Colossal permittivity in TiO2 co‐doped by donor Nb and isovalent Zr

Effect of isovalent Zr dopant on the colossal permittivity (CP) properties was investigated in (Zr + Nb) co‐doped rutile TiO₂ ceramics, i.e., Nb_0.5%Zr_xTi_1?xO₂. Compared with those of single Nb‐doped TiO₂, the CP properties of co‐doped samples showed better frequency‐stability with lower dielectric losses. Especially, a CP up to 6.4 × 10⁴ and a relatively low dielectric loss (0.029) of x = 2% sample were obtained at 1 kHz and room temperature. Moreover, both dielectric permittivity and loss were nearly independent of direct current bias, and measuring temperature from room temperature to around 100°C. Based on X‐ray photoelectron spectroscopy, the formation of oxygen vacancies was suppressed due to the incorporation of Zrions. Furthermore, it induced the enhancement of the conduction activation energy according to the impedance spectroscopy. The results will provide a new routine to achieve a low dielectric loss in the CP materials.     

High‐Q microwave ceramics of Li2TiO3 co‐doped with magnesium and niobium

In order to solve the problems of acceptor/donor individual doping in Li₂TiO₃ system and clarify the superiority mechanism of co‐doping for improving the Q value, Mg + Nb co‐doped Li₂TiO₃ have been designed and sintered at a medium temperature of 1260°C. The effects of each Mg/Nb ion on structure, morphology, grain‐boundary resistance and microwave dielectric properties are investigated. The substitution of (Mg_1/3Nb_2/3)⁴⁺ inhibits not only the diffusion of Li⁺ and reduction in Ti⁴⁺, but also the formation of microcracks in ceramics, which promotes the enhancement of Q value. The experiments reveal that Q × f value of Li₂TiO₃ ceramics co‐doped with magnesium and niobium is 113 774 GHz (at 8.573 GHz), which is increased by 113% compared with the pure Li₂TiO₃ ceramics. And the co‐doped ceramics have an appropriate dielectric constant of 19.01 and a near‐zero resonance frequency temperature coefficient of 13.38 ppm/°C. These results offer a scientific basis for co‐doping in Li₂TiO₃ system, and the outstanding performance of (Mg + Nb) co‐doped ceramics provides a solid foundation for widespread applications of microwave substrates, resonators, filters and patch antennas in modern wireless communication equipments.     

Preparation of Nb-doped TiO2 nanopowder by liquid-feed spray pyrolysis followed by ammonia annealing for tunable visible-light absorption and inhibition of photocatalytic activity

Visible-light absorbing TiO₂ with color tunability and negligible photocatalytic activity was developed by synthesizing Nb-doped TiO₂ using liquid-feed flame spray pyrolysis (LF-FSP) followed by NH₃ annealing. Nb-doped TiO₂ nanopowders with selected Nb contents were synthesized by changing ratios of metalloorganic precursors. By using Ti and Nb precursors whose thermal decomposition behavior is very similar, the maximum Nb doping concentration was improved compared to Nb-doped TiO₂ prepared by flame spray pyrolysis (FSP) in previous reports. As-synthesized Nb-doped TiO₂ nanopowders were white regardless of the Nb content, while all became colored after NH₃ annealing. In addition, the color of Nb-doped TiO₂ after NH₃ annealing varied from yellow to dark green depending on the Nb content. Light absorption characterization indicates that the color variation is attributable to the localized levels formed by interstitial N-doping and Nb-doping-induced Ti³⁺ formation resulting from reduction by H₂, formed as a result of NH₃ decomposition.     

Niobium and divalent‐modified titanium dioxide ceramics: Colossal permittivity and composition design

Colossal permittivity (CP) (ε_r=10⁴~10⁵) is attained in (A_1/3Nb_2/3)_xTi_1‐xO₂ (A=Ba²⁺, Ca²⁺, Zn²⁺, Mg²⁺) ceramics. Here, (Ca_1/3Nb_2/3)_xTi_1‐xO₂ material was studied as a typical example, and effects of Ca and Nb on their microstructure, dielectric properties and stability were studied. Both backscattering and elements mapping strongly confirmed the formation of secondary phases due to the addition of Ca and/or Nb. Secondary phases‐induced by Ca cannot affect dielectric properties of the ceramics when low Ca and Nb contents were doped, while secondary phases formed by Ca and Nb strongly affected their dielectric properties in a high doping level. In particular, their dielectric properties can be well modified by the optimization of sintering temperatures. In addition, the (Ca_1/3Nb_2/3)_xTi_1‐xO₂ ceramics with x=0.01 exhibited the optimum dielectric properties (ε_r=130500 and tan δ=0.19). Electron‐pinned defect‐dipoles may be suitable to explain CP phenomenon of this work. We believed that this profound investigation can benefit the development of new TiO₂ ceramics as a CP material.     

Surface barrier layer effect in (In + Nb) co‐doped TiO2 ceramics: An alternative route to design low dielectric loss

Giant dielectric permittivity (ε′) with low loss tangent (tanδ) was reported in (In + Nb) co‐doped TiO₂ ceramics. Either of electron‐pinned defect‐dipole or internal barrier layer capacitor model was proposed to be the origin of this high dielectric performance. Here, we proposed an effectively alternative route for designing low‐tanδ in co‐doped TiO₂ ceramics by creating a resistive outer surface layer. A pure rutile‐TiO₂ phase with a dense microstructure and homogeneous dispersion of dopants was achieved in (In + Nb) co‐doped TiO₂ ceramics prepared by a simple sol‐gel method. Two giant dielectric responses were observed in low‐ and high‐frequency ranges, corresponding to extremely high ε′≈10⁶‐10⁷ and large ε′≈10⁴‐10⁵, respectively. After annealing in air, a low‐frequency dielectric response disappeared and could be restored by removing the outer surface of the annealed sample, indicating the dominant electrode effect in the initial sample. Annealing can cause improved dielectric properties with a temperature‐ and frequency‐independent ε′ value of ≈1.9 × 10⁴ and cause a decrease in tanδ from 0.1 to 0.035. High dielectric performance in (In_0.5Nb_0.5)_xTi_1?xO₂ ceramics can be achieved by eliminating the electrode effect and forming a resistive outer surface layer.     

Facile preparation of flake-like blue TiO2 nanorod arrays for efficient visible light photocatalyst

Different kinds of oriented TiO₂ nanorod arrays have been actively pursuing in recent years, however, these fabrications relied on the substrates, such as fluorine-doped tin oxide glass (FTO), silicon wafer or other semiconductor precursor layer. Herein, a stable Ti³⁺ and oxygen vacancies doped blue TiO₂ flakes composed of oriented nanorod arrays were synthesized using a facile hydrothermal treatment in diluted hydrochloric acid solution. Such centimeter-scale flake-like TiO₂ product was obtained without any substrate. Since Ti³⁺ self-doped and/or oxygen vacancies TiO₂ could extend the absorption range of TiO₂ to visible light region, the blue TiO₂ sample exhibited excellent photocatalytic activity under visible light irradiation (photocatalytic degradation efficiency can nearly reach up to 100% within 60?min).     

Improved Pyroelectric Properties of CaBi4Ti4O15 Ferroelectrics Ceramics by Nb/Mn Co‐Doping for Pyrosensors

The pyroelectric properties of Nb(Mn)‐doped and Nb/Mn co‐doped CaBi₄Ti₄O₁₅ (CBT) bismuth layer‐structured ferroelectric ceramics were investigated. It was found that Nb/Mn co‐doping resulted in stronger enhancement of pyroelectric properties than that of single Nb or Mn doping. The mechanism of doping effect was explained by the distortion of the [BO₆] octahedra induced by the doped Nb and Mn cations occupying the B‐site of the pseudoperovskite structure. A large pyroelectric coefficient of 84.4 μC/m²K was obtained at room temperature for Nb/Mn co‐doped CBT (CBTN‐Mn) ceramics, higher than that of pure, Nb or Mn‐doped counterparts, being on the order of 35.9, 58.2, 44.0 μC/m²K, respectively. The enhanced pyroelectric coefficient together with reduced dielectric constant (99) and dielectric loss (0.002) led to greater improvement of figures of merit (FOMs), including FOMs for voltage responsivity (F_v ~ 3.95 × 10^?2 m²/C) and detectivity (F_d ~ 2.44 × 10^?5 Pa^?1/2), in CBTN‐Mn ceramics. Furthermore, the temperature variations of F_v and F_d were found to be 24% and 68%, respectively, over a broad temperature range from room temperature to 350°C, making CBTN‐Mn ceramics potential candidate for high‐temperature pyroelectric devices.     

Origin of Violet‐Blue Emission in Ti‐Doped Gahnite

ZnAl₂O₄ doped with Ti⁴⁺ (2%) was synthesized by the hydrothermal method at 220°C at pressure of 25 bars. An average grain size of the as‐prepared sample was 3 nm, the samples with biggest grain size were obtained after annealing at 300°C, 500°C, 600°C, 700°C, and 900°C, diameter of the latter was about 33 nm. IR spectroscopy indicated that ZnAl₂O₄ was partially inverted. The degree of the inversion decreases with increase in the annealing temperature but increases with increasing Ti⁴⁺ content. Absorption and emission spectra as well as emission decay profiles were recorded at 300 and 77 K. The observed spectra are due to charge‐transfer O^2??Ti⁴⁺ transitions. Color of the emission depends on the nanocrystal size and with increase in its diameter changes from violet to blue, accordingly the absorption bands exhibit redshift. The calculations based on Density Functional Theory confirmed the experimental results. 3d electrons of titanium ions form the bottom of the ZnAl₂O₄:Ti⁴⁺ conduction band, oxygen, aluminum or zinc vacancies create additional levels in the gahnite energy band gap. It was also found that in ZnAl₂O₄ aluminum or zinc vacancy induces magnetism with relatively high magnetic moment close to 1 μ_B per vacancy.     

Preparation,characterization and the antimicrobial properties of metal ion-doped TiO2 nano-powders

TiO₂ samples doped with lithium, sodium, magnesium, iron or cobalt were prepared by high-energy ball milling for different periods of time. The crystalline phase, chemical composition, crystalline size and photo-absorption were characterized by X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS) and Ultraviolet visible diffuse reflectance spectroscopy (UV - Vis - DRS), Fourier transform infrared spectroscopy (FT - IR) and scanning electron microscopy (SEM). The antimicrobial properties of the modified TiO₂ samples were evaluated with E. coli and S.aureus assays. The results of the XRD show that the TiOSO₄, Ti₃O₅, Li₂TiO₃ and NaTi₂O₄ phases appear along with Li, Na and Mg doped TiO₂. However, XPS spectra indicated that Ti exists as both Ti³⁺ and Ti⁴⁺ in Na-doped TiO₂ samples. Ti³⁺, due to its narrow band gap, is highly active in promoting visible light-induced photocatalytic activity. SEM images showed that the crystalline size of TiO₂ is reduced and has a common-round and hexagonal plate morphology after milling. The modified TiO₂ samples had the best antimicrobial activities after 3 h of milling. In particular, the antimicrobial rate of TiO₂ 5% doped with transition metals (Co, Fe) reached 100% against E. coli, but the antibacterial rate against S. aureus for Co and Fe dopants was 98.4% and 98.2%, respectively.     

High performance PPO/Ti3+/TiO2NT membrane/electrode for lithium ion battery

In order to improve the electrical conductivity and electrochemical performances of lithium ion battery, the electrodeposition of poly(phenylene oxide) (PPO) on Ti³⁺-doped TiO₂ nanotube arrays (Ti³⁺/TiO₂NT) was designed and achieved via self-doping of Ti³⁺ and the following electropolymerization of phenol monomers. The as-synthesized PPO/Ti³⁺/TiO₂NT membrane/electrode was investigated in terms of SEM, EDX, XPS, galvanostatic charge/discharge, cycle voltammetry (CV) and AC impedance. As expected, PPO film indeed grew onto the surface of Ti³⁺/TiO₂NT electrode via one-step electrodeposition; furthermore, PPO/Ti³⁺/TiO₂NT membrane/electrode delivered satisfactory rate performances and cycle stability, mainly attributed to the joint contributions from higher electrical conductivity of Ti³⁺/TiO₂NT electrode and the synergy effects between Ti³⁺/TiO₂NT electrode and loose PPO film.     

Long‐term oxidation and electrical behavior of Nb‐doped Ti3SiC2 as solid oxide fuel cell interconnects

Nb‐doped Ti₃SiC₂ compounds ((Ti_1‐xNb_x)₃SiC₂, x=0%, 2%, 5%, 7%, and 10%) as novel interconnect materials of intermediate temperature solid oxide fuel cell (IT‐SOFC) were studied in the simulated cathode atmosphere. The long‐term oxidation behaviors and area‐specific resistance (ASR) of these compounds have been investigated at 800°C up to 700 hours. Among these compounds, (Ti_0.95Nb_0.05)₃SiC₂ shows the best oxidation resistance and lowest postoxidation ASR (5.6 mΩ·cm² after exposure at 800°C in air for 700 hours), endowing it a great promising material in the application as interconnect of IT‐SOFC. After oxidation, Nb is mainly doped uniformly into the lattice of rutile‐TiO₂ (r‐TiO₂) grains formed on the tested compounds. Nb doping could decrease the concentrations of both oxygen vacancies and titanium interstitials in r‐TiO₂. As a result, the oxidation rate of (Ti,Nb)₃SiC₂ decreases remarkably, the structure of the oxide scale changes from a duplex layer of TiO₂ outer layer and TiO₂+SiO₂ mixture inner layer to a single mixture layer. Nb doping also increases the amount of semifree electrons, causing the significant reduce of the postoxidation ASR of (Ti,Nb)₃SiC₂.     

Effect of surface sites of TiO2 support on the formation of cobalt-support compound in Co/TiO2 catalysts

This study has addressed the effect of surface sites (as Ti⁴⁺ and Ti³⁺) of TiO₂ support on the formation of the cobalt-support compound on Co/TiO₂ catalyst. A 20% cobalt was prepared on each TiO₂ support having the different Ti⁴⁺/Ti³⁺ ratios covering on the surface (1.2, 1.3, 1.4, and 1.6). It can be concluded that the non-reducible compound as a Co-SCF preferred to form on the Co/TiO₂ catalyst when the most proportional site of surface TiO₂ became Ti⁴⁺. This is because the migration of cobalt cluster, which plays the significant role for creating Co-SCF, can be promoted by the increasing of Ti⁴⁺ sites resulting in the decrease of cobalt dispersion. The hyperfine patterns of ESR spectra demonstrated that the Co⁰(H_xTiO_y) was the simple chemical form of Co-SCF formed after standard reduction. Moreover, the structure of Co-SCF changed to be a higher non-uniform structure when the number of Ti⁴⁺ site on TiO₂ surface increased. The possible mechanism to form this compound has also been discussed.     

Minimization of defects in Nb-doped TiO2 photocatalysts by molten salt flux

Transition metal doping is a popular pathway to improve the photocatalytic performance of TiO₂. However, most dopants are aliovalent (e.g., Nb⁵⁺, W⁶⁺, and Fe³⁺), where defects are inevitably introduced (e.g., oxygen or Ti³⁺ defects). To minimize defects and hence ensure better separation of photogenerated charges, this work incorporates an Nb dopant into TiO₂ in a flux of molten salt. The molten salt flux not only induces recrystallization of TiO₂ nanoparticles into nanowires but also allows a + 4 valence dopant in the TiO₂ lattice to inhibit formation of Ti³⁺ or oxygen defects. This minimization of defects enhances the photocatalytic activity in the degradation of gaseous acetaldehyde. This enhancement is mainly attributed to the higher crystallinity of TiO₂ and better separation of charge carriers induced by equivalent Nb-doping.     

Influence of Nb2O5 addition on dielectric properties and diffuse phase transition behavior of BaZr0.2Ti0.8O3 ceramics

Nb₂O₅ doped Ba(Zr_0.2Ti_0.8)O₃ (short as BZT20) ceramics were prepared by a mixed-oxide method using a high-energy planetary ball mill and the influence of Nb₂O₅ addition on microstructure, dielectric properties and diffuse phase transition behavior of BZT20 ceramics were investigated. It was demonstrated that Nb⁵⁺entered the B-site of BZT20 ceramic and substituted for Ti⁴⁺, which caused the expansion and distortion of crystal lattice. BZT20 ceramics doped with 0.2 mol% Nb₂O₅ showed excellent dielectric property and lower diffusivity with ε_m=37,823 and γ=1.49. We supposed that the increase of dielectric constant and decrease of diffuseness parameter with increasing Nb₂O₅ content were caused by lattice disorder and unbalancing of cations induced by the substitution of Ti⁴⁺ by Nb⁵⁺ in the B sites of BZT20 ceramics. The Curie temperature decreased with the increase of Nb₂O₅ content, which can be attributed to enlarged distortion energy of the Nb doped BZT20 structure. Besides, grain size effect on the dielectric property and diffuse phase transition behavior of Nb₂O₅ doped BZT20 ceramics was also investigated.     

Large-size-mismatch co-dopants for colossal permittivity rutile TiO2 ceramics with temperature stability

Colossal permittivity (CP) in donor-accepter co-doped rutile TiO₂ has attracted significant interest. Here, the CP behavior of (Ta?+?La) co-doped rutile TiO₂ ceramics were studied, where the ionic radii of Ta⁵⁺ and La³⁺ are much larger than that of Ti⁴⁺. The ceramics with an extremely low doping exhibit colossal dielectric permittivity (～2.6?×?10⁴) with an acceptable low dielectric loss (<0.07) in the frequency range from 40 to 10⁶?Hz. The CP properties obtained in (Ta?+?La) co-doped TiO₂ ceramics show excellent temperature stability over a wide temperature range of 20–400?°C. The X-ray diffraction analysis and the density functional theory calculation illustrates that the La₂³⁺${\text{V}}_{\text{o}}^{??}$Ti₂³⁺ and Ta₂⁵⁺Ti³⁺Ti⁴⁺ defect complexes with the lowest energy are responsible for the enhanced dielectric properties. Moreover, the defect complex formed by large-size trivalent substitutions and oxygen vacancy is very stable, and assists in improving temperature stability of the dielectric properties of co-doped rutile TiO₂ ceramics.     

Photo Catalytic Degradation of Imidachloprid Under Solar Light Using Metal Ion Doped TiO2 Nano Particles: Influence of Oxidation State and Electronic Configuration of Dopants

Anatase TiO₂ was doped with metal ions like Th⁴⁺, V⁵⁺ and Mo⁶⁺ and tested for the degradation of imidachloprid under solar light. X-ray diffraction results inferred that all the dopants stabilized the anatase phase irrespective of their nature, oxidation state and ionic size. The undoped and transition metal ion doped TiO₂ were completely transformed to rutile phase at 700 °C while rare earth Th⁴⁺ doped sample completely transformed to rutile phase at 1,000 °C. The rare earth dopant stabilized the anatase phase by hindering the growth of crystallite size. Among the photo catalysts used, Th⁴⁺ (0.06%)-TiO₂ showed highest activity and its efficiency was 2.8 times higher than that of Degussa P-25. The Th⁴⁺ ion lowered the band gap of TiO₂ to 2.6 and 2.5 eV facilitating solar light absorption. Detrapping of the trapped charge carriers depends on electronic configuration and the oxidation state of the dopants.     

Effects of reoxidation on the dielectric and energy storage properties of Ce-doped (Ba,Sr)TiO3 ceramics prepared by hot-pressed sintering

Fully densified Ba_0.6Sr_0.34Ce_0.04TiO₃ ceramics with small grain microstructure (0.3−0.8 μm) and high insulation (∼10¹⁴Ω cm) could be successfully obtained by hot-pressed sintering (HPS) coupled with subsequent thermal annealing in oxygen atmosphere. The effects of annealing on the microstructure, dielectric, electric and energy storage properties were investigated with scanning electron microscope, X-ray photoelectron spectroscope, impedance and ferroelectric analyzers. The preferential oxidation of grain boundaries to gains resulted in the formation of electrically inhomogeneous microstructure, which could be much improved by increasing O₂ pressure during annealing process. The reduction of Ti⁴⁺ in the as-sintered HPS sample could be decreased with increasing the annealing temperature up to 1200 °C in flowing O₂ gas, leading to the decrease in dielectric relaxation and corresponding permittivity/loss above room temperature. Maximum energy density of 1.75 J/cm³ could be obtained under the field of 235 kV/cm with the efficiency of 85 %.     

Microstructure,optical and magnetic properties of TiO2/CuO nanocomposites synthesized by a two-step method

TiO₂/CuO composites in different ratios were prepared via a two-step method. X-ray diffraction and transmission electron microscopy results indicated that part of Cu²⁺ substituted Ti⁴⁺ in the TiO₂ lattice in the composite, leading to Cu²⁺-substituted sites in the TiO₂ lattice as well as Cu²⁺ species located in the CuO lattice. Scanning electron microscopy revealed a morphology change in the sample from a three-dimensional structure to a two-dimensional structure while forming an interface between TiO₂ and CuO. X-ray photoelectron spectroscopy and Raman spectra showed that there are oxygen vacancies (V_O) and Ti³⁺ in the lattice. UV–vis absorption spectra exhibited a widening of the absorption range and a decrease in the bandgap with increasing amount of CuO in the TiO₂/CuO composites. Additionally, the composites exhibited room-temperature ferromagnetism (RTFM), as can be explained by the indirect double-exchange model, which is related to V_O and the exchange interaction between the 3d orbitals of Ti³⁺ and Ti⁴⁺ at the interface.     

Ionic conductivity of tetragonal ZrO2 polycrystal doped with TiO2 and GeO2

The effects of the co-doping and the resultant co-segregation of 2 mol% TiO₂ and 2 mol% GeO₂ on the ionic conductivity and on the chemical bonding state in a tetragonal ZrO₂ polycrystal were investigated. The conductivity data and grain boundary microstructure showed that the doped Ti⁴⁺ and Ge⁴⁺ cations segregate along the grain boundary, and this segregation causes a reduction in the conductivity of both the grain interior and grain boundary and an increase in the activation energy of the grain boundary conductivity. Overall, the data indicate that the segregation retards the diffusion of oxygen anions. A first-principle molecular orbital calculation explains the retarded diffusion of the oxygen anion from a change in the covalent bonds around the dopant cations; an increase in the strength of the covalent bond between the oxygen and doped cation should work to suppress the diffusion of the oxygen anion.     

Structural and spectral properties of red light emitting Eu3+ activated TiO2 nanophosphor for white LED application

The structural and luminescent properties of Eu³⁺ doped TiO₂ nanophosphors synthesized by low cost combustion method were investigated. The X-ray diffraction analysis revealed that crystallite size decreases with doping concentration. Lattice volume expansion occurred due to the substitution of Ti⁴⁺ ions by larger ionic radii ions Eu³⁺. FESEM images showed prepared phosphors to be nano size spherical shaped particles. Energy band gap of 3 mol% Eu³⁺ doped samples decreased to 3.15 eV due to doping effect. The Eu³⁺ doped TiO₂ nanophosphors exhibited main red emission peak centered at 616 nm under 395 nm UV light excitation. Concentration quenching was observed at 3 mol% doping, that has been ascribed to dipole-dipole interaction. The covalent nature of Eu-O bond and environment around Eu³⁺ ions were discussed using Judd-Ofelt (J-O) intensity parameters. Internal quantum efficiency was calculated using excited state lifetime ⁵D₀ state of Eu³⁺ ion and J-O theory. The CIE colour coordinates and colour purity were calculated using the spectral energy distribution function. Low excited state life time indicated that Eu³⁺ doped TiO₂ can be used as red emitting phosphor for white light emitting diode applications.