Self-assembly of thiolated graphene oxide onto a gold surface and in the supramolecular order of discotic liquid crystals

Abstract

Graphene oxide can be covalently functionalized with thiol molecules to produce thiolated graphene oxide which shows interesting behaviour of self-assembly on gold surfaces and in the supramolecular structures of discotic liquid crystals. The formation of a self-assembled monolayer on the gold surface was confirmed by electrochemical, XPS, SEM, and grazing angle IR studies. Supramolecular nanocomposites of anthraquinone based discotic liquid crystals with thiolated graphene oxide were characterized using SAXS, POM, DSC and Cryo-SEM. Such self-assembling supramolecular nanocomposites open up new and interesting possibilities for fundamental studies as well as for optoelectronic applications.

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1. Introduction

Graphene, with its two-dimensional structure and fascinating physical properties, is emerging as a potential candidate for various device applications.^1–5 The chemical route for the preparation of graphene is by chemical exfoliation of graphite to graphene oxide by oxidation followed by the reduction of exfoliated graphene oxide with hydrazine. Interestingly, in recent times graphene oxide has also gained wide attention owing to its special physical and chemical properties and the ease with which it can be prepared and processed in solution. This is partly due to the structure of graphene oxide with oxygen functionalities such as epoxy, hydroxyl and carboxyl groups on the basal plane and edges hindering the close stacking of layers.^6,7 This prevents the sheets from aggregating and makes them easily dispersible in aqueous medium and this makes the individual layers to behave like macromolecules in solution. Graphene oxide has been shown to have the potential for device applications such as in organic solar cells^8,9 and organic light-emitting diodes.¹⁰ It is also examined as a saturable absorber in ultrafast lasers, fluorescent biosensor, cellular imaging and a drug delivery agent in cancer therapeutics.^11–15 The functional groups present on the surface and edges of graphene oxide viz. epoxy, hydroxyl, carbonyl and carboxyl, can be used to further modify the electronic properties of graphene oxide^16,17 and also to improve processability of graphene oxide sheets.¹⁸

Self-assembled graphene oxide layers on gold surface may open up several interesting possibilities for physical studies, including potential device applications in organic electronics and for biosensing. It was recently reported that it is possible to reduce graphene oxide immobilized on the surfaces by chemical treatment with hydrazine or by heat treatment, leading to the formation of graphene films.¹⁹ It may be pointed out that all the previous reports were aimed at adsorption of graphene oxide onto alkanethiol monolayers and their electrochemical studies.²⁰ For example, ionic self-assembly of negatively charged graphene oxide onto oppositely charged self-assembled monolayers (SAM)^21,22 and self-assembly of graphene oxide at water–air interface²³ have been demonstrated in literature.

Due to non-covalent interactions in functionalized disc-shaped molecules, supramolecular columnar structures are formed; these are referred to as discotic liquid crystals (DLCs). DLC's are of enormous scientific interest because of their extraordinary anisotropic charge migration properties. As these systems are of great fundamental and technological importance, significant research work is going on around the globe which has been reviewed in several articles, for example see ref. 24–44. A majority of discotic liquid crystals are derived from polycyclic aromatic cores such as, triphenylene, anthraquinone, coronene, phthalocyanine, etc., which possess strong π–π interactions favouring columnar stacking of the molecules. In the columnar mesophase of DLCs, aromatic cores are oriented in columns separated by aliphatic hydrocarbon chains. The intra-columnar (core–core) separation in a columnar mesophase is usually of the order of 0.35 nm while the intercolumnar (neighbouring columns) separation is generally in the range of 2–4 nm, depending on the length of flexible chains. Therefore, intracolumnar interactions are much stronger than intercolumnar interactions.

Previously we have reported the synthesis, characterization and physical properties of discotic liquid crystals doped with various metallic, semiconducting and carbonaceous nanostructures like, gold nanoparticles,^45,46 CdSe and CdTe quantum dots^47,48 carbon nanotubes,^49,50 functionalized graphene nanoparticles⁵¹ and functionalized graphene.⁵² The dispersion of these nanostructures in DLCs does not impart much effect on their mesomorphism but improves physical properties significantly.

Here, for the first time we show that by covalent functionalization of the graphene oxide sheets with thiol groups, it is possible to form self-assembled films of thiolated graphene oxide on gold substrates. Formation of monolayer was confirmed by X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM) and grazing angle infra-red (IR) studies, while electrochemical studies show interesting electron transfer barrier properties.

In our previous study we had shown that octadecylamine edge functionalized reduced graphene oxide formed an ordered sandwich like layered structure with anthraquinone discotic liquid crystal,⁵² here in this paper we explore the effect of surface functionalized graphene oxide on anthraquinone discotics which are characterized using small angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), polarized optical microscope (POM) and Cryo-SEM.

2. Experimental section

2.1. Chemicals and instruments

All the solvents and reagents used were of AR grade. Graphite powder (325 mesh), H₂SO₄ (conc. 80%), nitric acid (conc. 80%), sodium nitrate (NaNO₃), hydrochloric acid (conc. 80%), hydrogen peroxide (H₂O₂), potassium permanganate (KMnO₄), 3,7-dimethyloctylbromide, dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP) and 30% ammonia water (NH₃·H₂O), thiophenol, and hexadecanethiol were purchased from Aldrich. All aqueous solutions were prepared in ultrapure Millipore water.

Fourier transform infrared (FT-IR) spectra were recorded as KBr discs on Shimadzu FTIR-8400. X-Ray diffraction studies (XRD) were carried out on unoriented samples using Cu-Kα (l = 1.54 Å) radiation from a Rigaku Ultrax 18 rotating anode generator (5.4 kW) monochromated with a graphite crystal. The samples were held in sealed Lindemann capillary tubes (0.7 mm diameter) and the diffraction patterns were collected on a two-dimensional Mar research image plate. Elemental analysis was performed on Carlo-Erba Flash 1112 analyser. High Resolution Transmission Electron Microscope images of samples were recorded with JEOL JEM 2100 HRTEM. SEM images were recorded with Ultra plus FE-SEM. XPS of the samples were recorded in an ESCA-3 Mark II spectrometer (VG Scientific Ltd., England). Raman spectra were recorded with LabRAM HR High Resolution Raman spectrometer (Horiba Jobin Yvon, USA), using a He–Ne laser (λ = 632.8 nm). DSC of liquid crystals and composites were taken at the temperature range of 5 °C to 150 °C using Perkin-Elmer Pyris-1 DSC. Optical textures of mesophases were observed using Olympus BX-51 Polarized Optical Microscope provided with a heating stage (Mettler FP82HT) and a central processor (Mettler FP90).

2.2. Thiolation of graphene oxide

The graphite oxide (200 mg) obtained by the Hummers method (procedure for synthesis of graphene oxide is included in ESI†) is powdered and added to a mixture of thiol (was taken in excess to ensure complete functionalization) and potassium tert-butoxide in a molar ratio of 1 : 1.2. NMP (200 mL) which was added to above mixture and maintained at 50 °C for 15 minutes accompanied with stirring to ensure deprotonation. This mixture is sonicated for 5 minutes to disperse the graphene oxide in the reaction medium. The reaction is carried out at 50 °C accompanied by vigorous stirring for an hour. After the reaction, the unreacted thiolate ions are neutralised by treating with dilute H₂SO₄. The reaction mixture is taken in a separating funnel with water and dichloromethane mixture. The thiolated graphene oxide gets suspended in between the water and dichloromethane bilayer, the water and dichloromethane are separated. The thiolated graphene oxide present at the interface is collected and is filtered and washed with plenty of water and dichloromethane to remove ionic impurities and unreacted thiol. Thiolated graphene oxide was then dispersed in DCM by sonication and this was centrifuged at 5000 rpm, the top solution is drained, thiolated graphene oxide is dried and re-dispersed in water by sonication. The dispersion is centrifuged at 5000 rpm and the thiolated graphene oxide is collected and dried. Two thiol compounds, thiophenol and hexadecanethiol are used separately for functionalization of graphene oxide.

2.3. Procedure to form self-assembled thiolated graphene oxide films

The gold strip on which monolayer is to be formed is sonicated multiple times in Millipore water and acetone for cleaning and then is immersed in the solution of thiolated graphene oxide dispersed in ethanol by sonication and kept for monolayer formation for 24 hours, with the solution being frequently sonicated to ensure that dispersed thiolated graphene oxide does not settle down.

2.4. Procedure for electrochemical studies of self-assembled films

The gold disc electrode of area 0.008 cm² sealed in glass is used for the formation of monolayer. The electrode preparation and surface pre-treatment are similar to the procedure earlier reported,⁵³ as reported in ESI.† The electrochemical gold oxidation and reduction are carried out in 0.1 M HClO₄, this is carried out by cycling the potential between 0 and 1.4 V. Reductive stripping of thiolated graphene oxide modified gold electrode in 0.5 M KOH solution is carried out by cycling the potential between 0 and −1.4 V. Electrochemical characterizations are performed in a solution of 1 mM potassium ferrocyanide and 1 mM potassium ferricyanide with 1 M sodium fluoride as a supporting electrolyte. A conventional three-electrode electrochemical cell is used for cyclic voltammetry (CV) and impedance studies. A platinum foil of large surface area is used as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The cell is cleaned thoroughly before each experiment and kept in a hot air oven at 100 °C for at least 1 h before the start of the experiment. Cyclic voltammetry study is carried out by scanning the potential between −100 to 500 mV vs. SCE. The impedance measurements are carried out by applying an ac voltage of 10 mV amplitude at the formal potential of the redox couple. A frequency range of 100 kHz to 100 mHz is used for impedance measurements.

2.5. Preparation of composites

The rufigallol-based room temperature DLC namely 1,5-dihydroxy-2,3,6,7-tetrakis(3,7-dimethyloctyloxy)-9,10-anthraquinone (AQ) was synthesized as reported by our group⁵⁰ to disperse the thiol functionalized graphene oxide. Dispersion was carried out by sonicating a dichloromethane solution of AQ and thiolated graphene oxide for 30 min followed by removal of the solvent under vacuum. The dried composites were heated to isotropic state and cooled at the rate of 2 °C min⁻¹ to room temperature. Two composites of hexadecanethiol functionalized graphene oxide (HDT-GO) and thiophenol functionalized graphene oxide (TP-GO) were prepared. 1% by wt. of HDT-GO in AQ (1HDT-GO/AQ), 5% by wt. of HDT-GO in AQ (5HDT-GO/AQ), 1% by wt. of TP-GO in AQ (1TP-GO/AQ) and 5% by wt. of TP-GO in AQ (5TP-GO/AQ) were prepared.

3. Result and discussions

3.1. Characterization of functionalized graphene oxide

We have acquired the IR spectra of hexadecanethiol modified graphene oxide and thiophenol modified graphene oxide C–S stretching vibrations at 723 cm⁻¹ (Fig. S1†) and 721 cm⁻¹ (Fig. S2†) respectively indicating the presence of C–S bond, which is absent in IR spectra of graphene oxide (Fig. S3†). The rest of the IR spectrum of functionalized graphene oxide was similar to graphene oxide. Solid state ¹³C NMR spectra of hexadecanethiol functionalized graphene oxide (Fig. S4†) shows peaks around 70 ppm which can be attributed to sp³ carbon attached to oxygen moieties. There are also peaks from 125 ppm to 145 ppm assigned to aromatic carbons; these are typical of graphene oxide.^17,53 Additionally there are peaks around 30 ppm which can be assigned to methylene groups of hexadecanethiol bonded to graphene oxide. The elemental analysis data of all the three samples are given in the Table 1. The thiolated derivatives show the presence of sulfur in elemental analysis, while the unmodified graphene oxide does not show any sulphur content. The analysis values of graphene oxide are similar to values observed in literature.^54,55

Table 1 Elemental analysis data of graphene oxide (GO), hexadecanethiol functionalized graphene oxide (HDT-GO) and thiophenol functionalized graphene oxide (TP-GO)

Sample	C (%)	H (%)	N (%)	S (%)
GO	46.43	2.12	0	0
HDT-GO	50.316	1.89	0	0.23
TP-GO	51.4305	1.51	0	0.2625

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The functionalized and un-functionalized graphene oxide were characterized using XRD and HR-TEM. The XRD patterns of the graphene oxide shows a peak which corresponds to d-spacing of 0.877 nm. This d-spacing is attributed to distance between two graphene oxide layers having water molecules trapped in between them, which extend hydrogen bonding to oxide functionalities present on graphene oxide. The XRD of graphene oxide carried out at 110 °C show decreased d-spacing of 0.63 nm primarily due to removal of these sandwiched water molecules. The XRD patterns of hexadecanethiol and thiophenol modified graphene oxide do not show any similar prominent peak corresponding to the above spacing. This is expected as the hydrophobic alkyl chains present on the surface do not allow water molecule to form hydrogen bonds and the bulky substituents do not allow two graphene sheets to come close. XRD patterns of graphene oxide and modified graphene oxide are included in ESI (Fig. S5†).^2,56 The HRTEM of unmodified graphene oxide (Fig. S6†) shows graphene sheets with 4–5 μm in dimensions, in some places the layers show folding and overlapping on other sheets. The hexadecanethiol modified graphene oxide images (Fig. S7†), show the modified graphene oxide are smaller in size, same goes with the thiophenol modified graphene oxide. Overlapping of sheets is less in functionalized graphene oxide sheets (Fig. S7 and S8†). Raman spectra of unmodified graphene oxide shows D-band at 1340 cm⁻¹ and G-band at 1584 cm⁻¹. Hexadecanethiol modified graphene oxide shows D-band at 1340 cm⁻¹ and G-band at 1584 cm⁻¹. Thiophenol modified graphene oxide shows D-band at 1340 cm⁻¹ and G-band at 1584 cm⁻¹ (spectras are included in ESI, Fig. S9† (a–c)). The unmodified graphene oxide showed I_D/I_G of 0.939, while hexadecanethiol modified graphene oxide showed an I_D/I_G of 1.018 and similarly thiophenol modified graphene oxide showed an I_D/I_G of 1.033, there is a small increment to the I_D/I_G in functionalized sample compared to the pure graphene oxide, this can be due to imperfections created at the surface of the graphene oxide by the thiol functional groups.

3.2. Characterization of modified gold electrode

Modified gold electrode was characterized using XPS, grazing angle IR, and electrochemical analysis and FE-SEM. The XPS was recorded to characterize self-assembled thiolated graphene oxide on gold surface; C1s regions (Fig. 1a) showed three overlapping peaks typical of graphene oxide. The peak at binding energy of 285 eV corresponds to C–C and C–H bonds while the peak at 286.5 eV is due to the C–O bond, while peaks at 289–290 eV can be assigned to C=O and O=C–OH. The O1s spectra show a peak at 532.85 eV which can be assigned to C–OH. The peak at 162.25 eV (Fig. 1b) corresponding to S2p region is that of thiol bound to gold. The absence of any peak at 164 eV suggests the absence of unbound thiol.^19,57–61

Fig. 1 XPS spectra of hexadecanethiol functionalized graphene oxide monolayer on gold, (a) C1s region and (b) S2p region.

The hexadecanethiol functionalized graphene oxide monolayer on gold showed peak at 730 cm⁻¹ suggest the presence of C–S bond on gold surface (Fig. 2). The thiophenol modified graphene oxide monolayer on gold also showed peak at 742 cm⁻¹ assigned to the presence of C–S bond on gold surface. Bare gold electrode showed no presence of the above mentioned peaks (Fig. S10†).

Fig. 2 Grazing angle IR of hexadecanethiol functionalized graphene oxide on gold showing the sulfur peaks.

Electrochemistry is a well-established method for probing the self-assembled films of organic thiol molecules on noble metal surfaces.⁶² In order to confirm the presence of thiolated graphene oxide films on gold electrode we carried out electrochemical studies in 0.1 M HClO₄ by cycling the potential between 0 and 1.4 V. Fig. 3 shows the cyclic voltammogram of both unmodified gold electrode (a) and thiolated graphene oxide modified (b). From the CV it is observed that the peak corresponding to the gold oxidation occurs at 1.1 V while the modified electrode has a peak at 1.2 V suggesting that there is an inhibition of oxidation due to the presence of graphene oxide layer which causes the oxidation potential to shift to higher potential. The current due to gold oxide reduction peak of modified electrode (Fig. 3b) occurs at 0.83 V, is almost same as that of the bare gold electrode. This is due to the fact that the thiolated graphene oxide layer is desorbed after the first cycle when the potential is scanned up to 1.2 V. Fig. 4 shows different cycles of gold oxide stripping. Desorption of graphene oxide layer is also clear from the fact that during the first cycle, gold oxide stripping (Fig. 4a) happens at positive potential of 1.2 V, while during second and third cycles (Fig. 4b and c) the electrode behaves like bare gold electrode with peak potential at a more negative value of 1.1 V. It is known that application of positive potentials desorb the thiol monolayer due to formation of gold oxide.⁶²

Fig. 3 Cyclic voltammogram of (a) bare gold (red), and (b) thiophenol substituted graphene oxide (black) in 0.1 M HClO₄.

Fig. 4 Cyclic voltammogram of and thiophenol substituted graphene oxide modified gold electrode, (a) first cycle (black), (b) second cycle (red), (c) third cycle (blue) in 0.1 M HClO₄.

We carried out reductive stripping of thiolated graphene oxide modified gold electrode in 0.5 M KOH solution by cycling the potential between 0 and −1.4 V, Fig. 5 shows the stripping of modified electrode, first cycle (Fig. 5a) shows a broad desorptive stripping with a peak at −0.98 V corresponding to reductive stripping which is not seen in second (Fig. 5b) and third (Fig. 5c) cycles. The broad hump is due to gradual desorption with potential of several thiol groups of graphene oxide on gold electrode. Subsequent cycles do not show any desorption peak confirming the loss of graphene oxide film.

Fig. 5 Cyclic voltammogram of and thiophenol substituted graphene oxide modified gold electrode, (a) first cycle (black), (b) second cycle (red), (c) third cycle (blue) in 0.5 M KOH.

The blocking of thiolated graphene oxide modified gold electrode to electron transfer process was studied using cyclic voltammetry (Fig. 6) and electrochemical impedance spectroscopy (Fig. 7). The blocking properties of the self-assembled films of thiolated graphene oxide to electron transfer reactions have been evaluated by using the potassium ferrocyanide/ferricyanide as a redox probe. The CV of bare gold (Fig. 6a) shows reversible peaks for the redox couple while the thiolated graphene modified (Fig. 6b and c) electrodes show quasi reversible behaviour with a larger peak separation (ΔE) of 201 mV for hexadecanethiol substituted graphene oxide and 170 mV for thiophenol substituted graphene oxide and lower peak currents indicating that the self-assembled films hinders the electron transfer process; however the reaction is not completely blocked by the film. This can be due to the thin monolayer film of π electron rich graphene oxide which to some extent facilitates the electron transfer thereby preventing complete blocking of the redox process. It can also be seen that hexadecanethiol modified electrode shows higher level of blocking compared to thiophenol modified electrode as evidenced by the larger ΔE and lower peak current which can be attributed to long alkyl chains partially shielding the access of the redox probe to the graphene oxide surface.

Fig. 6 Cyclic voltammograms of (a) unmodified gold electrode (green line), (b) thiophenol functionalized graphene oxide modified gold electrode modified gold electrode (blue line), and (c) hexadecanethiol functionalized graphene oxide modified electrode (red line) in potassium ferrocyanide/ferricyanide system.

Fig. 7 Electrochemical impedance analysis and Nyquist plots for (a) bare gold (green), (b) thiophenol functionalized graphene oxide modified gold electrode (blue), (c) hexadecanethiol functionalized graphene oxide modified gold electrode (red).

The impedance plots of bare gold shows a straight line over a wide frequency window typical of a reaction which is fully under diffusion control process (Fig. 7a). The thiophenol functionalized graphene oxide modified gold electrode shows a semicircle at higher frequency representing a charge transfer process. The width of semicircle further increases in the case of hexadecanethiol functionalized graphene oxide modified gold electrode which is due to higher blocking nature of the long alkyl chains present on graphene oxide surface.

Randles type equivalent circuit was used to model the impedance plots obtained for the potassium ferrocyanide/potassium ferricyanide redox reaction. Table 2 lists the values of different components, viz., R_u, R_ct, C_dl, Q, and W, obtained by fitting the impedance data to equivalent circuits (the equivalent circuits are shown at the tops of the respective tables). Bare gold fitted well to the equivalent circuit model R(C(RW)). However, the impedance data of modified electrodes follows the equivalent circuit R(Q(RW)). As the impedance of the CPE is given by ZCPE = 1/Q(jω)n, when n = 1, purely capacitive behavior is expected (i.e., Q = C_dl). The values of n in the case of hexadecanethiol functionalized graphene oxide modified gold electrode and thiophenol functionalized graphene oxide modified gold electrode are 0.905 and 0.77, respectively, showing deviation from purely capacitive behavior.

Table 2 Values of different components of the equivalent circuit R(C(RW)) obtained for gold electrode and the equivalent circuit R(Q(RW)) obtained for thiophenol functionalized graphene oxide and hexadecanethiol functionalized graphene oxide modified gold electrode, from the fitting of impedance data for potassium ferrocyanide/potassium ferricyanide redox reaction in aqueous medium

Sample	Ru (Ω cm^2)	Q (Ω^−1 cm^−2)		Cdl (μF cm^−2)	Rct (Ω cm^2)	W (Ω cm^2)
		S^n	n
Bare gold	0.592	—	—	40.89	1.33	3.99 × 10^−7
TP-GO	0.505	1.93 × 10^−4	0.77	—	77.57	2.74 × 10^−7
HDT-GO	0.632	4.91 × 10^−5	0.905	—	177.09	3.93 × 10^−7

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FESEM image of bare gold strip and that of hexadecanethiol functionalized graphene oxide self-assembled to the gold strip are shown in Fig. 8a and b. The surface of bare gold is featureless as seen in Fig. 8a, whereas hexadecanethiol functionalized graphene oxide modified gold electrode shows layered sheets of graphene oxide present on its surface as seen in Fig. 8b showing clearly self-assembly of thiolated GO on gold electrode.

Fig. 8 FESEM images of (a) bare gold electrode, and (b) bare gold electrode modified with self-assembled layer of hexadecanethiol functionalized graphene oxide.

3.3. Characterization of nanocomposites of functionalized graphene oxide with anthraquinone discotic liquid crystal

Self-assembly of functionalized graphene oxide in discotic liquid crystal were studied using POM, DSC, SAXS and Cryo-SEM. The mesomorphic and thermal behaviour of nanocomposites formed by dispersing 1%, 5% HDT-GO and TP-GO in 1,5-dihydroxy-2,3,6,7-tetrakis(3,7-dimethyloctyloxy)-9,10-anthraquinone (AQ) was investigated by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). Fig. 9a–d shows the typical polarizing optical micrographic textures for the composites 1 HDT-GO/AQ, 5 HDT-GO/AQ and 1TP-GO/AQ, 5TP-GO/AQ respectively. All the samples exhibit classical textures of columnar mesophase at room temperature. These textures are typical of columnar hexagonal phase and look similar to the POM image of AQ (Fig. S11†). Unlike the pure AQ, doped composites cleared to isotropic state at much lesser temperatures. 1 HDT-GO/AQ composite melted at 80.1 °C, 5 HDT-GO/AQ composite melted at 82.4 °C, 1TP-GO/AQ composite melted at 72.2 °C, and 5TP-GO/AQ composite melted at 73.6 °C. This decrease in isotropic transition temperatures of the composites point towards disordered mesophase in composites. The surface functionalized graphene oxide induces non stabilizing interactions between the functionalities on the graphene. Hexadecanethiol functionalized graphene oxide melt at higher temperature compared to thiophenol functionalized graphene oxide, this reveals that van der Waals interaction between the alkyl chains of AQ and that of HDT-GO induces small amount of stabilization, which is absent in the case of TP-GO composites.

Fig. 9 Polarizing optical microscope image of the columnar phase of composites (a) 1% HDT-GO/AQ, (b) 5% HDT-GO/AQ, (c) 1% TP-GO/AQ and (d) 5% TP-GO/AQ (at room temperature, crossed polarizers).

Fig. 10a and b shows the DSC plots for the AQ, 1HDT-GO/AQ, 5HDT-GO/AQ and 1TP-GO/AQ, 5TP-GO/AQ nano-composites recorded at a rate of 10 °C min⁻¹. The enthalpy of the phase transition is decreases from 6.38 J g⁻¹ in pure compound to 0.65 J g⁻¹ and 0.49 J g⁻¹ in 1% HDT-GO/AQ and 5% HDT-GO/AQ respectively. Decrease in the enthalpy of transition is due to disorganization in mesophase induced by thiolated graphene oxide. We observe that there is a greater disorganization in the thiophenol functionalized graphene oxide composites, this is due to destabilizing interaction between, thiophenol groups present on graphene oxide surface and alkyl chains of AQ. Both the nanocomposites of 1% TP-GO/AQ and 5% TP-GO/AQ show very small enthalpy of transition to isotropic state and thus no observable peak is seen in their DSC trace. Which clearly reveals that the thiolated graphene-discotic system is disordered. The DSC data obtained are presented in Table 3.

Fig. 10 DSC curve of nano-composites of: (a) AQ (black), 1% HDT-GO/AQ and 5% HDT-GO/AQ, and (b) AQ, 1% TP-GO/AQ and 5% TP-GO/AQ.

Table 3 DSC results of undoped DLC AQ and 1% and 5% composites dispersed with hexadecanethiol (HDT-GO). Col_h = columnar hexagonal mesophase, I = isotropic

Composites	Thermal transition (°C)/Enthalpy (J g^−1)
	Heating scan	Cooling scan
AQ	Colh 116.86(6.38) I	I 112.38(6.01) Colh
HDT-GO/AQ 1%	Colh 89.20(0.65) I	I 79.39(0.065) Colh
5%	Colh 88.69(0.49) I	I 79.55(0.047) Colh

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The liquid crystalline phases of the nanocomposites were further studied by SAXS study. The SAXS pattern of composite and pure AQ are recorded at room temperature and the intensity vs. 2θ diffraction pattern for HDT-GO/AQ and TP-GO/AQ nano-composites are shown in Fig. 11a and b. In the small angle region the diffraction peaks in both nanocomposites follow the typical Bragg diffractions of AQ DLC with d-spacing in the ratio of 1: 1/√3: 1/2:1/√7, typical of 2-dimentional columnar hexagonal phase in small angle reason for first four peaks. The results suggests that the columnar hexagonal order of discotics is not affected on dispersion of thiolated graphene oxide.

Fig. 11 Intensity vs. 2θ plots for: (a) AQ, 1% HDT-GO/AQ and 5% HDT-GO/AQ and (b) AQ, 1% TP-GO/AQ and 5% TP-GO/AQ (SAXS pattern were recorded at room temperature).

Cryo-SEM images of the composites and pure liquid crystals were acquired to study the ordering of nanocomposite, we observe spherical aggregates in pure liquid crystalline system (Fig. 12a). Both the doped composites showed layered structure owing to functionalized graphene oxide sheets, one could observe the presence of liquid crystalline domains on these layered structures. The ordering of liquid crystals on the sheet is not very clear from these images. Cryo-SEM reveals the layered structures which are absent in pure liquid crystal (Fig. 12b–e). The layered structures observed in Cryo-SEM images of all the composites can be pictured as in the given 2D model (Fig. 13a and b).

Fig. 12 Cryo-SEM images of (a) AQ (b) 1% HDT-GO/AQ, (c) 1% TP-GO/AQ, (d) 5% HDT-GO/AQ and (e) 5% TP-GO/AQ.

Fig. 13 Schematic interpretation of self-assembly of (a) hexadecanethiol functionalized graphene oxide in AQ, (b) thiophene functionalized graphene oxide in AQ.

4. Conclusions

In conclusion we have shown for the first time that graphene oxide can be functionalized with thiol based molecules which can self-assemble onto gold surface and in the supramolecular order of columnar mesophase of discotic liquid crystal. The SEM images show presence of layered graphene oxide sheets on the gold strip. This monolayer film acting as blanket on the gold surface has potential application in various devices applications such as sensors, organic electronic devices etc. We have also studied the self-assembly of thiolated graphene oxide in supra-molecular structures of discotic liquid crystals, the observation reveals that thiolated graphene oxide due to its functionalization on the surface induces destabilization in columnar mesophase.

Acknowledgements

Thanks to Prof. A. K. Shukla, Dr A. Roy, A. Dhason, Mrs. K. N. Vasudha and Dr H. T. Srinivasa for their help in characterization.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra06713h

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