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Multitechnique Investigation of Mesoporous Titanosilicate Materials Prepared from Both the Self-Assembly and the Liquid Crystal Mechanisms K. Zimny,† C. Carteret,‡ M. J. Stebe,† and J. L. Blin*,† †
Equipe Physico-chimie des Colloïdes, UMR SRSMC N° 7565 Universite Henri Poincare-Nancy 1/CNRS, Faculte des Sciences, BP 239, F-54506 Vandoeuvre-les-Nancy cedex, France ‡ Laboratoire de Chimie Physique et Microbiologie pour l0 Environnement, UMR7564 Universite Nancy 1/CNRS 405, rue de Vandoeuvre, F-54600 Villers-les-Nancy, France
bS Supporting Information ABSTRACT: We have investigated the preparation of titanosilicate mesoporous materials from both the self-assembly and the liquid crystal templating mechanisms. A fluorinated surfactant has been used as a template. Different amounts of titanium have been incorporated in the silica matrix. The Ti contents have been estimated by elementary analysis. Samples have been characterized by SAXS; XRD; nitrogen adsorptiondesorption; and XPS, SEM, Raman, UV, and IR spectroscopies. SAXS analysis and nitrogen adsorptiondesorption have shown that the hexagonal array is lost when the titanium contents increase in the materials. In the meantime, the specific surface area decreases. However, when materials are prepared via the liquid crystal mechanism, the hexagonal structure is preserved for the higher amounts of titanium. As proven by XRD and Raman experiments, whatever the mechanism, anatase is formed. XPS analysis has shown different locations of TiO2 anatase according to the mechanism involved in the synthesis. If materials are prepared via the cooperative templating mechanism, TiO2 anatase is present on the surface, whereas for the liquid crystal templating mechanism, TiO2 anatase is rather located in the bulk. The latter also favors the substitution of Si4þ by Ti4þ. Photocatalytic activities of the titanosilicates have been estimated for the decomposition of methyl orange.
1. INTRODUCTION Titania is a semiconductor that has been widely used in the field of photocatalysis but also has received considerable attention for applications in electronics, electrochemical systems, including photoelectrochemical solar cells, electrocatalysis, optoelectronic sensor devices, and high-performance photocatalytic films.15 In almost all these applications, the crystal structure, particle size, surface area, and porosity of titania are important factors for the performance of these materials, especially for its photocatalytic activities.6,7 In particular, the increase of the photocatalyst surface area improves the efficiency of photocatalytic reactions.810 One way to increase the specific surface area consists of incorporating titanium into a mesoporous silica matrix. Indeed, mesoporous materials exhibit a mesopore ordering with a high specific surface area and uniform pore size distribution. The synthesis of the pure silica mesoporous molecular sieves consists of the condensation and polymerization of an inorganic source of silica around surfactant aggregates. Two main pathways lead to the introduction of the transition metal in the silica matrix. The first one, called the postsynthesis method, r 2011 American Chemical Society
consists of dispersing the transition metal after the achievement of the pure silica. For this reason, it can be adapted to a large variety of mesostructured silica, such as MCM-41 or SBA, and performed over a wide range of synthesis conditions.1419 Different strategies to disperse titanium, such as grafting or atom-planting, have been tested.1419 However, all these postsynthesis procedures are based on ion exchange with the residual hydroxyl groups at the surface of the pure silica matrix. For example, Lihitkar et al. have obtained TiO2 nanoparticles embedded within the MCM-41 mesopore by the impregnation method.16 The second approach is a direct method, and it involves the co-condensation of the metal precursor with a silicon one.14 The first synthesis of Ti-containing MCM-41 samples prepared through the one-step method was reported in 1994 by Corma et al. The obtained compounds have been used for the oxidation of sulfide or hex-1-ene by oxygen peroxide.20,21 Received: December 14, 2010 Revised: March 24, 2011 Published: April 12, 2011 8684
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The Journal of Physical Chemistry C Since, a series of Ti-substituted mesoporous silica materials have been synthesized by different groups.14,2227 For instance, using nonionic surfactants and titanocene dichloride under strong acid conditions, Melero et al. have reported the preparation of TiSBA-15 materials.24 The authors have controlled the synthesis conditions in order to prevent the formation of extraframework titanium species. Therefore, no photocatalytic activity can be noted, but the obtained materials show high catalytic activity and selectivity in the epoxidation of 1-octene using ethylbenzylhhydroperoxide as an oxidant. In a paper dealing with the preparation of Ti-SBA-15 by the direct synthesis method, Zhao et al. have shown that, at low content, titanium is incorporated into the framework of SBA-15. With the increase of titanium content, the TiO2 anatase particles are formed, located on the external surface of SBA-15.23 The recovered Ti-SBA-15 are active for the photocatalytic reaction in water treatment. Comparing the direct and the postsynthesis method, the first one results in a relatively homogeneous incorporation of the titanium, whereas the postsynthesis procedure leads to an increase of the titanium concentration on the surface.14 Depending on the surfactant concentration, two mechanisms can lead to the formation of the ordered mesoporous material. The first one is the selfassembly mechanism,1116,2830 also named cooperative templating mechanism (CTM). In this case, the building blocks are the micelles, so the CTM rather occurs at low surfactant concentrations. The second approach for the preparation of ordered mesostructures uses the liquid crystal phase and is labeled as the direct liquid crystal templating (LCT) pathway.3135 The inorganic precursors grow around the liquid crystal. After the polymerization and the condensation, the template can be removed, leaving a mesoporous material whose structure, pore size, and symmetry are determined by the liquid crystal scaffold. The CTM mechanism has been widely employed to obtain Ti-incorporated mesoporous materials, either by the postsynthesis or by the direct method. By contrast, to the best of our knowledge, only one group has considered the synthesis of titanium-incorporated mesoporous silica via the LCT pathway.36,37 In the paper reported here, mesoporous silica materials containing titanium have been prepared through both the selfassembly and the liquid crystal templating mechanisms by using, respectively, micelles and the liquid crystal phase of polyoxyethylene fluoroalkyl ether surfactants as building blocks. Titanium has been introduced into the silica matrix by the direct method. The quantity of titanium introduced into the silica matrix has been evaluated by elementary analysis. We have particularly investigated the effect of the synthesis pathway on the titanium location. The photocatalytic properties of the recovered materials toward the decomposition of methyl orange were also examined.
2. MATERIALS AND METHODS The used fluorinated surfactant, which was provided by DuPont, has an average chemical structure of C8F17C2H4(OC2H4)9OH. It is labeled as RF8(EO)9. The hydrophilic chain moiety exhibited a Gaussian chain length distribution, and the hydrophobic part is composed of a well-defined mixture of fluorinated tails. Tetramethoxysilane (TMOS) and titanium isopropoxide (TiOpr) used as the inorganic precursors were purchased from Aldrich. 2.1. Mesoporous Preparation. The materials have been prepared from a micellar solution of RF8(EO)9 at 10 wt % in water (CTM) or from a hexagonal liquid crystal phase (H1)
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containing 60 wt % of RF8(EO)9 in water (LCT). The surfactant/inorganic precursor molar ratio was fixed to 0.5 when the synthesis is performed via the CTM mechanism and to 0.12 when the LCT pathway is employed. Depending on the mechanism, the pH of the solution was adjusted with hydrochloric acid (HCl) either to 2 (CTM) or to 1.3 (LCT). In previous papers, we have shown that, under the conditions given above, the pure silica mesoporous materials, prepared from both the CTM38 and the LCT39 mechanisms, exhibit a hexagonal pore ordering. In both cases, the Ti/(Ti þ Si) molar fraction (noted as r) was varied from 0.02 to 0.20. For example, for r = 0.05 when the preparation is carried out through the CTM mechanism, 1.12 g of surfactant is dissolved in 10 g of a solution at pH 2. 0.037 g of titanium isopropoxide (TiOPr) and 0.374 g of TMOS are then added. If the LCT is used, 1.5 g of RF8(EO)9 is dissolved in 1 g of a solution at pH 1.3. 0.0167 g of titanium isopropoxide (TiOPr) and 1.7 g of TMOS are then added. In both cases, the obtained samples were sealed in Teflon autoclaves and heated at 80 °C during 1 day. The final products were recovered after ethanol extraction with a Soxhlet apparatus during 48 h. 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. Nitrogen adsorptiondesorption isotherms were obtained at 196 °C over a wide relative pressure range from 0.01 to 0.995 with a volumetric adsorption analyzer, TRISTAR 3000 manufactured by Micromeritics. The samples were degassed under vacuum for several hours at 320 °C before nitrogen adsorption measurements. The pore diameter and size distribution were determined by the BJH (Barret, Joyner, Halenda) method.40 Raman scattering spectra were collected on a JobinYvon 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 = 0.5) on an area of about 3 μm2. The laser power on the sample was approximately 10 mW. The spectral resolution was 3 cm1, with a wavenumber precision better than 1 cm1. The infrared spectra were recorded on a Fourier transform infrared spectrometer (PerkinElmer 2000), equipped with a KBr beam splitter and a DTGS detector. The spectra in diffuse reflectance (DRIFTS) mode were collected using a Harrick DRA-2CI equipment and a Harrick HVC-DRP cell. To perform the analysis, the mesoporous powder was first diluted in a KBr matrix (10 wt %). The sample was then kept inside an evacuated chamber (104 mbar). Reflectances Rs of the sample and Rr of pure KBr, used as a nonabsorbing reference powder, were measured under the same conditions. The mesoporous reflectance is defined as R = Rs/Rr. The spectra are shown in pseudoabsorbance (logR) mode. The DRIFT technique was employed, instead of the conventional transmission method, with pressed wafers to avoid destruction of mesoporous structures under pressure. Diffuse reflectance spectra were recorded between 200 and 600 nm at 1 nm increments with an integrated sphere attached to a Cary 5G UVvis-NIR spectrophotometer. The relative spectral reflectance R is defined as the ratio between the flux reflected by the sample, Rs, and that of a PTFE reference, Rr. The spectra are shown in pseudoabsorbance (logR) mode because the use of the KubelkaMunk function does not improve the quality of the calibration line. XPS spectra were collected on a Kratos Axis Ultra (Kratos Analytical, U.K.) spectrometer with a hemispherical energy analyzer and 8685
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using a monochromatic Al KR source (1486.6 eV). All spectra were recorded at a 90° takeoff angle, with the analyzed area being currently about 0.7 0.3 mm. Survey spectra were acquired with a 1.0 eV step and 160 eV analyzer pass energy and the highresolution regions with a 0.05 eV step and 20 eV pass energy (instrumental resolution better than 0.5 eV). The Shirley background-subtraction routine was used throughout, and the O(1s), Si(2p), and Ti(2p) binding energies were referenced to the C(1s) line situated at 284.6 eV, that is, the value generally accepted for adventitious carbon surface contamination. All these analyses have been performed after surfactant removal. 2.3. Decomposition of Methyl Orange. Methyl orange was chosen as the organic compound to evaluate the photocatalytic properties of the obtained materials. For this purpose, 50 mg of Table 1. Elementary Analysis: Quantity of Titanium in the Titanosilicates as a Function of the Ti/(Ti þ Si) Molar Ratio (r) in the Starting Solution r in the titanosilicate materials r in the starting solution
CTM
LCT
0.02
0.015
0.012
0.05
0.032
0.027
0.07
0.042
0.041
0.1
0.062
0.058
silica materials containing titanium have been added to 100 mL of a methyl orange solution (15 mg per mL). The reaction was carried out with continuous stirring in a glass flask. Samples were irradiated with UV light from a high-mercury lamp. The degradation process of methyl orange was monitored with a Cary 3G UVvis spectrophotometer.
3. RESULTS AND DISCUSSION Prior to any other analyses, the titanium amount that is really present in the sample has been measured by elementary analysis. As shown in Table 1, whatever the synthesis pathway, the quantity of titanium detected in the mesoporous materials linearly increases as a function of r. However, the determined value of r is lower than the targeted ratio. It means that a part of the titanium is lost during the synthesis. A fraction of titanium can be removed during the extraction step. 3.1. Structural and Textural Investigation. Figure 1 depicts the variation of the SAXS pattern with the Ti/(Ti þ Si) molar ratio (r) in the starting solution. Whatever the mechanism, the pure silica mesoporous materials exhibit a hexagonal mesopore ordering. Indeed, the peaks located at 5.2, 3.0, and 2.6 nm (Figure 1A, pattern a) or at 5.0, 2.8, and 2.5 nm (Figure 1B, pattern a), when the syntheses are, respectively, performed through the self-assembly and the liquid crystal templating pathway, can be attributed to the (110) and (200) reflections of the hexagonal structure. According to Bragg’s rule, the unit cell dimension (a0) can be deduced and is
Figure 1. Variation of the SAXS pattern with the initial Ti/(Ti þ Si) molar ratio (r). The titanosilicates are prepared through either the CTM (A) or the LCT (B) mechanism. The value of r is equal to (a) 0, (b) 0.02, (c) 0.05, (d) 0.07, (e) 0.1, and (f) 0.2. 8686
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Figure 2. Wide-angle X-ray diffraction pattern of samples prepared from the CTM mechanism. The value of r is equal to (a) 0, (b) 0.02, (c) 0.05, and (d) 0.07. The pattern of pure anatase (e) is given as a reference.
of about 6.0 and 5.8 nm, respectively, for the CTM and the LCT mechanisms. The higher intensity of the secondary reflections in Figure 1B, pattern a, indicates that mesoporous silica prepared via LCT, that is, from a hexagonal liquid crystal phase, presents a higher degree of organization than the one obtained by CTM, that is, from a micellar solution. When titanosilicates are prepared from this latter mechanism, the mesopore ordering is kept until r = 0.05 (Figure 2A, patterns b and c). However, the decrease of the (110) and (200) reflections intensity indicates that the disorganization of the channel array has begun. If the value of r reaches 0.07, no reflection is detected anymore on the SAXS patterns (Figure 1A, patterns df). That means that a transition from a hexagonal structure to a randomly oriented pore arrangement occurs. The same trend is noted when the syntheses are carried out from the LCT mechanism. The hexagonal structure is maintained until r = 0.07 (Figure 1B, patterns bd). The mesopore ordering is disturbed as soon as r = 0.05, and it is completely lost for r = 0.10 (Figure 1B, pattern e). Except for a large peak at around 2θ = 23°, characteristic of amorphous silica, no reflection is detected on the wide-angle X-ray diffraction pattern of the pure silica mesoporous material (Figure 2, pattern a). Upon the addition of titanium, as soon as r = 0.02, peaks that can be attributed to the (101), (004), (200), (105), (204), (220), (116), (215), and (224) of anatase appear (Figure 2, patterns bd). Their intensity increases with r. With the addition of titanium, the structural analysis evidences the formation of anatase. The pure silica materials exhibit a type IV isotherm (see the Supporting Information, S1), characteristic of mesoporous
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materials, according to the IUPAC classification.41 The pore diameter distribution, determined by using the BJH method, is quite narrow and centered at around 3.8 and 2.8 nm when the CTM and LCT mechanisms are, respectively, used. Concerning the titanosilicates, whatever the pathway employed for their synthesis, for values of r lower than 0.2, compounds display a type IV isotherm (see the Supporting Information, S1A,B). However, the adsorbed volume at saturation decreases with r, and the relative pressure for which capillary condensation takes place is spread out over a larger range of relative pressure. Because the p/p0 position of the inflection point is related to the pore diameter, it can be inferred that pore size becomes less homogeneous upon the addition of titanium. This is confirmed by the pore size distribution, which becomes broader when the value of r is changed from 0 to 0.1 (see the Supporting Information, S2A,B). In this range of Ti/(Ti þ Si) molar ratios, the maximum of the pore size distribution remains almost constant, respectively, to about 3.03.8 (Supporting Information, S2A) and 2.83.0 nm (Supporting Information, S2B). When r reaches 0.2, the isotherm is not type IV anymore, but rather type II (see the Supporting Information, S1), according to the IUPAC classification. This suggests an interparticular porosity and the loss of the inner mesoporosity of the compounds, which is also indicated by the pore size distribution. As a matter of fact, in the Supporting Information, S2Ad, no mesopore size distribution is detected for titanosilicates prepared from the CTM mechanism. The one of the sample obtained via the LCT route is very broad and centers at 3.8 nm, but the dV/dD value is very low (Supporting Information, S2Bd). The interparticular porosity can be related to the formation of anatase that has been evidenced by the wide angle XRD analysis. A decrease of the specific surface area (SBET) is noted with the addition of titanium (see the Supporting Information, S3). Nevertheless, this drop is less pronounced when syntheses are carried out from the LCT. For instance, if materials are prepared from the CTM mechanism, the specific surface area drops sharply from 1045 to 540 m2/g when r is varied from 0 to 0.2 (Supporting Information, S3A). For the LCT route, SBET lowers from 1100 to 700 m2/g when r is increased from 0 to 0.1, then keeps constant (Supporting Information, S3B). The decrease of the specific surface area upon the addition of titanium can be attributed to the appearance of the anatase phase. Whatever the mechanism used to prepare the titanosilicate materials, the addition of titanium involves a loss in mesopore ordering. The investigation of the RF8(EO)9/alcohol/water systems reveals that the micellar domain (L1) detected for an RF8(EO)9 content lower than 40 wt % in water is progressively expanded toward the higher surfactant concentrations with the addition of alcohol (see the Supporting Information, S4). For instance, at 20 °C, the micellar domain is extended up to 57 wt % of RF8(EO)9 for a weight percent of CH3OH equal to 20 wt % (Supporting Information, S4A). Thus, the alcohols produced during the hydrolysis of the inorganic precursors do not disturb the existence of micelles. Nevertheless, upon the presence of alcohol, the surfactant range of composition belonging to H1 is progressively reduced. For example, the hexagonal liquid crystal phase is completely melted when the CH3OH concentration reaches 15 wt % (Supporting Information, S4A). That is the reason why methanol and isopropanol are removed under vacuum when the syntheses are performed via the LCT route. So, from the surfactant point of view, all the conditions are together for the synthesis to occur through both mechanisms. To 8687
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3.2. Investigation of the Titanium Location and Photocatalytic Properties. The addition of titanium into a silica matrix
Figure 3. Infrared spectra of the materials prepared with a value of r equal to (a) 0, (b) 0.02, (c) 0.1, and (d) 1. The synthesis is carried out from the CTM (A) or the LCT (B) pathway.
explain the absence of mesopore ordering with the increase of the titanium loading, we should rather look at the difference of reactivity between the two alkoxides. Indeed, the hydrolysis rate of the titanium isopropoxide is high, and the tendency to fill the coordination sphere of Ti leads to irreversible formation of TiOTi bonds.42 The wide-angle X-ray analysis and the Raman experiments show that TiO2 clusters of anatase are formed. At low titanium content, these clusters do not disturb either the self-assembly or the liquid crystal template mechanisms. Hydrogen-bonding interactions between the oxygen atoms of the oxyethylene groups of the surfactant and OH groups of the hydrolyzed TMOS precursors are still formed, and after the surfactant removal, titanosilicates with a hexagonal channel array are obtained. With the increase of the titanium content, the anatase particles disturb the interactions between RF8(EO)9 and the hydrolyzed TMOS. The self-mechanism does not occur anymore, and only disordered materials are recovered. When the LCT is employed, the formation of anatase involves a collapse of the mesostructure. As observed by nitrogen adsorptiondesorption analysis for r = 0.2, the mesoporosity has almost completely disappeared and only an interparticular porosity is obtained.
by the direct method can lead to a partial substitution of Si4þ by Ti4þ, in particular, at low titanium content.14 In that case, SiOTi bonds are formed and a band at around 960 cm1 can be detected on the infrared spectrum.4345 Figure 3 depicts the evolution of the infrared spectra with the addition of titanium. All analyzed samples were outgassed under vacuum in order to eliminate physisorbed water. The spectra of the pure mesoporous silica, given as a reference, exhibit bands at 3745, 3340, 2980, 2940, 2905, 1170, 1070, and 960 cm1 (Figure 3A, spectra a). The broad absorption around 3340 involves the OH stretching mode of H-bonded silanol groups, whereas the sharp band at 3745 cm1 characterizes the free silanols.46 Bands detected at 2980, 2940, and 2905 cm1 are attributed to the stretching vibrations of the CH groups of the surfactant, which was not completely removed by the ethanol extraction. However, no evidence of CF absorption is observed on the FTIR spectrum. The quantity of RF8(EO)9, remaining after extraction, is very low. Below 1800 cm1, the spectrum is dominated by a broad and intense band with a maximum at 1070 cm1 and a shoulder at 1170 cm1, characteristic of the antisymmetric stretching vibrational mode of the SiOSi siloxane bridges. The less intense absorption at 960 cm1 is assigned to the SiO stretching of free silanols.46 In addition to the vibration related to the surfactant in the 25004000 cm1 domain, the spectrum of the pure titania synthesized in the same conditions than the ones used to obtain the titanosilicates exhibits bands at 3235 and 3625 cm1. They are attributed to the TiOH bonding.47 As the vibrations of the TiO2 skeleton48 occur below 800 cm1, they are not observed on the spectra reported in Figure 3A (spectrum d),B (spectra d). Whatever the synthesis pathway, the infrared spectra of the titanosilicates show similar features than the one of the pure silica. Upon the addition of titanium, it seems that the intensity of the band at 960 cm1 increases (Figure 3A (spectra ac),B (spectra ac)). This variation suggests that a part of the titanium is incorporated into the framework. However, as this band also occurs in the Tifree materials, it cannot be used as the only criterion to claim the incorporation of titanium into the framework. To confirm or not the substitution of a part of Si4þ by Ti4þ, X-ray photoelectron spectroscopy analyses have been performed. Figure 4 shows the Ti 2p and O 1s spectra of the titanosilicates and of pure anatase. When the samples are prepared from the CTM mechanism, two peaks at 533.1 and 529.6 eV are observed on the O 1s spectra (Figure 4Aa). They are, respectively, attributed to oxygen for a SiO249 and to a TiOTi environment.24 No binding energy at around 531.7 eV, characteristic of oxygen in the SiOTi linkage, is observed. Whatever the Ti/(Ti þ Si) molar ratio, the positions of the Ti 2p3/2 (458.5 eV) and Ti 2p1/2 (464.3 eV) are observed; they are characteristic of anatase (Figure 4Ab). This result is in agreement with the wide-angle XRD diffraction analysis; that is, the increase of the r value leads to the formation of anatase. By contrast, if the titanosilicate is synthesized via the LCT pathway, the Ti 2p3/2 regions show a doublet at 458.5 and 459.6 eV (Figure 4Bb). According to the literature, the contribution at the higher binding energy corresponds to a tetrahedral environment of titanium and refers to its accommodation into the silica framework.44 Thus, we can conclude that a part of Si4þ is substituted by Ti4þ, when LCT is employed. This substitution is also supported by the modification of the intensity of the free silanols noted in the infrared spectra. Indeed, as it can be seen in 8688
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Figure 4. O 1s (a) and Ti 2p (b) XPS spectra of the materials prepared with a value of r equal to (9) 0.02, (O) 0.07, (4) 0.1, and (-) 1. The synthesis is carried out from the CTM (A) or the LCT (B) pathway.
Figure 5. Variation of the titanium surface composition as a function of the initial Ti/(Ti þ Si) molar ratio. The synthesis is carried out from the CTM (9) or the LCT (O) pathway.
Figure 3, if the titanosilicate preparation is carried out via LCT, the intensity of the SiOH bond decreases as a function of r (Figure 3B, spectra ac), whereas it keeps constant for CTM (Figure 3A, spectra ac). Moreover, with the addition of titanium, the increase in intensity of the O 1s peak due to oxygen
in a TiOTi environment (529.6 eV) is less pronounced for the materials obtained by CTM. The XPS analysis also provides information about the surface composition. From Figure 5, we can see that the content of titanium at the surface is higher when samples are prepared through the CTM mechanism. As the elementary analysis has revealed that whatever the mechanism, the overall loadings of titanium are in the same range of order (Table 1), we can conclude that the CTM mechanism leads to the location of titanium at the surface, whereas it is rather located in the bulk for LCT. Thus, for the LCT pathway, the transition metal is located primarily in the central hydrocarbon regions of the surfactant cylinders. By contrast, when the preparations are carried out from the CTM mechanism, these clusters are rather located near the surfactant headgroup. In addition, in Figure 6, we show the Raman spectra of some titanosilicates. The commercial pure anatase is also shown for comparison. The Raman spectrum of commercial anatase exhibits a very intense band at 143 cm1 (Eg band) and three other bands at 394, 514, and 637 cm1 that correspond, respectively, to B1g, (A1g, B1g) and Eg modes. Whatever the synthesis pathway, upon addition of titanium, the Raman spectra of the recovered materials show all the anatase vibrations. The intensity of these bands increases with r. Thus, the Raman analysis confirms the formation of anatase with the addition of titanium. It has been reported that Raman scattering can be used for an estimation of a TiO2 crystallite size, by examining the 8689
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Figure 6. Raman spectra A: the materials are prepared from CTM with a value of r equal to (a) 0.02, (b) 0.05, (c) 0.07, and (d) 0.1. Raman spectra B: titanosilicates are synthesized via LCT with a value of r equal to (a) 0.02, (b) 0.07, and (c) 0.1. Spectra of commercial anatase (A, spectrum e, and B, spectrum d) with a particle size above 20 nm are given as a reference.
Eg mode,5052 which appears at 143 cm1 in bulk anatase and shifts to higher wavenumbers in nanocrystalline materials. The spectra presented in Figure 6 show the Eg band at 143 cm1 for the commercial anatase and at 150 cm1 for the titanosilicates. Therefore, nanosized anatase is formed. Swamy et al.53 have reported that the full width at half-maximum (fwhm) of the band at 143 cm1 can also be used to determine the size of the crystallites. For titanosilicates, the fwhm of this band is around 35 cm1. According to the study published by Swamy et al., that means that the size of the anatase particles is smaller than 10 nm. Diffuse reflectance UVvisible spectroscopy has also been employed to characterize the materials. A broad adsorption band, attributed to a O2Ti4þ charge transfer,24 is detected at 335 nm on the spectrum of pure titania (see the Supporting Information, S5A,B). Spectra of the titanosilicates prepared from both the CTM and the LCT pathways are similar; the adsorption occurs in a range between 230 and 310 nm. They exhibit a broad band that can be decomposed in two parts (Supporting Information, S5A (spectra ad),B (spectra ac)). The first contribution at around 230 nm arises from a ligand-tometal charge-transfer transition in isolated TiO4 units. It corresponds to coordinated Ti4þ species that are incorporated into the silica framework.15,23 The second contribution at 300 nm is blue shifted from the one of TiO2. This blue shift has been observed for titania nanoparticles24,44,49 on silica and corresponds to small TiO2 clusters through the quantum size effect. The UVvisible analysis evidences that, for both mechanisms, a part of the Si4þ is substituted by Ti4þ. However, XPS reveals that this substitution at the surface is more important for the LCT mechanism than for the CTM one. Indeed, one should be reminded that, for the latter at the surface, no substitution is observed by X-ray photoelectron spectroscopy.
Figure 7. Kinetics of the photocatalytic degradation of methyl orange solution containing titanosilicates. Materials are prepared from the CTM mechanism with (2) r = 0.05 and (3) r = 0.1 or from the LCT pathway with (9) r = 0.05 and (O) r = 0.1.
The photocatalytic activity of the titanosilicates has been tested by considering the decomposition of methyl orange. It should be noted that, prior to UV irradiation, the solution was stirred in the dark for 15 min until an adsorptiondesorption equilibrium was reached. Results are reported in Figure 7. From this figure, we can see that, for a given titanium loading, materials prepared from the CTM mechanism exhibit a higher activity than the ones obtained from the LCT pathway. For instance, for r = 0.05, if the titanosilicate is synthesized via the CTM mechanism, 32% of the methyl orange solutions is decomposed within 240 min, whereas during the same period, this value reaches 18% if the LCT pathway 8690
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The Journal of Physical Chemistry C is employed. The materials obtained by using both pathways have similar mesopore diameters; thus, the difference of activity cannot be attributed to a difference of mass transport of the methyl orange molecules inside the channels. As photocatalysis is a surface phenomenon, this photoactivity can be rather related to the location of the anatase. As a matter of fact, as indicated above, the proportion of anatase at the surface is more important when the CTM is used. It favors the interaction between TiO2 and the organic compound during the diffusion of reactive species along the mesoscale channel. By contrast, the amount of titania in the bulk is more important when the titanosilicate is prepared via LCT, which also favors the incorporation of Ti4þ into the silica framework. These Ti4þ species are not involved in the photocatalytic process.
4. CONCLUSIONS In this study, titanosilicates have been synthesized either in aqueous solutions of fluorinated micelles through the cooperative templating mechanism or with fluorinated hexagonal liquid crystals via the liquid crystal mechanism. The preparation of the materials has been carried out by using a mixture of tetramethylorthosilicate and titanium isopropoxide as inorganic sources and C8F17C2H4(OC2H4)9OH [RF8(EO)9] as a structure-directing agent. The investigation of the RF8(EO)9/CH3OH/water and RF8(EO)9/(CH3)2CH(OH)/water phase diagrams reveals that alcohols released during the hydrolysis of the inorganic precursors do not disturb the micellar phase, but they involve a melting of the hexagonal liquid crystal phase. Therefore, the produced methanol and isopropanol have been removed under vacuum when the LCT pathway is employed. Different Ti/(Ti þ Si) molar ratios have been considered. The amount of titanium that is really present in the sample has been determined by elementary analysis. Whatever the pathway, the wide-angle X-ray and the Raman analyses evidence that anatase is obtained. The formation of anatase, with the increase of titanium, involves a loss of the mesopore ordering and a collapse of the mesostructure. In the meantime, the specific surface area decreases. However, the hexagonal channel is maintained for higher titanium loading if the syntheses are performed from the LCT. The combination of the Raman and XPS results indicates that CTM rather leads to the formation of anatase at the surface. By contrast, LCT favors the location of anatase in the bulk and the substitution of Si4þ by Ti4þ at the surface. Finally, the photoactivity of the recovered materials has been tested by considering the photodegradation of methyl orange. Because of the formation of anatase at the surface, the titanosilicates obtained from the CTM mechanism exhibit a higher activity than the ones prepared through the LCT pathway. Moreover, from the SEM analysis, it appears that titanosilicate particles prepared from the CTM pathway have a smaller size than those prepared from the LCT pathway (see the Supporting Information, S6A,B). The smaller size of titanosilicate particles synthesized through the CTM pathway may also contribute to the increase of the photocatalyst activity. ’ ASSOCIATED CONTENT
bS
Supporting Information. Figures showing the variation of the nitrogen adsorptiondesorption isotherm, the mesopore size distribution, and the specific surface area with the initial molar ratio, r; temperaturecomposition phase diagram; diffuse reflectance UVvis spectra; and SEM micrographs. This material is available free of charge via the Internet at http://pubs.acs. org.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors would like to thank DuPont de Nemours Belgium for providing the fluorinated surfactants. Kevin Zimny thanks the “Region Lorraine’’ for the financial support of his Ph.D. We are also grateful to A. Renard for XPS experiments and to the “Plateforme de Microscopies des Interfaces, Institut Jean Barriol,’’ of the Nancy University, for providing access to spectroscopic facilities. ’ 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. J. Phys. Chem. B 2004, 108, 3713. (10) Sakatani, Y.; Grosso, D.; Nicole, L.; Boissiere, C.; Soller-Illia, G. J.; De, A. A.; Sanchez, C. J. Mater. Chem. 2006, 16, 77. (11) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (12) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCulle, S. B.; Higgins, J. B.; Schlender, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (13) Wan, Y.; Zhao, D. Chem. Rev. 2007, 107, 2821. (14) Li, J.; Zhou, C.; Xie, H.; Ge, Z.; Yuan, L.; Li, X. J. Nat. Gas Chem. 2006, 15, 164. (15) Berube, F.; Nohair, B.; Kleitz, F.; Kaliaguine, S. Chem. Mater. 2010, 22, 1988. (16) Lihitkar, N. B.; Abyaneh, M. K.; Samuel, V.; Pasricha, R.; Gosavi, S. W.; Kulkarni, S. K. J. Colloid Interface Sci. 2007, 314, 310. (17) Zhang, S. G.; Fujii, Y.; Yamashita, H.; Koyano, K.; Tatsumi, T.; Anpo, M. Chem. Lett. 1997, 26, 659. (18) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature 1995, 378, 159. (19) Yamagishi, K.; Namba, S.; Yashima, T. Stud. Surf. Sci. Catal. 1991, 60, 171. (20) Corma, A.; Camblor, M. A.; Esteve, P.; Martinez, A.; Perez-Pariente, J. J. Catal. 1994, 145, 151. (21) Corma, A.; Navarro, M. T.; Pariente, J. P. J. Chem. Commun. 1994, 147. (22) Galacho, C.; Ribeiro Carrott, M. M. L.; Carrott, P. J. M. Microporous Mesoporous Mater. 2007, 100, 312. (23) Li, G.; Zhao, X. S. Ind. Eng. Chem. Res. 2006, 45, 3569. (24) Melero, J. A.; Arsuaga, J. M.; De Frutos, P.; Iglesias, J.; Sainz, J.; Blazquez, S. Microporous Mesoporous Mater. 2005, 86, 364. (25) Zhang, W.-H.; Lu, J.; Han, B.; Li, M.; Xiu, J.; Ying, P.; Li, C. Chem. Mater. 2002, 14, 3413. (26) Chen, Y.; Huang, Y.; Xiu, J.; Han, X.; Bao, X. Appl. Catal., A 2004, 273, 185. (27) Alba, M. D.; Luan, Z. H.; Klinowski, J. J. Phys. Chem. 1996, 100, 2178. 8691
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