Spin conversion of positronium of ZIFs nanocrystalline
Authors: Li Chong-Yang , Li Meng-De , Wang Mei , Li Tao , Liu Jian-Dang , Ye Bang-Jiao , Chen Zhi-Quan ,   College of Electric Power, North China University of Water Resources and Electric Power, Zhengzhou 450045, China,   School of Physics Science and Technology, Wuhan University, Wuhan 430072, China,   School of Physics, University of Science and Technology of China, Hefei 230026, China,   School of Physics and Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, China
Published: May 2022

ZIFs crystal is composed of imidazolidyl bridging single metal ions, and its structure can be adjusted by flexibly selecting functional groups of imidazolidyl ligands, thereby possessing more new properties and functions. While, the pore structure and chemical environment of ZIFs crystals are closely related to their properties. In this work, ZIF nanocrystals are prepared by static reaction. The X-ray diffraction results confirm that the prepared crystals are typical of ZIF-8 crystals, and the regular rhomboidal structure can be observed by scanning electron microscopy. The N<sub>2</sub> adsorption-desorption test indicates that the ZIF crystal exhibits the larger specific surface area (2966.26 m<sup>2</sup>/g) and pore volume (3.01 cm<sup>3</sup>/g) . With the increase of Co content, specific surface area and pore volume of ZIFs crystal decrease, while the pore size remains nearly unchanged (around 12 Å). However, the pore size distribution calculated by N<sub>2</sub> adsorption/desorption isothermal curve does not show the ultra-micropore information of the six-membered ring composed of imidazole ligands (3.4 Å). The microstructure and surface properties of the crystal are investigated by positron annihilation lifetime and Doppler broadening. The positron lifetime spectrum has four components. The longer lifetimes <inline-formula><tex-math id="M8">\begin{document}$ {\tau }_{3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M8.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M9">\begin{document}$ {\tau }_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M9.png"/></alternatives></inline-formula> are the annihilation lifetimes of o-Ps in the microporous region and the regular angular gap of the crystal, respectively. With the increase of Co content, the lifetime <inline-formula><tex-math id="M10">\begin{document}$ {\tau }_{3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M10.png"/></alternatives></inline-formula> hardly changes, while the longer lifetime <inline-formula><tex-math id="M11">\begin{document}$ {\tau }_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M11.png"/></alternatives></inline-formula> decreases from 30.89 ns to 12.57 ns, and the corresponding intensities <inline-formula><tex-math id="M12">\begin{document}$ {I}_{3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M12.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M12.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M13">\begin{document}$ {I}_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M13.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M13.png"/></alternatives></inline-formula> decrease sharply from 12.93% and 8.15% to 3.68% and 0.54%, respectively. With the increase of Co content, the <i>S</i> parameter obtained by doppler broadening shows a continuous upward trend, and the p-Ps intensity also increases gradually, which is mainly due to the self-rotation effect of the electron element. Therefore, the decrease of <inline-formula><tex-math id="M14">\begin{document}$ {\tau }_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M14.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20220305_M14.png"/></alternatives></inline-formula> in ZIFs nanocrystal is probably due to the self-rotation effect of positronium and Co ion on the crystal surface.