One-dimensional Nanomaterials-PbWO₄, ZnO And CuO: Synthesis And Property Study

Because of their unique physical and chemical properties, nano-materials have awide application on the areas such as biomedicine, new energy, information technology,environmental protection, which is showing some great advantages. Since thecrystalline structures and morphologies of nano-materials will have a vital influence ontheir properties and applications, it is very important to transform the structures andmorphologies purposely, characterize the transformation process and investigate theinfluence of the transformation to the properties. In this thesis, three differentone-dimensional nano-materials—PbWO₄, ZnO and CuO were successfully synthesized,and their crystalline structures or morphologies were changed, whose influence to theproperties was investigated.The research contents and conclusions of this thesis include the following aspects:1) Using Na₂WO₄·2H₂O and Pb（NO₃）₂as the reactants, with the ammonia to tunethe PH of the reaction solution, PbWO₄nano-materials of raspite structure weresuccessfully synthesized, through the Composite-Molten-Salt （CMS） method, bykeeping the solution at160℃for24h. The as-synthesized materials are nanobelts grownalong [010] direction, with the width of0.5³μm and the length of hundreds of microns.Through the analysis of the growth mechanism, we think that ammonia is the chieffactor that enable the isotropic growth along [010] direction. In composite-molten-salt,Pb（NO₃）₂and NH3·H₂O will firstly react to get Pb（OH）₂seed, then Pb（OH）₂will reactwith WO₄2-to create the PbWO₄nucleuses, and produce OH-at the same time. Becausethe （010） surface of PbWO₄has the largest surface energy,（010） surface willpreferentially absorb OH-to lower the surface energy, and the absorbed OH-will reactwith Pb（NO₃）₂in the system. This growth process will repeat continuously, and finallyget the one-dimensional nanobelts grown along [010] direction. Using the in-situtransmission electron microscope （TEM）, the phase transformation of PbWO₄wasinvestigated. The as-synthesized PbWO₄nanobelts were inserted into TEM, and heatedin an in-situ manner. The change of the selected-area electron diffraction （SAED）pattern with the change of temperature was investigated. Heated until538℃, the changeof the diffraction pattern was observed for the first time. After indexing the SAEDpatterns, it is found that Raspite of the monoclinic structure has transformed to Scheeliteof tetrahedral structure; when the sample was heated to618℃, the sample transformed from single crystal nanobelts to those composed of many small nanorods. Through theanalysis of the nanorods’ composition and structure, the nanobelts are found to be WO₃of tetrahedral structure, which were formed because of the evaporation of Pb during theheating process. Such experiment and analysis has been carried on30nanobelts, andafter the statistic of the results, it is found that the transformation of PbWO₃crystalfrom Raspite to Scheelite is irreversible, and there exist three phase transformationrelationships:1) a_r=-b_s+c_s, b_r=c_s, c_r=b_s;2)（100）_s//（100）_r, c_s//（011）_r;3)（11|-0）_s//（001）_r,[（11）|-1]_s//[011|-]_r2) Highly oriented ZnO nanowire arrays have been synthesized using hydrothermalmethod. When the ZnO nanowire array is irradiated by a short KrF laser pulse（wavelength=248nm, pulse time=25ns）, the tips of the nanowires will reshape intospheres. After the characterization using SEM, TEM, HRTEM and EDS, it is found thatthe tips are highly crystallized ZnO hollow spheres, with an even diameter of²00nm,and the shell thickness of40nm. We analyzed the formation mechanism of the hollowZnO nanosphere and investigated the influence of laser intensity and target material tothe morphology after the laser ablation. It is found that both the suitable laser energydensity and discrete, highly-oriented ZnO nanowire are necessary for the formation ofhollow ZnO nanospheres. Because the tips of ZnO nanowires are decorated by hollowZnO nanosphere, and the top of the spheres are c-plane, which is extremely small, thetotal surface energy of ZnO nanowire arrays are reduced, which makes the surfacechange from super-hydrophobic to super-hydrophilic. Besides, because of the smallexposure of c-plane, the laser irradiated ZnO nanowires can act as the template toepitaxially grow fine （10nm） ZnO nanotips. Moreover, since the top-decoratednanospheres can reflect incident light, thus increase the absorption of light, the laserirradiated ZnO nanowire arrays will exhibit a better performance when used tofabricated dye-sensitized solar cells （DSSC）.3) Using CuCl2.2H2O and KOH as the reactants, CuO nanorods were synthesizedunder200℃（24h） through the Composite-Molten-Salt （CMS） method. For comparison,CuO nanostructure was also synthesized by the Composite-Hydroxide-Mediated （CHM）method under the same experimental condition, and SEM image shows that theas-prepared sample is3-dimensional nanoflower assembled by2-dimensionalnanoplates. CuO nanocrystals from both the methods have been applied to modify the graphite electrodes and fabricated nonenzymatic glucose sensors, which have beencompared with the bare graphite electrode. The electrochemical characteristics of thethree types of electrodes have been study by electrochemical impedance spectrum （EIS）,cyclic voltammetry （CV） and amperometry, and the results shows that the CuOnanostructure modified electrodes, compared to the bare graphite electrode, exhibitenhanced electrocatalytic properties for direct glucose oxidation and show highsensitivities and excellent slectivities. Moreover, the sample morphology can influencethe performance of its corresponding electrod. The electrode modified by CHM CuOwhich had a large specific surface area showed a high sensitivity （300μA/mM） towardsglucose, but a poor linear response because of absorbing of the intermediate products. Incontrast, the electrode modified by CMS CuO showed a lower sensitivity （150μA/mM）towards glucose, but a better stability and excellent linear response.