The morphology observations of the membranes were carried out on a FEI Tecnai G20 transmission electron microscope (TEM) and a HITACHI S-4700 scanning electron microscope (SEM). X-ray diffraction (XRD) patterns were recorded on an X’Pert PRO SUPER rA rotation anode X-ray diffractometer with Ni-filtered Cu-Ka radiation (l¼1.5418 ˚A). Raman spectra were recorded on a confocal Raman system (Horiba Jobin Yvon HR800, France) with an excitation of 632.8 nm laser light. UV–vis absorption spectra