Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Controlled Nucleation and Growth of Carbon Nanotubes(2024) Alibrahim, Ayman; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Single-walled carbon nanotubes (SWCNTs) exhibit exceptional electrical, mechanical, and optical properties, making them potential game changers for diverse applications. However, the synthesis of SWCNTs faces significant challenges, including low yield, inadequate control over catalyst particle size, and prevalent impurities. This dissertation focuses on elucidating SWCNTs' nucleation and growth mechanisms to address these challenging issues. First, I applied in-situ absorption spectroscopy to monitor the SWCNT production by chemical vapor deposition. Second, I investigated the factors affecting metal catalyst nucleation and introduced a confinement strategy that enabled a record-breaking growth rate of 4500 meters per hour for SWCNTs. Furthermore, I developed a novel “seed doping” technique to control the nucleation of metal catalysts, significantly reducing catalyst particle size and producing purer, smaller-diameter SWCNTs continuously. Finally, I explored the role of ethanol in enabling the controlled growth of double-walled carbon nanotubes by building on SWCNTs as templates.Item DISTRIBUTION AND VARIABILITY OF CARBON STOCKS IN MID-ATLANTIC TIDAL MARSH SOILS(2022) Kim, Jordan; Rabenhorst, Martin C; Environmental Science and Technology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tidal marsh wetlands have the capacity to store disproportionately large quantities of Cdespite their small areal extent. Good estimations of this “blue C” are now more critical than ever due to implications for the global C cycle and climate change, especially since C storage in tidal marshes has historically been understudied. In this study, we set out to measure, more accurately estimate, and conceptually model the C stocks in representative tidal marshes of the Mid-Atlantic region. We found that C storage differs significantly in marshes formed among various pedogeomorphic settings due to differences in pedogenic processes and soil morphology. Further, we have demonstrated that the mean C densities of particular soil materials can be used in conjunction with soil morphological descriptions to reliably estimate the C stocks in the absence of laboratory data. Finally, we augmented existing concepts of tidal marshes in the region by incorporating newly gained understandings of the spatial changes in morphology and C stocks across marshes within different pedogeomorphic settings.Item HYDROLOGIC DRIVERS OF SOIL ORGANIC CARBON STORAGE AND STABILITY IN FRESHWATER MINERAL WETLANDS(2019) Kottkamp, Anna Isabel; Palmer, Margaret; Tully, Katherine; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mineral wetlands comprise most of historic wetland loss, yet few studies focus on mineral wetland soil organic carbon (SOC). We explore SOC across continuous hydrologic gradients within and among seasonally flooded mineral wetlands. First, we quantify SOC stabilization (e.g., organo-mineral associations and aggregates) across a wetland–upland gradient. Second, we examine relationships between hydrologic regime and SOC stocks among wetlands. From wetland–upland, saturation was highly variable in the transition zone. Organo-mineral associations peaked in the transition zone while large macroaggregate SOC declined from wetland–upland. Across wetlands, indicators of drying (e.g., minimum water level and summertime recession rate) were more related to SOC than inundation duration. From wetland basin–upland, SOC stocks were significantly related to both mean water level and relative elevation. We highlight relationships between SOC and the dynamic hydrology of wetlands, emphasizing the need for research on how changing hydrologic regime may influence mineral wetland SOC.Item HYDROLOGICAL, BIOLOGICAL, AND GEOCHEMICAL RELATIONSHIPS AMONG CARBON, NITROGEN, AND BASE CATIONS IN RESTORED AND UNRESTORED URBAN STREAMS(2017) Doody, Thomas Rossiter; Kaushal, Sujay S; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Urban infrastructure changes hydrologic flowpaths of water into streams and alters ecosystem function. Geomorphic stream restoration is commonly implemented to stabilize channels, while ecosystem function, and nutrient retention are of secondary concern. This research investigated whether restoration alone significantly influences N uptake in streams and if significant hydrological, biological, and geochemical relationships exist between coupled biogeochemical cycles that should be considered when evaluating restorations. Carbon, nitrogen, base cations, and stream metabolism dynamics were investigated in six urban streams in Baltimore,MD. Nitrate tracer injections were used to quantify nitrogen uptake dynamics. Results did not show significant differences in nitrogen uptake based on restoration. Organic carbon, inorganic carbon, and nitrogen each have distinct but interrelated hydrological, biological, and geochemical relationships across all sites. These dynamic relationships may also significantly affect nitrogen uptake, but more spatiotemporal data are needed to quantify and understand variability among restored and unrestored sites.Item Ecosystem dynamics in tidal marshes constructed with fine grained, nutrient rich dredged material(2015) Staver, Lorie Winchell; Stevenson, J. Court; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An ecological study was undertaken of the tidal marshes at Poplar Island, a restoration project utilizing fine grained dredged material from the shipping channels in upper Chesapeake Bay. The goals of the study were to examine the effect of a nutrient rich substrate on vegetation development, elevation change, and nutrient cycling in the constructed tidal marshes. Specifically, I examined macrophyte production, nitrogen (N), carbon (C) and silicon (Si) budgets, as well as the success of a silica amendment in enriching plant tissue concentrations. Establishment of Spartina alterniflora and S. patens on fine grained dredged material was rapid, and growth peaked in the second year. Thereafter S. alterniflora dieback occurred sporadically during the growing season, but the causes remain unclear. Elevation change averaged 7.9 ±0.8 mm y-1 in the dredged material marshes (low marsh only), compared to 7.4 ±1.4 mm y-1 in a low nutrient onsite reference marsh. Elevation change was significantly correlated with biomass production suggesting that inputs of organic matter from high rates of aboveground biomass production on nutrient rich dredged material offset the reduced contribution of belowground biomass to vertical accretion. However, dieback may have a detrimental effect on vertical accretion, which is essential for keeping up with apparent sea level rise (13.6 and 11.0 mm y-1 at Baltimore and Solomon’s tide gauges, respectively) since elevation monitoring began at Poplar Island. The tidal flux study revealed that the marsh exports ~665 kg of N y-1, including 100 kg NH4+-N y-1, and 67,874 kg y-1 TSS, and imports 35 kg NO3-N y-1. Silicon is also exported on both a seasonal and annual basis, including 4,337 kg dissolved Si y-1 and 3,924 kg biogenic Si y-1, with highest exports in July, an overlooked benefit of dredged material restoration projects. Soil Si amendments increased plant tissue concentrations significantly, but this study did not show increased resistance to N related stress effects on the vegetation. Overall, this study suggests that when considering trajectories of vegetation development, nutrient exchanges and elevation change in constructed marshes, it is essential to consider the initial nitrogen content of the substrate.Item Heterogeneous Ordered Mesoporous Carbon/Metal Oxide Composites for the Electrochemical Energy Storage(2015) Hu, Junkai; Lee, Sang Bok; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The combination of high electronic conductivity, enhanced ionic mobility, and large pore volume make ordered mesoporous carbons (OMCs) promising scaffolds for active energy storage materials. However, mesoporous structures and material morphology need to be more thoroughly addressed. This dissertation discusses the effects of mesoporous structures and material morphologies on the electrochemical performance of OMC/Fe2O3 composites. In the first approach, Fe2O3 was embedded into 1D cylindrical (FDU-15), 2D hexagonal (CMK-3), and 3D bicontinuous (CMK-8) symmetries of mesoporous carbons. These materials were used as supercapacitors for a systematic study of the effects of mesoporous architecture on the structure stability, ion mobility, and performance of mesoporous composite electrodes. The results show that the CMK-3 and CMK-8 synthesized by hard template method can provide high pore volume, but the instability of their mesostructures hinders the total electrode performances upon oxide impregnation. In contrast, the FDU-15 from the soft template method can provide a stable mesostructure. However, it contains much smaller pore volume and surface area, leading to limited metal oxide loading and electrode capacitance. Based on these results, anodized aluminum oxide (AAO) and triblock copolymer F127 are used together as hard and soft templates to fabricate ordered mesoporous carbon nanowires (OMCNW) as a host material for Fe2O3 nanoparticles. The synergistic effects in the dual template strategy provide a high pore volume and surface area, and the structure remains stable even with high metal oxide loading amounts. Additionally, the unique nanowire morphology and mesoporous structure of the OMCNW/Fe2O3 facilitate high ionic mobility in the composite, leading to a large capacitance with good rate capability and cycling stability. I further evaluated this OMCNW/Fe2O3 as a lithium-ion battery (LIB) anode, which showed that the porous symmetry, material morphology, and structure stability are even more important in the rate and cycling performances of LIBs. This work helps further the understanding and optimization of porous structures and morphologies of heterogeneous composites for next generation electrochemical energy storage materials.Item Assessing Wetland Restoration on the Delmarva Peninsula using Vegetation Characteristics(2015) McFarland, Eliza Katherine; Baldwin, Andrew H; Environmental Science and Technology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)With wetland restoration, post-restoration monitoring is essential for determining developmental trajectories, particularly when comparing to natural reference systems. As part of the Mid-Atlantic Conservation Effects Assessment Project, 15 depressional wetlands on the Delmarva Peninsula of Maryland and Delaware were surveyed for above-ground vegetation and seed bank community composition, annual biomass production, and vegetation carbon content (10 restorations from prior-converted cropland (aged 5-31 years), and 5 natural forested depressions). Within each wetland, hydrologic zones (emergent, transition, upland) were also denoted and sampled. Restored wetlands showed more seed bank community similarity to natural wetlands than above-ground vegetation communities. Restorations also produced more annual herbaceous biomass than natural systems, and lower annual leaf litter biomass. After this period of post-restoration development, restored wetlands do not perform vegetation-related functions identical to their natural counterparts; however, these restorations are performing important vegetation-based functions that require yet more time to truly develop.Item Urea Hydrolysis in Soil Profile Toposequences: Mechanisms Relevant to Nitrogen Transport and Water Quality(2014) Fisher, Kristin A.; James, Bruce R.; Environmental Science and Technology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Urea has been linked to harmful algal blooms in surface waters, but numerous studies of its hydrolysis in agricultural soils have concluded that urea does not persist long enough to be transported to surface waters. This paradox in the published literature may be explained by our lack of knowledge regarding the soil chemical conditions that affect microbial urease activity in surface and subsurface horizons of soil profiles that lie between agricultural fields and surface waters, particularly in sandy Coastal Plain regions. Laboratory studies were conducted to determine the most influential soil chemical characteristics predicting rates of urea hydrolysis in six Maryland soils. Soils were sampled from both the A and B horizons of toposequences consisting of an agricultural field, a grassed field border, and a transitional zone adjacent to surface waters. A pH-adjustment experiment identified soil C and N as important predictors of urea hydrolysis. Analysis of microbial community composition and ureC genes across a toposequence found the greatest abundance of bacteria, fungi, and ureC genes in riparian A horizon soils, despite inhibitory conditions of low pH, low field-sampled moisture content, and high extractable metal concentrations. The high carbon content of A horizon riparian soils likely mediated these toxic characteristics. Of particular note was the significant correlation between ureC genes and rate of urea hydrolysis (r2 = 0.82), indicating that the presence of this gene may be useful as a biomarker for predicting rates of urea hydrolysis in other soils. An investigation into the effects of added C revealed that diverse soil C compounds influenced urea hydrolysis differently. In a 24 hr incubation, ascorbic and gallic acid acted as pro- and antioxidants with both enhancement and inhibition of hydrolysis, depending upon concentration, whereas benzoic and cinnamic acids likely enhanced hydrolysis as a result of being metabolized by soil microorganisms. A better understanding of the mechanisms controlling urea hydrolysis in diverse soils will help researchers and policymakers formulate defensible recommendations related to urea fertilizer and animal waste application so that urea-N can be efficiently used by crops and urea movement across the landscape and into surface waters can be minimized.Item Electronic Properties of Carbon Nanotubes studied in Field-Effect Transistor Geometries(2004-05-12) Dürkop, Tobias; Fuhrer, Michael S; PhysicsDue to their outstanding properties carbon nanotubes have attracted considerable research effort during the last decade. While they serve as an example of a 1-dimensional electron system allowing one to study fundamental quantum effects nanotubes-especially semiconducting nanotubes-are an interesting candidate for next-generation transistor application with the potential to replace silicon-based devices. I have fabricated nanotubes using chemical vapor deposition techniques with various catalysts and gas mixtures. The nanotubes produced with these techniques vary in length from 100 nm to several hundreds of micrometers. While data taken on shorter metallic and semiconducting devices show Coulomb blockade effects, the main part of this work is concerned with measurements that shed light on the intrinsic properties of semiconducting nanotubes. On devices with lengths of more than 300 um I have carried out measurements of the intrinsic hole mobility as well as the device-specific field-effect mobility. The mobility measured on these nanotube devices at room temperature exceeds that of any semiconductor known previously. Another important consideration in nanotube transistor applications are hysteresis effects. I present measurements on the time scales involved in some of these hysteresis effects and a possible application of the hysteresis for memory devices.