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BIOMIMETIC EXFOLIATION OF GRAPHENE AND ITS FEASIBILITY AS A DIELECTRIC MATERIAL
Typology: Thesis
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PRANAV
Master of Technology
In
Nanotechnology
Reg No: Year Exam Roll No:
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Central University of Jharkhand Ranchi (Established by an Act of Parliament of India, 2009) Centre for Nanotechnology Ratu- Lohardaga Road, Brambe- 835205 Ranchi, India
The dissertation entitled “ BIOMIMITIC EXFOLIATION OF GRAPHENE AND ITS FEASIBLITY AS A DIELECTRIC MATERIAL” by Mr. PRANAV (Reg No: CUJ/I/2010/INT/26) for partial fulfillment of the degree of MASTER OF TECHNOLOGY in NANOTECHNOLOGY, submitted in the year 2015 has been approved by committee consisting of following members:
Pro. (Dr.). R. K. Dey Centre Head Centre for Nanotechnology
Dr. G. P. Singh Assistant Professor Centre for Nanotechnology
Dr. A.S. Bhattacharya Assistant Professor Centre for Nanotechnology
Dr. Lawrence Kumar Assistant Professor Centre for Nanotechnology
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I certify that
a. The work contained in this thesis is original and has been done by myself under the general supervision of my supervisor/s.
b. The work has not been submitted to any other Institute for degree or diploma.
c. Whenever I have used materials (data, theory and text) from other sources, I have given due credit to them by citing them in the text of the thesis and giving their details in the reference section.
Signature of the Student
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CVD- Chemical Vapour Deposition.
GO- Graphite Oxide.
CNT- Carbon nanotube.
PVA- Poly Vinly alcohol.
PL- Photoluminescence.
FTIR- Fourier transform Infrared.
DLS- Differential Light Spectrometer.
PdI- Poly Dispersity Index.
SEM- Scanning Electron Microscopy.
TEM- Transmission electron microscopy.
SAED- Selected area electron diffraction.
TGA- Thermogravimetric analysis.
LCR- Inductance, Capacitance and resistance.
CL-Collagen.
GR-Graphite.
GC-Graphene collagen.
FWHM- Full width half maxima.
BLG- Bilayer graphene.
GM- Graphene methionine.
GA- Graphene arginine.
GT- Graphene tyrosine.
GGA- Graphene glutamic acid.
EDX- Energy dispersive X-ray
- 3.2.7. Thermo Gravimetric Analysis……………………………………………………………………………. - 3.2.8. Electro spinning Unit………………………………………………………………………………........... - 3.2.9. LCR meter………………………………………………………………………………………………………… xi
Figure 1.1: Graphite Crystal Structure Figure 1.2: Graphene mother of all graphitic forms of carbon material. Figure 1.3: Band structure of Single layer, Bilayer and Trilayer Graphene Figure 1.4: Structure of amino acid Figure 1.5: Structure of Methionine Figure 1.6: Structure of Tyrosine Figure 1.7: Structure of Arginine Figure 1.8: Structure of Glutamic acid Figure 1.9: Peptide Bond formation Figure 1.10:(a) alpha helix (b) beta plated structure of Protein Figure 1.11: Collagen Structure Figure 3.1: Schematic collagen based exfoliation of graphite into graphene. Figure 3.2: Schematic Amino Acid based exfoliation of graphite into graphene. Figure 3.3: Schematic representation of Raman effect Figure 3.6: Raman Spectrometer Figure 3.7: FTIR Spectrometer Figure 3.8: Scanning Electron microscopy Figure 3.9: Transmission electron microscopy Figure 3.10: High temperature- TGA Figure 3.11: Schematic of Electro spinning Unit Figure 4.1: Raman Spectra of Collagen Figure 4.2: Raman Spectra of Graphite Figure 4.3: Raman Spectra of GC Figure 4.4: 2D deconvoluted peak of GC Figure 4.5: Raman Spectra of GM,GGA,GA,GT Figure 4.6: 2D deconvoluted peak of GM,GGA,GA,GT Figure 4.7: Luminescence and Fluorescence of Collagen Figure 4.8: Luminescence and fluorescence of GC w.r.to Collagen Figure 4.9: Fluorescence of GA, GM,GT&GGA w.r.to their control. Figure 4.10: Luminescence of GA, GGA,GT&GM at variable excitation wavelength. Figure 4.11: FTIR of collagen and graphite Figure 4.12: FTIR of GC Figure 4.13: FTIR of GM,GGA,GA,GT Figure 4.14: TGA of Collagen and Graphite Figure 4.15: TGA of GC and GA,GGA,GM,GT Figure 4.16: FE-SEM of Graphite and collagen Figure 4.17: FE-SEM of GC Figure 4.18: FE-SEM of GGA Figure 4.19: FE-SEM of GA Figure 4.20: FE-SEM of GM Figure 4.21: FE-SEM of GT Figure 4.22: FE FE-SEM image of Graphene nanofiber (a). Bead
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There are several common synthesis methods of graphene, each method has certain advantages but most are neither scalable nor practical for broader applications. One of the methods for bulk production of graphene is by liquid phase exfoliation; most widely used method is the Hummer’s method which involves oxidation of graphite using strong oxidizing agents followed by its reduction to graphene using equally strong reductants. Chemically processed graphite oxide is attractive because it forms stable suspensions in water, however graphite oxide is an insulator and must be chemically or thermally reduced to reduced graphene oxide (rGO) which exhibits poor thermal and electrical properties compared to directly exfoliated graphene. The graphene produced has very large defects and use of concentrated acid evolves toxic gases. So direct exfoliation using surfactants overcomes these disadvantages and is a good method to exfoliate graphite into graphene. The surfactant used has to match the surface energy of graphite and this pulls apart the stacked layers and leads to graphite exfoliation. The defects are very less and the process is very easy and can be used for bulk production.
I have directly exfoliated graphene using collagen and amino acids. There is a US patent which states that collagen when dispersed in organic acids behave like a surfactant. In addition, I have used four different classes of amino acids (methionine, arginine, tyrosine and glutamic acid) to directly exfoliate graphite into graphene. Raman Spectra confirms the graphene formation. Also the D band intensities were very less, signifying graphene formation with lesser defects. The fluorescence spectra show the quenching phenomenon in collagen based graphene exfoliation and tyrosine based exfoliation proving opening up of a band gap. Fourier transform infrared spectrometer spectra confirm the chemical interaction between graphite and collagen, amino acids. Dynamic light spectrometer was done to check the chemical stability of the solution, zeta potential values are in the stable colloidal range but the poly dispersity index indicates a distribution of sizes. Scanning electron microscopy & Transmission electron microscopy images confirms the graphene sheets formation. These graphene-collagen composites were electro spun into nanofibers using polyvinyl alcohol, the electro spun sheets cut into shape, gold coated and probed and capacitance value was measured using LCR meter.
This study explored a new method of exfoliating graphite using acylated collagen (based on a US patent) and amino acids, characterized it in depth, electro spun the nanocomposite with the help of a polymer into nanofibers and explored its feasibility for usage as a material for super capacitance. Reports are also coming up on the usage of these graphene nanofibers as an anode material in Lithium ion batteries.
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1.1.Biomimetics
The term “Biomimetics” this was coined by Otto H Schmitt (Schmitt 1969), represents the studies and imitation of nature’s methods, mechanisms and processes. Nature’s capabilities are far superior in many areas to human capabilities, and adapting many of its features and characteristics can significantly improve our technology [1]. Creatures in nature, if viewed as engineering designs, have general features rather than designs with specifications that are exact duplicates. As opposed to man-made designs that require exact duplication, creatures are able to perform quite well while having an identity that distinguishes one member from another in the same species [2]. In contrast, our commercial products are sought to be duplicated as closely as possible to assure their quality and performance. The cell-based structure, which makes up the majority of biological creatures, offers the ability to grow with fault-tolerance and self-repair, while doing all of the things that are characteristic of biological systems[3]. If we are successful in making biomimetic structures that consist of multiple cells, we may be able to design devices and mechanisms that are currently considered science fiction. Emerging nanotechnologies increasingly enhance the potential of such capabilities [4]. Humans have learned much from nature and the results have helped surviving generations and continue to secure a sustainable future. Nature makes economic use of materials by optimizing the design of the entire structure or system to meet multiple needs. For example, feathers besides helping the bird fly insulate it from the environment. The many ways in which nature tries to design a system to suit a function is best illustrated with respect to fish. Fish reduce the drag as they swim both through chemical and structural devices. Some of them release substances, which make their skin slippery. In others, the body is designed to aid avoidance of turbulent floor around it during swimming. In some fish gill slits are formed and located on the body such that smooth flow of water around the fish is ensured. There are various methods of biomimetics but generally for nanomaterial synthesis I have used
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a single atomic lattice of hexagonally arranged sp^2 hybridized carbon atoms [9]. The quantum mechanical basis proposed to explain the band structure and electronic behavior of graphite was first carried out by Wallace in 1947 using the tight binding approximation [10]. Wallace’s work developed the description of the single hexagonal unit cell crystal structure of graphite and corresponding k-space Brillion zone. The tight binding method approximates the Hamiltonian operator for a crystal lattice as the Hamiltonian operator of a single atom within that lattice. In addition, the electron is assumed to be well localized about a given atomic site such that the wave function readily decays outside the range of the lattice constant of the crystal.
Fig1.1. Graphite Crystal Structure.( Molecules 2014 , 19 (9), 14582-14614). As a result, the wave functions of electrons moving through the crystal are approximated as the atomic orbitals of their corresponding atoms and there are no other electron interactions within the structure. Using the tight binding approximation, Wallace was able to qualitatively describe the band structure of the graphitic unit cell which explained some of the observed conductivity behavior in graphite crystals [10]. However, it was not for another half century that the modeled single layer graphite crystal would be successfully isolated and available to the scientific community. In 2004 single layer graphite, given the name graphene, was isolated and the electronic structure measured by, Andre Geim and Konstantin Novoselov. This work has since ignited substantial research in graphene across many fields of science and engineering on the extraordinary two-
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dimensional material and earned Geim and Novoselov the 2010 Nobel Prize in Physics [11-12].Graphene is also called the mother of all forms of carbon materials. Carbon, the sixth element of the periodic table, has 4 valence electrons and a ground state electron configuration of 1s^2 2s 2 p^2 [13]. However, in the hexagonal lattice of graphite, graphene, and other carbon allotropes, the valence electron wave functions are sp^2 hybridized such that every carbon atom in graphene has three covalent σ bonds with its nearest carbon neighbor in the trigonal xy plane [14].
Fig1.2. Graphene mother of all graphitic forms of carbon material. ( Nature, 2007, 6,185 )
These in-plane σ bonds form 120^0 angles while the remaining pz electron orbital forms a π bond whose electron density lies above and below the nodal xy plane. The resulting electron configuration is 1s^2 sp^1 sp^2 sp^3 2pz [15]. The aromatic π electron and the crystal structure are responsible for the extraordinary electronic and thermal properties of monolayer, Bilayer and few layer graphene. The structure of graphene, is a honeycomb net with a unit cell consisting of two triangular sub lattices A and B and although the honeycomb net is not itself a Bravais lattices, it can be represented as a two dimensional triangular Bravais lattice.
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Because of this electron band structure, graphene is considered a zero-gap semiconductor or semi-metal[20], however it is possible to induce a band gap within graphene by methods such as constraining the domain of graphene to a single layer nanoribbon [18,19] or by tuning the band gap in Bilayer graphene devices[20]. Similarly, various transport regimes have been identified in single layer and few layer graphene as studied by Morozov, the results of which indicate that few layer to multi-layer graphene behaves as a conductive metal [21]. There are also methods of altering the intrinsic properties of graphene by both positive and negative doping from surface adsorbed molecules and substrate interaction as well as atmospheric doping [22-24]. This doping behavior has consequently led to the construction of graphene P-N junctions in graphene electronic devices [25].
1.2.2. Properties of Graphene Electronic properties of graphene- Most of the experimental research on graphene focuses on the electronic properties. The graphene charge carriers are mass less and mimic relativistic properties. The most notable feature about the early work on graphene transistors was the ability to continuously tune the charge carriers from holes to electrons. At low temperatures and high magnetic fields, the exceptional mobility of graphene allows for the observation of the quantum hall effect for both electrons and holes. Due to its unique band structure, the graphene quantum Hall Effect exhibits a subtle difference from the conventional quantum Hall effect in that plateaus occur at half integers of 4 e 2/ h rather than the typical 4 e 2/ h [11, 26]. For more practical applications one would like to utilize the strong gate dependence of graphene for either sensing or transistor applications. Apart from that due to 2D the scattering of charge carrier is not takes place in graphene and has the presence of charge carrier concentration. Unfortunately, graphene has no band gap and correspondingly resistivity changes are small. Therefore, a graphene transistor by its very nature is plagued by a low on/off ratio. However one way around this limitation, is to carve graphene into narrow ribbons. By shrinking the ribbon the momentum of charge carriers in the transverse direction becomes quantized which results in the opening of a band gap.
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Mechanical properties of graphene- Graphite is unique in that the elastic constants in the direction perpendicular are vastly different than the elastic constants along the basal plane. Due to the presence of filled σ electron the mechanical strength of graphene is very high. Although the experimental mechanical properties of graphene are largely unexplored and the time is ripe to revisit some of the old assumptions about bulk graphite to determine how the elastic constants scale down to the atomic thicknesses. By working with single atomic layers or few atomic layers some of the uncertainties involved in working with large single crystals such as dislocations and defects are avoided. Thermal Properties- Thermal conductivity in suspended graphene is carried by ballistic phonons. Hence, the thermal conductance of graphene on a substrate will significantly downgrade, since the number of scattering channels increases. In this case, heat may be lost to the substrate. The thermal conductance also decreases with an increased number of graphene layers. However, the mechanism of heat transport across graphene interfaces in the cross-plane direction is currently unknown. Optical Properties- Single layer graphene posse’s high transparency. Graphene posses high range of absorption across visible light spectrum. And due to its atomic thickness it can be very useful for small screen devices and has true potential lies in the field of optoelectronics. The pristine graphene does not show fluorescence and luminescence phenomenon because band gap is not present. 1.2.3. Graphene Synthesis Method Basically there are two different approaches to preparing graphene [27]. On the one hand graphene can be detached from an already existing graphite crystal, the so- called exfoliation Methods, on the other hand the graphene layer can be grown directly on a substrate surface. Micromechanical Exfoliation method-In this method graphene is detached using adhesive tapes from the graphite crystal. After peeling the tape from graphite multi layer graphene remains present and by repeating that process and the tape gets attached on the substrate single layer graphene can be drawn [28]. However it is impossible to obtain graphene in bulk quantity using this method. Totally different approach in needed to produce graphene on any substrate. The size of the graphene obtained does not depend upon initial graphite crystal. There are two methods by which graphene can be produced in this fashion-