Optoelectronic devices possess the ability to generate, modulate, transmit and sense electromagnetic radiation in the visible, infrared and ultraviolet ranges. Recent technological advances have contributed to the development of colloidal metal halide perovskite (e.g. CsPbX3, X = Cl, Br, or I) nanocrystals of auspicious optoelectronics applicability in light-emitting diodes (LEDs), photodetectors, and solar cells. Unfortunately, the perovskite nanocrystals utilization is impeded by their inherent poor stability as a result of their ionic structure. As a consequence, they degrade upon exposure to several conditions such as high temperature, ultraviolet light, moisture and oxygen: with the last two being the most notorious. Surface modification of the perovskite nanocrystals provides significant stability improvements. However, the insulating shells used restrict the charge transport of the nanocrystals thus hampering their performance. To this effect, it is imperative to develop novel strategies for water-stable perovskite nanocrystals while maintaining good charge transport properties for optoelectronic and related applications.
Researchers led by Dr. Weiwei Zheng at Syracuse University, Syracuse in the United States developed a facile technique that could be utilized in fabricating a coating of titanium dioxide shell on nanometer-sized inorganic lead-halide nanocrystals. They hoped that their technique would yield an efficient protective layer for the inorganic lead-halide nanocrystals. Their work is published in the research journal, Advanced Functional Materials.
The research carried out by Zhi-Jun Li in the Zheng group commenced their experimental procedure by preparing the inorganic cesium lead bromide (CsPbBr3) nanocrystals. Then, titanium dioxide (TiO2) shell coated CsPbBr3 core/shell-structured nanocrystals are synthesized through the encapsulation of colloidal CsPbBr3 nanocrystals with titanium precursor, followed by calcination at 300 °C for five hours.
The authors observed that the nearly monodispersed inorganic cesium lead bromide/titanium dioxide (CsPbBr3/TiO2) core/shell nanocrystals showed excellent water stability for at least three months with the size, structure, morphology, and optical properties remaining identical. This was a groundbreaking observation since it represents the most water-stable inorganic shell passivated perovskite nanocrystals reported to date. In addition, the titanium dioxide shell coating was noted to effectively suppress anion exchange and photo-degradation, therefore dramatically improving the chemical stability and photostability of the core centimeter-sized inorganic lead-halide nanocrystals. More importantly, the encapsulating CsPbBr3 nanocrystals into the electrical conductive thin-layer TiO2 shell allows for efficient photoinduced charge transfer and enhanced photoelectric activity in real water testing.
“Given the promising use of perovskite nanocrystals in energy harvesting, the excellent environmental stability of the CsPbBr3/TiO2 core/shell nanocrystals provide new opportunities for the design and utilization of perovskites as a visible-light photocatalyst for solar energy conversion in aqueous solution.” Zheng told Advances in Engineering.
Dr. Weiwei Zheng and colleagues has successfully demonstrated a facile strategy to produce nearly monodispersed inorganic lead-halide/titanium dioxide core/shell nanocrystals by encapsulating the latter with a titanium precursor, followed by high temperature calcination. It has been seen that the titanium dioxide shell not only efficiently protects the nanocrystals from degradation, but also facilitates charge carrier transfer thereby exhibiting the most water-stable inorganic shell passivated perovskite nanocrystals reported to date. Altogether, this work has presented an excellent method that would enable for the design and utilization of perovskite nanocrystals as a visible-light photo-catalyst for solar energy conversion in aqueous solution.
Zhi-Jun Li, Elan Hofman, Jian Li, Andrew Hunter Davis, Chen-Ho Tung, Li-Zhu Wu, Weiwei Zheng. Photoelectrochemically Active and Environmentally Stable CsPbBr3/TiO2 Core/Shell Nanocrystals. Advanced Functional Materials 2018, volume 28. 1704288.
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