Ultrasound-assisted fast encapsulation of metal microparticles in SiO₂ via an interface-confined sol-gel method

Although the traditional Stoˇber process-based methods are widely used for encapsulation of metal nanoparticles in SiO₂, these time-consuming methods are not effective for coating metal microparticles with a uniform SiO₂ layer of desired thickness. Herein, an ultrasound-assisted, interface-confined sol-gel method is proposed for fast encapsulation of metal microparticles in SiO₂, and the encapsulation of Sn microparticles is chosen as an example to illustrate its feasibility. The proposed method involves covering metal microparticles with liquid films that contain water, alcohol, surfactant (Span-80) and catalyst (NH₄F) and then ultrasonically dispersing these particles into cyclohexane, where tetraethylorthosilicate (TEOS) is added. To ensure the hydrolysis-condensation reactions of TEOS occurring at the particle-cyclohexane interface so that the formed SiO₂ is coated on the particles, the microparticles should be well dispersed into cyclohexane with the liquid films being not broken away from their surfaces. It is found that the assistance of probe sonication and the addition of surfactant are crucial to achievement of a good dispersion of metal microparticles in cyclohexane. And using high-viscosity alcohol (namely glycerol), controlling the volume ratio of water to alcohol and the amount of water, and choosing a suitable ultrasonic power are essential for preventing the formation of free SiO₂ (namely SiO₂ that is not coated on the particles), which is a result that the liquid films escape from the particle surfaces under ultrasonic cavitation. Our results have also revealed that the thickness of SiO₂ layer can be adjusted by changing the reaction time or the total amount of water. In particular, the thickness of SiO₂ layer can be easily raised by simply repeating the encapsulation procedure. Compared with the traditional Stoˇber process-based methods, the proposed method is time-saving (reaction time: about 30 min vs. more than 12 h) and extremely effective for coating microparticles with a continuous, uniform SiO₂ layer of desired thickness.