Anatase-rutile Phase Research Articles

H2 and O2 photocatalytic production on TiO2 nanotube arrays: Effect of the anodization time on structural features and photoactivity

TiO2 nanotube (NT) arrays of different lengths were prepared by electrochemical anodization of ca. 10cm2 area titanium disks in NH4F–H2O–formamide solution for different times, followed by annealing at 450°C. After Pt deposition on the opposite side of the disk, the so obtained Ti-supported crystalline anodic oxides were employed as photoactive electrodes in a two compartments cell for separate H2 and O2 production through water photosplitting and characterized by SEM, XRD analysis and photocurrent measurements. The anodization time affected the phase composition and morphology of the growing NTs, which strictly influenced their photocatalytic activity. Short anodization times (40–60min) resulted in well aligned short tubes composed of anatase-rutile mixed phases. Longer anodization (>2.5h) yielded thicker NT arrays covered on top by a preferentially oriented anatase layer that limited their photoactivity. Photocurrent intensity measurements perfectly paralleled the water splitting activity results obtained with the different NT arrays. In particular, a square-shaped fast photoresponse was recorded with ordered and fully top-open nanotubular structures. On the other hand, clogged tubes not only yielded low current densities, but also showed delayed photocurrent transient signals due to the reduced mobility of the charge carriers within the preferentially oriented anatase layer. NT arrays obtained under optimized conditions had a ca. 80:20 anatase:rutile composition and ensured a hydrogen production rate of 83mmolh−1m−2 (i.e. 1.9NLh−1m−2) in the absence of any hole scavenger or external bias.

The influence of anatase-rutile mixed phase and ZnO blocking layer on dye-sensitized solar cells based on TiO2nanofiberphotoanodes

High performance is expected in dye-sensitized solar cells (DSSCs) that utilize one-dimensional (1-D) TiO2 nanostructures owing to the effective electron transport. However, due to the low dye adsorption, mainly because of their smooth surfaces, 1-D TiO2 DSSCs show relatively lower efficiencies than nanoparticle-based ones. Herein, we demonstrate a very simple approach using thick TiO2 electrospun nanofiber films as photoanodes to obtain high conversion efficiency. To improve the performance of the DSCCs, anatase-rutile mixed-phase TiO2 nanofibers are achieved by increasing sintering temperature above 500°C, and very thin ZnO films are deposited by atomic layer deposition (ALD) method as blocking layers. With approximately 40-μm-thick mixed-phase (approximately 15.6 wt.% rutile) TiO2 nanofiber as photoanode and 15-nm-thick compact ZnO film as a blocking layer in DSSC, the photoelectric conversion efficiency and short-circuit current are measured as 8.01% and 17.3 mA cm−2, respectively. Intensity-modulated photocurrent spectroscopy and intensity-modulated photovoltage spectroscopy measurements reveal that extremely large electron diffusion length is the key point to support the usage of thick TiO2 nanofibers as photoanodes with very thin ZnO blocking layers to obtain high photocurrents and high conversion efficiencies.

Structure and stability of pristine and Bi and/or Sb decorated titanate nanotubes

90–100nm long protonated titanate nanotubes were synthesized by hydrothermal method from anatase at 155°C. The bismuth and antimony affected crystallization of the tubular titanate structures during calcination was studied by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. It was shown that bismuth and antimony have different effects on the transformation; while in the presence of antimony oxide the trititanate–anatase–rutile phase transformation is hindered, the bismuth oxide promotes the crystallization process to rutile.

Evidence of oxygen vacancy induced room temperature ferromagnetism in solvothermally synthesized undoped TiO2 nanoribbons

We report on the oxygen vacancy induced ferromagnetism (FM) at and above room temperature in undoped TiO2 nanoporous nanoribbons synthesized by a solvothermal route. The origin of FM in as-synthesized and vacuum annealed undoped nanoribbons grown for different reaction durations followed by calcinations was investigated by several experimental tools. X-Ray diffraction pattern and micro-Raman studies reveal the TiO2(B), TiO2(B)-anatase, and anatase-rutile mixed phases of TiO2 structure. Field emission scanning electron microscopy and transmission electron microscopy observations reveal nanoribbons with uniform pore distribution and nanopits/nanobricks formed on the surface. These samples exhibit strong visible photoluminescence associated with oxygen vacancies and a clear ferromagnetic hysteresis loop, both of which dramatically enhanced after vacuum annealing. Direct evidence of oxygen vacancies and related Ti(3+) in the as-prepared and vacuum annealed TiO2 samples are provided through X-ray photoelectron spectroscopy analysis. Micro-Raman, infrared absorption and optical absorption spectroscopic analyses further support our conclusion. The observed room temperature FM in undoped TiO2 nanoribbons is quantitatively analyzed and explained through a model involving bound magnetic polarons (BMP), which include an electron locally trapped by an oxygen vacancy with the trapped electron occupying an orbital overlapping with the unpaired electron (3d(1)) of Ti(3+) ion. Our analysis interestingly shows that the calculated BMP concentration scales linearly with concentration of oxygen vacancies and provides a stronger footing for exploiting defect engineered ferromagnetism in undoped TiO2 nanostructures. The development of such highly porous TiO2 nanoribbons constitutes an important step towards realizing improved visible light photocatalytic and photovoltaic applications of this novel material.

Facile fabrication and characterization of multi-type carbon-doped TiO2 for visible light-activated photocatalytic mineralization of gaseous toluene

Carbon-doped titanium dioxide (C-TiO2) nanoparticles were synthesized by conventional mild oxidation of precursor titanium carbide (TiC) at 350 °C for 2 to 50 hours and more aggressive oxidation at the higher temperature of 400 to 600 °C for 2 hours in air. XRD and TEM studies indicated the formation of nano-sized C-TiO2 with mixed anatase–rutile phases. With prolonged oxidation time or increase in oxidation temperature, an initial decrease in crystallite size was unveiled due to cracking of TiC grains, renucleation of TiO2 and diffusion of carbon atoms. Raman, FTIR and XPS measurements revealed the presence of graphite-like carbon and the coexistence of substitutional and interstitial carbon in the TiO2 lattice. This multi-type carbon doping either served as a photosensitizer or resulted in additional electronic states above the valence band of the TiO2 lattice, directly responsible for the red shift of the absorption edge in the UV-vis absorption spectrum. The band structure and possible visible light photocatalytic mechanism of the C-TiO2 were thus elucidated. The synthesized C-TiO2 nanoparticles demonstrated improved photocatalytic performance for the mineralization of gaseous toluene in comparison to commercial P25 TiO2 under visible light irradiation. The C-TiO2 nanoparticles prepared at higher oxidation temperature with shorter time exhibited a more pronounced enhancement than those prepared by the mild oxidation process, providing a facile method for large-scale production of C-TiO2 suitable for indoor photocatalytic applications.

Influences of air calcination and steam activation on microstructure and photocatalytic activity of continuous TiO2 fibers

Effects of air calcination and steam activation, on phase transformation, microstructure and photocatalytic activity of continuous TiO2 fibers prepared by sol–gel method were studied. The fibers were characterized by XRD, SEM, and N2 adsorption–desorption. The photocatalytic activity was evaluated by photocatalytic degradation of formaldehyde. Results showed that 100% rutile fibers heat-treated in air and steam were obtained at 800°C and 600°C, respectively. The anatase grain sizes and special surface areas of the fibers heat-treated at 500°C in air and steam were 31.6nm, 22.7nm, 26.7m2/g and 32.3m2/g, respectively. The degradation rates of formaldehyde over the fibers heat-treated at 500°C in air and steam were 92.3% and 98.6% after 14h under UV irradiation, respectively. Compared with the air calcination, the steam activation promoted the anatase–rutile phase transformation, reduced the grain size, increased the special surface area, and improved photocatalytic activity of continuous TiO2 fibers.

Anatase–rutile phase transformation of titanium dioxide bulk material: a DFT + U approach

The anatase–rutile phase transformation of TiO2 bulk material is investigated using a density functional theory (DFT) approach in this study. According to the calculations employing the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional with the Vanderbilt ultrasoft pseudopotential, it is suggested that the anatase phase is more energetically stable than rutile, which is in variance with the experimental observations. Consequently, the DFT + U method is employed in order to predict the correct structural stability in titania from electronic-structure-based total energy calculations. The Hubbard U term is determined by examining the band structure of rutile with various values of U from 3 to 10 eV. At U = 5 eV, a theoretical bandgap for rutile is obtained as 3.12 eV, which is in very good agreement with the reported experimental bandgap. Hence, we choose the DFT + U method (with U = 5 eV) to investigate the transformation pathway using the newly-developed solid-state nudged elastic band (ss-NEB) method, and consequently obtain an intermediate transition structure that is 9.794 eV per four-TiO2 above the anatase phase. When the Ti–O bonds in the transition state are examined using charge density analysis, seven Ti–O bonds (out of 24 bonds in the anatase unit cell) are broken, and this result is in excellent agreement with a previous experimental study (Penn and Banfield 1999 Am. Miner. 84 871–6).

Relationship between N-doping induced point defects by annealing in ammonia and enhanced thermal stability for anodized titania nanotube arrays

Annealing in ammonia slows down the anatase–rutile phase transformation (ARPT) of TiO2 nanotube arrays (NTs) by anodization as compared with that of NTs annealed in oxygen. This improves the thermal stability of NTs indicated by SEM, XRD, Raman spectroscopy. The surface states of NTs annealed in ammonia identified by XPS as N 1s, O 1s and Ti 2p indicate that the retarded ARPT and enhanced thermal stability are closely related to nitrogen interstitials. At temperature below 550°C, the chemisorbed NH3 for γ-N on the surface of NTs is predominant. In contrast, at annealing temperature above 550°C, the oxygen vacancies compensating the charge balance of substitutional nitrogen result in the acceleration of ARPT and subsequently the collapse of nanotubular structure. Combining XPS with UV–vis diffuse reflection spectra, it is found that substitutional N impurities significantly enhance the visible light absorption of NTs.

Raman spectral analysis of TiO2thin films doped with rare-earth samarium

TiO(2) thin films doped with rare-earth samarium were prepared on a quartz plate by the sol-gel/spin-coating technique. The samples were annealed at 700 °C to 1100 °C, and the Raman spectra of the samples were obtained. Analyses of Raman spectra show that samarium doping can inhibit the anatase-rutile phase transition. Samarium doping can refine grains of TiO(2) thin films and increase the internal stress, thereby preventing lattice vibration. Nanocrystalline TiO(2) thin films obviously show the phonon confinement effect, i.e., the blueshift of characteristic Raman peak and full width at half-height increase, and the peak shapes asymmetrically broaden with a decrease in the grain sizes of the samples.

Preparation and characterization of nanocrystalline titania powders by sonochemical synthesis

Nanocrystalline mesoporous titania powders were synthesized by hydrolyzing titanium isopropoxide in ethanol–water mixtures which were ultrasonically treated without using any templates or chemicals. Titanium isopropoxide–ethanol mixture was added dropwise to a water–ethanol mixture placed in an ultrasonic bath. The properties of the sonochemically synthesized powder were compared with those of the powders prepared without ultrasonic treatment along with Degussa P-25 titania powder. The phase structure, crystallite size, surface area, particle size, powder density were determined and sintering behavior was analyzed in this work. The nanotitania powder prepared during ultrasonic induced hydrolysis (TiO2-U) was determined to be formed from a mixture of anatase and brookite phases at 25°C. The brookite phase in nanotitania powder prepared without ultrasonic treatment (TiO2-NoU) was detected at 70°C. The anatase–rutile phase transformation was completed in the 500–700°C range for both powders. The average crystallite sizes of the powders at 25°C were determined as 10 and 5nm for TiO2-NoU and TiO2-U, respectively. The surface area decreased from 238 to106m2/g for TiO2-NoU and from 287 to 82m2/g for TiO2-U when the calcination temperature was increased from 200 to 500°C. The evolution of the N2 adsorption–desorption behavior with calcination temperature and the corresponding pore size distributions/volumes was attributed to the formation of closely packed submicron aggregates during powder synthesis and calcination. The sintering behavior was concluded to be controlled by 7–10nm crystallites and the submicron aggregates. The determination of the densification behavior of titania powders prepared by different methods with various levels of dopants may prove to be very useful for a better understanding of the phase/pore structure evolution which is crucial for a significant number of applications.

Effect of nitrogen doping on anatase–rutile phase transformation of TiO2

N–TiO2 nanoparticles were prepared by hydrolysis of TiCl4 and characterized by means of XPS, SEM, XRD, TG-DSC and UV–vis DRS. The nitrogen was doped into TiO2 lattice successfully, and the temperature of anatase phase transforming to rutile phase was found to be increased by N-doping. By comparison of the crystal structure of titanium nitride, anatase and rutile, the mechanism for the transformation temperature increase was summarized as the effects of the similar structure of titanium nitride and anatase, the crystal aberration and the retained crystal growth induced by N-doping. The visible light photocatalytic activity of N-doped TiO2 was much higher in comparison with that of P25. The higher photocatalytic efficiency could be attributed to the synergetic effect of N-doping and appropriate crystallinity of anatase phase TiO2.

Phase transformations in the TiO2-NiO system

The anatase-rutile phase transition in fine-particle TiO2-NiO oxides has been studied using physicochemical characterization techniques (X-ray diffraction, differential thermal analysis, and mass spectrometry). The results demonstrate that NiO additions considerably increase the rate of the anataserutile polymorphic transformation. Nickel titanate formation depends on the procedures used to prepare both titanium dioxide and nickel oxide. In the temperature range 700–850°C, the anatase to rutile phase transition prevents nickel titanate formation.

Chemical and sensory changes of different dairy products during storage in packages containing nanocrystallised TiO2

SummaryThe chemical changes of three types of cheese, yoghurt and kefir were evaluated during storage in packages containing TiO2 with two different compositions of the mixed anatase–rutile phase. The influence of TiO2 packaging during storage on titratable acidity and fat content and on the external aspect was investigated. Fourier Transformed Infrared Spectroscopy (FTIR) analyses were performed to establish whether the TiO2 penetrated the food during storage. The results showed that the TiO2 nanoparticles deposited on polyethylene surface significantly decreased acidity and fat content of the investigated cheese and yoghurt types. During storage in TiO2 nanopackages, the change in chemical parameters is accentuated as the content of rutile grows from 2.8% to 6.9%. The sensory evaluation revealed that the dairy products stored for 11 days in TiO2 nanopackages present similar colour, texture and flavour with that of the references; the samples stored in polyethylene film were found to be unacceptable.

The effect of iron ions on the anatase–rutile phase transformation of titania (TiO2) in mica–titania pigments

Abstract Rutile TiO 2 -coated mica–titania pigments were prepared by hydrolysis of titanium tetrachloride in the presence of Fe 3+ . After calcination at 700 °C for 2 h, TiO 2 nanolayers in rutile phase were formed on the mica surfaces. The morphology and the anatase–rutile transformation were probed by scanning electronic microscopy (SEM) and X-ray diffraction (XRD) respectively. SEM micrographs show that the dopants enhance the growth of particles of TiO 2 thin layers. The change of lattice parameters confirms that Fe 3+ enter anatase structure and affect the anatase–rutile transformation. For the iron loading regime studied here, the anatase–rutile transformation is inhibited at low dopant levels with respect to undoped titania. While the anatase–rutile transformation is promoted as iron loading is increased. Moreover, synthesized pH value also has a pronounced effect on the anatase–rutile transformation and a highly acidic environment favors the formation of rutile.

Fabrication of Rice-Like Porous Anatase TiO2 with High Thermal Stability and Enhanced Photocatalytic Performance

Rice-like porous anatase TiO2 with high crystallinity has been successfully synthesized by means of a facile hydrothermal method with the inorganic precursor titanyl sulfate, followed by calcination at higher temperature. The structures of the prepared TiO2 nanocrystals are characterized in detail by using XRD, Raman spectra, N2 adsorption/desorption isotherms, X-ray photoelectron spectroscopy, SEM, and TEM. Experimental results indicate that the anatase crystalline structure could be maintained up to 800 °C, which is attributed to the protection of S-containing species that are produced in situ during the hydrolysis of titanyl sulfate. They hinder the undesirable grain growth and aggregation, inhibit the anatase–rutile phase transformation, and protect the porous structure from collapse during the calcination process. The rice-like porous anatase TiO2, after calcination at 700 °C, exhibits better photocatalytic activity than Degussa P 25 TiO2 in the degradation of highly toxic 2,4-dichlorophenol under UV light irradiation. This improvement was attributed to the porous structure, which offers more active sites and facilitates mass transport, and the high crystallinity that favors the separation of photogenerated electron–hole pairs, which is confirmed by analysis of surface photovoltage spectra.

Preparation and Characterization of TiO<sub>2</sub> Photocatalytic Thin Film Prepared by Sol-Gel Dip Coating

Titanium dioxide thin films have been prepared from tetrabutyl-orthotitanate solution and methanol as a solvent by sol-gel dip coating technique. The thermal properties of TiO2 gel were analyzed by thermogravimetry and differential thermal analysis (TG-DTA). Structural properties of TiO2 thin films at different heat treatment temperatures were analyzed by X-ray diffraction (XRD), Infrared Raman spectroscopy and scanning electron microscopy (SEM). The results show that the TiO2 thin films crystallize in anatase phase at 400 °C and the crystallinity of anatase phase was improved with increasing temperature. The TiO2 thin films crystallize into the anatase-rutile phase at 600 °C and further into the rutile phase at 800 °C. The photocatalytic performance of TiO2 film was tested for the degradation of 2×10-5 mol/L Rhodamine B. The degradation of Rhodamine B solutions reached 0.9×10-5 mol/L after irradiated for 8 h under UV light. The TiO2 thin film exhibited relatively high photocatalytic activity towards degrading Rhodamine B.

The Preparation and Visible Light Photocatalytic Activity of ZnO-TiO<sub>2</sub> Nanofibers

ZnO-TiO2 composite nanofibers were prepared by using the electrospun fibers of PVP/ZnSO4/ Ti(OiPr)4 composite as precursor, and through calcination treatment. The chemical components, structures and morphology were characterized by FT-IR, XRD, SEM, respectively. The photocatalytic activities of the catalysts were investigated by photocatalystic degradation of Rhodamine B under visible-light irradiation. It was found that the ZnO-TiO2 composite nanofibers photocatalyst calcined at 600 °C which was mixed the anatase phase and rutile phase, showed higher photocatalytic activities than those calcined at other two temperatures. This result was expected as a new morphology of photocatalysts to degrade the organic pollutants in the sun.

Distribution of Additives in Single-Modified TiO2

ABSTRACTIn the present work the influence of different modifiers, calculated to B2O3, CeO2, Sb2O3, ZnO, and ZrO2, on their distribution in TiO2 is investigated. The phase composition and phase transformation of prepared rutile-TiO2 is determined by the selective leaching method, ICP-AES, XRD and FT-IR techniques. The addition of Sb2O3 to TiO2 has no influence on the anatase–rutile phase transformation, CeO2 and ZrO2 act as inhibitors of the TiO2 phase transformation and the addition of ZnO or B2O3 to TiO2 accelerates rutile formation. It is observed that boron is located in TiO2 in the form of soluble B2O3, zinc partly reacts with titanium forming co-phase TiZn2O4 and antimony addition to TiO2 presumably causing the formation of a co-phase of Sb with Ti. Cerium forms a separate phase, CeO2, and reacts partly with titanium, probably creating co-phase, CexTi(1-x)O2 (for example Ce0.8Ti0.2O2). Zirconium addition in TiO2 forms separate ZrO2 phase and solid solution of Zr with Ti.

Bicontinuous nanodisc and nanospherical titania materials prepared by sol-gel process in reverse microemulsion

AbstractTiO2 nanoparticles with different shapes and sizes were synthesised by the sol-gel route in Water/Brij78/Hexane reverse microemulsions. The aqueous cores of these microemulsions were used as nanoreactors to control sol-gel reactions. We studied the effect of water/surfactant mole ratio (W 0) on the morphology, and textural properties of the final products. The materials thus obtained were characterised by different techniques. Thermogravimetric and differential thermal analysis (TG-DTA) was used to study the thermal behaviour of the products and X-ray diffraction (XRD) to identify the crystalline phases. The morphological and textural properties of the products were determined by scattering electron microscopy (SEM) and the Brunauer-Emmett-Teller (BET) method, respectively. We also studied the influence of thermal treatment on the structure and size of the TiO2 particles. The effect of W 0 on the anatase-rutile phase transition temperature was investigated.

Assembly of CeO 2–TiO 2 nanoparticles prepared in room temperature ionic liquid on graphene nanosheets for photocatalytic degradation of pollutants

CeO 2–TiO 2 nanoparticles were prepared by the sol–gel process using 2-hydroxylethylammonium formate as room-temperature ionic liquid and calcined at different temperatures (500–700 °C). CeO 2–TiO 2–graphene nanocomposites were prepared by hydrothermal reaction of graphene oxide with CeO 2–TiO 2 nanoparticles in aqueous solution of ethanol. The photocatalysts were characterized by X-ray diffraction, BET surface area, diffuse reflectance spectroscopy, scanning electron microscopy, and Fourier transformed infrared techniques. The results demonstrate that the room-temperature ionic liquid inhibits the anatase–rutile phase transformation. This effect was promoted by addition of CeO 2 to TiO 2. The addition of graphene to CeO 2–TiO 2 nanoparticles enhances electron transport and therefore impedes the charge recombination of excited TiO 2. The photodegradation results of the pollutants in aqueous medium under UV irradiation revealed that CeO 2–TiO 2–graphene nanocomposites exhibit much higher photocatalytic activity than CeO 2–TiO 2 and pure TiO 2. The photocatalytic activity of CeO 2–TiO 2–graphene nanocomposites decreases with additional increasing of the graphene content. Moreover, comparison of the photocatalytic activities of CeO 2–TiO 2–graphene with the other CeO 2–TiO 2–carbon demonstrates that CeO 2–TiO 2-graphene nanocomposites have the highest photocatalytic activity due to their unique structure and electronic properties. Chemical oxygen demand for solutions of the pollutants gave a good idea about mineralization of them.