Synthesis method enables small-diameter, high-density carbon nanowires

Carbynes, or long linear carbon chains (LLCC), have received considerable attention in recent years due to their exceptional properties. However, their properties have been difficult to test experimentally due to their low stability. To improve stability, LLCCs need to be encapsulated in small diameter carbon nanotubes (CNTs).

Now, researchers have developed a new method for synthesizing small-diameter single-walled carbon nanowires (SWCNWs) incorporating high-density LLCCs encapsulated in single-walled CNTs. Their research was published in Letters on Chemical Physics.

Carbon is known for existing in many different physical forms, or allotropes. It appears in three-dimensional (3D) forms such as graphite and diamond, in two-dimensional (2D) structures such as graphene, or even as linear carbon chains (LCC).

Among them, carbynes, extremely long chains of single carbon atoms, also known as long LCCs (LLCCs), have received considerable research attention. They are expected to have outstanding theoretical mechanical strength and thermal conductivitymaking them promising for a variety of applications in areas such as nanotechnology and energy storage.

However, experimentally, researchers have struggled to study the properties of LLCC in detail. This is because they are unstable under environmental conditions due to the high reactivity of the exposed carbon atoms.

One proven way to solve this problem is to insert LLCC into carbon nanotubes (CNTs), creating so-called carbon nanowires (CNWs).

Over the past decade, a promising method for synthesizing CNTs has been developed, in which small carbon molecules trapped inside CNTs are heated at a temperature high temperatures. OOK are the most stable inside single wall carbon nanotubes (SWNT) with a diameter of 0.7–0.8 nanometers (nm). But most previous attempts have resulted in single-walled CNWs (SWCNWs) with diameters larger than 0.9 nm.

Addressing this challenge, a research team led by Professor Takahiro Maruyama from the Department of Applied Chemistry at Meijo University, Japan, has developed a highly efficient method for synthesizing small-diameter SWCNW with high-density LLCC.

“It was recently shown that by encapsulating polyyne in SWCNTs, small diameter carbon nanowires can be obtained,” explains Professor Maruyama.

“Building on this, we synthesized single-walled carbon nanowires with even smaller diameters and also achieved significantly higher concentrations of LLCC.”

To create the nanowires, the team first mixed open-ended SWCNTs with an n-hexane solution containing purified polyine molecules at different concentrations. The mixture was then heated to 80°C for 24 hours in a high-pressure reactor, allowing polyine molecules to penetrate into the SWCNT.

The resulting polyyne-loaded SWCNTs (polyyne@SWCNTs) were then heated to 700°C under high vacuum for four hours, turning them into LLCCs@SWCNTs, or in other words, SWCNWs.

The team confirmed the efficient encapsulation of polyyne molecules into SWCNTs in the first step and subsequent formation of SWCNTs using Raman spectroscopy.

Experiments also showed that the concentration of LLCC in SWCN increases with increasing concentration of polyine molecules in the initial n-hexane solution. By optimizing this concentration, the researchers were able to synthesize SWCNW with a record high LLCC density.

Notably, the resulting SWCNW samples had a diameter of only 0.73–0.77 nm, which is much smaller than those reported in previous studies. Such a small diameter could be achieved due to the small size of the polyine molecules.

As Professor Maruyama explains: “In our experiments, the carbon source used was polyyne molecules with a thin linear shape, having a diameter almost the same as the van der Waals diameter of the carbon atoms. In contrast, previous studies used relatively larger precursor molecules, resulting in SWCNWs with diameters greater than 0.9 nm.”

Additionally, the ratio of L-band to G-band in the Raman spectrum, a measurement that reflects the amount and density of LLCCs, reached 3.6 for the optimized samples, which is the highest value recorded to date for SWCNWs of such small diameter.

“Our method of synthesis of smalldiameter SWCNW will help researchers study the precise properties of LLCC,” adds Professor Maruyama. “This could lead to breakthroughs in many areas, from nanotechnology to sensors and energy storage.”

Overall, this work represents an important step forward in LLCC research, bringing researchers closer to unlocking the full potential of long linear carbon chains.