TiO3 Solid Solution Research Articles

Sn-doped TiO2 modified carbon to support Pt anode catalysts for direct methanol fuel cells

Catalyst supports are known to play important role in governing overall catalyst activity and durability. Here, a new type of SnO2–TiO2 solid solution (TixSn1-xO2) support was prepared via a solvothermal method with substitution of Ti4+ by Sn4+ in the TiO2 lattice. Furthermore, the TixSn1-xO2 was combined with conventional carbon black (Vulcan XC-72) to prepare a hybrid support (TixSn1-xO2–C) for depositing Pt nanoparticles. The ratios of Sn vs. Ti in the solid-solution and TixSn1-xO2vs. XC-72 were systematically optimized in terms of their performance as supports for methanol oxidation. Compared to Pt/TiO2–C and commercial Pt/C catalysts, the best performing Pt/Ti0.9Sn0.1O2–C catalyst exhibited the highest activity, evidenced by methanol oxidation and CO stripping experiments. The well-dispersed Pt nanoparticles (2–3 nm) are mostly deposited on the boundaries of Ti0.9Sn0.1O2 and carbon blacks. Formation of the special triple junction structure can play an important role in improving Pt utilization with increased electrochemical active surface areas (ESA) of Pt. In addition, the enhanced activity for Pt supported on Ti0.9Sn0.1O2–C is due to high content of OH group on Ti0.9Sn0.1O2 along with the strengthened metal-supports interactions. Both promote the oxidation of poisoning CO absorbed on Pt active sites.

Defect structure of xSc2O3 · (1 − x)TiO2 (x = 0.4–0.5) solid solutions

X-ray diffraction characterization with monochromatic synchrotron X-rays has demonstrated that xSc2O3 · (1 − x)TiO2 (x = 0.4–0.5) solid solutions consist of a fluorite-like (Fm3m) disordered phase and a nanoscale (≃ 10–50 nm) pyrochlore-like (Fd3m) ordered phase of the same composition, coherent with the disordered phase. We have determined their lattice parameters. The formation of nanodomains with different degrees of order is shown to be caused by the internal strain due to the high density of structural defects in their unit cells. The materials obtained in this study possess enhanced sorption capacity and can be used as catalysts, catalyst supports, gas sensors, etc.

Defect structure of xY2O3 · (1 − x)TiO2 (x = 0.5–0.58) solid solutions

X-ray diffraction characterization with monochromatic synchrotron X-rays has demonstrated that xY2O3 · (1 − x)TiO2 (x = 0.5–0.58) solid solutions consist of a fluorite-like (Fm3m) disordered phase and a nanoscale (≃40–1000 nm) pyrochlore-like (Fd3m) ordered phase of the same composition, coherent with the disordered phase. In the composition range of the solid solutions (0.5 ≤ x ≤ 0.58), the lattice parameter of the fluorite-like phase follows Vegard’s law. The formation of nanodomains with different degrees of order is shown to be caused by the internal strain due to the high density of structural defects in their unit cells. The materials obtained in this study possess enhanced sorption capacity and can be used as catalysts, catalyst supports, gas sensors, etc.

Improvement of dielectric loss of Ba0.75Sr0.25TiO3 tunable material by La0.5Na0.5TiO3 addition

The structure and microstructure of (1 − x)Ba0.75Sr0.25TiO3–xLa0.5Na0.5TiO3 solid solution were investigated by XRD, Raman and microscopy analysis in this study. The temperature dependent dielectric properties and tunability under DC field were investigated at low frequencies with broad band dielectric spectrometer. Microwave dielectric properties were measured with a network analyzer. The compound demonstrated relaxor behavior when x ≤ 0.4 and incipient ferroelectric character when x ≥ 0.6, respectively. The dielectric permittivity (er) and tunability decreased, whereas the Q × f value increased with increasing x. The material with dielectric constant ~273, Q × f = ~1,700 GHz at 2.8 GHz and tunability ~9 % at E = 4,000 V/mm were obtained.

Microstructures and Microwave Dielectric Properties of Low-Temperature Fired Ca0.8Sr0.2TiO3-Li0.5Sm0.5TiO3 Ceramics with Bi2O3-2B2O3 Addition

The microstructures and microwave dielectric properties of xCa0.8Sr0.2TiO3-(1−x) Li0.5Sm0.5TiO3 (x = 0.25, 0.28, 0.3, 0.35 and 0.4) ceramics with about 20 wt.% Bi2O3-2B2O3 (BB) addition were investigated. All the composite ceramics prepared using the conventional mixed oxide route consisted of a primarily orthorhombic perovskite xCa0.8Sr0.2TiO3-(1−x)Li0.5Sm0.5TiO3 solid-solution phase and minor secondary phases ascribed to LiBO2 and BiB3O6. It was found that the addition of 20 wt.% Bi2O3-2B2O3 in xCa0.8Sr0.2TiO3-(1−x) Li0.5Sm0.5TiO3 (CSLSTx) phases decreased the optimal sintering temperature range to 1025–1050°C. In the optimal sintering temperature range, the highest values of bulk density (ρ), dielectric constant (er) and the product (Q.f) of quality factor (Q) and frequency (f) for the ceramics were obtained. With x increasing from 0.25 to 0.40, er increased from 94.9 to 116.6 and τf from −14.76 ppm/°C to 100.2 ppm/°C, while Q.f value slightly increased from 1725 GHz to 1745 GHz and then decreased from 1745 GHz to 1541 GHz. For the 100 CSLSTx-20BBO ceramics sintered at 1025°C, a near-zero τf ceramic with er of 100 and Q.f value of 1737 GHz was obtained at x = 0.28.

Photovoltaic properties of Zr(x)Ti(1-x)O2 solid solution nanowire arrays.

In this paper, Zr(0.05Ti(0.95)O2 solid solution nanowire arrays (NWs) were prepared by a low temperature hydrothermal method. The as-prepared NWs were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-visible (UV-vis) spectroscopy. With the doping of Zr content, the band gap of the composite can be varied in a wide range and excellent photoelectric properties of the arrays could be obtained. Moreover, a preliminary study on the photoelectric properties was conducted, which indicates potential applications of the arrays for fabricating high performance ultraviolet photodetectors.

Chemical-photo-electricity diagrams by Ohm’s law – A case study of Ni-doped TiO2 solutions in dye-sensitized solar cells

Unlike phase diagrams, which reveal only the chemical equilibrium in a system, starting from Ohm’s law, we constructed chemical-photo-electricity diagrams to display the relationship among materials chemistry, processing conditions of working electrodes, and photo-current/voltage of dye-sensitized solar cells (DSSCs). We derived new equations that quantify the above relationship by using the DSSC with Ni-doped TiO2 solid solution as an example. The same principles and procedures can be applied to construct chemical-photo-electricity diagrams for other solid solutions in not only photo-voltaic like solar cells but also general optoelectronic and electrochemistry research. Besides being a practical tool, these diagrams may also provide opportunities for a new theory that quantitatively couples chemistry with photo-electricity in semiconductors.

Modified Pechini Synthesis of Proton‐Conducting Ba(Ce,Ti)O3 and Comparative Studies of the Effects of Acceptors on its Structure, Stability, Sinterability, and Conductivity

The phase structure, chemical stability, sinterability, and electrical performance of the proton‐conducting Ba(Ce,Ti)O3 solid solution with a series of acceptors M (M = In, Y, Sm) synthesized by a modified Pechini method were systematically investigated. The substitution of cerium with titanium was proved as an effective way to improve the stability of BaCeO3. Especially for the BaCe0.95Ti0.05O3−δ sample doped with In, no change in the phase was found even after treatment in the atmosphere containing both CO2 and H2O at 700°C for 10 h. Thanks to the highly sinteractive powders with particle size of ~100 nm, dense ceramics were easily acquired. Moreover, compared with the undoped BaCe0.95Ti0.05O3 sample, In, Y, and Sm dopants further improved the sinterability of the solid solution. In particular, In played a role of sintering aid and led to the largest linear shrinkage of the ceramics. As to the electrical performance, the transport properties of the samples under various atmospheres were analyzed and compared. The impedance tests demonstrated the best electrical performance of the Y‐doped samples.

Investigation of some new hydro(solvo)thermal synthesis routes to nanostructured mixed-metal oxides

We present a study of two new solvothermal synthesis approaches to mixed-metal oxide materials and structural characterisation of the products formed. The solvothermal oxidation of metallic gallium by a diethanolamine solution of iron(II) chloride at 240°C produces a crystalline sample of a spinel-structured material, made up of nano-scale particles typically 20nm in dimension. XANES spectroscopy at the K-edge shows that the material contains predominantly Fe2+ in an octahedral environment, but that a small amount of Fe3+ is also present. Careful analysis using transmission electron microscopy and powder neutron diffraction shows that the sample is actually a mixture of two spinel materials: predominantly (>97%) an Fe2+ phase Ga1.8Fe1.2O3.9, but with a minor impurity phase that is iron-rich. In contrast, the hydrothermal reaction of titanium bis(ammonium lactato)dihydroxide in water with increasing amounts of Sn(IV) acetate allows nanocrystalline samples of the SnO2–TiO2 solid solution to be prepared directly, as proved by powder XRD and Raman spectroscopy.

Synchrotron X-ray diffraction study of nanostructured Er2O3-TiO2 (50–60 mol % Er2O3) solid solutions

Monochromatic synchrotron X-ray diffraction data demonstrate that single-crystal and polycrystalline xEr2O3 · (1 − x)TiO2 (x = 0.5–0.6) solid solutions consist of a fluorite-like disordered (Fm3m) phase and a nanoscale (40–1000 nm) pyrochlore-like ordered phase (Fd3m) of the same composition in the range 0.5 ≤ x ≤ 0.57, coherent with the disordered phase. Reducing the density of structural defects in the unit cell of Er3TiO6.5 (x = 0.6) leads to a structural transformation of the pyrochlore-like phase into a Ta2O3-type ordered phase (Ia3), derived from the fluorite phase. In the composition range of the solid solutions (0.5 < x < 0.6), the lattice parameter of the fluorite-like phase follows Vegard’s law. The formation of nanodomains with different degrees of order is shown to be caused by the internal strain due to the high density of structural defects in their unit cells.

Origin of Semiconducting Behavior of CaO Added BaTiO&lt;sub&gt;3&lt;/sub&gt;-(Bi&lt;sub&gt;1/2&lt;/sub&gt;Na&lt;sub&gt;1/2&lt;/sub&gt;)TiO&lt;sub&gt;3&lt;/sub&gt; Ceramics

An effect of addition of CaO on electrical resistivity in BaTiO3-(Bi1/2Na1/2)TiO3(BT-BNT) solid solution ceramics was investigated. Three kinds of CaO adding processes were applied. Extra CaO additive against stoichiometric composition is effective to give semiconductive property into the BT-BNT ceramics. However, the ceramics with CaO-inclusive composition such as Ba0.9(Bi0.5Na0.15Ca0.35)0.1TiO3had a high resistivity. From chemical composition analysis and structure analysis, it was concluded that Ca2+dominantly substituted for Na+in theAsite of perovskite-typeABO3structure and then that residual Bi3+acts a donor.

Effect of Er3+ doping on the thermal stability of TiO2 nanoparticulate xerogels

The effect of Er3+ doping on the thermal stability of TiO2 nanoparticulate xerogels prepared by a colloidal sol–gel route was investigated. It was found that the as-synthesized xerogels crystallize as anatase phase with crystallite sizes in the low nanoscale range ( 100 ± 5 m2/g). Nevertheless, it was also found that the Er3+ cations are deposited on the surface of TiO2 nanocrystallites thus resulting in xerogels with smaller and more uniform nanoaggregates. Most importantly, detailed analyses using X-ray thermo-diffractometry together with selective analyses by transmission electron microscopy, selected area electron diffractometry, and X-ray energy-dispersive spectrometry showed that the thermal stability of these TiO2 nanoparticulate xerogels increases with increasing Er3+ doping. Specifically, the nanocrystallite growth is slowed down, the onset temperature of the anatase-to-rutile phase transformation is delayed, and the anatase phase is retained up to greater temperatures as the Er3+ content increases, which are all phenomena attributable to the formation at low temperatures (i.e. ~200 °C) of solid solutions with Er3+ solutes within the TiO2 host. Moreover, these solid solutions also have lower thermal expansion coefficient than the undoped crystal structure. Finally, Er3+ doping increasingly promotes the precipitation at high temperatures of Er2Ti2O7 from the rutile TiO2 solid solution, with the precipitation temperature decreasing with increasing Er3+ doping content.

Zirconium Incorporation into CaTiO3 Perovskite Prepared from Xerogels and Implication for the Fate of (Ca,Sr)TiO3 Nuclear Waste Ceramics

CaTiO3 perovskite has been proposed as a ceramic waste form for immobilization of 90Sr. Nonradioactive coprecipitated xerogel powders with nominal atomic ratios of Ca:Zr:Ti = 0.75:0.25:1.00 were synthesized to mimic the fate of (Ca0.7590Sr0.25)TiO3 solid solution after complete decay of the Sr and its intermediate product Y to stable Zr when an excess B4+ (Ti and 90Zr) cations will present. Ca:Ti = 1.00:1.00 samples were used as a reference. The powders were heated to various conditions to explore the thermodynamic stability of its oxides. The heated Ca:Zr:Ti = 0.75:0.25:1.00 samples formed a major orthorhombic Ca(Zr1−xTix)O3 perovskite phase. The Ti/(Ti + Zr) ratio of the perovskite preserves its nominal ratio at 600°C. The Zr rejects from the Ca(Zr1−xTix)O3 with further increasing the temperature, following the formation of Ca–Ti–Zr–O secondary phases. This study indicates a tendency of the Zr to segregate from an original (Ca,Sr)TiO3 waste form when the stoichiometry is controlled by the conversion of Sr to Zr (in normal oxidation states).

Material constants of (Ba,Sr)TiO3 solid solutions

The phenomenological theory of solid solution is used to calculate the material constants (elastic compliances, susceptibilities, piezoelectric moduli, pyroelectric constants) of Ba x Sr1 − x TiO3 solid solutions over the entire range of concentrations x at room temperature. The phenomenological potential constants and the numerical values of the material constants are given for specific concentrations.

Effect of Reoxidation Temperature on Electrophysical Properties of High-T&lt;sub&gt;C&lt;/sub&gt; Barium Titanate-Based PTCR Ceramics

Ceramic samples of (1 x)BaTiO3–xNa0.5Bi0.5TiO3 system were prepared by sintering in reducing atmosphere of N2/H2 and were subsequently reoxidized in air. The influence of reoxidation temperature firing on the PTCR effect of (1 x)BaTiO3–xNa0.5Bi0.5TiO3 ceramics was investigated. The effect of Na0.5Bi0.5TiO3 concentration on resistivity and microstructure of the reoxidized samples was investigated by means of complex impedance spectroscopy and scanning electron microscopy. It has been found that the grain size decreases with the increase in Na0.5Bi0.5TiO3 content. The values of minimum ρmin and maximum ρmax resistivities of the samples were observed to increase with the increase in reoxidation temperature in the 600 – 1000°C temperature range. It was shown that with increasing in reoxidation temperature of (1-x)BaTiO3-xNa0.5Bi0.5TiO3 solid solutions, potential barrier at grain boundaries increases.

Emergence of central mode in the paraelectric phase of ferroelectric perovskites

Abstract

Dielectric response of (Ba,Sr)TiO3 thin films in a terahertz and IR ranges

Dependences of the dielectric response spectra on the thickness of the films based on the (Ba,Sr)TiO3 (BST) solid solutions, which were deposited on single-crystal MgO substrates, have been analyzed. Using the Lorentz and Lyddane-Sachs-Teller models, the mutual correspondence of longitudinal optical (LO) and transverse optical (TO) phonon frequencies has established. The giant LO-TO splitting of the lowest-frequency vibration and inversion of LO and TO frequencies of other phonons in the IR spectrum of BST films has been found for the first time.

Influence of In doping on the structure, stability and electrical conduction behavior of Ba(Ce,Ti)O3 solid solution

Pure perovskite structured proton conductive BaCe0.95−xInxTi0.05O3−δ (x=0.1, 0.2, 0.3, 0.4) powders were prepared by a modified Pechini method. The influences of indium doping on the performance of Ba(Ce,Ti)O3 solid solution, especially on the chemical stability and electrical conductivity were systematically investigated. With the increase of indium concentration, the chemical stability against the atmosphere containing CO2 and H2O (94%N2+3%CO2+3%H2O) was gradually enhanced. At the In doping level of 0.3 and 0.4, almost no any phase change is found in the powders at 700°C after 80–100h treatment, which is comparable to the widely-recognized BaCe0.4Zr0.4Y0.2O3−δ proton conductor. The specimen with the In content of 0.3 displayed the highest conductivity under all the atmospheres studied. Comparison between BaCe0.65In0.3Ti0.05O3−δ and BaCe0.4Zr0.4Y0.2O3−δ on the basis of electrochemical impedance spectroscopy analysis further demonstrated that the Ba(Ce,Ti)O3 solid solution containing proper indium content is more conductive and also a potential proton conductor to be used as electrolytes for solid oxide fuel cells.

Crystal structure, local structure, and defect structure of Pr-doped SrTiO3

X-ray diffraction studies showed that the structure of (Sr1−xPrx)TiO3 solid solutions at 300 K changes from the cubic Pm3m to the tetragonal I4/mcm with increasing x. The analysis of XANES and EXAFS spectra of the solid solutions revealed that Pr ions are predominantly in the 3+ oxidation state, they substitute for Sr atoms and are on-center regardless of the preparation conditions. The weak dependence of the lattice parameter in (Sr1−xPrx)TiO3 on the Pr concentration was explained by the competition between the relaxation of the Sr–O bond length, which results from the difference in ionic radii of Sr and Pr ions, and the repulsion of positively charged Pr3+ and Ti4+ ions. It was shown that the most important defects in the crystals are charged Sr vacancies and SrO planar faults; praseodymium does not enter the Sr sites in the planar faults.

Synthesis and electrical characteristics of (1 − x)BaTiO3–xK0.5Bi0.5TiO3 PTCR ceramics

Solid solutions of (1 − x)BaTiO3–xK0.5Bi0.5TiO3 were prepared by the solid state reaction technique. Samples were sintered in reducing atmosphere of N2/H2 in the temperature range 1100–1240 °C with subsequent oxidation at 700 °C. Phase composition and crystal structure were investigated by X-ray powder diffractometry (XRPD). It was shown that all samples of (1 − x)BaTiO3–xK0.5Bi0.5TiO3 (0 ≤ x < 0.4) exhibit a tetragonal structure at room temperature, the parameters a and c decrease with increasing x. With increasing x the Curie temperature of solid solutions (1 − x)BaTiO3–xK0.5Bi0.5TiO3 (0.1 ≤ x < 0.4) increases from 150 to 220 °C. The values of potential barriers at grain boundaries were calculated on the basis of Heywang model. With increasing x, the potential barrier at grain boundaries increases. It was shown that the grain size decreases with increasing the bismuth–potassium titanate content. The complex impedance results indicate that the grain boundary and the outer layer region, located between the boundary and the core of the grain, make a contribution to the positive temperature coefficient of resistance (PTCR) effect in (1 − x)BaTiO3–xK0.5Bi0.5TiO3 solid solutions. With increasing x ρmax increases due to an increase in potential barrier at grain boundaries.