Polycrystalline TiO2 Research Articles

Photocatalytic activity of V 5+, Mo 6+ and Th 4+ doped polycrystalline TiO 2 for the degradation of chlorpyrifos under UV/solar light

TiO 2 photocatalysts were prepared by doping transition metal ions like V 5+, Mo 6+and an inner transition metal Th 4+ in the concentration range of 0.02–0.1% and were characterized by various analytical techniques. The photocatalytic activities of these catalysts were studied for the mineralization of chlorpyrifos (CP) as a probe molecule. X-ray diffraction results showed only anatase phase irrespective of nature, oxidation state and concentration of these dopants. The photocatalytic activity of undoped TiO 2 showed a better activity under UV light compared to doped catalysts. Under solar light illumination, the Th 4+ (0.06%)–TiO 2 showed highest activity for the mineralization of CP. This was attributed to the prolonged separation of photogenerated electron–hole pairs, high specific surface area of the catalyst and high concentration of surface adsorbed water/hydroxyl groups. Further large shift in the absorption band (460 and 482 nm) due to the creation of mid band gap states by Th 4+ dopant and also by its small crystallite size additionally contributes to the enhancement of the degradation process. The degradation pathway was followed by UV–vis spectroscopic and GC–MS analysis.

Preparation and characterization of TiO 2 fiber with a facile polyorganotitanium precursor method

TiO 2 precursor fibers were prepared with a new facile polyorganotitanium precursor by a dry-spinning method. The synthesis process, structure of the precursor, and its evolution from the precursor to polycrystalline TiO 2 fibers were studied by 1H nuclear magnetic resonance ( 1H NMR), Fourier transformation infrared (FTIR), X-ray diffraction (XRD), and differential scanning calorimetry and thermal gravimetry analysis (DSC/TGA). The average diameter of the fibers is in the range of 10–20 μm from scanning electron microscopy (SEM) and the length can reach tens of centimeters. The BET surface area of the fibers calcined at 500 °C/2 h is around 160.3 m 2/g and decreases with temperature increase.

Dependence of photoinduced surface friction force variation on UV intensity and atmosphere in polycrystalline TiO 2 thin films

Anatase polycrystalline thin films and a plasma crystallization process were used to investigate the dependence of the photoinduced friction force variation for a TiO 2 surface on the UV intensity and atmosphere. Photoinduced friction force variation for UV illumination time was divisible into two stages, decreasing and increasing, where the switching time between these two stages and the degree of variation depended on the UV intensity and atmosphere. The decreasing stage was attributable to the photocatalytic decomposition of organic compound adsorbed onto the surface, whereas the increasing stage was attributed to the capillary effect or an increase in the adsorbed water layer. Results demonstrated that UV intensity and atmosphere controlled the friction force.

An EPR investigation of acetonitrile reactivity with superoxide radicals on polycrystalline TiO2

The interaction of acetonitrile with superoxide radicals over a polycrystalline TiO2 (Degussa P25) surface was investigated using continuous-wave electron paramagnetic resonance (cw-EPR) spectroscopy. For the first time, a thermally unstable radical intermediate has been observed following the low-temperature exposure of acetonitrile to surface-adsorbed O 2 − radicals. The radical intermediate has been identified as an [O 2 − ···CH3CN] type surface complex characterised by the g values of g 1 = 2.031, g 2 = 2.010 and g 3 = 2.003. This surface complex is thermally unstable and decomposes at temperatures of T > 240 K. A second oxygen-centred species was also observed following acetonitrile adsorption, characterised by the spin Hamiltonian parameters of g 1 = 2.028, g 2 = 2.010, g 3 = 2.004, A 1 = 1.2 mT, A 2 = 1.0 mT and A 3 = 1.0 mT, and was assigned to a hydroperoxy radical (HO 2 • ).

Tungstophosphoric acid supported on polycrystalline TiO2 for the photodegradation of 4-nitrophenol in aqueous solution and propan-2-ol in vapour phase

Abstract The present work reports the preparation of samples based on Millennium PC-500 TiO 2 loaded with different amounts of tungstophosphoric acid (up to 30% in weight) and their characterisation by means of different physicochemical techniques: powder X-ray diffraction, N 2 adsorption–desorption at −196 °C, transmission electron microscopy, UV–vis and FT-IR spectroscopies and thermogravimetric analyses. The acidity of samples was evaluated by FT-IR using pyridine as probe molecule, and the catalysts activity was tested for the photodegradation of 4-nitrophenol in aqueous solution and for the photo-oxidation of propan-2-ol in vapour phase. The results show a noticeable improvement (8 and 2.5 times faster in the photodegradation rate of 4-nitrophenol and propan-2-ol, respectively), in both regimes for these specific substrates, when tungstophosphoric acid is dispersed over the oxide up to a certain amount (20% in weight). A partial leaching of the polyoxometalate was also detected in solution for samples loaded with the largest amounts on this compound.

Self-assembled titania–silica–sepiolite based nanocomposites for water decontamination

This work describes the preparation of microchannel structured monolithic pieces by the “ice-segregation induced self-assembly” (ISISA) process. The monoliths exhibit a hierarchical structure composed of homogeneously mixed colloidal silica resulting from hydrolysis and condensation of an alkoxysilane precursor (e.g., TEOS) and TiO2 polycrystalline nanoparticles adsorbed onto sepiolite fibres. The combination of such a different species into a single macroporous structure provides a multifunctional material capable of water decontamination by pollutant removal from aqueous media (via adsorption on sepiolite) and subsequent elimination by UV irradiation (via photocatalytic oxidation on TiO2nanoparticles). The performance of the resulting materials has been studied using two organic compounds often present in wastewater such as p-nitrophenol (PNP) and methylene blue (MB).

Resistive Switching Properties of a Polycrystalline TiO2 Memory Cell with a WN Buffer Layer Inserted

Resistive Switching Properties of a Polycrystalline TiO2 Memory Cell with a WN Buffer Layer Inserted

Al electrode dependent transition to bipolar resistive switching characteristics in pure TiO2 films

Stable bipolar resistive switching was demonstrated in polycrystalline TiO2 films involving two different top and bottom Al electrodes of two different structures (Al/TiO2/Pt and Pt/TiO2/Al) after a forming process. With an Al electrode, the transition to bipolar resistive switching was clearly observed, together with counterclockwise and clockwise switching directions, which depended on the position of the Al electrode. The transition from unipolar to bipolar resistive switching seems to be attributable to the redox reaction and trap/detrap at the interfaces between the Al electrode and TiO2 layer due to the migration of oxygen ions and electrons. However, current level analysis of devices reveals that the forming process method basically leads to the formation of conducting paths inside the TiO2 layers. The electrical device properties of the two different structures, the effects of compliance currents, and the operation voltages are also analyzed.

Analysis of Dielectric Response of TiO2 in Terahertz Frequency Region by General Harmonic Oscillator Model

The terahertz range dielectric response of polycrystalline TiO2 (rutile) ceramics and rutile single crystals were investigated by analyzing infrared-active phonons by both Fourier-transform far-infrared (FT-FIR) reflectivity measurement and terahertz time-domain spectroscopy (THz-TDS). The dielectric functions at room temperature measured by THz-TDS are well expressed using the general harmonic oscillator model. We found that the imaginary part of the dielectric permittivity of the TiO2 ceramic is threefold higher than that of the rutile single crystal at 300 K. The large dielectric loss is attributed to the additional damping of the A2u mode or/and an increase in the phonon damping. The low-temperature dielectric loss of the rutile single crystals is exceedingly low. However, TiO2 ceramics shows little temperature dependence. These results indicate that extrinsic scattering significantly affects the dielectric loss of the TiO2 ceramics in the THz region.

Novel meso-substituted porphyrins: Synthesis, characterization and photocatalytic activity of their TiO 2-based composites

Two series of novel meso-substituted porphyrins, namely 5,10,15,20-tetra[4-(3-phenoxy)-propoxy]phenyl porphyrin, the structural analogue 5,10,15,20-tetra[2-(3-phenoxy)-propoxy]phenyl porphyrin and their Co(II) Cu(II) and Zn(II) complexes were synthesized. The compounds were characterized using various spectroscopic techniques and their molecular structure was proposed based on density functional theory calculations. The diverse properties of the porphyrin derivatives result from the different stereochemistry of the particular substituents at the meso site on the macrocycle and are controlled also by the coordinated metal. The 1H NMR spectrum of the free-base porphyrin showed a complicated spin-splitting. The photocatalytic activities in degradation of 4-nitrophenol were investigated using polycrystalline TiO 2 impregnated with the porphyrins and metalloporphyrins. The Cu(II) porphyrin was a more effective sensitizer than other metal containing compounds (M = Co, Zn) as well as the free-base. Photocatalytic activity was also influenced by spatial positions of the substitutions on the porphyrin molecules.

Room temperature liquefied petroleum gas (LPG) sensor based on p-polyaniline/n-TiO2 heterojunction

Abstract In the present work, we report on the performance of a room temperature (300 K) liquefied petroleum gas (LPG) sensor based on a p-polyaniline/n-TiO 2 heterojunction. The heterojunction was fabricated using electrochemically deposited polyaniline on chemically deposited TiO 2 on a stainless steel substrate. Both the methods (chemical bath deposition and electrodeposition) are simple, inexpensive and suitable for large-scale production. TiO 2 and polyaniline films were characterized for their structural as well as surface morphologies and LPG response was studied. The XRD analysis showed formation of polycrystalline TiO 2 while polyaniline exhibited amorphous nature. Morphological analysis using scanning electron microscopy (SEM) of the junction cross-section revealed formation of a diffusion free interface. The heterojunction showed the maximum response of 63% upon exposure to 0.1 vol% LPG at room temperature.

Optical properties of nanocrystalline titanium oxide

Optical emission properties of nanocrystalline TiO 2 powder and of polycrystalline TiO 2 obtained by powder sintering have been studied by cathodoluminescence in the scanning electron microscope. Radiative transitions in nanocrystalline TiO 2 powder were associated with shallow traps induced by surface oxygen vacancies. In polycrystalline TiO 2 the presence of shallow traps diminishes and the radiative transitions involve energy levels induced by Ti 3+ ions. A red shift of the infrared emission band of titanium ions appears to be determined by oxygen vacancies affecting the band gap energy.

Fabrication of mechanically robust, large area, polycrystalline nanotubular/porous TiO 2 membranes

This work describes the fabrication of self-standing, mechanically robust, polycrystalline (anatase) TiO 2 nanotube array membranes of uniform pore size distribution. The membranes are made as flat sheets with thicknesses ranging from approximately 4.4 μm to 1 mm. To evaluate biofiltration applications, we have used glucose, phenol-red, bovine serum albumin (BSA), and immunoglobulin (IgG) as model molecules for studying the diffusion characteristics through the nanotube array membranes. It is found that the natural log of the molecule diffusion coefficients scale linearly with molecular weight. To date, we have fabricated membranes up to 12.5 cm 2, a size limited only by the processing equipment.

Degradation of the drugs Gemfibrozil and Tamoxifen in pressurized and de-pressurized membrane photoreactors using suspended polycrystalline TiO 2 as catalyst

The performance of two configurations of catalytic membrane photoreactors (pressurized and de-pressurized) in batch and continuous systems for the degradation of two pharmaceuticals (Gemfibrozil and Tamoxifen), using suspended TiO 2 as catalyst, was investigated. The effects of parameters such as pH of the aqueous TiO 2 suspensions, pump flow rate and membrane clean-up, on the efficiency of the membrane photoreactor, were studied. In the batch pressurized photoreactor, catalyst and drugs rejections presented interesting values but a small or no-rejection to degradation products of both the drugs was evidenced. A good operating stability (steady-state maintained for more than 1.5 h) was observed in the continuous systems, reaching a steady state in ca. 120 min and a complete abatement of the Gemfibrozil together a constant retentate mineralization of 60%. A TOC rejection of about 62% in the pressurized photoreactor was obtained, while the no TOC rejection in the de-pressurized (submerged membrane) system, showed the necessity to identify a membrane selective to intermediate products. However, in both systems one common benefit was the retention of the suspended catalyst in the reaction ambient and the possibility to run continuous operations. The permeate flux (65.1 L h −1 m −2) was higher in the submerged membrane photoreactor showing it interesting for application purposes.

Random Circuit Breaker Network Model for Unipolar Resistance Switching

The random circuit breaker network model is proposed for unipolar resistance switching behavior. This model describes reversible dynamic processes involving two quasi-metastable states. The formation and rupture of conducting channels (see figure) in the polycrystalline TiO2 thin films may be analyzed by the self organized avalanche process in the random circuit breaker network model. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2089/2008/adma200702024_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Tailoring optical properties of pulsed laser deposited TiO2 films

The paper is dedicated to the investigation into optical properties of TiO2 thin films pulsed laser deposited at the temperature as low as 150 °C and subjected to the following heat treatment. Properties of obtained nanocrystalline films were compared to polycrystalline TiO2 films grown at elevated temperatures. The highest transmission and the best morphology in polycrystalline films have been obtained at the growth temperature of 300 °C. The two methods of post-annealing of amorphous films were employed: annealing at thermodynamically equilibrium conditions and rapid thermal annealing (RTA). RTA at 500 °C enables achievement of the best optical performance: smooth surface and high films transparency.

Electrical Characteristics of Fluorine Passivated MOCVD-TiO<sub>2</sub> Film on (NH<sub>4</sub>)<sub>2</sub>S<sub>x</sub> Treated GaAs

The high Dit is the major problem of III-V compound semiconductor MOSFET, which causes the pinning of the surface Fermi level near the middle of the energy gap. The GaAs with (NH4)2Sx treatment (S-GaAs) can remove the native oxides on GaAs and prevent it from oxidizing. The electrical characteristics of fluorinated polycrystalline TiO2 films deposited on p-type(100) S-GaAs were investigated. The fluorine from liquid phase deposition solution can passivate the grain boundary of polycrystalline TiO2 prepared by MOCVD. The leakage current through the grain boundaries was suppressed. The leakage current of MOCVD-TiO2/S-GaAs can be improved from 6.8 x 10-6 and 0.2 A/cm2 to 3.41 x 10-7 and 1.13 x 10-6A/cm2 under positive and negative electric fields at 1.5 MV/cm, respectively. Dit and k can be improved from 1.44 x 1012 cm-2eV-1 to 4.6 x 1011 cm-2eV-1 and 52 to 65, respectively. The effective oxide charges can be improved from 2.5 x 1012 C/cm-2 to 9.3 x 1011 C/cm-2.

Photoelectron Spectroscopy at the Solid–Liquid Interface of Dye–Sensitized Solar Cells: Unique Experiments with the Solid–Liquid Interface Analysis System SoLiAS at BESSY

At the synchrotron BESSY we run the experimental station SoLiAS, dedicated to solid–liquid interface analysis with soft X-ray induced photoelectron spectroscopy (SXPS). SoLiAS allows wet chemically prepared surfaces to be transferred to the ultra high vacuum without contact with ambient air. In addition in situ (co)adsorption of volatile solvent species onto liquid nitrogen cooled samples is possible. SoLiAS proves to be very useful in analyzing the chemical and electronic structure at the solid–liquid interface of dye–sensitized solar cells. The standard dye RuII(2,2?-bipyridil-4,4?-dicarboxylate)2(NCS)2 was adsorbed from ethanol solution under clean N2 atmosphere in an UHV-integrated electrochemical cell (EC). The standard solvent acetonitrile was adsorbed in situ from the gas phase. For comparison also the nonpolar solvent benzene was adsorbed. Ex situ sintered nanocrystalline anatase substrates as well as in situ deposited polycrystalline TiO2 samples were used, which show a similar distribution of two types of occupied surface states. Distinct reversible changes occur in synchrotron-induced photoelectron valence band and core level spectra when the solvent acetonitrile is adsorbed to pristine and dye-covered TiO2 substrates. TiO2 surface states are quenched and the line width of the dye S2p emission decreases strongly. Based on the experimental results the alignment of the photovoltaic relevant electronic states and a model on the dye–solvent interaction can be deduced that points to the promotion of vectorial charge transfer by increased dye orientation due to solvation.

The influence of titanium on the properties of zinc oxide films deposited by radio frequency magnetron sputtering

TiO 2-doped zinc oxide thin films were deposited on glass substrates by radio frequency (RF) magnetron sputtering with TiO 2-doped ZnO targets in an argon atmosphere. The structural properties of TiO 2-doped ZnO films doped with different TiO 2 contents were investigated. The experimental results show that polycrystalline TiO 2-doped ZnO films had the (0 0 2) preferred orientation. The deposition parameters such as the working pressure and substrate temperature of TiO 2-doped ZnO films were also investigated. The crystalline structure of the TiO 2-doped ZnO films gradually improved as the working pressure was lowered and the substrate temperature was raised. The lowest electrical resistivity for the TiO 2-doped ZnO films was obtained when the Ti addition was 1.34 wt%; its value was 2.50 × 10 −3 Ω cm, smaller than that found in previous studies. The transmittance of the TiO 2-doped ZnO films in the visible wavelength range was more than 80%. The optical energy gap was related to the carrier concentration, and was in the range of 3.30–3.48 eV.

TiO2 Nanotube Arrays of 1000 μm Length by Anodization of Titanium Foil: Phenol Red Diffusion

We report for the first time fabrication of self-aligned hexagonally closed-packed titania nanotube arrays of over 1000 μm in length and aspect ratio ≈10 000 by potentiostatic anodization of titanium. We describe a process by which such thick nanotube array films can be transformed into self-standing, flat or cylindrical, mechanically robust, polycrystalline TiO2 membranes of precisely controlled nanoscale porosity. The self-standing membranes are characterized using Brunauer−Emmett−Teller surface area measurements, glancing angle X-ray diffraction, and transmission electron microscopy. In initial application, such membranes are used to control the diffusion of phenol red.