Low temperature molten salt synthesis of SrTiO3 submicron crystallites and nanocrystals in the eutectic NaCl-KCl

An eutectic NaCl-KCl molten salts method has been developed for the synthesis of SrTiO₃ submicron crystallites and nanocrystals from SrO₂ and two kinds (submicron and nano-sized) of TiO₂ powders at 700 °C, which was much lower than that (generally > 1000 °C) of the conventional solid state reactions. The characterization results from X-ray diffraction and X-ray photoelectron spectroscopy revealed that the obtained products were pure perovskite phase SrTiO₃ without any contamination of Na, K, and Cl ions. The scanning electronic microscopy and transmission electron microscopy images disclosed that the starting TiO₂ played an important role in the morphology and size of the obtained SrTiO₃: while the product derived from TiO₂ submicron crystallites comprised faceted submicron crystallites of about 95-184 nm, that derived from TiO₂ nanocrystals comprised quadrate nanocrystals of about 20-61 nm. Besides, based on the experiments without the molten NaCl-KCl eutectic, at different temperatures and times, and using different kinds of TiO₂, the possible formation mechanism of SrTiO₃ submicron crystallites and nanocrystals was proposed.     

Preparation of nanostructured TiO2 ceramics by spark plasma sintering

The effect of spark plasma sintering (SPS) on the densification of TiO₂ ceramics was investigated using a nanocrystalline TiO₂ powder. A fully-dense TiO₂ specimen with an average grain size of ∼200 nm was obtained by SPS at 700 °C for 1 h. In contrast, a theoretical density specimen could only be obtained using conventional sintering above 900 °C for 1 h with an average grain size of 1-2 μm.     

Synthesis and characterization of beta barium borate thin films obtained from the BaO-B2O3-TiO2 ternary system

This work reports on the synthesis and the structural and optical characterization of beta barium borate (β-BBO) thin films containing 4, 8 and 16 mol% of titanium oxide (TiO₂) deposited on fused silica and silicon (0 0 1) substrates using the polymeric precursor method. The thin films were characterized by X-ray diffraction, Raman spectroscopy, atomic force microscopy and scanning electron microscopy techniques. The optical transmission spectra of the thin films were measured over a wavelength range of 800-200 nm. A decrease was observed in the band gap energy as the TiO₂ content was raised to 16 mol%. Only the β-BBO phase with a preferential orientation in the (0 0 l) direction was obtained in the sample containing 4 mol% of TiO₂ and crystallized at 650 °C for 2 h.     

Effects of post-deposition surface treatment on the optical, structural and hydrogen sensing properties of TiO2 thin films

TiO₂ thin films were prepared by DC reactive magnetron sputtering in a mixture of oxygen and argon on glass and oxidized silicon substrates. The effect of post-deposition annealing (300 °C, 500 °C and 700 °C for 8 h in air) on the structural and morphological properties of TiO₂ thin films is presented. In addition, the effect of Pt surface modification (1, 3 and 5 nm) on hydrogen sensing was studied. XRD patterns have shown that in the range of annealing temperatures from 300 °C to 500 °C crystallization starts and the thin film structure changes from amorphous to polycrystalline (anatase phase). In the case of samples on glass substrate, optical transmittance spectra were recorded. TiO₂ thin films were tested as sensors of hydrogen at concentrations 10,000-1000 ppm and operating temperatures within the 180-200 °C range. The samples with 1 nm and in particular with 3 nm of Pt on the surface responded to hydrogen fast and with high sensitivity.     

Low temperature synthesis and optical properties of CaTiO3 nanoparticles from Ca(NO3)2·4H2O and TiO2 nanocrystals

Choosing low-melting-point Ca(NO₃)₂·4H₂O and high-reactive-activity TiO₂ nanocrystals as the raw materials, a simple and cost-effective route was developed for the synthesis of CaTiO₃ nanoparticles at 600 °C, which is much lower than that (about 1350 °C) used in the conventional solid state reaction methods. X-ray diffraction, energy dispersive X-ray spectroscopy and field emission scanning electron microscopy revealed the formation of orthorhombic phase CaTiO₃ nanoparticles with oxygen-deficiency at the surface. UV-vis absorption spectrum of the as-obtained CaTiO₃ nanoparticles displayed an absorption peak centered at around 325 nm (3.8 eV), together with a tail at lower energy side. Room temperature photoluminescence spectrum of the as-obtained CaTiO₃ nanoparticles upon laser excitation at 325 nm demonstrated a strong and broad visible light emission ranging from about 527 to 568 nm, which may be originated from the surface states and defect levels.     

Controlled synthesis of Mn3O4 and MnCO3 in a solvothermal system

Controlled synthesis of Mn₃O₄ nanocrystals and MnCO₃ aggregates was achieved by a facile solvothermal method using different divalent manganese source in the solvent of N,N-dimethylformamide (DMF) with/without the introduction of poly(vinylpyrrolidone) (PVP). PVP was used as a co-reducing reagent in the controlled formation of MnCO₃ crystal. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED), Fourier transform infrared (FTIR) spectra, Raman spectrum and magnetic measurement. Higher process temperature and longer solvothermal time were favorable for the formation of MnCO₃ single phase using MnCl₂ as the manganese source. Mn₃O₄ nanocrystals were prepared at a relatively lower temperature. MnCO₃ aggregates consisted by small nanoparticles have a certain orientation, showing that the nanocrystals formed earlier through oriented aggregation. The size of Mn₃O₄ nanocrystals was 22.5 ± 7.3 nm and 7.3 ± 1.4 nm prepared using MnCl₂ and Mn(CH₃COO)₂, respectively, at 160 °C for 24 h. Raman spectra showed size-dependent characteristics. Smaller Mn₃O₄ nanoparticle resulted in a red-shift in Raman spectra. Magnetic property of the prepared Mn₃O₄ nanoparticle was influenced by the size distribution and crystallinity.     

Structure and CO gas sensing properties of electrospun TiO2 nanofibers

Titanium dioxide (TiO₂) nanofibers were fabricated by electrospinning a hybrid solution, which is a mixture of the TiO₂ sol precursor, polymer, and solvent. The structure and gas sensing properties of TiO₂ nanofibers were investigated. By calcining at 600 °C, the polymeric components were decomposed and a multi-layered random network structure of TiO₂ nanofibers was obtained. Polycrystalline TiO₂ nanofibers consist of tetragonal anatase and rutile TiO₂ phases. The diameter ranged from 400 nm to 500 nm and the grain size was about 15 nm. The TiO₂ nanofibers-based sensor exhibited response to CO concentration as low as 1 ppm at 200 °C.     

Effect of TiO2-SiO2 sol-gel coating on the cpTi-porcelain bond strength

The effects of TiO₂-SiO₂ sol-gel coating with different firing temperatures (300 °C, 500 °C, and 750 °C) on the cpTi-porcelain bond strength were investigated in the present study. Prior to applying the low-fusing dental titanium porcelain, the phase, surface morphology, surface roughness and static water contact angle of the intermediate layer were evaluated. The cpTi-porcelain bond strength was measured using the three-point flexure test according to ISO 9693 standard. Statistical analyses were made using one-way ANOVA and Dunnett-t test. Significantly higher bond strength of TiO₂-SiO₂/750 °C (specimens coated with TiO₂-SiO₂ sol-gel coating and fired at 750 °C for 1 h) when compared to the control group was observed (p < 0.05). No rutile phase was found in all the tested specimens coated with TiO₂-SiO₂ sol-gel coating. The surface morphology of the intermediate layer was apparently different with different firing temperatures. It was found that the static water contact angle of TiO₂-SiO₂/750 °C significantly decreased (p < 0.05). However, no markedly different R_a of TiO₂-SiO₂/500 °C and TiO₂-SiO₂/750 °C in comparison to that of the control group was observed (p > 0.05). The results show that the TiO₂-SiO₂ sol-gel coating fired at 750 °C for 1 h can notably improve the cpTi-porcelain bond strength and may be suitable for clinical use.     

Growth and characterization of epitaxial anatase TiO2(001) on SrTiO3-buffered Si(001) using atomic layer deposition

Epitaxial anatase titanium dioxide (TiO₂) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer grown by molecular beam epitaxy (MBE) to serve as a surface template. The growth of TiO₂ was achieved using titanium isopropoxide and water as the co-reactants at a substrate temperature of 225-250 °C. To preserve the quality of the MBE-grown STO, the samples were transferred in-situ from the MBE chamber to the ALD chamber. After ALD growth, the samples were annealed in-situ at 600 °C in vacuum (10^− 7 Pa) for 1-2 h. Reflection high-energy electron diffraction was performed during the MBE growth of STO on Si(001), as well as after deposition of TiO₂ by ALD. The ALD films were shown to be highly ordered with the substrate. At least four unit cells of STO must be present to create a stable template on the Si(001) substrate for epitaxial anatase TiO₂ growth. X-ray diffraction revealed that the TiO₂ films were anatase with only the (004) reflection present at 2θ = 38.2°, indicating that the c-axis is slightly reduced from that of anatase powder (2θ = 37.9°). Anatase TiO₂ films up to 100 nm thick have been grown that remain highly ordered in the (001) direction on STO-buffered Si(001) substrates.     

Properties of TiO2 prepared by acid treatment of BaTiO3

TiO₃ powders were prepared by acid treatment of BaTiO₃ and their properties were investigated. The BaTiO₃ powder was subjected to HNO₃ in concentrations ranging from 10⁻³ to 8 M at 90 °C for 0.5-6 h. Dissolution of BaTiO₃ and precipitation of TiO₂ occurred at acid concentrations of 2-5 M. BaTiO₃ dissolves completely to form a clear solution at reaction times of 0.5-1 h, but a rutile precipitate is formed after 2 h of acid treatment. By contrast, anatase is precipitated by adjusting the pH of the clear solution to 2-3 using NaOH or NH₄OH solution. The rutile crystals were small and rod-shaped, consisting of many small coherent domains connected by grain boundaries with small inclination angles and edge dislocations, giving them a high specific surface area (S_BET). With increasing HNO₃ concentration, the S_BET value increased from 100 to 170 m²/g while the crystallite size decreased from 25 to 11 nm. The anatase crystals obtained here were very small equi-axial particles with a smaller crystallite size than the rutile and S_BET values of about 270 m²/g (higher than the rutile samples). The photocatalytic activity of these TiO₂ was determined from the decomposition rate of Methylene Blue under ultraviolet irradiation. Higher decomposition rates were obtained with larger crystallite sizes resulting from heat treatment. The maximum decomposition rates were obtained in samples heated at 500-600 °C. The photocatalytic activity of the TiO₂ was found to depend more strongly on the sample crystallite size than on S_BET.     

Formation of SrTiO3 from Sr-oxalate and TiO2

SrTiO₃ powder has been prepared from Sr-oxalate and TiO₂ precursors, instead of using titanyl-oxalate. Sr-oxalate was precipitated from nitrate solution onto the surface of suspended TiO₂ powders. Crystallization of SrTiO₃ from the precursor was investigated by TGA, DTA and XRD analysis. It is evident that precursor, upon heating, dehydrates in two stages, may be due to the presence of two different types of Sr-oxalate hydrates. Dehydrated precursor then decomposes into SrCO₃ and TiO₂ mixture. Decomposition of SrCO₃ and simultaneous SrTiO₃ formation occur at much lower temperature, from 800 °C onwards, due to the fine particle size of the SrCO₃ and presence of acidic TiO₂ in the mixture. The precursor completely transforms into SrTiO₃ at 1100 °C. About 90 nm size SrTiO₃ crystallites are produced at 1100 °C/1 h, due to the lower calcination temperature and better homogeneity of the precursor.     

Structural, thermal and optical characterization of TiO2:ZrO2 thin films prepared by sol-gel method

We have studied the structural and optical properties of thin films of TiO₂, doped with 5% ZrO₂ and deposited on glass substrate (by the sol-gel method). The dip-coated thin films have been examined at different annealing temperatures (350 to 450 °C) and for various layer thicknesses (63-286 nm). Refractive index and porosity were calculated from the measured transmittance spectrum. The values of the index of refraction are in the range of 1.62-2.29 and the porosity is in the range of 0.21-0.70. The coefficient of transmission varies from 50 to 90%. In the case of the powder of TiO₂, doped with 5% ZrO₂, and aged for 3 months in ambient temperature, we have noticed the formation of the anatase phase (tetragonal structure with 14.8 nm grains). However, the undoped TiO₂ exhibits an amorphous phase. After heat treatments of thin films, titanium oxide starts to crystallize at the annealing temperature 350 °C. The obtained structures are anatase and brookite. The calculated grain size, depending on the annealing temperature and the layer thickness, is in the range (8.58-20.56 nm).     

Synthesis and electrophoretic deposition of hydrothermally synthesized multilayer TiO2 nanotubes on conductive filters

TiO₂ nanotubes were synthesized by the decomposition of titanium isopropoxide in water and the calcination at 450 °C for 2 h to form TiO₂ nanoparticles. The synthesized TiO₂ in anatase form nanoparticles were processed hydrothermally in 10 M NaOH solution at 130 °C for 24 h to obtain multilayer TiO₂ nanotubes. TEM analysis revealed that the diameters of the tubes were around 10 nm and they are in the length of 100 nm. Subsequently, colloidal suspensions containing 1% wt. Of TiO2 nanotubes were prepared with TEA and butanol and electrophoretic deposition (EPD) experiments were conducted in order to obtain coatings on Ni and carbon filters using a deposition time of 10 min. and an applied voltage of 65 V. It is also shown that multilayer TiO₂ nanotubes having outer diameter around 10 nm and inner diameters of 4.3 nm can be produced using the described technique. EPD is also shown to be an effective technique to coat three dimensional components, such as Ni and C filters for various applications including water and air purification systems.     

Investigation into real-time pressure sensing properties of SnO2, TiO2, and TiO2/ZnO thick films with interdigitated electrodes

The pressure sensing properties of nanocomposite SnO₂, TiO₂, and TiO₂/ZnO thick film capacitors with interdigitated electrodes are investigated. To form the dielectric layers, the metal oxides powders were respectively mixed with isopropanol, wet ball milled for 24 h, then the mixtures were dried at 120 °C and further the powders were placed under 2 tonnes of pressure to form pellets, which were fired at 1250 °C (rate of 5 °C/min) in a vacuum of 6 × 10⁻³ mbar for 5 h, followed by cooling (rate of 3 °C/min). After firing, the resultant nanopowders were mixed with 7 wt.% of polyvinyl butyral (binder) and suitable amount of ethylenglycolmonobutylether (solvent) to form the pastes. These were screen-printed over the Ag electrodes on alumina substrates to form SnO₂, TiO₂, and TiO₂/ZnO capacitor pressure sensors accordingly. The evaluation of pressure sensing properties of these sensors was performed using a HP 4192A Impedance Analyser, which recorded the changes in the values of the capacitances under different mechanical stresses. At the applied load of 5 kPa, the response times of 2.5 s, 5.6 s and 4 s were recorded for SnO₂, TiO₂, and TiO₂/ZnO sensors, respectively. In addition to instant response times, these pressure sensors have the advantage of being reusable, as their electrical properties were restored to the original value after annealing for 2 h at 80 °C. Moreover, one year later after the initial testing, the sensors were still operational and produced similar time responses to pressure.     

Raman spectroscopy and cathodoluminescence characteristics of order-disorder Ba(Zn1/3Ta2/3)O3 ceramics

The order-disorder transition in Ba(Zn_1/3Ta_2/3)O₃ (BZT) was characterized by using Raman spectroscopy, transmission electron microscopy (TEM), and cathodoluminescence (CL) microscopy. The 1:2 ordered structure of pure BZT ceramics was replaced by a 1:1 ordered structure at 1650 °C and the 1:1 ordered structure of BZT sintered at 1650 °C exhibited a 1:2 ordered structure when it was reannealed at 1500 °C for 12 h. The A_1g lines in the Raman spectrum of the sintered and reannealed samples were shifted to lower and higher wavenumbers, respectively. From the CL analysis, the 1:1 ordered BZT exhibited mainly three emission bands at around 533.2 (2.32 eV), 599.1 (2.07 eV), and 682.1 nm (1.81 eV), whereas the 1:2 ordered BZT exhibited mainly five bands at 346.4 (3.58 eV), 427.5 (2.90 eV), 520.9 (2.38 eV), 593.0 (2.09 eV), and 678.9 nm (1.82 eV). The strongest band originating from 2.32 to 2.38 eV was broadened, and the band center shifted towards a higher and lower wavelength in the 1:1 and 1:2 ordered BZT, respectively. Additional bands at around 346 and 427 nm in the grain interior of the annealed sample were strongly related to the 1:2 ordering of BZT.     

Phase transformation in semi-transparent TiO2 films irradiated with CO2 laser

Anatase (TiO₂) thin films were obtained by immersion of glass plates into a titanium sol-gel precursor followed by calcination at 450 °C for 3 h. The Raman results for the CO₂ laser irradiated TiO₂ films show that laser radiation is able to promote favorable changes of anatase phase in anatase/rutile mixtures. Nevertheless, the transformation process level depends on laser characteristics and scan speed of the radiation treatment.     

Morphology of CaF2 nanocrystals and elastic properties in transparent oxyfluoride crystallized glasses

An oxyfluoride glass with the composition of 25CaF₂-5CaO-20Al₂O₃-50SiO₂ (mol%) and crystallized glasses containing CaF₂ nanocrystals (10-70 nm) are fabricated. The size and morphology of CaF₂ nanocrystals is examined using transmission electron microscopy and atomic force microscopy (AFM), and elastic properties of crystallized glasses are evaluated using a cube resonance method. The large increase in the glass transition temperature in crystallized glasses suggests that the Al₂O₃-SiO₂ based glass network having a high thermal stability is created due to the formation of CaF₂ nanocrystals. It is suggested from AFM observations that the chemical bonding between CaF₂ nanocrystals and oxide glass matrix is weak. Young’s modulus (E) increases with increasing heat treatment temperature, i.e., E = 88.4 GPa for the glass and E = 93.3 GPa for the sample heat-treated at 700 °C for 1 h. The present study demonstrates that oxyfluoride crystallized glasses containing CaF₂ nanocrystals have good elastic (mechanical) properties, being available in practical device applications even from the mechanical point of view.     

Fabrication of TiO2 nanotubes by anodization of Ti thin films for VOC sensing

Ti thin films were anodized in aqueous HF (0.5 wt.%) and in polar organic (0.5 wt.% NH₄F + ethylene glycol) electrolytes to form TiO₂ nanotube arrays. Ti thin films were deposited on microscope glass substrates and then anodized. Anodization was performed at potentials ranging from 5 V to 20 V for the aqueous HF and from 20 V to 60 V for the polar organic electrolytes over the temperatures range from 0 to 20 °C. The TiO₂ nanotubes were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). It has been observed that anodization of the deposited Ti thin films with aqueous HF solution at 0 °C resulted in nanotube-type structures with diameters in the range of 30-80 nm for an applied voltage of 10 V. In addition, the nanotube-type structure is observed for polar organic electrolyte at room temperature at the anodization voltage higher than 40 V. The volatile organic compound (VOC) sensing properties of TiO₂ nanotubes fabricated using different electrolytes were investigated at 200 °C. The maximum sensor response is obtained for carbon tetrachloride. The sensor response is dependent on porosity of TiO₂. The highest sensor response is observed for TiO₂ nanotubes which are synthesized using aqueous HF electrolyte and have very high porosity.     

Sonochemical preparation of TiO2 nanoparticles in the ionic liquid 1-(3-hydroxypropyl)-3-methylimidazolium-bis(trifluoromethylsulfonyl)amide

Spherical shaped anatase nanoparticles (ø 5 nm) have been synthesized in the ionic liquid 1-(3-hydroxypropyl)-3-methylimidazolium-bis(trifluoromethanesulfonyl)amide from titanium tetraisopropoxide by ultrasound assisted synthesis under ambient conditions. XRD, EDX, TEM, XPS, Raman, UV–vis, PL and BET measurements have been employed for characterization of the nanostructure of as-prepared TiO₂. XRD and Raman measurements both show that the obtained material is crystalline with anatase structure. The morphology of TiO₂ nanoparticles was characterized by transmission electron microscopy (TEM). The bandgap of the TiO₂ nanocrystals estimated from XRD and UV–vis measurements is about 3.3 eV. The surface area of a typical sample is 177 m² g⁻¹. The synthesized anatase nanocrystals show good photocatalytic activity in the degradation of methylorange.     

NH4Cl-assisted low temperature synthesis of anatase TiO2 nanostructures from Ti powder

A simple, low temperature and low cost method, which was based on heating the mixture of Ti and NH₄Cl powders in air at 300 °C, has been developed for the controlled synthesis of anatase TiO₂ nanostructures including irregular nanoparticle aggregates, curved nanowires built up by the oriented attachment of nanoparticles, and nanoplates constructed with nanoparticles. The characterization results from X-ray diffraction and Raman spectra indicated that the as-obtained products were anatase TiO₂. Field emission scanning electron microscope images revealed that the products obtained for 3, 10 and 16 h comprised, in turn, irregular nanoparticle aggregates (8-55 nm), curved nanowires built up by the oriented attachment of nanoparticles (~ 9 nm), and nanoplates constructed with nanoparticles (~ 8 nm).