A group of scientists from the Massachusetts Institute of Technology and other universities for the first time in the world presented a scalable process for the production of graphene for use in ultrathin membranes necessary for filtering various molecules - salts, ions, proteins and nanoparticles. The new method allowed for 4 hours to make a graphene tape with a width of 1 centimeter and a length of 10 meters. The method can be scaled for continuous industrial production.
Graphene-based membranes can be useful for desalination, separation of biological materials, and other uses.
The director of the MIT Production Laboratory, John Hart, led the study, which was attended by scientists from universities in the United States, Singapore, and a specialist from Caltech, Andrei Vyatskikh, a former student at the Skolkovo Institute of Science and Technology. “In the past few years, scientists believe graphene is a promising material for creating ultra-thin membranes. Our study represents the world's first method of producing graphene for membranes that require characteristics such as seamlessness and high quality material, ”Hart notes.
Graphene is a honeycomb-like lattice formed by one layer of carbon atoms, it is durable and impermeable even to the smallest atom - helium. Scientists use this property to create membranes: nanopores of the required size, made in a graphene sheet, will allow specific molecules to pass through it.
The main method of graphene production is
chemical vapor deposition , in which copper foils are exposed to chemical compounds of carbon and other gases in order to precipitate carbon on it. This method so far allows to produce membranes only in laboratory conditions, in manual mode and in small batches.
For the production of membranes for commercial purposes, it is necessary to establish a continuous process - it is impossible to make a sufficient amount of material out of small pieces. Membranes for certain purposes can be quite large, which also becomes an impossible task for laboratory conditions.
Hart and his colleagues expect that the method they have created will make it possible to start production on an industrial scale and commercialize it. Scientists have combined in the production chain roll technology and chemical deposition from the gas phase.
Roll technology - the process of manufacturing electronic devices on flexible rolls of plastic or foil. In this case, the scientists took the basis of a long strip of copper foil with a width of less than 1 centimeter. The system consists of two coils, between which is placed a small oven. One coil unfolds a strip of foil that passes through two pipes in sequence.
In the first pipe is heated to the required temperature of 1000 degrees Celsius. In the second tube, the ribbon is affected by the ratio of methane and hydrogen gas, which are deposited on the heated foil to obtain graphene. Scientists note that at first graphene is deposited by islands, which then grow to a single leaf. “After leaving the oven, graphene completely covers the foil in a single layer, as if it were a long sheet of pizza,” says Hart.
The strip of material is wound on the second roll, after which the copper foil is etched and replaced with a polymer substrate with pores larger than the graphene pores, so that the material does not coagulate and does not lose shape. Scientists conducted diffusion tests with samples of membranes obtained at different tape feed rates. They filtered water, salts and other molecules, and as a result confirmed that the filtration capacity was comparable to the membranes obtained using standard manual production methods in laboratories.
The new system, while operating at a speed of 5 centimeters per minute, made it possible to produce about 10 meters of graphene tape in 4 hours. The process can be scaled by changing the feed rate of the foil and the width of the tape in order to obtain a material of various qualities necessary for use for different purposes.
Graphene is one of the promising materials for use in various fields. IDTechEx insight experts
predict a growing market for products using this material to $ 300 million by 2027. Scientists and commercial companies are using graphene in biochips to increase their sensitivity, in photosensors and other electronic devices. Scientists were able to increase the strength of silk silkworm thread,
feeding the insect with graphene. One of the obstacles to the commercial success of products based on two-dimensional carbon is the complexity of its production on an industrial scale, which researchers in different countries are working on today.
The scientific work published in the journal Applied Materials and Interfaces.
DOI: 10.1021 / acsami.8b00846 /