Metal sandwich solution

A technique for fusing different elements in layers to make a uniform and stable composite with predictable properties could open up routes to faster, smaller and more efficient energy storage devices, supercapacitors and wear-resistant and tough armored materials, according to a team at Drexel University in Philadelphia, Pennsylvania, USA. Babak Anasori and his colleagues at Drexel and Linköping University, Sweden, have demonstrated how to sandwich together two-dimensional sheets of molybdenum, titanium and carbon that would otherwise not stick together.

"By sandwiching one or two atomic layers of a transition metal like titanium, between monoatomic layers of another metal, such as molybdenum, with carbon atoms holding them together, we discovered that a stable material can be produced," Anasori explains. "It was impossible to produce a 2D material having just three or four molybdenum layers in such structures, but because we added the extra layer of titanium as a connector, we were able to synthesize them."

Metal sandwich solution

The team reports details in the journal ACS Nano and explains how each new combination of atom-thick layers offers new opportunities and properties. [Anasori et al, ACSNano, 2015, online; DOI: 10.1021/acsnano.5b03591]

The team begins with what they refer to as an ordered MAX phase material, M3AX2, for example Mo2TiAlC2, which can be separated into thin 2D sheets of metal and metal carbide by acid etching away the aluminum, to generate MXenes, named by analogy with graphene, etc. Ordered MXenes have the formula M'2M''C2 or M'2M''3C3, where M' and M'' are two different early transition metals and M′ layers sandwich M″ carbide layers. The team has now worked their way through the early transition metals making new MXenes of different compositions and testing their properties.

"We had reached a bit of an impasse, when trying to produce a molybdenum containing MXenes," Anasori explains. "By adding titanium to the mix we managed to make an ordered molybdenum MAX phase, where the titanium atoms are in center and the molybdenum on the outside. "Theoretical calculations (density functional theory, DFT) carried out by colleagues at Oak Ridge National Laboratory, suggest to the Drexel team that they could, in principle, make as many as 25 new materials with different combinations of transition metals, that they had assumed were not worth trying before.

The new layering method gives researchers a large number of possibilities for tuning the properties for a wide range of applications. Now, Anasori plans to make more materials by replacing titanium with vanadium, niobium, and tantalum and other metals, which could unearth a vein of new physical properties that support energy storage and other applications in thermoelectrics, batteries, catalysis, photovoltaics, electronic devices, structural composites and many other fields.

"We want to explore the limits of this approach to making new MAX phases and their MXenes experimentally," team leader Michel Barsoum told Materials Today." Just because DFT calculations predict something, does not always mean you will get it in the lab. Second, we need to now carefully characterize the 2D materials we made and find out in what applications they would be most suitable," he told us.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling popular science book "Deceived Wisdom".