Solvothermal Synthesis And Characterization Of Transition-metal Nitrides, Phosphides And Sulfides Nanocrystals

In this thesis, we first summarized the structure, character, application and preparation, especially introduced the history of hydrothermal/solvothermal methods of nanomaterials and the recent work in the field of the transition metal nitrides, phosphides and sulfides. Based on this, several transition metal nitrides, phosphides and sulfides were synthesized successfully by a simple mix-solvents sovothermal method, and their phase, morphology, optics and magnetism were characterized. The research conclusions provided some original and innovative results, and the major contents can be summarized as follows:1. Transition-metal nitrides attract much attention in the basic study and the application due to their special physical and chemical property.The novel solvothermal method was successfully developed to prepare iron nitride nanocrystals, using NaN₃ and NH₄Cl as mix-nitrogen source. The structure and properties of as-prepared nitride nanocrystals were also studied. Iron nitride nanocrystal was synthesized at 400°C on the reaction of FeCl₂·4H₂O (without treatment), NaN₃, NH₄Cl and metallic Na. XRD, TEM, HRTEM and EDS all were used to investigate the structure of as-prepared sample. The results show that iron nitride has two kinds of morphologies: one is the pure cube-like iron nitride, the other is core-shell structured nanocrystal (the core is iron nitride and the shell is some amorphous iron carbide). It is well known that iron nitride is a magnetic material and their magnetism property was measured. It is found that the maximal saturation density is much less than that of the reported values due to the existence of amorphous shell. Compared to direct nitridation reaction by NH₃ or N₂, iron source needn't be pre-treated and the reaction temperature is lower via the present approach.2. Transition metal phosphides are becoming a main study topic in new catalytic material field due to their similar properties to transition metal nitrides and carbides as well as their excellent reaction activity and catalytic selectivity for deep hydrodesulfurization (HDS) and hydrodenitrogenation (HDN). In this paper, a series of transition-metal phosphides were prepared using yellow phosphorous as P-source and the major contents can been summarized as follows:First, based on the disproportionation of elemental phosphorus in glycol/water, Ni₂P and Ni₁₂P₅ minicrystals were synthesized successfully at 160-180℃, using NiSO₄·6H₂O and yellow phosphorous as the precursor. XRD, TEM, HRTEM, SEM and SAED all proved that the hexagonal phase Ni₂P and the tetragonal phase Ni₁₂P₅ nanocrystals can be selectively synthesized and Ni₂P display spherical and dendrite-like morphology, Mi₁₂P₅ were composed of nanospheres. Based on the observation of TEM, the growth process of Ni₂P nanocrystals from nanospheres to dendrites was also studied. It is considered that a special solid-liquid-solid translation process and an Ostwald ripe process might responsible for the transition process. In addition, the optical property of nickel phosphides was also exhibited. In our experiment, the photo luminescence (PL) spectra were also exhibited. The PL spectra results show that the position of the emission peak of Ni₂P is close to that of Ni₁₂P₅, but the intensity is much weaker than that of Mi₁₂P₅.Similarly, based on the disproportionation of elemental phosphorus, Co₂P nanorods or nanoflowers were synthesized by a polymer-assistanted hydrothermal route, using cobalt chloride (CoCl₂·6H₂O) and yellow phosphorous as reagents. By controlling the temperature and the time of the system with the polymer surfactant, Co₂P nanorods and nanoflowers were synthesized at 190 and 220℃. The average diameter of these nanorods is about 18nm and the length is about 100-200nm. Then, the growth process was also studied, the results show that the smaller nanoparticles first formed and later aggregated into nanorods and nanoflowers due to the surface energy and the surfactant polyaerylamide (PAM).Next, based on the reactions between copper sulfate (CuSO₄·5H₂O) and yellow phosphorous, copper phosphides nanocrystals were prepared in the mixed solvent (glycol, ethanol and water) at 140-180℃. After the analyses of XRD, TEM, SAED, EDS and HRTEM on the as-prepared Cu₃P sample, the results show that as-prepared sample is the hexagonal phase and polycrystal. When the temperature is 140℃, the sample is some solid particles, but the yield of hollow spherical Cu₃P nanocrystals increased gradually with the increment of the temperature and the time, which may be related to the increased gas bubbles produced in the reaction system. So the formation process of hollow spheroid and possible mechanism were also discussed. Fourthly, tetragonal phase Cd₃P₂ minicrystal was synthesized in the mixed solvent (glycol and water), using CdCl₂·6H₂O and yellow phosphorous as the precursors. Under the TEM observations, it is found that the sample consists of a lot of the aggregated polygons. But SEM exhibits some flower-like microcrystals consisted of some small polyhedrons. Furthermore, the experimental parameters were studied and the results show that the temperature and time affects the morphology less between 140 and 180 °C, while the solvents have much influence. In addition, the fluorescence spectrum of as-prepared sample was also measured.3. It is well known that transition-metal sulfides have special photoelectric properties and they are widely applied semiconductor materials. Based on the complex solvothermal route, some metal sulfides as follows were synthesized by selecting the appropriate S-source and metal salts.First, NiS nanospheres and nanowires were prepared successfully, using nickel sulfate (NiSO₄·6H₂O) and thiourea as the precursors. By controlling the volume proportion between two components of the mixed solvents, the transformation between two phases(r-NiS and h-NiS) can be achieved. The morphologies of these two nickel sulfides nanocrystals are also different. The r-NiS nanocrystals are composed of some nanospheres with "sticks", while the h-NiS nanocrystals have two morphologies: nanosphers and nanowires. The diameter of nanowires is close to that of nanospheres with length of several to tens of micrometers. Based on the TEM observations their possible formation process was also discussed. In addition, the PL spectrum indicated that the wavelengths of the strong emission peaks for each nickel sulfide are different when being excitated at different wavelengths and the fluorescence intensity of h-NiS is always higher than that of r-NiS.Next, CoS_1.097 and Fe₃S₄ nanocrystals with irregular morphology were also synthesized successively in the mixed solvent (glycol and water) at 180 °C, using Co(NO)₃·6H₂O and yellow phosphorous as the precursor. The as-prepared sample is hexagonal phase and cubic phase, respectively. The diameter of CoS_1.097 is about several hundreds of nanometers, and that of Fe₃S₄ is only about tens of nanometers. For the solvothermal system, the solvent always affect the phase and the morphology much. So these parameters were also investigated. The results show that the solvent affect the product much when other parameter unchanged. On the other side, it is observed that CoS_1.097 has strong fluorescence intensity. Two emission peaks of cobalt sulfide change to one with the increment of the excision wavelength and the fluorescence intensity decreases too.