Evolution of Molybdenum Catalysts in Carbon Nanotube Synthesis

Single-walled carbon nanotubes (SWCNTs) consist of a single layer of graphene rolled into a cylindrical tube. Despite their nanometer size, they exhibit impressive physical properties. They are one of the strongest and stiffest materials in terms of tensile strength and elastic modulus. SWCNTs can be either metallic or semiconducting in their electrical behavior, which makes them useful in electronics and nanotechnology. They have an exceptionally high thermal conductivity, higher than that of diamond. SWCNTs are also chemically stable and can be functionalized with various chemical groups, making them useful in a range of applications, from chemical processing to drug delivery. SWCNTs can have applications in nanoelectronic devices, including transistors, diodes, and sensors. Their high strength-to-weight ratio makes them ideal for aerospace, automotive, and sporting goods industries. Moreover, they are used in the development of high-capacity batteries and supercapacitors, as well as in solar cells.

Molybdenum nanoparticles has been previously reported to be an important catalyst in the synthesis of SWCNTs as they significantly influence the growth, quality, and specific properties of the nanotubes. Molybdenum nanoparticles, due to their small size, have a high surface area-to-volume ratio. This property is essential for catalysis, as it provides more active sites for the chemical reactions to occur. Molybdenum maintains stability at high temperatures, which is a necessary condition for SWCNT synthesis. The most common method for producing SWCNTs is chemical vapor deposition. In this process, a   methane is decomposed at high temperatures in the presence of a catalyst. Molybdenum nanoparticles serve as a catalyst in this process, assisting in the breakdown of the carbon source and the formation of SWCNTs. The size and composition of molybdenum nanoparticles can influence the diameter and chirality of the SWCNTs. This is important for tailoring the electronic properties of the nanotubes, as these characteristics determine whether the SWCNTs will be metallic or semiconducting. By optimizing the conditions and the properties of the molybdenum nanoparticles, researchers can improve the yield and quality of the SWCNTs. In a new study published in the Journal Carbon led by Professor Haiming Duan, PhD candidate Xuan Chen and Biaobing Cao from the School of Physics Science and Technology at Xinjiang University focused on the understanding the behavior and transformation of the Molybdenum nanoparticles catalysts during the early stages of SWCNT growth is crucial for optimizing production processes and enhancing the quality of the nanotubes. They presented a comprehensive study on the behavior and evolution of molybdenum nanoparticles during the initial stage of catalytic growth of SWCNTs. The study emphasizes the significance of molybdenum as a refractory catalyst and investigated the transformation dynamics of molybdenum nanoparticles under varying conditions, including their phase changes and the effect of carbon atom deposition. The work incorporates computational methods like classical molecular dynamics and Density Functional Theory (DFT) calculations to elucidate these mechanisms. Key focus areas include the potential energy surface (PES) of molybdenum nanoparticles, Gupta potential, Lennard-Jones potential, many-body potential, and the interaction between molybdenum nanoparticles and carbon atoms. The findings provide valuable insights into the control and optimization of SWCNT growth, contributing to advancements in the field of nanotechnology and material science.

The research team showed that the phase transformation of molybdenum nanoparticles under varying conditions. The nanoparticles undergo transformations between solid and semi-liquid phases during the gradual deposition of carbon atoms. This phase change, occurring at temperatures below the melting point of the nanoparticles, is attributed to the elastic strain in the nanoparticles, which can be manipulated by carbon atoms. Understanding these transformations is essential for controlling the lifetime of nanoparticles in their semi-liquid state, a key factor in the growth of SWCNTs.

The authors’ demonstrated that Mo catalysts have profound implications for SWCNT synthesis. By adjusting experimental parameters, such as the feeding rate of carbon atoms, researchers can show greater control over the chirality and quality of the nanotubes produced. This can result in a more precise and efficient production methods, leading to the development of high-quality SWCNTs suitable for various applications in electronics, materials science, and nanotechnology. In conclusion, Professor Haiming Duan and colleagues showed the evolution mechanism of molybdenum catalysts in the initial stage of SWCNT growth is a complex interplay of material science, chemistry, and computational modeling.