Metal complexes and semiconductors are well-known photocatalysts for CO2 reduction under normal temperature and pressure conditions. However, these catalysts have several problems, including low selectivity for CO2 reduction and insensitivity to visible light. To overcome these issues, we have successfully developed a new photocatalytic system based on a p-type semiconductor linked with a metal complex electrocatalyst to carry out highly selective CO2 reduction using visible light (Fig. 2). We have named this system a "semiconductor/metal-complex hybrid catalyst." Due to the rapid electron transfer that takes place from the photo-excited semiconductor to the metal complex catalyst, the hybrid catalyst enables selective CO2 reduction with visible light. Such hybrid catalysts can be constructed from a wide variety of semiconductors and metal complexes.
MORIKAWA Senior Fellow Laboratory
Artificial photosynthesis is the process of capturing solar energy and storing it in the bonds of hydrocarbon molecules at ordinary temperatures and pressures, and is expected to be a next-generation technology for high energy density energy storage. In the first step of artificial photosynthesis, water is oxidized by sunlight, generating oxygen (O2), protons, and electrons. Then, carbon dioxide (CO2) is reduced by the generated electrons, and the reduction products react with the generated protons to produce hydrocarbon molecules. We proposed and developed a novel semiconductor/metal-complex hybrid photocatalyst that utilizes visible light energy for the conversion of CO2 into organic compounds in aqueous media.
We have recently developed a simplified artificial photosynthetic system that can generate the organic molecule formate, utilizing only CO2, water, and solar energy. The reaction is carried out by immersing a monolithic artificial photosynthetic device, which we have termed an “Artificial Leaf,” in a single-compartment reactor filled with an electrolyte and saturated with gaseous CO2, and irradiating the device with sunlight. A solar-to-chemical energy conversion efficiency of up to 4.6% has been achieved for the Artificial Leaf, which is comparable to the photosynthetic conversion efficiency of plants. Our next target is not only to improve the efficiency of the device, but also to develop technology for the synthesis of more practical and valuable organic species.
Fig.1 "Artificial Leaf" : Monolithic device for artificial photosynthesis.
- Sato, S., Arai, T., Morikawa, T., Uemura, K., Suzuki, T.M., Tanaka, H. and Kajino, T., Journal of the American Chemical Society, Vol. 133, No. 39 (2011), pp. 15240-15243. http://dx.doi.org/10.1021/ja204881d
- Arai, T., Sato, S., Kajino, T. and Morikawa, T., Energy & Environmental Science, Vol. 6, No. 4 (2013), pp. 1274-1282. http://dx.doi.org/10.1039/c3ee24317f
- Arai, T., Sato, S. and Morikawa, T., Energy & Environmental Science, Vol. 8, No. 7 (2015), pp. 1998-2002. http://dx.doi.org/10.1039/c5ee01314c
- 1. A new photocatalytic system for CO2 reduction utilizing a semiconductor/metal-complex hybrid photocatalyst
Fig. 2 Reaction mechanism for artificial photosynthesis utilizing a semiconductor/metal-complex hybrid photocatalyst.
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- Arai, T., Sato, S., Uemura, K., Morikawa, T., Kajino, T. and Motohiro, T., Chemical Communications, Vol. 46, No. 37 (2010), pp. 6944-6946. http://dx.doi.org/10.1039/c0cc02061c
- Suzuki, T. M., Tanaka, H., Morikawa, T., Iwaki, M., Sato, S., Saeki, S., Inoue, M., Kajino, T. and Motohiro, T., Chemical Communications, Vol. 47, No. 30 (2010), pp. 6944-6946. http://dx.doi.org/10.1039/c0cc02061c
- Sato, S., Morikawa, T., Kajino, T. and Ishitani, O., Angewandte Chemie-International Edition, Vol. 52, No. 3 (2011), pp. 8673-8675. http://dx.doi.org/10.1039/c1cc12491a
- 2. Nanostructured photoelectrode
To improve the efficiency of artificial photosynthetic systems, nanostructuring of the photoelectrode is one of the most important issues. We have developed high-surface-area oxide nanotube arrays that are visible light sensitive by co-doping with two different elements to control the band-gap energy. In particular, titanium dioxide nanotube arrays (TNT) co-doped with N and Fe generated the highest visible-light-induced photoelectrochemical water oxidation yield, and we also confirmed the evolution of oxygen from water splitting over co-doped TNT with a cobalt borate co-catalyst.
Fig. 3 Nitrogen and iron co-doped TNT arrays for visible-light-sensitive photoelectrochemical water oxidation.
- Suzuki, T. M., Kitahara, G., Arai, T., Matsuoka, Y. and Morikawa, T., Chemical Communications, Vol. 50, No. 57 (2014), pp. 7614-7616. http://dx.doi.org/10.1039/c4cc02571g
- 3. Development of powdered Z-scheme photocatalytic system
One of our aims is to achieve CO2 reduction in water under solar irradiation through the use of a powdered photocatalytic system. To this end, we constructed a Z-scheme photocatalyst consisting of a combination of a metal-complex catalyst, two kinds of visible-light-driven semiconductor photocatalysts, and reduced graphene oxide (RGO, solid electron mediator between both semiconductors). Although we have not yet achieved CO2 photoreduction in water, we have successfully demonstrated a powdered Z-scheme system for the overall photocatalytic water splitting under visible light irradiation. (collaboration with Professor Kudo)
Fig. 4 Artificial photosynthesis over powdered metal-complex/semiconductor hybrid photocatalysts mediated by reduced graphene oxide.
- Suzuki, T. M., Iwase, A., Tanaka, H., Sato, S., Kudo, A. and Morikawa, T., Journal of Materials Chemistry A, Vol. 3, No. 25 (2015), pp. 13283-13290. http://dx.doi.org/10.1039/c5ta02045j
- 4. Application of p-type iron-based semiconductors for artificial photosynthesis
Expanding the practical applications of artificial photosynthetic systems requires that their operation be based on low-cost semiconductors fabricated from earth-abundant elements. Iron-based semiconductor materials such as hematite (α-Fe2O3) and CaFe2O4 are promising candidates because they are abundant and can absorb a substantial amount of solar light. However, solar-energy conversion has not yet been accomplished using iron-based semiconductors because their solar-to-electron conversion efficiency is very low. One of our aims is therefore to enhance the solar conversion efficiency of these materials through techniques such as doping and constructing heterojunctions so that they may be used for artificial photosynthesis.
Fig. 5 Iron-based doped semiconductor photoelectrode with multiple heterojunction.
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- Morikawa, T., Arai, T. and Motohiro, T., Applied Physics Express, Vol. 6, No. 4 (2013). http://dx.doi.org/10.7567/APEX.6.041201
- Sekizawa, K., Nonaka, T., Arai, T. and Morikawa, T., ACS Applied Materials & Interfaces, Vol. 6, No. 14 (2014), pp. 10969-10973. http://dx.doi.org/10.1021/am502500y
- 5. Analyses of electronic structures and electron transfer mechanism for semiconductor/metal-complex hybrid catalyst
The reaction mechanisms for the new photocatalytic system consisting of semiconductor photosensitizers and metal-complex catalysts are still not well understood. By making full use of techniques such as time-resolved transient spectroscopy, quantum chemical calculations, and synchrotron-radiation-based electronic state measurements, we have been conducting analyses of the factors that govern the electronic states and electron transfer for the hybrid photocatalytic system.
Fig. 6 Electronic structure of semiconductor/metal-complex hybrid catalyst.
- Yamanaka, K., Sato, S., Iwaki, M., Kajino, T. and Morikawa, T., Journal of Physical Chemistry C, Vol. 115, No. 37 (2011), pp. 18348-18353. http://dx.doi.org/10.1021/jp205223k
- Akimov, A. V., Jinnouchi, R., Shirai, S., Asahi, R. and Prezhdo O. V., Journal of Physical Chemistry B, Vol. 119, No. 24(2014), pp.7186-7197. http://dx.doi.org/10.1021/jp5080658
- Akimov, A. V., Asahi, R., Jinnouchi, R. and Prezhdo, O. V., Journal of the American Chemical Society, Vol. 137, No. 35(2015), pp. 11517-11525. http://dx.doi.org/10.1021/jacs.5b07454