growth of carbon nanotubes over transition metal loaded on co-sba-15 and its application for high...

Journal ofMaterials Chemistry A

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aDepartment of Chemistry, Institute of Ca

University, Chennai, 600 025, India. E-mail

44-22200660; Tel: +91-44-22358653bElectrochemical Materials Science Division

Institute, Karaikudi, 630 006, India

† Electronic supplementary informa10.1039/c3ta00140g

Cite this: J. Mater. Chem. A, 2013, 1,5070

Received 10th September 2012Accepted 8th February 2013

DOI: 10.1039/c3ta00140g

www.rsc.org/MaterialsA

5070 | J. Mater. Chem. A, 2013, 1, 50

Growth of carbon nanotubes over transition metalloaded on Co-SBA-15 and its application for highperformance dye-sensitized solar cells†

Jayaraman Balamurugan,a Rangasamy Thangamuthub

and Arumugam Pandurangan*a

High quality MWCNT material based counter electrodes for dye-sensitized solar cells (DSSCs) were

fabricated using a novel route and their power-conversion efficiency was studied. Transition metals such

as Fe, Ni, V, Mn, Cr, Mo, Ru and Pd loaded on Co-SBA-15 molecular sieves were synthesized and tested

for the formation of MWCNTs at different temperatures (700–1000 �C) using a chemical vapour

deposition (CVD) method. This result showed that Fe/Co-SBA-15 and Ru/Co-SBA-15 systems are highly

suitable sources as catalysts for the growth of MWCNTs compared to other bimetallic systems. TEM and

Raman spectroscopy revealed that the synthesized MWCNTs were of high quality and well-graphitized.

The MWCNTs were applied to the counter electrode of dye-sensitized solar cells (DSSCs). Cyclic

voltammetry measurements proved that the catalytic activity of the MWCNT coated electrode towards

I3� reduction was significantly higher than that of the Pt coated electrode. Electrochemical impedance

measurement of the symmetric cell revealed that the charge transfer resistance of the MWCNT coated

electrode was less than that of the Pt coated electrode. Due to the low charge transfer resistance of the

synthesized MWCNTs, the DSSCs with MWCNTs as counter electrode gave better photoelectric

performance compared to DSSCs equipped with a conventional Pt counter electrode.

1 Introduction

Dye-sensitized solar cells (DSSCs) have attracted huge interestfrom researchers all over the globe because of their simplicity offabrication and cost-effectiveness compared with silicon basedphotovoltaic devices.1–5 In general, a DSSC comprises a dyesensitized TiO2 nanocrystalline lm as a working electrode, anelectrolyte containing iodide/triiodide (I�/I3

�) redox couple anda platinum (Pt) lm coated conductive glass as a counter elec-trode (CE). A key requirement for a high performance DSSC isthat the counter electrode needs to possess high catalyticactivity towards triiodide reduction and conductivity. Platinumis an efficient catalyst for triiodide reduction and a powerconversion efficiency of more than 11% has been achieved whenit was used as a counter electrode.6,7 However, the expensesinvolved in acquiring this rare noble metal limit the practicalapplications of this technology on large scale. Therefore,extensive research has been devoted to develop alternative

talysis and Petroleum Technology, Anna

: [email protected]; Fax: +91-

, CSIR-Central Electrochemical Research

tion (ESI) available. See DOI:

70–5080

functional materials to replace Pt as the counter electrode, inorder to reduce the overall cost and simultaneously retain theperformance of DSSCs.8,9 A variety of low-cost counter electrodematerials such as conducting polymers,10–17 carbonaceousmaterials18–29 and inorganic compounds4,8,30–32 have beenproposed to replace Pt in DSSCs. Among the Pt-free counterelectrode materials, multi-wall carbon nanotubes (MWCNTs)are potential candidates because of their high specic surfacearea, superior electronic conductivity and excellent mechanicalstrength.33 Recently, MWCNTs have been employed as a counterelectrode in DSSCs with improved performance of DSSCs.21,22,34

Carbon nanotubes (CNTs) can be synthesized using variousmethods, including laser-ablation, arc-discharge and chemicalvapor deposition (CVD).35–37 Among these methods, CVD ismoderately simple, economical and easy to scale up theproduction level when compared to laser-ablation and arc-discharge. In addition, CVD is the most economic way to obtainhigh purity CNTs with predictable properties, uniform diameterand good yield for their application purposes. Synthesis of CNTsis essentially a two-step process consisting of an initial catalystpreparation step followed by the real reaction for which thepresence of catalysts is vital. It is worth reiterating that carefulselection of the catalyst and the catalytic support improves theprocess yield signicantly. Interactions, either chemical orphysical, between the catalytic support and metal nanoparticleshave a tremendous inuence on the catalytic properties of the

This journal is ª The Royal Society of Chemistry 2013

http://dx.doi.org/10.1039/c3ta00140g

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Paper Journal of Materials Chemistry A

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nanoparticles. As stated by a rule of thumb, weak interactionslead to tip-growth mode whereas strong interactions lead tobase-growth.38 Investigations bymany researchers have revealedthat bimetallic catalysts are much more effective in facilitatingthe growth of CNTs by CVD than monometallic catalysts.39,40

Different researchers have tried various mixtures of transitionmetals (Fe–Co, Co–Ni, Fe–Ni) on various porous supports suchas MgO, Al2O3, SiO2, CaCO3 etc. and evaluated their catalyticactivities.40–43 It was reported that the yield and diameter ofsynthesized CNTs varied according to the size of transitionmetal particles on the surface of the support. Bimetallic cata-lysts like Co–Mo and Fe–Co have also been effectively utilizedfor growing CNTs.44,45 The use of transition metal catalysts overa suitable support is the favoured method for producing largeamounts of carbon material at moderate temperatures.46

Noble metals have been previously investigated as promotersfor cobalt based catalysts. Kogelbauer et al. studied the effect ofRu on Co/Al2O3 catalysts prepared by different methods andfound that Ru increased the reducibility and the dispersion ofcobalt.47 The results of CNT growth with the same catalyst buton different types of support suggest that substrates with largersurface areas, such as alumina and silica, will promote thenucleation and growth of CNTs.48,49 High surface area allowsthe carbon source atoms to diffuse readily on the surface of themetal catalysts. If an active catalyst metal species is presentwithin a ne pore, it will quickly form enough carbon to ll upthe pore and block the entrance of additional reactanthydrocarbons.

The discovery of silica based and metal substituted meso-porous materials, such as MCM-41, MCM-48, Santa BarbaraAmorphous-15 (SBA-15), etc., found potential applications inthe eld of catalysis, separation, adsorption and CNTsynthesis.50,51 Among the catalytic supports, SBA-15 is a silica-based mesoporous material with ordered uniform hexagonalchannels ranging from 3 to 30 nm with narrow pore sizedistribution.51 It is one of the most attractive catalyst supportswith high thermal and hydrothermal stability and a largesurface area of 600–1000 m2 g�1, allowing for the dispersion of alarge number of catalytically active species. Besides that,reducibility is favoured for the SiO2 supported Co catalystbecause the strength of the interaction between the cobalt andthe support is lower than other commonly used supports.52 Incomparison to other Co supported catalytic systems such asMCM-41, Co incorporated in SBA-15 offers additional advan-tages such as high surface area, uniform porosity, and higherthermal and hydrothermal stability. The present report eluci-dates the superior activity and stability of cobalt incorporatedSBA-15 catalysts in the selective synthesis of SWCNTs andMWCNTs. In addition, the second metal introduced into theCo-SBA-15 increases the yield of CNTs signicantly bypromoting the uniform dispersion of cobalt nanoparticles.

In the present investigation, the different transition metalssuch as Fe, Ni, V, Mn, Cr, Mo, Ru and Pd loaded on Co-SBA-15ordered mesoporous molecular sieves were synthesized. Thesecatalysts were evaluated qualitatively and quantitatively for thegrowth of CNTs. Fe/Co-SBA-15 and Ru/Co-SBA-15 are highlysuitable sources as catalysts for the growth of MWCNTs


compared to other bimetallic systems. The puried CNTs werephysico-chemically characterized by TEM, Raman spectra andTGA. In addition, it was demonstrated that the MWCNTs grownover Fe/Co-SBA-15 and Ru/Co-SBA-15 can be used as a counterelectrode for DSSCs with improved photoelectric performancecompared to conventional Pt counter electrode.

2 Experimental section2.1 Materials

Cobalt(II) acetylacetonate and tetraethylorthosilicate (TEOS)were purchased from Aldrich and used as the source for cobaltand silicon, respectively. Ferric nitrate, nickel nitrate, chro-mium nitrate, manganese acetate, vanadyl sulfate, ammoniummolybdate, ruthenium chloride and palladium chloride werepurchased fromMerck and used as a source of Fe, Ni, Cr, Mn, V,Mo, Ru and Pd, respectively. Triblock copolymer poly(ethyleneglycol)-block-poly(propylene glycol)-block-poly(ethylene glycol)(EO20PO70EO20, Pluronic P123;Mav ¼ 5800, Aldrich) was used asa structure-directing agent. Anhydrous toluene, ethanol,hydrochloric acid, acetonitrile and acetic acid were purchasedfrom Merck. Chloroplatinic acid (H2PtCl6), iodine, lithiumiodide, 4-tert-butyl-pyridine, ethyl cellulose, terpineol and N719dye [cis-di (thiocyanato)-N,N-bis(2,20-bipyridyl-4-carboxylic acid-4-tetrabutylammonium carboxylate) ruthenium(II)] werepurchased from Aldrich. All the reagents were used withoutfurther purication. Fluorine doped tin oxide (FTO) conductingglass plates (sheet resistance 15 Ohm sq�1) were purchasedfrom Xinyan Technology Ltd, HK. The FTO conducting glassplates were ultrasonicated thoroughly in acetone, ethanol anddistilled water for 15 min in each step to remove organicpollutants and other contamination.

2.2 Synthesis of the catalysts

The Co-SBA-15 (1, 3, 5 and 8 wt%) was synthesized following atwo-step method. Initially, mesoporous siliceous SBA-15 wassynthesized according to the procedure from literature.51,53 4 gof Pluronic P123 was dissolved in 30 g of distilled water and120 g of 2 MHCl. The mixture was continuously stirred until thecopolymer was completely dissolved. Then, 8.5 g of TEOS wasslowly added in drops. The mixture was then stirred for anadditional 10 min and transferred to polypropylene bottles. Thebottles were maintained at room temperature for 20 h. Subse-quently, the samples were aged at 100 �C in an autoclave for48 h. Aer cooling, the solids were washed and ltered withdeionized water. The as-synthesized samples were heated undernitrogen atmosphere from room temperature to 540 �C at aheating rate of 3 �C min�1 and then stabilized under nitrogenfor another hour following which the carrier gas was switchedfrom nitrogen to air for 5 h to remove the template at 540 �C.The mass ratio of each component during synthesis was2 : 4.38 : 4.25 : 70.62 (P123 : HCl : TEOS : H2O).

The Co-SBA-15 catalysts were synthesized following acontrolled graing process through atomic layer deposition(ALD) using cobalt(II) acetylacetonate precursor. Wang et al. hadearlier reported that only the samples graed using cobalt(II)

J. Mater. Chem. A, 2013, 1, 5070–5080 | 5071


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acetylacetonate showed high dispersion and high reductionstability.54 The cobalt precursor solution was prepared by dis-solving cobalt(II) acetylacetonate in 150 mL of anhydroustoluene for 3 h. The solution was reuxed at 110 �C for 3 h withN2 owing through the apparatus to completely dissolve theappropriate amount of cobalt precursors (1, 3, 5 and 8 wt%).The calcined SBA-15 was simultaneously suspended in 80 mL ofanhydrous toluene and reuxed in N2 atmosphere to removeany adsorbed water. The cobalt precursor solution was thenadded to the SBA-15 toluene suspension, and the mixture wasreuxed for another 3 h. N2 gas was purged to remove anyphysisorbed water and to prevent the contact of samples withmoisture in air. Thus, the N2 gas was continuously purgedthroughout the reaction. The reaction mixture was cooled,ltered and washed with toluene to ensure that there was nounreacted cobalt precursor le on the SBA-15. The solids werethen dried overnight at ambient temperature. The calcinationprocedure for the as-synthesized Co-SBA-15 was the same as forthe siliceous SBA-15.

Thus, the prepared catalysts when evaluated gravimetricallyfor yield percentage of MWCNTs revealed that 5 wt% of Co-SBA-15 showed optimum yield and hence, it was further used for thepreparation of bimetallic catalysts which were subjected todifferent studies. Fe/Co-SBA-15 catalysts with different Fecontents (0.1–0.4 wt%) were prepared by wet-impregnation of 5wt% Co-SBA-15 using an aqueous solution containing thedesired amount of ruthenium chloride. The catalysts were driedovernight in an oven at 120 �C followed by calcination at 400 �Cfor 5 h. In this study, the Fe-loaded samples were labeled as x%Fe/Co-SBA-15 with x standing for the iron weight percent in theCo-SBA-15, where wt% represents the nal Fe/Co content in thesample tested by ICP-AES analysis. Bimetallic Ni/Co-SBA-15, Cr/Co-SBA-15, Mn/Co-SBA-15, V/Co-SBA-15, Mo/Co-SBA-15, Ru/Co-SBA-15 and Pd/Co-SBA-15 catalysts were synthesized followingsimilar procedure as discussed above.

2.3 Catalytic performance

The growth process of CNTs was carried out in a simple CVDreaction assembly, consisting of a horizontal tubular furnaceand a quartz tube as reported previously.55 The catalyticactivities of Fe/Co-SBA-15, Ni/Co-SBA-15, Cr/Co-SBA-15, Mn/Co-SBA-15, V/Co-SBA-15, Mo/Co-SBA-15, Ru/Co-SBA-15 and Pd/Co-SBA-15 were measured by estimating the carbon deposit yield.The catalyst was rst prereduced at high temperature (700 �C)under pure hydrogen for 30 min and then purged with argon foranother 30 min to remove any hydrogen that may have beenadsorbed on the samples. Typically, 100 mg of pre-reducedcatalyst was placed uniformly on a quartz boat located at themiddle of the reactor. The furnace was heated from roomtemperature to reaction temperature under argon ow (110 mLmin�1), and then the feed was switched to equal volumetricratio of nitrogen and acetylene gas mixture when the desiredreaction temperature was reached. The CVD reaction wascarried out for 30 min, and then the furnace was cooled down toambient temperature under argon atmosphere.

5072 | J. Mater. Chem. A, 2013, 1, 5070–5080

2.4 Fabrication of counter electrode

The MWCNTs were mixed with a 5 wt% solution of ethylcellulose (EC) in terpineol by ultrasonication. The mixture wasfabricated on the transparent conduction oxide (FTO) glasssubstrate by a spin-coating method and dried at 300 �C for 30min to obtain the MWCNT counter electrodes. For comparison,a Pt electrode was prepared by a similar method.

2.5 Fabrication of the TiO2 photoanode

The TiO2 thin-lm photoanode was fabricated by a spin-coatingtechnique. The detailed procedure is as follows: 0.5 mL of aceticacid and 3 g TiO2 (P25, Degussa) powder were mixed in an agatemortar. Then, 2.5 mL distilled water and 15 mL ethanol wereintroduced dropwise into the agate mortar. The mixture wastransferred to a beaker using 25 mL of ethanol and stirred for1 h followed by ultrasonication for 30 min. Terpineol and ethylcellulose in ethanol was added to it. Subsequently, the abovemixture was kept for 24 h in an ultrasonic water bath at 28 �C toobtain a well-dispersed TiO2 paste. This paste was spin coatedon a cleaned FTO conducting substrate and then sintered at500 �C for 30 min. The TiO2 thin-lm is immersed in 50 mL of0.5 mM ethanolic N719 dye solution for 24 h in dark at roomtemperature. Then the lm was cleaned with ethanol and driedand used as a photoanode (working electrode) for DSSCs.

2.6 DSSCs assembly

A Surlyn thermosetting spacer was kept between photoanodeand counter electrode and both of them were clamped. Theactive area (0.25 cm2) of the cell was lled with an electrolyte byinjecting the mixture of 0.1 M LiI + 0.05 M I2 + 0.5 M 4-tert-butylpyridine (4-TBP) + 0.6 M MPI (1-methyl-3-propylimidazo-lium iodide) in acetonitrile through the pre-drilled holes in thecounter electrode. The holes were subsequently sealed with apolymer sheet.

2.7 Characterization of catalysts and MWCNTs

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 hydrouoric acid andnitric acid before the measurement. The X-ray diffractionpatterns of the samples were recorded with a PANalyticalX'Pertdiffractometer, using nickel-ltered Cooke radiation (l ¼ 1.54A) and a liquid nitrogen-cooled germanium solid-state detector.The diffractograms were recorded in the 2q range of 0.5–10� forthe catalyst and 5–80� for the CNTs. The peaks were identiedwith reference to the compilation of stimulated X-ray diffractionpatterns. The surface area, pore volume and pore size distri-bution were measured by N2 adsorption at �197 �C using anASAP-2010 porosimeter from Micromeritics Corporation. Thesamples were degassed at 200 �C and 1.3 � 10�3 Pa for about 8–10 h prior to the adsorption experiments. The mesopore volumewas estimated from the amount of N2 adsorbed at a relativepressure of 0.5 by assuming that all the mesopores were lledwith condensed nitrogen in the normal liquid state. Pore sizedistribution was estimated using the BJH algorithm




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(ASAP-2010) available as built-in soware from Micromeritics.Thermogravimetric analysis measurements were carried outunder atmospheric air using a high-resolution TA Instrument,SDT Q600. About 15 mg of the synthesized catalyst and CNTswere used for each experiment. The sample was heated in air atthe rate of 20 �C min�1 in the temperature range from 35 to1000 �C. The samples for TEM analysis were initially dispersedin ethanol or acetone by ultrasonicating for 30 min which wasallowed to settle. A drop of the supernatant liquid was thentransferred on to a carbon coated copper grid and mountedonto the TEM (JEOL 3010) operated at 300 kV and the micro-graphs recorded. Raman spectra were recorded with a Micro-Raman system RM 1000 Renishaw using a laser excitationwavelength of 532 nm (Nd–YAG), 0.5 to 1 mW, with a 1 mm focusspot in order to avoid photodecomposition of the samples. Inaddition to ICP-AES analysis, the presence of metal nano-particles in the SBA-15 and in the as-synthesized MWCNTs wasconrmed by XPS analysis. Shimadzu ESCA 3400 with Mg Kaprimary radiation (15 kV, 15 mA) was used for the XPSmeasurements. The XPS experiments were performed in anultra-high vacuum (10�8 Torr) aer cleaning the sample surfaceby an ion-gun etching process. The XPS data for the as-synthe-sized MWCNTs is discussed and that for the catalysts is given asESI.†

2.8 Electrochemical and photovoltaic measurements

The assembly and measurements of the dye-sensitized solarcells were carried out as described below. The electrochemicalexperiments were carried out using an electrochemical work-station (Autolab PGSTAT 302N). Cyclic voltammograms wererecorded in acetonitrile solution containing 10 mM LiI + 1 mMI2 + 0.1 M LiClO4, using a three electrode cell with Pt or CNTcoated FTO as the working electrode, a saturated calomel elec-trode as the reference electrode and a Pt counter electrode. ForEIS measurements, a thin layer symmetric cell was fabricated byclamping two identical MWCNT or Pt coated FTO electrodes toeach other with a thermosetting polymer (Surlyn, Dupont)spacer. The EIS measurements were performed under normalconditions (without exposure to a light source) over a frequencyrange of 0.1 Hz to 100 KHz with a perturbation amplitude of10 mV. The electrolyte, 0.1 mM LiI + 0.05 M I2 + 0.5 M 4-tertiarybutyl pyridine in acetonitrile was lled between the electrodesthrough the pre-drilled holes and sealed. The photovoltaicperformance parameters of DSSCs were measured under a lightintensity of 100 mW cm�2.

3 Results and discussion

The HRTEM images of calcined Co-SBA-15 (1, 3, 5 and 8 wt%)showed well-ordered hexagonal arrays of mesopores along withone-dimensional channels, which indicated a P6mm mesostruc-ture, as shown in Fig. S1(a–d).† TheHRTEM images of Fe/Co-SBA-15 and Ru/Co-SBA-15 with different magnications are shown inFig. 1a–c and d–f, respectively. The images showed the represen-tative morphology and distribution of Co, Fe/Co and Ru/Conanoparticles inside the pores of SBA-15 mesoporous molecular


sieves. From the images, it was observed that the pore structurewas regular with well-ordered arrays and the metallic nano-particles were present within the channels of SBA-15. The Co,Fe/Co and Ru/Co nanoparticles exhibited a narrow size distribu-tion in the rangeof 1–3nm. Thedistance between two consecutivecenters of hexagonal pores was estimated to be about 12 nmwhilethe thickness of the pore wall was around 4 nm.

The surface area, pore size and pore volume of Co-SBA-15 (1,3, 5 and 8 wt%), Fe/Co-SBA-15 and Ru/Co-SBA-15 materials weredetermined by N2 sorption isotherm and tabulated in Table 1.From the values, it was evident that the surface area, porediameter and pore volume decreased with an increase in the Co,Fe/Co and Ru/Co concentrations. The sorption isotherms andthe pore size distribution of the Co-SBA-15 (1, 3, 5 and 8 wt%),Fe/Co-SBA-15 and Ru/Co-SBA-15 are depicted in Fig. 2. From thegure, it was inferred that the mesoporous structure wasretained even aer the loading of Fe or Ru into the Co-SBA-15.The isotherms conformed to type IV with an H1 hysteresis loopwhich was typical for mesoporous materials with 2D-hexagonalstructure and having large pore size but narrow size distribu-tion. The sharp steps that were found to occur at a relativepartial pressure in the range of 0.6–0.8 correspond to thecapillary condensation of N2 and indicate the uniformity in thepores. Moreover, the narrow pore size distribution conrmedthe regularity of the pore diameter.

The XRD patterns of calcined Co-SBA-15 (1, 3, 5 and 8 wt%),Fe/Co-SBA-15 and Ru/Co-SBA-15 are shown in Fig. 3. It wasnoted that the XRD patterns of the catalysts were similar tothose of pure siliceous SBA-15 materials by Zhao et al.51 Threewell-resolved and two poorly resolved diffraction peaks could beobserved which are indexed to the (100), (110), (200), (210) and(300) reections, respectively of the hexagonal P6mm spacegroup. Therefore, the above results from HRTEM, N2 phys-isorption and XRD characterization suggested that the methodemployed in this study was a suitable method for the formationof mesoporous silica catalysts with highly ordered 2D hexagonalP6mm structure.

The concentrations of metal in Co-SBA-15 (1, 3, 5 and 8 wt%),Fe/Co-SBA-15 and Ru/Co-SBA-15 catalysts, determined usingICP-AES, are given in Table 1. The data indicated that the Si/M(M¼ Co, Fe/Co and Ru/Co) ratio was retained in all the sampleswhich suggested that there was no loss in the metal concen-tration that was added initially; in other words, leaching out ofthe metal particles was not observed from the mesoporousmolecular sieves. This observation resulted from the interactionbetween M and Si–OH species leading to the formation of M–

OH and as a consequence, a higher amount of M was retainedwithin the pores of the molecular sieves. As a result, the Si/Mratios of the synthesized material were very close to that of theinitial addition of the metal content.

The order of the activity of the catalysts for the synthesis ofMWCNTs at 800 �C was determined gravimetrically and was asfollows: 5 wt% Co-SBA-15 > 3 wt% Co-SBA-15 > 8 wt% Co-SBA-15> 1 wt% Co-SBA-15. The highest carbon deposition was achievedin 5 wt% Co-SBA-15 compared to the other monometallic SBA-15 catalysts. The efficiency of the optimized 5 wt% Co-SBA-15 inthe synthesis of MWCNTs at different temperatures (700, 800,

J. Mater. Chem. A, 2013, 1, 5070–5080 | 5073


Fig. 1 HRTEM images of Fe/Co-SBA-15 (a–c) and Ru/Co-SBA-15 (d–f) catalysts.

Fig. 2 N2 sorption isotherms of mesoporous molecular sieves: (a) 1 wt% Co-SBA-15, (b) 3 wt% Co-SBA-15, (c) 5 wt% Co-SBA-15, (d) 8 wt% Co-SBA-15, (e) Fe/Co-SBA-15 and (f) Ru/Co-SBA-15 catalysts.


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900 and 1000 �C) was studied. Themaximum yield was observedat 800 �C followed by 900 �C, 1000 �C and 700 �C (Table S1†).The explanation for the observation of lower activity at higherloading of Co is as follows. Higher levels of loading are associ-ated with irregular dispersion and result in the formation ofbulk and clustered metal particles. This decreases the numberof active sites and surface area thereby affecting the growth ofCNTs. The inuence of temperature on the growth of MWCNTswas also evaluated and it was shown that the amount of carbondeposited increased when the temperature was raised from700 �C to 800 �C, but a further increase in temperature wasassociated with a decrease in the carbon deposit yield. Thedecrease in carbon deposit yield observed at higher tempera-tures was due to the formation of carbonaceous impurities. Theenhancement in the growth of MWCNTs on 5 wt% Co-SBA-15with the use of a secondmetal (Fe, Ni, Cr, Mn, V, Mo, Ru and Pd)with 0.2 wt% loading was observed at different temperatures(800–1000 �C) (0.2 wt% of metal loading was chosen as it dis-played the best properties when compared with other wt ratios).Fig. 4 shows the effect of the reaction temperature on thecarbon deposit yield over Fe, Ni, Cr, Mn, V, Mo, Ru and Pd

Table 1 Structural and textural properties of the catalyst

CatalystUnit cell parameter ao

a

(nm)Surface areab

(m2 g�1) Pore sizeb (nm)Pore volumeb

(cm3 g�1)M contentc

(wt%)M contentd

(wt%)

Pure siliceous SBA-15 12.5 819.96 5.15 0.82 — —1 wt% Co-SBA-15 12.4 798.99 4.96 0.76 1 0.93 wt% Co-SBA-15 12.1 758.61 4.78 0.69 3 2.85 wt% Co-SBA-15 11.9 742.68 4.59 0.65 5 4.88 wt% Co-SBA-15 11.7 731.14 4.42 0.61 8 7.6Fe/Co-SBA-15 11.9 738.22 4.52 0.64 5.2 4.8Ru/Co-SBA-15 11.8 735.81 4.47 0.62 5.2 4.7

a The values obtained from XRD analysis. b The values obtained from N2 sorption studies. c M content used in the siliceous SBA-15 (M ¼ Co, Fe/Coand Ru/Co). d M content measured by ICP-AES analysis.

5074 | J. Mater. Chem. A, 2013, 1, 5070–5080 This journal is ª The Royal Society of Chemistry 2013


Fig. 3 X-ray diffraction patterns of mesoporous molecular sieves: (a) 1 wt% Co-SBA-15, (b) 3 wt% Co-SBA-15, (c) 5 wt% Co-SBA-15, (d) 8 wt% Co-SBA-15, (e) Fe/Co-SBA-15 and (f) Ru/Co-SBA-15 catalysts.


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loaded Co-SBA-15 catalysts for 20 min. The carbon deposit yieldwas found to follow the order: Ru/Co-SBA-15 > Fe/Co-SBA-15 >Ni/Co-SBA-15 > V/Co-SBA-15 > Pd/Co-SBA-15 > Mo/Co-SBA-15 >Cr/Co-SBA-15 > Mn/Co-SBA-15 at the same reaction temperatureof 800 �C. When compared with the other metal loaded cata-lysts, Fe/Co-SBA-15 and Ru/Co-SBA-15 showed the best andcomparable MWCNT growth at 800 �C (Table S1†). This obser-vation of excellent performance by the Fe/Co-SBA-15 and Ru/Co-SBA-15 could be attributed to the even dispersion of the metalparticles over the support and the existence of a higher numberof active sites. This suggests that the Fe or Ru addition gave riseto increased metal dispersion. Li et al. reported that theincreased metal dispersion observed in their study was possiblydue to the effect of Ru on the nucleation of cobalt clusters.56 Inaddition, Fe or Ru metal particles have the potential to stimu-late the growth of MWCNTs when present along with cobalt.From the above discussion, it is evident that the presence of

Fig. 4 Effect of reaction temperature on the carbon deposited yield over tran-sition metals loaded on Co-SBA-15 catalyst (catalyst: 100 mg, acetylene: 40 mLmin�1, argon: 110mLmin�1, hydrogen: 110mLmin�1 and reaction time: 20min).


small amounts of Fe or Ru enhances the growth of MWCNTssignicantly on the Co-SBA-15 mesoporous molecular sieves.

The morphology of the MWCNTs grown on Fe/Co-SBA-15and Ru/Co-SBA-15 was characterized using TEM and is shownin Fig. 5. It was observed that highly dense rope-like carbonnanostructures grew from the surface of the catalyst supportswhile a closer look with higher magnication showed that theypossessed a hollow core. From the TEM images correspondingto Fe/Co-SBA-15 catalysts (Fig. 5a–c), the MWCNTs wereobserved to possess a more uniform diameter than those grownover Ru/Co-SBA-15 (Fig. 5d–f). To further validate the observa-tions as regards the diameter distributions, more than 200MWCNTs per sample were selected and their outer diameterswere measured from the TEM images. It was found that boththe catalysts were not only associated with the highest yield butwith comparable morphologies. These results show that both Feand Ru affect the diameter distribution of the produced CNTs.In addition, it was also noted that the 0.2 wt% loading of Fe andRu facilitated the growth of MWCNTs with the smallest dia-meter and having the most narrow diameter distribution.

In general, Raman spectra are characterized by three mainsignals: G band, D band and radial breathing mode (RBM). TheG band appears at around 1200–1400 cm�1 and was ascribed totangential modes of a graphene sheet. The D band at around1400–1600 cm�1 was related to the defects in a graphene sheetand the presence of amorphous carbon. The RBM peaks origi-nate in CNTs with small diameters of less than 3 nm, and theirposition was strongly dependent on the nanotube diameter.Raman spectra of CNTs grown over Fe/Co-SBA-15 andRu/Co-SBA-15 system at the optimized reaction condition(temperature ¼ 800 �C; ow rate of acetylene ¼ 40 mL min�1

and time ¼ 20 min) are shown in Fig. 6. The Raman spectro-scopic measurements for MWCNTs grown over Fe/Co-SBA-15and Ru/Co-SBA-15 catalysts (designated as MWCNTs-1 andMWCNTs-2, respectively and the same will be followedthroughout the text) were conducted at an excitation wavelengthof 532 nm. It can be seen that the spectra of MWCNTs displayedtwo common peaks at 1330 cm�1/1326 cm�1 (MWCNTs-1/MWCNTs-2) and 1581 cm�1/1580 cm�1 (MWCNTs-1/MWCNTs-2), which are associated with disordered carbonaceous products(D-band) and tangential graphitized products (G-band) and arecharacteristic peaks for MWCNTs.57,58 The peak intensities ofthe G-band and D-band are denoted as IG and ID, respectively. Itis to be noted that the IG > ID values for the MWCNTs-1 weresimilar to that of MWCNTs-2.

Fig. 7a–d show the main photoemission signals of iron,cobalt, carbon and oxygen of as-grown MWCNTs on Fe/Co-SBA-15 bimetallic catalyst. The two distinct core level signals wereobserved at 710.43 and 723.73 eV in Fig. 7a, corresponding to Fe(2p3/2) and Fe (2p1/2), respectively. The doublet feature of the Fe(2p) spectrum is due to spin orbit coupling resulting in 2p3/2and 2p1/2 of Fe. The observed binding energy (710.43 eV) for Fe(2p3/2) is relatively higher than that of bulk iron (707 eV).59 Thesame shi was observed for Fe (2p1/2) as well. Similarly, adoublet peak was observed at 779.45 and 794.89 eV for Co (2p3/2)and Co (2p1/2), respectively (Fig. 7b). Moreover, the peaks wereslightly shied towards higher binding energy when compared

J. Mater. Chem. A, 2013, 1, 5070–5080 | 5075


Fig. 5 TEM images of purified MWCNTs grown over: (a–c) Fe/Co-SBA-15, (d–f) Ru/Co-SBA-15 catalyst.

Fig. 6 Raman spectra of MWCNTs-1 and MWCNTs-2.

Fig. 7 XPS spectra for (a) Fe, (b) Co, (c) C and (d) O of MWCNTs grown over Fe/Co-SBA-15.


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to the pure metallic state of Co. These changes in the bindingenergies of Fe and Co in SBA-15 are mainly attributed to theformation of Fe–Co alloy in association with the small size ofthe metal nanoparticles, which also causes a slight shi in thebinding energies.59 Fig. 7c and d show the core level signals ofcarbon of CNTs and oxygen of SBA-15. The XPS signal at 530.6eV represents the unbound state of oxygen at the surface andthe shouldering at about 532 eV represents the oxygen bondedwith Si in the SBA-15 matrix.

The XPS spectra for the MWCNTs grown over Ru/Co-SBA-15are shows in Fig. 8a–d. A strong and symmetric peak at 284.5 eVwas observed as shown in Fig. 8a which represents the Ru 3d5/2and the peak is slightly shied towards higher binding energywhen compared to bulk Ru (280 eV).60 Moreover, the Co (2p3/2)and Co (2p1/2) were observed at 780.7 and 794.9 eV and thesebinding energies are also relatively higher than that of pure bulkCo. As discussed in the previous case, the shis in the bindingenergies of Ru and Co are the indication of Ru–Co alloy

5076 | J. Mater. Chem. A, 2013, 1, 5070–5080

formation in the SBA-15 matrix. Fig. 8c and d show the XPSsignals of carbon of CNTs and oxygen of SBA-15. In this case, theXPS signal at 531.9 eV is dominant which represents the oxygenbonded with Si in the SBA-15 matrix and the weak shoulderingat lower energy side (530 eV) represents the unbound state ofoxygen at the surface. In addition, the XPS spectra for the Fe/Co-SBA-15 and Ru/Co-SBA-15 catalysts were shown in Fig. S2a–dand S3a–d.† The similar characteristics of bimetallic catalystswere observed with clear binding energy variations. As aconsequence, the presence of metallic nanoparticles within thechannels of SBA-15 was conrmed by the XPS analysis.

The thermogravimetric analyses of MWCNTs-1 andMWCNTs-2 are shown in Fig. S4.† It was observed that boththe samples displayed similar weight loss characteristics. Theinitial weight loss (in trace amounts) observed between thetemperature 40 and 100 �C was due to desorption of physically



Fig. 8 XPS spectra for (a) Ru, (b) Co, (c) C and (d) O of MWCNTs grown over Ru/Co-SBA-15.

Table 2 Electrochemical and photovoltaic parameters of DSSCs with MWCNTs-1, MWCNTs-2 and Pt counter electrodes

SampleRct(U cm2)

Light intensity(mW cm�2)

Jsc(mA cm�2) Voc (V) FF h (%)

MWCNTs-1 2.68 10 1.99 0.73 0.81 9.9150 8.26 0.76 0.80 9.74

100 15.36 0.78 0.79 9.38MWCNTs-2 2.76 10 1.92 0.74 0.81 9.79

50 8.19 0.76 0.80 9.67100 14.99 0.78 0.79 9.26

Pt 3.57 10 1.85 0.75 0.80 9.6850 7.71 0.77 0.79 9.42

100 14.65 0.79 0.79 9.17


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adsorbed moisture on the MWCNTs. The steep fall in weightobserved in the decomposition temperature range between 550and 650 �C was attributed to the oxidation of the MWCNTs. Itwas noted that there was no weight loss below the temperatureof 550 �C which proves the absence of carbonaceous impuritieslike amorphous carbon and microcrystalline carbon, since thecombustion of amorphous carbon usually occurs below 400 �C.Additionally, since no weight loss was observed beyond 650 �C,it was conrmed that the synthesized MWCNTs were of highpurity without any metallic particles. Furthermore, theMWCNTs have high thermal stability due to the presence ofnumerous well-graphitized walls.

The X-ray diffraction patterns of MWCNTs-1 and MWCNTs-2are shown in Fig. S5.† They showed that the intensity of thegraphitic peak (002) strong and prominent, while the interlayerspacing of the MWCNTs was closer to the interlayer spacing ofideal graphite crystals (0.3354 nm). The spectra displayed astrong peak at 2q ¼ 25.53�/25.73� (MWCNTs-1/MWCNTs-2) anda weak peak at 2q¼ 44. 17�/44.21� (MWCNTs-1/MWCNTs-2) andwere respectively assigned to the (002) and (100) planes ofgraphite.

CV measurements were carried out to evaluate the electro-catalytic activity of the counter electrodes for the reduction ofI3�. The oxidation and reduction peaks of I�/I3

� for the Pt,MWCNTs-1 and MWCNTs-2 counter electrodes were almostidentical (Fig. S6†), while the cathode peak potentials ofMWCNTs-1 and MWCNTs-2 were found to be more positivethan that of the Pt counter electrode. In addition, the cathodiccurrent densities of the MWCNTs-1 and MWCNTs-2 weresignicantly higher than that of Pt, demonstrating a muchfaster I�/I3

� reaction rate. The superior electrochemical activi-ties of the synthesized MWCNTs-1 and MWCNTs-2 were prob-ably due to their higher active surface areas.

Cyclic voltammetry of the I�/I3� system on the MWCNTs-1

and MWCNTs-2 with different scan rates was also studied(Fig. S7†). The cathodic and anodic peak current densities werefound to gradually shi to the negative and positive directions


respectively with increasing scan rates. Also, the excellent linearrelationship between the square root of the scan rate and peakcurrent density can be observed. This indicates the diffusionlimitation of the redox reaction on MWCNTs-1 and MWCNTs-2with no specic interaction between the I�/I3

� redox couple andeither MWCNTs-1 or MWCNTs-2.

Most importantly, the DSSCs with the MWCNTs-1 andMWCNTs-2 electrodes show good stability. This stability wasconrmed by the stability of the current–voltage curves overtime for the I�/I3

� redox system with the MWCNTs workingelectrode and Pt counter electrode (Fig. S8†). No detachment ofthe MWCNTs lms from the FTO glass was observed aer the200th cycle. It was observed that both the counter electrodesexhibited a stable cathodic peak current density as inferredfrom their CV. This indicated that the fabricated MWCNTs-1and MWCNTs-2 lm not only possessed excellent electro-chemical stability, but also were attached rmly onto the FTOsubstrate.

The value of charge transfer resistance (Rct), which wasdetermined using electrochemical impedance spectroscopy(EIS), plays a signicant role in the performance of the DSSCs.The Rct values of Pt, MWCNTs-1 and MWCNTs-2 are given inTable 2. Nyquist plots of MWCNTs-1, MWCNTs-2 and Pt counterelectrodes in a thin-layer symmetric cell conguration areshown in Fig. 9. The half value of the real component ofimpedance of an electrode at a high-frequency side semicircle isconsidered as its Rct.61 When the impedance data were ttedwith the equivalent circuit (inset in Fig. 9), Rct of MWCNTs-1and MWCNTs-2 counter electrodes were found to be 2.68 U cm2

and 2.76 U cm2, respectively. Under similar conditions, the Ptcounter electrode showed a higher value of 3.57 U cm2. Thederivation of such high electrocatalytic property in MWCNTselectrodes is still a subject of interest. Only a few reports haveemphasized that a large surface area enhances the electrontransfer kinetics of the counter electrodes.62

The measured photovoltaic parameters of DSSCs based onthree counter electrodes (MWCNTs-1, MWCNTs-2 and Pt) aresummarized in Table 2. It was determined that the optimumannealing temperature for the binding of MWCNTs over FTOwas 300 �C (results not discussed here). The photocurrentdensity–voltage (J–V) performance of the three DSSCs in thedifferent levels of simulated solar illumination is shown in

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Fig. 9 Impedance spectra of symmetric cells fabricated using MWCNTs-1,MWCNTs-2 and Pt coated FTO plates.

Fig. 10 (a) Photocurrent voltage (J–V) performance of MWCNTs-1 and Ptcounter electrode DSSCs at different levels of simulated solar illumination. (b)Photocurrent voltage (J–V) performance of MWCNTs-2 and Pt counter electrodeDSSCs at different levels of simulated solar illumination.


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Fig. 10a and b. Under 1 sun illumination (100 mW cm�2, airmass 1.5 G), the J–V curves of all three DSSCs were compared.The overall conversion efficiencies of DSSCs with MWCNTs-1and MWCNTs-2 counter electrodes showed 9.38% and 9.26%,

5078 | J. Mater. Chem. A, 2013, 1, 5070–5080

respectively, which was relatively improved when compared tothe efficiency (9.17%) of the DSSC prepared with platinumcounter electrode (Table 2). It is well known that the heteroge-neous rate constant (k0) of Pt electrode for the I�/I3

� redoxreaction is one order higher when compared to conductingpolymers/carbonaceous materials.63 Therefore, the observednominal enhanced efficiencies of DSSCs fabricated usingMWCNT-1 and MWCNT-2 based counter electrodes can beunderstood from their lower Rct values. Besides lower Rct, thelarge surface area of MWCNT-1 and MWCNT-2 can alsocontribute to the efficiency enhancement.

4 Conclusions

High quality MWCNT counter electrodes were successfullyfabricated for the photovoltaic application of dye-sensitizedsolar cells. MWCNTs were grown outside the mesopores of Fe/Co-SBA-15 and Ru/Co-SBA-15 molecular sieves with straight anduniform diameter along with a high degree of graphitization. Itwas shown that Fe/Co-SBA-15 and Ru/Co-SBA-15 molecularsieves were good carriers for the preparation of MWCNTs. Fromthe above observations, it is noteworthy that a small amount ofadded Fe or Ru plays a signicant role in enhancing the growthof MWCNTs. The XPS measurements show that carbon nano-tube formation occurred over the bimetallic Fe/Co-SBA-15 andRu/Co-SBA-15 catalysts. The excellent growth of MWCNTs overFe/Co-SBA-15 and Ru/Co-SBA-15 was proved by the TGA, TEMand Raman studies. These catalysts were suitable to produce theMWCNTs with 10–15 nm diameter and high density. The Fe orRu metals were dispersed in the catalysts mostly through theformation processes of solid solution with Co-SBA-15 andintermetallic species. The optimized reaction conditions for theformation of high quality MWCNTs with high yield were 40 mLmin�1 of acetylene at a temperature of 800 �C for 20 min ofreaction time. All the results suggested that mesoporousmolecular sieves with Fe or Ru loaded on the Co-SBA-15exhibited a good structural stability and are promising catalystsfor the growth of MWCNTs in large scale.

Their uniform controlled diameter, high density, high purityand good graphitization was found to be superior to thecommercial MWCNTs. The synthesized MWCNTs were used ascounter electrode material for DSSCs. The electrocatalyticability of the synthesized MWCNT based counter electrode wasfound to be higher than that of the Pt based counter electrode.The long-term stability of the fabricated MWCNT electrocatalystfor I3

� reduction was clearly improved by the chemical bindingbetween FTO and MWCNTs. Electrochemical impedancemeasurements of the DSSCs exposed that the Rct of MWCNTfabricated electrode was very small. The DSSCs with MWCNTs-1and MWCNTs-2 delivered a higher conversion efficiency (h) of9.38 and 9.26%, respectively. The conversion efficiency (h) of theDSSCs employing the MWCNTs-1 and MWCNTs-2 counterelectrodes are slightly higher than that of the cell with the Ptcounter electrode (h ¼ 9.17%). Thus, the high crystallineMWCNTs can serve as a potential candidate to replace theexpensive Pt for the counter electrode of DSSCs.




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Acknowledgements

One of the authors, J. Balamurugan, acknowledges the DST (SR/S5/NM-35/2005) and UGC-Meritorial Student Fellowship (603/PD6/2007), New Delhi for nancial support and is also thankfulto the UGC-DRS, DST-FIST, Department of Chemistry andInstitute of Catalysis and Petroleum Technology, Anna Univer-sity, Chennai, India for extending the instrumentation facilitiesto characterize the materials. The authors acknowledge DrMukannan Arivanandhan, Associate Professor, Nanodevicesand Nanomaterial Division, Research Institute of Electronics,Shizuoka University, Hamamatsu, Japan for providing XPSanalysis. Dr R. Thangamuthu is grateful to CSIR-CECRI and theDirector, Dr Vijayamohanan K. Pillai for the nancial support ofthis work through the OLP 0068 start-up project. The authorsthank the Director, CSIR-CECRI for permitting the collaborativeresearch work and his constant encouragement.

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