graphene - complex oxide ceramic nanocomposites
TRANSCRIPT
Graphene – complex oxide ceramic
nanocomposites
A dissertation submitted to
The University of Manchester
for the degree of
Master of Science
in the
Faculty of Engineering and Physical Science
2016
Yu Chen (9666871)
School of Materials
2
Contents
List of Tables ................................................................................................ 4
List of Figures .............................................................................................. 5
Abstract ......................................................................................................... 8
Declaration ................................................................................................... 9
Copyright Statement .................................................................................. 10
Acknowledgement ..................................................................................... 11
Chapter 1. Introduction ............................................................................. 12
Chapter 2. Literature Review ................................................................... 14
2.1 Development of Graphene Aerogels ................................................................. 14
2.1.1 Graphene Aerogels Synthesised by Mild Chemical Reduction .......... 14
2.1.1.1 Synthesis of Graphene Aerogels by Mild Chemical Reduction
................................................................................................. 14
2.1.1.2 Characterisation of Mild Chemically Reduced Graphene
Aerogels ................................................................................... 15
2.1.1.3 Incorporation of Emulsion Template ...................................... 18
2.1.2 Graphene Aerogels Synthesised by Ice-templating ............................ 19
2.1.2.1 Synthesis of Graphene Aerogels via Freeze-Casting .............. 19
2.1.2.2 Microstructural Architectures ................................................. 21
2.1.2.3 Mechanical Response ............................................................. 23
2.1.2.4 Electrical Conductivity ........................................................... 24
2.1.2.5 Absorption of Organics ........................................................... 25
2.2 Complex Oxide Ceramic – BFO ....................................................................... 26
2.2.1 Synthesis of BFO Nanoparticles ......................................................... 26
2.2.1.1 Synthesis of BFO Nanoparticles by Hydrothermal Method ... 27
2.2.1.2 Synthesis of BFO Nanoparticles by Sol-gel Method .............. 28
2.2.2 Structure and Morphology of BFO Nanoparticles .............................. 28
2.2.3 Optical and Photocatalytic Response of BFO Nanoparticles ............. 30
2.2.4 Magnetic Properties of BFO Nanoparticles ........................................ 31
2.3 Enhanced Properties of Graphene-BFO Nanocomposites.............................. 32
2.3.1 Synthesis of Graphene-BFO Nanocomposites ................................... 33
2.3.2 Characterisation of Phase and Microstructures .................................. 34
2.3.3 Bandgap Tuning and Enhanced Photocatalytic Performance ............. 35
2.4 Summary............................................................................................................... 37
3
Chapter 3. Materials & Methods .............................................................. 39
3.1 Chemicals and Materials .............................................................................. 39
3.2 Fabrication of 3D rGO Aerogels .................................................................. 39
3.2.1 Preparation of GO by Modified Hummers Method ............................ 40
3.2.2 Synthesis of rGO Aerogels by Emulsion-templating .......................... 40
3.2.3 Synthesis of rGO Aerogels by Ice-templating .................................... 41
3.3 Fabrication of rGO-BFO Nanocomposites .................................................. 43
3.3.1 Preparation of the rGO-BFO Mixture ................................................. 43
3.3.2 Annealing of the rGO-BFO Mixture ................................................... 45
3.4 Characterisation ................................................................................................... 45
3.5 Measurement of Photocatalytic Activity .......................................................... 46
Chapter 4. Results & Discussion ............................................................. 47
4.1 rGO Aerogels with 3D Cellular Structures ...................................................... 47
4.1.1 Emulsion-templating ........................................................................... 47
4.1.2 Ice-templating ..................................................................................... 50
4.1.3 Comparison of Two Approaches ......................................................... 55
4.2 rGO-BFO Nanocomposites ................................................................................ 56
4.2.1 Effect of Infiltration ............................................................................ 57
4.2.2 Effect of Annealing Conditions .......................................................... 58
4.2.3 Photocatalytic Activity ........................................................................ 60
Chapter 5. Conclusions & Future Work .................................................. 63
5.1 Conclusion ............................................................................................................ 63
5.2 Future Work.......................................................................................................... 64
References .................................................................................................. 65
4
List of Tables
Table 2.1 The effect of different reducing agents on the properties of as-prepared
graphene aerogels [22]. .......................................................................... 17
Table 2.2 Derived room temperature magnetic parameters [44]. ............................. 32
Table2.3 Effect of KOH concentration on crystallisation, bandgaps, and
photodegradation kinetic rate of graphene-BFO nanocomposites [9]. ....... 37
5
List of Figures
Figure 2.1 Schematic diagram of developing graphene aerogels by mild chemical
reduction [24] ............................................................................................ 15
Figure 2.2 XRD patterns of GO (curve 1), as-prepared graphene after reduction for 40
min (curve 2) and 3 h (curve 3), pristine graphite (curve 4) [22] ............. 15
Figure 2.3 (a) Image of a graphene aerogel. (b) SEM image of porous structure within
the graphene aerogels [22]......................................................................... 16
Figure 2.4 (a) Raman spectra of GO and as-prepared graphene aerogel. (b) TGA
measurement of GO, as-prepared graphene aerogel and 400℃ annealed
graphene aerogel [22] ................................................................................ 16
Figure 2.5 Fabrication of graphene aerogels by the assembly of GO at oil-water
interface under mild reduction condition [28] ........................................... 18
Figure 2.6 SEM images of emulsion-templated graphene aerogels with scale bars of
(a) 150μm, (b) 50μm, (c) 8μm, (d) 500nm [28] ........................................ 19
Figure 2.7 Assembly strategy of 3D graphene aerogels with controlled architectures
[2]............................................................................................................... 20
Figure 2.8 (a) Side view and (b) top view of GO-CNs after freeze-casting. (c)
Isotropic porous structure of GO-CNs with addition of 75 vol.% emulsions.
(d) Co-existence of the lamellar and porous structure of GO-CNs with a
low oil content of 25 vol.% [29] ................................................................ 21
Figure 2.9 The microstructure of materials under the influence of organic additives [2]
................................................................................................................... 22
Figure 2.10 Microstructure of rGO-CNs under the influence of different thermal
treatment temperatures [2] ......................................................................... 23
Figure 2.11 Mechanical response of rGO-CNs [2]…………………….…………………..……24
Figure 2.12 Electrical conductivity versus density ρ for rGO-CNs and other
carbon-based nanomaterials [2, 4, 26, 32-35] ........................................... 25
Figure 2.13 (a) Superhydrophobicity, (b) organics absorption capability, (c, d)
dimensional recovery of rGO-CNs [2] ...................................................... 26
Figure 2.14 Summary of various techniques used for the BFO synthesis [46] .......... 27
Figure 2.15 Schematic diagram of synthesis of BFO nanoparticles by sol-gel method
[48]............................................................................................................. 28
Figure 2.16 SEM images of BFO nanoparticles synthesised by (a) sol-gel method (b)
hydrothermal method [38, 49] ................................................................... 29
Figure 2.17 XRD patterns of BFO nanoparticles calcined at temperatures ranging
from 600 to 900℃ [50] ............................................................................. 29
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Figure 2.18 (a) UV-vis absorption spectra of BFO nanoparticles. (b) the square root of
Kubelka-Munk functions F(R) versus photon energy, where the dotted line
is the tangent of the linear part [37] .......................................................... 30
Figure 2.19 Photodegradation of CR under visible light by BFO nanoparticles with
different morphologies and size [38] ......................................................... 31
Figure 2.20 M-H hysteresis loops of the BFO nanoparticles with different size by
using a SQUID magnetometer [44] ........................................................... 31
Figure 2.21 Illustration of the formation of graphene-BFO nanocomposites via
hydrothermal method [8] ........................................................................... 33
Figure 2.22 Fabrication process of graphene-BFO nanocomposites [8] .................... 34
Figure 2.23 (a) XRD diffraction curves of graphene-BFO nanocomposites and GO. (b)
XPS curves of graphene-BFO nanocomposites with respect to different
bonds [8] .................................................................................................... 34
Figure 2.24 SEM images of (a) pure BFO nanoparticles, (b) graphene-BFO mixture
before centrifugation, (c) graphene-BFO nanocomposites [10] ................ 35
Figure 2.25 UV-vis absorption spectra of graphene-BFO nanocomposites (RGO-BFO)
and BiFeO3 (BFO) [8] ............................................................................... 36
Figure 2.26 (a) Absorption spectra of CR for pure BFO nanoparticles and
nanocomposites. (b) The photodegradation efficiency from BG4 to BG12
under visible light [9] ................................................................................ 36
Figure 3.1 Illustration of the transformation of graphite to reduced graphene oxide
[59]............................................................................................................. 39
Figure 3.2 Assembly strategy of rGO aerogels by emulsion-templating ................... 41
Figure 3.3 Assembly strategy of rGO aerogels by ice-templating [62] ...................... 42
Figure 3.4 Fabrication process of rGO-BFO nanocomposites ................................... 43
Figure 3.5 Flow diagram for the formation procedure of BFO nanoparticles ............ 44
Figure 3.6 Image of the Castable Vacuum System: the BFO solution was poured into
the mould and subsequently pumped into the chamber to impregnate the
rGO aerogels [63] ...................................................................................... 44
Figure 3.7 Schematic diagram of annealing in the tubular furnace [64] .................... 45
Figure 3.8 Schematic illustration of the photocatalytic mechanism of rGO-BFO
nanocomposites toward the degradation of CR........................................47
Figure 4.1 Illustration of emulsification process. (a) Three-dimensional perspective.
(b) Planar perspective [70] ........................................................................ 48
Figure 4.2 SEM images of emusion-templated rGO aerogels. (a-c) Overview of the
cellular architectures. (d) Morphology of cell wall. .................................. 49
Figure 4.3 XRD patterns of pristine graphite, GO aerogels and rGO aerogels. ......... 49
Figure 4.4 Rationale of freeze-casting. (a) The GO-sus is poured into a PTFE mould
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and placed onto the copper cold finger, which is cooled by a liquid
nitrogen bath. Temperature and cooling rate at the mould bottom are
controlled using a heater. (b) Following the arrows: Ice lamellae grow with
the decreasing of temperature, porosity is created after sublimation of ice
crystals. ...................................................................................................... 50
Figure 4.5 Formation process of lamellar structure: ice crystals grow more rapidly in
directions perpendicular to the c-axis [62]………………………….….……….…51
Figure 4.6 Ultralight and hydrophobic rGO aerogels. (a) rGO aerogel propped up on
a leaf. (b) The rGO aerogel float on the water due to hydrophobicity. ..... 51
Figure 4.7 Lamellar structure of ice-templated graphene aerogels. (a) Side view
(parallel to casting direction) and (b) top view (perpendicular to casting
direction) of GO aerogels produced by freeze-casting. (c) Side view and (d)
top view of rGO aerogels after thermal reduction at 600℃. (e,f) Wrinkled
wall of rGO aerogels. ................................................................................ 52
Figure 4.8 Shrinkage of samples after thermal reduction.. ......................................... 53
Figure 4.9 Density, mass loss, volume shrinkage of rGO aerogels after thermal
treatment at 200, 400, 600 and 800℃ respectively. .................................. 54
Figure 4.10 Raman spectra of the as-prepared GO aerogels and rGO aerogels reduced
at 200, 400 and 600℃. .............................................................................. 55
Figure 4.11 Comparison of rGO aerogels. .................................................................... 56
Figure 4.12 Effect of the amount of infiltrated layers. (a) XRD patterns of rGO-BFO
nanocomposites containing 1 BFO layer and 5 BFO layers. (b) Raman
spectra of as-prepared rGO aerogels, rGO-BFO nanocomposites with 1
BFO layer and 5 BFO layers. .................................................................... 57
Figure 4.13 Effect of annealing conditions. (a) Raman spectra (b) XRD patterns of
rGO-BFO nanocomposites annealed at different conditions: 600℃ 4 hours,
700℃ 4 hours, 700℃ 3 hours and 700℃ 2 hours. ................................... 58
Figure 4.14 SEM images of rGO-BFO nanocomposites annealed at different
conditions. ................................................................................................. 59
Figure 4.15 Image of decolourised CR solutions after catalytic effect by rGO-BFO
nanocomposites annealed for 600℃ 4 hours, 700℃ 4 hours, 700℃ 3
hours, 700℃ 2 hours and blank CR solution (From left to right). ............ 60
Figure 4.16 UV-vis absorption spectra of CR after 72 hours of irradiation.. .............. 61
Figure 4.17 Concentration of CR relative to its initial value (C/C0) after photocatalytic
effect by different rGO-BFO samples.. ..................................................... 61
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Abstract
Several studies have documented the fabrication of graphene-complex oxide ceramic
nanocomposites for photocatalyst applications in the visible range via hydrothermal method.
In this project, reduced graphene oxide with three-dimensional cellular architecture is
hybridised with perovskite-type BiFeO3 through a facile sol-gel process. The photocatalytic
activity of the resulting materials is then evaluated by the degradation of Congo red under
visible light irradiation.
Ultralight ice-templated reduced graphene oxide aerogels with a density of 3.15mg cm-3
are
prepared by a freeze-casting technique. The highly-ordered microstructure of products makes
them desirable for infiltration with BiFeO3 solution. XRD and Raman analysis demonstrates
that well-crystallised BiFeO3 can be achieved by increasing the annealing temperature,
whereas the lamellar structure of reduced graphene oxide is better preserved under a shorter
dwell time. Nanocomposites with BiFeO3 nanoparticles of 80-200 nm in diameter attached to
reduced graphene oxide flakes are successfully obtained. The degradation efficiency of Congo
red after exposure to visible light illumination for 72 hours reaches 63% by a sample annealed
at 700℃ for 3 hours. This result can be accredited to the combined effect of BiFeO3 with an
intrinsic bandgap responsive to visible light and the chemical bonding between BiFeO3 and
reduced graphene oxide.
This study has been one of the first attempts to combine reduced graphene oxide with BiFeO3
by a sol-gel method, which can be further applied to create more graphene-based technologies.
Furthermore, the findings presented in this dissertation add to our understanding of the origin
of photocatalytic performance in graphene-complex oxide ceramic nanocomposites.
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Declaration
No portion of the work referred to in the dissertation has been submitted in support of an
application for another degree or qualification of this or any other University or other Institute
of learning.
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Copyright Statement
i. The author of this dissertation (including any appendices and/or schedules to this
dissertation) 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 dissertation, either in full or in extracts and whether in hard or electronic
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regulations (see http://www.manchester.ac.uk/library/aboutus/regulations) and in The
University’s Guidance for the Presentation of Dissertations.
11
Acknowledgement
First, I would like to express my profound gratitude to Doctor Suelen Barg for her kindly
guidance and encouragement throughout this project. Second, I owe sincere and earnest
thankfulness to Miss Vildan Bayram for her continuous support and tireless patience.
Special thanks to Dr. Liang Qiao for his helpful suggestions and discussions during the
project, Dr. John Warren who provided technical support for XRD measurement, Mr. Michael
Faulkner for SEM images, Mr. Andy Wallwork for general experimental setups and Dr.
Zheling Li for his kind support and advice on Raman spectroscopy. My thanks also go to the
staff in School of Materials for their invaluable support.
I am obliged to many colleagues in the group, particularly, Ms. Kirstie Ryan, Mr. Yu Lu, Mr.
Yunyang Wang, Mr. Yaoshu Xie, Mr. Hezhuang Liu and Mr. Qihang Wang, for their fruitful
discussions. They are not only good colleagues but also faithful friends.
At last but not least, I would like to give my special appreciation to my family and all the
friends for their constant support and encouragement. Without their supporting and efforts, I
would not have the chance to study in the UK.
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Chapter 1. Introduction
Graphene is an atomic-scale two-dimensional carbon material [1] that has the potential to
create innovative solutions for more sustainable, efficient processes and products in the field
of information technology, energy, and the environment. However, to achieve this goal,
graphene will often need to be assembled into three-dimensional structures and to be
combined with other materials. In this context, complex oxide ceramics (e.g. perovskite
oxides) exhibit an extensive range of functional properties (e.g. magnetic, piezoelectric,
ferroelectric and photovoltaic) due to an intrinsic coupling among atomic level degrees of
freedom. The possibility of rationally combining graphene and complex oxide ceramics at
multiple scales will generate novel materials and properties that could represent a step
forward towards more efficient photovoltaic cells, filters, energy harvesting and self-powered
sensors, to name a few [4-6].
More specifically, perovskite-type BiFeO3 (BFO) with simultaneous electric ordering and a
small bandgap of ~2.2 eV is considered to be a promising candidate as an oxide photocatalyst.
Furthermore, the bandgap of BFO can be effectively reduced by elemental doping as well as
by preparing BFO with a larger specific area. Therefore, the marriage between graphene and
BFO is of great interest due to their significant impact on photocatalytic behaviours in the
visible range. Recent studies have successfully hybridised graphene with BFO via
hydrothermal treatment [8-10]. The rationale of enhanced photocatalytic activity lies in the
high electrical conductivity of graphene, the modulated bandgap of BFO, and the long
life-time of electron-hole pair generated from BFO [7]. From this perspective, the close
contact and interface coupling between graphene and BFO play a critical role in determining
the enhanced photoelectrochemical properties. In this contribution, the reduced graphene
oxide (rGO) [11-13] with desirable electrical conductivity [14], mechanical behaviour [15],
13
optical transparency, and chemical stability [16] can be assembled into three-dimensional
networks, consequently templating the morphology of BFO nanoparticles.
The overall aim of this project is to develop reduced graphene oxide-BiFeO3 (rGO-BFO)
macroscopic cellular nanocomposites by a sol-gel method and investigate its structure and
properties. The rGO aerogels with cellular architectures (e.g. foam-like or lamellar) are
synthesised via two distinct approaches: emulsion-templating and ice-templating [2]. Their
products have been compared in detail and subsequently impregnated [3] with BFO solutions,
following high-temperature sintering. The main challenge of this process is simultaneously
maintaining the cellular structure of rGO aerogels and obtaining well-crystallised BFO
nanoparticles. Finally, both the nanocomposites and the starting materials (graphene oxide)
are characterised by state-of-the-art techniques including SEM, XRD, and Raman
spectroscopy in order to correlate processing with the resulting materials’ properties. The
photocatalytic activity of rGO-BFO nanocomposites under visible light is investigated by the
degradation of Congo red.
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Chapter 2. Literature Review
2.1 Development of Graphene Aerogels
To date, a substantial amount of studies [2, 17] have demonstrated that three-dimensional (3D)
networks assembled by two-dimensional (2D) chemical modified graphene (CMG) could
advance its consequence for applications from bioengineering to energy technology and
sustainability. In order to achieve this objective, a number of methods have been intensively
developed such as microwave synthesis [18], hydrothermal [19], and sol-gel drying [20, 21],
various approaches can also be combined to improve the quality of 3D graphene aerogels.
Consequently, resulting materials with tunable structure, good mechanical response, high
electrical conductivity and energy absorption have been obtained.
2.1.1 Graphene Aerogels Synthesised by Mild Chemical Reduction
Although numerous methods have been conducted in the production of graphene aerogels, the
majority of them require specialist instruments, including high-pressure and low-temperature
processing conditions. Contrastingly, the self-assembly of graphene aerogels by mild
chemical reduction of graphene oxide (GO) under atmospheric pressure [22] is assumed to be
a facile approach for the preparation of graphene with 3D architecture on such a large scale.
2.1.1.1 Synthesis of Graphene Aerogels by Mild Chemical Reduction
In Chen and Yan’s study [22], the first stage of the mild chemical reduction approach was to
prepare the GO using the modified Hummers method [23], the GO was then dispersed into
water to form GO suspension with an addition of the reducing agent NaHSO3. The suspension
was heated at 95℃ for 3 hours without stirring, followed by the dialysis against deionised (DI)
water for as-prepared graphene hydrogels to eliminate the remaining inorganic compounds.
The graphene aerogels were finally attained after the expelling of the water via the
freeze-drying process.
This self-assembly mechanism of graphene aerogels can be accredited to the hydrophobic and
15
π-π stacking interaction of the reduced graphene oxide (rGO). The increasing hydrophobicity
originates from the reduction of GO by NaHSO3 and conclusively gives rise to the compact
3D architectures (Figure 2.1).
Figure 2.1 Schematic diagram of developing graphene aerogels by mild chemical reduction [24].
2.1.1.2 Characterisation of Mild Chemically Reduced Graphene Aerogels
Characterisation was implemented in order to explore the morphologies and the properties of
mild chemically reduced graphene aerogels. The XRD patterns of GO, graphite, and graphene
hydrogels that were reduced for 40 min and 180 min are displayed in Figure 2.2. The
disappearance of the peak in curve 2 demonstrates the successful exfoliation of multilayer
following the reduction of GO for 40 min. Alternatively, the peak in curve 3 corresponds to
the self-assembly mechanism resulting from the significant reduction of GO [25].
Figure 2.2 XRD patterns of GO (curve 1), as-prepared graphene after reduction for 40 min (curve 2)
and 3 hours (curve 3), pristine graphite (curve 4) [22].
The ultralight graphene aerogels (Figure 2.3(a)) can be achieved after the freeze-drying
procedure for as-prepared graphene hydrogels by removing the absorbed water. Figure 2.3(b)
exhibits the cellular structure of graphene aerogels with pore sizes of 3 – 6 μm.
aerogel
16
Figure 2.3 (a) Image of a graphene aerogel. (b) SEM image of porous structure within the graphene
aerogels [22].
The Raman spectra of GO and aerogels (Figure 2.4(a)) presents the degree of reduction by
using NaHSO3. For reduced graphene aerogels, the location of G band is close to that of pure
graphite, confirming the reduction of GO under atmospheric pressure. While, the similar
location of D bands for GO and graphene aerogels exposes the existence of defects in both of
the samples.
Figure 2.4(b) compares the TGA measurement result of GO, the as-prepared graphene
aerogels, and the graphene aerogels after annealing at 400℃. It has been verified that the
graphene aerogels has a high thermal stability in comparison with GO, which obtained a
reduction in mass of over 50% at 800℃.
Figure 2.4 (a) Raman spectra of GO and as-prepared graphene aerogel. (b) TGA measurement of GO,
as-prepared graphene aerogel and 400℃ annealed graphene aerogel [22].
17
The electrical conductivity of the resulting graphene aerogels is 87 S m-1
, comparable to that
of graphene aerogels formulated by the sol-gel method [20]. Additionally, the relationship
between the reducing agent and the properties of graphene aerogels was also considered by
introducing other types of reducing agents including Vitamin C, Na2S and HI. The electrical
conductivity, density and C/O ratio of as-prepared graphene hydrogels are listed in Table 2.1.
It is subsequently highlighted that the high electrical conductivity, density and low remaining
oxygen groups of hydrogels are reduced by using HI. In addition, a strong relationship
between the degree of reduction and electrical conductivity has been established, which
indicates that the reducing agent plays a crucial role in determining the properties of the
graphene aerogels.
Table 2.1 The effect of different reducing agents on the properties of as-prepared graphene
aerogels [22].
Correspondingly, Yang et al. [17] also reported the ambient pressure dried graphene aerogels
when using L-ascorbic acid as reducing agent. After a full reduction of 6 hours, most
functional groups can be removed [26], enabling the C/O elemental ratio of graphene aerogels
to rise to 9.08, demonstrating that L-ascorbic acid is additionally an efficient reducing agent.
The studies reviewed above provide a superficial method of producing the graphene aerogels
with outstanding electrical conductivity and hydrophobicity under atmosphere pressure.
However, the capillary action [27] caused by evaporation of water can lead to severe
shrinkage of pore structure within graphene aerogels during the drying process:
18
P = (-2γcos(θ))/r (1.1)
Where, P is the capillary pressure, γ is the surface tension, θ is the contact angle and r is the
pore radius.
2.1.1.3 Incorporation of Emulsion Template
The previously indicated equation clearly demonstrates that increasing the pore radius (r) of
GO suspension can be an applicable route to reduce the effect of capillary action. Efforts can
therefore be made by introducing a template that is subsequently eliminated to create porosity,
thereby providing shape controlled graphene aerogels. Additionally, the choice and amount of
the reducing agent, reduction temperature and duration should be carefully controlled.
Zhang et al. [28] exhibits the fabrication of 3D graphene aerogels by self-assembly at
oil-water interface under mild conditions. As shown in Figure 2.5, the cyclohexane as an oil
phase was added to the GO suspension, followed by the heat treatment at 70℃ for 12 hours to
thoroughly reduce the GO. During this procedure, functional groups were removed and
hydrophobic and π-π interactions of graphene provided the self-assembly mechanism to form
the cellular network. Finally, the remaining water and oil phase were both eliminated through
freeze-drying to produce graphene aerogels.
Figure 2.5 Fabrication of graphene aerogels by the assembly of GO at oil-water interface under mild
reduction condition [28].
Figure 2.6 demonstrates the highly ordered honeycomb-like microstructure of
19
emulsion-templated graphene aerogels. The shape and distribution of the pores are more
uniform, the size of pores has also been increased to tens of micrometres. Furthermore, the
density of 2.8 mg cm-3
is merely one tenth of graphene aerogels synthesised in the absence of
oil and emulsion. This data validates that the emulsion-templating is an effective way to
fabricate ultralight graphene aerogels with controlled and ordered cellular structure. The mild
reduction temperature (70℃) also contributes to the preservation of architecture.
Figure 2.6 SEM images of emulsion-templated graphene aerogels with scale bars of (a) 150μm, (b)
50μm, (c) 8μm, (d) 500nm [28].
2.1.2 Graphene Aerogels Synthesised by Ice-templating
As previously discussed, the fabrication of 3D graphene aerogels by mild chemical reduction
under atmospheric pressure is strongly desired for cost-effective and large-scale industrial
production. However, the challenge remains regarding how to achieve a tailored structure and
maintain its stability. Consequently, the method of developing reduced graphene oxide
cellular networks (rGO-CNs) via ice template is an ideal alternative and a versatile approach
that enables a controlled and tunable structure [2, 29].
2.1.2.1 Synthesis of Graphene Aerogels via Freeze-Casting
The assembly strategy of ice-templating approach is illustrated in Figure 2.7. In order to
commence this process, the aqueous GO suspensions (GO-sus) were prepared using the
(a)
(c)
(b)
(d)
20
modified Hummers method [30]. Meanwhile, various organic additives (such as sucrose or
PVA) were added to improve the surface wettability and activity of GO. There are organic
additives (sucrose) which also operates as a binder to stabilise the structure of networks
during the segregation with ice crystals. In one version of this method, the GO-sus were
directly poured into a cylindrical mould and then unidirectionally frozen by reducing the
temperature of the mould at a controlled rate between 1 to 10 K min-1
. Following the
freeze-drying to eliminate the ice crystals formed during the freeze-casting, GO-CNs with
lamellar structure will be left behind (Figure 2.8(a)).
Similarly, an extra emulsification step can be undertaken [29]. In the GO-sus, a hydrophobic
oil phase was homogeneously dispersed by hand-shaking in order to form GO emulsion
(GO-em) with low micrometer-scale droplets, these oil droplets act as a template to fabricate
cellular networks. The amphiphile GO could then self-assembly at water – oil interface. The
GO-em was subsequently moulded and unidirectionally frozen in cylindrically shaped moulds,
ice crystals formed during this solidification process and encapsulated the oil droplets,
subsequently controlling the alignment of GO within the water phase. The approximate 75
vol.% of oil composition within the GO-em was conducive to the fabrication of highly porous
structure once the ice crystals were removed after freeze-drying.
Figure 2.7 Assembly strategy of 3D graphene aerogels with controlled architectures. The procedure
consists of emulsification, freeze-casting, freeze-drying, and thermal reduction [2].
21
On completion of the freeze-drying, the final step is the reduction of the GO into rGO by
thermal treatment at high temperatures ranging from 300 to 2400℃, therefore, eliminating
residual functional groups and organic additives.
2.1.2.2 Microstructural Architectures
In comparison with the lamellar structure which resulted from freeze-casting, the additional
emulsion template gives rise to a densified porous microstructure by impeding the formation
of lamellar ice crystals (Figure 2.8).
Figure 2.8 (a) Side view and (b) top view of GO-CNs after freeze-casting. (c) Isotropic porous
structure of GO-CNs with addition of 75 vol.% emulsions. (d) Co-existence of the lamellar and porous
structure of GO-CNs with a low oil content of 25 vol.% [29].
Techniques including SEM and Raman spectroscopy were performed in order to explore how
organic additives and thermal treatment conditions impact the GO-CNs microstructure. It has
been certified that by adding organic additives, the cells of GO-CNs are prominently densified
and spherically shaped (Figure 2.9(a, b)). Contrastingly, thermal treatment results in wrinkled
rGO-CNs (Figure 2.9(a-d)), which have an effect on both additive-free GO-CNs and
additive-added GO-CNs, however the cell size of the rGO-CNs remains similar to the
non-reduced GO-CNs. Successively, rGO-CNs are lighter due to the elimination of functional
22
groups attached to GO-CNs. In addition, the characteristic Raman spectra (Figure 2.9(e))
imply that carbon source provided by the decomposition of organic additives at high
temperatures improves the recrystallisation of rGO-CNs.
Figure 2.9 The microstructure of materials under the influence of organic additives. SEM images: (a)
GO-CNs produced without additive; (b) GO-CNs produced with 5 wt.% organic additives; (c)
rGO-CNs produced after thermal treatment without additive; (d) rGO-CNs produced after thermal
treatment with 5 wt.% organic additives. Raman spectroscopy of rGO-CNs: (e) The peak of curves
labelled with ‘D’ and ‘G’ represents the intensity value of graphene and carbon allotropes respectively,
the specific value of D/G stands for the defect density in the carbon material [2].
Furthermore, results from SEM (Figure 2.10(a-d)) and Raman spectroscopy (Figure 2.10(e))
indicate that the crystalline quality of rGO-CNs was improved by additional thermal treatment
above 1000℃ in a graphite furnace. Markedly, the decrease of D/G intensity ratio (Figure
2.10(e)) with increasing annealing temperature suggests the restoration of sp2 network, and
the 2D peaks become more detectable following annealing at 2400℃, which can be
characterised as the existence of the graphene layers with less misorientation [31].
23
Figure 2.10 Microstructure of rGO-CNs under the influence of different thermal treatment
temperatures. SEM image of rGO-CNs after thermal reduction inside an tubular oven under high
vacuum (a, b) at 1000℃, with scale bars of 100 um and 2 um respectively; (c, d) at 2400 ℃ with scale
bars of 100 μm and 2 μm respectively; (e) Raman spectroscopy of GO-CNs: as-prepared and thermally
treated inside a tubular oven under a high vacuum at different temperatures. The peak of curves
labelled with ‘D’ and ‘G’ represents the intensity value of graphene and carbon allotropes. The specific
value of D/G stands for the defect density in the carbon material [2].
2.1.2.3 Mechanical Response
The compressive cycle testing was subsequently carried out to specifically analyse the
mechanical response of rGO-CNs. It can be concluded that the linear elastic response is
dominant in the first four cycles (Figure 2.11(a, b)). In addition, ‘yielding’ can be witnessed in
the testing curves, which has been associated with the density of rGO-CNs samples. Despite
the errors of measurement at low loads, the relationship between Young’s modulus and
density (Figure 2.11(c)) suggests that the denser rGO-CNs thermally reduced at the higher
temperature exhibit the brittle collapse in the compression process (Figure 2.11(b, d)).
Nonetheless, the rGO-CNs exhibit recovery during unloading provided the density < 100 mg
cm-3
. Meanwhile, the stress-strain curve gradually stabilises following the apparent
degradation in the first four cycles, exhibiting very good cycling performance. It also
considerable to note that higher annealing temperature improves the recrystallisation of
graphene, leading to less damage in the structure and contributing to the outstanding elastic
24
behaviour. In summary, this multi-cycle compressive test indicates that the fabrication
approach of this rGO-CNs is practicable with specific regards to the structural features.
Figure 2.11 Mechanical response of rGO-CNs. (a) Compressive curves tested for rGO-CNs (with a
density of 6.1mg cm-3
, the thermal annealing temperature of 300℃, additives addition of 1.2 wt.%). (b)
Compressive curves tested for rGO-CNs (with a density of 17mg cm-3
, the thermal annealing
temperature of 1000℃, additives addition of 2.5 wt.%). (c) Young’s modulus versus density for
different carbon-based material. (d) Collapse stress of several carbon-based porous materials as a
function of density [2].
2.1.2.4 Electrical Conductivity
The electrical conductivity of several 3D carbon nanomaterials including rGO-CNs are
illustrated in Figure 2.12. The rGO-CNs with an electrical conductivity of 0.9 S cm-1
established it as apparently superior to graphene elastomers [26] and previously reported
graphene aerogels [35].
25
Figure 2.12 Electrical conductivity versus density ρ for rGO-CNs and other carbon-based
nanomaterials [2, 4, 26, 32-35].
2.1.2.5 Absorption of Organics
The combination of ultralow density, high porosity and superhydrophobicity makes the
rGO-CNs float when in contact with water (Figure 2.13(a)). Whereas, the rGO-CNs show
very good wettability for organics and good recovery after immersion in the organic solvents
(Figure 2.13(b)). The rGO-CNs (4.3mg cm-1
density) could absorb organics reaching 113 to
276 times their own weight, the absorption capability was also found to be highly dependent
on their density, the rGO-CNs with lower density exhibits higher organic intake [2].
Owing to the mechanical and chemical stability, the structural integrity of rGO-CNs can be
maintained after repeating the absorption and extrusion of organics for several cycles (Figure
2.13 (c, d)), enabling them to be competitive candidates as organics absorbers.
rGO-CNs
26
Figure 2.13 (a) Superhydrophobicity, (b) organics absorption capability, (c, d) dimensional recovery
of rGO-CNs [2].
With considering given to all the evidence established, the rGO-CNs fabricated by this
versatile self-assembly strategy which combines the freeze-casting and freeze-drying have
been concluded to obtain unique architecture, superior mechanical response, appreciable
electrical conductivity, and high organics absorption capabilities. This consequently creates
new opportunities for increased efficiency of technological applications. However, this
approach can also be modified as nanopores and defects are still generated during the
elimination of functional groups upon completion of the thermal treatment.
2.2 Complex Oxide Ceramic – BFO
In recent years, perovskite-type BFO with a small bandgap of ~2.2 eV has received an
increasing amount of attention due to its fascinating physics such as multiferroics [36],
photovoltaic effect [37] and photocatalytic activity under visible light [38]. However, bulk
leakage and other nonstoichiometry related defects of BFO necessitate the development of
BFO nanostructure material, such as BFO thin film [39], BFO nanowires [40] and BFO
microcrystals [41].
2.2.1 Synthesis of BFO Nanoparticles
Synthesis of single-phase BFO nanoparticles (Figure 2.14) without impurities can be
27
challenging for conventional solid-state reaction, primarily due to the kinetics of phase
formation which often results in the appearance of impurities such as Bi2O3, Bi2Fe4O9 and
Bi25FeO40 [42, 43]. In this regard, novel wet chemical methods such as hydrothermal method
and sol-gel method which allow for the crystallisation of single-phase have been extensively
developed. Consequently, BFO nanoparticles with a desirable crystal structure, morphology
and intriguing properties have been successfully synthesised [44, 45], forging a focus for
applications including photocatalysts, ferroelectrics, and photovoltaics.
Figure 2.14 Summary of various techniques used for the BFO synthesis [46].
2.2.1.1 Synthesis of BFO Nanoparticles by Hydrothermal Method
The first step in the hydrothermal process is to prepare an aqueous solution consisting of
Fe(NO3)3·9H2O, Bi(NO3)3·5H2O, nitric acid, and distilled water. Following this, the mixture
was slowly dropped into KOH solution under mechanical stirring. The brown suspension was
then transferred to a 120 mL Teflon autoclave, where the hydrothermal treatment was
performed at 200℃, and the processing time was in accordance with the KOH concentration.
Once the autoclave naturally cooled to room temperature after heating, the final products were
collected by centrifugation, then rinsed with distilled water and dried at 70°C in the air before
any further utilisation [38, 47].
28
2.2.1.2 Synthesis of BFO Nanoparticles by Sol-gel Method
The synthesis of BFO nanoparticles by sol-gel method [47, 48] is outlined in Figure 2.15.
Firstly, the bismuth subnitrate (Bi5O(OH)9(NO3)4) and the iron nitrate nonahydrate
(Fe(NO3)3·9H2O) were separately dissolved in glacial acetic acid (CH3COOH) at the
stoichiometric molar ratio Bi:Fe = 1:1. Once the solutions became transparent under stirring,
ethylene glycol is added as a dispersant [37]. The mixture was further stirred for 30 min until
the sol became stable, after drying at 40℃ for a week, the Bi-Fe gel was achieved. Finally,
this precursor was calcined at temperatures ranging from 400 to 900℃ for 1-3 hours to
acquire the BFO nanoparticles.
Figure 2.15 Schematic diagram of synthesis of BFO nanoparticles by sol-gel method [48].
2.2.2 Structure and Morphology of BFO Nanoparticles
The morphology of BFO nanoparticles synthesised by sol-gel method and hydrothermal
method are compared in Figure 2.16. The uniform BFO single-phase nanoparticles can be
clearly observed, demonstrating that both of the methods are desirable for the development of
BFO nanoparticles in terms of the microstructure. Interestingly, the 100 nm size of particles
prepared by the sol-gel method is relatively smaller than 500nm-2μm achieved from the
hydrothermal method.
29
Figure 2.16 SEM images of BFO nanoparticles synthesised by (a) sol-gel method (b) hydrothermal
method [38, 49].
Figure 2.17 details the typical XRD patterns of the BFO nanoparticles calcined at
temperatures ranging from 500 to 900℃ for 2 hours. According to the result, the perovskite
BFO nanoparticles are found to be formed with some impurity phases after thermal treatment
at 500℃ and 750℃, the highly crystallised BFO nanoparticles require the heat treatment to be
above 800℃.
Figure 2.17 XRD patterns of BFO nanoparticles calcined at temperatures ranging from 600 to 900℃
[50].
(a)
30
2.2.3 Optical and Photocatalytic Response of BFO Nanoparticles
The optical absorption of the BFO nanoparticles that holds a significant role in determining
the bandgap of semiconductor catalyst has been investigated. As is presented in Figure 2.18(a),
the absorption spectra demonstrates that the present material can absorb a considerable
amount of visible light in the wavelength range of 400-560 nm [37, 51, 52]. According to the
Kubelka-Munk (K-M) theory [53], the bandgap can be estimated by the tangent line from the
plot of the equation (Figure 2.18(b)). The calculated value of 2.18 eV exhibits a latent
utilisation for photocatalyst under visible light.
Figure 2.18 (a) UV-vis absorption spectra of BFO nanoparticles. (b) the square root of Kubelka-Munk
functions F(R) versus photon energy, where the dotted line is the tangent of the linear part [37].
The photocatalytic response of BFO nanoparticles has been investigated by the
photodegradation of Congo red (CR) under visible light (Figure 2.19), the influence of the
particle size has also been revealed. Owing to its large size (20 μm), the BFO microspheres
illustrates negligible photocatalytic activity. In contrast, the BFO microcubes with 5 μm
particle size displays a detectable photocatalytic activity. In addition, the BFO submicrocubes
with 500 nm particle size could enable 40 % CR degradation after 3 hours irradiation under
visible light. Larger specific areas of nanoparticles may be liable for the higher efficiency.
This remarkable photocatalytic activity of BFO nanoparticles establishes it as a promising
candidate for photocatalyst under visible light compared with TiO2, which is only reactive for
UV irradiation.
31
Figure 2.19 Photodegradation of CR under visible light by BFO nanoparticles with different
morphologies and size: microspheres (20 μm), microcubes (5 μm), submicrocubes (500 nm) [38].
2.2.4 Magnetic Properties of BFO Nanoparticles
The magnetic response of the BFO nanoparticles annealed at 600℃ has been plotted as a
function of applied magnetics (Figure 2.20), the size effect has been additionally taken into
account. The M-H hysteresis loops indicate that the BFO nanoparticles with a size of 245 nm
reveal a weak magnetic response similar to that of the BFO bulk. While an appreciable
magnetic response has been demonstrated by the samples with a diameter of 95 nm and a
pronounced increase in magnetic performance can be achieved once the particle size
decreases to 62 nm or smaller. The magnetic behaviour as a function of particle size is plotted
in the inset of the figure.
Figure 2.20 M-H hysteresis loops of the BFO nanoparticles with different size by using a SQUID
magnetometer [44].
32
Furthermore, all the relevant parameters have been summarised in Table 2.2. This data
evidently indicates a strong relationship between magnetic properties and the size of the BFO
nanoparticles. This size-dependent magnetic property of BFO nanoparticles can be attributed
to the uncompensated spins at the particle surfaces, which is known to be associated with the
surface-to-volume ratio in nanostructures. Smaller BFO nanoparticles with increased specific
surface area give rise to enhanced overall magnetisation.
Table 2.2 Derived room temperature magnetic parameters [44].
2.3 Enhanced Properties of Graphene-BFO Nanocomposites
In recent years, graphene has been hybridised with SnO2 as anode materials for lithium-ion
batteries [54], with Al2O3 for heat transfer and thermal energy storage [55]. Substantial effort
has also been made to combine graphene with a number of semiconductors such as TiO2 for
photocatalysts [56]. Subsequently due to the high charge mobility of graphene and the
reduced electron-hole pair recombination rate from semiconductor nanoparticles [7], the
graphene-semiconductor photocatalysts are actively pursued for the degradation of organic
pollutants [57] and water splitting [58].
Multiferroic BFO has demonstrated an efficient photocatalytic response in the visible range in
comparison with TiO2. In this respect, it is of extreme interest to fabricate graphene-BFO
nanocomposites and investigate their photocatalytic performance under visible light (Figure
2.21).
33
Figure 2.21 Illustration of the formation of graphene-BFO nanocomposites via hydrothermal method
[8].
2.3.1 Synthesis of Graphene-BFO Nanocomposites
Tie Li et al. [8] have successfully synthesised the graphene-BFO nanocomposites through
hydrothermal method (Figure 2.22), where the BFO nanoparticles directly formed on the
graphene nanosheets. The preparation of BFO started from dissolving the Bi(NO3)3·5H2O and
Fe(NO3)3·9H2O in KOH solution on the basis of stoichiometric ratio [10]. GO was prepared
by modified Hummers method [30], the sonication procedure was then completed for GO
after an addition of Vitamin C, graphene nanosheets can therefore be homogeneous dispersed
in the water to form a GO solution. Once the two precursors were realised, the hydrothermal
process was performed to mix the two parts in an autoclave for 6 hours at 180℃.
Subsequently, centrifugation, the washing and drying steps were applied to complete the
whole fabrication process of graphene-BFO nanocomposites.
34
Figure 2.22 Fabrication process of graphene-BFO nanocomposites [8].
2.3.2 Characterisation of Phase and Microstructures
The structure of graphene-BFO nanocomposites was characterised through different
techniques including XRD, XPS, and SEM. In the XRD patterns of graphene-BFO
nanocomposites (Figure 2.23(a)), rhombohedrally distorted BFO single-phase can be found.
Successively, no typical pattern of GO was detected due to the exfoliation of reduced GO
during the hydrothermal process. Furthermore, XPS peaks (Figure 2.23(b)) at the different
binding energies indicate that the oxygenated functional groups (HO-C=O, C=O=C, C-OH)
attached on graphene sheets were successfully replaced by the Fe-O-C bonds. This is believed
to be the evidence of the reduced bandgap of the graphene-BFO nanocomposites [8].
Figure 2.23 (a) XRD curves of graphene-BFO nanocomposites and GO. (b) XPS curves of
graphene-BFO nanocomposites with respect to different bonds [8].
graphene-BFO
(a) (b)
35
As illustrated in Figure 2.24, the nucleation and growth of BFO nanoparticles on graphene
sheets can be clearly observed. With the presence of graphene nanosheets, the further growth
of BFO nuclei can be restricted during the hydrothermal process, consequently giving rise to a
substantially reduced particle size of 100 nm as compared with 15-20 μm for pure BFO
nanoparticles (Figure 2.24(a)). In addition, the modulated particle size of BFO can be ascribed
to the adsorption of –OH groups on graphene nanosheets, by which the amount of –OH
groups that contributes to the growth of BFO nanoparticles being considerably reduced.
Figure 2.24 SEM images of (a) pure BFO nanoparticles, (b) graphene-BFO mixture before
centrifugation, (c) graphene-BFO nanocomposites [10].
2.3.3 Bandgap Tuning and Enhanced Photocatalytic Performance
Once the microstructure and the chemical binding energy of graphene-BFO nanocomposites
were obtained, the role of graphene in defining the bandgaps and the optical absorption
behaviour was examined. Figure 2.25 exhibits the results attained from UV-vis diffuse
reflectance spectra, the graphene-BFO nanocomposites reveal significant higher optical
absorption in both UV range and visible range. Bandgaps derived from the UV-vis
measurement were 2.52 eV and 3.21 eV for pure BFO and graphene-BFO nanocomposites
respectively (inset of Figure 2.25), which additionally indicates that the optical absorption
capability was evidently changed.
36
Figure 2.25 UV-vis absorption spectra of graphene-BFO nanocomposites (RGO-BFO) and BiFeO3
(BFO) [8].
The photocatalytic property of graphene-BFO nanocomposites was measured by the
degradation of Congo red (CR) under visible light irradiation. Effect of –OH groups can also
be investigated by adjusting the concentration of –OH groups from 4M to 12M for samples
BG4 to BG12 correspondingly. Accordingly, enhanced photocatalytic performance has been
substantiated due to the decrease in bandgaps. As illustrated in Figure 2.26, following two
hours of irradiation, the percentage of decomposed CR increases from 40% for sample BG4
to 70% for sample BG12, the findings can be accredited to the change of –OH group
concentration, which mediates the formation of Fe-O-C bonds that transfer the
photo-generated electrons from BFO to graphene.
Figure 2.26 (a) Absorption spectra of CR for pure BFO nanoparticles and nanocomposites. (b) The
photodegradation efficiency from BG4 to BG12 under visible light [9].
(a) (b)
37
Therefore, the bandgap of BFO and the coupling between graphene with BFO are considered
to be responsible for enhanced photocatalytic performance of graphene-BFO nanocomposites
under visible light. These factors, in addition to the kinetics of photodegradation rate have
been summarised in Table 2.3. It is noteworthy to assert that the interaction between CR and
graphene may also have an effect on the photodegradation properties.
Table 2.3 Effect of KOH concentration on crystallisation, bandgaps, and photodegradation
kinetic rate of graphene-BFO nanocomposites [9].
2.4 Summary
Collectively, studies reviewed here suggest that both mild chemical reduction and
ice-templating method are believed to be relatively cost-effective and reliable techniques for
the production of graphene aerogels which postulates a wide range of functionalities. In
addition, the relationship between the processing route and the resulting materials properties
has been distinctly defined. However, prior studies have been unable to ascertain any
connection between the distinct fabrication approaches and the utilisation of graphene-based
nanocomposites. It is therefore recommended that further research based on these flexible
techniques is required to be completed in order to meet specific requirements of the
applications.
The evidence presented in this section verifies that graphene can be hybridised with BFO,
acceptable bandgaps and enhanced coupling between BFO nanoparticles with graphene can
be obtained by adjusting the concentration of –OH groups in the hydrothermal process. The
38
high charge mobility of graphene and lower recombination rate of electron-hole pairs as a
result enable the rGO-BFO nanocomposites with an outstanding photocatalytic performance
under visible light.
In significant comparison with the hydrothermal method, the sol-gel approach is preferable
for the synthesis of BFO nanoparticles with regard to size-dependent optical and magnetic
properties. In the interim, fabrication of rGO-BFO nanocomposites through this route has not
been reported at present. In this respect, the sol-gel method is certainly worthwhile in an
attempt to further improve the photocatalytic activity, detect the rationale of combining the
graphene with BFO, in addition to investigate other exceptional functional properties.
39
Chapter 3. Materials & Methods
3.1 Chemicals and Materials
Potassium permanganate (KMnO4), sodium nitrite (NaNO3), L-ascorbic acid, decane,
polyvinyl alcohol (PVA), 2-methoxyethanol (C3H8O2), ethanolamine (C2H7NO), bismuth
nitrate pentahydrate (Bi(NO3)3·5H2O), 99.99% iron nitrate nonahydrate (Fe(NO3)3·9H2O) and
congo red were purchased from Sigma-Aldrich. Sulfuric acid (H2SO4) and sucrose were
purchased from Fisher Chemical. Graphite was purchased from Graphexel. Ethanol without
further purification and distilled water were used for the sample preparation.
3.2 Fabrication of 3D rGO Aerogels
In recent times, extensive research has been carried out to revolutionise the production of
high-quality graphene. Thus far, one of the most cost-effective ways is through the reduction
of graphene oxide (GO) into reduced graphene oxide (rGO) while some imperfections are
created during the thermal treatment procedure (Figure 3.1) [60]. Furthermore, the reduction
process gives rise to the self-assembly mechanism of rGO, where the hydrophobicity is
considerably increased, along with the removal of functional groups and the re-formation of
sp2 carbon networks.
Figure 3.1 Illustration of the transformation of graphite to reduced graphene oxide [59].
Among various reported assembly methods, mild chemical reduction and ice-templating are
two of the most applicable approaches due to their simplicity and high efficiency. In addition,
40
owing to their flexibility, these methods can be customised for different applications. Hence,
both of the approaches were adopted for the current study and preferred products were
selected for combination with BFO.
3.2.1 Preparation of GO by Modified Hummers Method
The modified Hummers method is the most common method for the production of GO [61].
To begin this process, 3.8 g of NaNO3, 5 g of graphite powder and 22.5 g of KMnO4 were
carefully dissolved in 625 ml of H2SO4, while the compounds were continuously mixed for 4
hours in an ice bath to avoid excess heating due to the exothermic behaviour of the reactions.
When the viscosity of the mixture markedly increased, an extra 169 ml of H2SO4 was added.
The mixed solution was then maintained at room temperature with continuous magnetic
stirring for 5 days in order to ensure sufficient chemical reaction. Following this treatment,
the centrifugation was implemented to purify the solution by adding distilled water for the
neutralisation of acids and for the removal of the big particles. Finally, once the pH value of
the solution had reached 7, the preparation of GO was completed.
3.2.2 Synthesis of rGO Aerogels by Emulsion-templating
Previous studies [17, 22, 28] have demonstrated that 3D graphene aerogels can be prepared by
one-step mild chemical reduction under atmospheric pressure. The major advantage of using
this method is that the graphene aerogels can be produced on a large-scale since special
instruments and extreme reaction conditions are not required. Nevertheless, the capillary
action caused by the evaporation of liquid phase frequently leads to the collapse of the
cellular structure. To minimise this effect, the oil droplets as a template were used to maintain
the 3D architecture of the graphene aerogels.
Figure 3.2 displays the processing route of rGO aerogels by emulsion-templating. Once
obtained through the modified Hummers method, the GO (3.07 ml) was subsequently
dispersed in water (6.93 ml) to form GO suspension. Meanwhile, the non-toxic and efficient
reducing agent L-ascorbic acid was added. The suspension was then emulsified with the 25 ml
hydrophobic phase (decane) by hand-shaking and these two phases formed a homogeneous
41
GO emulsification (GO-em). The glass vessel containing the GO-em was then immersed in an
oil bath at 80℃ for 1 hour and finally the partially reduced GO was subject to oven drying at
60℃ for 3 days to remove the remaining liquid and complete the reduction.
Figure 3.2 Assembly strategy of rGO aerogels by emulsion-templating.
3.2.3 Synthesis of rGO Aerogels by Ice-templating
As is shown in Figure 3.3, freeze-casting combined with freeze-drying was utilised in the
ice-templating approach. The starting material GO was also achieved by using the modified
Hummers method and the GO suspension (16.9 mg/ml) was prepared by mixing the GO
(14.79 ml) with distilled water (33.98 ml) and organic additives (PVA: sucrose in a 1:1 weight
ratio). The GO-sus was then casted into cylindrical Teflon moulds and unidirectionally cooled
down to -60℃ with a 5℃ min-1
cooling rate. This was then followed with freeze-drying
which removes the ice crystals by directly sublimating them from liquid phase to gas phase
under the reduced surrounding pressure. Finally, the rGO aerogels were obtained after thermal
reduction within a tubular furnace at temperatures ranging from 200 to 800℃ for 20 min
under argon atmosphere.
42
Figure 3.3 Assembly strategy of rGO aerogels by ice-templating. (a) Flow chart of processing [2]. (b)
Schematic diagram of the freeze-casting technique [62] (the upper inset illustrates the temperature
variation of cold finger and sample while the lower inset plots the position of freezing front as a
function of time where the speed of freezing can be calculated by the tangent line of the curve).
(a)
(b)
43
3.3 Fabrication of rGO-BFO Nanocomposites
In order to obtain BFO nanoparticles with smaller size and establish a more detailed
understanding of the coupling between graphene and complex oxide ceramics, a novel sol-gel
method was employed to combine the rGO with BFO. Therein the rGO was utilised as a
substrate for nucleation and growth of BFO nanoparticles due to the large surface energy of
rGO, which enables the BFO nuclei to be absorbed on the surface of rGO flakes (Figure 3.4).
Figure 3.4 Fabrication process of rGO-BFO nanocomposites. From left to right: rGO aerogels
containing microscopic channels are infiltrated with BFO solution via a castable vacuum system,
followed by the high-temperature sintering in a tubular furnace. The growth of BFO particles is
confined by rGO aerogels [3].
3.3.1 Preparation of the rGO-BFO Mixture
A typical flow diagram depicting the formation process of BFO in the present work is shown
in Figure 3.5. Firstly, the bismuth nitrate pentahydrate (Bi(NO3)3·5H2O) and iron nitrate
nonahydrate (Fe(NO3)3·9H2O), weighed according to the stoichiometric ratio of 1:1 were
dissolved in the mixture of 2-methoxyethanol (C3H8O2) and ethanolamine. The solution (0.5
M) was stirred on a hotplate at 60℃ for 1 hour to ensure all the raw materials were fully
dissolved.
44
Figure 3.5 Flow diagram for the formation procedure of BFO nanoparticles.
The infiltration process was then carried out in the Buehler Cast N’ Vac castable vacuum
system (Figure 3.6). The prepared rGO aerogel was placed on the turntable within the vacuum
chamber while the BFO solution was poured into the mould and subsequently pumped into
the chamber to impregnate the cellular rGO aerogels. Meanwhile, the entrapped air in the
porous specimen was confirmed to be totally evacuated and the possibility of air entering was
likewise completely eliminated.
Figure 3.6 Image of the Castable Vacuum System: the BFO solution was poured into the mould and
subsequently pumped into the chamber to impregnate the rGO aerogels [63].
45
3.3.2 Annealing of the rGO-BFO Mixture
The rGO-BFO mixture was kept at 85℃ for 12 hours to obtain the dried BFO gel. Following
this drying treatment, the annealing process was respectively conducted at 400℃ for 4 hours
in air in the box furnace (Carbolite High-Temperature Box Furnace) and at 600-700℃ for 2–4
hours under argon atmosphere in the tubular furnace (LTF Tube Furnaces: 1200℃) (Figure
3.7). The ramp up/down rate during heat treatment was maintained at 3℃ min-1
.
Figure 3.7 Schematic diagram of annealing in the tubular furnace [64].
3.4 Characterisation
The structure and morphology of both the starting materials (GO, BFO nanoparticles) and the
resulting materials (rGO, rGO-BFO nanocomposites) were investigated by various
state-of-the-art techniques in order to find the optimal processing condition.
The phase constitutions were characterised by the X-ray diffraction (XRD) using a
PANanalytical XRD diffractometer in the 2θ range of 5-90°, with a step size of 0.05°. The
Raman spectra were collected using Renishaw 1000/2000 spectrometers equipped with
Olympus BH-2 microscope. The lasers used were HeNe laser (λ= 633 nm, Elaser=1.96 eV)
and Ar+ laser (λ= 514 nm, Elaser=2.41 eV) [65, 66]. The microstructural architecture of rGO
aerogels and the crystal morphologies of rGO-BFO nanocomposites were observed via
scanning electron microscopy (SEM) (Zeiss EVO50 VPSEM). Prior to the scanning, the
samples were coated with thin gold sputter in order to increase their electrical conductivity.
46
3.5 Measurement of Photocatalytic Activity
As is shown in Figure 3.8, the photocatalytic activity of rGO-BFO nanocomposites was
evaluated by degradation of Congo red (CR) under visible light (lamp, 75 W). Prior to
illumination, an amount of the rGO-BFO (20g L-1
) was dispersed in 50ml aqueous CR
solutions (0.1g L-1
) and the suspensions were magnetically stirred for 15 min. After irradiation
for 48 hours, the samples were filtered to separate the rGO-BFO particles before being
subjected to measurement by the UV-vis spectrophotometer. The degree of CR decomposition
was examined through the following expression:
D (%) = (1 -
) × 100% (3-1)
Where, A0 and A represent the initial absorbance of the CR solution and the value after
irradiation at λmax = 497 nm [67].
Figure 3.8 Schematic illustration of the photocatalytic mechanism of rGO-BFO nanocomposites
toward the degradation of CR [67].
47
Chapter 4. Results & Discussion
4.1 rGO Aerogels with 3D Cellular Structures
Emulsion-templating and ice-templating have been proved to be efficient strategies for the
fabrication of graphene aerogels with cellular architectures. However, each approach has its
drawbacks. The purpose of this project is to modify the two approaches in order to obtain
graphene aerogels with a controlled and stable structure, hence, successfully realising the
combination between graphene and complex oxide ceramics.
4.1.1 Emulsion-templating
The emulsion-templating method that is based on the mild chemical reduction is illustrated in
Figure 3.2. In order to diminish the capillary effect caused by the evaporation of liquid phase
in the reduction process, the utilisation of a secondary phase is considered to be a practical
technique for maintaining the 3D cellular architecture of rGO aerogels.
The amphiphilic GO here could be well dispersed into water and act as an emulsifier for the
decane/water mixture with the presence of the reducing agent L-ascorbic acid. The suspension
gradually turned dark since the reduction reaction started for 20minutes while the whole
reduction process at 80℃ for 1 hour allows the thorough removal of functional groups.
Meanwhile, this mild temperature (80℃) was conducive to the preservation of the porous
structure in the rGO aerogels. As a result, the increasing hydrophobic and π-π interaction of
the conjugated graphene enabled the self-assembly mechanism to be closely around the oil
droplets (Figure 4.1). Finally, the remaining oil and water were removed after being subject to
oven drying to form compact rGO aerogels with an average density of 18,mg,cm-1
, slightly
higher than previously reported emulsion-templated ones [17, 21, 22, 28, 68, 69].
48
Figure 4.1 Illustration of emulsification process. (a) Three-dimensional perspective. (b) Planar
perspective [2, 70].
Figure 4.2 presents the SEM images of rGO aerogels obtained via emulsion-templating. A
foam-like structure with interconnected pores is observed for the samples. Owing to a high oil
content (4:1 decane-to-water volume ratio), the size of the pores is approximately 100 μm, ten
times larger than previously reported graphene aerogels produced without an emulsion
template [22]. Consequently, the stable architecture of the rGO aerogels has been achieved by
this approach. However, it can be clearly observed that the distribution and the size of pores
within the rGO aerogels are not uniform, which implies that the influence of capillary action
has not been fully eliminated and that therefore the cell walls are wrinkled (Figure 4.2(d)).
The XRD patterns (Figure 4.3) provide the evidence on the elimination of major functional
groups. The peak of GO appears at 11.0°, while the peak of rGO appears at 23.1°,
corresponding to a d-spacing of 0.82 nm and 0.39 nm respectively. Another peak of rGO
appears at 42.7°, indicating the regeneration of graphitic microcrystals on the graphene plane
due to the reduction of GO [25]. The XRD analysis meanwhile demonstrates that the
L-ascorbic acid is a satisfactory reducing agent since the same degree of reduction was
obtained after 3-hour thermal reduction at 95℃ in Chen’s work [22] by using NaHSO3.
(a) (b)
49
Figure 4.2 SEM images of emulsion-templated rGO aerogels. (a-c) Overview of the cellular
architectures. (d) Morphology of cell wall.
Figure 4.3 XRD patterns of pristine graphite, GO aerogels and rGO aerogels.
(a) (b)
(c) (d)
50
4.1.2 Ice-templating
Apart from the above discussed emulsion-templating approach that uses oil droplets, hard
templates such as ice crystals can also be employed to control the architecture of rGO
aerogels. In this respect, a versatile technique that combines freeze-casting and thermal
reduction was performed to provide an accurate control of microstructure in the micrometre
scale. As illustrated in Figure 4.4, the GO suspension with an addition of organic additives
(sucrose and PVA) was subject to directional freeze-casting. The ice crystals grew more
rapidly along the direction of the temperature gradient and were subsequently sublimated by
freeze-drying, consequently creating continuous graphene oxide cellular networks (GO-CNs)
with a lamellar structure (Figure 4.5). As discussed in the literature review, the oil droplets
were alternatively incorporated by an extra emulsification step to produce a foam-like
microstructure [2]. However, the morphology of graphene networks has a negligible effect on
the fabrication and application of graphene-complex oxide ceramic nanocomposites in this
study and the emulsion template was therefore omitted to simplify the assembly process.
Figure 4.4 Rationale of freeze-casting. (a) The GO-sus is poured into a PTFE mould and placed onto
the cold plate, which is cooled by a liquid nitrogen bath. Temperature and cooling rate at the mould
bottom are controlled using a heater. (b) Following the arrows: Ice lamellae grow with the decreasing
of temperature, porosity is created after sublimation of ice crystals [62].
(a) (b)
51
Figure 4.5 Formation process of lamellar structure: ice crystals grow more rapidly in directions
perpendicular to the c-axis [62].
The thermal reduction was conducted at different temperatures between 200 and 800℃ for 20
minutes under argon atmosphere to remove functional groups and organic additives which
deteriorate the electrical conductivity of graphene. As a result, ultralight and hydrophobic
rGO aerogels were achieved (Figure 4.6).
Figure 4.6 Ultralight and hydrophobic rGO aerogels. (a) The rGO aerogel propped up on a leaf. (b)
The rGO aerogel floats on the water due to hydrophobicity.
Diverse organic additives play a critical role in this approach in maintaining the stability of
the lamellar structure. The PVA absorbed on the GO improves its wettability and surface
activity, which prevents excessive aggregation and which leads to an ultralow density of GO
aerogels ranging from 7.4 to 9.5mg,cm-1
. On the other hand, the sucrose reinforces the
structure of networks which can be easily affected by the elimination of ice crystals during
freeze-drying [2].
(a) (b)
52
Figure 4.7 Lamellar structure of ice-templated graphene aerogels. (a) Side view (parallel to casting
direction) and (b) top view (perpendicular to casting direction) of GO aerogels produced by
freeze-casting. (c) Side view and (d) top view of rGO aerogels after thermal reduction at 600℃. (e,f)
Wrinkled wall of rGO aerogels.
As is shown in Figure 4.7, a highly ordered lamellar structure with a honeycomb-like
cross-sectional morphology has been achieved in the samples. The average cell size of 25μm
is similar to previously reported carbon-based porous networks fabricated by freeze-casting
(a) (b)
(c)
(e)
(d)
(f)
53
[26, 71, 72]. The uniform size and shape of the cells demonstrates that the freezing rate was
very well controlled during the whole procedure. Despite the increasing π-π interaction, the
cell size of the rGO aerogels remained unchanged after thermal treatment at 600℃, as well as
the microstructure of the rGO aerogels.
The quantity of eliminated functional groups such as –OH and –COOH can be indicated by
the mass loss and volume shrinkage of samples after thermal reduction (Figure 4.8). As
summarised in Figure 4.9, there is a linear relationship between reduction temperature and
volume shrinkage or mass loss for reduction at 200-600℃. Surprisingly, when GO aerogels
were thermally reduced at 800℃, the mass loss and shrinkage are lower than that of the
samples reduced at 600℃, this unexpected result could be attributed to the burning of rubber
tube blocks which cannot sustain 800℃ of heating. Flaming particles filled out the pores
within rGO aerogels, leading to an additional weight and restriction of the shrinkage. Owing
to the temperature limitations of the equipment, the reduction temperature could not be further
increased. Nonetheless, the density of 3.15mg,cm-3
for rGO aerogels reduced at 600℃ is
within the range of 1.5 to 12mg,cm-3
from reported samples [29].
Figure 4.8 Shrinkage of samples after thermal reduction. Images of (a) GO aerogels and (b) 600℃
reduced rGO aerogels.
(a) (b)
54
Figure 4.9 Density, mass loss, volume shrinkage of rGO aerogels after thermal treatment at 200, 400,
600 and 800℃ respectively.
Due to the ultralow density and porous structure, the crystallinity of rGO aerogels can be
barely characterised by X-ray diffraction. In contrast, the presence of disorder in
sp2-hybridised carbon systems can result in resonance Raman spectra, making Raman
spectroscopy one of the most sensitive techniques for characterisation of carbon materials [65,
73-75]. There are three major bands in a Raman spectrum of graphene, namely D band, G
band, and 2D band. The D band is caused by the disordered structure of graphene, rarely
observed in graphite and high-quality graphene [76]. The G band arises from the first-order
scattering of the E2g phonon from sp2 carbon atoms and is sensitive to the number of layers
present in the sample. The strong peak in the range 2500 - 2800 cm-1
in the Raman spectra is
called 2D band, which is the signature for all kinds of sp2 carbon materials and can be only
detected in defect-free graphene samples [77]. Typically, the relative intensity ratio of D and
G peaks can be used to verify the reduction process. As illustrated in Figure 4.10, the value of
ID/IG boosts with the escalation of reduction temperature. This increase of ID/IG ratio is
commonly found in studies regarding the reduction of GO [78-81], which suggests that new
graphitic domains have been created and the π-π conjugated structure of graphene has been
partially restored, indicating the successful reduction with small defect concentration.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
0
1
2
3
4
5
6
7
8
reduced at
200℃
reduced at
400℃
reduced at
600℃
reduced at
800℃
density (mg/cm3)
mass loss (%)
shrinkage (%)
55
Figure 4.10 Raman spectra of the as-prepared GO aerogels and rGO aerogels reduced at 200, 400 and
600℃.
4.1.3 Comparison of Two Approaches
On completion of fabrication and characterisation, the rGO aerogels produced by
emulsion-templating and ice-templating approaches were methodically compared in terms of
the microstructure in order to select the preferable samples for following procedures (Figure
4.11).
The emulsion-templating strategy proposed in the current study produces rGO aerogels by
self-assembly at oil-water interface on the basis of a mild chemical reduction process (80℃),
which exhibits several unique advantages: First, the low concentration of GO (0.44 mg ml-1
)
in GO-em contributes to a high porosity and ultralow density. Secondly, this approach is
simple and energy-efficient since the whole reduction procedure can be conducted at mild
temperatures without the utilisation of special equipment. Thirdly, the additive-free resulting
materials could retain the intrinsic properties of graphene and the usage of L-ascorbic acid as
the reducing agent can be less hazardous compared with using toxic HI.
However, the collapse of pores caused by capillary action still unavoidably remains a
challenge for mild chemical reduction - based approaches despite the introduction of an
56
emulsion template. In contrast, the chemistry and architecture of materials can be very
precisely controlled in the ice-templating strategy presented here due to the flexibility and
scalability. In addition, the freeze-drying ensures the minimal distortion of structure after
segregation from the liquid phase. More specifically, considering the utilisation of a template
to shape the formation of BFO nanoparticles with an approximate 200 nm grain size, the
ice-templated rGO aerogels with correct cell size and highly-organised architecture at the
nanometre scale are unambiguously preferred.
Figure 4.11 Comparison of rGO aerogels. Macro-morphology of (a) emulsion-templated and (b)
ice-templated samples. Microstructure of (c) emulsion-templated and (d) ice-templated samples.
4.2 rGO-BFO Nanocomposites
In order to preferably impart high electrical conductivity and a low electron-hole pair
recombination rate, the ice-templated rGO aerogels with highly-ordered lamellar pores
synthesised by directional freeze-casting were chosen to be hybridised with BFO. Due to its
high organic absorption capability [2], the rGO aerogels can be fully infiltrated with the BFO
(c) (d)
(a) (b)
57
precursor solution containing 68 wt.% of 2-methoxyethanol (C3H8O2). Here, the rGO aerogels
act as a skeleton that templates the formation of BFO nanoparticles.
4.2.1 Effect of Infiltration
Figure 4.12(a) shows the XRD patterns of rGO-BFO nanocomposites containing 1 infiltrated
BFO layer and 5 infiltrated BFO layers after annealing in air at 400℃ for 4 hours. In spite of
some impurity phases such as Bi2O3 and Fe2O3, the majority of both samples are
perovskite-type BFO (R phase). More pronounced peaks can be observed in nanocomposites
with 5 BFO layers, demonstrating the proportionality relationship between the cycle of BFO
infiltration and the amount of crystallised BFO nanoparticles. The infiltration procedure is
therefore suggested to be repeated several times in order to fill more pores within rGO
aerogels.
Figure 4.12 Effect of the amount of infiltrated layers. (a) XRD patterns of rGO-BFO nanocomposites
containing 1 BFO layer and 5 BFO layers. (b) Raman spectra of as-prepared rGO aerogels, rGO-BFO
nanocomposites with 1 BFO layer and 5 BFO layers.
One unanticipated outcome is that the rGO was ‘burnt out’ in the air since the evidence of
graphene cannot be found in the Raman spectra (Figure 4.12(b)). The reaction between carbon
materials and oxygen at high temperatures has therefore been recognised as one of the
greatest challenges in this sintering procedure. In this regard, annealing must be carried out in
reducing atmosphere.
(a) (b)
58
4.2.2 Effect of Annealing Conditions
Apart from infiltration and annealing atmosphere, the annealing temperature and dwell time
can also be of significance in determining the crystallisation of BFO nanoparticles and the
preservation of hierarchical structure of rGO aerogels. Figure 4.13(a) displays the Raman
spectra of rGO-BFO nanocomposites annealed under different conditions. The D band for the
sample that annealed at 600℃ is relatively more pronounced than that of the sample annealed
at 700℃ for the same length of dwell time, this can be interpreted as the better preservation of
rGO flakes since more distortion of sp2 domain in the hexagonal graphitic layers of rGO have
been identified. Meanwhile, the G band can be only detected in the Raman spectrum of the
sample annealed for 2 hours, indicating that the breakage of sp2 C-C bonds and hierarchical
structure in rGO could be associated with the growth of BFO nanoparticles, which had a
remarkable influence after heat treatment for 2 hours. Interestingly, both the D band and G
band in the Raman spectra disappeared after annealing at 700℃ for 3 hours. One possible
implication of this is that a large number of C-C bonds were replaced by newly generated
bonds between rGO and BFO. It is worth mentioning that the D bands of the samples have
been found to have shifted from 1350 cm-1
to 1400 cm-1
as a result of the interaction between
rGO and BFO [82].
Figure 4.13 Effect of annealing conditions. (a) Raman spectra (b) XRD patterns of rGO-BFO
nanocomposites annealed at different conditions: 600℃ 4 hours, 700℃ 4 hours, 700℃ 3 hours and
700℃ 2 hours.
(a) (b)
59
Figure 4.13(b) shows the XRD patterns of rGO-BFO nanocomposites annealed at 600℃ for 4
hours and 700℃ for 2-4 hours. The intensity of rhombohedrally distorted perovskite
diffraction peaks increases prominently by increasing annealing temperature and dwell time,
demonstrating that unlike annealing in the air, the crystallisation of BFO with less impurity
under annealing in argon atmosphere requires a higher annealing temperature and more
reaction time [83]. Therefore, the single-phase perovskite structure has failed to be realised
due to the insufficient annealing temperature and dwell time applied in this study. As a
consequence, the kinetics of phase formation results in a lot of impurity phases including
Bi2O3, Fe2O3, and Bi2Fe4O9. In addition, as the majority of the impurity phases are Bi2O3, one
possible reason for this result can be the decomposition of unstable Bi(NO3)3·5H2O after the
long-time standing of the BFO precursor solution. Therefore, it is suggested an excessive
amount of Fe(NO3)3·9H2O is added to react with Bi(NO3)3·5H2O.
Figure 4.14 SEM images of rGO-BFO nanocomposites annealed at different conditions. (a) 600℃ 4
hours, (b) 700℃ 4 hours, (c) 700℃ 3 hours, (d) 700℃ 2 hours.
(d) (c)
(b) (a)
60
Figure 4.14 exhibits the microstructural morphologies of rGO-BFO nanocomposites, the
morphological evolution of particles at different annealing stages can be concluded from these
SEM images. The BFO nanoparticles ranging from 80-200 nm attach on the rGO flakes have
formed and particles with a larger size can be observed for samples annealed for 4 hours since
a longer dwell time allows for more growth of BFO nuclei. Driven by the large surface energy
of rGO, the nucleation takes place on the surface of rGO. The further growth of BFO is
limited by the steric effect of rGO aerogels and the migration of the seed particles is therefore
restricted, giving rise to a reduced size of BFO nanoparticles. The similar size and
morphology of BFO have also been achieved by Li et al [9] in their study on decorating
graphene nanosheets with BFO nanoparticles through a hydrothermal approach.
4.2.3 Photocatalytic activity
After 72 hours of irradiation, the CR was decomposed by rGO-BFO samples. Decolourised
solutions can be observed in Figure 4.15. More specifically, the solution containing rGO-BFO
annealed at 700℃ for 3 hours is almost transparent, suggesting the highest decomposition
efficiency among all the samples [84].
Figure 4.15 Image of decolourised CR solutions after catalytic effect by rGO-BFO nanocomposites
annealed for 600℃ 4 hours, 700℃ 4 hours, 700℃ 3 hours, 700℃ 2 hours and blank CR solution
(from left to right).
The UV-vis absorption spectra of CR in the presence of different samples are shown in Figure
4.16. The intensity of absorption peak at λ = 497 has changed under the photocatalytic effect
61
of rGO-BFO nanocomposites, demonstrating the degradation of CR [28]. The concentration
of CR is deduced from the Beer-Lambert Law [86]:
A = log
=εlc (4.1)
Where, A is the absorbance, I is the radiant intensity, ε is the absorptivity, l is the length of the
beam and c is the concentration of the absorbing species.
Figure 4.16 UV-vis absorption spectra of CR after 72 hours of irradiation. The absorption wavelengths
at 340 nm and 497 nm stem from the naphthalene rings [28] and the azo bonds.
Figure 4.17 Concentration of CR relative to its initial value (C/C0) after photocatalytic effect by
different rGO-BFO samples.
62
The removal efficiency of CR can therefore be expressed by the value of C/C0 as plotted in
Figure 4.17, where the lowest value is achieved by the solution containing rGO-BFO
nanocomposites annealed at 700℃ for 3 hours. This result, combined with the Raman
analysis indicates that the close surface contact and chemical bonding between rGO and BFO
are highly likely to be responsible for the exceptional photocatalytic activity under visible
light. In addition, it is possible that the interaction between CR and rGO via π-π stacking to be
another factor that gives rise to the degradation of CR [9, 10, 67]. However, It should be noted
that the thermal catalytic effect might also contribute to the degradation of CR since the
concentration of CR in the blank sample has also decreased. Therefore, the reaction
temperature is suggested to be kept at 0℃ to prevent the influence of heat.
63
Chapter 5. Conclusions & Future Work
5.1 Conclusion
This study set out to demonstrate a novel sol-gel method for the development of rGO-BFO
macroscopic cellular nanocomposites and to investigate the correlation between processing,
structure and photocatalytic properties of the resulting materials. Conclusions from a series of
analysis can be drawn as the following:
1. Ice-templated rGO aerogels produced via the freeze-casting technique exhibit a
highly-organised lamellar structure and superhydrophobicity, providing an effective approach
to decorate BFO particles onto the rGO flakes.
2. The outstanding chemical and structural stability of rGO aerogels upon thermal reduction
benefit the nucleation and growth of BFO nanoparticles templated by rGO flakes, which can
even be well-preserved after 4 hours of heat treatment at 700℃.
3. The infiltration process is recommended to be repeated multiple times in order to fill up the
voids within cellular rGO aerogels.
4. Increasing annealing temperature and dwell time is found to be an effective way to obtain
well-crystallised BFO nanoparticles, particularly for annealing under reducing atmosphere.
5. The superior photocatalytic performance under visible light could be obtained by varying
the heat treatment temperature and dwell time. Finally, 700℃ and 3 hours is considered to be
the optimal annealing condition in this work due to the close surface contact and chemical
bonding established between rGO and BFO.
Notwithstanding the instrumental limitations, the findings from this study substantiate the
feasibility of combining graphene with BFO via a sol-gel process, which can be easily
extended to the preparation of other grapheme-complex oxide ceramic nanocomposites.
64
5.2 Future Work
Considering the limitations of the timescale for this project, much information regarding the
structure and photocatalytic properties of rGO-BFO nanocomposites still remains unknown.
Based on the presented study, it is recommended that further research be undertaken
according to the following aspects:
1. Attempts can be taken to further increase the thermal reduction temperature for GO up to
1000℃, by which the functional groups can be more thoroughly removed, enabling the rGO
aerogels with better hydrophobicity and electrical conductivity to finally attract more BFO
nuclei to attach onto the surface of rGO flakes.
2. As the crystallinity of BFO is closely linked to the annealing temperatures, the rGO-BFO
can therefore be annealed at temperatures above 700℃, while the dwell time can be
accordingly adjusted in order to maintain the hierarchical structure of rGO aerogels and the
interaction between rGO and BFO.
3. The photo-generated electrons from BFO nanoparticles are believed to be transported by
the chemical bonding between rGO and BFO, which could be characterised by X-ray
photoelectron spectroscopy (XPS). Detailed information about the mobility of
photo-generated electrons and the oxidation state of elements obtained from XPS analysis
could validate the role of rGO in modulating the particle size and bandgaps of BFO.
4. It would also be interesting to explore the photodegradation of CR as a function of
irradiation time under visible light. Furthermore, it is strongly recommended that the links
between bandgaps of rGO-BFO nanocomposites which are the primary cause of the
photocatalytic performance and the photodegradation efficiency of CR are further
investigated.
65
References
1. Geim A, Novoselov K. The rise of graphene. Nature Materials. 2007; 6(3):183-191.
2. Barg S, Perez F, Ni N, do Vale Pereira P, Maher R, Garcia-Tuñon E et al. Mesoscale
assembly of chemically modified graphene into complex cellular networks. Nature
Communications. 2014; 5.
3. D'Elia E, Barg S, Ni N, Rocha V, Saiz E. Self-Healing Graphene-Based Composites
with Sensing Capabilities. Adv Mater. 2015; 27(32):4788-4794.
4. Chen Z, Ren W, Gao L, Liu B, Pei S, Cheng H. Three-dimensional flexible and
conductive interconnected graphene networks grown by chemical vapour deposition.
Nature Materials. 2011; 10(6):424-428.
5. Zhu Y, Murali S, Stoller M, Ganesh K, Cai W, Ferreira P et al. Carbon-Based
Supercapacitors Produced by Activation of Graphene. Science. 2011;
332(6037):1537-1541.
6. Niu Z, Chen J, Hng H, Ma J, Chen X. A Leavening Strategy to Prepare Reduced
Graphene Oxide Foams. Adv Mater. 2012; 24(30):4144-4150.
7. Yadav R, Baeg J, Oh G, Park N, Kong K, Kim J et al. A Photocatalyst–Enzyme Coupled
Artificial Photosynthesis System for Solar Energy in Production of Formic Acid from
CO2. J Am Chem Soc. 2012; 134(28):11455-11461.
8. Li T, Shen J, Li N, Ye M. Hydrothermal preparation, characterization and enhanced
properties of reduced graphene-BiFeO3 nanocomposite. Materials Letters. 2013;
91:42-44.
9. Li Z, Shen Y, Guan Y, Hu Y, Lin Y, Nan C. Bandgap engineering and enhanced interface
coupling of graphene–BiFeO3 nanocomposites as efficient photocatalysts under visible
light. J Mater Chem A. 2014; 2(6):1967-1973.
10. Li Z, Shen Y, Yang C, Lei Y, Guan Y, Lin Y et al. Significant enhancement in the visible
light photocatalytic properties of BiFeO3–graphene nanohybrids. J Mater Chem A. 2013;
1(3):823-829.
11. Yang N, Zhai J, Wang D, Chen Y, Jiang L. Two-Dimensional Graphene Bridges
Enhanced Photoinduced Charge Transport in Dye-Sensitized Solar Cells. ACS Nano.
2010; 4(2):887-894.
12. Tien H, Huang Y, Yang S, Wang J, Ma C. The production of graphene nanosheets
decorated with silver nanoparticles for use in transparent, conductive films. Carbon.
2011; 49(5):1550-1560.
13. Tang B, Hu G. Two kinds of graphene-based composites for photoanode applying in
dye-sensitized solar cell. Journal of Power Sources. 2012; 220:95-102.
14. Chen J, Jang C, Xiao S, Ishigami M, Fuhrer M. Intrinsic and extrinsic performance
66
limits of graphene devices on SiO2. Nature Nanotech. 2008; 3(4):206-209.
15. Lee C, Wei X, Kysar J, Hone J. Measurement of the Elastic Properties and Intrinsic
Strength of Monolayer Graphene. Science. 2008; 321(5887):385-388.
16. Wang X, Zhi L, Müllen K. Transparent, Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells. Nano Letters. 2008; 8(1):323-327.
17. Yang H, Zhang T, Jiang M, Duan Y, Zhang J. Ambient pressure dried graphene aerogels
with superelasticity and multifunctionality. J Mater Chem A. 2015; 3(38):19268-19272.
18. Sridhar V, Lee I, Yoon H, Chun H, Park H. Microwave synthesis of three dimensional
graphene-based shell-plate hybrid nanostructures. Carbon. 2013; 61:633-639.
19. Xu Y, Sheng K, Li C, Shi G. Self-Assembled Graphene Hydrogel via a One-Step
Hydrothermal Process. ACS Nano. 2010; 4(7):4324-4330.
20. Worsley M, Pauzauskie P, Olson T, Biener J, Satcher J, Baumann T. Synthesis of
Graphene Aerogel with High Electrical Conductivity. J Am Chem Soc. 2010;
132(40):14067-14069.
21. Hu H, Zhao Z, Wan W, Gogotsi Y, Qiu J. Ultralight and Highly Compressible Graphene
Aerogels. Adv Mater. 2013; 25(15):2219-2223.
22. Chen W, Yan L. In situ self-assembly of mild chemical reduction graphene for
three-dimensional architectures. Nanoscale. 2011; 3(8):3132.
23. Hummers W, Offeman R. Preparation of Graphitic Oxide. J Am Chem Soc. 1958;
80(6):1339-1339.
24. Lv W, Zhang C, Li Z, Yang Q. Self-Assembled 3D Graphene Monolith from Solution. J
Phys Chem Lett. 2015; 6(4):658-668.
25. Zhang B, Wang T, Liu S, Zhang S, Qiu J, Chen Z et al. Structure and morphology of
microporous carbon membrane materials derived from poly (phthalazinone ether sulfone
ketone). Microporous and Mesoporous Materials. 2006; 96(1-3):79-83.
26. Qiu L, Liu J, Chang S, Wu Y, Li D. Biomimetic superelastic graphene-based cellular
monoliths. Nature Communications. 2012; 3:1241.
27. Ling Z, Wang G, Dong Q, Qian B, Zhang M, Li C et al. An ionic liquid template
approach to graphene–carbon xerogel composites for supercapacitors with enhanced
performance. J Mater Chem A. 2014; 2(35):14329.
28. Zhang B, Zhang J, Sang X, Liu C, Luo T, Peng L et al. Cellular graphene aerogel
combines ultralow weight and high mechanical strength: A highly efficient reactor for
catalytic hydrogenation. Sci Rep. 2016; 6:25830.
29. Ni N, Barg S, Garcia-Tunon E, Macul Perez F, Miranda M, Lu C et al. Understanding
Mechanical Response of Elastomeric Graphene Networks. Sci Rep. 2015; 5:13712.
30. Hirata M, Gotou T, Horiuchi S, Fujiwara M, Ohba M. Thin-film particles of graphite
67
oxide 1. Carbon. 2004; 42(14):2929-2937.
31. Poncharal P, Ayari A, Michel T, Sauvajol J. Raman spectra of misoriented bilayer
graphene. Phys Rev B. 2008; 78(11).
32. Qian Y, Ismail I, Stein A. Ultralight, high-surface-area, multifunctional graphene-based
aerogels from self-assembly of graphene oxide and resol. Carbon. 2014; 68:221-231.
33. Worsley M, Kucheyev S, Satcher J, Hamza A, Baumann T. Mechanically robust and
electrically conductive carbon nanotube foams. Appl Phys Lett. 2009; 94(7):073115.
34. Zou J, Liu J, Karakoti A, Kumar A, Joung D, Li Q et al. Ultralight Multiwalled Carbon
Nanotube Aerogel. ACS Nano. 2010; 4(12):7293-7302.
35. Zhang X, Sui Z, Xu B, Yue S, Luo Y, Zhan W et al. Mechanically strong and highly
conductive graphene aerogel and its use as electrodes for electrochemical power sources.
Journal of Materials Chemistry. 2011; 21(18):6494.
36. Choi T, Lee S, Choi Y, Kiryukhin V, Cheong S. Switchable Ferroelectric Diode and
Photovoltaic Effect in BiFeO3. Science. 2009; 324(5923):63-66.
37. Gao F, Chen X, Yin K, Dong S, Ren Z, Yuan F et al. Visible-Light Photocatalytic
Properties of Weak Magnetic BiFeO3 Nanoparticles. ChemInform. 2007; 38(49).
38. Li S, Lin Y, Zhang B, Wang Y, Nan C. Controlled Fabrication of BiFeO3 Uniform
Microcrystals and Their Magnetic and Photocatalytic Behaviors. J Phys Chem C. 2010;
114(7):2903-2908.
39. Liu H, Liu Z, Liu Q, Yao K. Ferroelectric properties of BiFeO3 films grown by sol–gel
process. Thin Solid Films. 2006; 500(1-2):105-109.
40. Gao F, Yuan Y, Wang K, Chen X, Chen F, Liu J et al. Preparation and photoabsorption
characterization of BiFeO3 nanowires. Appl Phys Lett. 2006; 89(10):102506.
41. Du Y, Cheng Z, Xue Dou S, Attard D, Lin Wang X. Fabrication, magnetic, and
ferroelectric properties of multiferroic BiFeO3 hollow nanoparticles. J Appl Phys. 2011;
109(7):073903.
42. Morozov M, Lomanova N, Gusarov V. Specific Features of BiFeO3 Formation in a
Mixture of Bismuth (III) and Iron (III) Oxides. Russian Journal of General Chemistry.
2003; 73(11):1676-1680.
43. Valant M, Axelsson A, Alford N. Peculiarities of a Solid-State Synthesis of Multiferroic
Polycrystalline BiFeO3. Chemistry of Materials. 2007; 19(22):5431-5436.
44. Park T, Papaefthymiou G, Viescas A, Moodenbaugh A, Wong S. Size-Dependent
Magnetic Properties of Single-Crystalline Multiferroic BiFeO3 Nanoparticles. Nano
Letters. 2007; 7(3):766-772.
45. Lv Y, Xing J, Zhao C, Chen D, Dong J, Hao H et al. The effect of solvents and
surfactants on morphology and visible-light photocatalytic activity of BiFeO3
microcrystals. J Mater Sci: Mater Electron. 2014; 26(3):1525-1532.
68
46. Silva J, Reyes A, Esparza H, Camacho H, Fuentes L. BiFeO3 : A Review on Synthesis,
Doping and Crystal Structure. Integrated Ferroelectrics. 2011; 126(1):47-59.
47. Chen X, Qiu Z, Zhou J, Zhu G, Bian X, Liu P. Large-scale growth and shape evolution
of bismuth ferrite particles with a hydrothermal method. Materials Chemistry and
Physics. 2011; 126(3):560-567.
48. Xu J, Ke H, Jia D, Wang W, Zhou Y. Low-temperature synthesis of BiFeO3
nanopowders via a sol–gel method. Journal of Alloys and Compounds. 2009;
472(1-2):473-477.
49. Kim J, Kim S, Kim W. Sol–gel synthesis and properties of multiferroic BiFeO3.
Materials Letters. 2005; 59(29-30):4006-4009.
50. Tu Y, Chang C, Wu M, Shyue J, Su W. BiFeO3/YSZ bilayer electrolyte for low
temperature solid oxide fuel cell. RSC Advances. 2014; 4(38):19925.
51. Basu S, Martin L, Chu Y, Gajek M, Ramesh R, Rai R et al. Photoconductivity in BiFeO3
thin films. Appl Phys Lett. 2008; 92(9):091905.
52. Hengky C, Moya X, Mathur N, Dunn S. Evidence of high rate visible light
photochemical decolourisation of Rhodamine B with BiFeO3 nanoparticles associated
with BiFeO3 photocorrosion. RSC Advances. 2012; 2(31):11843.
53. P. Kubelka, F. Munk. An Article on Optics of Paint Layers. Tech. Z. Phys. 1931, 12, 593.
54. Yao J, Shen X, Wang B, Liu H, Wang G. In situ chemical synthesis of SnO2–graphene
nanocomposite as anode materials for lithium-ion batteries. Electrochemistry
Communications. 2009; 11(10):1849-1852.
55. Zhou M, Lin T, Huang F, Zhong Y, Wang Z, Tang Y et al. Highly Conductive Porous
Graphene/Ceramic Composites for Heat Transfer and Thermal Energy Storage. Adv
Funct Mater. 2012; 23(18):2263-2269.
56. Štengl V, Bakardjieva S, Grygar T, Bludská J, Kormunda M. TiO2-graphene oxide
nanocomposite as advanced photocatalytic materials. Chemistry Central Journal. 2013;
7(1):41.
57. Zhang N, Zhang Y, Xu Y. Recent progress on graphene-based photocatalysts: current
status and future perspectives. Nanoscale. 2012; 4(19):5792.
58. Xiong Z, Zhang L, Ma J, Zhao X. Photocatalytic degradation of dyes over
graphene–gold nanocomposites under visible light irradiation. Chemical
Communications. 2010; 46(33):6099.
59. The first order Raman spectrum of isotope labelled nitrogen-doped reduced graphene
oxide [Internet]. Utu.fi. 2016 [cited 6 August 2016]. Available from:
https://www.utu.fi/en/units/sci/units/chemistry/research/mcca/PublishingImages/GO%20
rGO.jpg
60. Lambert R. Types of graphene | The University of Manchester [Internet].
69
Graphene.manchester.ac.uk. 2016 [cited 3 August 2016]. Available from:
http://www.graphene.manchester.ac.uk/collaborate/types-of-graphene/
61. Reduced Graphene Oxide - What Is It? How Is It Created? [Internet]. Graphenea. 2016
[cited 3 August 2016]. Available from:
http://www.graphenea.com/pages/reduced-graphene-oxide#.V6H-gbgrLIU
62. Deville S, Saiz E, Nalla R K, et al. Freezing as a path to build complex composites.
Science, 2006, 311(5760): 515-518.
63. Cast N' Vac Castable Vacuum System | Buehler [Internet]. Shop.buehler.com. 2016
[cited 19 August 2016]. Available from: https://shop.buehler.com/node/1636
64. Chemical Vapor Deposition - Nanoscience & Nanotechnology [Internet].
Sites.google.com. 2016 [cited 16 August 2016]. Available from:
https://sites.google.com/site/nanomodern/Home/CNT/syncnt/cvd
65. Gong L. Deformation Micromechanics of Graphene Nanocomposites [PhD Thesis]. The
University of Manchester; 2016.
66. Li Z. Raman Spectroscopic Studies of the Mechanics of Graphene-based
Nanocomposites [PhD Thesis]. The University of Manchester; 2016.
67. Soltani TLee B. Sono-synthesis of nanocrystallized BiFeO3-reduced graphene oxide
composites for visible photocatalytic degradation improvement of bisphenol A.
Chemical Engineering Journal. 2016; 306:204-213.
68. Menzel R, Barg S, Miranda M, Anthony D, Bawaked S, Mokhtar M et al. Joule Heating
Characteristics of Emulsion-Templated Graphene Aerogels. Adv Funct Mater. 2014;
25(1):28-35.
69. Li Y, Chen J, Huang L, Li C, Hong J, Shi G. Highly Compressible Macroporous
Graphene Monoliths via an Improved Hydrothermal Process. Adv Mater. 2014;
26(28):4789-4793.
70. Thickett S, Wood N, Ng Y, Zetterlund P. Hollow hybrid polymer–graphene oxide
nanoparticles via Pickering miniemulsion polymerization. Nanoscale. 2014; 6(15):8590.
71. Kuang J, Liu L, Gao Y, Zhou D, Chen Z, Han B et al. A hierarchically structured
graphene foam and its potential as a large-scale strain-gauge sensor. Nanoscale. 2013;
5(24):12171.
72. Xie X, Zhou Y, Bi H, Yin K, Wan S, Sun L. Large-range Control of the Microstructures
and Properties of Three-dimensional Porous Graphene. Sci Rep. 2013; 3.
73. Malard L, Pimenta M, Dresselhaus G, Dresselhaus M. Raman spectroscopy in graphene.
Physics Reports. 2009; 473(5-6):51-87.
74. Ferrari A, Meyer J, Scardaci V, Casiraghi C, Lazzeri M, Mauri F et al. Raman Spectrum
of Graphene and Graphene Layers. Phys Rev Lett. 2006; 97(18)
75. Ferrari A, Basko D. Raman spectroscopy as a versatile tool for studying the properties
70
of graphene. Nature Nanotech. 2013; 8(4):235-246.
76. Characterization of Graphene Using Raman Spectroscopy [Internet]. AZoM.com. 2012
[cited 6 August 2016]. Available from:
http://www.azom.com/article.aspx?ArticleID=6271#3
77. Tang B, Guoxin H, Gao H. Raman Spectroscopic Characterization of Graphene. Applied
Spectroscopy Reviews. 2010; 45(5):369-407.
78. Stankovich S, Dikin D, Piner R, Kohlhaas K, Kleinhammes A, Jia Y et al. Synthesis of
graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon.
2007; 45(7):1558-1565.
79. Bose S, Kuila T, Mishra A, Kim N, Lee J. Dual role of glycine as a chemical
functionalizer and a reducing agent in the preparation of graphene: an environmentally
friendly method. Journal of Materials Chemistry. 2012; 22(19):9696.
80. Chen Y, Zhang X, Zhang D, Yu P, Ma Y. High performance supercapacitors based on
reduced graphene oxide in aqueous and ionic liquid electrolytes. Carbon. 2011;
49(2):573-580.
81. Wang H, Robinson J, Li X, Dai H. Solvothermal Reduction of Chemically Exfoliated
Graphene Sheets. J Am Chem Soc. 2009; 131(29):9910-9911.
82. Tiwari A, Syvajarvi M, Richardson R. Graphene materials.
83. Sakar M, Balakumar S, Saravanan P, Jaisankar S. Annealing temperature mediated
physical properties of bismuth ferrite (BiFeO3) nanostructures synthesized by a novel
wet chemical method. Materials Research Bulletin. 2013; 48(8): 2878-2885.
84. Bhoi Y, Pradhan S, Behera C, Mishra B. Visible light driven efficient photocatalytic
degradation of Congo red dye catalyzed by hierarchical CuS–Bi2CuxW1−xO6−2x
nanocomposite system. RSC Adv. 2016; 6(42):35589-35601.
85. Shi W, Liu X, Zhang T, Wang Q, Zhang L. Magnetic nano-sized cadmium ferrite as an
efficient catalyst for the degradation of Congo red in the presence of microwave
irradiation. RSC Adv. 2015; 5(63):51027-51034.
86. Swinehart D F. The beer-lambert law. J. Chem. Educ, 1962, 39(7): 333.