Electrochemical Quartz Crystal Microbalance Technique in Sonoelectrochemistry

摘要

Ultrasound has been increasingly applied in chemistry and electrochemistry in the past decades (1-4). The major effects of ultrasound in a liquid medium are cavitation associated with microstreaming and acoustic streaming (1, 4-12). Cavitation is the formation of bubbles in the liquid during the rarefaction cycle of the ultrasonic wave. Bubbles can grow over several periods and finally collapse in an adiabatic manner (so-called transient cavitation) (6, 13, 14). The collapse leads locally to very high temperatures and pressures ("hot spot"), high heating and cooling rates, and shock waves in the surrounding liquid (4, 5, 7,12,13, 15-18). Due to the extreme conditions hydroxyl and hydrogen radicals are formed (18-20). In the proximity of a surface, cavitation causes strong microstreaming either by replacement of the volume taken up by the bubble by liquid (21) or by the formation of a strong microjet during bubble collapse (8,13,17, 22-24). Acoustic streaming is convection induced in the liquid in the direction of the traveling wave by momentum transfer (24-26). The effects of ultrasound on electrochemistry are caused by surface cavitation leading to cleaning and activation of the electrode surface, possibly a local temperature increase, and to the extreme improvement in mass transport by acoustic streaming and microstreaming that surpasses conventional stirring methods and the rotating disc electrode by far (1-3, 24-30). This can cause a change from mass transport to charge transfer control, influence nucleation and growth processes, and alter selectivities if alternate electrode reactions are possible (1-3,24-30).