Comment on “In Situ and Ex Situ Infrared Studies of Nature and Structure of Thiol Monolayers Adsorbed on Cuprous Sulfide at Controlled Potential. Simulation and Experimental Results”

In situ and ex situ infrared reflection spectroscopy combined with spectral simulation were applied to the study of monolayers of ethyl xanthate (C 2 H 5 OCS 2 - ) formed at controlled potential on cuprous sulfide. Because cuprous sulfide is a semiconductor and shows low absorption in the infrared region, the reflection spectra were recorded at different incident angles and polarizations allowing full spectral characterization ofthe molecules in the adsorption layer. Detailed compositional and structural characterization is presented on the basis of the comparison of simulated and experimental spectroscopic results. Quantitative evaluation of the orientation of particular molecular groups in the adsorbed xanthate molecules and the thickness of the surface layers produced under different adsorption conditions are discussed in detail. The in situ cell configuration used in this study was tailored specifcally for the investigated system. Two major factors were considered in designing the in situ cell : the optimum of experimental sensitivity and, equally as important, the maximum confidence in the interpretation of experimental results. Spectral simulation of different in situ cell configurations enabled the determination of experimental conditions under which optical effects are less dependent on certain parameters which are difficult to precisely control in real experimental systems (uncollimated incident beam, surface roughness, real thickness of solution layer). Also, the verification of the quantitative results obtained from the in situ experiments is discussed. Three adsorption products were identified (i) cuprous ethyl xanthate complex, (ii) product(s) of surface-induced decomposition of the first adsorbed xanthate molecules, and (iii) xanthate oxidation product, dixanthogen ((C 2 H 5 OCS 2 ) 2 ), formed at very high potentials. The xanthate decomposition product is formed only at submonolayer coverage and its presence results in a low degree of structural order of the adsorbed cuprous ethyl xanthate molecules. Under conditions which favor slow adsorption kinetics (low concentration, open circuit potential) the decomposition product can be removed from the surface structure. During this process the surface diffusion and reorientation of the adsorbed xanthate molecules take place. This results in the formation of a well-ordered structure of the cuprous ethyl xanthate surface complex with the methyl group oriented toward the aqueous solution. Removal of the decomposition product and changes in the structural order of the adsorbed xanthate molecules on cuprous sulfide were observed when the sample was immersed in a low-concentration xanthate solution or into water. Under fast kinetic conditions, i.e., xanthate concentration above 1 x 10 -4 M and at potentiostatic control, the decomposition product is trapped at the interface within a multilayer structure of cuprous xanthate and cannot be easily removed. This surface structure shows a very low degree of structural order. Formation of close to monolayer coverage by cuprous xanthate complex is observed at a potential close to 0 mV, which agrees with the thermodynamic data. At lower potentials, between -60 and 0 mV (underpotential adsorption) only submonolayer coverage was produced.