Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 145 (2015) 117–124
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Bioinspired reduced graphene oxide nanosheets using Terminalia chebula seeds extract Sireesh Babu Maddinedi a, Badal Kumar Mandal a,⇑, Raviraj Vankayala b, Poliraju Kalluru b, Sreedhara Reddy Pamanji c a b c
Trace Elements Speciation Research Laboratory, Environmental and Analytical Chemistry Division, School of Advanced Sciences, VIT University, Vellore 632014, India Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC Department of Physics, Sri Venkateswara University, Tirupati, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A green eco-friendly synthesis
A simple and efficient method for the reduced graphene oxide production by the eco-friendly deoxygenation of graphene oxide using the seed extract of T. chebula. The phytochemicals in the T. chebula extract are responsible for the reduction and also stabilizes by functionalizing on the surface of the graphene sheets.
method for T. chebula mediated reduced graphene oxide. HPLC, FTIR and Raman results explained the stabilization of graphene sheets. Adsorption of polyphenols on the surface of graphene sheets stabilizes it.
O HO
O
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O O O
O
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O O O O O
O O
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a r t i c l e
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Article history: Received 24 October 2014 Received in revised form 30 January 2015 Accepted 6 February 2015 Available online 21 February 2015 Keywords: Graphene oxide Terminalia chebula Polyphenols Nanosheets
a b s t r a c t A green one step facile synthesis of graphene nanosheets by Terminalia chebula (T. chebula) extract mediated reduction of graphite oxide (GO) is reported in this work. This method avoids the use of harmful toxic reducing agents. The comparative results of various characterizations of GO and T. chebula reduced graphene oxide (TCG) provide a strong indication of the exclusion of oxygen containing groups from graphene oxide and successive stabilization of the formed reduced graphene oxide (RGO). The functionalization of reduced graphene oxide with the oxidized polyphenols causes their stability by preventing the aggregation. We also have proposed how the oxidized polyphenols are accountable for the stabilization of the formed graphene sheets. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Graphene is a novel class of material of significant interest with one atom thickness; possess exceptional electrical, thermal and ⇑ Corresponding author. http://dx.doi.org/10.1016/j.saa.2015.02.037 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
mechanical properties [1]. These extraordinary features offer great potential applications in different fields such as nanoelectronics [2], composites [3–7], fuel cells [8], super capacitor [9] as well as sensors [10] and catalysis [11]. However, several approaches have been reported for the synthesis of graphene such as micro mechanical exfoliation of graphite
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[12], chemical vapor deposition [13], epitaxial growth on electrically insulating surfaces [14], the exfoliation and reduction of graphite oxide [15–17]. Moreover, the lower yield production of the first three methods are not well suitable for the applications in bulk scale, whereas graphene oxide reduction has become the promising route for the large scale synthesis of graphene. Furthermore, several chemical methods have been developed using chemical reducing agents like hydrazine [18–20], sodium borohydride [21,22], hydroquinone [23] and dimethylhydrazine [24]. Due to the toxic and hazardous nature of some of these chemicals these approaches are less promising route for biological applications of the synthesized graphene. Therefore, the demand for the new, ecofriendly and easily scalable biosynthetic approaches for graphene synthesis is mounting up. During last few years, environmental friendly and green methods for the synthesis of graphene have been reported using different biomolecules [25]. For example, the uses of L-ascorbic acid [26], reducing sugar [27] and protein bovine serum albumin [28], and casein [29] have been used in the reduction of graphene oxide. In addition, some of the interesting alternative green methods include the use of green plants and their extracts for ecofriendly biosynthesis of graphene. Recently, different nanoparticles (NPs) such as gold, silver and platinum were effectively synthesized using plant extracts. Our group has shown that plant extracts can act as good reducing agents for the reduction of noble metal ions into their nanoparticles [30,31]. Generally, plant extracts contains different types of polyphenols which are converted to the corresponding oxidized quinone forms in the presence of reactive oxygen and hence these plant extracts have adequate potential to reduce GO. In this work, we have introduced a green method of graphene synthesis using Terminalia chebula (T. chebula) plant seed aqueous extracts as reducing agents. T. chebula is a plant used in ayurvedic medicine due to its antibacterial and strong cardio tonic properties. Natural polyphenols that are present in the extracts of T. chebula are responsible for the reduction and stabilization of graphene sheets. Experimental section Materials Graphite powder (100 mesh, 99.9995%), sodium nitrate (NaNO3), concentrated sulfuric acid (98%), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%), and all organic solvents were purchased from Sigma–Aldrich, Bangalore. Preparation of GO Graphene oxide was chemically prepared from natural graphite flakes using the mixture of concentrated sulfuric acid and KMnO4 by the modified Hummers method [32]. In brief, 0.5 g of graphite flakes and 0.5 g of sodium nitrate were stirred in 23 mL of 12.1 M H2SO4 (98%) with a magnetic stirrer for 15 min at 0–5 °C. Then 4 g of KMnO4 was slowly added into the above mixture by keeping the temperature below 20 °C. Then the mixture was allowed to stir at 40 °C for 90 min in water bath followed by dilution with 50 mL of double distilled water with vigorous stirring for 10 min. The resulting dark brown suspension further was treated slowly with 6 mL of 30% H2O2 solution followed by dilution with 50 mL double distilled water. The resulting GO nanoparticles were washed several times with deionized water to remove excess of manganese salt until neutrality was reached. Finally, the purified GO was dispersed in water (1 mg/mL) followed by exfoliation using ultrasonication in an ultrasonic bath for about 3 h. The formed stable dispersion was used for further experiments.
Preparation of plant seed extract T. chebula plant seeds were collected and washed with water and then dried under sunlight. 3.5 g of finely ground powder of T. chebula seeds was added to 200 mL of deionized water and heated at 90 °C on water bath for 1 h, cooled and filtered through 0.2 lm cellulose nitrate membrane filter paper. Preparation of reduced graphene oxide About 200 mL of GO solution was added to 200 mL aqueous extract of T. chebula seeds and mixed well by manual shaking. pH of the above solution was adjusted to 12 by using NH4OH. The resulting suspension was refluxed on a water bath at 90 °C for about 24 h. The completion of reduction of GO was confirmed by change in the color of GO from yellowish brown to black (also confirmed by UV–Vis analysis). A precipitate of reduced GO settled down due to the loss of oxygen moieties present in graphene oxide after the reduction. Characterization Fourier transform infrared (FTIR) study was carried out over the range of 4000–400 cm 1 in the diffuse reflectance mode at a resolution of 4 cm 1 by JASCO FTIR 4100 instrument using KBr pellets. Sample was prepared by mixing TCG powder with KBr powder and pelletised. UV–Visible spectra were analyzed by Jasco V-670 UV– Vis double beam spectrophotometer after dispersing the dried TCG sample in double distilled water and the spectra was measured against the double distilled water as blank. The X-ray diffraction (XRD) study was performed at room temperature by a Bruker D8 Advance diffractometer with Cu Ka radiation (k = 1.54 Å) over the range of 2h of 3–80° with a scanning rate of 4°/min and with a step size of 0.02°. The instrument was calibrated with lanthanum hexaboride (LaB6) prior to analysis. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis were done by using a Carl Zeiss SEM instrument attached to EVO MA 15 (Oxford Instrument). The solid TCG powder was spread over the surface of carbon tape adhered on a metallic disk. Images were taken at diverse magnification for the sample surface. A Simultaneous EDS spectrum was taken at selected areas on the TCG solid surface to get the surface atomic distribution. Raman spectral analysis was carried out by using a Senterra R200-L apparatus (Bruker Optics) by passing laser light of 532 nm wavelength. High resolution transmission electron microscope (HRTEM) images were taken with JEOL-2100 F electron microscope at an operating voltage of 200 kV. Samples were prepared by dispersing dried TCG in water (1 mg mL 1) under ultrasonic conditions which further placed a drop of it on a Lacey Carbon Coated Copper Grid and dried under vacuum. A simultaneous measurement of SEAD pattern was also carried out. Dynamic light scattering (DLS) analysis was carried out by Malvern Instruments. Sample was prepared by dispersing TCG powder in water by sonication (1 mg mL 1). HPLC analysis was done using Perkin Elmer 200 Series HPLC equipped with UV–Vis Detector (k = 192–700 nm) and a 200 Series pump. Mobile phase was prepared using 32% of acetonitrile and 0.1 M HCl and pH was adjusted to 3 using dilute HCl. Samples were prepared by mixing the extract with the mobile phase. High resolution X-ray photoelectron spectroscopy (HRXPS) measurements were taken with a PHI Quantera SXM, scanning X-ray microprobe (ULVac-PHI Inc.). Measurements were carried out by homogeneously depositing the dry TCG powder on the surface of silicon wafer pasted with a copper tape. Thermogravimetric analysis (TGA) was carried out for TCG, by using a thermal analyzer, TG50 (Simadzu) at heating rate of 10 °C/min with a nitrogen flow rate of 30 mL/min. Diffuse reflectance spectroscopic (DRS) studies were
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performed by using Jasco V-670 UV–Vis spectrophotometer with dried powder of TCG. Results and discussion The synthetic procedure involved in the preparation of graphene from graphene oxide is illustrated in Fig. 1, which is shown by the color change from brownish yellow to black. After the completion of reduction, reduced graphene oxide settled down as precipitate, suggesting the loss of oxygen containing moieties present in the graphene oxide. UV-Visible spectra analysis The reduction process of graphene oxide was examined by taking the UV–Visible absorption spectra of RGO as a function of time. UV analysis showed a maximum absorption peak of 232 nm of GO which was ascribed to the p–p⁄ transitions of the aromatic C–C bonds and a weak shoulder appeared at 300 nm due to n–p⁄ transitions of C@O bonds. Completion of reduction was confirmed by the establishment of new peak and a red shift of characteristic peak at 232 nm to 275 nm (Fig. 2), indicating the restoration of electronic conjugation of graphene sheets. The performance of reducing agent can be estimated by the maximum red shift value observed [33]. For instance, our T. chebula extract reduced graphene oxide showed an absorption peak at 275 nm, which is higher than that of hydrazine [34] and phenyl hydrazine [35] reduced graphene (270 nm), indicating the efficient reducing ability of polyphenols present in T. chebula extracts. The typical broad shoulder appeared at 367 nm may be due to the presence of polyphenols that are attached on the surface of graphene nanosheets which prevents agglomeration of graphene [36,37].
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FTIR study Further, Fourier transform infrared (FTIR) spectroscopic studies of pure GO and TC reduced GO samples were performed to know the extent of reduction (Fig. 3). It is observed that GO shows a broad band in the range of 3100–3400 cm 1 corresponding to the O–H stretching vibrations. The absorption band around 1722 cm 1 is due to C@O stretch of carbonyl group and a band at 1618 cm–1 corresponds to the O–H bending and epoxide ring vibrations. The narrower absorption peak around 1050 cm 1 is due to the C–O stretching vibrational peak of carboxylic (COOH) groups present on the surface of graphene oxide. The absorption band at around 1400 cm 1 may be due to the presence of tertiary alcohol (C–OH) group. The FTIR spectra of dried TCG shows a significant decrease in the intensities of absorption bands (O–H, C@O, C–O and tertiary C–OH bands) corresponding to the oxygen functionalities present in the graphene oxide, which establishes the deoxygenation and the results are similar to the results obtained from the graphene synthesized by using plant extracts [38]. XRD study Fig. 4 shows the XRD diffractogram which approves T. chebula extract induced reduction of GO to graphene. The GO shows a single basal reflection diffraction peak at 11.2° (0 0 2) with corresponding interlayer d-spacing of 0.77 nm, revealing the intercalation of water and other oxygen containing groups between the layers of graphite upon oxidation. Conversely, TCG formed after the reduction of GO with T. chebula extract showed a broad diffraction peak at 2h of 26.6° from (1 1 1) crystal plane with corresponding d-spacing of 0.33 nm. The disappearance of peak at 2h of 11.2° and a relatively decreased d-spacing value of graphene formed when compared with GO, indicating the complete removal of oxygen containing groups in GO after reduction. SEM and EDS analysis
Fig. 1. Schematic illustration of Terminalia chebula mediated reduction of graphene oxide.
1.4
We further investigated the morphology of GO and TCG by SEM analysis. Fig. 5A clearly showed the existence of stacked layers of solid GO with rough surface. However, the surface roughness of GO may be due to the oxidation of sheets. Compared to GO, the TCG also showed the layered structures but are thin and exists with lower height (Fig. 5B). The EDS spectrum of the synthesized GO and TCG showed the oxygen percentage of GO and TCG are 43.2, 24.8 atom% (as revealed in Fig. 5C and D), confirming the existence of oxygen in the T. chebula reduced graphene even after the reduction, which may be due to the surface oxidized polyphenols. This finding is further confirmed by XPS study.
275 nm
TEM analysis HRTEM microscopic images of TCG (which are shown in Fig. 6A–C) showed a transparent, silk like appearance of ultra thin graphene sheets after reduction. It is also shown that the edges of the suspended graphene films tend to fold back, permitting the cross-sectional view of the films. From Fig. 6B, it is clearly concluded the formation of three layered graphene sheets. The selected area electron diffraction (SAED) pattern of graphene shows a typical sharp polycrystalline ring pattern composed of many diffraction spots (Fig. 6D), representing the loss of long range organization between the formed graphene nanosheets.
1 232 nm Abs
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0.1 200
367 nm
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800
Wavelength [nm]
Fig. 2. UV–Visible spectra of (a) GO (green) (b) TCG (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
HPLC analysis HPLC analysis is performed to know the polyphenolic constituents present in the aqueous extract of T. chebula seeds.
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1402.25
1053.13
1708.93 3172.90
1618.28
1722.43
%T
3300.20
TC GO-2
4000 GO-2
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500 1/cm
Fig. 3. FTIR spectra of GO (blue) and dried TCG (pink). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. S1A is showing the presence of four different phytochemicals in T. chebula aqueous extract namely, ascorbic acid, gallic acid, pyrogallol and methyl gallate which are known by comparing with the retention times of the respective standards under the same instrumental conditions. Further, the presence of gallic acid as major constituent in T. chebula extract indicates its predominant role in the reduction of GO to TCG (Fig. S1A). Mechanism
Fig. 4. XRD patterns of GO and TCG.
The suggested mechanism for the reduction of GO by T. chebula extract is shown in Fig. S1C. The surface of the GO contains different oxygen containing groups such as epoxide, hydroxyl, and carbonyl [39]. Under basic conditions both the epoxide and carbonyl groups may be converted into hydroxyl groups. T. chebula plant extract contains four different types of polyphenols namely,
Fig. 5. SEM images and EDS spectra of GO (A, C) and TCG (B, D).
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Fig. 6. HR-TEM images of TCG showing the formation of few layer graphene (A–C) and the SAED pattern (D).
gallic acid, pyrogallol, ascorbic acid, methyl gallate (Fig. S1A and S1B), containing highly acidic hydrogen atoms, which easily disassociate to their anionic forms. The anionic forms of polyphenols react with the hydroxyl moiety of GO through a nucleophilic substitution 2 (SN2) mechanisms resulting in the formation of intermediate 1. The intermediate 1 is transformed to intermediate 2 with the elimination of water molecule which finally forms T. chebula reduced graphene oxide (Fig. S1C). Raman analysis Raman spectroscopy analysis was carried out to know the size of sp2 domains in the graphene structure containing both sp3 and sp2 network. The ratio of ID/IG obtained for both GO and TCG were 1.47, 2.63 respectively (Fig. 7). The increase of ID/IG ratio after reduction may be due to the higher adsorption of the oxidized polyphenols on the graphene surface, thus increasing the surface sp3 domains. Fig. 7 also showed a shift of about 10 cm 1 in G band of graphene oxide from 1594 cm 1 to 1584 cm 1 after reaction indicating a high degree of reduction. However, an increased ID/IG ratio of TCG compared to the GO further illustrated the formation of graphene crystalline domains after reduction [41].
by incubating 40 mg of T. chebula reduced graphene oxide in a hot furnace for about 2 h at 500 °C and are observed to be the loss of 90% by weight after the incubation period (data not shown) which is further confirmed by TGA results shown later. In addition, FTIR spectra were taken for the dried water extract of T. chebula before and after the reduction of GO, to find out the biomolecular capping of the formed graphene sheets. Fig. 8 showed the presence of intense infra red bands at 3446, 1400 cm 1 corresponding to the O–H and C–O stretching of phenolic groups indicating the rich content of polyphenols in T. chebula extract. However the reduced intensity of O–H stretching and the absence of C–O stretching vibrations at 1400 cm 1 with the appearance of new vibrational band at 1710 cm 1 (characteristics of ketone group) in the extract
Stabilization of TCG From the UV–Vis data (Fig. 2), it is confirmed that the presence of broad band at 362 nm indicates the presence of oxidized polyphenols on the surface of reduced graphene oxide, which are responsible for preventing the agglomeration of successive graphene sheets. Furthermore, the weight loss studies have been performed to know the quantity of biomolecules present on the surface of graphene sheets. These studies have been carried out
Fig. 7. Raman spectra of GO (black) and TCG (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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3446.79
1710.86
3446.79
1400.32
%T
4000 TCE AR
3000
2000
1500
1000
500 1/cm
Fig. 8. FTIR spectra of T. chebula leaf extract before reduction (green) and after reduction (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. (A) Dynamic light scattering spectrum of TCG aqueous dispersion, (B) TGA thermogram of TCG, (C) optical diffuse reflectance spectra of T. chebula reduced graphene oxide.
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after the reduction further confirmed the conversion of polyphenols into their quinone forms during the deoxygenation of GO. From these studies, it is assumed that the oxidized polyphenols interact with TCG sheets by p–p stacking interactions that generate electrostatic repulsions between the graphene layers resulting in the stabilization of individual graphene sheets. The p–p interactions between graphene sheets and the oxidized polyphenols have been represented in Fig. S2A.
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polyphenols present in T. chebula extract. It is also found that polyphenols in T. chebula are not only responsible for the deoxygenation, but also for stabilization of formed graphene nanosheets by preventing their aggregation. This paper further opens up an environmental friendly method to synthesize a biocompatible graphene in large scale by using naturally occurring polyphenols rich plant extracts. Acknowledgments
XPS study Further, high-resolution XPS C1s spectrum of prepared TCG (Fig. S2B and C) showed the presence of typical C1s peaks at 284.2, 286.2, 288, 289 eV corresponding to C@C, C–O–H, C@O, O–C@O respectively. However, the higher intense peak at 286.2 eV and the other peaks corresponding to the remaining oxygen functionalities are due to the biomolecular capping on the surface of TCG, which is also further supported by the presence of high intense C–O–H peak (533.1 eV) and other peak for C@O (532 eV) in the O1s spectrum [40].
Mr. S.B.M. greatly acknowledges the help of VIT University, Vellore-632014, India for the financial help and platform given to do this research. Also, he acknowledges the help from Department of Chemistry, National Tsing Hua University, Hsinchu-30013, Taiwan ROC and NIIST, CSIR, Thiruvananthapuram-695 019, India. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.02.037.
DLS analysis
References
We also employed DLS analysis to know the size distribution of TCG particles in aqueous dispersion by using the standard spherical model (Fig. 9A). After the prolonged sonication, the graphene sheets are broken into particles and the average particle size of the graphene particles present in the TCG dispersion measured by using DLS is 1014 nm.
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TGA Further to confirm the presence of oxygen containing groups, thermal stability of TCG was examined by TGA (Fig. 9B). Thermogram of as synthesized TCG exhibits a weight loss of 14.5% below 200 °C due to the loss of absorbed water, oxygen functionalities and the other biomolecules present on the surface of graphene sheets. Subsequently, it also exhibited a large weight loss of 86% at 500 °C, because of the pyrolysis of the remaining oxygen containing groups and also due to the burning of the ring carbon. This higher weight loss at 200 °C when compared to the graphene synthesized by using other green methods is clearly indicating the high accumulation of biomolecules on the surface of graphene sheets after reduction [41,42]. Band gap studies The optical band gap of RGO was calculated from UV–Visible spectra by the following equation as a = c (hm Ebulk)1/2/hm, where hm is the photon energy, a is absorption coefficient, c is a constant, and Ebulk is bulk ‘band gap’. Fig. 9C shows the plot drawn between hm versus (ahm)2 for the formed graphene, by using UV–Visible spectroscopy at the wavelength range from 200 nm to 2400 nm. The band gap was obtained by the extrapolation of a linear regression to the X-axis in the plot and the gap was found to be 3.74 eV. The value of band gap is observed to be in almost similar agreement with previous reports obtained by using aqueous leaf extracts of Colocasia esculenta and Mesuaferrea Linn and an aqueous peel extract of Citrus sinensis [39]. Conclusions In summary, we demonstrate a green, low-cost ecofriendly synthetic route for the synthesis of graphene by using polyphenols from T. chebula seed extracts. HPLC analysis identified the
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Bioinspired reduced graphene oxide nanosheets using Terminalia chebula seeds extract Sireesh Babu Maddinedi a, Badal Kumar Mandal a,⇑, Raviraj Vankayala b, Poliraju Kalluru b, Sreedhara Reddy Pamanji c a b c
Trace Elements Speciation Research Laboratory, Environmental and Analytical Chemistry Division, School of Advanced Sciences, VIT University, Vellore 632014, India Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC Department of Physics, Sri Venkateswara University, Tirupati, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A green eco-friendly synthesis
A simple and efficient method for the reduced graphene oxide production by the eco-friendly deoxygenation of graphene oxide using the seed extract of T. chebula. The phytochemicals in the T. chebula extract are responsible for the reduction and also stabilizes by functionalizing on the surface of the graphene sheets.
method for T. chebula mediated reduced graphene oxide. HPLC, FTIR and Raman results explained the stabilization of graphene sheets. Adsorption of polyphenols on the surface of graphene sheets stabilizes it.
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Article history: Received 24 October 2014 Received in revised form 30 January 2015 Accepted 6 February 2015 Available online 21 February 2015 Keywords: Graphene oxide Terminalia chebula Polyphenols Nanosheets
a b s t r a c t A green one step facile synthesis of graphene nanosheets by Terminalia chebula (T. chebula) extract mediated reduction of graphite oxide (GO) is reported in this work. This method avoids the use of harmful toxic reducing agents. The comparative results of various characterizations of GO and T. chebula reduced graphene oxide (TCG) provide a strong indication of the exclusion of oxygen containing groups from graphene oxide and successive stabilization of the formed reduced graphene oxide (RGO). The functionalization of reduced graphene oxide with the oxidized polyphenols causes their stability by preventing the aggregation. We also have proposed how the oxidized polyphenols are accountable for the stabilization of the formed graphene sheets. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Graphene is a novel class of material of significant interest with one atom thickness; possess exceptional electrical, thermal and ⇑ Corresponding author. http://dx.doi.org/10.1016/j.saa.2015.02.037 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
mechanical properties [1]. These extraordinary features offer great potential applications in different fields such as nanoelectronics [2], composites [3–7], fuel cells [8], super capacitor [9] as well as sensors [10] and catalysis [11]. However, several approaches have been reported for the synthesis of graphene such as micro mechanical exfoliation of graphite
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[12], chemical vapor deposition [13], epitaxial growth on electrically insulating surfaces [14], the exfoliation and reduction of graphite oxide [15–17]. Moreover, the lower yield production of the first three methods are not well suitable for the applications in bulk scale, whereas graphene oxide reduction has become the promising route for the large scale synthesis of graphene. Furthermore, several chemical methods have been developed using chemical reducing agents like hydrazine [18–20], sodium borohydride [21,22], hydroquinone [23] and dimethylhydrazine [24]. Due to the toxic and hazardous nature of some of these chemicals these approaches are less promising route for biological applications of the synthesized graphene. Therefore, the demand for the new, ecofriendly and easily scalable biosynthetic approaches for graphene synthesis is mounting up. During last few years, environmental friendly and green methods for the synthesis of graphene have been reported using different biomolecules [25]. For example, the uses of L-ascorbic acid [26], reducing sugar [27] and protein bovine serum albumin [28], and casein [29] have been used in the reduction of graphene oxide. In addition, some of the interesting alternative green methods include the use of green plants and their extracts for ecofriendly biosynthesis of graphene. Recently, different nanoparticles (NPs) such as gold, silver and platinum were effectively synthesized using plant extracts. Our group has shown that plant extracts can act as good reducing agents for the reduction of noble metal ions into their nanoparticles [30,31]. Generally, plant extracts contains different types of polyphenols which are converted to the corresponding oxidized quinone forms in the presence of reactive oxygen and hence these plant extracts have adequate potential to reduce GO. In this work, we have introduced a green method of graphene synthesis using Terminalia chebula (T. chebula) plant seed aqueous extracts as reducing agents. T. chebula is a plant used in ayurvedic medicine due to its antibacterial and strong cardio tonic properties. Natural polyphenols that are present in the extracts of T. chebula are responsible for the reduction and stabilization of graphene sheets. Experimental section Materials Graphite powder (100 mesh, 99.9995%), sodium nitrate (NaNO3), concentrated sulfuric acid (98%), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%), and all organic solvents were purchased from Sigma–Aldrich, Bangalore. Preparation of GO Graphene oxide was chemically prepared from natural graphite flakes using the mixture of concentrated sulfuric acid and KMnO4 by the modified Hummers method [32]. In brief, 0.5 g of graphite flakes and 0.5 g of sodium nitrate were stirred in 23 mL of 12.1 M H2SO4 (98%) with a magnetic stirrer for 15 min at 0–5 °C. Then 4 g of KMnO4 was slowly added into the above mixture by keeping the temperature below 20 °C. Then the mixture was allowed to stir at 40 °C for 90 min in water bath followed by dilution with 50 mL of double distilled water with vigorous stirring for 10 min. The resulting dark brown suspension further was treated slowly with 6 mL of 30% H2O2 solution followed by dilution with 50 mL double distilled water. The resulting GO nanoparticles were washed several times with deionized water to remove excess of manganese salt until neutrality was reached. Finally, the purified GO was dispersed in water (1 mg/mL) followed by exfoliation using ultrasonication in an ultrasonic bath for about 3 h. The formed stable dispersion was used for further experiments.
Preparation of plant seed extract T. chebula plant seeds were collected and washed with water and then dried under sunlight. 3.5 g of finely ground powder of T. chebula seeds was added to 200 mL of deionized water and heated at 90 °C on water bath for 1 h, cooled and filtered through 0.2 lm cellulose nitrate membrane filter paper. Preparation of reduced graphene oxide About 200 mL of GO solution was added to 200 mL aqueous extract of T. chebula seeds and mixed well by manual shaking. pH of the above solution was adjusted to 12 by using NH4OH. The resulting suspension was refluxed on a water bath at 90 °C for about 24 h. The completion of reduction of GO was confirmed by change in the color of GO from yellowish brown to black (also confirmed by UV–Vis analysis). A precipitate of reduced GO settled down due to the loss of oxygen moieties present in graphene oxide after the reduction. Characterization Fourier transform infrared (FTIR) study was carried out over the range of 4000–400 cm 1 in the diffuse reflectance mode at a resolution of 4 cm 1 by JASCO FTIR 4100 instrument using KBr pellets. Sample was prepared by mixing TCG powder with KBr powder and pelletised. UV–Visible spectra were analyzed by Jasco V-670 UV– Vis double beam spectrophotometer after dispersing the dried TCG sample in double distilled water and the spectra was measured against the double distilled water as blank. The X-ray diffraction (XRD) study was performed at room temperature by a Bruker D8 Advance diffractometer with Cu Ka radiation (k = 1.54 Å) over the range of 2h of 3–80° with a scanning rate of 4°/min and with a step size of 0.02°. The instrument was calibrated with lanthanum hexaboride (LaB6) prior to analysis. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis were done by using a Carl Zeiss SEM instrument attached to EVO MA 15 (Oxford Instrument). The solid TCG powder was spread over the surface of carbon tape adhered on a metallic disk. Images were taken at diverse magnification for the sample surface. A Simultaneous EDS spectrum was taken at selected areas on the TCG solid surface to get the surface atomic distribution. Raman spectral analysis was carried out by using a Senterra R200-L apparatus (Bruker Optics) by passing laser light of 532 nm wavelength. High resolution transmission electron microscope (HRTEM) images were taken with JEOL-2100 F electron microscope at an operating voltage of 200 kV. Samples were prepared by dispersing dried TCG in water (1 mg mL 1) under ultrasonic conditions which further placed a drop of it on a Lacey Carbon Coated Copper Grid and dried under vacuum. A simultaneous measurement of SEAD pattern was also carried out. Dynamic light scattering (DLS) analysis was carried out by Malvern Instruments. Sample was prepared by dispersing TCG powder in water by sonication (1 mg mL 1). HPLC analysis was done using Perkin Elmer 200 Series HPLC equipped with UV–Vis Detector (k = 192–700 nm) and a 200 Series pump. Mobile phase was prepared using 32% of acetonitrile and 0.1 M HCl and pH was adjusted to 3 using dilute HCl. Samples were prepared by mixing the extract with the mobile phase. High resolution X-ray photoelectron spectroscopy (HRXPS) measurements were taken with a PHI Quantera SXM, scanning X-ray microprobe (ULVac-PHI Inc.). Measurements were carried out by homogeneously depositing the dry TCG powder on the surface of silicon wafer pasted with a copper tape. Thermogravimetric analysis (TGA) was carried out for TCG, by using a thermal analyzer, TG50 (Simadzu) at heating rate of 10 °C/min with a nitrogen flow rate of 30 mL/min. Diffuse reflectance spectroscopic (DRS) studies were
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performed by using Jasco V-670 UV–Vis spectrophotometer with dried powder of TCG. Results and discussion The synthetic procedure involved in the preparation of graphene from graphene oxide is illustrated in Fig. 1, which is shown by the color change from brownish yellow to black. After the completion of reduction, reduced graphene oxide settled down as precipitate, suggesting the loss of oxygen containing moieties present in the graphene oxide. UV-Visible spectra analysis The reduction process of graphene oxide was examined by taking the UV–Visible absorption spectra of RGO as a function of time. UV analysis showed a maximum absorption peak of 232 nm of GO which was ascribed to the p–p⁄ transitions of the aromatic C–C bonds and a weak shoulder appeared at 300 nm due to n–p⁄ transitions of C@O bonds. Completion of reduction was confirmed by the establishment of new peak and a red shift of characteristic peak at 232 nm to 275 nm (Fig. 2), indicating the restoration of electronic conjugation of graphene sheets. The performance of reducing agent can be estimated by the maximum red shift value observed [33]. For instance, our T. chebula extract reduced graphene oxide showed an absorption peak at 275 nm, which is higher than that of hydrazine [34] and phenyl hydrazine [35] reduced graphene (270 nm), indicating the efficient reducing ability of polyphenols present in T. chebula extracts. The typical broad shoulder appeared at 367 nm may be due to the presence of polyphenols that are attached on the surface of graphene nanosheets which prevents agglomeration of graphene [36,37].
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FTIR study Further, Fourier transform infrared (FTIR) spectroscopic studies of pure GO and TC reduced GO samples were performed to know the extent of reduction (Fig. 3). It is observed that GO shows a broad band in the range of 3100–3400 cm 1 corresponding to the O–H stretching vibrations. The absorption band around 1722 cm 1 is due to C@O stretch of carbonyl group and a band at 1618 cm–1 corresponds to the O–H bending and epoxide ring vibrations. The narrower absorption peak around 1050 cm 1 is due to the C–O stretching vibrational peak of carboxylic (COOH) groups present on the surface of graphene oxide. The absorption band at around 1400 cm 1 may be due to the presence of tertiary alcohol (C–OH) group. The FTIR spectra of dried TCG shows a significant decrease in the intensities of absorption bands (O–H, C@O, C–O and tertiary C–OH bands) corresponding to the oxygen functionalities present in the graphene oxide, which establishes the deoxygenation and the results are similar to the results obtained from the graphene synthesized by using plant extracts [38]. XRD study Fig. 4 shows the XRD diffractogram which approves T. chebula extract induced reduction of GO to graphene. The GO shows a single basal reflection diffraction peak at 11.2° (0 0 2) with corresponding interlayer d-spacing of 0.77 nm, revealing the intercalation of water and other oxygen containing groups between the layers of graphite upon oxidation. Conversely, TCG formed after the reduction of GO with T. chebula extract showed a broad diffraction peak at 2h of 26.6° from (1 1 1) crystal plane with corresponding d-spacing of 0.33 nm. The disappearance of peak at 2h of 11.2° and a relatively decreased d-spacing value of graphene formed when compared with GO, indicating the complete removal of oxygen containing groups in GO after reduction. SEM and EDS analysis
Fig. 1. Schematic illustration of Terminalia chebula mediated reduction of graphene oxide.
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We further investigated the morphology of GO and TCG by SEM analysis. Fig. 5A clearly showed the existence of stacked layers of solid GO with rough surface. However, the surface roughness of GO may be due to the oxidation of sheets. Compared to GO, the TCG also showed the layered structures but are thin and exists with lower height (Fig. 5B). The EDS spectrum of the synthesized GO and TCG showed the oxygen percentage of GO and TCG are 43.2, 24.8 atom% (as revealed in Fig. 5C and D), confirming the existence of oxygen in the T. chebula reduced graphene even after the reduction, which may be due to the surface oxidized polyphenols. This finding is further confirmed by XPS study.
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TEM analysis HRTEM microscopic images of TCG (which are shown in Fig. 6A–C) showed a transparent, silk like appearance of ultra thin graphene sheets after reduction. It is also shown that the edges of the suspended graphene films tend to fold back, permitting the cross-sectional view of the films. From Fig. 6B, it is clearly concluded the formation of three layered graphene sheets. The selected area electron diffraction (SAED) pattern of graphene shows a typical sharp polycrystalline ring pattern composed of many diffraction spots (Fig. 6D), representing the loss of long range organization between the formed graphene nanosheets.
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Fig. 2. UV–Visible spectra of (a) GO (green) (b) TCG (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
HPLC analysis HPLC analysis is performed to know the polyphenolic constituents present in the aqueous extract of T. chebula seeds.
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1402.25
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Fig. 3. FTIR spectra of GO (blue) and dried TCG (pink). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. S1A is showing the presence of four different phytochemicals in T. chebula aqueous extract namely, ascorbic acid, gallic acid, pyrogallol and methyl gallate which are known by comparing with the retention times of the respective standards under the same instrumental conditions. Further, the presence of gallic acid as major constituent in T. chebula extract indicates its predominant role in the reduction of GO to TCG (Fig. S1A). Mechanism
Fig. 4. XRD patterns of GO and TCG.
The suggested mechanism for the reduction of GO by T. chebula extract is shown in Fig. S1C. The surface of the GO contains different oxygen containing groups such as epoxide, hydroxyl, and carbonyl [39]. Under basic conditions both the epoxide and carbonyl groups may be converted into hydroxyl groups. T. chebula plant extract contains four different types of polyphenols namely,
Fig. 5. SEM images and EDS spectra of GO (A, C) and TCG (B, D).
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Fig. 6. HR-TEM images of TCG showing the formation of few layer graphene (A–C) and the SAED pattern (D).
gallic acid, pyrogallol, ascorbic acid, methyl gallate (Fig. S1A and S1B), containing highly acidic hydrogen atoms, which easily disassociate to their anionic forms. The anionic forms of polyphenols react with the hydroxyl moiety of GO through a nucleophilic substitution 2 (SN2) mechanisms resulting in the formation of intermediate 1. The intermediate 1 is transformed to intermediate 2 with the elimination of water molecule which finally forms T. chebula reduced graphene oxide (Fig. S1C). Raman analysis Raman spectroscopy analysis was carried out to know the size of sp2 domains in the graphene structure containing both sp3 and sp2 network. The ratio of ID/IG obtained for both GO and TCG were 1.47, 2.63 respectively (Fig. 7). The increase of ID/IG ratio after reduction may be due to the higher adsorption of the oxidized polyphenols on the graphene surface, thus increasing the surface sp3 domains. Fig. 7 also showed a shift of about 10 cm 1 in G band of graphene oxide from 1594 cm 1 to 1584 cm 1 after reaction indicating a high degree of reduction. However, an increased ID/IG ratio of TCG compared to the GO further illustrated the formation of graphene crystalline domains after reduction [41].
by incubating 40 mg of T. chebula reduced graphene oxide in a hot furnace for about 2 h at 500 °C and are observed to be the loss of 90% by weight after the incubation period (data not shown) which is further confirmed by TGA results shown later. In addition, FTIR spectra were taken for the dried water extract of T. chebula before and after the reduction of GO, to find out the biomolecular capping of the formed graphene sheets. Fig. 8 showed the presence of intense infra red bands at 3446, 1400 cm 1 corresponding to the O–H and C–O stretching of phenolic groups indicating the rich content of polyphenols in T. chebula extract. However the reduced intensity of O–H stretching and the absence of C–O stretching vibrations at 1400 cm 1 with the appearance of new vibrational band at 1710 cm 1 (characteristics of ketone group) in the extract
Stabilization of TCG From the UV–Vis data (Fig. 2), it is confirmed that the presence of broad band at 362 nm indicates the presence of oxidized polyphenols on the surface of reduced graphene oxide, which are responsible for preventing the agglomeration of successive graphene sheets. Furthermore, the weight loss studies have been performed to know the quantity of biomolecules present on the surface of graphene sheets. These studies have been carried out
Fig. 7. Raman spectra of GO (black) and TCG (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. FTIR spectra of T. chebula leaf extract before reduction (green) and after reduction (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. (A) Dynamic light scattering spectrum of TCG aqueous dispersion, (B) TGA thermogram of TCG, (C) optical diffuse reflectance spectra of T. chebula reduced graphene oxide.
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after the reduction further confirmed the conversion of polyphenols into their quinone forms during the deoxygenation of GO. From these studies, it is assumed that the oxidized polyphenols interact with TCG sheets by p–p stacking interactions that generate electrostatic repulsions between the graphene layers resulting in the stabilization of individual graphene sheets. The p–p interactions between graphene sheets and the oxidized polyphenols have been represented in Fig. S2A.
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polyphenols present in T. chebula extract. It is also found that polyphenols in T. chebula are not only responsible for the deoxygenation, but also for stabilization of formed graphene nanosheets by preventing their aggregation. This paper further opens up an environmental friendly method to synthesize a biocompatible graphene in large scale by using naturally occurring polyphenols rich plant extracts. Acknowledgments
XPS study Further, high-resolution XPS C1s spectrum of prepared TCG (Fig. S2B and C) showed the presence of typical C1s peaks at 284.2, 286.2, 288, 289 eV corresponding to C@C, C–O–H, C@O, O–C@O respectively. However, the higher intense peak at 286.2 eV and the other peaks corresponding to the remaining oxygen functionalities are due to the biomolecular capping on the surface of TCG, which is also further supported by the presence of high intense C–O–H peak (533.1 eV) and other peak for C@O (532 eV) in the O1s spectrum [40].
Mr. S.B.M. greatly acknowledges the help of VIT University, Vellore-632014, India for the financial help and platform given to do this research. Also, he acknowledges the help from Department of Chemistry, National Tsing Hua University, Hsinchu-30013, Taiwan ROC and NIIST, CSIR, Thiruvananthapuram-695 019, India. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.02.037.
DLS analysis
References
We also employed DLS analysis to know the size distribution of TCG particles in aqueous dispersion by using the standard spherical model (Fig. 9A). After the prolonged sonication, the graphene sheets are broken into particles and the average particle size of the graphene particles present in the TCG dispersion measured by using DLS is 1014 nm.
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TGA Further to confirm the presence of oxygen containing groups, thermal stability of TCG was examined by TGA (Fig. 9B). Thermogram of as synthesized TCG exhibits a weight loss of 14.5% below 200 °C due to the loss of absorbed water, oxygen functionalities and the other biomolecules present on the surface of graphene sheets. Subsequently, it also exhibited a large weight loss of 86% at 500 °C, because of the pyrolysis of the remaining oxygen containing groups and also due to the burning of the ring carbon. This higher weight loss at 200 °C when compared to the graphene synthesized by using other green methods is clearly indicating the high accumulation of biomolecules on the surface of graphene sheets after reduction [41,42]. Band gap studies The optical band gap of RGO was calculated from UV–Visible spectra by the following equation as a = c (hm Ebulk)1/2/hm, where hm is the photon energy, a is absorption coefficient, c is a constant, and Ebulk is bulk ‘band gap’. Fig. 9C shows the plot drawn between hm versus (ahm)2 for the formed graphene, by using UV–Visible spectroscopy at the wavelength range from 200 nm to 2400 nm. The band gap was obtained by the extrapolation of a linear regression to the X-axis in the plot and the gap was found to be 3.74 eV. The value of band gap is observed to be in almost similar agreement with previous reports obtained by using aqueous leaf extracts of Colocasia esculenta and Mesuaferrea Linn and an aqueous peel extract of Citrus sinensis [39]. Conclusions In summary, we demonstrate a green, low-cost ecofriendly synthetic route for the synthesis of graphene by using polyphenols from T. chebula seed extracts. HPLC analysis identified the
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