| Numerous techniques have been reported to synthesize nano-sized particles, such as homogeneous precipitation, hydrothermal, pyrolysis, microemulsion, sonochemical, mechanochemical, sol-gel, and solution combustion. For these methods, extremely special conditions, long synthesis time, complex apparatus, organic solvents, or higher reactive temperature often are required. Now, we proide a simple and facilitate method to prepare nanomaterials, which has rarely been reported. This method is called "the gas-liquid chemical precipitate method", namely in a sealed vessel, the reactant or the pH regulator diffusing into the metal ion solution in the form of gas to form precipitates.In this thesis, the preparation and characterization methods are summarized firstly, and the formation mechanisms of nanoparticles and one-dimension nanostructures are proposed. Then the gas-liquid chemical precipitate method was used to synthesize several kinds of inorganic nanomaterials, such as oxides, sulfides, inorganic salts and hydrates.1,Cerium dioxide with fcc structure was formed using the gas-liquid chemical precipitation process at room temperature. TEM images showed two kinds of one-dimensional nano-structures CeO2 have been mainly obtained. CeO2 nanowires are 0.3～1.2μm in lengths and 5～20nm in diameters. CeO2 nanotubes have outer diameters in 10-40nm with lengths up to 2μm. The formation mechanism of the CeO2 crystal and its 1D nano-structures was discussed in terms of the nucleation and growth processes. The as-prepared CeO2 shows very strong UV absorption ability and the maximum absorption peak redshifts compared with that of CeO2 nanoparticles. In the room temperature PL spectrum, there appeared two emission peaks at 366nm and 400nm respectively. The latter was rarely reported, which should be resulted from the transition from defects to O 2p energy band.2,Nanosized sulfide semiconductors are at present the subjects of intense research due to their abroad applications. Various methods have been used for the preparation of sulfides, however, for the major methods, there have some limitation in practice for the use of noxious gas, e.g., H2S. In our experiments, Zn(Ac)2, CdCl2 and Pb(Ac)2 react with H2S gas in a sealed vessel respectively, which was produced by the reaction of sodium sulfide and sulfuric acid aqueous solution. The reactions happened in the sealed vessel, which made the danger of H2S gas reduced as little as possible. Using the Debye-Scherrer formula, the average crystalline sizes were determined as about 4.6 nm, 21.6 nm and 34.5 nm for ZnS, CdS and PbS, respectively. TEM images show some CdS and PbS nanoparticles prolonged to nanorods. From the UV-vis absorption spetra, it can be seen that the UV-vis absorption edges of the as-prepared ZnS, CdS and PbS nanoparticles are ca. 306 nm, 470 nm and 388 nm, which showed a very significant blue shift from the bulk materials. In the case of CdS and PbS, there were a little red-shifted in comparison to references. This should be due to the clubbed particles in the as-prepared CdS and PbS samples. In the PL spectrum of ZnS, two emission peaks centered at about 420 nm and 466.2 nm respectively appeared. The reason for the appearance of the former is worthy of further study.3,When the Pb(Ac)2 aqueous solution was added into the K2CrO4 aqueous solution, the bright yellow precipitates appeared immediately. We used HC1 and NH3 to adjust the pH value of the suspension, and found the structure, size and morphology of the initial precipitations changed. In the acid condition, when the ratio of [Pb2+]/[CrO42-] was equal to 1:1,1:5 or 1:10, the initial orthorhombic phase PbCrO4 would translate to the monoclinic phase PbCrO4. And, the smaller the ratio of [Pb2+]/[Cr42-], the larger the aspect ratios of the particles will be. In the basic condition, with the increase in the concentration of the hydroxyl ion, the products will be converted from the orthorhombic PbCrO4 to the monoclinic Pb2CrO5. The effects of reactive conditions on the size and morphology of the particles were studied. UV-Vis absorption and PL spectra are observed.4,β-Ni(OH)2 andβ-Co(OH)2 are the key materials in the preparation of cells. Because Co2+ is easily oxided to Co3+, the preparation ofβ—Co(OH)2 is difficult. Using ammonia as precipitator,β-Ni(OH)2 andβ-Co(OH)2 nanoparticles were successfully synthesized through the gas-liquid chemical precipitation process at room temperature. TEM images showed that the nanoparticle are 10nm in diameter for both samples. Because the reactive happened in a sealed vessel, when oxygen was used up, Co2+ would not be oxided to Co3+ further, so the final products were Co(OH)2.5,Nanocrystalline CeO2 powders were synthesized by the combustion reactions using citric acid and glycol as fuels and nitrate as an oxidant. The adiabatic flame temperatures in the auto ignition processes of the precursors were calculated theoretically. XRD measurements indicated that the powders produced in the combustion processes were cubic fluorite CeO2 phase. It was found that the higher the adiabatic flame temperature, the larger the crystallite size of the powders wound be. TEM images show that the powders synthesized using citric acid and citric acid -glycol as fuels are 5-10nm in diameter with good dispersion, and powders prepared using glycol as a fuel presents agglomerated particles. After calcination, the agglomeration in the powders prepared using glycol as a fuel is lessened, in the other hand, the agglomeration in the powders prepared using citric acid - glycol as fuel becomes severe. This is also be proved by the size distribution measurements. Blue shifts of the absorption peak were observed. Compared three combustion products, the samples prepared using the citric acid as fuel has the better dispersion before and after calcinations, and has better UV absorption ability. So, it is the better fuel.The gas-liquid chemical precipitation method has shown many advantages such as no need for organic solvent, low temperature (room temperature), low pollution and simple reaction technology. We have synthesized several kinds of inorganic nanomaterials using this method. However, this process should widely exist in the nature, due to the ammonia gas and sulfureted hydrogen easily produced from the decomposed bodies. The work in this thesis bring the experimental foundation to geochemistry, and a new thoughtway to prepare other kinds nanomaterials. |
PDF Full Text Request