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Photoelectrochemical behavior and Photocatalytic decomposition of Methylene Blue were studied on (Nb,Ti)O2 nanosheets electrode and (Nb,Ti)O2 particles produced from nanosheets, respectively. A detailed characterization of the materials show that Nb-substitution suppresses the transition from anatase to rutile. Depending on the oxygen partial pressure during the transformation, the Nb-substitution into TiO2 provokes different defect situations and also electronic properties. 1% Nb-substitution can drasticly increase the photocurrent and photocatalytic activity of Ti0.9O2 due to the formation of new defects and electron traps that can promote the separation of photoinduced holes and electrons. However, high concentration of electron traps produced by heavy Nb-doping can serve as efficient recombination centers that caused loss of photocatalytic activity of the samples.
Thin TiO2 films were grown on Si(001) and SiO2 substrates by reactive dc magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS) at temperatures ranging from 300 to 700 °C. Both dcMS and HiPIMS produce polycrystalline rutile TiO2 grains, embedded in an amorphous matrix, despite no postannealing taking place. HiPIMS results in significantly larger grains, approaching 50% of the film thickness at 700 °C. In addition, the surface roughness of HiPIMS-grown films is below 1 nm rms in the temperature range 300–500 °C which is an order of magnitude lower than that of dcMS-grown films. The results show that smooth, rutile TiO2 films can be obtained by HiPIMS at relatively low growth temperatures, without postannealing.
The kinetics of slow aggregation of monodisperse TiO2 nanocrystals in the acidic hydrosols at room temperature was studied for months by turbidimetry. The dependence of the initial rate of aggregation on the pH was calculated. The comparison of results obtained by turbidimetry and small angle X-ray scattering permits to suppose that very loose aggregates form at the low pH in HCl solution. The dependencies obtained in this work for room temperature can be taken into consideration at the tuning of TiO2 nanoparticles morphology through thermal treatment of hydrosols.
In this paper, titanium doped (2 wt. %) indium oxide (TIO) thin films deposited on quartz substrates by DC sputtering were presented. Dealt with different temperatures from 420°C to 620°C of post-annealing in vacuum for 40 minuets, the samples display different optical and electric properties. The deposited films exhibited polycrystalline in the preferred (222) and (440) orientation, with higher mobility (up to 48.6 cm2/VS) and lower resistivity (1.26 ×10-4Ω·cm) at the post-annealing temperature of 520°C. The average optical transmittance of the films is over 92% in a wavelength range from 300 to 1100 nm and the transmittance has only around 1.8% change with different post-annealing temperatures.
Nanosized titanium dioxide photocatalysts with anatase structure or mixture of anatase and rutile phases have been synthesized. Homogeneous precipitation of aqueous solutions containing TiOSO4 and ammonia and following treatment with H2O2 was used to prepare porous yellowish (Ti-Per) gel. The gel was lyophilized for 48h and (Ti-Per)LYO-powder was obtained. Single phase anatase TiO2 samples were prepared by heating of the (Ti-Per)LYO powder. The lamella-like particle morphology of TiO2 samples determined by SEM were stable in air up to 950°C.The structure evolution during heating of the starting (Ti-Per)LYO powder was studied by DTA and XRD analyses in overall temperature range of phase transformation. The morphology and microstucture characteristics were also obtained by HRTEM and BET methods. The photocatalytic activity of the sample titania TI-LYO/950 heated to 950ºC in air contained 78.4% anatase and 21.6% rutile was higher than standard Degussa P25. Titania sample TI-LYO/950 reveals the highest catalytic activity during the photocatalyzed degradation course of 4-chlorophenol in aqueous suspension under UV-irradiation.
The synthesis and application of environmentally benign, efficient and low cost heterogeneous catalysts is increasingly important for affordable and clean chemical technologies. Nanomaterials have been proposed to have new and exciting properties relative to their bulk counterparts due to the quantum level interactions that exist at nanoscale. These materials also offer enormous surface to volume ratios that would be invaluable in heterogeneous catalysis. Recent studies point at titanium dioxide nanomaterials as having strong potential to be applied in heterogeneous photocatalysis for environmental remediation and pollution control. This work reports the use of surface modified anatase TiO2 nanofibers with rhodium (Rh) nanoparticles in the photodegradation of rhodamine B (RH-B), an organic pollutant. The dimensions of TiO2 nanofibers were 150±50 nm in diameter and the size of the Rh nanoparticles was ~5 nm. The Rh-doped TiO2 catalyst exhibited an enhanced photocatalytic activity in photodegradation of rhodamine B under visible light irradiation, with 95 % degradation within 180 minutes reaction time. Undoped TiO2 did not show any notable phocatalytic activity under visible light.
Nanocrystalline anatase titania thin films were prepared by using two different precursor solutions, a highly acid solution (Sol-1) and a polymer-like solution (Sol-2), via the dip-coating technique on different substrates (<100>-Si wafer, fused silica and soda lime glass). The influence of the two sol-gel titania precursor solutions and of the substrate type on the film morphology, coating porosity, surface roughness, crystalline phases and grain size of the titania films were investigated. Our experimental results clearly indicate that the sol - composition and substrate type remarkably influence the microstructural/morphological properties of the titanium dioxide. They consequently modify the optical response and hydrophilic performances of the samples. The photocatalytic oxidations of the methylene blue in water of the samples grown on the glass substrate were monitored to investigate the influence of the sol-gel precursor solution on the photocatalytic activity of the titania coatings, and the results were put in relation with the hydrophilic and optical properties of the films. The outcome demonstrates that the optical properties and the hydrophilic and photocatalytic performances of nanocrystalline titania can be opportunely tailored tuning the size dimension of the crystalline domain according to the specific coating applications.
A highly reproducible two-step anodization method is reported to fabricate anatase TiO2 nanotube layers. The nanotube membrane fabricated by this method is highly uniform and crack-free. Large area nanotube membranes can be transferred completely onto transparent FTO electrodes without the need for damaging ultrasonic agitation or acid treatment for application in front-illuminated nanotube-based dye-sensitized solar cells. A 16 μm thin front-illuminated nanotube-based dye-sensitized solar cell produced using this method reaches an efficiency of 6.3% under 1 sun illumination AM1.5.
In-situ laboratory measurements in X-ray diffraction (XRD) high-temperature chamber and detailed XRD measurements at room temperature were used for the study of the thickness, temperature and time dependences of crystallization of amorphous TiO2 thin films. The films deposited by magnetron sputtering, plasma jet sputtering and sol-gel method were analyzed. Tensile stresses were detected in the first two cases. They are generated during the crystallization and inhibit further crystallization that also depends on the film thickness. XRD indicated quite rapid growth of larger crystallites unlike the sol-gel films when the crystallites grow mainly by increasing of annealing temperature.
In situ photodeposition techniques taking advantage of the TiO2 photocatalysis have been developed for coupling metal sulfide quantum dots (QDs) and TiO2 at a nonoscale. The coupled metal sulfide-TiO2 systems possess the following characteristics: (I) a large amount of metal sulfides can be directly formed on TiO2 during a fairly short period with excellent reproducibility, (II) the band energies of metal sulfides QDs are widely tunable by irradiation time, (III) metal sulfide QDs can be deposited on not only the external surfaces but also the inner ones of mesoporous TiO2 nanocrystalline films without pore-blocking, (IV) the simple solution-based technique at low temperature enables the low-cost production, (V) this technique has a wide possibility for coupling TiO2 and narrow gap metal sulfides. These unique features produce the excellent performances of the resulting heteronanojunaction systems as the photoanodes for QD-sensitized solar cells.
Titanium dioxide nanotubes (TiO2-NT) have been synthesized via an electrochemical anodization strategy followed by calcination under different temperatures to form TiO2 nanostructures of anatase and rutile crystal phases. The nanotube-on-Ti structure is further used as a substrate for calcium hydroxyapatite (HAp) coating. The effect of TiO2 morphology and crystal phases (i.e. amorphous, anatase and rutile) on the coating efficiency of HAp has been investigated in comparison with HAp coating on bare Ti metal. The HAp coated TiO2-NT have been studied using X-ray absorption near-edge structure (XANES) at the Ti K- and Ca K-edge. The results show that TiO2 of amorphous and anatase phases are of comparably good performance for HAp crystallization, and both are better than rutile TiO2, while HAp is hardly found on bare Ti. The implications of the findings are discussed.
TiO2 nanoparticles have been prepared by sol-gel process using titanium isopropoxide as a precursor with ethanol and water as solvents. The synthesis involves gel formation, digestion for 24h, drying at 100oC for 10h, and calcination in air at 500-800oC for 2h. The resulting powder has been studied with respect to phase(s), morphology, optical absorption and photo -luminescence (PL) behaviour. The calcination of dried sol-gel product at 500oC for 2h leads to formation of anatase phase that possesses a tetragonal structure (a = 3.785 Å, c = 9.514 Å, Z = 4), average crystallite size ~ 11 nm and band gap of 3.34 eV. Further, increasing the time (t) of calcination causes crystallite growth that follows the relation d = α – β exp (-t/τ), α = 18.1 nm, β = 9.6 nm and τ = 6.9h. However, calcination of sol-gel product at 800oC for 2h gives rise to a rutile phase (tetragonal a = 4.593Å, c = 2.959Å, Z = 2), average crystallite size ~ 25 nm and band gap of 3.02 eV. The anatase phase exhibits strong PL emission peaks (excitation wavelength 405 nm) at 2.06 and 1.99 eV due to defect levels within the energy band gap. This observation has been attributed to finite size effects occurring in nanoparticles.
Three-dimensionally ordered macro-/mesoporous (3DOM/m) TiO2 monoliths were fabricated by a dual-templating synthesis approach employing a combination of both colloidal crystal templating (hard-templating) and surfactant templating (soft-templating) techniques. Titania precursor, consisting of amphiphilic triblock copolymer Pluronic P123 as a mesopore-structure-directing agent and titanium tetraisopropoxide as a titanium source, was infiltrated into the void spaces of the poly(methyl methacrylate) (PMMA) colloidal crystal monolith. Subsequent thermal treatment produced 3DOM/m TiO2 monolith. The macropore walls of the prepared 3DOM/m TiO2 exhibit a well-defined mesoporous structure with narrow pore size distribution, and the mesopore walls are composed of nanocrystalline anatase TiO2. The material also shows a high surface area (171 m2/g), and large pore volume (0.402 cm3/g).
We present low-temperature inelastic neutron scattering spectra collected on two metal oxide nanoparticle systems, isostructural TiO2 rutile and SnO2 cassiterite, between 0-550 meV. Data were collected on samples with varying levels of water coverage, and in the case of SnO2, particles of different sizes. This study provides a comprehensive understanding of the structure and dynamics of the water confined on the surface of these particles. The translational movement of water confined on the surface of these nanoparticles is suppressed relative to that in ice-Ih and water molecules on the surface of rutile nanoparticles are more strongly restrained that molecules residing on the surface of cassiterite nanoparticles. The INS spectra also indicate that the hydrogen bond network within the hydration layers on rutile is more perturbed than for water on cassiterite. This result is indicative of stronger water-surface interactions between water on the rutile nanoparticles than for water confined on the surface of cassiterite nanoparticles. These differences are consistent with the recently reported differences in the surface energy of these two nanoparticle systems.
Recently, few tens of nanometer thin films of TiOx have been intensively studied in applications for organic solar cells as optical spacers, environmental protection and hole blocking. In this paper we provide initial measurements of optical and electrical properties of TiOx thin films and it’s applications in solar cell and sensor devices. The TiOx material was made through hydrolysis of the precursor synthesized from titanium isopropoxide, 2-methoxyethanol, and ethanolamine. The TiOx thin films of thickness between 20 nm to 120 nm were obtained by spin coating process. The refractive index of TiOx thin films were measured using an ellipsometric technique and an optical reflection method. At room temperature, the refractive index of TiOx thin film was found to be 1.77 at a wavelength of 600 nm. The variation of refractive index under various thermal annealing conditions was also studied. The increase in refractive index with high temperature thermal annealing process was observed, allowing the opportunity to obtain refractive index values between 1.77 and 2.57 at a wavelength 600 nm. The refractive index variation is due to the TiOx phase and density changes under thermal annealing.
The electrical resistance was measured by depositing a thin film of TiOx between ITO and Al electrode. The electrical resistivity of TiOx thin film was found to be 1.7×107 Ω.cm as measured by vertical transmission line method. We have also studied the variation of electrical resistivity with temperature. The temperature coefficient of electrical resistance for 60 nm TiOx thin film was demonstrated as - 6×10-3/°C. A linear temperature dependence of resistivity between the temperature values of 20 – 100 °C was observed.
The TiOx thin films have been demonstrated as a low cost solution processable antireflection layer for Si solar cells. The results indicate that the TiOx layer can reduce the surface reflection of the silicon as low as commonly used vacuum deposited Si3N4 thin films.
Colloidal precursor solutions, obtained from a mixture of titanium isopropoxide, isopropyl alcohol and silver nitrate, were used to fabricate amorphous TiO2 and Ag/TiO2 thin films by sol-gel process. The films were deposited on borosilicate substrates, which were heated at 400 °C for 30 minutes and cooled rapidly to the formation of amorphous coatings. The films were investigated by X-ray diffraction, scanning electron microscopy, atomic force microscopy and UV-vis spectroscopy. The thickness, roughness, refraction index, and particle size of the TiO2 and Ag/TiO2 films were determined and compared. Finally, hydrophobic-hydrophilic property was evaluated to the thin films produced.
The energy levels of the different charge states of an oxygen vacancy and titanium interstitial in rutile TiO2 were calculated using the screened exchange (sX) hybrid functional . The sX method gives 3.1 eV for the band gap of rutile TiO2, which is close to the experimental value. We report the defect formation energy of the oxygen deficient structure. It is found that the defect formation energies, for the neutral charge state, of oxygen vacancy and titanium interstitial are quite similar, 2.40 eV and 2.45 eV respectively, for an oxygen chemical potential of the O-poor condition. The similar size of these two calculated energies indicates that both are a cause of oxygen deficiency, as observed experimentally . The transition energy level of oxygen vacancy lies within the band gap, corresponding to the electrons located at adjacent titanium sites. The sX method gives a correct description of the localization of defect charge densities, which is not the case for GGA [3-6].
We report an template-free process to fabricate S-C-codoped and (I2)n-C-codoped meso/nanoporous TiO2 nanocrystallites. Methylene blue solutions are used as a model pollute to evaluate the sorption and photocatalytic activity of the samples under visible light radiation. The high photocatalytic activity in visible light region of our samples is attributed to numerous oxygen vacancies, large specific surface area and the continuous states in the band gap of TiO2 introduced by I2 or S doping.
The large intrinsic band gap in TiO2 has hindered severely its potential application for visible-light irradiation. We have used a passivated approach to modify the band edges of anatase-TiO2 by codoping of X (N, C) with transition metals (TM=W, Re, Os) to extend the absorption edge to longer visible-light wavelengths. It was found that all the codoped systems can narrow the band gap significantly; in particular, (N+W)-codoped systems could serve as remarkably better photocatalysts with both narrowing of the band gap and relatively smaller formation energies and larger binding energies than those of (C+TM) and (N+TM)-codoped systems. Our theoretical calculations help to rationalise experimental results and provide reasonably meaningful guides for experiment to develop more powerful visible-light photocatalysts.