Mesoporous TiO and Its Application Bull. KoreanChem. Soc. 2009, Vol. 30, No. 1 193
Synthesis of Mesoporous TiO
2and Its Application to Photocatalytic Activation of Methylene Blue and E. coli
Eun-Young Kim, DongSukKimJ and Byung-Tae Ahn*
Departmentof Chemistry, Ewha Womans University, Seoul 120-750, Korea. *E-mail: btahn@mm.ewha.ac.kr
，Korean Agencyfor Technology and Standards, Gwacheon427-723, Korea Received April 28, 2008, AcceptedNovember24, 2008
Mesoporous T1O2material was synthesized from diblock copolymers with ethylene oxide chains via a sol-gel process in aqueous solution. The properties of these materials were characterized with several analytical techniques including field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), wide angle X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis, and Barrett-Joyner-Halenda (BJH) analysis. The meso
porous TiO2 materials calcined at 400oC were found to have specific surface areas 212 m2g-1, average pore sizes 6.2 nm, and their average crystal sizes were found to be 8.2 nm. The photocatalytic activity of mesoporous TiO2was char
acterized with UV-Vis spectroscopy, and it was found to be 5.8 times higher than that of Degussa P25 TiO2(P25). For deactivation of Escherichia coli, mesoporous TiO2also has high photocatalytic inactivity than that of P25. Such a high photocatalytic activity is explained with large surface area and small crystal size with wormhole-like meso- porous structure.
Key Words: Synthesis,MesoporousTiO2, Photocatalyticdegradation, Inactivation,E. coli
Introduction
Titanium dioxide (TIO2) is a semiconductor witha large band gap of 3.2 eV, whichcanproduce thehydroxyl radical and super oxide under UV irradiation.1-3 Suchproperties allow their utilizationsforkeymaterials as photocatalyst,dye-sensi tizedsolarcells(DSSCs),photochromicdevices and environ mental pollution reductors.4-6 The photocatalytic activity of TiO2 has beenconsideredtobedeterminedbythe crystalline phase, crystal size,and specific surfacearea.To improve the photocatalyic activity of TiO2, small size andhighcrystallinity of nanoparticles have beenused. Small crystal size can lead to quantum size effect in semiconductor.7-9 Due to active site number, TiO2 with a largesurface area is expectedto exhibit outstanding photochemical activity.
Mesoporous TiO2 hasa large surface area because of its confined porous structure and high surface tovolume ratio, and so itshould inprinciple have high photochemical steri
lization, because of theimprovedaccesstotheactive sites of TiO2.10-12 However, much less research has been devoted to improvingthephotocatalyticactivation of mesoporousTiO2
comparedto that invested in nonporousorcommercialTiO2. Thelowcrystallinity of mesoporousTiO2 with an amorphous orsemicrystalline framework is thought to be the main reason for its negligiblephotocatalytic activity. Increasing the crystal linity throughcalcinationat high temperatures is notexpected tobe helpful because ofthe resulting collapse ofthe meso- porous framework and theconsequent loss of surfacearea.
In this study, we reportthe synthesisofmesoporousTiO2
through the sol-gelapproach of titanium(IV) precursor con
taining blockcopolymer in water solution. Theresultingma terialshavethesphericalmorphology viaaggregation ofana
tasephase single crystalline particles with large surface area andhigh crystallinity.Owing to their specific physical proper
ties, the photocatalytic degradation of methylene blue and
photochemical sterilization of mesoporous TiO2 is signifi cantly higher than that of P25.
Experiment
Synthesis of mesoporous T1O2. Titanium tetraisopropoxide (Ti(OiPr)4, 97%,Aldrich) and the diblockcopolymer surfac
tants Lutensol AT 50 (C16C18 fatty alcochol and 50 of PEO units) were used withoutfurther purification.3g of surfactant (1.2 mmol)was dissolvedin 100 mL of distilled water, and then 1.5 g (15.3mmol) of concentration 95% sulfuric acid was added. Next, titanium tetraisopropoxide (27.5 mmol) mixed with 2,4-pentanedione (27.5 mmol)was droppedinto surfac
tant solutionundermagnetic stirring. Theresultingsolutions werekept at 50oCfor 10h and then hydrothermaltreatment wascarriedoutat 90oC for about 24 h. The solids in thesolu tionswere filtered out, dried inair.Finally, the materialwas calcined at 400oC at a rate of 1oC min-1 inair.
Characteiization. The Field emission-gun scanning electron microscope (FE-SEM, JEOL JSM-6330F) and transmission electron microscope(TEM,JEOLJEM-2000EXII)was used to determinedthemorphology and porestructure of the meso- porous TiO2 particle. X-ray difraction (XRD,MAC/Sci. MXP 18XHF-22SRA with Cu Karadiation) was used tosurveythe crystal compositions and crystal sizes.The crystal size was calculated with theScherrer equation (⑦=K邳cos。),where
① is thecrystal size, K is usuallytaken as 0.89,入 is 0.154nm, which is thewavelength of the X-ray radiation,卩 is thefull width at half maximumintensity (FWHM), and9 isthedif fraction angle of the (101} peak for anatase(29 = 25.3o). The surface area and pore size distributionwasinvestigated with BET and BJH analysis with a Micrometrics ASAP 2000 appa ratus after thesamplesweredegassedat 200oC for 10 h.The specificsurface areaswere estimated byusingthe Brunauer- Emmett-Teller(BET)method, and thepore size distributions
194 Bull. KoreanChem.Soc.2009, Vol. 30, No.1 Eun-Young Kim et al.
were determined with theBarrett-Joyner-Halenda (BJH) meth od by using thenitrogendesorption branches of theisotherms.
Photocatalytic degradation of methylene blue. Thephoto- catalytic activity of themesoporousTiO2 sample wasinves tigated bymeasuring thedegradation of methylene blue(MB) with UV-irradiation, and compared with that of P25. Quartz cellcontained 20 mL of theaqueous 50 ppm MB solution and 20 mg of thecatalyst.In order to providesufficient UV radia tion, four UV-A (20W) lamps which the wavelengths range between 320 and 400 nm were used as lightsources. After30 minstirring indark, thesolutionwas then irradiated under UV light with continuous magnetic stirring. A fixed quantity of each MB solutionwastakenatregularintervals duringthe irra
diationtime and filteredthrough asyringe filtertoevaluatethe concentration of MB remaining in the solution.The UV-Vis absorption spectra of the taken solutions were obtained usinga Shimadzu2101PC UV-Vis scanning spectrophotometer in the range 500-700 nm (Zmax.= 664 nmfor methylene blue)
Photocatalytic inactivation ofE-Coli. Preparation of meso porous TO thin film: To make TiO2 thin film, doctor-blade coating system was used.In mortar,420 卩L of 2.4-pentane- dione (0.1M)wasdropped into1.2g of mesoporousTiO2 and mixedvigorously for30 min until homogeneityobtained.And then, each 500 卩L of distilledwaterwasadded 3 times with vigorously mixing. After 0.24 g of PEO (MW. 10,000) was mixed,0.24g ofPEG (MW. 100,000) wasmixedstepbystep.
The viscoussuspension wasspread onto a glassplate (5 cm 乂 5 cm) atroomtemperature using adhesive tape as a spacer to obtain a unique film thickness. After that, the mixedsolution wascoated with screenprinting method.13
Culture of micro-organism: Eschrichia coli (E. coli) was usedas standard micro-organism.The initialconcentration of E.coli was〜106 cells/mL. The 0.5 mL of thissolutionwas used as atest inoculum.
Photocatalytic reaction procedure: Before photocatalytic inactivation test, the mesoporous TiO2 coated sample was washed with 70%ethanol.Then thetestpieceswas put under UV light (BLB lamp, 1.0 mWcm-2) to initiate the photo- catalytic activity. The0.5 mL of 〜106 cells/mL suspension was dropped on themesoporousTiO2 coated sample, and thesteri lizedpolyethylene film which wascut 5x5 cmwas coveredon it. After UV-Airradiation(intensity is1.0 mWcm-2), the sam ple was taken at regular intervals during the illumination period. Forevaluation of inoculatedbacteria, add4.5mL of neutralization solution, massage the sample, and wash the cov
ering filmsufficiently with pipette. 1 mL of solutionwas with
drawn with pipette and diluted10-fold serially with phosphate buffered physiologicalsaline. Dilution ranged from 100 to 102 is suitable.Finally 0.1 mL sample of thedilutedsolutions was spread in order to count thenumber of colonies. Thenumber of colony forming units (CFUs) is determined after 24 h in
cubation at37oC.
ResultAnd Discussion
SEM and TEM Images Analyses. Figure 1 shows the FE- SEM and HR-TEM images of mesoporousTiO2 calcined at 400oC. The morphology of synthesized material hasspherical shape and its diameter was approx. 2 卩 m as shown in Figure
Figure 1. FE-SEM image of synthesized mesoporous (a), and a mag
nified FE-SEM image of a selected area of one particle (b). HR-TEM image of mesoporous TiO2 (c), and a magnified image of nanocrystal
line anatase (d).
1a. Such a spherical shape of mesoporous TiO2 can be ex pected to induce the surface tension of surfactant through sol-gelprocess. To survey whether the synthesized material has porous structure or not, the magnifiedFE-SEMimage of the one particle is investigated. It is observed that the dis
cernablepore arepresentatthesurface of mesoporous TiO2, andtheporestructureconsistsof a wormhole-likepore array as shown in Figure 2b. Approx. 10 nm of numerous TiO2 nano particles areaggregated and approx.2gmsize of mesoporous TiO2 issynthesized. TheFE-SEMimagesalsoshowthe evi dence for disordered, wormhole-likeframework porestructure.
To observethe pore sizeand its array ofmesoporous TiO2, HR-TEMimages of the mesoporous TiO2 wereinvestigatedas shown in Figure1c and1d. Theobservablesiteshows that the porestructuresarespotted with a non-orderedworm-hole like
Figuie 2. Wide angle X-ray diffraction patterns at different temper
ature: as-synthesized, calcined at 400oC and P25.
Mesoporous TiO and Its Application Bull. KoreanChem. Soc. 2009, Vol. 30, No. 1 195 structure as shown in Figure 1c. The porestructure is observed
atthespace between TiO2 nanoparticles,indicatingthe inter
particle porosityis due tothemesoporosity.This result iscon
sistent with theSEM image. The poresize of mesoporous TiO2
is estimatedabout 6 nm. Onthe further magnifiedHR-TEM image, aggregated numerous nanocrystalline TiO2 nano particles and mesopore structures formed were shown as in Figure 1d. The crystal phase of anatase structure is induced and its average crystalsize is about 8 nm.
From theFE-SEM and HR-TEManalyses, themechanism for the formation of mesoporous TiO2 can be analogized. As can be seen fromtheFE-SEMandHR-TEMimages, the meso- porous TiO2 probably arises from the aggregation of nano crystalline TiO2 particles. When the titanium precursorwas dissolved in surfactant solution, TiO2 nanoparticles can be formed viahydrolysis and condensation of precursor, and in teractionwiththe hydrophilicPEO blocks of surfactantmi celle only. At thisstate,the formed TiO2 nanoparticlesare sta
bilized with surfactant micelle. Further hydrolysis andcon
densation of titaniumprecursors lead tothe growth of larger TiO2 nanoparticles, which are slowly aggregated to one another. AsTiO2 nanoparticles are furtheraggregated,the TiO2 particles become microscopic particles, and finally form mi crospheresas shown Figure 1 images. Similaraggregation of nanoparticlesleadingto mesoporousspherewas also studied by Zhou et al. andYang etalW1
XRD Analysis.TodeterminetheTiO2 crystalcomposition, wideangleX-raydiffractionpatternsweremeasured as shown in Figure2. In caseof mesoporous TiO2 calcined at 400oC, the broaden diffractionpeak of anatase is observed and the crystal sizecalculatedbytheScherrerequation formula from anatase peak {101} is about 8.2nm. This result is similar with the HR-TEM image of Figure 1d. On the other hand, the dif fractionpeak of P25 has anatase and rutile structure, andits crystalsize is approx 25 nm, which issimilar result withrefer
ence data of P25.16
BET and BJH Analyses. The specific surfacearea andpo
rosity of themesoporousTiO2 wereinvestigated by using the N2 adsorption and desorption isothermsbefore andafter the calcination asshownin Figure3. All the isothermsof samples reveal thestepwise adsorption and desorption branch of type IV curves, indicating the presence ofmesoporous material havingathreedimensional(3D)intersectionaccording tothe IUPAC classification. A hysteresis loop witha stepwise ad sorption and desorption branch is observed at wide range of pressure (P/Po), and the surfacearea of mesoporousTiO2 cal cined at 400oC is 212 m2g-1 as shown in Figure 3a. However, a hysteresis loop witha stepwise adsorption and desorption branch is notobserved in case of P25. Thesurface area is50 m2g-1 as shown in Figure 3a. This result indicates that thesyn thesizedmaterial haswider mesoporousstructure.To analysis pore size and pore volume, the plots of the pore size dis
tribution are investigatedby desorption branch of the BJH methodas shown in Figure 3b. The averageporediameter of mesoporousTiO2 calcinedat400oC is 6.2 nm with relatively narrow pore size distribution.The pore volume of mesoporous TiO2 is 0.4 cm3g-1.However,thepore size distribution ofP25 is not observed and porevolume is not alsocounted. Such a
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300 250 200 150 100 50
0 4
. 6- 8.
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Degussa P25 TiO2
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Figure 3. Nitrogen adsorption and desorption isotherms of meso
porous T1O2 and Degussa P25 TiO2 (a). The pore size distributions of mesoporous TiO2 and P25 (b).
physicalproperties of largesurface area and high crystallinity with nanocrystalline aggregated is goodcandidatematerial for highphotocatalytic activity.
Photocatalytic Degradation ofMethylene Blue. Thephoto- catalytic activity of mesoporous TiO2 was carried out by the degradation of methylene blue solution under UV-irra- diation as shown in Figure 4. That of the mesoporous TiO2
was higher than that of commercial TiO2 P25. For the 400oC calcined mesoporous TiO2, the methylene blue was almost all degraded within 30 min under UV-irradiation, whereas the degradation time of commercial TiO2 P25 took about 60 min for 90% degradation of methylene blue. A plot of ln(C0/C) versus UV irradiation time, the slops of straight lines were represented in all materials, indicating that the degradationrate of methylene blue followed the first order process. The higher photocatalytic activity of mesoporous TiO2 can be explained in terms of its large surface area and small crystal size. A larger surface area is likely to result in better photocatalytic activity, because a larger surface area provides more active sites. These mesoporous TiO2
materials have lager surface areas (approx. 212 m2g-1) than the P25 (approx. 50 m2g-1). Therefore, mesoporous TiO2 is likely to have better photocatalytic activity than the P25.
Another factor that influences photocatalytic activity is crystal size. It is commonly accepted that a smaller crystal size corresponds to more powerful redox ability because smaller crystal induce a larger band gap due to the quan tum size effect, which arises due to a dramatic reduction
196 Bull. KoreanChem.Soc.2009, Vol. 30, No.1 Eun-Young Kim et al.
m畫
*8(。응 .) 은。。
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Figure 4. The relative concentrations of methylene blue (MB) as functions of UV irradiation time in the presence of mesoporous TiO2
and P25.
(s n LL。
a _E
>>
nJ
% s
0 30 60 90 120 150 180
Irradiation Time(min)
Figure 5. The concentrations ofE coli as functions of UV irradiation time in the presence of mesoporous TiO2 and P25.
in the number of free electrons. Sincethe mesoporous TiO2
was formedby theaggregation of nanocrystallineanatase with a smaller crystalsize (approx. 8.2 nm) than that ofP25 (approx.
25nm), its photocatalyticactivity isbetter.
Photocatalytic Inactivation ofE-coli Disinfection.The per
formance of mesoporous TiO2 asa photocatalyst is demon
strated usinga Film adhesion method in a recirculating air ex perimental facility. Escherichiacoliwas used asindexto de monstrate the disinfection efficiency of the photocatalysis process. Antibacterialactivity is measured byquantifying the survival of bacterial cells which have been heldinintimate contact for 60 min at RTwith asurface containing a meso- porous TiO2 or P25. The antibacterialeffect is measuredby comparingthe survival ofbacteria on a TiO2 coated surface with that achieved onan uncoated surface.Figure5 shows that of mesoporous TiO2 coatedsurface, P25 coated surface, and uncoated surface under UV-A irradiation. At same photo- catalytic degradationexperiment of methylene blue, a rela tively high rate of bacterial inactivation was observed for mesoporousTiO2 coated sample, whereas arelatively low rate of bacterialinactivation wasobserved for P25coated sample.
Whenthe bacteria wereexposedtoTiO2 in thedark no reduc
tionin viable countswasobserved.Inthecase of theP25 sam
ple, theefficacy of E. coli inactivation reached48% in 90min.
However,in thecase of themesoporousTiO2 coatedsample, theE. coli bacteria cells wereinactivated completely (approx.
98.5%) within30min of UV irradiation.These results indicate thatthemesoporous TiO2 hashighphotocatalyticactivity of methylene blue degradation as well asbacteria disinfection.
Such high photocatalyticdisinfection also is explaineddueto the large surface area and small crystal size of mesoporous TiO2. The bacteria werecontacted with large surface area of mesoporousTiO2 and easily contactthe OH radical induced UV-A irradiation, and consequently high reduction in bacterial numberwasobserved.Furthermore,thesmall crystalsizeand high crystallinity of mesoporous TiO2 give more chance to make high concentration of OH radical on the TiO2 coated sample. Therefore, the photocatalytic disinfection of meso- porousTiO2 has higher than that of P25.
Conclusion
Thisstudyhasshown that mesoporous TiO2 can besuccess fully synthesized with asol-gelapproach from a mixture of a block copolymer and atitaniumprecursor in aqueous solution.
450 oC calcined sample was approx. 2 伽,surfacearea : 212 m2g-1,pore size: 6.2nm, and crystalsize : 8.2 nm. The meso- porousTiO2 was foundtoexhibit higher photocatalytic activi
ties inthe degradations of methylene blue than the P25, in
dicating that the mesoporous TiO2 can be effectively used in photocatalysis. In addition, thephotocatalyticdisinfection of bacteriaalsohashigher reduction that nonporous TiO2 sample.
References
1. Huang, Z.; Maness, P. C.; Blake, D. M.; Wolfrum, E. J.;
Smolinksi, S. L.; Jacoby, W. A. J. Photochem. Photobiol. A 2000, 130, 163-170.
2. Cho, M.; Chung, H.; Choi, W.; Yoon, J. Water Res. 2004, 38, 1069-1077.
3. Fu, G.; Vary, P. S.; Lin, C. T. J. Phys. Chem. B 2005, 109, 8889
8898.
4. Calleja, G.; Serrano, D. P.; Sanz, R.; Pizzaro, P.; Garcia, A. Ind.
Eng. Chem. Res. 2004, 43, 2485-2492.
5. Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W.
Chem. Rev. 1995, 95, 69-96.
6. Stone, V. F.; Davis, R. J. Chem. Mater. 1998, 10, 1468-1474.
7. Antonelli, D. M.; Ying, J. Y. Angew. Chem. Int. Ed. Engl. 1995, 34, 2014-2017.
8. Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.;
Beck, J. S. Nature 1992, 359, 710-712.
9. Zhang, Y.; Weidenkaff, A.; Reller, A. Mater. Lett. 2002, 54, 375-381.
10. Zhan, S.; Chen, D.; Jiao, X.; Tao, C. J. Phys. Chem. B 2006, 110, 1199-11204.
11. Peng, T.; Zhao, D.; Dai, K.; Shi, W.; Hirao, K. J. Phys. Chem. B 2005, 109, 4947-4952.
12. Kim, D. S.; Kwak, S. Y. Appl. Catal. A 2007, 323, 110-118.
13. Yanagida, H.; Takata, M.; Nagai, M. J. Am. Cerm. Soc. 1981, 64, 34-35.
14. Zhou, Y.; Antonietti, M. J. Am. Chem. Soc. 2003, 125, 14960.
15. Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 3492.
16. Ma, Y.; Yao, J. Chemosphere 1999, 38, 2407.
