Dichlorocarbene-Functionalized Fluorographene: Synthesis and Reaction Mechanism

Emerging graphene derivatives as active 2D coordination platforms for single-atom catalysts

Single-atom catalysts (SACs) based on graphene derivatives are an emerging and growing class of materials functioning as two-dimensional (2D) metal-coordination scaffolds with intriguing properties. Recently, owing to the rich chemistry of fluorographene, new avenues have opened toward graphene derivatives with selective, spacer-free, and dense functionalization, acting as in-plane or out-of-plane metal coordination ligands. The particular structural features give rise to intriguing phenomena occurring between the coordinated metals and the graphene backbone. These include redox processes, charge transfer, emergence, and stabilization of rare or otherwise unstable metal valence states, as well as metal-support and metal-metal synergism. The vast potential of such systems has been demonstrated as enzyme mimics for cooperative mixed-valence SACs, ethanol fuel cells, and CO₂ fixation; however, it is anticipated that their impact will further expand toward diverse fields, e.g., advanced organic transformations, electrochemical energy storage, and energy harvesting.

Recent Advances in Fluorinated Graphene from Synthesis to Applications: Critical Review on Functional Chemistry and Structure Engineering

Fluorinated graphene (FG), as an emerging member of the graphene derivatives family, has attracted wide attention on account of its excellent performances and underlying applications. The introduction of a fluorine atom, with the strongest electronegativity (3.98), greatly changes the electron distribution of graphene, resulting in a series of unique variations in optical, electronic, magnetic, interfacial properties and so on. Herein, recent advances in the study of FG from synthesis to applications are introduced, and the relationship between its structure and properties is summarized in detail. Especially, the functional chemistry of FG has been thoroughly analyzed in recent years, which has opened a universal route for the functionalization and even multifunctionalization of FG toward various graphene derivatives, which further broadens its applications. Moreover, from a particular angle, the structure engineering of FG such as the distribution pattern of fluorine atoms and the regulation of interlayer structure when advanced nanotechnology gets involved is summarized. Notably, the elaborated structure engineering of FG is the key factor to optimize the corresponding properties for potential applications, and is also an up-to-date research hotspot and future development direction. Finally, perspectives and prospects for the problems and challenges in the study of FG are put forward.

Halogen Functionalization in the 2D Material Flatland: Strategies, Properties, and Applications

Given the electronegativity and bonding environment of halogen elements, halogenation (i.e., fluorination, chlorination, bromination, and iodination) serves as a versatile strategy for chemical modifications of materials. The combination of halogens and 2D materials has triggered extensive interests since the first report on graphene fluorination in 2008. Subsequently, scholars consistently conduct pre-, in-process, or posthalogenation modifications of emerging 2D materials to achieve desired properties and broad device applications. They also continuously explore the role of halogens in 2D material functionalization. The multiple advantages introduced by halogen decoration make 2D materials outstanding from each subclass. In this review, an overall retrospect is provided on the research advances in the area of 2D material halogenation, including experimental halogenation strategies, halogen-triggered novel physics and properties, and advanced applications across the studied objects. Future research directions in this area are also proposed.

Immobilization of molecular catalysts for artificial photosynthesis

Artificial photosynthesis offers a way of producing fuels or high-value chemicals using a limitless energy source of sunlight and abundant resources such as water, CO2, and/or O2. Inspired by the strategies in natural photosynthesis, researchers have developed a number of homogeneous molecular systems for photocatalytic, photoelectrocatalytic, and electrocatalytic artificial photosynthesis. However, their photochemical instability in homogeneous solution are hurdles for scaled application in real life. Immobilization of molecular catalysts in solid supports support provides a fine blueprint to tackle this issue. This review highlights the recent developments in (i) techniques for immobilizing molecular catalysts in solid supports and (ii) catalytic water splitting, CO2 reduction, and O2 reduction with the support-immobilized molecular catalysts. Remaining challenges for molecular catalyst-based devices for artificial photosynthesis are discussed in the end of this review.

Microwave Energy Drives "On-Off-On" Spin-Switch Behavior in Nitrogen-Doped Graphene

The established application of graphene in organic/inorganic spin-valve spintronic assemblies is as a spin-transport channel for spin-polarized electrons injected from ferromagnetic substrates. To generate and control spin injection without such substrates, the graphene backbone must be imprinted with spin-polarized states and itinerant-like spins. Computations suggest that such states should emerge in graphene derivatives incorporating pyridinic nitrogen. The synthesis and electronic properties of nitrogen-doped graphene (N content: 9.8%), featuring both localized spin centers and spin-containing sites with itinerant electron properties, are reported. This material exhibits spin-switch behavior (on-off-on) controlled by microwave irradiation at X-band frequency. This phenomenon may enable the creation of novel types of switches, filters, and spintronic devices using sp² -only 2D systems.

In Situ Radical Polymerization and Grafting Reaction Simultaneously Initiated by Fluorinated Graphene

Fluorinated graphene (FG) showed interesting electrochemical, electronic, and mechanical properties, as well as chemical reactivity for multifarious functionalization of graphene material. This work reported a free radical polymerization and grafting from polymerization of a styrene monomer directly initiated by FG, which simultaneously provided free polymers and functionalized graphene with polymer chains grafted. The FG exhibited an almost comparative initiation efficiency to equivalent commercial initiator azodiisobutyronitrile under similar conditions, resulting in a high yield of free polystyrene (40.9%) with a high molecular weight ( M_n = 114.7 kg/mol). It was demonstrated that FG-triggered polymerization presented some special characteristics, such as a long lifetime of chain radical centers even when the reaction was stopped and insensitivity to oxygen molecules. The mechanistic study indicated that the polymerization was initiated by single-electron transfer reaction between FG and a monomer leading to formation of primary radicals; in addition, FG also played an important role in chain transfer and termination reactions during the polymerization process.

The Friedel-Crafts reaction of fluorinated graphene for high-yield arylation of graphene

Herein, we report the Friedel-Crafts reaction of fluorinated graphene with aryl molecules including methylbenzene, chlorobenzene and polystyrene. The reaction achieved the high-yield arylation functionalization of graphene under mild reaction conditions and extends the range of the Friedel-Crafts reaction to the field of two-dimensional materials.

Reactivity of fluorographene is triggered by point defects: beyond the perfect 2D world

Preparation of graphene derivatives using fluorographene (FG) as a precursor has become a key strategy for the large-scale synthesis of new 2-D materials (e.g. graphene acid, cyanographene, allyl-graphene) with tailored physicochemical properties. However, to gain full control over the derivatization process, it is essential to understand the reaction mechanisms and accompanying processes that affect the composition and structure of the final products. Despite the strength of C-F bonds and high chemical stability of perfluorinated hydrocarbons, FG is surprisingly susceptible to reactions under ambient conditions. There is clear evidence that nucleophilic substitution on FG is accompanied by spontaneous defluorination, and solvent-induced defluorination can occur even in the absence of any nucleophilic agent. Here, we show that distributed radical centers (fluorine vacancies) on the FG surface need to be taken into account in order to rationalize the defluorination mechanism. Depending on the environment, these radical centers can react as electron acceptors, electrophilic sites and/or cause homolytic bond cleavages. We also propose a new radical mechanism of FG defluorination in the presence of N,N'-dimethylformamide (DMF) solvent. Spin-trap experiments as well as ¹⁹F NMR measurements unambiguously confirmed formation of N,N'-dimethylformyl radicals and also showed that N,N'-dimethylcarbamoyl fluoride plays a key role in the proposed mechanism. These findings imply that point defects in 2D materials should be considered as key factor determining their chemical properties and reactivity.

Chemistry of Graphene Derivatives: Synthesis, Applications, and Perspectives

The chemistry of graphene and its derivatives is one of the hottest topics of current material science research. The derivatisation of graphene is based on various approaches, and to date functionalization with halogens, hydrogen, various functional groups containing oxygen, sulfur, nitrogen, phosphorus, boron, and several other elements have been reported. Most of these functionalizations are based on sp³ hybridization of carbon atoms in the graphene skeleton, which means the formation of out-of-plane covalent bonds. Several elements were also reported for substitutional modification of graphene, where the carbon atoms are substituted with atoms like nitrogen, boron, and several others. From tens of functional groups, for only two of them were reported full functionalization of graphene skeleton and formation of its stoichiometric counterparts, fluorographene and hydrogenated graphene. The functionalization of graphene is crucial for most of its applications including energy storage and conversion devices, electronic and optic applications, composites, and many others.

Fluorographite to hydroxy graphene to graphene: a simple wet chemical approach for good quality graphene

Good quality graphene is prepared in a scalable manner from fluorographite by nucleophilic substitution of F with OH⁻ ions.

Radical mechanism of a nucleophilic reaction depending on a two-dimensional structure

The mechanism of nucleophilic substitution deserves more investigation to include more reaction systems such as two-dimensional (2D) materials. In this study, we used fluorinated graphene (FG) as a representative 2D material to reveal the in-depth mechanism of its defluorination and nucleophilic substitution reaction under attack of common nucleophiles to explore the chemistry of 2D materials and enrich the research on the nucleophilic substitution reaction. DFT calculations and electron paramagnetic resonance spectroscopy (EPR) demonstrated that defluorination of FG occurred via a radical mechanism after a single electron transfer (SET) reaction between the nucleophile and C-F bond, and a spin center was generated on the nanosheet and fluorine anion. Moreover, neither the S_N1 nor S_N2 mechanism was suggested to be appropriate for the substitution reaction of FG with a 2D structure due to the corresponding kinetics or thermodynamics disadvantage; hence, its nucleophilic substitution was proved to occur via a radical mechanism initiated by the defluorination step. The proposed substitution mechanism of FG demonstrates that nucleophilic substitution via a radical mechanism can also be applied to the attacking process of common nucleophiles without any particular conditions. Furthermore, it has been discovered that triethylamine without active hydrogen can be covalently attached to graphene nanosheets via a nucleophilic substitution reaction with FG; this further indicates a radical process for the nucleophilic substitution of FG rather than an S_N1 or S_N2 mechanism. The detailed process of the nucleophilic substitution reaction of FG was revealed to occur via a radical mechanism depending on the 2D structure of FG, which could also represent the typical characteristic of 2D chemistry.

Chemistry, properties, and applications of fluorographene

Fluorographene, formally a two-dimensional stoichiometric graphene derivative, attracted remarkable attention of the scientific community due to its extraordinary physical and chemical properties. We overview the strategies for the preparation of fluorinated graphene derivatives, based on top-down and bottom-up approaches. The physical and chemical properties of fluorographene, which is considered as one of the thinnest insulators with a wide electronic band gap, are presented. Special attention is paid to the rapidly developing chemistry of fluorographene, which was advanced in the last few years. The unusually high reactivity of fluorographene, which can be chemically considered perfluorinated hydrocarbon, enables facile and scalable access to a wide portfolio of graphene derivatives, such as graphene acid, cyanographene and allyl-graphene. Finally, we summarize the so far reported applications of fluorographene and fluorinated graphenes, spanning from sensing and bioimaging to separation, electronics and energy technologies.

Cyanographene and Graphene Acid: Emerging Derivatives Enabling High-Yield and Selective Functionalization of Graphene

Efficient and selective methods for covalent derivatization of graphene are needed because they enable tuning of graphene's surface and electronic properties, thus expanding its application potential. However, existing approaches based mainly on chemistry of graphene and graphene oxide achieve only limited level of functionalization due to chemical inertness of the surface and nonselective simultaneous attachment of different functional groups, respectively. Here we present a conceptually different route based on synthesis of cyanographene via the controllable substitution and defluorination of fluorographene. The highly conductive and hydrophilic cyanographene allows exploiting the complex chemistry of -CN groups toward a broad scale of graphene derivatives with very high functionalization degree. The consequent hydrolysis of cyanographene results in graphene acid, a 2D carboxylic acid with pK_a of 5.2, showing excellent biocompatibility, conductivity and dispersibility in water and 3D supramolecular assemblies after drying. Further, the carboxyl groups enable simple, tailored and widely accessible 2D chemistry onto graphene, as demonstrated via the covalent conjugation with a diamine, an aminothiol and an aminoalcohol. The developed methodology represents the most controllable, universal and easy to use approach toward a broad set of 2D materials through consequent chemistries on cyanographene and on the prepared carboxy-, amino-, sulphydryl-, and hydroxy- graphenes.

Towards stoichiometric analogues of graphene: graphane, fluorographene, graphol, graphene acid and others

Stoichiometric derivatives of graphene, having well-defined chemical structure and well-defined chemical bonds, are of a great interest to the 2D materials research.

Fluorinated graphenes as advanced biosensors - effect of fluorine coverage on electron transfer properties and adsorption of biomolecules

Graphene derivatives are promising materials for the electrochemical sensing of diverse biomolecules and development of new biosensors owing to their improved electron transfer kinetics compared to pristine graphene. Here, we report complex electrochemical behavior and electrocatalytic performance of variously fluorinated graphene derivatives prepared by reaction of graphene with a nitrogen-fluorine mixture at 2 bars pressure. The fluorine content was simply controlled by varying the reaction time and temperature. The studies revealed that electron transfer kinetics and electrocatalytic activity of CFx strongly depend on the degree of fluorination. The versatility of fluorinated graphene as a biosensor platform was demonstrated by cyclic voltammetry for different biomolecules essential in physiological processes, i.e. NADH, ascorbic acid and dopamine. Importantly, the highest electrochemical performance, even higher than pristine graphene, was obtained for fluorinated graphene with the lowest fluorine content (CF0.084) due to its high conductivity and enhanced adsorption properties combining π-π stacking interaction with graphene regions with hydrogen-bonding interaction with fluorine atoms.

Modelling of graphene functionalization

This perspective describes the available theoretical methods and models for simulating graphene functionalization based on quantum and classical mechanics.

Electrochemical Fluorographane: Hybrid Electrocatalysis of Biomarkers, Hydrogen Evolution, and Oxygen Reduction

Fluorographane (C1 Hx F1-x+δ )n is a new member of the graphene family that exhibits hydrophobicity and a large band gap that is tunable based on the level of fluorination. Herein, sensing and energy applications of fluorographane are reported. The results reveal that the carbon-to-fluoride ratio of fluorographane has a great impact on the electrochemical performance of the materials. Lowered oxidation potentials for ascorbic and uric acids, in addition to a catalytic effect for hydroquinone and dopamine redox processes, are obtained with a high fluoride content. Moreover, fluorographane, together with residual copper- and nickel-based doping, acted as a hybrid electrocatalyst to promote hydrogen evolution and oxygen reduction reactions with considerably lower onset potentials than those of graphane (starting material), which makes this a promising material for a broad range of applications.