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

摘要

The voltammetry induced growth of tin oxides on tin in the buffer solution of 0.18 mol L-1 Na2H2PO4 and 0.18 mol L-1 KH2PO4 (pH 8.7) has been studied. Ex-situ ellipsometric mea-surements were made in an order to determine thicknesses of the grown oxides. From these results the film volume per charge unit, Vf, was calculated for different charge den-sities of the film. This parameter was used to calculate the variable ionic resistivity of the film, ρf, 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-1 showed to be less dense for values of charge density below 50 C m-2, having Vf near 5.7x10-10 m3 C-1. For higher values of charge density, tin oxide films become denser, having Vf near 0.5x10-10 m3 C-1. The calculated values of the variable ionic resistivity of the film during voltammetric growth showed that ρf 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.