Graphite oxide

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

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 disperses in basic solutions 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.

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.[4] 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][5][6] 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.[7]

Graphite oxides demonstrate considerable variations of properties depending on degree of oxidation and synthesis method.[8] 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.[9] Hydration and solvation properties of Brodie and Hummers graphite oxides are also remarkably different.[10]

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).[11]

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.[12]

Structure

The structure and properties of graphite oxide depend on particular synthesis method and degree of oxidation.[8] 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:[8] 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 .[13][14][15] There is evidence of "buckling" (deviation from planarity), folding and cracking [16] of graphene oxide sheets upon deposition of the layers on a choice of substrate. 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.[13][14] 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 [14][17] The edges of each layer are terminated with carboxyl and carbonyl groups.[13] 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).[18] 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).[19]

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.[20] 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.[10] Complete removal of water from the structure seems difficult since heating at 60–80 °C results in partial decomposition and degradation of the material.

Exfoliation of graphite oxide at high temperature, screenshots from video available here:[21] Exfoliation results in tenfold increase of sample volume and formation of carbon powder with grains of few graphene layers thickness.[22]

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.[23] 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.[24] Additional methanol and ethanol monolayer is reversibly inserted into the structure of Brodie graphite oxide also at high pressure conditions.[23]

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.[25]

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.[22]

Characterization

(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).[8]

XRD, FTIR, Raman, XPS, AFM, TEM, SEM/EDX,[8][26][27] etc. are some common techniques to characterize GO samples.[28] 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.[8] 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.[8]

Surface Properties

It is also possible to modify the surface of graphene oxide to change it's properties.[27][29] Graphene oxide has unique surface properties which make it a very good surfactant material stabilizing various emulsion systems.[27][8] 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.[30][27]

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.[31]

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.[32] 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.[32]

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.[33] 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.[34] It is also proved that metal nanoparticles can greatly enhance the optical nonlinearity[35] and fluorescence[36] 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,[37] almost a semiconductor, with differential conductivity[8] between 1 and 5×10−3 S/cm at a bias voltage of 10 V.[37] 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.[38] 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.[39]

Partial reduction can be achieved by treating the suspended graphene oxide with hydrazine hydrate at 100 °C for 24 hours,[18] by exposing graphene oxide to hydrogen plasma for a few seconds,[37] or by exposure to a strong pulse of light, such as that of a Xenon flash.[40] 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.[41] However, the conductivity of the graphene obtained by this route is below 10 S/cm,[40] and the charge mobility is between 0.1 and 10 cm2/Vs.[37][42][43] These values are much greater than the oxide's, but still a few orders of magnitude lower than those of pristine graphene.[37] 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.[44] 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.[37]

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.[45]

Exposing a film of graphite oxide to the laser of a LightScribe DVD has also revealed to produce quality graphene at a low cost. [46]

Graphene oxide has also been reduced to graphene in situ, using a 3D printed pattern of engineered E. coli bacteria.[47]

Water purification

Graphite oxides were studied for desalination of water using reverse osmosis beginning in the 1960s.[48] In 2011 additional research was released.[49]

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.[50] The product was not expected to be released until 2020.[51]

Another study showed that graphite oxide could be engineered to allow water to pass, but retain some larger ions.[50] 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.[52]

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.[53]

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 [54]

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.[55]

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.[55]

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 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.[56]

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.[57]

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.[12]

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.[58] 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.[59] [60]

Flexible rechargeable battery electrode

Graphene oxide has been demonstrated as a flexible free-standing battery anode material for room temperature lithium-ion[61] and sodium-ion batteries.[62][63] It is also being studied as a high surface area conducting agent in lithium-sulfur battery cathodes.[64]

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.[65] 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.

See also

References

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