Oxide Film Formation and Oxygen Adsorption on Copper in Aqueous Media As Probed by Surface-Enhanced Raman Spectroscopy

The electrode potential-dependent formation of oxygen species on copper in noncomplexing aqueous media, encompassing oxide phase films and adsorbed oxygen/hydroxide, are explored at different pH values by means of surface-enhanced Raman spectroscopy (SERS). This technique provides a monolayer-sensitive in- situ vibrational probe, which can follow potential-dependent surface speciation on voltammetric or longer time scales. In alkaline NaClO4 electrolytes (pH 13), the cyclic voltammetric peaks associated with copper oxide phase-film formation and removal are correlated quantitatively with simultaneously acquired SER spectral sequences. The latter indicate the sequential formation of Cu2O and then mixed Cu2O/Cu(OH)2 layers, diagnosed by the appearance of metal−oxygen lattice vibrations at 625/525 and 460 cm-1, respectively. The potential-dependent speciation is in concordance with the Pourbaix diagram, certifying the “bulk-phase” nature of the films. The Raman band intensity−film thickness correlation (the latter deduced from the voltammetric Coulombic charges) indicate that the vibrational spectral responses are limited to the first 15−20 monolayers, consistent with earlier SERS observations and theoretical predictions. Weaker bands at ca. 800 and 460 cm-1 are discernible at more negative potentials, suggestive of hydroxide adsorption. Similar, although thinner, oxide films were deduced to form in neutral 0.1 M NaClO4. In the additional presence of chloride under these conditions, a potential-sensitive competition between the formation of a CuCl and a more passivating Cu2O phase film was evident from SERS. While oxide phase films are absent on copper in 0.1 M H2SO4 and 0.1 M HClO4, an adsorbed oxygen species was nonetheless detected from a broad SERS band at ca. 625 cm-1. This feature, which was deduced to involve oxygen rather than hydroxyl from an absence of a frequency shift upon H/D solvent isotopic substitution, is evident throughout most of the “polarizable potential” region on copper in acid, ca. −0.7 to −0.1 V vs SCE. The likely nature and reasons for its remarkable prevalence on copper in acidic media are discussed with reference to the recent literature.