Water-enhanced oxidation of graphite to graphene oxide with controlled species of oxygenated groups

Water-enhanced oxidation of graphite via a modified Hummers method can produce graphene oxide with controlled species of oxygenated groups.

mol −1 to remove the remaining acid and metal species. The resultant GO aqueous dispersion was then stirred overnight to exfoliate it to GO. Successively, the GO dispersion was then centrifuged at 3000 r.p.m. for 20 min twice to remove the unexfoliated particles.
Preparation of GO at 0 o C. Water (0 or 4 mL) was mixed with ice-cooled concentrated sulfuric acid (46 mL, < 5 o C) in a 250 mL flask setting in an ice bath with mechanical agitation at 200 r.p.m. After cooling the mixture to < 5 o C, 1 g of natural graphite powder was added, and then 3 g KMnO 4 was slowly put into the system within 30 min. Successively, the reaction mixture was mechanically stirred (300 r.p.m) for 12 or 48 h under water bath cooling to about 0 o C by refrigerator system. After oxidation, the reaction mixture was poured into 300 mL ice/water mixture to keep the temperature < 10 o C. The resulting solution was stirred for 15 min, and followed by the addition of 5 mL H 2 O 2 . The washing and dialysis processes were the same to those described above. Afterward, the GO dispersion was then centrifuged at 3000 r.p.m. for 20 min 5 times to remove the unexfoliated particles and used for further comparisons. To obtain large-size GO sheets, centrifugation rate of 2000 r.p.m. should be applied to avoid precipitation of lager GO sheets at higher centrifugation rates. The SEM images in Figure S18 demonstrate the large-size GO sheets synthesized at 0 o C for 48 h with adding 4 mL water and finally centrifuged at 2000 r.p.m.

Preparation of GO and rGO Papers.
The GO films were prepared by drying 5 mg mL −1 GO suspensions in plastic containers placed in a glass dryer at room temperature. The thickness of each GO film was controlled to be about 6−7 µm by adjusting the volume of GO suspension and the size of container. rGO films were obtained by chemical reduction of GO films by immersing them in a 1:1 (by volume) mixture of HI solution (57 wt. %) and ethanol sealed in a container at room temperature overnight. The rGO films were then repeatedly washed with water and ethanol, and finally dried at room temperature overnight for characterizations.

Measurements of GO yields.
A certain volume of purified GO aqueous dispersion was freeze-dried for over 48 h, and the weight of dried GO was used to calculated the concentration of GO (C GO in mg mL −1 ). The yield of GO (Y GO ) was calculated by Y GO = (C GO ×V GO /m Gr )  100%, where V GO is the total volume of purified GO dispersion (in mL), m Gr is the weight of feeding graphite powder (in mg).

UV-vis spectral studies on the oxidative solutions.
A certain volume of water (0, 4, 8, or 12 mL) was mixed with ice-cooled concentrated sulfuric acid (46 mL, < 10 o C) in a 250 mL flask setting in an ice bath with mechanical agitation at 200 r.p.m. After cooling down the solution to < 10 o C, 3 g of KMnO 4 was slowly added and stirred for 15 min under ice bath. Successively, the reaction system was transferred to a 40 o C oil bath and the stirring rate was increased to 300 r.p.m. Samples were taken out from the reaction system after heating for 0 or 2 h, and diluted for recording their UV-vis spectra.
Characterizations. X-ray photoelectron spectra (XPS) were carried out by the use of an ESCALAB 250 photoelectron spectrometer (ThermoFisher Scientific) with Al Kα (1486.6 eV) as the X-ray source set at 150 W and a pass energy of 30 eV for high resolution scan. Solid-state 13 C magic-angle spinning (MAS) NMR experiments were conducted on a Bruker Avance-III spectrometer (100.6 MHz 13 C, 400.1 MHz 1 H) and a 4 mm MAS rotor probe. Direct 13 C pulse spectra were acquired using a MAS spinning speed of 10,000 Hz and ~10,000 scans. Raman spectra and optical images were recorded on a LabRAM HR Evolution (Horiba Jobin Yvon) with a 514-nm laser.

Water-Enhanced Oxidation of Graphite to GO at 0 o C
Water-enhanced oxidation can greatly increase the yield of high-quality GO synthesized at 0 o C. For comparison, the GO samples were prepared by oxidation of graphite in the systems with addition of a mL (a = 0 or 4) water for b hours (b =12 or 48), and they were nominated as GO-a-0-bh. The yields of GO-4-0-bh were found to be much higher than those of the corresponding GO-0-0-bh, as reflected by the color difference of their dispersions ( Figure S13a). Actually, the yields of GO-0-0-12h and GO-0-0-48h were measured to be only ~0 and 1 ± 0.3 %. However, the yields of GO-4-0-12h and GO-0-0-48h were dramatically increased to 9 ± 1 and 60 ± 2 % ( Figure   S13b). It should be noted here, the GO-0-0-12h product did not have monolayer GO sheets, composing of partially exfoliated particles. GO-0-0-12h completely precipitated from its dispersion upon keeping undisturbed for several days.
Water-induced increase of the yield of GO-a-0-bh was caused by increasing its oxidation degree. As shown in Figure S14a, the relative content of oxidized carbon atoms increased remarkably with the increase of water volume from 43.7 % for GO-0-0-12h to 47.9 % for GO-4-0-12h, and from 45.5 % for GO-0-0-48h to 53.7 % for GO-4-0-48h. Raman spectral studies indicated that the I D /I G s of GO-a-0-bh samples decreased as 'a' changed from 0 to 4 ( Figure S14b), also reflecting the increase of oxidation degree. SEM images ( Figure S14c) showed that the sizes of GO-4-0-bh sheets are larger than those of GO-0-0-bh counterparts. The largest sheets of GO-4-0bh sheets are larger than 10 µm, while those of GO-0-0-bh are smaller than 7 µm. This is mainly due to that the higher oxidation degrees GO-4-0-bh sheets facilitate the exfoliation from their GrO precursors.
The TGA curves of GO-a-0-bh are shown in Figure S15. Among them, the curve of GO-0-0-12h exhibits two steps of thermal decomposition in the temperature range of 150 to 300 o C, indicating the presence of organosulfate groups. This incomplete hydrolysis of organosulfate was resulted from unexfoliated GrO, because the organosulfate groups in GrO matrix were inaccessible to water. The decomposition of organosulfate are not observed from the other TGA curves, indicating the corresponding GO samples have been well exfoliated and hydrolyzed. These TGA curves also indicate that the contents of functional groups in the GO samples are in the sequence of GO-0-0-12h < GO-4-0-12h ≈ GO-0-0-48h < GO-4-0-48h. XPS and Raman analysis also confirmed this observation.
The XRD pattern ( Figure S17b) of rGO-4-0-48h exhibits single characteristic (002) reflection peak at 2θ = 24.2 o , corresponding to a d-space of 3.68 Å. This d-space is smaller than those of rGO-n (3.71−3.74 Å) because of its lower content of the residual oxygenated groups.
Typical Raman spectrum of rGO-4-0-48h ( Figure S17c) features a stronger Dbands with respect to G-band (I D /I G = 1.72), indicating the restoration of graphitic structures. S2, S3 Furthermore, the G band of rGO-4-0-48h is a combination of a narrow G peak centered at ~1580 cm −1 and a broader blue-shifted peak (G+D'), having a higher intensity ratio of I G /I D' with respect to that of rGO-0. S4 Moreover, the intensity ratio (I 2D /I D+G ) of 2D band (~2690 cm −1 ) to D+G band (~2940 cm −1 ) of rGO-4-0-48h was measured to be 1.31, much higher than that (1.03) of rGO-0. S4 This result implies that the structural integrity of rGO-4-0-48h sheets is better than that of rGO-0 sheets.