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

Summary

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

Abstract

This paper presents a method to prepare charge-transfer chromophores using polyoxotungstate (PW₁₂O₄₀^3-), 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 PW₁₂O₄₀^3- content. Incorporation and structure retention of PW₁₂O₄₀^3- 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 MnO_x oxygen evolving catalysts, photocurrent measurements under visible light irradiation verified the sequential charge transfer, Mn → MMCT unit → electrode, and 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.

Introduction

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 issues^1,2,3,4. 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 reactions^5,6,7,8,9.

Our group extended the single atomic system to a polynuclear system utilizing polyoxometalate (POM) as the electron acceptor^10,11,12, 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 MnO_x 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.

Protocol

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.

1. Preparation of precursor polyvinyl alcohol (PVA) solution
   1. Add 3 g of polyvinyl alcohol 1000 (completely hydrolyzed) and a stirring bar to a 50 mL vial.
   2. Add 27 mL of water to the vial.
   3. Heat the vial to 70 °C within a water bath under stirring conditions to completely dissolve all polyvinyl alcohol particles.
2. Preparation of precursor polyacrylamide (PAM) solution
   1. Add 0.75 g of polyacrylamide (see Table of Materials) and a stirring bar to a 50 mL vial.
   2. Add 29.25 mL of water to the vial.
   3. Heat the vial to 70 °C within a water bath under stirring conditions to completely dissolve all polyacrylamide particles.
3. Preparation of mixture solution of POM and polymers
   1. 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.
   2. 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.
   3. Heat the vial to 70 °C within a water bath under vigorous stirring conditions and continue to stir for 6 h after leaching at 70 °C.
   4. Place a glass substrate (5 x 5 cm²) on a hot plate (approximately 100 °C) and drop 750 μL of the solution onto the substrate.
      NOTE: The solution should be kept hot during the dropping process to prevent the polymers from congealing.
   5. To dry the sample, keep it overnight in a dark condition at room temperature.
4. Cross-linking process for membrane sample
   1. 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.
   2. 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.
   3. After 30 min, replace the cross-linking solution with distilled water, and wash once.
   4. 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 MnO_x deposition), the peeling-off process should be omitted.
   5. 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.

2. Reaction of POM/polymer Membrane with Donor Metals (Ce³⁺ and Co²⁺)

1. Preparation of Ce³⁺ solution.
   1. Add 2.08 g of Ce(NO₃)₃ • 6H₂O and a stirring bar to a 50 mL vial.
   2. Add 30 mL of water to the vial and stir to dissolve all Ce(NO₃)₃.
2. Reaction of membrane sample with Ce³⁺
   1. Put the membrane sample in a Petri dish (approximately 9.5 cm diameter) and add Ce³⁺ solution until the membrane is completely immersed.
   2. Put the Petri dish into a pre-heated oven at 80 °C for 5 h.
   3. Replace the Ce³⁺ solution with distilled water and wash once.
   4. Store in distilled water in dark conditions.
3. Preparation of Co²⁺ solution
   1. Use the same preparation and reaction procedures as with Ce³⁺, except using 1.14 g of CoCl₂• 6H₂O instead of Ce(NO₃)₃ • 6H₂O. Samples reacted with Ce³⁺ and Co²⁺ are called POM/PVA/PAM/Ce and POM/PVA/PAM/Co, respectively.
   2. In this study, micro-Raman spectra of POM/PVA/PAM/Ce and POM/PVA/PAM/Co were examined to determine the molecular structure of PW₁₂O₄₀^3- after reaction with donor metal ions¹³. The optical properties of these samples were also examined using UV-Vis spectroscopy.

3. Deposition of MnO_x Water Oxidation Catalysts

NOTE: The preparation and deposition procedures of colloidal MnO_x follow those in Perez-Benito et al. 1989¹⁵ and Takashima et al. 2012¹⁶, respectively.

1. Preparation of colloidal MnO_x solution
   1. 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.
   2. Add 10 mL water to the beaker and stir to completely dissolve all KMnO₄.
   3. Add 50 mg of Na₂S₂O₃• 5H₂O and an oval-shaped stirring bar to a 500 mL round-bottom flask.
   4. Add 20 mL water to the flask and stir to completely dissolve all Na₂S₂O₃.
   5. Add 10 mL of KMnO₄ solution to the flask drop-wise under vigorous stirring.
      ​Note: KMnO₄ solution should be added slowly to prevent formation of aggregated MnO₂.
   6. After adding the KMnO₄ solution, immediately add 470 mL of distilled water to the flask.
   7. Keep stirring for over 2 h and then immediately use the solution for the next process (spraying).
2. Spraying MnO_x colloidal solution onto membrane sample
   1. Place the membrane samples onto a hot plate (approximately 60 °C) with masks made of silicone rubber to determine the area to be used for deposition.
      NOTE: For the deposition of MnO_x 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.
   2. Add 300 mL of the colloidal MnO_x 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.
   3. When the volume in the bottle decreases, add the remaining colloidal solution to the bottle.
   4. Store the samples in distilled water in dark conditions. After MnO_x-deposition, the samples are called POM/PVA/PAM/Ce/MnO_x or POM/PVA/PAM/Co/MnO_x.
   5. Electron transfer properties under visible-light irradiation of POM/PVA/PAM/Co/MnO_x 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.

Results

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 (

Discussion

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...

Materials

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