Preparation and characterization of water-soluble jingle-bell-shaped silica-coated cadmium sulfide nanoparticles

Treatment of surface silanol groups with hydrophilic compounds enabled silica-coated cadmium sulfide (SiO2/CdS) nanoparticles of core-shell morphologies to be dissolved in a water/methanol mixture. Size-selective photoetching of the particles was performed by irradiation with monochromatic light at 458 nm, resulting in a blue shift of its absorption onset because of the decreasing size of the CdS. TEM analyses revealed that the SiO2 shell structure was not shrunken by photoetching and that a void space (ca. 2.3 nm) was formed between the US core and the SiO2 shell to give a jingle-bell structure. The emission spectra of photoetched particles showed the development of band-gap emission when cadmium ion (Cd2+) was added and the pH was adjusted to 10, whereas the same treatment of original particles gave no peaks assigned to band-gap emission, indicating that the SiO2 shell was sufficiently porous for small ionic species such as Cd2+ to penetrate through the shell and that the photoetched US core incorporated in the SiO2 shell had a bare surface to be covered with a cadmium hydroxide layer. With increasing shell thickness, the rate of CdS photoetching was reduced as a result of decrease in the rate of scavenging of the photogenerated electrons in US by electron acceptors such as O-2 and methyl viologen (MV2+) in solution. The structure of the SiO2 layer was varied by surface modification of the US with both 3-mercaptopropionic acid (MPA) and 3-mercaptopropyltrimethoxysilane (MPTS) followed by hydrolysis of its trimethoxysilyl group. FT-IR spectra showed that MPA molecules, but not MPTS molecules, attached to the US core surface were eliminated by photoetching of the particles, resulting in the formation of windows in the SiO2 shell. The behavior of US emission quenching suggested that MV2+ is transported through the windows to the void space inside the SiO2 shell to reach the US core.