Immobilized TiO<Subscript>2</Subscript> nanoparticles produced by flame spray for photocatalytic water remediation

Anatase/rutile mixed-phase titanium dioxide (TiO₂) photocatalysts in the form of nanostructured powders with different primary particle size, specific surface area, and rutile content were produced from the gas-phase by flame spray pyrolysis (FSP) starting from an organic solution containing titanium (IV) isopropoxide as Ti precursor. Flame spray-produced TiO₂ powders were characterized by means of X-ray diffraction, Raman spectroscopy, and BET measurements. As-prepared powders were mainly composed of anatase crystallites with size ranging from 7 to 15 nm according to the synthesis conditions. TiO₂ powders were embedded in a multilayered fluoropolymeric matrix to immobilize the nanoparticles into freestanding photocatalytic membranes. The photocatalytic activity of the TiO₂-embedded membranes toward the abatement of hydrosoluble organic pollutants was evaluated employing the photodegradation of rhodamine B in aqueous solution as test reaction. The photoabatement rate of best performing membranes significantly overcomes that of membranes produced by the same method and incorporating commercial P25-TiO₂.     

Effect of Nb doping on morphology,crystal structure,optical band gap energy of TiO2 thin films

Nb–TiO₂ nanofibers and thin films were prepared using a sol–gel derived electrospinning and spin coating, respectively, by varying the Nb/Ti molar ratios from 0 to 0.59 to investigate the effect of Nb doping on morphology, crystal structure, and optical band gap energy of Nb–TiO₂. XRD results indicated that Nb–TiO₂ is composed of anatase and rutile phases as a function of Nb/Ti molar ratio. As the Nb/Ti molar ratio rose, the anatase to rutile phase transformation and the reduction in crystallite size occurred. The band gap energy of Nb–TiO₂ was changed from 3.25 eV to 2.87 eV when the anatase phase was transformed to rutile phase with increasing the Nb doping. Experimental results indicated that the Nb doping was mainly attributed to the morphology, the crystal structure, the optical band gap energy of Nb–TiO₂, and the photocatalytic degradation of methylene blue.     

On the spray-drying deposition of TiO2 photocatalytic films

The photocatalytic activity of TiO₂ films deposited on different substrates by the spray-drying method using suspensions of commercially available TiO₂ (Degussa P25 or Tronox) as starting material was studied. The influence of the type of the initial TiO₂, preparation conditions (temperature of the substrate during the film deposition, temperature of the post-deposition annealing), substrate material (glass, fused silica, stainless steel and graphite), the presence of additives in the spraying suspension (polyethylene glycol, ethylene glycol and acetylacetone) and its sonication before spraying on the morphology, size of crystallites and phase composition (rutile/anatase ratio) was studied. Optimal conditions for spray deposition of the films are suggested.     

Low-temperature preparation of F-doped TiO2 film and its photocatalytic activity under solar light

A novel and simple method for preparing F-doped anatase TiO₂ (defined as FTO) film with high photocatalytic activity was developed using titanium-n-butoxide and NH₄F as TiO₂ and fluorine precursors under mild condition, i.e. low temperature (lower than 373 K) and ambient pressure. The prepared samples were characterized by XRD, SEM, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectrum (DRS), photoluminescence spectrum (PL) and TG-DSC analysis. The photocatalytic activity was evaluated by decomposing X-3B under artificial solar light. The results showed that the crystallinity of TiO₂ was improved by F-doping. F⁻ ions can prevent the grain growth, and the transformation of anatase to rutile phase was also inhibited. The doped fluorine atoms existed in two chemical forms, and the ones incorporated into TiO₂ lattice might take a positive role in photocatalysis. Compared with surface fluorination samples, FTO film exhibited better photocatalytic activity. The high photocatalytic activity of FTO may due to extrinsic absorption through the creation of oxygen vacancies rather than the excitation of the intrinsic absorption band of bulk TiO₂. Furthermore, the FTO can be recycled with little photocatalytic activity depression. Without any further treatment besides rinsing, after 6 recycle utilization, the photocatalytic activity of FTO film was still higher than 79%.     

Structural transformation study of TiO2 nanoparticles annealing at different temperatures and the photodegradation process of eosin-Y

Hydrothermal method was used to prepare TiO₂ nanoparticles with annealing temperature at 500 °C–700 °C. The mixture of anatase-rutile phase was investigated by powerful tool of X-ray diffraction (XRD). The structural parameters of anatase and rutile mixture phaseTiO₂ nanoparticles were calculated from the Rietveld refinement. The transformation rate of rutile was increased linearly with an annealing temperature of 500 °C–700 °C. The spherical morphology of the anatase and rutile mixed phase were obtained by scanning electron microscope and transmission electron microscope. The spherical particle of the anatase and rutile TiO₂ shows with great aggregation with different size and within the range of few tens nm. The EDAX study revealed the presence of titanium and oxygen. The best photocatalytic activity was identified as the 87.04% of anatase and 12.96% of rutile mixer phase of TiO₂. Various factors could be involved for a better photocatalytic activity.     

Enhanced photocatalytic performance of TiO2 particles via effect of anatase–rutile ratio

In the present work anatase–rutile transformation temperature and its effect on physical/chemical properties as well as photocatalytic activity of TiO₂ particles were investigated. The characterisation of the synthesised and annealed TiO₂ particles were determined by X-Ray Powder Diffraction (XRD), scanning electron microscope (SEM), dynamic light scattering (DLS) and Brunauer–Emmett–Teller surface area analysis (BET). The refraction in the ultraviolet–visible (UV–vis) range was assessed using a dual-beam spectrophotometer. The photocatalytic performance of the particles was tested on methylene blue solution. The XRD data indicated that the percentage of rutile increased with the annealing temperature and almost 100% of anatase transformed to rutile at 1000 °C. In addition, the phase transformation was a linear function of annealing temperature so phase composition of TiO₂ can be controlled by changing the annealing temperature. The SEM and BET results presented the increase of agglomerate size and the decrease of specific surface area with the increasing annealing temperature. This proved that anatase has smaller particle size and higher surface area than rutile. The photocatalytic activity of the annealed TiO₂ powders reduced with the increase of annealing temperature. The samples annealed at 900 °C and 925 °C with anatase: rutile ratio of 92:8 and 77:23, respectively, showed the best activity. These results suggested that the photocatalytic activity of TiO₂ particles is a function of phase composition. Thus it can be enhanced by changing its phase composition which can be controlled by annealing temperature.     

Porous TiO2 microspheres with tunable properties for photocatalytic air purification

The synthesis of highly-crystalline porous TiO₂ microspheres is reported using ultrasonic spray pyrolysis (USP) in the presence of colloidal silica as a template. We have exploited the interactions between hot SiO₂ template particles surface and TiO₂ precursor that occur during reaction inside the droplets, to control the physical and chemical properties of the resulting particles. Varying the SiO₂ to titanium precursor molar ratio and the colloidal silica dimension, we obtained porous titania microspheres with tunable morphology, porosity, BET surface area, crystallite size, band-gap, and phase composition. In this regard, we have also observed the preferential formation of anatase vs. rutile with increasing initial surface area of the silica template. The porous TiO₂ microspheres were tested in the photocatalytic degradation of nitrogen oxides (NO_x) in the gas phase. USP prepared nanostructured titania samples were found to have significantly superior specific activity per surface area compared to a commercial reference sample (P25 by Evonik-Degussa).     

Hydrothermal synthesis of well-dispersed ultrafine N-doped TiO2 nanoparticles with enhanced photocatalytic activity under visible light

Ultrafine nitrogen-doped TiO₂ nanoparticles with narrow particle size distribution, good dispersion, and high surface area were synthesized in the presence of urea and PEG-4000 via a hydrothermal procedure. TEM observation, N₂ adsorption, XRD, UV-vis spectroscopy, the Raman spectroscopy and XPS analysis were conducted to characterize the synthesized TiO₂ particles. The synthesized TiO₂ particles were a mixture of 49.5% anatase and 50.5% rutile with a size of around 5 nm. The photocatalytic activities were tested in the degradation of an aqueous solution of a reactive Brilliant Blue KN-R under both UV and visible light. The synthesized TiO₂ particles showed much higher photocatalytic activity than a commercial P25 TiO₂ powder under both UV and visible light irradiations. The high performance is associated to N doping, the reduced particle size, good dispersion, high surface area, and a quantum size effect.     

Comparison of photocatalytic performance of anatase TiO2 prepared by low and high temperature route

Anatase TiO₂ was prepared by a facile sol-gel method at low temperature through tailoring the pH of sol-gel without calcination. As a control, anatase TiO₂ was also synthesized by the conventional sol-gel process, in which calcination at 500 °C was required to transform the amorphous oxide into highly crystalline anatase. As-prepared samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL). Their photocatalytic activities were evaluated by degradation of methyl orange under UV light irradiation. On the basis of experiment results, it could be concluded that TiO₂ prepared by low temperature route showed more advantages in small particle size, highly dispersion nature, abundance of surface hydroxyl groups, strong PL signal, and high photocatalytic activity over TiO₂ obtained by the conventional sol-gel process. Furthermore, the reason of the former possessing higher photocatalytic activity was discussed.     

Preparation, characterization and photocatalytic activity of the neodymium-doped TiO2 nanotubes

A series of Nd-TiO₂ powders have been prepared by the sol-gel technique with neodymium nitrate and tetra-n-butyl titanium as raw materials, and then Nd-TiO₂ nanotubes were fabricated by the hydrothermal method with a 10 mol l⁻¹ NaOH solution. The as-prepared Nd-TiO₂ nanotubes were characterized by TEM, XRD, DRS, and XPS, and their photocatalytic activity was also tested in the case of the degradation of methyl orange in water. TEM photograph showed that Nd-TiO₂ nanotubes were about 10-20 nm in diameter, with the lengths range from 100 to 300 nm. TiO₂ nanotubes contained anatase and rutile crystallites. However, 0.3% Nd-TiO₂ nanotubes contained anatase crystallites, and only little rutile crystallites, so it is shown that neodymium doping hindered the phase transformation from anatase into rutile. Nd doping increased the visible-light absorption ability of Nd-TiO₂ nanotubes, and a red shift for Nd-TiO₂ nanotubes appeared when compared to TiO₂ nanotubes. XPS analysis showed that two types of oxygen existed on the photocatalyst surface, including metal-O and hydroxyl group, and more hydroxyl group was on the surface of 2% Nd-TiO₂ nanotubes than on the surface of TiO₂ nanotubes. Nd doping enhanced the photocatalytic activities of Nd-TiO₂ nanotubes, and 0.3% Nd-TiO₂ nanotubes exhibited the highest photocatalytic activity.     

Influence of the oxygen concentration on the formation of crystalline phases of TiO<Subscript>2</Subscript> during the low-pressure arc-discharge plasma synthesis

The synthesis of titanium dioxide (TiO₂) nanoparticles with different percentage of anatase and rutile phases is investigated. The synthesis is performed by controlling the oxygen percentage in the gas mixture in the plasmachemical evaporation–condensation process employing a low-pressure arc discharge. In all our experiments, the pressure in the plasmachemical reactor and the average size of particles remain constant and are 60 Pa and 6 nm, respectively. The crystal structure of synthesized TiO₂ is studied using X-ray diffraction; the morphology of the particles is analyzed employing transmission electron microscopy. Using X-ray phase analysis, it is established that the concentration of the TiO₂ anatase phase decreases upon a decrease in the oxygen concentration in the gas mixture. It is shown that the TiO₂ anatase phase is more efficient for photocatalytic decomposition of methylene blue than the rutile phase.     

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, characterization of Au (or Pt)-loaded titania nanotubes and their photocatalytic activities for degradation of methyl orange

TiO₂ nanotubes were prepared by hydrothermal method and Au (or Pt) was loaded on TiO₂ nanotubes by photodeposition method. The photocatalysts were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and N₂ adsorption technique, respectively. The photocatalytic properties of the samples were also investigated. The results show that TiO₂ nanotubes with uniform diameter were prepared, and they have specific surface areas over 400 m²/g. The specific surface areas of TiO₂ nanotubes decrease with the increasing of calcining temperature, and crystalline phase of TiO₂ in the wall of nanotubes was transformed from anatase into rutile phase in calcination process. The photocatalytic activities of TiO₂ nanotubes are higher than that of nanosized TiO₂, and the photocatalytic activities of TiO₂ nanotubes were enhanced after loading Au (or Pt). After irradiation for 40 min under a 300 W of middle-pressure mercury lamp (MPML), the degradation rate of methyl orange solution using the Au/TiNT-500 (or Pt/TiNT-500) as a catalyst can reach 96.1% (or 95.1%). On the other hand, Au-loaded sample has evident adsorption peak in visible range, indicating that Au-loaded TiO₂ nanotubes are hopeful to become visible light photocatalyst.     

Curcumin-sensitized TiO2 for enhanced photodegradation of dyes under visible light

Curcumin was coated on P25 TiO₂ by using impregnation method from freshly prepared curcumin solution. The resulting products (Cur–TiO₂–P25) was studied by several techniques such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier-transformed infrared spectroscopy, specific surface area by the Brunauer–Emmett–Teller method, and UV–Vis diffused reflectance spectroscopy. Experimental results revealed that impregnation of curcumin at 0.5, 3, 5, and 7 wt% did not affect the native phase of anatase and rutile in P25 significantly, however, it caused red shift of absorption onset in all curcumin-coated samples. The Cur–TiO₂–P25 showed enhanced adsorption efficiency and increased photocatalytic activity under visible light with optimal result at 5 wt% curcumin content. Commercial anatase and rutile coated with curcumin (Cur–TiO₂–an and Cur–TiO₂–ru) were also prepared by the same method for the use in comparative studies of photodegradation of dyes. Cur–TiO₂–an and Cur–TiO₂–ru were also characterized with some selected equipment above but not as extensively as the Cur–TiO₂–P25. Curcumin coating helped improve photocatalytic efficiencies of P25 and anatase but not for rutile. The mechanism of photocatalytic reaction was proposed that under visible light irradiation, curcumin molecule could act as dye sensitizing agent that injected electron into the conduction band of TiO₂ leading to photodegradation of dyes.     

Effect of hydrothermal temperature on photocatalytic properties of TiO2 nanotubes

This work presents a study on the effect of hydrothermal temperature and structure on the photocatalytic activity of TiO₂ nanotubes (TNT) prepared using commercially available TiO₂ nanoparticles (P25). From the results, it was found that a higher hydrothermal temperature led to an increase in the specific surface area, total pore volume, and the size of mesopores in TNT. Moreover, the TNTs synthesized by the hydrothermal method had a new structure, which was very different from the anatase and rutile structures found in P25. The TNTs synthesized at 150 °C had the highest specific surface area of 371 m²/g. However, the TNTs synthesized at 180 °C exhibited the best photocatalytic efficiency and dye adsorption capacity, as compared to other TNTs, resulting from their well-developed mesopores.     

Photocatalytic properties of nanocrystalline TiO<Subscript>2</Subscript> thin films doped with Tb

In this work photocatalytic properties of TiO₂ thin films doped with different amount of Tb have been described. Thin films were prepared by high energy reactive magnetron sputtering process. Comparable photocatalytic activity has been found for all doped TiO₂ thin films, while different amounts of Tb dopant (0.4 and 2.6 at. %) results in either an anatase or rutile structure. It was found that the terbium dopant incorporated into TiO₂ was also responsible for the amount of hydroxyl groups and water particles adsorbed on the thin film surfaces and thus photocatalytic activity was few times higher in comparison with results collected for undoped TiO₂ thin films.     

Photocatalytic activity of dc magnetron sputter deposited amorphous TiO2 thin films

For photocatalytic thin film applications TiO₂ is one of the most important materials. The most studied TiO₂ crystal phase is anatase, though also rutile and brookite show good photoactivity. Usually anatase or a mixture of rutile and anatase is applied for powder or thin film catalysts. It has been claimed that amorphous films do not exhibit any or only a very low photocatalytic activity.We have deposited amorphous thin films by dc magnetron sputtering from sub-stoichiometric TiO_2−x targets. The coatings are transparent and show a photocatalytic activity half of that of a thin layer of spin-coated reference photocatalyst powder. Annealing the thin films to yield anatase crystallization more than doubles their photocatalytic activity. At the same film thickness these thin films show the same activity as a commercially available photocatalytic coating.The dependence of the photocatalytic activity on deposition parameters like gas pressure and sputter power is discussed. A decrease in film density, as deduced from the refractive index and the microstructure, resulted in an increase in photocatalytic activity. Film thickness has a marked influence on the photocatalytic activity, showing a strong increase up to 300-400 nm, followed by a much shallower slope.     

Phase transition rate of anatase during detonation synthesis of TiO2

TiO₂ powders were synthesized by two types of mixed explosives in a sealed reaction kettle. The phase and morphology of TiO₂ powders were obtained by X-ray diffractometry and transmission electron microscopy. Results indicate that powders obtained from metatitanic acid contained mixed explosive are mixed crystal of anatase and rutile. The phase transition rate of anatase increases from 22.9% to 93.3% with the rise of mass ratio of hexogen, and the grain size also enlarges gradually. The powder obtained from anatase contained mixed explosive is rutile, and the phase transition rate of anatase is 100%. Compared with that before detonation, the grain size of anatase after detonation significantly changes, from nanoscale to micronscale. Based on the calculation of detonation parameters, the phase transition process and grain growth during the synthesis of TiO₂ by means of detonation method are analyzed, and the nucleating collision–growth model is proposed.     

Structural characterizations of sol–gel synthesized TiO2 and Ce/TiO2 nanostructures

Mixed phase TiO₂ and Ce/TiO₂ samples were synthesized by a sol–gel method using different hydrolysis conditions. In pure TiO₂ samples, traditional X-ray diffraction (XRD) and Ti K-edge synchrotron X-ray absorption near edge structures (XANES) independently revealed their anatase/rutile phase ratios. XANES results further revealed a substantial amount of Ti atoms existed in other forms beside anatase and rutile TiO₂ in the sample synthesized by the low hydrolysis condition. An increase in the extent of the hydrolysis during the synthesis leads to an increased rutile ratio and a reduction in other forms. In Ce/TiO₂ samples, the crystal sizes were too small for XRD characterization. Only XANES could be used to characterize their phase ratios. It is found that adding Ce impedes rutile formation; leading to increased anatase ratio. The difference in the fundamental aspects of XRD and XANES techniques in providing the phase ratios is discussed.     

Solar light decomposition of DFP on the surface of anatase and rutile TiO2 prepared by hydrothermal treatment of microemulsions

The photocatalytic decomposition of diisopropylfluorophosphate (DFP) over nanostructured anatase and rutile TiO₂ powder was investigated by FTIR and XPS. Upon irradiation with artificial solar light DFP decomposed on both polymorphs as evidenced by FTIR. For both crystalline structures acetone and subsequently coordinated formate and carbonate were observed on the surface during the photocatalytic reaction as the isopropyl groups dissociated from DFP. XPS revealed that small amounts of phosphates and inorganic fluoride (TiF) gradually built up on both TiO₂ surfaces, while organic F was present only on the rutile phase. From repeated cycles of intermittent DFP adsorption and irradiation measurements, the decomposition rates and formation of residuals on the surface were deduced. It was found that the overall oxidation yield is higher on anatase than rutile. The oxidation rate decreases with increasing irradiation time, an effect that is more pronounced on rutile. We find that both the difference between the polymorphs and the initial decrease of the oxidation yield can largely be explained by variations in surface area rather than poisoning by PO_x or F species. In particular, we observe a dramatic decrease of the specific area of rutile as a function of photocatalytic oxidation cycle.