Revealing the Secret of Water-Assisted Carbon Nanotube Synthesis

Nov 4, 2008 - We elucidated the secret of water-assisted chemical vapor deposition (CVD) by elucidating the influence of water on the catalysts, throu...
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NANO LETTERS

Revealing the Secret of Water-Assisted Carbon Nanotube Synthesis by Microscopic Observation of the Interaction of Water on the Catalysts

2008 Vol. 8, No. 12 4288-4292

Takeo Yamada,† Alan Maigne,‡,§,| Masako Yudasaka,§,⊥ Kouhei Mizuno,† Don N. Futaba,† Motoo Yumura,† Sumio Iijima,† and Kenji Hata*,† Nanotube Research Center, National Institute of AdVanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, Laboratoire de Physique des Solides, UniVersite´ Paris-Sud, CNRS UMR 8502, 91405 Orsay, France, and JST/SORST, c/o NEC, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan Received July 7, 2008; Revised Manuscript Received October 17, 2008

ABSTRACT We elucidated the secret of water-assisted chemical vapor deposition (CVD) by elucidating the influence of water on the catalysts, through ex situ microscopic and spectroscopic analysis. We unambiguously showed that catalyst deactivation readily occurs due to carbon coating and that water acted to remove this coating and revive catalysts activity. This represents the central point of water-assisted CVD.

Historically, the low synthesis efficiency of single-walled carbon nanotubes (SWNTs) has been the bottleneck preventing the utilization of their full potential.1,2 The synthesis efficiency is the product of the catalyst activity (percent of active catalysts), catalyst lifetime, and SWNT growth rate. Until recently, no chemical vapor deposition (CVD) approach had simultaneously achieved high catalyst activity, long lifetime, and high growth rate. In particular, in conventional hydrocarbon CVD the catalyst activity was only several percent, the lifetime was less than one minute, while the growth rate was very high.3 Recently there have been numerous reports of various SWNT synthetic approaches with improved growth efficiency by use of plasmas, oxygencontaining gas sources, and alumina buffer layers below the catalysts. When SWNTs were grown by these approaches from catalysts deposited on a substrate, nanotubes grew vertically and aligned creating a bulk material, a so-called SWNT “forest.” 4-12 * To whom correspondence should be addressed. E-mail: [email protected]. † National Institute of Advanced Industrial Science and Technology (AIST). ‡ Universite ´ Paris-Sud. § JST/SORST. | Currently address: Gatan Inc., 5794 West Las Positas Blvd, Pleasanton, CA 94588. ⊥ Currently address: Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. 10.1021/nl801981m CCC: $40.75 Published on Web 11/04/2008

 2008 American Chemical Society

Plasma increases the decomposition of the carbon feedstock and thus improves the growth efficiency. Particularly, the use of a remote point-arc microwave plasma was shown to improve the lifetime.4 Moreover, Dai has reported that the growth efficiency of plasma enhanced CVD can be further improved by the addition of a small amount of oxygen in the growth ambient.5 In general, plasma-enhanced CVD has been successful to synthesize millimeter-scale SWNT forest; however typically growth times on the scale of hours are required. Another important approach is to use alcohol as the carbon source where alcohol improves both the catalyst activity and lifetime;13 however, this was offset by much slower growth rate than conventional hydrocarbon CVD. A different route to improve growth is to modify the catalyst. Introducing a thin Al2O3 support layer beneath the catalysts has been shown to improve the growth efficiency significantly.4,6-12 So far, the use of the Al2O3 support seems to be one of the important factors to achieve high growth efficiency to grow forest with millimeter scale. A different approach that simultaneously satisfied the above three issues adds a small and controlled level of water into the growth ambient of a hydrocarbon thermal CVD system (“water-assisted” or “super-growth” method).6 Through this way, the catalyst activity reached 84%,14 the lifetime exceeded 30 min, and the growth rate was over 250 µm/ min.15 As a result of this improved synthesis efficiency, vertically aligned SWNT forests with 2.5 mm height were grown in 10 min from Al2O3 supported Fe catalysts. The

super-growth did improve the growth efficiency on other catalyst systems such as Al2O3/Ni with height exceeding several 100 µm, demonstrating its generality. Many other groups have further developed the super-growth to achieve forests with height approaching a centimeter.8-11 However, despite all this concrete experimental evidence, the role of water in improving the synthesis efficiency has not been elucidated. Empirically it is known that the synthesis efficiency maximizes at 100-200 ppm water level and drops steadily at higher levels.15 For the case of oxygen-enhanced PECVD mentioned above,5 the oxygen level (10 000 ppm) was one hundred times higher than our water level, and the role of oxygen was interpreted as a scavenger of the radical hydrogen produced by the plasma enhanced decomposition. In contrast, the low water level in super-growth precludes the possibility of water directly reacting with the hydrocarbon source, a well-known phenomena called “steam reforming,” leaving the possibility of water reacting with the catalysts as the only reasonable explanation. The key to further understand and to improve synthesis to the next level requires a fundamental understanding of the role of water which can only be achieved through clarifying the interaction between water and the catalysts. This is possible only though microscopic observations to directly see the influence of water on the catalyst particles. In this letter, we investigated and elucidated the influence of water on the catalyst for SWNT synthesis through analyzing the structure and composition of individual catalysts exposed to normal growth conditions, water ambient, and water-assisted growth. From a series of direct ex situ microscopic observations, we proved unambiguously that water keeps the surface of the catalyst clean by removing a carbon coating and thus sustaining the ability of the catalyst to synthesize SWNTs. This effect directly explains the high catalyst activity, long lifetime, and high growth rate that characterizes water-assisted CVD and provides crucial insight to unlock new experimental frontiers for improved SWNT synthesis. We have used a rational approach, denoted “Ball-CVD”,16 where the nanotubes were synthesized from catalysts deposited onto 50 nm SiO2 nanoballs placed on a transmission electron microscope (TEM) grid. The use of the nanoballs increased the amount of peripheral area, which in turn increased the likelihood to find catalysts located at the edge of the nanoballs and protruded into vacuum, making them observable by TEM.16,17 In contrast, previous methods, such as CNTs grown from catalyst particles deposited onto the grid,18,19 grown CNTs deposited on a TEM grid,20 or in situ growth on a TEM grid,3,21,22 were severely limited in the number of observable and representative catalysts. More specifically, 50 nm diameter SiO2 nanoballs were deposited onto a Cu TEM grid, followed by sequential sputtering of a 50 nm SiO2 layer, for coating and protection of the TEM grid and 1.5 nm Fe thin film as a catalyst. This TEM grid was inserted into a CVD furnace and annealed to the growth temperature in a He/H2 ambient. By this process, the Fe thin film broke up into well-isolated individual Fe nanoparticles distributed across the surface of the SiO2 nanoballs. From Nano Lett., Vol. 8, No. 12, 2008

numerous researches, these Fe nanoparticles are well established to be excellent catalysts for SWNT growth. We first used the Ball-CVD approach to characterize the structure and composition of the catalysts that were exposed to the conventional hydrocarbon CVD ambient to investigate the origin of the low catalyst activity in normal CVD. The growth ambient was identical to that of normal hydrocarbon CVD (750 °C, ethylene 50 sccm, 1 min), except that the ethylene concentration was set higher than usual. This resulted in a very low yield of SWNTs proven by the Raman spectrum (Figure 1f) at 532 nm showing a weak G-band (due to graphitic carbon) at 1550 cm-1, compared to the high D-band (due to disordered carbon) at 1340 cm-1. This low yield of SWNTs facilitated the investigation of catalysts that failed to produce nanotubes (deactivated catalysts). By this approach, the deactivated catalyst population on the periphery surface of nanoballs was sufficient for efficient catalyst observation by ex situ TEM (Figure 1a). Importantly, high resolution TEM images (Figure 1a inset) revealed that the deactivated catalysts were coated by a layer of material. To examine the composition of this coating, dark-field imaging (Figure 1b) and electron energy loss spectroscopy (EELS) (Figure 1d) analysis were carried out with a scanning TEM (STEM) Hitachi HD-2300 equipped with a Gatan ENFINA spectrometer system. Dark field images discriminates heavy and light elements by their relative brightness (heavier being brighter) and revealed the existence of small nanoparticles composed from heavy elements protruding from the ball surface. The EELS spectrum (Figure 1d) taken from a small region surrounding a nanoparticle of Figure 1b, showed the existence of iron (Fe M peak at 57 eV) silicon (Si L peak at 100 eV) and carbon (C K peak at 284 eV). For the region surrounding the catalyst particles (Figure 1b), we used STEM-EELS spectrum imaging technique,23 which consists of measuring the EELS spectrum for each scanned pixel of the studied area. From this data, the amount of Fe, Si and C can then be quantified pixel by pixel and represented as an color map (Figure 1c) by assigning colors to each of those elements (Fe in green, Si in red, and C in blue). This elemental mapping showed that the particles in the dark field images and the coating layer were composed mainly from Fe and carbon, respectively. Practically all of the catalysts that failed to produce SWNTs were coated with carbon. Taken together, these results provides direct evidence that carbon coating of the catalyst prohibits the growth of SWNTs, which had been long suspected, but had not been proven.13 Furthermore, carbon coating of the catalysts also explains the low catalytic activity and probable short catalyst lifetime in normal hydrocarbon CVD. To directly explore the influence of water on the catalyst particles, the deactivated catalysts on the nanoballs were exposed to water at the growth temperature, and the structure and composition of the catalysts were characterized. The above-mentioned sample was reinserted into a CVD furnace (750 °C) and exposed to 100 ppm water level (5 min) as employed in standard water-assisted CVD. Subsequent ex situ dark-field images (Figure 2a) observation again showed bright protruding particles on the SiO2 nanoballs; however, 4289

Figure 2. (a) Dark-field image of the deactivated catalyst after exposed to 100 ppm water level (5 min) at the growth temperature (750 °C). (b) Elemental mappings of five arbitrarily chosen particles (Fe (green), Si (red), and C (blue)). (c) EELS spectra of each catalyst of panel b as indicated by the squares.

Figure 1. (a) HRSEM image of deactivated catalysts on the periphery surface of nanoballs. Left inset: the illustration. Right inset: bright field TEM image. (b) Dark field image of the deactivated catalysts. (c) Element mapping of the nanoball surface from EELS spectra of each pixel of panel b (Fe (green), Si (red), and C (blue)). (d) EELS spectrum taken from a small region surrounding a nanoparticle Figure 1b as indicated by the square. (e) HRSEM image deactivated catalysts exposed to normal hydrocarbon CVD. (f) Raman spectra and radial breathing mode (inset) of deactivated catalysts (black) and deactivated catalysts after normal hydrocarbon CVD (red).

in contrast to the deactivated particles, which were round, these particles were flat. EELS and elemental (Figure 2b,c) mappings on five arbitrarily chosen particles from Figure 2a were carried out to characterize the composition. Elemental mappings (Figure 2b) on these individual particles showed that these flat particles were composed mainly of Fe, and more importantly, the absence of the carbon coating on the particles. More quantitatively, individual EELS spectra (Figure 2c) averaged over the region indicated by the squares in Figure 2b showed similar intensities of Fe M and Si L peaks as measured before water exposure, though the carbon 4290

K peak ((K) 284.0-309.0 eV) was almost immeasurable. These results provide ex situ direct microscopic evidence of the interaction of water and the catalyst, that is, the role of water in “cleaning” the catalyst to expose the surface of the Fe catalyst particle. To determine whether the water-exposed “cleaned” catalysts possessed the ability to synthesize nanotubes, we carried out further CVD experiments that highlighted the effect of water to revive deactivated catalysts. The deactivation process, the water exposure process, and CVD were sequentially performed on catalysts containing nanoballs sample. Standard water-assisted growth conditions (H2O, 100 ppm; the career gas, He, H2; carbon source gas, C2H4; growth temperature, 750 °C; and 1 min growth time) were employed. An ex situ high-resolution scanning electron microscopy (HRSEM) image (Figure 3a) of this sample revealed numerous entangled nanotubes covering the nanoballs. The existence of SWNTs was proved by the radial breathing modes (RBM) in Raman spectroscopy (Figure 3b inset). Moreover, EELS mapping (Figure 3a inset) showed the Fe catalyst particles remained, still protruding from the ball surface, yet being covered with a massive amount of carbon that is derived from the SWNTs. This is significantly different from the carbon-coated catalysts observed in Figure 1. Despite experiencing a deactivation and a water exposure process, the synthesis efficiency of the catalysts was similar to that of a control catalyst sample that went directly to the CVD process. In contrast to TEM observation, Raman spectroscopy averages over a micrometer scale area thus providing semiquantitative information of the synthesis efficiency, in addition to the quality and the existence of SWNTs. Raman spectra (Figure 3b) of both samples showed similar G- and D-band signals and RBM profiles in terms of absolute intensity and relative intensities. This implies similar amounts (growth efficiencies) and size distributions of nanotubes. In contrast to these results, without the exposure to water no additional CNTs were produced by subsequent CVD on the deactivated catalysts. Deactivated catalysts were exposed Nano Lett., Vol. 8, No. 12, 2008

Figure 3. (a) HRSEM image of numerous entangled nanotubes covering the nanoballs grown by water-assisted CVD from waterexposed deactivated catalysts. Inset: elemental mapping of catalyst particles (Fe (green), Si (red), and C (blue)). (b) Raman spectra and radial breathing mode (inset) of growth from reactivated catalysts (blue) and control catalyst (green) that went directly to the CVD. (c) Scheme of the two competing reaction pathways for normal hydrocarbon CVD-SWNT growth (synthesis pathway) and carbon coating (deactivation pathway). (d) Scheme of the waterassisted CVD-reactivation pathway reducing the deactivation pathway to enhance the synthesis pathway.

to a CVD process that contains no water. The growth ambient was chosen to be identical to that of a typical hydrocarbon CVD (750 °C, ethylene 10 sccm, 1 min). The Raman spectra (Figure 1f) and HRSEM image (Figure 1e) remained unchanged from its previous state, and this is in sharp contrast to the dramatic change observed when water was involved. Comparison between identical growth conditions (ethylene 10 sccm) with and without water (see the Supporting Information Figure S1) showed a similar tendency with that presented in Figures 1 and 3. These results constitute our central experimental finding: water reactivates catalysts. The potency of this reactivation as demonstrated by the ability to raise the synthesis efficiency of a completely deactivated catalyst to the level of a new catalyst suggests that water can essentially reactivate each and every dead catalyst. This fundamental effect is central to the high catalyst activity, long lifetime, and high growth rate observed for water-assisted CVD. With this cultivated understanding of the SWNT growth mechanism, we define the key concept of super-growth as the following: The synthesis process of conventional hydrocarbon CVD involves two competing reaction pathways: one produces a SWNT (pathway one, synthesis) while the other results in a carbon coating and catalyst deactivation (pathway two, deactivation). The carbon coating has been widely neglected in the past, but is, in fact, the dominant process (Figure 3c). This is the reason why the synthesis efficiency has been low in hydrocarbon CVD. Nano Lett., Vol. 8, No. 12, 2008

The introduction of water in the growth ambient introduces a new reaction pathway of cleaning the catalyst particles by removing the carbon coating through oxidization (Figure 3d). The cleaning pathway reactivates the catalysts (pathway three, reactivation). Because of the reactivation pathway, the dominant pathway shifts to synthesis and results in a dramatic improvement in SWNT synthesis efficiency (Figure 3d). Such competing pathways are a standard approach to describe catalytic reactions exemplified by the growth of diamond and carbon fibers.24 As described in the introduction, water-assisted CVD is not the only CNT growth method that introduces oxygen into the CVD ambient. For example, both the HiPco25 and alcohol CVD processes use carbon feedstock containing oxygen. Because the oxygen is incorporated into the carbon feedstock molecule, the relative levels of carbon and oxygen is fixed. For example, for the HiPco process, the carbon/ oxygen ratio is fixed at 1:1. The central concept of waterassisted CVD is the separation of the carbon and oxygen sources to enable tuning and optimizing the levels of carbon and oxygen independently. Through this way, for standard water-assisted CVD the optimum relative level of water to carbon was found to be 1:2000,15 which is very different from that of the HiPco process. Since oxidized Fe catalysts are reported to show improved growth efficiency26 and as water can oxidize the Fe catalysts, there is a possibility that oxidized Fe catalysts is the origin of the improved growth efficiency of super-growth. A series of experiments were carried out to address this issue. First, Fe catalysts were exposed to water at 750 °C prior to growth to oxidize them, and then were exposed to a typical hydrocarbon CVD process without water. This CVD on oxidized Fe catalysts resulted in a low yield of CNTs (see the Supporting Information, Figure S2) compared to supergrowth. This result implies that oxidized Fe catalysts are not the key for super-growth. Second, the water supply was discontinued during a standard super-growth process while monitoring the height of a forest (see the Supporting Information, Figure S2) by an in situ telecentric optical system.27 When the supply of water was stopped, the growth rapidly terminated. These experimental results mean that a continuous supply of water during growth is necessary to maintain the catalyst activity. This phenomenon is readily explainable by our model because the carbon coating must be constantly removed during growth but not by the oxidized Fe catalyst model since the growth would be expected to continue as long as the chemical composition of the catalyst remains the same. In summary, we demonstrate an approach to directly ex situ observe microscopically the interaction of water on the catalyst to study the effect of water on carbon nanotube synthesis. As a result, we elucidated the following: (1) catalysts were coated with a carbon layer and deactivated in normal hydrocarbon CVD (no water); (2) catalysts were efficiently cleaned and reactivated by water exposure; and (3) catalyst activity was dramatically increased directly as a result of this reactivation. These findings represent the secret 4291

of the extremely high synthesis efficiency of water-assisted CVD from which the key concept of super-growth was defined. Supporting Information Available: Additional TEM images and growth curves data of the absence and the presence of water experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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Nano Lett., Vol. 8, No. 12, 2008