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Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11612−11620

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Novel and Highly Efficient Strategy for the Green Synthesis of Soluble Graphene by Aqueous Polyphenol Extracts of Eucalyptus Bark and Its Applications in High-Performance Supercapacitors Saikumar Manchala,†,‡ V. S. R. K. Tandava,† Deshetti Jampaiah,§ Suresh K. Bhargava,*,§ and Vishnu Shanker*,†,‡ †

Department of Chemistry, National Institute of Technology Warangal, Warangal, 506004 Telangana, India Centre for Advanced Materials, National Institute of Technology Warangal, Warangal, 506004 Telangana, India § Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Science, RMIT University, GPO BOX 2476, Melbourne, Victoria 3001, Australia

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‡

S Supporting Information *

ABSTRACT: The sustainable synthesis of high-quality graphene sheets is one of the hottest and most inspiring topics in the fields of science and engineering. While the graphene oxide (GO) chemical reduction method is widely used to synthesize graphene sheets, this route commonly includes highly hazardous reducing agents that are dangerous to both humans and the environment. In this context, here we describe a green, effective, and economical strategy for the synthesis of soluble graphene by using a Eucalyptus polyphenol solution that is obtained from a Eucalyptus bark extract. The reducing ability of polyphenol compounds present in the Eucalyptus bark extract is responsible for the reduction of exfoliated GO to soluble graphene under reflux conditions in an aqueous medium. The XRD, FT-IR, XPS, and UV−vis results demonstrate the effective removal of the oxygen functionalities in GO. TEM and AFM images show straight corroboration for the development of 1−4 layers of graphene. The stable and homogeneous dispersion of the E-graphene in various solvents, both aqueous and nonaqueous, confirms the powerful interactions between Eucalyptus polyphenol compounds and graphene. The electrochemical performances are evaluated by cyclic voltametry (CV) and galvanostatic charge−discharge (GCD). GCD results show that the E-graphene supercapacitor has a high specific capacitance of 239 F g−1 and a high energy density of 71 W h kg−1 at a current density of 2 A g−1. These characteristics demonstrate that this green approach has an excellent prospective not only in the fabrication of high-performance supercapacitors but also in the synthesis of graphene-based materials. KEYWORDS: Soluble graphene, Green synthesis, Eucalyptus bark, Polyphenols, Cyclic voltammetry, Supercapacitor

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tion11 of graphite, and solvothermal or chemical reduction of graphene oxide12−15 toward a variety of applications, i.e., supercapacitors,16,17 biosensors,18 nanoelectronics,19 ultra sensors,20 composites,21 Li-ion batteries,22 photocatalysis,23 and fuel cells.24 Of all the synthesis methodologies, the chemical reduction of graphene oxide has gained immense importance as it provides scope for the production of graphene at a low cost in bulk quantities. However, toxic or explosive chemicals including sodium borohydride,25 hydrazine hydrate, and dimethyl hydrazine26 are commonly used for this purpose, leading to a significant environmental and health concerns. Therefore, there is an immediate demand for ecofriendly

INTRODUCTION

In recent years, the development of efficient ecofriendly strategies for the synthesis of graphene is one of the most popular areas in the world of science. Graphene, a unique twodimensional nanocarbon material with sp2 hybridized planes/ thin sheets of carbon atoms tightly confined to a honeycomb lattice, has found significant applications in the field of material science and engineering because of its versatile properties like high surface area, high mechanical strength, and excellent electrical and thermal conductivities.1−3 The first discovery of graphene in 2004,4 using the Scotch tape peeling method, has revolutionized the field. Until now, multiple methods have been employed for the synthesis of graphene such as chemical vapor deposition/plasma enhanced chemical vapor deposition (CVD/PECVD),5,6 epitaxial growth,7 electric arc discharge,8 mechanical or ultrasonic exfoliation,9,10 chemical intercala© 2019 American Chemical Society

Received: March 16, 2019 Revised: May 4, 2019 Published: June 13, 2019 11612

DOI: 10.1021/acssuschemeng.9b01506 ACS Sustainable Chem. Eng. 2019, 7, 11612−11620

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Representation for the Synthesis of Soluble E-Graphene

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alternative methods for the graphene synthesis from GO without using hazardous reagents or expensive instruments. From the past decade, several efforts have been made toward the development of green reducing agents for GO that are environmentally safe and hazardless like sugar,27 ascorbic acid,28 heparin,29 wild carrot roots,30 aloe vera,31 vancomycin,32 and bovine serum albumin.33 However, in most of the cases, the formed product showed highly agglomerated morphologies without the addition of an external stabilizer. Recently, researchers have been interested in the synthesis of solution-processable graphene, due to its many applications such as polymer composites34 and transparent conductive thin films.35 In this context, Wang et al. reported the green synthesis of soluble graphene by using a tea polyphenol solution, and it was studied for the biorelated applications.36 Therefore, it still seems to be an attractive and highly desirable research interest to develop an effective and highly economical green reducing agent for the mass production of soluble graphene. In the present work, we have chosen the bark of the Eucalyptus globulus tree, whose overall 900 species and subspecies have been found worldwide so far of which 170 are prevalent in India.37 Commercially obtained Eucalyptus oil has found immense application in therapeutics, beverages, perfumes, and flavored foods and is also used to cure bronchitis, pneumonia, and other respiratory diseases. In Asian countries, hot water is used to obtain an extract from dried Eucalyptus leaves because of its antipyretic, antiinflammatory, and analgesic remedies for respiratory infections like cold, flu, and sinus congestion.38 Eucalyptus globulus bark extract is rich in 29 polyphenolic compounds. Moreover, these are said to be biologically active compounds due to having excellent oxidant properties and anticancer properties.39−42 To the best of our knowledge, several methods have been tried for the synthesis of graphene from GO using green reducing agents, but the results show poor C/O ratio. Nevertheless, we focused on developing an easily identifiable, abundant, efficient, and highly economical reducing agent for the bulk production of graphene, based on a Eucalyptus globulus bark extract, and applied it in the fabrication of supercapacitor.

EXPERIMENTAL DETAILS

Materials. Graphite powder (Loba, 99%); KMnO4 and NaNO3 (Finar, 99%); H2SO4, HCl, and H2O2 (SDFCL, 99%); ethanol (EtOH, Finar, 99%); methanol (MeOH, Finar, 99%); dimethylformamide (DMF, Finar, 99.5%); ethyl acetate (EtAC, Finar, 99%); nhexane (Finar, 95%); dimethyl sulfoxide (DMSO, Finar, 99%); acetone (Finar, 99%); and tetrahydrofuran (THF, Finar, 99%) were used as obtained. Eucalyptus globulus’s bark was obtained from the local forest area. Double distilled (DD) water was used throughout the experiments. Methods. Details of the Eucalyptus globulus sampling and extraction procedures are provided in the Supporting Information. GO was prepared using a modified Hummer’s method based on previous reports.43 A 100 mg portion of GO was taken in a 100 mL flask containing 50 mL of Eucalyptus polyphenol extract, and the contents were ultrasonicated for 1 h to obtain a uniform suspension. Then, the suspension was magnetically stirred for 24 h in an oil bath under reflux conditions. The temperature was maintained at 80−85 °C throughout the stirring. Finally, the solution mixture was kept aside for cooling to room temperature. After cooling, the acquired black product was washed with water several times and dried overnight in an oven. The procedure for the synthesis of graphene is represented schematically in Scheme 1. The acquired Eucalyptus polyphenols reduced graphene is referred to as E-graphene. Characterization. Powder X-ray diffraction (PXRD) measurements were performed on a PAN Analytical Advance X-ray diffractometer using Ni-filtered Cu Kα (λ = 1.5406 Å) radiation in a 2θ scan range between 10° and 60°. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images were recorded on a JEOL 1010 instrument operating at 100 kV. High-resolution TEM (HRTEM) and energy-dispersive X-ray spectroscopy (EDX) studies were carried out on a JEOL 2100F microscope with an operating voltage of 200 kV. The optical properties were obtained by using an Analytikjena SPECORD 205 UV−vis spectrophotometer (UV−vis). Fourier transform infrared (FT-IR) spectra were recorded using a PerkinElmer Spectrum 100 FT-IR spectrophotometer with a transmission method from 4000 to 400 cm−1 using a KBr pellet. Atomic force microscopy (AFM) images were recorded on a Bruker Dimension Icon instrument in tapping mode. The surface elemental analysis was carried out by using X-ray photoelectron spectroscopy (XPS) (Thermo Scientific) with an Al Kα (1486.7 eV) radiation source at room temperature under ultrahigh vacuum (10−8 Pa). Electrochemical Measurements. Electrochemical measurements were evaluated on a CH Instruments electrochemical analyzer (Model CHI760E) with a conventional 3-electrode electrochemical 11613


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ACS Sustainable Chemistry & Engineering system by using 0.2 M KCl as the electrolyte solution at different scanning rates. A glassy carbon electrode (GCE) modified with the synthesized samples served as working electrodes, while the Ag/AgCl (in saturated KCl) and Pt wire served as the reference and counter electrodes, respectively. The typical modified working GCE preparation was as follows: 2.5 mg of a synthesized sample was dispersed in 2.5 mL of isopropanol−water solution and 20 μL of Nafion solution (5 wt %), which was then sonicated well at room temperature (RT) for 1 h to make a uniform slurry. The resultant slurry was deposited as a thin film onto the surface of the GCE by the drop-casting method. Finally, the modified GCE was dried at RT for 24 h.

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RESULTS AND DISCUSSION Inspired by the development of green reducing agents and 12 principles of green chemistry to use an appropriate efficient green reducing agent to reduce GO to graphene, we desired to attempt the possibility of an aqueous extract of Eucalyptus polyphenols to synthesize graphene sheets from exfoliated GO. A simple reflux method was employed by adding GO to the aqueous extract of Eucalyptus polyphenols. The reducing ability of polyphenols present in the aqueous extract of Eucalyptus may play a major role to produce few-layer graphene sheets. UV−vis spectroscopy was performed to monitor the reduction of GO by Eucalyptus polyphenol solution. As presented in Figure 1, GO shows a sharp and

Figure 2. XRD of graphite, synthesized GO, and E-graphene.

The dspacing of GO being relatively larger than that of nanographite is due to the formation of oxygen functionalities and the intercalation of water molecules between the layers of nanographite. Nevertheless, the dspacing of graphene was appreciably decreased after reduction, indicating the removal of oxygen functionalities.45 Raman spectroscopy is a robust technique for the qualitative analysis of the synthesized nanocarbon materials such as GO and E-graphene. As presented in Figure 3, the Raman spectra

Figure 1. UV−vis spectra of GO and E-graphene.

strong absorption around 235 nm corresponding to the π−π* transitions, whereas E-graphene shows a broad absorption around 270 nm corresponding to the π−π* transitions. This specifies that most of the oxygen functionalities are removed from the surface of GO, and conjugation (CC) remains stored.44 In addition, the color of the solution changed to deep black from brownish, indicating the reduction of GO solution. PXRD was carried out, to characterize the interlayer spacings and atomic structures of the GO and E-graphene samples, and the results are shown in Figure 2. The nanographite powder shows a strong and characteristic diffraction peak at 26.6° (dspacing = 0.335 nm) with a basal reflection (002). After oxidation, the GO diffraction peak transfers to a lower angle of 12.2° (dspacing = 0.725 nm). Then, after reduction with Eucalyptus polyphenol extract, in the case of graphene, the diffraction peak at 12.2° fades out while a broad peak appearing at 25° (dspacing = 0.356 nm) was noticed.

Figure 3. Raman spectra of synthesized GO and E-graphene.

of GO revealed the presence of two major characteristic peaks. The first peak appearing at 1330 cm−1 is the D-band, which can correspond to the breathing mode of A1g symmetry. Moreover, according to the reported literature, the intensity of the D-band can refer to several defects. The second peak appearing at 1582 cm−1 is the G-band, which can be attributed to phonon mode of E2g symmetry of in-plane vibrations of all sp2 carbon atoms. To understand the role of Eucalyptus polyphenol solution for the reduction of GO, the Raman experiment was employed for the E-graphene over a certain period (4, 8, and 24 h) during synthesis. As can be seen from Figure 3, it was clear that the intensity ratios of D- and Gbands increased after GO reduction. The determined intensity 11614


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ACS Sustainable Chemistry & Engineering

AFM was performed, to determine the lateral size and thickness of the graphene. Figure 5 represents the AFM images and height profile of E-graphene. Previous reports have been reported after reduction of GO wherein the thickness/height of the graphene sheet decreased possibly due to the removal of oxygen functionalities from the GO plane.49 As presented in Figure 5, it is observed that the average thickness of Egraphene was found to be 1−4 nm, consistent with previous reports where each graphene layer contributes 1 nm,36,50 which indicates that the E-graphene is composed of 1−4 layers of graphene sheets, and to know the average thickness of Egraphene, 25 pieces are chosen randomly and analyzed; the thickness distributions are shown in Figure S2, and the average thickness of E-graphene is ∼4.07 nm, indicating that the Egraphene consists of 1−4 layers. The above results suggest that the synthesized E-graphene is mostly a combination of monolayer and multiple layers of sheets, as is commonly observed for nanostructured carbon materials. XPS analysis was carried out to assess the reduction efficiency of Eucalyptus polyphenols by reducing GO to Egraphene, by analyzing the C to O ratio in the reduced graphene oxide. As is represented in Figure 6a,b, the GO and E-graphene XPS spectra reveal the existence of C and O elements. As shown in Figure 6a, the XPS spectrum of C 1s was deconvoluted into four major peaks corresponding to C C (sp2, 284.8 eV), CO (epoxy/hydroxyl, 286.8 eV), CO (carbonyl, 287.7 eV), and OCO (carboxyl, 289.5 eV). In the case of E-graphene, the peak intensities corresponding to oxygen functionalities have been decreased, and one has completely disappeared, followed by the peak corresponding to sp2 carbon increasing, as shown in Figure 6b. Furthermore, the determined C to O ratio has been increased from GO (5.06) to E-graphene (10.93), indicating the effective removal of oxygen functionalities from GO. This C to O ratio of Egraphene is much higher than that of not only recently reported green reducing agents such as caffeic acid (7.15),45 tea polyphenols (3.1),36 gallic acid (3.89−5.28),50 L-ascorbic acid (5.7),51 natural cellulose (5.47),52 tannic acid (2.44),53 and baker’s yeast (5.9)54 but also hazardous reducing agents like hydrazine monohydrate (10.3).26 The above XPS results indicate that not only is the Eucalyptus polyphenol solution an effective green reducing agent for the reduction of GO to graphene, but it is also an ideal substitute for hydrazine monohydrate. Further, the FT-IR experiment was employed for the GO and E-graphene and shown in Figure S3. As is presented in Figure S3, the FT-IR spectra of GO disclose a broad band between 3000 and 3700 cm−1 along with a sharp peak at 1634 cm−1, which are correlated to the stretching and bending frequencies of OH groups present on the GO sheet, respectively. The bands noticed at 2921 and 2852 cm−1 correspond to the symmetric and asymmetric stretching frequencies of CH, respectively. Finally, the bands at 1248 and 1024 cm−1 are ascribed to the stretching frequencies of phenolic C−O and epoxy C−O−C groups, respectively. However, after the reduction of GO by the Eucalyptus polyphenol bark extract, the absorption band intensities corresponding to oxygen functionalities have been decreased significantly. On the basis of the detailed characterization of GO and Egraphene, the oxygen functionalities present on the surface of GO were effectively removed by Eucalyptus polyphenol extract. To explain the role of polyphenols in reducing the

ratio of D- and G-bands for the GO, E-graphene (4 h), Egraphene (8 h), and E-graphene (24 h) was 0.98, 1.03, 1.06, and 1.15, respectively. The intensity ratio increased from 0.98 to 1.15 confirming the formation of E-graphene, which is wellmatched with the previous reports.46 These results suggest that the increased D to G ratio can be attributed to the removal of oxygen moieties with an enhanced number of sp3 domains and decreased number of sp2 domains.47 Therefore, the higher intensity of the D-band suggests the creation of new and small isolated graphitic domains in reduced GO compared to that of fresh GO.26 Additionally, the E-graphene exhibited a peak at 2662 cm−1, which can be attributed to the two-phonon mode with opposite wave vectors (2D-band). Overall, it can be concluded that the Eucalyptus polyphenol solution can be responsible for the efficient removal of oxygen functionalities from GO and thereby the formation of E-graphene. Thus, it would be reasonable to consider Eucalyptus polyphenol solution as a promising reducing agent for the facile reduction of GO. A TEM examination was carried out to describe the morphological characteristics of E-graphene. As presented in Figure 4a,b, it is evident that the synthesis procedure formed

Figure 4. TEM (a, b), HRTEM (c), and SAED (d) images of synthesized E-graphene.

few-layer nanosheets along with scrolled, wrinkled, and transparent features which often accompany few-layer nanosheets. Moreover, the number of 2D nanosheets of graphene cannot be determined exactly, and there are no large graphitic aggregates present, indicating the efficient formation of fewlayer graphene. The HRTEM image of Figure 4c shows an interplanar distance (dspacing) of 0.4 nm, which is approximately close to the dspacing (i.e., 0.356 nm) obtained by PXRD and the SAED pattern as mentioned in Figure 4d, showing diffraction dots of the (002) plane of E-graphene sheets, which also preferably matches with the PXRD pattern of E-graphene.48 As shown in Figure S1, the EDX and elemental mapping images reveal the presence of C and O elements in the E-graphene. 11615


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Figure 5. Typical AFM images and line profiles of E-graphene.

Figure 6. High-resolution C 1s XPS spectra of GO (a) and E-graphene (b).

GO to E-graphene, a detailed mechanistic pathway has been proposed, as shown in Scheme 2. Eucalyptus bark extract composed mainly a mixture of “29” polyphenolic compounds39 thus results in a mixture of chemical structures which can be easily oxidized. Among these polyphenols, catechin and its derivatives, gallic acid and its derivatives, and caffeic acid were the predominant components. GO contains a variety of oxygen functionalities on its surface including epoxy, carboxylic, and hydroxyl groups. The epoxy and hydroxyl groups of GO react with hydroxyl groups of polyphenols to form rings followed by ring opening facilitating the reduction. The carboxylic groups of GO undergo condensation with hydroxyl groups of polyphenols to form an ester and further bond cleavage resulting in the formation of the reduced form of GO. The suspension nature of E-graphene in aqueous and nonaqueous solvents was also studied. All the solutions were prepared using completely dried E-graphene. As presented in

Figure S4, after sonication it is observed that the E-graphene has been dispersed effectively in water as well as all the other nonaqueous solvents, i.e., MeOH, EtOH, EtAC, DMF, nhexane, DMSO, acetone, and THF. After 15 days, except for THF and n-hexane, all the dispersions in polar solvents showed long-term stability with a negligible amount of precipitate settled. In the case of the n-hexane dispersion, the E-graphene was precipitated out completely, while in THF dispersion, the E-graphene was observed to precipitate out to a large extent. This can be explained by the Hansen solubility parameter (HSP);55−57 the hydrogen bonds, dipolar interactions, and dispersive intermolecular interactions between residual oxygen functionalities present on the graphene layers and solvent molecules are responsible for the stable dispersion of graphene in polar solvents like MeOH, EtOH, EtAC, DMF, DMSO, and acetone. It is noteworthy that the good dispersibility of Egraphene in various low-boiling-point polar solvents can 11616


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Scheme 2. Proposed Mechanistic Pathway for the Green Reduction of GO by Eucalyptus Polyphenol Solution

Table 1. Comparisons of Graphene, Synthesized by Different Green Reducing Agents reducing agents

category

UV−vis absorption (nm)

C/O ratio

ID/IG

ref

caffeic acid tea polyphenols gallic acid L-ascorbic acid natural cellulose tannic acid baker’s yeast Eucalyptus polyphenol extract

phenols phenols phenols organic acids polysaccharides phenols microorganisms phenols

270 271 270−273 264 269 274 263 270

7.15 3.1 3.89−5.28 5.7 5.47 2.44 5.9 10.93

1.15

45 36 50 51 52 53 54 present work

1.86−1.92 >1 1.53 1.15 1.44 1.15

C/O ratio: carbon to oxygen ratio; ID: intensity of D-band; IG: intensity of G-band.

capacitance (Cs), energy density (E), and power density (P) for the E-graphene electrode are calculated from the following eqs 1−3 at multiple current densities.60

facilitate the design and manufacture of composites and devices to a great extent.58 The combination of all the abovementioned qualities makes Eucalyptus-polyphenol-based reduction a cost-effective, ecofriendly, and efficient strategy to replace not only hazardous chemical reagents like hydrazine but also previously reported green reducing agents as shown in Table 1 in the large-scale preparation of solution-processable graphene from GO. The specific capacitance and electrochemical performances of synthesized GO and E-graphene were examined by GCD and CV. Figure 7a shows the CV curves of E-graphene with various scan rates. All curves look similar, which significantly indicates an outstanding rate capability with an increase of scan rates. Figure 7b shows the CV curves of GO and E-graphene at a scan rate of 100 mV s−1. The CV curves of E-graphene demonstrate improved conductivity compared to GO and also had a nearly rectangular shape without apparent redox-current peaks, remarkably suggesting that a typical capacitive character of an electrical double-layer (EDL) capacitance.59 Figure 7c shows the GCD curves of E-graphene with various current densities. All GCD curves of the E-graphene supercapacitor have a similar shape with capacitive behavior. The specific

Cs =

I Δt mΔV

(1)

E=

Cs × (ΔV )2 2

(2)

P=

E t

(3)

Here, I is the response current, Δt is the discharge time, V is the potential window, and m is the mass of the active material. Figure 7d shows that the 'Cs' of E-graphene at 2 A g−1 is 239 F g−1. The E-graphene possesses high Cs compared to that of several recent reports.59,61−65 Figure 7e shows the Ragone plot for the E-graphene electrode at multiple current densities. The highest energy density of 71 W h kg−1 and power density of 1800 W kg−1 can be observed. This clearly indicates that the Egraphene electrode possesses an outstanding combination of high power density and energy storage. 11617


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Figure 7. (a) CV curves of the E-graphene-modified GC electrode at different scan rates. (b) CV curves of synthesized GO- and E-graphenemodified GC electrodes at 100 mV s−1 scan rate. (c) GCD curves, (d) specific capacitance, and (e) Ragone plot of the E-graphene-modified GC electrode at different current densities.

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CONCLUSIONS A unique, efficient, and cheap green reducing agent having the ability to produce stable suspensions of graphene from GO was discovered in the form of Eucalyptus polyphenols. This method provides an excellent scope for the facile synthesis of graphene, because of its low cost, scalability, outstanding removal of oxygen functionalities from GO, and avoidance of the use of toxic solvents which can lead to hazardous wastes. The polyphenols present in the Eucalyptus bark extract behave as both a reducing agent and a stabilizer thus improving the

solubility of the resultant E-graphene in aqueous and different types of commercially attractive nonaqueous solvents. These findings suggest that Eucalyptus polyphenols bark extract is an ideal substitute for hazardous reducing agents like hydrazine, sodium borohydride, and dimethylhydrazine in the large-scale synthesis of solution-processable graphene. The biocompatible Eucalyptus polyphenols make the soluble E-graphene a propitious candidate in developing high-performance supercapacitor applications. It is also deduced that this approach has 11618


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great potentials for the low-cost and large-scale production of graphene-based materials from GO.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01506. Additional experimental details and figures including EDX elemental mapping, AFM thickness distribution, FT-IR spectra, and solubility studies of E-graphene (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]; [email protected]. ORCID

Deshetti Jampaiah: 0000-0003-3453-7669 Suresh K. Bhargava: 0000-0002-3127-8166 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Saikumar Manchala acknowledges the MHRD, Govt. of India, for providing a fellowship. The authors acknowledge the facilities and the scientific and technical assistance of the RMIT Microscopy & Microanalysis Facility (RMMF), a linked laboratory of Microscopy Australia. The authors are thankful to Dr Blake Plowman for his suggestions in improving the quality of this manuscript.

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