Molecular Precursors for CdS Nanoparticles: Synthesis and Characterization of Carboxylate−Thiourea or −Thiosemicarbazide Cadmium Complexes and Their Decomposition

Abstract

The reactions of cadmium acetate with picolinic (Hpic), 2,6-dipicolinic (H2pydc), or salicylic acid (H2sal) followed by the addition of thiourea (tu) or thiosemicarbazide (ths) yielded six new coordination complexes, including [Cd(Hsal)2(tu)2] (1a), [Cd(pic)2(tu)2]·0.5H2O (2a), [Cd(pic)2(ths)2]·2H2O (2b), [Cd(pydc)(tu)2] (3a), and [Cd(pydc)(ths)2(H2O)]·2H2O (3b). All of the compounds were characterized spectroscopically and by elemental analysis. Compounds 1a, 2a, 2b, 3a, and 3b formed well-defined crystals and were further characterized by single-crystal X-ray diffraction. Additionally, reaction of Cd(Hsal)2 with ths produced a compound 1b of uncertain composition that had bound ths as evidenced by ESI-MS. The most likely formula for this compound was Cd(sal)(ths)x, but Cd(Hsal)2(ths)x was also possible. In compounds 1a and 3a, the Cd(II) ion exhibits penta-coordinated geometry; in 1a, three carboxylate oxygen atoms and two sulfur atoms from thiourea complete the coordination sphere, whereas in 3a, two carboxylate oxygen atoms, one pyridyl nitrogen atom and two sulfur atoms from thiourea are within the coordination sphere. In 2a and 2b, the Cd(II) ions are hexa-coordinated, binding to two carboxyl oxygen atoms, two pyridyl nitrogen atoms, and two sulfur atoms in both cases. In 3b, the thiosemicarbazide acts as a chelating ligand, coordinating through both the sulfur atom and the primary nitrogen atom of NHNH2 group. In addition, two carboxylate oxygen atoms, one pyridyl nitrogen atom, and one water molecule are within the coordination sphere making the central Cd(II) ion hepta-coordinated. Although the complexes appeared to be slightly photosensitive they are stable under ambient conditions. The precursors were decomposed at 175 °C using N-cetyl trimethylammonium bromide, ethylenediamine, oleylamine, or hexadecylamine as surfactants. The CdS nanoparticles were explored by scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray powder diffraction (XRPD), and energy-dispersive spectroscopy (EDS). Solvothermal decompositions using CTAB in an aqueous solution gave rise to microsized flowerlike crystals; long-chain organic amines such as oleylamine or hexadecylamine led to spherical or ellipsoidal nanoparticles, ethylenediamine produced nanorods, and a mixed surfactant of tri-n-octylphosphine and oleylamine produced multipod structures. TEM studies revealed planar defects (such as polysynthetic and multiplet twinning) in the nanocrystals, which gives an explanation for mechanism of growth. Interestingly, linear arms that arise from polypodal branching may not necessarily be a single phase. Even straight arms may exhibit highly regular polysynthetic twinning. From XRPD studies, it was found that most of the nanostructures were of the stable hexagonal phase. However, in two cases, the nanostructures were found to be predominantly of a metastable orthorhombic phase. Variation of precursor slightly affected the morphology of the decomposition product.