Enhanced Photocatalytic Activity of HighlyCrystallized and Ordered Mesoporous TitaniumOxide Measured by Silicon Resonators

Jinmyoung Joo,† Jongmin Shim,† Hyejung Seo,† Namchul Jung,† Ulrich Wiesner,§

Jinwoo Lee,*,†,‡ and Sangmin Jeon*,†

Department of Chemical Engineering and School of Environmental Science and Engineering, Pohang University ofScience and Technology (POSTECH), Pohang, Kyungbuk 790-784, Korea and Department of Materials Science andEngineering, Cornell University, Ithaca, New York 14853

Ordered mesoporous TiO2 was synthesized using thecombined assembly of soft and hard chemistriesmethod and deposited as a film coating on a micro-cantilever sensor array along with two other types ofTiO2 film: one from nanoparticles and one preparedvia a sol-gel reaction. After loading methylene bluemolecules on the TiO2 films, the films were exposedto ultraviolet radiation. The photocatalytic decomposi-tion of methylene blue was monitored by measuringchanges in the resonance frequency of each cantilever.The mesoporous TiO2 film showed higher photocata-lytic activity than conventional TiO2 films fabricatedfrom nanoparticles or via a sol-gel reaction; thisdifference is attributed to the purely anatase crystallinemorphology of the mesoporous TiO2 film as well as itswell-organized pore structure. The three-dimensionallyinterconnected pore network facilitates the diffusionof methylene blue molecules to the photocatalyticallyactive sites of the mesoporous TiO2.

Since the discovery of light-induced water splitting on TiO2

surfaces,1 TiO2 has attracted attention as a promising photo-catalyst due to its excellent photocatalytic activity and long-term stability.2-6 Photocatalytic TiO2 coatings on glass, tiles,and filters endow the materials with antifogging, self-cleaning,and deodorizing functions, respectively.7-10 The mechanism of

the photocatalytic decomposition of various organic molecules onTiO2 surfaces has been reported elsewhere.5–7,11-13 When TiO2

is irradiated by photons whose energy exceeds the TiO2 bandgap (3.2 eV), electrons are excited from the valence band tothe conduction band, resulting in the formation of charge-carrierpairs, that is, a hole (h+) and an electron (e-). These chargecarriers can migrate to the surface and react with adsorbedmolecules, unless recombination occurs first. The hole typicallyoxidizes adsorbed water to a hydroxyl radical (•OH), whereasthe electron reduces adsorbed molecular oxygen to a super-oxide radical anion (•O2

-). These oxidizing radicals react withadsorbed organic molecules, inducing oxidative degradationto carbon dioxide and water.8,12–14

The photocatalytic activity of TiO2 is affected by its crystalstructure, because the defects in the crystal are usually thetrap sites for the recombination of light-generated electron-holepairs. Among the various TiO2 polymorphs, including anatase,rutile, and brookite, it is known that anatase crystallites havethe highest photocatalytic activity.5–7,15 The photocatalyticperformance of TiO2 can be further enhanced by increasing thesurface area, which allows greater contact between organicmolecules and photocatalytic active sites. Therefore, in efforts todevelop TiO2-based photocatalysts, it is important to synthesizepure-anatase TiO2 crystallites with a large surface area.16 Apromising candidate is highly crystallized mesoporous TiO2 witha well-defined pore structure. A number of research groups havetried to fabricate ordered mesoporous TiO2 using soft-template17–19

* Corresponding author. E-mail: jeons@postech.ac.kr (S.J.); jinwoo03@postech.ac.kr (J.L.). Phone: +82-54-279-2392. Fax: +82-54-279-5528.

† Department of Chemical Engineering, Pohang University of Science andTechnology (POSTECH).

‡ School of Environmental Science and Engineering, Pohang University ofScience and Technology (POSTECH).

§ Department of Materials Science and Engineering, Cornell University.(1) Fujishima, A.; Honda, K. Nature 1972, 238 (5358), 37–8.(2) Kiriakidou, F.; Kondarides, D. I.; Verykios, X. E. Catal. Today 1999, 54

(1), 119–130.(3) Zhang, F. L.; Zhao, J. C.; Shen, T.; Hidaka, H.; Pelizzetti, E.; Serpone, N.

Appl. Catal. B: Environ. 1998, 15 (1-2), 147–156.(4) Krysa, J.; Keppert, M.; Waldner, G.; Jirkovsky, J. Electrochim. Acta 2005,

50 (25-26), 5255–5260.(5) Macak, J. M.; Zlamal, M.; Krysa, J.; Schmuki, P. Small 2007, 3 (2), 300–

304.(6) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem.

Rev. 1995, 95 (1), 69–96.(7) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. Rev. 1995, 95 (3), 735–

758.

(8) Ollis, D. F.; Al-Ekabi, H. Photocatalytic purification and treatment of waterand air; Elsevier: Amsterdam and New York, 1993; p xiii, 820 p.

(9) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura,A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388 (6641), 431–432.

(10) Tatsuma, T.; Tachibana, S.; Miwa, T.; Tryk, D. A.; Fujishima, A. J. Phys.Chem. B 1999, 103 (38), 8033–8035.

(11) Serpone, N.; Pelizzetti, E. Photocatalysis: fundamentals and applications;Wiley: New York, 1989; p x, 650 p.

(12) Konstantinou, I. K.; Albanis, T. A. Appl. Catal. B: Environ. 2004, 49 (1),1–14.

(13) Rajeshwar, K.; Osugi, M. E.; Chanmanee, W.; Chenthamarakshan, C. R.;Zanoni, M. V. B.; Kajitvichyanukul, P.; Krishnan-Ayer, R. J. Photochem.Photobiol. C: Photochem. Rev. 2008, 9 (4), 171–192.

(14) Einaga, H.; Futamura, S.; Ibusuki, T. Appl. Catal. B: Environ. 2002, 38(3), 215–225.

(15) Mills, A.; LeHunte, S. J. Photochem. Photobiol. A: Chem. 1997, 108 (1),1–35.

(16) Wang, Y. Q.; Chen, S. G.; Tang, X. H.; Palchik, O.; Zaban, A.; Koltypin, Y.;Gedanken, A. J. Mater. Chem. 2001, 11 (2), 521–526.

Anal. Chem. 2010, 82, 3032–3037

10.1021/ac100119s 2010 American Chemical Society3032 Analytical Chemistry, Vol. 82, No. 7, April 1, 2010Published on Web 03/08/2010

or hard-template20,21 methods. However, nanostructured materialsproduced by the soft-template method generally have amorphousor semicrystalline morphology because the thermally unstable softtemplate limits heat treatment to temperatures below ∼400 °C, whichis not sufficient to produce crystalline mesoporous TiO2.22 On theother hand, although hard templates can be heated to tempera-tures that are sufficiently high for crystallization, this approachhas the disadvantage that hard template synthesis is time-consuming and complicated.20,21 To overcome these limitations ofthe soft- and hard-template methods, previously we developed amethod called combined assembly of soft and hard chemistries(CASH).23 The resulting mesoporous TiO2 was found to have a well-connected pore network and a highly crystalline anatase structure.

In this study, we synthesized a mesoporous TiO2 film using theCASH method and compared its photocatalytic activity with thoseof two other TiO2 films fabricated from nanoparticles or via asol-gel reaction. The photocatalytic activity was measured usingmicrocantilever arrays; to our knowledge, this is the first studyto use this approach to determine photocatalytic activity. Com-pared with conventional methods for the evaluation of photocata-lysts, such as ultraviolet-visible absorption spectroscopy (UV-vis)and gas chromatography (GC), the microcantilever array methodhas many advantages. First, microcantilever arrays measure theabsolute mass change due to the photodegradation of organicmolecules, whereas UV-vis and GC techniques measure thephotodegradation indirectly via color change and the amount ofproduced gases, respectively. For example, UV-vis spectroscopycannot distinguish between the complete photodegradation of dyemolecules and partial structural damage, because they inducesimilar color changes of the dye solution. Second, the sensitivityof microcantilever arrays is superior to those of UV-vis and GC.The resonance frequency, f, of an oscillating cantilever is relatedto the mass of the cantilever, m, as follows:

f ) 12π� k

m(1)

where k is the spring constant of the microcantilever. The changein mass, ∆m, can be calculated from

∆m ) k4π2( 1

f12 - 1

f02) (2)

where f0 is the initial resonance frequency and f1 is the resonancefrequency after the mass change. The mass sensitivity of themicrocantilever used in this study is about a few picograms inair. Compared to other mass sensors such as quartz crystalmicrobalances (QCM),24-26 this sensitivity is extremely high and

could be further increased using a cantilever with a higher springconstant. Further, the arrayed structure of the microcantilevers canbe utilized to evaluate multiple photocatalyst samples simultaneously,making the microcantilever technique highly efficient for the screen-ing of photocatalyst candidates.

EXPERIMENTAL SECTIONMaterials. Titanium tetraisopropoxide (TTIP), titanium(IV)

chloride, tetrahydrofuran (THF), chloroform, ethanol, and methyleneblue were obtained from Sigma-Aldrich. Nitric acid (60%) waspurchased from Matsunden Chemicals. TiO2 nanoparticles wereobtained from Degussa (P25, mean diameter of ∼30 nm). Thecomposition of the P25 nanoparticles as stated by the manufac-turer is 80% anatase and 20% rutile. Polyisoprene-polyethyleneoxideblock copolymer (PI-b-PEO) was synthesized using anionic po-lymerization techniques. Its number average molecular weight(Mn), polydispersity, and PEO fraction were determined to be30 000 g/mol, 1.13, and 0.06, respectively.23,27 Arrays of eight siliconcantilevers were purchased from Micromotive (Mainz, Germany).Each cantilever in the array was 450 µm long, 90 µm wide, and 5µm thick, with a spring constant of ∼3.3 N/m.

Preparation of TiO2 Films. To compare the photocatalyticactivities of TiO2 films having different crystalline structuresand surface areas, three types of TiO2 film were prepared: aP25-nanoparticle-derived TiO2 film (P25-TiO2), a sol-gel reac-tion-derived TiO2 film (sg-TiO2), and a mesoporous TiO2 film(meso-TiO2). Three cantilevers in an array were each coatedwith one of these three types of TiO2 film using microcapillarytubes, as shown in Figure 1. For the preparation of the P25-TiO2 film, P25 nanoparticles were dispersed in deionized water(3 mg/mL). A microcapillary tube was filled with this disper-sion, and a silicon cantilever was immersed in the tube for 1min. The controlled immersion depth of the cantilever insidethe capillary tube was ∼100 µm. The cantilever was then heatedat 200 °C for 30 min, in order to fix the nanoparticles on thecantilever surface.28 Heat treatment at this temperature doesnot affect the crystal structure of P25 nanoparticles. For thepreparation of sg-TiO2, aqueous nitric acid was added to TTIPin ethanol to yield a titania sol via hydrolysis. The molar ratioof the sol-gel reagents was TTIP/C2H5OH/H2O/HNO3 )1:100:10:0.04.26 After coating a cantilever with this sol using acapillary tube, the resultant film was heated to 530 °C at a rateof 1 °C/min and calcined for 2 h to obtain crystalline TiO2.

The procedure for the synthesis of meso-TiO2 can be foundelsewhere.23 In summary, 0.71 mL of titanium tetraisopropoxide(TTIP) and 0.21 mL of titanium(IV) chloride were added to0.2 g of PI-b-PEO block copolymer dissolved in 2 mL of THF.The mass ratio of TiCl4 to copolymer is about 2. Afterimmersing a cantilever in the solution using a capillary tube,the cantilever was dried at 50 °C overnight. The TiO2-coatedcantilever was heated to 530 °C at a rate of 1 °C/min andcalcined for 2 h in argon to convert the amorphous TiO2 to

(17) Antonelli, D. M.; Ying, J. Y. Chem. Mater. 1996, 8 (4), 874–881.(18) Boettcher, S. W.; Fan, J.; Tsung, C. K.; Shi, Q. H.; Stucky, G. D. Acc. Chem.

Res. 2007, 40 (9), 784–792.(19) Yang, P. D.; Zhao, D. Y.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D.

Chem. Mater. 1999, 11 (10), 2813–2826.(20) Jiao, F.; Bruce, P. G. Adv. Mater. 2007, 19 (5), 657.(21) Lee, J.; Kim, J.; Hyeon, T. Adv. Mater. 2006, 18 (16), 2073–2094.(22) Yang, P. D.; Zhao, D. Y.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D.

Nature 1998, 396 (6707), 152–155.(23) Lee, J.; Orilall, M. C.; Warren, S. C.; Kamperman, M.; Disalvo, F. J.; Wiesner,

U. Nat. Mater. 2008, 7 (3), 222–228.(24) Joo, J.; Lee, D.; Yoo, M.; Jeon, S. Sens. Actuators B: Chem. 2009, 138 (2),

485–490.

(25) Nakamura, Y.; Katou, Y.; Rengakuji, S. Electrochemistry 2004, 72 (6), 408–411.

(26) Hidaka, H.; Honjo, H.; Horikoshi, S.; Serpone, N. New J. Chem. 2003, 27(9), 1371–1376.

(27) Allgaier, J.; Poppe, A.; Willner, L.; Richter, D. Macromolecules 1997, 30(6), 1582–1586.

(28) Sakthivel, S.; Shankar, M. V.; Palanichamy, M.; Arabindoo, B.; Murugesan,V. J. Photochem. Photobiol. A: Chem. 2002, 148 (1-3), 153–159.

3033Analytical Chemistry, Vol. 82, No. 7, April 1, 2010

highly crystallized meso-TiO2. The carbon deposited on themeso-TiO2 due to the burning of PI-b-PEO was removed byheat treatment at 450 °C in air for 1 h and subsequent ultraviolet(UV) irradiation for 12 h.

Characterization of TiO2 Films. Transmission electronmicroscopy (TEM) and high-resolution TEM characterizationwere carried out using a JEOL EM-2010 electron microscope.X-ray diffraction (XRD) analysis was performed using a PANa-lytical X’Pert diffractometer (Cu KR radiation) with an X’Celeratordetector. The surface areas of the various TiO2 films weredetermined by recording the nitrogen adsorption-desorptionisotherms at 77 K using a Micromeritics Tristar II 3020 andanalyzing the data with the Brunauer-Emmett-Teller (BET)method. Small-angle X-ray scattering (SAXS) data were col-lected using an apparatus equipped with an 18 kW rotatinganode X-ray generator (Rigaku Co., Cu KR ) 1.542 Å) and aone-dimensional position-sensitive detector (M. Braun Co.).

Measurements of Photocatalytic Activity of Various TiO2

Films. To investigate the photocatalytic activities of the variousTiO2 films, a bare silicon cantilever and the cantilevers coatedwith P25-TiO2, sg-TiO2, and meso-TiO2 were immersed for 20min in capillary tubes containing 0.2 mM methylene blue inethanol solution, to allow the methylene blue molecules toadsorb on the TiO2 films. The bare silicon cantilever is usedhere as a reference cantilever, and methylene blue is a

representative organic compound.5,29 An optical method wasadopted to measure the resonance frequency of each cantileverafter excitation by an external piezoelectric actuator. In thismethod, a focused laser beam is reflected off the free end of thecantilever onto a position-sensitive detector, and the voltagechange due to the vibration of the cantilever is converted to aresonance peak using a fast Fourier transform (FFT) technique.The resonance peaks were fitted with Lorentzian curves, and thecorresponding resonance frequencies were calculated. The varia-tions in the resonance frequencies of the cantilevers under UVirradiation (λ ) 254 nm, 4.5 mW/cm2, Spectroline, NY) weremeasured every 10 min to monitor the photocatalytic degrada-tion of the dye molecules. A control experiment with a baresilicon cantilever under UV irradiation showed no measurablefrequency change, confirming that the mechanical character-istic of the cantilever is not affected by UV exposure.

RESULTS AND DISCUSSIONFigure 2a-c shows scanning electron microscopy (SEM)

images of various TiO2 films on the cantilevers. The P25-TiO2

film contains large macropores with a broad pore-size distribu-tion. In contrast, mesoscale pores are observed in the sg-TiO2

and meso-TiO2 films, although the meso-TiO2 film exhibits amore uniform and porous structure than the sg-TiO2. Figure2d shows XRD patterns for the various TiO2 films (JCPDS21-1272), indicating that the P25-TiO2 film consists of anataseand rutile crystallites whereas the sg-TiO2 and meso-TiO2 filmsconsist of purely anatase crystallites. The anatase crystallitesof sg-TiO2 and meso-TiO2 were produced by heat treatment at530 °C during the preparation of the films, which was notperformed for P25-TiO2. The calculated BET surface areas ofthe TiO2 films are 50, 41, and 65 m2/g for P25-, sg-, and meso-TiO2, respectively. Note that the surface area of sg-TiO2 is muchsmaller than that of meso-TiO2 even though they both haveanatase crystal structures. The smaller surface area of sg-TiO2

can be attributed to its structural collapse during heat treat-ment, which does not occur in meso-TiO2 due to its mechani-cally strong hard template.

Figure 3 shows the change in resonance frequency of eachcantilever due to its sequential coating with TiO2 film andmethylene blue. The frequency changes due to the TiO2 filmsare 262, 341, and 519 Hz for P25-TiO2, sg-TiO2, and meso-TiO2,respectively, which correspond to 3.8, 5.0, and 7.7 ng ofdeposited TiO2 calculated from eq 2. The subsequent changesin the resonance frequencies of the cantilevers due to theadsorption of methylene blue are 50, 116, 132, and 437 Hz for thebare silicon cantilever and P25-, sg-, and meso-TiO2-coatedcantilevers, respectively. The ratios of the mass of adsorbedmethylene blue to the mass of the TiO2 film are 0.44, 0.40, and0.84 for P25-, sg-, and meso-TiO2-coated cantilevers, respec-tively, which is proportional to the surface areas of the TiO2

films.Figure 4 shows the photocatalytic degradation of methylene

blue molecules adsorbed on a bare silicon cantilever and the P25-,sg-, and meso-TiO2-coated cantilevers when they are exposedto UV light. The normalized frequency shift is calculated with

(29) Zhang, T. Y.; Oyama, T.; Aoshima, A.; Hidaka, H.; Zhao, J. C.; Serpone, N.J. Photochem. Photobiol. A: Chem. 2001, 140 (2), 163–172.

Figure 1. (a) Optical microscope image and (b) SEM image of asilicon microcantilever array. Three of the cantilevers are coated withdifferent types of TiO2 film; white regions at the free ends of thesecantilevers represent the deposited TiO2 films. Cantilever 1: meso-TiO2;3: sg-TiO2;5: P25-TiO2;7: bare silicon cantilever (reference).Cantilevers2,4,6, and8 were broken by the capillary tube duringthe coating process.

3034 Analytical Chemistry, Vol. 82, No. 7, April 1, 2010

respect to each cantilever with TiO2 and methylene bluecoatings. The photoinduced hole oxidizes the methylene bluemolecules to methylene blue radicals (•MB+), which decom-pose fully to water and carbon dioxide by further reaction withO2.29 The control experiment with the bare silicon cantilevershows a slight change in the frequency due to the UV-induceddirect photolysis of methylene blue, which is negligiblecompared to the frequency changes observed for the TiO2-film-coated cantilevers. The smallest frequency shift among the TiO2

films is observed for P25-TiO2 due to its lower fraction ofanatase crystallites, which have higher photocatalytic activity;the fraction of anatase crystallites in the P25-TiO2 film is ∼80%whereas the sg-TiO2 and meso-TiO2 films are 100% anatase.

It is interesting to note that meso-TiO2 shows higher photo-catalytic activity than does sg-TiO2 despite the similarity of theiranatase structures. This could be attributed to their differentsurface areas: a TiO2 film with a larger surface area possesses

a greater number of active sites for the electron-hole chargeseparation. However, the ratio of the mass of adsorbedmethylene blue to the mass of sg-TiO2 is only half that of meso-TiO2 even though the surface area of sg-TiO2 is two-thirds thatof meso-TiO2, indicating that the number of active sites availablefor the photocatalytic decomposition of adsorbed methyleneblue molecules is proportionally greater in sg-TiO2 than inmeso-TiO2. Therefore, the higher photocatalytic activity ofmeso-TiO2 is attributed to its pore structure rather than itssurface area. Compared to the pores randomly formed in sg-TiO2, meso-TiO2 is known to have a well-ordered and intercon-nected pore network, which facilitates the dispersion of organicmolecules to the photocatalytically active sites.

Figure 5a,b displays TEM images of meso-TiO2, showingshort-range ordered hexagonal mesoporous structures with awall thickness of ∼10 nm and pore diameter of ∼25 nm. The

Figure 2. SEM images of various types of TiO2 films: (a) P25-TiO2, (b) sg-TiO2, and (c) meso-TiO2; (d) XRD patterns of TiO2 films: P25-TiO2

(green), sg-TiO2 (blue), and meso-TiO2 (pink). A and R indicate the anatase and rutile phases of TiO2, respectively.

Figure 3. Resonance frequency shifts due to the sequential loadingof TiO2 (blue) and methylene blue (red) on each cantilever. Note thatthe frequency change due to the methylene blue loading is withrespect to the TiO2-coated cantilever.

Figure 4. Photocatalytic degradation of methylene blue depositedon various TiO2 films, measured via a normalized cantilever frequencyshift: sg-TiO2 (b), P25-TiO2 ((), and meso-TiO2 (2), as well as directlydeposited on a silicon cantilever without TiO2 (9). Inset shows thecalculated mass of methylene blue on each cantilever before UVirradiation (dark blue) and after UV irradiation (red).

3035Analytical Chemistry, Vol. 82, No. 7, April 1, 2010

pore size is sufficiently large for the diffusion of organicmolecules inside the pores. The crystallite size in meso-TiO2

is estimated to be 12.3 nm using the Debye-Scherrer equationand the (101) anatase peak of the XRD pattern in Figure 2d.The similarity between the crystallite size and the wall thicknessindicates that the walls are highly crystallized rather than beingcomposed of nanocrystals embedded in an amorphous matrix.22

The SAXS experimental data for meso-TiO2 in Figure 5c show afirst-order peak and two broad higher order peaks. The first-orderpeak corresponds to d-spacing of 30.5 nm. The broad higher orderpeaks at higher q values correspond to the angular positions of31/2 and 41/2 of the first-order peak maximum. This peak patternis typically observed for wormhole-like-structured mesoporousmaterials, which are known to have three-dimensionally inter-connected and hexagonally ordered pore networks, facilitatingthe diffusion of guest molecules inside the pores.30,31 Thestructure of meso-TiO2 was further characterized by nitrogenadsorption-desorption experiments as shown in Figure 5d, where it is foundthat meso-TiO2 exhibits a type-IV nitrogen adsorption isothermand a calculated surface area of 65 m2/g. The pore size wasestimated by the Barrett-Joyner-Halenda (BJH) method to be25 nm, which is in agreement with the pore-size estimate fromTEM.

Figure 6 shows that the logarithms of the normalized frequencychanges during the photodegradation are linear with respect toUV-irradiation time, indicating that the photodegradation of

methylene blue follows pseudo-first-order kinetics. The rate ofphotocatalytic decomposition of dye molecules is typically fittedwith the Langmuir-Hinshelwood (L-H) kinetics model:6,12,32,33

r ) dCdt

) kKC1 + KC

(3)

where r is the dye-decomposition reaction rate, C is the concentra-tion of the dye solution, t is the UV irradiation time, k is thereaction rate constant, and K is the adsorption coefficient of thedye molecules. If the concentration of the dye solution is

(30) Ryoo, R.; Kim, J. M.; Ko, C. H.; Shin, C. H. J. Phys. Chem. 1996, 100 (45),17718–17721.

(31) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269 (5228),1242–1244.

(32) Houas, A.; Lachheb, H.; Ksibi, M.; Elaloui, E.; Guillard, C.; Herrmann, J. M.Appl. Catal. B: Environ. 2001, 31 (2), 145–157.

(33) Tang, W. Z.; An, H. Chemosphere 1995, 31 (9), 4157–4170.

Figure 5. (a) TEM image of mesostructured-TiO2 film and (b) its magnified image; (c) SAXS pattern of meso-TiO2 film; (d) nitrogenadsorption-desorption isotherms of mesostructured-TiO2 film, with the corresponding pore-size distribution (inset) calculated from the adsorptionisotherm using a BJH method.

Figure 6. Logarithm of normalized cantilever frequency shift rep-resenting photocatalytic degradation of methylene blue deposited onvarious TiO2 films: sg-TiO2 (b), P25-TiO2 ((), and meso-TiO2 (2), aswell as directly deposited on a silicon cantilever without TiO2 (9).

3036 Analytical Chemistry, Vol. 82, No. 7, April 1, 2010

sufficiently low (KC , 1), the equation can be simplified to anapparent first-order equation:

ln(C0

C ) ) kKt ) kappt (4)

where C0 is the initial dye concentration and kapp is the apparentfirst-order rate constant. Since the change in concentration ofdye can be calculated from the frequency shifts of thecantilever, eq 4 becomes

ln( ∆fload

∆fdecomp) ) kappt (5)

where ∆fload and ∆fdecomp are the resonance frequency changesdue to the initial loading and photodecomposition of methyleneblue, respectively. The apparent first-order rate constants, kapp,are calculated from the gradients to be 0.010, 0.016, and 0.031min-1 for P25-TiO2, sg-TiO2, and meso-TiO2, respectively,confirming that meso-TiO2 has superior photocatalytic activity.

CONCLUSIONIn the present study, we have used microcantilever arrays to

investigate the photocatalytic activity of TiO2 films; to ourknowledge, this represents the first time such an approach hasbeen used to determine photocatalytic activity. Three types of

TiO2 film were tested: one derived from nanoparticles, oneprepared via a sol-gel reaction, and mesoporous TiO2 obtainedvia the CASH method. Since the microcantilever measuresabsolute mass changes with unprecedented sensitivity, itprovides more direct information on the photocatalytic degra-dation of organic molecules than conventional methods suchas GC and UV-vis. Further, the arrayed structure of thecantilever sensors enables measurement of multiple samplessimultaneously, resulting in a highly efficient tool for thescreening of photocatalyst candidates. The results from thecantilever measurements and various analyses using SAXS,BET, and TEM suggest that the high photocatalytic activity ofthe mesoporous TiO2 can be attributed to its purely anatasecrystalline morphology as well as its well-organized porestructure.

ACKNOWLEDGMENTThis work was supported by Basic Science Research Program

through the National Research Foundation of Korea (NRF) grantfunded by Ministry of Education, Science, and Technology(20090073868).

Received for review January 15, 2010. Accepted February26, 2010.

AC100119S

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