Size and shape controlled synthesis of CdS and ZnS nanoparticles and their applications in photodegradation of organic dyes

Different size and shapes of CdS and ZnS nanoparticles have been synthesized, characterized and studied for photodegradation of organic dyes. In this study, effect of alkyl chain of dithiocarbamates was investigated for the size and shaped controlled CdS and ZnS nanoparticles. A thorough structural characterization of nanoparticles has been carried out by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM) images, EDS, XRD, and UV/vis spectroscopy. Results showed that the alkyl chain in dithiocarbamate complexes of zinc and cadmium systems plays a signicant role in the nal morphology of the CdS and ZnS nanoparticles. A photocatalytic degradation of UV-irradiated Methylene Blue solutions in the presence of ZnS and CdS nanoparticles was studied in order to examine the nanoparticles inuence on photodegradation rates.


Introduction
Fluorescent nanomaterials have attracted increasing attention due to their applications in visualizing biological processes, thermotherapy agents for the diagnosis of many diseases, drug delivery, and catalysts in organic transformations [1][2][3][4][5]. Amongst the various uorescent nanomaterials, CdS and ZnS nanoparticles are the rst discovered uorescent nanoparticles that extensively studied due to their unique electronic and optical properties which make them useful in optoelectronic applications [6][7][8][9][10][11].
Recent studies have demonstrated that due to high surface free energy of the CdS and ZnS nanoparticles, the particles are easily aggregated [12]. On the other hand, the optical properties of the CdS and ZnS nanoparticles can be controlled by changing their size and distribution [13]. Therefore, control of the particles sizes and their stability is an important challenge for the scientists.
Amongst the various reported methods for size and shaped controlled synthesis of CdS and ZnS nanoparticles, the use of single source precursors, such as metal complexes, containing both the sul de and the metal source has proven to be an effective method to high quality CdS and ZnS nanoparticles [14]. Cadmium or zinc dithiocarbamate complexes (dithiocarbamate anion is readily available by the reaction of amines with carbon disul de ligand) have been used as a single precursurs for the synthesis of CdS and ZnS nanoparticles [15]. Revaprasadu and co-workers studied the effect of piperidine and tetrahydroquinoline dithiocarbamate for the synthesis of CdS nanoparticles [16].
To the best our knowledge, there is no any report in the litreature for the systematic studies on the effect of various alkyl chains of dithiocarbamates in their cadmium and zinc complexes as a single precursors on the size and shape of synthesized CdS and ZnS nanoparticles. In this report, effect of alkyl chain of dithiocarbamates was investigated for the size and shaped controlled CdS and ZnS nanoparticles.
Different size and shapes of CdS and ZnS nanoparticles have been synthesized, characterized and studied for photodegradation of organic dyes.
2. Experimental 2.1 Materials: All chemicals were purchased from Merck, Fluka and Aldrich chemical companies and used without further puri cation. All yields refer to isolated products. Reactions were monitored by thinlayer chromatography (TLC) carried out on silica gel plates (SILG/UV 254, Merck) using UV light as the visualizing agent. Melting points were measured on an Electrothermal 9100 apparatus and are uncorrected. FT-IR spectra were recorded on a Bruker spectrophotometer. The NMR spectra were recorded on a Bruker AVANCE DMX400 spectrometer, operating at 400 MHz ( 1 H NMR) and 100 MHz ( 13 C NMR).

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 disul de 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 signi cant role in the nal 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) con rmed 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 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 signi cantly 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).

Conclusions
Herein we reported synthesis two different size and shapes of CdS and ZnS nanoparticles. In this study, effect of alkyl chain of dithiocarbamates was investigated for the size and shaped controlled CdS and ZnS nanoparticles. The complexes of zinc and cadmium were used for the synthesis of ZnS and CdS nanoparticles by hydrothermal method at 120°C and for 4 hrs. CdS nanoparticles (CdS-3c) exhibit pyramid-like morphology with the hexagonal base, and the ZnS nanoparticles (ZnS-3b) prepared by hydrothermal method of complex Zn-3b (ethyl group) exhibit cubic-like aggregate morphology. Other CdS and ZnS nanoparticles were easy agglomerated and particles morphologies were unclear. A photocatalytic degradation of UV-irradiated Methylene Blue solutions in the presence of ZnS and CdS nanoparticles was also studied in order to examine the nanoparticles in uence on photodegradation rates.

Declarations
Con icts of Interest: The authors declare no con ict of interest.