Metals when exposed to air under ambient conditions leads to the formation of self-limiting atomically thin oxide layer at the metal-air interface, which is considered to be a naturally occurring two-dimensional (2D) material. However, isolation of 2D metal oxides from the metal surface poses considerable challenges.
Researchers at RMIT University Australia, Queensland University of Technology, Australia and California NanoSystems Institute, University of California, USA have shown that it would be possible to synthesis atomically thin metal oxides (2D metal oxide) at room-temperature using liquid metals as reaction environment (Reference: Ali Zavabeti et al., A liquid metal reaction environment for the room-temperature synthesis of atomically thin metal oxides. Science, 2017; 358 (6361): 332 DOI: 10.1126/science.aao4249)
In this study galinstan (liquid metal alloy containing gallium, indium and tin) was used as a reaction environment. Galinstan alloyed ~1 wt % of elemental hafnium, aluminum, or gadolinium served as the precursors for the formation of their respective oxides (HfO2, Al2O3 and Gd2O3). The choice of these alloying elements were made on the basis of thermodynamic considerations (Gibbs free energy (ΔGf) value).
Two different methods were proposed for isolating the surface oxides; (i) van der Waals (vdW) exfoliation technique; and (ii) gas injection method.
The van der Waals (vdW) exfoliation technique is quite similar to the method for obtaining monolayer of graphene which involves touching the liquid metal droplet with a solid substrate. The liquid nature of the parent metal allows a clean delamination of the oxide layer (Fig. 1). This technique is suitable for the production of high-quality thin oxide sheets on substrates.
The second technique relies on the injection of pressurized air into the liquid metal, in which the metal oxide forms rapidly on the inside of air bubbles and rose through the liquid metal. When the released air bubbles pass through deionized water placed above the liquid metal, allows dispersion of the oxide sheets in the aqueous suspension. Subsequently, the suspension can be subjected to drop casting to prepare 2D metal oxide films on suitable substrates (Fig. 2). This technique is highly scalable and hence suitable for the synthesis of the target oxide nanosheets with high yield.
Fig. 1 Schematic representation of the van der Waals exfoliation technique. The pristine liquid metal droplet is first exposed to an oxygen-containing environment. Touching the liquid metal with a suitable substrate allows transfer of the interfacial oxide layer.
Fig. 2 Schematic representation of the gas injection method (left), photographs of the bubble bursting through the liquid metal (center), and an optical image of the resulting sheets drop-cast onto a SiO2/Si wafer (right)
The findings of the study indicate that oxide layers formed on liquid metals can be manipulated by an appropriate choice of alloying elements based on Gibbs free energy. The two method proposed to isolate the 2D nanosheets require simple experimental set-up and allows either a direct deposition on solid surfaces or formation of an aqueous suspension that can be drop cast over a variety of substrates. The methodology outlined in this study provides a novel pathway for the synthesis and easy isolation of 2D materials that was previously inaccessible.
The 2D materials, viz., HfO2, Al2O3 and Gd2O3, synthesized in this study hold promise for applications in energy storage, such as supercapacitors and batteries. HfO2 can be used as an ultrathin insulator dielectric material for the fabrication of field-effect transistors.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer Ali Zavabeti et al., Science, 2017; 358 (6361): 332 DOI: 10.1126/science.aao4249
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