1. Production of inorganic nanohybrids by the
templating of carbon and peptide nanostructures
A thesis submitted to The University of Manchester for the degree of
Doctor of Philosophy
In the Faculty of Engineering and Physical Sciences
2013
Yanning Li
School of Materials

2. 2
Table of Contents
List of tables…………………………………………………………………..…..8
List of figures………………………………………………………………..……9
List of abbreviations…………………………………………………………..…26
List of symbols………………………………………………………………..…29
Abstract…………………………………………………………………….……30
Declaration………………………………………………………………………31
Copyright………………………………………………………………………..32
Acknowledgements…………………………………………………………..…33
Chapter 1 Introduction…………………………………………………………..34
1.1 Overview ……………………………………………………………………34
1.2 Aims…………………………………………………………………………35
1.3 References……………………………………………………………...……37
Chapter 2 Literature Review……………………………………………….……40
2.1 Sol-gel chemistry……………………………………………………………40
2.2 CNT-Inorganic nanohybrids…………………………………………………43
2.2.1 Introduction to carbon nanotubes………………………………...……43
2.2.1.1 Structures………………………………………………….………43
2.2.1.2 Properties……………………………………………….…………44
2.2.1.3 Synthesis………………………………………………….………46
2.2.1.4 Applications………………………………………………………46
2.2.2 Functionalization of CNTs……………………………………….……47
2.2.2.1 Covalent functionalization………………………………..………48
2.2.2.2 Non-covalent functionalization…………………………...………51
2.2.3 CNT-inorganic nanohybrids ……………………………………….…57
2.2.3.1 Synthesis……………………………………………………….…58
2.2.3.2 CNT-SiO2 hybrids…………………………………………..……59
2.2.3.3 CNT-TiO2 hybrids………………………………………..………63
2.2.3.4 Inorganic nanotubes………………………………………………69
2.3 Peptide self-assembly and mineralization…………………………...………72

3. 3
2.3.1 Introduction……………………………………………………………72
2.3.2 Strategy for peptide self-assembly……………………………….……72
2.3.2.1 β-sheets and α helices………………………………………….…72
2.3.2.2 Peptide amphiphiles………………………………………………75
2.3.2.2.1 All-amino acid peptide amphiphiles……………………...……75
2.3.2.2.2 Lipidated peptides……………………………………………78
2.3.2.3 Aromatic short peptide derivatives………………………..………79
2.3.3 Controlled self-assembly of peptides…………………………….……81
2.3.3.1 PH/ionic strength triggered ………………………………………81
2.3.3.2 Enzyme triggered …………………………………………...……82
2.3.4 Mineralisation…………………………………………………….……84
2.3.4.1 Biomineralization…………………………………………………84
2.3.4.2 Biomimetic mineralization………………………………..………88
2.4 Graphene and graphene based nanocomposites……………………..………91
2.4.1 Introduction to graphene………………………………………………91
2.4.1.1 Structure and properties of graphene……………………..………91
2.4.1.2 Production of graphene…………………………………...………94
2.4.1.2.1 Micromechanical cleavage…………………………...………95
2.4.1.2.2 Liquid phase exfoliation………………………………..……95
2.4.2 Graphene based nanocomposites and nanohybrids ………………….101
2.5 References…………………………………………………………….……105
Chapter 3 Experimental Methods……………………………………….…..…127
3.1 Materials……………………………………………………………………127
3.2 Experimental procedure……………………………………………………127
3.2.1 Synthesis of alignedCNT arrays by injection CVD method…………127
3.2.2 Adsorption Study of the surfactants on CNTs………………….……128
3.2.2.1 Adsorption of the surfactants on aligned CNT arrays………..…128
3.2.2.2 Adsorption of the surfactants on randomly aligned CNT
networks…………………………………………………………………134
3.2.2.3 Desorption of the surfactants from CNT arrays in H2O…………135
3.2.2.4 Freundlich adsorption isotherm …………………………....……135

4. 4
3.2.2.5 Competitive binding from the Fmoc-AAs library on graphite…..136
3.2.2.6 Switchable surface chemistry……………………………..……..137
3.2.3 Synthesis of CNT-inorganic nanohybrids……………………………138
3.2.3.1 Synthesis of silica coated Fmoc-AA functionalized CNTs…..…138
3.2.3.2 Synthesis of TiO2 coated Fmoc-AA functionalized CNTs……...140
3.2.3.3 Combined sites…………………………………………….……142
3.2.4 Graphene and graphene based nanocomposites and nanohybrids..…143
3.2.4.1 GO-Inorganic nanohybrids ………………………………….…143
3.2.4.1.1 Preparation of aqueous dispersion of GO……………….…143
3.2.4.1.2 Preparation of GO-TiO2 nanohybrids………………………144
3.2.4.1.3 Preparation of GO-SiO2 nanohybrids………………………144
3.2.4.2 bwGO-Inorganic nanohybrids …………………………..………145
3.2.4.2.1 Preparation of bwGO dispersion……………………………145
3.2.4.2.2 Synthesis of bwGO-TiO2 nanohybrids…………………...…145
3.2.4.3 Exfoliated graphene (EG)-Inorganic nanohybrids………………146
3.2.4.3.1 Preparation of graphene dispersion ……………………...…146
3.2.4.3.2 Preparation of EG-TiO2 nanocomposites and nanohybrids…147
3.2.5 Mineralization of peptide self-assembled hydrogels…………………148
3.2.5.1 Fmoc-Y hydrogel preparation………………………………...…148
3.2.5.2 Fmoc-FY hydrogel preparation …………………………………148
3.2.5.3 Characterization …………………………………………………148
3.2.5.4 Silicification of Fmoc-Y gel ………………………………….…149
3.3 Analytical techniques………………………………………………………150
3.3.1 Scanning Electron Microscopy (SEM)………………………………150
3.3.2 Transmission Electron Microscopy (TEM)………….………………150
3.3.3 Energy Dispersive X-ray Spectroscopy (EDX)………………………153
3.3.4 Reversed-phase high-performance liquid chromatography (RP-HPLC)...153
3.3.5 Contact angle measurement………………………………………..…154
3.3.6 Raman spectroscopy …………………………………………………156
3.3.6.1 Background…………………………………………………...…156
3.3.6.2 Raman characterization of the exfoliated samples………………158

5. 5
3.3.7 Atomic Force Microscopy (AFM)……………………………………159
3.4 References …………………………………………………………………161
Chapter 4 Dynamic Interaction of Fmoc-AAs with CNTs…………………..…163
4.1 Introduction ……………………………………………………………..…163
4.2 Synthesis of aligned MWNT arrays by injection CVD method ………...…163
4.3. Interaction of surface modifiers with CNTs……………………………….166
4.3.1 Adsorption behavior of modifiers on CNT aligned CNT arrays….…166
4.3.2 Adsorption behavior of modifiers on randomly oriented CNT
Networks…………………………………………………………………170
4.3.3 Desorption behavior of the modifiers in excess of water……………171
4.3.4 Freundlich isotherm model …………………………………………172
4.3.5 Competitive binding from the Fmoc-AAs library on graphite………173
4.3.6 Switchable surface chemistry…………………………………..……176
4.4 Conclusion…………………………………………………………………178
4.5 References…………………………………………………………………180
Chapter 5 Synthesis of CNT-inorganic nanohybrids and the corresponding
inorganic NTs using Fmoc-AAs as surface modifier………………………….181
5.1 Introduction…………………………………………………………..……181
5.2. Synthesis of CNT-silica nanohybrids using Fmoc-AAs as surface
modifier………………………………………………………………….182
5.2.1 Synthesis and morphology characterization……………………….…182
5.2.2 Discussion on the role of Fmoc-AA functionalization in controlling the
morphology of the hybrids……………………………………..……189
5.2.3 Growing mechanism of silica coating on Fmoc-AA functionalized
CNTs……………………………………………………………………..…189
5.2.4 Kinetics for silica growth………………………………………….....191
5.2.5 Annealing………………………………………………………….…193
5.3 Synthesis of CNT-TiO2 nanohybrids using Fmoc-AAs as surface
modifier………………………………………………………………….…194
5.3.1 Synthesis and morphology characterization ……………………...…194
5.3.2 Mechanism for the formation of TiO2 coating on the functionalized

6. 6
CNTs………………………………………………………………………...…196
5.3.3Effect of CNT to TBOT ratio on the hybrid morphology……………198
5.3.4 Effect of modifier to CNT ratio on the hybrid morphology…………201
5.3.5Kinetics for TiO2 growth …………………………………………..…202
5.3.6 Synthesis of TiO2 NTs……………………………………………..…203
5.3.7 Phase transformation…………………………………………………211
5.3.8 Aligned arrays of TiO2 NTs …………………………………………214
5.4 Combined sites for catalyzing SiO2 and TiO2 deposition…………….……218
5.4.1 Synthesis of the biomimetic catalyst…………………………………219
5.4.2 Synthesis of SiO2 catalyzed by the combined sites……………….…220
5.4.3 Synthesis of TiO2 catalyzed by the combined sites……………….…221
5.5. Conclusion…………………………………………………………………223
5.6 References…………………………………………………………….……225
Chapter 6 Mineralization of peptide self-assembled hydrogels…………….…227
6.1 Introduction……………………………………………………………..…227
6.2 Enzymatic self-assembly of Fmoc-Y and Fmoc-FY hydrogels……………227
6.2.1 Fmoc-Y hydrogel…………………………………………………….227
6.2.2 Fmoc-FY hydrogel………………………………………………...…229
6.3 Silicification of hydrogel nanostructures…………………………………..231
6.3.1 Silicification of Fmoc-Y gel…………………………………………231
6.3.1.1 Silicification via vortexing TEOS in the diluted hydrogels
(Method 1)………………………………………………………….……231
6.3.1.2 Silicification via depositing TEOS/H2O mixture on hydrogels
(Method 2)………………………………………………...…….…….…234
6.4 Conclusion………………………………………………………………….238
6.5 References …………………………………………………………………239
Chapter 7 Graphene-Inorganic hybrids……………………………………...…240
7.1 GO-Inorganic nanohybrids…………………………………………………240
7.1.1 Characterization of GO dispersion…………………………….……240
7.1.2 Preparation of GO-TiO2 nanohybrids ………………………………242
7.1.3 Preparation of GO-SiO2 nanohybrids…………………………….…247

7. 7
7.2 bwGO-Inorganic nanohybrids………………………………………...……251
7.2.1 bwGO dispersion……………………………………………………251
7.2.2 bwGO-TiO2 nanohybrids……………………………………...……253
7.2.2.1 Reaction in aqueous solution……………………………….……253
7.2.2.2 Reaction in EtOH…………………………………………..……255
7.3 Exfoliated graphene-Inorganic nanohybrids…………………………….…260
7.3.1 Effect of sonication time and centrifuge speed on the concentration of
the graphene dispersion ……………………………………………260
7.3.2 Evidence for exfoliation to graphene ………………………...……262
7.3.2.1 Raman characterization of the exfoliated samples…………..…262
7.3.2.2 TEM characterization of the exfoliated samples ………………273
7.3.2.3 AFM characterization of the exfoliated samples…………….…278
7.3.3 Preparation of exfoliated graphene (EG)-TiO2 nanohybrids………284
7.3.3.1 Preparation of EG-TiO2 hybrids in aqueous solution…………..284
7.3.3.2 Preparation of EG-TiO2 nanohybrids in EtOH…………………287
7.4 Conclusions………………………………………………………………...288
7.5 References……………………………………………………………….…290
Chapter 8 General conclusions and future work ………………………………293
8.1 General conclusions …………………………………………………….…293
8.2 Recommendation for future work …………………………………………297
8.3 References…………………………………………………………….....…297
Total Word Count: 65700

8. 8
List of tables
Table 3.1 Calculated molar absorptivity ε for all the modifiers studied………134
Table 3.2 Conditions used for the preparation of graphene dispersions……….147
Table 4.1 Initial adsorption rate of the Fmoc-AAs on CNT arrays……………170
Table 4.2 Calculated adsorption capacity (k) and intensity (n) for Fmoc-AAs
adsorbed on CNT arrays. Note that the units for k depend on the value of n. The
quality of fit, R2
, was also given for each Fmoc-AA…………………………..173
Table 5.1 Measured SiO2 coating thickness based on TEM images…………...187
Table 5.2 Correlation of the adsorption equilibrium of the Fmoc-AAs on CNT
mats with the morphology of the hybrids ……………………………………..189
Table 5.3 Measured thickness of the TiO2 coating based on the TEM
observation……………………………………………………………………..201
Table 5.4 Measured inner diameter of the synthesized TiO2 NTs……………...205
Table 5.5 Measured wall thickness of the synthesized TiO2 NTs……………...205
Table 5.6 Measured inner diameter and wall thickness of the resultant TiO2
NTs …………………………………………………………………….………218
Table 7.1 Measured concentrations of graphene dispersions produced with
various sonication time and centrifuge speed …………………………………262

9. 9
List of figures
Figure 2.1 Schematic representation of sol-gel process of synthesis of
nanomaterials 7
…………………………………………………………………..41
Figure 2.2 The structures of (a) SWNTs and (b) MWNTs 15
……………………44
Figure 2.3 Schematic representation of a 2D graphene sheet with the lattice
vectors a1 and a2 and the roll-up vector Ch=na1+ma2. 18
…….………………….45
Figure 2.4 1,3-dipolar cycloaddition of an aminoethylene glycol linker to the
external surface of CNTs and the derivatization with N-protected glycine was
then obtained via amidation reaction. 85
................................................................49
Figure 2.5 Fabrication of a glucose biosensor based on CNT nanoelectrode
ensembles 89
……………………………………………………………………..50
Figure 2.6 Amine groups on a protein react with the anchored succinimidyl ester
to form amide bonds for protein immobilization.62
…………………………….52
Figure 2.7(a) TEM micrographs of MWNTs dispersed with Fmoc-W (trp).
Arrows indicate the edge of the lattice structure upon which Fmoc-W aggregates
are apparent; (b) Optimized structures of (i) Fmoc-G (gly) and (ii) Fmoc-W
bound to [6,6] SWNTs with close-up images that highlight the orientation and
arrangement of Fmoc and the aromatic W ring 47
……………………………….53
Figure 2.8 (i) SEM images of nano-1/SWNT fibres formed from a 100 μM
peptide/nanotube dispersion upon addition of no salt (A), 40 mM NaCl (B), and
120 mM NaCl (C). (ii) (A) SEM image of fibres formed from the addition of
0.0015% (by volume) DMF to a nano-1/SWNT dispersion. (B) Low-resolution

10. 10
TEM image of the same fibres observed in i(A). The small dark spheres are Fe
catalyst particles from the HiPco SWNT synthesis. (C) High-resolution TEM
image of the same fibres showing alignment of nanotubes. The large dark areas
are Fe particles 49
. ………………………………………………………………54
Figure 2.9 Proposed mechanism of nanotube isolation from bundle 120
…..……56
Figure 2.10 Schematic representations of the mechanisms by which surfactants
help disperse SWNTs. 101
…………………………………………………..……57
Figure 2.11. Scheme for the preparation of CNT–silica nanohybrids.196
……..…62
Figure 2.12 Scheme of the reaction between MWCNT-OH and AEAPS for the
following synthesis of silica coated MWCNTs. 197
……………………..……….63
Figure 2.13 Mechanism of photocatalysis on the surface of TiO2 in presence of
UV radiation. 216
…………………………………………………………………64
Figure 2.14 Schematic representation of a dye-sensitized solar cell based on
particulate TiO2. 217
……………………………………………………….……..65
Figure 2.15 Schematic representation of the electron path through a (a) percolated
and (b) oriented nanostructure. 220
…………………………….………………..66
Figure 2.16 Electron transport across nanostructured semiconductor films: (A) in
the absence and (B) in the presence of CNTs support. 222
……………..………..67
Figure 2.17 Left: Scheme of the beneficial role of benzyl alcohol in the in situ
coating of pristine CNTs with TiO2. One possible conformation of two BA molecules
on the CNT surface is shown in Scheme. Right: SEM images of TiO2 on CNTs after
conversion from anatase to rutile: A) no BA and B) with BA.172
…………………..68

11. 11
Figure 2.18 (a) Primary structures of the K2 and (QL)6 series of peptides showing
the comparative domain size. (b) Proposed model of nanofibre self-assembly
indicating hydrophobic packing region, axis of hydrogen bonding, and repulsive
positive charges. 271
…………………………………………………………..…74
Figure 2.19 Computer modelling of the designed self-assembling fibre 274
….…75
Figure 2.20 Potential pathway of V6D peptide nanotube formation.279
…………77
Figure 2.21 (A) chemical structure of a PA which includes three distinct regions:
a hydrophobic alkyl tail, a glycine containing region, and a charged head group.
(B) Three-dimensional representation of the regions within the PA nanofibre.
Region (a) is the hydrophobic core composed of aliphatic tails. Region (b) is the
critical β-sheet hydrogen bonding portion of the peptide. Region (c) is the
peripheral peptide region which is not constrained to a particular hydrogen
bonding motif and forms the interface with the environment. 282
………………79
Figure 2.22 Some of the possible modes of π-π interactions that contribute to the
emissions in the gel phase. 289
………………………………………………..…80
Figure 2.23 (A) A model structure was created of Fmoc-FF peptides arranged into an
anti-parallel β-sheet pattern (i) which then come together through π–π interactions
between the Fmoc groups (in orange) (ii) like a zipper to create a cylindrical structure
(iii & iv) (B) TEM image of the Fmoc-FF hydrogels composed of flat ribbons made
up of side-by-side packing of the fibrils. 292
…………………………………….....81
Figure 2.24 (A) Suspension of Fmoc-Leu2-OMe and inversion of glass vial
demonstrates self-supporting gel formation of Fmoc-Leu2 after ester hydrolysis
using subtilisin (Entry 1). (B) Proposed mechanism of Fmoc-peptide ester
hydrolysis that self-assembles to form higher-order aggregates through π–π
interlocked β-sheets. 305
…………………………………………………………83

12. 12
Figure 2.25 Solutions of Fmoc-Thr-OH and Leu-OMe. The inversion of the glass
vial demonstrates self-supporting gel formation of Fmoc-Thr-Leu-OMe via
reversed hydrolysis by thermolysin (entry 6). 305
……………………………….83
Figure 2.26 (i) Chemical structure of Nap-FFGEY. (ii) Reversible modification of
the peptide gelator by a phosphatase/kinase reaction. (iii) Optical images of (A)
gel formed initially (B) the solution obtained after adding a kinase to A (C) gel
restored after adding a phosphatase to B. 306
…………………………………….84
Figure 2.27 Proposed mechanism of silicon ethoxide condensation catalyzed by
silicatein α. 316
…………………………………………………………………...87
Figure 2.28 Proposed condensation reaction between silicic acid and serine on the
protein template of the silicalemma. Water by-product may be eliminated or
structurally incorporated into the forming frustule through hydrogen bonding with
the oxygens of silica. 318
………………………………………………………..88
Figure 2.29 Schematic of the interaction between two GNPs (B,C) capped with
imidazole and hydroxyl functionalities (A). (D) TEM image of silica product with
entrapped GNPs. Selected area electron diffraction (inset) indicating amorphous
nature of silica. 337
……………………………………………………………….91
Figure 2.30 Mother of all graphitic forms. Graphene is a 2D building material for
carbon materials of other dimensionalities.338
…………………….…….………92
Figure 2.31 Preparation of graphene by chemical reduction of GO synthesized by
Hummers’ method. …………………………………………………..…………97
Figure 2.32 Schematic model of a GO sheet, with -COOH hanging on the edge
and -O- and –OH decorate the basal plane. 388
………………………………….98

13. 13
Figure 2.33 Schematic representation of as-produced GO: large oxidatively
functionalized graphene-like sheets with surface-bound debris. Note that the
graphene-like sheets extend further than depicted. 394
……………..…………….98
Figure 2.34 TiO2-graphene composite and its response under UV-excitation.427
…104
Figure 3.1 Schematic diagram showing the set-up for the CVD synthesis of
aligned CNT arrays………………………………………………………….…128
Figure 3.2 (a) Molecular structures of the modifiers studied. (b) Scheme
illustrating the UV-Vis measurement of the adsorption of the surfactant on (c)
aligned CNT arrays (side-view) and (d) randomly aligned CNT networks……130
Figure 3.3 Calibration curves of all the modifiers studied.……………………131
Figure 3.4 Schematic illustration of the competitive binding from the library
solution of Fmoc-AAs on graphite……………………………….…………….137
Figure 3.5 Molecular structure of THEOS……………………..………………145
Figure 3.6 Schematic diagram of a TEM. 11
………………………..…………151
Figure 3.7 Ray path in a TEM operating in (a) image mode (b) diffraction mode.
12
………………………………………………………………………………..152
Figure 3.8 Schematic representation of reversed-phase HPLC. The most
hydrophilic components (orange) elute from the column first, followed by the less
hydrophilic components (green), and finally the most hydrophobic components
(blue). 13
……………………………………………………………….………..154

14. 14
Figure 3.9 Schematic of a liquid drop on a solid surface, where the
solid–vapor interfacial energy is denoted by γsv, the solid–liquid interfacial
energy is denoted by γsl, and the liquid–vapor interfacial energy is denoted by
γlv. 14
………………………………………………………………….………..155
Figure 3.10 Sessile drop method for determining the contact angle. The fitted
contour is shown in green. 15
………………………………..…………………156
Figure 3.11 (a) Typical Raman spectra for bulk graphite and monolayer graphene
obtained using a 514 nm laser. (b) Comparison of the D band at 514 nm at the
edge of bulk graphite and monolayer graphene. The fit of D1 and D2 components
of the D band of bulk graphite is shown. 18
……………………………………157
Figure 3.12 Measured 2D band for (a) monolayer, (b) bilayer, (c) trilayer, (d)
four-layer and (e) HOPG using a 514 nm laser. 20
……………………………..158
Figure 3.13 Schematic diagram of the beam deflection system in an atomic force
microscope, using laser and photodetector to measure the beam position. 25
…160
Figure 4.1 SEM images of CNT arrays grown at 760 ºC from a 5wt% ferrocene in
toluene solution on SiO2 substrate for 1h. (a) Cross-sectional image of the aligned
CNT arrays. (b) Close-up view of the CNTs from the arrays. (c) TEM image of
the pristine CNTs with dark particles presented both in the hollow cavity and the
walls of CNTs (indicated by arrows). Scale bar, 0.2 μm. (d) HRTEM image
showing the multilayered structure of a synthesized CNT with the lattice fringes
clearly visible. Scale bar, 5 nm. (e) The corresponding SAED pattern was indexed
to the (002), (100) and (004) planes of MWNTs……..…………..…………….165
Figure 4.2 Adsorption profiles of (a) Fmoc-Trp (c) Fmoc-Phe (e) Fmoc-Tyr (g)
Fmoc-His (i) Fmoc-Gly and (l) BA on aligned CNT arrays. (b,d,f,h,j)
Determination of the initial adsorption rate of the corresponding modifiers on the

15. 15
arrays. (k) Histogram showing the equilibrium loadings of the Fmoc-AAs on the
arrays.…………………………………………...……………………...………168
Figure 4.3 Adsorption profile of Fmoc-Trp on randomly aligned CNT
networks………………………………………………………………………..170
Figure 4.4 Desorption profiles of (a) Fmoc-Trp and (b) Fmoc-Phe from CNT
arrays in water…………………………………………………………………171
Figure 4.5 Plot of ln Q vs. ln C for the adsorption of Fmoc-Trp (red circles) and
Fmoc-Gly (blue triangles) on the arrays……………………………………….173
Figure 4.6 HPLC chromatogram of 0.4 mM of (a) Fmoc-Phe (b) Fmoc-Trp (c)
Fmoc-Tyr (d) Fmoc-Gly and (e) Fmoc-His. (f) The mixture of the 5 Fmoc-AAs
with the same volume ratio……………………………………………………175
Figure 4.7 (a) HPLC traces of the mixture consisting of the five Fmoc-AAs at 0 h
(upper) and after 173 h of competitive binding (lower). (b) Comparison of the
equilibrium loadings of the five Fmoc-AAs on graphite in individual adsorption
and competitive binding experiments………………………………..…………175
Figure 4.8 Displacement of Fmoc-Gly by Fmoc-Trp on HOPG surface………178
Figure 5.1 SEM images of (a) the product obtained from the control experiment
in which pristine CNTs were used as templates. (b) Silica coated Fmoc-Trp and (c)
Fmoc-His functionalized CNTs. (d) A mixture of partially coated and uncoated
CNTs in the presence of Fmoc-Tyr after reaction for 21 days. (e) EDX spectrum
of the product shown in (c). Note that the aluminum and some of the oxygen were
from the sample stub………………………………....………………………...183
Figure 5.2 TEM images of (a) pristine CNTs co-existed with isolated SiO2

16. 16
particles. Note. The image was over-focused as it was taken during early stage of
the PhD. Silica coated Fmoc-Trp functionalized CNTs after reaction for (b) 3
days and (c) 21 days. Silica coated Fmoc-His functionalized CNTs after reaction
for (d) 3 days and (e) 21 days. Partially coated Fmoc-Tyr functionalized CNTs
after reaction for (f) 3 days and (g) 21 days. Scale bar, (a) 100nm, (b) 20nm,
(c)-(g) 50nm……….……..……………………………………………….……186
Figure 5.3 (a) Line profile taken perpendicular to the tube axis direction. Inset:
Dark field STEM image of the hybrid NT. The direction of the scan was marked
by the arrow. The analysis was conducted with the help of Xiaofeng Zhao. (b)
Cross sectional view of a SiO2 coated CNT. The interaction of electron beam
with the edge and the centre of the hybrid tube was indicated by the red and
yellow line respectively. Blue colour: silica coating…………….…………..…188
Figure 5.4 Proposed catalytic mechanisms for silica templating………………190
Figure 5.5 SEM images of silica coated Fmoc-His functionalized CNTs obtained
after a growth time of (a) 3 days (b) 7 days and (c) 21 days. (d) Plot of the
diameter of the hybrid NT against the growth time. The average value was
calculated based on 50 separate measurements..………………..…………..…192
Figure 5.6 TEM images of silica coated Fmoc-Trp functionalized CNTs (a) before
and (b) after annealing at 200°C, and silica coated Fmoc-His functionalized CNTs
(c) before and (d) after annealing under the same
condition……………………………………………….………………………193
Figure 5.7 SEM images of (a) the product obtained using pristine CNTs as
templates. TiO2 coated CNTs in the presence of (b) Fmoc-Trp (c) Fmoc-His (d)
Fmoc-Tyr and (e) BA. (f-h) EDX spectra measured for the hybrids shown in (b-d).
Note the Al signal was originated from SEM stub, and Pt signal was originated
from the conductive coating on the SEM sample to reduce charging effect. The

17. 17
considerably stronger C signal in (h) was due to the application of a thin layer of
carbon on the SEM sample as the conductive coating………….……..………195
Figure 5.8 TEM images of (a) the product obtained using pristine CNTs as
templates. TiO2 coated CNTs in the presence of Fmoc-Trp with the CNT
concentration of (b) 30 wt% and (c) 12 wt%. TiO2 coated CNTs in the presence
of Fmoc-His with the CNT concentration of (d) 30 wt% and (e) 12 wt%. A
cluster of TiO2 nanoparticles were deposited on the smooth surface of the TiO2
coating in (e). TiO2 coated CNTs in the presence of Fmoc-Tyr with the CNT
concentration of (f) 30 wt% and (g) 12 wt%. TiO2 coated CNTs in the presence of
BA with the CNT concentration of (h) 30 wt% and (i) 12 wt%. The arrows
indicated the uncoated part of CNTs. Note. This was different from the cracks
resulting from the drying effect. (j) SAED pattern taken from the sample shown
in (f). (k) XRD pattern of the as-produced CNT-TiO2 nanohybrids. C: CNT. For
(c), (e), (g) and (i), scale bar = 200 nm. For (a), (b), (d), (f) and (h), scale bar =
100 nm.….……………………………………………...………………………199
Figure 5.9 SEM images of the structures produced with the addition of (a)
undiluted and (b) diluted Fmoc-His solutions (by a factor of 10)…………….202
Figure 5.10 SEM images of TiO2 coating growing on Fmoc-Trp functionalized
CNTs at different reaction times of (a) 10 min (b) 1 h and (c) 6.5 h. (d) Plot of the
diameter of the hybrid NT against the growth time. The average value was
calculated based on 50 separate measurements……..……….…………………203
Figure 5.11 SEM images of TiO2 nanotubes produced from (a) TiO2 coated
Fmoc-His functionalized CNTs (30 wt%) and (b) TiO2 coated Fmoc-Tyr
functionalized CNTs (12 wt%). (c) EDX spectrum of the hybrid after calcination
at 550 ºC. Note. Pt signal was originated from the conductive coating on the SEM
sample. Scale bar, (a) 500nm, (b) 1μm…...……………………………………204

18. 18
Figure 5.12 TEM images of the calcined hybrids. (a) In the presence of Fmoc-Trp
and 30wt% of CNTs. (b) In the presence of Fmoc-Trp and 12wt% of CNTs. (c) In
the presence of Fmoc-His and 30wt% of CNTs. (d) In the presence of Fmoc-His
and 12wt% of CNTs. (e) In the presence of Fmoc-Tyr and 30wt% of CNTs. (f) In
the presence of Fmoc-Tyr and 12wt% of CNTs. (g) In the presence of BA and
30wt% of CNTs. (h) In the presence of BA and 12wt% of CNTs. (i-l) SAED
patterns taken from the samples shown in (b-d) and (f) respectively (upper half)
which confirmed the polycrystalline anatase phase of the NTs by showing
excellent agreement with those simulated from JCPDS 21-1272 (lower half). The
SAED patterns were indexed to the (101), (004), (200) and (211) planes of
anatase phase. (m) XRD pattern taken from the sample shown in (d). A: anatase.
For (a), (e) and (g), scale bar = 100 nm and for (b), (c), (d), (f) and (h), scale bar =
200 nm……...………………………………………………………………….208
Figure 5.13 HRTEM image of a synthesized TiO2 NT showing the lattice spacing
of 0.35 nm, corresponding to the (101) crystal planes of anatase. Scale bar,
10nm……………………………………………………………………………210
Figure 5.14 (a) TEM images of the hybrids after heat treatment in Ar at 900 ºC
followed by in air at 550 ºC with the ramp rate of 20 ºC/min. Scale bar, 20 nm. (b)
XRD pattern taken from the sample shown in (a). (c) TEM image of the hybrids
after heat treatment in Ar at 800 ºC followed by in air at 550 ºC with the ramp
rate of 20 ºC/min. Scale bar, 100 nm. (d) SAED pattern (upper half) taken from
the sample shown in (c). The pattern was indexed to the (101), (004), (200) and
(211) planes of anatase phase. (e) TEM image of the hybrids after heat treatment
in air at 400 ºC followed by in Ar at 800 ºC with a ramp rate of 20 ºC/min. Scale
bar, 100 nm. (f) XRD pattern taken from the sample shown in (e). A: anatase, R:
rutile, C: CNT. (g) TEM image of the hybrids after heat treatment in air at 400 ºC
followed by in Ar at 800 ºC with a ramp rate of 1 ºC/min. Scale bar, 200 nm. (h)
SAED pattern (upper half) taken from the sample shown in (g). The SAED

19. 19
pattern was indexed to the (110), (111), (210), (211) and (220) planes of rutile
phase. ……………………………………………….………………..…….…..213
Figure 5.15 SEM images of (a) the product obtained from the control experiment
where as-produced CNT mat was used as templates. TiO2 NT arrays produced in
the presence of (b) Fmoc-Trp (c) Fmoc-His (d) Fmoc-Tyr and (e) BA……….216
Figure 5.16 TEM images of (a) the product obtained from the control experiment.
TiO2 NTs produced in the presence of (b) Fmoc-Trp (c) Fmoc-His and (d)
Fmoc-Tyr. (e) Collapsed NT structures obtained in the presence of BA. The red
arrow in (b) and (c) indicated the open ends of the TiO2 NTs. Note. CNT
templates were not completely removed after calcination as indicated by the black
arrows in (c). (f) XRD pattern taken from the sample shown in (b). Scale bar, (a-e)
200 nm………………………………………………………………………….217
Figure 5.17 SEM images showing (a) bundled fibers and (b) spherical aggregates
formed in the combined solutions. (c,d) Magnified images of the aggregates
shown in (a) and (b) respectively. (e) Fmoc-His f-CNTs and (f) Fmoc-Tyr
f-CNTs…………………………………………………………………………219
Figure 5.18 (a,b) SEM image of silica coated combined catalyst after heat
treatment. (c) EDX spectrum of the sample shown in (a)……………...………221
Figure 5.19 SEM images of (a) TiO2 nanorods coated CNT bundles (b) TiO2
nanorods coated individual CNTs (c) TiO2 nanorods coated CNT bundles after
heat treatment and (d) TiO2 particles formed on Si wafer. (e) and (f) EDX
spectrum of the sample shown in (a) and (c) respectively…………….……….222
Figure 6.1 (a) Schematic representation of the enzymatic dephosphorylation of
Fmoc-Y(p)-OH to Fmoc-Y. The corresponding optical images for Fmoc-Y(p)-OH
precursor solution before enzyme addition and the self-supporting hydrogels

20. 20
formed were also shown. (b) Negatively stained TEM image of the diluted
Fmoc-Y hydrogel. (c,d) Negatively stained TEM image of the undiluted
hydrogel.. …………………………………………………………..…………..228
Figure 6.2 (a) AP catalyzed dephosphorylation reaction of Fmoc-FpY and a
schematic representation of the supramolecular transition from micelles to fibres2
.
(b) Negative stained TEM image showing the Fmoc-FY self-assembled
nanofibrils. Scale bar, 100 nm. (c) HPLC trace of the conversion of Fmoc-FpY to
Fmoc- FY as a function of time. The gelation point is marked with an arrow. (d)
Fluorescence emission spectra of the solution of Fmoc-FpY and the hydrogel of
Fmoc-FY………………………………………………………………………230
Figure 6.3 TEM images of silica coating on Fmoc-Y self-assembled
nanostructures after reaction for (a) 1 h, (b) 2 h, (c,d) 4 h and (e) 5 h. Scale bar,
100 nm. (f) EDX spectrum of the mineralized peptide nanofibrils. (f) EDX
spectrum of the silicified fibrils…………………………………………...……232
Figure 6.4 Silicification process of Fmoc-Y hydrogel…………………………234
Figure 6.5 SEM analysis on (a) the upper aqueous phase and (c) the lower
hydrogel phase. (b) EDX spectrum of (a)……………..……………………….236
Figure 6.6 Unstained TEM images of (a) the network of silicified hydrogel
nanofibrils that were derived from the resulting clear gel. Scale bar, 100 nm. (b)
Fmoc-Y self-assembled hydrogel. Scale bar, 200 nm …………………………238
Figure 7.1 (a) SEM image of aggregated GO sheets. (b) TEM image of single
layer GO sheet with folds present at both sides (indicated by arrows). Scale bar,
100 nm. (c) Corresponding SAED pattern taken from the region marked by the
dashed box in (b). The pattern was labeled with Miller-Bravais indices. (d)
Intensity profile plot along the line between the arrows shown in (c). (e) Lower

21. 21
magnification TEM image of GO sheets with the folds indicated by arrows. Scale
bar, 200 nm. (f) Corresponding SAED pattern taken from the region marked by
the dashed box in (e) showing three superimposed hexagonal patterns indicated
by yellow, red and blue colors………………………………………………….241
Figure 7.2 (a) TEM image of GO-TiO2 nanohybrids produced with lower TBOT
concentration for 4 h. Inset corresponds to the SAED pattern taken from the
region marked by the red dashed box. (b) A magnified image of the region shown
in the orange dashed box in (a). (c) EDX spectrum of (a). Note that Cu signal is
originated from the TEM grid. (d) TEM image of the hybrids produced with
lower TBOT concentration for 7 d. (e) TEM image of the hybrids produced with
higher TBOT concentration for 4 h. (f) Corresponding SAED pattern taken from
the region marked by the dashed box in (e) and the diffraction spots are labeled
using Miller-Bravais indices. (g) Intensity profile plot along the line between the
arrows shown in (f).…………………………………………………………….244
Figure 7.3 TEM images of the thermally treated nanohybrids obtained from (a)
the reaction with lower TBOT concentration for 4h and (b) the reaction with
higher TBOT concentration for 4h. The inset in (a) and (b) showed the
corresponding SAED patterns which were indexed to (c,e) GO (labeled using
Miller (hkl) indices) and (d,f) anatase TiO2 respectively. Note that the upper half
in (c)-(f) showed the experimental data while the lower half in (c) and (e) showed
the diffraction pattern of GO, and that in (d) and (f) showed the simulated
diffractions for anatase according to JCPDS 21-1272…………………………246
Figure 7.4 SEM images of (a) highly aggregated GO sheets. (b) GO-SiO2
nanohybrids with layered structure (indicated by arrows along the edges). (c)
Higher magnification image showing the partial separation of two hybrid sheets.
(d) EDX spectrum of the sample shown in (b)…………………………………248

22. 22
Figure 7.5 (a) Low magnification TEM image of silica coated GO sheets. (b) A
magnified TEM image showing the ripples present on the GO sheet (indicated by
the arrow). The SAED pattern taken from the region marked by the dashed box
was labeled using Miller-Bravais indices………………………………………249
Figure 7.6 (a) TEM image of porous silica sheets obtained from the calcination of
GO-SiO2 hybrids. (b) Corresponding SAED pattern taken from the sample shown
in (a)……………………………………………………………………………250
Figure 7.7 Photographs of the aqueous dispersion of bwGO (a) in the absence and
(b) in the presence of Fmoc-Trp. The dispersions were allowed to stand for 35
days…………………………………………………………………………….252
Figure 7.8 (a) TEM image of bwGO sheets deposited from the dispersion in
Fmoc-Trp solution. (b) The corresponding SAED pattern taken from the sample
shown in (a). The pattern was labeled using Miller (hkl) indices……………...253
Figure 7.9 TEM image of bwGO-TiO2 nanohybrids prepared in aqueous solution.
The arrows indicate the wrinkles present in bwGO sheets. (b) The corresponding
SAED pattern taken from the sample shown in (a)……………………………254
Figure 7.10 Raman spectra for (a) bwGO deposited from the dispersion in
Fmoc-Trp solution (b) anatase TiO2 and (c) annealed bwGO-TiO2 nanohybrids
prepared in aqueous solution. The spectra were taken using a 633 nm HeNe laser.
Note that the peak at around 520 cm-1
was attributed to the SiO2/Si
substrate…………………………………………………………………….…..255
Figure 7.11 (a) SEM image of bwGO-TiO2 nanohybrids prepared in EtOH. (b)
EDX spectrum. Pt signal is originated from Pt coating on the SEM sample to
reduce charging effect……….…………………………………………………256

23. 23
Figure 7.12 (a) TEM image of bwGO-TiO2 nanohybrids prepared in EtOH with
the addition of H2O. (b) Corresponding SAED pattern taken from the region
marked by the dashed box in (a). (c) TEM image of bwGO-TiO2 nanohybrids
prepared in EtOH with the addition of Fmoc-Trp solution. (d) Corresponding
SAED pattern taken from the region marked by the dashed box in (c). The pattern
was labeled using Miller-Bravais indices. (e) Intensity profile plot along the line
between the arrows shown in (d)………………………………………………257
Figure 7.13 Schematic illustration of the synthesis of bwGO-TiO2 nanohybrids in
(a) EtOH and (b) aqueous solution…………………………………………..…258
Figure 7.14 Raman spectra for (a) bwGO deposited from the dispersion in
Fmoc-Trp solution (b) anatase TiO2 and (c) annealed bwGO-TiO2 nanohybrids
prepared in EtOH. The spectra were taken using a 633 nm HeNe laser………260
Figure 7.15 Digital images of the graphene dispersions prepared under various
conditions………………………………………………………………………261
Figure 7.16 Raman spectra for (a) the starting graphite powder and the flakes
deposited from the dispersions prepared with (b) 1 h (c) 6 h and (d) 12 h of
sonication followed by centrifugation at 3000 rpm respectively. The spectra were
measured on SiO2/Si substrate and in all cases the excitation wavelength was
633nm. D, G, 2D and D’ bands are indicated in the Figure. All the spectra were
normalized to have the similar G band intensity and offset for
clarity…………………………………….…………………………….……….264
Figure 7.17 Raman spectra for (a) the starting graphite powder and the flakes
deposited from the dispersions prepared with centrifugation at (b) 500 rpm (c)
3000 rpm and (d) 6000 rpm following 6 h of sonication respectively. The spectra
were measured on SiO2/Si substrate and in all cases the excitation wavelength
was 633 nm. D, G, 2D and D’ bands are indicated in the Figure. All the spectra

24. 24
were normalized to have the similar G band intensity and offset for
clarity………………………………………………………………………...…265
Figure 7.18 Histograms and normal distribution of the 2D band position for
varying sonication time and centrifuge speed…………………………………266
Figure 7.19 Mean 2D band position as a function of (a) sonication time and (b)
centrifuge speed. The data for the starting graphite powder was also shown for
comparison……………………………………………………………………267
Figure 7.20 Histograms and normal distribution of the 2D bandwidth for varying
sonication time and centrifuge speed…………………………………………267
Figure 7.21 Mean 2D bandwidth as a function of (a) sonication time and (b)
centrifuge speed………………………………………………………….……268
Figure 7.22 Histograms and normal distribution of I2D/IG ratio for varying
sonication time and centrifuge speed…………………………………………269
Figure 7.23 Mean I2D/IG ratio as a function of (a) sonication time and (b)
centrifuge speed………………………………………………………………270
Figure 7.24 Histograms and normal distribution of ID/IG ratio for varying
sonication time and centrifuge speed…………………………………………271
Figure 7.25 Mean ID/IG ratio as a function of (a) sonication time and (b)
centrifuge speed. The ratio for the starting graphite was also shown for
comparison…………………………………………………………………….272
Figure 7.26 Plot of ID/IG ratio against 2D band position for varying sonication
time and centrifuge speed. The data for the starting graphite was also shown for

25. 25
comparison. The direction of the arrow corresponds to flakes of fewer layer and
smaller size……………………………………………………………………273
Figure 7.27 Representative TEM images of graphene flakes deposited from the
dispersions prepared with various sonication time and centrifugation speed….274
Figure 7.28 (a-e) Histograms and normal distribution of the flake area for varying
sonication time and centrifuge speed. (f) Mean flake area as a function of
sonication time. (g) Mean flake area as a function of centrifuge speed………277
Figure 7.29 AFM characterization of the exfoliated flakes……………………281
Figure 7.30 TEM images of the EG-TiO2 nanocomposites……………………285
Figure 7.31 TEM images of EG-TiO2 nanohybrids prepared in EtOH………..287

26. 26
List of abbreviations
1D 1- dimensional
2D 2-dimensional
3D 3- dimensional
AFM Atomic force microscopy
Al Aluminum
Ala or A Alanine
ALD Atomic layer deposition
AP Alkaline phosphatase
APTES 3-aminopropyltriethoxyysilane
Asn or N Asparagine
Asp or D Aspartic acid
BA Benzyl alcohol
BSA Bovine serum albumin
bwGO Base-washed graphene oxide
C Carbon
CCG Chemically converted graphene
CD Circular dichroism
CMGs Chemically modified graphenes
CNT Carbon nanotube
Cu Copper
CVD Chemical vapour deposition
Cys or C Cysteine
D2O Deuterium oxide
dH2O Deionized H2O
DLS Dynamic light scattering
DMF Dimethylformamide
EDX Energy dispersive x-ray
EG Exfoliated graphene
EtOH Ethanol
FEGSEM Field emitter gun scanning electron microscope
FGSs Functionalized graphene sheets
Fmoc-AA N-(fluorenyl-9-methoxycarbonyl) terminated amino acid
Fmoc-FY Fmoc-Phenylalanine-Tyrosine
Fmoc-FpY Fmoc-Phenylalanine-Tyrosine (phosphate)
Fmoc-Y Fmoc-Tyrosine
Fmoc-Y(p)-OH Fmoc-Tyrosine (phosphate)-OH
FT-IR Fourier transform infrared spectroscopy
FWHM Full width at half maximum
Glu or E Glutamic acid
Gly or G Glycine
GNPs Golden nanoparticles
GO Graphene oxide

27. 27
GS Graphene sheets
HA Hydroxyapatite
HeNe Helium–neon
HiPco High-pressure decomposition of carbon oxide
His or H Histidine
HOPG Highly orientated pyrolytic graphite
HPLC High performance liquid chromatography
HRTEM High-resolution transmission electron microscopy
H-bonding Hydrogen bonding
iTO in-plane transverse optical
Leu or L Leucine
LO longitudinal optical
Lys or K Lysince
MWNTs Multi-walled nanotubes
NaOH Sodium hydroxide
NMP N-Methyl-2-Pyrrolidone
NMR Nuclear magnetic resonance
NT Nanotube
O Oxygen
OD Oxidative debris
PA Peptide amphiphile
PECS Precision Etching Coating System
Phe or F Phenylalanine
Pt Platinum
QDs Quantum dots
RGO Reduced graphene oxide
rpm Revolutions per minute
SAED Selected area electron diffraction
SAF Self-assembling fibre
SDS Sodium dodecyl sulfate
SDBS Sodium dodecyl benzene sulfonate
SEM Scanning electron microscopy
Ser or S Serine
Si Silicon
SiO2 Silicon dioxide
STEM Scanning transmission electron microscopy
SWNTs Single-walled nanotubes
TBOT Tetrabutyl titanate
TEM Transmission electron microscopy
TEOS Tetraethyl orthosilicate
THEOS Tetrakis (2-hydroxyethyl) orthosilicate
Thr or T Threonine
Ti Titanium
TiO2 Titanium dioxide
Trp or W Tryptophan
Tyr or Y Tyrosine

28. 28
UV-Vis Ultraviolet-visible light spectroscopy
Val or V Valine
wt% Weight%
XRD X-ray diffraction

29. 29
List of Symbols
A Absorbance
a1, a2 Lattice vectors of graphene sheet
b Path length
C Equilibrium concentration of the solute in solution
c Concentration
Ch Chiral vector
d Crystal size
I Intensity
I2D Raman intensity for 2D band
IA Integrated intensity of anatase (101) peak
ID Raman intensity for D band
IG Raman intensity for G band
IR Integrated intensity of rutile (110) peak
K Shape factor
k Adsorption capacity constant
ki Initial adsorption rate
n Adsorption intensity constant
(n, m) Indices defining the nanotube structure
Q Amount of the solute adsorbed per unit weight of the
adsorbent
R2
Correlation coefficient
S Surface area
T Translation vector
t Time
V Volume
WR Percentage of rutile
β Full width at half maximum intensity
ε Molar absorptivity
λ Wavelength
θ Angle

30. 30
Abstract
Silica and titania nanoparticles have been produced by using carbon nanotubes
(CNTs) and graphene as templates in a sol-gel reaction. A range of Fmoc
terminated amino-acids (Fmoc-AAs) were studied as surface modifiers to
encourage the templating on the nanocarbons. After annealing the deposited
structures, the carbon templates were either left in place to give hybrid structures
or oxidized to leave pure inorganic nanoparticles.
Absorption studies were initially conducted to identify Fmoc-AAs that would
bind well to the CNTs. Fmoc-Trp had the best affinity for CNTs out of the amino
acids studied. The fully reversible nature of the binding process was
demonstrated via the desorption of Fmoc-AAs from CNTs in water. The
equilibrium data were found to be well described by the Freundlich isotherm
model. The competitive binding from a library of Fmoc-AAs on graphite was
developed to efficiently identify the strongest binding candidate.
The synthesis of CNT-SiO2 and CNT-TiO2 nanohybrids were successfully
demonstrated. The morphology of the hybrids was found to be dependent on the
CNT:precursor and Fmoc-AA:CNT ratios. Fmoc-AAs were believed to play a
dual role: (1) electrostatically stabilizing the NT dispersion and (2) the
functionalities from the side chains of the amino acids providing binding sites for
SiO2 and TiO2 deposition. Uniform anatase nanotubes (NTs) were synthesized
after calcination of the CNT-TiO2 nanohybrids. Both the inner diameter and wall
thickness of the synthesized TiO2 NTs were controlled by the dimension of CNT
templates and the ratio of CNT:precursor. The transition from anatase to rutile
phase was found to be affected by heating temperature, pre-treatment and ramp
rate. A simple route towards the production of TiO2 NT arrays was also
demonstrated by using aligned CNT arrays as templates in the presence of the
Fmoc-AAs.
Graphene based nanohybrids were synthesized in the presence of graphene oxide
(GO), Fmoc-Trp stabilized base-washed graphene oxide (bwGO) and exfoliated
graphene via the sol-gel process. It was found that the morphology of the
products was highly dependent on the reaction media. Graphene dispersions were
prepared by direct exfoliation of graphite in Fmoc-Trp solution. Raman, TEM
and AFM analyses suggested the dispersion comprised of mainly few layer
graphene (<5 layers) with a broad size distribution and that the defects introduced
during sonication were predominately associated with the formation of new flake
edges due to sonication-induced cutting.
A preliminary study was conducted on the silicification of Fmoc-Y and Fmoc-FY
self-assembled hydrogels. The presence of a high density of –OH group on the
nanofibers’ surface was found to promote silica deposition.

31. 31
Declaration
No portion of the work referred to in the thesis has been submitted in support of
an application for another degree or qualification of this or any other university or
other institute of learning.

32. 32
Copyright Statement
i. The author of this thesis (including any appendices and/or schedules to this
thesis) owns certain copyright or related rights in it (the “Copyright”) and s/he
has given The University of Manchester certain rights to use such Copyright,
including for administrative purposes.
ii. Copies of this thesis, either in full or in extracts and whether in hard or
electronic copy, may be made only in accordance with the Copyright, Designs
and Patents Act 1988 (as amended) and regulations issued under it or, where
appropriate, in accordance with licensing agreements which the University has
from time to time. This page must form part of any such copies made.
iii. The ownership of certain Copyright, patents, designs, trade marks and other
intellectual property (the “Intellectual Property”) and any reproductions of
copyright works in the thesis, for example graphs and tables (“Reproductions”),
which may be described in this thesis, may not be owned by the author and may
be owned by third parties. Such Intellectual Property and Reproductions cannot
and must not be made available for use without the prior written permission of
the owner(s) of the relevant Intellectual Property and/or Reproductions.
iv. Further information on the conditions under which disclosure, publication and
commercialisation of this thesis, the Copyright and any Intellectual Property
and/or Reproductions described in it may take place is available in the University
IP Policy (see http://documents.manchester.ac.uk/DocuInfo.aspx?DocID=487), in
any relevant Thesis restriction declarations deposited in the University Library,
The University Library’s regulations (see
http://www.manchester.ac.uk/library/aboutus/regulations) and in The University’s
policy on Presentation of Theses

33. 33
Acknowledgements
The author would like to thank Prof. Ian Kinloch for his guidance and support
throughout. Further thanks go to Prof. Rein Ulijn who enabled the collaboration
and has offered guidance and encouragement at times of need.
Thanks also go to Chris, Polly, Alan, Gary, Andy, Xiaofeng for help with SEM,
UV-Vis, TEM, XRD, Raman, AFM and EDX linescan, Dr. Sarah Haigh for help
with TEM and discussion on diffraction pattern interpretation, Kate Thornton for
help with preparation of Fmoc-Tyr-OH hydrogel, Sangita Roy and Louise
Birchall for help with HPLC and fluorescence spectroscopy
Most of all, the author is indebted to her family for all their support and much
needed funding to complete the degree.

34. 34
Chapter 1 Introduction
1.1 Overview
Carbon nanotubes (CNTs) and more recently graphene have attracted
considerable interest owing to their unusual combination of electronic, thermal
and mechanical properties. Such remarkable properties have opened up a world of
possible applications, including photochemical, catalytic and electrochemical
technologies.
CNT-inorganic hybrid materials combine the physicochemical properties of
CNTs with the advantages of their inorganic components, leading to new
functionalities that do not exist in either building block 1,2
. For example, the
hybrid materials exhibit a significant synergistic effect through size domain
effects and charge transfer processes across the CNT-inorganic interface.
Dielectric materials such as SiO2 and TiO2 are of particular interest amongst the
inorganic compounds. Owing to the high biocompatibility, hydrophilic nature and
easy surface functionalization of SiO2, CNT-SiO2 hybrids have found an
extensive range of applications, such as in biotechnology 3-5
, nanoelectric devices
and reinforcement materials in composites 6-8
. TiO2 has been extensively studied
as a highly active semiconductor photocatalyst material for applications in solar
energy conversion9
, environmental purification10-20
and dye-sensitized solar
cells21-23
. The combination of CNTs with the well-established photoactivity of
TiO2 has increased the charge-transfer efficiency, which further enhances the
photocatalytic activity 24-26
.
Typically, inorganic compounds are coated onto CNTs using a sol-gel process
due to the mild reaction conditions (room temperate, near neutral pH etc.). The
morphology of the coating depends significantly on the surface chemistry of
CNTs, as the surface groups act as both catalysts and structural directors. Eder et
al. 27
employed benzyl alcohol as a surfactant to coat pristine CNTs with TiO2.
They assumed that the benzene ring of the surfactant adsorbed on CNT surface

35. 35
via - stacking interactions, while the hydroxyl groups activated the hydrolysis
of the titanium precursor. Based upon this assumption, there should be a family of
surface modifiers which could enhance the inorganic templating process on the
nanotubes. For example, N-(fluorenyl-9-methoxycarbonyl) terminated amino
acids (Fmoc-AAs) are cheap and have previously been shown by the research
group 28
to bind well to CNTs and the amino acid group gives 26 different
functional motifs to explore for the templating reaction. The importance of the
amino acids is highlighted by studies on the biomineralization process of
silicateins in a marine sponge 29
. The site-directed mutagenesis results have found
that both serine and histidine residues were required for the efficient catalysis of
the siloxane polymerization. Several synthetic counterparts have been developed.
For instance, peptide based self-assembled supramolecular structures have been
demonstrated to mimic the catalytic activity of silicateins for the templating of
silica 30, 31
.
Graphene-TiO2 hybrid materials showed improved photocatalytic activity
compared with CNT-TiO2 attributed to the higher dye adsorption capacity and
enhanced charge separation and transportation properties. However, for such
applications to be achieved, suitable routes for graphene manufacturer have to be
developed. High-quality graphene has been produced by liquid-phase exfoliation
which includes the reduction of exfoliated GO 32,33
and sonication-assisted direct
exfoliation of graphite in solution34-39
. Solvent exfoliation is particularly
attractive as it produces relatively defect-free graphene and can either be done in
a solvent such as NMP or in a surfactant solution. In particular for the latter, it
may be possible to select a surfactant that both enables exfoliation and can direct
templating.
1.2 Aims
Thus, this thesis initially aims to identify suitable Fmoc-AA surface modifiers for
CNTs and understand the adsorption process for these Fmoc-AA modifiers. The

36. 36
Fmoc-AA coated CNTs will then be used as templates in the sol-gel deposition of
silica and titania, and their performance compared to that of the published benzyl
alcohol. The Fmoc-AA functionalized nanotubes also allow an attempt at
mimicking the catalytic active site of silicatein. The identified successful
Fmoc-AA will then be used as a surfactant to exfoliate graphene from graphite
and as a surface modifier in the production of inorganic-graphene hybrids using a
range of graphene materials.
More explicitly, the thesis aims to
(1) Study the non-covalent functionalization of CNTs through the adsorption of a
library of aromatic Fmoc-AAs on both aligned CNT arrays and randomly
aligned CNT networks.
(2) Synthesize silica and titania based nanohybrids via an in-situ sol-gel process
employing the Fmoc-AA functionalized CNTs as templates. Herein,
Fmoc-Trp, Fmoc-His and Fmoc-Tyr which render the templates’ surface
with the functionalities that have been reported to catalyze silica and titania
deposition were investigated as surface modifiers. The surface modifier is
expected to serve two purposes: helps to colloidally stabilize the CNT
dispersion as well as to promote the deposition of silica and titania on CNTs.
The role of the surface chemistry of CNTs in controlling the coating
morphology was also investigated.
(3) Prepare graphene dispersion by direct exfoliation of graphite in Fmoc-AA
solution and subsequently produce graphene–TiO2 nanohybrids employing the
exfoliated flakes as templates via sol-gel process. The degree of exfoliation
and quality of the exfoliated flakes was characterized by Raman spectroscopy,
TEM and AFM.
(4) Conduct a preliminary study on the silicification of Fmoc-Y and Fmoc-FY

37. 37
self-assembled hydrogels. Both of the gels were prepared through an enzyme
catalyzed dephosphorylation. The presence of a high density of –OH group
on the nanofibers’ surface was expected to promote silica deposition.
1.3 References
1. Y. Zhang et al., Reinforcement of silica with single-walled carbon nanotubes
through covalent functionalization, J. Mater. Chem., 2006, 16, 4592.
2. M. Bottini et al., Non-destructive decoration of full-length multi-walled carbon
nanotubes with variable amounts of silica gel nanoparticles, Carbon, 2006, 44,
1301.
3. D. T. Mitchell et al., Smart Nanotubes for Bioseparations and Biocatalysis,
J. Am. Chem. Soc., 2002, 124, 11864.
4. S. Kapoor et al., Ultrasound-Triggered Controlled Drug Delivery and
Biosensing Using Silica Nanotubes, J. Phys. Chem. C, 2009,113, 7155.
5. C. C. Chen et al., Preparation of Fluorescent Silica Nanotubes and Their
Application in Gene Delivery, Adv.Mater., 2005, 17, 404.
6. M. Olek et al., Nanomechanical properties of silica-coated multiwall carbon
nanotubes-poly(methyl methacrylate) composites, Langmuir, 2005, 21, 3146.
7. J. Wang et al., Utilization of silane functionalized carbon nanotubes-silica
hybrids as novel reinforcing fillers for solution styrene butadiene rubber,
Polym. Compos., 2013, 34, 690.
8. W. Zhang et al., Polymer Nanocomposites Using Urchin-Shaped Carbon
Nanotube-Silica Hybrids as Reinforcing Fillers, Macromol. Rapid Commun.,
2004, 25, 1860.
9. S. Lee et al., Two-Step Sol-Gel Method-Based TiO2 Nanoparticles with
Uniform Morphology and Size for Efficient Photo-Energy Conversion Devices,
Chem. Mater., 2010, 22, 1958.
10. I. Sopyana et al., An efficient TiO2 thin-film photocatalyst: photocatalytic
properties in gas-phase acetaldehyde degradation, J. Photochem. Photobiol. A:
Chem., 1996, 98, 79.
11. Q. Li et al., Antimicrobial nanomaterials for water disinfection and microbial
control: potential applications and implications, Water Res., 2008, 42, 4591.
12. M. L. Sauer et al., Photocatalyzed Oxidation of Ethanol and Acetaldehyde in
Humidified Air, J. Catal., 1996, 158, 570.
13. T. N. Obee et al., TiO2 Photocatalysis for Indoor Air Applications: Effects of
Humidity and Trace Contaminant Levels on the Oxidation Rates of
Formaldehyde, Toluene, and 1,3-Butadiene, Environ. Sci. Technol., 1995, 29,
1223.
14. C.H. Aoa et al., Photodegradation of volatile organic compounds (VOCs) and
NO for indoor air purification using TiO2: promotion versus inhibition effect of
NO, Appl. Catal. B: Environ., 2003, 42, 119.
15. L. Petrov et al., Photocatalytic properties of modified TiO2 coatings for

38. 38
purification of waste water and air, J. Environ. Prot. Ecol., 2007, 8, 881.
17. I. Salema et al., Recent Studies on the Catalytic Activity of Titanium,
Zirconium, and Hafnium Oxides, Catal. Rev. Sci. Eng., 2003, 45, 205.
18. Z. Liu et al., Efficient Photocatalytic Degradation of Gaseous Acetaldehyde
by Highly Ordered TiO2 Nanotube Arrays, Environ. Sci. Technol., 2008, 42, 8547.
19. O. Carp et al., Photoinduced reactivity of titanium dioxide, Prog. Solid State
Chem., 2004, 32, 33.
20. T. Hirakawa et al., Mechanism of Photocatalytic Production of Active
Oxygens on Highly Crystalline TiO2 Particles by Means of Chemiluminescent
Probing and ESR Spectroscopy, J. Phys. Chem. B, 2001, 105, 6993.
21. M. Grätzel et al., A low-cost, high-efficiency solar cell based on
dye-sensitized colloidal TiO2 films, Nature, 1991, 353, 737.
22. M. Grätzel et al., Photoelectrochemical cells, Nature, 2001, 414, 338.
23. M. Adachi et al., Formation of Titania Nanotubes and Applications for
Dye-Sensitized Solar Cells, Journal of the Electrochemical Society, 2003, 150,
G488.
24. Y. Yu et al., Enhancement of photocatalytic activity of mesoporous TiO2 by
using carbon nanotubes, Appl. Catal. A: Gen., 2005, 289, 186.
25. W. Wang et al., Visible light photodegradation of phenol on
MWNT-TiO2 composite catalysts prepared by a modified sol–gel method, J. Mol.
Catal. A: Chem., 2005, 235, 194.
26. W. Wang et al., Photocatalytic degradation of phenol on MWNT and titania
composite catalysts prepared by a modified sol–gel method, Appl. Catal., B, 2005,
56, 305.
27. D. Eder et al., Carbon–Inorganic Hybrid Materials, Adv.Mater., 2008, 20,
1787.
28. B. G. Cousins et al., Enzyme-Activated Surfactants for Dispersion of Carbon
Nanotubes, Small, 2009, 5, 587.
29. Y. Zhou et al., Efficient catalysis of polysiloxane synthesis by silicatein a
requires specific hydroxyl and imidazole functionalities, Angew.Chem.Int.Ed.,
1999, 38, 780.
30. J. N. Cha et al., Biomimetic synthesis of ordered silica structures mediated by
block copolypeptides, Nature, 2000, 403, 289.
31. V. M. Yuwono et al., Peptide Amphiphile Nanofibers Template and Catalyze
Silica Nanotube Formation, Lagmuir, 2007, 23, 5033.
32. S. Stankovich et al., Synthesis of graphene-based nanosheets via chemical
reduction of exfoliated graphite oxide, Carbon, 2007, 45, 1558.
33. S. Dubin et al., A One-Step, Solvothermal Reduction Method for Producing
Reduced Graphene Oxide Dispersions in Organic Solvents, ACS Nano, 2010, 4,
3845.
34. J. N. Coleman et al., Liquid-Phase Exfoliation of Nanotubes and Graphene,
Adv. Funct.Mater., 2009, 19, 3680.
35. S. De et al., Flexible, transparent, conducting films of randomly stacked
graphene from surfactant-stabilized, oxide-free graphene dispersions, Small, 2010,
6, 458.

39. 39
36. J. N. Coleman et al., High-yield production of graphene by liquid-phase
exfoliation of graphite, Nature Nanotechnol., 2008, 3, 563.
37. U. Khan et al., High-concentration solvent exfoliation of graphene, Small,
2010, 6, 864.
38. M. Lotya et al., Liquid Phase Production of Graphene by Exfoliation of
Graphite in Surfactant/Water Solutions, J. Am. Chem. Soc., 2009, 131, 3611.
39. M. Lotya et al., High-concentration, surfactant-stabilized graphene
dispersions, ACS Nano, 2010, 4, 3155.

40. 40
Chapter 2 Literature review
2.1 Sol-gel chemistry
The sol–gel process is the most popular technique for the production of glasses
and ceramic materials due to its low reaction temperatures compared to melting
glass or firing ceramics. Also, as a wet-chemical technique, it has many
advantages over other conventional "powder" routes, including 1
: (1) The
rheological properties of sols and gels allow the production of various forms of
products including ultrafine powders, thin films, fibers and monoliths depending
on the processing conditions 2-4
. (2) Easy deposition of good quality coatings onto
a variety of substrates. (3) Better control over the whole process and the synthesis
of "tailor-made" materials. (4) Production of high-purity materials at mild
reaction conditions which are highly desired in some applications such as
bioencapsulation and sensors 5,6
.
The sol-gel process involves the formation of a colloidal suspension (sol) and the
transition of the liquid “sol” into a wet and continuous network (gel). Removal of
the liquid from the sol yields the gel, and the sol-gel transition controls the
particle size and shape. Calcination of the gel then produces the oxide. The gel
phase can be processed by various drying methods to develop materials with
distinct properties. Subsequent drying under supercritical conditions converts the
gel into a low-density, highly porous aerogel, while drying induced by heating
typically results in a xerogel (low temperature) or a dense ceramic (high
temperature) (Figure 2.1)7
.

41. 41
Figure 2.1 Schematic representation of sol-gel process of synthesis of nanomaterials
7
.
Two reactions are typically used in the sol-gel process: (1) there is an initial
hydrolysis reaction through which the alkoxide group (-OR) of the precursor is
replaced by the hydroxyl group (Equation 2.1). For example, the mechanism is
based on the nucleophilic attack to the central Si atom in silica production; (2)
this is then followed by water or alcohol condensation reactions (Equation 2.2
and 2.3), in which two hydrolyzed species (monomeric and polymeric silica
reacting units) link together to form siloxane bonds (Si-O-Si) with the elimination
of water or alcohol. Under most conditions, polycondensation commences before
hydrolysis is complete. However, conditions such as, pH, H2O/Si molar ratio, and
catalyst can force completion of hydrolysis before condensation begins 8
.
Additionally, because water and alkoxides are immiscible, a mutual solvent such
as an alcohol is utilized9
. With the presence of this homogenizing agent,
hydrolysis is facilitated due to the miscibility of the alkoxide and water.

42. 42
Typical sol-gel processes require strong acid or base for accelerating the
hydrolysis of the precursors. It is generally found that the alkaline conditions
usually favor the formation of “particulate” sols, whereas the acidic conditions
produce weakly branched “polymeric sols”. For example, the kinetics and
mechanism of silica-particle formation by the base-catalyzed hydrolysis of TEOS
in alcohol media have been studied extensively10
. It was found that the dilute
NaOH-catalyzed hydrolysis of TEOS had a first-order dependence on the
concentrations of both TEOS and hydroxyl ion (OH-
). While for
ammonia-catalyzed reaction (Stöber process), both the rates of silica-particle
growth and TEOS hydrolysis were first order with almost the same specific rate
constant, indicating that silica-particle growth was reaction-controlled by the
hydrolysis of TEOS.
Typical precursors for the sol-gel synthesis of oxide materials include metal
alkoxides and metal salts11,12
, among which the most versatile precursors are
undoubtedly alkoxides because they react readily with water. Alkoxide materials
consist usually of a metal or metalloid element surrounded by the reactive ligands.
Sol-gel methods using metal alkoxides usually produce fine and spherical oxide
particles of uniform size. However, the disadvantage of such water reactivity is
that tight control of the reaction conditions is required.
(2.2)
(2.1)
(2.3)

43. 43
The morphology and properties of a particular sol-gel inorganic network are
related to a number of factors that influence the rate of hydrolysis and
condensation reactions, such as, pH, temperature, reagent concentrations, alkyl
groups in the alkoxide, type of solvent, catalyst adopted and its concentration,
H2O/alkoxides molar ratio and drying 1, 9,13,14
. Among the factors listed above, pH,
nature and concentration of catalyst, H2O/Si molar ratio, and temperature have
been identified as the most important.
2.2 CNT-Inorganic nanohybrids
2.2.1 Introduction to Carbon Nanotubes
2.2.1.1 Structures
Carbon nanotubes (CNTs) are the 1D allotrope of carbon and are formed by
predominantly sp2
-bonded carbon atoms arranged in a honeycomb lattice. CNTs
are generally classified as either single-walled nanotubes (SWNTs) or
multi-walled nanotubes (MWNTs). A SWNT can be visualized as a single layer of
graphene sheet rolled into a seamless cylindrical tube with a diameter of 1–2 nm.
While a MWNT consists of several concentric and closed graphene tubules with
an overall diameter of ~10 to 100 nm and a length of up to centimeters. The
interlayer distance between the tubules is approximately 0.34 nm, similar to the
interlayer spacing in HOPG. Both types are displayed in Figure 2.2. CNTs can be
either open-ended or closed by a cap which in ideal models is described as a
hemispherical fullerene-type cap.

44. 44
Figure 2.2 The structures of (a) SWNTs and (b) MWNTs 15
.
2.2.1.2 Properties
Ever since their discovery in 1991 by Iijima 16
, CNTs have drawn considerable
research attentions in the field of nanoscience and nanotechnology owing to their
rich electrical properties 17
, high mechanical strength and excellent chemical and
thermal stability. CNTs possess high aspect ratio and large specific surface areas
attributed to their hollow geometry. While MWNTs are purely metallic, SWNTs
can be either metallic or semiconducting depending primarily on their diameter
and chirality.
The chirality is defined as the symmetry of a nanotube’s wall. A SWNT can be
considered as a rolled-up graphene sheet and is characterized by the way the
graphene sheet is conceptually rolled up to form it (Figure 2.3), i.e. the chiral
vector 18
:
Ch=na1+ma2=(n, m) (2.4)
Where Ch is the chiral vector, a1 and a2 are unit vectors, n and m are integers
denote the number of unit vectors along two directions in the crystal lattice of
graphene. The length of Ch determines the tube diameter and the angle between
Ch and the (n,0) lattice vector, the chiral angle θ, determines the chirality.
(a) (b)

45. 45
Tubes having n = m (θ= 30°) are called armchair NTs and those with m = 0 (θ= 0°)
are called zigzag NTs. Otherwise, they are called chiral NTs. Both armchair and
zigzag NTs have a high degree of symmetry. All the armchair tubes are metallic
and for zigzag and chiral NTs, when (n−m)/3 is an integer, the tubes are metallic
and otherwise semiconducting 19,20
.
The situation in MWNT is complicated as their properties are determined by
contribution of all individual shells having different chiralities. However, it has
been reported for small diameter MWNTs that only one concentric tube needs to
be metallic for the overall electronic properties to be essentially metallic 21
. In
large MWNTs quantum confinement is lost in the circumference.
Figure 2.3 Schematic representation of a 2D graphene sheet with the lattice vectors
a1 and a2 and the roll-up vector Ch=na1+ma2. The achiral cases, (n,0) zigzag and (n,
n) armchair are indicated with dashed lines. The translation vector T is along the
nanotube axis and defines the 1D unit cell. The shaded boxed area represents an
unrolled unit cell, defined by T and Ch. 18

46. 46
2.2.1.3 Synthesis
There are three main methods for nanotube synthesis; electric arc discharge22
,
laser ablation23
, and chemical vapour deposition (CVD) 24
. Although the former
two methods generally produce CNTs with fewer structural defects, they tend to
suffer from low yield issues and thus proves infeasible for mass production 25
. On
the other hand, CVD shows great promise for possible industrial scaled-up due to
the relatively low growth temperature, high yields and high purities of the
synthesized CNTs 26
. It is also capable of growing nanotubes directly onto the
desired substrate26
, whereas the nanotubes produced from the other routes must
be subsequently processed and deposited in the required morphology. CVD
technique allows the growth of aligned CNTs of various packing densities which
may be useful for applications such as electrodes. Positional control of growth
has been achieved by patterned pre-deposition of the catalyst. In addition, this
method allows greater control over the morphology of CNTs by manipulating the
reaction parameters, such as reaction temperature, catalyst concentration and
reaction time 26-28
.
In CVD technique, the nanotubes are grown from carbon containing gaseous
compounds (i.e. hydrocarbon) which are reacted with a metal catalyst at moderate
temperatures (≤ 1000 °C). The catalyst is present either in-situ from a precursor
or pre-produced on a substrate. However, this method is not without drawbacks.
Residual metal catalyst particles tend to remain in the CNT structures which limit
some of their applications, therefore post-production treatments are required to
purify the nanotubes.
2.2.1.4 Applications
The unique physical and chemical properties of CNTs have led to their diverse
use as supercapacitors29
, reinforcement materials of polymers and ceramics 30-32
,
electromechanical actuators 33
, field emission devices 34
, gas sensors 35,36
and
nanosize probe tips for AFM37
. Recently their bioapplication as biosensor

47. 47
materials has attracted increasing interests due to their ability to enhance the
electroactivity of biomolecules and to promote the electron-transfer between the
biomolecules’ active site and the electrochemical transducer.38-41
CNTs can also
act as supports for metal and semiconductor catalysts thanks to their high aspect
ratio 42-45
.
To take advantages of the remarkable properties of CNTs, a popular solution is to
prepare composite materials based on CNTs and various other materials ranging
from ceramics, polymers to biomolecules 46
. However, the as-produced CNTs
tend to be chemically inert due to their inherently hydrophobic nature which
provides little attractive interaction with the inorganic compounds. Thus, it is
necessary to modify their surface chemistry in order to achieve good interfacial
bonding with the matrix in the composites.
2.2.2 Functionalization of CNTs
Due to the hydrophobic nature of pristine CNTs, they tend to aggregate into
bundles in solvents held together by the strong van der Waals forces. This
bundling is a significant barrier to their processing and also perturbs the
electronic structure of the tubes. Functionalizaiton of CNTs has opened up the
possibilities of dispersing 47,48
and self-assembly of the nanocarbons49
which
allows for the generation of useful architectures50
.
Two strategies are generally reported towards the functionalization of CNTs: (1)
covalent functionalization through attachment of chemical groups to the sidewall
of CNTs51,52
and (2) non-covalent adsorption of various functional molecules,
such as surfactants and polymers. Both non-covalent and covalent
functionalization have been reported to improve the solubility of CNTs 53
which
is necessary for their characterization and manipulation.
Recently, increasing interests have been focused on the functionalization of CNTs

48. 48
with biomolecules as motivated by the prospects of using nanotubes as new types
of biosensor materials 54-56
. Carbohydrates57
, proteins 55,56
, peptides58,59
, and
single strand DNA55
have been demonstrated to modify CNTs through either
covalent 60
or non-covalent way49,61-63
. The modification of nanotubes by these
biomolecules, as well as their analogs and precursors (such as oligosaccharide,
amino acids and peptide, etc.) represents a significant step toward the application
of CNTs in the field of biotechnology and the transfer of biomolecular
self-assembly techniques to nanomaterials.
2.2.2.1Covalent functionalization
Covalent functionalization of CNTs provides more control over the location and
density of the attached groups than the non-covalent adsorption and thus leads to
more robust and predictable conjugates. Considerable progress has been made on
the open-end 64,65
and sidewall 52,66
modifications of CNTs using covalent
chemistry. Reactions that are usually employed to introduce chemical
functionalities onto CNTs include cycloadditions 67,68
nucleophilic additions 69
,
ozonolysis70
, halogenation 71
and radical additions 72-74
.
The most popular example of covalent functionalization involves the oxidation of
CNTs in strong acids, such as HNO3
75,76
and HNO3/H2SO4 mixture 64,77
. Acid
treatment introduces oxygen-containing functional groups (-COOH and –OH)
onto the sidewalls and open ends of CNTs 76,78
which significantly enhance their
aqueous solubility and facilitates further functionalization 79-83
. Fu et al. 84
have
developed a milder route for attaching bovine serum albumin (BSA) protein to
CNTs via the esterification of nanotube-bound carboxylic acids by oligomeric
polyethylene glycol compounds followed by the ester-to-amide transformation
reactions with BSA protein. The entire conjugation procedure did not subject the
protein to any damaging experimental conditions, therefore, the method may be
valuable for the preparation of conjugates involving more fragile biological
species.

49. 49
However, this method suffers from a major disadvantage of cutting the CNTs in
short lengths, making them useless for some applications. Soluble full-length
CNTs have been reported using the 1,3-dipolar cycloaddition of an
aminoethylene glycol linker to the external surface of CNTs and the
derivatization with N-protected glycine was then obtained via amidation reaction
(Figure 2.4). 85
Figure 2.4 1,3-dipolar cycloaddition of an aminoethylene glycol linker to the
external surface of CNTs and the derivatization with N-protected glycine was then
obtained via amidation reaction. 85
Biofunctionlization of CNTs has also been employed in the fabrication of
nanoscale biosensors base on enzyme-CNT86
, DNA-CNT87
or antibody-CNT
conjugates88
. Yu et al. 41
attached myoglobin and horseradish peroxidase
covalently onto the ends of vertically oriented SWNTs forest arrays, which were
used as electrodes. Results suggested that the “trees” in the nanotube forest
behaved electrically similar to a metal, conducting electrons from the external
circuit to the redox sites of the enzymes.
Lin et al. 89
have demonstrated a novel glucose biosensor based on CNT
nanoelectrode ensembles (NEEs) for the selective and sensitive detection of
glucose. Glucose oxidase (GOx) was covalently immobilized on CNT NEEs via
carbodiimide chemistry by forming amide linkages between their amine residues
and carboxylic acid groups on the CNT tips (Figure 2.5). The biosensor

50. 50
effectively performed a selective electrochemical analysis of glucose in the
presence of common interferents (e.g., acetaminophen, uric and ascorbic acids),
avoiding the generation of an overlapping signal from such interferers. Such an
operation eliminates the need for permselective membrane barriers or artificial
electron mediators, thus greatly simplifying the sensor design and fabrication.
Figure 2.5 Fabrication of a glucose biosensor based on CNT nanoelectrode
ensembles: (A) Electrochemical treatment of the CNT NEEs for functionalization
(B) Coupling of the enzyme (GOx) to the functionalized CNT NEEs. 89
Covalent functionalization has been shown to introduce structural defects to
CNTs’ sidewall which lead to a disruption of the nanotubes’ delocalized π system
and consequently compromises their electronic and mechanical properties 90
. This
will in turn lead to a significantly poorer performance of CNT-based composites.
To circumvent this problem, non-covalent modification of CNTs which do not
significantly alter their properties is required for the development of high
performance CNT-based hybrids. Nevertheless, the covalent route offers a

51. 51
convenient and controllable means of tethering molecular species.
2.2.2.2Non-covalent functionalization
In contrast to covalent functionalization, non-covalent binding, which utilizes π-π
stacking,62,91,92
hydrophobic interactions93,94
, electrostatic interaction 95
and
hydrogen bonding has relatively less impact on the structural and functional
properties of CNTs.
Stable CNT dispersions in both aqueous and organic solutions have been
achieved through immobilization of ionic 96-98
and nonionic surfactants 99,100
respectively. This solubilization process opens the door to solution chemistry on
pristine CNTs. Commonly employed surfactants include the anionic sodium
dodecyl sulfate (SDS) 101-104
and sodium dodecyl benzene sulfonate (SDBS) 105,106
,
cationic CTAB98
and nonionic surfactants such as Triton X-100 and Tween 80.
Synthetic aromatic ligands such as pyrenyl 107-112
, porphyrin 113
as well as phenyl
groups are known to interact strongly with the sidewalls of CNTs via π-π stacking
interaction. The interaction is typically weaker for phenyl groups as compared
with the ligands with higher aromaticities. However, their smaller size favors the
higher density of the groups that can be immobilized on CNTs. Dai et al. 62
have
reported the non-covalent functionalization of the sidewalls of SWNTs with a
bifunctional molecule, 1-pyrenebutanoic acid, succinimidyl ester (Figure 2.6,
molecule 1), and subsequent immobilization of various biological molecules onto
nanotubes with a high degree of control and specificity.

52. 52
Figure 2.6 Amine groups on a protein react with the anchored succinimidyl ester to
form amide bonds for protein immobilization.62
There has been increasing interest on using biologically based surfactants, which
then opens up the biochemistry tool kit to nanotechnology. Biomolecules such as
DNA112
, polysaccharides57,114
and peptides62,91,92
have been reported for
functionalization of CNTs. Among the biosurfactants, peptides are of particular
interest owing to their designable chemistry. Phage display study has identified
the CNT binding peptide sequences which were invariably rich in aromatic amino
acids such as histidine (H) and tryptophan (W), with W, in particular, interacting
strongly with the nanotube surface 115
. It was suggested that the aromatic rings in
these amino acid residues contributed to the observed affinities through π-π
stacking interactions92
. However, these literatures tend to focus on the long chain
peptides which usually contain 12 or more residues that are expensive to produce.
As a cost-effective alternative, recent studies have reported using synthetic
aromatic ligands combined with aromatic amino acids for CNT dispersion.
Cousins et al. 47
have demonstrated the use of N-(fluorenyl-9-methoxycarbonyl)
terminated aromatic amino acids (Fmoc-AAs) as surfactants for preparing
homogeneous CNT dispersions (Figure 2.7a). (It should be noted that the author
of this thesis was a co-author on this paper.) Fmoc was selected as a particularly
promising ligand since it is used commonly as a protecting group in solid-state

53. 53
peptide synthesis and it is known to be able to self-assemble into nanofibres via
π-π stacking interactions 116
. The turbidity study of the dispersions of CNTs in the
Fmoc-AA solutions revealed the comparable ability of these biosurfactants to
disperse CNTs to those achieved by using commonly used surfactants such as
SDS and SDBS. The molecular interactions between the ligand and nanotube
surface were then confirmed by quantum mechanical modelling and it was found
that both the aromatic fluorenyl rings and the aromatic rings in the side chains of
the amino acid were stacked on the surface of CNTs to maximize their π-stacking
interactions (Figure 2.7b).
(a) (b)
Figure 2.7(a) TEM micrographs of MWNTs dispersed with Fmoc-W (trp). Arrows
indicate the edge of the lattice structure upon which Fmoc-W aggregates are
apparent; (b) Optimized structures of (i) Fmoc-G (gly) and (ii) Fmoc-W bound to
[6,6] SWNTs with close-up images that highlight the orientation and arrangement
of Fmoc and the aromatic W ring 47
.
Although the approaches described above increase the solubility of CNTs, they
have not been generally adapted to control the assembly of the solubilized CNTs
into higher order architectures that are necessary for realizing many of their
applications. Dieckmann et al.49,50
have designed an amphiphilic α-helical peptide
(“nano-1”) not only to coat and solubilize CNTs into water, but also to control the
self-assembly of the peptide-coated nanotubes into supramolecular structures
through peptide-peptide interactions between adjacent peptide-wrapped
nanotubes. The CD measurements suggested that the α-helical conformation of
the peptide is stabilized in the presence of the nanotubes through the interaction
of the hydrophobic face of the helix with the nanotube surface. Electron

54. 54
microscopy and polarized Raman studies revealed that the peptide-coated
nanotubes assemble into fibres with the nanotubes aligned along the fibre axis.
Most importantly, the size and morphology of the fibres can be controlled by the
addition of either salt in different concentrations or the amphiphilic additive DMF
which can affect the peptide-peptide charge interactions (Figure 2.8). This study
helps to realize the transfer of biomolecular self-assembly techniques to
nanomaterials.
(i) (ii)

55. 55
Figure 2.8 (i) SEM images of nano-1/SWNT fibres formed from a 100 μM
peptide/nanotube dispersion upon addition of no salt (A), 40 mM NaCl (B), and 120
mM NaCl (C). (ii) (A) SEM image of fibres formed from the addition of 0.0015%
(by volume) DMF to a nano-1/SWNT dispersion. (B) Low-resolution TEM image of
the same fibres observed in i(A). The small dark spheres are Fe catalyst particles
from the HiPco SWNT synthesis. (C) High-resolution TEM image of the same fibres
showing alignment of nanotubes. The large dark areas are Fe particles 49
.
These investigations have contributed to the understanding of the nonspecific
interactions between CNTs and biomolecules, and the current knowledge on
non-specific protein–nanotube interactions has already been applied to the
development of biosensors but they have also revealed the complexity of the
issue. Researches based on the molecular level are required to further understand
the interactions.
Polymer wrapping has also been reported for CNT dispersion without destroying
their electrical character117,118
. The wrapping of SWNTs with polymers that bear
polar side-chains, such as polyvinylpyrrolidone (PVP) or polystyrenesulfonate
(PSS), leads to stable solutions of the corresponding SWNT/polymer complexes
in water 117
. The thermodynamic driving force for complex formation is the need
to avoid unfavorable interactions between the apolar tube walls and water. It is
thought that multi-helical wrapping of the tubes with the polymers is most
favorable for reasons of strain. A nonionic surfactant or polymer’s ability to
suspend nanotubes appears to be due mostly to the size of the hydrophilic group,
with higher molecular weights suspending more nanotube material because of
enhanced steric stabilization with longer polymeric groups119
.
An “unzipping” mechanism for nanotube isolation from a bundle with the
combined assistance of ultrasonication and surfactant adsorption has been
proposed as shown in Figure 2.9120
. The role of ultrasonic treatment is likely to
provide high local shear, particularly to the nanotube bundle end (ii). Once spaces
or gaps between the bundle and individual nanotubes at the bundle ends are
formed, they are propagated by surfactant adsorption (iii), ultimately separating
the individual nanotubes from the bundle by either steric stabilization or

56. 56
electrostatic repulsions (iv).
Figure 2.9 Proposed mechanism of nanotube isolation from bundle (i) obtained by
ultrasonication and surfactant stabilization. Ultrasonic processing “fray” the
bundle end (ii), which then becomes a site for additional surfactant adsorption. This
latter process continues in an “unzippering” fashion (iii) that terminates with the
release of an isolated, surfactant-coated NT in solution (iv).120
Several mechanisms have been proposed for the stabilization of CNT dispersion
by surfactants. O'Connell et al. 96
have suggested the formation of SDS
cylindrical micelles around SWNT (Figure 2.10a) or the hemimicellar adsorption
of the surfactants on the tubes (Figure 2.10b) while Richard et al. 121
suggested
the formation of helices or double helices, and Yurekli et al.101
suggested that the
structureless random adsorption with no preferential arrangement of the head and
tail groups of the surfactants is responsible for the stabilization of the dispersions
(Figure 2.10c).

57. 57
Figure 2.10 Schematic representations of the mechanisms by which surfactants help
disperse SWNTs. (a) SWNT encapsulated in a cylindrical surfactant micelle: right:
cross section; left: side view. (b) Hemimicellar adsorption of surfactant molecules
on a SWNT. (c) Random adsorption of surfactant molecules on a SWNT.101
2.2.3 CNT-inorganic nanohybrids
During the past decades, CNT based hybrid materials have been extensively
reported owing to their potential in applications such as photocatalysis122,123
,
electrocatalysis124-127
, gas and biosensing128-131
, supercapacitors132-135
and field
emission device 136-141
.
The first CNT based nanohybrid was produced by opening the capped tube ends
of MWNTs and then filling the hollow cavities with lead particles 142
. Later,
SWNTs were filled with RuCl3
143
. Although a wide range of compounds have
been successively encapsulated into both SWNTs and MWNTs, few have
exploited their potentials in application and have been mainly used by electron
microscopists to understand crystallization in restricted volumes.

58. 58
Alternatively, a wide range of inorganic compounds have been anchored onto the
surface of CNTs for the preparation of hybrid materials. Among the inorganic
components, the most frequently studied are semiconductor oxide nanoparticles
such as SiO2
144-147
, Al2O3
148-150
, SnO2
151-153
, ZnO154-156
and TiO2
157-163
. Of
particular interest are dielectric materials such as silica and TiO2. TiO2 exists in
nature as three polymorphic forms, namely rutile, anatase and brookite, amongst
which, the most important being rutile and the metastable anatase phases. Both of
the phases have tetragonal structures. The properties and applications of TiO2 are
greatly dependent on their crystalline phase, particle size, and morphology, which
could be controlled by varying the reaction conditions 164
. A number of studies
have reported the improved photocatalytic activity of CNT-TiO2 hybrids as
compared to the individual component for the oxidative degradation of organic
compounds 122,123,165
.
2.2.3.1 Synthesis
The most important challenge in synthesizing such hybrid materials is optimizing
the interface between CNTs and the inorganic components. In general, two
strategies have been adopted for the synthesis of CNT-inorganic hybrids;
1. ex-situ techniques where the preformed inorganic components are
directly attached to the surface of CNTs,
2. in-situ techniques where the inorganic components form directly on the
surface of pristine or functionalized CNTs.
The ex-situ route is mainly used for the deposition of metal nanoparticles67,166
and
semiconductor QDs 167
. Surfactants 168
are usually employed as the linking agents
in this approach which utilize both covalent 67,167,169
and non-covalent
interactions 169-176
.
Although the ex-situ route holds the advantage of producing inorganic
components with desired structures and dimensions, it requires the chemical
modification of either CNTs or inorganic compounds for their attachment.

59. 59
Furthermore, the in-situ route allows more flexibility of the morphology of the
deposited inorganic components as either discrete units in the form of
nanoparticles or a continuous film on CNTs, while the ex-situ way is typically
restricted to the formation of monolayers of nanoparticles. The presence of CNTs
also prevents the growth of crystals during crystallization and
phase-transformation thus provides an efficient way of synthesizing nanohybrids
with high specific surface area.
The in-situ techniques include (1) hydrothermal techniques132,148,155,159
(2) sol-gel
process 177
(3) electrochemical methods 178-181
and (4) gas phase deposition 182-184
.
The main advantage of hydrothermal technique is that it enables the formation of
crystalline phase without the need for post-annealing and calcinations. However,
it typically requires high temperatures 132,148,155
. Jitianu et al. 159
have compared
the morphologies of TiO2 coating on CNTs obtained from both sol-gel process
and hydrothermal methods and found that the coating produced with
hydrothermal method is less uniform and the nanotubes surface is partially
damaged due to the oxidizing medium of deposition.
To overcome the above problems, sol-gel process has been widely employed as
an alternative method to prepare CNT-inorganic nanohybrids benefiting from its
benign reaction conditions. Sol-gel process on both covalently152,161,185-188
and
non-covalently147, 189, 190
functionalized CNTs have been reported. During the
sol-gel process, CNT surface chemistry plays an important role in inducing
inorganic compound deposition as well as in controlling the structure and
property of the deposited coatings172
.
2.2.3.2 CNT-SiO2 hybrids
Silica-CNT hybrids are of great interest due to their potential in the development
of nanoscale sensors and electric devices 191,192
as well as optical, magnetic, and
catalytic applications 193
. CNT-SiO2 hybrids also combine the bioactivity of silica
and the conductivity of CNTs which facilitate their biomedical applications 194
. In