Department of Physics, ‡Department of Mechanical and Nuclear Engineering, and §Department of Chemical Engineering, The Pennsylvania State University , University Park, Pennsylvania 16802.
Nano Lett. 2014;14(4):1836-42. doi: 10.1021/nl404533k. Epub 2014 Mar 10.
Oriented attachment (OA) of nanocrystals is now widely recognized as a key process in the solution-phase growth of hierarchical nanostructures. However, the microscopic origins of OA remain unclear. We perform molecular dynamics simulations using a recently developed ReaxFF reactive force field to study the aggregation of various titanium dioxide (anatase) nanocrystals in vacuum and humid environments. In vacuum, the nanocrystals merge along their direction of approach, resulting in a polycrystalline material. By contrast, in the presence of water vapor the nanocrystals reorient themselves and aggregate via the OA mechanism to form a single or twinned crystal. They accomplish this by creating a dynamic network of hydrogen bonds between surface hydroxyls and surface oxygens of aggregating nanocrystals. We determine that OA is dominant on surfaces that have the greatest propensity to dissociate water. Our results are consistent with experiment, are likely to be general for aqueous oxide systems, and demonstrate the critical role of solvent in nanocrystal aggregation. This work opens up new possibilities for directing nanocrystal growth to fabricate nanomaterials with desired shapes and sizes.
各向异性附着（OA）被广泛认为是分级纳米结构在溶液相中生长的关键过程。然而，OA 的微观起源仍不清楚。我们使用最近开发的 ReaxFF 反应力场进行分子动力学模拟，以研究各种二氧化钛（锐钛矿）纳米晶体在真空中和潮湿环境中的聚集。在真空中，纳米晶体沿其接近方向合并，形成多晶材料。相比之下，在水蒸气存在的情况下，纳米晶体通过 OA 机制重新定向并聚集形成单晶或孪晶。它们通过在聚集的纳米晶体的表面羟基和表面氧之间创建氢键的动态网络来实现这一点。我们确定 OA 在最有可能离解水的表面上占主导地位。我们的结果与实验一致，可能对水性氧化物体系具有普遍性，并证明了溶剂在纳米晶体聚集中的关键作用。这项工作为指导纳米晶体生长以制造具有所需形状和尺寸的纳米材料开辟了新的可能性。
