Cao Boxiao, Xu Enshi, Li Tianshu
Department of Civil and Environmental Engineering , George Washington University , Washington , D.C. 20052 , United States.
ACS Nano. 2019 Apr 23;13(4):4712-4719. doi: 10.1021/acsnano.9b01014. Epub 2019 Mar 25.
The freezing of water mostly proceeds via heterogeneous ice nucleation, a process in which an effective nucleation medium not only expedites ice crystallization but also may effectively direct the polymorph selection of ice. Here, we show that water confined within a hydrophobic slit nanopore exhibits a freezing behavior strongly distinguished from its bulk counterpart. Such a difference is reflected by a strong, non-monotonic pore-size dependence of freezing temperature but, more surprisingly, by an unexpected stacking ordering of crystallized two-dimensional ice containing just a few ice layers. In particular, confined trilayer ice is found to exclusively crystallize into a well-ordered, hexagonal stacking sequence despite the fact that nanopore exerts no explicit constraint on stacking order. The absence of cubic stacking sequence is found to be originated from the intrinsically lower thermodynamic stability of cubic ice over hexagonal ice at the interface, which contrasts sharply the nearly degenerated stability of bulk hexagonal and cubic ices. Detailed examination clearly reveals that the divergence is attributed to the inherent difference between the two ice polymorphs in their surface phonon modes, which is further found to generically occur at both hydrophobic and hydrophilic surfaces.
水的冻结主要通过异质冰核化进行，在这个过程中，有效的成核介质不仅能加速冰的结晶，还可能有效地指导冰的多晶型选择。在这里，我们表明，限制在疏水狭缝纳米孔内的水表现出与其本体状态截然不同的冻结行为。这种差异体现在冻结温度对孔径的强烈非单调依赖性上，但更令人惊讶的是，体现在仅包含几层冰层的结晶二维冰的意外堆积有序性上。特别是，尽管纳米孔对堆积顺序没有明确限制，但发现受限的三层冰仅结晶成有序的六边形堆积序列。发现立方堆积序列的缺失源于界面处立方冰相对于六边形冰本质上较低的热力学稳定性，这与本体六边形冰和立方冰几乎简并的稳定性形成鲜明对比。详细研究清楚地表明，这种差异归因于两种冰多晶型在其表面声子模式上的固有差异，进一步发现这种差异普遍存在于疏水和亲水表面。
