Design of Metal-to-Metal Charge-Transfer Chromophores for Visible

Aug 14, 2017 - Voltammograms were obtained with a commercial potentiostat and potential programmer (HZ-5000, Hokuto Denko), and using a Pt wire as the...
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Design of Metal-to-Metal Charge-Transfer Chromophores for VisibleLight Activation of Oxygen-Evolving Mn Oxide Catalysts in a Polymer Film Akira Yamaguchi,*,†,‡,§ Toshihiro Takashima,¶ Kazuhito Hashimoto,†,⊥ and Ryuhei Nakamura‡,□ †

Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Biofunctional Catalyst Research Team, RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan § Department of Materials Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan ¶ Clean Energy Research Center, University of Yamanashi, Kofu, Yamanashi 400-8511, Japan □ Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan ‡

S Supporting Information *

ABSTRACT: To construct photoresponsive unidirectional charge transfer units for the activation of oxygen-evolving manganese oxide (MnOx) catalyst, metal-oxide nanoclusters consisting of cerium (CeIII) or cobalt (CoII) ions and Keggintype polyoxotungstate (PW12O403−) were synthesized in a polymer matrix as visible-light-absorbing chromophores. The utilization of the polymer matrix enabled the molecularly dispersed PW12O403− states and was advantageous to achieve product-separable energy conversion systems. The reaction of PW12O403− with Ce(NO3)3 or CoCl2 in the polymer matrix generated the new broad absorption tails extending from the UV to visible region, assignable to metal-to-metal charge transfer (MMCT) transitions of oxo-bridged binuclear WVI−O−CeIII and WVI−O−CoII units. Cyclic voltammetry analysis of the oxo-bridged binuclear units in the polymer membrane revealed that the CoIII/CoII couple had a 300 mV more positive redox potential than that of CeIV/CeIII and was capable of extracting electrons from MnOx catalyst. Although visible-light irradiation of the polymer membrane having WVI−O−CoII units resulted in negligible photocurrent generation, a clear anodic photocurrent response assigned to photoinduced WVI−O−CoII → WV−O−CoIII transition was observed after the coupling of MnOx catalysts to WVI−O−CoII units. This finding demonstrated that the generation of anodic photocurrent is derived from the activation of MnOx catalyst by the photogenerated CoIII through confined WVI−O−CoII linkages. The system in this work based on POM and polymer, and its synthetic method provide us a novel methodology to develop artificial photosynthetic systems with spatially and energetically optimized components.

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INTRODUCTION

One promising approach for visible-light chromophores composed of purely inorganic redox centers with controllable redox and optical properties is oxo-bridged metal-to-metal charge-transfer (MMCT) units proposed by Frei and coworkers.3−5 Heterobinuclear units linked by μ-oxo bridges offer flexible control of the redox potential and absorption wavelength based on the selection of the grafted ions in the MMCT unit. Using this approach, they fabricated various oxobridged MMCT units on the surface of mesoporous silica (MCM-41 and SBA-15)3−5 and demonstrated that the visiblelight irradiation of ZrIV−O-CoII MMCT units coupled to IrOx clusters on the surface of SBA-15 triggered water oxidation by

Developing charge transfer units that absorb visible-light and promote multielectron transfer reactions, such as water oxidation, hydrogen evolution, and carbon dioxide reduction, is necessary for constructing efficient solar-to-chemical energy conversion systems. One of the principal challenges for constructing charge transfer units is the fabrication of an inorganic chromophore that (1) is able to promote multielectron transfer reactions, (2) has tunable redox properties similar to metal−organic systems,1,2 and (3) is highly stable under light irradiation. In addition, the spatially and energetically optimized arrangement of each components (lightabsorbing sites, reduction/oxidation sites) is also highly demanding to achieve separation of reduction/oxidation products, and efficient charge transportation. © 2017 American Chemical Society

Received: April 24, 2017 Revised: July 26, 2017 Published: August 14, 2017 7234

DOI: 10.1021/acs.chemmater.7b01669 Chem. Mater. 2017, 29, 7234−7242

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Chemistry of Materials

precipitate was collected by filtration and was then washed with diethyl ether (Wako). The electrochemical properties of the synthesized K5BW12O40 sample were identical to those previously reported.14,23 A self-standing POM/polymer membrane was fabricated by blending a POM precursor, poly(vinyl alcohol) (PVA), and polyacrylamide (PAM) as linkers following the procedure reported by Helen et al.,24 although the total amount of POM in the membrane was increased to promote electron hopping between POM molecules. H3PW12O40 was added to a mixture of 10 wt % aqueous PVA (poly(vinyl alcohol) 1000, completely hydrolyzed; Wako) and 2.5 wt % PAM (Polysciences, Inc.) at a weight ratio of POM/(POM + polymers) of 0.8. After stirring at 70 °C for 6 h, one drop of the solution (750 μL) was placed on a clean glass substrate (5 × 5 cm2) and dried at room temperature. The glass substrate was immersed in a cross-linking reagent consisting of 72 vol % distilled water, 24 vol % acetone (Wako), 2 vol % 25% glutaraldehyde (Glu) solution (Wako), and 2 vol % conc. HCl (Wako) for 30 min, and was then washed once with distilled water. The membrane that formed on the surface of the glass substrate was removed and stored in distilled water. The sample was designated as POM/PVA/PAM. Samples containing different POMs were also fabricated using an identical procedure, with the exception that H4SiW12O40 (designated as Si-POM/PVA/PAM) and K5BW12O40 (B-POM/PVA/PAM) were used as POM precursors. The reaction of the POM/PVA/PAM samples with Ce(NO3)3 or CoCl2 to construct MMCT units was performed as follows. The POM/PVA/PAM sample was immersed in aqueous 160 mM Ce(NO3)3·6H2O (Wako) or CoCl2·6H2O (Wako) solution for 5 h at 80 °C, and was then washed once with distilled water. The resulting samples were designated POM/PVA/PAM/Ce and POM/PVA/ PAM/Co, respectively. MnOx water-oxidation catalysts were deposited on the POM/PVA/ PAM, POM/PVA/PAM/Ce, and POM/PVA/PAM/Co membranes using a spray deposition method. A colloidal MnOx solution was prepared by reducing KMnO4 (Wako) with a stoichiometric amount of Na2S2O3·5H2O (Wako) at room temperature. The solution was sprayed onto the membrane samples using an automatic spray gun (ST-6, Lumina), resulting in the formation of δ-MnO2.25 The POM/ PVA/PAM, POM/PVA/PAM/Ce, and POM/PVA/PAM/Co samples combined with MnOx were designated POM/PVA/PAM/MnOx, POM/PVA/PAM/Ce/MnOx, and POM/PVA/PAM/Co/MnOx, respectively. Film thickness was characterized using a profilometer (DEKTAK6M, Ulvac), and molecular structures were determined by microRaman and FT-IR spectroscopy. Micro-Raman spectra were recorded using a Raman microscope (Senterra, Bruker) and a 532 nm laser operated at 20 mW. FT-IR spectra of the membrane samples were recorded with a FT-IR spectrometer (Vertex 70, Bruker) equipped with a deuterated triglycine sulfate detector and operated in transmission mode at room temperature. For the measurements, the sample was mounted in a homemade transmission IR vacuum cell equipped with KBr windows. UV−vis spectra of the samples were measured with a UV−vis spectrometer (UV-2550, Shimadzu). For electrochemical measurements, the membrane samples were fabricated on a clean conducting glass substrate (indium tin oxide [ITO]-coated glass; resistance: 20 Ω/square; size: 30 × 30 mm2; SPD Laboratory, Inc.). Voltammograms were obtained with a commercial potentiostat and potential programmer (HZ-5000, Hokuto Denko), and using a Pt wire as the counter electrode and a Ag/AgCl/KCl (sat.) electrode as the reference electrode. The electrolyte solution (0.1 M Na2SO4) was prepared from high-purity Milli-Q water (18 MΩ−1 cm−1) and reagent-grade chemicals. All measurements were conducted at 30 °C. Current vs time curves (I−t curves) for the membrane samples fabricated on the ITO electrode were obtained in 0.1 M Na2SO4 electrolyte (pH 10, adjusted with 1 M NaOH) under visible-light irradiation and an applied potential of +1100 mV (vs a standard hydrogen electrode [SHE]) after 20 min of Ar bubbling. The light source was a 300 W Xe lamp (Asahi Spectra, MAX-302) equipped with a UV cutoff filter (Y-47, transparent at wavelengths > 450 nm; Asahi Glass). The action spectrum was obtained in the same

the IrOx cluster and the reduction of CO2 to CO at Zr sites.6 They further demonstrated that the cuprous oxide cluster serves as the catalyst for CO2 reduction and the total activity of the all-inorganic photochemical reaction system depended on the nature of the electron acceptor site.7 To facilitate photoinduced multielectron transfer utilizing oxo-bridged MMCT units, our group has constructed multinuclear MMCT systems using polyoxometalates (POMs) as acceptor sites of the excited electrons,8−10 because POMs are attractive multielectron transfer catalysts for controlling the catalytic properties of artificial photosynthetic systems11−13 and for the flexible design of purely inorganic chromophores due to their diverse molecular properties, including charge, redox potential,14−17 and multielectron-storing ability.18 We demonstrated the effectiveness of this approach using CuPW11O39/Ce as a POM-based MMCT system for the efficient two electron reduction of O2.8−10 Independently, Hill et al. demonstrated MMCT from ReI to WVI using POM-supported [Re(CO)3]+ complexes ([P4W35O124{Re(CO)3}2]16‑),19 and they revealed that the absorption property such as absorption wavelength of MMCT from central CoII to WVI in [CoW11O39]10‑ was altered by metal substitution.20 Mizuno et al. also induced intramolecular MMCT (CeIII → WVI) using tetra-n-butylammonium (TBA)6 [{Ce(H2O)}2 {Ce(CH3 CN)}2 (μ4-O)(γ-SiW10O36) 2] with a Pt cocatalyst to promote H2 evolution.21 Collectively, these reports demonstrate the utility of POM-based MMCT systems as a platform for photodriven multielectron transfer reactions. The next step of the exploration of the cluster-based MMCT units for solar-to-energy conversion system is the unidirectional alignment of the light-absorbing site and reduction/oxidation sites, and the coupling with water oxidation catalysts. In this report, we constructed POM-based MMCT units (PW12O40/ Ce and PW12O40/Co), and subsequently the MMCT units were coupled to a water oxidation catalyst. A polymer matrix was used as a scaffold for the MMCT units to achieve the appropriate alignment of each component for unidirectional electron transfer and product separation in the total energy conversion system. In addition, utilization of the polymer matrix makes it possible to fabricate itself onto an electrode to investigate the redox potential of the each component in detail by electrochemical techniques, which is a powerful tool for energy analysis. We adopted a manganese oxide (MnOx) as a water-oxidation catalyst due to its inexpensiveness and abundance. The structural, optical, and electrochemical properties of the MMCT/MnOx assembly were investigated by microRaman, FT-IR, diffuse reflectance UV−vis, and cyclic voltammetry. The photoactivation and the electron transfer properties of the MMCT/MnOx system were examined by photocurrent measurements.

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EXPERIMENTAL SECTION

For all synthetic procedures, all chemical reagents were used as received without further purification. P-centered (12 tungstophosphoric acid n-hydrate, H3PW12O40; Wako), Si-centered (tungstosilicic acid hydrate, H4SiW12O40; Sigma-Aldrich), and B-centered POMs (K5BW12O40) were used as precursors for the MMCT unit. K5BW12O40 was synthesized using a previously reported procedure.22 Briefly, 6 M HCl (6 mL) was added to a 10 mL solution containing sodium tungstate (VI) dehydrate (10 g; Wako) and boric acid (0.5 g; Wako), and the resulting solution was boiled for 24 h. Following filtration with 1 μm pore size filter paper, the filtrate was adjusted to pH 2 with 6 M HCl and was then further boiled for 30 min. After adding KCl (2 g; Wako) to the boiled solution, the resulting white 7235

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Scheme 1. Schematic Outline of the Synthetic and Examination Procedures of POM-Based MMCT Units in the Polymer Matrixa

a

(1) Fabrication of POM/PVA/PAM by mixing POM (H3PW12O40), PVA, and PAM, followed by cross-linking of PAM with Glu. (2) The MMCT units were constructed by reacting the POM/PVA/PAM membranes with Ce(NO3)3 or CoCl2. The structures of PW12O403− clusters in the polymer matrix were characterized using micro-Raman and FT-IR spectroscopy. The redox potentials of the acceptor (PW12O403−) and donor sites (CeIII and CoII) of the MMCT units were measured by cyclic voltammetry and differential pulse voltammetry. (3) The MnOx water-oxidation catalyst was deposited on the MMCT units, and (4) photoelectrochemical measurements under visible-light irradiation were performed to monitor activation of the MnOx water-oxidation catalyst by the POM-based MMCT units. Schematic illustration of the energy levels for the MMCT unit and the onset potential of O2 evolution by MnOx at pH 10 are shown together with state density distributions for the one-electron-transfer redox couples CeIV/ CeIII and CoIII/CoII. Visible-light (λ > 450 nm) irradiation induced MMCT (CeIII or CoII → WVI), and the generated holes and excited electrons were collected by the MnOx catalyst and electrode, respectively. electrolyte using a band-pass filter with a half-width of 10−15 nm to obtain monochromatic light.

donor metal centers so as to activate the linked oxidation catalysts efficiently, this work adopted two metal ions (CeIII and CoII) as the electron donating sites to control the oxidation potential of the MMCT units and compare their capability for photoactivation of MnOx catalysts. Because the standard redox potential of the CoIII/CoII couple (1.92 V) is more positive than that of the CeIV/CeIII couple (1.71 V), CoII is the better candidate as an electron donating site to activate MnOx catalysts. Raman/IR Spectroscopic Characterization of MMCT Unit in a Polymer Matrix. The molecularly dispersed state and the retention of the structure of PW12O403− clusters in the polymer matrix are of crucial importance for utilizing POMs as a component of MMCT units to construct the redox-tunable chromophore.8−10 To confirm that the H3PW12O40 molecules were intact and molecularly dispersed as shown in Scheme 1-1, and to determine if oxo-bridged binuclear WVI−O−CeIII and WVI−O−CoII units were formed within the polymer matrix without altering the structure of the PW12O40 molecules (Scheme 1-2), micro-Raman and FT-IR spectroscopic measurements were conducted.

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RESULTS AND DISCUSSION Synthetic Strategy. In this work, the construction of POM-based MMCT units was attempted toward the photoactivation of MnOx water oxidation catalysts using a polymer matrix as a scaffold to align the MMCT units and MnOx catalysts. As shown in Scheme 1-1, POM/polymer composite membranes (thickness: 100 μm) were prepared by cross-linking using poly(vinyl alcohol) (PVA) and polyacrylamide (PAM) as base polymers and glutaraldehyde (Glu) as a cross-linking reagent24 because the high aggregation propensity and high acidity of H3PW12O40 prevented molecularly dispersed, selfstanding membranes from forming when Nafion or polyvinyl acetate were used as base polymers. To fabricate POM-based MMCT units, the second metals (Ce(NO3)3 and CoCl2), which possess electron donating properties, were reacted with the framework W (W = Ot groups) of the PW12O403− sites. As the selection of metals offers the flexibility required for tuning the redox potentials of the 7236

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solvent molecules, and polymers. Upon the introduction of PW12O403− into the polymer matrix, peaks for νs(WOt) and [δ(O2c1−WO) and δ(W−O2c1−W)] were red-shifted from 1013 to 1007 and 221 to 216 cm−1, respectively (Figure 1a, trace 1). The observed red shifts reflected an increase in the distance between the PW12O403− clusters and the Coulombic interaction between the WOt moiety of PW12O403− and hydroxyl groups of PVA.24,27 These micro-Raman observations indicate that PW12O403− molecules did not form the network structure as observed in crystalline H3PW12O40. In the Raman spectra of Nafion- and polyvinyl acetate (PVAc)-based samples, which were not self-standing membranes due to the high crystallinity of POM, the νs(WOt) band (1010 cm−1) was observed at a similar position to that of the Raman spectrum of crystalline H3PW12O40 (1013 cm−1), further supporting that PW12O403− was molecularly dispersed in PVA/PAM system. Upon reaction with Ce(NO3)3 or CoCl2, PW12O403− retained the original structure as shown in Scheme 1-2, as evidenced by the identical Raman characteristics of POM/PVA/PAM/Ce (Figure 1a, trace 2) and POM/PVA/PAM/Co (Figure 1a, trace 3) samples to those of POM/PVA/PAM samples except for a slight shift of the band attributable to the WOt moiety. No bands characteristic of PW11O397− (971 and 964 cm−1) were observed in the spectra of the POM/PVA/PAM/Ce and POM/PVA/PAM/Co samples. The absence of peaks in the region of 300−900 cm−1 indicates neither CeO2 nor Co oxides were formed.28,29 The molecular dispersion of PW12O403− was further confirmed by FT-IR spectroscopy, which is a strong method to examine the coordination environment such as hydrogen bonding between PW12O403− and the polymer matrix. FT-IR spectra of the POM/PVA/PAM membrane sample and sample lacking POM (PVA/PAM) are shown in Figure 1b. The bands at 2940 and 1661 cm−1 in the spectrum for PVA/PAM (Figure 1b, trace 1) were assigned to C−H alkyl stretching and CO stretching of the acrylamide unit, respectively.24 The spectrum also exhibited C−H and CO stretching bands for aldehyde groups of Glu cross-linked to PVA at 2873 and 1716 cm−1, respectively. Upon reaction with POM, bands originating from Glu (2873 and 1716 cm−1) and PAM (1661 cm−1) in the PVA/ PAM spectrum shifted to 2865, 1664, and 1612 cm−1, respectively (Figure 1b, trace 2). The observed peak shifts indicate that the POMs formed hydrogen bonds with the C O groups of the cross-linking reagent, PAM, and probably the O−H groups of PVA (Scheme 1-1). FT-IR spectroscopy also revealed that the interaction between POM and donor metal ions. As shown in Figure 1c, the peak assigned to the WOt (t = terminal) vibration of POM (980 cm−1) was shifted to a higher energy region after reaction with CoCl2, indicating the WO moiety of POM interacts with the Co ion. All Raman and FT-IR results taken together, and as previous works exhibited that the electronic interaction between POM and metal ions provides MMCT unit,8,30 the molecularly dispersed PW12O40/Ce and PW12O40/ Co MMCT units are constructed in the polymer matrix. Their energetic properties, namely, optical properties and redox potentials, will be discussed in the following sections. Optical Properties of the MMCT Unit. To examine whether the PW12O403−/Ce and PW12O403−/Co assemblies within the POM/PVA/PAM/Ce and POM/PVA/PAM/Co samples function as the MMCT units as shown in Scheme 1-2, UV−vis spectroscopic measurements were conducted. Before reaction with Ce(NO3)3 and CoCl2, the UV−vis diffuse

Raman spectroscopy was used for analyzing the molecular structure of POM in polymer matrixes because this method is sensitive to Keggin units and the Raman signals in the spectral fingerprint region are not affected by the polymer support. Micro-Raman spectra of the POM/PVA/PAM and crystalline H3PW12O40 as a reference are shown in Figure 1a (traces 1 and

Figure 1. (a) Micro-Raman spectra of the membrane samples before (POM/PVA/PAM, trace 1) and after reaction with Ce(NO3)3 (POM/ PVA/PAM/Ce, trace 2) or CoCl2 (POM/PVA/PAM/Co, trace 3). The spectrum of crystalline H3PW12O40 is also shown as a reference (trace 4). Laser intensity was 20 mW. (b) FT-IR spectra of PVA/PAM (trace 1) and POM/PVA/PAM (trace 2) membrane samples. (c) FTIR spectra of POM/PVA/PAM (trace 1) and POM/PVA/PAM/Co (trace 2).

4). The Raman spectrum of POM/PVA/PAM (Figure 1a, trace 1) is essentially identical to that of crystalline H3PW12O40 (Figure 1a, trace 4), indicating that PW12O403− maintained its original Keggin structure during membrane fabrication, as shown in Scheme 1-1. No bands characteristic of PW11O397− (971 and 964 cm−1), which is the hydrolysis product of PW12O403−,8 were observed. The observed peaks at 1007, 992, 236, and 216 cm−1 in the Raman spectrum of POM/PVA/PAM were assigned to the symmetrical and asymmetrical stretching vibration bands of terminal W = O moiety (νs(WOt), νas(WOt)), the combination of a symmetric stretching mode of W, four-coordinated oxygen, and bending motion of a W− O2c2−W bond ([ν(W−O) and δ(W−O−W)]), and the coupling band between the two bending modes ([δ(O2c1− WO) and δ(W−O2c1−W)]), respectively.26 The molecular dispersion of PW12O403− in polymer matrix was supported by the peak shifts of the Raman bands for W Ot moieties upon the introduction of PW12O403− into the polymer matrix because the positions of the vibration bands associated with the WOt moiety are strongly dependent on the interaction of PW12O403− with its neighboring anions, 7237

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absorption in the visible-light region extending to 650 nm was newly observed in the spectrum (Figure 2b, trace 2 and inset) in addition to the d−d transition absorption of CoII (530 nm, Figure 2b, trace 3). The appearance of the new absorption bands in the visible-light region after the treatment of POM/ PVA/PAM with Ce(NO3)3 and CoCl2 agrees well with the findings reported by Frei and Han for binuclear Ti−O−Ce and Ti−O−Co MMCT units constructed on the surface of mesoporous silica (MCM-41).31,32 Electrochemical Properties of the Donor/Acceptor Sites in MMCT Units. In the previous section, UV−vis spectroscopy confirmed that visible-light irradiation can induce MMCT in the PW12O40/Ce and PW12O40/Co moieties. Here whether the MMCT excitation can promote the target reactions is examined in terms of the energy level of each component. Energy levels of the electron acceptor (PW12O403−) and donor (CeIII and CoII) sites of MMCT units and the transporting property of the excited electrons are critical parameters to promote the photoactivation of the water oxidation catalyst and to achieve the unidirectional electron transfer. For this purpose, electrochemical measurement is an essential tool because it can determine the redox potential of the system. In the present work, cyclic voltammetry and differential pulse voltammetry for POM/PVA/PAM, POM/ PVA/PAM/Ce, and POM/PVA/PAM/Co fabricated on an ITO electrode were used to (1) examine whether PW12O403− retains its redox properties in the polymer matrix, (2) determine the redox potential of the electron donor (CeIII and CoII) sites, and (3) confirm that electrons are conducted through the membrane by intermolecular hopping. To confirm that (1) the multielectron redox properties of PW12O403− were maintained in the polymer matrix, the redox properties of the POM/PVA/PAM membrane sample were investigated by examining the cyclic voltammograms (CVs) and differential pulse voltammograms (DPVs) (Figure S2a and b, respectively, trace 1). Several redox peaks were observed in the potential region more negative than 0.1 V (vs SHE) in both the CV and DPV, a finding that reflects the multielectron oxidation/reduction property of PW12O403−. When the potential was shifted to the redox region of PW12O403−, the color of the sample changed from colorless to blue, a response that is indicative of intervalence charge-transfer between WV and WVI (heteropoly blue).33 Thus, PW12O403− was electrochemically active and retained its redox properties, even within the polymer matrix. The retention of the electrochemical multielectron redox properties of PW12O403− in the polymer matrix was further supported by the CVs and DPVs for membrane samples prepared with SiW12O404− and BW12O405− as POM reagents (Figure S2a and b, traces 2 and 3). As the redox potential of POMs is influenced by the central atom,14,23 altering the central atom from P to Si or B resulted in a shift of the first reduction potential of the membranes from 0.07 V for the POM/PVA/ PAM sample to −0.26 V for the B-POM/PVA/PAM sample. Altenau et al. examined the redox potential of various POMs (XW12O40n−, X = P, Si, Ge, Fe, B, Co, H2, or Cu and XMo12O40n−, X = P or Si) and reported that the first redox potential showed anion charge dependence over a range of 600 mV, from 0.2 V (vs SHE) for PW12O403− to −0.4 V for CuW12O407−.14,23 The degree of the potential shift observed here was identical to that for the homogeneous solution of H3PW12O40, H4SiW12O40, and K5BW12O40 (Figure S2c) and the POM/PVA/PAM, Si-POM/PVA/PAM, and B-POM/

reflectance spectra of the POM/PVA/PAM membrane sample (Figure 2, trace 1) exhibited an absorption assigned to a

Figure 2. (a) UV−vis diffuse reflectance spectra of the POM/PVA/ PAM membrane samples before (trace 1) and after reaction with Ce(NO3)3 (POM/PVA/PAM/Ce, trace 2). The spectrum of the POM-lacking membrane sample after reaction with Ce(NO3)3 is also shown as a reference (trace 3). (b) UV−vis diffuse reflectance spectra of the POM/PVA/PAM membrane samples before (trace 1) and after reaction with CoCl2 (POM/PVA/PAM/Co, trace 2). Because the POM-lacking membrane sample (PVA/PAM) did not react with CoCl2, the UV−vis absorption spectrum of an aqueous solution of CoCl2 was used as a reference (trace 3). The insets are the subtraction spectra before and after reaction with Ce(NO3)3 or CoCl2.

HOMO−LUMO transition (O 2p → W 5d) of PW12O403− in the UV region (λ < 400 nm). Although a slight absorption in the visible region extending to the near-infrared region was also observed and was attributed to the minor aggregation of POMs, the POM/PVA/PAM sample showed no prominent absorption in the visible-light region. Upon reaction with Ce(NO3)3 and CoCl2, the colorless POM/PVA/PAM sample changed to pale yellow and pink in color, respectively. The UV−vis diffuse reflectance spectra for POM/PVA/PAM/Ce also exhibited visible-light absorption in the region of 400−600 nm (Figure 2a, trace 2 and inset). In contrast, the POM-lacking sample (PVA/PAM/Ce) displayed no absorption in the visible-light region (Figure 2a, trace 3). The possibility that CeO2 attributed to the absorption in the visible region was excluded by the absence of Raman bands corresponding to CeO2 in the micro-Raman spectrum.28 Therefore, the new visible-light absorption was most likely attributable to MMCT from CeIII to WVI. In addition to PW12O403−/Ce MMCT units, the formation of the PW12O403−/Co MMCT unit was also confirmed by UV−vis spectroscopy measurements of POM/PVA/PAM/Co, as 7238

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Chemistry of Materials

the reaction with Ce3+, the electrochemical response of POM is retained as observed in the potential region more negative than 0.5 V with the shift of the peak positions. A stable voltammogram after several cycles and the retention of peaks originating from POM in micro-Raman spectra after CV measurement (Figure S3) indicate the POM/Ce MMCT unit is highly stable during the electrochemical measurement. The absence of the Raman signal corresponding to CeO2 excludes the possibility that the CV signal is assigned to the oxidation of noncoordinated Ce3+. For initiating the photoactivation of a MnOx water-oxidation catalyst, the electron donor site of the MMCT units requires sufficient oxidative energy to promote the water oxidation reaction on MnOx catalysts. To examine the effect of the redox potential of the donor metal site on the activation of a MnOx catalyst, the POM/PVA/PAM membrane sample was alternatively reacted with CoCl2. Figure 3b shows the CV of POM/ PVA/PAM after treatment with CoCl2 (POM/PVA/PAM/ Co). In contrast to POM/PVA/PAM (broken line), the oxidation current increased in the potential region more positive than +1.8 V. As the standard redox potential of the CoIII/CoII couple is +1.92 V and because no cathodic process was active, the observed current increase was assigned to the electrochemical water oxidation reaction at the Co site. PW12O403− was excluded as the water oxidation site because a negligible current was observed at potential regions more positive than +1.8 V for the sample lacking the Co site (POM/ PVA/PAM) (Figure 3b, broken line). Based on these findings, the redox potential of CoIII/CoII in POM/PVA/PAM/Co was shown to be ≥ +1.8 V. Because this redox potential is more positive than that of the CeIV/CeIII couple, CoIII generated by photoinduced MMCT from CoII to WVI has a greater capacity to activate MnOx for oxygen evolution from water than CeIV. These results are consistent with the absorption maximums of MMCT observed in the UV−vis spectra of 420 nm for POM/ PVA/PAM/Ce and 395 nm for POM/PVA/PAM/Co. The alteration of the redox properties of both the electron acceptor (PW12O403−) and donor (CeIII and CoII) sites of MMCT units demonstrates that an MMCT unit with a suitable redox potential for target reactions can be formed within the polymer matrix using this synthetic approach. Again, the retention of the redox behavior of POM was observed after the reaction with Co2+ in voltammograms even after several cycles, and microRaman spectra after CV measurements (Figure S3). Then, (3) the transfer properties of excited electrons through POM/PVA/PAM membrane were examined. To assess the electron conductivity of the POM/PVA/PAM membrane sample, intermolecular electron transfer by the H3PW12O40 molecules in POM/PVA/PAM samples with different membrane thicknesses was measured using CVs. Plots of the cathodic peak currents in CVs for POM/PVA/PAM samples of different thicknesses as a function of the amount of H3PW12O40 are shown in Figure S4. The peak currents exhibited a linear increase with a zero intercept as the amount of H3PW12O40 increased. The maximum amount of H3PW12O40 in the monolayer of POM/PVA/PAM sample was estimated to be 0.2 nmol cm−2. This value was based on the estimation that H3PW12O40 molecules have a diameter of 1 nm,34 and corresponds to a Coulomb number of 38.6 μC cm−2, which was based on the two-electron reduction property of PW12O403−. As the observed peak current in the voltammogram of POM/PVA/PAM exceeds this Coulomb value, the observed linear increase in the peak currents indicates that electron

PVA/PAM samples. Thus, the results of the electrochemical measurements for the Si-POM/PVA/PAM and B-POM/PVA/ PAM membrane samples confirm that the redox property of PW12O403− is maintained in the polymer matrix. Because the redox potential of POMs is largely affected by the external environment, like pH, the hydrogen-bonding network with PVA evidenced by FT-IR (Figure 1b) resulted in the difference of the peak positions of POMs between aqueous solution and polymer systems. As H3PW12O40 is able to store six electrons within its framework, it can promote multielectron transfer reactions. Next, because (2) determining the redox potential of the electron donor metal site in the MMCT unit (CeIII or CoII) formed in the polymer matrix is essential for POM/PVA/ PAM/Ce and POM/PVA/PAM/Co to achieve the efficient activation and electron transfer in photoinduced water oxidation systems, CV measurements were conducted for POM/PVA/PAM/Ce and POM/PVA/PAM/Co films fabricated on an ITO electrode (Figure 3). Following the reaction of POM/PVA/PAM with Ce(NO3)3, a new redox couple with a midpoint potential of 1.5 V was observed at pH 10 (Figure 3a, solid line). The observed redox peak was assigned to the CeIV/ CeIII couple, indicating the redox potential of the electron donor metal site of POM/PVA/PAM/Ce was 1.5 V. Even after

Figure 3. CVs of membrane samples after reaction with (a) Ce(NO3)3 (POM/PVA/PAM/Ce) and (b) CoCl2 (POM/PVA/PAM/Co) (solid lines) under Ar. The voltammogram for POM/PVA/PAM is shown as a control (broken line). The electrolyte was 0.1 M Na2SO4 aq. (pH 10), and the scan rate was 100 mV/s. The arrows in panel (a) indicate the scan direction. 7239

DOI: 10.1021/acs.chemmater.7b01669 Chem. Mater. 2017, 29, 7234−7242

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Chemistry of Materials hopping occurred between molecules of PW12O403−.35−39 The POM/PVA/PAM sample used in subsequent analyses had a PW12O403− amount of 2.6 μmol cm−2 and a thickness of 100 μm. Photoinduced Charge-Transfer Properties. As discussed above, POM/MMCT assemblies fabricated in this work satisfy the energy and electron transport requirements to photoactivate the water oxidation catalysts. The capacity of the MMCT unit to photoactivate MnOx catalysts (Scheme 1-3) was examined using a photoelectrochemical technique under visible-light illumination. The POM/PVA/PAM/Ce and POM/PVA/PAM/Co samples were fabricated on ITO electrodes and then coupled with the MnOx water-oxidation catalyst using a spray technique.40 Micro-Raman spectra after the deposition of MnOx shows the retention of the POM structure (Figure 4) and the absence of CeO2 and Co oxides. The I−t

Figure 4. Micro-Raman spectra of the membrane samples before (POM/PVA/PAM/Co, trace 2, POM/PVA/PAM/Ce, trace 4) and after the deposition of MnOx (POM/PVA/PAM/Co/MnOx, trace 1, POM/PVA/PAM/Ce/MnOx, trace 4).

Figure 5. (a) I−t curves for (1) POM/PVA/PAM/Co/MnOx, (2) POM/PVA/PAM/Ce/MnOx, (3) POM/PVA/PAM/Co, and (4) POM/PVA/PAM/MnOx samples fabricated on an ITO electrode and irradiated with visible-light. The light source was a 300 W Xe lamp equipped with a 450 nm cutoff filter to prevent UV absorption. (b) Action spectrum for POM/PVA/PAM/Co/MnOx (filled squares). The solid line represents the differential UV−vis diffuse reflectance spectrum after subtracting trace 1 from trace 2 in Figure 2b.

curves of POM/PVA/PAM/Co/MnOx (trace 1), POM/PVA/ PAM/Ce/MnOx (trace 2), POM/PVA/PAM/Co (trace 3), and POM/PVA/PAM/MnOx (trace 4) membrane samples at +1100 mV (vs SHE) are shown in Figure 5a. For the trimetallic sample composed of W (POM), Co, and Mn (POM/PVA/ PAM/Co/MnOx, trace 1), a photocurrent was observed under visible-light irradiation (λ > 450 nm; Y-47 cutoff filter, Asahi Glass), indicating that the excited electrons were conducted through the membrane sample and collected by the ITO electrode. Small magnitude of photocurrent may be attributed to the small absorption coefficient of the MMCT unit because the electronic interaction between W and Co in the MMCT unit is weak. The origin of the spike observed in POM/PVA/ PAM/Co/MnOx when light switched on is unclear, but it may be attributable to the charge accumulation at the interface between electrolyte solution and sample.41 The gradual decrease of dark current may be attributed to the surface accumulation of proton. Because water oxidation activity of MnOx largely deteriorated under neutral pH,40 the acidification of the surface resulted in the decrease of the dark water oxidation current. As a control experiment, photoelectrochemical measurements were performed for other dimetallic samples fabricated without MnOx or Co (POM/PVA/PAM/Co, trace 3 and POM/PVA/PAM/MnOx, trace 4). The photocurrent generated by these dimetallic samples was only approximately 10% of that produced by the trimetallic samples, clearly indicating that the metal centers of the MMCT unit and MnOx catalysts play a crucial role in the visible light-induced chargetransfer reaction. Notably, the photocurrent generated by the POM/PVA/PAM sample reacted with Ce(NO3)3 (POM/

PVA/PAM/Ce, trace 2) was only approximately 25% of that produced by the sample treated with CoCl2 (POM/PVA/ PAM/Co, trace 1). The reduction in the photocurrent reflected the less positive potential of CeIV compared with CoIII as revealed with electrochemical techniques, and indicates that altering the metal ion in the MMCT donor sites allows for flexible tuning of the redox potential of the membrane system. To confirm that the photocurrent originated from MMCT excitation, the action spectrum for the POM/PVA/PAM/Co/ MnOx sample was examined (Figure 5b). The action spectrum of the photocurrent (Figure 5b filled squares) corresponds to the CoII → WVI MMCT absorption (Figure 5b solid line), which appeared after the reaction with CoCl2. The d−d transition of CoII at 530 nm does not contribute to the action spectrum of the POM/PVA/PAM/Co/MnOx sample. MicroRaman spectroscopy conducted after the photoelectrochemical measurements revealed the POM structure is retained (Figure S6) and neither CeO2 nor Co oxides were formed. These results indicate that MMCT excitation triggered the photocurrent generation observed in the I−t curves (Figure 5a). Visible-light irradiation of POM/PVA/PAM/Co/MnOx induces MMCT (CoII → WVI) and the transfer of electrons at PW12O403− sites to the ITO electrode by intermolecular hopping between PW12O403− molecules. Further, the generated holes at Co sites are transferred to MnOx. In our previous report, an O2 evolution current on the MnOx electrode was 7240

DOI: 10.1021/acs.chemmater.7b01669 Chem. Mater. 2017, 29, 7234−7242

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Chemistry of Materials observed in potential regions more positive than +1.5 V.40 Because the redox potential of the Co site in POM/PVA/ PAM/Co is more positive than +1.8 V (Figure 3b), the holes generated at Co sites possess sufficient oxidizing power to promote the oxidation of water by the MnOx catalyst (Scheme 1-4).

Present Address

CONCLUSIONS In this work, we anchored a MnOx multielectron wateroxidation catalyst to a POM-based MMCT chromophore to promote photoactivation of MnOx catalyst. To align the chromophore and MnOx catalyst for promoting unidirectional photoinduced electron transfer, we first fabricated POM-based MMCT units in a cross-linked polymer matrix as a scaffold, and then coupled the MMCT units with the MnOx water-oxidation catalyst. Photoactivation of the MnOx catalysts and the sequential electron transfer from MnOx to the electrode were demonstrated by photocurrent measurements under visiblelight irradiation. As the functional parts of the POM/PVA/ PAM/Co/MnOx are composed of the inorganic molecules, results in this work offer a synthetic strategy for the redoxtunable chromophores with robustness. The membrane-based system opens up doors for the product-separable energy conversion system. Namely, reduction products evolve from one side of the membrane, while oxidation products from the other side. Furthermore, as POMs possess extremely rich redox and multielectron transfer catalytic properties for CO 2 reduction and H2 evolution reactions,11,12 and because POMbased MMCT units coupled with MnOx water-oxidation catalysts are able to regulate unidirectional electron transfer reactions due to the alignment of the catalyst and chromophore, solar-to-chemical energy-conversion systems with desirable redox and optical properties can be designed and fabricated at the molecular level using POM-based MMCT chromophores.

ACKNOWLEDGMENTS We thank Dr. K. Tajima of RIKEN for the surface profile measurements. A. Y. received financial support from the Global Center of Excellence for Mechanical Systems Innovation program of the University of Tokyo and the University Tokyo Grant for Ph.D. Research.

⊥

K.H.: National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan.

Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01669. Micro-Raman spectra of the samples used Nafion and PVAc as base polymer; CVs and DPVs for POM/PVA/ PAM membrane samples prepared with various POMs; micro-Raman spectra of the membrane samples before and after the CV measurement; peak current (at 0.07 V) vs POM contents plot; I−t curve for POM/PVA/PAM/ Co/MnOx sample fabricated on an ITO electrode and irradiated with visible-light at rest potential; microRaman spectra of POM/PVA/PAM/Co/MnOx sample before and after photoelectrochemical measurement (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-(0)3-5734-2527. Fax: +81-(0)3-5734-3368. E-mail: [email protected]. ORCID

Akira Yamaguchi: 0000-0002-3550-4239 Ryuhei Nakamura: 0000-0003-0743-8534 7241

DOI: 10.1021/acs.chemmater.7b01669 Chem. Mater. 2017, 29, 7234−7242

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Chemistry of Materials

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