Effects of annealing temperature on the photocatalytic activity of N-doped TiO2 thin films

The effects of annealing temperature on the photocatalytic activity of nitrogen-doped (N-doped) titanium oxide (TiO₂) thin films deposited on soda-lime-silica slide glass by radio frequency (RF) magnetron sputtering have been studied. Glancing incident X-ray diffraction (GIAXRD), Raman spectrum, scanning electron microscopy (SEM), atomic force microscopy (AFM) and UV-vis spectra were utilized to characterize the N-doped TiO₂ thin films with and without annealing treatment. GIAXRD and Raman results show as-deposited N-doped TiO₂ thin films to be nearly amorphous and that the rutile and anatase phases coexisted when the N-doped TiO₂ thin films were annealed at 623 and 823 K for 1 h, respectively. SEM microstructure shows uniformly close packed and nearly round particles with a size of about 10 nm which are on the slide glass surface for TiO₂ thin films annealed at 623 K for 1 h. AFM image shows the lowest surface roughness for the N-doped TiO₂ thin films annealed at 623 K for 1 h. The N-doped TiO₂ thin films annealed at 623 K for 1 h exhibit the best photocatalytic activity, with a rate constant (k_a) of about 0.0034 h⁻¹.     

Aerosol assisted chemical vapour deposited (AACVD) of TiO2 thin film as compact layer for dye-sensitised solar cell

Compact TiO₂ has been introduced onto the surface of an indium tin oxide glass slide (ITO), using an aerosol-assisted chemical vapour deposition method. This serves as a blocking layer for a dye-sensitised solar cell (DSSC). The thickness of the compact TiO₂ could be controlled by deposition time. X-ray diffraction and Raman spectroscopy analyses reveal that the compact TiO₂ is made up of mixed anatase and rutile phases. The field emission scanning electron microscopy image displays a pyramidal morphology of the compact TiO₂. A layer of P25 paste was then smeared onto the compact TiO₂-modified ITO, using the doctor's blade method. A post-treatment procedure was applied to remove the contaminants from the prepared hybrid film, by immersing in a hydrochloric acid solution. The photoelectrochemical measurements and J–V characterisation of the hybrid film show an approximately fourfold increase in photocurrent density generation (114.22 µA/cm²), and approximately 25% enhancement of DSSC conversion efficiency (4.63%), compared to the acid-treated P25 paste alone (3.68%).     

Nafion-TiO2 composite DMFC membranes: physico-chemical properties of the filler versus electrochemical performance

TiO₂ nanometric powders were prepared via a sol-gel procedure and calcined at various temperatures to obtain different surface and bulk properties. The calcined powders were used as fillers in composite Nafion membranes for application in high temperature direct methanol fuel cells (DMFCs). The powder physico-chemical properties were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and pH measurements. The observed characteristics were correlated to the DMFC electrochemical behaviour. Analysis of the high temperature conductivity and DMFC performance reveals a significant influence of the surface characteristics of the ceramic oxide, such as oxygen functional groups and surface area, on the membrane electrochemical behaviour. A maximum DMFC power density of 350 mW cm⁻² was achieved under oxygen feed at 145 °C in a pressurized DMFC (2.5 bar, anode and cathode) equipped with TiO₂ nano-particles based composite membranes.     

Improved performance of dense TiO2/CdSe coupled thin films by low temperature process

Dense TiO₂ and TiO₂/CdSe coupled nanocrystalline thin films were synthesized onto ITO coated glass substrate by chemical route at relatively low temperature (≤100 °C). TiO₂ films were nanocrystalline and crystallinity disappears after CdSe deposition as evidenced by X-ray powder diffraction. Surface morphology and physical appearance of films were studied from SEM and actual photo-images, reveals dense nature of TiO₂ (10-12 nm spherical grains, faint violet) and CdSe (80-90 nm spherical grains, deep brown), respectively. Presence of two absorption edges in UV spectra implies existence of separate phases rather than composite formation. TiO₂ film was found to have higher water contact angle (71°) than TiO₂/CdSe (61°) and CdSe (56°). I-V and stability tests of photo-electrochemical cells were performed with TiO₂ and TiO₂/CdSe film electrodes (under light of illumination intensity 80 mW/cm²) in lithium iodide as an electrolyte using two-electrode system.     

Hydrothermal synthesis of S-doped TiO2 nanoparticles and their photocatalytic ability for degradation of methyl orange

A simple synthesis route to nanocrystalline S-doped TiO₂ photocatalysts by a hydrothermal method at 180 °C was developed and the photocatalytic activity of the obtained powders for the degradation of methyl orange was studied. The products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The phase composition (anatase/rutile ratio) and the photocatalytic activity of the final materials were found to be markedly influenced by the amount of the incorporated sulphur. On increasing the S-dopant amount, the anatase/rutile ratio and the photocatalytic activity of the as-prepared powders increased.     

Anatase TiO2 spheres with high surface area and mesoporous structure via a hydrothermal process for dye-sensitized solar cells

Spherical pure anatase TiO₂ spheres with a mesoporous structure and high surface area of up to 116.5 m² g⁻¹ were prepared by a simple urea-assisted hydrothermal process and investigated as dye-sensitized solar-cell electrodes. Although the particle diameters of the prepared TiO₂ spheres ranged from 0.4 to 1.3 μm, due to the high specific surface area, mesoporous TiO₂ sphere electrode was obtained with enhanced light harvesting and a larger amount of dye loading. An overall light conversion efficiency of 7.54% under illumination of simulated AM 1.5G solar light (100 mW cm⁻²) was achieved using the mesoporous TiO₂ spheres electrode, which was significantly higher than a commercial Degussa P25 TiO₂ nanocrystals electrode (5.69%).     

Photocatalytic degradation of methylene blue with TiO2 nanoparticles prepared by a thermal decomposition process

The generation of TiO₂ nanoparticles by the thermal decomposition of titanium tetraisopropoxide (TTIP) was carried out experimentally using a tubular electric furnace at various synthesis temperatures (700-1300 °C) and TTIP heating temperatures (80-110 °C). The photocatalytic activity of the resulting TiO₂ nanoparticles was examined by measuring the rate of methylene blue decomposition. The TiO₂ nanoparticles were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) measurements and transmission electron microscopy (TEM). The crystallite size and crystallinity increased with increasing synthesis temperature and TTIP heating temperature. A TTIP heating temperature and synthesis temperature of 95 °C and 900 °C, respectively, were found to be the optimal synthesis conditions. The primary particle diameter obtained under optimum synthesis conditions was considerably smaller than the commercial photocatalyst (Degussa, P25). The specific surface areas were more than 134.4 m² g^− 1. Under the optimal conditions, the photocatalytic activity for methylene blue was higher than that of the commercial photocatalyst.     

Lithium-ion battery anode properties of TiO2 nanotubes prepared by the hydrothermal synthesis of mixed (anatase and rutile) particles

From mixed (anatase and rutile) bulk particles, anatase TiO₂ nanotubes are synthesized in this study by an alkaline hydrothermal reaction and a consequent annealing at 300-400 °C. The physical and electrochemical properties of the TiO₂ nanotube are investigated for use as an anode active material for lithium-ion batteries. Upon the first discharge-charge sweep and simultaneous impedance measurements at local potentials, this study shows that interfacial resistance decreases significantly when passing lithium ions through a solid electrolyte interface layer at the lithium insertion/deinsertion plateaus of 1.75/2.0 V, corresponding to the redox potentials of anatase TiO₂ nanotubes. For an anatase TiO₂ nanotube containing minor TiO₂(B) phase obtained after annealing at 300 °C, the high-rate capability can be strongly enhanced by an isotropic dispersion of TiO₂ nanotubes to yield a discharge capacity higher than 150 mAh g⁻¹, even upon 100 cycles of 10 C-rate discharge-charge operations. This is suitable for use as a high-power anode material for lithium-ion batteries.     

Photocatalytic functional coatings of TiO2 thin films on polymer substrate by plasma enhanced atomic layer deposition

We prepared photocatalytic TiO₂ thin films which exhibited relatively high growth rate and low impurity on polymer substrate by plasma enhanced atomic layer deposition (PE-ALD) from Ti(NMe₂)₄ [tetrakis (dimethylamido) Ti, TDMAT] and O₂ plasma to show the self-cleaning effect. The TiO₂ thin films with anatase phase and bandgap energy about 3.3 eV were deposited at growth temperature of 250 °C and the photocatalytic effects were compared with commercial Activ glass. From contact angles measurement of water droplet and photo-induced degradation test of organic liquid, TiO₂ thin films with anatase phases showed superhydrophilic phenomena and decomposed organic liquid after UV irradiation. The anatase TiO₂ thin film on polymer substrate showed highest photocatalytic efficiency after 5 h UV irradiation. We attribute the highest photocatalytic efficiency of TiO₂ thin film with anatase structure to the formation of suitable crystalline phase and large surface area.     

Microwave-hydrothermal synthesis of extremely high specific surface area anatase for decomposing NOx

Apparently C-doped and undoped or pure nanoparticles of anatase were synthesized using a microwave hydrothermal process in the temperature range of 140–180 °C for 1 h from several Ti precursors, such as Ti ethoxide, Ti isopropoxide and Ti oxysulfate. Nanoparticles of anatase samples were characterized by powder X-ray diffraction, transmission electron microscopy (TEM) and photocatalytic activity measurements. Results showed that nanoparticles in the size range of 4–17 nm of anatase were obtained in all cases with surface areas in the range of 151–267 m²/g. The photocatalytic activity of the prepared titanias was measured using methylene blue (MB) and NO_x molecules. Because MB has very strong adsorption on the samples, photocatalytic degradation under either solar light or black light irradiation was found to be very limited. However, the DeNO_x abilities of carbon-doped titanias were higher than those of Degussa P25 commercial titania sample and undoped or pure titanias especially under irradiation by long wavelength or visible light (>500 nm).     

Electrophoretic deposition of nanocrystalline TiO2 films on Ti substrates for use in flexible dye-sensitized solar cells

Nanocrystalline TiO₂ films were prepared on flexible Ti-metal sheets by electrophoretic deposition followed by chemical treatment with tetra-n-butyl titanate (TBT) and sintering at 450 °C. X-ray diffraction (XRD) analysis indicates that TBT treatment led to the formation of additional anatase TiO₂, which plays an important role in improving the interconnection between TiO₂ particles, as well as the adherence of the film to the substrate, and in modifying the surface properties of the nanocrystalline particles. The effect of TBT treatment on the electron transport in the nanocrystalline films was studied by intensity-modulated photocurrent spectroscopy (IMPS). An increase in the conversion efficiency was obtained for the dye-sensitized solar cells with TBT-treated nanocrystalline TiO₂ films. The cell performance was further optimized by designing nanocrystalline TiO₂ films with a double-layer structure composed of a light-scattering layer and a transparent layer. The light-scattering effect of the double-layer nanocrystalline films was evaluated by diffuse reflectance spectra. Employing the double-layer nanocrystalline films as the photoelectrodes resulted in a significant improvement in the incident photo-to-current conversion efficiency of the corresponding cells due to enhanced solar absorption by light scattering. A high conversion efficiency of 6.33% was measured under illumination with 100 mW cm⁻² (AM 1.5) simulated sunlight.     

Mesoporous polyaniline or polypyrrole/anatase TiO2 nanocomposite as anode materials for lithium-ion batteries

A functional composite as anode materials for lithium-ion batteries, which contains highly dispersed TiO₂ nanocrystals in polyaniline matrix and well-defined mesopores, is fabricated by employing a novel one-step approach. The as-prepared mesoporous polyaniline/anatase TiO₂ nanocomposite has a high specific surface area of 224 m² g⁻¹ and a predominant pore size of 3.6 nm. The electrochemical performance of the as-prepared composite as anode material is investigated by cyclic voltammograms and galvanostatic method. The results demonstrate that the polyaniline/anatase nanocomposite provides larger initial discharge capacity of 233 mAh g⁻¹ and good cycle stability at the high current density of 2000 mA g⁻¹. After 70th cycles, the discharge capacity is maintained at 140 mAh g⁻¹. The excellent electrochemical performance of the polyaniline/TiO₂ nanocomposite is mainly attributed to its special structure. Furthermore, it is accessible to extend the novel strategy to other polymer/TiO₂ composites, and the mesoporous polypyrrole/anatase TiO₂ is also successfully fabricated.     

Enhanced photocatalytic activity of Au-buffered TiO2 thin films prepared by radio frequency magnetron sputtering

Au-buffered TiO₂ thin films have been prepared by radio frequency magnetron sputtering method. The structural and morphological properties of the thin films were characterized by X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The photocatalytic activity of the samples was evaluated by the photodecomposition of methylene blue. The Au-buffer thin layer placed between the TiO₂ thin films significantly enhanced photocatalytic activity by 50%. Annealing the Au-buffered TiO₂ thin film at 600 °C decreased the film roughness, but it increased the surface area and anatase crystalline size, enhancing the photocatalytic activity.     

Facile synthesis of mesoporous Ag-loaded TiO2 thin film and its photocatalytic properties

Ag nanoparticles highly dispersed into TiO₂ thin films are synthesized via a remarkably simple one-pot route in the presence of a P123 triblock copolymer as template directing and reducing agents, where the reduction of Ag⁺ to Ag⁰ by in situ heat-induced reduction through the oxidation of template at 400 °C and the controlled polymerization of TiO₂ take place simultaneously. The obtained mesoporous Ag/TiO₂ films deposited on soda-lime glass were optically transparent and crack-free. SEM and Kr adsorption clearly prove that Ag/TiO₂ films at different Ag contents are mesoporous with large surface area and regularly ordered mesopores and the thickness of the obtained films is ∼280 ± 20 nm. The pristine TiO₂ film exhibits a specific surface area of 63 cm²/cm² and specific pore volume of 0.013 mm³/cm² that it is decreased to 42 cm²/cm² and 0.010 mm³/cm² respectively as a result of Ag-loaded mesoporous TiO₂. The newly prepared photocatalysts Ag/TiO₂ films were evaluated for their photocatalytic degradation of 2-chlorophenol as a model reaction. It was found that the meso-ordered Ag/TiO₂ films are more photoactive 8 times than nonporous commercial photocatalysts Pilkington Glass Activ™. The recycling tests indicated that Ag/TiO₂ films was quite stable during that liquid-solid heterogeneous photocatalysis since no significant decrease in activity was observed even after being used repetitively for 10 times, showing a good potential in practical application. In general, the cubic mesoporous Ag/TiO₂ nanocomposites are stable and can be recycled without loss of their photochemical activity.     

Removal of the herbicide Bentazon from contaminated water in the presence of synthesized nanocrystalline TiO2 powders under irradiation of UV-C light

A solution-based processing method has been used to synthesize nanocrystalline TiO₂ powders by controlling the hydrolysis of TiCl₄ in an aqueous solution in both anatase and rutile phases. The primary particle sizes of the powders were in the range of 5-15 nm. To determine the crystal phase composition and size of the prepared photocatalysts, X-ray diffraction (XRD) measurements were used. We also studied the photocatalytic removal of the herbicide, Bentazon, from contaminated water in the presence of synthesized nanocrystalline TiO₂ powders under UV light illumination (30 W). The removal efficiency of Bentazon was 16% when the photolysis was carried out in the absence of TiO₂ and it was negligible in the absence of UV light. We have studied the influence of the basic operational parameters such as the different kinds of TiO₂, amount of TiO₂, irradiation time and initial concentration of Bentazon on the photocatalytic removal efficiency of Bentazon. Our results indicated that 99% removal of the herbicide from the solution containing 15 ppm of Bentazon after selecting desired operational parameters could be achieved in a relatively short time, about 90 min. A kinetic model was successfully established for the prediction of removal of Bentazon by the UV/TiO₂ system with any concentration of the herbicide. In this work, we also compared the photocatalytic activity between the commercial TiO₂ and synthesized nanocrystalline TiO₂ powders. The photocatalytic activities of different photocatalysts were tested using the herbicide solution.     

Photocatalytic oxidation of toluene to benzaldehyde over anatase TiO2 hollow spheres with exposed {001} facets

A new series of anatase TiO₂ hollow structures were prepared by a facile hydrothermal process. When the hydrothermal time was increased from 20 min to 72 h, the resulting TiO₂ solid spheres gradually transformed into TiO₂ hollow spheres with higher surface crystallinity and exposed {001} facets. The as-prepared TiO₂-72 h sample exhibited the highest activity comparing to other TiO₂-based samples and commercial product Degussa P-25 towards the selective photocatalytic oxidation of toluene to benzaldehyde. Such great photocatalytic performance was mainly attributed to enhanced UV-adsorption and better charge separation efficiency due to higher surface crystallinity of TiO₂-72 h.     

Tailoring high-surface-area nanocrystalline TiO2 polymorphs for high-power Li ion battery electrodes

The crystallization and morphology of brookite and anatase titania (TiO₂) were controlled using the urea-mediated hydrolysis/precipitation route in the presence of the Ti³⁺ ions. Without the strong complexing agents and the non-hydrothermal conditions, simple alterations to the urea concentration led to the synthesis from brookite nanorods to anatase nanoflowers at a low temperature below 100 °C, whereas the BET specific surface area evolved from 102 to 268 m² g⁻¹, respectively. A possible formation mechanism was also proposed for these TiO₂ nanostructures. The excellent reversible capacity and rate capability were achieved for the anatase nanoflowers because of the small crystallite size and significantly large surface area.     

Synthesis of a TiO2 ceramic membrane containing SrCo0.8Fe0.2O3 by the sol-gel method with a wet impregnation process for O2 and N2 permeation

A SrCo_0.8Fe_0.2O₃ impregnated TiO₂ membrane (TiO₂-SrCo_0.8Fe_0.2O₃ membrane) was successfully prepared using a sol-gel method in combination with a wet impregnation process. The membrane was subjected to a single gas permeance test using oxygen (O₂) and nitrogen (N₂). The TiO₂ membrane was immersed in the SrCo_0.8Fe_0.2O₃ solution, dried and then calcined to affix SrCo_0.8Fe_0.2O₃ into the membrane. The effect of the acid/alkoxide (H⁺/Ti⁴⁺) molar ratio of the TiO₂ sol on the TiO₂ phase transformation was investigated. The optimal molar ratio was found to be 0.5, which resulted in nanoparticles with a mean size of 5.30 nm after calcination at 400 °C. The effect of calcination temperature on the phase transformation of TiO₂ and SrCo_0.8Fe_0.2O₃ was investigated by varying the calcination temperature from 300 to 500 °C. X-ray diffraction spectroscopy (XRD) and Fourier transform infrared (FTIR) analysis confirmed that a calcination temperature of 400 °C was preferable for preparing a TiO₂-SrCo_0.8Fe_0.2O₃ membrane with fully crystallized anatase and SrCo_0.8Fe_0.2O₃ phases. The results also showed that polyvinyl alcohol (PVA) and hydroxypropyl cellulose (HPC) were completely removed. Field emission scanning electron microscopy (FESEM) analysis results showed that a crack-free and relatively dense TiO₂ membrane (∼0.75 μm thickness) was created with a multiple dip-coating process and calcination at 400 °C. The gas permeation results show that the TiO₂ and TiO₂-SrCo_0.8Fe_0.2O₃ membranes exhibited high permeances. The TiO₂-SrCo_0.8Fe_0.2O₃ membrane developed provided greater O₂/N₂ selectivity compared to the TiO₂ membrane alone.     

Synthesis of calcium phosphate-based composite nanopowders by mechanochemical process and subsequent thermal treatment

One-step mechanochemical process followed by thermal treatment has been used to produce calcium phosphate-based composite nanopowders. Effects of milling and subsequent heat treatment on the phase transition as well as structural features were investigated. The products were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) techniques. The results revealed that the dominant phases after mechanical activation were hydroxyapatite, anatase (TiO₂), and periclase (MgO), while after thermal annealing process at 700 °C hydroxyapatite along with geikielite (MgTiO₃) and periclase (MgO) were the major phases. In addition, decomposition of hydroxyapatite to tricalcium phosphate (β-TCP) occurred after heat treatment in the range 900–1100 °C which resulted in the formation of tricalcium phosphate-based composite nanopowders. Evaluation of structural features of the samples calculated by X-ray diffraction profiles analysis indicated that the average crystallite size of hydroxyapatite after 10 h of milling and subsequent heat treatment at 700 °C were about 21 and 34 nm, respectively. TEM and SEM studies exhibited that the considerable morphological changes at temperatures ≥900 °C had to be ascribed not only to grain growth, but also for the transformation of hydroxyapatite to β-TCP.     

Anatase TiO2-pillared hexaniobate mesoporous nanocomposite with enhanced photocatalytic activity

Mesoporous anatase TiO₂-pillared hexaniobate has been successfully prepared by an exfoliation-restacking route. The resulting nanocomposite was characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscope, thermogravimetric analysis, UV-Vis spectroscopy and N₂ adsorption-desorption measurements. It was reveal that the obtained material has a gallery height of about 2 nm, a specific surface area of about 170 m²/g and a wide pore size distribution with two extrema at about 2 and 3 nm. The mesoporous material exhibits an enhanced photocatalytic activity in the degradation of acid red G under ultraviolet light irradiation, attributed to its high surface area, mesoporosity and the electronic coupling between the host and the guest components. A photoexcitation model in the semiconductor-semiconductor pillared photocatalyst was proposed based on the results of XPS and UV-Vis.