Electrophoretic Crystallization of Ultrathin High-performance Metal-organic Framework Membranes

This method can help answer the key question in the synthesis of polycrystalline MOF films such as how to reproducibly synthesize MOF membranes. The main advantage of this technique is that we can control the heterogeneous nucleation over a wide range of substrates leading to ultrathin, pinhole-free MOF films. With the help of crystal engineering involving controlled nucleation and growth we aim to simplify the synthesis protocol of MOF films.

First cut a high purity copper foil into four by four centimeter pieces. For ease of clamping the cut foil with a set of copper crocodile clips draw a line 0.5 centimeters from one of the edges of each square foil. Then flatten each foil using a cylindrical roller on a clean surface.

Clean the copper foils thoroughly by bath sonication in acetone for 15 minutes followed by bath sonication in isopropanol for 15 minutes. Then dry the copper foils in a glass dish. Carefully position the desired substrate on the center of the copper electrode using tape.

Rinse the substrate electrode assembly for one minute with water followed by isopropanol and again with water. Following this attach a bare copper electrode to an anode. Then attach the substrate electrode assembly to a cathode.

Place the two electrodes in a 100 milliliter glass beaker and adjust the separation between them to one centimeter. Add 31.6 grams of zinc hexahydrate solution and 35 grams of benzimidazole solution to a 100 milliliter beaker and stir for 30 seconds at room temperature to form the precursor sol. Transfer the precursor sol to the beaker containing the electrodes.

Then immerse both the electrodes up to the 3.5 centimeter mark by adjusting the height of the beaker. Carry out electrophoretic deposition with the deposition voltage of one volt for four minutes by turning on the power of the power source. At the end of the deposition lower the beaker slowly to avoid disturbing weak adhesion between the freshly deposited nuclei and the substrate.

After drying the substrate transfer it to a microscopic glass slide using tape to hold the substrate in place. For crystal growth mix 31.6 grams of zinc hexahydrate solution and 35 grams of 2-Methylimidazole solution in a 100 milliliter beaker. Place the microscopic glass slide with the substrate vertically in the precursor solution and leave it undisturbed for 10 hours at 30 degrees Celsius.

After 10 hours of crystal growth rinse the substrate with water for 30 minutes. Then dry the substrate in a clean atmosphere. To prepare the sealant thoroughly mix an equal proportion of epoxy and hardener and leave the mixture for one hour.

Place the ZIF-8 membrane on a 24 millimeter wide steel disc with a five millimeter diameter hole at the center. Apply the epoxy along the edges of the substrate and subsequently cover the substrate except for the five millimeter diameter hole at the center. After allowing the epoxy to dry overnight use a stereomicroscope to scan the membrane along with the known reference scale.

Use graphical software to calculate the exposed area of the membrane from the scanned image. Next place the steel disc with the membrane in the stainless steel permeation cell and place the cell in an oven. Ensure a leak tight fit by placing Viton O-rings above and below the steel disk and fastening the screws.

Set the gas flow rates on the feed and the sweep sides to 30 milliliters per minute. To remove the absorbed water during synthesis heat the membrane cell using hydrogen as the feed gas and argon as this sweep gas at 130 degrees Celsius for two hours. Maintain a pressure of 0.1 megapascal at the feed and the sweep side by adjusting the needle valves on the on the retentate and the permeate side, respectively.

Change the gas flow in the feed side to target gas and set the gas flow rate to 30 milliliters per minute. Set the temperature of the oven to the desired temperature. Then calculate the permeance in Excel once a steady state is established according to the data from the mass spectrum.

The SEM images and XRD patterns show that the ZIF-8 nuclei film is compact and demonstrates that the ENACT method is quite efficient in controlling the heterogenous nucleation density over a substrate. Post growth the film morphology observed by SEM is compact and pinhole free and appears to be highly intergrown. The surface and cross sectional morphologies of the AAO support, ZIF-8/AAO membrane, PAN support, and ZIF-8/PAN membrane are shown here.

The gas permanence data of ZIF-8/AAO and ZIF-8/PAN membranes show that their hydrogen propane selectivity are 2700 and 190, respectively, proving that the MOF film is nearly defect free. The ZIF-8/AAO membrane shows ultra high hydrogen permeance because of the ultra thin thickness. While attempting this procedure it is important to wait for the crystal induction to occur before applying electrophoretic deposition.

For example crystal induction can be ratified by studying the precursor sol in a transmission electron microscope. This protocol allows one to precisely control the heterogeneous nucleation density and the thickness of the nuclei film by simply controlling the electric field and the electrophoretic deposition time and can be extended to synthesize polycrystalline films of various structures. After the development of this technique it paved the way for us to conduct systematic studies on engineering MOF films of thickness less than 100 nanometers.

Don't forget working with methylimidazole and zinc nitrate can be hazardous and precautions such as wearing gloves and operating in a fume hood should always be taken while performing this procedure.