effective synthesis of carbon nanotubes of high purity over cr–ni–sba-15 and its application in...

Cite this: RSC Advances, 2013, 3,4321

Effective synthesis of carbon nanotubes of high purityover Cr–Ni–SBA-15 and its application in highperformance dye-sensitized solar cells3

Received 27th November 2012,Accepted 17th January 2013

DOI: 10.1039/c3ra23081c

www.rsc.org/advances

Jayaraman Balamurugan,a Rangasamy Thangamuthub

and Arumugam Pandurangan*a

The synthesis of carbon nanotubes (CNTs) by chemical vapour deposition (CVD) using 2D hexagonal

ordered mesoporous SBA-15 supported mono and bimetallic catalysts using chromium (Cr) and nickel (Ni)

were prepared by a post-synthesis grafting process. The structure of the synthesized well graphitized CNTs

with a uniform diameter was investigated by transmission electron microscopy, X-ray diffraction,

thermogravimetry and Raman spectroscopy. Studies performed to evaluate the quality and quantity of the

synthesized CNTs provide evidence in support of the superior catalytic nature of the bimetallic (Cr–Ni)

catalyst over the mono metallic (Cr/Ni) catalysts. The CNTs synthesized using Cr–Ni–SBA-15 catalysts were

coated on fluorine doped tin oxide conductive glass by spin-coating and evaluated as a counter electrode

for dye-sensitized solar cells (DSSCs). It was observed that the counter electrode based on CNTs exhibited a

photo conversion efficiency of 9.34%, which was slightly higher than that observed with a conventional Pt

counter electrode (9.09%). The lower charge transfer resistance and higher electrocatalytic activity of the

CNT counter electrode over the Pt counter electrode was confirmed by electrochemical impedance

spectroscopy and cyclic voltammetry, respectively. The studies showed that the CNTs synthesized over Cr–

Ni–SBA-15 could be employed as a counter electrode in DSSCs as a replacement for Pt.

1. Introduction

Dye-sensitized solar cells (DSSCs) were first reported byO’Regan and Gratzel in 1991.1 Since then, DSSCs haveattracted considerable attention as an alternative to conven-tional silicon solar cells because of the low fabrication cost.2

Moreover, the solar energy conversion efficiency of DSSCs witha platinum (Pt) counter electrode was found to be more than11%.3 In general, the role of the Pt counter electrode in DSSCsis the reduction of I3

2 to I2 due to its high electrocatalyticactivity. However, the scarcity along with the high cost of Pt isan impediment which needs to be overcome through suitablealternatives to improve the stability of the cell and lower theproduction cost. Some investigators have reported on Pt freecounter electrodes for DSSCs using different forms of carbonmaterials.4–9

Carbon nanotubes (CNTs) are a well known alternative forPt counter electrodes in DSSCs due to their large surface area,

high conductivity and chemical stability.1 Even though thecost of CNTs is lower than that of Pt, a lack of adhesion to thetransparent conducting oxide substrate poses a long-termstability risk.5 However, stability of the carbon-based counterelectrodes have been remarkably improved by adoptingnanocomposite electrode fabrication processes.10 Some ofthe methods previously employed for the fabrication of CNTcounter electrodes include doctor-blade, drop casting andspray coating.11,12 However, these methods are inconvenientsince the irregular arrangement of CNTs and their non-uniform dispersion adversely affect the performance of theDSSCs. Among these methods, spin-coating has been widelyused for device fabrication. For the growth of CNTs, the metalnano particles need to be fine with a diameter ranging from 5–7 nm, which act as the catalyst at high temperatures (from700–1000 uC). To synthesize such fine particles, inorganicmaterials with a high surface area have been frequentlyutilized as the support material.13 The support material helpsto form a number of metal particles and prevent theirsintering at high temperatures. The two main factors criticalfor the growth of CNTs over the supported catalysts are (i) thepresence as well as the size of the mesopores in the supportand (ii) the interaction between the metal particles and thesupport material.

aDepartment of Chemistry, Institute of Catalysis and Petroleum Technology, Anna

University, Chennai-600 025, India. E-mail: [email protected];

Fax: +91-44-22200660; Tel: +91-44-22358653bElectrochemical Materials Science Division, CSIR-Central Electrochemical Research

Insititute, Karaikudi-630 006, India

3 Electronic supplementary information (ESI) available: See DOI: 10.1039/c3ra23081c

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This journal is � The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 4321–4331 | 4321

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Porous materials have been extensively studied to preparefine metal nano particles in CVD. The main concerns includethe preparation of a number of metal particles inside themesopores and control of the nanotube diameter by the size ofthe mesopores. Ciuparu et al.13 reported the synthesis of singlewalled carbon nanotubes (SWCNTs) with a uniform diameterover Co metal-supported MCM-41. Zhao et al.14 reported thesynthesis of uniform diameter and high quality multi-walledcarbon nanotubes (MWCNTs) using a Ni–MCM-41 catalyst.Wang et al.15 have synthesized SWCNTs with a uniformdiameter (0.89–1.6 nm) over highly dispersed Co–SBA-15 witha pore size between 6.7 and 7.0 nm. In addition to the presenceof mesopores, the metal–support interaction is believed to playan important role in the growth of the nanotubes as the yieldof nanotubes strongly depend on the support material. Thepositive effect of the addition of Cr to a Co–MCM-41 catalyst inthe growth of SWCNTs was reported by other investigators.16

SBA-15 is an attractive support material because of itspotential to facilitate large-scale synthesis of CNTs which areof high quality. In addition, the silica support can be removedby treatment with relatively mild aqueous hydrofluoric acidsolution and subsequently washing with hydrochloric acid.17

There is a wide range of literature available on the metal–support interaction, which has been studied using substratessuch as SiO2, MCM-41 and SBA-15 as model systems.15,18,19

These studies show that the support material significantlyinfluences the catalytic activity of metal catalysts and thusunderlines the importance of the metal–support interaction.

The supported metal oxide catalyst was synthesized by apost-synthesis grafting method. This consists of two steps: (i)irreversible adsorption of the complex by hydrogen bonding orby a ligand exchange mechanism and (ii) decomposition of thegrafted complex in an oxygen-containing atmosphere atelevated temperatures. The process of this grafting method isvisualized in Scheme 1. There are two possible mechanisms for

the adsorption of the complexes: (i) through hydrogen bondingbetween an acetylacetonate ligand and the surface hydroxyls or(ii) by ligand exchange in which a covalent metal–oxygensupport bond is formed while an acetylacetonate ligand (Hacac)is lost. Van der Voort et al. have proved that the hydrogenbonding mechanism for the adsorption of VO(acac)2 on silicatakes place.20 Some more research work also confirmed thismechanism for the adsorption of metal acetylacetonate.21,22

Wang et al.15 reported the catalytic activity of highly dispersedCo particles supported on SBA-15. It was unclear as to how thepresence of the mesopores or the metal support interactioncontributed to reduce the actual particle size of the metalnanoparticles, which in turn influences the growth of CNTs. Themono metallic Fe–MCM-41, Co–SBA-15 and bimetallic Fe/Cr–MCM-41, Mn–Co–MCM-41 are reported as catalysts for CNT growth.23–26

However, the resultant CNTs derived from such methods have notbeen investigated as counter electrodes in DSSCs.

The present study focuses on the synthesis of CNTs by CVDusing low-cost acetylene as a carbon source and Cr–SBA-15, Ni–SBA-15 and Cr–Ni–SBA-15 mesoporous molecular sieves ascatalytic templates. These catalysts were characterized by X-raydiffraction, nitrogen (N2) adsorption isotherms and TEM, whichare used to evaluate the structure of Cr, Ni and Cr–Niincorporated SBA-15. Furthermore, the CNTs were characterizedby TEM, Raman spectra, X-ray diffraction and thermogravimetricanalysis. The CNTs thus optimized were used as the counterelectrode in DSSCs and their performance was compared withthat of DSSCs with a conventional Pt counter electrode.

2. Experimental

2.1 Synthesis of catalysts

The siliceous SBA-15 was synthesized according to an earlierreport.27 In a typical synthesis, 4 g triblock poly(ethylene

Scheme 1 Plausible mechanism of Mn+ (acac)n grafted on SBA-15.

4322 | RSC Adv., 2013, 3, 4321–4331 This journal is � The Royal Society of Chemistry 2013

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glycol)–poly(propylene glycol)–poly(ethylene glycol)(EO20PO70EO20, Pluronic P123, Aldrich) was dissolved in amixture of 30 mL distilled water and 120 mL 2 M hydrochloricacid (Merck) under stirring. 8.5 g tetraethyl orthosilicate(TEOS, 98%, Aldrich) was added dropwise to the solution at 40uC. After being stirred continuously for 24 h, the mixture wastransferred to a teflon lined stainless steel autoclave andplaced in an oven at 100 uC for 48 h. The precipitate wasfiltered, washed with distilled water, ethanol and dried at 80uC for 12 h. The obtained powder was heated from roomtemperature to 550 uC at a ramp rate of 1 uC min21 under N2

flow for 1 h and then calcined at 550 uC under N2 flow for 6 hand in atmospheric air to eliminate the template fromsiliceous SBA-15 for 2 h.

The Cr, Ni and Cr–Ni grafted SBA-15 catalysts weresynthesized following the controlled post-synthesis graftingprocess. The most suitable sources for Cr and Ni arechromium(III) acetylacetonate and nickel(II) acetylacetonate(Aldrich, 99%), since these precursors show a higher disper-sion and a higher reduction stability than other sources. Thecalcined pure siliceous SBA-15 was suspended in 80 mLanhydrous toluene and refluxed for 3 h under N2 flow toremove adsorbed water. The Cr precursor solution wasprepared by dissolving chromium(III) acetylacetonate in 150mL anhydrous toluene. The solution was refluxed at 110 uC for3 h under N2 flow and used for grafting. The Cr precursorsolution was added into the siliceous SBA-15 toluene suspen-sion and the mixture was refluxed for 3 h under N2 flow toremove the physisorbed water and to prevent contact withmoisture. Thus, the nitrogen flow was continued throughoutthe reaction. The reaction mixture was cooled, filtered andwashed with toluene to ensure no unreacted metal precursorwas left on the SBA-15. The solid was then dried overnight inambient air to yield Cr–SBA-15. The Cr–SBA-15 was calcinedfollowing the procedure of siliceous SBA-15. Similarly, theprocedure was followed to synthesize Ni–SBA-15 and Cr–Ni–SBA-15.

2.2 CNTs synthesis and purification

CNTs were synthesized by using CVD according to thefollowing procedure. The catalyst (100 mg) was placed in aquartz tube of 30 mm diameter and heated to temperaturesranging from 700–1000 uC with a constant heating rate of 5 uCmin21 under argon flow (110 mL min21). After reaching therequired temperature, the argon gas was switched to a mixtureof acetylene (40 mL min21) and hydrogen (110 mL min21) atatmospheric pressure. The reaction time was varied from 10–40 min. After the reaction, the system was cooled to roomtemperature under argon flow (110 mL min21). The followingeqn (1) was used for determination of the percentage of carbondeposited from the catalytic decomposition of acetylene.

Carbon deposit yield (%) = (mtot 2 mcat)/mcat 6 100 (1)

Where mcat and mtot are the mass of the catalyst before andafter the reaction, respectively.

In order to separate the template and CNTs, the synthesizedimpure CNTs were ultrasonicated for 1 h with 40% hydro-fluoric acid at ambient temperature and the obtained CNTs

were separated by filtration. The CNTs were then washed withdistilled water and dried at 80 uC for 24 h. The sample wassubjected to reflux at 120 uC in 4 M nitric acid for 12 h in orderto remove the amorphous carbon and residual metal particles.In the final step of the purification process, the sample wassintered in air at 400 uC for 60 min to remove amorphouscarbon from the CNTs. The purified CNTs were characterizedby TEM, X-ray diffraction, thermogravimetric analysis andRaman spectroscopy.

2.3 Fabrication of the counter electrode and working electrode

About 100 mg purified CNTs were ultrasonically dispersed interpineol containing 5 wt.% ethylcellulose for 1 h.28 Theresulting mixture was spin-coated onto a fluorine doped tinoxide (FTO) glass substrate at 1000 rpm for 20 s, dried at 120uC for 20 min and sintered at 300 uC for 20 min and the abovecoating was repeated to get required thickness. A platinizedcounter electrode was prepared by spin coating using 5 mMH2PtCl6 dispersed in terpineol containing 5 wt.% ethylcellu-lose. The spin coated thin-film was heated at 385 uC for 20min.

A TiO2 thin film photo-anode (working electrode) isfabricated by a spin coating technique for DSSCs. The detailedfabrication practice is as follows: 0.5 mL acetic acid and 3 gTiO2 (P25, Degussa) powder is mixed in an agate mortar for 5min. 2.5 mL double distilled water and 15 mL ethanol areintroduced drop by drop into the agate mortar. The abovemixture is transferred into a beaker using 25 mL ethanol andstirred for 1 h followed by ultrasonication for 30 min.Terpineol and ethyl cellulose in ethanol is added into thebeaker. Subsequently, the above mixture is kept for 24 h in anultrasonic water bath at 28 uC to obtain a well-dispersed TiO2

paste. This paste is spin coated on a cleaned FTO conductingsubstrate with the guidance of 3 M scotch tape at 1000 rpm for20 s and dried at 60 uC for 10 min in a hot air oven. The TiO2

spin coated FTO is sintered at 500 uC for 30 min. The filmthickness was about 8–10 mm. The fabricated TiO2 electrodewas immersed into a solution of 0.25 mM N-719 dye(Ruthenium, RuL2 (NCS)2) (N719, Aldrich) in ethanol for 12h at room temperature. The active area of the cell wasmaintained as 0.25 cm2.

2.4 DSSC assembly

The DSSC was assembled using a nanocrystalline TiO2 thin-film as the working electrode and CNTs or Pt as a counterelectrode. The two electrodes were sandwiched together with a60 mm thick Surlyn polymer film (Solaronix). A liquidelectrolyte was filled into the cell. The liquid electrolyte wasa mixture of 0.6 M 1-methyl-3-propylimidazolium iodide (MPI),0.1 M anhydrous lithium iodide (LiI), 0.05 M iodide (I2) and 0.5M tert-butylpyridine (TBP) in acetonitrile.

2.5 Characterization

The amount of the metal loaded into the SBA-15 was analyzedand determined by ICP-AES (Perkin Elmer OPTIMA 3000). Thesample was dissolved in a mixture of hydrofluoric acid andnitric acid before the measurement. The X-ray diffractionpatterns of the samples were recorded with a PANalyticalX’Pert diffractometer, using nickel-filtered Cooke radiation (l


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= 1.54 Å) and a liquid nitrogen-cooled germanium solid-statedetector. The diffractograms were recorded in the 2h range0.5–10u for the catalyst and 5–80u for the CNTs, respectively.The peaks were identified with reference to the compilation ofsimulated X-ray diffraction patterns. The surface area, porevolume and pore size distribution were measured by N2

sorption at 2197 uC using an ASAP-2010 porosimeter fromMicromeritics Corporation. The samples were degassed at 200uC and 1.3 6 1023 Pa 8–10 h prior to the adsorptionexperiments. The mesopore volume was estimated from theamount of N2 adsorbed at a relative pressure of 0.5 byassuming that all the mesopores were filled with condensednitrogen in the normal liquid state. The pore size distributionwas estimated using the BJH algorithm (ASAP-2010) availableas built-in software from Micromeritics. Thermogravimetricanalysis measurements were carried out in air using a high-resolution TA Instrument, SDT Q600. About 15 mg of thesynthesized catalyst and CNTs were used for each experiment.The sample was heated in air at a rate of 20 uC min21 in thetemperature range from 35 to 1000 uC. SEM was performed onJEOL with a beam energy of 4 kV. The samples were suspendedin ethanol or acetone and the specimen stub was dipped intothe liquid and then removed. The powder obtained was evenlydeposited onto the surface of the stub when ethanolevaporated. This specimen was coated with gold for 2 minusing an ion sputter coater. The samples for TEM analysiswere initially dispersed in ethanol or acetone by ultrasonicat-ing for 30 min, which was then allowed to settle. A drop of thesupernatant liquid was then transferred onto a carbon coatedcopper grid and mounted onto the TEM (JEOL 3010) operatedat 300 kV and the micrographs recorded. Raman spectra wererecorded with a Micro-Raman system RM 1000 Renishawusing a laser excitation wavelength at 532 nm (Nd-YAG), 0.5 to1 mW, with a 1 mm focus spot in order to avoid photo-decomposition of the samples.

Electrochemical experiments were carried out using anelectrochemical workstation (Autolab PGSTAT 302N). Cyclicvoltammograms were recorded in a three electrode cellassembly. Pt or CNT coated FTO was used as the workingelectrode, saturated calomel electrode and Pt were used as thereference and counter electrodes, respectively. The electrolyteis composed of 10 mM LiI, 1 mM I2, and 0.1 M lithiumperchlorate (LiClO4) in acetonitrile solution.29 For the EISmeasurement, a thin layer symmetric cell was fabricated byclamping two identical CNTs or Pt coated FTO electrodes oneach other with parafilm laboratory film as a spacer. Theelectrolyte composition of 0.1 mM LiI, 0.05 M I2 and 0.5 M4-tertiary butyl pyridine in acetonitrile was filled between thetwo electrodes before clamping. The measurement was carriedout using an AC impedance analyzer from 0.1 Hz to 100 kHzwith a perturbation amplitude of 10 mV. DSSC was fabricatedby clamping a dye-sensitized TiO2 electrode and a CNT or Ptcounter electrode. A surlyn thermosetting spacer was keptbetween the photo-anode and counter electrode. Beforeclamping, the active area (0.25 cm2) was filled with the sameelectrolyte used in the EIS measurements. The photovoltaicperformance of the DSSCs was measured under a lightintensity of 100 mW cm22.

3. Results and discussion

3.1 X-ray diffraction pattern of catalysts

Fig. 1 shows the low-angle X-ray diffraction patterns of puresiliceous SBA-15, Cr–SBA-15, Ni–SBA-15 and Cr–Ni–SBA-15.The samples exhibited three well resolved peaks, which wereindexed as (1 0 0), (1 1 0) and (2 0 0) characteristics of thep6mm hexagonal mesostructure of SBA-15.30 With an increasein the metal loading, the intensity of the (1 0 0) peakweakened, which was a result of the blocking of SBA-15mesopores by metal particles and a decline of the long-rangeorder of the hexagonally arranged porosity. However, the (1 10) and (2 0 0) peaks were still detectable for all the samples.The results indicated that the SBA-15 structure was retainedafter grafting Cr, Ni and Cr–Ni into SBA-15 mesoporousmolecular sieves. We propose the following mechanism toexplain how the grafting of metal (MLCr or Ni or Cr–Ni) ontoSBA-15 silica pore walls occurs. As shown in Scheme 1, metalacetylacetonate (Mn+(acac)n) is anchored onto the SBA-15support via silanol groups and subsequently the adsorbedMn+(acac)n decomposes to form MOx species.

Fig. 2 shows the wide angle X-ray diffraction patterns of puresiliceous SBA-15, Cr–SBA-15, Ni–SBA-15 and Cr–Ni–SBA-15. Asshown in Scheme 1, metal grafting via this modified post-synthesis grafting process relies on silanol groups. Therefore,it is reasonable to suggest that metal species are incorporatedinto the SBA-15 channels due to the preferential reactionbetween metal precursors and silanol groups on the pore wallsurfaces. It was observed that the samples had a broad peak,suggesting that the synthesized SBA-15 was amorphous innature. There were no X-ray diffraction peaks assigned tocrystalline metal oxide in Cr–SBA-15, Ni–SBA-15 and Cr–Ni–SBA-15 catalysts, which indicated the high dispersion of Cr, Niand Cr–Ni into the SBA-15 framework.

3.2 N2 sorption isotherm of catalysts

N2 sorption isotherms of pure siliceous SBA-15, Cr–SBA-15, Ni–SBA-15 and Cr–Ni–SBA-15 are given in Fig. 3. The Cr, Ni and

Fig. 1 Low-angle X-ray diffraction patterns of mesoporous catalysts.


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Cr–Ni surface densities of each catalyst were calculated basedon their surface area and metal loading. It was observed thatpure siliceous SBA-15 exhibited a high surface area, large porediameter and high pore volume. The loading of Cr, Ni and Cr–Ni grafted into SBA-15 caused a significant decline in itssurface area. It was obvious that the surface area decrease wasdue to the blocking of SBA-15 pores by metal particles. Withthe increase of metal loading, there was an extension of the Cr,Ni and Cr–Ni surface density and a slight decrease in theaverage pore diameter and pore volume. Similar results havebeen reported.30,31 It was observed that the N2 sorptionisotherms were similar in the samples with a hysteresis loopappearing in the relative pressure ranging from 0.65–0.85 andit belongs to type IV isotherms. The presence of well orderedcylindrical pores in the resultant materials is also confirmed.

The pore size distribution of SBA-15 was narrow, butbroadening occurred when SBA-15 was loaded with Cr, Ni

and Cr–Ni, resulting in a drop in the pore diameter and porevolumes, as shown Fig. S1, ESI.3

3.3 ICP-AES analysis of the catalysts

The wt% of Cr, Ni and Cr–Ni content of the material preparedby grafting was estimated by ICP-AES and the results are givenin Table 1. From the result it was established that more than96% of the metal particles were loaded into the silica SBA-15mesopores.

3.4 TEM images of catalysts

The TEM technique is a powerful tool for the directobservation of pore architectures of porous solid materials.In recent years, a number of investigations on catalystpreparation and characterization have provided novel insightsinto the dependence of catalytic activity on the morphologyand porosity of the catalyst particles.32–34 The TEM images ofCr–SBA-15, Ni–SBA-15 and Cr–Ni–SBA-15 catalysts are shownin Fig. 4. The results are in agreement with previousreports.33,34 A morphological evaluation of the SBA-15 particleswas carried out only after the loading of Cr, Ni and Cr–Ni. Thisimplied that the interaction between Cr, Ni and Cr–Ni andsilica were strong. It can be proposed that during calcinationof Cr, Ni and Cr–Ni precursors in oxygen at 550 uC,chromium(III) acetylacetonate, nickel(II) acetylacetonate andchromium(III) acetylacetonate–nickel(II) acetylacetonate inter-act with the Si–OH of siliceous SBA-15 to form Si–O–M (MLCr/Ni/Cr–Ni) and M–O–M linkages due to the weakening of Si–O–Si bonds and the breaking of long interconnected chains. Thenumber of Si–O–M and M–O–M linkages increases as the Cr,Ni and Cr–Ni loading increases.

The generation of hexagonal silica was due to the contrac-tion of Si–O–M and M–O–M linkages when the Cr, Ni and Cr–Ni oxide particles were being formed and the change in porestructure was due to the formation of the hexagonal structureof SBA-15. Such variation in the surface morphology and porestructure at higher loadings of Cr, Ni and Cr–Ni deservesfurther investigation. From the SEM observation (Fig. S2, ESI3),the silica SBA-15 exhibited a typical long-chain structure ofseveral micrometers in length and the diameter of the porestructure was regular and highly ordered with pore size anduniform wall thickness. The Cr, Ni and Cr–Ni grafting intoSBA-15 breaks the long-chain architecture into separate unitsof banana-like rods, which are well in agreement with Liuet al.32 but the well-defined mesoporous channels were stillretained after grafting of Cr, Ni and Cr–Ni into SBA-15.

3.5 Optimization conditions for the synthesis of CNTs with ahigh yield

The growth of CNTs was carried out using Cr–SBA-15, Ni–SBA-15 and Cr–Ni–SBA-15 catalysts at 800 uC for 20 min in thepresence of acetylene with a flow rate at 40 mL min21. It isillustrated in Fig. 5 that the activity of the catalysts was foundto be in the order: Cr–Ni–SBA-15 . Ni–SBA-15 . Cr–SBA-15during the growth of CNTs.

The highest carbon deposit yield was about 344% whileusing Cr–Ni–SBA-15, compared to mono-metallic Cr–SBA-15and/or Ni–SBA-15 catalysts. Whether the metal migratesthrough the silica framework as an anion or a metal atom isFig. 3 Nitrogen sorption isotherms of mesoporous catalysts.

Fig. 2 Wide angle X-ray diffraction patterns of mesoporous catalysts.


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not yet understood and the clarification of this mechanism isbeyond the scope of this study. However, once the reducedamount of atoms reached the pore wall surface they started tonucleate into clusters, which increased in size before theyinitiated the growth of CNTs or other types of carbon on theirsurface. Under certain reaction conditions, very small metalclusters which are not active for CNT growth would be presentin the pores. It was also noted that the growth of metal clustersprobably ceased once their surface was covered with carbon.Therefore, the optimum size and the size distribution of themetal clusters are controlled by the relative rates of severalphysical and chemical processes that affect the structure andstate of the catalysts during CNT synthesis. This is likely theminimum size required for the metallic metal cluster toinitiate the growth of CNTs.

In order to investigate the effect of temperature on theactivity of the catalyst, CNTs were synthesized over a Cr–Ni–SBA-15 catalyst at temperatures of 700, 800, 900 and 1000 uC.The yields of CNTs were 258%, 344%, 309% and 281% attemperatures of 700, 800, 900 and 1000 uC, respectively. As thetemperature increased, the average diameter of the CNTs

increased from 8–10 nm and their diameter distributionbecame broader. The yield first increased with the temperatureup to 800 uC and then decreased with a further increase intemperature. The decrease might be associated with the highvaporization of the carbon formed from decomposition ofacetylene as well as agglomeration of catalyst particles at 900and 1000 uC. This observation clearly showed that themaximum CNT yield was obtained at 800 uC. The smallerpore size in SBA-15 due to reducibility of the metal in theframe work is also another factor that affects the maximumyield. As the metal cluster initiates the growth of CNTs, it iscovered with the carbon, but it cannot be further sintered intometallic clusters. At 800 uC, before metallic cluster formation,the metal nanoparticles initiate the growth of CNTs. As thesize of the metal clusters increases, their selectivity for CNTgrowth decreases and also leads to the formation of a surfacegraphite layer, which makes them inactive for acetylenedecomposition.

It is well known that the flow rate of acetylene plays a vitalrole in determining the yield of CNTs and also other carbonimpurities. The temperature and time were fixed as 800 uC and20 min, respectively and the flow rate of acetylene was variedfrom 10 to 80 mL min21, as shown in Fig. S3, ESI.3 Theoptimized flow rate was found to be 40 mL min21, where thecarbon yield was 344% of CNTs with a few impurities like

Table 1 Structural and textural properties of mesoporous catalysts

CatalystUnit cellparameter ao (nm)a

Surface area(m2 g21)b

Pore size(nm)b

Pore volume(cc g21)b

M contentc

(wt.%)M contentd

(wt.%)

Pure Si–SBA-15 12.4 826 27 0.79 — —Cr–SBA-15 11.8 760 24 0.65 6 5.8Ni–SBA-15 11.9 756 24 0.64 6 5.7Cr–Ni–SBA-15 11.7 759 24 0.63 6 5.8

a The values obtained from X-ray diffraction analysis. b The values obtained from N2 sorption studies. c M content used in the siliceous SBA-15 (MLCr, Ni and Cr–Ni). d M content measured by ICP-AES analysis.

Fig. 4 TEM images of mesoporous catalysts: (a) Cr–SBA-15, (b) Ni–SBA-15 and(c) Cr–Ni–SBA-15.

Fig. 5 Effect of reaction temperature on the amount of carbon deposit yieldedover Cr–SBA-15, Cr–Ni–SBA-15 and Ni–SBA-15 catalyst (catalyst: 100 mg, C2H2:40 mL min21, argon: 110 mL min21, hydrogen: 110 mL min21 and reactiontime: 20 min).


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amorphous carbon, graphitic impurities and non carbonac-eous impurities. It was expected that the amorphous carbonwould not be formed when a low flow rate of acetylene (i.e. lessthan 40 mL min21) is employed. From these results it can befound that some amount of amorphous carbon, graphiticimpurities and non carbonaceous impurities are present evenat this low flow rate of acetylene, since some metal particleswere not bound to SBA-15. The amount of amorphous carbon,graphitic impurities and non carbonaceous impurities werefound to increase when the acetylene flow rate was increasedabove 40 mL min21 (i.e. 50, 60, 70 and 80 mL min21

respectively), which was probably due to the reduction ofmetal oxides to a metallic form, leading to the formation oflarger particles.

3.6 TEM images of CNTs

The TEM images of purified CNTs synthesized over Cr–Ni–SBA-15 are shown in Fig. 6. The TEM study shows that CNTssynthesized over a bimetallic Cr–Ni–SBA-15 catalyst dependedon the size of the metal particles and the diameter of thenanotubes varied from 8–10 nm, whereas the length of thenanotubes reached several microns. The nanotubes grow dueto the finely dispersed metal particles located at their ends.For larger metal particles the diameter of the CNTs changesduring their growth. The initial diameter of such nanotubes is.20 nm. However, eventually it decreases to 8–10 nm. Itshould be specially noted that the internal volume of the CNTsbecomes partially filled with the metal. Thus, the CNTssynthesized over the Cr–Ni–SBA-15 catalyst are quite uniformwith respect to their diameters.

3.7 Raman spectra of CNTs

In general, Raman spectra are characterized by three mainsignals: G band, D band and radial breathing mode (RBM).

Only two Raman bands appear in the high wave numberregion at 1305.21 cm21 and 1580.99 cm21, indicating that theCNTs synthesized in this study are most likely to be multi-walled carbon nanotubes (MWCNTs). The absence of radialbreathing modes in the lower wave number region (below 400cm21) also confirms the formation of MWCNTs. The RBMpeaks originate in CNTs with small diameters of less than 3nm, and their position was strongly dependent on thenanotube diameter. The Raman spectrum of CNTs synthesizedover Cr–Ni–SBA-15 at the optimized reaction conditions (T =800 uC; flow rate of acetylene = 40 mL min21 and time = 20min) is shown in Fig. 7. This feature is quite distinct from thatof MWCNTs, which showed no low frequency modes. Anotherregion in the high frequency displayed two characteristicbands such as the G band and D band, which were related tothe graphite in-plane vibration with an E2g symmetry intralayermode35 and the defect on the nanotubes, respectively.36

Therefore, the relative intensity of the D band was propor-tional to the amount of defects and other carbon impurities inthe CNT sample. Fig. 7 shows that the intensity of the D banddecreases drastically with the increase in quality of the CNTsdue to the preferential removal of amorphous carbon. In otherwords, few defects were generated on the side wall of the CNTsexcept at the ends. Thus the increase of the G–D ratio wasessentially assigned to the reduced amount of carbonimpurities in the sample. The purified CNTs exhibited a betterelectrical performance than impure and defective CNTs.

3.8 Thermogravimetric analysis of CNTs

The thermogravimetric analysis of the purified CNTs is shownin Fig. 8. Thermogravimetric analysis provides a straightfor-ward characterization method in which the thermal stability ofCNTs can be measured qualitatively. The weight loss duringheat treatment is due to the burning of carbonaceousimpurities and amorphous carbon. Purified CNTs with fewerresidues exhibit the highest decomposition rate at round 550–600 uC. The thermal stability of CNTs depends on the defectson the side walls and the impurities in the sample. The highquality of CNTs with few defects and low amounts of

Fig. 6 TEM images of purified CNTs synthesized over Cr–Ni–SBA-15 withdifferent magnifications.

Fig. 7 Raman spectrum of purified CNTs synthesized over Cr–Ni–SBA-15.


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impurities contributed to the high thermal stability.Quantitative characterization of the thermogravimetric analy-sis with Raman spectroscopy indicated that high purity CNTscan be achieved.

3.9 X-ray diffraction pattern of CNTs

The X-ray diffraction patterns of the purified CNTs are shownin Fig. 9. The diffraction pattern shows a strong peak at 2h =24.90u and a weak peak at 2h = 43.32u, which are assigned to(002) and (100) diffraction patterns of typical graphite. Thecarbonaceous impurities and metal particle peaks are absentin the CNTs from the X-ray diffraction pattern. This resultindicates that the CNTs are a highly crystalline and well-graphitized structure.

3.10 Electrochemical measurement

The relationship between ion diffusivity and reaction kineticsof an electrochemical system is analyzed by cyclic voltamme-

try. Two pairs of redox waves are observed in Fig. 10a. The twoanodic and cathodic peaks represent the following reactions(eqn (2) and (3)):

3I2 « I32 + 2e2 (2)

I32 + 2e2 « 3I2 (3)

The relative positive pair was assigned to the redox reactionof I2/I3

2 and the negative was associated with the reaction ofI3

2/I2. The redox current density of I32/I2 in CNT electrode

was much higher than those in the Pt electrode, indicatingthat the former electrode has a stronger electrocatalytic activitytowards the reduction of I2 to I2.

Fig. 10b shows 200 consecutive cyclic voltammograms inacetonitrile solution containing 0.1 M LiClO4, 10 mM LiI, 1mM I2. The cyclic voltammograms showed no change andexhibited stable peak currents, indicating the CNT fabricatedFTO possessed excellent electrochemical stability.

Electrochemical impedance spectra (EIS) was used toevaluate the catalytic activity of the CNT or Pt counterFig. 9 X-ray diffraction of purified CNTs synthesized over Cr–Ni–SBA-15.

Fig. 10 (a) Cyclic voltammograms for Pt or CNT electrodes in 0.1 mM LiI, 0.05 MI2 and 0.5 M 4-tertiary butyl pyridine in acetonitrile at a scan rate of 50 mV s21;(b) 200 consecutive cyclic voltammograms of the CNT electrode in 0.1 mM LiI,0.05 M I2 and 0.5 M 4-tertiary butyl pyridine in acetonitrile. Scan rate 50 mV s21.

Fig. 8 Thermogravimetric analysis of purified CNTs synthesized over Cr–Ni–SBA-15.


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electrode towards the reduction of I32 to I2. The EIS of CNT/Pt

based symmetric cells are compared in Fig. 11. It was seen thatthe charge-transfer resistance (Rct) of the Pt electrode washigher than the Rct of the CNT electrodes, as presented inTable 2. The Rct change in this study essentially refers to thecomponent of resistance against electron transfer from thecounter electrode to I3

2. The smallest Rct value of the CNTsassociates well with the highest Jsc value of the correspondingDSSCs. The EIS results indicate that the CNTs efficientlycatalyze the reduction of I3

2 to I2. The high surface area of theCNTs improves the catalytic activity of the counter electrode.

3.11 Photovoltaic performance of dye-sensitized solar cells

Fig. 12 shows the current–voltage performance of CNT and Ptcounter electrode DSSCs in the different levels of simulatedsolar illumination. Under 1 sun illumination (100 mW cm22,air mass 1.5 G), the j–V curves of DSSCs based on CNTs andstandard Pt counter electrode values were compared. The opencircuit potential (Voc), short circuit current density (Jsc), FF andconversion efficiency (g) of cells are listed in Table 2. It isknown that the fill factor (FF) depends on the resistances ofthe photoanode, electrolyte and counter electrode. Theincreased fill factor of the DSSCs with a CNT electrode,compared with those of cells with a Pt counter electrode, isdue to a higher charge-transfer rate and lower sheet resistance,which is supported by EIS measurements. The performance ofDSSCs fabricated using our MWCNT based counter electrodeis comparable with a conventional Pt counter electrode and

out-performs most of the carbon based counter electrodesreported in the literature.37–40 The origin of such a highelectrocatalytic activity of CNT electrodes can be attributed tothe large surface area provided by the CNTs.41–43 Theadvantages of our approach are that device grade MWCNTscan be conveniently synthesized by an economically viable low-cost route using a Cr–Ni–SBA-15 catalyst.

4. Conclusions

A bimetallic Cr–Ni–SBA-15 mesoporous catalyst was preparedby post-synthesis grafting for the effective synthesis of CNTs. Ahigh yield with more uniform CNTs was obtained under theoptimized conditions at 800 uC with a 20 min reaction period,argon and acetylene gas flow rates of 110 and 40 mL min21,respectively. The maximum carbon deposit yield was about344% for Cr–Ni–SBA-15 when compared with other catalysts.The TEM images showed the purified CNTs were free fromamorphous carbon, graphitic impurities, silica and metalparticles. The high quality of the purified CNTs wasdetermined by the high G–D ratio and good thermal stabilityfrom the Raman spectroscopy and thermogravimetric analysis,respectively. The Cr–Ni incorporated SBA-15 framework wasfound to be thermally stable at high temperature (800 uC),allowing slow reduction and nucleation of metallic clusters

Fig. 11 Impedance spectra of CNT/Pt symmetric cell.

Table 2 Electrochemical and photovoltaic parameters of DSSCs with a CNT/Pt counter electrode

Sample Light intensity (mW cm22) Rct (V) Jsc (mA cm22) Voc (V) FF g (%)

CNTs 10 1.98 1.97 0.72 0.71 9.8350 8.61 0.75 0.75 9.64100 15.64 0.76 0.79 9.34

Pt 10 2.29 1.76 0.74 0.73 9.4650 8.20 0.76 0.74 9.23100 15.05 0.77 0.78 9.09

Fig. 12 Photocurrent voltage (j–V) performance of DSSCs fabricated using CNTand Pt counter electrodes at different levels of simulated solar illumination.


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with a high carbon yield and high selectivity of CNTs. Theseresults suggest that the Cr–Ni dispersed in SBA-15 would be apromising catalytic template for the synthesis of high qualityCNTs. Investigation of the performance of DSSCs with counterelectrode fabrication using the synthesized CNTs by spin-coating were also studied. When the pure, uniform diameterand well-graphitized CNTs were used as a counter electrode,the conversion efficiency of the cell achieved is 9.34% underthe 1 sun conditions. The large surface area and highelectronic conductivity of CNTs might have contributed tothe lower Rct value. Thus, the CNTs synthesized by this methodcould be employed as a counter electrode in DSSCs byeffectively replacing Pt.

Acknowledgements

One of the authors, J. Balamurugan, acknowledges the DST(SR/S5/NM-35/2005) and UGC (603/PD6/2007), New Delhi forthe award of a Research Fellowship and is thankful to theDepartment of Chemistry & Institute of Catalysis andPetroleum technology, Anna University, Chennai. Dr R.Thangamuthu is grateful to CSIR-CECRI and the Director DrVijayamohanan K. Pillai for the financial support of this workthrough OLP 0068 start-up project. The authors thank theDirector, CSIR-CECRI, for permitting the collaborativeresearch work and his constant encouragement.

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effective synthesis of carbon nanotubes of high purity over cr–ni–sba-15 and its application in...

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