Green synthesis of reduced graphene oxide using ball milling

Today, finding superior methods for synthesis of graphene oxide (GO) and reduced graphene oxide (rGO) from commercial graphite, is still a challenge.The global industry requires the use of synthesis methods that are price-competitive and also use processes that are environmentally compatible.GO is a substrate used for several chemical transformations, including its reduction to related materials, such as rGO and graphene.The rGO has been used recently to increase catalytic activity in the o-nitroaniline reduction to NaBH 4 and Raman activity in test molecules as p-aminothiophenol [1].Other authors showed that the rGO in combination with carbon nanotubes has the properties of supercapacitors [2].Mao et al.[3] obtained a gas sensor type field defect transistor of high sensitivity at low concentrations, using sheets of rGO.On the other hand, rGO linked with Cu 2 O nanocrystals is capable of removing dye pollution from wastewater [4].An efficient method for synthesizing GO was improved by Hummers et al. [5] in 1958, who used KMnO 4 and H 2 SO 4 .The current synthesis methods are based on modifications and improvements of this method.Among these, the ones to be emphasized are those that use reduction agents (e.g., hydrazine, hydroquinone, sodium borohydride, and ascorbic acid) [5][6][7][8][9].In new materials synthesis, the search for environmentally friendly methods has motivated several authors to explore the use of bio-reducers obtained from plants, fruits, fungi, and bacteria [10][11][12][13][14].This has led us to consider, among various options, the Opuntia ficus-indica (OFI) plant extract for its significant potential.This plant is a strong reducer and stabilizer used in nanomaterials synthesis.By using this OFI extract, we can dispense with using H 2 SO 4 , HNO 3 , or H 3 PO 4 ; as well as the common oxidants (KMnO 4 or KClO 4 ) for rGO synthesis.On the other hand, the plant extract has traces of Mn, Cl, P, K, S, Se, etc. [15].In this research, we are proposing a green synthesis method by which to generate rGO from commercial graphite, methanol, and OFI extract as reducing agent, in a high energy process of wet ball milling.Young OFI cladodes were cleaned and separated from the transparent external cuticle.Then, they were cut into small pieces and added to deionized water at 80C until the color changed from green to yellow (about an hour).Next, the heated solution was passed through a filter containing small pieces of young leaves, to obtain a viscous, whitish solution.Then, 2.0 mg of commercial graphite (Sigma Aldrich, USA; 99.995% purity level), 1.0 mL of methanol, 2.0 mL of OFI extract, and 40 mL of deionized water were mixed in a beaker.Afterwards, the mixture was put in a zircon oxide vial and subjected to a high energy ball milling (HEBM) process at 250 r/min.This was done intermittently every 30 min for 8 h.Subsequently, 20 mL of a solution of 0.01 M ascorbic acid was added and the milling process continued for 8 h.The mixture changed its tone from black to gray.At the end of the process, samples were dried at room temperature.The synthesis process using high-energy milling