Novel electrochemical technique: Grain control in preparation of polycrystalline BaMoO4 film

BaMoO₄ films have been prepared by a novel electrochemical technique without impressed current on molybdenum substrates at room temperature in barium hydroxide aqueous solution. In order to form desired surface morphology and grain size, the BaMoO₄ crystal grains have been controlled effectively by adjusting ion-concentration and adding adequate surfactant. The as-prepared BaMoO₄ films have been characterized by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). SEM and XRD results indicate that the morphology, grain size, phase and crystallinity were significantly developed.     

Fabrication of AMoO4 (A = Ba, Sr) film on Mo substrate by solution reaction assisted ball-rotation

Alkaline earth molybdates, such as BaMoO₄ and SrMoO₄, films have been successfully fabricated on a Mo metal substrate in AOH (A = Ba, Sr) solutions by a ball-rotation-assisted solution reaction, at room temperature. The dissolution of Mo was mainly controlled by the concentration of the H₂O₂ oxidizing agent and ball-rotation to form MoO₄²⁻ in the solution. AMoO₄ was deposited on the substrate by the reaction between MoO₄²⁻ and A²⁺ ions without any high energy or high-temperature treatment. Also, the mass transport of alkaline earth ions onto the solid/solution interface was improved as a result of the vigorous solution agitation by the ball-rotation. Therefore, the rate of deposition of the AMoO₄ films was accelerated by the ball-rotation. A decrease in the grain size of the film was observed with an excessive ball-rotation.     

Efficiency enhancement by aluminum addition to CaTiO3:Pr phosphor thin films

The mechanism of enhancement of the red emission efficiency from CaTiO₃:Pr³⁺ thin film by Al addition has been investigated. Al-ions have been attracting interest as a sensitizer to improve the luminescent efficiency of phosphors. Also, influence of Al-doping on the crystallization, surface morphology and luminescent properties of CaTiO₃:Pr³⁺ thin films have been discussed. CaTiO₃:Pr³⁺ and Al-doped CaTiO₃:Pr³⁺ films were grown using pulsed laser deposition technique on Al₂O₃ (0001) substrates under different substrate temperatures and oxygen pressures. The crystalline phase and surface morphology of the films were very dependent on the oxygen pressure and substrate temperature and they affected the luminescent brightness of the films. The crystalline structure and microstructure of these films have been characterized by X-ray diffraction and electron microscopy and their luminescent properties have been evaluated at room temperature using a luminescence spectrometer and excitation by a broadband incoherent ultraviolet light source with a dominant excitation wavelength of 325 nm. In particular, the incorporation of Al³⁺ ions into CaTiO₃ lattice could induce a remarkable increase of photoluminescence. The enhancement of luminescence for Al-doped films may result not only from the improved crystallinity but also from the reduced internal reflections caused by rougher surfaces. Also, the luminescent intensity and surface roughness of the films exhibited similar behavior as a function of oxygen pressure.     

Synthesis of BaMoO<Subscript>4</Subscript> hollow spheres

Based on the “polymer-cation templates” technique, well-defined BaMoO₄ hollow spheres with an average diameter of 2.3 μm have been synthesized via a simple precipitation reaction between BaCl₂ and (NH₄)₆Mo₇O₂₄ · 4H₂O at room temperature in the presence of poly(methacrylic acid). The size of the hollow spheres could be tuned down to 1.5 μm by decreasing the concentrations of Ba²⁺ or MoO₄²⁻ ions in the reaction solution. Compared with current fabrication routines for hollow spheres, this method is facile and cost-effective as no surfactant is needed, and can be applied to produce other inorganic hollow-structured materials.     

Controlled synthesis and photoluminescence properties of hierarchical BaXO4 (X = Mo, W) nanostructures at room temperature

Controlled synthesis of hierarchical Barium molybdate (BaMoO₄) nanostructures with different morphologies, such as peanut-like, cube-like and flower-like, was successfully achieved in aqueous solution at room temperature. The obtained products were characterized by a scanning electron microscope (SEM) and an X-ray power diffractometer (XRD). The morphologies of the obtained products were found to be greatly dependent on reaction time, EDTA concentration and the [Ba²⁺]/[MoO₄²⁻] ratio. This controllable method could be readily extended to produce hierarchical Barium tungstate (BaWO₄) nanostructures with peanut-like, dumbbell-like, sphere-like and flower-like morphologies. The photoluminescence (PL) properties of the obtained BaMoO₄ and BaWO₄ nanostructures exhibited strong dependence on the morphologies and sizes, respectively.     

Facile synthesis of BaMoO4 and BaMoO4/BaWO4 heterostructures with type -I band arrangement and enhanced photoluminescence properties

A series of BaMoO₄ and BaMoO₄/BaWO₄ phosphors were successfully prepared via a polyacrylamide gel method and low temperature calcination technology. The effects of sintering temperature and mass percentage of BaMoO₄/BaWO₄ on the phase purity, functional group, surface morphology, charge state, photoluminescence properties and photocatalytic activity of the prepared products were studied in detail. The results indicate that the BaMoO₄ phosphor is a scheelite tetragonal structure with high crystallinity. The photoluminescence spectra indicates that the phosphors have a strong blue emission peak at 440 nm with excitation wavelength of 282 nm for the BaMoO₄ phosphor, and three emission peaks at 400, 440 and 460 nm with excitation wavelength of 284 nm for the BaMoO₄/BaWO₄ phosphors. These photoluminescence behaviors can be ascribed to the ¹T₂→¹A₁ transition, Jahn–Teller distorted tetrahedral symmetry of [MoO₄]^2- and surface defect. Photocatalytic experiments further confirmed that the BaMoO₄/BaWO₄ phosphors exhibit a high recombination rate of electron hole pairs. The result further indicates that the type-I band arrangement structure of BaMoO₄/BaWO₄ phosphors is beneficial to enhance the photoluminescent properties of single-phase phosphors. This study provides a novel route for preparing the type-I band arrangement structure composite phosphors with high photoluminescent properties and potential applications in light emitting devices, optoelectronic devices, laser devices and white pigments.     

Pr3+ doped BaMoO4 octahedron to shuttle-like microcrystals: synthesis and luminescence properties

Octahedron to shuttle-like microcrystals of highly ordered BaMoO₄:Pr³⁺ have been successfully prepared through a facile hydrothermal method assisted with PVP (K30) (polyvinylpyrrolidone). Comprehensive structural, morphological studies like X-ray diffraction, scanning and transmission electron microscopy were employed to characterize the as synthesized microcrystals. In the hydrothermal process, a very small amount of PVP was added which not only acts as a surfactant to facilitate the formation of BaMoO₄:Pr³⁺, but also acts as a surface capping agent. By varying the reaction time, octahedron to shuttle-like morphologies has been obtained and rest of the parameters like, molar ratio and temperature are kept as constant have crucial influence on the shape evolution and microstructures. Photoluminescence (PL) studies on BaMoO₄:Pr³⁺ showed strong red emission upon UV illumination, and this implied potential application in the luminescent field.     

New chemical methods for the deposition of Cu1·8S and TlSe thin film

New chemical methods for the deposition of thin film of Cu_1·8S and TlSe have been developed. The deposition of Cu_1·8S thin film has been performed by thiourea, ammonia and Cu²⁺ ions at room temperature, while TlSe thin films are obtained from triethanolamine as complexing agent, ammonia, sodium selenosulphate solution and Tl¹⁺ ions at room temperature. The electrical resistance, mobility, carrier concentration and optical band gap have been measured.     

Room temperature ferromagnetism in laser ablated Zn ferrite thin films

Nanocrystalline ZnFe₂O₄ was synthesized using sol–gel method. The room temperature ferromagnetic behavior was observed in ZnFe₂O₄ thin films fabricated by pulsed laser deposition (PLD) technique. The ZnFe₂O₄ nanoparticles and target material used for the fabrication of thin films was observed to have paramagnetic behavior at room temperature. The possible ferromagnetic behavior observed in ZnFe₂O₄ thin films has been explained in terms of random distribution of Zn²⁺ and Fe³⁺ ions at tetrahedral (A) and octahedral [B] sites. The blocking temperature T_B was observed in the case of both nanoparticles and thin films.     

Solution synthesis of crystallized AMO4 (A=Ba, Sr, Ca; M=W, Mo) film at room temperature

A well-crystallized AMO₄ (A=Ba, Ca, Sr; M=W, Mo) films have been prepared at room temperature through a simple solution reaction in respective alkaline solution at higher pH ranging from 12–14. Adopting the corrosion principle for oxidation of metal substrate, these double oxide films were carried out in presence of chemical driving force without any special apparatus or devices. Hydrogen peroxide was used to enhance the dissolution rate of metal substrates. The driving force for the film formation and growth were high concentration of A²⁺, MO₄²⁻ ions with high pH conditions. Average grain sizes of 8–10 μm with bipyramidal shaped particle were grown to the thickness of about 10–14 μm after 3–6 hours treatment. The crystallization of AMO₄ was characterized by three-dimensional nucleation. This work demonstrates the possibility of fabrication of functional ceramic films directly from the aqueous solution in a single step by solution reactions.     

Catalytic behavior of AMoOx (A = Ba, Sr) in oxidation of 2-propanol

Perovskite-type oxides, BaMoO₃ and SrMoO₃, were prepared by reduction of scheelite-type oxides, BaMoO₄ and SrMoO₄, in H₂ flow at 873 K and characterized by XRD, TG, SEM, TPR, NH₃-TPD, UV-vis DRS and BET measurement. The catalytic activity of these alkaline-earth molybdenum oxide catalysts was tested for oxidation of 2-propanol with gaseous oxygen under atmospheric pressure. Dehydration to propylene was mainly promoted over the scheelite-type with Mo⁶⁺, while oxidative dehydrogenation to acetone was mainly promoted over the perovskite-type with Mo⁴⁺, and selectivity to acetone was much higher over BaMoO₃ than over SrMoO₃. Both perovskite-type oxide catalysts underwent oxidation to some degree during the catalytic reaction, so that they also contained some Mo⁶⁺. We concluded that the high selectivity to acetone resulting from oxidative dehydrogenation during 2-propanol conversion is related to the constantly changing oxidation state of the catalyst, resulting in coexistence of Mo⁶⁺ octahedra and Mo⁴⁺ octahedra on the AMoO₃ oxides.     

The roles of B-site ions in lead strontium zirconate titanate thin films for electrically tunable device applications

B-site modification lead strontium zirconate titanate Pb_0.4Sr_0.6Zr_xTi_1 − xO₃ (PSZT, x = 0-0.7) thin films were prepared on Pt/TiO₂/SiO₂/Si substrates by a sol-gel method. The XRD results indicate that paraelectric PSZT thin films at room temperature are obtained as x approaches 0.2. The temperature-dependent dielectric and hysteresis loop measurements reveal that the thin films have diffuse phase transition characteristics and relaxor-like behavior with nano-polar regions in the paraelectric films at room temperature. The Curie temperature of the PSZT thin films varies with the Zr contents, exhibiting a complex trend. This can be attributed to two competitive factors: higher mobility of Ti⁴⁺ than Zr⁴⁺ and smaller open space left for the displacement of Ti ions with the increase of Zr content. The further increase of the Zr contents leads to the simultaneous decrease of dielectric constant, dielectric loss and tunability. PSZT (x = 0.4) thin film shows the largest figure of merit of 24.3 with a moderate tunability of 55.8% and a dielectric loss of 0.023. This suggests that B-site ions have different roles in modifying the electrically tunable performance of PSZT thin films for tunable microwave device applications.     

A study of the infrared absorption spectra of thin amorphous films of molybdenum trioxide

The spectra of thin amorphous films of MoO₃ deposited by vacuum evaporation have been studied within the spectral range 4000–400 cm^–1 by the Fourier transform infrared technique. Some samples in the thickness range 100 to 400 nm were investigated at room temperature. Four samples were investigated in the substrate deposition temperature range 293 to 543 K. Some samples were annealed in vacuum in the temperature range 473 to 673 K and their IR spectra were recorded as soon as the samples were cooled to room temperature. It has been observed that annealing of the samples at 473 K or above and also heating the substrates in the temperature range 473 K or above, results in the formation of molybdenum species of lower oxidation state than the normal Mo(VI), i.e. Mo(V). This new oxidation state is formed by electron transfer from the oxygen 2p to the molybdenum 4d level.     

Controlled synthesis and photoluminescence properties of BaXO4 (X = W, Mo) hierarchical nanostructures via a facile solution route

Shape-controlled synthesis of BaWO₄ hierarchical nanostructures has been achieved in a mixed solvent of water and ethanol at room temperature. By simply adjusting the volume ratio of C₂H₅OH and H₂O (R ratio), the size and shape of BaWO₄ nanostructures, such as shuttle-like and ellipsoid-like, are successfully controlled. This simple method has been extended to synthesize BaMoO₄ hierarchical nanostructures. Both BaWO₄ and BaMoO₄ hierarchical nanostructures exhibited new green emission peaks at 558 and 560 nm, respectively.     

Irradiation induced effects on Ni3N/Si bilayer system

The irradiation effect in Ni₃N/Si bilayers induced by 100 MeV Au ions at fluence 1.5 × 10¹⁴ ions/cm² was investigated at room temperature. Grazing incidence X-ray diffraction determined the formation of Ni₂Si and Si₃N₄ phases at the interface. The roughness of the thin film was measured by atomic force microscopy. X-ray reflectivity was used to measure the thickness of thin films. X-ray photoelectron spectroscopy has provided the elemental binding energy of Ni₃N thin films. It was observed that after irradiation (Ni 2p_3/2) peak shifted towards a lower binding energy. Optical properties of nickel nitride films, which were deposited onto Si (100) by ion beam sputtering at vacuum 1.2 × 10⁻⁴ torr, were examined using Au ions. In-situ I–V measurements on Ni₃N/Si samples were also undertaken at room temperature which showed that there is an increase in current after irradiation.     

Photoluminescence of TiO2 films co-doped with Tb/ Gdand energy transfer from TiO2/Gd to Tb ions

Nanometer TiO₂ thin films doped with different concentration of Tb were prepared by sol-gel method and characterized by X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. XRD results show preferentially oriented (101) anatase films. TEM image indicates that the TiO₂ films consist of TiO₂ grains with diameter about 15 nm. Under room temperature, strong visible luminescence of Tb³⁺ ions due to intra-4f shell transitions are obtained and the PL intensity is found to have a well matching relation with the doping concentration of Tb³⁺ ions. Concentration quenching of PL occurs when Tb³⁺ concentration exceeds a certain value (9.2 mol%). Furthermore, the luminescence intensity is improved obviously after co-doping with Gd³⁺ ions because of the sensitization effects of Gd³⁺ ions to Tb³⁺ ions in TiO₂ system. The energy transfer mechanism from TiO₂ and Gd³⁺ ions to Tb³⁺ ions was proposed.     

Synthesis of BaMoO4:Er/Yb particles by an MAM method and their upconversion photoluminescence properties

Er³⁺-doped BaMoO₄ (BaMoO₄:Er³⁺) and Er³⁺/Yb³⁺ co-doped BaMoO₄ (BaMoO₄:Er³⁺/Yb³⁺) particles were successfully synthesized by a cyclic microwave-assisted metathetic (MAM) method, and show fine and homogeneous morphology with particle sizes of 0.5–1 μm. At 980-nm excitation, BaMoO₄:Er³⁺ and BaMoO₄:Er³⁺/Yb³⁺ particles exhibited a strong 525-nm emission band and a weak 550-nm emission band in the green region. The Raman spectrum of BaMoO₄:Er³⁺/Yb³⁺ particles indicated the appearance of additional peaks at higher frequencies (390 and 505 cm⁻¹) and at lower frequencies (218 and 255 cm⁻¹).     

Room temperature oxidation rate constants of ultra- thin (discontinuous) molybdenum films

Following the method developed by Fehlner, an attempt is made to determine the room temperature oxidation rate constants of ultra-thin (discontinuous) molybdenum films. Thin films of molybdenum of thickness less than 100 Å, corresponding to resistance of 0.55–12.0 Mω, were deposited onto glass substrates at pressures of less than 10^?8 Torr and were exposed to oxygen pressures of 10^?7?10^?5 Torr at room temperature. The oxidation rate constants were calculated from the slope of log R_dcversus log t plots using the electron tunneling conduction model of Neugebauer and Webb. Values of the logarithmic oxidation rate constants obtained ((0.05?0.18) × 10^?9 cm) are compared with those for various refractory metal films and agree fairly well with those published in the literature for discontinuous metal films.     

Effect of metal ion dopants on photochemical properties of anatase TiO2 films synthesized by a modified sol-gel method

Anatase TiO₂ films were successfully synthesized by a modified sol-gel method wherein peroxo titanic acid solution was derived from TiCl₄/ethanol/water solution at room temperature. The as-prepared films were further surface-doped by photodeposited Fe₂O₃ and Cr₂O₃ to improve its physicochemical properties. The phase and structure of the films were investigated by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. The physicochemical properties of the films were also measured. The results show that both hydrophilicity and photocatalytic activity of the films were remarkably improved by doping transition metal ion Fe³⁺. In case of Cr³⁺ doped films, hydrophilicity was also significantly enhanced but photocatalytic activity for methyl orange under UV irradiation was still comparable with the undoped films.     

Molten salt synthesis of barium molybdate and tungstate microcrystals

Pure microcrystalline barium molybdate BaMoO₄ and barium tungstate BaWO₄ materials were prepared by molten flux reaction using alkali metal nitrates as reaction media. The obtained crystals have rhombic shape and expose mostly (111) crystallographic planes. Their mean size depends on the flux temperature and the nature of the alkali metal cation. Monomeric molybdate and tungstate used as precursors yield target products already at 673 K whereas if polymerized ammonium oxosalts were used, then higher temperatures were necessary to obtain barium salts. The optimal temperature for the preparation of pure crystals with well defined shape was found to be near 773 K. UV–visible spectra have been measured to precise energy gaps in these important d⁰ transition metal compounds. The values of Eg for these two mixed oxides are 4.3 eV for BaMoO₄ and 3.8 eV for BaWO₄. Such values contradict to what can be expected from the known data on their structure and the relative electronegativity of W and Mo ions. The possible explanations of this observation are commented.