Tuning Electrical Properties of Graphene with Different π-Stacking

Article pubs.acs.org/JPCC

Tuning Electrical Properties of Graphene with Different π‑Stacking Organic Molecules Manash Jyoti Deka and Devasish Chowdhury* Material Nanochemistry Laboratory, Physical Sciences Division, Institute of Advanced Study in Science and Technology, Paschim Boragaon, Garchuk, Guwahati-781035, India

ABSTRACT: Tuning of the electrical properties of graphene and functionalized graphene is very important to its use in optoelectronic devices. In this work, we study the electrical properties of graphene and graphene with different π-stacking organic molecules. The different π-stacking organic molecules used in the present study were hemin, 1-amino 2-naphthol 4-sulfonic acid, and ferrocene. The noncovalently functionalized reduced graphene oxides were characterized by UV−visible spectroscopy, Fourier transformed infrared spectroscopy, atomic force microscopy, scanning electron microscopy, thermogravimetric analysis, and Raman spectroscopy. The noncovalently functionalized reduced graphene oxide show higher ac conductivity than graphene oxide (GO). The enhancement of conductivity shown can be attributed to higher mobility, and the density of π-electron and higher surface area of hybrid nanocomposites system. The reason is supported because that interaction of rGO with non-πsystem like 18-crown-6 did not help in increasing the ac conductivity of the system. Thus, the electrical properties of graphene can be tuned through noncovalent interaction with π-stacking organic molecules.

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INTRODUCTION Graphene, a new allotrope of carbon hybrid nanomaterial family, draws the intense attention of scientific community recently due to its extraordinary properties, unique band gap1,2 structure, and having remarkable application potential. Because of its unique 2D hexagonal structure and geometry, it exhibits many exciting properties like high thermal conductivity,3 large specific surface area, high Young’s modulus,4 quantum hall effect,5,6 long-range ballistic transport,7,8 and sensitivity toward adsorption of individual gas molecules,9 carrier mobility,10 and so forth. These fascinating properties enable graphene as an ideal promising material for a wide range of applications, ranging from nanoelectronics,11 energy sector,12−14 catalysis,15,16 sensors,17,18 surface coating,19 and engineering of nanocomposites and biomaterials,20,21 quantum physics,22 and so forth. Chemical modification and stabilization of graphene, such as covalent and noncovalent functionalization approaches play a vital role to change the electrical properties of graphene by tuning the band gap.23−25 One of the methods employed to tune the electronic properties is through noncovalent πstacking interaction. The π-effects or π-stacking interactions are a type of noncovalent interaction that involves π-systems. These kinds of interaction occur in many systems such as © XXXX American Chemical Society

vertical nucleobase pair stabilization in the double helical structure of DNA and RNA, intercalation drug into DNA, aggregation of porphyrin, packing of aromatic molecules in crystals, the tertiary fabric of the protein and in many host− guest systems, and so forth.26,27 Zhang et al. showed theoretically that noncovalent stacking with aromatic molecules through π−π interaction can tune the band gap of graphene.28 Dong et al. through Raman analysis showed that the doping level (or Fermi energy) of graphene film can be modulated by aromatic molecules.29 Moreover graphene-based π-stacking system has been used extensively for various applications. For example, Teng Xue et al. have successfully synthesized a catalyst of graphene supported-hemin for biomimetic oxidation.16 Polypyrrole-based hemin-reduced graphene oxide nanocomposites hybrid system for electrocatalysis for the reduction of hydrogen peroxide was modified by Huang et al. recently.15 Feng and co-workers investigated the stacking of polycyclic aromatic hydrocarbons as a prototype for graphene multilayers using density functional theory.30 Received: December 18, 2015 Revised: January 22, 2016

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DOI: 10.1021/acs.jpcc.5b12403 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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nanocomposites system. For a comparison, an ac conductivity study of graphene with other non-π-stacking molecules was also investigated and discussed.

Avinash et al. reported the covalent modification and functionalization of graphene using ferrocene.31 However, the electrical properties on chemical modification of graphene with different π-stacking molecules has not been studied in detail. Here we synthesize and measure the ac impedance of noncovalently functionalized reduced graphene oxide. The ac impedance involves measuring both the electronic component (due to movement of electron) and ionic component (due to movement of ions) to obtain total conductivity of the system. In most cases, chemical functionalization and π-stacking interaction of graphene involves ionic component32 so measurement of the ac impedance and hence ac conductivity becomes important. In our previous report,24 we have successfully exfoliated and covalently incorporated different functional groups on graphene nanosheets and investigated the ac electrical conductivity with different functionality and the lateral dimension. We have investigated that larger size lateral dimension sheets possess higher ac electrical conductivity in comparison to lower lateral size dimension. On covalent functionalization, the planar structure of graphene was distorted, and delocalization of π-electron was also hindered, which causes lower ac conductivity in the case of functionalized graphene. In this work, an attempt has been made to carry out detailed study on electrical properties of graphene oxide and noncovalently functionalized reduced graphene oxide hybrid nanocomposites. Commercially available graphite nanopowder (GnP) is exfoliated and oxidized with the previously reported method like CNT oxidation.33 The system was then in a single step reduced and interacted with different π-stacking molecules through noncovalent interaction. Different π-stacking molecules used in this study were hemin (Hmn), 1-amino 2-napthol 4-sulfonic acid (Nphl), and ferrocene (Fcn). We have randomly selected these π-stacking organic molecules to study the noncovalent interaction with rGO and their effect on ac electrical conductivity. Hemin is a well-known protoporphyrin IX-Fe(III) complex, containing a ferric ion with a chloride ligand that is the active part of hemoglobin. Porphyrins are electron-rich, planar, aromatic systems, having notably high extinction coefficients in the visible region.34−36 Nphl is a naphthalene homologue of phenol, having free −NH2 and − SO3H groups belongs to polycyclic aromatic hydrocarbons family.37 Bhandari et al. have successfully constructed an electrostatic charge dissipation material of polyaniline in the presence of Nphl.38 Zhang et al. used different naphthalene sulfonic acids as dopants for polyaniline to prepare polyaniline nanotubes.39 Ferrocene is a metallocene compound and consists of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Ferrocene possesses some important properties such as generation of stable redox states, reversibility and regeneration at a low potential, and its thermal and chemical stability.40,41Applications of Fcn mainly involve electrochemical sensor detections for biomolecules,42 detection of protein biomarker,43 photoinduced electron transfer,44 and so forth. These reduced graphene oxide hybrid nanocomposites show higher ac electrical conductivity (lower impedance) when compared with graphene oxide. As noncovalent functionalization of graphene mainly based on π-stacking interaction in hybrid structures has less hindered influence on its structure and hybridization thus improves its electrical properties. The enhancement of conductivity shown is due to higher mobility, and the density of π-electron and higher surface area of hybrid

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EXPERIMENTAL SECTION Chemicals Used. Graphite nanopowder and ferrocene (C10H10Fe) were purchased from Sisco Research Laboratories, hemin (C34H32ClFeN4O4) and 18-crown-6(C12H24O6) were purchased from Sigma-Aldrich. 1-Amino-2-naphthol-4-sulfonic acid (C10H9NO4S), hydrazine hydrate (H6N2O), sulfuric acid (H2SO4), nitric acid (HNO3), and hydrochloric acid (HCl) were purchased from Merck, India. All the chemicals were used as obtained without further purification. The water used throughout the experiments was from a Milli-Q water purification system. Exfoliation and Oxidation of Graphite Nanopowder (GnP) Using H2SO4/HNO3. Commercially available graphite nanopowder (GnP) was exfoliated and oxidized in a single step using a simple chemical reaction. Here, an acid mixture, that is, a 3:1 mixture of H2SO4/HNO3,was used as an oxidizing agent followed by a simple sonication and centrifugation technique at room temperature to separate the exfoliated oxidized graphene sheets. Typically 0.1 g of GnP was dispersed in a 3:1 acid mixture of concentrated H2SO4 (98 wt %)/HNO3 (16 M). This mixture was then sonicated for 24 h followed by dilution and centrifugation at 2500 rpm for 30 min to separate the lighter exfoliated GO platelets. The supernatant solution was then separated from the heavier particles and analyzed separately. The heavier particles were collected and redispersed in water, and the process of centrifugation and dispersion was repeated several times. The resultant dispersion after a few cycles was then evaporated at a temperature of 100 °C to obtain the solid product. One Step Reduction and Noncovalent Functionalization of GO with Different π-Stacking Molecules (i.e., Hemin, Ferrocene, and 1-Amino-2-Naphthol-4-Sulfonic Acid). Asprepared GO was transferred into ethanol by centrifuge and diluted to 0.5 mg/mL and the dispersions were ultrasonicated for 1 h. Twenty milliliters of GO suspension was then mixed in a 50 mL round-bottom flask with 20 mL of 0.5 mg/mL hemin(Hmn) dispersion in ethanol under vigorous stirring. After 30 min, 75 μL hydrazine hydrate (35%) were added into this dispersion. After another 30 min of vigorous stirring, the dispersion was refluxed at 75 °C for 3.5 h. The same procedure was employed for other π-stacking molecules, that is, Nphl refluxed at 80 °C for 7 h, Fcn refluxed at 90 °C for 5 h, and the non-π-stacking molecule (18-crown-6) to form reduced graphene oxide hybrid nanocomposites of these molecules. Three reduced graphene oxide hybrid nanocomposites were prepared with each π-stacking molecules. The ratios used for formation of nanocomposites are 1:1, 1:2, and 1:5. The nanocomposites were designated in the paper as rGO-Hmn1, rGO-Hmn2, and rGO-Hmn3, respectively for 1:1, 1:2, and 1:5 compositions for reduced graphene oxide-hemin nanocomposite. Similar nomenclature is given to reduced graphene oxide-1-amino-2-naphthol-4-sulfonic acid nanocomposite, rGO-Nphl1, rGO-Nphl 2, and rGO-Nphl3, and reduced graphene oxide ferrocene nanocomposites, rGO-Fcn1, rGOFcn2, and rGO-Fcn3. Characterization. The characterization of GO and noncovalently functionalized reduced graphene oxide hybrid nanocomposites were carried out by UV−visible absorption spectrophotometer (Shimadzu, UV-2600), dynamic light B

DOI: 10.1021/acs.jpcc.5b12403 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Schematic Illustration of Preparation of Noncovalently Functionalized Reduced Graphene Oxide Hybrid Nanocomposites from Graphite Nanopowder

scattering (DLS, Malvern Zetasizer Nano series, Nano ZS90), Fourier transform infrared spectrophotometer (FTIR, Nicolet 6700), scanning electron microscopy (SEM, Carl Zeiss ∑igmaVP), and atomic force microscopy (AFM, NTEGRA PRIMA NT-MDT). Raman analysis was performed using laser microRaman system (Horiba Jobin Vyon, Model LabRam HR) using 514 nm laser at room temperature. The thermogravimetric analysis (TGA) thermograms were recorded by PerkinElmer 4000 and were done in the range of 35−700 °C at a heating rate of 10 °C/min and with nitrogen flow rate of 20 mL/min. All ac impedance measurements were recorded at room temperature between a frequency range of 42 Hz to 5 MHz at the constant voltage of 1 V at ambient conditions using an impedance analyzer Hioki 3532−50. The samples were taken in the form of a thin film sandwiched in between two symmetric stainless steel (SS) electrodes. The geometry of the cell for measurement of conductivity was SS|GO/noncovalent functionalized rGO|SS. The experiment was carried out in a relative humidity of 55%.

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Figure 1. Stacked UV−visible spectra of (A) rGO-Hmn nanocomposite and Hmn, (B) rGO-Nphl nanocomposite and Nphl, (C) rGO-Fcn nanocomposites and Fcn, and (D) rGO-18-crown-6 nanocomposites and 18-crown-6.

RESULTS AND DISCUSSION Commercially available GnP was oxidized and exfoliated using simple one-step chemical reactions like carbon nanotube oxidation using HNO3/H2SO4 and then it simultaneously reduced and interacted with different π-stacking molecules through noncovalent interaction. The π-stacking molecules used in the study were Hmn, Nphl, and Fcn. The one step reduction and noncovalent functionalization of GO with different π-stacking molecules resulted in successful synthesis of hemin functionalized reduced graphene oxide hybrid (rGO-Hmn), 1-amino 2-napthol 4-sulfonic acid functionalized reduced graphene oxide hybrid (rGO-Nphl), and ferrocene-functionalized reduced graphene oxide hybrid (rGO-Fcn) nanocomposites. Schematic representation of the protocol followed for the preparation of noncovalently functionalized reduced graphene oxide hybrid nanocomposites is shown in Scheme 1. UV−visible Characterization. UV−visible spectroscopy studies were performed on rGO-Hmn, rGO-Nphl, and rGOFcn nanocomposites to investigate the π−π interactions. The stacked absorption spectra are shown in Figure 1. It is evident from Figure 1A that for free hemin dispersion the main absorption peak corresponding to soret band at 397 cm−1 is due to π−π * transition. After the π−π interactions with rGO,

the soret band of hemin is bathochromically shifted. The new peaks are appeared at 403, 404, and 406 nm for rGO-Hmn1, rGO-Hmn2, and rGO-Hmn3 nanocomposites, respectively, which confirm successful π−π interactions between rGO and hemin. Figure 1B shows that 1-amino-2-naphthol-4-sulfonic acid has two distinct peaks: one near 305 nm and other near 348 nm. A peak near 305 nm can be assigned to π−π* transition of the CC bond. The other peak at 348 nm can be assigned to n−π* transition of the hydroxyl (−OH) groups. After incorporation with rGO, these two peaks are also shifted bathochromically for rGO-Nphl1, rGO-Nphl2, and rGO-Nphl3 nanocomposites. Figure 1C shows that free ferrocene dispersion has two distinct peaks, one near 323 nm and the other near 440 nm. The peak near 323 nm can be assigned to intraligand π−π* transition and near 440 nm involving both ligand and the metal−ligand charge transfer transition. After noncovalent interactions with rGO, the peaks near 323 nm disappeared gradually and peaks near 440 nm shifted bathochromically. It is true for all compositions, namely, rGO-Fcn1, rGO-Fcn2 and rGO-Fcn3 nanocomposites, which C

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The Journal of Physical Chemistry C indicates the successful interaction and π−π stacking of ferrocene with rGO. We have also studied the π−π interaction of GO with different non π-stacking molecules like 18-crown-6. Stacked UV−visible spectra clearly shows that there is no change in soret band after reduction and incorporation with rGO, which indicates that there is an absence of any interaction of rGO with 18-crown-6. AFM and SEM Analysis. Figure 2A shows the representative AFM tapping-mode image GO deposited onto a silicon wafer

substrate. The images show that the average lateral dimension of the exfoliated GO is ∼1.2 μm. Figure 2B shows the average cross-section height and the thickness of the sheet obtained is ∼1.5 nm, which correlates to approximately 3−4 layers of GO. The additional thickness might have been generated from the oxygen-containing functional groups such as epoxy, carboxylate, and hydroxyl groups on the GO surface. Figure 2C shows the representative SEM image of exfoliated GO nanosheets.The image also confirms that GO has a typical lateral size of ∼1.2 μm.The size of the exfoliated GO nanosheets is also determined from DLS measurement. DLS data show the size of GO nanosheets at ∼1 μm (Figure 2 D). The lateral dimension ⟨L⟩ for GO dispersion is calculated from the DLS measurement45 using the formula ⟨L⟩ = (0.07 ± 0.03)aDLS(1.5 ± 0.15) . Thus, the lateral dimension of GO nanosheets are found to be 1269 ± 12 nm. FTIR Study. FTIR spectroscopy was carried out on all the samples to demonstrate the successful reduction and noncovalent functionalization of GO sheets with different πstacking molecules. Figure 3A shows the stacked FTIR spectra of rGO-Hmn1 nanocomposite, GO, and Hmn. GO shows four prominent peaks appearing near 3450, 1720,1620, and 1194 cm−1, which are due to hydroxyl groups (−OH), carboxylic acid group (−COOH), alkenes (−CC−), and C−O groups, respectively, on the graphene sheet. Free hemin exhibits three main characteristics peaks. The peaks appear at 1728, 1585, and 1191 cm−1. The peak at 1728 cm−1 corresponds to CO stretching vibration of the carboxylic acid groups. One characteristic peak is observed at 1585 cm−1for γ10 (Cα−Cß) band of porphyrin skeleton, and another peak is at 1191 cm−1 due to surface bound carboxylate (−CO−) of hemin. It is also interesting to note that upon reduction and noncovalent interaction with GO to form rGOHmn1, these peaks are shifted to 1670, 1563, and 1123 cm−1,

Figure 2. (A) Representative AFM tapping mode image of GO deposited on silicon wafer substrate. (B) Cross-section measurements taken along the line, indicating a sheet thickness of 1.5 nm, which is equivalent to approximately 3−4 layers of GO. (C) Representative SEM images of GO and (D) size distribution curve as obtained from DLS technique.

Figure 3. Stacked FTIR spectra of (A) rGO-Hmn1, GO, and Hmn, (B) rGO-Nphl1, GO, and Nphl, and (C) rGO-Fcn1, GO, and Fcn. D

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Figure 4. TGA thermograph of (A) rGO-Hmn, (B) rGO-Nphl, and (C) rGO-Fcn nanocomposites at various concentrations and compared with GO.

organic molecules. It has been already reported in literature that after reduction some oxygenated functional groups may remain to attach to the graphene nanosheets. It is tough to remove all the oxygenated functionalities from GO nanosheets using chemical reduction. But the peaks may be shifted with GO due to change in the structural conformation after reduction and functionalization.46−49 TGA Analysis. Thermal stability study was carried out on GO and reduced graphene oxide functionalized hybrid nanocomposites. Figure 4 shows the TGA curves of rGO-Hmn, rGO-Nphl, and rGO-Fcn nanocomposites when compared with GO. In all the cases, the weight loss below 100 °C can be attributed to the deintercalation of water. Weight loss from 160−300 °C in GO ascribes from the thermal decomposition of oxygenated functional groups. It is evident from the thermogram that all reduced graphene oxide functionalized hybrid nanocomposites exhibit higher thermal stability compared to GO. All these nanocomposites that show weight loss of 10% below 200 °C illustrated the presence of the less oxygen-containing functional groups on hybrid nanocomposites. The weight loss region (280−500 °C) for all the hybrid nanocomposites could be due to decomposition of these πstacking molecules. In summary the rGO-Hmn, rGO-Nphl, and rGO-Fcn nanocomposites show higher thermal stability when compared with GO. Raman Analysis. Raman spectra were recorded for GO, rGO-Nphl1, rGO-Hmnl, and rGO-Fcn1 nanocomposites. Figure 5 shows the stacked Raman spectra of GO and reduced graphene oxide functionalized hybrid nanocomposites. The summary of the characteristic peaks D, G, and 2D is tabulated in Table 1. Existance of the band near 1575−1590 cm−l (Gband) is due to the graphitic structure (sp2 hybridized) and E2g

respectively, which confirms the successful noncovalent interaction with hemin. Figure 3B shows the stacked FTIR spectra of rGO-Nphl1 nanocomposite, GO and Nphl. Free 1amino-2-naphthol-4-sulfonic acid dispersion shows four prominent peaks. The peak near 3290 cm−1 is due to hydroxyl group (−OH−) and a peak at 3145 cm−1 is due to N−H stretching vibration. One characteristic peak at 1640 cm−1 is due to −C C− stretching vibration and the peak at 1340 cm−1 is due to SO stretching vibration. After reduction and formation of hybrid nanocomposites, the hydroxyl (−OH−) peak is reduced in intensity, and other peaks are shifted to 2950, 1660, and 1376 cm−1 respectively. Similarly, Figure 3C shows stacked FTIR spectra of rGO-Fcn1, GO, and Fcn. It is evident from the spectra that in free ferrocene, there are three main characteristic peaks. The peak near 3090 cm−1 is due to sp2 −C−H− stretching vibration, the peak near 1645 cm−1 is due −CC− stretching vibration, and a sharp peak at 1003 cm−1 is for sp2 C−H in plane bending vibration. In the case of rGO-Fcn1 nanocomposites, these peaks appeared at 3080, 1672, and 1126 cm−1. It is also interesting to note that upon reduction and noncovalent functionalization of GO with these π-stacking molecules, all the significant peaks are reduced in intensity due to the gradual removal of the oxygenated functional groups from the hybrid nanocomposites that confirms the successful reduction and noncovalent interaction. Note that we did hydrazine hydrate reduction of GO to form rGO and then interacted with different π-stacking organic molecules. It was generally observed that reduced graphene oxide functionalized hybrid nanocomposites show pronounced sharp, prominent peaks that occur near 1500 cm−1 and this may be due to some remaining oxygenated functional groups after reduction and functionalization with different π-stacking E

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in electrical conductivity of 18-crown-6 after reduction and incorporation with rGO, as 18-crown-6 is a non-π-stacking molecule, and there is an absence of π−π electron interaction with rGO. This clearly demonstrates the role of interaction of π-stacking molecules used in the present study viz. hemin, ferrocene, and 1-amino-2-naphthol-4-sulfonic acid in increasing the conductivity of the nanocomposites. Nyquist Diagram. Figure 7 shows the Nyquist plot of GO and reduced graphene oxide functionalized hybrid nanocomposites of different π-stacking molecules. In Nyquist diagram, Z′(Z cos θ) is plotted against Z″(Z sin θ) where Z′ is the real part and Z″ is the imaginary part of impedance Z, and θ is the phase angle. The intersection point of the two semicircles of the Nyquist plot correlates with the bulk resistance (Rb) of the sample. The semicircle portion at higher frequencies relates to the electron-transfer-limited process while the linear part at lower frequencies corresponds to the diffusion process. The decrease in semicircle diameter reflects the decrease in interfacial electron-transfer resistance of the system. For reduced graphene oxide functionalized hybrid nanocomposites, the semicircles are compressed corresponding to higher conductivity value of hybrid nanocomposites. This is true for all reduced graphene oxide hybrid nanocomposites thus demonstrating higher ac conductivity shown by rGO-Fcn, rGOHmn, and rGO-Nphl. Mechanistic Insight. We put forward a simple mechanism to explain the enhancement of ac conductivity (lower impedance) of rGO-Hmn, rGO-Nphl, and rGO-Fcn in comparison to GO. As on oxidation, the planar structure is distorted, and delocalization is also hindered gradually due to an introduction of oxygenated bulky functional groups on the graphene sheets.50,51 It is already reported that on functionalization electrical conductivity of MWCNT decreases due to physical structure defects and unbalanced polarization effect is due to severe condition during chemical treatment.52 Again in the next step, as we have reduced the GO to rGO, it gradually regenerates it is planar and hexagonal structures having alternate −C−C− and −CC− bonds and also delocalized of sp2 hybridized carbon electrons. Thus, π-system of rGO and different π-stacking molecules interacted making the band gap energy somewhat lesser in comparison to GO, which is responsible for enhancement in ac conductivity in the case of reduced graphene oxide hybrid nanocomposites. A schematic representation of the probable mechanistic pathway for the difference in conductivity between GO and different reduced graphene oxide functionalized hybrid nanocomposites is shown in Scheme 2. Another probable reason for the enhancement of conductivity shown is due to higher mobility and the density of π-electron and higher surface area of hybrid nanocomposites system compared to GO. This mechanism is supported by the fact that interaction of rGO with a non-π-system like 18-crown6 did not help in increasing the ac conductivity of the system.

Figure 5. Stacked Raman spectra of GO, rGO-Nphl1, rGO-Hmn1, and rGO-Fcn1 nanocomposite.

Table 1. Calculation of ID/IG Ratio of GO and Reduced Graphene Oxide Functionalized Hybrid Nanocomposite sample name

D-band (cm−1)

G-band (cm−1)

2D-band (cm−1)

ID/IG

GO rGO-Nphl1 rGO-Hmn1 rGO- Fcn1

1381 1357 1350 1355

1585 1575 1581 1578

2725 2722 2715 2724

0.23 0.30 0.91 0.16

vibrational mode of sp2 bonded carbon. The emergence of a peak near 1365−1390 cm−1(D-band) originated from a disordered structure in the carbon (sp3 hybridized). The ID/ IG ratio indicated the amount of disorder present in the nanosheets. It is evident from Table 1 upon reduction and functionalization the degree of disorder changed that confirmed the existence of localized sp3 defects in the sp2 carbon moiety. Moreover on reduction and noncovalent functionalization, all the bands were shifted toward lower wavenumbers with respect with GO and 2D band intensities that also decreased; this indicates the successful reduction and noncovalent interaction of GO with different π-stacking molecules. Alternating Current Impedance Study. Impedance spectroscopy was carried out on GO, rGO-Nphl, rGO-Hmn, and rGO-Fcn nanocomposites. The plots were recorded in the frequency range between 42 Hz to 1 MHz at 1 V, which is the default voltage of the system. The measurements were carried out at room temperature and at relative humidity of 55%. The samples were taken in the form of a thin film sandwiched in between two symmetric stainless steel (SS) electrodes. The geometry of the cell for measurement of conductivity was SS| GO/noncovalent functionalized rGO|SS. Figure 6A−C shows log Z (impedance) versus log f (frequency) of GO and rGO-Hmn, rGO-Nphl, and rGO-Fcn nanocomposites, respectively. These nanocomposites show higher electrical conductivity in comparison to GO. GO shows a lower value of electrical conductivity due to structural disorder upon incorporation of different bulky groups on it. However, on interaction with different π-stacking molecules, total surface area of hybrid nanocomposites increases and there is also enhancement in π-electron mobility in the system due to the regeneration of planar structure of rGO after reduction. This is the reason for the higher electrical conductivity shown by noncovalently functionalized reduced graphene oxide hybrid nanocomposites. It is also interesting to note that the order of ac conductivity follows the order rGO-Fcn > rGO-Hmn > rGO-Nphl > GO. Impedance studies were carried out on 18crown-6 (which do not have π-electron mobility) functionalized reduced graphene oxide and log Z versus log f plot is shown in Figure 6 E. It is evident from Figure 6E, that there is no change

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CONCLUSION In this work, electrical properties of graphene and graphene with different π-stacking organic molecules were studied. Different π-stacking organic molecules viz. Hmn, Nphl, and Fcn were used successfully to synthesize rGO-Hmn, rGO-Nphl, and rGO-Fcn nanocomposites. These noncovalently functionalized reduced graphene oxide nanocomposites show higher ac conductivity than GO. The enhancement of conductivity shown can be attributed to higher mobility and the density of π-electron and higher surface area of hybrid nanocomposites F

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Figure 6. log Z versus log f plot of (A) GO and rGO-Hmn, (B) GO and rGO-Nphl, (C) GO and rGO-Fcn, (D) GO, rGO-Nphl3, rGO-Hmn3, and rGO-Fcn3, and (E) GO and rGO-18-crown-61.

Scheme 2. Schematic Representation of the Probable Mechanism for Difference in Conductivity of GO and Reduced Graphene Oxide Functionalized Hybrid Nanocomposites

Figure 7. Nyquist plot of GO, rGO-Hmn1, rGO-Nphl1, and rGOFcn1.

system. The reason is supported by the fact that the interaction of rGO with non-π-system like 18-crown-6 did not help in increasing the ac conductivity of the system. Thus, the electrical

properties of graphene can be tuned through noncovalent interaction with π-stacking organic molecules. G

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91 361 2279909. Tel: +91 361 2912073. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Science and Engineering Research Board (SERB), New Delhi, for Project Grant SB/S1/ PC- 69/2012 and the BRNS, Mumbai, Grant 34/14/20/2014BRNS. M.J.D. wants to thank SERB, New Delhi, for fellowship.

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DOI: 10.1021/acs.jpcc.5b12403 J. Phys. Chem. C XXXX, XXX, XXX−XXX