Germanate nanowires/nanorods with different lengths were synthesized and used as additives for the fabrication of polymer composite membranes for high-flux water filtration. We for the first time demonstrated that at a small nanowire/nanorod loading (e.g., <0.5 wt % on the basis of poly(ether sulfone)), the length of germinate nanowires was a key parameter in determining their migration and diffusion in the polymer solution, and thus affecting polymer precipitation in the membrane formation process. In particular, short Ca2Ge7O16 nanowires with an average length of 138.7 nm and an average diameter of 12.7 nm, and Zn2GeO4 nanorods with an average length of 400 nm and an average diameter of 18.7 nm quickly diffused out of the membrane, leading to a higher pore density on the active layer in comparison with the pristine membranes. The addition of short Ca2Ge7O16 nanowires resulted in greater pore sizes than the addition of Zn2GeO4 nanorods because the out-diffusion of the former was faster than that of the latter. In contrast, the addition of long Ca2Ge7O16 nanowires with lengths of several tens to hundreds of micrometers and an average of 27.3 nm was not effective in promoting the pore formation because of partial embedment of nanowires. Poly(ether sulfone) composite membranes prepared by adding a small amount of Zn2GeO4 nanorods exhibited dramatically enhanced water permeation without losing rejection property. For example, the poly(ether sulfone) (PES) composite membrane prepared with 0.3 wt % Zn2GeO4 nanorods exhibited the highest flux, 1294.5 LMH, which was 3.5 times of the pristine (PES) membrane (384.2 LMH). Our work provides a new strategy for developing high-performance ultrafiltration membranes for practical industrial filtration applications.
Keywords: germanate nanowires; high flux; nanoporous structure; polymer; ultrafiltration membranes.