Undoped Titania Research Articles

Structural investigations on TiO 2 and Fe-doped TiO 2 nanoparticles synthesized by laser pyrolysis

Undoped and Fe-doped TiO 2 nanopowders with Fe/Ti (atomic ratio) precursor concentration ranging from 7% up to 25% have been prepared by the IR laser pyrolysis technique. A sensitized mixture of TiCl 4 and Fe (CO) 5 was used as titanium and iron precursor, respectively. Reference undoped titania samples with a major concentration of anatase phase (about 90%) were obtained by the same technique by using very high flows of the oxidizing agent (air). The effects of the iron-dopant concentration on the essential structural properties of the resultant powders such as the phase formation, the crystallinity, the average particle size and distributions were systematically investigated by X-ray diffraction, Raman spectroscopy and transmission electron microscopy. The decrease of the TiO 2-anatase crystalline phase, the simultaneous increase of the amorphous phase and the decrease in size of particle mean diameter appear as main effects induced by the Fe-dopant concentration.

Photocatalytic Activity of Titania and Zirconia-Doped Titania Nanocrystals Prepared by Surfactant-Assisted Templating Method

The photocatalytic activity of undoped and ZrO2-doped titania nanocrystals, prepared by a surfactant-assisted templating method (SATM), was investigated based on the formation of tri-iodide (I3 -) species in titania suspension in KI solution under UV-irradiation. Optimum dopant concentration was found to be 0.5mol% ZrO2-doping in titania. The effects of dopant concentrations and gelation conditions (gelation at 80oC, and gelation under hydrothermal treatment) on the photocatalytic activity of the synthesized titania-based nanocrystals are also presented.

Antimicrobial function of Nd 3+-doped anatase titania-coated nickel ferrite composite nanoparticles: A biomaterial system

The present study describes and makes a relative comparison of the antimicrobial function of undoped and neodymium-doped titania coated-nickel ferrite composite nanoparticles processed by uniquely combining the reverse micelle and chemical hydrolysis approaches. This methodology facilitates the formation of undoped and doped photocatalytic titania shells and a magnetic ferrite core. The ferrite core is needed to help in the removal of particles from the sprayed surface using a small magnetic field. Doping of the titania shell with neodymium significantly enhances the photocatalytic and anti-microbial function of the core–shell composite nanoparticles without influencing the magnetic characteristics of the nickel ferrite core. The increased performance is believed to be related to the inhibition of electron–hole recombination and a decrease in the band gap energy of titania. The retention of magnetic strength ensures controlled movement of the composite nanoparticles by the magnetic field, facilitating their application as removable anti-microbial photocatalyst nanoparticles. The consistent behavior of the composite nanoparticles points to the viability of the synthesis process adopted.

UV and Visible Light Acrylate Photopolymerisation Initiated by Nitrogen- or Carbon-Doped Titanium Dioxide

Summary Nitrogen and carbon doped titania powders initiate acrylate polymerisation upon UV excitation. Time resolved photovoltage measurements afford surface charge carrier lifetimes of 7–8 ms for undoped and N-doped titania, whereas 12 ms are observed for the C-doped semiconductor. These values correlate with the observation that the polymerisation rate constants are similar for titania or N-doped titania but about two times larger for the C-doped material. Wavelength dependent polymerisation experiments indicate that visible light polymerisation is possible only with carbon-doped titania. This suggests that hole relaxation to dopant-centered surface states is much more efficient in nitrogen-doped titania. These experimental findings are in accord with the assumption that polymerisation is initiated by hole oxidation of the acrylate.

Highly sensitive gas sensor based on integrated titania nanosponge arrays

Highly sensitive gas sensors were fabricated using nanostructured titania pad arrays. Nanostructured titania (NST) formed is sponge-like consisting of interconnected nanoscale wires and walls, which are made up of anatase nanocrystals. Electrical characterization indicates that contacts were ohmic and NST was highly sensitive to O2. Variations of hundreds of oxygen molecules over a 20μm NST square pad sensing element were detected at 250°C. The NST-based sensor operates at lower temperatures, has fast response time, and superior sensitivity relative to oxygen sensors based on porous undoped titania reported in the literature.

Optical properties of in situ doped and undoped titania nanocatalysts and doped titania sol–gel nanofilms

In this paper we present spectroscopic properties of doped and undoped titanium dioxide (TiO 2 ) as nanofilms prepared by the sol–gel process with rhodamine 6G doping and studied by photoacoustic absorption, excitation and emission spectroscopy. The absorption spectra of TiO 2 thin films doped with rhodamine 6G at very low concentration during their preparation show two absorption bands, one at 2.3 eV attributed to molecular dimmer formation, which is responsible for the fluorescence quenching of the sample and the other at 3.0 eV attributed to TiO 2 absorption, which subsequently yields a strong emission band at 600 nm. The electronic band structure and optical properties of the rutile phase of TiO 2 are calculated employing a fully relativistic, full-potential, linearized, augmented plane-wave (FPLAPW) method within the local density approximation (LDA). Comparison of this calculation with experimental data for TiO 2 films prepared for undoped sol–gels and by sputtering is performed.

Unprecedented ultra-high hydrogen gas sensitivity in undoped titania nanotubes

A highly ordered array of micron-length undoped titania nanotubes exhibits anunprecedented variation in electrical resistance of about 8.7 orders of magnitude(50 000 000 000%), at room temperature, when exposed to alternating atmospheres ofnitrogen containing 1000 ppm hydrogen and air. This represents the largest known changein electrical properties of any material, to any gas, at any temperature. The nanotubearrays were fabricated using anodic oxidation of titanium foil in a pH 4.0 electrolytecontaining potassium fluoride, sodium hydrogen sulfate monohydrate and sodium citratetribasic dihydrate. The dramatic change in resistance is believed to be due to the highlyactive surface states on the nanoscale walls of the tubes, high surface area of the nanotubearchitecture, and the well-ordered geometry allowing for hydrogen-sensitive tube-to-tubeelectrical connections.

Microstructure and photocatalytic properties of nanostructured TiO 2 and TiO 2–Al coatings elaborated by HVOF spraying for the nitrogen oxides removal

This paper deals with the study of the photocatalytic behaviours of TiO 2 and TiO 2–10 wt.% Al coatings elaborated by high-velocity oxygen fuel (HVOF) spraying using spray-dried nanosized feedstock powders. In the HVOF flame, the powders were injected by two methods: internal injection, i.e. as in conventional HVOF process and external injection, i.e. outside the torch nozzle. Scanning electron microscopy and X-ray diffraction were employed to characterize the microstructure and the crystalline structure of the TiO 2-based coatings. The photocatalytic activities were evaluated by the nitrogen oxides removal and compared with that of the initial powders. It was found that the amount of anatase in the coatings depends on the nature of the powder and also on the type of injection method. The TiO 2 coatings, as well as, the TiO 2–Al coatings elaborated using the external powder injection presented a better photocatalytic activity (the nitrogen oxides conversion rates were about 25–42% for NO and 14–18% for NO x ) than those obtained by the conventional HVOF process (<5%). Besides, TiO 2 coatings containing aluminium particles had an enhanced photocatalytic activity than those of un-doped titania deposits.

Doping of TiO 2 with nitrogen to modify the interval of photocatalytic activation towards visible radiation

A simple method for the doping of titanium dioxide with nitrogen was developed. By this technique, commercial titanium dioxide was thermally treated under a nitrogen enriched atmosphere. The N doped TiO 2 presented a slight displacement in the absorption edge of light, towards the visible radiation range. Phenol degradation in water solution was used to compare photocatalytic activity of doped titania. With the treatment at 600 °C, the photocatalitic activity of the N–TiO 2 was improved, and the changes of anatase to rutile phases ratio as well as specific surface area were minimal. The higher activity obtained after TiO 2 treatment could be correlated with the modification in radiation absorption spectra, since the radiation used for the determination of the photocatalytic activity was in the edge of the UV–vis radiation. Treatments at higher temperature shown almost the same activity of undoped titania, but sintering reduced the surface area per gram of TiO 2, thus the photocatalytic activity of doped surface was more active (moles of phenol transformed/m 2 h).

Electrochemical Mass Spectroscopic and Surface Photovoltage Studies of Catalytic Water Photooxidation by Undoped and Carbon-Doped Titania

Carbon-doped TiO2, demonstrated as an efficient photocatalyst in visible light photooxidation of organic compounds, was prepared with different doping concentrations and investigated via differential electrochemical mass spectroscopy (DEMS) and capacitive surface photovoltage (SPV) measurements in the form of thin layer electrodes. In all cases the total photocurrent as well as the surface photovoltage of the doped materials decreased markedly in relation to the undoped one. No detectable oxygen evolved from the doped electrodes in acidic solution under UV-light excitation. Since an oxidation of formic acid is still apparent, it is concluded that this oxidation occurs via isolated, catalytically poorly active trap states within the forbidden energy region. The existence of these states is confirmed by capacitive SPV measurements.

Physical and photocatalytic properties of zinc ferrite doped titania under visible light irradiation

Visible light responsive zinc ferrite doped titania photocatalyst (TiO2(ZnFe2O4)) was prepared by sol–gel method and was calcined at different temperatures. Diffuse reflectance spectroscopy (DRS) results show that the absorption edge of TiO2(ZnFe2O4) has moved to the visible spectrum range in comparison with the undoped titania. The photocatalytic experimental result demonstrates that TiO2(ZnFe2O4) powder can effectively photodegrade methyl orange (MO) under visible light irradiation. Effect of calcination temperature on the photocatalytic activity of TiO2(ZnFe2O4) was also investigated. Zeta potential measurements indicate that the isoelectric point (IEP) of the TiO2(ZnFe2O4) powder shifts to lower pH units with the increase of calcination temperature. Brunauer–Emmett–Teller (BET) surface area decreases with the rise of calcination temperature. It is concluded that the surface charge and BET surface area are the key factors for the higher photoactivity of zinc ferrite doped titania photocatalyst calcined at 400°C.

The effects of manganese doping on UVA absorption and free radical generation of micronised titanium dioxide and its consequences for the photostability of UVA absorbing organic sunscreen components.

The effect of manganese doping on the free radical generation rate, free radical scavenging and UVA absorption properties of micronised sunscreen grade titania has been studied with respect to enhancement of the UVA photostability of test sunscreen formulations containing the organic UVA absorber Parsol 1789. Manganese doping has been shown to increase the UVA:UVB absorption ratio of titania, reduce free radical generation rates by over 90%, and provide free radical scavenging behaviour. Adding manganese-doped titania to a test formulation incorporating Parsol 1789 shows that manganese doping increases UVA attenuation stability by up to 3 times the amount achieved by comparable commercial undoped titania materials. HPLC data shows this to be related to an improved stabilisation of the organic sunscreen components. Manganese doped titania shows improved efficacy over undoped titania in sunscreen formulations containing organic UV absorbers.

Influence of ZrO 2 addition on microstructure and liquid permeability of mesoporous TiO 2 membranes

The influence of up to 30 mol% zirconia doping on microstructure, liquid permeability, thermal and chemical stability of mesoporous composite titania/zirconia membranes was investigated. It was demonstrated that zirconia retards the phase transformation of anatase until at least 700 °C. The pore size decreased gradually to 3.6 nm by addition of up to 20 mol% zirconia, while nitrogen sorption experiments showed that the porosity, BET surface area and pore connectivity increased. Chemical stability tests showed that undoped titania and composite titania/zirconia materials were stable in the pH range 1–13 at room temperature. The mechanical strength of titania membranes was significantly improved by addition of zirconia. The permeability of titania and titania/zirconia composite membranes was eight times larger than that of γ-alumina. The permeability was not affected by the level of zirconia doping or by the nature of the permeating liquid.

Structural and spectroscopic studies of iron (III) doped titania powders prepared by sol-gel synthesis and hydrothermal processing

Iron-doped titania powders, of microfine dimensions and iron loadings of up to 5.64 wt.% were prepared by a sol-gel route. Gels, obtained by hydrolysis of titanium alkoxide in an aqueous solution of iron (II) chloride, were subjected to thermal treatment by heating in air or hydrothermal processing in an autoclave. The evolution of the crystalline component was determined by X-ray powder diffraction, and the local environment of the dopant was probed by electron spin resonance spectroscopy. Mössbauer spectroscopy was used to determine the oxidation state of the dopant. Over the iron-loading regime studied here, the anatase-rutile transition is inhibited at low dopant levels with respect to undoped titania. The effect becomes less pronounced as iron-loading is increased. Indeed the crystalline composition of the powders as a function of firing temperature is dependent not only on iron loading level but also on its dispersion within the titania matrix. This response is discussed in the context of the solution-based synthetic routes employed here.

High Temperature Stabilisation of Pores in Sol-Gel Titania in Presence of Silica

Sol-gel titania was prepared by the hydrolysis and condensation reaction of acetic acid modified titanium isopropoxide. Partially hydrolysed tetraethoxy silane was further added to this precursor in order to get titania-silica mixed oxides. The silica content was varied from 1% to 50%. The gels were characterised by thermal analysis, FTIR and XRD analysis. Surface area of titania with out any additive was 320 m2/g. This got considerably reduced to 56 m2/g on tempering at 550°C for a period of one hour and further to 29 m2/g on tempering for 32 hours at the same temperature. However, in the case of silica added samples the surface area becomes constant after one hour of tempering and remains nearly at the same value even up to 32 hours. While undoped titania retains only ∼10% of its initial surface area when tempered at 550°C over a period of 32 hours, a titania-1% silica composition retains up to 26% under identical conditions. This trend further increases with increase in silica content and for a composition titania-50% silica, retention of surface area is as high as ∼80%. Untempered titania and titania-silica samples are highly micro porous in nature. On tempering, titania samples become mesoporous. However a major fraction of the micro porosity is retained in the case of the titania-silica compositions, even after tempering for 32 hours at 550°C, which also contributes to their high surface area. The results of this study are discussed.

Calculation of Zeta Potential from Electrokinetic Measurements on Titania Plugs

The O'Brien theory developed for the electrical conductivity and electro-osmosis in porous plugs consisting of nonconducting particles has been applied for the calculation of ζ-potentials from conductivity and streaming potential measurements at two pH values, 3.3 and 6.5 or 5.8, on plugs composed of titania (undoped and doped with W6+and Li+ions) particles. The particles have been prepared both by precipitation (continuous crystallization at constant supersaturation) and by the sol-gel method. It was found that O'Brien's theory is also applicable inn-type semiconductor particles, such as titania (doped and undoped). Specifically, for the correct determination of ζ-potentials measured in solutions of relatively low ionic strength, it is necessary to take into account surface conduction behind the shear plane, for all samples measured at pH 3.3, a value considerably lower than the isoelectric point (i.e.p.) of titania. Under these conditions the surface charge of the suspended titania particles is high, and consequently the contribution of surface conductance to the conductivity of the plug is significant. The effect of surface conduction behind the shear plane on the calculation of ζ-potential is enhanced for the Li+-doped preparations due to the increase of surface conductivity caused by doping with Li+. Measurements of the streaming potential for the samples prepared with the sol-gel method were made at pH 6.5 (5.8 for the undoped titania); i.e., near the pH corresponding to the i.e.p., correct values of ζ-potential may be obtained in suspensions even at low ionic strength without taking into account the polarization of the double layer due to surface conduction, by applying the primitive Smoluchowski theory. This treatment is valid because at pH values near the i.e.p. the surface charge of the particles is very low, and consequently surface conductance is negligible compared with the conductivity of the plug.

The Dielectric Loss of Single Crystal and Polycrystalline TiO2

ABSTRACTThe dielectric loss of single crystal and polycrystalline TiO2 has been studied. In polycrystalline TiO2 the dielectric loss is determined by both the microstructure and by the oxygen stoichiometry. Experiments have been carried out to determine the influence of both the microstructure (particularly porosity) and the oxygen stoichiometry. The TiO2 powder has been doped with partially stabilised zirconia, an oxygen ion conductor, in order to modify the oxygen stoichiometry. Sintered discs have been examined for loss as a function zirconia doping, pore volume and as a function of temperature. The behaviour of the doped and undoped titania powders is significantly different. Since many microwave dielectric materials contain Ti eg Ba-Ti-O, Ba-Nd-Ti-O, (Ba-RE-Ti-O, RE=Rare Earth), Zr-Sn-Ti-O etc it is essential to understand the role of the titanium, particularly as it can exist in mixed valence states, and the role of oxygen and its influence on the dielectric loss.

Textural evolution and phase transformation in titania membranes: Part 1.—Unsupported membranes

Textural evolution in sol–gel derived nanostructured unsupported titania membranes has been studied using differential scanning calorimetry (DSC), differential thermal analysis (DTA), thermal gravimetry (TG), X-ray diffraction (XRD) and N2 adsorption. The anatase-to-rutile phase transformation kinetics were studied using the Avrami model. The precursor gel had a surface area of ca. 165 m2 g–1, which after heat treatment at 600 °C for 8 h reduced to zero. Undoped titania-gel layers transformed to more than 95% rutile after calcination at 600 °C for 8 h. The causes of surface-area reduction and pore growth were anatase crystallite growth and the enhanced sintering of rutile during transformation. Lanthanum oxide was identified as a suitable dopant for shifting the transformation temperature to ca. 850 °C. Lanthanum oxide doped titania showed an improved stability of porous texture compared to that of the undoped titania membranes.

The photocatalysed reduction of aqueous sodium carbonate using platinized titania

Abstract Precipitated titania, prepared from TiCl 4 or TiCl 3 was photoplatinized. It was suspended in 1 M Na 2 CO 3 solution and irradiated with UV-visible (mercury lamp) or visible (xenon lamp) light. The photoproduced electrons in the conduction band of the semiconductor reduced carbonate ions to CH 3 OH (determined spectrophotometrically), carbon (determined by burning to CO 2 which was trapped and titrated), HCHO (determined spectrophotometrically by the Hantzsch reaction) and HCOO − ions (detected similarly after magnesium-HCl reduction). Doping with chromium and/or manganese ions produced lower yields of products, although the doped samples were more light absorbing than the undoped titania. Prior vacuum heat treatment of a chromium-doped sample or loading with RuO 2 necessitated longer platinization times, but neither treatment led to any significant changes in the yield of reduction products. Two anatase pigments behaved similarly in photocatalysing the reduction of carbonate. When one of these samples was irradiated with a more powerful mercury lamp, there was a marked increase in the yield of products. A tentative explanation is suggested for the observed order of yields: CH 3 OH > C > HCHO ≈ HCOO − .