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- All Subjects: Materials Science
- Genre: Doctoral Dissertation
Description
Geopolymers, a class of X-ray amorphous, ceramic-like aluminosilicate materials are produced at ambient temperatures through a process called geopolymerization. Due to both low energy requirement during synthesis and interesting mechanical and chemical properties, geopolymers are grabbing enormous attention. Although geopolymers have a broad range of applications including thermal/acoustic insulation and waste immobilization, they are always prepared in monolithic form. The primary aim of this study is to produce new nanostructured materials from the geopolymerization process, including porous monoliths and powders.
In view of the current interest in porous geopolymers for non-traditional applications, it is becoming increasingly important to develop synthetic techniques to introduce interconnected pores into the geopolymers. This study presents a simple synthetic route to produce hierarchically porous geopolymers via a reactive emulsion templating process utilizing triglyceride oil. In this new method, highly alkaline geopolymer resin is mixed with canola oil to form a homogeneous viscous emulsion which, when cured at 60 °C, gives a hard monolithic material. During the process, the oil in the alkaline emulsion undergoes a saponification reaction to decompose into water-soluble soap and glycerol molecules which are extracted to yield porous geopolymers. Nitrogen sorption studies indicates the presence of mesopores, whereas the SEM studies reveals that the mesoporous geopolymer matrix is dotted with spherical macropores. The method exhibits flexibility in that the pore structure of the final porous geopolymers products can be adjusted by varying the precursor composition.
In a second method, the geopolymerization process is modified to produce highly dispersible geopolymer particles, by activating metakaolin with sodium silicate solutions containing excess alkali, and curing for short duration under moist conditions. The produced geopolymer particles exhibit morphology similar to carbon blacks and structured silicas, while also being stable over a wide pH range.
Finally, highly crystalline hierarchical faujasite zeolites are prepared by yet another modification of the geopolymerization process. In this technique, the second method is combined with a saponification reaction of triglyceride oil. The resulting hierarchical zeolites exhibit superior CO2-sorption properties compared to equivalent commercially available and currently reported materials. Additionally, the simplicity of all three of these techniques means they are readily scalable.
In view of the current interest in porous geopolymers for non-traditional applications, it is becoming increasingly important to develop synthetic techniques to introduce interconnected pores into the geopolymers. This study presents a simple synthetic route to produce hierarchically porous geopolymers via a reactive emulsion templating process utilizing triglyceride oil. In this new method, highly alkaline geopolymer resin is mixed with canola oil to form a homogeneous viscous emulsion which, when cured at 60 °C, gives a hard monolithic material. During the process, the oil in the alkaline emulsion undergoes a saponification reaction to decompose into water-soluble soap and glycerol molecules which are extracted to yield porous geopolymers. Nitrogen sorption studies indicates the presence of mesopores, whereas the SEM studies reveals that the mesoporous geopolymer matrix is dotted with spherical macropores. The method exhibits flexibility in that the pore structure of the final porous geopolymers products can be adjusted by varying the precursor composition.
In a second method, the geopolymerization process is modified to produce highly dispersible geopolymer particles, by activating metakaolin with sodium silicate solutions containing excess alkali, and curing for short duration under moist conditions. The produced geopolymer particles exhibit morphology similar to carbon blacks and structured silicas, while also being stable over a wide pH range.
Finally, highly crystalline hierarchical faujasite zeolites are prepared by yet another modification of the geopolymerization process. In this technique, the second method is combined with a saponification reaction of triglyceride oil. The resulting hierarchical zeolites exhibit superior CO2-sorption properties compared to equivalent commercially available and currently reported materials. Additionally, the simplicity of all three of these techniques means they are readily scalable.
ContributorsMedpelli, Dinesh (Author) / Seo, Dong-Kyun (Thesis advisor) / Herckes, Pierre (Committee member) / Petuskey, William (Committee member) / Arizona State University (Publisher)
Created2015
Description
Transparent conductive oxides (TCO) comprise a class of materials that exhibit unique combination of high transparency in the visible region along with high electrical conductivity. TCOs play an important role as transparent electrodes for optoelectronic devices such as solar cell panels, liquid crystal displays, transparent heat mirrors and organic light emitting devices (OLED). The most commonly used transparent electrodes in optoelectronic applications is indium tin oxide (ITO) due to its low resistivity (~ 10−4 Ω-cm) and high transmittance (~ 80 %). However, the limited supply of indium and the growing demand for ITO make the resulting fabrication costs prohibitive for future industry. Thus, cost factors have promoted the search for inexpensive materials with good electric-optical properties.
The object of this work is to study the structure-property-processing relationship and optimize a suitable transparent electrode with the intent to optimize them for flexible optoelectronics applications. The work focuses on improved processing of the mixed oxide (indium gallium zinc oxide, IGZO) thin films for superior optical and electrical properties. The study focuses on two different methods of post-deposition annealing-microwave and conventional. The microwave annealing was seen to have the dual advantage of reduced time and lower temperature, as compared to conventional annealing. Another work focuses on an indium free transparent composite electrode (TCE) where a very thin metal layer is inserted between the two TCO layers. A novel Nb2O5/Ag/Nb2O5 multilayered structure can exhibit better electrical and optical properties than a single layered TCO thin film. The focus for low cost alternative leads to a TiO2/metal/TiO2 based TCE. A systematic study was done to understand the effect of metal thickness and substituting different metals (Ag, Cu or Au) on the opto-electrical properties of the TCEs. The TiO2/Ag/TiO2 with mid Ag thickness 9.5 nm has been optimized to have a sheet resistance of 5.7 Ohm/sq. average optical transmittance of 90 % at 550 nm and figure of merit with 61.4 ×10-3 Ω-1. The TCEs showed improved optical and electrical properties when annealed in forming gas and vacuum. These dielectric/metal/dielectric multilayer TCEs have lower total thickness and are more efficient than a single-layer ITO film.
The object of this work is to study the structure-property-processing relationship and optimize a suitable transparent electrode with the intent to optimize them for flexible optoelectronics applications. The work focuses on improved processing of the mixed oxide (indium gallium zinc oxide, IGZO) thin films for superior optical and electrical properties. The study focuses on two different methods of post-deposition annealing-microwave and conventional. The microwave annealing was seen to have the dual advantage of reduced time and lower temperature, as compared to conventional annealing. Another work focuses on an indium free transparent composite electrode (TCE) where a very thin metal layer is inserted between the two TCO layers. A novel Nb2O5/Ag/Nb2O5 multilayered structure can exhibit better electrical and optical properties than a single layered TCO thin film. The focus for low cost alternative leads to a TiO2/metal/TiO2 based TCE. A systematic study was done to understand the effect of metal thickness and substituting different metals (Ag, Cu or Au) on the opto-electrical properties of the TCEs. The TiO2/Ag/TiO2 with mid Ag thickness 9.5 nm has been optimized to have a sheet resistance of 5.7 Ohm/sq. average optical transmittance of 90 % at 550 nm and figure of merit with 61.4 ×10-3 Ω-1. The TCEs showed improved optical and electrical properties when annealed in forming gas and vacuum. These dielectric/metal/dielectric multilayer TCEs have lower total thickness and are more efficient than a single-layer ITO film.
ContributorsDhar, Aritra (Author) / Alford, Terry L. (Thesis advisor) / Petuskey, William (Thesis advisor) / Krause, Stephen (Committee member) / Chizmeshya, Andrew (Committee member) / Arizona State University (Publisher)
Created2015
Description
In-situ environmental transmission electron microscopy (ETEM) is a powerful tool for following the evolution of supported metal nanoparticles under different reacting gas conditions at elevated temperatures. The ability to observe the events in real time under reacting gas conditions can provide significant information on the fundamental processes taking place in catalytic materials, from which the performance of the catalyst can be understood. The first part of this dissertation presents the application of in-situ ETEM studies in developing structure-activity relationship in supported metal nanoparticles. In-situ ETEM studies on nanostructures in parallel with ex-situ reactor studies of conversions and selectivities were performed for partial oxidation of methane (POM) to syngas (CO+H2) on Ni/SiO2, Ru/SiO2 and NiRu/SiO2 catalysts. During POM, the gas composition varies along the catalyst bed with increasing temperature. It is important to consider these variations in gas composition in order to design experiments for in-situ ETEM. In-situ ETEM experiments were performed under three different reacting gas conditions. First in the presence of H2, this represents the state of the fresh catalyst for the catalytic reaction. Later in the presence of CH4 and O2 in 2:1 ratio, this is the composition of the reacting gases for the POM reaction and this composition acts as an oxidizing environment. Finally in the presence of CH4, this is the reducing gas. Oxidation and reduction behavior of Ni, Ru and NiRu nanoparticles were followed in an in-situ ETEM under reacting gas conditions and the observations were correlated with the performance of the catalyst for POM. The later part of the dissertation presents a technique for determining the gas compositional analysis inside the in-situ ETEM using electron energy-loss spectroscopy. Techniques were developed to identify the gas composition using both inner-shell and low-loss spectroscopy of EELS. Using EELS, an "operando TEM" technique was successfully developed for detecting the gas phase catalysis inside the ETEM. Overall this research demonstrates the importance of in-situ ETEM studies in understanding the structure-activity relationship in supported-metal catalysts for heterogeneous catalysis application.
ContributorsChenna, Santhosh (Author) / Crozier, Peter A. (Thesis advisor) / Carpenter, Ray (Committee member) / Sieradzki, Karl (Committee member) / Petuskey, William (Committee member) / Arizona State University (Publisher)
Created2011
Description
The inexorable upsurge in world’s energy demand has steered the search for newer renewable energy sources and photovoltaics seemed to be one of the best alternatives for energy production. Among the various photovoltaic technologies that emerged, organic/polymer photovoltaics based on solution processed bulk-heterojunctions (BHJ) of semiconducting polymers has gained serious attention owing to the use of inexpensive light-weight materials, exhibiting high mechanical flexibility and compatibility with low temperature roll-to-roll manufacturing techniques on flexible substrates. The most widely studied material to date is the blend of regioregular P3HT and PC61BM used as donor and acceptor materials. The object of this study was to investigate and improve the performance/stability of the organic solar cells by use of inexpensive materials. In an attempt to enhance the efficiency of organic solar cells, we have demonstrated the use of hexamethyldisilazane (HMDS) modified indium tin oxide (ITO) electrode in bulk heterojunction solar cell structure The device studies showed a significant enhancement in the short-circuit current as well as in the shunt resistance on use of the hexamethyldisilazane (HMDS) layer. In another approach a p-type CuI hole-transport layer was utilized that could possibly replace the acidic PEDOT:PSS layer in the fabrication of high-efficiency solar cells. The device optimization was done by varying the concentration of CuI in the precursor solution which played an important role in the efficiency of the solar cell devices. Recently a substantial amount of research has been focused on identifying suitable interfacial layers in organic solar cells which has efficient charge transport properties. It was illustrated that a thin layer of silver oxide interfacial layer showed a 28% increase in power conversion efficiency in comparison to that of the control cell. The optoelectronic properties and morphological features of indium-free ZnO/Ag/MoOx electrodes was also studied. Organic solar cells on these composite electrodes revealed good optical and electrical properties, making them a promising alternative indium free and PEDOT:PSS-free organic solar cells. Lastly, inverted solar cells utilizing zinc oxide and yttrium doped zinc oxide electron transport was also created and their device properties revealed that optimum annealing conditions and yttrium doping was essential to obtain high efficiency solar cells.
ContributorsDas, Sayantan (Author) / Alford, Terry L. (Thesis advisor) / Petuskey, William (Thesis advisor) / Buttry, Daniel (Committee member) / Krause, Stephen (Committee member) / Arizona State University (Publisher)
Created2015
Description
Polycrystalline magnetite thin films were deposited on large area polymer substrates using aqueous solution based spin-spray deposition (SSD). This technique involved the hydrolysis of precursor salt solutions at low temperatures (70-100°C). The fundamental mechanisms and pathways in crystallization and evolution of the film microstructures were studied as a function of reactant chemistry and reactor conditions (rotation rate, flow rates etc.). A key feature of this method was the ability to constantly supply fresh solutions throughout deposition. Solution flow due to substrate rotation ensured that reactant depleted solutions were spun off. This imparted a limited volume, near two-dimensional restriction on the growth process. Film microstructure was studied as a function of process parameters such as liquid flow rate, nebulizer configuration, platen rotation rate and solution chemistry. It was found that operating in the micro-droplet regime of deposition was a crucial factor in controlling the microstructure.
Film porosity and substrate adhesion were linked to the deposition rate, which in-turn depended on solution chemistry. Films exhibited a wide variety of hierarchically organized microstructures often spanning length scales from tens-of-nanometers to a few microns. These included anisotropic morphologies such as nanoplates and nanoblades, that were generally unexpected from magnetite (a high symmetry cubic solid). Time resolved studies showed that the reason for complex hierarchy in microstructure was the crystallization via non-classical pathways. SSD of magnetite films involved formation of precursor phases that subsequently underwent solid-state transformations and nanoparticle self-assembly. These precursor phases were identified and possible reaction mechanisms for the formation of magnetite were proposed. A qualitative description of the driving forces for self-assembly was presented.
Film porosity and substrate adhesion were linked to the deposition rate, which in-turn depended on solution chemistry. Films exhibited a wide variety of hierarchically organized microstructures often spanning length scales from tens-of-nanometers to a few microns. These included anisotropic morphologies such as nanoplates and nanoblades, that were generally unexpected from magnetite (a high symmetry cubic solid). Time resolved studies showed that the reason for complex hierarchy in microstructure was the crystallization via non-classical pathways. SSD of magnetite films involved formation of precursor phases that subsequently underwent solid-state transformations and nanoparticle self-assembly. These precursor phases were identified and possible reaction mechanisms for the formation of magnetite were proposed. A qualitative description of the driving forces for self-assembly was presented.
ContributorsVadari Venkata, Kaushik Sridhar (Author) / Petuskey, William (Thesis advisor) / Carpenter, Ray (Committee member) / McCartney, Martha (Committee member) / Chan, Candace (Committee member) / Arizona State University (Publisher)
Created2018
Description
The larger tolerance to lattice mismatch in growth of semiconductor nanowires (NWs) offers much more flexibility for achieving a wide range of compositions and bandgaps via alloying within a single substrate. The bandgap of III-V InGaAsP alloy NWs can be tuned to cover a wide range of (0.4, 2.25) eV, appealing for various optoelectronic applications such as photodetectors, solar cells, Light Emitting Diodes (LEDs), lasers, etc., given the existing rich knowledge in device fabrication based on these materials.
This dissertation explores the growth of InGaAsP alloys using a low-cost method that could be potentially important especially for III-V NW-based solar cells. The NWs were grown by Vapor-Liquid-Solid (VLS) and Vapor-Solid (VS) mechanisms using a Low-Pressure Chemical Vapor Deposition (LPCVD) technique. The concept of supersaturation was employed to control the morphology of NWs through the interplay between VLS and VS growth mechanisms. Comprehensive optical and material characterizations were carried out to evaluate the quality of the grown materials.
The growth of exceptionally high quality III-V phosphide NWs of InP and GaP was studied with an emphasis on the effects of vastly different sublimation rates of the associated III and V elements. The incorporation of defects exerted by deviation from stoichiometry was examined for GaP NWs, with an aim towards maximization of bandedge-to-defect emission ratio. In addition, a VLS-VS assisted growth of highly stoichiometric InP thin films and nano-networks with a wide temperature window from 560◦C to 720◦C was demonstrated. Such growth is shown to be insensitive to the type of substrates such as silicon, InP, and fused quartz. The dual gradient method was exploited to grow composition-graded ternary alloy NWs of InGaP, InGaAs, and GaAsP with different bandgaps ranging from 0.6 eV to 2.2 eV, to be used for making laterally-arrayed multiple bandgap (LAMB) solar cells. Furthermore, a template-based growth of the NWs was attempted based on the Si/SiO2 substrate. Such platform can be used to grow a wide range of alloy nanopillar materials, without being limited by typical lattice mismatch, providing a low cost universal platform for future PV solar cells.
This dissertation explores the growth of InGaAsP alloys using a low-cost method that could be potentially important especially for III-V NW-based solar cells. The NWs were grown by Vapor-Liquid-Solid (VLS) and Vapor-Solid (VS) mechanisms using a Low-Pressure Chemical Vapor Deposition (LPCVD) technique. The concept of supersaturation was employed to control the morphology of NWs through the interplay between VLS and VS growth mechanisms. Comprehensive optical and material characterizations were carried out to evaluate the quality of the grown materials.
The growth of exceptionally high quality III-V phosphide NWs of InP and GaP was studied with an emphasis on the effects of vastly different sublimation rates of the associated III and V elements. The incorporation of defects exerted by deviation from stoichiometry was examined for GaP NWs, with an aim towards maximization of bandedge-to-defect emission ratio. In addition, a VLS-VS assisted growth of highly stoichiometric InP thin films and nano-networks with a wide temperature window from 560◦C to 720◦C was demonstrated. Such growth is shown to be insensitive to the type of substrates such as silicon, InP, and fused quartz. The dual gradient method was exploited to grow composition-graded ternary alloy NWs of InGaP, InGaAs, and GaAsP with different bandgaps ranging from 0.6 eV to 2.2 eV, to be used for making laterally-arrayed multiple bandgap (LAMB) solar cells. Furthermore, a template-based growth of the NWs was attempted based on the Si/SiO2 substrate. Such platform can be used to grow a wide range of alloy nanopillar materials, without being limited by typical lattice mismatch, providing a low cost universal platform for future PV solar cells.
ContributorsHashemi Amiri, Seyed Ebrahim (Author) / Ning, Cun-Zheng (Thesis advisor) / Petuskey, William (Committee member) / Yu, Hongbin (Committee member) / Arizona State University (Publisher)
Created2018
Description
Owing to their special characteristics, group III-Nitride semiconductors have attracted special attention for their application in a wide range of optoelectronic devices. Of particular interest are their direct and wide band gaps that span from ultraviolet to the infrared wavelengths. In addition, their stronger bonds relative to the other compound semiconductors makes them thermally more stable, which provides devices with longer life time. However, the lattice mismatch between these semiconductors and their substrates cause the as-grown films to have high dislocation densities, reducing the life time of devices that contain these materials. One possible solution for this problem is to substitute single crystal semiconductor nanowires for epitaxial films. Due to their dimensionality, semiconductor nanowires typically have stress-free surfaces and better physical properties. In order to employ semiconductor nanowires as building blocks for nanoscale devices, a precise control of the nanowires' crystallinity, morphology, and chemistry is necessary. This control can be achieved by first developing a deeper understanding of the processes involved in the synthesis of nanowires, and then by determining the effects of temperature and pressure on their growth. This dissertation focuses on understanding of the growth processes involved in the formation of GaN nanowires. Nucleation and growth events were observed in situ and controlled in real-time using an environmental transmission electron microscope. These observations provide a satisfactory elucidation of the underlying growth mechanism during the formation of GaN nanowires. Nucleation of these nanowires appears to follow the vapor-liquid-solid mechanism. However, nanowire growth is found to follow both the vapor-liquid-solid and vapor-solid-solid mechanisms. Direct evidence of the effects of III/V ratio on nanowire growth is also reported, which provides important information for tailoring the synthesis of GaN nanowires. These findings suggest in situ electron microscopy is a powerful tool to understand the growth of GaN nanowires and also that these experimental approach can be extended to study other binary semiconductor compound such as GaP, GaAs, and InP, or even ternary compounds such as InGaN. However, further experimental work is required to fully elucidate the kinetic effects on the growth process. A better control of the growth parameters is also recommended.
ContributorsDíaz Rivas, Rosa Estela (Author) / Mahajan, Subhash (Thesis advisor) / Petuskey, William (Committee member) / Crozier, Peter (Committee member) / Arizona State University (Publisher)
Created2010
Description
Perovskite solar cells are one of the rising stars in the solar cell industry. This thesis explores several approaches to enhance the properties of the perovskite layer and the solar cell devices in which they operate. They include studies of different antisolvent additives during spin coating of triple cation perovskites, the use of surfactants to improve the quality of perovskite film microstructures, the applicability of a new fabrication process, and the value of post-deposition thermal and chemical annealing processes.This thesis experimentally analyzes different antisolvents, viz., ethyl acetate, isopropyl alcohol, toluene, and chlorobenzene. It focuses on the antisolvent-assisted crystallization method to achieve homogenous nucleation of the perovskite film. Of all the antisolvents, ethyl acetate-treated films gave the best-performing device, achieving a power conversion efficiency of 15.5%.
This thesis also analyzes the effects of mixed antisolvents on the qualities of triple-cation perovskites. Different solution concentrations of chlorobenzene in ethyl acetate and isopropyl alcohol in ethyl acetate are optimized for optimal supersaturation to achieve enlarged perovskite grains. Evaluations are discussed in the context of solution polarity and boiling point of the antisolvents, where 25% chlorobenzene in ethyl acetate antisolvent mixture shows the best film properties.
Another study discusses a new fabrication process called electrical field-assisted direct ink deposition for large-scale printing of perovskite solar cells. This process involves the formation of nanodroplets under an electrical field deposited onto ITO/glass substrates. As a result, smooth Poly (3,4-ethylene dioxythiophene) polystyrene sulfonate layers are
ii
produced with an average effective electrical resistivity of 4.15104 0.26 -m compared to that of spin-coated films.
A successive chapter discusses the studies of the electrical field-assisted direct ink deposition of the photoactive CH3NH3PbI2 (MAPbI3) layer. Its focus is on the post-deposition chemical annealing of the MAPbI3 films in methylamine gas, termed as methylamine gas-assisted healing and growth of perovskite films. This treatment improved the smoothness, reduced porosity, increased density, and generated more uniform grain sizes. Moreover, it improved the inter-grain boundary contacts by eliminating secondary, fine-grained boundary structures. Mechanisms behind the initial liquefaction of the MAPbI3 film's subsequent re-solidification are discussed.
ContributorsGogoi, Banashree (Author) / Alford, Terry (Thesis advisor) / Petuskey, William (Thesis advisor) / Gould, Ian (Committee member) / Li, Jian (Committee member) / Arizona State University (Publisher)
Created2023
Description
Recent advancements in the field of light wavefront engineering rely on complex 3D metasurfaces composed of sub-wavelength structures which, for the near infrared range, are challenging to manufacture using contemporary scalable micro- and nanomachining solutions. To address this demand, a novel parallel micromachining method, called metal-assisted electrochemical nanoimprinting (Mac-Imprint) was developed. Mac-Imprint relies on the catalysis of silicon wet etching by a gold-coated stamp enabled by mass-transport of the reactants to achieve high pattern transfer fidelity. This was realized by (i) using nanoporous catalysts to promote etching solution diffusion and (ii) semiconductor substrate pre-patterning with millimeter-scale pillars to provide etching solution storage. However, both of these approaches obstruct scaling of the process in terms of (i) surface roughness and resolution, and (ii) areal footprint of the fabricated structures. To address the first limitation, this dissertation explores fundamental mechanisms underlying the resolution limit of Mac-Imprint and correlates it to the Debye length (~0.9 nm). By synthesizing nanoporous catalytic stamps with pore size less than 10 nm, the sidewall roughness of Mac-Imprinted patterns is reduced to levels comparable to plasma-based micromachining. This improvement allows for the implementation of Mac-Imprint to fabricate Si rib waveguides with limited levels of light scattering on its sidewall. To address the second limitation, this dissertation focuses on the management of the etching solution storage by developing engineered stamps composed of highly porous polymers coated in gold. In a plate-to-plate configuration, such stamps allow for the uniform patterning of chip-scale Si substrates with hierarchical 3D antireflective and antifouling patterns. The development of a Mac-Imprint system capable of conformal patterning onto non-flat substrates becomes possible due to the flexible and stretchable nature of gold-coated porous polymer stamps. Understanding of their mechanical behavior during conformal contact allows for the first implementation of Mac-Imprint to directly micromachine 3D hierarchical patterns onto plano-convex Si lenses, paving the way towards scalable fabrication of multifunctional 3D metasurfaces for applications in advanced optics.
ContributorsSharstniou, Aliaksandr (Author) / Azeredo, Bruno (Thesis advisor) / Chan, Candace (Committee member) / Rykaczewski, Konrad (Committee member) / Petuskey, William (Committee member) / Chen, Xiangfan (Committee member) / Arizona State University (Publisher)
Created2022
Description
Non-stoichiometric oxides play a critical role in many catalytic, energy, and sensing technologies, providing the ability to reversibly exchange oxygen with the ambient environment through the creation and annihilation of surface oxygen vacancies. Oxygen exchange at the surfaces of these materials is strongly influenced by atomic structure, which varies significantly across nanoparticle surfaces. The studies presented herein elucidate the relationship between surface structure behaviors and oxygen exchange reactions on ceria (CeO2) catalyst materials. In situ aberration-corrected transmission electron microscopy (AC-TEM) techniques were developed and employed to correlate dynamic atomic-level structural heterogeneities to local oxygen vacancy activity.
A model Ni/CeO2 catalyst was used to probe the role of a ceria support during hydrocarbon reforming reactions, and it was revealed that carbon formation was inhibited on Ni metal nanoparticles due to the removal of lattice oxygen from the ceria support and subsequent oxidation of adsorbed decomposed hydrocarbon products. Atomic resolution observations of surface oxygen vacancy creation and annihilation were performed on CeO2 nanoparticle surfaces using a novel time-resolved in situ AC-TEM approach. Cation displacements were found to be related to oxygen vacancy creation and annihilation, and the most reactive surface oxygen sites were identified by monitoring the frequency of cation displacements. In addition, the dynamic evolution of CeO2 surface structures was characterized with high temporal resolution AC-TEM imaging, which resulted in atomic column positions and occupancies to be determined with a combination of spatial precision and temporal resolution that had not previously been achieved. As a result, local lattice expansions and contractions were observed on ceria surfaces, which were likely related to cyclic oxygen vacancy activity. Finally, local strain fields on CeO2 surfaces were quantified, and it was determined that local strain enhanced the ability of a surface site to create oxygen vacancies. Through the characterization of dynamic surface structures with advanced AC-TEM techniques, an improvement in the fundamental understanding of how ceria surfaces influence and control oxygen exchange reactions was obtained.
A model Ni/CeO2 catalyst was used to probe the role of a ceria support during hydrocarbon reforming reactions, and it was revealed that carbon formation was inhibited on Ni metal nanoparticles due to the removal of lattice oxygen from the ceria support and subsequent oxidation of adsorbed decomposed hydrocarbon products. Atomic resolution observations of surface oxygen vacancy creation and annihilation were performed on CeO2 nanoparticle surfaces using a novel time-resolved in situ AC-TEM approach. Cation displacements were found to be related to oxygen vacancy creation and annihilation, and the most reactive surface oxygen sites were identified by monitoring the frequency of cation displacements. In addition, the dynamic evolution of CeO2 surface structures was characterized with high temporal resolution AC-TEM imaging, which resulted in atomic column positions and occupancies to be determined with a combination of spatial precision and temporal resolution that had not previously been achieved. As a result, local lattice expansions and contractions were observed on ceria surfaces, which were likely related to cyclic oxygen vacancy activity. Finally, local strain fields on CeO2 surfaces were quantified, and it was determined that local strain enhanced the ability of a surface site to create oxygen vacancies. Through the characterization of dynamic surface structures with advanced AC-TEM techniques, an improvement in the fundamental understanding of how ceria surfaces influence and control oxygen exchange reactions was obtained.
ContributorsLawrence, Ethan Lee (Author) / Crozier, Peter A. (Thesis advisor) / Lin, Jerry (Committee member) / Liu, Jingyue (Committee member) / Petuskey, William (Committee member) / Arizona State University (Publisher)
Created2019