Self-cleaning MOF: realization of extreme water repellence in coordination driven self-assembled nanostructures

Bio-inspired self-cleaning surfaces have found industrial applications in oil-water separation, stain resistant textiles, anti-biofouling paints in ships etc. Interestingly, self-cleaning metal-organic framework (MOF) materials having high water contact angles and corrosion resistance have not been realized so far. To address this issue, we have used the fundamentals of self-assembly to expose hydrophobic alkyl chains on a MOF surface. This decreases the surface free energy and hence increases hydrophobicity. Coordination directed self-assembly of dialkoxyoctadecyl-oligo-(p-phenyleneethynylene)dicarboxylate (OPE-C ) with Zn^II in a DMF/H₂O mixture leads to a three dimensional supramolecular porous framework {Zn(OPE-C₁₈)·2H₂O} (NMOF-1) with nanobelt morphology. Inherently superhydrophobic and self-cleaning NMOF-1 has high thermal and chemical stability. The periodic arrangement of 1D Zn-OPE-C₁₈ chains with octadecyl alkyl chains projecting outward reduces the surface free energy leading to superhydrophobicity in NMOF-1 (contact angle: 160-162°). The hierarchical surface structure thus generated, enables NMOF-1 to mimic the lotus leaf in its self-cleaning property with an unprecedented tilt angle of 2°. Additionally, superhydrophobicity remains intact over a wide pH range (1-9) and under high ionic concentrations. We believe that such a development in this field will herald a new class of materials capable of water repellent applications.