Suspension high velocity oxy-fuel spraying of a rutile TiO2 feedstock: Microstructure,phase evolution and photocatalytic behaviour

Nano-structured TiO₂ coatings were produced by suspension high velocity oxy fuel (SHVOF) thermal spraying using water-based suspensions containing 30 wt% of submicron rutile powders (~180 nm). By changing the flame heat powers from 40 kW to 101 kW, TiO₂ coatings were obtained with distinctive microstructures, phases and photocatalytic behaviour. Spraying with low power (40 kW) resulted in a more porous microstructure with the presence of un-melted nano-particles and a lower content of the anatase phase; meanwhile, high powers (72/101 kW) resulted in denser coatings and rougher surfaces with distinctive humps but not necessarily with a higher content of anatase. Linear sweep voltammetry (LSV) was used to evaluate the photocatalytic performance. Surprisingly, coatings with the lowest anatase content (~20%) using 40 kW showed the best photocatalytic behaviour with the highest photo-conversion efficiency. It was suggested that this was partially owing to the increased specific surface area of the un-melted nano-particles. More importantly, the structural arrangement of the similarly sized TiO₂ nano-crystallites between rutile and antase phases also created catalytic “hot spots” at the rutile−anatase interface and greatly improved the photo-activity.     

Preparation,characterization and photocatalytic activity evaluation of micro- and mesoporous TiO2/spherical activated carbon

The influences of heat-treatment temperature and activation time on the properties of TiO₂ supported on spherical activated carbon (TiO₂/SAC) were investigated. Nano-sized TiO₂ was dispersed on the spherical activated carbon with the size of 10–30 nm. Some anatase phase of TiO₂ was transformed to rutile phase of TiO₂ with an increase of heat-treatment temperature. All of the TiO₂/SAC photocatalysts had microporous structure, with the mesopore volume increasing over an activation time of 6 h. The TiO₂/SAC photocatalysts obtained at activation times of 6 h and 9 h were observed synergistic effects between adsorption and photocatalysis in the removal of humic acid.     

Kinetics of anatase transition to rutile TiO2 from titanium dioxide precursor powders synthesized by a sol-gel process

Kinetics of anatase transition to rutile TiO₂ from titanium dioxide precursor powders synthesized by a sol-gel process have been studied using differential thermal analysis (DTA), X-ray diffraction, transmission electron microscopy (TEM), selected area electron diffraction (SAED), nano beam electron diffraction (NBED) and high resolution TEM (HRTEM). The DTA result shows residual organic matter decomposed at 436 K. The transition temperature for amorphous precursor powders converted to anatase TiO₂ occurred at 739 K. Moreover, the full anatase transition to rutile TiO₂ occurred at 1001 K. The activation energy of anatase TiO₂ formation was 128.9 kJ/mol. On the other hand, the activation energy of anatase transition to rutile TiO₂ was 328.4 kJ/mol. Mesoporous structures can be observed in the TEM image.     

TiO2 nanopowders via radio-frequency thermal plasma oxidation of organic liquid precursors: Synthesis and characterization

TiO₂ nanopowders have been synthesized via Ar/O₂ thermal plasma oxidation of titanium butoxide (TBO) solutions stabilized with diethanolamine (DEA). Experiments were conducted by varying the O₂ input in the plasma sheath (10–90 L/min) and the DEA/TBO molar ratio (R), while keeping the plasma generation power at 25 kW and the reactor pressure at 500 Torr. The resultant powders are mixtures of the anatase and rutile polymorphs in the studied range, whose anatase content and crystallite size exhibit weak dependence on the O₂ input at a fixed R. Increasing R decreases the anatase content, signifying the role of CO gas, generated via oxidation of the organic precursor, on the phase structure. FE-SEM and TEM analysis show that the resultant powders contain majority of nanoparticles (<50 nm) and some large spheres (>100 nm), whose size and/or number tends to decrease at a higher O₂ input, leading to gradually increased specific surface area. Raman spectroscopy reveals no significant differences in the crystallite size and oxygen-vacancy concentration of the nanocrystals by varying the O₂ input.     

Ordered mesoporous carbon–TiO2 materials for improved electrochemical performance of lithium ion battery

A series of ordered mesoporous carbon–TiO₂ (OMCT) materials with various weight percentages of TiO₂ (50–75 wt%) were synthesized by evaporation-induced self-assembly and in-situ crystallization at various calcination temperatures (600–1200 °C) to evaluate the Li-ion storage performance. The OMCT has ordered 2D hexagonal mesoporous structures and the TiO₂ nanocrystals with different phases are embedded into the frameworks of carbonaceous matrix. The reversible capacity of OMCT is highly dependent on the phase and content of TiO₂, and the anatase TiO₂ is a superior crystalline phase to rutile and TiN for Li-ion insertion. The OMCT₆₅ which contains 35 wt% carbon and 65 wt% TiO₂ shows a high capacity of 500 mAh g^?1 at 0.1C after 80 cycles. In addition, OMCT₆₅ exhibits a good cyclability and rate capability. The reversible capacity remains at 98 mAh g^?1 at a high rate of 5C, and then recoveries to 520 mAh g^?1 at 0.1C after 105 cycles. The excellent reversible capacity and rate capability of OMCT₆₅ are attributed to the embedment of well-dispersed anatase TiO₂ nanocrystals into the specific porous structure of OMCT, which can not only facilitate the fast Li-ion charge transport but can also strengthen the carbon–TiO₂ co-constructing channels for lithiated reactions.     

Synergic effect of cation doping and phase composition on the photocatalytic performance of TiO2 under visible light

The synergic effect of cation doping and phase composition for the further improvement of the photocatalytic activity of TiO₂ under visible light is reported for the first time. Fe^3 + and Sn^4 + co-doped TiO₂ with optimized phase composition were synthesized through a simple soft-chemical solution method. The visible-light-driven photocatalytic activity of Fe^3 + and Sn^4 + co-doped TiO₂ was 5 times of that of Evonik P25 TiO₂ using degradation of methylene blue as model reaction. The synthesized photocatalysts were characterized by powder X-ray diffraction, UV–Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, ¹¹⁹Sn Mössbauer spectroscopy, and X-ray absorption fine structure spectroscopy. It is indicated that Sn^4 + doping can facilitate the phase transition from anatase to rutile. The different ratios of anatase and rutile can be achieved by tuning the amount of Sn^4 + doped into the lattice. Furthermore, the doping of Sn^4 + into TiO₂ lattice can stabilize the phase composition when Fe^3 + is co-doped. In the Fe^3 + and Sn^4 + co-doped TiO₂, Sn^4 + is mainly used to tune and stabilize the phase composition of TiO₂ and Fe^3 + acts as a doping cation to narrow the band gap of TiO₂. Both band gap and phase composition of TiO₂ can be tuned effectively by the simultaneous introduction of Fe^3 + and Sn^4 +. The synergic effect of optimized phase composition (anatase/rutile = 25/75) and narrowed band gap should be the two main reasons for the promoted photocatalytic activity of TiO₂ under visible light.     

Synthesis and photocatalysis of TiO2 hollow spheres by a facile template-implantation route

Process variables such as reaction temperature (55 to 90 °C), calcination temperature (450 to 750 °C), and concentration of TiCl₄ precursor (26 to 105 mM) have been examined in order to tailor the surface area, crystallite size, and the anatase/rutile ratio of the polycrystalline TiO₂ microcapsules prepared by a template-implantation route in heptane solvent. The hollow capsules are all non-aggregating with nanoporous shell structure. Among the process variables examined, the Brunauer–Emmett–Teller (BET) surface area and the anatase/rutile ratio are found critically dependent on the reaction temperature, in which a reduced reaction temperature (from 90 to 55 °C) leads to a higher BET value (from 8.4 to 36.4 m^?2 g^?1), a predominant anatase phase (weight fraction of the anatase phase increases from 0.20 to 0.84), and an improved photodegradation of aqueous methylene blue (MB) dye under UV exposure (the degradation rate increases from 0.5×10^?2 to 5.5×10^?2 min^?1).     

Synthesis of phase- and shape-controlled TiO2 nanoparticles via hydrothermal process

The TiO₂ nanoparticles with anatase (5.7–12.7 nm), rutile (5.4–8.8 nm), mixed (4.4–8.6 nm) phase were individually prepared using the hydrothermal method. The structure and shape of the particles could be controlled by careful alterations of the hydrothermal conditions. Herein, the TiO₂ nanoparticles were successfully synthesized by employing Ti-isopropoxide as the titanium source into hydrochloric acid solution at mild conditions. The crystal structures such as anatase, rutile and mixed phase of TiO₂ nanoparticles were determined by means of concentration of hydrochloride. Especially, we observed that the rutile TiO₂ crystallites were grown into one-dimensional nanostructures, especially, nanowires, with increasing reaction time. The mechanism of the crystallization of the nanoparticles and the growth habit of TiO₂-rutile structure were discussed.     

Decomposition of nonionic surfactant on a nitrogen-doped photocatalyst under visible-light irradiation

A visible-light-active N-containing TiO₂ photocatalysts were prepared from crude amorphous titanium dioxide by heating amorphous TiO₂ in gaseous NH₃ atmosphere. The calcination temperatures ranged from 200 to 1000 °C, respectively. UV–vis/DR spectra indicated that the N-doped catalysts prepared at temperatures <400 °C absorbed only UV light (E_g = 3.3 eV), whereas samples prepared at temperatures ≥400 °C absorbed both, UV (E_g = 3.10–3.31 eV) and vis (E_g = 2.54–2.66 eV) light. The chemical structure of the modified photocatalysts was investigated using FT-IR/DRS spectroscopy. All the spectra exhibited bands indicating nitrogen presence in the catalysts structure. The photocatalytic activity of the investigated catalysts was determined on a basis of a decomposition rate of nonionic surfactant (polyoxyethylenenonylphenol ether, Rokafenol N9). The most photoactive catalysts were those calcinated at 300, 500 and 600 °C. For the catalysts heated at temperatures of 500 and 600 °C Rokafenol N9 removal was equal to 61 and 60%, whereas TOC removal amounted to 40 and 35%, respectively. In case of the catalyst calcinated at 300 °C surfactant was degraded by 54% and TOC was removed by 35%. The phase composition of the most active photocatalysts was as follows: (a) catalyst calcinated at 300 °C—49.1% of amorphous TiO₂, 47.4% of anatase and 3.5% of rutile; (b) catalyst calcinated at 500 °C—7.1% of amorphous TiO₂, 89.4% of anatase and 3.5% of rutile; (c) catalyst calcinated at 600 °C—94.2% of anatase and 5.8% of rutile.     

Phase development in the catalytic system V2O5/TiO2 under oxidising conditions

The target of this work was to investigate phase development in the catalyst system consisting of TiO₂ (anatase) and V₂O₅ (Shcherbinaite). Thus, a set of V₂O₅/TiO₂ specimens was prepared by ball milling and exposed to subsequent annealing in air in the temperature range from 400 to 700 °C. The XRD-results showed the presence of anatase and shcherbinaite as the only phases up to 525 °C. For temperatures above 525 °C the peak intensities were diminishing and rutile as a new TiO₂-phase occurred. Peak intensities and positions were shifted. No loss of oxygen or vanadium was detected. The reaction involves the formation of a rutile solid solution containing VO_x species. XPS studies showed an oxidation state of 4.75 for V in the rutile solid solution as compared to 4.65 in the shcherbinaite. A rutile solid solution once formed could not be re-transformed.The rutile solid solution was first found at 525 °C < T < 550 °C for compositions of 3 mol% < V₂O₅ < 5 mol%. The phase field for rutile solid solutions extends to 10 mol% < V₂O₅ < 12.5 mol% at 675 °C. For very high V₂O₅ concentrations (95 mol% V₂O₅) a eutectic reaction was found at 631 °C. The DTA runs showed a widened endothermic melting peak and a very sharp crystallization peak on cooling. A shcherbinaite structure remained with shifted peak intensities and positions due to the alloying of Ti-ions.SEM inspections showed that the rutile formation and the eutectic reaction both cause a substantial grain growth and a loss of surface area. The catalytic activity is entirely lost when the rutile formation occurs. The knowledge of phase relations helps to find the appropriate processing conditions and to understand the aging phenomena of catalysts.     

Preparation of Ni1−xMnxFe2O4 ferrites by sol–gel method and study of their cation distribution

The spinel ferrite nanoparticles of the system Ni_1?xMn_xFe₂O₄ with x=0.0, 0.1, 0.3, 0.5, 0.7 and 0.9 were prepared by sol–gel auto combustion technique using chlorides of Ni, Mn and Fe as a source with citric acid as chelating agent. The structure of the ferrite materials and the particle size were determined by XRD, it was observed that the structure was a single phase, face centered cubic with lattice parameter ranging from 8.365 Å to 8.394 Å and the particle size ranging from 23.86 Å to 38.30 Å. The lattice parameter showed a linear dependence on concentration in accordance with the Vegard's law. By analyzing XRD patterns, the cation distribution over A and B-sites was estimated through the R-Factor method. The magnetic moment for each sample was determined from cation distribution on the two sites. An enhancement in the net magnetic moment was observed with gradual increase in the Mn content.     

Anatase–rutile transformation of TiO2 sol–gel coatings deposited on different substrates

Titanium dioxide is widely used in a lot of applications. The properties of TiO₂ strongly depend on its phase composition. The transformation temperature between phases is influenced by a lot of factors. One of them is a type of substrate under the TiO₂ film. In presented work, thin films of TiO₂ were deposited by the sol–gel method on silicon, stainless steel (304 L) and Co–Cr–Mo alloy (Vitallium). The process of anatase–rutile phase transformation was investigated by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) studies of deposited coatings. The results were compared with anatase–rutile transformations temperature of TiO₂ powders obtained by analogous sol–gel process. The temperature of anatase–rutile phase transformation changed in the range of 700–1000 °C and strongly depends on a kind of substrate. It was found that anatase–rutile transformation of TiO₂ coating proceeded at a higher temperature than rutilization of titania powders.     

Enhanced photocatalytic degradation of rutile/anatase TiO2 heterojunction nanoflowers

Rutile/anatase TiO₂ heterojunction nanoflowers were prepared via a facile one-step hydrothermal approach using titanium tetrachloride and urea as the raw materials, cetyl trimethyl ammonium bromide (CTAB) as the template. The prepared TiO₂ nanoflowers were characterized by XRD, SEM, TEM and BET analyses. The photocatalytic performance of the as-prepared TiO₂ samples for methyl blue degradation under simulated solar light was investigated. TiO₂ heterojunction nanoflowers with mixed rutile/anatase phase (prepared with 3 mmol CTAB) give the highest photocatalytic activity. In addition, TiO₂ nanoflowers show excellent stability after 9 cycles under the same conditions. These results suggested that the mixed phase anatase/rutile TiO₂ heterojunction nanoflowers have great potential for the future photodegradation of real dye waste water.     

A model of growth of titanium dioxide crystals with simultaneous transformation from anatase to rutile

This study simulates the combination of crystal growth by evaporation–condensation with the potential for a phase change once the crystals have reached a critical size.Crystal growth has been simulated as a two-dimensional Monte Carlo process in which the probability of accepting or rejecting an individual evaporation–condensation step is given by the Metropolis algorithm. Crystals, taken to be of the anatase form of TiO₂, which have grown larger than a specified critical size may transform irreversibly to a second phase, taken to be rutile. The probability of conversion increases with increasing crystal size. For each critical size there exists a limited range over which the transformation kinetics follow the familiar relationship α = 1 ? exp(?ktⁿ) (where α is the fractional conversion at time, t). The dependence of the mean size of the two polymorphs and of the rutile size-spread (rms) on critical size have been investigated. The most significant aspect of the plots of the normalized size-spread, rms/mean-size, is its gradual increase as the rutile fraction increases from 0.75 to 0.98.     

Novel carbon diamond-like phases LA5, LA7 and LA8

In this work, the structure and properties of seven diamond-like carbon phases obtained by linking the graphene layers were calculated using DFT and PM3 methods. The LA5 (Cmca), LA7 (Cmcm), and LA8 (I4₁/amd) diamond-like phases were predicted and studied in this work for the first time. Values of the unit cell parameters of the predicted phases are: a = 4.337 Å, b = 5.024 Å, and c = 4.349 Å for LA5 phase; a = 4.942 Å, b = 4.808 Å, and c = 4.390 Å for LA7 phase; and a = 4.906 Å and c = 4.960 Å for LA8 phase. For these LA5, LA7, and LA8 phases, various structure characteristics, densities, cohesive energies, bulk moduli, electronic densities of states and X-ray patterns were calculated. The comparative analysis showed that the diamond-like phase properties depend on the extent of their structure deformation relative to the cubic diamond structure.     

Characteristics and catalytic properties of Co/TiO2 for various rutile:anatase ratios

This present study revealed a dependence of rutile:anatase ratios in titania on the characteristics and catalytic properties of Co/TiO₂ catalysts during CO hydrogenation. In this study, Co/TiO₂ catalysts were prepared using various titania supports consisting of various rutile:anatase ratios of titania. In order to identify the characteristics, all catalyst materials were characterized using XRD, SEM/EDX, TPR, and hydrogen chemisorption. CO hydrogenation (H₂/CO = 10/1) was also performed to determine the overall activity and selectivity. It was found that both activity and selectivity were altered by changing the rutile:anatase ratios in the titania support.     

Fabrication and photocatalytic performance of Sn-doped titania hollow spheres using polystyrene as template

Sn-doped anatase hollow spheres were fabricated using a template method involving polystyrene spheres as core and anatase coating as shell. The synthesis route included the preparation of PS spheres, followed by their coating by Sn-doped TiO₂ sol-gel precursor and subsequent removal of the PS cores by pyrolysis and recrystallization at 500 °C for 2 h. The observation of minor amounts of rutile suggests that Sn promotes the anatase → rutile phase transformation. At doping levels of ≤ 1.0 mol% Sn, the unsaturated solubility and increasing defect densities enhanced nucleation. At 1.0–2.0 mol% Sn, the solubility remained unsaturated but increasing Sn incorporation reduced crystallinity owing to lattice deformation and partial amorphization. At 2.0–3.0 mol% Sn, solid solution saturation occurred, resulting in excess dopant precipitation, leading to grain boundary pinning and partial blockage of surface-active sites. Ionic radii, thermodynamic, phase equilibria, intervalence charge transfer, and defect chemistry considerations suggest that Sn⁴⁺ exhibits substitutional solid solubility in the TiO₂ lattice. The photocatalytic performance was in the order 1.0 > 1.2 > 1.5 ≈ 0.7 > 2.0 > 0.0 > 3.0 mol% Sn. This ranking is consistent with the dominant role of crystallinity such that, at ≤ 1.0 mol% Sn, the performance increased owing to enhanced nucleation from low defect density and increasing crystallinity while, at 1.0–2.0 mol% Sn, the performance decreased from increased lattice strain and effective partial amorphization, and, at 2.0–3.0 mol% Sn, it decreased from maximal lattice strain and blockage of active sites.     

Slurry explosive detonation synthesis and characterization of 10 nm TiO2

TiO₂ was prepared by detonating a slurry explosive made of Ti precursor, ammonium nitrate, cyclotrimethylenetrinitramine (RDX), and polystyrene (EPS). X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectrometry, Fourier transform infrared spectroscopy, and UV–vis diffuse reflection spectroscopy revealed that the sample was composed of mixed crystals of rutile and anatase TiO₂ with irregular spherical shapes and 10 nm particle size. The minimum energy gap of the sample was 2.9 eV. An ideal TiO₂ explosive was prepared from a precursor/ammonium nitrate/RDX ratio of 1:1:0.6 and 2 g of EPS as a density modifier.     

Structural determination and microwave properties of (x)Re(Co1/2Ti1/2)O3–(1 − x)CaTiO3 (Re = La and Nd) solid solutions

Solid solutions of (x)Re(Co_1/2Ti_1/2)O₃–(1 − x)CaTiO₃ (Re = La and Nd, abbreviated to xLCT and xNCT, respectively) where x = 0, 0.25, 0.5, 0.75 and 1 have been fabricated using solid state synthesis. Samples have been examined using X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy (TEM) and their dielectric properties measured at microwave (MW) frequencies. Formation of single phase solid solutions were confirmed by XRD and the measured lattice parameters varied linearly from LCT (a = 5.66 Å, b = 7.867 Å and c = 5.494 Å) and NCT (a = 5.636 Å, b = 7.914 Å and c = 5.461 Å) to CT (a = 5.596 Å, b = 7.731 Å and c = 5.424 Å). XRD and TEM confirmed both in-phase and antiphase rotations of O-octahedra consistent with an a⁻a⁻c⁺ tilt system across the entire solid solution series. Electron diffraction revealed that LCT and NCT have reflections associated with B-site cation ordering which is absent for x ≤ 0.75. MW dielectric measurements showed that LCT and NCT were highly insulating with microwave quality factor (Qf₀) values of 39,000 and 34,000, respectively. Compositions anticipated to have a zero temperature coefficient of resonant frequency (τ_f) are 0.48LCT-CT and 0.52NCT-CT with ɛ_r = 45 and Qf₀ ∼ 5000 and ɛ_r = 43 and Qf₀ ∼ 4000, respectively.     

Porcelain containing anatase and rutile nanocrystals

The aim of this work is to study the dielectric of two porcelains containing TiO₂ in the form of anatase and rutile. TiO₂ was added in compositions by means of raw kaolin with a relative high quantity of anatase, or the addition of anatase powder (10 wt%) in the initial mixture. An alternative porcelain containing kaolin–anatase mixture was obtained by a preliminary firing at 1300 °C. Beside kaolin, compositions also contain quartz and alkaline feldspar.The microstructural observations show various crystalline phases and micropores, which also have an effective role in affecting the properties. The dielectric characterization of fired porcelain, in the frequency range of 10⁵–10⁹ Hz, shows that permittivity value can be increased from 7.19 to 8.41, depending in TiO₂ crystal type, morphology and content. Permittivity depends also on mullite, quartz and cristobalite, quantities, but the role of TiO₂ phase is predominant. The macroscopic permittivity of porcelains can be calculated using a mixing rule, which fit accurately experimental results.