Proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) are two types of low-temperature fuel cells (LTFCs) that operate at temperatures less than 100 °C and are appealing for portable, transportation, and stationary applications. However, commercialization has been hampered by several problems such as cost, efficiency, and durability. New electrocatalysts must be developed that have higher oxygen reduction reaction (ORR) activity, lower precious metal loadings, and improved durability to become commercially viable. This dissertation investigates the development and use of new electrocatalysts for the ORR. Core-shell (shell@core) Pt@Pd[subscript x]Cu[subscript y]/C electrocatalysts, with a range of initial compositions, were synthesized to result in a Pt-rich shell atop a Pd[subscript x]C[subscript y]-rich core. The interaction between core and shell resulted in a delay in the onset of Pt-OH formation, accounting in a 3.5-fold increase in Pt-mass activity compared to Pt/C. The methanol tolerance of the core-shell Pt@PdCu₅/C was found to decrease with increasing Pt-shell coverage due to the negative potential shift in the CO oxidation peak. It was discovered that Cu leached out from the cathode has a detrimental effect on membrane-electrode assembly performance. A spray-assisted impregnation method was developed to reduce particle size and increase dispersion on the support in a consistent manner for a Pd₈₈W₁₂/C electrocatalyst. The spray-assisted method resulted in decreased particle size, improved dispersion and more uniform drying compared to a conventional method. These differences resulted in greater performance during operation of a single DMFC and PEMFC. Additionally, Pd₈₈W₁₂/C prepared by spray-assisted impregnation showed DMFC performance similar to Pt/C with similar particle size in the kinetic region while offering improved methanol tolerance. Pd₈₈W₁₂/C also showed comparable maximum power densities and activities normalized by cost in a PEMFC. Lastly, the activation of aluminum as an effective reducing agent for the wet- chemical synthesis of metallic particles by pitting corrosion was explored along with the control of particle morphology. It was found that atomic hydrogen, an intermediate, was the actual reducing agent, and a wide array of metals could be produced. The particle size and dispersion of Pd/C produced using Al was controlled using PVP and FeCl₂ as stabilizers. The intermetallic Cu₂Sb was similarly prepared with a 20 nm crystallite size for potential use in lithium-ion battery anodes. Lastly, it was found that the shape of Pd produced with Al as a reducing agent could be controlled to prepare 10 nm cubes enclosed by (100) facets with potentially high activity for the ORR. / text

The objective of this research is to develop a new class of miniaturized sensors on-catheter technology through the integration of functional nanomaterials and flexible microsystems, with high sensitivity, fast recovery time, reduced form factor, for in situ blood pressure and flow monitoring with minimal invasiveness. Real-time endovascular pressure measurement techniques are crucial to evaluate the hemodynamics, which indicates the physiological state of the cardiovascular system. Current technology relies on fluid filled catheter coupled to remote transducers to measure endovascular pressures and gradients. The fluid filled catheters are bulky, inherently inaccurate due to the tubing mechanical resonance, and with low signal integrity due to the vibration noises from the environment. Silicon based conventional pressure sensors have complications due to issues of catheter stiffness, biocompatibility or small form factor integration. We propose a paradigm shift in designing the endovascular pressure sensing technology, through developing compact flexible sensing structures using nanoengineered piezoelectric polymers which can be integrated on catheters without consuming the internal lumen space. We focused on designing novel nanostructures using PVDF-TrFE (Polyvinyledene fluoride trifluoroethylene), with well controlled [Beta]-crystalline phase to significantly improve the resulting sensor performance. The research objectives include: (1) Thin-film structures for higher piezoelectric effect without any mechanical stretching or poling requirements, (2) High density highly-aligned electrospun nanofibers through electrospinning towards enhanced sensitivity; (3) Core-shell electrospun nanofiber for tapping the near [Beta]-crystalline phase formation and high cyrstallinity by virtue of inherent stress and stretching involved in the fabrication procedure. For pressure sensor design and characterization, we worked on two main form factors designs: thin-film, and aligned electrospun nanofiber based sensors patterned on catheter tips which are ready to be deployed in intra-vascular environment. Testing results showed promising results from PVDF based pressure sensors. The average sensitivity of the PVDF sensors was found to be four times higher than commercial pressure sensor while the PVDF sensor had five fold shorter response time than commercial pressure sensor, making the PVDF sensors highly suitable for real-time pressure measurements using catheters. / text

The science and technology of catalysis is more important today than at any other time in our history due to the grand energy and environment challenges we are facing. With the explosively growth of computation power nowadays, computer simulation can play an increasingly important role in the design of new catalysts, avoiding the costly trail-and-error attempts and facilitating the development cycle. The goal to inverse design of new materials with desired catalytic property was once far off, but now achievable. The major focus of this dissertation is to find the general rules that govern the catalytic performance of a nanoparticle as the function of its structure. Three types of multi-metallic nanoparticles have been investigated in this dissertation, core-shell, random alloy and alloy-core@shell. Significant structural rearrangement was found on Au@Pt and Pd@Pt nanoparticle, which is responsible for a dramatic improvement in catalytic performance. Nonlin- ear binding trends were found and modeled for random alloy nanoparticles, providing a prescription for tuning catalytic activity through alloying. Studies of ORR on Pd/Au random alloy NP and hydrogenation reaction on Rh/Ag random alloy NP revealed that binding on individual ensemble should be in- vestigated when large disparity of adsorbate affinity is presented between two alloying elements. In the alloy-core@shell system, I demostrated a general linear correlations between the adsorbate binding energy to the shell of an alloy-core@shell nanoparticle and the composition of the core. This relation- ship allows for interpolation of the properties of single-core@shell particles and an approach for tuning the catalytic activity of the particle. A series of promising catalysts were then predicted for ORR, HER and CO oxidation. As a first attempt to bridge the material gap, bimetallic nano clus- ter supported on CeO₂(111) was investigated for CO oxidation. A strong support-metal interaction induces a preferential segregation of the more reac- tive element to the NC-CeO₂ perimeter, generating an interface with the Au component. (Au-Cu)/CeO₂ was found to be optimal for catalyzing CO oxida- tion via a bifunctional mechanism. O₂ preferentially binds to the Cu-rich sites whereas CO binds to the Au-rich sites. A method called distributed replica dynamics (DRD) is proposed at last to utilize enormous distributed computing resources for molecular dynamics simulations of rare-event in chemical reac- tions. High efficiency can be achieved with an appropriate choice of N [subscript rep] and t [subscript rep] for long-time MD simulation. / text

A novel lithographic fabrication method is used to fabricate nanopillars arrays of anisotropic Ag and TCO electrodes. Optical and electrical properties of the electrodes including bandgap, free carrier concentration, resistivity and surface plasmon frequency of different electrodes can be tuned by adjusting the dimensions and geometry of the pillars. Given the ability to tune the nonlocal responses of the plasmonic field enhancements, we attempt to determine the nature of the effective refractive index profile within the visible wavelength region for multi-layer hybrid nanostructures. Knowledge of the effective optical constants of the obtained structure is critical for various applications. nanopillars of TCO\Ag core shell structures have been successfully fabricated. The Maxwell-Garnett mixing law has been used to determine the optical constants of the nanostructure based on spectroscopic ellipsometry measurements. Simulated reflection spectra indicate a down shift in the Brewster angle of the pillars resulting from the reduction in the effective refractive index of the nanostructure. Two plasmonic resonances were observed, with one in the visible region and the other in the IR region. Plasmon hybridization model is used to describe the behavior of metal and metal oxide core shell nanostructured electrodes. Different charge density distributions around the pillars determine the plasma frequency which depends on the core and surrounding media dielectric constants. Finite Difference Time Domain (FDTD) simulation of different structures agree well with experiment and help us to understand electric field behavior at different structures with different geometries and dielectric constants. Plasmonic Ag nanopillar arrays are effective substrates for surface enhanced Raman spectroscopy (SERS). An enhancement factor up to 6 orders of magnitude is obtained. Monolayers of C60 is deposited on the Ag nanopillars and the interface of C60/Ag is studied which is important in optoelectronic devices. Electron delocalization between C60 and Ag is confirmed.

No description available.

The diverse field of material research has been steadily expanding with a great help from computational physics, especially in the investigation of the fundamental properties of materials. This has driven the computational physics to become one of the main branches of physics, allowing for density functional theory (DFT) to develop as one of the cornerstones of material research. Nowdays, DFT is the method of choice in a great variety of studies, from fundamental properties, to materials modelling and searching for new materials. In this thesis, DFT is employed for the study of a small part of this vast pool of applications. Specifically, the microscopic characteristics of Zn1-xCdxS alloys are studied by looking into the evolution of the local structure. In addition, the way to model the growth of graphene on Fe(110) surface is discussed. The structural stability of silicon nanocrystals with various shapes is analysed in detail, as well. DFT is further used in studying different properties of semiconductor nanocrystals. The size evolution of the character of the band gap in silicon nanocrystals is investigated in terms of changes in the character of the states around the band gap. The influence of various surface impurities on the band gap, as well as on the electronic and optical properties of silicon nanocrystals is further studied. In addition, the future use of silicon nanocrystals in photovoltaic devices is examined by studying the band alignment and the charge densities of silicon nanocrystals embedded in a silicon carbide matrix. Furthermore, the electronic and optical properties of different semiconductor nanocrystals is also investigated. In the case of the CdSe/CdS and CdS/ZnS core-shell nanocrystals the influence of the nanocrystal size and different structural models on their properties is analysed. For silicon nanocrystal capped with organic ligands, the changes in the optical properties and lifetimes is thoroughly examined with changes in the type of organic ligand.

Vapor grown carbon nanofibers (VGCNFs) developed in 1980s are being widely used for reinforcing composites. Carbon nanotubes (CNTs) discovered in early 1990 can be classified as single wall carbon nanotubes (SWNTs), double wall carbon nanotube (DWNTs) and multi wall carbon nanotubes (MWNTs) depending on the number of grapheme layer forming the carbon nanotube. Polyacrylonitrile (PAN), a commercially important polymer is a predominant precursor for carbon fiber. Carbonized and activated PAN/SWNT films can find application as electrochemical supercapacitor electrodes. This study is focused on the structure, processing and properties of polyacrylonitrile/carbon nanotube (CNT) composite films. PAN/SWNT (60/40) composite film have been processed with unique combination of tensile strength, modulus, electrical conductivity, dimensional stability, low density, solvent resistance, and thermal stability. PAN molecular motion above the glass transition temperature (Tg) in the composite film is significantly suppressed, resulting in high PAN/SWNT storage modulus above Tg. The specific modulus of PAN/VGCNF composite films is consistent with the predictions of the Halpin-Tsai equation up to 20% VGCNF loading. The magnitude of modulus and other property enhancement is reduced as the nanofiber loading is increased (up to 40%). Further increase in nanofiber loading (> 40%) in composite results in modulus and tensile strength lower than those of control PAN. Electrical percolation was observed at 3.1 vol% VGCNF loading. PAN/CNT composite films were processed using SWNTs, DWNTs, MWNTs and VGCNFs to study the effect of various nanotubes on the composite properties. PAN/CNT films have been characterized by wide angle X-ray diffraction (WAXD), Raman spectroscopy, and scanning as well as transmission electron microscopy. Films have also been characterized for electrical conductivity, tensile and dynamic mechanical properties. Mechanical property results have been analyzed in terms of the nanotube surface area determined by nitrogen gas adsorption. PAN/CNT composite films and fibers are characterized using solid state 1HNMR spin lattice relaxation time (T1). With the addition of nanotubes, the T1 values for the PAN matrix generally decreased, and the reduction mechanism is discussed. The optical anisotropy of SWNT in PAN/SWNT composites was observed in their polarized infrared spectra and analyzed using the effective medium theory.

Two types of core/shell nanoparticles (CS-NPs) generation based on laser ablation are developed in this study, namely, double pulse laser ablation and laser ablation in colloidal solutions. In addition to the study of the generation mechanism of CS-NPs in each scheme, the optical properties of designed CS-NPs are determined with UV-VIS-NIR spectroscopy and EM field simulation. In the first scheme, which is double pulse laser ablation, two laser beams are fired in a sequence on two adjacent targets with different material. We have successfully demonstrated the generation of Sn/Glass, Zn/Glass, Zn/Si, Ge/Si, and Cu/Zn CS-NPs. Key factors affecting the generation of CS-NPs are (1) surface tensions of the constructing materials affecting the associated Gibbs free energy of CS-NPs, (2) physical properties of selected background gases (i.e., He and Ar), (3) delay time between two laser pulses, and (4) the amount of laser energy. The second scheme examined for the generation of CS-NPs is through laser ablation of solid targets in colloidal solutions. Compared to the double pulse laser ablation, this second approach provides better control of the size and shape of the resulting CS-NPs. Two colloidal solutions, namely, Au and SiO2 colloidal solution are applied in the second scheme. Key factors affecting the formation of CS-NPs with the second scheme and are (a) the adhesion energy between the shell and the core material, (b) the diameter of the core and (c) the laser ablation time and the laser energy. Red shift of absorption peaks are measured in both SiO2/Au and SiO2/Ag colloids compared with pure nanoparticles (NPs). The amount of red-shift is very sensitive to the shell thickness of the CS-NPs. The same red shift is reproduced with the corresponding full wave analysis. The observed red shift can be attributed to the additional surface plasmon resonance at the interface of metal/dielectric of the CS-NPs compared with pure nanoparticles. Through adjusting the material and size combination, the absorption peak of the CS-NPs can be tuned in a limit range around the intrinsic absorption peak of the metal of the CS-NPs. The freedom of adjusting the absorption peak makes CS-NPs is favorable in bio and optical applications.

This thesis describes the synthesis, characterisation and catalytic testing of multifunctional immobilised metal nanoparticle in metal-organic framework (MOF) materials. Combining the activity of metal nanoparticles with the porosity and Lewis acidity of metal-organic frameworks provides a single catalytic material which can perform multi-step reactions. Strategies to immobilise the metal nanoparticles within the metal-organic frameworks have been investigated. Immobilisation has been achieved by applying three different methodologies. First, deposition of metal nanoparticle precursors within mesoporous MOFs is discussed. Chapter 3 shows the effectivity of the double solvents deposition technique to achieve dispersed and small nanoparticles of around 2.7 nm. The best system combined Pd nanoparticles with MIL-101(Cr). This system was further investigated in tandem reductive amination catalysis, discussed in Chapter 4, to investigate the activity and selectivity provided by these multifunctional catalysts. Another immobilisation technique was performed by coating Pd decorated SiO2 spheres with a MOF layer. Using this technique, MOF was grown cyclically in solution, providing tuneable shell thicknesses of MOF on the metal nanoparticle decorated oxide spheres. While the homogeneity of the MOF shell needs more optimisation, it was determined that the surface charge on the spheres played an important role in the growth of MOF in the desired location. Finally, the third immobilisation technique is the core-shell growth of MOF on colloidal metal nanoparticles. Polymer-capped metal nanoparticles with well-defined shapes were synthesised and characterised. From here, the optimisation of conditions for core-shell growth of UiO-66 and MIL-100(Sc) were investigated. Conditions which provided the desired core-shell morphology were found for both MOF types. These materials were then subsequently used in tandem reductive amination catalysis and a more straightforward styrene hydrogenation. It was shown that the metal nanoparticles remain active catalysts within either MOF shell and the MOF shell stabilises the metal nanoparticle and acts as a Lewis acid catalyst.

abstract: With a recent shift to a more environmentally conscious society, low-carbon and non-carbon producing energy production methods are being investigated and applied all over the world. Of these methods, fuel cells show great potential for clean energy production. A fuel cell is an electrochemical energy conversion device which directly converts chemical energy into electrical energy. Proton exchange membrane fuel cells (PEMFCs) are a highly researched energy source for automotive and stationary power applications. In order to produce the power required to meet Department of Energy requirements, platinum (Pt) must be used as a catalyst material in PEMFCs. Platinum, however, is very expensive and extensive research is being conducted to develop ways to reduce the amount of platinum used in PEMFCs. In the current study, three catalyst synthesis techniques were investigated and evaluated on their effectiveness to produce platinum-on copper (Pt@Cu) core-shell nanocatalyst on multi-walled carbon nanotube (MWCNT) support material. These three methods were direct deposition method, two-phase surfactant method, and single-phase surfactant method, in which direct deposition did not use a surfactant for particle size control and the surfactant methods did. The catalyst materials synthesized were evaluated by visual inspection and fuel cell performance. Samples which produced high fuel cell power output were evaluated using transmission electron microscopy (TEM) imaging. After evaluation, it was concluded that the direct deposition technique was effective in synthesizing Pt@Cu core-shell nanocatalyst on MWCNTs support when a rinsing process was used before adding platinum. The peak power density achieved by the rinsed core-shell catalyst was 618 mW.cm-2 , 13 percent greater than that of commercial platinum-carbon (Pt/C) catalyst. Transmission electron microscopy imaging revealed the core-shell catalyst contained Pt shells and platinum-copper alloy cores. Rinsing with deionized (DI) water was shown to be a crucial step in core-shell catalyst deposition as it reduced the number of platinum colloids on the carbon nanotube surface. After evaluation, it was concluded that the two-phase surfactant and single-phase surfactant synthesis methods were not effective at producing core-shell nanocatalyst with the parameters investigated. / Dissertation/Thesis / M.S.Tech Technology 2012