Surface chemistry of sulphides. Part 1.Infra-red study of molybdenum and germanium sulphides and of their reaction with H₂, H₂O, thiophene and ethanethiol

Abstract

SH stretching vibrations on the surfaces of reduced GeS_x(x⋍1.4) and MoS₂ have been observed in the infra-red domain of 2500-2700 cm⁻¹. In GeS_x, a single broad band at 2575 cm⁻¹ shifts towards higher frequencies on heating to 200°C. In MoS₂ two bands are observed at 2640 and 2500 cm⁻¹; they decrease in intensity at increasing temperature. Chemisorption of an electron donor, such as H₂O, on reduced GeS_x produces a doubling of the SH band, with a high-frequency component above 2700 cm⁻¹ and a low-frequency component around 2500 cm⁻¹. The same observation has been made by adsorbing thiophene below 200°C. The study of the CH stretching region permits the assignment of the infra-red features in this temperature range to 2-point (or 4-point) attachment of thiophene, and eventually, to butene attached by the double bond. Above 200°C, in GeS_x and from 20-400°C on MoS₂, the infra-red spectra show the transformation of chemisorbed thiophene into 1- or 2-butene and into acetylenic and vinyledenic radicals. The same radicals are observed on adsorbing ethanethiol on GeS_x. At low temperature, however, ethyl surface groups are probably more abundant. Conductivity measurements performed during the chemisorption of thiophene on GeS_x support the conclusion that the adsorbate transfers electrons to the solid at low temperature.