Infrared spectroscopic studies of monothiol ligand adsorption on CdS nanocrystal films in aqueous solutions

Probing the surface chemistry of thiol ligand binding to cadmium chalcogenide nanocrystals is important to clarify factors involved in quantum dot stability and surface functionalization. Deposited CdS nanocrystal films have been used in this work as model quantum dot surfaces for ligand adsorption studies. The adsorption of mercaptoacetic acid, mercaptopropionic acid, and mercaptoethanol, from aqueous solution to CdS thin films, has been studied by in situ infrared spectroscopy. The absence of a S-H stretch absorption for the adsorbed species shows that adsorption occurs via the deprotonated thiol group, and the spectrum of the adsorbed carboxylic acid species closely resembles those of the solution ligands. Adsorption of mercaptoacetic acid and of mercaptopropionic acid resulted in pKa(COOH) decreases of 1.5 and 0.5, respectively. Significant changes in the spectrum of mercaptoethanol upon adsorption have been observed, but the present uncertainty in mercaptoethanol spectral interpretation does not provide structural inferences. Adsorption isotherms determined from the spectral data indicate strong thiol adsorption to CdS. The adsorption isotherms have been fitted to both Langmuir and Freundlich equations, with the latter providing a better fit. This may be attributed to a change in the probability of adsorption to vacant surface sites due to the increased CdS surface negative charge as the surface coverage increases.