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Impact of Photosensitizing Multilayered Structure on Ruthenium(II)Dye-Sensitized TiO2‑Nanoparticle Photocatalysts Sogo Furugori,† Atsushi Kobayashi,*,†,‡ Ayako Watanabe,† Masaki Yoshida,† and Masako Kato*,† †

Department of Chemistry, Faculty of Science, Hokkaido University, North-10 West-8, Kita-ku, Sapporo 060-0810, Japan Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan

‡

S Supporting Information *

ABSTRACT: To improve the efficiency of photoinduced charge separation on the surface of dye-sensitized TiO2 nanoparticles, we synthesized the Ru(II)-photosensitizerimmobilized, Pt-cocatalyst-loaded TiO 2 nanoparticles RuCP 2 @Pt−TiO 2 , RuCP 2 −Zr−RuP 6 @Pt−TiO 2 , and RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2 (RuCP2 = [Ru(bpy)2(mpbpy)]2−, RuP4 = [Ru(bpy)(pbpy)2]6−, RuP6 = [Ru(pbpy) 3 ] 10− , H 4 mpbpy = 2,2′-bipyridine-4,4′-bis(methanephosphonic acid), and H4pbpy = 2,2′-bipyridine4,4′-bis(phosphonic acid)) using phosphonate linkers with bridging Zr4+ ions. X-ray fluorescence and ultraviolet−visible absorption spectra revealed that a layered molecular structure composed of Ru(II) photosensitizers and Zr4+ ions (i.e., RuCP2−Zr−RuP6 and RuCP2−Zr−RuP4−Zr−RuP6) was successfully formed on the surface of Pt−TiO2 nanoparticles, which increased the surface coverage from 0.113 nmol/cm2 for singly layered RuCP2@Pt−TiO2 to 0.330 nmol/cm2 for triply layered RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2. The photocatalytic H2 evolution activity of the doubly layered RuCP2−Zr−RuP6@Pt−TiO2 was three times higher than that of the singly layered RuCP2@Pt−TiO2, whereas the activity of triply layered RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2 was less than half of that for RuCP2@Pt−TiO2. The photosensitizing efficiencies of these Ru(II)-photosensitizer-immobilized nanoparticles for the O2 evolution reaction catalyzed by the Co(II)-containing Prussian blue analogue [CoII(H2O)2]1.31[{CoIII(CN)6}0.63{PtII(CN)4}0.37] decreased as the number of Ru(II)-photosensitizing layers increased. Thus, crucial aspects of the energy- and electron-transfer mechanism for the photocatalytic H2 and O2 evolution reactions involve not only the Ru(II)-complex-TiO2 interface but also the multilayered structure of the Ru(II)-photosensitizers on the Pt−TiO2 surface.

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INTRODUCTION To effectively address global energy concerns, solar water splitting is one of the most promising reactions to generate clean and renewable energy resources (i.e., dihydrogen and dioxygen) from water without the formation of environmental pollutants.1−7 From the pioneering work on TiO2 by Honda and Fujishima,1 extensive efforts have been devoted to develop various types of solar water splitting systems, including not only heterogeneous photocatalysts based on semiconductor materials8−15 but also homogeneous photocatalytic systems based on molecular materials.16−27 Recently, an extraordinary heterogeneous solar water splitting device was developed by Nocera and co-workers.10,11 This wireless device, which is composed of heterogeneous O2 and H2 evolution catalysts attached to both surfaces of a central solar cell, exhibits high solar-to-fuel efficiency (2.5%). From the viewpoint of reducing the energy costs required to fabricate solar water splitting devices, a photocatalyst sheet developed by Domen and co-workers is also noteworthy.15 The photocatalyst sheet composed of two © 2017 American Chemical Society

particle-based hydrogen and oxygen evolution photocatalysts embedded on a thin Au-conducting layer shows good solar-tohydrogen efficiency (1.5%), and its solution-based preparation technique is thought to be effective in reducing the cost for device preparation. Homogeneous photocatalytic systems composed of molecular photosensitizers and catalysts have also been widely studied for several decades not only to achieve a highly efficient photocatalytic system but also to investigate the fundamental steps in the solar water splitting reaction.4,5,16−27 Among them, a molecular photosensitizer [Ru(bpy)3]2+ (bpy = 2,2′bipyridine) and its analogues have been used widely to drive the photocatalytic H2 and O2 evolution reactions in the presence of various sacrificial reagents.28−33 For example, Sun and co-workers reported a supramolecular photocatalyst Received: May 7, 2017 Accepted: July 12, 2017 Published: July 25, 2017 3901

DOI: 10.1021/acsomega.7b00566 ACS Omega 2017, 2, 3901−3912

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Scheme 1. (a) Molecular Structures of Three Ru(II) Photosensitizers and (b) Schematic Representations of Three Types of Ru(II)-Photosensitizer-Immobilized Pt−TiO2 Nanoparticles

photosensitizers on ZrO2 nanocrystalline films.68 These fascinating studies motivated us to prepare a semiconductornanoparticle-based photocatalyst with a functional multilayered molecular structure. In this work, we newly prepared Ru(II)photosensitizer-immobilized Pt-cocatalyst-loaded TiO2 nanoparticle (Pt−TiO2) photocatalysts to improve the photoinduced charge separation efficiency by the formation of a photosensitizing multilayered structure on the surface of Pt− TiO2 nanoparticles. Three different Ru(II) photosensitizers (Scheme 1a: RuCP2 = [Ru(bpy)2(mpbpy)]2−, RuP4 = [Ru(bpy)(pbpy)2]6−, and RuP6 = [Ru(pbpy)3]10−; H4mpbpy = 2,2′-bipyridine-4,4′-bis(methanephosphonic acid) and H4pbpy = 2,2′-bipyridine-4,4′-bis(phosphonic acid)) with two to six phosphonate linkers were used to regulate precisely the redox potentials in both the ground state (Ru(III)/Ru(II)) and the photoexcited state (Ru(III)/Ru(II)*).64 Zr4+ ions were used to form multilayered structures on the Pt−TiO 2 nanoparticles, which is possible because of their ability to bind the phosphonate groups tightly.69−73 In this paper, we report on the synthesis and photocatalytic performance of three different Pt−TiO2 nanoparticles with photosensitizing singly, doubly, and triply layered structures (Scheme 1b: RuCP2@Pt− TiO2, RuCP2−Zr−RuP6@Pt−TiO2, and RuCP2−Zr−RuP4− Zr−RuP6@Pt−TiO2, respectively). Although the approach of employing TiO2 nanoparticles as an electron mediator to connect the H2-evolving catalyst and the molecular photosensitizer (so-called “through particle mechanism”)74−82 has been reported previously, the effect of the photosensitizing multilayered structure has scarcely been examined. Notably, we found that the photocatalytic activity of the doubly layered RuCP2−Zr−RuP6@Pt−TiO2 nanoparticle for the H2 evolution reaction was remarkably higher than that of the singly layered RuCP2@Pt−TiO2 nanoparticle, indicating that the energy- and electron-transfer dynamics in the Ru(II)-photosensitizing layers play a crucial role on the photocatalytic activity for both the O2 and H2 evolution reactions.

constructed from two [Ru(bpy)3]-type photosensitizing units connected with the highly active molecular O2-evolving catalyst [Ru(6,6′-dcbpy)(pic)2] (6,6′-H2dcbpy = 2,2′-bipyridine-6,6′dicarboxylic acid; pic = 4-picoline). Compared with a molecular-based multicomponent system, the activity of the supramolecular photocatalyst was enhanced considerably by the connection between the photosensitizing units and the catalytic center.35−39 A similar supramolecular photocatalyst for the H2 evolution reaction was developed by Sakai and co-workers by coupling a Ru(II) photosensitizer with a Pt(II) water reduction catalyst; relative to a multicomponent molecular-based system, higher photocatalytic activity was also observed.40,41 These works clearly indicate the importance of electron transfer, viz., the connection between the molecular photosensitizer and the catalyst. However, in these half reactions, sacrificial reagents also elicit negative and complicated effects on the photocatalytic activity.42−44 Recently, dye-sensitized photoelectrosynthesis cells (DSPECs) have been developed extensively not only to avoid the effect of sacrificial regents but also to develop more practical devices for solar fuel production.45−54 The well-known molecular photosensitizer [Ru(bpy)3]2+ is used widely as the sensitizing dye in DSPECs because of the efficient electron injection from the surface-bound photoexcited [Ru(bpy)3]2+* species to the conduction band of the semiconductor substrate (typically TiO2). Relatedly, it has recently been established that the bonding interactions between the [Ru(bpy)3]-type dye and the semiconductor substrate play a crucial role in the electron injection dynamics.55−68 For example, Durrant and co-workers found that a photoexcited electron of the surface-bound Ru(II) dye [Ru(4,4′-dcbpy)2(NCS)2] (4,4′-H2dcbpy = 2,2′-bipyridine4,4′-dicarboxylic acid) was promptly injected from the metal-toligand charge-transfer (MLCT) excited state to the conduction band of the TiO2 substrate on the picosecond time scale.57 Recently, Meyer and co-workers reported that the introduction of an electron-accepting or electron-donating molecular layer between the semiconductor substrate and the [Ru(bpy)3]-type dye greatly improved the lifetime of the charge-separated state.66,67 They also reported that the energy transfer occurs in a bilayered structure composed of two different Ru(II)

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RESULTS AND DISCUSSION Synthesis and Characterization. Energy-dispersive X-ray fluorescence (XRF) spectra were measured to estimate the 3902


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Pt−TiO2, although it was observed clearly and its intensity increased significantly in the order of RuCP2−Zr−RuP6@Pt− TiO2 < RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2. Overall, these XRF spectra are indicative of the successive immobilization of the Ru(II) photosensitizers on the Pt−TiO2 nanoparticles by the Zr4+-ion linkers. In addition, TEM images of RuCP2@Pt−TiO2 and RuCP2−Zr−RuP6@Pt−TiO2 (Figure S1) suggest that the size of the Pt cocatalyst loaded on the TiO2 nanoparticles barely changed in response to the immobilization of the Ru(II) complexes. We estimated the amount of each element in the Ru(II)-complex-immobilized nanoparticles based on these XRF spectra. The estimated molar ratios of Ru to Pt (Ru/Pt) of RuCP2@Pt−TiO2, RuCP2−Zr−RuP6@ Pt−TiO2, and RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2 were 0.318, 0.477, and 0.964, respectively, clearly indicating that the amount of the immobilized Ru complexes on the Pt−TiO2 nanoparticles increased in the order of RuCP2@Pt−TiO2 < RuCP 2 −Zr−RuP 6 @Pt−TiO 2 < RuCP 2 −Zr−RuP 4 −Zr− RuP6@Pt−TiO2 (Figure S2). This result also indicates that Zr4+ ions play a key role in the successive immobilization of phosphonate-functionalized Ru(II) photosensitizers on the Pt− TiO2 nanoparticles. To fully investigate the amount of immobilized Ru(II) complexes on the Pt−TiO2 nanoparticle surface, as well as the surface coverage per unit area (details given in the Supporting Information), ultraviolet−visible (UV−vis) absorption spectra of the supernatant solutions were recorded following the immobilization of RuCP2, RuP4, and RuP6 (Figure S3). The results are summarized in Table 1. The estimated surface coverage of the immobilized Ru(II) complexes per unit area of RuCP2−Zr−RuP6@Pt−TiO2 and RuCP2−Zr−RuP4−Zr− RuP6@Pt−TiO2 is approximately 1.5 and 2.9 times larger than that of RuCP2@Pt−TiO2, respectively. This result agrees quantitatively with the results of the XRF spectra. The occupied area of the TiO2 surface per one Ru(II) complex molecule (i.e., molecular footprint) was also calculated based on the amount of immobilized Ru(II) complexes per unit area. The occupied area of RuCP2 in RuCP2@Pt−TiO2 was estimated to be 1.47 nm2. This value corresponds approximately to the molecular footprint of RuCP2 (1.66 nm2, Figure S4) estimated by singlecrystal X-ray diffraction analysis, suggesting that the TiO2 nanoparticle surface is almost fully covered by RuCP 2 molecules to form a RuCP2-photosensitizing molecular layer on the Pt−TiO2 nanoparticle surface. However, the amount of immobilized RuP6 in RuCP2−Zr−RuP6@Pt−TiO 2 and RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2 was slightly less than the amount of immobilized RuCP2 in RuCP2@Pt−TiO2. This is likely caused by the larger molecular footprint of RuP6 (2.09 nm2, Figure S5) compared with that of RuCP2 (1.66 nm2,

amount of Ru(II) complexes immobilized on the Pt−TiO2 nanoparticle surface (Figure 1). Two characteristic peaks

Figure 1. XRF spectra of Pt−TiO2 (black), RuCP2@Pt−TiO2 (red), RuCP2−Zr−RuP6@Pt−TiO2 (blue), and RuCP2−Zr−RuP4−Zr− RuP6@Pt−TiO2 (green) nanoparticles.

assigned to Pt Lα and Lβ originating from the Pt cocatalyst on the TiO2 nanoparticle were clearly observed for all three Ru(II)-complex-immobilized nanoparticles. The intensities of these peaks were almost comparable with that of the Pt−TiO2 nanoparticles, indicating that the Pt cocatalyst did not detach from the TiO2 surface during immobilization of the Ru(II) photosensitizers. The amount of Pt on the TiO2 nanoparticles was estimated to be ca. 5.1 wt % by the intensity ratio of the Kα peak of Ti to the Lα peak of Pt. Transmittance electron microscopy (TEM) images revealed that Pt nanoparticles with diameters ranging from 2 to 5 nm were loaded uniformly on the TiO2 nanoparticle surface (Figure S1). In the XRF spectra of the three Ru(II)-complex-immobilized nanoparticles, the peaks assigned to Ru Kα radiation were observed clearly, indicating that the Ru(II) complex was successfully immobilized on the Pt−TiO2 nanoparticle surface. It is noteworthy that the intensity ratio of Ru Kα radiation to that of Pt Lα increased in the order of RuCP2@Pt−TiO2 < RuCP2−Zr−RuP6@Pt− TiO2 < RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2. In addition, Zr Kα radiation was barely observed for Pt−TiO2 or RuCP2@

Table 1. Amount of Immobilized Ru(II) Complex on the Pt−TiO2 Nanoparticle Surface RuCP2@Pt−TiO2 RuCP2−Zr−RuP6@Pt−TiO2

RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2

a

amount of immobilized Ru(II) complex (nmol/1 mg TiO2)a

surface coverage (nmol/cm2)

116 64.3 114 178 96.7 153 88.3 338

0.113 0.0627 0.111 0.174 0.0942 0.150 0.0861 0.330

1st layer RuCP2 1st inner layer RuP6 2nd outer layer RuCP2 total 1st inner layer RuP6 2nd mid layer RuP4 3rd outer layer RuCP2 total

Estimated based on the absorbance observed in the UV−vis spectra of each supernatant solution. 3903


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Figure 2. (a) Diffuse reflectance spectra in the solid state and (b) emission spectra of the dispersed solution in water (2.5 μM of the Ru(II) complex, λex = 458 nm) of RuCP2@Pt−TiO2 (red lines), RuCP2−Zr−RuP6@Pt−TiO2 (blue dotted lines), and RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2 (green chain lines) at 298 K.

diameters of the three Ru(II)-complex-immobilized nanoparticles are in the range of 600−730 nm, which is comparable with that of TiO2 and Pt−TiO2 nanoparticles. Thus, the dispersibility of these nanoparticles in water changed very little by the immobilization of the Ru(II) photosensitizers. Therefore, the Ru(II) complexes were uniformly immobilized on the Pt−TiO2 surface without any growth of Pt−TiO2 nanoparticle aggregates. In addition, particles smaller than Pt−TiO2 were not observed in the DLS measurements, suggesting that the nanoparticles composed of only molecular Ru(II) photosensitizers or Zr4+ ions did not precipitate separately during the immobilization reactions. UV−vis diffuse reflectance and emission spectra were measured to investigate the photophysical properties of the three Ru(II)-complex-immobilized Pt−TiO2 nanoparticles. As shown in Figure 2, both 1MLCT absorption and 3MLCT emission bands derived from the Ru(II) photosensitizers were observed in all three Ru(II)-complex-immobilized nanoparticles. Clearly, these results provide additional evidence that the Ru(II) complexes were immobilized on the Pt−TiO2 surface. Interestingly, even when the same Ru(II)-complex concentration in the suspended aqueous solution is used (note that the Ru(II)-complex concentration is not uniform on account of the immobilization process, but the amounts in the suspensions are constant at 2.5 μM), the emission intensities of the Ru(II)-photosensitizer-immobilized Pt−TiO2 nanoparticles decreased significantly in the order of RuCP2@Pt−TiO2 > RuCP 2 −Zr−RuP 6 @Pt−TiO 2 > RuCP 2 −Zr−RuP 4 −Zr− RuP6@Pt−TiO2. As mentioned in the Introduction, photoexcited electrons of Ru(II) photosensitizers are known to be injected rapidly to the conduction band of TiO2, which quenches 3MLCT phosphorescence.55−68 Thus, one possible reason for the emission intensity decrease is that the electron injection efficiency from the Ru(II) photosensitizer to TiO2 is improved by increasing the number of immobilization cycles of the Ru(II) complex. We previously reported that the shorterlived emission species than RuCP2 was observed for the RuCP2@TiO2 nanoparticle without the Pt cocatalyst in the solid state.83 Considering that all three Ru(II)-photosensitizer-

Figure S4) on account of its six phosphonic acid groups. In fact, the molecular size of RuP6 (679 Å) estimated by the singlecrystal X-ray diffraction analysis is larger than that of RuCP2 (534 Å),83 which is consistent with the smaller amount of immobilized RuP6 than RuCP2. In addition, the electrostatic repulsion between the negatively charged phosphonate groups may also contribute to the lower surface coverage by RuP6. Interestingly, the amount of immobilized RuCP2 in RuCP2− Zr−RuP6@Pt−TiO2 is almost identical to that in RuCP2@Pt− TiO2, suggesting that the number of Zr4+ ions bound by the RuP6 phosphonate groups is large enough to immobilize RuCP2 to form a second photosensitizing layer on the outer edge of the nanoparticle. However, in the case of RuCP2−Zr− RuP4−Zr−RuP6@Pt−TiO2, the amount of immobilized RuCP2 in the outer layer was smaller than that in RuCP2@ Pt−TiO2, which is probably due to the smaller number of phosphonate groups in RuP4 than RuP6. Thus, these results clearly indicate that self-assembled, layered molecular structures composed of Ru(II) photosensitizers and Zr4+ ions have been successfully constructed on the Pt−TiO2 nanoparticles. In the infrared (IR) spectra of the three Ru(II)-complex-immobilized nanoparticles (Figure S6), the symmetric stretching band of the P−OH group at 950 cm−1, which was clearly observed in RuP6, RuP4, and RuCP2, almost disappeared following immobilization with concomitant formation of symmetric and asymmetric stretching modes of the P−O bond at 1000−1200 cm−1. These results suggest that Zr4+ ions were coordinated to the deprotonated phosphonate groups of the phosphonatefunctionalized Ru(II) photosensitizers during the immobilization reaction.58 The powder X-ray diffraction (PXRD) patterns of all Ru(II)-photosensitizer-immobilized Pt−TiO2 nanoparticles were almost identical to that of nonimmobilized Pt− TiO2 nanoparticles (Figure S7), suggesting that any crystalline impurities were not formed in the immobilization reactions of the Ru(II) photosensitizers and Zr4+ cations. The particle diameter distribution was also measured by dynamic light scattering (DLS) (see Figure S8) to investigate the influence of the immobilized Ru(II) photosensitizers on the dispersibility of the Pt−TiO2 nanoparticles in water. The average particle 3904


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reduction catalyst (Figure S9) probably because of the low emission quenching efficiency of [Ru(bpy)3]2+ by H2A under the acidic condition.84 Therefore, the immobilization of the Ru(II) photosensitizer on the TiO2 nanoparticles by the phosphonate linkers is a crucial component for the photocatalytic H2 evolution reaction. Notably, the amount of H2 evolution, apparent quantum yield (Φ), TON, and turnover frequency (TOF) of RuCP2−Zr−RuP6@Pt−TiO2 were approximately three times larger than that of RuCP2@Pt− TiO2, whereas those of RuCP2−Zr−RuP4−Zr−RuP6@Pt− TiO2 were much less than those of RuCP2@Pt−TiO2. These results indicate that the insertion of an inner RuP6 layer between RuCP2 and the TiO2 nanoparticle surface via Zr4+ ions greatly enhances the photocatalytic activity of H2 evolution; by contrast, further insertion of a middle RuP4 layer between the inner RuP6 and outer RuCP2 layers significantly suppresses the activity. The enhancement may be due to improved charge separation efficiency in RuCP2−Zr−RuP6@Pt−TiO2 because the amount of Ru(II) photosensitizers in these three reactions is equal (100 μM); that is, the optical absorption intensities are almost comparable among the three reaction solutions. The suppression of the photocatalytic activity of H2 evolution by the insertion of a RuP4 middle layer may be related to energytransfer processes among the three different Ru(II) photosensitizers. To clarify why the photocatalytic H2 evolution activity depends so strongly on the structure of the Ru(II)photosensitizing layer immobilized on the Pt−TiO2 nanoparticle surface, emission quenching experiments were performed in the presence of the sacrificial electron donor H2A (Figure S10). Using the same molar ratio of Ru(II) photosensitizer to H2A as that in the photocatalytic H2 evolution reaction, the emission quenching efficiency was found to decrease in the same order as that of the photocatalytic H2 evolution activity (i.e., RuCP2−Zr−RuP6@ Pt−TiO2 > RuCP2@Pt−TiO2 > RuCP2−Zr−RuP4−Zr− RuP6@Pt−TiO2). Thus, the formation of a multilayered structure composed of Ru(II) photosensitizers and Zr4+ ions on the Pt−TiO2 surface greatly affects the photoinduced charge separation process among the TiO2 and Ru(II) photosensitizers. As mentioned in the Introduction, Meyer and co-workers already reported that the energy-transfer processes occur from photoexcited RuCP2 ([RuCP2]*) to RuP6 to generate [RuP6]* species in the RuCP2−Zr−RuP6 bilayer structure formed on a nanocrystalline ZrO2 film.68 This energy transfer from RuCP2 to RuP6 would be thermodynamically favorable because the emission energy of RuP6 (1.91 eV = 650 nm) is slightly smaller than that of RuCP2 (1.97 eV = 630 nm),64 which can be attributed to the stabilized π* energy levels on the 2,2′-bipyridyl ligands by the directly attached electron-withdrawing phosphonate substituents. Thus, in the case of RuCP2−Zr−RuP6@Pt−TiO2, the energy transfer could occur in the same direction from the outer-layered [RuCP2]* to the inner-layered RuP6 (process (a) shown in Scheme 2) to generate [RuP6]* on the Pt−TiO2 surface, followed by electron injection from [RuP6]* (which can be generated by both energy transfer and/or direct photoexcitation) to the conduction band of TiO2 (process (b) in Scheme 2). This electron injection is known to be favorable because the redox potential of Ru(III)/Ru(II)* in RuP6 is more negative [−0.69 V vs normal hydrogen electrode (NHE)] 64 than the conduction band minimum of TiO2 (−0.41 V vs NHE at pH = 2.2).85,86 Another possible pathway to inject an electron to

immobilized Pt−TiO2 nanoparticles were hardly emissive in the solid state, the Pt cocatalyst loading may improve the emission quenching efficiency. A second plausible reason is due to energy-transfer quenching among the immobilized Ru(II) complexes. Photocatalytic H2 Evolution. Photocatalytic water reduction was investigated using the three Ru(II)-compleximmobilized nanoparticles (100 μM as Ru(II) complex) RuCP 2 @Pt−TiO 2 , RuCP 2 −Zr−RuP 6 @Pt−TiO 2 , and RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2 as the photocatalytic water reduction catalysts in the presence of 0.5 M L-ascorbic acid (H2A) as the sacrificial electron donor in aqueous solution (pH = 2.2). The amount of H2 evolution versus time is plotted in Figure 3, and the results are summarized in Table 2. Blue

Figure 3. Photocatalytic water reduction reaction driven by RuCP2@ Pt−TiO2 (red open circles), RuCP2−Zr−RuP6@Pt−TiO2 (blue closed squares), and RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2 (green closed triangles) (100 μM of the Ru(II) complex) in a 0.5 M Lascorbic acid sacrificial electron donor aqueous solution (pH = 2.2) under an Ar atmosphere. A blue LED light (λ = 470 ± 10 nm) was used as the irradiation source.

Table 2. Results of the Photocatalytic H2 Evolution Reactions photocatalyst

H2 (μmol)a

Φb (%)

TONa,c

TOFc

RuCP2@Pt−TiO2 RuCP2−Zr−RuP6@Pt−TiO2 RuCP2−Zr−RuP4−Zr−RuP6@Pt− TiO2

92.0 256 43.4

1.35 3.77 0.64

368 1022 174

52.6 146 24.5

a

After 6 h irradiation. bApparent quantum yield estimated by eq 2. TON and TOF (for an initial 1 h irradiation) were calculated based on the Ru(II) photosensitizer. c

light-emitting diode (LED) light (λ = 470 ± 10 nm) was irradiated to photoexcite the Ru(II) photosensitizers. In all three reactions, the amount of H2 evolution increased linearly with the irradiation time and the estimated turnover numbers (TONs) per one Ru(II) photosensitizer were much greater than 1, indicating that the Ru(II)-complex-immobilized Pt− TiO2 nanoparticles act as photocatalysts for the H2 evolution reaction. By contrast, a negligible amount of H2 was observed in the reaction system containing [Ru(bpy)3]2+ as the molecular photosensitizer and Pt−TiO2 nanoparticles as the water 3905


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Scheme 2. Schematic Diagram Showing a Plausible Energy- and Electron-Transfer Mechanism of RuCP2−Zr−RuP6@Pt−TiO2a

a

The redox potentials of the Ru(II) photosensitizers and L-ascorbic acid (H2A) and the position of conduction band minimum of TiO2 were inferred from the literature.63,91,92

TiO2 is the reductive quenching of [RuCP2]* by H2A. However, the efficiency would be negligible because of the relatively small reductive quenching efficiency of [Ru(bpy)3]2+ by H2A at pH = 2.2.84 Thereafter, the generated hole in [RuP6]+ could be transferred to the RuCP2 located at the outer edge of the RuCP2−Zr−RuP6@Pt−TiO2 nanoparticle (process (c) in Scheme 2) because of the slightly lower Ru(III)/Ru(II) redox potential of RuCP2 (1.22 V vs NHE) than that of RuP6 (1.43 V vs NHE)64 to generate a charge-separated state in the nanoparticle (i.e., [RuCP2]+−Zr−RuP6@Pt−[TiO2(e−)]). In this way, the excited electron in the conduction band of TiO2 is spatially separated from the hole localized in the outer-layered RuCP2, resulting in suppression of charge recombination and thus a longer lifetime of the charge-separated state. In the case of RuCP2@Pt−TiO2, however, charge recombination would occur more easily because of back electron transfer from [TiO2(e−)] to [RuCP2]+ because the hole is localized on RuCP2, which is directly bound to the Pt−TiO2 surface. This hypothesis is consistent with the results from the emission measurements; the emission intensity of RuCP2−Zr−RuP6@ Pt−TiO2 was weaker than that of RuCP2@Pt−TiO2 (Figure 2b), and the quenching efficiency of RuCP2−Zr−RuP6@Pt− TiO2 by H2A is higher than that of RuCP2@Pt−TiO2 (Figure S10). Thus, the formation of a long-lived, spatially chargeseparated state in [RuCP2]+−Zr−RuP6@Pt−[TiO2(e−)] could improve not only the efficiency of electron injection from RuP6 to TiO2 via energy transfer from RuCP2 but also the reactivity of the sacrificial electron donor by hole migration from the inner-layer [RuP6]+ to the outer-layer RuCP2, leading to an enhancement of the photocatalytic activity of the H2 evolution reaction. By contrast, the catalytic activity of H2 evolution of the triply layered RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2 was significantly lower than that of RuCP2@Pt−TiO2, despite the presence of three different Ru(II) photosensitizers immobilized on the Pt−TiO2 surface, as well as the doubly layered RuCP2− Zr−RuP6@Pt−TiO2. Unfortunately, we have not yet determined the cause of the lower photocatalytic activity of RuCP2− Zr−RuP4−Zr−RuP6@Pt−TiO2, but one plausible factor can be envisioned. The reason is that RuP4 introduced as a photosensitizing middle layer negatively affects the direction of

energy transfer. Considering the emission energies of the three photosensitizers [i.e., RuP6, RuP4, and RuCP2 are 1.91 eV (650 nm), 1.88 eV (660 nm), and 1.97 eV (630 nm), respectively],64 thermodynamically favorable energy transfers in the RuCP2− Zr−RuP4−Zr−RuP6@Pt−TiO2 nanoparticles would be limited in the following two directions: (i) from the outer RuCP2 layer to the middle RuP4 layer and (ii) from the inner RuP6 layer to the middle RuP4 layer (Scheme S1). In other words, the photoexcitation energy would concentrate on RuP4 in the middle layer, but the photoexcited [RuP4]* could not inject the electron to TiO2 because the distance from the TiO2 surface to RuP4 would be too far for direct injection owing to the presence of the RuP6 inner layer (i.e., it would be associated with a high electron-transfer barrier). In addition, indirect electron injection via the reduction of RuP6 would also be difficult because the Ru(III)/Ru(II)* redox potential of RuP4 (−0.74 V vs NHE) is not negative enough to reduce RuP6 (Ru(II)/Ru(I), −1.29 V vs NHE).64 Photocatalytic O2 Evolution. Photocatalytic water oxidation was investigated using the three Ru(II)-compleximmobilized nanoparticles (100 μM as Ru(II) complex) RuCP 2 @Pt−TiO 2 , RuCP 2 −Zr−RuP 6 @Pt−TiO 2 , and RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2 as the photosensitizers and a Co(II)-containing Prussian blue analogue [CoII(H2O)2]1.31[{CoIII(CN)6}0.63{PtII(CN)4}0.37] (hereafter CoPt−PBA) as the water oxidation catalyst34 in the presence of 5 mM Na2S2O8 as the sacrificial electron acceptor in a 40 mM phosphate buffer aqueous solution (pH = 7.3). The amount of evolved O2 versus irradiation time is plotted in Figure 4, and the results are summarized in Table 3. The induction period for the initial 5 min was due to the location of the O2 detector that is placed on the head of the sample vial. Although the concentrations of the Ru(II) photosensitizers in these three reactions are identical to those used in the photocatalytic H2 evolution reactions discussed above, the TON decreased in the order of RuCP2@Pt−TiO2 > RuCP2− Zr−RuP6@Pt−TiO2 > RuCP2−Zr−RuP4−Zr−RuP6@Pt− TiO2, suggesting that the photosensitizing efficiency for the O2 evolution reaction is suppressed by the formation of additional Ru(II)-photosensitizing layers on Pt−TiO2 via the Zr4+ linkages. Likewise, emission quenching experiments in the 3906


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the subsequent electron injection to TiO2 would suppress the electron-transfer efficiency from the Ru(II) photosensitizer to S2O82− (Scheme S2). Therefore, the reactivity with Na2S2O8 decreased as the number of Ru(II) complex layers increased, resulting in lower water oxidation activities for RuCP2−Zr− RuP6@Pt−TiO2 and RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2 versus RuCP2@Pt−TiO2. In other words, the multilayered structure composed of Ru(II) photosensitizers and Zr4+ bridging ions on Pt−TiO2 nanoparticles acts as a photoinduced charge separator in which electrons tend to migrate to the inside of the TiO2 nanoparticles.

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CONCLUSIONS We synthesized the Ru(II)-photosensitizer-immobilized, Ptcocatalyst-loaded TiO2 nanoparticles RuCP2@Pt−TiO 2 , RuCP2−Zr−RuP6@Pt−TiO2, and RuCP2−Zr−RuP4−Zr− RuP6@Pt−TiO2 using phosphonate linkers with bridging Zr4+ ions and investigated their photocatalytic activities for both the water oxidation and reduction reactions. The photocatalytic O2 evolution activity in the presence of a CoPt−PBA catalyst decreased as the number of Ru(II)photosensitizing layers on the Pt−TiO2 nanoparticles increased (RuCP2@Pt−TiO2 > RuCP2−Zr−RuP6@Pt−TiO2 > RuCP2− Zr−RuP4−Zr−RuP6@Pt−TiO2). Notably, the photocatalytic performance of the doubly layered nanoparticle RuCP2−Zr− RuP6@Pt−TiO2 for the H2 evolution reaction was approximately three times higher than that of the singly layered RuCP2@Pt−TiO2 nanoparticle, whereas the activity of the triply layered nanoparticle RuCP2−Zr−RuP4−Zr−RuP6@Pt− TiO2 was the lowest among the three nanoparticle systems investigated. A crucial factor for the photocatalytic activity of these sensitized nanoparticle systems is the electron- and energy-transfer processes in the Ru(II)-photosensitizing layers on the Pt−TiO2 nanoparticles. A characteristic feature was observed for the doubly layered nanoparticle RuCP2−Zr− RuP6@Pt−TiO2. Specifically, the photoexcited energy transfer from the outer [RuCP2]* to the inner RuP6 and the hole transfer from the electron-injected [RuP6]+ to the outer RuCP2 are key factors for high photocatalytic H2 evolution activity. However, these two processes suppress the reactivity when S2O82− is used as the sacrificial electron acceptor, resulting in a lower photocatalytic performance for the O2 evolution reaction in the presence of CoPt−PBA. The photosensitizing efficiency for O2 evolution and the photocatalytic activity of H2 evolution of the triply layered RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2 system were remarkably lower than those of the other Ru(II)complex-immobilized nanoparticles, which is probably caused by the detrimental effect of the RuP4 middle layer to the energy-transfer processes. This work clearly demonstrates that the immobilization of multilayered structures composed of molecular Ru(II) photosensitizers and Zr4+ linkages on Pt− TiO2 surfaces is a simple and promising method to improve the photoinduced charge separation efficiency for solar fuel production. Moreover, this study establishes that the key factors for higher charge separation efficiency include the direction of the photoexcited energy transfer and the subsequent electron transfers in the layered structure. The next important step to achieve water splitting without the use of sacrificial reagents must address how to transfer the hole located on the H2-evolving photocatalyst surface to the O2evolving catalyst. Further studies on controlling the electronand energy-transfer processes in Ru(II)-photosensitizing multi-

Figure 4. Photocatalytic water oxidation reaction driven by RuCP2@ Pt−TiO2 (red solid line), RuCP2−Zr−RuP6@Pt−TiO2 (blue broken line), RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2 (green chain line) (100 mM of the Ru(II) complex), and CoPt−PBA (1 mg) in a 5 mM Na2S2O8 sacrificial electron acceptor in 40 mM phosphate buffer aqueous solution (pH = 7.3) under an Ar atmosphere. A blue LED light (λ = 470 ± 10 nm) was used as the irradiation source.

Table 3. Results of the Photocatalytic O2 Evolution Reactions photosensitizer 2

RuCP @Pt−TiO2 RuCP2−Zr−RuP6@Pt−TiO2 RuCP2−Zr−RuP4−Zr−RuP6@ Pt−TiO2

O2 (μmol)a

Φb (%)

TONa,c

TOFc

3.20 2.11 0.543

0.565 0.373 0.0959

25.6 16.9 4.34

0.436 0.299 0.0617

a

After 60 min irradiation. bApparent quantum yield estimated by eq 1. TON and TOF (for an initial 15 min irradiation) were calculated based on the Ru(II) photosensitizer. c

presence of various Na2S2O8 concentrations (Figure S11) suggest that the emission quenching efficiency also decreased in the same order of RuCP2@Pt−TiO2 > RuCP2−Zr−RuP6@ Pt−TiO2 > RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2. Given that the surfaces of all three Ru(II)-photosensitizer-immobilized nanoparticles are covered by the same molecular photosensitizer (i.e., RuCP2) and that their average sizes are comparable (see Figure S8), their distinct reactivities in response to the Na2S2O8 sacrificial electron acceptor could not be due to differences in the photosensitizer sizes or surface conditions. As mentioned in the previous section, the photoexcited energy transfer among the Ru(II) photosensitizers could play an important role in the charge separation process. In the case of RuCP2−Zr−RuP6@Pt−TiO2, [RuP6]* in the inner layer could inject an excited electron to TiO 2 after direct photoexcitation or the energy transfer could originate from [RuCP2]* in the outer layer. Although this charge separation process should positively influence H2 evolution, it would negatively influence the O2 evolution reaction in the presence of an electron acceptor such as S2O82−; a key process to drive this photocatalytic water oxidation reaction is the electron transfer from the Ru(II) photosensitizer on the Pt−TiO2 nanoparticles to the sacrificial electron acceptor S2O82−. However, energy transfer among the Ru(II) complexes and 3907


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RuCP2 (3 mL), and then stirred continuously overnight to immobilize RuCP2 to the Zr4+ ions bound to the phosphonate groups of RuP6. Finally, the obtained RuCP2−Zr−RuP6@Pt− TiO2 nanoparticles were isolated by ultracentrifugation (50 000 rpm; 15 min), washed twice with a 0.1 M HClO4 aqueous solution (ca. 6 mL), and then dried in air at 298 K for 1 day. The amounts of RuCP2 and RuP6 immobilized on the Pt− TiO2 nanoparticle surface were estimated by using XRF and UV−vis absorption spectroscopy of the supernatant solution (see the “Synthesis and Characterization” section and Supporting Information). Preparation of RuCP2−Zr−RuP4−Zr−RuP6@Pt−TiO2. The immobilization of RuP6 and binding of Zr4+ ions were conducted by the same method outlined above to prepare the Zr−RuP6@Pt−TiO2 nanoparticles. The dried Zr−RuP6@Pt− TiO2 nanoparticles were dispersed in acidified water (3 mL) by the addition of a 60% HClO4 aqueous solution (50 μL). To this dispersion, a 2.5 mM aqueous solution of RuP4 (3 mL) was added and then stirred continuously overnight to form RuP4immobilized RuP4−Zr−RuP6@Pt−TiO2 nanoparticles. After isolation by ultracentrifugation (50 000 rpm; 15 min) and washing twice with a 0.1 M HClO4 aqueous solution (ca. 6 mL), the obtained nanoparticles were dispersed again in methanol (3 mL), added to a 20 mM methanol solution of ZrCl2O·8H2O (3 mL), and then stirred continuously for 1 h to form the Zr−RuP4−Zr−RuP6@Pt−TiO2 nanoparticles. After isolation by ultracentrifugation (50 000 rpm; 15 min) and washing twice with methanol (ca. 6 mL), the obtained precipitates were dispersed in acidified water (3 mL) by the addition of a 60% HClO4 aqueous solution (50 μL), added to a 2.5 mM aqueous solution of RuCP2 (3 mL), and then stirred continuously overnight. Finally, the target RuCP2−Zr−RuP4− Zr−RuP6@Pt−TiO2 nanoparticles were isolated by ultracentrifugation (50 000 rpm; 15 min), washed twice with a 0.1 M HClO4 aqueous solution (ca. 6 mL), and then dried at 298 K in air for 1 day. The amounts of RuCP2, RuP4, and RuP6 immobilized on the Pt−TiO2 nanoparticle surface were estimated by XRF and UV−vis absorption spectroscopy of the supernatant solution (see the “Synthesis and Characterization” section and the Supporting Information). Measurements. UV−vis absorption spectra and diffuse reflectance spectra were recorded on a Shimadzu UV-2400PC spectrophotometer. The obtained reflectance spectra were converted to absorption spectra using the Kubelka−Munk function F(R∞). Luminescence spectra were recorded on a JASCO FP-6600 spectrofluorometer at 298 K. Each sample solution was deoxygenated by N2 bubbling for 30 min at 298 K. IR spectra were recorded on a JASCO FT-IR 4100 spectrophotometer using KBr pellets. PXRD patterns were recorded on a Bruker D8 ADVANCE diffractometer equipped with a graphite monochromator using Cu Kα (λ = 1.54187 Å) radiation and a one-dimensional LynxEye detector. DLS analysis was conducted using an OTSUKA ELSZ-1000SCl analyzer. TEM was recorded on a JEOL 2010 FasTEM microscope (200 kV). Energy-dispersive XRF spectra were recorded on a Bruker S2 PUMA analyzer. Single-crystal X-ray structure analysis of RuP6 was conducted using a Rigaku XtaLAB P200 diffractometer with graphite monochromated Cu Kα radiation and a rotating anode generator. Diffraction data were collected and processed using the CrysAlisPro program.90 The structure was solved by the direct methods using SIR2011,91 and structural refinements were conducted using the full-matrix least-squares method with SHELXL-2013.92 Non-

layered structures and the development of a new hole-transfer mediator are now in progress.

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EXPERIMENTAL SECTION Synthetic Procedures. All starting materials were used as received from commercial sources, and solvents were used without any further purification. TiO2 nanoparticles (ca. 15 nm in diameter) were purchased from the Sakai Chemical Industry Co. Ltd. Unless otherwise stated, all reactions were performed in air. The Co(II)-containing Prussian blue analogue34 [CoII(H2O)2]1.31[{CoIII(CN)6}0.63{PtII(CN)4}0.37] and the Ru(II) molecular photosensitizers (RuCP2, RuP4, and RuP6)87,88 were synthesized using previously reported methods. The Ptcocatalyst-loaded TiO2 nanoparticles (hereafter Pt−TiO2) were prepared using a previously reported photodeposition method.89 Single-Crystal Preparation of RuP6. The chloride salt of RuP6 (22.2 mg) was suspended in water (6 mL), and the pH was adjusted to 6 using a 1 M NaOH aqueous solution to dissolve the suspensions. A 5 M HCl aqueous solution (0.6 mL) was then added to this red solution. Red RuP6 crystals began to form after being kept at 298 K for 5 days (yield: 18.7 mg, 72%). One of the crystals was used for X-ray diffraction analysis, whereas the remaining crystals were dried under vacuum. Elemental analysis (%) calcd for C30H30Cl2N6O18P6Ru1·2H2O: C, 31.16; H, 2.96; N, 7.27; found: C, 31.66; H, 2.70; N, 7.38. Preparation of RuCP2@Pt−TiO2. The powder of Pt−TiO2 nanoparticles (30 mg) was dispersed in acidified water (3 mL) by the addition of a 60% HClO4 aqueous solution (50 μL). A 2.5 mM aqueous solution of RuCP2 (3 mL) was added to the Pt−TiO2-dispersed solution and then stirred continuously overnight to immobilize RuCP2 on the Pt−TiO2 surface. The obtained RuCP 2 -immobilized Pt−TiO 2 nanoparticles (RuCP2@Pt−TiO2) were collected by ultracentrifugation (50 000 rpm; 15 min), and the supernatant solution was removed. After washing twice with a 0.1 M HClO4 aqueous solution (ca. 6 mL), RuCP2@Pt−TiO2 was dried in air at 298 K for 1 day. The amount of RuCP 2 immobilized on the Pt−TiO 2 nanoparticle surface was estimated by using XRF and UV−vis absorption spectroscopy of the supernatant solution (see the “Synthesis and Characterization” section and Supporting Information). Preparation of RuCP2−Zr−RuP6@Pt−TiO2. The powder of Pt−TiO2 nanoparticles (30 mg) was dispersed in acidified water (3 mL) by the addition of a 60% HClO4 aqueous solution (50 μL). To this dispersed solution, a 2.5 mM aqueous solution of RuP6 (3 mL) was added and then stirred continuously overnight. After ultracentrifugation (50 000 rpm; 15 min) to isolate the RuP6-immobilized Pt−TiO2 nanoparticles (RuP6@Pt−TiO2), the obtained RuP6@Pt−TiO2 precipitates were washed twice with a 0.1 M HClO4 aqueous solution (ca. 6 mL) and then dispersed in methanol (3 mL). To this dispersed solution, a 20 mM methanol solution of ZrCl2O· 8H2O (3 mL) was added and stirred continuously for 1 h to bind the Zr4+ ions to the phosphonate groups of the surfaceimmobilized RuP6 species. Unreacted Zr4+ ions were removed by ultracentrifugation (50 000 rpm; 15 min) and washing twice with methanol (ca. 6 mL); the sample was then dried under vacuum at 298 K. The obtained Zr4+-attached nanoparticles (i.e., Zr−RuP6@Pt−TiO2) were dispersed again in acidified water (3 mL) by the addition of a 60% HClO4 aqueous solution (50 μL), added to a 2.5 mM aqueous solution of 3908


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hydrogen atoms were refined anisotropically, whereas H atoms were refined using the riding model. All calculations were performed using the CrystalStructure crystallographic software package.93 The crystallographic data are summarized in Table S1 and deposited in the Cambridge Crystallographic Data Centre (CCDC-1542141). Although several alerts (levels A and B) are reported, these are due to the highly disordered crystal water molecules. Photocatalytic Water Oxidation Reaction. Under dark conditions, a phosphate buffer solution (40 mM, pH = 7.3) containing a Ru(II) photosensitizer (100 μM of the Ru(II) complex) and water oxidation catalyst (1 mg CoPt−PBA) was placed in a Pyrex vial (volume ca. 5 mL) with a small magnetic stirring bar and covered with a rubber septum. To this mixed solution, a Na2S2O8 aqueous solution (50 mM) was injected using a syringe, and the resultant solution was deoxygenated by bubbling with an Ar gas for 30 min. A robust O2 sensor probe (Pyro Science, FireSting O2 oxygen meter) was equipped on top of the septum to detect the oxygen concentration within the vial’s headspace. The vial was irradiated from the bottom with a blue LED lamp (λ = 470 ± 10 nm; 160 mW; Opto Device Lab. Ltd., OP6-4710HP2). The temperature was controlled at 293 K using a homemade aluminum watercooling jacket with a water-circulating temperature controller (EYELA CCA-1111). The TON and TOF were estimated from the amount of evolved O2, which required four photoredox cycles of the Ru(II) photosensitizer to oxidize one water molecule. The apparent quantum yield (Φ) was calculated using the following equation.94 Φ = Ne/Np = 4NO2 /Np

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00566. UV−vis absorption spectra of the supernatant solutions to estimate the adsorbed amount of Ru(II) photosensitizers on Pt−TiO2, TEM images, IR spectra, PXRD patterns, particle size distribution, and emission quenching (PDF) X-ray crystallographic data of RuP6 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.K.). *E-mail: [email protected] (M.K.). ORCID

Atsushi Kobayashi: 0000-0002-1937-7698 Author Contributions

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

This study was supported by JST-PRESTO (no. JPMJPR12C3), the Shimadzu Science Foundation, the Shorai Science and Technology Foundation, the Inamori Foundation, the Murata Science Foundation, Grant-in-Aid for Scientific Research (C)(26410063), and Artificial Photosynthesis (no. 2406) from MEXT, Japan. Notes

(1)

The authors declare no competing financial interest.

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Here, Ne stands for the number of reacted electrons, NO2 is the number of the evolved O2 molecules, and Np is the number of incident photons. Photocatalytic Water Reduction Reaction. Under dark conditions, an L-ascorbic acid aqueous solution (0.5 M, pH = 2.2) containing a Ru(II) photosensitizer (100 μM of the Ru(II) complex) was placed into a homemade Schlenk flask-equipped quartz cell (volume: 265 mL) with a small magnetic stirring bar. Each sample flask was doubly sealed with rubber septa. This mixed solution was deoxygenated by Ar bubbling for 1 h. The flask was then irradiated from the bottom with a blue LED lamp (λ = 470 ± 10 nm; 160 mW; Opto Device Lab. Ltd., OP6-4710HP2). The temperature was controlled at 293 K using a homemade aluminum water-cooling jacket with a watercirculating temperature controller (EYELA CCA-1111). The gas samples (0.3 mL) for each analysis were collected from the headspace using a gastight syringe (Valco Instruments Co. Inc.). The amount of evolved H2 was determined using a gas chromatograph (Agilent 490 Micro Gas Chromatograph). The TON and TOF were estimated from the amount of evolved H2, which requires two photoredox cycles of the Ru(II) photosensitizer to reduce one water molecule. The apparent quantum yield (Φ) was calculated using the following equation.94 Φ = Ne/Np = 2NH2 /Np

Article

ACKNOWLEDGMENTS The authors thank Dr. K. Sawaguchi-Sato, Prof. Y. Hasegawa, Dr. S. Noro (Hokkaido Univ.), and Dr. Y. Shibata (Bruker AXS) for their valuable support with the TEM, SCXRD, and XRF measurements. The authors also thank Mr. Yoshimura and Mr. Ogawa (Hokkaido Univ.) for their kind help with emission lifetime measurement in the revision process.

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