Functionalized periodic mesoporous organosilica fibers with longitudinal pore architectures under basic conditions

Microporous and Mesoporous Materials 92 (2006) 201–211 www.elsevier.com/locate/micromeso

Functionalized periodic mesoporous organosilica fibers with longitudinal pore architectures under basic conditions M.A. Wahab a, Ichiro Imae b, Yusuke Kawakami b, Il Kim a, Chang-Sik Ha b

a,*

a Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, South Korea Graduate School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), Ishikawa 923-1292, Japan

Received 11 August 2005; received in revised form 17 December 2005; accepted 26 December 2005 Available online 23 February 2006

Abstract Hierarchically organized functionalized periodic mesoporous organosilica (PMO) fibers with longitudinal pore architectures have been prepared via a simple one-pot synthesis procedure from the co-condensation of 1,2-bis(triethoxysilyl)ethane (BTESE) with either of two kinds of trialkoxysilanes, (3-triethoxysilylpropyl) isocyanate (ICP), or 1-[3-(trimethoxysilyl)propyl] urea (UREDO) under basic conditions. The influence of organosilica source and temperature on the formation and the internal pore architecture of the functionalized PMO fibers were investigated. The pore channels in both types of PMO nanofibers are hexagonally packed, in which pore channels are aligned parallel to fiber axis. The diameters of the fibers range from 50 to 300 nm, and the lengths are up to 7 lm. Electron microscopy and X-ray diffraction investigations were carried out to elucidate the morphological and structural features of the functionalized PMO fibers. The formation of well-condensed and interconnected organosiloxane network was proven by 29Si CP/MAS NMR spectroscopy. How the nature of organic groups has impacted on the surface areas and pore volumes were evaluated by N2 isotherms. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Periodic mesoporous organosilica; Fiber; Morphology; Pore; Functionalization

1. Introduction Since the discovery of periodic mesoporous organosilicas (PMOs) in 1999 [1], many efforts have been made to functionalize PMOs for more versatile applications [1–4]. The nature of inorganic or organic precursors and the variations in the synthesis conditions are crucial factors for the efficient design of PMO materials with desired architectures as well as properties. Among the wide variety of silica mesophases, the controlled morphologies of MCM-41 and SBA-15 have been studied extensively [5–9] but far less effort has been exerted on the morphological control of PMOs [1,3,4]. Many silica mesostructures have been prepared successfully under acidic conditions, including mesoporous fibers, spheres, rods and tubules, with sizes ranging from the microscopic to the macroscopic scale [5–9]. As far *

Corresponding author. Tel.: +82 51 510 2407; fax: +82 51 514 4331. E-mail address: [email protected] (C.-S. Ha).

1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.12.016

as PMOs are concerned, sporadic success on the control of morphologies—such as rods, gyroids and spheres—has been reported when synthesized under basic conditions [1a,3,4]. However, it remains a key challenge to prepare multifunctional hierarchical PMO fibers. Mesostructured materials that possess designed nanopores and nanometer-scale (or larger) morphologies comprise an important new class of materials for potential applications in sieving, catalysis and nanofluidics [6a]. The orientation of the pores in these materials is an important issue that affects their applications. For example, if the pores in a nanoscale mesoporous fiber are aligned with the fiber axis, they could function as nanofluidic channels in applications such as electrophoresis systems and biologically sensitive transistors [6a,10]. The incorporation and alignment of chemical species [10], such as polymers, into these nanofibers could offer opportunities for studying the transport properties of individual or highly aligned polymer chains, which will provide important information

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towards optimizing polymer-based transistors and diodes. Hierarchically organized fibrous meso-materials are strong candidates for embedding advanced materials [5c]. In addition, mesoporous silica fibers having diameters of 1–100 lm have already been employed in mechanical drawing and two-phase reactions [6a,d,e,11]. Dye-doped mesoporous fibers presenting such an organization, due to the presence of a molecular dye within the hexagonally ordered mesophase and the homogenous cylindrical morphology of the fibers, could be used as high surface area optical waveguides or as a new type of laser materials [12]. The formation of metal-functionalized PMO rods and organic functionalized PMOs with various morphologies has also been described [4b]. Recently, we have demonstrated a simple and flexible route for the synthesis of well-defined rod-type morphologies and different degree of curvatures of functionalized PMOs, which are highly dependent on the types of organic precursors [4a]. This efficient method afforded well-defined morphologies (rod, worm-like, spiral) with functional groups-dependant controlled pore sizes ranging from 3.38 to 2.38 nm without adding any additives [4a]. Importantly, it has been suggested that various silica sources, surfactant, temperature, pH, and the ionic strength of the solution have strong influences on the growth of different morphologies and shapes. To date, various morphologies of mesoporous materials have been achieved under acidic conditions, but the hierarchically organized well-defined fiber organization of functionalized PMOs with longitudinal pore architectures have not been realized yet under basic conditions. In this paper, we report a simple and facile route to the hierarchical organization of functionalized PMO fibers synthesized under basic conditions. These functionalized PMO fibers consist of hexagonally organized pores and possess longitudinal pore architectures in which the pore channels are oriented parallel to the fiber axis. We found that the pore architectures of the functionalized PMO fibers are always positioned longitudinally and are insensitive to elevated temperatures. The resulting periodic mesoporous structures were well characterized by X-ray diffraction (XRD), N2 sorption, solid-state 29Si cross-polarizing magic angle (CP/MAS) nuclear magnetic resonance spectroscopy (NMR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analyzer (TGA). 2. Experimental 2.1. Materials 1,2-Bis(triethoxysilyl)ethane (BTESE, 96%), (3-triethoxysilylpropyl) isocyanate (ICP), 1-[3-(trimethoxysilyl)propyl] urea (UREDO), non-ionic surfactant, Brij-30 (C12E4) and cetyltrimethylammonium bromide (CTAB) purchased from Aldrich, were used as supplied. Other chemicals [sodium hydroxide (NaOH), hydrochloric acid (HCl), methanol] and deionized water were also used.

2.2. Simple direct assembly route to functionalized PMO fibers The assemblies of surfactant molecules via direct selfassembly route were formed under basic conditions using our previously published protocol [4a,b], followed by the addition of BTESE and ICP or UREDO. In a typical procedure, a surfactant mixture (CTAB/Brij30 = 5.74, w/w) was dissolved into the alkaline solution (water/6 M NaOH = 12.4, w/w) while stirring at 52 ± 3 °C. BTESE (10 mmol) was then added into the surfactant solution with vigorous stirring. The stirring was continued for a few hours until a white precipitate appeared. After aging for a few days at 80 °C, the resultant powders were washed with a copious amount of a methanol/water solution and then dried at room temperature. Then we systematically varied the mole composition of either of two different organosilanes with respect to BTESE to prepare the functionalized PMO fibers. In Table 1, the sample code was designated as PMO-Yx, where Y is one of the two different co-precursors, ICP or UREDO and x is 0.1 mmol fraction of the Y (for more details, see Table 1). We also prepared functionalized PMO fibers at 75 °C. Table 1 lists the detailed mole fractions and the synthesis variables. 2.3. Surfactant extraction According to the previously described method [4a,b], the solvent extraction method was conducted for all of the samples using a binary mixture of hydrochloric acid/methanol. For example, 0.5 g of PMO-icp15 sample was stirred for 12 h at 60 °C in a solution of HCl (5 g) and methanol (150 g). The extracted samples were separated by filtration, washed with methanol and water, and dried in air. 2.4. Measurements and characterization The XRD patterns were collected using a Rigaku Miniflex (0.05 kV) instrument using a Cu Ka radiation source Table 1 Effect of functional groups on structural and textural properties of solvent-extracted pure PMO and functionalized PMO materials Sample code

d 100 a (nm)

BET surface area (m2/g)

Total pore volume (cm3/g)

Pore size (nm)

Pure PMO PMO-icp5 PMO-icp15 PMO-icp25 PMO-icp40

5.10 4.90 4.77 4.42 Broad

1071 649 599 473 384

0.93 0.55 0.50 0.41 0.34

3.38 3.36 3.34 3.17 2.89

PMO-uredo5 PMO-uredo15 PMO-uredo25 PMO-uredo40

4.41 4.45 4.86 Broad

656 574 556 355

0.71 0.52 0.48 0.31

3.05 3.05 3.15 2.60

a nk = 2d100 sin h. Mixture of 10x mmol of Y and 10(1 x) mmol of BTESE (where x = 0.05, 0.15, 0.25 and 0.40) were used for the functionalized PMO fibers, where x indicates mole fraction. Y is one of two different precursors (ICP or UREDO).

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(d)

Intensity (a.u.)

with 0.01° 2h steps and a step time of 1 s. All samples were scanned under the same conditions in the range of 2h = 1.5–5°. Nitrogen sorption isotherms were measured using a Micromeritics ASAP2010 apparatus at 77 K and applying the BET (Brunaumer, Emmett and Teller) and BJH (Barret–Joyner–Halenda) method to the experimental values. The pore-size distributions were obtained from the desorption branch of the isotherm by the BJH method. All the samples were dehydrated at 150 °C for 24 h prior to nitrogen adsorption. SEM (Hitachi S-4200) was used to observe fiber morphology of the solvent extracted samples. Prior to measurements, samples were mounted on a carrier made from glassy carbon and coated with a film of gold. TEM images were obtained using a JEOL JEM2010 microscope operating at 200 kV. The sample for TEM imaging was prepared by dispersing a large number of product particles by ultrasonication in methanol by vibrating and then pouring the solution into a holey carbon grid. The solid-state 29Si CP/MAS NMR spectra were recorded using a Varian Inc., 400 MHz UNITY INOVA spectrometer with the following experimental conditions: 5.0 s delay time, 4.6 ls pulse width, 2.5 ms the length of the contact pulse, 4 kHz MAS rate, and 1024 times the number of acquisitions. The relative intensities for the pure PMO and functionalized PMOs were calculated according to the curve fitting method [4a]. The weight loss curves were obtained on a Perkin Elmer, TGA 7 at a heating rate of 10 °C/min under a nitrogen atmosphere.

203

(c)

(b)

(a)

2

3

4

5

Two theta (degrees) Fig. 1. XRD patterns of solvent-extracted pure PMO (a), PMO-icp5 (b), PMO-icp15 (c), and PMO-icp25 (d).

3. Results and discussion

The X-ray diffraction patterns for the samples (both series, PMO-icp and PMO-uredo) containing different proportion of ICP or UREDO to pure BTESE are compared in Figs. 1 and 2, respectively, after the surfactant extraction. The three peaks in Figs. 1a and 2a for the pure PMO could be indexed as (1 0 0), (1 1 0), and (2 0 0) reflections, suggesting the presence of a periodic arrangement of channels in a 2-D hexagonal symmetry [1a,b,3b,c,4a,b,13]. The three reflection peaks were observed for both series (PMO-icp and PMO-uredo) when x = 0.15, which could also be indexed (1 0 0), (1 1 0), and (2 0 0) according to the 2-D hexagonal symmetry, indicating a well-defined 2-D hexagonal functionalized PMOs [1a,b,4a]. The XRD patterns in Figs. 1d and 2d show poorly resolved (1 1 0) and (2 0 0) reflections, whereas one broad (1 0 0) and very weak (1 1 0) reflection were observed when x = 0.25 for both series, similar to the other reported works on the functionalized mesoporous silica [1b,3a–c,5e,8b,14–18], and functionalized PMO materials [1b,d,3,4]. Such XRD patterns are due to the formation of disordered PMO materials [1b,3b,4a–e]. This change due to the nature of functional groups in functionalized PMOs is accompanied by systematic changes in textural properties such as surface areas and pore volumes [4a]. More degree of

(d) Intensity (a.u.)

3.1. Effect of molecular structure of functional groups on XRD patterns

(c)

(b)

(a) 2

3

4

5

Two theta (degrees) Fig. 2. XRD patterns of solvent-extracted pure PMO (a), PMO-uredo5 (b), PMO-uredo15 (c), and PMO-uredo25 (d).

disordering in Fig. 2d was observed for UREDO containing PMO materials compared to the ICP containing PMO materials in Fig. 1d, because UREDO is basic in nature, which can contribute to increase the pH of the resulting synthesis medium when the amount of the UREDO was increased from 0.05 to 0.25 and 0.40 (data not shown here). That kind of disordered structure has already been reported for cyanopropyl and bridged amine functionalized PMO

materials [4a,c,d] and other functionalized materials [8b,15,6]. Similar results have also been observed in literatures [15,16]. Both series showed almost amorphous characteristics when x = 0.40. The reflections (1 0 0) with reduced intensity for the two series at all composition ranges are related to a decrease in mesophase order [4a–d,5e,8b]. The d100 spacing for PMO-icp series was decreased but that for PMO-uredo series was increased when x (see Table 1) was increased from 0.05 to 0.25. This result was reported to be due to the interaction between employed functional groups and the hydrophobic part of the surfactant molecules [1b,3,4a–d,5d,e,8b,9,17,18]. This can be also explained by the steric repulsion of the introduced large organosilane such as ICP, which forces the headgroups apart in the micelle, causing the curvature of micelle assembly to increase and thus decreasing its diameter [4a,5e]. The hydrolysis of organosilica at the interface of this contracted micelle therefore results in a mesostructure with diminished lattice spacing [4a,5e]. As is suggested previously [16,19], the condensation rate of silica precursors becomes faster than its hydrolysis rate as pH is increased, leading to the incomplete hydrolysis of the silica precursors. Thus, organosilanes can change the conformation of micelle structure if incorporated organosilanes are partly solubilized in the hydrophobic core of micelle of surfactant molecules, leading to an increase in micelle dimension [20]. Wahab et al., Mercier and Pinnavaia found similar results for the functionalized mesoporous silicas and functionalized PMOs, respectively [4a,5e]. This results in the occlusion of at least some organosilanes within the framework walls of the mesostructure causing an increase in the wall thickness. For both series, the presence of only three hydrolyzable Si–OC2H5 bonds in such intraframework trialkoxysilane is likely to impede the cross-linking of the silica network, resulting in the observed perturbation in the framework ordering [4a,5e]. The degree of organosilane loading into the mesostructure of PMO also directly manifests an influence on their structural and surface properties in Table 1. 3.2. Effect of functional groups on textural properties of functionalized PMO fibers The N2 adsorption–desorption isotherms of the pure PMO and functionalized PMO samples are shown in Figs. 3a and 4a. The pure PMO displays a type IV isotherm with H1 hysteresis loop, which is a characteristic of highly ordered PMO materials with cylindrical mesochannels [1–4]. Both PMO-icp series and PMO-uredo series also exhibited type IV isotherms with H1 hysteresis loop, wellconsistent with ordered mesoporous organosilicas [1–4]. Such a sharp hysteresis is believed to be related to the capillary condensation associated with ordered mesopore channels [1–4,21]. Functionalization of PMOs could modulate the channel structures and preludes the change of hysteresis loop. This kind of small change of hysteresis loop in Figs. 3a and 4a has already been observed in the functionalized

Volume adsorbed N2 (cm3/g STP) (a.u.)

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PMO-icp25 PMO-icp15 PMO-icp5 PMO-pure

0.0

0.2 0.4 0.6 0.8 Relative pressure (P/P0)

a

PMO-ICP series PMO-icp40

Pore volume (cm3.g-1) (a.u.)

204

PMO-icp25

PMO-icp15 PMO-icp5 PMO-pure

5 b

10 15 20 25 Pore diameters (nm)

30

Fig. 3. BET isotherms (a) and pore size distributions (b) of solvent extracted PMO-icp series.

mesoporous materials [1b–d,3,4,5d,e,9,15–18], and also indicates that the amount of ICP or UREDO present during synthesis via direct co-condensation method has a little effect on the isotherms of the functionalized PMOs. All functionalized PMOs showed a gradual decrease in surface areas and pore volumes when x goes from 0.05 to 0.40. In particular, the pore sizes for PMO-icp series are found to be systematically decreased with increasing the amount of ICP. These observations demonstrate the ability of the direct assembly approach for the controlled loading of organic functional groups within the pore channels of the PMOs, allowing the formation of compounds in which both chemical function and structural properties can be simultaneously tuned [4a]. The surface properties such as BET surface areas, pore volumes, and pore diameters are listed in Table 1, indicating that textural properties are found to be dependant on the types of incorporated organic functional groups. A systematic decreases in both BET surface areas and pore volumes are noted for both series when x goes from 0.05 to 0.25, whereas the pore sizes remained almost unchanged until x = 0.25 and then the pore size decreased again when x = 0.40. Mercier and Pinnavaia [5e] incorporated various organic groups with tetraethoxysilane (TEOS). They found that d100 spacings are increased, whereas surface areas, pore volumes, and pore diameters are decreased. Similarly, the occlusion of some

Volume adsorbed N2 (cm3/g STP) (a.u.)

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curves, and XRD patterns discussed above summarize that various organosilanes have various disruptive effects on the formation of functionalized PMOs because of their different nature (reactivity, size, and configuration) [1b,d,3,4], chemical properties and steric hindrance and matching effect with the surfactant molecules [4a–d,5d,e,22].

PMO-uredo25 PMO-uredo15 PMO-uredo5 PMO-pure

3.3. Organic functionality by

0.0

Pore volume (cm3.g-1) (a.u.)

PMO-UREDO series PMO-uredo40 PMO-uredo25

Si NMR spectroscopy

PMO-uredo15

C

5

10 15 20 25 Pore diameters (nm)

OSi

OH

PMO-uredo5

Si

PMO-pure

(b)

29

The 29Si CP/MAS NMR spectra of the resulting functionalized materials are shown in Fig. 5 and in Table 2. The spectra for the two series basically exhibit two kinds of signals, namely signals attributed to Tn groups in the range of 59.47 to 59.50 ppm (T2), and 68.42 to 68.89 (T3) ppm stemming from the functional groups C–Si(OSi)2(OH) and C–Si(OSi)3 framework sites, respectively, which are characteristic of cross-linked organosiloxane species [1a–d,4a–c], demonstrating the incorporation of the functional groups within the framework walls of the

0.2 0.4 0.6 0.8 Relative pressure (P/P0)

(a)

205

C

OSi

Si

OSi

3 OSi

OSi

T

30 T2

Fig. 4. BET isotherms (a) and pore size distributions (b) of solvent extracted PMO-uredo series.

(d)

ICP or UREDO organosilane within the framework walls of the mesostructure, causes an increase in wall thickness [4a,5e]. However, both PMO-icp and PMOuredo series have almost similar degree of pore blocking and disorderness with respect to textural properties although the PMO-uredo series contain a little larger hydrophilic functional groups that may have more impact on textural properties that the PMO-icp series does. The pore size distribution curves of the pure PMO and functionalized PMOs are shown in Figs. 3b and 4b. It is seen that both PMO-icp series and PMO-uredo series have broad pore size distribution with reduced intensity compared to the pure PMO, indicating lower degree of mesoorder obtained for the functionalized PMOs. As can be seen also in Figs. 3b and 4b, the pore sizes for PMO-icp series have a narrower distribution, whereas pore size distribution curves of PMO-uredo series wider as UREDO organosilanes were used. This discrepancy is due to the nature of different sizes and molecular configuration of employed organic precursors, which causes the different pore size distribution curves and ultimately different degree of pore blockage [4a,5e]. When samples made with x = 0.40 for both series, these results are consistent with the formation of distorted pore network with very little uniformity in the pore channel structure. The conclusion made on pore size distribution will be discussed later again with TEM images. The N2 isotherms, pore size distribution

(c) (b) (a)

0

-20

-40

-60 -80 -100 Chemical shift (ppm)

-120

-140

Fig. 5. Solid-state 29Si NMR spectra of (a) as-synthesized pure PMO, (b) solvent extracted PMO, (c) PMO-uredo15, and (d) PMO-icp15.

Table 2 The peak assignments and the fraction of Tn groups obtained from 29Si CP MAS spectra Relative intensitya

Peak position T Spectrum Spectrum Spectrum Spectrum

a b c d

2

58.81 58.15 59.47 59.50

T

3

T2

T3

66.64 65.21 68.42 68.89

40.5 49.6 26.4 24.4

59.5 50.4 73.6 75.9

The values are obtained from Fig. 5. Note: T1 group is not found in the spectra. a The fraction of Tn groups was obtained by curve fitting method from 29 Si CP MAS NMR spectra (Fig. 5).

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mesostructures. The different chemical shifts were observed because of the existence of different organic groups [4a,5d,e]. Moreover, the apparent increase in the Tn signals of functionalized PMO derivatives with respect to the pure PMO indicates the incorporation of organic functional groups into the PMOs, which also confirms the existence of the covalent bond between the organic groups and the silica atoms. These results suggest that functionalization of the mesostructure proceeds via the simultaneously cooperative direct assembly of both BTESE and trialkoxyorganosilanes at the structure–micelle interface. Their relative intensities obtained by curve fitting method of the spectra are summarized in Table 2. These results are similar to the functionalized mesoporous materials by Mercier and Pinnavaia [5e]. This discrepancy observed in Table 2 for the functionalized mesostructures can be explained by the decrease in curvature for the larger pore functionalized mesostructure, which can allow, on the grounds of the reduced steric repulsion, the packing of a greater number of organic moieties on the pore wall surface. The presence of Qn sites (Si(OSi)n(OH)4 n, n = 2–4) in the range from 90 to 120 ppm of 29Si NMR spectra in Fig. 5c and d indicates some carbon–silicon bond cleavage of organosilane precursors during synthesis or solvent extraction, though the residual in the Q-range is trivial

[4a–c]. The apparent retention of hydroxyl groups on the surface of the incorporated PMO-icp and PMO-uredo mesostructure may be useful, in the preparation of multifunctional mesostructured materials, where the subsequent grafting of different chemical species on the surface Si–OH may be possible. 3.4. Functionalized PMO nanofibers with longitudinal pore architectures Despite major achievements in supramolecular chemistry to achieve controlled self-assembly of organic molecules, leading to organized mesostructures with diverse morphologies [1–8], until now functionalized fibrous PMO mesostructured solids with longitudinal pore architectures have not been found yet. Fig. 6 shows SEM images of pure PMO, functionalized PMO-icp5, and functionalized PMO-uredo5, where the contents of ICP and UREDO are 5 wt.%, respectively. Fig. 6b and c show typical images of the functionalized PMO fibers grown at 55 °C. For both functionalized PMO series, most of the PMO fibers have diameters in the range of 200–450 nm and lengths of 4–5 lm, whereas the pure PMO shows nano-rod (i.e. nonfibrous) morphology. Figs. 7 and 8 show SEM and TEM

Fig. 6. SEM images of functionalized (a) pure PMO, (b) PMO-icp5, and (c) PMO-uredo5.

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207

Fig. 7. SEM (a, b) and TEM (c, d) images of functionalized PMO-icp15, (c) is the pore direction and (d) is channel direction. (b) is higher magnification of (a). Arrow in the TEM image indicates fiber axis.

images of PMO-icp15 and PMO-uredo15, where the contents of ICP and UREDO are 15 wt.%. TEM images show that the pore channels of these PMO fibers are aligned parallel to the fiber axis (Figs. 7d and 8d); that is, they have longitudinal pore channel architectures. The pore channels are continuous and all toward the same direction. The TEM images of the samples (PMO-icp15 and PMOuredo15) clearly indicate that the uniform pores are packed hexagonally (Figs. 7c and 8c), while pore nanochannels of the functionalized PMO fibers are aligned parallel to the fiber axis with longitudinal pore architecture (Figs. 7d and 8d) [6a,f]. The structure was disrupted from periodically hexagonal pore arrays to disordered pores by the incorporation of higher amount of organosilica into PMOs (not shown). SEM images of higher magnification (Figs. 7b and 8b) display fibers having widths of 200–450 nm and lengths of 4.5–5.0 lm for the PMO-icp series, whereas the PMO-uredo series comprise slightly thinner fibers (width: 80–200 nm; length: 4–5 lm). The fibers are assembled in bundles having widths within the range 0.3–0.4 lm and lengths of up to 3–5 lm. The SEM images indicate that the fibers have smooth surfaces without branching or inter-

connecting or kinks. The functionalized PMO fibers seem to be quite flexible; some fibers are bent or flat, while others exhibit overlapped images; non-impressive amorphous stick-like particles are also present. In the SEM images, the fibers exhibit a defect of a p/3 disclination around the longitudinal axis [5b]; such defects are observed rarely in mesoporous silica materials [4a,b,5a,b]. Of particular interest is the observation that the samples prepared using the UREDO moiety display a substantially different texture in their fibrous assemblies, although they possess similar lengths, than do the ICP-moiety-containing PMO samples, which exist in much-thinner fibrous bundles and bent morphologies. This finding implies that the aggregation of fibers and/or fibrous bundles at the nano/macroscale is markedly dependent on the nature of the organic moiety. For both series when x = 0.15, we have also investigated temperature effect for growing PMO fibers. The samples mainly consist of fibers of 150–250 nm wide and 6–7 of micrometers long, indicating again high stability of PMO fibers at higher temperature up to 75 °C in Fig. 9, TEM images show that the pore channels of these fibers are aligned parallel to the fiber axis. Similar SEM and TEM

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Fig. 8. SEM (a, b) and TEM (c, d) images of functionalized PMO-uredo15, (c) is the pore direction and (d) is channel direction. (b) is higher magnification of (a). Arrow in the TEM image indicates fiber axis.

images have previously been obtained under acidic conditions, where pore channels of obtained fibers are also aligned parallel to fiber axis [6a,f]. Meanwhile, the growth of differently shaped particles stresses the role of topological defects and left some important clues for the formation of particles [1a,4a,b,5a,b]. Ozin and co-workers proposed that mesoporous silica fibers can be thought of +p/3 disclinations around the longitudinal C3-axis [5a,b]. The rate of condensation and hydrolysis of precursors as well as the rate of aggregation of charged species to liquid–crystal membranes are also important factors, which will maintain the formed special liquid–crystal membranes and then bend them into fibers/tubules [23]. Mann and co-workers [7b] used the fibrous organization of bacteria (bacillus subtilis) as a substrate for the deposition of MCM-41 type materials. The pore channels of the resulting mesoporous silica fibers that grow at the oil– water interfaces are highly organized and the pore channels are oriented parallel to the fiber axis [6a,f]. Lin and coworkers [5d] reported for the functionalized mesoporous materials that the strong interaction between non-polar organic groups and the hydrophobic carbon tails of surfactant molecules led to rod-like morphologies, where the interactions are not favored by the hydrophilic trialkoxysil-

anes, yielding particles with randomly oriented pore structures. Average rod-shaped particles were also observed where the disruption of the packing mechanism was indeed minimized [5d]. A similar explanation was made by Mann and co-workers [7b], who observed the growth of the functionalized mesoporous silica particle in the direction that is perpendicular to the pore-alignment upon the introduction of amine-containing organoalkoxysilane. Sanchez and coworkers suggested that fibrous porous silicas or functionalized silicas could be created by the interaction of the fibrous aggregates built from organic molecules and the growing silica network under basic conditions [24]. Moreau et al. [25] explored the fibrous morphologies for the 1,12-diurea segments containing bridged silsesquioxane due to the supramolecular interactions of urea groups by H-bond and those of long hydrocarbon chains which result from hydrophobic properties. Very recently, we have described that vinyl, cyanopropyl, ethyl, and glycidoxy propyl organosilica showed rod-like morphologies with different degree of curvatures for functionalized PMOs due to the favorable interactions between organic groups and tails of surfactant molecules, where bridged amine led to the worm-like particles under same conditions [4a]. It is quite

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209

Fig. 9. TEM images of PMO-icp15 (a), and PMO-uredo15 (b) synthesized at 75 °C. SEM images of PMO-icp15 (c), and PMO-uredo15 (d), respectively. Arrows in TEM images indicate fiber axis direction.

interesting that ICP and UREDO gave fibrous morphologies under the same protocol used [4a,b]. These morphologies strongly suggest different interactions between the organosilicas and surfactant molecules, which arise on the nature of employed organosilica, as supported by previous observations [3c,4a–e,5,23–28]. 3.5. Weight loss behavior by thermogravimetric analysis The conclusion drawn from the NMR spectra may be supported also by the results of the TGA analysis in Fig. 10. The weight loss of about 1.1% is demonstrated

below 150 °C, attributable to the loss of small amounts of water adsorbed on the materials surface [4a–d]. This is followed by a weight loss of 3.6% between 150 °C and 365 °C due to the small amounts of organosilane and negligible amount of surfactant left in the samples. In this range, the decomposition of trialkoxysilanes (UREDO or ICP) also overlaps with that of ethane fragment, while the two contributions cannot be resolved. This indicates almost complete removal of surfactant molecules by extraction method. Thereafter, an additional weight loss in the range of 9.6–16.2% is observed between 365 °C and 750 °C, probably resulting from decomposition of

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Acknowledgments

Physiosorbed water

100

Organic fragments

The work was supported by the National Research Laboratory Program, the Center for Integrated Molecular Systems, and the Brain Korea 21 Project.

Weight loss (%)

in the hybrid pore wall Surfactant +

90

organosilane

(b)

References

(a) PMO-uredo15

80

(b) PMO-icp15

(a)

Condensation of the silicate walls 150

300

450 600 Temperature (ºC)

750

900

Fig. 10. The weight loss behaviors for functionalized (a) PMO-uredo15 and (b) PMO-icp15.

the organic components of the hybrid pores’ wall in the functionalized PMOs [4a–d]. With the support of T2 sites in Fig. 5c and d, the additional weight loss occurred again at higher temperature range 740–950 °C due to further condensation of the silicate walls as observed in the other mesoporous silicate and functionalized PMOs [4a–d]. The PMO-cps15 sample showed higher thermal stability compared PMO-uredo15, due to the difference of structures of UREDO compared to ICP. 4. Conclusions Hierarchically organized functionalized periodic mesoporous organosilica (PMO) fibers with longitudinal pore architectures have been prepared via a simple one-pot synthesis procedure from the co-condensation of 1,2-bis (triethoxysilyl)ethane (BTESE) with either of two kinds of trialkoxysilanes, (3-triethoxysilylpropyl) isocyanate (ICP), or 1-[3-(trimethoxysilyl)propyl] urea (UREDO) under basic conditions. The influence of organosilica source and temperature on the formation and the internal pore architecture of the functionalized PMO fibers were investigated. The pore channels in both types of PMO nanofibers are hexagonally packed, in which pore channels are aligned parallel to fiber axis. The diameters of the fibers range from 50 to 300 nm, and the lengths are up to 7 lm. We have demonstrated the first functionalized hierarchically organized hexagonal PMO fibers under basic conditions that possess pore channels oriented parallel to the fibre axis and the pores are packed hexagonally. The pore architectures of these PMOs nanofibers with aligned parallel orientation to fiber axis could be synthesized at 75 ± 3 °C. The pore architecture is utmost significant for producing hierarchically organized fibrous materials in which the nanofibres having pore channels aligned parallel to the fiber axis could function as nanoscale conveyors for transporting and delivering chemical and biological species [6a,f,27]. In addition, these multifunctional materials may find applications in the fields of catalysis, chemosensing and chromogenic discrimination [6a,f,28].

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