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In situ Nanoscale Structural Determination and Construction of Functional Phases at Solid/Liquid Interfaces
Prof. Kohei Uosaki has contributed much to the understanding of a wide variety of phenomena at solid/liquid interfaces, which are not only important in fundamental science but also play key roles in many practical processes such as fuel cell, battery, electrodeposition, corrosion and many biological processes. He played a leading role in establishing the field of "Electrochemical Surface Science" by carrying out fundamental investigation on in situ real time nanoscale determination of geometric, electronic and molecular structures and construction of functional phases at electrode/electrolyte interfaces
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Prof. Kohei Uosaki
Hokkaido University
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in atomic/molecular resolution, utilizing single crystalline substrates and novel techniques such as STM, AFM, second harmonic generation (SHG), sum frequency generation (SFG), electro-and photo-luminescence, surface x-ray scattering (SXS), and x-ray absorption fine structure (XAFS).
He has published more than 300 scientific papers (total citation exceeds 6000), more than 50 review articles, and more than 40 books and book chapters on various aspects of electrochemistry, and filed 20 patents.
His contributions are briefly summarized as follows.
1 .Formation, Structure and Functionalities of Organic Monolayer on Metal and Semiconductor Electrodes
He has been working on surface modification of electrode by organic molecules since 1978 when he worked with Prof. Hill in Oxford on electrochemistry of cytchrome-c but his major contributions are on self-assembled monolayers (SAMs) of thiol molecules on gold and organic mono- and multi-layers on hydrogen terminated Si(111) with various functions.
He showed that thiol SAM formation on Au(111) with and without potential control is not a simple adsorption but more dynamic process accompanied with surface reconstruction lifting and diffusion of gold atoms by QCM, in situ STM as well as surface x-ray diffraction.
He synthesized a number of novel thiol molecules and formed SAMs with various functions such as redox, proton-coupled redox, electrochemically generated luminescence, second harmonic generation (SHG), and photoinduced electron transfer on gold. For example, he demonstrated more than 10% quantum efficiency for photoinduced up-hill electron transfer at a gold electrode modified with ferrocene-porphyrin-thiol SAM. It was the highest efficiency at the SAM modified metal electrode at that time. This work has been cited 200 times by now.
He also constructed organic multi-layer of viologen with dispersed Pt nanoclusters, which shows high photoelectrochemical hydrogen evolution efficiency with high stability.
2. Fundamental Understanding of Interfacial Processes at Single Crystalline Metal and Semiconductor Surfaces
He is one of the pioneers who started to apply scanning probe microscopy, STM and AFM, to electrochemistry. While most of the SPM studies using STM were limited to underpotential deposition (UPD) on single crystalline metal surface, i.e., static structure, he investigated more dynamic and interesting systems such as metal deposition and dissolution. Furthermore, knowing the limitation of SPM, he used other complimentary in situ techniques such as SXS and XAFS. For example, he has shown that Pd deposition on Au(111) and Au(100) proceeded layer-by-layer fashion with adsorbed Pd complex on the surface, giving pseudomorphic structure with unique electrocatalytic activity. He also studied dissolution of Au(111) electrode in solutions containing chloride ion.
To understand interfacial processes, not only geometric structure but also electronic and molecular structures of the interface to be determined. In this respect, he employed SHG and SFG spectroscopy. Since they are based on second order non-linear optical processes, they are inhibited in non-centrosymmetric media and provide interface specific information. He used SHG to interfacial electronic structure of Au electrode drastically changed by submonolayer deposition of Pd. He also followed the electrochemical Te deposition on Au and probed CO on Pt surface in situ real time during anodic oxidation of HCHO and demonstrated that surface chirality can be evaluated by SHG.
He used SFG to determine the structure of water at electrode surface, one of the most important issues in electrochemistry, and showed that interfacial water structure strongly depends on the nature of the electrode and the surface charge.
He investigated not only processes at metal electrodes but also at semiconductor electrodes. He studied photoelectrochemical hydrogen evolution at p-type single crystalline semiconductors and tried to correlate photoelectrochemical characteristics with various physical properties of the semiconductors. He suggested the importance of the potential drop in the double layer and charge transfer step at the interface, which was neglected initially but accepted later.
He then investigated the effects of surface modification of semiconductor electrode by metal and metal ions and showed they may act as surface recombination center and/or catalyst.
Other important result he obtained is Cu deposition on GaAs. Using AFM, XAFS and light scattering, he clarified the deposition mechanism from very initial stage to multilayer deposit.
In his study, he always tries to probe the structure of the electrode surface in situ using as many techniques as possible such as luminescence, impedance, STM, AFM, and XAFS.
In addition to the fundamental study on semiconductor electrochemistry, he also studied electrochemical deposition of CdTe and CdSexTe1-x and recently achieved epitaxial growth of CdTe on Si(111) by pulsed illumination.
3. Construction of Multilayers of Metal and Semiconductor Nanoclusters
Although surface modification is very useful for controlling the properties of electrode, often electrocatalysis requires higher number of active atoms/molecules and photoelectrochemical application needs more molecules to absorb photons. Thus, three dimensional assemblies are very important.
He constructed multilayers of gold nanoclusters modified with anionic SAM using cationic polymer as binder and showed they act as three-dimensional electrode up to 20 layers. Furthermore, he developed a way to modify the Au nanocluster surface within the multilayer by a foreign metal so that catalytic activity can be tuned.
He also constructed multilayers of CdS nanoclusters by various techniques and showed that photoelectrochemical behaviors of the multi-layers reflect the quantum size effect of the CdS nanoclusters. He measured carrier and charge transfer dynamics of CdS dispersion and CdS multilayers and proposed a model for photoelectrochemical processes at CdS/electrolyte interfaces.
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