Solid state NMR study of water binding on the surface of CdS nanoparticles

CdS nanoparticles precipitated from aqueous solution were studied by H-1 NMR. The nanoparticles were deliberately not capped by any surface termination agent. The samples had porous structure. Proton high spinning spec,cd magic angle spinning (MAS) NMR spectra revealed that there are three abundant proton species in nanoparticle samples prepared with an excess of Cd, having different chemical shifts: a relatively narrow peak due to hydroxyl groups and two broader lines resulting from adsorbed water molecules with different chemical environments. The relative intensities of the lines were temperature-dependent, reflecting a complicated chemical equilibration processes. A small fraction of water molecules (similar to 5%) experiences anisotropic dynamic motion as deduced from the analysis of the dipolar sideband patterns. The main part of the spectrum was due to rapidly exchanging protons. The exchange between protons of the same type occurs on a time scale not much longer than 1 mu s. The exchange between different lines was studied by 2D exchange MAS spectroscopy. The analysis of the proton-proton correlation spectra as a function of the mixing time led us to the conclusion that the nanocrystalline surface is covered by clusters of water. The clusters are well separated from each other and are formed by protons occupying sites with the same chemical environment. The size of the pores was also estimated to be of the order of a few nanometers.