Figure 4: Solid-state NMR study.

1D ¹³C MAS solid-state NMR spectra of (a) GO, (b) EG-1 h and (c) EG-5 h. All the spectra were obtained at a ¹³C NMR frequency of 125.78 MHz with 15 kHz MAS for the samples of (a) 33 mg, (b) 38 mg, (c) 45 mg. The ¹³C MAS spectra were obtained with direct ¹³C excitation by a π/2-pulse followed by a rotor-synchronous echo. The peaks marked by * denote spinning sidebands. The recycle delay was 2 s, and the experimental times were (a) 3 h and (b,c) 7 h. A Lorentz broadening of 300 Hz was applied to improve the signal-to-noise ratios. ¹³C T₁ values of the samples were found to be 0.5–0.6 s. The ¹³C π/2-pulse widths are (a) 3.65 μs and (b,c) 5.75 μs.