TiO2 Grains Research Articles

Effect of thermal treatment on TiO2 varistor properties in different atmospheres

TiO2 is a typical nonstoichiometric compound and defects such as oxygen vacancies in TiO2 grains and at TiO2 grain boundaries are very sensitive to the oxygen partial pressure in the environment. In this paper, commercial TiO2 doped with Nb2O5 and Bi2O3 was used as the raw material to prepare TiO2 varistors. The samples were thermally treated in three kinds of atmospheres: H2, vacuum, and O2 at 600°C. In the two low oxygen partial pressure atmospheres of vacuum and H2, due to the volatilization of lattice oxygen in TiO2, the oxygen vacancy concentration and the number of semiconducting grains increase, while the height and width of the potential barrier and the varistor voltage decrease. After the oxygen atmosphere thermal treatment, the acceptor state density and the height and width of the potential barriers increase, the conductivity decreases, and the varistor voltage improves significantly as a result of oxygen accumulation at grain boundaries.

Facile synthesis of tunable carbon modified mesoporous TiO2 for visible light photocatalytic application

In this paper, we describe a simple and novel approach for preparing tunable carbon-modified mesoporous TiO2 photocatalysts by combining the in-situ carbonization of PAA-Ti/TiO2, hydrothermal reaction process and post-calcination treatment. The synthesized carbon-modified mesoporous TiO2 powders were of high crystallinity, large specific surface area and good visible light response. The carbon species were formed by the carbonization of polyacrylate (PAA). The presence of carbonates was subsequently confirmed by the XPS spectra, which significantly narrow down the band gap of TiO2. The organic group in polyacrylate served as the carbon source and carbon resulted from in-situ carbonization treatment could help to inhibit the excessive growth of TiO2 grain and enlarge the pore structure of TiO2. The amount of carbon species could be feasibly modulated by adjusting the post-calcination temperature and the surface area of the photocatalyst was enlarged further after the partial removal of carbon species. The carbon-modified mesoporous TiO2 powders exhibit excellent reproducibility and photocatalytic performance under visible light irradiation.

Size controlled nanocrystalline TiO<SUB align="right">2 supported on mesostructured SBA-15 for better photocatalytic performance

Nanocrystalline TiO2 was prepared using sol gel hydrolysis and silica SBA-15 using organic solvent method. The synthesised SBA-15 served as the TiO2 support to reduce the TiO2 grain size and well-dispersed TiO2 for enhancement of photocatalytic activity. The synthesised titania/silica composites were characterised by X-ray diffraction, FTIR, TEM, UV-Visible spectroscopy, etc. TEM micrographs showed that the organic solute template method could prepare the titania-silica composite, and successfully embedded titania in SBA-15 channel. The mesoporous structure restrained the titania grain size within 5–7 nm which caused blue shift identified by UV-Vis spectra. The photo catalytic test of this material has been studied using methylene blue which shows improved performance than TiO2. Titania loading on the silica matrix decreases the surface area of the support and there is systematic decrease in their pore volume values, as titania content increases.

Modification of Charge Trapping at Particle/Particle Interfaces by Electrochemical Hydrogen Doping of Nanocrystalline TiO2

Particle/particle interfaces play a crucial role in the functionality and performance of nanocrystalline materials such as mesoporous metal oxide electrodes. Defects at these interfaces are known to impede charge separation via slow-down of transport and increase of charge recombination, but can be passivated via electrochemical doping (i.e., incorporation of electron/proton pairs), leading to transient but large enhancement of photoelectrode performance. Although this process is technologically very relevant, it is still poorly understood. Here we report on the electrochemical characterization and the theoretical modeling of electron traps in nanocrystalline rutile TiO2 films. Significant changes in the electrochemical response of porous films consisting of a random network of TiO2 particles are observed upon the electrochemical accumulation of electron/proton pairs. The reversible shift of a capacitive peak in the voltammetric profile of the electrode is assigned to an energetic modification of trap states at particle/particle interfaces. This hypothesis is supported by first-principles theoretical calculations on a TiO2 grain boundary, providing a simple model for particle/particle interfaces. In particular, it is shown how protons readily segregate to the grain boundary (being up to 0.6 eV more stable than in the TiO2 bulk), modifying its structure and electron-trapping properties. The presence of hydrogen at the grain boundary increases the average depth of traps while at the same time reducing their number compared to the undoped situation. This provides an explanation for the transient enhancement of the photoelectrocatalytic activity toward methanol photooxidation which is observed following electrochemical hydrogen doping of rutile TiO2 nanoparticle electrodes.

Rapid expansion of TiO2 layers processed by supercritical solutions for dye sensitized solar cell applications

Abstract In this work, anatase-type TiO2 layers were prepared by deposition of precursor solution including titanium isopropoxide, ethanol and acetic acetylacetone on a FTO substrate using rapid expansion of supercritical solutions (RESS). The spraying to obtain a particulate coating of TiO2 was carried out at 65 °C and 180 bar of supercritical carbon dioxide environment. Then, thermal annealing at 550 °C for 2 h was applied to coated substrates to obtain desired phase composition. The structural and optical properties of the film were evaluated by XRD, SEM and UV-Vis. SEM observation showed that the photoanode film has porous microstructure with interconnected spherical TiO2 grains. TiO2 film has a band gap of 2.6 eV.

Effects of sintering temperature on sensing properties of V2O5-WO3-TiO2 electrode for potentiometric ammonia sensor

V2O5-WO3-TiO2 mixed metal oxide catalyst was synthesized by impregnation method for fabricating sensing electrodes of potentiometric ammonia sensor in this paper. The sensing electrodes were sintered from 650°C to 900°C to obtain different electrode microstructures. Crystalline structures of the sensing materials were analyzed by X-ray powder diffraction, and the sensing electrode microstructures were characterized by environmental scanning electron microscopy. Sintering temperature strongly affects the microstructure and the sensitivity of the sensing electrode. With the increment of sintering temperature, the slope of semi-logarithmic characteristic curves increased but began to fall when temperature was more than 750°C. The effect of sintering temperature on the response and recovery property has similarities, the sensors sintered at 750°C and 700°C show the shortest response/recovery times. The increase of the sintering temperature is beneficial for a stronger bond between sensing electrode and YSZ electrolyte, and this could increase the amount of three-phase boundary, and improve the sensor performance. High temperature promotes the V2O5 volatilization, anatase TiO2 grains growing up and the anatase to rutile phase transition, further cause the reduction of the electrode specific surface area. The volatilization of V2O5 and the destruction of high dispersion V2O5 resulted from the growing up of the carrier TiO2, are responsible for the degraded properties of sensing electrode. The suitable operating temperature for this type NH3 sensor is 550°C based on the compromise of various factors which could influence the sensor performance.

Effect of SrO on the electrical barrier formation and microstructure of TiO2 varistors

TiO2-based varistor materials with SrO dopants were prepared by the mechanical mixed oxides synthesis technique using conventional sintering at 1400 °C. IV and microstructural characterization were performed. The composition range of 0.50–2.00 mol % SrO was studied and compared to pure TiO2. Experimental evidence shows that small amounts of SrO improve the nonlinear properties of the samples significantly. Optimal varistor characteristics α = 5.50 and Eb = 345 V/cm, were obtained with 1.00 mol % SrO-doped TiO2. SrO in larger amounts causes the formation of precipitates of the SrTiO3 layer on the microstructure, being deleterious to the electrical properties. Therefore, dopants such as SrO or TiO2 play a special role in the morphology of the grain boundary and nonlinear response of these materials. An atomic defect model based on the double barrier Shottky type can be adopted to explain the formation of electrical barriers in TiO2 grain boundaries.

Effect of different nano-carbon reinforcements on microstructure and properties of TiO2 composites prepared by spark plasma sintering

In this study, Titania (TiO2) based composites reinforced with 2wt% of various carbon nanomaterials were prepared using spark plasma sintering (SPS). Prior to SPS, the samples were ball milled. The reinforcements used in the composites were graphene nanoplatelets (GNP), carbon nanotubes (CNT) and single walled carbon nanohorns (SWNH). The ball milled powders and SPS compacts were characterized using various techniques. Mechanical and photocatalytic properties of the SPS composites were evaluated and compared for different nano-carbon reinforced TiO2 composites. X-ray diffraction and Raman spectroscopy studies confirmed that the milled powders comprised of Anatase phase which transformed into Rutile phase during SPS. Nano transformation twins were observed in Rutile grains. Fractured surfaces showed that the reinforcements were well bonded with the TiO2 grains and the SWNH reinforcement resulted in comparatively finer grain size. Nanoindentation studies showed that the hardness and elastic modulus of GNP reinforced composites was 78% and 30% respectively higher compared to TiO2 matrix. The hardness and modulus of the CNT reinforced TiO2 increased by 22% and 5.4% respectively while that of the SWNH reinforced TiO2 increased by 11% and 23% respectively. GNP and CNT reinforced TiO2 exhibited superior photocatalytic activity for the degradation of methylene blue dye compared to pure TiO2.

Investigation of Ag-TiO2 nanostructures photocatalytic properties prepared by modified dip coating method

In this article, titanium dioxide and silver nanostructures were deposited on glass substrates using modified sol–gel methods and dip-coating technique. The films were characterised chemically and physically using different techniques (TLC, UV–Vis and XRD) and tested for environmental applications regarding degradation of aromatic hydrocarbons. The photocatalytic activity of the TiO2 nanostructures is tested with different small concentrations of phenol in water and reaction mechanisms discussed. Considerable enhancement is observed in the photodegradation activity of Ag-modified (3 wt.%) TiO2 compared to unmodified TiO2 nanostructures for phenol concentrations within the pseudo-first-order Langmuir–Hinshelwood (LH) model for reaction kinetics. The pseudo-first-order global degradation rate constant increased from <0.005 min−1 for TiO2 to 0.013 min−1 for 3 mol% Ag-modified TiO2. The enhancement is attributed to the incorporation of Ag which promotes the generation of reactive oxygen species and increases the carrier recombination life-time. In addition, Ag has been observed to extend the absorption to the visible region by its surface plasmon resonances and to suppress the anatase–rutile phase transformation. Moreover, TiO2 grain size prepared was found to be 10 nm which maximises the active surface area. For phenol initial concentrations as low as 0.0002 M, saturation trend in the degradation process occurred at 0.00014 M and the reaction rate can be fitted with half-order LH kinetics.

Growing TiO2 nanowires by solid–liquid–solid mechanism including two factors (Ti and O)

Identifying the growth factors of nanowires helps in controlling their structure and morphology and determining their optimal growth conditions. This study investigates the effect of titanium substrate in growing TiO2 nanowires (NWs) with evaporation method. It reveals that the titanium in substrate is indeed the main source of growth. Using the substrate as the only source of growth with regulated amount of accessible oxygen in the furnace, NWs with lengths ranging from 1 to 70 µm were obtained. The results of the experiments show that the nanowires’ growth is mainly controlled by diffusion of titanium atoms from the substrate through TiO2 grain boundaries and surface diffusion toward NWs’ tips rather than adsorption from vapor phase. The solid–liquid–solid mechanism including two factors (Ti and O) is proposed and discussed for growth of TiO2 NWs.

Preparation, characterization, and kinetic and thermodynamic studies of mixed-phase TiO2 nanoparticles prepared by detonation method

Although detonation method has been reported to prepare TiO2 nanoparticles, phase transformation and crystal growth of detonation-prepared TiO2 nanoparticles have not been explored extensively. In this study, mixed-phase TiO2 nanoparticles, consisting of anatase (34.8wt%, ∼9.4nm) and rutile (65.2wt%, ∼18.2nm), were prepared by the detonation method. The phase transformation in the TiO2 sample at different annealing temperatures was modeled to understand the transformation behaviors. Thermal treatment experiments of the detonation-prepared TiO2 nanoparticles indicated that the initial process involved an anatase-to-rutile transformation. The TiO2 grains coarsened drastically after thermal treatment at 600°C for 1h. In contrast to a normal transformation, the anatase-to-rutile transformation was followed by conversion of rutile to anatase in large crystals. Moreover, smaller TiO2 particles could gain more kinetic energy and combine rapidly with the increase in temperature, forming larger particles. Anatase was transformed completely to rutile after thermal treatment at 720°C for 3.5h. The transformation temperature from anatase to rutile of the detonation-prepared TiO2 nanoparticles was approximately 80°C lower than that of TiO2 prepared by conventional methods. Therefore, thermal treatment may be employed to control the particle size and phase content of detonation-prepared TiO2 nanoparticles.

Band gap manipulation of cerium doping TiO2 nanopowders by hydrothermal method

Cerium doped TiO2 nanopowders were prepared by hydrothermal method with cerium nitrate. The samples were characterized by high-resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), ultraviolet–visible light diffuse reflectance spectra. Results show that the crystal of cerium dioxide can be observed in the grains of TiO2. The morphology of TiO2 prepared by hydrothermal methods changed from nanosheets to nanopowders by cerium doping. Meanwhile cerium doping can significantly reinforce the absorption of visible light, and adjust the absorption cut-off wavelength, from 373 nm to 499 nm for the undoped and the 25% doped 499 nm samples respectively. With the increase in doping ratio of cerium, the band gaps also decreased from 3.32 to 2.48 eV. Based on our observations, it can be concluded that cerium oxide can improve the absorptivity of TiO2 and may have potential applications in the areas of solar cells and photocatalysis.

SiO2/TiO2 double-shell hollow particles: Fabrication and UV–Vis spectrum characterization

Well-defined SiO2/TiO2 double-shell hollow particles (STDSHPs) were successfully fabricated by a sol–gel method with avoiding the particle aggregations and the generation of freestanding TiO2 particles. Comparing with the conventional solid TiO2 particles, STDSHPs showed a novel hollow structure, a much smaller TiO2 grain size (9nm), and a larger specific surface area (113m2/g). In the UV–vis spectrum characterizations, STDSHPs showed significant light harvesting capability in both UV- and visible-light range. STDSHPs also showed stronger per-weight UV absorbance capability than solid TiO2 particles and commercial TiO2 photocatalyst P25, which could be attributed to their novel hollow structure—and thus the absence of light-inaccessible TiO2 particles. Moreover, we analyzed the narrowed band-gap of STDSHPs by the Brus’ effective-mass model (EMM).

Preparation of a La/N co-doped TiO2 film electrode with visible light response and its photoelectrocatalytic activity on a Ni substrate

To prevent the recombination of photoelectron–hole pairs and to extend the adsorption in the visible-light region, a TiO2 coating co-doped with lanthanum and nitrogen on nickel foam was successfully prepared through sol–gel and dip coating methods using the precursors butyl titanate (TBOT) for Ti, lanthanum trioxide for La and urea for nitrogen. The photoelectrocatalytic properties of the prepared samples in the degradation of malachite green were examined under visible light. The phase composition, crystalline structure and optical properties of the La/N co-doped TiO2(Ni) photo-electrodes were studied using different measurement methods, including XRD, SEM, UV–vis DRS, Raman and FTIR. The results indicated that the co-doped TiO2(Ni) photo-electrode not only effectively restrained the growth of the TiO2 grains, but also inhibited the transformation of the titania phase from anatase to rutile. The photo-response of the modified samples was substantially extended from 400 nm to 480 nm, and the band gap narrowed from 3.2 eV to 2.79 eV. The co-doped TiO2(Ni) samples exhibited higher photocatalytic activity as a result of the synergistic effects of the La and N co-doping. The degradation rate of malachite green by the La0.2%N20%/TiO2(Ni) photo-electrode was 23.5% higher than that of the un-doped sample; this result indicated that the extended range of responsive wavelengths greatly expanded the possible applications of the modified electrode. When a low bias voltage was applied to the La0.2%N20%/TiO2(Ni) photo-electrodes, the electrodes exhibited even better photoelectrochemical properties and photoelectrocatalytic activity than a La0.2%N20%/TiO2(Ni) film under visible light irradiation.

Self-development of hollow TiO2 nanoparticles by chemical conversion coupled with Ostwald ripening

In this communication, the TiO2 materials with porous and hollow nanostructures were successfully synthesized through one-pot solvothermal process without any surfactant and template. Our study demonstrated that, the chemical conversion could induce the self-development of hollow TiO2 nanostructures coupled with the physical Oswald ripening process. In particular, the chemical conversion took place progressively from the particle surface to the core of an assembly, generating the converted TiO2 materials. The exterior TiO2 grains were quickly transformed to be a solid and stable surface shell of spheres. Continuous chemical conversion process for inner grains of spheres would lead to the formation of loose package of grains and thus could facilitate would the outward migration and deposition of inner grains onto the stable exterior shell of spheres. The depletion of interior material resulted in generation and development of hollow core structure. This study can be basis for the fabrication of hollow structures of different solid materials by one-pot solvothermal process.

Effect of V2O5-content on electrode catalytic layer morphology and mixed potential ammonia sensor performance

SCR catalysts V2O5–WO3–TiO2 (VWT) with various V2O5 content (0–5wt.%) were prepared by impregnation method in this work. Crystalline structures of the synthesized powder were analyzed by X-ray powder diffraction. Different VWT catalysts were used as sensing materials to prepare mixed potential ammonia sensors based on 5YSZ (ZrO2 doped 5mol% Y2O3) solid electrolyte. The sensing electrodes were characterized by XRD and environmental scanning electron microscopy. NH3 sensitivity was tested at 550°C for the sensors with different V2O5 content sensing electrodes. With the increment of V-content, the slope of semi-logarithmic characteristic curves increased but began to fall when V-content was more than 2wt.%. High-contented V2O5 promoted anatase TiO2 grains growing up to be rod-shaped and block-shaped morphology in sintering, leading to the reduction of specific surface area. The decreased dispersion of V2O5, resulting from the growing up of the carrier TiO2, was responsible for the reduced activity of sensing electrode catalytic. The suitable content of V2O5 in VWT catalysts for NH3 sensors is 1–2wt.% based on various factors.

Pt-decorated TiO2 Electrocatalysts for PEFCs

For polymer electrolyte fuel cell (PEFC) electrocatalysts, carbon corrosion is one of the important technological issues to be solved. TiO2 is a possible alternative support material. In this study, vapor-grown carbon fiber (VGCF-H) conductive fillers are applied as the built-in electron-conductive backbone, on which Nb-doped TiO2 is coated and Pt nanoparticles are then impregnated. FESEM images of Pt/Ti0.95Nb0.05O2/VGCF-H reveal selective impregnation of Pt nanoparticles on the TiO2 grains rather than the VGCF-H conductive fillers. This conductive-filler-integrated electrocatalyst exhibits relatively higher electrochemical surface area (ECSA) and oxygen reduction reaction (ORR) activity, compared to the electrocatalysts without conductive fillers.

The smallest comet 81P/Wild 2 dust dances around the CI composition

AbstractThe bulbous Stardust track #80 (C2092,3,80,0,0) is a huge cavity. Allocations C2092,2,80,46,1 nearest the entry hole and C2092,2,80,47,6 about 0.8 mm beneath the entry hole provide evidence of highly chaotic conditions during capture. They are dominated by nonvesicular low‐Mg silica glass instead of highly vesicular glass found deeper into this track which is consistent with the escape of magnesiosilica vapors generated from the smallest comet grains. The survival of delicate (Mg,Al,Ca)‐bearing silica glass structures is unique to the entry hole. Both allocations show a dearth of surviving comet dust except for a small enstatite, a low‐Ca hypersthene grain, and a Ti‐oxide fragment. Finding scattered TiO2 fragments in the silica glass could support, but not prove, TiO2 grain fragmentation during hypervelocity capture. The here reported dearth in mineral species is in marked contrast to the wealth of surviving silicate and oxide minerals deeper into the bulb. Both allocations show Fe‐Ni‐S nanograins dispersed throughout the low‐Mg silica glass matrix. It is noted that neither comet Halley nor Wild 2 had a CI bulk composition for the smallest grains. Using the analogs of interplanetary dust particles (IDPs) and cluster IDPs it is argued that a CI chondritic composition requires the mixing of nonchondritic components in the appropriate proportions. So far, the fine‐grained Wild 2 dust is biased toward nonchondritic ferromagnesiosilica materials and lacking contributions of nonchondritic components with Mg‐Fe‐Ni‐S[Si‐O] compositions. To be specific, “Where are the GEMS”? The GEMS look‐alike found in this study suggests that evidence of GEMS in comet Wild 2 may still be found in the Stardust glass.

Hollow spheric Ag–Ag2S/TiO2 composite and its application for photocatalytic reduction of Cr(VI)

Hollow spheric Ag–Ag2S was prepared by in-situ chemical transforming of sacrificial Cu2S templates with AgNO3 solution. Hollow spheric Ag–Ag2S/TiO2 composites were then prepared by assembling TiO2 grains around the Ag–Ag2S spheres with hydrothermal treatment of Ag–Ag2S with Ti precursor at 180°C. X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) were adopted to characterize the as-prepared composite. Ag–Ag2S/TiO2 composite has obvious absorption at both UV and visible regions, and shows prior photocatalytic activity for the reduction of Cr(VI) under both UV and visible irradiation. Particularly, the Ag–Ag2S/TiO2-5 and Ag–Ag2S/TiO2-10 show the highest activity for the photocatalytic reduction of Cr(VI) under UV and visible irradiation, respectively. The Schotty barrier between Ag and Ag2S (and TiO2 as well) and the heterojunction between the Ag2S and TiO2 are suggested as the main reasons that enhance the photocatalytic reduction of Cr(VI). The photocatalytic activity of Ag–Ag2S/TiO2 composite is maintained well after being recycled several times.

Development of an aqueous TiO2 paste in terms of morphological manipulation of nanostructured photoanode electrode of dye-sensitized solar cells

A novel simple synthetic procedure for improving cell efficiency and reducing the production cost of TiO2 dye-sensitized solar cells (DSSCs) by modification and optimization of homemade formulated paste is reported. This is achieved in terms of morphological manipulation of deposited monolayer TiO2 films by controlling three processing parameters of paste formulation. These parameters are tailored to obtain a paste with proper viscosity suitable for spin-coating technique and to achieve uniform, homogeneous, and crack-free films with good connections between TiO2 grains and porous structure. Photovoltaic measurements show that TiO2 monolayer DSSC fabricated from the optimized paste formulation (i.e., T7B5G12) has the highest power conversion efficiency (PCE) and short-circuit current (J SC) of 5.36 ± 0.04 % and 14.29 ± 0.02 mA/cm2, respectively. The double-layer DSSCs are also fabricated to enhance electron transport rate and light scattering of the cells. These cells are composed of T7B5G12 film as the under-layer and mixtures of various morphologies (i.e., nanoparticles, regular nanowires, corn-like nanowires, and microparticles) as the over-layers. An enhancement of PCE and J SC up to 7.05 ± 0.024 % and 16.67 ± 0.04 mA/cm2, respectively, is achieved for double-layer DSSC containing a mixture of nanoparticles and corn-like nanowires, as the over-layer. The photovoltaic improvement of double-layer cells is related to tailoring the light scattering, photogenerated charge carriers, and dye-loading parameters.