One-step synthesis of Sn-based phase change material capsules via TiO2 nanoparticles-stabilized metal-in-salt emulsions

Conventional synthesis of stable metal oxide-encapsulated metal particles typically requires two-stage processing: metal particle formation and oxide coating via a sol-gel method. However, this biphasic protocol is often time-consuming and environmentally unfriendly. Herein, a novel, one-step approach that enables concurrent metal particle formation and oxide encapsulation is proposed to synthesize TiO₂-encapsulated Sn-based particles. The proposed method involves ultrasonic emulsification of molten Sn or Sn alloys (e.g. Sn-Zn-Cu) within a LiCl-KCl-CsCl eutectic melt containing TiO₂ nanoparticles, followed by rapid emulsion solidification and salt matrix dissolution in water. Our results demonstrate that Sn-based capsules with diameters ranging from hundreds of nanometers to several micrometers and TiO₂ shell thicknesses typically in the range of 80–120 nm were successfully synthesized via such emulsion-based system, where the capsule size decreases with the concentration of TiO₂ nanoparticles. The as-prepared Sn-2.0Zn-1.5Cu (in wt%) phase change capsules exhibit significantly reduced undercooling (∼7 °C vs. ∼88 °C for Sn capsules of comparable size) and their phase change properties remain stable after 300 thermal cycles, thereby showing great potential for thermal energy storage applications. With short emulsification durations (<10 min) and recyclable chloride salts, this Pickering-type metal-in-salt emulsion methodology might serve as a new pathway for rapid and environmentally friendly synthesis of metal-based micro/submicro-capsules across diverse material systems by varying metal alloy compositions and selecting suitable nanoparticle stabilizers.