TiO2‐Decorated Graphite Nanoplatelet Nanocomposites for High‐Temperature Sensor Applications

Temperature and/or composition mapping inside high temperature energy conversion and storage devices are challenging, yet of critical importance to improve the material design for optimum performance. Here, the great potential of TiO₂ nanoparticle (NP)‐decorated graphite nanoplatelet (GNP) nanocomposites as high temperature thermal senors or gas sensors is reported. Effects of the GNP substrate on phonon confinement in Raman spectrum, grain growth, and phase stability of anatase TiO₂ NPs at high temperatures are systematically studied. Thermally sensitive Raman signatures, indicating the ultrafast grain growth of TiO₂ NPs in response to short thermal shock treatments (0.1–25 s) at high temperatures, are exploited for high temperature thermal sensing applications. A very high accuracy of nearly 98% in temperature measurements is demonstrated for a given short‐time thermal exposure. Thermal stability of anatase TiO₂ NPs against transformation into the rutile phase in TiO₂‐GNP nancomposites is substantially increased by controlling the surface area of the substrate, which would significantly improve the performance of TiO₂‐based high temperature gas sensors.     

Effect of SBA-15 microporosity on the inserted TiO2 crystal size determined by Raman spectroscopy

The effect of SBA-15 microporosity on the crystal size of TiO₂ was investigated employing SBA-15 materials with high (SBA-15-HM) and low (SBA-15-LM) microporosities (14.2% and 4.7% of microporous volume, respectively). TiO₂ phase was incorporated inside SBA-15 using internal hydrolysis method over a wide range of loadings (7–63 wt%). At all loadings, TiO₂ inside SBA-15 pores was in the form of anatase nanocrystals as found in characteristic Raman spectra. The crystal size of TiO₂ anatase phase was determined by Raman spectroscopy using a correlation between Raman peak position and peak width and TiO₂ crystal size. The correlation was established based on the set of unsupported TiO₂ samples with the crystals size in the range 5–120 nm (BET and XRD). Using this correlation, it was found that the crystal size of TiO₂ inside SBA-15 with high microporosity was lower than inside SBA-15 with low microporosity. This is a direct proof of the effect of wall microporosity on the dispersion of TiO₂ inside SBA-15. Due to the higher TiO₂ dispersion, TiO₂/SBA-15-HM adsorbed more vanadia than TiO₂/SBA-15-LM at the same TiO₂ loadings. As a result, V₂O₅/TiO₂/SBA-15-HM displayed higher activity than V₂O₅/TiO₂/SBA-15-LM in NO SCR with ammonia.     

An efficient visible light photocatalyst prepared from TiO<Subscript>2</Subscript> and polyvinyl chloride

An efficient visible light photocatalyst has been prepared from TiO₂ nanoparticles and a partly conjugated polymer derived from polyvinyl chloride (PVC). It was characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), UV–visible diffuse reflectance spectroscopy (UV–Vis DRS), Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The visible light photocatalytic activity of the as-prepared photocatalyst was evaluated by the photocatalytic degradation of Rhodamine B (RhB) under visible light irradiation. The XPS, FT-IR, and Raman spectra show that the partly conjugated polymer derived from PVC exists on the surface of the TiO₂ nanoparticles. The UV–Vis DRS, XRD, and TEM results reveal that the modification of the partly conjugated polymer can obviously improve the absorbance of the TiO₂ nanoparticles in the range of visible light and hardly affect their size and crystallinity. The visible light photocatalytic activity of the as-prepared TiO₂ nanocomposites is higher than that of commercial TiO₂ (Degussa P25) and comparable with those of visible light photocatalysts reported in the literature. Their visible light photocatalytic stability is also good. The reasons for their excellent visible light photocatalytic activity and the major factors affecting their photocatalytic activity are discussed.     

Preparation N-F-codoped TiO2 nanorod array by liquid phase deposition as visible light photocatalyst

An efficient method for the preparation of N-F-codoped visible light active TiO₂ nanorod arrays is reported. In the process, simultaneous nitrogen and fluorine doped TiO₂ nanorod arrays on the glass substrates were achieved by liquid phase deposition method using ZnO nanorod arrays as templates with different calcination temperature. The as-prepared samples were characterized by Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV-vis absorption spectra measurements. It was found that calcination temperature is an important factor influencing the microstructure and the amount of N and F in TiO₂ nanorod arrays samples. The visible light photocatalytic properties were investigated using methylene blue (MB) dye as a model system. The results showed that N-F-codoped TiO₂ nanorod arrays sample calcined at 450 °C demonstrated the best visible light activity in all samples, much higher than that of TiO₂ nanoparticles and P25 particles films.     

Undoped visible-light-sensitive titania photocatalyst

Colorful rutile TiO₂ was prepared by heating Ti₂O₃ at 550–900 °C to develop novel visible-light-sensitive and eco-friendly photocatalysts for environmental remediation under visible-light irradiation. The colors of the prepared samples, which ranged from grayish green to yellowish off-white via yellow differed from the reported colors of reduced TiO₂, such as blue and black. The TiO₂ prepared in this study was characterized by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and UV–Visible spectroscopy. These measurements showed that the TiO₂ contained Ti³⁺-interstitial sites. The TiO₂ was sensitive to visible light, and calculation of the band diagram demonstrated that this visible-light absorption is caused mainly by formation of Ti³⁺-interstitial sites in rutile TiO₂. Among the prepared samples, the TiO₂ prepared by heating Ti₂O₃ at 700 °C shows the highest photocatalytic activity under visible-light irradiation. In addition, the sample was further and mildly ground using a bead-milling machine. The ground sample possessed higher surface area and better photocatalytic activity.     

Synthesis and characterization of CdS doped TiO2 nanocrystalline powder: A spectroscopic study

This report aimed to study the effect of CdS doping in TiO₂ on the phase transformation of TiO₂ from anatase to rutile using X-ray diffraction (XRD) and Raman spectroscopy. CdS-doped TiO₂ nanocomposites have been prepared and characterized using Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). We have observed that contrary to bare TiO₂, phase transformation of TiO₂ from anatase to rutile is hindered when doped with CdS at high temperature. Raman spectroscopy is found to be more sensitive for detection of the surface of TiO₂ as compared to XRD.     

Raman spectral analysis of TiO₂ thin films doped with rare-earth samarium

TiO₂ thin films doped with rare-earth samarium were prepared on a quartz plate by the sol-gel/spin-coating technique. The samples were annealed at 700?°C to 1100?°C, and the Raman spectra of the samples were obtained. Analyses of Raman spectra show that samarium doping can inhibit the anatase-rutile phase transition. Samarium doping can refine grains of TiO₂ thin films and increase the internal stress, thereby preventing lattice vibration. Nanocrystalline TiO₂ thin films obviously show the phonon confinement effect, i.e., the blueshift of characteristic Raman peak and full width at half-height increase, and the peak shapes asymmetrically broaden with a decrease in the grain sizes of the samples.     

Gel-hydrothermal synthesis of carbon and boron co-doped TiO2 and evaluating its photocatalytic activity

Carbon and boron co-doped TiO₂ photocatalysts were prepared firstly by the gel-hydrothermal method, that is, synthesized through sol-gel process followed by hydrothermal in the glucose solution. The prepared photocatalysts were characterized by XRD, Raman spectra, TEM, N₂ physical adsorption, XPS, and UV-vis absorption spectra. It was found that the co-doped TiO₂ has a larger BET surface areas and a narrower band gap than undoped TiO₂. The experimental results show that the coke carbon generated on the carbon doped TiO₂ surface act as a photosensitizer and has the photosensitization effect under the visible light. Except for carbon sensitization effect, the boron and carbon co-doped TiO₂ has synergistic effect which is responsible for effective photo-degradation of 2,4-dichlorophenol in the visible light irradiation.     

Removal of fluoride from aqueous solution by polypyrrole/Fe3O4 magnetic nanocomposite

Highly efficient visible light TiO₂ photocatalyst was prepared by the sol-gel method at lower temperature (≤300 °C), and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and differential scanning calorimetry-thermogravimetric analysis (DSC-TGA). The effects of the heat treatment temperature and time of the as-prepared TiO₂ on its visible light photocatalytic activity were investigated by monitoring the degradation of methyl orange solution under visible light irradiation (wavelength ≥ 400 nm). Results show that the as-prepared TiO₂ nanoparticles possess an anatase phase and mesoporous structure with carbon self-doping and visible photosensitive organic groups. The visible light photocatalytic activity of the as-prepared TiO₂ is greatly higher than those of the commercial TiO₂ (P-25) and other visible photocatalysts reported in literature (such as PPy/TiO₂, P3HT/TiO₂, PANI/TiO₂, N-TiO₂ and Fe³⁺-TiO₂) and its photocatalytic stability is excellent. The reasons for improving the visible light photocatalytic activity of the as-prepared TiO₂ can be explained by carbon self-doping and a large amount of visible photosensitive groups existing in the as-prepared TiO₂. The apparent optical thickness (τ_app), local volumetric rate of photo absorption (LVRPA) and kinetic constant (k_T) of the photodegradation system were calculated.     

Photocatalytic mechanism of high-activity anatase TiO<Subscript>2</Subscript> with exposed (001) facets from molecular-atomic scale: HRTEM and Raman studies

Anatase TiO₂ with a variant percentage of exposed (001) facets was prepared under hydrothermal processes by adjusting the volume of HF, and the photocatalytic mechanism was studied from atomic-molecular scale by HRTEM and Raman spectroscopy. It was revealed that: 1) From HRTEM observations, the surface of original TiO₂ with exposed (001) facets was clean without impurity, and the crystal lattice was clear and completed; however, when mixed with methylene blue (MB) solution, there were many 1 nm molecular absorbed at the surface of TiO₂; after the photocatalytic experiment, MB molecules disappeared and the TiO₂ lattice image became fuzzy. 2) The broken path of the MB chemical bond was obtained by Raman spectroscopy, i.e., after the irradiation of the light, the vibrational mode of C-N-C disappeared due to the chemical bond breakage, and the groups containing C-N bond and carbon rings were gradually decomposed. Accordingly, we propose that the driving force for breaking the chemical bond and the disappearance of groups is from the surface lattice distortion of TiO₂ during photocatalyzation.     

Investigation on the structure of TiO2 films sputtered on alloy substrates

Titanium dioxide (TiO₂) films have been successfully deposited on metal alloy substrates by radio-frequency magnetron reactive sputtering in an Ar+O₂ gas mixture. The effects of gas total pressure on the structure and phase transition of TiO₂ films were studied by X-ray diffraction and Raman spectra. It is suggested that the film structure changes from rutile to anatase while work gas total pressure changes from 0.2 to 2 Pa. The structure of TiO₂ films is not affected by the film thickness.     

Oxidative degradation of phthalic acid using TiO<Subscript>2</Subscript> photocatalyst

TiO₂ is one of the well-known multipurpose material for different applications such as gas sensors, catalysis, solar cells, optoelectronic devices, etc. TiO₂ is widely used as photocatalyst due to interest optical and electrical properties. In the present work, TiO₂ photocatalyst is used to study the degradation of phthalic acid under ultraviolet light illumination. The prepared TiO₂ photocatalyst is characterized by different characterization techniques such as X-ray diffraction, Raman spectroscopy, scanning electron microscopy. The end result shows that the degradation percentage of phthalic acid using TiO₂ photocatalyst has reached 41.93% after 320 min. under ultraviolet light illumination. The amount of mineralization of phthalic acid is confirmed by chemical oxygen demand measurements.     

Innovative one-step immobilization of TiO2 on polymer material by the sol–gel method under IL/MW conditions

An innovative one-step immobilization of titanium dioxide (TiO₂) nano-particles on organic polymer (PMMA) substrate at ambient condition is reported in this article. This immobilization can be achieved by the sol–gel method under ionic liquid/microwave heating conditions. In this method, a sol–gel reaction is conducted at specific sites of the polymer surface. These sites are the tiny cavities of the rough surface resulting from the softening and swelling effect of an alcohol, such as isopropyl alcohol, on the polymer surface under microwave irradiation. The roughness of the polymer surface is an important factor for the effective immobilization. In addition, ionic liquid can induce low temperature surface anatase crystallization of immobilized titanium dioxide in a short time. From the field emission scanning electron microscopy and energy dispersive spectroscopy observation, the TiO₂ particles could be effectively immobilized on the PMMA substrate. Raman spectra analysis data showed that the immobilized TiO₂ was anatase phase. The experimental data also shows that the immobilized TiO₂ prepared by this novel method has good immobilization stability and photocatalytic water treatment performance.     

Visible-light-responsive Ag–Si codoped anatase TiO2 photocatalyst with enhanced thermal stability

To utilize visible light more efficiently and enhance the photocatalytic performance of TiO₂, Ag–Si/TiO₂ photocatalyst was synthesized via a two-step method. The obtained materials were characterized by XRD, Raman, TEM, HRTEM, BET, TG–DTA, XPS, ICP as well as UV–vis DRS. All photocatalyst materials held an anatase phase confirmed by XRD, Raman and HRTEM. The Ag–Si/TiO₂ photocatalysts possessed high thermal stability and the phase transformation was retarded to about 900 °C revealed by XRD and TG–DTA. The Ag–Si/TiO₂ particles synthesized via the nonaqueous method were highly monodispersed and the particles size became smaller compared to the un-doped TiO₂, resulting in the enlargement of surface area. In addition, UV–vis light absorption shifted to visible region after Ag doping. XPS results demonstrated that Si weaved into the matrix of TiO₂ and enriched in the surface layer, while Ag dispersed on the surface of TiO₂ particles. The Ag dopant suppressed the recombination of photogenerated electrons and holes, Si enlarged the surface of photocatalysts. Silver and silicon co-doping improved the visible photocatalytic activity, which was evaluated by Rhodamine B (RhB) degradation. The photocatalytic activity of the obtained Ag–Si/TiO₂ sample was much more higher than those of pure TiO₂ and Ag/TiO₂, reaching the maximum at the Ag and Si content of 0.5 mol% and 20.0 mol%, respectively. The improved visible photocatalytic activity may be attributed to the synergetic effects of codoping by silver and silicon.     

Graphene Wrapped TiO2 Based Catalysts with Enhanced Photocatalytic Activity

Reduced graphene oxide (RGO) wrapped titanium dioxide nanocrystals (TiO₂ NCs@RGO) with oxygen vacancies (Vo) and Ti³⁺ defects have been synthesized by electrostatically wrapping GO around TiO₂ NCs followed by thermal annealing at 400 °C. Transmission electron microscope observations have shown that TiO₂ NCs@RGO has a unique crystalline core/crystalline shell structure, which is different from the original amorphous TiO₂ covered TiO₂ NCs. Raman spectroscopy, X‐ray photoelectron spectroscopy, and electron paramagnetic resonance have demonstrated that Vo‐Ti³⁺ species are more readily formed in TiO₂ NCs@RGO than in TiO₂ NCs. As a result, TiO₂ NCs@RGO exhibits enhanced optical absorption in a wide wavelength range from visible light to near IR and red‐shifted absorption edge. In the photocatalytic degradation reaction of methyl orange, the photodegradation rate constant for TiO₂ NCs@RGO is 2.4 times higher than that of TiO₂ NCs. The enhanced photocatalytic performance can be attributed to the improved charge separation at the interface of TiO₂ NCs and RGO layer and the enhanced optical absorption in visible light region due to the donor levels of the defects such as Vo‐Ti³⁺ species. This work establishes a new method for preparing Vo defect contained TiO₂ catalysts.     

Electrochromic properties of TiO2 anatase thin films prepared by a dipping sol-gel method

Electrochromic TiO₂ anatase thin films on ITO were prepared by the sol-gel dipping method using a solution of titanium tetraisopropoxide, diethanolamine and ethanol. The films were transparent in the visible range and can be colored in a solution of LiClO₄ in propylene carbonate. The transmittances of the colored films were found to be strongly dependent on the Li⁺ inserted charge. Combining the experimental data obtained from in situ Raman and in situ transmittance spectra with the data from chronoamperometic measurements, it was demonstrated that the fully colorated state of the TiO₂ anatase films is Li_0.5TiO₂ with a crystalline structure of Imma space group symmetry. In the Raman spectra this coloration state exhibits five characteristic bands at 176, 224, 316, 531 and 629 cm⁻¹.     

Effect of TiCl4 concentration on the structure of its aqueous solutions and on the properties and photoactivity of derived nanocrystalline titania

The influence of the concentration of aqueous TiCl₄ solution on the phase formation, morphology and particle size of the titanium dioxide hydrolysis product was investigated by XRD and TEM. Significant features, observed in the Raman spectra of the TiCl₄ solutions with a concentration >3 M, demonstrated that the TiCl₄ had hydrolysed. As the formal concentration of TiCl₄ decreased from 4.98 to 1 M, the Raman spectra changed qualitatively. Despite the changes in the Raman spectra of these precursor solutions, the TiO₂ product was mainly rutile in all cases. However, at low TiCl₄ concentrations small amounts of anatase were also observed. Electron microscopy suggested that the anatase particles were significantly smaller than the rutile and also indicated increasing aggregation of the product from the more dilute TiCl₄ solutions. The optical properties and photoactivities of the TiO₂ powders prepared at different concentrations were also investigated. The powder synthesized from 5 M TiCl₄ showed the highest UV extinction. The photoactivity of the product, determined by the photocatalytic oxidation of propan-2-ol (isopropanol) to propanone (acetone), was not significantly modified by changes in the concentration of the starting TiCl₄. The possibility that the relatively low area of most rutiles contributes to the reported photocatalytic activity of rutile being lower than that of anatase is discussed.     

Electrochromic investigation of sol-gel-derived thin films of TiO2-V2O5

Thin films of TiO₂ and TiO₂-V₂O₅ were obtained by dip-coating sol-gel technique. Sols were prepared from titanium ethoxide and inorganic V₂O₅ sol received by dissolution of vanadium pentoxide in hydrogen peroxide. Sol-gel TiO₂ and TiO₂-V₂O₅ films are deposited on conductive glass substrates. TiO₂ and TiO₂-V₂O₅ systems were characterized by FTIR and Raman spectroscopy. Optical transmittance measurements were carried out. Electrochromic characterization was recorded by cyclic voltammetry using three-electrode arrangement. All samples demonstrated electrochromic effect.     

In situ investigation of chemical reactions between BaCO3 and anatase or rutile TiO2

The most popular method of preparation of BaTiO₃, which is one of the most widely used ferroelectric materials for multi-layered ceramic capacitors, is the solid-state reaction process between powdered barium carbonate (BaCO₃) and titanium dioxide (TiO₂) at high temperature. The influence of the different structural forms of TiO₂ used (i.e. rutile or anatase) on the reaction process and on the crystallinity of synthesized BaTiO₃ is only known on empirical grounds. The solid-state reaction between BaCO₃ and either TiO₂ anatase or TiO₂ rutile was investigated by in situ by X-ray diffraction and micro Raman scattering measurements. The formation of both barium oxycarbonate BaO_x(CO₃)_1−x and of a very small amount of Ba₂TiO₄ were detected in the samples as intermediate phase before the formation of BaTiO₃. The Raman spectra of the final product obtained in each case is essentially the tetragonal BaTiO₃ containing small amounts of non-reacting BaCO₃ and of hexagonal barium titanate. The tetragonality of the final product was found to be slightly better for BaTiO₃ synthesized from rutile and BaCO₃ than from anatase and BaCO₃, whereas the average particle sizes are essentially the same for both products.     

Fabrication of TiO<Subscript>2</Subscript>/ZnO composite nanofibers with enhanced photocatalytic activity

TiO₂/ZnO composite nanofibers have been successfully prepared by electrospinning technique. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, Raman spectrum, X-ray photoelectron spectroscopy and UV–Vis diffuse reflectance spectroscopy, were used to characterize the as-synthesized nanofibers. The photocatalytic studies revealed that the TiO₂/ZnO nanofibers exhibited enhanced photocatalytic efficiency of photodegradation. Additionally, the recycling experiment of TiO₂/ZnO nanofibers had been done, demonstrating that TiO₂/ZnO nanofibers have high efficiency and stability.