Direct Synthesis and the Morphological Control of Highly Ordered Two

10130

J. Phys. Chem. C 2008, 112, 10130–10140

Direct Synthesis and the Morphological Control of Highly Ordered Two-Dimensional P6mm Mesoporous Niobium Silicates with High Niobium Content P. Srinivasu,† C. Anand,† S. Alam,† K. Ariga,‡ S. B. Halligudi,† V. V. Balasubramanian,† and A. Vinu*,† International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1, Namiki, Tsukuba, 305-0044, Japan, and Supermolecules Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ReceiVed: January 12, 2008; ReVised Manuscript ReceiVed: April 11, 2008

Here we demonstrate the preparation of highly ordered two-dimensional P6mm mesoporous niobium silicates with unprecedented loadings of niobium in the silica framework (up to nSi/nNb ratio of 6.1) and highly winding morphologies maintaining regularly aligned arrays of nanochannels through a simple adjustment of the molar water to hydrochloric acid ratio (nH2O/nHCl) in the synthesis gel using nonionic surfactant as a template in a highly acidic medium. X-ray diffraction (XRD), N2 adsorption, high-resolution scanning electron microscopy (HRSEM), energy-dispersive spectrometry (EDS), and high-resolution transmission electron microscopy (HRTEM) results provided strong evidence of the presence of highly ordered two-dimensional structure in the NbSBA-15 materials with a very high content of niobium and winding morphologies, whereas UV-visible diffuse reflectance spectroscopy (UV-vis DRS) confirmed the presence of tetrahedrally coordinated niobium in the silica framework. Moreover, the loading of Nb in the silica framework can be controlled by a simple adjustment of the nH2O/nHCl and the amount of Nb source in the synthesis gel. Thus, a hitherto unachieved loading of niobium in the silica framework with a controlled manner, considered by many researchers to be impossible to achieve for the highly ordered mesoporous SBA-15 materials because of the highly acidic synthesis condition and the framework connectivity constraints, has been reported. We also found that the Nb incorporation could be a way to control the morphlogy of the NbSBA-15 materials. The morphology of the materials changed from rod to semispherical to “winding road” shape with decreasing the nSi/nNb from 22.5 to 6.1. Moreover, the detailed mechanism on the morphological control as a function of Nb content has also been proposed. This simple one-step technique to control the morphology, which would be of general interest for the application requiring the materials with different morphology, and Nb loading in a highly acidic medium synthesis have not been reported so far. Introduction Unusual phenomena such as quantum effects have been extensively reported in dimensionally controlled nanosized systems, parts of which are highly expected to be practically used in advanced applications with the aid of highly sophisticated technology, so-called nanotechnology.1 Nanoscience and nanotechnology require technical developments for fabrication of nanoscale environments with reliable structural precision together with versatile functionalizaion of their interior. A series of technologies on mesoporous materials2 would satisfy these demands, because mesoporous materials such as mesoporous silica and carbon can provide arrays of regular nanospaces with incredible structural precision in a desirable range (unexplored dimension, 10-9 to 10-7 m). Although considerable research has so far been done for synthesis3 and functionalization4 of mesoporous materials, several important challenges still remain unexplored. Some of them can be assigned as (i) techniques for free immobilization of heteroatoms in a framework of mesoporous materials and (ii) hierarchic structure control of * To whom correspondence should be addressed. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044. Phone: +81-29-860-4563. Tel: +81-29-860-4667. E-mail: [email protected]. † International Center for Materials Nanoarchitectonics. ‡ Supermolecules Group.

nanochannel arrays. In this article, we present novel advances of both aspects. Concerning point (i), one of the findings described in this report is the successful immobilization of a large amount of heteroatom (niobium in this case) to the frameworks of desired mesoporous silica structures. Incorporation of metallic heteroatoms in the framework of mesoporous silica materials that are neutral is one of the forefront targets in this field, because it realistically leads to the development of novel types of catalysts.5 While the postsynthetic grafting procedures for heteroatom incorporation often cause pore blocking, which limits the diffusion of reactant molecules inside the mesoporous channels, the direct synthesis avoids the pore blocking problem because of the uniform distribution of the heteroatoms in the entire framework. However, synthesis of mesoporous silicates such as SBA-15 and KIT-6, which have larger pores and are highly advantageous for the diffusion of large reactant molecules, require strong acidic conditions that are not ideal for the incorporation of oxo-species of transition metals because of their high solubility in acidic media. For example, incorporation of niobium into the silica framework of SBA-15 is not widely conducted, although niobium has high potential in its catalytic performance and specific interactions with biomaterials.6 Recently, we reported a novel but simple technique to incorporate heteroatoms into an SBA-15 framework with realizing success-

10.1021/jp800292a CCC: $40.75  2008 American Chemical Society Published on Web 06/03/2008

Highly Ordered 2D P6mm Mesoporous Nb Silicates ful preparation of AlSBA-15,7 FeSBA-15,8 and Ti-SBA-15,9 where adjustment of acidity was made in the synthesis gel by the simple adjustment of the molar water-to-hydrochloric acid ratio (nH2O/nHCl) resulted in sufficiently high contents of the heteroatoms in the SBA-15 framework. In this paper, using this methodology, we report the successful preparation of hexagonally ordered niobium-substituted SBA-15 (NbSBA-15) with a very high niobium content (nSi/nNb ratio of up to 6.1). With regard to point (ii), one of the more significant advances in the current research is the macroscopic morphological control of mesopore arrays upon heteroatom doping. Although macroscopic morphological control of mesoporous materials10 has also been focused on as an important subject in controlling catalysis and medical applications, the preparation of winding, twisted, and helical arrays was recently initiated, as seen in a report by Che et al.11 In their pioneering work, use of chiral structuredirecting reagent was believed to be a crucial step in the formation of helical nanochannels. However, several reports12 followed, using anionic, cationic surfactants, and ionic liquids as structure-directing reagents, revealing that twisted and helical structures can be tuned by various factors, and chirality in synthetic components is not always necessary for these structures. Therefore, the preparation of hierarchically controlled mesoporous silica with winding, twisted, and helical nanochannels is not well explored, where big challenges are still awaited. In this report, we strikingly demonstrate that the incorporation of a heteroatom (Nb) into the silica framework of hexagonally ordered mesoporous silica SBA-15 can lead to macroscopic morphological control of nanochannel array, i.e., highly winding structures of mesopore arrays can be successfully prepared in Nb-substituted SBA-15. As described above, we report here two distinct progresses in mesoporous technology: (i) successful doping of a high content of Nb in an SBA-15 framework and (ii) the formation of a highly winding structure of a nanochannel array in NbSBA15 structures. Both of the above features are disclosed here, and the presented novel concepts will be generalized in facile metal incorporation in mesoporous silicates and their morphological control. Experimental Section Materials. Niobium (V) chloride and tetraethylorthosilicate (TEOS, Merck) were used as the source for niobium and silicon, respectively. Triblock copolymer poly(ethylene glycol)-blockpoly(propylene glycol)-block-poly(ethylene glycol) (Pluronic P123, molecular weight ) 5800, EO20PO70EO20) was purchased from Aldrich and used as a structure-directing template. Preparation of the Materials. NbSBA-15 materials with various nSi/nNb ratios were prepared by the following procedure: Amphiphilic triblock copolymer Pluronic P123 (4 g) was dispersed in 30 g of water and stirred for 4 h. Subsequently, 70 mL of 0.29 M HCl was added and stirred for 2 h (pH ) ca. 2.2). Then 9.0 g of TEOS and an appropriate amount of niobium (V) chloride were added directly into the homogeneous solution under stirring. The first set of samples was prepared by changing the nH2O/nHCl, and the samples are denoted as NbSBA-15(xH) where x denotes the nH2O/nHCl ratio (x ) nH2O/nHCl). For this set of samples, the nSi/nNb ratio in the gel was fixed to 10. Another set of the samples was prepared by varying the nSi/nNb ratio in the gel using the constant nH2O/nHCl of 276. The samples were labeled NbSBA-15(y) where y denotes the nSi/nNb ratio. The resulting gel was aged at 40 °C for 24 h and finally heated to 100 °C for 48 h. The molar gel composition of the synthesis mixture was 1:0.035-0.15:0.016:0.46:127 TEOS/Nb2O5/P123/

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10131 HCl/H2O. Pure siliceous SBA-15 was synthesized using the same procedure (1:0.016:5.54:190 TEOS/P123/HCl/H2O) in the absence of niobium. The obtained solids were finally calcined in flowing air at 540 °C to decompose the triblock copolymer. Characterization. The powder X-ray diffraction (XRD) patterns of NbSBA-15 samples were collected on a Rigaku diffractometer using Cu KR (λ ) 0.154 nm) radiation. The diffractograms were recorded in the 2θ range of 0.8 to 10° with a 2θ step size of 0.01° and a step time of 10 s. Nitrogen adsorption and desorption isotherms were measured at -196 °C on a Quantachrome Autosorb 1 sorption analyzer. The samples were outgassed for 3 h at 250 °C under vacuum in the degas port of the adsorption analyzer. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) model. The pore size distributions were obtained from the adsorption branch of the nitrogen isotherms by the Barrett-Joyner-Halenda (BJH) method. The morphology and elemental mapping of the materials were observed on a Hitachi S-4800 field emission scanning electron microscope (HR-FESEM) with energy-dispersive X-ray (EDX) spectroscopy using an accelerating voltage of 10 kV. The highresolution transmission electron microscopy (HRTEM) images and the elemental mapping were obtained with an advanced field emission electron microscope JEOL JEM-2100F featuring ultrahigh resolution (0.1 nm) and rapid data acquisition. The preparation of samples for HRTEM analysis involved sonication in ethanol for 5 min and deposition on a copper grid. The accelerating voltage of the electron beam was 200 kV. UV-visible diffuse reflectance spectra (UV-vis DRS) were measured with a Perkin-Elmer Lambda 18 spectrometer equipped with a Praying-Mantis diffuse reflectance attachment. BaSO4 was used as a reference. Elementary analysis was done using an Analyst AA 300 spectrometer. 29Si magic-angle spinning (MAS) NMR spectra were recorded at a resonance frequency of 99.361 MHz using a pulse length of 3 µs, a recycle delay of 30 s, and about 2000 scans to obtain a good signal-to-noise ratio. The signals were referenced to external tetramethylsilane (TMS, 0 ppm). 93Nb MAS NMR spectra were recorded at a resonance frequency of 122.413 MHz using short pulse lengths of 2.1 µs, a recycle delay of 100 ms, and about 600 000 scans to obtain a better signal-to-noise ratio. The signals were referenced to an external solution of NbCl5 in dry acetonitrile (chemical shift -49 ppm). The rotation frequency was 12 kHz for all spectra. All NMR spectra were measured on a Bruker MSL 500 spectrometer. Results and Discussion SBA-15 mesoporous silica molecular sieves with large pore diameter apparently have advantages in several applications, including catalysis, separation, and drug delivery, as compared with small-pore mesoporous silica such as MCM-41. Nevertheless, heteroatom incorporation into an SBA-15 framework by the direct synthesis method is much more difficult than that into other mesoporous materials, namely MCM-41, MCM-48, or HMS because the synthesis of SBA-15 requires highly acidic conditions, under which oxo-species of heteroatoms are generally less stable, and the unavoidable electrostatic repulsion between heteroatom ions and silica species suppresses favorable contact between them. In fact, although there are several reports on the preparation of NbMCM-41,13 which can be synthesized under basic conditions, reports on the preparation of niobiumsubstituted SBA-15 are very limited. For instance, Jaroniec and co-workers reported the preparation of niobium-containing mesoporous silicates with pore diameters in the range of 2.0 to 10.3 nm under acidic medium using triblock copolymer as a

10132 J. Phys. Chem. C, Vol. 112, No. 27, 2008

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TABLE 1: Synthesis Conditions and Physicochemical Properties of NbSBA-15 Samples nSi/nNb materials SBA-15 NbSBA-15(28) NbSBA-15(10) or NbSBA-15(276H) NbSBA-15(7) NbSBA-15(137H) NbSBA-15(68H) NbSBA-15(33H)

gel

product (AAS)

product (EDS)

nH2O/nHCl

a0 (nm)

ABET (m2.g-1)

dp, BJH (nm)

Vp, (cm3.g-1)

28.0 10.0

22.5 9.1

22.8 9.5

276 276 276

10.10 11.85 11.81

910 958 738

9.2 8.8 8.9

1.25 1.30 1.07

7.0 10 10 10

6.1 17.1 19.2 28.5

6.8 17.5 19.4 29.0

276 137 68 33

11.65 10.80 10.50 10.40

711 855 905 925

8.8 8.9 9.0 9.0

0.96 1.11 1.17 1.20

structure-directing agent.14 Unfortunately, the nSi/nNb ratio of their materials was very high, in the range of 110 to 239. Unlike these previous examples, we report here on the successful preparation of NbSBA-15 with surprisingly high niobium content using our unique synthesis strategy in which the nH2O/nHCl in the synthesis gel is tuned to control the incorporation of heteroatoms in the silica framework of SBA15 in a highly acidic medium. NbSBA-15(xH) materials were synthesized at nH2O/nHCl values of 276, 137, 68, and 33 while keeping a constant nSi/nNb ratio of 10 in the synthesis gel in order to study the effect of solution pH on the niobium ion incorporation, structural order, and the textural parameters of the NbSBA-15(xH) materials. Table 1 summarizes the details of the chemical composition and the textural parameters of the NbSBA-15 materials prepared at different nH2O/nHCl ratios. As can be seen from Table 1, the nSi/nNb ratio of the calcined NbSBA-15 samples decreases from 28.5 to 9.1 upon increasing the nH2O/nHCl ratio from 33 to 276. As can be seen in Table 1, the nSi/nNb ratio of the NbSBA-15 materials prepared at an nH2O/ nHCl of 276 is lower than that added in the synthesis gel. This indicates that a part of the Si atoms in the synthesis mixture is not involved in the mesostructure formation. Further, the above condition also supports a preferential incorporation of Nb atoms as compared to Si in the walls of the NbSBA-15 material. On the other hand, the nSi/nNb ratio of the NbSBA-15 materials prepared at nH2O/nHCl values below 276 is significantly higher than that present in the starting synthesis gel. This could be mainly due to either the higher solubility of the Nb source or the repulsion between the positively charged inorganic precursors in a highly acidic medium. The detailed mechanism for this trend will be explained in the next section. Shown in Figure 1 are the low-angle powder XRD patterns of the calcined NbSBA-15(xH) materials prepared at different nH2O/nHCl ratios along with the pure silica SBA-15 material. All the samples display XRD patterns with at least three wellresolved peaks, which are indexed to the (100), (110), and (200)

Figure 1. Powder XRD patterns of calcined NbSBA-15 materials prepared at different nH2O/nHCl ratios and pure SBA-15: (a) SBA-15, (b) NbSBA-15(33H), (c) NbSBA-15(68H), (d) NbSBA-15(137H), and (e) NbSBA-15(276H).

of the hexagonal space group of P6mm. As can be seen in Figure 1, the shape of the XRD pattern of the NbSBA-15(xH) materials is quite similar to that of the pure silica SBA-15 material. In addition, their observation is clearly indicative of a highly ordered two-dimensional mesostructure with hexagonal pore systems. It must be noted that the XRD patterns of the NbSBA15 materials prepared at different nH2O/nHCl ratios become better resolved with a significant shift of the XRD peaks at higher 2θ angles upon the calcination to decompose the polymeric surfactant. The intensity of the XRD patterns increases significantly as a result of the removal of the intercalated polymeric template, indicating that the atomic arrangements occur in the Nb-O-Si framework in the mesoporous wall structure during the calcination process (see Supporting Information, Figure 1S). Upon further inspection of the XRD patterns of calcined samples, it is inferred that the unit cell parameter increases significantly as the nH2O/nHCl ratios increases. Moreover, the unit cell parameter of the calcined samples, which shows an increasing tendency with the Nb content, is much higher than that of the pure siliceous SBA-15. This could be mainly attributed to the fact that the atomic radius of Nb5+ (0.62 Å) is larger compared to that of Si4+ (0.40 Å), assuming the coordination number of both the atoms is 4, leading to a longer Nb-O distance. These observations suggest that the interaction between the Nb and Si at higher nH2O/nHCl is significantly high, which supports the effective incorporation of Nb in the mesoporous silica framework of SBA-15. On the other hand, the specific surface area and the specific pore volume of the NbSBA-15(xH) materials decrease upon increasing the nH2O/ nHCl and niobium content (Table 1), which may be attributed to the formation of a small amount of Nb2O5 nanoclusters in the mesoporous channels of NbSBA-15 with a high Nb content. The morphology and the elemental composition of the NbSBA-15 materials prepared at different nH2O/nHCl ratios were obtained by HRSEM. The HRSEM images of NbSBA-15 materials synthesized at different nH2O/nHCl are shown in Figure 2. It can be seen that the morphology of the materials can be controlled by varying the nH2O/nHCl in the synthesis gel. The shape, length, and diameter of the NbSBA-15 particles are significantly affected by simple adjustment of the nH2O/nHCl ratio. It is worth noting that the length of the rod-like particle decreases significantly upon increasing the nH2O/nHCl ratio. When the nH2O/ nHCl in the synthesis gel is low, rod-like particles can be obtained with a yield of almost 90% and have a length as much as several hundred micrometers that are composed of a bundle of small rods, whereas the diameter of the particles is relatively uniform with a diameter of ca. 0.7 µm. This shows that a high-quality rod-like morphology, which is typically observed for the pure mesoporous silica SBA-15, is retained for NbSBA-15(33H). On the other hand, particles with the twisted morphologies are obtained for NbSBA-15(276H). These twisted shape particles appear like semicircular rods and have a length of ca. 0.85 µm

Highly Ordered 2D P6mm Mesoporous Nb Silicates

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10133

Figure 2. HRSEM images of calcined NbSBA-15 materials prepared at different nH2O/nHCl ratios: (a) NbSBA-15(33H), (b) NbSBA-15(68H), (c) NbSBA-15(137H), and (d) NbSBA-15(276H).

with a diameter of 0.7 µm. The reason for these interesting features will be further discussed in the later part of the manuscript. Energy-dispersive spectrometry (EDS) analysis was also performed to further characterize the surface and the composition of the NbSBA-15 materials prepared under the different synthesis conditions. The EDS patterns of the calcined NbSBA-15 samples synthesized at different nH2O/nHCl ratios are shown in Figure 3a. Pronounced peaks of Nb, Si, and O are clearly seen in the EDS spectra, suggesting that the samples are indeed pure niobiumsilicates. The absence of the Cl and C peaks in the EDS patterns of the calcined NbSBA-15 samples reveals that the polymeric template and the hydrochloric acid added in the synthesis gel are completely removed from the as-synthesized samples during the calcination process, irrespective of the synthesis condition, and the calcined samples are totally free from Cl and C. It is also found that the intensity of the Nb peak significantly increases upon increasing the nH2O/ nHCl ratio, suggesting an increase in the amount of Nb in the silica framework. These observations are quite consistent with the quantitative information of Nb in the SBA-15 obtained from the atomic absorption spectroscopy (AAS) analysis. The reason for the difference in the amount of Nb incorporation can be explained as follows: the nH2O/nHCl in the synthesis gel, which also directly relates the gentle control of the solution pH, is the key to obtain SBA-15 materials with a high amount heteroatom content in the silica framework. From the above data, it is clearly found that the nH2O/nHCl of 276 is the best synthesis condition that gives a solution pH of above 2, which is above the isoelectric point of silica, and certainly within the acceptable pH range for SBA-15 synthesis. Moreover, at a solution pH above 2, silica species are negatively charged because the isoelectric point of the silica is around 2, resulting in favorable interaction with the cationic oxo-species of niobium. In addition, the decrease of the local H+ ion concentration in

Figure 3. (Top) HRSEM-EDS patterns of calcined NbSBA-15 materials prepared at different nH2O/nHCl ratios: (a) NbSBA-15(33H), (b) NbSBA-15(68H), (c) NbSBA-15(137H), and (d) NbSBA-15(276H). (Bottom) HRSEM-EDS patterns of calcined NbSBA-15 materials with different nSi/nNb ratios prepared at a nH2O/nHCl ratio of 276: (a) NbSBA15(28), (b) NbSBA-15(10), and (c) NbSBA-15(7).

the synthesis mixture also supports the formation of stable niobium hydroxyl species, which can strongly bind with anionic silica species, resulting in a significant improvement in the Nb incorporation in the SBA-15 silica framework. On the other hand, the lower nH2O/nHCl decreases the pH of the synthesis gel


Srinivasu et al.

Figure 4. Powder XRD patterns of as-synthesized NbSBA-15 materials with different nSi/nNb ratios prepared at an nH2O/nHCl of 276 along with the as-synthesized pure silica SBA-15: (a) SBA-15, (b) NbSBA-15(28), (c) NbSBA-15(10), and (d) NbSBA-15(7).

Figure 5. Powder XRD patterns of calcined NbSBA-15 materials with different nSi/nNb ratios prepared at the nH2O/nHCl of 276 along with the pure calcined silica SBA-15: (a) SBA-15, (b) NbSBA-15(28), (c) NbSBA-15(10), and (d) NbSBA-15(7).

below the isoelectric point of the silica, where the cationic silica species are dominant, and might suppress the interaction between the Si-OH and Nb-OH species in the synthesis gel. Moreover, the solubility of the Nb source is greatly enhanced at lower pH in the synthesis gel, resulting in a lower Nb incorporation in the SBA-15 silica framework when the lower nH2O/nHCl in the synthesis gel is used. From these observations, it has been found that the nH2O/nHCl in the synthesis gel plays a critical role in controlling the amount of Nb content in the sample, and an nH2O/ nHCl of 276 is the best condition to achieve a higher loading of Nb in the mesoporous silica matrix of NbSBA-15. To study the effect of nSi/nNb ratio on the structural order, textural parameters, morphology, and the niobium content, the NbSBA-15 materials with different nSi/nNb ratios were prepared by varying the amount of Nb source in the synthesis mixture with a fixed nH2O/nHCl of 276. The elemental compositions of NbSBA-15 materials synthesized with different nSi/nNb ratios are also listed in Table 1. It is interesting to note that the nSi/ nNb ratio of the calcined NbSBA-15 materials is lower as compared to that added in the synthesis gel, suggesting a preferential incorporation of Nb atoms as compared to silicon. Moreover, the amount of Nb in the final product significantly increases with an increase of the amount of Nb source in the synthetic gel. It is worth noting that NbSBA-15 materials with an nSi/nNb ratio of 6.1 can be successfully achieved at an nH2O/ nHCl of 276 in the synthesis gel. The elemental composition of the NbSBA-15 materials was also obtained from EDS analysis, and the corresponding patterns are displayed in Figure 3b. The peaks for the elements Nb, Si, and O are clearly seen in the EDS spectra, and the intensity of the Nb peaks increases upon increasing the Nb source added in the synthesis mixture. It is also observed that the nSi/nNb ratio obtained from the EDS for all the NbSBA-15(x) samples is in close agreement with that obtained from the AAS. This indicates that the Nb content of SBA-15 materials indeed increases upon increasing the quantity of Nb source added in the synthesis mixture. These observations lead to the conclusion that NbSBA-15 material with an nSi/nNb ratio up to 6.1 can be successfully achieved by simply adjusting the nH2O/nHCl and the nSi/nNb ratio in the synthesis gel. To the best of our knowledge, this is the first demonstration of the preparation of NbSBA-15 materials with a high content of niobium in the silica framework of SBA-15. The powder XRD patterns of both the as-synthesized and calcined NbSBA-15 samples with different Nb contents are shown in Figures 4 and 5, respectively. The powder XRD patterns of all the as-synthesized NbSBA-15 samples with different nSi/nNb ratios show at least three well-resolved peaks, which are indexed to the (100), (110), and (200) reflections of

the hexagonal space group of P6mm. The observation is in good agreement with the XRD pattern of pure hexagonally ordered SBA-15 material reported by Zhao et al.,15 indicating that the NbSBA-15 materials possess a well-ordered two-dimensional mesoporous structure with a hexagonal porous network. Interestingly, the intensity of the (100) reflection of NbSBA-15 with different nSi/nNb ratios except NbSBA-15(68H) increases with decreasing nSi/nNb ratio. This indicates that the structural order was significantly improved with the Nb incorporation into the silica framework of SBA-15. The improvement in the structural order of the NbSBA-15 materials upon increasing the Nb content could be due to the generation of chloride anions from the Nb source added in the synthesis gel, which are highly necessary for the formation of hexagonal phase through the surfactant-silica assembly bridged by anionic species, which is best represented by an S0H+X-I+ pathway (nonionic polymeric surfactant, (S0), halogen anions (X-) and the protonated inorganic SiO2 species (I+)). The presence of a large amount of chloride anions in the synthesis gel would significantly increase the interaction between the polymeric template and the silicate species and help to neutralize the protonated micellar charge. These factors would strongly stimulate the formation of more stable and small cylindrical micelles, which are critical for obtaining the wellordered hexagonal structure. Thus the intensity of the powder XRD patterns of the NbSBA-15 materials increases upon increasing the Nb content. It is also observed that, when the loading of Nb in the silica framework of NbSBA-15 materials is increased, the intensity of the higher order reflection (200) of the as-synthesized materials increases significantly with the concomitant decrease of the (110) reflection, indicating an increase in the wall thickness of the as-synthesized NbSBA-15 material with a high Nb content due to the incomplete condensation of the Nb-OH and Si-OH species in the mesoporous wall structure. After calcination of the NBSBA-15 samples, all four diffraction peaks can be observed, which are indexed to the (100), (110), (200), and (210) reflections of the hexagonal space group of P6mm, with much higher intensity as compared to those of the as-synthesized samples, indicating that the NbSBA-15 materials are highly stable even after the calcination, and the hexagonal structure is completely retained. The rise in the peak intensity can also be attributed to the cross-linking of silanol and NbOH groups, as well as the greater scattering density contrast and reduced X-ray absorbance after the polymeric surfactant template removal from the hexagonally ordered mesostructured niobium silicates. In addition, the peaks are shifted to a higher angle upon calcination, indicating that the calcination leads to contraction of unit cell size accompanied by the condensation


Figure 6. Nitrogen adsorption-desorption isotherms of calcined NbSBA-15 materials with different nSi/nNb ratios prepared at the nH2O/ nHCl of 276 along with the pure silica SBA-15 (closed symbols: adsorption; opened symbols: desorption): (1) SBA-15, (2) NbSBA15(28), (9) NbSBA-15(10), and (b) NbSBA-15(7).

of silanol and niobium hydroxo species. Similar reduction in the unit cell size has been previously reported for metalsubstituted mesoporous materials by many researchers.16 On the other hand, when the calcined NbSBA-15 materials with different nSi/nNb ratios are compared, a significant shift in the peaks to a higher angle is observed, indicating that the unit cell constant of the calcined NbSBA-15 materials decreases upon increasing the Nb content. The unit cell constant is calculated to be 11.85, 11.81, and 11.65 nm for the calcined NbSBA15(28), NbSBA-15(10), and NbSBA-15(7) samples, respectively (Table 1). The reduction in the unit cell constant for NbSBA15(7) may be attributed to the formation of a small quantity of nanosized Nb2O5 clusters in the mesoporous channels of SBA15. When the unit cell constants of NbSBA-15 with different nSi/nNb ratios and SBA-15 are compared, it is found that the unit cell constant of the NbSBA-15 materials is much larger than that of the pure silica material. This could be mainly due to the difference in the atomic radius of the Nb and Si cations, which has been clearly explained in the previous section. All these results clearly indicate that the Nb atoms are successfully incorporated in the silica framework of SBA-15 without affecting its well-ordered hexagonal porous networks. For further understanding of pore structures of NbSBA-15 materials, the nitrogen adsorption-desorption isotherms of the corresponding samples were closely investigated (Figure 6). They exhibit isotherms of type IV of the IUPAC classification featuring a narrow step due to capillary condensation of N2 in the primary mesopores. Filling of the mesopores of the NbSBA15 materials occurred at P/Po ) 0.5 - 0.95, which is reflected as a steep increase in the isotherm. All isotherms exhibit a H1type hysteresis loop that is mainly due to the capillary condensation and desorption of nitrogen, which strongly suggests the presence of large mesopores in the NbSBA-15 materials. The BJH pore size distribution profiles shown in Figure 7, which were calculated from the adsorption branch of the isotherms, reveal that a change in acid concentration virtually does not affect the pore size distribution, although the nSi/nNb ratio varies between 6.1 and 22.5. It should be mentioned that the amount of nitrogen adsorbed decreases upon increasing the Nb loading. Analyzed pore geometries are summarized in Table 1, where high BET surface area (ca. 711-958 m2 g-1) and large specific pore volume (0.96-1.30 cm3 g-1) can be seen together with a large pore diameter of ca. 8.8-8.9 nm for all the corresponding samples. It is found that an increase of the Nb loading in the SBA-15 materials causes a significant decrease in the values of the specific surface area and the specific pore volume of the NbSBA-15 materials, whereas the pore diameter


Figure 7. BJH adsorption pore size distributions of calcined NbSBA15 materials with different nSi/nNb ratios prepared at an nH2O/nHCl of 276 along with the pure silica SBA-15: (3) SBA-15, (4) NbSBA15(28), (0) NbSBA-15(10), and (O) NbSBA-15(7).

Figure 8. UV-vis DRS of NbSBA-15 materials with different nSi/ nNb ratios prepared at an nH2O/nHCl of 276 (A: as-synthesized; B: calcined): (a) NbSBA-15(28), (b) NbSBA-15(10), and (c) NbSBA15(7). Spectra are shifted from each other for clearer display.

of the materials is almost constant, irrespective of the Nb loading in the silica framework of SBA-15. The decrease in the specific pore volume and the specific surface area of the NbSBA-15 may be attributed to either the formation of a small amount of nanosized niobium oxide nanoclusters in the mesoporous channels of SBA-15 or a reduction in the sample quality because of the large size of the Nb atoms, which may create the defect in the niobium silicates of the mesoporous walls. These observations are quite consistent with the results obtained from the XRD measurements. For more specific information about the nature and the coordination of Nb atoms in the SBA-15 framework, UV-vis DRS was performed. Spectra of as-synthesized and calcined samples are shown in Figure 8, panels A and B, respectively. All the samples show a sharp band centered around 200 nm and a broad shoulder centered around 245 nm that are shifted toward a higher wavelength by about 2-5 nm for the calcined samples. Moreover, a gradual shift in the peak at 200 nm to higher wavelength is also observed with increasing the loading of Nb content. It is interesting to note that the intensity of both the bands increases monotonically with increasing the loading of Nb in the silica framework. The peak at 200 nm is typically associated with Nb-O bonds, which is unambiguously assigned to the ligand-to-metal charge transfer transition in the NbO4 tetrahedra units. This charge transfer occurs with electron excitation from the oxygen to an unoccupied orbital of the Nb


Figure 9. (A) 29Si and (B) 93Nb MAS NMR spectra of the calcined NbSBA-15 materials: (a) NbSBA-15(28), (b) NbSBA-15(10), and (c) NbSBA-15(7).

ions surrounded by the oxygen. This gives the direct evidence that the Nb atoms are tetrahedrally coordinated to both in the as-synthesized and calcined NbSBA-15 samples. Similar results have also been reported in niobium-substituted MCM-4117 and AlPOs.18 It should be mentioned that a remarkable increase in the intensity of the peaks is observed for the samples after the calcination. This could be mainly due to the fact that the calcination results in a complete condensation of the unreacted Nb-OH and Si-OH species, which eventually increase the percentage of mononuclear or oligomeric NbO4 tetrahedra species in the calcined samples. As the peaks at 200 and 245 nm are originated from these species, the intensity of the peaks is significantly increased upon calcination. The broadband centered around 245 nm may also be assigned to a low-energy charge transfer transition between the tetrahedral oxygen ligands and the central Nb ions in the mononuclear tetrahedral NbO4 with the higher coordination number. The increase of the coordination number of Nb is mainly caused by the adsorption and coordination of water molecules or other ligands or the formation of Nb2O5 nanoclusters, which is increased with an increase of Nb loading in the mesoporous silica matrix.19 This is also clearly reflected by the significant rise in the intensity of the broad peak for the NbSBA-15 with higher loading of Nb. The absence of the peak at the visible region in the UVspectra suggests that the samples are free from large size Nb2O5 clusters in the mesoporous channels because of the strong interaction between the Nb-OH and Si-OH species at pH higher than the isoelectric point of silica, which prevents the formation of large niobium oxide clusters on the porous channels. Solid-state 29Si and 93Nb NMR measurements were carried out to confirm the nature and the coordination of Nb in the SBA15 samples. 29Si and 93Nb NMR spectra of NbSBA-15 with different Nb contents are shown in Figure 9A and 9B, respectively. 29Si NMR spectra of all the samples show two well-resolved peaks, at -102 and -92.5 ppm. The peak at -102 ppm can be assigned to a Si(1Nb) species whereas the peak at -92.5 ppm can be attributed to a Si(2Nb) species. Similar

Srinivasu et al. spectra have also been reported for the zeolites where aluminum is tetrahedrally coordinated with Si atoms.20 93Nb NMR spectra of the samples were performed using different NMR methods, but only a single pulse excitation led to results. It is rather difficult to obtain well-resolved spectra for Nb because there exists a series of problems that influence the quality of the spectra. In materials with a low point symmetry such as SBA15, it is hard to detect the isotropic signal that belongs to Nb species and not to spinning sidebands and quadrupolar splitting peaks, respectively. Furthermore, paramagnetic centers strongly influence the signal-to-noise ratio of the spectra, leading to shortened relaxation times and line broadening, respectively. However, we could obtain the 93Nb spectra for the samples with high Nb content. As can be seen in Figure 9B, at least three peaks were found for the NbSBA-15(7) and NbSBA-15(10) samples, which can be attributed to NbO6 octahedrons linked with SiO4 tetrahedra to form a three-dimensional framework. According to the different chemical environment of Nb species, the chemical shifts are distributed over a wide range between 1000 and -900 ppm. It must be noted that the NbSBA-15 sample with low Nb content shows only poor resolution, which may be due to the low point symmetry of SBA-15 and/or to the small amount of niobium absorbed into the framework. From the 29Si and 93Nb MAS NMR results, one can conclude that the NbSBA-15 material was formed, wherein Nb was mostly incorporated into the silica framework of the SBA-15. A surprising finding obtained was unusual morphology control of NbSBA-15 materials, as demonstrated by SEM observations. SEM images of NbSBA-15 materials at two levels of magnifications are shown in Figure 10 together with the corresponding images of pure siliceous SBA-15. Macroscopic morphologies of these materials highly depend on Nb content. Pure siliceous SBA-15 essentially possesses straight rod-like morphology without any twisted or curved shape morphology, as seen in Figure 10B. With increasing the niobium content in the SBA-15 silica framework, a remarkable difference in the shape and the size of the NbSBA-15 particles is observed. Incorporation of a small amount of niobium in SBA-15 (NbSBA-15(28)) induces gentle curvatures in the rod-like structure (Figure 10D). However, when the nSi/nNb ratio is decreased from 22.5 to 6.1, the morphology of the materials is drastically changed from rod-like particles to curved-shape particles with a concomitant decrease in the particle size. As can be seen in Figure 10F,H, NbSBA-15(10) and NbSBA-15(7), the materials with very high Nb contents, exhibit winding road morphology with multiple kinked points. A similar tendency has also been observed for the samples prepared at different nH2O/nHCl ratios, in which the content of the Nb in NbSBA-15 increases upon increasing the nH2O/nHCl ratios in the synthesis gel. It is important to note that the winding directions of most of the objects switched alternately. Nonchirality in synthetic components would result in an equal existence of winding senses, and structural strain would be efficiently released by alternate arrangement of these winding senses. To confirm the winding road morphology in the NbSBA-15 materials, HRTEM of the NbSBA-15 samples with different Nb contents was observed. Figure 11 shows the HRTEM images of NbSBA-15 samples with different Nb content. It can also be clearly seen that the shape of the particles changes from rod to curved shape to winding road morphologies upon increasing the content of Nb (Figure 11). Regardless of Nb content (degree of winding), all the mesopores are aligned as highly regular arrays of pores with narrow pore size distributions. Regular hexagonal arrangement of the mesopores can be seen in cross-



Figure 10. HRSEM images of the calcined SBA-15 and NbSBA-15 materials with different nSi/nNb ratios prepared at an nH2O/nHCl of 276: (A,B) SBA-15; (C,D) NbSBA-15(28); (E,F) NbSBA-15(10); (G,H) NbSBA-15(7).

sectional views of rod-like structures at low Nb content and winding structures at high Nb content (Figure 11B,D,F). These pores run parallel to the long axis of each object (Figure 11A,C,E). These observations support the fact that the NbSBA15 materials with high Nb content indeed possess particles with a winding road shape. It should also be mentioned that these winding morphologies are not the cause of the formation of disordered wormhole-like mesopore structures, as demonstrated by TEM observations (Figure 11). The large difference in the shape and the size of the mesoporous particles upon increasing the loading of Nb can be explained by the cooperative selfassembly of the inorganic precursors and the polymeric surfactants through the S0H+X-I+ pathway. It is well documented that the changes in the morphology of the polymeric surfactant

micelles are directly related to the spontaneous curvature of the polymeric surfactant layers.21 In the case of the synthesis of a pure siliceous SBA-15 system, the adsorption of silicate ions, driven either by hydrogen bonding or charge matching, on the surface of the hydrophilic chain of the surfactants results in a rearrangement of the micellar morphology, followed by the condensation of silicate ion-covered micelles into ordered phase.22 This process also reduces the intermicellar repulsion and induces a reduction in the spontaneous curvature of the polymeric surfactant layers. The reduction in the curvature of the surfactant layer is caused by the adsorption of silicate ions on the hydrophilic chain of the polymeric surfactant, which reduces the hydration level of the hydrophilic polymeric chain. Moreover, the cross-linking of the neighboring block copolymers


Srinivasu et al.

Figure 11. HRTEM images of the calcined NbSBA-15 materials with different nSi/nNb ratios prepared at an nH2O/nHCl of 276: (A,B) NbSBA15(28); (C,D) NbSBA-15(10); (E,F) NbSBA-15(7).

by the silicate ions also reduces the curvature of the surfactant layers and the effective headgroup area of the hydrophilic chain. It has been also reported that the reduction in the spontaneous curvature of the surfactant layer leads to increase in the length of the micelles and rod like morphology.21 When the foreign atom Nb is added, the adsorption and the condensation of the inorganic precursors may be disturbed because of the difference in the size and the valency of the atoms. This process results in the increase of the spontaneous curvature of the micelles due to the strain in the polymeric inorganic species, which increases with increasing the loading of the Nb atoms. Thus, the curve shaped particles with small particle size are observed for NbSBA-15 samples with a high Nb content. Winding structure can be seen everywhere in mesoporous rods, which is due to homogeneous distribution of niobium atoms in the silica framework. This feature was well confirmed in the elemental mapping shown in Figure 12. All the composition of atoms (Si, O, and Nb) can be clearly seen in the elemental mapping, which are uniformly distributed throughout the samples. It is also interesting to see the winding structure in the NbSBA-15 materials with a high Nb content where the Nb atoms are distributed uniformly, even in surface curvature.

Conclusions In this paper, we strikingly demonstrate the successful incorporation of Nb into an SBA-15 silica framework with very high content (nSi/nNb ratio of up to 6.1) through a simple adjustment of the molar water-to-hydrochloric acid ratio in the synthesis gel, which controls the pH of the synthesis mixture. The simplicity of the proposed concept will allow us to generalize this methodology to free doping of heteroatoms into silica frameworks of SBA-15 materials, which, however, require a certain acidity for their preparative conditions. The obtained materials have been unambiguously characterized by several sophisticated techniques such as XRD, N2 adsorption, HRTEM, HRSEM, EDS, elemental mapping, and UV-vis spectroscopy. The characterization results reveal that the NbSBA-15 materials with two-dimensional hexagonally ordered mesopores can be prepared, and the loading of Nb in the materials can be easily controlled by simply adjusting the molar water-to-hydrochloric acid ratio. UV-vis results indicate that most of the Nb atoms in the NbSBA-15 materials prepared at higher molar water-tohydrochloric acid ratios occupy the tetrahedral position in the silica framework walls, which are highly critical for the redox catalysis. A more surprising finding in this research is the



Figure 12. Elemental mapping of the calcined NbSBA-15 materials with different nSi/nNb ratios prepared at an nH2O/nHCl of 276: (A) NbSBA15(28); (B) NbSBA-15(10); (C) NbSBA-15(7).

10140 J. Phys. Chem. C, Vol. 112, No. 27, 2008 morphology of the NbSBA-15 materials, which changes from rod-like morphology to highly winding morphologies, maintaining regularly aligned arrays of nanochannels upon increasing the nSi/nNb ratio from 22.5 to 6.1. The detailed mechanism of the formation of NbSBA-15 materials with different morphologies has also been proposed. Several methodologies, including the use of chiral surfactants and ionic liquids, have been reported so far for the preparation of winding, twisted, and helical mesopore arrays, which are not yet established as a general strategy. Our method to prepare highly winding nanochannels is based on simple doping of heteroatoms in well-known SBA15 materials. The technique proposed in this paper does not request large modification in synthetic conditions from preexisting knowledge, and thus it can be generally applicable to a wide range of mesoporous materials. The incorporation of Nb in the silica framework could be a way to control the morphology of the arrays of nanopores, which would open a new field of nanofluidic sciences where devices with nanometerscale channels are used for molecular sensing, molecular separation, drug release control, and nanoreactors. Acknowledgment. This work was financially supported by Japan Science and Technology under the Strategic Program for Building an Asian Science and Technology Community Scheme. Supporting Information Available: Powder XRD patterns of as-synthesized NbSBA-15 materials prepared at different nH2O/nHCl ratios and pure SBA-15. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49–68. (b) Gimzewski, J. K.; Joachim, C. Science 1999, 283, 1683–1688. (c) Drexler, K. E. Sci. Am. 2001, 285, 74–75. (d) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787–792. (f) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293–346. (2) (a) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988–992. (b) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710–712. (c) 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.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834–10843. (d) Inagaki, S.; Fukushima, A.; Kuroda, K. J. Chem. Soc. Chem. Commun. 1993, 680, 682. (3) (a) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865–867. (b) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242– 1244. (c) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548–552. (d) Yang, P. D.; Zhao, D. Y.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152–155. (e) Zhao, D. Y.; Huo, Q. S.; Feng, J. L.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024–6036. (f) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 10712–10713. (g) Ciesla, U.; Schu¨th, F. Microporous Mesoporous Mater. 1999, 27, 131–149. (h) Ariga, K. J. Nanosci. Nanotechnol. 2004, 4, 23–34. (i) Okabe, A.; Fukushima, T.; Ariga, K.; Niki, M.; Aida, T. J. Am. Chem. Soc. 2004, 126, 9013–9016. (j) Vinu, A.; Ariga, K.; Mori, T.; Nakanishi, T.; Hishita, S.; Golberg, D.; Bando, Y. AdV. Mater. 2005, 17, 1648–1652. (k) Vinu, A.; Terrones, M.; Golberg, D.; Hishita, S.; Ariga, K.; Mori, T. Chem. Mater. 2005, 17, 5887–5890. (l) Vinu, A.; Miyahara, M.; Sivamurugan, V.; Mori, T.; Ariga, K. J. Mater. Chem. 2005, 15, 5122–5127. (4) (a) Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923–926. (b) Davis, M. E. Nature 2002, 417, 813–821. (c) Okabe, A.; Fukushima, T.; Ariga, K.; Aida, T. Angew. Chem., Int. Ed. 2002, 41, 3414–3417. (d) Mal, N. K.; Fujiwara, M.; Tanaka, Y. Nature 2003, 421, 350–353. (e) Zhang, Q.; Ariga, K.; Okabe, A.; Aida, T. J. Am. Chem. Soc. 2004, 126, 988–989. (f) Vinu, A. Hossain, K. Z.; Ariga, K. J. Nanosci. Nanotechnol. 2005, 5, 347–371. (g) Vinu, A.; Miyahara, M.; Ariga, K. J. Nanosci. Nanotechnol. 2006, 6, 1510–1532. (5) (a) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321–323. (b) Maschneyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature

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