2.5.1. Characterization of membrane structure
The morphology of the membrane surfaces was characterized using a Gemini500 (Zeiss, Germany) SEM instrument. The cross-sectional morphology of the membranes was analyzed using a SU8220 (Hitachi, Japan) SEM apparatus. The chemical structure of the thin surface layers was studied using a Nicolet FTIR (Thermo Scientific, USA) spectrometer in attenuated total reflectance (ATR) mode. The cross-sections of the TFN-(Zr/Ti) membranes were further investigated using a JEM-2010 (JEOL, Japan) transmission electron microscopy (TEM) device. To prepare the membrane samples for TEM analysis, we embedded them in E44 epoxy resin. Subsequently, 100 nm thick sections of the membranes that were cut using an EM UC7 (Leica, Germany) ultramicrotome were transferred to a TEM grid. The roughness of the membrane surfaces was measured using a MultiMode V (Veeco, USA) atomic force microscopy (AFM) instrument.