Development of artificial photosynthesis is prospected on the basis of its history, the three milestones in late 20th century, and recent advances in biological approach, molecular catalysts, and semiconductors chemistry. Photon-flux-density problem to be resolved in getting through one of the bottleneck issues is discussed as well as renewable energy factor (REF) as one of the most crucial points to be considered even in the early stage of fundamental research.
Artificial photosynthesis is expected to solve energy, environment, and resources issues. Water splitting and CO2 reduction of artificial photosynthesis have extensively been studied using photocatalyst materials. Various metal oxide and sulfide photocatalysts developed by the authorʼs original strategies are introduced. These heterogeneous photocatalyst materials have been applied to a single particle system, Z-schematic system, and photoelectrochemical systems for water splitting and CO2 reduction using water as an electron donor.
Photocatalytic water splitting using semiconductor materials has attracted considerable interest due to its potential for clean production of H2 from water by utilizing abundant solar light. We have developed a new type of photocatalysis system that can split water into H2 and O2 under visible light irradiation, which was inspired by the two-step photoexcitation (Z-scheme) mechanism of natural photosynthesis in green plants. The introduction of a Z-scheme mechanism with appropriate redox couples reduces the energy required to drive each photocatalysis process, extending the usable wavelengths significantly from that in conventional water splitting systems. The key in constructing such Z-scheme systems is to clarify and utilize the reactivities of redox couples based on their adsorption onto the photocatalyst surfaces, on which photogenerated carriers (electrons and holes) react with the redox couples.
Photocatalytic conversion of CO2 to hydrocarbon fuel is of great significance in solving both energy and environmental issues. However, the reaction remains very inefficient due to the kinetic limitations of multiple e−/H+ transfer processes and the limited abilities of traditional semiconductors to activate thermodynamically stable CO2 molecules. A more flexible utilization strategy of solar energy beyond the conventional framework of photocatalysis is needed for realizing a highly efficient CO2 conversion. In this article, we introduce our recent works on surface-plasmon-enhanced photodriven CO2 reduction, and discuss how to take the advantages of the unique functions of nanometals in different types of catalytic processes to improve the efficiency of solar-energy utilization for more practical artificial photosynthesis.
Reduction of CO2 producing high-energy compounds using water as an electron donor and sun light as an energy source has been investigating as useful technology for solving both depletion of the fossil resources and the global warming problem. Our group has successfully developed several types of hybrid photocatalysts consisting of semiconductors and metal complexes, which have both efficient CO2 reduction ability supplied by the metal-complex unit and strong oxidation power of semiconductors. In this paper, our recent progresses of the visible-light-driven hybrid photocatalytic and photoelectrochemical systems for CO2 reduction are introduced: (1) hybrids consisting C3N4 and Ru(II) mononuclear complexes, (2) semiconductors, e.g., TaON, and a Ru(II)-Ru’(II) binuclear complex, (3) a photoelectrochemical cell comprising a photocathode of the Ru(II)-Re(I) binuclear complex immobilized on p-type semiconductor NiO and a CoOx/TaON photoanode. First two systems can photocatalyze CO2 reduction using methanol as an electron donor, and the third photoelectrochemical system can reduce CO2 using water as a reductant.
In recent years, solar fuel or chemical production based on the photoreduction or fixation of CO2, the so-called “artificial photosynthesis” has received considerable attention. Thus, the development of an effective catalyst for the conversion of CO2 to useful organic molecules is desirable. Biocatalysts for CO2 reduction and conversion are useful catalyst for the artificial photosynthesis system. In this review, two types of artificial photosynthesis systems for CO2 reduction and conversion consisting of the visible-light sensitizer and biocatalyst are introduced. One is the artificial photosynthesis with visible-light sensitizer and biocatalyst for CO2 photoreduction to formic acid or methanol. The other one is the artificial photosynthesis with visible-light sensitizer, and novel electron carrier molecule and biocatalyst for the carbon-carbon bond formation from CO2 as a feedstock.
A technology for SnO2 coating, which keep surface hardness and does not peel with repeated alkaline washing was developed to provide lightweight returnable glass bottles. This advanced coating method enables us to produce lightweight returnable bottles with about 20% weight reduction, which have nearly the same strength as conventional ones. It was realized by controlling the surface temperature of the bottle immediately after forming and using a chamber devised to get a uniform film thickness. The coating is formed by hydrolysis reaction of SnCl4. Coating conditions with high durability are as follows. (1) The temperature on an outer surface of the bottle ranges from 500 to 700oC. (2) The coating thickness of SnO2 films is in the range of 40 to 100 nm.