Surface photoelectric and photodegradation activities of AgNbO3 synthesized by solvothermal method

Silver niobate (AgNbO₃) with different morphologies is obtained by annealing the precursors prepared at different temperatures by solvothermal method at 800 °C. Scanning electron microscope images of the samples show the formation process of the morphologies. The weakest photovoltaic and the highest photocatalytic properties of the sample obtained from the precursors prepared at 190 °C are possibly caused by the synergetic effects between several kinds of particles, and between AgNbO₃ and Nb₂O₅. Both the temperature and the amount of Nb₂O₅ are crucial for the morphologies, the photoelectric and photocatalytic properties of the samples.     

Effect of annealing temperature on the microstructures and photocatalytic property of colloidal ZnO nanoparticles

Colloidal ZnO nanoparticles were prepared in ethanol solutions and annealed at different temperatures (150-500 °C) subsequently. The size, morphology and surface characteristics of ZnO nanoparticles were examined by TEM, XRD, UV-vis absorption spectrum and FTIR technique. With the increase of annealing temperature, the mean size of ZnO nanoparticles was increased from 10 to 90 nm, while the bonding structure of acetate groups coordinating with zinc ions evolved from unidentate to bidentate type. The UV-induced degradation results of methyl orange verified that the photocatalytic process of colloidal ZnO nanoparticles without annealing and the sample annealed at 150 °C was unstable for the weakly bonding unidentate type of acetate groups. However, the sample annealed above 150 °C demonstrated their photocatalytic stability in the whole catalytic process for the stable bidentate bonding type of acetate groups. In addition, the change of particle size in the annealing process significantly affected the catalytic activity of photocatalysts. ZnO nanoparticles annealed at 300 °C would be a prospective photocatalysts with a high catalytic activity and stability compared with the other samples.     

Preparation of nitrogen-doped titanium dioxide with visible-light photocatalytic activity using a facile hydrothermal method

Nitrogen-doped TiO₂ (N-TiO₂) nanoparticles have been successfully prepared via a direct and simple hydrothermal reaction of a commercial Degussa P25 with triethanol amine as solvent and nitrogen source. As-prepared N-TiO₂ was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet-visible light (UV-vis) absorption spectra, electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS) techniques. The results confirm that hydrothermal reaction is an effective way to incorporate nitrogen into the TiO₂ lattice, especially nitrogen substitute for titanium. The nitrogen concentration in TiO₂ can be as high as 21% (molar ratio), which is described as Ti_1−yO_2−xN_x+y (in this paper, x=0.36, y=0.27, i.e., Ti_0.73O_1.64N_0.63). The chemical statuses of N have been assigned to N-Ti-O and O-N-O in the TiO₂ lattice as identified by XPS. Photocatalytic degradation of methyl orange has been carried out in both UV-vis (simulated solar light) and the visible region (λ>400 nm). N-TiO₂ exhibits higher activity than the Degussa P25 TiO₂ photocatalyst, particularly under visible-light irradiation. This study has developed a promising and practical pathway to new nitrogen-doped photocatalysts.     

Catalyst efficiency, photostability and reusability study of ZnO nanoparticles in visible light for dye degradation

Polydispersed ZnO nanoparticles (ZnO1000 and ZnO600) with two different windows of particle size distributions (∼120 and 30 nm) were synthesized using citrate gel route and different annealing treatments (1000 and 600 °C, respectively). Photocatalytic efficiency of these samples was compared with TiO₂ in its commercial form-P25, on two dyes, Methylene blue (MB) and Methyl orange (MO). The X-ray diffraction data showed wrutzite ZnO and anatase and rutile phases of P25. UV-visible absorbance spectra of ZnO1000 showed broad absorption range from UV-to-visible (from 382 to 700 nm), as against sharp absorption peaks in UV range for both ZnO600 and P25. The microstructural morphology as seen through scanning electron micrographs showed ZnO1000 with tetrapod-like structures while the ZnO600 showed almost spherical morphologies. Upon subjecting these catalysts to dye solutions in sunlight it was found that both the dyes were completely decolorised within 20 min by ZnO1000, as against partial decolorisation by ZnO600 and P25 ( 53% and 78% for MO and 77% and 88% for MB samples). The effect of catalyst loading (from 125 mg to 1 g) on decolorisation showed that ZnO1000 had good efficiency for all concentrations which was followed by P25 and then by ZnO600. Small perturbations are attributed to the competition between sunlight scattering-induced, reduced irradiation field and the exposed surface area offered by catalyst, which work as active sites for decolorisation. The reusability of the catalysts when studied on fresh dye samples (4 trials), the decolorisation efficiency decreased merely from 99.2% to 99.12% for ZnO1000 as compared to ZnO600 (53.3% to 19.94%) and P25 (78.3% to 31.42%), indicating the efficient reusability of ZnO1000. The effective half life of the catalysts, in terms of number of reuses, were calculated and found to be ∼3 for both ZnO600 and P25 and was >3000 for ZnO1000, which justifies its extremely high reuse. The byproduct analysis (compared with standards prescribed by World Health Organisation (WHO) and Central Pollution Control Board of India (CPCB)) showed cleavage of the chromophore and of other bonds with opening of benzene rings, indicating degradation of the dyes in concurrence with decolorisation, in the stipulated time. Further, cytotoxicity studies performed on SiHa cell lines showed non-toxicity of the byproducts with ZnO1000 as compared to ZnO600 and P25.     

Sorption and decomposition of crude oil using exfoliated graphite/ZnO composites

Exfoliated graphite/ZnO composites (EG/ZnO) were prepared by impregnating expandable graphite with Zn(OH)₂, abruptly expanding at 700 °C for 40 s, and heating at 500 °C for 3 h. The composites were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), nitrogen adsorption and mercury porosimetry. The sorption capacity of the composites for spilled crude oil was measured and under UV irradiation the decomposition of the absorbed crude oil was investigated. The results showed that the composites provided with the adsorption and photocatalysis capacity for crude oil at the same time. The sorption capacity of the composites decreased gradually on increasing the ZnO content of the composites. Moreover, the decomposition ratio of the absorbed crude oil increased on increasing the ZnO content or decreasing the weight ratio of crude oil to composites.     

Synergistic effect of efficient adsorption g-C3N4/ZnO composite for photocatalytic property

Novel g-C₃N₄/ZnO composite photocatalyst was synthesized from an oxygen-containing precursor by direct thermal decomposition urea in air without any other templates assistance. Different percentages of g-C₃N₄ were hybridized with ZnO via the monolayer-dispersed method. The prepared g-C₃N₄/ZnO composites were characterized by XRD, SEM, UV–vis diffuse reflectance spectra (DRS), FT-IR, TEM and XPS. The composites showed much higher efficiency for degradation of Rhodamine B (RhB) than ZnO under UV and visible light irradiation. Especially, the photocatalytic efficiency was the highest under UV light irradiation when the percentage of g-C₃N₄ was 6%. The improved photocatalytic activity may be due to synergistic effect of photon acquisition and direct contact between organic dyestuff and photocatalyst. Then, effective separation of photogenerated electron–hole pairs at the interface of g-C₃N₄ is an important factor for improvement of photocatalytic activity. This work indicates that g-C₃N₄ hybrid semiconductors photocatalyst is a promising material in pollutants degradation.     

TiO2 nanoparticles with high ability for selective adsorption and photodegradation of textile dyes under visible light by feasible preparation

Particular TiO₂ nanoparticles with high selective photocatalytic oxidation of anionic dyes are prepared by a feasible hydrothermal method. Moreover, its photocatalytic selectivity can be easily switched to cationic dyes by a simple post-treatment in ammonia solution, which makes the prepared TiO₂ have bi-directional selectivity in dye photodegradation. Based on the photocatalytic performances and the structure and surface characteristics of the catalyst, the bi-directional selectivity of the catalysts is found to be closely related to the adsorption selectivity. The adsorption selectivity originates from surface charge groups, which are introduced during the preparation and post-treatment progresses. This study provides a facile and economical approach towards selective degradation of dyes with high efficiency by the special TiO₂ nanoparticles synthesized through a simple hydrothermal method, which may be used practically in the future.     

Aqueous synthesis and enhanced photocatalytic activity of ZnO/Fe2O3 heterostructures

We report a facile synthesis of ZnO/Fe₂O₃ heterostructures based on the hydrolysis of FeCl₃ in the presence of ZnO nanoparticles. The material structure, composition, and its optical properties have been examined by means of transmission electron microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and diffuse reflectance UV–visible spectroscopy. Results obtained show that 2.9 nm-sized Fe₂O₃ nanoparticles produced assemble with ZnO to form ZnO/Fe₂O₃ heterostructures. We have evaluated the photodegradation performances of ZnO/Fe₂O₃ materials using salicylic acid under UV-light. ZnO/Fe₂O₃ heterostructures exhibited enhanced photocatalytic capabilities than commercial ZnO due to the effective electron/hole separation at the interfaces of ZnO/Fe₂O₃ allowing the enhanced hydroxyl and superoxide radicals production from the heterostructure.     

Spray drying: An alternative synthesis method for polycationic oxide compounds

Synthesis of polycationic compounds by the spray-drying technique is an interesting alternative in the domain of aqueous precursor synthesis methods. Spray drying yields high quality samples with good reproducibility. The possibility of scaling up for production of large quantities with fast processing time is well established by the commercial availability of powders of various compositions. In this paper, we have discussed the advantages and limitations of this method and demonstrated its interest by synthesizing a few polycationic compounds selected for their attractive properties of thermoelectricity [Bi_1.68Ca₂Co_1.69O₈, La_0.95A_0.05CoO₃ (A=Ca, Sr, Ba)] or magnetoresistance [La_0.70A_0.30MnO₃ (A=Sr, Ba)]. We have confirmed the quality of these samples by reporting their structure, magnetic and transport properties.     

Effect of chromium substitution in Ca4Mn3O10

Ca₄Mn_3−xCr_xO₁₀ compounds were synthesized in order to investigate the role of an isoelectronic substitution in the layered manganite. Induced structural changes are mainly described as a distortion of the two types of octahedra in the n=3 RP structure. The results indicate that Cr³⁺ is not the only significant valence state for chromium ions. Electrical and magnetic characterization allow to conclude that chromium does not favour the double exchange mechanism in these compounds.     

Superparamagnetic-like behavior and spin-orbit coupling in (Co,Zn)RE4W3O16 tungstates (RE=Nd, Sm, Eu, Gd, Dy and Ho)

Magnetic susceptibility measurements carried out on (Co,Zn)RE₄W₃O₁₆ compounds revealed a disordered state of magnetic moments above 4.2 K for all compounds under study, and a weak response to magnetic field and temperature for ZnSm₄W₃O₁₆ and ZnEu₄W₃O₁₆ samples. The temperature independent component of magnetic susceptibility has a negative value for ZnGd₄W₃O₁₆ and a positive one for the rest of the tungstates, indicating a domination of van Vleck contribution. The magnetization isotherms of majority of the tungstates under study revealed both spontaneous magnetic moments and hysteresis characteristic for the superparamagnetic-like behavior with blocking temperature T_B∼30 K, except for ZnEu₄W₃O₁₆. Fitting procedure of the Landé factor revealed that the stronger the orbital contribution, the weaker the superparamagnetic effect, namely for ZnRE₄W₃O₁₆. In case of CoRE₄W₃O₁₆ a significant participation of the Co²⁺ moment in the spontaneous magnetization was observed.     

Effect of sintering temperature on structure and nonlinear dielectric properties of Ba0.6Sr0.4TiO3 ceramics prepared by the citrate method

Ba_0.6Sr_0.4TiO₃ ceramics were prepared by a citrate precursor method. The structure and nonlinear dielectric properties of the resulting ceramics were investigated within the sintering temperature range 1200-1300 °C. Adopting fine Ba_0.6Sr_0.4TiO₃ powder derived from the citrate method was confirmed to be effective in reducing the sintering temperatures required for densification. The ceramic specimens sintered at 1230-1280 °C presented relative densities of around 95%. A significant influence of sintering temperature on the microstructure and nonlinear dielectric properties was detected. The discrepancy in nonlinear dielectric behavior among the specimens sintered at different temperatures was qualitatively interpreted in terms of the dielectric response of polar micro-regions under bias electric field. The specimens sintered at 1230 and 1250 °C attained superior nonlinear dielectric properties, showing relatively low dielectric losses (tan δ) of 0.24% and 0.22% at 10 kHz together with comparatively large figure of merits (FOM) of 121 and 142 at 10 kHz and 20 kV/cm, respectively.     

Rapid microwave-assisted hydrothermal synthesis of Bi12TiO20 hierarchical architecture with enhanced visible-light photocatalytic activities

Flower-like Bi₁₂TiO₂₀ hierarchical nanostructures composed of numerous nanobelts were synthesized at 180 °C within 1 h by a microwave-assisted hydrothermal method in the presence of cetyltrimethylammonium bromide (CTAB) for the first time. The as-prepared products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and ultraviolet–visible (UV–vis) absorption spectroscopy. Furthermore, the hierarchical Bi₁₂TiO₂₀ nanostructures exhibited higher photocatalytic activities in the degradation of Rhodamine B under visible-light irradiation than that of the samples prepared without CTAB. In addition, the role of CTAB cationic surfactant has been investigated thoroughly and a possible mechanism is proposed.     

Photocatalytic properties of lanthanide tungstates Ln2W2O9 (Ln=La,Pr, Nd,Sm, and Gd)

Lanthanide tungstates, Ln₂W₂O₉ (Ln=La, Pr, Nd, Sm, and Gd), were prepared via the polymerized complex method at 1273 K for 2 h, and their photocatalytic activities for hydrogen and oxygen evolution were investigated. Pt-loaded Gd₂W₂O₉ exhibited activity for H₂ evolution from an aqueous methanol solution under light irradiation (λ>300 nm). The remaining Ln₂W₂O₉ were inactive for H₂ evolution due to the influence of the Ln elements and their crystal structures. All Ln₂W₂O₉ were inactive for O₂ evolution from an aqueous AgNO₃ solution due to the lack of O₂ evolution sites on the surface.     

Solution-combusting preparation of mono-dispersed Mn3O4 nanoparticles for electrochemical applications

Manganese oxide (Mn₃O₄) nanoparticles with average diameter of 15 nm were prepared using alcohol solution of manganese chloride as starting material via a facile solution-combusting method. The flame core zone was chosen to prepare mono-dispersed and high crystalline products, which were employed to modify glassy carbon electrode and detect dopamine via cyclic voltammetry. The results exhibited excellent electrochemical sensitivity. A linear relationship between the concentration of dopamine and its oxidation peak current was obtained by differential pulse voltammetry, which will find wide application in the biological detection.     

Preparation and characterization of SrSnO3 nanorods

Perovskite strontium stannate (SrSnO₃) nanorods were prepared by annealing the precursor SnSr(OH)₆ nanorods at 600 °C for 3 h. The precursor nanorods were hydrothermally synthesized at 160 °C for 16 h using Sr(NO₃)₂ and SnCl₄·5H₂O as starting materials in the presence of surfactant cetyltrimethyl ammonium bromide (CTAB). As-prepared samples were characterized by X-ray diffraction (XRD), thermogravimetric-differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and infrared ray spectroscopy (IR). The results show that the as-synthesized powders are made of SrSnO₃ one-dimensional nanorods of about 0.2-1 μm length and 100-150 nm diameter. Possible formation mechanism of SrSnO₃ with nanorod structure under certain conditions was preliminarily analyzed, in which it was thought that CTAB played an important role in the formation process of the nanorod structure. Electrochemical performance of the samples versus Li metal was also evaluated for possible use in lithium-ion batteries.     

Influence of initial pH on the particle size and fluorescence properties of the nano scale Eu(III) doped yttria

Europium doped ytrrium oxide (Eu:Y₂O₃) was synthesized by a chemical wet method in the presence of tween-80 and ?-caprolactam in pH range 4-10. It has been observed that the variation in surface area, pore size, and pore volume of the final product, was strongly dependent on the initial pH of the solution. The powder with a large surface area (∼230 m²/g) and low pore diameter (∼16 nm) was obtained when the powder was processed at pH ∼4. The crystallite sizes of the powders processed at pH ∼4 and 10, were found to be 35 and 198 nm, respectively. The structural, chemical and thermal studies of the powders were characterized by X-ray diffraction (XRD), Fourier transformed infrared spectrophotometer (FTIR), Carbon analyzer and Thermogravimetry (TGA). High resolution transmission electron microscopic (HRTEM) study of heat treated powders showed a polygonal morphology with particle size of 40 nm when powder was derived at pH ∼4. Observations of fluorescence suggested that the ⁵D₀→⁷F₂ transition within europium was found to be highly dependent on the initial pH.     

Chemical reactions in cracks of aluminum crystals: Generation of hydrogen from water

Pure hydrogen is generated from water molecules which are dissociated by specific aluminum particles called activated Al powder. Reaction mechanism of Al atoms with H₂O molecules is investigated in micro-cracks of Al crystals. It becomes obvious that hydrogen atoms exist in Al crystal mainly in states of AlH₃ hydrides. It is concluded that virgin walls of micro-cracks right after the creation provide virtually Al radical atoms of (Al−) or (Al=) with one or two free bonds, which react with H₂O molecules via surface diffusion resulting in producing AlH₃ and eventually in producing H₂. The production of H₂ seems to be a result of micro-tribochemical reactions in cracks, which are produced by mechanical crushing of Al crystals in water; tips of cracks as stress-focused points play a major role to create AlH_3. Peculiar environments of nano-spaces in micro-cracks surrounded by reactive atoms enable us to realize unusual chemical reactions at low temperatures as exemplified in the present paper.     

Recent development and application of Li4Ti5O12 as anode material of lithium ion battery

Lithium-ion batteries with both high power and high energy density are one of the promising power sources for electric devices, especially for electric vehicles (EV) and other portable electric devices. One of the challenges is to improve the safety and electrochemical performance of lithium ion batteries anode materials. Li₄Ti₅O₁₂ has been accepted as a novel anode material of power lithium ion battery instead of carbon because it can release lithium ions repeatedly for recharging and quickly for high current. However, Li₄Ti₅O₁₂ has an insulating character due to the electronic structure characterized by empty Ti 3d-states, and this might result in the insufficient applications of LTO at high current discharge rate before any materials modifications. This review focuses first on the present status of Li₄Ti₅O₁₂ including the synthesized method, doping, surface modification, application and theoretical calculation, then on its near future development.     

Magnetic susceptibility of vanadium garnets NaPb2Co2V3O12 and NaPb2Ni2V3O12

Vanadium garnets NaPb₂Co₂V₃O₁₂ and NaPb₂Ni₂V₃O₁₂ have been successfully synthesized. The X-ray diffraction experiments indicate that these compounds have the garnet structure of cubic symmetry of space group with the lattice constant of 12.742 Å (NaPb₂Co₂V₃O₁₂) and 12.666 Å (NaPb₂Ni₂V₃O₁₂), respectively. The magnetic susceptibility of NaPb₂Ni₂V₃O₁₂ shows the Curie-Weiss paramagnetic behavior between 4.2 and 350 K. The effective magnetic moment μ_eff of NaPb₂Ni₂V₃O₁₂ is 3.14 μ_B due to Ni²⁺ ion at A-site and the Weiss constant is −3.67 K (antiferromagnetic sign). For NaPb₂Co₂V₃O₁₂, the simple Curie-Weiss law cannot be applicable. The ground state is the spin doublet and the first excited state is spin quartet , according to Tanabe-Sugano energy diagram on the basis of octahedral crystalline symmetry. This excited spin quartet state just a bit higher than ground state influences strongly the complex temperature dependence of magnetic susceptibility for NaPb₂Co₂V₃O₁₂.