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Double-base propellants, derived from nitrocellulose and nitroglycerin, are a combined solid fuel and oxidiser system. They present smokeless combustion, and are typically utilised for small rocket motor applications. In order to provide stability in combustion performance, ballistic modifiers, which modify the burn-rate properties in three distinct ways, are added to the formulation. However, these additives are lead-based, which poses personal and environmental safety concerns. Moreover, impending European legislation will soon ban their use. Despite years of experimental research, no viable alternative currently exists, and so the impending ban presents a considerable challenge for our defence industries. For this reason, the main aim of this thesis is to develop a better understanding of the fundamental role played by lead as a ballistic modifier. Catalysis with the lead-based ballistic modifier is known to occur at the solid/gas-phase boundary of the propellant, which is known as the burning surface. It is assumed throughout this work that the lead additives, presented as metal salts in the propellant formulation, will decompose to lead oxide in the high temperatures (> 300 °C) of the propellant flame. This is taken as the baseline catalytic model. As such, in this work two computational models have been employed; one to investigate catalysis in the solid-state, the other in the gas-phase. The lead additives are known to generate (i) super-rate, (ii) plateau-burn and (iii) mesa-burning effects, and it is known that carbon-soot, which builds up and is subsequently lost from the burning surface, plays an essential (but unknown) role in these burn-rate phenomena. Thus the interaction of carbon with lead oxide is a recurring theme throughout this work. Looking first to the solid-state, Chapter 3 documents a comparative study of the properties of several metal oxides, namely lead, tin and bismuth oxide. Tin and bismuth oxide are ballistic modifiers which demonstrate super-rate burning, but fail to produce plateau- and mesa-rate burning. This chapter examines the chemical reactivities of each metal oxide through computation of their electronic band gaps, surface energies and surface work functions, to deduce any unique properties that separates the behaviour of lead oxide from the other metal oxides. A layer of amorphous carbon is also bound to the stable surfaces of each metal oxide to ascertain whether any significant differences in bond strength and surface integrity arise. Chapter 4 turns its attention to investigate the formulation of industry-standard ballistic modifiers, which are derived from a blend of lead and copper salts. Here the structures of stable small metal oxide clusters which could form in the gas-phase above the burning surface are investigated with respect to their interaction with carbon. The individual roles of each metal in terms of the burn-rate effects are accounted for, and a phenomenological model is proposed that accounts for the three burn-rate effects. Finally, Chapter 5 presents a continuation of the narrative from Chapter 4, and widens the gas-phase discussion to include tin and bismuth oxide. The results obtained further validate the catalytic model presented in Chapter 4. Thus overall, the work reported in this thesis provides an atomistic interpretation of ballistic modifiers in double-base propellants, routed in first principles simulation, that provides a new platform for the continued search for lead-free additives.

In the manufacturing process of float glass often atmospheric pressure chemical vapour deposition (APCVD) reactors are integrated on-line for the deposition of functional thin solid films. Such functional films have applications in architectural glass, flat panel displays and solar cells. As glass moves downstream in the process, the thin film is deposited at temperatures between 500 to 700°C. The high temperatures make it difficult to monitor the deposition process and thin film quality control is commonly done at the end of the line or at lower temperatures. A time delay therefore exists between the point of thin film deposition and subsequent quality control, which can lead to large quantities of defective product being produced before faults are detected. It is therefore desirable to monitor in the APCVD reactor for rapid feedback of unexpected deviations from desired process conditions, reaction progress and fault detection. High uniformity of film properties across the substrate are important, but APCVD reactors are often empirically designed and the detailed chemical reaction mechanism is unknown. This leads to inefficient gas flow patterns and precursor utilization as well as difficulties in the design of new reactors. The APCVD deposition of tin oxide from the mono-butyl-tin tri-chloride (MBTC) is an example of such a process. Optical monitoring instruments in-situ and in-line on the APCVD reactor provided rapid feedback about process stability and progress non-invasively. Near infrared diode laser absorption spectroscopy (NIR-LAS) monitored the concentration of the reaction species hydrogen chloride (HCl) in-situ and spatially in the coating zone. A mid-infrared grating absorption spectrometer (IR-GAS) with novel pyro-electric array detector monitored the concentration of precursor entering the coating system simultaneously. In combination these instruments provide the means for rapid process feedback. Fourier transform infrared absorption spectroscopy (FTIR) was used to investigate the unknown decomposition pathway of the precursor to find the yet unknown key tin radical that initiates film growth. Stable species forming during MBTC decomposition over a temperature range of 170 to 760°C were investigated but the tin intermediate remains unknown. Computational fluid dynamics (CFD) is routinely employed in research and industry for the numerical simulation of CVD processes in order to predict reactor flow patterns, deposition rates, chemical species distribution or temperature profiles. Two and three dimensional models with complex geometries and detailed reaction models exist. A three dimensional computational fluid dynamics (CFD) model of the used APCVD reactor was built using the Fluent CFD software. The numerical simulation included a chemical model that predicted qualitatively the chemical species distribution of hydrogen chloride in the gas phase. This was confirmed through comparison with NIR-LAS results. Design shortcomings due to inefficient flow patterns were also identified. In combination the optical tools developed provide the means for safe and efficient manufacturing of thin films in APCVD reactors. CFD simulations can be used to increase precursor utilization and film uniformity in the development of new reactor designs.

Metal oxides exhibit a wide range of chemical and electronic properties, making them an extremely interesting subject for numerous applications in modern electronics. The primary goal of this research is to develop metal-oxide-based electronic devices, including thin-film transistors (TFTs), resistance random-access memory (RRAM) and planar nano-devices. This research requires different processing techniques, novel device design concepts and optimisation of materials and devices. The first experiments were carried out to optimise the properties of zinc oxide (ZnO) semiconductors, in particular the carrier concentration, which determines the threshold voltage of the TFTs. Thermal annealing is one common method to affect carrier concentration and most work in the literature reports performing this process in a single-gas environment. In this work, however, annealing was carried out in a combination of air and nitrogen, and it was found that the threshold voltage could be tuned over a wide range of pre-determined values.Further experiments were undertaken to enhance the carrier mobility of ZnO TFTs, which is the most important material quality parameter. By optimising deposition conditions and incorporating a high-k gate dielectric layer, the devices showed saturation mobility values over 50 cm2/Vs at a low operating voltage of 4 V. This is, to our knowledge, one of the highest field-effect mobility values achieved in ZnO-based TFTs by room temperature sputtering. As an important type of metal-oxide-based novel memory devices, which have been studied intensively in the last few years, RRAM devices were also explored. New materials, such as tin oxide (SnOx), were tested, exhibiting bipolar-switching operations and a relatively large resistance ratio. As a novel process variation, anodisation was performed, which yielded less impressive results than SnOx, but with a potential for ultra-low-cost manufacturing. Finally, novel planar nano-devices were explored, which have much simpler structures than conventional multi-layered transistors and diodes. Three types of ZnO-based nano-devices (a side-gated transistor, a self-switching diode and a planar inverter) were fabricated using both e-beam lithography and chemical wet etching. After optimisation of the challenging wet etching procedure at nanometre scale, ZnO nano-devices with good reproducibility and reliability have been demonstrated.

Perovskite solar cells (PSCs) are a promising alternative to silicon-based photovoltaics. However, PSCs face several challenges due to shortcomings in their stability, module efficiency, and scaled production. Although PSCs is still a young field of research, significant attention has been given to demonstrating power conversion efficiencies that are on par with traditional silicon. With that target reached, converting the laboratory demonstration into practical materials to increase access and abundance of solar energy are among the next large targets for the field. This comes with material challenges for perovskite and their companion charge transport layers (CTLs). Among the charge transport materials often used, NiOx nanoparticles have stood out as a leading material for an efficient, stable, and scalable CTL. In this dissertation the scalable deposition of NiOx is addressed through compositional design and solvent engineering. Two NiOx nanoparticle formulations have been developed and deposited by scalable coating methods to fabricate functional PSC devices. Inks were analyzed by zeta potential, TGA, and theoretical solvent analysis. Film quality was characterized by SEM and J-V analysis. Using compositional design, NiOx nanoparticles were successfully suspended in a nonpolar solvent system. It was found that the addition of large ligands was unnecessary for particle stabilization in a chlorobenzene-based solution, and that the presence of these ligands significantly obstructed charge extraction. Through further work with solvent engineering, a water based NiOx suspension that did not require additional ligands was developed. This NiOx formulation allowed for rapid large area coating on both blade coating and slot die coating systems. Utilizing intense pulsed light (IPL) annealing, processing time was further decreased with an increase in device performance. When used in a slot die coating system with IPL we were able produce a champion device PCE of 12.23%. In this dissertation the stability of the novel SnO2/Ag interface was also explored. Perovskite ion migration is a leading source of degradation with the formation of AgI a common by product. A series of imidazoles were evaluated to identify structural features that contribute to an effective blocking layer. It was found that imidazoles with conjugated planer systems and hydrogen bonding heteroatoms were able to prevent ion migration and retain device performance

As society continues to use fossil fuels as its main source of energy, both global climate change and fuel source depletion are pressing concerns. Sunlight is the most ubiquitous energy source available, but is unreliable as an on-demand source of energy. However, using solar energy to produce hydrogen gas via electrolysis for use in fuel cells is a promising clean energy alternative for electricity production. The most direct method of solar powered water electrolysis is a photoelectrochemical (PEC) device, where a photovoltaic is submersed in an aqueous electrolyte to produce hydrogen and oxygen upon irradiation of the photoanode. This photoanode is very susceptible to failure due to the corrosive conditions used for water electrolysis, and therefore needs a transparent, yet conductive anti-corrosive protection layer. Fluorine doped tin oxide is a transparent conductive oxide, and is a potential protective layer for a PEC device due to its good stability under harsh environmental conditions. One of the most commonly used inexpensive deposition methods for tin oxide thin films is spray pyrolysis with tin chloride precursors at elevated temperatures of 350 to 550 °C. As these temperatures are too high for deposition onto the photovoltaic device used in this research, other precursors as well as solution additives that can lower the decomposition temperature were explored in this work. Films were successfully deposited at 250 to 280 °C using dibutyltin diacetate and the solution additive erbium chloride. The corrosion stability of these films was compared to films produced with the commercially used tin tetrachloride precursor. It was found that films prepared with dibutyltin diacetate showed much stronger resistance to corrosion than those obtained from tin tetrachloride. The films were analyzed with scanning electron microscopy, energy dispersive X-ray spectroscopy and powder X-ray diffraction. Film topographies were strongly affected by the precursor, and chloride incorporation was observed for the films produced with tin tetrachloride. Such ion incorporation can change the stress state of a thin film and may affect its corrosion resistance. Strain measurements using powder X-ray diffraction were carried out in order to analyze the deposited films’ levels of stress. The results showed that the films deposited from tin tetrachloride were more stressed, which is the likely cause of the higher corrosion susceptibility of these films. Additionally, the solution additive erbium chloride and its interaction with the precursor dibutyltin diacetate was investigated using infrared spectroscopy and single crystal X-ray diffraction. It was found that erbium chloride reacts with dibutyltin diacetate to form compounds that contain small pre-formed tin oxide moieties. One of these compounds, C36H78Sn4O8, was successfully prepared by direct synthesis from appropriate precursors. This material decomposed at significantly lower temperatures, and could be used for direct deposition of tin oxide films at 230 to 240 °C. Finally, a preliminary study into cation doping of tin oxide films was carried out to evaluate their response to the oxygen evolution reaction. The resultant films were tested with linear sweep voltammetry and it was found that the incorporation of cobalt (III) into the tin oxide films potentially lowers the onset of oxidation for the oxygen evolution reaction.

Metal halide perovskite solar cells (PSCs) have attracted tremendous attention because of their rapid development. To enhance the power conversion efficiency (PCE) of PSCs, significant research efforts have focused on the optimization of electron transport layer (ETL). SnO2 has been extensively used as ETL due to its excellent electron transport properties. The optimization of the fabrication of SnO2 and passivation of the structural defects are essential to the improved performance of ETL. This thesis aims to (i) investigate the fabrication of pristine SnO2 thin film and characterize its material properties and (ii) develop a novel fabrication of reduced graphene oxide (RGO) modified SnO2 thin film as ETL and characterize its material properties. The first part was achieved by investigating the effects of UV-ozone treatment on fabrication of SnO2 thin films as well as the effects of precursor concentration and heating temperature on the resultant properties of the SnO2 thin films. The results showed that UV-ozone pretreatment is essential for depositing a continuous SnO2 thin film. A high precursor concentration resulted in low roughness and high n-type defects in SnO2 thin film, which would decrease the electrical conductivity while a high temperature of heat treatment resulted in increased crystallinity and a decrease in oxygen vacancies and residual Cl. The second part was achieved by investigating the effects of (i) preparation time of the RGO-SnO2 precursor and (ii) the concentration of RGO on the properties of RGO-SnO2 thin films. The RGO-SnO2 thin film was successfully fabricated by spin coating a precursor solution of SnCl2 in ethanol (95%) mixed with graphene oxide (GO) followed by heating at low temperatures. By conducting RGO modification, RGO-SnO2 thin films with high crystallinity and low oxygen vacancy contents were achieved resulting in a high electrical conductivity. The study showed that 3 h of preparation time for the precursor and addition amounts of 2.62 wt% RGO would result in the best properties of the RGO-SnO2 thin films. This ETL has a surface roughness Ra of

Non-volatile memories with larger capacity, faster operation speed, lower power consumption, smaller size and simpler fabrication process are strongly desired in the big data era. Recently, oxide-based random-access memories (OxRAMs) have been considered as a promising solution to meet these requirements. By achieving multiple resistance states in metal oxides, the memory capacity can be multiplied at the same cell size hence the development of metal oxide switching materials is important for achieving high-performance memory devices. In this thesis, a transparent metal oxide (SnO2) was selected for developing resistive switching-based RRAMs. Three strategies for improving the resistive switching behaviour were proposed and investigated, based on the solution-processed SnO2 thin films, which covered the following aspects: a.     Defects engineering: to facilitate multi-level resistance states in metal oxides, defects engineering via cation doping has been investigated. The possibility of forming cationic and ionic defects in SnO2 by doping has been explored through density functional theory. Cationic defects Mn3+ interstitials in Mn-doped SnO2 were synthesized through a new solution-processed approach and by altering the doping level, multi-resistance states in Mn-doped SnO2 have been achieved successfully. b.     Layer design: to alter the migration and redox process of defects in metal oxides, an alternative multi-layer structure was designed. By inserting pure SnO2 layer as the ionic defects diffusion barrier, the migration and redox of ionic defects in the Mn-doped SnO2 layer can be modified. Stable multi-level resistive switching, high On/Off ratio, excellent retention and low operation voltage have been achieved. c.       Graphene/metal oxide nanocomposites: graphene can be used as electrodes or buffer layer in RRAMs which can increase the device flexibility or modify the switching mechanism. However, due to the high hydrophobic behaviour of graphene, the fabrication of uniform and dense graphene derived thin film through solution-process is challenging. In addition, the lack of bandgap also limits the application of graphene in various electronic devices. A facile approach (UV irradiation) was employed to modify the hydrophilicity and open the bandgap of graphene. The UV treatment can induce H2O dissociative absorption on graphene, which effectively enhanced the interface interaction between graphene and SnO2. This significantly improved the thin film quality of graphene/SnO2 for resistive switching, with more stable switching behaviour and lower SET/RESET voltages. More importantly, according to the density functional theory calculation results, bandgap of graphene can be opened to 0.75 eV. This work provides a systematic study on strategies which improve the metal oxide resistive switching behaviour for high density data storage devices. The proposed strategies of defect engineering, layer design and graphene/metal oxide composites may provide new insights to develop metal oxide-based devices microelectronic devices.

Gas sensors are indispensable aspects of our life as it warns us about the dangerous gases in our environment. Semiconducting oxide gas sensors are by far the most popular type of sensors, because of their simple processing and low fabrication cost. The early metal oxide-based sensor materials however often exhibit several undesirable characteristics, such as poor selectivity, sensitivity to moisture, long-term signal drift and, slow response time. Hence, the development of fast-responding gas sensors with high sensitivity and selectivity is highly desirable. The introduction of nanotechnology has attracted large interests in gas-sensing research, largely because nanoscale particles offer a larger high surface area to volume ratio and enhanced functionalities compared to bulk particles. In particular, the synthesis of metal oxide nanostructures with controlled morphology is highly attractive because the properties of nanostructure depend not only on their composition, but also on their structure, phase, shape, size, and size distribution. Many efforts have been carried out to improve the „3S‟: sensitivity, selectivity, and stability of semiconductor gas sensors by utilizing metal oxide nanostructures. However, some challenges still exist in both synthesis and fundamental understanding of the gas-sensing mechanism of nanoscale metal oxides. This thesis aims to explore the use of nanostructures based on n-type semiconducting oxides as gas sensor materials for the detection of VOCs and to develop different ways to enhance the sensitivity of these metal oxide nanostructures through means of structural iii control, surface engineering and introduction of additives such as metal oxides and noble metals. Our research method involves the use of various microscopy, diffraction, and spectroscopy techniques to characterize the achieved metal oxide nanostructures or nanocomposites. To gain a further insights understanding of the metal oxide/gas interactions during the sensing process, it is important to use multi-scale theoretical methods validated by experimental techniques. The findings will benefit the design and construction of gas sensors with desirable properties/performance (sensitivity, selectivity and stability) for potential applications in environmental monitoring and detection.

Energy independence requires that the nation reduce its dependence on foreign oil imports. This can be achieved through electrification of transportation vehicles if proper battery technology can be developed. In addition, the renewable energy sources such as solar and wind tend to be intermittent with time scales ranging from seconds to hours. So, a suitable energy storage technology is essential for integrating renewable sources for base load generation. Lithium ion battery technology is promising; however, the big challenge limiting its widespread implementation is with capacity, durability and safety. A dramatic advancement is needed in terms of materials used for both electrodes and electrolytes. Several materials such as tin, tin oxide (SnO2), cobalt oxide (CoO3), iron oxide (Fe2O3), intermetallic alloys and semiconductors like silicon (Si) and germanium (Ge) potentially provide much higher theoretical capacity compared to conventionally used carbon based materials for anodes. Although most of these materials have favorable characteristics, they come at the expense of enormous volume changes associated with lithium alloying as a result of which the material integrity is lost. One-dimensional nanowires are believed to have better charge transport and strain relaxation properties but mostly unproven. In this dissertation, a generic hybrid architecture concept involving one-dimensional nanowires covered with nanoclusters IS proposed for improving the durability of anodes with high capacity retention. Specifically, this concept is demonstrated with metal-nanocluster-covered metal oxide nanowires using Sn/SnO2 system. The results showed that Sn nanocluster covered SnO2 nanowires exhibited a capacity retention of ~800 mAhg-1 for up to 100 cycles, the highest reported until now. In this study, the presence of well-spaced nanoclusters provides adequate room for metal volume expansion on lithiation preventing cluster coalescence leading to stable material structure while the metal oxide base provides various channels for electron conductivity. Cyclic voltammetric studies are conducted to understand the fundamental behavior of mono layers of nanoscale and micron scale tin clusters supported on both metallic substrates and hybrid architectures. The results suggest that tin clusters with sizes less than 50nm undergo complete de-lithiation while larger clusters exhibit incomplete delithiation due to diffusion limitation. The hybrid architecture concept can also be extended to other high capacity materials systems using unique carbon structures and molybdenum oxide nanowire arrays as base materials. In this direction, carbon microtubes (CMTs) are synthesized in large quantities and tested for their lithiation and de-lithiation characteristics. CMTs are micron sized tubes with 50nm walls comprised of random nanographite domains. The results indicated that CMTs exhibited capacity retention of ~440 mAhg-l, higher than the theoretical capacity of graphite. More importantly, CMTs show excellent rate capability of ~135 mAhg-1 at rates as high as 5C which makes them ideal as base materials in hybrid architectures. Another material of interest is molybdenum oxide (MoO3) which has excellent theoretical capacity and stability. Nanowire arrays are grown on conducting substrates providing direct charge conduction pathways eliminating the use of conducting polymer, generally used in powder based electrodes. These arrays show good capacity retention of ~630 mAhg-1 along with rate capability. In addition, the capacity retention below 0.7 V is ~500 mAhg-1 , which is better than the performance of any other MoO3 based materials and hence, makes the material viable for practical application as electrodes. Technologically, the proposed concept of hybrid architectured materials involving I-D materials with nanoclusters should result in the development of new materials architectures for high capacity, high rate and durable anodes. Scientifically, for the first time, the study showed fundamental differences in the lithiationlde-lithiation behavior of tin clusters at nanoscale which could apply to several other material systems. In addition, the interesting aspects involved in high capacity retention and durability have been aptly studied and understood for further application in other material systems.

Submicron tin oxide (SnO2) was obtained from thermal decomposition of tin oxalate (SnC2O4) precipitated at room temperature from amixture of solutions of tin (II) chloride (SnCl2) and oxalic acid (H2C2O4). Aqueous precipitation of SnC2O4 was firstly investigated by parametersvariation of starting material concentrations, addition methods and mixing times. Upon calcination, SnO2 powder tacky and subsequent grinding was found to cause nanosized SnO2 particles to agglomerate into plates. Aqueous–alcohol precipitation was then developed, based on the previously conducted aqueous precipitation Stabilisation of SnO2 suspensions was found to be better in aqueous rather than non–aqueous media, as determined by zeta potential analysis and sedimentation tests. A detailed concept of the effects of zeta potential and sedimentation (enhanced sedimentation region (ESR)) on colloidal processing, i.e., suspension stability, was introduced. Two systems, Sn–Al–O and Sn–Si–O, were investigated at their invariant temperatures and ternary phase diagrams, which haven’t been reported elsewhere, were constructed (at nine isothermal temperatures each). The binary diagram for the system SnO2–SiO2, which has not been reported in the literature, was constructed. The systems compatibilities were confirmed experimentally at 1000oC, with incidental finding of micron–sized fibres of single crystal SnO2 with preferential [110] growth direction obtained. It was also deduced that 1000oC can be used for SnO2 coatings sintering without undesired reaction or mutual solubility. Successful electrophoretic deposition (EPD) of commercial SnO2 powder on dense sapphire was obtained by the use of pH 2 SnO2 suspension, but not with pH 9 suspensions leading to a review of the basis for EPD requirements in terms of suspension properties. Thus, another conceptual approach to EPD processing and setup was proposed in terms of zeta potential, suspension stability and net particle charge. Obtained homogeneous deposition of commercial SnO2 powders contradicted the findings of published works of EPD on insulating dense substrates. Thus critical factors in the design of EPD processing on dense insulating substrates and associated mechanisms responsible for the deposition were developed. However, EPD of synthesised SnO2 powders yielded inhomogeneous coatings, even with voltage application of up to 30 V. Microcell effects, which were deduced based on localised particle leaching in the suspension, were proposed. Although deposition was relatively unsuccessful, this demonstrated possibility of aqueous EPD with the usage of high voltages without occurrence of water electrolysis which hasn’t been observed in literature.

In the current work, a new method for the synthesis of nanocomposite materials is demonstrated. The technique is based on the hypothesis that staged decomposition and nucleation of multi-phase precursors and products can be used to create nanocomposite materials in a combustion system. As part of this work, a novel particle feed system (PFS) has been developed to inject solid-phase particle precursor materials into the reaction zone of a multi-element diffusion flame burner (a Hencken burner). Nanocomposite materials designed at the molecular level have considerable potential to improve numerous applications. In particular, tin dioxide (SnO 2) materials are excellent active materials for gas-sensing applications. Tin dioxide was selected for the demonstration studies because gas sensor applications utilizing SnO2 show strong sensitivity to nanocomposite properties, such as the dopant material and the size of the tin dioxide crystallites. In the study, several dopant materials (Au, Pd, CuxO, Alx Oy) were examined as well as multiple precursors for the same additive (e.g. gold acetate and metallic gold). The resulting materials were analyzed using a variety of techniques including: Transmission Electron Microscopy (TEM) imaging, TEM Energy Dispersive Spectroscopy (EDS), Scanning Electron Microscopy X-ray Energy Dispersive Spectroscopy (SEM XEDS), and X-ray Diffractometry (XRD). The results of the materials analyses provide valuable information on the composition, phase, size, and distribution of the additives in the SnO 2 matrix. A large range of control of the SnO2 crystallite size was demonstrated from the smallest dimension of 4 nm obtained using a high-temperature tetra-methyl tin (TMT)/gold acetate reactant precursor combination to the largest dimension of 11 nm using a TMT/pure metallic palladium reactant precursor combination. The results demonstrate the precursor properties and the burner operating conditions can be used to control the composite properties such as the additive loading and morphology. The nanocomposite morphology and other materials characteristics were also used to identify the physical and chemical pathways important in the mixed-phase combustion synthesis system. For example, the gold acetate precursor particles were irregular in shape and greater than 1 mum in size (approximate diameter). However, the gold particles identified in the nanocomposites using TEM imaging and elemental analyses indicate spherical gold particles with an average diameter of 83 nm. As evidenced by these results, significant particle restructuring occurs consistent with phase change and decomposition of the solid phase precursor.

Tin oxide derived from sol-gel yielded nano-size crystals. Tin oxide (cassiterite) crystals were present in the sol-gel dried at 100° C, and crystallization rapidly increased at 700° C and above. At temperatures below 700° C, the grain growth kinetics showed a weaker time dependence (grain growth exponent n of 6-7.4) with an activation energy of 20-25 kJ/mol, and no growth anisotropy was observed. At 1100° C, n was about 4, consistent with pore control by surface diffusion as a dominant growth mechanism. The results at high temperatures (950 and 1100° C) showed faster growth along the direction normal to (101) than the [110] direction. No significant grain growth was observed for 10-day anneals at 300° C after the material had been calcined at 600 or 800° C. Spin-coating of sol-gel materials provides continuous, porous, polycrystalline thin films with nanometer particle size. Particle and pore size increase with heating temperature and time. The films show (101) texture, and this texture becomes enhanced with increasing heat-treatment temperature. For the same fabrication conditions, films on platinum electrodes exhibited more cracks than those on polycrystalline alumina substrates. Rougher substrates resulted in more cracks. The critical film thickness for cracking on alumina substrate with 970 nm roughness was less than that for alumina with 280 nm roughness. Most of the cracks were located in the valleys between alumina grains. Cracking most likely occurs during drying, but these cracks enlarged and widened during subsequent calcining at 700° C. Doping with Ru suppressed SnO2 nucleation from the gel materials and slowed down the grain growth (n=8.1-8.9) up to 700° C. Ru doping also reduced the (101) texture of the thin film and the peak intensity relationship returned to the values for SnO2 powder (i.e. I101/I110 = 0.75) at 800° C. Doping with Pt increased SnO2 grain size due to heterogeneous nucleation in the initial stage of heating. The grain size difference between Pt-doped and undoped SnO2 is larger for 1 h heat-treatment than for 24 h heat-treatment at 500-800° C. For heat-treatments from 1 h up to 24 h, Pt doping significantly suppressed SnO2 grain growth (n=7.6-11.9), resulting in preferential grain growth in the direction normal to (101). Two kinds of surfactants were studied to modify the substrate. Thiol (-SH) surfactant attached only to screen-printed platinum ink via S-Pt bonds, while trichlorosilyl (-SiCl3) bonding group surfactant attached to both alumina and platinum ink. The hydrophilic sulfonate-modified surfaces promoted film adherence and resistance to ultrasonic agitation. The hydrophobic methyl-modified Pt surfaces exhibited reduced film adhesion.

Acid/Base characterizations of metal oxide surfaces are often used to explain their catalytic behavior. However, the vast majority of these studies have been performed on powders or supported oxides, and there is very little information available in the literature on the interaction of acid/base probe molecules with well-defined oxide surfaces of known coordination geometry and oxidation state. The well-defined, single crystal surfaces of Cu₂O (111), SnO₂ (110), and Cr₂O₃ (101̲2) were investigated for their acid/base properties by the interactions between the probe molecules and the well-defined surface features. The adsorption of NH₃ at cation sites was used to characterize the Lewis acidity of SnO₂ (110) and Cu₂O (111) surfaces. The adsorption of CO₂, a standard acidic probe molecule, was used to characterize the Lewis basicity of the oxygen anions on SnO₂ (110), Cu₂O (111) , and Cr₂O₃ (101̲2) surfaces. BF₃, while not a standard probe molecule, has been tested as a probe of the Lewis basicity of the oxygen anions on SnO₂ (110) and Cr₂O₃ (101̲2). By studying probe molecules on well-defined metal oxide surfaces with known coordination geometry and oxidation state, an overall evaluation of NH₃, CO₂, and BF₃ as probe molecules can be made using the surfaces studied. NH₃ probed differences in Lewis acidity of Sn cations on SnO₂ (110), which had differences in coordination environments and oxidation states. But, NH₃ adsorption failed to provide any direct information on differences in Lewis acidity of Cu cations in different local coordination geometries on Cu₂O (111). CO₂ is a poor probe of the Lewis basicity of oxygen anions on the metal oxide surfaces studied here. CO₂ does not strongly adsorb to either SnO₂ (110) or Cu₂O (111). On Cr₂O₃ (101̲2), CO₂ does interact with oxygen sites but in two different coordinations, which vary with surface condition, making a comparison of basicity difficult. In the cases studied here, CO₂ either does not adsorb, or it does not provide a clear set of results that can be related simply to Lewis basicity. BF₃ seems to be a much better probe of the Lewis basicity than CO₂ for the well-defined metal oxide surfaces studied here. On SnO₂ (110) and Cr₂O₃ (101̲2), the boron atom of BF₃ directly interacts with oxygen sites by accepting their electrons. BF₃ thermal desorption seems to provide a direct measure of the Lewis basicity of different surface oxygen species as long as they are thermally-stable in vacuum.

Tin dioxide (SnO2) with rutile type structure is a wide band n-type semiconductor which exhibits unique electronic and optical properties. In application of this material as gas sensors, a film form of SnO2 provides high surface area to volume ratio and leads to high sensitivity and fast responses. It has been found that the substrate material, the deposition conditions and the annealing procedure may directly influence the microstructure of thin films, hence control gas sensing properties. This thesis describes a concerted effort to study the microstructure-property relationship in SnO 2 thin film sensors. Our studies help to elucidate the effects of microstructure on sensor performance and provide some fundamental understanding of sensor design principals. Thin films with different microstructures were obtained by using two deposition techniques, namely pulsed laser deposition (PLD) and electron-beam evaporation, and a variety of substrates, such as Al2O3(1¯012), Al2O3(112¯0) and Al2O3(0001). The obtained SnO2 thin films include single crystalline films, compact epitaxial films with different grain boundary density, and porous films with rough surface. Property measurements reveal that single crystal has low gas sensitivity and the performance of compact films depends on the grain boundary density. On the other hand, porous films exhibit high sensitivity. Based on the experimental results, a model is proposed to interpret the observed phenomena in terms of depletion and grain boundary. We also investigated the effects of film thickness and additives, both bulk and surface, on gas sensitivity. Among the three films examined with thickness of 20nm, 60nm and 100nm, the thinnest film showed better sensitivity than the thicker ones. Dopants influence the sensitivity through the modification of depletion region. Trivalent additives (acceptor type) result in increased depletion layer thickness, hence improve sensor performance. On the contrary, films doped by pentavalent additives (donor type) only have very low gas sensitivity due to the thin depletion layer in the films. We believe that the volume ratio of the depletion region over the total volume is a key factor in determining sensitivity. Addition of surface Pt, even with a thickness of 1nm, is an effective way for improving gas sensitivity.