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Low Temperature Growth of SWNTs on a Nickel Catalyst by Thermal Chemical Vapor Deposition

Maoshuai He1 (!), Alexander I. Chernov2, Elena D. Obraztsova2, Jani Sainio3, Emma Rikkinen1, Hua Jiang4,

Zhen Zhu4, Antti Kaskela4, Albert G. Nasibulin4, Esko I. Kauppinen4, 5 (!), Marita Niemelä1, and Outi Krause1

1 Department of Biotechnology and Chemical Technology, School of Science and Technology, Aalto University, P.O. Box 16100, FI-00076

Aalto, Finland 2 A.M. Prokhorov General Physics Institute RAS, 38 Vavilov Street, 119991 Moscow, Russia 3 Department of Applied Physics, School of Science and Technology, Aalto University, P.O. Box 11100, FI-00076 Aalto, Finland 4 Nanomaterials Group, Department of Applied Physics and Center for New Materials, School of Science and Technology, Aalto University,

P.O. Box 15100, FI-00076 Aalto, Finland 5 VTT Biotechnology, P.O. Box 1000, FI-02044 Espoo, Finland Received: 24 September 2010 / Revised: 2 November 2010 / Accepted: 28 November 2010 © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2010

ABSTRACT Single-walled carbon nanotubes (SWNTs) have been grown on a silica-supported monometallic nickel (Ni) catalyst at temperatures ranging from as low as 450 °C to 800 °C. Different spectroscopic techniques, such as Raman, photoluminescence emission (PLE), and ultra violet–visible–near infrared (UV–vis–NIR) absorption spectroscopy were used to evaluate the diameter and quality of the SWNTs grown over the Ni catalyst at different temperatures. The analysis revealed that high quality SWNTs with a very narrow diameter distribution were obtained at a growth temperature of 500 °C. In the PLE and absorption spectra, differences were observed between the SWNTs grown on Ni and those grown on cobalt (Co). This result expands the potential of growing a specific (n, m) tube species with relatively high abundance by tuning the catalyst composition. Furthermore, the prerequisites for the low temperature growth of SWNTs over a monometallic transition metal catalyst have been elucidated. KEYWORDS Single-walled carbon nanotubes, synthesis, low temperature, nickel catalyst

1. Introduction

Since their discovery in 1993 [1, 2], single-walled carbon nanotubes (SWNTs) have been the focus of intensive research in nanomaterial science because of their unique electronic and optical properties. These intrinsic properties are sensitive to the SWNT chirality which is identified by the chiral indices (n,m) [3]. Most of the existing SWNT synthesis techniques, such as

arc-discharge [1, 2], laser ablation [4], and chemical vapor deposition (CVD) [5] with a growth temperature in the 700–4000 °C range, have been optimized for bulk production of SWNTs with a wide distribution of (n,m) species. Nevertheless, to revolutionize the field of molecular electronics, it is necessary to directly grow SWNTs with some specific chiralities [6]. Selective synthesis of SWNTs over some bimetallic catalysts, such as CoMo [7], FeRu [8], FeNi [9], or FeCu [10] has

Nano Res. 2010, 3(9): 676–684 ISSN 1998-0124DOI 10.1007/s12274-010-0088-3 CN 11-5974/O4Research Article

Address correspondence to Maoshuai He, maoshuai.he@hut.fi; Esko I. Kauppinen, esko.kauppinen@hut.fi

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been empirically developed to obtain SWNTs with a narrow diameter distribution and only a few SWNT species. A drawback of most bimetallic catalysts [7, 8], however, is that the presence of two metals might complicate the SWNT purification process. Thus, it is crucial to develop a monometallic catalyst system for the selective synthesis of SWNTs [11, 12].

Meanwhile, to achieve a high degree of compatibility with standard semiconductor processes, it is highly desirable to grow SWNTs at as low a temperature as possible [13–15]. Plasma CVD, in which plasma was introduced to dissociate the carbon sources, has been adopted for low temperature growth of SWNTs [13, 14]. The introduced plasma might, however, also damage the SWNTs and thus, result in SWNTs of poor quality, as indeed has been indicated by the high disorder- induced (D) mode intensity in the Raman spectra [14]. Assisted by an alumina buffer layer, Cantoro et al. reported growth of SWNTs at 350 °C with thermal CVD in a cold wall reactor [15]. However, both the plasma CVD and the alumina-assisted thermal CVD techniques provide little control over the SWNT diameter and chirality.

Ni catalysts have been widely used in hydrogen (H2) generation from various carbon feed stocks [16, 17], such as methane (CH4) and ethane (C2H6). During the H2 production process, carbon fiber or filament growth on a small Ni particle has been reported at a tem- perature of 450 °C [16]. Recently, Ziebro et al. have demonstrated the synthesis of carbon nanotubes over a zeolite-supported Ni catalyst by CH4 CVD in the temperature range 400–500 °C [18]. However, due to the relatively large diameter and wide diameter distributions of Ni particles, only multi-walled carbon nanotubes (MWNTs) with a diameter ranging from 8 to 63 nm were produced.

In this article, we demonstrate the growth of SWNTs on a SiO2-supported Ni (Ni/SiO2) catalyst using carbon monoxide (CO) as carbon source by the thermal CVD method. Raman, UV–vis–NIR absorption, and photo- luminescence excitation/emission (PLE) spectroscopy were used to characterize the final products. Further- more, we investigated the mechanisms of the low temperature SWNT growth, which it should be noted does not require high temperature pre-reduction in H2.

2. Experimental

2.1! Preparation of Ni/SiO2 catalyst and carbon nanotubes

The Ni/SiO2 catalyst was prepared at low pressure (6–10 kPa) by chemisorption of Ni(II) acetylacetonate (Ni(acac)2, Aldrich, 95%) onto a SiO2 (Grace 432) support using an atomic layer deposition (ALD) technique. The deposition process is similar to that described for preparing an alumina-supported Ni catalyst [19]. The SiO2 support was first preheated in nitrogen (N2, 99.999%) at 400 °C for 4 h to desorb any water present on the support, and the solid Ni(acac)2 was then vaporized at 200 °C and reacted with the support surface functional groups for 6 h. After purging the reaction chamber with N2 to blow away non-reacted precursors, the catalyst was further annealed at 450 °C in air to remove the residual acac ligands.

The set up used for carbon nanotube growth has been described elsewhere [10]. Growth of carbon nanotubes was carried out under atmospheric pressure in the temperatures range 450 °C to 800 °C. The Ni/SiO2 catalyst was heated to the growth temperature at a rate of 70 °C/min with an Ar flow rate of 50 standard cubic centimeters per minute (sccm). The Ar flow was switched off once the growth temperature was reached, and 50 sccm CO was then introduced into the CVD reactor for 10 min. After carbon nanotube growth, the system was flushed with Ar and cooled to room temperature.

2.2! Characterization of Ni/SiO2 catalyst and carbon nanotubes

2.2.1! X-ray diffraction (XRD)

XRD measurements were performed with a PANalytical (X'pert Pro) diffractometer to compare the crystal structures of the SiO2 support and Ni/SiO2 catalyst.

2.2.2! X-ray photoelectron spectroscopy (XPS)

The contents and chemical states of Ni in both as- prepared and reduced Ni/SiO2 catalysts were examined by XPS. To characterize the reduced catalyst, the Ni/SiO2 catalyst was first reduced in flowing H2 (5% in N2) at 500 °C for 1 h and then transferred under an inert atmosphere into the measurement chamber for

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characterization. Photoelectrons were excited by an Al K! (1486.6 eV) X-ray source and the spectra were collected using a Surface Science SSX-100 ESCA spec- trometer and an electrostatic hemispherical analyzer.

2.2.3! H2 temperature-programmed reduction (TPR)

TPR measurements of the Ni/SiO2 catalyst were performed using an Altamira Instruments AMI-200R catalyst characterization system. Approximately 100 mg of catalyst was loaded into a “U”-shape quartz cell and preheated at 150 °C in Ar for 2 h. After cooling to 40 °C, 10.3% H2 in Ar was introduced into the system and the catalyst was heated linearly to 850 °C at a rate of 10 °C/min. The H2 consumed was monitored by analyzing gases in the outlet with a Hiden mass spectrometer (MS).

2.2.4! Scanning electron microscope (SEM) characterization

SEM (JEOL JSM-7500F) measurements were performed on carbon nanotubes after removal of the SiO2 support.

2.2.5! High resolution transmission electron microscope (TEM) characterization

As-grown SWNTs were sonicated in acetone and collected on a carbonized copper grid for TEM (JEOL JEM-2200FS 2x CS corrected) characterization.

2.2.6! Raman characterization

All Raman measurements were performed on as- produced carbon nanotubes using a HORIBA Jobin Yvon LabRAM 300 spectrometer with an excitation wavelength of 633 nm (He–Ne) or 514 nm (Ar ion)

lasers. Laser power was kept below 0.02 mW (50× objective, ~2 µm spot size) for all Raman measure- ments to minimize the heating effect.

2.2.7! Ultra violet–visible–near infrared (UV–vis–NIR) absorption

As-grown SWNT samples were introduced into an aqueous solution of sodium cholate hydrate (2 wt.%) surfactant and sonicated using a tip sonicator with an 80 W output power for 1 h. After sonication, the dispersions were centrifuged at 50,000 g for 60 min to precipitate residual metallic particles and large bundles. UV–vis–NIR absorption spectra were recorded with a PerkinElmer Lambda-950 spectrometer.

2.2.8! Photoluminescence excitation/emission characterization

The photoluminescence excitation and emission spectra were recorded using a liquid nitrogen cooled InGaAs charge-coupled device (CCD) detector (HORIBA Jobin Yvon NanoLog system) in the range 860–1400 nm.

3. Results and discussions

3.1! Nature of the Ni/SiO2 catalyst

The crystalline species in the Ni/SiO2 catalyst were studied by XRD (Fig. 1(a)). Only weak and broad reflection peaks at 42.9° and 62.5°, which can be assigned as either cubic NiO (JCPDS card 78"0429) or Ni2SiO4 (JCPDS card 14-0481), were observed. The crystallite size of Ni-containing phase was difficult to

Figure 1! (a) XRD patterns of the Ni/SiO2 catalyst and the SiO2 support. (b) H2-TPR profiles of the Ni/SiO2 catalyst

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determine due to the weak, broad, and partially overlapping peaks.

The TPR profile (Fig. 1(b)) can reflect the essential features of the reducibility of the catalyst and help in identifying the Ni-containing species. A peak related to the evolution of H2O was observed at a temperature of ~290 °C in the H2O MS signal curve. However, the peak cannot be attributed to catalyst reduction because no significant change in H2 MS intensity was observed. Thus, the low temperature H2O formation peak is probably related to the desorption of H2O from the catalyst. Reduction of the catalyst started at around 400 °C where a decrease of H2 signal together with an increase of H2O signal was observed. Meanwhile, the reduction of the Ni/SiO2 catalyst continued until temperatures well above 700 °C, which indicated the high possibility of Ni2SiO4 formation [20] during the catalyst preparation process.

The content of Ni on SiO2 was determined by XPS, which showed that the atomic ratio of Ni to Si was 0.07. The Ni/SiO2 catalyst, which had been pretreated in flowing H2 at 500 °C, was studied by XPS to determine the chemical state of the surface Ni species (Fig. 2). In the non-reduced Ni/SiO2 catalyst, Ni was found in an oxidized state with a binding energy of 856.5 eV which could correspond to either NiSiO3 or Ni2SiO4 [21]. The reduced catalyst showed increased Ni 2p intensity below the binding energy of 853 eV, which is ascribed to metallic Ni [22]. The XPS results

Figure 2! Ni 2p XPS spectra of as-prepared and reduced Ni/SiO2 catalysts. The red dashed line denotes the peak position of metallic Ni

are thus, in agreement with the H2-TPR profiles of the Ni/SiO2 catalyst.

3.2! Carbon nanotube characterization

SEM and high-resolution transmission electron microscopy (HRTEM) were used to characterize the morphologies and structures of SWNTs. Figure 3(a) is a typical SEM image of carbon nanotubes grown on the Ni/SiO2 catalyst at 600 °C after removal of the SiO2 support. The TEM image (Fig. 3(b)) clearly shows that high quality SWNTs were obtained even at the relatively low growth temperature. As-synthesized carbon nano- tubes were characterized by Raman spectroscopy with the working wavelengths of 633 nm and 514 nm. The radial breathing modes (RBMs), which are intrinsic features of SWNTs, were observed in the Raman spectra on the carbon nanotubes grown at 450 °C (Fig. 4(a)). The relatively high intensity of the D mode implies that either the quality of the SWNTs is poor, or there also exist other carbon forms in admixtures. With increasing growth temperature, the intensity ratio of G to D modes (IG /ID) increased accordingly. For the SWNTs grown at 500 °C, the intensity ratio was 28, and the IG /ID ratio rose to 33 for the SWNTs grown at 800 °C.

According to the formula of Bandow et al. [23], the diameter of SWNTs grown at 500 °C is only ~0.8 nm, and the prominent RBM peaks centered at 279 cm–1 and 294 cm–1 can be assigned to (7,5) and (8,3) tubes, respectively. The small diameter of SWNTs grown at 500 °C was also confirmed by the 514 nm laser excited Raman spectra (Fig. 4(b)), in which only RBMs with frequencies higher than 265 cm–1 were observed. With the increasing growth temperature, as reported in many previous works [7–11], the diameters of the SWNTs increased. In the Raman spectra of the SWNTs grown at 800 °C, the RBM frequencies downshifted to the range 130–190 cm–1, which is characteristic of SWNT diameters in the range 1.2–1.7 nm.

This finding is in good agreement with the PLE maps of suspensions of SWNTs (Fig. 5). Strong exciton energy transfer, where the excitation of large band gap tubes leads to emission from the smaller band SWNTs [24, 25], is shown in the PLE maps. This indicates aggregation of SWNTs in the suspension because of the difficulty in debundling SWNTs. For

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the suspension of SWNTs grown at 500 °C, only small diameter (smaller than 0.9 nm) semiconducting SWNTs, such as (6,4), (6,5), (8,3), (7,5), (8,4), and (7,6) were observed. Large diameter SWNTs were observed in the product grown at higher temperatures. For instance, PLE emissions from (10,2), (9,4), (8,7), (9,5), and (8,6) tubes were observed in the suspension of SWNTs grown at 600 °C. Compared to the Co-catalyzed SWNTs grown at 600 °C [12], the SWNTs grown on Ni have relatively large diameter and wide diameter distri- bution. While considering that the reduction of Ni/SiO2

occurs at a temperature approximately 100 °C lower than that for Co/SiO2 [12], we noticed that the PLE intensities of SWNT species grown on Ni at 500 °C or 600 °C look quite similar to those of the suspension of SWNTs grown on a Co catalyst at 600 °C or 700 °C, respectively (Fig. 5(d)). For SWNTs grown at low temperatures (Ni at 500 °C and Co at 600 °C), (6,5), and (7,5) tubes are the predominant species in both suspensions. This could be anticipated because (6,5) and (7,5) tubes have relatively low nanotube growth energy barriers compared to other tube species [26].

Figure 3! (a) SEM image of carbon nanotubes grown on a Ni/SiO2 catalyst at 600 °C, taken after removal of the SiO2 support. (b) TEM image of SWNTs grown on a Ni/SiO2 catalyst at 600 °C

Figure 4! (a) Raman spectra of carbon nanotubes grown on a Ni/SiO2 catalyst at different temperatures, with the spectra obtained by excitation with a 633 nm laser. (b) RBMs of Raman spectra excited by a 514 nm laser on carbon nanotubes grown on a Ni/SiO2 catalyst at different temperatures

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However, some differences exist in the PLE inten- sities of SWNTs grown on Ni and Co catalysts. For example, the Ni-catalyzed SWNTs grown at 600 °C show lower emission intensity of (7,5) and (7,6) tubes, and higher emission intensity of (8,4) tubes than SWNTs grown on Co at 700 °C. The differences cannot be explained by growth temperature induced diameter change of SWNTs [23], because the diameter of the (8,4) tubes is larger than that of the (7,5) tubes and smaller than that of the (7,6) tubes. Differences were also observed in the UV–vis–NIR absorption spectra between the suspensions of the SWNTs grown on Ni at 600 °C and the suspension of the SWNTs grown on Co at 700 °C (Fig. 6). For example, the absorption of (8,3) tubes grown on the Ni catalyst is much more predominant than that of SWNTs grown on Co. This implies that the SWNT species grown on Ni are

Figure 6! The UV–vis–NIR optical absorption spectra of dispersions of SWNTs grown on a Ni/SiO2 catalyst at 600 °C and dispersions of SWNTs grown on Co/SiO2 at 700 °C

Figure 5! Photoluminescence spectra of suspensions of SWNTs grown on (a) a Ni/SiO2 catalyst at 500 °C; (b) a Ni/SiO2 catalyst at600 °C; (c) a Co/SiO2 catalyst at 700 °C. (d) Normalized PLE intensity of SWNTs grown on different catalysts at different temperatures

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different from those grown on Co also taking the tem- perature effect into account. Therefore, this observation could lead to the potential control of the SWNT chirality by fine tuning the catalyst composition [27].

3.3! Carbon nanotube growth mechanisms

ALD is a self-limiting gas phase deposition process based on the chemical reaction between the precursor and the surface functional groups on the support. Once the functional groups are saturated, the reaction will terminate and the excess precursors will be flushed out of the reactor. This process allows the preparation of a highly dispersed catalyst at the appropriate deposition temperature. For the Ni(acac)2 precursor, a deposition temperature of 200 °C is found to be suitable for preparing the Ni/SiO2 catalyst. If the temperature is too low, the activation energy of the precursor cannot be overcome. If the temperature is too high, the precursor might decompose and result in a catalyst with poor dispersion. Furthermore, the air calcination process after deposition not only removes the residual acac ligands, but also favors the reaction between the catalyst and the support [20].

Similar anchoring mechanisms [12, 28] to those suggested for the carbon nanotubes grown on Co catalyst are anticipated to be valid also on the Ni/SiO2 catalyst. Numerous studies [20, 29] have been devoted to the reduction of SiO2-supported nickel oxide to Ni due to its many applications in industry. Transition metal oxides supported on the surface without confinement are usually reduced at a temperature below 500 °C [20, 28, 30]. Once metal ions are reduced, the resulting metallic clusters tend to migrate and aggregate to form large particles, which usually catalyze the formation of carbonaceous deposits such as MWNTs and graphitic layer encapsulated particles instead of SWNTs at low growth temperatures. To directly grow small diameter SWNTs at low tem- peratures without high temperature reduction in H2, besides the absence of large diameter particles, several other prerequisites should be satisfied: The initial temperature of reduction should be low, the catalyst should be reduced to the metallic state to some extent at the growth temperature, and anchoring sites which function like the Mo species in the CoMoCAT

catalyst [7] should be present. Based on their TEM and magnetic studies, Bonneviot et al. deduced that the Ni+ and Ni2+ ions can function as anchoring sites and favor the formation of highly dispersed small metallic Ni particles [29]. The TPR profile of our Ni/SiO2 catalyst suggests that the catalyst is first reduced forming metallic clusters, which can be stabilized by unreacted Ni2+ thus, preventing aggregation. Because reduction starts at low temperature (~400 °C), the growth of SWNTs can occur on the small metallic Ni clusters at temperatures as low as 450 °C. With increasing temperature, the possibility of forming large diameter clusters, which can further induce the growth of larger diameter SWNTs, increases. It is also noteworthy that the use of CO as a carbon source is important for the low temperature growth of SWNTs. Although CH4 alone is also found to give growth of SWNTs over a Ni/SiO2 catalyst, the quality of the produced SWNTs is poor, because the reactive hydrogen species produced from CH4 dissociation are detrimental to the formation of SWNTs [31].

4. Conclusions

The reduction of the ALD-prepared Ni/SiO2 catalyst starts at low temperature and proceeds to full reduction at high temperature. This catalyst favors the synthesis of small diameter SWNTs with narrow diameter distributions at relatively low temperatures. Both the PLE and the UV–vis–NIR absorption spectra of the SWNTs grown on the Ni catalyst were different from those of the SWNTs grown on the Co catalyst. These results provide evidence for the potential of controlling the SWNT chirality by fine tuning the catalyst compositions.

Acknowledgements

This work was supported by Teknologian ja inno- vaatioiden kehittämiskeskus (the Finnish Funding Agency for Technology and Innovation), Academy of Finland (project Nos. 128445 and 128495), CNB-E pro- ject in Aalto University through the Multidisciplinary Institute of Digitalization and Energy (MIDE) program, European FP6 Project 033350 and RFBR-09-02-91231

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project. We thank Ms. Sonja Kouva and Mr P. V. Fedotov for experimental support.

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