Dr. Hiroaki Misawa pioneered the development of photochemical reaction field in which light and molecular systems interact with each other in a highly efficient way by using metal nanoparticles. This inventive new area in photochemistry accelerates not only excitation process of molecules but also chemical processes such as electron transfer reaction. His major achievements in this field are summarized below.
1. Pioneering the development of photochemical reaction on metal nanostructures and a method for direct observation of localized surface plasmon by a time-resolved photoemission electron microscope
Dr. Misawa pioneered the development of plasmon-enhanced photochemical reaction field which enables strong interactions between light and molecular systems by "near field enhancement". The origin of such field-enhancement is "localized surface plasmon (LSP)" generated by the interaction between electromagnetic field of incident light and free electrons on the surface of metal nanostructures. As enhancement of near field greatly depends on the size, shape and arrangement of metal nanoparticles, he developed a methodology for creating fine metal nanostructures at a high spatial resolution of approximately 2 nm by applying semiconductor nanofabrication technology, and realized a photochemical reaction field that enables stable enhancement of near field. Besides, he is the first to discover that, if gaps between gold nanostructures are reduced to several nanometers by precisely controlling their size, shape and arrangement, near field enhancement owing to LSP occurs significantly in the neighborhood of gaps, resulting in a significant increase in the two-photon induced emission intensity of gold.
Furthermore, Dr. Misawa developed a method for visualizing LSP and measuring its dephasing time, which are fundamental for evaluating the properties of the photochemical reaction field. He succeeded in exciting LSP on gold nanostructures by combining femtosecond laser and a photoemission electron microscope, and directly imaged LSP at the spatial resolution of 10 nm or less from spatial distribution of photoemission electrons. In the above research area, he has also developed an original system for observing the interference of LSP excited on gold nanostructures, which was by injecting phase-controlled pump-probe having 7 fs pulse width into a photoemission electron microscope and controlling temporal delay between the pump and probe pulses. This system enabled him to measure extremely short dephasing time of 10 fs or less, which is fundamentally important for photochemical studies using LSP. While weak interaction between light and molecules has long been a barrier in accomplishing efficient photochemical reactions, Dr. Misawa's realization of a photochemical reaction field that excites molecules highly efficiently not only changes the situation but also becomes the foundation of a new area in chemistry, namely plasmon-induced photochemistry.
2. Discovery of two-photon induced reaction by incoherent light using the near field enhancement effect
Dr. Misawa developed a photochemical reaction field in which nanogaps manifesting near field enhancement are precisely arranged in gold nanostructures. These nanostructures effectively enhance the near field, enabling two-photon induced reaction by faint incoherent light. He was the first in the world to find that two-photon polymerization reaction can be induced by filling this photochemical reaction field with a resist material, which polymerizes only by ultraviolet irradiation, and irradiating it with faint incoherent visible and near-infrared light from a halogen lamp. Furthermore, he revealed that two-photon induced reaction similar to solid state photochemical reaction can be induced using photochromic molecules in the solution phase. These findings show for the first time that two-photon induced reaction, which has so far been achieved only by using high-intensity light sources such as lasers, can be induced by a faint incoherent light source by taking advantage of near field enhancement.
3. Acceleration of chemical reaction processes by near field enhancement and its application to light energy conversion systems
Focusing on electron transfer reaction on metal/semiconductor electrode interface and oxidation-reduction reaction based on it, Dr. Misawa revealed for the first time that near infrared light can be stably converted into electrical energy by using plasmonic photoelectrode created by precisely tethering gold nanostructures on a n-type titanium dioxide semiconductor single-crystal substrate. Furthermore, by tracing photochemical reactions in the electrodes, he revealed the oxidative decomposition of water and the generation of oxygen and hydrogen peroxide. This was a pioneering achievement by Dr. Misawa, which clearly demonstrates that LSP in gold nanostructure accelerates chemical reactions.
Dr. Misawa furthered his studies on artificial photosynthesis that generates hydrogen and oxygen by the decomposition of water by the said plasmonic photoelectrode. He developed an artificial photosynthesis system by using n-type strontium titanate as a semiconductor substrate, placing gold nanoparticles on one side for creating an oxidation site, and affixing a thin platinum plate on the other side via ohmic joint for creating a reduction site. The fact that water can be decomposed into hydrogen and oxygen stoichiometrically at different spatially separated sites and that light of any wavelength in the visible range can be used are highly valuable for not only studies in the photochemical reaction fields but also artificial photosynthesis.
Dr. Misawa also revealed that by using ruthenium microparticles as reduction cocatalyst and light of any wavelength in the visible range as the energy source, atmospheric nitrogen can be converted into ammonia. In addition, he revealed that ammonia can be obtained with ~100% selectivity by using zirconium/zirconium oxide cocatalyst. These findings blaze new methods for energy-efficient photocatalytic production of ammonia using solar light, water, and nitrogen gas, which are entirely different from conventional methods of ammonia synthesis. These achievements by Dr. Misawa extend the application of LSP photochemical reaction field for the activation of molecular systems to the "reaction field" that accelerates chemical reactions, thus opening a new door to photochemical studies using LSP.
As described above, Dr. Hiroaki Misawa has made significant contributions to fundamental chemistry by independently creating plasmon-enhanced photochemical reaction field on metal nanostructure and accomplishing highly energy-efficient reactions such as artificial photosynthesis. Furthermore, he has developed a measurement method for evaluating properties of plasmon-enhanced photochemical reaction field. The leading role he played in pioneering new areas in chemistry is outstanding and highly evaluated internationally. His accomplishments momentously impact on a wide range of research areas including photochemistry, physical chemistry, catalysis, and energy science. Therefore, his achievement is acknowledged to deserve the CSJ Award.