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Recently, Li Can's team at the State Key Laboratory of Catalysis and the National Laboratory of Clean Energy of Dalian Institute of Chemical Physics, Chinese Academy of Sciences has made new progress in the research of photoanodes for photoelectrochemical decomposition of water in silicon-based semiconductor materials. The interfacial donor defect level in the crystalline silicon photoelectrode is one of the factors restricting the efficiency of the photoelectrode. The interfacial energy band structure of the heterojunction is finely controlled, and the charge separation and water oxidation efficiency of the photoelectrode are effectively improved. The relevant research results were published in full text in the "American Chemical Society" (Tingting Yao, Can Li, et al, J. Am. Chem. Soc., 2016, DOI: 10.1021/jacs.6b07188).
The solar catalytic water decomposition reaction is one of the ideal ways to solve energy problems and environmental pollution in the future. In semiconductor-based photoelectrocatalysts, photocarriers are effectively separated and migrated to the surface of the photocatalyst to participate in the water decomposition reaction, which is the key to improving photocatalytic efficiency. Li Can's team has long been committed to solving this key issue and has successively proposed "heterojunction" (J. Am. Chem. Soc., 2008, DOI: 10.1021/ja8007825) and "Anomaly knot". Chem. Int. Ed., 2012, DOI: 10.1002/anie.201207554) and “Separation of charge between crystal planes†(Nat. Commun., 2013, DOI: 10.1038/ncomms2401) and other strategies to promote photo-generated charge separation. However, because the charge transfer mainly occurs at the junction interface, the photovoltaic characteristics of the photoelectrode are often closely related to the interface energy band structure and properties. However, as of now, the mechanism of charge separation and transport at the interface level of a silicon-based photoelectrode is not clear.
In this work, the researchers found that the donor state energy level existing at the interface of the n-Si/ITO Schottky junction prevented the photogenerated holes from reaching the electrode surface and participated in the oxidation reaction, and confirmed the energy level position of the donor state defect; A thin layer of TiOx is introduced between n-Si and ITO, and its discrete energy level is finely controlled, which effectively reduces the interfacial donor state energy level and realizes the near-theory of the Schottky barrier height of n-Si/TiOx/ITO. The value greatly improves the separation and transmission efficiency of photogenerated charge. Second, the research also confirmed the dual role of surface-loaded NiOOH, on the one hand, as a traditional cognitive water oxidation catalyst to promote the injection efficiency of photogenerated charge; on the other hand, the surface work function of the Schottky electrode was modified, and the potential barrier was increased. The height and built-in electric field strength increase the available surface light voltage by an additional 215%. The initial potential of the water oxidation reaction of n-Si/TiOx/ITO/NiOOH photoelectrode was reduced from about 1.1-1.2V reported in the literature to 0.9V, which is the lowest value reported in the literature. At the same time, a photoelectron charge injection efficiency of more than 90% and a charge separation efficiency of nearly 100% were achieved at a wide electrode potential. The results of this study are applicable not only to silicon-based photoanodes but also to the application of other semiconductor-based optoelectronic devices. This work is also a new progress in the use of monocrystalline silicon for photoelectrochemical decomposition of water following the team's research on amorphous silicon (ChemSusChem, 2015, DOI: 10.1002/cssc.201501004).
The work was funded by the "973" project of the Ministry of Science and Technology, the National Natural Science Foundation, and the iChEM.
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