A Vibrational Structural Analysis of Benzotriazole Adsorption and Phase Film Formation on Copper Using Surface-Enhanced Raman Spectroscopy

The adsorption of benzotriazole (BTAH) on copper in aqueous electrolytes is examined by means of a vibrational analysis utilizing surface-enhanced Raman spectroscopy (SERS) as a function of solvent deuteration, pH, and electrode potential in order to examine adsorbate speciation, surface bonding, and the potential-dependent onset of Cu I BTA phase film formation. In acidic media at pH > 1, the monoprotonated acid BTAH constitutes the predominant adsorbate, being replaced by the diprotonated BTAH 2 + species by pH 0, and the basic BTA - form in alkaline electrolytes. The analysis of adsorbate speciation is facilitated by the presence of several coupled ring modes yielding vibrational bands in the 1050−1350 cm - 1 region that are sensitive to deuteration of the triazole ring nitrogen(s). Insight into the adsorbate molecular geometry and mode of surface attachment is obtained by examining the adsorption-induced alterations in frequency, bandwidth, and relative intensity of the various benzotriazole ring modes. These changes are especially prevalent for triazole ring vibrations and some deuteration-sensitive modes, the latter apparently associated with Fermi resonance effects. The former torsional/bending ring vibrations also exhibit potential-dependent frequencies (i.e., an electrochemical Stark effect), reflecting copper−triazole electronic interactions. Taken together, the SER spectral features for BTAH and BTA - suggest surface attachment via a pair of triazole nitrogens with a tilted (or vertical) orientation; the single nitrogen binding site anticipated for BTAH 2 + is consistent with the milder vibrational perturbations observed for this adsorbate. The transformation from adsorbed benzotriazole to a Cu I BTA film at higher potentials is readily diagnosed from the SER spectra, including the observed lack of a D/H vibrational effect on the latter species. The structure of benzotriazole films in air is also examined; emersion of adsorbed BTAH from solution spontaneously yields a Cu I BTA layer via a coupled O 2 -induced electrochemical pathway. The thermal stability of copper-benzotriazole films in ambient-pressure O 2 and N 2 from 25 to 300 °C was also probed with SERS.