Facile synthesis of graphene using a biological method

Abstract

A new, facile, low cost, environmentally safe process is demonstrated for the production of few layer graphene by liquid phase exfoliation of graphite using extracts of medicinal plants in water. Plant extracts possibly function as bio-surfactants by creating a barrier in the aggregation by adsorbing on to the exposed surfaces of the graphite, weakening the attraction between the layers created by the van der Waals forces and allowing the graphite to slowly exfoliate in the form of undamaged flakes.

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The exceptional electrical and thermal properties, combined with its mechanical stiffness, strength and elasticity, make graphene a fascinating material.^1,2 Graphene is produced by scotch tape peeling,³ chemical vapor deposition,⁴ chemical reduction or thermal treatment of graphene oxide,^5–7 solvent or chemical/biological surfactant assisted exfoliation of graphite with sonication.^8–12 At present, the chemical reduction or thermal treatment of graphene oxide (GO) by the Hummer's method is the most frequently used practice for the scalable production of few layer graphene.^5–7 However, there is a probability of occurrences of structural defects within the graphene sheets which can compromise some of the properties and the unique morphology of the pristine two dimensional hexagonal carbon lattices due to harsh oxidization steps involved in the process.⁸ In addition, costs involved in process, safety, and environmental considerations are high due to multistep process comprising the use of concentrated acids in oxidization and the harsh chemicals to reduce GO. Therefore, researchers are looking for the easily scalable processes to produce graphene with low basal plane and edge defects.

The processes involving sonication of graphite with solvent or surfactants to produce graphene flakes with low defect concentration have been proven to be promising.^9–12 Aromatic donor containing organic solvents such as ortho-dichlorobenzene, n-methylpyrrolidone, and benzylamine assisted exfoliation resulted in stable dispersions through extended low power bath sonication, though these solvents are expensive and require special handling.^9,13 Diverse surfactants have also been found useful for large scale production.¹⁴ The use of sodium cholate with longer sonication periods of 400 hours exhibited good exfoliation of graphite and yield of graphene.¹⁰ However, toxicity issues involved in use of some surfactants can limit their use due to their properties of bioaccumulation, adsorption to proteins, disruption of enzyme function, and organ damage.¹⁵ The additional cost required for waste water treatments to restrict mammalian exposure could reduce the value of the process.¹² Therefore, there is need for development of eco-friendly approaches of graphene synthesis. Recently, use of gum Arabic as a green alternative for the exfoliation of graphite to produce graphene by sonication was investigated.¹² The self-assembling hydrophobin Vmh2 protein extracted from the fungus Pleurotus ostreatus was reported for biofunctionalized defect-free graphene synthesis through the liquid phase ultrasonic exfoliation of raw graphitic material.¹¹ Different approaches are used for synthesis of diverse nanomaterials.^16–18 Plant extracts have been demonstrated to be valuable for biosynthesis of variety of nanomaterials and the phyto-synthesized nanomaterials display beneficial characteristics as compared to other synthesis modes for diverse applications.^19–26 Using plants for nanomaterial synthesis can be advantageous over other biological processes because it eliminates the elaborate process of maintaining cell cultures and purifying specific components; and can also be suitably scaled up for large-scale nanomaterial synthesis.^20,21,27 In this regard, use of plant extracts for synthesis of graphene from graphite via liquid phase ultrasonic exfoliation in low boiling solvents like water will be promising new approach.

In the present study, we demonstrate a novel, low cost, environmentally safe process for the production of few layer graphene by liquid phase exfoliation of graphite using extracts of medicinal plants. The synthesized materials by a selected plant were characterized by UV-visible spectroscopy, Raman spectroscopy, and high-resolution transmission electron microscopy (TEM). To the best of our knowledge, this is the first report on direct biosynthesis of graphene from graphite using plant extracts.

The scheme of plant extract mediated liquid phase exfoliation of graphite to graphene is summarized in Fig. 1a–e. The approach included low power sonication of expanded graphite (Samjung C & G, Korea, 1–10 g L⁻¹) in plant extracts (50 g L⁻¹) for 24 hours by maintaining the temperature lower than 30 °C with continuous flow of water in the ultrasonication bath (JAC-Ultrasonic 4020P). After sonication, the dispersion was left to sit overnight to enable separation of large unstable graphite aggregates. The stably dispersed solution of graphene was collected and centrifuged at 1500 rpm for 90 min to get graphene solution which was used for further testing.

Fig. 1 The scheme of plant extract mediated graphene synthesis using liquid phase exfoliation of graphite by sonication at low power. (a) Plant materials (50 g L⁻¹) were boiled in deionized water for fifteen minutes, (b) plant extracts were filtered through Whatman number one filter paper, (c) expanded graphite (1–10 g L⁻¹) was added in plant extracts to form graphite solution, (d) the mixture was sonicated for 24 hours using extended low power ultrasonication and the stably dispersed supernatant after centrifugation at 1500 rpm for 90 min was collected, (e) supernatants of 24 hour old liquid phase exfoliated colloidal dispersion prepared using (1) sodium cholate; extracts of plant (2) Xanthium strumarium, (3) Kalopanax pictus (4) Diospyros kaki (5) Pinus strobes, (f) Tyndall effect on diluted graphene solution prepared using Xanthium strumarium.

The stable black dispersions were seen for Xanthium strumarium and Artemisia princeps plant extracts; similar to 1 g L⁻¹ sodium cholate (Acros organics, USA) assisted exfoliation using the same graphite precursor as per previously reported method of Lotya et al.,¹⁰ which was run as a control. The ability of various plant extracts to form stable graphene dispersion was different (Table 1, Fig. 1e). Moderate black dispersions were observed for Alnus species, Hovenia dulcis, Kalopanax pictus, Leonurus cardiac, Magnolia Kobus, Morus species, and Prunus species plant extracts. The stable graphene dispersions were not observed for some plant extracts as the exfoliated material settled at the bottom. Tyndall effect generated due to scattering of the laser light of the stable particles was observed for the diluted graphene prepared using X. strumarium plant extract (Fig. 1f).

Table 1 Properties of graphite solution after 24 hour sonication with plant extracts and sodium cholate in water

Scientific name	Plant part used	Spectra detected in UV-Vis spectrophotometry	Colour of supernatant
Acer species	Leaves	365	Transparent
Alnus species	Leaves	329	Moderate black
Artemisia princeps	Leaves	359, 267	Black
Castanea crenata	Leaves	239	Transparent
Chionanthus retusus	Leaves	327, 280	Transparent
Diospyros kaki	Leaves	264	Faint black
Hovenia dulcis	Bark	279	Moderate black
Kalopanax pictus	Stem	331, 296, 235	Faint black
Kalopanax pictus	Leaves	266	Moderate black
Leonurus cardiaca	Leaves	391, 296, 257	Moderate black
Magnolia kobus	Leaves	348, 236	Moderate black
Malus species	Leaves	456, 280	Transparent
Morus species	Leaves	327, 236	Moderate black
Nelumbo nucifera	Flower	293	Transparent
Nelumbo nucifera	Leaves	316	Transparent
Pinus strobus	Cone	452, 239	Transparent
Pinus strobus	Needles	273, 235	Transparent
Prunus species	Leaves	395, 296, 233	Moderate black
Taxus cuspidata	Leaves	277, 223	Transparent
Xanthium strumarium	Fruits and seeds	268	Black
Sodium cholate	—	268	Black

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UV-Vis spectrophotometric analyses observations for graphene prepared using X. strumarium plant extracts and sodium cholate are similar to the studies of Chabot et al.,¹² who reported the appearance of the peak centered at 268 nm and a nearly constant absorbance above 600 nm for graphene prepared by gum Arabic by sonication assisted approach (Fig. 2a). The characteristic peak from plant extracts revealed minimal absorbance at 268 nm demonstrating that graphene from different production technology has characteristic peak at UV range.

Fig. 2 (a) UV-visible spectra of graphene prepared using Xanthium strumarium plant extract and sodium cholate, (b) Raman spectra of expanded graphite and graphene prepared using Xanthium strumarium plant extract.

Raman analyses (NT-MDT, NTEGRA) revealed the defect ratio (I_d/I_g) of 0.62 for graphene produced by X. strumarium plant extract, which is in the range of earlier reported studies for other surfactant based exfoliation methods.^{10,12–14,28–30} The D-band (I_d, 1350 cm⁻¹) in graphite is negligible compared to the high G-band (I_g, 1580 cm⁻¹) and a moderately intense 2D band is visible at higher wave numbers.¹² The low increase in the D-band indicates the mild exfoliation process which leads to very few basal plane defects and only moderate levels of edge defects. The low number of edge defects for the graphene flakes further supports the unaltered graphitic character of the basal plane. The defect ratio for graphene synthesized by plant extracts is much less than the defect ratio reported by Chabot et al.¹² for reduced graphene oxide (r-GO) samples (I_d/I_g of 1.31) prepared by Hummer's method. The basal and edge defects in r-GO samples by Hummer's method are reported to be created due to the harsh oxidation process.¹² The low defect ratio in the present study indicates low amount of the basal and edge defects in graphene produced by plant extract. Raman analysis has also been shown in literature to be an effective means for determining flake thickness of graphene materials.^1,31 The occurrence of D-band at 1342 cm⁻¹ in this study suggests presence of few layer graphene as per literature.³² A characteristic shift in the 2D peak position and shape is indicative of the transition from graphite to graphene materials. Interestingly, 2D peak present in graphite demonstrated the expected shift in the peak position and shape (Fig. 2b). The peak broadening and 20–30 cm⁻¹ shift in the literature suggests that the graphene flakes are composed of layers between 5–20 layers in thickness.^1,12 The 2D peak in this study for graphene prepared by X. strumarium plant extract was found at 2687 cm⁻¹. The 2D peak centered in the range of 2650–2690 cm⁻¹ designates the presence of 5–10 layer graphene in the samples.^12,33 This indicates that the graphene prepared by plant extract in this study is few layer graphene. The number of graphene layers was estimated to be 5.573 using the empirical equations proposed by Paton et al.,³⁴ further indicating synthesis of few layer graphene.

Representative micrographs of many TEM images taken on 200 kV FE-TEM (JEM-2100F HR, Jeol ltd.) revealed the well-defined layer structure of the few layer graphene (Fig. 3a–c and e) as observed in earlier studies by other researchers.^10,11 Selected area electron diffraction (SAED) pattern of graphene is illustrated in Fig. 3d, which exhibits crystalline structure of graphene. The inner six member ring comes from the (1100) plane, while the six brilliant points are related to the [0001] diffractions and retain the hexagonal symmetry of the [0001] diffraction pattern.³⁵ The diffraction pattern images indicate that the resulting graphene has been restored into the hexagonal graphene framework.

Fig. 3 TEM micrographs (a–c and e) and SAED pattern (d) of liquid phase exfoliated graphene samples prepared using Xanthium strumarium plant extract.

The yield of stably dispersed graphene produced by Xanthium strumarium plant extract from initial 10 g graphite after freeze drying was found to be 0.62 g. Thermo-gravimetric analysis (TGA) was used to detect the amount of plant material remaining in the derived graphene powders. The TGA curve of the graphene powder indicates two stages of degradation separated by a transition region (Fig. 4a). The first stage which occurs between 200–400 °C is in agreement with the bulk degradation temperatures of plant material similar to gum Arabic.¹² The remaining plant material burns away slowly as the temperature increases to 660 °C suggesting approximately 31% plant material mass left in the dried graphene powder.

Fig. 4 (a) Thermo-gravimetric analysis and (b) electrical conductivity results for graphene powder produced using Xanthium strumarium plant extract.

The graphene powder was compressed into thin wafers and subjected to 4-probe analysis for measuring electrical conductivity. The low defect concentration in the graphitic structure of the graphene pellet enabled us to achieve electrical conductivity around 80 S cm⁻¹ at 258 MPa (Fig. 4b). Other researchers previously reported comparable values between 15–100 S cm⁻¹ by other liquid-phase dispersion techniques.^9,10,12,36 The electrical conductivities of a single graphene sheet are significantly more (theoretical in-plane conductivity ∼10⁶ S cm⁻¹)³⁷ than experimentally measured conductivities of graphene films. This suggests that the resistance of the film is dominated by the resistance of the inter-particle junctions.³⁸ As per report of Chabot et al.,¹² the electrical conductivity of the graphene film produced by the current process has higher electrical conductivity than that of the r-GO film.

A new, simple, economical, eco-friendly, and scalable strategy to produce few-layer graphene flakes through mild sonication with the use of eco-friendly plant extracts is demonstrated in the present study. The exfoliation of initial graphite into few-layer graphene flakes occurred in plant extract assisted sonication approach as indicated by the formation of dark black color of the graphene dispersions and the results of Raman peak shift and high resolution TEM. The barrier in the aggregation may be created by adsorption of plant extracts to the exposed surfaces of the graphite and allowing the graphite to slowly exfoliate in the form of undamaged flakes. The functioning of the plant extract in exfoliation may be similar to the surfactants. Plant extracts are rich source of bioactive chemicals.^19,39 X. strumarium is a medicinal plant and extracts of the whole plant, especially fruits, seeds, leaves, and roots have been applied in traditional medicine.^40,41 Phytochemical constituents like sesquiterpene lactones, glycoside, phenols, and polysterols have been studied for various biological activities.⁴¹ The stress is applied on the graphite particles due to strong sonophysical energy in low power sonication, which is transferred throughout the sp² hybridized carbons in the graphene planes. The bioactive phytochemical constituents in the plant extracts combined with sonication result in weakening the attraction between the layers created by the van der Waals forces that hold the graphene sheets together. The plant metabolites may be intercalated in the layers and overcoming the 0.35 nm spacing of the graphite planes. This study lays the groundwork for other researchers to screen different plant extracts for synthesis of graphene and other familiar or unfamiliar carbon materials; optimize process for mass production and recycle the partially exfoliated flakes to further increase yield. As the synthesis route involves use of eco-friendly medicinal plant extracts, the produced materials may be less-toxic, biocompatible, and useful for diverse applications including biomedical applications.

Acknowledgements

This research was supported by the National Research Foundation of Korea (NRF-2012R1A1A2006375 and NRF-2013R1A2A2A01067117). We acknowledge Yongjae Jeong and Chorong Choi for their technical assistance. We thank Prof. Shichoon Lee, Sun A Moon, and Han-Min Kim for FE-TEM, TGA, and electrical conductivity analyses.

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