Fig. 4: Changes in the structure and OFGs upon ripening.

a, b Band intensity changes of iGO, mGO and tGO with the ripening time. The horizontal axes are expressed as the logarithm of the reduced time (t_red). a \(\pi -{\pi }^{*}\) transitions. b \({{{{{\rm{n}}}}}}-{\pi }^{*}\) transitions. Ripening temperature: black solid line and circles, 298 K; green solid line and squares, 308 K; blue solid line and diamonds, 333 K; red solid line and triangles, 348 K. Band intensities of the non-ripened GO colloid are shown with open circle. Error bars in a, b represent the reproducibility in the band intensity-determination of at least three measurements. c Total oxygen content determined by XPS. Black circles, GO; brown square, rGO. Solid and dashed lines represent the changes upon ripening and reduction, respectively. d Compositional changes of the oxygen-containing functional groups determined from the deconvoluted C1s spectra. The fraction of carbon atoms not bonded to oxygen atoms is excluded. Oxygen-containing functional groups: red lines and triangles, epoxy group (C–O–C); blue lines and squares, hydroxyl group (C–OH); green lines and circles, carbonyl group (C=O); purple lines and diamonds, carboxyl group (COOH). Solid and dashed lines represent the changes upon ripening and reduction, respectively. The horizontal axes in c, d are expressed as the logarithm of the ripening time of GO ripened at 348 K. The error bars in c, d represent the standard deviation of three measurements at different positions. Brown and gray zones represent the transition regions.