Synthesis and characterization of anisotropic cadmium and lead sulfide nanostructures from single source molecular precursors and an insight to their growth mechanisms.

摘要

This thesis details the synthesis and characterization of anisotropic cadmium and lead sulfide nanostructures from single-source molecular precursors. Six new precursors were synthesized for cadmium and lead sulfide each, by the reaction of the appropriate metal acetate with picolinic (HPic), 2,6-dipicolinic (H2dipic) or salicylic acid (H2sal) followed by the addition of thiourea (th) or thiosemicarbazide (ths). The precursors for CdS are [Cd(Hsal)2(tu)2] (Cd1a), [Cd(Hsal) 2(ths)2]·nH2O (Cd1b), [Cd(pic) 2(tu)2]·0.5H2O (Cd2a), [Cd(pic) 2(ths)2]·2H2O (Cd2b), [Cd(dipic)(tu) 2] (Cd3a) and [Cd(dipic)(ths)2(H2O)]·2H 2O (Cd3b) and the precursors for PbS are [Pb(Hsal) 2(th)2] (Pb1a), [Pb(Hsal)2(ths) 2] (Pb1b), [Pb(pic)2(th)2] ( Pb2a), [Pb(pic)2(ths)2] (Pb2b), [Pb(dipic)(th)(H2O)]2·2H2O ( Pb3a) and [Pb(dipic)(ths)2]·H2O ( Pb3b). All of the compounds were characterized spectroscopically and by elemental analysis. Cd1a, Cd2a, Cd2b, Cd3a, Cd3b, Pb2b Pb3a and Pb3b formed well-defined crystals and were characterized by single crystal X-ray diffraction.;The precursors were decomposed at or around 170 °C using n-cetyltrimethylammonium bromide (CTAB), sodium dodecylsulphate (SDS), ethylenediamine, oleic acid, oleylamine, trioctylamine or hexadecylamine as surfactants. Systematic variations of surfactants gave small spherical nanoparticles, micro-sized flowers, multipods and nanorods for CdS and nanocubes, truncated nanocubes, hexapods, octahedrons and dendritic stars for PbS. From XRPD studies it was found that most of the CdS nanostructures were of the stable hexagonal phase. However, in two cases the nanostructures were found to be predominantly of a metastable orthorhombic phase. For PbS system, all the decompositions yielded pure crystalline galena. For CdS system, TEM studies revealed planar defects (such as polysynthetic and multiplet twinning) in the nanocrystals, which gave an explanation for mechanism of growth. For PbS system, in order to elucidate the effect of single source precursors on the mechanism of growth of nanoparticles, the decomposition results were compared with PbS nanostructures synthesized from multiple-source precursors, lead acetate and thiourea or thiosemicarbazide. It was found that in the reactions of multiple source precursors, acidic components in the reaction mixture (oleic acid, acetic acid) led to etching and crystal splitting, which played a crucial role in the formation of anisotropic nanostructures.