Fig. 1(b) depicts the reaction schemes for the synthesis of a TiO2–GO photocatalytic membrane. First, TiO2 nanoparticles were attached to the PSf base membrane by the Ti–O bond formed between Ti4 þ and sulfonic groups and/or by the hydrogen bond between the hydroxyl groups of TiO2 and sulfonic groups [46]. Next, GO was deposited on the TiO2 layer via Ti–O and/or hydrogen bond between ti and the carboxyl groups of GO. Finally, GO was partially reduced and bonded to TiO2 by the ethanol–UV treatment [44,45]. Note that the reduction of GO can be easily verified by the color change (from light brown to dark black) of the membrane surface [16].