Optical Second Harmonic Generation Studies of Electrochemical Interfaces
Author(s)
Baten, Shah Mohammad Abdul
Type
Thesis
Abstract
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.
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.
Date Issued
2012-02
Date Awarded
2012-03
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Wilde, Paul
Taylor, Alan
Publisher Department
Chemistry
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)