Rice Scientists Use ‘Flashing’ to Create Valuable 2-D Boron Nitride

An illustration compares flakes of hexagonal boron nitride, top, and turbostratic boron nitride, bottom, the latter synthesized through the flash Joule heating process developed at Rice. Image courtesy of the Tour Group, Rice University.

Chemists from Rice University (Houston, Texas, USA) have adapted an instant process known as “flashing,” commonly used to synthetize substances like graphene, to create boron nitride, a two-dimensional (2-D) material nanomaterial that is highly valued for its thermal and chemical stability. 

As used by the lab of Rice chemist James Tour, the flashing process exposes a precursor to rapid heating and cooling to produce 2-D materials such as pure boron nitride and boron carbon nitride—both of which, until now, had been hard to create in bulk and nearly impossible to produce in easily soluble form. 

The lab’s report in Advanced Materials details how flash Joule heating, a technique introduced by the Tour lab in 2020, can be tuned to prepare purified, microscopic flakes of boron nitride with varying degrees of carbon. Experiments with the material showed that these flakes can be used as part of a powerful anticorrosive coating. 

“Boron nitride is a highly sought 2-D material,” Tour says. “To be able to make it in bulk, and now with mixed amounts of carbon, makes it even more versatile.” 

At the nanoscale, boron nitride comes in several forms, including a hexagonal configuration that looks like graphene but with alternating boron and nitrogen atoms instead of carbon. Boron nitride is soft, so it’s often used as a lubricant and as an additive to cosmetics, and is also found in ceramics and metal compounds to improve their ability to handle high heat. 

Rice chemical engineer Michael Wong recently reported that boron nitride is an effective catalyst in helping to destroy per- and polyfluroalkyl substances (PFAS), a dangerous “forever chemical” found in the environment and in humans. 

Flash Joule heating involves stuffing source materials between two electrodes in a tube and sending a quick jolt of electricity through them. For graphene, the materials can be just about anything containing carbon, such as food waste and used plastic car parts. The process has also successfully isolated rate earth elements from coal fly ash and other feedstocks. 

In experiments led by Rice graduate student Weiyin Chen, the lab fed ammonia borane (BH3NH3) into the flash chamber with varying amounts of carbon black, depending on the desired product. The sample was then flashed twice, first with 200 volts to degas the sample of extraneous elements and again with 150 volts to complete the process, with a total flashing time of less than a second. 

Microscopic images showed the boron nitride flakes are turbostratic—that is, misaligned like badly stacked plates—with weakened interactions between them that makes the flakes easy to separate. They are also easily soluble, which led to the anticorrosion experiments. The lab mixed flash boron nitride with polyvinyl alcohol (PVA), painted the compound on copper film, and exposed the surface to electrochemical oxidation in a bath of sulfuric acid. 

The flashed compound proved more than 92-percent better at protecting the coper than PVA alone or a similar compound with commercial hexagonal boron nitride. Microscopic images showed the compound created “tortuous diffusion pathways for corrosive electrolytes” to reach the copper, and also prevented metal ions from migrating. 

Chen said the conductivity of the precursor can be adjusted not only by adding carbon, but also with iron and tungsten. According to him, the lab sees potential for flashing additional materials. 

“Precursors that have been used in other methods, such as hydrothermal and chemical vapor deposition, can be tried in our flash method to see if we can prepare more products with metastable features,” Chen says. “We’ve demonstrated flashing metastable phase metal carbides and transition metal dichalcogenides, and this part is worth more research.”

Source: Rice University, https://news.rice.edu