The bipyridine adducts of N-phenyldithiocarbamato complexes of Zn(II) and Cd(II); synthesis, spectral, thermal decomposition studies and use as precursors for ZnS and CdS nanoparticles

Highlights

• 2,2′ Bipyridyl adducts of bis(N-phenyl dithiocarbamate) complexes were synthesized and characterised.

• TGA showed that the compounds decomposed to give their respective metal sulphides as final residue.

• ZnS and CdS nanoparticles were prepared from the metal complexes.

• The synthesized particles showed blue shifted band edge.

Abstract

Bipyridine adducts of N-phenyldithiocarbamato complexes, [ML¹₂L²] (M = Cd(II), Zn(II); L¹ = N-phenyldithiocarbamate, L² = 2,2′ bipyridine), have been synthesized and characterised. The decomposition of these complexes to metal sulphides has been investigated by thermogravimetric analysis (TGA). The complexes were used as single-source precursors to synthesize MS (M = Zn, Cd) nanoparticles (NPs) passivated by hexadecyl amine (HDA). The growth of the nanoparticles was carried out at two different temperatures: 180 and 220 °C, and the optical and structural properties of the nanoparticles were studied using UV–Vis spectroscopy, photoluminescence spectroscopy (PL), transmission emission microscopy (TEM) and powdered X-ray diffraction (p-XRD). Nanoparticles, whose average diameters are 2.90 and 3.54 nm for ZnS, and 8.96 and 9.76 nm for CdS grown at 180 and 220 °C respectively, were obtained.

Introduction

Dithiocarbamates (DTC) are products of the reaction between carbon disulphide and primary or secondary amines in basic medium [1]. They possess good complexing properties, which have been attributed to the presence of two-electron donor sulphur atoms, which determine the nature of binding to metals and also the stability of the resulting complex. The complexing capacity with various metal ions resulted in dithiocarbamates being used in several areas, such as in industry, agriculture, medicine and in analytical chemistry [2], [3]. Metal dithiocarbamates, specifically the group 12 metal complexes, have been utilised in materials chemistry as precursor molecules for the synthesis of the II–VI semiconductor nanoparticles.

Most reports on the synthesis of the II–VI semiconductor nanoparticles from the dithiocarbamates involved the dithiocarbamates from the secondary amines, (N,N′-dialkyl and N-alkyl-N-phenyl dithiocarbamato complexes). There is a significant void in reported research on using the dithiocarbamates synthesized from the primary amines. The reason has been attributed to the less volatile nature, and also the chemical reactivity of the N-alkyldithiocarbamato species, which makes them less stable than the N,N dialkylated groups [4]. O’Brien et al. reported the synthesis of good quality organically capped ZnS and CdS nanocrystals, using different Zn and Cd complexes of dithiocarbamates as precursors [5], [6], [7]. Our recent reports on N-alkyl-N-phenyl dithiocarbamato complexes indicate they are good precursors for ZnS and CdS nanoparticles [8], [9], [10]. Some N-phenyl dithiocarbamato complexes of Zn(II) and Cd(II) have been reported, and their thermal properties under nitrogen atmosphere were also investigated [11], [12].

In our continued search for good precursors for the synthesis of semiconductor nanoparticles via single-source precursors, zinc and cadmium compounds obtained from the mixed complexes of N-phenyldithiocarbamato and bipyridine ligands are hereby explored as single source precursors for the synthesis of ZnS and CdS nanoparticles. Incorporation of the neutral donor ligand can improve the volatility of the complexes by removing the metal coordination sites, thus limiting the degree of aggregation of these species [13]. The phase and morphology of the resultant nanoparticles are often influenced by the precursor molecule, the solvent, as well as the thermolysis temperature [14].

ZnS and CdS nanoparticles (NPs) have been the subject of great interest due to their use in various applications such as light emitting applications [15], in solar cells [16], catalysis, and bio-imaging [17], [18] photoelectric conversion in solar cells and other optical devices [19]. Different methods of nanoparticles synthesis such as microwave irradiation [20], direct reaction of gaseous H₂S with metal cations in solutions or on solid surfaces [21], solvothermal routes [22], [23], hydrothermal routes [24], and the γ-irradiation route [25] have been reported. In an ideal synthetic route, the products should be pure, monodispersed, crystalline, and well stabilized from the surrounding chemical environment by a capping agent [26]. The use of single source precursors, containing both the metal and chalcogenide source, has proven quite effective for the synthesis of high quality nanoparticles with narrow size distribution [26], [27], [28].

In this work, we report the synthesis, spectral and thermal characterisation of the bipyridine adducts of Zn(II) and Cd(II) bis(N-phenyldithiocarbamate) complexes. The complexes were used as precursors for the synthesis of ZnS and CdS nanoparticles in the presence of hexadecyl amine as the capping group. The effect of growth temperature on the properties of the prepared nanoparticles was investigated and will be discussed.