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Because of the worldwide energy and environmental issues, the research on the use of solar photocatalysis to decompose water to produce hydrogen and reduce carbon dioxide has attracted widespread attention in the international academic community. Photocatalytic decomposition of water is considered to be a “Holy Grail†problem in the field of chemical science. Once a breakthrough is achieved, it is expected to affect the world energy landscape.
The research team of the Department of Solar Energy of the National Laboratory for Clean Energy of the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, led by Li Can, Chinese Academy of Sciences, has long been engaged in the research of artificial light synthesis of solar fuels. In recent years, it has made series of research progress, and it has for the first time proposed an “abnormal knot†in the world. , “Separation of photogenerated charge between crystal planes†and other strategies to promote charge separation (Angew. Chem. Int. Ed., 2008; Angew. Chem. Int. Ed., 2012; Nature Commun., 2013; Energy Environ. Sci., 2014; Angew. Chem. Int. Ed., 2015), systematically proposed the concept of “bifunctional cocatalysts†for collaborative promotion (Acc. Chem. Res., 2013), and successfully assembled the first natural-artificial hybrid system to achieve light Catalytic decomposition of water (Nature Commun., 2014) and other original achievements have been widely recognized by the academic community at home and abroad.
In solar energy photocatalytic water splitting, the energy band structure of many semiconductor photocatalysts meets the requirement of decomposing hydrogen from the production of hydrogen by thermodynamics, but it cannot actually achieve water decomposition. In the end, what causes this problem is a long-term problem. The challenging problems in this area of ​​research have not been well resolved. Li Can's research team has conducted a series of researches on this issue. After several years of hard work, he has made new progress recently. The relevant results are published in the energy chemistry journal Energy & Environmental Science (Rengui Li, Yuxiang Weng and Can Li et al. , Energy Environ. Sci., 2015, 8, 2377-2382).
TiO2 is the most typical model semiconductor photocatalyst in photocatalytic research. It satisfies the requirements of water decomposition in thermodynamics completely. However, it is difficult to realize the decomposition of water-produced hydrogen by using TiO2 for the photocatalytic decomposition of water. The research team selected this model of photocatalyst as a research object and systematically investigated the photocatalytic water decomposition properties of different phases TiO2 (anatase, rutile, and brookite). It was found that only rutile TiO2 can completely decompose water under light excitation, H2 and O2 conform to the stoichiometric ratio, and neither anatase nor brookite TiO2 can completely decompose water, and only H2 can be generated during the reaction. Cannot generate O2. However, when the anatase and brookite phase TiO2 is treated with strong light for a long time, it can achieve decomposing water while obtaining H2 and O2. Furthermore, various characterization methods such as transient absorption spectroscopy (TAS), electron spin resonance (EPR), time-resolved infrared-excitation energy scanning spectroscopy (TIRA-EESS) and theoretical calculation (DFT) were found. The large differences in the water properties of decomposition are determined by both thermodynamic and kinetic factors.
In terms of thermodynamics, there are many bound state energy levels near the valence band of TiO2 in the anatase and brookite phases, which reduce the ability of photocatalytic water oxidation. When strong light treatment gradually eliminates these bound state energy levels, complete decomposition of aquatic products can be achieved. Hydrogen produces oxygen, and there is no bound state energy level near the valence band of rutile TiO2. The valence band energy level has enough ability to realize the decomposition of water. In terms of kinetics, the oxidation water process of different phase TiO2 undergoes different reaction intermediate pathways. The photogenerated holes of TiO2 in the anatase and brookite phases first oxidize water to generate hydroxyl radicals, while the photogenerated holes in the rutile phase TiO2 pass through superoxide radical intermediate species, and the theoretical calculation results further confirm the above result. For anatase and brookite-phase TiO2, the presence of bound energy levels near the valence band on the thermodynamics reduces the water oxidation capacity, which affects the kinetic process, resulting in photogenerated holes generating hydroxyl radicals via two-electron processes. The reaction intermediates; and for rutile TiO2, thermodynamics has a sufficiently high energy to oxidize water, kinetically undergo the superoxide radical process of a four-electron process and then release oxygen, thermodynamic factors and kinetic factors determine the light together. Catalytic decomposition of water in this complex process. This work systematically explained why TiO2 photocatalyst completely meets the requirements of water decomposition in thermodynamics but does not realize the cause of water decomposition. It not only promotes people's understanding of photocatalytic water decomposition mechanism, but also the research strategy is expected to be further extended to other areas. With practical potential on semiconductor materials.
This work was done in cooperation with the research team of the Chinese Academy of Sciences’ Physics Institute, and supported by the National Natural Science Foundation of China and the “973†project of the Ministry of Science and Technology.
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