Enhanced Photocatalytic Activity of Highly Crystallized and Ordered

Mar 8, 2010 - interconnected pore network facilitates the diffusion of methylene ... the highest photocatalytic activity.5–7,15 The photocatalytic p...
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Anal. Chem. 2010, 82, 3032–3037

Enhanced Photocatalytic Activity of Highly Crystallized and Ordered Mesoporous Titanium Oxide 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 of Science and Technology (POSTECH), Pohang, Kyungbuk 790-784, Korea and Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853 Ordered mesoporous TiO2 was synthesized using the combined assembly of soft and hard chemistries method and deposited as a film coating on a microcantilever sensor array along with two other types of TiO2 film: one from nanoparticles and one prepared via a sol-gel reaction. After loading methylene blue molecules on the TiO2 films, the films were exposed to ultraviolet radiation. The photocatalytic decomposition of methylene blue was monitored by measuring changes in the resonance frequency of each cantilever. The mesoporous TiO2 film showed higher photocatalytic activity than conventional TiO2 films fabricated from nanoparticles or via a sol-gel reaction; this difference is attributed to the purely anatase crystalline morphology of the mesoporous TiO2 film as well as its well-organized pore structure. The three-dimensionally interconnected pore network facilitates the diffusion of methylene blue molecules to the photocatalytically active sites of the mesoporous TiO2. Since the discovery of light-induced water splitting on TiO2 surfaces,1 TiO2 has attracted attention as a promising photocatalyst due to its excellent photocatalytic activity and longterm 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 * Corresponding author. E-mail: [email protected] (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 and Technology (POSTECH). ‡ School of Environmental Science and Engineering, Pohang University of Science 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.

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the photocatalytic decomposition of various organic molecules on TiO2 surfaces has been reported elsewhere.5–7,11-13 When TiO2 is irradiated by photons whose energy exceeds the TiO2 band gap (3.2 eV), electrons are excited from the valence band to the conduction band, resulting in the formation of charge-carrier pairs, that is, a hole (h+) and an electron (e-). These charge carriers can migrate to the surface and react with adsorbed molecules, unless recombination occurs first. The hole typically oxidizes adsorbed water to a hydroxyl radical (•OH), whereas the electron reduces adsorbed molecular oxygen to a superoxide radical anion (•O2-). These oxidizing radicals react with adsorbed organic molecules, inducing oxidative degradation to carbon dioxide and water.8,12–14 The photocatalytic activity of TiO2 is affected by its crystal structure, because the defects in the crystal are usually the trap sites for the recombination of light-generated electron-hole pairs. Among the various TiO2 polymorphs, including anatase, rutile, and brookite, it is known that anatase crystallites have the highest photocatalytic activity.5–7,15 The photocatalytic performance of TiO2 can be further enhanced by increasing the surface area, which allows greater contact between organic molecules and photocatalytic active sites. Therefore, in efforts to develop TiO2-based photocatalysts, it is important to synthesize pure-anatase TiO2 crystallites with a large surface area.16 A promising candidate is highly crystallized mesoporous TiO2 with a well-defined pore structure. A number of research groups have tried to fabricate ordered mesoporous TiO2 using soft-template17–19 (8) Ollis, D. F.; Al-Ekabi, H. Photocatalytic purification and treatment of water and 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. 10.1021/ac100119s  2010 American Chemical Society Published on Web 03/08/2010

or hard-template20,21 methods. However, nanostructured materials produced by the soft-template method generally have amorphous or semicrystalline morphology because the thermally unstable soft template limits heat treatment to temperatures below ∼400 °C, which is not sufficient to produce crystalline mesoporous TiO2.22 On the other hand, although hard templates can be heated to temperatures that are sufficiently high for crystallization, this approach has the disadvantage that hard template synthesis is timeconsuming and complicated.20,21 To overcome these limitations of the soft- and hard-template methods, previously we developed a method called combined assembly of soft and hard chemistries (CASH).23 The resulting mesoporous TiO2 was found to have a wellconnected pore network and a highly crystalline anatase structure. In this study, we synthesized a mesoporous TiO2 film using the CASH method and compared its photocatalytic activity with those of two other TiO2 films fabricated from nanoparticles or via a sol-gel reaction. The photocatalytic activity was measured using microcantilever arrays; to our knowledge, this is the first study to use this approach to determine photocatalytic activity. Compared with conventional methods for the evaluation of photocatalysts, such as ultraviolet-visible absorption spectroscopy (UV-vis) and gas chromatography (GC), the microcantilever array method has many advantages. First, microcantilever arrays measure the absolute mass change due to the photodegradation of organic molecules, whereas UV-vis and GC techniques measure the photodegradation indirectly via color change and the amount of produced gases, respectively. For example, UV-vis spectroscopy cannot distinguish between the complete photodegradation of dye molecules and partial structural damage, because they induce similar color changes of the dye solution. Second, the sensitivity of microcantilever arrays is superior to those of UV-vis and GC. The resonance frequency, f, of an oscillating cantilever is related to the mass of the cantilever, m, as follows:

f)

1 2π

mk

(1)

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

(

k 1 1 - 2 4π2 f12 f0

)

(2)

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

could be further increased using a cantilever with a higher spring constant. Further, the arrayed structure of the microcantilevers can be utilized to evaluate multiple photocatalyst samples simultaneously, making the microcantilever technique highly efficient for the screening of photocatalyst candidates. EXPERIMENTAL SECTION Materials. Titanium tetraisopropoxide (TTIP), titanium(IV) chloride, tetrahydrofuran (THF), chloroform, ethanol, and methylene blue were obtained from Sigma-Aldrich. Nitric acid (60%) was purchased from Matsunden Chemicals. TiO2 nanoparticles were obtained from Degussa (P25, mean diameter of ∼30 nm). The composition of the P25 nanoparticles as stated by the manufacturer is 80% anatase and 20% rutile. Polyisoprene-polyethyleneoxide block copolymer (PI-b-PEO) was synthesized using anionic polymerization techniques. Its number average molecular weight (Mn), polydispersity, and PEO fraction were determined to be 30 000 g/mol, 1.13, and 0.06, respectively.23,27 Arrays of eight silicon cantilevers were purchased from Micromotive (Mainz, Germany). Each cantilever in the array was 450 µm long, 90 µm wide, and 5 µm thick, with a spring constant of ∼3.3 N/m. Preparation of TiO2 Films. To compare the photocatalytic activities of TiO2 films having different crystalline structures and surface areas, three types of TiO2 film were prepared: a P25-nanoparticle-derived TiO2 film (P25-TiO2), a sol-gel reaction-derived TiO2 film (sg-TiO2), and a mesoporous TiO2 film (meso-TiO2). Three cantilevers in an array were each coated with one of these three types of TiO2 film using microcapillary tubes, as shown in Figure 1. For the preparation of the P25TiO2 film, P25 nanoparticles were dispersed in deionized water (3 mg/mL). A microcapillary tube was filled with this dispersion, and a silicon cantilever was immersed in the tube for 1 min. The controlled immersion depth of the cantilever inside the capillary tube was ∼100 µm. The cantilever was then heated at 200 °C for 30 min, in order to fix the nanoparticles on the cantilever surface.28 Heat treatment at this temperature does not affect the crystal structure of P25 nanoparticles. For the preparation of sg-TiO2, aqueous nitric acid was added to TTIP in ethanol to yield a titania sol via hydrolysis. The molar ratio of the sol-gel reagents was TTIP/C2H5OH/H2O/HNO3 ) 1:100:10:0.04.26 After coating a cantilever with this sol using a capillary tube, the resultant film was heated to 530 °C at a rate of 1 °C/min and calcined for 2 h to obtain crystalline TiO2. The procedure for the synthesis of meso-TiO2 can be found elsewhere.23 In summary, 0.71 mL of titanium tetraisopropoxide (TTIP) and 0.21 mL of titanium(IV) chloride were added to 0.2 g of PI-b-PEO block copolymer dissolved in 2 mL of THF. The mass ratio of TiCl4 to copolymer is about 2. After immersing a cantilever in the solution using a capillary tube, the cantilever was dried at 50 °C overnight. The TiO2-coated cantilever was heated to 530 °C at a rate of 1 °C/min and calcined for 2 h in argon to convert the amorphous TiO2 to (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.

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representative organic compound.5,29 An optical method was adopted to measure the resonance frequency of each cantilever after excitation by an external piezoelectric actuator. In this method, a focused laser beam is reflected off the free end of the cantilever onto a position-sensitive detector, and the voltage change due to the vibration of the cantilever is converted to a resonance peak using a fast Fourier transform (FFT) technique. The resonance peaks were fitted with Lorentzian curves, and the corresponding resonance frequencies were calculated. The variations in the resonance frequencies of the cantilevers under UV irradiation (λ ) 254 nm, 4.5 mW/cm2, Spectroline, NY) were measured every 10 min to monitor the photocatalytic degradation of the dye molecules. A control experiment with a bare silicon cantilever under UV irradiation showed no measurable frequency change, confirming that the mechanical characteristic of the cantilever is not affected by UV exposure.

Figure 1. (a) Optical microscope image and (b) SEM image of a silicon microcantilever array. Three of the cantilevers are coated with different types of TiO2 film; white regions at the free ends of these cantilevers represent the deposited TiO2 films. Cantilever 1: mesoTiO2; 3: sg-TiO2; 5: P25-TiO2; 7: bare silicon cantilever (reference). Cantilevers 2, 4, 6, and 8 were broken by the capillary tube during the coating process.

highly crystallized meso-TiO2. The carbon deposited on the meso-TiO2 due to the burning of PI-b-PEO was removed by heat treatment at 450 °C in air for 1 h and subsequent ultraviolet (UV) irradiation for 12 h. Characterization of TiO2 Films. Transmission electron microscopy (TEM) and high-resolution TEM characterization were carried out using a JEOL EM-2010 electron microscope. X-ray diffraction (XRD) analysis was performed using a PANalytical X’Pert diffractometer (Cu KR radiation) with an X’Celerator detector. The surface areas of the various TiO2 films were determined by recording the nitrogen adsorption-desorption isotherms at 77 K using a Micromeritics Tristar II 3020 and analyzing the data with the Brunauer-Emmett-Teller (BET) method. Small-angle X-ray scattering (SAXS) data were collected using an apparatus equipped with an 18 kW rotating anode X-ray generator (Rigaku Co., Cu KR ) 1.542 Å) and a one-dimensional position-sensitive detector (M. Braun Co.). Measurements of Photocatalytic Activity of Various TiO2 Films. To investigate the photocatalytic activities of the various TiO2 films, a bare silicon cantilever and the cantilevers coated with P25-TiO2, sg-TiO2, and meso-TiO2 were immersed for 20 min in capillary tubes containing 0.2 mM methylene blue in ethanol solution, to allow the methylene blue molecules to adsorb on the TiO2 films. The bare silicon cantilever is used here as a reference cantilever, and methylene blue is a 3034

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RESULTS AND DISCUSSION Figure 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 distribution. In contrast, mesoscale pores are observed in the sg-TiO2 and meso-TiO2 films, although the meso-TiO2 film exhibits a more uniform and porous structure than the sg-TiO2. Figure 2d shows XRD patterns for the various TiO2 films (JCPDS 21-1272), indicating that the P25-TiO2 film consists of anatase and rutile crystallites whereas the sg-TiO2 and meso-TiO2 films consist of purely anatase crystallites. The anatase crystallites of sg-TiO2 and meso-TiO2 were produced by heat treatment at 530 °C during the preparation of the films, which was not performed for P25-TiO2. The calculated BET surface areas of the TiO2 films are 50, 41, and 65 m2/g for P25-, sg-, and mesoTiO2, respectively. Note that the surface area of sg-TiO2 is much smaller than that of meso-TiO2 even though they both have anatase crystal structures. The smaller surface area of sg-TiO2 can be attributed to its structural collapse during heat treatment, which does not occur in meso-TiO2 due to its mechanically strong hard template. Figure 3 shows the change in resonance frequency of each cantilever due to its sequential coating with TiO2 film and methylene blue. The frequency changes due to the TiO2 films are 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 of deposited TiO2 calculated from eq 2. The subsequent changes in the resonance frequencies of the cantilevers due to the adsorption of methylene blue are 50, 116, 132, and 437 Hz for the bare silicon cantilever and P25-, sg-, and meso-TiO2-coated cantilevers, respectively. The ratios of the mass of adsorbed methylene blue to the mass of the TiO2 film are 0.44, 0.40, and 0.84 for P25-, sg-, and meso-TiO2-coated cantilevers, respectively, 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 exposed to 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 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 loading of TiO2 (blue) and methylene blue (red) on each cantilever. Note that the frequency change due to the methylene blue loading is with respect to the TiO2-coated cantilever.

respect to each cantilever with TiO2 and methylene blue coatings. The photoinduced hole oxidizes the methylene blue molecules to methylene blue radicals (•MB+), which decompose fully to water and carbon dioxide by further reaction with O2.29 The control experiment with the bare silicon cantilever shows a slight change in the frequency due to the UV-induced direct photolysis of methylene blue, which is negligible compared to the frequency changes observed for the TiO2-filmcoated cantilevers. The smallest frequency shift among the TiO2 films is observed for P25-TiO2 due to its lower fraction of anatase 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 photocatalytic activity than does sg-TiO2 despite the similarity of their anatase structures. This could be attributed to their different surface areas: a TiO2 film with a larger surface area possesses

Figure 4. Photocatalytic degradation of methylene blue deposited on various TiO2 films, measured via a normalized cantilever frequency shift: sg-TiO2 (b), P25-TiO2 ((), and meso-TiO2 (2), as well as directly deposited on a silicon cantilever without TiO2 (9). Inset shows the calculated mass of methylene blue on each cantilever before UV irradiation (dark blue) and after UV irradiation (red).

a greater number of active sites for the electron-hole charge separation. However, the ratio of the mass of adsorbed methylene blue to the mass of sg-TiO2 is only half that of mesoTiO2 even though the surface area of sg-TiO2 is two-thirds that of meso-TiO2, indicating that the number of active sites available for the photocatalytic decomposition of adsorbed methylene blue molecules is proportionally greater in sg-TiO2 than in meso-TiO2. Therefore, the higher photocatalytic activity of meso-TiO2 is attributed to its pore structure rather than its surface area. Compared to the pores randomly formed in sgTiO2, meso-TiO2 is known to have a well-ordered and interconnected pore network, which facilitates the dispersion of organic molecules to the photocatalytically active sites. Figure 5a,b displays TEM images of meso-TiO2, showing short-range ordered hexagonal mesoporous structures with a wall thickness of ∼10 nm and pore diameter of ∼25 nm. The Analytical Chemistry, Vol. 82, No. 7, April 1, 2010

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

pore size is sufficiently large for the diffusion of organic molecules inside the pores. The crystallite size in meso-TiO2 is estimated to be 12.3 nm using the Debye-Scherrer equation and the (101) anatase peak of the XRD pattern in Figure 2d. The similarity between the crystallite size and the wall thickness indicates that the walls are highly crystallized rather than being composed of nanocrystals embedded in an amorphous matrix.22 The SAXS experimental data for meso-TiO2 in Figure 5c show a first-order peak and two broad higher order peaks. The first-order peak corresponds to d-spacing of 30.5 nm. The broad higher order peaks at higher q values correspond to the angular positions of 31/2 and 41/2 of the first-order peak maximum. This peak pattern is typically observed for wormhole-like-structured mesoporous materials, which are known to have three-dimensionally interconnected and hexagonally ordered pore networks, facilitating the diffusion of guest molecules inside the pores.30,31 The structure of meso-TiO2 was further characterized by nitrogen adsorptiondesorption experiments as shown in Figure 5d, where it is found that meso-TiO2 exhibits a type-IV nitrogen adsorption isotherm and a calculated surface area of 65 m2/g. The pore size was estimated by the Barrett-Joyner-Halenda (BJH) method to be 25 nm, which is in agreement with the pore-size estimate from TEM. Figure 6 shows that the logarithms of the normalized frequency changes during the photodegradation are linear with respect to UV-irradiation time, indicating that the photodegradation of (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.

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

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

kKC dC ) dt 1 + KC

(3)

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

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

ln

( )

C0 ) kKt ) kappt C

(4)

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

ln

(

)

∆fload ) kappt ∆fdecomp

(5)

where ∆fload and ∆fdecomp are the resonance frequency changes due to the initial loading and photodecomposition of methylene blue, respectively. The apparent first-order rate constants, kapp, are calculated from the gradients to be 0.010, 0.016, and 0.031 min-1 for P25-TiO2, sg-TiO2, and meso-TiO2, respectively, confirming that meso-TiO2 has superior photocatalytic activity. CONCLUSION In the present study, we have used microcantilever arrays to investigate the photocatalytic activity of TiO2 films; to our knowledge, this represents the first time such an approach has been used to determine photocatalytic activity. Three types of

TiO2 film were tested: one derived from nanoparticles, one prepared via a sol-gel reaction, and mesoporous TiO2 obtained via the CASH method. Since the microcantilever measures absolute mass changes with unprecedented sensitivity, it provides more direct information on the photocatalytic degradation of organic molecules than conventional methods such as GC and UV-vis. Further, the arrayed structure of the cantilever sensors enables measurement of multiple samples simultaneously, resulting in a highly efficient tool for the screening of photocatalyst candidates. The results from the cantilever measurements and various analyses using SAXS, BET, and TEM suggest that the high photocatalytic activity of the mesoporous TiO2 can be attributed to its purely anatase crystalline morphology as well as its well-organized pore structure. ACKNOWLEDGMENT This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by Ministry of Education, Science, and Technology (20090073868).

Received for review January 15, 2010. Accepted February 26, 2010. AC100119S

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