Controllable Modification of Optical Properties of Graphene Oxide

Controllable generation of band gaps in graphene is one of the important current tasks in modern nanoelectronics. To investigate whether this can be achieved by controllable functionalization, we have studied the changes in optical properties of graphene oxide in water suspension upon ozone-induced oxidation of reduced graphene oxide (RGO). The reduced graphene oxide, whose properties strongly resemble graphene, was produced by oxidizing graphite using the Hummers method followed by a hydrazine reduction. Exposure of the resulting RGO to ozone for the periods of 5 to 35 minutes caused a dramatic bleaching of the sample’s visible and ultraviolet absorption and the concurrent appearance of strong visible fluorescence in previously nonemissive samples. These observed spectral changes suggest a functionalization-induced band gap opening. The ozone treatment also caused solubilization of RGO in water. Such fluorescence emission from graphene oxide was found to be highly pH-dependent. Sharp and structured excitation and emission features resembling the spectra of molecular fluorophores were present near 500 nm in basic conditions. In acidic conditions fluorescence spectra reversibly broadened and shifted to 680 nm, while the excitation spectra remained similar in shape and position, suggesting the excited state protonation of the emitting species in acidic media. Oxygen-containing addends resulting from the ozonation treatment were detected by XPS and ATR FTIR spectroscopy and computational modeling was used to relate their presence to optical transitions in localized graphene oxide fluorophores. Further research on controlled ozonation-induced functionalization of graphene will be directed towards producing graphene-based optoelectronic devices with tailored and controllable optical properties.