The influence of light on the properties of paramagnetic centers in TiO<Subscript>2</Subscript> nanocrystals doped with nitrogen

Samples of nitrogen-doped nanocrystalline titanium dioxide were studied by electron-paramagnetic resonance technique. It was found that the doping of a material with an admixture of nitrogen leads to the appearance of nitrogen-containing paramagnetic centers, viz., N^⊙ and NO^⊙ radicals. The recharge effect of these centers was observed during illumination. These results may be useful for the development of photocatalytic filters based on doped nanocrystalline titanium dioxide.     

Influence of Defects on Photoconductivity and Photocatalytic Activity of Nitrogen-Doped Titania

Samples of nitrogen-doped titanium dioxide (anatase, 0.2 ≤ N ≤ 1.0 wt%) prepared by the sol–gel method were investigated using X-band electron paramagnetic resonance spectroscopy, photoconductivity, and photocatalysis measurements. N· and NO· paramagnetic defects in N-TiO₂ have been observed; their concentrations and spin-Hamiltonian parameters were calculated. An increase both in the rate of the generation of free charge carriers and in the rate of photocatalysis was found in N-TiO₂ in contrast with non-doped titania under visible light. The correlation of the density of the observed radicals with the photoconductivity and photocatalysis data is discussed.     

The amounts of hydroxyl radicals generated by titanium dioxide and 3.5% hydrogen peroxide under 405-nm diode laser irradiation

A new bleaching agent consisting of a titanium dioxide photocatalyst and 3.5% hydrogen peroxide has recently been developed for in-office bleaching and has proven to react well with irradiated light around a wavelength of 400 nm. This study was conducted to evaluate the efficacy of dental bleaching with a 405-nm diode laser irradiation on titanium dioxide and 3.5% hydrogen peroxide by measuring the generation of hydroxyl radicals. The amount of hydroxyl radicals generated from a titanium dioxide and 3.5% hydrogen peroxide irradiated by a 405-nm diode laser were measured using electron spin resonance spin-trapping techniques. The irradiation conditions of the laser were a spot size of 1 mm, output powers of 100, 200, 400 mW, and 600 mW, and irradiation times of 10, 20, 30, and 60 s. Nonirradiated samples were used as controls. DMPO-OH adduct spectra and DMPO-O ₂ ^? adduct spectra were observed when the bleaching agent was irradiated by laser light. The amount of hydroxyl radicals generated changed in accordance with adjustments in the irradiation power and irradiation time. In the nonirradiated controls, hydroxyl radicals were generated in far smaller amounts. The 405-nm diode laser is effective in inducing the generation of hydroxyl radicals and shows promise as a bleaching light source for use in combination with bleaching agents composed of titanium dioxide and low concentrations of hydrogen peroxide.     

Synthesis, characterization and application of iodine modified titanium dioxide in phototcatalytical reactions under visible light irradiation

Iodine doped titanium dioxide has been successfully prepared by simple hydrolysis of tetrabutyl titanate in the presence of iodic acid. The adopted method allowed for the production of spherical iodine doped titaniun dioxide nanoparticles with varied amount of iodine content. Analysis by X-ray diffraction, Raman, transmission electron microscopy as well as UV-vis DRS revealed that titanium dioxide nanostructures were doped with iodine which existed in two different valence states I⁵⁺ and I⁻. The iodine in the form of I⁵⁺ is believed to have doped into the lattice whereas I⁻ was well dispersed on the surface of TiO₂ probably as iodine adducts hence rendering it to be highly absorbing in visible light region. The I-TiO₂ exhibited improved photocatalytic activity toward degradation of acid orange 7 (AO7), methyl orange (MO) and 2,4-dichlorophenol (2,4-DCP) under visible light over the pristine TiO₂ prepared by the same method. High catalytic properties are attributed to iodine doping which led to high specific surface area, absorption in visible region as well as alleviation of charge carrier recombination. The most probable route undertaken in the degradation of AO7 is through indirect oxidation by the hydroxyl radicals.     

Synthesis and characterization of TiO2 thin films coated on metal substrate

Nanostructured titanium dioxide (TiO₂) thin films have been prepared on metal substrates using a facile layer-by-layer dip-coating method. The phase structure and morphologies of preparing samples were characterized by means of X-ray powder diffraction (XRD) and field-emission scanning electron microscopy (FESEM). The results confirm that films are highly crystalline anatase TiO₂ and free from other phases of titanium dioxide. Scanning electron microscopy (SEM) shows that the nanoparticles are sintered together to form a compact structure. The electrical properties of samples were investigated by cutternt-voltage analysis, the result indicates that a rectifying junction between the nanocrystalline TiO₂ film and metal substrate was formed. The photoelectrochemical characteristics recorded under 1.5 AM illumination indicates that the as-fabricated thin film electrode possesses the highest photocurrent density at 450 °C, which is 1.75 mA/cm² at 0 V vs. Ag/AgCl.     

Influence of Fe ions in characteristics and optical properties of mesoporous titanium oxide thin films

Fe-doped mesoporous titanium dioxide (M-TiO₂-Fe) thin films have been prepared on indium tin oxide (ITO) glass substrates by sol–gel and spin coating methods. All films exhibited mesoporous structure with the pore size around 5–9 nm characterized by small angle X-ray diffraction (SAXRD) and further confirmed by high resolution transmission electron microscopy (HRTEM). Raman spectra illustrated that lower Fe-doping contributed to the formation of nanocrystalline of M-TiO₂-Fe thin films. X-ray photoelectron spectroscopy (XPS) data indicated that the doped Fe ions exist in forms of Fe³⁺, which can play a role as e⁻ or h⁺ traps and reduce e⁻/h⁺ pair recombination rate. Optical properties including refractive indices/n, energy gaps/E_g and Urbach energy width/E₀ of the thin films were estimated and investigated by UV/vis transmittance spectra. The presence of Fe content extended the light absorption band and decreased the values of n, implying enhanced light response and performance on dye-sensitized solar cells (DSSC). The optimum Fe content in M-TiO₂-Fe thin films is determined as 10 mol%, for its compatibility of well crystalline and well potential electron transfer performance.     

The optoelectronic properties of nitrogen- and carbon-doped nanocrystalline titania

Carbon- and nitrogen-doped nanocrystalline titania specimens were studied by optical spectroscopy and electron paramagnetic resonance. The doping of this material was established to result in the appearance of additional absorption in the visible spectral region. The doping with admixture nitrogen was revealed to lead to the formation of nitrogen-containing paramagnetic sites, namely, N· and NO· radicals. The effect of the recharging of the detected sites under light was revealed. The obtained results could be useful for the development of photocatalytic filters on the basis of doped nanocrystalline titania.     

Magnetic properties of oxygenated carbon nitride films from RF-discharge in an acetylene, nitrogen, and oxygen mixture environments

Properties of oxygenated carbon nitride films have attracted the attention of physics researchers due to their magnetic and physical properties, as well as for their usefulness in the industry. The free radicals were investigated using electron paramagnetic resonance applied in the study of spin concentration due to the different mechanism of preparation of carbon nitride films by RF-discharge with different kinds of plasma. Unpaired spin concentrations, in the order of 10²⁰ per cm³, were measured and their time recombination dependency was important in those films. The films were grown by plasma enhanced chemical vapor deposition using mixtures of hydrocarbons, N₂ and O₂ in different proportions.     

Interaction of electrochemically deposited aluminium nanoparticles with reactive gases

Metastable induced electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) were used to study the interaction of nanocrystalline aluminium with oxygen and carbon monoxide, respectively. High resolution scanning electron microscopy (HRSEM) was used to investigate the morphology of the nanocrystalline aluminium films. These films were prepared by electrodeposition from the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide containing 1.6 Mol per litre AlCl₃ in an argon filled glove box.Only a slight oxidation under exposure to oxygen and carbon monoxide was observed. After carbon monoxide dosage, no significant amount of carbon contamination was detected on the sample. These results indicate that the nanocrystalline aluminium is rather inert.     

EPR study of the illumination effect on properties of paramagnetic centers in nitrogen-doped TiO<Subscript>2</Subscript> active in visible light photocatalysis

An electron paramagnetic resonance study of nitrogen-doped TiO₂ samples has been carried out in the dark and under illumination with light at different wavelengths. N^· and NO^· paramagnetic species were detected in the samples and their dynamics under illumination of TiO₂ was investigated. Photosensitivity of the nitrogen-doped TiO₂ and its photocatalytic activity under visible-light irradiation increases through the creation of additional levels in the band gap of TiO₂ caused by doping.     

EPR Study of the Orientation Distribution Function of HO2 • Radicals Ordered by Light Irradiation

Partial orientational alignment of HO₂ ^• radicals in the matrix of glassy hydrogen peroxide was created by light irradiation. The orientation distribution function of the radicals was found by analyzing the angular dependence of electron paramagnetic resonance spectra. The direction of the dipole transition moment of HO₂ ^• in the molecular frame of reference was determined. Authors' address: Natalia A. Chumakova, Chemistry Department, Moscow State University, Leninskiye Gory 1/3, Moscow 119899, Russian Federation     

Photoactive lead ions in ferroelectric Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> and paramagnetic resonance

The recharging of lead matrix ions upon exposure to light in pure, doped, and nonstoichiometric lead germanate crystals has been studied using electron paramagnetic resonance. It has been shown that the maximum concentration of metastable Pb³⁺ ions is achieved in crystals doped with chlorine, fluorine, titanium, and in samples with excess lead oxide. The annealing activation energy and the parameters of the superhyperfine interaction of Pb³⁺ paramagnetic centers have been determined.     

Application of benzophenones to elucidate the photochemical reaction pathways of hydrogen peroxide with dimethylsulfoxide

Benzophenone ((C₆H₅)₂CO) and decafluorobenzophenone ((C₆F₅)₂CO) were applied to elucidate the photochemical reaction pathway of hydrogen peroxide (H₂O₂) with dimethylsulfoxide (DMSO). When a solution of benzophenone in DMSO was excited with the 355 nm laser light, three transient species were observed in the time-resolved electron paramagnetic resonance spectra: benzophenone ketyl (C₆H₅)₂COH^⋅, methyl^⋅CH₃, and methylsulfinic methyl^⋅CH₂SOCH₃ radicals. However, when decafluoro-benzophenone was used with DMSO, only ketyl and methylsulfinic methyl radicals were observed under the same experimental conditions. When the reaction of benzophenone and DMSO was carried out in the presence of H₂O₂, different time profiles of^⋅CH₃ radicals were observed. In the reaction of decafluorobenzophenone-DMSO-H₂O₂, the time profiles of the radicals were not affected by the presence of H₂O₂. Thus, these results verify that^⋅CH₃ radicals are regenerated in a cyclic pathway, in which^⋅CH₃ radicals attack H₂O₂. The regeneration pathway allows us to observe f-pair polarization throughout the lifetime of^⋅CH₃ radicals, which last several microseconds, an order of magnitude longer than theT ₁ relaxation time of^⋅CH₃ radicals.     

Sonosynthesis of nano TiO2 on wool using titanium isopropoxide or butoxide in acidic media producing multifunctional fabric

This study presents a novel idea to prepare nanocrystalline structure of TiO₂ under ambient pressure at 60–65 °C using in situ sonochemical synthesis by hydrolysis of either titanium isopropoxide or titanium butoxide in an acidic aqueous solution. The nano titanium dioxide coated wool fabrics possess significant antibacterial/antifungal activity and self-cleaning property by discoloring Methylene blue stain under sunlight irradiation. This process has no negative effect on cytotoxicity and tensile strength of the sonotreated fabric even reduces alkaline solubility and photoyellowing and improves hydrophilicity. More titanium isopropoxide or titanium butoxide as a precursor led to higher photocatalytic activities of the treated fabrics. Also introducing more ethanol improved the adsorption of TiO₂ on the wool fabric surface leading to enhanced photocatalytic activity. EDS and XRD patterns, SEM images, X-ray mapping confirmed the presence of nano TiO₂ particles on the fabric surface. The role of both solvent and precursor concentrations on the various properties of the fabric was investigated and the optimized conditions were obtained using response surface methodology.     

Titanium dioxide-coated nanofibers for advanced filters

This article reports on titanium dioxide (TiO₂)-coated nanofibers deposited on a filter surface by the electrospinning process. After depositing a micrometer-thick film of polyamide 11 nanofibers on polypropylene fabric, TiO₂ nanoparticles can be directly electrosprayed onto the nanofibers. X-ray diffraction and Raman spectroscopy showed minimal change in the phase composition (anatase and rutile) and no change in the particle size of nanocrystalline TiO₂ after coating. Scanning electron microscopy demonstrated that nanofibers were uniformly coated by titanium dioxide nanoparticles without agglomeration. TiO₂-coated filters showed excellent photocatalytic-bactericidal activity and photo-induced hydrophilicity.     

Local structure of titanium pair centers in SrF2 crystals: EPR and ENDOR spectroscopic studies

The local structure of titanium pair centers in SrF₂: Ti crystals is investigated using electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy. It is found that titanium pair centers with spin moment S=2 and tetragonal symmetry of the magnetic properties are formed in SrF₂: Ti cubic crystals under certain growth conditions and during annealing. The tensor components of the fine and ligand hyperfine structures in the EPR and ENDOR spectra are determined. A model of the Ti⁺-Ti³⁺ paramagnetic dimer is proposed. This model provides an adequate interpretation of both the ferromagnetic nature of the exchange interaction and the observed displacements of four ligands in the first coordination sphere of titanium impurity ions in directions perpendicular to the impurity ion-ligand bonds.     

High temperature stability of nanocrystalline anatase powders prepared by chemical vapour synthesis under varying process parameters

Systematic variation in the high temperature stability of nanocrystalline anatase powders prepared by chemical vapour synthesis (CVS) using titanium (IV) isopropoxide under varying flow rates of oxygen and helium was obtained by progressively shifting the decomposition product from C₃H₆ to CO₂. The as-synthesised powders were characterised by high temperature X-ray diffraction (HTXRD), simultaneous thermo-gravimetric analyses (STA), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). It was observed that the anatase to rutile transformation temperature progressively increased for samples synthesised at higher O₂/He flow rate ratios. The improved anatase stability was attributed to the presence of incorporated carbon within the titania structure and confirmed by a high temperature carbon desorption peak.     

Investigation of photocatalytic reaction for thetitanium dioxide-phosphor composite

The present study examined the photocatalytic reaction of titanium dioxide (TiO₂)-coated, phosphor composite particles. Nanocrystalline TiO₂ layers were directly coated on the alkaline earth aluminate phosphor (CaAl₂O₄:Eu²⁺,Nd³⁺) particles by a sol-gel processing method and their photocatalytic reaction was analyzed according to the degradation of methylene blue aqueous solution under visible light irradiation. Compared with pure TiO₂, the TiO₂-coated phosphor powders showed a different photocatalytic mechanism and much faster photocatalytic reactivity under visible irradiation than that of pure TiO₂, which was almost negligible. The mechanism of the photocatalytic reactivity for the TiO₂-phosphor composite was discussed in terms of the energy band structure and phosphorescence. In addition, the TiO₂-coated phosphor powders were characterized by X-ray diffraction and transmission electron microscopy.     

Synthesis,characterization, and photocatalytic properties of core/shell mesoporous silica nanospheres supporting nanocrystalline titania

The facile bulk synthesis of silica nanospheres makes them an attractive support for the transport of chemical compounds such as nanocrystalline titanium dioxide. In this contribution we present a promising route for the synthesis of mesoporous silica nanospheres (m-SiO₂) with diameter in range 200 nm, which are ideal supports for nanocrystalline titanium dioxide (TiO₂). The detailed microscopic and spectroscopic characterizations of core/shell structure (m-SiO₂/TiO₂) were conducted. Moreover, the photocatalytic potential of the nanostructures was investigated via phenol decomposition and hydrogen generation. A clear enhancement of photoactivity in both reactions as compared to commercial TiO₂-Degussa P25 catalyst is detected.     

Dissymmetrization in X-irradiated RbTiOPO4 crystal

Electron paramagnetic resonance has been used to study the hole and electron paramagnetic centers formed in X-irradiated RbTiOPO₄, the crystals of the KTP family. X-irradiation of RbTiOPO₄ crystals at 77 K produced an oxygen hole center and four different trivalent titanium electron centers I₁, II, III and IV. Theg-tensors, their principal values and axes for the defects were calculated and compared with those for KTiOPO₄ centers. X-irradiation at 300 K produced another two oxygen hole centers and three electron centers I₁, I₂ and II. EPR spectra of the center II revealed dissymmetrization, i.e., irregular distribution of growth defects, between the physically equivalent sites lowering the point group symmetry of the local environment of paramagnetic centers Ti³⁺.