In Situ Ftirs Study of Glycine Effects on Cobalt Electrodeposition on Gold Electrodes

INTRODUCTION

The presence of certain chemical species (additives) in plating baths can significantly influence the metal electrodeposition process. Thus, the electrodeposits may show different properties, due to altered morphology, structure or composition. Glycine is an additive present in many electrolytic solutions for electrodeposition of Cobalt thin films and alloys (which are interesting due to their magnetic properties). The properties of some Co-based alloys obtained from glycine-containing baths are well documented [1,2], but more fundamental-oriented studies of the exact role of the aminoacid are lacking.

In this paper, we report some spectroelectrochemical studies using external reflection FTIR spectroscopy, method that can be used to obtain microscopic information on the interfacial region, assess the possibility of glycine adsorption at negative electrode potentials, and verify possible alterations in composition of solutions containing cobalt and glycine.

EXPERIMENTAL

The spectroelectrochemical experiments were carried out in solutions containing CoSO₄ (0.050 mol L^-1), Na₂SO₄ (0.30 mol L^-1) and glycine (0.20 mol L^-1), with pH values of 3, 4, and 6. A gold electrode (8 mm diameter) was used as working electrode. The electrode potentials are referred to a Ag|AgCl (3M KCl) reference electrode, which was separated from solution via Luggin capillary. The spectroelectrochemical cell had a flat CaF₂window at its bottom, to which the gold electrode was pressed against.

Experiments consisted of a potential step from +0.20 V (E₀) to the desired sample potential (E_S, with E_S<E₀), where spectra were acquired. The absorbance unit is given by A = -log(I/I₀), with I₀being the radiation intensity at the reference potential (+0.20 V).

RESULTS AND DISCUSSION

The hydrogen evolution reaction (HER) imposed a lower limit for the electrode potentials that could be studied. At these limit potentials, the H₂evolution starts to disturb the thin layer. In our system, the potential limits were -0.55 V for pH 3, and -0.75 V for pH 6. Within these limits, no signs of adsorption of glycine on the Au electrode were detected with the experimental conditions and configuration utilized.

Changes in composition of the solution in the thin layer were detected and depended on pH, as expected. For solutions in pH 3 the HER is strongly intensified in comparison to glycine-free solutions, this being attributed to appearance of a pH buffer, as a significant fraction of glycine is in the cation form (protonated carboxyl). During the potential steps, negative absorption bands assigned to the cation and positive bands assigned to the zwitterion form were observed in the spectra, confirming perturbation of the equilibrium. The use of glycine in this pH value is not common in plating baths, but may be useful in Co alloys electrodeposition according to some authors [2].

For pH 4, signs of glycine coordination to Co²⁺ are observed. The absorption bands assigned to symmetric and asymmetric carboxyl stretching in the zwitterion appear in lower wavenumbers when sufficiently negative potentials are applied (-0.60 V, Figure 1), this evidences complex formation [3]. In the initial pH value of 4, cobalt-glycine complexes are absent. Therefore, the increase in thin layer pH should be substantial (at least to pH 5) during electrode polarization. Similarly, in pH 6 solutions, the infrared spectra show an increase in the concentration of cobalt-glycine complexes at negative potentials.

CONCLUSIONS

In the range of electrode potentials investigated, glycine molecules do not undergo redox reactions, and no signs of adsorption on polycrystalline Au electrode were detected. For low pH values in which a considerable amount of glycine is in its cation form, the HER is greatly enhanced. In pH 4, the increase of pH in the thin layer during potential step to -0.60 V was high enough to promote Co²⁺coordination with glycine. In pH 6, a rise in the concentration of the complexes was detected, probably also due to HER.

ACKNOWLEDGEMENTS

We thank Professor Mauro C. dos Santos and Júlio C. M. da Silva for useful discussions. We thank Fapesp and CNPq for the financial support.

REFERENCES

1. J. C. Wei, M. Schwartz e K. Nobe. J. Electrochem. Soc. 155(10), D660 (2008).

2. O. Ergeneman, K. M. Sivaraman, S. Pané, E. Pellicer, A. Teleki, A. M. Hirt, M. D. Baró, B. J. Nelson, Electrochim. Acta 56, 1399 (2011).

3. K. Nakamoto, Y. Morimoto, A. E. Martell, J. Am. Chem. Soc. 83, 4528 (1961).