State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China.
Chemistry. 2012 Jul 16;18(29):8957-63. doi: 10.1002/chem.201201108. Epub 2012 Jun 22.
In this study, quasi-cubic and hexagonal bipyramid α-Fe(2)O(3) polyhedrons with high-index facets exposed were controllably synthesized by applying metal ions Zn(2+) or Cu(2+) as structure-directing agents. The growth of the α-Fe(2)O(3) nanostructures with high-index facets were induced by metal ions without the addition of any other surfactants. The quasi-cubic form controlled by Zn(2+) looks like a cube but has an angle of approximately 86° bound by (012), (10-2), and (1-12) facets, whereas the hexagonal bipyramid form controlled by Cu(2+) has a sixfold axis bound by {012} facets. Magnetic measurements confirm that these two kinds of nanocrystals display shape- and surface-dependent magnetic behaviors. The hexagonal bipyramid iron oxide nanocrystals show a lower Morin transition temperature of 240 K and might be spin-canted ferromagnetically controlled at room temperature, and the ferromagnetism disappears at low temperature. The quasi-cubic nanocrystals have a splitting between FC curve and ZFC curve from the highest experimental temperature and no Morin transformation occurs; this indicates that they would be defect ferromagnetically controlled at low temperature. The reported metal-ion-directing technique could provide a universal method for shape- and surface-controlled synthesis of nanocrystals with high-index facets exposed.
在这项研究中，通过应用金属离子 Zn(2+)或 Cu(2+)作为结构导向剂，可控合成了具有高指数晶面暴露的类六方双锥和类立方α-Fe(2)O(3)多面体。在没有添加任何其他表面活性剂的情况下，金属离子诱导了具有高指数晶面的α-Fe(2)O(3)纳米结构的生长。由 Zn(2+)控制的类立方形式类似于立方体，但具有约 86°的角，由（012）、（10-2）和（1-12）晶面限定；而由 Cu(2+)控制的六方双锥形式具有由{012}晶面限定的六重轴。磁性测量证实，这两种纳米晶体表现出形状和表面依赖的磁行为。六方双锥氧化亚铁纳米晶体显示出较低的 Morin 转变温度 240 K，可能在室温下呈自旋倾斜铁磁控制，而在低温下铁磁性消失。类立方纳米晶体在最高实验温度下，FC 曲线和 ZFC 曲线之间存在分裂，并且不会发生 Morin 转变；这表明它们在低温下可能通过缺陷铁磁控制。所报道的金属离子导向技术为具有高指数晶面暴露的纳米晶体的形状和表面控制合成提供了一种通用方法。
