Low-firing Li<Subscript>2</Subscript>ZnTi<Subscript>3</Subscript>O<Subscript>8</Subscript> microwave dielectric ceramics with BaCu(B<Subscript>2</Subscript>O<Subscript>5</Subscript>) additive

Phase purity, microstructure, sinterability and microwave dielectric properties of BaCu(B₂O₅)-added Li₂ZnTi₃O₈ ceramics and their cofireability with Ag electrode were investigated. A small amount of BaCu (B₂O₅) can effectively reduce the sintering temperature from 1075°C to 925°C, and it does not induce much degradation of the microwave dielectric properties. Microwave dielectric properties of ε _r = 23·1, Q × f = 22,732 GHz and τ _f = − 17·6 ppm/°C were obtained for Li₂ZnTi₃O₈ ceramic with 1·5 wt% BaCu(B₂O₅) sintered at 925°C for 4 h. The Li₂ZnTi₃O₈ +BCB ceramics can be compatible with Ag electrode, which makes it a promising microwave dielectric material for low-temperature co-fired ceramic technology application.     

Preparation of Fe-doped TiO2 nanotube arrays and their photocatalytic activities under visible light

Fe-doped TiO₂ nanotube arrays have been prepared by the template-based liquid phase deposition method. Their morphologies, structures and optical properties were investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and UV-vis absorption spectroscopy. Their photocatalytic activities were evaluated by the degradation of methylene blue under visible light. The UV-vis absorption spectra of the Fe-doped TiO₂ nanotube arrays showed a red shift and an enhancement of the absorption in the visible region compared to the undoped sample. The Fe-doped TiO₂ nanotube arrays exhibited good photocatalytic activities under visible light irradiation, and the optimum dopant amount was found to be 5.9 at% in our experiments.     

Preparation and photoelectrochemical characterization of WO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> nanotube array electrode

WO₃/TiO₂ nanotube array electrode was fabricated by incorporating WO₃ with TiO₂ nanotube array via a wet impregnation method using ammonium tungstate as the precursor. TiO₂ and WO₃/TiO₂ nanotube arrays were characterized by field emission scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray analysis. In order to characterize the photoelectrochemical properties of WO₃/TiO₂ electrode, electrochemical impedance spectroscopy, and steady-state photocurrent (i _ss) measurement at a controlled potential were performed in the supporting electrolyte containing different concentrations of glucose. The photoelectrochemical characterization results reveal that WO₃/TiO₂ nanotube array electrode possesses a much higher separation efficiency of the photogenerated electron–hole pairs and could generate more photoholes on the electrode surface compared with the pure TiO₂ nanotube array electrode. The i _ss for glucose oxidation at WO₃/TiO₂ nanotube array electrode is much higher than that at the pure TiO₂ nanotube array electrode.     

Effect of low level substitution of Sr–Ba on transport and magnetic behaviour of La<Subscript>0·67</Subscript>Ca<Subscript>0·33</Subscript>MnO<Subscript>3</Subscript>

In this paper we report the investigation of transition metal oxide compound, La_0·67Ca_0·25Sr_0·04Ba_0·04MnO₃ (LCSBMO), along with La_0·67Ca_0·33MnO₃ (LCMO), synthesized by sol–gel route under identical conditions. The effect of simultaneous low level substitution of large size ions such as Sr²⁺ and Ba^2 + for Ca^2 + ions on the electronic transport and magnetic susceptibility properties are analysed and compared apart from microstructure and lattice parameters. The temperature dependent electrical transport of the polycrystalline pellets of LCSBMO and LCMO when obeying the well studied law, r = r₀ + r₂   T²\rho = \rho_{0} + \rho_{2} \;T^{2} for T < T _MI , is observed to differ by more than 50% from the values of ρ ₀ and ρ ₂, with the former compound showing enhanced electrical conductivity than the latter. Similarly in fitting the adiabatic small polaron model for resistivity data of both the samples for T > T _MI , the polaron activation energy is found to differ by about 11%. In addition, the temperature dependent a.c. magnetic susceptibility study of the compounds shows a shift of about 6% in the paramagnetic to ferromagnetic transition temperature (285 K for LCSBMO and 270 K for LCMO).     

Synthesis of nano-MoS<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> composite and its catalytic degradation effect on methyl orange

A nano-MoS₂/TiO₂ composite was synthesized in H₂ atmosphere by calcining a MoS₃/TiO₂ precursor, which was obtained via a quick deposition of MoS₃ on anatase nano-TiO₂ under a strong acidic condition. The obtained nano-MoS₂/TiO₂ composite was characterized by X-ray diffraction spectroscopy, Brunauer–Emmett–Teller (BET) surface area, scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive spectrometry, ultraviolet–visible spectroscopy, and Fourier transform infrared spectroscopy. The results show that the composite had a high BET surface area because of its small size and irregularly layered structure. MoS₂ in the composite was composed of typical layered structures with thicknesses of 2–8 nm and lengths of 10–40 nm. The composite contained a wide and intensive absorption at 400–700 nm, which is in the visible light region, and presented a positive catalytic effect on removing methyl orange from the aqueous solution. The catalytic activity of the composite was influenced by the initial concentration of methyl orange, the amount of the catalyst, the pH value, and the degradation temperature. In addition, the composite catalyst could be regenerated and repeatedly used via filtration three times. The deactivating catalyst could be reactivated after catalytic reaction by heating at 450 °C for 30 min in H₂.     

Visible light photoelectrocatalysis with salicylic acid-modified TiO2 nanotube array electrode for p-nitrophenol degradation

This research focused on immersion method synthesis of visible light active salicylic acid (SA)-modified TiO₂ nanotube array electrode and its photoelectrocatalytic (PEC) activity. The SA-modified TiO₂ nanotube array electrode was synthesized by immersing in SA solution with an anodized TiO₂ nanotube array electrode. Scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), UV–vis diffuse reflectance spectrum (DRS), and Surface photovoltage (SPV) were used to characterize this electrode. It was found that SA-modified TiO₂ nanotube array electrode absorbed well into visible region and exhibited enhanced visible light PEC activity on the degradation of p-nitrophenol (PNP). The degradation efficiencies increased from 63 to 100% under UV light, and 79–100% under visible light (λ > 400 nm), compared with TiO₂ nanotube array electrode. The enhanced PEC activity of SA-modified TiO₂ nanotube array electrode was attributed to the amount of surface hydroxyl groups introduced by SA-modification and the extension of absorption wavelength range.     

Functionalization of electrospun magnetically separable TiO<Subscript>2</Subscript>-coated SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanofibers: strongly effective photocatalyst and magnetic separation

Magnetically separable TiO₂-coated SrFe₁₂O₁₉ electrospun nanofibers were obtained successfully by means of sol–gel, electrospinning, and coating technology, followed by heat treatment at 550–650 °C for 3 h. The average diameter of the electrospun fibers was 500–600 nm. The fibers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM). The optimized calcining temperature was determined by XRD and the analysis of decolorizing efficiency of methylene blue (MB) under UV–vis irradiation. The photocatalytic activity of the TiO₂-coated SrFe₁₂O₁₉ fibers was investigated using ultraviolet–visible absorbance by following the photooxidative decomposition of a model pollutant dye solution, MB in a photochemical reactor. In contrast to pure TiO₂ fibers, the TiO₂-coated SrFe₁₂O₁₉ fibers have higher absorption in 250–750 nm wavelength regions. The presence of SrFe₁₂O₁₉ not only broadened the response region of visible-light, but also enhanced the absorbance for UV light. The decolorizing efficiency of MB under UV–vis irradiation was up to 98.19%, which was a little higher than that of Degussa P25 (97.68%). Furthermore, these fibers could be recollected easily with a magnet in a photocatalytic process and had effectively avoided secondary pollution of treated water.     

Preparation and photocatalytic activity of N-doped TiO2 nanotube array films

The N-doped TiO₂ nanotube array films were fabricated directly by one-step electrochemical anodic oxidation of Ti foils in an HF electrolyte containing ammonium and nitrate ions. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDX), and ultraviolet–visible (UV–vis) absorption spectroscopy, respectively. The photocatalytic activities were evaluated by the degradation of methyl orange (MO) under visible light irradiation. The results showed that N dopant was successfully introduced into the TiO₂ nanotube array films. The N-doped TiO₂ nanotube array films showed a red shift and an enhancement of the absorption in the visible light region compared to the undoped sample. The photocatalytic activities of the N-doped TiO₂ samples were much higher than those of the undoped sample. A maximum enhancement of photocatalytic activity was achieved for the N-doped TiO₂ sample prepared in 0.07 M HF electrolyte containing 1.0 M NH₄NO₃, and 81% of MO was degraded in 150 min under visible light irradiation.     

Dye-sensitized solar cells using TiO<Subscript>2</Subscript> nanoparticles transformed from nanotube arrays

Dye-sensitized solar cells (DSSCs) were fabricated using TiO₂ mesoporous layers obtained by very simple method—transformation of TiO₂ nanotube (NT) films grown by electrochemical oxidation to nanoparticle (NP) films. This transformation is based on thermal annealing of TiO₂ NT arrays formed by anodization of titanium foil in fluorine ambient. Performance of DSSCs fabricated with different size NPs was studied in the range from 35 to 350 nm. Highest nominal efficiency (9.05%) was achieved for DSSC with NP size 65 nm while the lowest nominal efficiency (1.48%) was observed for DSSCs with NP size 350 nm. The dependence of the solar cell parameters with NP size is discussed.     

Temperature-controlled assembly and morphology conversion of CoMoO<Subscript>4</Subscript>·3/4H<Subscript>2</Subscript>O nano-superstructured grating materials

The regular and homogeneous single-crystal CoMoO₄·3/4H₂O nanorods, with the diameters ca. 100–300 nm and lengths ca. 8–15 μm, have been successfully prepared by a simple and facile precipitation method. Their morphology conversion from broom-like to cage-like structure has been firstly reported through controlling the reaction temperature. The broom-like microbunches were obtained at 50 °C while at 80 °C, dispersive nanorods can be prepared. As the temperature reached 90 °C, the morphology of the products converted to cage-like microspheres. SEM results show that the reaction temperature has a critical role in both the formation of the products and their morphologies. The UV–visible diffuse reflectance absorbance spectra of the products display two intense, broad absorbance bands cover almost the whole ultraviolet and visible region except for a narrow region around 450 nm, which is in the region for purple light. Based on the experimental results, a possible formation mechanism was also proposed. The synthesis strategy is simple, facile, mild, and has a good reproducibility. The as-prepared products may have potential applications in optics, catalysis, and grating materials.     

Electrical and magnetic properties of (BiNa)<Subscript>1/2</Subscript>(FeV)<Subscript>1/2</Subscript>O<Subscript>3</Subscript>

Potential multiferroic material, (BiNa)_1/2(FeV)_1/2O₃, synthesized using solid-state route is investigated. The phase formation was confirmed by X-ray diffraction and surface morphology by scanning electron microscopy (SEM). Structural data reveal the single phase formation corroborated by SEM. The grain distribution is uniform with an average grain size of 3·6 μm. Electrical properties were investigated in a frequency range (1 kHz–1 MHz) by complex impedance spectroscopy (CIS) technique. The material showed negative temperature coefficient of resistance (NTCR) reflecting semiconductor behaviour. A.C. conductivity was found to obey Johnscher’s law. Conductivity mechanism is discussed and activation energy estimated (1·17 eV) for the conduction process is associated with Fe^3 +→ Fe^2 + variable state. The M–H curve showed the presence of ferromagnetism in the studied material.     

Photocatalytic reduction of methylene blue by TiO2 nanotube arrays: effects of TiO2 crystalline phase

TiO₂ nanotube arrays were synthesized by anodization of Ti metal sheets followed by thermal annealing at elevated temperatures from 400 to 600 °C. Scanning electron microscopic measurements showed that dense arrays of nanotubes were produced with the inner diameter about 100 nm, wall thickness 35 nm, and length about 10 μm. X-ray diffraction measurements showed that the as-prepared nanotubes were largely amorphous, whereas thermal annealing led to the formation of well-defined anatase crystalline phase. More interestingly, at 470 °C, the brookite crystalline phase also started to emerge, which became better defined at 500 °C and disappeared eventually at higher temperatures, a phenomenon that has not been observed previously in TiO₂ nanotube arrays prepared by anodization. The impacts of the TiO₂ nanocrystalline structure on the photocatalytic activity were then examined by using the reduction of methylene blue in water as an illustrating example. Upon exposure to UV lights, the visible absorption profiles of methylene blue exhibited apparent diminishment. Based on these spectrophotometric measurements, the corresponding pseudo-first-order rate constant was estimated, and the sample thermally annealed at 500 °C was found to exhibit the highest activity. The strong correlation between the TiO₂ crystalline characteristics and photocatalytic performance suggests that the synergistic coupling of the anatase and brookite crystalline domains led to effective charge separation upon photoirradiation and hence improved photocatalytic activity, most probably as a consequence of the vectorial displacement at the nanoscale junctions between these crystalline grains that impeded the dynamics of electron–hole recombination. These results demonstrate the significance of nanoscale engineering in the manipulation of oxide photocatalytic performance.     

Co/Mn co-doped TiO<Subscript>2</Subscript> nanotube arrays for enhanced photoelectrochemical properties: experimental and DFT investigations

In this study, we used first principle calculations to investigate the electronic properties of TiO₂ modified with eight different elements. Co/Mn co-doped TiO₂ nanotube arrays, which had the smallest band gap, were subsequently prepared using electrochemical anodization followed by atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM) measurements. The results showed that the TiO₂ nanotube arrays were highly ordered and well aligned. Finally, the photoconversion efficiency was measured using photoelectrochemical experiments and, under the same conditions, the photoconversion efficiency under visible light increased approximately three times from 9.35% for the undoped TiO₂ nanotubes to 21.25% for the Co/Mn co-doped TiO₂ nanotubes. These results indicate that Co/Mn co-doped TiO₂ nanotube arrays can improve the efficiency of visible-light utilization and could be a promising material in such fields as photocatalysis and solar cells.     

Hydrothermal synthesis of hemisphere-like F-doped anatase TiO<Subscript>2</Subscript> with visible light photocatalytic activity

Hemisphere-like F-doped anatase TiO₂ has been synthesized by hydrothermal treatment of TiF₄ aqueous solution in the presence of starch at 130 °C for 10 h, and then calcined at 450 °C for 2.5 h in air. The as-synthesized product has been investigated by photocatalytic reaction test and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), and UV–Vis diffuse reflectance spectra (DRS). The results showed that fluorine was successfully doped into the TiO₂ hemispheres. The F-doped TiO₂ hemispheres showed high visible light activity in degradation of acid orange II, which could be attributed to the creation of oxygen vacancies and good crystallinity.     

Phase developments and dielectric responses of barium substituted four-layer CaBi<Subscript>4</Subscript>Ti<Subscript>4</Subscript>O<Subscript>15</Subscript> Aurivillius

In this paper, mixed Ca–Ba oxide Ca_1 − x Ba _x Bi₄Ti₄O₁₅ (CBBT) ceramics, fabricated by the improved traditional ceramics process were investigated by doping concentrations of Ba ion up to x =\emph{x} {=} 0·9 (in steps of 0·1). At room temperature, an orthorhombic crystal system was confirmed using XRD, and their parameter was obtained using the Rietveld method. Dielectric properties and phase transitions were studied and are explained in terms of lattice response of these ceramics. A shift in ferroelectric–paraelectric phase transition (T_C\emph{T}_{C}) to lower temperatures and a corresponding decrease in permittivity peak with increasing concentration of Ba^2 + are also observed. The ferroelectric–paraelectric phase transition of CBBT compounds is of normal type in nature, differing from the relaxor characteristic of BBT. The decrease of orthorhombicity in the lattice structure by the larger Ba^2 + ion incorporation, indicating an approach of a and b, results in lower Curie temperature. Appearance of anomalous loss peaks of Ba-rich compounds at 530°C reveals a phase transition development trend from ferroelectric orthorhombic structure to the paraelectric orthorhombic structure. Relationship of polarization with lattice response is discussed.     

Preparation and characterization of solid-state sintered aluminum-doped zinc oxide with different alumina contents

Aluminum-doped zinc oxide (AZO) ceramics with 0− 2 ·5 wt.% alumina (Al₂O₃) content were prepared using a solid-state reaction technique. It was found that AZO grains became finer in size and more irregular in shape than undoped ZnO as the Al₂O₃ content increased. Addition of Al₂O₃ dopant caused the formation of phase transformation stacking faults in ZnO grains. The second phase, ZnAl₂O₄ spinel, was observed at the grain boundaries and triple junctions, and inside the grains. In this study, a 3-inch circular Al₂O₃ (2 wt.%)-doped ZnO ceramic target sintered at 1500°C for 6 h has a relative density of 99·8% with a resistivity of 1·8 × 10^− 3 Ω-cm. The AZO film exhibits optical transparency of 90·3% in the visible region and shows an electrical resistivity of 2·5 × 10^− 3 Ω-cm.     

Electrochemically induced Ti<Superscript>3+</Superscript> self-doping of TiO<Subscript>2</Subscript> nanotube arrays for improved photoelectrochemical water splitting

We report a facile electrochemical reduction method to synthesize Ti³⁺-self-doped TiO₂ nanotube arrays (TNTs), where the effects of reduction duration and potential on the photoelectrochemical performance were systematically investigated. The X-ray photoelectron spectroscopy and electron paramagnetic resonance spectra confirmed the presence of Ti³⁺ in the TNTs. Under the optimum reduction condition, the Ti³⁺-self-doped TNTs exhibited remarkably enhanced photocurrent density and photoconversion efficiency, which were nearly 3.1 and 1.75 times that of pristine TNTs, respectively. The enhancement of PEC performance is due to the improved electrical conductivity, accelerated charge transfer rate at the TNTs/electrolyte interface, as well as the improved visible light response, which is elucidated by electrochemical impedance spectra, Mott–Schottky, and UV–Vis diffuse reflection spectra.     

Effects of samarium dopant on photocatalytic activity of TiO<Subscript>2</Subscript> nanocrystallite for methylene blue degradation

Sm³⁺-doped TiO₂ nanocrystalline was synthesized by a sol–gel auto-combustion method and characterized by X-ray diffraction, Brunauer-Emmett-Teller method (BET), UV–vis diffuse reflectance spectroscopy (DRS), and also photoluminescence (PL) emission spectroscopy. The photocatalytic activity of Sm³⁺–TiO₂ catalyst was evaluated by measuring degradation rates of methylene blue (MB) under either UV or visible light. The results showed that doping with the samarium ions significantly enhanced the photocatalytic activity for MB degradation under UV or visible light irradiation. This was ascribed to the fact that a small amount of samarium dopant simultaneously increased MB adsorption capacity and separation efficiency of electron-hole pairs. The results of DRS showed that Sm³⁺-doped TiO₂ had significant absorption between 400 nm and 500 nm, which increased with the increase of samarium ion content. The adsorption experimental demonstrated that Sm³⁺–TiO₂ had a higher MB adsorption capacity than undoped TiO₂ and adsorption capacity of MB increased with the increase of samarium ion content. It is found that the stronger the PL intensity, the higher the photocatalytic activity. This could be explained by the points that PL spectra mainly resulted from surface oxygen vacancies and defects during the process of PL, while surface oxygen vacancies and defects could be favorable in capturing the photoinduced electrons during the process of photocatalytic reactions, so that the recombination of photoinduced electrons and holes could be effectively inhibited.     

Low-temperature sintering and microwave dielectric properties of TiO<Subscript>2</Subscript>-based LTCC materials

Temperature stable high-K LTCC material was prepared. The influence of fabrication process on the crystalline phases, microstructures and microwave dielectric properties of TiO₂-Bi₂O₃-CuO ceramics were investigated. The crystalline phases and microstructures of TiO₂-Bi₂O₃-CuO ceramics were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. It was found that rutile TiO₂ phase and Bi₂Ti₄O₁₁ phase co-existed in the TiO₂-Bi₂O₃-CuO ceramics. Separate TiO₂ grains and Bi₂Ti₄O₁₁ grains distributed uniformly in the ceramic matrix. The composition 0.92TiO₂-0.08Bi₂Ti₄O₁₁ with 2 wt% CuO addition that was sintered at 900 °C for 2 h showed high dielectric constant (ε_r ~ 81), high quality factor (Q × f ~ 3,500 GHz) and near zero temperature coefficient of resonant frequency (τ_f ~ −5.1 ppm/°C), meanwhile the compatibility test showed that it could co-fire with silver electrode. The processing-microstructure-property interrelationship was also studied.     

Photocatalytic activity of polypyrrole/TiO<Subscript>2</Subscript> nanocomposites under visible and UV light

A series of polypyrrole (PPy)/titanium dioxide (TiO₂) nanocomposites were prepared in different polymerization conditions by ‘in situ’ chemical oxidative polymerization. The nanocomposites were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectra (FT-IR), X-ray photoelectron spectroscopy spectra (XPS), and UV–Vis diffuse reflectance spectra. The photocatalytic degradation of methyl orange (MO) was chosen as a model reaction to evaluate the photocatalytic activities of TiO₂/PPy catalysts. The results show that a strong interaction exists at the interface between TiO₂ and PPy, the deposition of PPy on TiO₂ nanoparticles can alleviate their agglomeration, PPy/TiO₂ nanocomposites show stronger absorbance than neat TiO₂ under the whole range of visible light. The obtained PPy/TiO₂ nanocomposites exhibit significantly higher photocatalytic activity than the neat TiO₂ on the degradation of MO aqueous solution under visible and UV light illumination. The reasons for improving the photocatalytic activity were also discussed.