Enhanced Photocatalytic Production of H2 on Mesoporous TiO2

Oct 3, 2007 - Materials Research Bulletin 2016 76, 370-375 ..... Template free synthesis of locally-ordered mesoporous titania and its application in ...
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J. Phys. Chem. C 2007, 111, 15244-15250

Enhanced Photocatalytic Production of H2 on Mesoporous TiO2 Prepared by Template-Free Method: Role of Interparticle Charge Transfer Narayanan Lakshminarasimhan, Eunyoung Bae, and Wonyong Choi* School of EnVironmental Science and Engineering, Pohang UniVersity of Science and Technology (POSTECH), Pohang 790-784, Korea ReceiVed: July 6, 2007; In Final Form: August 14, 2007

Efficient charge separation and transport are important factors in achieving high efficiency in TiO2 photocatalysis and dye-sensitized TiO2 applications. Mesoporous TiO2 (meso-TiO2) consisting of compactly packed nanoparticles can be a promising candidate for such purposes. In the present study, meso-TiO2 exhibiting an enhanced photocatalytic activity for H2 evolution was synthesized by a simple and facile, template-free nonhydrothermal method. The photocatalytic activity for H2 evolution of meso-TiO2 was the highest compared to nonporous colloidal-TiO2 and commercial Degussa P25 (P25) and Hombikat UV-100 (HBK) samples. The highest activity of meso-TiO2 was further supported by photocurrent generation and photoluminescence. Under visible light irradiation, the dye-sensitized TiO2 system also exhibited a higher activity for H2 production with meso-TiO2 compared with colloidal-TiO2. The activity of meso-TiO2 exhibited a unique dependence on Pt cocatalyst loading. Under both UV and visible light irradiation, the highest activity for H2 production was obtained around 0.1 wt % Pt and further increase reduced the activity, whereas other nonporous TiO2 samples exhibited a typical saturation behavior with increasing Pt load. The enhanced photocatalytic activity of mesoTiO2 is ascribed to the compact and dense packing of TiO2 nanoparticles forming a uniform agglomerate, which enables efficient charge separation through interparticle charge transfer. Finally, the present simple synthesis method we developed is advantageous over other methods because it eliminates the use of templates and a hydrothermal process, which is highly favored for the scale-up production of the meso-TiO2 photocatalyst.

Introduction Titanium dioxide nanoparticles as a photocatalyst find diverse applications ranging from the remediation of polluted water and air to H2 production.1-6 One of the key and popular research areas related to TiO2 is its synthesis that enables the control of shape, size, morphology, and crystallinity. Since the photocatalytic activity of TiO2 nanoparticles often depends on such structural parameters, the structure-activity relation in TiO2 photocatalysis is an interesting issue. Mesoporous materials have received great attention ever since the discovery of MCM-41, and the synthesis of non-siliceous mesoporous materials is of growing interest for their application in different fields.7,8 Several studies have reported the enhanced photocatalytic activity of mesoporous TiO2 (meso-TiO2), in which the mesoporous structure is advantageous in enhancing the adsorption (or desorption)of reactants (or products).9-12 The photocatalytic activity of meso-TiO2 has been tested for H2 production as well.13 The meso-TiO2 structure consisting of compactly packed nanoparticles can be advantageous in making the interparticle charge transfer (electron and hole hopping) facile to enhance the overall photoefficiency. This kind of ordered nanoparticle assembly in meso-TiO2 significantly enhanced the performance of dye-sensitized solar cells (DSSC).14-18 A similar effect might be expected for meso-TiO2 as a photocatalyst in which the compact packing provides facile charge transfer while the reactants freely diffuse through the mesopores as illustrated in Figure 1. * Corresponding author. Fax: +82-54-279-8299, E-mail: wchoi@ postech.ac.kr.

Figure 1. Schematic illustration showing the compact packing of TiO2 nanoparticles with creating mesopores and facilitating the charge-pair separation.

There are several methods reported in the literature for the synthesis of meso-TiO2 such as sol-gel, sonochemical, hydrothermal, and microwave hydrothermal methods.19-22 Although most synthesis processes involve the use of templates and hydrothermal treatment, the preparation of meso-TiO2 can be achieved in principle by compact packing of uniform nanoparticles without the need of the template. The resulting structure must contain pores that are smaller than one particle diameter. It has been demonstrated that TiO2 particles can be compactly packed with a uniform distribution of pore volume in a solution with pH above or below its isoelectric point (∼5.5).23-25 The packing of TiO2 particles and the concurring pore formation are governed by the electrostatic potential that can be adjusted by controlling the ionic strength. Therefore, it is possible to synthesize meso-TiO2 consisting of nanoparticles by a controlled

10.1021/jp0752724 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007

Photocatalytic Production of H2 on Mesoporous TiO2 hydrolysis of titanium alkoxide precursor in the presence of an electrolyte (e.g., KCl). Based on the above reasoning, we have successfully synthesized meso-TiO2 in a simple and reproducible method without the use of templates and hydrothermal processing. Such a method is highly advantageous for practical scale-up production. Here we also demonstrated that meso-TiO2 with densely packed nanoparticles (prepared by a facile synthesis process) exhibited an enhanced photocatalytic activity for H2 production compared with colloidal and commercial TiO2 samples in both UV and visible light (dye-sensitized) systems. The meso-TiO2 required a minimum loading of Pt cocatalyst, and its enhanced photocatalytic activity was investigated in various ways and discussed. Experimental Section Synthesis. Mesoporous TiO2 was synthesized as follows: 4.4 mL (0.015 mol) of titanium tetraisopropoxide (TTIP; Junsei 98%) was added to 100 mL of ethanol (J.T. Baker, 99.9%) containing 0.4 mL of 0.01 M aqueous KCl (∼pH 8.5) under vigorous stirring. A white precipitate was formed immediately, and the stirring was continued at a moderate speed for 6 h. After the stirring, the precipitate was collected by filtering and washed thoroughly with distilled water several times. The washed precipitate was dried overnight in an oven at 85 °C. The dried powder was calcined at 450 °C for 1 h in a muffle furnace. The heating and cooling rate was 3 °C/min. This calcination resulted in white powder. The thorough washing of the precursor and the heating condition were critical to obtaining pure white product; otherwise contamination by carbon from the precursor occurred. Colloidal-TiO2 nanoparticles were synthesized by a controlled hydrolysis of TTIP. In a typical procedure, a mixture of 30 mL of TTIP and 5 mL of 2-propanol was added to 180 mL of distilled water and then 2 mL of nitric acid was added. The mixture was heated at 80 °C for 8 h, and then the solvent was evaporated using a rotavap to get the final product. The commercial TiO2 samples, Degussa P25 (P25) and Hombikat UV-100 (HBK), were used as received. Characterization. The thermogravimetric analysis of the dried precursor of meso-TiO2 was carried out in flowing air (Perkin-Elmer, DTA 1700). Low- and wide-angle powder X-ray diffraction (XRD) were carried out for the phase analysis using Cu KR radiation (MAC Science Co., M18XHF). The N2 sorption studies were carried out at 77 K using a Micromeritics ASAP 2000 instrument. Diffuse reflectance UV-visible absorption spectra were recorded using a spectrophotometer (Shimadzu UV-2401PC) with an integrating sphere attachment. BaSO4 was used as the reference. The field emission scanning electron micrographs (FE-SEM) were obtained using a Jeol JSM-7401F microscope. The transmission electron micrographs (TEM) of meso-TiO2 and colloidal-TiO2 nanoparticles were obtained using a Jeol JEM-2100F microscope. Photoluminescence of powder samples was recorded at room temperature using a fluorescence spectrometer (Shimadzu, RF-5410PC). Photocatalytic Activity Measurements. In the UV irradiation experiments, the H2 evolution was tested in aqueous CH3OH solution (10% v/v). The reactor used was a Pyrex bottle with a total volume of 95 mL. A 450 W Xe arc lamp (Oriel) was used as the light source, and the light passed through a 10 cm IR water filter and a UV cutoff filter (λ > 300 nm). Platinum (0.1 wt %) was used as a cocatalyst and deposited on the surface of TiO2 by an in situ photodeposition method using H2PtCl6 solution.26 After photodeposition of Pt for 30 min, the reactor was sealed with a rubber septum and purged with N2 gas for 45 min before initiating the H2 evolution experiment. The

J. Phys. Chem. C, Vol. 111, No. 42, 2007 15245 evolved H2 was detected by a gas chromatograph (GC, HP6890N) with a thermal conductivity detector (TCD) using N2 as the carrier gas. In the case of visible light sensitized experiments, a ruthenium bipyridyl complex, [Ru(4,4′-dicarboxy-2,2′-bipyridine)3]Cl2 (RuL3), was used as a sensitizer.27 The Pt/TiO2 powder was redispersed in distilled water (0.5 g/L) under sonication, and then the ruthenium sensitizer was added. The typical sensitizer concentration added to the suspension was 10 µM, and all sensitizers were completely adsorbed on TiO2. EDTA (10 mM) was added in the suspension as an electron donor to regenerate the oxidized sensitizers. The visible light activities of the Pt/ TiO2/RuL3 complex were tested for H2 production. The light source was a 450 W Xe arc lamp (Oriel). The light passed through a 10 cm IR water filter and a UV cutoff filter (λ > 420 nm) and then was focused onto a 30 mL reactor. The aqueous suspension containing the sensitized catalyst and EDTA (10 mM) at pH 3 was deaerated by N2 sparging before irradiation, and the reactor was sealed from ambient air during the irradiation. The production of H2 was monitored by GC with a TCD using N2 as the carrier gas. Photoelectrochemical Measurements. The photocurrent collection experiments were carried out using a three-electrode assembly. A Pt plate (1 × 1 cm2), graphite rod, and standard calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. Methyl viologen (MV2+) (1 mM) was used as an electron shuttle, KNO3 (0.1 M) was used as the electrolyte, and CH3OH (10% v/v) was used as the hole scavenger as in the H2 evolution experiment. N2 gas was continuously purged through the suspension before and during the experiment. The suspension was magnetically stirred during the UV light irradiation (λ > 300 nm). The photocurrents were collected by applying a potential (+0.1 V vs SCE) to the Pt working electrode using a potentiostat (EG&G 263A2) that was connected with a computer. Results and Discussion Physical Property Characterization. The thermogravimetric analysis (TGA) results of the dried precursor of meso-TiO2 are shown in Figure 2a. From the TGA curve, the observed weight loss of 17.9% is completed around 400 °C. The differential thermogravimetric analysis (DTA) curve shows an endothermic peak at 157 °C which is due to the loss of adsorbed water. The very sharp and narrow exothermic peak at 372 °C is ascribed to the burning of organic residue. Based on these data, we could confirm that the calcination temperature of 450 °C is sufficient to crystallize the amorphous precursor into crystalline TiO2. Powder X-ray diffractograms of the obtained TiO2 in the low and wide angle are shown in Figure 2b. There are no diffraction lines observed in the low angles, indicating that there is no ordered arrangement of the mesopores in TiO2. The wide-angle powder XRD pattern shows broad peaks, and all these reflections are identified as being due to the anatase phase of TiO2. The crystallite size (d) was calculated using the Scherrer formula:

d)

0.9λ B cos θ

where λ is the wavelength of the X-ray used (1.5406 Å), B is the full width at half-maximum (fwhm, in radians), and θ is the diffraction angle of the reflection. Using this, a crystallite size of 18 nm was obtained from the fwhm of the (101) diffraction line. Compared with commercial samples, meso-TiO2

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Figure 3. N2 adsorption-desorption BET isotherms. The inset shows the pore size distribution of meso-TiO2.

Figure 2. (a) TG and DTA curves of dried precursor of meso-TiO2 and (b) powder XRD pattern of meso-TiO2. The inset shows the lowangle XRD pattern.

has a crystallinity in between that of P25 and HBK samples (see Figure S1 in the Supporting Information). The Brunauer-Emmett-Teller (BET) method was used to calculate the surface area (SA). We could reproducibly synthesize meso-TiO2 with BET SA of 70 ( 5 m2 g-1. The average particle size corresponding to the BET SA was 22.3 nm as calculated assuming spherical particles using the relation dBET ) 6/(F(SA)), where F is the density of anatase (3.84 g cm-3).25 A good agreement between the particle sizes obtained from the XRD line broadening and BET SA indicates that the agglomeration of primary nanoparticles in meso-TiO2 takes place without merging at the grain boundaries. The N2 adsorption-desorption isotherms are shown in Figure 3. The results show that the obtained isotherm is of type IV with H2 type hysteresis. These are the characteristics of mesoporous solids.28 The pore size distribution (obtained from the desorption branch of the isotherm) is centered around the diameter of 3.9 nm for the meso-TiO2. In general, uniformly agglomerated particles usually have a narrow distribution of pores, all with a certain fraction (typically 1/2-1/5) of the

primary particle size.29 The pore size of this meso-TiO2 agrees very well with this, and it can be concluded that meso-TiO2 consists of densely packed primary nanoparticles with uniform agglomeration. The N2 adsorption-desorption isotherms of colloidal-TiO2 nanoparticles and commercial TiO2 samples are different compared to that of meso-TiO2 (Figure S2, Supporting Information). From the shape of the isotherms, it is clear that the colloidal-TiO2 and commercial TiO2 samples are nonporous with no uniform agglomerates of the particles. FE-SEM images of meso-TiO2, P25, and HBK samples and a TEM image of colloidal-TiO2 are shown in Figure 4. The obtained meso-TiO2 consists of spherical secondary particles whose sizes range over 0.5-1 µm. These secondary particles are made up of densely packed smaller primary nanoparticles (10-15 nm; Figure S3, Supporting Information), which induces the mesoporous structure. SEM images of P25 and HBK and a TEM image of colloidal-TiO2 are also compared in Figure 4. All these TiO2 samples consist of loose and unconsolidated nanoparticles, and there is no dense packing of the primary nanoparticles as observed in meso-TiO2. These images are also consistent with the absence of mesopores in colloidal-TiO2 and commercial TiO2 samples as evidenced in the N2 adsorptiondesorption isotherm. The diffuse reflectance UV-visible absorption spectra of all TiO2 samples show strong absorption in the UV region (Figure S4, Supporting Information). Meso-TiO2 Formation without Template. In general, the synthesis of mesoporous materials involves the use of templates as a structure-directing agent and the later removal of the template by calcination. Sometimes, the surfactant-assisted solgel method takes more than 1 week.13 The incomplete removal of the template at a given temperature may lead to contamination of the product, which may have an adverse effect on catalytic properties. Indeed, there are some reports in which the use of templates was avoided. Liu et al. synthesized meso-TiO2 without using any template by controlled hydrolysis of titanium nbutoxide at acidic pH, but the preparation took more than 2 weeks for gelation and drying in vacuo.30 In other cases, ultrasonication9 and hydrothermal processing21 were essentially used in the absence of template. Though the hydrothermal method removes the template and induces crystallinity at low temperatures, a simpler calcination step is highly preferred for practical scale-up preparation. Our new synthesis of meso-TiO2 can be done within a day through a template-free and nonhydrothermal route.

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Figure 4. (a, b) FE-SEM images of meso-TiO2, (c) magnified view of (b), (d) P25, and (e) HBK; (f) TEM image of colloidal-TiO2. The inset in (c) shows the lattice image of meso-TiO2 obtained from HR-TEM.

The formation of meso-TiO2 can be understood on the basis of the classic work of Barringer and Bowen.23-25 The mesoporous structure originates from the state of dispersion of the precursor particles in the solution followed by its agglomeration. To get uniformly packed dense nanoparticles, the coagulation of the nanoparticles should occur in a controlled way. The coagulation is induced by the van der Waals attraction and can be prevented electrostatically.29 The electrostatic stabilization depends on the presence of an electrical double layer that is formed by adsorbed counterions or the protonation/deprotonation of the surface hydroxyl group. When the attractive interaction prevails near the isoelectric point (IEP, ∼pH 5.5), TiO2 nanoparticles are highly aggregated in a disordered state and the aggregates do not have any mesoporous structure. At pHs well above or below the IEP, the TiO2 nanoparticles surrounded by the electrical double layer are repulsively stabilized. At the same time, the electrostatic repulsive force can be controlled by adjusting the ionic strength. That is, adding KCl electrolyte mitigates the repulsive force, which enables the dense compact packing of TiO2 nanoparticles in a controlled way as shown in Figure 4a. H2 Evolution under UV Light. To evaluate the photocatalytic activity of the synthesized meso-TiO2, the photocatalytic H2 evolution was carried out in aqueous methanol solution under UV irradiation. The H2 production activity of the meso-TiO2 was compared with that of nonporous and irregularly agglomerated TiO2 samples, i.e., colloidal-TiO2 nanoparticles (BET SA, 163 m2 g-1; crystallite size 7.5 nm), P25 (BET SA, 50 m2 g-1; crystallite size 26 nm), and HBK (BET SA, 348 m2 g-1; crystallite size 8 nm). The BET SA and the crystallite size (70 m2 g-1, 18 nm) of meso-TiO2 are in between those of P25 and HBK TiO2. Therefore, by comparison with these samples, the effect of the mesoporous structure on the photocatalytic H2

Figure 5. Time course of H2 evolution by different TiO2 photocatalysts; [TiO2] ) 1 g/L (with 0.1 wt % Pt); 10% (v/v) CH3OH in water; λ > 300 nm.

production can be inferred. With 0.1 wt % Pt loaded on these TiO2 samples, the H2 production linearly increased with the irradiation time, and the activity order is meso-TiO2 > HBK > P25 > colloidal-TiO2 (Figure 5). Considering the fact that the SA of meso-TiO2 is comparable to that of P25 and much lower than that of HBK, the SA-normalized activity is the highest with meso-TiO2. In particular, it is noted that colloidal-TiO2 is almost completely inactive for H2 production despite its high SA. The colloidal-TiO2 is in the least aggregated state (Figure 4f) among four samples. The most plausible explanation for the inactivity of colloidal-TiO2 is that the charge-pair separation through the interparticle charge transfer is hindered because of the lack of the particle-to-particle connectivity. In such a case,

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Figure 6. Amount of evolved H2 as a function of Pt loading; [TiO2] ) 1 g/L; 10% (v/v) CH3OH in water; λ > 300 nm. The amount of H2 evolved is after 5 h reaction.

charge-pair recombination should be the fastest in colloidalTiO2 with the lowest activity. Based on this reasoning, the order of H2 production activity observed in Figure 5 can be directly related to the degree of interparticle connectivity. The more compactly aggregated, the higher the H2 production activity of TiO2. Therefore, the highest activity of meso-TiO2 can be ascribed to the most efficient charge separation through the interparticle charge transfer within the compactly packed aggregates. The relationship between the activity and the interparticle connectivity will be further discussed and supported below (regarding Figures 7 and 9). Figure 6 shows the dependence of the H2 production activity on the Pt loading among three TiO2 samples: meso-TiO2, P25, and HBK. While the activity of nonporous TiO2 rapidly increases and then reaches the saturation region with increasing Pt load up to 1.0 wt %, the activity of meso-TiO2 is maximal at 0.1 wt % and further increase of Pt loading (up to 1 wt %) gradually decreases the activity. The fact that the activity of meso-TiO2/Pt is optimized at such a low Pt loading is of practical importance because the use of expensive platinum can be minimized in the preparation process. Such a unique behavior of meso-TiO2 seems to be related to the mesoporous structure and will be discussed below. The photocatalytic activity of TiO2 samples was also measured in terms of the photocurrent collected in the illuminated suspension (Figure 7). MV2+ was used as an electron shuttle in the aqueous TiO2 suspension with the three-electrode assembly.31,32 The general trend agrees well with that of Figure 5 (H2 production activity): the photocurrent generation is highest with meso-TiO2 and negligible with colloidal-TiO2. Note that the photocurrent was obtained with bare TiO2 samples (no Pt deposition). This indicates that the highly enhanced photoactivity of meso-TiO2 should not be related to the special interaction between Pt and TiO2, but ascribed to the retarded charge-pair recombination through the interparticle charge separation. Hence the dense packing of TiO2 nanoparticles in meso-TiO2 plays an important role in the efficient charge separation and higher photocatalytic activity as depicted schematically in Figure 1. The retarded recombination in meso-TiO2 is also supported by the photoluminescence (PL) spectra. The PL spectra of the TiO2 samples are compared in Figure 8. The emission bands at 3.10 and 3.39 eV (indicated by arrows) correspond to the indirect and direct transitions of TiO2, respectively.33,34 The PL intensity

Lakshminarasimhan et al.

Figure 7. Photocurrent collected by MV2+ electron shuttle in N2saturated TiO2 suspensions; [TiO2] ) 1 g/L (without Pt), 10% (v/v) CH3OH in water; [MV2+] ) 1 mM; [KNO3] ) 0.1 M; applied potential ) +0.1 V (vs SCE); λ > 300 nm.

Figure 8. Photoluminescence emission spectra of various TiO2 powder samples.

is directly related to the electron-hole recombination process.35 Under the same intensity of excitation irradiation, the lower emission intensity implies that more charge carriers are trapped, recombined via a radiationless path, or transferred at the particle surface. In terms of the photogenerated conduction band electrons, the above process can be expressed by eq 1, where

[e-]ex ) [e-]trap + [e-]nonrad + [e-]rad + [e-]transfer (1) each species represents the electrons that are initially excited (ex), trapped at defect sites (trap), recombined via a radiationless path (nonrad), recombined with PL (rad), or transferred/reacted at the interface (transfer). The photocatalytic activity (or quantum yield, Φ ) [e-]transfer/[e-]ex) is higher with higher [e-]transfer. Therefore, higher photocatalytic activity is related to lower emission intensity provided that the term of [e-]trap + [e-]nonrad remains relatively constant. The PL intensities decrease in the order HBK > P25 > colloidal-TiO2 ≈ meso-TiO2. This is consistent with meso-TiO2 having the highest photocatalytic activity as shown in Figures 5 and 7. The case of colloidalTiO2 seems to be an exception to the above assumption: lower

Photocatalytic Production of H2 on Mesoporous TiO2

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emission intensity (i.e., lower [e-]rad) despite its lower activity (i.e., lower [e-]transfer) than HBK and P25. This implies that the term of [e-]trap + [e-]nonrad in colloidal-TiO2 is higher than in other samples. The colloidal-TiO2 nanoparticles have the lowest crystallinity in terms of the XRD intensity (see Figure S1, Supporting Information) and may possess a larger number of defect sites, which increases the term of [e-]trap + [e-]nonrad. Between P25 and HBK, the PL intensity of P25 is lower than that of HBK although the H2 production and photocurrent activities were lower with P25. It should be noted that P25 has a mixed crystallinity of anatase and rutile. Although the charge separation at the anatase/rutile interface may increase [e-]transfer with accumulating electrons in the rutile phase and hence lowering PL, the rutile phase is inactive for H2 production. H2 Evolution under Visible Light in Dye-Sensitized TiO2 System. If it were the dense packing of TiO2 nanoparticles responsible for the enhanced photocatalytic activity for H2 evolution under UV light, then we would expect a similar behavior under visible light in the dye-sensitized TiO2 system. As the mesoporous TiO2 electrode increased the efficiency of DSSC in previous studies,14-18 H2 production with meso-TiO2 can also be enhanced in the dye-sensitized system. In this case, the activity should be directly related to the electron transport since holes are not involved. The oxidized dyes may be compared to holes but they are immobilized on the TiO2 surface, unlike valence band holes that can hop from particle to particle. We have carried out the H2 production experiments with colloidal-TiO2 and meso-TiO2 under visible light irradiation using a Ru complex (RuL3) as the sensitizer, which can be described by the following reactions.27

RuIIL3-TiO2/Pt + hν f RuIIL3*-TiO2/Pt (dye excitation) (2) RuIIL3*-TiO2/Pt f RuIIIL3-TiO2(e-)/Pt (electron injection) (3) RuIIIL3-TiO2(e-)/Pt f RuIIL3-TiO2/Pt (recombination) (4) RuIIIL3-TiO2(e-)/Pt f RuIIIL3-TiO2/Pt(e-) (electron trapping at Pt) (5) RuIIIL3-TiO2/Pt(e-) + H+ f RuIIIL3-TiO2/Pt + 0.5H2 (H2 production) (6) RuIIIL3-TiO2/Pt + EDTA f RuIIL3-TiO2/Pt + EDTAox (dye regeneration) (7) The time course of H2 evolution in the visible light sensitized system is shown in Figure 9a. The amount of H2 evolved is higher with meso-TiO2 compared to colloidal-TiO2. This result is in good agreement with the photocatalytic activity observed under UV light irradiation. The higher activity of meso-TiO2 in this case is fully ascribed to the role of hopping elelctrons within the organized aggregates because the holes are not generated in the dye-sensitized system. On the colloidal-TiO2, the injected electrons (reaction 3) are rapidly recombined (reaction 4) because the electron is mostly confined in an isolated nanoparticle. On the other hand, the injected electrons in meso-TiO2 hop from nanoparticle to nanoparticle within a

Figure 9. (a) Time course of H2 evolution by colloidal-TiO2 and mesoTiO2 in the sensitized system. (b) Amount of H2 produced (after 3 h reaction) by colloidal-TiO2 and meso-TiO2 as a function of Pt loading; [TiO2] ) 0.5 g/L (with 0.1 wt % Pt in (a)); [RuL3]i ) 10 µM; [EDTA] ) 10 mM; λ > 420 nm.

secondary agglomerated particle to escape recombination and the activity is enhanced. Since meso-TiO2 exhibited a unique behavior with Pt loading, i.e., a smaller amount of Pt is sufficient to attain the highest photocatalytic activity, the dependence of H2 evolution activity on the amount of Pt loaded was also tested in the dye-sensitized system; the results are shown in Figure 9b. In this case, mesoTiO2 showed the highest activity with 0.05 wt % Pt loading and further increase in Pt content decreased the amount of H2 evolution. This behavior is similar to the result observed under UV irradiation where the highest activity was observed with 0.1 wt %. In the case of colloidal-TiO2, the dependence of H2 evolution on the Pt loading showed a behavior similar to that observed with commercial TiO2 samples; i.e., the amount of H2 evolved increased with increasing Pt content. With mesoTiO2, the unique effect of Pt loading is due to the presence of densely packed secondary structure. More Pt loading (above 0.05-0.1 wt %) on meso-TiO2 may shield inner TiO2 nanoparticles from UV light, and the higher amount of Pt deposited may block the pores, affecting the diffusion of reactants (H2O and electron donors). Alternatively, more than needed Pt loading may create the surface defect sites that act as a charge recombination center. It was reported that the photocatalytic

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Lakshminarasimhan et al. gram of MOST/KOSEF (Grant R11-2000-070-06004-0), and the BK21 program. Supporting Information Available: Powder XRD patterns, N2 adsorption-desorption BET isotherms, HR-TEM image of meso-TiO2, and diffuse reflectance UV-visible absorption spectra of various TiO2 samples (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 10. Schematic illustration showing the hopping electron transfer in meso-TiO2 to remote Pt sites under UV or visible light.

activity of meso-TiO2 loaded with Au for phenol oxidation and Cr(VI) reduction exhibited an optimum loading at 0.5% Au, beyond which the activity decreased,36 which is similarly compared with this work. Here we propose a general explanation for why the activity of meso-TiO2 is highly enhanced. The primary nanoparticles in meso-TiO2 are densely packed with 4 nm pores created within the aggregate, while those in colloidal-TiO2, HBK, and P25 are little or loosely agglomerated (hence no mesopores). The transport of charge carriers through the grain boundaries within the TiO2 agglomerate is efficient.37 Therefore, the interparticle transport of charge carriers should depend on the connectivity among primary TiO2 nanoparticles. While the interparticle connection in colloidal-TiO2, P25, and HBK is rather random, loose, and anisotropic, that in meso-TiO2 is highly ordered, compact, and isotropic. This implies that the charge separation (i.e., the photocatalytic activity) is most efficient with mesoTiO2. The photocurrent collection data presented in Figure 7 and the activity comparison with the dye-sensitized TiO2 system (Figure 9) strongly support this claim. With such a good connectivity among primary nanoparticles in meso-TiO2, a minimum loading of Pt can be sufficient (Figures 6 and 9b) because the photogenerated electrons can be efficiently transported to remote Pt sites as shown schematically in Figure 10. Thus the densely packed TiO2 nanoparticles forming a uniform agglomerate are responsible for the enhanced photocatalytic activity of meso-TiO2 prepared by a template-free nonhydrothermal method which is a simple and fast approach for scaleup production of mesoporous TiO2 photocatalyst. Conclusions The present work shows the photocatalytic activity for H2 evolution is markedly enhanced with meso-TiO2 compared to nonporous TiO2 samples. The enhanced photocatalytic activity of meso-TiO2 is due to the compact and dense packing of TiO2 nanoparticles forming an uniform agglomerate, which enables efficient charge separation through interparticle charge transfer. The unique photocatalytic properties of meso-TiO2 were demonstrated with photocatalytic H2 evolution under both UV irradiation on bare TiO2 and visible light on dye-sensitized TiO2. On the other hand, the present synthesis method we developed is facile, fast,and reproducible to obtain crystalline meso-TiO2. The synthesis method is advantageous over other methods in two ways: it is a template-free and nonhydrothermal process. Hence, this method is of great practical value for scaling up the synthesis of the meso-TiO2 photocatalyst. Acknowledgment. This work was supported by the KOSEF Nano R&D program (Grant 2005-02234), the SRC/ERC pro-

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