Thiourea-urea metal (Cd & Ni) chalcogenide (O & S) complexes for the synthesis and characterization of metal chalcogenide nanoparticles

Abstract

The understanding of the fundamental properties and potential use of semiconductor materials in nanotechnology has stimulated the interest of many researchers. Coordination compounds containing ligands with chalcogenide atoms as donors have received considerable attention. Among these chalcogenide ligands, thiourea and urea have been extensively used previously to form single source precursors for the synthesis of group II–VI semiconductor nanoparticles. The synthesis and study of semiconductor nanocrystals has become a subject area of considerable research interest because they have potentially useful applications such as biomedical imaging, sensing, light-emitting diodes (LEDs), photovoltaics and displays.

In this work, special attention has been given to the synthesis and characterization of cadmium and nickel chalcogenides nanoparticles because of their interesting and unique optical properties, using a single source precursor method. The mixed chalcogenide sources are also explored for their complexes or simple reactions to produce semiconductor nanoparticles. The advantages of single molecular precursors over other existing methods for the synthesis of metal chalcogenide nanoparticles has proven to be a more efficient route for the synthesis of high-quality nanocrystals. Cadmium and nickel complexes of urea and thiourea were successfully synthesized by refluxing metal salts of cadmium and nickel with thiourea and urea at 30-40 °C for an hour and the complex mixture was cooled at room temperature. The synthesized complexes were washed with methanol and acetone to remove impurities and dried in air. All other complexes were synthesized using the same refluxing synthesis method mentioned above. The characterization of all complexes was done using Fourier-Transform infrared spectroscopy, thermogravimetric analysis and CARLO ERBA elemental analysis. Nickel thiourea and cadmium thiourea complexes were found to be coordinating with the center metal through a sulfur atom, and the urea complexes through an oxygen atom. This observation of the coordination of metals with ligands makes these complexes suitable for use as the single source precursor for the synthesis of metal chalcogenides nanoparticles. The six resulted complexes from above synthesis are of cadmium sulphide, cadmium oxide, and nickel sulphide, nickel oxide. The TGA showed that all the complexes were stable at room temperature, as they decomposed around 200 °C, which makes these complexes suitable to be used for the synthesis of nanoparticles.

The synthesized complexes reported in this study were used as a single source molecular precursor in the preparation of cadmium oxide, cadmium sulfide, nickel oxide, and nickel sulfide nanoparticles. The precursors were thermalized under nitrogen gas while refluxing at 160 °C for 1 hour, using HDA as the capping agent and TOP was used as the solvent. Crystalline semiconducting nanoparticles were obtained as the end product for all complexes, the resulting nanoparticles were washed with acetone to remove any impurities. The UV-Vis spectra of all the nanoparticles were blue-shifted, with their PL spectra red-shifted from the maximum absorption peak due to change in size of particles from bulk to nano-size. XRD pattern of CdS nanoparticles from Cd-thiourea suggested that a hexagonal phase of CdS was formed, and TEM analysis showed large particle sizes that were polydispersed with the dominance of cubic and rod shaped particles. The XRD patterns of CdS nanoparticles from cadmium thiourea and urea mixture showed the mixture of hexagonal and cubic phase nanoparticles with the predominance of cubic phase and its TEM images shows small particles size ranging from less than 50 nm, and the particles were polydispersed with the predominance of spherical nanoparticles. The XRD pattern of CdO nanoparticles showed the cubic phase nanoparticles with the existence of broad peaks indicating small particle size distribution. The TEM images of CdO nanoparticles confirmed the XRD data showing small particle size distribution with a size average of 6.8 nm. The NiS nanoparticles synthesized from Ni-thiourea complex showed narrow peaks with hkl indices indicating hexagonal phase. Particles has no clear morphology due to the agglomeration of the nanoparticles that can be caused by the instability of the nanoparticles because of their high surface area. A cluster of particles can be observed from the TEM images, making it difficult to determine the particle size and shape of NiS particles from Ni-thiourea complex. NiS particles from nickel thiourea and urea mixture also showed XRD patterns of the hexagonal plane and TEM showed small size conjugated nanocrystals. The TEM of NiO indicated a spherical morphology, existence of other morphology rather than spherical can also be observed from the image. The average particle size was 8 nm, and XRD pattern showed the cubic phase of NiO nanocrystals.