Effect of C doping on the structural and optical properties of sol–gel TiO2 thin films

Undoped and C-doped TiO₂ thin films have been prepared by sol–gel process. Their structure and optical properties have been investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV–vis spectroscopy. It has been observed that C dopants retard the transformation from anatase-to-rutile phase. Namely, C doping effect is attributed to the anatase phase stabilization. The optical analyses show that the optical band gap of anatase C-doped TiO₂ decreases with increasing amount of C. Also, it is founded that C dopants have been shown to make TiO₂ have a visible light photoresponse.     

Influences of the iron ion (Fe)-doping on structural and optical properties of nanocrystalline TiO2 thin films prepared by sol-gel spin coating

Titanium dioxide (TiO₂) thin films doping of various iron ion (Fe³⁺) concentrations were deposited on silicon (Si) (100) and quartz substrates by sol-gel Spin Coating technique followed by a thermal treatment at 600 °C. The structure, surface morphology and optical properties, as a function of the doping, have been studied by X-ray diffractometer (XRD), Raman, ultraviolet-visible (UV-vis) and Spectroscopic Ellipsometry (SE). XRD and Raman analyzes of our thin films show that the crystalline phase of TiO₂ thin films comprised only the anatase TiO₂, but the crystallinity decreased when the Fe³⁺ content increased from 0% to 20%. During the Fe³⁺ addition to 20%, the phase of TiO₂ thin film still maintained the amorphous state. The grain size calculated from XRD patterns varies from 29.3 to 22.6 nm. The complex index and the optical band gap (E_g) of the films were determined by the spectroscopic ellipsometry analysis. We have found that the optical band gap decreased with an increasing Fe³⁺ content.     

Preparation of Co:TiO2 thin films by crossed-beam pulsed laser deposition

Co:TiO₂ thin films were deposited using two interacting plasmas produced from different targets, TiO₂ and cobalt. By keeping constant the laser ablation conditions on the TiO₂ target and changing them on the Co target, it was possible to vary in a controlled way the Co content in the films. The cobalt plasma parameters, such as the ion kinetic energy and plasma density, were determined for each deposition condition in an attempt to correlate them with the material’s properties. The cobalt ion mean kinetic energy was varied from 36 to 789 eV, resulting in films with Co content from 1.2 up to 5.1 at.%, respectively, revealing that the cobalt content can be controlled by the Co⁺ kinetic energy. The study of the optical properties showed that the optical band gap decreased from 2.9 to 2.0 eV as the Co content increased. Raman spectroscopy was used to characterize the microstructure of the deposits, and the obtained results suggest the formation of two coexisting phases: TiO₂ in its rutile phase and CoTiO₃. It was found that as the Co⁺ energy increases, the CoTiO₃ phase develops in a greater quantity. XPS measurements confirm the Raman spectroscopy results.     

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.     

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     

Phase growth control in low temperature PLD Co: TiO2 films by pressure

This paper reports on the structural and optical properties of Co-doped TiO₂ thin films grown onto (0001)Al₂O₃ substrates by non-reactive pulsed laser deposition (PLD) using argon as buffer gas. It is shown that by keeping constant the substrate temperature at as low as 310 °C and varying only the background gas pressure between 7 Pa and 70 Pa, it is possible to grow either epitaxial rutile or pure anatase thin films, as well as films with a mixture of both polymorphs. The optical band gaps of the films are red shifted in comparison with the values usually reported for undoped TiO₂, which is consistent with n-type doping of the TiO₂ matrix. Such band gap red shift brings the absorption edge of the Co-doped TiO₂ films into the visible region, which might favour their photocatalytic activity. Furthermore, the band gap red shift depends on the films’ phase composition, increasing with the increase of the Urbach energy for increasing rutile content.     

Electronic and optical properties of pure and Mo doped anatase TiO2 using GGA and GGA+U calculations

Comparative GGA and GGA+U calculations for pure and Mo doped anatase TiO₂ are performed based on first principle theory, whose results show that GGA+U calculation provide more reliable results as compared to the experimental findings. The direct band gap nature of the anatase TiO₂ is confirmed, both by using GGA and GGA+U calculations. Mo doping in anatase TiO₂ narrows the band gap of TiO₂ by introducing Mo 4d states below the conduction band minimum. Significant reduction of the band gap of anatase TiO₂ is found with increasing Mo doping concentration due to the introduction of widely distributed Mo 4d states below the conduction band minimum. The increase in the width of the conduction band with increasing doping concentration shows enhancement in the conductivity which may be helpful in increasing electron–hole pairs separation and consequently decreases the carrier recombination. The Mo doped anatase TiO₂ exhibits the n-type characteristic due to the shifting of Fermi level from the top of the valence band to the bottom of the conduction band. Furthermore, a shift in the absorption edge towards visible light region is apparent from the absorption spectrum which will enhance its photocatalytic activity. All the doped models have depicted visible light absorption and the absorption peaks shift towards higher energies in the visible region with increasing doping concentration. Our results describe the way to tailor the band gap of anatase TiO₂ by changing Mo doping concentration. The Mo doped anatase TiO₂ will be a very useful photocatalyst with enhanced visible light photocatalytic activity.     

Influence of dopant level on structural, optical and magnetic properties of Co-doped anatase TiO2 nanoparticles

Cobalt-doped TiO₂ nanoparticles were synthesized by sol-gel method. The associated structural, optical, compositional and magnetic properties of the nanoparticles as a function of cobalt concentration have been systematically studied. The X-ray powder diffraction reveals that all samples have pure anatase phase tetragonal system and the lattice parameter analysis indicated that Co ions may substitute into the lattice of TiO₂. The average particle size is 15 nm, when found through transmission electron microscope. Optical spectroscopy measurement showed that the bandgap value decreases upon increasing Co concentration. The magnetic measurements revealed that the enhanced room temperature ferromagnetism (RTFM) strongly depends on the doping content.     

Effect of cobalt doping on structural,optical and redox properties cerium oxide nanoparticles

Cobalt-doped ceria nanoparticles were synthesized using the polyol method under co-precipitation hydrolysis. The structural, morphological, optical and redox properties were observed to investigate the influence of different concentration of cobalt ion doping on the prepared CeO₂ nanomaterials in terms of X-ray diffraction, field-emission transmission electron microscopy, thermogravimetric analysis, Fourier-transform infrared spectroscopy, UV/vis absorption spectroscopy and temperature program reduction techniques. The optical band gap energy was calculated from the optical absorption spectra for doped ceria nanoparticles, which have been found to be 2.68, 2.77, and 2.82 eV for the 2, 4, and 7 mol% Co ion-doped CeO₂ nanoparticles, respectively. As observed, the band gap energies increases as the doping Co ion concentrations increased, which could be due to significant increased oxygen vacancies with Co doping. The synergistic interaction between Co and CeO₂ was the main factor responsible for high catalytic activity of cobalt-doped CeO₂ model catalysts.     

Synthesis and characterisations of Au-nanoparticle-doped TiO2 and CdO thin films

In the present study, pure and gold nanoparticle (Au NP)-doped titanium dioxide (TiO₂) and cadmium oxide (CdO) thin film were prepared by the sol–gel method, and the effect of Au NP doping on the optical, structural and morphological properties of these thin films was investigated. The prepared thin films were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet–visible–near infrared (UV–Vis–NIR) spectra. While the optical band increases from 3.62 to 3.73 for TiO₂ thin films, it decreases from 2.20 to 1.55 for CdO thin films with increasing Au doping concentration. Analysis of XRD indicates that the intensities of peaks of the crystalline phase have increased with the increasing Au NP concentrations in all thin films. SEM images demonstrate that the surface morphologies of the samples were affected by the incorporation of Au NPs. Consequently, the most significant results of the present study are that the Au NPs can be used to modify the optical, structural and morphological properties of TiO₂ and CdO thin films.     

Correlation between microstructure and optical properties of nano-crystalline TiO2 thin films prepared by sol-gel dip coating

Titanium dioxide thin films have been prepared from tetrabutyl-orthotitanate solution and methanol as a solvent by sol-gel dip coating technique. TiO₂ thin films prepared using a sol-gel process have been analyzed for different annealing temperatures. Structural properties in terms of crystal structure were investigated by Raman spectroscopy. The surface morphology and composition of the films were investigated by atomic force microscopy (AFM). The optical transmittance and reflectance spectra of TiO₂ thin films deposited on silicon substrate were also determined. Spectroscopic ellipsometry study was used to determine the annealing temperature effect on the optical properties and the optical gap of the TiO₂ thin films. The results show that the TiO₂ thin films crystallize in anatase phase between 400 and 800 °C, and into the anatase-rutile phase at 1000 °C, and further into the rutile phase at 1200 °C. We have found that the films consist of titanium dioxide nano-crystals. The AFM surface morphology results indicate that the particle size increases from 5 to 41 nm by increasing the annealing temperature. The TiO₂ thin films have high transparency in the visible range. For annealing temperatures between 1000 and 1400 °C, the transmittance of the films was reduced significantly in the wavelength range of 300-800 nm due to the change of crystallite phase and composition in the films. We have demonstrated as well the decrease of the optical band gap with the increase of the annealing temperature.     

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.     

Annealing temperature effect on the properties of mercury-doped TiO2 films prepared by sol-gel dip-coating technique

This work presents the annealing temperature effect on the properties of mercury (Hg)-doped titanium dioxide (TiO₂). Thin films and polycrystalline powders have been prepared by sol-gel process. The structure, surface morphology and optical properties, as a function of the annealing temperature, have been studied by atomic force microscopy (AFM), Raman, reflectance and ellipsometric spectroscopies. In order to determine the transformation points, we have analyzed the xerogel-obtained powder by differential scanning calorimetry (DSC). Raman spectroscopy shows the crystalline anatase and rutile phases for the films annealed at 400 °C and 1000 °C respectively. The AFM surface morphology results indicate that the particle size increases from 14 to 57 nm by increasing the annealing temperature. The complex index and the optical band gap (E_g) of the films were determined by the spectroscopic ellipsometry analysis. We have found that the optical band gap decreases by increasing the annealing temperature.     

掺Cd氧化锌的电子结构及相结构稳定性的第一性原理研究

采用基于密度泛函理论结合投影缀加平面波方法的VASP软件包,在考虑所有掺杂原子构型的前提下,对Cd掺杂ZnO合金的晶格常数、禁带宽度、电子态密度和形成焓进行了计算,分析了Cd含量和掺杂原子构型对纤锌矿wz-Zn_1-xCd_xO合金的电子结构和结构稳定性的影响.计算结果表明:随着Cd含量的不断增加,纤锌矿ZnCdO合金的平均晶格常数a,c均线性增加,但c/a的比值不会发生显著的变化;纤锌矿ZnCd 关键词： 密度泛函理论 ZnCdO合金 电子结构 形成焓     

Improved visible light photocatalytic activity of TiO<Subscript>2</Subscript> nano powders with metal ions doping for glazed ceramic tiles

Self-cleaning and anti-bacterial activities of the photo-catalyst titanium dioxide make it a superior compound for use in the ceramics and glass industry. In order to achieve high self-cleaning efficiency for building products, it is important that Titania is present as anatase phase. Moreover, it is desirable that the particle sizes are in Nano-range, so that a large enough surface area is available for enhanced catalytic performance. In the present paper, Cobalt and Nickel co-doped (4%mol Ni and 4%mol Co doped TiO₂) and un-doped TiO₂ Nano powders have been prepared by sol–gel technique. They were calcined at the temperatures in the range of 475–1075 °C. Ni/Co co-doped TiO₂ postponed the anatase to rutile transformation of TiO₂ by about 200–300°C, such that before calcination at 775°C, no rutile was detected for 4 mol% Ni/Co co-doped TiO₂. A systematic decreasing on crystallite size and increasing on specific surface area of Ni/Co co-doped TiO₂ were observed. Photo-catalytic activity of anatase polymorph was measured by the decomposition rate of methylene blue under visible light. The results showed enhanced catalysis under visible light for Ni/Co co-doped TiO₂ as compared to pure TiO₂. The enhanced performance was attributed to surface chemistry change associated with a slight shift in the band gap. Depending on the temperatures ranging from 475 to 1075 °C, band gap energy of Ni and Co doped TiO₂ crystals decreased. For all samples there is a general reduction of the band gap energy from 3.00 to 2.96 eV.     

Intermediate Ce3+ defect level induced photoluminescence and third-order nonlinear optical effects in TiO2–CeO2 nanocomposites

We report on the linear and nonlinear optical studies of TiO₂–CeO₂ nanocomposites. It was found that the band gap of the nanocomposite can be tuned by varying Ce/Ti content. Nonlinear absorption characteristics of these samples were studied by employing open aperture Z-scan technique using an Nd:YAG laser (532 nm, 7 ns, 10 Hz). It has been observed that as the CeO₂ amount increases, band gap of the nanocomposites decreases and the reason proposed for the change in band gap is the smudging of localised states of Ce³⁺ into the forbidden energy gap, thus acting as the intermediate state. Fluorescence studies confirmed the above argument. Nonlinear investigation revealed that with increase in the CeO₂ amount, the two-photon absorption coefficient increased due to the modification of TiO₂ dipole symmetry. Suitable candidature of the nanocomposites for the fabrication of nonlinear optical devices was proved by determining the optical limiting threshold.     

Structure, magnetic and optical properties of polycrystalline Co-doped TiO2 films

Co-doped TiO₂ films were fabricated under different conditions using reactive facing-target magnetron sputtering. Co doping improves the transformation of TiO₂ from anatase phase to rutile phase. The chemical valence of doped Co in the films is +2. All the films are ferromagnetic with a Curie temperature above 340 K. The average room-temperature moment per Co of the Co-doped TiO₂ films fabricated at 1.86 Pa decreases from 0.74 μ_B at x=0.03 to 0.02 μ_B at x=0.312, and decreases from 0.54 to 0.04 μ_B as x increases from 0.026 to 0.169 for the Co-doped TiO₂ films fabricated at 0.27 Pa. The ferromagnetism originates from the oxygen vacancies created by Co²⁺ dopants at Ti⁴⁺ cations. The optical band gaps value (E_g) of the Co-doped TiO₂ films fabricated at 1.86 Pa decreases linearly from 3.35 to 2.62 eV with the increasing x from 0 to 0.312. For the Co-doped TiO₂ films fabricated at 1.86 Pa, the E_g decreases linearly from 3.26 to 2.53 eV with increasing x from 0 to 0.350.     

Investigation of phase segregation in Zn1−xMgxO systems

The present work reports on the synthesis of the Zn_1?xMg_xO (x = 0, 0.02, 0.05, 0.10, 0.15 and 0.20) samples by sol–gel method and the investigations on their structural, morphological and optical properties. X-ray diffraction (XRD) data analysis confirms the formation of pure ZnO phase below 10% Mg doping and MgO related phases appears in 10% doped sample indicating that phase segregation of MgO starts at x ≥ 0.10 samples. The phase segregation observed through XRD analysis is also supported by results from Scanning Electron Microscopy (SEM), Raman spectroscopy and photoluminescence studies. Furthermore, the enhancement in optical band gap, with Mg doping, from 3.1 ± 0.1 eV to 3.5 ± 0.1 eV has been observed through UV–Vis spectroscopic analysis. Above results have been discussed on the basis of defects level observed through Raman and photoluminscence studies.     

First-principles study of electronic structures and optical properties of Cu, Ag, and Au-doped anatase TiO2

We perform first-principles calculations to investigate the band structure, density of states, optical absorption, and the imaginary part of dielectric function of Cu, Ag, and Au-doped anatase TiO₂ in 72 atoms systems. The electronic structure results show that the Cu incorporation can lead to the enhancement of d states near the uppermost of valence band, while the Ag and Au doping cause some new electronic states in band gap of TiO₂. Meanwhile, it is found that the visible optical absorptions of Cu, Ag, and Au-doped TiO₂, are observed by analyzing the results of optical properties, which locate in the region of 400-1000 nm. The absorption band edges of Cu, Ag, and Au-doped TiO₂ shift to the long wavelength region compared with the pure TiO₂. Furthermore, according to the calculated results, we propose the optical transition mechanisms of Cu, Ag, and Au-doped TiO₂. Our results show that the visible light response of TiO₂ can be modulated by substitutional doping of Cu, Ag, and Au.     

The influence of nickel dopant on the microstructure and optical properties of SnO2 nano-powders

The microstructure and optical properties of Ni-doped SnO2 nano-powders are studied in detail. By Ni-doping, not only the grain size reduces, but also the grain shape changes from nano-rods to spherical particles. The crystallization becomes better with annealing temperature increasing. The band gap energy decreases as nickel doping level increases. The sp-d hybridization and alloying effect due to amorphous SnO2-x phase should be responsible for the band gap narrowing effect. Nickel dopant does not change the photoluminescence （PL） peak positions.