AWO₄ (A=Mn, Cd, Ca, Zn) Nanoparticles: Size And Doping Effect On Lattice Structure And Properties

The lattice structure and physi-chemical properties of nanomaterials are closely related to their particle size and surface modification. Indeed, properties are always different from the bulk when the physical dimension comes down to nanoscale. ABO4 type materials had a great application in many areas, such as phosphors, laser crystals, catalysis, and multiferroic materials. In this thesis, we conducted a detailed study of size and doping effect on lattice structure and properties of AWO4 (A=Mn, Cd, Ca, Zn) nanoparictls. The main results can be summarized as follows:(1) Using citric acid as the capping agent, MnWO4 nanocrystals were prepared to show particle sizes ranging from 8 to 29 nm under hydrothermal conditions. The grain-growth kinetics for MnWO4 nanoparticles was determined to quantitatively follow the equation, where D is the particle size at given reaction time,t, and reaction temperature, T. Systematic sample characterization using combined techniques of X-ray diffraction, transmission electron microscope, selected area electron diffraction, Barret-Emmett-Teller technique, Fourier transformed infrared spectra, UV-visible diffuse reflectance spectra, and Raman spectra indicates that particle size reduction led to an apparent lattice expansion, which is followed by lowering of lattice symmetry and band-gap broadening. Different from the bulk counterparts, new vibration modes were observed in both Infrared and Raman spectra at about 913 cm-1 and 930 cm-1, respectively, which intensified monotonously with particle size reduction, leading to a picture that MnWO4 nanoparticles were terminated by a surface disordered layers. All these size-dependent physical properties were closely related to the surface disorder and the relevant absorbates.(2) CdWO4 nanocrystals with controlled particle size and crystallinity were successfully synthesized via a simple hydro thermal method using citric acid as the capping agent. By systematic sample characterization using X-ray powder diffraction, transmission electron microscope, selected area electron diffraction, Barret-Emmett-Teller technique, Fourier transformed infrared spectra, UV-visible diffuse reflectance spectra, and photoluminescence spectra, all as-prepared CdWO4 samples were demonstrated to crystallize in a pure-phase of monoclinic wolframite structure. With varying the reaction temperature from 160 to 220℃, particle size was controlled to grow from 11 to 21 nm. With particle size reduction, CdWO4 nanostructure showed a lattice expansion, as is followed by a surprisingly lowered lattice symmetry, band gap broadening, and redshift of Au vibration mode. Photocatalytic activity of CdWO4 nanocrystals was examined by monitoring the degradation of methyl orange dye in an aqueous solution under UV radiation of 254 nm. High crystallinity CdWO4 nanostructures with relatively small particle size showed an optimum photocatalytic performance. Consequently, systematic control over semiconductor nanostructures is proved to be useful, in some cases likely general, in achieving the advanced photocatalytic properties for technological uses.(3) Ca1-xZnxWO4 nanocrystals were prepared at room temperature by co-precipitation method using citric acid as a capping agent. As prepared samples have a scheelite structure, and the average sizes of spherical-like particles were about 6nm. With increasing the zinc content, the lattice volume of Ca1-xZnxWO4 nanocrystals decreased, and band edge shift from 4.99 eV for x=0 to 3.98 eV for x=0.104. IR spectra revealed that nanocrystals surfaces were hydration and also caped with citric acid. The photo luminescence spectra show that emission intensity was decreased dramatically with increasing of zinc content. Impedance spectroscropy of Ca1-xZnxWO4 nanocrystals shows that the resistivity of the samples decreased with increasing of zinc content. The Solid solution limit of znic in CaWO4 was estimated to be around 10%.