Morphological effects of single-layer graphene oxide in the formation ...

Author's personal copy J Nanopart Res DOI 10.1007/s11051-011-0459-z

RESEARCH PAPER

Morphological effects of single-layer graphene oxide in the formation of covalently bonded polypyrrole composites using intermediate diisocyanate chemistry Raymond L. D. Whitby • Alina Korobeinyk • Sergey V. Mikhalovsky • Takahiro Fukuda • Toru Maekawa

Received: 20 December 2010 / Accepted: 9 June 2011 Ó Springer Science+Business Media B.V. 2011

Abstract Single-layer graphene oxide (SLGO) possesses carboxylic and hydroxyl groups suitable for reactions with aliphatic or aromatic diisocyanate molecules. TEM analysis reveals that aliphatic diisocyanate molecules caused SLGO to scroll into star-like formations, whereas aromatic diisocyanate molecules retained SGLO in a flat-sheet morphology. TGA confirms the stabilisation of the formed urea and urethane groups on SLGO, but the onset of sheet pyrolysis occurs at a lower temperature due to isocyanate reactions with anhydride and epoxide groups embedded in the sheet. Pendant isocyanate groups act as bridging units to facilitate the attachment of pyrrole molecules, which are then used as anchor sites for the covalent polymerisation of pyrrole to polypyrrole (PPy). The use of FeCl3 as the polymerisation catalyst generated both covalent and free PPy, but also iron hydroxide nanoparticles were observed decorating the SLGO surface. When using ammonium persulfate as a catalyst and dodecylbenzenesulfonate as a dopant, free PPy could be removed under treatment with solvents to leave a R. L. D. Whitby (&) A. Korobeinyk S. V. Mikhalovsky Nanoscience & Nanotechnology Group, University of Brighton, Lewes Road, Brighton BN2 4GJ, UK e-mail: [email protected] T. Fukuda T. Maekawa Bio-Nano Electronics Research Centre, Toyo University, 2100 Kujirai, Kawagoe, Saitama 350-8585, Japan

purely covalent system. Discrete regions of SLGO were observed decorated with nanoparticles of PPy along the edge or across the surface of individual sheets. It was found that the flexibility of the SLGO sheet and the type of diisocyanate used directly affected the electrical resistance of the final composite. Keywords Single-layer graphene oxide Diisocyanate chemistry Polypyrrole Morphology changes Thermogravimetric analysis

Introduction The chemistry of nanocarbon materials has generated significant interest with the wide-scale production of carbon nanotubes (dos Santos et al. 2010; Tasis et al. 2006) and fullerenes and its application to other nanocarbons, such as nanodiamond (Behler et al. 2009), nano carbon black (Oh et al. 2009) and nanoribbons (Cai et al. 2010), is increasing. Nanomaterial-based composites have been developed to release the enhancement of key properties at the nanoscale for the macroscale, which is useful for electronics, mechanical reinforcement, acoustics and chemical catalysis (Englert 2007; Karn et al. 2009; Lee et al. 2001; Lee et al. 2010; Whitby et al. 2004). Understandably, the emergence of new carbon

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Author's personal copy J Nanopart Res

nanomaterials will garner similar focus. To that end single-layer graphenes (Novoselov et al. 2004), a single atom-thick sheet of sp2 hybridised carbons, represent a material that features many augmentations over carbon nanotubes, exhibiting increased electrical and thermal conductivity. However, the incorporation of SLG into composite materials requires a degree of functionalisation and chemical processing to achieve covalent grafting. Oxidised SLG (SLGO) provides a suitable material for chemical treatment due to its convenient production methods (Marcano et al. 2010) though by comparison to SLG a number of the properties are reduced in performance (Soldano et al. 2010). SLGO exhibits carboxylic, lactone and phenolic groups around the periphery and across the surface of the sheet (Gao et al. 2009). Carboxylic groups are commonly targeted molecules on carbon nanomaterials for chemical coupling (Wang et al. 2010a); however, the carboxylic reactivity of SLGO under conventional thionyl chloride or carbodiimide reactions does present drawbacks (Whitby et al. 2011). SLGO treated with the thionyl chloride are chemically etched thus losing part of their functionality, but do react with amines for covalent attachment. When treated with carbodiimide, SLGO agglomerated into spider-like clusters and exhibit almost no further reactivity with amines. Therefore, alternative approaches are desired. Conducting polymers are materials that similarly garner interest in the research and industry communities and their applications penetrate into numerous fields (Kumar and Sharma 1998). Polypyrrole (PPy) exhibits sp2 hybridised carbons and therefore exhibit electrical conductivity, which can be controlled according to the type and degree of the dopant added (Wang et al. 2001). Polymerisation of pyrrole can be orchestrated through electrical or chemical catalysis. In particular, iron (III) trichloride-mediated polymerisation has a potential advantage in that the catalyst may also be incorporated as a dopant (Hulea et al. 2005). Alternatively, ammonium persulfate oxidative polymerisation also has advantages in that dopants can be used to render the final PPy solvent soluble, which is useful for purification and controlled casting (Lee et al. 1995). Isocyanates and diisocyanates have been previously used in the functionalisation and composite synthesis of graphene oxide platelets (Kim et al. 2010; Stankovich et al. 2006; Hua et al. 2010; Kim et al.

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2011; Liu et al. 2009; Wang et al. 2010b; Xu et al. 2008; Yang et al. 2011; Zhang et al. 2009). Herein, we report the generation of PPy from acidic functional groups on SLGO, where diisocyanate molecules operate as a bridging unit, to generate solvent insoluble and soluble PPy. The final composites are characterised by TEM, TGA and IR analysis to monitor the reaction formation of PPy on SLGO.

Experimental SLGO–isocyanate SLGO (CheapTubes Inc) was purified by sonicating in distilled water and methanol, spin centrifuging, washing repeatedly in methanol and drying from acetone in vacuo. Dried SLGO (25 mg) was dispersed in dimethylformamide (DMF) (25 mL) to which either excess 1,6-diisocyanatohexane (hexamethylene diisocyanate, HDI) (6.2 mmol) (1) or 2,4-diisocyanato-1-methylbenzene (toluene diisocyanate, TDI) (6.9 mmol) (2) was added and the mixture was refluxed under nitrogen (80 °C, 4–8 h). SLGO–isocyanate was isolated under centrifugation and repeatedly washed in dry DMF. SLGO–pyrrole SLGO–isocyanate (1) or (2) was dispersed in dry DMF and the temperature lowered to 0–2 °C. Excess pyrrole (7.2 mmol) was added and allowed to stir (4 h) after which the temperature was raised to 20 °C (12 h). SLGO–pyrrole (3) for the HDI system and (4) for the TDI system was isolated by centrifugation and washed in DMF. SLGO–PPy SLGO–pyrrole (3) or (4) (20 mg) was dispersed in dichloromethane (DCM), an excess of pyrrole added (15 mmol) and stirred vigorously at 0–2 °C. Anhydrous iron (III) trichloride (20 mg) was dispersed into dry DCM (20 mL) and slowly added to the SLGO– pyrrole mixture and stirred for 8 h. The mixture was washed with toluene, then methanol and finally a mixture of water and methanol and dried from acetone to obtain a solvent insoluble SLGO–PPy with either a hexane bridge (5) or a toluene bridge (6).


Alternatively, SLGO–pyrrole (3) or (4) (20 mg) was dispersed into water containing dodecylbenzenesulfonate (DBSA) (0.2 mmol) and pyrrole (15 mmol) and cooled to 0–2 °C. A solution of ammonium persulfate (APS) was added to achieve a ratio of 0.2 for APS to pyrrole. The mixture was allowed to stir for 8 h before being terminated with the addition of methanol. The mixture was washed in an excess of water, then methanol. Free PPy was removed by dissolving in DCM and SLGO–PPy was separated through centrifugation and repeated washing with DCM. The final SLGO–PPy sample with either a hexane bridge (7) or a toluene bridge (8) was washed in toluene–methanol and then methanol–water and finally dried from acetone.

Results and discussion In order to facilitate the formation of PPy, pyrrole is anchored to SLGO using diisocyanate molecules, which couple one end to carboxylic groups to form amides (Blagbrough et al. 1986) or to alcohol groups to form urethanes (Farkas and Strohm 1965) (Scheme 1, step i). The characteristic IR spectra of SLGO (Fig. 1a) indicate the presence of the oxygencontaining functional groups; bands at 1063, 1227 and 1385 cm-1 corresponds to C–O vibration of alkoxide, epoxy and hydroxyl group and the band at 1728 cm-1 to the C=O stretching vibration in –COOH group. The sp2 carbon network of SLGO is revealed via the C=C stretching vibration at 1630 cm-1 and C–C vibrations at 989 cm-1. After reaction with HDI (1), the IR spectra undergo certain changes in peak positions (Fig. 1b) corresponding to the covalent coupling. The band at 1728 cm-1 of carboxyl groups on SLGO is transformed into a wide band with maximum at 1699 cm-1 corresponding to an amide I vibration in an urethane group (Zhang et al. 2008). A new band appears at 1559 cm-1, which indicates the combined C–N stretching and CHN deformation vibrations. The band at 3313 cm-1 corresponds to the N–H stretching vibrations in an amide II group that has been shifted to a higher frequency and confirms formation of the urethane structure. Characteristic peaks of the asymmetric NCO stretching vibration also appear at 2269 cm-1. IR spectroscopy confirms the reaction of HDI to either hydroxyl and/or carboxyl groups on SLGO,

Fig. 1 FTIR spectra of (a) SLGO, (b) SLGO–HDI (1), (c) SLGO–pyrrole (3), (d) SLGO–PPy (5) and (e) SLGO– PPy (7)

resulting in pendant isocyanate species available for further chemical reactions. A similar change in the IR profile was also observed for the TDI system. Boehm titrations, which are useful for analysing chemical reactions with acidic groups on the surface of carbon nanomaterials (Wang et al. 2009), reveal that SLGO reacts with isocyantes with a conversion of 78% for HDI and 72% for TDI, demonstrating the possibility of steric hindrance of the toluene group in proximity to the surface acidic groups during the reaction. It is likely that additional HDI and TDI units have been added to the system due to its reactions with hydroxyl groups, which are not detected through Boehm titrations unless in a phenolic environment. Thermogravimetric analysis (TGA) was conducted to study the thermal stability of the sample after each stage of the reaction. The thermal profile of pure SLGO shows two significant mass losses. The first (*42%) at 150–250 °C, which corresponds to the decomposition of acidic groups that are easier to remove from the carbon surface, possibly carboxylic groups (Shen et al. 2010). There is a 10% mass loss

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Author's personal copy J Nanopart Res Scheme 1 Synthesis strategy for SLGO–PPy involves (i) coupling to diisocyanate molecules, (ii) attachment of pyrrole and (iii) polymerisation to PPy

between 220 and 660 °C, which may be due to removal of acidic groups containing within the lattice rings, such as epoxy or ether bonded oxygen. The second (*43%) at 630–740 °C corresponds to pyrolysis of the carbon backbone (Jeong et al. 2009), which occurs at a lower temperature than pure graphite (Viculis et al. 2005) or multi-walled carbon nanotubes (Ebbesen et al. 1994; Whitby et al. 2001), given that there are fewer van der Waals forces operating between individual graphene sheets with their surrounding layers. TGA profiles of chemically reacted SLGO show significant differences when compared with unreacted SLGO. The TGA curve for SLGO–isocyanate (1) shows a small mass loss (*8%) in the temperature region 120–250 °C, which might be due to the presence of unreacted carboxylic groups. Given the maximum reactivity of SLGO-COOH groups typically occurs with around 80% conversion (Whitby et al. 2011), a small number of free carboxylic groups are expected. A larger mass loss in the temperature region 280–450 °C is due to the decomposition of organic diisocyanate molecules (Zhang et al. 2009) that are attached to the SLGO sheet. Herein, the conversion of carboxyl and hydroxyl into urea and urethane groups has increased the thermal stability of the intermediate bridging groups by around 120 °C. However, the stability of the remaining sheet is severely impacted with the absence of the mass loss at 630–740 °C as featured for pure SLGO. Herein, isocyanate molecules are known to react with anhydride and epoxide groups and their reactivity with similar groups, which are likely to be embedded within the SLGO sheet, may cause facile fracturing and premature degradation of the lattice. The pendant isocyanate group was subsequently reacted with pyrrole (Scheme 1, step ii) (Papadopoulos

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1972) and then the composite was polymerised in excess pyrrole using FeCl3 or DBSA/APS (Scheme 1, step iii). A control test showed that pyrrole was not immobilised onto the surface of SLGO through van der Waals or p–p bonding interactions, thus could only participate through covalent bonding through available isocyanate groups. The aromatic nature of TDI would allow for electrical conductivity between the SLGO sheet and the PPy chain, however, its arrangement of isocyanate groups may incur steric hindrance effects when bonding to acidic groups across the surface of the SLGO sheet and to a lesser degree at the periphery of the sheet. Therefore, aliphatic HDI was also used to overcome the possible steric effects of TDI. It was found that the flexibility of the SLGO (Fig. 2a) sheet is sufficient to allow conformational changes to occur, whereby the pendant isocyanate group can bond to another acidic group on the same SLGO sheet, i.e., intrasheet bridging. This was prominent when an aliphatic diisocyanate was used and the resulting SLGO–isocyanate (1) has wrapped into spider-like clusters (Fig. 2b). It is possible that intersheet bridging has also occurred albeit to a lesser extent. However, when using an aromatic diisocyanate, the resulting SLGO–isocyanate (2) predominantly retains a near planar geometry due to steric hindrance preventing the SLGO sheet wrapping to both sides of the toluene molecule (Fig. 2c). The polymerisation of pyrrole into PPy, using either the HDI or TDI synthesis route, results in a series of interlinked spherical particles, ranging from 50 to 500 nm in diameter (Fig. 3a), which is consistent with previous studies of PPy. In SLGO–PPy (5), a significant portion of SLGO retains its spider-like conformation, though a number of sheets are found disentangled and flattened and the presence of PPy is not immediately obvious (Fig. 3b). However, on


Fig. 2 TEM image of a SLGO appearing as flat sheets occasionally overlapping with adjacent sheets and frequently possess wrinkles (inset). b SLGO–isocyanate (1) has assumed a spider-like morphology through intrasheet bonding of the aliphatic diisocyanate and dark contrast due to agglomeration.

c SLGO–isocyanate (2) retains a flat sheet conformation though frequently possessing more than one layer, thus giving a darker contrast image than SLGO. Scale bars are 2, 5 and 1 lm, respectively

Fig. 3 TEM image of a free PPy (scale bar 500 nm), b SLGO–PPy (5) (scale bar 200 nm), exhibiting c domains of PPy wrapped within or on the surface of SLGO (scale bar 100 nm) and d unwrapped SLGO sheets that are etched (scale bar 200 nm) and exhibit particles of iron oxide across the

surface (inset, scale bar 50 nm), e SLGO–PPy (6) (scale bar 500 nm) where f large SLGO sheets lie flat and are extensively decorated with PPy (scale bar 500 nm) and g SLGO has wrapped into a tight bundle and PPy particles are found in and on the SLGO surface (scale bar 200 nm)

higher magnification in the scrolled areas of SLGO, PPy particles can be observed either encapsulated by the SLGO sheet or decorating the surface (Fig. 3c). The location of PPy particles appears random in relation to the SLGO sheets and their size is usually found within 20–100 nm. Visually, the coverage of SLGO with PPy is insufficient to obtain percolation

across the sheets, revealing around 5% surface coverage in the HDI-mediated reaction with PPy. Moreover, closer examination of the unwrapped SLGO sheets shows their edges to be irregular over shorter lengths than is typical for pure SLGO and the sheet size has reduced in size (Fig. 3d). The surface is decorated with a myriad of black spots ranging in size

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up to 7 nm in diameter, distributed randomly across the surface of the sheet, which are domains of iron hydroxide (Fig. 3d, inset). These are formed from the drying of electrostatically attracted Fe3? cations to the acidic sites distributed across the surface and periphery of SLGO, which confirms similarities of its structure to the Lerf model of oxide group distribution on graphitic surfaces (Lerf et al. 1998). Trapping of the FeCl3 catalyst in this manner may have inhibited the polymerisation of PPy resulting in the lower degree of coverage found. Moreover, it also indicates that the formation of SLGO–isocyanate (1) is possibly retarded due to the spider-like geometry inhibiting complete conversion of the surface acidic groups, leaving many sites free to bind to Fe3? ions during the attempted polymerisation. This is confirmed in the analysis of SLGO–PPy (6). Herein, SLGO is still largely in a planar state and extensively decorated with PPy, covering around 25% of the surface of the sheets (Fig. 3e), which is higher than that of the HDI–PPy system probably due to the absence of reaction hindrance stemming from conformational changes in SLGO. The distribution of PPy particles is predominantly interconnected yet random across the surface of SLGO, though free PPy particles are observed present on the TEM grid away from SLGO. The extent of coverage appears to approach percolation and is located along the periphery and the surface of SLGO (Fig. 3f). Certain areas of the SLGO sheet have partly scrolled and PPy particles appear located within or on the surface of these areas (Fig. 3g). Few iron hydroxide particles are found decorating the surface of SLGO. FTIR was used to monitor the reactions of SLGO– isocyanate in the attachment of PPy. After the reaction of SLGO–isocyanate (1) with pyrrole to form SLGO–pyrrole (3), the IR spectra (Fig. 1c) shows a weak peak at 2281 cm-1 corresponding to the NCO stretching vibration of residual isocyanate groups, which fully disappear in the growth of PPy (Fig. 1d). The band of C–H and N–H in-plane deformation vibrations occurs at 1080 cm-1 for SLGO–pyrrole, which increases in intensity for SLGO–PPy (3) centred at 1040 cm-1. The band of C–H out-of-plane deformation vibrations of the pyrrole ring has a maximum at 924 cm-1 for SLGO–PPy (3) though is not present for monopyrrole attachment to SLGO–isocyanate. A similar pattern of transformations were also recorded in the IR spectra

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for SLGO–isocyanate (2) transforming to SLGO–PPy (6) (not shown). The iron (III) trichloride polymerisation of PPy methodology does have additional drawbacks in that PPy is produced both covalently attached to and freely detached from SLGO and due to the insolubility of PPy, purification of the composite is currently unachievable. This makes ascertainment of the bonding occurrence between PPy and SLGO difficult. Therefore, the formation of solvent-soluble PPy is necessary to remove free PPy from the SLGO–PPy composite. TEM analysis of SLGO–PPy (7) reveals a scrolled tip of an SLGO sheet where PPy particles are interlinked and emanate from the tip (Fig. 4a). PPy particles are also observed decorating the surface of the SLGO sheet (Fig. 4a, inset). For SLGO–PPy (8) clusters of PPy particles are found along the periphery (Fig. 4b) or across the surface (Fig. 4b, inset) of flat SLGO sheets. In both cases, no free PPy was found in the sample and no electrostatic attraction of PPy to SLGO could be discerned. Therefore, the APScatalytic polymerisation of pyrrole reveals the extent of covalent bonding of PPy to SLGO. When comparing SLGO–PPy (5) and solventsoluble SLGO–PPy (7), minor differences were noted in the IR spectra. The appearance of additional peaks between 600 and 1400 cm-1 is consistent with the presence of the surfactant species (Fig. 1e) and is similar to that previously reported for PPy–DBSA (Omastova et al. 2003). The TGA profile of SLGO–pyrrole (3) and SLGO– PPy (5) (result not shown) are similar, both reveal minor differences when compared with SLGO– isocyanate (1) (Fig. 5). Pure PPy has a similar TGA profile to SLGO that has been transformed with diisocyanate molecules, such that the major mass loss that is ascribed to the degradation of PPy or SLGO occurs between 300 and 600 °C. Therefore, any calculation of the PPy grafting ratio to SLGO by this technique is difficult to ascertain. The only clear difference obtained in this study is when a surfactant is added into the PPy network to generate solventsoluble PPy, the residual mass will depend on the degree and type of surfactant added (Omastova et al. 2003). Herein, the residual mass of the system after 600 °C is 9% (Fig. 5) and attributed to the concentration of DBSA used in the synthesis procedure. The coverage with APS-catalysed PPy is far smaller than the FeCl3-catalytic route, achieving less


Fig. 4 TEM image of a SLGO–PPy (7) where the spider-like SLGO has a number of PPy particles decorating the middle and ends (inset) of the sheet and b SLGO–PPy (8) where the SLGO

Fig. 5 TGA profiles of SLGO, SLGO–isocyanate (1), SLGO– pyrrole (3) and soluble SLGO–PPy (7) reveal the transformation of thermal stability under chemical reaction with the initial diisocyanate molecules

than 1% surface coverage in both the TDI and HDI systems. This is speculated for the HDI system to be due to the low reactivity due to the sheet’s conformational structure due to intrasheet bridging and for the TDI system possibly indicating steric hindrance in the attachment of pyrrole. In both cases, fewer pyrrole sites would be available for covalent polymerisation. Two probe resistance measurements across SLGO and SLGO–PPy compressed films were taken. Purified SLGO possessed a resistivity of *807 kX cm, which is consistent with other studies where SLGO was ascertained to be an insulator due to the high degree of acidic groups covering the surface and periphery that form an insulating barrier between adjacent sheets (Jung et al. 2008). The resistance dropped to *3.1 kX cm for

remain flat with PPy particles decorating the edges and middle (inset) of the sheet. Scale bars 200 nm

SLGO–PPy (5) and *22.4 kX cm for SLGO–PPy (6), which is still far greater than that of pure PPy, *2 X cm (Park et al. 2002). In both samples, PPy was present in unattached and attached forms and given that SLGO acts as an insulator it is understandable that the continuity of electron propagation across the sample will be hindered by SLGO domains. The difference in resistance values may therefore reflect the conformational state in which SLGO is present. For SLGO–PPy (5), SLGO retains it scrolled morphology, which reduces the area of sheet interaction with PPy allowing smaller hindrance. In comparison, the sheets in SLGO–PPy (6) remain flat and proffer maximum area of interruption, thus possessing a higher resistance value. It was not possible to obtain resistance measurements for SLGO–PPy (7) and (8), which might be due to the electrostatic attraction of ammonium cations to acidic sites on SLGO keeping the system electrically insulated.

Conclusions SLGO–PPy composites could be covalently generated through a variety of well-established chemical techniques, however, the flexibility of SLGO does present a challenge due to the conformational changes that occur through intrasheet bonding, particularly when using aliphatic diisocyanates. This can be (partly) obviated by using aromatic diisocyanates, but their steric hindrance limits coupling of the

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pendant pyrrole required to anchor the PPy chain. Nevertheless, PPy was covalently coupled to the surface and periphery of chemically functionalised SLGO, as demonstrated through the generation of soluble PPy where the free polymer could be removed from the composite. The conformation of SLGO within the sample has been shown to affect the overall electrical resistance of the composite. Acknowledgments We thank the RCUK Academic Fellowship UK and the Marie-Curie Industry-Academia Partnerships and Pathways Agreement (FP7-PEOPLE-IAPP2009-251429-UNCOS).

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