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Title: Vertically aligned few layered graphene (FLG) nanoflakes : synthesis and applications
Author: Soin, Navneet
Awarding Body: University of Ulster
Current Institution: Ulster University
Date of Award: 2011
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This thesis focuses on the synthesis and characterisation of catalyst free, vertically aligned, Few Layered Graphene Nanoflakes (FLGs) and investigates associated field emission properties and inherent and applied electrochemical properties. The catalyst-free synthesis of FLGs was carried out using the Microwave Plasma Enhanced Chemical Vapour Deposition (MPECVD) technique on heavily doped Si substrates. To understand their growth mechanism, an in-depth study of FLGs was carried out using a combination of Raman spectroscopy, X-ray diffraction (XRD) and electron microscopy. These were studied over a range of growth times. The XRD analysis revealed that the increase in the growth time, promoted an enhanced vertical orientation of the flakes and was accompanied by the structural transformation of nanocrystalline turbostratic graphite to highly-ordered few layered graphene nanoflakes. The transformation involves the release of internal stress at nanocrystalline turbostratic graphite nucleation sites as measured by Raman spectroscopy, where a 5 cm-1 reduction in the G band position was observed. The Nitrogen doping of FLGs was carried out using in-situ low pressure Electron Cyclotron Resonance (ECR) plasma. As compared to pristine FLGs, the N doped FLGs displayed a significant improvement in the field emission characteristics where the turn-on field reduced from 1.94 Vμm-1 for pristine FLGs to 0.85 Vμm-1 for N doped FLGs. This was accompanied by an increase in the emission current density, where the pristine FLGs showed a maximum current density of 17μAcm-2 (at 2.1 Vμm-1), while the N-doped FLGs displayed 297 μAcm-2 (at 1.08 Vμm-1). The change in the field emission behaviour of pristine and N doped FLGs is explained in terms of microstructural changes as well as a reduction in the work function, probed using Valence Band X-ray Photoelectron Spectroscopy. Due to the large number of exposed edge planes and associated high electronic Density of States (DOS), the FLGs exhibited excellent reversible electrochemical behaviour. The FLGs demonstrated fast electron transfer rates, with near ideal peak-to-peak separation of 60 mV for redox peaks, thus exhibiting ideal Nerstian behaviour. The quantification of edge plane sites revealed that with the increasing growth time, the contributing sites actually reduce. The ECR plasma treatment provided a facile route to prepare N-doped FLGs for applications such as Oxygen Reduction Reaction (ORR) and H2O2 reduction. The increase in the defect density, localised electronic DOS and grafting of favourable N-moieties such as pyridinic nitrogen, increased the ORR activity of N-doped FLGs as compared to the pristine FLGs. Excellent long term stability was also observed for these materials in chrono-amperometric studies, making them a potential candidate for metal-catalyst free ORR electrodes. Nitrogen doped FLGs also exhibited excellent sensitivity towards the reduction of H2O2 at much lower applied potentials as compared to pristine FLGs. The FLGs provide an ideal, high surface area platform for sputter deposition of Pt and RuO2 nanoparticles for methanol oxidation and supercapacitor applications, respectively. Addition of pseudo-capacitive RuO2 nanoparticles to FLGs resulted in a significant enhancement of capacitance of RuO2-FLG nanocomposite structures. The RuO2-FLG nanocomposites not only exhibited higher capacitance but also significant improvements in charging-discharging characteristics as well as long operational stability. Ultra-low catalyst loadings of Pt on FLGs exhibited high performance for efficient oxidation of methanol with high mass specific current densities and a high tolerance towards CO poisoning. The enhancement in the electrochemical behaviour was attributed to the synergistic effects of FLGs, which resulted in a high dispersion of the catalyst nanoparticles, thereby increasing their effective surface area and rate of electron transfer through the FLGs.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available