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Immobilization of Electrocatalytically Active Gold Nanoparticles on Nitrogen-Doped Carbon Fiber Electrodes ABSTRACT Recently, immobilization of single metal nanoparticles on nanometer-sized electrodes has been demonstrated as a means to electrochemically probe the relationship between nanoparticle structure and function. Such studies of individual, isolated nanoparticles enable investigation of electrochemical behavior and electrocatalytic properties in the absence of complicating factors like interparticle distance and nanoparticle loading that are typically associated with collections of particles distributed on electrode supports. However, interpretation of electrochemical data obtained from single nanoparticle immobilization experiments can also be difficult since the underlying nanoelectrode platform can sometimes contribute to the measured current or the immobilization strategy may have adverse effects on electron transfer. Here we report immobilization of gold nanoparticles on relatively catalytically inert carbon fiber ultramicro- and nanoelectrodes through a modification method based on recently reported soft nitriding process found to be effective in attaching ligand-free ultrasmall noble metal catalysts to activated carbons. X-ray photoelectron spectroscopy results reveal that the nitriding of carbon fibers provides 3.5 times increase in surface nitrogen content, introducing mostly pyridinic and amine nitrogen groups. The nitrogen-containing surface sites proved to be beneficial to the deposition of gold nanoparticles (AuNPs), as sodium borohydride reduction of tetrachloroaurate resulted in attachment of AuNPs on nitrided carbon fiber ultramicroelectrodes (N-CF-UMEs) in as little as 10 seconds while immobilization of AuNPs on unmodified CF-UMEs required at least 12 hours. A recently reported electrochemical method was employed to characterize immobilized AuNPs, and AuNP size was found to be directly related to deposition time. AuNPs immobilized on N-CF-UMEs also exhibited electrocatalytic activity towards methanol oxidation. Reduction of electrode size will enable this strategy to be employed to investigate electrochemical behavior of individual gold nanoparticles, while the ligand-free nature of the immobilized particles also provides the opportunity to investigate effects of surface capping agents on electrocatalytic properties.

The operating characteristics of non-volatile memory for modern requirement are high-density , low power consumption, fast read and write speed, and good reliability. The floating gate memory generated leakage path in the tunnel oxide during the trend of scaling down, which will result in the loss of all stored charge to the silicon substrate. As the data retention time and endurance are taken into consideration, the thickness of tunnel oxide exist a physical limit, owing to the demand of high-density capacities. RRAM is offered as an option in the next generation non-volatile memories, due to the following advantages: (1) simple structure and easy to process, and low cost ; (2) less restrictive in the scaling-down process; (3) with the multi-bit data storage features; (4) high speed operation; (5) Repeat write and read is more than one million. In the thesis, we use a simple and low-temperature process to form the silicon germanium oxide (Si-Ge-O) RRAM and silicon germanium oxide RRAM with nitrogen doping between the electrode and silicon-germanium oxide interface. By sputtering at argon and oxygen (Ar/O2), and sputtering at argon and ammonia (Ar/NH3) with silicon-germanium target to form silicon germanium oxide RRAM and silicon germanium oxide (Si-Ge-O)/silicon germanium oxnitride (Si-Ge-O-N) RRAM. By informing a SiGeON layer between the interface of electrode and silicon-germanium oxide improve the stability of write voltage and endurance reliability. In addition, both silicon and germanium are useful as materials in the optoelectronics industry and extensively studied in material science. Based on the two materials, the smiting characterizations of RRAM will be improved in the read-write stability and operation reliability.

Screen-printed carbon electrodes (SPCEs) have garnered much attention as sensors due to their simplicity and relatively low cost. However, to impart necessary selectivity and sensitivity for specific applications, modification of the SPCE surface, which can involve time-consuming procedures or costly equipment/materials, is typically required. Here, a simple nitrogen-doping process based on NH4OH was used to modify SPCEs prepared from commercially available ink for electrochemical detection of H2O2, a common target for biosensing strategies and indicator of cell stress. XPS studies showed that NH4OH treatment of SPCEs led to a nearly 5-fold increase in surface nitrogen content (from 0.28% to 1.34%). Compared to SPCEs, nitrogen-doped SPCEs (N-SPCEs) demonstrated enhanced current and lower onset potentials for H2O2 reduction. Amperometric detection of H2O2 at an applied potential of -0.4 V (vs. Ag/AgCl) using N-SPCEs also exhibited a wider linear range, lower detection limit, and higher sensitivity than SPCEs.

Carbon fiber ultramicroelectrodes (CF-UMEs) are commonly used as electrochemical probes and sensors due to their small size, fast response, and high signal-to-noise ratio. Surface modification strategies are often employed on CF-UMEs to improve their selectivity and sensitivity for desired applications. However, many modification methods are cumbersome and require expensive equipment. In this study, a simple approach known as soft nitriding is used to prepare nitrogen-doped CF-UMEs (N-CF-UMEs). Nitrogen groups introduced via soft nitriding act as electrocatalytic sites for the breakage of O-O bonds during the reduction of peroxides like H2O2, a common target of biosensing strategies. Voltammetric studies confirm that, compared to CF-UMEs, N-CF-UMEs possess enhanced electrocatalytic activity towards H2O2 reduction as evidenced by an increase in current and positive shift in onset potential for the reaction. N-CF-UMEs also proved capable for amperometric detection of H2O2, exhibiting good linear response from 0.1 to 5.6 mM at -0.4 V vs. Ag/AgCl.

Over the last decade, the interstitial alloying of niobium has proven to be essential for enabling superconducting radiofrequency (SRF) cavities to operate more efficiently at high accelerating gradients. The discovery of "nitrogen doping" was the first readily accessible avenue of interstitial alloying in which researchers saw an increase in cavity performance. However, the serendipitous nature of the discovery led to additional research to fundamentally understand the physics behind the increase in cavity performance. This knowledge gap is bridged by materials characterization. Secondary ion mass spectrometry (SIMS) is a characterization technique which has become a staple of SRF cavity characterization that details elemental concentration profiles as a function of depth into the niobium surface with submicron resolution. SIMS has been widely used by the semiconductor industry for decades but has found less application in other fields due to the difficulty to produce reproducible data for polycrystalline materials. Much effort has been given to reduce the uncertainty of SIMS results to as low as 1% - 2% for single crystals. However, less attention has been given to polycrystalline materials with uncertainty values reported between 40% - 50% The sources of uncertainty were found to be deterministic in nature and therefore could be mitigated to produce reliable results. This dissertation documents the efforts to reduce SIMS method uncertainty which has been further used to solve mysteries regarding the characterization of SRF cavities which include predictive modeling of oxygen diffusion as well as the identification of contaminants resulting from cavity furnace treatments. / Doctor of Philosophy / Particle accelerators find many uses in society in which their complexity depends on their intended purpose. These purposes vary from projecting an image as in cathode ray tube (CRT) TVs, to creating high energy x-rays for life saving cancer treatments, to researching the very fundamental principles of science and physics. The later uses particle accelerators which are very large, spanning multiple miles, and run at extremely high energies. To keep operational costs reasonable, these instruments need to run as efficiently as possible. Therefore, superconducting radiofrequency (SRF) niobium cavities are used and are responsible for propulsion of charged particles. Although, niobium SRF cavities can pass incredibly high currents with very little surface resistance, these high-end particle accelerators push the operational boundaries of efficiency. To improve the efficiency of these cavities, an optimal concentration of impurities, such as oxygen and nitrogen, are added to the niobium surface. The addition of an impurity level that is too high or too low causes the resistance to increase which translates to higher operational costs. Therefore, it is important to accurately determine the concentration of impurities within the niobium and with high depth resolution. Secondary ion mass spectrometry is a characterization method that uses a primary ion beam to impact the surface of a material to remove atoms at the very surface. The ejected atoms are then ionized into a secondary beam which can then be detected to determine the concentration and to identify the species which was removed. Historically, this instrument has yielded results with 40% - 50% uncertainty for polycrystalline metals, such as niobium, which do not sputter evenly. With SRF cavity performance depending on accurate quantitation of impurities, a more robust method is needed. Therefore, this dissertation identifies issues which cause high uncertainties for polycrystalline materials and in addition offers mitigation strategies to reduce uncertainty to below 10%. These methods were then applied to solve real problems aimed to improve the production of niobium SRF cavities.

Formaldehyde is a ubiquitous and hazardous indoor air pollutant and reducing concentrations in indoor environments is a public health priority. The goals of this doctoral work were to advance analytical methods for continuous monitoring of formaldehyde at very low concentrations (sub-20 ppb[subscript v]) and to improve fundamental, mechanistic understanding of how structural and chemical properties of activated carbon influence removal of formaldehyde from indoor environments. To achieve these goals, emerging sensor-based technology was evaluated for its ability to detect and quantify ppb[subscript v]-level formaldehyde concentrations on a continuous basis at relative humidity levels characteristic of residential indoor environments. Also, a combination of spectroscopic and selective titration techniques was employed to characterize molecular-level structural and chemical properties of traditional and chemically treated granular activated carbon (GAC). In addition to selecting two different commercially available GACs for study, design and preparation of a laboratory-prepared, chemically treated GAC was pursued to create nitrogen-doped GAC with desirable surface chemical properties. Performance of all GACs was evaluated with respect to formaldehyde removal through a series of packed bed column studies. With respect to continuous formaldehyde monitoring, a method detection limit for emerging sensor technology was determined to be approximately 2 ppb[subscript v], and for relative humidity levels characteristic of indoor environments (> 40%), quantitative, continuous formaldehyde measurements less than 10 ppb[subscript v] were robust. The two commercially available GACs tested were both capable of removing formaldehyde; however, the GAC with greater density of basic surface functional groups and greater electron-donating potential (Centaur) removed twice as much formaldehyde (on a GAC mass basis) as the less basic GAC (BPL). A laboratory-prepared GAC (BPL-N) was successfully created to contain pyridinic and pyrrolic nitrogen, which was associated with increased surface density of basic functional groups, as well as with increased electron-donating potential. BPL-N exhibited better removal capacity for formaldehyde than BPL and Centaur. Furthermore, packed bed column studies of BPL-N and BPL formaldehyde removal performance yielded evidence to support the hypothesis that electron-donating potential, especially nitrogen functional groups at the BPL-N surface, promote catalytic removal of gas-phase formaldehyde via oxidation. / text

With growing demand for cardiovascular implants, improving the performance of artificial blood-contacting devices is a task that deserves close attention. Current prostheses made of fluorocarbon polymers such as expanded polytetrafluoroethylene (ePTFE) suffer from early thrombosis and require periodic replacement. A great number of attempts have already been made to improve blood compatibility of artificial surfaces, but only few of them found commercial implementation. One of the surfaces under intensive research for cardiovascular use is amorphous carbon-based coatings produced by means of the plasma-assisted deposition. However, this class of coatings can be produced using various techniques leading to a number of coatings with different properties. Carbon coatings produced in different plasmas may be of hard diamond-like type or soft graphite-like type, doping with different elements also changes the surface structure and properties. Taking this into account, the search for blood-compatible coating requires the understanding of surface composition and structure and its influence on blood-compatibility. This work attempts to advance our knowledge of this field. Here, commercial PTFE thin film was used as a working material, which composition corresponds to the composition of modern ePTFE vascular grafts and which compatibility with blood we tried to improve by deposition of nitrogenated amorphous carbon (a-CN) coatings in the plasma. Biocompatibility was assessed by a number of tests including the interaction with whole blood and various cells such as platelets, endothelial cells, neutrophils, and fibroblasts. Most of tests showed the blood compatibility of coated surface is better than that of untreated PTFE. Physico-chemical and morphological properties of coated surfaces were studied in parallel using x-ray photoemission spectroscopy (XPS), electron energy loss spectroscopy (EELS), x-ray absorption spectroscopy (XAS), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM). Some correlation between the structure of coatings and blood compatibility was inferred. It was found that at first nitrogen incorporation into amorphous carbon film stimulates blood compatibility. However, when nitrogen fraction increases over 23-25 %, no further improvement but reduction of blood compatibility was observed. Conclusion is made that for best biomedical performance, nitrogen percentage in a-CN coatings must be adjusted to the optimum value.

With growing demand for cardiovascular implants, improving the performance of artificial blood-contacting devices is a task that deserves close attention. Current prostheses made of fluorocarbon polymers such as expanded polytetrafluoroethylene (ePTFE) suffer from early thrombosis and require periodic replacement. A great number of attempts have already been made to improve blood compatibility of artificial surfaces, but only few of them found commercial implementation. One of the surfaces under intensive research for cardiovascular use is amorphous carbon-based coatings produced by means of the plasma-assisted deposition. However, this class of coatings can be produced using various techniques leading to a number of coatings with different properties. Carbon coatings produced in different plasmas may be of hard diamond-like type or soft graphite-like type, doping with different elements also changes the surface structure and properties. Taking this into account, the search for blood-compatible coating requires the understanding of surface composition and structure and its influence on blood-compatibility. This work attempts to advance our knowledge of this field. Here, commercial PTFE thin film was used as a working material, which composition corresponds to the composition of modern ePTFE vascular grafts and which compatibility with blood we tried to improve by deposition of nitrogenated amorphous carbon (a-CN) coatings in the plasma. Biocompatibility was assessed by a number of tests including the interaction with whole blood and various cells such as platelets, endothelial cells, neutrophils, and fibroblasts. Most of tests showed the blood compatibility of coated surface is better than that of untreated PTFE. Physico-chemical and morphological properties of coated surfaces were studied in parallel using x-ray photoemission spectroscopy (XPS), electron energy loss spectroscopy (EELS), x-ray absorption spectroscopy (XAS), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM). Some correlation between the structure of coatings and blood compatibility was inferred. It was found that at first nitrogen incorporation into amorphous carbon film stimulates blood compatibility. However, when nitrogen fraction increases over 23-25 %, no further improvement but reduction of blood compatibility was observed. Conclusion is made that for best biomedical performance, nitrogen percentage in a-CN coatings must be adjusted to the optimum value.

This dissertation discusses the influence of nitrogen doping (N-doping) on the electronic and catalytic properties of carbon nanotubes (CNTs). These properties have been studied using a variety of techniques, in order to both qualitatively and quantitatively analyze the relationship between the nitrogen concentration and observed properties. Chapter 1 provides a general overview of CNTs and N-doping and details some of the previous research from our group. Chapter 2 discusses the assembly and characterization of free-standing electrode mats, which are used in order to understand the intrinsic physicochemical properties of the material without relying on the secondary influence of another conductive support. Raman microscopy, X-Ray photoelectron spectroscopy, scanning and scanning-tunneling electron microscopy, as well as electrochemical methods were all used to demonstrate the viability of the mat electrodes for further experiments. Chapter 3 addresses the examination of a range of nitrogen concentrations in order to better understand the effects of nitrogen concentration on the electrochemical and electrical properties such as the differential capacitance, density of states at the Fermi level (D(E[subscript F])), bulk conductivity and work function. These properties were studied using a variety of techniques, including UV-photoelectron spectroscopy, electrochemical impedance spectroscopy and conductive four point probe. Chapter 4 investigates the inherent catalysis of the nitrogen doped CNTs (N-CNTs) with respect to O2 reduction, and a complex mechanism is proposed. Electrochemical methods such as cyclic and linear sweep voltammetries as well as thermo-gravimetric analysis and gasometric analysis were all employed to determine heterogeneous decomposition rates as well as to detect intermediates of the O₂ reduction reaction. Chapter 5 discusses the electrocatalytic degradation of free cyanide (CN⁻) at the N-CNT mat electrodes. These results both provide further support for the mechanism discussed in Chapter 4, and present the opportunity for a potential application of N-CNTs for environmental purposes. Specifically, spectroscopic and electrochemical methods, in conjunction with theoretical models show both that the presence of CN⁻ does not inhibit O2 reduction, and that it can be effectively converted to cyanate (OCN⁻) at the N-CNT electrodes. Future work involving the assembly and characterization of transparent N-CNT films is discussed in Chapter 6. / text