Nanocrystalline Mesoporous Titania Research Articles

Noble metal free Fe and Cr dual-doped nanocrystalline titania (Ti1−x−yMx+yO2) for high selective photocatalytic conversion of benzene to phenol at ambient temperature

Metal (Fe and Cr)-incorporated mesoporous nanocrystalline titania (Ti1−x−yMx+yO2, or TiMxO2) was synthesized using a simple solution combustion method. Structural characterization, microscopy, and spectroscopy techniques confirmed the incorporation of Fe and Cr into the TiO2 lattice. Photocatalytic benzene oxidation to phenol was investigated under ambient conditions, and under solar stimulator (AM1.5 or one sun condition) and ultraviolet (UV) irradiation. Compared to pristine TiO2, TiO2 doped with 2 at% of Fe (Ti0.98Fe0.02O2) exhibited a higher catalytic activity for benzene-to-phenol oxidation, highlighting the importance of the introduction of the Fe dopant. Introduction of Fe and Cr into the TiO2 (Ti0.98Fe0.01Cr0.01O2) lattice further enhanced the oxidation of benzene to phenol under UV light and gave a phenol yield that was ∼2 times higher than that given by Ti0.98Fe0.02O2. The high photocatalytic activity of Ti1−x−yMx+yO2 and good photocurrent behaviour were mainly ascribed to the generation of an electron trapping level composed of Fe3+ and Cr3+ in the TiO2 conduction band, disordered mesoporosity, high mobility, and a short diffusion length for charge carriers.

Ordered mesoporous nanocrystalline titania: A promising new class of photocatalyic materials

We report the photocatalytic activity of uniform-sized nanocrystalline mesoporous titania synthesized via surfactant-mediated hydrothermal method. All the prepared catalysts were systematically characterized by various analytical, spectroscopic and imaging techniques. The characterization results indicate that all the catalysts under investigation possess uniform-sized nanocrystalline particles having mesoporous pore structure with narrow pore size distribution. The phase transformation occurs from anatase, brookite to rutile phase while increasing the calcination temperature. The performance of mixed phase titania for the photocatalytic degradation of 4-chlorophenol was analyzed. These catalysts show much higher activity than commercial titania (Aeroxide; P-25) owing to synergistic effect of stable mesoporous structure and mixed crystalline phases of titania.

Microstruture and surface characteristics evolution of mesoporous multiple spin-coated titania films

Multilayer nanocrystalline mesoporous titania films were synthesized using the layer-by-layer deposition and surfactant-assisted sol-gel methods on glass substrates. The thickness, morphology and surface roughness of the films were systematically investigated using field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). Multilayer films were manufactured by repeated spin coating followed by heat treatment at 250°C for 1h after each deposition, and a final calcination at 400°C for 4h. Multilayer films with completely uniform thicknesses between 470nm–1.85μm and without clear interfaces were obtained by 1–6 coating cycles. The X-ray diffraction (XRD) analysis demonstrated 9.60nm anatase crystallites. Interestingly, AFM analysis indicated that the root mean square (rms) surface roughness of the films was significantly reduced from 3.11 to 1.03nm by increasing the deposited layers from 1 to 6. Moreover, power spectral density (PSD) analysis revealed that the roughness exponent and fractal dimension values of all films fall within the range of 0.48–0.53 and 2.47–2.52, respectively. This suggests that while the first titania layer exhibits a coarse microstructure, it acts as an intermediate layer in the formation of improved microstructures by eliminating the lattice mismatch between the glass substrate and subsequent titania depositions. Altogether, our results demonstrate the formation of self-affine surfaces with quite similar dynamics evolution of roughness formation either on the glass substrate or titania layer.

Mesoporous TiO2 powders as host matrices for iron nanoparticles. Effect of the preparation procedure and doping with Hf

The aim of the current investigation is to follow the formation of iron phase in mesoporous nanocrystalline titania and Hf-doped titania powders and its influence on the physicochemical and catalytic properties of the obtained materials. The parent host matrices were synthesized by using either template-assisted hydrothermal approach or homogeneous precipitation with urea. The iron modifications were prepared by incipient wetness impregnation with Fe(NO3)3.9H2O. Nitrogen physisorption, X-ray diffraction, TEM analysis, Moessbauer spectroscopy, UV–Vis spectroscopy and TPR with hydrogen were the techniques used for samples characterization and their catalytic behaviour was studied in methanol decomposition and ethyl acetate oxidation as catalytic tests. The hydrothermal treatment, the iron modification and the doping with Hf lead to the appearance of a significant absorption in the visible region for the studied samples, which together with the red shift of the absorption edge could be advantageous because of increased yield of photogenerated electron–hole pairs. The presence of very highly dispersed and uniform iron oxide particles with sizes below 4 nm and superparamagnetic behaviour are most beneficial for the catalytic activity in ethyl acetate total oxidation. On the other hand, the iron introduction retards the methanol conversion by blocking some of the supports active sites and alone or together with Hf changes the selectivity to predominant methane formation.

Optimizing the preparation parameters of mesoporous nanocrystalline titania and its photocatalytic activity in water: Physical properties and growth mechanisms

Titania nanomaterial with an anatase structure and 5.6nm crystallite size and 280.7m2g−1 specific surface areas had been successfully prepared by sol–gel/hydrothermal route. The effect of pH as a type of autoclave and calcination was studied. Crystallite size and phase composition of the prepared samples were identified. X-ray diffraction analyses showed the presence of anatase with little or no rutile phases. The crystallite size of the prepared TiO2 with acidic catalyst was both smaller than that prepared with basic catalyst, and was increasing after acidic calcinations by a factor 4–5. Basic calcinations produced a specific increase of 1.5. Rutile ratio and the particle size were increased after calcination at 500°C. However, TiO2 powder synthesized using a basic catalyst persisted the anatase phase and a loosely aggregation of particles. Anatase TiO2 as prepared with acidic catalyst in Teflon lined stainless steel autoclave demonstrated the highest photocatalytic activity for degradation of 2,6-dichlorophenol-indophenol under ultraviolet irradiation with t½ 0.8min.

Preparation and characterization of nanocrystalline titania powders by sonochemical synthesis

Nanocrystalline mesoporous titania powders were synthesized by hydrolyzing titanium isopropoxide in ethanol–water mixtures which were ultrasonically treated without using any templates or chemicals. Titanium isopropoxide–ethanol mixture was added dropwise to a water–ethanol mixture placed in an ultrasonic bath. The properties of the sonochemically synthesized powder were compared with those of the powders prepared without ultrasonic treatment along with Degussa P-25 titania powder. The phase structure, crystallite size, surface area, particle size, powder density were determined and sintering behavior was analyzed in this work. The nanotitania powder prepared during ultrasonic induced hydrolysis (TiO2-U) was determined to be formed from a mixture of anatase and brookite phases at 25°C. The brookite phase in nanotitania powder prepared without ultrasonic treatment (TiO2-NoU) was detected at 70°C. The anatase–rutile phase transformation was completed in the 500–700°C range for both powders. The average crystallite sizes of the powders at 25°C were determined as 10 and 5nm for TiO2-NoU and TiO2-U, respectively. The surface area decreased from 238 to106m2/g for TiO2-NoU and from 287 to 82m2/g for TiO2-U when the calcination temperature was increased from 200 to 500°C. The evolution of the N2 adsorption–desorption behavior with calcination temperature and the corresponding pore size distributions/volumes was attributed to the formation of closely packed submicron aggregates during powder synthesis and calcination. The sintering behavior was concluded to be controlled by 7–10nm crystallites and the submicron aggregates. The determination of the densification behavior of titania powders prepared by different methods with various levels of dopants may prove to be very useful for a better understanding of the phase/pore structure evolution which is crucial for a significant number of applications.

Molecular oxygen-assisted oxidative dehydrogenation of ethylbenzene to styrene with nanocrystalline Ti1−xVxO2

Oxidative dehydrogenation of ethylbenzene to styrene has been studied with vanadium-incorporated mesoporous nanocrystalline titania (Ti1−xVxO2) and molecular oxygen between 440 and 530 °C. Incorporation of V in TiO2 lattice framework has been achieved by simple solution combustion method. Incorporation of V in TiO2 lattice has been confirmed by X-ray diffraction, XPS and Raman spectra and other physicochemical analysis. High ethyl benzene conversion and stable styrene yield has been observed with 10% V-containing rutile phase titania at 500 °C. However, stable but relatively lower styrene yield has been observed with 2 and 5% V-containing catalysts between 440 and 500 °C. Highest selectivity is observed with lower vanadium content. Comparable activity has been observed under similar experimental conditions with four times higher air-flow than that of O2. In order to understand the structure activity relationship, spent catalysts were analyzed by all physico-chemical methods. Although there is a phase change from anatase to rutile Ti1−xVxO2 within 1 h of reaction, higher activity is primarily attributed to the ionic V5+ in Ti1−xVxO2 lattice, which prevents agglomeration to V2O5. It is to be underscored the reactivity is retained at the cost of textural properties and phase change from anatase to rutile, which is essential for the reaction.

Synthesis, characterization and application of sol–gel derived mesoporous TiO 2 nanoparticles for dye-sensitized solar cells

Nanocrystalline mesoporous titania of anatase crystal phase were prepared by sol–gel route by varying calcination (400 °C and 600 °C) conditions, and the photo-electrochemical properties were investigated for dye-sensitized solar cell applications. The TTIP precursor in n-heptane solvent with ratio of water to TTIP (5:1) was found to be effective substrate for the working electrodes. The overall conversion efficiency of 7.59% was achieved under 1 sun irradiation with open circuit voltage of 0.77 V, current density of 17.00 mA/cm 2 and FF of 51.12. The high efficiency of the 400 °C calcined sample were attributed to its mesopores, high BET surface area (80.1 m 2/g) and large pore volume of prepared titania substrate which provide better surface for the absorption of dye, improves light harvesting efficiency and better charge injection. The prepared samples were characterized by XRD, small angle XRD, FE-SEM, TEM, IPCE, I– V curve, BET surface area and BJH plot techniques.

Mesoporous gadolinium doped titania photocatalyst through an aqueous sol–gel method

A surfactant-free, aqueous sol–gel synthetic route to a thermally stable gadolinium doped mesoporous nanocrystalline titania is reported for the first time. The mesoporosity was revealed using BET surface area measurements and the pores are stable at temperatures as high as 800 °C. The 2 mol% gadolinium doped titania calcined at 800 °C, displayed a specific surface area of 22 m 2 g −1and a total pore volume 0.142 cm 3 g −1, whereas undoped titania is non-porous and has a surface area value as low as 0.3364 m 2 g −1 and total pore volume of 0.00096 cm 3 g −1. The thermal stability of the anatase phase up to 800 °C was achieved due to the reduction of crystallite size as a result of gadolinium doping in the titania lattice. Transmission electron micrographs showed that the crystallite size of 2 mol% gadolinium doped titania is 20 nm at 800 °C, whereas undoped titania had a crystallite size of 43 nm at the same temperature. The Bronsted acidity of doped titania was further studied using 2,6-dimethyl pyridine adsorption studies. Finally the photocatalytic performance of this mesoporous TiO 2 was analysed using methylene blue degradation and revealed a significantly higher activity than the undoped titania, a factor attributed to the mesoporosity, higher surface area and thermal stability of anatase phase.

Rapid microwave synthesis of mesoporous TiO2 for electrochromic displays

The fabrication of paper quality electrochromic displays based on the viologen modified TiO2 electrodes (Vio2+/TiO2) requires a cost-effective, energy efficient and rapid synthesis of mesoporous TiO2 with high yield in short reaction time. A straightforward and industrially viable process for the preparation of mesoporous nanocrystalline titania (meso-nc-TiO2) for NanoChromics™ display device applications by the use of microwave synthesis is presented here. Spherical aggregates of meso-nc-TiO2 were rapidly achieved using titanium butoxide, deionised water and common alcohols (isopropanol, ethanol and butanol) at comparatively low microwave power intensity (300 W) for 2 min irradiation. The material has been characterised by a range of different techniques such as XRD, Raman spectroscopy, SEM and BET surface area analysis. These materials possess surface areas up to 240 m2 g−1, which is significantly higher than similar traditional sol–gel or commercial samples. This meso-nc-TiO2 prepared was used as the working electrode for an electrochromic display device with Sb doped SnO2 as the counter electrode material on an ITO coated conducting glass. A working prototype of a NanoChromics™ display was successfully fabricated using this approach.

Synthesis and Photovoltaic Properties of Mesophorous TiO2 for the Dye-Sensitized Solar Cell

Nanocrystalline mesoporous titania was synthesized by using the surfactant assisted templating method with different temperatures. The energy conversion efficiency of 5.9% was achieved in the dye sensitized solar cell (DSSC) based on the 400°C sintered TiO2 electrode with treatment of TiCl4 aqueous solution. We found that the amount of dye adsorbed in TiO2 surface was affected significantly with the temperature during the sintering process.

Study on the effect of different acids on the structure and photocatalytic activity of mesoporous titania

Nanocrystalline mesoporous titania was synthesized via a combined sol–gel process with surfactant-assisted templating method using cetyltrimethyl ammonium bromide (CTAB) as the structure-directing agent. The process was catalyzed by different acid (hydrochloric acid, nitric acid, sulfuric acid, or phosphoric acid). The prepared samples were characterized by XRD, TEM, BET and FT-IR. The photocatalytic activity of the samples was determined by degradation of phenol in aqueous solution. Results showed that different acid had different effect on the structure and crystal phase of the samples. The sample adjusted by phosphoric acid showed highest surface area and photocatalytic activity. The formation mechanism of the samples catalyzed by different acid was also discussed.

Microstructure, optical and optoelectrical properties of mesoporous nc-TiO 2 films by hydrolysis-limited sol–gel process with different inhibitors

Mesoporous nanocrystalline titania (nc-TiO 2) films were synthesized by sol–gel route using poly (alkylene oxide) block copolymer as template and tetrabutyl orthotitanate as titanium source. Influence of acetylacetone (AcAc), glacial acetic acid, HCl and AcAc-HCl as inhibitors on hydrolysis/condensation reaction was investigated. The mesopore structure, crystalline phase and optical characteristics were analyzed by N 2 adsorption, X-ray diffraction, transmission electron microscopy and UV–VIS–NIR spectrum. Photoaction measurements were performed on dye-sensitized solar cells (DSSCs) using the mesoporous nc-TiO 2 as photoanodes. The results show that the formed films are anatase phase and mesoporous structure. Different restraining mechanisms of these inhibitors to hydrolysis/condensation process lead to different mesoporous structure whose specific surface area (S BET) varies from 34 to 176 m 2/g, and crystal size is from 5.6 to 38.2 nm. The mesoporous photoanode with high S BET and large crystalline grains may be responsible for increasing conversion efficiency of the DSSCs.

Synthesis of mesoporous nanocrystalline titania powders by nonhydrolitic sol–gel method

Mesoporous titania with well-crystallized anatase phase was synthesized by nonhydrolytic sol–gel process combined with solvothermal treatment. Titanium-isopropoxide, titanium-tetrachloride, and carbon-tetrachloride were used for synthesis under nitrogen atmosphere in a glove box. The obtained mixture was heated in an autoclave at 140 ∘C for 3 h and 30 h. XRD analysis showed that dry gels contained anatase phase with low crystallinity, while powders obtained by calcination of dry gels at 500 ∘C for 3 h consisted of pure anatase with high crystallinity. SEM analysis of dried gels and powders shows agglomerates of spherical particles of the size ∼300 nm. The TEM micrographs confirmed the nanoscale size of the titania powders, with the primary particle size of about 20 nm. Titania powders exhibit maximum pore diameter of 13.2 nm and 9.5 nm for gelation time 3 h and 30 h, respectively. The photocatalytic activity of such synthesized titania is superior to that of commercial photocatalyst Degussa P-25.

Template Free Synthesis of Mesoporous TiO<SUB>2</SUB> with High Wall Thickness and Nanocrystalline Framework

A simple procedure to prepare nanocrystalline mesoporous titania (meso-TiO2) is reported without any templating agent and it possesses a high BET surface area and a large pore wall thickness (11.3 nm) than that of meso-TiO2 prepared by other methods. Nanocrystalline meso-TiO, also has been synthesized with hexadecylamine template for comparison through known procedure. The meso-TiO2 materials were characterized by X-ray diffraction, FT-IR, UV-Vis absorbance spectra, thermal analysis, SEM, HRTEM and textural properties through N2 adsorption-desorption isotherms. Spherical shape particles in a range of few hundred nanometers are obtained in the template free method. Above systematic characterization provides direct indications toward the mechanism of formation of meso-TiO2 in the template free method. A comparison of the physical and textural properties indicates a possibility of coarse-tuning of the textural characteristics of mesoporous TiO2 by adopting different preparation methods.

Optical Properties of Hybrid Dendritic–Mesoporous Titania Nanocomposite Films

New hybrid optical sensors have been prepared by grafting specifically designed fluorescent, functionalised, phosphorus-containing dendrimers onto a nanocrystalline mesoporous titania thin film formed by evaporation-induced self-assembly. The structural characterisation and optical behaviour of these new fluorescent probes have been studied both in solution and after being grafted onto an inorganic network, which resulted in the discovery of improved probing selectivity in the solid state. This new hybrid sensor exhibits high sensitivity to phenolic OH moieties (especially those from resorcinol and 2-nitroresorcinol), which induce the quenching of fluorescence more efficiently in the solid state than in solution. This effect is a result of the increased spatial proximity of the fluorescent molecules, which is induced by pore confinement that makes the formation of hydrogen bonds between the hydroxyl moieties of the quenchers and the carbonyl groups of the dendrimer easier.

Visible‐Light Photocatalysis in Titania‐Based Mesoporous Thin Films

Photocatalytic activity in the visible part of the solar spectrum (442 nm) is demonstrated for highly organized mesoporous nanocrystalline titania thin films doped with thiourea. This property, which is of interest for the further development of photovoltaic devices operating in sunlight, is revealed by monitoring the photodegradation of methylene blue (see figure) upon contact with an N-doped TiO2 film as a function of irradiation time and film thickness.

Synthesis and photocatalytic activity for water-splitting reaction of nanocrystalline mesoporous titania prepared by hydrothermal method

Nanocrystalline mesoporous TiO 2 was synthesized by hydrothermal method using titanium butoxide as starting material. XRD, SEM, and TEM analyses revealed that the synthesized TiO 2 had anatase structure with crystalline size of about 8 nm. Moreover, the synthesized titania possessed a narrow pore size distribution with average pore diameter and high specific surface area of 215 m 2/g. The photocatalytic activity of synthesized TiO 2 was evaluated with photocatalytic H 2 production from water-splitting reaction. The photocatalytic activity of synthesized TiO 2 treated with appropriate calcination temperature was considerably higher than that of commercial TiO 2 (Ishihara ST-01). The utilization of mesoporous TiO 2 photocatalyst with high crystallinity of anatase phase promoted great H 2 production. Furthermore, the reaction temperature significantly influences the water-splitting reaction.

Influence of synthesis parameters on morphology and phase composition of porous titania layers prepared via water based chemical solution deposition

In this work, thick nanocrystalline mesoporous titania layers are synthesized via chemical solution deposition using a water based citratoperoxo-Ti(IV)-precursor solution. The aqueous citratoperoxo-Ti(IV)-precursor solution is modified by the addition of polyvinyl alcohol (PVA), which acts as a thickener and pore forming agent. Layers are tape casted onto ITO-coated glass substrates and are thermally processed. The influence of process parameters like Ti(IV)-concentration, blade thickness, crystallization temperature and time on the film's phase composition, morphology and thickness are investigated by means of X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), variable angle spectroscopic ellipsometry (VASE), atomic force microscopy (AFM) and profilometry. It is shown that the Ti(IV)-concentration and heat treatment influence the size and shape of the grains of which the films are composed, the film morphology (porosity, surface roughness) and the layer thickness, but no influence on the phase formation is observed. In all cases phase pure anatase layers are obtained.

Dye-sensitized solar cell made of mesoporous titania by surfactant-assisted templating method

Nanocrystalline mesoporous titania was obtained by surfactant-assisted templating method using tetraisopropyl orthotitanate modified with acethylacetone and laurylamine hydrochloride as template. This material was applied for the electrode of dye-sensitized solar cell. The mesoporous TiO2 (MP-TiO2) cells exhibited higher short-circuit photocurrent density and solar energy conversion efficiency compared to P25 (a typical commercial titania powder) cells. The incident photon to current conversion efficiency spectrum of MP-TiO2 can be improved by using the cell made with 5% P25 additive. Double-layer titania cells were fabricated to further improve cell performance by increasing the film thickness and light scattering. The solar conversion efficiency up to 8.06% was obtained by using the double-layer titania cell sintered at 450 °C for 2 h.