Development of multifunctional photoactive self-cleaning glasses

Glasses for architecture must have many functions in addition to their transparency. For example, the glasses with the functions, of self-cleaning, light control, UV reduction, anti-bacterial, energy conversion, and so on, will be used in buildings in the near future. This paper reviews some results on multifunctional photoactive glasses based on multi-layer coatings containing TiO₂ film and other functional coatings developed by us recently. The self-cleaning of glasses can be realized by coating the photoinduced super-hydrophilic nanoporous thin films based on TiO₂ photocatalysts via sol–gel route. A new method to enhance the photocatalytic activity of TiO₂ thin films direct coated on soda-lime glass was developed by treating the films in acidic solutions. The films also have good photoinduced anti-bacterial properties. The doping of a small amount of silver into the TiO₂ porous film can enhance its anti-bacteria effect without UV light irradiation. The TiO₂ thin films by appropriate heat-treatment can operate as self-cleaning glass in the visible light region. The UV reduction self-cleaning glasses are prepared by magnetron sputtering two layers of TiO₂–CeO₂ and TiO₂ thin films on soda-lime glasses. The TiO₂–CeO₂ thin films can cut all of UV light through adjusting the ratio of TiO₂ and CeO₂. The TiO₂/TiN/TiO₂ type multi-layer coated on glass substrate can act as low-E self-cleaning glass. The potential water-repellent coating based on TiO₂ is discussed finally.     

Formation of nanoporous and nanocrystalline anatase films by pyrolysis of PEO–TiO2 hybrid films

Poly(ethylene oxide)–titania (PEO–TiO₂) organic–inorganic hybrid films were synthesized through a sol–gel process and spin coating. The mixed precursor sols were aged for 6 and 12 h, respectively, prior to the spin coating. Hybrid films were crystallized to the anatase phase first and then further heat treated at elevated temperatures for different time periods to analyze the anatase-to-rutile phase transformation. Quantitative X-ray diffraction (Q-XRD) results were employed for Johnson–Mehl–Avrami (JMA) kinetic analyses and the Avrami exponent (n) was determined for each film. The Arrhenius plots were created based on the JMA plots and the activation energy (Q) values for the phase transformation were determined for each film. The difference in the phase transformation kinetics in the films was discussed based upon the difference in the activation energy values and thus the molecular structures of each hybrid film. Nanoporous and nanocrystalline anatase films were successfully achieved through the retarded phase transformation during the pyrolysis of the organic–inorganic hybrid films. Owing to the maximized surface area these nano-structured anatase films are highly expected to show the enhanced photocatalytic efficiency compared to common TiO₂ films as well as sintered bulk forms.     

Study of the phase transition and the thermal nitridation of nanocrystalline sol-gel titania films

Nanocrystalline titania films were prepared by a complexing agent-assisted sol-gel method and converted to titanium nitride by a thermal nitridation process. The effect of acetylacetone (AcAc), diethanolamine (DEA) and acid catalysts (HCl and HNO₃) on the structure and morphology of the heat-treated titania films and on their nitridation products was examined by FTIR spectroscopy, X-ray diffraction (XRD) and atomic force microscopy (AFM). The carbothermal reduction of titania during the nitridation process with the formation of carboxynitrides has been considered. The results showed that the oxide to nitride transition strongly depends on the complexing agent used to prepare the titania films. The XRD results indicated the dependence of the lattice parameter of the nitridation product on the complexing agent or acid catalyst: AcAc and DEA lead to TiN_x with a lattice parameter α close to the theoretical value, while with HCl the lattice parameter was found sensibly lower showing the presence of an oxynitride. These results are accounted for by the effect of complexing agents and acid catalysts on the size of both TiO₂ and TiN grains and the different reactivity of the anatase and rutile phases. The possibility of tailoring the composition and morphology of TiN films by using complexing agents is envisaged.     

Morphological alteration of anatase titania nanostructures depend on the amount of Na ion intercalation

Controllable preparation of different nanoscale‐shaped titania materials was realized by hydrothermal treatment of anatase TiO₂ in an alkaline solution. The gradually changing morphologies and microstructures of titanium dioxide were investigated by powder X‐ray diffraction, scanning electron microscopy and transmission electron microscopy. And a top‐down path is illuminated to have an insight to the morphological evolution from nanoparticle to nanosheet by adjusting the concentrations of Na ion in the aqueous solution. The results of photocatalytic experiments indicated that the TiO₂ nanobelts exhibited enhanced photocatalytic performance, due to their lower electron‐hole recombination rate confirmed from the photoluminescence spectra. This study suggests that the photocatalysis efficiency of nanocrystals can be significantly improved by the shape‐dependent morphological transformation.     

High performance visible light responsive photocatalysts for environmental cleanup via solution processing

Fabrication of nitrogen-doped titania and/or strontium titanate nanoparticles via soft chemical reactions and their performance for environmental cleanup under visible light irradiation were introduced. Nitrogen-doped anatase, brookite and rutile with high specific surface area can be selectively prepared by the solvothermal reactions in mixed aqueous solution of titanium trichloride-hexamethylenetetramine. Nitrogen-doped strontium titanate can be prepared by mechanochemical reaction of strontium carbonate, titania and hexamethylenetetramine using a planetary ball-mill. Nitrogen-doped titania shows excellent photocatalytic activity for the nitrogen monoxide destruction under visible light irradiation. The photocatalytic activity of nitrogen-doped titania is in the order anatase > brookite > rutile. The photocatalytic activity of nitrogen-doped titania and strontium titanate can be improved by co-doping with higher valence metal ions to reduce oxygen vacancy and/or coupling with Fe₂O₃ and Pt to retard quick recombination of photo-induced electron and hole by the heterogeneous electron transfer.     

Hydroxyapatite/Anatase Photocatalytic Core–Shell Composite Prepared by Sol‒Gel Processing

The conditions for preparing hydroxyapatite/anatase Ca₁₀(PO₄)₆(OH₎₂/TiO₂ nanocomposites with a core–shell structure by sol?gel processing have been optimized. The photocatalytic activity of these nanocomposites is close to that of the commercial photocatalysts based on titanium dioxide (precursor Ti(OBut)₄ concentration 40 vol %, hydroxyapatite/TiO₂ sol ratio 1: 2, annealing temperature 500°C). The photocatalytic activity of hydroxyapatite/TiO₂ composites with different anatase contents has been estimated for the first time from the singlet oxygen yield. It is shown that the degree of modification of apatite particle surface affects significantly the structural characteristics of the hydroxyapatite/TiO₂ composite. An increase in the relative anatase content reduces the sizes of apatite crystallites and increases their specific surface.     

Photo-chargeable titanium/vanadium oxide composites

Titanium and vanadium oxides were prepared by a sol-gel technique and supercritical drying and evaluated for light energy conversion and storage in a wet-type photoelectrochemical cell. Ultraviolet light irradiation to the nanocrystalline titania aerogel (anatase) produced significant photocurrents, and the maximum incident photon-to-current conversion efficiency (IPCE) attained 37.1%. From cyclic voltammetry, the vanadium oxide gel was shown capacitive for charge storage, associated with the shcherbinaite structure and V⁴⁺/V⁵⁺ redox pair. Photopotential responses revealed that coupled TiO₂/V₂O₅ composites not only were photo-chargeable but also exhibited a greater discharging capacity than the TiO₂ or V₂O₅ alone. The discharging capacity was remarkably reduced in the presence of dissolved oxygen. Results demonstrate that in the hybrid TiO₂/V₂O₅ system the V₂O₅ serves to accumulate photoelectrons generated by the illuminated TiO₂ during the photo-charging process. A schematic energy diagram that describes the band structure of the composite semiconductor is proposed.     

Surface enhanced Raman scattering in an amorphous matrix for Raman amplification

In order to improve the efficiency of Raman Amplifiers, the Surface Enhanced Raman Scattering (SERS) effect of an amorphous matrix of TiO₂ was studied. First, optimisation of the amorphous layer quality was performed by depositing thin films on glass substrates at different temperatures. Then, thin films of amorphous TiO₂ were deposited on silicon commercial gold SERS substrates (Klarite®) by a dip-coating process. The SERS effect was demonstrated by the great difference of Raman intensities of the amorphous TiO₂ matrix dip-coated on active and inactive parts of Klarite® substrate under 633 nm and 780 nm laser excitations in the tail of the Surface Plasmon Resonance band of gold nanoparticles.     

Effects of ion exchange and calcinations on the structure and photocatalytic activity of hydrothermally prepared titanate nanotubes

Titanate nanotubes (TiNTs) were prepared by alkaline hydrothermal processing. The TiNTs are thermodynamically unstable and easily transformed to the titania phase by heat or acid treatment. These phase transformations are affected by the preparation conditions. In this study, we investigated the effects of using the washing process to modify the sodium content of the TiNTs. After an alkaline hydrothermal process was used to prepare the TiNTs, the resulting suspensions were washed with weak acid solution and distilled water until the pH value of the wash solution reached approximately 1 or 7, and the products were identified as H‐TiNTs or Na‐TiNTs, respectively. The characteristics and photocatalytic activities of the H‐TiNTs and Na‐TiNTs were compared for various calcination temperatures. The H‐TiNTs were transformed completely to anatase‐type TiO₂ by dehydration during calcination, while the crystallinity of the Na‐TiNTs increased with calcination temperature. However, the photocatalytic H₂ production rates on calcined H‐TiNTs were much higher than on Na‐TiNTs, which could be attributed to the crystalline anatase phase.     

Synthesis and characterization of visible light responsive N-TiO2 mixed crystal by a modified hydrothermal process

N doped TiO₂ with anatase and rutile mixed crystal were prepared by using tetrabutyl titanate as the precursor via a modified hydrothermal process and calcination at 320 °C. The microstructure and morphology of samples were characterized by XRD, UV-vis-DRS, FTIR and XPS. The results showed that N-TiO₂ particles were crystallized to anatase and rutile mixed crystal structure; they were presented narrow particle size distribution, and the average particle size was ca. 13.5 nm calculated from XRD results. It was found that the N-doped TiO₂ particles showed strong visible-light absorption and high photocatalytic activity for the mineralization of Rhodamine B under irradiation by visible light (400-500 nm). The high visible-light photocatalytic activity of the obtained N-doped TiO₂ might result from the synergetic effect of nitrogen doping and the mixed lattice structure of N-TiO₂. Possible mechanism of N-TiO₂ mixed crystal formed under hydrothermal conditions was discussed.     

Characterization of ramiform precipitates formed on SiO2–TiO2 gel coatings by electric field hot water treatment

The shape of the precipitates on sol–gel derived SiO₂–TiO₂ coatings at the negative electrode changed from granular to ramiform by applying an electric field to the substrates during a hot water treatment, whereas such changes in the shape of titania nanocrystals with the electric field were not observed at the positive electrode. The granular and ramiform precipitates were identified as anatase (TiO₂) and hydrated titania (n(TiO₂) · mH₂O), respectively. The ramiform shape of the titania precipitates became significant with increasing the applied voltage, while the coatings gradually became dark-colored due to the reduction of Ti⁴⁺ to Ti³⁺. The coatings with ramiform precipitates showed an excellent wettability for water.     

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.     

Substrate effect on microstructure and optical performance of sputter‐deposited TiO2 thin films

This study deals with the role of the different substrates on the microstructural, optical and electronical properties of TiO₂ thin films produced by conventional direct current (DC) magnetron sputtering in a mixture of pure argon and oxygen using a Ti metal target with the aid of X–ray diffractometer (XRD), ultra violet spectrometer (UV–vis) and atomic force microscopy (AFM) measurements. Transparent TiO₂ thin films are deposited on Soda lime glass, MgO(100), quartz and sitall substrates. Phase purity, surface morphology, optical and photocatalytic properties of the films are compared with each other. It is found that the amplitude of interference oscillation of the films is in a range of 77‐89%. The transmittance of the film deposited on Soda lime glass is the smallest while the film produced on MgO(100) substrate obtains the maximum transmittance value. The refractive index and optical band gap of the TiO₂ thin films are also inferred from the transmittance spectra. The results show that the film deposited on Soda lime glass has the better optical property while the film produced on MgO(100) substrate exhibits much better photoactivity than the other films because of the large optical energy band gap. As for the XRD results, the film prepared on MgO(100) substrate contains the anatase phase only; on the other hand, the other films contain both anatase and rutile phases. Furthermore, AFM images show that the regular structures are observed on the surface of all the films studied. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical properties of Rh6G molecules ordering in the silica- and titanium dioxide mesoscopic sol-gel films

The sol-gel method has been employed for the fabrication high quality mesostructured hybrid films. The effect of the nature of the matrix on the absorption and fluorescence spectra was found. It was shown that the aggregation of the dye molecules is significantly weaker in the SiO₂-P123/Rh6G based hybrid films. In case of TiO₂-P123/Rh6G films, the loss of isosbestic points in the absorption spectra in a TiO₂ matrix suggests the formation of higher aggregates and spectra concern mostly the red shift of the fluorescence maximum. Optical properties and exited state dynamic of Rh6G make the film a good candidate for producing photonic materials.     

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.     

Preparation and characterization of ZnO-TiO2 films obtained by sol-gel method

The sol-gel route has been applied to obtain ZnO-TiO₂ thin films. For comparison, pure TiO₂ and ZnO films are also prepared from the corresponding solutions. The films are deposited by a spin-coated method on silicon and glass substrates. Their structural and vibrational properties have been studied as a function of the annealing temperatures (400-750 °C). Pure ZnO films crystallize in a wurtzite modification at a relatively low temperature of 400 °C, whereas the mixed oxide films show predominantly amorphous structure at this temperature. XRD analysis shows that by increasing the annealing temperatures, the sol-gel Zn/Ti oxide films reveal a certain degree of crystallization and their structures are found to be mixtures of wurtzite ZnO, Zn₂TiO₄, anatase TiO₂ and amorphous fraction. The XRD analysis presumes that Zn₂TiO₄ becomes a favored phase at the highest annealing temperature of 750 °C. The obtained thin films are uniform with no visual defects. The optical properties of ZnO-TiO₂ films have been compared with those of single component films (ZnO and TiO₂). The mixed oxide films present a high transparency with a slight decrease by increasing the annealing temperature.     

Photocatalytic TiO2 films prepared by chemical vapor deposition at atmosphere pressure

Nanometer semiconductor titanium dioxide thin films have excellent properties of photocatalysitic degeneration. The glass coated TiO₂ films is called self-cleaning glass. In this paper, chemical vapor deposition (CVD) method is employed to deposit TiO₂ films on glass. The effect of spacing between nozzle and glass and the effect of substrate temperature on deposition rate are studied. The crystalline structure at different deposition temperatures is analyzed by XRD and anatase was found. And the microstructure of the TiO₂ films is also determined by SEM. It shows that the size of crystal grain increases with the temperature.     

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.     

Solvothermal synthesis of mesoporous slabstone‐like TiO2 and its photodegradation preference in a methyl orange‐rhodamine B mixture solution

Mesoporous slabstone‐like anatase TiO₂ micro‐nanometer composite structure has been successfully synthesized by a facile solvothermal method at 180 °C using polyethylene glycol (PEG) as a structure‐directing agent, followed by calcination at 400 °C for 2 h. The crystal structure and morphology of the product were characterized by XRD, SEM, TEM and HRTEM. Its BET specific surface area was obtained from N₂ adsorption‐desorption isotherm measurement. Rhodamine B (RB) aqueous solution was used to evaluate the photocatalytic activity of the as‐prepared TiO₂ under simulated sunlight irradiation and compared with that of commercial TiO₂ (P25). A RB and methyl orange (MO) coexisting solution was chosen to investigate the photodegradation preference of the slabstone‐like TiO₂ on these two dyes. The results show that the photocatalytic activity of the as‐prepared TiO₂ is much higher than that of P25, and MO is the preferential degradation species in the MO‐RB mixture solution.     

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.