This journal is©The Royal Society of Chemistry 2014 Chem. Commun.

Cite this:DOI: 10.1039/c3cc47056c

Positive graphene by chemical design: tuningsupramolecular strategies for functional surfaces†

Caroline Hadad,a Xiaoxing Ke,b Mauro Carraro,c Andrea Sartorel,c

Carla Bittencourt,d Gustaaf Van Tendeloo,b Marcella Bonchio,*c Mildred Quintanae

and Maurizio Prato*a

A diazonium based-arylation reaction was efficiently used for the

covalent addition of 4-amino-N,N,N-trimethylbenzene ammonium to

stable dispersions of few layer graphene (FLG) yielding an innovative

FLG platform with positive charges to immobilize inorganic polyanions.

The outstanding physical and chemical properties of graphene1

hold great potential in the preparation of emerging materialswith advanced functions.2 Graphene provides a 2D confinedspace to anchor active sub-units and trigger enhanced optical,mechanical, catalytic, and/or magnetic properties of the composites.Some examples are found in the preparation of energy materials,3

sensors,4 reinforced polymer matrices,5 drug delivery systems,6 andcleaning materials for the removal of metal particles and organicpollutants from water.7 Graphene-integrated functional effectorsinclude inorganic compounds, organic molecules, polymers, metals,metal oxides, and magnetic nanoparticles. A widely used procedurefor the synthesis of graphene composites exploits graphene oxide(GO).8 This material consists of graphene-like sheets, chemicallyfunctionalized with moieties such as carboxyl, hydroxyl andepoxides, which stabilize the graphene layers in polar solventslike water and allow its interaction with other materials.2

Unfortunately, oxidation disrupts the electronic structure ofthe carbon lattice making GO an insulator rather than a semi-metal. Alternatively, ultrasonication-based procedures can beused to prepare stable dispersions of few layer graphene (FLG)on a preparative scale,9 and to enable an efficient chemicalmodification by covalent organic reactions yielding functionalizedFLG (f-FLG).10 f-FLG has the advantage of being easily tuned forintegration into new composite materials since tailored graphenearchitectures can be specifically designed to host other functionalmolecules or nanostructures avoiding major structural damage tothe p-surface compared to GO.11 Herein, we report a successfulsynthetic protocol to introduce a vast coverage of permanentpositive charges that disrupt the hydrophobic surface of pristinegraphene. The aim is twofold: (i) to increase solubility and disper-sion by inter-layer electrostatic repulsion and (ii) to intercalatefunctional anions by complementary charge interaction.12,13

For this purpose, we have expressly synthesized the positivelycharged 4-amino-N,N,N-trimethylbenzene ammonium (3), see ESI.†Then, FLG was covalently functionalized with 3 (see ESI†) by adiazonium-based arylation reaction14 expected to have an impacton the electronic structure of FLG by doping and opening a bandgap.15 As the final step, the positively charged f-FLG has beenexposed to a totally inorganic polyanionic cluster, in order to probeits surface anchoring properties. In particular we have investigatedthe electrostatic immobilization of the ruthenium polyoxometalate{Ru4(m-O)4(m-OH)2(H2O)4[g-SiW10O36]2}10� (Ru4POM),12 Scheme 1.The resulting material, Ru4POM@f-FLG, is relevant for the mole-cular design of nanostructured multi-electron catalysts enablingoxygen evolution from aqueous solutions by direct water oxidationor H2O2 dismutation.13 The in situ production of nascent gas canbe conveniently exploited with surface anti-fouling/self-cleaningactivity, and avoiding the graphene-flake aggregation/clustering.

FLG was produced by the ultrasonication of graphite in DMF(1 g in 200 mL) at room temperature (see ESI† for the detailedprotocol). The concentration of FLG in the dispersion wasdetermined by UV-vis spectroscopy16 to be 0.3 mg mL�1. Thisdispersion was stable for several weeks. The covalent reactionwith 3 (2.5 equiv. per FLG carbon) was performed in aqueous

a Center of Excellence for Nanostructured Materials (CENMAT) and INSTM, Unit of

Trieste, Dipartimento di Scienze Chimiche e Farmaceutiche, University of Trieste,

Piazzale Europa 1, I-34127 Trieste, Italy. E-mail: prato@units.it;

Fax: +39 040 52572; Tel: +39 040 558 7883b Electron Microscopy for Materials Research (EMAT), University of Antwerp,

Groenenborgerlaan 171, B-2020 Antwerp, Belgiumc ITM-CNR and Dipartimento di Scienze Chimiche, University of Padova,

via F. Marzolo 1, 35131 Padova, Italy. E-mail: marcella.bonchio@unipd.it;

Fax: +39 049 8275239; Tel: +39 049 8275670d Chemistry of Interaction Plasma Surface (ChiPS), University of Mons,

Rue du Parc 20, B-7000 Mons, Belgiume Instituto de Fısica, Universidad Autonoma de San Luis Potosı, Manuel Nava 6,

Zona Universitaria, 78290 San Luis Potosi, SLP, Mexico. Fax: +52 444 8133874;

Tel: +52 444 8262362 ext. 130

† Electronic supplementary information (ESI) available: Complete experimentaldetails of the synthesis of 4-amino-N,N,N-trimethylbenzene ammonium iodide (3),NMR and ESI-MS spectra, TGA, Raman, TEM images and X-ray photoelectronspectroscopy analysis of graphene compounds. See DOI: 10.1039/c3cc47056c

Received 15th September 2013,Accepted 6th November 2013

DOI: 10.1039/c3cc47056c

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DMF (DMF/H2O, 5 : 1) in the presence of 3-methylbutyl nitrite(5 equiv. per FLG carbon) at 80 1C overnight. After cooling downto room temperature, the suspension was diluted with DMF(100 mL), filtered (PTFE membrane, 0.45 mm) and washedseveral times with DMF and MeOH.

The successful covalent functionalization of f-FLG was verified bydifferent characterization techniques, such as thermogravimetricanalysis (TGA), transmission electron microscopy (TEM), andRaman spectroscopy.17 The temperature modulated curves forFLG and f-FLG (see ESI,† Fig. S10 and S11) indicate that FLGs havenot been damaged by the ultrasonication treatments, whereas f-FLGpresents a weight loss of 6.7% at 450 1C. This weight loss isattributed to the functional groups attached to the FLG surface.TEM analysis shows no significant differences in the morphology off-FLG compared to FLG (ESI,† Fig. S13–S15).

The self-assembly of the Ru4POM@f-FLG nanohybrid wasinduced by the straightforward metathesis in water. f-FLG(1 mg) was added to an aqueous solution of Ru4POM(10�4 mol L�1, 2 mL) at pH 4. The mixture was sonicated for2 min and let under stirring overnight. Ru4POM@f-FLG waswashed several times yielding a stable suspension in water(0.1 mg mL�1, pH 4). X-Ray photoelectron spectroscopy (XPS),Raman spectroscopy, TEM, and HRTEM techniques wereinstrumental for the material characterization.

The non-covalent binding of the Ru4POM@f-FLG nanohybridwas detected by XPS (see ESI,† Fig. S16–S19). The W 4f peakcharacteristic of the Ru4POM confirms its attachment on f-FLG.The N 1s peak shifted about 0.4 eV towards lower bindingenergies after the grafting of the Ru4POM. The C 1s peak can bereproduced using 5 components indicating the presence of carbonatoms at 5 different chemical environments: component 1 centeredat 283.4 eV was associated with carbon atoms in the hexagonalunits near defect-like vacancies.18 Component 2 at 284.6 eV isgenerated by photoelectrons emitted from carbon atoms in theCQC bonding.19 Components 3, 4 and 5 at 285.1 eV, 286.0 eVand 287.8 eV are due to carbon atoms in 3.20

As expected, Raman spectra in Fig. 1 show the specificfeatures of Ru4POM in the Ru4POM@f-FLG nanohybrid andthe signal peaks corresponding to D, G and 2D peaks of f-FLG at1350, 1586 and 2715 cm�1, respectively.11,17

The ratio of the intensities of the D and G bands (ID/IG) isoften used as an indication of the chemical modificationextent.11 The ID/IG ratios in FLG, f-FLG, and Ru4POM@f-FLGturn out to be 0.29, 0.46 and 0.43, respectively. The increase inthe D band signal observed for the modified graphene surfacecompared to that of pristine FLG is consistent with the covalentfunctionalization of the carbon lattice (Fig. S12, ESI†).7 The Ru-(H2O)stretching modes are clearly recognized at 250–400 cm�1,together with the tetraruthenate core vibrations at 450–500 cm�1

(Fig. 1, inset).12

Finally, the morphology of the Ru4POM@f-FLG nanohybridwas investigated by TEM and HRTEM showing the inorganiccluster domains well-distributed all over the graphene surface(Fig. 2a and b). High resolution images acquired with a dose ofapproximately 1000 e� �2 s�1 allow the inspection of theinorganic domains anchored on the positive graphene nano-flakes with atomic resolution (Fig. 2c). Aberration-correctedTEM is able to image the Ru4POM at the atomic scale whereas anaccelerating voltage of 80 kV reduces the beam damage to thegraphene-based nanohybrids. Simulation of the Ru4POM molecularstructure addresses its conformational mobility upon 2D confine-ment on f-FLG, as a function of time (Fig. 2d). Interestingly, duringthe acquisition time of 37 s, only 5 different projections areidentified from the image series, which indicate a strong electro-static adhesion to the nano-carbon surface, and a limited mobility.The latter depends on the nature of the graphene-linker. Indeed,a spatially protruding dendrimer, with multiple ammoniumterminals, displaying a long-range, flexible grip on the molecularanion, allowed the registration of over 25 different orientationsduring 70 s, under the same experimental conditions.21

The benzene ammonium linker 3 is likely to act with ashorter-radius, and combined surface-assisted confinement ofthe Ru4POM is likely to take place, limiting the conformationalfreedom. Polyoxometalate ions have been shown to interactwith the electronic density of the GO platform in response toH-bonding type interactions.22 Our study confirms thatthe interplay of electrostatic forces and p-electron clouds ongraphene results in the tight immobilization of the inorganicpolyanion, displaying a lower motion frequency and remainingstatics for a longer time.

Scheme 1 Synthesis of Ru4POM@f-FLG. Reagents and conditions: (a)4-amino-N,N,N-trimethylbenzene ammonium, 3-methylbutyl nitrite, DMF/H2O at 80 1C; (b) Ru4POM, H2O (pH 4) at r.t.

Fig. 1 Raman spectra of FLG (black curve), Ru4POM@f-FLG (red curve),and Ru4POM (blue curve). In the inset, the superimposed spectral featuresof Ru4POM@f-FLG and Ru4POM up to 1000 cm�1 are shown.

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The resulting hybrid material retains the catalytic activitytypical of molecular Ru4POM, which is known to decomposeH2O2 to dioxygen.13 Indeed, addition of aqueous H2O2 (35% v/v)to the hybrid–graphene suspension results in the expectedoxygen evolution (see ESI,† Fig. S20). Noteworthily, the hybridremains stable as no clustering/precipitation is observed, andTEM analysis confirms the integrity and morphology of therecovered material (see ESI,† Fig. S21).

A novel nano-graphene platform, featuring positive charges, isshown to anchor metal oxide anions using a post-functionalizationprotocol. The positive graphene offers a valid alternative and iscomplementary to the widely used GO material, which usuallydisplays a negative surface. The positively charged f-FLG is readilyavailable via covalent addition of 4-amino-N,N,N-trimethylbenzeneammonium salt, by a diazonium based-arylation reaction. Tunablegraphene platforms, with tailored functionalities, are now at reach.

Financial support of Trieste University, the EU-FP7 ERC-2008-AdG-227135, MIUR (Cofin prot. 2010N3T9M4 and FIRBprot. RBAP11ETKA) and Fondazione CARIPARO (NanoMode,2010) is acknowledged. MQ thanks CONACYT project 166914.MQ and CB acknowledge the project 193257 Bilateral Coopera-tion Mexico (CONACYT) – Belgium (F.R.S.– FNRS). XK and GVTacknowledge funding from the EU-FP7-ERC Grant (No 246791).We acknowledge the participation in the COST Action NanoTPand FRC-FNRS 2.4577.11F (CHEMOGRAPHENE).

Notes and references1 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang,

S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004,306, 666; A. K. Geim, Science, 2009, 324, 1530.

2 S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas,E. J. Zimney, E. A. Stach, R. D. Piner, S. B. T. Nguyen and R. S. Ruoff,Nature, 2006, 442, 282.

3 M. Quintana, A. Montellano-Lopez, S. Rapino, F. M. Toma, M. Iurlo,M. Carraro, A. Sartorel, C. Maccato, X. Ke, C. Bittencourt, T. Da Ros,G. Van Tendeloo, M. Marcaccio, F. Paolucci, M. Prato andM. Bonchio, ACS Nano, 2013, 7, 811.

4 J. Huang, C. Zong, H. Shen, M. Liu, B. Chen, B. Ren and Z. Zhang,Small, 2012, 8, 2577.

5 C. Wan and B. Chen, J. Mater. Chem., 2012, 22, 3637.6 L. Zhang, J. Xia, Q. Zhao, L. Liu and Z. Zhang, Small, 2010, 6,

537.7 V. Chandra, J. Park, Y. Chun, J. W. Lee, I.-C. Hwang and K. S. Kim,

ACS Nano, 2010, 4, 3979; L. Fan, C. Luo, M. Sun and H. Qiu, J. Mater.Chem., 2012, 22, 24577; H. Sun, L. Cao and L. Lu, Nano Res., 2011,4, 550.

8 D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii, Z. Sun,A. Slesarev, L. B. Alemany, W. Lu and J. M. Tour, ACS Nano, 2010,4, 4806.

9 J. N. Coleman, Acc. Chem. Res., 2013, 46, 14.10 M. Quintana, E. Vasquez and M. Prato, Acc. Chem. Res., 2013,

46, 138; L. Rodrıguez-Perez, M. A. Herranza and N. Martın, Chem.Commun., 2013, 49, 3721; V. Georgakilas, M. Otyepka,A. B. Bourlinos, V. Chandra, N. Kim, K. C. Kemp, P. Hobza,R. Zboril and K. S. Kim, Chem. Rev., 2012, 112, 6156; J. Malig,N. Jux and D. M. Guldi, Acc. Chem. Res., 2013, 46, 53; A. Hirsch,J. M. Englert and F. Hauke, Acc. Chem. Res., 2013, 46, 87.

11 M. Quintana, A. Montellano, A. Esau del Rio Castillo, G. VanTendeloo, C. Bittencourt and M. Prato, Chem. Commun., 2011,47, 9330; M. Quintana, M. Grzelczak, K. Spyrou, B. Kooi, S. Bals,G. Van Tendeloo, P. Rudolf and M. Prato, Chem. Commun., 2012,48, 12159; M. Quintana, K. Spyrou, M. Grzelczak, W. R. Browne,P. Rudolf and M. Prato, ACS Nano, 2010, 4, 3527.

12 A. Sartorel, M. Carraro, G. Scorrano, R. De Zorzi, S. Geremia,N. D. McDaniel, S. Bernhard and M. Bonchio, J. Am. Chem. Soc.,2008, 130, 5006; M. Carraro, A. Sartorel, G. Scorrano, C. Maccato,M. H. Dickman, U. Kortz and M. Bonchio, Angew. Chem., Int. Ed.,2008, 47, 7275.

13 F. M. Toma, A. Sartorel, M. Iurlo, M. Carraro, P. Parisse, C. Maccato,S. Rapino, B. R. Gonzalez, H. Amenitsch, T. Da Ros, L. Casalis,A. Goldoni, M. Marcaccio, G. Scorrano, G. Scoles, F. Paolucci,M. Prato and M. Bonchio, Nat. Chem., 2010, 2, 826; F. M. Toma,A. Sartorel, M. Iurlo, M. Carraro, S. Rapino, L. Hoober-Burkhardt,T. Da Ros, M. Marcaccio, G. Scorrano, F. Paolucci, M. Bonchio andM. Prato, ChemSusChem, 2011, 4, 1447; A. Sartorel, M. Carraro,F. M. Toma, M. Prato and M. Bonchio, Energy Environ. Sci., 2012,5, 5592.

14 J. Park and M. Yan, Acc. Chem. Res., 2013, 46, 181; A. Sinitskii,A. Dimiev, D. A. Corley, A. A. Fursina, D. V. Kosynkin and J. M. Tour,ACS Nano, 2010, 4, 1949; J. R. Lomeda, C. D. Doyle, D. V. Kosynkin,W. F. Hwang and J. M. Tour, J. Am. Chem. Soc., 2008, 130, 16201.

15 R. Sharma, J. H. Baik, C. J. Perera and M. S. Strano, Nano Lett., 2010,10, 398.

16 Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Sun, S. De,I. T. McGovern, B. Holland, M. Byrne, Y. K. GunKo, J. J. Boland,P. Niraj, G. Duesberg, S. Krishnamurthy, R. Goodhue, J. Hutchison,V. Scardaci, A. C. Ferrari and J. N. Coleman, Nat. Nanotechnol., 2008,3, 563.

17 A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri,F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth andA. K. Geim, Phys. Rev. Lett., 2006, 97, 187401.

18 A. Barinov, H. Ustunel, S. Fabris, L. Gregoratti, L. Aballe, P. Dudin,S. Baroni and M. Kiskinova, Phys. Rev. Lett., 2007, 99, 046803.

19 I. Suarez-Martinez, C. P. Ewels, X. Ke, G. Van Tendeloo, S. Thiess,W. Drube, A. Felten, J.-J. Pireaux, J. Ghijsen and C. Bittencourt, ACSNano, 2010, 4, 1680.

20 A. P. Dementjev, A. De Graaf, M. C. M. Van de Sanden,K. I. Maslakov, A. V. Naumkin and A. A. Serov, Diamond Relat.Mater., 2000, 9, 1904.

21 X. Ke, S. Turner, M. Quintana, C. Hadad, A. Montellano-Lopez,M. Carraro, A. Sartorel, M. Bonchio, M. Prato, C. Bittencourt andG. Van Tendeloo, Small, 2013, DOI: 10.1002/smll.201300378.

22 J. Sloan, Z. Liu, K. Suenaga, N. R. Wilson, P. A. Pandey,L. M. Perkins, J. P. Rourke and I. J. Shannon, Nano Lett., 2010,10, 4600.

Fig. 2 (a) TEM, (b) HRTEM images of Ru4POM@f-FLG, (c) HRTEM imagesof Ru4POM@f-FLG from a time sequence and (d) projections from a timesequence of Ru4POM@f-FLG and their corresponding quantified orientations.

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