An equifield strength model has been established to elucidate the formation mechanism for the highly ordered alumina pore arrays and titanium oxide nanotubular arrays prepared via a common electrochemical methodology, anodisation. The fundamentals of the equifield strength model was the equilibrium between the electric field driven oxidation rate of the metal and electric field enhanced dissolution rate of oxide. During the anodic oxidation of metal, pore initiation was believed to generate based on dissolution rate difference caused by inhomogeneity near the metal/oxide interface. The ionic nanoconvection driven by the electric force exerted on the space charge layer in the vicinity of electrolyte/oxide interface is established to be the main driving force of the pore ordering at the early stage of the anodisation. While the equifield strength requirement governs the following formation of the single pore and the self-ordering of random distributed pore arrays during the anodisation process. Hexagonal patterned Al2O3 nanopore arrays and TiO2 nanotubular arrays have been achieved by anodisation of corresponding metal substrates in proper electrolytes. The two characteristic microstructural features of anodic aluminium oxide (AAO) and anodic titanium oxide (ATO) were investigated using scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The observations of the hemispherical electrolyte/oxide and oxide/metal interfaces, uniform thickness of the oxide layer, as well as self-adjustment of the pore size and pore ordering can be well explained by the equifield strength model. Field enhanced dissociation of water is extremely important in determination of the porosity of anodic metal oxide. The porosity of AAO and ATO films was found to be governed by the relative dissociation rate of water which is dependent on anodisation conditions, such as electrolyte, applied voltage, current density and electric field strength. Using an empirical method, the relations between the porosity of the AAO (ATO) films and the anodisation parameters, such as electric field strength, current density and applied voltage, have been established. Besides, the extent that an external electric field can facilitate the heterolytic dissociation of water molecule has been estimated using quantum-chemical model computations combined with the experimental aspect. With these achievements, the fabrication of anodic metal oxide films can be understood and controlled more precisely. Additionally, the impacts of other factors such as the electrolyte type and the temperature effect on the morphology of the anodic products were also investigated. Some important experimental evidences on the pore diameters variation with applied voltage in the anodisation of aluminium and the titanium were obtained for future investigation of the anodic metal oxide formation processes.

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

Proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) are attractive power sources as they offer high conversion efficiencies with low or no pollution. However, the most commonly used platinum electrocatalyst is expensive and the world supply of Pt is limited. In addition, the slow oxygen reduction and methanol oxidation kinetics as well as the poisoning of the Pt catalyst at the cathode resulting from methanol permeation from the anode through the Nafion membrane to the cathode lead to significant performance loss. Also, the electrocatalyst utilization in the electrodes also needs to be improved to reduce the overall cost of the electrocatalysts and improve the fuel cell performance. This dissertation explores nanostructured Pt alloys with lower cost and higher catalytic activity than Pt for oxygen reduction in PEMFC to understand the effect of synthesis and structure on the catalytic activity, methanol tolerant Pt/TiOx nanocomposites for oxygen reduction in DMFC, nanostructured Pt-Ru alloys for methanol oxidation in DMFC, and improvement in the utilization of Pt by optimizing the membrane-electrode assembly (MEA) fabrication. From a systematic investigation of a series of Pt-M alloys (M = Fe, Co, Ni, and Cu), the catalytic activity of Pt-M alloys is correlated with the extent of atomic ordering. More ordered Pt alloys exhibit higher catalytic activity than disordered Pt alloys. The higher activity of the ordered Pt alloys is found to relate to various factors including the Pt-Pt distance, Pt: 5d orbital vacancy, {100} planar density and surface atomic configuration. The catalytic activity of the Pt alloys is also influenced by the synthesis method. Low temperature solution methods usually result in smaller particle size and higher surface area, while high temperature routes result in larger particle size and lower surface area but with a greater extent of alloying. Pt/TiOx/C nanocomposites exhibit higher performance than Pt for oxygen reduction in DMFC. The nanocomposites show higher electrchochemical surface area, lower charge transfer resistance, and higher methanol tolerance than Pt. Pt-Ru alloy synthesized by a reverse microemulsion method exhibits higher catalytic surface area than the commercial Pt-Ru. The higher catalytic activity is attributed to a better control of the particle size, crystallinity, and microstructure. Membrane-electrode assemblies (MEAs) fabricated by a modified thin film method exhibit much higher electrocatalyst utilization efficiency and performance than the conventional MEAs in PEMFC. Power densities of 715 and 610 mW/cm2 are obtained at a Pt loading of, respectively, 0.1 and 0.05 mg/cm2 and 90 oC. The higher electrocatalyst utilization is attributed to the thin catalyst layer and a better continuity of the membrane/catalysts layer interface compared to that in the conventional MEAs. / text

A better understanding of how the nanoscale environment affects the mechanical properties of materials, in particular metallic nanoparticles and nanocrystalline metals is vital to the development of next generation materials. Of special interest is obtaining a fundamental understanding of the inverse Hall-Petch Effect in nanocrystalline metals, and nanoindentation in individual nanoparticles. Understanding these subjects is critical to understanding how the mechanical properties of materials are fundamentally affected by nanoscale dimensions. These topics have been addressed by a combination of theoretical modeling and in-situ nanoindentation transmission electron microscopy (TEM) analysis. Specifically, the study of the inverse Hall-Petch effect in nanocrystalline metals will be investigated by a thorough review of the literature followed by a proposed novel theoretical model that better explains the experimentally observed behavior of nanocrystalline metals. On the other hand, the nanoindentation of individual nanoparticles is a very new research topic that has yet to aggregate a large body of experimental data. In this context, in-situ TEM nanoindentation experiments on silver nanoparticles will be first performed to determine the mechanisms of deformation in these nanostructures. A theoretical explanation for the observed deformation mechanisms will be then developed and its implications will be discussed. In addition to nanoparticles, this study will also provide unique and valuable insight into the deformation mechanisms of nanopillars, a growing area of research despite much controversy and speculation about their actual mechanisms of deformation. After studying the novel behavior of both nanocrystalline metals and nanoparticles, useful applications of both classes of materials will be explored. The discussion of applications will focus on utilizing the interesting behaviors explored in the dissertation. Of particular interest will be applications of nanoparticles and nanocrystalline materials to coatings, radiation resistance and super-plastic materials. / text

A resonant cavity operating in TE011 mode was used to study the adsorption response of single walled carbon nanotubes (SWCNTs) and other nanomaterials for different types of gas molecules. The range of the frequency signal as a probe was chosen as geometry dependent range between 9.1 -9.8 GHz. A highly specific range can be studied for further experiments dependent on the type of molecule being investigated. It was found that for different pressures of gases and for different types of nanomaterials, there was a different response in the shifts of the probe signal for each cycle of gassing and degassing of the cavity. This dissertation suggests that microwave spectroscopy of a complex medium of gases and carbon nanotubes can be used as a highly sensitive technique to determine the complex dielectric response of different polar as well as non-polar gases when subjected to intense electromagnetic fields within the cavity. Also, as part of the experimental work, a range of other micro-porous materials was tested using the residual gas analysis (RGA) technique to determine their intrinsic absorption/adsorption characteristics when under an ultra-high vacuum environment. The scientific results obtained from this investigation, led to the development of a chemical biological sensor prototype. The method proposed is to develop operational sensors to detect toxin gases for homeland security applications and also develop sniffers to detect toxin drugs for law enforcement agency personnel.

Nanocomposites with expandable smectites such as montmorillonite layered silicates (MLS) in polymer matrices have attracted extensive application interest. Numerous MLS concentrations have been used with no particular justification. Here, we investigate the effects of MLS dispersion within the matrix and on mechanical performance. The latter is resolved through a three-prong investigation on rate dependent tensile results, time dependent creep results and the influence of a sharp notch in polypropylene (PP) nanocomposites. A fixed concentration of maleated polypropylene (mPP) was utilized as a compatibilizer between the MLS and non-polar PP. Analysis of transmission electron micrographs and X-ray diffraction patterns on the surface and below the surface of our samples revealed a unique skin-core effect induced by the presence of clay. Differential scanning calorimetric and polarized optical microscopic examination of spherulites sizes showed changes in nucleation and growth resulting from both the maleated PP compatibilizer and the MLS. These structural changes resulted in a tough nanocomposite, a concept not reported before in the PP literature. Nonlinear creep analysis of the materials showed two concentrations 3 and 5 % wt of PP, which reduced the compliance in the base PP. The use of thermal wave imaging allowed the identification of ductile failure among materials, but more important, aided the mapping of the elastic and plastic contributions. These are essential concepts in fracture analysis.

Ce travail a été réalisé dans le cadre d’une étude sur les matériaux massifs thermoélectriques nanostructurés, composés dont les propriétés peuvent potentiellement être améliorées grâce aux nombreuses interfaces. Il concerne la réalisation et la caractérisation de composites à matrices skutterudites (CoSb3 ou In0,4Co4Sb12) contenant différents taux d’inclusions nanométriques de PbTe ou de ZnO. Des techniques de métallurgie des poudres et de fracturation laser en milieu liquide ont été conjuguées à divers moyens de caractérisation (RX, MEB, MET) pour mettre au point le protocole d’élaboration des matériaux. L’observation des microstructures des composites a révélé que l’étape de dispersion des particules dans le matériau est la plus délicate de la préparation. Celles-ci forment des amas localisés aux joints de grains. La porosité est également relativement importante lorsque le taux d’inclusions est élevé en particulier avec ZnO. Les propriétés électriques (pouvoir thermoélectrique, résistivité électrique, effet Hall) et thermiques (conductivité thermique) ont été mesurées sur une large gamme de température (2-800 K) puis corrélées aux microstructures. L’analyse des résultats a permis de montrer que le PbTe tend à dégrader les propriétés électriques des deux matrices du fait notamment d’une réaction des nanoparticules avec les skutterudites. Par contre, l’ajout de ZnO semble plus prometteur même s’il est difficile de conclure définitivement quant à son rôle réel compte tenu de la complexité des microstructures (défauts, pores, joints de grains). Un modèle théorique développé afin de mieux comprendre l’impact des tailles de particules sur les propriétés thermiques a mis en évidence qu’il est plus intéressant de travailler avec des skutterudites partiellement remplies plutôt qu’avec CoSb3 puisque les inclusions affectent majoritairement les phonons les moins énergétiques. Les tendances qui se dégagent de cette étude vont plutôt dans le sens d’une détérioration des propriétés avec ce type de nanostructuration lorsque les taux de nanoparticules excèdent quelques pourcents mais l’utilisation de quantités plus faibles et une porosité mieux maîtrisée pourrait avoir un effet positif / This work was carried out as part of a study on nanostructured bulk thermoelectric materials, compounds whose properties can potentially be improved with many interfaces. It is related to the synthesis and characterization of skutterudites (CoSb3 or In0.4Co4Sb12) containing nanoinclusions of PbTe or ZnO. Techniques of powder metallurgy and laser fragmentation in liquid medium were combined to X-rays diffraction analyses, SEM and TEM observations to develop the experimental procedure for the preparation of materials. The microstructures of the composites show that the dispersion step is the most difficult part of the preparation as it leads to agglomerates located at the grain boundaries. The porosity is also relatively large when the quantity of inclusions is high especially with ZnO. The electrical and thermal properties (thermoelectric power, electrical resistivity, Hall effect, thermal conductivity) have been measured over a wide temperature range (2-800 K) and correlated with the microstructures. The results analysis showed that PbTe tends to degrade the electrical properties of the two matrixes because of a reaction between the nanoparticles and the skutterudites. Contrarily, the addition of ZnO seems more promising although it is difficult to conclude definitively on its effectiveness given the complexity of the microstructures (defects, pores, grain boundaries). A theoretical model developed to better understand the impact of the particle sizes on the thermal properties showed that it is more interesting to work with partially filled skutterudites instead of pure CoSb3 since the inclusions mostly affect the scattering of long wavelength phonons. The trends that emerge from this study are rather in the sense of a deterioration of the thermoelectric properties with this type of nanostructure when the quantities of nanoparticles exceed a few percentage. The use of smaller quantities and a better controlled porosity should be considered

The key challenge for a nanomaterial based sensor is how to synthesize in bulk quantity and fabricate an actual device with insightful understanding of operational mechanisms during performance. I report here effective, controllable methods that exploit the concepts of the "green approach" to synthesize two different one-dimensional nanostructures, including tin oxide nanowires and carbon nanotubes. The syntheses are followed by product characterization and sensing device fabrications as well as sensor performance understanding at the molecular level. Sensor-analyte response and recovery kinetics are also presented. The first part of the thesis describes bulk-scale synthesis and characterization of tin oxide nanowires by the molten salt synthetic method and the nanowire doping with antimony (n-types) and lithium. The work builds on the success of using n-doped SnO2 nanoparticles to selectively detect chlorine gas at room temperature. Replacing n-doped nanoparticles with n-doped nanowires reduces the number of inter-particle electron hops between sensing electrodes. The nanowire based sensors show unprecedented 5 ppb detectability of corrosive Cl2 gas concentration in air. At the higher range, 10 ppm of Cl2 gas leads to a 250 fold increase in the device resistance. During sensor recovery, FT-IR studies show that dichlorine monoxide (Cl2O) and chlorine dioxide (ClO2) are the desorbing species. Long term stability of devices is affected by lattice oxygen vacancies replaced by chlorine atoms. Bulk-scale synthesis of multiwall carbon nanotube (MWCNTs) was achieved by a novel inexpensive synthetic method. The green chemistry method uses the non-toxic and easy to handle solid carbon source naphthalene. The synthesis is carried out by simply heating naphthalene and organometallic precursors as catalysts in a sealed glass tube. Synthesis at 610º C leads to MWCNTs of 50 nm diameter and lengths exceeding well over microns. MWCNT doping is attempted with nitrogen (n-type) and boron (p-type) precursors. Palladium nanoparticles decorated on as-synthesized MWCNTs are employed for specific detection of explosive hydrogen gas with concentrations far below the explosive concentration limits. During performance, the sensor exhibits abnormal response behaviors at hydrogen gas concentrations higher than 1%. A model of charge carrier inversion, brought about by reduction of MWCNT by hydrogen molecules dissociated by Pd nanoparticles is proposed.

Monodispersed microgels composed of poly-acrylic acid (PAAc) and poly(N-isopropylacrylamide) (PNIPAM) interpenetrating networks were synthesized by 2-step method with first preparing PNIPAM microgel and then polymerizing acrylic acid that interpenetrates into the PNIPAM network. The semi-dilute aqueous solutions of the PNIPAM-PAAc IPN microgels exhibit an inverse thermo-reversible gelation. Furthermore, IPN microgels undergo the reversible volume phase transitions in response to both pH and temperature changes associated to PAAc and PNIPAM, respectively. Three applications based on this novel hydrogel system are presented: a rich phase diagram that opens a door for fundamental study of phase behavior of colloidal systems, a thermally induced viscosity change, and in situ hydrogel formation for controlled drug release. Clay-polymer hydrogel composites have been synthesized based on PNIPAM gels containing 0.25 to 4 wt% of the expandable smectic clay Na-montmorillonite layered silicates (Na-MLS). For Na-MLS concentrations ranging from 2.0 to 3.2 wt%, the composite gels have larger swelling ratio and stronger mechanical strength than those for a pure PNIPAM. The presence of Na-MLS does not affect the value of the lower critical solution temperature (LCST) of the PNIPAM. Surfactant-free hydroxypropyl cellulose (HPC) microgels have been synthesized in salt solution. In a narrow sodium chloride concentration range from 1.3 to 1.4 M, HPC chains can self-associate into colloidal particles at room temperature. The microgel particles were then obtained in situ by bonding self-associated HPC chains at 23 0C using divinyl sulfone as a cross-linker. The volume phase transition of the resultant HPC microgels has been studied as a function of temperature at various salt concentrations. A theoretical model based on Flory-Huggins free energy consideration has been used to explain the experimental results. Self-association behavior and conformation variation of long chain branched (LCB) poly (2-ethyloxazoline) (PEOx) with a CH3-(CH2)17 (C18) modified surface are investigated using light scattering techniques in various solvents. The polymer critical aggregation concentration (cac) strongly depends on solvent polarity, decreasing as the solvent becomes more hydrophobic.

This dissertation demonstrates the application of a vapor phase method to synthesize supported and unsupported nanoparticle catalysts for CO oxidation. The method is based on the Laser Vaporization/Controlled Condensation (LVCC) technique. The first part of this dissertation presents the vapor phase synthesis and characterization of gold nanoparticles supported on a variety of oxide supports such as CeO2, TiO2, CuO and MgO.The results indicate that Au nanoparticles supported on CeO2 exhibit higher catalytic activity than Au supported on other oxides. The high activity of the Au/CeO2 catalyst is attributed to the strong interaction of Au with CeO2. The results also indicate that 5% Au loading on CeO2 has higher activity than 2% Au or 10% Au. When comparing the catalytic activity of Au/CeO2 prepared by physical (LVCC) and chemical (deposition-precipitation)methods, it was found that the catalytic activity is higher for Au/CeO2 prepared by the deposition-precipitation method.The effect of alloying Au and Cu nanoparticles on the catalytic activity for low temperature CO oxidation was also investigated. The unsupported Au-Cu alloy nanoparticle catalyst exhibits higher catalytic activity than the activities of the individualcomponents and their physical mixtures. The XRD data of Au-Cu alloy taken after the catalysis test indicates the formation of CuO within the bimetallic nanoparticles, whichimproves the catalytic activity of Au-Cu alloy nanoparticle.The second part of this dissertation investigates the gas phase reactions of Au+ and Cu+ with CO, O2 and H2O molecules using the Laser Vaporization ionization, High-Pressure Mass Spectrometry (LVI-HPMS) technique. The gas phase reactions resulting from the interactions of Au+ with CO and O2 molecules are investigated. Although multiple additions of CO and O2 molecules on Au+ have been observed at room temperature, no evidence was found of the production of CO2. This is attributed to the presence of water molecules which effectively replace the oxygen molecules on Au+ at room temperature.Finally, the role of the metal cations Au+ and Cu+ in initiating the gas phase polymerization of butadiene and isoprene vapors was investigated.