Optical Second Harmonic Generation Studies of Electrochemical Interfaces

Electrochemical oxidation of metal electrodes in aqueous media has been examined by in situ optical second harmonic generation (SHG), a nonlinear optical technique of second order. Simultaneous SHG and cyclic voltammetry experiments were carried out on Cu electrodes in alkaline media. The SHG response was found to be sensitive as Cu was oxidised first to Cu2O and then to CuO. The formation of Cu2O causes resonant contribution to the SHG signals due to the band-gap of Cu2O. As the upper layer, CuO is formed; it generates an electric-field induced second harmonic (EFISH) response. When the oxides layers are reduced, the SHG intensity changes markedly. The correlation of oxidation and reduction charge with the SHG signal intensity is found to be a useful method to investigate the oxide formation. The oxidation of Au electrodes in acidic media has been investigated. The initial growth of the compact (or α-) oxide causes a decrease in SHG intensity which is suggested to be the effect of the oxide growth on surface plasmons originated from the d-electrons in Au. When the thicker hydrous (or β-) oxide layers are formed at constant potentials on top of the α-oxide, the reductions of them are accompanied by remarkable sharp peak in the SHG response that appear just before any reduction charges begin to pass. As the β-oxide layer gets thicker with longer oxidation time and then reduced, the initial peak evolves into two before merging to form one broad peak. The SHG peaks are suggested to be linked to an order-disorder transition phenomenon at the α-/β-oxide interface; or it can also be due to the presence of the surface traps. These investigations have shown that the SHG technique with its inherent submonolayer sensitivity can be a useful technique to study electrochemical oxidation of metal electrodes