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On determining the entrance size of cage-like pores in mesoporous silica films by positron annihilation lifetime spectroscopy
Chemical Physics Letters 590 (2013) 97–100
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On determining the entrance size of cage-like pores in mesoporous silica films by positron annihilation lifetime spectroscopy Chunqing He a,⇑, Bangyun Xiong a, Wenfeng Mao a, Yoshinori Kobayashi b, Toshitaka Oka b, Nagayasu Oshima b, Ryoichi Suzuki b a b
Key Laboratory of Nuclear Solid State Physics Hubei Province, School of Physics and Technology, Wuhan University, Wuhan 430072, China National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
a r t i c l e
i n f o
Article history: Received 27 July 2013 In final form 15 October 2013 Available online 23 October 2013
a b s t r a c t Pore entrance size of cage-like pores in mesoporous silica films is difficult to be determined by conventional techniques. A simple expedient is proposed by using positron annihilation lifetime spectroscopy (PALS) based on a slow positron beam. Because of the nature of positronium (Ps, the bound state of a positron and an electron) in mesoporous silica, almost no Ps annihilates in the smaller connecting channels of cages. By trimethylsilylation of the silica, an appreciable fraction of Ps can be trapped and annihilate in the channels, which renders the possibility to estimate the pore entrance size from Ps lifetime in it. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, mesoporous silica/hybride silica thin films have attracted great interests of many researchers, because of their promising applications in the next generation of insulator interlayers in ultra larger scale integrated circuits (LSI), gas sensors, molecular encapsulation, separation and catalysis [1–4], etc. However, the most important information of porous thin films, i.e., the pore size and the entrance size of cage-like pores are difficult to be accessed by conventional techniques not only due to the disadvantages of the techniques themselves but also the inaccessible of thin films on substrate. To date, only two methods have been used to determine the pore entrance size in silica with cage-like structures [5,6]. One is electron crystallography. This method is powerful, providing the solution of the entire mesostructure, but requires extensive high-resolution transmission electron microscopy (TEM) imaging from different directions [5]. It is restricted to highly ordered samples with appreciable ordered domain sizes, whereas some well-defined silicas with cage-like mesoporous structures exhibit appreciable structural disorder. The local nature of TEM imaging does not allow one to obtain a definite answer about the extent of occurrence of pore entrance structure defects. The other method is nitrogen adsorption, which can roughly estimate the pore entrance size by measuring the nitrogen isotherms of a series of silica samples silylized by various organosilanes with different lengths of organic segments [6]. Positron annihilation lifetime spectroscopy (PALS) has emerged as a very useful tool to evaluate the size of mesopores in thin mesoporous thin films due to the development of both the slow posi⇑ Corresponding author. Fax: +86 27 68752370. E-mail address: [email protected] (C. He). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.10.042
tron beam techniques, which enables applications of PALS to depth-profiling [7,8], and the calibration of positronium (Ps, a bound state of an electron and its antiparticle, positron) lifetime to the pore size [9–13]. After a positron being injected into thin films, it may annihilate with electrons directly or form Ps in either the spin anti-parallel state (para-Ps/p-Ps, singlet state) or the spinparallel state (ortho-Ps/o-Ps, triplet state). The intrinsic lifetime of p-Ps via 2c-annihilation is 0.125 ns, while that of o-Ps via 3c-annihilation in a vacuum is 142 ns. In mesoporous silica films, energetic o-Ps loses kinetic energy (a few eV) by collisions with the pore surface during its diffusion in the film and is preferentially localized in the pores, undergoes 2c-annihilation with the electrons from the pore surface as well as 3c-annihilation in vacuum spaces. The o-Ps lifetime is shortened down to a few nanoseconds depending on the pore size. Recently, our experimental results showed that in mesoporous silica film with pores consisting of connected cage-like pores, Ps atom may diffuse in the connecting channels and finally is preferentially to localize in the larger cages [14]. Thus, it seems difficult to use PALS to determine the entry size of the cage-like pores in mesoporous silica films because Ps atom tends to localize and annihilate in the larger cage-like pores but not in the connecting channels. In this Letter, we propose a simple method to estimate the pore entrance size of cage-like pores in mesoporous silica films using positron annihilation lifetime spectroscopy (PALS) based on a slow positron beam. 2. Materials and methods Silica films with cage-like pores were synthesized via a sol–gel process [15] using triblock copolymers with BASF surfactants F88 and F127 as the structure-directing agents and tetraethoxysilane
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[Si(OCH2CH3)4, TEOS] as the network skeleton precursor, respectively. For comparison, a silica film with random channel-like pores was prepared with F38. The coating solutions were prepared by the addition of ethanol solutions of triblock copolymers to silica sols made by an acid-catalyzed process. The precursor sols were dipcoated on polished Si (1 0 0) wafers. The deposited films were cured in a furnace at 100 °C for 3 h and subsequently calcined at 450 °C for 3 h in order to decompose the polymer templates. Mesoporous silica films thus prepared with F38, F88 and F127 are denoted as F38-TEOS, F88-TEOS and F127-TEOS, respectively. In order to modify the pore surfaces, silylation of these films was performed by immersing them in dehydrated pyridine with 15 v.% trimethylchlorosilane and keeping at 100 °C for a few hours under stirring, followed by washing in toluene and hexane several times. The modified films are referred to m-F38-TEOS, m-F88-TEOS and mF127-TEOS. The total open porosities of F38-TEOS, F88-TEOS and F127-TEOS were estimated to be 37%, 39% and 40%, respectively, by ellipsometric porosimetry combined with heptane adsorption. The transmission electron microscope observations and heptane adsorption isotherms of these films suggest that the pores are likely channel-like in F38-TEOS and cage-like in others [14]. Fourier Transform Infrared (FTIR) spectra for the as-prepared silica films and the trimethylsilylated ones were recorded in the range of 450–4000 cm1 using a FTIR spectroscopy (Perkin–Elmer Instruments). Nitrogen adsorption isotherms were measured at 77 K for F38-TEOS and m-F38-TEOS for the purpose of checking the change of pore size upon surface modification. Positron annihilation lifetime spectra were measured at an incident positron energy of 2 keV for 200–300 nm thick films capped with 20 nm-thick nonporous SiO2, using PALS based on the intense pulsed positron beam at the National Institute of Advanced Industrial Science and Technology (AIST) [8,16,17]. Prior to the PALS experiments, each sample was baked for 30 min at 250 °C under N2 gas flow to remove possible adsorbates, for instance, water molecules. Each positron annihilation lifetime spectrum was recorded with a total count of 4–6 106. All spectra were analyzed using the LT program [18] with either discrete components or continuous distribution for o-Ps lifetime in mesopores.
3. Results and discussion 3.1. Surface modification and reduction in pore size Silylation of all as-prepared porous silica films were analyzed by FTIR measurements. Because the FTIR spectra for the silica films prepared with different porogens were similar, here for instance, only those of the films prepared with F127 are shown in Figure 1.
Absorbance (arb. unit)
Si-CH3
1350 1300 1250 1200
OH
CH
Si-O-Si
Si-CH3
m-F127-TEOS F127-TEOS
4000
3000 2000 Wave number (cm-1)
1000
Figure 1. FTIR spectra for F127-TEOS and m-F127-TEOS.
It can be seen that the absorption band of –OH stretching (3400 cm1) of SiOH dramatically is weakened after surface modification by trimethylsilylation. Simultaneously, some new absorption peaks due to CH stretching, Si–CH3 deformation (as shown in the inset) and Si–CH3 rocking appear around 2970, 1258 and 850 cm1, respectively, for m-F127-TEOS. These results confirm that the pore surface groups (–OH) of as-prepared porous silica films were successfully substituted by more bulky –OSi(CH3)3 groups. Thus, one can expect unchanged pore structures but smaller size for pores in the modified silica films. Nitrogen adsorption isotherm is often used to measure the pore size of mesoporous materials. However, it is difficult to conduct nitrogen adsorption measurements for thin film samples not only because such experiment requires enough mass of film sample but also because the mass of the film could not be determined precisely. Many pieces of thin film samples of F38-TEOS and m-F38TEOS were used for nitrogen adsorption measurements, in which a reference mass of 1 mg was used for calculating the relative adsorbed nitrogen volume and pore size distribution of the film samples. Figure 2 displays nitrogen adsorption/desorption isotherms and pore size distributions of F38-TEOS and m-F38-TEOS, respectively. The nitrogen adsorption–desorption loops of both films show type I isotherms, indicating the pores are likely cylindrical pores [19]. After grafting –OSi(CH3)3 groups on pore surface, the total pore volume decreases remarkably. Further, the peak of BJH pore size distribution for m-F38-TEOS shifts to a smaller value by 0.5 nm, which is consistent with the published literatures [20– 22] on a reduction of 0.5 nm in pore size upon surface trimethylsilylation of mesoporous silicas. 3.2. Positron annihilation characteristics in pores upon surface modification For the silica films without pore surface modification, positron annihilation lifetime spectra were well resolved into a sum of 5 exponential terms; however, they were well decomposed into 6 lifetime components for the films with cages-like pores after surface modification by trimethylsilylation. The long-lived components with lifetimes longer than 20 ns were attributed to the annihilation of o-Ps in the mesopores. These long-lived o-Ps lifetimes for all films are listed in Table 1, details of other shorter oPs lifetimes (s3 2 ns and s4 9 ns) corresponding to o-Ps annihilation in the micropores can be found in our previous paper [23]. The longest o-Ps lifetimes in F38-TEOS, F88-TEOS, and F127-TEOS are around 47, 57 and 68 ns, respectively. The mesopore size of F38-TEOS was calculated to be 3.0 nm assuming rectangular pores, whereas the mesopore sizes of F88-TEOS and F127-TEOS were 4.3 and 5.6 nm, respectively, assuming cubic pores, in both cases based on the rectangular Tao–Eldrup model with d = 0.18 nm for the thickness of a positron–electron overlapping layer on pore surface [12,13]. These results reveal that almost no o-Ps atom annihilates in the connecting channels of cage-like pores in F88-TEOS and F127-TEOS, hence the entry size of the cages seems to be inaccessible by PALS. As shown in Table 1, it is worth noting that the longest o-Ps lifetimes are elongated after trimethylsilylation due to the weaker interaction between Ps and organic –CH3 groups than –OH groups [24,25], although the trimethylsilylation of silica reduces the pore size by 0.5 nm. Such an increment in Ps lifetime is often observed in relatively smaller mesopores with varying chemical environments [25–27]. Simultaneously, it is interesting to note that a new long-lived Ps lifetime of 26 ns with an intensity of 3% appears after the modification for the silica films (F88-TEOS and F127-TEOS) with cage-like pores but not for the film (F38-TEOS) with channel-like pores. It is necessary to mention that for all silica films the o-Ps intensities in the mesopores (I5 + I6) decrease while
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[×10-4] 4 3 2 1
0
0
F38-TEOS m-F38-TEOS
0.2 0.4 0.6 0.8 1 Relatively Pressure P/P0
Relative pore distribution
Relative Volume (Arb. Unit)
Nitrogen adsorption
3
2.5 nm 2.0 nm
F38-TEOS m-F38-TEOS
2 1 0 0
2
4 6 Pore size (nm)
8
Figure 2. Nitrogen adsorption isotherms and BJH pore size distributions for F38-TEOS and m-F38-TEOS.
Table 1 Lifetimes and intensities (s5, I5, s6, I6) of o-Ps in mesopores of the as-prepared silica films and the corresponding trimethylsilylated films. Films
o-Ps lifetimes
Pore
s5/ns, I5/%
s6/ns, I6/%
A
F38-TEOS m-F38-TEOS
46.5 ± 0.1, 27.7 ± 0.1 50.6 ± 0.1, 24.6 ± 0.1
B
F88-TEOS m-F88-TEOS
55.6 ± 0.1, 25.6 ± 0.1 25.9 ± 1.2, 3.0 ± 0.1
65.3 ± 0.4, 10.1 ± 0.1
F127-TEOS m-F127-TEOS
68.3 ± 0.1, 24.0 ± 0.1 26.8 ± 0.4, 3.2 ± 0.1
84.5 ± 0.4, 7.9 ± 0.1
C
Channel-like Cage-like Cage-like
ture and result in a decrement in o-Ps diffusion length in them. For F127-TEOS, the trimethylsilylation of pore surfaces induces the one o-Ps lifetime distribution situated at 68 ns split into two o-Ps lifetime distributions with much lower intensities located 28 ns and 84.5 ns, respectively. The comparable o-Ps intensities of I5 3% and I6 8–10% make the LT program capable to resolve these two longest lifetime distributions. The new lifetime component (s5) with distinguishable intensity appears for m-F127-TEOS and m-F88-TEOS because more Ps atoms tend to reside and annihilate in the connecting channels between cage-like pores. It is known that the zero-point energy of Ps in an isolated pore with diameter d can be expressed as,
E0 ¼ P
the total o-Ps intensities ð iP3 Ii Þ remain essentially unchanged upon the trimethylsilylation. This implies o-Ps atoms are formed in the matrix of silica, and some of them diffuse into the mesopores and annihilate therein. The increment of o-Ps intensities in micropores (I3 and I4, data are not shown here) could be interpreted as that the trimethylsilylation of mesopore surfaces induces the closure of a portion of micropores and/or the formation of some new ones around the sites of silylation on the pore surfaces. Figure 3 depicts o-Ps lifetime distributions in F38-TEOS, F127-TEOS and the corresponding ones with surface modification. For F38TEOS, after trimethylsilylation the peak position of o-Ps lifetime dispersion for mesopores shifts to a relative larger value meanwhile it becomes broader. The broadening o-Ps lifetime distribution indicates the pore size is more heterogeneous after trimethylsilylation, which might enhance the pore surface curva-
2 1 nm 753 meV 2 d 2MPs d
p2 h 2
where MPs is the rest mass of the Ps atom. Accordingly, Ps zeropoint energy is much lower in a larger pore. Therefore, o-Ps atom can easily move from small pores to larger pores [14]. On the contrary, the o-Ps diffusion from larger pores to smaller ones requires significant activation energy that depends on the size of smaller pores and is unlikely. In as-prepared silica films with F88 and F127, Ps atoms in the connecting channels can easily diffuse to the much larger open space of its neighboring cage-like pore, as shown in Figure 4a. This renders it negligible for the possibility of o-Ps annihilation in the connecting channels between two neighboring cage-like pores. After trimethylsilylation, the pore surface becomes covered with a monolayer of ligands of well-defined structure and the size of pore/channel is reduced by about 0.5 nm, meanwhile the pore surface curvature become more higher.
Probability density f(τ)dτ
-2 [×10 ] 1
0.8
F38-TEOS m-F38-TEOS
F127-TEOS m-F127-TEOS
0.6 0.4 0.2 0 0
ð1Þ
20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 o-Ps lifetime τ (ns)
Figure 3. Lifetime distributions of o-Ps in mesopores of F38-TEOS, F127-TEOS and the corresponding surface modified ones.
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Figure 4. Behaviors of o-Ps formed in the connecting channels of as prepared silica films with cage-like pores and the trimethylsilylated ones.
Because of the change of physicochemical environment, the Ps lifetime and its intensity in the cage are elongated and decreased [25], respectively. As demonstrated in Figure 4b, after surface modification it is less probable for Ps in the pore-connecting channels to diffuse into the larger cages because of a ‘blocking’ effect of the grafted larger bulk groups resulted from lower Ps diffusivity in the channels with reduced size and higher surface curvature. An appreciable fraction (ca. 3%) of o-Ps therefore is confined in the connecting channels during diffusion and annihilates with electrons from the surrounding atoms therein, resulting in a new o-Ps lifetime component. However, in the film with channel-like pores, o-Ps atom can diffuse and annihilate at a pre-favorable site in the well connected pores no matter whether the surface is trimethylsilylated or not. Therefore, according to the above picture the appearance of new lifetime components of 26 ns in the trimethylsilylated silica films with cage-like pores can be safely attributed to o-Ps annihilation in the connecting channels, which corresponds to 2.0 nm in channel size according to RTE model with a modified parameter d = 0.15 nm for the pore surface decorated with organic –CH3 groups [25]. Taking account of the reduction of the pore size (0.5 nm) after the surface trimethylsilylation, the pore entrance size of the as prepared silica is 2.5 nm. Furthermore, if excluding the positron–electron overlapping layer (for unmodified silica, d = 0.18 nm in thickness) on the pore surface, the pore entrance size is 2.1 nm. These values are in good agreement with the pore entrance size (2.3 nm) of SBA-16 prepared with F127 in a similar process estimated by electron crystallography [5]. Nevertheless, it is necessary to mention that in the present films the connecting channels between two cage-like pores are 2.5 nm in diameter and 12 nm long [14]; in case the connecting channels are short enough for a Ps atom tunneling from one cage to its neighboring one, PALS is not capable to detect the pore entry size of cage-like pores due to localization of o-Ps in the cage-like pores rather than in the connecting channels. 4. Conclusion The present work demonstrates that the silylation of silica with cage-like pores makes PALS applicable to determine the size of pore entrance as well as the cage-like pores themselves in mesoporous silica. The pore size can be obtained from the lifetime of o-Ps annihilation in mesopores. After the trimethylsilylation of silica, a fraction of o-Ps tends to be trapped and annihilate in the connecting channels because of the change of physicochemical environment, thereby reflecting the size of pore entrance.
Acknowledgments This work was supported in part by National Natural Science Foundation of China (Grant Nos. 10975108 & 11375132), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and the Fundamental Research Funds for the Central Universities under Grant No. 2012202020218. Dr. K. Ito is appreciated for his helpful comments and assistance in gas adsorption experiments. C. He is grateful to Prof. J. Kansy for providing the LT program and for fruitful discussions. References [1] L.-X. Dai, Y.-H. Teng, K. Tabata, E. Suzuki, T. Tatsumi, Microporous Mesoporous Mater. 43 (2001) 573. [2] H.M.A. Hunter, P.A. Wright, Microporous Mesoporous Mater. 43 (2001) 361. [3] G. Zheng, H. Zhu, Q. Luo, Y. Zhou, D.Y. Zhao, Chem. Mater. 13 (2001) 2240. [4] R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Adv. Mater. 13 (2001) 677. [5] Y. Sakamoto et al., Nature 408 (2000) 449. [6] M. Kruk, V. Antochshuk, J.R. Matos, L.P. Mercuri, M. Jaroniec, J. Am. Chem. Soc. 124 (2002) 768. [7] D.W. Gidley, H.G. Peng, R.S. Vallery, Annu. Rev. Mater. Res. 36 (2006) 49. [8] R. Suzuki, T. Ohdaira, T. Mikado, Radiat. Phys. Chem. 58 (2000) 603. [9] V.P. Shantarovich, J. Radioanal. Nucl. Chem. 210 (1996) 357. [10] T. Goworek, K. Ciesielski, B. Jasinska, J. Wawryszczuk, Chem. Phys. 230 (1998) 305. [11] K. Ito, H. Nakanishi, Y. Ujihira, J. Phys. Chem. B 103 (1999) 4555. [12] D.W. Gidley, W.E. Frieze, T.L. Dull, A.F. Yee, E.T. Ryan, H.M. Ho, Phys. Rev. B 60 (1999) R5157. [13] T.L. Dull, W.E. Frieze, D.W. Gidley, J. Sun, A.F. Yee, J. Phys. Chem. B 105 (2001) 4657. [14] C. He, S.J. Wang, Y. Kobayashi, T. Ohdaira, R. Suzuki, Phys. Rev. B 86 (2012) 075415. [15] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [16] R. Suzuki, T. Mikado, H. Ohgaki, M. Chiwaki, T. Yamazaki, Y. Kobayashi, Phys. Rev. B 49 (1994) 17484. [17] R. Suzuki, T. Mikado, M. Chiwaki, H. Ohgaki, T. Yamazaki, Appl. Surf. Sci. 85 (1995) 87. [18] J. Kansy, Nucl. Instrum. Methods Phys. Res. A 374 (1996) 235. [19] M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169. [20] C.P. Jaroniec, M. Kruk, M. Jaroniec, A. Sayari, J. Phys. Chem. B 102 (1998) 5503. [21] D.H. Park, N. Nishiyama, Y. Egashira, K. Ueyama, Ind. Eng. Chem. Res. 40 (2001) 6105. [22] S. Tanaka, J. Kaihara, N. Nishiyama, Y. Oku, Y. Egashira, K. Ueyama, Langmuir 20 (2004) 3780. [23] C. He, M. Muramatsu, T. Ohdaira, N. Oshima, A. Kinomura, R. Suzuki, Y. Kobayashi, Radiat. Phys. Chem. 76 (2007) 204. [24] C. He et al., Phys. Rev. B 75 (2007) 195404. [25] C. He, T. Oka, Y. Kobayashi, N. Oshima, T. Ohdiara, A. Kinomura, R. Suzuki, Appl. Phys. Lett. 91 (2007) 024102. [26] S.Y. Huang, S.J. Tao, J. Chem. Phys. 54 (1971) 4902. [27] R. Zaleski, W. Dolecki, A. Kierys, J. Goworek, Microporous Mesoporous Mater. 154 (2012) 4902.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
On determining the entrance size of cage-like pores in mesoporous silica films by positron annihilation lifetime spectroscopy Chunqing He a,⇑, Bangyun Xiong a, Wenfeng Mao a, Yoshinori Kobayashi b, Toshitaka Oka b, Nagayasu Oshima b, Ryoichi Suzuki b a b
Key Laboratory of Nuclear Solid State Physics Hubei Province, School of Physics and Technology, Wuhan University, Wuhan 430072, China National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
a r t i c l e
i n f o
Article history: Received 27 July 2013 In final form 15 October 2013 Available online 23 October 2013
a b s t r a c t Pore entrance size of cage-like pores in mesoporous silica films is difficult to be determined by conventional techniques. A simple expedient is proposed by using positron annihilation lifetime spectroscopy (PALS) based on a slow positron beam. Because of the nature of positronium (Ps, the bound state of a positron and an electron) in mesoporous silica, almost no Ps annihilates in the smaller connecting channels of cages. By trimethylsilylation of the silica, an appreciable fraction of Ps can be trapped and annihilate in the channels, which renders the possibility to estimate the pore entrance size from Ps lifetime in it. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, mesoporous silica/hybride silica thin films have attracted great interests of many researchers, because of their promising applications in the next generation of insulator interlayers in ultra larger scale integrated circuits (LSI), gas sensors, molecular encapsulation, separation and catalysis [1–4], etc. However, the most important information of porous thin films, i.e., the pore size and the entrance size of cage-like pores are difficult to be accessed by conventional techniques not only due to the disadvantages of the techniques themselves but also the inaccessible of thin films on substrate. To date, only two methods have been used to determine the pore entrance size in silica with cage-like structures [5,6]. One is electron crystallography. This method is powerful, providing the solution of the entire mesostructure, but requires extensive high-resolution transmission electron microscopy (TEM) imaging from different directions [5]. It is restricted to highly ordered samples with appreciable ordered domain sizes, whereas some well-defined silicas with cage-like mesoporous structures exhibit appreciable structural disorder. The local nature of TEM imaging does not allow one to obtain a definite answer about the extent of occurrence of pore entrance structure defects. The other method is nitrogen adsorption, which can roughly estimate the pore entrance size by measuring the nitrogen isotherms of a series of silica samples silylized by various organosilanes with different lengths of organic segments [6]. Positron annihilation lifetime spectroscopy (PALS) has emerged as a very useful tool to evaluate the size of mesopores in thin mesoporous thin films due to the development of both the slow posi⇑ Corresponding author. Fax: +86 27 68752370. E-mail address: [email protected] (C. He). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.10.042
tron beam techniques, which enables applications of PALS to depth-profiling [7,8], and the calibration of positronium (Ps, a bound state of an electron and its antiparticle, positron) lifetime to the pore size [9–13]. After a positron being injected into thin films, it may annihilate with electrons directly or form Ps in either the spin anti-parallel state (para-Ps/p-Ps, singlet state) or the spinparallel state (ortho-Ps/o-Ps, triplet state). The intrinsic lifetime of p-Ps via 2c-annihilation is 0.125 ns, while that of o-Ps via 3c-annihilation in a vacuum is 142 ns. In mesoporous silica films, energetic o-Ps loses kinetic energy (a few eV) by collisions with the pore surface during its diffusion in the film and is preferentially localized in the pores, undergoes 2c-annihilation with the electrons from the pore surface as well as 3c-annihilation in vacuum spaces. The o-Ps lifetime is shortened down to a few nanoseconds depending on the pore size. Recently, our experimental results showed that in mesoporous silica film with pores consisting of connected cage-like pores, Ps atom may diffuse in the connecting channels and finally is preferentially to localize in the larger cages [14]. Thus, it seems difficult to use PALS to determine the entry size of the cage-like pores in mesoporous silica films because Ps atom tends to localize and annihilate in the larger cage-like pores but not in the connecting channels. In this Letter, we propose a simple method to estimate the pore entrance size of cage-like pores in mesoporous silica films using positron annihilation lifetime spectroscopy (PALS) based on a slow positron beam. 2. Materials and methods Silica films with cage-like pores were synthesized via a sol–gel process [15] using triblock copolymers with BASF surfactants F88 and F127 as the structure-directing agents and tetraethoxysilane
98
C. He et al. / Chemical Physics Letters 590 (2013) 97–100
[Si(OCH2CH3)4, TEOS] as the network skeleton precursor, respectively. For comparison, a silica film with random channel-like pores was prepared with F38. The coating solutions were prepared by the addition of ethanol solutions of triblock copolymers to silica sols made by an acid-catalyzed process. The precursor sols were dipcoated on polished Si (1 0 0) wafers. The deposited films were cured in a furnace at 100 °C for 3 h and subsequently calcined at 450 °C for 3 h in order to decompose the polymer templates. Mesoporous silica films thus prepared with F38, F88 and F127 are denoted as F38-TEOS, F88-TEOS and F127-TEOS, respectively. In order to modify the pore surfaces, silylation of these films was performed by immersing them in dehydrated pyridine with 15 v.% trimethylchlorosilane and keeping at 100 °C for a few hours under stirring, followed by washing in toluene and hexane several times. The modified films are referred to m-F38-TEOS, m-F88-TEOS and mF127-TEOS. The total open porosities of F38-TEOS, F88-TEOS and F127-TEOS were estimated to be 37%, 39% and 40%, respectively, by ellipsometric porosimetry combined with heptane adsorption. The transmission electron microscope observations and heptane adsorption isotherms of these films suggest that the pores are likely channel-like in F38-TEOS and cage-like in others [14]. Fourier Transform Infrared (FTIR) spectra for the as-prepared silica films and the trimethylsilylated ones were recorded in the range of 450–4000 cm1 using a FTIR spectroscopy (Perkin–Elmer Instruments). Nitrogen adsorption isotherms were measured at 77 K for F38-TEOS and m-F38-TEOS for the purpose of checking the change of pore size upon surface modification. Positron annihilation lifetime spectra were measured at an incident positron energy of 2 keV for 200–300 nm thick films capped with 20 nm-thick nonporous SiO2, using PALS based on the intense pulsed positron beam at the National Institute of Advanced Industrial Science and Technology (AIST) [8,16,17]. Prior to the PALS experiments, each sample was baked for 30 min at 250 °C under N2 gas flow to remove possible adsorbates, for instance, water molecules. Each positron annihilation lifetime spectrum was recorded with a total count of 4–6 106. All spectra were analyzed using the LT program [18] with either discrete components or continuous distribution for o-Ps lifetime in mesopores.
3. Results and discussion 3.1. Surface modification and reduction in pore size Silylation of all as-prepared porous silica films were analyzed by FTIR measurements. Because the FTIR spectra for the silica films prepared with different porogens were similar, here for instance, only those of the films prepared with F127 are shown in Figure 1.
Absorbance (arb. unit)
Si-CH3
1350 1300 1250 1200
OH
CH
Si-O-Si
Si-CH3
m-F127-TEOS F127-TEOS
4000
3000 2000 Wave number (cm-1)
1000
Figure 1. FTIR spectra for F127-TEOS and m-F127-TEOS.
It can be seen that the absorption band of –OH stretching (3400 cm1) of SiOH dramatically is weakened after surface modification by trimethylsilylation. Simultaneously, some new absorption peaks due to CH stretching, Si–CH3 deformation (as shown in the inset) and Si–CH3 rocking appear around 2970, 1258 and 850 cm1, respectively, for m-F127-TEOS. These results confirm that the pore surface groups (–OH) of as-prepared porous silica films were successfully substituted by more bulky –OSi(CH3)3 groups. Thus, one can expect unchanged pore structures but smaller size for pores in the modified silica films. Nitrogen adsorption isotherm is often used to measure the pore size of mesoporous materials. However, it is difficult to conduct nitrogen adsorption measurements for thin film samples not only because such experiment requires enough mass of film sample but also because the mass of the film could not be determined precisely. Many pieces of thin film samples of F38-TEOS and m-F38TEOS were used for nitrogen adsorption measurements, in which a reference mass of 1 mg was used for calculating the relative adsorbed nitrogen volume and pore size distribution of the film samples. Figure 2 displays nitrogen adsorption/desorption isotherms and pore size distributions of F38-TEOS and m-F38-TEOS, respectively. The nitrogen adsorption–desorption loops of both films show type I isotherms, indicating the pores are likely cylindrical pores [19]. After grafting –OSi(CH3)3 groups on pore surface, the total pore volume decreases remarkably. Further, the peak of BJH pore size distribution for m-F38-TEOS shifts to a smaller value by 0.5 nm, which is consistent with the published literatures [20– 22] on a reduction of 0.5 nm in pore size upon surface trimethylsilylation of mesoporous silicas. 3.2. Positron annihilation characteristics in pores upon surface modification For the silica films without pore surface modification, positron annihilation lifetime spectra were well resolved into a sum of 5 exponential terms; however, they were well decomposed into 6 lifetime components for the films with cages-like pores after surface modification by trimethylsilylation. The long-lived components with lifetimes longer than 20 ns were attributed to the annihilation of o-Ps in the mesopores. These long-lived o-Ps lifetimes for all films are listed in Table 1, details of other shorter oPs lifetimes (s3 2 ns and s4 9 ns) corresponding to o-Ps annihilation in the micropores can be found in our previous paper [23]. The longest o-Ps lifetimes in F38-TEOS, F88-TEOS, and F127-TEOS are around 47, 57 and 68 ns, respectively. The mesopore size of F38-TEOS was calculated to be 3.0 nm assuming rectangular pores, whereas the mesopore sizes of F88-TEOS and F127-TEOS were 4.3 and 5.6 nm, respectively, assuming cubic pores, in both cases based on the rectangular Tao–Eldrup model with d = 0.18 nm for the thickness of a positron–electron overlapping layer on pore surface [12,13]. These results reveal that almost no o-Ps atom annihilates in the connecting channels of cage-like pores in F88-TEOS and F127-TEOS, hence the entry size of the cages seems to be inaccessible by PALS. As shown in Table 1, it is worth noting that the longest o-Ps lifetimes are elongated after trimethylsilylation due to the weaker interaction between Ps and organic –CH3 groups than –OH groups [24,25], although the trimethylsilylation of silica reduces the pore size by 0.5 nm. Such an increment in Ps lifetime is often observed in relatively smaller mesopores with varying chemical environments [25–27]. Simultaneously, it is interesting to note that a new long-lived Ps lifetime of 26 ns with an intensity of 3% appears after the modification for the silica films (F88-TEOS and F127-TEOS) with cage-like pores but not for the film (F38-TEOS) with channel-like pores. It is necessary to mention that for all silica films the o-Ps intensities in the mesopores (I5 + I6) decrease while
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[×10-4] 4 3 2 1
0
0
F38-TEOS m-F38-TEOS
0.2 0.4 0.6 0.8 1 Relatively Pressure P/P0
Relative pore distribution
Relative Volume (Arb. Unit)
Nitrogen adsorption
3
2.5 nm 2.0 nm
F38-TEOS m-F38-TEOS
2 1 0 0
2
4 6 Pore size (nm)
8
Figure 2. Nitrogen adsorption isotherms and BJH pore size distributions for F38-TEOS and m-F38-TEOS.
Table 1 Lifetimes and intensities (s5, I5, s6, I6) of o-Ps in mesopores of the as-prepared silica films and the corresponding trimethylsilylated films. Films
o-Ps lifetimes
Pore
s5/ns, I5/%
s6/ns, I6/%
A
F38-TEOS m-F38-TEOS
46.5 ± 0.1, 27.7 ± 0.1 50.6 ± 0.1, 24.6 ± 0.1
B
F88-TEOS m-F88-TEOS
55.6 ± 0.1, 25.6 ± 0.1 25.9 ± 1.2, 3.0 ± 0.1
65.3 ± 0.4, 10.1 ± 0.1
F127-TEOS m-F127-TEOS
68.3 ± 0.1, 24.0 ± 0.1 26.8 ± 0.4, 3.2 ± 0.1
84.5 ± 0.4, 7.9 ± 0.1
C
Channel-like Cage-like Cage-like
ture and result in a decrement in o-Ps diffusion length in them. For F127-TEOS, the trimethylsilylation of pore surfaces induces the one o-Ps lifetime distribution situated at 68 ns split into two o-Ps lifetime distributions with much lower intensities located 28 ns and 84.5 ns, respectively. The comparable o-Ps intensities of I5 3% and I6 8–10% make the LT program capable to resolve these two longest lifetime distributions. The new lifetime component (s5) with distinguishable intensity appears for m-F127-TEOS and m-F88-TEOS because more Ps atoms tend to reside and annihilate in the connecting channels between cage-like pores. It is known that the zero-point energy of Ps in an isolated pore with diameter d can be expressed as,
E0 ¼ P
the total o-Ps intensities ð iP3 Ii Þ remain essentially unchanged upon the trimethylsilylation. This implies o-Ps atoms are formed in the matrix of silica, and some of them diffuse into the mesopores and annihilate therein. The increment of o-Ps intensities in micropores (I3 and I4, data are not shown here) could be interpreted as that the trimethylsilylation of mesopore surfaces induces the closure of a portion of micropores and/or the formation of some new ones around the sites of silylation on the pore surfaces. Figure 3 depicts o-Ps lifetime distributions in F38-TEOS, F127-TEOS and the corresponding ones with surface modification. For F38TEOS, after trimethylsilylation the peak position of o-Ps lifetime dispersion for mesopores shifts to a relative larger value meanwhile it becomes broader. The broadening o-Ps lifetime distribution indicates the pore size is more heterogeneous after trimethylsilylation, which might enhance the pore surface curva-
2 1 nm 753 meV 2 d 2MPs d
p2 h 2
where MPs is the rest mass of the Ps atom. Accordingly, Ps zeropoint energy is much lower in a larger pore. Therefore, o-Ps atom can easily move from small pores to larger pores [14]. On the contrary, the o-Ps diffusion from larger pores to smaller ones requires significant activation energy that depends on the size of smaller pores and is unlikely. In as-prepared silica films with F88 and F127, Ps atoms in the connecting channels can easily diffuse to the much larger open space of its neighboring cage-like pore, as shown in Figure 4a. This renders it negligible for the possibility of o-Ps annihilation in the connecting channels between two neighboring cage-like pores. After trimethylsilylation, the pore surface becomes covered with a monolayer of ligands of well-defined structure and the size of pore/channel is reduced by about 0.5 nm, meanwhile the pore surface curvature become more higher.
Probability density f(τ)dτ
-2 [×10 ] 1
0.8
F38-TEOS m-F38-TEOS
F127-TEOS m-F127-TEOS
0.6 0.4 0.2 0 0
ð1Þ
20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 o-Ps lifetime τ (ns)
Figure 3. Lifetime distributions of o-Ps in mesopores of F38-TEOS, F127-TEOS and the corresponding surface modified ones.
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Figure 4. Behaviors of o-Ps formed in the connecting channels of as prepared silica films with cage-like pores and the trimethylsilylated ones.
Because of the change of physicochemical environment, the Ps lifetime and its intensity in the cage are elongated and decreased [25], respectively. As demonstrated in Figure 4b, after surface modification it is less probable for Ps in the pore-connecting channels to diffuse into the larger cages because of a ‘blocking’ effect of the grafted larger bulk groups resulted from lower Ps diffusivity in the channels with reduced size and higher surface curvature. An appreciable fraction (ca. 3%) of o-Ps therefore is confined in the connecting channels during diffusion and annihilates with electrons from the surrounding atoms therein, resulting in a new o-Ps lifetime component. However, in the film with channel-like pores, o-Ps atom can diffuse and annihilate at a pre-favorable site in the well connected pores no matter whether the surface is trimethylsilylated or not. Therefore, according to the above picture the appearance of new lifetime components of 26 ns in the trimethylsilylated silica films with cage-like pores can be safely attributed to o-Ps annihilation in the connecting channels, which corresponds to 2.0 nm in channel size according to RTE model with a modified parameter d = 0.15 nm for the pore surface decorated with organic –CH3 groups [25]. Taking account of the reduction of the pore size (0.5 nm) after the surface trimethylsilylation, the pore entrance size of the as prepared silica is 2.5 nm. Furthermore, if excluding the positron–electron overlapping layer (for unmodified silica, d = 0.18 nm in thickness) on the pore surface, the pore entrance size is 2.1 nm. These values are in good agreement with the pore entrance size (2.3 nm) of SBA-16 prepared with F127 in a similar process estimated by electron crystallography [5]. Nevertheless, it is necessary to mention that in the present films the connecting channels between two cage-like pores are 2.5 nm in diameter and 12 nm long [14]; in case the connecting channels are short enough for a Ps atom tunneling from one cage to its neighboring one, PALS is not capable to detect the pore entry size of cage-like pores due to localization of o-Ps in the cage-like pores rather than in the connecting channels. 4. Conclusion The present work demonstrates that the silylation of silica with cage-like pores makes PALS applicable to determine the size of pore entrance as well as the cage-like pores themselves in mesoporous silica. The pore size can be obtained from the lifetime of o-Ps annihilation in mesopores. After the trimethylsilylation of silica, a fraction of o-Ps tends to be trapped and annihilate in the connecting channels because of the change of physicochemical environment, thereby reflecting the size of pore entrance.
Acknowledgments This work was supported in part by National Natural Science Foundation of China (Grant Nos. 10975108 & 11375132), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and the Fundamental Research Funds for the Central Universities under Grant No. 2012202020218. Dr. K. Ito is appreciated for his helpful comments and assistance in gas adsorption experiments. C. He is grateful to Prof. J. Kansy for providing the LT program and for fruitful discussions. References [1] L.-X. Dai, Y.-H. Teng, K. Tabata, E. Suzuki, T. Tatsumi, Microporous Mesoporous Mater. 43 (2001) 573. [2] H.M.A. Hunter, P.A. Wright, Microporous Mesoporous Mater. 43 (2001) 361. [3] G. Zheng, H. Zhu, Q. Luo, Y. Zhou, D.Y. Zhao, Chem. Mater. 13 (2001) 2240. [4] R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Adv. Mater. 13 (2001) 677. [5] Y. Sakamoto et al., Nature 408 (2000) 449. [6] M. Kruk, V. Antochshuk, J.R. Matos, L.P. Mercuri, M. Jaroniec, J. Am. Chem. Soc. 124 (2002) 768. [7] D.W. Gidley, H.G. Peng, R.S. Vallery, Annu. Rev. Mater. Res. 36 (2006) 49. [8] R. Suzuki, T. Ohdaira, T. Mikado, Radiat. Phys. Chem. 58 (2000) 603. [9] V.P. Shantarovich, J. Radioanal. Nucl. Chem. 210 (1996) 357. [10] T. Goworek, K. Ciesielski, B. Jasinska, J. Wawryszczuk, Chem. Phys. 230 (1998) 305. [11] K. Ito, H. Nakanishi, Y. Ujihira, J. Phys. Chem. B 103 (1999) 4555. [12] D.W. Gidley, W.E. Frieze, T.L. Dull, A.F. Yee, E.T. Ryan, H.M. Ho, Phys. Rev. B 60 (1999) R5157. [13] T.L. Dull, W.E. Frieze, D.W. Gidley, J. Sun, A.F. Yee, J. Phys. Chem. B 105 (2001) 4657. [14] C. He, S.J. Wang, Y. Kobayashi, T. Ohdaira, R. Suzuki, Phys. Rev. B 86 (2012) 075415. [15] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [16] R. Suzuki, T. Mikado, H. Ohgaki, M. Chiwaki, T. Yamazaki, Y. Kobayashi, Phys. Rev. B 49 (1994) 17484. [17] R. Suzuki, T. Mikado, M. Chiwaki, H. Ohgaki, T. Yamazaki, Appl. Surf. Sci. 85 (1995) 87. [18] J. Kansy, Nucl. Instrum. Methods Phys. Res. A 374 (1996) 235. [19] M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169. [20] C.P. Jaroniec, M. Kruk, M. Jaroniec, A. Sayari, J. Phys. Chem. B 102 (1998) 5503. [21] D.H. Park, N. Nishiyama, Y. Egashira, K. Ueyama, Ind. Eng. Chem. Res. 40 (2001) 6105. [22] S. Tanaka, J. Kaihara, N. Nishiyama, Y. Oku, Y. Egashira, K. Ueyama, Langmuir 20 (2004) 3780. [23] C. He, M. Muramatsu, T. Ohdaira, N. Oshima, A. Kinomura, R. Suzuki, Y. Kobayashi, Radiat. Phys. Chem. 76 (2007) 204. [24] C. He et al., Phys. Rev. B 75 (2007) 195404. [25] C. He, T. Oka, Y. Kobayashi, N. Oshima, T. Ohdiara, A. Kinomura, R. Suzuki, Appl. Phys. Lett. 91 (2007) 024102. [26] S.Y. Huang, S.J. Tao, J. Chem. Phys. 54 (1971) 4902. [27] R. Zaleski, W. Dolecki, A. Kierys, J. Goworek, Microporous Mesoporous Mater. 154 (2012) 4902.