Effect of oxalic acid (complexing agent) on anodic dissolution of Cobalt in hydrogen peroxide solutions: mechanism and kinetic analysis by electrochemical impedance spectroscopy

Abstract

The anodic dissolution of Cobalt in H₂O₂ solution is investigated in the presence and absence of a complexing agent, oxalic acid, using various techniques including electrochemical impedance spectroscopy (EIS). Anodic polarization measurements of both the solutions show that active dissolution occurs in the potential range of 0 to 600 mV w.r.t open circuit potential (OCP) and the addition of oxalic acid enhances the dissolution of Co by forming highly soluble Cobalt complexes. The mechanistic reaction pathway of Co dissolution at metal-solution interface is investigated by performing EIS measurements at various overpotentials under anodic conditions. EIS spectra exhibit two loops; capacitance (higher frequency) followed by inductance (lower frequency) at all the overpotentials and it was modeled by a multi-step mechanism with 3 intermediate adsorbed species. The dissolution via both chemical and electrochemical steps is considered in the proposed model. From the parameters obtained, dominance of $Co_{ad}^{2 + }$ species on the Co metal surface is observed for both the systems. The oxides/hydroxides formed on the Co surface on the addition of oxalic acid to H₂O₂ are higher than using only H₂O₂, thus properly justifying the role of a complexing agent in a CMP slurry. Products formed on exposure of Co to H₂O₂ and H₂O₂-oxalic acid solution at pH 9 are analyzed using X-ray photoelectron spectroscopy (XPS) analysis. The results confirm the formation and dominance of Co-oxalate complexes in H₂O₂—oxalic acid system.

Graphical abstract

Appendix

• 1. The steady state mass balance of the adsorbed species, ${Co}_{ad}^{+}$, ${Co}_{ad}^{2+}$, and ${Co}_{ad}^{3+}$ for the proposed mechanism are given by the following expressions:

  $$k_{1} (1 - \theta_{1ss} - \theta_{2ss} - \theta_{3ss} ) + k_{ - 2} \theta_{2} = k_{2} \theta_{1ss} + k_{ - 1} \theta_{1ss}$$

  (41)
  $$k_{2} \theta_{1ss} + k_{ - 3} \theta_{3ss} = (k_{3} + k_{5} + k_{ - 2} )\theta_{2ss}$$

  (42)
  $$k_{3} \theta_{2ss} = \left( {k_{4} + k_{ - 3} } \right)\theta_{3ss}$$

  (43)

• 2. $\frac{{{\text{d}}\theta_{1} }}{{{\text{d}}V}}$, $\frac{{{\text{d}}\theta_{2} }}{{{\text{d}}V}}$ and $\frac{{{\text{d}}\theta_{3} }}{{{\text{d}}V}}$ of Eq. (36) can be determined by applying Taylor’s approximation, and expanding the mass balance equation (unsteady state). It is to be mentioned that in order to maintain linearity the terms of higher order are neglected.

  $$\frac{{d\theta_{1} }}{dV} = \frac{{AK\left( {F_{1} J_{1} - GK} \right) + BIF_{1} J_{1} - BIGK + BJ_{1} \left( {KH - F_{1} I} \right)}}{{D_{1} K\left( {KH - F_{1} I} \right) + KE\left( {AK + BI} \right)}}$$

  (44)
  $$\frac{{d\theta_{2} }}{dV} = \frac{{KE\left( {\frac{{d\theta_{1} }}{dV}} \right) - F_{1} J_{1} + GK}}{{KH - F_{1} I}}$$

  (45)
  $$\frac{{d\theta_{3} }}{dV} = \frac{{IKE\left( {\frac{{d\theta_{1} }}{dV}} \right) - IF_{1} J_{1} + IGK - J_{1} \left( {KH - F_{1} I} \right)}}{{K\left( {KH - F_{1} I} \right)}}$$

  (46)

Here,

$$n = 1$$

(47)

$$A = k_{1} - k_{ - 2}$$

(48)

$$B = k_{1}$$

(49)

$$\begin{aligned}C =& k_{1} b_{1} - \big( {k_{1} b_{1} + k_{2} b_{2} + k_{ - 1} b_{ - 1} } \big)\theta_{1ss}\\& - \left( {k_{1} b_{1} - k_{ - 2} b_{ - 2} } \right)\theta_{2ss} - k_{1} b_{1} \theta_{3ss}\end{aligned}$$

(50)

$$D_{1} = k_{1} + k_{2} + k_{ - 1} + j\omega \tau$$

(51)

$$E = k_{2}$$

(52)

$$F_{1} = k_{ - 3}$$

(53)

$$G = k_{2} b_{2} \theta_{1ss} - \left( {k_{3} b_{3} + k_{5} b_{5} + k_{ - 2} b_{ - 2} } \right)\theta_{2ss} + k_{ - 3} b_{ - 3} \theta_{3ss}$$

(54)

$$H = k_{3} + k_{5} + k_{ - 2} + j\omega \tau$$

(55)

$$I = k_{3}$$

(56)

$$J_{1} = \left( {k_{4} b_{4} + k_{ - 3} b_{ - 3} } \right)\theta_{3ss} - k_{3} b_{3} \theta_{2ss}$$

(57)

$$K = k_{4} + k_{ - 3} + j\omega \tau$$

(58)