Synthesis of Nd:YVO4 Thin Films by a Sol-Gel Method

Nd: YVO₄ powders and thin films were successfuly synthesized by the sol–gel method using metal alkoxides. A homogeneous and stable solution was prepared by the reaction of Y(OEt)₃, VO(O i Pr)₃, and Nd(OEt)₃ in 2-methoxyethanol. The precursor was a mixture of vanadium and yttrium double alkoxide. Precursor films were prepared by dip coating and crystallization to single-phase YVO₄ at 500°C. Nd:YVO₄ films were crystallized with (200) preferred orientation on glass substrates, which showed the characteristic optical absorption of neodymium.     

Preparation, Characterization, and Photocatalytic Properties of CaNb2O6 Nanoparticles

CaNb₂O₆ nanoparticles with a size range of 30–50 nm were synthesized by heat treatment at 600°C after a solvothermal process and their optical and photocatalytic properties were investigated. The prepared powders were characterized by X-ray powder diffractometer, field-emission scanning electron microscope, transmission electron microscope, UV-Vis diffuse reflectance spectroscopy, Fluorescence spectroscopy, and Raman spectroscopy. Compared with a powder of the same material prepared by a solid-state reaction (SS) method, the nanoparticles exhibited a higher Brunauer–Emmett–Teller (BET) surface area, more efficient light absorption, and enhanced photocatalytic activity for producing H₂ from pure water under UV irradiation. The photoluminescence spectra revealed that a radiative recombination process is dominant in the powder prepared by the SS method (strong blue emission at 300 K) under UV light irradiation, while no obvious emission was observed in the nanoparticles. This decrease of the radiative recombination as well as the higher optical absorption ability and higher BET surface area resulting from the reduced dimensionality led to enhanced photocatalytic activity of the nanoparticles.     

Destabilization of Zirconia Thermal Barriers in the Presence of V2O5

Impurities, such as vanadium, degrade the operating performance of yttria-stabilized zirconia thermal-barrier coatings. V₂O₅ reacts preferentially with Y₂O₃, forms YVO₄, and leads to the destabilization of zirconia thermal-barrier material. A model experiment has been designed to monitor the destabilization of zirconia thermal barriers by directly exposing thin films of yttria-stabilized zirconia to V₂O₅ vapor. The growth of YVO₄ from yttria-stabilized zirconia and the destabilization of cubic yttria-stabilized zirconia into tetragonal and/or monoclinic zirconia polymorphs are monitored by selected-area diffraction and energy-dispersive X-ray spectroscopy in the transmission electron microscope. A special crystallographic orientation relation between YVO₄ and cubic zirconia is discussed.     

CNTs/Ta3N5 Nanocomposite with Enhanced Photocatalytic Activity Under Visible Light Irradiation

Carbon nanotubes (CNTs)/Ta₃N₅ nanocomposite is presented as a novel photocatalyst working under visible light irradiation. The results of Fourier transform infrared analyses and transmission electron microscopy indicate that a good interfacial combination has formed between CNTs and Ta₃N₅ nanoparticles. The CNTs/Ta₃N₅ nanocomposite shows strong absorption in the visible light range compared with pure Ta₃N₅ and has a bandgap energy of 2.01 eV. The photocatalytic experiments show that the nanocomposite has a higher photocatalytic activity than Ta₃N₅ nanoparticles.     

Direct Formation and Photocatalytic Performance of Anatase (TiO2)/Silica (SiO2) Composite Nanoparticles

Anatase (TiO₂)/silica (SiO₂: 23.9–27.7 mol%) composite nanoparticles were directly synthesized from (i) the reaction of titanyl sulfate (TiOSO₄) and sodium metasilicate (Na₂SiO₃) under mild hydrothermal conditions, (ii) the acidic precursor solutions of TiOSO₄ and tetraethylorthosilicate (TEOS) by thermal hydrolysis, and (iii) the metal alkoxides, i.e., tetraisopropoxide (TTIP) and TEOS, by the sol–gel method. Their photocatalytic activities were evaluated by measurements of the relative concentration of methylene blue after UV irradiation. The as-prepared TiO₂/SiO₂ composite nanoparticles showed far more improved photocatalytic activity than the pure anatase-type TiO₂. The composite nanoparticles formed from (i) TiOSO₄ and Na₂SiO₃ as well as those from (ii) TiOSO₄ and TEOS showed fairly good photocatalytic activity, and it was better than that of those synthesized from (iii) the metal alkoxides, which was suggested to be due to the difference in crystallinity of the anatase.     

Optical and Photocatalytic Properties of Spinel ZnCr2O4 Nanoparticles Synthesized by a Hydrothermal Route

In this paper, a simple hydrothermal route has been developed to synthesize ZnCr₂O₄ nanoparticles. Experimental results show that the as-prepared ZnCr₂O₄ nanoparticles have an average particle size of <5 nm. The ZnCr₂O₄ nanoparticles have a direct band gap about 3.46 eV and exhibit blue emission in the range of 300–430 nm, centered at 358 nm when excited at 220 nm. Furthermore, the nanoparticles show apparent photocatalytic activities for the degradation of methylene blue under UV light irradiation.     

Preparation, Characterization, and Photocatalytic Activity of N, S-Codoped TiO2 Nanoparticles

The doping of titanium dioxide (TiO₂) with various metal or nonmetal elements has been considered as an effective strategy to extend the photoactive wavelength region to visible light. In this paper (nitrogen [N] and sulfur [S])-codoped anatase TiO₂ nanoparticles were prepared via a sol–gel route, followed by a heat treatment at elevated temperatures. The as-prepared samples were extensively characterized by X-ray diffraction, UV–Vis absorption spectroscopy, and X-ray photoelectron spectroscopy. The N, S-codoped TiO₂ nanoparticles showed a strong visible light absorption and exhibited an enhanced photocatalytic activity for the degradation of methylene blue as compared with the pure, N- or S-doped TiO₂ under either UV light or solar light irradiation.     

Sol–Gel Synthesis and Photocatalytic Activity of CeO2/TiO2 Nanocomposites

Uniform CeO₂ / TiO₂ composite nanoparticles with different Ce/Ti molar ratios have been successfully synthesized via the sol–gel method. The samples were characterized using differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The surface state analysis by means of X-ray photoelectron spectroscopy (XPS) shows that the Ti element mainly exists as a chemical state of Ti⁴⁺, while the Ce element exists as a mixture of Ce³⁺ and Ce⁴⁺ oxidation states. The photocatalytic degradation of methyl orange (MeO) in CeO₂ / TiO₂ suspension was investigated. The results indicate that the CeO₂/TiO₂ nanocomposites show higher photocatalytic activity than pure TiO₂. Photodegradation of MeO can be improved by increasing the Ce/Ti molar ratio in the initial 15 min.     

Photocatalytic Activity of (Copper, Nitrogen)-Codoped Titanium Dioxide Nanoparticles

(Copper, Nitrogen)-codoped titanium dioxide (TiO₂) nanoparticles have been prepared via a sol–gel route, followed by heat treatment at an elevated temperature. Such (Cu, N)-codoped TiO₂ nanoparticles showed a strong absorption in the visible region and a red shift in the band gap transition and exhibited an enhanced photocatalytic activity as compared with the pure, N-doped and Cu-doped TiO₂ for xylenol orange decomposition.     

Morphologies-Controlled Synthesis and Optical Properties of Bismuth Tungstate Nanocrystals by a Low-Temperature Molten Salt Method

Well-crystallized bismuth tungstate (Bi₂WO₆) powders with different morphologies were successfully synthesized via a low-temperature molten salt method. The powders were characterized by X-ray diffraction, transmission electron microscopy, and UV-Vis, respectively. It was found that the variation of morphology of the obtained Bi₂WO₆ powder mainly depends on the different reaction temperatures and the weight ratio of LiNO₃–NaNO₃ salt to precursor. In addition, the UV-Visible absorption spectra showed that the synthesized powders had strong light absorption properties not only in the ultraviolet light but also in the visible light region.     

Two-Dimensional Mapping of Er3+ Photoluminescence in CaF2 Crystal Lines Patterned by Lasers in Oxyfluoride Glass

The laser-induced crystallization method is applied to an oxyfluoride glass with the composition of 41.5SiO₂–21.3Al₂O₃–4.8CaO–12.6NaF–16.4CaF₂–2.9NiO–0.5ErF₃ (mol%), and the lines consisting of CaF₂ nanocrystals (diameter: ∼20 nm) are patterned on the glass surface. It is found from micro-photoluminescence (PL) spectra of Er³⁺ ions that Er³⁺ ions are incorporated into CaF₂ nanocrystals formed by laser (continuous-wave Yb:YVO₄ fiber laser with a wavelength of 1080 nm) irradiations. Two-dimensional mappings of the PL intensity for the ⁴S_3/2→⁴I_15/2 transition of Er³⁺ ions are measured for the surface and cross section of the patterned lines. It is found that two phases giving different PL intensities are formed in the laser-irradiated region, suggesting that the center part of the laser-irradiated region consists of Er³⁺-doped CaF₂ nanocrystals and the surrounding of the center part gives the fluoride ion rich coordination state for Er³⁺ ions. The formation mechanism of Er³⁺-doped CaF₂ nanocrystals is related to the temperature distribution of the laser-irradiated region.     

Electron Magnetic Resonance Investigation of Gadolinium Diffusion in Ceria Powders

The electron magnetic resonance (EMR) technique was used to investigate the diffusion of gadolinium in ceria (CeO₂) powders. The EMR absorption intensity was measured for several annealing times and three different temperatures of isothermal annealing: 1273, 1323, and 1373 K. The activation temperature for diffusion, calculated from the experimental data using a theoretical model based on the Fick equation, was found to be E _A=485±5 kJ/mol. This value is very close to the one obtained for the diffusion of gadolinium in an oxide with a similar crystal structure, UO₂, using the secondary ion mass spectrometry technique, E _A=492 kJ/mol. On the other hand, it is much larger than the one obtained for the diffusion of gadolinium in yttrium vanadate (YVO₄), E _A=177 kJ/mol, a compound with a different crystal structure.     

Methanol–Water System for Solvothermal Synthesis of YVO4:Eu with High Photoluminescent Intensity

A methanol–water mixed solvent was used as a reaction medium for the preparation of Eu³⁺-doped YVO₄ phosphor materials. These were synthesized by a solvothermal method at 150°–300°C using a 10 vol% solution of water in methanol as the reaction medium followed by calcination at 1000°–1200°C. The phase composition and optical properties of the products were characterized by X-ray diffraction, scanning electron microscope, and photoluminescence spectroscopy. The powders obtained were composed of spherical particles ∼0.5 μm in size, with an internal structure that was different for samples prepared under subcritical and supercritical conditions of methanol. After the calcination, the powders obtained at 240°–300°C retained the initial raspberry-like morphology, whereas the morphology of samples prepared at 150°–210°C changed significantly due to noticeable sintering. The fluorescence intensity exhibited by the prepared samples was higher than the fluorescence intensity shown by one of the best commercial YVO₄:Eu phosphors having a large particle size.     

Photoactive and Adsorptive Niobium-Doped Anatase (TiO2) Nanoparticles: Influence of Hydrothermal Conditions on their Morphology, Structure, and Properties

Anatase-type TiO₂ nanoparticles with adsorptivity and improved photocatalytic activity for the decomposition of methylene blue (MB) in its aqueous solution, which contained up to 10 mol% niobium by forming solid solutions with niobium oxide, were directly synthesized from precursor solutions of TiOSO₄ and NbCl₅ under three hydrothermal conditions in the absence and presence of urea and aqueous ammonia at 180°C for 5 h. The influence of the hydrothermal conditions on the crystallite growth, morphology, specific surface area, adsorptivity, and photocatalytic activity of niobium-doped TiO₂ was investigated. The crystallite growth of anatase was enhanced by the presence of the niobium component. The 10 mol% niobium-doped TiO₂ that was prepared under the hydrothermal condition in the presence of urea had fine crystallites (11 nm) and high specific surface areas (135 m²/g), which showed the most enhanced photocatalytic activity and the highest adsorptivity. The hydrothermal treatment under weak basic conditions and formation of solid solutions with niobium oxide brought about a considerable increase in the adsorption of MB for the anatase-type TiO₂.     

Synthesis of AlN Nanopowder from γ-Al2O3 by Reduction–Nitridation in a Mixture of NH3–C3H8

Aluminum nitride (AlN) powders were synthesized by gas reduction–nitridation of γ-Al₂O₃ using NH₃ and C₃H₈ as the reactant gases. AlN was identified in the products synthesized at 1100°–1400°C for 120 min in the NH₃–C₃H₈ gas flow confirming that AlN can be formed by the gas reduction–nitridation of γ-Al₂O₃. The products synthesized at 1100°C for 120 min contained unreacted γ-Al₂O₃. The ²⁷A1 MAS NMR spectra show that Al–N bonding in the product increases with increasing reaction temperature, the tetrahedral AlO₄ resonance decreasing prior to the disappearance of the octahedral AlO₆ resonance. This suggests that the tetrahedral AlO₄ sites of the γ-Al₂O₃ are preferentially nitrided than the AlO₆ sites. AlN nanoparticles were directly formed from γ-Al₂O₃ at low temperature because of this preferred nitridation of AlO₄ sites in the reactant. AlN nanoparticles are formed by gas reduction–nitridation of γ-Al₂O₃ not only because the reaction temperature is sufficiently low to restrict grain growth, but also because γ-Al₂O₃ contains both AlO₄ and AlO₆ sites, by contrast with α-Al₂O₃ which contains only AlO₆.     

Sol–Gel Synthesis and Characterization of Ag and Au Nanoparticles in SiO2, TiO2, and ZrO2 Thin Films

Silver and gold nanoparticles were synthesized by the sol–gel process in SiO₂, TiO₂, and ZrO₂ thin films. A versatile method, based on the use of coordination chemistry, is presented for stabilizing Ag⁺ and Au³⁺ ions in sol–gel systems. Various ligands of the metal ions were tested, and for each system it was possible to find a suitable ligand capable of stabilizing the metal ions and preventing gold precipitation onto the film surface. Thin films were prepared by spin-coating onto glass or fused silica substrates and then heat-treated at various temperatures in air or H₂ atmosphere for nucleating the metal nanoparticles. The Ag particle size was about 10 nm after heating the SiO₂ film at 600°C and the TiO₂ and ZrO₂ films at 500°C. After heat treatment at 500°C, the Au particle size was 13 and 17 nm in the TiO₂ and ZrO₂ films, respectively. The films were characterized by UV–vis optical absorption spectroscopy and X-ray diffraction, for studying the nucleation and the growth of the metal nanoparticles. The results are discussed with regard to the embedding matrix, the temperature, and the atmosphere of the heat treatment, and it is concluded that crystallization of TiO₂ and ZrO₂ films may hinder the growth of Ag and Au particles.     

Low-Temperature Synthesis of NiFe2O4 by a Hydrothermal Method

Nickel ferrite (NiFe₂O₄) nanoparticles were successfully synthesized via a hydrothermal process and characterized by X-ray diffraction and transmission electron microscope techniques. The effects of reaction temperature, holding time, and RH ratio (isopropyl alcohol/water) were discussed. The NiFe₂O₄ nanoparticles could be obtained at 60°C within 3 h. The crystallization of the spinel ferrites was promoted by the increase in reaction temperature, holding time, and RH ratio.     

Formation of Small-Sized CdSxSe1-x Crystals in Sol-Gel-Derived SiO2 Glasses

Silica glasses doped with small-sized CdS_x-Se_1-x crystals were prepared by the sol-gel method. Gels synthesized by the hydrolysis of Si(OC₂H₅)₄, in the presence of CdSeO₄ with NH₄SCN dissolved in HNO₃ or NH₄OH, were heated in H₂-N₂ atmosphere. The pH value of solutions for CdSeO⁴ and NH⁴SCN primarily determined the fraction of anions in CdS^xSe^1-x crystals. The anion content in crystals was dependent on the pH value of the solvent and/or heat-treatment temperature, and the sulfur fraction changed from 0.1 to 0.6. The optical absorption spectra were red-shifted as the selenium content and the crystal size increased, and the emission spectra showed a sharp band near the absorption edge position and a broad tail extending into the long wavelength. The optical band gap energies increased reciprocally proportional to the square of the crystal size.     

Molten Salt Synthesis of Acicular Ba2NaNb5O15 Seed Crystals

Well-developed acicular Ba₂NaNb₅O₁₅ (BNN) seed crystals have been successfully prepared by the reaction between BaCO₃, Nb₂O₅ and molten NaCl salt. The effects of the calcination temperature, time, and weight ratio of the oxide mixture to salt on the morphology of BNN particles were investigated. Uniform acicular seeds with high aspect ratio could be achieved at 1200°C for 4 h with the salt-to-oxide mixture of 2:1.     

Enhanced Visible Light-Driven Photocatalytic Performance of La-Doped TiO2−xFx

La-doped TiO_{2− x} F _x (La–TiO_{2− x} F _x ) powders were prepared by the sol–gel method. X-ray diffraction results showed that La efficiently inhibited grain growth. X-ray photoelectron spectroscopy spectra revealed that La₂O₃ and O–Ti–F bonds have formed, the La₂O₃ maintained the high surface area of TiO_{2− x} F _x after calcination at a temperature above 500°C, while the O–Ti–F bonds increased the oxidation potential of the photogenerated hole in the valence band. The UV-vis spectroscopy of the La–TiO_{2− x} F _x showed that the presence of intraband gap states was likely responsible for its absorption of visible light. When the molar ratios of La and F to Ti were 1.5:100 and 5:100, respectively, and calcined at 500°C, the photocatalytic degradation rate of methylene blue over La–TiO_{2− x} F _x was about 1.5 times higher than that of F-doping TiO₂.