Schematic of the proposed mechanism for BNNO formation.
(a) Scanning electron microscope image of a cluster of BNNOs, surrounded by BN flakes, from the ablated product. (b) Transmission electron microscope image of a cluster of BNNOs.
Ablano group photo.Researchers have devised a simple, scalable means of harvesting hexagonal boron nitride (h-BN) nano-onions using a one-pot, non-toxic, catalyst-free procedure [Zhang et al., Materials Today (2023), https://doi.org/10.1016/j.mattod.2023.05.007].
Like graphite, h-BN is a two-dimensional, layered compound with exceptional lubrication properties and, thanks to its biocompatibility, potential as a protein carrier in drug delivery systems. But while h-BN nanostructures can be synthesized by a variety of methods, no route combining the virtues of simplicity, scalability, safety, and rapid reaction times specifically yields hollow, multi-walled nanoparticles. These particles, known as ‘nano-onions’ (BNNOs), are promising as biocompatible, non-toxic carriers for targeted tumor-selective drug delivery. The hollow spherical architecture could also be useful as additives to liquid lubricants, in hydrogen storage, and wastewater treatment.
“This material is known to be bio-compatible, lightweight, stable in air up to 900°C, a great thermal conductor, and an electrical insulator,” says Hui Tong Chua, head of Chemical Engineering at The University of Western Australia, who led the effort with Jeffrey M. Gordon of Ben-Gurion University of the Negev.
Until now, however, synthesis of these potentially useful particles has required long reaction times, toxic oxide precursors or catalysts, and complex multistep procedures that are not readily amenable to scale-up. Together with colleagues at the Technical University of Denmark and Taiyuan University of Technology in China, the researchers have found that BNNOs can be harvested after thermal exfoliation and lamp ablation of bulk precursor material.
“This is the first time, to our knowledge, that anyone has synthesized h-BN nano-onions at a promising scale, with a straightforward procedure and without toxic reagents or contaminants,” points out Chua.
The method simply involves placing h-BN powder in a sealed, evacuated quartz ampoule and exposing it to a xenon short arc discharge lamp, commonly used as a solar simulator, in an optical system to concentrate the light to a focal point. A range of differently sized and shaped particles are produced, from 10-nm polyhedrons to 60-nm multi-walled spheres, without any detectable contaminants. The team believes that the nano-onion architecture evolves from the initial thermal exfoliation into nano-platelets, which then grow outwards before folding over and forming the distinctive nano-cage structure. The only drawback of the simple process is that it is currently limited to a yield of 5-10%.
Nevertheless, the technology is now being commercialized through a university spin-out, Ablano Pty. Ltd., and the team hopes to improve the yield and assess the material for lubrication applications.