Yoshiharu Mukouyama

Periodic and chaotic oscillations of the electrochemical potential of p-Si in contact with an aqueous (CuSO4+HF) solution, caused by electroless Cu deposition

Periodic and chaotic oscillations were observed for the potential of p-type Si(111) immersed in an aqueous (HF+CuSO(4)) solution, accompanied by electroless Cu deposition on p-Si. They were, to our knowledge, the first examples of open-circuit potential oscillations observed for semiconductor electrodes. The oscillations appeared only when the Cu deposit formed a continuous porous film composed of mutually connected submicrometer-sized particles. Besides, the Si surface was kept flat within the size less than 50 nm even after the prolonged oscillation for a few hours, though the Si surface should be etched considerably with HF for this time. A plausible model is proposed for the periodic oscillation, in which interestingly coupling of autocatalytic shift in the flat-band potential of Si (U(fb)) caused by the change in the coverage of the Si oxide and the connection and disconnection of the Cu film with the Si surface plays the key role. The appearance of the chaotic oscillation is also explained by taking into account an oscillation-coupled change in the HF or Cu(2+) concentration near the Si surface.

Transient chaotic behavior during simultaneous occurrence of two electrochemical oscillations

We have found that a chaotic oscillation transiently appears when two kinds of current oscillations, named oscillation A and oscillation α, occur simultaneously. Oscillations A and α appear during the reduction of H2O2 and S2O82− respectively on the Pt electrode. When the electrode potential is stepped from the rest potential to a potential where both the oscillations appear, a period-1 oscillation first appears, and then a period-doubling bifurcation cascade occurs. After that, the transient chaos appears, which is followed by a sequence of mixed-mode oscillations (MMOs). The appearance of the chaotic behavior and the MMOs can be explained on the basis of the reported mechanisms for oscillations A and α. Both the oscillations are caused by an N-shaped negative differential resistance (N-NDR) due to the formation of under-potential deposited H (upd-H), indicating that a positive feedback mechanism works during the simultaneous occurrence of oscillations A and α. On the other hand, negative feedback mechanisms for oscillations A and α include the surface concentration of H2O2 and S2O82−, respectively. We can, then, conclude that the combination of the positive feedback mechanism and the two negative feedback mechanisms of different time scales gives rise to the chaotic behavior and the MMOs, which has been verified by numerical simulations.