(Nanowerk Highlight) Carbon nanotubes (CNTs) have lengthy been hailed as a revolutionary materials with the potential to rework industries starting from electronics to aerospace. These cylindrical constructions of carbon atoms, simply nanometers in diameter, possess extraordinary properties together with distinctive energy, electrical conductivity, and thermal conductivity. Nonetheless, realizing the complete potential of CNTs has been hampered by challenges in producing them at scale with ample size, alignment, and structural perfection.
Since their discovery in 1991, researchers have made regular progress in CNT synthesis strategies. Early methods like arc discharge and laser ablation may produce small portions of high-quality nanotubes however weren’t scalable. Chemical vapor deposition (CVD) emerged as a extra promising strategy for large-scale manufacturing. In CVD, carbon-containing gases are decomposed at excessive temperatures within the presence of steel catalyst particles, from which the nanotubes develop.
A key purpose has been to supply ultralong CNTs – these measuring centimeters and even meters in size. Such ultralong tubes are extremely fascinating for purposes like high-strength fibers and electrical transmission strains. Nonetheless, rising ultralong CNTs has confirmed extraordinarily difficult. Most CVD strategies produce tangled mats of quick nanotubes. Numerous methods have been explored to advertise aligned development, together with utilizing patterned catalyst substrates and making use of electrical fields. Whereas these have enabled development of vertically-aligned CNT “forests” as much as millimeters tall, producing well-aligned horizontal ultralong CNTs remained elusive.
A breakthrough got here in 2004 when researchers demonstrated the “flying catalyst” CVD technique may produce horizontally-aligned ultralong CNTs as much as a number of centimeters lengthy. On this strategy, catalyst particles and carbon precursors are launched within the fuel part, permitting steady development because the catalyst particles float by way of the reactor. Nonetheless, yields remained extraordinarily low – sometimes lower than 100 nanotubes per millimeter of substrate width.
Over the previous 20 years, researchers have investigated methods to spice up ultralong CNT yields, exploring components like fuel circulate charges, catalyst composition, and development temperatures. A significant advance got here in 2021 with the event of the “substrate interception and direction strategy” (SIDS). This technique makes use of a substrate to intercept floating catalyst particles and nanotubes, initiating aligned development. SIDS elevated yields by 2-3 orders of magnitude in comparison with earlier strategies. Nonetheless, additional enhancements in yield and uniformity have been nonetheless wanted to allow sensible purposes of ultralong CNTs.
Now, researchers from Tsinghua College have developed an revolutionary strategy that takes ultralong CNT development to new heights. Their work, printed within the journal Superior Supplies (“Floating Bimetallic Catalysts for Growing 30 cm-Long Carbon Nanotube Arrays with High Yields and Uniformity”), introduces a way for synthesizing “floating bimetallic catalysts” (FBCs) that dramatically improves each the yield and uniformity of ultralong CNT arrays.
Schematic illustration of the in situ synthesis technique of floating bimetallic catalysts and the next development of ultralong carbon nanotubes by way of substrate interception and course technique. (Picture: Tailored from DOI:10.1002/adma.202402257 with permission from Wiley-VCH)
The important thing innovation is the in-situ formation of bimetallic catalyst nanoparticles by concurrently vaporizing two totally different steel precursors. The researchers used ferrocene as an iron supply, paired with numerous steel acetylacetonates to introduce a second steel. This allowed them to supply a variety of bimetallic catalysts combining iron with components like copper, nickel, cobalt, and chromium.
Among the many numerous mixtures examined, iron-copper (FeCu) catalysts confirmed notably exceptional efficiency. Utilizing optimized FeCu catalysts, the researchers achieved ultralong CNT arrays with a record-breaking areal density of roughly 8,100 nanotubes per millimeter. This represents a significant leap ahead in comparison with earlier strategies, which usually produced lower than 100 nanotubes per millimeter.
Past simply boosting general yield, the FeCu catalysts additionally considerably improved the uniformity of the CNT arrays. The researchers discovered that FeCu catalysts exhibited a “lifetime” 3.4 instances longer than pure iron catalysts. This prolonged catalyst lifetime permits nanotubes to develop to larger lengths earlier than terminating, leading to extra uniform arrays.
To reveal the potential of their technique, the researchers grew an impressively lengthy and dense CNT array measuring 30 centimeters in size. Even on the far finish of this array, the nanotube density remained round 90 nanotubes per millimeter – nonetheless increased than the utmost density achievable with conventional strategies.
The researchers performed detailed characterization of the CNTs produced utilizing their FeCu catalysts. They discovered the nanotubes have been primarily single-walled, double-walled, and triple-walled, with only a few defects. This excessive structural high quality is essential for sustaining the distinctive properties of CNTs over lengthy lengths.
To grasp the mechanisms behind the improved efficiency of FeCu catalysts, the researchers developed a kinetic mannequin and carried out molecular dynamics simulations. They discovered that including copper to iron catalysts creates a trade-off between two necessary components: catalyst fluidity and carbon solubility.
Copper lowers the melting level of the catalyst nanoparticles, growing their fluidity. This improved fluidity enhances carbon diffusion by way of the catalyst, which is useful for CNT development. Nonetheless, copper additionally decreases the catalyst’s capacity to dissolve carbon. At low copper concentrations, the advantages of elevated fluidity outweigh the drawbacks of decreased carbon solubility. However past an optimum level (round 11.4% copper on this research), additional will increase in copper content material start to hinder development.
This work represents a major advance within the area of ultralong CNT synthesis. The power to supply dense, uniform arrays of high-quality nanotubes at lengths of 30 centimeters and past opens up new potentialities for CNT purposes. The strategy’s versatility in creating numerous bimetallic catalysts additionally gives a robust new software for researchers to optimize CNT development for particular purposes.
Whereas challenges stay in scaling up manufacturing and exactly controlling nanotube properties, this analysis brings us nearer to realizing the complete potential of ultralong CNTs. As synthesis strategies proceed to enhance, we might quickly see ultralong CNTs enabling transformative applied sciences like ultra-lightweight structural supplies, long-distance energy transmission strains with minimal losses, and high-performance digital gadgets.
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