Layer-by-layer accumulation of cadmium sulfide core-silica shell nanoparticles and size-selective photoetching to make adjustable void space between core and shell

Layer-by-layer accumulation of monolayers of silica-coated cadmium sulfide (CdS) was achieved through repeated monolayer deposition-hydrolysis cycles using US particles (average diameter: 5 nm) modified with 3-mercaptopropyltrimethoxysilane (MPTS) and glass substrates. Absorption spectroscopic analyses of the resulting yellow films revealed that each layer had almost the same thickness, the estimated density of which corresponded to ca. 66% of that for close hexagonal packing of nanoparticles. Monochromatic light irradiation at 488, 458, or 436 nm onto the film immersed in oxygen- saturated aqueous methylviologen solution caused a decrease in the size of the US core depending on the irradiation wavelength, while atomic force microscopic analyses suggested that the size of the silica shell of the immobilized nanoparticles was almost unchanged by irradiation, i.e., the immobilized silica-coated US nanoparticles had void spaces between the photoetched core and the silica shell, and their sizes could be regulated by choosing the wavelength of irradiation light. This size-selective photoetching was applied to nanoparticulate films that had been heat-treated at 473, 573 or 673 K to observe the blue-shift of the absorption edge of US to the irradiation wavelength. The amount of CdS remaining in the film after the photoetching process depended on the temperature of heat treatment. The largest amount among as-prepared and heat-treated samples was obtained at 473 K and decreased with an increase in temperature. The growth of particles, i.e., the diminution of particle number, and/or the diminution of number of separated independent shells may account for this dependence. (C) 2003 Elsevier Science B.V. All rights reserved.