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Nontronites as catalyst for synthesis of carbon nanotubes by catalytic chemical vapor deposition.
Applied Clay Science 114 (2015) 170–178
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Research paper
Nontronites as catalyst for synthesis of carbon nanotubes by catalytic chemical vapor deposition. Štefan Kavecký a, Jana Valúchová b, Mária Čaplovičová c,d, Stefan Heissler e, Pavol Šajgalík b, Marián Janek f,g,⁎ a
Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Račianska 75, SK-83102 Bratislava, Slovakia Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, SK-84536 Bratislava, Slovakia Comenius University, Faculty of Natural Sciences, Department of Geology of Mineral Deposits, Mlynská dolina, 842 15 Bratislava, Slovakia d Slovak University of Technology, Centre for Nanodiagnostics, Vazovova 5, 812 43 Bratislava, Slovak Republic e Karlsruhe Institute of Technology, KIT; Institute of Functional Interfaces, IFG; Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany f Slovak University of Technology, Faculty of Chemical and Food Technology, Radlinského 9, SK-81237 Bratislava, Slovakia g Comenius University, Faculty of Natural Sciences, Department of Physical and Theoretical Chemistry, Mlynská dolina CH1, SK-84215 Bratislava, Slovakia b c
a r t i c l e
i n f o
Article history: Received 19 January 2015 Received in revised form 31 May 2015 Accepted 1 June 2015 Available online xxxx Keywords: Nontronites Sampor and Washington Hematite nanoparticles Carbon nanotubes MWCNT CVD
a b s t r a c t Multiwall carbon nanotubes (MWCNT) were synthesized by catalytic chemical vapor deposition (CCVD) in a horizontal tube reactor using acetylene as the carbon source in a gas mixture with argon at 700 °C and 30 minute reaction time. As a catalyst was used the sodium forms of natural platy nontronite particles Sampor or Washington and their iron modified forms. Additionally, hydrothermally synthesized hematite α-Fe2O3 and/or its heterocoagulates with nontronite particles were used to test the catalytic activity. Catalyst nanoparticles were used to modify the conditioned Si wafer surface used as the catalyst support. Before CNT growth the catalyst nanoparticles were activated by applying a hydrogen stream in the tube reactor at 700 °C. The effects of catalyst type and the reaction conditions on MWCNT growth such as C2H2/Ar ratio, time and reaction temperature were investigated. The growth of MWCNT was affected by the density of catalyst particles covering the surface e.g. for hematite, the amount deposited on a silicon surface. Depending on the type of catalyst located on the Si substrate, the bamboo-like, curly shaped and straight individual MWCNT were formed. The quality of the synthesized MWCNT was investigated using Raman spectroscopy. According to the cation exchange to Fe-forms, the iron content in nontronites was increased by about 14.5 wt.%. However, by the addition of hematite particles, the iron content was increased by about 13.0 wt.% of the total iron present. Raman spectroscopy has proved that good quality ordered graphite structure of the carbon sheets in CNT was also achieved by using pure Na-forms of natural nontronites applicable as the low-cost catalyst nanoparticles. Therefore, no ion exchange modification using iron salt is necessary for this type of material. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNT) and graphene sheets are considered to be some of the most promising nano-materials of future technical applications, due to their unique physical–chemical properties. They are disposable as graphene based tubes, which are rolled in cylindrical form either as single-walled carbon nanotubes (SWCNT) consisting of a single graphene cylinder, or as multi-walled carbon nanotubes (MWCNT) composed of several co-centric graphene cylinders (Dresselhaus and
⁎ Corresponding author at: Department of Physical and Theoretical Chemistry, Comenius University, Faculty of Natural Sciences, Mlynská dolina CH1, SK-84215 Bratislava, Slovakia. Tel.: +421 2 60296418; fax: +421 2 60296231. E-mail address: [email protected] (M. Janek).
http://dx.doi.org/10.1016/j.clay.2015.06.001 0169-1317/© 2015 Elsevier B.V. All rights reserved.
Dresselhaus, 2002). The SWCNT exhibit high flexibility and toughness, extremely high strength and Young's modulus (≥1 TPa), chemical inertness, magnetic, thermal- and electro-conductive properties (Treacy et al., 1996; Thostenson et al., 2001; Robertson, 2004). They can be utilized as bulk materials for applications such as catalysts, adsorbents support, gas separation and storage etc., and also as single nano-electronic devices placed in defined positions of electronic circuits such as THz devices (Cao and Rogers, 2008). Synthesis of CNT has been successfully developed and tested by several methods, such as direct current arc discharge evaporation (Iijima, 1991), laser ablation (Paradise and Goswami, 2007), electrolysis (Kinloch et al., 2003) or chemical vapor deposition (CVD). The thermal CVD (Park et al., 2002), plasma-enhanced CVD (PECVD) (Park et al., 2003) and catalytic CVD (CCVD) (Li et al., 2005) have become dominant in the synthesis of CNT. Although the CVD method is frequently used because of the relatively low cost of CNT production, it typically produces
Š. Kavecký et al. / Applied Clay Science 114 (2015) 170–178 Table 1 Chemical composition of pristine nontronite Sampor (SA) and ferruginous smectite Washington (SWa-1). wt.%
SiO2
Fe2O3
Al2O3
MgO
CaO
loi⁎
SA SWa-1
45.4 50.4
32.9 27.0
3.0 9.2
0.1 1.3
3.4 2.7
11.2 8.5
⁎ Loss of ingnition.
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MWCNT or poor quality SWCNT. The catalytic CVD methods are based on high-temperature catalyzed gas phase carbonation reaction of low molecular weight hydrocarbon substances such as acetylene (Sato et al., 2003), methane (Jeong et al., 2005), ethylene (Hu et al., 2001), propylene (Hao et al., 2003), ethanol (Iskandar et al., 2009), benzene (Bai et al., 2003) or carbon monoxide (Tang et al., 2001). This process involves the decomposition of a carbon feedstock gas during the interaction with catalytic particles and growth of carbon nanotubes at the solid catalyst/gaseous interface. If the catalyst used is able to form carbide, the efficiency of the nanotubes growth
Fig. 1. Schematic representation of the CVD experimental equipment used for synthesis of CNT.
Fig. 2. SEM and corresponding TEM images of nontronites used for CNT growth.
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is decreased as reaction time proceeds due to formation of the respective carbide (Harris, 1999). The choice of a suitable catalyst is supposed to be crucial to the successful growth of CNT by the CVD method. Ivanov et al. (1995) found that different transition metals showed different catalytic activity and different quality of the synthesized nanotubes. Transition metal elements such as Fe, Ni, Co, Cu, Mo, Pd and/or their alloys are effective growth catalysts (Vajtai et al., 2002; Shajahan et al., 2004; Song et al., 2009; Hermes et al., 2011). Commonly tested metals include Fe, Co and Ni, because: (i) high carbon solubility in selected metals at higher temperatures; and (ii) high carbon diffusion rates in these metals, which are favorable for CNT growth. Therefore Fe, Co, Ni, and their alloys are the most widely used transition metal catalysts for the production of CNT (Ding et al., 2008). At the same time a very thin coating layer of metal catalyst using the above mentioned metals can be prepared on a suitable substrate. The coating is transformed into islands of catalyst particles of nanometer size, either before or during the CVD reaction (Lee and Park, 2001; Park et al., 2003). Various catalyst supports, such as silica, metal oxides, and different types of zeolites and iron-modified clay minerals have already been employed, and depending on the catalytic particles and synthesis conditions used, CNT with different morphology and diameter size were obtained (Esmaieli et al., 2009; Fu et al., 2012; Pastorkova et al., 2012; Santangelo et al., 2012; Manikandan et al., 2013). The role and fate of the exchangable iron in montmorillonite were investigated by Bakandritsos et al. (2006). For practical application the availability of low-cost CNT with desired dimensions and purity remains a challenge for their further practical utilization. At the same time, the growth mechanism of CNT during the CVD process is still not quite sufficiently understood. The process requires further optimization due to several multiple parameters affecting the synthesis which influence the final product. Nevertheless, the CCVD as a potential low-cost technique to grow CNT by adjusting reaction parameters e.g. carbon source, reaction temperature, gas pressure and flow rate, size and type of catalyst, catalyst concentration etc. is able to produce higher amounts of CNT (Nerushev et al., 2001). The CCVD method is also supposed to be suitable for preparation of hybrid materials based on CNT on different supports, where surface located growth is required. It is known that colloidal particles can be easily used for modification of support surfaces (Chopra and Hinds, 2004; Ji et al., 2007). Therefore this study focused on utilization and comparison of natural nontronites and synthetic hematite as interface modification nanoparticles for local growth of CNT at required supports. For this purpose, synthetic hematite particles prepared by the forced hydrolysis method were used. Such particles can be prepared as almost monodispersed sols with 3D particle diameters ranging from 10 to 200 nm and relatively narrow particle size distribution. The natural nontronites are considered as a low-cost variety of clay minerals with 2D lateral particle diameters in the range comparable to synthetic hematite particles, but with single layer thickness typical for smectites, i.e. 1 nm. Hence, the objective of this work was to test the natural platy nontronite particles and their combination with synthesized spherical hematite for modification of surfaces suitable for pre-processing in further CNT growth applications. For this purpose the hematite and/or nontronite particles were activated by hydrogen to facilitate the decomposition of acetylene and growth of CNT under specific conditions of CCVD process.
parent Ca-forms were transformed to their Na-forms (NaSA, NaSWa) using a solution of 1 M sodium chloride (NaCl, Lachema Czech Republic, p.a. purity) and washed free of excess ions. In addition, the iron saturated forms were prepared from the respective clay using a solution of 0.33 M iron chloride (FeCl3 6H2O Lachema Czech Republic, p.a. purity) with pH adjusted to 4.0 to suppress hydrolysis. The dispersions were washed free of excess ions until a negative silver nitrate test was obtained for chloride anions and denoted FeSA and FeSWa. The cation exchange to Feforms increased the total iron content in the nontronites by about 14.5 wt.% as calculated from exchangeable cation content in Table 1
2. Experimental 2.1. Clay catalyst preparation Nontronites Sampor (SA) and Washington (SWa-1, sometimes also denoted in the literature as ferruginous smectite) as clay minerals with high iron content were used in this study. The clays were Ca saturated before fractionation to b2 μm, washed free of excess ions and dried at 60 °C. SA and SWa-1 were found by chemical analysis to contain ~33% and 27% of iron oxide, respectively (Table 1). To prepare more stable dispersions,
Fig. 3. TEM images of a) synthesized hematite nanoparticles with the corresponding SAED pattern inset; b) magnetite particles upon hydrogen reduction of hematite with the corresponding SAED pattern inset; c) more detailed image of magnetite single crystal formed from hematite upon hydrogen reduction. The relevant spot SAED pattern is inset.
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after complete cation exchange. In a second step, heterocoagulates containing hematite (see below) and SA or SWa-1 particles were prepared in water dispersion using 0.05 wt.% of the Na-form of clay which was consequently homogenized with hematite dispersion at a concentration of 1.85 × 10−2 g L−1 in an ultrasonic bath for 60 min producing HeSA and HeSWa samples. By the addition of hematite particles, the total iron content in nontronites increased by about 13.0 wt.%, as calculated from mass fraction of both components used for heterocoagulate preparation (Ji et al., 2004) To test the catalytic activity of these types of clays, the catalyst substrate was modified using the respective sample containing clay mineral on conditioned Si wafer. The Si wafer plates were cut to a size of ~15 × 10 mm and treated with diluted 10% nitric acid (Slavus Slovakia 65%, p.a. purity) for 48 h, to improve the surface wettability. The 0.05% aqueous dispersions of the respective nontronite were prepared in a glass vial, homogenized for 20 min in ultrasound bath and stirred on a magnetic stirrer for 30 min. After this time while stirring, about 20 μL of dispersion was transferred on the Si wafer plates by micropipette. The excess dispersion which spread on the surface was carefully sucked by paper tissue at the Si wafer edge and followed by drying at room temperature in a dessicator. Removal of excess dispersion was necessary to suppress the solid particles pre-concentration at the solid–liquid–gas interface during drying. By this procedure, the following catalyst supports were prepared by the above mentioned dispersion casting: i) the Na-form of the respective clay was applied to the Si substrate; ii) the Fe-form of the respective clay was applied to the Si substrate; and iii) the dispersions of the respective clay heterocoagulate of and/or hematite were applied to the Si substrate.
2.2. Hematite catalyst preparation Colloidal hematite particles (α-Fe2O3) were prepared by the forced hydrolysis method (Matijevič, 1993) using solutions of 17.8 mM FeCl3 (purity as above) and 3.75 mM HCl (Slavus Slovakia 35%, p.a. purity). Both solutions were added to a Pyrex bottle equipped with a back cooler
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and heated at 125 °C in an oil bath. A magnetic stirrer ensured a constant rate of solution agitation. The temperature was kept constant with a precision better than ±1 °C. All chemicals used were pro analysis quality and de-ionized water was used for all solutions. The hydrolysis was performed for 12 h, after which the solid was separated from the supernatant liquid by centrifugation after coagulation of the prepared batch dispersion with 0.1 M NaCl. The coagulate was washed free of excess salt by repeated washing using de-ionized water in a centrifuge until the conductivity of the supernatant water was less than 10 μS cm−1. The solid content of hematite particles in the final parent dispersion was determined gravimetrically to be 1.85 × 101 g L−1 and the pH of the dispersion was adjusted by addition of 1M HCl to about 4.5. In this case the dispersion casting used different dilutions of the parent hematite dispersion (1.85 × 101 g L−1) applied for the Si wafer surface modification (1.85 × 10−1, 1.85 × 10−2 and 1.85 × 10−3 g L−1), thereby ensuring different surface loads of catalyst (e.g. substrate with the lowest concentration of dispersion used for modification will be denoted HeSi−3). The Fe2O3 activation using gaseous hydrogen at 700 °C should lead to the formation of Fe3O4 and subsequently to metal iron particles precipitation in the first stage of CNT growth. X-ray diffraction (XRD) was used to detect formerly present Fe2O3 nanoparticles at the Si substrate, and upon their hydrogen reduction, only the Fe3O4 phase was detected.
2.3. Synthesis of CNT The sample holder loaded with four pieces of Si wafer modified with solid catalyst particles of different loads was placed in the CVD flow reactor, equipped with a mass flow controllers to meter flow rates of the gases used (Fig. 1). The reactor consists of a quartz tube 800 mm long and 24 mm internal diameter enclosed in a tubular furnace from Nabertherm GmbH (Germany) equipped with temperature controller B150. The heating ramp was selected starting at room temperature to the final reaction temperatures of 500°, 550°, 600° and 700 °C, respectively. The highest synthesis temperature was selected with respect to
Fig. 4. SEM images of Si substrates top view after deposition and reduction of hematite particles using dispersion concentration in g L−1 (a) 1.85 × 101, (b) 1.85 × 10−1, (c) 1.85 × 10−2 and (d) 1.85 × 10−3.
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thermal decomposition of acetylene above 800 °C, to avoid deposition of carbonaceous products of decomposition in the synthesis oven. The heating rate was 6.0 °C min−1 under a flow of gas consisting of hydrogen/argon equal to 175/125 mL min−1. The temperature near the Si wafer surface which was covered with selected catalyst particles was controlled with a thermocouple inserted into the quartz tube as is shown in Fig. 1. After stabilization of the reaction temperature, the argon flow was turned off for 60 min to ensure effective catalyst reduction under a hydrogen flow 175 mL min−1. After this time, the hydrogen flow was turned off and the flow of argon with acetylene was turned on. Two series of different acetylene to argon flow rates were tested. The first included acetylene flow at 25 mL min−1 combined with argon flow at 150 mL min−1, total gas mixture flow rate 175 mL min−1 at ambient atmospheric pressure was completed after 30, 60 or 120 min. The second tests used an acetylene flow of 50 mL min−1 combined with argon flow at 125 mL min−1, with total gas mixture flow rate 175 mL min−1. The results obtained showed that a reaction time of 30 min was sufficient for growth of CNT. Therefore a reaction time of 30 min was used for all further studies. After the synthesis, the reactor was cooled under argon flow until the temperature reached 200 °C, before exposing the synthesized CNT to the air, to avoid their oxidation at the elevated temperatures. As was found in the experiments, changing the acetylene to argon ratio also had very little effect on the quality of
the results achieved for corresponding reaction times, therefore the synthesis of CNT on clay catalyst was carried out using the best synthesis conditions found. 2.4. Techniques A scanning electron microscope EVO 40 (Karl-Zeiss Jena, Germany) operating at 20 kV was used for primary investigation of the morphology of the substrates with applied catalyst and synthesized CNT. A piece of sample was stuck to a conventional sample holder and covered with gold in a commercial sputtering unit. The sample and holder were then transferred to the microscope chamber and images obtained at magnifications up to 40k times. For examination by transmission electron microscopy (TEM) a JEOL JEM 2000FX instrument at accelerating voltage of 160 kV, equipped with energy-dispersive (EDS) detector was used. A powder sample mixed with ethanol was sonicated for 10 min to obtain dispersion. A drop of dispersion was positioned on holey carbon film covering the TEM Cu grid. After drying in air, the sample was examined by TEM. Raman spectra of synthesized CNT were measured by Micro Raman spectroscopy on a Bruker Senterra Raman spectrometer (Bruker Optics, Ettlingen, Germany). As excitation source a green diode pumped solidstate NdYAG laser (frequency doubled, wavelength: 532 nm) was used.
Fig. 5. SEM and corresponding TEM images of MWCNT grown using catalyst nanoparticles, lines: a) NaSA; b) FeSA and c) HeSA.
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The excitation beam as well as the Raman backscattering radiation was guided through a 20× objective, NA 0.45 (OLYMPUS, Tokyo, Japan), to the spectrograph. The spectra were obtained with a resolution of 9 cm−1 in the range of 75 cm−1 to 4100 cm−1 at 10 mW laser power. The integration time was 120 s with 2 co-additions (2 times 60 s). The background signal was collected on every occasion prior to the sample measurement. The spectra were evaluated using the Bruker OPUS® software version 7.2. 3. Results and discussion 3.1. Characterization of catalyst particles by TEM and SEM The solid catalyst particles of nontronites spread on the surface of the Si wafer revealed only typical flat morphology with slightly uneven surfaces (Fig. 2), while the morphology of heterocoagulated samples was similar to those published elsewhere (Ji et al., 2004) with aggregates of particles observed on the wafer surface. For illustration, a TEM image of pristine hematite particles synthesized by the forced hydrolysis method is shown in the Fig. 3a. The typical rhombohedral shape of nanoparticles with an average size of about 20 nm can be distinguished. The inset represents a selected area electron diffraction (SAED) pattern from pristine hematite particles, confirming the presence of a hematite
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phase (JCPDS No. 33-664, a = 0.50356 nm, c = 1.37489 nm). The innermost SAED rings can be indexed to the 012, 104, 110, 113 reflections of hematite. Good crystallinity of the hematite nanoparticles is evident from sharp rings in the corresponding SAED pattern. However, a small fraction of nanoparticles also formed polycrystalline aggregates. The hematite particles upon reduction with hydrogen are shown in Fig. 3b. It is evident from comparison of the relevant SAED patterns that the original hematite has transformed upon hydrogen reduction. The size and morphology of the nanoparticles have also changed. The ring SAED pattern indicates that heating the particles caused transformation of hematite to magnetite with a = 0.8397 nm in agreement with JCPDS No. 75-449. The innermost rings in the SAED pattern in the inset of Fig. 3b can be assigned to the 220, 311, 400, and 440 reflections of magnetite. It is evident from Fig. 3b that particles have also an uneven size. Some particles have a size from about 10 nm to 25 nm, but others range from 100 nm to 250 nm. A more detailed image of one well-faceted magnetite nanocrystal around 100 nm in size is shown in Fig. 3c. The relevant SAED pattern (in inset) indicates the single-crystal character of this particle. SEM images on Figs. 4a–d show the top view of hematite deposited on the Si wafer substrates with typical morphology after annealing in the reducing atmosphere. Upon reduction, the catalyst formed aggregates obviously formed from several reduced pristine hematite particles which
Fig. 6. SEM and corresponding TEM images of MWCNT grown using catalyst nanoparticles, lines: a) NaSWa; b) FeSWa and c) HeSWa.
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sintered during the catalyst activation. The surface coverage of activated catalyst particles can be seen on the SEM images of respective samples upon their hydrogen reduction (Fig. 4a–d). The deposition of hematite dispersions with concentrations 1.85 × 101 and 1.85 × 10−1 g L−1 resulted in full surface coverage of the Si support by the reduced particle aggregates. However, lower concentration ensured homogeneous spreading of catalyst aggregates at the Si surface (Fig. 4b). For even lower hematite concentration in the dispersions e.g. 1.85 × 10−2 and 1.85 × 10−3 g L−1, the morphology of deposited aggregates changed to isolated islands containing catalyst with framboidal aggregates of deposited nanoparticles (Fig. 4c,d).
HeSWa heterocoagulates used for the synthesis and their internal diameters indicate the presence of MWCNT. It can be expected that catalytic activity of natural clay particles is induced upon their reduction with hydrogen, thus forming regularly distributed activated structural iron catalyst islands similar to those reported by Gournis et al. (1992). When stacks of platy particles are reduced during their activation, domain areas of various sizes with catalytically active iron were produced, thanks to homogeneous spreading of 1 nm thick nontronite layers at the Si surface. This enabled the growth of straight CNT with different diameters. 3.3. CNT grown using pure synthetic hematite particles
3.2. CNT grown using natural clay particles Comparison of the sodium form of nontronite SA with its iron form and hematite heterocoagulate is shown in the SEM images (Fig. 5). It is clearly distinguishable that the most uniform CNT were produced on the wafer surface with relatively even surface coverage by nontronite particles (Fig. 5a). The presence of aggregates resulted in the synthesis of CNT with different diameters (Fig. 5b,c). Uniform surface coverage by the sodium form of nontronite is ensured by very good delamination of the nontronite particle to single layers in aqueous dispersions (Güven, 1992). Interestingly, the sodium form of SWa-1 produced CNT with different diameters (Fig. 6a), which were also observed on the corresponding iron form and hematite heterocoagulate (Fig. 6b,c). The SEM/TEM images (Figs. 5,6) shows well developed, straight CNT with an outer diameter about 45–70 nm for the sodium form of SA and SWa-1. The lengths of a few individual straight CNT on the SEM and TEM images were up to 10 μm. This linear dimension was essentially higher than for the observed curly shaped bundles of nanotubes having the length just a few micrometers if pure hematite was used as catalyst. For the iron form of SA and SWa-1, straight CNT with different diameters and wall thickness were synthesized, e.g. for FeSWa, MWCNT with diameters around 50 or 100 nm were observed. The TEM experiments show that thickness CNT was comparable for either HeSA or
Corresponding SEM images of randomly oriented CNT upon their growth on substrates with different hematite surface coverage are shown in the Figs. 7a,b. The SEM images showed that the growth of CNT is not uniform with respect to their length and diameter. Two different types of nanotubes (Figs. 7a,b) can be distinguished on SEM images: i) the curly shaped bundles of nanotubes and ii) individual relatively straight nanotubes. The formation of CNT on the Si substrates correlates to surface coverage of deposited and reduced colloidal hematite particles (Figs. 4c,d). The straight CNT (HeSi−2 and HeSi−3) were found with increasing ratio in products at the lowest hematite concentration. The TEM outer diameter for straight CNT synthesized at lower hematite concentrations was up to 60–80 nm and inner diameter of individual straight CNT about 20 nm. The lengths of individual straight CNT observed on the SEM and TEM images were comparable to the lengths of CNT synthesized on natural clay particles, i.e. up to 10 μm. In comparison, curly shaped bundles of nanotubes having the length just a few micrometers (1–3 μm) and diameter 150–300 nm were mostly observed if two highest hematite concentrations had been used as the catalyst (HeSi1, HeSi−1). The TEM experiments proved that prepared nanotubes frequently have bamboo-like MWCNT morphology. The dimensions of the bamboo compartment cavities in an individual tube are not always uniform, as the consequence of the local environment fluctuation during growth of the tubes (Lee and Park, 2001; Yuan
Fig. 7. SEM and corresponding TEM images of MWCNT grown using hematite particles as catalyst; lines: a) HeSi−2; b) HeSi−3.
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et al., 2003; He et al., 2007; Sengupta and Jacob, 2010). Similarly, as observed by Zhang et al. (2007), the TEM/SEM images revealed the presence of iron particles mostly on the tips of the observed CNT. The diameter of CNT increased for longer reaction times; however, their quality was reduced as the amount of low-quality carbon increased. During the longest reaction times, the worse quality of CNT is also achieved because the catalyst particles start to be deactivated as they are covered by carbon deposits. 3.4. Raman spectroscopy The Raman spectra of carbon-based materials exhibit two main firstorder peaks (Fig. 8). The so-called G-line (1575–1601 cm−1) is a characteristic feature of the graphitic layers and corresponds to the tangential vibration of the carbon atoms and indicates the presence of well-developed CNT. The second characteristic mode is a typical sign for defective graphitic structures and involves scattering from defects breaking the basic symmetry of the graphene sheets, observed in sp2 carbons containing porous impurities or other symmetry-breaking defects by D-line (1335–1348 cm−1) (Tan et al., 1997; Qian et al., 2003). Comparison of
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the intensity ratios, e.g. the IG/ID ratio, is therefore useful to compare both the purity and the defect density as a measure of the quality of the bulk samples. Lee et al. (2002) reported the IG/ID ratio around unity to be an indicator of good crystallinity of graphite sheets. Raman spectra determined in the experiments presented in Fig. 8 showed typical bands at about 1336 cm−1 (D band) and 1579 cm−1 (G band), while the intensity ratio changed in the synthesized samples according to different catalyst and the surface loads used. As can be seen from Table 2, the ratio IG/ID changed depending on the respective catalyst used under similar conditions. Due to the 5 μm spot diameter of the excitation beam using the 20× objective, the measured sample area is around this size. The spectrum gives the information integral of this area. Indeed there is an influence of nanoparticles on the scattering behavior in Raman spectroscopy. In fact this influence is more a point for amorphous phase, but this should not to be a critical factor in our experiments according to the Gouadec and Colomban (2007). The best IG/ID ratio obtained for CNT on pure hematite particles was found for the lowest surface catalyst density (HeSi−3) on the Si substrate used and achieved the value ~1.07. The smallest IG/ID value 0.75 was obtained at the highest surface catalyst density (HeSi1) on Si substrate, where only curly shaped bundles of CNT were grown. An interesting observation from this study is that using either pure Na- or Feform of SA applied on Si substrate resulted in relatively comparable values of IG/ID with HeSi−3, around 1.06. Samples based on SWa-1 resulted in a performance comparable to the second-best HeSi− 2 or even better than HeSi− 3 sample in the Na-form (0.93) or Fe-form (1.11), respectively. However, the best performance with respect to the observed IG/ID values gave catalysts in the form of heterocoagulated material, hence the combination of hematite nanoparticles with the SA and SWa-1 nontronites used, with values of 1.39 and 1.15, respectively (Table 2). These results clearly indicate that good quality of ordered graphite structure of the carbon sheets in CNT can also be achieved by using pure natural nontronites applicable as the low-cost catalyst. Their application requires suitable substrate modification and sufficient fraction of straight CNT can be synthesized. 4. Conclusions The CVD method in the flow system of acetylene/argon was used to grow MWCNT on a Si substrate using natural nontronites and synthesized hematite particles as catalyst nanoparticles. The CNT were synthesized in the temperature range 500 °C to 700 °C, while the best MWCNT were grown at the highest temperature. The sodium forms of nontronite particles from the localities Sampor and Washington and their iron forms were used as catalyst. In addition, hydrothermally synthesized hematite α-Fe2O3 and/or its heterocoagulates with nontronite particles were used to test the catalytic activity. The CNT growth required activation of the catalyst nanoparticles using a stream of hydrogen at 700 °C. Depending on the type of catalyst located on the Si substrate, bamboo-like, curly shaped and straight individual MWCNT were formed. The straight MWCNT attained lengths up to ten micrometers and CNT with outer diameter between 40 and 150 nm were prepared. By the cation exchange
Table 2 Ratios of Raman G-, vs. D-bands observed in synthesized CNT. Sample 1
Fig. 8. Raman spectra of synthesized MWCNT using the following catalysts deposited on Si substrate: NaSA; FeSA; HeSA; NaSWa; FeSWa; HeSWa; HeSi−2; and HeSi−3.
HeSi HeSi−1 HeSi−2 HeSi−3 NaSA FeSA HeSA NaSWa FeSWa HeSWa
IG,(~1585 cm−1)/ID,(~1340 cm−1) 0.75 0.88 0.94 1.07 1.06 1.06 1.39 0.93 1.11 1.15
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of nontronites to Fe-forms, the iron content in nontronites was increased by about 14.5 wt.%. However, by the addition of hematite particles, the iron content in nontronites increased by about 13.0 wt.%. Raman spectroscopy proved that good quality of ordered graphite structure of the carbon sheets in CNT was also achieved by using sodium forms of natural nontronites as the low-cost catalyst nanoparticles. Acknowledgments The financial support of the Slovak Grant Agency for Science VEGA (grant No. 1/0943/13, 2/0179/13) and of the Slovak Research and Development Agency APVV (grant APVV-0291-11) is greatly appreciated. Our gratitude belongs also to Dr. D.C. Bain who has helped to improve the language and style of the final version of the manuscript. References Bai, S., Li, F., Yang, Q.H., Cheng, H.M., Bai, J.B., 2003. Influence of ferrocene/benzene mole ratio on the synthesis of carbon nanostructures. Chem. Phys. Lett. 376, 83–89. Bakandritsos, A., Simopoulos, A., Petridis, D., 2006. Iron changes in natural and Fe(III) loaded montmorillonite during carbon nanotube growth. Nanotechnol. 17, 1112–1117. Cao, Q., Rogers, J.A., 2008. Random networks and aligned arrays of single-walled carbon nanotubes for electronics device applications. Nano Res. 1, 259–272. Chopra, N., Hinds, B., 2004. Catalytic size control of multiwalled carbon nanotube diameter in xylene chemical vapor deposition. Inorg. Chim. Acta 357, 3920–3926. Ding, F., Larsson, P., Larsson, J.A., Ahuja, R., Duan, H., Rosen, A., Bolton, K., 2008. The importance of strong carbon–metal adhesion for catalytic nucleation of single-walled carbon nanotubes. Nano Lett. 8, 463–468. Dresselhaus, M.S., Dresselhaus, G., 2002. Intercalation compounds of graphite. Adv. Phys. 51, 1–186. Esmaieli, M., Khodadadi, A., Mortazavi, Y., 2009. Catalyst support and pretreatment effects on carbon nanotubes synthesis by chemical vapor deposition of methane on iron over SiO2, Al2O3 or MgO. IJCRE-Int. J. Chem. React. Eng. 7, 1542–6580. http://dx.doi. org/10.2202/1542-6580.1927 (ISSN (Online)). Fu, H., Du, M., Zheng, Q., 2012. Effect of iron concentration on the growth of carbon nanotubes on clay surface. ACS Appl. Mater. Interfaces 4 (4), 1981–1989. Gouadec, G., Colomban, Ph., 2007. Raman spectroscopy of nanomaterials: how spectra relate to disorder, particle size and mechanical properties. Prog. Cryst. Growth Charact. Mater. 53, 1–56. Gournis, D., Karakassides, M.A., Bakas, T., Boukos, N., Petridis, D., 1992. Catalytic synthesis of carbon nanotubes on clay minerals. Carbon 40, 2641–2646. Güven, N., 1992. Clay–water interface and its rheological implications. In: Güven, N., Pollastro, R.M. (Eds.), Clay minerals workshop lectures. vol 4, pp. 81–126 (Boulder, USA). Hao, Y., Qunfeng, Z., Fei, W., Weizhong, Q., Guohua, L., 2003. Agglomerated CNTs synthesized in a fluidized bed reactor: agglomerate structure and formation mechanism. Carbon 41, 2855–2863. Harris, P.J.F., 1999. Carbon Nanotubes and Related Structures—New Materials for the Twenty-first Century. Cambridge University Press, Cambridge. He, C., Zhao, N., Shi, C., Du, X., Li, J., 2007. TEM studies of the initial stage growth and morphologies of bamboo shaped carbon nanotubes synthesized by CVD. J. Appl. Comp. 433, 79–83. Hermes, N.A., Lansarin, M.A., Perez-Lopez, O.W., 2011. Catalytic decomposition of methane over M–Co–Al catalysts (M = Mg, Ni, Zn, Cu). Catal. Lett. 141, 1018–1025. http://dx.doi.org/10.1007/s10562-011-0611-5. Hu, W., Gong, D., Chen, Z., Yuan, L., Saito, K., Grimes, C.A., Kichambare, P., 2001. Growth of well-aligned carbon nanotube arrays on silicon substrates using porous alumina film as nanotemplate. Appl. Phys. Lett. 79, 3083–3085. Iijima, S., 1991. Helical microtubules of graphitic carbon. Nature 354, 56–58. Iskandar, F., Kim, S.G., Nandiyanto, A.B.D., Kaihatsu, Y., Ogi, T., Okuyama, K., 2009. Direct synthesis of h-BN/MWCNT composite particles using spray pyrolysis. J. Alloys Compd. 471, 166–171. Ivanov, V., Fonseca, A., Nagy, J.B., Lucas, A., Lambin, P., Bernaerts, D., Zhang, X.B., 1995. Catalytic production and purification of nanotubules having fullerene-scale diameters. Carbon 33, 1727–1738.
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Nontronites as catalyst for synthesis of carbon nanotubes by catalytic chemical vapor deposition. Štefan Kavecký a, Jana Valúchová b, Mária Čaplovičová c,d, Stefan Heissler e, Pavol Šajgalík b, Marián Janek f,g,⁎ a
Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Račianska 75, SK-83102 Bratislava, Slovakia Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, SK-84536 Bratislava, Slovakia Comenius University, Faculty of Natural Sciences, Department of Geology of Mineral Deposits, Mlynská dolina, 842 15 Bratislava, Slovakia d Slovak University of Technology, Centre for Nanodiagnostics, Vazovova 5, 812 43 Bratislava, Slovak Republic e Karlsruhe Institute of Technology, KIT; Institute of Functional Interfaces, IFG; Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany f Slovak University of Technology, Faculty of Chemical and Food Technology, Radlinského 9, SK-81237 Bratislava, Slovakia g Comenius University, Faculty of Natural Sciences, Department of Physical and Theoretical Chemistry, Mlynská dolina CH1, SK-84215 Bratislava, Slovakia b c
a r t i c l e
i n f o
Article history: Received 19 January 2015 Received in revised form 31 May 2015 Accepted 1 June 2015 Available online xxxx Keywords: Nontronites Sampor and Washington Hematite nanoparticles Carbon nanotubes MWCNT CVD
a b s t r a c t Multiwall carbon nanotubes (MWCNT) were synthesized by catalytic chemical vapor deposition (CCVD) in a horizontal tube reactor using acetylene as the carbon source in a gas mixture with argon at 700 °C and 30 minute reaction time. As a catalyst was used the sodium forms of natural platy nontronite particles Sampor or Washington and their iron modified forms. Additionally, hydrothermally synthesized hematite α-Fe2O3 and/or its heterocoagulates with nontronite particles were used to test the catalytic activity. Catalyst nanoparticles were used to modify the conditioned Si wafer surface used as the catalyst support. Before CNT growth the catalyst nanoparticles were activated by applying a hydrogen stream in the tube reactor at 700 °C. The effects of catalyst type and the reaction conditions on MWCNT growth such as C2H2/Ar ratio, time and reaction temperature were investigated. The growth of MWCNT was affected by the density of catalyst particles covering the surface e.g. for hematite, the amount deposited on a silicon surface. Depending on the type of catalyst located on the Si substrate, the bamboo-like, curly shaped and straight individual MWCNT were formed. The quality of the synthesized MWCNT was investigated using Raman spectroscopy. According to the cation exchange to Fe-forms, the iron content in nontronites was increased by about 14.5 wt.%. However, by the addition of hematite particles, the iron content was increased by about 13.0 wt.% of the total iron present. Raman spectroscopy has proved that good quality ordered graphite structure of the carbon sheets in CNT was also achieved by using pure Na-forms of natural nontronites applicable as the low-cost catalyst nanoparticles. Therefore, no ion exchange modification using iron salt is necessary for this type of material. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNT) and graphene sheets are considered to be some of the most promising nano-materials of future technical applications, due to their unique physical–chemical properties. They are disposable as graphene based tubes, which are rolled in cylindrical form either as single-walled carbon nanotubes (SWCNT) consisting of a single graphene cylinder, or as multi-walled carbon nanotubes (MWCNT) composed of several co-centric graphene cylinders (Dresselhaus and
⁎ Corresponding author at: Department of Physical and Theoretical Chemistry, Comenius University, Faculty of Natural Sciences, Mlynská dolina CH1, SK-84215 Bratislava, Slovakia. Tel.: +421 2 60296418; fax: +421 2 60296231. E-mail address: [email protected] (M. Janek).
http://dx.doi.org/10.1016/j.clay.2015.06.001 0169-1317/© 2015 Elsevier B.V. All rights reserved.
Dresselhaus, 2002). The SWCNT exhibit high flexibility and toughness, extremely high strength and Young's modulus (≥1 TPa), chemical inertness, magnetic, thermal- and electro-conductive properties (Treacy et al., 1996; Thostenson et al., 2001; Robertson, 2004). They can be utilized as bulk materials for applications such as catalysts, adsorbents support, gas separation and storage etc., and also as single nano-electronic devices placed in defined positions of electronic circuits such as THz devices (Cao and Rogers, 2008). Synthesis of CNT has been successfully developed and tested by several methods, such as direct current arc discharge evaporation (Iijima, 1991), laser ablation (Paradise and Goswami, 2007), electrolysis (Kinloch et al., 2003) or chemical vapor deposition (CVD). The thermal CVD (Park et al., 2002), plasma-enhanced CVD (PECVD) (Park et al., 2003) and catalytic CVD (CCVD) (Li et al., 2005) have become dominant in the synthesis of CNT. Although the CVD method is frequently used because of the relatively low cost of CNT production, it typically produces
Š. Kavecký et al. / Applied Clay Science 114 (2015) 170–178 Table 1 Chemical composition of pristine nontronite Sampor (SA) and ferruginous smectite Washington (SWa-1). wt.%
SiO2
Fe2O3
Al2O3
MgO
CaO
loi⁎
SA SWa-1
45.4 50.4
32.9 27.0
3.0 9.2
0.1 1.3
3.4 2.7
11.2 8.5
⁎ Loss of ingnition.
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MWCNT or poor quality SWCNT. The catalytic CVD methods are based on high-temperature catalyzed gas phase carbonation reaction of low molecular weight hydrocarbon substances such as acetylene (Sato et al., 2003), methane (Jeong et al., 2005), ethylene (Hu et al., 2001), propylene (Hao et al., 2003), ethanol (Iskandar et al., 2009), benzene (Bai et al., 2003) or carbon monoxide (Tang et al., 2001). This process involves the decomposition of a carbon feedstock gas during the interaction with catalytic particles and growth of carbon nanotubes at the solid catalyst/gaseous interface. If the catalyst used is able to form carbide, the efficiency of the nanotubes growth
Fig. 1. Schematic representation of the CVD experimental equipment used for synthesis of CNT.
Fig. 2. SEM and corresponding TEM images of nontronites used for CNT growth.
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is decreased as reaction time proceeds due to formation of the respective carbide (Harris, 1999). The choice of a suitable catalyst is supposed to be crucial to the successful growth of CNT by the CVD method. Ivanov et al. (1995) found that different transition metals showed different catalytic activity and different quality of the synthesized nanotubes. Transition metal elements such as Fe, Ni, Co, Cu, Mo, Pd and/or their alloys are effective growth catalysts (Vajtai et al., 2002; Shajahan et al., 2004; Song et al., 2009; Hermes et al., 2011). Commonly tested metals include Fe, Co and Ni, because: (i) high carbon solubility in selected metals at higher temperatures; and (ii) high carbon diffusion rates in these metals, which are favorable for CNT growth. Therefore Fe, Co, Ni, and their alloys are the most widely used transition metal catalysts for the production of CNT (Ding et al., 2008). At the same time a very thin coating layer of metal catalyst using the above mentioned metals can be prepared on a suitable substrate. The coating is transformed into islands of catalyst particles of nanometer size, either before or during the CVD reaction (Lee and Park, 2001; Park et al., 2003). Various catalyst supports, such as silica, metal oxides, and different types of zeolites and iron-modified clay minerals have already been employed, and depending on the catalytic particles and synthesis conditions used, CNT with different morphology and diameter size were obtained (Esmaieli et al., 2009; Fu et al., 2012; Pastorkova et al., 2012; Santangelo et al., 2012; Manikandan et al., 2013). The role and fate of the exchangable iron in montmorillonite were investigated by Bakandritsos et al. (2006). For practical application the availability of low-cost CNT with desired dimensions and purity remains a challenge for their further practical utilization. At the same time, the growth mechanism of CNT during the CVD process is still not quite sufficiently understood. The process requires further optimization due to several multiple parameters affecting the synthesis which influence the final product. Nevertheless, the CCVD as a potential low-cost technique to grow CNT by adjusting reaction parameters e.g. carbon source, reaction temperature, gas pressure and flow rate, size and type of catalyst, catalyst concentration etc. is able to produce higher amounts of CNT (Nerushev et al., 2001). The CCVD method is also supposed to be suitable for preparation of hybrid materials based on CNT on different supports, where surface located growth is required. It is known that colloidal particles can be easily used for modification of support surfaces (Chopra and Hinds, 2004; Ji et al., 2007). Therefore this study focused on utilization and comparison of natural nontronites and synthetic hematite as interface modification nanoparticles for local growth of CNT at required supports. For this purpose, synthetic hematite particles prepared by the forced hydrolysis method were used. Such particles can be prepared as almost monodispersed sols with 3D particle diameters ranging from 10 to 200 nm and relatively narrow particle size distribution. The natural nontronites are considered as a low-cost variety of clay minerals with 2D lateral particle diameters in the range comparable to synthetic hematite particles, but with single layer thickness typical for smectites, i.e. 1 nm. Hence, the objective of this work was to test the natural platy nontronite particles and their combination with synthesized spherical hematite for modification of surfaces suitable for pre-processing in further CNT growth applications. For this purpose the hematite and/or nontronite particles were activated by hydrogen to facilitate the decomposition of acetylene and growth of CNT under specific conditions of CCVD process.
parent Ca-forms were transformed to their Na-forms (NaSA, NaSWa) using a solution of 1 M sodium chloride (NaCl, Lachema Czech Republic, p.a. purity) and washed free of excess ions. In addition, the iron saturated forms were prepared from the respective clay using a solution of 0.33 M iron chloride (FeCl3 6H2O Lachema Czech Republic, p.a. purity) with pH adjusted to 4.0 to suppress hydrolysis. The dispersions were washed free of excess ions until a negative silver nitrate test was obtained for chloride anions and denoted FeSA and FeSWa. The cation exchange to Feforms increased the total iron content in the nontronites by about 14.5 wt.% as calculated from exchangeable cation content in Table 1
2. Experimental 2.1. Clay catalyst preparation Nontronites Sampor (SA) and Washington (SWa-1, sometimes also denoted in the literature as ferruginous smectite) as clay minerals with high iron content were used in this study. The clays were Ca saturated before fractionation to b2 μm, washed free of excess ions and dried at 60 °C. SA and SWa-1 were found by chemical analysis to contain ~33% and 27% of iron oxide, respectively (Table 1). To prepare more stable dispersions,
Fig. 3. TEM images of a) synthesized hematite nanoparticles with the corresponding SAED pattern inset; b) magnetite particles upon hydrogen reduction of hematite with the corresponding SAED pattern inset; c) more detailed image of magnetite single crystal formed from hematite upon hydrogen reduction. The relevant spot SAED pattern is inset.
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after complete cation exchange. In a second step, heterocoagulates containing hematite (see below) and SA or SWa-1 particles were prepared in water dispersion using 0.05 wt.% of the Na-form of clay which was consequently homogenized with hematite dispersion at a concentration of 1.85 × 10−2 g L−1 in an ultrasonic bath for 60 min producing HeSA and HeSWa samples. By the addition of hematite particles, the total iron content in nontronites increased by about 13.0 wt.%, as calculated from mass fraction of both components used for heterocoagulate preparation (Ji et al., 2004) To test the catalytic activity of these types of clays, the catalyst substrate was modified using the respective sample containing clay mineral on conditioned Si wafer. The Si wafer plates were cut to a size of ~15 × 10 mm and treated with diluted 10% nitric acid (Slavus Slovakia 65%, p.a. purity) for 48 h, to improve the surface wettability. The 0.05% aqueous dispersions of the respective nontronite were prepared in a glass vial, homogenized for 20 min in ultrasound bath and stirred on a magnetic stirrer for 30 min. After this time while stirring, about 20 μL of dispersion was transferred on the Si wafer plates by micropipette. The excess dispersion which spread on the surface was carefully sucked by paper tissue at the Si wafer edge and followed by drying at room temperature in a dessicator. Removal of excess dispersion was necessary to suppress the solid particles pre-concentration at the solid–liquid–gas interface during drying. By this procedure, the following catalyst supports were prepared by the above mentioned dispersion casting: i) the Na-form of the respective clay was applied to the Si substrate; ii) the Fe-form of the respective clay was applied to the Si substrate; and iii) the dispersions of the respective clay heterocoagulate of and/or hematite were applied to the Si substrate.
2.2. Hematite catalyst preparation Colloidal hematite particles (α-Fe2O3) were prepared by the forced hydrolysis method (Matijevič, 1993) using solutions of 17.8 mM FeCl3 (purity as above) and 3.75 mM HCl (Slavus Slovakia 35%, p.a. purity). Both solutions were added to a Pyrex bottle equipped with a back cooler
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and heated at 125 °C in an oil bath. A magnetic stirrer ensured a constant rate of solution agitation. The temperature was kept constant with a precision better than ±1 °C. All chemicals used were pro analysis quality and de-ionized water was used for all solutions. The hydrolysis was performed for 12 h, after which the solid was separated from the supernatant liquid by centrifugation after coagulation of the prepared batch dispersion with 0.1 M NaCl. The coagulate was washed free of excess salt by repeated washing using de-ionized water in a centrifuge until the conductivity of the supernatant water was less than 10 μS cm−1. The solid content of hematite particles in the final parent dispersion was determined gravimetrically to be 1.85 × 101 g L−1 and the pH of the dispersion was adjusted by addition of 1M HCl to about 4.5. In this case the dispersion casting used different dilutions of the parent hematite dispersion (1.85 × 101 g L−1) applied for the Si wafer surface modification (1.85 × 10−1, 1.85 × 10−2 and 1.85 × 10−3 g L−1), thereby ensuring different surface loads of catalyst (e.g. substrate with the lowest concentration of dispersion used for modification will be denoted HeSi−3). The Fe2O3 activation using gaseous hydrogen at 700 °C should lead to the formation of Fe3O4 and subsequently to metal iron particles precipitation in the first stage of CNT growth. X-ray diffraction (XRD) was used to detect formerly present Fe2O3 nanoparticles at the Si substrate, and upon their hydrogen reduction, only the Fe3O4 phase was detected.
2.3. Synthesis of CNT The sample holder loaded with four pieces of Si wafer modified with solid catalyst particles of different loads was placed in the CVD flow reactor, equipped with a mass flow controllers to meter flow rates of the gases used (Fig. 1). The reactor consists of a quartz tube 800 mm long and 24 mm internal diameter enclosed in a tubular furnace from Nabertherm GmbH (Germany) equipped with temperature controller B150. The heating ramp was selected starting at room temperature to the final reaction temperatures of 500°, 550°, 600° and 700 °C, respectively. The highest synthesis temperature was selected with respect to
Fig. 4. SEM images of Si substrates top view after deposition and reduction of hematite particles using dispersion concentration in g L−1 (a) 1.85 × 101, (b) 1.85 × 10−1, (c) 1.85 × 10−2 and (d) 1.85 × 10−3.
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thermal decomposition of acetylene above 800 °C, to avoid deposition of carbonaceous products of decomposition in the synthesis oven. The heating rate was 6.0 °C min−1 under a flow of gas consisting of hydrogen/argon equal to 175/125 mL min−1. The temperature near the Si wafer surface which was covered with selected catalyst particles was controlled with a thermocouple inserted into the quartz tube as is shown in Fig. 1. After stabilization of the reaction temperature, the argon flow was turned off for 60 min to ensure effective catalyst reduction under a hydrogen flow 175 mL min−1. After this time, the hydrogen flow was turned off and the flow of argon with acetylene was turned on. Two series of different acetylene to argon flow rates were tested. The first included acetylene flow at 25 mL min−1 combined with argon flow at 150 mL min−1, total gas mixture flow rate 175 mL min−1 at ambient atmospheric pressure was completed after 30, 60 or 120 min. The second tests used an acetylene flow of 50 mL min−1 combined with argon flow at 125 mL min−1, with total gas mixture flow rate 175 mL min−1. The results obtained showed that a reaction time of 30 min was sufficient for growth of CNT. Therefore a reaction time of 30 min was used for all further studies. After the synthesis, the reactor was cooled under argon flow until the temperature reached 200 °C, before exposing the synthesized CNT to the air, to avoid their oxidation at the elevated temperatures. As was found in the experiments, changing the acetylene to argon ratio also had very little effect on the quality of
the results achieved for corresponding reaction times, therefore the synthesis of CNT on clay catalyst was carried out using the best synthesis conditions found. 2.4. Techniques A scanning electron microscope EVO 40 (Karl-Zeiss Jena, Germany) operating at 20 kV was used for primary investigation of the morphology of the substrates with applied catalyst and synthesized CNT. A piece of sample was stuck to a conventional sample holder and covered with gold in a commercial sputtering unit. The sample and holder were then transferred to the microscope chamber and images obtained at magnifications up to 40k times. For examination by transmission electron microscopy (TEM) a JEOL JEM 2000FX instrument at accelerating voltage of 160 kV, equipped with energy-dispersive (EDS) detector was used. A powder sample mixed with ethanol was sonicated for 10 min to obtain dispersion. A drop of dispersion was positioned on holey carbon film covering the TEM Cu grid. After drying in air, the sample was examined by TEM. Raman spectra of synthesized CNT were measured by Micro Raman spectroscopy on a Bruker Senterra Raman spectrometer (Bruker Optics, Ettlingen, Germany). As excitation source a green diode pumped solidstate NdYAG laser (frequency doubled, wavelength: 532 nm) was used.
Fig. 5. SEM and corresponding TEM images of MWCNT grown using catalyst nanoparticles, lines: a) NaSA; b) FeSA and c) HeSA.
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The excitation beam as well as the Raman backscattering radiation was guided through a 20× objective, NA 0.45 (OLYMPUS, Tokyo, Japan), to the spectrograph. The spectra were obtained with a resolution of 9 cm−1 in the range of 75 cm−1 to 4100 cm−1 at 10 mW laser power. The integration time was 120 s with 2 co-additions (2 times 60 s). The background signal was collected on every occasion prior to the sample measurement. The spectra were evaluated using the Bruker OPUS® software version 7.2. 3. Results and discussion 3.1. Characterization of catalyst particles by TEM and SEM The solid catalyst particles of nontronites spread on the surface of the Si wafer revealed only typical flat morphology with slightly uneven surfaces (Fig. 2), while the morphology of heterocoagulated samples was similar to those published elsewhere (Ji et al., 2004) with aggregates of particles observed on the wafer surface. For illustration, a TEM image of pristine hematite particles synthesized by the forced hydrolysis method is shown in the Fig. 3a. The typical rhombohedral shape of nanoparticles with an average size of about 20 nm can be distinguished. The inset represents a selected area electron diffraction (SAED) pattern from pristine hematite particles, confirming the presence of a hematite
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phase (JCPDS No. 33-664, a = 0.50356 nm, c = 1.37489 nm). The innermost SAED rings can be indexed to the 012, 104, 110, 113 reflections of hematite. Good crystallinity of the hematite nanoparticles is evident from sharp rings in the corresponding SAED pattern. However, a small fraction of nanoparticles also formed polycrystalline aggregates. The hematite particles upon reduction with hydrogen are shown in Fig. 3b. It is evident from comparison of the relevant SAED patterns that the original hematite has transformed upon hydrogen reduction. The size and morphology of the nanoparticles have also changed. The ring SAED pattern indicates that heating the particles caused transformation of hematite to magnetite with a = 0.8397 nm in agreement with JCPDS No. 75-449. The innermost rings in the SAED pattern in the inset of Fig. 3b can be assigned to the 220, 311, 400, and 440 reflections of magnetite. It is evident from Fig. 3b that particles have also an uneven size. Some particles have a size from about 10 nm to 25 nm, but others range from 100 nm to 250 nm. A more detailed image of one well-faceted magnetite nanocrystal around 100 nm in size is shown in Fig. 3c. The relevant SAED pattern (in inset) indicates the single-crystal character of this particle. SEM images on Figs. 4a–d show the top view of hematite deposited on the Si wafer substrates with typical morphology after annealing in the reducing atmosphere. Upon reduction, the catalyst formed aggregates obviously formed from several reduced pristine hematite particles which
Fig. 6. SEM and corresponding TEM images of MWCNT grown using catalyst nanoparticles, lines: a) NaSWa; b) FeSWa and c) HeSWa.
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sintered during the catalyst activation. The surface coverage of activated catalyst particles can be seen on the SEM images of respective samples upon their hydrogen reduction (Fig. 4a–d). The deposition of hematite dispersions with concentrations 1.85 × 101 and 1.85 × 10−1 g L−1 resulted in full surface coverage of the Si support by the reduced particle aggregates. However, lower concentration ensured homogeneous spreading of catalyst aggregates at the Si surface (Fig. 4b). For even lower hematite concentration in the dispersions e.g. 1.85 × 10−2 and 1.85 × 10−3 g L−1, the morphology of deposited aggregates changed to isolated islands containing catalyst with framboidal aggregates of deposited nanoparticles (Fig. 4c,d).
HeSWa heterocoagulates used for the synthesis and their internal diameters indicate the presence of MWCNT. It can be expected that catalytic activity of natural clay particles is induced upon their reduction with hydrogen, thus forming regularly distributed activated structural iron catalyst islands similar to those reported by Gournis et al. (1992). When stacks of platy particles are reduced during their activation, domain areas of various sizes with catalytically active iron were produced, thanks to homogeneous spreading of 1 nm thick nontronite layers at the Si surface. This enabled the growth of straight CNT with different diameters. 3.3. CNT grown using pure synthetic hematite particles
3.2. CNT grown using natural clay particles Comparison of the sodium form of nontronite SA with its iron form and hematite heterocoagulate is shown in the SEM images (Fig. 5). It is clearly distinguishable that the most uniform CNT were produced on the wafer surface with relatively even surface coverage by nontronite particles (Fig. 5a). The presence of aggregates resulted in the synthesis of CNT with different diameters (Fig. 5b,c). Uniform surface coverage by the sodium form of nontronite is ensured by very good delamination of the nontronite particle to single layers in aqueous dispersions (Güven, 1992). Interestingly, the sodium form of SWa-1 produced CNT with different diameters (Fig. 6a), which were also observed on the corresponding iron form and hematite heterocoagulate (Fig. 6b,c). The SEM/TEM images (Figs. 5,6) shows well developed, straight CNT with an outer diameter about 45–70 nm for the sodium form of SA and SWa-1. The lengths of a few individual straight CNT on the SEM and TEM images were up to 10 μm. This linear dimension was essentially higher than for the observed curly shaped bundles of nanotubes having the length just a few micrometers if pure hematite was used as catalyst. For the iron form of SA and SWa-1, straight CNT with different diameters and wall thickness were synthesized, e.g. for FeSWa, MWCNT with diameters around 50 or 100 nm were observed. The TEM experiments show that thickness CNT was comparable for either HeSA or
Corresponding SEM images of randomly oriented CNT upon their growth on substrates with different hematite surface coverage are shown in the Figs. 7a,b. The SEM images showed that the growth of CNT is not uniform with respect to their length and diameter. Two different types of nanotubes (Figs. 7a,b) can be distinguished on SEM images: i) the curly shaped bundles of nanotubes and ii) individual relatively straight nanotubes. The formation of CNT on the Si substrates correlates to surface coverage of deposited and reduced colloidal hematite particles (Figs. 4c,d). The straight CNT (HeSi−2 and HeSi−3) were found with increasing ratio in products at the lowest hematite concentration. The TEM outer diameter for straight CNT synthesized at lower hematite concentrations was up to 60–80 nm and inner diameter of individual straight CNT about 20 nm. The lengths of individual straight CNT observed on the SEM and TEM images were comparable to the lengths of CNT synthesized on natural clay particles, i.e. up to 10 μm. In comparison, curly shaped bundles of nanotubes having the length just a few micrometers (1–3 μm) and diameter 150–300 nm were mostly observed if two highest hematite concentrations had been used as the catalyst (HeSi1, HeSi−1). The TEM experiments proved that prepared nanotubes frequently have bamboo-like MWCNT morphology. The dimensions of the bamboo compartment cavities in an individual tube are not always uniform, as the consequence of the local environment fluctuation during growth of the tubes (Lee and Park, 2001; Yuan
Fig. 7. SEM and corresponding TEM images of MWCNT grown using hematite particles as catalyst; lines: a) HeSi−2; b) HeSi−3.
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et al., 2003; He et al., 2007; Sengupta and Jacob, 2010). Similarly, as observed by Zhang et al. (2007), the TEM/SEM images revealed the presence of iron particles mostly on the tips of the observed CNT. The diameter of CNT increased for longer reaction times; however, their quality was reduced as the amount of low-quality carbon increased. During the longest reaction times, the worse quality of CNT is also achieved because the catalyst particles start to be deactivated as they are covered by carbon deposits. 3.4. Raman spectroscopy The Raman spectra of carbon-based materials exhibit two main firstorder peaks (Fig. 8). The so-called G-line (1575–1601 cm−1) is a characteristic feature of the graphitic layers and corresponds to the tangential vibration of the carbon atoms and indicates the presence of well-developed CNT. The second characteristic mode is a typical sign for defective graphitic structures and involves scattering from defects breaking the basic symmetry of the graphene sheets, observed in sp2 carbons containing porous impurities or other symmetry-breaking defects by D-line (1335–1348 cm−1) (Tan et al., 1997; Qian et al., 2003). Comparison of
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the intensity ratios, e.g. the IG/ID ratio, is therefore useful to compare both the purity and the defect density as a measure of the quality of the bulk samples. Lee et al. (2002) reported the IG/ID ratio around unity to be an indicator of good crystallinity of graphite sheets. Raman spectra determined in the experiments presented in Fig. 8 showed typical bands at about 1336 cm−1 (D band) and 1579 cm−1 (G band), while the intensity ratio changed in the synthesized samples according to different catalyst and the surface loads used. As can be seen from Table 2, the ratio IG/ID changed depending on the respective catalyst used under similar conditions. Due to the 5 μm spot diameter of the excitation beam using the 20× objective, the measured sample area is around this size. The spectrum gives the information integral of this area. Indeed there is an influence of nanoparticles on the scattering behavior in Raman spectroscopy. In fact this influence is more a point for amorphous phase, but this should not to be a critical factor in our experiments according to the Gouadec and Colomban (2007). The best IG/ID ratio obtained for CNT on pure hematite particles was found for the lowest surface catalyst density (HeSi−3) on the Si substrate used and achieved the value ~1.07. The smallest IG/ID value 0.75 was obtained at the highest surface catalyst density (HeSi1) on Si substrate, where only curly shaped bundles of CNT were grown. An interesting observation from this study is that using either pure Na- or Feform of SA applied on Si substrate resulted in relatively comparable values of IG/ID with HeSi−3, around 1.06. Samples based on SWa-1 resulted in a performance comparable to the second-best HeSi− 2 or even better than HeSi− 3 sample in the Na-form (0.93) or Fe-form (1.11), respectively. However, the best performance with respect to the observed IG/ID values gave catalysts in the form of heterocoagulated material, hence the combination of hematite nanoparticles with the SA and SWa-1 nontronites used, with values of 1.39 and 1.15, respectively (Table 2). These results clearly indicate that good quality of ordered graphite structure of the carbon sheets in CNT can also be achieved by using pure natural nontronites applicable as the low-cost catalyst. Their application requires suitable substrate modification and sufficient fraction of straight CNT can be synthesized. 4. Conclusions The CVD method in the flow system of acetylene/argon was used to grow MWCNT on a Si substrate using natural nontronites and synthesized hematite particles as catalyst nanoparticles. The CNT were synthesized in the temperature range 500 °C to 700 °C, while the best MWCNT were grown at the highest temperature. The sodium forms of nontronite particles from the localities Sampor and Washington and their iron forms were used as catalyst. In addition, hydrothermally synthesized hematite α-Fe2O3 and/or its heterocoagulates with nontronite particles were used to test the catalytic activity. The CNT growth required activation of the catalyst nanoparticles using a stream of hydrogen at 700 °C. Depending on the type of catalyst located on the Si substrate, bamboo-like, curly shaped and straight individual MWCNT were formed. The straight MWCNT attained lengths up to ten micrometers and CNT with outer diameter between 40 and 150 nm were prepared. By the cation exchange
Table 2 Ratios of Raman G-, vs. D-bands observed in synthesized CNT. Sample 1
Fig. 8. Raman spectra of synthesized MWCNT using the following catalysts deposited on Si substrate: NaSA; FeSA; HeSA; NaSWa; FeSWa; HeSWa; HeSi−2; and HeSi−3.
HeSi HeSi−1 HeSi−2 HeSi−3 NaSA FeSA HeSA NaSWa FeSWa HeSWa
IG,(~1585 cm−1)/ID,(~1340 cm−1) 0.75 0.88 0.94 1.07 1.06 1.06 1.39 0.93 1.11 1.15
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