PVDF membranes with simultaneously enhanced permeability and selectivity by breaking the tradeoff effect via atomic layer deposition of TiO2

Polyvinylidene fluoride (PVDF) is one of the most extensively used membrane materials, but PVDF membranes suffer deteriorating performances because of the intrinsic hydrophobic nature of PVDF. Significant efforts have been made to improve the hydrophilicity of PVDF membranes; however, there is frequently a tradeoff between the permeability and selectivity of the modified PVDF membranes. In this work, we broke the tradeoff effect by the atomic layer deposition (ALD) of TiO2 onto the PVDF membranes using TiCl4 and water as precursors. The vaporized precursors accessed the small membrane pores and produced a conformal and uniform thin layer of TiO2 that was tightly adhered to the pore walls. We progressively reduced the membrane pores by repeatedly cycling the deposition process until the membrane pores were completely blocked. The deposition of TiO2 enhanced the hydrophilicity and fouling resistance of the PVDF membranes, which was more evident at higher ALD cycle numbers. We simultaneously achieved greatly improved water flux and retention at 120 ALD cycles as a result of the competing effect of the increased hydrophilicity and reduced pore sizes. Moreover, the water flux could be sensitively tuned by changing the exposure time of the precursors. The thermal stability of the TiO2-deposited membranes was enhanced at a low ALD cycle number of 30.