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

This method can help answer key questions in the solar-to-chemical energy conversion field such as artificial photosynthesis. The main advantage of this technique is that we can fabricate flexible, durable, and redox tunable photo-functional materials. First add two milliliters of polyvinyl alcohol solution and two milliliters of polyacrylamide solution to a 50 milliliter vial.

Then add a triangular shaped stir bar and one gram of phosphotungstic acid to the vial. This cross-linking method was reported by another group and we modified the polyoxotungstate and its amount. Utilizing this method we can fabricate a membrane with an acid-type polyoxometalate instead of a salt-type.

This is a key species to construct a photoresponsive membrane. Heat the vial to 70 degrees Celsius in a water bath under vigorous stirring and continue to stir for six hours after leaching at 70 degrees Celsius. Place a glass substrate on a hot plate pre-heated to 100 degrees Celsius and drop 750 microliters of the solution on to the substrate.

To dry the sample store it in the dark overnight at room temperature. To prepare the crosslinking reagent add 72 milliliters of distilled water, 24 milliliters of acetone, 2 milliliters of 25%glyceraldehyde solution, and 2 milliliters of hydrochloric acid to a 100 milliliter vial. Place the glass substrate with the sample in a 9.5 centimeter Petri dish and add the crosslinking reagent until the membrane is completely immersed.

After 30 minutes replace the crosslinking reagent with distilled water and wash once. If necessary peel the membrane from the glass substrate using a spatula and store it in distilled water in the dark. Add 2.08 grams of cerium nitrate hexahydrate and a stir bar to a 50 milliliter vial.

Then add 30 milliliters of water to the vial and stir to dissolve the solid. Next place the membrane in a 9.5 centimeter Petri dish and add the cerium nitrate solution until the membrane is completely immersed. Place the Petri dish into a pre heated oven at 80 degrees Celsius for five hours.

After cooling to room temperature replace the cerium nitrate solution with distilled water and wash once. Store the membrane and distilled water in the dark. To prepare a membrane with cobalt as the donor metal use the same preparation and reaction procedures as previously described except using 1.14 grams of cobalt chloride hexahydrate.

Place the membranes onto a hot plate at 60 degrees Celsius with masks made of silicone rubber to determine the area to be used for deposition. Add 300 milliliters of a previously prepared colloidal manganese oxide solution to a 500 milliliter bottle connected to an automated spray gun above the hot plate and spray the solution onto the membranes. After manganese oxide deposition store the samples and distilled water in the dark.

Retention of the POM structure in the polymer matrix was confirmed by micro-Raman and FT-IR spectroscopy. 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.

In addition it was confirmed by UV-Vis spectroscopy and photo electro chemical measurements because charge transfer is achieved when all components are adequately aligned. In particular the spectroscopic and photoelectrochemical results confirmed one directional charge transfer under light irradiation which thus supports the validity of our synthetic method. The production of oxygen on manganese oxide was monitored using an indirect electrochemical method employing a rotating disk-ring electrode system where the reduction current corresponding to oxygen reduction could be observed under light irradiation.

After its development this technique paves the way for researchers in the field of photochemistry to fabricate a redox-tunable and stable photosystem to trigger many photochemical reactions.