Scientists at Skolkovo Institute of Science and Technology (Skoltech) have found a way to increase the yield of single-walled carbon nanotube film production. The materials are promising for solar cells, LEDs, flexible and transparent electronics, smart textiles, medical imaging, toxic gas detectors, filtration systems, and more. By adding hydrogen gas and carbon monoxide to the reaction chamber, the team managed to almost triple carbon nanotube yield compared to growth promoters, without compromising quality. Until now, low yield has been the bottleneck limiting the potential of that manufacturing technology, otherwise known for high product quality. Conceptually, nanotubes are a form of carbon in which sheets of graphene are seamlessly rolled into hollow cylinders. They vary in length, diameter, and so-called chirality (how the honeycomb pattern is skewed), as well as whether the tube is single-walled or has other, wider tubes around it, making it multiwalled. The properties of carbon nanotubes vary widely based on those parameters. Chirality, for example, controls their electrical conductivity. Carbon nanotubes are manufactured as powder, thin films, fibers, and other forms, depending on the application they are intended for. Schematic representation of hydrogen’s effect in CO-based CVD synthesis of nanotubes. Courtesy of Ilya Novikov et al./Chemical Engineering Journal. Owing to their superb mechanical, electrical, optical, and thermal properties, carbon nanotubes are used in diverse products and technologies, from tear-resistant car tires and composite materials for wind turbine blades to flexible touchscreens and lithium-ion battery components. The main applications of single-walled carbon nanotubes in the form of thin films are in electronic and optical devices, components, and solutions, particularly those intended to be flexible, stretchable, wearable, and transparent. Among them are lasers, light-emitting diodes and displays, solar cells, cables, transistors, mechanical, chemical, and light sensors, gas and liquid filtration systems, antistatic coatings, and even drug delivery vehicles. The main technology for manufacturing single-walled carbon nanotube (SWCNT) films — and indeed most other forms of carbon nanotubes — is known as chemical vapor deposition (CVD) and encompasses several techniques that are variations on the same basic process. Among such variations, floating catalyst (aerosol) CVD is used for the production of thin films, because it can be done in a single step. In this method, gaseous flows of carbon source (carbon feedstock for growing nanotubes, such as hydrocarbons, carbon monoxide, ethanol, etc.) and catalyst precursor (typically, precursor of iron nanoparticles — for example, ferrocene) are introduced into the high-temperature reactor. The high temperature decomposes the precursor into catalytic nanoparticles followed by the decomposition of carbon source and deposition of carbon on their surface, the formation of fullerene hemisphere-like cap (known as yarmulke mechanism), and nanotube growth. At the outlet of the reactor, nanotubes are filtered simultaneously forming a 2D network on the filter surface — the thin SWCNT film. “The choice of carbon source depends on the desired properties of nanotubes,” said study co- assistant professor Dmitry Krasnikov, assistant professor at Skoltech. “Carbon monoxide provides high product quality suitable for optics and electronics applications, but at the cost of a rather modest yield.” To solve this problem, researchers typically use growth promoters — additional compounds in the CVD reactor that increase nanotube growth or improve catalyst activation and/or lifetime. Typically, these are sulfur compounds, weak oxidants such as carbon dioxide or water, or additional carbon sources. Nevertheless, all these options have their drawbacks. According to Ilya Novikov, the main author of the publication who has recently defended his Ph.D. thesis devoted to nanotube synthesis at Skoltech, current solutions couldn’t significantly improve the productivity of carbon dioxide-based synthesis. Two- and threefold increases in yield were typical, while sulfur was shown to be infective for the carbon dioxide-based process. The researchers considered hydrogen as a possible growth promoter. “In previous works, it was found that its introduction into the CO atmosphere could trigger an extra reaction producing carbon in addition to the Boudouard reaction (the CO disproportionation: CO + CO → C + CO2) — CO hydrogenation (CO + H2 → C + H2O),” Novikov said. “We concluded it might work in our case too.” After the thorough investigation of hydrogen’s effect on SWCNT synthesis yield, as well as the properties of the nanotube product, the authors found a fifteenfold increase in the synthesis productivity at 10% volume concentration of H2 without deterioration of nanotube film structural properties and performance as a transparent conductor. Schematic representation of hydrogen’s effect in different temperature regimes. Courtesy of Ilya Novikov et al./Chemical Engineering Journal. “Having studied the mechanisms involved in nanotube growth by optical spectroscopy and electron microscopy methods and performed a detailed study of the thermodynamics of the process, we concluded that carbon monoxide hydrogenation is indeed responsible for such a remarkable effect,” said Albert Nasibulin, head of the Laboratory of Nanomaterials at Skoltech. “Moreover, to explain its influence on the process in detail, we examined different temperature regimes for nanotube synthesis in addition to varying hydrogen concentration,” Krasnikov added. The researchers observed, unexpectedly, two different phenomena, Krasnikov said. In the low-temperature regime, hydrogen significantly improved catalyst activation (the fraction of iron particles active for catalysis), thereby boosting yield. In the high-temperature regime, it enhanced nanotube growth, resulting in longer nanotubes with higher conductivity of the films. Nasibulin said that the study solves two problems simultaneously. First, a considerable improvement in synthesis productivity significantly extends the applications of CO-based aerosol CVD processes and brings the method closer to industry-level nanotube production. Second, the researchers found fundamental mechanisms behind nanotube growth based on carbon dioxide disproportionation, which is expected to deepen understanding of nanotube CVD synthesis more broadly. The research was published in Chemical Engineering Journal (www.doi.org/10.1016/j.cej.2023.146527).
