Professor, Chemistry and Chemical Engineering, Graduate School of Engineering, Hiroshima University
Use of undesirable fine particles as new materials
—Synthesis and measurement of nanoparticles—
Prof. Okuyama says, “Fine particles floating in a gas, that are called aerosol particles, are thought to be unclean, undesired substances because they could cause asthma and allergic reactions.” He had been conducting basic research on the behavior of aerosol particles. However, after he returned to Japan from the US in 1980, he decided to study fine particles as new materials. As materials become smaller, from the sub-micron scale to the nanometer scale, their electric and optical properties change extensively. He targeted his research on the development of new materials, making use of new functions, when fine particles are scaled down to the nanometer scale. Various types of nanoparticles developed by Prof. Okuyama are now used in a wide range of fields. A composite material made of nanoparticles of TiO2 and SiO2 is used in cosmetics because TiO2 and SiO2 nanoparticles are able to block ultraviolet light, while visible light can pass through the particles. For electronic materials, a composite of polymers and nanoparticles that have high heat-resistance property could be applicable for LSI packaging, photoluminescence materials for use in display materials, etc.
Generally, there are three methods for synthesizing such particles; a solid phase method, a vapor-phase method and a liquid-phase method. However, among these methods, the solid phase method has limits, in terms of producing nanometer scale particles. The liquid phase method and the vapor phase method are suitable methods for the synthesis of nanoparticles. In the conventional spray pyrolysis (CSP), a liquid-phase method, droplets are sprayed to a heated furnace and fine particles are produced through evaporation of the solvent and chemical reactions. They are finally sintered to make their shapes and structures almost uniform. However, it is difficult to produce uniform-sized nanometer scale particles using the CSP method. Therefore, he developed the salt-assisted spray pyrolysis (SASP) to overcome limitations associated with the CSP method. In the SASP method, salts such as NaNO3, KNO3, LiNO3 and KCl are added to the spray solution since salt promotes nucleation and prevents nanoparticles from agglomerating. As a result, uniform-sized nanoparticles are produced. The diameters of nanoparticles produced by the SASP method range from several nm to several ten nm. These sizes are about one-30th to one-80th those of fine particles produced by CSP. For using nanoparticles in commercial applications, it is indispensable to develop a new manufacturing method for rapidly and inexpensively producing highly functional nanoparticles. Besides the SASP method, Prof. Okuyama is also working on the development of a low-pressure spray pyrolysis since this would permit the production of nanoparticles 100 times faster than CSP as well as the SASP method.
Prof. Okuyama is also studying the preparation of porous structures using nanoparticles, and has devised a method for producing porous structured SiO2 fine particles and porous SiO2 films with ordered holes using polystyrene latex (PSL) particles as a template and silica nanoparticles. When a colloidal solution of SiO2 nanoparticles and PSL particles is sprayed and dried at low temperatures, silica particles precipitate around self-organized PSL particles and fill spaces between particles. When these particles are heated, the PSL particles burn and disappear and only SiO2 fine particles with ordered holes remain. These particles may be used in many applications, including carriers for the controlled releases of drugs. To prepare porous SiO2 films, a substrate is soaked in the colloidal solution and lifted at a constant speed. The films may be used to form photonic crystals, low dielectric constant films, catalyst films and others.
Prof. Okuyama also stresses the importance of measurements. He says, “In evaluating the manufacturing processes of nanoparticles, we do not know how or under what conditions nanoparticles nucleate and grow unless we are able to measure accurately the sizes of suspended nanoparticles being formed.” Prof. Okuyama has developed a differential mobility analyzer using electrostatic force to measure particle size distributions. The accurate sizes of nanoparticles and ions with a diameter of 1 nm to several hundred nm can be measured using his equipment. “Developing a method for measuring the sizes of nanoparticles is an important aspect of nanotechnology. It is very important to develop equipment yourself that will satisfy your own needs, which off-the-shelf machinery cannot meet.” This is his motto and that of the members of his research team as well.
Prof. Okuyama encourages young researchers to attend quality universities abroad as early as possible to conduct research jointly with young professors who are not much older than they are. He says that if the researchers conduct joint studies with young professors, they will be able to contact them for many years thereafter. Prof. Okuyama attended the California Institute of Technology as a visiting researcher every summer from 1985 to 1991 and is still in contact with Caltech professors John Seinfeld and Richard Flagan, who are well known for their work on atmospheric aerosols. In addition, students whom Prof. Okuyama instructed at Caltech are now professors at various universities. He also advises young researchers to send their papers to authoritative journals, even if those journals do not completely match their own fields. He says that if their papers appear in those journals, they can become very confident in their abilities as researchers and will be motivated through unexpected questions from reviewers of their papers. He believes researchers need to be active in order to receive such feedback.
