The Reducing Agents in Sonochemical Reactions without Any Additives

It has been experimentally reported that not only oxidation reactions but also reduction reactions occur in aqueous solutions under ultrasound without any additives. According to the numerical simulations of chemical reactions inside an air or argon bubble in water without any additives under ultrasound, reducing agents produced from the bubbles are [Formula: see text] (which becomes superoxide anion ([Formula: see text]) in liquid water), [Formula: see text] , and [Formula: see text] (which becomes [Formula: see text] in liquid water). In addition, [Formula: see text] sometimes works as a reducing agent. As the reduction potentials of [Formula: see text] and [Formula: see text] (in strongly alkaline solutions for [Formula: see text]) are higher than those of [Formula: see text] radicals, which are usually used to reduce metal ions, [Formula: see text] and [Formula: see text] generated from cavitation bubbles are expected to reduce metal ions to produce metal nanoparticles (in strongly alkaline solutions for [Formula: see text] to work). It is possible that the superoxide anion ([Formula: see text]) also plays some role in the sonochemical reduction of some solutes. In strongly alkaline solutions, hydrated electrons ([Formula: see text]) formed from [Formula: see text] atoms in liquid water may play an important role in the sonochemical reduction of solutes because the reduction potential is extremely high. The influence of ultrasonic frequency on the amount of [Formula: see text] atoms produced from a cavitation bubble is also discussed.