Chemists have doubled the members of a family of buzzy 2D materials, and even jammed a record nine metals into one of them. The feat, published today in Science1, has excited researchers because it opens the door to designing a multitude of weird but useful substances.
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The materials are so complex that, at this point, it’s impossible to simulate them with computer models, says Max Hamedi, a physicist at the KTH Royal Institute of Technology in Stockholm. Scientists will need to test their properties in the laboratory, he adds — a tantalizing prospect. “Maybe we will get some properties that are very surprising and that we couldn’t predict.”
The newly expanded family of materials, called MXenes (pronounced ‘max-eens’), has previously caused a stir because their high electrical conductivity and other characteristics suggest that they might one day be used in technologies such as next-generation batteries and coatings that protect against electromagnetic interference. Not only that, these materials can be dispersed in water, so they can be sprayed or painted onto surfaces.
Snazzy sandwiches
The first MXene ever synthesized, a 2D sheet of titanium carbide, was reported in 20112 by a team co-led by Yury Gogotsi, a nanomaterials scientist at Drexel University in Philadelphia, Pennsylvania. Unlike the iconic 2D material graphene, which is a single layer of carbon atoms, MXenes contain several layers of metal and carbon or nitrogen atoms. In the case of titanium carbide, for instance, there are two ‘bread’ layers of titanium atoms that sandwich a sheet of carbon.
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But scientists don’t have tight control over which metals end up in which layers when more are added. Certain metals ‘like’ to be in a particular layer owing to atomic properties, such as the size of each atom and its hunger for electrons, says Babak Anasori, a materials engineer at Purdue University in West Lafayette, Indiana, and co-author of the latest study.
So although a person making a sandwich can stack up layers of cheese, lettuce, pickles and other ingredients in the order of their choosing, chemists making MXenes yield control to nature. To create these materials, scientists start by heating ingredients in a furnace to make crystals. In the resulting materials, certain metals always choose to be in the inner layers, whereas others are drawn to the outside.
Owing to these constraints, some metals, including tungsten, zirconium and hafnium, have been challenging to incorporate, Anasori says. And scientists have not been able to make MXenes with certain metals on the inner or outer layers: for instance, molybdenum likes to go on the outside of the sandwich, whereas titanium likes to be enveloped. Materials’ preferences for a certain chemical ordering to achieve the lowest energy configuration possible is a thermodynamic property called enthalpy.
