Exploring the World of Surfaces

Hiroshi Kondoh (Professor, Department of Chemistry)

Everything has a surface. When we look at an object, we often judge what it is by its surface rather than by looking directly at its contents. In a sense, we unconsciously recognize objects through their "surfaces." However, if we consciously observe the surface of an object, we may discover an unexpected world unfolding. For example, when a drop of water falls on a lotus leaf, we can see it roll off as a bead of water. Observing the surface of the lotus leaf under a microscope reveals that it is covered with small bumps spaced 20 to 30 microns apart. These bumps play a crucial role in making the water droplets roll off.

Furthermore, examining surfaces with a microscopic eye capable of seeing atoms and molecules reveals the fascinating world of the nanoscale. In addition to microscopes, we are engaged in research to investigate the behavior of molecules on surfaces using powerful X-rays called synchrotron radiation X-rays (Figure 1).

Figure 2 shows a microscope image observed after exposing a gold surface to the vapor of a certain organic molecule. Although one might expect the molecules to adhere randomly, as they collide with the surface randomly, they actually self-assemble into a highly ordered arrangement. When we examine the molecular arrangement with synchrotron radiation X-rays, we find that, as schematically shown in the figure, the molecules lie flat on the surface, forming "molecular nanowires" where the sulfur atoms, indicated by black circles, are aligned in two rows.

You are probably aware that "catalysts" often play a very important role when chemical reactions occur. Many catalytic reactions are reactions on the surface of the catalyst. On a catalyst surface, chemical reactions that would never occur by simply mixing the reactants can happen easily. We are also conducting research to investigate the mechanisms of chemical reactions on catalyst surfaces using synchrotron radiation X-rays.

Figure 3 shows a simulation of the progress of the detoxification (oxidation) reaction of carbon monoxide on the surface of a platinum catalyst, which is used to detoxify automobile exhaust gas. When there is too much carbon monoxide, a phenomenon called "poisoning" occurs, which degrades the catalyst's performance. The simulation shows how this happens.

Figure 3: A reaction simulation of the oxidation of carbon monoxide (O + CO → CO2↑) on a platinum surface.

Chemical reactions involving surfaces are not limited to catalytic reactions. Chemical reactions on the surfaces of cometary ice and cosmic dust are thought to play an important role in the material evolution of the universe. Furthermore, chemical reactions on the surfaces of biological membranes and tissues are essential for life activities. Understanding the behavior of molecules on surfaces is a profound and rewarding theme, both as a fundamental science and in its connection to practical applications.