Letter pubs.acs.org/JPCL
In Situ TEM Observations on the Sulfur-Assisted Catalytic Growth of Single-Wall Carbon Nanotubes Lili Zhang, Peng-Xiang Hou, Shisheng Li, Chao Shi, Hong-Tao Cong, Chang Liu,* and Hui-Ming Cheng Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China S Supporting Information *
ABSTRACT: The effect of sulfur on the catalytic nucleation and growth of single-wall carbon nanotubes (SWCNTs) from an iron catalyst was investigated in situ by transmission electron microscopy (TEM). The catalyst precursor of ferrocene and growth promoter of sulfur were selectively loaded inside of the hollow core of multiwall CNTs with open ends, which served as a nanoreactor powered by applying a voltage inside of the chamber of a TEM. It was found that a SWCNT nucleated and grew perpendicularly from a region of the catalyst nanoparticle surface, instead of the normal tangential growth that occurs with no sulfur addition. Our in situ TEM observation combined with CVD growth studies suggests that sulfur functions to promote the nucleation and growth of SWCNTs by forming inhomogeneous local active sites and modifying the interface bonding between catalysts and precipitated graphitic layers, so that carbon caps can be lifted off from the catalyst particle. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
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supplied by the thermal dissociation of the preloaded Fc. Thus, the nucleation and growth of CNTs from the iron catalyst with sulfur can be studied by in situ TEM observation. By comparing the processes with and without sulfur addition, the effect of sulfur on the growth of CNTs was investigated. The results of a combined in situ TEM study and parallel ex situ CNT growth show that sulfur functions in promoting the nucleation and growth of SWCNTs by forming inhomogeneous localized active sites and modifying the interface bonding between the catalyst and precipitated graphitic layers. The direct evidence obtained would be beneficial to understand the functions of sulfur during CNT growth. CNTs with both ends open were prepared by an anodic aluminum oxide (AAO) technique, the detailed process of which was described in an earlier report.18 The diameter and shell thickness of the CNTs used are ∼55 and ∼10 nm, respectively (Figure 1a−d). Fc was placed inside of the hollow core of the CNTs by a vapor deposition method (for details, see the Experimental Methods section). The CNTs loaded with Fc (denoted Fc@CNT) were then manipulated with a scanning transmission microcopy (STM)−TEM holder. An individual CNT was picked out and bridged between a gold wire and a piezo-driven gold probe, and a voltage was then applied. The configuration is schematically shown in Figure 1e. When a current is passed through the assembly, Joule heating provides the necessary high temperature for catalyst formation and CNT
ransitional metals (such as iron, cobalt, and nickel) are the most widely used catalysts for carbon nanotube (CNT) production. It had been found that aside from these metal catalysts, the addition of a small amount of growth promoter, such as sulfur (or sulfur-containing compounds), would significantly improve the CNT (or carbon nanofiber) growth efficiency.1−3 The use of sulfur leads not only to a higher yield and quality but also to a tunable CNT structure.4,5 It has been reported that the addition of sulfur affects the wall number,6 morphology,7,8 and even conductivity9 of the CNTs obtained. There is no doubt that revealing the working mechanism of sulfur would provide valuable hints for the structure- and property-controlled growth of CNTs. However, very little progress has been made on revealing the detailed function of sulfur during CNT growth due to the complex catalytic growth process, high temperature, and small size of the catalyst particles and CNTs. Some assumptions have been proposed explaining the roles of sulfur, whereas there is still controversy. Although postsynthesis transmission electron microscopy (TEM) observations on Fe−S catalysts and CNTs have been performed,7,10−12 a convincing demonstration of the role that a sulfur growth promoter plays is still absent. In this study, we investigate the nucleation and growth process of CNTs from an iron catalyst with and without a sulfur growth promoter by using an in situ TEM technique, which allows real time observations on the morphology and structure changes of the catalyst and the CNTs grown.13−17 We used a multiwall CNT with both ends open as a nanofurnace or a nanoreactor and filled it with both sulfur and ferrocene (Fc). Then, a voltage was applied to the CNT inside of the TEM to achieve a high temperature. Carbon atoms and iron catalyst are © 2014 American Chemical Society
Received: February 26, 2014 Accepted: March 31, 2014 Published: March 31, 2014 1427
dx.doi.org/10.1021/jz500419r | J. Phys. Chem. Lett. 2014, 5, 1427−1432
The Journal of Physical Chemistry Letters
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nucleation. The whole process was observed in real time and recorded. Figure 1a shows a typical bright-field (BF) TEM image of the Fc@CNT before Joule heating. It can be seen that some amorphous material is randomly distributed inside of the CNT, while the outer surface of the CNT is very clean, suggesting that the Fc existed only inside of the hollow core. A typical (Energy Dispersive X-ray) EDX spectrum of the Fc@ CNT sample shows C, O, and Fe peaks. The C and Fe are from CNT and filled Fc (C10H10Fe) molecules, while oxygen is originated from oxygen-containing functional groups formed on the CNT surface during synthesis process. For comparison, mixed Fc and S (with a Fc to S weight ratio of 5:1) was placed inside of CNTs using a similar method, and a BF TEM image of the sample configuration with a voltage applied is given in Figure S1 (Supporting Information). A representative BF TEM image of the resultant structure (denoted Fc−S@CNT) is shown in Figure 1c. Spots with darker contrast were observed, and the EDX spectrum clearly shows a sulfur signal (Figure 1d); the distribution of Fc and sulfur was further charaterized by dark-field TEM and line-scanning EDX spectra; see Figure S2 (Supporting Information). Figure 2 shows structural changes of a Fc@CNT with a voltage applied. Figure 2a,b shows that as the Fc filling was decomposed into iron and carbon species by Joule heating, Fe catalyst nanoparticles (NPs) started to form. While most of the NPs are smaller than 2 nm, some large particles with diameters of 5−10 nm can also be observed, as indicated by white arrows. The fast Fourier transform (FFT) image of the largest particle in Figure 2b shows no diffraction spots or rings, suggesting that the NP is in a melting or partial melting state. An obvious profile difference between the left and right sides of a particle
Figure 1. Typical BF TEM images of a Fc@CNT (a) and a Fc−S@ CNT (c); the corresponding EDX spectrum is shown in (b) and (d). (e) Schematic showing a biased CNT with both ends open filled with either Fc or Fc−S for the in situ TEM study.
Figure 2. Sequence of BF TEM images recording the formation of NPs and their structural evolutions to nucleate CNTs. (a) A Fc@CNT without biasing. (b) Appearance of NPs after the Joule heating. The inset is a FFT image of the largest NP. We denoted t = 0 s when the particle started to evolve. (c−e) Changes in the morphology and size of the NPs. The inset in (e) is a FFT image of the NP inside of the rectangule. (f,g) Higher magnification TEM images showing the formation of CNTs and carbon caps from a NP. Panels (b−e) have the same scale as panel (a). 1428
dx.doi.org/10.1021/jz500419r | J. Phys. Chem. Lett. 2014, 5, 1427−1432
The Journal of Physical Chemistry Letters
Letter
can be observed in Figure 2c (at 21 s), on which a graphitic layer precipitates after 39 s (Figure 2d). Except for large particles, a medium size particle was also observed in Figure 2e (denoted with a white arrow). It migrated upright, and a SWCNT was formed (Figure 2e, tube wall marked by dotted lines). As the morphology of the catalyst particle evolved, a short CNT with three domes lifted off (Figure 2e,f, 49 s). Finally, the growth of the CNT terminated by necking down (Figure 2g, 110 s). On the same particle as that shown in Figure 2f, another single-layer carbon cap tangentially grew from the reshaped catalyst particle, while another carbon dome appeared on its surface. These growth processes are in accordance with those deduced from the general vapor−liquid−solid mechanism19 for CNT growth from transition-metal catalysts. Except for large particles, a medium size particle in Figure 2e (denoted with a white arrow) also catalyzes the growth of a CNT. However, no CNT grown from small NPs (
