What sort of research are you engaged in?

When a chemical reaction is implemented under appropriate conditions, an interesting phenomenon described as “self-organization” occurs, wherein a regular structure is formed naturally. Fig. 1 is a photograph of a porous alumina film created when two aluminum sheets are placed in a sulfuric acid aqueous solution, and a 25 V voltage is applied to the aluminum sheets so that one side of the aluminum is electrochemically oxidized (this is referred to as “anodizing”).

Fig. 1: Alumina oxide film, with honeycomb-formation pores, created naturally through anode oxidation.

The image was observed using a scanning electron microscope, with a few nanometer (1 millionth of a millimeter) resolution. A regular honeycomb arrangement of pores, with a pore size of approximately 30 nanometers, has naturally been formed. We still do not fully understand how this regular structure is naturally formed, but materials that self-organize at this nanometer scale are now considered important in the growing field of nanotechnology, which is expected to provide a wide range of applications in future. We use the simple method of anodizing to create regularly structured, nanometer-sized materials, and are engaged in research to discover new properties and better processes.

 

How will these materials be useful in the future?

Fig. 2: Carbon nanofibers produced using the alumina oxide film in Fig. 1.

These alumina oxide film pores, for example, can be used in the formation of carbon nanofibers with diameters measured in nanometers (Fig. 2). Graphite is one example of a carbon material, which comprises the electrode material in the lithium ion rechargeable batteries used in cellular phones and notebook computers. Lithium ions pass in and out between the layers of graphite, causing the battery to charge and discharge. Replacing this graphite with carbon nanofibers is expected to facilitate swifter charge and discharge rates. Their use as the electrodes in the new and highly anticipated fuel cells may allow a reduction in the quantity of extremely expensive platinum required. Rechargeable batteries and fuel cells are required for next-generation automobiles (electric and fuel cell vehicles), but they will need to perform better and be safer than those currently available. We are engaged in research into the development of materials that will enhance battery performance.

 

Fig. 3: Micro-cone shaped oxide film created using anode oxidation Fig. 3: Micro-cone shaped oxide film created using anode oxidation

 

Furthermore, being able to skillfully create hierarchically rough surfaces in the nanometer and micrometer scales on the surface of metal allows us to control wettability for water and oil. Surfaces that conventionally repel water (hydrophobic surfaces) can have their water repellent property improved even further by the introduction of surface roughness, while surfaces that are typically easily wetted (hydrophilic surfaces) can be made even easier to wet down. If we can create a surface that completely repels water or oil, said surface would remain completely clean. There would be no need to wash it periodically with detergents, which would have a positive impact on the environment.

Fig. 3 shows a surface formed from micrometer (1/1000 of a millimeter)-sized cone shapes, with the cones consisting of oxide in the shape of nanometer-sized fibers. This surface has been formed by anodizing of a metal surface. This is also an oxide film, formed by anodizing, with naturally formed ordered morphology. The oxide is easily wet using water, and this surface is super-hydrophilic (water droplet contact angle is 0°). If the surface is covered with an organic monolayer film so that it repels water, as shown in Fig. 4, the water droplets become almost spherical, and simply fall off the surface. This is a state known as “super water-repellent”. It is very difficult to create a surface that repels oil (“super oil-repellent” surface), but a doctoral student in our laboratory has recently created a surface that repels both oil and water. If we can control the wettability of the surface of materials used in various settings, then not only will we be able to create surfaces that do not get dirty, we may also be able to create surfaces that collect water from the air in dry climates, such as deserts, allowing us to prevent metallic materials from rusting, for example.

 

What is your dream at the moment?

One of them is that the materials we create here go on to be widely used throughout the world. Anodizing research is not a new field, but recently new discoveries are being made relatively frequently and are gathering a lot of attention. I hope to develop this discipline even further in the future. Finally, we have a lot of students working on research alongside us, including some overseas students. I hope that they will all go on to play active roles in the world.