Synthesis and Photoactivity of Ordered Mesoporous Titania with a

Article pubs.acs.org/JPCC

Synthesis and Photoactivity of Ordered Mesoporous Titania with a Semicrystalline Framework K. Zimny,† T. Roques-Carmes,‡ C. Carteret,§ M. J. Stébé,† and J. L. Blin*,† †

Equipe Physico-chimie des Colloïdes, UMR SRSMC No. 7565, Université Henri Poincaré - Nancy 1/CNRS, Faculté des Sciences, BP 70239, F-54506 Vandoeuvre-les-Nancy Cedex, France ‡ Laboratoire Réactions et Génie des Procédés, UPR 3349 CNRS, Nancy-Université, 1 rue Grandville, BP 20451, 54001 Nancy Cedex, France § Laboratoire de Chimie Physique et Microbiologie pour l’Environnement UMR7564, Université Nancy 1/CNRS 405, rue de Vandoeuvre, F-54600 Villers-lès-Nancy, France ABSTRACT: Mesoporous titania with a high mesopore ordering and a high surface area have been synthesized by a new surfactant templating process that combined both the evaporation-induced self-assembly method and the liquid crystal templating pathway. The precipitation of titania in the hybrid mesophase is activated by an NH3 treatment. We have investigated the influence of the surfactant concentration and the surfactant/titanium precursor (TiOPr) molar ratio on the properties of mesostructured titania. The recovered materials have been characterized by SAXS measurements, nitrogen adsorption−desorption analysis, and Raman spectroscopy. Results clearly evidence that the lower the surfactant concentration, the better the mesopore ordering. The surfactant/TiOPr molar ratio also affects the structure of the recovered material. Indeed, the mesopore ordering is detected only for ratios lower than 0.0245. The optimal ratio has been found equal to 0.015. The obtained mesoporous titania exhibited high thermal stability, and the transformation of the amorphous titania walls into nanosized anatase walls occurred at 350 °C without the collapse of the mesostructure. The photocatalytic activity of the calcinated materials has been tested on the photodegradation of methyl orange. The results indicate that the photocatalytic activity considerably depends on the calcination temperature. The maximum methyl orange degradation (96% after 180 min) is observed at a calcination temperature of 450 °C. Despite the larger surface area of the materials calcinated at lower temperature, the higher degradation at a higher calcination temperature implies that crystallinity plays a major role in the photocatalytic activity.

1. INTRODUCTION Titania is a semiconductor that has received considerable attention for applications in electronics, electrochemical systems, including photoelectrochemical solar cells, electrocatalysis, optoelectronic sensor devices, and high performance photocatalytic films.1−5 In almost all of these applications, the crystal structure, particle size, surface area, and porosity of titania are important factors for the performance of these materials, especially for photocatalytic activities.6,7 In particular the increase in the photocatalyst surface area improves the efficiency of photocatalytic reactions.8−10 Thus, many efforts have been made to reach this goal. One way to enhance the titania-specific surface area consists of dispersing titania into a silica matrix.11−17 The obtained SiO2−TiO2 composite exhibits a high thermal stability and mechanical strength; both of these properties come from silica, and good optical and catalytic properties are provided by the TiO2 phase. Different methods have been reported for the preparation of silica materials containing titanium. Among them the sol−gel route in the presence of a surfactant appears to be a very promising way to obtain photocatalysts.11 These materials can provide simultaneously enhanced photocatalytic and thermal properties. For © 2012 American Chemical Society

example, in a study dealing with the preparation of Ti−SBA-15 by the direct synthesis method, Zhao et al.16 have shown that at low titania content, titanium is incorporated into the framework of SBA-15. With the increase in titanium content, TiO2 anatase particles are formed and become located on the external surface of SBA-15.23 The recovered Ti−SBA-15 is active for the photocatalytic reaction in water treatment, but the photocatalytic activity decreases with titanium content because of aggregation of TiO2 particles. Another approach to increase the titania-specific surface area deals with the development of synthesis methods to prepare a pure mesoporous titania material. However, compared to silica, it is difficult to obtain TiO2 with stable mesostructure, and the main challenge is to preserve the pore ordering upon surfactant removal. As a matter of fact, the templated synthesis of mesoporous TiO2 usually leads to the formation of amorphous walls, and the heat treatment performed to activate the titania crystallization often results in a collapse of the ordered Received: December 23, 2011 Revised: February 20, 2012 Published: February 26, 2012 6585

dx.doi.org/10.1021/jp212428k | J. Phys. Chem. C 2012, 116, 6585−6594

The Journal of Physical Chemistry C

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in an oven at 40 °C during 12 h. Afterward, they were placed in a well-closed glass vessel during 12 h. The atmosphere of the vessel was saturated by ammonia vapor to allow the precipitation of TiO2. Finally, almost all of the surfactant species in the product channels were removed by ethanol extraction. This method can avoid a possible structural collapse caused during high-temperature calcination to free the pores. 2.2. Characterization. SAXS measurements were carried out using a home-built apparatus, equipped with a classical tube (λ = 1.54 Å). The X-ray beam was focused by means of a curved gold/silica mirror on the detector placed at 527 mm from the sample holder. Samples for transmission electron microscopy (TEM) analysis were prepared by crushing some material in ethanol. Afterward a drop of this slurry was dispersed on a holey carbon-coated copper grid. A Philips CM20 microscope, operated at an accelerating voltage of 200 kV, was used to make the TEM images. N2 adsorption and desorption isotherms were determined on a Micromeritics TRISTAR 3000 sorptometer at −196 °C. The pore diameter and the pore size distribution were determined by the BJH (Barret, Joyner, Halenda) method.36 Raman scattering spectra were collected on a Jobin-Yvon T64000 spectrometer equipped with an optical microscope in confocal mode. The excitation beam (514.5 nm) was focused using a long-frontal 50× objective (numerical aperture 0.85) on an area of about 3 μm2. The laser power on the sample was approximately 10 mW. The spectral resolution was 3 cm−1, with a wavenumber precision better than 1 cm−1. 2.3. Decomposition of Methyl Orange. Methyl orange was chosen as a model organic compound to evaluate the photoactivity of the prepared mesoporous TiO2 at different calcination temperatures. Photocatalytic and adsorption experiments were performed at room temperature in a static batch photoreactor open to the air. This reactor consisted in a cylindrical quartz cell with 100 mL capacity. A magnetic stirrer guaranteed an oxygenation from atmospheric air and a satisfactory mixing of the reaction mixture with the TiO2 suspension. The irradiation was assured by artificial light using a mercury vapor lamp, emitting in the near-UV. The lamp was positioned outside the reactor and was parallel to it. The lamp and the reactor were inserted in an ellipsoidal closed base cylinder made of polished aluminum, which allowed the maximum amount of light reflected on the wall of the metallic system to be concentrated in the reactor. The lamp and the reactor were positioned along the two parallel local axes of the elliptically cylinder. The intensity of the UV-light reaching the middle of the solution was measured with an UV radiometer and indicated a spectral response centered at 365 nm with a half bandwidth of 12 nm. The mean value of the radiation power impinging on the reacting suspension was estimated to be Iincident = 10−5 Einstein L−1 s−1. The solution was prepared by dissolving the methyl orange powder in ultrapure water and aerating before use. The photocatalytic experiments were performed at pH 6−7. Note that the pH was not initially modified or controlled in the reactor. We used a standard initial methyl orange concentration of 15 mg L−1 while the titania concentration was equal to 0.5 g L−1. In each experiment, prior to UV irradiation, the aqueous TiO2 suspension containing methyl orange was magnetically stirred in the dark for 1 h until adsorption/desorption equilibrium was reached. This was enough to reach an equilibrated adsorption concentration. The solution was then

mesostructure. This is the reason why most of the reported studies on mesoporous titania are focused on films,18−23 which are mainly synthesized through the evaporation-induced selfassembly method (EISA). Using this technique, we have recently shown that liquid crystals of a polyoxyethylene fluoroalkyl ether surfactant can be used as a template for the preparation of new TiO2 films.21 The films have been selfassembled on a solid substrate by dip-coating using TiCl4 as the titanium source. Their GI-SAXS patterns are characteristic of a 2-D hexagonal structure, in which tubular rods of the fluorinated surfactant are packed hexagonally and aligned parallel to the substrate. Even if most papers deal with mesoporous titania films, some attempts have been made to prepare bulk ordered titania through the template-based method. On the basis of their amphiphilic nature that provides well-organized micelles around which the titania framework can be assembled by electrostatic interactions,24,25 ionic surfactants were used initially. Indeed, in 1995 Antonelli et al.24 reported the first example of mesoporous titania by using tetradecyl phosphate as surfactant and titanium acetylacetonate trisisopropoxide as Ti source. Acetylacetone acts as a hydrolysiscontrolling agent of the titanium precursor. The obtained material adopts a hexagonal pore ordering, but the structure partially collapses after the pores are freed from the surfactant by calcination at 350 °C. Since this date, other groups have developed different strategies11,26−32 such as the acid−base concept,26 the use of mixed inorganic precursor,27 or hydrolysis-controlling agents.28 Among them the synthesis in nonaqueous media seems to be the most successful way.11 However, in most cases, the synthesis of mesoporous titania needs several days (as long as 15 days) and the obtained TiO2 presents a local organization. In this paper, we have developed a simple and effective route for synthesizing ordered mesoporous titania with a high surface area in a short synthetic period (3 days). Our approach is inspired by the evaporation-induced self-assembly method and the liquid crystal templating pathway that can be employed for the synthesis of silica mesoporous materials.33−35 In addition to understanding the effect of the synthesis parameters on the formation of the ordered mesoporous titania, we have investigated, in detail, the effect of both the surfactant concentration and the surfactant/ titanium precursor molar ratio on the structural and textural properties of mesoporous titania. Contrary to that for the silica mesoporous materials, this kind of study is barely reported in the literature for titania. After crystallization of the amorphous mesoporous walls, the photocatalytic properties of the recovered materials toward decomposition of methyl orange were also examined.

2. EXPERIMENTAL SECTION The triblock copolymers P123 (EO)20(PO)70(EO)20, employed as a structure-directing agent, and titanium isopropoxide (37%), used as an inorganic precursor, were purchased from SigmaAldrich. 2.1. Mesoporous Titania Preparation. First, 1 g of surfactant was dissolved in 20 g of ethanol under stirring at room temperature. Then x g of a hydrochloric acid solution (35% Prolabo), y g of titanium isopropoxide (TiOPr), and water were added. The amount of water was fixed in order to keep an acidity of 6 N, and x and y were varied, respectively, from 0.5 to 2.5 g and from 1 to 4 g. The obtained mixture was directly evaporated under vacuum to remove ethanol and 2propanol released by hydrolysis of TiOPr. Samples were dried 6586



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irradiated under UV light with continuous magnetic stirring to keep the suspension homogeneous. In order to determine the change in methyl orange concentration during UV irradiation, 3 mL aliquots of the suspension were withdrawn at appropriate times from the reactor using a glass syringe. The withdrawn liquid was separated from TiO2 particles by filtration through a 0.45 μm PTFE Millipore filter and then, if necessary, centrifuged. A first sample was taken at the end of the dark adsorption period, just before the light was turned on, in order to determine the concentration of dye in solution (nonadsorbed). The methyl orange concentration was determined with a spectrophotometer (UV/vis spectrophotometer, carry 5G UV−vis-NIR). When methyl orange was dissolved in distilled water, the UV−visible spectrum showed two absorption maxima. The first band was observed at 270 nm and the second band, more intense, at 458 nm. The photocatalytic degradation of methyl orange solution was followed by measuring the absorbance value at λ = 458 nm, which allowed access to the methyl orange concentration. Analytical uncertainty for methyl orange concentration was mainly due to the filtration step and has been evaluated to be 1.5 μg L−1. No significant change in the methyl orange concentration could be observed when the photolysis was carried out in the absence of TiO2, and the photodegradation efficiency was also negligible in the absence of UV light (but in the presence of TiO2). It should be noted that during this step a weak adsorption of methyl orange on the mesoporous titania occurred.

3. RESULTS AND DISCUSSION 3.1. Effect of the Synthesis Parameters on the Ordered Mesoporous Titania Formation. To begin our study, the surfactant concentration and the P123/TiOPr molar ratio are fixed to 33 wt.% and 0.019, respectively. Results dealing with the optimization of these parameters will be presented below. One key parameter for the formation of titania with a mesopore ordering concerns the control of the titanium precursor hydrolysis and condensation. Indeed, precursors such as titanium alkoxides or titanium chloride exhibit a high reactivity toward hydrolysis and condensation. As a consequence, dense TiO2 with a poor mesopore arrangement is often recovered. To overcome this drawback, a large amount of ethanol is used to prevent the formation of a gel and, in the initial step, the preparation is carried out in very acidic conditions. As a matter of fact, the high concentration of HCl allows the control of the hydrolysis−condensation reactions and avoids the precipitation of a titanium oxide phase. After the evaporation of the solvent and the 2-propanol produced during the hydrolysis of TiOPr, peaks located at 13.7, 7.8, and 6.6 nm are observed on the SAXS pattern (Figure 1Aa). They can be attributed to the (100), (110), and (200) reflections of the hexagonal structure. Therefore, the hexagonal hybrid mesophase is formed. However, due to the dissolution of both the organic and inorganic species, after washing with ethanol, no material can be recovered. To avoid this phenomenon and to induce the condensation reaction of the inorganic precursor, a treatment under NH3 atmosphere is performed. As shown in Figure 1Ab and 1Ac, the hexagonal mesopore ordering is maintained after the drying of the gel (Figure 1Ab) and the surfactant removal after the TiOPr condensation (Figure 1Ac). According to Bragg’s law, the unit cell dimension (a0 = 2d100/ 31/2), which is the sum of the pore diameter and the thickness

Figure 1. A: SAXS patterns of TiO2 prepared with P123 (a) after solvent evaporation, (b) after treatment with NH3, and (c) after surfactant removal. B: TEM micrographs and high resolution scanning electron micrographs of mesoporous TiO2 obtained after treatment with NH3 and surfactant removal. C: Nitrogen adsorption−desorption isotherms and the pore size distribution of the recovered titania.

of the pore wall, can be calculated. Its value (a0) varies from 16.3, for the hexagonal hybrid mesophase, to 13.7 nm, for the ordered mesoporous tiania. The variation of a0 indicates that the pore diameter or/and the wall thickness decrease during the ammonia treatment. The mesopore ordering is further confirmed by the transmission electron microscopy (TEM) images (Figure 1B). Indeed, either the honeycomb-like arrangement (Figure 1Ba) or the hexagonal stacking (Figure 1Bb) of the channels is evidenced by the TEM analysis. The diffraction electron patterns exhibit 6-fold symmetry, and the measured angles between two bright spots are very close to 60° (Figure 1Ba insert). Two light spots (Figure 1Bb insert) are present, which indicates the parallelism of well-oriented channels. The Raman spectra show that the titania frameworks possess amorphous walls (spectra not reported here). No spectral feature due to the anatase or rutile is detected. From nitrogen adsorption−desorption measurements, we can observe that the recovered sample exhibits a type IV isotherm (Figure 1C), characteristic of mesoporous materials according to the IUPAC classification.37 The specific surface area and pore volume values are respectively 370 m2/g and 0.52 cm3/g. The pore diameter distribution determined by using the BJH 6587



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Figure 2. Scheme illustrating the different steps of the synthesis of ordered mesoporous titania.

method is quite narrow and centered at ca. 9.8 nm (Figure 1C, insert). The wall thickness, deduced by subtracting the pore size from the dimension of the unit cell, is equal to 3.9 nm. The proposed preparation method allows the formation of ordered mesoporous titania with amorphous walls. The synthesis scheme is summarized in Figure 2. We have investigated the effect of the variation of both the P123/TiOPr molar ratio and the surfactant concentration on the amorphous titania mesopore ordering. The P123/ethanol/ water phase diagram is reported in the literature.38 The micellar domain (L1) detected for P123 content lower than 27 wt %, without alcohol, is progressively expanded toward the higher surfactant concentrations with addition of ethanol. The cubic, the hexagonal, and the lamellar liquid crystal phases progressively appear with an increase in the surfactant concentration. Upon the addition of ethanol, the surfactant range of composition of the liquid crystal domains is progressively reduced. The synthesis of the mesoporous titania is performed under strong acidic conditions, so we have first checked that the limits of the liquid crystal domains are not modified. No variation of these limits has been detected under strong acidic conditions. Mesoporous materials are thus prepared with a P123 concentration between 28.5 and 66.6 wt.% in a 12 M hydrochloric acid solution in order to cover the overall range of surfactant concentration belonging to the cubic (I1) and the hexagonal (H1) domains of the P123/water system. In a previous study dealing with the preparation of mesoporous silica materials through the liquid crystal pathway,35 we have evidenced that the surfactant/inorganic precursor molar ratio is a key parameter to obtain a mesopore ordering. Indeed, the mesopore ordering has been detected only for ratios located in the range between 0.119 and 0.175. An increase in the surfactant/silica molar ratio results in materials with a disordered channel array.35 In the study reported here, the P123 concentration is maintained constant at 40 wt.% while the P123/TiOPr molar ratio (R′) has been varied from 0.05 to 0.012. The evolution of the materials' SAXS patterns as a function of R′ is depicted in Figure 3. Among the investigated molar ratios of surfactant/TiOPr, the mesopore ordering is detected only for ratios located in the range between 0.012 and 0.019. Indeed, the reflections, characteristics of the hexagonal structure, are evidenced in Figure 3a−d. The repetition distance corresponding to the (100) reflection increased from 10.1 to 11.4 nm when R′ is changed from 0.012 to 0.019. Thus, the cell parameter a0 decreases with the quantity of TiOPr used for the synthesis. The SAXS patterns of the material prepared with an R′ value between 0.033 and 0.024 exhibit only a single broad reflection (Figure 3e,f), which indicates the formation of a disordered structure. If R′ is further

Figure 3. SAXS pattern of the materials synthesized with a surfactant/ TiOPr molar ratio (R′) equal to (a) 0.012, (b) 0.014, (c) 0.016, (d) 0.019, (e) 0.024, (f) 0.033, and (g) 0.050.

increased, no line is observed in the SAXS pattern (Figure 3g), indicating that the recovered compounds exhibit a complete, randomly oriented pore structure. Nitrogen adsorption− desorption isotherms and the corresponding BJH pore size distributions (insert of Figure 4), obtained from an analysis of the adsorption branch of the isotherm, are shown in Figure 4. Except for the sample prepared with R′ = 0.05, all the recovered titania material exhibit a type IV isotherm, characteristic of mesoporous materials according to the IUPAC classification.37 A H1 type hysteresis loop in which adsorption and desorption branches are steep is observed for the samples obtained with R′ lower than 0.024. However, the value of the relative pressure, at which the capillary condensation occurs, increases with R′. Because the p/p0 position of the inflection point is related to the pore diameter according to Kelvin’s equation, this observation suggests that the pore diameter of molecular sieves is increased when the P123/TiOPr molar ratio varies from 0.012 to 0.019. This is confirmed by the pore size distribution, for which the maximum is shifted toward higher value when the value of R′ is increased from 0.012 to 0.019 (Figure 4a−d). When the transition from a hexagonal to a disordered channel 6588



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Figure 4. Nitrogen adsorption−desorption isotherms and the pore size distribution (insert) of the materials synthesized with a surfactant/TiOPr molar ratio (R′) equal to (a) 0.012, (b) 0.014, (c) 0.016, (d) 0.019, (e) 0.024, (f) 0.033, and (g) 0.050.

hexagonal pore ordering is recovered for P123 concentrations between 28 and 50 wt.% (Figure 5a−d). In addition, the slight increase in the second reflection intensity observed in the SAXS pattern of the material prepared with the lower concentrations of P123 suggests that the mesopore ordering is enhanced when the surfactant concentration in the acidic solution is decreased. The cell parameter is progressively increased from 11.5 to 15.8 nm as the surfactant concentration is lowered. This corresponds to a swelling of the liquid crystals by the acidic solution. As a matter of fact, when water content is raised, the thickness of the film which separates the surfactant cylinder also increases. Figure 5B shows the nitrogen adsorption−desorption isotherms and the pore size distribution (insert of Figure 5B) of the materials obtained with different P123 concentrations. A IV-type isotherm is obtained regardless of the surfactant content. When the water content is increased, i.e., the surfactant concentration in the mixture is decreased, the relative pressure for which the capillary condensation takes place is shifted toward higher values. This indicates that an enlargement of the mean pore diameter occurs when the water content is raised. This increase in pore diameter is further confirmed by the pore size distribution (insert of Figure 5B), the maximum of which is shifted from 6 to 9.7 nm when the

array occurs, the relative pressure at which the capillary condensation takes place is spread out over a larger range of relative pressures and the pore size distribution becomes broader (Figure 4e,f). Finally, the sample synthesized with a P123/TiOPr molar ratio equal to 0.05 exhibits an isotherm, which is intermediate between type I and IV (Figure 4g). According to Dubinin,39 this kind of isotherm is characteristic of supermicroporous materials, i.e., the pore size is located at the limit between micro- and mesoporous domain. This is confirmed by the pore size distribution (Figure 4g, insert). For the titania having a hexagonal structure, the wall thickness, deduced by subtracting the pore size determined by the BJH method from the dimension of the unit cell, decreases from 5.6 to 3.5 when R′ is changed from 0.012 to 0.024. Thus, we can assume that the surfactant cylinders are squeezed by the progressive addition of titanium. This phenomenon involves an increase in the wall thickness and a decrease in the mesopore diameter. Fixing the P123/TiOPr molar ratio to 0.016, Figure 5A depicts the variation of the SAXS pattern with the surfactant weight percentage in the aqueous solution. While a disordered structure is formed when the materials are prepared with a surfactant weight percent equal to 66 wt.% (Figure 5Ae), the 6589



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Figure 5. SAXS patterns (A), nitrogen adsorption−desorption isotherms (B), and the pore size distribution (insert of B) of the materials synthesized with a surfactant concentration equal to (a) 28, (b) 33, (c) 40, (d) 50, and (e) 66 wt.%.

P123 concentration is progressively decreased from 66 to 28 wt.% . A similar behavior has already been observed for mesoporous silica prepared through the liquid crystal templating mechanism.35 The condensation of the hydrolyzed species around the surfactant cylinders involves an expansion of the cylinders, and, as a consequence, the pore diameter of the obtained mesoporous materials is reduced. The wall thickness increases from 4.0 to 6.1 nm when the P123 concentration is decreased from 66 to 28 wt.%. Thus, we can assume that the titanium species occupy the space between the surfactant cylinders. The overall variation of the mesopore ordering, the specific surface area, and the pore diameter as a function of both the surfactant concentration and the P123/TiOPr molar ratio are depicted in Figure 6. Depending on the features of the SAXS

pattern, the obtained titania have been gathered in three groups: (i) disordered, meaning that no peak is observed on the SAXS pattern, (ii) wormhole-like when only a broad reflection is detected, and (iii) ordered. Figure 6A shows that whatever the surfactant concentration, the mesoporous titania synthesized with a P123/TiOPr molar ratio higher than 0.0245 exhibit either a disordered or a very poor ordered channel array. Except the materials prepared with a 28 wt.% of P123, the best mesopore ordering is reached for R′ = 0.015. The SAXS analysis shows that there is an optimum range of R′ that leads to the formation of hexagonal mesoporous TiO2. This result is in perfect agreement with those previously reported in the literature. For example, in a paper dealing with the preparation of wormhole-like mesoporous titania by using P123 as surfactant and TBOT as titanium source, Hung et al.40 have 6590



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specific surface area decreases to 290 m2/g when R′ reaches 0.012 (wormhole-like structure). For R′ = 0.015, which, regarding the mesopore ordering, corresponds to the optimal P123/TiOPr molar ratio, whatever the surfactant concentration, the specific surface area is constant to 400 m2/g. The variation of the mesopore diameter follows the same tendency as that described for the specific surface area (Figure 6C). Mesoporous titania have some features similar to those of mesoporous silica prepared through the liquid crystal mechanism.35 Indeed, for both cases, the mesopore ordering is obtained for a well-defined range of surfactant/inorganic precursor molar ratios. Note that the hexagonal liquid crystal phase is composed of infinite cylinders packed in a hexagonal array. In the case of direct systems, the cylinders are filled by the hydrophobic chains which are covered by both head groups and water. Thus, we can assume that when the surfactant/ inorganic molar ratio is too high, the quantity of the inorganic precursor added is not sufficient to interact with all the cylinders. By contrast, once the R′ value becomes too low, the formation of a disordered structure results from a precipitation of the excess of the inorganic precursor as a nonbulk templated phase. In the case of the mesoporous titania materials, we have also observed that the mesopore wall thickness is increased when the liquid crystals are swelled by water. The titanium species replace progressively the water molecules of the film located between the surfactant cylinders. Therefore, we can conclude that the pore network is less damaged by the surfactant removal when the wall thickness is high. According to Sanchez et al., the formation of the ordered mesopore network when the water content is increased, i.e. when the surfactant concentration is decreased, can also be attributed to the formation of hydrophilic species that can interact through hydrogen bonds with the oxyethylenic groups.41 3.2. Photoacatalytic Activity. This study has been performed for a mesoporous titania synthesized with a 33 wt. % of P123 and a 0.016 P123/TiOPr molar ratio. Among the common crystalline forms of titania, anatase is generally recognized to be the most active phase. One way to transform the amorphous TiO2 to anatase consists of calcination. To get the anatase phase, the recovered amorphous titania were first heated under nitrogen from 20 °C to the final temperature (ranging from 150 to 550 °C) at a rate of 1 °C/min. Then, they were kept at this temperature under oxygen for 2 h. After calcination above 350 °C, the Raman spectra of heat-treated mesoporous titania show all the anatase bands (Figure 7B). These bands were assigned according to the results published in the literature.42,43 The mesopore ordering is maintained until 500 °C, and the complete collapse of the mesostructure appears at 550 °C (Figure 7A). The specific surface area and the pore diameter decrease progressively upon calcination (Figure 8). Nevertheless, when the anatase appears, the specific surface area still maintains about 80% of its initial value. The dependence of the calcination temperature of the mesoporous TiO2 on the time course of the photocatalytic degradation of methyl orange is illustrated in Figure 9. The investigated calcination temperatures are 350, 400, 450, and 500 °C. Each curve shows the bulk concentration of methyl orange as a function of the irradiation time. The bulk concentration of methyl orange decreases with the reaction time. For noncalcinated titania, no degradation is observed over a long period and irradiation for 180 min produces no appreciable reduction in the methyl orange concentration. For TiO2 calcinated at 350 and 400 °C, there is some methyl orange degradation, but the

Figure 6. Variation of the mesopore ordering (A), the specific surface area (B), and the mesopore diameter (C) as a function of both the surfactant concentration and the P123/TiOPr molar ratio.

reported that the wormhole-like structure is recovered only for P123/TBOT molar ratios located in the range from 0.013 to 0.025. Concerning the effect of the surfactant concentration, it appears that the lower the P123 concentration, the better the mesopore ordering. It should also be noted that although the P123/ethanol/water phase diagram38 presents a cubic liquid crystal phase, under the conditions reported here, only a hexagonal pore ordering is recovered for the mesoporous titania. Globally, a higher specific surface area is emphasized for the materials having a hexagonal mesopore ordering (Figure 6B). For a given surfactant concentration, an increase in the specific surface area is noted when the value of R′ is increased. By contrast, for a fixed P123/TiOPr molar ratio, a decrease in its value is observed when the surfactant concentration is raised (Figure 6B). For example, the value of the specific surface area of the titania prepared with 50 wt.% of P123 increases from 220 to 130 m2/g when R′ is changed from 0.05 (disordered structure) to 0.015 (hexagonal pore ordering). Then, the 6591



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Figure 9. Change in the concentration of methyl orange as a function of the time of irradiation for noncalcinated and calcinated mesoporous TiO2 at various temperatures.

Figure 7. A: Evolution of the SAXS pattern as a function of the calcination temperature. (a) As-prepared titania, (b) 150, (c) 250, (d) 350, (e) 400, (f) 450, and (g) 550 °C. B: Raman spectra of (a) the asprepared titania and the heat-treated titania at (b) 350 °C, (c) 400 °C, (d) 450 °C. (e) Spectrum of commercial anatase with particle size above 20 nm given as reference.

and 125 m2 g−1, respectively), their photocatalytic activity is lower. This indicates that the degree of crystallinity of TiO2 is a more significant factor in the photocatalytic activity, because the degree of crystallinity of the anatase phase of TiO2 calcinated at 450 and 500 °C is higher than those of TiO2 calcinated at 350 and 400 °C. This phenomenon has been described in the literature.44,45 It was mentioned that higher crystallinity enhanced drastically the photocatalytic activity of TiO2 materials.44,45 In addition, the amorphous phase contained to some extent in the materials calcinated at 350 and 400 °C has been reported to play a negative effect on the photocatalytic activity.45 This is due to the amorphous phase, which usually comprises numerous defects, i.e., impurities, dangling bonds, and microvoids that can behave as recombination centers for the photoinduced electron/hole (e−/h+) pairs.45 In the same way, the imperfect crystallization accompanied by the fairly small crystallite size of noncalcinated titania is considered to favorably increase the probability of mutual e−/h+ recombination at both surface and bulk traps. In contrast, the use of the nanocrystalline mesoporous TiO2 with uniform pore size and high crystallinity, obtained at calcination temperatures of 450 and 500 °C, decreases the number of lattice defects and then facilitates the electron transport for reacting with water and methyl orange molecules adsorbed on the TiO2 surface along the mesopore structure, leading to much better photocatalytic performance.46 Therefore, we conclude that the photocatalytic activity of mesoporous TiO2 is mainly dependent on the crystallinity and the crystallite size. The increment of the photocatalytic activity owing to the increase in crystallinity (less number of lattice defects) exceeds the decrement of the photocatalytic activity owing to the decrease in the surface area. The decay in the methyl orange concentration [MeO] appears linear to the irradiation time (t) (Figure 9). Then the rate constant can be obtained from the regression analysis of the linear curve in the plot of [MeO] versus t. The reaction rates were determined 150 min after initiation of the photocatalytic reactions. The reaction time was sufficiently high to obtain different values for the degradation rates under different conditions. Table 1 summarizes the rate of photocatalytic reaction for different calcination temperatures. As expected, the rate of photocatalytic degradation is affected by the calcination temperature. The degradation rate increases with the calcination temperature to reach a maximum at about 450 °C. No significant difference is observed between the samples calcinated at 450 and 500 °C.

Figure 8. Variation of the specific surface area (A) and the mesopore diameter (B) as a function of the calcination temperature.

efficiency of the degradation is inferior to that of the TiO2 calcinated at higher temperatures. In the presence of TiO2 calcinated at 450 and 500 °C, the methyl orange solutions are almost fully degraded within 180 min under UV light. The results indicate that the photocatalytic activity considerably depends on the calcination temperature. All the calcinated materials have photocatalytic activities better than those of noncalcinated ones. The maximum methyl orange degradation is observed at calcination temperatures of 450 and 500 °C. Generally, large surface area is likely to exhibit better photocatalytic activity, because a large surface area provides more active sites for adsorbing methyl orange molecules. However, in the present study, large surface area does not give better photocatalytic activity. Although TiO2 calcinated at 350 and 400 °C (275 and 250 m2 g−1, respectively) have a surface area larger than that of TiO2 calcinated at 450 and 500 °C (175 6592



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4. CONCLUSIONS A relatively simple and effective route has been developed for the synthesis of mesoporous titania with a high mesopore ordering and thermal stability in a short synthetic period (3 days). Mesoporous TiO2 has been synthesized by a surfactant templating process that combined the evaporation-induced selfassembly method, the liquid crystal templating pathway, and basic atmosphere treatment. Materials have been prepared in an alcoholic acidic media. These conditions prevent the precipitation of TiO2, which is induced by a treatment under NH3 atmosphere. The obtained titania exhibit a hexagonal pore ordering with amorphous walls. The influence of the synthesis conditions on the properties of mesoporous materials has been investigated. Results show that the best mesopore ordering is reached for R′ = 0.015. When the surfactant/TiOPr molar ratio exceeds 0.0245, the quantity of TiOPr added is not sufficient enough to interact with all the cylinders of the H1 phase. By contrast, at low ratios, the hexagonal liquid crystal phase is diluted into an amorphous titania matrix. As a consequence, a disordered structure is formed, which results from a precipitation of the excess titania as a nonbulk templated phase. The transformation of the amorphous titania walls into nanosized anatase walls occurs without the mesostructure collapse after calcination at 350 °C. Finally, the photocatalytic activity of the calcinated materials has been tested on the photodegradation of methyl orange. Results show that crystallinity plays a significant role in the photocatalytic activity.

Table 1. Effect of the Calcination Temperature of the Mesoporous TiO2 on the Rates of Methyl Orange Degradation temperature of calcinations (°C)

degradation rates (mg/L min)

coefficient of linear regression (R2)

not calcinated 350 400 450 500

0.0094 0.0536 0.0550 0.0973 0.0929

0.759 0.993 0.992 0.985 0.996

Our results are in good agreement with other studies which reported that the rate of photocatalytic degradation of many pollutants is a function of the TiO2 calcination temperature.47 Because no significant difference can be observed between the mesoporous TiO2 calcinated at 450 and 500 °C, in terms of degradation efficiency and degradation rates, it is interesting to investigate the evolution of the oxidation product formed during the photocatalytic reaction. Recall that the UV−visible spectrum of methyl orange shows two absorption maxima at 270 and 458 nm. The absorption peak at 458 nm is related to the nitrogen to nitrogen double bond (−NN−) of the dye. The absorption peak at 270 nm corresponds to the aromatic cycle48 and is used to monitor the presence of byproduct. Figure 10 shows the absorbance corresponding to the main

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Kevin Zimny thanks the “Region Lorraine” for the financial support of his Ph.D. REFERENCES

(1) Hoffmann, M.; Martin, S.; Choi, W.; Bahnemann, D. Chem. Rev. 1995, 95, 69. (2) Fox, M.; Dulay, M. Chem. Rev. 1993, 93, 341. (3) Linsebigler, A.; Lu, G.; Yates, J. Chem. Rev. 1995, 95, 735. (4) Thurston, T.; Wilcoxon, J. J. Phys. Chem. B 1999, 103, 11. (5) Chen, X.; Mao, S. S. Chem. Rev. 2007, 37, 1. (6) Li, Z.; Hou, B.; Xu, D.; Wu, D.; Sun, Y. J. Colloid Interface Sci. 2005, 288, 149. (7) Kim, D. S.; Han, S. J.; Kwak, S. Y. J. Colloid Interface Sci. 2007, 316, 85. (8) Ohtani, B.; Ogawa, Y.; Nishimoto, S. J. Phys. Chem. B 1997, 101, 3746. (9) Cassiers, K.; Linssen, T.; Mathieu, M.; Bai, Y. Q.; Zhu, H. Y.; Cool, P.; Vansant, E. F. Phys. Chem. B 2004, 108, 3713. (10) Sakatani, Y.; Grosso, D.; Nicole, L.; Boissière, C.; Soller-Illia, G. J.; De, A. A.; Sanchez, C. J. Mater. Chem. 2006, 16, 77. (11) Ismail, A. A.; Bahnemann, D. W. J. Mater. Chem. 2011, 21, 11686. (12) Lihitkar, N. B.; Abyaneh, M. K.; Samuel, V.; Pasricha, R.; Gosavi, S. W.; Kulkarni, S. K. J. Colloid Interface Sci. 2007, 314, 310. (13) Calleja, G.; Serrano, D. P.; Sanz, R.; Pizarro, P. Microporous Mesoporous Mater. 2008, 111, 429. (14) Zimny, K.; Caretert, C.; Stébé, M. J.; Blin, J. L. J. Phys. Chem. C 2011, 115, 8684. (15) Shi, K. Y.; Chi, Y. J.; Yu, H. T.; Fu, H. G. J. Phys. Chem. B 2005, 109, 2546. (16) Li, G.; Zhao, X. S. Ind. Eng. Chem. Res. 2006, 45, 3569.

Figure 10. Time course of the absorbance measured at λ = 270 nm corresponding to the oxidation product formed during the photocatalytic reaction using mesoporous TiO2 calcinated at 450 and 500 °C.

intermediate (detected at λ = 270 nm) as a function of the irradiation time. The decrease in intensity of absorption peaks of methyl orange at λ = 458 nm indicates a rapid degradation of the nitrogen to nitrogen double bond. This emphasizes that the nitrogen to nitrogen double bond is the most active site for oxidative attack. In parallel, the absorption peak at λ = 270 nm first increases up to a maximum and then decays with the reaction time. The absorbance shows a maximum level for 1 h of illumination. The mesoporous TiO2 degrades continuously the methyl orange molecules with a simultaneous formation of the aromatic intermediate. This also indicates that the intermediate oxidation product experiences the photocatalytic reaction. The complete degradation of the methyl orange dye and also the oxidation product is performed after 180 min for the mesoporous TiO2 calcinated at 450 °C, while a very small amount of byproduct remains in the solution for the powder calcinated at 500 °C. 6593



Article

(17) Wang, X. H.; Li, J. G.; Kamiyama, H.; Moriyoshi, Y.; Ishigaki, T. J. Phys. Chem. B 2006, 110, 6804. (18) Grosso, D.; De A. A. Soller-Illia, G. J.; Babonneau, F.; Sanchez, C; Albouy, P. A.; Brunet-Bruneeau, A.; Balkenade, A. R. Adv. Mater. 2001, 13, 1085. (19) Yu, J.; Yu, J. C.; Ho, W.; Jiang, Z. New J. Chem. 2002, 26, 607. (20) Peng, Z.; Shi, Z.; Liu, M. Chem. Commun. 2000, 2125. (21) Henderson, M. J.; Zimny, K.; Blin, J. L.; Delorme, N.; Bardeau, J. F.; Gibaud, A. Langmuir 2010, 26, 1124. (22) Henderson, M. J.; Gibaud, A.; Bardeau, J. F.; White, J. W. J. Mater. Chem. 2006, 16, 2478. (23) Meng, X.; Kimura, T.; Ohji, T.; Kato, K. J. Mater. Chem. 2009, 19, 1894. (24) Antonelli, M. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1995, 34, 2014. (25) On, D. T. Langmuir 1999, 15, 8561. (26) Tian, B.; Liu, X.; Tu, B.; Yu, C.; Fan, J.; Wang, L.; Xie, S.; Stucky, G. D.; Zhao, D. Nat. Mater. 2003, 2, 159. (27) Tian, B.; Yang, H.; Liu, X.; Xie, S.; Yu, C.; Fan, J.; Tu, B.; Zhao, D. Chem. Commun. 2002, 1824. (28) Li, H.; Shi, J. L.; Liang, J.; Li, X.; Li, L.; Ruan, M. Mater. Lett. 2008, 62, 1410. (29) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152. (30) Sung, C. C.; Fung, K. Z.; Hung, I. M.; Hon, M. H. Solid State Ionics 2008, 179, 1300. (31) Shibata, H.; Ogura, T.; Ohkubo, T.; Sakai, H.; Abe, M. J. Am. Chem. Soc. 2005, 127, 16396. (32) Kao, L. H.; Hsu, T. C.; Cheng, K. K. J. Colloid Interface Sci. 2010, 341, 359. (33) Attard, G. S.; Glyde, J. C.; Göltner, C. G. Nature 1995, 378, 366. (34) El-Safty, S. A.; Kiyozumi, Y.; Hanaoka, T.; Mizukami, F. J. Phys. Chem. C 2008, 112, 5476. (35) Zimny, K.; Blin, J. L.; Stébé, M. J. J. Phys. Chem. C 2009, 113, 11285. (36) Barret, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 37. (37) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (38) Soni, S. S.; Brotons, G.; Bellour, M.; Narayanan, T.; Gibaud, A. J. Phys. Chem. B 2006, 110, 15157. (39) Dubinin, M. M. In Progress in Surface and Membrane Science 9; Cadenhead, D.A., Ed.; Academic Press: New York, 1975; p 1. (40) Hung, I. M.; Wang, Y.; Huang, C. F.; Fan, Y. S.; Han, Y. J.; Peng, H. W. J. Eur. Ceram. Soc. 2010, 30, 2065. (41) Soler-Illia, G. J. A. A.; Scolan, E.; Louis, A.; Albouy, P. A.; Sanchez, C. New J. Chem. 2001, 25, 156. (42) Kelly, S.; Pollak, F. H.; Tomkiewicz, M. J. Phys. Chem. B 1997, 101, 2730. (43) Zhang, W. F.; He, Y. L.; Zhang, M. S.; Yin, Z.; Chen, Q. J. Phys. D: Appl. Phys. 2000, 33, 912. (44) Stone, V. F. Jr.; Davis, R. J. Chem. Mater. 1998, 10, 1468. (45) Ohtani, B.; Ogawa, Y.; Nishimoto, S. J. Phys. Chem. B 1997, 101, 3746. (46) Sreethawong, T.; Suzuki, Y.; Yoshikawa, S. Catal. Commun. 2005, 6, 119. (47) Kim, D. S.; Kwak, S. Y. Appl. Catal., A 2007, 323, 110. (48) Tasaki, T.; Wada, T.; Fujimoto, K.; Kai, S.; Ohe, K.; Oshima, T.; Baba, Y.; Kukizaki, M. J. Hazard. Mater. 2009, 162, 1103.

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Synthesis and Photoactivity of Ordered Mesoporous Titania with a

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