Phase transformation and nanocrystallite growth behavior of 2 mol% yttria-partially stabilized zirconia (2Y-PSZ) powders

Two mol% yttria-partially stabilized zirconia (2Y-PSZ) precursor powders were obtained through a co-precipitation process using ZrOCl₂.8H₂O and Y(NO₃)₃.6H₂O as starting materials. Phase transformation and crystallite growth behavior have been investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). XRD results show that the crystal structure to be composed of coexisting tetragonal ZrO₂ (t-ZrO₂) and monoclinic ZrO₂ (m-ZrO₂) when the 2Y-PSZ freeze dried precursor powders was calcined at 773–1273 K for 2 h. The fraction of m-ZrO₂ content is lower than 3.0 % when the calcination temperature is lower than 1073 K, whereas m-ZrO₂ content rapidly increases to 8.7 % with the increase of calcination temperature to 1273 K. The crystallite size of t-ZrO₂ increases from 12.3 to 30.2 nm when calcination temperature increased from 773 to 1273 K. In addition, the activation energy of t-ZrO₂ and m-ZrO₂ crytallite growth in 2Y-PSZ freeze dried precursor powders are 29.2 and 21.8 kJ/mol, respectively.     

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.     

Effects of SiO2 addition on TiO2 crystal structure and photocatalytic activity

A series of TiO₂–SiO₂ mixtures – having the following stoichiometry Ti_1?xSi_xO₂, with x = 0, 0.1, 0.3 and 0.5 atoms per formula unit – were prepared by using precursor oxides and fired at three temperatures (900, 1000 and 1200 °C). The modifications in the structure and, consequently, on the photocatalytic activity, induced by the addition of SiO₂ into the TiO₂ powder, were thoroughly investigated by using various analytical techniques: X-ray powder diffraction, electron microscopy (FE-SEM and TEM), XPS, FT-IR, DRS and BET analysis. The results underlined as essentially no solid solution occurs between the two crystalline end-members. Nevertheless, silica addition caused a retarding effect on anatase-to-rutile phase transformation and on the crystallite growth.The photocatalytic activity of the powders was assessed in gas phase and the results were explained by taking into account the anatase and rutile relative amounts in the samples, their crystallite size, the surface hydroxyl groups adsorbed on the photocatalysts and the surface area of the mixtures.     

Sol–gel auto-igniting synthesis and structural property of cerium-doped titanium dioxide nanosized powders

Anatase-type TiO₂ (titania) doped with cerium up to 5 mol% was directly formed as nanometer-sized particles from TiO(NO₃)₂–Ce(NO₃)₂–NH₄NO₃–citric acid complex compound system by sol–gel auto-igniting synthesis process. The precursor gel was characterized by infrared spectroscopy and TG/DSC analysis. The XPS measurement showed that Ce(III) was easily oxidized to Ce(IV) at 550 °C and above. The XRD data, XPS spectra, and TEM selected-area diffraction patterns confirmed that cerium(IV) formed a solid solution in the anatase-type TiO₂ powders. Doping of CeO₂ into TiO₂ shifted the phase transformation from anatase- to rutile-type structure to a high temperature. On the other hand, CeO₂ was segregated on the surface of TiO₂ and the rutile formation was accelerated during phase transformation from anatase to rutile at elevated temperature. When the cerium content was increased in the anatase phase, onset of optical absorption shifted to longer wavelengths, and absorption in the UV-light region and in the visible-light region over 400–500 nm clearly appeared in the diffuse reflectance spectra of the as-prepared Ce-doped TiO₂.     

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.     

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.     

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.     

Probing the oxidation behavior of Ti2AlC MAX phase powders between 200 and 1000 °C

Oxidation of commercial Ti₂AlC MAX phase powders at 200–1000 °C has been investigated by XRD, XPS, SEM, STA and TGA coupled with FTIR. These powders are a mixture of Ti₂AlC, Ti₃AlC₂, TiC and Ti_1.2Al_0.8. Oxidation at 400 °C led to disappearance of carbide phases from Ti 2p, Al 2p and C 1s XPS spectra. At 600 °C, powders changed from dark grey to light grey with a significant volume increase due to crack formation. Powders were severely oxidized by detecting rutile with minor anatase TiO₂. At 800 °C, α-Al₂O₃ was detected while anatase transformed into rutile TiO₂. The cracks were healed and disappeared. At 1000 °C, the Ti₂AlC powders were fully oxidized into rutile TiO₂ and α-Al₂O₃ with a change of powder color from light grey to yellow. FTIR detected the release of C as CO₂ from 200 °C onwards but with additional CO above 800 °C.     

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.     

Physical properties of V-doped TiO2 nanoparticles synthesized by sonochemical-assisted process

V-doped TiO₂ nanoparticles were synthesized by sonochemical process using titanium isopropoxide as a titanium source, vanadyl acetylacetonate as a dopant source. Sonication was conducted using sonic horn operated at 20 kHz for 20 min until the completely precipitated product was reached. The as-synthesized precipitates with various vanadium dopant (1–5 mol %) were calcined at 500–1000 °C for 4 h. The relevant physical properties of the nanoparticles were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and transmission electron microscope (TEM). The anatase phase TiO₂ nanoparticles can be synthesized by sonochemical process. Post calcinations process results in the anatase-to-rutile phase transformation and the enhancement in crystallinity with increasing temperature. The results also indicate good incorporation of V ions in TiO₂ lattices and significant effect of V dopant on alternation of interplanar spacing of TiO₂.     

Photocatalytic activity of titanium dioxide coatings: Influence of the firing temperature of the chemical gel

Heterogeneous photocatalysis can be exploited for the decomposition of micro-organisms which have developed on the surfaces of building materials. In this work, the efficiency of titanium dioxide coatings on fired clay products is examined. The sol–gel method is used to synthesize a fine TiO₂ powder with a specific surface area of 180 m² g^?1. Thermal treatment of the chemical gel at 340 °C leads to crystallisation in the anatase phase and with further temperature increase, crystallite growth. For thermal treatments in the range 580–800 °C, there is a progressive transition from anatase to rutile. However, despite a decrease in specific surface area of the powder attributed to aggregation/agglomeration, the coherent domain size deduced from X-ray diffraction measurements remains almost constant at 23 nm. Once the transition is completed, increase of thermal treatment temperature above 800 °C leads to further crystallite growth in the rutile phase. The thermally treated titania powders were then sprayed onto fired clay substrates and the photocatalytic activity was assessed by the aptitude of the coating to degrade methylene blue when exposed to ultraviolet light. These tests revealed that the crystallite size is the important controlling factor for photocatalytic activity rather than the powder specific surface area or the anatase/rutile polymorph ratio.     

Preparation of high active nanometer TiO2 sonocatalyst by partial transition crystal in hydrogen peroxide solution under ultrasonic irradiation

The transition crystal nanometer TiO₂ sonocatalyst with high sonocatalytic activity was prepared utilizing the method of ultrasonic irradiation in hydrogen peroxide solution. The sonocatalytic activity of transition crystal nanometer TiO₂ powder was validated through the degradation of acid red B and azo fuchsin solutions by ultrasonic irradiation, respectively. The results show that the sonocatalytic activity of the transition crystal nanometer TiO₂ powder is obviously higher than ones of both original nanometer rutile and anatase TiO₂ powders. The degradation ratios of acid red B and azo fuchsin in the presence of the transition crystal nanometer TiO₂ catalyst surpass 96.5% and 85.3% within 40 min ultrasonic irradiation, respectively. At the same conditions, the degradation ratios are 62.5% and 45.0% in the presence of original nanometer anatase TiO₂ powders, 73.5% and 59.5% in the presence of original nanometer rutile TiO₂ powders, respectively, while the corresponding degradation ratios are only 29.8% and 14.2% in the absence of any TiO₂ catalyst, respectively. The degradation processes of both acid red B and azo fuchsin solutions are the pseudo-first-order reaction.     

One-step preparation of size-defined aggregates of TiO2 nanocrystals with tuning of their phase and composition

One-step route based on the thermal decomposition of the double salt (NH₄)₂TiO(SO₄)₂ (ammonium titanyl sulfate, ATS) is presented to prepare size-defined aggregates of Ti-based nanoparticles with structural hierarchy. The component of Ti-based networks is tunable from anatase/rutile TiO₂, nitrogen-doped TiO₂, TiN_xO_1−x, to TiN depending on the atmospheres and reaction temperatures. The as-prepared Ti-based powders were characterized by X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectra (DRS), and BET surface area techniques. It is found that TiO₂ in the predominant rutile phase could be achieved by the thermal decomposition of ATS in flowing Ar gas. Furthermore, the nitrogen-doped TiO₂, TiN_xO_1−x solid solution and TiN were prepared by the thermal decomposition of ATS in flowing NH₃ gas by varying the temperatures. The network of anatase TiO₂ with a specific surface area up to 64 m² g⁻¹ contains large mesopores with a mean diameter of ca. 15 nm, and the large pore size allows more accessible surface and interface available for the photocatalytic degradation of large-molecule dyes. The photocatalytic activity of the prepared TiO₂ and nitrogen-doped TiO₂ under UV–vis light irradiation is compared to Degussa P-25 using the photocatalytic degradation of methylene blue (MB) as a model reaction. The anatase TiO₂ nanoparticles derived from one-step route show the highly efficient photocatalytic activity for the degradation of MB in comparison with Degussa P-25. The presence of large-sized rutile in the TiO₂ powder decreases the specific surface area and thus the powder exhibits a lower photocatalytic activity.     

Bismuth titanates candidates for high permittivity LTCC

Bi₂O₃–TiO₂ composites are known to possess attractive microwave dielectric properties. However, producing LTCC analogues with equally promising dielectric properties is problematic. Here, we show that judicious choice of both TiO₂ starting powders and dopants can produce composites with excellent properties. Three TiO₂ powders were evaluated: 1 μm-anatase, 1 μm-rutile and a nanosized (30 nm) mixture of 75–25 anatase-rutile. The best dielectric properties were obtained by using uncalcined nanosized anatase/rutile with Bi₂O₃ powder. By doping this Bi₂O₃–TiO₂ powder mixture with 0.112 wt.% CuO dielectric properties of Q × f = 9000 GHz, ɛ_r = 80 and τ_f = 0 ppm/K (at 300 K) were obtained at a sintering temperature of 915 °C.     

Formation of anatase TiO2 nanoparticles by simple polymer gel technique and their properties

Pure anatase nano-TiO₂ powders were successfully prepared by a simple polymer gel technique using poly-(vinylpyrrolidone) (PVP) as the polymer. The products were systematically characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), UV–visible spectroscopy and photoluminescence studies. The XRD and XPS results indicate that the prepared powder had a pure anatase nano-TiO₂ structure with lattice parameters a and c of 0.378 and 0.951 nm, respectively. The particle size analysed by TEM ranged between 7 and 12 nm. The maximum UV absorption for the TiO₂ nanoparticles was below 400 nm with an estimated direct band gap (Eg) of 3.55 eV. The photoluminescence peaks of the nanopowder were observed at 391 and 468 nm. The nanosized materials were produced using a simple and cost effective polymer gel technique.     

Tunable TiO2 (anatase and rutile) materials manufactured by mechanical means

Anatase and rutile are two naturally found titanium dioxide phases with attractive dielectric, catalytic, and photo-catalytic characteristics. Anatase and rutile are photo-catalytically active in the UV region, since their band gaps are 3.2 eV and 3.75 eV, respectively. In this work is proposed a cost-effective, easy to launch methodology for modification of the TiO₂ bandgap. Such modifications will make the oxides photo-catalytically active in a wider optical range from the visible wavelengths to an extended UV spectrum. The proposed methodology is based on mechanical means such as mixing and milling. Various ratios of anatase:rutile were investigated and milled from 0 (mixing only) to 50 h using high energy mills. The results on mixing and milling show that it is possible to modify the bandgap of the TiO₂ from 2.53 eV to 4.04 eV. The characterization was conducted by means of X-ray diffraction, Raman spectroscopy, Scanning electron microscopy, and optical spectroscopy.     

Preparation of spherical nanoparticles of LaAlO3 via the reverse microemulsion process

Spherical LaAlO₃ nanoparticles in a reverse microemulsion consisting of solution (water phase), Tween-80 and Span-80 (surfactant), n-butanol (cosurfactant, and cyclohexane (oil phase) were prepared. Precursor powders and calcined powders were characterized by differential thermal analysis (DTA), thermogravimetry analysis (TG), X-ray diffraction (XRD) and transmission electron microscopy (TEM). A pure perovskite LaAlO₃ formed when the precursor hydroxides calcined at 800 °C for 2 h. The particle size was about 50 nm and the shape of the monodisperse particles is spherical. The reverse microemulsion process can dramatically lower the crystallization temperature of LaAlO₃ about 700 °C than the classical solid-state reaction method.     

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.     

Molten salt synthesis of SrTiO3 nanocrystals using nanocrystalline TiO2 as a precursor

A molten salt method was proposed to synthesize SrTiO₃ nanocrystals in the eutectic NaCl–KCl at 700 °C for 6 h, by using the homemade TiO₂ nanocrystals and commercial Sr(NO₃)₂ powder as raw materials. Besides, a control experiment with the commercial TiO₂ submicron-sized crystallites as a precursor was also conducted. The structure and composition of the obtained products were characterized by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The results from XRD and XPS revealed the preparation of pure cubic phase SrTiO₃ powders. The TEM observation demonstrated that the nanocrystalline TiO₂ precursor played an important role in the current molten salt synthesis of SrTiO₃ nanocrystals, which were likely formed by the template formation mechanism.     

TiO2 nanofibers doped with rare earth elements and their photocatalytic activity

In the present study rare earth doped (Ln³⁺–TiO₂, Ln = La, Ce and Nd) TiO₂ nanofibers were prepared by the sol–gel electrospinning method and characterized by XRD, SEM, EDX, TEM, and UV-DRS. The photocatalytic activity of the samples was evaluated by Rhodamine 6G (R6G) dye degradation under UV light irradiation. XRD analysis showed that all the synthesized pure and doped titania nanofibers contain pure anatase phase at 500 °C but at 700 °C it shows both anatase and rutile phase. XRD result also shows that Ln³⁺-doped titania probably inhibits the phase transformation. The diameter of nanofibers for all samples ranges from 200 to 700 nm. It was also observed that the presence of rare-earth oxides in the host TiO₂ could decrease the band gap and accelerate the separation of photogenerated electron–hole pairs, which eventually led to higher photocatalytic activity. To sum up, our study demonstrates that Ln³⁺-doped TiO₂ samples exhibit higher photocatalytic activity than pure TiO₂ whereas Nd³⁺-doped TiO₂ catalyst showed the highest photocatalytic activity among the rare earth doped samples.