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.     

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.     

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.     

Integration of individual TiO2 nanotube on the chip: Nanodevice for hydrogen sensing

Titania (TiO₂) exists in several phases possessing different physical properties. In view of this fact, we report on three types of hydrogen sensors based on individual TiO₂ nanotubes (NTs) with three different structures consisting of amorphous, anatase or anatase/rutile mixed phases. Different phases of the NTs were produced by controlling the temperature of post‐anodization thermal treatment. Integration of individual TiO₂ nanotubes on the chip was performed by employing metal deposition function in the focused ion beam (FIB/SEM) instrument. Gas response was studied for devices made from an as‐grown individual nanotube with an amorphous structure, as well as from thermally annealed individual nanotubes exhibiting anatase crystalline phase or anatase/rutile heterogeneous structure. Based on electrical measurements using two Pt complex contacts deposited on a single TiO₂ nanotube, we show that an individual NT with an anatase/rutile crystal structure annealed at 650 °C has a higher gas response to hydrogen at room temperature than samples annealed at 450 °C and as‐grown. The obtained results demonstrate that the structural properties of the TiO₂ NTs make them a viable new gas sensing nanomaterial at room temperature. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

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.     

Influence of the oxygen concentration on the formation of crystalline phases of TiO<Subscript>2</Subscript> during the low-pressure arc-discharge plasma synthesis

The synthesis of titanium dioxide (TiO₂) nanoparticles with different percentage of anatase and rutile phases is investigated. The synthesis is performed by controlling the oxygen percentage in the gas mixture in the plasmachemical evaporation–condensation process employing a low-pressure arc discharge. In all our experiments, the pressure in the plasmachemical reactor and the average size of particles remain constant and are 60 Pa and 6 nm, respectively. The crystal structure of synthesized TiO₂ is studied using X-ray diffraction; the morphology of the particles is analyzed employing transmission electron microscopy. Using X-ray phase analysis, it is established that the concentration of the TiO₂ anatase phase decreases upon a decrease in the oxygen concentration in the gas mixture. It is shown that the TiO₂ anatase phase is more efficient for photocatalytic decomposition of methylene blue than the rutile phase.     

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.     

Crystal structure mediates mode of cell death in TiO2 nanotoxicity

Certain properties that nanoparticles possess differentiate them from their bulk counterparts, and these characteristics must be evaluated prior to nanoparticle studies and include: size, shape, dispersion, physical and chemical properties, surface area, and surface chemistry. Early nanotoxicity studies evaluating TiO₂ have yielded conflicting data which identify either size or crystal structure as the mediating property for nano-TiO₂ toxicity. However, it is important to note that none of these studies examined size with the crystal structure composition controlled for or examined crystal structure while controlling the nanoparticle size. The goal of this study was to evaluate the role of size and crystal structure in TiO₂ nanotoxicity while controlling for as many other nanoproperties as possible using the HEL-30 mouse keratinocyte cell line as a model for dermal exposure. In the size-dependent studies, all the nanoparticles are 100% anatase, and aggregate sizes were determined in order to take into account the effect of agglomeration on size-dependent toxicity. In addition, varying crystal structures were assessed while the size of the nanoparticles was controlled. We were able to identify that both size and crystal structure contribute to cytotoxicity and that the mechanism of cell death varies based on crystal structure. The 100% anatase TiO₂ nanoparticles, regardless of size, induced cell necrosis, while the rutile TiO₂ nanoparticles initiated apoptosis through formation of reactive oxygen species (ROS).     

Nanostructured TiO<Subscript>2</Subscript> Films with a Mixed Phase for Perovskite Solar Cells

A series of thin films made with TiO₂ nanoparticles with a varied anatase/rutile phase ratio is prepared on conducting glass substrates using a spin-coating method. The structure, morphology, and optical properties of TiO₂ nanopowders and thin films fabricated are studied using powder X-ray diffraction, scanning electron microscopy, and optical spectroscopy. The TiO₂ nanostructured films created are used as photoelectrodes for the fabrication of perovskite solar cells (PSCs). The photovoltaic characteristics of PSCs under AM1.5 light illumination (1000 W/m²) under ambient conditions are studied. It is shown that the best efficiency of solar-to-electrical energy conversion, namely, 9.3%, is obtained for the PSC with a photoelectrode based on a TiO₂ film with an anatase/rutile mixed phase ratio of 86/14%.     

Phase transition rate of anatase during detonation synthesis of TiO2

TiO₂ powders were synthesized by two types of mixed explosives in a sealed reaction kettle. The phase and morphology of TiO₂ powders were obtained by X-ray diffractometry and transmission electron microscopy. Results indicate that powders obtained from metatitanic acid contained mixed explosive are mixed crystal of anatase and rutile. The phase transition rate of anatase increases from 22.9% to 93.3% with the rise of mass ratio of hexogen, and the grain size also enlarges gradually. The powder obtained from anatase contained mixed explosive is rutile, and the phase transition rate of anatase is 100%. Compared with that before detonation, the grain size of anatase after detonation significantly changes, from nanoscale to micronscale. Based on the calculation of detonation parameters, the phase transition process and grain growth during the synthesis of TiO₂ by means of detonation method are analyzed, and the nucleating collision–growth model is proposed.     

Optical investigation on sulfur-doping effects in titanium dioxide nanoparticles

The sulfur-doping (S-doping) effects in TiO₂ nanoparticles are investigated by means of Raman spectroscopy and UV–Vis spectroscopy with different S-doping levels (10 and 50%). Raman spectra indicate that the rutile and anatase phases dominate for the low S-doped (10%) and high S-doped (50%) TiO₂ nanoparticles, respectively. The variation of phase with different S-doping levels has been ascribed to the different S-doping processes into TiO₂ nanoparticles. In addition, an extra absorption band is observed in both the S-doped TiO₂ nanoparticles. With increasing S-doping level from 10 to 50%, the extra absorption band shows a blue-shift from 470 to 445 nm, which may be ascribed to the variation of phase from rutile to anatase for TiO₂.     

Observation of carbon‐facilitated phase transformation of titanium dioxide forming mixed‐phase titania by confocal Raman microscopy

Confocal Raman microscopy, a relatively new and advanced technique, is found to be suitable for imaging the chemical morphology below the submicrometer scale. It has been employed to probe the phase transformation of carbon‐containing titania (TiO₂) nanopowder and titania thin film subjected to laser annealing. The observation of phase transformation from the anatase phase to the rutile phase at high laser power annealing is attributed to carbon inclusion inside or on the surface of titania. Upon annealing, carbon could react with the oxygen of titania and create oxygen vacancies favoring the transformation from the anatase to the rutile phase. This study provides evidence for the carbon‐assisted phase transformation for creating carbon‐containing mixed‐phase titanium dioxide by laser annealing. We explicitly focus on the presence of carbon in the phase transformation of TiO₂ using confocal Raman microscopy. In all of the investigated samples, mixed anatase/rutile phases with carbon specifically was found at the rutile site. X‐ray diffraction (XRD), scanning electron microscopy (SEM) and energy‐dispersive spectroscopy (EDS) studies have been performed in addition to Raman mapping to verify the mixed‐phase titania formation. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of Nb doping on morphology,crystal structure,optical band gap energy of TiO2 thin films

Nb–TiO₂ nanofibers and thin films were prepared using a sol–gel derived electrospinning and spin coating, respectively, by varying the Nb/Ti molar ratios from 0 to 0.59 to investigate the effect of Nb doping on morphology, crystal structure, and optical band gap energy of Nb–TiO₂. XRD results indicated that Nb–TiO₂ is composed of anatase and rutile phases as a function of Nb/Ti molar ratio. As the Nb/Ti molar ratio rose, the anatase to rutile phase transformation and the reduction in crystallite size occurred. The band gap energy of Nb–TiO₂ was changed from 3.25 eV to 2.87 eV when the anatase phase was transformed to rutile phase with increasing the Nb doping. Experimental results indicated that the Nb doping was mainly attributed to the morphology, the crystal structure, the optical band gap energy of Nb–TiO₂, and the photocatalytic degradation of methylene blue.     

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.     

Preparation of F-doped titania nanoparticles with a highly thermally stable anatase phase by alcoholysis of TiCl4

Pure anatase is a metastable phase and inclined to (transform) be transformed into rutile structure under heating over than 500 °C, which limits its suitability for high-temperature applications. Hitherto much research efforts have been made to increase the stability temperature of anatase structure. However, metallic doping usually introduced metallic oxides into titania at high temperature, and many nonmetallic doping are not competent for increasing the stability temperature of anatase structure up to 900 °C. In this study, F-doped anatase TiO₂ nanoparticles were conveniently prepared via the alcoholysis of TiCl₄ and the as-prepared product shows very high stability temperature up to 1000 °C before being transformed into rutile structure phase. On the basis of XPS results of F-doped titania annealed at different temperature, it is learned that the F atoms were anchored on the crystal planes of anatase in favor of decreasing the energy faces of anatase and stabilizing the anatase structure till annealed at 1300 °C all the anatase were transformed into rutile phase.     

Heterojunctions of TiO2 nanoparticle film and c-Si with different Fermi level positions

The photovoltaic properties of heterojunctions of titanium dioxide (TiO₂) nanoparticle films with single crystal silicon (c-Si) substrates with different Fermi level (E _f) positions were studied. The TiO₂ nanoparticles of rutile and anatase structures were studied without any sintering process. To clarify the photovoltaic properties, the characteristics of the heterojunction solar cells of TiO₂ nanoparticle films with p-Si and n-Si substrates were investigated, where several Si substrates with different resistivities were used. The I–V characteristics of p-Si/TiO₂ heterojunction showed the rectifying behavior and photovoltaic effect. The n-Si/TiO₂ heterojunction also showed good rectifying characteristics; however, the conversion efficiency was extremely lower than that of p-Si/TiO₂ heterojunction. The conversion efficiencies of various Si/TiO₂ (rutile) heterojunction solar cells against the Fermi level E _f of c-Si showed the maximum in the p-doped region. The photovoltaic properties of the Si/TiO₂ heterojunction also depended on the crystal structure of TiO₂, and the conversion efficiency of anatase is higher than that of rutile, which was attributed to the higher carrier mobility of anatase.     

Structural characterizations of sol–gel synthesized TiO2 and Ce/TiO2 nanostructures

Mixed phase TiO₂ and Ce/TiO₂ samples were synthesized by a sol–gel method using different hydrolysis conditions. In pure TiO₂ samples, traditional X-ray diffraction (XRD) and Ti K-edge synchrotron X-ray absorption near edge structures (XANES) independently revealed their anatase/rutile phase ratios. XANES results further revealed a substantial amount of Ti atoms existed in other forms beside anatase and rutile TiO₂ in the sample synthesized by the low hydrolysis condition. An increase in the extent of the hydrolysis during the synthesis leads to an increased rutile ratio and a reduction in other forms. In Ce/TiO₂ samples, the crystal sizes were too small for XRD characterization. Only XANES could be used to characterize their phase ratios. It is found that adding Ce impedes rutile formation; leading to increased anatase ratio. The difference in the fundamental aspects of XRD and XANES techniques in providing the phase ratios is discussed.     

锐钛矿相和金红石相TiO2:Nb的光电性能研究

采用基于密度泛函理论的第一性原理计算了锐钛矿相和金红石相TiO₂:Nb的晶体结构、电子结构和光学性质. 结果表明, 在相等的摩尔掺杂浓度下(6.25%), 锐钛矿相TiO₂:Nb的导带底电子有效质量小于金红石相TiO₂:Nb, 且前者室温载流子浓度是后者的两倍左右, 即具有更大的施主杂质电离率, 从而解释了锐钛矿相TiO₂:Nb比金红石相TiO₂:Nb具有更优异电学性能的实验现象. 光学计算也表明锐钛矿相在可见光区有更大的透过率, 从而在理论上解释了锐钛矿相TiO₂:Nb比金红石相TiO₂:Nb更适于做透明导电材料的原因. 计算结果与实验数据能较好符合. 关键词： 2:Nb')" href="#">TiO₂:Nb 第一性原理 电子结构 光学性能     

Effect of ambient pressure on the crystalline phase of nano TiO2 particles synthesized by a dc thermal plasma reactor

The synthesis of nanoparticles of titanium dioxide (TiO₂) with varying percentages of anatase and rutile phases is reported. This was achieved by controlling the operating pressure in a transferred-arc, direct current thermal plasma reactor in which titanium vapors are evaporated, and then exposed to ambient oxygen. The average particle size remained around 15 nm in each case. The crystalline structure of the as-synthesized nanoparticles of TiO₂ was studied with X-ray diffraction analysis; whereas the particle morphology was investigated with the help of transmission electron microscopy. The precursor species responsible for the growth of these nanoparticles was studied with the help of optical emission spectroscopy. As inferred from the X-ray diffraction analysis, the relative abundance of anatase TiO₂ was found to be dominant when synthesized at 760 Torr, and the same showed a decreasing trend with decreasing chamber pressure. The study also reveals that anatase TiO₂ is a more effective photocatalytic agent in degrading methylene blue by comparison to its rutile phase.     

Adsorption study of 4‐MBA on TiO2 nanoparticles by surface‐enhanced Raman spectroscopy

In this paper, the adsorption of 4‐mercaptobenzoic acid (4‐MBA) on TiO₂ nanoparticles was studied mostly by surface‐enhanced Raman spectroscopy (SERS) and UV‐vis spectroscopy, at different pH values as well as under different temperatures and concentrations. The results show that the 4‐MBA molecules are bonded to the TiO₂ surface both through the sulfur atoms and COO⁻ groups at neutral or alkaline pH, but only through the sulfur atom at acidic pH. Furthermore, the 4‐MBA molecules possess high adsorptive stability on TiO₂ at a comparatively high temperature (150 °C). Concentration‐dependent SERS experiments show that the saturation concentration for 4‐MBA adsorbed on TiO₂ is about 10⁻³ M in natural case (pH = 6). Copyright © 2009 John Wiley & Sons, Ltd.