Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University , DK-8000 Aarhus C, Denmark.
ACS Nano. 2014 Oct 28;8(10):10704-14. doi: 10.1021/nn5044096. Epub 2014 Oct 6.
The formation and growth of maghemite (γ-Fe2O3) nanoparticles from ammonium iron(III) citrate solutions (C(6)O(7)H(6) · xFe(3+) · yNH(4)) in hydrothermal synthesis conditions have been studied by in situ total scattering. The local structure of the precursor in solution is similar to that of the crystalline coordination polymer Fe(H(2)cit(H2O), where corner-sharing [FeO(6)] octahedra are linked by citrate. As hydrothermal treatment of the solution is initiated, clusters of edge-sharing [FeO(6)] units form (with extent of the structural order <5 Å). Tetrahedrally coordinated iron subsequently appears, and as the synthesis continues, the clusters slowly assemble into crystalline maghemite, giving rise to clear Bragg peaks after 90 s at 320 °C. The primary transformation from amorphous clusters to nanocrystallites takes place by condensation of the clusters along the corner-sharing tetrahedral iron units. The crystallization process is related to large changes in the local structure as the interatomic distances in the clusters change dramatically with cluster growth. The local atomic structure is size dependent, and particles smaller than 6 nm are highly disordered. The final crystallite size (<10 nm) is dependent on both synthesis temperature and precursor concentration.
在水热合成条件下，通过原位全散射研究了从柠檬酸铁(III)铵溶液（C(6)O(7)H(6)·xFe(3+)·yNH(4))中形成和生长磁赤铁矿（γ-Fe2O3）纳米粒子。溶液中前体的局部结构类似于结晶配位聚合物Fe(H(2)cit(H2O)，其中通过柠檬酸连接角共享[FeO(6)]八面体。随着溶液水热处理的开始，边缘共享[FeO(6)]单元的簇形成（结构有序度<5 Å）。随后出现四配位的铁，并且随着合成的进行，簇缓慢组装成结晶磁赤铁矿，在 320°C 下 90 秒后产生清晰的布拉格峰。从无定形簇到纳米晶的主要转变是通过沿角共享四面体铁单元的簇缩合发生的。结晶过程与局部结构的巨大变化有关，因为随着簇的生长，簇中的原子间距离发生剧烈变化。局部原子结构是尺寸依赖性的，小于 6nm 的颗粒高度无序。最终的结晶粒度（<10nm）取决于合成温度和前体浓度。
