There is intense interest in graphene in fields such as physics, chemistry, and materials science, among others. Interest in graphene's exceptional physical properties, chemical tunability, and potential for applications has generated thousands of publications and an accelerating pace of research, making review of such research timely. Here is an overview of the synthesis, properties, and applications of graphene and related materials (primarily, graphite oxide and its colloidal suspensions and materials made from them), from a materials science perspective.

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Graphene and Graphene Oxide: Synthesis, Properties, and Applications

Approaches, Derivatives and Applications Vasilios Georgakilas,† Michal Otyepka,‡ Athanasios B. Bourlinos,‡ Vimlesh Chandra, Namdong Kim, K. Christian Kemp, Pavel Hobza,‡,§,⊥ Radek Zboril,*,‡ and Kwang S. Kim* †Institute of Materials Science, NCSR “Demokritos”, Ag. Paraskevi Attikis, 15310 Athens, Greece ‡Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University Olomouc, 17. listopadu 12, 771 46 Olomouc, Czech Republic Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, v.v.i., Flemingovo naḿ. 2, 166 10 Prague 6, Czech Republic

Functionalization of Graphene: Covalent and Non-Covalent Approaches, Derivatives and Applications

Graphene has attracted tremendous research interest in recent years, owing to its exceptional properties. The scaled-up and reliable production of graphene derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO), offers a wide range of possibilities to synthesize graphene-based functional materials for various applications. This critical review presents and discusses the current development of graphene-based composites. After introduction of the synthesis methods for graphene and its derivatives as well as their properties, we focus on the description of various methods to synthesize graphene-based composites, especially those with functional polymers and inorganic nanostructures. Particular emphasis is placed on strategies for the optimization of composite properties. Lastly, the advantages of graphene-based composites in applications such as the Li-ion batteries, supercapacitors, fuel cells, photovoltaic devices, photocatalysis, as well as Raman enhancement are described (279 references).

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Graphene-based composites

Graphene based materials: Past, present and future

Recent advances in graphene based polymer composites

Soluble graphene covalently functionalized with poly(vinyl alcohol) (PVA) has been synthesized by simple esterification reaction of carboxylic groups in graphite oxide using two different synthetic strategies. Graphene has emerged to be a promising material due to its novel electrical transport properties. However, the use of graphene is limited by the lack of an effective method for large-scale production. Several chemical methods have been explored to obtain soluble graphene including the reduction of graphite oxide (GO) in a stabilization medium and covalent modification by amidation of the carboxylic groups, nucleophilic substitution to epoxy groups, and the diazonuim salts coupling. These modified graphenes provide opportunities for researchers to employ them in designing new materials, such as polymer nanocomposites. In such materials, controlling the interfacial interaction between the filler and the polymer is crucial to control the properties. Although graphene-based polymeric nanocomposites have been developed, polymer chains have not been used to covalently modify graphene. Nevertheless, polymer-bounded carbon nanotubes (CNT) and fullerene (C60) 8 have been widely studied. PVA-functionalized CNTs have been obtained by esterification reaction and used as fillers in nanocomposites. In that sense, the presence of COOH in the edge planes of GO can be exploited to anchorage PVA by using a similar way. In this Communication, we report the covalent functionalization of graphene sheets with PVA (Scheme 1). We employed two synthetic strategies. The first one involves the direct esterification of GO, while the second one goes through its acyl chloride derivative (GOCl) (Supporting Information). The obtained products, named as GO-es-PVA and GOCl-es-PVA, are soluble in DMSO and water with the aid of heat, similar to PVA and PVACNT. In the present research a special interest has been paid to the isolation and characterization of the PVA-functionalizedGO. First, an appropriate elimination of the no reacted GO was achieved by centrifugation, which completely removes the GO and leaves the PVA-functionalized GO (see Supporting Information). A comprehensive spectroscopic analysis was carried out to ensure that the esterification reactions were successfully completed. The HNMR spectrum of the soluble GO-es-PVA andGOCles-PVA in DMSO-d6 are compared with that of neat PVA (Figure S2). The latter exhibits the polymer backbone signals at ∼3.82 ppm (methine) and ∼1.38 ppm (methylene) and the hydroxyl signals at 5-4 ppm, from which the stereoregularity of PVA is estimated as isotactic (mm):hetero (mr): syndiotactic (rr) of roughly 2:5:3. Upon the attachment to GO the PVA proton signals become a littlewider butmaintain similar chemical shifts (Figure S2). The signals are less broadened than in PVACNT suggesting that, in principle, a lower degree of functionalization was achieved in our case. It is interesting to pay special attention to the hydroxyl protons resonances which are resolvable in terms of configurational sequences (Figure 1). Several authors have concluded that the hydrogen-bonding tendency, which is dominant for hydroxyl units in a meso configuration, predominantly determines the hydroxyl proton shielding. Since hydrogen bonding leads to downfield shifts, the chemical shifts of hydroxyl sequences resonances increase when passing from iso (4.7 ppm) to hetero (4.5 ppm) and fromhetero to syndio (4.2 ppm) triad.On the other hand, it has been demonstrated that the reactivity of substituent groups along the polymer chain can be influenced by tactic sequence where the substituent atmm sequence exhibited higher reactivity than those at the rr counterpart. By a mere inspection of the evolution of the H spectrum, we can easily observe an increase in the rr signal in detriment of the mm one for both GO-es-PVA and GOCl-es-PVA which suggests that esterification reaction occur at isotactic configuration. Surprisingly, as shown in Figure 1, a new signal at 4.2 ppm upfield, very closed to the rr triads of unmodified PVA, is clearly observed in spite of the low degree of esterification. This signal can be related to hydroxyl protons next to acetate groups as have been reported for esterification of PVA. By integration of this H NMR signal we can evaluate, within the experimental uncertainties, the degree of functionalization, obtaining a modification of around 1.8%. Interestingly, this value is reasonably low due to the huge volume of graphitic laminates (GLs) and is in agreement with the decrease ofmm triad content, as consequence of being less sterically hindered internally than those at syndiotactic counterpart, specifically in the incorporation of the GLs. The FTIR spectra of GO-es-PVA and GOCl-es-PVA retained most of the bands of PVA, although some of them changed in intensity or even disappeared due to the modification, and show new bands (Figure 1). The development of the band at 1715 cm suggests the presence of new carbonyl species. This band is most often related to the CdO stretching motions of COOHgroups situated at the edges of theGO lamellae and has low intensity due to high aspect ratio of GO which makes the relative amount of edge sites very small. Similarly, in our case the band at 1715 cm can be attributed to CdO stretching of ester groups. Indeed, this band was also observed in the esterification reaction of PVA and glycerol, suggesting that esterification is also taking place in our system. This is confirmed by the marked increase in the band around 1640 cm, which appears weakly in PVA and has been assigned to adsorbedwater. However, this is a strong band centered at 1628 cm in the spectrum of the GO (Figure S1), and although may also be due to adsorbed water, it contains a significant contribution from the skeletal vibration of nonoxidized graphitic domains. In addition, some interesting changes in the relative intensities of the characteristics bands of PVA in the 1200-1000 cm region can be observed. These bands are attributed to the C-O of doubly H-bonded OH in crystalline regions (1144 cm) and C-O unbonded in amorphous zones (1096 cm). The intensity ratio of these bands (I1144/I1096) diminishes markedly for the esterified products, suggesting a large decrease in the degree of crystallinity of the modified polymer. Raman data further confirm the presence of GLs covalently bonded to PVA chains (Figure S3). The Raman spectra for *Corresponding author: Fax þ34 915 644 853; Tel þ34 915 622 900; e-mail horacio@ictp.csic.es. D ow nl oa de d by C SI C M L O R A T A M A Y O Q U IM O R G o n Se pt em be r 1, 2 00 9 | h ttp :// pu bs .a cs .o rg

Polymeric Modification of Graphene through Esterification of Graphite Oxide and Poly(vinyl alcohol)

Poly(vinylalcohol)/reduced graphite oxide nanocomposites have been synthesised by reducing graphite oxide in the presence of the polymer matrix and coagulating the system with 2-propanol. It has been observed that some interactions occur between the polymer and the reduced graphite oxide layers, mainly by hydrogen bonding. These interactions are responsible for a remarkable change in the thermal behaviour of the nanocomposites. In addition, high electrical conductivity has been achieved at concentrations beyond 7.5 wt% of reduced graphite oxide (∼0.1 S cm−1), with a percolation threshold between 0.5 and 1 wt%.

Synthesis of poly(vinyl alcohol)/reduced graphite oxide nanocomposites with improved thermal and electrical properties

Covalent binding of polymers to graphene represents an interesting alternative for the development of novel composite materials with a compendium of interfacial interactions. Through covalent linking, the concept of interface changes from a traditional view of interactions between components, such as van der Waals, hydrogen bonding, and so on, that is to say, at a polymer-filler interface, to a single compound concept where graphene forms an integral part of the polymeric chains. This feature article provides an overview of the strategies currently employed to functionalize graphene with polymers. We focus on the grafting-from and grafting-to methods used to bind polymers to graphene. The advantages and drawbacks, as well as the influence of each method on the final properties, are highlighted.

Recent advances in the covalent modification of graphene with polymers.

High-quality few layer graphene produced by electrochemical intercalation and microwave-assisted expansion of graphite

The preparation of nanocomposites of reduced graphene oxide (RGO) with poly(vinyl chloride) (PVC) is reported. Covalent modification of PVC with RGO and reduction/precipitation of graphite oxide and isocyanates-modified GO in the presence of PVC are studied. Among these methods it is proposed that only the former produces nanocomposites with enhanced final properties. The other method failed because it did not produce nanocomposites with different properties to the original polymer, probably due to the lack of effective interfacial interactions. The RGO-chemically modified PVC samples have different thermal and mechanical properties with respect to the parent polymer. A discussion on the dependence of the properties on the method of preparation is given. Furthermore, preliminary results based on the effect of the aspect ratio and surface chemistry of the filler are also given.

Gerardo Rodríguez Martínez

Papers

Polymeric Modification of Graphene through Esterification of Graphite Oxide and Poly(vinyl alcohol)

Synthesis of poly(vinyl alcohol)/reduced graphite oxide nanocomposites with improved thermal and electrical properties

Recent advances in the covalent modification of graphene with polymers.

High-quality few layer graphene produced by electrochemical intercalation and microwave-assisted expansion of graphite

Importance of Covalent Linkages in the Preparation of Effective Reduced Graphene Oxide−Poly(vinyl chloride) Nanocomposites