New catalyst grows 30 cm-long carbon nanotube arrays

(Nanowerk Spotlight) Carbon nanotubes (CNTs) have long been hailed as a revolutionary material with the potential to transform industries ranging from electronics to aerospace. These cylindrical structures of carbon atoms, just nanometers in diameter, possess extraordinary properties including exceptional strength, electrical conductivity, and thermal conductivity. However, realizing the full potential of CNTs has been hampered by challenges in producing them at scale with sufficient length, alignment, and structural perfection.
Since their discovery in 1991, researchers have made steady progress in CNT synthesis methods. Early techniques like arc discharge and laser ablation could produce small quantities of high-quality nanotubes but were not scalable. Chemical vapor deposition (CVD) emerged as a more promising approach for large-scale production. In CVD, carbon-containing gases are decomposed at high temperatures in the presence of metal catalyst particles, from which the nanotubes grow.
A key goal has been to produce ultralong CNTs - those measuring centimeters or even meters in length. Such ultralong tubes are highly desirable for applications like high-strength fibers and electrical transmission lines. However, growing ultralong CNTs has proven extremely challenging. Most CVD methods produce tangled mats of short nanotubes. Various strategies have been explored to promote aligned growth, including using patterned catalyst substrates and applying electric fields. While these have enabled growth of vertically-aligned CNT "forests" up to millimeters tall, producing well-aligned horizontal ultralong CNTs remained elusive.
A breakthrough came in 2004 when researchers demonstrated the "flying catalyst" CVD method could produce horizontally-aligned ultralong CNTs up to several centimeters long. In this approach, catalyst particles and carbon precursors are introduced in the gas phase, allowing continuous growth as the catalyst particles float through the reactor. However, yields remained extremely low - typically less than 100 nanotubes per millimeter of substrate width.
Over the past two decades, researchers have investigated ways to boost ultralong CNT yields, exploring factors like gas flow rates, catalyst composition, and growth temperatures. A major advance came in 2021 with the development of the "substrate interception and direction strategy" (SIDS). This method uses a substrate to intercept floating catalyst particles and nanotubes, initiating aligned growth. SIDS increased yields by 2-3 orders of magnitude compared to previous methods. However, further improvements in yield and uniformity were still needed to enable practical applications of ultralong CNTs.
Now, researchers from Tsinghua University have developed an innovative approach that takes ultralong CNT growth to new heights. Their work, published in the journal Advanced Materials ("Floating Bimetallic Catalysts for Growing 30 cm-Long Carbon Nanotube Arrays with High Yields and Uniformity"), introduces a method for synthesizing "floating bimetallic catalysts" (FBCs) that dramatically improves both the yield and uniformity of ultralong CNT arrays.
Schematic illustration of the in situ synthesis process of floating bimetallic catalysts and the subsequent growth of ultralong carbon nanotubes via substrate interception and direction strategy.
Schematic illustration of the in situ synthesis process of floating bimetallic catalysts and the subsequent growth of ultralong carbon nanotubes via substrate interception and direction strategy. (Image: Adapted from DOI:10.1002/adma.202402257 with permission from Wiley-VCH)
The key innovation is the in-situ formation of bimetallic catalyst nanoparticles by simultaneously vaporizing two different metal precursors. The researchers used ferrocene as an iron source, paired with various metal acetylacetonates to introduce a second metal. This allowed them to produce a wide range of bimetallic catalysts combining iron with elements like copper, nickel, cobalt, and chromium.
Among the various combinations tested, iron-copper (FeCu) catalysts showed particularly remarkable performance. Using optimized FeCu catalysts, the researchers achieved ultralong CNT arrays with a record-breaking areal density of approximately 8,100 nanotubes per millimeter. This represents a major leap forward compared to previous methods, which typically produced less than 100 nanotubes per millimeter.
Beyond just boosting overall yield, the FeCu catalysts also significantly improved the uniformity of the CNT arrays. The researchers found that FeCu catalysts exhibited a "lifetime" 3.4 times longer than pure iron catalysts. This extended catalyst lifetime allows nanotubes to grow to greater lengths before terminating, resulting in more uniform arrays.
To demonstrate the potential of their method, the researchers grew an impressively long and dense CNT array measuring 30 centimeters in length. Even at the far end of this array, the nanotube density remained around 90 nanotubes per millimeter - still higher than the maximum density achievable with traditional methods.
The researchers conducted detailed characterization of the CNTs produced using their FeCu catalysts. They found the nanotubes were primarily single-walled, double-walled, and triple-walled, with very few defects. This high structural quality is crucial for maintaining the exceptional properties of CNTs over long lengths.
To understand the mechanisms behind the improved performance of FeCu catalysts, the researchers developed a kinetic model and performed molecular dynamics simulations. They found that adding copper to iron catalysts creates a trade-off between two important factors: catalyst fluidity and carbon solubility.
Copper lowers the melting point of the catalyst nanoparticles, increasing their fluidity. This improved fluidity enhances carbon diffusion through the catalyst, which is beneficial for CNT growth. However, copper also decreases the catalyst's ability to dissolve carbon. At low copper concentrations, the benefits of increased fluidity outweigh the drawbacks of reduced carbon solubility. But beyond an optimal point (around 11.4% copper in this study), further increases in copper content begin to hinder growth.
This work represents a significant advance in the field of ultralong CNT synthesis. The ability to produce dense, uniform arrays of high-quality nanotubes at lengths of 30 centimeters and beyond opens up new possibilities for CNT applications. The method's versatility in creating various bimetallic catalysts also provides a powerful new tool for researchers to optimize CNT growth for specific applications.
While challenges remain in scaling up production and precisely controlling nanotube properties, this research brings us closer to realizing the full potential of ultralong CNTs. As synthesis methods continue to improve, we may soon see ultralong CNTs enabling transformative technologies like ultra-lightweight structural materials, long-distance power transmission lines with minimal losses, and high-performance electronic devices.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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