Synthesis and characterisation of graphene hybrid nanoarchitechures for potential sensing applications Mahesh Vaka 213188021 Master of Science (Higher Degree by Research) Submitted in fulfilment of the requirements for the degree of Master of Science (Higher Degree by Research) School of Life and Environmental Sciences Deakin University November 2015 V V
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Synthesis and characterisation of graphene hybrid nanoarchitechures for potential sensing applications
Mahesh Vaka 213188021
Master of Science (Higher Degree by Research)
Submitted in fulfilment of the requirements for the degree of
Master of Science (Higher Degree by Research)
School of Life and Environmental Sciences
Deakin University
November 2015
V V
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Acknowledgement
First of all, I would like to say thanks to Deakin University for giving me the opportunity to
undertake my Master of Science project at the School of Life & Environmental Science.
Foremost, I would like to show my honour and express my gratitude to my supervisor Dr.
Wenrong Yang for his patience, guidance, helping, sharing his scientific knowledge and
constant encouragement throughout the project.
It’s honour to acknowledge Dr. Colin Barrow, my Co-supervisor who funded me for all the
chemicals and equipment’s which I required during the course of my project.
It’s my pleasure to thank Dr. Xavier Conlan, myCo-supervisor who helped with his valuable
suggestions and encouragement.
Special thanks to Motilal and Tej for helping with my thesis and for their valuable time and
great suggestions and supportive throughout my Master of Science.
Hearty thanks to Dr. Munish Puri and his lab members who helped with the equipment.
Sincere thanks to Dr. Nguyen Dam Nam who helped me with EIS data and valuable
suggestions with experimental data.
I would like to thank all my friends, especially my close friends (Ranjith, Shashank, Aditya
Potti, Thanuja, Mrudhula and Lavanya) and lab members who helped me in times and support
me and boosts my spirit throughout my project.
Last but not least, I dedicate this work and my career to my Dad, Mom, Grandfather and
brothers who have been with me all the time and supporting me both financially and
emotionally.
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Conference poster presentation
Mahesh Vaka, Motilal Mathesh, Da Li, Alireza Dijabi, Colin Barrow, Conlan, X and Wenrong
Yang
The application of Graphene based materials for electrochemical actuators presented at 19th
Australia and New Zealand Electrochemistry Symposiumheld at Melbourne on 2013.
Mahesh, V, Motilal, M, Li, D,Barrow, C J, Conlan, X and Yang, W*
A highly sensitive strain gauge using a hybrid graphene material presented at RACI national
congress held at Adelaide on December 2014.
Manuscripts under preparation
Mahesh et.al; Efficient electron transfer pathways: New class of graphene based electrodes.
1.9.3.2 Drug Delivery ................................................................................................................................. 25
1.9.3.4 Cancer Therapy .............................................................................................................................. 26
Figure 1Armchair (blue) and zigzag (red) edges in monolayer graphene8. ............................................. 3 Figure 2 Nanocomposites Design Space11 ................................................................................................... 4 Figure 3 Different graphene forms17. ............................................................................................................. 6 Figure 4 Sp2-hybridization of carbon atoms monolayer model in graphene. The appropriate crystalline structure of graphene is a hexagonal grid19. .............................................................................. 7 Figure 5 Graphical representation of the protocol for the fabrication of a Graphene/SAM altered gold electrode and the heterogeneous ET mechanism on the Graphene altered electrode32. ................... 10 Figure 6 Diagrammatic illustration of four electro modified with SAMs of AET, AHT, AOT, and AUT and then subsequently altered with a monolayer of gold nanoparticles33.............................................. 11 Figure 7 Voltammetric responses of an Au/SAM modified graphene electrode in the solution of 1 M KCl containing 10 mM Ru(NH3)6 3+. Potential scanning rate: 10 mV/s40. ............................................. 12 Figure 8 Actuation of the bilayer paper sample as a function of relative humidity (%), a) 12, b) 25, c) 49, d) 61, e) 70, and f) 90. White-arrowed side: surface of graphene oxide layer44. ........................... 14 Figure 9 Illustrates the Change in resistance with the strain for a NP film which is functionalized by4-NTP52. ............................................................................................................................................................... 16 Figure 10 Graphene Field Effect Transistor70 ............................................................................................. 19 Figure 11 Alternating layers of graphene and tin are used to create a nanoscale composite for renewable lithium ion batteries79 .................................................................................................................. 20 Figure 12 Graphene based ultracapacitor71 ............................................................................................... 21 Figure 13 Represents protein detection by graphene-gold nanoparticle conjugates.86 ....................... 22 Figure 14 Graphene materials used in different sensors to detect biomolecules98 .............................. 24 Figure 15 Graphene material used in cancer therapy112 .......................................................................... 27 Figure 16 Represents the self-assembly of hybrid graphene film (Mahesh, Visual Molecular Dynamics). ....................................................................................................................................................... 35 Figure 17 Pictorial representation of graphene hybrid fabrication (Mahesh, PowerPoint). ................. 35 Figure 18 Surface properties: a) UV-Vis for Gr, AuNPs, AuNPs coated cystamine and Gr-AuNPs coated cystamine. b) Zeta potential Gr and Gr-functionalised gold NPs in aqueous dispersion. ...... 37 Figure 19 SEM image: Cross-section of graphene hybrid film. ............................................................... 38 Figure 20 Schematic representation graphene hybrid actuator a,b,c) Response of gr-hybrid actuator before and after applying different potential range ± 0.6, d) Front view of gr hybrid actuator (Mahesh, Power Point). ................................................................................................................................. 38 Figure 21 Deflection ∂ (swelling of the film) of the graphene hybrid actuator as a function of time, wave potential at ± 0.6 V. a) functionalised gold NPs sandwiched between graphene layers. b) thiourea functionalised gold NPs sandwiched between graphene layers. ............................................. 39 Figure 22 Actuator response on applying ± 0.6 V. a,b) Current as a function of time arising from charging and discharging current. c,d) Anson plot represents charging and discharging current. .... 40 Figure 23 Cyclic voltammograms of the graphene hybrid film before stability test and after 2000 cycles ................................................................................................................................................................ 41 Figure 24 Pictorial representation of the hybrid film (Mahesh, PowerPoint) ......................................... 45 Figure 25 SEM images a.) Cross-section of Gr hybrid film b.) Shows side view for Gr hybrid film. .. 46 Figure 26 TEM images a.) AuNPs coated Cyst b.) Shows the functionalised gold NPs on the Gr sheet. ................................................................................................................................................................ 47 Figure 27 Raman spectra of functionalized gold nanoparticles before and after binding on graphene sheets and cystamine. ................................................................................................................................... 48 Figure 28 a.) FTIR spectra of Gr, Gr hybrid and cystamine sulfate hydrate.......................................... 49
xi
Figure 29 a.) Relative change in resistance with respective to applied force. The sensitivity factor at the highest point shows 0.005 mN-1. b.) Detection of current response while loading and unloading of pressure by pressing. c) Plot shows current response for Tap and release. d) Plot for current response as a function of time for applied pressure. ................................................................................ 51 Figure 30 Schematic representation of CRGO’s self-assembly process on to the gold electrode (Mahesh, Power Point). ................................................................................................................................. 54 Figure 31 a) CVs of different COOH terminated thiols modified SAM electrode b) CVs of different CH3 terminated thiols modified SAM electrode in 1 M KCl of 10 mM Fe (CN)6
3-. ................................. 55 Figure 32A Cyclic voltammograms of a.) MBA with CRGO’s with different reduction times, b.) MHA with CRGO’s with different reduction times, and c.) MOA with CRGO’s with different reduction times and d.) MUA with CRGO’s with different reduction times in 10 mM Fe(CN)6
3- in1 M KCl solution. The scan rate is 100 mV/s..................................................................................................................................... 57
List of Tables Table 1 Electrochemical data obtained for different SAMs on gold electrode before and after modification with CRGO’s with different reduction times from impedance plots in 10 mM Fe(CN)6
3-
in1 M KCl solution. .......................................................................................................................................... 62 Table 2 Electrochemical parameters for SAM-modified Au electrode before and after adsorption of CRGO’s with different reduction times. ....................................................................................................... 64
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Abstract
Graphene’s (Gr) unique properties such as mechanical, electrical, thermal and
electrochemical properties have made it an interesting material in the current field of
research. Graphene derivatives such as graphene oxide (GO) and chemically reduced
graphene oxide (CRGO) provides a vast scope of chances to fabricate graphene hybrid
materials for different applications. Graphene can be used synergistically by functionalisation
with other materials such as metal nanoparticles, metal oxides, polymers and peptides for
novel devices. First of all, this project focuses on graphene hybrid material and various
methods to synthesise graphene hybrid material, particularly with functionalised AuNPs with
graphene can be seen in different applications such as actuators, sensitive pressure sensors
and also functionalized AuNPs along with self-assembly onto the alkanethiol modified Au
electrode. Due to the presence of functional groups on the surface of graphene, it can be
coated with functionalised AuNPs or other materials through covalent or non-covalent
interactions. The hybrid graphene actuators shows change in deflection with different
potential upon subjected into the electrolyte. The hybrid graphene pressure sensor exhibits
high sensitivity by applying different forces. Based on charge transfer process and electron
transfer (ET) kinetics, carbon hybrid nanomaterials have shown improvement in the
sensitivity, efficiency in senor applications. Secondly, different CRGO’s are self-assembled
on to the alkanethiol modified Au electrode in a controlled manner. This shows step by step
change in the charge transfer between different CRGO’s with respective to different carbon
chain length of –COOH and –CH3 terminated thiols. This hybrid materials could be used in
different applications such as actuators, highly sensitive pressure sensor leading to
development of new class of graphene electrodes to improve the efficient electron transfer
pathways.
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Chapter 1- Review of literature
1.1 Introduction
Nanotechnology can be defined as the technology which is developed with particles of
dimensions 10-9 m and application of physical, chemical and biological molecules at the
scale which goes from single atoms or molecules to the submicron dimensions, as well as
combination of the resulting nanostructures into high architecture frameworks1.
Carbon has been known and studied from ancient times. It has two allotropes namely,
graphite and diamonds. Utilization of graphite dates back to 6000 years, but the utilization of
graphene, single layer of graphiteis just 50 years old and was studiedfor its high conductivity2.
From that point forward graphite as single atom was a captivating field for examination. In
2010, Andre Geim and Konstantin Novoselov won the Nobel Prize for discovery of single
layer of carbon which was stable, as conductive as copper, and as small as not even helium
molecule could go pass through it3. Since, then graphene and carbon nanotubes has been
broadly studied because of its electrical, mechanical, thermal and structural properties. This
nano size graphene is cutting edge material to be utilized as biosensor. The graphene film
thickness could be controlled from tens of nanometers to 10 μm4. Graphene sheets are
usually prepared by filtration process using isopore membrane filter. Based on the above
properties graphene can be used as a composite with other materials such as AuNPs, CNTs,
polymers in biosensor, super capacitors and memory cell applications5-7. Figure 1 shows the
structure of graphene.
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Figure 1Armchair (blue) and zigzag (red) edges in monolayer graphene8.
1.2 Hybrid materials
The hybrid nanomaterials are designed through assembly of different molecules on the
carbon nanomaterial. The carbon nanomaterial has been covered by metals, biomolecules
and polymers as a second material which shows huge potential in future applications such
as senors, energy storage and supercapacitors9, 10. A hybrid material is generally defined as
a material which comprises of two moieties that are mixed on the molecular scale. Usually
one of the material is active and the other one is inactive in nature. Category I hybrids
represents the weak interaction between the two compounds like van der Waals, electrostatic
interactions or hydrogen bonding. Category II hybrids illustrate strong covalent interactions
between the two components11.
1.3 Nanocomposites
Nanocomposites is the combination of carbon nanomaterial with other materials in which one
acts as a filler (nanowires, nanotubes, nanoparticles) and other acts as a support or matrix
(ceramics, polymers).The main concept of the nanocomposite development is to integrate
specific properties of one material into another to achieve high performance and also to
enhance the electrical/ mechanical properties9, 12. Usually nanocomposites are prepared by
random mixing of two different materials and hence exhibiting non-uniform properties. This
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materials can be used in different industrial application such as Li-ion batteries, aerospace
materials and flame retardants due to large scale availability13-15.
Figure 2Nanocomposites Design Space11
1.4 Development of Hybrid Materials
The association of inorganic and polymeric materials with carbon nanomaterial has resulted
in development of hybrid materials, which could be the future for multifunctional composite.
The properties of the hybrid material depends on the energy transfer process and charge
between the two layers. Managing the nature of the two layers and increasing the distance
between two layers is the most interesting point to fabricate perfect hybrid nanomaterial9. The
main approaches for fabricating hybrid nanomaterials are divided into ex situ and in situ
methods. In case of ex situ each compound is fabricated individually with particular shape
and dimensions modified with functional group and bound together with the help of covalent
5 | P a g e
or non-covalent interactions. In the other case, one compound is directly synthesized on the
surface of other compound directly16.
1.4.1 Graphene
Graphene is single atom thick molecule, which comprises of sp2bonded carbon atoms in a
hexagonal lattice and has a honeycomb like structure, and is a general building square for
the graphitic materials of every other dimensionalities. Graphene could be wrapped into 0D
fullerenes, built into 1D nanotubes or stacked into 3D graphite. Graphene has been
contemplated hypothetically for about sixty years and has been depicted as part of diverse
carbon-based materials. Following, forty years it has been understood that graphene likewise
offers a fantastic dense matter simple to (2+1) – dimensional quantum electrodynamics 4-6,
which drives graphene into a flourishing hypothetical toy model. Likewise, despite the fact
that graphene was known as the interior piece of 3D materials, it was constantly assumed
not to exist in free state in the era of bended structure like residue, fullerenes and
nanotubes17.
All of a sudden, the vintage model has transformed into reality, when the free-standing
graphene was discovered unexpectedlyand the investigations confirmed that its charge
transporters were doubtlessly masslems Dirac fermions. There was no look back for
graphene, after that point.
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Figure 3Different graphene forms17.
1.4.2 Graphene History
Once graphene was discovered, it had to be characterised to learn about the nature and
properties of 2D crystals. It has been showcased that the electronic structures create with the
layer numbers, drawing nearer as far as possible for graphite as of now at 10 layers.
Additionally, just graphene and, to the great estimate, its bilayer has straightforward
electronic spectra: they are both the zero-gap semiconductors with one sort of electrons and
one kind of gaps. For three and more layers, the spectra will get progressively troublesome.
Different charge transporters show up and the valence groups begin to eminently cover. This
will permit one to separate between single-, twofold, and couple of (3to <10) layer graphene
as three various types of 2D crystals. Structures which are thicker must be considered, to
every one of the purposes and reasons, as thin films of graphite18. From the experimental
view point, such type of definition is also sensible. The screening length in graphite is only
≈5Å and so one should distinguish between the surface and the bulk even for films as thin as
5 layers19.
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Figure 4Sp2-hybridization of carbon atoms monolayer model in graphene. The appropriate crystalline structure
of graphene is a hexagonal grid19.
In 2004, Andre Geim and Novoselov open a new approach for providing high quality of
graphene through peeling of graphite until graphene was seen20. The graphene properties
grabbed much attention among scientists and technologists, who focused mainly on
functionalizing graphene, controlling the layers of graphene on the substrate and
investigation of potential applications. In late 20th century research on graphene had started
slowly due to its superior properties when exfoliated from graphite to graphene layers. Due
to increase in research there was a need for increased production of high quality graphene.
In one of the approaches, graphene was exfoliated from graphite, by inserting molecules into
the atomic planes which separates the graphene layers in the matrix. A mixture of graphene
stacks can be produced after the removal of these molecules. However, these methods didn’t
produce perfect graphene monolayers, and resulted in few layers of graphene. Several
attempts have been made to synthesize graphene likewise carbon nanotubes, by chemical
vapour deposition on metal surfaces21-23. Due to the high quality of graphene synthesised
CVD has become the most promising technique to produce graphene these days19, 24. But
the limitation for this method was its high cost of production. Hence, liquid exfoliation methods
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were established by reducing graphene oxide into graphene25, which resulted in decrease in
cost of production, together with increase in product.
1.4.3 Graphene with Metals
In general, fabrication of metal-matrix composites reinforced by graphene inclusions (sheets,
nanoplatelets) is very challenging. There has been few successful studies in this area which
are listed below, below are some of the examples focused on fabrication of metal graphene
nanocomposites with enhanced mechanical properties:
Wang and co-workers fabricated Al-graphene nanocomposites showing a dramatic
enhancement in strength, as compared to their graphene-free counterpart. This was achieved
by a novel approach based on flake powder metallurgy.
Wang with co-workers performed tensile tests with specimens of 5mmdiameter and
25mmgauge length machined from extruded rods consisting of Al-matrix reinforced by 0.3
wt. % graphene nanosheets. A good correlation was observed between the theoretical and
experimental values for mechanical strength of the Al-graphene nanocomposites. Based on
their results, Wang with co-coworkers concluded that reinforcement by graphene nanosheets
is most effective for Al-matrix materials and have a huge potential for applications26. There
still remains huge room of space for improvement of strength and other mechanical
characteristics exhibited by Al-matrix nanocomposites due to their strengthening by graphene
nanosheets.
1.4.4 Graphene with Thiols
In recent years, because of the advancement and development of commercial ventures, the
levels of contamination of water with heavy metals has increased. Contamination of water
bodies is a big issue for biological network and furthermore for the human life. Among these
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different heavy metal contaminations, mercury stands out to be most hazardous because of
its poisonous nature. Various types of adsorbents have been utilized to remove Hg2+ from
the modern wastewaters. But still, there is a need to have improved absorbents. Addressing
this issue, a thiol-functionalized magnetite/graphene oxide (MGO) was synthesized for
effective adsorption of Hg2+by a two stage response27. It showcased a more prominent
adsorption ability as compared to graphene oxide and MGO separately, because of the
consolidated adsorption of thiol groups and magnetite nanocrystals. The absorption capacity
increased to 289.9 mg g−1with solution of 100 mg l-1 Hg2+ concentration. The adsorption of
Hg2+ by thiol-functionalized MGO fits well with the Freundlich isotherm and follows pseudo-
second-order reaction. Thiol-funcionalized adsorbents demonstrated a specific binding of
Hg2+ because of the complexation of Hg2+with thiol groups when they are in close vicinity.
Iron oxide nanocrystalsenhanced the absorption capacities because of their high specific
surface area28, 29. Another advantage of using iron oxide was the ease of removal of
adsorbate from wastewater by application of magnetic force. The presence of oxygen
functional groups on graphene oxide allows binding of such metal oxides and grafting of
organic groups to its surface30, 31. In this study, the reserachers havebound Fe3O4
nanoparticles on graphene oxide and grafted thiol groups on the Fe3O4/graphene oxide
(MGO). The thiol-functionalized MGO represented moderately higher Hg2+ adsorption
capacity. The adsorbent can be separated from the water with straightforward process and
reused after it was exchanged over with H+27.
Shao et.al, has examined the electrochemical behavior of the graphene sheets which was
bound to the SAM’s on a gold electrode. The electrochemical behavior of
graphene/SAM/modified electrode was investigated. The gold electrode was modified with
C18SAM followed by controllable adsorption of graphene onto the SAM modified Au electrode.
The graphene/SAM/Au electrode was successfully characterised by using atomic force
10 | P a g e
microscopy (AFM), scanning electron microscopy (SEM) and ruthenium hexaammine
(Ru(NH3)63+). Further the electrochemical studies showed that the electron transfer (ET) can
be blocked by the SAM on the Au electrode which can be restored by immobilisation of
graphene sheets32.
Figure 5Graphical representation of the protocol for the fabrication of a Graphene/SAM altered gold electrode
and the heterogeneous ET mechanism on the Graphene altered electrode32.
Gooding’s group has also observed the influence of SAM chain also will play a crucial role in
determining the charge transfer resistance. The gold electrode was modified with four
different alkanethiols with different chain lengths (n= 2, 6, 8 and 11). The SAM modified
electrode showed good blocking effect as the carbon chain length increased. After the binding
of gold nanoparticles onto the alkanethiols of four different chain lengths in the presence of
ruthenium hexaammine (Ru(NH3)63+), the faradic electrochemistry was restored and similar
to the bare gold electrode. The charge transfer kinetics was observed to be insensitive to the
chain length and also the electron transfer between the redox couple and the nanoparticles
was due to the rate limiting step rather than the electron tunneling across the SAM33. The
rate limiting step is determined as the slowest step of a chemical reaction which determines
the rate at which overall reaction takes place.
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Figure 6Diagrammatic illustration of four electro modified with SAMs of AET, AHT, AOT, and AUT and then
subsequently altered with a monolayer of gold nanoparticles33.
In, summary these findings will open a new route in the area of graphene-thiol chemistry and
the blocked electrodes have potentially used in sensing for systems to determine the desired
electrochemistry at the electrode and also in other electrochemical processes.
1.5 Graphene Nanoelectrodes
The electrochemical methods has grabbed much attention due to its properties such as
sensitivity and fast response time with very low cost. Nano-electrodes have emerged in the
field of sensors, electronics when compared to macro-electrodes due to increased mass
transport, increased faradic current at the electrode surface and reduced IR drop34-37. In
electrochemical studies, graphene plays a key role due to its physicochemical properties and
high electric conductivity which can potentially make this carbon material as a new kind of
electrode material with potential applications in biosensing and electrochemical sensors38, 39.
The assembly and the electrochemical studies of graphene electrodes has been carried out
by many research groups. Bo Zhang et.al; has demonstrated reduced graphene oxide of
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nano to submicrometer size were obtained from graphene oxide by hydrazine reduction
process and separated by centrifugation. The reduced graphene oxides are self-assembled
onto the SAM modified Au ultramicroelectrode to form a monolayer. After the successful
assembly of reduced graphene oxides onto the electrode, the charge transfer dynamics and
also the electron transfer kinetics at the double layer were studied. The voltammetric
response of the r-GO electrode was determined after immobilization of GO flakes. The
electron transfer of Fe (CN)63- shows a slight increase in electron transfer rate for nanometer
sized reduced graphene oxide when compared to large ones40.
Figure 7Voltammetric responses of an Au/SAM modified graphene electrode in the solution of 1 M KCl
containing 10 mM Ru(NH3)6 3+. Potential scanning rate: 10 mV/s40.
Chenzhong Yu et.al; successfully demonstrated the procedure to assemble the reduced
graphene oxide film onto the SAM modified Au electrode in a controlled manner. They
showed the moderate decrease of electron transfer resistance of redox couple at the reduced
graphene oxide onto the electrode with the extend of time. Further, depending on the
immersion time, the electrochemical studies revealed that the GNF/SAM/Au electrode have
tunable dimension from nanoelectrode to conventional electrode. Moreover the GNF/SAM/Au
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electrode shows good electrocatalytic activity towards uric acid, dopamine and ascorbic
acid41.
In summary, these graphene nanoelectrodes in future can be used in electrochemical
investigations and more practical applications such as electroanalysis in vivo and in vitro.
1.6 Graphene based material for actuators
An actuator is a device which converts one form of energy such as thermal, electrical, light
into mechanical form. Under these external stimuli, actuators can undergo change in shape,
volume and other mechanical properties, in order to convert one form of energy into
mechanical energy42.
The ordinary activation materials such as piezoelectric, ferroelectric and conducting polymer
materials experience problems related to lower adaptability, higher driving voltages and lower
vitality. In contrast, graphene shows astounding mechanical, electrical, and optical angles
and chemical security, which has provoked graphene to be studied as an actuation incitation
material. Different actuation components and the required future improvements has been
reported below. Graphene subordinate materials with composites of different superior quality
components which are similar in material abundance, mechanical quality together with more
prominent actuation execution are anticipated to have more prominent probability for the
application in the cutting edge actuators43.
Ruoff et.al; have demonstrated a bilayer actuator, assembled layer by layer by vacuum
filtration process. Fabrication of graphene oxide and multiwalled CNT bilayer film showed
fascinating actuation response towards humidity or temperature. The MWCNT was
electrically conductive and the other side graphene layer was electrically insulated. The
functional groups such as -OH and -COOH on graphene sheet makes it sensitive to humidity
compared to CNTs. The bilayer paper was investigated as function of humidity at room
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temperature. As, the temperature changed, the bilayer paper curls in two different directions
as shown in the Figure 744.
Figure 8 Actuation of the bilayer paper sample as a function of relative humidity (%), a) 12, b) 25, c) 49, d) 61,
e) 70, and f) 90. White-arrowed side: surface of graphene oxide layer44.
Raguse et.al; have successfully prepared a nanoparticle actuator based on gold
nanoparticles functionalised with short bifunctional cystamine hydrochloride. The surface
charge on the gold nanoparticles can be manipulated which helps to carry out the actuation
mechanism by applying different potentials. Based on the surface charge the actuator
showed forward and backward deflection by applying different voltage ± 0.6 V45.
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1.7 Graphene in touch screen technology
Presently, the touch screen technology has observed a huge overhaul with the introduction
of the graphene-based innovations. Modern touch sensitive screens may utilize indium tin
oxide, a substance that is transparent but carries the electrical currents. One of the
drawbacks of indium tin oxide is its cost, they are expensive and has been put to application
only by few. So replacing the indium tin oxide with the graphene-based compounds would
facilitate for flexible and cheap paper-thin computers and television screens.
Touch sensors are the most sophisticated and emerging area of nanotechnology and are
gaining a great deal of attention due to their applications in human robotics,46 therapeutics47
and diagnostics.48 Sensation of touch is defined as the applied pressure over the specified
area of physical contact between the device and the object. For designing a flexible sensor,
it is necessary to pay special attention to the way the NPs are inserted on to the flexible
sensor. Change in thickness, morphology49 and density50 of the films affect the sensitivity,
selectivity and the overall functionality of NP-based sensor. Different supporting layers have
different adhesion properties corresponding to NP films which can alter the sensing signal51
as well as the sensor’s life. Several biomimetic sensors and strain gauges were successfully
developed by functionalization of gold nanoparticles with different peptides. The change in
resistance, with respect to the change in pressure, plays a key role in designing a flexible
sensor. The behavior of a device depends mostly on parameters like particle size,
interparticle distance and the conductance of linker molecule. Herrmann et al.52 have
successfully developed a sensitive strain gauge by functionalization of gold nanoparticles
with 4-nitrothiophenol (4-NTP). They observed that the sensitivity of the change in resistance
was relative to the change in pressure. To date, no one has completely explored the usage
of functionalized gold nanoparticles in the area of touch sensors and has great future in the
field of medicine, robotics and for the detection of environmental changes.
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Figure 9 Illustrates the Change in resistance with the strain for a NP film which is functionalized by4-NTP52.
1.8 Other applications of Graphene
Graphene has pulled in much consideration from scientists because of its intriguing
mechanical, electrochemical and electronic properties. Graphene, a solitary nuclear layer of
sp2-fortified carbon atoms firmly stuffed in a two dimensional (2D) honeycomb cross section,
has evoked extraordinary enthusiasm all through established researchers since its revelation.
As a novel nanomaterial, graphene has one of a kind electronic, optical, warm, and
mechanical properties. Graphene and its subordinates have indicated extraordinary
possibilities in numerous fields, for example, nanoelectronics, designing nanocomposite
The electrochemical studies showed that the adsorption of CRGO’s with different reduction
times on to the electrode surface shows different electron transfer pathways which can
influence the electron transfer efficiency and also the rate of charge transfer. There is a strong
electrostatic interaction between COOH terminated SAM and CRGO’s and hydrophobic
interaction between CH3 terminated SAM and CRGO’s which shows a consistent effect on
the charge transfer resistance (Rct) and the apparent rate (kapp). The kinetics of ET activity
between the CRGO’s/SAM/Au electrode and redox species in the solution is attributed to
charge transfer being confined to CRGO’s with different reduction times. An increase of the
carbon chain in both COOH terminated thiols and CH3 terminated thiols lost the conductivity
due to long chain and high molecular weight which affects the conductivity. An increase in
the reduction time of graphene oxideslead to reduced number of oxygen functional groups
on their surface which significantly improves their conductivity.
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Chapter 6 – Summary and future perspective
This chapter outlines the major findings in this project, which concludes the results of
graphene hybrid material and functionalised AuNPs self-assembly with graphene, which
opens a new route for the development of highly sensitive devices and actuators. On the
other hand, immobilization of different CRGO’s sheet on –COOH and –CH3 terminated
alkanethiols modified Au electrode showed an efficient electron transfer pathway which
paves path for new class of graphene electrode. This chapter also covers the future scope
from the current findings.
The assembly of graphene and its derivatives, which are cost-effective and easy to
synthesise, have made them favorable material for fabricating nanohybrids which could
be integrated with functionalised AuNPs and thiols. Fabrication of graphene hybrid
material have been done by sequential filtration of solution through vacuum filtration.
Incorporation of graphene into other materials has the promising advantage of improving
durability, mechanical strength, charge transport and other properties. In this thesis,
functionalised AuNPs sandwiched between graphene layers has been studied for
actuators and sensing applications.
6.1 A graphene hybrid Actuator
The graphene hybrid was successfully synthesised and the interactions were studied.
The binding of functionalised AuNPs and graphene is due to electrostatic interaction
between the positively charged functionalised AuNPs and negatively charged graphene
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sheets. This graphene hybrid film showed good potential to be used as actuator for
artificial muscle application. The successful fabrication of graphene hybrid also improved
the stability of the device. Graphene hybrid actuator showed good movement with
application of different potentials and also, the charging and discharging of current
through the films in the process of deflection was observed by Anson plot. The
interactions between the functionalised AuNPs and graphene showed improvement in the
deflection, durability and life time of the device.
6.2 A highly sensitive pressure sensor based on graphene
hybrid
The graphene hybrid device was prepared by sequential filtration of solution through
vacuum filtration. To check the sensitivity of the device, good electronic and
electrochemical properties; surface uniformity and free standing film is preferred.
Moreover the device should be flexible and the fabrication process should be economical.
The addition of functionalised AuNPs to graphene showed tremendous improvement in
sensitivity and exhibits faster response with different weights. The hybrid device showed
a good response with minute forces, together with good response for tap and release
applications for real time monitoring devices. Further, the graphene hybrid can be used
as highly sensitive sensor for robotics and medical applications.
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6.3 A new class of graphene electrode
The self-assembly of different CRGO’s onto the –COOH, -CH3 modified Au electrode
opens a new route in the development of new class of graphene electrodes for sensing
applications. Adetailed study of alkanethiols with two different end groups and their effect
on the Au electrode and type of interactions between different CRGO’s were taken into
consideration. The modification of SAM modified Au electrode with different CRGO’s in a
controlled manner is shown in chapter 5. Sufficient time had to be given to bind different
CRGO’s onto the SAM modified Au electrode. We clearly demonstrated the blocking
effect on the SAM modified Au electrode in the presence and absence of different
CRGO’s. The charge transfer resistance was found to be insensitive to the length of the
SAM. This new class of graphene electrodes showed significant changes in the rate of
charge transfer for different CRGO’s. Finally, CH3 terminated alkanethiol modified Au
electrode with different CRGO’s showed efficient charge transfer rate compared to COOH
terminated alkanethiols.
6.4 Future Perspective
The graphene hybrid materials has wide range of applications and definitely play an
important role in the future development of advanced hybrid materials for touch sensor,
actuators, energy storage and biosensor applications. Binding of graphene with other
materials helps to improve the conductivity, charge transport mechanism and mechanical
strength. Incorporation of one material into different functionalised materials can be useful
in obtaining new properties. A new method of approach for fabrication has been
developed for the construction of hybrid materials. Still particular efforts are required to
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design and develop methods for the self-assembling hybrid nanostructures rather than
random mixing, which will drive a constant demand for optimized hybrid properties.
Future research on functionalised AuNPs sandwiched between graphene layers can be
helpful to improve performance and conductivity, which should be tested with different
applications. Investigation on the efficient electron transfer of different CRGO’s self-
assembled on –COOH, -CH3 modified Au electrode need to be carried out to further
investigate the step by step change in electron transfer with different CRGO’s. These
hybrid materials have the potential to for a wide range of applications such as sensors,
energy storage, touch screens, catalysis and photovoltaics.
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