaviour of these silica hydride materials prepared using supercritical carbon dioxide and using organic solvent are explored in chapter 3. Chapter 4 focusses on the preparation of octadecylsilica using mono-, di- and trifunctional alkoxysilanes in supercritical carbon dioxide and in anhydrous toluene. The surface coverage of octadecyl groups, as calculated using thermogravimetric analysis and elemental analysis, is highest when a trifunctional alkoxysilane is reacted with silica in supercritical carbon dioxide. A novel silica stationary phase is discussed in chapter 5 which displays selectivity for analytes based on their hydrogen bonding capabilities. The phase is also highly selective for barbituric acid and may have a future application in the solid phase extraction of barbiturates from biological samples.

Synthesis and characterisation of novel superficially porous silica based stationary phases for use in liquid chromatography

The concept of pellicular particles was suggested by Horváth and Lipsky over fifty years ago. The reasoning behind the idea of these particles was to improve column efficiency by shortening the pathways analyte molecules can travel, therefore reducing the effect of the A and C terms. Several types of shell particles were successfully marketed around this time, however with the introduction of high quality fully porous silica under 10 μm, shell particles faded into the background. In recent years a new generation of core shell particles have become popular within the separation science community. These particles allow fast and efficient separations that can be carried out on conventional HPLC systems. Chapter 1 of this thesis introduces the chemistry of chromatographic stationary phases, with an emphasis on silica bonded phases, particularly focusing on the current state of technology in this area. The main focus is on superficially porous silica particles as a support material for liquid chromatography. A summary of the history and development of these particles over the past few decades is explored, along with current methods of synthesis of shell particles. While commercial shell particles have a rough outer surface, Chapter 2 focuses on the novel approach to growth of smooth surface superficially porous particles in a step-by-step manner. From the Stöber methodology to the seeded growth technique, and finally to the layer-bylayer growth of the porous shell. The superficially porous particles generated in this work have an overall diameter of 2.6 μm with a 350 nm porous shell; these silica particles were characterised using SEM, TEM and BET analysis. The uniform spherical nature of the particles along with their surface area, pore size and particle size distribution are examined in this chapter. I discovered that these smooth surface shell particles can be synthesised to give comparable surface area and pore size in comparison to commercial brands. Chapter 3 deals with the bonding of the particles prepared in Chapter 2 with C18 functionality; one with a narrow and one with a wide particle size distribution. This chapter examines the chromatographic and kinetic performance of these silica stationary phases, and compares them to a commercial superficially porous silica phase with a rough outer surface. I found that the particle size distribution does not seem to be the major contributor to the improvement in efficiency. The surface morphology of the particles appears to play an important role in the packing process of these particles and influences the Van Deemter effects. Chapter 4 focuses on the functionalisation of 2.6 μm smooth surface superficially porous particles with a variety of fluorinated and phenyl silanes. The same processes were carried out on 3.0 μm fully porous silica particles to provide a comparison. All phases were accessed using elemental analysis, thermogravimetric analysis, nitrogen sorption analysis and chromatographically evaluated using the Neue test. I observed comparable results for the 2.6 μm shell pentaflurophenyl propyl silica when compared to 3.0 μm fully porous silica. Chapter 5 moves towards nano-particles, with the synthesis of sub-1 μm superficially porous particles, their characterisation and use in chromatography. The particles prepared are 750 nm in total with a 100 nm shell. All reactions and testing carried out on these 750 nm core shell particles are also carried out on 1.5 μm fully porous particles in order to give a comparative result. The 750 nm core shell particles can be synthesised quickly and are very uniform. The main drawback in their use for HPLC is the system itself due to the backpressure experienced using sub – 1 μm particles. The synthesis of modified Stöber particles is also examined in this chapter with a range of non-porous silica and shell silica from 70 nm – 750 nm being tested for use on a Langmuir – Blodgett system. These smooth surface shell particles have only been in existence since 2009. The results displayed in this thesis demonstrate how much potential smooth surface shell particles have provided more in-depth optimisation is carried out. The results on packing studies reported in this thesis aims to be a starting point for a more sophisticated methodology, which in turn can lead to greater chromatographic improvements.

Single-crystal micro/nanostructures and thin films of lamellar molybdenum oxide by solid-state pyrolysis of organometallic derivatives of a cyclotriphosphazene

The solid-state pyrolysis of organometallic derivatives of a cyclotriphosphazene is demonstrated to be a new, simple and versatile solid-state templating method for obtaining single-crystal micro- and nanocrystals of transition and valve metal oxides. The technique, when applied to Mo-containing organometallics N3P3[OC6H4CH2CN·Mo(CO)5]6 and N3P3[OC6H4CH2CN·Mo(CO)4 py]6, results in stand-alone and surface-deposited lamellar MoO3 single crystals, as determined by electron and atomic force microscopies and X-ray diffraction. The size and morphology of the resulting crystals can be tuned by the composition of the precursor. X-ray photoelectron and infrared spectroscopies indicate that the deposition of highly lamellar MoO3 directly on an oxidized (400 nm SiO2) surface or (100) single-crystal silicon surfaces yields a layered uniphasic single-crystal film formed by cluster diffusion on the surface during pyrolysis of the metal-carbonyl derivatives. For MoO3 in its layered form, this provides a new route to an important intercalation material for high energy density battery materials.

Fabrication of sub-10 nm lamellar structures by solvent vapour annealing of block copolymers

Fabrication of nanoscale patterns through the bottom-up approach of self-assembly of phase-separated block copolymers (BCP) holds promise for nanoelectronics applications. For lithographic applications, it is useful to vary the morphology of BCPs by monitoring various parameters to make “from lab to fab” a reality. Here I report on the solvent annealing studies of lamellae forming polystyrene-blockpoly( 4-vinylpyridine) (PS-b-P4VP). The high Flory-Huggins parameter (χ = 0.34) of PS-b-P4VP makes it an ideal BCP system for self-assembly and template fabrication in comparison to other BCPs. Different molecular weights of symmetric PS-b-P4VP BCPs forming lamellae patterns were used to produce nanostructured thin films by spin-coating from mixture of toluene and tetrahydrofuran(THF). In particular, the morphology change from micellar structures to well-defined microphase separated arrangements is observed. Solvent annealing provides a better alternative to thermal treatment which often requires long annealing periods. The choice of solvent (single and dual solvent exposure) and the solvent annealing conditions have significant effects on the morphology of films and it was found that a block neutral solvent was required to realize vertically aligned PS and P4VP lamellae. Here, we have followed the formation of microdomain structures with time development at different temperatures by atomic force microscopy (AFM). The highly mobilized chains phase separate quickly due to high Flory-Huggins (χ) parameter. Ultra-small feature size (~10 nm pitch size) nanopatterns were fabricated by using low molecular weight PSb- P4VP (PS and P4VP blocks of 3.3 and 3.1 kg mol-1 respectively). However, due to the low etch contrast between the blocks, pattern transfer of the BCP mask is very challenging. To overcome the etch contrast problem, a novel and simple in-situ hard mask technology is used to fabricate the high aspect ratio silicon nanowires. The lamellar structures formed after self-assembly of phase separated PS-b-P4VP BCPs were used to fabricate iron oxide nanowires which acted as hard mask material to facilitate the pattern transfer into silicon and forming silicon nanostructures. The semiconductor and optical industries have shown significant interest in two dimensional (2D) molybdenum disulphide (MoS2) as a potential device material due to its low band gap and high mobility. However, current methods for its synthesis are not ‘fab’ friendly and require harsh environments and processes. Here, I also report a novel method to prepare MoS2 layered structures via self-assembly of a PS-b-P4VP block copolymer system. The formation of the layered MoS2 was confirmed by XPS, Raman spectroscopy and high resolution transmission electron microscopy.