Controlled synthesis of reduced graphene oxide sheets on large scale using thermal exfoliation

Abstract

Oxidized graphene-based materials have been widely studied due to its excellent properties like, mechanical, electrical, high chemical stability, high thermal conductivity, and large specific area. Particularly, graphene oxide (GO) and reduced graphene oxide (rGO) has grabbed much attention for wide range of applications in the field of batteries, supercapacitors, sensors, drug delivery, catalysis,  opto-electronic devices and so on.[1] Graphene oxide is nothing but a layers of carbon sheets heavily decorated with oxygen function groups such as hydroxyl, carbonyl, carboxyl and epoxy groups. Variety of methods has been developed by the researchers for the synthesis of graphene oxide which includes, brodie’s method, Staudeumeir method and hummers and its modified method.[2–4] Despite of many attempts large scale production of graphene oxide and reduced graphene oxide with equivalent conductivity is great challenge to achieve. Similarly for reducing oxygen functional groups from the material, many strategies have been employed such as chemical, solvothermal, photo-reduction, electrochemical, and thermal. Amongst all, thermal reduction is one of the simple, easy and less time consuming for the large-scale production of reduced graphene oxide.

Here, we report on synthesis and characterisation of graphene oxide and reduced graphene oxide. Graphene oxide was prepared by using modified hummer’s method as it is recommended for large scale production with less impurities and more defects in the carbon basal plane.[5] The prepared graphene oxide was reduced via thermal reduction at 300 º C for 1 minute. The resultant product was characterised to study structural, chemical and optical properties. The structural analysis was done using X-ray diffraction in Bragg Brentano geometry. Single hexagonal phase of the material was confirmed. Field-emission scanning electron microscopy was used to investigate the surface morphologies of the prepared powder. Fig 1 (A) displays cross view of graphene oxide sheet and (B) top view of thermally exfoliated reduced graphene oxide. The cross-view image of graphene oxide sheet (fig 1 (A)) clearly shows stacking of graphene layers while thermally exfoliated reduced graphene oxide displays comparative distance between the stacked layers. The chemical composition of the samples was estimated by using energy dispersive x-ray analysis in order to determine the proportion of carbon and oxygen in the synthesized materials. To study the defects and degree of disorder induced during the synthesis was analysed using Raman spectroscopy. The presence of D and G band in samples revealed successful synthesis of graphene oxide via modified hummer’s method. Similarly, the decrease in the intensity of D band of thermally exfoliated GO shows comparative reduction of oxygen functional groups. Further to study the degree of disorder in the materials, ID/IG were calculated.[6] The optical properties were examined by using UV-Vis-NIR spectroscopy in the diffused reflectance spectroscopy mode. The band gap estimated from the UV-Vis. Spectroscopy was found to be 3.9 eV and 3.2 eV for GO and rGO. As much as 1 g of graphene oxide and reduced graphene oxide powders is derivable by such thermal exfoliation technique in a single batch with controlled crystallographic phase purity and high degree of reproducibility. These findings are a demonstration of scalable synthesis of graphene derivatives for its functional applications.