Exploring Crystal Growth within Liquid Metal in Their Natural States

Liquid metal offers fascinating properties and an unconventional method for dissolving metallic solutes. It provides a new approach for tuning and engineering metallic crystal features formed through this pathway, thus bringing immense value as a functional material for diverse applications.

This study unravels three-dimensional spatial observations of metallic crystals formed inside the liquid metal via an X-ray micro-computed tomography system (X-ray micro-CT/ XCT), showcasing the novelty of this phenomenon. We harness the distinctive ability of liquid metal, where environmental boundaries play a significant role in shaping and tailoring the metallic crystal formed within. The temperature gradient of the cooling rate and chemical composition in the reaction medium influence the crystal morphologies and intermetallic phases. In this study, platinum (Pt) is used as the solute metal, while gallium (Ga) and a eutectic mixture of Ga and indium (In), known as EGaIn, are used as the liquid metal solvent systems. The analysis results show that different crystal morphologies form inside the liquid metal droplets. Their spatial arrangement tends to be at the interface, which may act as a substrate rather than in the core of the droplets.

Furthermore, the extraction process of metallic crystal formed inside the liquid metal droplet is also demonstrated. A potential was applied to the liquid metal system, significantly decreasing its surface tension and facilitating the extraction of crystals from the droplets. Simultaneously, a vacuum-driven force was applied to assist the filtration process. The extracted crystals were subsequently washed to remove any residual liquid metal solvents. Scanning Electron Microscope (SEM) and X-ray diffraction (XRD) analysis were performed to characterize the extracted crystals. SEM imaging reveals distinct morphologies between the two liquid metal system solvents. The Ga solvent produces rod-shaped crystals, while the EGaIn solvent system results in polyhedron-shaped crystals. The XRD results show different phases, predominantly Ga₂Pt for Ga solvent and Ga₇Pt₃ for EGaIn solvent.These results align with the density functional theory and ab-initio molecular dynamics (DFT-AIMD) theoretical calculations, indicating that Pt in Ga solvent tends to have stronger interaction than in the EGaIn solvent system.

Finally, a proof-of-concept application, the hydrogen evolution reaction (HER), is conducted to reveal the potential of liquid metal systems in enhancing and tailoring the specific characteristics of metallic crystals for catalytic activities. The crystals formed from different liquid metal solvent systems exhibit distinct catalytic activities. A similar observation was obtained with crystals formed under different cooling crystals, resulting in varying electrochemical activities. The experiment was conducted in two different electrolyte media, acid and alkaline, with the acidic media providing better HER performance. In one of the crystal systems, the HER activity (V vs. RHE) was comparable to that of commercial Pt on carbon support (Pt/C).

In short, our works demonstrate the synthesis process to quality control, followed by catalysis applications via the liquid metal-based pathway, addressing the pressing demand for providing low-energy-cost and sustainable technologies.