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A facile method for preparing thiocyanato-functionalized porous silica nanospheres
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A facile method for preparing thiocyanato-functionalized porous silica nanospheres Jie Li a,b , Lianxi Chen a,∗ , Xi Li a , Chaocan Zhang b , Zhenhui Liu a a b
School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, Wuhan 430070, China School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China
a r t i c l e
i n f o
Article history: Received 7 November 2014 Received in revised form 16 February 2015 Accepted 3 March 2015 Keywords: Thiocyanato Functionalization Silica nanospheres Porous Micellar template
a b s t r a c t In this study, we present a facile method to prepare thiocyanato-functionalized porous silica nanospheres. Thiocyanato functionalized silica shells were coated on positively charged cetyltrimethylammonium bromide (CTAB) micelles via hydrolysis and condensation of (3-thiocyanatopropyl)triethoxysilane (TCPTES), the CTAB cores were removed subsequently to form thiocyanato-functionalized porous silica nanospheres. We demonstrate that the contents of the thiocyanato groups within the functionalized porous silica nanosphere frameworks gradually diminished as a function of hydrothermal treatment time at 100 ◦ C until complete removal, confirmed by TGA and FTIR spectra. The data indicate that extended operation at relatively elevated temperatures may lead to the decomposition of the thiocyanato functional groups. In addition, at a lower CTAB concentration (0.0009 M), non-porous thiocyanato functionalized silica nanospheres were formed. However, increasing the CTAB concentration to 0.01 M resulted in porous nanospheres inferring that a CTAB concentration threshold is needed to form thiocyanato-functionalized porous silica nanospheres. © 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction The design and synthesis of organic/inorganic hybrid silica materials are especially attractive in recent decades, because they combine the versatility of the organic chemistry with the advantage of inorganic species (Meng et al., 2009; Contessotto et al., 2009; Arkhireeva, Hay, & Manzano, 2005; Sharma, Das, & Maitra, 2004). Among the various hybrid silica materials, silica nanospheres with porous structures are particularly noteworthy because of their low density, low toxicity, high biocompatibility, large specific surface areas, and excellent thermal and mechanical stability, and thus have been widely applied in the fields of catalysis, separation/adsorption, biomedicine, and controlled drug release (Deng & Marlow, 2012; Teng, Wang, Li, & Zhang, 2011; Yang, Liu, Li, Liu, & Yang, 2011; Trilla, Cattoen, Blanc, Man, & Pleixats, 2011; Sasidharan et al., 2011). There are two common functionalization strategies that can be adopted when functionalizing porous silica spheres: post-synthetic grafting of functional siloxanes and in situ co-condensation (Burkett, Sims, & Mann, 1996; Lim, Blanford, & Stein, 1997;
∗ Corresponding author. Tel.: +86 027 87756662; fax: +86 027 87758214. E-mail address: [email protected] (L. Chen).
Anwander et al., 2000; Yao et al., 2005; Wei et al., 2005; Choi et al., 2005; Lin et al., 2009; Zarabadi-Poor, Badiei, Fahlman, Arab, & Ziarani, 2011; Han, Chen, Wang, Gao, & Che, 2011; Vathyam et al., 2011; Liu, Yan, Zhang, Yang, & Bai, 2012). In the former method, organic functional groups are directly grafted onto the surface of porous silica materials. It may lead to a nonhomogeneous distribution of the organic groups within the pores and a lower degree of occupation. In extreme cases (e.g., with very bulky grafting species), complete closure of the pores can be caused (pore blocking). Compared with the post-synthetic grafting method, co-condensation synthesis involves the simultaneous condensation of corresponding silica and organosiloxane precursors, which can result in a decrease in the long-range order with increasing (R O)3 SiR concentration in the reaction mixture. For this reason, optimizing control of the added functional siloxane is necessary for facile preparation of organic-functionalized porous silica materials when using one silica precursor only. In this study, we adopted a simple one-step method using (3thiocyanatopropyl)triethoxysilane (TCPTES) as the sole precursor and the cationic surface active agent, cetyltrimethylammonium bromide (CTAB), as a template to synthesize thiocyanatofunctionalized porous silica nanospheres under alkaline conditions. The porous structure and the size of the spherical particles can be tailored as a function of hydrothermal treatment time and CTAB
http://dx.doi.org/10.1016/j.partic.2015.03.002 1674-2001/© 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Li, J., et al. A facile method for preparing thiocyanato-functionalized porous silica nanospheres. Particuology (2015), http://dx.doi.org/10.1016/j.partic.2015.03.002
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Scheme 1. The schematic diagram of the formation mechanism of thiocyanato-functionalized porous silica nanospheres.
concentration. It is well known that surfactant molecules form micelles of uniform size (typically in the range from several to several tens of nanometers) and well-defined shape, e.g., spherical or lamellar (Mandal & Kruk, 2011). The micelles can further aggregate forming lyotropic liquid crystal structures, such as cylinders or spheres. The reports of a new surfactant-templating mechanism in the 1990s led to a new family of porous silicas, referred to as mesoporous silicas, with excellent long-range ordering (Beck et al., 1992; Kresge, Leonowicz, Roth, Vartuli, & Beck, 1992; Inagaki, Fukushima, & Kuroda, 1993; Zhao, Huo, Feng, Chmelka, & Stucky, 1998). Extending the one-step method of utilizing a sole functional siloxane – in this case TCPTES – in the absence of a silica source to generate alternative functionalized porous silica nanospheres may provide a general method to allow the direct synthesis of tailored functionality to the nanospheres. To the best of our knowledge, the current work on the one-step synthesis of organic-functionalized porous silica nanospheres can effectively avoid the drawbacks
of the above-mentioned post-modification and co-condensation strategies. Beyond that, the key point of this method lies in that the organic functional groups exist in the entire silica network including the inner core and outer shell. Furthermore, compared with the former two methods, the one-step method offers a higher and more uniform surface coverage of functional groups and better control of the surface properties of the resultant materials.
Experimental Reagents and materials TCPTES was purchased from Aladdin (purity 98%, Shanghai, China). Ethanol (95%), HCl (38%), and NH3 ·H2 O solutions (28 wt%), CTAB (99%) were all purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Water (resistivity:
Fig. 1. (a) Optical photograph and (b) TEM micrograph of thiocyanato-functionalized porous silica nanospheres with 0-h hydrothermal treatment; (c) the nitrogen adsorption–desorption isotherm and (d) corresponding pore size distribution curve. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. FTIR spectra of (a) TCPTES and (b) thiocyanato-functionalized porous silica nanospheres.
18.25 M cm) used to prepare standard solutions was obtained from a Milli-Q System (Millipore, Bedford, MA, USA). Synthesis In a typical synthesis procedure, the desired amount of CTAB (0.0009, 0.005, and 0.01 M) was dissolved in 50 mL of water at 30 ◦ C under vigorous stirring. After the surfactant was fully dissolved, 1 mL NH3 ·H2 O and 2 mL of TCPTES were sequentially added drop wise into the solution under stirring. The resultant mixture was maintained at the same temperature and continued stirring for an additional 24 h. The molar ratio of the sol–gel was TCPTES/CTAB/NH3 ·H2 O/H2 O = 1:(0.006–0.07):7:385. Finally, a yellow precipitate from the sol–gel was observed after the hydrolysis was complete. The solution along with the yellow precipitate prepared was then transferred to an autoclave and subjected to hydrothermal treatment at 100 ◦ C under static conditions for different times (0, 12, 24, and 48 h). After the desired condensation time, a yellow solid was recovered via filtration, washed several times with water and ethanol, and air-dried at 50 ◦ C. The solid products were collected and then acid extracted (HCl) by refluxing in ethanol for 12 h to completely remove the surfactant. The materials were filtered, washed with water and then air dried at 50 ◦ C to obtain the final surfactant-free products. The final yellow solid product was denoted as thiocyanato-functionalized porous silica nanospheres.
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Fig. 3. FTIR spectra of thiocyanato-functionalized porous silica nanospheres synthesized with different hydrothermal treatment times: (a) 0 h, (b) 12 h, (c) 24 h, and (d) 48 h.
to determine the organic content of the samples. Measurements were conducted in air from room temperature to 800 ◦ C at a scan rate of 25 ◦ C/min. Results and discussion Synthesis of thiocyanato-functionalized porous silica nanospheres In this study, a fundamental condition for the synthesis of thiocyanato-functionalized porous silica nanospheres is that an attractive interaction between the template and the silica precursor is produced to ensure inclusion of the structure directing agent (SDA) without phase separation taking place. According to the suggestion of Huo et al. (1994a, 1994b), if the reaction takes place under basic conditions (whereby the silica species are present as anions) and cationic quaternary ammonium surfactants are used as the SDA, the synthetic pathway is termed S+ I− (S: surfactant; I: inorganic species). In this study, thiocyanato-functionalized porous silica nanospheres were obtained using this synthetic strategy, their typical synthesis is schematically illustrated in Scheme 1. A thiocyanato functionalized silica shell was directly formed on
Characterization Nitrogen sorption measurements were performed at 77 K on an ASAP 2020 system (Micromeritics, USA). The Brunauer–Emmett–Teller (BET) model was utilized to calculate the specific surface area at relative pressures of P/Po = 0.05–0.15. The pore volume and pore size distribution were derived from the desorption or adsorption branches of the isotherms using the Barrett–Joyner–Halenda (BJH) model. The total pore volume was estimated from the uptake of adsorbate at a relative pressure of P/Po = 0.99. Transmission electron microscopy (TEM, JEM-2100F STEM/EDS, JEOL, Japan) was used to observe particle morphology and measuring the geometric parameters of the particles. Fourier transform infrared (FTIR) spectra were collected with a Nexus 470 IR spectrometer (Nicolet, USA) with KBr pellets. Thermogravimetric analysis on a TGA-7 instrument (Perkin–Elmer, USA) was used
Fig. 4. TGA analysis of thiocyanato-functionalized porous silica nanospheres synthesized with different hydrothermal treatment times.
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Table 1 Textural properties of the functionalized porous nanospheres subjected to different hydrothermal treatment times at 100 ◦ C. No.
CTAB concentration (M)
Hydrothermal treatment time (h)
a Average spherical particle size (nm)
b
BET specific surface area (m2 /g)
c Total pore volume (cm3 /g)
1 2 3
0.01 0.01 0.01
0 12 48
23 22 16
13 1.1 345
0.017 0.004 0.46
a b c
Measured by transmission electron microscopy. BET specific surface area calculated within the relative pressure range of P/Po = 0.05–0.15. Total pore volume measured at a relative pressure of P/Po = 0.99.
the surface of each CTAB micelle by the hydrolysis and condensation of TCPTES with the aid of an ammonia catalyst, forming a core–shell structure, denoted as CTAB/thiocyanato functionalized silica composite. Finally, the cationic surfactant cores composed of CTAB were extracted completely using an acid/ethanol mixture to yield thiocyanato-functionalized porous silica nanospheres. This proposed approach is highly reproducible and scalable. Grams of product can be easily prepared in this facile one-step reaction after the template removal process. As indicated in Fig. 1(a), the photograph shows the filter paper containing approximately 1 g of thiocyanato-functionalized porous silica nanospheres synthesized without any hydrothermal treatment. It is clearly observed that the final product has a light yellow color, in contrast to the white porous silica nanospheres reported previously. This may be related to the thiocyanato group residing within the framework of the porous silica nanospheres. In addition, the corresponding TEM micrograph is shown in Fig. 1(b). It is found that the
thiocyanato-functionalized porous silica nanospheres are bonded together as a result of the poor dispersion. Furthermore, the average diameter of each sphere is around 23 nm calculated from the TEM micrograph. Fig. 1(c) and (d) presents the nitrogen adsorption–desorption isotherm and corresponding pore size distribution, respectively, of the thiocyanato-functionalized porous silica nanospheres subjected to 0-h hydrothermal treatment. The desorption isotherm showed a capillary condensation step appearing at a relative pressure range of 0.7–0.95 P/Po corresponding to a broad pore size distribution centering at 10.8 nm as calculated by the BJH method. The BET specific surface area and pore volume were calculated to be 13 m2 /g and 0.017 cm3 /g, respectively. The textural properties of the functionalized porous nanospheres are shown in Table 1. The presence of thiocyanato group in thiocyanatofunctionalized porous silica nanospheres can be directly proved by FTIR spectra (Fig. 2). In the TCPTES spectrum, the band at
Fig. 5. TEM micrographs of thiocyanato-functionalized porous silica nanospheres synthesized with different hydrothermal times: (a) 0 h, (b) 12 h, (c) 24 h, and (d) 48 h.
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Fig. 6. N2 adsorption ()–desorption (䊉) isotherms curves (left column) and pore size distributions (right column) of thiocyanato-functionalized porous silica nanospheres obtained at different hydrothermal treatment times: (a)–(b) 0 h, (c)–(d) 12 h, and (e)–(f) 48 h.
1004 cm−1 relates to the Si O C2 H5 bond (Fig. 2(a)), which diminishes in the spectrum of the thiocyanato-functionalized porous silica nanospheres (Fig. 2(b)). This is the result of the thiocyanto siloxane undergoing hydrolysis and condensation during the reaction. Similarly, the bands arising from Si O Si bonds appear within the range of 1139–1037 cm−1 in the spectrum of thiocyanato-functionalized porous silica nanospheres. These results show that the ethoxy groups of TCPTES underwent hydrolysis under alkaline conditions to form silanol groups that later dehydrated and condensed to yield the siloxane networks. Additionally, the bands attributed to Si C at 1347 and 1309 cm−1 , C3 H6 S at 1255 cm−1 , and SCN at 2151 cm−1 are present in the spectra of TCPTES and thiocyanato-functionalized porous silica nanospheres (Li et al., 2011; Fan & Sun, 2012). The FTIR results confirm that the thiocyanato groups have been incorporated, by covalent bonds, into the thiocyanato-functionalized porous silica nanospheres.
Effect of the hydrothermal treatment time Recent reports indicate that hydrothermal treatment is an important parameter in the resulting structures and morphologies of mesoporous materials. To further investigate the mesostructure of the final sample, the influence of varying hydrothermal treatment times on the formation of thiocyanato-functionalized porous silica nanospheres has been studied in detail. FTIR spectra of the samples having different hydrothermal treatment times for 0, 12, 24, and 48 h are shown in Fig. 3. The data show that the bands assigned to Si C at 1342 and 1307 cm−1 , C3 H6 S at 1259 cm−1 , and SCN at 2151 cm−1 are present in the spectra of all the products. However, the peak intensity of thiocyanato groups within thiocyanato-functionalized porous silica nanospheres frameworks gradually reduces, to the point of almost zero intensity, with increasing hydrothermal treatment times from 0 to 48 h at 100 ◦ C. The data infer that extended hydrothermal treatment at elevated
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Fig. 7. TEM images of thiocyanato-functionalized porous silica nanospheres synthesized with different CTAB concentrations: (a) 0.0009 M, (b) 0.005 M, (c) and (d) 0.01 M, all before hydrothermal treatment.
temperatures can easily lead to the decomposition of the thiocyanato groups within thiocyanato-functionalized porous silica nanospheres. TGA measurements on surfactant extracted samples having different hydrothermal treatment times of 0, 12, 24 and 48 h are displayed in Fig. 4. The decreasing mass loss as a function of increasing hydrothermal treatment time is complementary to the FTIR data. It can be seen that the sample having no hydrothermal treatment provides the greatest organic mass loss because there has been more of the functional siloxane incorporated into the porous nanosphere. As hydrothermal treatment increases, the mass loss decreases as the functionality decomposes and is no longer incorporated within the framework of the material corresponding to the diminishing 2151 cm−1 peak from FTIR. In addition, the corresponding TEM micrographs of thiocyanato-functionalized porous silica nanospheres are shown in Fig. 5. It was found that particle dispersion improved with increasing hydrothermal treatment time. The mean diameter of thiocyanato-functionalized porous silica nanospheres gradually decreased from 23 to 16 nm (Table 1). With the decomposition of the functional thiocyanto groups increasing with hydrothermal treatment time, this is effectively a decomposition of the main framework as the organosiloxane was the sole source of silica. It can therefore be expected that if the structural integrity of the main framework is compromised, then the degree of necking between particles will reduce improving the dispersion of individual particles. Furthermore, the hybrid organic/inorganic shell would also be expected to decrease in dimension resulting in smaller particle sizes. This is confirmed by TEM analysis.
To better understand the resulting textural properties of the materials, Fig. 6 shows the representative nitrogen adsorption–desorption isotherms and corresponding pore size distributions of samples synthesized under the different hydrothermal treatment times. Materials subjected to hydrothermal treatment time of 12 h resulted in non-porous materials with a BET specific surface area and pore volume calculated to be 1.1 m2 /g and 0.004 cm3 /g, respectively (Fig. 6(c) and (d)). The surface area of the products is reduced when compared with materials synthesized with hydrothermal treatment times of 0 and 48 h (Table 1). The rationale behind this is that for a hydrothermal treatment time of 12 h, at the elevated temperature, this leads to decomposition, in part, of the thiocyanato groups within the thiocyanato-functionalized porous silica nanospheres. The structural integrity of the framework, in this material, may be damaged because of Si C or C SCN bond cleavage. In contrast, when hydrothermal treatment time increased to 48 h, the nitrogen adsorption–desorption isotherm shows a type iv isotherm with a hysteresis loop at the relative pressures P/Po = 0.9–0.5 (Fig. 6(e)). Broad capillary condensation at the higher relative pressures corresponds to the mesoporous nature of the material, Fig. 6(f), resulting in a broad pore size distribution curve, as calculated by the BJH method. The BET specific surface area and pore volume were calculated to be 345 m2 /g and 0.46 cm3 /g, respectively. FTIR and TGA data show that the contents of the thiocyanato groups within thiocyanato-functionalized porous silica nanospheres significantly reduce as a function of increasing hydrothermal
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treatment time, particularly at 48 h. We propose that at this extended hydrothermal time, the silica framework may ‘self-heal’ to form a new silica network structure. A significant portion of decomposed thiocyanato groups will egress from the pores resulting in increased specific surface area and pore volume.
Effect of concentration of CTAB Based on the current synthesis, the formation of cationic surfactant CTAB micelles in a pure water solution plays an essential role in preparing thiocyanato-functionalized porous silica nanospheres. Herein, the influence of CTAB concentration on thiocyanato-functionalized porous silica nanosphere formation without hydrothermal treatment has been investigated. Fig. 7 shows the TEM micrographs of different concentrations of CTAB. Previous reports have shown that the critical micellar concentration (CMC) of CTAB in a pure water system is approximately 0.0009 M (Li, Han, Zhang, Wang, & Song, 2006; Bielawska, ´ ´ Janczuk, & Zdziennicka, 2013). From Fig. 7(a), it is Chodzinska, clearly observed that solid spheres with diameters of 119 nm were obtained as the CTAB concentration was at the CMC value. Increasing the CTAB concentration to 0.005 M, irregular spherical particles with a porous shell were formed and the calculated average diameter was approx. 59 nm (Fig. 7(b)). At this point, the system is believed to be at the transition between a nonporous nanosphere to a porous nanosphere. However, completely porous silica nanospheres with diameters of 23 nm were generated as the CTAB concentration was increased to 0.01 M (Fig. 7(c)), inferring that the generation of porous structures is closely related to the CTAB concentration. At lower CTAB concentrations, the surfactant molecules exist as individual entities or as small clusters within a system where the concentration not being sufficient. The interaction between the TCPTES molecules and CTAB molecules is less frequent, resulting in the formation of non-porous thiocyanato-functionalized silica nanospheres. In contrast, the CTAB micelles formed at a higher CTAB concentration will interact more frequently with TCPTES. TCPTES molecules and its oligomer will first interact with the surface of the CTAB micelles via an electrostatic interaction, and the CTAB/thiocyanato-functionalized porous silica nanocomposites will be formed by further hydrolysis and condensation. Consequently, thiocyanato-functionalized porous silica nanospheres can be obtained after extraction of the CTAB cores. In addition, excess CTAB molecules may hinder the transport of the TCPTES molecules to the micelles resulting in a smaller shell and hence smaller particles.
Conclusions In this paper, a facile sol–gel process method for the preparing thiocyanato-functionalized porous silica nanospheres based on positively charged CTAB micelles acting as a template has been developed. Thiocyanato-functionalized porous silica nanospheres with spherical particle sizes of less than 23 nm have been successfully prepared by cooperative assembly between positively charged CTAB cationic surfactants and negatively charged silica particles under alkaline conditions. Additionally, the porous structures and morphology of the samples can be easily controlled as a function of hydrothermal treatment time and CTAB concentration. The synthesis approach adopted in this study can likely be extended as a general approach to synthesize various organic functionalized hollow nanostructures by the use of adequate structure directing agents as templates.
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Acknowledgements We are grateful for the financial support of the Nature Science Foundation of Hubei Province (No. 2014CFB862) and State Key Laboratory of Natural and Biomimetic Drugs (No. K20140214). This work was partially supported by the National Natural Science Foundation of China (No. 51273155). This work was also financially supported by the Guangdong Well-SilicaSol Co., LTD, China. References Anwander, R., Nagl, I., Widenmeyer, M., Engelhardt, G., Groeger, O., Palm, C., et al. (2000). Surface characterization and functionalization of MCM-41 silicas via silazane silylation. The Journal of Physical Chemistry B, 104, 3532–3544. Arkhireeva, A., Hay, J. N., & Manzano, M. (2005). Preparation of silsesquioxane particles via a nonhydrolytic sol–gel route. Chemistry of Materials, 17, 875–880. Beck, J. S., Vartuli, J. C., Roth, W. J., Leonowicz, M. E., Kresge, C. T., Schmitt, C. T., et al. (1992). A new family of mesoporous molecular sieves prepared with liquid crystal templates. 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A facile method for preparing thiocyanato-functionalized porous silica nanospheres Jie Li a,b , Lianxi Chen a,∗ , Xi Li a , Chaocan Zhang b , Zhenhui Liu a a b
School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, Wuhan 430070, China School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China
a r t i c l e
i n f o
Article history: Received 7 November 2014 Received in revised form 16 February 2015 Accepted 3 March 2015 Keywords: Thiocyanato Functionalization Silica nanospheres Porous Micellar template
a b s t r a c t In this study, we present a facile method to prepare thiocyanato-functionalized porous silica nanospheres. Thiocyanato functionalized silica shells were coated on positively charged cetyltrimethylammonium bromide (CTAB) micelles via hydrolysis and condensation of (3-thiocyanatopropyl)triethoxysilane (TCPTES), the CTAB cores were removed subsequently to form thiocyanato-functionalized porous silica nanospheres. We demonstrate that the contents of the thiocyanato groups within the functionalized porous silica nanosphere frameworks gradually diminished as a function of hydrothermal treatment time at 100 ◦ C until complete removal, confirmed by TGA and FTIR spectra. The data indicate that extended operation at relatively elevated temperatures may lead to the decomposition of the thiocyanato functional groups. In addition, at a lower CTAB concentration (0.0009 M), non-porous thiocyanato functionalized silica nanospheres were formed. However, increasing the CTAB concentration to 0.01 M resulted in porous nanospheres inferring that a CTAB concentration threshold is needed to form thiocyanato-functionalized porous silica nanospheres. © 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction The design and synthesis of organic/inorganic hybrid silica materials are especially attractive in recent decades, because they combine the versatility of the organic chemistry with the advantage of inorganic species (Meng et al., 2009; Contessotto et al., 2009; Arkhireeva, Hay, & Manzano, 2005; Sharma, Das, & Maitra, 2004). Among the various hybrid silica materials, silica nanospheres with porous structures are particularly noteworthy because of their low density, low toxicity, high biocompatibility, large specific surface areas, and excellent thermal and mechanical stability, and thus have been widely applied in the fields of catalysis, separation/adsorption, biomedicine, and controlled drug release (Deng & Marlow, 2012; Teng, Wang, Li, & Zhang, 2011; Yang, Liu, Li, Liu, & Yang, 2011; Trilla, Cattoen, Blanc, Man, & Pleixats, 2011; Sasidharan et al., 2011). There are two common functionalization strategies that can be adopted when functionalizing porous silica spheres: post-synthetic grafting of functional siloxanes and in situ co-condensation (Burkett, Sims, & Mann, 1996; Lim, Blanford, & Stein, 1997;
∗ Corresponding author. Tel.: +86 027 87756662; fax: +86 027 87758214. E-mail address: [email protected] (L. Chen).
Anwander et al., 2000; Yao et al., 2005; Wei et al., 2005; Choi et al., 2005; Lin et al., 2009; Zarabadi-Poor, Badiei, Fahlman, Arab, & Ziarani, 2011; Han, Chen, Wang, Gao, & Che, 2011; Vathyam et al., 2011; Liu, Yan, Zhang, Yang, & Bai, 2012). In the former method, organic functional groups are directly grafted onto the surface of porous silica materials. It may lead to a nonhomogeneous distribution of the organic groups within the pores and a lower degree of occupation. In extreme cases (e.g., with very bulky grafting species), complete closure of the pores can be caused (pore blocking). Compared with the post-synthetic grafting method, co-condensation synthesis involves the simultaneous condensation of corresponding silica and organosiloxane precursors, which can result in a decrease in the long-range order with increasing (R O)3 SiR concentration in the reaction mixture. For this reason, optimizing control of the added functional siloxane is necessary for facile preparation of organic-functionalized porous silica materials when using one silica precursor only. In this study, we adopted a simple one-step method using (3thiocyanatopropyl)triethoxysilane (TCPTES) as the sole precursor and the cationic surface active agent, cetyltrimethylammonium bromide (CTAB), as a template to synthesize thiocyanatofunctionalized porous silica nanospheres under alkaline conditions. The porous structure and the size of the spherical particles can be tailored as a function of hydrothermal treatment time and CTAB
http://dx.doi.org/10.1016/j.partic.2015.03.002 1674-2001/© 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
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Scheme 1. The schematic diagram of the formation mechanism of thiocyanato-functionalized porous silica nanospheres.
concentration. It is well known that surfactant molecules form micelles of uniform size (typically in the range from several to several tens of nanometers) and well-defined shape, e.g., spherical or lamellar (Mandal & Kruk, 2011). The micelles can further aggregate forming lyotropic liquid crystal structures, such as cylinders or spheres. The reports of a new surfactant-templating mechanism in the 1990s led to a new family of porous silicas, referred to as mesoporous silicas, with excellent long-range ordering (Beck et al., 1992; Kresge, Leonowicz, Roth, Vartuli, & Beck, 1992; Inagaki, Fukushima, & Kuroda, 1993; Zhao, Huo, Feng, Chmelka, & Stucky, 1998). Extending the one-step method of utilizing a sole functional siloxane – in this case TCPTES – in the absence of a silica source to generate alternative functionalized porous silica nanospheres may provide a general method to allow the direct synthesis of tailored functionality to the nanospheres. To the best of our knowledge, the current work on the one-step synthesis of organic-functionalized porous silica nanospheres can effectively avoid the drawbacks
of the above-mentioned post-modification and co-condensation strategies. Beyond that, the key point of this method lies in that the organic functional groups exist in the entire silica network including the inner core and outer shell. Furthermore, compared with the former two methods, the one-step method offers a higher and more uniform surface coverage of functional groups and better control of the surface properties of the resultant materials.
Experimental Reagents and materials TCPTES was purchased from Aladdin (purity 98%, Shanghai, China). Ethanol (95%), HCl (38%), and NH3 ·H2 O solutions (28 wt%), CTAB (99%) were all purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Water (resistivity:
Fig. 1. (a) Optical photograph and (b) TEM micrograph of thiocyanato-functionalized porous silica nanospheres with 0-h hydrothermal treatment; (c) the nitrogen adsorption–desorption isotherm and (d) corresponding pore size distribution curve. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. FTIR spectra of (a) TCPTES and (b) thiocyanato-functionalized porous silica nanospheres.
18.25 M cm) used to prepare standard solutions was obtained from a Milli-Q System (Millipore, Bedford, MA, USA). Synthesis In a typical synthesis procedure, the desired amount of CTAB (0.0009, 0.005, and 0.01 M) was dissolved in 50 mL of water at 30 ◦ C under vigorous stirring. After the surfactant was fully dissolved, 1 mL NH3 ·H2 O and 2 mL of TCPTES were sequentially added drop wise into the solution under stirring. The resultant mixture was maintained at the same temperature and continued stirring for an additional 24 h. The molar ratio of the sol–gel was TCPTES/CTAB/NH3 ·H2 O/H2 O = 1:(0.006–0.07):7:385. Finally, a yellow precipitate from the sol–gel was observed after the hydrolysis was complete. The solution along with the yellow precipitate prepared was then transferred to an autoclave and subjected to hydrothermal treatment at 100 ◦ C under static conditions for different times (0, 12, 24, and 48 h). After the desired condensation time, a yellow solid was recovered via filtration, washed several times with water and ethanol, and air-dried at 50 ◦ C. The solid products were collected and then acid extracted (HCl) by refluxing in ethanol for 12 h to completely remove the surfactant. The materials were filtered, washed with water and then air dried at 50 ◦ C to obtain the final surfactant-free products. The final yellow solid product was denoted as thiocyanato-functionalized porous silica nanospheres.
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Fig. 3. FTIR spectra of thiocyanato-functionalized porous silica nanospheres synthesized with different hydrothermal treatment times: (a) 0 h, (b) 12 h, (c) 24 h, and (d) 48 h.
to determine the organic content of the samples. Measurements were conducted in air from room temperature to 800 ◦ C at a scan rate of 25 ◦ C/min. Results and discussion Synthesis of thiocyanato-functionalized porous silica nanospheres In this study, a fundamental condition for the synthesis of thiocyanato-functionalized porous silica nanospheres is that an attractive interaction between the template and the silica precursor is produced to ensure inclusion of the structure directing agent (SDA) without phase separation taking place. According to the suggestion of Huo et al. (1994a, 1994b), if the reaction takes place under basic conditions (whereby the silica species are present as anions) and cationic quaternary ammonium surfactants are used as the SDA, the synthetic pathway is termed S+ I− (S: surfactant; I: inorganic species). In this study, thiocyanato-functionalized porous silica nanospheres were obtained using this synthetic strategy, their typical synthesis is schematically illustrated in Scheme 1. A thiocyanato functionalized silica shell was directly formed on
Characterization Nitrogen sorption measurements were performed at 77 K on an ASAP 2020 system (Micromeritics, USA). The Brunauer–Emmett–Teller (BET) model was utilized to calculate the specific surface area at relative pressures of P/Po = 0.05–0.15. The pore volume and pore size distribution were derived from the desorption or adsorption branches of the isotherms using the Barrett–Joyner–Halenda (BJH) model. The total pore volume was estimated from the uptake of adsorbate at a relative pressure of P/Po = 0.99. Transmission electron microscopy (TEM, JEM-2100F STEM/EDS, JEOL, Japan) was used to observe particle morphology and measuring the geometric parameters of the particles. Fourier transform infrared (FTIR) spectra were collected with a Nexus 470 IR spectrometer (Nicolet, USA) with KBr pellets. Thermogravimetric analysis on a TGA-7 instrument (Perkin–Elmer, USA) was used
Fig. 4. TGA analysis of thiocyanato-functionalized porous silica nanospheres synthesized with different hydrothermal treatment times.
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Table 1 Textural properties of the functionalized porous nanospheres subjected to different hydrothermal treatment times at 100 ◦ C. No.
CTAB concentration (M)
Hydrothermal treatment time (h)
a Average spherical particle size (nm)
b
BET specific surface area (m2 /g)
c Total pore volume (cm3 /g)
1 2 3
0.01 0.01 0.01
0 12 48
23 22 16
13 1.1 345
0.017 0.004 0.46
a b c
Measured by transmission electron microscopy. BET specific surface area calculated within the relative pressure range of P/Po = 0.05–0.15. Total pore volume measured at a relative pressure of P/Po = 0.99.
the surface of each CTAB micelle by the hydrolysis and condensation of TCPTES with the aid of an ammonia catalyst, forming a core–shell structure, denoted as CTAB/thiocyanato functionalized silica composite. Finally, the cationic surfactant cores composed of CTAB were extracted completely using an acid/ethanol mixture to yield thiocyanato-functionalized porous silica nanospheres. This proposed approach is highly reproducible and scalable. Grams of product can be easily prepared in this facile one-step reaction after the template removal process. As indicated in Fig. 1(a), the photograph shows the filter paper containing approximately 1 g of thiocyanato-functionalized porous silica nanospheres synthesized without any hydrothermal treatment. It is clearly observed that the final product has a light yellow color, in contrast to the white porous silica nanospheres reported previously. This may be related to the thiocyanato group residing within the framework of the porous silica nanospheres. In addition, the corresponding TEM micrograph is shown in Fig. 1(b). It is found that the
thiocyanato-functionalized porous silica nanospheres are bonded together as a result of the poor dispersion. Furthermore, the average diameter of each sphere is around 23 nm calculated from the TEM micrograph. Fig. 1(c) and (d) presents the nitrogen adsorption–desorption isotherm and corresponding pore size distribution, respectively, of the thiocyanato-functionalized porous silica nanospheres subjected to 0-h hydrothermal treatment. The desorption isotherm showed a capillary condensation step appearing at a relative pressure range of 0.7–0.95 P/Po corresponding to a broad pore size distribution centering at 10.8 nm as calculated by the BJH method. The BET specific surface area and pore volume were calculated to be 13 m2 /g and 0.017 cm3 /g, respectively. The textural properties of the functionalized porous nanospheres are shown in Table 1. The presence of thiocyanato group in thiocyanatofunctionalized porous silica nanospheres can be directly proved by FTIR spectra (Fig. 2). In the TCPTES spectrum, the band at
Fig. 5. TEM micrographs of thiocyanato-functionalized porous silica nanospheres synthesized with different hydrothermal times: (a) 0 h, (b) 12 h, (c) 24 h, and (d) 48 h.
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Fig. 6. N2 adsorption ()–desorption (䊉) isotherms curves (left column) and pore size distributions (right column) of thiocyanato-functionalized porous silica nanospheres obtained at different hydrothermal treatment times: (a)–(b) 0 h, (c)–(d) 12 h, and (e)–(f) 48 h.
1004 cm−1 relates to the Si O C2 H5 bond (Fig. 2(a)), which diminishes in the spectrum of the thiocyanato-functionalized porous silica nanospheres (Fig. 2(b)). This is the result of the thiocyanto siloxane undergoing hydrolysis and condensation during the reaction. Similarly, the bands arising from Si O Si bonds appear within the range of 1139–1037 cm−1 in the spectrum of thiocyanato-functionalized porous silica nanospheres. These results show that the ethoxy groups of TCPTES underwent hydrolysis under alkaline conditions to form silanol groups that later dehydrated and condensed to yield the siloxane networks. Additionally, the bands attributed to Si C at 1347 and 1309 cm−1 , C3 H6 S at 1255 cm−1 , and SCN at 2151 cm−1 are present in the spectra of TCPTES and thiocyanato-functionalized porous silica nanospheres (Li et al., 2011; Fan & Sun, 2012). The FTIR results confirm that the thiocyanato groups have been incorporated, by covalent bonds, into the thiocyanato-functionalized porous silica nanospheres.
Effect of the hydrothermal treatment time Recent reports indicate that hydrothermal treatment is an important parameter in the resulting structures and morphologies of mesoporous materials. To further investigate the mesostructure of the final sample, the influence of varying hydrothermal treatment times on the formation of thiocyanato-functionalized porous silica nanospheres has been studied in detail. FTIR spectra of the samples having different hydrothermal treatment times for 0, 12, 24, and 48 h are shown in Fig. 3. The data show that the bands assigned to Si C at 1342 and 1307 cm−1 , C3 H6 S at 1259 cm−1 , and SCN at 2151 cm−1 are present in the spectra of all the products. However, the peak intensity of thiocyanato groups within thiocyanato-functionalized porous silica nanospheres frameworks gradually reduces, to the point of almost zero intensity, with increasing hydrothermal treatment times from 0 to 48 h at 100 ◦ C. The data infer that extended hydrothermal treatment at elevated
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Fig. 7. TEM images of thiocyanato-functionalized porous silica nanospheres synthesized with different CTAB concentrations: (a) 0.0009 M, (b) 0.005 M, (c) and (d) 0.01 M, all before hydrothermal treatment.
temperatures can easily lead to the decomposition of the thiocyanato groups within thiocyanato-functionalized porous silica nanospheres. TGA measurements on surfactant extracted samples having different hydrothermal treatment times of 0, 12, 24 and 48 h are displayed in Fig. 4. The decreasing mass loss as a function of increasing hydrothermal treatment time is complementary to the FTIR data. It can be seen that the sample having no hydrothermal treatment provides the greatest organic mass loss because there has been more of the functional siloxane incorporated into the porous nanosphere. As hydrothermal treatment increases, the mass loss decreases as the functionality decomposes and is no longer incorporated within the framework of the material corresponding to the diminishing 2151 cm−1 peak from FTIR. In addition, the corresponding TEM micrographs of thiocyanato-functionalized porous silica nanospheres are shown in Fig. 5. It was found that particle dispersion improved with increasing hydrothermal treatment time. The mean diameter of thiocyanato-functionalized porous silica nanospheres gradually decreased from 23 to 16 nm (Table 1). With the decomposition of the functional thiocyanto groups increasing with hydrothermal treatment time, this is effectively a decomposition of the main framework as the organosiloxane was the sole source of silica. It can therefore be expected that if the structural integrity of the main framework is compromised, then the degree of necking between particles will reduce improving the dispersion of individual particles. Furthermore, the hybrid organic/inorganic shell would also be expected to decrease in dimension resulting in smaller particle sizes. This is confirmed by TEM analysis.
To better understand the resulting textural properties of the materials, Fig. 6 shows the representative nitrogen adsorption–desorption isotherms and corresponding pore size distributions of samples synthesized under the different hydrothermal treatment times. Materials subjected to hydrothermal treatment time of 12 h resulted in non-porous materials with a BET specific surface area and pore volume calculated to be 1.1 m2 /g and 0.004 cm3 /g, respectively (Fig. 6(c) and (d)). The surface area of the products is reduced when compared with materials synthesized with hydrothermal treatment times of 0 and 48 h (Table 1). The rationale behind this is that for a hydrothermal treatment time of 12 h, at the elevated temperature, this leads to decomposition, in part, of the thiocyanato groups within the thiocyanato-functionalized porous silica nanospheres. The structural integrity of the framework, in this material, may be damaged because of Si C or C SCN bond cleavage. In contrast, when hydrothermal treatment time increased to 48 h, the nitrogen adsorption–desorption isotherm shows a type iv isotherm with a hysteresis loop at the relative pressures P/Po = 0.9–0.5 (Fig. 6(e)). Broad capillary condensation at the higher relative pressures corresponds to the mesoporous nature of the material, Fig. 6(f), resulting in a broad pore size distribution curve, as calculated by the BJH method. The BET specific surface area and pore volume were calculated to be 345 m2 /g and 0.46 cm3 /g, respectively. FTIR and TGA data show that the contents of the thiocyanato groups within thiocyanato-functionalized porous silica nanospheres significantly reduce as a function of increasing hydrothermal
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treatment time, particularly at 48 h. We propose that at this extended hydrothermal time, the silica framework may ‘self-heal’ to form a new silica network structure. A significant portion of decomposed thiocyanato groups will egress from the pores resulting in increased specific surface area and pore volume.
Effect of concentration of CTAB Based on the current synthesis, the formation of cationic surfactant CTAB micelles in a pure water solution plays an essential role in preparing thiocyanato-functionalized porous silica nanospheres. Herein, the influence of CTAB concentration on thiocyanato-functionalized porous silica nanosphere formation without hydrothermal treatment has been investigated. Fig. 7 shows the TEM micrographs of different concentrations of CTAB. Previous reports have shown that the critical micellar concentration (CMC) of CTAB in a pure water system is approximately 0.0009 M (Li, Han, Zhang, Wang, & Song, 2006; Bielawska, ´ ´ Janczuk, & Zdziennicka, 2013). From Fig. 7(a), it is Chodzinska, clearly observed that solid spheres with diameters of 119 nm were obtained as the CTAB concentration was at the CMC value. Increasing the CTAB concentration to 0.005 M, irregular spherical particles with a porous shell were formed and the calculated average diameter was approx. 59 nm (Fig. 7(b)). At this point, the system is believed to be at the transition between a nonporous nanosphere to a porous nanosphere. However, completely porous silica nanospheres with diameters of 23 nm were generated as the CTAB concentration was increased to 0.01 M (Fig. 7(c)), inferring that the generation of porous structures is closely related to the CTAB concentration. At lower CTAB concentrations, the surfactant molecules exist as individual entities or as small clusters within a system where the concentration not being sufficient. The interaction between the TCPTES molecules and CTAB molecules is less frequent, resulting in the formation of non-porous thiocyanato-functionalized silica nanospheres. In contrast, the CTAB micelles formed at a higher CTAB concentration will interact more frequently with TCPTES. TCPTES molecules and its oligomer will first interact with the surface of the CTAB micelles via an electrostatic interaction, and the CTAB/thiocyanato-functionalized porous silica nanocomposites will be formed by further hydrolysis and condensation. Consequently, thiocyanato-functionalized porous silica nanospheres can be obtained after extraction of the CTAB cores. In addition, excess CTAB molecules may hinder the transport of the TCPTES molecules to the micelles resulting in a smaller shell and hence smaller particles.
Conclusions In this paper, a facile sol–gel process method for the preparing thiocyanato-functionalized porous silica nanospheres based on positively charged CTAB micelles acting as a template has been developed. Thiocyanato-functionalized porous silica nanospheres with spherical particle sizes of less than 23 nm have been successfully prepared by cooperative assembly between positively charged CTAB cationic surfactants and negatively charged silica particles under alkaline conditions. Additionally, the porous structures and morphology of the samples can be easily controlled as a function of hydrothermal treatment time and CTAB concentration. The synthesis approach adopted in this study can likely be extended as a general approach to synthesize various organic functionalized hollow nanostructures by the use of adequate structure directing agents as templates.
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Acknowledgements We are grateful for the financial support of the Nature Science Foundation of Hubei Province (No. 2014CFB862) and State Key Laboratory of Natural and Biomimetic Drugs (No. K20140214). This work was partially supported by the National Natural Science Foundation of China (No. 51273155). This work was also financially supported by the Guangdong Well-SilicaSol Co., LTD, China. References Anwander, R., Nagl, I., Widenmeyer, M., Engelhardt, G., Groeger, O., Palm, C., et al. (2000). Surface characterization and functionalization of MCM-41 silicas via silazane silylation. The Journal of Physical Chemistry B, 104, 3532–3544. Arkhireeva, A., Hay, J. N., & Manzano, M. (2005). Preparation of silsesquioxane particles via a nonhydrolytic sol–gel route. Chemistry of Materials, 17, 875–880. Beck, J. S., Vartuli, J. C., Roth, W. J., Leonowicz, M. E., Kresge, C. T., Schmitt, C. T., et al. (1992). A new family of mesoporous molecular sieves prepared with liquid crystal templates. 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Please cite this article in press as: Li, J., et al. A facile method for preparing thiocyanato-functionalized porous silica nanospheres. Particuology (2015), http://dx.doi.org/10.1016/j.partic.2015.03.002