ZIFs纳米晶体中电子偶素的自旋转换

Spin conversion of positronium of ZIFs nanocrystalline

ZIFs晶体由咪唑基桥接单金属离子构成,可通过咪唑酯连接物灵活选取合适的官能团对其结构进行调控,因而被赋予更多新的性质和功能.ZIFs晶体中孔结构及其化学环境与其性能紧密相关.本文采用静置法制备了ZIFs纳米晶体.X射线衍射结果证实制备的晶体为典型的ZIF-8晶体,扫描电子显微镜图可观察到其规则的菱型结构.N_(2)吸附-脱附测试表明ZIFs晶体具有较大的比表面积和孔容,分别为2966.26 m^(2)/g和3.01 cm^(3)/g.随着Co摩尔含量的增大,ZIFs晶体比表面积和孔体积逐渐减小,但是其孔径大小几乎稳定保持在12?左右.而N_(2)吸附-脱附等温线计算得到的孔径分布未显示咪唑配体组成的六元环的超微孔信息(3.4?).此外,利用正电子湮没寿命和多普勒展宽对晶体的微观结构和表面性能进行了研究.正电子的寿命谱有4个分量.较长寿命τ3,τ4分别是o-Ps在其微孔区域和晶体规则棱角间隙处的湮没寿命.随Co摩尔含量增大,其寿命τ3几乎没有变化,而较长寿命τ4从30.89 ns降至12.57 ns,其对应强度I3,I4也分别从12.93%和8.15%急剧下降至3.68%和0.54%.随Co摩尔含量的增大,多普勒展宽得到的S参数呈连续上升趋势,进一步多高斯拟合表明p-Ps强度也逐渐增大,这主要是由于电子偶素发生了自旋转换效应.因此,ZIFs纳米晶体中τ4下降很可能是由于正电子偶素与晶体表面Co离子发生了自旋转换效应.

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 N2 adsorption-desorption test indicates that the ZIF crystal exhibits the larger specific surface area(2966.26 m2/g) and pore volume(3.01 cm3/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 ?).However,the pore size distribution calculated by N2 adsorption/desorption isothermal curve does not show the ultra-micropore information of the six-membered ring composed of imidazole ligands(3.4 ?).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 τ3 and τ4 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 τ3 hardly changes,while the longer lifetime τ4decreases from 30.89 ns to 12.57 ns,and the corresponding intensities I3 and I4 decrease sharply from 12.93%and 8.15% to 3.68% and 0.54%,respectively.With the increase of Co content,the S 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 τ4 in ZIFs nanocrystal is probably due to the self-rotation effect of positronium and Co ion on the crystal surface.