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Preparation of hexagonal platy particles of nanoporous silica using hydrotalcite as morphology template
Journal of Colloid and Interface Science 312 (2007) 311–316 www.elsevier.com/locate/jcis
Preparation of hexagonal platy particles of nanoporous silica using hydrotalcite as morphology template Naoki Shimura a , Makoto Ogawa a,b,∗ a Graduate School of Science and Engineering, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan b Department of Earth Sciences, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan
Received 30 September 2006; accepted 28 February 2007 Available online 15 March 2007
Abstract Hexagonal platy composite particles with a hydrotalcite core and a nanoporous silica shell with a thickness of ca. 100 nm were synthesized by the reaction of a Mg–Al hydrotalcite with a homogeneous aqueous solution containing tetraethoxysilane, hexadecyltrimethylammonium chloride, ammonia and methanol at 3 ◦ C. The calcination of the products at 500 ◦ C in air led to the composite particle with a Mg/Al mixed oxide core and a nanoporous silica shell. Hexagonal platy particles of nanoporous silica with a pore diameter of 2.3 nm and BET surface area of 700 m2 (g of silica)−1 were obtained by removing the Mg/Al mixed oxide core. © 2007 Elsevier Inc. All rights reserved. Keywords: Nanoporous silicas; Core shell particles; Hydrotalcite; Supramolecular templating approach
1. Introduction Morphosyntheses of porous silicas have been investigated for practical applications as well as for basic scientific interests. Sol–gel process is a promising way to design the morphology of different materials, and films and fibers of porous silica have been synthesized so far. It has also been reported the precipitation of nanoporous silicas with such morphologies as spherical particles and films from homogeneous solutions [1,2]. Hybrid particles composed of a silica shell on a variety of particles is another class of morphology controlled silicas. Hybrid particles composed of calcium carbonate with silica shell were successfully transformed to hollow mesoporous silica particles retaining the original morphology of the core calcium carbonate particle [3]. Thus, the deposition of silica layer on various particles is worth investigating for the preparation of hybrid core shell particles as well as hollow silica particles [4]. The formation of surfactant templated mesoporous silica layers on calcium carbonate, latex and iron oxides are successful ex-
* Corresponding author. Fax: +81 3 3207 4950.
E-mail address: [email protected] (M. Ogawa). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.02.091
amples [3,5,6]. It seems possible to use various particles as the morphology template. Hydrotalcite is a Mg–Al-layered double hydroxide (LDH, 3+ n− general formula of M2+ 1−x Mx (OH)2 (A )x/n ·mH2 O) consisting of positively charged brucite-like layers and exchangeable interlayer anions [7–10]. Hydrotalcite and its analogous compounds are expected as catalysts and adsorbents. There are several reports on the preparation of nonporous silica coated hydrotalcite [11–14]. In the present study, the coating of hydrotalcite with nanoporous silica layers was investigated. Hydrotalcite particle surface modification with nanoporous silica layers can be a way to impart additional functions such as molecular sieving. Since hydrotalcite is easily dissolved in acidic solution, it is a possible morphology template candidate for the preparation of hollow platy particles. Recent progress in the morphosyntheses of hydrotalcite motivates us to investigate the deposition of silica layer on hydrotalcite particle [15–19]. Very recently, we have reported a novel approach to deposit thin mesostructured silica–surfactant layers on hydrotalcite from basic solutions containing tetraethoxysilane, alkyltrimethylammonium chloride, methanol and ammonia [20]. This paper reports the detailed experimental procedure for the preparation and characterization of hexagonal platy particles of nanoporous silica
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using hydrotalcite particles as morphology template. The reconstruction of the hydrotalcite from the calcined silica–Mg/Al mixed oxide product was also investigated.
After the leaching, the product was washed with deionized water and dried at 60 ◦ C in a Teflon-vessel for a day. The washed product was named as L-C-C-HT.
2. Experimental
2.3. Characterization
2.1. Materials
Scanning electron micrographs (SEM) were obtained on a Hitachi S-2380N scanning electron microscope. Prior to SEM measurements, samples were coated with gold. X-ray diffraction (XRD) was performed on a RAD IB diffractometer (Rigaku) using monochromatic CuKα radiation, operated at 40 kV and 20 mA. TG–DTA curves were recorded on a Rigaku TG-8120 instrument at a heating rate of 10 ◦ C min−1 and using α-alumina as the standard material. Nitrogen adsorption isotherms were measured at −196 ◦ C on a Belsorp TCV (BEL Japan Inc.). Prior to adsorption measurements, samples were dried at 120 ◦ C under vacuum for 3 h. Infrared (IR) spectra of the samples were recorded on a Shimadzu FT-IR 8200PC Fourier transform infrared spectrophotometer by the KBr disk method. CHN analysis was performed on a Perkin Elmer 2400 II instrument. Mg2+ and Al3+ contents in C-HT were determined from the leached amount of Mg2+ and Al3+ from C-CHT by ICP using a Rigaku SPECTRO CIROS CCD. Characterization data of the products (C-HT, C-C-HT, and L-C-C-HT) was obtained from two samples prepared by same procedure: One sample was used for XRD except for L-C-C-HT, SEM, and FT-IR spectra. CHN analysis, ICP, TG–DTA curves, XRD patterns of L-C-C-HT, and nitrogen adsorption isotherms were obtained from another sample.
Urea was obtained from Wako Pure Chemical Industries, Ltd. Magnesium chloride hexahydrate, aluminum chloride hexahydrate, methanol, hydrochloric acid, and 28% aqueous ammonia solution were obtained from Kanto Chemical Co., Inc. Tetraethoxysilane (abbreviated as TEOS) and hexadecyltrimethylammonium chloride (abbreviated as CTAC) were obtained from Tokyo Kasei Kogyo Co. Ltd. All the chemicals were used without further purification. 2.2. Sample preparation Hydrotalcite (HT, Mg0.67 Al0.33 (OH)2 (CO2− 3 )0.19 ·mH2 O, which was derived from inductively coupled plasma emission spectroscopy (ICP; Mg: 20.7 mass% and Al: 11.0 mass%), Thermogravimetric–differential thermal analysis (TG–DTA), and CHN analysis (C: 2.7 mass% and N: 0.0 mass%) was prepared by the reported method [15], where 0.03 M magnesium chloride aqueous solution (40 mL), 0.03 M aluminum chloride aqueous solution (10 mL), and 0.3 M urea aqueous solution (10 mL) were mixed (molar Mg:Al:urea ratio was 4:1:10) and the mixture was allowed to react in a Teflon-lined container (100 mL, Taiatsu Glass Ind. Co.) at 150 ◦ C for 24 h. After aging at room temperature for a day, the precipitate was collected by centrifugation (3500 rpm, 3 min) followed by washing with deionized water. Product was dried at 60 ◦ C for a day. HT (40.0 mg) was mixed with an aqueous mixture of CTAC (0.106 g), deionized water (8.9 g), methanol (100 mL) and 28% aqueous ammonia solution (3.6 g) and the mixture was aged at 3 ◦ C for a day. To this latter solution TEOS (0.184 mL) was added, and the mixture was shaken for 3 s. The mixture was subsequently aged at 3 ◦ C for 2.0, 2.5 and 3 h (this time was named as reaction time). In order to avoid sedimentation of the HT during the reaction, the mixture was shaken for 3 s every 30 min. The molar ratio of TEOS:CTAC: water:methanol:ammonia was 1:0.4:774:3000:72. After the aging, products were separated by centrifugation (3500 rpm, 3 min). The product was washed with methanol and dried at 60 ◦ C for a day. The sample prepared with CTAC (reaction time = 2.5 h) was named as C-HT. In order to remove the surfactant, the C-HT was calcined in air at 500 ◦ C for 1 h (heating rate of 150 ◦ C h−1 ). The calcined particle was designated as C-C-HT. The reconstruction of the HT was examined by soaking the C-C-HT in deionized water at room temperature for 14 days. The sample was named as RC-C-HT. C-C-HT was washed with aqueous hydrochloric acid (pH 1.0) at room temperature for 24 h in order to remove the core particles composed of mixed magnesium and aluminum oxides.
3. Results and discussion Fig. 1 shows the XRD patterns and the SEM images of the products. The preparation of hexagonal platy particles of hydrotalcite was confirmed by XRD (Fig. 1b; d values of (003), (006), (101), (009), (012) and (015) were 0.76, 0.38, 0.26, 0.25, 0.23 and 0.19 nm, respectively), TG–DTA (Fig. 2a) and SEM microscopy (Fig. 1a). After reaction of the hydrotalcite with the solution for 2.5 h, the sample weight increased by ca. 20%, suggesting the deposition of silica on the HT particles. Fig. 1c shows the SEM image of the coated particle (C-HT), where hexagonal plates similar to the original hydrotalcite were observed. The thickness of the platy particles (C-HT) was 300– 500 nm, which was larger than that of the HT (100–300 nm) used. The surface of the C-HT particle was smooth and homogeneous, and there are no cracks. In the infrared spectrum of C-HT (data not shown) [20], absorption bands due to Si–O stretching of silica (at around 1180 and 1080 cm−1 ) [21] and Si–O stretching of silanol groups (at around 960 cm−1 ) [21– 23], in addition to those of HT (3450, 3020, 1350, 870, 780, 680 and 560 cm−1 ) have been observed. In the XRD patterns (Figs. 1b and 1d), all the diffraction peaks did not change during the reaction, suggesting that hydrolyzed TEOS was not intercalated into the interlayer space of HT [24]. It is difficult to think that Cl− was intercalated due to the size of Cl− at the present condition [25,26]. Thus, it was shown the successful coating of HT with a thin layer of the silica–CTA+ hybrid.
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Fig. 1. SEM images (a, c, e and g) and XRD patterns (b, d, f and h) of HT (a and b), C-HT (c and d), C-C-HT (e and f) and L-C-C-HT (g and h). (Reaction time = 2.5 h.)
Fig. 2. TG–DTA curves of (a) HT and (b) C-HT prepared for 2.5 h.
The TG–DTA curves of HT and C-HT are shown in Fig. 2. Exothermic peaks accompanying the weight losses in the corresponding TG curve were observed at around 150–500 ◦ C for the DTA curve of HT (Fig. 2a). The observed thermal behavior is typical of hydrotalcite [15,27–29]. In the DTA curve of C-HT (Fig. 2b), exothermic reactions, probably due to the oxidative decomposition of CTA+ , were observed at around 250– 350 ◦ C [30] in addition to those characteristic of HT. The chemical composition of C-HT was derived from the CHN elemental analysis (C: 2.9 mass% and N: 0.0 mass%), TG–DTA and ICP (Mg: 16.6 mass% and Al: 9.0 mass%), assuming that the chemical composition of the hydrotalcite in C-HT was the same to that of HT. Additionally, C/Al molar ratios were increased from 0.56 to 0.72 (CHN analysis and ICP), indicating that the deposited layers contained CTA+ . The amount of SiO2 was derived from the TG–DTA results (Fig. 2): The weight of the sample heated at 800 ◦ C was ascribed to those of SiO2 and calcined HT. Because the weight of HT was decreased by ca. 40 mass% (Fig. 2a), the weight of HT in C-HT at 800 ◦ C was estimated as ca. 47 mass% of the original sample weight. Thus, the remaining weight (ca. 16 mass%) was ascribed to the weight of SiO2
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Fig. 3. FT-IR spectra of the C-HT prepared for 2.0 (red line) and 2.5 h (black line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. SEM image of the as-synthesized product prepared for 3.0 h.
in C-HT. As a result, the chemical composition of C-HT was 2− CTA+ 0.003 –(SiO2 )0.27 –Mg0.67 Al0.33 (OH)2 (CO3 )0.19 ·mH2 O. When the synthesis was conducted in the absence of HT, spherical particles are formed [31]. The particle size increased at longer aging times [32]. Though the present system was heterogeneous nucleation, it was thought that a continuous CTA+ – SiO2 layer formed, and the layer became thicker as the reaction time after the addition of TEOS was longer. FT-IR and TG– DTA suggested that the as-coated products prepared by the aging for 2.0 h were CTA+ –SiO2 –HT composites. The FT-IR spectra indicated that the deposition of silica increased as the aging time was longer (Fig. 3). This result suggested that the silica layers of the product prepared by aging for 2.0 h were thinner than the ones by aging for 2.5 h. Further aging (over 3 h) resulted in the formation of spherical particles by homogeneous nucleation, as shown by the SEM image given in Fig. 4. In our separate study on the preparation of nanoporous silica spherical particles from solutions similar to those used in the present study [32], it was shown that CTA+ played a role as porogen. Accordingly, we expect that the removal of CTA+ will lead to the formation of nanoporous silica. Judging from the TG–DTA results (Fig. 2b), C-HT was calcined in air at 500 ◦ C to decompose CTA+ . The SEM image and the XRD pattern of
Fig. 5. Nitrogen adsorption isotherms of Mg–Al mixed oxide (diamond), C-C-HT (square), and L-C-C-HT (circle). (Reaction time = 2.5 h.)
C-C-HT are shown in Figs. 1e and 1f. The platy morphology did not change by calcination, as shown in Fig. 1e. The SEM image showed no cracks on the particle surface. The XRD pattern of C-C-HT (Fig. 1f) showed two broad diffraction peaks at 0.21 and 0.15 nm ascribable to (400) and (440) reflections of a mixed oxide phase of magnesium and aluminum with the MgO type structure, which formed by the thermal decomposition of HT [33,34]. All the infrared absorption bands were ascribable to those of silica [21] and Mg–Al mixed oxides which contained remaining carbonate (1400, 860 and 650 cm−1 ) [28,35, 36]. Thus, C-C-HT consisted of silica and mixed oxides of magnesium and aluminum. The nitrogen adsorption isotherm of C-C-HT is shown in Fig. 5 (square). The BJH pore size distribution [37] is shown in Fig. 6 (square). The BET surface area [38] and the pore diameter [37] of C-C-HT were 140 m2 g−1 and 2.5 nm, respectively. Though it was reported that the calcination of hydrotalcite at 300–600 ◦ C led to porous materials [7,39], the nitrogen adsorption isotherm indicated that the present mixed oxide of Mg–Al made from HT was not porous (Fig. 5, diamond). It was shown that the shell became porous upon CTA+ removal. The amount of silica in the product increased from ca. 16 mass% (calculated from chemical composition of C-HT) to ca. 24 mass% by calcination because the weight of the hydrotalcite as core decreased (ca. 40 mass%) (TG–DTA curves; Fig. 2a) and CTA+ was removed. Considering the composition of C-C-HT, the BET surface area of silica layer was ca. 580 m2 (g of silica)−1 , which is a value comparable to surfactant templated nanoporous silicas. It is well known that the XRD patterns of surfactant templated nanoporous silicas with highly ordered nanostructures show diffraction peaks due to nanostructures [40]. The XRD patterns did not show peaks ascribable to ordered nanostructures as shown in Figs. 1d and 1f. These results suggested that the nanostructure of the C-HT shell was disordered, which was similar to that of the spherical silica particles synthesized under reaction conditions similar to those of the present study [31].
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It is known that calcined hydrotalcites (MgO type mixed oxides) uptake various anions in water to reform layered structures [7–9]. The present calcined particle reconstructed the HT structure by soaking in water. The diffraction peaks ascribable to HT appeared in the XRD pattern of R-C-C-HT (Fig. 7c). The FT-IR spectrum of R-C-C-HT showed the absorption band (1400 cm−1 ) ascribable to CO2− 3 [28] (Fig. 7d). Thus, C-C-HT 2− adsorbed CO3 to reconstruct the HT structure. The SEM images showed hexagonal platy particles with small fans (Figs. 7a and 7b), suggesting that magnesium and aluminum were dissolved into the water (pH = ca. 6), and hydrotalcite was de-
Fig. 6. Pore size distributions of C-C-HT (square) and L-C-C-HT (circle). (Reaction time = 2.5 h.)
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posited on the hexagonal platy particles of the nanoporous silica. C-C-HT is a kind of core–shell particle, where mixed oxides of Mg and Al (core) were covered with a nanoporous silica layer. By removing the core, hollow particles of nanoporous silica are formed. It was reported that needle like calcium carbonate particles with the size of 3–5 µm were used as template to form nanoporous silica tubes [3]. Such hollow silica particles may find applications in the area of drug delivery and adsorbents. The variation of the thickness, shape and size of those hollow particles are worth investigating to find practical applications. Silica is not soluble to acidic solution while the mixed oxide is soluble, so that the core was easily removed from the hybrid particle by leaching in acidic solution. Fig. 1g shows the scanning electron micrograph of the product (L-C-C-HT) washed with hydrochloric acid. The platy morphology was retained even after the washing. SEM images of broken particles showed that the particles were hollow as shown in Fig. 8. The cross-sectional view of L-C-C-HT revealed that the thickness of the shell was ca. 100 nm. All the absorption bands observed
Fig. 8. SEM image of broken L-C-C-HT. (Reaction time = 2.5 h.)
Fig. 7. SEM images (a and b), XRD pattern (c) and FT-IR spectrum (d) of R-C-C-HT. (Reaction time = 2.5 h.)
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in the IR spectrum of L-C-C-HT (data not shown) [20] were ascribed to silicas [21–23]. The diffraction peaks ascribable to the mixed oxide of Mg and Al were not observed in the XRD pattern (Fig. 1h). Thus, the mixed oxide of Mg and Al was successfully removed from the calcined product. The nitrogen adsorption isotherm of L-C-C-HT is shown in Fig. 5 (circle). The BET surface area [38] and the BJH pore size [37] (Fig. 6, circle) were 700 m2 g−1 and 2.3 nm, respectively. The pore diameter was slightly decreased if compared with those of C-C-HT (2.5 nm). Thus, hydrotalcite was successfully utilized as a morphology template for the preparation of nanoporous silica. To our knowledge, hexagonal platy silica hollow particles have never been obtained, though hexagonal platy particles have been prepared by using morphology template (protein crystal) [41]. The platy morphology may open up new opportunities for silica applications, for example, in cosmetic products. Since LDHs with various particle size have been prepared previously [7,9,16,17, 19], it is possible to prepare nanoporous silica platy particles with various sizes by utilizing the present procedure. The present successful preparation of silica-coated hydrotalcites and hexagonal platy particles of nanoporous silicas motivate us to deposit nanoporous silica layers on particles with other compositions and morphology. Further studies on the preparation and characterization of nanoporous silica-coated particles and hollow nanoporous silica particles are underway in our laboratory. 4. Conclusions Hexadecyltrimethylammonium–silica nanocomposite layer was deposited on hydrotalcite by reaction in a homogeneous aqueous solution containing tetraethoxysilane, hexadecyltrimethylammonium chloride, methanol and ammonia. The core– shell particle was further converted to composite particles with a nanoporous silica shell and a magnesium aluminum mixed oxide core. XRD patterns and FT-IR spectrum suggested that this particle adsorbed CO2− 3 to reconstruct hydrotalcite structures. Hexagonal hollow platy nanoporous silica particles (particle size of several micrometers) with the pore size of 2.3 nm and a BET surface area of 700 m2 g−1 were obtained by removing the core. Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government, by CREST, by Tokuyama Science Foundation and by Waseda University Grant for Special Research Projects (2005B-073, 2005B-074).
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Preparation of hexagonal platy particles of nanoporous silica using hydrotalcite as morphology template Naoki Shimura a , Makoto Ogawa a,b,∗ a Graduate School of Science and Engineering, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan b Department of Earth Sciences, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan
Received 30 September 2006; accepted 28 February 2007 Available online 15 March 2007
Abstract Hexagonal platy composite particles with a hydrotalcite core and a nanoporous silica shell with a thickness of ca. 100 nm were synthesized by the reaction of a Mg–Al hydrotalcite with a homogeneous aqueous solution containing tetraethoxysilane, hexadecyltrimethylammonium chloride, ammonia and methanol at 3 ◦ C. The calcination of the products at 500 ◦ C in air led to the composite particle with a Mg/Al mixed oxide core and a nanoporous silica shell. Hexagonal platy particles of nanoporous silica with a pore diameter of 2.3 nm and BET surface area of 700 m2 (g of silica)−1 were obtained by removing the Mg/Al mixed oxide core. © 2007 Elsevier Inc. All rights reserved. Keywords: Nanoporous silicas; Core shell particles; Hydrotalcite; Supramolecular templating approach
1. Introduction Morphosyntheses of porous silicas have been investigated for practical applications as well as for basic scientific interests. Sol–gel process is a promising way to design the morphology of different materials, and films and fibers of porous silica have been synthesized so far. It has also been reported the precipitation of nanoporous silicas with such morphologies as spherical particles and films from homogeneous solutions [1,2]. Hybrid particles composed of a silica shell on a variety of particles is another class of morphology controlled silicas. Hybrid particles composed of calcium carbonate with silica shell were successfully transformed to hollow mesoporous silica particles retaining the original morphology of the core calcium carbonate particle [3]. Thus, the deposition of silica layer on various particles is worth investigating for the preparation of hybrid core shell particles as well as hollow silica particles [4]. The formation of surfactant templated mesoporous silica layers on calcium carbonate, latex and iron oxides are successful ex-
* Corresponding author. Fax: +81 3 3207 4950.
E-mail address: [email protected] (M. Ogawa). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.02.091
amples [3,5,6]. It seems possible to use various particles as the morphology template. Hydrotalcite is a Mg–Al-layered double hydroxide (LDH, 3+ n− general formula of M2+ 1−x Mx (OH)2 (A )x/n ·mH2 O) consisting of positively charged brucite-like layers and exchangeable interlayer anions [7–10]. Hydrotalcite and its analogous compounds are expected as catalysts and adsorbents. There are several reports on the preparation of nonporous silica coated hydrotalcite [11–14]. In the present study, the coating of hydrotalcite with nanoporous silica layers was investigated. Hydrotalcite particle surface modification with nanoporous silica layers can be a way to impart additional functions such as molecular sieving. Since hydrotalcite is easily dissolved in acidic solution, it is a possible morphology template candidate for the preparation of hollow platy particles. Recent progress in the morphosyntheses of hydrotalcite motivates us to investigate the deposition of silica layer on hydrotalcite particle [15–19]. Very recently, we have reported a novel approach to deposit thin mesostructured silica–surfactant layers on hydrotalcite from basic solutions containing tetraethoxysilane, alkyltrimethylammonium chloride, methanol and ammonia [20]. This paper reports the detailed experimental procedure for the preparation and characterization of hexagonal platy particles of nanoporous silica
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using hydrotalcite particles as morphology template. The reconstruction of the hydrotalcite from the calcined silica–Mg/Al mixed oxide product was also investigated.
After the leaching, the product was washed with deionized water and dried at 60 ◦ C in a Teflon-vessel for a day. The washed product was named as L-C-C-HT.
2. Experimental
2.3. Characterization
2.1. Materials
Scanning electron micrographs (SEM) were obtained on a Hitachi S-2380N scanning electron microscope. Prior to SEM measurements, samples were coated with gold. X-ray diffraction (XRD) was performed on a RAD IB diffractometer (Rigaku) using monochromatic CuKα radiation, operated at 40 kV and 20 mA. TG–DTA curves were recorded on a Rigaku TG-8120 instrument at a heating rate of 10 ◦ C min−1 and using α-alumina as the standard material. Nitrogen adsorption isotherms were measured at −196 ◦ C on a Belsorp TCV (BEL Japan Inc.). Prior to adsorption measurements, samples were dried at 120 ◦ C under vacuum for 3 h. Infrared (IR) spectra of the samples were recorded on a Shimadzu FT-IR 8200PC Fourier transform infrared spectrophotometer by the KBr disk method. CHN analysis was performed on a Perkin Elmer 2400 II instrument. Mg2+ and Al3+ contents in C-HT were determined from the leached amount of Mg2+ and Al3+ from C-CHT by ICP using a Rigaku SPECTRO CIROS CCD. Characterization data of the products (C-HT, C-C-HT, and L-C-C-HT) was obtained from two samples prepared by same procedure: One sample was used for XRD except for L-C-C-HT, SEM, and FT-IR spectra. CHN analysis, ICP, TG–DTA curves, XRD patterns of L-C-C-HT, and nitrogen adsorption isotherms were obtained from another sample.
Urea was obtained from Wako Pure Chemical Industries, Ltd. Magnesium chloride hexahydrate, aluminum chloride hexahydrate, methanol, hydrochloric acid, and 28% aqueous ammonia solution were obtained from Kanto Chemical Co., Inc. Tetraethoxysilane (abbreviated as TEOS) and hexadecyltrimethylammonium chloride (abbreviated as CTAC) were obtained from Tokyo Kasei Kogyo Co. Ltd. All the chemicals were used without further purification. 2.2. Sample preparation Hydrotalcite (HT, Mg0.67 Al0.33 (OH)2 (CO2− 3 )0.19 ·mH2 O, which was derived from inductively coupled plasma emission spectroscopy (ICP; Mg: 20.7 mass% and Al: 11.0 mass%), Thermogravimetric–differential thermal analysis (TG–DTA), and CHN analysis (C: 2.7 mass% and N: 0.0 mass%) was prepared by the reported method [15], where 0.03 M magnesium chloride aqueous solution (40 mL), 0.03 M aluminum chloride aqueous solution (10 mL), and 0.3 M urea aqueous solution (10 mL) were mixed (molar Mg:Al:urea ratio was 4:1:10) and the mixture was allowed to react in a Teflon-lined container (100 mL, Taiatsu Glass Ind. Co.) at 150 ◦ C for 24 h. After aging at room temperature for a day, the precipitate was collected by centrifugation (3500 rpm, 3 min) followed by washing with deionized water. Product was dried at 60 ◦ C for a day. HT (40.0 mg) was mixed with an aqueous mixture of CTAC (0.106 g), deionized water (8.9 g), methanol (100 mL) and 28% aqueous ammonia solution (3.6 g) and the mixture was aged at 3 ◦ C for a day. To this latter solution TEOS (0.184 mL) was added, and the mixture was shaken for 3 s. The mixture was subsequently aged at 3 ◦ C for 2.0, 2.5 and 3 h (this time was named as reaction time). In order to avoid sedimentation of the HT during the reaction, the mixture was shaken for 3 s every 30 min. The molar ratio of TEOS:CTAC: water:methanol:ammonia was 1:0.4:774:3000:72. After the aging, products were separated by centrifugation (3500 rpm, 3 min). The product was washed with methanol and dried at 60 ◦ C for a day. The sample prepared with CTAC (reaction time = 2.5 h) was named as C-HT. In order to remove the surfactant, the C-HT was calcined in air at 500 ◦ C for 1 h (heating rate of 150 ◦ C h−1 ). The calcined particle was designated as C-C-HT. The reconstruction of the HT was examined by soaking the C-C-HT in deionized water at room temperature for 14 days. The sample was named as RC-C-HT. C-C-HT was washed with aqueous hydrochloric acid (pH 1.0) at room temperature for 24 h in order to remove the core particles composed of mixed magnesium and aluminum oxides.
3. Results and discussion Fig. 1 shows the XRD patterns and the SEM images of the products. The preparation of hexagonal platy particles of hydrotalcite was confirmed by XRD (Fig. 1b; d values of (003), (006), (101), (009), (012) and (015) were 0.76, 0.38, 0.26, 0.25, 0.23 and 0.19 nm, respectively), TG–DTA (Fig. 2a) and SEM microscopy (Fig. 1a). After reaction of the hydrotalcite with the solution for 2.5 h, the sample weight increased by ca. 20%, suggesting the deposition of silica on the HT particles. Fig. 1c shows the SEM image of the coated particle (C-HT), where hexagonal plates similar to the original hydrotalcite were observed. The thickness of the platy particles (C-HT) was 300– 500 nm, which was larger than that of the HT (100–300 nm) used. The surface of the C-HT particle was smooth and homogeneous, and there are no cracks. In the infrared spectrum of C-HT (data not shown) [20], absorption bands due to Si–O stretching of silica (at around 1180 and 1080 cm−1 ) [21] and Si–O stretching of silanol groups (at around 960 cm−1 ) [21– 23], in addition to those of HT (3450, 3020, 1350, 870, 780, 680 and 560 cm−1 ) have been observed. In the XRD patterns (Figs. 1b and 1d), all the diffraction peaks did not change during the reaction, suggesting that hydrolyzed TEOS was not intercalated into the interlayer space of HT [24]. It is difficult to think that Cl− was intercalated due to the size of Cl− at the present condition [25,26]. Thus, it was shown the successful coating of HT with a thin layer of the silica–CTA+ hybrid.
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Fig. 1. SEM images (a, c, e and g) and XRD patterns (b, d, f and h) of HT (a and b), C-HT (c and d), C-C-HT (e and f) and L-C-C-HT (g and h). (Reaction time = 2.5 h.)
Fig. 2. TG–DTA curves of (a) HT and (b) C-HT prepared for 2.5 h.
The TG–DTA curves of HT and C-HT are shown in Fig. 2. Exothermic peaks accompanying the weight losses in the corresponding TG curve were observed at around 150–500 ◦ C for the DTA curve of HT (Fig. 2a). The observed thermal behavior is typical of hydrotalcite [15,27–29]. In the DTA curve of C-HT (Fig. 2b), exothermic reactions, probably due to the oxidative decomposition of CTA+ , were observed at around 250– 350 ◦ C [30] in addition to those characteristic of HT. The chemical composition of C-HT was derived from the CHN elemental analysis (C: 2.9 mass% and N: 0.0 mass%), TG–DTA and ICP (Mg: 16.6 mass% and Al: 9.0 mass%), assuming that the chemical composition of the hydrotalcite in C-HT was the same to that of HT. Additionally, C/Al molar ratios were increased from 0.56 to 0.72 (CHN analysis and ICP), indicating that the deposited layers contained CTA+ . The amount of SiO2 was derived from the TG–DTA results (Fig. 2): The weight of the sample heated at 800 ◦ C was ascribed to those of SiO2 and calcined HT. Because the weight of HT was decreased by ca. 40 mass% (Fig. 2a), the weight of HT in C-HT at 800 ◦ C was estimated as ca. 47 mass% of the original sample weight. Thus, the remaining weight (ca. 16 mass%) was ascribed to the weight of SiO2
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Fig. 3. FT-IR spectra of the C-HT prepared for 2.0 (red line) and 2.5 h (black line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. SEM image of the as-synthesized product prepared for 3.0 h.
in C-HT. As a result, the chemical composition of C-HT was 2− CTA+ 0.003 –(SiO2 )0.27 –Mg0.67 Al0.33 (OH)2 (CO3 )0.19 ·mH2 O. When the synthesis was conducted in the absence of HT, spherical particles are formed [31]. The particle size increased at longer aging times [32]. Though the present system was heterogeneous nucleation, it was thought that a continuous CTA+ – SiO2 layer formed, and the layer became thicker as the reaction time after the addition of TEOS was longer. FT-IR and TG– DTA suggested that the as-coated products prepared by the aging for 2.0 h were CTA+ –SiO2 –HT composites. The FT-IR spectra indicated that the deposition of silica increased as the aging time was longer (Fig. 3). This result suggested that the silica layers of the product prepared by aging for 2.0 h were thinner than the ones by aging for 2.5 h. Further aging (over 3 h) resulted in the formation of spherical particles by homogeneous nucleation, as shown by the SEM image given in Fig. 4. In our separate study on the preparation of nanoporous silica spherical particles from solutions similar to those used in the present study [32], it was shown that CTA+ played a role as porogen. Accordingly, we expect that the removal of CTA+ will lead to the formation of nanoporous silica. Judging from the TG–DTA results (Fig. 2b), C-HT was calcined in air at 500 ◦ C to decompose CTA+ . The SEM image and the XRD pattern of
Fig. 5. Nitrogen adsorption isotherms of Mg–Al mixed oxide (diamond), C-C-HT (square), and L-C-C-HT (circle). (Reaction time = 2.5 h.)
C-C-HT are shown in Figs. 1e and 1f. The platy morphology did not change by calcination, as shown in Fig. 1e. The SEM image showed no cracks on the particle surface. The XRD pattern of C-C-HT (Fig. 1f) showed two broad diffraction peaks at 0.21 and 0.15 nm ascribable to (400) and (440) reflections of a mixed oxide phase of magnesium and aluminum with the MgO type structure, which formed by the thermal decomposition of HT [33,34]. All the infrared absorption bands were ascribable to those of silica [21] and Mg–Al mixed oxides which contained remaining carbonate (1400, 860 and 650 cm−1 ) [28,35, 36]. Thus, C-C-HT consisted of silica and mixed oxides of magnesium and aluminum. The nitrogen adsorption isotherm of C-C-HT is shown in Fig. 5 (square). The BJH pore size distribution [37] is shown in Fig. 6 (square). The BET surface area [38] and the pore diameter [37] of C-C-HT were 140 m2 g−1 and 2.5 nm, respectively. Though it was reported that the calcination of hydrotalcite at 300–600 ◦ C led to porous materials [7,39], the nitrogen adsorption isotherm indicated that the present mixed oxide of Mg–Al made from HT was not porous (Fig. 5, diamond). It was shown that the shell became porous upon CTA+ removal. The amount of silica in the product increased from ca. 16 mass% (calculated from chemical composition of C-HT) to ca. 24 mass% by calcination because the weight of the hydrotalcite as core decreased (ca. 40 mass%) (TG–DTA curves; Fig. 2a) and CTA+ was removed. Considering the composition of C-C-HT, the BET surface area of silica layer was ca. 580 m2 (g of silica)−1 , which is a value comparable to surfactant templated nanoporous silicas. It is well known that the XRD patterns of surfactant templated nanoporous silicas with highly ordered nanostructures show diffraction peaks due to nanostructures [40]. The XRD patterns did not show peaks ascribable to ordered nanostructures as shown in Figs. 1d and 1f. These results suggested that the nanostructure of the C-HT shell was disordered, which was similar to that of the spherical silica particles synthesized under reaction conditions similar to those of the present study [31].
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It is known that calcined hydrotalcites (MgO type mixed oxides) uptake various anions in water to reform layered structures [7–9]. The present calcined particle reconstructed the HT structure by soaking in water. The diffraction peaks ascribable to HT appeared in the XRD pattern of R-C-C-HT (Fig. 7c). The FT-IR spectrum of R-C-C-HT showed the absorption band (1400 cm−1 ) ascribable to CO2− 3 [28] (Fig. 7d). Thus, C-C-HT 2− adsorbed CO3 to reconstruct the HT structure. The SEM images showed hexagonal platy particles with small fans (Figs. 7a and 7b), suggesting that magnesium and aluminum were dissolved into the water (pH = ca. 6), and hydrotalcite was de-
Fig. 6. Pore size distributions of C-C-HT (square) and L-C-C-HT (circle). (Reaction time = 2.5 h.)
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posited on the hexagonal platy particles of the nanoporous silica. C-C-HT is a kind of core–shell particle, where mixed oxides of Mg and Al (core) were covered with a nanoporous silica layer. By removing the core, hollow particles of nanoporous silica are formed. It was reported that needle like calcium carbonate particles with the size of 3–5 µm were used as template to form nanoporous silica tubes [3]. Such hollow silica particles may find applications in the area of drug delivery and adsorbents. The variation of the thickness, shape and size of those hollow particles are worth investigating to find practical applications. Silica is not soluble to acidic solution while the mixed oxide is soluble, so that the core was easily removed from the hybrid particle by leaching in acidic solution. Fig. 1g shows the scanning electron micrograph of the product (L-C-C-HT) washed with hydrochloric acid. The platy morphology was retained even after the washing. SEM images of broken particles showed that the particles were hollow as shown in Fig. 8. The cross-sectional view of L-C-C-HT revealed that the thickness of the shell was ca. 100 nm. All the absorption bands observed
Fig. 8. SEM image of broken L-C-C-HT. (Reaction time = 2.5 h.)
Fig. 7. SEM images (a and b), XRD pattern (c) and FT-IR spectrum (d) of R-C-C-HT. (Reaction time = 2.5 h.)
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in the IR spectrum of L-C-C-HT (data not shown) [20] were ascribed to silicas [21–23]. The diffraction peaks ascribable to the mixed oxide of Mg and Al were not observed in the XRD pattern (Fig. 1h). Thus, the mixed oxide of Mg and Al was successfully removed from the calcined product. The nitrogen adsorption isotherm of L-C-C-HT is shown in Fig. 5 (circle). The BET surface area [38] and the BJH pore size [37] (Fig. 6, circle) were 700 m2 g−1 and 2.3 nm, respectively. The pore diameter was slightly decreased if compared with those of C-C-HT (2.5 nm). Thus, hydrotalcite was successfully utilized as a morphology template for the preparation of nanoporous silica. To our knowledge, hexagonal platy silica hollow particles have never been obtained, though hexagonal platy particles have been prepared by using morphology template (protein crystal) [41]. The platy morphology may open up new opportunities for silica applications, for example, in cosmetic products. Since LDHs with various particle size have been prepared previously [7,9,16,17, 19], it is possible to prepare nanoporous silica platy particles with various sizes by utilizing the present procedure. The present successful preparation of silica-coated hydrotalcites and hexagonal platy particles of nanoporous silicas motivate us to deposit nanoporous silica layers on particles with other compositions and morphology. Further studies on the preparation and characterization of nanoporous silica-coated particles and hollow nanoporous silica particles are underway in our laboratory. 4. Conclusions Hexadecyltrimethylammonium–silica nanocomposite layer was deposited on hydrotalcite by reaction in a homogeneous aqueous solution containing tetraethoxysilane, hexadecyltrimethylammonium chloride, methanol and ammonia. The core– shell particle was further converted to composite particles with a nanoporous silica shell and a magnesium aluminum mixed oxide core. XRD patterns and FT-IR spectrum suggested that this particle adsorbed CO2− 3 to reconstruct hydrotalcite structures. Hexagonal hollow platy nanoporous silica particles (particle size of several micrometers) with the pore size of 2.3 nm and a BET surface area of 700 m2 g−1 were obtained by removing the core. Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government, by CREST, by Tokuyama Science Foundation and by Waseda University Grant for Special Research Projects (2005B-073, 2005B-074).
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