UV absorption and photoluminescence of nanometer-sized ZnS prepared by different routes

Nanosized ZnS passivated by chitosan was successfully synthesized by coordination transformation method through solution route (chitosan-Zn²⁺ complex solution co-precipitated with Na₂S solution) and film route (chitosan-Zn²⁺ complex film immersed in Na₂S solution). UV absorption of nanosized ZnS showed a clear blue-shift of the onset wavelength for both kinds of the samples while the shift amount through film route is bigger than that through solution route. Dimension and the energy gap of samples by either route depended slightly on the ratio of nitrogen atoms within the chitosan to Zn ions over the experimental range. Excitation-emission spectrum manifested that the emission spectra of nanosized ZnS samples prepared by both routes have shown broad emission bands, while the emission peaks of the samples by solution route are more complicated than those by film route. The emission mechanisms concerning to the peaks found in the samples by solution route were suggested to excitons energy level, trap-hole energy level generated by S vacancy and Cl^-impurity energy level, respectively. However, for the samples of nanosized ZnS prepared by film route, only single but broad and strong emission peak at 406nm was observed, corresponding to S vacancy energy level.