Studies On Synthesis, Morphology And Properties Of The Bimetallic Sulfides

Because of its rich composition, varied spatial structure, and unique optical, electrical, magnetic and other properties, bimetallic sulfides have caused the wide interest of scientists. The study of bimetallic sulfides always was an integral part of sulfur chemistry. In recent years, experimental design, morphology, structure and chemical properties of bimetallic sulfides are one of the very active research areas. It has gone through years of change, and now entered a new era, namely, from the simple synthesis to the new phase of structure control and directional assembly. Relative to the traditional functional materials, such as ZnS and PbS, one more metal atom in bimetallic sulfides will enrich connections between atoms, and lead to the diversification of the structure, further give various electrical, magnetic and other properties. In addition, the research of morphology, especially on nano-size, has been a popular topic in materials science, since the morphology, size, surface condition and other changes will affect the physical and chemical properties of the materials. In summary, closely around the features, experimental design of the new bimetallic sulfide and assembly of controlled morphology, now are main task about bimetallic sulfide. This paper selects two representative bimetallic sulfides system of Cu-Ta-S and misfit-layered sulfides to study. 1. Cu-Ta-S system is an important topic of inorganic materials. CuTaS3 is a typical Cu-Ta-S compound. However, the presence of the unusually short Cu-Ta bond (～2.8 A) and still some lack of knowledge about the electronic properties, make the valence descriptions to assign either (d0)Ta5+ and (d10)Cu+ or (d1)Ta4+ and (d9)Cu2+Here we report the synthesis of high-purity needle-like CuTaS3 single crystals with a chemical vapor transport (CVT) process. The investigation of crystallographic data shows the special double chains of edge-sharing TaS6 octahedra. For the first time, the detailed XPS analysis of CuTaS3 was undertaken and the classical oxidation states can be described by Cu+ Ta5+ 3S2-. In addition, Raman spectra of CuTaS3 excited with different excitation wavelengths have been obtained. Although the information about vibrational modes were discussed by factor group analysis in terms of structure characteristics, further lattice dynamic investigations are obligatory to make precise assignments for the Raman modes. The thermochemistry research of needle-like CuTaS3 shows that CuTaS3 could retain stably at 420℃in atmosphere. Therefore, CuTaS3 will be applied widely in insulator, adiabatic materials and electronic devices, because of its large size, well crystallization and high chemical stability.2. Misfit-layered compounds (MS)x(NbS2)m and (MS)x(TaS2)m (M=Sn, Pb, Bi) are the pioneers in early research due to their application as superconduct and lubricate materials. Previous studies showed that Pb-Nb-S, Bi-Nb-S system appears anomalous tubular morphology, but the exact cause is not conclusive. This paper selects (PbS)1.14NbS2 as the main object of observation, while (PbS)1.14(NbS2)2, (PbS)1.13TaS2 and (SnS)1.17NbS2 as the subordinate ones.Here, we report the preparation and characterization of curl-like (PbS)1.14NbS2 and (PbS)1.14(NbS2)2 through a CVT process at 750℃and 850℃respectively for 7 days with each metal and sulfur as reactants. The morphology of curl-like (PbS)1.14NbS2 and (PbS)1.14(NbS2)2 was first discussed during chemical reaction. The parameters of the reaction, such as temperature, time and ratio of reactants, were studied to understand the effects of crystal growth. Higher temperature (>900℃) was unfavorably for curl-like products. Curl-like products were obtained with more sulfur, although too much sulfur could lead to explosion failures. In addition, the CVT experiments of Pb-Nb-S system, show that the stage-two (PbS)1.14(NbS2)2is prior to generate curl-like products than the stage-one (PbS)1.14NbS2. The XRD, XPS, Raman and other tests were undertaken for the flake and tubular (PbS)1.14NbS2, indicating that the two kinds morphology of (PbS)1.14NbS2 are different in the electronic structure, crystal structure and so on.For first time, the curled morphology was found in Sn-Nb-S and Pb-Ta-S systems. At 750℃, curled-like products obtained in Pb-Ta-S system, which were inexistent at lower or higher temperature. The curled-like Sn1.17NbS3.17 came out at 850℃, and disappeared at 900℃The observations of the growth nuclei suggests a formation and growth mechanism based on the spontaneous curling up of thin triangular flakes as a consequence of the lattice parameter difference between the constituent subsystems.3. The traditional synthesis method for misfit-layered sulfides (MS)x(TS2)m is CVT method, and the general conditions for reaction are:heating temperature of 700-1000℃and reaction time of 7-30 days. In recent years, high pressure and high temperature technology is widely used in physics, chemistry, material and geology.High pressure and high temperature technology has made remarkable achievements, but the direct synthesis of misfit-layered compounds under high pressure is poor. The bond lengths and layer structure of misfit-layered sulfides must be affected by pressure changes, to fit the value of x. In this thesis, (PbS)1.18TiS2 and (PbS)1.18(TiS2)2 were obtained under high pressure in Quick-press 3.0 piston-cylinder equipment and its structure and composition were preliminarily studied. (PbS)1.18TiS2 was got under 1 GPa and 800℃for 18h and (PbS)1.18 (TiS2)2 was got under 1.5 GPa and 850℃for 12 h. The two phases obtained under high pressure have smaller crystal lattice parameters in comparetion with the CVT ones. This indicates that the misfit-layered sulfides have been compressed in molecule grade. Pressure, temperature and time have been studied.Raman spectra of (PbS)1.18TiS2 and (PbS)1.18(TiS2)2 excited with different excitation wavelengths have been obtained. And the information about vibrational modes were discussed by factor group analysis in terms of structure characteristics. Thermogravimetric analysis was used to study the thermal stability of the recovered sample. (PbS)1.18TiS2 sample exhibited an oxidation resistance in air up to 450℃.4. The sulfide nanostructures can be fabricated directly on metal substrates via a simple gas-solid route. The defining characteristic of this growth strategy is that metal acts both as the substrate and the metal source for growing of the nanostructures. The sulfide nanostructures are grown in-situ and adhered on the metal substrate without using templates or catalysts. The assembly of nanostructures on the underlying substrate has the virtues of simplicity, efficiency, and low cost. In this thesis, Ta foils and Nb foils were respectively used as the Ta and Nb source as well as substrate for the growth of Cuo.65TaS2 hexagonal nanoplates and (PbS) 1.14NbS2 nanobelts.To preparation of Cuo.65TaS2 hexagonal nanoplates, the needle-like CuTaS3 single crystals were raw materials. The tubes with reactants are evacuated and sealed, then heated at 800℃for 8 h. The products are hexagonal nanoplates with a border length of～400 nm and thickness range 20-30 nm.To preparation of (PbS)1.14NbS2 nanobelts, the metal substrates Nb foils is the only source of Nb. The tubes with reactants are evacuated and sealed, then heated at 700℃for 5 h. The products are nanobelts with a thickness of 40-80 nm, a width of 100-200 nm, and the length of 15μm. In summary, we have demonstrated three effective approachs of vapor transport method, high pressure method and metal substrate CVT method, for the preparation of needle-like CuTaS3 single crystal, tubular (PbS)1.14(NbS2)m(m=1,2) and (PbS)1.13TaS2, Cu0.65TaS2 hexagonal nanoplates, (PbS)1.14NbS2 nanobelts. We also studied the structure, morphology and optical properties of products. In conclusion, our works provide examples and information to the research of bimetallic sulfides, and develop the application of the high pressure and high temperature and CVT methods in materials synthesis.