Elucidating structure-property-performance relationships of plasma modified tin(IV) oxide nanomaterials for enhanced gas sensing applications

2017 Spring.Includes bibliographical references.This dissertation examines structure-property-performance relationships of plasma modified tin(IV) oxide (SnO2) nanomaterials to successfully and efficiently create sensitive targeted gas sensors. Different project aspects include (1) materials characterization before and after plasma modification, (2) plasma diagnostics with and without a SnO2 nanomaterial, (3) sensor performance testing, and ultimately (4) elucidation of gas-surface relationships during this project. The research presented herein focuses on a holistic approach to addressing current limitations in gas sensors to produce desired capabilities for a given sensing application. Strategic application of an array of complementary imaging and diffraction techniques is critical to determine accurate structural information of nanomaterials, especially when also seeking to elucidate structure-property relationships and their effects on performance in specific applications such as gas sensors. In this work, SnO2 nanowires and nanobrushes grown via chemical vapor deposition (CVD) displayed the same tetragonal SnO2 structure as revealed via powder X-ray diffraction (PXRD) bulk crystallinity data. Additional characterization using a range of electron microscopy imaging and diffraction techniques, however, revealed important structure and morphology distinctions between the nanomaterials. Tailoring scanning transmission electron microscopy (STEM) modes and combining these data with transmission electron backscatter diffraction (t-EBSD) techniques afforded a more detailed view of the SnO2 nanostructures. Indeed, upon deeper analysis of individual wires and brushes, we discovered that despite a similar bulk structure, wires and brushes grew with different crystal faces and lattice spacings. Had we not utilized multiple STEM diffraction modes in conjunction with t-EBSD, differences in orientation related to bristle density would have been overlooked. Thus, it is only through methodical combination of several analysis techniques that precise structural information can be reliably obtained. To begin considering what additional features can affect gas sensing capabilities, we needed to understand the driving force behind SnO2 sensors. SnO2 operates widely as a gas sensor for a variety of molecules via a mechanism that relies on interactions with adsorbed oxygen. To enhance these interactions by increasing surface oxygen vacancies, commercial SnO2 nanoparticles and CVD-grown SnO2 nanowires were plasma modified by Ar/O2 and H2O(v) plasmas. Scanning electron microscopy (SEM) revealed changes in nanomaterial morphology between pre- and post-plasma treatment using H2O plasma treatments but not when using Ar/O2 plasmas. PXRD patterns of the bulk SnO2 showed the Sn4+ is reduced by H2O and not Ar/O2 plasma treatments. X-ray photoelectron spectroscopy (XPS) indicated Ar/O2 plasma treatment increases oxygen adsorption with increasing plasma power and treatment time, without changing Sn oxidation. With the lowest plasma powers and treatment times, however, H2O plasma treatment results in nearly complete bulk Sn reduction. Although both plasma systems increased oxygen adsorption over the untreated (UT) materials, there were clear differences in the tin and oxygen species as well as morphological variations upon plasma treatment. Given that H2O plasma modification of SnO2 nanomaterials resulted in reduction of Sn+4 to Sn0, this phenomenon was further explored. To develop a deeper understanding of the mechanism for this behavior, gas-phase species were detected via optical emission spectroscopy (OES) during H2O plasma processing (nominally an oxidizing environment), both with and without SnO2 substrates in the reactor. Gas-phase species were also detected in the reducing environment of H2 plasmas, which provided a comparative system without oxygen. Sn* and OH* appear in the gas phase in both plasma systems when SnO2 nanowire or nanoparticle substrates are present, indicative of SnO2 etching. Furthermore, H2 and H2O plasmas reduced the Sn in both nanomaterial morphologies. Differences in H* and OH* emission intensities as a function of plasma parameters show that plasma species interact differently with the two SnO2 morphologies. The H2O plasma gas-phase studies found that under most plasma parameters the ratio of reducing to oxidizing gas-phase species was ≥1. The final consideration in our holistic approach relied on sensor performance studies of SnO2 nanomaterials. Resistance was recorded as a function temperature for UT, Ar/O2 and H2O plasma treated nanoparticles and nanowires exposed to air, carbon monoxide (CO), or benzene (C6H6). Resistance data were then used to calculate sensor response (Rair/Rgas) and sensitivity (Rair/Rgas > 1 or Rgas/Rair > 1). Specifically, Ar/O2 and H2O plasma modification increase CO and C6H6 sensitivity under certain conditions, but H2O plasma was more successful at increasing sensitivity over a wider range of plasma parameters. In particular, certain H2O plasma conditions resulted in increased sensitivity over the UT nanomaterials at 25 and 50 °C. Overall, H2O plasma appears to be more effective at increasing sensitivity than Ar/O2 plasma. Furthermore, although certain treatments and temperatures for nanoparticles had greater CO or C6H6 sensitivity than nanowires, nanowire sensitivity was less temperature dependent than nanoparticle sensitivity. Prior materials characterization data were combined with resistance data to elucidate specific structure-property-performance relationships for the different UT and plasma treated materials

Plasma catalysis using low melting point metals.

Plasma catalysis is emerging as one of the most promising alternatives to carry out several reactions of great environmental importance, from the synthesis of nanomaterials to chemicals of great interest. However, the combined effect of a catalyst and plasma is not clear. For the particular case of 1-D nanomaterials growth, the low temperatures synthesis is still a challenge to overcome for its scalable manufacturing on flexible substrates and thin metal foils. Herein, the use of low-melting-point metal clusters under plasma excitation was investigated to determine the effectiveness in their ability to catalyze the growth of 1-D nanomaterials. Specifically, plasma catalysis using Gallium (Ga) was studied for the growth of silicon nanowires. The synthesis experiments using silane in hydrogen flow over Ga droplets in the presence of plasma excitation yielded tip-led growth of silicon nanowires. In the absence of plasma, Ga droplets did not lead to silicon nanowire growth, indicating the plasma-catalyst synergistic effect when using Ga as catalyst. The resulting nanowires had a 1:1 droplet diameter to nanowire diameter relationship when the droplet diameters were less than 100 nm. From 100 nm to a micron, the ratio increased from 1:1 to 2:1 due to differences with wetting behavior as a function of droplet size. The growth experiments using Ga droplets derived from the reduction of Gallium oxide nanoparticles resulted in silicon nanowires with size distribution similar to that of Gallium oxide nanoparticles. Systematic experiments over 100 ºC – 500 ºC range suggest that the lowest temperature for the synthesis of silicon nanowires using the plasma-gallium system is 200 ºC. A set of experiments using Ga alloys with aluminum and gold was also conducted. The results show that both Ga rich alloys (Ga-Al and Ga-Au) allowed the growth of silicon nanowires at a temperature as low as 200 ºC. This temperature is the lowest reported when using either pure Al or Au. The estimated activation energy barrier for silicon nanowire growth kinetics using Al-Ga alloy (~48.6 kJ/mol) was higher compared to that using either pure Ga or Ga-Au alloy (~34 kJ/mol). The interaction between Ga and hydrogen was measured experimentally by monitoring pressure changes in a Ga packed batch reactor at constant temperature. The decrease of the pressure inside the reactor when the Ga was exposed to plasma indicated the absorption of hydrogen in Ga. The opposite effect is observed when the plasma is turned off suggesting that hydrogen desorbed from Ga. This experimental observation suggests that Ga acts as hydrogen sink in the presence of plasma. The formation of Ga-H species in the Ga surface and in the bulk as intermediate is suggested to be responsible for the dehydrogenation of silyl radicals from the gas phase and subsequently for selective dissolution of silicon into molten Ga. The proposed reaction mechanism is also consistent with the experimentally determined activation barrier for growth kinetics (~34 kJ/mol). In addition, theoretical simulations using VASP (Vienna Ab-initio Simulation Package) were used to study atomic hydrogen – molten Ga interactions. The simulation results suggest significant interaction of atomic hydrogen with molten Ga through formation of Ga-H species on the surface and fast diffusion through bulk Ga while supporting the proposed model to explain the Plasma-Ga synergistic effect. Finally, plasma synthesis of silicon nanotubes using sacrificial zinc oxide nanowire thin film as a template was investigated for lithium ion battery anode applications. The silicon nanotube anode showed high initial discharge capacity during the first cycle of 4600 mAh g−1 and good capacity retention (3600 mAh g−1 after 20 cyles). The silicon nanotubes preserved their morphology after cycling and the observed performance was attributed to the change in phase from nanocrystalline silicon hydrogenated (nc-Si:H) to amorphous silicon hydrogenated (a-Si:H) during lithiation. This dissertation demonstrated the plasma synergism with molten metals during vapor-liquid-solid growth of silicon nanowires. A model based on atomic hydrogen interactions with molten metals under plasma excitation has been proposed and validated through systematic experimental studies involving Ga and its alloys with gold and aluminum and theoretical studies involving first principles computations. Finally, the plasma-Ga system has been used to grow successfully silicon nanowires on various technologically useful substrates at temperatures as low as 200 ºC

Plasma versus thermal activation of the Phillips catalyst

Silica supported chromium oxide catalysts, known as Phillips catalysts, are used in the production of over 40% of the world's high-density polyethylene. The original catalyst comprised CrO(_3) impregnated onto silica. Due to the carcinogenic nature of chromium(VI), chromium(m) catalyst precursors which are oxidised during calcination are now preferred. Two such precursors have been employed throughout the studies reported in this thesis; one is prepared by the aqueous impregnation of a silica support with basic chromium(in) acetate, whilst the other comprises a dry-blended mixture of chromium(m) acetylacetonate with silica. Calcination of the two precursors has been studied using a combination of temperature-programmed quadrupole mass spectrometry and infrared spectroscopy. The chromium(III) acetylacetonate precursor is postulated to disperse near its melting point and react via an acetate intermediate. Both precursors may therefore be expected to produce the same catalyst following calcination. The study of subsequent CO reduction of these calcined catalysts by quadrupole mass spectrometry supports this observation. The reduction is found to proceed via a Langmuir-Hinshelwood mechanism, both precursors demonstrating the same behaviour. Activation energies for the catalyst reduction have been determined from the corresponding Arrhenius plots. Quadrupole mass spectrometry techniques have identified 1-hexene production during the early stages of polymerization using the CO reduced catalysts. This indicates the formation of a chromacyclopentane intermediate species which may also be involved in the mitiation of polymerization. The continuous fragmentation of the catalyst support and polymer growth have been investigated using contact mode and phase-imaging atomic force microscopy. Non-equilibrium plasma oxidation of the two catalyst precursors has been studied by quadrupole mass spectrometry. An active catalyst is obtained from the chromium(m) acetate catalyst, however the dry-blended chromium(in) acetylacetonate precursor is unable to achieve the dispersion required, and the oxidised species are inactive for ethylene polymerization

Summaries of FY 1997 Research in the Chemical Sciences

The objective of this program is to expand, through support of basic research, knowledge of various areas of chemistry, physics and chemical engineering with a goal of contributing to new or improved processes for developing and using domestic energy resources in an efficient and environmentally sound manner. Each team of the Division of Chemical Sciences, Fundamental Interactions and Molecular Processes, is divided into programs that cover the various disciplines. Disciplinary areas where research is supported include atomic, molecular, and optical physics; physical, inorganic, and organic chemistry; chemical energy, chemical physics; photochemistry; radiation chemistry; analytical chemistry; separations science; heavy element chemistry; chemical engineering sciences; and advanced battery research. However, traditional disciplinary boundaries should not be considered barriers, and multi-disciplinary efforts are encouraged. In addition, the program supports several major scientific user facilities. The following summaries describe the programs

D-Glucose Oxidation by Cold Atmospheric Plasma-Induced Reactive Species

The glucose oxidation cascade is fascinating; although oxidation products have high economic value, they can manipulate the biological activity through posttranslational modification such as glycosylation of proteins, lipids, and nucleic acids. The concept of this work is based on the ability of reactive species induced by cold atmospheric plasma (CAP) in aqueous liquids and the corresponding gas-liquid interface to oxidize biomolecules under ambient conditions. Here, we report the oxidation of glucose by an argon-based dielectric barrier discharge plasma jet (kINPen) with a special emphasis on examining the reaction pathway to pinpoint the most prominent reactive species engaged in the observed oxidative transformation. Employing d-glucose and d-glucose-13C6solutions and high-resolution mass spectrometry and ESI-tandem MS/MS spectrometry techniques, the occurrence of glucose oxidation products, for example, aldonic acids and aldaric acids, glucono- and glucaro-lactones, as well as less abundant sugar acids including ribonic acid, arabinuronic acid, oxoadipic acid, 3-deoxy-ribose, glutaconic acid, and glucic acid were surveyed. The findings provide deep insights into CAP chemistry, reflecting a switch of reactive species generation with the feed gas modulation (Ar or Ar/O2with N2curtain gas). Depending on the gas phase composition, a combination of oxygen-derived short-lived hydroxyl (•OH)/atomic oxygen [O(3P)] radicals was found responsible for the glucose oxidation cascade. The results further illustrate that the presence of carbohydrates in cell culture media, gel formulations (agar), or other liquid targets (juices) modulate the availability of CAP-generated species in vitro. In addition, a glycocalyx is attached to many mammalian proteins, which is essential for the respective physiologic role. It might be questioned if its oxidation plays a role in CAP activity

Aero-Thermal Characterization Of Silicon Carbide Flexible Tps Using A 30kw Icp Torch

Flexible thermal protection systems are of interest due to their necessity for the success of future atmospheric entry vehicles. Current non-ablative flexible designs incorporate a two-dimensional woven fabric on the leading surface of the vehicle. The focus of this research investigation was to characterize the aerothermal performance of silicon carbide fabric using the 30 kW Inductively Coupled Plasma Torch located at the University of Vermont. Experimental results have shown that SiC fabric test coupons achieving surface temperatures between 1000°C and 1500°C formed an amorphous silicon dioxide layer within seconds after insertion into air plasmas. The transient morphological changes that occurred during oxidation caused a time dependence in the gas / surface interactions which may detrimentally affect the in-flight performance. Room temperature tensile tests of the SiC coupons have shown a rapid strength loss for durations less than 240 seconds due to oxidation. Catastrophic failure and temperature spikes were observed on almost all SiC coupons when exposed to air plasmas at heat fluxes above 80 W/cm2. Interestingly, simulation of entry into the Mars atmosphere using a carbon dioxide plasma caused a material response that was vastly different than the predictable silica layer observed during air plasma exposure

The 2017 Plasma Roadmap: Low temperature plasma science and technology

Journal of Physics D: Applied Physics published the first Plasma Roadmap in 2012 consisting of the individual perspectives of 16 leading experts in the various sub-fields of low temperature plasma science and technology. The 2017 Plasma Roadmap is the first update of a planned series of periodic updates of the Plasma Roadmap. The continuously growing interdisciplinary nature of the low temperature plasma field and its equally broad range of applications are making it increasingly difficult to identify major challenges that encompass all of the many sub-fields and applications. This intellectual diversity is ultimately a strength of the field. The current state of the art for the 19 sub-fields addressed in this roadmap demonstrates the enviable track record of the low temperature plasma field in the development of plasmas as an enabling technology for a vast range of technologies that underpin our modern society. At the same time, the many important scientific and technological challenges shared in this roadmap show that the path forward is not only scientifically rich but has the potential to make wide and far reaching contributions to many societal challenges.I Adamovich et al 2017 J. Phys. D: Appl. Phys. 50 32300