Raman spectroscopy has been used for investigating the gold oscillatory electrodissolution processes in a 2 mol dm-3 HCl solution. The vibration bands for Au–Cl-, AuCl-4 and Au–O(H) as a function of potential, the spatial profile of AuCl-4 concentration in the diffusion layer, and the temporal evolution of AuCl-4 during the current oscillations have all been measured in situ by a confocal Raman spectroscope. These experimental results indicate that the transition between active and passive states of gold accounts for the current oscillations in a very narrow potential range. A crossing cycle in the cyclic voltammogram that means a pair of overlapping positive and negative feedbacks corresponds to the active–passive transition. A possible oscillatory electrodissolution mechanism has been proposed on the basis of the experimental results.

Electrochemical quartz crystal microbalance (EQCM) has been employed to study the potential oscillatory mechanism for the IO- 3 reduction accompanying periodic hydrogen evolution. The new experimental results that were obtained by in situ EQCM monitoring clearly demonstrate the all key steps involved in the oscillation: the diffusion-limited depletion of IO-3 by reduction, the formation, growth and departure of hydrogen bubbles on the surface, and the convection-induced replenishment of IO-3 by the hydrogen evolution. In addition to the frequency response to the surface mass change as reported in the literature, our study first shows that simultaneous frequency responses to the changes of density and viscosity in the diffusion layer during the oscillation can also provide meaningful and even decisive information on the oscillatory mechanism for the oscillators involving the coupling of electrochemical reactions with diffusion and convection mass transfer.

In situ Raman spectroscopic studies, in combination with electrochemical measurements, further testify that the electrochemical reactions,i.e., iodate reduction and periodic hydrogen evolution, coupled with alternately predominant diffusion and convection mass transfer of iodate,account for the potential oscillation that appears under galvanostatic reduction of iodate over its limiting current in alkaline solution. The diffusion-limited depletion and the convection-enhanced replenishment of the iodate consist of a pair of positive and negative feedback steps between the bistable states (iodate reduction with and without hydrogen evolution). This mechanism is applicable to the same category of oscillators originating from such a coupling. The limiting diffusion concentration profile and the concentration variation of iodate in the diffusion layer during the oscillation by diffusion-limited depletion and by convection-enhanced replenishment through hydrogen evolution have been measured directly by using in situ Raman spectroscopy for the first time. A crossing cycle in the cyclic voltammogram that displays the bistability and the positive and negative feedbacks can be obtained only when the scan is reversed at a potential where hydrogen evolution takes place, and hydrogen evolution is thus mainly to induce the convection feedback of the reactant after its surface concentration depletes to zero by diffusion-limited reduction, rather than purely an additional current carrier. No oscillation can occur by simply removing the convection feedback with another pure current carrier instead of hydrogen evolution. The other model on the basis of negative differential resistance (NDR) fails to reflect the convection feedback step required for this category of oscillators.

Geometrical effects on the oscillatory behavior during the reduction of iodate in an alkaline solution have been studied in detail. We have found that the geometrical factors greatly affect the oscillatory behavior when the iodate concentration is smaller, and chaos appears on the electrodes with smaller size or smoother surface, whereas at higher iodate concentration, the influence from the geometrical factors is minor and periodic behavior occurs instead, though some differences still exist in the waveforms with different roughness. The geometrical factors affect mainly the convection mass transfer of the iodate induced by the hydrogen evolution,in addition to the different adsorption.