透射电子显微镜下原位观察石墨烯液体池中水的辐解和凝结

利用石墨烯液体池技术,将液体水束缚在两层石墨烯之间,实现透射电子显微镜下纳米尺度液相反应的原位动态观察。通过对电子束的精确调控来控制水的辐解和凝结行为:若先在高电子剂量率下辐照液体,我们发现回到低剂量率后一系列纳米气泡在水中有序地析出并发生长大。界面反应是纳米气泡生长的限制因素,且新生的气泡的生长会抑制既有气泡的长大行为。进一步分析表明气泡内的气体处于致密的压缩态,体系内总的分子数随时间近似线性增加。而持续以相对适中的恒定电子剂量率作用,水辐解产生的气泡中又出现纳米水滴的凝结,并重复地长大/消失。该结果对于研究纳米限域环境下气/液界面反应等重要过程具有参考价值,同时有助于深入理解液体透射电镜下电子束效应对实验的影响。

In Situ TEM Observation of Radiolysis and Condensation of Water via Graphene Liquid Cell

Water is involved in many important physical, chemical, and biological phenomena. However, not much is known about water at the nanoscale level, such as about its radiolysis and condensation, because of the experimental difficulties of imaging liquid at such a length-scale. The newly developed graphene liquid cell (GLC) technique facilitates imaging dynamic events in a liquid medium with unprecedented resolution while sustaining the most realistic liquid condition achievable under electron-beam irradiation. The graphene liquid cell was fabricated by the wet transfer of monolayer graphene synthesized by chemical vapor deposition to a graphene-supported transmission electron microscope (TEM) grid, and the water islands were naturally captured during the wet transfer process. Compared with traditional, commercial silicon nitride cells, the fabrication of graphene liquid cells required higher expertise and thus related research reports are limited. Here, we used graphene liquid cells for in situ TEM observation of dynamic behaviors of entrapped water between two layers of graphene. The radiolysis and condensation processes of water could be modulated by controlling the electron dose rate. We showed that a high electron dose rate yields supersaturated concentration of gas molecules in a liquid system, and the excess gas then dissolve out in the form of a series of nanobubbles at a low dose rate. From The quantitative and statistical analyses of the dynamic processes of the confined liquid showed that the growth of the nanobubbles is limited by interface reactions and the newly formed nanobubble inhibits the growth of the existing one. The gas molecules inside the nanobubbles are in a "dense gas" phase and the density number ratio of the gas molecules inside each nanobubble decreases during the growth process of each nanobubble. The total number of gas molecules increases approximately linearly with time. A fixed middle dose rate leads to the condensation of droplets, with repeat growth/dissolution processes on the inner wall of graphene. The contact angles at the water-graphene interface are less than 90°, suggesting that the scrolled graphene is hydrophilic. Using in situ GLC-TEM imaging, we directly observed the dynamic processes involved in beam-induced bubble formation in liquid and nanodroplet condensation from vapor at the preferential sites. Some unexpected and so-far undiscovered phenomena involving both nanobubbles and nanodroplets were observed and investigated in detail, which increased our knowledge on the behaviors of the nanoconfined liquid. Our results presented here would serve as an important reference to understand the vapor/liquid interface transition in a nanoconfined space and electron-water interactions in liquid cell TEM. The experiment and method reported here may help find further applications of the new graphene liquid cell technique. The use of graphene liquid cell will also help a wide range of studies on reactions and process dynamics in material science and biochemistry that either have not been explored or need more detailed understanding.