Dependence of photocatalytic activity of ZnxCd1-xS quantum dot composition

Abstract

Aqueous colloidal dispersions containing ZnxCd1-xS quantum dots (QDs) of different x compositions were prepared by precipitating zinc and cadmium acetates with sodium sulphide, in the presence of a cetyltrimethylammonium bromide stabilizer. Ultraviolet-visible absorption spectroscopy was used to determine the transition energies of the QDs, which in turn were used to calculate their sizes, which depended on their composition. The QD size decreased with increasing Zn content. The photocatalytic activity of the ZnxCd1-xS QDs was studied by the decomposition of methylene blue under ultraviolet irradiation, at a maximum intensity at 365 nm (3.4 eV). Three different photocatalytic activity regions were observed, which depended on the Zn content. The quantum levels of the QDs could be excited by incident irradiation, and influenced the resulting photocatalytic activity. Maximum photocatalytic activity was achieved at x = 0.6, where the QD transition energy was equal to the irradiation photon energy. The photocatalytic efficiency of the QDs depended on their surface area and arrangement of quantum levels, because of the quantum size effect.