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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.[1]
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.[2] Graphene oxide sheets have been used to prepare a strong paper-like material, and have recently attracted substantial interest as a possible intermediate for the manufacture of graphene. However, as of 2010 this goal remains elusive since graphene obtained by this route still has many chemical and structural defects.
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.[3] In 1957 Hummers and Offeman developed a safer, quicker, and more efficient process, using a mixture of sulfuric acid H2SO4, sodium nitrate NaNO3, and potassium permanganate KMnO4, which is still widely used (as of 2009).[1] Recently, a safer and better method was developed using sulfuric acid H2SO4, phosphoric acid H3PO4, and potassium permanganate KMnO4.[4]
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).[5]
The structure and properties of graphite oxide depend on particular synthesis method and degree of oxidation. 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: carbonyl (=CO), hydroxyl (-OH), phenol groups attached to both sides.[6][7] There is evidence of "buckling" (deviation from planarity) of the layers. 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.[6][7] 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 [7][8] The edges of each layer are terminated with carboxyl and carbonyl groups.[6] X-ray photoelectron spectroscopy shows the presence of carbon atoms in non-oxygenated ring contexts (284.8 eV), in C-O (286.2 eV), in C=O (287.8 eV) and in O-C=O (289.0 eV).[9]
Graphite oxide is easily hydrated, 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. [10] The bulk product absorbs moisture from ambient air proportionally to humidity. Complete removal of water from the structure seems difficult since heating at 60–80 °C results in partial decomposition and degradation of the material. Similar to water, graphite oxide also easily incorporates other polar solvents, e.g. alcohols. Separation of graphite oxide layers is proportional to the size of alcohol molecule; additional monolayer is inserted into the structure at high pressure conditions. [11]
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.[12]
Graphite oxide has attracted much interest recently 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,[15] almost a semiconductor, with differential conductivity between 1 and 5×10−3 S/cm at a bias voltage of 10 V.[15] However, being hydrophilic, graphene 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.[16] 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.[17]
Partial reduction can be achieved by treating the suspended graphene oxide with hydrazine hydrate at 100 °C for 24 hours,[9] or by exposing graphene oxide to hydrogen plasma for a few seconds,[15] or by exposure to a strong pulse of light, such as that of a Xenon flash.[18] However, the conductivity of the graphene obtained by this route is below 10 S/cm,[18] and the charge mobility is between 2 to 200 cm2/(V·s) for holes and 0.5 to 30 cm2/(V·s) for electrons.[15] These values are much greater than the oxide's, but still a few orders of magnitude lower than those of pristine graphene.[15] 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.[15]
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.