Yasuaki Einaga - Diamond Electrodes: Fundamentals and Applications

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The application development of utilizing diamond as a chemical electrode has made rapid progress in both fundamental physical properties and application development in the last 1020 years. In particular, this diamond electrode was mainly used for waste water treatment and purification in the early years for environmental improvement, but in these days, this is utilized for electrochemical analysis and sensors such as environmental monitoring and biomedical measurement. Then, it has been found that the application range is extremely diverse, including as an electrode for ozone generation, an electrode for synthesizing organic compounds that lead to drug discovery, and an electrode for synthesizing useful chemicals by CO2 reduction, etc. In that sense, it is becoming more attractive than before. On the other hand, the diamond electrode that contributes to such applications exhibits superior electrochemical properties that are unique compared to conventional electrode materials, so the fundamental studies on physical properties of the electrode interface and the correlation between physical properties and electrochemical properties are attracting increasing attention.

In particular, since 2011, national projects for Strategic Basic research Programs on diamond electrodes (CREST (20112015) and ACCEL (20142020)), whose director is Prof. Yasuaki Einaga, have been adopted by Japan Science and Technology Agency (JST). Then, we could accelerate the diamond electrodes research with relevant active researchers. In the meantime, although research activity on diamond electrodes has been increasing year by year worldwide, this book especially focuses on the attractive development of diamond electrodes on fundamentals and applications in Japan related to these national projects.

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Lastly, I would very much like to acknowledge Prof. Akira Fujishima (Distinguished Professor, Tokyo University of Science), who is my supervisor and led me to this research area of diamond electrochemistry. Furthermore, I would like to thank Prof. Kohei Tamao (Research supervisor of JST-CREST) and Prof. Yoichiro Matsumoto (Chair, R&D Management Committee of JST-ACCEL and President of Tokyo University of Science) for giving us a chance to accelerate the researches on diamond electrochemistry. Finally, I would like to thank all the authors for their efforts and great contributions.

A first example of electrochemical measurement using diamond as an electrode is a study by Iwaki et al. (RIKEN) in 1983. They are investigating the electrochemical properties as a semiconducting electrode. However, the conductivity was due to the damaged surface by ion implantation into insulating diamond []. Although this is the first example of using diamond as a working electrode, it is a little different from diamond electrode (basically consisting of sp3 carbon) in this book, which is doped with impurities to make it conductive. In fact, research into this direction has not been pursued since then.

On the other hand, diamond materials have been actively studied and developed as wide bandgap semiconductor. From the viewpoint, photoelectrochemical properties as a semiconducting diamond are discussed after 1987. At first, Pleskov et al. observed the photocurrent by photoexciting semiconducting diamond []. Most of the diamond used at the periods, which was called Diamond electrode, was mainly p-type semiconducting diamond with a low boron doping level.

Then, several companies in Europe have begun commercializing water treatment systems using diamond electrodes. In terms of industrialization of diamond electrodes, this application to wastewater treatment and water purification is the first one []. Around the periods, the number of reports for fundamental researches and application of electrochemical analysis has begun to increase along with application to the waste water treatment.

Since then, also, the number of research papers has continued to increase since 2000, and most of them are related to electrochemical analysis such as sensor applications []. Especially around this time, Fujishima and Einaga have mainly focused on highly boron-doped diamond, which exhibits metal-like conductivity, and has been developing them for electrochemical applications. Especially in these days, for many applications, the highly boron-doped diamond with metal-like conductivity can be developed as boron-doped diamond (BDD) electrodes.