Catalyst nanoparticle growth dynamics and their influence on product morphology in a CVD process for continuous carbon nanotube synthesis
Introduction
Individual CNTs have exceptional mechanical, thermal and electrical properties, with tensile strengths up to 100 GPa, thermal conductivities up to 3000 W m−1 K−1 and electrical resistivities as low as 5 × 10−6 Ω cm [1], [2], [3]. However, the ability to translate the superior properties of individual CNTs into a macro-scale CNT material for bulk applications remains an unsolved challenge, with even the strongest materials only able to capture one-hundredth of the available individual CNT tensile strength [4], [5]. To achieve resistivities of even 1.5 × 10−5 Ω cm requires several post-treatment steps, involving strong acids and iodine doping [6].
There are a number of different techniques to assemble individual CNTs into macro material products, such as spinning CNT fibers from a liquid-crystal phase and spinning fibers or pulling a continuous film from forest-grown CNTs [7], [8]. However, spinning a film or fiber from a floating catalyst chemical vapor deposition (FCCVD) method is the most attractive as an industrially scalable route, by virtue of it being a one-step, continuous, gas-phase process and has already been adopted by several companies looking to exploit these advantages. The process, first described in detail by Li et al. involves the continuous, controlled injection of a hydrocarbon source, an iron source (typically ferrocene vapor) and a sulfur source into a tubular reactor at temperatures above 1000 °C in a reducing H2 atmosphere [9]. Thermal decomposition of the iron and sulfur sources leads to the nucleation of catalyst nanoparticles which act as a catalytic surface for CNT growth once sufficient carbon is available from the decomposition of the reactants. The role of sulfur is still under investigation but current research suggests it conditions the iron nanoparticles by affecting the carbon diffusivity at the surface and stimulating CNT growth [10], [11]. As CNTs begin to grow, they preferentially bundle due to Van der Waals forces and these bundles intertwine to form an aerogel. The aerogel is mechanically drawn from the reactor tube onto a winding mechanism for continuous collection.
Understanding the impact of parameters such as the choice of carbon and catalyst precursors, the nature and ratio of sulfur-containing compounds to other reactants and the reaction temperature on CVD grown CNTs is increasingly well-documented [12]. However, extrapolating this information to the more complex continuous spinning of bulk CNT products from FCCVD processes is not straightforward. Parametric studies on how the morphology of bulk CNT products are affected by variables such as the carbon source, sulfur source and bulk flow rates indicate that while these factors do give some control, it is their interaction with the catalyst nanoparticles which is key to product purity [13], [14], [15]. The synthesis of undesired carbon structures for instance is reported to be influenced by secondary parameters including all of the above factors, with the control of sulfur being crucial [16]. Control of the formation of the iron-based catalyst nanoparticles is widely recognized as a primary parameter in controlling the diameter, purity, yield, crystalline quality, entanglements, chirality and number of walls of the CNTs in the final product and hence is an important factor in optimizing the bulk material properties [11], [17]. The diameter of the catalyst nanoparticles closely correlates with the diameter of the CNTs, however it has recently been shown that in some CVD systems only 1% of iron based nanoparticles lead to CNT growth [18]. Some of the additional iron contributes to the co-synthesis of undesirable impurities such as graphitically-encapsulated nanoparticles, defective nanotubes and large diameter carbon tubules. These impurities are enmeshed in the CNT aerogel and disrupt the mechanical and electrical properties of the final product.
While some real-time analytical techniques such as TEM, XPS and Raman have been used to study the CVD growth of CNTs on substrates, studies of the relationship between catalyst nanoparticle formation and the synthesis of CNTs and impurities in FCCVD systems have principally relied on ex-situ post-experimental characterization [19], [20], [21]. An exception to this is the use of aerosol measurement techniques applied to both control catalytic nanoparticle size distributions prior to injection into a CVD system and to measure the synthesized CNTs at the exit [22], [23], [24].
In order to maximize the uptake of catalytic nanoparticles for CNT growth, and minimize or prevent the formation of unwanted side products, a much clearer understanding of the in-situ nanoparticle formation process is crucial for the industrial development of this FCCVD method. In the process, the catalyst nanoparticles initially nucleate from a vapor phase, which is created as the iron precursor (ferrocene) and sulfur precursor (thiophene) decompose. Besides nucleation; coagulation, surface growth, thermophoresis and diffusion affect nanoparticle growth. An experimentally-based analysis of the catalytic nanoparticle formation in real time along the axis of a tubular furnace will provide information on how these nanoparticles grow and where in the reactor bulk CNT material is generated. The present study investigates the production of catalyst nanoparticles within a CNT reactor in real time using a sampling system, which allows extraction and size distribution analysis of the catalyst particles from the hot reactor at multiple axial locations. Coupling the particle size distributions with velocity and thermal profiles, together with FTIR analysis of decomposition species synthesized from reactant inputs, provides new insight into the catalyst nanoparticle formation factors which influence both CNT and impurity formation.
Section snippets
Experimental methods
The in situ experiments described in this paper were carried out in a horizontal tube furnace at ambient pressure and variable furnace temperatures. A schematic of the setup is shown in Fig. 1. Ferrocene (Acros, purity 98%) (∼0.5 mass%) and Thiophene (Sigma Aldrich, purity ≥ 99%) (∼3 mass%) as precursors diluted in a hydrogen (purity grade hydrogen N5.0, BOC) bulk flow of usually 0.5 slpm entered the reactor tube (40 mm ID and 700 mm length) through a showerhead injector which ensured a uniform
Flow and temperature profile in the reactor
The flow and temperature profile within the reactor (40 mm ID and 700 mm length) affect reactant decomposition, as well as nanoparticle nucleation, diffusion and thermophoretic phenomena. As shown in Fig. 2, the temperature difference between wall and centerline is small (ΔTmax ∼ 98 °C) relative to the axial gradients, and the radial gradient changes from negative to positive down the centerline of the reactor. After injecting the reactants through a uniform flow injector reaching 50 mm into
Conclusion
Infrared spectrometry results demonstrate that the catalyst particle precursors (ferrocene and thiophene) decompose independently from one another within narrow temperature dependent zones, indicating that the onset of the decomposition of each species is primarily thermally rather than catalytically driven.
Catalyst nanoparticles nucleate and grow as a result of the precursor decomposition. Size distribution measurements along the reactor axis show a distinctive, temperature dependent variation
Acknowledgments
The authors thank Qflo Ltd for providing funding towards this research, C. Hoecker additionally thanks Churchill College Cambridge for financial support, M. Bajada gratefully acknowledges financial support through the 'Master it! Scholarship Scheme'. C. Hoecker and F. Smail contributed equally to this work.
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