[论文解读] Two-dimensional Ca-Cl crystals under ambient conditions observed directly by cryo-electron microscopy

Lei Zhang, Guosheng Shi+15 more|arXiv (Cornell University)|Dec 18, 2018

Graphene research and applications被引用 3

一句话总结

本研究通过在还原氧化石墨烯（rGO）膜上使用冷冻电子显微镜，在常温常压下直接观测到二维Ca-Cl晶体，其中钙离子为单价Ca⁺。这些晶体由于Ca²⁺与石墨烯芳香环之间强烈的阳离子-π相互作用而表现出金属性行为，为催化、储氢以及基于功能化石墨烯异质结构的原子尺度晶体管提供了潜在应用。

ABSTRACT

Recently, we report the direct observation, under ambient conditions, of Na2Cl and Na3Cl as two-dimensional (2D) Na-Cl crystals, together with regular NaCl, on reduced graphene oxide membranes and on the surfaces of natural graphite powders from salt solutions far below the saturated concentration. However, what are these abnormal stoichiometries for high valence ions, such as calcium ions and copper ions still remain unknown. Here, using cryo-electron microscopy, we report the direct observation of two-dimensional (2D) Ca-Cl crystals on reduced graphene oxide (rGO) membranes, in which the calcium ions are only monovalent (i.e. +1). Remarkably, metallic properties rather than insulating are displayed by those CaCl crystals. We note that such CaCl crystals are obtained by simply incubating rGO membranes in salt solutions below the saturated concentration, under ambient conditions. Theoretical studies show that the formation of those abnormal crystals is attributed to the strong cation-pi interactions of the Ca2+ ions with the aromatic rings in the graphitic surfaces. Since strong cation-pi interactions also exist between other metal ions (such as Mg2+, Fe2+, Co2+, Cu2+, Cd2+, Cr2+ and Pb2+) and graphitic surfaces, similar 2D crystals with abnormal valence state of the metal cations and corresponding abnormal properties are highly expected. The 2D crystals with monovalent calcium ions show unusual electronic properties, and can be applicated in catalyzer, hydrogen storage, high-performance conducting electrodes and sensors. These findings also produce functionalized graphene including compact "graphene-metallic CaCl-insulating CaCl2" junction that can serve as transistors down to the atomic scale, and other devices for magnetic, optical and mechanical applications.

研究动机与目标

• 研究在常温常压下，高氧化态阳离子（如Ca²⁺）形成异常二维金属氯化物晶体的机制。
• 确定在rGO膜上形成的此类二维Ca-Cl晶体的电子性质。
• 探讨阳离子-π相互作用在稳定二维体系中金属离子异常价态中的作用。
• 评估基于这些新型二维晶体的功能化石墨烯基器件的潜在应用。

提出的方法

• 采用冷冻电子显微镜直接成像在还原氧化石墨烯（rGO）膜上形成的二维Ca-Cl晶体。
• 将rGO膜在低于饱和浓度的CaCl₂溶液中于常温常压下孵育。
• 通过理论计算分析Ca²⁺与rGO中芳香碳环之间阳离子-π相互作用的贡献。
• 开展电子结构分析，以评估所观测到的CaCl晶体的金属性质。
• 将阳离子-π相互作用的对比分析扩展至其他二价金属离子（如Mg²⁺、Fe²⁺、Cu²⁺），以预测类似二维晶体的形成。

实验结果

研究问题

• RQ1 在rGO膜上，常温常压下能否形成具有单价Ca⁺离子的二维Ca-Cl晶体？
• RQ2 所观测到的二维CaCl晶体的电子性质是金属性还是绝缘性？
• RQ3 Ca²⁺与石墨烯表面之间的阳离子-π相互作用在稳定这些异常二维晶体相中的作用程度如何？
• RQ4 其他二价金属离子是否也能在石墨烯基底上形成具有异常价态的类似二维晶体？

主要发现

• 通过冷冻电子显微镜在常温常压下直接观测到rGO膜上形成的具有单价Ca⁺离子的二维Ca-Cl晶体。
• 所观测的二维CaCl晶体表现出金属性电子性质，而非体相CaCl₂的绝缘行为。
• 这些晶体的形成归因于Ca²⁺离子与rGO晶格中芳香环之间强烈的阳离子-π相互作用。
• 理论建模证实，阳离子-π相互作用稳定了二维CaCl相，并使其表现出单价Ca⁺特性。
• 预测其他二价阳离子（如Mg²⁺、Fe²⁺、Cu²⁺）也会因类似的阳离子-π相互作用而形成具有异常价态的类似二维晶体。
• 该体系可实现紧凑的“石墨烯-金属CaCl-绝缘CaCl₂”结，适用于原子尺度晶体管和多功能纳米器件。