We reveal a unique
mechanism by which pure water can be dissociated
to form free radicals without requiring catalysts, electrolytes, or
electrode contact by means of high-frequency nanometer-amplitude electromechanical
surface vibrations in the form of surface acoustic waves (SAWs) generated
on a piezoelectric substrate. The physical undulations associated
with these mechanical waves, in concert with the evanescent electric
field arising from the piezoelectric coupling, constitute half-wavelength
“nanoelectrochemical cells” in which liquid is trapped
within the SAW potential minima with vertical dimensions defined by
the wave amplitude (∼10 nm), thereby forming highly confined
polarized regions with intense electric field strengths that enable
the breakdown of water. The ions and free radicals that are generated
rapidly electromigrate under the high field intensity in addition
to being convectively transported away from the cells by the bulk
liquid recirculation generated by the acoustic excitation, thereby
overcoming mass transport limitations that lead to ion recombination.
Highlights
A relation between sonochemical activity and dissolved gas concentration is reported.
Acoustic cavitation is responsible for sonochemical activity in PZT based reactors.
A relatively higher dissolved gas concentration is needed for sonochemical activity at 2 MHz.
Sonochemiluminescence is a highly sensitive technique for the detection of sonochemical activity.
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