Hollow Metal-Incorporated Monodispersed Mesoporous Silica

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Hollow metal-incorporated monodispersed mesoporous silica spheres Kazuhisa Yano* Toyota Central R&D Labs., Inc., Nagakute, Aichi, 480-1192 [email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

Expansion of mesopores and concomitant incorporation of metal species into mesoporous silica particles was achieved through the hydrothermal treatment of monodispersed mesoporous silica spheres (MMSS) in an aqueous metal salt solution. Mesopore size was increased to 24 nm from 2.2 nm while retaining monodispersed spherical shape. Surprisingly, hollow spheres were produced when Mg salt was used. Incorporation of metal species led to drastic improvement in adsorption performance. The amount of Rhodamine B adsorbed was increased by 27 times.

Introduction A variety of applications has been reported for mesoporous silica which has nanometer-sized mesopores.1 Due to large specific surface area and uniformity in mesopores, it can be useful as new types of catalysts,2,3 adsorbents,4,5 and host materials.6,7 Depending on synthetic conditions, its shape changes to fibers,8-10 sponge like membranes,11,12 rod like powders,13 films,14 polyhedral particles,15,16 and spheres.17-21 Among these shapes, spheres are the most advantageous for various applications such as chromatography22-25 and cosmetics.26 ACS Paragon Plus Environment

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We have been conducting researches on the synthesis and applications of monodispersed mesoporous silica spheres (MMSS).27 Due to high uniformity in size, a colloidal crystal, usually made from Polystyrene or silica particles, can also be fabricated from MMSS.28 The colloidal crystal acts as a photonic crystal. Emission control and laser oscillation have been demonstrated with MMSS colloidal crystals by incorporating nanoparticles into mesopores.

29,30

In addition, monodispersed mesoporous

carbon spheres templated from MMSS are capable of incorporating anodic materials such as tin oxides, and can be applied to a capacitor or a Lithium ion battery anode. 31,32 In case, MMSS is used as a host, the size of guest molecules incorporated into mesopores is restricted depending on the mesopore size. MMSS with 2.5 nm mesopores is produced by using octadecyltrimethyammonium as a surfactant. There have been some reports for pseudomorphic transformation of silica spheres with disordered porosity to an ordered one by post hydrothermal treatment.33-35 Then, the post treatment with a swelling reagent was conducted on MMSS, and mesopores were expanded to 5.5 nm.

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Recently, we have found that the hydrothermal treatment of

MMSS in acidic aqueous solution leads to drastic change of mesopore structure; the pore diameter was expanded to ca. 20 nm. In acidic condition, silica dissolves slightly and the resolution and reprecipitation of silica proceeds slowly, eventually mesopore restructuring occurs while retaining monodispersed spherical shape. 37 We will report here on the effect of metal species on the pore-expansion of MMSS. Hydrothermal treatment of MMSS is conducted in a metal salt solution. It is interesting to know whether metal species are incorporated into mesoporous silica while mesopores are expanded. Some metal species are tested and it is found that Al and Mg can be incorporated. Surprisingly, when Mg salt is used, MMSS changes to hollow spheres. It is demonstrated that replacement of high valency Si to lower valency metal species produce electron rich mesoporous silica that adsorbs cationic compounds more largely.

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Experimental Materials.

Monodispersed

mesoporous

silica

spheres

(MMSS)

were

obtained

using

cetyltrimethylammonium chloride as a template, and used without calcination.28,29 Iron chloride hexahydrate, copper chloride dihydrate, aluminum nitrate nonahydrate, magnesium chloride hexahydrate, and 2 N HCl were purchased from Wako Pure Chemical Co (Japan). All materials were used as received.

Hydrothermal treatment of MMSS with a metal salt. A typical treatment procedure is as follows. A 1 g of MMSS without calcination was dispersed in 60 mL of water containing a metal salt. FeCl3·6H2O, CuCl2·2H2O, Al(NO3)3·9H2O, and MgCl2·6H2O were used as metal salts. The mixture was heated at 423 K for 3 days by using an autoclave with Teflon container. After rinsing with distilled water and subsequent filtration, the powder was dried at 313 K and then calcined in air at 823 K for 6h.

Characterization. Powder x-ray diffraction measurement was carried out with a Rigaku Ultima IV xray diffractometer using Cu-Kα radiation. Scanning electron micrographs (SEMs) were obtained with a Hitachi SU-3500. The surface of a sample was coated with gold before the measurement. The average particle diameter was calculated from diameters of 50 particles in an SEM image. Since only parts of SEMs are shown in figures, particles not appeared in the figures were also examined. Elemental analysis was done on energy dispersive X- ray (EDX) analysis integrated in the Hitachi SU-3500. Transmission electron micrograph was obtained with a Jeol-2100F TEM using an acceleration voltage of 200 kV. To obtain a direct image of the internal structure of PS-Mg-4, particles embedded in epoxy resin was sectioned using an electron beam, and then observed by TEM. Nitrogen adsorption isotherm was measured using a BELSORP mini II (Bell Japan) at 77 K. The sample was evacuated at 353 K under 103

mmHg before each measurement. A pore diameter was calculated by the BJH method for the

desorption branch. Solid state 27Al-NMR spectrum was recorded on a Bruker BioSpin AVANCE400 at 104.261 MHz with spinning 4 kHz using pulses at 2-s intervals. ACS Paragon Plus Environment

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Results and Discussion Effect of metal species We have previously reported that acidic condition is inevitable to keep spherical shape unchanged during the hydrothermal treatment for the pore-expansion of MMSS.37 This is because the surfactant liberated from MMSS shows basicity. Under a basic condition, the solubility of silica so increases that restructuring of MMSS mesopores cannot proceed with keeping spherical shape. This suggests that metal salt solution should be acidic to keep original shape of MMSS. Fe, Cu, Mg and Al are chosen as metal species, and their strong acid salts, such as FeCl3·6H2O, CuCl2·2H2O, MgCl2·6H2O and Al(NO3)3·9H2O, are used for the hydrothermal treatment. The equimolar amount of metal salt to silica is used in the hydrothermal treatment. The samples are denoted as PS-Fe, PS-Cu, PS-Mg, and PS-Al, respectively depending on metal species used for the hydrothermal treatment. It is investigated whether expansion of mesopores and concomitant incorporation of metal species into mesoporous silica occur or not. SEMs were taken for MMSSs treated with various metal salts, and images are shown in Figure 1. When the hydrothermal treatment was done with Fe salt, spherical or columnar bigger particles are observed other than monodispersed spherical silica particles (Figure 1(a)). In case Cu salt was used, a big particle consisting of nanoparticles is seen other than silica particles in Figure 1(b). The XRD patterns of the samples are shown in Figure S1, and from the peaks, the existence of iron oxide or copper oxide is confirmed. In the case of PS-Mg and PS-Al, only monodispersed spherical particles are seen (Figure 1(c), (d)). EDX mappings are shown in Figure S2 in Supporting Information. Si and Fe elements are not located at the same position (Figure S2(a)). But Fe and O elements have the same location. The spherical or columnar bigger particles are identified as an iron oxide. The location of Cu is also different from that of Si’s (Figure S2(b)). However, Mg and Al are located at the same position of Si (Figure S2(c), (d)). This indicates that Mg and Al are incorporated into mesoporous silica. The reason why Mg and Al are incorporated could be explained by considering ionic radius and basicity. The ionic ACS Paragon Plus Environment

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radius of 4-coordinaton Si4+ is 0.26 Å. Meanwhile, those for Fe3+, Cu2+, Mg2+, and Al3+ are 0.49, 0.57, 0.57, and 0.39 Å, respectively. From these values, it can be deduced that only Al3+ could be replaced with Si4+. On the other hand, among these metal species, only Mg shows basicity. Mg can be incorporated into mesoporous silica through ionic interaction between acidic silicate and MgO as occurred in Magnesium silicate.

Pore expansion with Al salt First, the effect of Al ion concentration was investigated. The molar ratio of the Al salt used for the hydrothermal treatment against Si moles in MMSS was changed from 0.1 to 10. The sample conditions, pH values, and adsorption properties are listed in Table I. SEM images are shown in Figure 2. Regardless of Al/Si ratios, all particles are spherical, and monodispersity is retained. To further investigate the internal structure of Al-treated particles, TEM images were taken for PS-Al-4, and the images are in Figure 3. White and black contrast is seen through a particle, indicating mesopores are homogeneously distributed in the particle. Ten nm-sized silica domains are observed in a high resolution image (Figure 3(b)). When MMSS was hydrothermally treated with HCl, ordered hexagonal structure completely disappeared and instead nano-sized silica particles generated in the sphere.37 The same change occurred when MMSS was hydrothermally treated with an Al salt. Figure S3 (Supporting Information) shows images of EDX-mappings. Si and Al elements are located at the same position at any Al/Si ratios, indicating Al is incorporated homogenously into particles. Al/Si ratios obtained by EDX are 0.078, 0.23, 0.43, and 0.057 for PS-Al-1, 2, 3, and 4, respectively. With increasing the amount of Al salt, Al/Si ratio also increases. However, when the Al/Si ratio of 10 was employed, the ratio obtained with EDX dropped drastically. It can be explained by the increase in the acidity of the solution. Since Al(NO3)3 is acidic, the use of large amounts of the Al salt led to large decrease of pH (ca. 2) in the solution. As a result, Al ions were stabilized in the solution, and the amount of Al ions hybridized into mesoporous siilca were highly reduced.


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XRD patterns are shown in Figure S4 (Supporting Information). The peak at 2.8° which corresponds to the diffraction of 100 facet of hexagonal alignment is detected when the amount of Al salt is lower, indicating that some of the mesopores are kept unchanged. Since Al ions are said to suppress silica dissolution, mesopore transformation was restricted under some Al concentration. However, the peak at 2.8° was mostly disappeared at higher Al/Si ratio although some ordered mesoporosity is retained (Figure S4(e)). To evaluate mesopore structure, nitrogen adsorption-desorption measurements were conducted, and isotherms are shown in Figure 4. The large uptake of nitrogen gas around P/P0 of 0.2 is observed for MMSS. This uptake is observed in the isotherms for PS-Al-1, 2, and 3, indicating the mesopores originated from MMSS still remain in those samples. This uptake disappears in the isotherm of PS-Al-4. The results agree well with those of XRD measurement. Adsorption properties determined from the isotherms are summarized in Table I. The average mesopore diameter of PS-Al-4 is 10.2 nm (Figure S6 (a)). It is revealed that mesopore structure changed drastically during the hydrothermal treatment. To confirm the introduction of Al into the skeleton,

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Al-NMR measurement was conducted and an

NMR chart is shown in Figure 5. The peaks around 0 ppm and 50 ppm are attributed to 6-coordinate and 4-coordinate species, respectively. If Al is incorporated into the skeleton, it should be in 4-coordination state. In case Al2O3 or extraframework hydrated Al3+ is formed, 6-coordination state is dominant. The ratio of 4/6-coordination state is calculated to be 71/29 from the chart. It is concluded that the most of Al is incorporated into the silica skeleton.

Pore expansion with Mg salt The effect of Mg ion concentration was also examined. The molar ratio of Mg salt used for the hydrothermal treatment against Si molars in MMSS was changed from 0.1 to 5. The sample conditions, Mg/Si ratios are listed together with adsorption properties in Table I. SEM images are shown in Figure 6. When the amount of Mg salt is small (PS-Mg-1, 2), collapsed particles are observed. This can be explained by the effect of basicity. All of the solutions before hydrothermal treatment were basic. ACS Paragon Plus Environment

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Especially, at lower Mg salt concentrations, pH values were nearly ten. In this case, silica dissolution proceeds drastically. Since the solutions became acidic after hydrothermal treatment, silica dissolution could be suppressed and spherical shape was kept. Mg/Si ratio should be more than one to suppress the collapse of particles. XRD patterns are shown in Figure S5. The two peaks at 35° and 60° are observed in all patterns. Those peaks are not from MgO because the main peaks of MgO should be at 42° and 62°. In case Forserite (Mg2SiO4) is formed, main peak should be at 23°. It was reported by other researchers that peaks at 35° and 62° are observed during the crystallization of amorphous MgSiO3.38 Although the crystal structure has not been identified yet, the formation of oxide compound containing Mg and Si is mentioned. To investigate the internal structure of the sample, PS-Mg-4 powders were embedded in epoxy resin, and a film was sliced from the resin. Figure 7 shows dark field STEM images of the sliced PS-Mg-4. The center parts of bigger particles are dark, mentioning hollow morphology. Although a few big white particles are seen in the images, these particles are considered to be uncut particles. Black part was not seen in smaller particles because they are cut near the surface of the particle. The particle diameters of PS-Mg series are ca. 0.75 µm, which is bigger than the original MMSS diameter (0.72 µm). If it is assumed that core part dissolves and precipitates at the external side of particles, the diameter of hollow space can be estimated to be 365 nm. From the images in Figure 7, it is obvious that the value is reasonable. The reason why this transformation occurs is unknown. Since the phenomenon is interesting, it should be investigated in detail in future. Figure 8 shows the results of EDS mapping of PS-Mg-4. It is well understood that O, Mg, and Si elements are distributed homogenously throughout a particle. The ratio of Mg/Si is estimated to be 0.07. SEM observation shows the corruption of some particles when the hydrothermal treatment was conducted in lower Mg concentration (Figure 6). This suggests that mesopores be more expanded in such a condition. Nitrogen adsorption-desorption isotherms are shown in Figure 9. The position at which steep increase occurs shifts to higher P/P0 value with decreasing Mg concentration. The result ACS Paragon Plus Environment

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demonstrates that mesopores are more expanded at lower Mg concentration. The inverse dependency of metal concentration on mesopore size against Al is observed in case Mg was used.

Adsorption capacity Through the hydrothermal treatment with metal ions, it is clarified that pore-expansion together with metal incorporation occurs concomitantly. Since ion valencies of Si, Al and Mg are 4, 3 and 2, respectively, the replacement of Si to Al or Mg should lead to the increase of electrons in mesoporous silica. It is highly expected that cationic compounds be more adsorbed onto the pore-expanded mesoporous silicas. To confirm this, an adsorption test of Rhodamine B was conducted because we had previously used Rhodamine B adsorbed mesoporous silica for the emission control.29 The adsorbed amount of Rhodamine B is small, and Rhodamine B is thought to be appropriate to distinguish adsorption performance of samples. Transmission spectra of Rhodamine B solutions after adsorption tests are shown in Figure 10. The transmittance at 542 nm decreases as PS-Mg-4＞PS-Al-4＞MMSS＞ HT-MMSS. The adsorbed amount of Rhodamine B for those materials is estimated, and values are summarized in Table II together with adsorption properties of tested samples. MMSS is mesoporous silica of which pores are not expanded. HT-MMSS is pore-expanded MMSS by using HCl as an expander.

37

MMSS and HT-MMSS do not contain any metal species other than Si. The big difference

between them is the specific surface area. In general, it can be said that the amount of Rhoadamine B adsorbed should be proportional to the specific surface area. PS-Al-4 and PS-Mg-4 adsorb more than ten times of Rhodamine B than HT-MMSS in spite of little difference in specific surface areas. The adsorbed amount of Rhodamine B for PS-Al-4 or PS-Mg-4 is more than twice the value of MMSS even though the specific surface area of MMSS is five times larger than that of PS-Al-4 or PS-Mg-4. If the adsorbed amount is compared with per surface area basis, PS-Mg-4 adsorbs Rhodamine B 27 times more than MMSS. It is demonstrated that the incorporation of Al or Mg into mesoporous silica dramatically increases adsorption capacity of cationic compounds.


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Conclusions In conclusion, hydrothermal treatment of MMSS in an aqueous solution with an Al or Mg salt led to large expansion of mesopore size as well as incorporation of metal species into mesoporous silica, concomitantly. The effect of metal salt concentration on the incorporation of metal species was entirely different for Al and Mg. In the case of Al, a lot of amount of Al salt was needed to expand mesopores largely. However, mesopores were largely expanded when smaller amount of Mg ions was used. Furthermore, it was found that hollow spheres were formed during hydrothermal treatment. It was also demonstrated that adsorption capacity of cationic compounds was dramatically increased. Mgincorporated MMSS adsorbed 27 times of Rhodamine B if compared to MMSS.

Acknowledgements The author would like to thank Mr. Yusuke Akimoto and Mr. Noritomo Suzuki from Toyota Central R & D Labs. Inc. for TEM observations.

Supporting Information XRD patterns for Cu and Fe treated samples, SEM images and corresponding element mappings for PSFe, PS-Cu, PS-Mg, and PS-Al, SEM images and corresponding element mappings for HT-MMSS treated with different Al/Si ratio, XRD patterns for HT-MMSS treated with different Al/Si ratio, XRD patterns for HT-MMSS treated with different Mg/Si ratio, Nitrogen adsorption-desorption isotherms and corresponding pore size distribution curves for PS-Al-4 and PS-Mg-4, αS-plot for PS-Al-4, PS-Mg-4 and MMSS. This information is available free of charge via the Internet at http://pubs.acs.org/.


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29. Yamada, Y.; Yamada, H.; Nakamura, T.; Yano, K. Manipulation of the Spontaneous Emission in Mesoporous Synthetic Opals Impregnated with Fluorescent Guests. Langmuir, 2009, 25, 1359913605, 30. Yamada, H.; Nakamura, T.; Yamada, Y.; Yano, K. Colloidal-Crystal Laser Using Monodispersed Mesoporous Silica Spheres. Adv. Mater. 2009, 21, 4134-4138. 31. Tatsuda, N.; Yano, K. Pore Size Control of Monodispersed Starburst Carbon Spheres. Carbon 2013, 51, 27-35. 32. Chen, J.; Yano, K. Highly Monodispersed Ti n Oxide/Mesoporous Starbust Carbon Composite as High-Performance Li-Ion Battery Anode. ACS Appl. Mater. Interfaces 2013, 5, 7682-7687. 33. Martin, T.; Galarneau A.; Di Renzo, F.; Fajula, F.; Plee, D. Morphological Control of MCM-41 by Pseudomorphic Synthesis. Angew. Chem. Int. Ed. 2002, 41, 2590-2592. 34. Francois, F. Engineering of mesostructured silicas by pseudomorphism. Dalton. Trans. 2007, 3, 291-294. 35. Cheol, Y-W.; Stein, A. Solvent Effects on Morphologies of Mesoporous Silica Spheres Prepared by Pseudomorphic Transformations. Chem. Mater. 2011, 23, 1761-1767. 36. Mizutani, M.; Yamada, Y.; Nakamura, T.; Yano, K. Anomalous Pore Expansion of Highly Monodispersed Mesoporous Silica Spheres and Its Application to the Synthesis of Porous Ferromagnetic Composite. Chem. Mater. 2008, 20, 4777-4782. 37. Yano, K.; Nishi, T. A Novel Route to Highly Monodispersed Mesoporous Silica Spheres Consisting of Nano-Sized Particles. Microporous Mesoporous Mater. 2012, 158, 257-263.


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38. Matsuno, J.; Tsuchiyama, A.; Koike, C.; Chihara, H.; Imai, Y.; Murata, K.; Takahashi, R.; Kohara, S.; Kitajima, Y.; Yoshiasa, A.; Seto. Structural Modification in Amorphous MgSiO3 with Heat Treatments. Preprints of 44th Lunar and Planetary Science Conference 2013, 2199.


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Table I Preparation and Properties of Samples Exp. No.

Metal Salt

Me/Si

Me/Si

added determined

pH

pH

before after heating heating

Specific surface area

Pore volume

Pore diameter

[ml/g]

[nm]

2

by EDX

[m /g]

PS-Al-1

Al3(NO3)39H2O

0.1

0.078

4.22

2.83

1027

0.84

2.0, 4.1

PS-Al-2

Al3(NO3)39H2O

0.5

0.23

3.68

2.58

907

0.67

3.8

PS-Al-3

Al3(NO3)39H2O

1

0.43

3.42

2.67

726

0.57

2.1, 3.8

PS-Al-4

Al3(NO3)39H2O

10

0.057

1.95

1.78

237

0.54

10.2

PS-Mg-1

MgCl26H2O

0.1

0.065

9.94

5.86

116

0.47

-*

PS-Mg-2

MgCl26H2O

0.5

0.068

9.57

5.24

133

0.48

24.1

PS-Mg-3

MgCl26H2O

1

0.075

9.31

5.05

147

0.58

24.4

PS-Mg-4

MgCl26H2O

5

0.075

8.45

4.55

201

0.73

19.1

* Pore diameter cannot be evaluated properly due to large pore size.

Table II Physical properties and the adsorbed amount of RhB of Samples Exp. No.

Metal Species

Me/Si

Specific surface Area

Mean pore diameter

Pore volume

[m2/g]

[nm]

[ml/g]

RhB adsorbed RhB adsorbed [mg/g]

[µg/m2]

MMSS

-

-

1607

2.2

0.79

0.31

0.19

HT-MMSS

-

-

279

18.9

0.67

0.06

0.22

PS-Al-4

Al

0.057

237

10.2

0.54

0.67

2.83

PS-Mg-4

Mg

0.075

201

19.1

0.73

1.04

5.17


15

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Figure captions Figure 1. SEM images of hydrothermally treated MMSSs. (a) PS‐Fe, (b) PS‐Cu, (c) PS‐Mg, (d) PS‐Al. Figure 2. SEM images of HT‐MMSSs treated with different Al/Si ratio. (a) PS‐Al‐1, (b) PS‐Al‐2, (c) PS‐Al‐3, (d) PS‐Al‐4. Figure 3. TEM images of PS‐Al‐4 with (a) lower and (b) higher magnification. Figure 4. SEM images and corresponding oxygen, silicon, and aluminum mappings for HT‐MMSS treated with different Al/Si ratio. (a) PS‐Al‐1, (b) PS‐Al‐2, (c) PS‐Al‐3, (d) PS‐Al‐4. Figure 5. Nitrogen adsorption–desorption isotherms for HT‐MMSS treated under different Al/Si ratios. (a) PS‐Al‐1, (b) PS‐Al‐2, (c) PS‐Al‐3, (d) PS‐Al‐4. Figure 6. SEM images of HT‐MMSSs treated with different Mg/Si ratio. (a) PS‐Mg‐1, (b) PS‐Mg‐2, (c) PS‐Mg‐3, (d) PS‐Mg‐ 4. Figure 7. DF‐STEM image (a) and magnified one (b) of PS‐Mg‐4. Figure 8. DF‐STEM image (a) and corresponding EDS mappings of PS‐Mg‐4 for (b) Oxygen, (c) Mg, and (d) Si. Figure 9. Nitrogen adsorption–desorption isotherms for HT‐MMSS treated under different Mg/Si ratios. (a) PS‐Mg‐1, (b) PS‐Mg‐2, (c) PS‐Mg‐3, (d) PS‐Mg‐4. Figure 10. Fig. 13. Visible spectra of Rhodamine B solution (a) before the adsorption experiment, and after treated with (b) MMSS, (c) HT‐MMSS, (d) PS‐Al‐4, and (d) PS‐Mg‐4.


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(a)

(b)

(c)

(d)

Figure 1. SEM images of hydrothermally treated MMSSs. (a) PS-Fe, (b) PS-Cu, (c) PS-Mg, (d) PS-Al.


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(a)

(b)

(c)

(d)

Figure 2. SEM images of HT-MMSSs treated with different Al/Si ratio. (a) PSAl-1, (b) PS-Al-2, (c) PS-Al-3, (d) PS-Al-4.


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(a)

(b)

100 nm

Figure 3. TEM images of PS-Al-4 with (a) lower and (b) higher magnification.


(e)

Amount adsorbed / cm3STPg-1

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200

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(d)

(c) (b)

(a)

P/P0 Figure 4. Nitrogen adsorption–desorption isotherms for MMSS and HT-MMSS treated under different Al/Si ratios. (a) MMSS, (b) PS-Al-1, (c) PS-Al-2, (d) PS-Al-3, (e) PS-Al-4. ACS Paragon Plus Environment

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4-Al 6-Al *

  ppm Figure 5. 27Al-NMR chart for PS-Al-4. * Spinning side band.


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(a)

(b)

(c)

(d)

Figure 6. SEM images of HT-MMSSs treated with different Mg/Si ratio. (a) PS-Mg-1, (b) PS-Mg-2, (c) PS-Mg-3, (d) PS-Mg-4. ACS Paragon Plus Environment

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(a)

(b)

2 m

500 nm

Figure 7. DF-STEM image (a) and magnified one (b) of PS-Mg-4.


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(a)

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(b)

250 nm

DF

(c)

250 nm

O K

250 nm

Si K

(d)

250 nm

Mg K

Figure 8. DF-STEM image (a) and corresponding EDS mappings of PS-Mg-4 for (b) Oxygen, (c) Mg, and (d) Si. ACS Paragon Plus Environment

Amount adsorbed / cm3STPg-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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200

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(d)

(c)

(b)

(a)

P/P0 Figure 9. Nitrogen adsorption–desorption isotherms for HT-MMSS treated under different Mg/Si ratios. (a) PS-Mg-1, (b) PS-Mg-2, (c) PS-Mg-3, (d) PS-Mg-4. ACS Paragon Plus Environment

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(e) (d) (c) (b) (a)

Figure 10. Visible spectra of Rhodamine B solution (a) before the adsorption experiment, and after treated with (b) HT-MMSS, (c) MMSS, (d) PS-Al-4, and (d) PS-Mg-4. ACS Paragon Plus Environment

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Graphical contents entry


Hollow Metal-Incorporated Monodispersed Mesoporous Silica

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