Fluorographene (or perfluorographane, graphene fluoride) is a fluorocarbon derivative of

graphene Graphene () is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure.

.''Properties of Fluorinated Graphene Films'' It is a two dimensional

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...

sheet of sp³ hybridized

s, with each carbon atom bound to one fluorine. The chemical formula is . In comparison, Teflon ( polytetrafluoroethylene), -(CF₂)_n-, consists of carbon "chains" with each carbon bound to two fluorines. Unlike fluorographene, graphene is unsaturated ( sp² hybridized) and completely carbon. The

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

analogue to fluorographene is sp³ hybridized

graphane Graphane is a two-dimensional polymer of carbon and hydrogen with the formula unit (CH)n where ''n'' is large. Partial hydrogenation results in hydrogenated graphene, which was reported by Elias et al in 2009 by a TEM study to be "direct evidence ...

. Similar to other fluorocarbons (e.g.

perfluorohexane Perfluorohexane (), or tetradecafluorohexane, is a fluorocarbon. It is a derivative of hexane in which all of the hydrogen atoms are replaced by fluorine atoms. It is used in one formulation of the electronic cooling liquid/insulator Fluorinert ...

), fluorographene is highly insulating. Fluorographene is thermally stable, resembling polytetrafluoroethylene; however, chemically it is reactive. It can be transformed back into graphene by reaction with

potassium iodide Potassium iodide is a chemical compound, medication, and dietary supplement. It is a medication used for treating hyperthyroidism, in radiation emergencies, and for protecting the thyroid gland when certain types of radiopharmaceuticals are u ...

at high temperatures. During reactions of fluorographene with

NaOH Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...

and NaSH simultaneous reductive defluorination and substitution are observed. The reactivity of fluorographene represents a facile way towards graphene derivatives.

Preparation

The material was first created in 2010 by growing graphene on

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

foil exposed to

xenon difluoride Xenon difluoride is a powerful fluorinating agent with the chemical formula , and one of the most stable xenon compounds. Like most covalent inorganic fluorides it is moisture-sensitive. It decomposes on contact with water vapor, but is otherw ...

at 30 °C. It was discovered soon after that fluorographene could also be prepared by combining cleaved graphene on a

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile me ...

grid while being exposed to xenon difluoride at 70 °C. Also in 2010 Withers ''et al''. described exfoliation of fluorinated graphite (monolayer, 24% fluorination) and Cheng ''et al''. reported reversible graphene fluorination. Stoichiometric fluorographene was also prepared by chemical exfoliation of graphite fluoride. It was also shown that graphene fluoride can be transformed back into graphene via reaction with iodine, which forms graphene iodide as a short lived intermediate.

Structure

The structure of fluorographene can be derived from the structure of graphite monofluoride , which consists of weakly bound stacked fluorographene layers, and its most stable conformation (predicted for the monocrystal) contains an infinite array of trans-linked cyclohexane chairs with

covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...

C–F bonds in an AB stacking sequence.Karlicky F, Datta KKR, Otyepka M, Zboril R ''Halogenated Graphenes: Rapidly Growing Family of Graphene Derivatives''. ACS Nano, 2013, 7 (8), pp 6434–6464 The estimated C-F distance is 136-138 pm, C-C distance is 157-158 pm and the C-C-C angle is 110°.Karlicky F, Otyepka M 'Band Gaps and Optical Spectra of Chlorographene, Fluorographene and Graphane from G0W0, GW0 and GW Calculations on Top of PBE and HSE06 Orbitals''. J. Chem. Theory. Comput., 2013, 9 (9), Possible fluorographene conformations have been extensively investigated computationally.

Electronic properties

Fluorographene is considered a wide gap

semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

, because its I-V characteristics are strongly nonlinear with a nearly gate-independent resistance greater than 1 GΩ. In addition,

fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

and NEXAFS measurements indicate band gap higher than 3.8 eV. Theoretical calculations show that estimation of fluorographene band gap is rather challenging task, as GGA functional provides band gap of 3.1 eV, hybrid (HSE06) 4.9 eV, GW 8.1 eV on top of PBE 8.1 or 8.3 eV on top of HSE06. The optical transition calculated by the Bethe-Salpeter equation is equal to 5.1 eV and points to an extremely strong exciton binding energy of 1.9 eV. It has recently been demonstrated that using fluorographene as a passivation layer in Field Effect Transistors (FETs) featuring a graphene channel, carrier mobility increases significantly.

Reaction

Fluorographene is susceptible for

nucleophilic substitution In chemistry, a nucleophilic substitution is a class of chemical reactions in which an electron-rich chemical species (known as a nucleophile) replaces a functional group within another electron-deficient molecule (known as the electrophile). The ...

and reductive defluorination, which makes it an extraordinary precursor material for synthesis of numerous graphene derivatives. Both of these channels can be used to chemically manipulate fluorographene, and they can be tuned by suitable conditions, e.g., solvent.Matochová D, Medved M, Aristides B, Steklý T, Zbořil R, Otyepka M ''2D Chemistry - Chemical Control of Graphene Derivatization.'' J. Phys. Chem. Lett., 2018, 9 (13), pp 3580–3585 . In 2010 it was shown that fluorographene can be transformed to graphene by treatment with KI. Nucleophiles can substitute the fluorine atoms and induce partial or full defluorination.Dubecký M, Otyepková E, Lazar P, Karlický F, Petr M, Čépe K, Banáš P, Zbořil R, Otyepka M ''Reactivity of Fluorographene: A Facile Way toward Graphene Derivatives''. J. Phys. Chem. Lett., 2015, 6 (8), pp 1430–1434 The fluorographene reactivity is triggered by point defects.Medved M, Zoppellaro G, Ugolotti J, Matochová D, Lazar P, Pospíšil T, Bakandritsos A, Tuček J, Zbořil R, Otyepka M ''Reactivity of fluorographene is triggered by point defects: beyond the perfect 2D world.'' Nanoscale, 2018, 10, pp 4696-4707 The knowledge on fluorographene reactivity can be used for synthesis of new graphene derivatives, which contain i) mixture of F and other functional groups (like, e.g., thiofluorographene containing both -F and -SH Urbanová V, Holá K, Bourlinos AB, Čépe K, Ambrosi A, Loo AH, Pumera M, Karlický F, Otyepka M, Zbořil R ''Thiofluorographene-Hydrophilic Graphene Derivative with Semiconducting and Genosensing Properties''. Adv. Mater., 2015, 27 (14), pp 2305–2310 ) or ii) selectively only the functional group (and any -F groups).

Alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloa ...

and

aryl In organic chemistry, an aryl is any functional group or substituent derived from an aromaticity, aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar ...

groups can be selectively attached to graphene using

Grignard reaction The Grignard reaction () is an organometallic chemical reaction in which alkyl, allyl, vinyl, or aryl-magnesium halides (Grignard reagent) is added to a carbonyl group in an aldehyde or ketone. This reaction is important for the formation of ...

with fluorographene and this reaction leads to high-degree of graphene functionalization.Chronopoulos DD, Bakandritsos A, Lazar P, Pykal M, Čépe K, Zbořil R, Otyepka M ''High-Yield Alkylation and Arylation of Graphene via Grignard Reaction with Fluorographene''. Chem. Mater., 2017, 29 (3), pp 926–930 Very promising and selective graphene derivative cyanographene (graphene nitrile) was synthesized by reaction of NaCN with fluorographene. This material was further used for synthesis of graphene acid, i.e., graphene functionalized by -COOH groups over its surface, and it was shown that this graphene acid can be effectively conjugated with

amines In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent such ...

and alcohols. These findings open new door for high-yield and selective graphene functionalization.Bakandritsos A, Pykal M, Blonski P, Jakubec P, Chronopoulos DD, Poláková K, Georgakilas V, Cepe K, Tomanec O, Ranc V, Bourlinos AB, Zbořil R, Otyepka M ''Cyanographene and Graphene Acid - Emerging Derivatives Enabling High-Yield and Selective Functionalization of Graphene''. ACS Nano, 2017, 11 (3), pp 2982–2991

Other halogenated graphenes

Recent studies have also revealed that, similar to fluorination, full chlorination of graphene can be achieved. The resulting structure is called chlorographene. However other theoretical calculations questioned stability of chlorographene under ambient conditions. Also graphene can be fluorinated or halofluorinated by CVD-method with fluorocarbons, hydro- or halofluorocarbons by heating while in contact of carbon material with fluoroorganic substance to form partially fluorinated carbons (so called Fluocar materials). An overview on preparation, reactivity and properties of halogenated graphenes in available in ACS Nano journal free of charge.

References