Two-Dimensional Nanocomposites Based on Chemically Modified Graphene

Dongqing Wu,[a] Fan Zhang,[a] Ping Liu,[a] and Xinliang Feng*[a, b]

� 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2011, 17, 10804 – 1081210804

DOI: 10.1002/chem.201101333

Introduction

Two-dimensional (2D) nanostructures, which possess a highdegree of anisotropy with nanoscale thickness and infinitelength in other dimensions, have attracted tremendous at-tention due to their unique morphology associated withprominent physical properties and potential applications,such as in electronics, sensing, catalysis, energy storage, andconversion.[1] Various organic and inorganic 2D nanostruc-tures, such as porphyrin, C60, titanium oxide, and transition-metal nanosheets, have been reported recently.[2] In general,there are two strategies to prepare 2D nanosheets. One isthe top-down method, which typically involves the delami-nation of bulk materials with layered structures;[2a,b] howev-er, the bottleneck of this method is the extremely low yieldof product. The other method is based on the bottom-upfabrication, in which the nanosheets can be obtained fromthe anisotropic assembly of inorganic or organic precursorsin a 2D manner. In the past few years, the latter approachhas shown great promise for the convenient and reproduci-ble fabrication of 2D nanomaterials with a large diversity offunctions.[2c–i]

The discovery of graphene has triggered intensive re-search work in the last few years, since this one-atom-thickhexagonal carbon sheet possesses unique 2D morphologyand intriguing physical properties.[1] It is undeniable that thepristine defect-free graphene exhibits unprecedented high

charge carrier mobility, mechanical robustness, thermal con-ductivity, and so on.[1] Nevertheless, the cost-effective syn-thesis of high-quality graphene on a large scale, both byphysical and chemical means, remains a major challenge.Graphite oxide, readily accessible by the oxidation of graph-ite with strong acid and oxidant, has been known for morethan hundred years.[3] The exfoliation of graphite oxide gen-erates single-layer graphene oxide (GO), which can be fur-ther subjected to chemical reduction to produce reducedgraphene oxide (RGO) with partial recovery of its electricalproperties.[4] In this regard, GO, RGO, and their functionalderivatives, generally termed as chemically modified graphe-nes (CMGs),[5] have been attractive to chemists and materialscientists due to their easy accessibility and processability.[6]

Due to the termination of hydroxyl and carboxyl groupsin the plane or at the edge, CMGs are negatively chargedand thus have good dispersibility in aqueous solution, giventhat the steric hindrance and electrostatic repulsion causedby these oxygen-containing substitutions can effectively pre-vent the re-aggregation of CMGs. In addition, these oxygen-based functional groups can serve as “anchor points” tocouple with organic and inorganic species through covalentor non-covalent interactions. Similar to pristine graphene,CMGs also possess 2D character with a large aspect ratio,high specific surface area (theoretical value of 2600 m2 g�1

for graphene), and excellent flexibility. Therefore, theseunique characteristics qualify CMGs as a promising tem-plate for the anisotropic assembly with organic or inorganiccomponents in solution. Towards this end, 2D sandwich-likenanocomposites incorporated with graphene sheets havebeen successfully synthesized[7] that have a large aspectratio, high surface area, and high monodispersity. The gra-phene sheet in 2D nanocomposites may thus offer an addi-tional platform for the fast transportation of charge carriers,which can lead to enhanced performance in various applica-tions, such as catalysis, sensing, supercapacitors, batteries,and fuel cells. In this Concept article, we will summarize dif-ferent bottom-up approaches for the fabrication of 2D gra-phene-based nanosheets. These 2D nanosheets should haveindividual dispersibility and can be distinctly visualized bymicroscopy technology. Nanocomposites of graphene thatsuffer from strong aggregation will not be included. The typ-ical physical properties and potential applications of 2D gra-phene-based materials will be discussed in context. It is an-ticipated that this Concept article will arouse more attentiontowards graphene-based 2D nanomaterials and encouragefuture work to push forward the advancement of this emerg-ing area.

Fabrication of CMG-Based 2D Nanocompositeswith Organic Components

Chemically modified graphene can be viewed as a 2D poly-mer containing extended aromatic frameworks and multiple

Abstract: The multiple functional groups and uniquetwo-dimensional (2D) morphology make chemicallymodified graphene (CMG) an ideal template for theconstruction of 2D nanocomposites with various organ-ic/inorganic components. Additionally, the recoveredelectrical conductivity of CMG may provide a fast-elec-tron-transport channel and can thus promote the appli-cation of the resultant nanocomposites in optoelectronicand electrochemical devices. This Concept article sum-marizes the different strategies for the bottom-up fabri-cation of CMG-based 2D nanocomposites with small or-ganic molecules, polymers, and inorganic nanoparticles,which represent the new directions in the developmentof graphene-based materials.

Keywords: graphene · graphene oxide · nanocompo-sites · nanostructures

[a] Dr. D. Wu, Dr. F. Zhang, Dr. P. Liu, Dr. X. FengCollege of Chemistry and Chemical EngineeringShanghai Jiao Tong University, 800 Dongchuan RoadShanghai, 200240 (P. R. China)

[b] Dr. X. FengMax Planck Institute for Polymer Research, Ackermannweg 1055128, Mainz (Germany)E-mail : feng@mpip-mainz.mpg.de

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CONCEPT

functional groups. Therefore, the introduction of specific or-ganic species at the edge or on the in-plane surface ofCMGs can be achieved through covalent bonds or noncova-lent forces, such as aromatic and ionic interactions. In thissection, 2D nanocomposites of CMGs with organic compo-nents will be preferentially discussed according to the differ-ent driving forces for their formation.

Noncovalent interactions : Owing to the p–p interactions be-tween aromatic units, aromatic molecules such as polycyclicaromatic hydrocarbons (PAHs) and conjugated polymershave the capability to directly intercalate with CMGs. Tokeep the good dispersibility of resultant materials in aque-ous solution, aromatic molecules containing anionic groupsare frequently employed to exert the electrostatic repulsionand thus prevent the re-aggregation of the graphene sheets.Recently, we have reported the fabrication of 2D nanocom-posites of RGO with the sodium salt of pyrene-1-sulfonicacid (PyS, electron donor, Figure 1) or the diasodium salt of3,4,9,10-perylenetetracarboxylic diimide bisbenzenesulfonicacid (PDI, electron acceptor, Figure 1).[8] Both PyS and PDIhave extended aromatic backbones that can strongly immo-bilize them onto the graphene surface by means of p–p in-teractions. In addition, the negative charges on both mole-cules can stabilize the graphene dispersion through electro-static repulsion forces. After the suspension was depositedon mica, mainly single or double layer RGO sheets were at-tained. Atomic force microscopy (AFM) investigations indi-

cated that the thickness of a single layer RGO-PyS orRGO-PDI was �1.7 nm (Figure 1). On this basis, it was sup-posed that the aromatic molecules mainly arranged face-onon both sides of RGO sheet in a sandwich-like manner. Dueto this stacking feature, the significant charge-transfer ef-fects between donor/acceptor and RGO led to tunable elec-tronic properties of the nanocomposites. It was also remark-able to note that the thermal annealing of nanocompositefilms at high temperature resulted into thermal reduction ofRGO sheets with dramatically increased conductivities(>1100 S cm�1 at 1000 8C), about twice as high as that ofRGO (517 Scm�1).

In a similar approach, nanocomposites of conjugatedpolymers and CMG sheets can be also prepared throughp–p interactions. For example, Shi et al. reported the use ofsulfonated polyaniline (SPANI) as the stabilizer for the re-duction of GO sheets and obtained a homogeneous blackdispersion of SPANI/RGO sheets as the product.[9] Later,Niu et al. obtained highly conductive graphene-based nano-composites by reducing a mixture of poly ACHTUNGTRENNUNG[2,5-bis(3-sulfona-topropoxy)-1,4-ethynylphenylene-alt-1,4-ethynylphenylene]and GO with hydrazine monohydrate.[10] For both cases,sandwich-like structures of CMGs with conjugated polymerson both sides were attained.

Given the negatively charged nature of CMGs, positivelycharged organic molecules can favor assembly with CMGsthrough ionic interactions. Shi et al. prepared RGO com-plexes with positively charged 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin (TMPyP) in aqueous solution.[11] Themain driving forces for the complex formation consist in theelectrostatic and p–p stacking interactions between TMPyPsand RGO. A large bathochromic shift in the absorptionspectrum of TMPyP was observed when RGO suspensionwas added. This can be reasonably explained by the flatten-ing effect of TMPyP molecules when they are attached tothe surface of RGO sheets (Figure 2).

Covalent bonds : The functionalization of carbon nanotubes(CNTs) by covalent anchoring of organic molecules hasbeen well established in the last decades.[12] This strategy hasbeen successfully adapted to fabricate functionalized gra-phene materials. As a typical example, Gao et al. demon-strated the functionalization of GO by means of one-step ni-trene chemistry by simply mixing as-prepared GO withfunctional azides in N-methyl-2-pyrrolidone (NMP) at160 8C for 18 h (Scheme 1).[13] The highly reactive nitrene in-termediates were generated upon heating the azides at ele-vated temperature, and some of them were bound to thegraphene framework by cycloaddition to form aziridinerings. Meanwhile, thermal reduction of GO sheets proceed-ed during the reaction process.

By applying the above method with different azides re-agents, various types of electrically conductive 2D nanocom-posites, such as hydroxyl-, carboxyl-, amino-, bromine-,long-alkyl-chain-, polystryene-, and poly(ethylene glycol)-functionalized graphene could be obtained. These 2D nano-composites show good dispersibility in polar or nonpolar

Figure 1. a) Schematic illustration of aqueous dispersions of 2D nano-composites of RGO-PyS or RGO-PDI. AFM images and cross-sectiongraphs of b) RGO-Pys and c) RGO-PDI dispersion dip-coated on mica.

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solvents, which make them easily processable for fabricatingmore complex nanocomposites with organic/inorganic com-ponents by the available solution techniques (Scheme 1).

When polymer is covalently grafted on the graphene sur-face, the resultant CMG polymer sheet can be regarded as2D macromolecular brushes incorporating a flat graphenebackbone. Recently, Gao at al. reported another strategy forthe synthesis of 2D-CMG-based macromolecular brushes byusing free-radical polymerization (FRP).[14] The widely usedFRP cannot only be simply operated, but it can also be usedfor most vinyl monomers. During the reactions, the radicalsgenerated from the vinyl monomers, including acrylates,methacrylates, styrenics, acrylamides, and 4-vinylpyridine(VP), could directly react with the carbon–carbon doublebonds in GO/RGO to form the CMG-based 2D macromo-

lecular brushes. The growing process of the brushes wasclearly demonstrated by AFM (Figure 3), showing that thegraphene sheets remain isolated from each other after poly-mer grafting. It was interesting to note that the molecularweight, height, stability, and dispersibility of glycidyl metha-crylate (GMA)-functionalized GO brushes (GO-g-PGMA)could be tailored through the reaction time (Figure 3). Inaddition, various 2D macromolecular brushes could be ob-tained, which ranged from polar to apolar, water-soluble tooil soluble, acidic to basic, and from functional to commonpolymers, thus facilitating the design, synthesis, and applica-tion in biomimetic coatings and nanocomposites.

Fabrication of CMG-Based 2D Nanocompositeswith Inorganic Components

Inorganic nanoparticles including metal and metal oxidenanoparticles as well as quantum dots (QDs) have attractedenormous attention due to their unique catalytic, magnetic,biologic, and optoelectronic properties.[15] The assembly ofinorganic nanoparticles on the surface of conductive CMGsnot only avoids the agglomeration of nanoparticles, but alsobenefit their applications in which conductivity needs to bea significant concern. To integrate their unique features, fab-rication of 2D nanocomposites composed of CMGs and in-organic nanomaterials has been intensively pursued in thepast few years. One of the most common strategies to con-struct these sandwich-like 2D nanocomposites is to directlyassembly CMGs with pre-prepared inorganic nanoparticles.On the other hand, the in situ growth of inorganic nanopar-ticles after the adsorption of their precursor salts on the sur-face of CMGs offers an alternative approach towards theCMG based inorganic nanocomposites. Thereby, variousCMGs including GO, RGO, modified GO/RGO and exfoli-ated graphene have been explored as suitable 2D supportsfor such purposes.

GO or RGO as a 2D support : By sonicating the mixture ofGO and the colloidal suspension of the corresponding metaloxide nanoparticles, Kamat et al. obtained TiO2/GO[16] andZnO/GO[17] nanosheets. In their work, GO sheets were ap-plied as the adsorbent of the pre-prepared nanoparticles.The driving force for the sandwich-like 2D nanocompositeformation may originate from the physisorption, electrostat-ic binding, and charge-transfer interactions between GOand inorganic nanoparticles; however, this was not specifiedin this work. The morphology investigations of these nano-composites indicate that the distribution of metal oxidenanoparticles on GO is not homogeneous, which can be dueto the fact that the direct binding of inorganic nanoparticleson the surface of GO lacks selectivity.

As discussed in pervious section, the oxygenated groupsgive rise to the negatively charged nature of GO and thebinding with positively charged species. The precursors forinorganic nanoparticles are usually metal salts. Therefore,the electrostatic adsorption of inorganic salts on GO, associ-

Figure 2. a) Schematic illustration of TMPyP adsorbed on the RGOsheet, b) absorption spectra recorded during the titration of aqueous so-lution of TMPyP (1 mm, 3 mL) with various volumes of RGO dispersion(0.25 mg mL�1). Copyright � 2009, American Chemical Society.

Scheme 1. General strategy for the preparation of functionalized GOs bynitrene chemistry and the further modifications. Copyright � 2010,American Chemical Society.

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CONCEPTChemically Modified Graphene

ated with in situ chemical conversion, to give the corre-sponding nanoparticles represents a controllable and repro-ducible means to produce CMG-based 2D inorganic nano-composites.[18] For example, Yang et al. transformed the car-boxylic acid groups of GO to carboxylate anions by treat-ment with NaOH.[18a] After addition of iron salts withFeIII/FeII to the condensed suspension of GO, 2D nanocom-posites of Fe3O4 nanoparticles on GO were preferentiallyformed upon the additional treatment with aqueous NaOHsolution. Despite the occurrence of aggregation in some oc-casions, the size of Fe3O4 nanoparticles on GO was typicallyin the range of 2 to 4 nm with a narrow size distribution,which suggested that a large proportion of Fe3O4 nanoparti-cles were immobilized on the GO surface with strong bind-ing capability.

The in-situ growth approach can be also applied to pro-duce 2D QDs/RGO nanocomposites. Cao and co-workersfirst mixed GO and CdACHTUNGTRENNUNG(CH3COO)2·2 H2O in dimethylsulfox-ide (DMSO) and then heated the suspension in an autoclaveat 180 8C for 12 h.[19] During the thermal treatment, DMSOcould serve as both a solvent and source of sulfur. As theresult, the reduction of GO and the deposition of CdS onRGO occurred simultaneously, which led to a one-pot prep-aration of CdS/RGO. Transmission electron microscopy(TEM) images (Figure 4) revealed that the nanocompositesconsist of 2D nanosheets with decoration of individually iso-lated CdS QDs on RGO. In the X-ray diffraction pattern ofCdS/RGO, there are three main peaks at scattering anglesof 26.506, 43.960, and 52.1328, corresponding to the (111),(220), and (311) crystal planes of CdS, respectively. Thisresult shows that the CdS QDs on the graphene sheet are ofa blende structure (JCPDS 10-0454).

Modified GO/RGO as a 2D support : In spite of its ready ac-cessibility for the fabrication of 2D nanocomposites,GO/RGO suffers from a disadvantage, as the distribution ofinorganic nanoparticles on GO/RGO is strongly dependenton the number and density of oxygen-containing functionalgroups, which in turn unavoidably influences the deposition

density and uniformity of inor-ganic nanoparticles. Alterna-tively, the modification of GO/RGO with suitable organic sur-factants or stabilizer agentsmay provide a complementaryapproach for the fabrication ofgraphene-based 2D nanocom-posites. To a certain extent, onemay expect that not only theaggregation of the 2D nano-sheets can be hampered bychoosing the right combinationsystem, but also the functional-ized graphene surface may ad-ditionally promote the con-trolled nucleation and growthof inorganic nanoparticles.

Inspired by the colloidal stabilization of CNTs usinganionic surfactants,[20] Aksay et al. prepared 2D TiO2/GOnanocomposites based on thermally expanded GO whichwas decorated with sodium dodecyl sulfate (SDS) by meansof hydrophobic interactions.[21] It was believed that the hy-drophobic tails of SDS could adsorb onto the graphene sur-face and, in this way, the hydrophobic graphene resided inthe hydrophobic domains of the SDS micelles, leading to fa-vorable dispersion of graphene sheets in aqueous solution(Figure 5). On the other hand, the hydrophilic head groupsof SDS could interact with TiCl3, thus serving as the molecu-lar template for the controlled nucleation and growth ofTiO2 nanoparticles. As the result, nanocrystalline TiO2 withcontrolled crystalline phase (i.e. , rutile and anatase) werehomogeneously deposited on the graphene sheet throughcooperative interactions between the surfactant, graphene,and nanocrystalline TiO2.

As one of the most frequently used polymer stabilizersfor CNTs, poly(N-vinyl-2-pyrrolidone) (PVP) can be alsoadopted to functionalize CMGs by means of hydrophobicinteractions. The resulting PVP/CMG can form highly stablecolloidal suspensions in water, ethanol, and dimethylform-amide.[22] Wang et al. used this advantage to fabricate high-quality Pt-on-Pd bimetallic nanodendrites supported onRGO sheets (TP-BNGN).[22] In their work, PVP-functional-ized RGO was firstly prepared through the reduction of GO

Figure 3. AFM height images of GO and GO-g-PGMA 2D brushes at different reaction time (size for allimages: 1 mm x 1 mm). Copyright � 2010, American Chemical Society.

Figure 4. a) TEM image of a CdS/RGO nanocomposite with denselycoated CdS QDs. b) TEM image of a CdS/RGO nanocomposite sparselycoated with CdS QDs, showing natural wrinkles of a single graphenesheet. c) High-resolution TEM image of CdS crystals on a graphenesheet.

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with hydrazine in the presence of PVP. Single-crystal Pdnanoparticles with a size of about 3 nm were then anchoredon the PVP-functionalized RGO through the in situ reduc-tion of H2PdCl4. In the third step, Pd-supported RGO wasused as seeds to direct the dendritic growth of Pt upon thereduction of K2PtCl4 by ascorbic acid in an aqueous solu-tion. Remarkably, both AFM and TEM images demonstratethat the monodispersed 2D nanosheets comprise Pt-on-Ptbimetallic nanodendrites with an average size of 15 nm thatare uniformly distributed on the functionalized graphenesurface (Figure 6). Moreover, the number of Pt branches onnanodendrites could be easily adjusted by modifying the re-action parameters, such as the concentration of precursors,thus providing a facile means to tune their electrocatalyticactivity in methanol oxidation.

In another strategy, positively charged small moleculesand polymers can be used as linkers to couple CMGs withinorganic nanoparticles, both of which are negativelycharged, which may benefit the fabrication of 2D nanocom-posites. For instance, cationic polyelectrolyte-functionalizedCMG sheets were obtained by modifying RGO with poly-ACHTUNGTRENNUNG(diallyldimethyl ammonium chloride) (PDDA) associatedwith PVP as a stabilizer.[23] It turned out that citrate-cappedgold and Au@Pd hybrid nanoparticles with high loadingcould be uniformly deposited on PDDA-functionalized gra-phene sheets.

Very recently, we reported the fabrication of 2D sand-wich-like graphene-based mesoporous silica (GM-silica)sheets with cationic surfactant cetyl trimethylammoniumbromide (CTAB)-functionalized GO as the template(Figure 7).[24] In this work, CTAB was adapted to modifyGO in alkaline solution through strong electrostatic interac-tions. Thereby, CTAB could not only provide the templatefor the mesoporous silica formation through the hydrolysisof tetraethylorthosilicate (TEOS), but it also allowed thetight coupling between GO and the inorganic species. Thus,GO-based mesoporous silica (GOM-silica) nanosheets withcould be attained in high yield; these sheets underwent fur-

ther transformation into graphene-based mesoporous silicananosheets by thermal treatment. The resultant nanosheetspossess a large aspect ratio, mesoporous structure, high sur-face area, and high monodispersity. Microscopy images un-ambiguously reveal that the resulting nanosheets preservethe exact same morphology as GO. Figure 7 f and 7g furtherdemonstrate a uniform thickness of (28�1) nm with lowroughness for GOM-silica nanosheets. Notably, the thicknessof the sheets could be tuned by simply adjusting the ratio of

Figure 5. a) Illustration of preparing a TiO2/GO nanocomposite fromSDS-modified graphene sheets. (b) TEM and (c) SEM image of a rutileTiO2/GO nanocomposite. (d) TEM image of an anatase TiO2/GO nano-composite. Copyright � 2010, American Chemical Society.

Figure 6. AFM images of a) RGO/Pd nanosheets and b) RGO/bimetallicnanodendrite nanosheets. c) TEM and d) HRTEM images of the RGO/bimetallic nanodendrite nanosheets. The circled parts in panel d) denotePd nanoparticles. Copyright � 2009, American Chemical Society.

Figure 7. Graphene oxide based mesoporous silica (GOM-silica) sheets.a) Fabrication process for GOM-silica sheets. b),c) SEM and d),e) TEMimages reveal the flat GOM-silica sheets. f) AFM image and g) thicknessanalysis taken around the white line in f) reveal a thickness of 28 nm forGOM-silica sheets.

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CONCEPTChemically Modified Graphene

GO to TEOS during the fabrication process. More impor-tantly, the graphene-based mesoporous silica nanosheetscould further serve as template to prepare graphene-basedmesoporous carbon and metal oxide nanosheets by meansof nanocasting technology. We believe that this method willcertainly broaden the accessibility and application of differ-ent graphene-based 2D nanocomposites.

Biomacromolecules with multiple functional groups alsoshow potential in the fabrication of CMG-based 2D nano-composites. In a recent report, GO and RGO were decorat-ed with thiolated d(GT)29SH DNA oligomers by means ofp–p interactions, similar to the case of DNA wrapping oncarbon nanotubes.[25] The thiol groups tagged on DNAstrands could then act as the anchor points for the 2D self-assembly of pre-prepared gold nanoparticles on GO orRGO.[26] Later, the authors used the Tyr residues of bovineserum albumin (BSA) as both reductant and stabilizer forGO.[27] It turned out that the resultant BSA-RGO nanocom-posites could be further used as template for the uniform2D assembly of pre-synthesized Au, Pt, Pd, Ag, and latex(polystyrene sphere) nanoparticles due to the strong interac-tions between multiple thiol, amine, and imidazole groupson BSA potine and the nanoparticles.

Exfoliated graphene as a 2D support : Different from previ-ous approaches that use GO, RGO, or modified GO/RGOas the template for the synthesis of 2D nanocomposites, Daiet al. reported a step-wise strategy to cover high-quality gra-phene sheets with Ni(OH)2·0.75 H2O nanoparticles withcrystalline structure.[28] In the first step, small nanoparticlesof Ni(OH)2·0.75 H2O were grown on exfoliated graphenesheets after the adsorption of corresponding metal salt pre-cursors in N,N-dimethylformamide (DMF)/water (10:1). Af-terwards, the nanocomposites were hydrothermally treatedat 180 8C in water. It was interesting to find that the smallparticles of Ni(OH)2·0.75 H2O transformed into hexagonalnanoplates during the hydrothermal process. The morpholo-gy, size, and crystallinity of resulting nanocrystals were de-pendent on the oxidation degree of graphene. For the gra-phene with lower oxidation degree, large, single-crystallinehexagonal Ni(OH)2 nanoplates could be obtained thatshowed a typical size of several hundred nanometers andthickness of less than 10 nm (Figure 8). XRD revealed thatthe nanoplates were crystalline b-Ni(OH)2 (Figure 8 d). Incontrast, for highly oxidized GO, small Ni(OH)2 nanoparti-cles remained at their original positions after the hydrother-mal treatment. It was assumed that the large amount ofoxygen functional groups, such as carboxylic, hydroxyl, andepoxy groups, interacted strongly with the anchored inor-ganic nanoparticles, which therefore hampered the diffusionand recrystallization of nanoparticles.[29]

Properties and Applications

Due to their unique structural features such as large aspectratio, ultrathin thickness, and high specific surface area, 2D

nanomaterials may open up enormous opportunities for ap-plications across the fields of biology, medicine, chemistry,and physics. Regarding the CMG-based 2D nanocomposites,the recovered electrical conductivity of graphene may pro-vide additional electron-transport pathways and can thuspromote their applications in optoelectronic and electro-chemical devices. In the following section, we will specifical-ly discuss a few examples of CMG-based 2D nanocompo-sites for the applications in electrochemical energy storageand conversion, as well as catalysis and sensing.

Supercapacitors : The electric double-layer capacitor(EDLC), also known as supercapacitor, is an electrochemi-cal capacitor with much higher energy density than the tra-ditional electrolytic capacitors. Dai et al. investigated theirNi(OH)2/GS nanocomposite as the electrode material forelectrochemical pseudocapacitors. It turned out that thelightly oxidized, highly conductive graphene sheets withsingle-crystalline Ni(OH)2 hexagonal nanoplates exhibited ahigh specific capacitance of �1335 Fg�1 at a current densityof 2.8 A g�1 and �953 F g�1 at 45.7 Ag�1 with excellent cy-clability, outperforming that of Ni(OH)2 nanoparticlesgrown on highly oxidized GO and the simple mixture ofNi(OH)2 nanoplates with graphene sheets.[29a] This work sug-gests that the quality of graphene as well as the morphologyand crystallinity of the 2D nanomaterials are all the criticalissues required to achieve high electrochemical performancematerials for supercapcaitors.

Lithium-ion batteries : Lithium-ion batteries are currentlythe best portable energy storage device for the consumerelectronic market. In the last decades, carbonaceous materi-

Figure 8. a) SEM image of Ni(OH)2·0.75 H2O particles uniformly coatedon GS after the first step of growth at 80 8C. b) SEM image of Ni(OH)2/GS after the second step of simple hydrothermal treatment of the prod-uct depicted in a) at 180 8C. c) TEM image of hexagonal Ni(OH)2 nano-plates formed on top of GS. d) XRD spectrum of a packed thick film ofhexagonal Ni(OH)2 nanoplates on GS. Copyright � 2010, AmericanChemical Society.

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als have been widely used as the anode materials for lithi-um-ion batteries. The electrochemical performances of lithi-um storage including the charge/discharge capacity, columb-ic efficiency, and cycle stability, depend strongly upon themicrostructure of the carbon. CMG-based 2D nanocompo-sites are very attractive anode materials for lithium storagedue to their unique flat nanostructures and electronic prop-erties. For instance, a high first reversible capacity of915 mAh g�1 at a rate of C/5 (one lithium per six formulaunits (LiC6) was achieved in the case of graphene-basedmesoporous carbon (GM-C) nanosheets.[24] It was furtherfound that the reversible capacities of GM-C sheets werestabilized at about 770 mAh g�1, delivering 84 % capacity re-tention after 30 cycles (Figure 9). GM-C sheets also exhibit-ed excellent rate performance in stark contrast to the tradi-tional non-graphitic carbon, which typically show continuousand progressive capacity decay along with cycling processesat various rates. Such good electrochemical performances ofGM-C nanosheets for lithium storage was attributed to theirhigh surface area, thin thickness, and numerous mesopores,which were favorable for the accessibility of the electrolyte,rapid diffusion of lithium ions, and host uptake. Further-more, the graphene layer within each nanosheet could act asmini-current collectors homogeneously dispersed in the elec-trode, which facilitated the fast transport of electrons duringthe charge–discharge processes owing to the high electricalconductivity of graphene.

In another case, Mn3O4/RGO 2D nanocomposites wereprepared by two-step solution-phase reactions and exhibiteda high specific capacity up to �900 mAh g�1[29b] (close to thetheoretical capacity of Mn3O4), good rate capability, andcycle stability. The improved electrochemical performanceof Mn3O4/RGO nanocomposites was ascribed to the inti-mate interaction between graphene and Mn3O4 nanoparti-

cles,[29b] enabling the charge carriers to be effectively andrapidly transported from Mn3O4 to the current collectorthrough the highly conducting graphene network. On theother hand, the aggregation of Mn3O4 nanoparticles wasprevented by RGO sheets, which led to the increased cyclestability. Therefore, these results clearly suggest that gra-phene-based 2D nanocomposites can be promising candi-dates for high-capacity, low-cost, and environmentallyfriendly anodes for lithium ion batteries.

Sensor and other applications : CMG-based 2D nanocompo-sites are also good candidates as sensing materials for gas,ions, and biomolecules, since the target molecules can beeasily accessed due to their large specific surface areas. Fora typical example, Shi and co-workers found that the coordi-nation reaction between 2D TMPyP/RGO nanocompositesand CdII ions proceeded completely in 8 min under ambientconditions, while pure TMPyP required about 20 h to reachits equilibrium under the same conditions.[11] Compared withpure TMPyP, the selectivity of the 2D TMPyP/RGO probetowards CdII ions was also improved. The enhanced perfor-mance was attributed to the efficient anchoring of TMPyPon RGO sheets, which led to flattening feature of TMPyPon graphene.

CMG-based 2D nanocomposites have also been used inmany other fields such as catalysis[22] and solar cells.[8] Inthese cases, not only the outstanding electronic propertiesbut also the unique 2D morphology play pivotal role on theperformance of the resulting nanocomposites. It is no doubtthat by taking advantages of these 2D nanocomposite mate-rials, new applications will be further developed in the nearfuture.

Summary and Outlook

In this Concept article, the recent developments in thebottom-up fabrication of various CMG-based 2D nanocom-posite materials are described. The above results undoubt-edly indicate that the 2D nanocomposites based on CMGsheets are very attractive to material scientists due to theirstriking features and potential applications. Either the pre-prepared or in situ grown organic and inorganic nanostruc-tures can be loaded on the surface of CMG sheets throughcovalent or non-covalent forces. The diversified choices ofthe available molecular building blocks and fabricationmethods will certainly guarantee the synthesis of new CMG-based 2D nanocomposites with unique properties. Neverthe-less, it is undeniable that there remain some importantissues need to be overcome during the fabrication of 2Dnanomaterials for which the nature of CMG in solution canbe the major reason. For example, the shape and size of theCMG-based 2D nanocomposites are strongly governed bythe morphology of CMG substrates, while the production ofCMGs with desired size and shape constitutes a great chal-lenge.[30] In addition, the attachment of organic or inorganiccomponents on the surface of CMGs is more like a random

Figure 9. Electrochemical performance of GM-C sheets for lithium ionstorage. a) Lithium insertion and extraction in GM-C sheets, where gra-phene acts as mini-current collectors during discharge and charge pro-cesses, facilitating the rapid diffusion of electrons during cycling process-es. b) First two discharge–charge curves (black: first discharge, red: firstcharge, green: second discharge, blue: second charge) and c) cycle perfor-mance (black: discharge, red: charge) of GM-C sheets at a rate of C/5.The inset is the rate capability of GM-C sheets at various rates of C/5,1C, 5C, 10C, and 20C.

Chem. Eur. J. 2011, 17, 10804 – 10812 � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org 10811

CONCEPTChemically Modified Graphene

process. The uniform assembly of various nanostructures onCMGs with designed patterns will call for the rational selec-tion of fabrication methods and careful control over theprocessing conditions. Since the morphology of 2D nanoma-terials is the key factor that determines the performance inapplications,[7] it is predictable that more effort will be de-voted to the morphology control of CMG-based 2D nano-composites in the near future. The work on CMG-based 2Dnanocomposites is currently an exciting field that interfacesdisciplines such as chemistry, physics, materials science, andengineering. It is indeed exciting to stand at the point wherewe see one of the fastest developing research fields of gra-phene chemistry, which will surely benefit the human societyin near future owing to the excellent performance of thesefunctional materials.

Acknowledgements

The authors acknowledge the support by the “985 project” of ShanghaiJiao Tong University and BASF.

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Published online: August 18, 2011

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