Synthesis and characterization of N-alkylthiourea and N, N -dialkylthiourea complexes of Cu, Cd and Pb, and their use as single source precursors for the synthesis of metal sulfide nanoparticles
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Date
2005
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University of Zululand
Abstract
A number of complexes of various thiourea derivatives with different metals have been synthesized and characterized by a combination of spectroscopic studies, elemental analysis, melting points and X-ray crystallography. Infrared and NMR spectroscopy studies reveal the interaction between a metal and the ligands, thioureas, is mainly through bonding to sulfur. A series of cadmium(II) complexes with N-alkyl/aryl and N, N'-dialkyl/aryl thioureas (RNHCSNHR'; where R = R* = CH3, CH2CH3, CeHs and /or R' = H) have been synthesized and characterized. The structures of [CdCl3(CS(NH2)(NHCH3)3]n (I) and CdCl2(CS(NH2)NHCH2CH3)2 (II), CdCl2(CS(NH2)-NHCeHs^ (III) and CdCI2(CS(NH-CH3)2)2 (IV) were determined by single crystal X-ray methods. Complex I have a six coordinate polymeric chain with a CdCl3S3 core, which together with complex III (a monomer) gave a distorted octahedral geometry. Complexes II and IV are monomers with a distorted tetrahedral geometry around the cadmium(II) ion. The lH (nuclear magnetic resonance) NMR spectra in deuterated dimethyl sulfoxide (CD3SOCD3) at room temperature reveal the broadened NH peaks in the lower field region (6.0 - 10.0 ppm) and the presence of both syn and aw/z'-isomers for the N-alkylthioureas. All the spectral data obtained is consistent with the coordination of ligands by sulphur atom to the metal ion. Lead complexes were prepared in aqueous media to give white powder in a four coordinate form. Unlike cadmium and lead, copper gave multinuclear complexes resulting from the reduction of copper (II) to copper (I) by the thiourea ligands to form specifically tetranuclear copper complexes with thiourea behaving as the bridging ligands. Thermal behavior of these complexes both in air and under inert atmosphere gave residues after decomposition, which was predominantly metal sulfides, with little impurities. The presence of the metal sulfide was established by the XRD analysis of the residue from the thermo gravimetric analysis.
These alkyl substituted thiourea complexes of Cd, Pb and Cu were easy to prepare, stable in air, inexpensive, obtained in good yields and decompose at temperatures about 200 -220 °C, which is convenient to thermal decomposition of precursors in the high boiling solvents or the suitable capping agents to prepare surface capped metal sulfide nanoparticles. Some of the complexes have been used as single-source molecular precursors in the preparation of copper sulfide, cadmium sulfide and lead sulfide nanoparticles by a process driven by thermal decomposition. Good crystalline particles were obtained from thermolysis of the precursors in tri-n-octylphosphine oxide (TOPO) as a solvent and capping agent, at different temperatures. All nanoparticles, CdS, PbS and CuxSy, were prepared from alkylthiourea complexes, cleanly decomposing to give average size ranges 4 - 26 nm at reasonably lower temperatures. The small size of these particles is a consistent property with the observed blue shift in the band edge of their optical spectra. Photoluminescence spectra of most samples were broadened with a red shift in relation to the band edge indicating the broad size distribution. Some relative quantity of bulk particles of copper sulfide and lead sulfide were obtained from the large yield of the products as insoluble particles in hexadecylamine (HDA) and tri-n-octylphosphine oxide (TOPO). X-ray powder diffraction (powder XRD) patterns are consistent with predominantly hexagonal phase although copper sulfide showed evidence of various stoichiometric ratios. These phases were also confirmed by the images from the transmission electron microscope (TEM), although the CdS nanoparticles showed instability due to quick agglomeration in TOPO. Their patterns were confirmed by the XRD to be predominantly hexagonal. The morphology for most of the particles were close to those of thin needle-like plates, although the collapse is more of spheres which is a preferred mode of arrangement of the CdS particles when they settle on the carbon coated copper grids. High resolution transmission electron microscope (HRTEM) revealed the lattice fringes of all different types of nanoparticles including CdS, which tend to give agglomerates, hence it was difficult to determine particle sizes from the TEM images. All solution studies viz. ultraviolet-visible spectroscopy, photoluminescence and copper-coated grid-TEM were all consistent with particles in the nano-size regime. The copper and lead complexes of these thiourea derivatives have been used to prepare PbS and CuxSy, and gave crystalline, cubic or rod-shaped nanoparticles. CdS, PbS and CuxSy nanoparticles showed quantum confinement effects with a blue shift in their absorption spectra. The materials were further characterized by HRTEM images, XRD and selected area electron diffraction (SAED) patterns.
Description
Thesis submitted to the Faculty of Science and Agriculture in fulfillment for the degree, Doctor of Philosophy, in the Department of Chemistry, at the University of Zululand, 2005.
Keywords
Nanoparticles, Transition metals