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.     

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.     

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.     

An investigation of phase transformation of titania slag using microwave heating

The influences of microwave heating on the phase transformation of titania slag were systematically investigated. The thermal stability, surface chemical functional groups and microstructure of the titania slag before and after microwave heating, at a temperature of 950?°C for 60 min, were also analyzed using thermogravimetry and differential thermal analysis (TG-DSC), Fourier transform infrared spectroscopy (FT-IR) spectrum and scanning electron microscope (SEM), respectively. The TG-DSC analysis revealed that the phase transformation of the titania slag from anatase TiO₂ to rutile TiO₂ occurred between 750 and 1000 °C. The FT-IR rustles demonstrate that the banding form of Ti⁴⁺, Ti³⁺ and Ti²⁺ ions and the methyl groups on the surface of the titania slag has changed and a new chemical bond Ti–OH was formed. The results of SEM showed that a large number of regulation rutile TiO₂ crystals were found on the surface of the microwave-treated samples and the synthetic rutile has been synthesized successfully using microwave heating.     

Surface enhanced Raman scattering of nano diamond using visible‐light‐activated TiO2 as a catalyst to photo‐reduce nano‐structured silver from AgNO3 as SERS‐active substrate

In this work, we demonstrate nano‐structured silver particles photo‐reduced from silver nitride (AgNO₃) solution using visible‐light‐activated titanium dioxide (TiO₂), which can be a convenient and effective substrate for surface enhanced Raman spectroscopy (SERS) observation. Visible‐light‐activated carbon‐containing TiO₂ nanoparticles are applied to photo‐reduce and form nano‐structured silver from AgNO₃ upon visible‐light illumination. Photo‐reduced nano‐structured silver is used as an active substrate for SERS studies of TiO₂ as well as nano diamond and TiO₂. The photo reduction of AgNO₃ and SERS observation can be obtained by simultaneously using the same visible laser excitation. The coexistence of the anatase phase with small admixture of the rutile phase in the TiO₂ can be observed using SERS. The carbon structure in the carbon‐containing TiO₂ was determined to be sp² type carbon bonding by SERS. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis of nanocrystalline titania in pure water by pulsed Nd:YAG Laser

Synthesis of titania (TiO₂) nanoparticles (NPs) has been performed with pulsed laser ablation (PLA) approach by irradiating a 1064 nm Nd:YAG laser pulses on the titanium target immersed in pure water. A systematic characterization on the products, synthesized in different laser pulse energies, illustrated the conspicuous dependence of crystalline phase and size distribution of the NPs on this parameter. Emission spectroscopy of the induced plasma was exploited to justify the formation of titania NPs through the synthesis process, as well as the emergence of rutile phase beside the anatase by increasing the laser pulse energies. In addition, UV-vis optical absorption and Raman spectroscopy, associated with X-ray diffraction (XRD) were employed to quantitatively determine the crystalline phases of the products. Morphological observations by means of transmission electron microscopy (TEM), demonstrating the spherical shape of the synthesized NPs, was utilized to investigate the variation of particle size distribution with the laser pulse energy.     

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 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.     

Phase transformation at the surface of TiO<Subscript>2</Subscript> single crystal irradiated by femtosecond laser pulse

Phase transformation of a titanium oxide crystal irradiated by a femtosecond laser from rutile to anatase was studied by Raman spectroscopy. In the case of the high temperature phase of TiO₂ single crystal rutile, irradiated by the 120 fs, 800 nm, 250 kHz femtosecond laser with an average power of 300 mW for a short time, the intensity of Raman active mode E_g (446 cm^-1) of TiO₂ would decrease, while that of A_1g (611 cm^-1) increased, which indicated the color-center-defect-cluster was formed. After the longer irradiation time (less than 600 s), four new Raman active modes would occur, so a part of rutile in the irradiated region was transformed into anatase phase. As the irradiation time increased, the component of anatase increased to a constant, while that of rutile decreased. By this means, we can selectively induce anatase on the rutile surface through controlling the femtosecond laser exposure region. We suggest that this technique can be applied to fabricate micro patterns of anatase. PACS 52.38.Dx; 36.20.Ng; 36.40.Ei; 42.62.-b; 42.65.Dr     

Structural study of TiO2 thin films by micro-Raman spectroscopy

The Raman spectroscopy method was used for structural characterization of TiO₂ thin films prepared by atomic layer deposition (ALD) and pulsed laser deposition (PLD) on fused silica and single-crystal silicon and sapphire substrates. Using ALD, anatase thin films were grown on silica and silicon substrates at temperatures 125–425 °C. At higher deposition temperatures, mixed anatase and rutile phases grew on these substrates. Post-growth annealing resulted in anatase-to-rutile phase transitions at 750 °C in the case of pure anatase films. The films that contained chlorine residues and were amorphous in their as-grown stage transformed into anatase phase at 400 °C and retained this phase even after annealing at 900 °C. On single crystal sapphire substrates, phase-pure rutile films were obtained by ALD at 425 °C and higher temperatures without additional annealing. Thin films that predominantly contained brookite phase were grown by PLD on silica substrates using rutile as a starting material.     

The application of Raman and anti-stokes Raman spectroscopy for in situ monitoring of structural changes in laser irradiated titanium dioxide materials

The use of Raman and anti-stokes Raman spectroscopy to investigate the effect of exposure to high power laser radiation on the crystalline phases of TiO₂ has been investigated. Measurement of the changes, over several time integrals, in the Raman and anti-stokes Raman of TiO₂ spectra with exposure to laser radiation is reported. Raman and anti-stokes Raman provide detail on both the structure and the kinetic process of changes in crystalline phases in the titania material. The effect of laser exposure resulted in the generation of increasing amounts of the rutile crystalline phase from the anatase crystalline phase during exposure. The Raman spectra displayed bands at 144 cm⁻¹ (A1g), 197 cm⁻¹ (Eg), 398 cm⁻¹ (B1g), 515 cm⁻¹ (A1g), and 640 cm⁻¹ (Eg) assigned to anatase which were replaced by bands at 143 cm⁻¹ (B1g), 235 cm⁻¹ (2 phonon process), 448 cm⁻¹ (Eg) and 612 cm⁻¹ (A1g) which were assigned to rutile. This indicated that laser irradiation of TiO₂ changes the crystalline phase from anatase to rutile. Raman and anti-stokes Raman are highly sensitive to the crystalline forms of TiO₂ and allow characterisation of the effect of laser irradiation upon TiO₂. This technique would also be applicable as an in situ method for monitoring changes during the laser irradiation process.     

稀土钇、镧掺杂TiO2薄膜的拉曼谱分析

采用溶胶-凝胶法在石英玻璃衬底上用旋涂法制备了未掺杂、掺杂钇和掺杂镧的TiO₂薄膜样品,对样品在700—1100 ℃范围内进行退火处理,并对样品的拉曼光谱进行了分析.分析表明:随着退火温度的升高,未掺杂TiO₂薄膜发生了从锐钛矿相经混相最终向金红石相的转换,掺杂钇和掺杂镧对TiO₂薄膜的晶相转换起阻碍作用,掺杂镧的阻碍作用更强;稀土掺杂能使TiO₂薄膜晶粒细化,并使晶粒内部应力增大从而阻碍晶格振动,掺杂镧比掺杂钇的效果 关键词： 2薄膜')" href="#">TiO₂薄膜 稀土掺杂 拉曼光谱 溶胶-凝胶     

Development of the IR laser pyrolysis for the synthesis of iron–doped TiO2 nanoparticles: Structural properties and photoactivity

The preparation of TiO₂-based nanoparticles of closely controlled sizes and purity gives rise to considerable interest in the frame of environmental applications, e.g. in photocatalysis. When nanoparticles instead of their bulk counterpart are used the synthesis method plays a fundamental role in defining specific structural properties. Between the different gas-phase synthesis techniques, the CO₂ laser pyrolysis is a versatile method allowing for the preparation of nanostructures of various chemical compositions. Here we demonstrate that pure and Fe–doped TiO₂ nanoparticles with rather low Fe concentration may be prepared by applying the sensitized IR laser pyrolysis to a gas mixtures containing titanium tetrachloride, air and iron pentacarbonyl (vapors). The structures of TiO₂-based particles were systematically investigated by X-ray diffraction, transmission electron microscopy, high-resolution electron microscopy, selected area electron diffraction and X-ray Photoelectron Spectroscopy. Depending on the synthesis parameters, the nanoparticle system contains mixtures of anatase and rutile, with a preponderance of the anatase phase. Higher rutile proportion was found in the iron-doped samples. Mean particle diameters of around 14 nm and 12 nm were estimated for undoped and doped anatase titania, respectively. From UV–Vis diffuse reflectance spectra, higher absorbance and red shifted absorption were evidenced at higher amount of doped iron. Preliminary evaluation tests of the UV photoactivity of samples were performed by using the scanning electrochemical microscopy for determining the evolution of the oxygen consumption in the presence of IV-chlorophenol. They show that the undoped nano titania samples perform better than the reference P25 Degussa sample. A drop of the nano-titania photoactivity as a consequence of Fe doping was observed. Possible reasons of this effect are tentatively discussed.     

表面修饰的二氧化钛纳米材料的结构相变和光吸收性质

采用胶体化学法制备表面修饰的二氧化钛纳米材料，并使用XRD，TEM，UV-vis光谱等手段研究表面修饰的二氧化钛纳米微粒的结构相变和光吸收性质.结果表明，表面修饰可以改变二氧化钛的晶化行为、加快锐钛矿→金红石的相变进程、引起二氧化钛纳米粒子的光吸收带边大幅度红移.光吸收系数与光子能量之间关系的计算分析显示，在吸收带边附近，二氧化钛纳米微粒溶胶及二氧化钛纳米薄膜的(αhν)^1/2vs hν(间接)和(αhν)² vs hν(直接)均呈线性关系，其间接和直接光学带隙能可以分别通过外推这种线性关系来测量. 关键词： 二氧化钛纳米材料 结构相变 表面改性 光吸收     

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.     

TiO_2晶型及其相变的高温拉曼光谱研究

本文测量了锐钛矿型和金红石型TiO2常温至1923K的高温拉曼光谱,观察了锐钛矿型TiO2在1373K～1473K间发生相变,不可逆转化为金红石型TiO2,分析了特征峰随温度变化的规律以及两种结构相的温度依赖性。并为不同晶型TiO2的研究、生产和应用提供重要的实验依据。     

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.     

Phase transformations and crystallization kinetics in electrospun TiO2 nanofibers in air and argon atmospheres

The effects of atmospheric air and argon environments on thermal-induced phase transformations in electrospun TiO₂ nanofibers have been investigated in situ using synchrotron radiation diffraction. Diffraction results showed that the as-synthesized TiO₂ nanofibers were initially amorphous, but crystallized to form anatase and rutile after thermal annealing in air or argon at elevated temperatures. The crystallization temperature of anatase was delayed by 100 °C in argon relative to in air, and the transformation of anatase into rutile occurs faster in argon atmosphere than in air due to the formation of oxygen vacancies. Non-linear strains formed in both polymorphs and the substantial elevation of rutile thermal expansion pointed to strain anisotropy in the rutile phase and the concomitant fibre breakage.     

Laser-induced crystalline phase transition from rutile to anatase of niobium doped TiO2

The investigation of the laser-induced crystalline phase transition of niobium doped TiO₂ single crystal is discussed in this paper. The TiO₂ single crystal was studied by using Raman spectroscopy and electron backscatter diffraction before and after irradiation by 266?nm?ns laser at the threshold intensity of 56?MW/cm². Experimental results show an evidence of anatase phase formation from rutile single crystal. Moreover, the analysis of the experimentally obtained and calculated crystallographic texture indicates that the converted TiO₂ layer has a polycrystalline structure with the preferred orientations. According to the Raman spectroscopy the amount of anatase phase increases with the number of laser pulses indicating the dose-dependent conversion process.     

Comparing the Contribution of Visible‐Light Irradiation,Gold Nanoparticles,and Titania Supports in Photocatalytic Nitroaromatic Coupling and Aromatic Alcohol Oxidation

Under visible‐light irradiation, gold nanoparticles (Au NPs) supported by titania (TiO₂) nanofibers show excellent activity and high selectivity for both reductive coupling of nitroaromatics to corresponding azobenzene or azoxylbenzene and selective oxidation of aromatic alcohols to corresponding aldehydes. The Au NPs act as active centers mainly due to their localized surface plasmon resonance (LSPR) effect. They can effectively couple the photonic energy and thermal energy to enhance reaction efficiency. Visible‐light irradiation has more influence on the reduction than on the oxidation, lowering the activation energy by 24.7 kJ mol^?1 and increasing the conversion rate by 88% for the reductive coupling, compared to merely 8.7 kJ mol^?1 and 46% for the oxidation. Furthermore, it is found that the conversion of nitroaromatics significantly depends on the particle size and specific surface area of supported Au NPs; and the catalyst on TiO₂(B) support outperforms that on anatase phase with preferable ability to activate oxygen. In contrast, for the selective oxidation, the effect of surface area is less prominent and Au NPs on anatase exhibit higher photo‐catalytic activity than other TiO₂ phases. The catalysts can be recovered efficiently because the Au NPs stably attach to TiO₂ supports by forming a well‐matched coherent interface observed via high‐resolution TEM.