Abstract
This work reports on the fabrication, optimization and characterization of ultrathin films containing submicrometer particles (sMPs) of the hydrophilic and water stable UiO-66-COOH(Zr) metal organic framework (MOF). MOF particles of ≈200 nm have been synthesized and assembled at the air–water interface by the Langmuir–Blodgett technique. The use of different solvents, mixtures of solvents and surfactants has been investigated in order to improve the stability of MOF dispersions and reduce particle aggregation. The compact MOF/surfactant films containing 10 wt % octadecylphoshonic acid (ODP) have been deposited on substrates of different nature by Langmuir–Blodgett (LB) and Langmuir–Schaefer (LS) methods, showing that the presence of even only one MOF/ODP monolayer can increase the water contact angle of highly hydrophilic substrates such as mica or glass up to 120°. These films were characterized by scanning electron microscopy, grazing incidence X-ray diffraction, Fourier transform infrared spectroscopy and atomic force microscopy, revealing the formation of a continuous film where ODP molecules adopt an almost vertical position and cover MOF particles. Moreover, the presence of MOF particles significantly enhances the surface roughness and allows ultrathin, hydrophobic coverage to be obtained. Finally, it has been shown that the crystallinity and the porosity of the MOF remains almost unaltered in MOF/ODP films.
Highlights
Metal organic frameworks (MOFs) are well-known, crystalline, porous materials formed by metal ions and organic ligands coordinated in a pre-designed manner to form pores and/or channels of desired dimensions [1]
Langmuir films of pure MOF submicrometer particles (sMPs) obtained at the air–water interface using diluted suspensions in chloroform showed a lack of reproducibility, which could be ascribed to poor suspension stability, mostly associated with particle aggregation
It can be observed that the CO2 adsorption capacity of the pure UiO-66COOH(Zr) film is similar to that of mixed film UiO-66COOH(Zr) + 10 wt % octadecylphosphonic acid (ODP), which is in good agreement with the results reported for other Zr-based MOFs that do not result in significantly reduced adsorption capacity after modification with ODP [34]
Summary
Metal organic frameworks (MOFs) are well-known, crystalline, porous materials formed by metal ions (or metallic clusters) and organic ligands coordinated in a pre-designed manner to form pores and/or channels of desired dimensions [1]. To study the hydrophobic character of LB, LS and RLS films, water contact angle (WCA) values were measured on glass and mica substrates before and after coating with one monolayer mixed film MOF + 10 wt % ODP deposited at 30 mN·m−1.
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