How can collapsing bubbles driven by ultrasound help make nanoscale metals and carbides?

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Sonochemistry arises from acoustic cavitation: the formation, growth, and implosive collapse of bubbles in a liquid. A bubble’s implosive collapse generates a localized hot spot at approximately 5000 K and approximately 1800 atm, with heating and cooling rates exceeding 10⁷ K/s.

The researchers used these extreme conditions to develop a new synthesis technique for amorphous and nanostructured inorganic materials. They irradiated volatile organometallic precursors with high-intensity ultrasound in low-volatility solvents under argon. This produced high-surface-area, amorphous solids made up of agglomerates of nanometer-scale clusters.

For Fe and Co, the technique formed nanostructured metals and alloys that were non-crystalline according to X-ray, neutron, and electron-beam diffraction. For Mo and W, it produced metal semicarbides, such as Mo₂C.

These nanostructured solids were active heterogeneous catalysts for hydrocarbon reforming and CO hydrogenation. Polymeric ligands, such as polyvinylpyrrolidone, or oxide supports, such as alumina or silica, could trap the initially formed nanoscale clusters as colloids or supported catalysts.