Silver-Doping Induced Lattice Distortion in TiO2 Nanoparticles

The Ag-doping effects on Ti02 nanoparticles are investigated by means of x-ray diffraction （XRD） and Raman scattering spectroscopy. XRD and Raman results indicate that Ag-doping stabilizes the rutile phase in TiO2. We find an Ag-doping induced lattice expansion in both anatase and rutile phases. The Ag-doping has different influences on the lattice distortion for anatase and rutile phases, that is, the e/a-value for the anatase phase decreases with 0.5% Ag-doping and then increases with 1~ Ag-doping while that for the rutile phase shows a gradual increase with increasing Ag-doping. We have ascribed the different variations of lattice distortion due to Ag-doping to the change of interracial interaction between the anatase and rutile phases induced by different Ag concentrations.     

Optical properties of anatase and rutile titanium dioxide: Ab initio calculations for pure and anion-doped material

The optical properties of rutile and anatase titanium dioxide (TiO₂) are calculated from the imaginary part of the dielectric function using pseudopotential density functional method within its generalized gradient approximation (GGA) and a scissors approximation. The fundamental absorption edges calculated for the unit cell of both rutile and anatase are consistent with experimentally reported results of single crystal rutile and anatase TiO₂ and with previous theoretical calculations. A significant optical anisotropy is observed in the anatase structure which holds promise for investigating the band gap modification with better visible-light response and provides a reliable foundation for addressing the effect of impurities on the fundamental absorption edge/band gap of anatase TiO₂. Further calculations on the electronic structure and the optical properties of C-, N-, and S-doped anatase TiO₂ are performed. The results are analyzed and discussed in terms of optical anisotropy and scissors approximations.     

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.     

Oxygen defect dependent variation of band gap,Urbach energy and luminescence property of anatase,anatase–rutile mixed phase and of rutile phases of TiO2 nanoparticles

TiO₂ nanoparticles are prepared by a sol–gel method and annealed both in air and vacuum at different temperatures to obtain anatase, anatase–rutile mixed phase and rutile TiO₂ nanoparticles. The phase conversion from anatase to anatase–rutile mixed phase and to rutile phase takes place via interface nucleation between adjoint anatase nanocrystallites and annealing temperature and defects take the initiate in this phase transformation. The samples are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–vis and photoluminescence spectroscopy (PL). Anatase TiO₂ exhibits a defect related absorption hump in the visible region, which is otherwise absent in the air annealed samples. The Urbach energy is very high in the vacuum annealed and in the anatase–rutile mixed phase TiO₂. Vacuum annealed anatase TiO₂ has the lowest emission intensity, whereas an intense emission is seen in its air annealed counterpart. The oxygen vacancies in the vacuum annealed samples act as non-radiative recombination centers and quench the emission intensity. Oxygen deficient anatase TiO₂ has the longest carrier lifetime. Time resolved spectroscopy measurement shows that the oxygen vacancies act as efficient trap centers of electrons and reduce the recombination time of the charge carriers.     

Polyol-assisted synthesis of TiO2 nanoparticles in a semi-aqueous solvent

TiO₂ (anatase and rutile) nanoparticles with an average crystallite size of 20-40 nm have been prepared at room temperature by polyol-mediated synthesis technique in a semi-aqueous solvent medium using titanium iso-propoxide as the titanium source, acetone as the oil phase and ethylene glycol as the stabilizer. Phase and microstructure of the resultant materials have been characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Photocatalytic degradation of acetaldehyde using TiO₂ nanoparticles was investigated by gas-chromatography technique.     

Raman study of phase transformation of TiO2 rutile single crystal irradiated by infrared femtosecond laser

Titanium dioxide (TiO₂) rutile single crystal was irradiated by infrared femtosecond (fs) laser pulses with repetition rate of 250 kHz and phase transformation of rutile TiO₂ was observed. Micro-Raman spectra show that the intensity of E_g Raman vibrating mode of rutile phase increases and that of A_1g Raman vibrating mode decreases apparently within the ablation crater after fs laser irradiation. With increasing of irradiation time, the Raman vibrating modes of anatase phase emerged. Rutile phase of TiO₂ single crystal is partly transformed into anatase phase. The anatase phase content transformed from rutile phase increased to a constant with increasing of fs pulse laser irradiation time. The study indicates the more stable rutile phase is transformed into anatase phase by the high pressure produced by fs pulse laser irradiation.     

Investigation on SERS of different phase structure TiO2 nanoparticles

In this paper, anatase and rutile TiO₂ nanoparticles as well as their mixed crystal phase structure TiO₂ nanoparticles were synthesized by a sol‐hydrothermal method, and were served as active substrates for surface‐enhanced Raman scattering (SERS) study. The results show that the 4‐mercaptobenzoic acid probe molecules exhibit different degree SERS enhancements on the surface of different phase structure TiO₂ nanoparticles. The mixed crystal structure TiO₂ with an appropriate proportion of anatase and rutile phase is favourable to SERS enhancement of adsorbed molecules. These are mainly attributed to the contributions of the TiO₂‐to‐molecule charge transfer mechanism and the mixed crystal effect. Copyright © 2015 John Wiley & Sons, Ltd.     

Structural and thermoluminescence properties of undoped and Fe-doped-TiO<Subscript>2</Subscript> nanopowders processed by sol–gel method

In this study, we report on the nanocrystalline powders of TiO₂ and Fe-doped TiO₂ (anatase and rutile phases) prepared by sol–gel method. The X-ray diffraction and Raman spectroscopy measurements indicated the presence of anatase or rutile phase in nanopowders. TEM micrographs showed 10 and 112 nm average particle sizes for anatase and rutile, respectively. Furthermore, their thermoluminescence properties were analyzed.     

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.     

Maintaining particle size in the transformation of anatase to rutile titania nanostructures

We study the temperature-dependent transformation of two distinctly synthesized TiO₂ nanoparticles from the anatase to the rutile phase. These studies are carried out over the temperature range extending from room temperature to an excess of 800 °C where the anatase to rutile conversion is found to occur. Results obtained for both a sol-gel-generated nanocolloid (3-20 nm) and a sol-gel-generated micelle nanostructure (∼40 nm) are evaluated. While the TiO₂ nanocolloid structures aggregate to form larger crystallites as a function of increasing temperature with sizes comparable to the sol-gel-generated micelle structures, the resulting anatase crystallites, which are of a diameter 40-50 nm, appear to transform to comparable or slightly smaller rutile structures at 800 °C. This is in contrast to the transformation to larger rutile structures, observed for larger anatase particles. The importance of kinetic effects is considered as it enhances the rate of anatase to rutile conversion. These characteristics are established using a combination of Raman spectroscopic, X-ray diffraction, and scanning electron microscopy. The relative playoffs of the Raman and X-ray diffraction techniques are considered as they are used for the analysis of particles at the nanoscale, especially when phase transformations are evaluated.     

热处理对爆轰合成的纳米TiO_2混晶的结构相变的影响

采用爆轰法制备了纳米TiO2混晶体,初步研究了不同煅烧温度(600℃和720℃)和不同煅烧时间(1 h,2 h,3.5 h和5 h)对其微结构和结构相变行为的影响,并应用热动力学理论讨论了从锐钛矿相到金红石相的结构相变过程和相变机理.研究表明:随着煅烧温度的升高和煅烧时间的增加,纳米TiO2的粒径逐渐增大,混晶中金红石相的含量逐渐提高.与常规方法制备的纳米TiO2不同的是,在相同煅烧温度和煅烧时间下金红石相的平均生长速率明显低于锐钛矿相.锐钛矿相完全相变为金红石的温度也明显低于常规方法报道的相变温度.该研究会对控制纳米TiO2晶体尺寸和批量合成提供一定的理论和实验指导.     

Origin of visible-light-induced photocatalytic properties of S-doped anatase TiO2 by first-principles investigation

Electronic structures and optical absorption spectra of S-doped anatase TiO₂ are calculated by means of the first-principles pseudopotential total energy method. Electronic structure analyses demonstrate that isolated S 3p states which appear on the upper edge of the valence band cause the narrowing of the band gap of S-doped TiO₂. These changes of electronic structure exert great influence on the optical responses of S-doped TiO₂. Our results are in line with recent experimental observations that the redshift of optical absorption could be attributed to these isolated S 3p states on the upper edge of the valence band of S-doped TiO₂.     

Fabrication and characterization of facing-target reactive sputtered polycrystalline TiO2 films

Polycrystalline TiO₂ films were fabricated using dc facing-target reactive sputtering at different sputtering pressures. The films deposited consist of pure anatase phase or a mixture of anatase and rutile and increasing rutile content to some extent deteriorates the crystallinity of the anatase. It was found that the plasma heating effect, which plays the role of substrate heating, is an important factor for the crystallinity of the films in the case of without substrate heating. The roughness of the films increases monotonically with the increase of the sputtering pressure, which can be ascribed to the decrease in the mobility of the impinging particles. UV-vis transmission measurements reveal that the pure anatase films have higher transmittance than those having mixed phases of anatase and rutile. The band gap value decreases from ∼3.35 to 3.29 eV owing to the increase in the fraction of rutile phase.     

Evolution of structural and optical properties of rutile TiO2 thin films synthesized at room temperature by chemical bath deposition method

Nanocrystalline thin films of TiO₂ were prepared on glass substrates from an aqueous solution of TiCl₃ and NH₄OH at room temperature using the simple and cost-effective chemical bath deposition (CBD) method. The influence of deposition time on structural, morphological and optical properties was systematically investigated. TiO₂ transition from a mixed anatase–rutile phase to a pure rutile phase was revealed by low-angle XRD and Raman spectroscopy. Rutile phase formation was confirmed by FTIR spectroscopy. Scanning electron micrographs revealed that the multigrain structure of as-deposited TiO₂ thin films was completely converted into semi-spherical nanoparticles. Optical studies showed that rutile thin films had a high absorption coefficient and a direct bandgap. The optical bandgap decreased slightly (3.29–3.07 eV) with increasing deposition time. The ease of deposition of rutile thin films at low temperature is useful for the fabrication of extremely thin absorber (ETA) solar cells, dye-sensitized solar cells, and gas sensors.     

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.     

S掺杂对锐钛矿相TiO2电子结构与光催化性能的影响

采用基于第一性原理的平面波超软赝势方法研究了掺杂不同价态S的锐钛矿相TiO₂的晶体结构、杂质形成能、电子结构及光学性质.计算结果表明硫在掺杂体系中的存在形态与实验中的制备条件有关;掺杂后晶格发生畸变、原子间的键长及原子的电荷量也发生了变化,导致晶体中的八面体偶极矩增大; S 3p态与O 2p态、Ti 3d态杂化而使导带位置下移、价带位置上移及价带宽化,从而导致TiO₂的禁带宽度变窄、光吸收曲线红移到可见光区.这些结果很好地解释了S掺杂锐钛矿相TiO₂在可见光下具有优良的光催化性能的内在原因.根据计算结果分析比较了硫以不同离子价态掺杂对锐钛矿相TiO₂电子结构和光催化性能影响的差别. 关键词： 2')" href="#">锐钛矿相TiO₂ S掺杂 第一性原理 光催化性能     

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.     

Contact potential barriers and characterization of Ag-doped composite TiO2 nanotubes

Ag-doping TiO₂ composite nanotubes (Ag-TNTs) were synthesized by alkaline fusion followed by hydrothermal treatment. The microstructure and morphology of the materials were characterized by XRD, TEM, XPS, SPS (surface photovoltage spectroscopy), FISPS (electric field-induced surface photovoltage spectroscopy) and Raman spectroscopy. First-principles calculations based on density-functional theory (DFT) showed the formation of several impurity levels near the top of the valence band in the band gap (E_g) of rutile TiO₂ due to Ag doping. A “double junction” is proposed, involving a Schottky junction and p–n junction (denoted as “Ag-p–n junction”) occurring between the Ag particles and the nanotube surface, as well as forming inside TiO₂ nanotubes, respectively. The strongly built-in electric field of the junctions promotes the separation of photo-holes and photoelectrons, enhancing the photocatalytic efficiency. XRD results indicated that the composite Ag-TNTs exist as a mixture of anatase and rutile phases. XPS results showed that Ti⁴⁺ is the primary state of Ti. Raman spectral analysis of Ag-TNTs revealed the presence of a new peak at 271 cm⁻¹. The red-shift of the absorption light wavelength of Ag-TNTs was 0.16 eV (20 nm) due to a considerable narrowing of E_g by the existing impurity levels.     

Effect of Ag doping on the crystallization and phase transition of TiO2 nanoparticles

TiO₂ nanoparticles doped with different Ag contents were prepared by a modified sol–gel method, using titanium tetraisopropoxide and silver nitrate as precursors and 2-propanol as solvent. Silver was incorporated into the TiO₂ matrix via decomposition of AgNO₃ during thermal treatment in different atmospheres. Effects of Ag doping on the crystallization and phase transition of the TiO₂ nanoparticles were studied using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Raman spectroscopy techniques. While air annealing incorporates silver into TiO₂ matrix in silver oxide form, annealing in nitrogen incorporates metallic silver into TiO₂. Formation of silver oxide increases the thermal stability of the TiO₂ particles. Silver oxide affects the crystallization process of TiO₂ particles and the temperature of transition form anatase to rutile. On the other hand, presence of metallic silver in the samples annealed in nitrogen atmosphere decreases the temperature of phase transition of TiO₂ nanoparticles.     

Effects of hydroxyls on the structural and room temperature ferromagnetic properties of Co doped SnO2 nanoparticles

Co doped SnO₂ nanoparticles have been prepared via a wet chemical method with different precipitation processes. The structure and morphology of Co doped SnO₂ nanoparticles demonstrate that the nanoparticles are in a rutile single phase and uniform, respectively. X-ray photoelectron spectroscopy shows that the Co dopants are in 2+ oxidation valence state and doped ∼2 atm% in SnO₂ nanoparticles. Moreover, Raman spectroscopy further confirms that Co doped SnO₂ nanoparticles have single phase crystallinity without forming any extra modes related to secondary phases. The magnetic measurements reveal that all nanoparticles exhibit room temperature ferromagnetism (RTFM) due to the presence of disorders and defects introduced by hydroxyls in the crystal structure. In addition, it has been clearly observed that the saturated magnetic moments are strongly affected by the precipitation processes which control the incorporation of hydroxyls into the lattice.