Mechanism of selenidation of iron and nickel films

Abstract

The interaction of H₂Se with films of iron and nickel has been studied between –80 and 250°C. Rapid dissociative adsorption, followed by desorption of H₂, occurred on both metals. The average composition of the surface phase was expressed as H_nSe and n decreased with increase of both surface coverage and temperature. The composition of the surface phase at –80°C corresponded to H_0.88Se on iron and H_1.8Se on nickel. Some molecular adsorption was observed on nickel at this temperature. The completion of the surface layer on both metals at 25°C was followed by the formation of bulk selenides. Extensive incorporation of the gas took place at a rate which was faster, for a given temperature and pressure, on nickel than on iron. Above 25°C the H₂Se was converted quantitatively into selenide and gaseous hydrogen. Activation energies for the incorporation process were determined as the thickness of the selenide layer increased.

The rate of the reaction depended on the first power of the H₂Se pressure. The condensation coefficient A_X increased as the activation energy E_X increased on both metals. A linear relationship existed between log A_X and E_X. The slow-step of the interaction was considered to be dissociation of the H₂Se molecule at the selenide surface and the variation in A_X is interpreted in terms of the absolute rate theory where the transition state gains mobility during selenidation. Presorbed oxygen on iron and nickel films considerably enhanced the subsequent H₂Se chemisorption at –80°C. No hydrogen evolved unless the film was heated above 150°C.