Fabrication of hollow spheres composed of nanosized ZSM-5 crystals via laser ablation

Microporous and Mesoporous Materials 86 (2005) 14–22 www.elsevier.com/locate/micromeso

Fabrication of hollow spheres composed of nanosized ZSM-5 crystals via laser ablation Chunrong Xiong, Decio Coutinho, Kenneth J. Balkus Jr.

*

Department of Chemistry, The University of Texas at Dallas, P.O. Box 830688, Richardson, TX 75083-0688, USA Received 17 December 2004; accepted 17 May 2005 Available online 24 August 2005

Abstract Zeolite spheres with a core-shell structure were fabricated by a combination of pulsed laser deposition (PLD) and vapor-phase crystallization. Spherical mesoporous DAM-1 or SBA-15 was used as the silicon source and substrate. After vapor phase treatment hollow spherical shells consisting of nanosized ZSM-5 crystals were formed. In addition, the size of the ZSM-5 crystals in the shell ranges from 5 to 500 nm which can be adjusted by changing the crystallization time (3–5 days) or the PLD coating thickness. The deposition rate was 13 nm/min at 120 mJ and 6.5 nm/min at 70 mJ. The ZSM-5 core–shell structure was characterized by XRD, IR, SEM and N2 absorption. The dissolution of the mesoporous silica substrate and transport of the resulting silica species to the growing ZSM-5 film during vapor-phase crystallization is discussed in terms of a solution mediated transport mechanism. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Hollow sphere; Nanosized ZSM-5; DAM-1 and SBA-15; Laser ablation

1. Introduction There is growing interest in the preparation of organic or inorganic hollow spheres due to their core–shell structure [1–16], which may have potential application in areas such as drug delivery, catalysis, acoustic insulation, composite materials etc. [1–24]. The methods currently employed to fabricate hollow spheres mainly involve precipitation, chemical vapor deposition and layer-by-layer templating (LbL) [7,10,17,25–28] based on the electrostatic attraction between nanoparticles and oppositely charged substrates. There are a variety of potential shell materials, such as polymers [7,8], metal oxides [11], metals [1,5], and more recently zeolites [6,12,15,16]. Zeolites are crystalline, microporous aluminosilicates that are widely used in catalysis, separations, and *

Corresponding author. Tel.: +1 972 883 2659; fax: +1 972 883 2925. E-mail address: [email protected] (K.J. Balkus). 1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.05.052

adsorption. Although, there has been progress in controlling the zeolite form, including monoliths [29], foams [30], beads [31], hollow fibers [32] and spheres [33], only recently have core shell structures been prepared [15,16]. In one case, zeolite nanoparticles were assembled onto polymer spheres to form a core–shell structure, subsequent removal of the polymer core results in a zeolite hollow sphere [6,12]. Dong et al. [15,16] employed nanoscale ZSM-5 seeds (60 nm) to coat mesoporous silica spheres. Subsequently, vapor-phase crystallization resulted in dissolution of the mesoporous silica core and formation of hollow ZSM-5 spheres. This approach is interesting because guest species may be encapsulated in the mesoporous sphere which may then be used to functionalize the interior of the hollow spheres, enhancing the potential applications in catalysis and drug delivery. We have now extended this strategy to include the application of the pulsed laser deposition (PLD) in the preparation of zeolite films [34–37]. The PLD method involves irradiation of a pressed zeolite target with a

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pulsed excimer laser beam (248 nm). This results in a plume of fragments that evenly coat a substrate positioned some distance from the target. We have developed patented technology for evenly coating small objects by vibrating them in the plume [42]. The coated particles are then subjected to a hydrothermal treatment to recrystallize the PLD zeolites coating. In the present case, the ZSM-5 fragments were deposited by PLD onto mesoporous DAM-1 and SBA-15 spheres using a vibrating pan [42]. The result is a well-adhered uniform coating of zeolite fragments which can be viewed as protozeolite seeds [43–45]. The nature of the fragments is unknown but they are proposed to contain structural building units (SBU). The vapor-phase treatment of these coated DAM-1 and SBA-15 spheres resulted in dissolution of the DAM-1 and crystallization of ZSM5 to form hollow spheres.

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2.3. Pulse laser deposition of ZSM-5 on DAM-1 and SBA-15

2. Experimental

A ZSM-5 target was prepared according to literature procedure [36]. A mixture of 4.0 g of ZSM-5 and 2.0 g of ferrocene was pressed into pellets 2.5 cm in diameter and 3 mm in thickness. The target was placed in a controlled atmosphere chamber positioned 2.5 cm above a dish containing 0.3 g DAM-1 or SBA-15 spheres at an angle of 40°. The dish was vibrated using a pancake vibrator of the variety often found in cell phones and pagers as described in Ref. [42]. A COMPEX 100 (Lamda Physik Lasertechnik) excimer laser (248 nm, KrF*) was used to ablate the target at a frequency of 10 Hz. A computer-controlled rastering mirror (Oriel) was used to reflect and raster the beam. The spot size of the laser beam was focused on an area of 0.001 cm2 by a focusing lens. The oxygen background pressure was controlled in a range of 210–250 mTorr prior to laser deposition at room temperature.

2.1. Materials

2.4. Vapor-phase crystallization

Tetraethoxysilane (TEOS, Aldrich), hydrochloric acid (Mallinckrodt), vitamin E TPGS (Eastman), triethylamine (TEA, Aldrich), ethylenediamine (EDA, Aldrich), ethyl alcohol (Aldrich), H-ZSM-5 (Si/Al = 56 mol/mol, Sud-Chemie AG), Pluronic P123 (BASF) and cetyltrimethylammonium bromide (CTAB, Aldrich) were used as-received.

DAM-1 and SBA-15 spheres coated with ZSM-5 fragments were placed in a small cup made of stainless steel screen which was held in a Teflon lined autoclave 4 cm above a liquid mixture of 6 ml of TEA, 0.2 ml of EDA and 1.5 ml of H2O. The autoclave was then sealed, and heated at 145 °C for 3–7 days. The resulting spheres were washed with deionized water, dried at 90 °C for 10 h, and calcined at 550 °C for 12 h. Additionally, a blank experiment was conducted by direct VP treatment of DAM-1 spheres without a PLD coating of ZSM-5 for 4 days.

2.2. Preparation of spherical DAM-1 and SBA-15 DAM-1 spheres were synthesized according to a literature procedure [37]. In a typical preparation, 1.8 g of Vitamin E TPGS were dissolved in a solution containing water, HCl, formamide and ethanol. The solution was stirred for 4 h at room temperature. Then TEOS was added to the homogeneous mixture with stirring at room temperature for 30 min, and aged at room temperature for 4 days. The final molar composition of the gel was 617H2O:54.4HCl:107.8HCONH2O:24Vitamin E TPGS:52.9ethanol:1.0TEOS. The product DAM-1 spheres were isolated by vacuum filtration, washed with deionized water, then dried at 90 °C for 12 h and calcined at 550 °C for 15 h. SBA-15 spheres were prepared by using the cationic surfactant CTAB as a co-surfactant with P123 under acidic conditions. In a typical synthesis, 0.35 g of P123 block copolymer and 0.3 g of CTAB were dissolved with stirring in a 4 M HCl solution for 20 min. Then 1.8 g of TEOS was added with stirring at room temperature for 16 h. The gel was then crystallized at 40 °C for 24 h without stirring. The SBA-15 spheres were recovered by suction filtration, then dried at 90 °C and calcined at 550 °C.

2.5. Characterization The mesoporous silica spheres and the hollow zeolite spheres were characterized by powder X-ray diffraction (XRD) (Scintag XDS 2000 X-ray diffractometer with CuKa radiation). The morphologies were evaluated by scanning electron microscopy (Philips XL60 microscope and LEO field emission SEM) from Au/Pd coated samples. FT-IR spectra were recorded using an avatar 360 FT-IR from KBr pellets, Surface area and pore size were measured using a Quantachrome Autosorb I, from N2 absorption and DFT analysis.

3. Results and discussion Fig. 1a shows the XRD pattern of the calcined DAM-1 spheres which exhibit a broad peak at 2h = 1.1°, typical of the disordered wormhole pore structure previously observed for DAM-1 spheres [38]. The mesoporous DAM-1 spheres were in range of

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Intensity

16

b

a 0.9

1.9

2.9

3.9

4.9

2 Theta/Degree Fig. 1. XRD patterns of (a) DAM-1 and (b) SBA-15.

3–6 lm in diameter, as shown in Fig. 2. Meanwhile, spherical SBA-15 particles were also prepared, and the XRD pattern shown in Fig. 1b suggests a poorly ordered mesoporous structure. The calcined DAM-1 and SBA-15 were then coated with ZSM-5 fragments by PLD. In order to understand the transformation of a PLD coating to a ZSM-5 hollow sphere, one needs to know

Fig. 2. SEM images of DAM-1 and SBA-15 spheres.

the thickness of the PLD film. However, obtaining a cross-section view of a PLD film on a microsphere is difficult. Therefore, as shown in Fig. 3, the thickness of PLD coatings of ZSM-5 on silicon wafers was evaluated at 120 mJ and 70 mJ of laser energy output. When the coating time was 45 min, the thickness of the PLD layer at 120 mJ was 600 nm, whereas the thickness was 300 nm at 70 mJ. Thus at 120 mJ the deposition rate was 13 nm/min and at 70 mJ the deposition rate was 6.5 nm/min. The ZSM-5 PLD films are mostly amorphous to X-rays (shown in Figs. 4 and 5). However, analysis of the PLD particles by N2 adsorption revealed a surface area similar to the target ZSM-5 but with a ˚ ) (shown in Table 2). This is simsmaller pore size (3.7 A ilar to ZSM-5 precursor materials described as protozeolites where no apparent ZSM-5 characteristic peaks was observed by X-ray diffraction but other characterization reveals the presence of basic building units [46]. Compared with original surface morphology of the DAM-1 sphere, the PLD layer appears rougher and the fragment size varies in range from 20 to 50 nm (shown in Fig. 4).

Fig. 3. SEM images of cross-sections of PLD coating layer on silica wafers, (a) 70 mJ of energy output, 45 min of coating time, (b) 120 mJ of energy output, 45 min of coating time.

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Table 1 Conditions for preparation of ZSM-5 coated DAM-1 samples Samples

A B C D SBA-15

Fig. 4. (a) Surface of DAM-1 spheres, and (b) PLD coating layer on DAM-1 at 70 mJ of energy output for 45 min.

f Intensity

e d c b a 5

15

25

35

45

2 Theta/Degree Fig. 5. XRD patterns of samples: (a) DAM-1 coated with ZSM-5 flagments, (b) sample A, (c) sample B, (d) sample C, (e) sample D, (f) ZSM-5 (Si/Al = 56 mol/mol).

In the present study, four samples were prepared from DAM-1 spheres by the combination of PLD coating and VP crystallization. The synthesis parameters are listed in Table 1. Fig. 5 shows the X-ray diffraction pat-

PLD operation conditions Frequency (Hz)

Energy output (mJ/pulse)

Coating time (min)

10 10 10 10 10

65 65 65 120 70

20 20 20 20 20

VP crystallization time (days)

3 4 5 3 3.5

terns of the samples prepared by varying the crystallization time and the energy output during the PLD coating. The XRD patterns clearly indicate that crystals with the MFI structure formed in all samples. The fragments in the PLD film can act as protozeolite seeds to aid in formation of the ZSM-5 structure during VP crystallization. With an increase in the crystallization time, there was an increase in the degree of crystallinity. Sample B was treated one day longer than sample A (3 days) and the degree of crystallinity improved by 500%. The growth rate of MFI crystals continued to improve after 5 days (sample C) to nearly 100%. The crystallization rate was also affected by the PLD coating thickness which can be adjusted by controlling the laser energy output. Sample D, was coated using a laser energy of 120 mJ, and the degree of crystallinity after 3 days was 30% compared to 10% for sample A prepared using a laser energy of 65 mJ. The blank experiment using DAM-1 without a PLD layer did not exhibit any MFI X-ray diffraction peaks, which is consistent with the requirement of a PLD coating of protozeolite like fragments for vapor-phase crystallization to form hollow spheres. Compared with the ZSM-5 (Si/Al = 56) used as the target, the reflections for sample C shift 0.06° to higher angle. This reflects a smaller unit cell because the Si/Al ratio (718) in the generated ZSM-5 crystals increased with VP treatment time by consuming more silica from the DAM-1 core. The crystallinity of ZSM5 core–shell structures obtained after 4 days by the PLD seeding method are similar to the hollow spheres prepared by the electrostatic assembly method after 2 days [15,16]. However, the PLD method has the distinct advantage of controlling the protozeolite coating thickness by simply changing the ablation time. The infrared spectra ranging from 400 to 1450 cm 1 are shown in Fig. 6. The framework vibration at 550 cm 1 is indicative of MFI-type zeolites and associated double five-membered ring units [39]. The band near 1200 cm 1 is also present in double five-membered ring containing structures [40,46]. Although the ZSM-5 fragments collected during PLD are largely amorphous, the FTIR spectrum for the PLD coating reveals weak shoulders at 1200 and 550 cm 1 consistent with the MFI structural building units which is also similar to

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Transmittance %

Fragments produced by PLD ZSM-5

D C B A

1300

1100

900

700

500

300

Wavenumber (cm-1) Fig. 6. IR spectra of samples A–D, ZSM-5, and PLD fragments.

the IR data reported for ZSM-5 protozeolite seeds [43– 46]. The intensity ratio of the 550–450 cm 1 bands is indicative of the degree of crystallinity [39]. The band at 1100 cm 1 is assigned to the internal tetrahedral asymmetric stretching vibration of Si(Al)O4. When the Si/Al ratio decreases, the band shifts to higher wavenumber. For sample C coated at 70 mJ, the band appeared at 1090 cm 1, compared with 1100 cm 1 for sample D coated at 120 mJ, because more alumina was introduced on the sphere surface from target. The Si/ Al ratio was 718 for sample C, and 368 for sample D. The Si/Fe ratio in the target was 6.1, and EDS revealed the Si/Fe ratio was 78.1 for sample C, and 40 for sample D. Compared with the seed method [15,16], one may adjust the Si/Al ratio of the ZSM-5 shell by changing PLD coating thickness and/or by using a ZSM-5 target with different aluminum content. Again the thickness of a seeded film is hard to quantitatively control using just electrostatic assembly of nanocrystals [15,16]. The SEM images in Fig. 7 for sample A after 3 days of VP crystallization reveal a few crystals less than 200 nm dispersed on the surface of the spheres. Most of the surface appears amorphous but rougher than the starting PLD film. Considering the calculated PLD film thickness was 130 nm, the crystals must be growing at the expense of the DAM-1 core. Fig. 8 shows the SEM images of sample B which was vapor phase treated for 4 days. In this case, the surface is clearly composed of small crystals ranging in size from a 200 nm to 500 nm. Many crystals exhibit the typical coffin shaped morphology of ZSM-5. Fig. 8 also reveals that the DAM-1 core has been consumed to give hollow shells. The great increase in crystal growth must be at the expense of the DAM-1. After 5 days (sample C shown in Fig. 9), the ZSM-5 crystals increased in size to 1.0 lm. The increase in crystal size appears to consume the smaller spheres generating ring-like structures of ZSM-5. Interestingly, for the electrostatic assembly method [16], even when 200 nm sized silicalite-1 was

Fig. 7. SEM images of sample A spheres after a 3 day vapor-phase treatment.

used as seeds, the shell thickness was only 350 nm after complete consumption of the silica core, and all the ZSM-5 crystals located on the surface of shell. Sample D started with a PLD film nearly two times as thick as sample A, in this case after 3 days of vapor-phase treatment, the spheres appear to be more crystalline as shown in Fig. 10. These images reveal that the DAM-1 core has partially dissolved at this point. Thus, besides conveniently adjusting Si/Al ratio by controlling PLD coating thickness, the VP crystallization rate can be adjusted by varying the PLD film. The N2 adsorption results for samples A through D listed in Table 2 are consistent with the XRD and SEM results which show an increase in crystallinity with an increase in VP treatment time. Interestingly, after 3 days of vapor-phase treatment, there is no evidence of DAM-1 remaining. In a blank experiment using DAM-1 spheres without a PLD coating, the vapor-phase treatment for 3 days resulted in a completely amorphous material. Thus it is reasonable that remnants of DAM-1 are not observed for the PLD coated samples. If the PLD coating is continuous, how does the dissolution of the DAM-1 substrate occur? There are two

C. Xiong et al. / Microporous and Mesoporous Materials 86 (2005) 14–22

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Fig. 9. SEM images of sample C spheres after a 5 day vapor-phase treatment. Fig. 8. SEM images of sample B spheres after a 4 day vapor-phase treatment.

possible mechanisms proposed for crystal growth. The first is the ‘‘solution mediated transport’’ mechanism [41], in which an amorphous gel is dissolved providing reactants for nucleation that ultimately grows into crystals. The other mechanism is the ‘‘solid phase transformation’’ mechanism [42], in which an amorphous gel is converted into crystals. In our case, the solution mediated transport mechanism is more likely since at autog-

enous pressure and 145 °C, the water and amine likely condense on the sphere surface or into the mesopores. Some of the PLD layer and DAM-1 can be dissolved in the liquid phase. The fragments coated on surface of DAM-1 by PLD act as crystal seeds and grow from the nutrients in the liquid phase. Interestingly, we found that there were fiber-like structures with nanosized ZSM-5 crystals on DAM-1 surface (shown in Figs. 7 and 8). These fibers may have resulted from migration of silica on the surface during growth of ZSM-5 by

C. Xiong et al. / Microporous and Mesoporous Materials 86 (2005) 14–22

Intensity

20

5

15

25

35

45

55

2 Theta/Degree Fig. 11. XRD pattern of sample made from DAM-1 spheres containing Vit E TPGS.

Fig. 10. SEM images of sample D spheres after a 3 day vapor-phase treatment.

consumption of silica. Since the DAM-1 substrate appears to be dissolved after 3 days, the continued growth of ZSM-5 crystals may be a good example of Ostwald ripening, transforming nanosized ZSM-5 into micron sized crystals. Unfortunately, this process ultimately resulted in loss of the core–shell structure. Zeolite core–shell structures have been prepared with metal encapsulated inside [16]. It may be possible to include guest molecules in DAM-1 via the vitamin E TPGS template. Therefore, as-synthesized DAM-1 was

Table 2 Physiochemical properties of samples and comparison experiments Samples DAM-1 Blank experiment ZSM-5 Sample A Sample B Sample C ZSM-5 fragments collected during PLD

BET SA (m2/g)

Pore volume (cm3/g)

Pore size ˚) (A

808 66 345 145 250 330 316

0.36 0.30 0.19 0.24 0.20 0.20 0.13

21.7 171 5.5 5.1 5.3 5.5 3.7

Fig. 12. SEM images of sample made from DAM-1 spheres containing Vit E TPGS.

coated with ZSM-5 by PLD coating and vapor phase treated for 4 days. The XRD pattern shown in Fig. 11 confirms growth of the MFI phase, however, the crystallinity is less than sample B (30% vs 70%). The Vit E TPGS template may hinder the migration of silica

Intensity

C. Xiong et al. / Microporous and Mesoporous Materials 86 (2005) 14–22

b

a 5

15

25

35

45

55

2 Theta/Degree Fig. 13. XRD patterns of samples (a) sample made from SBA-15 spheres, (b) ZSM-5 (Si/Al = 56 mol/mol).

21

Another variable that could affect regrowth of the ZSM-5 shell is the substrate size. Therefore, the DAM-1 spheres were replaced with SBA-15 spheres with 1–2 lm in diameter. In this case, the laser energy was 70 mJ, and VP crystallization period was 3.5 days. The XRD pattern appears to be nearly 100% crystalline ZSM-5 (Fig. 13). The small spheres seem to dissolve and recrystallize faster than bigger DAM-1 spheres, in part because the amount of PLD coating per unit volume of silica substrate is greater for small spheres. The SEM images in Fig. 14 reveals a shell composed of uniform nanosized ZSM-5 crystals. These results imply the PLD coating technique can be widely applied to other mesoporous silica particles in the fabrication of core– shell structures. 4. Conclusion Spherical shells composed of nanosized ZSM-5 crystals were fabricated from spherical DAM-1 or SBA-15 coated with PLD film. The PLD coating is composed of protozeolite like fragments which reorganize to form crystalline ZSM-5. The PLD technique offers some advantages over conventional seeding methods such as the convenient PLD coating of 3-D objects and control over the coating thickness. By changing the crystallization time or the thickness of the coated fragments, we might effectively control the growth of the zeolites crystals composing the shell. However, excessive growth of ZSM-5 crystals could result in breakdown of the spherical shells. At vapor-phase treatment times of 3 days or less the ZSM-5 crystals are 5 nm or less. However, as the VPT time increased to 5 days or more the crystals grew to 500 nm. Acknowledgment We thank the Robert A. Welch foundation for financial support. References

Fig. 14. SEM images of sample made from SBA-15 spheres.

species during VP treatment. The SEM images (shown in Fig. 12) reveal a partial core–shell structure but no obvious coffin shaped crystals of ZSM-5. Nevertheless, the fabrication of core–shell structure from DAM-1 containing template is possible and may allow for inclusion of guest species inside the hollow zeolite spheres.

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