- Email: [email protected]
Synthesis of mesoporous silica by a surface charge reversal route
Journal of Colloid and Interface Science 349 (2010) 473–476
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis of mesoporous silica by a surface charge reversal route Wei-Min Zhang a,b, Jia Liu a,b, Zhong-Xi Sun a,b,*, Bao-Qiang Fan a, Zhen-Dong Yang a, Wills Forsling c a
School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, University of Jinan, Jinan 250022, China c Division of Chemistry, Department of Chemical Engineering and Geosciences, Luleå University of Technology, SE-97 187 Luleå, Sweden b
a r t i c l e
i n f o
Article history: Received 28 January 2010 Accepted 19 May 2010 Available online 23 May 2010 Keywords: Mesoporous silica Adsorption Surface charge reversal
a b s t r a c t Pore size adjustable mesoporous silica was synthesized by adsorption of varying amounts of sodium dodecyl benzenesulfonate at the surface of silica activated by zinc ion via a novel surface charge reversal route. The pore size and volume can be adjusted from 5.9 to 13.76 nm and 0.88 to 1.08 cm3 g1, respectively, with increasing the SDBS concentration from 0.77 to 3.08 mmol L1. Adsorption of Zn2+ as a function of pH and N2 adsorption/desorption isotherms demonstrated that the metal ions such as Zn2+ could be readily removed with dilute nitric acid without apparent collapse of the pore structure at the proper range of SDBS concentration. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Since 1992, the synthesis of ordered mesoporous materials using surfactant micelles as templates has been extensively studied [1]. The self-assembly of a meso-structured silica has been promoted by electrostatic charge-matching pathways including S+I and S+XI+ (S+, cationic surfactants; I or I+, anionic or cationic silicates; and X, counteranions), and by electrically neutral pathways, including N0I0 and S0I0 (N0 or S0, nonionic or neutral amine surfactants; and I0, neutral silica species) [2–18]. For example, well-ordered mesoporous silica [1–4] has been prepared by applying cationic quaternary ammonium surfactants under both basic and acidic conditions, which indicated that the formation of mesoporous silica is induced by electrostatic interactions. Highly ordered HMS [6–8], SBA, and FDU families [9–12] as well as MSU-X [13,14] have been produced utilizing primary amine micelles or some block copolymers as templates through hydrogen bonding or electrostatic interactions. Although hexagonal, cubic, and lamellar mesoporous silica with larger pores could be prepared via these methods, the high-cost and complex synthetic procedure inhibit these materials from wider applications. Utilizing mass-produced anionic surfactants for the synthesis of the mesoporous silica has been strongly desired since the discovery of M41S. However, the electrostatic repulsion obstructs the negatively charged silica to react with the functional group of an anionic surfactant. Gao et al. reported an anionic surfactant templating route to mesoporous silica by introducing an additional amino-propylsiloxane or quaternized aminopropylsiloxane on the
inorganic precursors to make the surface positively charged to facilitate anionic surfactant adsorption [19]. As well known, multivalent metal ions are effective activators for quartz in mineral flotation, which can reverse the surface charge by specific adsorption and then permit the adsorption of the anionic surfactant like oleate [20]. The surface functional groups of quartz and silica are basically the same, i.e., protonated or deprotonated silanol groups due to pH value. The complexation models of metal ions at the surface silanol group were well established [21]. The surface charge reversal (SCR) principle by complexation of metal ions at the surfaces of inorganic precursors would possibly be useful in the self-assembly of inorganic mesoporous materials; however, such studies have not been reported so far. In this paper, we prepared tunable mesoporous silica via complexation of zinc ions at the silica surface and a selfassembly process. As a commercial and well-known anionic surfactant, SDBS has the advantages of being a highly potent detergent and in addition inexpensive. The novel strategy presented here reflects the use of a divalent metal ion, Zn2+, to reverse the negative charge of silica surface, making it accessible for SDBS. In addition, the pore diameter and the pore volume can be adjusted by adding variable amounts of SDBS. It is likely that this SM2+I surface charge reversal (SCR) pathway may open a new route for synthesizing inorganic mesoporous materials. If the zinc ions can act as a bridge between the silica surface and the anionic surfactant, other metal ions may also work in the similar way. Furthermore zinc ions are easily available, less toxic, and inexpensive.
2. Material and methods * Corresponding author at: School of Chemistry and Chemical Engineering, University of Jinan, Jinan, Shandong, China. Fax: +86 (0) 531 87161600. E-mail address: [email protected] (Z.-X. Sun). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.05.069
Sodium n-dodecyl benzenesulfonate (SDBS) was used as a template. Tetraethyl orthosilicate (TEOS), ammonia solution
W.-M. Zhang et al. / Journal of Colloid and Interface Science 349 (2010) 473–476
(25%), nitric acid, ethanol, and zinc nitrate hexahydrate used in the experiments were of analytical grade and double-distilled water was used in all solution preparations. In a typical preparation, 0.1395 g SDBS and 0.0155 g Zn(NO3)2 6H2O were carefully mixed with 20 mL 2% ammonia solution to make an opaline solution. Then, 10 mL TEOS and 100 mL ethanol were added dropwise simultaneously into the mixture. The pH value was kept at 6–7 by adding dilute nitric acid. Thus, a colloidal suspension of silica–Zn2+–SDBS was obtained after continuous magnetic stirring for 0.5 h. Finally, the mixture of silica–Zn2+–SDBS was dried at 105 °C for 24 h and calcined in air at 600 °C for 6 h to remove the organic template. In order to test the easiness of the zinc ion removal a sample was also rinsed with dilute nitric acid. The powders were further characterized using a D/MAX-c X-ray diffractometer using CuKa radiation (wavelength 1.542 Å). A continuous mode was used for collecting data from 1° to 8 ° of 2h at a scanning rate of 0.004° s1. In order to study the functions of the zinc ions, zeta potential measurements were performed of the colloidal suspension of silica–Zn2+ under different pH values at a constant ionic strength of 0.01 mol L1 KNO3. Ten milliliters of the silica–Zn2+ colloidal suspension was added to each of a set of 20-mL containers. A solution of 0.1 mol L1 HNO3 or 0.1 mol L1 NaOH was used to adjust the pH of each sample to a value within the range of 3–8. The samples were shaken for 3 h and left overnight at ambient temperature. Then the zeta potential of each sample was measured by a zeta electrophoresis apparatus (JS94H, Shanghai, China). For comparison, zeta potential measurements of the silica colloidal suspension were also performed without adding zinc ions. To prepare the samples for batch adsorption experiments, each of a 25 mL container was filled with 5.0 mL of 0.5 mol L1 NaNO3 solution and 1.25 mL of 0.01 mol L1 zinc nitrate solution and 0.01 g mesoporous silica. The amount of 0.1 mol L1 HNO3 or 0.1 mol L1 NaOH was used to adjust pH to the desired pH value between 3 and 8, and distilled water was added to adjust the total volume to 25 mL. The containers were shaken for 3 h at room temperature and left overnight. After separation by a centrifuge, pH measurements were conducted and the concentrations of residual zinc ions were measured by a TAS-900 flame atomic absorption spectrometer (AAS). The amount of zinc adsorbed by mesoporous silica was determined by subtracting the residual quantities from the initial zinc ion concentration. The residual concentrations of Zn2+ on the final products before and after rinsing with dilute nitric acid and water were also determined by the AAS method. Specific surface areas were measured by the N2/BET method using a Quantachrome, NOVA 2000e surface area and pore size analyzer. N2 adsorption/desorption isotherms were obtained at 77 K by a static adsorption procedure. Samples were degassed at 150 °C in a vacuum below 103 Torr for 12 h before the Brunauer–Emmett–Teller (BET) specific surface area measurements. The specific surface area of sample was calculated using the BET equation, in N2 relative pressures (P/P0) between 0.05 and 0.3. The total pore volumes (Vp) were determined at a P/P0 value of 0.995. The mean pore diameters (Dp) were calculated with the Barret–Joyner–Helenda (BJH) equation using desorption data of the isotherm.
3. Results and discussion The results of a low-angle X-ray diffraction diagram for as-prepared silica activated by Zn2+ are presented in Fig. 1. A diffraction peak at 2h of 0.94° combined with a shoulder around 2h of 1.76° can be observed in the low-angle X-ray diffraction diagram, which reveals the mesoporous nature of the as-prepared silica sample.
Intensity/a.u.
474
1
2
3
4
5
6
7
Fig. 1. Low-angle X-ray diffraction diagram for as-prepared silica activated by Zn2+.
The BET surface areas, pore size, and the total pore volume calculated by the Barret–Joyner–Halenda (BJH) method using the desorption data for samples prepared with different amounts of SDBS are summarized in Table 1. The BET surface area of the samples was found to be 521.11, 459.43, 348.99, and 295.96 m2 g1, respectively, which decreases with increasing amounts of SDBS from 0.77 to 8.27 mmol L1. A comparatively narrow pore size distribution centered at 5.9, 10.42, and 13.76 nm, respectively, was obtained, increasing with the SDBS concentration from 0.77 to 3.08 mmol L1, but dropping abruptly to 3.43 nm at the SDBS concentration of 8.27 mmol L1. Nitrogen adsorption/desorption isotherms and the corresponding pore size distributions are shown in Fig. 2. The samples exhibited type IV isotherms with hysteresis, which are characteristic of mesoporous materials. The function of SDBS can be described as a pore size expander. The increase of the pore size and volume with increasing SDBS concentration might be due to a solubilization effect of SDBS into the hydrophobic parts of silica–Zn2+–surfactant aggregates. Some pores, however, may collapse when SDBS reached 8.27 mmol L1. Fig. 3 displays the N2 adsorption/desorption isotherms for the samples prepared with SDBS of 8.27 mmol L1. The notable increase of the BET specific area after rinsing with nitric acid of 0.1 mol L1 may be attributed to the removal of Zn2+ from the silica surface, and the small alteration of the pore volume and pore size reflected that rinsing with nitric acid has little effect on the pore structure of silicas. Zeta potentials of silica colloids in the presence and absence of Zn2+ are shown in Fig. 4. The pzc of pH 3.3 for silica colloids without Zn2+ at a constant ionic strength of 102 mol L1 KNO3 is in good agreement with literature data [22]. The addition of Zn2+ into silica colloids did not exert a significant shift in the pzc of silica. However, the zeta potentials for silica colloids in the presence of Zn2+ ions exhibited much less negative values than those of the corresponding colloids in the absence of zinc ions, indicating that the surface electrical charge for silica was significantly affected by adding zinc ions [23]. Similar to the complexation of metal ions at the silica surface, the adsorption of zinc ions can be expressed as Table 1 BET surface area, pore diameter, and pore volume data for mesoporous silica samples prepared with different amounts of SDBS. SDBS concentrations (103 mol L1)
BET surface area (m2 g1)
Pore size (nm)
Pore volume (cm3 g1)
0.77 1.54 3.08 8.27 (before rinsing) 8.27 (after rinsing) 9.24
521.11 459.43 348.99 295.96 337.42 104.71
5.90 10.42 13.76 3.43 3.44 1.84
0.88 0.94 1.08 0.38 0.40 0.31
475
W.-M. Zhang et al. / Journal of Colloid and Interface Science 349 (2010) 473–476
b
800 700
SDBS=0.77mM SDBS=1.54mM SDBS=3.08mM
Volume/cc/g
600
4.0 3.5
Pore Volume/cc/g
a
500 400 300 200 100 0
SDBS=0.77mM SDBS=1.54mM SDBS=3.08mM
3.0 2.5 2.0 1.5 1.0 0.5 0.0
0.0
0.2
0.4
0.6
0.8
1.0
0
5
10
15
Relative Pressure p/p°
20
25
30
35
40
45
50
Pore Diameter/nm
Fig. 2. Nitrogen adsorption/desorption isotherms (a) and the corresponding pore size distribution and (b) of mesoporous silica samples prepared with varying amounts of SDBS as template.
Before washing After washing
250
Volume/cc/g
Concentration of residual Zn2+ -5 /10 mol/L
300
200 150 100 50 0.0
0.2
0.4
0.6
0.8
10
Zeta Potential/m V
0
6 pH
4
8
10
12
-20 -30 -40 -50 -60
Fig. 4. Zeta-potential datum of silica colloids in the presence (N) and absence (4) of Zn2+ at the constant ionic strength of 0.01 mol L1 KNO3 as a function of pH.
þ
BSiOH þ Zn2þ ¼ BSiOZn þ Hþ
1.6 1.4 1.2 1.0 4
5
6
7
8
9
pH
Fig. 3. Nitrogen adsorption/desorption isotherms of sample before and after rinsing with dilute nitric acid of pH 1.
2
1.8
3
1.0
Relative Pressure/p/p°
-10
2.0
ð1Þ
Consequently, the positive charges were brought to the surfaces and the surfaces became much less negatively charged or even positively charged when the metal ion concentration were high enough. The near-zero zeta-potential at pH of 7.0 is possibly due to the adsorption of free zinc ions or the first hydrolysis products which may be insufficient to lead to charge reversal. It is not surprising that the surfaces carry both positive and negative surface complexes and the resulting surface charge depends on the competition between charged surface complexes. On the condition that
Fig. 5. The residual concentration of zinc ions in the supernatant of the silica–Zn2+ colloids suspension as a function of pH.
the solubility products of the interfacial precipitates of Zn(OH)2 17 are the same as bulk Zn(OH)2 (K H ), we calculated sp ¼ 1:2 10 the initial pH value of 6.96 of the interfacial precipitation, approximately equal to 7.0. After pH 7 the zeta potential decreased with increasing pH, reflecting a partial coating of zinc hydroxide on the colloidal silica. These results indicated that the maximum adsorption of free zinc ions probably occurred at about pH 7.0, as could be confirmed by batch adsorption experiments. Fig. 5 displays the concentrations of residual zinc ions in the supernatant of the suspension as a function of pH. The amount of zinc adsorbed by silica colloids was determined from the difference between the blank and sample values. The adsorbed quantities of zinc ions at the colloidal silica surface increased with increasing pH and the adsorption peak occurred in the range of pH 6.5–7.0. This result is related to the surface acidity of silica and to the predominant species of Zn2+ in aqueous solution. The steep decrease in the concentration of Zn2+ above pH 7.0 is possibly attributed to a surface precipitation of Zn(OH)2. We assume that the anionic surfactants SDBS have optimum opportunities to approach the surface of silica at about pH 6.5–7. The SDBS adsorption mechanism can be explained by the attractive interactions between the negative functional group of SDBS and the surface species „SiOZn+ at the silica particles according to þ
BSiOZn þ C12 H25 OSO3 ¼ BSiOZnO3 SOC12 H25 2+
ð2Þ
AAS results exhibit that the added Zn can be completely removed by a dilute nitric acid solution of pH 1 without affecting the mesoporous structure. It is not suprising that the adsorbed
W.-M. Zhang et al. / Journal of Colloid and Interface Science 349 (2010) 473–476
OH OH OH O O O Si Si Si Si Si Si + O O O O O O O -H O O O O O O O
aration of mesoporous silica by replacing Zn2+ with other divalent metal ions such as Cu2+ and Co2+ (see Supplementary material).
O O O Si Si Si O OO OO OO
476
Acknowledgments 2+
SDBS
Financial support from Chinese Natural Science Foundation (Nos. 20677022 and 20537020) and University of Jinan (B0400) is gratefully acknowledged.
SiO2
Appendix A. Supplementary material
Zn calcination
acidic rinsing
Fig. 6. The schematic presentation of the SCR route for the formation of mesoporous silica.
Zn2+ at the surface of silica is possible to desorb with decreasing pH according to Eq. (1). Considering the results of the zeta potential measurements and the batch adsorption experiments, the formation mechanism for mesoporous silica can be outlined as follows: (1) At pH < 7 and in the presence of Zn2+ ions there is a significant and definite trend in the zeta potential data toward less negative values of SiO2 with increasing pH. (2) pH around 7.0 coincides with the maximum adsorption of zinc ions. Thus, micelles formed by anionic surfactant SDBS are most favorable for approaching the silica surfaces, templating the formation of mesoporous silica. The proposed mechanism is visualized schematically in Fig 6. Zn2+ ions are adsorbed at the silica surface with a peak at pH 7.0 to make SCR. Then the silica surface is associated with the anionic functional group of the SDBS micelles, which are operating as the pore structure directing agents. By calcining the SiO2–Zn2+–SDBS composites, the mesoporous silica was obtained. This method has also been demonstrated to be effective in the prep-
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2010.05.069. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
C.T. Kresge, M.E. Leonowicz, W.J. Roth, et al., Nature 359 (1992) 710. J.S. Beck, J.C. Vartuli, W.J. Roth, et al., J. Am. Chem. Soc. 114 (1992) 10834. A. Monnier, F. Schüth, Q. Huo, et al., Science 261 (1993) 1299. S. Inagaki, Y. Fukushima, K. Kuroda, J. Chem. Soc. Chem. Commun. (1993) 680. Q.S. Huo, D.I. Margolese, U. Ciesla, et al., Nature 368 (1994) 317. P.T. Tanev, M. Chibwe, T.J. Pinnavaia, Nature 368 (1994) 321. P.T. Tanev, T.J. Pinnaviaia, Science 267 (1995) 865. P.T. Tanev, T.J. Pinnaviaia, Chem. Mater. 8 (1996) 2068. D.Y. Zhao, Q.S. Huo, J.L. Feng, et al., J. Am. Chem. Soc. 120 (1998) 6024. D.Y. Zhao, J.L. Feng, Q.S. Huo, et al., Science 279 (1998) 548. C.Z. Yu, Y.H. Yu, D.Y. Zhao, Chem. Commun. (2000) 575. C.Z. Yu, Y.H. Yu, L. Miao, D.Y. Zhao, Microporous Mesoporous Mater. 44 (2001) 65. S.A. Bagshaw, Chem. Commun. (1999) 271. S.A. Bagshaw, Chem. Commun. (1999) 1785. Y. Hoshikawa, H. Yabe, A. Nomura, et al., Chem. Mater. 22 (2010) 12. F. Schuth, W. Schmidt, Adv. Mater. 14 (2002) 629. J. Gu, W. Fan, A. Shimojima, T. Okubo, Small 10 (2007) 1740. Y. Wan, D.Y. Zhao, Chem. Rev. 107 (2007) 2821. C.B. Gao, H.B. Qiu, W. Zeng, Chem. Mater. 18 (2006) 3904. A.M. Gaudin, A. Rizo-Patron, AIME 153 (1943) 469. P.W. Schindler, B. Fiirst, R. Dick, et al., J. Colloid Interface Sci. 55 (1976) 469. W. Janusz, J. Patkowski, S. Chibowski, J. Colloid Interface Sci. 266 (2003) 259. R.O. James, T.W. Healy, J. Colloid Interface Sci. 40 (1972) 53.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis of mesoporous silica by a surface charge reversal route Wei-Min Zhang a,b, Jia Liu a,b, Zhong-Xi Sun a,b,*, Bao-Qiang Fan a, Zhen-Dong Yang a, Wills Forsling c a
School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, University of Jinan, Jinan 250022, China c Division of Chemistry, Department of Chemical Engineering and Geosciences, Luleå University of Technology, SE-97 187 Luleå, Sweden b
a r t i c l e
i n f o
Article history: Received 28 January 2010 Accepted 19 May 2010 Available online 23 May 2010 Keywords: Mesoporous silica Adsorption Surface charge reversal
a b s t r a c t Pore size adjustable mesoporous silica was synthesized by adsorption of varying amounts of sodium dodecyl benzenesulfonate at the surface of silica activated by zinc ion via a novel surface charge reversal route. The pore size and volume can be adjusted from 5.9 to 13.76 nm and 0.88 to 1.08 cm3 g1, respectively, with increasing the SDBS concentration from 0.77 to 3.08 mmol L1. Adsorption of Zn2+ as a function of pH and N2 adsorption/desorption isotherms demonstrated that the metal ions such as Zn2+ could be readily removed with dilute nitric acid without apparent collapse of the pore structure at the proper range of SDBS concentration. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Since 1992, the synthesis of ordered mesoporous materials using surfactant micelles as templates has been extensively studied [1]. The self-assembly of a meso-structured silica has been promoted by electrostatic charge-matching pathways including S+I and S+XI+ (S+, cationic surfactants; I or I+, anionic or cationic silicates; and X, counteranions), and by electrically neutral pathways, including N0I0 and S0I0 (N0 or S0, nonionic or neutral amine surfactants; and I0, neutral silica species) [2–18]. For example, well-ordered mesoporous silica [1–4] has been prepared by applying cationic quaternary ammonium surfactants under both basic and acidic conditions, which indicated that the formation of mesoporous silica is induced by electrostatic interactions. Highly ordered HMS [6–8], SBA, and FDU families [9–12] as well as MSU-X [13,14] have been produced utilizing primary amine micelles or some block copolymers as templates through hydrogen bonding or electrostatic interactions. Although hexagonal, cubic, and lamellar mesoporous silica with larger pores could be prepared via these methods, the high-cost and complex synthetic procedure inhibit these materials from wider applications. Utilizing mass-produced anionic surfactants for the synthesis of the mesoporous silica has been strongly desired since the discovery of M41S. However, the electrostatic repulsion obstructs the negatively charged silica to react with the functional group of an anionic surfactant. Gao et al. reported an anionic surfactant templating route to mesoporous silica by introducing an additional amino-propylsiloxane or quaternized aminopropylsiloxane on the
inorganic precursors to make the surface positively charged to facilitate anionic surfactant adsorption [19]. As well known, multivalent metal ions are effective activators for quartz in mineral flotation, which can reverse the surface charge by specific adsorption and then permit the adsorption of the anionic surfactant like oleate [20]. The surface functional groups of quartz and silica are basically the same, i.e., protonated or deprotonated silanol groups due to pH value. The complexation models of metal ions at the surface silanol group were well established [21]. The surface charge reversal (SCR) principle by complexation of metal ions at the surfaces of inorganic precursors would possibly be useful in the self-assembly of inorganic mesoporous materials; however, such studies have not been reported so far. In this paper, we prepared tunable mesoporous silica via complexation of zinc ions at the silica surface and a selfassembly process. As a commercial and well-known anionic surfactant, SDBS has the advantages of being a highly potent detergent and in addition inexpensive. The novel strategy presented here reflects the use of a divalent metal ion, Zn2+, to reverse the negative charge of silica surface, making it accessible for SDBS. In addition, the pore diameter and the pore volume can be adjusted by adding variable amounts of SDBS. It is likely that this SM2+I surface charge reversal (SCR) pathway may open a new route for synthesizing inorganic mesoporous materials. If the zinc ions can act as a bridge between the silica surface and the anionic surfactant, other metal ions may also work in the similar way. Furthermore zinc ions are easily available, less toxic, and inexpensive.
2. Material and methods * Corresponding author at: School of Chemistry and Chemical Engineering, University of Jinan, Jinan, Shandong, China. Fax: +86 (0) 531 87161600. E-mail address: [email protected] (Z.-X. Sun). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.05.069
Sodium n-dodecyl benzenesulfonate (SDBS) was used as a template. Tetraethyl orthosilicate (TEOS), ammonia solution
W.-M. Zhang et al. / Journal of Colloid and Interface Science 349 (2010) 473–476
(25%), nitric acid, ethanol, and zinc nitrate hexahydrate used in the experiments were of analytical grade and double-distilled water was used in all solution preparations. In a typical preparation, 0.1395 g SDBS and 0.0155 g Zn(NO3)2 6H2O were carefully mixed with 20 mL 2% ammonia solution to make an opaline solution. Then, 10 mL TEOS and 100 mL ethanol were added dropwise simultaneously into the mixture. The pH value was kept at 6–7 by adding dilute nitric acid. Thus, a colloidal suspension of silica–Zn2+–SDBS was obtained after continuous magnetic stirring for 0.5 h. Finally, the mixture of silica–Zn2+–SDBS was dried at 105 °C for 24 h and calcined in air at 600 °C for 6 h to remove the organic template. In order to test the easiness of the zinc ion removal a sample was also rinsed with dilute nitric acid. The powders were further characterized using a D/MAX-c X-ray diffractometer using CuKa radiation (wavelength 1.542 Å). A continuous mode was used for collecting data from 1° to 8 ° of 2h at a scanning rate of 0.004° s1. In order to study the functions of the zinc ions, zeta potential measurements were performed of the colloidal suspension of silica–Zn2+ under different pH values at a constant ionic strength of 0.01 mol L1 KNO3. Ten milliliters of the silica–Zn2+ colloidal suspension was added to each of a set of 20-mL containers. A solution of 0.1 mol L1 HNO3 or 0.1 mol L1 NaOH was used to adjust the pH of each sample to a value within the range of 3–8. The samples were shaken for 3 h and left overnight at ambient temperature. Then the zeta potential of each sample was measured by a zeta electrophoresis apparatus (JS94H, Shanghai, China). For comparison, zeta potential measurements of the silica colloidal suspension were also performed without adding zinc ions. To prepare the samples for batch adsorption experiments, each of a 25 mL container was filled with 5.0 mL of 0.5 mol L1 NaNO3 solution and 1.25 mL of 0.01 mol L1 zinc nitrate solution and 0.01 g mesoporous silica. The amount of 0.1 mol L1 HNO3 or 0.1 mol L1 NaOH was used to adjust pH to the desired pH value between 3 and 8, and distilled water was added to adjust the total volume to 25 mL. The containers were shaken for 3 h at room temperature and left overnight. After separation by a centrifuge, pH measurements were conducted and the concentrations of residual zinc ions were measured by a TAS-900 flame atomic absorption spectrometer (AAS). The amount of zinc adsorbed by mesoporous silica was determined by subtracting the residual quantities from the initial zinc ion concentration. The residual concentrations of Zn2+ on the final products before and after rinsing with dilute nitric acid and water were also determined by the AAS method. Specific surface areas were measured by the N2/BET method using a Quantachrome, NOVA 2000e surface area and pore size analyzer. N2 adsorption/desorption isotherms were obtained at 77 K by a static adsorption procedure. Samples were degassed at 150 °C in a vacuum below 103 Torr for 12 h before the Brunauer–Emmett–Teller (BET) specific surface area measurements. The specific surface area of sample was calculated using the BET equation, in N2 relative pressures (P/P0) between 0.05 and 0.3. The total pore volumes (Vp) were determined at a P/P0 value of 0.995. The mean pore diameters (Dp) were calculated with the Barret–Joyner–Helenda (BJH) equation using desorption data of the isotherm.
3. Results and discussion The results of a low-angle X-ray diffraction diagram for as-prepared silica activated by Zn2+ are presented in Fig. 1. A diffraction peak at 2h of 0.94° combined with a shoulder around 2h of 1.76° can be observed in the low-angle X-ray diffraction diagram, which reveals the mesoporous nature of the as-prepared silica sample.
Intensity/a.u.
474
1
2
3
4
5
6
7
Fig. 1. Low-angle X-ray diffraction diagram for as-prepared silica activated by Zn2+.
The BET surface areas, pore size, and the total pore volume calculated by the Barret–Joyner–Halenda (BJH) method using the desorption data for samples prepared with different amounts of SDBS are summarized in Table 1. The BET surface area of the samples was found to be 521.11, 459.43, 348.99, and 295.96 m2 g1, respectively, which decreases with increasing amounts of SDBS from 0.77 to 8.27 mmol L1. A comparatively narrow pore size distribution centered at 5.9, 10.42, and 13.76 nm, respectively, was obtained, increasing with the SDBS concentration from 0.77 to 3.08 mmol L1, but dropping abruptly to 3.43 nm at the SDBS concentration of 8.27 mmol L1. Nitrogen adsorption/desorption isotherms and the corresponding pore size distributions are shown in Fig. 2. The samples exhibited type IV isotherms with hysteresis, which are characteristic of mesoporous materials. The function of SDBS can be described as a pore size expander. The increase of the pore size and volume with increasing SDBS concentration might be due to a solubilization effect of SDBS into the hydrophobic parts of silica–Zn2+–surfactant aggregates. Some pores, however, may collapse when SDBS reached 8.27 mmol L1. Fig. 3 displays the N2 adsorption/desorption isotherms for the samples prepared with SDBS of 8.27 mmol L1. The notable increase of the BET specific area after rinsing with nitric acid of 0.1 mol L1 may be attributed to the removal of Zn2+ from the silica surface, and the small alteration of the pore volume and pore size reflected that rinsing with nitric acid has little effect on the pore structure of silicas. Zeta potentials of silica colloids in the presence and absence of Zn2+ are shown in Fig. 4. The pzc of pH 3.3 for silica colloids without Zn2+ at a constant ionic strength of 102 mol L1 KNO3 is in good agreement with literature data [22]. The addition of Zn2+ into silica colloids did not exert a significant shift in the pzc of silica. However, the zeta potentials for silica colloids in the presence of Zn2+ ions exhibited much less negative values than those of the corresponding colloids in the absence of zinc ions, indicating that the surface electrical charge for silica was significantly affected by adding zinc ions [23]. Similar to the complexation of metal ions at the silica surface, the adsorption of zinc ions can be expressed as Table 1 BET surface area, pore diameter, and pore volume data for mesoporous silica samples prepared with different amounts of SDBS. SDBS concentrations (103 mol L1)
BET surface area (m2 g1)
Pore size (nm)
Pore volume (cm3 g1)
0.77 1.54 3.08 8.27 (before rinsing) 8.27 (after rinsing) 9.24
521.11 459.43 348.99 295.96 337.42 104.71
5.90 10.42 13.76 3.43 3.44 1.84
0.88 0.94 1.08 0.38 0.40 0.31
475
W.-M. Zhang et al. / Journal of Colloid and Interface Science 349 (2010) 473–476
b
800 700
SDBS=0.77mM SDBS=1.54mM SDBS=3.08mM
Volume/cc/g
600
4.0 3.5
Pore Volume/cc/g
a
500 400 300 200 100 0
SDBS=0.77mM SDBS=1.54mM SDBS=3.08mM
3.0 2.5 2.0 1.5 1.0 0.5 0.0
0.0
0.2
0.4
0.6
0.8
1.0
0
5
10
15
Relative Pressure p/p°
20
25
30
35
40
45
50
Pore Diameter/nm
Fig. 2. Nitrogen adsorption/desorption isotherms (a) and the corresponding pore size distribution and (b) of mesoporous silica samples prepared with varying amounts of SDBS as template.
Before washing After washing
250
Volume/cc/g
Concentration of residual Zn2+ -5 /10 mol/L
300
200 150 100 50 0.0
0.2
0.4
0.6
0.8
10
Zeta Potential/m V
0
6 pH
4
8
10
12
-20 -30 -40 -50 -60
Fig. 4. Zeta-potential datum of silica colloids in the presence (N) and absence (4) of Zn2+ at the constant ionic strength of 0.01 mol L1 KNO3 as a function of pH.
þ
BSiOH þ Zn2þ ¼ BSiOZn þ Hþ
1.6 1.4 1.2 1.0 4
5
6
7
8
9
pH
Fig. 3. Nitrogen adsorption/desorption isotherms of sample before and after rinsing with dilute nitric acid of pH 1.
2
1.8
3
1.0
Relative Pressure/p/p°
-10
2.0
ð1Þ
Consequently, the positive charges were brought to the surfaces and the surfaces became much less negatively charged or even positively charged when the metal ion concentration were high enough. The near-zero zeta-potential at pH of 7.0 is possibly due to the adsorption of free zinc ions or the first hydrolysis products which may be insufficient to lead to charge reversal. It is not surprising that the surfaces carry both positive and negative surface complexes and the resulting surface charge depends on the competition between charged surface complexes. On the condition that
Fig. 5. The residual concentration of zinc ions in the supernatant of the silica–Zn2+ colloids suspension as a function of pH.
the solubility products of the interfacial precipitates of Zn(OH)2 17 are the same as bulk Zn(OH)2 (K H ), we calculated sp ¼ 1:2 10 the initial pH value of 6.96 of the interfacial precipitation, approximately equal to 7.0. After pH 7 the zeta potential decreased with increasing pH, reflecting a partial coating of zinc hydroxide on the colloidal silica. These results indicated that the maximum adsorption of free zinc ions probably occurred at about pH 7.0, as could be confirmed by batch adsorption experiments. Fig. 5 displays the concentrations of residual zinc ions in the supernatant of the suspension as a function of pH. The amount of zinc adsorbed by silica colloids was determined from the difference between the blank and sample values. The adsorbed quantities of zinc ions at the colloidal silica surface increased with increasing pH and the adsorption peak occurred in the range of pH 6.5–7.0. This result is related to the surface acidity of silica and to the predominant species of Zn2+ in aqueous solution. The steep decrease in the concentration of Zn2+ above pH 7.0 is possibly attributed to a surface precipitation of Zn(OH)2. We assume that the anionic surfactants SDBS have optimum opportunities to approach the surface of silica at about pH 6.5–7. The SDBS adsorption mechanism can be explained by the attractive interactions between the negative functional group of SDBS and the surface species „SiOZn+ at the silica particles according to þ
BSiOZn þ C12 H25 OSO3 ¼ BSiOZnO3 SOC12 H25 2+
ð2Þ
AAS results exhibit that the added Zn can be completely removed by a dilute nitric acid solution of pH 1 without affecting the mesoporous structure. It is not suprising that the adsorbed
W.-M. Zhang et al. / Journal of Colloid and Interface Science 349 (2010) 473–476
OH OH OH O O O Si Si Si Si Si Si + O O O O O O O -H O O O O O O O
aration of mesoporous silica by replacing Zn2+ with other divalent metal ions such as Cu2+ and Co2+ (see Supplementary material).
O O O Si Si Si O OO OO OO
476
Acknowledgments 2+
SDBS
Financial support from Chinese Natural Science Foundation (Nos. 20677022 and 20537020) and University of Jinan (B0400) is gratefully acknowledged.
SiO2
Appendix A. Supplementary material
Zn calcination
acidic rinsing
Fig. 6. The schematic presentation of the SCR route for the formation of mesoporous silica.
Zn2+ at the surface of silica is possible to desorb with decreasing pH according to Eq. (1). Considering the results of the zeta potential measurements and the batch adsorption experiments, the formation mechanism for mesoporous silica can be outlined as follows: (1) At pH < 7 and in the presence of Zn2+ ions there is a significant and definite trend in the zeta potential data toward less negative values of SiO2 with increasing pH. (2) pH around 7.0 coincides with the maximum adsorption of zinc ions. Thus, micelles formed by anionic surfactant SDBS are most favorable for approaching the silica surfaces, templating the formation of mesoporous silica. The proposed mechanism is visualized schematically in Fig 6. Zn2+ ions are adsorbed at the silica surface with a peak at pH 7.0 to make SCR. Then the silica surface is associated with the anionic functional group of the SDBS micelles, which are operating as the pore structure directing agents. By calcining the SiO2–Zn2+–SDBS composites, the mesoporous silica was obtained. This method has also been demonstrated to be effective in the prep-
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2010.05.069. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
C.T. Kresge, M.E. Leonowicz, W.J. Roth, et al., Nature 359 (1992) 710. J.S. Beck, J.C. Vartuli, W.J. Roth, et al., J. Am. Chem. Soc. 114 (1992) 10834. A. Monnier, F. Schüth, Q. Huo, et al., Science 261 (1993) 1299. S. Inagaki, Y. Fukushima, K. Kuroda, J. Chem. Soc. Chem. Commun. (1993) 680. Q.S. Huo, D.I. Margolese, U. Ciesla, et al., Nature 368 (1994) 317. P.T. Tanev, M. Chibwe, T.J. Pinnavaia, Nature 368 (1994) 321. P.T. Tanev, T.J. Pinnaviaia, Science 267 (1995) 865. P.T. Tanev, T.J. Pinnaviaia, Chem. Mater. 8 (1996) 2068. D.Y. Zhao, Q.S. Huo, J.L. Feng, et al., J. Am. Chem. Soc. 120 (1998) 6024. D.Y. Zhao, J.L. Feng, Q.S. Huo, et al., Science 279 (1998) 548. C.Z. Yu, Y.H. Yu, D.Y. Zhao, Chem. Commun. (2000) 575. C.Z. Yu, Y.H. Yu, L. Miao, D.Y. Zhao, Microporous Mesoporous Mater. 44 (2001) 65. S.A. Bagshaw, Chem. Commun. (1999) 271. S.A. Bagshaw, Chem. Commun. (1999) 1785. Y. Hoshikawa, H. Yabe, A. Nomura, et al., Chem. Mater. 22 (2010) 12. F. Schuth, W. Schmidt, Adv. Mater. 14 (2002) 629. J. Gu, W. Fan, A. Shimojima, T. Okubo, Small 10 (2007) 1740. Y. Wan, D.Y. Zhao, Chem. Rev. 107 (2007) 2821. C.B. Gao, H.B. Qiu, W. Zeng, Chem. Mater. 18 (2006) 3904. A.M. Gaudin, A. Rizo-Patron, AIME 153 (1943) 469. P.W. Schindler, B. Fiirst, R. Dick, et al., J. Colloid Interface Sci. 55 (1976) 469. W. Janusz, J. Patkowski, S. Chibowski, J. Colloid Interface Sci. 266 (2003) 259. R.O. James, T.W. Healy, J. Colloid Interface Sci. 40 (1972) 53.