[Paper Review] 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 applicationsMaterials Science3 citations

TL;DR

This study reports the direct observation of two-dimensional Ca-Cl crystals with monovalent Ca⁺ ions under ambient conditions using cryo-electron microscopy on reduced graphene oxide (rGO) membranes. The crystals exhibit metallic behavior due to strong cation-π interactions between Ca²⁺ and graphitic aromatic rings, enabling potential applications in catalysis, hydrogen storage, and atomic-scale transistors with functionalized graphene heterostructures.

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.

Motivation & Objective

• To investigate the formation of abnormal 2D metal-chloride crystals with high-valence cations like Ca²⁺ under ambient conditions.
• To determine the electronic properties of such 2D Ca-Cl crystals formed on rGO membranes.
• To explore the role of cation-π interactions in stabilizing unusual valence states of metal ions in 2D systems.
• To assess the potential for functionalized graphene-based devices using these novel 2D crystals.

Proposed method

• Cryo-electron microscopy was used to directly image 2D Ca-Cl crystals formed on reduced graphene oxide (rGO) membranes.
• rGO membranes were incubated in CaCl₂ solutions below saturation concentration under ambient conditions.
• Theoretical calculations were performed to analyze the contribution of cation-π interactions between Ca²⁺ and aromatic carbon rings in rGO.
• Electronic structure analysis was conducted to evaluate the metallic nature of the observed CaCl crystals.
• Comparative analysis of cation-π interactions was extended to other divalent metal ions (e.g., Mg²⁺, Fe²⁺, Cu²⁺) to predict similar 2D crystal formation.

Experimental results

Research questions

• RQ1 Can two-dimensional Ca-Cl crystals with monovalent Ca⁺ ions form under ambient conditions on rGO membranes?
• RQ2 What is the electronic nature (metallic vs. insulating) of the observed 2D CaCl crystals?
• RQ3 To what extent do cation-π interactions between Ca²⁺ and graphitic surfaces stabilize these abnormal 2D crystal phases?
• RQ4 Can similar 2D crystals with abnormal valence states form for other divalent metal ions on graphitic substrates?

Key findings

• Two-dimensional Ca-Cl crystals with monovalent Ca⁺ ions were directly observed on rGO membranes under ambient conditions using cryo-electron microscopy.
• The 2D CaCl crystals exhibit metallic electronic properties rather than insulating behavior, contrary to bulk CaCl₂.
• The formation of these crystals is attributed to strong cation-π interactions between Ca²⁺ ions and aromatic rings in the rGO lattice.
• Theoretical modeling confirms that cation-π interactions stabilize the 2D CaCl phase and enable monovalent Ca⁺ character.
• Similar 2D crystals with abnormal valence states are predicted for other divalent cations (e.g., Mg²⁺, Fe²⁺, Cu²⁺) due to comparable cation-π interactions.
• The system enables the creation of compact 'graphene-metallic CaCl-insulating CaCl₂' junctions, suitable for atomic-scale transistors and multifunctional nanodevices.