Dynamics of catalyst particle formation and multi-walled carbon nanotube growth in aerosol-assisted catalytic chemical vapor deposition
Introduction
The unique one-dimensional structure of carbon nanotubes (CNTs) made of single or multiple rolled-up graphene sheets, opened new avenues toward a potential fashioning of carbon objects at nanoscale, and allowed over-passing new limits on material properties in a lot of fields [1], [2], [3], [4]. In order to take benefit of these properties, production of well characterized CNTs is crucial, which implies a good understanding and control of their growth along the whole synthesis process. Among all the synthesis techniques developed so far [5], catalytic chemical vapor deposition (CCVD) appears as the most promising one [6] for scaling up at industrial levels due to its versatility and its moderated operating temperature. CCVD methods allow to produce as well single-walled [7], [8] and multi-walled CNTs (MWCNTs) [9], [10], [11] either entangled or aligned as in a carpet. These techniques are based on the use of catalyst nanoparticles, which diameter was shown to directly control CNT diameter [3], [12], [13], [14], [15]. Great emulation is active around two catalyst feeding methods. One consists in a two-step process with pre-formation of the catalyst particles on substrates, then introduction of carbon precursors to induce the CNT growth [16], [17], [18]. This method allows controlling the size of the nanoparticles which is one of the essential parameters to synthesize single-walled nanotubes. The other method is based on the simultaneous and continuous introduction of both catalyst and carbon precursors. This one-step technique [9], [10], [19], [20] is simpler to operate and generally gives high growth rate of long and aligned MWCNTs, which could be interpreted by prevention of catalyst from poisoning due to the continuous feeding in catalyst precursors [21], contrary to the two-step technique for which catalyst source is not renewed involving catalyst deactivation [22].
However, the one-step growth mechanism is more complicated to understand since catalyst particle formation, nucleation and growth of CNTs take place simultaneously in the reactor. Also, the reactants are submitted to a strongly inhomogeneous temperature profile along the reactor which is expected to act differently on the different precursors. In particular, detailed understanding of catalyst nanoparticle formation is important for the control of their sizes and for a smart scaling-up of the process. The different parametric studies available in the literature are difficult to compare due to the differences between experimental set-ups and precursors [23], [24]. A critical step appears to be the transition between gas and solid phase occurring during the nucleation and growth of catalyst particles. The few papers discussing this point suggest that a homogeneous nucleation is possible in the gas phase due to the very low iron saturated vapor pressure [25]. Computational fluid dynamics (CFD) simulation [26] predicts that homogeneous nucleation is the dominant mechanism for producing iron particles in a CVD reactor.
In this paper, our objective is to give new insights into the aerosol-assisted CCVD process we developed [10], [27] using a solution of ferrocene dissolved in toluene as liquid precursor. We have shown previously that the process begins by the formation of Fe-based nanoparticles on the reactor walls initiating a MWCNT base-growth mechanism [19], and that carbon precursor diffuses through the whole carpet to react only on the nanoparticles located on the substrates [21]. More recently, we have shown that the nature of catalyst particles is iron carbide and that they are probably molten supersaturated carbon–metal particles during the CNT growth [28]. The aim of this study is to understand how and where are formed these catalyst nanoparticles and how they govern the nanotube growth all along the CVD reactor. The main purpose is to check if a homogeneous nucleation is really occurring. Our strategy is based on ex-situ analyses of the reaction products at a given location along the reactor, that is to say as a function of the temperature, and also as a function of the iron vapor pressure in the gas phase, which can be varied by changing parameters such as ferrocene content in the aerosol or carrier gas flow rate.
Section snippets
Experimental
The synthesis set-up and standard procedure have been described previously [10], [27]. Briefly, the precursor mixture is made of ferrocene dissolved in toluene, the resulting solution is nebulized through an aerosol generator and the aerosol obtained is carried by an argon flow in the reactor through an aerosol generator (Fig. 1). The aerosol pyrolysis takes place in a cylindrical quartz reactor (17 mm internal diameter) introduced in a 45-cm long tubular furnace whose temperature profile is
Effect of location along the reactor
Weighable samples of MWCNTs can be collected from 13 to 37 cm. Fig. 3 reports three typical SEM micrographs of CNT material grown from optimized synthesis parameters and collected either at the entrance of the isothermal area (in section 3) or at its exit (in sections 6 and 7). These samples are all composed of carpets made of aligned MWCNTs, with very good cleanness, the number of by-products being very low, as usually observed with this method [27], [30]. The first striking observation is the
Discussion
The first striking result to discuss is the high difference between the surface density of iron and of collected products as a function of location along the reactor (Fig. 4a). The highest iron density is obtained before the isothermal area in a section where the mean temperature is around 820 °C (Fig. 2), and then this density strongly decreases, while the maximum of products is obtained in the isothermal area. In order to understand this behavior, we have to consider the chemical
Conclusion
Our results give new insights into the MWCNT growth mechanisms involved in an aerosol-assisted CCVD reactor taking into account its thermal gradient. From the analysis of the species collected all along the reactor, we have elucidated how catalyst particles are formed from the injected ferrocene as catalyst precursor, and determined the consequence on CNT growth and characteristics. Iron-based catalyst nanoparticles are formed by homogeneous nucleation in the gas phase with a high rate at a
Acknowledgements
The authors thank A. Gohier and J. Marie for their contributions in the synthesis work, P. Bonnaillie, S. Poissonnet, DEN/DMN/SRMP, CEA-Saclay and C. Berthier INSTN, UESMS, CEA-Saclay for their help in SEM observations, J.M. Verbavatz, DSV, CEA-Saclay for support in TEM analyses, D. Neff and P. Dillmann, DSM/SIS2M/LAPA CEA-Saclay for Raman facilities.
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