Seminars & Speakers

Towards Sustainable Photolysis of Water for Production of Solar Fuels
Speaker Prof. Yoon Sung Nam
Affiliation KAIST
Date September 6, 2017
Time 4:00 pm - 6:00 pm
Venue EB2(#104) E205
Sponsor UNIST-Energy and Chemical Engineering
Host Prof. Jung Ki Ryu
Contact 052-217-2564
Phone 052-217-3552
Plasmonic gold (Au) nanostructures are great attractive for visible-light-driven water splitting due to their excellent chemical stability and strong visible light absorption via localized surface plasmon resonance (LSPR). The absorbed light in the Au nanostructures can be converted to hot carriers via the decay of LSP that can be subsequently utilized for photocatalytic water splitting. Here I introduce a plasmonic photoanode comprising three-dimensional (3D) porous network of colloidal gold nanoparticles (AuNPs) coupled with an oxygen evolving catalyst. The 3D Au nanostructured photoanode is prepared by the layer-by-layer (LbL) assembly of negatively charged AuNPs and cationic polyethyleneimine (PEI) on a titanium oxide (TiO2) thin layer. First, the hot carrier transfer in the photoanodes is investigated depending on the interfacial properties under visible light illumination. The photocurrents increase with the removal of the PEI layer by calcination because the PEI layer prevents the hot carrier transfer from AuNPs to TiO2. The multi-layered assembly varies the structural and optical properties of the Au nanostructures that are associated with the generation and injection of hot carriers. The unique 3D porous networks of the multi-layered Au nanostructures are retained during thermal annealing with a silica layer that is removed by etching, which absorb a broadband solar spectrum from visible to NIR region. Furthermore, IrO2 hydrosols are assembled on the multi-layered Au nanostructures as water oxidation catalysts. Plasmonic photocurrents significantly increase with the increased number of AuNP layers under visible light illumination. The results indicate that the hot carrier generation in the Au due to the SPR increases with the enhanced light absorption. This study suggests that the optimized plasmonic nanostructures based on AuNPs will provide a promising light-harvesting platform for sustainable and efficient solar water splitting in the visible region.
             In the second part of this talk, I discuss the applications of plasmonic nanostructures to the photocatalytic CO2 conversion for the production of an alternative clean fuel in a sustainable manner using solar energy. I introduce a self-assembled, monopipyridine ruthenium complex with an aromatic ring and two cis-oriented chloride ions, on a plasmonic half-dome-shaped Au/TiO2 heterostructure for the visible light-driven CO2 conversion to formic acid. The plasmonic Au/TiO2 platform allows the ruthenium complex to run the catalytic cycle effectively in aqueous solutions under room temperature and ambient pressure. The plasmonic gold nanostructure generate hot electrons by surface plasmon resonance, and induce photothermal effect efficiently when visible light is employed. Moreover, the acidic electrolyte condition reduce energy demand of the most high energy requiring reaction step. As a result, the photocatalyst exhibits 12 times higher turnover frequency (1,200 h-1 at 450 W) and over 10-fold greater turnover number (~11,700) than the state-of-the-art ruthenium-based photocatalyst in similar environment. This ruthenium complex also exhibited a superior selectivity towards formic acid (>94%), the highest to date. The fast reaction rate was determined to be obtained by proton-mediated low energy intermediate instead of molecular hydrogen insertion. Besides, the uninterchangeable cis-configuration of the ruthenium complex resulted in remarkable selectivity. The improved catalytic activities were studied by experimental and computational approaches, and this work demonstrates that the plasmon-enhanced photocatalytic CO2 conversion through the self-assembly of molecular catalysts is a promising strategy to produce useful resources utilizing the solar energy in a sustainable way.
 
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