Graphene is a planar monolayer sheet of sp2-hybridized carbons arranged on
a two-dimensional hexagonal lattice. It is considered as the basic building unit of
other carbon allotropes, i.e. 0D fullerenes, 1D carbon nanotubes, and 3D graphite.
Following the definition, graphene can be divided into three different types: singlelayer
graphene (SG), bilayer graphene (BG), and few-layer graphene (FG, number of
layers ≤10). Since Geim and his co-workers at Manchester University first obtained
single-layer graphene by the micro-mechanical cleavage approach, there has been an
explosion of interest both in theoretical studies and experimental investigations.
Long-range π-conjugation in graphene yields exotic physical properties of graphene,
such as ultrahigh carrier mobility (~200 000 cm 2V-1 S-1 at electron densities of 2
×1011 cm-2), superior high Young's moduli (0.5-1 TPa), large spring constants (1-5
Nm-1), the ambipolar field effect, and the room-temperature quantum confinement
effect. These properties give graphene great potential in applications such as: field
effect transistors (FET), high-speed and radio frequency logic devices, transparent
electrodes, solar cells, supercapacitors, and rechargeable lithium ion batteries.
Though graphene has been extensively studied for 8 to 9 years, achieving
facile, environmental benignity and large-area growth of graphene is still the main
task for researchers. Furthermore, some preliminary results have shown that
graphene can greatly improve both the capacity and cycling stability of lithium ion
batteries. In this thesis, we developed two different methods to synthesize graphene
from solid carbon sources (polymethylmetacrylate and polydimethylsiloxane) with
plasma enhanced chemical vapor deposition (CVD). Two facile N-doping
approaches (ex-situ and in-situ) were demonstrated to efficiently incorporate nitrogen atoms into graphene lattices. Meanwhile, a facile one pot hydrothermal
method has been proposed to synthesize laminate-structured graphene sheets-Fe3O4
nanocomposites (GNS-Fe3O4), which reveals high capacity LIB performance.
Furthermore, a unique three-dimensional graphene scaffold has been fabricated to
support Si and Sn nanostructures, which shows outstanding integrative
electrochemical properties, excellent cyclability and rate performance.
In the first section, microwave plasmas were employed to synthesize singleor
double-layer graphene sheets on copper foils using a solid carbon source,
polymethylmetacrylate (PMMA). The utilization of reactive plasmas enables
graphene growth at reduced temperatures when compared with conventional thermal
chemical vapor deposition processes. The effects of substrate temperature on
graphene quality were revealed based on Raman analysis. Moreover, a facile
approach to incorporate nitrogen into graphene by plasma treatment in a
nitrogen/hydrogen gas mixture was demonstrated, and most of the nitrogen atoms
were verified to be pyridine-like in carbon networks.
The second section reports a novel method to achieve in situ nitrogen-doped
graphene by microwave plasma assisted chemical vapor deposition (CVD) using
PDMS (polydimethylsiloxane) as a solid carbon source. The percentage of nitrogen
in the graphene lattice was observed to be controllable by adjusting the flow rate of
nitrogen during the CVD process. The nitrogen atoms doped into graphene lattice
were mainly in the forms of pyridinic and pyrrolic structures as confirmed by X-ray
photoelectron spectroscopy. Moreover, first-principles calculations show that the
incorporated nitrogen atoms can lead to p-type doping of graphene. This in situ
approach provides a promising strategy to prepare graphene with controlled
electronic properties.
In the third section, we developed a facile one pot hydrothermal method to
synthesize laminate-structured graphene sheets-Fe3O4 nanocomposites (GNS-Fe3O4).
Fe3O4 nanoparticles were distributed densely and homogeneously in the graphene
matrix. Galvanostatic charge/discharge cycling of the GNS-Fe3O4 nanocomposites
exhibited a reversible specific capacity of over 1200 mAh/g at 100 mA/g without
palpable fading for 50 cycles in the voltage range of 0.01-3.0 V. A cell for the rate
capacity test reveals a high current density of 946 mAh/g at a cycling rate of 1000
mA/g, which could be recovered in full to 1359 mAh/g at 100mA/g after 50 cycles.
The laminated structure of nanocomposites could prevent the agglomeration of
Fe3O4 nanoparticles and the restacking of graphene sheets, effectively releasing the
strain caused by volume expansion of Fe3O4 nanoparticles and facilitating
ion/electron transportation within the electrode and at the electrode/electrolyte
interface.
In the fourth section, we proposed three-dimensional (3D) silicon (Si) thin
films supported on a graphene scaffold to act as an anode electrode for lithium-ion
batteries. The as-prepared Si anode exhibited a gravimetric capacity as high as 2495
mAh g-1 at C/3 rate, and capacity retention of 84% after 500 cycles compared with
that after 50 cycles. Meanwhile, specific capacities of 1732 and 1264 mAh g-1 were
demonstrated after 1200 cycles at rates of 1C and 3C, respectively. Such high
specific capacities, excellent cyclability, and rate performance could have
contributed to the highly porous 3D architecture of graphene scaffold, which
provides not only good electrical conductivity but also mechanical flexibility. The
results presented here have paved a new way to synthesize Si-graphene hybrid
materials using microwave plasma-enhanced chemical vapor deposition as robust
and scalable Si-based anodes for lithium ion batteries.
In the fifth section, we report the development of a Sn nanoparticledecorated
three-dimensional (3D) foothill-like graphene architecture as the anode
electrode in LIBs. 3D graphene scaffold not only acts as an effective pathway for
electron/lithium ions transport, but also efficiently buffers the huge volume
expansion/shrinkage of Sn particles in the charge/discharge cycling processes.
Electrochemical measurements demonstrated that the Sn@3D graphene anode
delivered a reversible capacity of 662 mAhg-1 at a current density of 992 mAg-1 after
4000 cycles and 1129 mAhg-1 at 330 mAg-1 after 400 cycles. Its capacity is three
times higher than that of the conventional graphite anode, suggesting that the novel
Sn@3D graphene structure brings about a significant improvement in the overall
performance of a LIB in the aspects of capacity, cycle life, and rate capacity.
| Date of Award | 2 Oct 2013 |
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| Original language | English |
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| Awarding Institution | - City University of Hong Kong
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| Supervisor | Wenjun ZHANG (Supervisor) |
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