Defects in the lattice structure of hexagonal boron nitride can be detected with photoluminescence. Researchers shine a light with a color or energy on the material and get a different color from the defect. In addition, the figure shows hydrogen bubbles being generated from these defects that contain catalyst atoms (gray and dark spheres attached to the vacancies). Credit: Elizabeth Flores-Gomez Murray, Yu Lei and Kazunori Fujisawa.Hexagonal boron nitride (hBN), known for its stability, chemical inertness and physical robustness, can be made ‘active’ by ball milling at extremely low temperatures, according to researchers [Lei et al., Materials Today (2021), https://doi.org/10.1016/j.mattod.2021.09.01].
Like two-dimensional graphene, hBN comprises hexagonal layers of covalently bonded boron and nitrogen atoms held together by weak van der Waals forces. The strong covalent bonds and wide bandgap mean that hBN is highly inert, making functionalization very difficult.
Defect engineering can tailor the properties of hBN through treatment with plasma or ball milling. But these are high energy processes, the latter of which introduces contamination over extended periods. Instead, researchers from The Pennsylvania State University, Binghamton University, Michigan State University, Centro de Investigación Cientifica de Yucatán, Shinshu University and Indian Institute of Technology Indore took ball milling to cryogenic temperatures (77 K) to reduce processing time and suppress undesirable side reactions.
“We created nitrogen vacancies in hBN via a cryomilling process in the presence of liquid nitrogen and found that these vacancies provide unique reactivity,” explains Mauricio Terrones, Verne M. Willaman Professor of physics and professor of chemistry and materials science at Penn State, who led the work with Jose L. Mendoza-Cortes.
Cryomilling introduces defects into bulk hBN in minutes. The process breaks down the material, creating surfaces and edges covered with dangling bonds ready to adsorb any nearby atoms. The vacancies create what is known as ‘defective’ – but much more chemically reactive – BN (or d-BN).
“To our surprise, these vacancies are able to reduce metal salts into single platinum atoms or sub-nanometer clusters of other metals,” says Terrones.
According to the researchers’ theoretical calculations, the band structure and shift in the Fermi energy level confines some metals, like Pt, as single atoms at vacancies and others like Au, Ag, Fe and Cu as sub-nanoclusters.
“This also explains how different defects affect the band structure and made us realize which defects are favorable for hosting single atoms or a cluster of atoms,” explains Mendoza-Cortes. “This is going to be a guiding principle going forward.”
The ability to trap metal atoms or clusters makes d-BN an ideal catalyst support but could be useful in other applications such as sensing or quantum information processing.
“This is something entirely new,” adds Terrones. “We tested the catalytic performance of these metal-hBN systems and found that Ag-Pt overperforms when compared with Pt or PtC catalysts [for the hydrogen evolution reaction].”
Different vacancies, produced with or without oxygen, bring a variety of useful properties, including photoluminescent emission in the visible region and the generation of free radicals.
“Fundamentally, we want to address the question of how we can use nano-sized defects to change a material completely,” says first author Yu Lei. “The unique heterostructure of BN and metal nanostructures [could also] be used for bio-applications.”