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

A superhydrophobic self-cleaning MOF nanostructure has been synthesized using a unique ligand design strategy and coordination directed self-assembly. The material has hierarchical surface roughness and is stable under extreme corrosive conditions.

Tetrahydrofuran was pre-dried using standard procedure and all other reagents, solvents were of reagent grade and used without further purification.

Physical Measurements
Infrared spectral studies were done by making samples with KBr pellets using Bruker FT-IR spectrometer. Thermal stability of the NMOF-1 is analyzed using Mettler Toledo TGA 850 instrument under inert atmosphere in the temperature range of 25-1000 °C at a heating rate of 3

Adsorption Measurements
Porosity measurements were carried out using QUNATACHROME QUADRASORD-SI analyser at 77 K for N 2 and 195 K for CO 2 . In the sample tube the adsorbent samples (∼100-150 mg) were placed which had been prepared at 170 °C under a 1×10 -1 Pa vacuum for about 12 h prior to measurement of the isotherms. Helium gas (99.999% purity) at a certain pressure was introduced in the gas chamber and allowed to diffuse into the sample chamber by opening the valve. The amount of gas adsorbed was calculated from the pressure difference (P cal -P e ), where P cal is the calculated pressure with no gas adsorption and P e is the observed equilibrium pressure.
All the operations were computer-controlled. Solvent vapor adsorptions were carried out at 298K

Contact Angle Measurements
Contact angles were measured using an indigenous set up coupled with a Logitech camera for capturing the images. Contact angles were also measured using dedicated contact angle analyzer, OCA30 from Data Physics instrument (GmbH, Germany). 4 μL of the sessile water droplets were employed for measuring the static contact angles. A minimum of ten measurements were made. Scheme S1. Synthetic scheme for the fabrication of NMOF-1.

EDAX Spectrum
Fig. S1 EDAX analysis of nanobelts showing the presence of Zn II metal ion in NMOF-1.

Superhydrophobicity models
Young's equation (cosθ = (γ sv -γ sl )/ γ lv where θ is the contact angle between the solid-liquid interface, γ sv,sl and lv are the corresponding surface tensions between the solid, liquid and air interfaces) is used to describe the wettability on a smooth surface. On a rough surface, the same is explained by two models: Wenzel and Cassie Baxter. 4 According to the Wenzel model, the water droplet on a rough surface is spherical in shape and wets the surface. As a consequence, it will not roll off the surface under the slightest disturbance. A transition from the Wenzel to the Cassie-Baxter model occurs when we consider that rough textures on a surface trap air-pockets in between. Therefore the water droplet cannot assume a spherical shape and rests on the rough texture with air-pockets trapped in between. This results in a low adhesion of the droplet to the surface and hence on the smallest perturbance, water rolls off the surface. It is theorized and verified experimentally that low surface energy along with a hierarchical surface roughness is essential to generate such superhydrophobic structures. A hierarchical structure implies the presence of roughness at two regimes: micro and nano. This leads to water contact angles >150° giving rise to superhydrophobicity and self-cleaning applications.

PXRD analysis before and after self-cleaning experiments
Fig. S15 PXRD pattern of NMOF-1 before (red) and after (blue) the study of self-cleaning property with water.