Scientists are working hard to make graphene (thin and hard carbon layer material) into ultra-thin films for the preparation of water filters with better performance and longer life to rapidly clean up pollutants in large volumes of water bodies. .
The unique properties of graphene make it an ideal material for water filtration or seawater desalination. However, before large-scale application, one disadvantage must be overcome. That is, the preparation of single atomic layer graphene film is an ultra-fine operation process, and it is easy to destroy the material structure during the preparation process so that there is a structural defect in the finished product. Some of these defects leaked.
Currently, engineers from the Massachusetts Institute of Technology (MIT), the Oak Ridge National Laboratory, and the King Fahd University of Petroleum and Minerals (KFUPM) chemical deposition and Aggregation technologies combine to design a solution that can fix this flaw. The team then used a solution that had been previously obtained to build small but regular holes in the material, the size of the aperture allowing only water to pass through.
By combining chemical deposition and polymerization techniques, researchers have been able to produce graphene films that are relatively large in size (one-cent coin size) and have no structural defects. The size of the membrane material is critical: when used as a filter membrane, the finished product must be sized to be in the centimeter range or larger.
In the experiment, the researchers allowed the water to pass through the graphene membrane after they had been double-improved (repaired + pored), and then compared the filtration effect with the commonly used seawater desalination membrane. It was found that graphene membranes can filter out most of macromolecule contaminants (such as magnesium sulfate and dextran).
Rohit Karnik, an associate professor of mechanical engineering at the Massachusetts Institute of Technology, said: “At least on the laboratory scale, we have been able to repair graphene membrane defects. We must know that before the molecular filtration of graphene in the macroscopic field could not be achieved. If we can further The optimization of the preparation process may not require the repair of defects in the future, but I personally think that it is impossible to obtain this kind of “perfect graphene”, and the leakage of contaminant impurities will always occur.The double improvement (repair+porosity) is example.
Very fine transfer process
O’Hern said: “The membranes that currently convert seawater into fresh water are relatively thick (200 nanometers thick). In contrast, graphene membranes are only about three angstroms in size (600 times thinner than existing membranes). This makes it more efficient for filtering water.”
O’Hern and Karnik have been working on how to make graphene membranes for the past few years. In 2009, the team began to try to coat graphene on the surface of copper (this metal supports graphene on its surface with a wide range of modifications). However, water cannot pass through the metal copper, so it is necessary to transfer the graphene to the porous layer later.
However, O’Hern discovered that the transfer process causes graphene cracks. In addition, intrinsic defects are also generated during the growth of graphene, which can affect the filtration of the membrane.
Holes in graphene membranes are “blocked”
In order to solve the above problems, the team came up with a technology that first solves the internal defects and then resolves the defects caused by the transfer process. The researchers used a technique known as atomic layer deposition to solve intrinsic defects. The graphene film was placed in a vacuum chamber, and then the cerium-containing chemicals (which usually did not react with graphene) were shaken. Add to defect site. However, if the chemical comes into contact with the small opening of the graphene, the higher surface energy at this location will cause the chemical and the opening to stick together.
After several rounds of atomic layer deposition experiments, researchers found that germanium dioxide can be successfully deposited into the nanoscale defects of graphene. However, O’Hern realized that it would take more time to fill larger holes and cracks (a few hundred nanometers) using the same method.
He and his colleagues came up with a second technique to solve larger defects. An interfacial polymerization method was chosen, which is often used to make membranes. After filling in the inherent defects in graphene, the researchers invaded the membrane into the interface between the two solutions (water bath and water-immiscible organic solvent, such as oil).
The two solutions dissolved two different molecules, respectively, to react to produce a polyamide fiber. As long as the graphene film is placed at the interface between the two solutions, the polyamide fiber is generated and filled into cracks and holes (because only these two parts can fully contact each other), and finally fills the remaining defects perfectly.
The researchers used a technique developed last year to further etch away those small and regular holes in graphene that are small enough to allow water molecules to pass through, but they can “reject” large pollutants. . In the experiment, they tested the filtration effect of membranes on aqueous solutions containing some macromolecules (including salts). They found that membranes can filter out 90% of macromolecules, but they cannot filter out salts, and the salt penetration rate Faster than water.
Preliminary tests have shown that although the technology of filling defects and controlling permeability still needs further improvement, graphene will still be a viable alternative to existing filtration membranes.