Synthesis, characterization and photodegradation study of mixed-phase titania hollow submicrospheres with rough surface

Titania hollow submicrospheres with mixed phase (anatase-brookite or anatase-rutile) were synthesized via the combination of hydrothermal treatment and calcination of submicrospheres consisting of a polystyrene core and an amorphous TiO₂ shell. After hydrothermal treatment, amorphous titania shell could be transformed to anatase-brookite shell consisting of loose packed titania nanocrystals, which could be further converted to anatase-brookite (below 700 °C) or anatase-rutile titania (700-800 °C) hollow spheres with rough surface via calcination. The loose packing of titania nanocrystals not only inhibited the transformation temperature from anatase to rutile, but also provided titania hollow submicrospheres with high photodegradation activity of Rhodamine B. The photocatalytic activity of titania hollow spheres increased firstly then decreased when the calcination temperature was varied in the range of 450-800 °C, while hollow spheres obtained via calcinating at 700 °C exhibited the highest photocatalytic activity, which was five times higher than that of counterpart without hydrothermal treatment.     

Effect of calcination temperature on porous titania prepared from industrial titanyl sulfate solution

Porous titania with anatase phase was prepared via thermal hydrolysis of industrial titanyl sulfate solution, and subsequent calcination at different temperature. The as-prepared powders were characterized by TG, DSC, XRD, particle size distribution test, N₂ adsorption-desorption isotherm and SEM. Raising calcination temperature could accelerate the crystal growth, enlarge its specific surface area, pore diameter, and improve its photocatalytic activity. While too high temperature could destroy the pore structure even collapse. The optimal calcination temperature is 550 °C, and the obtained sample showed good thermal stability and high photocatalytic activity, with specific surface area of 189.3 m²/g and higher degree of crystallinity, and its photocatalytic degradation rate of methylene blue was of 95.76%.     

Synthesis and characterization of nanostructured Ag on porous titania

In this work, porous titania was prepared on bulk Ti by chemical oxidation, and then nanostructured silver (Ag) was deposited on titania surface by ion beam sputtering. After annealing treatment, Ag/TiO₂ composites were characterized using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results indicated that a nano-porous titania layer with mean pore size of 150 nm and thickness of 1 μm was formed by chemical oxidation at 80 °C for 45 min. There were three Ag species (Ag (0), Ag (1+), and Ag (2+)) on composites surface after annealing treatment, and metallic Ag content achieved maximum value with annealing temperature of 500 °C in air. Ag showed high thermal stability being partly attributed to the inhibiting the diffusion of Ag by the underlying porous titania.     

Effect of calcination temperature on the morphology and surface properties of TiO2 nanotube arrays

Well-ordered TiO₂ nanotube arrays were prepared by electrochemical anodization of titanium in aqueous electrolyte solution of H₃PO₄ + NH₄F at a constant voltage of 20 V for 3 h, followed by calcined at various temperatures. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and Photoluminescence (PL) were used to characterize the samples. The results showed that the as-prepared nanotube arrays before being calcined were amorphous and could transform to anatase phase at a heat treatment temperature higher than 400 °C. As the calcination temperatures increased, crystallization of anatase phase enhanced and rutile phase appeared at 600 °C. However, further increasing the calcination temperature would cause the collapse of nanotube arrays. PL intensity of the nanotube arrays annealed at 500 °C was the lowest, which was probably ascribed to better crystallization together with fewer surface defects of the nanotube arrays.     

Effect of annealing on TiO2 nanoparticles

TiO₂ nanoparticles have been prepared by simple chemical precipitation method and annealed at different temperatures. The as-prepared TiO₂ are amorphous, and they transform into anatase phase on annealing at 450 °C, and rutile phase on annealing at 900 °C. The X-ray diffraction results showed that TiO₂ nanoparticles with grain size in the range of 21–24 nm for anatase phase and 69–74 nm for rutile phase have been obtained. FESEM images show the formation of TiO₂ nanoparticles with small size in structure. The FTIR and Raman spectra exhibited peaks corresponding to the anatase and rutile structure phases of TiO₂. Optical absorption studies reveal that the absorption edge shifts towards longer wavelength (red shift) with increase of annealing temperature.     

Preparation of titania hollow spheres by catalyst-free hydrothermal method and their high thermal stabilities

TiO₂ hollow spheres have been prepared by hydrothermal method using carbon spheres as hard templates based on template-directed deposition and calcination in order to remove templates. The morphology and structure of samples were systematically characterized by using various techniques, including XRD, zeta analyzer, SEM, TEM, DRS and FTIR. In this approach, the anatase phase was retained for temperatures up to 900 °C. Moreover, negative charged titania is deposited onto the negative charged surface of carbon spheres, which is proved by nanoparticle size analyzer. Therefore, a possible formation mechanism of TiO₂ hollow spheres was proposed. TiO₂ hollow spheres calcined at 550 °C exhibited the superior photocatalytic activity for the degradation of Rhodamine B, 2.9 times greater than that of Degussa P25. Furthermore, thermal stability of TiO₂ hollow spheres was examined. Fortunately, we found that hollow structures could still be visible distinctly after calcining at 900 °C.     

Synthesis of titania/carbon nanocomposites by polymeric precursor method

Here we describe a single chemical route to obtain highly dispersed nanometric Ni particles embedded in titania/carbon matrixes (amorphous and crystalline). The synthesis of these nanocomposites is based on a polymeric precursor method. The metallic Ni nanoparticles (1-15 nm) were obtained in a single process. We also present the results of photocatalytic experiments involving a series of nanocrystalline composites based on TiO₂/carbon with embedded Ni nanoparticles as nanocatalysts for rhodamine 6G degradation in aqueous solution and investigate the effects of the structure and properties of the nanocomposites on their photocatalytic applications. The effect of the different annealing treatments on the formation of TiO₂ nanophases (anatase and/or rutile), the size of Ni particles and the role of the residual carbon phase on the final solid are also described.     

Synthesis of coprecipitated strontium hexaferrite nanoparticles in the presence of polyvinyl alcohol

Strontium hexaferrite (SrFe₁₂O₁₉) nanoparticles were synthesized by the chemical coprecipitation method and using polyvinyl alcohol (PVA) as a protective agent. The synthesized samples were characterized by differential thermal analysis, X-ray diffraction, scanning and transmission electron microscopy, particle size analyzer, sedimentation test and vibrating sample magnetometer. In the presence of PVA, the single-phase SrFe₁₂O₁₉ nanoparticles were obtained at low temperature of 650 °C. The average particle size of SrFe₁₂O₁₉ precursor was 15 nm, which increased to 61 nm after calcination at 650 °C. The magnetic measurements indicated that PVA decreased coercivity from 4711 to 3216 Oe with particle size reduction. The results showed that PVA as a protective agent could be effective in decreasing the particle size, calcination temperature and coercivity of SrFe₁₂O₁₉ nanoparticles.     

Thermostable photocatalytically active TiO<Subscript>2</Subscript> anatase nanoparticles

Anatase is the low-temperature (300–550 °C) crystalline polymorph of TiO₂ and it transforms to rutile upon heating. For applications utilizing the photocatalytic properties of nanoscale anatase at elevated temperatures (over 600 °C) the issue of phase stabilisation is of major interest. In this study, binary TiO₂/SiO₂ particles were synthesized by a flame aerosol process with TiCl₄ and SiCl₄ as precursors. The theoretical Si/Ti ratio was varied in the range of 0.7–1.3 mol/mol. The synthesized TiO₂/SiO₂ samples were heat treated at 900 and 1,000 °C for 3 h to determine the thermostability of anatase. Pyrogenic TiO₂ P25 (from Evonik/Degussa, Germany) widely applied as photocatalyst was used as non-thermostabilized reference material for comparison of photocatalytic activity of powders. Both the non-calcinated and calcinated powders were characterized by means of XRD, TEM and BET. Photocatalytic activity was examined with dichloroacetic acid (DCA) chosen as a model compound. It was found that SiO₂ stabilized the material retarding the collapse of catalyst surface area during calcination. The weighted anatase content of 85% remains completely unchanged even after calcination at 1,000 °C. The presence of SiO₂ layer/bridge as spacer between TiO₂ particles freezes the grain growth: the average crystallite size increased negligibly from 17 to 18 nm even during the calcination at 1,000 °C. Due to the stabilizing effect of SiO₂ the titania nanoparticles calcinated at 900 and 1,000 °C show significant photocatalytic activity. Furthermore, the increase in photocatalytic activity with calcination temperature indicates that the titania surface becomes more accessible either due to intensified cracking of the SiO₂ layer or due to enhanced transport of SiO₂ into the necks thus releasing additional titania surface.     

Study of surface-bulk mass transport and phase transformation in nano-TiO2 using hyperfine interaction technique

Phase transition from anatase to rutile for the 70 nm TiO₂ crystallite has been investigated by the time differential perturbed angular correlation (TDPAC) technique. The study involved the annealing of the TiO₂ nanocrystals, adsorbed with the nuclear probe (¹⁸¹Hf/¹⁸¹Ta) at trace level, at different temperatures for different durations. The TDPAC measurement was also supported by XRD measurement where the width of the peaks increases with the increase in annealing temperature indicating a crystal growth. The samples annealed up to 823 K for 4 h showed no phase transition, except for the growth of the crystallites. However, it showed phase transition at the same temperature (823 K), when annealed for longer duration, indicating the slower kinetics of the phase transition process. Further the sample, when annealed at 1123 K for 4 h, showed phase transition. It has also been observed that the ¹⁸¹Hf tracer, adsorbed on 70 nm anatase TiO₂, diffuses from surface to bulk during the phase transition process and the extent of diffusion in anatase differs from that in rutile phase. However, surface to bulk mass-transfer is found to play a significant role in the phase transition process.     

The effects of activated carbon supports on the structure and properties of TiO2 nanoparticles prepared by a sol-gel method

TiO₂-coated activated carbon (TiO₂/AC) composites and pure TiO₂ powders were prepared by a sol-gel method using tetrabutylorthotitanate as a precursor. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA), X-ray photoelectron spectrum (XPS) and nitrogen absorption. The photoactivity of samples was evaluated by methylene blue (MB) degradation. The analysis results show that compared with pure TiO₂ powders, the spherical-shaped TiO₂ particles are well-dispersed in the AC matrix and the size of the resulting TiO₂ crystallites decreases to below 40 nm with increasing phase transformation temperature. The AC matrix creates anti-calcination effects and shows interfacial energy effects that control the growth of the TiO₂ particles, baffle the anatase to rutile phase transition, and cumber the TiO₂ particles to agglomerate. Compared with the surface areas of TiO₂ powders, the combination of TiO₂ and AC forms composites with high surface areas which are slightly affected by calcination temperature. By AC support, the photoactivity of TiO₂ is increased in MB photocatalytic course, possible because active carbon increases photocatalytic activity of TiO₂ particles by producing high concentration of organic compound near TiO₂, and small-size TiO₂ particles are well-dispersed on the surface of AC.     

Synthesis and characterization of nanocrystalline titanium nitride powder from rutile and anatase as precursors

In this research nanosized titanium nitride powder was synthesized through reaction of titanium oxide with ammonia gas. The reaction was carried out at a very slow heating rate. Two different TiO₂ starting powders contained rutile and anatase phase and differed in initial particle size and surface area. The crystallite size of TiN powders synthesized at 1000 °C was obtained about 40 nm for anatase sample. Surface area and particle size were found to be 19 m²/g, 70 nm for rutile sample and 31 m²/g, 39 nm for anatase sample, respectively. The rutile sample showed an increasing trend in surface area during conversion to the nitride, whilst the anatase sample followed an adverse trend. TiN powder synthesized from anatase had the highest surface area and smallest particle size due to the specification of initial precursor.     

Effect of annealing temperature on nanocrystalline TiO2 thin films prepared by sol–gel dip coating method

Nanocrystalline TiO₂ thin films have been prepared by sol–gel dip coating method. X-ray diffraction results showed the formation of anatase phase TiO₂ with grain size in the range of 18–26 nm. The HRTEM studies show that the average particle size of TiO₂ is 24 nm. The EDX analysis confirmed that Ti and O elements are present in the samples. The optical absorption spectra reveal that the absorption edge shifts towards longer wavelength side with increase in annealing temperature.     

The removal of sodium dodecylbenzene sulfonate surfactant from water using silica/titania nanorods/nanotubes composite membrane with photocatalytic capability

This paper reports experimental results on removal of sodium dodecylbenzene sulfonate (SDBS), using silica/titania nanorods/nanotubes composite membrane with photocatalytic capability. This multifunctional composite membrane has been successfully prepared from colloidal X-silica/titania sols (X denotes molar percent of silica) by the sol-gel technique. The prepared nanorods/nanotubes composite membranes were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), scanning probe microscope (SPM) and UV-vis diffuse reflectance spectra (DRS). XRD patterns confirmed that the embedding of amorphous silica into nanophase titania matrix helped to increase the thermal stability of titania and control the size of titania particles. The small size titania particles with anatase phase played an important role in formation of silica/titania nanorods/nanotubes composite membranes with photocatalytic capability. The percentage of anatase phase titania reached 93% when 20%-silica/titania nanorods/nanotubes composite membrane calcined at 400 °C for 2 h. Most (95%) of the pore volume was located in mesopores of diameters ranging from 1.4 to 10 nm. The experimental results showed that the removal of SDBS achieved 89% after 100 min by combining the photocatalysis with membrane filtration techniques. Although the SDBS was not completely decomposed by photocatalysis, the degradation of the SDBS helped to enhance composite membrane flux and prevent membrane fouling. It was possible to treat successfully surfactant wastewater using multifunctional silica/titania nanorods/nanotubes composite membrane by means of a continuous process; this could be interesting for industrial applications.     

Maintaining particle size in the transformation of anatase to rutile titania nanostructures

We study the temperature-dependent transformation of two distinctly synthesized TiO₂ nanoparticles from the anatase to the rutile phase. These studies are carried out over the temperature range extending from room temperature to an excess of 800 °C where the anatase to rutile conversion is found to occur. Results obtained for both a sol-gel-generated nanocolloid (3-20 nm) and a sol-gel-generated micelle nanostructure (∼40 nm) are evaluated. While the TiO₂ nanocolloid structures aggregate to form larger crystallites as a function of increasing temperature with sizes comparable to the sol-gel-generated micelle structures, the resulting anatase crystallites, which are of a diameter 40-50 nm, appear to transform to comparable or slightly smaller rutile structures at 800 °C. This is in contrast to the transformation to larger rutile structures, observed for larger anatase particles. The importance of kinetic effects is considered as it enhances the rate of anatase to rutile conversion. These characteristics are established using a combination of Raman spectroscopic, X-ray diffraction, and scanning electron microscopy. The relative playoffs of the Raman and X-ray diffraction techniques are considered as they are used for the analysis of particles at the nanoscale, especially when phase transformations are evaluated.     

Structure and magnetic properties of nanocrystalline cobalt ferrite powders synthesized using organic acid precursor method

Nanocrystalline octahedra of cobalt ferrite CoFe₂O₄ powders were synthesized using the organic acid precursor route. The effect of the calcination temperature, Fe³⁺/Co²⁺ molar ratio, calcination time and type of organic acid (oxalic, benzoic and tartaric acids) on the formation, crystallite size, microstructure and magnetic properties was studied systematically. The Fe³⁺/Co²⁺ molar ratio was varied from 2 to 1.739 while the annealing temperature was controlled from 400 to 1000 °C for various periods from 0.5 to 2 h. The resulting powders were investigated using X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). XRD results indicate that a well crystallized, single spinel cobalt ferrite phase was formed for the precursors annealed at 600-800 °C for 2 h, using oxalic and tartaric acids as precursors for Fe³⁺/Co²⁺ molar ratio 1.818. The crystallite size of as-formed powders was in the range of 38.0-92.6 nm at different operating conditions. The calcination temperature and Fe³⁺/Co²⁺ molar ratio have a significant effect on the microstructure of the produced cobalt ferrite. The microstructure of the produced powders was found to be octahedra-shaped. The crystalline, pure cobalt ferrite powders with magnetic properties having a maximum saturation magnetization (76.1 emu/g) was achieved for the single phase at Fe³⁺/Co²⁺ molar ratio 1.818 and annealing temperature of 600 °C for 2 h using tartaric acid precursor.     

Characterization and corrosion studies of titania-coated NiTi prepared by sol-gel technique and steam crystallization

Nickel titanium (NiTi) was dip-coated with titania via the sol-gel route using titanium butoxide (Ti(OC₄H₉)₄) as precursor. The as-coated titania film was crystallized to form anatase by treatment in steam at 105 °C. The crystallized film was relatively thick (about 750 nm) and even. Atomic force microscopy (AFM) revealed that the film was dense with a surface roughness of about 3 nm, and was composed of particles of about 100 nm. X-ray diffractometry (XRD) showed that these particles were composed of nanocrystallites of a few nanometers. Nanoindentation tests of the titania film indicated that the film was tough, possibly due to the nano-size of the crystallites. The mean hardness H and elastic modulus E of the coating were about 1.5 and 70 GPa, respectively. Direct pull-off test recorded a mean coating-substrate bonding strength larger than 17 MPa. Electrochemical impedance spectroscopic (EIS) study and cyclic polarization tests showed that the corrosion resistance of the coated NiTi samples in Hanks’ solution was increased by about two orders of magnitude compared with the substrate. Taken together, the present study showed that steam crystallization is a feasible low-temperature treatment method for sol-gel derived titania coating on NiTi in biomedical applications.     

Synthesis and bioactivity of highly ordered TiO2 nanotube arrays

The highly ordered TiO₂ nanotube arrays were fabricated by potentiostatic anodization of Ti foils in fluorinated dimethyl sulfoxide (DMSO). TiO₂ nanotube arrays are formed using a 40 V anodization potential for 24 h, with a length of 12 μm, diameter of 170 nm and aspect ration of about 70. The as-prepared nanotubes are amorphous, but can be crystallized as the heat treatment temperature increases. Anatase phase appears at a temperature of about 300 °C, then transforms to rutile phase at about 600 °C. After heat treatment at 500 °C and soaking in SBF for 14d, a thick apatite layer of about 13 μm covers the whole surface of TiO₂ nanotube arrays, indicating their excellent in vitro bioactivity, which is mainly attributed to their high specific surface area and the anatase phase.     

Preparation and characterization of nitrogen-doped TiO2 nanoparticles by the laser pyrolysis of N2O-containing gas mixtures

Nitrogen-doped TiO₂ nanoparticles have been prepared by the IR laser pyrolysis technique. A sensitized mixture of TiCl₄ (vapors) and N₂O was used as titanium and nitrogen precursors, respectively. The structural properties of the resultant N-doped nanoparticles such as the phase formation and the average particle size and distributions were investigated by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. The phase composition varied from almost pure anatase to mixtures of rutile and anatase. A decrease of the mean particle diameters from about 18 nm in case of the almost pure anatase sample to about 13 nm in case of the anatase-rutile mixture is observed. XPS analysis suggests and interstitial character of the doping process.     

Preparation and characterization of iodine-doped mesoporous TiO2 by hydrothermal method

Iodine-doped mesoporous TiO₂ (I/TiO₂) was prepared by hydrothermal method, using tetrabutyl titanate as precursor, potassium iodate as iodine sources. The as-prepared I/TiO₂ catalysts were characterized by UV-vis, XRD, TEM, BET, TG/DTA, XPS and photoluminescence (PL) spectroscopy. Production of OH radicals on the I/TiO₂ surface was detected by the PL technique using terephthalic acid as a probe molecule. The effects of hydrothermal reaction temperature, calcination temperature and iodine doping content on the structure and properties of the catalysts were investigated. The results showed that iodine-doped TiO₂ calcinated at 300 °C have good anatase crystal. The optimal hydrothermal conditions have been determined to be that reaction temperature 120 °C, calcinated temperature 300 °C and added 1.16 mmol iodine dopants. The average particle size of I/TiO₂ synthesized under optimal condition (I-3 sample) is about 3.9 nm. The I-3 photocatalyst contains 100% anatase crystalline phase of TiO₂. BET specific surface area of I-3 sample is184.8 m² g⁻¹ and is 3.67 times that of pure TiO₂ (50.37 m² g⁻¹). Iodine in I/TiO₂ catalyst mainly exists in the form of I₂, and photoactivity of I/TiO₂ catalyst in visible light comes from photosensitize of I₂. I/TiO₂ catalysis shows very high efficiency for the degradation of phenol under visible light.