Sonochemistry Creates Nanospheres, Crystals

Utilizing high-intensity ultrasound, investigators have devised hollow nanospheres and the first hollow nanocrystal. These nanospheres could be used in drug delivery, microelectronics, and as catalysts for making environmentally friendly fuels.

"We use high-intensity ultrasound to generate nanoparticles of molybdenum disulfide or molybdenum oxide, which bind to the surface of tiny silica spheres that are much smaller than red blood cells,” said Dr. Ken Suslick, a professor of chemistry at University of Illinois at Urbana-Champaign (USA)."After heating the spheres to produce uniform coatings, we use hydrofluoric acid to etch away the silica, leaving hollow shells of the desired material.” The ultrasound used consisted of sound waves above 18,000 cycles per second, too high-pitched to be heard by humans.

Hollow nanospheres created from molybdenum disulfide could serve as an excellent catalyst for removing sulfur-containing compounds from gasoline and other fossil fuels. "Molybdenum-disulfide is a layered material, but its catalytic activity occurs at its edges,” Dr. Suslick said. "By distorting and breaking up the layers, hollow nanospheres offer increased edge-surface area, as well as access to both inner and outer shell surfaces.”

However, additional processing of hollow spheres made from molybdenum oxide results in the atypical formation of hollow crystals that resemble condensed cubes. After heating a second time in a process called thermal annealing, the hollow molybdenum oxide spheres are converted into single crystal cubes with spherical hollow cavities. The sonochemical method could be easily applied to other material systems to devise further kinds of hollow, nanostructured particles, according to Dr. Suslick.

Sonochemistry processing occurs from acoustic cavitation, the formation, growth, and implosion of small gas bubbles in a liquid blasted with sound. The breakdown of these bubbles creates intense local heating, forming a hot spot in the liquid with a transient temperature of approximately 9,000o F, the pressure total of about 1,000 atmospheres, and the duration of approximately one billionth of a second. To roughly compare, these values correspond to the temperature of the surface of the sun, the pressure of the bottom of the ocean, and the lifetime of a lightning strike.




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