Studies on synthesis of reduced graphene oxide (RGO) via green route and its electrical property

Highlights

• RGO-1 and RGO-2 were synthesized by green method using different phytoextracts.

• M. indica L., S. tuberosum L. phytoextracts act as reducing and stabilizing agents.

• Conjugated structure of graphene is established following partial reduction of GO.

• Electrical conductivities of RGO-1 and RGO-2 are higher than GO.

Abstract

An environmentally friendly method has been applied for the preparation of reduced graphene oxide (RGO). This method was developed by using polyphenols that contained a phytoextract of Mangifera indica L. along with Solanum tuberosum L. as reducing agents since they are non-toxic and naturally available. The phytoextracts used in the production of RGO was set between 60 and 70 °C. Graphene oxide (GO) was prepared by modified Hummer’s method as reported in earlier findings. Structural and morphological studies demonstrate that the part of the oxygen functionalities in GO can be removed by following green reduction. Characterizations of the resulting product have been done by X-ray diffraction, FTIR, UV–vis and Raman spectroscopy. FESEM, TEM, EDX spectrum, TGA, DLS and Zeta potential measurements of the samples have also been carried out to study the morphological, thermal and surface charge characteristics. Electrical conductivity was also measured to check the extent of reduction of GO to RGO.

Introduction

Graphene is a two dimensional nanostructure with single layer carbon atoms firmly packed into a honey comb crystal lattice [1]. Due to graphene’s unique mechanical, thermal, catalytic, electrical and optical properties [1], it has attracted tremendous attention in recent years [2], [3], [4], [5]. Graphene has the ability to be used in a wide range of fields including bio-sensing [6], drug delivery [7], catalysis [8] and energy storage [9]. This is why it can be used in a wide range of applications such as nano-electronics [10], Li-ion batteries [4], thin-film transistors [11] and solar cells [12], [13]. Generally the production of individual graphene sheets in bulk quantity proves to be a significant challenge but the problem can be solved by the chemical reduction [14], [15], [16], [17] (using hydrazine, di-methyl hydrazine, hydroquinone, sodium borohydride etc.) of graphene oxide (GO). The reason being is that it has a low cost of production of reduced graphene oxide (RGO) however, the reducing chemicals are either non-environmentally friendly, poisonous or both. The presence of trace amounts of such toxic agents could have detrimental effect, especially for bio-related applications [18], [19]. Even in the case of metal/hydrochloric acid reduction of GO, metal particles may remain as impurities and tendency of π-π stacking between chemically reduced GO sheets may form irreversible aggregation [20]. In this context, the employment of green technology for the reduction of GO have been reported to overcome the above problem.

Alternatively few other methods for the preparation of RGO such as the exfoliation of GO under strong alkaline conditions and lower temperature, chemical vapor deposition (CVD) [21], synthesis using biomolecules as reducing agents such as ascorbic acid [22], [23], amino acid [24], sodium citrate [25], glucose [26], bovine serum albumin [27] etc. have also been reported. However, flash photo reduction [28], hydrothermal dehydration [16] and solvothermal reduction [29] processes lead to irreversible aggregation due to strong Vander Waals forces between RGO sheets which hamper its processibility. To avoid these difficulties, various surface modifications of GO have been introduced using small organic molecules, biomolecules and polymers or surfactants such as poly(N-vinyl-2-pyrrolidone) [30] and poly(sodium-4-styrene sulfonate) [15], etc. in order to improve the dispersibility of RGO sheets and prevent them from aggregation. Various phytoextracts such as carrot roots [31], green tea [32], bacteria (Escherichia coli) [33], C. sinensis peel (Orange), S. aromaticum (Cloves), S. oleracea (Spinach), R. damascene (Rose), P. serrulata (Cherry) etc. [60] have also been used to produce RGO. Although the degree of reduction through green methods is lower than that of the chemical methods, they are widely accepted in biological and biomedical fields. But some of the reported green methods suffer from limitations of high time consumption (e.g. 48–72 h) [19], relatively poor stability [25], [34] and poor solubility (e.g. 0.1 mg/ml) [23] which may not be propitious to obtain a large quantity of RGO in order to store them for a long time.

In this present work we have used two different types of aqueous phytoextracts such as Mangifera indica L. (mango) leaves extract and Solanum tuberosum L. (potato) extract as reducing agents, which are commonly available, eco-friendly, non-hazardous and have low environmental impacts [31]. These phytoextracts contain many phenolic compounds (Fig. 1) such as caffeic acid, chlorogenic acid, gallic acid, protocatechuic acid, salicylic acid, vanillic acid [35], [36], [55], [56], [57] etc. and have a large number of hydroxyl groups, which endow them in having mild reducing properties [58], [59]. This helps in the partial removal of the oxygen containing functionalities from GO during reduction in order to restore electronic conjugation in RGO. The presence of these phytoextracts will also help to prevent the extensive agglomeration in an aqueous dispersion of the resulting RGO through electrostatic repulsion interactions of the negative charge densities of many carboxylic groups. The green synthesized RGO is highly dispersible in water.