ABSTRACT
In this work, we focus on understanding the morphology and photocatalytic properties of CeO2 nanocrystals (NCs) synthesized via a microwave-assisted solvothermal method using acetone and ethanol as solvents. Wulff constructions reveal a complete map of available morphologies and a theoretical-experimental match with octahedral nanoparticles obtained through synthesis using ethanol as solvent. NCs synthesized in acetone show a greater contribution of emission peaks in the blue region (â¼450 nm), which may be associated with higher Ce3+ concentration, originating shallow-level defects within the CeO2 lattice while for the samples synthesized in ethanol a strong orange-red emission (â¼595 nm) suggests that oxygen vacancies may originate from deep-level defects within the optical bandgap region. The superior photocatalytic response of CeO2 synthesized in acetone compared to that of CeO2 synthesized in ethanol may be associated with an increase in long-/short-range disorder within the CeO2 structure, causing the Egap value to decrease, facilitating light absorption. Furthermore, surface (100) stabilization in samples synthesized in ethanol may be related to low photocatalytic activity. Photocatalytic degradation was facilitated by the generation of ·OH and ·O2- radicals as corroborated by the trapping experiment. The mechanism of enhanced photocatalytic activity has been proposed suggesting that samples synthesized in acetone tend to have lower e'âh· pair recombination, which is reflected in their higher photocatalytic response.
ABSTRACT
This work reports the setting up of the X-ray diffraction and spectroscopy beamline at the Brazilian Synchrotron Light Laboratory for performing total scattering experiments to be analyzed by atomic pair distribution function (PDF) studies. The results of a PDF refinement for Al2O3 standard are presented and compared with data acquired at a beamline of the Advanced Photon Source, where it is common to perform this type of experiment. A preliminary characterization of the Pb1-xLaxZr0.40Ti0.60O3 ferroelectric system, with x = 0.11, 0.12 and 0.15, is also shown.
ABSTRACT
In this paper, we report our initial research to obtain hexagonal rod-like elongated silver tungstate (α-Ag(2)WO(4)) microcrystals by different methods [sonochemistry (SC), coprecipitation (CP), and conventional hydrothermal (CH)] and to study their cluster coordination and optical properties. These microcrystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinements, Fourier transform infrared (FT-IR), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies. The shape and average size of these α-Ag(2)WO(4) microcrystals were observed by field-emission scanning electron microscopy (FE-SEM). The optical properties of these microcrystals were investigated by ultraviolet-visible (UV-vis) spectroscopy and photoluminescence (PL) measurements. XRD patterns and Rietveld refinement data confirmed that α-Ag(2)WO(4) microcrystals have an orthorhombic structure. FT-IR spectra exhibited four IR-active modes in a range from 250 to 1000 cm(-1). XANES spectra at the W L(3)-edge showed distorted octahedral [WO(6)] clusters in the lattice, while EXAFS analyses confirmed that W atoms are coordinated by six O atoms. FE-SEM images suggest that the α-Ag(2)WO(4) microcrystals grow by aggregation and the Ostwald ripening process. PL properties of α-Ag(2)WO(4) microcrystals decrease with an increase in the optical band-gap values (3.19-3.23 eV). Finally, we observed that large hexagonal rod-like α-Ag(2)WO(4) microcrystals prepared by the SC method exhibited a major PL emission intensity relative to α-Ag(2)WO(4) microcrystals prepared by the CP and CH methods.
ABSTRACT
Intense and broad visible photoluminescent (PL) band was observed at room temperature in disordered Pb(Zr0.53Ti0.47)O3 powders. Structural order-disorder was evaluated by different methods. XANES results pointed to the presence of different coordination modes of disordered Ti powders, and in the ordered sample the local structure around titanium atoms is characteristic of the structurally ordered PZT with only TiO6 units. Only samples containing simultaneous structural order and disorder in their network present the intense visible PL emission at room temperature.
ABSTRACT
The structure of Pb(1-x)Ba(x)Zr(0.65)Ti(0.35)O(3) (PBZT) ceramic materials with 0.00 < or = x < or = 0.40 was studied using synchrotron X-ray powder diffraction data. According to the Rietveld refinements, the structure of PBZT ceramics with x = 0.00, 0.10 and 0.20 at room temperature was rhombohedral R3c. A phase transition from rhombohedral to cubic was observed at 543 and 463 K for x = 0.10 and 0.20, respectively. The refinement for the compositions x = 0.30 and x = 0.40 showed a cubic structure from 10 to 450 K, in good agreement with the dielectric properties of these samples.
ABSTRACT
A series of W/Ni/Al2O3 catalysts was prepared by coprecipitation of nickel and aluminum and impregnation of tungsten. The samples were characterized by XRD, XPS and XAS techniques. XRD measurements detected two phases: NiO as the main phase and NiAl2O4 in lower amount; no phase containing tungsten was detected. XPS measurements indicated the presence of Al2O3, NiAl2O4, and Al2(WO4)3 phases on the catalyst surfaces. The XANES spectra of catalysts were rather similar for all of the samples. According to the EXAFS results, at the short range level, tungsten does not form any bound with Ni atoms. The nickel first coordination shell in the catalysts containing tungsten is similar to that encountered in COP 1 catalyst without tungsten. The lower Ni-Ni coordination number observed for all catalysts, when compared to the NiO phase, can be attributed to the nickel-aluminum interaction.