High-resolution transmission electron microscopy image of sub-nanofilaments of an anatase-based Ti carbo-oxide. Each bright spot is a Ti atom. Image taken by Dr P. Persson and illustrated by Patricia Lyons.Researchers have devised a simple means of producing 2D nanomaterials at ambient conditions from inexpensive green precursors [Badr et al., Materials Today (2022), https://doi.org/10.1016/j.mattod.2021.10.033]. Typically, 2D materials like graphene and MXenes are produced by exfoliating or etching solid layered bulk materials. The process can be slow and involved, which renders the production of 2D materials in large quantities challenging and expensive. Now a team from Drexel University, Worcester Polytechnic Institute, Tulane University, Murata Manufacturing Co., Université Grenoble Alpes, the European Synchrotron Radiation Facility, and Linköping University have converted ten titanium carbides, nitrides, borides, phosphides, and silicates into 2D flakes.
“We show that one can inexpensively make extremely fine nanomaterials by the kilogram,” says Michel W. Barsoum. “The fundamental problem with nanomaterials and the main reason, with a few exceptions, they remain a niche market is because it is complicated and expensive to make them in bulk.”
The new approach simply involves immersing a solid precursor powder into tetramethylammonium hydroxide (TMAH) solution and gently heating to a temperature of 50-85°C. The TMAH acts as a near-universal solvent, dissolving the precursor to release Ti atoms that react with C and O in the solvent/water. Sub-nano filaments – just 10 x 10 atoms in cross-section – are not only produced but also self-assemble into stacked sheets of C-containing anatase-based layers. The TMAH acts as both a solvent and templating agent. In this manner, the researchers produced flakes and films of anatase-based Ti carbo-oxides, as well as other 2D materials, some of which have unique properties.
“We [have] discovered a recipe to make nano- and sub-nanomaterials that is almost foolproof, inexpensive, single-pot, eco-friendly and massively scalable,” points out Barsoum.
As a result of the sub-nanometer scale of the filaments, the indirect band gap is in the 4 eV range, according to the researchers’ measurements, which is the highest reported for an anatase-based material. Nevertheless, some films, such as those derived from TiC and the MAX phases are weakly conductive, while those from TiO2 and TiB2 are not. Although the conductivities of these 2D nanomaterials are orders of magnitude larger than typical oxides, they are much lower than MXenes. More work is needed to understand the conductive behavior fully.
The materials have extraordinarily high surface areas and could prove useful as electrodes for lithium-ion and lithium-sulfur batteries. The stability of the nanofilaments in aqueous environments could enable other applications in water remediation and biomedicine because of their ability to reduce the viability of some cancer cells.
“I consider this discovery revolutionary rather than evolutionary,” comments Barsoum. “The entire process is a breakthrough in the field of scaling up the production of nanomaterials.”