Oscillatory Electrodeposition of Cu/Cu2o: A Study on the Influence of Ligands in Cu(II) Complexes
Authors: Pinto Maria Rodrigues , Pereira Guilherme Bueno , Kitagaki Bianca Tieme , Queiroz Adriana , Nagao Raphael 
Published: July 2021

Self-organization is a powerful tool for the synthesis of new materials, among a wide variety of processes that exhibit this phenomenon, electrochemical ones are promising candidates in which the main contributions can be divided between electrodeposition and electrodissolution systems.1 Potential and/or current oscillations have already been observed in several electrodeposition reactions and culminated in the formation of different structures under micro and nanoscale such as multilayers, spiral waves and dendrites.Periodic potential oscillations related to alternate electrodeposition of Cu and Cu/Cu2O, recorded in the form multilayered architecture were first reported by Switzer and collaborators,2 and further explored in the presence of other ligands such as tartrate and citrate.
The oscillations that occur under galvanostatic regime are detected in a broad range of pH and current densities values, that directly affect the period and amplitude of the oscillations. Therefore, as the material morphology and properties are directed related to the dynamical behavior of the system, it is of major importance to understand deeply the role of the experimental variables on the system. Although direct relationships between experimental parameters, especially at short range studies, are consistently reported at the literature, multivariate statistical analysis revealed a more complex dependence on the frequency including quadratic terms.3

Concerning the chemical influence of the ligand over the oscillatory electrodeposition of Cu/Cu2O, we have investigated the effect of lactate (Figs. A and C) and tartrate (Figs. B and D) ligands in the oscillation period (Figs. A and B) and amplitude (Figs. C and D).4 The potential oscillations at the Cu(II)-lactate and Cu(II)-tartrate systems are detected in different ranges of pH and current densities values for each system, that affect the period and amplitude of the oscillations as portrayed at the diagrams in Figs. A to D. Although a strict parallel of the systems can not be performed, the main characteristics are the period and amplitude variation when the lactate is replaced by tartrate in the reaction media. On balance, the potential oscillations in the Cu(II)−tartrate system exhibit larger periods and lower amplitude values in relation to the Cu(II)−lactate one. Beyond that, there is a significant suppression of the oscillation domain in the tartrate system, that can be connected to an apparent inhibition effect, as a result of the stronger adsorption of tartrate anions and/or its metallic complexes. In fact, the presence of a poisoning species on the surface, that acts as spectator of the reaction, can induce the increase of the oscillation period and suppress the Hopf domain, as previously reported.
The oscillation mechanism introduced by Switzer et al., further detailed by Leopold et al. and updated by our group, taking into account recent available structural information (Figs. E and F), describes the alternated deposition of copper and copper oxide, and considers pH interfacial variation as being an essential step for the oscillation mechanism. Supposing that only a buffering effect can alter the oscillation period, as stated before by previous investigations, the buffering capacity found in the lactate complex (Fig. E) should be less effective when compared to the tartrate one (Fig. F). This can be explained from the ligand structures: the second carbonyl and hydroxy groups available for protonation/deprotonation (tartrate vs. lactate complex) lead to an increase in the buffering capacity, besides the concentration of these species in solution. Since the oscillating period is reduced when the solution pH is increased, a longer period observed for the tartrate complex means that this system possesses a higher buffering capacity compared to that for the lactate complex.
Even though of significant importance, this mechanism does not account for the anion adsorption and/or the respective complexes dissolved in the electrolyte and as observed experimentally, which is an important consideration to be made in the description of the oscillatory electrodeposition. The surface blocking and buffering effect are enhanced when tartrate molecules are utilized as the ligand (in comparison with lactate system), and both mechanisms seem to control the dynamic properties of the electrochemical oscillator simultaneously. Therefore, we show that chemical structures or functions of the reactants could be altered to induce an autonomous growth of a target material, giving rise to a bottom-up synthesis approach. This is of great value especially since Cu and Cu2O interfaces are employed in many fields, such as electrocatalysis in the CO2 electroreduction.
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[3] B. T. Kitagaki, et al. Phys. Chem. Chem. Phys.
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[4] M. R. Pinto, ei al. J. Phys.
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Figure 1