Effective Synthesis of Well-Graphitized Carbon Nanotubes on

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Effective Synthesis of Well-Graphitized Carbon Nanotubes on Bimetallic SBA-15 Template for Use as Counter Electrode in DyeSensitized Solar Cells Jayaraman Balamurugan,† Arumugam Pandurangan,*,† Rangasamy Thangamuthu,‡ and Sakkarapalayam Murugesan Senthilkumar‡ †

Department of Chemistry, Institute of Catalysis and Petroleum Technology, Anna University, Chennai 600 025, India Electrochemical Materials Science Division, CSIR-Central Electrochemical Research Institute, Karaikudi 630 006, India

‡

ABSTRACT: The fabrication of dye-sensitized solar cells (DSSCs) using well-graphitized carbon nanotubes (CNTs) as a counter electrode has been described in this study. The well-graphitized CNTs were synthesized at different temperatures (700, 800, and 900 °C) using bimetallic Fe-V catalyst supported on Santa Barbara Amorphous-15 (SBA-15). The molecular ratios between the two metals were varied in the catalytic template, and their effect on the distribution of the synthesized CNTs was studied. Cyclic voltammetry and electrochemical impedance spectroscopy revealed that the CNTs had higher electrochemical activity for the I3−/I− redox reaction and a smaller charge transfer resistance than the platinum (Pt) electrode. Energy conversion efficiency of the CNTs was compared with Pt counter electrode. These results indicated that the CNTs have high surface conductivity, high active surface area, and good catalytic activity and can potentially replace Pt as counter electrode for application in DSSCs. coating,15 and spin coating.13 Even though the cost of CNTs is lower than that of Pt, lack of adhesion to the transparent conducting oxide substrate poses a long-term stability risk.7 However, stability of the carbon based counter electrodes has been remarkably improved by adopting the nanocomposite electrode fabrication processes.16 CNTs consist of concentric graphene cylinders ranging from several to tens of nanometers in diameter and up to tens of micrometers in length. Over the years, the synthesis of CNTs, including single-wall carbon nanotubes (SWCNTs) and multiwall carbon nanotubes (MWCNTs), has been comprehensively developed. Among the many methods available for the synthesis of CNTs, chemical vapor deposition (CVD) appears to be a promising and a viable method because of its low cost ability to better control the structure when compared with other methods such as arc-discharge and laser ablation. In the controlled growth of CNTs by CVD, the preparation of nanoparticle catalysts is one of the key factors. All over the world, there is a general agreement on the role of the metal catalysts such as Ni, Fe, Co, V, Cr, Mo, and their alloys in the growth of CNTs. It was found that the bimetallic catalysts are much more effective for the growth of the CNTs by CVD than single metals. The bimetallic combination also proved to be better candidates for CNTs synthesis. Another explanation for the growth mechanism comes from noting the use of support. Zeolite and mesoporous molecular sieves are usually used as catalyst supports for the growth of CNTs.17 Since the discovery of silica based and metal substituted mesoporous materials,

1. INTRODUCTION Dye-sensitized solar cells (DSSCs) are considered as a potential candidate for the next generation solar cells owing to their high energy conversion efficiency, low cost materials, and superficial fabrication methods.1,2 DSSCs commonly consist of a TiO2 working electrode, an electrolyte, and a counter electrode. The counter electrode, an essential component in the DSSCs,3 functions to transfer electrons from the external circuit and back to the redox electrolyte for I3− reduction. Noble metals, such as platinum (Pt), often serve as counter electrode because of their high electrochemical activity for I3− reduction. However, Pt is a noble metal, is relatively expensive, and shows poor stability in a corrosive iodide/tri-iodide redox system.4 The cost of Pt counter electrode represents more than 40% of the whole DSSC cell cost, regardless of its preparation method. Therefore, it is highly desirable to develop a low-cost alternative material for counter electrodes with high electrocatalytic activity and chemical stability. As more cost-effective materials for counter electrode, a variety of carbons have been evaluated,5 which include carbon black,6 activated carbons,7 graphene,8 and carbon nanotubes (CNTs).9 More recently, nanostructured carbons such as mesoporous carbon, graphene, and CNTs have been found to possess attractive structural properties that are valuable for DSSCs. A counter electrode with high electrochemical activity is a very important factor for achieving a high photo conversion efficiency of DSSCs as well as for maintaining a large scale application. CNTs are wellknown materials that could function as alternatives for Pt counter electrode for DSSCs due to their large surface area, high conductivity, chemical stability, and low cost (CNTs are 165,000% cheaper (per kg) than Pt).10 CNT based counter electrodes have been fabricated by different methods, namely, doctor blade,11 screen printing,12,13 drop coating,14 spray © 2012 American Chemical Society

Received: Revised: Accepted: Published: 384

August 17, 2012 December 5, 2012 December 10, 2012 December 10, 2012 dx.doi.org/10.1021/ie3021973 | Ind. Eng. Chem. Res. 2013, 52, 384−393

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acidic conditions. Anhydrous toluene, ethanol, hydrochloric acid, acetonitrile, and acetic acid were purchased from Merck. H2PtCl6, iodine, lithium iodide, 4-tert-butyl-pyridine, ethyl cellulose, terpineol, and N719 dye [cis-di (thiocyanato)-N,N-bis (2,2′-bipyridyl-4-carboxylic acid-4-tetrabutylammonium carboxylate) ruthenium(II)] were purchased from the Aldrich. All the reagents were used without further purification. Fluorine-doped tin oxide (FTO) conducting glass plates (sheet resistance 15 Ohm sq−1) were purchased from Xinyan Technology Ltd., HK. The FTO glass substrates were ultrasonicated thoroughly in acetone, ethanol, and double distilled water for 15 min in each step to remove organic pollution and other contamination. 2.2. Preparation of Catalysts. Bimetallic Fe-V-SBA-15 catalysts with different Fe-V mass ratios were synthesized along with the monometallic Fe-SBA-15, V-SBA-15 catalysts. In this paper, 3:1, 1:1, and 1:3 ratios are discussed because they best illustrate the range of behaviors. These catalysts have been labeled as Fe-V-SBA-15 3:1, Fe-V-SBA-15 1:1, and Fe-V-SBA15 1:3. All catalysts had a total of 6% metal loading. Among the catalysts, Fe-V-SBA-15 1:1 with 6% metal loading was used because it was observed to give maximum yield of CNTs (up to 329%) without causing structural loss in the SBA-15 framework. The catalyst synthesis was a two step process: In the first step, pure siliceous SBA-15 was synthesized according to the literature.21 In brief, 4 g of Pluronic P123 was combined with 120 mL of 2N HCl and 30 mL of double distilled water in a polypropylene bottle and stirred at 40 °C until it was completely dissolved. To this solution, 8.5 g of TEOS was added under vigorous stirring. The contents were stirred for 24 h at 40 °C then aged in an oven at 100 °C for 24 h. The above product was filtered and washed with distilled water and ethanol, resulting in a white powder. The powder was then heated at a constant ramping rate from room temperature to 540 °C over 5 h under N2 and held for 1 h under the same conditions, followed by calcination at 540 °C for 6 h under air to remove the residual organic template materials. In a second step, the pure siliceous SBA-15 sample, obtained from the first step, was suspended in anhydrous toluene and refluxed under an inert N2 atmosphere for 5 h to remove the adsorbed moisture. Subsequently, iron(III) acetylacetonate and vanadium(III) acetylacetonate in equal ratios were dissolved in anhydrous toluene and added dropwise to SBA-15 suspension. The grafting process was performed under an inert N2 atmosphere by refluxing at 110 °C for 8 h, involving the reaction of iron(III) acetylacetonate and vanadium(III) acetylacetonate with surface silanol groups of the calcined pure siliceous SBA-15. The reaction mixture was cooled, filtered, and washed with toluene to ensure that there was no unreacted iron and vanadium precursor left on the SBA-15. The solids were then dried overnight in ambient air. The calcination procedure for the synthesized Fe-V-SBA-15 1:1 was the same as that followed for the siliceous SBA-15. The calcined samples were denoted as wt % Fe-V-SBA-15, where wt % represents the final Fe-V content in the sample tested by inductive coupled plasma-atomic emission spectroscopy (ICP-AES) analysis. Bimetallic Fe-V-SBA-15 3:1 and Fe-V-SBA-15 1:3 and monometallic Fe-SBA-15 and V-SBA-15 catalysts were synthesized following a similar procedure described above by a controlled postsynthesis grafting process. 2.3. CNTs Production and Purification. CNTs were synthesized by acetylene disproportionation using the bimetallic Fe-V-SBA-15 3:1, Fe-V-SBA-15 1:1, and Fe-V-SBA-15 1:3 catalysts. For the growth of CNTs, about 100 mg of the catalyst

such as M41S, a broad research progress has been achieved in the synthesis and characterization of ordered mesoporous materials, the motivation for which lies in their high surface area, large and uniform pore size distribution, and potential applications in the field of catalysis, separation, adsorption,18 and CNTs synthesis.19 Among these materials, Santa Barbara Amorphous-15 (SBA-15), a polymer template silica with hexagonally ordered mesopores has attracted much attention lately. Its popularity is due to the larger pore size, thicker pore walls, and higher hydrothermal stability compared to the well recognized mobile composite matter No. 41 (MCM-41), which is surfactant template ordered mesoporous material.20 Further, the polymer template employed to obtain SBA-15, poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide), is biodegradable and cheaper than the surfactants used in the synthesis of MCM-41. Another attractive feature of SBA-15 is the existence of micropores interconnecting hexagonally ordered mesopores, which make it highly suitable for use as a catalyst because these interconnections facilitate the diffusion of carbon inside the porous structure.21 The improved hydrothermal and thermal stabilities of SBA-15 render them as one of the most promising catalytic materials. However, these materials consisting of pure silica frameworks are of limited use for various catalytic applications because of their lack of active sites and ion exchange capacity. Incorporation of heteroatoms such as Al, Ti, Cu, and V within the silica framework of mesoporous materials such as MCM-41 and MCM-48 has been implemented in order to create catalytically active sites, ionexchange capacity, and hence their catalytic activity.22 However, it is very difficult to introduce the metal ions into the silica framework of SBA-15 directly due to the facile dissociation of M−O−Si bonds under the strong acidic hydrothermal conditions. The isomorphous substitution of heteroatom in all silica mesoporous SBA-15 could lead to the formation of useful catalysts for the reactions involving bulkier molecules. Thus, a number of direct synthesis and postsynthesis attempts have been made to incorporate Al, V, Zr, and Ti in SBA-15 framework.23 Because of the strong acidic media involved during the synthesis, the postsynthesis route was found to be more effective over direct synthesis. Recently, a few efforts have been made using metal substituted (loading) SBA-15 mesoporous molecular sieve as catalytic template to synthesize CNTs.24,25 Consequently, SBA-15 materials are very promising candidates as catalyst supports, and the study of this topic is worth exploring further. In the present investigation, the authors endeavor to fabricate the DSSCs using a counter electrode made of well-graphitized CNTs by the spin-coating method and compared its performance with DSSCs having a Pt counter electrode in the same device fabrication conditions. Herein, we report the successful synthesis of CNTs with uniform diameter by the use of bimetallic Fe-V-SBA-15 catalyst. In the present study, CNTs were also synthesized over monometallic Fe-SBA-15 and VSBA-15 for comparison.

2. MATERIALS AND METHODS 2.1. Reagents and Chemicals. Iron(III) acetylacetonate, vanadium(III) acetylacetonate, and tetraethylorthosilicate (TEOS) were purchased from Aldrich and used as the source for iron, vanadium, and silicon, respectively. Triblock copolymer poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic P123; Mav = 5800, Aldrich) was used as a structure-directing agent under 385

dx.doi.org/10.1021/ie3021973 | Ind. Eng. Chem. Res. 2013, 52, 384−393


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mL of 0.5 mM ethanol N719 dye solution for 24 h in the dark at room temperature. Then, it was cleaned with ethanol and dried in an oven to be readily used as a photoanode or working electrode in DSSCs. The sensitized photoanode was assembled with CNTs/Pt counter electrode using a hot melting polymer spacer (Syrlyn, DuPont) before injecting the electrolyte through the hole into the counter electrode. The hole was then subsequently sealed with a polymer sheet. 2.6. Characterization Techniques. The amount of the metal loaded into the SBA-15 was analyzed and determined by inductive coupled plasma-atomic emission spectroscopy (ICPAES, Perkin-Elmer OPTIMA 3000). The samples were dissolved in a mixture of HF and HNO3 before the measurement. The XRD patterns of catalysts as well as CNTs were recorded with a PANalytical X’Pert diffractometer, using nickel-filtered Cu Kα radiation (λ = 1.54 Å) and a liquid nitrogen-cooled germanium solid-state detector. The diffractograms were recorded in the 2θ range of 0.5−10° for the catalyst and 10−80° for the CNTs. The peaks were identified with reference to compilation of simulated XRD powder patterns. The surface area, pore volume, and pore size distribution were measured by N2 sorption at −197 °C using an ASAP-2010 porosimeter from Micrometrics Corporation. The mesopores volume was estimated from the amount of N2 adsorbed at a relative pressure of 0.5 by assuming that all the mesopores were filled with condensed nitrogen in the normal liquid state. Pore size distribution was estimated using the Barrett−Joyner− Halenda (BJH) algorithm (ASAP-2010) available as built-in software from Micrometrics. TGA measurements were carried out under atmospheric air using a high-resolution TA Instrument, SDT Q600. About 15 mg of the CNTs was used in each experiment. The sample was heated in air at a heating rate of 10 °C min−1 in the temperature range from 35 to 1000 °C. The samples for TEM analysis were prepared by initially dispersing the CNTs and catalysts in ethanol or acetone by ultrasonication for 30 min and then allowed to settle. A drop of the supernatant liquid was then transferred onto a carbon coated copper grid and mounted onto the transmission electron microscope (JEOL 3010) operated at 300 kV, and the micrographs were recorded. Raman spectra were recorded with a Micro-Raman system RM 1000 Renishaw using a laser excitation wavelength of 532 nm (Nd:YAG), 0.5−1 mW, with 1 μm focus spot in order to avoid photodecomposition of the samples. The electrochemical experiments were carried out using an electrochemical workstation (Auto lab PGSTAT 302N). Cyclic voltammograms were recorded in a three electrode cell with Pt/CNTs coated FTO as working electrode, a standard calomel electrode as reference electrode, and a Pt as counter electrode. The electrolyte was composed of 10 mM LiI, 1 mM I2, and 0.1 M LiClO4 in acetonitrile solution.28 EIS spectra were recorded using symmetrical thin-layer cells composed of two identical electrodes separated by the electrolyte. The polymer spacer used in this case (Surlyn, Dupont) was 60 μm thick, and the electrolyte was injected using a vacuum backfilling process. The electrolyte composition was 0.1 mM LiI, 0.05 M I2, and 0.5 M 4-tertiary butyl pyridine in acetonitrile. The measurement was carried out using the AC impedance analyzer from 0.1 Hz to 100 kHz with perturbation amplitude of 10 mV. DSSCs were fabricated by clamping a dye-sensitized TiO2 electrode and CNTs/Pt counter electrode. Before clamping, the active area (0.25 cm2) was filled with similar electrolyte used in EIS

was spread over a quartz boat and placed at the center of the quartz tube in a temperature programmed CVD. The metalsupported catalyst was first prereduced at high temperature (650 °C) in pure hydrogen for 40 min and then purged with argon for another 30 min to remove any hydrogen. Immediately, the temperature was raised to the growth temperature in the presence of argon only and maintained for 30 min. The temperature of the furnace was then raised to the desired reaction temperatures (700, 800, and 900 °C) in the presence of argon flow. At this point, acetylene (40 mL/ min) mixed with argon gas (110 mL/min) was flown over the catalyst for 20 min before cooling in argon atmosphere to room temperature. For comparison purposes, the synthesis of CNTs was also carried out on monometallic catalyst Fe-SBA-15 and V-SBA-15. The weight of the carbon deposit yield along with the catalyst was determined. The following equation was used for the determination of the yield percentage of carbon deposit from the catalytic decomposition of acetylene. Carbon deposit yield (%) = (m tot − m cat)/m cat × 100 (1)

where mcat and mtot are the mass of the catalyst before and after the reaction, respectively. The synthesized carbon materials were treated with 10 N hydrofluoric acid at ambient temperature in order to remove the silica phase. Then, the sample was immersed in 6 N nitric acid solution to remove the metal catalysts, graphitic impurities, and amorphous carbon from large quantities of the as synthesized CNTs.26,27 In the final step of the purification process, the sample was air oxidized at 400 °C for 30 min during which the amorphous carbon and other graphitic impurities were removed. Finally, the purified CNTs were characterized by high resolution transmission electron microscopy (HRTEM), Raman spectra, thermogravimetric analysis (TGA), and X-ray diffraction (XRD). 2.4. Fabrication of Counter Electrode. CNTs were mixed with a 5 wt % solution of ethyl cellulose in terpineol by ultrasonication. This mixture was coated on an FTO glass substrate by a spin coating method and dried at 120 °C for 10 min in an air oven and subsequently sintered at 300 °C for 30 min. For comparison, Pt counter electrode was also prepared by the same method. H2PtCl6 was mixed with a 5 wt % solution of ethyl cellulose in terpineol by ultrasonication. This mixture was coated on an FTO glass substrate by a spin coating method and sintered at 385 °C for 15 min in an air oven. 2.5. Fabrication of Photovoltaic Device. The following method was employed for the fabrication of the photoanode. Three grams of TiO2 (P25, Degussa) powder and 0.5 mL of acetic acid was mixed in an agate mortar. Double distilled water (2.5 mL) and 15 mL of ethanol was consequently added dropwise into the agate mortar. The above mixture was then transferred into a beaker using 25 mL of ethanol and stirred for 1 h followed by ultrasonication for 30 min. Terpineol and ethyl cellulose in ethanol was then added into the beaker. Subsequently, the above mixture was kept for 24 h in an ultrasonic water bath at 28 °C to obtain a well-dispersed TiO2 paste. This paste was then coated onto the FTO glass substrate (0.25 cm × 0.25 cm) by the spin-coating technique and dried at 120 °C for 10 min. This step was repeated several times until the desired film thickness was reached. The TiO2 coated films were gradually heated to 500 °C and sintered at this temperature for 30 min. The TiO2 film employed in this study was of 8−10 μm thickness. This film was immersed in 50 386



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measurement. The photovoltaic performance parameters of DSSCs were measured under light intensity of 100 mW cm−2.

3. RESULTS AND DISCUSSION 3.1. Morphology of the Catalyst. Figure 1 compares HRTEM images obtained from different orientation of

Figure 2. (a) Low-angle X-ray diffraction patterns of the catalysts. (b) High-angle X-ray diffraction patterns of the catalysts.

Figure 1. HRTEM images of (a,b) bimetallic Fe-V-SBA-15 1:1, (c) FeV-SBA-15 3:1, (d) Fe-V-SBA-15 1:3, (e) monometallic Fe-SBA-15, and (f) V-SBA-15 catalysts.

calcined materials showed a broad diffraction peak at 2θ around 15−35°, which corresponded to amorphous silica walls of mesoporous materials. In addition, no crystalline phase appeared when the metal content was 6 wt %. It suggested that either the catalytic metal particles were uniformly dispersed within the SBA-15 silica matrix or no metal particles were grafted. However, the TEM analysis already confirmed that the metal ions were well dispersed in all of the catalyst. ICP-AES analysis and N2 sorption isotherm are listed in Table 1. All of the bimetallic Fe-V-SBA-15 3:1, Fe-V-SBA-15 1:1, and Fe-V-SBA-15 1:3 and monometallic Fe-SBA-15 and VSBA-15 catalysts present almost the same XRD pattern and N2 sorption isotherm. 3.3. N2 Sorption Isotherm of the Catalyst. As a primary measurement of the physical properties, nitrogen physisorption was conducted for the all the catalysts. Nitrogen sorption isotherms and corresponding pore size distribution are reported in Figure 3a. The Fe-V-SBA-15 with different molar composition of the metal, Fe-SBA-15 and V-SBA-15 mesoporous molecular sieves, exhibit type IV isotherms with H1 hysteresis loops, typical of mesoporous materials,20 while the sharp increase in the 0.6−0.8 P/P0 range were attributed to the capillary condensation within mesopores. The textural properties of the catalyst are also summarized in Table 1. The unit cell parameters (a0) and pore size distributions (PSDs) are independent of the metal loadings. On the other hand, the surface area, pore volume, and pore diameter decreased gradually with the metal loading in siliceous SBA-15 materials, while the trend for wall thickness was in the reverse. These results indicate that bimetallic and monometallic catalysts were incorporated in the SBA-15 silica framework without significantly blocking the mesopores. The

bimetallic Fe-V-SBA-15 1:1 with other ratios of bimetallic FeSBA-15 and monometallic Fe-SBA-15 and V-SBA-15. The HRTEM images of all the samples clearly showed the presence of highly ordered hexagonal arrays of one-dimensional mesopores, characteristic of SBA-15 silica support materials, for all catalyst supported silica matrix prepared by the atomic layer deposition (ALD) method. No evidence of bulk Fe2O3 and V2O5 crystallites condensed on the SBA-15 surface was observed, which further confirmed that the Fe and V species were thoroughly dispersed on the SBA-15 mesopore walls. 3.2. XRD Pattern of the Catalyst. The low-angle XRD patterns in Figure 2a show that SBA-15 exhibited three well resolved reflections at 2θ of 0.5 to 3°, including one strong (100) peak and two weak peaks (110) and (200). Grafting of bimetallic catalyst and monometallic catalyst into SBA-15 silica framework decreases the intensity which becomes noticeable when the metal content is 6 wt %, although the hexagonal regularity of the SBA-15 support was still maintained. The decrease in peak intensity suggested the irregular organization at long-range order of the mesoporous structure arising from either the incorporation of metal species into the channels or the collapse of the mesopores SBA-15. These results complement the HRTEM images which clearly show that highly ordered hexagonal mesoporous channels of SBA-15 were well sustained for all samples with metal loading. The above results suggest that the procedure employed for the grafting of metal into SBA-15 preserves the long-range order of the mesoporous structure probably due to the anhydrous conditions and hydrophobicity of the SBA-15 pore wall surface. The wide range angle X-ray diffraction patterns of the catalyst are shown in Figure 2b. In the wide-angle region, 387



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Table 1. Structural and Textural Properties of the Catalysts catalyst

unit cell parameter ao (nm)a

surface area (m2/g)b

pore size (nm)b

pore volume (cc/g)b

M contentc (wt %)

M contentd (wt %)

pure Si-SBA-15 Fe-SBA-15 Fe-V-SBA-15 3:1 Fe-V-SBA-15 1:1 Fe-V-SBA-15 1:3 V-SBA-15

12.6 11.9 12.4 12.0 11.9 12.1

810.32 753.06 750.68 751.17 739.89 746.61

25.18 24.56 24.52 24.49 24.46 24.48

0.81 0.68 0.69 0.66 0.68 0.67

6 6 6 6 6

5.7 5.8 5.8 5.6 5.7

a The values obtained from XRD analysis. bThe values obtained from N2 sorption studies. cM content used in the siliceous SBA-15 (M = Fe, V, and Fe-V). dM content measured by ICP-AES analysis.

Figure 4. Effect of reaction temperature on the amount of carbon deposited yield over the catalysts (catalyst, 100 mg; acetylene, 40 mL/ min; argon, 110 mL/min; hydrogen, 110 mL/min; reaction time, 20 min).

in controlling the growth density and diameter of the CNTs. In order to investigate the influence of reaction temperature, a series of experiments was carried out from 700 to 900 °C with interval of 100 °C under the conditions of constant growth time of 20 min and acetylene with a flow rate of 40 mL/min. As the temperature increased, the carbon deposition yield also increased rapidly and reached the maximum of 329% at 800 °C. Thus, at the lower growth temperature, for example, at 700 °C, the nanotubes formed were less dense due to not only the slow rate of carbon diffusion in the metal but also the slow carbon deposition rate. Another reason for the less dense CNTs could be the insufficient quantity of the metallic nanoparticles. The average diameter of CNTs was found to be 8−10 nm. The yield first increased dramatically with the temperature up to 800 °C which then witnessed a decrease with further increase in temperature. This decrease could be associated with the high vaporization of the carbon formed from decomposition of acetylene as well as agglomeration of catalyst particles at 900 °C. The above observation clearly indicated that the maximum carbon deposit yield was achieved at 800 °C. The decreases in the carbon deposit yield at the temperatures higher than 800 °C could also probably be due to the effect of the thermodynamic equilibrium of the acetylene decomposition reaction, which is disfavored at higher temperatures. The decrease of pore size in SBA-15 due to reducibility of the metal in the framework is also another factor which affects the maximum yield. The flow rate of acetylene plays a vital role in determining the yield of CNTs as well as other carbon impurities. Hence, the reaction temperature and time were maintained at the optimized conditions of 800 °C and 20 min, respectively, while the acetylene flow rate (10−60 mL/min) was varied. The optimized flow rate was found to be 40 mL/ min with the carbon yield being 329% of CNTs and associated

Figure 3. (a) N2 sorption isotherms of the catalysts. (b) Pore size distribution of the catalysts.

pore size distribution desorption isotherm branches are shown in Figure 3b. The pure siliceous SBA-15 showed a very sharp pore size distribution as obtained by dV/d (log D) (cm3/g/Ǻ ) according to the pore diameter. The pore size distribution in all catalysts, however, showed a monomodel shape, in which a smaller mesopore distribution was produced. 3.4. Optimization of Conditions for the Synthesis of CNTs with Higher Yield. CNTs grown over bimetallic Fe-VSBA-15 3:1, Fe-V-SBA-15 1:1, and Fe-V-SBA-15 1:3 and monometallic Fe-SBA-15 and V-SBA-15 catalysts and the carbon deposit yields are shown in Figure 4. It was noted that the CNTs grown over Fe-V-SBA-15 1:1 catalysts showed the narrowest diameter and the highest density. Fe-V-SBA-15 1:1 catalyst has greater ability to decompose unsaturated hydrocarbons like acetylene than the other catalyst. It possessed the optimum amount of metal to form metallic clusters leading to the growth of CNTs with desirable properties compared to the other catalysts. The variation of Fe and V composition in the catalysts was effective 388



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with less impurities like amorphous carbon. Contrary to the expectation that the amorphous carbon would not be formed at low acetylene flow rates (i.e., 10−30 mL/min), some amounts of amorphous carbon were observed and were attributed to the presence of some metal particles which were not bound to SBA-15. The amount of amorphous carbon was found to increase when the acetylene flow rate was increased above 40 mL/min (i.e., 50 and 60 mL/min, respectively), which may probably be due to the reduction of metal oxides to a metallic form, leading to the formation of larger particles. 3.5. Morphological Study of CNTs. Figure 5a−c shows the TEM images obtained for CNTs synthesized at the

Figure 6. TGA patterns of CNTs grown at various temperatures (700−900 °C) using Fe-V-SBA-15 1:1 as a catalyst.

Figure 7 shows the XRD patterns for the CNTs grown at 700, 800, and 900 °C under acetylene atmosphere with a flow

Figure 5. (a−c) TEM images and (d) HRTEM image of CNT synthesis over Fe-V-SBA-15 1:1 catalyst.

optimum temperature, 800 °C. It was clearly observed that the purified CNTs were actual one-dimensional tubular material, and the tube tip was opened. In addition, they were observed to possess uniform diameter which suggested that CNTs grown over bimetallic Fe-V-SBA-15 1:1 were of high quality. Figure 5d shows the HRTEM images of the wall of purified CNTs grown over bimetallic Fe-V-SBA-15 1:1. It was evident that the grown CNTs were MWCNTs having considerably thick tube walls and were well-graphitized consisting of ≈15 concentric shells of graphite layer. The walls were about 5 nm thick, and the central core was ≈5 nm. Herein, it was noted that, in the present preparation method of CNTs, the temperature of 800 °C favored the growth of thin, well-ordered, and well-graphitized tubes. 3.6. TGA and XRD Patterns of the CNTs. Figure 6 shows the TGA curves for the CNTs grown at 700, 800, and 900 °C under acetylene atmosphere with a flow rate of 40 mL/min. From the TG curves, the temperature at the initial weight loss was about 520 °C and the temperature at the final weight loss was about 660 °C. For the CNTs grown at 800 °C, the weight loss curve shifted toward a higher temperature, and the amount of residual catalyst decreased when compared to those grown at 700 and 900 °C. These results showed that most of the catalyst used for the growth of CNTs had been removed from the raw product during the purification by mild acid treatment. The remaining catalyst could be assigned to the metallic particles that were encapsulated by carbon layers and thus protected from acid attack. The TGA result indicated that the synthesized CNTs had high quality while their purity could reach up to 99.4% under these reaction conditions.

Figure 7. XRD of purified CNTs grown at various temperatures (700−900 °C) using Fe-V-SBA-15 1:1 as a catalyst.

rate of 40 mL/min. For the CNTs grown at 800 °C, the XRD results indicated that the intensity of graphite (002) increased and the interlayer spacing of the CNTs was close to the interlayer spacing of ideal graphite crystal (0.3354 nm). The spectrum displayed a strong peak at 2θ = 25.83° and a weak peak at 2θ = 43.71°, which were assigned to (002) and (100) diffraction patterns of a typical graphite, respectively. It was also observed that the intensity of the (002) plane of graphite reached the maximum intensity at 800 °C. Consequently, the optimum temperature to synthesize CNTs observed in this study was 800 °C. From the above TGA and XRD results, it was inferred that the optimum reaction temperature enhanced the graphitization degree of the synthesized CNTs and resulted in lower residual catalyst. 3.7. Raman Spectra of the CNTs. Figure 8 shows the Raman spectra taken for the CNTs grown at 700, 800, and 900 °C under acetylene atmosphere with a flow rate of 40 mL/min. All spectra showed mainly two bands at 1284 cm−1 (D band) 389



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3.8. Redox Behavior and Stability of CNTs. Figure 9a shows cyclic voltammograms (CVs) of I3−/I− redox system for

Figure 8. Raman spectra of CNTs grown at various temperatures (700−900 °C) using Fe-V-SBA-15 1:1 as a catalyst.

and 1541 cm−1 (G band). The G band indicates original graphite features, whereas the D band represents disorder features of graphite sheets.29 In general, the G band of MWCNTs will be appearing at ∼1580 cm−1; however, deviations were also reported in the literature30 as we observed in our case. In order to carry out line shape analysis on Raman spectra, we fitted G and D bands. The relative intensity of the D band was proportional to the amount of defects and other carbon impurities in the CNT sample. The intensity of the D band decreased dramatically with the increase in the purity of CNTs due to the preferential removal of amorphous carbon. Further, it was observed that few defects were generated on the side wall of CNTs except at their ends. The qualitative estimate of the structural purity of the nanotubes could be obtained by calculating peak intensity ratio of ID/IG. All spectra showed stronger G band peak intensities than their corresponding D band. This observation indicated that there was a higher degree of crystalline perfection of the CNTs grown on the Fe-V-SBA15 1:1 catalyst. This observation is in line with the information obtained from the TEM and HRTEM images. The linear relationship between the inverse of the in-plane crystallite dimension and the intensity ratio of the D band to the G band (ID/IG) was prominent.31,32 As the growth temperature increased from 700 to 900 °C, ID/IG also increased, indicating that the degree of long-range ordered crystallite perfection decreased with increasing growth temperature. For the CNTs grown at 800 °C, the intensity of the G band was higher than those grown at 700 and 900 °C. The higher intensity could be possibly obtained because of the growth of high quality CNTs. In contrast, it was clearly seen that the D band became much broader in the case of the CNTs grown at 900 °C which was due to the formation of disordered crystalline perfection of CNTs. The calculated peak intensity ratio ID/IG was found to be

Effective Synthesis of Well-Graphitized Carbon Nanotubes on

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