Nature of the Catalyst Particles in CCVD Synthesis of Multiwalled Carbon Nanotubes Revealed by the Cooling Step Study

Abstract

The true chemical nature and physical state of the catalyst particles in Catalytic Chemical Vapor Deposition (CCVD) synthesis of carbon nanotubes are the subject of intense discussions, as it is one of the keys to understand their growth mechanisms. The CCVD method considered in this article involves pyrolysis of mixed liquid aerosols and leads to the synthesis of large carpets of multiwalled nanotubes (MWNTs) partially filled with iron-based materials. The experimental approach consists in studying the influence of the cooling procedure applied at the end of the synthesis. Both slow standard cooling or quenching were performed, and the structure and chemical state of the iron-based particles were compared through complementary local and global investigations involving X-ray diffraction, electron microscopy, electron diffraction, as well as electron energy loss spectroscopy. We clearly demonstrate that iron-based catalyst particles are carbon-rich and oxygen-free in quenched samples, and that they oxidize during the slow cooling step. It is inferred that they are very probably molten supersaturated carbon-metal particles during the NT growth

Structure in nascent carbon nanotubes revealed by spatially resolved Raman spectroscopy

“…However, such oxide phases are not the catalytic phases in the process we use, where nanoparticles giving rise to CNT growth were shown to be cementite nanoparticles [22]. We have shown that the occurrence of oxides is due to iron oxidation during the cooling step [15], and the present results indicate that the oxidation is not complete for all iron-based particles since Fe 3 O 4 is detected. In addition, in our study no oxygen-containing promoters such as traces of water have been added to the reactive gas phase.…”

“…For this sample grown during the early stages and thus exhibiting a low amount of carbon, Raman spectroscopy is sensitive to both iron-based and carbon phases whereas XRD is mainly sensitive to iron-based phases. mass ratio = 4/3, figure 2 (e)) [11,15]. This is the first XRD identification of α-Fe 2 O 3 in such samples, due probably to a too low content of such phase in previous studies [11,15,17,19].…”

“…Mo-Kα (λ=0.711 Å) beam are performed in order to measure iron phases [15]. The intense X- In figure 1 (a) (sample 1), we observe a layer of iron-based catalyst nanoparticles covering the whole silicon wafer surface, and constituting the main part of the sample.…”

“…Ex-situ physico-chemical analyses of samples synthesized under precise conditions are focused on the size and shape of catalyst particles as well as on their physical state and their chemical composition and structure [11,[14][15][16]. The analysis of carbon in such samples has been also investigated by different techniques such as electron microscopy (SEM or TEM), Raman spectroscopy, X-Ray Diffraction or X-Ray Photoelectron Spectroscopy [11,15,[17][18][19]. Nevertheless, complementary information regarding global quantitative carbon structure changes occurring on nanotubes near the catalyst particles, from the CNT nucleation step up to the CNT growth step, still need to be completed.…”

“…Our previous works regarding A-MWNT carpet synthesised by aerosol-assisted CCVD (Combustion Chemical Vapor Deposition) process studied through ex-situ analysis demonstrated that (i) different iron-based phases are occurring in the same sample [15,19], (ii) iron oxide is formed during the cooling step [15], (iii) nanotubes are obtained through a base-growth mechanism [11,20]. X-Ray diffraction allows one to quantify the alignment degree of nanotubes in macroscopic carpets [19,21].…”