Name: Preparation of Polyoxometalate-based Photo-responsive Membranes for the Photo-activation of Manganese Oxide Catalysts (Video) | JoVE Journal
Uploaded: 2018-08-07T14:30:00.000+00:00
Duration: 5 min 47 s
Description: Learn to fabricate flexible, redox-tunable polyoxometalate-polymer composite membranes for solar-to-chemical energy conversion. This method enables photo-activation of manganese oxide catalysts for artificial photosynthesis.

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5 minutes

A. Y. received financial support from the Global Center of Excellence for Mechanical Systems Innovation program of the University of Tokyo and from the University Tokyo Grant for Ph.D. Research. This work is partly supported by JSPS KAKENHI Grant-in-Aid for Young Scientists (B) (17K17718).

It is advisable to refer to all relevant material safety data sheets (MSDS) prior to using chemicals, as some used in these syntheses are highly acidic and corrosive. In addition, one polymer used in this work (polyacrylamide) may contain the carcinogenic monomer, acrylamide. The use of personal protective equipment (safety glasses, gloves, lab coat, full-length pants, closed-toe shoes) is required to prevent injuries from chemicals or heat. After conducting the cross-linking process, membrane samples should be stored in water in dark conditions to avoid drying and the occurrence of any unnecessary photochemical reactions.
1. Preparation of POM/polymer Composite Membrane
Note: The synthesis procedure follows that reported in the article by Helen et al.14, except that the amount of POM was altered.
<ol>
	<li>Preparation of precursor polyvinyl alcohol (PVA) solution
	<ol>
		<li>Add 3 g of polyvinyl alcohol 1000 (completely hydrolyzed) and a stirring bar to a 50 mL vial.</li>
		<li>Add 27 mL of water to the vial.</li>
		<li>Heat the vial to 70 &#176;C within a water bath under stirring conditions to completely dissolve all polyvinyl alcohol particles.</li>
	</ol>
	</li>
	<li>Preparation of precursor polyacrylamide (PAM) solution
	<ol>
		<li>Add 0.75 g of polyacrylamide (see Table of Materials) and a stirring bar to a 50 mL vial.</li>
		<li>Add 29.25 mL of water to the vial.</li>
		<li>Heat the vial to 70 &#176;C within a water bath under stirring conditions to completely dissolve all polyacrylamide particles.</li>
	</ol>
	</li>
	<li>Preparation of mixture solution of POM and polymers
	<ol>
		<li>Add 2 mL of PVA solution and 2 mL of PAM solution to a 50 mL vial. 
		NOTE: As these polymer solutions have high viscocities, it is essential to accurately measure their volumes.</li>
		<li>Add a triangular-shaped stirring bar and 1 g of H3PW12O40 to the vial. 
		CAUTION: H3PW12O40 is a highly acidic material and should be stored in refrigerator. When working with it, use plastic materials and not metal ones.</li>
		<li>Heat the vial to 70 &#176;C within a water bath under vigorous stirring conditions and continue to stir for 6 h after leaching at 70 &#176;C.</li>
		<li>Place a glass substrate (5 x 5 cm2) on a hot plate (approximately 100 &#176;C) and drop 750 &#956;L of the solution onto the substrate. 
		NOTE: The solution should be kept hot during the dropping process to prevent the polymers from congealing.</li>
		<li>To dry the sample, keep it overnight in a dark condition at room temperature.</li>
	</ol>
	</li>
	<li>Cross-linking process for membrane sample
	<ol>
		<li>Add 72 mL of distilled water, 24 mL of acetone, 2 mL of 25% glutaraldehyde solution, and 2 mL of HCl to a 100 mL vial. This mixture is called the cross-linking reagent.</li>
		<li>Put the glass substrate with the sample in a Petri dish (approximately 9.5 cm diameter) and add the cross-linking reagent until the membrane is completely immersed.</li>
		<li>After 30 min, replace the cross-linking solution with distilled water, and wash once.</li>
		<li>Peel the membrane from the glass substrate using a spatula and store it in distilled water in dark conditions.This POM/polymer membrane is called POM/PVA/PAM. 
		NOTE: If the membrane is used for the next process (reaction with donor metals or MnOx deposition), the peeling-off process should be omitted.</li>
		<li>Use micro-Raman and FT-IR spectroscopy with POM/PVA/PAM to determine the chemical structure of the membrane13. Take an FT-IR spectrum of PVA/PAM (lacking POM) sample prepared by the same method as a reference.</li>
	</ol>
	</li>
</ol>
2. Reaction of POM/polymer Membrane with Donor Metals (Ce3+ and Co2+)
<ol>
	<li>Preparation of Ce3+ solution.
	<ol>
		<li>Add 2.08 g of Ce(NO3)3 &#8226; 6H2O and a stirring bar to a 50 mL vial.</li>
		<li>Add 30 mL of water to the vial and stir to dissolve all Ce(NO3)3.</li>
	</ol>
	</li>
	<li>Reaction of membrane sample with Ce3+
	<ol>
		<li>Put the membrane sample in a Petri dish (approximately 9.5 cm diameter) and add Ce3+ solution until the membrane is completely immersed.</li>
		<li>Put the Petri dish into a pre-heated oven at 80 &#176;C for 5 h.</li>
		<li>Replace the Ce3+ solution with distilled water and wash once.</li>
		<li>Store in distilled water in dark conditions.</li>
	</ol>
	</li>
	<li>Preparation of Co2+ solution
	<ol>
		<li>Use the same preparation and reaction procedures as with Ce3+, except using 1.14 g of CoCl2&#8226; 6H2O instead of Ce(NO3)3 &#8226; 6H2O. Samples reacted with Ce3+ and Co2+ are called POM/PVA/PAM/Ce and POM/PVA/PAM/Co, respectively.</li>
		<li>In this study, micro-Raman spectra of POM/PVA/PAM/Ce and POM/PVA/PAM/Co were examined to determine the molecular structure of PW12O403- after reaction with donor metal ions13. The optical properties of these samples were also examined using UV-Vis spectroscopy.</li>
	</ol>
	</li>
</ol>
3. Deposition of MnOx Water Oxidation Catalysts
NOTE: The preparation and deposition procedures of colloidal MnOx follow those in Perez-Benito et al. 198915 and Takashima et al. 201216, respectively.
<ol>
	<li>Preparation of colloidal MnOx solution
	<ol>
		<li>Add 39.4 mg of KMnO4 and a stirring bar to a 30 mL beaker. 
		NOTE: CAUTION: KMnO4 is a highly oxidative material; when working with it, use plastic spoons and not metal ones.</li>
		<li>Add 10 mL water to the beaker and stir to completely dissolve all KMnO4.</li>
		<li>Add 50 mg of Na2S2O3&#8226; 5H2O and an oval-shaped stirring bar to a 500 mL round-bottom flask.</li>
		<li>Add 20 mL water to the flask and stir to completely dissolve all Na2S2O3.</li>
		<li>Add 10 mL of KMnO4 solution to the flask drop-wise under vigorous stirring. 
		&#8203;Note: KMnO4 solution should be added slowly to prevent formation of aggregated MnO2.</li>
		<li>After adding the KMnO4 solution, immediately add 470 mL of distilled water to the flask.</li>
		<li>Keep stirring for over 2 h and then immediately use the solution for the next process (spraying).</li>
	</ol>
	</li>
	<li>Spraying MnOx colloidal solution onto membrane sample
	<ol>
		<li>Place the membrane samples onto a hot plate (approximately 60 &#176;C) with masks made of silicone rubber to determine the area to be used for deposition. 
		NOTE: For the deposition of MnOx catalysts, the membrane samples should be placed on glass substrates. If the membranes have not been peeled off from the substrate, put them onto a hot plate as they are. Keep the surface of the membrane wet during the deposition process. The mask also works as a weight to prevent the membranes from flying off due to the high air pressure from the spray gun.</li>
		<li>Add 300 mL of the colloidal MnOx solution to a 500 mL bottle connected to an automated spray gun (see Table of Materials) above the hot plate and spray the solution onto the membranes.</li>
		<li>When the volume in the bottle decreases, add the remaining colloidal solution to the bottle.</li>
		<li>Store the samples in distilled water in dark conditions. After MnOx-deposition, the samples are called POM/PVA/PAM/Ce/MnOx or POM/PVA/PAM/Co/MnOx.</li>
		<li>Electron transfer properties under visible-light irradiation of POM/PVA/PAM/Co/MnOx were monitored using photoelectrochemical measurements at a particular electrode potential. To conduct the photoelectrochemical experiment, the membrane sample was fabricated on transparent Sn-doped In2O3 (ITO) electrodes.</li>
	</ol>
	</li>
</ol>

<ol><li>Fujishima, A., Honda, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Electrochemical+Photolysis+of+Water+at+a+Semiconductor+Electrode%5BTitle%5D)+AND+%22Nature%22%5BJournal%5D)">Electrochemical Photolysis of Water at a Semiconductor Electrode.</a> Nature. 238, 37-38 (1972).</li>
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<li>Bard, A. J., Fox, M. A. <a target="_blank" href="https://pubs.acs.org/doi/abs/10.1021/ar00051a007">Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen.</a> Accounts of Chemical Research. 28, 141-145 (1995).</li>
<li>Lewis, N. S., Nocera, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Powering+the+Planet%3A+Chemical+Challenges+in+Solar+Energy+Utilization%5BTitle%5D)+AND+%22Proceedings+of+the+National+Academy+of+Sciences+of+the+United+States+of+America%22%5BJournal%5D)">Powering the Planet: Chemical Challenges in Solar Energy Utilization.</a> Proceedings of the National Academy of Sciences of the United States of America. 103, 15729-15735 (2006).</li>
<li>Lin, W., Frei, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Anchored+Metal-to-Metal+Charge-Transfer+Chromophores+in+a+Mesoporous+Silicate+Sieve+for+Visible-Light+Activation+of+Titanium+Centers%5BTitle%5D)+AND+%22The+Journal+of+Physical+Chemistry+B%22%5BJournal%5D)">Anchored Metal-to-Metal Charge-Transfer Chromophores in a Mesoporous Silicate Sieve for Visible-Light Activation of Titanium Centers.</a> The Journal of Physical Chemistry B. 109, 4929-4935 (2005).</li>
<li>Lin, W., Frei, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Photochemical+CO2+Splitting+by+Metal-to-Metal+Charge-Transfer+Excitation+in+Mesoporous+ZrCu%28I%29-MCM-41+Silicate+Sieve%5BTitle%5D)+AND+%22Journal+of+the+American+Chemical+Society%22%5BJournal%5D)">Photochemical CO2 Splitting by Metal-to-Metal Charge-Transfer Excitation in Mesoporous ZrCu(I)-MCM-41 Silicate Sieve.</a> Journal of the American Chemical Society. 127, 1610-1611 (2005).</li>
<li>Lin, W., Frei, H. <a target="_blank" href="https://www.sciencedirect.com/science/article/pii/S1631074805003000">Bimetallic redox sites for photochemical CO2 splitting in mesoporous silicate sieve.</a> Comptes Rendus Chimie. 9, 207-213 (2006).</li>
<li>Kim, W., Yuan, G., McClure, B. A., Frei, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Light+Induced+Carbon+Dioxide+Reduction+by+Water+at+Binuclear+ZrOCoII+Unit+Coupled+to+Ir+Oxide+Nanocluster+Catalyst%5BTitle%5D)+AND+%22Journal+of+the+American+Chemical+Society%22%5BJournal%5D)">Light Induced Carbon Dioxide Reduction by Water at Binuclear ZrOCoII Unit Coupled to Ir Oxide Nanocluster Catalyst.</a> Journal of the American Chemical Society. 136, 11034-11042 (2014).</li>
<li>Kim, W., Frei, H. <a target="_blank" href="https://pubs.acs.org/doi/abs/10.1021/acscatal.5b01306">Directed Assembly of Cuprous Oxide Nanocatalyst for CO2 Reduction Coupled to Heterobinuclear ZrOCoII Light Absorber in Mesoporous Silica.</a> ACS Catalysis. 5, 5627-5635 (2015).</li>
<li>Takashima, T., Nakamura, R., Hashimoto, K. <a target="_blank" href="https://pubs.acs.org/doi/abs/10.1021/jp9047049">Visible Light Sensitive Metal Oxide Nanocluster Photocatalysts: Photo-Induced Charge Transfer from Ce(III) to Keggin-Type Polyoxotungstates.</a> The Journal of Physical Chemistry C. 113, 17247-17253 (2009).</li>
<li>Takashima, T., Yamaguchi, A., Hashimoto, K., Nakamura, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Multielectron-transfer+Reactions+at+Single+Cu%28II%29+Centers+Embedded+in+Polyoxotungstates+Driven+by+Photo-induced+Metal-to-metal+charge+Transfer+from+Anchored+Ce%28III%29+to+Framework+W%28VI%29%5BTitle%5D)+AND+%22Chemical+Communications%22%5BJournal%5D)">Multielectron-transfer Reactions at Single Cu(II) Centers Embedded in Polyoxotungstates Driven by Photo-induced Metal-to-metal charge Transfer from Anchored Ce(III) to Framework W(VI).</a> Chemical Communications. 48, 2964-2966 (2012).</li>
<li>Takashima, T., Nakamura, R., Hashimoto, K. <a target="_blank" href="https://www.researchgate.net/publication/257367219_Visible-Light-Absorbing_Lindqvist-Type_Polyoxometalates_as_a_Building_Block_for_All-Inorganic_Photosynthetic_Assemblies">Visible-Light-Absorbing Lindqvist-Type Polyoxometalates as Building Blocks for All-Inorganic Photosynthetic Assemblies.</a> Electrochemistry. 79, 783-786 (2011).</li>
<li>Yamaguchi, A., Takashima, T., Hashimoto, K., Nakamura, R. <a target="_blank" href="https://pubs.acs.org/doi/abs/10.1021/acs.chemmater.7b01669">Design of Metal-to-metal Charge-transfer Chromophores for Visible-light Activation of Oxygen-Evolving Mn Oxide Catalysts in a Polymer Film.</a> Chemistry of Materials. 29, 7234-7242 (2017).</li>
<li>Helen, M., Viswanathan, B., Murthy, S. S. <a target="_blank" href="https://onlinelibrary.wiley.com/doi/abs/10.1002/app.31940">Poly(vinyl alcohol)-polyacrylamide Blends With Cesium Salts of Heteropolyacid as a Polymer Electrolyte for Direct Methanol Fuel Cell Applications.</a> Journal of Applied Polymer Science. 116, 3437-3447 (2010).</li>
<li>Perez-Benito, J. F., Brillas, E., Pouplana, R. <a target="_blank" href="https://pubs.acs.org/doi/abs/10.1021/ic00302a002">Identification of a Soluble Form of Colloidal Manganese(IV).</a> Inorganic Chemistry. 28, 390-392 (1989).</li>
<li>Takashima, T., Nakamura, R., Hashimoto, K. <a target="_blank" href="https://pubs.acs.org/doi/abs/10.1021/ja206511w">Mechanism of pH-Dependent Activity for Water Oxidation to Molecular Oxygen by MnO2 Electrocatalysts.</a> Journal of the American Chemical Society. 134, 1519-1527 (2012).</li>
<li>Bridgeman, A. J. <a target="_blank" href="https://www.sciencedirect.com/science/article/pii/S0301010402009783">Density Functional Study of the Vibrational Frequencies of α-Keggin Heteropolyanions.</a> Chemical Physics. 287, 55-69 (2003).</li>
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</ol>

The authors have nothing to disclose.

It is critical to apply the cross-linking method introduced by Helen et al.14 to develop a self-standing membrane. When polyvinyl acetate was applied as the base polymer in this study, aggregation of H3PW12O40 occurred, which prevented formation of the self-standing membrane. However, when fabrication of the membrane was attempted utilizing Nafion as the base polymer, there was no progression of the reaction with Ce3+ and Co2+, althou...

The development of solar energy conversion systems using artificial photosynthesis or solar cells is necessary to enable the provision of alternative energy sources that can ameliorate global climate and energy issues1,2,3,4. Photo-functional materials can be broadly categorized into two groups, semiconductor-based systems and organic molecule-based systems. Although many different system types have been developed, improvements still need to be made because semiconductor systems suffer from a lack of precise charge transfer control, and organic molecule systems are not adequately durable with respect to photo-irradiation. However, the use of inorganic molecules as charge transfer unit components can improve these respective issues. For example, Frei et al. developed oxo-bridged metal systems grafted on the surface of mesoporous silica which can induce the metal-to-metal charge transfer (MMCT) by photo-irradiation and trigger photochemical redox reactions5,6,7,8,9.
Our group extended the single atomic system to a polynuclear system utilizing polyoxometalate (POM) as the electron acceptor10,11,12, with the expectation that use of the polynuclear system would be advantageous in the induction and control of the multi-electron transfer reaction, which is an important concept in energy conversion. In the protocol described here, we present the detailed method used to prepare the POM-based MMCT system, which works in a polymer matrix as we recently reported13. The membrane-type configuration is favorable for product separation between anodic and cathodic reaction products. The cross-linking method was applied, which enabled formation of a self-standing membrane, even with high POM contents. Photoelectrochemical measurements proved that appropriate selection of the donor metal is key to triggering the target. The POM/donor metal system works as a photo-sensitizer to activate multi-electron transfer catalysts under visible light irradiation. Although this work utilizes MnOx as a multi-electron transfer catalyst for the water oxidation reaction, this photo-functional system is also applicable for use with other types of reactions by utilizing various POMs, donor metals, and catalysts.

<table><tbody><tr><td>Poly(vinyl Alcohol) 1000, Completely Hydrolyzed</td><td>Wako</td><td>162-16325</td><td></td></tr><tr><td>Polyacrylamide, Mv 6,000,000</td><td>Polyaciences, Inc.</td><td>2806</td><td>May contain carcinogenic monomer, acrylamide.</td></tr><tr><td>12 Tungsto(VI)phosphoric Acid n-Hydrate</td><td>Wako</td><td>164-02431</td><td>Highly acidic</td></tr><tr><td>Acetone 99.5 + %(GC)</td><td>Wako</td><td>012-00343</td><td></td></tr><tr><td>25% Glutaraldehyde Solution</td><td>Wako</td><td>079-00533</td><td></td></tr><tr><td>Hydrochloric Acid 35-37%</td><td>Wako</td><td>080-01066</td><td></td></tr><tr><td>Cerium(III) Nitrate Hexahydrate 98 + %(Ti)</td><td>Wako</td><td>031-09732</td><td></td></tr><tr><td>Cobalt(II) Chloride Hexahydrate 99 + %(Ti)</td><td>Wako</td><td>036-03682</td><td></td></tr><tr><td>Pottasium Permanganate 99.3 + %(Ti)</td><td>Wako</td><td>167-04182</td><td>Highly oxydative</td></tr><tr><td>Sodium Thiosulfate Pentahydrate 99 + %(Ti)</td><td>Wako</td><td>197-03585</td><td></td></tr><tr><td>Automatic spray gun</td><td>Lumina</td><td>ST-6</td><td></td></tr></tbody></table>

preparation of polyoxometalate-based photo-responsive membranes for the photo-activation of manganese oxide catalysts

Preparation of Polyoxometalate-based Photo-responsive Membranes for the Photo-activation of Manganese Oxide Catalysts

This paper presents a method to prepare charge-transfer chromophores using polyoxotungstate (PW12O403-), transition metal ions (Ce3+ or Co2+), and organic polymers, with the aim of photo-activating oxygen-evolving manganese oxide catalysts, which are important components in artificial photosynthesis. The cross-linking technique was applied to obtain a self-standing membrane with a high PW12O403- content. Incorporation and structure retention of PW12O403- within the polymer matrix were confirmed by FT-IR and micro-Raman spectroscopy, and optical characteristics were investigated by UV-Vis spectroscopy, which revealed successful construction of the metal-to-metal charge transfer (MMCT) unit. After deposition of MnOx oxygen evolving catalysts, photocurrent measurements under visible light irradiation verified the sequential charge transfer, Mn &#8594; MMCT unit &#8594; electrode, and&#160;the photocurrent intensity was consistent with the redox potential of the donor metal (Ce or Co). This method provides a new strategy for preparing integrated systems involving catalysts and photon-absorption parts for use with photo-functional materials.

Retention of the POM structure in the polymer matrix was confirmed by FT-IR and micro-Raman spectroscopy (Figure 1); vibration peaks corresponding to the Keggin structure of POM were observed, and peaks of the polymers were found to be shifted due to hydrogen bonding with POM. Spectroscopic analysis was very useful for determining successful construction of the charge transfer unit, and this was also confirmed by the apparent color change of the samples (<str...

Here, we present a protocol to prepare charge transfer chromophores based on a polyoxometalate/polymer composite membrane.

This paper presents a method to prepare charge-transfer chromophores using polyoxotungstate (PW12O403-), transition metal ions (Ce3+ or Co2+), and organic ...

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Learn to fabricate flexible, redox-tunable polyoxometalate-polymer composite membranes for solar-to-chemical energy conversion. This method enables photo-activation of manganese oxide catalysts for artificial photosynthesis.

Preparation of Polyoxometalate-based Photo-responsive Membranes for the Photo-activation of Manganese Oxide Catalysts

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