A novel approach towards high-performance composite photocatalyst of TiO2 deposited on activated carbon

TiO₂ photocatalysts deposited on activated carbon (TiO₂/AC) were prepared by dip-hydrothermal method at 180 °C using peroxotitanate as a precursor, then calcinated at 300-800 °C. The samples were characterized by X-ray diffraction, scanning electron microscopy, Raman spectroscopy and the nitrogen absorption. Their photocatalytic activity was evaluated by degradation of methyl orange (MO). The results showed that TiO₂ particles of anatase type were well deposited on the activated carbon surface. TiO₂/AC calcinated at 600 °C exhibited the best photocatalytic performance. For the comparison, the same photocatalysis experiment was carried out for two mixtures of commercial TiO₂ (Degussa P25) with AC and synthetic TiO₂ with AC. It was found that the composite catalyst TiO₂/AC was better than the two mixtures. Besides, different from fine powdered TiO₂, the granular TiO₂/AC photocatalysts could be easily separated from the bulk solution and reused; indeed, its photocatalytic ability was hardly decreased after a five-cycle for MO degradation. The kinetics of the MO degradation fitted well the Langmuir-Hinshelwood model.     

Effect of post heat treatment on microstructure and photocatalytic activities of TiO2 nanoribbons

Low-dimensional TiO₂ nanoribbons were synthesized by a simple one-step hydrothermal method. The TiO₂ nanoribbons were calcined over the temperature range 200-800 °C in order to enhance their photocatalytic properties by altering their crystal phase and increasing crystallization. Effects of hydrothermal temperature, calcinated temperature and calcination time on the formation of nanostructures have been observed and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The (BET) specific surface area of the samples which with different post treatments were determined by N₂ absorption-desorption experiment. In addition, photocatalytic activities of the nanoribbons were evaluated by photodegradation of organic dyes methyl orange under the radiation of UV light. The results reveal that the post-treatments have great effects on the microstructures and the photocatalytic activities of TiO₂ nanoribbons.     

Characterization and activity of visible-light-driven TiO2 photocatalyst codoped with lanthanum and iodine

The novel visible-light-activated La/I/TiO₂ nanocomposition photocatalyst was successfully synthesized using precipitation-dipping method, and characterized by X-ray powder diffraction (XRD), the Brunauer-Emmett-Teller (BET) method, transmission electron microscopy (TEM), thermogravimetry-differential scanning calorimetry (TG-DSC) and UV-vis diffuse reflectance spectroscopy (UV-vis DRS). The photocatalytic activity of La/I/TiO₂ was evaluated by studying photodegradation of reactive blue 19 as a probe reaction under simulated sunlight irradiation. Photocatalytic experiment results showed that the maximum specific photocatalytic activity of the La/I/TiO₂ photocatalyst appeared when the molar ratio of La/Ti was 2.0 at%, calcined at 350 °C for 2 h, due to the sample with good crystallization, high BET surface area and small crystal size. Under simulated sunlight irradiation, the degradation of reactive blue 19 aqueous solution reached 98.6% in 80 min, which showed La/I/TiO₂ photocatalyst to be much higher photocatalytic activity compared to standard Degussa P25 photocatalyst. The higher visible light activity is due to the codoping of lanthanum and iodine.     

Formation of impurity bands in iodine cation substitutionally doped TiO2 and its effects on photoresponse and photogenerated carriers

First-principles calculations based on the plane-wave pseudopotential (PWP) method were performed to investigate the electronic structure of iodine cation substitutionally doped anatase TiO₂ (referred to as I_sTiO₂). A 2×2×1 anatase supercell, in which one Ti atom was substituted by one iodine atom, was constructed for calculations. The calculated results reveal that iodine dopant contributes part of its 5s states to form one isolated impurity band in band gap and part of its 5p states to form some impurity bands at the bottom of conductive bands. The visible light response of I_sTiO₂ should be the consequence of the excitation between the two types of impurity bands (2.77 eV, about 450 nm). According to the distribution of impurity states, iodine dopant is considered to serve as a recombination center or an efficient trap for photogenerated carriers. This deduction was partly approved by water photosplitting under visible light irradiation.     

Oriented single crystalline TiO2 nano-pillar arrays directly grown on titanium substrate in tetramethylammonium hydroxide solution

Oriented single crystalline titanium dioxide (TiO₂) nano-pillar arrays were directly synthesized on the Ti plate in tetramethylammonium hydroxide (TMAOH) solution by one-pot hydrothermal method. The samples were characterized respectively by means of field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). Results showed that the TiO₂ nano-pillar with a tetrahydral bipyramidal tip grew vertically on the titanium substrate. HRTEM and Raman results confirmed that the TiO₂ nano-pillar arrays were single crystalline anatase. The controls of morphology, size, and orientation of the nano-pillar could be achieved by varying the solution concentration and hydrothermal temperature. Furthermore, the special morphology of the TiO₂ nano-pillar arrays was caused by the selectively absorption of the tetramethylammonium (TMA) through hydrogen bonds on the lattice planes parallel to (0 0 1) of anatase TiO₂. Less grain boundaries and direct electrical pathway for electron transferring were crucial for the superior photoelectrochemical properties of the single anatase TiO₂ nano-pillar arrays. This approach provides a facile in situ method to synthesize TiO₂ nano-pillar arrays with a special morphology on titanium substrate.     

Surface modification of nanocrystalline anatase with CTAB in the acidic condition and its effects on photocatalytic activity and preferential growth of TiO2

The nanocrystalline anatase TiO₂, which was synthesized by a sol-hydrothermal process in advance, has successfully modified with cetyltrimethylammonium bromide (CTAB) in the acidic condition as well as in the basic condition. On the basis of the measurements of infrared spectrum and X-ray photoelectron spectroscopy of the resulting TiO₂, together with the phase-transfer experiments, it is suggested that the modification mechanism in the acidic condition is closely related to Br⁻. Interestingly, compared with un-modified TiO₂, the modified TiO₂ exhibits high photocatalytic activity for degrading Rhodamine B (RhB) solution, especially for that modified in the acid. The enhanced photocatalytic activity of modified TiO₂ in the acid is attributed to the role that the Br⁻ can easily capture photo-induced holes and then form active Br, consequently effectively inducing photocatalytic oxidation reactions, based on the surface photovoltage responses of the resulting TiO₂. After that, a one-pot sol-hydrothermal route at the temperature as low as 80 °C is developed to directly synthesize CTAB-modified nanocrystalline TiO₂ with a little preferred growth along 〈0 0 1〉 direction, which can be easily dispersed in the organic system and possess good photocatalytic performance. This work provides a feasible strategy to further improve the photocatalytic performance of nanocrystalline anatase and to synthesize TiO₂ nanocrystals with preferential growth.     

Fabrication of titanium dioxide nanofibers via anodic oxidation

In this work, titanium dioxide (TiO₂) nanofibers were obtained from anodic oxidation process. A piece of titanium sheet was anodized in 1.0 M sodium chloride (NaCl) solution, at 30 V. The as-anodized sample was calcinated at 400 °C for one hour. Subsequently the sample was characterized using field-emission scanning electron microscopy (FE-SEM), high resolution X-ray diffraction (HR-XRD), and Raman spectroscopy. Interestingly, results showed rutile phase dominates over anatase phase, which is rarely reported. In addition, the possible chemical reactions that lead to the formation of nanofibers were proposed. It was found that the nanofibers having an average length of 3 μm, also diameter of 83 nm.     

Effect of heat treatment on morphology, crystalline structure and photocatalysis properties of TiO2 nanotubes on Ti substrate and freestanding membrane

Highly ordered titanium oxide (TiO₂) nanotubes were prepared by electrolytic anodization of titanium electrodes. Morphological evolution and phase transformations of TiO₂ nanotubes on a Ti substrate and that of freestanding TiO₂ membranes during the calcinations process were studied by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction microscopy. The detailed results and mechanisms on the morphology and crystalline structure were presented. Our results show that a compact layer exists between the tubular layer and Ti substrate at 600 °C, and the length of the nanotubes shortens dramatically at 750 °C. The freestanding membranes have many particles on their tubes during calcinations from 450 to 900 °C. The TiO₂ nanotubes on the Ti substrate transform to rutile crystals at 600 °C, while the freestanding TiO₂ membranes retain an anatase crystal with increasing temperature to 800 °C. The photocatalytic activity of TiO₂ nanotubes on a Ti substrate annealed at different temperatures was investigated by the degradation of methyl orange in aqueous solution under UV light irradiation. Due to the anatase crystals in the tubular layer and rutile crystals in the compact layer, TiO₂ nanotubes annealed at 450 °C with pure anatase crystals have a better photocatalytic activity than those annealed at 600 °C or 750 °C.     

Photooxidation of different organic dyes (RB, MO, TB, and BG) using Fe(III)-doped TiO2 nanophotocatalyst prepared by novel chemical method

The nano-structured Fe(III)-doped TiO₂ photocatalysts with anatase phase have been developed for the oxidation of non-biodegradable different organic dyes like methyl orange (MO), rhodamine B (RB), thymol blue (TB) and bromocresol green (BG) using UV-Hg-lamp. The different compositions of Fe_xTi_1−xO₂ (x = 0.005, 0.01, 0.05, and 0.1) nanocatalysts synthesized by chemical method (CM), have been characterized by X-ray diffraction (XRD), UV-vis diffuse reflectance spectra, specific surface area (BET), transmission electronic microscopy (TEM) analysis, XPS, ESR and zeta potential. From XRD analysis, the results indicate that all the compositions of Fe(III) doped in TiO₂ catalysts gives only anatase phase not rutile phase. For complete degradation of all the solutions of the dyes (MO, RB, TB, and BG), the composition with x = 0.005 is more photoactive compared all other compositions of Fe_xTi_1−xO₂, and degussa P25. The decolorization rate of different dyes decreases as Fe(III) concentration in TiO₂ increases. The energy band gap of Fe(III)-doped TiO₂ is found to be 2.38 eV. The oxidation state of iron has been found to be 3+ from XPS and ESR show that Fe³⁺ is in low spin state.     

Stabilization of the anatase phase in TiO2(Fe, PEG) nanostructured coatings

Multilayered TiO₂(Fe³⁺, PEG) films were deposited on glass and SiO₂/glass substrates by sol-gel dipping method. The influence of Fe³⁺ and PEG(polyethylene glycol) concentrations, the number of layers, the thermal treatment time and the temperature on the optical and microstructural properties of the TiO₂ films were studied.As-deposited TiO₂(Fe³⁺, PEG) films were very porous, but after the thermal treatment at 500 °C, the PEG decomposed and burned out to porosity decreasing. Homogeneous nanostructured films were obtained, where the amorphous and the anatase phases coexist. XRD analysis showed that no rutile phase is observed in the films deposited on SiO₂/glass as compared with those deposited directly on glass and that the presence of the anatase phase in the films without PEG is more evident in the three-layers film. The XRD intensity of the main peak of anatase from 25° decreases with the increase of PEG concentration.The optical gap of the TiO₂(Fe³⁺, PEG) films is found in 2.52-2.56 eV range and does not essentially depend on the PEG content.     

Optical properties of ITO/TiO2 single and double layer thin films deposited by RPLAD

Indium tin oxide (ITO) and titanium dioxide (TiO₂) single layer and double layer ITO/TiO₂ films were prepared using reactive pulsed laser ablation deposition (RPLAD) with an ArF excimer laser for applications in dye-sensitized solar cells (DSSCs). The films were deposited on SiO₂ substrates either at room temperatures (RT) or heated to 200-400 °C. Under optimized conditions, transmission of ITO films in the visible (vis) range was above 89% for films produced at RT and 93% for the ones deposited at higher temperatures. Increasing the substrate temperature from RT to 400 °C enhances the transmission of TiO₂ films in the vis-NIR from about 70% to 92%. High transmission (≈90%) was observed for the double layer ITO/TiO₂ with a transmission cut-off above 900 nm. From the transmission data, the energies gaps (E_g), as well as the refractive indexes (n) for the films were estimated. n ≈ 2.03 and 2.04, respectively for ITO films and TiO₂ film deposited at 400 °C in the visible region. Post-annealing of the TiO₂ films for 3 h at 300 and 500 °C was performed to enhance n. The refractive index of the TiO₂ films increases with the post-annealing temperature. The direct band gap is 3.6, 3.74 and 3.82 eV for ITO films deposited at RT, 200, and 400 °C, respectively. The TiO₂ films present a direct band gap of 3.51 and 3.37 eV for as deposited TiO₂ films and when annealed at 400 °C, respectively. There is a shift of about 0.1 eV between ITO and ITO/TiO₂ films deposited at 200 °C. The shift decreases by half when the TiO₂ film was deposited at 400 °C. Post-annealing was also performed on double layer ITO/TiO₂.     

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.     

Synthesis of anatase TiO2 nanowires by modifying TiO2 nanoparticles using the microwave heating method

Anatase TiO₂ nanowires with a diameter of 5-10 nm and length of 500 nm to 2 μm have been successfully synthesized by modifying TiO₂ nanoparticles (P25) using the microwave heating method. The microwave power, reaction pressure, and reaction time for the synthesis of TiO₂ nanowires were 500 W, 0.5-3.0 MPa (corresponding to a temperature range of 175-260), and 40-70 min, respectively. X-ray diffraction (XRD), field-emission scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and the BET techniques were used to investigate the phase structures, morphologies, and specific surface areas of the TiO₂ nanowires. The effects of reaction time, pressure, and different post-treatment processes on the microstructures of TiO₂ nanowires were discussed. It has been shown that the microwave heating method is efficient in transforming TiO₂ nanoparticles to anatase TiO₂ nanowires.     

Low temperature synthesis of iodine-doped TiO2 nanocrystallites with enhanced visible-induced photocatalytic activity

Iodine-doped TiO₂ nanocrystallites (denoted as I-TNCs) were prepared via a newly developed triblock copolymer-mediated sol-gel method at a temperature of 393 K. I-doping, crystallization and the formation of porous structure have been simultaneously achieved. The obtained particles were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and UV-vis spectrophotometer. The results indicated that the as-prepared I-TNCs possessed a diameter of ca. 5 nm with anatase crystalline structure and a specific surface area of over 200 m² g⁻¹. The presence of iodine expanded the photoresponse in visible light range, and led to enrich in surface hydroxyl group on the TiO₂ surface. Compared with the commercial photocatalyst P25, the I-TNCs significantly enhanced the photocatalytic efficiency in the degradation of rhodamine B and 2,4-dichlorophenol, and the I-TNCs with 2.5 mol% doping ratio exhibited the best photocatalytic activity.     

Point defects and orientation-dependent transport of matter and charge in iron-containing olivines

The variation of the oxygen content, x_O, of synthetic fayalite (Fe₂SiO₄) single crystals was investigated thermogravimetrically at 1130 °C as a function of the oxygen activity, a_O2 (= P_O2/P_O2° ≈ f_O2/f_O2° with P_O2° ≈ f_O2° = 1 bar ≈ 1 atm). It was found that x_O varies less in fayalite single crystals than in polycrystalline Fe₂SiO₄ studied earlier. The majority defects are most likely cation vacancies, (V_Me²⁺)″, ferric ions on M-sites, (Fe³⁺_Me²⁺), and ferric ions on Si-sites, (Fe³⁺_Si⁴⁺)′. Furthermore, the diffusion of iron in synthetic olivine single crystals ((Fe_xMg_1 − x)₂SiO₄) was studied at 1130 °C as a function of orientation, oxygen activity, and cationic composition. The observed oxygen activity dependencies suggest that cations move via different types of cation vacancies, most likely isolated vacancies, (V_Fe²⁺)″, and possibly neutral associates, {2(Fe³⁺_Me²⁺) ⋅ (V_Me²⁺)^′ ? ′}^x, the latter being minority defects. In addition, the electrical conductivity, σ, of fayalite single crystals was investigated as a function of orientation and oxygen activity within the stability field of fayalite at 1130 °C. The observed oxygen activity dependencies are compatible with (V_Me²⁺)^′ ? ′, (Fe³⁺_Me²⁺), and (Fe³⁺_Si⁴⁺)′ being the majority point defects at high a_O2 and with h^⋅ and e′ as the majority defects at low a_O2. The electrical conduction in fayalite is governed by contributions of electrons and holes. This extended point defect model for fayalite is also compatible with data for the variation of the oxygen content and for the iron tracer diffusion.     

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.     

Photocatalytic behavior of heavy La-doped TiO2 films deposited by pulsed laser deposition using non-sintered target

We have investigated the control of photocatalytic behavior under deposited conditions of non-sintered target of different molar ratios with TiO₂ and La₂O₃ from 1:0 to 1:2 for heavily La doping, and post-annealing temperature from 600 °C to 1000 °C for crystallizing by pulsed laser deposition. We have successfully crystallized heavily La-doped TiO₂ films with post-annealing temperature over 800 °C and with molar ratio of TiO₂:La₂O₃ over 1:1 on a quartz substrate. Heavily La-doped TiO₂ films are observed the decomposition of methylene blue and a water-splitting reaction in photocatalytic behavior under Xe light irradiation. When stoichiometric La-doped TiO₂ (TiO₂:La₂O₃ = 1: 1) is synthesized with heat-treatment at 900 °C, the best results are obtained under photocatalytic behavior and pure La₂Ti₂O₇ crystalline were obtained.     

Synthesis and characterization of TiO2/Fe2O3 core-shell nanocomposition film and their photoelectrochemical property

TiO₂/Fe₂O₃ core-shell nanocomposition film has been fabricated via two-step method. TiO₂ nanorod arrays are synthesized by a facile hydrothermal method, and followed by Fe₂O₃ nanoparticles deposited on TiO₂ nanorod arrays through an ordinary chemical bath deposition. The phase structures, morphologies, particle size, chemical compositions of the composites have been characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and ultraviolet-visible (UV-vis) spectrophotometer. The results confirm that Fe₂O₃ nanoparticles of mean size ca. 10 nm coated on the surface of TiO2 NRs. After depositing Fe₂O₃, UV-vis absorption property is induces the shift to the visible-light range, the annealing temperature of 600 °C is the best condition for UV-vis absorption property of TiO₂/Fe₂O₃ nanocomposite film, and increasing Fe content, optical activity are enhanced one by one. The photoelectrochemical (PEC) performances of the as-prepared composite nanorods are determined by measuring the photo-generated currents under illumination of UV-vis light. The TiO₂ NRs modified by Fe₂O₃ show the photocurrent value of 1.36 mA/cm² at 0 V vs Ag/AgCl, which is higher than those of unmodified TiO₂ NRs.     

Theoretical study of CO adsorption on the illuminated TiO2 (0 0 1) surface

A quantum modeling of the CO adsorption on illuminated anatase TiO₂ (0 0 1) is presented. The calculated adsorption energy and geometries of illuminated case are compared with the ground state case. The calculations were achieved by using DFT formalism and the BH and HLYP. Upon photoexcitation, an electron-hole pair is generated. Comparing of natural population in the ground state and the exited state, shows that an electron is trapped in a Ti⁴⁺ ion and a hole is localized in an oxygen ion. The photoelectron helps generation of a CO₂ molecule on the TiO₂ surface. As shown by optimization of these systems, the CO molecule adsorbed vertically on the TiO₂ (0 0 1) surface in the ground state case while the CO molecule made an angle of 134.3° to this surface at the excited state case. Based on the here used model the obtained adsorption energy was 0.36 eV which is in excellent agreement with the reported experimental value. In the present work the C-O stretch IR frequencies are calculated which are 1366.53 and 1423.16 cm⁻¹. These results are in good agreement with the earlier reported works for the surface carbonaceous compounds, and oxygenated carbon species.     

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.