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Cite this: Chem. Commun., 2014, 50, 710 Received 7th October 2013, Accepted 6th November 2013
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Mutually ordered self-assembly of discotic liquid crystal–graphene nanocomposites† Avinash B. Shivanandareddy, Suvratha Krishnamurthy, V. Lakshminarayanan and Sandeep Kumar*
DOI: 10.1039/c3cc47685e www.rsc.org/chemcomm
The room temperature anthraquinone discotic 1,5-dihydroxy2,3,6,7-tetrakis(3,7-dimethyloctyloxy)-9,10-anthraquinone (RTAQ) self-assembles in the presence of octadecylamine functionalized graphene (f-graphene) into an ordered sandwich like structure, where the discotic molecules form columnar structures on graphene sheets. Cryo-SEM and SEM images provide evidence for this ordering. This behaviour is also supported by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and conductivity studies of nanocomposites.
Nature creates fascinating supramolecular architectures using the principles of order and mobility.1 Several bio-systems, e.g., DNA, proteins, enzymes, etc., are created by hierarchical organization of small prototype discrete molecular building blocks by molecular recognition and supramolecular interactions.2,3 In materials science, liquid crystals (LCs) are excellent examples exhibiting counterintuitive combination of order and mobility.4–9 The hierarchical self-assembly of disc-shaped molecules leads to the formation of discotic liquid crystals (DLCs).10 Due to the property of one-dimensional charge migration brought about by their columnar structure, they are also referred to as molecular wires. They find applications in devices like photovoltaic solar cells, light emitting diodes, thin film transistors, sensors, etc.11–19 Graphene, the one-atom-thick, two-dimensional (2D) carbon allotrope, is the most well-known nanomaterial with exotic optical and electronic properties.20 Graphite can be chemically oxidized to obtain graphene oxide (GO),21 which unlike graphene is nonconducting, but has the potential to find wider applications due to its relative ease of synthesis and solution processability. The graphene oxide suspension when reduced using hydrazine, sodium borohydride, ascorbic acid, etc., forms reduced graphene oxide, which shows high charge mobility and therefore can be used in
Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore, 560080, India. E-mail: [email protected]; Fax: +91 80 23610492; Tel: +91 80 23610122 † Electronic supplementary information (ESI) available: Experimental section, Fig. S1–S9 and Table S1. See DOI: 10.1039/c3cc47685e
710 | Chem. Commun., 2014, 50, 710--712
device applications.22 Recently, GO was used to prepare discotic lyotropic LCs.23–25 On the other hand, discotic chromonic LCs can be used to prepare graphene sheets.26 Most of the DLCs are derived from polycyclic aromatic hydrocarbons (PAH) and, therefore, graphene may be considered as the ¨llen and co-workers have synthesized a mother of DLCs. Mu number of large aromatic cores, which are considered as nanographenes.11 Appropriate substitution of such large PAH cores with flexible aliphatic chains yields discotic liquid crystalline nanographene molecules. Increasing the size of the discotic core, as expected, increases the carrier mobility in these materials.11 We have recently put forward a different approach to improve electronic properties of DLCs by dispersing various nanomaterials.27 Previously we have described the method of dispersing carbon nanotubes,28,29 gold nanoparticles30,31 and quantum dots32 in various monomeric and polymeric DLCs. Here we report for the first time the dispersion of functionalized graphene in the supramolecular order of thermotropic DLCs. Graphene oxide (GO) is prepared by following the Hummers method21 and functionalized with octadecylamine (f-graphene) as reported by Haddon et al. (see ESI†).33 During this step reduction of GO occurs, with some defects, due to the refluxing of GO in thionyl chloride.34 The functionalization and reduction of graphene oxide are confirmed by IR spectroscopy, elemental analysis, and X-ray diffraction. IR spectra of f-graphene (Fig. S2, ESI†) show peaks at 3728 cm1 and 1690 cm1 which correspond to amide NH and amide carbonyl stretching respectively. The absence of peaks at around 1200 cm1 for f-graphene indicates the absence of epoxy groups. Elemental analysis of f-graphene (Table S1, ESI†) shows a marked increase in carbon content of graphene oxide after functionalization and reduction. The measured values are similar to those of reduced graphene oxide.35 The XRD intensity vs. 2y plot of GO (Fig. S3, ESI†) showed a peak at 11 degrees which corresponds to the spacing between two graphene oxide sheets of 8 Å. After the functionalization (and reduction) of graphene oxide the peak shifted to 23.8 degrees, which corresponds to a spacing of 3.3 Å (Fig. S4, ESI†). This confirms the reduction of graphene oxide leading to the formation of closely spaced layers of
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reduced graphene oxide. Raman spectra of GO (Fig. S5, ESI†) show a D band at 1340 cm1 and a G band at 1592 cm1. The f-graphene shows a D band at 1337 cm1 and a G band at 1591 cm1. We prepared a rufigallol-based room temperature DLC namely 1,5-dihydroxy-2,3,6,7-tetrakis(3,7-dimethyloctyloxy)-9,10-anthraquinone (RTAQ) to disperse the functionalized graphene (f-graphene) in it. The compound is synthesized following the procedure designed by Kumar et al.29 Dispersion was carried out by sonicating a dichloromethane solution of DLC and f-graphene for 30 min followed by removal of the solvent under vacuum. Two composites containing 1% f-graphene (1GrAQ) and 5% f-graphene (5GrAQ) were prepared. These DLC–graphene nanocomposites were analysed by UV-Vis spectroscopy, POM, DSC, XRD, Raman spectroscopy, and conductivity studies. UV-Vis spectra of the RTAQ are compared with composites 1GrAQ and 5GrAQ respectively (Fig. S6, ESI†). Both the pure and doped samples exhibit peaks at the same wavelength, indicating the absence of any strong charge transfer complex formation between f-graphene and RTAQ. The XRD patterns of all the compounds are recorded at room temperature and the intensity vs. 2y derived from the diffraction pattern is shown in Fig. 1D. In the small angle region four peaks pffiffiffi pffiffiffi in the ratio of 1 : 1= 3 : 1=2 : 1= 7 are seen with a strong first peak while the remaining three are weak. The ratios show that the mesophase has columnar hexagonal ordering. In the wide angle region there are two broad peaks with one centered at 4.8 Å due to liquid like chains on the periphery of the discotic molecule. The other one is a small peak at 3.34 Å corresponding to core–core separation of discotic molecules within the column in the mesophase. The doping of octadecylamine functionalized reduced graphene oxide does not change the order, with both the composites showing mesomorphism. Both composites exhibit an optical texture typical of the columnar hexagonal mesophase similar to RTAQ under a polarizing optical microscope (POM); at lower concentration (1%) aggregates of reduced graphene oxide sheets are not visible under the POM (Fig. 1A), but at
Fig. 1 (A) Polarizing optical microscope image of 1GrAQ. The photograph was taken under crossed polarizers at 200 magnification. (B) Differential scanning calorimetry of RTAQ (red), 1GrAQ (blue) and 5GrAQ (black) all recorded at a scan rate of 10 1C min1. (C) The variation of measured DC conductivity values as a function of temperature for RTAQ (black), 1GrAQ (red) and 5GrAQ (green). (D) One-dimensional intensity vs. 2y plot of RTAQ (blue), 1GrAQ (red) and 5GrAQ (green).
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Fig. 2 (a) Cryo-SEM image of RTAQ. (b) SEM image of f-graphene showing layered structures forming crumpled bundles. (c) SEM image of RTAQ (shows no prominent features). (d) SEM image of 1GrAQ showing a layered arrangement. (e) Cryo-SEM image of the 1GrAQ composite also showing a layered structure. (f) Enlarged portion of (e) showing graphene layers covered by a layer of discotics.
higher concentration (5%) these aggregates are evident, indicating that a larger amount of f-graphene could not be uniformly distributed in the discotic system. Differential scanning calorimetry (DSC) of RTAQ, 1GrAQ and 5GrAQ are recorded at a rate of 10 1C min1 to find the transition temperature and enthalpy associated with it (Fig. 1B). The parent RTAQ compound shows a sharp clearing peak at 115.6 1C. The composites 1GrAQ and 5GrAQ show a broad clearing peak at 117.3 1C and 116.7 1C respectively. An increase in clearing temperatures indicates that the doping of f-graphene stabilizes the discotic mesophase. This is due to enhanced local ordering of discotics on the surface of f-graphene owing to the p–p stacking between their aromatic cores. The Cryo-SEM images of RTAQ showed spherical structures (Fig. 2a). The SEM image of f-graphene shows crumpled sheets with layered structures (Fig. 2b), these structures are similar to images obtained for reduced graphene oxide.36,37 The SEM image of RTAQ shows no prominent features (Fig. 2c) while SEM images of 1GrAQ composites showed a layered structure (Fig. 2d). The Cryo-SEM images of composite 1GrAQ (Fig. 2e and f) also showed a layered structure unlike that of pure RTAQ, though the amount of f-graphene doped was only 1%. Layers observed in Cryo-SEM images of 1GrAQ are more ordered than those of f-graphene. The observation of layered morphology in the entire scanned region of the composite in conjunction with the POM results indicates the absence of any phase segregation of discotics. This suggests that these discotic molecules are arranged in the interlayer spacing of the f-graphene sheets. Discotic liquid crystals act like glue binding to f-graphene due to p–p interaction between aromatic cores. Upon closely observing the
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Fig. 3 Schematic illustration of sandwich like self-assembly of discotic liquid crystal–graphene nanocomposites.
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composites show higher conductivity compared to pure DLC. The f-graphene fillers act as bridges across defects resulting in efficient charge transport. The creation of sandwich like self-assembly with DLCs may widen the applicability of these nanocomposites in materials science. Ordered structures can be simply achieved through self-organization of a variety of interacting molecular components. The creation of such molecular buildings via selfassembly of LCs may lead to the induction of dynamically functional properties such as facile charge and energy migration in self-organized systems.
Notes and references Cryo-SEM images of 1GrAQ composites, one can see domains of liquid crystals present on f-graphene layers (Fig. 2f), which are absent in f-graphene SEM images. We carried out EDX analysis on the composite. The presence of both the compounds (RTAQ and f-graphene) was confirmed by EDX analysis. EDX analysis showed in most places the presence of carbon and oxygen of 1,5-dihydroxy-2,3,6,7-tetrakis(3,7-dimethyloctyloxy)9,10-anthraquinone (Fig. S8, ESI†). However, there were spots which showed the presence of nitrogen which arises due to octadecylamine functionalized graphene (Fig. S9, ESI†). High voltage (15 kV) necessary for EDX analysis caused local heating of the sample which resulted in melting of the liquid crystal and we could not fix a single spot for analysis as the composites were softening in the main region under study. Based on SEM, Cryo-SEM images, DSC and POM results, we propose a sandwich like structure, where discotic molecules arrange themselves between f-graphene sheets as shown in the model (Fig. 3). The conductivity measurements of the pure compound and composites were carried out to evaluate the effect of graphene doping on the semiconducting properties of DLC. The pure RTAQ sample showed a conductivity of 3.43 108 Sm1 at 41 1C. The sample 1GrAQ showed a conductivity of 6.8 105 Sm1 at the same temperature (41 1C). The conductivity decreases with an increase in temperature and finally reached a value of 2.98 107 Sm1 in the isotopic phase. Sample 5GrAQ shows a conductivity of 1.9 104 Sm1 at 41 1C which decreases to about 105 Sm1 at lower temperature (Fig. 1C). The enhancement of conductivity can be ascribed to ordered arrangement of discotics and f-graphene; f-graphene acts as a conductive filler which bridges the defects within the columnar matrix thus enhancing conductivity by three to four orders of magnitude. To explain the decrease in conductivity with temperature, we recorded the XRD pattern of RTAQ as a function of temperature (Fig. S7, ESI†). The core–core distance increases (from 3.37 Å to 3.44 Å) as the temperature increases from room temperature to 80 1C. This change in the core to core distance leads to a decreased overlap between discotic cores, which results in a decrease of conductivity with an increase in temperature. In conclusion, we have shown that f-graphene can be efficiently dispersed in the supramolecular order of DLCs. These composites show higher transition temperatures compared to pure DLC due to enhanced ordering of discotics on the surface of reduced graphene oxide owing to strong p–p interaction. Cryo-SEM images of the composites show that discotics and graphene are mutually ordered into a sandwich like structure. We have also found that the
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1 H. Ringsdorf, B. Schlarb and J. Venzmer, Angew. Chem., Int. Ed., 1988, 27, 113–158. 2 J. M. Lehn, Supramolecular chemistry: concepts and perspectives, Wiley-VCH, 1995. 3 G. Whitesides and B. Grzybowski, Science, 2002, 295, 2418–2421. ¨rtz, M. Draper, A. Hall, S. Sia, 4 J. Goodby, I. Saez, S. Cowling, V. Go G. Cosquer, S.-E. Lee and E. Raynes, Angew. Chem., Int. Ed., 2008, 47, 2754–2787. 5 T. Kato, Science, 2002, 295, 2414–2418. 6 C. Tschierske, Chem. Soc. Rev., 2007, 36, 1930–1970. 7 V. Percec, Philos. Trans. R. Soc. London, Ser. A, 2006, 364, 2709–2719. 8 D. Bruce, J. Goodby, J. Sambles and H. Coles, Philos. Trans. R. Soc. London, Ser. A, 2006, 364, 2567–2571. 9 H. Yu and T. Ikeda, Adv. Mater., 2011, 23, 2149–2180. 10 S. Kumar, Chem. Soc. Rev., 2006, 35, 83–109. ¨llen, Chem. Rev., 2007, 107, 718–747. 11 J. Wu, W. Pisula and K. Mu 12 L. Li, S.-W. Kang, J. Harden, Q. Sun, X. Zhou, L. Dai, A. Jakli, S. Kumar and Q. Li, Liq. Cryst., 2008, 35, 233–239. 13 S. Sergeyev, W. Pisula and Y. Geerts, Chem. Soc. Rev., 2007, 36, 1902–1929. 14 R. J. Bushby and K. Kawata, Liq. Cryst., 2011, 38, 1415–1426. 15 Q. Sun, L. Dai, X. Zhou, L. Li and Q. Li, Appl. Phys. Lett., 2007, 91, 253505. 16 X. Zhou, S. Kang, S. Kumar, R. R. Kulkarni, S. Z. D. Cheng and Q. Li, Chem. Mater., 2008, 20, 3551–3553. 17 X. Zhoua, S. Kangb, S. Kumar and Q. Li, Liq. Cryst., 2009, 36, 269–274. 18 Q. Li, Self-Organized Organic Semiconductors: From Materials to Device Applications, Wiley, 2011. 19 Q. Li, Liquid Crystals Beyond Displays: Chemistry, Physics, and Applications, Wiley, 2012. 20 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666–669. 21 S. W. Hummers and E. R. Offeman, J. Am. Chem. Soc., 1958, 80, 1339. 22 D. Chen, H. Feng and J. Li, Chem. Rev., 2012, 112, 6027–6053. 23 J. Kim, T. Han, S. Lee, J. Kim, C. Ahn, J. Yun and S. Kim, Angew. Chem., Int. Ed., 2011, 50, 3043–3047. 24 B. Dan, N. Behabtu, A. Martinez, J. S. Evans, D. V. Kosynkin, J. M. Tour, M. Pasquali and I. I. Smalyukh, Soft Matter, 2011, 7, 11154–11159. 25 Z. Xu and C. Gao, Nat. Commun., 2011, 2, 571. 26 F. Guo, A. Mukhopadhyay, B. W. Sheldon and R. H. Hurt, Adv. Mater., 2011, 23, 508–513. 27 H. Bisoyi and S. Kumar, Chem. Soc. Rev., 2011, 40, 306–319. 28 S. Kumar and H. K. Bisoyi, Angew. Chem., Int. Ed., 2007, 46, 1501–1503. 29 H. K. Bisoyi and S. Kumar, J. Mater. Chem., 2008, 18, 3032–3039. 30 S. Kumar and V. Lakshminarayanan, Chem. Commun., 2004, 1600–1601. 31 S. Kumar, S. Pal, P. S. Kumar and V. Lakshminarayanan, Soft Matter, 2007, 3, 896–900. 32 S. Kumar and L. Sagar, Chem. Commun., 2011, 47, 12182–12184. 33 S. Niyogi, E. Bekyarova, M. Itkis, J. McWilliams, M. Hamon and R. Haddon, J. Am. Chem. Soc., 2006, 128, 7720–7721. 34 N. Kumar, H.-J. Choi, Y. Shin, D. Chang, L. Dai and J.-B. Baek, ACS Nano, 2012, 6, 1715–1723. 35 I. Moon, J. Lee, R. Ruoff and H. Lee, Nat. Commun., 2010, 1, 73. 36 D. R. Dreyer, S. Park, C. W. Bielawski and R. S. Ruoff, Chem. Soc. Rev., 2010, 39, 228–240. 37 S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff, Carbon, 2007, 45, 1558–1565.
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Cite this: Chem. Commun., 2014, 50, 710 Received 7th October 2013, Accepted 6th November 2013
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Mutually ordered self-assembly of discotic liquid crystal–graphene nanocomposites† Avinash B. Shivanandareddy, Suvratha Krishnamurthy, V. Lakshminarayanan and Sandeep Kumar*
DOI: 10.1039/c3cc47685e www.rsc.org/chemcomm
The room temperature anthraquinone discotic 1,5-dihydroxy2,3,6,7-tetrakis(3,7-dimethyloctyloxy)-9,10-anthraquinone (RTAQ) self-assembles in the presence of octadecylamine functionalized graphene (f-graphene) into an ordered sandwich like structure, where the discotic molecules form columnar structures on graphene sheets. Cryo-SEM and SEM images provide evidence for this ordering. This behaviour is also supported by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and conductivity studies of nanocomposites.
Nature creates fascinating supramolecular architectures using the principles of order and mobility.1 Several bio-systems, e.g., DNA, proteins, enzymes, etc., are created by hierarchical organization of small prototype discrete molecular building blocks by molecular recognition and supramolecular interactions.2,3 In materials science, liquid crystals (LCs) are excellent examples exhibiting counterintuitive combination of order and mobility.4–9 The hierarchical self-assembly of disc-shaped molecules leads to the formation of discotic liquid crystals (DLCs).10 Due to the property of one-dimensional charge migration brought about by their columnar structure, they are also referred to as molecular wires. They find applications in devices like photovoltaic solar cells, light emitting diodes, thin film transistors, sensors, etc.11–19 Graphene, the one-atom-thick, two-dimensional (2D) carbon allotrope, is the most well-known nanomaterial with exotic optical and electronic properties.20 Graphite can be chemically oxidized to obtain graphene oxide (GO),21 which unlike graphene is nonconducting, but has the potential to find wider applications due to its relative ease of synthesis and solution processability. The graphene oxide suspension when reduced using hydrazine, sodium borohydride, ascorbic acid, etc., forms reduced graphene oxide, which shows high charge mobility and therefore can be used in
Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore, 560080, India. E-mail: [email protected]; Fax: +91 80 23610492; Tel: +91 80 23610122 † Electronic supplementary information (ESI) available: Experimental section, Fig. S1–S9 and Table S1. See DOI: 10.1039/c3cc47685e
710 | Chem. Commun., 2014, 50, 710--712
device applications.22 Recently, GO was used to prepare discotic lyotropic LCs.23–25 On the other hand, discotic chromonic LCs can be used to prepare graphene sheets.26 Most of the DLCs are derived from polycyclic aromatic hydrocarbons (PAH) and, therefore, graphene may be considered as the ¨llen and co-workers have synthesized a mother of DLCs. Mu number of large aromatic cores, which are considered as nanographenes.11 Appropriate substitution of such large PAH cores with flexible aliphatic chains yields discotic liquid crystalline nanographene molecules. Increasing the size of the discotic core, as expected, increases the carrier mobility in these materials.11 We have recently put forward a different approach to improve electronic properties of DLCs by dispersing various nanomaterials.27 Previously we have described the method of dispersing carbon nanotubes,28,29 gold nanoparticles30,31 and quantum dots32 in various monomeric and polymeric DLCs. Here we report for the first time the dispersion of functionalized graphene in the supramolecular order of thermotropic DLCs. Graphene oxide (GO) is prepared by following the Hummers method21 and functionalized with octadecylamine (f-graphene) as reported by Haddon et al. (see ESI†).33 During this step reduction of GO occurs, with some defects, due to the refluxing of GO in thionyl chloride.34 The functionalization and reduction of graphene oxide are confirmed by IR spectroscopy, elemental analysis, and X-ray diffraction. IR spectra of f-graphene (Fig. S2, ESI†) show peaks at 3728 cm1 and 1690 cm1 which correspond to amide NH and amide carbonyl stretching respectively. The absence of peaks at around 1200 cm1 for f-graphene indicates the absence of epoxy groups. Elemental analysis of f-graphene (Table S1, ESI†) shows a marked increase in carbon content of graphene oxide after functionalization and reduction. The measured values are similar to those of reduced graphene oxide.35 The XRD intensity vs. 2y plot of GO (Fig. S3, ESI†) showed a peak at 11 degrees which corresponds to the spacing between two graphene oxide sheets of 8 Å. After the functionalization (and reduction) of graphene oxide the peak shifted to 23.8 degrees, which corresponds to a spacing of 3.3 Å (Fig. S4, ESI†). This confirms the reduction of graphene oxide leading to the formation of closely spaced layers of
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reduced graphene oxide. Raman spectra of GO (Fig. S5, ESI†) show a D band at 1340 cm1 and a G band at 1592 cm1. The f-graphene shows a D band at 1337 cm1 and a G band at 1591 cm1. We prepared a rufigallol-based room temperature DLC namely 1,5-dihydroxy-2,3,6,7-tetrakis(3,7-dimethyloctyloxy)-9,10-anthraquinone (RTAQ) to disperse the functionalized graphene (f-graphene) in it. The compound is synthesized following the procedure designed by Kumar et al.29 Dispersion was carried out by sonicating a dichloromethane solution of DLC and f-graphene for 30 min followed by removal of the solvent under vacuum. Two composites containing 1% f-graphene (1GrAQ) and 5% f-graphene (5GrAQ) were prepared. These DLC–graphene nanocomposites were analysed by UV-Vis spectroscopy, POM, DSC, XRD, Raman spectroscopy, and conductivity studies. UV-Vis spectra of the RTAQ are compared with composites 1GrAQ and 5GrAQ respectively (Fig. S6, ESI†). Both the pure and doped samples exhibit peaks at the same wavelength, indicating the absence of any strong charge transfer complex formation between f-graphene and RTAQ. The XRD patterns of all the compounds are recorded at room temperature and the intensity vs. 2y derived from the diffraction pattern is shown in Fig. 1D. In the small angle region four peaks pffiffiffi pffiffiffi in the ratio of 1 : 1= 3 : 1=2 : 1= 7 are seen with a strong first peak while the remaining three are weak. The ratios show that the mesophase has columnar hexagonal ordering. In the wide angle region there are two broad peaks with one centered at 4.8 Å due to liquid like chains on the periphery of the discotic molecule. The other one is a small peak at 3.34 Å corresponding to core–core separation of discotic molecules within the column in the mesophase. The doping of octadecylamine functionalized reduced graphene oxide does not change the order, with both the composites showing mesomorphism. Both composites exhibit an optical texture typical of the columnar hexagonal mesophase similar to RTAQ under a polarizing optical microscope (POM); at lower concentration (1%) aggregates of reduced graphene oxide sheets are not visible under the POM (Fig. 1A), but at
Fig. 1 (A) Polarizing optical microscope image of 1GrAQ. The photograph was taken under crossed polarizers at 200 magnification. (B) Differential scanning calorimetry of RTAQ (red), 1GrAQ (blue) and 5GrAQ (black) all recorded at a scan rate of 10 1C min1. (C) The variation of measured DC conductivity values as a function of temperature for RTAQ (black), 1GrAQ (red) and 5GrAQ (green). (D) One-dimensional intensity vs. 2y plot of RTAQ (blue), 1GrAQ (red) and 5GrAQ (green).
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Fig. 2 (a) Cryo-SEM image of RTAQ. (b) SEM image of f-graphene showing layered structures forming crumpled bundles. (c) SEM image of RTAQ (shows no prominent features). (d) SEM image of 1GrAQ showing a layered arrangement. (e) Cryo-SEM image of the 1GrAQ composite also showing a layered structure. (f) Enlarged portion of (e) showing graphene layers covered by a layer of discotics.
higher concentration (5%) these aggregates are evident, indicating that a larger amount of f-graphene could not be uniformly distributed in the discotic system. Differential scanning calorimetry (DSC) of RTAQ, 1GrAQ and 5GrAQ are recorded at a rate of 10 1C min1 to find the transition temperature and enthalpy associated with it (Fig. 1B). The parent RTAQ compound shows a sharp clearing peak at 115.6 1C. The composites 1GrAQ and 5GrAQ show a broad clearing peak at 117.3 1C and 116.7 1C respectively. An increase in clearing temperatures indicates that the doping of f-graphene stabilizes the discotic mesophase. This is due to enhanced local ordering of discotics on the surface of f-graphene owing to the p–p stacking between their aromatic cores. The Cryo-SEM images of RTAQ showed spherical structures (Fig. 2a). The SEM image of f-graphene shows crumpled sheets with layered structures (Fig. 2b), these structures are similar to images obtained for reduced graphene oxide.36,37 The SEM image of RTAQ shows no prominent features (Fig. 2c) while SEM images of 1GrAQ composites showed a layered structure (Fig. 2d). The Cryo-SEM images of composite 1GrAQ (Fig. 2e and f) also showed a layered structure unlike that of pure RTAQ, though the amount of f-graphene doped was only 1%. Layers observed in Cryo-SEM images of 1GrAQ are more ordered than those of f-graphene. The observation of layered morphology in the entire scanned region of the composite in conjunction with the POM results indicates the absence of any phase segregation of discotics. This suggests that these discotic molecules are arranged in the interlayer spacing of the f-graphene sheets. Discotic liquid crystals act like glue binding to f-graphene due to p–p interaction between aromatic cores. Upon closely observing the
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Fig. 3 Schematic illustration of sandwich like self-assembly of discotic liquid crystal–graphene nanocomposites.
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composites show higher conductivity compared to pure DLC. The f-graphene fillers act as bridges across defects resulting in efficient charge transport. The creation of sandwich like self-assembly with DLCs may widen the applicability of these nanocomposites in materials science. Ordered structures can be simply achieved through self-organization of a variety of interacting molecular components. The creation of such molecular buildings via selfassembly of LCs may lead to the induction of dynamically functional properties such as facile charge and energy migration in self-organized systems.
Notes and references Cryo-SEM images of 1GrAQ composites, one can see domains of liquid crystals present on f-graphene layers (Fig. 2f), which are absent in f-graphene SEM images. We carried out EDX analysis on the composite. The presence of both the compounds (RTAQ and f-graphene) was confirmed by EDX analysis. EDX analysis showed in most places the presence of carbon and oxygen of 1,5-dihydroxy-2,3,6,7-tetrakis(3,7-dimethyloctyloxy)9,10-anthraquinone (Fig. S8, ESI†). However, there were spots which showed the presence of nitrogen which arises due to octadecylamine functionalized graphene (Fig. S9, ESI†). High voltage (15 kV) necessary for EDX analysis caused local heating of the sample which resulted in melting of the liquid crystal and we could not fix a single spot for analysis as the composites were softening in the main region under study. Based on SEM, Cryo-SEM images, DSC and POM results, we propose a sandwich like structure, where discotic molecules arrange themselves between f-graphene sheets as shown in the model (Fig. 3). The conductivity measurements of the pure compound and composites were carried out to evaluate the effect of graphene doping on the semiconducting properties of DLC. The pure RTAQ sample showed a conductivity of 3.43 108 Sm1 at 41 1C. The sample 1GrAQ showed a conductivity of 6.8 105 Sm1 at the same temperature (41 1C). The conductivity decreases with an increase in temperature and finally reached a value of 2.98 107 Sm1 in the isotopic phase. Sample 5GrAQ shows a conductivity of 1.9 104 Sm1 at 41 1C which decreases to about 105 Sm1 at lower temperature (Fig. 1C). The enhancement of conductivity can be ascribed to ordered arrangement of discotics and f-graphene; f-graphene acts as a conductive filler which bridges the defects within the columnar matrix thus enhancing conductivity by three to four orders of magnitude. To explain the decrease in conductivity with temperature, we recorded the XRD pattern of RTAQ as a function of temperature (Fig. S7, ESI†). The core–core distance increases (from 3.37 Å to 3.44 Å) as the temperature increases from room temperature to 80 1C. This change in the core to core distance leads to a decreased overlap between discotic cores, which results in a decrease of conductivity with an increase in temperature. In conclusion, we have shown that f-graphene can be efficiently dispersed in the supramolecular order of DLCs. These composites show higher transition temperatures compared to pure DLC due to enhanced ordering of discotics on the surface of reduced graphene oxide owing to strong p–p interaction. Cryo-SEM images of the composites show that discotics and graphene are mutually ordered into a sandwich like structure. We have also found that the
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