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Jan 8, 2011 - Yuanzheng Yue ... diameter and the length are about 100 nm to 1 μm and .... 1a that the as-prepared silica nanotubes have a smooth surface.
J Sol-Gel Sci Technol (2011) 58:334–339 DOI 10.1007/s10971-011-2397-8

ORIGINAL PAPER

Formation and characterization of mesostructured silica nanotubes Qifang Lu • Cuiqing Wang • Suwen Liu Yuanzheng Yue



Received: 3 October 2010 / Accepted: 3 January 2011 / Published online: 8 January 2011 Ó Springer Science+Business Media, LLC 2011

Abstract The multi-walled mesoporous silica nanotubes are prepared using cetyltrimethylammonium bromize (CTAB) as the surfactant micellar template and tetraethylorthosilicate (TEOS) as the silica precursor via a one-step wet chemical approach. The synthesized tubes are found to be double/triple walled and of amorphous nature. Their diameter and the length are about 100 nm to 1 lm and about 0.1–20 lm, respectively. The specific surface area approaches 1,488 m2/g. Based on the transmission electron microscopy analysis, it is inferred that the formation of the double/triple walled silica nanotubes is associated with the lamellar curling mechanism. A striking photoluminescence effect is detected in the mesostructured silica nanotubes. These nanotubes are expected to be a promising material for various applications such as gas storage, catalyst, or catalyst supports. Keywords Mesostructure  Silica nanotubes  Characterization

Q. Lu (&)  S. Liu  Y. Yue Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, Shandong Institute of Light Industry, 250353 Jinan, People’s Republic of China e-mail: [email protected] C. Wang Lunan Research Institute of Coal Chemistry, 272000 Jining, People’s Republic of China Y. Yue Section of Chemistry, Aalborg University, 9000 Aalborg, Denmark

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1 Introduction The combination of mesostructure and one-dimensional silica nanotubes has attracted increasing research interests because of their potential applications and hierarchical structures with at least three different length scales and well-defined macroscopic forms [1]. The synthesis of hollow silica tubules with the coaxial cylindrical MCM-41 type mesoporous channels in the tube walls was first reported by Lin and Mou [2]. Since then, many researchers have contributed to the study of mesoporous silica nanotubes regarding both synthesis approach and property tailoring. The silica nanotubes with mesoporous structure and a very high degree of structural particularity of hierarchical organization have at least three different kinds of surfaces: the inner and outer tube surface and the internal surface of the mesostructured nanotube walls. All these surfaces could be differently functionalized and act together in an integrated chemical system. Different surface functionalization enables the scientists to design the environment of nanoreactors [3, 4]. Moreover, the modification of the properties either by filling or coating the nanotubes has been carried out to meet the particular application, for example, in drug-delivery systems, molecular separation, single-DNA sensing, pollutants decomposition, hydrogen fuel, gas sensors, and solar energy conversion devices [5, 6]. To date, several approaches have been reported for the preparation of mesostructured silica nanotubes and helical tubular structures, spicules, such as surfactant-mediated method [7–12], molecular templating [13–15], catalytic synthesis [16], hydrothermal process [1, 17], sol–gel transcription method [18, 19], biomimetic synthesis[20] and porous anodic alumina membranes [21]. However, up to now it is still a big challenge to prepare the mesostructured silica nanotubes, particularly, the double/triple-walled

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silica nanotubes with tube diameter for targeted performances. To the best of our knowledge, the multi-walled (e.g. double/triple-walled) mesostructured silica nanotubes have not been developed. The present work is devoted to syntheses and characterization of the mesostructured silica nanotubes. To do so, a one-phase route will be applied: Cetyltrimethylammonium bromize (CTAB) is used as the surfactant micellar template, while tetraethylorthosilicate (TEOS) is used as the silica precursor. The morphologies and formation processes of mesostructured silica nanotubes will be investigated by monitoring the evolution of the shape and size of the intermediate products with help of high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), N2 adsorption isotherm and small-angle X-ray diffraction (SAXD). Finally, the tubes will be characterized with respect to the photoluminescence effect.

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patterns of the assynthesized and calcined samples were recorded on an X-ray diffractometer (Rigaku D/Max 2200PC) with a graphite monochromator and Cu Ka radiation (k = 0.15148 nm) in the range of 1–10° at room temperature while the voltage and electric current were held at 28 kV and 20 mA. A transmission electron microscope (TEM, Model H-800) and a high-resolution TEM (HRTEM, GEOL-2010) were applied to observe the morphology and cross section of the products. Nitrogen adsorption–desorption data were measured with an ASAP 2010 micromeritics apparatus at liquid nitrogen temperature (T = -196 °C). Isotherms were evaluated with the Varrett-Joyner-Halenda (BJH) theory to give the surface areas of the nanotubes. The PL spectrum of the mesostructured silica nanotubes was measured by a Hitachi M-850 fluorescence spectrophotometer at room temperature under ambient atmosphere. The mesostructured silica nanotubes were pressed into the thin slices, and the excitation wavelength was 365 nm.

2 Experimental methods 2.1 Synthesis of mesostructured silica nanotubes

3 Results and discussion

Mesostructured silica nanotubes were prepared with cetyltrimethylammonium bromize (CTAB) surfactant and tetraethylorthosilicate (TEOS) silica precursor. All chemicals were used as received without further purification. In a typical synthesis, 1.0 g CTAB was first added to the mixture of 5 g distilled water and 20 g 2 M hydrochloride acid which was added to adjust the pH value in a Teflon container, and the mixture were stirred until CTAB was dissolved completely. Then, 3 g formamide was added, and further stirred for 4 h. The use of formamide in the synthesis was to adjust the acidity and ionic strength to favor the formation of silica nanotubes [9]. Finally, 2.5 g TEOS was added dropwise, and stirred at room temperature for 2 h. The mixed solution was transferred to a 100 mL plastic beaker. The beaker was closed and kept in an isothermal oven at 60 °C in a quiescent state for 24 h. During this period of time, some white flocculates suspended in the growth solution were observed. The products were isolated by centrifugation and followed by repeatedly washing with deionized water and absolute ethanol. The obtained solids were dried at 100 °C in air. The obtained products were calcined in an airatmosphere tube furnace at a rate of 1 °C min-1 from room temperature to 550 °C. After calcined at 550 °C for 5 h, the final product was cooled to room temperature in the furnace.

The morphologies of the as-prepared and calcined silica nanotubes are shown in Fig. 1a–f. It can be seen in Fig. 1a that the as-prepared silica nanotubes have a smooth surface and the one-dimensional structures have been formed prior to calcination. The outer diameter ranges from 100 to 500 nm and a length up to several hundreds micrometers. After calcination, the one-dimensional structures are still preserved and the ultrasonic wave dispersion may make the long nanotubes broken seen in Fig. 1b and c, respectively. Figure 1d and e show the HRTEM images of the calcined silica nanotubes at 500 °C for 5 h. It is found that the nanotubes have smooth surfaces and sharp walls, and exhibit uniformity in both the diameter and the wall thickness along the tube’s axis. However, the uniformity of the outer diameter and the length of the nanotubes are not high. By examining a large number of samples, it is detected that the diameter of the tubes is around 100–500 nm and the length of the nanotubes is hundreds of nanometers. It is remarkable that some silica nanotubes possess single walled structure, while some have double/ triple-walled structures as shown in Fig. 1e. The total wall thickness of the double/triple-walled nanotubes is less than 30 nm as shown in Fig. 1d and e. The cross section of a triple-walled silica nanotube (seen in Fig. 1f) indicates the existence of the triple-walled silica nanotubes. The thickness of each wall is nearly constant and of the order of 5 nm and the distance between two layers is around 2 nm that is approximately equal to the pore size of the mesoporous silica.

2.2 Characterization The pH value was measured with a pH electrode placed in the solution during the reaction. X-ray diffraction (XRD)

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Fig. 1 SEM images of as-prepared and calcined mesostructured silica nanotubes: a as-prepared; c calcined. TEM images of calcined mesostructured silica nanotubes: b low magnification; d high magnification; e, f cross section of the triple walled nanotube

Figure 2 demonstrates the HRTEM images of the cross section of the silica nanotubes. It can be seen that the nanotubes have a hollow nature and the end of nanotubes is either open or closed. The ultrasonic wave dispersion may make the closed nanotubes broken into open ones during sample preparation for observation of HRTEM. Such a hollow structure is the channel for encapsulating foreign elements or compounds by capillary filling [22]. The crosssectional shape of the closed end is quadrate.

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The amorphous nature of the tube walls is revealed by the wide angle X-ray diffraction (XRD) spectra, since only a broad typical amorphous silica peak exists around 2h = 22–238. The low angle XRD patterns of the as-prepared and calcined samples are shown in Fig. 3, where sharp peaks appear between 2 and 38. This confirms that both as-synthesized and calcined materials are the mesostructured silica. However, the peak of calcined nanotubes is sharper and stronger than that of the as-prepared nanotubes which is due to the higher scattering contrast between the pore walls and the inside of the pores, caused by the burning-out of the templating organic species. In addition, the former one is centered at a higher 2h value compared to the latter one [23]. The appearance of the only one peak provides the additional information about disorder effects on the mesoscale pore structure. [24]. Figure 4 shows the representative N2 adsorption and desorption isotherms, in which the type IV adsorption isotherms typical of mesoporous materials is obseved [25]. The corresponding BJH (Barret-Joyner-Halenda) pore size distribution (PSD) curve for the silica nanotubes is displayed in Fig. 4. The capillary condensation step occurring in the relative pressure range of 0.2–0.4 is probably attributed to the capillary condensation and nitrogen adsorption in the cavities of the mesoporous of the tubes [26]. The BJH analyses show that the sample with a sharp mesoporous size distribution exhibits the pore size of about 2.6 nm. The Brunauer–Emmett–Teller (BET) surface area of the calcined silica nanotubes is 1,488 m2/g. The mesostructured silica nanotubes calcined at 500 °C for 5 h can emit stable, brightness visible light at room temperature (Fig. 5). The PL spectrum consists of a main, intense peak at 435 nm (2.85 eV) with a shoulder at 460 nm (2.70 eV) and a shoulder peak at 410 nm (3.03 eV). The violet light emission at 410 nm is ascribed to some intrinsic diamagnetic defect centers, such as the twofold-coordinate silicon lone-pair centers (O–Si–O), whereas the blue light emission at 460 nm is attributed to the neutral oxygen vacancy (Si–O–Si). The PL feature of the samples studied in this work is similar to that of the silica nanotubes prepared by a reverse-microemulsionmediated sol–gel method [22]. The PL properties of such nanotubes may be of great interest for both fundamental study and applied research, for example, opening up potential applications in high-resolution optical heads of scanning near-field optical microscope and the integrated optical devices [18]. In order to make the formation mechanism of the nonsingle-walled mesoporous silica nanotubes clear, a small quantity of nanoribbon, curled nanoribbon, helical nanoribbon and incomplete nanotubes are also observed as shown in Fig. 6.

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Fig. 2 HRTEM images of the cross section of the silica nanotubes: a Hollow nature of the nanotubes with the closed end. b Hollow nature of the nanotubes with the open end

Fig. 3 Small-angle XRD patterns of mesostructured silica nanotubes: (a) As-prepared nanotubes; (b) Calcined nanotubes at 550 °C for 5 h

Fig. 4 N2 adsorption and desorption isotherms and the corresponding BJH pore size distribution curve (inset) at -196 °C for the sample obtained after calcination in air at 550 °C for 5 h. P is the pressure of nitrogen in the gas phase and P0 is the saturation pressure. The unit of cc/g refers to cubic centimeters of N2 gas at standard temperature (0 °C) and pressure (1 atm) per gram of water

Fig. 5 Photoluminescence (PL) spectrum of mesostructured silica nanotubes calcined at 500 °C for 5 h (excitation wavelength at 365 nm)

Figure 6a and b show the HRTEM images of the mesoporous silica nanoribbons. It is seen that the silica nanoribbons are either parallel or vertical to each other and the average width of the ribbons is about 50 nm. The interspace between the nanoribbons is clearly observed and distinguishable (seen in Fig. 6a). The end of the nanoribbon is straight and not turnup. There coexist two kinds of cross-sectional shapes of the mesoporous silica nanoribbons (seen in Fig. 6b). One has a closed semicircle end, and another has an open end. It is also seen that an undulant-wave-like silica nanoribbon is attached to a straight one in Fig. 6c and the tilted edge of a straight nanoribbon denoted by the white rectangle as seen in Fig. 6d, which indicates the curly character of the mesoporous silica nanoribbons. The mesoporous silica nanotubes always coexist with the silica ribbon as illustrated in Fig. 6e and f, suggesting the link between the nanotubes and nanoribbons. The silica nanoribbon with a curled end was often observed, e.g., as shown in Fig. 6e. These nanoribbons

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Fig. 6 HRTEM images of the mesoporous silica nanoribbons (a–d) and incomplete nanotubes (e, f)

Fig. 7 SEM image a and TEM image b of the coexistence of the nanotubes and nanoribbons

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demonstrate the initial stage of the curling. Furthermore, a nanocone with a half-tube and -ribbon is observed (Fig. 6e). In the zone indicated by letter A, the nanoribbon begins to curl (become tube-like) and this is a transitional stage of the formation of the nanotubes. In the zone marked by B, the incompletely formed tubular structures are distinctly demonstrated. The one end denoted by C is a complete ribbon. Figure 6f is a magnified image of the section marked by the white square in Fig. 6e. Figure 7 further indicate the coexistence of the nanotubes, nanoribbons and twisted nanoribbons. The diameter of the silica nanotubes is of the order of 50 nm which is well consistent with the result of TEM. The existed states of the silica nanoribbons are either irregular curled or twisted helical structure (seen in Fig. 7a). Furthermore, there stand the curved nanobelts, along with the already rolled up nanotubes (seen in Fig. 7b).

4 Conclusions The mesotructured silica nanotubes have been successfully fabricated with the diameter of the tubes in the range of 100 nm to 1 lm by using a one-step wet chemical approach. This simple procedure allows not only for understanding the morphological change of silica nanotubules, but also for generating hierarchical structures of the silica nanotube materials for various applications. The double/triple-walled mesostructured silica nanotubes exhibit striking photoluminescence effect. This study provides a new insight into the molecular factors governing inorganic–organic and macro- and mesophase formation. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 50872076), the Open Research Fund Program of State Key Laboratory of Crystal materials (Grant No. KF0905), and the Ministry of Education of Shandong Province (Grant No. J09LD23); Authors also thank the Analytical Center of Shandong Institute of Light Industry and Shandong University for the technical support.

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References 1. Wang L, Shan Z, Zhang Z, Wei F, Xiao FJ (2009) Colloid Interface Sci 335:264–267 2. Lin HP, Mou CY (1996) Science 273:765–768 3. Jung J, Kobayashi H, Shinkai S, Shimizu T (2002) Chem Mater 14:1445–1447 4. Kleitz F, Wilczok U, Schu¨th F, Marlow F (2001) Phys Chem Chem Phys 3:3486–3489 5. Xu J, Li X, Zhou W, Ding L, Jin Z, Li YJ (2006) Porous Mater 13:275–279 6. Lu Q, Chen D, Jiao X (2005) Mater Chem 17:4168–4173 7. Kleitz F, Marlow F, Stucky GD, Schu¨th F (2001) Chem Mater 13:3587–3595 8. Marlow F, Kleitz F (2001) Micropor Mesopor Mater 44:671–677 9. Yang SM, Sokolov I, Coombs N, Kresge CT, Ozin GA (1999) Adv Mater 11:1427–1431 10. Harada M, Adachi M (2000) Adv Mater 12:839–843 11. Adachi M, Harada T, Harada M (2000) Langmuir 16:2376–2384 12. Kim M, Sohn K, Kim J, Hyeon T (2003) Chem Commun 5:652–654 13. Zollfrank C, Scheel H, Greil P (2007) Adv. Mater. 19:984–987 14. Jung JH, Shinkai S, Shimizu T (2003) Chem Rec 3:212–224 15. Jung JH, Shinkai S, Shimizu T (2002) Nano Lett 2:17–20 16. Jiang Z, Xie T, Yuan XY, Geng BY, Wu GS, Wang GZ, Meng GW, Zhang LD (2005) Appl Phys A 81:477–479 17. Lin H, Mou C, Liu S (2005) Adv Mater 12:103–106 18. Ciosan E, Bando Y, Wada K, Cheng LL, Pirouz P (2002) Appl Phys Lett 80:491–493 19. Jung J, Lee S, Yoo J, Yoshida K, Shimizu T, Shinkai S (2003) Chem Eur J 9:5307–5310 20. Jensen M, Keding R, Ho¨che T, Yue YJ (2009) Am Chem Soc 131:2717–2721 21. Wu Y, Cheng G, Katsov K, Sides SW, Wang J, Tang J, Fredrickson GH, Moskovits M, Stucky GD (2004) Nat Mater 3:816–822 22. Jang J, Yoon H (2004) Adv Mater 16:799–802 23. Kleitz F, Schmidt W, Schu¨th F (2003) Micropor Mesopor Mater 65:1–29 24. Marlow F, Kleitz F, Wilczok U, Schmidt W, Leike I (2005) Chem Phys Chem 6:1269–1275 25. Sing KSW, Everett PH, Haul RAW, Moscou L, Pierrotti RA, Rouque´rol J, Siemieniewska T (1985) Pure Appl Chem 57: 603–619 26. Ciesla U, Schu¨th F (1999) Micropor Mesopor Mater 27:131–149

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