One-Pot Microwave-Assisted Synthesis of Graphene/Layered Double Hydroxide (LDH) Nanohybrids
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A facile and rapid method to synthesize graphene/layered double hydroxide (LDH) nanohybrids by a microwave technique is demonstrated. The synthesis procedure involves hydrothermal crystallization of Zn–Al LDH at the same time in situ reduction of graphene oxide (GO) to graphene. The microstructure, composition, and morphology of the resulting graphene/LDH nanohybrids were characterized. The results confirmed the formation of nanohybrids and the reduction of graphene oxide. The growth mechanism of LDH and in situ reduction of GO were discussed. The LDH sheet growth was found to prevent the scrolling of graphene layers in resulting hybrids. The electrochemical properties exhibit superior performance for graphene/Zn–Al LDH hybrids over pristine graphene. The present approach may open a strategy in hybridizing graphene with multimetallic nano-oxides and hydroxides using microwave method.
KeywordsGraphene LDHs Nanostructures Microwave Composite materials
The combination of multidimensional nanomaterials leads to the formation of hierarchical composites that can take full advantages of each kind of components, which is an effective way for the preparation of multifunctional materials with exceptional properties. Recently, nanocarbon (e.g., carbon nanotubes and graphene sheets) has emerged as the most powerful material and was used in multifunctional hybrids for various applications [1, 2, 3]. Among nanocarbon materials, graphene has attracted a great deal of interests because of its single-atom thick, unique, and extensively conjugated structure, which exhibits intriguing properties like excellent electrical, thermal conductivities, and high stiffness [4, 5]. However, their inter-structural affinity leads to an irreversible agglomeration during their synthesis that amends intrinsic properties of graphene sheets, therefore confining their applicability . The retention of layered structure is vital for graphene nanosheets because most of their unique properties are primarily associated with individual sheets. Hence, to overcome this issue, hybridizing graphene with substrates like metals, metal oxides, and polymers are being practiced for various applications [7, 8].
Recently, decorating layered metal hydroxides like layered double hydroxides (LDHs) with different metal compositions have been studied in order to diminish the restacking interactions and to limit the aggregation in graphene nanosheets suitable for electrochemical applications [9, 10]. LDHs are brucite-like solids that are mainly constituted by two metals typically having 2 + (MII) and 3 + (MIII) or 4 + (MIV) oxidation states, octahedrally surrounded by oxo bridges and hydroxyl groups. The structure is organized as nanometer-thick layers that bear an excess of positive charge equivalent to the number of trivalent or twice the tetravalent metal compensated by anions that are located in the intergallary spaces . Because of their high surface area, variable metal compositions, and anion exchange property, LDH materials have been widely employed in a large set of applications [12, 13]. Especially, the LDHs composed of transition metals were explored as promising electrode materials in electrochemical field because of their relatively low cost, high redox activity, and environmentally friendly nature [14, 15]. Hence, hybridizing LDH nanosheets with large surface area in conjunction with thermo-electro conductive graphene can endow hybrid nanocomposites new multifunctional properties [16, 17]. However, most of the reported synthesis methods use hydrothermal process, which involves the use of high temperature aging over a long period of time, i.e., a process consuming high energy and time. Therefore, an alternative approach in synthesizing such multifunctional hybrids based on a rapid and facile method is highly desirable.
Microwave irradiation is often applied for the rapid synthesis of inorganic solids and organic synthetic reactions [18, 19]. The use of a microwave technique in LDHs synthesis over conventional hydrothermal process is gaining importance, and has shown to be a reliable technique to achieve highly crystalline layered structures. The microwave heating showed an enhancement of the crystallization rate of solids by improving the dissolution/recrystallization mechanism (Ostwald ripening), without the segregation of side phases . However, few recent reports have shed light on the use of microwave technique in preparation of graphene [21, 22]. Therefore, to the best of our knowledge, no studies related to the preparation of graphene/LDH hybrids using the microwave synthesis method have been reported.
In this work, we present a one-step synthesis method to synthesize graphene/Zn–Al LDH hybrids through microwave-assisted growth of 2D LDHs with simultaneous in situ reduction of graphene oxide (GO) to graphene under hydrothermal conditions. In this facile and rapid synthetic procedure, the exfoliated GO was reduced to graphene using in situ hydrolyzed urea (ammonia). Simultaneously, the Zn–Al LDH platelets were formed in situ and hybridized with graphene. The resulting hybrids were characterized using various physicochemical characterization techniques. Furthermore, we also demonstrated the use of graphene/Zn–Al LDHs for electrochemical applications by studying their cyclic voltammetry (CV) and galvanostatic charge/discharge measurements.
2.1 Synthesis of Reduced Graphene Oxide (RGO)/Zn–Al LDH Hybrids
All the chemicals were of analytical reagent grade and without any further purification. GO was prepared from expanded graphite by a modified Hummers method according to our previous report . Exfoliation of GO was achieved by ultrasonication of the dispersion in an ultrasonic bath (Brandsonic 2210 E-MTH).
2.2 Characterization Techniques
The surface morphology, structure, and composition of graphene and graphene–LDH nanohybrids were characterized by transmission electron microscopy (TEM, Philips CM200 with a tungsten filament ken with CCD Gatan digital camera) and X-ray diffraction (XRD, Siemens D5000). The Raman spectra were obtained by a Renishaw Raman system Model 3000 spectrometer equipped with an integral microscope (Olympus BH2-UMA). Radiation from a He–Ne laser (633 nm) was used as the excitation source. The FT-IR was recorded on a Bruker Infrared Spectrometer using 32 scans and a 4 cm−1 resolution. Thermogravimetric analysis (TGA) of the resulting hybrids was studied by a TA instruments Q500 TGA at a heating rate of 10°C min−1 under inert atmosphere. X-ray photoelectron spectroscopy (XPS) was used to control the elemental composition of the samples. All reported spectra were recorded at a 90° take-off angle relative to the substrate with a VG ESCALAB 220iXL spectrometer using the monochromatized Al Kα radiation (1486.6 eV). The electrochemical properties of the nanohybrids were measured in an aqueous system (electrolyte: 30 % KOH). CV curves were measured with an electrochemistry workstation (Princeton PARSTAT2273). Galvanostatic charge/discharge measurement was conducted with a charge–discharge tester (PCBT-100-32D, Wuhan Lixing Testing Equipment Co., Ltd. China). The galvanostatic charge–discharge tests were performed on a BTS-5 V/10 mA battery-testing instrument (Neware, China) at room temperature. A two-electrode cell was assembled for cyclic testing of cell. During the cycling process, the cells were charged at 1C for 60 min and discharged at 1C down to 1.4 V cut-off voltages.
3 Results and Discussion
Figure 1 shows the schematic representation of RGO/LDH nanohybrids synthesis using microwave technique. The colloidal dispersion containing mixture of GO and Zn–Al metal salts was magnetically stirred to enable their adsorption within expanded interlayer spacing of GO sheets. This eventually allows metal cations to interact with oxygenated moieties from GO through electrostatic attraction. Urea was then added to the above mixture, which yielded ammonia upon hydrolysis as shown in the following reaction : Open image in new window
When evolved, ammonia increased the pH, resulting in the controlled precipitation of metal ions. Therefore, the use of microwave activation is expected to energize the metal cations, and through their inherent ionic conduction to achieve a uniform bulk during heating the materials. Moreover, the excess ammonia present in the system was also used as a reducing agent for GO reduction at higher temperature under microwave irradiation. During microwave heating, well-crystalized LDH nanoplatelets were thereby synthesized and closely interacted with the RGO layers. These nanoplatelets also prohibit the stacking of graphene sheets by van der Waals force as evidenced hereafter.
3.2 Structure and Morphology
Figure 3 shows the TEM images of graphene and graphene/LDH nanohybrids. The wavy and scrolled multilayer sheets were seen in case of RGO (Fig. 3a). However, the morphological features of RGO/LDH nanohybrids (Fig. 3b) show graphene sheets intimately interacting with the round-shaped LDH platelets, which somehow resembles silk blanket. Therefore, the microwave-assisted hybridization of graphene with LDHs can control the morphology of graphene sheets, which are less scrolled and display high surface area.
3.3 TGA Result
3.4 Electrochemical Behaviors
Microwave-assisted method for hybridizing graphene with LDHs is presented. This efficient and rapid process involves the in situ reduction of GO to graphene with simultaneous growth of 2D Zn-containing LDHs using urea hydrolysis under microwave conditions. The resulting hybrids show improved electrochemical-specific capacitance, which attained maximum of 428 F g−1, almost 20 % higher than that of pristine graphene. Moreover, the RGO/Zn–Al LDH hybrids demonstrated good cycling stability. Hence, LDH nanosheets in conjunction with thermo-electroconductive graphene endow the resulting hybrid nanocomposites with new multifunctional properties. The effectiveness of the method described above for fabricating graphene/LDH nanohybrids could be of high importance for the preparation of other graphene-based hybrid nanocomposite electrode materials. Similarly, the use of microwave in fabrication of graphene/LDH nanohybrids provides a novel method for the development of new multifunctional nanocomposites on the basis of the existing nanomaterials.
The authors gratefully acknowledge “Région Wallonne,” and European Community (FEDER, FSE) and F.R.S.-FNRS (Belgium) for financial support. This research has been also funded by the Interuniversity Attraction Poles Programme (P7/05) initiated by the Belgian Science Policy Office. The authors are also thankful to Dr. M. Kasture, University of Pune, for electrochemical studies.
- 11.V. Rives (ed.), Layered Double Hydroxides: Present and Future Nova Science, NewYork, 2011Google Scholar
- 12.L. Feng, X. Duan, Applications of layered double hydroxides. Layered Double Hydroxides. 119, 193–223 (2006). doi: 10.1007/430_007
- 16.J. Memon, J.H. Sun, D.L. Meng, W.Z. Ouyang, M.A. Memon, Y. Huang, S.K. Yan, J.X. Geng, Synthesis of graphene/Ni–Al layered double hydroxide nanowires and their application as an electrode material for supercapacitors. J. Mater. Chem. A 2(14), 5060–5067 (2014). doi: 10.1039/c3ta14613h CrossRefGoogle Scholar
- 26.U. Costantino, F. Marmottini, M. Nocchetti, R. Vivani, New synthetic routes to hydrotalcite-like compounds - Characterisation and properties of the obtained materials. Eur. J. Inorg. Chem. 10, 1439–1446 (1998). doi: 10.1002/(SICI)1099-0682(199810)1998:10<1439:AID-EJIC1439>3.0.CO;2-1 CrossRefGoogle Scholar
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