Adsorption of mercaptobenzothiazole and similar molecules on cadmiumsulfide: A theoretical study

The structure of the adsorbate systems formed by mercaptobenzothiazole (MBT ) and analogue molecules on the CdS(1010) surface is studied quantum-chemic ally using density functional theory. Preliminary calculations of the free adsorptive molecules indicate an energetic preference of their thione form compared to the thiol form. For the anions of the adsorptive molecules, the role of the endocyclic nitrogen and the exocyclic sulfur as possible donor atoms is examined by means of known chelate complexes. Clusters with 24 an d 28 atoms that are saturated by point charges have been developed as surfa ce models. Geometry optimizations show that the structure of the adsorbate systems is dominated by the formation of two coordinative bonds from the do nor atoms of the adsorptive anions to two adjacent cadmium atoms of the sur face. It results that the molecular plane of the adsorptives is tilted with respect to the normal of the crystal face. The calculated tilt angle for t he MBT adsorbate agrees with angle-dependent XANES measurements, the only s tructural information presently available from experiment. It is found that the tilt angle changes with the variation of the heteroatom in the five-me mbered ring of the adsorptives. The molecule-surface interactions lending t o these structural differences are analyzed. Further, the relaxation of the surface is included in the investigation. It becomes obvious that the dire ction of the relaxation of the free surface is reversed by the formation of the adsorbate bonds.