Hydrophilic modification of polyvinylidene fluoride membranes by ZnO atomic layer deposition using nitrogen dioxide/diethylzinc functionalization

Thanks to the controllable deposition process, atomic layer deposition (ALD) has been applied for hydrophilic modification of organic membranes. Sometimes the enhanced hydrophilicity is at the expense of sacrificing membrane flux due to granular deposits generated on the hydrophobic membrane pores. This study proposed an innovative process of nitrogen dioxide (NO₂) and diethylzinc (DEZ) functionalization combined with Zinc oxide (ZnO) hydrophilic modification to realize the enhancement of membrane hydrophilicity and separation performance. Results showed that an ultrathin "ZnO-like" layer was formed on membrane surface and pore walls after 10 cycles of NO₂ and DEZ using ALD, and a more uniform and conformal layer was formed on the activated membrane after modification. More importantly, the growth rate of ZnO modified layer on activated membranes was higher than that of direct deposited membranes. After 200 ALD cycles, the water contact angle decreased from 140° of original membrane to 67° and 20° respectively for the direct modified membrane and NO₂/DEZ activated membrane with further modification. Furthermore, compared with direct modified membranes, a maximum water flux of 5261.4 L m^-2 h^-1 bar^-1 and superior separation performance with BSA retention of 97% were achieved for activated membranes at only 100 ALD cycles of modification. Further exploration of the mechanism indicated that the "ZnO-like" layer could accelerate the subsurface nucleation and growth of modified layer. The pattern of uniform surface growth was realized at lower ALD cycles, achieving improvement of water flux and separation performance simultaneously. The pretreatment method applied in this study could be of great potential in surface activation of other membranes.