[Image above] Imaginative illustration of the high-speed, large-area nanofilm deposition technique developed by Nagoya University researchers. Credit: Minoru Osada
Time and again, countries making the transition from an agricultural to industrialized economy have grappled with the tradeoff between economic growth and increased environmental pollution. Because of this established pattern, some countries that have reaped the benefits of industrialization are now hesitant to support other emerging economies through this transition due to purported concerns about the environment.
But as the United Nations, World Bank, and other global entities have pointed out, today’s developing nations have the benefit of hindsight thanks to the path forged by established economies that struggled through this transition. As such, it is possible for industrialization to occur in a much more environmentally friendly manner.
Green chemistry is one of the frameworks that can help ensure sustainable manufacturing. This set of 12 principles focuses on minimizing or eliminating the use and generation of hazardous substances during materials processing.
Many traditional manufacturing sectors have made use of green chemistry practices. But emerging industrial fields, such as nanomaterials, can benefit from these practices as well.
Currently, nanomaterials are often produced using chemical vapor deposition (CVD) techniques. However, these processes are inherently energy intensive, and they can sometimes produce toxic byproducts.
Applying green chemistry principles to CVD processes can make this manufacturing approach more sustainable. But the technique remains limited in its ability to allow damage-free and highly efficient transfer of layered 2D films onto arbitrary substrates.
As an alternative to CVD, researchers have investigated using solution-based deposition techniques to create well-packed, multilayered nanosheets on a mass scale. But to date, these approaches have been hindered by long processing times, complicated deposition operations, and limited coating sizes.
In a recent paper, researchers from Nagoya University in Japan described a new simple solution-based deposition technique that not only overcomes these challenges but does so with reduced environmental effects.
Their solution to solution-based deposition involves the well-known tears of wine phenomenon. This phenomenon is named after the thin film of liquid that creeps along the inside of a glass of wine. It is the result of the alcohol in wine evaporating faster than the water, which creates an interfacial tension gradient that pulls the wine upward.
In their method, the researchers created water/ethanol suspensions of 2D nanosheets with a cationic surfactant (tetrabutylammonium hydroxide). Then, with an ordinary pipette, they injected the suspension onto water in a petri dish.
As the ethanol evaporated, a surface tension gradient formed between regions with higher and lower ethanol concentrations. This gradient caused a spontaneous spreading of the nanosheet suspension from the center (low surface tension) to the edges (high surface tension), resulting in the formation of large, uniform monolayers of the nanomaterials within seconds.
In a Nagoya University press release, senior author Minoru Osada, professor in the Institute of Materials and Systems for Sustainability, compares the spontaneous alignment and packing of the nanosheets to ice floes coming together on the surface of water. He adds that the resulting nanosheet film can then be easily transferred onto a substrate, “completing the deposition process in as little as one minute.”
The researchers successfully applied the technique to various nanosheet compositions and structures, including metal oxides, hexagonal boron nitride, and molybdenum disulfide. The technique also worked when using different organic solvents, including acetone, formamide, and isopropyl alcohol.
In addition to its versatility of composition, the method required only a small amount of suspension: 10 μL of the suspension was used for 4-inchϕ deposition. This situation stands in contrast to those of previous deposition techniques, which typically wasted more than 90% of the suspension for monolayer coating.
In the press release, Osada emphasized other environmental benefits of this method. For example, “This technology … enables thin film production on various substrates at room temperature and in an aqueous solution process, without the need for vacuum film-forming equipment or expensive tools, which are common in conventional thin film processes,” he says.
The paper, published in Small, is “Ultrafast 2D nanosheet assembly via spontaneous spreading phenomenon” (DOI: 10.1002/smll.202403915).
