Visible Light-Driven Photoenergy Storage and Photocatalysis Using

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Visible Light-Driven Photoenergy Storage and Photocatalysis Using Polyoxometallates Coupled With a Ru Complex Kota Kumamoto, Kenta Tsuchibashi, Azzah Dyah Pramata, Masayoshi Yuasa, Kengo Shimanoe, and Tetsuya Kida J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 10, 2017

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The Journal of Physical Chemistry

Visible Light-Driven Photoenergy Storage and Photocatalysis Using Polyoxometallates Coupled with a Ru Complex Kota Kumamotoa, Kenta Tsuchibashib, Azzah Dyah Pramatab, Masayoshi Yuasac, Kengo Shimanoec, Tetsuya Kidac,d* a

Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka, Japan

b

Department of Applied Chemistry and Biochemistry, Graduate School of Science and Technology, Kumamoto University, Kurokami, Kumamoto, Japan c

Department of Energy and Material Sciences, Faculty of Engineering Science, Kyushu University, Kasuga, Fukuoka, Japan

d

Division of Materials Science, Faculty of Advanced Science and Technology, Kumamoto University, Kurokami, Kumamoto, Japan

ACS Paragon Plus Environment

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ABSTRACT

Polyoxometallates (POMs) have been attracting much attention as homogeneous molecular catalysts because of their excellent photocatalytic activities. However, the poor sensitivities of POMs to visible light limit their utilization of solar energy. Here, we studied photoinduced electron transfer (PET) from a Ru complex to POMs such as SiW10O368-, W10O324-, SiW12O404-, and PMo12O403- to use them for photoenergy storage and photocatalysis driven by visible light. A hydrophobic Ru complex ([Ru(nbpy)3]2+; nbpy = 4,4’-dinoyl-2,2’-bipyridyl) was coupled with POMs that were hybridized with dioctadecyldimethylammonium (DODA) in chloroform. Photoluminescence (PL) quenching and lifetime measurements indicate that PET efficiently occurred from the Ru complex to the POMs/DODA hybrids in chloroform by excitation with visible light. The PET led to the formation of one-electron reduced POMs that store photoexcited electrons. The stored/charged electrons can be discharged in a subsequent reaction that can proceed under dark conditions. The Ru complex-POM hybrid system in chloroform was used to reduce metal ions in a water phase at a liquid/liquid interface under visible light irradiation. The one-electron reduced POM that was formed by PET could reduce metal ions to produce metal particles, suggesting the applicability of this system for photocatalytic reactions under visible light irradiation.


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Introduction Metal oxide clusters of polyoxometalates (POMs) have been widely utilized in versatile areas including catalysis, electrochemistry, analytical sciences, and medicine.

1-8)

In particular,

photocatalytic and electrochemical applications of POMs have been successful due to their excellent active and reversible redox properties, diverse compositions, and tunable structures. One of the recent promising applications includes photocatalytic decomposition of water into H2 and O2 using POM-based homogeneous catalysts under UV light irradiation.9-14) The use of photogenerated electron-hole pairs in POMs for chemical reactions under solar irradiation leads to various green processes. Using the high activity of photogenerated electrons in POMs, reduction of noble metal ions is also possible under light irradiation.15, 16) The water-soluble properties of POMs enables selective photorecovery of noble metals such as Au, Pd, and Ag from waste solutions without deactivation of the catalytic activity by metal deposition, in contrast to cases with heterogeneous catalysts such as TiO2. We have developed a new photorecovery system using POM hybrids that combine POMs with a cationic surfactant. Negatively charged POMs are easily hybridized with a cationic surfactant via electrostatic attraction. The hybridized hydrophobic POM catalysts that are dissolved in an organic solution shows good photocatalytic activity for the reduction of noble metal ions at the interface between aqueous and organic phases under UV irradiation.17,

18)

The two-phase reaction system was

beneficial in recycling the homogeneous catalyst of POMs after reactions, improving the practical feasibility of this system. However, POMs usually work only under UV illumination because of their wide bandgaps, limiting their use in real environments under sunlight or weak room light. To tackle this problem, we attempted to couple the hydrophobic POM hybrids with a sensitizer that serves as a light-


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harvesting antenna to capture visible light. Previously, we reported that hydrophobic CuInS2 nanocrystals that were capped with long-chain amines served as a sensitizer to make POM hybrids sensitive to visible light in an organic solution. We found that an efficient electron transfer occurred from CuInS2 nanocrystals to POMs such as SiW12O40 and W10O32 under visible light irradiation, generating a form of POMs that is one-electron reduced by photoexcited electrons.19) It is known that reduced POMs are chemically reactive but can stably accommodate electrons in the absence of electron acceptors such as oxygen. Thus, using the POM’s ability to store electrons leads to a photoenergy charging system; the captured electrons can be discharged in the dark to cause desired reductive reactions to occur. In this study, to expand the applicability of this system in which photogenerated electrons can be accumulated and stored in POMs, we used a Ru complex as a light-harvesting antenna that captures visible light and transfers photoexcited electrons to POMs. So far, Ru complexes such as [Ru(bpy)3]Cl2 (bpy: bipyridine) have been widely used for catalysis and electroluminescing, sensing, light-emitting, and photovoltaic devices because of their excellent optical characteristics including bright emission from the excited triplet state and high sensitivity to visible light.20-25) Among their applications, the sensitization of wide bandgap semiconductors with a Ru complex has attracted much attention for solar energy utilization. Work that greatly pioneered the field is the development of dye-sensitized solar cells that are based on the TiO2/Ru complex system.25) In recent years, POMs such as α-SiW9O34 and Mn4(H2O)2(VW9O34)2 were coupled with a Ru complex to split water into hydrogen and oxygen under visible light irradiation.26,

27)

For

photochemical applications, several other systems that combine POMs with Ru complexes have been reported.28-37) Fay et al. reported a visible-light-induced photoreduction of POMs in DMF using [Ru(bpy)3]Cl2 as a sensitizer.38) Attempts have also been made to intensively generate a


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reduced form of POMs for photoenergy storage using Ru or Ir complexes.39, 40) However, only a few studies have reported the reduction of POMs by visible light irradiation in organic solutions.38) It is noteworthy that discharging electrons stored in a reduced form of POMs in an organic solution would enable various catalytic organic reactions under dark. To sensitize the hydrophobic POM hybrids in an organic solvent, we designed and synthesized a hydrophobic Ru complex with longer chains. For this purpose, RuCl3 was reacted with 4,4’dinoyl-2,2’-bipyridyl (nbpy) to produce hydrophobic [Ru(nbpy)3]Cl2. We carefully studied photoinduced electron transfer (PET) that moved electrons from the Ru complex to POM hybrids having different compositions and structures and tracked the formation of reduced POMs during visible light irradiation. The POM/Ru complex system was also used in an organic solvent to cause photocatalytic reactions such as noble ion reduction under visible light.

Experimental section Synthesis of [Ru(nbpy)3]Cl2 0.74 mmol of RuCl3 and 2.15 mmol of 4,4’-dinoyl-2,2’-bipyridyl (nbpy) were dissolved in 100 mL of dried EtOH in a three-neck flask. The reaction mixture was stirred and refluxed for 2 days under Ar flow. The cooled crude was refined using silica gel column chromatography with EtOH. The refining process the [Ru(nbpy)3]Cl2 (0.41 mmol) red solid in 55 % yield. The product was characterized by 1H-NMR, FT-IR, UV-vis, cyclic voltammetry, PL, and PL lifetime analyses. The 1H-NMR and FT-IR spectra of the product are shown in Figures S1 and S2, respectively. MALDI-TOF MS m/z: 1327.299 for [M ‒ HCl ‒ Cl-]+.


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Synthesis of POM hybrids Four kinds of hydrophobic POM hybrids that were based on SiW12O40, PMo12O40, W10O32, and SiW10O36 were synthesized. For the synthesis, SiW12O40.26H2O, Na3PMo12O40.nH2O, and DODA (dimethyldioctadecylammonium bromide) were purchased from WAKO Pure Chemical Industries Ltd. and used without further purification. W10O324- was synthesized via polycondensation of WO42- by maintaining the pH of a Na2WO4 solution (100 mL, 0.4 mmol/L) at 2 with hydrogen chloride under stirring in the same manner that was reported by Renneke et al.41) K8SiW10O36 was synthesized by the method that was reported by Canny et al.42) Typically, the hybrids were synthesized as follows: 1.0 mmol of SiW12O40.26H2O was dissolved in 100 mL of distilled water. Then, a solution containing 4.0 mmol of dioctadecyldimethylammonium bromide (DODA), as a surfactant, in 100 mL of EtOH was added to the SiW12O40.26H2O solution, and the mixed solution was stirred for 1 h at room temperature. Other hybrids such as W10O32/4DODA, SiW10O36/8DODA and PMo12O40/3DODA were prepared by the same manner noted above. The precipitates that formed were washed with distilled water and dried under vacuum at room temperature. FT-IR and UV-vis absorption spectra of the hybrids are shown in Figures S3 and S4, respectively.

CV measurements Electrochemical measurements were performed with a potentiostat (BAS; ALS/DY2325) employing a three-electrode cell under an Ar atmosphere. A glassy carbon and a platinum wire were used as the working and the counter electrodes, respectively. The working electrode was polished with aqueous alumina slurries. An Ag/Ag+ electrode was used as the reference electrode for cyclic voltammetry (CV) of the Ru complex in a dehydrated acetonitrile (CH3CN) solution


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with tributylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte. The ferrocene/ferrocenium redox couple (Fc/Fc+）was used as an internal standard (E1/2 = 0.69 V vs. NHE in acetonitrile). Before the measurements, dissolved oxygen was removed by Ar bubbling. The scan rate was 50 mV/s. For CV of POMs, an Ag/AgCl electrode was used as the reference electrode in an aqueous buffer solution in which 2 mM POMs was dissolved. An aqueous solution containing 0.5 M CH3COONa/CH3COOH was used for SiW10O36, SiW12O40, and PMo12O40, while a solvent containing 0.5 M Na3C6H5O7/C6H8O7 was used for W10O32. Before measurements, dissolved oxygen was removed by Ar bubbling. The scan rate was 5~100 mV/s.

PL quenching measurements PL intensity of the Ru complex was measured in the presence of POMs with a spectrometer (FP-6000, JASCO Co.). 6×10-5 M [Ru(nbpy)3]Cl2 was added to a designated amount of POM hybrid in chloroform that was deaerated in a quartz cuvette by Ar bubbling. The molar ratio of the Ru complex to POM hybrid was set in the range from 1:4 to 1:0.2.

PL lifetime measurements PL lifetime measurements were carried out with a fluorescence lifetime spectrometer (Quantaurus-Tau C11367-01, Hamamatsu Photonics). 6×10-5 M [Ru(nbpy)3]Cl2 was added to a designated amount of POM hybrid in chloroform in a quartz cuvette in Ar. The molar ratio of the Ru complex to POM hybrid was set to 1:0.2. The lifetime was calculated using curve-fitting software (U11487, Hamamatsu Photonics) until the goodness of fit parameter χ2 was close to unity.


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Photoenergy storage measurements The hybrid catalyst (2.0×10-4 M) was dissolved in 4 mL of chloroform in a quartz cuvette with a screw cap and a septum with a designated amount of the Ru complex. 18 mg of triethanolamine (TEOA) was also added as the sacrificial agent. The reaction system was degassed with Ar. The solution was irradiated using a 150 W Xe lamp (PEC-L01, Peccell Technologies Inc.) with 420 or 310 nm optical cut-off filters. Changes in the solution absorbance were monitored using a spectrometer (V-650, JASCO).

Photoreduction of AuCl46 mL of chloroform solution that contained 2.0×10-5 M [Ru(nbpy)3]Cl2, 3.2×10-4 M hybrid photocatalyst, and 2.0×10-2 M TEOA, as the sacrificial reagent, was prepared and put in a 30 mL quartz glass reactor with a septum. 6 mL of a HAuCl4 aqueous solution (15 mM) was added to the solution. The prepared two-phase solution was irradiated from the lateral side of the reactor with a 150 W Xe lamp equipped with a cut-off filter (λ > 420 nm). The concentration of reduced AuCl4- in the reaction system during the photocatalytic reaction was monitored by UV-vis spectrometry to evaluate the catalytic activity of the hybrid that was coupled with the Ru complex. The morphology and crystal structure of recovered Au particles were analyzed by FESEM (JSM-6340F, JEOL).

Results and discussion. Electron transfer from Ru complex to POM


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Figure 1 shows the UV-vis absorption and photoluminescence (PL) emission properties of the synthesized [Ru(nbpy)3]Cl2 complex in chloroform. The Ru complex had a maximum absorption at 462 nm due to the dπ-π* 1MLCT transition and showed red photoluminescence that peaked at 612 nm when excited at 462 nm. Its PL properties such as lifetime and quantum yield were further characterized and are shown in Table 1. The decay curve in Figure S5 was well-fit by a single exponential decay profile for pure species. The estimated lifetime of 653 ns is close to the value reported for [Ru(bpy)3]2+ (600 ns).27) Thus, this long-lived emission at 612 nm is ascribable to radiative recombination from the

3

MLCT state. The results revealed that

synthesized [Ru(nbpy)3]2+ had optical characteristics that are similar to those of [Ru(bpy)3]2+.43) The electrochemical properties were also examined by cyclic voltammetry (CV), as shown in Figure 2. The Ru complex exhibited reversible anodic and cathodic peaks that are ascribable to ruthenium- and ligand-centered oxidation and reduction. Table 2 summarizes the redox potentials of [Ru(nbpy)3]Cl2 that were measured by CV. A cathodic wave near -1.0 V (vs. NHE) was assigned to reduction of the nbpy ligand. On the other hand, peaks near +1.5 V (vs. NHE) were attributed to Ru3+/Ru2+ processes. It has been reported that the redox potential of Ru3+/Ru2+ in ground-state [Ru(bpy)3]2+ in CH3CN is +1.53 V (vs. NHE)44,45), which is close to the redox potential estimated here (+1.50 V) for Ru3+/Ru2+ in [Ru(nbpy)3]2+. The validity of this assignment is also supported by the fact that [Ru(nbpy)3]2+ showed optical characteristics that are similar to those of [Ru(bpy)3]2+, as shown above. Peaks at +1.2, +0.6, and -0.4 V are ascribable to impurities. The NMR spectrum (Figure S2) shows the presence of impurities in these samples, but the amount may be less than 10 %. CV measurements of the electrolyte solution (Figure S6) also suggest the presence of impurities in it. When comparing the determined redox potential of Ru3+/Ru2+ with that of [Ru(bpy)3]2+, [Ru(nbpy)3]2+ has the more negative potential of the


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Ru3+/Ru2+ process. The long alkyl chains of nbpy should have a greater electron-donating tendency, making the LUMO in [Ru(nbpy)3]2+ more negative due to the increased electron density of this π-conjugated compound. For the emission peak observed in the Ru complex at 77K, which is shown in Figure S7, the 0–0 transition energy (E00) was estimated to be 2.060 eV. An excited-state oxidation potential, E1/2 (Ru3+/Ru*2+), was then obtained to be -0.560 V (vs. NHE) according to the following equation: 46,47) E1/2 (Ru3+/Ru*2+) = E1/2 (Ru3+/Ru2+) – E00

(1)

Figure 1. UV-vis absorption and PL properties of [Ru(nbpy)3]Cl2 in CH3CN.

Table 1. Optical properties of [Ru(nbpy)3]Cl2. Measurements were performed in chloroform deaerated by Ar bubbling. Absorption

Emission Lifetime / µs

maximum / nm

Quantum yield

peak / nm


10

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462

610

0.653

0.029

Figure 2. Cyclic voltammogram of [Ru(nbpy)3]Cl2 in CH3CN.

Table 2. Redox potentials of [Ru(nbpy)3]Cl2. Measurements were performed in CH3CN deaerated by Ar bubbling. Epa / V (vs.

Epc / V (vs.

E1/2 / V (vs. ∆Ep / mV

NHE)

NHE)

NHE)

Ru3+/Ru2+

1.550

1.450

1.500

100

nbpy/nbpy-

-1.062

-1.112

-1.087

50

(Ru2+/Ru+)

It is important to examine the electron transfer from the excited Ru complex to hybridized POMs to confirm the ability of the Ru complex to harvest light for POMs. If an electron transfer occurs, PL emission from the Ru complex should be quenched. Thus, we carefully examined the PL intensity of the Ru complex in chloroform in the presence of different POM hybrids. We


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observed that obvious PL quenching occurred when the Ru complex was mixed with POMs. Figure 3 shows the intensity of PL emission at 610 nm from [Ru(nbpy)3]2+ as a function of the molar ratio of the POM to the Ru complex. In any case, the emission intensity decreased dramatically with an increased molar ratio. It should be noted that the quenching efficiency depended on the type of POMs. PMo12O40/3DODA showed the best quenching ability, while for SiW10O36/8DODA, PL emission was not completely quenched even when a large amount of the POM was mixed with the Ru complex.

Figure 3. Dependence of PL intensity on the molar ratio of POMs to [Ru(nbpy)3]Cl2 in chloroform.

To obtain more evidence for the electron communication between [Ru(nbpy)3]2+ and POM hybrids, we measured the photoluminescence lifetime of [Ru(nbpy)3]2+ coupled with the POM hybrids in a chloroform solution. Figure 4 shows the decay of 610 nm emission from [Ru(nbpy)3]2+ in the presence of the different POM hybrids. Apparently, the PL emission


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intensity more sharply decreased when the Ru complex was mixed with the POMs. We used the following equation to fit the decay curves and estimate PL lifetimes.

Gt exp-t/ exp-t/

(4)

The decay traces had two components: a fast (τ1) and a slow (τ2) decay. Table 3 summarizes the estimated photoluminescence lifetimes of [Ru(nbpy)3]2+/POMs. The slow decay component (τ2), ranging from 534.6 to 625.8 ns, was comparable to that observed for [Ru(nbpy)3]2+ (653.0 ns). Thus, the slow component is from single [Ru(nbpy)3]2+ ions that have a long phosphorescence lifetime due to emission from the triplet excited state. The τ2 value slightly decreased in the presence of POMs. In contrast, the fast decay component (τ1) significantly reduced in lifetime from 653.0 to 0.797-4.940 ns. This component is ascribable to the PL emission from [Ru(nbpy)3]2+/POM pairs. The significantly shortened lifetime is proof of an efficient electron transfer from the Ru complex to POMs.


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Figure 4. PL decay curves of [Ru(nbpy)3]Cl2 in the presence of POM hybrids in chloroform. PL at 612 nm was monitored after the sample was excited with a 405 nm light source.

Table 3. Estimated PL lifetimes of [Ru(nbpy)3]Cl2 coupled with different POM hybrids. Measurements were performed in chloroform deaerated by Ar bubbling. χ2

Acceptor

τ1

τ2

τaverage

None

653.0

-

-

1.054

SiW10O368-

0.797

625.8

623.4

1.199

W10O324-

4.940

534.6

528.6

1.137

SiW12O404-

0.819

535.1

533.4

1.446

PMo12O403-

2.816

587.0

584.3

1.219

The Stern-Volmer model is often used to study quenching phenomena. However, this model did not fit the above results, suggesting that the excited state deactivation is not collision-


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controlled. On the other hand, the Perrin model, which is represented by the following formula, gave a better fit to the experimental data, as shown in Figure 5 (a) and (b).

exp

(1)

Here, N is Avogadro’s number, V (= 4/3πr3) is the volume of the quenching sphere, and [Q] is the quencher concentration. The Perrin model for static quenching assumes that luminescent molecules are quenched if the quencher is within a sphere of radius r, and any quenchers located outside this sphere do not quench the emission at all48). Although this model is often used for systems where luminescent molecules are immobilized in a solid support49-51), it has also been adapted to explain a quenching phenomenon of luminescent molecules in fluid media52,

53)

.

Nevertheless, the better fit of the results indicate that there is an electrostatic interaction between the excited Ru complex (cations) and POMs (anions). The formation of ion-pair complexes is also suggested. The slope of the fitted lines changed at certain quencher concentrations, as shown in Figure 5 (a) and (b). The intersection of the two curves probably corresponds to the point where the concentration of the ion-pair complexes was saturated. The fitting parameters r, the quenching radius, was calculated from the first slopes and are listed in Table 4. The quenching sphere radius of POMs decreased in the series PMo12O403- (47.6 nm) > SiW12O404- (41.3)> W10O324- (27.9) > SiW10O368- (14.5 nm). It is suggested that a POM that has a larger quenching sphere radius can more efficiently quench the emission from the Ru complex according to the Perrin model. There is also the possibility that dynamic quenching between the Ru complex and POMs occurred simultaneously, because the lifetime of the slow decay component (τ2) decreased slightly, as shown in Table 3. The changes of the slope of the fitting lines at certain POM concentrations also suggest this possibility. A mixed static and dynamic model may lead to a much better fit.


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Figure 5. Perrin plots for the quenching of PL from [Ru(nbpy)3]Cl2 with POMs in chloroform.

The observed quenching behavior of the POMs depends on their ability to accept electrons. To gain more insight into a pathway for photoinduced electron transfer (PET), we measured the redox potentials that describe the formation of one-electron reduced POMs by cyclic voltammetry (CV). The measurements were carried out in water without DODA to obtain clear, well-defined CV peaks of POMs because the solubility of POM/DODA was low in conventional solvents that are typically used for CV measurements such as DMSO and acetonitrile. Representative CV curves of the POMs are shown in Figure S8. The POMs exhibited several redox peaks for one-, two- or three-electron oxidation and reduction. Table 3 summarizes the estimated redox potentials for the one-electron processes. The peak currents for one-electron processes varied linearly with the square root of the scan rate (Figure S9), in good agreement with the following equation and indicating that the POMS have good reversible redox characteristics. 2.69

10# $%/& A()*/& + */&


(2)

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where ip is the maximum current, n is the number of electrons transferred in the redox event, A is the electrode area, D is the diffusion coefficient, C is the concentration, and ν is the scan rate. The redox potentials for the one-electron reduction ascended in the following order: SiW10O368- (-0.480 V) > W10O324- (-0.352 V) > SiW12O404- (0.014 V) > PMo12O403- (0.500 V). As estimated, the redox potential for the excited-state oxidation (Ru3+/Ru*2+) of [Ru(nbpy)3]2+ is 0.560 V, which corresponds to the energy of its 3MLCT state. This value is more negative than the reduction potentials of all of the POMs, allowing for the oxidative quenching of excited [Ru(nbpy)3]2+ with the POMs. Figure 6 shows the energy diagram with the redox potentials of the POMs and the excited-state oxidation potential of the Ru complex. The redox potentials of POMs that are hybridized with DODA in organic solutions might be slightly different from those measured in water, but the order of their redox potentials is probably the same as that shown above. The results indicate that the energy gap between the excited state of the Ru complex and the reduction potential of PMo12O403- was the largest among these POMs. The driving force of the PET (∆G) was determined from this energy gap, as also shown in Table 4. It is obvious that the electron transfer from [Ru(nbpy)3]2+ to PMo12O403- is thermodynamically most favorable, resulting in efficient PL quenching. In contrast to that POM, SiW10O36 showed poor PLquenching properties. Because the reduction potential of SiW10O36 is the most negative and the energy gap is small, the back electron transfer may be probable, leading to incomplete quenching. The good quenching properties of W10O32 and SiW12O40 can be explained as follows: The formation of one-electron reduced POM by UV light occurs according to the following equation; 01

POM S 23 POM 4 S 5 ox


(3)

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where S and S+ (ox) represent an electron donor and its oxidized state, respectively. Particularly, W10O32 and SiW12O40 can easily form their one-electron reduced forms (POM-), which are stable in the absence of electron accepters. Due to this ability to accumulate charge, W10O32 and SiW12O40 limit the back electron transfer and thus showed good PL-quenching behavior, as shown in Figure 3.

Table 4. Quenching sphere radii, redox potentials of POMs and driving force for PET. CV measurements were performed in distilled water containing electrolytes that was deaerated by Ar bubbling. r (nm)

Epa / V

Epc / V

E1/2 / V

∆Ep

/

∆G

/

Acceptor (vs. NHE)

(vs. NHE)

(vs. NHE)

mV

eV

SiW10O368-

14.5

-0.431

-0.529

-0.480

98

-0.080

W10O324-

27.9

-0.301

-0.402

-0.352

102

-0.208

SiW12O404-

41.3

+0.046

-0.019

+0.014

66

-0.574

PMo12O403-

47.6

+0.532

+0.468

+0.500

64

-1.060


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Figure 6. Energy diagram for the excited state of [Ru(nbpy)3]2+ and the redox potentials of POMs.

Photoenergy storage in POM/Ru complex system As previously reported, irradiating a W10O324- hybrid with UV light in the presence of a sacrificial donor can produce one-electron reduced W10O325-.17, 54) The reduced POM shows blue color, indicating that photoexcited electrons are captured/stored in the POM. However, the photoreduction of POMs by visible light is difficult because they have less visible-light sensitivity, as shown in Figure S4. Here, we attempted to form reduced POM with visible light by using the Ru complex as a light-harvesting antenna. Figure 7 (a) and (b) show the UV-vis absorption spectra of POM/DODA coupled with [Ru(nbpy)3]Cl2 in a chloroform solution that contained triethanolamine (TEOA) as an electron donor, after it had been irradiated with visible light for 10-90 min. For W10O32, the MLCT absorption at approximately 450 nm decreased with


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increasing irradiation time, while a new broad peak appeared at approximately 780 nm, as shown in Figure (a). The decrease in the MLCT absorption suggests the oxidation of the metal center from Ru(ΙΙ) to Ru(ΙΙΙ). Figure 8 shows the dependence of the absorbance at 780 nm on visible light irradiation time. The intensity of the absorbance at 780 nm increased with an increasing visible light irradiation time. In contrast, without the light antenna, no change was observed in the absorbance at 780 nm. As shown in Figure S10 (a), a similar broad peak was seen at 780 nm for W10O32/4DODA after UV light irradiation. Thus, the appearance of the broad peak at 780 nm indicates the formation of reduced W10O32 by PET from [Ru(nbpy)3]Cl2 under visible light irradiation. The formation of reduced POMs driven by visible light follows the processes in eqs 5-7: 01

Runbpy% &5 23 Ru∗ nbpy% &5

(5)

Ru∗ nbpy% &5 POM → Runbpy% %5 POM 4

(6)

Runbpy% %5 S → Runbpy% &5 S 5 ox

(7)

As demonstrated above, photoexcited electrons are transferred to POM (W10O324-) to form POM- (W10O325-); the photoenergy is stored as a reduced POM that is stable in the absence of electron accepters. The solution containing the reduced POM quickly turned from blue to transparent when electron acceptors such as oxygen were introduced into the system. This result means that the stored electrons can be subsequently discharged under dark. Currently, we are studying the extraction of electrical current from this system.


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Figure 7. UV-vis absorption spectra of POM-[Ru(nbpy)3]Cl2 in chloroform after visible light irradiation (λ > 420 nm). (a) W10O32/4DODA, (b) SiW10O36/8DODA.

Figure 8. Dependence of absorbance at 780 nm on visible light irradiation time for the W10O32/4DODA-[Ru(nbpy)3]Cl2 system in chloroform.

In contrast to the case for W10O32, the formation of a one-electron reduced form was not confirmed for SiW10O36, as shown in Figure 7 (b). The absorbance at 780 nm was not changed, even after light irradiation in the presence of the light antenna. It is probable that the highly charged SiW10O36 is not able to accommodate extra electrons in its structure. However, a new peak at around 5approximately 550 nm appeared in the spectra after visible light irradiation. For SiW10O36, no such peak appeared after UV irradiation, as shown in Figure S10 (b), suggesting that the appearance of the new peak at approximately 550 nm is not due to electron transfer to or


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excitation in SiW10O36. It is possible that SiW10O36 transfers excited electrons back to the Ru complex due to its strong electron-donating characteristics. Odobel et al. also observed such a reverse electron transfer in a perylene-POM conjugate system in which no charge accumulation was observed in the POM55). This back electron transfer may be responsible for the appearance of the new absorbance at 550 nm. The formation of a charge-transfer complex that absorbs at 550 nm might be possible. However, there is no direct evidence to support this hypothesis. Further studies are necessary to clarify the reason for this phenomenon.

Photocatalytic metal reduction under visible light The results in this report confirm that the POM hybrids become visible light sensitive when coupled with [Ru(nbpy)3]2+. We next used the hybrid materials for photocatalytic reactions at a liquid/liquid interface under visible light irradiation (λ > 420 nm). Photoinduced metal ion recovery was carried out as a model reaction. A chloroform solution containing the POM hybrids and [Ru(nbpy)3]2+ was in contact with an aqueous solution that contained AuCl4-. This two-phase system was irradiated with a Xe lamp (150 W) with a 420 nm cut-off filter under static conditions without stirring. The gold ion concentration in the aqueous phase was measured with UV-vis spectrometry and monitored with time. As shown in the quenching results (Figure 3), the quenching occurred more efficiently when the molar ratio of [Ru(nbpy)3]2+ to POM was higher than 1: 1. Thus, in order to efficiently transfer photoexcited electrons in the Ru complex to POMs, the [Ru(nbpy)3]2+ to POM ratio was set to 1:10. Figure 9 shows the time dependence of the gold ion concentration in the water phase. The gold ion concentration progressively decreased with an increasing light irradiation time, suggesting that gold ion reduction occurred to produce gold particles. Indeed, gold films composed of gold


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particles were formed at the interface, as shown in scanning electron microscope (SEM) images (Figure S11). It is noted that when only using the Ru complex, the gold reduction did not proceed efficiently, as shown in Figure S12; in addition, the reduction of gold ions was not completed within 30 h. It was confirmed that the gold ion reduction also occurred in the presence of TEOA without the catalysts (Ru(nbpy)3/POM), as shown in Figure S13. The oxidation potential of TEOA as the sacrificial donor is more negative (0.611 V vs. NHE) than the redox potential of AuCl4- (0.99 V vs. NHE), leading to gold reduction without the catalysts. However, the reduction of gold by TEOA was retarded in the presence of the Ru complex, probably due to the photocatalytic decomposition of TEOA by the Ru complex. It is suggested that the hydrophobic Ru complex cannot reach the water phase and thus shows poor gold ion reduction activity. Nevertheless, the results shown in Figure 9 indicate that SiW10O36/8DODA and W10O32/4DODA have higher activities for the gold ion reduction than TEOA and the Ru complex do. Notably, the SiW10O36/8DODA hybrid recovered 99 % of the gold from the aqueous solution after 24 h of light irradiation and showed the highest photocatalytic activity. Because of the hydrophilic nature of POMs, the gold reduction at the liquid/liquid interface proceeded more efficiently. As discussed above, SiW10O36 may have a tendency to back transfer photoexcited electrons to the light antenna. The results here suggest that the presence of accepters with more positive redox potentials circumvents the back-electron transfer, allowing photocatalytic cascade reactions to proceed under visible light. The activity of the POMs descended in the order SiW10O368- > W10O324- > SiW12O404- > PMo12O403-. This tendency is in good agreement with their redox potentials. The more negative the POM’s redox potential, the higher its catalytic reduction activity is. However, the gold recovery rate of PMo12O40 was lower


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Page 24 of 47

than that without catalysts. It is probable that PMo12O40 can store photoexcited electrons more stably by changing Mo6+ to Mo5+, limiting the transfer of the trapped electrons to AuCl4-.

Figure 9. Gold ion concentration in the aqueous phase as a function of visible light (λ > 420 nm) irradiation time for two-phase photocatalytic reactions. Gold ion reduction was performed using the POM hybrids coupled with [Ru(nbpy)3]Cl2. The catalysts (Ru(nbpy)3/POM) were dissolved in a chloroform solution that contained TEOA as the sacrificial donor.

On the basis of the above results, in combination with the PL quenching and the POM reduction results, we postulate the reaction scheme in Figure 10. Photoexcitation of the Ru complex induces electron transfer to POM/DODA in the organic phase. Then, the POM is reduced to form a one-electron reduced POM that stores photoexcited electrons (photoenergy charging). The reduced POM/DODA subsequently reacts with metal ions to form metal particles at the liquid/liquid interface (photoenergy discharging). One major advantage of the developed system is that the homogeneous catalysts of POMs and Ru complexes can be easily recycled. This two-phase photocatalytic reaction system can be applied to other visible light-induced photocatalytic reactions such as water splitting and water-phase organic synthesis/decomposition by using the high reducing power of SiW10O36.


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Figure 10. Reaction scheme involving PET, photoenergy charging, and discharging reactions in the POM hybrid-Ru complex system.

Conclusions Using the hydrophobic Ru complex [Ru(nbpy)3]2+, we attempted to sensitize wide bandgap polyoxometalates (POMs) that were hybridized with a cationic surfactant, in chloroform. The photoluminescence (PL) from the Ru complex was efficiently quenched by POMs such as SiW10O368-, W10O324-, SiW12O404-, and PMo12O403-. The quenching ability of the POMs was dependent on their redox potentials. The energy diagram of the excited state of the Ru complex suggests that an electron transfer from the 3MLCT state to the POMs can occur, leading to an oxidative quenching of the excited Ru complex by the POMs. We confirmed that a photoinduced electron transfer (PET) to the W10O32 hybrid produced one-electron reduced POMs (POMs-) that were stable in the absence of electron acceptors such as oxygen. The formation of stable but reactive POMs- by visible light irradiation can possibly be utilized for photoenergy storage due to the electron reservoir properties of POMs. The POM hybrid-Ru complex system was also applied to photocatalytic reduction of noble metals at a liquid/liquid interface. The SiW10O36 hybrid combined with the Ru complex showed good photocatalytic activity under visible light irradiation because of the high reducing power of SiW10O36.


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ASSOCIATED CONTENT Supporting Information. FT-IR, NMR spectra, a photoluminescence decay profile, and a PL spectrum of the Ru complex at 77K. FT-IR, UV-vis absorption spectra, and cyclic voltammetry curves of POM hybrids. UVvis absorption spectra of the POM hybrids after UV irradiation. SEM images of gold particles formed by the photocatalysis of the hybrids. Results of control experiments on photocatalytic gold reduction at a liquid/liquid interface.

AUTHOR INFORMATION Corresponding Author *Prof. T. Kida, Faculty of Advanced Science and Technology, Kumamoto University Kumamoto 860-8555, Japan [email protected]

Author Contributions The manuscript was written with contributions from all authors. All authors have given approval of the final version of the manuscript.


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ACKNOWLEDGMENT This work was supported by the Environment Research and Technology Development Fund (No. 3K133008) of the Ministry of the Environment, Japan. We thank Prof. Ihara and Prof. Takafuji of Kumamoto University for their help in performing the PL lifetime measurements.


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TOC Graphic


36

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FIg.1 346x240mm (96 x 96 DPI)



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Fig.2 429x317mm (96 x 96 DPI)


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Fig.3 337x260mm (96 x 96 DPI)



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Fig.5 481x199mm (130 x 130 DPI)


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Fig.5 333x243mm (96 x 96 DPI)



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Fig.6 389x422mm (96 x 96 DPI)


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Figure 7 484x197mm (130 x 130 DPI)



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Figure 8 218x171mm (130 x 130 DPI)


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Figure 9 300x238mm (96 x 96 DPI)



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Figure 10 699x231mm (96 x 96 DPI)


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TOC revised 123x58mm (240 x 240 DPI)


Visible Light-Driven Photoenergy Storage and Photocatalysis Using

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