[Image above] Scanning electron microscopy images, from left, of the starting material, after phosphoric acid treatment, and final purified boron nitride nanotubes. Credit: Martí group, Rice University
From television screens to cancer treatments to crop protection, nanomaterials have played a role in revolutionizing almost every industrial domain during the past 20 years. Advancements in nanoscale imaging techniques enabled this adoption, but now the challenge is ensuring proper quality when producing nanomaterials in bulk.
As explained in an open-access article in The AAPS Journal, synthesizing nanoparticles often requires multiple process steps involving multicomponent systems. So even though reproducible, well-characterized components may be achieved on a small scale, “once beyond the early prototype, the reproducibility and consistency of the constructs remain a constant challenge for the scale-up and manufacturing process,” the article states.
Boron nitride nanotubes (BNNTs) are an emerging class of nanomaterials for which scalable synthesis methods are yet to be developed. BNNTs share some similarly interesting physical properties with carbon nanotubes, such as tensile strength and Young’s modulus, but they are electrically insulating and can resist oxidation at higher temperatures.
Because of their chemical inertness, BNNTs require high temperatures and/or pressures to be synthesized in bulk. This extreme processing environment results in the formation of non-nanotube boron structures, such as amorphous boron, hexagonal boron nitride, boron nitride cages, and other less ordered boron nitride materials.
Researchers have explored various processes to eliminate the unwanted boron nitride forms, including chemical etching, functionalization, and physical separation techniques. While several methods can effectively remove amorphous boron, the other forms are more chemically similar to BNNTs and thereby more difficult to remove.
To date, wet-thermal etching is the most effective method for removing these other forms by targeting morphological differences. While the cylindrical structure of BNNTs allows for seamless connections between the atoms, hexagonal boron nitride and boron nitride cages contain unconnected atoms, which makes them more susceptible to reaction with external stimuli.
In a recent paper, researchers at Rice University described a new wet-thermal etching method that uses a phosphoric/hydrochloric acid solution. Their new method, as explained in a Rice University press release, was inspired by a 2013 study at The Pennsylvania State University, which showed phosphoric acid acted as a boron nitride wetting agent.
The Rice University researchers believed this wetting property may allow them to selectively remove the unwanted boron nitride forms. But upon heating the boron nitride nanomaterials in solution, they were surprised to see a pyramid structure had formed.
This observation indicated a chemical reaction had taken place, against the researchers’ initial assumptions. So, they revised their hypothesis and conducted more experiments to tune the reaction to destroy only the unwanted boron nitride forms.
Their final process resulted in mass yields of up to 29% purified nanotubes, which is significantly greater than the 5% achieved with conventional steam etching setups. However, they note this result is highly dependent on the concentration of impurities in the starting material.
“This work could be a stepping stone to much better building materials both in terms of strength and in terms of sustainability,” says lead author Kevin Shumard, chemistry doctoral student at Rice University, in the press release.
The paper, published in Chemistry of Materials, is “Reactivity of boron nitride nanomaterials with phosphoric acid and its application in the purification of boron nitride nanotubes” (DOI: 10.1021/acs.chemmater.3c01424).