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Effect of Agglomerated State in Mesoporous TiO2 on the Morphology of Photodeposited Pt and Photocatalytic Activity Narayanan Lakshminarasimhan,† Alok D. Bokare, and Wonyong Choi* School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea S Supporting Information *

ABSTRACT: Two mesoporous TiO2 samples (M1-TiO2 and M2-TiO2) with different morphologies were synthesized, and the photocatalytic and photoelectrochemical properties of both TiO2 and their photoplatinized counterparts (0.05, 0.1, and 1.0 wt % of Pt) were systematically investigated. Electron microscopic analysis showed that M1-TiO2 consists of densely packed nanoparticles forming spherical secondary particles (0.5 to 1.0 μm), whereas M2-TiO2 is made up of loosely agglomerated nanoparticles. Subsequently, this morphological difference led to the formation of different Pt clusters (photodeposited on them): large Pt nanoparticles on M1TiO2 versus well-dispersed smaller Pt nanoparticles on M2TiO2. The photocatalytic activities of platinized M1-TiO2 and M2-TiO2 were investigated for H2 production and 4-chlorophenol degradation. Whereas M1-TiO2 exhibited the highest photoactivity with 0.1 wt % Pt loading, the activity of M2-TiO2 increased with increasing Pt loading (up to 1.0 wt %). The critical role of surface Pt morphology on the photocatalytic behavior of M1-TiO2 and M2-TiO2 was investigated using electrochemical impedance spectroscopy and photocurrent measurements. In the case of M1-TiO2, an increase in Pt cluster size enhanced the charge-transfer resistance and reduced the interfacial electron transfer efficiency, whereas the same loading of Pt on M2-TiO2 effectively enhanced the interfacial charge transfer. This dissimilar interfacial charge-transfer kinetics for M1-TiO2 and M2-TiO2 indicates that the TiO2 microstructure controls the photodeposited Pt morphology, which subsequently affects the photocatalytic activity. This study reveals that the agglomerated state of TiO2 nanoparticles can be an important parameter in determining the photocatalytic activity in both the suspension and film states.

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INTRODUCTION The overall efficiency of TiO2-based photocatalysis strongly depends on the competition among charge-carrier recombination, trapping, and interfacial charge transfer.1 Combining noble metals such as Pt or Au with TiO2 has been demonstrated to increase the photocatalytic efficiency.2,3 The electric field at the metal/TiO2 Schottky barrier and the low Fermi level of the metal drive the photogenerated electrons from TiO2 conduction band (CB) to the metal phase, leaving the valence band (VB) holes freely available for oxidation of substrates (e.g., pollutants).4,5 The rapid electron transfer to the noble metal facilitates the charge separation and subsequently the interfacial charge transfer with enhancing the photocatalytic efficiency. In particular, Pt has been intensively studied for this purpose because its high work function allows an efficient electron trapping. Pt-loaded TiO2 (Pt/TiO2) generally exhibits enhanced activities for the oxidation of organic compounds and hydrogen generation compared with bare TiO2.6−12 The presence of Pt often has negligible or negative effects on the photocatalytic activity.13−16 The positive (or negative) effect of Pt on the TiO2 photocatalyticactivity depends on various parameters such as © 2012 American Chemical Society

the kind of the target substrate, the oxidation state of Pt, the morphology, and mass of deposited Pt.9,17−20 The size of the Pt cluster also critically influences the electronic properties of Pt/ TiO2. The homogeneous dispersion of the cluster on the TiO2 surface is also essential for the high photocatalytic efficiency.21 Furthermore, as the number and size of the Pt clusters increase (high Pt loading), metal particles can act as recombination centers and affect the TiO2-to-Pt and/or Pt-to-electrolyte electron transfer.22 Pt clusters with size 320 nm) was used as the light source for H2 production experiments. The photoreactor was sealed with a rubber septum and purged with N2 gas for 30 min before initiating the irradiation. The evolved H2 was detected by a gas chromatograph (GC, HP6890N) with a thermal conductivity detector using N2 as carrier gas. In the case of 4-CP degradation experiments, 30 mL of 4-CP aqueous solution (100 μM) was mixed with M-TiO2/xPt (0.5 g/L) [x = 0, 0.05, 0.1, and 1.0 wt %] and continuously stirred in the dark for 30 min to establish an adsorption equilibrium. A 300 W Xe-arc lamp (Oriel) with a 10 cm IR water filter and a UV cutoff filter (λ> 320 nm) was used as the light source. All experiments were carried out under air-equilibrated conditions and at ambient temperature. Sample aliquots were withdrawn from the reactor at constant time intervals during the illumination and filtered through a 0.45 μM PTFE syringe filter (Millipore). The degradation of 4-CP was monitored using high-performance liquid chromatography (HPLC, Agilent 1100 series) equipped with a diode array detector and a ZORBAX 300SB C18 column (4.6 × 150 mm). The eluent consisted of 0.1% phosphoric acid + acetonitrile (4:1 by volume). The flow rate was 1.0 mL/min, and the detection

metals Rh (and Pt) and Au, which is ascribed to the strong metal−support interaction (SMSI), and shows that the photodeposition method alone resulted in such SMSI among the different methods of noble metal deposition.24 However, during the photodeposition process, the influence of TiO2 morphology (or the agglomerated state) on the Pt cluster properties (size and distribution) has not been completely understood. Many catalyst parameters (e.g., size, surface area, crystallinity) influence the photocatalytic activity, and the agglomerated state of TiO2 nanoparticles is one of them. Although most TiO2 photocatalysts are present as the agglomerated nanoparticles in both the slurry and film states, the role of the agglomeration on the photocatalytic activity is little understood. The agglomeration state of TiO2 nanoparticles is also a crucial factor in determining the formation of photodeposited noble metal nanoparticles.25 Recently, our group reported that densely packed TiO2 nanoparticles in a mesoporous structure showed an enhanced photocatalytic activity for H2 production with low Pt loading through efficient interparticle electron transfer when compared with other commercial TiO 2 samples.26 For a better understanding of Pt photodeposition dependent on the agglomerated structure of TiO2 nanoparticles, in the present study, we synthesized two different mesoporous TiO2 samples (M1-TiO2 and M2-TiO2) that consist of agglomerated TiO2 nanoparticles. Although both M1-TiO2 and M2-TiO2 are mesoporous, they exhibited different particle size, surface area, and structural morphology (agglomerated state). The effects of Pt loading on the photocatalytic activity of M1-TiO2 and M2TiO2 were assessed in terms of hydrogen production and 4chlorophenol (4-CP) degradation, which was markedly different between M1-TiO2 and M2-TiO2. The crucial role of mesoporous TiO2 morphology in deciding the cluster size and dispersion of Pt is highlighted in relation with the photocatalytic activity.

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EXPERIMENTAL SECTION Chemicals. Titanium tetraisopropoxide (TTIP, Junsei, 97%), titanium butoxide (Ti(OBu)4; Sigma), hexachloroplatinic acid (H2PtCl6, Sigma), 4-CP (Sigma), ethanol (J. T. Baker, 99%), methanol (Daejung), and acetonitrile (Merck, HPLC grade) were used as received without further purification. All solutions were prepared in ultrapure water (18 MΩ cm) prepared by a Barnstead purification system. Synthesis of Mesoporous TiO2 (M1-TiO2 and M2-TiO2). Two mesoporous TiO2 samples (M1-TiO2 and M2-TiO2) with different morphologies were synthesized by template-free methods. M1-TiO2 was synthesized according to the procedure described elsewhere.26 Typically, 4.4 mL of TTIP was added to 100 mL of ethanol containing 0.4 mL aqueous KCl (0.01 M) and stirred vigorously. A white precipitate resulted in 10 min, and the stirring continued for 6 h, followed by filtering and thorough washing with distilled water. The washed precipitate was dried overnight in an oven. The obtained dried precipitate was ground and calcined at 450 °C for 1 h with a heating rate of 3 °C/min to get the final product. M2-TiO2 was synthesized by a hydrolysis-precipitation protocol, wherein 10 mL of Ti(OBu)4 was mixed with 10 mL ethanol under continuous stirring. The mixed Ti(OBu)4−ethanol solution was slowly added dropwise into another solution containing 10 mL ethanol and 8 mL deionized water under vigorous stirring. After the formation of a white suspension, the stirring was continued 17532

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wavelength was set at 228 nm. Chloride ion generation was monitored and quantified with an ion chromatograph (IC, Dionex DX-120) equipped with a Dionex IonPacAS14 (4 mm × 250 mm) column and a conductivity detector. The eluent solution was 3.5 mM Na2CO3/1 mM NaHCO3. Photoelectrochemical Measurements. The photocurrent collection experiments were carried out in the TiO2 suspension using a three-electrode assembly.27 A Pt plate (1 × 1 cm2), a graphite rod, and a standard calomel electrode (SCE) were used as a working, a counter, and a reference electrode, 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. Ar gas was continuously purged through the suspension before and during the experiment. The suspension was magnetically stirred during the UV light irradiation (λ > 320 nm). The photocurrents were collected by applying a potential (+0.1 V vs Ag/AgCl) to the Pt working electrode using a potentiostat (EG&G 263A2) that was connected with a computer. Photocurrents were also obtained with bare and platinized (1.0 wt %) M1(or M2)-TiO2/FTO electrodes immersed in aqueous electrolyte solution (0.1 M, NaClO4). For preparing the TiO2/FTO electrode, a cleaned FTO glass (Pilkington, TEC8) was coated with TiO2 film using Carbowax as a binder. The TiO2-coated FTO plate was then dried for 20 min in air and then calcined at 450 °C for 1 h to burn off the organic binder. The TiO 2/FTO electrode, a Ag/AgCl electrode, and a Pt wire were immersed in a cell as a working, a reference, and a counter electrode, respectively. Photocurrent as a function of time, was measured in aqueous NaClO4 (0.1 M) solution with an applied potential of +0.1 V (vs Ag/AgCl) using a potentiostat (Gamry, Reference 600, Potentiostat/ Galvanostat/ZRA) connected to a computer. Electrochemical impedance spectroscopy (EIS) measurements were carried out in above configuration of cell under irradiation (λ> 320 nm) in a frequency range between 1 MHz and 0.1 Hz at different applied biases.

Figure 1. N2 adsorption−desorption isotherms of (a) M1-TiO2 and (b) M2-TiO2. The insets show the pore size distribution.

Table 1. Surface Area, Pore Size and Pore Volume of M1TiO2 and M2-TiO2 with Different Pt Loadings sample M1TiO2

M2TiO2

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Pt content (wt%)

BET surface area (m2g−1)

pore size (nm)

pore volume (cm3g−1)

0.0 0.05 0.1 1.0 0.0 0.1 1.0

76 34 33 34 123 119 118

3.9 4.0 4.0 4.0 4.0 4.0 4.0

0.103 0.047 0.045 0.046 0.313 0.316 0.279

increase in Pt loading. However, Pt deposition did not alter the pore size, which remains constant around 4 nm. In contrast with this behavior observed with M1-TiO2, Pt loading did not alter BET SA, pore size, and pore volume in M2-TiO2. The FE-SEM images of M1-TiO2 and M2-TiO2 are compared on the same scale of magnification in Figure 2. A distinct difference in their morphology is observed. M1-TiO2 consists of spherical secondary particles whose sizes are in the range 0.5 to 1.0 μm. These secondary particles are made up of densely packed smaller primary nanoparticles (15−20 nm), which induces the mesoporous structure, as confirmed in our previous work.26 M2-TiO2 consists of loose and unconsolidated nanoparticles forming mesopores within this assembly. To find the morphology of the deposited Pt (1.0 wt %) on M1-TiO2 and M2-TiO2, we used HR-TEM technique, and the images obtained are shown in Figure 3. The Pt nanoparticles are clearly seen as dark spots due to their higher electron density, and the Pt particle size is larger in M1-TiO2 (Figure 3a) than that in M2-TiO2 (Figure 3f). To confirm further the Pt morphology in the other region of sample, the high-angle annular dark field (HAADF) scanning images of a different regions of M1-TiO2 and M2-TiO2 are presented in Figure 3b, g. In M1-TiO2, irregularly shaped clusters of Pt nanoparticles and smaller Pt particles are seen together, whereas uniformly sized smaller Pt nanoparticles are present on M2-TiO2. The elemental mapping clearly shows the difference in Pt dispersion on M1-TiO2 (Figure 3c−e) and M2-TiO2 (Figure 3h−j).

RESULTS AND DISCUSSION Physical Property Characterization. Powder XRDs of the obtained M1-TiO2 and M2-TiO2 showed broad peaks, and all of the diffraction peaks were ascribed to the anatase phase of TiO2 (see Figure S1, Supporting Information). The intense and sharp peaks in M1-TiO2 indicate higher crystallinity than that of M2-TiO2. The crystallite sizes calculated from the broadening of (101) diffraction line using Scherrer formula are 18 and 10 nm for M1-TiO2 and M2-TiO2, respectively. The Brunauer−Emmet−Teller (BET) method was used to calculate the surface area (SA), and the obtained SAs were 76 and 123 m2 g−1 for M1-TiO2 and M2-TiO2, respectively. The higher SA obtained with M2-TiO2 is due to the smaller particle size. The pore characteristics of M1-TiO2 and M2-TiO2 were obtained from the N2 adsorption−desorption isotherms, as shown in Figure 1. The results show that the obtained isotherms are of type IV with H2 type hysteresis, which is characteristic of mesoporous solids.28The pore sizes obtained with M1-TiO2 and M2-TiO2 range from 2 to 4 nm, as shown in the insets of Figure 1. The BET SA, pore size, and pore volume characteristics of M1-TiO2 and M2-TiO2 with various Pt loadings are listed in Table 1 (see Figure S2, Supporting Information for isotherms). Pt loading (0.05 wt %) significantly decreased the BET SA as well as pore volume in M1-TiO2, and there is no further decrease in these parameters with further 17533

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Figure 2. FE-SEM images of M1-TiO2 (a,b) and M2-TiO2 (c,d). Figure 4. Diffuse reflectance UV−visible absorption spectra of (a) M1TiO2 and (b) M2-TiO2 with different Pt loadings (x = 0.0, 0.05, 0.1, and 1.0 wt %).

surface plasmon resonance (LSPR) of Pt nanoparticles that depends on the size, shape, and dielectric environment of the nanoparticles.29 This effect is clearly observed with M1-TiO2/ xPt, whereas the same behavior was not observed with M2TiO2/xPt. Because of this LSPR effect, M1-TiO2/Pt exhibited much darker color than M2-TiO2/Pt. This difference could be due to the difference in the morphologies of Pt deposited over M1-TiO2 and M2-TiO2. The large Pt clusters formed over M1TiO2is responsible for the absorption in the visible region. In the case of M2-TiO2 with larger surface area (120 m2g−1), the well-dispersed Pt nanoparticles do not induce light scattering in the visible region. The difference in the morphology of Pt (cluster vs dispersed) depending on the morphology of TiO2 will be discussed in the next section. To confirm the oxidation state of deposited Pt, XPS analysis was done. The results showed the Pt 4f band located at around 71.0 and 74.3 eV for both M1-TiO2 and M2-TiO2 (see Figure S4 Supporting Information), which corresponds to the Pt(0) state.30,31 This confirms that the photodeposited Pt is primarily in the zerovalent metallic state on both M1-TiO2 and M2-TiO2. Formation of Pt Clusters on Mesoporous TiO 2 Supports with Different Morphology. The Pt morphologies of M1-TiO2 and M2-TiO2 are different, which seems to be related to the difference in the formation mechanism of Pt nanoparticles during photodeposition. The metal deposition proceeds through nucleation and growth mechanism. For noble-metal deposition over TiO2, it is necessary that the TiO2 nanoparticles must be well-dispersed in the solution with their surfaces exposed to metal ions. In a previous study, the dispersion of Pd on TiO2 was controlled by changing the state of TiO2 dispersion by adjusting the solution pH.25 Under basic conditions, the deposited Pd nanoparticles were smaller in size and well-dispersed compared with the one deposited at pH 6− 8 (close to isoelectric point of TiO2). The first step in this photodeposition is the formation of metal nucleus through the reduction of adsorbed metal ions (eq 2). This metal nucleus then becomes a reservoir of photoinduced electrons (electron

Figure 3. HR-TEM images (a,f), HAADF images (b,g), and elemental mapping (c−e, h−j) of M1-TiO2/Pt and M2-TiO2/Pt, respectively (with 1.0 wt % Pt loading).

The normalized diffuse reflectance UV−vis absorption spectra of M1-TiO2 and M2-TiO2 with 0, 0.05, 0.1, and 1.0 wt % Pt loading are shown in Figure 4. The absorption spectra of M1-TiO2 and M2-TiO2 with Pt loading are markedly different. As for M1-TiO2, the absorption in the visible region increased with Pt loading, which could be ascribed to localized 17534

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the reduction of Ag+ ions into Ag clusters.32 Within a secondary spherical particle composed of densely packed nanoparticles (M1-TiO2 in Scheme 1), the internal nanoparticles are not efficiently in contact with the Pt precursors in the electrolyte. Therefore, the electrons need to migrate onto the external surface where the Pt precursors are available. As a result, the nucleation sites are limited to the external surface, and the growth of large Pt nanoparticles is favored under such condition. On M2-TiO2 with loosely packed TiO2nanoparticles (see Scheme 1), most nanoparticles are exposed to the Pt precursors in the electrolyte and can initiate the nucleation of Pt clusters on them. Under this condition, the accumulation of electrons on a distant TiO2 nanoparticle should be less efficient, and the growth of Pt clusters is limited. Therefore, the photodeposition of Pt on M2-TiO2 results in more uniform distribution with smaller size of Pt as evidenced from HR-TEM (see Figure 3f). The morphology of Pt reflects that of TiO2 support, on which it is photodeposited. Photocatalytic Activity. To investigate the role of Pt morphology on the photocatalytic activity, H2 production and 4-CP degradation were tested under UV light. Figure 5 shows

trapping, eq 4) to accelerate the metal photodeposition by subsequent reduction. Hence the nucleation and growth kinetics of noble metal are crucial in determining the metal morphology, size distribution, and dispersion over the TiO2 surface. If the nucleation occurs simultaneously on many different surface active sites on TiO2 nanoparticles, then uniform dispersion of the smaller metal clusters over TiO2 surface will result. Instead, if the nucleation sites are fewer, then particle growth will proceed on the limited sites, generating larger sizes of noble metals (eq 5). TiO2 + hν → eCB− + hVB+ (photoexcitation)

(1)

PtCl 6 2 − + eCB− → → → → Pt nuc 0 (Pt nucleus formation) (2)

CH3OH + hVB+ → oxidized products (hole reaction) (3)

Pt nuc 0 + eCB− → e− (in Pt nuc) (electron trapping)

(4)

e− (in Pt nuc) + PtCl 6 2 − → → → → (Pt0)n (Pt cluster formation)

2e− (in Pt nuc) + 2H+ → H 2 (H 2 production)

(5) (6)

In M1-TiO2, the photogenerated electron transfer to a distant TiO2 nanoparticle can be efficient because of its morphology of densely packed TiO2 nanoparticles.26 Hence, during the photodeposition process of Pt on M1-TiO2, the electrons can migrate from its excitation site to a distant TiO2 nanoparticle on which more and more Pt ions are reduced, forming larger Pt clusters (Scheme 1). This is similar to the antenna mechanism in which the photogenerated electrons are transported through several nanoparticles and accumulated at a distant TiO2 nanoparticle, which serves as an electron pool for

Figure 5. Time course of H2 production activity for M1-TiO2 and M2TiO2 with different Pt loadings (x = 0.1 and 1.0 wt %). [catalyst] = 1 g/L; [CH3OH] = 10%(v/v); λ > 320 nm cutoff filter.

Scheme 1. Schematic Illustration Showing the Effect of TiO2 Support Morphology (Agglomerated State) on the Pt Photodeposition and Growth Kineticsa

the time course of photocatalytic H2 production from aqueous CH3OH solution under UV irradiation over M1-TiO2 and M2TiO2 with different Pt loadings (0.1 and 1.0 wt %). With M1TiO2, the photocatalytic H2 production activity decreased when the Pt amount increased from 0.1 to 1.0 wt %. In contrast, the photocatalytic H2 production activity increased with Pt loading in the case of M2-TiO2, which is similar to the Pt dependence exhibited by commercial TiO2 samples (P25 and Hombikat) in our previous study.26 The difference in the Pt loading effect between M1-TiO2 and M2-TiO2 may be related to the different roles of Pt such as a pore blocker, a light shielder, a recombination center, and a catalyst. The pore blocking effect by Pt may be working in the initial loading of Pt (0 → 0.05%) on M1-TiO2 because a marked decrease in both BET surface area and pore volume was observed, but more loading of Pt did not decrease them further (see Table 1). In the case of M2TiO2, there was no change in BET surface area, pore volume, and pore size with different loadings of Pt (see Table 1). These results indicate that the pore blocking effect is not very responsible for the observed differences in the Pt-dependent photocatalytic activity of M1-TiO2 and M2-TiO2. The light shielding effect (absorption + scattering) of Pt may also play some role. The light shielding by Pt should be higher for M1TiO2 because the Pt loaded on M1-TiO2 absorbs/scatters more

a

With densely packed TiO2 nanoparticles in M1-TiO2, the electron accumulates at remote particle where large Pt clusters form whereas in M2-TiO2, Pt deposits on individual TiO2 nanoparticle surface. 17535

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photons than Pt on M2-TiO2 (compare Figure 4a,b). Considering the fact that the Pt size ( M2-TiO2/Pt > M1-TiO2 > M2TiO2. In both M1-TiO2 and M2-TiO2, the Pt deposition increased the resistance. The enhanced resistance of M-TiO2/ Pt electrodes is consistent with the observation that the Pt

Scheme 2. Proposed Model Showing the Generation of Hot Electrons in TiO2 by Photons with Higher Energy than Band Gap Energy, Their Thermalization, and Hot Electron Injection into Pt Nanoparticlesa

a

Electron mean free path in smaller Pt nanoparticle is efficient in transferring the electrons from TiO2 conduction band to interface, and this process becomes retarded when the Pt nanoparticle size becomes larger than EMP.

path (7.8 nm), and the photocatalytic activity increased with increasing the Pt content to 1.0 wt %. Photoelectrochemical Behavior of Platinized Mesoporous TiO2. To understand the influence of Pt loading on the electron transfer at the TiO2-solution interface, we collected the photocurrent in the photocatalyst suspension using methyl viologen as an electron shuttle.2,27 Methanol was used as a hole scavenger to simulate the experimental condition of photocatalytic H2 production. The photocurrent profiles are shown in Figure 8a. In general, the photocurrent is enhanced by Pt

Figure 8. (a) Electron shuttle-mediated photocurrent collected in suspensions of bare and Pt deposited (1.0 wt %) M1-TiO2 and M2TiO2; [catalyst] = 0.5 g/L; [KNO3] = 0.1 M; [MV2+] = 1.0 mM; [CH3OH] = 10% (v/v); applied potential of +0.1 V (vs Ag/AgCl); λ > 320 nm. (b) Photocurrent generation on the catalyst electrodes coated with M1-TiO2/Pt (1.0 wt %) and M2-TiO2/Pt (1.0 wt %); [NaClO4] = 0.1 M; applied potential of +0.1 V (vs Ag/AgCl); λ > 320 nm, continuously Ar purged. 17537

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CONCLUSIONS Although most TiO2 nanoparticles employed as a photocatalyst are actually present in the agglomerated form, its effect on the photocatalytic activity has been rarely investigated and little understood. Unlike the primary properties of nanoparticles such as surface area, particle size, and crystallinity, the secondary property like the agglomeration is difficult to be defined and controlled. The present study is focused on understanding the different Pt effects exhibited by two different mesoporous TiO2 samples (M1-TiO2 and M2-TiO2) that consist of agglomerated nanoparticles. The morphology of Pt nanoparticles photodeposited on mesoporous TiO2 was different depending on the substrate TiO2 morphology (agglomerated state), which subsequently led to dissimilar effects on the photocatalytic activity (measured for both H2 production and 4-CPdegradation). Large Pt clusters were formed on the dense agglomerate of TiO2 nanoparticles in M1TiO2 because the interparticle electron transfers are efficient within the agglomerate and the nucleation sites of Pt are largely limited on the external surface of the secondary particles. With loosely agglomerated TiO2 nanoparticles in M2-TiO2, a good dispersion of smaller Pt clusters was obtained. This difference in the Pt morphology, larger clusters versus well-dispersed smaller clusters, is responsible for the different Pt effect on the photocatalytic activity. The fact that M1-TiO2 with larger Pt clusters is less active than M2-TiO2 with smaller Pt clusters can be related to the multiple effects of Pt. The large Pt clusters shield and scatter more photons, more efficiently thermalize hot electrons injected from TiO2, and trap and hold more electrons compared with smaller Pt clusters. In other words, M1-TiO2 with larger Pt clusters is less efficient in the interfacial electron transfer, has higher charge-transfer resistance, and consequently exhibits lower photocatalytic activity. As a result, the Pt loading on TiO2 has an optimal value for the maximal photocatalytic activity. This work demonstrated that the optimal loading and size of Pt are different depending on the morphology of substrate TiO2. Therefore, the secondary structural property of the TiO2 nanoparticles (i.e., the agglomerated state) plays a central role in defining the photoactivity of mesoporous Pt/TiO2 catalysts.

Figure 9. EIS Nyquist plots of bare and Pt (1.0 wt %) deposited M1TiO2 and M2-TiO2 electrodes obtained under an applied voltage of (a) −0.7 and (b) −0.2 V (vs Ag/AgCl) in aqueous NaClO4 (0.1 M) solution. The solution was continuously Ar purged and was irradiated under λ > 320 nm.

deposition on M-TiO2 reduces the photocurrent density, as shown in Figure 8b. This behavior is contrary to the case of the photocurrent collection in the suspension (shown in Figure 8a) in which the platinization of TiO2 enhances the interfacial electron transfer rate. However, this is because of the different ways of electron transfer between the suspension and the electrode systems. In the photocatalyst suspension, the interfacial electron transfer can occur at any nanoparticle (in contact with the electrolyte) within the agglomerate. Therefore, the surface Pt traps electrons and subsequently accelerates the electron transfer to the electrolyte. The electron transfer within the mesoporous TiO2 electrode should be directed from excited nanoparticles all the way to the FTO substrate through crossing numerous grain boundaries to generate photocurrent in a photoelectrochemical cell. Any surface Pt particles in the middle of the electron path hold the electrons, retard the photocurrent generation, and increase the charge transfer resistance. Although the role of Pt as an electron sink is the same in both suspension and electrode systems, Pt on the particle suspension enhances the interfacial electron transfer, whereas that on the electrode reduces the photocurrent (Figure 8b). Between M1-TiO2/Pt and M2-TiO2/Pt, M1-TiO2/Pt exhibits a larger charge-transfer resistance, and this could be ascribed to larger clusters of Pt that can hold more electrons and hinder the electron transfer more efficiently within the electrode. Our study shows that the photodeposited Pt morphology depends on the support TiO2 morphology and reveals that the Pt morphology sensitively influences the charge transfer behaviors and consequently the photocatalytic activity.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray diffraction patterns, N2 adsorption−desorption isotherms, X-ray photoelectron spectra, and linear sweep voltammograms of M1-TiO2 and M2-TiO2 samples. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +82-54-279-8299. E-mail: [email protected]. Present Address †

Functional Materials Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi 630 006, Tamil Nadu, India. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by KOSEF NRL program (no. R0A2008-000-20068-0),the KOSEF EPB center (grant no. R1117538

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(34) Ji, X.; Zuppero, A.; Gidwani, J. M.; Somorjai, G. A. Nano Lett. 2005, 5, 753−756. (35) Frese, K. W., Jr.; Chen, C. J. Electrochem. Soc. 1992, 139, 3234− 3243. (36) Gomes, W. P.; Vanmaekelbergh, D. Electrochim. Acta 1996, 41, 967−973. (37) Gimenez, S.; Dunn, H. K.; Rodenas, P.; Santiago, F. F.; Miralles, S. G.; Barea, E. M.; Trevisan, R.; Guerrero, A.; Bisquert, J. J. Electroanal. Chem. 2012, 668, 119−125. (38) Santiago, F. F.; Belmonte, G. G.; Bisquert, J.; Zaban, A.; Salvador, P. J. Phys. Chem. B 2002, 106, 334−339.

2008-052-02002),the Global Frontier R&D Program on Center for Multiscale Energy System (2011-0031571), and KCAPat Sogang Univ. (NRF-2011-C1AAA001-2011-0030278) funded by MEST through NRF.

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