Graphene oxide
Encyclopedia
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 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.
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. 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.
chemist Benjamin C. Brodie in 1859, by treating graphite with a mixture of potassium chlorate
and fuming nitric acid
. 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). Recently, a safer and better method was developed using sulfuric acid
H2SO4, phosphoric acid
H3PO4, and potassium permanganate
KMnO4.
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).
groups (bridging oxygen atoms), other functional groups experimentally found are: carbonyl
(=CO), hydroxyl
(-OH), phenol
groups attached to both sides. 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. 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 The edges of each layer are terminated with carboxyl and carbonyl
groups. 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).
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.
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.
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
.
, a material with extraordinary electronic properties. Graphite oxide itself is an insulator, almost a semiconductor
, with differential conductivity between 1 and 5×10−3 S/cm at a bias voltage of 10 V. 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. 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.
Partial reduction can be achieved by treating the suspended graphene oxide with hydrazine
hydrate at 100 °C for 24 hours, or by exposing graphene oxide to hydrogen plasma
for a few seconds, or by exposure to a strong pulse of light, such as that of a Xenon flash. However, the conductivity of the graphene obtained by this route is below 10 S/cm, and the charge mobility
is between 2 to 200 cm2/(V·s) for holes and 0.5 to 30 cm2/(V·s) for electrons. These values are much greater than the oxide's, but still a few orders of magnitude lower than those of pristine graphene. 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.
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
.
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
in variable ratios, obtained by treating graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
with strong oxidizers. The maximally oxidized bulk product is 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.
The bulk material disperses in basic
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
solutions to yield monomolecular sheets, known as graphene oxide by analogy to graphene
Graphene
Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite and the suffix -ene by Hanns-Peter Boehm, who described single-layer...
, the single-layer form of graphite. 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.
History and preparation
Graphite oxide was first prepared by OxfordOxford
The city of Oxford is the county town of Oxfordshire, England. The city, made prominent by its medieval university, has a population of just under 165,000, with 153,900 living within the district boundary. It lies about 50 miles north-west of London. The rivers Cherwell and Thames run through...
chemist Benjamin C. Brodie in 1859, by treating graphite with a mixture of potassium chlorate
Potassium chlorate
Potassium chlorate is a compound containing potassium, chlorine and oxygen atoms, with the molecular formula KClO3. In its pure form, it is a white crystalline substance. It is the most common chlorate in industrial use...
and fuming nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
. In 1957 Hummers and Offeman developed a safer, quicker, and more efficient process, using a mixture of sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
H2SO4, sodium nitrate
Sodium nitrate
Sodium nitrate is the chemical compound with the formula NaNO3. This salt, also known as Chile saltpeter or Peru saltpeter to distinguish it from ordinary saltpeter, potassium nitrate, is a white solid which is very soluble in water...
NaNO3, and potassium permanganate
Potassium permanganate
Potassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt consisting of K+ and MnO4− ions. Formerly known as permanganate of potash or Condy's crystals, it is a strong oxidizing agent. It dissolves in water to give intensely purple solutions, the...
KMnO4, which is still widely used (as of 2009). Recently, a safer and better method was developed using sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
H2SO4, phosphoric acid
Phosphoric acid
Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...
H3PO4, and potassium permanganate
Potassium permanganate
Potassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt consisting of K+ and MnO4− ions. Formerly known as permanganate of potash or Condy's crystals, it is a strong oxidizing agent. It dissolves in water to give intensely purple solutions, the...
KMnO4.
Recently a mixture of H2SO4 and KMnO4 has been used to cut open carbon nanotubes lengthwise, resulting in microscopic flat ribbons of graphene
Graphene nanoribbons
Graphene nanoribbons , often abbreviated GNRs, are thin strips of graphene or unrolled single-walled carbon nanotubes...
, a few atoms wide, with the edges "capped" by oxygen atoms (=O) or hydroxyl groups (-OH).
Structure
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 epoxideEpoxide
An epoxide is a cyclic ether with three ring atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in...
groups (bridging oxygen atoms), other functional groups experimentally found are: carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
(=CO), hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
(-OH), phenol
Phenols
In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of a hydroxyl group bonded directly to an aromatic hydrocarbon group...
groups attached to both sides. 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. Scanning tunneling microscopy shows the presence of local regions where oxygen atoms are arranged in a rectangular pattern with lattice constant
Lattice constant
The lattice constant [or lattice parameter] refers to the constant distance between unit cells in a crystal lattice. Lattices in three dimensions generally have three lattice constants, referred to as a, b, and c. However, in the special case of cubic crystal structures, all of the constants are...
0.27 nm × 0.41 nm The edges of each layer are terminated with carboxyl and carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
groups. X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material...
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).
Graphite oxide is easily hydrated
Hydration reaction
In organic chemistry, a hydration reaction is a chemical reaction in which a hydroxyl group and a hydrogen cation are added to the two carbon atoms bonded together in the carbon-carbon double bond which makes up an alkene functional group. The reaction usually runs in a strong acidic, aqueous...
, 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.
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.
Graphite oxide exfoliates and decomposes when rapidly heated at moderately high temperatures (~280–300 °C) with formation of finely dispersed amorphous carbon
Amorphous carbon
Amorphous carbon or free, reactive carbon, is an allotrope of carbon that does not have any crystalline structure. As with all glassy materials, some short-range order can be observed...
, somewhat similar to activated carbon
Activated carbon
Activated carbon, also called activated charcoal, activated coal or carbo activatus, is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions.The word activated in the name is sometimes replaced...
.
Graphene manufacture
Graphite oxide has attracted much interest recently as a possible route for the large-scale production and manipulation of grapheneGraphene
Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite and the suffix -ene by Hanns-Peter Boehm, who described single-layer...
, a material with extraordinary electronic properties. Graphite oxide itself is an insulator, almost a semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
, with differential conductivity between 1 and 5×10−3 S/cm at a bias voltage of 10 V. However, being hydrophilic
Hydrophile
A hydrophile, from the Greek "water" and φιλια "love," is a molecule or other molecular entity that is attracted to, and tends to be dissolved by water. A hydrophilic molecule or portion of a molecule is one that has a tendency to interact with or be dissolved by, water and other polar substances...
, graphene oxide disperses readily in water, breaking up into macroscopic flakes, mostly one layer thick. Chemical reduction of these flakes would yield a suspension
Suspension (chemistry)
In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
of graphene flakes. It was argued that the first experimental observation of graphene
Graphene
Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite and the suffix -ene by Hanns-Peter Boehm, who described single-layer...
was reported by Hanns-Peter Boehm
Hanns-Peter Boehm
Hanns-Peter Boehm is a German chemist and professor emeritus at Ludwig-Maximilians-Universität in Munich, Germany. Boehm is considered a pioneer of graphene research.- Biography :...
in 1962. In this early work the existence of monolayer reduced graphene oxide flakes was demonstrated. The contribution of Boehm was recently acknowledged by Andre Geim
Andre Geim
Andre Konstantin Geim, FRS is a Dutch-Russian-British physicist working at the University of Manchester. Geim was awarded the 2010 Nobel Prize in Physics jointly with Konstantin Novoselov for his work on graphene...
, the Nobel Prize winner for graphene research.
Partial reduction can be achieved by treating the suspended graphene oxide with hydrazine
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...
hydrate at 100 °C for 24 hours, or by exposing graphene oxide to hydrogen plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
for a few seconds, or by exposure to a strong pulse of light, such as that of a Xenon flash. However, the conductivity of the graphene obtained by this route is below 10 S/cm, and the charge mobility
Electron mobility
In solid-state physics, the electron mobility characterizes how quickly an electron can move through a metal or semiconductor, when pulled by an electric field. In semiconductors, there is an analogous quantity for holes, called hole mobility...
is between 2 to 200 cm2/(V·s) for holes and 0.5 to 30 cm2/(V·s) for electrons. These values are much greater than the oxide's, but still a few orders of magnitude lower than those of pristine graphene. Inspection with the atomic force microscope
Atomic force microscope
Atomic force microscopy or scanning force microscopy is a very high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit...
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
Raman spectroscopy
Raman spectroscopy is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system.It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range...
of graphene oxide.
Related materials
DispersedDispersion (chemistry)
A dispersion is a system in which particles are dispersed in a continuous phase of a different composition . See also emulsion. A dispersion is classified in a number of different ways, including how large the particles are in relation to the particles of the continuous phase, whether or not...
graphene oxide flakes can also be sifted out of the dispersion (as in paper manufacture
Papermaking
Papermaking is the process of making paper, a substance which is used universally today for writing and packaging.In papermaking a dilute suspension of fibres in water is drained through a screen, so that a mat of randomly interwoven fibres is laid down. Water is removed from this mat of fibres by...
) and pressed to make an exceedingly strong graphene oxide paper
Graphene Oxide Paper
Graphene oxide paper or graphite oxide paper is a composite material fabricated from graphite oxide.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.The starting...
.