Tailoring TiO2 membranes for nanofiltration and tight ultrafiltration by leveraging molecular layer deposition and crystallization

Abstract As an advanced gas phase deposition technique, molecular layer deposition (MLD) has recently emerged as a precisely controllable method to fabricate organic-inorganic hybrid materials such as metalcones and hybrid films. Porous structures can be prepared by thermally removing the organic constituents of the metalcones deposited by MLD. Herein, we revealed that there is a crystallization-induced volume shrinkage during thermal treatment of metalcone films. By taking advantage of both the chemical degradation of organic moieties of metalcone films and the crystallization-induced volume shrinkage, we built microporous TiO 2 layers on porous ceramic substrates and obtained nanofiltration and tight ultrafiltration membranes with tunable molecular-weight-cut-off (MWCO) in the range between 630 Da and 3050 Da. Firstly, 50 MLD cycles of titanicone (TiEG) were deposited on the near-surface of plate ceramic ultrafiltration membranes to achieve a compact layer, which was then subjected to calcination at different temperatures to remove the organic constituents. The bare membrane with a water permeance of ∼130 L m −2  h −1 bar −1 and MWCO of ∼63 kDa was upgraded from ultrafiltration to nanofiltration with ∼7 L m −2  h −1 bar −1 permeance and ∼630 Da MWCO after MLD and calcination at 250 °C. Moreover, as the microporous TiO 2 was transformed from amorphous to crystalline with rising calcination temperatures, there was higher volume shrinkage and larger effective pore sizes. Consequently, both water permeance and MWCOs of the membranes were increased. For instance, the membranes after calcination at 400 °C showed a tight ultrafiltration performance of ∼30 L m −2  h −1 bar −1 permeance and ∼3050 Da MWCO. This work provides a simple and effective way to fabricate ceramic membranes with tunable performances between ultrafiltration, tight ultrafiltration, and nanofiltration.