Graphite oxide

Graphite oxide, formerly called graphitic oxide or graphitic acid, is a compound of carbon, oxygen, and hydrogen in variable ratios, obtained by treating graphite with strong oxidizers. The maximally oxidized bulk product is a yellow solid with C:O ratio between 2.1 and 2.9, that retains the layer structure of graphite but with a much larger and irregular spacing.[2]

The bulk material spontaneously disperses in basic solutions or can be dispersed by sonication in polar solvents to yield monomolecular sheets, known as graphene oxide by analogy to graphene, the single-layer form of graphite.[3] Graphene oxide sheets have been used to prepare strong paper-like materials, membranes, thin films, and composite materials. Initially, graphene oxide attracted substantial interest as a possible intermediate for the manufacture of graphene. The graphene obtained by reduction of graphene oxide still has many chemical and structural defects which is a problem for some applications but an advantage for some others.[4]

Graphite oxide
Structure proposed in 1998[1] with functional groups. A: Epoxy bridges, B: Hydroxyl groups, C: Pairwise carboxyl groups.

History and preparation

Graphite oxide was first prepared by Oxford chemist Benjamin C. Brodie in 1859, by treating graphite with a mixture of potassium chlorate and fuming nitric acid.[5] He reported synthesis of "paper-like foils" with 0.05 mm thickness. In 1957 Hummers and Offeman developed a safer, quicker, and more efficient process called Hummers' method, using a mixture of sulfuric acid H2SO4, sodium nitrate NaNO3, and potassium permanganate KMnO4, which is still widely used, often with some modifications.[2][6][7] Largest monolayer GO with highly intact carbon framework and minimal residual impurity concentrations can be synthesized in inert containers using highly pure reactants and solvents.[8]

Graphite oxides demonstrate considerable variations of properties depending on degree of oxidation and synthesis method.[9][10] For example, temperature point of explosive exfoliation is generally higher for graphite oxide prepared by Brodie method compared to Hummers graphite oxide, the difference is up to 100 degrees with the same heating rates.[11] Hydration and solvation properties of Brodie and Hummers graphite oxides are also remarkably different.[12]

Recently a mixture of H2SO4 and KMnO4 has been used to cut open carbon nanotubes lengthwise, resulting in microscopic flat ribbons of graphene, a few atoms wide, with the edges "capped" by oxygen atoms (=O) or hydroxyl groups (-OH).[13]

Graphite (Graphene) oxide (GO) has also been prepared by using a "bottom-up" synthesis method (Tang-Lau method) in which the sole source is glucose, the process is safer, simpler, and more environmentally friendly compared to traditionally “top-down” method, in which strong oxidizers are involved. Another important advantage of Tang-Lau method is control of thickness, ranging from monolayer to multilayers by adjusting growth parameters.[14]

Structure

The structure and properties of graphite oxide depend on particular synthesis method and degree of oxidation.[9][10][10] It typically preserves the layer structure of the parent graphite, but the layers are buckled and the interlayer spacing is about two times larger (~0.7 nm) than that of graphite. Strictly speaking "oxide" is an incorrect but historically established name. Besides oxygen epoxide groups (bridging oxygen atoms), other functional groups experimentally found are:[9] carbonyl (C=O), hydroxyl (-OH), phenol, for graphite oxides prepared using sulphuric acid (e.g. Hummers method) also some impurity of sulphur is often found, for example in a form of organosulfate groups .[15][16][17][18][19][20] The detailed structure is still not understood due to the strong disorder and irregular packing of the layers.

Graphene oxide layers are about 1.1 ± 0.2 nm thick.[15][16] Scanning tunneling microscopy shows the presence of local regions where oxygen atoms are arranged in a rectangular pattern with lattice constant 0.27 nm × 0.41 nm [16][21] The edges of each layer are terminated with carboxyl and carbonyl groups.[15] X-ray photoelectron spectroscopy shows presence of several C1s peaks, their number and relative intensity depending on particular oxidation method used. Assignment of these peaks to certain carbon functionalization types is somewhat uncertain and still under debates. For example, one of interpretations goes as following: non-oxygenated ring contexts (284.8 eV), C-O (286.2 eV), C=O (287.8 eV) and O-C=O (289.0 eV).[22] Another interpretation using density functional theory calculation goes as following: C=C with defects such as functional groups and pentagons (283.6 eV), C=C (non-oxygenated ring contexts) (284.3 eV), sp3C-H in the basal plane and C=C with functional groups (285.0 eV), C=O and C=C with functional groups, C-O (286.5 eV), and O-C=O (288.3 eV).[23]

Graphite oxide is hydrophilic and easily hydrated when exposed to water vapor or immersed in liquid water, resulting in a distinct increase of the inter-planar distance (up to 1.2 nm in saturated state). Additional water is also incorporated into interlayer space due to high pressure induced effects.[24] Maximal hydration state of graphite oxide in liquid water corresponds to insertion of 2-3 water monolayers, cooling the graphite oxide/H2O samples results in "pseudo-negative thermal expansion" and below freezing point of water media results in de-insertion of one water monolayer and lattice contraction.[12] Complete removal of water from the structure seems difficult since heating at 60–80 °C results in partial decomposition and degradation of the material.

GO exfoliation
Exfoliation of graphite oxide at high temperature, screenshots from video available here:[25] Exfoliation results in tenfold increase of sample volume and formation of carbon powder with grains of few graphene layers thickness.[26]

Similar to water, graphite oxide also easily incorporates other polar solvents, e.g. alcohols. However, intercalation of polar solvents occurs significantly different in Brodie and Hummers graphite oxides. Brodie graphite oxide is intercalated at ambient conditions by one monolayer of alcohols and several other solvents (e.g. dimethylformamide and acetone) when liquid solvent is available in excess. Separation of graphite oxide layers is proportional to the size of alcohol molecule.[27] Cooling of Brodie graphite oxide immersed in excess of liquid methanol, ethanol, acetone, and dimethylformamide results in step-like insertion of additional solvent monolayer and lattice expansion. The phase transition detected by X-ray diffraction and DSC is reversible; de-insertion of solvent monolayer is observed when sample is heated back from low temperatures.[28] Additional methanol and ethanol monolayer is reversibly inserted into the structure of Brodie graphite oxide also at high pressure conditions.[27]

Hummers graphite oxide is intercalated with two methanol or ethanol monolayers already at ambient temperature. The interlayer distance of Hummers graphite oxide in excess of liquid alcohols increases gradually upon temperature decrease, reaching 19.4 and 20.6 Å at 140 K for methanol and ethanol, respectively. The gradual expansion of the Hummers graphite oxide lattice upon cooling corresponds to insertion of at least two additional solvent monolayers.[29]

Graphite oxide exfoliates and decomposes when rapidly heated at moderately high temperatures (~280–300 °C) with formation of finely dispersed amorphous carbon, somewhat similar to activated carbon.[26]

Characterization

Figure 1 high resolution
(A) Image of fractionated GO, (B) XRD, (C) Raman, and (D) FTIR spectra of GO (black), more oxidized GOw fraction (blue), and less oxidized GOe fraction (red).[9]

XRD, FTIR, Raman, XPS, AFM, TEM, SEM/EDX,[9][30][31] etc. are some common techniques to characterize GO samples.[32] Since the distribution of oxygen functionalities on GO sheets is polydisperse, fractionation method can be used to characterize and separate GO sheets on the basis of oxidation.[9] Different synthesis methods give rise to different types of graphene oxide. Even the different batches from similar oxidation method can have differences in their properties due to variations in purification or quenching processes.[9]

Surface properties

It is also possible to modify the surface of graphene oxide to change its properties.[31][33] Graphene oxide has unique surface properties which make it a very good surfactant material stabilizing various emulsion systems.[31][9] Graphene oxide remains at the interface of the emulsions systems due to the difference in surface energy of the two phases separated by the interface.[34][31]

Relation to water

Graphite oxides absorb moisture proportionally to humidity and swells in liquid water. The amount of water absorbed by graphite oxides depends on the particular synthesis method and shows a strong temperature dependence.

Brodie graphite oxide absorbs selective methanol from water/methanol mixtures in certain range of methanol concentrations.[35]

Membranes prepared from graphite oxides (recently more often named as "graphene oxide" membranes) are vacuum tight and impermeable to nitrogen and oxygen, but are permeable to water vapors. The membranes are also impermeable to "substances of lower molecular weight". Permeation of graphite and graphene oxide membranes by polar solvents is possible due to swelling of the graphite oxide structure.[36] The membranes in swelled state are also permeable by gases, e.g. helium.

The interlayer distance of dried graphite oxides was reported as ~6–7 Å but in liquid water it increases up to 11–13 Å at room temperature. The lattice expansion becomes stronger at lower temperatures. The inter-layer distance in diluted NaOH reached infinity, resulting in dispersion of graphite oxide on single-layered graphene oxide sheets in solution. Graphite oxide can be used as a cation exchange membrane for materials such as KCl, HCl, CaCl2, MgCl2, BaCl2 solutions. The membranes were permeable by large alkali ions as they are able to penetrate between graphene oxide layers.[36]

Applications

Optical nonlinearity

Nonlinear optical materials are of great importance for ultrafast photonics and optoelectronics. Recently, the giant optical nonlinearities of graphene oxide (GO) has been proved useful for a number of applications.[37] For example, the optical limiting of GO are indispensable to protect sensitive instruments from laser-induced damage. And the saturable absorption can be used for pulse compression, mode-locking and Q-switching. Also, the nonlinear refraction (Kerr effect) is crucial for functionalities including all-optical switching, signal regeneration, and fast optical communications.

One of the most intriguing and unique properties of GO is that its electrical and optical properties can be tuned dynamically by manipulating the content of oxygen-containing groups through either chemical or physical reduction methods. The tuning of the optical nonlinearities have been demonstrated during its entire laser-induced reduction process through continuous increase of the laser irradiance and four stages of different nonlinear activities have been discovered, which may serve as promising solid state materials for novel nonlinear functional devices.[38] It is also proved that metal nanoparticles can greatly enhance the optical nonlinearity[39] and fluorescence[40] of graphene oxide.

Graphene manufacture

Graphite oxide has attracted much interest as a possible route for the large-scale production and manipulation of graphene, a material with extraordinary electronic properties. Graphite oxide itself is an insulator,[41] almost a semiconductor, with differential conductivity[9] between 1 and 5×10−3 S/cm at a bias voltage of 10 V.[41] However, being hydrophilic, graphite oxide disperses readily in water, breaking up into macroscopic flakes, mostly one layer thick. Chemical reduction of these flakes would yield a suspension of graphene flakes. It was argued that the first experimental observation of graphene was reported by Hanns-Peter Boehm in 1962.[42] In this early work the existence of monolayer reduced graphene oxide flakes was demonstrated. The contribution of Boehm was recently acknowledged by Andre Geim, the Nobel Prize winner for graphene research.[43]

Partial reduction can be achieved by treating the suspended graphene oxide with hydrazine hydrate at 100 °C for 24 hours,[22] by exposing graphene oxide to hydrogen plasma for a few seconds,[41] or by exposure to a strong pulse of light, such as that of a Xenon flash.[44] Due to the oxidation protocol, manifold defects already present in graphene oxide hamper the effectiveness of the reduction. Thus, the graphene quality obtained after reduction is limited by the precursor quality (graphene oxide) and the efficiency of the reducing agent.[45] However, the conductivity of the graphene obtained by this route is below 10 S/cm,[44] and the charge mobility is between 0.1 and 10 cm2/Vs.[41][46][47] These values are much greater than the oxide's, but still a few orders of magnitude lower than those of pristine graphene.[41] Recently, the synthetic protocol for graphite oxide was optimized and almost intact graphene oxide with a preserved carbon framework was obtained. Reduction of this almost intact graphene oxide performs much better and the mobility values of charge carriers exceeds 1000 cm2/Vs for the best quality of flakes.[48] Inspection with the atomic force microscope shows that the oxygen bonds distort the carbon layer, creating a pronounced intrinsic roughness in the oxide layers which persists after reduction. These defects also show up in Raman spectra of graphene oxide.[41]

Large amounts of graphene sheets may also be produced through thermal methods. For example, in 2006 a method was discovered that simultaneously exfoliates and reduces graphite oxide by rapid heating (>2000 °C/min) to 1050 °C. At this temperature, carbon dioxide is released as the oxygen functionalities are removed and explosively separates the sheets as it comes out.[49]

Exposing a film of graphite oxide to the laser of a LightScribe DVD has also revealed to produce quality graphene at a low cost. [50]

Graphene oxide has also been reduced to graphene in situ, using a 3D printed pattern of engineered E. coli bacteria.[51]

Water purification

Graphite oxides were studied for desalination of water using reverse osmosis beginning in the 1960s.[52] In 2011 additional research was released.[53]

In 2013 Lockheed Martin announced their Perforene graphene filter. Lockheed claims the filter reduces energy costs of reverse osmosis desalination by 99%. Lockheed claimed that the filter was 500 times thinner than the best filter then on the market, one thousand times stronger and requires 1% of the pressure.[54] The product was not expected to be released until 2020.[55]

Another study showed that graphite oxide could be engineered to allow water to pass, but retain some larger ions.[54] Narrow capillaries allow rapid permeation by mono- or bilayer water. Multilayer laminates have a structure similar to nacre, which provides mechanical strength at water free conditions. Helium cannot pass through the membranes in humidity free conditions, but penetrates easily when exposed to humidity, whereas water vapor passes with no resistance. Dry laminates are vacuum-tight, but immersed in water, they act as molecular sieves, blocking some solutes.[56]

A third project produced graphene sheets with subnanoscale (0.40 ± 0.24 nm) pores. The graphene was bombarded with gallium ions, which disrupt carbon bonds. Etching the result with an oxidizing solution produces a hole at each spot struck by a gallium ion. The length of time spent in the oxidizing solution determined average pore size. Pore density reached 5 trillion pores per square centimeter, while retaining structural integrity. The pores permitted cation transport at short oxidation times, consistent with electrostatic repulsion from negatively charged functional groups at pore edges. At longer oxidation times, sheets were permeable to salt but not larger organic molecules.[57]

In 2015 a team created a graphene oxide tea that over the course of a day removed 95% of heavy metals in a water solution [58]

One project layered carbon atoms in a honeycomb structure, forming a hexagon-shaped crystal that measured about 0.1 millimeters in width and length, with subnanometer holes. Later work increased the membrane size to on the order of several millimeters.[59]

Graphene attached to a polycarbonate support structure was initially effective at removing salt. However, defects formed in the graphene. Filling larger defects with nylon and small defects with hafnium metal followed by a layer of oxide restored the filtration effect.[59]

In 2016 engineers developed graphene-based films that can filter dirty/salty water powered by the sun. Bacteria were used to produce a material consisting of two nanocellulose layers. The lower layer contains pristine cellulose, while the top layer contains cellulose and graphene oxide, which absorbs sunlight and produces heat. The system draws water from below into the material. The water diffuses into the higher layer, where it evaporates and leaves behind any contaminants. The evaporate condenses on top, where it can be captured. The film is produced by repeatedly adding a fluid coating that hardens. Bacteria produce nanocellulose fibers with interspersed graphene oxide flakes. The film is light and easily manufactured at scale.[60]

Coating

Optically transparent, multilayer films made from graphene oxide are impermeable under dry conditions. Exposed to water (or water vapor), they allow passage of molecules below a certain size. The films consist of millions of randomly stacked flakes, leaving nano-sized capillaries between them. Closing these nanocapillaries using chemical reduction with hydroiodic acid creates “reduced graphene oxide” (r-GO) films that are completely impermeable to gases, liquids or strong chemicals greater than 100 nanometers thick. Glassware or copper plates covered with such a graphene "paint” can be used as containers for corrosive acids. Graphene-coated plastic films could be used in medical packaging to improve shelf life.[61]

Related materials

Dispersed graphene oxide flakes can also be sifted out of the dispersion (as in paper manufacture) and pressed to make an exceedingly strong graphene oxide paper.[14]

Graphene oxide has been used in DNA analysis applications. The large planar surface of graphene oxide allows simultaneous quenching of multiple DNA probes labeled with different dyes, providing the detection of multiple DNA targets in the same solution. Further advances in graphene oxide based DNA sensors could result in very inexpensive rapid DNA analysis.[62] Recently a group of researchers, from university of L'Aquila (Italy), discovered new wetting properties of graphene oxide thermally reduced in ultra-high vacuum up to 900 °C. They found a correlation between the surface chemical composition, the surface free energy and its polar and dispersive components, giving a rationale to the wetting properties of graphene oxide and reduced graphene oxide.[63] [64]

Flexible rechargeable battery electrode

Graphene oxide has been demonstrated as a flexible free-standing battery anode material for room temperature lithium-ion[65] and sodium-ion batteries.[66][67] It is also being studied as a high surface area conducting agent in lithium-sulfur battery cathodes.[68] The functional groups on graphene oxide can serve as sites for chemical modification and immobilization of active species. This approach allows for the creation of hybrid architectures for electrode materials. Recent examples of this have been implemented in lithium ion batteries, which are known for being rechargeable at the cost of low capacity limits. Graphene oxide-based composites functionalized with metal oxides and sulfides have been shown in recent research to induce enhanced battery performance. This has similarly been adapted into applications in supercapacitors, since the electronic properties of graphene oxide allow it to bypass some of the more prevalent restrictions of typical transition metal oxide electrodes. Research in this field is developing, with additional exploration into methods involving nitrogen doping and pH adjustment to improve capacitance.[69] Additionally, research into reduced graphene oxide sheets, which display superior electronic properties akin to pure graphene, is currently being explored. Reduced graphene oxide applications greatly increases the conductivity and efficiency, while sacrificing some flexibility and structural integrity.[70][71][72]

Graphene oxide lens

The optical lens has been playing a critical role in almost all areas of science and technology since its invention about 3000 years ago. With the advances in micro- and nanofabrication techniques, continued miniaturization of the conventional optical lenses has always been requested for various applications such as communications, sensors, data storage and a wide range of other technology-driven and consumer-driven industries. Specifically, ever smaller sizes, as well as thinner thicknesses of micro lenses, are highly needed for subwavelength optics or nano-optics with extremely small structures, particularly for visible and near-IR applications. Also, as the distance scale for optical communications shrinks, the required feature sizes of micro lenses are rapidly pushed down.

Recently, the excellent properties of newly discovered graphene oxide provide novel solutions to overcome the challenges of current planar focusing devices. Specifically, giant refractive index modification (as large as 10^-1), which is one order of magnitude larger than the current materials, between graphene oxide (GO) and reduced graphene oxide (rGO) have been demonstrated by dynamically manipulating its oxygen content using direct laser writing (DLW) method. As a result, the overall lens thickness can be potentially reduced by more than ten times. Also, the linear optical absorption of GO is found to increase as the reduction of GO deepens, which results in transmission contrast between GO and rGO and therefore provides amplitude modulation mechanism. Moreover, both the refractive index and the optical absorption are found to be dispersionless over a broad wavelength range from visible to near infrared. Finally, GO film offers flexible patterning capability by using the maskless DLW method, which reduces the manufacturing complexity and requirement.

As a result, a novel ultrathin planar lens on a GO thin film has been realized recently using the DLW method.[73] The distinct advantage of the GO flat lens is that phase modulation and amplitude modulation can be achieved simultaneously, which are attributed to the giant refractive index modulation and the variable linear optical absorption of GO during its reduction process, respectively. Due to the enhanced wavefront shaping capability, the lens thickness is pushed down to subwavelength scale (~200 nm), which is thinner than all current dielectric lenses (~ µm scale). The focusing intensities and the focal length can be controlled effectively by varying the laser powers and the lens sizes, respectively. By using oil immersion high NA objective during DLW process, 300 nm fabrication feature size on GO film has been realized, and therefore the minimum lens size has been shrink down to 4.6 µm in diameter, which is the smallest planar micro lens and can only be realized with metasurface by FIB. Thereafter, the focal length can be reduced to as small as 0.8 µm, which would potentially increase the numerical aperture (NA) and the focusing resolution.

The full-width at half-maximum (FWHM) of 320 nm at the minimum focal spot using 650 nm input beam has been demonstrated experimentally, which corresponding to the effective numerical aperture (NA) of 1.24 (n=1.5), the largest NA of current micro lenses. Furthermore, ultra-broadband focusing capability from 500 nm to as far as 2 µm have been realized with the same planar lens, which is still a major challenge of focusing in infrared range due to limited availability of suitable materials and fabrication technology. Most importantly, the synthesized high quality GO thin films can be flexibly integrated on various substrates and easily manufactured by using the one-step DLW method over a large area at a comparable low cost and power (~nJ/pulse), which eventually makes the GO flat lenses promising for various practical applications.

Energy Conversion

Photocatalytic Water Splitting is an artificial photosynthesis process in which water is dissociated into hydrogen (H2) and oxygen (O2), using artificial or natural light. Methods such as photocatalytic water splitting are currently being investigated to produce hydrogen as a clean source of energy. The superior electron mobility and high surface area of graphene oxide sheets suggest it may be implemented as a catalyst that meets the requirements for this process. Specifically, graphene oxide’s compositional functional groups of epoxide (-O-) and hydroxide (-OH) allow for more flexible control in the water splitting process. This flexibility can be used to tailor the band gap and band positions that are targeted in photocatalytic water splitting. Recent research experiments have demonstrated that photocatalytic activity of graphene oxide containing a band gap within the required limits has produced effective splitting results, particularly when used with 40-50% coverage at a 2:1 hydroxide:epoxide ratio. When used in composite materials with CdS (a typical catalyst used in photocatalytic water splitting), graphene oxide nanocomposites have been shown to exhibit increased hydrogen production and quantum efficiency.

Hydrogen Storage

Graphene oxide is also being explored for its applications in hydrogen storage. Hydrogen molecules can be stored among the oxygen-based functional groups found throughout the sheet. This hydrogen storage capability can be further manipulated by modulating the interlayer distance between sheets, as well as making changes to the pore sizes. Research in transition metal decoration on carbon sorbents to enhance hydrogen binding energy has led to experiments with titanium and magnesium anchored to hydroxyl groups, allowing for the binding of multiple hydrogen molecules.

See also

References

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1859 in science

The year 1859 in science and technology involved some significant events, listed below.

Camille Petit

Camille Petit is an Associate Professor of Chemistry at Imperial College London. She designs and characterises functional materials for environmental sustainability.

Deborah Chung

Deborah Duen Ling Chung (professionally known as D.D.L. Chung, Chinese: 鍾端玲 or 黛博拉 • D • L • 钟; born 1952) is an American scientist and university professor.

Graphene

Graphene (/ˈɡræfiːn/) is an allotrope of carbon in the form of a single layer of atoms in a two-dimensional hexagonal lattice in which one atom forms each vertex. It is the basic structural element of other allotropes, including graphite, charcoal, carbon nanotubes and fullerenes. It can also be considered as an indefinitely large aromatic molecule, the ultimate case of the family of flat polycyclic aromatic hydrocarbons.

Graphene has a special set of properties which set it apart from other allotropes of carbon. In proportion to its thickness, it is about 100 times stronger than the strongest steel. Yet its density is dramatically lower than any other steel, with a surfacic mass of 0.763mg per square meter. It conducts heat and electricity very efficiently and is nearly transparent. Graphene also shows a large and nonlinear diamagnetism, even greater than graphite, and can be levitated by Nd-Fe-B magnets. Researchers have identified the bipolar transistor effect, ballistic transport of charges and large quantum oscillations in the material.Scientists have theorized about graphene for decades. It has likely been unknowingly produced in small quantities for centuries, through the use of pencils and other similar applications of graphite. It was originally observed in electron microscopes in 1962, but only studied while supported on metal surfaces. The material was later rediscovered, isolated and characterized in 2004 by Andre Geim and Konstantin Novoselov at the University of Manchester. Research was informed by existing theoretical descriptions of its composition, structure and properties. High-quality graphene proved to be surprisingly easy to isolate, making more research possible. This work resulted in the two winning the Nobel Prize in Physics in 2010 "for groundbreaking experiments regarding the two-dimensional material graphene."The global market for graphene is reported to have reached $9 million by 2012, with most of the demand from research and development in semiconductor, electronics, battery energy and composites.

Graphene chemistry

Graphene is the only form of carbon (or solid material) in which every atom is available for chemical reaction from two sides (due to the 2D structure). Atoms at the edges of a graphene sheet have special chemical reactivity. Graphene has the highest ratio of edge atoms of any allotrope. Defects within a sheet increase its chemical reactivity. The onset temperature of reaction between the basal plane of single-layer graphene and oxygen gas is below 260 °C (530 K). Graphene combusts at 350 °C (620 K). Graphene is commonly modified with oxygen- and nitrogen-containing functional groups and analyzed by infrared spectroscopy and X-ray photoelectron spectroscopy. However, determination of structures of graphene with oxygen- and nitrogen- functional groups requires the structures to be well controlled.

Contrary to the ideal 2D structure of graphene, chemical applications of graphene need either structural or chemical irregularities, as perfectly flat graphene is chemically inert. In other words, the definition of an ideal graphene is different in chemistry and physics.

Graphene placed on a soda-lime glass (SLG) substrate under ambient conditions exhibited spontaneous n-doping (1.33 × 1013 e/cm2) via surface-transfer. On p-type copper indium gallium diselenide (CIGS) semiconductor itself deposited on SLG n-doping reached 2.11 × 1013 e/cm2.

Graphene oxide paper

Graphene oxide paper or graphite oxide paper is a material fabricated from graphite oxide. Micrometer thick films of graphene oxide paper are also named as graphite oxide membranes (in the 1960s) or (more recently) graphene oxide membranes. The membranes are typically obtained by slow evaporation of graphene oxide solution or by the filtration method.

The material has exceptional stiffness and strength, due to the intrinsic strength of the two-dimensional graphene backbone and to its interwoven layer structure which distributes loads.

Graphene production techniques

A rapidly increasing list of graphene production techniques have been developed to enable graphene's use in commercial applications.

Isolated 2D crystals cannot be grown via chemical synthesis beyond small sizes even in principle, because the rapid growth of phonon density with increasing lateral size forces 2D crystallites to bend into the third dimension. However, other routes to 2d materials exist:

Fundamental forces place seemingly insurmountable barriers in the way of creating [2D crystals]... The nascent 2D crystallites try to minimize their surface energy and inevitably morph into one of the rich variety of stable 3D structures that occur in soot.

But there is a way around the problem. Interactions with 3D structures stabilize 2D crystals during growth. So one can make 2D crystals sandwiched between or placed on top of the atomic planes of a bulk crystal. In that respect, graphene already exists within graphite... One can then hope to fool Nature and extract single-atom-thick crystallites at a low enough temperature that they remain in the quenched state prescribed by the original higher-temperature 3D growth.

The early approaches of cleaving multi-layer graphite into single layers or growing it epitaxially by depositing a layer of carbon onto another material have been supplemented by numerous alternatives. In all cases, the graphite must bond to some substrate to retain its 2d shape.

Graphitic acid

Graphitic acid may refer to:

Graphite oxide

Mellitic acid

History of graphene

Single-layer graphene was explored theoretically by P. R. Wallace in 1947. It was first unambiguously produced and identified in 2004, by the group of Andre Geim and Konstantin Novoselov, though they credit Hanns-Peter Boehm and his co-workers for the experimental discovery of graphene in 1962. Boehm et al. introduced the term graphene in 1986.

Hummers' method

Hummers' method is a chemical process that can be used to generate graphite oxide through the addition of potassium permanganate to a solution of graphite, sodium nitrate, and sulfuric acid. It is commonly used by engineering and lab technicians as a reliable method of producing quantities of graphite oxide. It is also able to be revised in the creation of a one-molecule-thick version of the substance known as graphene oxide.

Layer by layer

Layer-by-layer (LbL) deposition is a thin film fabrication technique. The films are formed by depositing alternating layers of oppositely charged materials with wash steps in between. This can be accomplished by using various techniques such as immersion, spin, spray, electromagnetism, or fluidics.

LightScribe

LightScribe is an optical disc recording technology that was created by the Hewlett-Packard Company. It uses specially coated recordable CD and DVD media to produce laser-etched labels with text or graphics, as opposed to stick-on labels and printable discs. Although HP is no longer developing the technology, it is still maintained and supported by a number of independent enthusiasts.

The LightScribe method uses the laser in a way similar to when plain data are written to the disc; a greyscale image of the label is etched onto the upper side of the disc. In the beginning, the discs were available only in a sepia color but today are available in many monochromatic colors.

The purpose of LightScribe is to allow users to create direct-to-disc labels (as opposed to stick-on labels), using their optical disc writer. Special discs and a compatible disc writer are required. Before or after burning data to the read-side of the disc, the user turns the disc over and inserts it with the label side down. The drive's laser then etches into the label side in such a way that an image is produced.

Nicholas A. Kotov

Nicholas A. Kotov (born August 29, 1965, Moscow, USSR), is the Joseph B. and Florence V. Cejka Professor of Chemical Engineering at the University of Michigan in Ann Arbor, MI. He is known for his pioneering contributions to biomimetic nanostructures. In this large interdisciplinary research fields, he has been working on the topics of layered biomimetic nanocomposites, self-assembly of nanoparticles, and chiral nanostructures.

Oxocarbon

An oxocarbon or oxide of carbon is a chemical compound consisting only of carbon and oxygen.The simplest and most common oxocarbons are carbon monoxide (CO) and carbon dioxide (CO2) with IUPAC names carbon(II) oxide and carbon(IV) oxide respectively. Many other stable (practically if not thermodynamically) or metastable oxides of carbon are known, but they are rarely encountered, such as carbon suboxide (C3O2 or O=C=C=C=O) and mellitic anhydride (C12O9).

While textbooks will often list only the first three, and rarely the fourth, a large number of other oxides are known today, most of them synthesized since the 1960s. Some of these new oxides are stable at room temperature. Some are metastable or stable only at very low temperatures, but decompose to simpler oxocarbons when warmed. Many are inherently unstable and can be observed only momentarily as intermediates in chemical reactions or are so reactive that they can exist only in the gas phase or under matrix isolation conditions.

The inventory of oxocarbons appears to be steadily growing. The existence of graphene oxide and of other stable polymeric carbon oxides with unbounded molecular structures suggests that many more remain to be discovered.

Pressure-induced hydration

Pressure-induced hydration (PIH), also known as “super-hydration”, is a special case of pressure-induced insertion whereby water molecules are injected into the pores of microporous materials. In PIH, a microporous material is placed under pressure in the presence of water in the pressure-transmitting fluid of a diamond anvil cell.Early physical characterization and initial diffraction experiments in zeolites were followed by the first unequivocal structural characterization of PIH in the small-pore zeolite natrolite (Na16Al16Si24O80 x 16H2O), which in its fully super-hydrated form, Na16Al16Si24O80 x 32H2O, doubles the amount of water it contains in its pores.

PIH has now been demonstrated in natrolites containing Li, K, Rb and Ag as monovalent cations as well as in large-pore zeolites, pyrochlores, clays and graphite oxide.Using the noble gases Ar, Kr, and Xe as well as CO2 as pressure-transmitting fluids, researchers have prepared and structurally characterized the products of reversible, pressure-induced insertion of Ar Kr, and CO2 as well as the irreversible insertion of Xe and water.

Rachid Yazami

Rachid Yazami is a Moroccan scientist and engineer. He is best known for his critical role in the development of the lithium-ion battery, as the inventor of the graphite anode (negative pole) of lithium-ion batteries. He is also known for his research on fluoride ion batteries.

Research in lithium-ion batteries

Research in lithium-ion batteries has produced many proposed refinements of lithium-ion batteries. Areas on research interest have focused on improving energy density, safety, rate capability, cycle durability, flexibility, and cost.

Rodney S. Ruoff

Rodney S. "Rod" Ruoff (born 1957) is an American physical chemist and nanoscience researcher. He is one of the world experts on carbon materials including carbon nanostructures such as fullerenes, nanotubes, graphene, diamond, and has had pioneering discoveries on such materials and others. Ruoff received his B.S. in Chemistry from the University of Texas at Austin (1981) and his Ph.D. in Chemical Physics at the University of Illinois-Urbana (1988). After a Fulbright Fellowship at the MPI fuer Stroemungsforschung in Goettingen, Germany (1989) and postdoctoral work at the IBM T. J. Watson Research Center (1990–91), Ruoff became a staff scientist in the Molecular Physics Laboratory at SRI International (1991-1996). He is currently UNIST Distinguished Professor at the Ulsan National Institute of Science and Technology (UNIST), and the director of the Center for Multidimensional Carbon Materials (CMCM), an Institute for Basic Science (IBS) Center located at UNIST.

Timothy M. Swager

Timothy M. Swager (born 1961) is an American Scientist and the John D. MacArthur Professor of Chemistry at the Massachusetts Institute of Technology and the director of the Deshpande Center for Technological Innovation. His research is at the interface of chemistry and materials science, with specific interests in carbon nanomaterials, polymers, and liquid crystals. He is a member of the National Academy of Sciences and American Academy of Arts and Sciences.

Common oxides
Exotic oxides
Polymers
Compounds derived from oxides

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