Fig. 3

Characterization of surface functional groups of GO and GOP membranes. XPS spectra of GOP membrane in C 1s region (a) and N 1s region (b); c ATR-FTIR spectra of GO and GOP membranes, showing the new carbon-nitrogen peak appeared and the original carboxyl and carbon double bond shifted; d XRD patterns of GO and GOP membranes at dry and wet states, respectively, and the corresponding d-spacing