In situ Raman spectroscopy of zinc electrodes in alkaline solutions

Electronic properties of the passive film formed on nominally smooth Zn electrodes in aqueous KOH solutions were investigated using in situ Raman spectroscopy. Measurements performed in 1 M KOH, in which the potential, E, of Zn electrodes was stepped from E = -1.6 V (fully reduced state) into the passive region, i.e., E > -1.2 V vs. saturated calomel electrode, revealed a significant increase in the intensity of the Raman peak attributed to the E₁ longitudinal optical (LO) phonon modes of ZnO at ca. 565 cm^-1, with respect to that of one of the E₂ modes of the same material at 436 cm^-1, as the passivation potential was made more positive. In analogy with observations made for sputtered ZnO films under high vacuum conditions (G. J. Exarhos and S. K. Sharma, Thin Solid Films, v. 270, p. 27-32, 1995; and M. Tzolov et al., Thin Solid Films, v. 379, p. 28-36, 2000), such unusually intense LO phonon features have been ascribed to the presence of interstitial Zn and oxygen deficiency in the lattice, affording rather convincing evidence that the electronic characteristics of Zn passive films in alkaline solutions are influenced by the conditions under which the films are formed. A similar behavior was found for electrochemically formed films in 0.1 M KOH solutions, in which case small amounts of Ag were electrodeposited on the bare Zn surfaces prior to potential step passivation to enhance the otherwise weak Raman signals found for the Zn passive film in this medium.