Preparation and photocatalytic activity of alkali titanate nano materials A2TinO2n+1 (A = Li, Na and K)

Photocatalysts nano A₂Ti_nO_2n+1 (A = Li, Na, K) were prepared successfully by novel hydrothermal synthesis process. Powders were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-vis) absorption spectra and field-emission scanning electron microscope (FE-SEM) measurements. These results showed that the compositions of lithium, sodium and potassium titanates were Li₂TiO₃, Na₂Ti₃O₇ and K₂Ti₈O₁₇, respectively. The nano crystals of Li₂TiO₃ were self-assembled as snowflakes while that of Na₂Ti₃O₇ and K₂Ti₈O₁₇ were nanorods. Photocatalytic properties of alkali titanates were also investigated. The results indicated that alkali titanates as prepared have higher photocatalytic activities compared with P25 TiO₂ in the degradation of chloroform under UV light irradiation. A combination of K₂Ti₈O₁₇ and NiO produces a photocatalyst effective for the degradation of chloroform in aqueous solution. The framework of the tunnel structure was suitable for accommodating cocatalysts such as NiO to induce a strong interaction between the active species and cocatalysts. Na₂Ti₃O₇ has high photocatalytic activity under visible-light irradiation due to its strong absorption in the visible light region. The photocatalytic properties of Li₂TiO₃ are inferior to that of Na₂Ti₃O₇ and K₂Ti₈O₁₇ due to its mono-perovskite structure.     

TiO2(B) a new form of titanium dioxide and the potassium octatitanate K2Ti8O17

TiO₂(B) a new form of titanium dioxide, the octatitanate K₂Ti₈O₁₇ and intergrowths between K₂ Ti₄ O₉ and K₂ Ti₈ O₁₇ have been prepared by hydrolysis of K₂ Ti₄ O₉ followed by heating at 500°C. TiO₂ (B) and K₂Ti₈ O₁₇ have the host covalent framework of the bronzes Na_x TiO₂ and K₃Ti₈O₁₇ respectively.     

Non-stoichiometric titanates A2−2xBa3xTi6−xO13 with a tunnel structure

From the partial study of the systems “BaOTiO₂A₂Ti₆O₁₃” three series of non-stoichiometric titanates A_2?2xBa_3xTi_6?xO₁₃ were isolated for A = Na, K, Rb with x ranging from 0 to about 0.2. The structural evolution of these oxides, which are isostructural of Na₂Ti₆O₁₃ (1) was studied and discussed.     

Ionic conductivity of Li4GeO4, Li2GeO3 and Li2Ge7O15

The ionic conductivity of polycrystalline samples of three lithium germanates: Li₄GeO₄, Li₂GeO₃, and Li₂Ge₇O₁₅, has been determined using a c techniques and complex plane analysis. Conductivities at 400°C are 8.7 × 10^?5, 1.5 × 10^?5, and $\text{1.4 × 10}{}^{\mathrm{?7}}\text{(Ω·}\text{cm}\text{)}{}^{\mathrm{?1}}$ respectively. The conductivity of Li₄GeO₄ rises appreciably in the range 700–750°C.     

La-substitution Bi2Ti2O7 thin films grown by chemical solution deposition

(La_0.05Bi_0.95)₂Ti₂O₇ (LBTO) thin films had been successfully prepared on P-type Si substrate by chemical solution deposition method. The structural properties of the films were studied by X-ray diffraction. The phase of (La_0.05Bi_0.95)₂Ti₂O₇ is more stable than the phase of Bi₂Ti₂O₇ without La substitution. The films exhibited good insulating properties with room temperature resistivities in the range of 10¹²-10¹³ Ω cm. The dielectric constant of the film annealed at 550 °C at 100 kHz was 157 and the dissipation factor was 0.076. The LBTO thin films can be used as storage capacitors in DRAM.     

Metals built up from ReO3 type blocks and P2O7 groups : The bronzes AxP8W32O112 (A = K,Rb, Tl)

The electron transport and magnetic properties of the A_xP₈W₃₂O₁₁₂ bronzes (A = K, Rb, Tl) have been studied as a function of temperature. These compounds are characterized by a Pauli paramagnetism. The evolution of conductivity, ranging from 1260 (Ωcm)^?1 to 1610 (Ωcm)^?1 at 300K is characteristic of a metal. Owing to the presence of ReO₃ type slabs, these properties can be interpreted by the band model conduction developed by Goodenough for Na_xWO₃ bronzes.     

Lithium insertion into Li2Ti3O7

Li₂Ti₃O₇ with the ramsdellite-type structure undergoes lithium insertion reactions with n-BuLi. Li_2+xTi₃O₇ phases form with x = 0.5 and 1.0 at room temperature and at 50°C, respectively. The ESR spectrum of Li₃Ti₃O₇ confirms the partial reduction of Ti⁴⁺ ions to Ti³⁺. The electrical conductivity of the fully lithiated phase is several orders of magnitude higher than that of the host compound, suggesting charge hopping in the mixed valent lithiated compound.     

Les phases SrLa2Al2O7 et SrGd2Al2O7

SrLa₂Al₂O₇ and SrGd₂Al₂O₇ belong to the structural type Sr₃Ti₂O₇. In SrGd₂Al₂O₇ strontium and gadolinium occupy respectively the 12 and 9 coordinated sites, whereas in SrLa₂Al₂O₇ the homologous cations are statistically distributed. The fluorescence spectra of both phases activated by Eu³⁺ ions show that the only possible position of Eu³⁺ is the 9 coordinated site.     

Luminescent properties of Gd2Ti2O7: Eu phosphor films prepared by sol-gel process

Gd₂Ti₂O₇: Eu³⁺ thin film phosphors were fabricated by a sol-gel process. X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting films. The results of XRD indicated that the films began to crystallize at 800 °C and the crystallinity increased with the elevation of annealing temperatures. Uniform and crack free phosphor films were obtained, which mainly consisted of grains with an average size of 70 nm. The doped Eu³⁺ showed orange-red emission in crystalline Gd₂Ti₂O₇ phosphor films due to an energy transfer from Gd₂Ti₂O₇ host to them. Both the lifetimes and PL intensity of the Eu³⁺ increased with increasing the annealing temperature from 800 to 1000 °C, and the optimum concentrations for Eu³⁺ were determined to be 9 at.%. of Gd³⁺ in Gd₂Ti₂O₇ film host.     

Ionic conductivity and crystallographic data for Na2-xCa1-xAlxSiO4 and Na2-xCaSi1-xPxO4 carnegieite phases

Single crystals of Na₂CaSiO₄ prepared at 30 kbar and 1500°C are cubic with $\text{a}\text{= 22.456}\text{A}\text{?}$. The polycrystalline compositions Na_1.9Ca_0.9Al_0.1SiO₄ and Na_1.8CaSi_0.8P_0.2O₄ can also be indexed using $\text{a}\text{= 22.458}\text{and}\text{22.471}\text{A}\text{?}$, respectively. The Ca carnegieites Na₂CaSiO₄, Na_1.9Ca_0.9Al_0.1SiO₄, and Na_1.8CaSi_0.8P_0.2O₄ have Na ion conductivities of only 5?8 × 10^?6 (ω-cm)^?1 at 300°C.     

V-doping effects on ferroelectric properties of K0.5Bi4.5Ti4O15 thin films

V-doped K_0.5Bi_4.5Ti₄O₁₅ (K_0.5Bi_{4.5 − x/3}Ti_4 − xV_xO₁₅, KBTiV-x, x = 0.00, 0.01, 0.03, and 0.05) thin films were prepared on a Pt(111)/Ti/SiO₂/Si(100) substrate by a chemical solution deposition method. The thin films were annealed by using a rapid thermal annealing process at 750 °C for 3 min in an oxygen atmosphere. Among them, KBTiV-0.03 thin film exhibited the most outstanding electrical properties. The value of remnant polarization (2P_r) was 75 μC/cm² at an applied electric field of 366 kV/cm. The leakage current density of the thin film capacitor was 5.01 × 10⁻⁸ at 100 kV/cm, which is approximately one order of magnitude lower than that of pure K_0.5Bi_4.5Ti₄O₁₅ thin film capacitor. We found that V doping is an effective method for improving the ferroelectric properties of K_0.5Bi_4.5Ti₄O₁₅ thin film.     

Mode de formation et stabilite des phases AnBnO3n+2 mises en evidence dans les systemes La2Ti2O7 - CaTiO3, Nd2Ti2O7 - CaTiO3 et Ca2Nb2O7 - CaTiO3

The stability of the A_nB_nO_3n+2 phases, whose structure is derived from the perovskite structure, was investigated through X-Ray diffraction, in the following systems: La₂Ti₂O₇ - CaTiO₃, Nd₂Ti₂O₇ - CaTiO₃ and Ca₂Nb₂O₇ - CaTiO₃. Some of the phases quenched from the liquid are metastable and include vacancies overA and B sites. A model of formation of these phases is proposed.     

第一性原理对CaTiO3和CaCu3Ti4O12的对比研究

通过Ca替换CaCu_3Ti_4O_(12)晶胞中的所有Cu,建立了包含TiO6八面体扭转的CaTiO3;通过Cu替换CaTiO32×2×2超胞中3/4的Ca,建立了不包含CuO_4正方形的CaCu_3Ti_4O_(12)。采用Materials Studio软件的CASTEP模块,对比了上述晶体和标准晶体成键状况、能带结构、态密度及介电函数,分析了TiO6八面体扭转和CuO_4正方形的影响,发现了Cu-O键或CuO_4正方形对CaCu_3Ti_4O_(12)光频介电常数的关键性作用。研究结果提供了通过内禀机制优化CaCu_3Ti_4O_(12)材料介电性能的新途径。     

Syntheses of hollandite type Rb2Cr8O16, K2Cr2V6O16 and K2V8O16

Hollandite-type compounds, Rb₂Cr₈O₁₆, K₂Cr₂V₆O₁₆ and K₂V₈O₁₆, were synthesized under high P-T conditions up to 1200°C and 7GPa. The structural refinement using a single crystal of Rb₂Cr₈O₁₆ confirms that the structure is similar to that of K₂Cr₈O₁₆. Magnetic measurements indicate that Rb₂Cr₈O₁₆ is ferromagnetic below 295K, K₂Cr₂V₆O₁₆ paramagnetic down to 77K and K₂V₈O₁₆ has susceptibility anomaly at 175K. These compounds are all semiconductive and show discontinuities in temperature-resistivity curves at points corresponding to magnetic anomalies.     

Study on properties of BaxSmyTi7O20 ceramics with CaO-SiO2-B2O3 glass

The effect of CaO-SiO₂-B₂O₃ (CSB) glass addition on the sintering temperature and dielectric properties of Ba_xSm_yTi₇O₂₀ ceramics has been investigated using X-ray diffraction, scanning electron microscopy and differential thermal analysis. The CSB glass starts to melt at about 970 °C, and a small amount of CSB glass addition to Ba_xSm_yTi₇O₂₀ ceramics can greatly decrease the sintering temperature from about 1350 to about 1260 °C, which is attributed to the formation of liquid phase. It is found that the dielectric properties of Ba_xSm_yTi₇O₂₀ ceramics are dependent on the amount of CSB glass and the microstructures of sintered samples. The product with 5 wt% CSB glass sintered at 1260 °C is optimal in these samples based on the microstructure and the properties of sintering product, when the major phases of this material are BaSm₂Ti₄O₁₂ and BaTi₄O₉. The material possesses excellent dielectric properties: ?_r = 61, tan δ = 1.5 × 10⁻⁴ at 10 GHz, temperature coefficient of dielectric constant is −75 × 10⁻⁶ °C⁻¹.     

Mechanochemical-molten salt synthesis of Na2Ti6O13 nanobelts

A novel route for the preparation of Na₂Ti₆O₁₃ nanobelts by mechanochemical treatment of the TiCl₄-Na₂SO₄·10H₂O-Na₂CO₃ mixture for 5 min followed by molten salt synthesis is described. The mixture of the amorphous TiO₂ and NaCl-Na₂SO₄·xH₂O-Na₂CO₃ salt matrix was generated during milling. The molten salt synthesis of the Na₂Ti₆O₁₃ nanobelts with lengths up to 0.5-2 μm and with a width in the range of 20-250 nm was carried out in the temperature range of 700-800 °C for 1 h. The partial formation of titanate nanotubes was observed after annealing at 600 °C and washing procedure. X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential thermal analysis (DTA) were used to characterize the formation and structure of Na₂Ti₆O₁₃ nanobelts.     

Formation and structural characterization of potassium titanates and the potassium ion exchange property

In the present work, K₂Ti₂O₅, K₂Ti₄O₉ and K₂Ti₆O₁₃ are synthesized by solid state method. Their structures and morphologies are characterized by X-ray diffraction, Raman spectra and scanning electron microscopy. The binding energies of K, Ti and O in potassium titanates were then evaluated by X-ray photoelectron spectroscopy and compared with those in K/TiO₂. Finally the corresponding K ion exchange properties are investigated by synthesizing NO oxidation catalysts with Co(NO₃)₂ precursor. It is found that the binding energy of K in K₂Ti₂O₅ is much higher than those in K₂Ti₄O₉and K₂Ti₆O₁₃, and because of which, it shows quite different catalytic performances. Compared with other potassium titanates, the K in K₂Ti₂O₅ is much easier to be exchanged out.     

Photocatalytic hydrogen production over Na2Ti2O4(OH)2 nanotube sensitized by CdS nanoparticles

Na₂Ti₂O₄(OH)₂ nanotubes were obtained by hydrothermal reaction of TiO₂ with concentrated NaOH solution. CdS nanoparticles were then decorated on Na₂Ti₂O₄(OH)₂ nanotubes through partial ion-exchange method. The composite photocatalysts were characterized by X-ray diffraction (XRD), ultraviolet-visible spectra (UV-vis), transmission electron microscope (TEM), etc. The results showed that CdS nanoparticles of 5-6 nm were anchored on the surface of the Na₂Ti₂O₄(OH)₂ nanotubes. Under irradiation of visible light (λ ≥ 430 nm), the prepared CdS/Na₂Ti₂O₄(OH)₂ showed high photoactivity for hydrogen production.     

Properties of La-substituted Na0.5Bi4.5Ti4O15 ferroelectric thin films

Ferroelectric Na_0.5La_0.5Bi₄Ti₄O₁₅ (NaLaBTi) thin films were prepared by a chemical solution deposition method. The NaLaBTi thin films annealed at 750 °C under oxygen atmosphere were randomly oriented polycrystalline. Electrical properties of the NaLaBTi thin films were compared to Na_0.5Bi_4.5Ti₄O₁₅ thin films and better properties were observed in the NaLaBTi thin films. Remnant polarization (2P_r) and coercive electric field (2E_c) were 43 µC/cm² and 204 kV/cm at an applied electric field of 478 kV/cm, respectively. Leakage current density was 1.95 × 10^− 6 A/cm² at 100 kV/cm. Dielectric constant and dielectric loss were 805 and 0.05 at 1 kHz, respectively. Switchable polarization was suppressed by 15% after 1.44 × 10¹⁰ switching cycles.     

Synthesis and field effect characteristics of Na2Ti3O7 nanowires

Na₂Ti₃O₇ nanowires with diameters of about 80-130 nm and lengths up to several tens of micrometers are synthesized via a simple hydrothermal method and characterized by the field-emission scanning electron microscopy and X-ray diffraction. Back-gate field-effect transistors based on these nanowires are fabricated on indium tin oxide glass substrates with polymethyl-methacrylate-co-glyciclyl-methacrylate as the gate insulator layers. Typical p-type semiconductor material properties are observed in our investigations. The field-effect mobility is about 0.1 cm²/Vs. The capacitance per unit area of the dielectric is 3.43 nF/cm² (dielectric constant, k = 3.9). The on/off ratio is around 10³ at the conduction of 10 V.