Concentration-dependent mesostructure of surfactant-templated mesoporous silica thin film

Thin Solid Films 494 (2006) 320 – 324 www.elsevier.com/locate/tsf

Concentration-dependent mesostructure of surfactant-templated mesoporous silica thin film Sang-Bae Jung, Hyung-Ho Park * Department of Ceramic Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Ku, Seoul, 120-749, South Korea Available online 12 September 2005

Abstract Surfactant-templated mesoporous silica thin films by using Brij-76 block copolymer were prepared from self-assembly of organic – inorganic species and the concentration-dependent mesostructure of the films was investigated. Silica sol with various aging time was spin-coated to check the possibility of structural transition from polycondensation reaction. X-ray diffraction and glazing incidence small-angle X-ray scattering measurement revealed that mesoporous silica film exhibits body-centered cubic structure depending on the surfactant molar ratio. By varying the concentration of the block copolymer, a mesoporous silica film with different pore size and pore ordering could be prepared. Degradation of mechanical properties of the film was found to be related with the increase of the porosity of mesoporous silica film, i.e., pore size and network wall thickness and the degree of pore ordering. D 2005 Elsevier B.V. All rights reserved. Keywords: Low-k; Ordered; Mesoporous silica films; Body-centered cubic; Brij-76; Surfactant concentration; Glazing incidence small-angle X-ray scattering

1. Introduction There has been an increasing interest in ordered mesoporous materials for applications in separation, catalysis, chemical sensing, optical coatings, and intermetal dielectrics [1– 3]. One of the synthesis procedures of mesoporous materials involves the formation of organic– inorganic composites by self-assembly, where organic surfactant forms a micelle surrounded by inorganic on a mesoscopic scale and serves as a sacrificial template for mesopores. Depending on the nature of templates, composition, and synthesis conditions, mesoporous materials form 2D-hexagonal, 3D-hexagonal or cubic mesostructures [4]. Mesoporous thin film can be formed by the evaporation-induced self-assembly process (EISA). From the homogeneous solution of silica and surfactant dissolved in alcohol below critical micelle concentration, the

* Corresponding author. Tel.: +82 2 2123 2853; fax: +82 2 365 5882. E-mail address: [email protected] (H.-H. Park). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.160

preferential evaporation of the solvent during dip or spin coating drives cooperative self-assembly of the silica and surfactant [5– 7]. Up to now, studies on the determination and adjustment of the mesostructure and the tailoring of pore size have been widely carried out. However, mechanical properties of mesoporous silica films depending on the pore structure have not been thoroughly investigated. Mechanical properties of mesoporous films are strongly dependent on the porosity, mesostructure, wall thickness, pore size, and the degree of pore ordering [8]. In this work, a composition range exhibiting an identical mesostructure and relationship between mechanical properties and porosity and/or degree of pore ordering were studied. The effect of sol aging on the mesostructure, i.e., the possibility of structural transition was examined. For these purposes, ordered mesoporous silica films using Brij-76 block copolymer (C18H37(OCH2CH2)10OH) were prepared by EISA process [9]. Mesoporous silica films with different porosity and degree of pore ordering were prepared by varying block copolymer/tetraethoxysilane (TEOS) molar ratio.

S.-B. Jung, H.-H. Park / Thin Solid Films 494 (2006) 320 – 324

3. Results and discussion In acidic silica sol, hydrolysis reaction is completed right after the addition of the catalyst while polycondensation reaction proceeds slowly [12]. In this experiment, the polycondensation reaction of silicic acid proceeds simultaneously with sol aging. In order to investigate the effect of sol-aging time on the formation of the mesostructure, the dspacing of major diffraction peak with aging time was measured by XRD for the as-prepared and mesoporous silica films with Brij-76/TEOS molar ratio of 0.05 and given in Fig. 1(a). Interplanar spacing of the as-prepared film gradually increases with sol-aging time. Mesoporous silica film shows the same behavior with anisotropic shrinkage of d-spacing after calcination. It is well known that the micelle structure templated by the block copolymer is made up of a hydrophobic core surrounded by hydrophilic polyethylene oxide (PEO) chains and silica wall. Due to the solubility of PEO chains into silica framework, a part of PEO chains may form an intermediate region between the hydrophobic block and silica, corona region [13]. The polycondensation

60

d spacing (A)

as-prepared film mesoporous film °

The precursor solution was prepared with a two-step process. The detailed experimental procedure has been given elsewhere [9]. The final composition of TEOS/ ethanol/H2O/HCl/Brij-76 was 1:20:5:0.01:x (x = 0.05, 0.07, 0.09 and 0.1). After 1 day aging, silica sol was spindeposited at 3000 rpm for 30 s. The as-prepared film was exposed to air for 2 h and the mesoporous silica film was formed by calcination at 400 -C with a heating rate of 1 -C/ min. Especially, a sol of Brij-76/TEOS molar ratio of 0.05 was aged up to 20 days in order to investigate the effect of sol aging on the mesoporous film structure. For the investigation of the mesoporous film structure, X-ray diffraction (XRD) patterns were collected using synchrotron radiation at 3C2 beamline of Pohang Light Source (PLS) in Korea [10]. The porosity of film was calculated by measuring the critical angle of film using specular X-ray reflectivity at 3C2 [11]. Glazing incidence small-angle X-ray scattering (GISAXS) measurements were ˚ and Dk/ also carried out at 4C2 beamline (k = 1.54 A k = 5  10 4) consisting of Si (111) double-crystal monochromators, ion chambers, and a two-dimensional positionsensitive detector with 1242  1152 pixels. The sample-todetector distance was 1.31 m and the incident angle of primary beam to sample surface was chosen to cover the whole film thickness range. Mechanical properties of the film were investigated by continuous stiffness measurement using a MTS nanoindentor XP. In this technique, a small oscillating force is applied to the Berkovich indenter, and the amplitude and phase shift of the oscillations are measured. This allows a continuous measurement of hardness and modulus even for extremely thin-layered materials.

(a)

50

40

30 0

5

10

15

20

Sol aging time (day) (b) as-prepared film mesoporous film

1.0

Relative intensity

2. Experimental procedure

321

0.8 0.6 0.4 0.2 0.0 0

5

10

15

20

Sol aging time (day) Fig. 1. (a) d-spacing and (b) relative intensity of major diffraction peak in the XRD measurements of as-prepared and mesoporous silica films with Brij-76/TEOS molar ratio of 0.05 by varying sol-aging time.

reaction reduces the charge density of the silicic acid and the interaction between silica and block copolymer. As a result, PEO chains are driven into the hydrophobic core region and the hydrocarbon chains elongate in order to reduce the interaction with PEO chains. Therefore, increased d-spacing in Fig. 1(a) was attributed to the elongated hydrocarbon chains [14]. Fig. 1(b) represents the changes in the XRD peak intensities of the as-prepared and mesoporous silica films as a function of sol aging. It was observed that the diffraction peak intensity was maximized with the film prepared using 1 day aged sol. Structuration by block copolymer was determined from the charge density matching between inorganic silicic acid and hydrated block copolymer mediated by negative chlorine ion [13]. After 1 day aging, the charge density of the silicic acid was reduced, and therefore, organic– inorganic structuration was impeded due to a strong interaction in inorganic – inorganic or organic –organic. However, even with a variation in solaging time, the mesostructure of film was not changed, and it was confirmed by GISAXS observation. The increase of block copolymer concentration may retard the polycondensation reaction, and possible structural transition from the polycondensation reaction could be depressed.

322

S.-B. Jung, H.-H. Park / Thin Solid Films 494 (2006) 320 – 324

greatly different, as much as 22%, which proves the anisotropic shrinkage of the film during the calcination. Fig. 3 shows the XRD spectra of the as-prepared and mesoporous silica films with various Brij-76/TEOS molar ratio. In Fig. 3(a), d-spacing of (011) peak increased with a molar ratio from 0.05 to 0.07 and maintained constant over 0.07. The increase in d-spacing was mainly attributed to the length of the hydrocarbon chain when the molar ratio increased from 0.05 to 0.07. In the case of the molar ratio of 0.09, it was considered that thinning of the silica wall and lengthening of the hydrocarbon chain simultaneously affected the d-spacing. In Fig. 3(b), interplanar spacing for mesoporous silica film increases with the increase in molar ratio from 0.05 to 0.07 due to the increase of pore diameter. Due to the thermal contraction of the thin silica wall during the calcination, a decrease of d-spacing with the molar ratio of 0.09 was found. Fig. 4 shows the porosity variation depending on the Brij-76/TEOS molar ratio. Porosity values of mesoporous silica film fabricated with molar ratios of 0.05 and 0.07 were 37.6% and 51.2%, respectively. This increased porosity could be related to the increased mesopore size as given in Fig. 3(b). Although the d-spacing of the mesoporous film with molar ratio of 0.09 was smaller than that of 0.07, the film shows a slightly increased porosity of 52.8% due to the thinning of the silica wall as described previously. Fig. 5 shows the elastic modulus and hardness of the mesoporous silica film according to Brij-76/TEOS molar ratio. Due to the hard substrate effect, elastic modulus and hardness steadily rose as indentation depth increases. So,

Fig. 2 shows the GISAXS patterns of the as-prepared films and mesoporous silica film using a sol aged for 1 day with varying the block copolymer concentration. In GISAXS pattern, elliptic spots on faint ring could be clearly seen. The faint ring corresponds to domains with the same structure but random orientation and elliptic spots show the mesostructure of the film. The extra spot located in the upper part of main spot was caused by the reflection effect at substrate surface [15]. In Fig. 2(a) and (b), GISAXS pattern was indexed with BCC structure [16]. This BCC mesostructure was not maintained with block copolymer concentration of 0.1. Worm-like micelle and lamellar phase coexist as given in Fig. 2(c). Fig. 2(d) corresponds to the mesoporous silica film with molar ratio of 0.05 and the pattern could be indexed with BCC structure as in the case of the as-prepared film. Up to now, hexagonal-closed packed structure and face-centered cubic structure have been proposed as a possible three-dimensional micellar structure for mesoporous silica prepared using the Brij-76 block copolymer [17]. In the case of the mesoporous film with one-dimensional shrinkage of the BCC structure along b011, a structural transformation into face-centered orthorhombic (FCO) or base-centered monoclinic could be induced [18,19]. For example, (011), (101), and (01-1) in Fig. 2(a) can be assigned as (020), (111), and (020) of FCO ˚ , b = 89.4 A ˚ , and with lattice parameters of a = 77.5 A ˚ c = 114.2 A. In Fig. 2(d), the lattice parameter was obtained ˚ , b = 73.4 A ˚ , c = 108.5 A ˚ with FCO structure. a as a = 74.1 A and c axis values parallel to the film surface were slightly different while b axis values normal to the film surface were

(a)

(b)

0.2

0.2

011

011 0.15

0.15

qz

qz

101

0.1 0.05

0.1

101

0.05

011

011

0

0 0.05 0.1 0.15

0

0.05 0.1 0.15

0.05 0.1 0.15

qy

(c)

0.05 0.1 0.15

(d) 011

20 0.2

0.2

0.15

qz

0

qy

0.15 10

0.1 0.05

qz

101

0.1 011

0.05

0

0 0.05 0.1 0.15

0

qy

0.05 0.1 0.15

0.05 0.1 0.15

0

0.05 0.1 0.15

qy

Fig. 2. GISAXS patterns of as-prepared silica films with Brij-76/TEOS molar ratio of (a) 0.05, (b) 0.07 and (c) 0.1 by using the sol aged for 1 day and (d) after calcination of film (a), i.e., mesoporous silica film.

S.-B. Jung, H.-H. Park / Thin Solid Films 494 (2006) 320 – 324

(a)

323

(a) Brij-76/TEOS = 0.05 Brij-76/TEOS = 0.07 Brij-76/TEOS = 0.09

80

Elastic modulus (GPa)

Intensity (arb. unit)

Brij-76/TEOS = 0.05 Brij-76/TEOS = 0.07 Brij-76/TEOS = 0.09

60 40 20 0 0

0

1

2

3

4

5

50 100 Indentation depth (nm)

6

(b)

2θ

4 Brij-76/TEOS = 0.05 Brij-76/TEOS = 0.07 Brij-76/TEOS = 0.09

Intensity (arb. unit)

Brij-76/TEOS = 0.05 Brij-76/TEOS = 0.07 Brij-76/TEOS = 0.09

Hardness (GPa)

(b) 3 2 1 0 0

0

1

2

3

4

5

6

2θ Fig. 3. XRD spectra of (a) as-prepared and (b) mesoporous silica films for various Brij-76/TEOS molar ratio.

porosity ( )

mechanical properties were extracted from the flat region in Fig. 5 and are summarized in Table 1. The elastic modulus and hardness values of mesoporous silica films were reduced with the increase in the molar ratio. Mechanical properties of the porous material are strongly dependent on porosity, pore wall rigidity, and the degree of pore ordering. When the molar ratio increases from 0.05 to 0.07, a degradation of mechanical properties is mainly caused by 54 52 50 48 46 44 42 40 38 36 0.05

0.07

0.09

Brij-76/TEOS molar ratio Fig. 4. Porosity of mesoporous silica films as a function of Brij-76/TEOS molar ratio.

50 100 Indentation depth (nm)

Fig. 5. (a) Elastic modulus and (b) hardness of mesoporous silica films with various Brij-76/TEOS molar ratio.

the increase in porosity. However, with the case of molar ratio of 0.09, a decrease in silica wall thickness and an increase in pore disordering as shown in Fig. 3(b) might be the main causes of degradation.

4. Conclusions Mesoporous silica films with the same mesostructure but different pore size and ordering were prepared by adjusting the Brij-76/TEOS molar ratio. A long aging period before spin coating increases the hydrocarbon chain length in the micelle and destroys pore ordering. However, structural transition does not occur even after 20 days of sol aging before spin coating. GISAXS patterns of mesoporous silica film with Brij-76/TEOS molar ratio of 0.05– 0.09 were indexed with a BCC structure while a structural transition into mixed lamellar structure was observed with the molar ratio over 0.1. When the molar ratio increases from 0.05 to 0.07, the porosity of the film was increased with pore size Table 1 Elastic modulus and hardness values of mesoporous silica films with various Brij-76/TEOS molar ratio Brij-76/TEOS molar ratio Elastic modulus [GPa] Hardness [GPa]

0.05 14.4 1.25

0.07 12.16 0.95

0.09 6.32 0.45

324

S.-B. Jung, H.-H. Park / Thin Solid Films 494 (2006) 320 – 324

and a degradation of mechanical properties was observed. However, the mechanical properties of the mesoporous silica film with molar ratio of 0.09 were severely degraded due to disordered mesopores and thinned silica wall, although the porosity values were nearly constant between the cases of molar ratio 0.07 and 0.09.

Acknowledgements The authors acknowledge the financial support from KISTEP (M1-0214-00-0228). Experiments at PLS were supported in part by the MOST and POSTECH.

References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] K. Maex, M.R. Baklanov, D. Shamiryan, F. Iacopi, S.H. Brongersma, Z.S. Yanovitskaya, J. Appl. Phys. 93 (2003) 8793. [3] S.B. Jung, H.H. Park, Thin Solid Films 420 – 421 (2002) 503. [4] Q. Huo, D.I. Margolese, G.D. Stucky, Chem. Mater. 8 (1996) 1147. [5] K.J. Edler, S.J. Roser, Int. Rev. Phys. Chem. 20 (2001) 387.

[6] D. Grosso, F. Cagnol, G.J. de A.A. Soler-Illia, E.L. Crepaldi, H. Amenitsch, A. Brunet-Bruneau, A. Bourgeois, C. Sanchez, Adv. Funct. Mater. 14 (2004) 309. [7] S. Besson, T. Gacoin, C. Ricolleau, C. Jacquiod, J.P. Boilot, J. Mater. Chem. 13 (2003) 404. [8] R.A. Pai, R. Humayun, M.T. Schulberg, A. Sengupta, J.N. Sun, J.J. Watkins, Science 303 (2004) 507. [9] S.B. Jung, C.K. Han, H.H. Park, Appl. Surf. Sci. 244 (2005) 47. [10] B.J. Park, S.Y. Rah, Y.J. Park, K.B. Lee, Rev. Sci. Instrum. 66 (1995) 1722. [11] J. Bolze, M. Lee, H.S. Youn, Langmuir 17 (2001) 6683. [12] C.J. Brinker, G.W. Scherer, Sol-Gel Science, 2nd ednR, Academic Press, San Diego, 1990, p. 213. [13] M. Imperor-Clerc, P. Davidson, A. Davidson, J. Am. Chem. Soc. 122 (2000) 11925. [14] K. Flodstrom, V. Alfredsson, Microporous Mesoporous Mater. 59 (2003) 167. [15] H. Miyata, T. Suzuki, A. Fukuoka, T. Sawada, M. Watanabe, T. Noma, K. Takada, T. Mukaide, K. Kuroda, Nat. Mater. 3 (2004) 651. [16] B. Smarsly, D. Grosso, T. Brezesinski, N. Pinna, C. Boissiere, M. Antonietti, C. Sanchez, Chem. Mater. 16 (2004) 2948. [17] Y. Sakamoto, I. Dı´az, O. Terasaki, D. Zhao, J.P. Pariente, J.M. Kim, G.D. Stucky, J. Phys. Chem., B 106 (2002) 3118. [18] S. Besson, C. Ricolleau, T. Gacoin, C. Jacquiod, J.P. Boilot, Microporous Mesoporous Mater. 60 (2003) 43. [19] E.L. Crepaldi, G.J. de A.A. Soler-Illia, D. Grosso, F. Gagnol, F. Ribot, C. Sanchez, J. Am. Chem. Soc. 125 (2003) 9770.