3.1 Preparation and Characterization of Controlled Sizes of CdS and ZnS Nanoparticles
The stable ammonium salts of the dithiocarbamates 2 were obtained by the reaction of the secondary amines 1 with carbon disulfide in aqueous medium at low temperature. The zinc and cadmium complexes M-3 were obtained in high yield at room temperature by the reaction of compounds 2 and the respective metal salts in water (Scheme 1).
The complexes of zinc and cadmium M-3 were used for the synthesis of ZnS-3 and CdS-3 nanoparticles by hydrothermal method at 120°C and for 4 hrs. SEM and TEM analysis showed that the alkyl chain in dithiocarbamate complexes of zinc and cadmium systems M-3 plays a significant role in the final size and morphology of the CdS-3 and ZnS-3 nanoparticles. The morphologies of nanoparticles were studied by SEM. Observations of the SEM images of CdS-3 and ZnS-3 nanoparticles reveals that variation of the alkyl groups on the complexes of M-3 gave particles with various size and morphologies (see supporting information). When Cd-3c complex was used for the synthesis CdS nanoparticles, the CdS nanoparticles (CdS-3c) exhibit pyramid-like morphology with the hexagonal base, and the particles are fused to form aggregates (Fig. 1a). The ZnS nanoparticles (ZnS-3b) prepared by hydrothermal method of complex Zn-3b (ethyl group) exhibit cubic-like aggregate morphology (Fig. 1b). TEM analysis of nanoparticles of CdS-3c and ZnS-3b were also used to obtain direct information about the structure and morphology of nanoparticles (Fig. 1c and 1d). TEM showed that the mean diameter of CdS-3c and ZnS-3b nanoparticles is about 60 and 24 nm respectively with a good uniform size distribution. Other CdS and ZnS nanoparticles were easy agglomerated and particles morphologies were unclear (see supporting information).
The EDS analysis for the CdS-3c and ZnS-3b are shown in Figs. 2a and 2b. The elemental analysis (EDS) confirmed the presence of Cd, Zn, and S in the nanoparticles (Fig. 2).
The powder X-ray diffraction patterns for the CdS-3c and ZnS-3b are shown in Figs. 3a and 3b. CdS-3c nanoparticles show the predominant planes of hexagonal phase on their XRD pattern. ZnS-3b nanoparticles show the peaks indexed to 1 1 1, 2 2 0, and 3 1 1 on their XRD pattern. The peaks are appearing due to reflection from the (1 1 1), (2 2 0) and (3 1 1) planes of the cubic phase of the zinc blended type patterns of ZnS.
The sizes and shapes of nanoparticles influence their optical band gaps. The optical band gaps of CdS-3c and ZnS-3b nanoparticles are calculated and found 4.15 and 2.36 eV respectively (see supporting information). The obtained values of the band gap of ZnS-3b nanoparticles are higher than CdS-3c (2.36 eV). The obtained values of the band gap of other type of CdS and ZnS nanoparticles were shown in supporting information.
3.2 Photocatalytic degradation of Methylene Blue in the presence of CdS-3c and ZnS-3b
Metal sulfide nanoparticles have been used widely in photodegradation of dyes. In order to consider the photoactivity of the prepared CdS and ZnS nanoparticles, photodegradation of methylene blue (MB) in water was examined under blue LED (see experimental section). The decreasing concentration of MB in the presence of nanoparticles was studied to evaluate the activity of the CdS-3c and ZnS-3b nanoparticles. To detect the concentration of MB in the solution the absorption peak of MB at 665 nm was chosen as the monitored parameter. Figures 4a and 4b show the absorption peak significantly decreases in intensity during the reaction between MB and CdS-3c and ZnS-3b nanoparticles under LED lamp. Results showed that the MB degradation rate in the presence CdS-3c is higher than ZnS-3b nanoparticles (The MB degradation rate results of other type of CdS and ZnS nanoparticles were shown in supporting information).