Visible light-sensitive yellow TiO2−xNx and Fe-N co-doped Ti1−yFeyO2−xNx anatase photocatalysts

Nitrogen substituted yellow colored anatase TiO_2−xN_x and Fe-N co-doped Ti_1−yFe_yO_2−xN_x have been easily synthesized by novel hydrazine method. White anatase TiO_2−δ and N/Fe-N-doped samples are semiconducting and the presence of ESR signals at g ∼1.994-2.0025 supports the oxygen vacancy and g∼4.3 indicates Fe³⁺ in the lattice. TiO_2−xN_x has higher conductivity than TiO_2−x and Fe/Fe-N-doped anatase and the UV absorption edge of white TiO_2−x extends in the visible region in N, Fe and Fe-N co-doped TiO₂, which show, respectively, two band gaps at ∼3.25/2.63, ∼3.31/2.44 and 2.8/2.44 eV. An activation energy of ∼1.8 eV is observed in Arrhenius log resistivity vs. 1/T plots for all samples. All TiO₂ and Fe-doped TiO₂ show low 2-propanol photodegradation activity but have significant NO photodestruction capability, both in UV and visible regions, while standard Degussa P-25 is incapable in destroying NO in the visible region The mid-gap levels that these N and Fe-N-doped TiO₂ consist may cause this discrepancy in their photocatalytic activities.     

Luminescence in some tantalate host lattices

The luminescence of the following systems are reported: ScTa_1−xNb_xO₄, Li₃Ta_1−xNb_xO₄, CaTa_2−2xNb_2xO₆ and Mg₄Ta_2−2xNb_2xO₉. The dependence of the luminescence properties on x becomes more pronounced if the distance between the pentavalent ions becomes shorter. In these systems the NbO₆ group gives an efficient blue emission at 300°K. The compound Mg₄Ta₂O₉ gives an efficient ultraviolet emission at 300°K. At 77°K both Mg₄Nb₂O₉ and Mg₄Ta₂O₉ show in addition a weaker emission peaking at wavelengths longer than the wavelength of the room temperature emission. This low-temperature emission can be excited by radiation with wavelengths longer than that of the absorption edge. The Mn²⁺ ion in Mg₄Ta₂O₉Mn acts as an efficient activator with a deep-red emission.     

New synthesis of excellent visible-light TiO2−xNx photocatalyst using a very simple method

An excellent visible-light-responsive (from 400 to 550 nm) TiO_2−xN_x photocatalyst was prepared by a simple wet method. Hydrazine was used as a new nitrogen resource in this paper. Self-made amorphous titanium dioxide precursor powders were dipped into hydrazine hydrate, and calcined at low temperature (110 °C) in the air. The TiO_2−xN_x was successfully synthesized, following by spontaneous combustion. The photocatalyst was characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), transmission electron microscope (TEM), UV-Vis diffuse reflectance spectrometer (DRS), and X-ray photoelectron spectroscopy (XPS). Analysis of XPS indicated that N atoms were incorporated into the lattice of the titania crystal during the combustion of hydrazine on the surface of TiO₂. Ethylene was selected as a target pollutant under visible-light excitation to evaluate the activity of this photocatalyst. The newly prepared TiO_2−xN_x photocatalyst with strong photocatalytic activity and high photochemical stability under visible-light irradiation was firstly demonstrated in the experiment.     

Synthesis, structure and electrical properties of Cu3.21Ti1.16Nb2.63O12 and the CuOx-TiO2-Nb2O5 pseudoternary phase diagram

Subsolidus phase relations in the CuO_x-TiO₂-Nb₂O₅ system were determined at 935 °C. The phase diagram contains one new phase, Cu_3.21Ti_1.16Nb_2.63O₁₂ (CTNO) and one rutile-structured solid solution series, Ti_1−3xCu_xNb_2xO₂: 0<x<0.2335 (35). The crystal structure of CTNO is similar to that of CaCu₃Ti₄O₁₂ (CCTO) with square planar Cu²⁺ but with A site vacancies and a disordered mixture of Cu⁺, Ti⁴⁺ and Nb⁵⁺ on the octahedral sites. It is a modest semiconductor with relative permittivity ∼63 and displays non-Arrhenius conductivity behavior that is essentially temperature-independent at the lowest temperatures.     

Influence of Oxygen Nonstoichiometry on the Spectral Properties of Solid Solutions LaNb2−2xTa2xVO9−δ (x=0-0.4) and LaTa2−2xNb2xVO9−δ (x=0-0.1)

Phase equilibria in the LaVO₄-Nb₂O₅-Ta₂O₅ system were analyzed. New solid solutions LaTa_2−2xNb_2xVO_9−δ (x=0-0.1) and LaNb_2−2xTa_2xVO_9−δ (x=0-0.4) were detected in this system. The structures of the vanadate-niobate LaNb₂VO₉ and vanadate-tantalate LaTa₂VO₉ are not known. The structures of the vanadate-tantalate LaTa₂VO₉ and LaTa₂VO₉-based solid solutions are similar to the structure of LaTa₇O₁₉, which refers to the hexagonal crystal system. The influence of the oxygen nonstoichiometry δ(x) on crystallochemical characteristics and spectral properties of these solid solutions were examined by the X-ray phase analysis, IR and radio spectroscopic methods. A correlation between the nonstoichiometry δ(x) and the volume of a unit cell V(x) of solid solutions LaTa_2−2xNb_2xVO_9−δ was found. The IR spectrum of LaTa₂VO_9−δ transformed in going from δ=0 to δ≠0. Two types of VO₄ tetrahedra were formed in solid solutions LaNb_2−2xTa_2xVO_9−δ depending on δ(x).     

Formation of crystalline TiO2−xNx and its photocatalytic activity

Amorphous precursors to nitrogen-doped TiO₂ (NTP) and pure TiO₂ (ATP) powders were synthesized by hydrolytic synthesis and sol-gel method (SGM), respectively. Corresponding crystalline phases were obtained by thermally induced transformation of these amorphous powders. From FT-IR and XPS data, it was concluded that a complex containing titanium and ammonia was formed in the precipitate stage while calcination drove weakly adsorbed ammonium species off the surface, decomposed ammonia bound on surface of precipitated powder and led to substitution of nitrogen atom into the lattice of TiO₂ during the crystallization. The activation energies required for grain growth in amorphous TiO_2−xN_x and TiO₂ samples were determined to be 1.6 and 1.7 kJ/mol, respectively. Those required for the phase transformation from amorphous to crystalline TiO_2−xN_x and TiO₂ were determined to be 129 and 142 kJ/mol, respectively. A relatively low temperature was required for the phase transformation in NTP sample than in ATP sample. The fabricated N-doped TiO₂ photocatalyst absorbed the visible light showing two absorption edges; one in UV range due to titanium oxide as the main edge and the other due to nitrogen doping as a small shoulder. TiO_2−xN_x photocatalyst demonstrated its photoactivity for photocurrent generation and decomposition of 2-propanol (IPA) under visible light irradiation ().     

Temperature and composition dependent structural investigation of the defect perovskite series Sr1−xTi1−2xNb2xO3, 0≤x≤0.2

The crystal structure of the defect perovskite series Sr_1−xTi_1−2xNb_2xO₃ has been investigated over a range of temperatures using high-resolution synchrotron X-ray diffraction, neutron diffraction and electron diffraction. Three distinct regions were observed: 0<x≤0.125 was a solid solution of Sr_1−xTi_1−2xNb_2xO₃ with minor SrTiO₃ intergrowth, 0.125<x≤0.2 was a pure Sr_1−xTi_1−2xNb_2xO₃ solid solution adopting the cubic perovskite type structure (Pm3¯m) and for x>0.2 Sr_0.8Ti_0.6Nb_0.4O₃ and Sr₃TiNb₄O₁₅ formed a two phase region. The cubic structure for Sr_0.8Ti_0.6Nb_0.4O₃ was stable over the temperature range 90-1248 K and the thermal expansion co-efficient was determined to be 8.72(9)×10⁻⁶ K⁻¹. Electron diffraction studies revealed diffuse scattering due to local scale Ti/Nb displacements and slightly enhanced octahedral rotations that did not lead to long range order. The octahedral rotations were observed to ‘lock-in’ at temperatures below ∼75 K resulting in a tetragonal structure (I4/mcm) with anti-phase octahedral tilting about the c-axis.     

Evolution and characterization of fluorite-like nano-SrBi2Nb2O9 phase in the SrO-Bi2O3-Nb2O5-Li2B4O7 glass system

Transparent glasses of various compositions in the system (100−x)Li₂B₄O₇−x(SrO-Bi₂O₃-Nb₂O₅) (where x=10, 20, 30, 40, 50 and 60, in molar ratio) were fabricated via splat quenching technique. The glassy nature of the as-quenched samples was established by differential thermal analyses. X-ray powder diffraction (XRD) and transmission electron microscopic studies confirmed the amorphous nature of the as-quenched and crystallinity in the heat-treated samples. Fluorite phase formation prior to the perovskite SrBi₂Nb₂O₉ phase was analyzed by both the XRD and high-resolution transmission electron microscopy. Dielectric and the optical properties (transmission, optical band gap and Urbach energy) of these samples have been found to be compositional dependent. Refractive index was measured and compared with the values predicted by Wemple-Didomemenico and Gladstone-Dale relations. The glass nanocomposites comprising nanometer-sized crystallites of fluorite phase were found to be nonlinear optic active.     

Preparation of titania microfiltration membranes supported on porous Ti–Al alloys

A TiO₂ membrane supported on a planar porous Ti–Al alloy was prepared by combination of electrophoretic deposition and dip-coating. In the electrophoretic deposition process, the membrane thickness increased linearly with the square root of the deposition time, while increased with decrease of the suspension viscosity. The perfect TiO₂/Ti–Al composite membrane was obtained by further dip-coating modification. SEM images showed that the surface of the membrane was defect-free. XRD result indicated that rutile TiO₂ still remained in the membrane bulk as the main phase, while a new phase titanium oxides with the form of Ti_xO_y, where y is less than 2x, was also observed. The supported TiO₂/Ti–Al composite membrane had an average pore size of 0.28 μm, a thickness of 40 μm or so and a pure water flux of 3037 L m⁻² h⁻¹ bar⁻¹.     

BiOBr/g-C3N4 S型异质结无H2O2光-自芬顿高效催化降解RhB

通过焙烧-超声混合法成功地制备了BiOBr/g-C₃N₄S型异质结复合光催化剂。采用多种表征手段对样品物理属性进行了表征,包括X射线多晶粉末衍射仪(XRD)、扫描电子显微镜(SEM)、X射线光电子能谱(XPS)、紫外可见漫反射光谱(UV-Vis DRS)。研究了所制备样品有/无Fe³⁺的光-自芬顿催化/光催化降解罗丹明B(RhB)性能。通过捕获实验确定了光催化反应中的主要活性物种,提出了光-自芬顿反应的降解机理。研究结果表明,BiOBr/g-C₃N₄S型异质结能原位生成H₂O₂,添加Fe³⁺后,H₂O₂被原位活化成活性物种且光生电流和载流子分离效率获得显著提高。该光-自芬顿过程能高效降解RhB,其反应速率常数为0.208 min^-1,约为无Fe³⁺光催化反应速率常数的5.3倍,在光-自芬顿循环使用过程中表现出良好的稳定性。Fe...     

Sur de nouveaux germanates et silicates de type bénitoïte

Five new “benitoïte” type germanates: ATaGe₃O₉ (A = K, Rb, Tl), RbNbGe₃O₉, and the silicate BaSnSi₃O₉ were synthesized. Solid solutions A(Ta_1−xNb_x) Ge₃O₉, ATa(Ge_3−ySi_y)O₉ (A = K, Rb, Tl) and BaSn(Ge_3−ySi_y)O₉ were also studied. Positions of atoms in the hexagonal cell were determined from intensity of X-ray powder diffraction patterns.     

Synthesis and characterization of Ti1−2xNbxNixO2−x/2 solid solutions

Doped-rutile has been traditionally used in ceramic pigments for its intense optical properties. In this paper, we compare the classical ceramic synthesis of Ti_1−2xNb_xNi_xO_2−x/2 system with the sol-gel methodology, which allows a reduction of the anatase-rutile transformation temperature. The composition was optimised in order to obtain a unique rutile phase with the minimum amount of pollutant Ni(II) and enhanced chromatic coordinates. Incorporation of the doping ions in the rutile structure was corroborated by XRD and Rietveld refinements. The species responsible for the colour mechanism were studied by different techniques. UV-VIS spectroscopy showed the characteristic features of Ni²⁺ ions, whose existence was corroborated by EPR and magnetic measurements. From these results, (Ni,Nb)doped-TiO₂ powder samples can be now shaped as thin films, monoliths, etc. by using sol-gel methodology without modifying their properties. This study introduces new possibilities of coloured TiO₂-based solid solutions in new combined advanced applications (colouring agent and photocatalyst, etc.).     

Electrochromism in Materials Prepared by the Sol-Gel Process

Electrochromism is defined as the persistent but reversible optical change (usually transmission) produced electrochemically. The preparation by the sol-gel process of thin films made of amorphous or crystalline nanoparticles of WO₃, V₂O₅, Nb₂O₅, TiO₂, CeO₂, Fe₂O₃ and mixed compounds such as WO₃−TiO₂, CeO₂−TiO₂, CeO₂−SnO₂, have opened remarkable new opportunities for obtaining electrochromic layers exhibiting large optical transmission variation in the UV, visible or infrared range and acceptable kinetics under H⁺ or Li⁺ insertion. In this paper we give an overview of what has been recently achieved in this field, with emphasis for cathodic electrochromic coatings of Nb₂O₅ and TiO₂ composition. Finally we stress the future developments in this fast growing field.     

V2.38Nb10.7O32.7: A V2O5-Nb2O5 mixed oxide tunnel structure related to the tetragonal tungsten bronzes

A crystal structural model for the orthorhombic compound V_2.38Nb_10.7O_32.7, which is known as “V₂Nb₉O_27.5”, was developed by means of selected area electron diffraction (SAED), Rietveld refinement and high resolution electron microscopy (HREM). The metastable compound is obtained by thermal decomposition of freeze-dried precursors as chain-like agglomerated nanoparticles or by reaction of V₂O₅ with fresh-precipitated Nb₂O₅ as more compact micro-scaled crystals. With the latter, it was possible to identify its structure for the first time (space group Cmmm). The tetragonal tungsten bronze (TTB)-type structure shows high potential for ionic intercalation, since easily reducible [V⁵⁺₂O²⁻] units are implemented in the tunnels of a rigid niobium oxide framework.     

Synthesis and characterization of rare-earth doped SrBi2Nb2O9 phase in lithium borate based nanocrystallized glasses

Glass composites comprising of un-doped and samarium-doped SrBi₂Nb₂O₉ nanocrystallites are fabricated in the glass system 16.66SrO-16.66[(1−x)Bi₂O₃-xSm₂O₃]-16.66Nb₂O₅-50Li₂B₄O₇ (0?x?0.5, in mol%) via the melt quenching technique. The glassy nature of the as-quenched samples is established by differential thermal analyses. Transmission electron microscopic studies reveal the presence of about 15 nm sized spherical crystallites of the fluorite-like SrBi_1.9Sm_0.1Nb₂O₉ phase in the samples heat treated at 530 °C. The formation of layered perovskite-type un-doped and samarium-doped SrBi₂Nb₂O₉ nanocrystallites with an orthorhombic structure through the intermediate fluorite phase is confirmed by X-ray powder diffraction and micro-Raman spectroscopic studies. The influence of samarium doping on the lattice parameters, lattice distortions, and the Raman peak positions of SrBi₂Nb₂O₉ perovskite phase is clarified. The dielectric constants of the perovskite SrBi₂Nb₂O₉ and SrBi_1.9Sm_0.1Nb₂O₉ nanocrystals are relatively larger than those of the corresponding fluorite-like phase and the precursor glass.     

Pyrochlore formation, phase relations, and properties in the CaO-TiO2-(Nb,Ta)2O5 systems

Phase equilibria studies of the CaO:TiO₂:Nb₂O₅ system confirmed the formation of six ternary phases: pyrochlore (A₂B₂O₆O′), and five members of the (110) perovskite-slab series Ca_n(Ti,Nb)_nO_3n+2, with n=4.5, 5, 6, 7, and 8. Relations in the quasibinary Ca₂Nb₂O₇−CaTiO₃ system, which contains the Ca_n(Ti,Nb)_nO_3n+2 phases, were determined in detail. CaTiO₃ forms solid solutions with Ca₂Nb₂O₇ as well as CaNb₂O₆, resulting in a triangular single-phase perovskite region with corners CaTiO₃-70Ca₂Ti₂O₆:30Ca₂Nb₂O₇-80CaTiO₃:20CaNb₂O₆. A pyrochlore solid solution forms approximately along a line from 42.7:42.7:14.6 to 42.2:40.8:17.0 CaO:TiO₂:Nb₂O₅, suggesting formulas ranging from Ca_1.48Ti_1.48Nb_1.02O₇ to Ca_1.41Ti_1.37Nb_1.14O₇ (assuming filled oxygen sites), respectively. Several compositions in the CaO:TiO₂:Ta₂O₅ system were equilibrated to check its similarity to the niobia system in the pyrochlore region, which was confirmed. Structural refinements of the pyrochlores Ca_1.46Ti_1.38Nb_1.11O₇ and Ca_1.51Ti_1.32V_0.04Ta_1.10O₇ using single-crystal X-ray diffraction data are reported (Fd3m (#227), a=10.2301(2) Å (Nb), a=10.2383(2) Å (Ta)), with Ti mixing on the A-type Ca sites as well as the octahedral B-type sites. Identical displacive disorder was found for the niobate and tantalate pyrochlores: Ca occupies the ideal 16d position, but Ti is displaced 0.7 Å to partially occupy a ring of six 96g sites, thereby reducing its coordination number from eight to five (distorted trigonal bipyramidal). The O′ oxygens in both pyrochlores were displaced 0.48 Å from the ideal 8b position to a tetrahedral cluster of 32e sites. The refinement results also suggested that some of the Ti in the A-type positions may occupy distorted tetrahedra, as observed in some zirconolite-type phases. The Ca-Ti-(Nb,Ta)-O pyrochlores both exhibited dielectric relaxation similar to that observed for some Bi-containing pyrochlores, which also exhibit displacively disordered crystal structures. Observation of dielectric relaxation in the Ca-Ti-(Nb,Ta)-O pyrochlores suggests that it arises from the displacive disorder and not from the presence of polarizable lone-pair cations such as Bi³⁺.     

Enzyme mimics of spinel-type CoxNi1−xFe2O4 magnetic nanomaterial for eletroctrocatalytic oxidation of hydrogen peroxide

A series of spinel-type Co_xNi_1−xFe₂O₄ (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) magnetic nanomaterials were solvothermally synthesized as enzyme mimics for the eletroctrocatalytic oxidation of H₂O₂. X-ray diffraction and scanning electron microscope were employed to characterize the composition, structure and morphology of the material. The electrochemical properties of spinel-type Co_xNi_1−xFe₂O₄ with different (Co/Ni) molar ratio toward H₂O₂ oxidation were investigated, and the results demonstrated that Co_0.5Ni_0.5Fe₂O₄ modified carbon paste electrode (Co_0.5Ni_0.5Fe₂O₄/CPE) possessed the best electrocatalytic activity for H₂O₂ oxidation. Under optimum conditions, the calibration curve for H₂O₂ determination on Co_0.5Ni_0.5Fe₂O₄/CPE was linear in a wide range of 1.0 × 10⁻⁸–1.0 × 10⁻³ M with low detection limit of 3.0 × 10⁻⁹ M (S/N = 3). The proposed Co_0.5Ni_0.5Fe₂O₄/CPE was also applied to the determination of H₂O₂ in commercial toothpastes with satisfactory results, indicating that Co_xNi_1−xFe₂O₄ is a promising hydrogen peroxidase mimics for the detection of H₂O₂.     

Structural and magnetic characterisation of Aurivillius material Bi2Sr2Nb2.5Fe0.5O12

The n=3 Aurivillius material Bi₂Sr₂Nb_2.5Fe_0.5O₁₂ is investigated and combined structural refinements using neutron powder diffraction (NPD) and X-ray powder diffraction data (XRPD) data reveal that the material adopts a disordered, tetragonal (I4/mmm) structure at temperatures down to 2 K. Significant ordering of Fe³⁺ and Nb⁵⁺ over the two B sites is observed and possible driving forces for this ordering are discussed. Some disorder of Sr²⁺ and Bi³⁺ over the M and A sites is found and is consistent with relieving strain due to size mismatch. Highly anisotropic thermal parameters for some oxygen sites suggest that the local structure may be slightly distorted with some rotation of the octahedra. Magnetic measurements show that the material behaves as a Curie-Weiss paramagnet in the temperature range studied with no evidence of any long-range magnetic interactions. Solid solutions including Bi_3−xSr_xNb₂FeO₁₂, Bi₂Sr_2−xLa_xNb₂FeO₁₂ and Bi₂Sr₂Nb_3−xFe_xO₁₂ were investigated but single-phase materials were only successfully synthesised for a narrow composition range in the Bi₂Sr₂Nb_3−xFe_xO₁₂ system.     

BiOBr/g-C3N4 S型异质结无H2O2光-自芬顿高效催化降解RhB

通过焙烧-超声混合法成功地制备了BiOBr/g-C₃N₄ S型异质结复合光催化剂。采用多种表征手段对样品物理属性进行了表征,包括X射线多晶粉末衍射仪(XRD)、扫描电子显微镜(SEM)、X射线光电子能谱(XPS)、紫外可见漫反射光谱(UV-VisDRS)。研究了所制备样品有/无Fe³⁺的光-自芬顿催化/光催化降解罗丹明B (RhB)性能。通过捕获实验确定了光催化反应中的主要活性物种,提出了光-自芬顿反应的降解机理。研究结果表明,BiOBr/g-C₃N₄ S型异质结能原位生成H₂O₂,添加Fe³⁺后,H₂O₂被原位活化成活性物种且光生电流和载流子分离效率获得显著提高。该光-自芬顿过程能高效降解RhB,其反应速率常数为0.208 min^-1,约为无Fe³⁺光催化反应速率常数的5.3倍,在光-自芬顿循环使用过程中表现出良好的稳定性。Fe³⁺的加入促进了光生电荷的分离和H₂O₂的活化,超氧阴离子自由基(·O₂^-)、空穴和羟基是光-自芬顿催化过程中的主要活性物种,且·O₂^-作用更大。     

Solid-state synthesis in the system Na0.8NbyW1−yO3 with 0?y?0.4: A new phase, Na0.5NbO2.75, with perovskite-type structure

Series of compounds in the system Na_xNb_yW_1−yO₃ were prepared according to the appropriate molar ratio of Na₂WO₄, WO₃, WO₂ and Nb₂O₅ with x=0.80 and 0.0?y?0.4 at 600 °C in evacuated silica glass tubes. These compounds were investigated by X-ray powder diffraction, optical microscopy, microprobe analysis, Raman and optical microspectroscopy. A y-dependent separation into three distinct coloured crystallites with cubic perovskite-type structures is observed: (i) red-orange crystallites with composition Na_xWO₃ with slightly decreasing x (i.e. 0.8-0.72) with increasing nominal y, (ii) bluish solid solution of composition Na_xNb_yW_1−yO₃ and (iii) white crystallites of a new phase having defect perovskite-type structure with composition Na_0.5NbO_2.75.