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Creating new functional materials by controlling structures of nanomaterials

Yoshiyuki Kuroda
Assistant Professor, Waseda Institute for Advanced Study

Creating materials that no one has seen before

I specialize in the field of inorganic material chemistry, and my research focuses on the chemical synthesis of ceramic and metallic materials. The development of nanotechnology has made it possible to create and process new materials by using chemical reactions to manipulate nano-level molecular structures, which are smaller than a micron and cannot be handled by machine processing. My goal is to create matter that no one has ever seen before. I would like to create materials that have fine and beautiful structures. From beautiful structures, we can draw out rules that are easy to understand and apply.

My research currently rests on two main pillars: 1) the synthesis of nanostructured materials using colloidal crystals, and 2) the synthesis of clay mineral materials that exhibit new utility in different fields such as the environment. In fact, I first did research in these two areas in the laboratory of Dr. Kazuyuki Kuroda, who is the leader of the laboratory I currently belong to and who was also my professor during my undergraduate years. At that time, my research theme was the synthesis of clay minerals that have an ordered structure. Although I completed this research in about a year, I think all the research I have engaged in since shares a common foundation with that study, even though the materials I researched differed. In retrospect, this is all thanks to Professor Kuroda's selection of an apt research theme. Afterwards, I proceeded to earn my master's and doctoral degrees, and I gained understanding of material synthesis from different aspects. I now engage in a variety of research from my distinct point of view. My first research theme is synthesis methods for nanospace materials. Synthesis of porous materials that have hierarchically integrated, regular-shaped hollow spaces increases the application potency of catalyst materials that absorb or separate nano-level materials (Figure 1). A method called template synthesis, in which a mold is formed and metal is poured into it, is used for nano-level materials as well. For example, a template of colloidal crystal, in which silica particles are regularly arranged, is formed, and then a precursor made of ionized gold is poured into this template. By reducing the gold ions to metal and removing the template by dissolving it, a gold porous material having fine holes with a regular pattern can be obtained (Figure 2). By controlling the design, such as by combining particles of different sizes, I can make various functional materials.

In this research, I recently succeeded in synthesizing new materials. I discovered that a thin, plate-like material can be generated through a reaction that breaks down the template of a nanoparticle assembly. To tell the truth, when this phenomenon first occurred, I thought the experiment had merely failed and had no interest in it for some time. However, I later started to wonder why such material was created at that time. I then repeated the experiment to reproduce it. As a result, I discovered that when the deposition speed of gold is slowed down, it expands as the reactions are concentrated in fewer locations, which in turn causes the template to split open in a certain direction. The gold flows into the crack and forms a plate-like material (Figures 3 and 4). By contrast, with a high deposition speed, reactions occur in many locations simultaneously, and the gold is dispersed throughout the template, so this phenomenon does not occur.

Figure 1. Nano-level porous materials

Figure 2. Synthesis of three-dimensional porous material from silica particle template

New synthesis method that "breaks down" the template's nanostructure

In this research, I recently succeeded in synthesizing new materials. I discovered that a thin, plate-like material can be generated through a reaction that breaks down the template of a nanoparticle assembly. To tell the truth, when this phenomenon first occurred, I thought the experiment had merely failed and had no interest in it for some time. However, I later started to wonder why such material was created at that time. I then repeated the experiment to reproduce it. As a result, I discovered that when the deposition speed of gold is slowed down, it expands as the reactions are concentrated in fewer locations, which in turn causes the template to split open in a certain direction. The gold flows into the crack and forms a plate-like material (Figures 3 and 4). By contrast, with a high deposition speed, reactions occur in many locations simultaneously, and the gold is dispersed throughout the template, so this phenomenon does not occur.

 

Figure 3. Deposition at a slow rate forms a plate-like material.

Figure 4. Gold is concentrated at the reaction locations, and a cleavage plane is formed on the template.

I also discovered that materials with different patterns can be created based on the template structure (Figure 5). In addition, I geometrically analyzed what kind of array structure makes it easier for a template to break, and I discovered that templates are fragile and easy to break where the number of bonds between particles is the fewest—for example, where they are arranged in tetragonal or hexagonal patterns (Figure 6). At present, this has been confirmed to be a special phenomenon that can only occur on a nano-scale, and it only occurs with gold, which has a fast crystal growth rate. This material synthesis is based on a concept of changing the template structure, which differs from the basic approach taken by conventional template synthesis methods. Further, it has been highly regarded internationally as an invention that opens up new possibilities for nano-level synthesis methods. It also received the 2015 Waseda Research Award (High-Impact Publication).

Figure 5. Regularity of space varies depending on the template structure.

Figure 6. Geometrical analysis of easily breakable structures

     
Increasing negative ion absorption capacity of clay minerals

In my research on clay minerals, my other area of expertise, I am working to synthesize materials with high functionality, such as selectively removing toxic substances. Clay has high water retention and absorption properties, and its particles have a flat, thin, wafer-like structure. Clay is an environmentally friendly material used in ways such as absorbing toxic ions between its particles for disposal. Although most clays absorb positive ions, I am working on the synthesis of clay minerals that absorb negative ions, which would be useful in the treatment of toxic substances such as hexavalent chromium, arsenic, and selenium.

However, in the case of negative ions, there is a problem where carbonate ions, CO2 dissolved in water, are absorbed before the target ions. As a solution, Professor Atsushi Yamazaki’s group (Department of Resources and Environmental Engineering, Waseda University) discovered that the effects of CO2 can be minimized by using microcrystals of layered double hydroxide (LDH) (Figures 7 and 8).

Figure 7. The negative ion exchange capacity is reduced by the effects of CO2.

Figure 8. Reduction of particle size improves negative ion exchange.

From the perspective of synthetic chemistry, I am working on a method for synthesizing even finer particles. So far, I have discovered that LDH with a size of 10 nm can be synthesized simply by mixing magnesium chloride, aluminum chloride, and tripodal ligands. The mixture is then heated to 80°C, so that the size of LDH crystals is uniform. The size of these crystals can be controlled by changing the concentration. This method makes it possible to create materials that improve the target material's absorption capacity without almost any CO2 effects. My current research focuses on this new synthesis method (Figure 9).

Figure 9. Highly functional LDH was successfully synthesized.

Sticking to a basic synthetic process

Since I was a child, I have always loved crafts. I was very excited when I mixed things and saw their colors change. In the chemistry club in high school, my club instructor’s motto was, think for yourself what you want to do and how to do it. In my second year of high school, I became discouraged because there were fewer club members than before, and I asked my instructor if there were any fun experiments that we could do. However, the instructor said, "It would be meaningless if I just told you what to do." This was a terribly embarrassing moment when realized that research was meaningless unless I thought for myself. This marked my first discovery as a researcher.

Pursuing applied chemistry and material chemistry was a very natural course for me. I have the confidence that I would enjoy doing anything in these fields. I would like to continue creating new materials that nobody has seen before while remembering my initial intentions. Rather than taking advantage of cutting-edge, complex synthesis methods, I would like to pursue research of basic synthesis methods that can be used by anyone with little knowledge in the field, from high school students to developing countries where there are no advanced facilities.

Yoshiyuki Kuroda
Assistant Professor, Waseda Institute for Advanced Study

Yoshiyuki Kuroda graduated from Waseda University's Faculty of Science and Engineering, Department of Applied Chemistry, in 2006. He completed his master' doctoral degree in Engineering at the Graduate School of Advanced Science and Engineering, Department of Applied Science. He served as a JSPS (Japan Society for the Promotion of Science) Research Fellow (DC1) at Waseda University from 2008 to 2011, and as a Postdoctoral Fellow at the School of Engineering, The University of Tokyo, from 2011 to 2014, when he assumed his current position.
Selected Honors: 2015 Waseda Research Award (High-Impact Publication), 69th CerSJ Award for advancements in ceramic science and technology from the Ceramic Society of Japan, 2015 Outstanding Performance Award of JX Nippon Oil & Energy Encouragement Research for Young Scientists, 2010 Oral Presentation Award of NRF A3 Foresight Program Seminar, and 2007 Best Paper Award of Journal of the Ceramic Society of Japan.