alázs Rétia, Károly Mogyorósib, András Dombib,c, Klára Hernádia,∗
Department of Applied and Environmental Chemistry, Faculty of Sciences and Informatics, University of Szeged, Rerrich tér 1, H-6720 Szeged, HungaryResearch Group of Environmental Chemistry, Institute of Chemistry, Faculty of Sciences and Informatics, University of Szeged, Dóm tér 7, H-6720 Szeged,ungaryInstitute of Material Sciences and Engineering, University of Szeged, Tisza Lajos krt. 103, H-6720, Hungary
r t i c l e i n f o
rticle history:eceived 21 February 2013eceived in revised form 4 September 2013ccepted 1 October 2013vailable online 10 October 2013
eywords:iO2
arbon nanotubeshotocatalysis
a b s t r a c t
Titanium dioxide multiwall carbon nanotube (TiO2/MWCNT) composite photocatalysts were synthesizedby the hydrolysis of titanium containing precursor adsorbed on the surface of MWCNT. Annealing wasapplied to convert amorphous titania oxide-hydroxide to crystalline material. The prepared photocata-lysts were characterized with TEM, XRD and Raman spectroscopy. XRD and Raman results showed thatonly anatase-phase titanium dioxide is present in the samples. Generally this is the preferred phase asit is photocatalytically more active than all other TiO2 phases. TEM investigations revealed that the pre-pared photocatalysts have well defined structures; the MWCNTs were decorated with TiO2 nanoparticles.The photocatalysts were tested in aqueous-phase photocatalytic reactions using sol–gel prepared TiO2
as references. The choice of model compounds was based on their aromatic and polar characteristics.
aterial science
henolxalic acid
The concentration of the model compounds (phenol and oxalic acid) was measured by HPLC technique.These investigations showed that photocatalytic activity does depend on the model compound and thepercentage of MWCNT content. The sample containing 5% MWCNT was the most efficient for oxalic aciddecomposition under UV irradiation while the sample containing 1% MWCNT performed the best in phe-nol decomposition under our experiment conditions. For both model compounds, there are MWCNTscontaining samples which are better than the similar way prepared TiO2 reference samples.
. Introduction
Advanced materials and new technologies are essential for envi-onmental friendly methods to remove toxic compounds in waternd air. Many oxidation methods are available to deal with theseroblems. One of these frequently investigated oxidation meth-ds is photocatalytic oxidation [1]. In the recent decade transitionetal oxide based photocatalysis has been studied in details [2].
he pioneer of these metal oxides was titanium dioxide since 19723]. Titanium dioxide particularly its anatase phase, a semicon-uctor metal-oxide is an ideal photocatalytic candidate [4] as itsand-gap (3.1–3.2 eV ≈ � = 390 nm) is not very far from visible lightbsorption. Its absorption overlaps with the sunshine radiationt the surface of Earth [5]. Moreover, it is cheap, non-toxic andiocompatible and it is available in large amount. These unique
roperties combined with multiwall carbon nanotubes (MWCNT)ffer a highly desirable composite material [6] for the decompo-ition of hazardous chemicals. Heterogeneous photocatalysis is a
phenomenon in which a semiconductor promotes reactions whenirradiated with photons of energy equal or higher than its band-gap.In this process valence band electrons are excited to the conductionband leaving holes behind in the valence band. The photogenera-ted electrons and holes can migrate and trap on the surface ofthe semiconductor to take role in reduction and oxidation reac-tions of the adsorbed molecules or species that are close to theactive surface. Electron–hole recombination at the surface, grainboundaries or in the bulk competes with the process mentionedabove and thus partly limits the quantum yield. The mechanism ofenhancement of MWCNT-TiO2 nanocomposites is not well under-stood yet and not even many research aims to investigate this [7].There are two main theories to explain the enhancement. First isproposed by Hoffmann et al. [8] which consider the MWCNTs aselectron sink. Second is considered by Wang et al. [9] they pro-pose that MWCNTs are playing the role of photosensitizer injectingphotogenerated electron to the TiO2 conduction band while thepositively charged MWCNTs scavenge electron from the valance
band of TiO2 leaving a hole onto the TiO2. In this theory the pho-togenerated electrons are transferred to the MWCNT and the holeremains on the TiO2 particle. Because MWCNTs are good adsorbentsdue to their high specific surface area, it is important to take into
onsideration the significance of this aspect too. P. K. Wong andis co-workers pointed out that the adsorptive property of theiO2/MWCNT composite had important role in photoactivity [10].
In this work TiO2/MWCNT nanocomposites were synthetizedith different MWCNT content by sol–gel method. After anneal-
ng anatase nanoparticles obtained with this method, in additionWCNTs were decorated with the amorphous titania particles and
eat treated under identical conditions in parallel syntheses. Thequeous phase degradation experiments with phenol and oxaliccid revealed that the best nanocomposite photocatalysts contain-ng MWCNTs could better perform than the bare TiO2 prepared withhe same sol–gel method proving the positive effect of MWCNTs onhe photocatalytic activity.
. Experimental
.1. Materials
Multiwall carbon nanotubes were prepared via catalytic vapourecomposition method in a rotary oven [11] in nitrogen flow at20 ◦C using acetylene as carbon source and Fe,Co/CaCO3 catalyst.he synthesis process uses CaCO3 as catalyst support thereforehere is a high selectivity of multiwall carbon nanotube forma-ion without carbonaceous particles and amorphous carbon [12].atalyst and the support were removed with washing the productith diluted hydrochloric acid and subsequently with deionizedater until neutral pH was achieved. The average diameter of theultiwall carbon nanotubes was 20–60 nm while average lengthsere about few hundred nanometers to couple of micrometres
Fig. 1) and the specific surface area is 182 m2/g. Ti(OCH2CH3)4Fluka) was used as the titania precursor. Absolute ethanol (VWRrolabo) was used as solvent without purification. Phenol (VWRxtra pure) and oxalic acid dihydrate (Scharlau) were analyti-al purity. Hydrochloric acid (Molar) of 2 mol dm−3 was used.ethanol (VWR HiperSolv Chromanorm), sulphuric acid and MilliQater (18.2 M� cm) were used through the experiments and torepare eluents for the HPLC measurements. Commercially avail-ble and popular Aeroxide P25 (Evonik) was used as photocatalyticeference material.
.2. Sample preparation
MWCNTs were dried at 110 ◦C for 1 h before use. Firstly cal-ulated amount of 2 mol dm−3 hydrochloric acid solution (whicholume is equal to the calculated volume of water) was addednto ethanol and in this solution MWCNTs were dispersed with
Fig. 1. TEM micrograph of pristine MWCNT.
eneral 469 (2014) 153– 158
ultrasonication for 30 min. MWCNT content varied 0% (referenceTiO2), 1%, 5% and 10% by weight to the calculated total final weightof the catalyst after annealing. The Ti(OCH2CH3)4 was added intoethanol and homogenized with ultrasonication for 15 min to formthe precursor solution. The dispersion of MWCNTs was stirred vig-orously while the precursor solution was added drop by drop. Themolar ratios were n (TiO2):n (H2O):n (ethanol) = 1:3:30. After theaddition of the precursor solution the beaker was covered withparafilm and left to be stirred for 1 h. After stirring the sampleswere kept at 80 ◦C until all the solvent evaporated. All sampleswere dispersed in water, filtered and washed with distilled wateruntil neutral pH. Samples were annealed at 400 ◦C for 4 h in air andgrinded into fine powder in an agate mortar.
2.3. Characterization of samples
To examine the morphology of the samples and determine theaverage particle size and size distribution, transmission electronmicroscopy (TEM) (Philips CM10) was used. A small amount ofsample was dispersed in ethanol with the aid of ultrasonicationfor 10 min than small drops of the suspension were dropped ontoa carbon film covered Cu TEM grid (Electron Microscopy Science,CF200-Cu). Crystal structure identification and particle size esti-mation was done with X-ray powder diffraction (XRD) technique(Rigaku MiniFlex II Diffractometer) using Cu K� radiation. Singlepoint BET measurements (BEL Japan, Inc. BELCAT-A) were carriedout at 77 K using pre-treatment of heating the samples at 300 ◦Cfor 30 min in helium flow (50 cm3/min). Raman spectra were taken(Thermo Scientific DXR Raman Microscope) using 532 nm laser. Theconcentrations of substrates were determined with HPLC technique(Merck Hitachi). To remove catalyst particles, samples were cen-trifuged and filtered using Whatman Anotop 25 (0.02 �m) syringefilter. The concentration of phenol was determined in the cen-trifuged and filtered samples using LiChrospher RP-18e column,eluent was the mixture of V(methanol):V(H2O) = 5:9. In case ofoxalic acid sample preparation was the same but GROM ResinZH column was used with eluent of 18.3 mmol dm3 of sulphuricacid solution. Thermogravimetric analysis (Netzsch STA 409 PC)has been performed in oxygen flow (40 mL/min) with 20 ◦C/minheating rate using ∼100 mg sample. The UV–vis spectra of thesolid samples were taken by Avantes AvaSpec-ULS2048 type spec-trophotometer.
2.4. Photocatalytic test
Aqueous phenol and oxalic acid degradation experiments werecarried out in continuously stirred batch type reactors (beakers)having the same dimensional parameters at ambient tempera-ture. All photodegradation experiments were carried out at thesame time using a multi-spot stirrer. UV irradiation were used,3 × 40 W fluorescent UV lamps (Lightech) with emission maximumat � = 365 nm were placed horizontally above the beakers. The UVlamps were 20 cm above the reactors. Mass concentration of thecatalyst was 1 g/L in 100 cm3. The initial concentration of phenolwas 5 × 10−4 mol dm−3 while it was 5 × 10−3 mol dm−3 for oxalicacid so the chemical oxygen demand would be similar for bothsubstrates. The pH of the solutions were not modified, the pH wereevaluated to 6.6 and 2.1 of the phenol and oxalic acid, respectively.Before irradiation with UV light the suspension of the catalyst wasleft to be stirred in dark for 30 min to reach sorption equilibrium.Immediately before the start of UV irradiation sample was taken
and from this point samples were taken in each hour for 3 h. Mea-surements were done in triplicate to ensure reproducibility andthe standard deviation was found to be ±3%. The temperature ofthe suspensions was checked before and after experiment and the
B. Réti et al. / Applied Catalysis A: General 469 (2014) 153– 158 155
/1% M
cm
3
3
fgbc
Fig. 2. TEM micrograph of prepared TiO2 nanoparticles (a), TiO2
hange was found to be ∼1 ◦C. Aeroxide P25 was used as a com-ercial reference material.
. Results and discussion
.1. Synthesis of nanocomposite photocatalysts
The prepared photocatalysts containing MWCNT were uni-
ormly grey although with increasing MWCNT content theyet darker. TEM investigation revealed that TiO2 nanoparticlesounded onto the surface of MWCNTs (Fig. 2). Individual nanoparti-les are dominant although some small aggregates are present in all
Fig. 3. Particle size distribution of prepared TiO2 nanoparticles (a), TiO2/1
produced samples. From the TEM images particle size distributionwas determined for all samples (Fig. 3). Quite narrow size distri-butions of nanoparticles were achieved with the used preparationtechnique for the bare titania and nanocomposite. BET measure-ments showed that with increasing MWCNT content the surfacearea of the samples is also increasing. Particle size distribution andspecific surface area of the samples are summarized in Table 1.
3.2. Effect of annealing
Firstly all prepared samples were analyzed with thermogravi-metric method to confirm the theoretically calculated MWCNT
Fig. 5. X-ray diffraction of the prepared samples.
cates better absorption in the UV and visible region thus morephotons can be utilized in the photocatalytic reactions. The spectraof TiO2/10% MWCNT show very similar shape to the pure MWCNTand the characteristic TiO2 spectral structure is missing suggesting
Fig. 4. Thermogravimetric analysis of the samples.
ontent (Fig. 4). In oxygen flow, all volatile and combustible mate-ials leave the system only the TiO2 remains so one can determinehe TiO2 amount and estimate the carbon content. During the mea-urement the bare TiO2 sample lost 3.7% of its weight. If we takehis in count the MWCNT content is found to be in good agree-
ent with the theoretical values (1.3%, 6.2% and 11.1% for 1%, 5%nd 10%, respectively). The little positive difference is caused byhe adsorbed solvent and water. In Fig. 4 three major regions areresent. The first one is between 60 and 180 ◦C which is attributedo the desorption of adsorbed water and ethanol. The secondne is between 180 and 450 ◦C in which temperature range theemaining chemisorbed organic matter burns away or simply leavehe system. The third region is between 450 and 750 ◦C where the
WCNTs combust. On the DTA curve (not shown here) exother-ic change was observed around 550 ◦C which can be attributed
o the anatase to rutile phase transition [13]. According to thesebservations the temperature of annealing must be under 450 ◦C.
All samples were annealed at 400 ◦C for 4 h in order to obtainrystalline anatase phase. This is important because from thehotocatalysis point of view anatase phase is favourable. 400 ◦Cas selected as calcination temperature because this temperature
s high enough to transform the as-prepared amorphous titania-xide-hydroxide into anatase titanium dioxide, but not too high tourn out the MWCNT in air. XRD and Raman spectroscopic inves-igations were carried out to study crystal phases in the samples.natase phase was identified by XRD technique (Fig. 5) by the char-cteristic reflections at 2� equal to 25.32◦, 36.68◦, 37.88◦, 39.03◦,8.08◦, 54.02◦, 55.10◦ and 69.16◦. These reflections can be assignedo anatase (1 0 1), (1 0 3), (0 0 4), (1 1 2), (2 0 0), (1 0 5), (2 1 1), (1 1 8)nd (1 1 6) crystal planes, respectively. MWCNTs reflection (0 0 2)ppears around 26.08◦ (2�) as small shoulder on the right ofhe anatase (1 0 1) reflection. With the Scherrer equation averagearticle size was calculated. These results are in good agreementith the average particle size values obtained from TEM images
Table 1).Raman investigation confirmed (Fig. 6) the presence of anatase
hase TiO as well as the MWCNTs in the samples. Anatase
2aman active modes are present at 143 cm−1, 198 cm−1, 395 cm−1,17 cm−1 and 639 cm−1. In TiO2/MWCNT samples with increas-
ng MWCNT content the characteristic peaks (mainly G and D)
Fig. 6. Raman spectra of the prepared samples.
of MWCNT are increasing in intensity. The G and D peaks can beobserved at 1344 cm−1 and 1572 cm−1, respectively, and a shoulderis present at 1610 cm−1 moreover peaks assigned to the MWCNTare also present at 2440 cm−1, 2680 cm−1, 3210 cm−1. XRD andRaman spectroscopy proved that anatase phase TiO2 is present inthe samples.
3.3. Optical properties
The optical properties of the samples were determined byUV–vis spectrometry shown in Fig. 7. The samples show red shiftwith increasing MWCNT content. The increasing absorbance indi-
Fig. 7. UV–vis spectra of the solid photocatalysts.
B. Réti et al. / Applied Catalysis A: G
Fig. 8. Phenol decomposition curves.
tdc
3
stnsaLctfiviE
−
TR
Fig. 9. Oxalic acid decomposition curves.
hat the absorption of the MWCNT is dominant over the TiO2. Thisominant absorption of MWCNT in the case of TiO2/10% MWCNTan hinder the photocatalytic efficiency.
.4. Photocatalytic degradation
Degradation of phenol and oxalic acid was studied using theame type of photocatalysts. The photolysis of both the phenol andhe oxalic acid under the applied UV irradiation was found to beegligible. Photocatalysts performed differently depending on theubstrate (Figs. 8 and 9). In most cases heterogeneous photocat-lytic degradation reactions can be described with the simplifiedangmuir–Hinshelwood model [14] when the model compoundoncentration is low, although the explicit mathematical descrip-ion is rather difficult because too many parameters determine thenal model compound concentration. The apparent rate constantalues (kapp) were calculated considering pseudo first order kinet-cs according to Eq. (2) which is the transformed, integral form of
q. (1)
dc
dt= kapp · c (1)
able 2esults of the photocatalytic test reactions.
Sample Phenol decomposition
kapp [10−3 min−1] Decompos
TiO2 1.62 26.6
TiO2/1% MWCNT 2.95 41.4
TiO2/5% MWCNT 1.39 23.0
TiO2/10% MWCNT 1.25 22.7
eneral 469 (2014) 153– 158 157
ct = c0 · e−kapp·t (2)
Aeroxide P25 achieved the highest activity(kapp = 6.49 × 10−3 min−1) in case of phenol followed by TiO2/1%MWCNT, while TiO2/5% MWCNT possessed the best activity(kapp = 6.22 × 10−3 min−1) in oxalic acid degradation. Compared tothe MWCNT containing nanocomposite samples the Aeroxide P25showed quite low photocatalytic activity in oxalic acid decompo-sition. The presence and quantity of MWCNT clearly influencedthe photocatalytic performance of the samples although themechanism of the enchantment is still not clear, however severaltheories exist [15] yet none of them is clarified. The results aresummarized in Table 2.
The difference of photocatalytic activity of the TiO2/MWCNTdepending on the substrate can be attributed to the differentadsorption properties of oxalic acid and phenol. While oxalate forma complex with surface Ti atoms the phenol poorly adsorb ontothe TiO2 [16–18]. Hence oxalate has better contact with TiO2 andcan react directly with generated holes [19] while phenol degradesmainly because of the produced OH• radicals [20]. The differentrate of photooxidation can be related to the above mentioned sorp-tion differences and forms. These effects can contribute to theenhanced oxalic acid degradation rate compared to phenol. Forboth substrates, MWCNT containing composite performed bettertherefore it is obvious that MWCNTs has a positive effect on thephotocatalytic activity compared to the homemade TiO2 reference.This is because of enhanced adsorption properties (�–� stacking ofMWCNTs and phenol) and MWCNT behaving as electron sinks thuslowering the recombination rate of the separated charge carriers[2,15].
The following mechanism can be proposed: with utilizing MWC-NTs in the system the absorption properties has changed due totheir high surface area. Because their structure, MWCNTs possessexcellent electronic properties therefore the electron can be trans-ferred from the conduction band of TiO2 to the MWCNT because oftheir lower Fermi level. The electrons generated by photon irra-diation can move to MWCNTs and the holes can move to thevalence band of TiO2. The above mentioned processes can hin-der the electron–hole recombination, so they improve the overallphotocatalytic activity.
4. Conclusion
TiO2/MWCNT photocatalysts were prepared with sol–gelmethod varying the composition. TEM micrographs showed thatnanoparticles decorate the surface of the MWCNTs. XRD and Ramaninvestigations proved that the desired, photocatalytically activeanatase phase was obtained by annealing. Anatase nanoparticleswith average diameter around 12 nm formed onto the surfaceof the MWCNTs. Prepared photocatalysts were tested in phenoland oxalic acid degradation under UV irradiation. TiO2/MWCNTphotocatalysts showed decreasing activity with increasing MWCNT
content in phenol degradation (10% < 5% < 1% MWCNT content) andAeroxide P25 was the most efficient however sample containing1 weight% of MWCNT had better performance than the referenceTiO2. TiO2/MWCNT samples had better performance (10% < 1% < 5%
Oxalic acid decomposition
ed [%] kapp [10−3 min−1] Decomposed [%]
3.49 48.14.28 54.96.22 68.62.66 40.6
1 is A: G
MnMiltattprotcip
A
Sasttt
[
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[
[[[
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58 B. Réti et al. / Applied Catalys
WCNT content) in oxalic acid degradation compared to phe-ol and using the nanocomposite photocatalyst (namely TiO2/5%WCNT) better efficiency was achieved than Aeroxide P25. Both
n phenol and oxalic acid degradation TiO2/10% MWCNT had theowest photoactivity. The UV–vis absorption experiments revealedhat high MWCNT content is not favourable due to MWCNTs lightbsorption and shadowing effect. Low MWCNT content is advan-ageous in the point of photocatalyst efficiency. We propose thathe MWCNTs act as conductive wires that can transfer and storehotogenerated electrons thus increase the lifetime of the sepa-ated charge carriers and increase the photocatalytic performancef the composite samples. The MWCNTs also play role of absorbentowards the model compounds. These results showed that TiO2ombined with MWCNTs prepared via CVD method is a promis-ng nanocomposite photocatalyst for oxalic acid and phenol typeollutant decomposition.
cknowledgements
This work was supported by grants from Switzerland throughwiss Contribution (SH/7/2/20). Special thanks to Prof. László Forrónd his research group in École Polytechnique Fédérale de Lau-
anne (EPFL) to provide us the multiwall carbon nanotubes. K.M.hanks the financial support of the Hungarian Research Founda-ion (OTKA PD78378) and the János Bolyai Research Scholarship ofhe Hungarian Academy of Sciences.
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eneral 469 (2014) 153– 158
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