Ionic self-assembly of porphyrin micro-nano structures for solar hydrogen evolution
This research project develops self-assembled porphyrin micro-nano structures and their metal nanocomposites for efficient solar H2 evolution. The project is composed of three inter-related tasks. The first task is to develop the synthetic methods for preparing the binary porphyrin micro-nano structures by ionic self-assembly under various aqueous solution conditions. Although the porphyrin molecules have been widely studied in artificial photosynthesis, the porphyrin monomers absorb light in a limited range of the solar spectrum and are unstable in solution. In using ionic selfassembly of two oppositely charged porphyrin molecules, we successfully synthesized the more robust micro/nano clover-like structures to replace the porphyrin monomer, which also have red-shifted light absorption due to J-aggreate formation. In addition, the microscale porphyrin structures can be designed with photochemical and catalytic properties, which is useful in solar energy harvesting and conversion to desired hydrogen. Effect of reaction time, growth temperature, ionic strength, and metal interchange on the self-assembly process of these micro-nano structures is studied. I have also investigated the ability of these porphyrin micro-nano structures to photocatalytically reduce aqueous gold(I) and platinum(IV) complexes to initiate and form nanostructures decorated with metal nanostructures on their surfaces. Photocatalytic reduction of both gold(I) thiosulfate and thiourea complexes lead to the metal-clover porphyrin nanocomposites. The platinized micro-nano clovers catalyze H2 generation utilizing visible light and a sacrificial electron donor at pH 3. The photoconductive ZnTPPS4-/SnT(N-EtOH-4-Py)P4+ porphyrin micro-nano structures are able to deliver electrons directly to the platinum nanoparticles at the surface with sufficient reducing power to generate hydrogen directly. However, the efficiency of this process is greatly improved when a soluble electron (and proton) relay molecule such as methylviologen is included in the artificial photosynthesis system. Stable production of hydrogen has been observed for more than two weeks. A comparative hydrogen evolution study of all of the clover-like structures from the Zn and Sn complexes of TPPS and T(N-EtOH-4-Py)P indicate that the porphyrin solids produce hydrogen more efficiently and durably than individual porphyrins in solution, suggesting that the collective properties of the micro-nano solids enhance the H2 production reaction and stablize the porphyrin light-harvesting materials.
Light Harvesting, Solar Hydrogen Evolution, Porphyrin, Ionic Self-Assembly, Water Splitting, Artificial Photosynthesis; Water--Electrolysis., Nanostructured materials., Nanocomposites (Materials), Porphyrins., Self-assembly (Chemistry)
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