Sonosynthesis of various types of microand nano-materials

Author(s): Ze’ev Porat, Vijay B. Kumar and Aharon Gedanken

Ultrasonic energy was used as a means of formation of mirco- and nano-materials from the liquid state. These include pure metals, alloys, oxides, carbon dots and protein spheres. We first focused on the formation of micro-and nano spheres of low-melting point metals (mp<400 ºC) metals, starting from their molten state under silicone oil. Ultrasonic irradiation of the system induces shear forces in the oil that rapidly disperse the molten metal into numerous tiny spheres. These forces are created during the collapse of the gas bubbles in the cavitation process. The spheres did not recombine to bulk metals after sonication was ceased, although the surrounding temperature was still higher than their melting points, probably due to the formation of a coating carbon layer on the spheres, as a result of reactions of the metals with the liquid media under the extreme local conditions of pressure and temperature that develop during the short time of the bubble collapse. This method was applied to several metals (Pb, Zn, In, Sn, Ga, Bi, Hg), as-well as two eutectic alloys (Au-Ge and Au-Si). We also applied this method to molten Ga (mp 29.7ºC) under warm water and extended it to aqueous solution containing various metal ions or organic compounds that could interact with the formed Ga particles in various ways. Ultrasonic irradiation of organic liquids, such as polyethylene glycol (PEG) 400, yields carbon dots (C-dots). During sonication, the collapsing gas bubbles in the liquid act as micro-reactors in which extreme transient conditions induce some decomposition of the PEG and formation of C-dots. Their average diameter is 2-9 nm, depending on the preparation conditions. When the PEG contained molten Ga or Sn, metal doped C-dots were formed, which may be applicable as antibacterial agent (Ga@C-dots) or in photocatalysis (Sn@C-dots).