Defects in the lattice structure of hexagonal boron nitride can be detected with photoluminescence. Researchers shine a light with a specific wavelength or color on the material and get a different color reflected from the defect. In addition, the image shows hydrogen bubbles being generated from defects that contain platinum atoms (gray and dark spheres attached to the vacancies). Image: Elizabeth Flores-Gomez Murray, Yu Lei and Kazunori Fujisawa, Penn State.Demonstrating that a material thought to be chemically inert, hexagonal boron nitride (hBN), can be turned chemically active holds potential for producing a new class of catalysts with a wide range of applications, according to an international team of researchers.
Like graphene, hBN is a layered material with monolayers that can be exfoliated. However, there is a key difference between the two.
“While hBN shares similar structure as graphene, the strong polar bonds between the boron and nitride atoms makes hBN unlike graphene, in that it is chemically inert and thermally stable at high temperature,” said Yu Lei, a postdoctoral scholar in physics at Penn State and first co-author of a paper on this work in Materials Today.
If hBN was chemically active and not inert, there would be more uses for it, including as a useful, cost-efficient catalyst support material similar to graphene. This would be useful for practical applications such as catalytic converters in gasoline-powered automobiles or the conversion of carbon dioxide into useful products.
“The catalytic converter in your gasoline car has the precious metal platinum in it to process the conversion of harmful gases into less harmful gases,” explained Jose Mendoza-Cortes, assistant professor of chemical engineering and materials science at Michigan State University. “However, this is expensive because you need to put in a lot of platinum atoms for the catalysis. Now imagine that you only need to put one or two, and still get the same performance.”
Platinum is used as a catalyst for many types of practical chemical reactions, and the platinum atoms that perform the conversion are usually on the surface of the catalyst, while the ones below are just there for structural support. “In this study, we have used defective hBN as structural support, which is cheaper, while exposing most of the platinum atom for performing chemical reactions,” Mendoza-Cortes said.
The defects in the hBN are the key to the material’s chemical activity. The researchers made defects, in the form of tiny holes, in the material via a process called cryomilling, which involves supercooling a material and then reducing it via cryogenic grinding.
The holes are so small that they can only hold one or two atoms of precious metal at a time. By applying a metal salt, the researchers were able to deposit metal nanostructures as small as one or two atoms onto the hBN substrate, due to the reactivity of the hole-filled hBN.
"Since boron nitride doesn't react with anything, then you can use this 'holey' hBN as a support for catalysts if you reduce a platinum, gold or silver salt into single atoms and place them in defects (holes) on the boron nitride surface,” said Maurico Terrones, professor of physics and professor of chemistry and materials science at Penn State. “This is something entirely new, and that's what we demonstrated here.”
This demonstration is significant, as it was previously believed that a material that is so inert could never become chemically active.
“The most difficult part of this project was to convince the research community that material that is as inert as hBN can be activated to have chemical reactivity, and serve as the catalyst support,” Lei said. “During the process of reviewing our study, additional experiments that were suggested by the reviewers improved the work and help to convince the community.”
These experiments involved using high-end equipment in the Materials Characterization Lab (MCL), part of the Materials Research Institute at Penn State. Computational and theoretical calculations were done at the Materials, Processes and Quantum Simulation Center (MUSiC) Lab and the Institute for Cyber-Enabled Research at Michigan State University.
“So, we wanted to know what type of defects we had in the material, and how can we demonstrate that we have the defects and it’s not something else?” Terrones said. “So, we did all these various very detailed characterizations, including synchrotron radiation, to demonstrate that what we had was in fact single-atom platinum, and not platinum clusters.”
Beyond experiments, the team also used modeling to prove their concept. “We showed and proved computationally and experimentally that we can make holes so small that they can hold only one or two atoms of precious metals at the time,” Mendoza-Cortes said.
The potential applications for this chemically active hBN are many and varied, including more cost-effective catalysts, energy storage and sensors. In addition, the technique could also be used for activating other inert materials or using other (precious) metals.
“I think we are showing that material that is supposed to be inert can be activated by creating and controlling defects on the material,” Terrones said. “We demonstrated that the necessary chemistry happens at the atomic level. If it works for boron nitride, it should work for any other material.”
This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.