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Graphene nanosheets produced via controlled detonation of hydrocarbonsNepal, Arjun January 1900 (has links)
Doctor of Philosophy / Physics / Christopher M. Sorensen / We demonstrated that gram quantities of pristine graphene nanosheets (GNs) can be produced via detonation of a hydrocarbon. This one-step and catalyst-free method is eco-friendly and economical for the production of GNs. The hydrocarbons detonated were C₂H₂, C₂H₄, C₃H₈ and CH₄ in the presence of O₂. The carbon products obtained from the detonation were analyzed by XRD, TEM, XPS and Raman spectroscopy. Depending upon the ratio of O₂ to C₂H₂, the GNs of size up to ~ 250 nm, SSA up to ~ 200 m²/g and yield up to 70% with 2-3 layers' stack have been obtained so far. N₂O was determined as a good alternative to O₂ as an oxidizer to produce GNs by detonating C₂H₂ with it.
A two-color pyrometer was designed and calibrated to measure the temperature of the detonation of hydrocarbons. The measured detonation temperatures were in between 2700 K and 4300 K. Along with the high detonation temperature, the composition of precursor hydrocarbon was observed to be crucial as well to determine its suitability to detonate with oxidizer to produce GNs. The hydrocarbons C₂H₂ and C₂H₄ were determined as the suitable precursors to produce GNs whereas detonation of C₃H₈ yields mere amorphous carbon soot and CH₄ gives no solid carbon while detonated with O₂. It has been proposed that the hydrocarbons with C/H≥0.5 are suitable for GNs production by detonation method.
Highly oxidized graphene nanosheets (OGNs) were produced by solution-based oxidation of GNs prepared via a controlled detonation of acetylene at O₂/C₂H₂=0.8. The produced OGNs were about 250 nm in size and hydrophilic in nature. The C/O ratio was dramatically reduced from 49:1 in the pristine GNs to about 1:1 in OGNs, as determined by X-ray photoelectron spectroscopy. This C/O in OGNs is the least ever found in all oxidized graphitic materials that have been reported. Thus, the OGNs produced from the detonated GNs with such high degree of oxidation herein yields a novel and promising material for future applications.
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Investigations on Graphene/Sn/SnO2 Based Nanostructures as Anode for Li-ion BatteriesThomas, Rajesh January 2013 (has links) (PDF)
Li-ion thin film battery technology has attracted much attention in recent years due to its highest need in portable electronic devices. Development of new materials for lithium ion battery (LIB) is very crucial for enhancement of the performance. LIB can supply higher energy density because Lithium is the most electropositive (-3.04V vs. standard hydrogen electrode) and lightest metal (M=6.94 g/mole). LIBs show many advantages over other kind of batteries such as, high energy density, high power density, long cycle life, no memory effect etc. The major work presented in this thesis is on the development of nanostructured materials for anode of Li-ion battery. It involves the synthesis and analysis of grapheme nanosheet (GNS) and its performance as anode material in Li ion battery. We studied the synthesis of GNS over different substrates and performed the anode studies. The morphology of GNS has great impact on Li storage capacity. Tin and Tin oxide nanostructures have been embedded in the GNS matrix and their electrochemical performance has been studied.
Chapter 1 gives the brief introduction about the Li ion batteries (LIBs), working and background. Also the relative advantages and characterization of different electrode materials used in LIBs are discussed.
Chapter 2 discusses various experimental techniques that are used to synthesize the electrode materials and characterize them.
Chapter3 presents the detailed synthesis of graphene nanosheet (GNS) through electron cyclotron resonance (ECR) microwave plasma enhanced chemical vapor deposition (ECR PECVD) method. Various substrates such as metallic (copper, Ni and Pt coated copper) and insulating (Si, amorphous SiC and Quartz) were used for deposition of GNS. Morphology, structure and chemical bonding were analyzed using SEM, TEM, Raman, XRD and XPS techniques. GNS is a unique allotrope of carbon, which forms highly porous and vertically aligned graphene sheets, which consist of many layers of graphene. The morphology of GNS varies with substrate.
Chapter 4 deals with the electrochemical studies of GNS films. The anode studies of GNS over various substrates for Li thin film batteries provides better discharge capacity. Conventional Li-ion batteries that rely on a graphite anode have a limitation in the capacity (372 mAh/g). We could show that the morphology of GNS has great effect in the electrochemical performance and exceeds the capacity limitation of graphite. Among the electrodes PtGNS shown as high discharge capacity of ~730 mAh/g compare to CuGNS (590 mAh/g) and NiGNS (508 mAh/g) for the first cycle at a current density of 23 µA/cm2. Electrochemical impedance spectroscopy provides the various cell parameters of the electrodes.
Chapter 5 gives the anodic studies of Tin (Sn) nanoparticles decorated over GNS matrix. Sn nanoparticles of 20 to 100nm in size uniformly distributed over the GNS matrix provides a discharge capacity of ~1500 mAh/g mAh/g for as deposited and ~950 mAh/g for annealed Sn@GNS composites, respectively. The cyclic voltammogram (CV) also shows the lithiation and delithiation process on GNS and Sn particles.
Chapter 6 discusses the synthesis of Tinoxide@GNS composite and the details of characterization of the electrode. SnO and SnO2 phases of Tin oxide nanostructures differing in morphologies were embedded in the GNS matrix. The anode studies of the electrode shows a discharge capacity of ~1400 mAh/g for SnO phase (platelet morphology) and ~950 mAh/g for SnO2 phase (nanoparticle morphology). The SnO phase also exhibits a good coulumbic efficiency of ~95%.
Chapter 7 describes the use of SnO2 nanowire attached to the side walls of the GNS matrix. A discharge capacity of ~1340 mAh/g was obtained. The one dimensional wire attached to the side walls of GNS film and increases the surface area of active material for Li diffusion. Discharge capacity obtained was about 1335 mAhg-1 and the columbic efficiency of ~86% after the 50th cycle.
The research work carried out as part of this thesis, and the results have summarized in chapter 8.
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