Synthesis And Growth Mechanism Of Transition And Rare Earth Metal Sulfides

It is well know that the base of property research and application is the synthesis of high quality compound. The aim of synthesis and preparation is not only to obtain the samples which are pure and well crystal, but also with the right morphology, size and assembly. In order to meet these requirements, various methods have been developted by studying the parameters carefully for optimal results. Therefore, it is very important to search for simple, green and efficiency methods to promote materials development.The transition metal trisul fides TS3 (T=Ti, Zr, Hf, Nb, Ta) of Group IVB and VB arc compounds with similar structure, chemical and physical properties. In general, they are thin fibres or rod-like, which show high anisotropism. This is due to the crystal structure, where infinite chains of MS6 (M=Zr, Hf) trigonal prisms sharing faces run parallel to the crystallographic b-axis. Neighboring chains are offset by b/2 and connected laterally into sheets through metal-chalcogen bonds. All layers are weakly held together in a two-dimensional structure only by van der Waals'forces related to (S2)2-. Therefore, they trend to form one- dimensional structure, and have chain-like and layered characters simultaneously. The transition metal disulfides of Group IVB and VB build up by strongly bonded S-T-S two-dimensional layers loosely connect to one another by weak van der Waals'forces. Thus, it can be found that both the metal trisulfides and disulfides of Group IVB and VB have a layer structure and strong anisotropism, which facilitate the process of intercalation and, therefore, reveal interesting chemical and physical properties. Then transition metal sulfides become one of the most actively researched fields.The transition metal sulfides of Group IVB and VB are diamagnetic semiconductors, and the resistance in room temperature are about 10～103Ωcm. The main applications of these compounds are the cathodes of rechargeable battery and field emission materials. As the development of the nanosciences, it is noticed that the nanomaterials influence every science areas including the areas above. The capability of the rechargeable battery can be improved, when Li ions combine and release fast with the cathodes in small size. Moreover, the low- dimensional materials of high aspect ratios and top curve radius show great field emission effect under a low voltage. Therefore, the facile synthesis of these compounds have attracted much attention in recent years.The transition metal sulfides of Group IVB and VB conventionally synthesize via vapor reaction and solvothermal route. However, most of these preparations require additional catalysts, transport media or templates. It is extremely important to control the atmosphere throughout the process to control the morphology. Ordinarily, during the CVT process:(1) fine stoichiometric compound grow under lower temperature; (2) large crystals are obtained affer long time reaction, or else nanostructure come out; (3) small temperature gradients bring high yield and slow rate of the crystal growth. Thus, it can be seen that exact parameters selected stongly influnce the purity, quality and size of the sulfides.Here, we we report the preparation and characterization of high-purity ZrS3, HfS3 and NbS3 with nanobelt structures through a facile, cost-effective, and green CVT process at 600℃for 15 and 3 days respectively with each metal and sulfur as reactant. The smooth nanobelts are 2 mm～5 mm in length and 50～120 nm in thickness. TEM images show all the ZrS3,HfS3 and NbS3 nanobelts grow along the [010] direction. In order to know the effects of thermodynamic parameters, a series of experiments with various reaction parameters were designed and conducted. During the synthetic process, reaction temperature and duration was found to be the two most important factors. Based on the temperature gradients, when the temperature below 600℃, the temperature of the cool end of quartz-glass tube was below the boiling point of the sulfur (bp 444.6℃), which means that it was easy for sulfur to be deposited there and unfavorable for nanobelts growth. When the temperature is too high, the crystal grow fast. Then the nanobelts finally become large crystal. The yield and length of nanobelts increase significantly with prolonged reaction time. In the present system, the formation of the trisuldes nanobelts can be interpreted by vapor-solid (VS) mechanism. In addition, Raman spectra at room temperature are also measured in comparison with large crystals, where a slight red shift and peak broadening are observed. Such phenomenon in nanobelts is believed to be ascribed to the phonon confinement effect as Ge nanowires. It is noteworthy that the shift and broadening of all the Raman peaks are not same, which is more obviously for the modes related to a-axis and c-axis. Because this phenomenon are more apparent with smaller crystal sizes, the result is consistent with the growth direction of nanobelts.The investigation of transition metal sulfide as field emission has been new, and the reports are few. However, they present great field emission effect, which must bring a fervency study in the future. The field emission arrays are the most importent part of the device, which are formed by millions of small one-dimensional nano emitter. Therefore, two problems, grow one-dimensional nanomaterials orderly and keep excellent conduction with the electrode, become the key point of the field emission devices.In this paper, the metal foil and sulfur are used as reactant. The one-dimensional nano sulfide grow on the foil directly, which highly decrease the resistance induced by plaster or daubing. In order to improve the activity of the reactant, the metal foil are scrubed. The tubes with reactants are evacuated and sealed, then heated at 350℃for 5 h. The product are nanowires arrays with the diameter 20～80 nm and length several micrometers. Observing the experiments under different condition, it is found that:(1) NbS3 molecules were synthesized by the reaction of Nb atoms on the foil surface and S; (2) some NbS3 molecules diffused to gas phase; (3) NbS3 molecules in vapor phase worked as the materials for the NbS3 nucleus on the surface of metal foil, and grew into nanowires; (4) as the reaction time prolonged, the nanowires grew to appropriate size, and nanowires arrays appeared. High temperature will increase the density and size of the nanowires. Then NbS2 nanowires arrays are obtained, after NbS3 nanowires arrays templates are heated under vacuum. The NbS2 nanowires retain the morphology and size of NbS3 nanowires. Otherwise, we also take some experiment to detect the influence of the scrubing, which reveals that scrubing is an important factor for both reaction active and morphology. The NbS2 film is obtained by using the metal fiol without scrubing, which is some micrometer thick and influenced by reaction temperature. Raman spectrums of all products are measured, and shifting and broaden of the Raman peaks are observed which induced by phonon confinement effect. The success of the synthesis of NbS3 and NbS2 nanowires arrays by CVT method will provide a new route to obtain grow nanomaterials orderly, which may be appled widely.High pressure and High temperature technology is widely used in physics, chemistry, material and geology. The conventional application of this technology is the synthesis of super hard materials, however, more other aspects come out, such as synthesis of other materials and the property research under high pressure. The capsule of the high pressure device is relative sealed and reductive. This can restrain the diffusion and leak of the gas, and then keep the reactant contact compactly with each other. Moreover, BN crucible could give enough space to allow the vapor product left the system, which make the reaction run toward to aim diretion. As a result the high pressure could be used to the unstable reactant avoiding the impurty induced by complex reaction. High pressure could also decrease the long extent diffusion and rate of crystal growth, then make the product nanomaterials. After quenching, nanomaterials in large bulk will be achieved.Here, we syhthesized the TiS3 nanobelt and Nd2S3 via a high pressure and high temperature process. TiS3 got under 2GPa,600℃for 60h can be indexed as monoclinic phase, which has smaller crystal lattice parameter in comparetion with the CVT ones. This indicate that the TiS3 has been compressed in molecule grade. Pressure, temperature and time have been studied. We find that the decomposion temperature of compound can be increased under high pressure. Then the TiS3 is reserved at 800℃, which decompose at 632℃. This means high pressure could extend the temperature range of sythesis. Prolonged time can make complete reaction. Differently, Nd2S3 was synthesized by Nd2O3 and S. Pure product were obtained.We synthezied transition and rare earth metal sulfides via CVT and high pressure and high temperature processes. Particular investigation of reaction parameter was conducted, and Raman spectroscopy was studies related to the product structure and size. It is expected that our work will offer references to the synthesis and preparation of nano devices and be useful for the research about application.