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 orga....

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 P....

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