Excitation wavelength dependent UV fluorescence of dispersed modified graphene oxide: Effect of pH

Modified graphene oxide (GO) was synthesized by controlled oxidation using different oxidizing agents with variable proportion, which induced chemical functionalizations in the GO sheets and thereby modulated the band gap. For the first time, an aqueous dispersion of modified GO exhibits a broad fluorescence in the UV region as a result of modulation in electronic band structure due to the small isolated sp2 carbon clusters containing about 10 aromatic rings isolated within sp3C-O matrix and the presence of oxygen containing functional groups at the GO surface. Observed UV fluorescence was found to be dependent upon excitation wavelength as a result of excitation of different moieties. UV fluorescence, obtained by exciting the dispersed GO at different wavelength, was dependent upon pH of the medium. With increase in pH, excitation at 240 nm exhibits a 32 nm blue-shift whereas excitation at 280 nm shows a 12 nm blue-shift. We investigated the origin of this blue-shift on the basis of different functional groups embedded in modified GO surface, movement of basal planes of GO and shape modulation of GO sheets.