Electrocrystallization of cadmium on anodically formed titanium oxide

Abstract

The electrocrystallization of Cd on previously anodized Ti substrates was studied by means of electrochemical techniques and SEM images in three different solutions of CdSO₄ (2, 10, and 50 mM). Voltammetric characterization showed the typical behavior of metal electrodeposition on conducting substrates, and potentials for electrodeposition of Cd were identified. However, the response obtained at different potentials in current transients exhibited an abnormal behavior suggesting the influence of TiO₂ on the process of electrocrystallization. The characterization of the obtained electrodeposits by SEM images allowed relating electrochemical with morphological changes, in particular, variations in the crystal size and shape as well as formation of presumably Cd branches on the substrate surface. This behavior only allowed the analysis of current transients of the electrodeposits obtained in 2 mM CdSO₄ solution using general equation for diffusion-controlled 3D growth and relating parameters of Cd crystal growth on anodized Ti electrode with the observed morphological changes.

Appendix

Procedure for non-linear adjustment

Equations (3) and (7) were used to make non-linear adjustments of experimental current transients. This appendix shows, just by way of example, the procedure of non-linear adjustment used for Eq. (3).

Equation (3) was parameterized in the following manner:

$$ {I_{{3\mathrm{DG}-\mathrm{DC}(t)}}}=\left( {P1\times \frac{{P{2^{{{1 \left/ {2} \right.}}}}}}{{{t^{{{1 \left/ {2} \right.}}}}}}} \right)\left( {1-\exp \left\{ {-P2\times P3\times P4\left[ {t-\frac{{1-\exp \left( {-P5\times t} \right)}}{P5 }} \right]} \right\}} \right) $$

(8)

where:

$P1=\frac{nFc }{{{\pi^{{{1 \left/ {2} \right.}}}}}}$, P2 = D, P3 = πk, P4 = N _o, P5 = A

The development of non-linear adjustments was performed using Statistica software version 5, which requires initial values for every parameter shown in Eq. (8); these parameters were estimated taking into consideration the following:

• For P1: the values of n, F and c do not vary during the experimentation, so P1 = constant value = 0.218.

• For P2: the average of D values estimated by Cottrell equation was used as initial value (1.97 × 10⁻⁵).

• For P3: since k does not vary during the experimentation, P3 was maintained constant at a value of 0.080.

• For P4 and P5: in order to estimate the initial values of these parameters, we considered equations defining i _m (maximum current) for instantaneous nucleation (Eq. (9)) and progressive diffusion-limited 3D nucleation (Eq. (10)).

$$ {i_{{{{\mathrm{m}}_{{3\mathrm{D}-\mathrm{iDC}}}}}}}=0.6382nFDc{{\left( {k{N_o}} \right)}^{{{1 \left/ {2} \right.}}}} $$

(9)

$$ {i_{{{{\mathrm{m}}_{{3\mathrm{D}-\mathrm{pDC}}}}}}}=0.4615nF{D^{{{3 \left/ {4} \right.}}}}c{{\left( {k\prime {N_o}A} \right)}^{{{1 \left/ {4} \right.}}}} $$

(10)

$$ k\prime =\frac{4}{3}k $$

(11)