Oxygen-promoted catalyst sintering influences number density, alignment, and wall number of vertically aligned carbon nanotubes

Modifying the Molecular Structure of Carbon Nanotubes through Gas-Phase Reactants

Current approaches to carbon nanotube (CNT) synthesis are limited in their ability to control the placement of atoms on the surface of nanotubes. Some of this limitation stems from a lack of understanding of the chemical bond-building mechanisms at play in CNT growth. Here, we provide experimental evidence that supports an alkyne polymerization pathway in which short-chained alkynes directly incorporate into the CNT lattice during growth, partially retaining their side groups and influencing CNT morphology. Using acetylene, methyl acetylene, and vinyl acetylene as feedstock gases, unique morphological differences were observed. Interwall spacing, a highly conserved value in natural graphitic materials, varied to accommodate side groups, increasing systematically from acetylene to methyl acetylene to vinyl acetylene. Furthermore, attenuated total reflectance Fourier-transfer infrared spectroscopy (ATR-FTIR) illustrated the existence of intact methyl groups in the multiwalled CNTs derived from methyl acetylene. Finally, the nanoscale alignment of the CNTs grown in vertically aligned forests differed systematically. Methyl acetylene induced the most tortuous growth while CNTs from acetylene and vinyl-acetylene were more aligned, presumably due to the presence of polymerizable unsaturated bonds in the structure. These results demonstrate that feedstock hydrocarbons can alter the atomic-scale structure of CNTs, which in turn can affect properties on larger scales. This information could be leveraged to create more chemically and structurally complex CNT structures, enable more sustainable chemical pathways by avoiding the need for solvents and postreaction modifications, and potentially unlock experimental routes to a host of higher-order carbonaceous nanomaterials.

Developments and advances in in situ transmission electron microscopy for catalysis research

Recent developments and advances in in situ TEM have raised the possibility to study every step during the catalysts' lifecycle. This review discusses the current state, opportunities and challenges of in situ TEM in the realm of catalysis.

Isolating the Roles of Hydrogen Exposure and Trace Carbon Contamination on the Formation of Active Catalyst Populations for Carbon Nanotube Growth

Limited understanding of the factors influencing the yield of carbon nanotubes (CNTs) relative to the number of catalyst particles remains an important barrier to their large-scale production with high quality, and to tailoring CNT properties for applications. This lack of understanding is evident in the frequent use of Edisonian approaches to give high-yield CNT growth, and in the sometimes-confusing influence of trace residues on the reactor walls. In order to create conditions wherein CNT yield is reproducible and to enable large-scale and reliable CNT synthesis, it is imperative to understand-fundamentally-how these common practices impact catalytic activity and thus CNT number density. Herein, we use ambient pressure-X-ray photoelectron spectroscopy (AP-XPS) to reveal the influence of carbon and hydrogen on the coupling between catalyst reduction and CNT nucleation, from an iron catalyst film. We observe a positive correlation between the degree of catalyst reduction and the density of vertically aligned CNTs (forests), verifying that effective catalyst reduction is critical to CNT nucleation and to the resulting CNT growth yield. We demonstrate that the extent of catalyst reduction is the reason for low CNT number density and for lack of self-organization, lift-off, and growth of CNT forests. We also show that hydrocarbon byproducts from consecutive growths can facilitate catalyst reduction and increase CNT number density significantly. These findings suggest that common practices used in the field-such as reactor preconditioning-aid in the reduction of the catalyst population, thus improving CNT number density and enabling the growth of dense forests. Our results also motivate future work using AP-XPS and complementary metrology tools to optimize CNT growth conditions according to the catalyst chemical state.

Synthesis Procedure of Highly Densely Packed Carbon Nanotube Forests on TiN

The goal of this research was to obtain high-density single-walled carbon nanotube forests (SWNTs) on conductive substrates for different applications, including field emission. For this, dip-coating was chosen as the catalyst deposition method, to subsequently grow SWNTs by Alcohol Catalytic Chemical Vapor Deposition (AC-CVD). Si (100) was chosen as the substrate, which was then coated with a TiN thin film. By sputtering with Ar, it was possible to generate alternating TiN and Si lanes, with a different wettability and, therefore, a different affinity for the catalysts. As a result, the Mo-Co catalyst was mainly deposited on TiN and not on sputtered-Si, which allowed the selective growth of SWNT forests on the TiN conductive surfaces. These as-synthesized SWNTs were used for field emission measurements in a high vacuum chamber.

Carbon Nanotubes and Related Nanomaterials: Critical Advances and Challenges for Synthesis toward Mainstream Commercial Applications

Advances in the synthesis and scalable manufacturing of single-walled carbon nanotubes (SWCNTs) remain critical to realizing many important commercial applications. Here we review recent breakthroughs in the synthesis of SWCNTs and highlight key ongoing research areas and challenges. A few key applications that capitalize on the properties of SWCNTs are also reviewed with respect to the recent synthesis breakthroughs and ways in which synthesis science can enable advances in these applications. While the primary focus of this review is on the science framework of SWCNT growth, we draw connections to mechanisms underlying the synthesis of other 1D and 2D materials such as boron nitride nanotubes and graphene.

Carbon Dioxide Promotes Dehydrogenation in the Equimolar C2 H2 -CO2 Reaction to Synthesize Carbon Nanotubes

The equimolar C₂ H₂ -CO₂ reaction has shown promise for carbon nanotube (CNT) production at low temperatures and on diverse functional substrate materials; however, the electron-pushing mechanism of this reaction is not well demonstrated. Here, the role of CO₂ is explored experimentally and theoretically. In particular, ¹³ C labeling of CO₂ demonstrates that CO₂ is not an important C source in CNT growth by thermal catalytic chemical vapor deposition. Consistent with this experimental finding, the adsorption behaviors of C₂ H₂ and CO₂ on a graphene-like lattice via density functional theory calculations reveal that the binding energies of C₂ H₂ are markedly higher than that of CO₂ , suggesting the former is more likely to incorporate into CNT structure. Further, H-abstraction by CO₂ from the active CNT growth edge would be favored, ultimately forming CO and H₂ O. These results support that the commonly observed, promoting role of CO₂ in CNT growth is due to a CO₂ -assisted dehydrogenation mechanism.

Process-morphology scaling relations quantify self-organization in capillary densified nanofiber arrays

Model-informed experiments reveal that cellular pattern formation in capillary-densified aligned carbon nanotube arrays is governed not only by their height, but also by substrate adhesion strength.