Applied Mechanics Laboratory, Department of Engineering Mechanics, and Center for Nano and Micro Mechanics, Tsinghua University , Beijing 100084, China.
ACS Nano. 2017 Nov 28;11(11):11152-11161. doi: 10.1021/acsnano.7b05419. Epub 2017 Nov 2.
Recent experimental studies have revealed unconventional phase and transport behaviors of water confined within lamellar graphene oxide membranes, which hold great promise not only in improving our current understanding of nanoconfined water but also in developing high-performance filtration and separation applications. In this work, we explore molecular structures and diffusive dynamics of water intercalated between graphene or graphene oxide sheets. We identify the monolayer structured water between graphene sheets at temperature T below T = ∼315 K and an interlayer distance d = 0.65 nm, which is absent as the sheets are oxidized. The non-continuum collective diffusion of water intercalation between graphene layers facilitates fast molecular transport due to reduced wall friction. This solid-like structural order of intercalated water is disturbed as T or d increases to a critical value, with abnormal declines in the coefficients of collective diffusion. Based on a patched model of graphene oxide sheets consisting of spatially distributed pristine and oxidized regions, we conclude that the non-continuum collective diffusion of intercalated water can explain fast water permeation through graphene oxide membranes as reported in recent experimental studies, in stark contrast to the conventional picture of pressure-driven continuum flow with boundary slip, which has been widely adopted in literature but may apply only at high humidity or in the fully hydrated conditions.
最近的实验研究揭示了层状氧化石墨烯膜中受限水的非传统相态和输运行为，这不仅有望增进我们对纳米受限水的现有理解，还有望开发高性能过滤和分离应用。在这项工作中，我们探索了插层在石墨烯或氧化石墨烯片之间的水分子的分子结构和扩散动力学。我们在温度 T 低于 T = ∼315 K 和层间距 d = 0.65nm 的情况下，确定了在石墨烯片之间存在的单层结构水，而当片层被氧化时，这种结构则不存在。由于壁面摩擦的减少，插层在石墨烯层之间的水分子的非连续集体扩散促进了快速的分子输运。随着 T 或 d 增加到一个临界值，插层水中的这种类似固体的结构有序性被打乱，集体扩散系数异常下降。基于由空间分布的原始和氧化区域组成的氧化石墨烯片的补丁模型，我们得出结论，插层水中的非连续集体扩散可以解释最近的实验研究中报道的氧化石墨烯膜中快速的水渗透，这与传统的边界滑移驱动的压力连续流观点形成鲜明对比，后者在文献中被广泛采用，但可能仅适用于高湿度或完全水合的条件。
