Can defects transform inert materials into useful materials,


image: Defects in the structure of the hexagonal boron nitride lattice can be detected by photoluminescence. The researchers project a light with a color or an energy on the material and obtain a color different from the defect. In addition, the figure shows hydrogen bubbles generated from these defects which contain catalyst atoms (gray and dark spheres attached to vacancies).
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Credit: Elizabeth Flores-Gomez Murray, Yu Lei and Kazunori Fujisawa, Penn State

Demonstrating that a material thought to be always chemically inert, hexagonal boron nitride (hBN), can be made chemically active holds the potential for a new class of catalysts with a wide range of applications, according to an international team of researchers.

HBN is a layered material and monolayers can be exfoliated like in graphene, another two-dimensional material. However, there is an essential difference between the two.

“Although hBN shares a similar structure to graphene, the strong polar bonds between the boron and nitride atoms make hBN different from graphene in that it is chemically inert and thermally stable at high temperatures,” said Yu Lei, postdoctoral researcher in physics at Penn State and first co-author of the study published in Materials today.

If hBN were chemically active and not inert, this would allow other uses, notably as a useful and economical catalyst support similar to graphene. This would be useful for practical applications like in a gasoline automobile or to convert carbon to help reduce greenhouse gases into other products.

“The catalytic converter in your gasoline car contains platinum, a precious metal, to process the conversion of harmful gases into less harmful gases,” said Jose Mendoza-Cortes, assistant professor of chemical engineering and materials science at Michigan. State University. “However, it is expensive because you have to put a lot of platinum atoms for catalysis. Now imagine that you only need to put in one or two and you still get the same performance.

Platinum is also used as a catalyst for many other types of practical chemical reactions, and the platinum atoms that do the conversion are usually on the surface, while the ones below are just there as a structural support.

“In this study, we used defective hBN as a structural support, which is cheaper, while exposing most of the platinum atom to perform chemical reactions,” Mendoza-Cortes said.

The defects of hBN are the key to the chemical activity of the material. Researchers have made flaws, tiny holes, in materials through a process called cryogrinding, which involves supercooling a material and then shrinking it by cryogenic grinding.

The holes are so small that they can only hold one or two atoms of a precious metal at a time. By mixing a metal salt, nanostructures as small as one or two atoms on the hBN substrate can be deposited, due to the reactivity of the hBN filled with holes.

“As boron nitride does not react with anything, then you can use this” holed “hBN as a catalyst support if you reduce a salt of platinum, gold or silver to single atoms and place them in defects (holes ) on boron nitride surface, “said Maurico Terrones, Verne M. Willaman professor of physics and professor of chemistry and materials science at Penn State.” It’s something entirely new, and that’s what that we have demonstrated here.

Demonstrating this was important, because it was previously believed that such an inert material could never become chemically active.

“The most difficult part of this project was convincing the research community that a material as inert as hBN can be activated to have chemical reactivity and serve as a catalyst support,” said Lei. “During the review process of our study, additional experiences that were suggested by the reviewers improved the work and helped convince the community.”

The experiments involved the use of high-end equipment in the Materials Characterization Laboratory (MCL), which is part of the Materials Research Institute at Penn State. Computational and theoretical calculations were performed at the Materials, Processes and Quantum Simulation Center (MUSiC) laboratory and at the Institute for Cyber-Enabled Research at Michigan State University.

“So we wanted to know what kind of flaws we have in the material and how can we demonstrate that we have the flaws and that it’s not something else? Terrones said. “So we did all of these various very detailed characterizations, including synchrotron radiation, to show that what we had was actually single atom platinum, not clumps of platinum. “

Beyond the experiments, the team also used modeling to prove their concept.

“We have shown and proven by computer and experimentally that we can make holes so small that they can only hold one or two precious metal atoms at the time,” Mendoza-Cortes said.

The potential for applications of chemically active hBN is varied, including more cost effective catalysts, energy storage and sensors. In addition, it is possible that their technique is used to activate other inert materials or use other (precious) metals.

“I think we are showing that a material that is supposed to be inert can be activated by creating and controlling defects on the material,” Terrones said. “We have shown that the necessary chemistry occurs at the atomic level. If it works for boron nitride, it should work for any other material.

Along with Lei, Mendoza-Cortes and Terrones, other authors of the study include from the Indian Institute of Technology Indore, first co-author Srimanta Pakhira, associate professor of physics. From Penn State, the study’s authors include Kazunori Fujisawa, assistant research professor of physics; He Liu, doctoral research assistant in chemistry at the time of the study; Tianyi Zhang, doctoral student in materials science and engineering at the time of the study; Archi Dasgupta, research assistant graduated in chemistry at the time of the study; Ke Wang, MCL scientific staff member; Jeff Shallenberger, associate director of MCL; and Ana Laura Elías, physics research professor at the time of the study. From the Yucatan Scientific Research Center, study authors include Cynthia Guerrero-Bermea, postdoctoral researcher. From the University of Texas at El Paso, the study’s authors include Luis M. Martinez, graduate student in physics, and Srinivasa Rao Singamaneni, assistant professor of physics. From the University of Shinshu (Japan), the study’s authors include Rodolfo Cruz-Silva, specially appointed professor in the Faculty of Engineering, and Morinobu Endo, distinguished professor emeritus.

The study was partially funded by the National Science Foundation. This work was supported in part by computer resources and services provided by the Institute for Cyber-Enabled Research at Michigan State University.


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