Synthesis of nanosized TiO2/SiO2 particles in the microemulsion and their photocatalytic activity on the decomposition of p-nitrophenol

Nanosized pure TiO₂ particles were prepared by hydrolysis of TTIP in the sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. TiO₂/SiO₂ nanoparticles were also prepared from TEOS as a silicon source and TTIP as a titanium source. These particles were characterized by TEM, XRD, FT-IR, BET, TGA and DTA. From thermal analysis and XRD analysis, the anatase structure of pure titania appeared in the 300–600 °C calcination temperature range and the rutile structure was showed above 700 °C. However, no rutile phase was observed for the TiO₂/SiO₂ particles up to 800 °C. The crystallite size decreased and the surface area of TiO₂/SiO₂ particles monotonically increased with an increase of the silica content. From FT-IR analysis, the band for Ti–O–Si vibration was observed and the band intensity for Si–O–Si vibration increased with an increase of the silica content. The micrographs of TEM showed that the TiO₂/SiO₂ nanoparticles had a spherical and a narrow size distribution. In addition, TiO₂/SiO₂ particles showed higher photocatalytic activity than pure TiO₂ and the TiO₂/SiO₂ (90/10) particles showed the highest activity on the photocatalytic decomposition of p-nitrophenol.     

Hydrothermal synthesis of titanium dioxides from peroxotitanate solution using different amine group-containing organics and their photocatalytic activity

Nanosized TiO₂ particles were prepared by hydrothermal method of the amorphous powders which were precipitated in an aqueous peroxotitanate solution using different amine group-containing organics. The physical properties of prepared nanosized TiO₂ particles were investigated. We also examined the activity of TiO₂ particles as a photocatalyst for the decomposition of orange II. The TiO₂ particles calcined at 400 °C were shown to have a stable anatase phase which has no organic compounds. The particles size of titania particles decreased from 15 to 10 nm as the carbon chain length increased. The titania nanoparticles were shown to have a polygonal shape prepared using NH₄OH and tetramethylammonium hydroxide (TMAOH) as additives, however, the micrographs showed the spherical and narrow size distribution prepared using tetraethyl-ammonium hydroxide (TEAOH) and tetrabutylammonium hydroxide (TBAOH). The titania particles prepared using TEAOH as an amine group-containing organic showed the highest activity on the photocatalytic decomposition of orange II. In addition, the titania particles calcined at 500 °C showed the highest activity on the photocatalytic decomposition of orange II.     

Preparation and characterization of sol–gel derived mesoporous titania spheroids

The formation of mesoporous spherical titania particles via hydrolysis of pure titanium tetra-isopropoxide in n-heptane solution upon the application of a slow stirring rate is described. Calcination of the dry hydrolysis product produced pure anatase at 400–600°C, and rutile at 800°C. Nitrogen adsorption results indicate high surface area (S_BET 132 m²/g) and uniform mesopores peaking at 10 nm for the material calcined at 400°C. Upon calcination at 600°C, the pore size remained at 10 nm, whereas the S_BET value was decreased. The material calcined at 400°C was found by scanning electron microcopy to be shaped into spherical particles about 2 μm in diameter. Sizes of the spherical particles were unchanged at 400°C and up to 800°C. This was ascribed to the spherical morphology of the particles which prevented primary particles from growing beyond the boundary of the host aggregate even when the rutile phase transition occurred at 800°C.     

Photoactivity of anatase–rutile TiO2 nanocrystalline mixtures obtained by heat treatment of homogeneously precipitated anatase

Nanosized titanium dioxide photocatalysts with varying amount of anatase and rutile phases have been synthesized. Homogeneous precipitation of aqueous solutions containing TiOSO₄ with urea was used to prepare porous spherical clusters of anatase TiO_2. Photoactive titania powders with variable amount of anatase and rutile phases were prepared by heating of pure anatase in the temperatutre range 800–1150 °C. The structure evolution during heating of the starting anatase powders was studied by XRD analysis in overall temperature range of phase transformation. The morphology and microstucture characteristics were also obtained by HRTEM, BET and BJH. The spherical particle morphology of TiO₂ mixtures determined by SEM was stable in air up to 900 °C. The photocatalytic activity of the sample titania TIT85/825 heated to 825 °C in air, contained 77.4% anatase and 22.6% rutile was higher than that nanocrystalline anatase powder. Titania sample TIT85/825 reveals the highest catalytic activity during the photocatalyzed degradation of 4-chlorophenol in aqueous suspension.     

INFLUENCE OF CALCINATION ON MICROSTRUCTURES AND PHOTOACTIVITIES OF ALKOXIDE-DERIVED TiO2 NANOPARTICLES PREPARED IN W/O MICROEMULSIONS

Ultrafine titania particles were synthesized by hydrolysis of titanium tetraisoproxide (TTIP) in the nanodroplets of water/NP-5/cyclohexane microemulsions. The as-prepared particles were amorphous, transformed into the anatase phase at 450°C, and completely into the rutile phase at 700°C. The amorphous-to-anatase phase transition temperature decreased with increasing water/surfactant molar ratio. With increasing temperature from 500 to 900°C, the crystallite size increased about twice from 11.7 to 24.4 nm, while the size of the secondary particles, agglomerates of the primary panicles, increased by a factor of about 10. The particles grew largely by intra-agglomerate densification below 700°C, whereas they grew by interagglomerate densification above 700°C. The anatase phase formed at 500-600°C showed considerable photoactivity for the degradation of phenol, whereas both the amorphous phase at 300°C and the rutile phase at 700°C were almost inactive for this reaction.     

Effect of the redox treatment of Pt/TiO2 system on its photocatalytic behaviour in the gas phase selective photooxidation of propan-2-ol

Several titania systems were synthesized by the sol–gel method using two different titanium precursors (titanium isopropoxide or tetrachloride) and diverse ageing methods (magnetic stirring, sonication, reflux and microwave radiation). Screening of such different synthetic conditions led us to choose titanium isopropoxide as the titanium precursor and sonication as the method of choice for ageing the gel. Application of the method to the synthesis of a platinum-doped system resulted in a solid with a BET surface area of 57 m²/g and consisting of 100% anatase titania. The system was submitted to different oxidative and reductive treatments in order to study the effect of such treatments on catalytic performance in gas-phase selective photooxidation of propan-2-ol. Interestingly, both oxidation and reduction at 850 °C led to an increase in molar conversion and selectivity to acetone as compared to calcination at 500 °C. So much so, that oxidation at 850 °C either in synthetic air flow or in static air resulted in better catalytic performance than Degussa P25, despite the fact that our catalysts consisted in very low surface area (6–8 m²/g) rutile titania specimens. XPS analyses of the systems showed that thermal treatment at 850 °C resulted in electron transfer from titania to Pt⁰ particles through the so-called strong metal-support interaction (SMSI) effect. Furthermore, the greater the SMSI effect, the better the catalytic performance. Improvement in photocatalytic activity is explained in terms of avoidance of electron–hole recombination through the electron transfer from titania to platinum particles.     

Synthesizing and Comparing the Photocatalytic Properties of High Surface Area Rutile and Anatase Titania Nanoparticles

Rutile titania nanocrystalline particles with high specific surface areas were directly prepared by thermal hydrolysis of titanium tetrachloride aqueous solution. The as-prepared rutile titania powder was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller surface area analysis, and Fourier transform Raman and IR spectroscopies. Neither anatase nor amorphous titania could be detected in this titania powder by XRD, Raman spectroscopy, and high-resolution TEM. In the phenol degradation reaction, the rutile titania powder with an initial crystalline size of 7 nm was found to have higher photocatalytic activity than that of anatase titania with the same specific surface area. The rutile titania powders calcined at 300° and 450°C also showed a relatively high photocatalytic property. The high activity of the as-prepared rutile titania was attributed to the abundance of hydroxy groups in the powder, as was proven by thermogravimetric analysis data, which provided more active sites for the degradation reaction.     

Hydrophilic/hydrophobic conversion of Ni-doped TiO2 thin films on glass substrates

Nickel-doped titanium oxide thin films were prepared on soda–lime–silica glass substrates by using a metal naphthenate. Films prefired at 500 °C for 10 min were finally annealed at 600 °C for 30 min in air. Contact angle measurement was used for analyzing hydrophilic/hydrophobic conversion. NiTiO₃, rutile and anatase peaks were obtained for the film after nickel doping. The film containing nickel showed a shift towards the visible in the absorption threshold.     

Control of Phase and Pore Structure of Titania Powders Using HCl and NH4OH Catalysts

Porous titania powders were prepared by hydrolysis of titanium tetraisopropoxide (TTIP) and were characterized at various calcination temperatures by nitrogen adsorption, X-ray diffraction, and microscopy. The effect of HCl or NH₄OH catalysts added during hydrolysis on the crystallinity and porosity of the titania powders was investigated. The HCl enhanced the phase transformations of the titania powders from amorphous to anatase as well as anatase to rutile, while NH₄OH retarded both phase transformations. Titania powders calcined at 500°C showed bimodal pore size distributions: one was intra-aggregated pores with average pore diameters of 3–6 nm and the other was interaggregated pores with average pore diameters of 35–50 nm. The average intra-aggregated pore diameter was decreased with increasing HCl concentration, while it was increased with increasing NH₄OH concentration.     

Structural peculiarities of TiO2 and Pt/TiO2 catalysts for the photocatalytic oxidation of aqueous solution of Acid Orange 7 Dye upon ultraviolet light

TiO₂ (anatase) with different microstructure was synthesized by thermal hydrolysis of the titanyl sulfate and studied by X-ray powder diffraction, high resolution transmission electron microscopy and UV–vis diffuse reflectance spectroscopy. The effect of titanium dioxide structure, regular or distorted, on the photocatalytic degradation of Acid Orange 7 Dye (AO7) in water upon ultraviolet light was studied. It was found that synthesized TiO₂ possesses a relatively high reactivity when illuminated but also show different adsorption in the dark. The relationship between these behaviors depends on the real structure of the catalysts. Catalysts with a perfect structural ordering formed after heating at temperature higher than 500 °C show better photocatalytic performance. Small amount of Pt added into the TiO₂ structure was found to improve further the catalyst reactivity. Pt-modified titania catalysts oxidize AO7 more efficiently than P-25 Degussa TiO₂. Doping effect of Pt on the structural and photocatalytic properties of the samples is discussed.     

Hydrothermal synthesis of titanium dioxide using acidic peptizing agents and their photocatalytic activity

We have prepared TiO₂ nanoparticles by the hydrolysis of titanium tetraisopropoxide (TTIP) using HNO₃ as a peptizing agent in the hydrothermal method. The physical properties of nanosized TiO₂ have been investigated by TEM, XRD and FT-IR. The photocatalytic degradation of orange II has been studied by using a batch reactor in the presence of UV light. When the molar ratio of HNO₃/TTIP was 1.0, the rutile phase appeared on the titania and the photocatalytic activity decreased with an increase of HNO₃ concentration. The crystallite size of the anatase phase increased from 6.6 to 24.2 nm as the calcination temperature increased from 300 °C to 600 °C. The highest activity on the photocatalytic decomposition of orange II was obtained with titania particles dried at 105 °C without a calcination and the photocatalytic activity decreased with increasing the calcination temperature. In addition, the titania particles prepared at 180 °C showed the highest activity on the photocatalytic decomposition of orange II. This paper was prepared at the 2004 Korea/Japan/Taiwan Chemical Engineering Conference held at Busan, Korea between November 3 and 4, 2004.     

Carbon-modified TiO2 photocatalyst by ethanol carbonisation

The new photocatalysts based on commercially available titanium dioxide powders: Tytanpol A11 (Police, Poland), pure anatase and P-25 (Degussa, Germany) containing about 20% rutile were modified by carbon via ethanol carbonisation. Titanium dioxides were heated at different temperature from 150 to 400 °C for 1 h in an atmosphere of ethanol vapour. The photocatalytic activity of carbon-modified TiO₂ was studied by oxidation of phenol in water under UV and artificial solar light irradiation. With increasing of carbon content in TiO₂ photocatalysts the activity for phenol decomposition under UV light was decreasing but that under visible light was stable. Turbidity of the slurry solution decreased with increasing of carbon content for all prepared photocatalysts because of the change of their surface character from hydrophilic to hydrophobic.     

Preparation of nanosized TiO2 particles via ultrasonic irradiation and their photocatalytic activity on the decomposition of 4-nitrophenol

TiO₂ nanoparticles were prepared by hydrolysis of TTIP (titanium tetraisopropoxide) using an ultrasonication technique coupled with a sol-gel method. The physical properties of nanosized TiO₂ were investigated. The photocatalytic degradation of 4-nitrophenol was studied by using a batch reactor in the presence of UV light. The crystallite size of the anatase phase is increased with an increase of R_EtOH ratio (EtOH/H₂O molar ratio). The particles’ crystallite size prepared with and without ultrasonic irradiation is marginally different. Those particles prepared with ultrasonic irradiation show a higher activity on the photocatalytic decomposition of 4-nitrophenol compared to those prepared without ultrasonic irradiation. The photocatalytic activity decreases with an increase of R_EtOH ratio. In addition, the photocatalytic activity shows the highest value on the titania particle calcined at 500 ‡C. This paper was presented at the 2004 Korea/Japan/Taiwan Chemical Engineering Conference held at Busan, Korea between November 3 and 4,2004.     

Sol–gel derived mesoporous titania nanoparticles: Effects of calcination temperature and alcoholic solvent on the photocatalytic behavior

In this study, titania nanoparticles were successfully prepared by a sol–gel process, employing titanium (IV) tetraisopropoxide (Ti[OCH(CH₃)₂]₄; TIP) as a starting precursor. Either ethanol or isopropanol was used as an alcoholic solvent. The as-synthesized mesoporous titania was calcined at different temperatures in the range of 300–700 °C. The aim of this study is to investigate the effects of calcination temperature and the types of solvent on the photocatalytic behavior of titania. The results indicate that crystallinity and crystalline phase are important factors influencing the degree of photocatalytic activity of titania. It was found that the photocatalytic property of titania consisting predominantly of anatase crystallites has been markedly improved in the degradation of methylene blue under UVC light. Compared to ethanol, enhanced photocatalytic activity is obtained with isopropanol solvent through the thermal stability of anatase phase.     

Preparation of a porous nanocrystalline TiO2 layer by deposition of hydrothermally synthesized nanoparticles

In this work, a porous nanocrystalline anatase TiO₂ layer is prepared by tape casting a viscous dispersion of nanoparticles. Phase pure anatase titanium dioxide nanoparticles with a particle size of 10–20 nm are prepared by a very simple low temperature (100 °C) hydrothermal synthesis route in a pressure vessel, using only water as the medium and Ti(IV)-isopropoxide as starting material without additives. The size, shape and phase composition of the particles are studied by means of X-ray diffraction and transmission electron microscopy. A dispersion of the as-prepared nanoparticles with a narrow particle size distribution, confirmed by photon correlation spectroscopy, is prepared. After increasing the viscosity of this dispersion by addition of hydroxypropyl cellulose, anatase titanium dioxide layers are tape cast on a transparent conducting metal oxide substrate. Pores are induced by burning out the organic additive at 450 °C. The morphology and the final phase composition of the deposited TiO₂ layers are examined by X-ray diffraction and scanning electron microscopy.     

Solution blow spun titania nanofibers from solutions of high inorganic/organic precursor ratio

In this work titania nanofibers were produced from different precursor solutions by solution blow spinning. Hydrophilic polyvinylpyrrolidone, hydrophobic poly(vinyl acetate) and amorphous and semi-crystalline poly(lactic acid) polymers were used with green-solvents and titanium isopropoxide as the inorganic precursor. Hybrid nanofibers with high inorganic loading content were successfully produced from all precursor solutions. The fibers were calcined at different annealing temperatures for evaluation of phase transitions. The minimum temperature to obtain pure titania fiber was found to be 500 °C, as assessed by thermal characterization. Anatase was the unique polymorph formed at this annealing temperature. This is of paramount importance due to its photocatalytic character. Calcination at high temperatures showed that rutile slightly appeared at 600 °C for the polyvinylpyrrolidone-based system and showed a strong peak at 700 °C for all systems, co-existing with anatase as minor phase. The use of amorphous and semicrystalline poly(lactic acid) polymers did not influence the anatase crystal size and phase conversion. This was found to be dependent on the annealing temperature and medium acidity, as currently found in powder synthesis.     

Porous titania microspheres with uniform wall thickness and high photoactivity

Highly porous titania particles were prepared by depositing thin films of titania, using alternating reactions of TiCl₄ and hydrogen peroxide, on poly(styrene-divinylbenzene) (PS-DVB) template particles via atomic layer deposition (ALD) at 77 °C. The composition of the titania films was verified by XPS analysis and the titania films were directly observed by TEM. TGA/DSC was used to study the thermal decomposition of the polymer template. Porous titania particles with uniform wall thicknesses were successfully obtained after the template PS-DVB was removed by oxidation in air at 400 °C for 24 h. Verification of the resulting porous structure of the titania particles was done by cross-sectional SEM and nitrogen adsorption–desorption analysis. Porous titania particles were treated at different temperatures. XRD analysis was used to determine the microstructure and phase transformation of titania at elevated temperatures. The photocatalytic activity of these porous titania particles was studied by methylene blue decomposition under UV light at room temperature and was found to be comparable to that of commercial anatase titania nanoparticles (~20 nm). Depositing Na₂SO₄ on TiO₂ retarded the TiO₂ phase transformation from anatase to rutile during calcination and, thus, greatly increased the photoactivity of the porous titania particles.     

Investigation of the selective catalytic reduction of NO by NH3 on Fe-ZSM5 monolith catalysts

Photocatalytic nitric oxide (NO) decomposition and reduction reactions, using carbon monoxide (CO) as a reducing gas, have been investigated over Degussa P25 titanium dioxide photocatalysts, using a continuous flow reactor. The effects of thermal pretreatment temperature and reaction gas composition on the activity and selectivity of the decomposition and reduction reactions have been evaluated. Prepared materials were characterised by X-ray diffraction (XRD), N₂ physisorption, transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) and findings from these techniques were used to explain the observed photocatalytic properties. XRD and TEM results indicated that for the pretreatment temperatures used (70, 120, 200, 450 and 600 °C) there was no appreciable change in the phase composition and the original composition of ca. 77 vol.% anatase and 23 vol.% rutile was maintained even after treatment at 600 °C. It was found that the photocatalytic activity for both the decomposition and reduction reactions decreased with increasing pretreatment temperature. This was attributed to the removal of surface hydroxyl species that act as active sites for reaction. For the decomposition reactions the only products observed were nitrogen and nitrous oxide and the selectivity for nitrogen formation remained constant (ca. 23%) regardless of the pretreatment temperature. The presence of CO in the reaction gas had a dramatic effect on the selectivity of the reactions with nitrogen selectivities as high as 65% being observed. It was found that as the CO/NO ratio increased the selectivity for nitrogen formation increased.     

反胶束法纳米TiO_2的UV-vis和TEM研究

用石油醚为溶剂的反胶束体系溶胶-凝胶法合成了纳米TiO2,采用UV-vis光谱跟踪纳米TiO2粒子粒径的变化,研究了影响纳米TiO2溶胶粒子大小的因素,采用TEM表征了纳米TiO2的粒子大小及其粒径分布。结果表明,合成的反胶束纳米TiO2的UV-vis光谱吸收边λonset为344.8~363.2 nm,与锐钛型纳米TiO2的吸收边λonset=385 nm相比,紫外可见光谱发生“蓝移”,反胶束纳米TiO2粒子的半径为5~6.5 nm;TEM表明,纳米TiO2粒子的粒径为15~55 nm,粒径分布较窄,粒径分布指数SD I为1.19~1.29;FTIR谱图表明,TiO2粒子为表面具有一定数量钛羟基的水合TiO2。     

Effect of surfactant in a modified sol on the physicochemical properties and photocatalytic activity of crystalline TiO<Subscript>2</Subscript> nanoparticles

This study describes the effect of amphiphilic organic molecules (surfactants) in a sol on the physicochemical properties and photocatalytic activity of crystalline TiO₂ nanoparticles prepared via a simple sol–gel route at high temperatures from 400 to 800 °C. Addition of polyoxyethylenesorbitan surfactant and polyethylene oxide and polypropylene oxide triblock copolymer as particle size inhibitors and pore directing agents into a stable titania sol affected the physicochemical properties of TiO₂ nanoparticles such as their crystallographic structure, morphology, and defect structure. With the addition of the surfactants, the ratio of anatase and rutile crystal phases of TiO₂ was controlled and an active anatase crystal phase was maintained during heat treatment up to 800 °C. Decrease in the sintering rate and inhibition in crystal growth were also observed, which resulted in higher surface area and inhibition of crystallite aggregation. Bulk defects in TiO₂ were reduced while surface defects were increased as a result of the addition of surfactants. These physicochemical properties of TiO₂ nanoparticles were correlated with photocatalytic degradation of 4-chlorophenol in water. The results revealed that high crystallinity, anatase crystal phase, high specific surface area, surface defects, and segregated morphology of TiO₂ nanoparticles, which were induced by the addition of surfactants, were more advantageous for enhancing photocatalytic destruction of the model organic compound tested in the study.