Engineers 'sandwich' atomic layers to make new materials for energy storage

The scientists whose job it is to test the limits of what nature--specifically chemistry-- will allow to exist, just set up shop on some new real estate on the Periodic Table. Using a method they invented for joining disparate elemental layers into a stable material with uniform, predictable properties, Drexel University researchers are testing an array of new combinations that may vastly expand the options available to create faster, smaller, more efficient energy storage, advanced electronics and wear-resistant materials. Led by postdoctoral researcher Babak Anasori, PhD, a team from Drexel's Department of Materials Science and Engineering created the material-making method, that can sandwich 2-D sheets of elements that otherwise couldn't be combined in a stable way. And they proved its effectiveness by creating two entirely new, layered two-dimensional materials using molybdenum, titanium and carbon. layered nanomaterial Drexel University engineers have created a layered material of molybdenum and titanium by using a new process they invented to etch a MAX phase into a two-dimensional, layered MXene. (Image: Drexel University) "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 said. "It was impossible to produce a 2-D 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." The discovery, which was recently published in the journal ("Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes)"), is significant because it represents a new way of combining elemental materials to form the building blocks of energy storage technology--such as batteries, capacitors and supercapacitors, as well as superstrong composites--like the ones used in phone cases and body armor. Each new combination of atom-thick layers presents new properties and researchers suspect that one, or more, of these new materials will exhibit energy storage and durability properties so disproportional to its size that it could revolutionize technology in the future. "While it's hard to say, at this point, exactly what will become of these new families of 2-D materials we've discovered, it is safe to say that this discovery enables the field of materials science and nanotechnology to move into an uncharted territory," Anasori said. Mastering Materials Combining two-dimensional sheets of elements in an organized way to produce new materials has been the goal of Drexel nanomaterials researchers for more than a decade. Imposing this sort of organization at the atomic level is no easy task. "Due to their structure and electric charge, certain elements just don't 'like' to be combined," Anasori said. "It's like trying to stack magnets with the poles facing the same direction--you're not going to be very successful and you're going to be picking up a lot of flying magnets." But Drexel researchers came up with a clever way to circumvent this chemistry challenge. It starts with a material called a MAX phase, which was discovered by Distinguished Professor Michel W. Barsoum, PhD, head of the MAX/MXene Research Group, more than two decades ago. A MAX phase is like the primordial ooze that generated the first organisms--all the elements of the finished product are in the MAX phase, waiting for the researchers to impose some order. That order was imposed by Michel W. Barsoum, PhD and Yury Gogotsi, PhD, Distinguished University and Trustee Chair professor in the College of Engineering and head of the Drexel Nanomaterials Group, when they first created a stable, two-dimensional, layered material called MXene in 2011. To create MXenes, the researchers selectively extract layers of aluminum atoms from a block of MAX phase by etching them out with an acid. "Think of MXene synthesis like separating layers of wood by dunking a plywood sheet into a chemical that dissolves the glue," Anasori said. "By putting a MAX phase in acid, we have been able to selectively etch away certain layers and turn the MAX phase into many thin 2-D sheets, which we call MXenes." As far as energy storage materials go, MXenes were a revelation. Prior to their discovery, graphene, which is a single sheet of carbon atoms, was the first two-dimensional material to be touted for its potential energy storage capabilities. But, as it was made up of only one element, carbon, graphene was difficult to modify in form and therefore had limited energy storage capabilities. The new MXenes have surfaces that can store more energy. An Elemental Impasse Four years later, the researchers have worked their way through the section of the Periodic Table with elements called "transition metals," producing MAX phases and etching them into MXenes of various compositions all the while testing their energy storage properties. Anasori's discovery comes at a time when the group has encountered an obstacle on its progress through the table of elements. "We had reached a bit of an impasse, when trying to produce a molybdenum containing MXenes," Anasori said. "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. The Next Frontier Now, with the help of theoretical calculations done by researchers at the FIRST Energy Frontier Research Center at the Oak Ridge National Laboratory, Drexel's team knows that, in principle, it can use this method to make as many as 25 new materials with combinations of transition metals, such as molybdenum and titanium, that previously wouldn't have been attempted. "Having the possibility to layer different elements at the thinnest form of material known to the scientific community leads to exciting new structures and allows unprecedented control over materials properties," Barsoum said. "This new layering method gives researchers an unimaginable number of possibilities for tuning materials' properties for a variety of high-tech applications." Anasori plans to make more materials by replacing titanium with other metals, such as vanadium, niobium, and tantalum, which could unearth a vein of new physical properties that support energy storage and other applications. "This level of structural complexity, or layering, in 2-D materials has the potential to lead to many new structures with unique control over their properties," Gogotsi said. "We see possible applications in thermoelectrics, batteries, catalysis, solar cells, electronic devices, structural composites and many other fields, enabling a new level of engineering on the atomic scale."
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Recipe book for colloids

Researchers from Jülich have, together with colleagues from Austria, Italy, Colombia and the USA, developed a model system for so-called soft colloids. The model gives us a better understanding of correlations between the atomic structure of colloids and their perceptible material properties. These findings could lead to new approaches for the targeted development of innovative colloid materials. The results have just been published in the journal ("Dynamic phase diagram of soft nanocolloids"). neutron scattering Using neutron scattering, researchers were able to study the structure of their samples. The size of the “rings” in the image can, for example, define the distance between two colloid particles. (Image: Forschungszentrum Jülich) Colloids are nano- or micrometer-sized finely dispersed particles or droplets. Soft colloids are made up of flexible materials, for example, polymers, such as proteins and synthetic molecules. In nature, soft colloids are found in cells, for instance. In industry, they are used among other things in food processing, cosmetics and emulsion paints or in oil production to achieve the necessary flow properties. In paint manufacture, for example, they ensure that products are easy to apply yet do not run off surfaces. The model system developed by researchers from the Jülich Centre for Neutron Science is made up of water and block copolymers – thread-like molecules with both a hydrophilic and a hydrophobic component. In water, the polymer threads arrange themselves in a star shape, with the hydrophilic ends pointing outwards, and the hydrophobic pointing inwards. If the hydrophilic component is large, only a few molecules will bundle themselves loosely together and their physical behaviour resembles that of threads. The bigger the hydrophobic component is, the more polymers will clump together and dense, hard spheres are formed. Until now, there have always been separate physical models for threads and spheres, which would predict in each case whether the resulting solution would be liquid or glassy. Aided by their scientific investigations and, among other things, by neutron scattering experiments, the researchers have now succeeded in combining both models and have developed a comprehensive phase diagram which describes the material properties depending on the structure and concentration of the colloid – producing a recipe book for colloids, so to speak. In effect, they found a connecting parameter which essentially decides whether the model colloid solution will be liquid or glassy: the so-called interaction length. This corresponds approximately to the radius in which the colloids can interact with each other, and depends among other things on how many molecules a colloid is composed of as well as the concentration strength of the colloids. A special feature of the model colloids made these findings possible: their softness can be tuned very finely over a large area by altering the length ratio between the hydrophilic and hydrophobic components of the molecule threads. The fact that the basic ingredients always remain the same makes it simpler to distinguish fundamental correlations.
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