Raman spectroscopy study of the phase transformation on nanocrystalline titania films prepared via metal organic vapour deposition

Raman spectroscopy (RS) was used to study the phase transformations of nanocrytalline TiO₂ thin films. The films were grown by a vertical-flow cold-wall metal organic chemical vapour deposition system, using Ti(C₁₀H₁₄O₅) as the source reagent, at different substrate temperatures. The results indicate that the anatase phase is present at around 550 °C and the rutile phase starts to form at 620 °C. The anatase phase completely transforms into the rutile phase at 680 °C. We have demonstrated that RS can be used as a powerful nondestructive technique for a quick and efficient determination of the phase of TiO₂ thin films.     

Synthesis and characterization of anatase and rutile TiO2 nanorods by template-assisted method

Well-aligned anatase and rutile TiO₂ nanorods and nanotubes with a diameter of about 80–130 nm have successfully been fabricated via sol-gel template method. The prepared samples were characterized by using thermogravimetric (TG) and differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). The XRD results indicated that the TiO₂ nanorods were crystallized in the anatase and rutile phases, after annealing at 400–800 °C for different periods of time from 0.2 to 10 h.     

Microwave-assisted synthesis and characterization of Ti1 − xVxO2 (x = 0.0–0.10) nanopowders

Ti_1 ? xV_xO₂ (x = 0.0–0.10) nanopowders were successfully synthesized by a microwave-assisted sol–gel technique and their crystal structure and electronic structure were investigated. The products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and UV–Vis spectroscopy. The results revealed that TiO₂ powders maintained the anatase phase for calcination temperature below 600 °C, but gradually changed to the rutile phase above 800 °C. The formation of the rutile phase was completed at 1000 °C. For Ti_1 ? xV_xO₂ (x = 0.05) powders, the phase transformation appeared at 600 °C. The absorption edge of Ti_1 ? xV_xO₂ (x > 0) powders broadened to the visible region with increasing V concentration and a strong visible light absorption was obtained with 10% V doping. V doping and subsequent coexistence of both anatase and rutile phases in our Ti_1 ? xV_xO₂ nanoparticles are considered to be responsible for the enhanced absorption of visible light up to 800 nm.     

Titanium and iron oxides produced by sol–gel processing of [FeCl{Ti2(OPri)9}]: structural,spectroscopic and morphological features

The first structurally characterised titanium and iron isopropoxide, [FeCl{Ti₂(OPrⁱ)₉}] (1), has been used as a single-source precursor for TiO₂/Fe₂TiO₅ composites prepared by the sol–gel route. Two distinct hydrolysis and condensation conditions were employed, followed by drying and thermal treatment up to 1000 °C. Product composition and oxide phase transitions were characterised by powder X-ray diffractometry and Raman, electron paramagnetic resonance, Mössbauer and Fourier-transformed infrared spectroscopies. A mixture of nanometric-size TiO₂ (anatase, 3.6–5.8 nm) and amorphous iron(III) oxide was obtained up to 500 °C, while TiO₂ (rutile), α-Fe₂O₃ (hematite) and Fe₂TiO₅ (pseudobrookite) were found at 700 °C. At 1000 °C, only rutile and pseudobrookite were observed. These results suggest that 1 behaves as a type III single-source precursor. Powders calcined at 1000 °C were analysed for surface morphology, microstructure and elemental composition by scanning electron microscopy/energy dispersive X-ray spectroscopy. Results suggest no phase segregation on a sub-micrometer level. Different morphologies were observed for the materials produced by the N₂ route, and this could relate to early crystal growth in an oxygen-deficient environment.     

High-energy ball milling of intermetallic Ti-Cu alloys for the preparation of oxide nanoparticles

Copper and titanium oxides in the nano-size range show unique chemical and physical properties and thus have been intensively considered for novel and smart applications. In this work, oxide nanoparticles were prepared by high-energy ball milling of Ti-Cu alloys followed by a controlled oxidation process. Alloys of the Ti-Cu system Ti-50Cu, Ti-57Cu, and Ti-65Cu (in wt.%) prepared by arc melting were selected considering they provide different starting brittle intermetallic phases before milling. Microstructural investigation indicated that Ti-50Cu was composed of Ti₂Cu and TiCu, while Ti-57Cu was single-phase TiCu. Ti-65Cu was dual-phase and consisted of Ti₃Cu₄ and Ti₂Cu_3. A mean particle size below 10 nm was achieved after high-energy ball milling for all compositions. The oxidation process was then investigated in two temperature ranges. At high oxidation temperatures of 700–800 °C, a complete oxidation took place leading to oxides TiO₂-rutile and CuO in all alloys. However, at a low oxidation temperature (350 °C), partial oxidation occurred and different oxides were obtained. Ti-50Cu was the most promising alloy and led to a mix of TiO₂ (rutile and anatase), CuO, Cu₂O, and Ti₃Cu₃O. After long exposure to thermal oxidation, the resulting oxides remained in the nanometric range with a particle size distribution showing a D50 of approximately 6 nm.     

Magnetic properties of the highly iron-doped rutile TiO2 nano crystals

The highly iron-doping to the rutile TiO₂ nano-crystals up to Fe/Ti = 0.2 (Ti_0.833Fe_0.167O_2−δ) was investigated by magnetic and high-pressure annealing study. The magnetic data clearly confirmed the substitution and absence of ferromagnetic order. Instead, presence of anti-ferromagnetic interactions was detected. The high-pressure annealed pellets were revealed highly electrically insulating, reflecting electrical nature of the doped crystals. The result provides a great contrast to what was observed for the thin film form of the doped ferromagnetic TiO₂.     

Structural and optical properties of Er-doped Y2Ti2O7 thin films by sol-gel method

Er³⁺-doped Y₂Ti₂O₇ and Er₂Ti₂O₇ thin films were fabricated by sol-gel spin-coating method. A well-defined pyrochlore phase Er_xY_2-xTi₂O₇ was observed while the annealing temperature exceeded 800 °C. The average transmittance of the Er_xY_2-xTi₂O₇ thin films annealed at 400 to 900 °C reduces from ∼ 87 to ∼ 77%. The refractive indices and optical band gaps of Er_xY_2-xTi₂O₇ (x = 0-2) annealed at 800 °C/1 h vary from 2.20 to 2.09 and 4.11 to 4.07 eV, respectively. The ∼ 1.53 μm photoluminescence spectrum of Er³⁺ (5 mol%)-doped Y₂Ti₂O₇ thin films annealed at 700 °C/1 h exhibits the maximum intensity and full-width at half maximum (∼ 60 nm).     

Synthesis and characterization of pure anatase TiO<Subscript>2</Subscript> nanoparticles

Pure anatase TiO₂ nanoparticles were synthesized by microwave assisted sol–gel method and further characterized by powder X-ray diffraction (XRD), energy dispersive x-ray analysis (EDAX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV–Visible spectrophotometer, SEM images showed that TiO₂ nanoparticles were porous structure. The XRD patterns indicated that TiO₂ after annealed at 300 °C for 3 h was mainly pure anatase phase. The crystallite size was in the range of 20–25 nm, which is consistent with the results obtained from TEM images. Microwave heating offers several potential advantages over conventional heating for inducing or enhancing chemical reactions.     

Effect of substrate type, dopant and thermal treatment on physicochemical properties of TiO2–SnO2 sol–gel films

Thin nanocrystalline TiO₂–SnO₂ films (0–50 mol% SnO₂) were prepared on quartz and stainless steel substrates by sol–gel coating method. The obtained films were investigated by XRD, Raman spectroscopy and XPS. The size of the nanocrystallites was determined by XRD–LB measurements. We ascertained that the increase of treatment temperature and concentration of SnO₂ in the films favour the crystallization of rutile phase. The substrate type influences more substantially the phase composition of the TiO₂–SnO₂ films. It was established that a penetration of elements took place from the substrate into the films. TiO₂ films deposited on quartz substrate include a Si which stabilizes anatase phase up to 600 °C. The films which are deposited on stainless steel substrate and treated at 700 °C show the presence of significant quantity of rutile phase. This phenomenon could be explained by the combined effect of Sn dopant as well as Fe and Cr, which also are penetrated in the films from the steel substrate. The titania films doped up to 10 mol% SnO₂ on stainless steel possess only 12–17 nm anatase crystallites, whereas the TiO₂–(10–50 mol%) SnO₂ films contain very fine grain rutile phase (4 nm).     

Structural investigation of thin TiO2 films prepared by evaporation and post‐heating

Thin films of TiO₂ have been prepared by reactive evaporation of Ti₂O₃ at substrate temperatures from 150 °C to 350 °C and by post‐heating at 150 °C to 850 °C. The mass density of the films increases with increasing substrate and annealing temperature. The crystalline structure of the film prepared at 350 °C is anatase and becomes rutile upon annealing at 850 °C. All other films are amorphous as‐prepared and become anatase upon annealing above 250 °C. The crystallinity is higher for films prepared at lower temperature and does not increase with annealing temperature. Coatings with reproducible optical properties are obtained when deposited and post‐annealed at 250 °C.     

Study on crystallinity and morphology controlling of titania using acrylamide gel method and their photocatalytic properties

In this study, polyacrylamide gel method was used for preparation of pure and mixed phase TiO₂ nanoparticles. The influence of synthesis conditions on the physicochemical properties of products was investigated. It was found that the type of acid, which was used for acidifying the precursor solution together with calcination temperature can affect the phase structure, crystalline size, morphology and thereby photocatalytic activity of obtained TiO₂ nanoparticles. Different trends were observed during the phase transformation, particle growth, shift in energy of band gap with the change in tensile strain to compressive strain of the prepared TiO₂ nanomaterial. X-ray diffraction (XRD) showed that prepared nanocrystals, which were calcined at 450 °C have pure anatase and anatase–rutile mixed structures. The prepared samples having crystallite size between 5 nm and 60 nm were observed at different calcination temperatures. In addition, the photocatalytic activities of the prepared samples were evaluated by monitoring the degradation of Cresol Red (CR). The results show that the photocatalyst (TEC_I), exhibits the highest photocatalytic efficiency where 94.7% of CR can be decomposed after UV exposure for 75 min.     

Deformation-induced reactions of ZnO and TiO2

Reactions of ZnO and TiO₂ (anatase and rutile) in different mole ratios induced by high-energy ball milling were studied by X-ray diffraction. It was found that three main reactions could involve during high-energy ball milling: (1) (4 ? X)ZnO + (2 + Y) TiO₂ (anatase or rutile) → Zn_4?X Ti_2+Y O₈; (2) ZnO + TiO₂ (rutile) → ZnTiO₃, and (3) TiO₂ (anatase) → TiO₂ (II) → TiO₂ (rutile). Cubic Zn_4?X Ti_2+Y O₈ nanocrystals with an average crystal size of about 15 nm can be prepared by high-energy ball milling, which could be an attractive process to fabricate material in industrial scale. No decomposition of ZnTiO₃ into Zn₂TiO₄ and rutile was detected during milling. Anatase shows higher reaction activity than rutile and favours the formation of Zn_4?X Ti_2+Y O₈ while rutile favours the formation of ZnTiO₃. During the anatase-to-rutile transformation a transient metastable phase, TiO₂ (II) which is a high-pressure phase of TiO₂, is detected.     

High-temperature preparation and electrochemical properties of TiO2 anatase phase-mounted carbon aerogels

TiO₂ anatase phase-mounted carbon aerogels (CAs) were synthesized via the sol–gel polymerization of a mixture of resorcinol, formaldehyde, and tetrabutyl orthotitanate, followed by gelation, supercritical drying, and carbonization in N₂ atmosphere. Ethanol was selected as the solvent in the sol–gel polymerization process. The morphology and microstructural characteristics of the TiO₂-mounted carbon aerogel sample were obtained using thermogravimetry, transmission electron microscopy, scanning electron microscopy, N₂ adsorption–desorption, and X-ray diffraction methods. The total pore volume and average pore sizes increased with TiO₂ addition. Titania phase in anatase form was found to be more homogeneously distributed in the CAs at 900 °C. The anatase–rutile transformation did not take place at carbonization temperatures from 500 to 1100 °C and cyclic voltammetry measurements showed that the TiO₂-mounted CAs exhibit good reversibility and high specific capacity as electrode materials, and the anatase in the carbon structure were not involved in the redox reactions. Electrochemical impedance spectroscopy demonstrated that the CA electrode behaves as an excellent capacitor with low resistance.     

Occurrence and stability of ferromagnetism in chemically synthesized cobalt doped TiO<Subscript>2</Subscript>

We report the development of ferromagnetism in ∼30 nm sized well-characterized Ti_1−x Co _x O₂ powders with x = 0.00015–0.006 and its absence for x > 0.006. In addition, these studies show the effect of Co doping on the structural stability and anatase to rutile phase transformation. X-ray diffraction data of samples synthesized by a wet chemical method and annealed at 450 °C indicate a limited solubility of ∼1.2% for Co in the anatase TiO₂ matrix, and with further increase, the CoTiO₃ phase is formed along with increased presence of rutile TiO₂. The bandgap (∼3.23 eV) of the anatase TiO₂ remained almost unchanged for x < 0.006, but decreased rapidly for x ≥ 0.006 approaching 2.8 eV for x = 0.03. The magnetic data from Ti_1−x Co _x O₂ samples with x = 0.006 showed a coercivity H _c ∼ 150 Oe and a weak magnetic moment of 0.2 μ_B/ion at 300 K. The ferromagnetism of Ti_0.994Co_0.006O₂ with open hysteresis loops continue up to a high superparamagnetic blocking temperature T _B ∼ 675 K, above which a superparamagnetic behavior was observed. Systematic changes in the structural, magnetic and optical properties suggest that Co doping is an excellent method to tailor the physical properties of TiO₂ nanoparticles.     

Synthesis of titanium carbide and TiC–SiO2 nanocomposite powder using rutile and Si by mechanically activated sintering

High-energy ball milling was applied with subsequent heat treatment for synthesizing nanoparticles of TiC powders by the carbothermic and carbosilisisothermic reduction of titanium oxide (rutile type). The milling procedure involved milling of TiO₂/C and TiO₂/Si/C powders at room temperature in an argon atmosphere. The progress of the mechanically induced solid state reaction was monitored using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD results showed that TiC nanoparticles were duly synthesized in the TiO₂/C system at 1700 °C in 60-h milled samples. In the non-milled samples, although heated at the same temperature, only a minor amount of a lower degree of titanium oxide (Ti₃O₅) was observed to form. Further, in other non-milled samples, but with Si initially present, despite heating to 1550 °C no TiC phase was detected. However, using Si as a reducing agent accompanied by graphite, after 60 h ball milling, only Si remained as a distinguishable crystalline phase. Further, heat treatment of activated powders by forming the interphase compounds (such as Ti₃Si₅ and Ti₅Si₃) remarkably decreased the synthesis temperature to 900 °C for the 60 h milled samples.     

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

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

Visible photoluminescence from Er-doped TiO2 nanofibres by electrospinning

Er³⁺-doped TiO₂ nanofibres were fabricated with electrospinning method followed by annealing in air at 420, 600, 800 and 1000 °C, respectively. The obtained nanofibres are relatively straight and have an average diameter of ∼ 75 nm. X-ray diffraction measurements showed that the crystal structure transforms from anatase to rutile phase with the increase of annealing temperature. Visible photoluminescence peaking at 528.1, 566.6 and 669.3 nm is detected which is ascribed to ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions and the PL intensities increase with the increase of annealing temperature. Meanwhile at high annealing temperatures, near-infrared photoluminescence peaking at 815 nm due to the defect states associated with Ti³⁺ ions is also found. The strong green photoluminescence of Er³⁺ ions may have potential applications in one-dimensional luminescent nanodevices.     

Characterization of titania thin films grown by dip-coating technique

A dip-coating technique was employed to prepare anatase phase of titania thin films. Fluorine doped tin oxide substrates were used to prepare titania thin films. The samples were annealed at 550 °C for 18 h. X-ray diffraction results revealed the amorphous and anatase phases of TiO₂ for as-synthesized and annealed samples, respectively. The crystallite size of anatase TiO₂ thin films was almost 25 nm for annealed samples. UV–visible confirmed the energy band gap 3.86 and 3.64 eV for as-prepared and calcinated titania thin films. The reduction in the energy band gap could be due to the change in crystallization and agglomeration of small grains after calcination. The morphology of the prepared films was investigated by field emission scanning electron microscopy which demonstrated the agglomeration of spherical particles of TiO₂ with average particle size of about 30 nm. The molecular properties (chemical bonding) of the samples were investigated by means of Fourier Transform Infrared (FTIR) spectroscopy. FTIR analysis exhibited the formation of titania, functional group OH, hydroxyl stretching vibrations of the C–OH groups, bending vibration mode of H–O–H, alkyl C–H stretch, stretching band of Ti–OH, CN asymmetric band stretching, and C=O saturated aldehyde.     

Synthesis and photocatalytic activity of nanocrystalline TiO2-SrO composite powders under visible light irradiation

Nano-sized homogeneously distributed TiO₂-20, -40, -60 wt.% SrO composite powders were successfully synthesized by a sol-gel method. The as-received amorphous TiO₂—20 wt.% SrO composite powders were crystallized with anatase TiO₂ at around 750 °C. As calcination temperatures increased, the anatase TiO₂ crystalline phase was transformed to rutile TiO₂ at about 900 °C, whereas nano-sized, squarish SrTiO₃ phase was detected. The peaks obtained after calcining at 1050 °C mainly exhibited the rutile TiO₂ and SrTiO₃ phases. However, a small number of SrO₂ peaks were also detected. For the comparison of photocatalytic activity depending on light sources, TiO₂-SrO composite powders were tested in phenol degradation. TiO₂-60 wt.% SrO composite powder showed good visible light photoactivity for the photo-oxidation of phenol.