Comparison of Fabrication Methods of Metal-Organic Framework Optical Thin Films

Homogeneous metal-organic frameworks (MOFs)-based optical thin films have attracted increasing attention, since they can potentially be used as active components in optical/opt-electrical devices, and how to fabricate MOF thin films with high quality is the premise of practically using them. Herein, five fabrication methods of MOF films are systematically investigated and compared from the aspects of appearance, reflectivity, micro-morphology, surface roughness, and optical properties of the films. The famous robust Zr-based MOF, UiO-66 (UiO = University of Oslo) is chosen as a model, and the five methods are spin-coating, dip-coating, self-assembly, direct growth, and the stepwise layer by layer growth method. This study provides fundamental support for the application of MOFs in the optical field.


Synthesis of UiO-66 nanocrystals
The synthesis of UiO-66 nanocrystals was performed according to literature 1, 2 .
49.8 mg of H2BDC and 30 μL of TEA were dissolved in 5 mL of DMF while 66.8 mg of ZrCl4 and 1.38 mL of acetic acid were dissolved in 5 mL of DMF separately. The solutions of H2BDC and ZrCl4 were combined in 20 mL vial, capped and placed in 85º C oven for a day. The resultant UiO-66 nanocrystals were washed three times with DMF using a centrifuge (4,400 rpm for 20 min) and sonication, and then sequentially immersed in methanol for three 24 hrs periods. Finally, UiO-66 nanocrystals were activated by removing the solvent under vacuum for 12 hrs at room temperature, and the resultant products were as raw materials for subsequent MOFs film preparation by spin coating, dip-coating and self-assembly.

Synthesis of Zr-precursor
Synthesis of Zr-precursor, Zr6O4(OH)4(MAA)12 was performed according to literature 3,4 . MMA (1.4 mL) was added to 2 mL Zr(PrO)4 solution, with addition of a drop of water. Reaction mixture was stirred for 10 minutes using a magnetic stirrer in open flask and left to stand for one hour before the volume of the solution was reduced to 1/4 of a starting volume. The formed colorless solid was filtered under vacuum and washed once with 2 mL of i-propanol. Products, colorless crystalline materials were subjected to PXRD and FTIR-ATR analyses prior to further stepwise layer-by-layer growth.

Pre-treatment of substrates
Silicon wafers were cut into small pieces (~2 cm × 2 cm) which were used as substrates for fabrication of MOF thin films. The substrates were pre-cleaned with soap and water and subsequently treated with piranha solution (H2SO4/H2O2, volumetric ratio 7:3) at 70 °C for one hour. After thoroughly rinsing with deionized water, the wafers were dried under air flow and stored in ethanol. For spin-coating, dip-coating, self-assembly deposition and direct growth, the substrates were dried under nitrogen flow before use. For layer-by-layer growth, the substrates were functionalized with H2BDC by incubating in a DMF solution containing 33 mM H2BDC at room temperature for 3 hours, and then cleaned with ethanol and dried with a stream of nitrogen gas before use.

Spin-coating method
The MOF optical thin films were fabricated by spin-coating 200 µL of UiO-66 nanocrystals alcoholic suspensions with MOFs content of 4.3 wt.% onto the substrate at 3000 rpm for 60 s. The films were annealed at 200°C for 20 min after coating, and denoted as OTF-SP.

Dip-coating method
The MOF optical thin films were fabricated by dip-coating UiO-66 nanocrystals alcoholic suspensions (4.3 wt%) at room temperature and under ambient atmospheric conditions, using withdrawal speed of 1 mm· s -1 . The films were annealed at 200°C for 20 min after coating. The deposition process was performed once, trice and five times, and the corresponding thin films were denoted as OTF-DP-1, OTF-DP-3 and OTF-DP-5, respectively.

Self-assembly method
Before assembly, the surface of UiO-66 nanocrystals were functionalized with PVP according to literature and redispersed in water/ethanol (v:v=1:1) solution. The self-assembly was carried out on water-air interface according to a previously reported procedure 5 . Briefly, an 8 mm×35 mm glass slide was leaned against the rim of a Petri dish (14 cm in diameter and 1.5 cm in depth) and the Petri dish was then carefully filled up with DI water. Subsequently, 100 ul UiO-66 suspension (10 mg/ml) was dropped on the glass slide which spread quickly on water surface. After 5 min, 2-3 drops of SDS solution (2wt%) were added to consolidate the UiO-66 film so that a close-packed monolayer was formed. Afterwards, the monolayer was transferred onto the silicon substrate. Finally, the transferred film was dried at 200°C for 20 min.
The process was repeated for one, two, or three times and the corresponding thin films were denoted as OTF-SA-1, OTF-SA-2 and OTF-SA-3, respectively.

Direct growth
The composition ratio of the precursor solution is ZrCl4:H2BDC:H2O:acetic acid:DMF = 1:1:1:500:1500. In detail, ZrCl4 (0.0933 g) and H2BDC(0.0655 g) were dissolved in 46.2 mL of DMF. Then, 11.5 mL of acetic acid and 0.0072 mL of water were added to the solution and further mixed at room temperature for 20 minutes. A silicon substrate was then introduced horizontally into a Teflon stainless steel autoclave and the precursor solution was added. The reaction was performed in a 100 o C oven for 24 hours. After solvothermal synthesis, the resulting MOFs film was immersed in water and washed several times with water. The resulting film was finally dried at 200°C for 20 min. the according thin films were denoted as OTF-DG.

Layer-by-layer growth
The growth were carried out on thermostatic coating machine (PTL-OV5P, MTI).
The functionalized substrate was dipped successively in (1) an ethanolic Zr-precursor

Characterization
Scanning electron microscopy (SEM) images were obtained using a S-4800 electron microscope (Hitachi, Japan). The samples were sputtered with a thin layer of Au prior to imaging. Fourier transform infrared (FTIR) spectra were obtained using a Spectra Two spectrophotometer (PerkinElmer, USA) from 4000 cm -1 to 400 cm -1 with am attenuated total reflection (ATR) accessory. The X-ray diffraction (XRD) patterns were collected using a Ttr III type X-ray diffractometer (Rigaku, Japan) in θ-θ geometry with a graphite-monochromated CuKα radiation source. The N2 adsorption-desorption isotherm of the samples at liquid nitrogen temperature (78K) and gas saturation vapor tension range was measured by a BEL Mini sorption instrument. Thermogravimetric analysis (TGA) was completed by a STA6000 thermogravimetric analyzer (Perkin Elmer, Waltham, MA, U.S.) at a scanning rate of 10 °C min −1 under N2 atmosphere from 30 to 700 °C. A fiber spectrometer (USB2000+, Ocean optics, USA) coupled to optical microscope was used to measure the specular reflectance in the 400-900 nm range at the normal incidence. The surface profiles were characterized using a profilometer (Talysurf PGI 1240, Taylor-Hobson, UK) with a diamond stylus. Ellipsometry measurements performed with a XLS-100