Tailoring high-surface-area nanocrystalline TiO2 polymorphs for high-power Li ion battery electrodes

The crystallization and morphology of brookite and anatase titania (TiO₂) were controlled using the urea-mediated hydrolysis/precipitation route in the presence of the Ti³⁺ ions. Without the strong complexing agents and the non-hydrothermal conditions, simple alterations to the urea concentration led to the synthesis from brookite nanorods to anatase nanoflowers at a low temperature below 100 °C, whereas the BET specific surface area evolved from 102 to 268 m² g⁻¹, respectively. A possible formation mechanism was also proposed for these TiO₂ nanostructures. The excellent reversible capacity and rate capability were achieved for the anatase nanoflowers because of the small crystallite size and significantly large surface area.     

Lithium-ion battery anode properties of TiO2 nanotubes prepared by the hydrothermal synthesis of mixed (anatase and rutile) particles

From mixed (anatase and rutile) bulk particles, anatase TiO₂ nanotubes are synthesized in this study by an alkaline hydrothermal reaction and a consequent annealing at 300-400 °C. The physical and electrochemical properties of the TiO₂ nanotube are investigated for use as an anode active material for lithium-ion batteries. Upon the first discharge-charge sweep and simultaneous impedance measurements at local potentials, this study shows that interfacial resistance decreases significantly when passing lithium ions through a solid electrolyte interface layer at the lithium insertion/deinsertion plateaus of 1.75/2.0 V, corresponding to the redox potentials of anatase TiO₂ nanotubes. For an anatase TiO₂ nanotube containing minor TiO₂(B) phase obtained after annealing at 300 °C, the high-rate capability can be strongly enhanced by an isotropic dispersion of TiO₂ nanotubes to yield a discharge capacity higher than 150 mAh g⁻¹, even upon 100 cycles of 10 C-rate discharge-charge operations. This is suitable for use as a high-power anode material for lithium-ion batteries.     

Effects of tungsten thickness and annealing temperature on the electrical properties of W-TiO2 thin films

TiO₂ thin films were prepared by RF magnetron sputtering onto glass substrates and tungsten was deposited onto these thin films (deposition time 15-60 s) to form W-TiO₂ bi-layer thin films. The crystal structure, morphology, and transmittance of these TiO₂ and W-TiO₂ bi-layer thin films were investigated. Amorphous, rutile, and anatase TiO₂ phases were observed in the TiO₂ and W-TiO₂ bi-layer thin films. Tungsten thickness and annealing temperature had large effects on the transmittance of the W-TiO₂ thin films. The W-TiO₂ bi-layer thin films with a tungsten deposition time of 60 s were annealed at 200 °C-400 °C. The band gap energies of the TiO₂ and the non-annealed and annealed W-TiO₂ bi-layer thin films were evaluated using (αhν)^1/2 versus energy plots, showing that tungsten thickness and annealing temperature had major effects on the transmittance and band gap energy of W-TiO₂ bi-layer thin films.     

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.     

Magnetic and antibacterial studies of sol-gel dip coated Ce doped TiO2 thin films: Influence of Ce contents

Ce-doped TiO₂ thin films were synthesized by sol-gel dip coating route to evaluate the effect of Ce doping percentage on properties of TiO₂. X-ray diffraction spectra revealed both anatase and brookite phases, and Ce doping favours the anatase–brookite transformation of TiO₂ films. The optical constants of the thin films were achieved by evaluating spectroscopic UV-VIS-NIR spectrophotometry data. The band gap of the Ce doped TiO₂ was reduced from ~3.93 eV to ~3.79 eV with an increase in Ce doping percentage. All films have shown ferromagnetic behaviors which increase with the increase in Ce content due to enhancement in the bound magnetic polaron. Higher Ce doping increases the oxygen vacancies and saturation magnetization. Boost magnetic properties stem from the generation of the interaction between the Ce ion and an oxygen vacancy. The study showed that the antimicrobial activity of Ce doped TiO₂ is ineffective. Hence doping of Ce can modify the properties of TiO₂ and are used in LEDs, magneto-optical devices and solar cells.     

Effect of Ni Doping on the Structural and Optical Properties of TiO<Subscript>2</Subscript> Nanoparticles at Various Concentration and Temperature

TiO₂ nanopowders doped by Ni were prepared by sol–gel method. The effects of Ni ion (transition metal ion) doping on the physical structural and optical properties of TiO₂ have been investigated by X-ray diffraction (XRD), scanning electron microscopy and UV–Vis absorption spectroscopy. XRD results suggest that adding impurities has a significant effect on anatase phase stability, crystallinity, and particle size of TiO₂. The phase transformation from anatase to rutile was inhibited by Ni ion doped TiO₂ at temperatures 675 °C. The lowest band gap value (2.83 eV) was obtained for TiO₂-4%Ni sample calcined at 675 °C.     

Effect of 2-propanol and water contents on the crystallization and particle size of titanium dioxide synthesized at low-temperature

Two series of titanium dioxide, TiO₂, powder were prepared at a temperature of 50 °C without any catalyst. The effects of 2-propanol and water contents on the formation of crystalline powder mixture of anatase and brookite were systematically studied. The characteristics of produced powder were determined by employing X-ray diffraction, transmission electron microscopy, nitrogen adsorption test and Fourier transform infrared spectroscopy.     

Electrical and optical properties of a polyblend electrolyte

A potassium ion conducting polyblend electrolyte based on polyvinyl pyrrolidone (PVP)+polyvinyl alcohol (PVA) complexed with KBrO₃ was prepared using solution cast technique. The electrical conductivity increased with increasing dopant concentration. Optical absorption studies were made in the wavelength range (200-600 nm) on pure (PVP+PVA) and KBrO₃ doped (PVP+PVA) films. The absorption edge was observed at 5.13 eV for undoped (PVP+PVA) while it ranged from 4.88 to 5.0 eV for differently KBrO₃-doped samples. The direct band gaps for undoped and KBrO₃ doped (PVP+PVA) films were found to be, respectively, 5.05 and 4.95, 4.86 and 4.90 eV while the indirect band gaps were 5.03 and 4.88, 4.79 and 4.83 eV, respectively. The absorption edge and the band gaps moved towards lower energies as the dopant concentration was increased up to 20 wt% of the dopant. For further increase in dopant concentration these values started increasing again. This is explained in terms of formation of charge transfer complexes between the dopant and the host matrix. The thermal properties of these films were investigated with differential scanning calorimetry (DSC). The variation in film morphology is examined by scanning electron microscopic examination.     

Effect of brookite phase on the anatase–rutile transition in titania nanoparticles

Nanosized TiO₂ powders were prepared from the precipitation in the TiCl₄ precursor under various pH values. The prepared titania existed in the form of nanocrystalline anatase with some brookite, which was evidenced by X-ray diffraction analysis and Raman spectroscopy. The average crystallite sizes of the TiO₂ particles heat treated at 450 °C for 2 h are in the range of 7–9 nm. The lattice constant c of anatase increased with increasing the synthesized pH value, whereas the volume fraction of the brookite phase increased with decreasing the synthesized pH value. The beginning and ending temperatures for the anatase–rutile transformation were found to decrease with increasing the volume fraction of the brookite phase. The brookite phase in the powder is responsible for enhancing the anatase–rutile transition.     

Hydrothermal synthesis of S-doped TiO2 nanoparticles and their photocatalytic ability for degradation of methyl orange

A simple synthesis route to nanocrystalline S-doped TiO₂ photocatalysts by a hydrothermal method at 180 °C was developed and the photocatalytic activity of the obtained powders for the degradation of methyl orange was studied. The products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The phase composition (anatase/rutile ratio) and the photocatalytic activity of the final materials were found to be markedly influenced by the amount of the incorporated sulphur. On increasing the S-dopant amount, the anatase/rutile ratio and the photocatalytic activity of the as-prepared powders increased.     

The improvement of electron transport rate of TiO2 dye-sensitized solar cells using mixed nanostructures with different phase compositions

Dye-sensitized solar cells (DSCCs) in the form of mixed nanostructures containing TiO₂ nanoparticles and nanowires with different weight ratios and phase compositions are reported. X-ray diffraction and field emission scanning electron microscopy analyses revealed that the synthesized TiO₂ nanoparticles had average crystallite size in the range 21–39 nm, whereas TiO₂ nanowires showed diameter in the range 20–50 nm. The indirect optical band gap energy of TiO₂ nanowires, anatase- and rutile-TiO₂ nanoparticles was calculated to be 3.35, 3.28 and 3.17 eV, respectively. The power conversion efficiency of the solar cells changed with nanowire to nanoparticle weight ratio, reaching a maximum at a specific value. An increase of 4.3% in cell efficiency was achieved by introducing 10 wt% nanowire into the as-synthesized TiO₂ nanoparticles (WP1 cell). Furthermore, an increase of 27.6% in cell efficiency was achieved by using crystalline anatase-TiO₂ nanoparticles rather than as-synthesized TiO₂ nanoparticles in WP1 solar cell. It was found that the power conversion efficiency and short circuit current of WP1 cell were decreased down to around 30.8% and 39.1%, respectively using rutile nanoparticles rather than anatase nanoparticles. The improvement of cell efficiency was related to rapid electron transport and less recombination of photogenerated electrons, as confirmed by electrochemical impedance spectroscopy.     

TiO2 based nanopowder coatings over stainless steel plates for UV-C photocatalytic degradation of methylene blue

Immobilizing the photocatalyst in a water treatment process design is essential; however, the immobilization method may affect the photoactivity of the photocatalyst. In this work, photocatalyst powders were successfully coated on 316 L stainless steel plates by a novel brush coating method. Three combinations of photocatalyst mixtures (pure TiO₂ anatase, anatase doped with WO₃, or TiO₂ rutile) were and annealed at different temperatures between 460-540°C. The ~ 10 μm thick coatings demonstrated full plate coverage and strong adhesion and adequate durability. Surface roughness increased with annealing temperature. The doping and annealing process enabled band gap reduction to the visible light spectrum for all coatings, with the smallest band gap being 2.48 eV (1 eV = 1.6 × 10⁻¹⁹ J). Subsequent methylene blue degradation tests under UV-C showed that the coatings annealed in 460°C exhibited the best performance and with the highest degradation rate constant of 5.59 hours⁻¹.     

Carbon-coated anatase: the role of the carbon layer for photocatalytic performance

Carbon-coated anatase-type TiO₂ was prepared by the heat treatment of powder mixtures of photocatalyst TiO₂ (ST-01) with different carbon precursors, poly(vinyl alcohol), hydroxyl propyl cellulose and poly(ethylene terephthalate), at a temperature between 700 and 900 °C for 1 h in an Ar gas flow. Since the carbon layers formed on the TiO₂ particles were porous, the samples prepared showed a high adsorptivity, in addition to the photoactivity of TiO₂. The carbon coating was shown to suppress the phase transition from photoactive anatase to much less active rutile, but seemed to reduce the amount of UV radiation reaching the surface of the TiO₂ particles. A balance among different factors controlled by the carbon layer on the TiO₂ particles was required to get high photocatalytic performance, i.e., high rate constant for the photodecomposition of methylene blue. On the sample prepared at 850 °C with a carbon content of about 3.5 wt%, the highest rate constant in the present work was obtained, in which the transition from anatase to rutile was suppressed and carbon layer was thin enough to transmit UV rays.     

Properties of sol-gel SnO2/TiO2 electrodes and their photoelectrocatalytic activities under UV and visible light illumination

A visible light active binary SnO₂-TiO₂ composite was successfully prepared by a sol-gel method and deposited on Ti sheet as a photoanode to degrade orange II dye. Titanium and SnO₂ can promote the development of rutile phase of TiO₂ and inhibit the formation of anatase phase of TiO₂. Formation of SnO₂ crystalline is insignificant even when the calcination temperature increases to 700 °C. Heterogenized interface between SnO₂ and TiO₂ inhibits growth of TiO₂ linkage and leads to the particle-filled surface morphology of SnO₂-containing films. The carbonaceous, Ti-O-C bonds and Ti³⁺ species are likely to account for the photoabsorption and photoelectrocatalytic (PEC) activity under visible light illumination. The electrode with 30% SnO₂ exhibits higher photocurrent when compared with those in the region of 0-50%. The 600 °C-calcined SnO₂-TiO₂ electrode indicates higher activity when compared with those at 400, 500, 700 and 800 °C. PEC degradation of orange II follows the Langmuir-Hinshelwood model and takes place much effectively in a solution of pH 3.0 than those in pH 7.0 and pH 11.0.     

Effects on the size of nano-TiO2 powders prepared with sol-emulsion-gel method

Small and uniform nano-TiO₂ particles were obtained by sol-emulsion-gel method. Optimum preparation conditions include the surfactant concentration at the critical micelle concentration (CMC) and the pH value at 8-9. The calcinations could improve the crystallization of TiO₂ powders and accelerated the phase transformation from amorphous phase to anatase or rutile. Seen from the TEM, the spherical and uniform nano-TiO₂ particles were obtained as calcined at 500 °C, and the gradual shape change of the particles in the process of calcination was noticed: Formed sphere → Rod → Sphere → Distorted sphere.     

Investigation of lithium diffusion in nano-sized rutile TiO2 by impedance spectroscopy

The kinetics of the electrochemical lithium insertion reaction in nano-sized rutile TiO₂ has been investigated using ac impedance spectroscopy. The experimental data are obtained for a rutile compound synthesized via a solution technique and characterized by a morphology corresponding to spherical particles made of a large number of very thin nanorods 20 nm thick. The results are discussed as a function of the Li content x for 0 < x ≤ 0.8 in Li_xTiO₂, the temperature over the range 10-50 °C and the number of discharge-charge cycles. The significant linear decrease of the chemical diffusion coefficient D_Livs. the lithium content and the high values of D_Li found in the composition range 0 < x ≤ 0.5 are discussed and related with the electrochemical behaviour of the nano-sized material. From comparison with the bulk material, a promoting effect of the morphology on the kinetic characteristics is evidenced. For the first time an experimental evaluation of the activation energy for Li diffusion in nano-sized rutile TiO₂ is obtained; the value of 0.35 eV being much lower than that reported from computational experiments for the micro-sized oxide. This work also demonstrates a new system takes place from the second cycle, characterized by a significant improvement of Li diffusion by a factor five and allowing high rates to be used.     

Effect of annealing on UV-visible absorption and photoluminescence behavior of liquid phase deposited TiO2 nanorods

In this work, anatase and rutile TiO₂ nanorods were fabricated using one-step liquid phase deposition process, followed by heat treatment in the range 300-800°C. The direct and indirect band gap of the TiO₂ nanorods was estimated form optical absorption data which illustrated a red shift at higher temperatures owing to the different nature of excitons in anatase and rutile phases. The photoluminescence (PL) spectra revealed the presence of two main emission bands consisting of four peaks. It was found that two high-energy peaks located at 2.95-3.30 eV could be generated from exciton transitions from the conduction band to the valence band of TiO₂ nanorods, while two low-energy peaks located at 2.43-2.64 eV may arise from surface state transitions. The PL intensity firstly increased with temperature and at 500°C reached a maximum value, then decreased through increasing temperature up to 800°C. These variations in the intensity of PL emission could be explained in terms of changes in phase structure, crystallinity, and amount of the oxygen vacancies, which are all dependent to the annealing temperature based on X-ray diffractometer and X-ray photoelectron spectrometer studies. These results indicated that annealing temperature allows to manipulate the properties of TiO₂ nanorods for opto-electronic applications.     

Brookite and anatase nanomaterial polymorphs of TiO2 synthesized from TiCl3

Synthesis of anatase and brookite was achieved under mild conditions in aqueous solution. In addition, mixtures of brookite and rutile as well as mixtures of the anatase and rutile polymorphs were observed at different temperatures. It was observed that temperatures above 80 °C produced anatase and brookite phases, exclusively. The samples prepared using the acetate synthesis showed on average bandgap around 2.95–3.0 eV and brookite was observed. Whereas the samples prepared using the sulfate synthesis showed and bandgap from 3.1 to 3.2 eV and anatase was observed. In addition, the average grain size of the brookite and anatase phase synthesized at 100 °C were 9.7 and 12 nm, respectively, as determined from XRD.     

Mn-doped titania thin films prepared by spin coating

TiO₂ thin films doped with ≤7 mol% Mn (metal basis) were deposited on F-doped SnO₂-coated (FTO) glass substrates by spin coating. The structural, morphological, and optical properties of the films were investigated by glancing angle X-ray diffraction (GAXRD), laser Raman microspectroscopy, field emission scanning electron microscopy (FESEM), and ultraviolet–visible spectroscopy (UV–VIS). Mn doping of TiO₂ (anatase) extended the optical absorption edge to longer wavelengths (lower photon energies) significantly lowering the band gap from 3.32 eV (undoped) to 2.90 (7 mol% Mn). The absorption edges of all films were sharp and the transparencies in the visible region were in the range 60–75%. All of the films were homogeneous, fully dense, and essentially crack-free.     

Photocatalytic Nb2O5-doped TiO2 nanoparticles for glazed ceramic tiles

TiO₂ nanoparticles are currently used as coating for self-cleaning building products. In order to achieve high self-cleaning efficiency for outdoor applications, it is important that titania is present as anatase phase. Moreover, it is desirable that the particle sizes are in nano-range, so that a large enough surface area is available for enhanced catalytic performance. In this work, TiO₂ nanoparticles doped with 0–5 mol% Nb₂O₅ were synthesized by co-precipitation. Nb₂O₅ postponed the anatase to rutile transformation of TiO₂ by about 200 °C, such that after calcination at 700 °C, no rutile was detected for 5 mol% Nb₂O₅-doped TiO₂, while undoped TiO₂ presented 90 wt% of the rutile phase. A systematic decreasing on crystallite size and increasing on specific surface area of TiO₂ were observed with higher concentration of Nb₂O₅ dopant. Photocatalytic activity of anatase polymorph was measured by the decomposition rate of methylene blue under ultraviolet and daylight illumination and compared to commercial standard catalyst (P25). The results showed enhanced catalysis under UV and visible light for Nb₂O₅-doped TiO₂ as compared to pure TiO₂. In addition, 5 mol% Nb₂O₅-doped TiO₂ presented higher photocatalytic activity than P25 under visible light. The enhanced performance was attributed to surface chemistry change associated with a slight shift in the band gap.