Effect of Oxalate on The Growth of Cuprous Oxide Layers on Copper Electrodes. Ellipsometric and Isoelectric Point Study

Jorge O. Zerbino,^1,* Rosa M. Torres Sánchez² and Marija G. Sustersic³

¹ Instituto de Investigaciones Fisicoquimicas Teóricas y Aplicadas (INIFTA), UNLP, Sucursal 4, C.C. 16, (1900) La Plata (Argentina)
² Centro de Tecnología de Recursos Minerales y Cerámica. CETMIC. CIC. C.C. 49, (1987) M. B. Gonnet- (Argentina)
³ Faultad de Ingeniería. FICES .UNSL. 25 de Mayo 384 (5730) Villa Mercedes, San Luis (Argentina)
* Corresponding author: E-mail: jzerbino@inifta.unlp.edu.ar

Abstract
The effect of the addition of oxalate to the growth of a cuprous oxide layer on copper electrodes was analysed at potential near that of the open circuit, in borax solutions (7 < pH < 9) by cyclic voltammetry, ellipsometry and surface charge techniques. The oxide formation is explained as a sequence of Cu₂O layer growth, ippl, cationic defect accumulation and Cu(II) adsorption on the oxide/solution interface, and a dissolution/precipitation step similar to the mechanism previously reported in oxalate free solutions. The oxalate adsorption at pH = 9 increases the dissolution rate and a greater thickness of the outer layer, oppl, is obtained. Nevertheless, the oxalate adsorption at pH = 7 decreases the cationic defect on the cuprous oxide/electrolyte interface, promoting the Cu₂O growth. For copper particles immersed in solutions of pH between 7 and 9, the measured isoelectric point values, iep, (11.8 < iep < 11.5) shifts in the presence of oxalate to pH between 11.6 and 11.0, respectively. This shift in the iep to a lower pH value indicates oxalate adsorption on the Cu/ Cu₂O particles.

Keywords: Copper oxide, oxalate, ellipsometry, isoelectric point.