A Metal-Organic Framework Nanosheet-Assembled Frame Film with High Permeability and Stability.

Abstract The engineering of metal–organic frameworks (MOFs) into membranes and films is being investigated, to transform laboratory‐synthesized MOFs into industrially viable products for a range of attractive applications. However, rational design and construction of highly permeable MOF thin films, without trade‐offs in terms of structural mechanical stability, remains a significant challenge. Herein, a simple, general strategy is reported to prepare thin MOF nanosheet (NS)‐assembled frame film via heteroepitaxial growth from metal hydroxide film. As the thin MOF NS‐assembled film significantly enhances the permeability of mass though the film, the resultant gold nanoparticle (Au NP)@MOF film exhibits much higher catalytic efficiency than the Au NP@MOF bulk film. Meanwhile, the unique framework of the MOF NS‐assembled film resists torsion and collapse, so the composite catalyst exhibits long‐term stability.


Characterizations
The products were characterized by XRD (model D/MAX2500; Rigaku, Tokyo, Japan) with Cu-Kα radiation at a scanning rate of 3° min -1 . The Fourier transform-infrared (FT-IR) spectra were measured using a Nicolet 6700 instrument (Thermo Fisher Scientific, Waltham, MA, USA). The morphologies were characterized by a S-4800 scanning electron microscope (Hitachi, Tokyo, Japan) equipped with energy-dispersive X-ray spectroscopy (EDX) analysis functionality. The transmission electron microscopy (TEM) images were obtained using H-800 (Hitachi) and JEM-2010 (JEOL, Tokyo, Japan) instruments at an accelerating voltage of 200 kV. The Au content was quantified by an Optima 7300 DV inductively coupled plasma-atomic emission spectrometer (ICP-AES; PerkinElmer, Waltham, MA, USA). The surface area and pore diameter were evaluated using a physisorption analyzer (model ASAP 2020M; Micromeritics, Norcross, GA, USA) at −196°C. Prior to the measurements, samples were degassed in vacuo at 180°C for at least 8 h. The S BET was calculated using adsorption data at P/P 0 of 0.05-0.30. The pore size distributions (PSDs) were derived from the adsorption branches of the isotherms using the Barrett-Joyner-Halenda (BJH) model. The total pore volume (V t ) was estimated from the adsorbed amount at P/P 0 of 0.995. The TGA was carried out using a Pyris 1 TGA (PerkinElmer) with a nitrogen flow of 10 mL min -1 .

Preparation of Zinc hydroxide nanostrands film
Zinc hydroxide precursor were synthesized following a synthesis strategy described by Peng et al. [1] Equal volumes of 4 mM copper nitrate solution and 2.0 mM aminoethanol solution were rapidly mixed and aged at room temperature for 30 min. Filtering 60 mL of the mixture solution through a nylon 66 microporous membrane left a white thin film on the membrane.

Preparation of CuBDC bulk and NAF films on silicon wafer
First, the silicon wafers were sonicated for 15 min in acetone, ethanol and deionized water, respectively. The CuBDC film on Nylon 66 microporous membrane was heat at 100 o C for 15 min, then it was immersed in a cold acetone solution instantaneously. The CuBDC film was detached from the Nylon 66 microporous membranes spontaneously. Finally, the self-standing CuBDC film was deposited on the silicon wafer.

Preparation of CuBDC NS film on silicon wafer
First, the silicon wafers were sonicated for 15 min in acetone, ethanol and deionized water, respectively. The CuBDC NS films were deposited onto the aforementioned cleaned substrates through the Langmuir-Schäfer method. [2][3][4][5] Briefly, the synthesized CuBDC NSs were first dispersed in ethanol to obtain a colloidal suspension with a concentration of 1.0 mg mL −1 . Then the suspension was gently dropped onto the surface of water in a watch glass. After the CuBDC NSs spontaneously spread to form a thin film on water, the film was transferred onto a silicon wafer via the Langmuir-Schäfer method. Finally, the film-coated solid substrate was immersed into ethanol to remove the loosely deposited nanosheets prior to blowing it with N 2 . The aforementioned procedure is defined as one deposition cycle. By repeating the aforementioned deposition procedure, MOF nanosheet films could be obtained.  This result show that the CuBDC nanosheets are connected to each other to form a integrate membrane, and the CuBDC NAF could be easily deposited on any substrate which is highly desirable for practical applications.  In the initial time, the surface of copper hydroxide nanostrands thin film is quite flat and hundreds of copper hydroxide nanostrands can be see clearly. After the copper hydroxide nanostrands thin film was dipped into organic ligand solution, the morphology of the parent copper hydroxide nanostrands would readily evolve into blurry. Some small MOFs sprout emerged and stacked on the surface of film after 20 min of reaction at room temperature. When the process was prolonged to 2 h, it was observed that the nanosheets have covered the surface of the film and assembled into frame. The copper hydroxide nanostrands thin film has transformed into nanosheet assembled frame completely at 6 h.      The SEM images demonstrated that the CuBDC NAF structure tend to emerged at low concentration of organic ligands (0.01 g L -1 -0.2 g L -1 ) while the CuBDC NAF structure formed at high concentration of organic ligands (10 g L -1 -50 g L -1 ).

Supplementary Note 2. The mechanisms for the preparation of CuBDC crystals:
For the low concentration of ligands, as dissolution and coordination rate are too low for the concentration of This process was considered as nucleation of CuBDC crystals from the solution, which were deposited onto the surface of the templates. Afterwards, the Cu(H 2 BDC) 2+ concentration remained low (~0.5 ppm), indicating that the whole process takes place without any release of metal ions into the solution. As shown in Fig. 4j, the pH values were also tracked during the reaction, and were found to gradually decrease as the concentration of Cu(H 2 BDC) 2+ decreased, which can be attributed to an acid-base interaction between the Cu(OH) 2 and H 2 BDC.      Table 2 in the Supporting Information).