Surface properties of precipitated CdS nanoparticles studied by NMR

CdS and CdS:Mn nanoparticles were studied by Cd-113 and proton NMR. Nanoparticle samples were synthesized by precipitation of CdS from an aqueous solution of Na2S and CdSO4. The nanoparticles were not deliberately capped by any surface-termination agent. The NMR spectra of CdS nanoparticles prepared with an excess of Cd consist of three frequency bands: a bulk line corresponding to the inner Cd atoms with full sulfur coordination, a broad band due to surface Cd atoms, and a sharp line of CdSO4 in solution. The assignment of these lines is accomplished by experiments on S-rich samples, by the study of the temperature dependence of the spectra, and by comparing the chemical shift values of the lines with tabulated chemical shift values of different cadmium compounds. A significant amount of water was found in the samples. This water is trapped in microscopic pores resulting in a lowering of the water-ice phase transition and a significant shortening of the proton T-1 and T-2 relaxation times. The NMR spectra of CdS:Mn do not show any additional spectral structure. Mn2+ ions remain in the included water and/or are located at the nanocrystallite surface and do not appear to be incorporated inside the nanoparticles.