Ultrathin hydrophobic films based on the metal organic framework UiO-66-COOH(Zr)

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.

Keywords: Langmuir–Blodgett (LB) films; UiO-66-COOH(Zr); hydrophobic coating; metal organic framework (MOF); surface modification.

Figure 1

Surface pressure–area (π–A) isotherms obtained using different UiO-66-COOH(Zr) + ODP suspensions in the MOF concentration range 0.025–0.11 mg·mL⁻¹. ODP content in all suspensions is 0.8 µmol·mL⁻¹. Note that relative ODP wt % changes because the mass amount of ODP for all the dispersions is the same but MOF concentration changes.

Figure 2

SEM images of Langmuir–Blodgett films transferred at: (a) 25 mN·m⁻¹, (b) 30 mN·m⁻¹, (c) 35 mN·m⁻¹ and (d) 45 mN·m⁻¹. Spreading suspensions were 0.05 mg·mL⁻¹ UiO-66-COOH(Zr) + 10 wt % ODP mixtures. The scale bars correspond to 5 µm.

Figure 3

CO₂ adsorption isotherms for drop-cast films: pure UiO-66-COOH(Zr) (blue triangles), pure ODP (black squares) and mixture UiO-66-COOH(Zr) + 10 wt % ODP (red circles).

Figure 4

GIXRD pattern of an LB sample of UiO-66-COOH(Zr) + 10 wt % ODP (black line). For comparison purposes, the experimental powder diffraction of UiO-66-COOH(Zr) (PXRD, blue line) and ODP (PXRD, red line) is included.

Figure 5

AFM images of a pure ODP LB film (a,b) and a mixed MOF/ODP LB film (c,d), transferred onto mica substrates at a surface pressure of 30 mN·m⁻¹. RMS values of the images are: (a) 5.70 nm, (b) 6.41 nm, (c) 40.25 nm, (d) 61.28 nm. The films were prepared using suspensions containing 0.05 mg·mL⁻¹ of UiO-66-COOH(Zr) + 10 wt % ODP.