Ellipsometric measurement of thickness of tin oxides grown by voltammetry in phosphate solution of pH 8.7

Abstract

<p class="PaperAbstract"><span lang="EN-US">The voltammetry induced growth of tin oxides on tin in the buffer solution of 0.18 mol L<sup>-1</sup> Na<sub>2</sub>H<sub>2</sub>PO<sub>4</sub> and 0.18 mol L<sup>-1</sup> KH<sub>2</sub>PO<sub>4</sub> (pH 8.7) has been studied. Ex-situ ellipsometric mea­surements were made in an order to determine thicknesses of the grown oxides. From these results the film volume per charge unit, </span><span lang="EN-US">V</span><sub><span lang="EN-US">f</span></sub><span lang="EN-US">, was calculated for different charge den­sities of the film. This parameter was used to calculate the variable ionic resistivity of the film, </span><span lang="EN-US">ρ</span><sub><span lang="EN-US">f</span></sub><span lang="EN-US">, considered by the Ohmic model for the case of voltammetric growth of oxides on metals having a previously existing continuous film. Tin oxide films grown at 2 mV s<sup>-1</sup> showed to be less dense for values of charge density below 50 C m<sup>-2</sup>, having </span><span lang="EN-US">V</span><sub><span lang="EN-US">f</span></sub><span lang="EN-US"> near <br /> 5.7x10<sup>-10</sup> m<sup>3</sup> C<sup>-1</sup>. For higher values of charge density, tin oxide films become denser, having </span><span lang="EN-US">V</span><sub><span lang="EN-US">f</span></sub><span lang="EN-US"> near 0.5x10<sup>-10</sup> m<sup>3</sup> C<sup>-1</sup>. The calculated values of the variable ionic resistivity of the film during voltammetric growth showed that </span><span lang="EN-US">ρ</span><sub><span lang="EN-US">f</span></sub><span lang="EN-US"> passes through a minimum (justifying the maximum in current densities). This behavior was also found by other authors in the cases of Zn, Nb, Ni and galvanized steel sheets.</span></p>

Voltammetric growth of tin oxides in borate solution of pH 8.7

“…At the same time, the representation of ρf vs. charge density of the film presents a minimum. This behavior was also found by other authors in the cases of Zn, Nb, Ni and galvanized steel sheets [12][13][14][15][16][17].…”

“…In our recent publication [12] we applied the ohmic model to describe the induced growth of tin oxides by voltammetry in phosphate buffer solution pH 8.7. The thicknesses of the grown oxides were measured by ex-situ ellipsometry to determine the film volume per charge unit, Vf, for different charge densities of the film.…”

“…The ohmic model developed by D'Alkaine [12][13][14][15][16][17] presented in earlier work, describes the relation between the current density and the film overpotential during the growth of passivating film under voltammetric conditions. The following equation should be valid for peak or plateau transient conditions during voltammetric growth of a film:…”

“…In order to calculate the values of ρf using Eq. (4), many authors [12][13][14][15][16][17] applied the ohmic model, considering the volume per charge unit (Vf) as the constant equal to:…”

“…Ellipsometry is an interesting technique to be considered for the thickness measurement [21]. Ellipsometry is an indirect technique, which allows one to access layer thickness in ex-situ [12,22], and in-situ [19,20,23] experiments, which is the main interest in voltammetric measurements. In an in-situ experiment it can also monitor layer growth [19,20].…”