Photocatalytic properties of TiO2/ZnO thin film

ABSTRACT

TiO₂, ZnO and ZnO/TiO₂ thin films have been prepared by radio frequency magnetron sputtering method under different temperatures. Their photo catalytic activities have been investigated. The structural of the thin films were characterized by X-ray diffraction and Raman spectroscopy. The photo catalytic activities of TiO₂ and ZnO/TiO₂ samples were evaluated by the photo decomposition of methylene blue. We note that the structural proprieties of the thin films showed a perfect crystallization along the (002) for ZnO, Rutile (110) for TiO₂ and Anatase (101) for TiO₂. The experimental results show that the bilayer ZnO/TiO₂ were the most efficient photo catalysts compared to the layer of TiO₂. This increased catalytic effect can attributed to the interface between the ZnO layer and the TiO₂ one, which modify significantly the chemical potential of the bilayer.     

Microstructure and percolation threshold characteristics of reactive sputtered Au-TiO2 granular composite films

The Au/TiO₂ composite films were prepared by using reactive co-sputtering technique. The size and shape of the embedded Au particles and the absorption spectra of the composite films were investigated by using SEM, XRD, and UV-VIS-NIR spectrophotometer, respectively. The average size of Au particles and the electrical conductivity decrease as the sputtering pressure increases. The normalized conductivity of the films deposited at five different pressures with the Au concentration in the range of 0.15-0.91 were measured. The percolation threshold increases from 0.21 to 0.90 as the sputtering pressure increases from 2 × 10⁻² Torr to 9.5 × 10⁻² Torr.     

Controllable synthesis of TiO2 nanoparticles employing substrate/dielectrophoresis/sol‐gel

Due to size‐dependent catalytic selectivity, the size and special morphology are of great importance to applications of TiO₂. The synthesis method of size and morphology control has been in need of innovation. In this study, TiO₂ nanoparticles(TiO₂‐NPs) with well‐defined morphology and homogenous size were synthesized using a novel method, in which bamboo substrate, dielectrophoresis (DEP) technology and a sol‐gel process were combined(substrate/ DEP/ sol‐gel). Powder X‐ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were used to characterize the TiO₂‐NPs. Further study showed that, with this combined method, the size and the uniformity of TiO₂‐NPs can be controlled by changing the voltage of DEP. The number and arrangement of TiO₂ nanorods can be controlled by changing the voltage. Substrate/ DEP/ sol‐gel proved to be an efficient way to form special morphologies of TiO₂‐NPs. A visible‐light catalytic activity experiment showed that among three preparation methods, the substrate/ DEP/ sol‐gel method made TiO₂‐NPs with the highest catalytic activity for degradation of methyl orange. TiO₂‐NPs produced by the DEP/ sol‐gel process presented higher catalytic activity than TiO₂‐NPs produced by only a sol‐gel process.     

Characterization of HF-catalyzed silica gels doped with Degussa P25 titanium dioxide

SiO₂/TiO₂ composites were synthesized by adding Degussa P25 TiO₂ to a liquid sol that was catalyzed by HNO₃ and HF acids. Various composites were synthesized by altering the mass loading of TiO₂ and concentration of HF added to the liquid sol before gelation. The resulting materials were characterized by SEM, nitrogen adsorption-desorption, streaming potential, XRD, diffuse reflectance and TiO₂ surface area analyses. Approximate characteristics include an isoelectric point of 3, TiO₂ particle size of 30 nm, and a band gap energy of 3.2 eV. Small variations in these properties were noted for the different composites. Physical characteristics were largely affected by HF concentration and TiO₂ loading. Nitrogen adsorption-desorption isotherms were type IV for all materials and exhibited trends of decreased pore volume with an increase in TiO₂ loading and an increase in pore diameter with increased HF concentration. Surface areas of the composites ranged from 167 to 630 m²/g. Available TiO₂ surface area of the composite was also dependent upon TiO₂ loading and increased as the mass composition of TiO₂ increased but was not largely affected by HF concentration.     

Studies on a possible growth mechanism of silver nanoparticles loaded on TiO2 thin films by photoinduced deposition method

The TiO₂ thin films loaded with silver nanoparticles were prepared on soda-lime glass substrates by a photoinduced deposition method. The TiO₂ films immersed in AgNO₃ solution were vertically irradiated by UV light with center wavelength of 365 nm for 60 h. The as-produced films were characterized by X-ray diffraction (XRD), UV–Vis spectroscopy, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The studies show that the film after UV excitation is composed of anatase phase TiO₂ and metallic silver with face centered cubic structure. A possible growth mechanism of silver nanoparticles on TiO₂ thin films under UV irradiation was proposed. The charge carriers of TiO₂ semiconductor are generated by photoexcitation. Owing to the conduction band position of TiO₂ which is above the standard potential of Ag⁺/Ag, the generated electrons could transfer from the conduction band to Ag⁺ adsorbed on the surface of the TiO₂ films. Therefore, the Ag⁺ was finally reduced into a Ag atom, which could preferentially localize in the grain boundaries of TiO₂ particles due to high surface free energy there. With the irradiation time extended, silver nanoparticles were shaped into certain morphologies on the surface of the TiO₂ films.     

Glass-ceramics in the system BaO/TiO2(ZrO2)/Al2O3/B2O3 and their thermal expansion

Glasses in the system BaO/TiO₂(ZrO₂)/Al₂O₃/B₂O₃ were melted from the raw materials with and without the addition of platinum. They were crystallized in two steps at 720 and 780 °C. TiO₂ does not act as nucleating agent in this system, but widely prevents crystallization. Samples additionally doped with platinum show the crystallization of BaAl₂B₂O₇. However, samples containing equimolar quantities of TiO₂ and ZrO₂ showed the crystallization of this phase without the addition of platinum. The thermal expansion coefficients of samples with TiO₂-concentration ⩽7 mol% or TiO₂/ZrO₂-concentrations ⩽5 mol% exhibit zero or even slightly negative thermal expansion coefficients.     

Pt immobilization on TiO2‐embedded carbon nanofibers using photodeposition

Currently, the use of fuel cell electrodes containing Pt catalysts has been limited due to technological problems in this system, primarily the system's high cost. The improvement of Pt catalyst use has been achieved by changes in the Pt immobilization method. In this study, we have studied Pt immobilization on carbon nanofiber composites using the photodeposition method. First, we prepared the carbon nanofibers, which were homogeneously embedded TiO₂ using the electrospinning technology. These TiO₂‐embedded carbon nanofiber composites (TiO₂/CNFs) were then immersed in a Pt precursor solution and irradiated with UV light. The obtained Pt‐deposited TiO₂/CNFs contained Pt that was immobilized on the carbon nanofibers, and the Pt particle size was 2‐5 nm. The XPS spectra showed that the amount of Pt increased with an increasing UV irradiation time. The current densities and total charge also increased with an increase in the UV irradiation time, possibly due to an increase of active specific area by finely dispersed Pt nanoparticles. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Doping of a high calcium oxide metaphosphate glass with titanium dioxide

This study investigates the effect of doping a high calcium oxide containing metaphosphate glass series (CaO)₄₀(Na₂O)₁₀(P₂O₅)₅₀ with TiO₂ (1, 3, and 5 mol%). TiO₂ incorporation increased the density and glass transition temperature while reduced the degradation rate (5 mol% in particular) by twofold compared with (CaO)₃₀ system reported previously. This has been confirmed by ion release and the minimal pH changes. TiP₂O₇, NaCa(PO₃)₃ and CaP₂O₆ phases were detected for all TiO₂-containing ceramics. XPS showed that the surface is composed of Ca, P, and Ti. Ti was recognized mainly as TiO₂, but its total amount was lower than theoretical values. ³¹P magic angle spinning (MAS) NMR showed a downfield shift of the ³¹P lineshape with increasing TiO₂, interpreted as an effect of the titanium cation rather than an increase in the phosphate network connectivity. FTIR showed that incorporation of TiO₂ increased the strength of the phosphate chains, and the O/P ratio while introducing more Q¹ units into the structure at the expense of the Q² units. There were no differences, however, in surface topography roughness and free energies between these glasses. These results suggested that TiO₂ and CaO were acting synergistically in producing glasses with controllable bulk and structural properties.     

Synthesis of TiO2 nanorings and nanorods on TCO substrate by potentiostatic anodization of titanium powder

Various TiO₂ nanostructures, such as nanorings, nanorods were synthesized via potentiostatic anodization of titanium powder under different conditions. The morphology of the obtained TiO₂ nanostructures can be easily tuned by varying applied voltage. The crystal structure, compositional information and morphological structures were characterized by X‐ray diffraction (XRD), X‐ray photoelectron spectra (XPS) and field emission scanning electronic microscopy (FESEM). XRD and XPS analysis confirmed the anodization products were TiO₂. A possible formation mechanism was suggested on the basis of the shape evolution of TiO₂ nanostructures observed by FESEM. The results revealed that the applied voltage played an important role in the formation of various nanostructures. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of TiO2SiO2 glasses by X-ray absorption spectroscopy

The coordination and nearest-neighbor bond distances of Ti in a series of TiO₂SiO₂ glasses have been quantitatively determined using a combination of XANES and EXAFS measurements about the Ti K-edge at 4966 eV. The glasses covering the range 0.012 to 14.7 wt% TiO₂ were prepared by flame hydrolysis of predetermined mixtures of SiCl₄ and TiCl₄ vapors. At TiO₂ concentrations below ～0.05 wt%, Ti is found in a rutile-like octahedral coordination. With increased TiO₂ content in the glass, a two-site model applies, in which Ti is found predominately in a fourfold site. About ～9 wt% TiO₂, t6he sixfold/fourfold ratio increases appreciably and eventually at ～15 wt% TiO₂, crystalline TiO₂ segregates out as a second phase. The average TiOSi bond angle in these glasses was estimated to be ～159 ° which is slightly larger than the most probable value of 152 degrees for SiOSi in vitreous SiO₂. Within the accuracy of the edge shift measurements all Ti in the glass is in 4+ valence. Finally, various physical properties such as density, optical transparency and thermal expansion are correlated in light of the new structural data for this interesting binary silicate glass system.     

Molecular orientation and dielectric anisotropy properties of 4-cyano-4′-n-heptylbiphenyl-TiO2 liquid crystal composite

The molecular orientation and the dielectric anisotropy of the nematic liquid crystal (LC) 4-cyano-4′-n-heptylbiphenyl (7CB) and of TiO₂-doped 7CB have been investigated. The dielectric properties of the LCs exhibit a relaxation peak that shifts to lower frequencies with increasing voltages. The relaxation frequencies of 7CB and 7CB/TiO₂ liquid crystals were calculated and found to decrease as the bias voltage increases. This is attributed to molecular reorientation. The dielectric anisotropy of the LCs changes from the positive type to negative type and the static electric permittivity and dielectric anisotropy values were found to be lower for the 7CB/TiO₂ system.     

Synthesis and Photocatalytic Property of Hectorite/(Pt,TiO2) and H4Nb6O17/(Pt,TiO2) Nanocomposites

Abstract

TiO₂ and Pt have been intercalated in hectorite and H₄Nb₆O₁₇. The height of TiO₂ and Pt pillars was less than 0.8 nm and the band gap energy of TiO₂ pillars was ca. 3.3 eV. Both hectorite/TiO₂ and H₄Nb₆O₁₇(Pt, TiO₂) were capable of hydrogen evolution following irradiation from a high pressure mercury are (λ > 290 nm) in the presence of methanol as a sacrificial hole acceptor and the hydrogen evolution was enhanced by co-incorporation of Pt, although hectorite and hectorite/Pt did not show photocatalytic activity. Incorporation of Pt or Pt and TiO₂ in the interlayer of H₄Nb₆O₁₇ has resulted in enhanced photo evolution of hydrogen, however, TiO₂ alone in the interlayer of H₄Nb₆O₁₇ showed adverse photocatalytic activity.     

Photoelectrochemical solar cells based on TiS2/TiO2(TiOxSy) photoelectrodes

A new photoelectrode of TiS₂/TiO₂ alloy with a possible new TiO_xS_y phase has been synthesized for the first time in the present investigation. This photoanode exhibits improved photoelectrochemical (PEC) response as compared to either TiS₂ or TiO₂ photoelectrode. With this new photoelectrode high PEC conversion efficiency is evidenced by large photovoltage (840 mv) and significantly high photocurrent density (40 mA/cm²). The photovoltage obtained in the present system is significantly high and the photocurrent density is the highest reported so far for the TiO₂ based systems. It has been suggested that the enhancement in photoactivity in the present system is due to the reduced band gap of TiO_xS_y alloy material. Such a band gap reduction is thought to be facilitated by the downward movement of the valence band of titanium disulphide which is made up of chalcogen atoms through substitution of sulphur atoms by oxygen atoms.     

Influence of Fe2O3 Doping on TiO2 Electrode for Enhancement Photovoltaic Efficiency of Dye-Sensitized Solar Cells

In this work, we report for the first time the improvement of the photovoltaic characteristics of dye-sensitized solar cells (DSSCs) by doping TiO₂ with Fe₂O₃. DSSCs were fabricated using various percentages of Fe₂O₃-doped TiO₂ composite nanoparticles. The Fe₂O₃-doped DSSCs exhibited a maximum conversion efficiency of 5.76% because of the effective electron transport. DSSCs based on Fe₂O₃-doped TiO₂ films showed better photovoltaic performance than cells fabricated with only TiO₂ nanoparticles. This result was attributed to the prevention of recombination between electrons in the TiO₂ conduction band with the dye or electrolytes. A mechanism was suggested based on impedance results, which indicated improved electron transport at the interface of the TiO₂/dye/electrolyte.     

Physicochemical studies of phosphate based P2O5-Na2O-CaO-TiO2 glasses for biomedical applications

Glasses P₂O₅-Na₂O-CaO-TiO₂ with different TiO₂ contents and fixed P₂O₅ (45 wt%) and CaO (24 wt%) have been prepared employing the normal melting and annealing technique. Measurements such as ultrasonic velocity, attenuation, solubility and pH have been carried out in all the compositions of the glasses. It is interesting to note that the above measured ultrasonic parameters exhibit an abnormal behavior (minimum) at 0.5 wt% of TiO₂ content, beyond which an increase in these parameters with increasing TiO₂ content is observed. The maximum pH values and Ca²⁺ ion release have been observed for the TiO₂ free glass those compositions with and the low TiO₂(?1.0 wt%) content. As the content of the TiO₂ increases, the solubility of the glasses decreases. The observed weight loss reveals two stages of phosphate dissolution kinetics i.e. the first stage, in which the weight loss is proportional to t^1/2, and a second stage in which a linear behavior is observed.     

Microstructural and photocatalytic properties of sol–gel-derived vanadium-doped mesoporous titanium dioxide nanoparticles

The vanadium (V)-doped mesoporous titanium dioxide (TiO₂) nanoparticles at low V/Ti ratios ranging from 0 to 2 wt% were prepared using hydrolytic sol–gel method in the presence of tri-block copolymer Pluronic F127. The microstructures of TiO₂ in terms of morphology, crystallization, chemical states of species, surface area, and band gap were characterized by SEM, TEM, XRPD, XPS, surface area analyzer, and UV–Vis spectrophotometer, respectively. SEM images showed that the V-doped TiO₂ nanoparticles were porous structures, and the surface areas and pore sizes ranged from 86 ± 9 to 96 ± 15 m²/g and from 12 ± 4 to 15 ± 2 nm, respectively. The XRPD patterns indicated that V-doped mesoporous TiO₂ after calcination at 500 °C was mainly anatase phase, and the crystallite sizes were in the range 14–16 nm, which are consistent with the results obtained from SEM images. XPS spectra and HRTEM images showed that vanadia was doped both on the surface and in the lattice of anatase TiO₂. A slight red-shift in wavelength absorption was observed when V/Ti ratio increased from 0 to 2 wt%. Methylene blue (MB) was further used as the target compound to examine the photocatalytic activity of V-doped mesoporous TiO₂ nanocatalysts under illumination of solar simulator or UV light. Addition of vanadium ions slightly decreased the photocatalytic activity of TiO₂ toward the decolorization of MB under the illumination of UV light at 305 nm. However, a 1.6–1.8 times increase in rate constants for MB photodegradation was observed when 0.5–1.0 wt% V-doped TiO₂ was illuminated with sunlight at AM 1.5.     

Fabrication of free‐standing TiO2 nanotube membranes with through‐hole morphology

In the present work, we show a simple and robust ex‐situ method to fabricate free‐standing membranes consisting of vertically oriented, both‐side‐open TiO₂ nanotube arrays. In this method, self‐organized TiO₂ nanotube membranes with different thickness ranging from seven to tens of micrometers could be easily separated from the metallic Ti substrate by applying a reverse‐bias voltage at the end of anodization. The stress developing at the TiO₂ nanotubes/Ti interface during H₂ gas generation facilitates the separation of the TiO₂ membranes. This procedure leads to an intact, free‐standing TiO₂ nanotube membrane with closed bottoms. After exposing the TiO₂ membrane to HF vapor, the barrier layer at the closed bottoms was etched away, and then a free‐standing TiO₂ membrane with through‐hole morphology was obtained. The bottom‐opening process is a slow barrier layer thinning process. Meanwhile, it is found that the mean diameter of tube bottoms monotonously decreases with the increasing the etching time.     

Influence of TiO2 film on photo-catalytic property of enamels

Ti(OC₄H₉)₄–C₂H₅OH–H₂O was used as matrix solution of titanium dioxide (TiO₂) film. Through sol–gel process, enamels were coated with TiO₂ film. Using methyl-orange as the simulative pollutant solution of photo-catalytic efficiency, we studied photo-catalytic activities of the enamels with and without TiO₂ film. Crystal structure and microstructure of the TiO₂ film were analyzed by means of scanning electron microscope (SEM) and X-ray diffract meter (XRD). Results showed that the photo-catalytic efficiency was greatly improved by the TiO₂ film coated on the enamel surface, and crystal of the TiO₂ film was anatase with imperfect crystal structure at the baking temperature of 450 °C. The TiO₂ film consisted of a lot of very small particles (0.01–0.05 μm and/or 10–50 nano), those particles had very small size and very large surface dimension as well as very high chemical activity, which made TiO₂ film have excellent photo-catalytic degradation.     

Doped-metal oxide nanoparticles for use as photocatalysts

Nano-sized doped-metal oxides such as Fe-doped TiO₂, WO₃-doped ZnO and Fe-doped CeO₂ were synthesized by the modified sol-gel/impregnation, flame spray pyrolysis and homogeneous precipitation/impregnation methods respectively. The crystalline phase, particles size, and crystallinity of nano-sized powder were analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Specific surface area of the sample was examined by the Brunauer, Emmett and Teller (BET) adsorption–desorption of nitrogen gas. The photocatalytic activity of Fe-doped TiO₂, WO₃/ZnO, Fe-doped CeO₂ nanoparticles was examined by studying the mineralization of methanol, sucrose, glucose, oxalic acid and formic acid under UV and visible light irradiations in a pyrex spiral photoreactor. It was found that doped metal oxides could improve the photocatalytic activity of the pure metal oxides.     

Mn, Mg, and Zn ilmenite group titanates: A reconnaissance rietveld study

Ecandrewsite-geikielite (Zn_1/2Mg_1/2TiO₃), ecandrewsite-pyrophanite (Zn_1/2Mn_1/2TiO₃), pyrophanite-geikielite (Mg_1/2Mn_1/2TiO₃), and ecandrewsite-pyrophanite-geikielite (Zn_1/3Mn_1/3Mg_1/3TiO₃) titanates were synthesized at 900–1000°C at ambient pressure. All of the compounds adopt R— ilmenite-type structures. These structures were refined by the Rietveld method from X-ray powder diffraction data. Unit cell parameters and unit cell volumes decrease with decreasing average radii of the A ²⁺ cation. All structures consist of distorted AO ₆ and TiO₆ octahedra. In common with pyrophanite and ilmenite, the distortion of the AO ₆ octahedra is less than that of the TiO₆ octahedra. Data are given for the volumes and distortion indexes of all coordination polyhedra. Published in Russian in Kristallografiya, 2006, Vol. 51, No. 3, pp. 417–424. The text was submitted by the authors in English.