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Synthesis and ch

aDepartment of Chemistry, Sathyabama U

Salai, Chennai-600 119, India. Fax: +91 44bCentre for Nano Science and Nano Tech

Sathyabama University, Jeppiaar Nagar,

India. E-mail: chennanml@yahoo.comcDepartment of Inorganic Chemistry, Guindy

600 025, India

† Electronic supplementary information (Ecotton ber powder XRD, cotton FESEMcomposites, pore volume graphs, absorpDRS (smoothed) and XRD patterns of thelectronic supplementary information. Se

Cite this: RSC Adv., 2015, 5, 56391

Received 15th April 2015Accepted 8th June 2015

DOI: 10.1039/c5ra06812f

www.rsc.org/advances

This journal is © The Royal Society of C

aracterization of carbonmicrotube/tantalum oxide composites and theirphotocatalytic activity under visible irradiation†

K. Chennakesavulu*ab and G. Ramanjaneya Reddyc

Carbon microtubes (CMTs) were synthesized by the combustion of cotton fibers in an open air

atmosphere at an optimized temperature. The combustion of the cotton fibers leads to the formation

of CMTs by a self rolling approach. In situ synthesis of the CMT : Ta2O5 composite was done in an

alkaline ethanolic medium. The formation of the CMTs and composites was characterized by RAMAN,

DRS/UV-Visible, FTIR, XPS, XRD, BET, N2-adsorption isotherms, FESEM/EDX, TEM/EDX and particle

size analysis. The composites were effectively employed in the photo oxidation of xylenol orange

(XO) and methyl orange (MO). The composites catalytic activity and the adsorbed dyes removal

percentage were determined by a spectrophotometric method. The CMT : Ta2O5 composites

attained a higher photocatalytic activity due to the formation of nascent oxygen during degradation,

which enhances the photocatalytic performance. The kinetic parameters obey the pseudo-first order

reaction, which may be due to the constant amount of the catalyst and the concentration of the dye

solution. The catalytic activity of the recycled composites was compared with the catalytic activity of

the fresh catalyst.

Introduction

Carbon microtubes are a new form of carbon in addition tomore conventional forms such as graphite, diamonds, fuller-enes, carbon nanotubes (CNTs), carbon nanospheres, meso-porous carbon, carbon microcoils and carbon nanofoams.CNTs are rolled graphene layers which have a diameter lessthan 1 mm. The synthesis of CNTs and their applications arewidely investigated. But, the disadvantages of CNTs are thecommercial availability in large quantities with the requireddiameter, dispersion and the solubility of the CNTs in aqueousmedium. The functionalization of CNTs with proteins, cells,inorganic clusters is also a tedious process. In spite of all theadvantages of CNTs, the development of inexpensive, stable,efficient, band gap tunable surface active carbon based mate-rials is still a major challenge. CMTs have a hollow tubular

niversity, Jeppiaar Nagar, Rajiv Gandhi

24503814; Tel: +91 4424503814

nology, International Research Centre,

Rajiv Gandhi Salai, Chennai-600 119,

Campus, University of Madras, Chennai-

SI) available: The particles size graphs,/EDX, FESEM/TEM-EDAX spectras of

tion plots of XO and MO, the Raman,e recovered catalysts are given in thee DOI: 10.1039/c5ra06812f

hemistry 2015

structure with a diameter ranging from 10 mm to 150 mm.1

CMTs can act as another substitute for CNTs. CMT synthesisand physico-chemical characterization needs to be established.In the 1960s rst Roger Bacon from the Union Carbidecompany reported the formation of graphite whiskers duringthe pyrolysis of carbon monoxide at 1800 �C.2,3 Microtubes canbe applied in the elds of gas sensors and microuidics. CNTfabrication methods such as the arc discharge method andpulse laser ablation cannot be used to fabricate carbonmicrotubes (CMTs). The synthesis of CMTs with a largerdiameter was a complex and time consuming process. CMTshave the potential of becoming a key material in mesoscalescience. CMTs have been synthesized from a natural renewableprecursor (coconut oil) by chemical vapour deposition. TheCMTs were synthesized in a nitrogen atmosphere at atemperature of 1175 �C and at a ow rate of 100 mL min�1

using ferrocene as the catalyst.4 Also CMTs have been synthe-sized by the carbonization of poly (acrylonitrile)bers,5

catkins,6 bucky paper (gas pressure enhanced CVD),7 activatedcarbon,8 reed bristles,9 propane,10 acetylene (PE-CVD),11 theuff of the chinar tree,12 urea,13 graphite,14 plant biomass,15 andpolyvinyl butyral powder.16 Carbon/metal hybrid microtubes ofvery high aspect ratio have been synthesized via a self-rollingapproach of polymer multilayers. These microtubes areexpected to have potential applications in areas such asmicrouidic devices and catalysis.17 Voloshin et al. investigatedthe properties of virtual CMTs as a function of their diameterand the number of atoms constituting a ring or a spiral turn in

RSC Adv., 2015, 5, 56391–56400 | 56391

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the microtube. In order to describe and evaluate the propertiesof CMTs they used for the rst time the electron densitydistribution method.18

Carbon composites are widely used in the elds of catalysis,sensors, high thermal conductive materials and as adsorbentsfor toxic metals and organic pollutants. In the eld of catalysis,carbon composites with metals, metal complexes and metaloxides have been widely reported.19,20 Silica/CNTs coupled withmetal oxides such as TiO2, ZrO2, Al2O3 and ZnO have also beenused as photocatalysts.21,22 In relation to the metal oxides, therehas been a growing interest in various elds about tantalumand niobium oxides and their composites. Niobium oxide andtantalum oxide are good visible light absorbents and theseoxides are potential catalytic materials for heterogeneouscatalysis reactions and organic transformations.23,24 Ta2O5 is agood semiconducting material, with a high dielectric constantand a high index of refraction, which has found importantapplications such as dental imaging, CT scans and X-rays,coatings, plastics, nanowires, textiles, nanobers, alloys, cata-lyst applications and as a better material for environmentalpolluted water treatment.25 The Ta2O5/TiO2 photocatalyst wasprepared by a simple solid–solid reaction technique withdifferent Ta : Ti ratios and used in the degradation of methy-lene blue.26 Tantalum (oxy)nitrides were synthesized by thenitridation of Ta2O5 and were added to a photo-Fenton-likesystem to enhance Fe3+ reduction and atrazine degradationunder visible light.27 A Ta2O5–IrO2 thin lm was used as theelectrocatalyst in the electrochemical degradation of lignin.28 Agraphene, Zn–Al–In mixed metal oxide/carbon nanotubecomposite showed a high photocatalytic performance. Even ata low CNT percentage, the CNTs were ascribed to the syner-gistic effects of supporting, electron–hole recombination andthe e� trapping effect of the transition metals.29 MWCNTreinforced ZnO composites have been utilized as an anodematerial for Li-ion batteries, microwave absorbing materials,nanoresonators and as ammonia sensors. MWCNT decoratedZnO nanostructures have been used for the photocatalytic,sonocatalytic and sonophotocatalytic degradation of rhoda-mine-B.30 In situ synthesized ZnO : Ta2O5/Nb2O5 compositeshave been used as photocatalysts in the degradation of chlor-ophenols, cationic and anionic dyes. The degradation ofcationic, anionic and nonionic dyes by carbon composites isdifficult under UV/Visible light. Tantalum oxides could act asan active catalyst with high surface area carbon materials.Carbon materials were used to minimize electron–holerecombination as well as the relative number of active sitespresent on the surface of the heterogeneous catalyst.31 Thesynthesis of conducting materials like CMTs from insulatingcotton materials by an inexpensive and rapid process hasn’tbeen reported yet. Composites of CMTs and tantalum oxidesand their usage in photocatalysis under UV/Visible light alsohasn’t been reported yet.

In the present investigation, the carbon source for the CMTsis surgical cotton and the rst approach involves the open airatmosphere combustion of cotton to CMTs. This study revealsthe synthesis of CMTs and their composites with Ta2O5, whichis a precursor of tantalum chloride. The CMT : Ta2O5 composite

56392 | RSC Adv., 2015, 5, 56391–56400

formation with different ratios of tantalum oxide, wasconrmed by various characterization techniques. Thetantalum doped CMTs may restrict electron–hole recombina-tion during dye degradation and higher photocatalytic activity(removal percentage) was observed for CMT : Ta2O5 compositestowards the removal of XO and MO dyes when compared toCMTs. The optical absorption spectral studies were used todetermine the photocatalytic activity and the kinetic parame-ters. The reused catalyst’s photocatalytic activity was comparedwith the photocatalytic activity of the fresh catalyst.

ExperimentalMaterials

Ammonia (Merck Pvt. Ltd, India), xylenol orange tetra sodiumsalt, methyl orange (Lobachemie, India), tantalum penta chlo-ride (Sigma-Aldrich Pvt. Ltd, India), cotton ber and otheranalar grade chemicals were used without further purication.Millipore water was used throughout the work.

Physicochemical measurements and characterization

The FTIR spectra were recorded on a FTIR Perkin-Elmer 8300spectrometer with a KBr disk. The UV-Vis absorption spectra ofthe liquid samples were recorded on a Perkin Elmer Lambda-35spectrophotometer. The UV-Visible Diffuse Reectance Spectral(UV-Vis/DRS) analyses were carried out on a JASCO-V-670 UV-visible spectrophotometer. The Raman spectra were recordedon a NANO PHOTON11i confocal Raman microscope using aHe–Ne laser emitting at 532 nm. The crystalline nature of theCMT : Ta2O5 composites was ascertained by powder X-raydiffraction using a Rigaku XRD-Smart Lab with Cu-Ka1 radia-tion (l ¼ 1.5418 A). The N2-adsorption, desorption isothermsand the Brunauer–Emmett–Teller (BET) surface area measure-ments at �196 �C were carried out on a Micrometrics ASAP(Model 2020) surface area analyzer with nitrogen and heliumgases with a purity of 99.99%. The FESEM was obtained on aFESEM-SUPRA 55-CARL ZEISS scanning electron microscope.The XPS analysis was carried out on a XM1000 Omicron nano-technology XPS system with a Al-Ka monochromatic wave-length. The samples were made into pellets and were used forthe X-ray photoelectron spectroscopic (XPS) studies. The highresolution XPS traces were deconvoluted using the Gaussianstatistical analysis by using origin-7 soware. The HRTEManalysis was carried out by using a FEI TECNAI G2 (T-30)transmission electron microscope with an accelerating voltageof 250 kV. A CILAS particle size analyzer model 1180 was used tomeasure the particle size of the CMTs and their composites.

Preparation of the carbon microtubes

A known amount of cotton was well dispersed in a glass tray.The cotton was allowed to burn for two to ve minutes. Duringthe combustion, CMTs formation was observed and thetemperature was monitored by a thermocouple, which was xedto the glass tray. The same method was repeated ve times andthe reproducibility was conrmed. The combustion tempera-ture was monitored to be at approx. 450 �C. During the

This journal is © The Royal Society of Chemistry 2015

Fig. 1 Raman spectra of the (a) CMTs, (b) CMT : Ta1, (c) CMT : Ta2 and(d) CMT : Ta3.

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combustion, the cotton bers rolled themselves under differ-ential aeration resulting in the CMTs.

Preparation of the CMT : Ta2O5 composites

0.3 mM tantalum chloride was well dispersed in 200mL ethanolsolution, in a double neck round bottom ask, 1.2 M ammoniasolution was added dropwise in the above solution at roomtemperature and a xed amount (0.1 g) of CMTs was added tothis solution. The resulting solids were centrifuged andrepeatedly washed with milli-Q water. The black threads wereheated in a furnace at 300 �C for 4 h. The pure CMTs andcomposites prepared with 1%, 3% and 7% of tantalum oxide inthe CMTs were represented as CMT : Ta1, CMT : Ta2 andCMT : Ta3 respectively.

Photocatalytic degradation of the XO under visible lightirradiation

The photocatalytic activity of the CMTs, CMT : Ta1, CMT : Ta2and CMT : Ta3 for the removal of XO under visible light wasperformed in a cylindrical glass reactor. 100 mL of 0.1 mmolaqueous dye solution was added to 0.1 g of each catalyst (CMTs,CMT : Ta1, CMT : Ta2, CMT : Ta3) separately. The reactionconditions were optimized in the dark at room temperature andthe irradiation started under visible light (>420 nm). Theremoval percentage and consequent spectral changes at pre-determined time intervals were monitored by the UV-Visibleabsorption spectra at 436 � 2 nm for 4 h. The percentageconversion is calculated from eqn (1).

A ¼ 3cl (1)

where 3 ¼ molar extinction coefficient [M�1 cm�1], c ¼ sampleconcentration, and l ¼ path length of cuvette (1 cm).

Photocatalytic degradation of MO under visible lightirradiation

The photocatalytic activity of the CMTs, CMT : Ta1, CMT : Ta2and CMT : Ta3 for the removal of the MO under visible light wasperformed in a cylindrical glass reactor. 100 mL of 0.2 mmolaqueous dye solution was added to 0.1 g of each catalyst (CMTs,CMT : Ta1, CMT : Ta2, CMT : Ta3) separately. The reactionconditions were optimized in the dark at room temperature andthe irradiation started under visible light (>420 nm). Theremoval percentage and consequent spectral changes at pre-determined time intervals were monitored by the UV-Visibleabsorption spectra at 464 � 2 nm for 2 h. The percentageconversion is calculated from eqn (1).

Results and discussionRaman spectral studies

Raman spectra of the CMTs, CMT : Ta1, CMT : Ta2 andCMT : Ta3 are shown in Fig. 1. The peaks at 1330 cm�1 and1573 cm�1 indicate the D-band and G-band respectively. Thecharacteristic bands were also observed for graphite and carbonnanotubes.32 The composites had the same structural integrity

This journal is © The Royal Society of Chemistry 2015

as the CMT framework. The peaks in the region of 110–480 cm�1 and 500–910 cm�1 were due to the presence of Ta2O5

in the composites, but these peaks were absent in the case of thepure CMTs. The peaks at 740 cm�1 and 870 cm�1 were due tothe octahedral TaO6 modes. The peaks at 410 cm�1 and210 cm�1 indicate Ta–O units. The peaks at 465 cm�1 and572 cm�1 indicate the terminal Ta–O bond vibrations. Thepeaks at 735–821 cm�1 were due to the Ta2O5 moiety in thecomposites. The Raman shi around 910 cm�1 was due to thestretching mode of the O]Ta–O–H bond vibration.33 TheRaman spectral analyses clearly suggest the formation andstability of tantalum oxide with the CMT framework.

DRS/UV-Visible spectral analysis

DRS/UV-Visible spectra of the CMTs, CMT : Ta1, CMT : Ta2 andCMT : Ta3 are shown in Fig. 2. It was found that the absorptionspectra of the CMTs and the CMT : Ta2O5 composites werequite different. The CMTs showed a peak at 236 nm, whereasthe CMT : Ta2O5 composites showed a peak in the region of310–335 nm and at a higher percentage of tantalum oxide, aslight broadening of the peak was observed. The absence of apeak in the region of 400–800 nm clearly indicates that thetantalum has a d0 conguration.34

FTIR spectral analysis

Fig. 3 shows typical FTIR spectra of the CMTs, CMT : Ta1,CMT : Ta2 and CMT : Ta3. The strong and broad peak around3400 cm�1, corresponded to the stretching frequency of O–Hgroups. The peak at 1638 cm�1 was due to the in planebending vibrations of the Ta–OH groups. The peakaround 3500–3650 cm�1 was due to stretching vibrations ofthe HO–Ta]O(O2H) units. CMT : Ta2O5 exhibited peaks inthe range of 450–1000 cm�1 and this was due to the vibrationmodes of Ta2O5, but there was no peak for the pure CMTs.The peaks at 890 cm�1, indicated the Ta–O–Ta mode ofvibrations. The peak at 452 cm�1 was due to the T]O modeof vibration. A peak at 889–902 cm�1 corresponded to theTaO6 octahedra units and Ta suboxides of the composites.35

The CMT : Ta2O5 composites showed similar vibration modes,

RSC Adv., 2015, 5, 56391–56400 | 56393

Fig. 2 DRS/UV-Visible spectra of the (a) CMTs, (b) CMT : Ta1, (c)CMT : Ta2 and (d) CMT : Ta3.

Fig. 3 FTIR spectra of the (a) CMTs, (b) CMT : Ta1, (c) CMT : Ta2 and (d)CMT : Ta3.

Fig. 4 Powder XRD patterns of the (a) CMTs, (b) CMT : Ta1, (c)CMT : Ta2 and (d) CMT : Ta3.

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and it conrmed the formation of tantalum oxide with theCMTs.

XRD analysis

The XRD patterns of the CMTs, CMT : Ta1, CMT : Ta2 andCMT : Ta3 are shown in Fig. 4. The peaks at the 2-theta valuesof 25.9� and 43.5� corresponded to the (002) and (100) faces ofthe graphite diffraction patterns respectively. These peaks arewell matched with earlier literature values (JCPDS Card no.: 23–64).8,18,36,37 From Fig. S1† the cotton XRD showed a higherintensity peak at 22.3� and a low intensity peak at 15.9� sug-gesting that morphological changes occurred during thecombustion process and therefore the XRD patterns of thecotton ber and the CMTs are not identical. The new peaks at

56394 | RSC Adv., 2015, 5, 56391–56400

22.7�, 28.3� and 36.1� were due to the formation of Ta2O5 withthe carbon microstructures.38 Hence the XRD studiesconrmed the crystalline nature of the microtubes and theCMT : Ta2O5 composites.

X-ray photoelectron spectroscopy (XPS) analysis

XPS is a powerful technique to investigate the electronicproperties of species formed on the surface. XPS reveals theelectronic environment, oxidation state, binding energy andspin multiplicity. The bonding properties can inuence thebinding energy of the metal oxide composites. The XPS traces ofthe CMTs and CMT : Ta3 are shown in Fig. 5. It revealed thepresence of C, O and Ta. The XPS survey graphs of thecomposites showed a carbon signal (285.10 eV) for the carbon1s peak. The highly intense broad bands around 531 eVconrmed the presence of oxygen atoms. For CMT : Ta3, C 1s, O1s, Ta 4p and Ta 4f peaks appear at 285.5, 531.2, 233.02 and 30eV respectively.39 The green line in Fig. 5(a) showed the peaks at233.02 and 240.8 eV were related to Ta 4d5/2 and Ta 4d3/2respectively. The peak tting spectra of Ta 4f for the compositeshowed the values at 25.8 eV and 30.04 eV corresponded to 4f7/2and 4f5/2 respectively. The XPS survey graph of the parent CMTwas given as the black line in Fig. 5(a). It clearly suggested thatpresence of carbon elements. From Fig. 5(c) it was seen that theTa 4f core level spectra deconvoluted into two peaks, thetantalum 4f peaks showed that the higher binding energy valuewas due to the formation of Ta2O5 with the carbon micro-structures. Fig. 5(d) shows the 4p core spectra deconvolutedinto four peaks, but the two low intensity shake up satellitepeaks indicate the various oxidation states of tantalum. The O1s core level XPS spectral traces show an asymmetric nature andthe deconvoluted spectral trend suggests that there was morethan one type of oxygen atom bonded to Ta.40 The O 1s peaks ofthe CMTs and CMT : Ta3 XPS are shown in Fig. 5(e) and (f)respectively. For CMT : Ta3, the O 1s peak deconvoluted intofour peaks, but in the case of the CMTs only two deconvolutedpeaks were observed. Here the oxygen molecules binding

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energy was higher for CMT : Ta3, when compared to the CMTs.CMT : Ta3 exhibited peaks at 533.4, 531.8, 530.1 and 529.2 eVfor Ta–OH, Ta]O, Ta–O and Ta–O(H2O) respectively.41 It indi-cated that the tantalum has a +5 oxidation state (Ta+5) and thetantalum was present in the form of oxides and suboxides. Inthis O 1s spectra the presence of a peak at 533.4 eV, wasascribed to oxygen coupled to hydroxyls or hydrated molecules.The CMT : Ta2O5 composites may have contained O–H groups.It was consistent with the FTIR and DRS results. The XPSspectral analysis also conrmed the formation of Ta2O5 for theCMT : Ta2O5 composites and tantalum in the +5 oxidation stateand the stability of the composites.

FESEM and TEM analysis

This microscopic analysis is evidence for the formation ofmicrotubes during the combustion of the cotton bers. The

Fig. 5 (a) XPS survey curves of CMTs (black line) and CMT : Ta3 (green lineTa 4f spectrum and (d) Ta 4p spectrum of the CMT : Ta3 composites; O

This journal is © The Royal Society of Chemistry 2015

ber sheets would have been twisted by a self rolling process at450 �C. The twisting of sheets would have been dependent onthe direction and density of oxygen. Fig. 6(a) conrmed theformation of the CMTs and that the outer diameter rangedfrom 2 to 5 mm and had a length on the millimetre scale.42,43

From Fig. 6(d) and (g) it also showed the Ta2O5 formed evenlyon the surfaces of the CMTs. The microtube nature was notchanged during the formation of CMT : Ta2O5 composite. TheTEM images and the SAED patterns of the CMTs, CMT : Ta2and CMT : Ta3 are given in Fig. 6(b) & (c), (e) & (f), (h) & (i). Thisclearly suggests that the crystalline nature of CMTs with stableconnement.44 The FESEM analysis is evidence for theformation of CMTs.

The FESEM/TEM/EDAX (Fig. S2–S4†) of the CMTs andCMT : Ta2 clearly suggested the presence of carbon and oxygenin the CMTs, whereas the CMT : Ta1 composite had Ta, C and

), (b) C 1s spectra of the CMTs (black line) and CMT : Ta3 (green line), (c)1s spectrum of (e) CMTs and (f) CMT : Ta3 composites.

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O. Silica was also present in negligible traces. The electronmicroscopy study conrmed that the microtube formation atoptimized condition and that the crystallinity was not changedduring the formation of the CMT and tantalum oxidecomposites. Hence, only oxygen played a key role in theformation of CMTs.

Surface area measurements

The Brunauer–Emmett–Teller (BET) surface area, N2-adsorption/desorption isotherms, and the pore volume studyfor the CMTs, CMT : Ta1, CMT : Ta2 and CMT : Ta3 are given inFig. 7. These composites had type-II isotherms, but a steepincrease was observed above P/P0 ¼ 0.7. The BET surface areaof the CMTs, CMT : Ta1, CMT : Ta2 and CMT : Ta3 catalystswere found to be 270, 223, 164 and 132 m2 g�1 respectively.The surface area values were reduced and this may have beendue to the formation of the CMT : Ta2O5 composites. The poresize distribution curves of the samples were evaluated fromthe adsorption branches of the isotherms and given inFig. S5.† The average pore diameter of the CMTs, CMT : Ta1,CMT : Ta2 and CMT : Ta3 was 22 A, 19 A, 21 A and 14 Arespectively. The BET data satisfactorily correlated with theXRD results and the discrepancy between the BET and XRDdata could have been due to the complicated geometry of thepolycrystalline nature of the composites. Hence, the surfacearea was reduced gradually by the increasing tantalum oxidepercentage, and this also meant that the amount of catalytic

Fig. 6 FESEM images of (a) CMTs, (d) CMT : Ta1 and (g) CMT : Ta3. TEM imCMTs, (f) CMT : Ta2 and (i) CMT : Ta3.

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active sites also increased. Hence, the composite could behaveas a good surface active catalyst in the oxidation reaction.

Particle size analysis

A CILAS particle size analyzer model 1180 was used to measurethe particle size. Depending on the size of the particle, a certainquantity of CMTs was dispersed in ethanol, for particles of 1mm in size 0.05 g of the sample is required and for largerparticles (>400 mm) around 5.0 g of the sample is required. Toavoid the agglomeration of particles, an ultrasonic vibrator wasswitched on during the measurement. The suspension waspumped through a detector and a laser beam. The equipmentanalyzed the particle size and also the weight percent of thefraction. The histogram is shown below Fig. S6.† At a diameterof 10%, the particle size was 5 mm, at a diameter of 50% theparticle size was below 10 mm and at a diameter of 80% theparticle size was 20 mm, the mean diameter of the CMTs was 16mm. Whereas the average particle size of CMT : Ta1, CMT : Ta2and CMT : Ta3 was 17.5 mm, 19.8 mm and 23.5 mm respectively.The surface area reduced, when the particle size increased.

Photocatalytic studies: mechanistic issues involved in theremoval of adsorbed the XO and MO dyes

The photocatalytic removal of the adsorbed XO and MO withCMTs, CMT : Ta1, CMT : Ta2 and CMT : Ta3 under visible lightirradiation had been studied. The removal percentages of theadsorbed dye and the kinetic parameters were evaluated from

ages of (b) CMTs, (e) CMT : Ta2 and (h) CMT : Ta3. SAED patterns of (c)

This journal is © The Royal Society of Chemistry 2015

Fig. 7 N2-adsorption and desorption isotherms of (a) CMTs, (b)CMT : Ta1, (c) CMT : Ta2 and (d) CMT : Ta3.

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eqn (1) and (2). The rate of reaction followed the rst order rateequation.

ln

�Ct

C0

�¼ �kt (2)

where Ct ¼ the concentration of the dye at different time, C0 ¼the initial concentration, t ¼ time and k ¼ the rate constantin min�1.

The removal efficiency was calculated by optical absorptionspectral analysis. Aliquot samples of the reaction mediumwere collected and the consequent absorption changes wererecorded at 436 � 2 nm for the XO dye (464 � 2 nm in case ofMO). The spectral changes are given in Fig. S7 and S8.†CMT : Ta1, CMT : Ta2 and CMT : Ta3 had a better degradationperformance, when compared to the CMTs. From Fig. 8(i) theremoval percentages of XO are 45%, 62%, 80% and 100% forCMTs, CMT : Ta1, CMT : Ta2 and CMT : Ta3 respectively.However in the case of MO, the percentages for MO removalfrom Fig. 8(iii) are 11%, 38%, 66% and 100% respectively. Theactive sites present in the catalyst are responsible for thehigher catalytic performance.

The surface area of the active catalysts and the particle sizeof the photocatalysts also have an impact on the photooxida-tion reactions. Here a gradual increase in themean particle sizewith a corresponding decrease in the surface area of activecatalysts may lead to induced charge transfer in the HOMO–LUMO energy levels which retards electron–hole recombina-tion. The reaction was carried out with a xed concentration forall of the reactants. The catalytic reactions exhibit pseudo-rstorder kinetic parameters. Fig. 8(ii) and (iv) show the negativeslope in all cases. The rate constants of XO were 0.0055 min�1,0.0060 min�1, 0.0081 min�1 and 0.025 min�1 for the CMTs,CMT : Ta1, CMT : Ta2 and CMT : Ta3 respectively, while in thecase of the MO dye the rate constants are 0.00009min�1, 0.0082min�1, 0.0087 min�1 and 0.0091 min�1 for the CMTs,CMT : Ta1, CMT : Ta2 and CMT : Ta3 respectively. During thedegradation under visible light, the photogenerated holes ande�s may react with surface hydroxyl groups and adsorbed wateror O2 to generate the active oxidative ionic radicals (O2c

� andcOH) in the reaction medium. The catalytic activity ofCMT : Ta2O5 composite could be enhanced due to the tubularform and tantalum oxide. Here the visible light passes throughthe tubes and degrades the dye molecules quickly. The O2c

�

and cOH radicals are very reactive and quickly oxidize organicspecies at the CMTs’ surface.45,46 The Lewis acid sites enhancethe acidic nature of catalyst and formation of active radicalslimits the dye removal time.47 Under visible light irradiation,e�s are excited from the valence band to the conduction bandof the CMT : Ta2O5 catalysts and create a charge vacancy in h+

in the V.B. During degradation, the generation of the cOHradicals along with the formation of hydrogen peroxidesincrease the catalytic activity towards the degradation of dyes.48

The generated peroxy radicals react with the oxides of Taleading to the formation of nascent oxygen. This happens whenthe reduction of Ta(V) to lower oxidation states occurs. Thisresults in highly active oxygen for the degradation of XO andMO under visible light irradiation.49 The pentavalent oxidation

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Fig. 8 The removal percentage of XO (i) and MO (iii) and the kinetic plots of XO (ii) and MO (iv); (a) CMTs, (b) CMT : Ta1, (c) CMT : Ta2 and (d)CMT : Ta3 composites.

Fig. 9 The proposed radical-ions mechanism for XO and MOdegradation.

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state of Ta provides multiple e�s to vary the electricalconductivity and so more e�s are generated and thereforeenhance the catalytic activity. Ta+5 induced the formation ofoxygen vacancy sites and tantalum oxide predominantly showsmore acidic behaviour than redox behaviour. Hence, the activesite present on the CMTs modied with highly conductiveactive tantalum oxides can lead to the degradation of organicpollutants.50,51 The proposed radical ion mechanism is shownFig. 9. The organics degrade to carbon dioxide, water and otherless toxic minerals such as ammonia, nitrates andsulphates.52–55

Each recovered catalyst from the reaction was collected bynano-ltration and washed with water, followed by an ethanolmixture and then dried at 200 �C for 2 h. The extensively puri-ed recovered catalysts were subjected to Raman spectroscopy,powder XRD and DRS/UV-Visible spectroscopy (Fig. S9–S12†).This spectral analysis conrmed the stability of the photo-catalyst. Thus the recovered catalysts’ spectral data is providedin the ESI.† The overall percentage of removal for the bothadsorbed orange dyes was studied for the three cycles and givenin Fig. 10. It revealed the degradation and efficiency of thereused catalysts were comparatively the same as the freshcatalyst.

56398 | RSC Adv., 2015, 5, 56391–56400 This journal is © The Royal Society of Chemistry 2015

Fig. 10 Overall percentage of conversion for the recovered photo-catalysts for the degradation of adsorbed (a) XO and (b) MO dyes.

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Conclusions

The novel economical production of CMTs was achieved inambient conditions. The physico-chemical characterizationalso conrms the proposed differential aeration mechanism forthe formation of CMTs. The microscopic and surface studiesreveal the CMTs may be promising materials in the elds ofcatalysis and sustainable energy conversion. The CMT : Ta2O5

composite materials were synthesized by a simple in situmethod. The formation of the CMTs and CMT : Ta2O5

composites was conrmed by FESEM, TEM, XPS and XRDanalysis. The RAMAN, FTIR and DRS spectral data also revealsthe formation of CMT/Ta2O5 composites. All of the studiessuggest the stability and effective coupling of CMTs and Ta2O5.The composite materials were used as photocatalysts in thedegradation of adsorbed XO and MO. The composites showhigher photocatalytic activity and the reused catalysts showcomparable activity with the fresh catalysts. This presentinvestigation may lead to improve the research interest towardsthe development of low cost semiconducting materials byrefractory metals for sustainable energy conversion and eco-friendly materials for catalysis. These composites have goodvisible light absorption capacity, they could be applicable asphotocatalysts under visible light.

Acknowledgements

The authors are grateful to Ch. Narendra Reddy, post doctoralfellow, IICT-CSIR, Hyderabad, India, for his support during theexperimental work. The authors are thankful to G. BhaskarRaju, Chief Scientist, CSIR-NML Madras Complex, Chennai,India, for his support in surface area measurements. Theauthors are grateful to Col. Dr Jeppiaar, Chancellor of Sathya-bama University and Dr T. Sasipraba, Dean, SathyabamaUniversity, Chennai, India, for their constant support.

This journal is © The Royal Society of Chemistry 2015

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