Synthesis And Characterization Of Low-dimensional Bismuth Using Chemical Methods

In recent years, much attention has been paid to nanomaterials because it's unique physical and chemical properties. Nanomaterials, based on its nano-scale particle size, have surface effect, volume effect, quantum size effect and macroscopic quantum tunneling effect, which are widely used in building materials, ceramic materials, insulation materials, sensing materials and other fields.Semi-metal bismuth (Bi) is a kind of heavy particle, which has prospective application in thermoelectric field because it's unique property of phonon scattering ability and low thermal conductivity. Furthermore, Bi has the biggest diamagnetism, the smallest thermal conductivity (except Hg) and the biggest resistance among all metal materials; a strong tendency to anisotropic Fermi surface; lower carrier concentration and move fast; the smallest effective electron mass (about 9.11×10-34 kg), long free path and the largest hall effect. All those properties make bismuth the ideal material for the investigation of quantum confinement effect, volume effect and giant magnetoresistance. Bismuth nano-materials is also a magnetic material and thermoelectric conversion material because its magnetoresistance and thermoelectric effect. Therefore, the preparation and characterization of low-dimensional semi-metal bismuth were selected as the subject of the dissertation.In our study, nanospheres, nanobelts, onion ball structure and nanobelts on silicon substrate, thin film of bismuth were prepared via hydrothermal, solvothermal and spin coating-reduction method. XRD,SEM,EDS,TEM,HRTEM,I-V measurement were used to characterize the nano bismuth.Experimental results and conclusions of the study are listed as follows:1. Preparation and characterization of bismuth nanoparticles. The nanoparticles were prepared via solvothermal synthesis method. Put the bismuth nitrate solution, which were used as precursor, into autoclave under 180℃. Bismuth nanoparticles with different morphologies were obtained at different reaction time. Constitution and morphology of the as prepared sample were characterized by XRD and transmission scanning electron microscopy. Results showed that 12 h for reaction time is a nodal point for the formation of Bi nanoparticles in low concentration of the precursor solution. Crystallinity of nanoparticles increased gradually as the reaction time increases. Along with increasing concentation of the precursor solution, the concentration of growth unit was also increased, thereby the radius of nanoparticles became larger. The nanobelts structure showed regular geometric shape when the precursor solution concentration was bigger than lOmmol/L.Ⅰ-Ⅴcurve measurement on the nanobelts showed the resistivity was 1.58×10-3Ωm, an order magnitude larger than that of the bulk bismuth. This was because the size effect of Bi nanobelts and the result showed the transforming trend of the bismuth from metal to semiconductor. It indicated that Bi nanomaterials showed different physical properties with the bulk bismuth along with the decreasing size.2. Preparation and characterization of Bi "onion ball" structure. Hydrothermal method was used to prepare the Bi "onion ball" structure. Bismuth nitrate suspension, (111) oriented silicon substrate, overdosed ammonium hydroxide and deionized water were put into autoclave under 180℃. Experimental samples were recovered at different reaction time. We can find that the quantity of nanoparticles was increased with increasing reaction time. Based on the corrosion pits in the silicon substrate, it can be inferred that silicon atoms of the substrate was involved in the formation of nanospheres. XRD measurement of samples recovered at 12 h reaction time showed that the hexagonal Bi nanoparticles are prepared. HETEM image of crystalline fracture showed the Bi nanospheres had layered structure just like the structure of "onion ball". So we can conclude the growth mechanism of Bi "onion ball" as follows:In the early stage of the reaction, ammonium hydroxide dissociated into ammonia ions and hydroxide ions; the hydroxide ions may react with silicon atom in substrate rapidly and generated silicon hydroxide. The silicon hydroxide is extremely active and easy to dissolve, so the atom on the surface of Si substrate will be "dissolved'gradually, formed the etch pits aforementioned and released hydrogen. The hydrogen atom can reduce the bismuth hydroxide adsorbed on the substrate surface under a special temperature to form bismuth, the structure was constructed around the bismuth core and the "onion ball" layer structure was thereby formed eventually.3. Preparation and characterization of Bi nanobelts on silicon. The Bi-nanobelts on silicon were prepared via hydrothermal method in autoclave under 180℃. Bismuth hydroxide was used as the raw material and the Bi nanobelts were synthesized on root position of silicon substrate surface. Linear nano-structure with length longer than 60 nm was observed on the surface of silicon substrate. Some nanobelts were found embedded into silicon, while the others embedded on the silicon surface.Root position Growth Mechanism was proposed to explain the growth of Bi nanobelts on silicon substrate. When the system was heated, bismuth hydroxide and silicon atoms reacted on the surface of the wafer, formed silicon hydroxide and bismuth.Bi core were thereby formed gradually. (100) then gradually appeared as the preferred plane on the silicon surface. The front tip of (100) nanobelts would grow perpendicularlly in the surface of silicon before it bent and formed free end when it encountered obstacles. The free end would then separate from the Si wafer. When the growth plane (100) faces was parallel to the silicon wafer, the nanobelts grow-freely from the silicon wafer because of the original angel with the plane. Furthermore, we also measured the I-V curve of the three-layer Bi nanobelts, the resistance is 2×104Ω, corresponding to the resistivity of 0.27×10-3Ωm, larger than that of the bulk bismuth 0.12×10-3Ωm.4. Preparation and characterization of bismuth nanofilms. Spin coating- reduction method were used to prepare the Bi nanofilms. Bismuth nitrate solution was dissolved into propylene glycol to obtain the precursor solution. Precursor solution was spinned on to a silicon substrate and then heated up to 200℃to prepare precursor thin film. The precursor film was then reduced in a hygrazine circumstance under 230℃for 24 h to prepare bismuth nanofilms. According to scanning electron microscope image, along with the increasing times of spin coating-reduction nanoparticles spreaded on the silicon substrate surface gradually and eventually covered the surface of the substrate. Uniform nanofilms were thereby prepared. XRD results indicated that (001) is the preferred orientation of the thin film. Diffraction transmission electron microscopy images showed that the samples were of the hexagonal structure. I-V curve measurement showed the the resistivity of the sample was 0.46×10-4Ωm, larger than that of the bulk bismuth.