Remarkable Enhancement of Photocatalytic Hydrogen Evolution

Article pubs.acs.org/JPCC

Remarkable Enhancement of Photocatalytic Hydrogen Evolution Efficiency Utilizing An Internal Cavity of Supramolecular Porphyrin Hexagonal Nanocylinders Under Visible-Light Irradiation Taku Hasobe,*,† Hayato Sakai,† Kentaro Mase,§ Kei Ohkubo,§ and Shunichi Fukuzumi*,§,‡ †

Department of Chemistry, Faculty of Science and Technology, Keio University, Yokohama 223-8522, Japan Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan ‡ Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea §

S Supporting Information *

ABSTRACT: An efficient visible light-induced hydrogen evolution system has been developed by using supramolecular porphyrin hexagonal nanocylinders that encapsulate Pt-colloids-deposited TiO2 nanoparticles (Pt/ TiO2) in the internal cavity. First, porphyrin nanocylinders structurally controlled by encapsulated Pt/TiO2 are prepared via a solvent mixture technique. The bar-shaped structure composed of Pt/TiO2 and zinc mesotetra(4-pyridyl)porphyrin [ZnP(Py)4] is formed with the aid of a surfactant: cetyltrimethylammonium bromide (CTAB) in a DMF/H2O mixture solution [denoted as Pt/TiO2−ZnP(Py)4 nanorods]. In scanning electron microscopy (SEM) measurements, ZnP(Py)4 pristine hexagonal nanocylinder with a large hollow structure [denoted as ZnP(Py)4 nanocylinder] was observed, whereas the hollow hole was completely closed in case of Pt/TiO2− ZnP(Py)4 nanorods. X-ray diffraction (XRD) analyses also revealed that ZnP(Py)4 alignment in the nanorod was based on the stacked-assemblies of ZnP(Py)4 coordinated hexagonal formations. These results clearly indicate that Pt colloids-deposited TiO2 nanoparticles (Pt/ TiO2) were successfully encapsulated within a ZnP(Py)4 hexagonal nanocylinder. Pt/TiO2−ZnP(Py)4 also shows a broadened absorption in the visible region because of aggregation of ZnP(Py)4. Then, Pt/TiO2−ZnP(Py)4 exhibited efficient hydrogen evolution under visible light irradiation, whereas no hydrogen was evolved in the case of Pt/TiO2 without ZnP(Py)4. In addition, the hydrogen evolution efficiency of Pt/TiO2−ZnP(Py)4 nanorods per unit weight of Pt was two orders magnitude greater than that of the nonencapsulated system: Pt/TiO2 and ZnP(Py)4 nanocylinder composites [Pt/TiO2 + ZnP(Py)4 composites]. Finally, the photodynamics of the excited state of Pt/TiO2−ZnP(Py)4 nanorods was examined by femtosecond time-resolved transient absorption spectroscopy to clarify the photocatalytic mechanism.

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INTRODUCTION Solar-to-fuel energy conversion based on clean, inexpensive, and earth-abundant materials is potentially a viable option to satisfy the demand for huge renewable energy source that can be stored and used on demand.1−5 In particular, considerable attention has been drawn toward hydrogen evolution from water by solar irradiation due to its potential application.6−9 Noble metals such as platinum (Pt) have been identified to be the most active cocatalyst for this reaction. However, due to the scarce and expensive material, the heavy use is impeditive. Consequently, extensive efforts have been devoted to reduce the platinum loading and an intense search for nonplatinum or nonprecious alternative metal in the catalytic system.10−15 Synthesis of efficient photosensitizers and photocatalysts such as semiconducting materials and molecular compounds utilizing visible light are of great importance for practical application.16−18 Especially, concerning the hydrogen evolution systems, construction of molecular-based systems of donor− © 2013 American Chemical Society

acceptor assemblies combined with hydrogen evolution catalysts have also been widely investigated.12,19−32 Although these systems are originally composed of three-component units such as a photosensitizer, an electron relay, and a hydrogen evolving catalyst,33−35 recent continuous efforts have also been made to develop the organized systems.4,10,19,36 Self-assembly is a natural and spontaneous process occurring mainly through noncovalent bonds and interactions such as van der Waals forces, hydrogen and metal-coordination bondings.37,38 These bondings play a complementary role toward covalent ones since nanometer or larger dimensional organization is not available by the use of the covalent bond alone. The possibility of controlling macroscopic structures of the resulting species through a proper choice of the molecular Received: January 12, 2013 Revised: February 12, 2013 Published: February 12, 2013 4441

dx.doi.org/10.1021/jp400381h | J. Phys. Chem. C 2013, 117, 4441−4449

The Journal of Physical Chemistry C

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Figure 1. A schematic illustration of organization procedure of Pt/TiO2−ZnP(Py)4 nanorods in this study. The electron micrograph images show (A) Pt/TiO2 (TEM), (B) ZnP(Py)4 nanocylinder (SEM), and (C) Pt/TiO2−ZnP(Py)4 nanorods (SEM). The crystal structure of ZnP(Py)4 ‘Hexagonal Formation’ was analyzed by the reported data.69

(Py)4 nanorod per unit weight of Pt was found to be more than 2 orders of magnitude greater than that of nonencapsulated Pt/ TiO2 and ZnP(Py)4 nanocylinder composites [Pt/TiO2 + ZnP(Py)4 composites], whereas no hydrogen evolution was observed in the case of reference Pt/TiO2 without ZnP(Py)4 under visible light irradiation (λ > 420 nm). Ways to improve photocatalytic behavior of Pt/TiO2−ZnP(Py)4 nanorods as compared to the reference nonorganized systems are discussed in this study.

components opens a way to design and synthesize materials capable of exhibiting specific properties and functions. Therefore, such a supramolecular technique is also considered to be one of the useful strategies to control the functionalities of light energy conversion such as light-harvesting (LH) and charge-separation (CS) properties in aggregate states.39,40 Porphyrins used as an electron donor as well as a sensitizer are also suitable for efficient electron transfer with small reorganization energies.41−47 In addition, the aggregated state guarantees increased absorption cross sections and an efficient use of the solar spectrum for energy conversion.48−55 Based on the above concept, numerous reports have appeared discussing photoinduced charge separation and light energy conversion in porphyrin-based photosynthetic reaction center models, so far.41,46,56−58 We have previously reported a series of highly organized supramolecular photovoltaic cells composed of donor moieties (D) such as porphyrins and acceptor moieties (A) such as fullerenes, which demonstrate drastic enhancement of photovoltaic properties as compared to the corresponding nonorganized system.49,59−68 Especially, bar-shaped formation composed of two different D and A molecules with separated inside and outside layers shows a possibility for a novel zeolitelike photoreaction cavity with efficient visible light absorption.60 However, photocatalytic hydrogen evolution with such organized layer materials with D and A molecules has yet to be explored. We report herein a new type of photocatalytic hydrogen evolution system utilizing an internal cavity of self-assembled porphyrin hexagonal nanocylinders composed of zinc mesotetrakis-(4-pyridyl)porphyrin [ZnP(Py)4] under visible light irradiation (Figure 1). Pt colloids-deposited TiO2 nanoparticles (Pt/TiO2) were successfully encapsulated within a ZnP(Py)4 hexagonal nanocylinder [denoted as Pt/TiO2−ZnP(Py)4 nanorods]. The hydrogen evolution efficiency of Pt/TiO2−ZnP-

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EXPERIMENTAL SECTION General Information. ZnP(Py)4 and cetyltrimethylammonium bromide (CTAB) was purchased from Aldrich. All solvents and reagents of the best grade available were purchased from commercial suppliers and were used without further purification. All experiments were performed at room temperature. Plasma-atomic emission spectroscopy (ICP-AES) and electron probe microanalysis (EPMA) were performed by SHIMAZU ICPS-7000 and EPMA-8705, respectively. Preparation of ZnP(Py)4 Nanocylinders. ZnP(Py)4 nanocylinder was prepared as following procedure. 0.8 mL of 0.25 mM ZnP(Py)4 in DMF solution was injected into 10 mL of continuously stirred 0.80 mM CTAB (cetyltrimethylammonium bromide) H2O solution at room temperature (final concentration; [ZnP(Py)4] = 0.02 mM). Then, the suspended solution was centrifuged at 10,000 rpm and washed by H2O (3 times) to remove CTAB and ZnP(Py)4. The resulting green color solution was employed for analysis of the structures. Preparation of Supramolecular Porphyrin Hexagonal Nanocylinders That Encapsulate Pt-Colloids-Deposited TiO2 Nanoparticles [Pt/TiO2−ZnP(Py)4 Nanorods]. Preparation of Pt colloids-deposited TiO2 nanoparticles (Pt/TiO2) was performed according to a reported method.70 Pt/TiO2− ZnP(Py)4 nanorod was prepared by the same method as 4442



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420 nm at room temperature. After the solution was stirred for 1 min in the dark, the gas in the headspace was analyzed using a Shimadzu GC-14B gas chromatograph (detector, TCD; column temperature, 50 °C; column, active carbon with 60− 80 mesh particle size; carrier gas, N2) to quantify the evolved hydrogen.

ZnP(Py)4 nanocylinder. 0.8 mL of 0.25 mM ZnP(Py)4 with Pt/ TiO2 in DMF solution was injected into 10 mL of continuously stirred 0.80 mM CTAB H2O solution at room temperature (final concentrations; [ZnP(Py)4] = 0.02 mM). Then, the suspended solution was centrifuged at 10,000 rpm and washed by H2O (3 times) to remove CTAB, ZnP(Py)4, and Pt/TiO2. The greenish colors residue was collected and dried under vacuum. Finally greenish colors solid powder was used for ICPAES analysis. Electron Micrograph Measurements. Transmission electron micrograph (TEM) measurements were recorded on Tecnai spirit (FEI company) by applying a drop of the sample to a copper grid. TEM images were recorded on a transmission electron microscope an accelerating voltage of 120 kV for imaging. SEM images of porphyrin assemblies were recorded using a Hitachi S-4700 scanning electron microscope. X-ray Diffraction Measurements. X-ray diffraction (XRD) measurements were carried out with a BRUKERDiscover using filtered Cu Kα radiation. The sample for XRD analysis was prepared by drying suspension liquid over a glass substrate in air. Steady-State Spectroscopic Measurements. Steadystate absorption spectra in the visible and near-IR regions were measured on a Perkin-Elmer (Lamda 750) UV−vis-NIR spectrophotometer. Steady-state fluorescence spectra were measured on a Perkin-Elmer (LS-55) spectrofluorophotometer equipped with a photomultiplier tube having high sensitivity in the 400−800 nm region. Femtosecond Laser Flash Photolysis. Femtosecond transient absorption spectroscopy experiments were conducted using an ultrafast source: Integra-C (Quantronix Corp.), an optical parametric amplifier: TOPAS (Light Conversion Ltd.), and a commercially available optical detection system: Helios provided by Ultrafast Systems LLC. The source for the pump and probe pulses was derived from the fundamental output of Integra-C (λ = 786 nm, 2 mJ/pulse and fwhm = 130 fs) at a repetition rate of 1 kHz. 75% of the fundamental output of the laser was introduced into a second harmonic generation (SHG) unit: Apollo (Ultrafast Systems) for excitation light generation at λ = 393 nm, while the rest of the output was used for white light generation. The laser pulse was focused on a sapphire plate of 3 mm thickness, and then white light continuum covering the visible region from λ = 410 to 800 nm was generated via self-phase modulation. A variable neutral density filter, an optical aperture, and a pair of polarizer were inserted in the path in order to generate stable white light continuum. Prior to generating the probe continuum, the laser pulse was fed to a delay line that provides an experimental time window of 3.2 ns with a maximum step resolution of 7 fs. In our experiments, a wavelength at λ = 393 nm of SHG output was irradiated at the sample cell with a spot size of 1 mm diameter where it was merged with the white probe pulse in a close angle (

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RESULTS AND DISCUSSION Preparation and Characterization of Pt/TiO2−ZnP(Py)4 Nanorods. Preparation of Pt colloids-deposited TiO2 nanoparticles (Pt/TiO2) was performed according to a reported method (Figure 1A).70 The typical Pt loading on TiO2 was estimated to be 3.5 wt % by the electron probe microanalysis (EPMA), which is very similar to the reported one.70 Then, Pt/ TiO2−ZnP(Py)4 nanorods were formed with the aid of a surfactant, cetyltrimethylammonium bromide (CTAB), in mixed DMF/H2O. The detailed organization procedure for ZnP(Py)4 and Pt/TiO2 is shown in Figure 1. A mixture of the appropriate ratio of ZnP(Py)4 (0.25 mM) to Pt/TiO2 (0.5 g L−1) in DMF was injected into 12.5 times volume of H2O, and the solution with CTAB (0.80 mM) was stirred at room temperature for 30 min. In the Supporting Information (Figure S1), time-dependent organization between Pt/TiO2 and ZnP(Py)4 is shown by SEM. After injecting, we can see a lot of Pt/TiO2 nanoparticles and ZnP(Py)4 flake assemblies, separately (Figure S1A). In the diffusion process of DMF into H2O, the Zn−N axial coordination induced the initial hexagonal and subsequent flake aggregates of ZnP(Py)4. The organization process of ZnP(Py)4 moieties is originated from a Zn−N axial coordination for hexagonal formation and π−π interaction for the subsequent flake organization as shown in hexagonal structures of ZnP(Py)4 (Figure 1C and Figure 3B). The details are discussed in the section of XRD measurements (vide inf ra). Then, the suspended solution was centrifuged at 10,000 rpm and washed by H2O (3 times) to remove CTAB, ZnP(Py)4, and Pt/TiO2. The self-assembled structures can be maintained for many hours. The final concentration of ZnP(Py)4 was 0.02 mM in DMF/H2O (2/25, v/v). In this case, Pt loading on Pt/TiO2−ZnP(Py)4 nanorods was estimated to be 1.4 × 10−2 wt % from inductively coupled plasma-atomic emission spectroscopy (ICP-AES). This indicates that the relative amount of Pt loading on TiO2− ZnP(Py)4 nanorods (1.4 × 10−2 wt %) is much smaller than that on pristine TiO2 (3.5 wt %). The ZnP(Py)4 nanocylinders show a bar-shaped structure with a large hollow hole (Figure 1B), whereas the hole is completely closed in Pt/TiO2−ZnP(Py)4 nanorods (Figure 1C). To perform the quantitative discussion about structural sizes of these assemblies, we calculated several hundred samples as shown in Figure 2. The Pt/TiO2−ZnP(Py)4 nanorods have the size of 230 ± 30 nm in length and 59 ± 8 nm in outside diameter as shown by the SEM analysis (Figure 2). As compared to ZnP(Py)4 nanocylinders (262 ± 38 nm in length and 57 ± 9 nm in outside diameter in Figure 2), these results indicate that the size of ZnP(Py)4 hexagonal assemblies remains the same irrespective of the absence or presence of encapsulated Pt/TiO2. X-ray Diffraction (XRD) Measurements. To examine the internal structures, we also measured X-ray diffraction (XRD) patterns of these assemblies. In Figure 3, the pattern a was obtained from simulated pattern from the reported single crystal structure of ZnP(Py)4.69 The patterns b, c, d, and e were also derived from ZnP(Py)4 nanocylinders, TiO2−ZnP(Py)4 4443



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(patterns c and d) are approximately the same as that of ZnP(Py)4 nanocylinders (pattern b). This suggests that ZnP(Py)4 internal alignment of the nanorods have quite similar structures to ZnP(Py)4 nanocylinders since TiO2 or Pt/TiO2 is encapsulated within a ZnP(Py)4 nanocylinder as shown in Figure 1. Additionally, in Pt/TiO2−ZnP(Py)4 nanorods (pattern d), the characteristic peaks originated from Pt/TiO2 (pattern e) were not observed because of encapsulation of Pt/ TiO2. We can also see strong diffraction peaks of a and b axes such as (110) and (220) in patterns b-d by comparing with a simulated pattern from the reported single crystal structure of ZnP(Py)4 (pattern a). In contrast, the diffraction intensity based on the c axis is very weak. Considering the unit cell structure (monoclinic structure: dihedral angle: 90° in axes a−c and b−c),69 the growth direction of nanorods is in the c axis direction. As shown in Figure 3B, this direction is attributable to the stacked ZnP(Py)4 hexagonal formation, which contributes to the final macroscopic bar-shaped structure (vide supra). Thus, based on the above SEM and XRD data, we can conclude that such Pt/TiO2-encapsulated organization within a ZnP(Py)4 hexagonal nanocylinder mainly occurs in this system. Steady-State Spectroscopic Measurements. Electronic absorption spectroscopy was used to probe the formation of ZnP(Py)4 aggregate structures as well as the electronic interactions between the individual components in the nanocylinders and nanorods (Figure 4). In measurements of

Figure 2. Length and diameter-distributions analyzed by SEM images. (A) Diameter and (B) length of Pt/TiO2−ZnP(Py)4 nanorods and (C) diameter and (D) length of ZnP(Py)4 nanocylinders.

Figure 4. Steady-state absorption spectra of (a) Pt/TiO2−ZnP(Py)4 nanorods, (b) ZnP(Py)4 nanocylinders, and (c) 1 μM ZnP(Py)4 monomer in DMF.

absorption spectra, we employed an integrating sphere to avoid a scattering effect on the apparent absorption. The absorption spectra of Pt/TiO2−ZnP(Py)4 nanorods (spectrum a) and ZnP(Py)4 nanocylinders (spectrum b) exhibited much broader and more intense absorption in the visible region than that of the corresponding monomer: ZnP(Py)4 in DMF (spectrum c) because of aggregation. The Soret and Q bands of Pt/TiO2− ZnP(Py)4 nanorods and ZnP(Py)4 nanocylinders (spectra a and b) became split, broadened, and red-shifted by approximately 40 nm as compared to the corresponding bands of monomer solution (spectrum c). These results suggest that an electronic interaction occurs in the ZnP(Py)4 nanorods. Further, this sharp splitting of the Soret band may be attributable to the exciton coupling.71 The broadened and red-shifted Q-bands mainly suggest the occurrence of the related J-type interaction in the nanorods.72 As discussed above, length direction of the nanorod assembly is c axis. In c axis direction, head-to-tail structure of ZnP(Py)4 moieties was

Figure 3. (A) XRD patterns of (a) a simulated pattern from the single crystal structure of ZnP(Py)4, (b) ZnP(Py)4 nanocylinders, (c) TiO2− ZnP(Py)4 nanorods, (d) Pt/TiO2−ZnP(Py)4 nanorods, and (e) Pt/ TiO2. (B) Hexagonal formation of ZnP(Py)4 analyzed by single crystal structures. The data were adopted from the following reference paper.69

nanorods, Pt/TiO2−ZnP(Py)4 nanorods, and Pt/TiO2, respectively. The pattern of self-assembled ZnP(Py)4 nanocylinder (pattern b) is consistent with that of crystal structures of ZnP(Py)4 (pattern a), which are based on Zn−N axial coordination for hexagonal formation and π−π interaction for the stacking formation (Figure 3B). Moreover, the patterns of TiO2−ZnP(Py)4 nanorods and Pt/TiO2−ZnP(Py)4 nanorods 4444



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The similar amount of hydrogen evolution (∼6 μmol) was observed in the case of ascorbic acid used as a sacrificial electron donor instead of NADH. To check the encapsulation effect of Pt/TiO2 within ZnP(Py)4 hexagonal nanocylinders on the hydrogen evolution properties, we separately prepared nonencapsulated Pt/TiO2 and ZnP(Py)4 nanocylinders mixed H2O solutions [denoted as Pt/TiO2 + ZnP(Py)4 composites]. Since the amount of Pt loading was already estimated (vide supra), we can compare the amounts of hydrogen evolution per unit weight of Pt (1g) between Pt/TiO2−ZnP(Py)4 nanorods and Pt/TiO2 + ZnP(Py)4 composites (Figure 5B). The hydrogen evolution efficiency of Pt/TiO2−ZnP(Py)4 nanorods (∼60 mol/1g Pt) is two orders magnitude greater than that of Pt/TiO2 + ZnP(Py)4/Pt composites (∼0.3 mol/1g Pt). Femtosecond Transient Absorption Measurements. In order to gain insights into the photocatalytic mechanism, the photodynamics of the excited state of Pt/TiO2−ZnP(Py)4 nanorods was examined by femtosecond time-resolved transient absorption spectroscopy. Time-resolved transient absorption spectra of Pt/TiO2−ZnP(Py)4 nanorods in H2O containing CTAB are shown in Figure 6A, where the strong

mainly observed as shown in Figure 3B. Thus, such a broad absorption in the visible region is useful for the efficient solar energy conversion. Additionally, the excitation energy of ZnP(Py)4 was determined to be 2.07 eV from the absorption and fluorescence spectra (Supporting Information Figure S2). Photocatalytic Hydrogen Evolution. Figure 5A shows the time course of hydrogen evolution in the reaction system

Figure 5. (A) Time dependence of hydrogen evolution under photoirradiation (λ > 420 nm) to the aqueous buffer solution at pH 4.5 containing NADH (2.5 × 10−2 M). ● Pt/TiO2−ZnP(Py)4 nanorods, □ TiO2−ZnP(Py)4 nanorods, and ▲ ZnP(Py)4 nanocylinders. (B) Time dependence of hydrogen evolution per unit weight of Pt (1 g) under photoirradiation (λ > 420 nm) to the aqueous solution at pH 4.5 containing ascorbic acid (3.8 × 10−2 M). ● Pt/TiO2−ZnP(Py)4 nanorods and ■ nonencapsulated Pt/TiO2 + ZnP(Py)4 composites.

composed of dihydronicotinamide adenine dinucleotide (NADH) as a sacrificial electron donor and Pt/TiO2− ZnP(Py)4 nanorods as a photocatalyst, respectively, under visible light irradiation (λ > 420 nm). The amount of evolved hydrogen was determined by gas chromatography. No hydrogen was evolved in the case of TiO2−ZnP(Py)4 nanorods and ZnP(Py)4 nanocylinders, whereas the efficient hydrogen evolution was observed using Pt/TiO2−ZnP(Py)4 nanorods. It should be emphasized that no hydrogen evolution was observed in pristine Pt/TiO2 without ZnP(Py)4 under visible light irradiation (λ > 420 nm). This indicates that ZnP(Py)4 hexagonal nanocylinders act as a photoreaction cavity for hydrogen evolution. The total volume of evolved hydrogen is approximately 6 μmol. The stoichiometric volume of evolved hydrogen would be 75 μmol if the sacrificial electron donor NADH was completely consumed for hydrogen evolution, as is shown by a dashed line in Figure 5A. The H2 evolution less than the stoichiometry results from the decomposition of NADH under low pH conditions although ZnP(Py)4 hexagonal nanocylinders are relatively stable for several hours. This may also lead to the short duration time (∼6 h).

Figure 6. (A) Femtosecond transient absorption spectra of Pt/TiO2− ZnP(Py)4 nanorods in H2O. (B) Time profiles of (a) Pt/TiO2− ZnP(Py)4 nanorods, (b) TiO2−ZnP(Py)4 nanorods, and (c) ZnP(Py)4 nanocylinders at 740 nm. Excitation wavelength is 393 nm.

ground-state absorption bleaching at 570 and 620 nm due to ZnP(Py)4 was observed upon the laser pulse excitation at 393 nm. The broad positive absorption appears within 5 ps at 450− 550 and 630−750 nm. These bands are assigned to the singlet excited state of porphyrins.61,73 The broad absorption shows monotonous decrease with increasing time. In the case of ZnP(Py)4 nanocylinders and TiO2−ZnP(Py)4 nanorods, the similar trends were observed as shown in the Supporting Information (Figure S3). The time profiles of Pt/TiO2− ZnP(Py)4 nanorods (trace a), TiO2−ZnP(Py)4 nanorods (trace 4445



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Scheme 1. Schematic Illustration for Mechanism of Hydrogen Evolution

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CONCLUSIONS In conclusion, we have demonstrated a new type of visible lightinduced photocatalytic hydrogen evolution system utilizing supramolecular porphyrin hexagonal nanocylinders. First, Pt colloids-deposited TiO2 nanoparticles (Pt/TiO2) were successfully encapsulated within a ZnP(Py)4 hexagonal nanocylinders. The morphology of Pt/TiO2−ZnP(Py)4 nanorods and the reference systems were investigated with SEM, TEM, and XRD, from which the average length and diameter of the nanocylinders and nanorods were evaluated. Pt/TiO2 nanoparticlesencapsulated ZnP(Py)4 hexagonal nanocylinders exhibited efficient hydrogen evolution under visible light irradiation, whereas no hydrogen was evolved in the case of Pt/TiO2 without ZnP(Py)4. In addition, drastic enhancement of hydrogen evolution per unit weight of Pt was observed as compared to the nonencapsulated system: Pt/TiO2 + ZnP(Py)4 composites. Such supramolecular strategies will pave a new way for developing efficient and future photocatalytic systems.

b), and ZnP(Py)4 nanocylinders (trace c) at 740 nm are shown in Figure 6B. In all traces, two different decay components are clearly observed. The initial fast component with the rate constant k > 1011 s−1 was omitted because this may be attributable to the singlet−singlet annihilation between the excited porphyrins.74,75 The rate constants of the next slow decay components (kqn) for Pt/TiO2−ZnP(Py)4 nanorods, TiO2−ZnP(Py)4 nanorods, and ZnP(Py)4 nanocylinders were determined to be 9.6 × 109 s−1, 1.2 × 1010 s−1, and 1.6 × 109 s−1, respectively. The faster kqn values of Pt/TiO2−ZnP(Py)4 nanorods and TiO2−ZnP(Py)4 nanorods as compared to that of ZnP(Py)4 nanocylinders suggest occurrence of photoinduced electron-transfer from 1ZnP(Py)4* to TiO2 in the nanorods. Finally, it should be noted that assignment of radical species [i.e., ZnP(Py)4•+] is not clear due to the ultrafast carrier trap in aggregate states in the transient spectra of aggregated forms. The similar trend was previously observed in other cases.76 Mechanism of Photocatalytic Hydrogen Evolution. Based on the above results and electrochemical data of ZnP(Py)4 (Supporting Information Figure S4), hydrogen evolution may be initiated by photoinduced electron injection from the porphyrin singlet excited state [ZnP(Py)4•+/1ZnP(Py)4* = −1.09 V vs. SCE] to the conduction band of TiO2 (∼−0.7 V vs SCE)77 in Pt/TiO2−ZnP(Py)4 nanorods as shown in Scheme 1. The injected electrons migrate toward Pt nanoparticles on TiO2 surface to reduce H+ to H2. The oxidized porphyrins [ZnP(Py)4•+] undergo the electrontransfer reduction with NADH, because the one-electron oxidation potential of NADH (E0ox = 0.76 V vs SCE)78,79 is less positive than that of the one-electron oxidation potential of ZnP(Py)4 [ZnP(Py)4•+/ZnP(Py)4 = 0.98 V vs SCE]. The oneelectron oxidation potential of ascorbic acid is reported to be E0ox = 0.29 V vs SCE.80 Alternatively photoinduced electron transfer from NADH to the singlet excited state of porphyrin [1ZnP(Py)4*/ZnP(Py)4•− = +0.94 V vs SCE] is energetically feasible. In this case, however, the observed decay rate constant of the porphyrin singlet excited state with NADH is estimated as ∼2.5 × 108 s−1 at the concentration of NADH (2.5 × 10−2 M) assuming that the rate constant of bimolecular photoinduced electron transfer is diffusion-limited value (∼1010 M−1 s−1). Judging from the ultrafast quenching process in Figure 6, photoinduced electron transfer from 1ZnP(Py)4* to TiO2 is likely to be the initial event for photocatalytic hydrogen evolution.

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ASSOCIATED CONTENT

S Supporting Information *

Time-dependent organization of Pt/TiO2 and ZnP(Py)4 by SEM, steady-state fluorescence spectra of ZnP(Py)4 nanocylinders, femtosecond time-resolved transient absorption spectra of TiO2−ZnP(Py)4 nanorods and ZnP(Py)4 nanocylinders, and cyclic voltammogram of ZnP(Py)4. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.H.), [email protected]. osaka-u.ac.jp (S.F.). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grant-in-Aid for Scientific Research (Nos. 23108721 and 23681025 to T.H., No. 23750014 to K.O., and No. 20108010 to S.F.) and the Science Research Promotion Fund from the Promotion and Mutual Aid 4446



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Corporation for Private Schools from MEXT, Japan and KOSEF/MEST through WCU project (R31-2008-000-100100), Korea. H.S. gratefully acknowledges the support of a Japan Society for the Promotion of Science (JSPS) Fellowship.

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