Photochemical shape control of cadmium sulfide nanorods coated with an amorphous silica thin layer.

The surface of cadmium sulfide nanorods was modified by 3-mercaptopropyltrimethoxysilane, followed by the hydrolysis of trimethoxysilyl groups to form a silica shell structure (SiO2/CdS[rod]). Size-selective photoetching was applied to SiO2/CdS[rod] to modify the size of the CdS rod core. The absorption spectra were blue-shifted by irradiation of monochromatic light, and finally absorption onset agreed with the wavelength of irradiation light. These facts indicated that CdS rod particles were photoetched to smaller ones until the irradiated photons were no longer absorbed by the photoetched particles and that the SiO2 shell layer surrounding the CdS rod core prevented coalescence between photoetched particles. Changes in the wavelength of irradiation light from 488 to 436 nm caused a decrease in rod width from 3.5 to 2.3 nm along with remarkable decrease in the length of rod from 14 to 4.2 nm, suggesting that the photoetching rate was dependent on the kind of crystal faces and that the photocorrosion reactions at the tips of the CdS rod, that is, on (001) and/or (001) faces, were faster than those on other faces that appeared on the sides of the rod. This technique enabled control of CdS rod shape by selecting the wavelength of irradiation light.