Synthesis and morphological investigation of ordered SBA-15-type mesoporous silica with an amphiphilic triblock copolymer template under various conditions

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Abstract

Finely ordered SBA-15-type mesoporous silica materials were synthesized by templating with an amphiphilic block copolymer under various conditions, and their internal two-dimensional hexagonal mesostructures were confirmed by X-ray diffraction patterns, transmission electronic microscopy and nitrogen adsorption–desorption isotherms. Evaluation on morphological dependence of the synthetic conditions demonstrated that, for the samples synthesized through a one-step route, a variety of morphologies (e.g. short columns, spheres, hexagonal platelets, etc.) were achieved by means of adjusting acid species, reaction temperature, acidity and the concentration of inorganic salt. For the samples synthesized through a two-step route under mildly acidic conditions, the morphologies can be controlled by acidity of the solutions, and exhibits the irregularly faceted particles, the spherical ones, and the aggregated spheres by varying the pH from 1.02 to 2.61.

Introduction

Mesoporous silica materials with highly ordered periodic porous structure have been extensively studied in recent decade due to their potential applications in catalysts, nano-reactors, separations, chemical sensors, and low dielectric and optical coating [1], [2], [3], [4], [5], [6]. With the development in synthesis strategies during the last few years, mesoporous silica materials with various controllable morphologies have been receiving much attention. Because, for a variety of practical uses, the fabrication of desired morphologies for mesoporous silica is as important as the control of its internal structure and porosity. Furthermore, the control of the particle morphology could open up new possibilities for these applications. For example, the mesoporous silica materials with spheres morphology may be used as a stationary in high-performance liquid chromatography [7], while ones having faceted single crystal morphology may be used as good matrix materials for making micron-lasers due to their well-defined internal optical gallery mode [8], [9]. For this reason, many efforts have been devoted to the synthesis of the mesoporous materials with uniform morphology and defined size, and there have been several literatures on the synthesis of mesoporous material with emphasis on the morphology control [10], [11], [12], [13], [14], [15], [16]. By far, the mesoporous materials with various morphologies have been obtained in acidic or basic medium, such as spheres [17], [18], rod-like powers [19], [20], films [21], [22], hollow tubules [23], fibers [13], [24], monolithic [25], particles with complex forms [26], [27], and inerratic crystal [10]. Using ionic surfactants as templates, Ryoo et al. reported the synthesis of MCM-48 crystals (Ia3d) with rhombic dodecahedral shape in 1998 [16]. Tatsumi and co-workers obtained a particular morphology of SBA-1 having 54 or more crystal faces by using decyltrimethylammonium bromide as cationic surfactant [12]. Lin and co-workers reported the synthesis of different single crystal shaped SBA-1 mesoporous silica from C18TMACl-sodium silicate–H2SO4–H2O at pH value between 1.0 and 2.0 [11].

Compared with the ionic surfactants, the interaction between the nonionic block copolymer surfactant and inorganic species (S0H+)(XI0) is much weaker than that of ionic surfactants under acidic conditions (S+XI+) and under basic conditions (S+I), where S, I, and X represent organic micelles, inorganic species, and halide anion, respectively. So it is more difficult to synthesize mesoporous crystals using the nonionic block copolymer surfactant. Zhao and co-workers reported the synthesis of SBA-16 single crystals with rhombic dodecahedral shape (∼1 μm) using a nonionic block copolymer as template with an inorganic salt under an acidic condition [10]. Kwon and co-workers described a microwave synthesis of rhombic dodecahedral or deca-octahedral shape of SBA-16 mesoporous [15]. In 2005, Mou and co-workers synthesized the faceted single crystals of mesoporous silica SBA-16 from a ternary surfactant system [28]. These mesoporous silica materials with well-defined morphological architectures may have potential versatile applications in separations, catalyst carrier, nano-reactors, sensors, electro-optic devices and as templates for the generation of three-dimensional (3D) carbon, metal, and other nano-objects.

As one of the most important mesoporous silica materials, SBA-15 with two-dimensional (2D) hexagonal mesostructure (p6mm) has attracted lots of attention due to its large pores, thick pore walls, and better thermal and hydrothermal stability. Many literatures have reported the morphology control of highly ordered SBA-15 by using poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) triblock copolymer (EO20–PO70–EO20, Pluronic P123) as template. Besides P123, Pluronic P104 (EO27–PO61–EO27) is another nonionic block copolymer surfactant that can be used as template to synthesize SBA-15-type mesoporous silica. However, only few articles regarding the morphologies control of SBA-15-type mesoporous silica by using the P104 template were presented [18], [29], [30]. It should be pointed out that the interaction between organic micelles and inorganic species affects intensively the mesostructures of the final mesoporous material during the self-assembly process. Lots of investigations on the mechanism of the mesostructure formation revealed that the synthetic routes of mesoporous silica materials were obtained on the basis of the interactions of the type (S+I), (S+XI+), (SI+), (S0I0), (S0H+)(XI+), and (S0FI+), where F is fluoride ion and X is halide anion. Many studies indicated that several factors (e.g. temperature, inorganic salt, pH, acid species, etc.) could affect the interaction between organic micelles and inorganic species, and consequently influence the final structure of the mesoporous materials during the self-assembly process [10], [30], [31]. The variation of the hydrolysis of tetraethoxysilane (TEOS) and condensation rates of the silica precursor as a function of pH value has been fully understood from the general sol–gel science knowledge [32], [33]. Based on these reasons, it is possible to obtain the mesoporous silica with the desired morphology by controlling the hydrolysis and condensation rates of the silica precursor.

In our former studies, through a static self-assembly strategy in a strongly acidic condition (2 M HCl), we synthesized SBA-15-type mesoporous silica with the morphology of uniform platelets with equilaterally hexagonal structure by using P104 as template [34]. However, such a strongly acidic condition cannot favor the industrial synthesis of the mesoporous silica materials like SBA-15. So we also developed a two-step synthetic route by separating the hydrolysis and the condensation steps of silica source in the controlled pH range higher than the isoelectric point of silica (pH 2–5), and synthesized successfully the highly ordered SBA-15-type and SBA-16-type mesoporous silica materials under mildly acidic conditions [35], [36]. The present work investigates the influence of synthetic condition parameters including the acid species, concentration of inorganic salt, and temperature on the morphology of the SBA-15-type mesoporous silica synthesized with a P104 surfactant template through the one-step route under strongly acidic conditions, and also evaluates the effect of pH value on morphology of the SBA-15-type mesoporous silica synthesized through the two-step route under mildly acidic conditions. The obtained SBA-15-type mesoporous silica materials with various morphologies are expectable to be used in some research fields such as fabrication of various mesoporous carbon, nano-reactors and other new possibilities applications.

Section snippets

Chemicals and synthesis

Pluronic P104 was commercially obtained from BASF. TEOS, potassium sulfate (K2SO4), hydrochloric acid (HCl) and nitric acid (HNO3) were supplied by Beijing Chemical Reagents Company. All chemicals were of reagent quality and used as received without further purification.

The mesoporous silica samples were prepared by templating with nonionic P104 surfactant through the traditional one-step and novel two-step synthetic routes in strongly and mildly acidic solutions, respectively. A typical

Results and discussion

The highly ordered SBA-15-type mesoporous silica with different morphologies was synthesized through the ordinary one-step route and the novel two-step one that has been described in our former study [32]. In present work, we synthesized the SBA-15-type mesoporous silica by using P104 surfactant as template through above two routes, and some synthetic conditions (such as temperature, concentration of inorganic salt, acid species, etc.) were adjusted so as to investigate the influence of these

Conclusion

Highly ordered SBA-15-type mesoporous silicas with various morphologies have been synthesized under different conditions, and their 2D hexagonal mesostructures were confirmed by the XRD patterns and the TEM images. For the materials synthesized through the one-step route, their morphologies are dependent on the reaction temperatures, the concentration of inorganic salt and the acid species. The short column-shaped particles, the finely defined hexagonal platelets and the spheres can be obtained

Acknowledgements

The authors greatly appreciate the financial support by the National Nature Science Foundation of China (Grant no.: 50573006 and 5463002) and the Major Project for Polymer Chemistry and Physics Subject Construction from Beijing Municipal Education Commission (XK100100640).

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