Graphene oxide
Further information: Graphite oxide
By dispersing oxidized and chemically processed graphite in water, and using paper-making techniques, the monolayer flakes form a single sheet and bond very powerfully. These sheets, called graphene oxide paper have a measured tensile modulus of 32 GPa.[124] The peculiar chemical property of graphite oxide is related to the functional groups attached to graphene sheets. They even can significantly change the pathway of polymerization and similar chemical processes.[125] Graphene Oxide flakes in polymers also shown enhanced photo-conducting properties.[126] A team at Manchester University led by Andre Geim published findings showing that graphene-based membranes are impermeable to all gases and liquids (vacuum-tight). However, water evaporates through them as quickly as if the membranes were not there at all.[127]
Chemical modification
Photograph of single-layer graphene oxide undergoing high temperature chemical treatment, resulting in sheet folding and loss of carboxylic functionality, or through room temperature carbodiimide treatment, collapsing into star-like clusters.
Soluble fragments of graphene can be prepared in the laboratory[128] through chemical modification of graphite. First, microcrystalline graphite is treated with a strongly acidic mixture of sulfuric acid and nitric acid. A series of steps involving oxidation and exfoliation result in small graphene plates with carboxyl groups at their edges. These are converted to acid chloride groups by treatment with thionyl chloride; next, they are converted to the corresponding graphene amide via treatment with octadecylamine. The resulting material (circular graphene layers of 5.3 angstrom thickness) is soluble in tetrahydrofuran, tetrachloromethane and dichloroethane. Refluxing single-layer graphene oxide (SLGO) in solvents leads to size reduction and folding of the individual sheets as well as loss of carboxylic group functionality, by up to 20%, indicating thermal instabilities of SLGO sheets dependant on their preparation methodology. When using thionyl chloride, acyl chloride groups result, which can then form aliphatic and aromatic amides with a reactivity conversion of around 70–80%.
Boehm titration results for various chemical reactions of single-layer graphene oxide, which reveal reactivity of the carboxylic groups and the resultant stability of the SLGO sheets after treatment.
Hydrazine reflux is commonly used for reducing SLGO to SLG(R), but titrations show that only around 20–30% of the carboxylic groups are lost leaving a significant number of COOH groups available for chemical attachment. Analysis of SLG(R) generated by this route reveals that the system is unstable and using a room temperature stirring with HCl (< 1.0 M) leads to around 60% loss of COOH functionality. Room temperature treatment of SLGO with carbodiimides leads to the collapse of the individual sheets into star-like clusters, which exhibited poor subsequent reactivity with amines (ca. 3–5% conversion of the intermediate to the final amide).[129] It is apparent that conventional chemical treatment of carboxylic groups on SLGO generates morphological changes of individual sheets that leads to a reduction in chemical reactivity, which may potentially limit their use in composite synthesis. Therefore, other types of chemical reactions have been explored. SLGO has also been grafted with polyallylamine, cross-linked through epoxy groups. When filtered into graphene oxide paper, these composites exhibit sheets increased stiffness and strength relative to unmodified graphene oxide paper.[130]
Full hydrogenation from both sides of graphene sheet results in graphane, but partial hydrogenation leads to hydrogenated graphene[131]
Thermal properties
The near-room temperature thermal conductivity of graphene was measured to be between (4.84±0.44) × 103 to (5.30±0.48) × 103 W·m−1·K−1. These measurements, made by a non-contact optical technique, are in excess of those measured for carbon nanotubes or diamond. The isotopic composition, the ratio of 12C to 13C, has a significant impact on thermal conductivity, where isotopically pure 12C graphene has higher conductivity than either a 50:50 isotope ratio or the naturally occurring 99:1 ratio.[132] It can be shown by using the Wiedemann-Franz law, that the thermal conduction is phonon-dominated.[133] However, for a gated graphene strip, an applied gate bias causing a Fermi energy shift much larger than kBT can cause the electronic contribution to increase and dominate over the phonon contribution at low temperatures. The ballistic thermal conductance of graphene is isotropic.[134][135]
Potential for this high conductivity can be seen by considering graphite, a 3D version of graphene that has basal plane thermal conductivity of over a 1,000 W·m−1·K−1 (comparable to diamond). In graphite, the c-axis (out of plane) thermal conductivity is over a factor of ~100 smaller due to the weak binding forces between basal planes as well as the larger lattice spacing.[136] In addition, the ballistic thermal conductance of a graphene is shown to give the lower limit of the ballistic thermal conductances, per unit circumference, length of carbon nanotubes.[137]
Despite its 2-D nature, graphene has 3 acoustic phonon modes. The two in-plane modes (LA, TA) have a linear dispersion relation, whereas the out of plane mode (ZA) has a quadratic dispersion relation. Due to this, the T2 dependent thermal conductivity contribution of the linear modes is dominated at low temperatures by the T1.5 contribution of the out of plane mode.[137] Some graphene phonon bands display negative Grüneisen parameters.[138] At low temperatures (where most optical modes with positive Grüneisen parameters are still not excited) the contribution from the negative Grüneisen parameters will be dominant and thermal expansion coefficient (which is directly proportional to Grüneisen parameters) negative. The lowest negative Grüneisen parameters correspond to the lowest transversal acoustic ZA modes. Phonon frequencies for such modes increase with the in-plane lattice parameter since atoms in the layer upon stretching will be less free to move in the z direction. This is similar to the behavior of a string, which, when it is stretched, will have vibrations of smaller amplitude and higher frequency. This phenomenon, named "membrane effect", was predicted by Lif****z in 1952.[139]
Mechanical properties
As of 2009, graphene appears to be one of the strongest materials ever tested. Measurements have shown that graphene has a breaking strength over 100 times greater than a hypothetical steel film of the same (incredibly thin) thickness,[140] with a tensile modulus (stiffness) of 1 TPa (150,000,000 psi).[141] However, the process of separating it from graphite, where it occurs naturally, will require some technological development before it is economical enough to be used in industrial processes,[142] though this may be changing soon.[143] Graphene is very light, weighing only about 0.77 milligrams per square meter. The Nobel announcement illustrated this by saying that a 1 square meter graphene hammock would support a 4 kg cat but would weigh only as much as one of the cat's whiskers, at 0.77 mg about 0.001% of the weight of 1m2 of paper.[144]
Graphene paper or GP has recently been developed by a research department from the University of Technology Sydney by Guoxiu Wang, that can be processed, reshaped and reformed from its original raw material state. Researchers have successfully milled the raw graphite by purifying and filtering it with chemicals to reshape and reform it into nano-structured configurations, which are then processed into sheets as thin as paper, according to a university statement. Lead researcher Ali Reza Ranjbartoreh said: 'Not only is it lighter, stronger, harder and more flexible than steel, it is also a recyclable and sustainably manufacturable product that is eco-friendly and cost effective in its use.' Ranjbartoreh said the results would allow the development of lighter and stronger cars and planes that use less fuel, generate less pollution, are cheaper to run and ecologically sustainable. He said large aerospace companies have already started to replace metals with carbon fibres and carbon-based materials, and graphene paper with its incomparable mechanical properties would be the next material for them to explore.[citation needed]
Using an atomic force microscope (AFM), the spring constant of suspended graphene sheets has been measured. Graphene sheets, held together by van der Waals forces, were suspended over SiO2 cavities where an AFM tip was probed to test its mechanical properties. Its spring constant was in the range 1–5 N/m and the Young's modulus was 0.5 TPa, which differs from that of the bulk graphite. These high values make graphene very strong and rigid. These intrinsic properties could lead to using graphene for NEMS applications such as pressure sensors and resonators.[145]
As is true of all materials, regions of graphene are subject to thermal and quantum fluctuations in relative displacement. Although the amplitude of these fluctuations is bounded in 3D structures (even in the limit of infinite size), the Mermin-Wagner theorem shows that the amplitude of long-wavelength fluctuations will grow logarithmically with the scale of a 2D structure, and would therefore be unbounded in structures of infinite size. Local deformation and elastic strain are negligibly affected by this long-range divergence in relative displacement. It is believed that a sufficiently large 2D structure, in the absence of applied lateral tension, will bend and crumple to form a fluctuating 3D structure. Researchers have observed ripples in suspended layers of graphene,[30] and it has been proposed that the ripples are caused by thermal fluctuations in the material. As a consequence of these dynamical deformations, it is debatable whether graphene is truly a 2D structure.[1][69
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