High efficient ultraviolet photocatalytic activity of BiFeO<Subscript>3</Subscript> nanoparticles synthesized by a chemical coprecipitation process

Single-crystalline BiFeO₃ nanoparticles have been synthesized through a simple chemical coprecipitation process using bismuth and iron nitrates. By employing both X-ray diffraction and electron diffraction, the nanoparticles were unambiguously identified to have a rhombohedrally distorted perovskite structure. X-ray photoelectron spectroscopy investigation shows that Fe element exists as the Fe³⁺ valence state in the BiFeO₃ nanoparticles. UV–Vis absorption spectrum indicates that the absorption cut-off wavelength of the nanoparticles is about 580 nm, corresponding to the energy bandgap of 2.10 eV. The BiFeO₃ nanoparticles exhibited an efficient ultraviolet photocatalytic activity, more than 92% of methyl orange was decolorized after 260 min UV irradiation. Unexpectedly, the BiFeO₃ nanoparticles do not show any efficient visible light photocatalytic activity, although the nanoparticles absorb visible light in the wavelength range of 400–580 nm.     

Photocatalytic activities of hollow and hierarchical Bi<Subscript>3.15</Subscript>Nd<Subscript>0.85</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> microspheres synthesized through a hydrothermal process

Hollow and hierarchical Bi_3.15Nd_0.85Ti₃O₁₂ (BNdT) microspheres of 0.5–1.2 μm in diameter were synthesized through a hydrothermal process. Each hollow microsphere is constructed across by many single-crystalline BNdT nanoplates with in-plane dimension of ~400 nm. The BNdT nanoplates are of layered perovskites. The UV–visible absorption characteristics demonstrated that the band gap of the BNdT microspheres is 3.33 eV. The hierarchical microspheres exhibit significant photocatalytic activity. Up to 75% methyl orange was decolorized after UV irradiation for 210 min, whereas lower than 10% methyl orange decolorized using BNdT powders prepared from single crystals as catalyst. The photocatalytic decolorization of methyl orange solution is a pseudo-first-order reaction and its kinetics can be expressed as ln(C/C ₀) = kt.     

Effects of addition of BiFeO<Subscript>3</Subscript> on phase transition and dielectric properties of BaTiO<Subscript>3</Subscript> ceramics

Samples of xBiFeO₃–(1 − x)BaTiO₃ (x = 0, 0.02, 0.04, 0.06, 0.07 and 0.08) were synthesized by solid state reaction technique and sintered in air in the temperature range 1,220–1,280 °C for 4 h. X-ray diffraction data showed that 2–8 mol% BiFeO₃ can dissolve into the lattice of BaTiO₃ and form single perovskite phase. The crystal structure changes from tetragonal to cubic phase at room temperature when 8 mol% of BiFeO₃ was added into BaTiO₃. Scanning electron microscope images indicated that the ceramics have compact and uniform microstructures, and the grain size of the ceramics decreases with the increase of BiFeO₃ content. Dielectric constants were measured as functions of temperatures (25–200 °C). With rising addition of BiFeO₃, the Curie temperature decreases. For the sample with x = 0.08, the phase transition occurred below room temperature. The boundary between tetragonal and cubic phase of the BiFeO₃–BaTiO₃ system at room temperature locates at a composition between 7 and 8 mol% of BiFeO₃. The diffusivity parameter γ for compositions x = 0.02 and x = 0.07 is 1.21 and 1.29, respectively. The relaxor-like behaviour is enhanced by the BiFeO₃ addition.     

Ultra-thin BiFeO3 nanowires prepared by a sol–gel combustion method: an investigation of its multiferroic and optical properties

Perovskite-type polycrystalline BiFeO₃ nanowires, with 150 nm in length and 10 nm in diameter, were synthesized using a sol–gel combustion method at a relative low reactive temperature. The BiFeO₃ nanowires exhibit a remarkably high saturation magnetization of 4.22 emu/g finite coercivity (177 Oe), and a enhanced Mr/Ms value about 0.22, which is independent on the synthesize temperature. The permittivity constant (ε′) and dielectric loss (0.01 at 0.4 MHz) of BiFeO₃ nanowires are very low as compared to reported BiFeO₃ bulk and film. In addition, BiFeO₃ nanowires reveal a wide band gap of 2.5 eV measured from the UV–visible diffuse reflectance spectrum, which may be useful as a photoelectrode material and photocatalytic decomposition of contamination.     

Low-temperature synthesis of Sr<Subscript>0.3</Subscript>Ba<Subscript>0.7</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> ultrafine powder and its characteristics

Ultrafine strontium barium niobate (Sr_0.3Ba_0.7Nb₂O₆, SBN30) powders were prepared by urea method starting from a precursor solution constituting of Sr (NO₃)₂, Ba (NO₃)₂, NbF₅, urea and polyvinyl alcohol (PVA) as surfactant. Their structural behavior and morphology were examined by means of X-ray diffractometry (XRD) and Scanning electron microscopy (SEM). The results showed that the SBN30 powders crystallized to a pure tetragonal phase at annealing temperatures as low as 750 °C. The average particle size of SBN powders subjected to 750 °C was of the order of 150–300 nm. With increasing calcination temperature,however, the average particle size of the calcined powders increased. The SBN30 ceramic prepared from urea method can be sintered at temperature as low as 1,225 °C. The transition temperature from the ferroelectric phase to the paraelectric phase and the relative dielectric permittivity of the SBN30 powder were less than the corresponding values of the bulk ceramic. The permittivity and loss tangent (tan δ) at room temperature (1 kHz) was found to be 930 and below 0.025.     

Microemulsion-based synthesis and luminescence of nanoparticulate CaWO<Subscript>4</Subscript>, ZnWO<Subscript>4</Subscript>, CaWO<Subscript>4</Subscript>:Tb,and CaWO<Subscript>4</Subscript>:Eu

Blue emitting CaWO₄ and greenish blue emitting ZnWO₄ nanoparticles are synthesized via microemulsion techniques applying a cationic (CTAB) as well as a non-ionic surfactant (TritonX-100). The influence of the surfactant on particle size and shape is studied. Scanning electron microscopy and dynamic light scattering confirm the presence of uniform and non-agglomerated nanoparticles, 60–80 nm in diameter. Photoluminescence confirms [WO₄]²⁻-related broad-band emission with its maximum at 440 nm (CaWO₄) and 420 nm (ZnWO₄). The highest quantum yield (QY) is observed for nanoscaled CaWO₄ with a value of 23–25%. Doping of CaWO₄ and ZnWO₄ with Tb³⁺ and Eu³⁺ was performed and in the case of CaWO₄:Tb and CaWO₄:Eu and results in the emission of green and red light, again with comparably high QYs (17–19%).     

Functionalization of electrospun magnetically separable TiO<Subscript>2</Subscript>-coated SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanofibers: strongly effective photocatalyst and magnetic separation

Magnetically separable TiO₂-coated SrFe₁₂O₁₉ electrospun nanofibers were obtained successfully by means of sol–gel, electrospinning, and coating technology, followed by heat treatment at 550–650 °C for 3 h. The average diameter of the electrospun fibers was 500–600 nm. The fibers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM). The optimized calcining temperature was determined by XRD and the analysis of decolorizing efficiency of methylene blue (MB) under UV–vis irradiation. The photocatalytic activity of the TiO₂-coated SrFe₁₂O₁₉ fibers was investigated using ultraviolet–visible absorbance by following the photooxidative decomposition of a model pollutant dye solution, MB in a photochemical reactor. In contrast to pure TiO₂ fibers, the TiO₂-coated SrFe₁₂O₁₉ fibers have higher absorption in 250–750 nm wavelength regions. The presence of SrFe₁₂O₁₉ not only broadened the response region of visible-light, but also enhanced the absorbance for UV light. The decolorizing efficiency of MB under UV–vis irradiation was up to 98.19%, which was a little higher than that of Degussa P25 (97.68%). Furthermore, these fibers could be recollected easily with a magnet in a photocatalytic process and had effectively avoided secondary pollution of treated water.     

Luminescence property of Ca<Subscript>3</Subscript>(VO<Subscript>4</Subscript>)<Subscript>2</Subscript>:Eu<Superscript>3+</Superscript> dependence on molar ratio of Ca/V and solution combustion synthesis temperature

Red emitting phosphor Ca₃(VO₄)₂:Eu³⁺ was prepared by citric acid-assisted solution combustion method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and fluorescence spectrophotometer. The influences of Ca to V molar ratio and synthesis temperature on phase composition, morphology, grain size, photofluorescence properties, and ultraviolet–visible diffuse reflectance spectra (UV–Vis DRS) of as-synthesized samples were investigated. The results indicate that Ca to V molar ratio play a key role for the changing of phase composition, excitation spectrum, and luminescence intensity. The sample prepared at 900 °C, keeping Ca/V = 3:2.2, has the highest photoluminescence intensity. The possible causes of the effects on photoluminescence mechanism were also discussed in this work.     

Solubility and coprecipitation of barium and strontium nitrates in HNO<Subscript>3</Subscript> solutions and multicomponent systems

The solubility of Ba(NO₃)₂ and Sr(NO₃)₂ in HNO₃ solutions at 25–95°C is characterized by the power dependence on the total concentration of the nitrate ion with the exponent for Ba(NO₃)₂ equal to −2 in ∼9 M HNO₃ and −6 in more concentrated acid solutions. The latter exponent is also characteristic of the more soluble Sr(NO₃)₂ throughout the examined range of HNO₃ concentrations. In strongly acidic solutions, Ba(NO₃)₂ coprecipitates with Sr(NO₃)₂. The solubility curve for Ba(NO₃)₂ in NH₄(Na)NO₃ solutions suggests formation of a double salt, whereas in UO₂(NO₃)₂ solutions the dependence is the same as in HNO₃ solutions.     

Visible-light activities of Gd<Subscript>2</Subscript>O<Subscript>3</Subscript>/BiVO<Subscript>4</Subscript> composite photocatalysts

Gd₂O₃/BiVO₄ composite photocatalysts were hydrothermal synthesized and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and UV–vis diffusion reflectance spectra; all the composite photocatalysts exhibited enhanced photocatalytic activities than the pure BiVO₄ for degradation of methyl orange under visible-light irradiation. The improved activity of composites was discussed and ascribed to the electron-scavenging effect of dopants.     

Synthesis of nano-MoS<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> composite and its catalytic degradation effect on methyl orange

A nano-MoS₂/TiO₂ composite was synthesized in H₂ atmosphere by calcining a MoS₃/TiO₂ precursor, which was obtained via a quick deposition of MoS₃ on anatase nano-TiO₂ under a strong acidic condition. The obtained nano-MoS₂/TiO₂ composite was characterized by X-ray diffraction spectroscopy, Brunauer–Emmett–Teller (BET) surface area, scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive spectrometry, ultraviolet–visible spectroscopy, and Fourier transform infrared spectroscopy. The results show that the composite had a high BET surface area because of its small size and irregularly layered structure. MoS₂ in the composite was composed of typical layered structures with thicknesses of 2–8 nm and lengths of 10–40 nm. The composite contained a wide and intensive absorption at 400–700 nm, which is in the visible light region, and presented a positive catalytic effect on removing methyl orange from the aqueous solution. The catalytic activity of the composite was influenced by the initial concentration of methyl orange, the amount of the catalyst, the pH value, and the degradation temperature. In addition, the composite catalyst could be regenerated and repeatedly used via filtration three times. The deactivating catalyst could be reactivated after catalytic reaction by heating at 450 °C for 30 min in H₂.     

Temperature-controlled assembly and morphology conversion of CoMoO<Subscript>4</Subscript>·3/4H<Subscript>2</Subscript>O nano-superstructured grating materials

The regular and homogeneous single-crystal CoMoO₄·3/4H₂O nanorods, with the diameters ca. 100–300 nm and lengths ca. 8–15 μm, have been successfully prepared by a simple and facile precipitation method. Their morphology conversion from broom-like to cage-like structure has been firstly reported through controlling the reaction temperature. The broom-like microbunches were obtained at 50 °C while at 80 °C, dispersive nanorods can be prepared. As the temperature reached 90 °C, the morphology of the products converted to cage-like microspheres. SEM results show that the reaction temperature has a critical role in both the formation of the products and their morphologies. The UV–visible diffuse reflectance absorbance spectra of the products display two intense, broad absorbance bands cover almost the whole ultraviolet and visible region except for a narrow region around 450 nm, which is in the region for purple light. Based on the experimental results, a possible formation mechanism was also proposed. The synthesis strategy is simple, facile, mild, and has a good reproducibility. The as-prepared products may have potential applications in optics, catalysis, and grating materials.     

Fabrication of hierarchical <Emphasis Type="Italic">β</Emphasis>-Co(OH)<Subscript>2</Subscript> microspheres via hydrothermal process

Hierarchical β-Co(OH)₂ microspheres with 20–50 μm diameter assembled from nanoplate building blocks were successfully fabricated via a hydrothermal process in the presence of a cation surfactant cetyltrimethylammonium bromide (CTAB). The products are characterized in detail by multiform techniques: X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis. The effect of CTAB and pH value on the β-Co(OH)₂ morphology was also investigated. When pH value is maintained at 9, an appropriate added amount of CTAB (3 g) is the crucial prerequisite for the formation of this interesting morphology. In this experiment, pH value of the solution and the cation surfactant CTAB together results in the formation of hierarchical β-Co(OH)₂ microsphere structures assembled from nanoplates.     

Magnetic and photocatalytic properties of nanocrystalline ZnMn<Subscript>2</Subscript>O<Subscript>4</Subscript>

The present study describes the synthesis of ZnMn₂O₄ nanoparticles with the spinel structure. These oxide nanoparticles are obtained from the decomposition of metal oxalate precursors synthesized by (a) the reverse micellar and (b) the coprecipitation methods. Our studies reveal that the shape, size and morphology of precursors and oxides vary significantly with the method of synthesis. The oxalate precursors prepared from the reverse micellar synthesis method were in the form of rods (micron size), whereas the coprecipitation method led to spherical nanoparticles of size, 40–50 nm. Decomposition of oxalate precursors at low temperature (∼ 450°C) yielded phase pure ZnMn₂O₄ nanoparticles. The size of the nanoparticles of ZnMn₂O₄ obtained from reverse micellar method is relatively much smaller (20–30 nm) as compared to those made by the co-precipitation (40–50 nm) method. Magnetic studies of nanocrystalline ZnMn₂O₄ confirm antiferro-magnetic ordering in the broad range of ∼ 150 K. The photocatalytic activity of ZnMn₂O₄ nanoparticles was evaluated using photo-oxidation of methyl orange dye under UV illumination and compared with nanocrystalline TiO₂. Dedicated to Prof. C N R Rao on his 75th birthday     

Synthesis and characterization of pure anatase TiO<Subscript>2</Subscript> nanoparticles

Pure anatase TiO₂ nanoparticles were synthesized by microwave assisted sol–gel method and further characterized by powder X-ray diffraction (XRD), energy dispersive x-ray analysis (EDAX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV–Visible spectrophotometer, SEM images showed that TiO₂ nanoparticles were porous structure. The XRD patterns indicated that TiO₂ after annealed at 300 °C for 3 h was mainly pure anatase phase. The crystallite size was in the range of 20–25 nm, which is consistent with the results obtained from TEM images. Microwave heating offers several potential advantages over conventional heating for inducing or enhancing chemical reactions.     

An alternative approach of solid-state reaction to prepare nanocrystalline KNbO<Subscript>3</Subscript>

A simple preparation of KNbO₃ powders was proposed by an alternative approach of solid-state reaction. Stoichiometric niobium oxalate and potassium acetate were mixed in water and then dried. It was demonstrated that an ion-exchange reaction occurred with the formation of K[NbO(C₂O₄)₂]·nH₂O intermediate. The single-phase KNbO₃ powders were synthesized when K[NbO(C₂O₄)₂]·nH₂O intermediate was calcined between 500 and 800 °C for 3 h. KNbO₃ powders obtained at 500 °C are determined as orthorhombic structure with an average particle size of 20–50 nm by X-ray diffraction, scanning electron microscope (SEM), and transmission electron microscopy (TEM) analysis. The morphologies of KNbO₃ obtained at different temperatures were observed by SEM and TEM analysis. The average band gap energy is estimated to be 3.16 eV by UV–vis diffuse reflectance spectra.     

Study on carbon quantum dots/BiFeO<Subscript>3</Subscript> heterostructures and their enhanced photocatalytic activities under visible light irradiation

Carbon quantum dots/Bismuth ferrite (CQDs/BiFeO₃) composite materials were successfully synthesized by a facile hydrothermal treatment of Fe(NO₃)₃·9H₂O, Bi(NO₃)₃·5H₂O and CQDs solutions. The structural and optical characteristics of the composite materials were characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, transmission electron microscopy and ultraviolet–visible absorption, respectively. The photocatalytic activities of pure BiFeO₃, CQDs and CQDs/BiFeO₃ composite materials had also been carried out by using Rhodamine B as test stuff. The experimental results indicated that for QDs/BiFeO₃ composite materials, the CQDs were attached to the surfaces of BiFeO₃ materials, CQDs and BiFeO₃ belong to different phase. Owing to the heterojunction formed at the interface between CQDs and BiFeO₃ materials together with CQDs as an electron reservoir, the photocatalytic activities of CQDs/BiFeO₃ composite materials were significantly improved. Especially, the CQDs/BiFeO₃ composite sample with 3.3 wt% CQDs has the highest degradation rate, which was about 7.3, 3.7 times higher than those of pure BiFeO₃ and CQDs, respectively. Moreover, the mechanism of RhB degradation catalyzed by CQDs/BiFeO₃ composite materials was also thoroughly explained.     

Effect of hydrothermal duration on synthesis of WO<Subscript>3</Subscript> nanorods

Among different type of transition metal oxides, tungsten trioxide (WO₃) is a suitable candidate for electronic device fabrication due to its n-type property and wide band gap. Herein, one-dimensional tungsten trioxide (WO₃) nanorods were achieved from an aqueous solution of sodium tungstate dihydrate (Na₂WO₄·2H₂O) and sodium chloride (NaCl) in an acidic media by a time-optimized hydrothermal synthesis in autoclave at 180°C or different synthesis durations. For studying morphology and size of obtained powder, X-ray diffraction (XRD), scanning electron microscope (SEM), and high resolution transmission electron microscope (HRTEM) were applied. Finally, WO₃ nanorods of about 2–3 μm in length and 100–200 nm in diameter were obtained during 3 h hydrothermal process.     

Al-doped Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Nanoparticles and Their Magnetic Properties

Al-doped Fe₃O₄ nanoparticles were synthesized for the first time via the Composite-Hydroxide-Mediated (CHM) method from Fe₃O₄ and Al₂O₃ without using any capping agent. The synthesis technique was one-step and cost effective. The obtained products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersion spectroscopy (EDS). Samples with a tunable size of 500–1500 nm, 200–800 nm, and 100–700 nm could be obtained by adjusting the reaction time and temperature. Magnetic property of the as-synthesized Al-doped Fe₃O₄ nanoparticles was investigated. Magnetic hysteresis loops measured in the field range of −10 kOe<H<10 kOe, indicated the ferromagnetic behavior with coercivity (H _c) of 470 and 110 Oe and remanence magnetization (M _r) of 13 and 6.4 emu/g at the temperature of 5 and 300 K, respectively. The saturation intensity (M _s) was 46.1 emu/g at 5 K, while it was about 43.6 emu/g at 300 K.     

Hydrothermal synthesis of hemisphere-like F-doped anatase TiO<Subscript>2</Subscript> with visible light photocatalytic activity

Hemisphere-like F-doped anatase TiO₂ has been synthesized by hydrothermal treatment of TiF₄ aqueous solution in the presence of starch at 130 °C for 10 h, and then calcined at 450 °C for 2.5 h in air. The as-synthesized product has been investigated by photocatalytic reaction test and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), and UV–Vis diffuse reflectance spectra (DRS). The results showed that fluorine was successfully doped into the TiO₂ hemispheres. The F-doped TiO₂ hemispheres showed high visible light activity in degradation of acid orange II, which could be attributed to the creation of oxygen vacancies and good crystallinity.