Spectroscopic studies of large sheets of graphene oxide and reduced graphene oxide monolayers prepared by Langmuir–Blodgett technique

Abstract

Graphene oxide (GO) sheets prepared by chemical exfoliation were spread at the air–water interface and transferred to silicon substrates by Langmuir–Blodgett technique as closely spaced monolayers of 20–40μm size. Hydrazine exposure followed by annealing in vacuum and argon ambient results in the formation of reduced graphene oxide (RGO) monolayers, without significantly affecting the overall morphology of the sheets. The monolayer character of both GO and RGO sheets was ascertained by atomic force microscopy. X-ray photoelectron spectroscopy supported by Fourier transform infrared spectroscopy revealed that the reduction process results in a significant decrease in oxygen functionalities, accompanied by a substantial decrease in the ratio of non-graphitic to graphitic (sp2 bonded) carbon in the monolayers from 1.2 to 0.35. Raman spectra of GO and RGO monolayers have shown that during the reduction process, the G-band shifts by 8–12cm−1 and the ratio of the intensities of D-band to G-band, I(D)/I(G) decreases from 1.3±0.3 to 0.8±0.2, which is in tune with the smaller non-graphitic carbon content of RGO monolayers. The significant decrease in I(D)/I(G) has been explained by assuming that substantial order is present in precursor GO monolayers as well as RGO monolayers obtained by solid state reduction.