Photocatalytic "memory" effect of metal has been made progress

Photocatalytic technology has broad application prospects in the field of environmental protection. In recent years, researchers have developed a series of high-efficiency visible light photocatalytic materials that have greatly improved the efficiency of the use of solar energy, reduced the cost increase and operational risks brought about by the use of ultraviolet radiation, and are conducive to the practical application of photocatalytic technology. However, existing high-efficiency visible light photocatalytic materials cannot generate electron-hole pairs after losing the energy supply of the external light source, so that active groups cannot be generated, the reactivity thereof is rapidly lost, and pollutants in the environment cannot be further processed . Therefore, the existing high-efficiency visible light photocatalytic materials cannot use the solar energy to continuously process the pollutants in the environment, and it is necessary to configure an auxiliary light source besides the solar light energy so as to be continuously reactive in the night. This will bring about two problems. On the one hand, the auxiliary light source system inevitably increases costs and energy consumption. On the other hand, many environmental pollution treatments are not suitable for uninterrupted lighting conditions.

Institute of Metal Research, Chinese Academy of Sciences, Shenyang Li Qi, researcher of the Environmental Materials Research Department of the National (Joint) Laboratory for Materials Science, and his research team focused on this issue and proposed a photocatalytic “memory” effect storage based on the study of high-efficiency visible light photocatalytic materials. The high-energy photoelectrons produced under light conditions produce active groups by releasing these stored electrons after the light is turned off, thereby making it possible to maintain activity for a longer period of time in the absence of light. This will be able to make full use of solar energy and general lighting sources to continuously treat environmental pollutants all the time, greatly enhancing the effect of photocatalytic technology on environmental pollution, reducing treatment costs and energy consumption, and making photocatalytic technology It is of great significance to obtain new applications in a wider range of environmental protection. Under the guidance of this idea, they developed the first generation of photocatalytic material system with "memory" effect - noble metal palladium oxide nanoparticles modified nitrogen-doped titanium dioxide (Advanced Materials, 2008, 20, 3717; Journal of Materials Chemistry, 2010 , 20, 1068). The photogenerated electrons stored on palladium oxide nano-particles are released by light shutoff to generate superoxide and hydroxyl reactive groups, and successful removal of various environmental contaminants in the dark is successfully achieved.

Based on the previous work, they have made new advances in the study of photocatalytic “memory” effects through the study of photocatalytic “memory” effect mechanisms and effective active groups. They found that precious metal modifications are not required for photocatalytic "memory" effects. According to the energy band structure of semiconductor materials, by selecting the material with electronic storage capacity and matching the appropriate material structure and energy band, the enrichment, storage and illumination of photoelectron under light can be achieved in a variety of photocatalytic nanomaterial systems. After the release, the high-efficiency photocatalytic water purification material with “memory” effect with better performance and lower cost is obtained.

Under the guidance of the research results of this mechanism, they developed a second-generation photocatalytic material with a “memory” effect: a titanium dioxide “nano-island” modified cuprous oxide nanospheres photocatalytic material. On the basis of the synthesis of cuprous oxide nanospheres, discontinuous titanium dioxide "nano-islands" are formed on cuprous oxide nanospheres by adsorption, controlled hydrolysis and solvothermal reaction. In this material system, an n-type titanium dioxide "nano-island" is built on a p-type cuprous oxide nanosphere to form a nano-pn junction structure.

The p-type cuprous oxide nanospheres can absorb visible light and generate electron-hole pairs. The conduction band energy level of TiO2 nano-island is lower than that of cuprous oxide nano-spheres, and photo-generated electrons can transfer from cuprous oxide to TiO2; at the same time, the built-in electric field generated by the interface of nano pn junction also promotes photo-electron oxidation. The cuprous oxide is transferred to titanium dioxide, thereby more effectively separating electron-hole pairs. While achieving effective separation of electron-hole pairs, Ti4+ on titanium dioxide nano islands can be reduced to Ti3+ to store photoelectrons (ie, light energy); after light is turned off, superoxide and hydroxy reactive groups are generated by releasing electrons. Clusters produce a photocatalytic "memory" effect (ACS Applied Materials & Interfaces, 2014, 6, 5629).

By studying the active groups that may produce a photocatalytic "memory" effect, they further found that the generation of photocatalytic "memory" effects does not necessarily require the formation of superoxide-reactive groups by single-electron oxygen reduction in this process. The electrons stored in the modified component can also generate hydrogen peroxide through a two-electron oxygen reduction reaction, which can also have a photocatalytic “memory” effect. Therefore, the energy requirements for storing electrons can be greatly reduced, and the bottom conductivity of the modified component of the conduction band can be reduced to only need to be less than the potential of the two-electron oxygen reduction reaction. For example, semiconductor materials such as SnO2, WO3, CuWO4, BiWO6, CeO2, etc. whose conduction band bottom position is higher than the oxygen two-electron reduction potential position may become modified elements in a material system having a photocatalytic "memory" effect, thereby making it possible to have The material system of this effect has been greatly expanded. In addition, the lower conduction band potential modifying component expands the difference in the conduction band bottom potential between the light absorption functional component and the storage electron functional component, which is favorable for the enhancement of the system's ability for photo-induced electron transfer, and its performance is expected to be further improved. Under the guidance of this understanding, they developed a third-generation photocatalytic material with a "memory" effect - tin oxide nanoparticle-modified cuprous oxide nanocube single crystal photocatalytic material. In this system, Cu2O nanocubic single crystals generate photogenerated electron-hole pairs under visible light irradiation. Under the pn junction built-in electric field and matching energy band structure, the photo-electron transfer and enrichment in the SnO2 nanoparticles, thereby enhancing its visible-light electron-hole separation efficiency and improve its photocatalytic activity. After the visible light is turned off, the enriched photo-generated electrons undergo a two-electron reduction reaction with oxygen, thereby generating an active group H2O2 in the dark, producing a photocatalytic "memory" effect. The study found that this material system has a strong ability to continuously generate active H2O2 through electron release in the dark, and can still generate H2O2 24 hours after the light is turned off, so that it can have a photocatalytic "memory" effect for a long period of time (Scientific Reports, 2016 , 6, 20878).

Their studies show that photocatalytic material systems with "memory" effects are diverse, have different mechanisms of action, effective active groups, and their "memory" effects can be regulated and optimized through material design, and photocatalytic "memory". The study of effects has broad space for development. The above research work has been supported by the National Natural Science Foundation of China, the Ministry of Education's Overseas Returnees Research Foundation Project, the Chinese Academy of Sciences Youth Innovation Promotion Project, the introduction of outstanding scholar projects, and the State-Owned (joint) Laboratory Fundamental Frontier Innovation Project of Shenyang Materials Science. .

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