Microemulsion-assisted hydrothermal preparation and infrared radiation property of TiO2 nanomaterials with tunable morphologies and crystal form

TiO₂ nanomaterials with adjustable crystalline phases and morphologies were prepared via a mild condition of microemulsion-assisted hydrothermal method. The effects of various reaction conditions, such as the species of inhibitor acid, the concentrations of acid and the reaction temperatures on the crystal forms and morphologies of the TiO₂ nanoparticles were investigated. The particles were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and Fourier transformed infrared spectrometer. Experimental results showed that the inhibitor acid of hydrochloric acid in the high concentration was conducive to the formation of rutile TiO₂. Nitric acid led to the formation of anatase TiO₂. The crystallinity of the TiO₂ was improved and the particle size was increased with the increase of the reaction temperature and the acid concentration. The morphologies of the TiO₂ nanoparticles also depended on the species and the concentration of the acids. TiO₂ nanoparticles with various structures of irregular granular, spherical, cosh and donuts-like were presented. Moreover, the infrared emissivity values of the TiO₂ nanoparticles were measured at wavelength of 8–14 μm. The irregular TiO₂ granular with single crystal phase exhibited the optimal infrared radiation property owing to the enhanced scattering ability and attenuation effect.     

Facile synthesis of monodispersed nanocrystalline anatase TiO2 particles with large surface area and enhanced photocatalytic activity for degradation of organic contaminant in wastewaters

Monodispersed nanocrystalline anatase TiO₂ particles have been successfully synthesized via a facile hydrothermal process, combined with encircling hexamethylenetetramine protectors. The products are characterized in detail by X-ray diffraction, Raman spectra, and transmission electron microscope. Experimental results indicate that the catalysts possess monodispersed nanocrystalline anatase structure with small particles (∼6 nm) and large surface area (286 m² g⁻¹), which are attributed to encircling hexamethylenetetramine protectors for inhibiting the undesirable grain growth. The obtained monodispersed nanocrystalline anatase TiO₂ particles exhibit better photocatalytic activity than that of Degussa P25 TiO₂ for degradation of highly toxic 2,4-dichlorophenol in wastewaters under UV irradiation, which is ascribed to the small particle size and large surface area offering more active sites, and the improved crystallinity in favor of the separation of photogenerated electron–hole pairs.     

Morphology and crystalline phase-controllable synthesis of titania nanoparticles via acrylamide gel method and their photocatalytic properties

Titanium dioxide nanoparticles with average particle size of about 5 nm to 60 nm were readily synthesized via a simple, fast and low cost method; polyacrylamide gel method. Furthermore, the effect of the used acid and solvent together with calcination temperature, on crystallite size, morphology, band gaps of resultant material were investigated. The products were characterized by means of thermo gravimetric and differential thermal analysis (TG-DTA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and UV–vis diffuse reflectance spectroscopy analysis. The XRD results show that the presence of different anions in the precursor solution leads to the formation of samples with different anatase/rutile ratios. Their photocatalytic activities were evaluated by photocatalytic degradation of Yellow GX aqueous solution under ultraviolet radiation. The results show that the photocatalyst (TEPC_I), containing 79% anatase exhibits the highest photocatalytic activity, which is due to a synergistic effect between anatase and rutile.     

Preparation and characterization of TiO2 thin film by thermal oxidation of sputtered Ti film

Titanium dioxide (TiO₂) thin films were successfully prepared on quartz substrate by thermal oxidation of sputtered titanium film in air. The structure, composition, morphology and optical properties of oxidized TiO₂ films were characterized by Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy and UV-visible spectroscopy. Meanwhile, the photocatalytic activity of the films was evaluated on the basis of the degradation of methyl orange solution under UV irradiation. Ti films after oxidation present mainly in TiO₂ form with a larger amount of adsorbed O₂, and oxidation temperature has a strong impact on the crystal structure and properties of the films. A phase transformation of anatase to rutile for oxidized TiO₂ films occurred in the temperature range of 700–800 °C. The energy band gap of oxidized TiO₂ films decreased first and then increased with annealing temperature. Furthermore, TiO₂ film oxidized at 600 °C exhibited the best photocatalytic activity due to suitable crystal phase and size. These results might contribute to the synthesis of metal oxide thin films with expectant structural morphology and properties by thermal oxidation methods.     

Synthesis of TiO2 thin films with highly efficient surfaces using a sol–gel technique

Three different strong acid catalysts were used in a simple sol–gel synthesis to produce TiO₂ thin films with increased homogeneity and enhanced photocatalytic activity on their mesoporous surfaces. Various techniques were used to characterize the samples, including UV–visible spectrophotometry, X-ray diffraction, micro-Raman spectrometry, photobleaching, scanning electron microscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The band gaps varied from 3.73 to 3.75 eV and the transmittance was >80%. An anatase phase was obtained in all the samples and the crystal size varied from 20 to 45 nm as a function of the annealing temperature. The increase in the efficiency of the surface of the TiO₂ thin films was evaluated by photodegradation of methylene blue in water. The results showed that the acid catalysts used in the synthesis had an important effect on the morphology and photocatalytic activity of the thin films, resulting in more efficient surfaces. Synthesis with hydrofluoric acid produced thin films with a homogenous mesoporous structure and improved the photodegradation of the methylene blue dye to 92% in 2.5 h.     

Efficient dye-sensitized solar cells based on CNTs and Zr-doped TiO2 nanoparticles

The light scattering, harvesting and adsorption effects in dye-sensitized solar cells (DSSCs) are studied by preparation of coated carbon nanotubes (CNTs) with TiO₂ and Zr-doped TiO₂ nanoparticles in the forms of mono- and double-layer cells. X-ray diffraction (XRD) analysis reveals that the phase composition of Zr-doped TiO₂ electrode is a mixture of anatase and rutile phases with major rutile content, whereas it is the same mixture with major anatase content for coated CNTs with TiO₂. Furthermore, the average crystallite size of Zr-doped TiO₂ electrode is slightly decreased with Zr introduction. Field emission scanning electron microscope (FE-SEM) images show that the porosity of Zr-doped TiO₂ electrodes is higher than that of undoped electrode, enhancing dye adsorption. UV–visible spectroscopy analysis reveals that the absorption onset of Zr-doped TiO₂ electrodes is slightly shifted to longer wavelength (the red-shift) in comparison with that of undoped TiO₂ electrode. Moreover, the band gap energy of TiO₂ nanoparticles is decreased by Zr introduction, enhancing light absorption. It is found that electron injection of monolayer TiO₂ electrode is improved by introduction of 0.025 mol% Zr, resulted in enhancement of its power conversion efficiency (PCE) up to 6.81% compared with 6.17% for pure TiO₂ electrode. Moreover, electron transport and light scattering are enhanced by incorporation of 0.025 wt% coated CNTs with TiO₂ in the over-layer of double layer electrode. Therefore, double layer solar cell composed of 0.025 mol% Zr-doped TiO₂ nanoparticles as the under-layer and mixtures of these nanoparticles and 0.025 wt% coated CNTs with TiO₂ as the over-layer shows the highest PCE of 8.19%.     

High photocatalytically active cocoons-like TiO2/SiO2 synthesized by hydrothermal process and subsequent calcination at 900 °C

TiO₂/SiO₂ nanoparticles with cocoons-like morphology were synthesized by one-pot hydrothermal process and subsequent calcination at 900 °C, using tetrabutyl titanate and tetraethoxysilane as starting materials. The effects of calcination temperature on the morphology, crystalline phase, band gap, microtexture as well as photocatalytic activity of TiO₂/SiO₂ were investigated by TEM, XRD, TG, IR, DRS and N₂ adsorption-desorption isothermals. The calcination caused the shrinkage of highly porous SiO₂ gel shell grafted on TiO₂ crystallite and led to the formation of cocoons-like structure, and the crystallite size and the anatase phase of TiO₂ can be maintained up to 900 °C and 1100 °C, respectively. The TiO₂/SiO₂ photocatalyst calcined at 900 °C showed a higher photocatalytic activity compared with commercial P25.     

Synthesis,characterization and optical properties of nanostructured ZnWO4

The ZnWO₄ nanorods were fabricated by template-free hydrothermal route and characterized by X-ray diffraction, scanning electron microscopy, Raman, fluorescence and UV–visible spectra. The ZnWO₄ nanorods were descended from ZnWO₄ nanosheets with the increase of the pH value. The correlations among these optical spectra were studied and the results showed that the ZnWO₄ nanorods had good crystallinity and few oxygen-vacancy defects comparing with ZnWO₄ nanosheets. Fluorescence emission and UV–visible transmission spectra of ZnWO₄ nanorods were increased, while the fluorescence excitation and UV absorption spectra were decreased in the wavelength region of 250–325 nm. This phenomenon was reversed in the wavelength region of 325–400 nm. The ZnWO₄ nanorods displaying superior photocatalytic activity were suitable to be photocatalytic materials which could absorb selectively ultraviolet in the wavelength of 325–400 nm for their low recombination probability of light excited electron–hole.     

Low temperature TiO2 rutile phase thin film synthesis by chemical spray pyrolysis (CSP) of titanyl acetylacetonate

Rutile phase TiO₂ thin films have been synthesized using chemical spray pyrolysis of titanyl acetylacetonate TiAcAc in ethanol at 500 °C. The first part of the paper focuses on the thermal decomposition behavior of the precursor by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) coupled with differential scanning calorimetry (DSC). The second part of the paper focuses on the evolution of TiO₂ thin films and their structural transformation with substrate temperature. XRD revealed amorphous TiO₂ thin film at low substrate temperatures (<350 °C) and on high substrate temperatures anatase (3.84 g/cm³) or rutile (4.25 g/cm³) crystalline structure was obtained. The lattice constant, grain size, microstrain and the dislocation density of the film were obtained from the peak width. FTIR spectra of both anatase and rutile TiO₂ revealed stretching vibration of the Ti–O bond for tetrahedral and octahedral surroundings of the titanium atom. Scanning electron micrograph showed the compactness of the rutile film.     

Synthesis based structural and optical behavior of anatase TiO2 nanoparticles

The effect of synthesis method on optical and photoconducting properties of titanium dioxide (TiO₂) nanoparticles has been investigated. Sol–gel and co-precipitation methods have been employed to prepare pure anatase phased TiO₂ nanoparticles calcinated at different temperatures below 500 °C. The optimized value of average crystallite size is within the range of 19−21 nm for a common calcination temperature of 400 °C for both the methods. The redshift in optical band gap of 0.9 eV has been observed for the sample synthesized by co-precipitation method with respect to the sol–gel method. The photoluminescence spectrum exhibits broad visible emission in both routes of synthesis while photoconductivity shows fast growth and decay of photocurrent in TiO₂ prepared by co-precipitation method as compared to TiO₂ prepared by the sol–gel method under visible illumination. Crystal structure based Rietveld refinement of X-ray diffraction data, scanning electron microscopy as well as photoluminescence and photoconductivity measurements were performed to characterize nanocrystalline anatase TiO₂.     

Room-temperature synthesis of nanocrystalline titanium dioxide via electrochemical anodization

This paper presents a study of the growth of nanoporous anatase and rutile phases of titanium dioxide (TiO₂) subjected to electrochemical anodization at room temperature without post-thermal treatment, using sulfuric acid as the electrolyte. Effects of the applied voltage on the morphological, structural, and photoelectrochemical (PEC) properties were examined. Images from field emission scanning electron microscopy reveal that pore size could be manipulated by changing the anodization voltage. In addition, X-ray diffraction (XRD) results indicate that anatase and rutile phases of TiO₂ appeared in samples subjected to minimum anodization voltages of 100 V and 150 V. The Scherrer method was used to calculate the mean crystallite size, and the interplanar d-spacing formula was used to obtain the in-plane and out-of-plane strains. XRD measurements reveal that the amount of anatase and rutile crystallinity and their mean crystallite sizes were affected significantly by the anodization voltage. Results of the PEC studies reveal that the photocurrent density and photoconversion efficiency increased with increasing anodization voltage. In addition, the synthesized nanoporous TiO₂ showed stable photoresponse where only a small decay of photocurrent density is observed in numerous on-to-off illumination cycles.     

Structural phase-dependent resistivity of intrinsic-extrinsic co-doped transparent titanium dioxide films

The authors report a method of enhancing the conductivity of TiO₂ films by controlling their structural phases. Thin films of Nb:TiO₂ (TNO) were prepared on glass and silicon substrates by RF sputtering with varying Nb content at 200 °C. It is shown that fine control over the structural phases of TiO₂ is critical for achieving low resistivity. The resistivity values of the films doped with oxygen vacancies and Nb⁺⁵ decreased from 3.8 × 10⁻¹ to 4.1 × 10⁻³ Ω cm when the weight percent of rutile in anatase-rutile phase mixture decreases from 52.8% to 32%. Furthermore, the lowest resistivity value of 2.37 × 10⁻³ Ω cm was obtained for the doped TiO₂ films having single phase anatase structure. The physical processes responsible for the diverse electrical properties are discussed and are associated with the growth conditions. Our result indicates that highly conductive doped-TiO₂ film can be obtained by controlling the anatase phase formation via the growth temperature. The obtained results can significantly contribute to the development of transparent electrodes by RF sputtering, a suitable technique for coating large area substrates.     

Hydrothermal synthesis of WO3 films on the TiO2 substrates and their photochromic properties

Tungsten oxide (WO₃) films have been prepared on the synthesized TiO₂ substrates from a sodium tungsten precursor via a hydrothermal method. X-ray diffraction, scanning electron microscopy and transmission electron microscopy analyses were used to investigate the effect of precursor concentrations on the structures and morphologies of the films. Ordered WO₃ films were successfully synthesized on the as-grown TiO₂ substrates. With the concentrations increasing from 0.001 M to 0.024 M, the morphologies of the films changed from multi-layer laminated structure to ladder-shaped lamellar structure finally columnar structure. The results also showed that with an increase in precursor concentration, the observed absorptions at 365 nm of the films increased until precursor concentration of 0.016 M, and then decreased with higher concentration. The film obtained with precursor concentration of 0.016 M on the TiO₂ substrate had the best photochromic properties.     

WO3/TiO2 nanocomposites: Salt–ultrasonic assisted hydrothermal synthesis and enhanced photocatalytic activity

A series of WO₃/TiO₂ composite photocatalysts were fabricated via a facile salt–ultrasonic assisted hydrothermal process. The obtained samples were characterized by X-ray diffraction, scanning eletron microscopy, energy dispersive X-ray spectroscopy and UV–vis diffused reflectance spectroscopy. It was confirmed that anatase TiO₂ and monoclinic WO₃ coexisted in the composites. The photocatalytic activity of as-prepared WO₃/TiO₂ composites for degradation of Rhodamin B (RhB) under visible light irradiation was investigated. The results showed that WO₃/TiO₂ composites have a higher photocatalytic activity than those of pure TiO₂ and pure WO₃. First-principle calculations based on density functional theory were performed to explore the electronic structure and illustrate the photocatalytic mechanism of WO₃/TiO₂. The calculated energy gap was 2.53 eV, which was close to the experimental observation (2.58 eV). Due to the combination of WO₃/TiO₂, the photoinduced electrons and holes transfer between the WO₃ and TiO₂ in opposite directions, thus providing sufficient charge separation, which contributed to the photocatalytic activity enhancement.     

Anatase titania nanorods by pseudo-inorganic templating

A novel, facile and cost-effective single step aqueous sol–gel method for the synthesis of anatase TiO₂ nanorods without the assistance of structure-directing organic/inorganic templates is reported. We specifically demonstrate a pseudo-inorganic templating method using ammonium iron (III) sulfate. Highly thermal stable anatase TiO₂ nanorods were obtained using titanium oxysulfate and ammonium iron (III) sulfate as precursors. The structural, microstructural and chemical analyses of the nanorods synthesized, strongly supported the pseudo inorganic templating role of ammonium iron (III) sulfate on the formation of nanorod morphology. The materials have been characterized using different techniques such as TEM, XRD, BET surface area measurement, diffuse reflectance spectra and XPS. TEM study confirmed the rod shape of nano-anatase TiO₂, having a diameter in the range of 20–40 nm and a length of 100 nm. XPS investigation showed that along with Fe³⁺, nitrogen and sulfur were also been doped into the TiO₂ lattice from the single source dopant precursor ammonium iron (III) sulfate. Moreover, UV/vis diffuse reflectance spectra of nanorods showed red-shifts towards visible light and a corresponding decrease in band-gap energies consistent with an n-type doping of the anatase TiO₂ matrix. This aqueous sol–gel synthesis of anatase nanorod with pseudo inorganic templating explores the advantages of inexpensive precursors, control over the powder morphology and optical properties, and distribution of the dopants over TiO₂ at nano level due to homogeneous mixing of the precursors. Finally the photocatalytic analysis showed that the TiO₂ nanorod have two times higher activity than the commercially available Degussa P 25.     

Synthesis and properties of Fe3O4/SiO2/TiO2 nanocomposites by hydrothermal synthetic method

Fe₃O₄/SiO₂/TiO₂ nanocomposites with well-defined core-shell structures were successfully prepared by a facile hydrothermal synthetic method for titania coating on Fe₃O₄/SiO₂ magnetic core. The as-prepared Fe₃O₄/SiO₂/TiO₂ composite particles were characterized by X-ray diffraction (XRD),transmission electron microscopy (TEM),Fourier transform infrared spectroscopy (FT-IR). The results showed that Fe₃O₄/SiO₂/TiO₂ was well crystallized at 140 °C with well-defined core-shell structures. Fe₃O₄/SiO₂/TiO₂ nanocomposites as well as pure TiO₂ showed good photocatalytic performance in the decolorization of methyl orange aqueous solution. Magnetic core coated by SiO₂ and TiO₂ layer still retained the good magnetic properties (Ms:3.04 emu g^-1) to facilitate catalyst recovery using external magnetic field.     

Enhanced photocatalytic performance of platinized CdS/TiO2 by optimizing calcination temperature of TiO2 nanotubes

TiO₂ nanotubes were prepared by hydrothermal treatment of TiO₂ powder in NaOH aqueous solution and then calcined at various temperatures. The post-calcination treated TiO₂ nanotubes were decorated with CdS by wetness impregnation and subsequently sulfurization to fabricate CdS/TiO₂ composites. The photocatalytic performance of CdS/TiO₂ composites toward hydrogen production from water splitting was investigated. The results show that the calcination temperature of TiO₂ nanotubes has a significant effect on the photocatalytic performance of CdS/TiO₂. With the increase of calcination temperature from 300 to 500 °C, the crystallinity of TiO₂ nanotubes is increased resulting in the enhanced photocatalytic performance of CdS/TiO₂. When the calcination temperature is higher than 500 °C, TiO₂ nanotubes gradually transform into nanorods and finally completely collapse, which leads to the decrease of photocatalytic performance of CdS/TiO₂. The CdS/TiO₂ composite with TiO₂ nanotubes calcined at 500 °C exhibits the highest hydrogen evolution rate, which could be attributed to its 1 D nanotubular structure and good crystallinity.     

Preparation and photocatalytic activity of Ag/bamboo-type TiO2 nanotube composite electrodes for methylene blue degradation

Photocatalysis phenomena in TiO₂ have been intensively investigated for its potential application in environmental remediation. The present work reports improved photocatalytic degradation of methylene blue dye in aqueous solution by using bamboo-type TiO₂ nanotubes deposited with Ag nanoparticles via electrochemical deposition. The photocatalytic processes are performed on Ag-modified TiO₂ bamboo-type nanotube arrays, Ag-modified smooth-walled nanotube arrays, and bare smooth-walled nanotube arrays. Both Ag-modified bamboo-type and smooth-walled nanotube arrays show improved photocatalytic degradation efficiencies (64.4% and 52.6%) compared to smooth-walled TiO₂ nanotubes of the same length (44.4%), due to the enhanced electron–hole seperation and more surface area provided by bamboo ridges. The photocatalytic activity and kinetic behavior of Ag-modified bamboo-type nanotube arrays are also optmized by tuning pulse deposition time of Ag nanoparticles. Bamboo-type nanotubes deposited with Ag nanoparticles via pulse deposition time of 0.5 s/1.5 s shows the highest methylene blue degradation efficiency of 78.5%, which represents 21.9% and 76.8% enhancement of efficiency compared to those of bare bamboo-type and smooth-walled nanotubes, respectively, indicating that a proper amount of Ag nanoparticles on TiO₂ can maximize the photocatalytic processes. In addition, overly long pulse deposition time will not further increase photocatalytic activity due to agglomeration of Ag paticles. For example, when the pulse deposition time is increased to 2 s/6 s, Ag-modified bamboo-type nanotube array exhibits a lower photocatalytic degradation efficiency of 62.9%.     

Sol–gel fabrication and enhanced optical properties,photocatalysis, and surface wettability of nanostructured titanium dioxide films

TiO₂ photocatalytic film, annealed at temperatures of 500 °C and 700 °C, was prepared on SiO₂ pre-coated glass via sol–gel technique for photocatalytic purposes and effects of catalyst-type on its properties were investigated by an X-ray diffractometer (XRD), Scanning Electron Microscope, UV–vis spectrophotometer, and contact angle measurements. The XRD results showed that present phases depend upon catalyst used in the solution and phase transformation behaves in a temperature-dependent manner. For the layers derived from sols containing acidic catalysts, the anatase structure dominated and exhibited much better photocatalytic activity. The results indicated that the sample derived from sol comprises H₂SO₄ as catalyst, and exhibits anatase grains with the lowest size. This could be the reason for its better photocatalytic activity. Finally, samples derived from sol containing acidic catalysts showed superhydrophilicity and superior cleaning ability.     

The environment effect on the electrical conductivity and photoconductivity of anatase TiO2 nanoplates with silver nanoparticles photodeposited on {101} crystal facets

The electrical conductivity and the transient photoconductivity of Ag/TiO₂ nanoplates were studied at 300 K, both in vacuum and in air. A solvothermal method was used for the preparation of anatase TiO₂ nanoplates and the embedding of Ag nanoparticles was achieved via the photoreduction of AgNO₃ under UVA irradiation. The particle size was controlled by varying the time of illumination and the obtained Ag nanoparticles sizes were in a range of 5–20 nm. The Ag/TiO₂ nanoplates were characterized by X-ray diffraction (XRD) and transition electron microscopy (TEM). The electrical conductivity and the photoconductivity responses were investigated. The influence of environment was also discussed. For some periods of UVA illumination time, photoconductivity reaches higher values in comparison with the pure TiO₂ anatase nanoplates. In air, the influence of the adsorbed on the surface oxygen is obvious, resulting in high recombination rates.