Background: Reduced graphene oxide (rGO) is often prepared from graphene oxide (GO) by using various reducing agents like hydrazine hydrate, dimethylhydrazine, sodium borohydride, hydroquinone etc. In view of toxicity of chemical reagents, recent trends are directed towards the use of nature-derived biomolecules as the reducing source. The present study describes a comparative evaluation of different properties of reduced graphene oxides (rGOs), prepared by using chemical and biological reducing sources, and investigated their catalytic functions in the reduction of nitrobenzene to aniline.
Methods: GO is prepared from graphite powder following modified Hummers oxidation and subsequent exfoliation by sonication. In order to obtain rGO, four different plants leaf extracts viz. Adathoda Vasika (Malabar nut), Azadirachta Indica (Neem), Camellia Sinensis (Tea), Moringa Oleifera (Drumstick), and a fungi extract of Volvereilla Volvacea (Mushroom) are chosen primarily from the local sub-Himalayan region, all are edible to human beings as well as have long been used as traditional medicines to combat with various diseases. The plant leaves are washed carefully with distilled water, kept in air oven at 80 oC for two days, and then the dry leaves are crushed to fine powder. The plant leaf-extracts are obtained by stirring a suspension of each type of finely grinded leaf powder (500 mg in 80 mL of DI water) overnight at room temperature and filtering the extract to remove any dust plant leaves. Each rGO is denoted as rGO-AV, rGO-AI, rGO-CS, brGO-MO and rGO-VV, based on the abbreviation of sources. The rGO-HH stands for rGO obtained by using hydrazine hydrate.
Results: Each rGO was characterized by UV-Vis, FT-IR, Raman spectroscopic techniques, and surface morphological aspects were studied by powder XRD, Scanning and Transmission electron microscopic images (SEM and TEM). The acidic nature (pH) of each rGO in aqueous suspension as well as cation-exchange capacity was measured by potentiometric titration in the absence and presence of an electrolyte respectively. Finally, the catalytic ability was evaluated in the reduction of nitrobenzene to aniline at room temperature monitored by UV-Vis spectrophotometer. While textural aspects of various rGOs are fairly similar, various physicochemical properties like pH, cation-exchange ability etc are found to be different for rGOs obtained by using different phyto-extracts. Moreover, there is significant variation observed in their catalytic activity in the reduction of nitrobenzene.
Conclusion: Present studies establish that the rGO can be efficiently prepared by various nature-derived aqueous extracts. Although the textural aspects of rGOs display marginal changes, their catalytic activity in the reduction of nitrobenzene to aniline greatly differs, as studied by UV-Vis spectrophotometric kinetic measurements. By comparison, it is found that rGOs obtained by using plant leaf extract of Adathoda Vasika, (rGO-AV) and an edible mushroom extract of Volvereilla Volvacea, (rGO-VV) exhibit significantly better catalytic efficiency than others.