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Hanna Colostate 0053A 16025.Pdf (7.277Mb) DISSERTATION EXPLORING GAS-PHASE PLASMA CHEMISTRY AND PLASMA-SURFACE INTERACTIONS: PROGRESS IN PLASMA-ASSISTED CATALYSIS Submitted by Angela R. Hanna Department of Chemistry In partial fulfilment of the requirements For the Degree of Doctor of Philosophy Colorado State University Fort Collins, Colorado Spring 2020 Doctoral Committee: Advisor: Ellen R. Fisher Nancy E. Levinger Anthony Rappe R. Mark Bradley Copyright by Angela R. Hanna 2020 All Rights Reserved ABSTRACT EXPLORING GAS-PHASE PLASMA CHEMISTRY AND PLASMA-SURFACE INTERACTIONS: PROGRESS IN PLAMSA-ASSISTED CATALYSIS The fundamental aspects of inductively coupled plasma chemistry was investigated, focusing on the interdependence of gas-phase and surface reactions for process improvement. Different project aspects included (1) gas-phase energetics and kinetics determination with and without substrates (i.e., catalysts) via spectroscopy; (2) material characterization before and after plasma modification; and (3) elucidation of gas-surface relationships whereby both the substrate and plasma are impacted by their interaction. The research presented herein focused on a holistic approach to plasma diagnostics, specifically addressing current limitations in pollution abatement strategies. To enhance our understanding of plasma phenomena, optical spectroscopy was employed to provide insight into foundational plasma properties. This dissertation begins with a review of the applicability of optical spectroscopy as a non-intrusive, versatile diagnostic tool to explore non- thermal discharges. Specifically, time-resolved optical emission spectroscopy (TR-OES) was utilized to provide kinetic information about excited-state species formation, ultimately lending mechanistic insight into a range of plasma reactions. In addition, optical emission and broadband absorption spectroscopies (OES and BAS, respectively) were combined for the determination of rotational and vibrational temperatures (TR and TV, respectively) for both excited and ground state species. This comprehensive approach can and should be applied globally, across a wide range of plasma systems (i.e., clean gas, etching, depositing) and ii materials (i.e., semiconductors, catalysts, polymers). The two platforms explored here were plasma-assisted catalysis systems containing NxOy species and fluorocarbon (FC) plasmas, utilizing a range of precursors to evoke either etching or depositing conditions. An understanding of how energy is dispersed into rotational, vibrational, and translational modes of a gas-phase molecule by means of plasma-stimulated decomposition of a precursor lends critical insight into molecule formation mechanisms, decomposition pathways, and overall plasma chemistry. Specifically, energetic distributions for excited and ground state CFx (x = 1, 2) radicals within fluorinated plasma systems were measured via OES and BAS. Elevated excited state TV for CF radicals within CxFy discharges suggest that vibrational modes are preferentially excited over other degrees of freedom. In CxFy plasma systems, TR for the radicals equilibrate to the plasma gas temperature and remain independent of changing plasma parameters. Generally, CxFy precursors with lower fluorine-to-carbon (F/C) ratios tend to deposit FC films, where higher F/C ratios tend to etch surfaces. Here, zeolite modification via CxFy and H2O(v) plasmas was investigated, along with the fabrication of various zeolite constructs (i.e., native, pellets, and electronspun fibers). Inductively coupled plasmas tuned the wettability of microporous zeolites, evaluated by static and dynamic water contact angle goniometry. Zeolite modification, such as the formation of fluorocarbon films or surface functionalization, was verified via X-ray photoelectron spectroscopy and scanning electron microscopy. OES was used to probe gas-phase species, gleaning how the material intrinsically changes the plasma environment. Our studies revealed correlations between gas-phase spectroscopic analyses, the gas-surface interface, and the resulting plasma modified surface properties. iii A current area of interest to the plasma community, plasma-assisted catalysis (PAC), seeks to understand the synergistic coupling of a plasma with a catalyst for improved pollution abatement. Prior to probing the plasma-material interface within PAC systems, energetic and kinetic processes in NxOy plasmas were elucidated. Energy partitioning between degrees of freedom and multiple molecules (i.e., N2 and NO) formed within NxOy plasma systems (N2, N2O, N2/O2) was investigated at various applied rf powers and system pressures. TR and TV for both molecules (regardless of state) showed a strong positive correlation with applied plasma power, as well as a negative correlation with system pressure. Analogous to the excited state CF radical study, in all cases, TV values are significantly higher than TR for NO and N2 species. These studies characterized the gas-phase plasma chemistry without the added complexity of a material. The impact of adding a catalyst (i.e., zeolites) on plasma energetics within an N2 low- temperature, radio frequency plasma was investigated, where OES-related studies entailed in situ measurements of steady-state plasma-substrate interactions through the determination of N2 TR and TV. N2 plasmas were selected for this study to minimize system intricacy and number of plasma species. The presence of micro-structured zeolites within the plasma significantly decreases N2(g) vibrational temperature, suggesting these materials promote vibrational relaxation within the discharge upon interaction with a catalytic substrate. In addition to evaluating the spectroscopic characteristics of the N2 discharge, material morphology and chemical composition were assessed before and after plasma exposure. Zeolite substrates maintained a porous, interconnected network, although small amounts of surface nitrogen incorporation occurred at high applied powers. Building upon this foundation, OES was utilized to examine the impact of Pt and zeolite iv catalysts on gas-phase species densities, plasma energetics, reaction kinetics, and plasma-catalyst configurations within an N2O plasma. By studying Pt materials with two different morphologies and size scaling (i.e., foil and nanopowder), the role of material structure on resulting plasma chemistry was revealed. The concentration of excited state NO substantially decreased at high powers in the presence of Pt nanopowder and micro-structured zeolites. All catalytic materials studied herein significantly decreased N2 TV in the plasma, with little impact on rotational thermalization pathways. Pt nanopowder further enhanced NO decomposition within a two- stage configuration (i.e., plasma and substrate are physically separate), compared to the single- stage system. Material characterization conducted after N2O plasma exposure revealed the plasma effectively poisons both Pt materials through oxidation, resulting in poorer performance in the single-stage system. Alternatively, zeolites, remained relatively unchanged by N2O plasma exposure, hence further decomposed NO within the single-stage system. These studies exemplify the need for a holistic approach to solving challenges presented in the plasma- catalysis and fluorocarbon plasma community. v TABLE OF CONTENTS ABSTRACT .................................................................................................................................... ii LIST OF TABLES ......................................................................................................................... xi LIST OF FIGURES ..................................................................................................................... xiii CHAPTER 1. Introduction...............................................................................................................1 1.1 Plasma Chemistry and Physics ......................................................................................1 1.2 Motivations for Fluorocarbon Investigations .................................................................4 1.3 Motivations for Plasma-Assisted Catalysis Studies .......................................................6 1.4 Overview of Research ..................................................................................................10 REFERENCES ..................................................................................................................13 CHAPTER 2. Experimental Methods ............................................................................................16 A.1 General Information .....................................................................................................16 A.2 Gas –Phase Diagnostics ...............................................................................................20 A.3 Gas –Surface Interface .................................................................................................36 A.4 Substrate Preparation and Fabrication .........................................................................39 A.5 Material Characterization.............................................................................................41 REFERENCES ..................................................................................................................45 CHAPTER 3. Investigating Recent Developments and Applications of Optical Plasma Spectroscopy: A Review ................................................................................................................47
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