Rational Modification of a Metallic Substrate for CVD Growth of Carbon Nanotubes

Citation: Li, X., Baker-Fales, M., Almkhelfe, H., Gaede, N. R., Harris, T. S., & Amama, P. B. (2018). Rational Modification of a Metallic Substrate for CVD Growth of Carbon Nanotubes. Scientific Reports, 8(1). https://doi.org/10.1038/s41598-018-22467-7Growth of high quality, dense carbon nanotube (CNT) arrays via catalytic chemical vapor deposition (CCVD) has been largely limited to catalysts supported on amorphous alumina or silica. To overcome the challenge of conducting CNT growth from catalysts supported on conductive substrates, we explored a two-step surface modification that involves ion beam bombardment to create surface porosity and deposition of a thin AlxOy barrier layer to make the surface basic. To test the efficacy of our approach on a non-oxide support, we focus on modification of 316 stainless steel (SS), a well-known inactive substrate for CNT growth. Our study reveals that ion beam bombardment of SS has the ability to reduce film thickness of the AlxOy barrier layer required to grow CNTs from Fe catalysts to ∼ 5 nm, which is within the threshold for the substrate to remain conductive. Additionally, catalysts supported on ion beam-damaged SS with the same AlxOy thickness show improved particle formation, catalyst stability, and CNT growth efficiency, as well as producing CNTs with higher quality and density. Under optimal reaction conditions, this modification approach can lead to CNT growth on other nontraditional substrates and potentially benefit applications that require CNTs be grown on a conductive substrate

Carbon Nanotube Fabrication at Industrial Scale: Opportunities and Challenges

Careful research on different materials reveals that the material properties are mostly affected by the size of it. Material size down to nanometer scale exhibits some remarkable properties, resulting in unique physical and chemical characteristics. In todays world of nanotechnology, carbon nanotubes (CNTs) have become a high priority material because of their exclusive structure, novel characteristics with enormous potential in many technological applications. Till date chemical vapor deposition (CVD) is the preferred and widely used technique among different CNT growth methods, because of its potential advantage to produce CNTs of high purity, large yield with ease of scale up and low setup cost. This article provides an overview of different CVD methods for industrial scale fabrication of CNTs. The influence of material aspect, viz. catalyst type, catalyst support, and growth control aspect, viz. process temperature, pressure, catalyst concentration, are discussed. Additionally, possible growth mechanisms concerning CNT formation are described. Finally, the key challenges of the process are addressed with future perspective.Comment: carbon nanotubes, chemical vapor depositio

Controlling the Tube Diameter of SWCNTs Using High Melting Point Promotors

A particular control of the diameter of Single Walled-Carbon Nanotube (SWCNT) using Chemical vapor Deposition (CVD) system will enable many promising applications in different fields. Here we demonstrate the growth of SWCNT with good control of diameter (1.5 nm ± 0.7) using a high melting temperature metal (Ru) as a catalyst promotor with the main catalyst Co at 850˚C via CVD. We hypothesis that using high melting temperature metal as a promotor, like Ru can limit the mobility/change in the shape of the formed metal nanoparticles and eventually decrease the effects of Ostwald ripening (OR). FTS-GP is used as a carbon precursor. The results have been verified by high-resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM) and multi-excitation Raman.</jats:p

Scalable carbon nanotube growth and design of efficient catalysts for Fischer-Tropsch synthesis

Doctor of PhilosophyDepartment of Chemical EngineeringPlacidus B. AmamaThe continued depletion of fossil fuels and concomitant increase in greenhouse gases have encouraged worldwide research on alternative processes to produce clean fuel. Fischer-Tropsch synthesis (FTS) is a heterogeneous catalytic reaction that converts syngas (CO and H₂) to liquid hydrocarbons. FTS is a well-established route for producing clean liquid fuels. However, the broad product distribution and limited catalytic activity are restricting the development of FTS. The strong interactions between the active metal catalyst (Fe or Co) and support (Al₂O₃, SiO₂ and TiO₂) during post-synthesis treatments of the catalyst (such as calcination at ~500°C and reduction ~550°C) lead to formation of inactive and unreducible inert material like Fe₂SiO₄, CoAl₂O₄, Co₂SiO₄. The activity of FTS catalyst is negatively impacted by the presence of these inactive compounds. In our study, we demonstrate the use of a modified photo-Fenton process for the preparation of carbon nanotube (CNT)-supported Co and Fe catalysts that are characterized by small and well-dispersed catalyst particles on CNTs that require no further treatments. The process is facile, highly scalable, and involves the use of green catalyst precursors and an oxidant. The reaction kinetic results show high CO conversion (85%), selectivity for liquid hydrocarbons and stability. Further, a gaseous product mixture from FTS (C1-C4) was utilized as an efficient feedstock for the growth of high-quality, well-aligned single-wall carbon nanotube (SWCNT) carpets of millimeter-scale heights on Fe and (sub) millimeter-scale heights on Co catalysts via chemical vapor deposition (CVD). Although SWCNT carpets were grown over a wide temperature range (between 650 and 850°C), growth conducted at optimal temperatures for Co (850°C) and Fe (750°C) yielded predominantly SWCNTs that are straight, clean, and with sidewalls that are largely free of amorphous carbon. Also, low-temperature CVD growth of CNT carpets from Fe and Fe–Cu catalysts using a gaseous product mixture from FTS as a superior carbon feedstock is demonstrated. The efficiency of the growth process is evidenced by the highly dense, vertically aligned CNT structures from both Fe and Fe–Cu catalysts even at temperatures as low as 400°C–a record low growth temperature for CNT carpets obtained via conventional thermal CVD. The use of FTS-GP facilitates low-temperature growth of CNT carpets on traditional (alumina film) and nontraditional substrates (aluminum foil) and has the potential of enhancing CNT quality, catalyst lifetime, and scalability. We demonstrate growth of SWCNT carpets with diameter distributions that are smaller than SWCNTs in conventional carpets using a CVD process that utilizes the product gaseous mixture from Fischer-Tropsch synthesis (FTS-GP). The high-resolution transmission electron microscopic (HR-TEM) and Raman spectroscopic results reveal that the use of a high melting point metal as a catalyst promoter in combination with either Co (1.5 nm ± 0.7) at 850ºC or Fe (1.9 nm ± 0.8) at 750ºC yields smaller-diameter SWCNT arrays with narrow diameter distributions. Scalable synthesis of carbon nanotubes (CNTs), carbon nanofibers (CNFs), and onion like carbon (OLC) in a batch reactor using supercritical fluids as a reaction media is demonstrated. The process utilizes toluene, ethanol, or butanol as a carbon precursor in combination with ferrocene that serves as a catalyst precursor and a secondary carbon source. The use of supercritical fluids for growth does not only provide a route for selective growth of a variety of carbon nanomaterials, but also provides a unique one-step approach that is free of aggressive acid treatment for synthesis of CNT-supported metallic nanoparticle composites for catalysis and energy storage applications

Supercritical Fluids as Reaction Media for Scalable Production of Carbon Nanomaterials

We have demonstrated scalable and selective synthesis of carbon nanotubes (CNTs), carbon nanofibers (CNFs), and onion-like carbons (OLCs) in a batch reactor using supercritical fluids (SCFs) as reaction media. The process utilizes toluene and alcohols (ethanol, propanol, and butanol) as carbon precursors in combination with ferrocene. Growth with supercritical toluene at 600 °C in the absence of water yields large diameter CNTs while introduction of 92.5 mmol/L of water enhances product yield by 50%, promoting formation of smaller diameter CNTs and decorating the exterior surface of CNTs with Fe nanoparticles. At 400 and 500 °C, in the absence of water, supercritical toluene produces mainly OLCs and CNFs, respectively. For alcohols, a gradual evolution of the morphology of nanocarbons forms from mainly OLCs to tube-like structures as the ratio of C/O atoms increases, possibly due to a decrease in the tendency of graphitic sheets to minimize their energies by curling into onion-like structures as chain length increases. This study provides a framework for utilizing SCF reaction media in a batch reactor to achieve scalable and selective growth of different nanocarbons and nanocarbon–metal nanocomposites

Hydrothermal synthesis of carbon nanotube–titania composites for enhanced photocatalytic performance

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