Interpretation of Negative Resistance Observed in Electrochemical Impedance during Copper Electrodeposition Containing Thiourea

Abstract

The effect of the thiourea on the electrochemical impedance during copper electrodeposition was investigated using a rotating ring disk electrode. The rest potential of the copper electrode measured in the solution containing thiourea was shifted to less noble due to the adsorption of thiourea. The cathode polarization curve of the copper electrode demonstrated that the current density of the copper electrode decreased remarkably because of an increase in the concentration of thiourea. The Nyquist plot of impedance spectrum in the presence of the thiourea described the capacitive loop, which was related to the time constant of the charge transfer resistance and the electric double layer capacitance, in the high frequency range and a part of the large locus crossing the imaginary axis in the low frequency range. This low frequency locus is related to negative resistance which contributes to "leveler" effect. In order to clarify the negative resistance, the theoretical equations of the Faraday impedance were derived. We proposed the four steps of the copper electrodepostion reactions in the presence of thiourea.

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