Multifunctionality in graphene decorated with cobalt ...

Materials and Design 101 (2016) 204–209

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Multifunctionality in graphene decorated with cobalt nanorods O. Mondal a, S. Mitra b, A. Datta c,⁎, D. Chakravorty b, M. Pal d a

Department of Physics, M.U.C. Women's College, Burdwan 713104, India MLS Prof's Unit, Indian Association for the Cultivation of Science, Kolkata 700032, India c University School of Basic and Applied Sciences, Guru Gobind Singh Indraprastha University, New Delhi 110075, India d Sensor and Actuator Division, CSIR-Central Glass and Ceramic Research Institute, Kolkata 700032, India b

a r t i c l e

i n f o

Article history: Received 1 November 2015 Received in revised form 16 March 2016 Accepted 25 March 2016 Available online 7 April 2016 Keywords: Cobalt-graphene nanocomposites Superparamagnetic Photoluminescence

a b s t r a c t Creating multifunctionality is always fascinating and required for various advance applications. We have prepared cobalt-graphene nanocomposite by co-reduction of graphite oxide and a divalent cobalt salt in water, at room temperature, by sodium borohydride. Microstructural study by high resolution transmission electron microscope (HRTEM) delineates that the nanocomposites consists of few layer of graphene sheets decorated by cobalt nanorods. The composite shows unusual photoluminescence in visible range due to defect decoration by cobalt nanorods, in spite of showing expected absorbance and magnetic properties. Functional nanomaterials with both magnetic and luminescence properties are important for application in biological systems. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Graphene is an interesting two-dimensional sheet which consists of single layer of sp2 hybridized carbon atoms. Graphene has been attracting extensive scientific interest due to its extraordinary electronic, mechanical and thermal properties. The extraordinary properties include high carrier mobility exceeding 104 cm2/Vs [1], high thermal conductivity of 103 W/mK [2], extremely high surface area of 2630 m2/g [3], and large value of Young's modulus ~1 TPa [4].A great deal of scientific attention has been generated towards the optical properties of graphene because of the insight they provide into the excited states of this remarkable material, and potential that they offer for novel applications [5,6]. Perfectly exfoliated single layer graphene sheet is a zero band gap semiconductor, and is therefore not likely to show any photoluminescence. In such a material, carriers can fully relax through rapid electronelectron and electron-phonon interactions which are much faster processes compared to light emission. However, if ultrafast pump-probe processes are applied, some light emission can be observed in graphene [7]. Therefore designing photoluminescent graphene based nanocomposite material is a major challenge of optoelectronics. Recently, glucose-derived water-soluble crystalline graphene quantum dots (GQDs) with an average diameter as small as 1.65 nm (5 layers) were prepared by a facile microwave-assisted hydrothermal method, which exhibited deep ultraviolet (DUV) emission of 4.1 eV, the shortest emission wavelength among all the solution-based quantum dots. The emission wavelength is independent of the size of the GQDs. The unique ⁎ Corresponding author. E-mail address: [email protected] (A. Datta).

http://dx.doi.org/10.1016/j.matdes.2016.03.130 0264-1275/© 2016 Elsevier Ltd. All rights reserved.

optical properties of the GQDs are attributed to the self-passivated layer on the surface of the GQDs [8]. Green luminescent graphene quantum dots (GQDs) with a uniform size of 3, 5, and 8.2(±0.3) nm in diameter were prepared electrochemically from MWCNTs in propylene carbonate by using LiClO4 at 90 °C, whereas similar particles of 23(±2) nm were obtained at 30 °C under identical conditions [9]. In fact, the combination of such an extraordinary material with functional nanomaterials may lead to interesting nanocomposites for a variety of applications. For this reason, composites comprising of graphene (G) decorated with metal [10,11], metal oxides [12–14] and sulphides [15,16]have also attracted a great deal of attention recently. Yang et al synthesized Co-graphene nanocomposites (Co-G) by pyrolysis of organometallic-graphene composite precursor at 800 °C in Ar atmosphere [17]. Ji et al synthesized Co-G through in situ reduction in ethylene glycol medium at 110 °C in N2 atmosphere [18]. Chen et al synthesized porous Co-G nanocomposite by annealing graphene/Co3O4 nanocomposite at 350 °C under Ar/H2 gas flow [19]. Nevertheless, there is no single report of preparation of Co-G nanocomposite at room temperature in ordinary atmosphere. In this paper, we report the synthesis of Co-G nanocomposite by co-reduction of graphite oxide (GO) and salt of Co2 + in solution, at room temperature and in ordinary atmosphere. We have observed strong violet-blue emission in the composite, which is not observed till date in metal-G nanocomposites. A Co-CoO-graphene nanocomposite is recently reported to have very high catalytic activity for electrochemical reduction of oxygen [20]. Synthesis of this composite is quite different from our procedure and the shape and size as well as the Co-CoO nanoparticle decoration of G sheets is different. However, this readily shows the importance of these classes of materials.

O. Mondal et al. / Materials and Design 101 (2016) 204–209

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2. Experimental details

3. Results and discussion

2.1. Materials

3.1. Morphological study

Natural flake graphite was purchased from Loba Chemie. All the other reagents used in the experimental process were analytical grade, purchased from Merck and used without further purification.

The morphology of Co-G nanocomposite was examined using transmission electron micrographs (TEM). A typical micrograph is presented in Fig. 2(a). The figure shows a thin graphene sheet embedded with Co nanoparticles (higher electron density portions on the G sheet). It is clearly visible that the G sheet is very thin with folded edges. The weak contrast between the background and G sheet confirms the thinness of the sheet. Interestingly, the shape of the Co nanoparticles are not spherical but are in form of one-dimensional rod-like structure of average diameter ~3 nm and average length ~30 nm (Fig. 2(b)). The high resolution transmission electron micrograph (HR-TEM) of Co-G sheet confirms the rod-like structure of Co nanoparticles (Fig. 3(a), (b)) and shows the lattice fringe with d-spacing 0.21 nm corresponding to (111) plane of Co. The HR-TEM also depicts that the non-agglomerated nature of nanorods.

2.2. Methods GO was prepared from natural graphite using modified Hummers method [21]. In the typical synthesis of Co-G, 0.04 g GO was dispersed in 30 ml water using ultrasonication to form stable dispersion. 30 ml aqueous solution of hydrated cobalt nitrate (Co(NO 3 )2 ·6H2 O) (a maximum of 5 mM) was formed separately. Two solutions were mixed under constant stirring condition to form homogeneous solution and kept in an ultrasonicator. Subsequently, sodium hydroxide (NaOH) solution and 0.14 g sodium borohydride (NaBH4) was added. After 30 min, a black precipitate was formed which was collected by centrifugation and washed several times with water and ethanol. The precipitate was left to dry in vacuum and the dried powder was used for further characterization. The flowchart of synthesis procedure is presented in Fig. 1. For comparison, graphene was also synthesized by reducing GO with NaBH4under similar conditions.

2.3. Characterization The synthesized samples were characterized using different techniques. Fourier transform infra-red (FTIR) and UV–Vis spectra of the samples were recorded using Shimadzu FTIR 8400S and Varian Cary 5000 UV–Vis–NIR spectrophotometers respectively. The morphology was visualized using JEOL 2010 Transmission Electron Microscope (TEM) operated at 200 kV and equipped with a detector for energy dispersive X-ray spectroscopy study. X-Ray diffraction (XRD) patterns of the powder samples were obtained using Rigaku Mini Flex Benchtop X-Ray diffractometer. Magnetic properties were investigated using a superconducting quantum interference device (SQUID) magnetometer (supplied by Quantum Design, USA) over the temperature range 2–300 K. Photoluminescence spectrum of Co-G nanocomposite at room temperature was recorded on F-2500 FL spectrometer, Hitachi.

3.2. X-ray diffraction study To confirm the purity of the cobalt phase in the nanocomposite, XRD patterns of the samples were recorded. Fig. 4 presents the XRD patterns of GO, G and Co-G. The peaks in the sample Co-G can be indexed to cubic phase of Co (JCPDS No-15-0806) and graphene. This further strengthens our argument that the Co is present in metallic form as peaks corresponding to oxide are absent. The intensity of peaks of Co phase is relatively less compared to that of graphene, as the fraction of Co is relatively low compared to graphene. The elemental composition of Co-G nanocomposite was determined from energy dispersive spectrum (EDS) and is presented in Fig. 4(b). The spectrum confirms the presence of elements C, Cu and Co. The element Cu is attributed to the carbon coated copper grid on which the sample for TEM was cast. The weight fraction of Co compared to C was found to be within 10%. The result of XRD study corroborates well with the result of EDS. 3.3. FT-IR spectroscopy The FT-IR spectra of GO, G and Co-G are presented in Fig. 5. The FT-IR spectrum of GO shows the presence of C\\O (alkoxy) stretching bond (~ 1065 cm− 1), the C\\O (epoxy) stretching peak at (~ 1227 cm− 1), C=C stretching bond (~ 1634 cm− 1) and stretching mode of C=O (~1725 cm−1) [22]. The peak at 1390 cm−1 corresponds to deformation of O\\H group of adsorbed water molecules respectively. It can be clearly seen that the characteristic absorption bands of oxide groups in GO (~ 1725 cm− 1) is almost absent in G and Co-G, confirming the complete reduction of GO. The peaks corresponding to C=C and C\\C aromatic bonds (~1629 cm−1 and ~1500 cm−1) remain the most prominent signatures in G and Co-G. The presence of peak in the range 650– 570 cm−1 in all the three samples corresponds to C\\H out-of-plane bending for substituted benzene ring, which further confirms the presence of aromatic rings in the samples. 3.4. UV–Visible absorbance spectroscopy

Fig. 1. Flowchart of synthesis procedure.

Fig. 6 shows the UV–Visible optical absorption spectra of GO, G and Co-G nanocomposite dispersed in ethanol. The peak within the range 255–270 nm is present in all the three samples and corresponds to π → π* transitions of aromatic C=C rings [23]. The peak at 256.4 nm in GO shifted towards red on reduction, as is observed in G and Co-G nanocomposite. The presence of this particular peak in Co-G confirms that carbon network is not interrupted in presence of Co nanoparticles. The Co-G nanocomposite shows an additional broad peak at around 398.2 nm which is attributed to surface plasmon resonance (SPR) of Co nanoparticles [24], arises from oscillation of conduction band electrons in presence of interacting electromagnetic

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Fig. 2. Transmission electron micrograph of (a) Co-G nanocomposite, (b) G sheets showing embedded Co nanorods.

field. The combined effect of graphenic structure and Co nanoparticles is reflected in the optical spectrum of Co-G nanocomposite. 3.5. Photoluminescence properties Fig. 7 shows the room temperature photoluminescence spectrum of Co-G nanocomposite after exciting the system with wavelength 250 nm. Various emissions in UV and visible region are observed in Co-G sample. The PL peaks at 326, 350 and 381 nm originates from radiative recombination of electron–hole pair which are generated at the defect sites of graphene [23]. Additionally, strong violet emission (~ 402 nm) and blue emission (~ 426 nm) are observed. The room temperature photoluminescence spectrum of G is presented in the Inset of Fig. 7. The sample does not exhibit luminescent property which is obvious since graphene has no band gap and photoluminescence is not expected from relaxed charge carriers. But there are various reports on photoluminescence of reduced GO [23,25,26] in UV region which arises due to manipulation of relative fraction of sp2 and sp3 hybridized domains. Pasricha et al have shown that Ag nanoparticles may get trapped inside two monolayers of GO, while forming the Ag-GO nanocomposite, resulting into a more stable structure emitting stable PL in the visible range, at similar position to that of graphite powder [27]. Simple graphite powder is known to show discrete sharp peaks in the visible region. GO can show similar kind of photoluminescence when a nanocomposite is prepared with silver, attributed to destabilized system, while changing from bulk to nano form. Although, by this method, there is large increase of surface area of the resultant composite, it could still retain characteristic photoluminescence of graphite structure.

Metallic nanoparticles interact with the GO sheets through physisorption, electrostatic binding, or through charge-transfer interactions [10], which then modify the local electronic structure. Under favorable condition, this leads to photoluminescence in GO. A broad photoluminescence (PL) are usually observed from pure GO in solid as well as liquid phase [28], the origin of which is not yet clearly understood. However, by fine tuning of the relative sp2/sp3 hybridization it was shown to be possible to produce blue luminescence from GO, even without metal nanoparticle decoration [23]. It is understood, that in order to generate photoluminescence, one needs localization of sp2 structure, either by sp3 network, or by defects, with local modification of the defect levels with metal nanoparticle. Distortion of sp2 network may also produce confinement of well crystalline sp2 island, while G is created by reduction. The better crystalline the sp2 network is, less is the chance to generate photoluminescence. Optoelectronic properties of carbon related materials mostly depend on π states of sp2 hybridization [29]. In GO, π and π* states of sp2 clusters are localized within the band gap of σ and σ* states created by sp3 network. The structure of GO is more like a nanocomposite of these two network, with sp2 island with crystalline nature, sitting inside sp3 network. The localized electron-hole recombination based on the relative size of these hybridizations generates tunable photoluminescence. The graphene based QDs [8,9] showing photoluminescence also indicate, confinement of well crystallized sp2 crystal structure. As these QDs are water soluble/dispersed, so by some passivation they are rendered from collapsing on each other. In our study, when chemically reduced GO is produced from such system by borohydride treatment, sp3 hybridization along with epoxide and hydroxyl group may get removed to a large extent. But this restoration

Fig. 3. High resolution transmission electron micrographs of Co nanorod showing (a) crystal planes, (b) (111) plane of Co.


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Fig. 4. (a) XRD patterns of GO, G and Co-G nanocomposite. (b) energy dispersive spectrum of Co-G nanocomposite.

process by itself may leave behind distortion and defects, resulting into localization. The advantage with GO is its solution processability. When chemically reduced GO is produced from GO in water, two disadvantages appear in the resulting materials. The graphene sheets tend to collapse on each other as the surface of graphene sheet is highly hydrophobic [30]. As a result, quick precipitation due to agglomeration happens for the chemically reduced GO in water due to strong van der Waal interaction and multilayer reduced GO is produced. Usually, collapse of many graphene sheets onto one another, provides faster path for charge carrier to relax. The strong reduction process induced by sodium borohydride (NaBH4) in the composite system is known to introduce various kinds of topological defects in the restored sp2 islands in the sp3 limited carbon structure [31]. Among these defects, the most likely candidate for blue emission is the pentagonal rings, which occurs more frequently [32]. Another possible source may be due to effectively much smaller multiple sp2 islands, which results into this violet and blue emission based on the effective size [23], as in order to produce such emissions much smaller sp2 islands are required. Random electrostatic attachment may also promote formation of such islands. Additionally, there might be some plasmon induced modulation of the emission, where these sp2 islands or pentagonal ring type or other similar defects may act very similar to fluorescent molecules whose emission spectra strongly responds to the aspect ratio of the nanorod. The intensity of this plasmon-induced peak increases as the size of the nanorod is increased [33]. The decoration of G sheet by cobalt nanorods actually reduces the collapsing tendency, keeping the composite materials suspended in water for longer time. We would also like to point out that this soft

As the modification is different in our case compared to the previously reported Co-G or Co-CoO-G nanocomposites [10.18–20], a different magnetic property is expected compared to these reports. Magnetic properties of the Co-G and G were studied by using zero-field-cooling (ZFC) and field-cooling (FC) procedures and field-dependent magnetization measurements. Measurements of the ZFC, FC magnetization as a function of temperature were performed between 2 K and 300 K under an applied field of 500 Oe and 1000 Oe for Co-G and G respectively and the results are shown in Fig. 8. Bifurcation of ZFC and FC curve is observed in the temperature range 2–9 K suggesting the presence of unblocked ferromagnetic state in this range. It reflects the superparamagnetic nature of the nanocomposite. The blocking temperature (TB) determined from ZFC and FC study is found to be around 8.8 K, whereas, the G synthesized by chemical reduction of GO is diamagnetic in the whole temperature range. Thus by embedding Co nanoparticles on graphene sheets superparamagnetic property can be induced which may be used as contrast agent for MRI.

Fig. 5. FT-IR spectra of GO, G and Co-G nanocomposite.

Fig. 6. UV–Vis absorbance spectra of GO, G and Co-G nanocomposite.

method is an excellent process to stabilize nanoparticles without functionality on the surface during the formation of nanocomposites [27]. In the resulting nanocomposite, Co nanoparticles are therefore creating a situation, where this relaxation could not take place. As a result, the localization of the network could promote the photoluminescence. Additionally, these embedded Co nanoparticles modify the electronic states of graphene inducing photoluminescence in the nanocomposite, although they themselves are not photoluminescent. The modification of electronic states by Co nanoparticles is confirmed from photoluminescence and UV–Vis absorbance spectra. 3.6. Magnetic properties

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Fig. 7. Room temperature photoluminescence spectrum of Co-G nanocomposite. Inset: Spectrum of G.

The M versus H plot (Fig. 9) below TB proves the ferromagnetic behavior of the composite at lower temperatures The M-H loop at 2 K have finite coercivity of 590 Oe (Inset of Fig. 7) whereas the M-H loops above 10 K does not show any coercivity or remanent magnetization which further confirm the superparamagnetic nature of the samples. In this context, we would like to add a comment that Ji et al have observed the graphene nanosheets are well decorated with ~ 3 nm Co nanoparticles [19]. Almost no free Co nanoparticles were detected outside the graphene sheets, indicating the perfect combination between Co nanoparticles and G nanosheet are very densely populated. Although the particle size is small enough to show superparamagnetism, the nanocomposite was found to show room temperature ferromagnetism. This might have resulted from clustering of Co nanoparticles. This is the reason we kept the concentration of Co-nanoparticles rather low deliberately. Our intention is to create a stable fluid suspension of the composite, which may lead to a very interesting ferrofluid based on water. We are currently working on the optimization of parameters for such a purpose. In this context, we would like to comment that although it appears from the TEM picture that the graphene sheets in our samples are not decorated by Co nanorods homogeneously, but they are clustered in some areas, but they are not agglomerated. In case of agglomeration, the close interactions of various nanorods would have forced the system to be ferromagnetic in nature [19]. Further magnification hints towards some gaps among the nanorods,

Fig. 9. M versus H loops of Co-G nanocomposite. Inset: Magnified view of M-H loop at 2 K.

which is supported by the existence of superparamagnetic response. We would also extrapolate our argument further by noting that a real agglomeration would have been detrimental to its photoluminescence behavior, where instead of surface plasmon based enhancement, we would have observed decrease or even quenching of the phenomena. Although, a much better control is shown in the work by Guo et al. [20] on Co-CoO nanoparticles which show superparamagnetism at room temperature, unfortunately, the magnetic property of the nanocomposite is not reported [10]. Thus we can see that a small variation in preparation techniques for these kinds of nanocomposites can lead to large variations in microstructure, optical and magnetic properties as well. In fact, large simulation effort is needed to address that how metal nanoclusters will decorate the G sheets in these composites. 4. Conclusion In summary, we have prepared Co-graphene nanocomposite by co-reduction of GO and Co2+ salt solution at room temperature which produces unusual strong violet and blue emission. The FT-IR and UV– Vis spectra of the composite are evidence of the presence of graphenic structure in the nanocomposite. The composite is superparamagnetic in nature with blocking temperature around 8.8 K. The Co-graphene nanocomposite has strong possibility to find applications in optoelectronics, catalysis, biological labeling and MRI contrast enhancer. Acknowledgements M. Pal acknowledges Council for Scientific and Industrial Research (CSIR), New Delhi, for financial support. Authors also thank Prof. P. Kumbhakar, NIT Durgapur and Dr. S Chatterjee, IIT BHU for extending the emission measurement (PL) facility. Authors also acknowledge Dr. B. Satpati, SINP Kolkata for extending the TEM facility and Neelam Singh, USABS New Delhi for help in experimental methods. References

Fig. 8. ZFC-FC curve of Co-G nanocomposite and G sheets in presence of 500 Oe field. Inset: Magnified form of ZFC-FC curve of Co-G nanocomposite.

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