Graphene oxide-Laponite hybrid from highly stable

Applied Clay Science 132-133 (2016) 105–113

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Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Research paper

Graphene oxide-Laponite hybrid from highly stable aqueous dispersion Devesh K. Chouhan a, T. Umasankar Patro a,⁎, G. Harikrishnan b, Sanjay Kumar b, Siddharth Gupta c, G. Sudheer Kumar a, Hagai Cohen d, H. Daniel Wagner e a

Department of Materials Engineering, Defence Institute of Advanced Technology, Girinagar, Pune 411025, Maharashtra, India Department of Chemical Engineering, Indian Institute of Technology Kharagpur, West Bengal 721302, India Department of Applied Chemistry, Indian Institute of Technology (Banaras Hindu University), Varanasi, Uttar Pradesh 221005, India d Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel e Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel b c

a r t i c l e

i n f o

Article history: Received 22 March 2016 Received in revised form 21 May 2016 Accepted 24 May 2016 Available online 2 June 2016 Keywords: Partial reduction Cation exchange reaction Electrostatic interaction Stable dispersion

a b s t r a c t A simple method for preparation of hybrid of graphene oxide (GO) and Laponite (Lap), obtained by solvent evaporation from their highly stable aqueous dispersions is reported. The dispersion up to ~1 mg/ml of GO in 1% Lap dispersion, i.e., 10:1 of Lap:GO was found to be stable without flocculation for several months; lower mass ratios of Lap to GO than this showed marginal flocculation with time. The electrostatic interaction between cations present in the interlayers of Lap and the functional groups of GO is envisaged to be the cause for the stable dispersion, which was confirmed by the presence of cations; viz., Na+ and small amounts of K+ and Mg2+ in the aqueous filtrate of the hybrid. Their interaction was further confirmed by higher absorption of GO in aqueous Lap dispersion than that in water using UV–vis spectroscopy. The resulting hybrid material was found to be partially reduced and self-assembled to form layered structure in its dry state. The hybrids further showed improved electrical conductivity (~0.01 S/cm) upon chemical reduction. The present study demonstrates a facile method for preparation of a new hybrid material and greener pathway for GO reduction; though partially. This hybrid has potential as multifunctional filler for clay polymer nanocomposites. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Nanocomposites of carbon and inorganic clay are important materials due to their wide range of applications; such as electrodes for Li ion batteries (Sandí et al., 1999; Duclaux et al., 2000), supercapacitor (Duclaux et al., 2000), electrocatalysis and preparation of carbon nanofibers (Fernández-Saavedra et al., 2004). In these clay polymer nanocomposites (CPN), the inorganic clay mineral (e.g., montmorillonite (Sonobe et al., 1991), taeniolite (Bandosz et al., 1996), sepiolite (Sandí et al., 1999)) is used as a template and the carbon source (e.g., furfuryl alcohol (Sonobe et al., 1991), polyacrylonitrile (Fernández-Saavedra et al., 2004), sucrose (Bakandritsos et al., 2004)) is intercalated into interlayer spaces of the clay mineral and subsequently pyrolyzed to obtain CPN (Sonobe et al., 1991). However, in recent years with the invention of graphene (Novoselov et al., 2004; Geim and Novoselov, 2007) and its derivatives, hybrid materials having graphene as one of the major components have attracted a great deal of research attention due to their versatile and tunable properties (Nethravathi et al., 2008a, 2008b; Nethravathi et al., 2010; Zhao et al., 2012; Dey and Raj, 2013; Wang et al., 2013a, 2013b). Often, these materials exhibit improved catalytic activity (Liang et al., 2011), better sensing properties (Xiao et al., 2012; ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (T.U. Patro).

http://dx.doi.org/10.1016/j.clay.2016.05.023 0169-1317/© 2016 Elsevier B.V. All rights reserved.

Zhang et al., 2015) and improved energy storage (Xiao et al., 2011; Giri et al., 2014), compared to their individual constituents. Evolution of these properties has been attributed to the extraordinary properties of graphene and its chemical coupling with wide range of nanomaterials. The advantages of these nanocomposites are their easier processing methods over their conventional counterparts and moreover in the former nanocomposites, carbon precursor is not required. A significant research effort has been made towards preparing such nanocomposites. For instance, graphene oxide (GO)-silica nanocomposites prepared through sol-gel process and subsequent reduction to graphene-silica nanocomposites were found to exhibit electrically conducting (~0.5 S/cm) and optically transparent coatings on glass substrates (Watcharotone et al., 2007). Clay-graphene nanomaterials were prepared by intercalating liquid caramel into various clay minerals (montmorillonite, sepiolite) followed by heat-treatment at elevated temperatures and the resulting materials showed promising hydrogen storage (Garcia et al., 2013). Adsorption of DNA and Cytochrome C onto the reduced GO-amino clay hybrids, reported by Achari et al. (2013) showed promising catalytic and biomedical applications. Various strategies have been adopted to prepare graphene/metal oxide nanocomposites from GO-layered double hydroxide nanocomposites (Nethravathi et al., 2008a, 2008b; Rajamathi et al., 2010; Zhao et al., 2012; Dey and Raj, 2013; Wang et al., 2013a, 2013b). Exfoliated and porous nanocomposites of GO and smectite was produced by heat-

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D.K. Chouhan et al. / Applied Clay Science 132-133 (2016) 105–113

treatment followed by leaching out of clay by acid treatment (Nethravathi et al., 2008a, 2008b). Among these, hybrids of GO and clays are particularly important, as these nanocomposites offer a possibility of having a layer-to-layer (LTL) interaction by electrostatic interaction between mobile interlayer cations of clays and polar functional groups of GO. Further, the similarity of their morphology (both having layered structures), intercalation and exfoliation properties may contribute to the LTL interaction. Recently, Yoo et al. (2014) found that a multilayer approach of Lap and GO coating on a polyester transparent sheet significantly lowered oxygen permeability as compared to the coatings made by individual components. However, they did not observe any reduction of GO in presence of Lap. Besides the numerous applications of these hybrid materials, they can also potentially serve as new functional fillers for CPN, which might impart synergistic effects, improved interfacial bonding and multifunctional properties in nanocomposites. It is worth-noting that most of the techniques used for hybrid material preparation involve several steps and are relatively complex in nature. Moreover, the interaction between the constituents of hybrid is not well-understood; particularly between a synthetic hectorite and GO. Here, a simple two-step method was adopted for preparation of Lap-GO hybrid and their chemical interaction was studied. GO is chemical derivative of graphene and synthesized by wellknown Hummers method (Hummers and Offeman, 1958). GO contains a mixture of sp2 and sp3 C atoms. The sp3 C atoms in GO arise due to attachment of functional groups; like phenolic, epoxide, carbonyl, etc. Reduction of GO eliminates the functional groups on graphitic backbone to produce a compound which closely resembles graphene (Dreyer et al., 2011). Reduced graphene oxide (RGO), however due to defects on its graphitic plane, exhibits lower electrical conductivity than the graphene produced directly from graphite. Nevertheless, RGO has been widely studied in the context of various applications; such as polymer nanocomposites (Stankovich et al., 2006), sensors (Wang et al., 2013a, 2013b), supercapacitors (Dreyer et al., 2011) due to its large throughput and ease in synthesis by simple reduction of GO. Significant research efforts have been carried out in the past decade for developing better, greener and newer methods for reduction of GO. Among these, hydrazine treatment of GO perhaps is the most popular method of reduction for its ease and efficacy (Stankovich et al., 2007). Alcohols were also used to prepare RGO in an attempt to make the process greener (Dreyer et al., 2011). Strong reducing agent like sodium borohydrate was found to reduce GO effectively with sheet resistance of 2.6 × 103 Ω/sq and electrical conductivity of 45 S/m (Shin et al., 2009). Successful efforts were also made to reduce GO using hydroquinone (Wang et al., 2008), ascorbic acid (Zhang et al. 2010), formamidinesulfinic acid (Ma et al., 2013), urea (Lei et al., 2012), sulfur containing compounds (Chen et al., 2010), thiourea (Yanzhen et al., 2011), using microwaves (Bourlinos et al., 2009), etc. In this study, a solid non-toxic and environmentally-benign clay mineral was serendipitously found to partially reduce GO. Lap, a synthetic hectorite with a generic formula: Na+ 0.7(Si8Mg5.5Li0.3) O20(OH)4)0.7, disperses easily in water and forms transparent solution at low concentrations below percolation threshold of network formation (Ramsay, 1986). At higher concentrations (N2 wt%) Lap makes a gel with a 3D network of particles (Ruzicka et al., 2010). Lap surface is negatively charged, which is originated by the isomorphous substitution of Mg2+ by Li+ at octahedral sites and the deficiency in positive charges are compensated by the electrostatically bound sodium cations (Na+) (Pinnavaia, 1983). Lap is used as an additive in cosmetic products, household cleaners, industrial surface, antistatic coatings and agrochemical products, etc. On account of the numerous uses of Lap, its hybrid with GO is further expected to increase its application realm by imparting unique physical and electrical properties. Stabilization of multi-walled carbon nanotubes (CNT) (Loginov et al., 2012) and graphene (Alhassan et al., 2012) by Lap aqueous dispersion has been reported at small mass fractions of CNT and graphene. A significant

research effort has also been devoted to synthesize water soluble graphene or to prepare stable colloidal dispersions of graphene in water (Bourlinos et al., 2009; Alhassan et al., 2012; Achari et al., 2013). In this context, Lap is chosen to be an ideal system to make a hybrid with GO and subsequent preparation of water-soluble RGO by exploiting its unique intercalation and cation exchange properties. In this article, a two-fold study is reported: (i) the formation of a highly stable graphene oxide-Lap dispersion in water by simple ultrasonication and subsequent hybrid preparation of Lap and GO by solvent evaporation and (ii) chemical reduction of the resulting hybrid in order to produce an electrically conducting hybrid. In this process, a partial reduction of GO in the hybrid is observed. A possible mechanism of the interaction between the layers of GO and Lap is postulated. 2. Materials and methods 2.1. Materials Lap XLS (Rockwood Additives Ltd., UK) a synthetic clay mineral modified by an inorganic polyphosphate dispersing agent and with an aspect ratio of ~ 25 was used in this study. The density and surface area of Lap XLS were 1 g/cm3 and 300 m2/g, respectively, as provided by the supplier. The average diameter and thickness of individual layer were ~ 25 nm and ~ 1 nm, respectively. GO was prepared using the modified Hummers method from natural graphite (100 μm mesh size, SD Fine Chem. Ltd., Mumbai, India) (Hummers and Offeman, 1958). De-ionized water (Nanopore, 18 MΩ, pH ~ 5.6) was used throughout the experiments. 2.2. Lap-GO dispersion and hybrid preparations 1 g of Lap was thoroughly mixed in 100 ml of water for 24 h at room temperature using a magnetic stirrer. GO was added to the above Lap dispersion in different mass ratios; such that the final dispersions contained the Lap and GO mass ratios of 10:1, 10:3, 5:2 and 2:1, which correspond to 1, 3, 4 and 5 mg/ml of GO in Lap dispersions, respectively. The above dispersions were ultra-sonicated using a low intensity bath sonicator (20 ± 3 kHz, Electrosonic Industries, India) for 1 h. The resulting dispersions were black and uniform in color and they are termed as Lap-GO. About 100 ml of the above dispersions were refluxed with 1 ml of hydrazine monohydrate (Thomas Baker, India) at 80 °C for 20 h in order to prepare reduced Lap-GO dispersion. The dispersions were allowed to keep unperturbed for various time periods and the photographs were taken. Both Lap-GO and reduced LapGO hybrids were prepared by evaporating the solvent (water) in an oven at 70 °C. Reduced Lap-GO powder was washed repeatedly with water to ensure complete removal of hydrazine hydrate. The black powder was gently crushed using a mortar-pestle to make fine powder before performing characterizations. The crushing was gentle enough to have minimal mechanical damage to GO sheets. The 10:1 Lap-GO hybrid was characterized by various techniques; because this sample has minimum amount of GO in it; which showed excellent colloidal stability without flocculation for considerable time period, i.e., several months. Other dispersions, which contain higher amounts of GO, i.e., 3, 4, 5 mg/ml in Lap dispersions, showed flocculation with time. The measured pH values for all the dispersions are presented in the Supporting Information (Table S1). 2.3. Ultrafiltration of cations using membrane Polyether sulfone dialysis membranes (Permionics Membranes Pvt. Ltd., Vadodara, India) of 10 kDa (pore size: 2–3 nm) with permeability of (3.16 ± 0.1) × 10−11 m/Pa∙s were used to filter the loosely bound interlayer cations present in Lap. Small pore size of the membrane ensured that the Lap particles (size ~ 25 nm) were not in filtrated through the membrane. This experiment was performed in order to


understand the electrostatic interaction between Lap and GO through possible cation exchange reaction. The membranes were folded into conical shape and submerged into a 100 ml beaker containing DI water so that only one side of the membrane was in contact with water (Mondal et al., 2013). Small amount of dispersion was slowly transferred onto the membrane and the water in the beaker was slowly agitated using magnetic stirring. The stirring was continued for 3– 4 days. The ions, which transferred to the DI water by diffusion through the membrane, were detected by ion-chromatography. 2.4. Characterization UV–vis spectroscopy (Specord 210 Plus, Analytik Jena, Germany) was carried out in the wavelength between 200 and 1000 nm. Ion-chromatography (883 Basic IC plus 1, Metrohm) was carried out on liquid samples. Wide-angle X-ray diffraction (WAXD) was carried out using a Cu-Kα radiation with an X-ray source of wavelength 1.54 Å using an X-ray Diffractometer (Rigaku TTRAX III, Japan). The samples were scanned between 1–30° at a scan rate of 0.5°/min. The experiments were carried out using a rotating anode and, the voltage and current were set at 50 kV and 200 mA, respectively. High resolution transmission electron microscopy (HRTEM) (FEI Tecnai G2, USA) images were taken at an acceleration voltage of 300 kV. Selected area electron diffraction (SAED) patterns were taken in HRTEM. Small quantity of Lap-GO (~1 mg) was dispersed in 10 ml of DI water by means of bath sonication. Then two-three drops were placed on carbon-coated Cu grid and then air-dried. Field emission scanning electron microscopy (FESEM) was performed on a Carl Zeiss (SIGMA, Germany) instrument. The sample was placed on a silicon wafer by drop-casting and coated with an Au– Pd sputtering prior to imaging. X-ray photoelectron spectroscopy (XPS) measurements were carried out in a Kratos AXIS ULTRA system, using a monochromatic Al Kα X-ray source (hν = 1486.6 eV) at 75 W and detection pass energies ranging between 20 and 80 eV. FTIR spectroscopy (Nicolet 6700) was performed for all the samples using KBr pallets in the wavelength range of 400–4000 cm−1 at a resolution of 2 cm−1 and at a scan rate of 128 min−1. The changes in GO functionality in the presence of Lap was studied using a micro-Raman Spectroscope (Horiba LABRAM–HR 800) with a laser source of wavelength, λ = 633 nm. The samples were drop-casted on silicon wafer from its aqueous solutions after ultrasonication. At least five spectra were taken for each case. Impedance studies were performed at 23 °C on a broad band dielectric spectrometer (Novocontrol, Germany) with Alpha-A analyzer in the frequency range between 0.1 Hz and 10 MHz. About 600 mg of the powder samples were pressed with a load of 3 ton and the load was held for 3 min in a stainless steel die to make discs of diameter 20 mm using a hydraulic press (ATLAS Auto T25, Specac, UK). Conducting electrodes were made on the opposite faces of the discs using silver paste.

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In order to further confirm the GO dispersion in Lap dispersion, UV– vis spectroscopy was performed on GO dispersion in de-ionized water and in 0.5% aqueous Lap dispersion at a concentration of 0.5 mg/ml. The above dispersions were kept unperturbed and UV–vis spectra were recorded intermittently to investigate the colloidal stability of GO with time in the above media. The spectra for Lap, GO and Lap-GO dispersions immediately after ultrasonication and one month after ultrasonication are shown in Fig. 1a and b, respectively. The aqueous Lap dispersion showed almost no absorbance or 100% transmittance in the entire range of wavelength. As seen in Fig. 1, Lap-GO showed a significant higher absorbance as compared to aqueous GO dispersion in both the cases: immediately after sonication and one month after sonication, which is further confirmed by the digital photographs (insets of Fig. 1a,b). Lap-GO showed marginal decrease in absorbance after one month; however the absorbance is still higher than that of aqueous GO dispersion (Fig. 1b). This clearly indicates that GO is more stable in Lap dispersion than in water. GO showed an absorbance peak centered at ~ 234 nm, which corresponds to π-π* transition of GO (Xu et al., 2013). Interestingly, in case of Lap-GO, this peak became intense and more prominent without the peak position (~ 236 nm) being shifted much; this is likely due to partial reduction of GO in presence of Lap, which is discussed later on. The stability of GO in Lap dispersion may be attributed to the electrostatic interaction between Lap and GO

3. Results and discussion 3.1. GO dispersion GO is known to make stable dispersions in water in strong alkaline pH (Li et al., 2008; Fan et al., 2008). However, GO being acidic in nature due to presence of carboxylic groups, pH of the solution needs to be made alkaline in order to overcome the attractive interlayer van der Waal forces. GO formed a stable colloidal dispersion in Lap dispersion at a mass ratio of Lap:GO ~ 10:1 and higher without pH adjustments. The former dispersion is stable even for more than a year. This is most likely due to the intrinsic alkaline nature of aqueous Lap solution (pH N 9), which is responsible for maintaining the interlayer electrostatic repulsion and facilitate the Lap and GO interaction. Further, it was observed that once the dual dispersion undergoes a gel formation, then the gel completely prevents GO flocculation (see Supporting Information).

Fig. 1. UV–vis spectroscopy of Lap, GO and Lap-GO dispersions in water (a) immediately after ultrasonication, (b) one month after ultrasonication. The inset images show the respective dispersions. It can be clearly seen that sedimentation of GO dispersion is more prominent one month after sonication as compared to that of Lap-GO.

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due to abundance of functional groups in both the materials. Moreover, the rich intercalation chemistry of clay may also contribute to their interaction (Pinnavaia, 1983). Further, the Lap-GO co-dispersion remained stable for several months with little sedimentation. Furthermore, the Lap-GO co-dispersion was treated with hydrazine monohydrate at 80 °C for 20 h to prepare reduced GO and it was found that the reduced GO did not sediment substantially as generally seen in case of GO when reduced in presence of only water (see Supporting Information). Digital images of the various dispersions immediately after sonication and after two weeks of sonication are presented in Supporting Information (Fig. S3). All the dispersions looked black and uniform after sonication, indicating a good dispersion. As seen in Fig. S3, after two weeks of ultrasonication, there was marginal color change particularly in GO and reduced Lap-GO dispersions. The color change is more prominent in reduced Lap-GO dispersions, which contains higher amount of GO (4 and 5 mg/ml) (Fig. S3b). This is mainly due to flocculation of GO with time at higher mass contents in Lap dispersions. The flocculation is most likely due to unavailability of sufficient Lap surface area to support the GO layers particularly in higher mass ratios of GO in Lap dispersions, which can be corroborated by the fact that the surface area of GO is much larger than that of Lap particles, as shown in TEM and atomic force microscopy (AFM) images (see Supporting Information) and discussed in subsequent sections. Further, reduction process makes GO more hydrophobic due to elimination of functional groups leading to decreased electrostatic interaction. However, the change is not visibly noticeable in Lap-GO dispersions. Alhassan et al. (2012) found that graphene directly obtained from graphite by turbulent mixing at 22,000 rpm was arrested by Lap colloidal dispersion in gelling regime using a very high Lap to graphene ratio, i.e., 360:1. This is expected in their study because of the strong interlayer van der Waals forces and highly hydrophobic nature of graphene, which prevented the graphene exfoliation in large quantities (Loginov et al., 2012). In contrast, the intrinsic nature of water solubility of graphene oxide facilitates the dispersion of it in Lap dispersions. Hence, in the present study the Lap to GO mass ratio is much lower, i.e., 10:1. That means much higher amount of GO can be easily dispersed in Lap dispersion. However further increase of GO content in Lap dispersion showed marginal sedimentation of GO with time. Moreover, lower concentrations of GO (b10:1) were found to be stable. Therefore, Lap to GO in the ratio of 10:1 was considered to be optimum ratio for stable dispersion of GO in Lap dispersion. Maximum GO concentration up to 5 mg/ml in 1% Lap dispersion, where the Lap to GO ratio was 2:1, was made and found stable for few days then it showed slow sedimentation. Reduction of Lap-GO with hydrazine monohydrate also showed stable dispersions in water. A maximum of 3 mg/ml of reduced GO was found to be stable in 1% aqueous Lap dispersion after two weeks of sonication. Reduced Lap-GO dispersions with 4 mg/ml and 5 mg/ml of reduced GO showed marginal flocculation as inferred by the color change in the images (Fig. S3b). 3.2. Formation of Lap-GO hybrid In WAXD patterns of Lap, GO and Lap-GO hybrid obtained after solvent evaporation (Fig. 2), Lap showed a reflection at 2θ ~6.3°, which corresponds to the (001) plane of Lap with basal distance, d(001) = 14.0 Å. Similarly GO is also a layered compound with a reflection at 2θ ~11.2°. The corresponding basal distance, d(001) is 7.9 Å. In case of Lap-GO nanocomposites, the (001) reflection is observed at 2θ ~6.8° with d(001) = 13.0 Å, which is close to the basal distance of Lap. However, the (001) reflection of GO in Lap-GO is either shifted to 2θ ~ 6.8° or completely suppressed. From the above observations, it is apparent that the layer structure of GO is disrupted leading to possible intercalation of GO into Lap layers and delamination of GO layers in Lap-GO. However the complete delamination of Lap platelets in Lap-GO could be ruled out due to the fact that the presence of strong (001) reflection which is

Fig. 2. WAXD of Lap, GO and Lap-GO hybrid.

observed at almost the same position as that for (001) reflection of Lap. Further, this reflection of nanocomposite became narrower and more intense than the usual highest intensity reflection at 2θ ~20.1° observed for Lap. Therefore, significant alternation of layered morphology of GO is quite plausible, which is further corroborated by HRTEM studies as discussed in subsequent section. Nonetheless, layered structure of hybrid is evident by the presence of strong (001) reflection. Note that the mass fraction of GO in Lap-GO was only ~10 wt%. Further the specific surface area or aspect ratio of GO is significantly higher as compared to that of Lap (see Supporting Information). The average particle size of Lap is found to be ~ 26.6 nm with a standard deviation of ± 3 nm (see Supporting Information). The thickness of the GO layer is found to be ~ 1.2 nm and the average area of GO platelets is estimated to be 0.44 (± 0.24) μm2 as obtained from AFM (Fig. S1 in Supporting Information). The TEM image of Lap-GO nanocomposite clearly indicates the overlapping of GO and Lap layers with each other (Fig. 3a), which is a possible indication of nano-level interaction between the layers of the Lap and GO. The hexagonal array of bright spots in SAED pattern (Fig. 3b), where each bright spot representing the family of {1100} plane perpendicular to the (0001) basal plane clearly suggests the presence of GO sheets at the location X (Wang et al., 2013a, 2013b). The 60° angle between concentric hexagons made by brighter spots in Fig. 3b suggests the AB stacking of GO layers. The amorphous ring pattern in SAED at location Y (Fig. 3c) indicates the presence of Lap at that location. Further, the resemblance of the location Y in Fig. 3a with that shown in TEM image of Lap in Fig. S2 (Supporting Information) confirms the presence of Lap. This clearly indicates stacking of Lap and GO layers. In fact, Lap particles are likely to adsorb onto GO surface (Fig. 3a). Similar results were observed by Yoo et al. (2014) while preparing Lap-GO coating material for oxygen barrier. Further, the layered morphology is clearly evident from FESEM image of Lap-GO (Fig. 3d). Some peculiar ellipticalshaped structures with major and minor axes of ~ 0.7 (± 0.1) μm and ~ 0.5 (± 0.1) μm, respectively, as seen in FESEM image (shown by arrows in Fig. 3d) are found to be ubiquitous in the hybrid. These structures may have arisen due to self-organization and electrostatic interaction between Lap and GO layers. 3.3. Partial reduction of GO in Lap-GO In XPS wide spectra (Fig. S7 in Supporting Information), Lap shows major binding energy peaks at 89, 102.5, 306.5, 351.5, 385, 532, 1072.5 and 1303.5 eV, which are assigned to Mg 2s, Si 2p, K 2p, Ca 2p, K 2s, O 1s, Na 1s and Mg 1s species, respectively. These peaks confirm


Fig. 3. HRTEM of (a) Lap-GO nanocomposite, (b) SAED pattern at location X, showing the hexagonal basal plane (0001) of graphitic structure, (c) SAED pattern at location Y, showing the polycrystalline ring pattern and amorphous nature of Lap and (d) FESEM image of Lap-GO powder showing oval-shaped layered morphology (shown by arrows).

the elements present in the Lap, which is a sodium magnesium silicate. The high intensity O 1s peak seen in the spectra is due to the presence of hydroxyl groups attached to Si atoms and also due to the contribution from physisorbed water. These groups derive the hydrophilic nature of Lap. XPS survey spectrum for GO shows peaks for O 1s and C 1s species at ~532 eV and ~286 eV, respectively (Fig. S7). Curve fitting of the peaks was performed using Gaussian-Lorentzian line-shapes; the results are shown in Fig. 4. The C1s components of GO, Fig. 4a, at 283.7, 284.9, 286.9, 288, 288.7 eV, are assigned to C_C, C–C and C–H, C–OH or C– O–C, O_C, O_C–OH groups, respectively, indicating the presence of hydroxyl, epoxide, carbonyl groups on graphitic backbone (Stankovich et al., 2007). On the other hand Lap does not have C-species in its molecular structure, which is confirmed by the very minor presence of C 1s peak, attributed to some surface contamination (see survey spectra of Lap in Fig. S7). Lap-GO shows a strong C 1s peak (Fig. 4b). The curve fitting yields five major peaks referred to C–C and C–H (284.8 eV), C– OH or C–O–C (286.8 eV), O_C–OH (288.9 eV) bonds and a π-π* (289.7 eV) shake-up feature. Notably, the intensities of the C–O–C or

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C–OH and C_O peaks (Fig. 4b) at 286.8 eV and 288.9 eV, respectively, for Lap-GO are reduced significantly with respect to corresponding peaks in GO. The complete XPS data are presented in Table S2 of the Supporting Information. The results clearly indicate the partial elimination of oxygen containing functional groups; such as hydroxyl, epoxy and carboxylic groups from the graphitic plane of graphene oxide in Lap-GO. The presence of the peaks with lesser intensity apparently suggests only partial reduction of GO. Moreover, the XPS spectrum for LapGO closely resembles that of reduced graphene oxide, commonly seen in the literature (Fan et al., 2008; Yang et al., 2009). The reduction of GO in presence of Lap is further supported by the O 1s XPS spectra of GO (Fig. 4c) and Lap-GO (Fig. 4d). The O 1s spectrum for GO was observed at ~ 532 eV, whereas the spectra for Lap-GO showed a marginal shift (~ 0.7 eV) to a lower binding energy. This may have arisen due to interaction between layers of Lap and GO. The absence of carboxyl or carbonyl O 1s components is consistent with the elimination of these functional groups from GO as reflected by the C 1s line. However, the presence of C–OH peak in O 1s spectrum confirms the presence of phenolic groups, which is also supported by the presence of C–OH group (~286.8 eV) in C 1s spectrum of Lap-GO with reduced intensity. Yang et al. (2009) reported that complete removal of C–OH groups from reduced graphene oxide could not be achieved even after heat-treating GO at 1000 °C for 30 min in Ar atmosphere. Moreover in the present case the signature of hydroxyl group in LapGO could arise both from adsorbed water in the material and Si–OH and Mg–OH groups of Lap (Bippus et al., 2009). These results are further corroborated by FTIR studies. Lap shows a broad characteristic FTIR band between 3700 and 3000 cm−1 (Fig. 5a), which is due to the O–H stretching of closely overlapping bands; such as Si–OH stretching, Mg–OH stretching and O–H stretching of physisorbed water (Bippus et al., 2009). The bands at ~1634 cm−1 and ~976 cm−1 for Lap are assigned to O–H bending and Si–O stretching, respectively. On other hand, GO showed a broad O–H stretching band between 3600 and 2900 cm−1, which is an overlapping band of C–OH and O–H stretching of interlayer water. Apart from this, GO also showed strong bands at ~1718 cm−1 and ~1040 cm−1, which are assigned to C_O and C–O stretching, respectively. These bands indicate the presence of functional groups; like hydroxyl, carbonyl or carboxyl and epoxy groups in GO. In contrast, the bands corresponding to C_O and C–O stretching disappeared in FTIR spectrum of Lap-GO (Fig. 5a), which contains ~ 9 wt% of GO in Lap-GO nanocomposite. Higher mass fractions (up to 90 wt%) of GO in Lap-GO nanocomposites have also been prepared by the similar method mentioned above and carried out FTIR; however the results were similar (Figures not shown). Interestingly, there is a significant blueshift in C_C stretching band from ~1614 cm−1 for GO to ~1657 cm−1 for Lap-GO, which is not well-understood. This shift may be due to interaction between Lap and GO. Further, the broad O–H stretching band (3700–3000 cm− 1) showed a blueshift in case of Lap-GO with respect to GO. The shifting of O–H stretching in Lap-GO may be due to the hydrogen bonding between the hydroxyl groups of Lap with the remaining CO functional groups of GO. Furthermore, the O–H stretching in Lap-GO appears to be an overlapping band of Si–OH, Mg–OH and interlayer water of Lap. The above results clearly indicate the presence of reduced GO and interaction between Lap and GO in Lap-GO hybrid. Raman spectroscopy was carried out to further investigate the reduction of GO (Tuinstra and Koenig, 1970). GO displayed the Raman characteristic peaks at 1574 cm−1 (G band) and 1326 cm−1 (D band) (Fig. 5b), which are associated with tangential C–C bond stretching and disordered graphitic structure, respectively (Tuinstra and Koenig, 1970). Lap-GO showed marginal blue-shifts in the Raman peaks and are observed at 1336 cm−1 and 1585 cm−1 for D and G band, respectively. The marginal shift of G band could be due to two possible reasons: (i) partial reduction of GO in Lap-GO (Kudin et al., 2008) and (ii) electrostatic interaction between GO and Lap, which may be corroborated by the fact that GO makes a stable colloidal dispersion in Lap

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Fig. 4. XPS (a–b) C1s and (c–d) O 1s spectra for GO and Lap-GO, respectively.

Fig. 5. (a) FTIR spectra for Lap, GO and Lap-GO, (b) Raman spectra of GO and Lap-GO.

dispersion. Moreover, the upshift of G band is also likely due to chemical doping and adsorption of Lap particles onto GO (Ferrari et al., 2006; Yoo et al., 2014). Generally the decrease in intensity ratio of D band to G band (ID/IG) is an indicator of GO reduction (Kudin et al., 2008). However, in the present case the average ID/IG ratios for GO and Lap-GO are found to be almost same, i.e., ~1.2. This indicates that even though GO has partially reduced in Lap dispersion; however the aromaticity or six-membered carbon ring of graphite has not been obtained in the process of reduction (Kudin et al., 2008). The peak at 2640 cm−1 in GO is referred to 2D band, which is a second-order overtone of D band (Ferrari et al., 2006). The 2D band showed a blue-shift from 2646 cm−1 for GO to 2655 cm−1 for Lap-GO (inset in Fig. 5b). Further, the intensity of the 2D band is suppressed in Lap-GO than that in GO, which indicates that there is a significant change in number of layers of GO in the nanocomposite (Tuinstra and Koenig, 1970). Further the broad nature of the 2D band suggests the presence of multilayer graphene in both GO and Lap-GO (Tuinstra and Koenig, 1970). However exact number of layers could not be determined due to the broad nature of the peak. A holistic appraisal of the results obtained from the above characterization techniques indicated a specific interaction; most likely an electrostatic interaction between these two components leading to the formation of a new functional hybrid material. In this context, a simple hypothesis is proposed for the mechanism of partial reduction of GO in Lap-GO, which is elucidated in Fig. 6. Lap layers readily exfoliate and make clear dispersion in water because of osmotic swelling (Harikrishnan et al., 2012) and the interlayer Na+ ions, which are loosely bound by electrostatic forces, are likely to have electrostatic interaction with the electron-rich oxygen containing functional groups; such as –OH, –COOH, C–O–C of GO, resulting in partial elimination of these groups. Moreover, the alkaline nature (pH ~10) of Lap aqueous dispersion may also be partially responsible for GO reduction, as GO is reported to undergo deoxygenation in strong alkaline (KOH and NaOH) solution (Fan et al., 2008). The presence of Na+ along with small amounts of K+ and Mg2+ cations in the filtrate of Lap-GO dispersion as revealed by ion-chromatography (Fig. S8 in Supporting Information) indicates the likely elimination of functional groups. Ultrasonication is likely to contribute further to the reduction process. It is important to mention here that the components of Lap-GO hybrid were not separated by simple ultrasonication or solvent treatment.


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Fig. 6. Schematic representation of electrostatic interaction between interlayer cations of Lap and functional groups of GO. Lap is represented by the discs.

This is again a plausible indication of strong interaction between them. Moreover, the amphiphilic nature of GO may also contribute greatly to the bonding of Lap and GO to form the hybrid (Kim et al., 2010). 3.4. Electrical properties The AC conductivity was found to increase with frequency for Lap (Fig. 7), which is a typical behavior of dielectric material (Zhang et al., 2007). However, the Lap-GO hybrids showed marginal increase in conductivity with frequency up to ~10 Hz followed by a frequency independent behavior up to ~ 106 Hz. At higher frequencies (N 106 Hz), the values showed marginal increase with frequency. This could be due to presence of GO in these hybrids. Similar conductivity behavior was observed for only GO as well (Fig. 7). Although the conductivities of LapGO hybrids are lower than that of pure GO; but the nature of the curves for Lap-GO and GO remains the same. Upon chemical reduction of LapGO hybrid with hydrazine monohydrate in order to reduce GO thereby resulted in increase of the AC conductivity by three orders of magnitude (Fig. 7). The conductivity of chemically reduced only GO was found to be in order of ~10−2 S/cm. It is interesting to note that the reduced Lap-GO hybrids regardless of their GO content showed the electrical conductivity values comparable to that of chemically reduced pure GO; notably

the lowest content of GO in Lap-GO was only 9 wt% among the compositions studied here. This indicates that only ~ 9 wt% of reduced GO is sufficient to make a conducting path in a relatively less expensive Lap. Further, in case of hybrid with Lap:GO ratio of 5:2, the conductivity is higher than that of reduced GO (Fig. 7). The low values of conductivity in these cases are possibly due to the large number of defects in the GO backbone as revealed by Raman spectra. The synergistic effects of the individual layers could be manifested in different ways in different applications. The hybrid material synthesized in the present study is a potential candidate as a functional filler for nanocomposites and for charge storage mainly due to their huge surface area obtained as a result of separation of GO into few layers and their excellent electrical properties. Graphene oxide was reported to enhance the overall mechanical properties of polymers (Liang et al., 2009; Rafiee et al., 2009; Zhao et al., 2010; Bortz et al., 2011), whereas Lap was shown to exhibit excellent ion transport properties (Lutkenhaus et al., 2007) besides mechanical properties (Patro and Wagner, 2011). Further, the resulting material has potential for oxygen barrier (Yoo et al., 2014) and conducting coatings. Furthermore, in a very recent study, Alhwaige et al. (2016) reported significant improvements in mechanical properties of poly(vinyl alcohol) aerogels using Lap-graphene platelet hybrid filler and found that the hybrid fillers showed synergistic affects on the mechanical properties and glass transition temperatures of the nanocomposite aerogels by increasing the content of graphene platelets in the hybrid. 4. Conclusions

Fig. 7. AC conductivity as a function of frequency for GO, Lap-GO and reduced Lap-GO at different mass ratios of Lap and GO.

A hybrid of graphene oxide and Lap was prepared from a highly stable dual aqueous dispersion, which were stable for several months without flocculation. The colloidal stability of the dispersion was likely originated from the electrostatic interaction between interlayer cation of Lap and functional groups of graphene oxide and partly due to hydrogen bonding, and adsorption of Lap particles onto graphene oxide surface. Graphene oxide was found to be partially reduced in Lap dispersion. Further, graphene oxide layers are found to be partially delaminated in the Lap-GO hybrid. The partial reduction is also one of the likely causes of hybrid formation. Reduced graphene oxide hybrids, prepared by hydrazine hydrate treatment, were found to make stable dispersions in aqueous Lap at high mass fractions — up to 3 mg/ml. The resulting hybrid showed improved electrical properties. The current findings direct a new pathway for reduction of graphene oxide without any thermal and chemical assistance in addition to the preparation of a

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promising graphene based hybrid materials. Further, owing to the simplicity of the hybrid preparation technique and the versatile colloidal chemistry, these hybrids may open up new avenues in preparing multifunctional materials for various applications. Acknowledgements TUP would like to thankfully acknowledge the funding from DST, Govt. of India, under the Fast Track Project for Young Scientists (SB/FT/ CS-043/2012) and build-up grant from DIAT(DU) (DIAT/F/MATE/TUP/ 4845). The authors would like to thank Dr. H. S. Panda, DIAT, Mr. Asish Goutam, DIAT and Mr. Ishank Singh, IIT(BHU), Varanasi for assistance and fruitful interactions. The authors would like to acknowledge the DRDO-DIAT Nano program for various characterizations. HDW is the Livio Norzi Professor in Materials Science. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.clay.2016.05.023. References Achari, A., Datta, K.K.R., De, M., Dravid, V.P., Eswaramoorty, M., 2013. Amphiphilic aminoclay–RGO hybrids: a simple strategy to disperse a high concentration of RGO in water. Nanoscale 5, 5316–5320. Alhassan, S.M., Qutubuddin, S., Schiraldi, D.A., 2012. Graphene arrested in Laponite–water colloidal glass. Langmuir 28, 4009–4015. Alhwaige, A.A., Herbert, M.M., Alhassan, S.M., Ishida, H., Qutubuddin, S., Schiraldi, D.A., 2016. Laponite/multigraphene hybrid-reinforced poly(vinyl alcohol) aerogels. Polymer 91, 180–186. Bakandritsos, A., Steriotis, T., Petridis, D., 2004. High surface area montmorillonite–carbon composites and derived carbons. Chem. Mater. 16, 1551–1559. Bandosz, T.J., Jagiełło, J., Putyera, K., Schwarz, J.A., 1996. Pore structure of carbon-mineral nanocomposites and derived carbons obtained by template carbonization. Chem. Mater. 8, 2023–2029. Bippus, L., Jaber, M., Lebeau, B., 2009. Laponite and hybrid surfactant/Laponite particles processed as spheres by spray-drying. New J. Chem. 33, 1116–1126. Bortz, D.R., Heras, E.G., Gullon, I.M., 2011. Impressive fatigue life and fracture toughness improvements in graphene oxide/epoxy composites. Macromolecules 45, 238–245. Bourlinos, A.B., Georgakilas, V., Zboril, R., Steriotis, T.A., Stubos, A.K., Trapalis, C., 2009. Aqueous-phase exfoliation of graphite in the presence of polyvinylpyrrolidone for the production of water-soluble graphenes. Solid State Commun. 149, 2172–2176. Chen, W., Yan, L., Bangal, P.R., 2010. Chemical reduction of graphene oxide to graphene by sulfur-containing compounds. J. Phys. Chem. C 114, 19885–19890. Dey, R.S., Raj, C.R., 2013. A hybrid functional nano scaffold based on reduced graphene oxide–ZnO for the development of an amperometric biosensing platform. RSC Adv. 3, 25858–25864. Dreyer, D.R., Murali, S., Zhu, Y., Ruoff, R.S., Bielawski, C.W., 2011. Reduction of graphite oxide using alcohols. J. Mater. Chem. 21, 3443–3447. Duclaux, L., Frackowiak, E., Gibinski, T., Benoit, R., Beguin, F., 2000. Clay/carbon nanocomposites as precursors of electrode materials for lithium-ion batteries and supercapacitors. Mol. Cryst. Liq. Cryst. 340, 449–454. Fan, X., Peng, W., Li, Y., Wang, S., Zhang, G., Zhang, F., 2008. Deoxygenation of exfoliated graphite oxide under alkaline conditions: a green route to graphene preparation. Adv. Mater. 20, 4490–4493. Fernández-Saavedra, R., Aranda, P., Ruiz-Hitzky, E., 2004. Templated synthesis of carbon nanofibers from polyacrylonitrile using sepiolite. Adv. Funct. Mater. 14, 77–82. Ferrari, A.C., Meyer, J.C., Scardaci, V., Casiraghi, C., Lazzeri, M., Mauri, F., Piscanec, S., Jiang, D., Novoseloy, S.K., Roth, S., Geim, A.K., 2006. Raman spectrum of graphene and graphene layers. Phys. Rev. Lett. 97, 187401. Garcia, C.R., Carvajal, J.P., Murcia, A.B., Darder, M., Aranda, P., Amoros, D.C., Hitzky, E.R., 2013. Clay-supported graphene materials: application to hydrogen storage. Phys. Chem. Chem. Phys. 15, 18635–18641. Geim, A.K., Novoselov, K.S., 2007. The rise of graphene. Nat. Mater. 6, 183–191. Giri, S., Ghosh, D., Das, C.K., 2014. Growth of vertically aligned tunable polyaniline on graphene/ZrO2 nanocomposites for supercapacitor energy‐storage application. Adv. Funct. Mater. 24, 1312–1324. Harikrishnan, G., Singh, S.N., Lindsay, C.I., Macosko, C.W., 2012. An aqueous pathway to polymeric foaming with nanoclay. Green Chem. 14, 766–770. Hummers, J.W.S., Offeman, R.E., 1958. Preparation of graphitic oxide. J. Am. Chem. Soc. 80, 1339. Kim, J., Cote, L.J., Kim, F., Yuan, W., Shull, K.R., Huang, J., 2010. Graphene oxide sheets at interfaces. J. Am. Chem. Soc. 132, 8180–8186. Kudin, K.N., Ozbas, B., Schniepp, H.C., Prud'homme, R.K., Aksay, I.A., Car, R., 2008. Raman spectra of graphite oxide and functionalized graphene sheets. Nano Lett. 8, 36–41. Lei, Z., Lu, L., Zhao, X.S., 2012. The electrocapacitive properties of graphene oxide reduced by urea. Energy Environ. Sci. 5, 6391–6399.

Li, D., Muller, M.B., Gilje, S., Kaner, R.B., Wallace, G.G., 2008. Processable aqueous dispersions of graphene nanosheets. Nat. Nanotechnol. 3, 101–105. Liang, J., Huang, Y., Zhang, L., Wang, Y., Ma, Y., Guo, T., Younsheng, T.C., 2009. Molecular‐ level dispersion of graphene into poly(vinyl alcohol) and effective reinforcement of their nanocomposites. Adv. Funct. Mater. 19, 2297–2302. Liang, Y., Li, Y., Wang, H., Zhou, J., Wang, J., Regier, T., Dai, H., 2011. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat. Mater. 10, 780–786. Loginov, M., Lebovka, N., Vorobiev, E., 2012. Laponite assisted dispersion of carbon nanotubes in water. J. Colloid Interface Sci. 365, 127–136. Lutkenhaus, J.L., Olivetti, E.A., Verploegen, E.A., Cord, B.M., Sadoway, D.R., Hammond, P.T., 2007. Anisotropic structure and transport in self-assembled layered polymer–clay nanocomposites. Langmuir 23, 8515–8521. Ma, Q., Song, J., Jin, C., Li, Z., Liu, J., Meng, S., Zhao, J., Guo, Y., 2013. A rapid and easy approach for the reduction of graphene oxide by formamidinesulfinic acid. Carbon 54, 36–41. Mondal, S., Rai, C., De, S., 2013. Identification of fouling mechanism during ultrafiltration of stevia extract. Food Bioprocess Technol. 6, 931–940. Nethravathi, C., Rajamathi, J.T., Ravishankar, N., Shivakumara, C., Rajamathi, M., 2008a. Graphite oxide-intercalated anionic clay and its decomposition to graphene–inorganic material nanocomposites. Langmuir 24, 8240–8244. Nethravathi, C., Viswanath, B., Shivakumara, C., Mahadevaiah, N., Rajamathi, M., 2008b. The production of smectite clay/graphene composites through delamination and co-stacking. Carbon 46, 1773–1781. Nethravathi, C., Rajamathi, M., Ravishankar, N., Basit, L., Felser, C., 2010. Synthesis of graphene oxide-intercalated α-hydroxides by metathesis and their decomposition to graphene/metal oxide composites. Carbon 48, 4343–4350. Novoselov, K.S., Geim, A.K., Morozov, S.V., Jiang, D., Zhang, Y., Dubonos, S.V., Grigorieva, I.V., Firsov, A.A., 2004. Electric field effect in atomically thin carbon films. Science 306, 666–669. Patro, T.U., Wagner, H.D., 2011. Layer-by-layer assembled PVA/Laponite multilayer freestanding films and their mechanical and thermal properties. Nanotechnology 22, 455706. Pinnavaia, T.J., 1983. Intercalated clay catalysts. Science 220, 365–371. Rafiee, M.A., Rafiee, J., Wang, Z., Song, H., Yu, Z.Z., Koratkar, N., 2009. Enhanced mechanical properties of nanocomposites at low graphene content. ACS Nano 3, 3884–3890. Ramsay, J.D.F., 1986. Colloidal properties of synthetic hectorite clay dispersions. J. Colloid Interface Sci. 109, 441. Ruzicka, B., Zulian, L., Zaccarelli, E., Angelini, R., Sztucki, M., Moussaid, A., Ruocco, G., 2010. Competing interactions in arrested states of colloidal clays. Phys. Rev. Lett. 104, 085701. Sandí, G., Carrado, K.A., Winans, R.E., Johnson, C.S., Csencsits, R., 1999. Carbons for lithium battery applications prepared using sepiolite as inorganic template. J. Electrochem. Soc. 146, 3644–3648. Shin, H.J., Kim, K.K., Benayad, A., Yoon, S.M., Park, H.K., Jung, I.S., Jin, M.H., Jeong, H.K., Kim, J.M., Choi, J.Y., Lee, Y.H., 2009. Efficient reduction of graphite oxide by sodium borohydride and its effect on electrical conductance. Adv. Funct. Mater. 19, 1987–1992. Sonobe, N., Kyotani, T., Tomita, A., 1991. Formation of graphite thin film from polyfurfuyl alcohol and polyvinyl acetate carbons prepared between the lamellae of montmorillonite. Carbon 29, 61–67. Stankovich, S., Dikin, D.A., Dommett, G.H.B., Kohlhaas, K.M., Zimney, E.J., Stach, E.A., Piner, R.D., Nguyen, S.T., Ruoff, R.S., 2006. Graphene-based composite materials. Nature 442, 282–286. Stankovich, S., Dikin, D.A., Piner, R.D., Kohlhaas, K.A., Kleinhammes, A., Jia, Y., Wu, Y., Nguyen, S.T., Ruoff, R.S., 2007. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 45, 1558–1565. Tuinstra, F., Koenig, J.L., 1970. Raman spectrum of graphite. J. Chem. Phys. 53, 1126–1130. Wang, X., Meng, G., Zhu, C., Huang, Z., Qian, Y., Sun, K., Zhu, K., 2013a. Nanomaterials: a generic synthetic approach to large-scale pristine-graphene/metal-nanoparticles hybrids. Adv. Funct. Mater. 23, 5771–5777. Wang, H., Xia, B., Yan, Y., Li, N., Wang, J.Y., Wang, X., 2013b. Water-soluble polymer exfoliated graphene: as catalyst support and sensor. J. Phys. Chem. B 117, 5606–5613. Wang, G., Yang, J., Park, J., Gou, X., Wang, B., Liu, H., Yao, J., 2008. Facile synthesis and characterization of graphene nanosheets. J. Phys. Chem. C 112, 8192–8195. Watcharotone, S., Dikin, D.A., Stankovich, S., Piner, R., Jung, I., Dommett, G.H.B., Ruoff, R.S., 2007. Graphene-silica composite thin films as transparent conductors. Nano Lett. 7, 1888–1892. Xiao, F., Li, Y., Zan, X., Liao, K., Xu, R., Duan, H., 2012. Growth of metal–metal oxide nanostructures on freestanding graphene paper for flexible biosensors. Adv. Funct. Mater. 22, 2487–2494. Xiao, J., Wang, X., Yang, X.Q., Xun, S., Liu, G., Koech, P.K., Liu, J., Lemmon, J.P., 2011. Electrochemically induced high capacity displacement reaction of PEO/MoS2/graphene nanocomposites with lithium. Adv. Funct. Mater. 21, 2840–2846. Xu, S., Yong, L., Wu, P., 2013. One-pot, green, rapid synthesis of flower-like gold nanoparticles/reduced graphene oxide composite with regenerated silk fibroin as efficient oxygen reduction electrocatalysts. ACS Appl. Mater. Interfaces 5, 654–662. Yang, D., Velamakanni, A., Bozoklu, G., Park, S., Stoller, M., Pine, R.D., Stankovich, S., Jung, I., Field, A.D., Ventrice, J.A.C., Ruoff, S.R., 2009. Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and micro-Raman spectroscopy. Carbon 47, 145. Yanzhen, L., Yongfeng, L., Yonggang, Y., Yuefang, W., Maozhang, W., 2011. Reduction of graphene oxide by thiourea. J. Nanosci. Nanotechnol. 11, 10082–10086. Yoo, J.T., Lee, S.B., Lee, C.K., Hwang, S.W., Kim, C.R., Fujigaya, T., Nakashima, N., Shim, J.K., 2014. Graphene oxide and Laponite composite films with high oxygen-barrier properties. Nanoscale 6, 10824–10830.

D.K. Chouhan et al. / Applied Clay Science 132-133 (2016) 105–113 Zhang, R., Alecrim, V., Hummelgård, M., Andres, B., Forsberg, S., Andersson, M., Olin, H., 2015. Thermally reduced kaolin-graphene oxide nanocomposites for gas sensing. Sci. Rep. 5, 7676. Zhang, C.S., Ni, Q.Q., Fu, S.Y., Kurashiki, K., 2007. Electromagnetic interference shielding effect of nanocomposites with carbon nanotube and shape memory polymer. Comput. Sci. Technol. 67, 2973–2980. Zhang, J., Yang, H., Shen, G., Cheng, P., Zhang, J., Guo, S., 2010a. Reduction of graphene oxide via L-ascorbic acid. Chem. Commun. 46, 1112–1114.

113

Zhao, X., Zhang, Q., Chen, D., Lu, P., 2010b. Enhanced mechanical properties of graphenebased poly(vinyl alcohol) composites. Macromolecules 43, 2357–2363. Zhao, M.Q., Zhang, Q., Huang, J.H., Wei, F., 2012. Hierarchical nanocomposites derived from nanocarbons and layered double hydroxides - properties, synthesis, and applications. Adv. Funct. Mater. 22, 675–694.