Positron Annihilation Lifetime in Ordered Porous Silica SBA-3 - The

J. Phys. Chem. C 2008, 112, 8779–8783

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Positron Annihilation Lifetime in Ordered Porous Silica SBA-3 Masanori Koshimizu,*,† Keisuke Shimokita,† Haoshen S. Zhou,‡ Itaru Honma,‡ and Keisuke Asai‡ Department of Applied Chemistry, Graduate School of Engineering, Tohoku UniVersity, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan, and National Institute of AdVanced Industrial Science and Technology, AIST, Tsukuba Central 2, Tsukuba, Ibaraki 305-8568, Japan ReceiVed: NoVember 11, 2007; ReVised Manuscript ReceiVed: February 19, 2008

To clarify the correlation between the positron lifetime and the pore size in mesoporous materials, the positron lifetime profiles of SBA-3 were measured. SBA-3 has ordered cylindrical pores, and the pore size can be controlled using surfactant molecules of different length as templates. Thus, SBA-3 is appropriate as a model material in order to establish the correlation. The lifetime of the longest component was attributed to annihilations in the intergrain space. The lifetime of the second longest component increased for larger pores, and this component is attributed to the annihilation of o-Ps in the primary cylindrical pores. The lifetime component corresponding to the primary pores was obtained because the sizes of the grains were greater than or comparable to the diffusion length of o-Ps; this is in contrast to the case of MCM-41 where the grain size is very small. The effect of the escape of o-Ps from the primary pores was also observed in the intensities of the longest and the second longest components. The relation between the positron lifetime and the pore size was compared to those in several theoretical models, and the cylindrical or channel models agreed rather well with the experimental results, although significant deviations were found for the SBA-3-12 and SBA3-18 samples. These deviations can be attributed to the possible irregularities in the sample and the escape of o-Ps from the primary pores. These results clearly show that the escape of o-Ps from the primary pores is an important process in analysis of the pore structure by using positron lifetime measurements. Introduction The positron annihilation lifetime measurement of porous materials has attracted considerable attention in materials science. A positron, which is an antiparticle of an electron, annihilates with an electron in matter and emits γ-ray photons. In insulating solids, including porous silica, some positrons form positroniums (Ps’s) before annihilation. Positronium is a hydrogen-atom-like bound state of an electron-positron pair. Depending on the mutual spins of the electron and the positron, there are two Ps substates, para-positronium (p-Ps) and orthopositronium (o-Ps), which are singlet and triplet substates, respectively. In the cavity of a porous solid, o-Ps annihilates with a lifetime that depends on the cavity size. By establishing a quantitative correlation between the positron lifetime and the pore size the pore size can be determined by measuring the positron lifetime. For the structural analysis of porous materials, the advantages in using the positron lifetime measurement over several conventional methods such as N2 adsorption isotherm measurements or transmission electron microscopy (TEM) observations are as follows: (i) closed pores can also be detected, (ii) the chemical properties of the inner surface of pores can be analyzed, and (iii) the pore structure of thin films can be analyzed by using a pulsed positron beam. For micropores, the model proposed by Tao is commonly used for obtaining the pore size from the positron lifetime.1–3 This model considers micropores with spherical shapes. Regarding mesopores, several models have been proposed for obtaining * To whom correspondence should be addressed. Fax: +81-22-795-7219. E-mail: [email protected]. † Tohoku University. ‡ National Institute of Advanced Industrial Science and Technology.

Figure 1. XRD scans of SBA-3 with various surfactant chain lengths.

a correlation between the positron lifetime and the sizes of mesopores with various shapes.4–7 In addition to the theoretical considerations, several experimental studies have been performed for establishing a quantitative correlation between the positron lifetime and the mesopore size.4–6,8 However, most of the experimental reports are for materials possessing mesopores with broad size distributions or irregular shapes. These characteristics of the mesopores are not compatible with the presented models where the mesopore is assumed to be regular in size and shape. Thus, a material possessing mesopores with a uniform size and shape is desirable as a model system. Since the 1990s, novel mesoporous materials have been developed by using surfactant molecules as templates.9,10 These materials possess uniform mesopores aligned in an orderly manner in cubic or hexagonal forms. Several researchers have reported the positron annihilation characteristics in such a

10.1021/jp710786x CCC: $40.75  2008 American Chemical Society Published on Web 05/27/2008

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Figure 2. Nitrogen adsorption-desorption isotherms for SBA-3 with various surfactant chain lengths. For clarity, the origin is shifted upward by 200, 400, and 600 cm3 STP g-1 in the isotherms of SBA-3-14, SBA-3-16, and SBA-3-18, respectively.

Koshimizu et al. lifetime measurement of porous thin films.7,15 Several papers have been published on the annihilation characteristics of o-Ps in the template in the mesopores and/or the degradation and desorption processes of the template surfactants.16–20 Quite recently, Zaleski et al. detected the lifetime component corresponding to annihilation in the primary mesopore; however, its intensity is very low.21 In this paper, we present the results of the positron lifetime measurement of SBA-3, which possess cylindrical pores of uniform size, similar to MCM-41. SBA-3 is synthesized under acidic conditions22,23 by using the same surfactant molecules used in the synthesis of MCM-41, which is in contrast to the systhesis of MCM-41 under basic conditions. It has to be mentioned that the powder particles of SBA-3 are considerably larger than those of MCM-41; this affects the escaping tendency of o-Ps from the primary pores to the intergrain space, which is discussed later. The aim of this study is to evaluate the correlation between the positron lifetime and the cylindrical pore size by using SBA-3 as a model porous material. Experimental Methods

Figure 3. Pore size distributions of SBA-3-14, SBA-3-16, and SBA3-18.

TABLE 1: Pore Diameters and BET Surface Areas of SBA-3 with Various Surfactant Chain Lengthsa SBA-3-12 SBA-3-14 SBA-3-16 SBA-3-18

pore diameter (nm)

BET surface area (m2 g-1)

(1.0) 1.38 ( 0.37 1.80 ( 0.27 2.44 ( 0.28

650 780 1020 1060

a The pore size is defined as the maximum pore size in the pore size distribution. The pore diameter of SBA-3-12 is estimated to be smaller than that of SBA-3-14 by roughly 0.4 nm, which is the common value of the difference in the pore diameters for surfactants that differ in the number of carbon atoms by 2. The experimental error in pore diameter is taken as the half-width at the half-maximum of the pore size distribution.

mesoporous material, MCM-41. This porous solid is known to have uniform mesopores with a cylindrical shape, and it can be a model system. In the early stages of the positron annihilation studies for MCM-41, it was reported that the lifetime component corresponding to the mesopore of MCM-41 was obtained,11,12 although details of the structural properties of the samples were not depicted. Later, it was pointed out that the longest lifetime component of approximately 100 ns corresponds to positron annihilation in the intergrain space13,14 because the size of the grain particles is approximately 50-120 nm,14 which is considerably smaller than the diffusion length of o-Ps (more than 1 µm15 or several micrometers7) obtained by the positron

All the reagents were purchased and used without further purification. N-Alkyltrimethylammonium bromide (CnH2n+1N(CH3)3Br, abbreviated as Cn hereafter) was used as the template; it was purchased from Wako for n ) 12, 14, and 16 and from Aldrich for n ) 18. HCl aqueous solution (37%) was purchased from Wako. Tetraethyl orthosilicate (TEOS, min. 95.0%) was purchased from Wako. SBA-3 with various pore sizes was synthesized by the following procedure.23 At first, Cn was dissolved in water at room temperature for n ) 12, 14, and 16. For n ) 18, C18 was dissolved in water at 323 K under stirring for 1 h, owing to its low solubility at room temperature. After the addition of HCl, TEOS was added dropwise The molar ratio of the final solution is 1 TEOS:0.12 Cn:9.0 HCl:162 H2O. After stirring for 1 h, the white precipitate formed was filtered and washed with distilled water. The precipitate was then dried at 373 K for 3 h and calcined in air at 823 K for 24 h to remove the template. The duration of calcination is significantly longer than that mentioned in a previous report23 in order to avoid a decrease in the positron lifetime due to residual carbon precipitates in the pore; such a decrease was reported in ref 14. The samples obtained by using Cn as the templates are hereafter designated as SBA-3-n. The pore structure of the samples was analyzed by measuring N2 adsorption-desorption isotherms at 77 K (BELSORP-mini II, Bel Japan, Inc.). The pore size distribution was obtained by the Barrett-Joyner-Halenda (BJH) method.24 The periodicity of the samples was examined by X-ray diffraction (XRD) measurements (RINT2200, RIGAKU). The shape and size of the powder particles were observed by scanning electron microscopy (SEM) (LEO1420, Carl Zeiss). Positron lifetime measurements were performed by using a conventional fast-fast measuring system.22 Na sealed in a layer of polyimide sheet (POSK-22, Isotope Products Laboratories) was used as the positron source. The activity of the positron source was approximately 300 kBq. Pilot-U was used as a scintillator, and the scintillation signals detected by PMTs (H3378-51, HAMAMATSU) were transmitted to constant fraction discriminators (583A, ORTEC). The time difference between the start and the stop signals was converted into pulse height by a time-to-amplitude converter (566, ORTEC). The lifetime data were collected in a multichannel analyzer, and the positron lifetime profile was obtained. The total number of counts was approximately 2 million for each lifetime profile.

Positron Annihilation in SBA-3

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Figure 4. SEM micrographs of the powder particles of SBA-3.

The time resolution of the measuring system was approximately 350 ps. The obtained lifetime profile was decomposed into the sum of several lifetime components by using the computer program PATFIT.25 No correction was made for the lifetime components of the positron source. This does not affect the long lifetime components, which are the main subject in this paper. Results and Discussion Structural Properties of the Samples. Figure 1 shows the XRD patterns of SBA-3-n. To the best of our knowledge, the synthesis of SBA-3 has been reported only for C16. For SBA3-16, the dominant peak in the XRD pattern coincides with the (100) peak that has been previously reported.23 Thus, it is indicated that this sample has cylindrical pores similarly to the samples reported in ref 23. For other samples, the dominant peak was observed at a smaller angle as the surfactant molecule increases in chain length. However, (110) and (200) peaks are almost absent in the XRD patterns. Therefore, the samples have periodicities corresponding to the size of the surfactant molecules and ordered cylindrical pores, although hexagonal ordering is rather weak. The N2 adsorption-desorption isotherms recorded at 77 K are presented in Figure 2. For SBA-3-12, the isotherm is of type I according to the IUPAC definition, indicating the presence of microporosity in the material. For the other samples, clear adsorption steps are observed in the isotherms due to capillary condensation; these steps appear at a higher relative pressure for long surfactant molecules. This result indicates that a larger pore size was obtained for longer surfactant molecules. From the desorption branches of the isotherms, pore size distributions were obtained for samples with n ) 14, 16, and 18 by using the BJH method,24 as shown in Figure 3. A monodisperse distribution was obtained for each sample. The Brunauer-Emmett-Teller (BET) surface area and pore size for each sample are listed in Table 1. The pore size is defined as the maximum pore size in the pore size distribution. As the number of the carbon atoms of the surfactant molecule increases

by 2, the pore size increases by approximately 0.4-0.6 nm. Tentatively, the pore size of the SBA-3-12 is assumed to be smaller than that of SBA-3-14 by 0.4 nm. It should be emphasized that the adsorption isotherms have no extra condensation step at a relative pressure of unity; this is in contrast to the case of MCM-4114 where a condensation step is observed at this relative pressure due to capillary condensation in the intergrain space. This difference suggests that the sizes of the powder particles of SBA-3 are significantly greater than those of MCM-41. The above argument is further verified by SEM measurements. The SEM micrographs for typical powder particles of the samples are shown in Figure 4. For SBA-3-12 and SBA3-14, the size of the powder particle exceeds 10 µm. For SBA3-16 and SBA-3-18, powder particles consist of many grains of several micrometers and approximately 1 µm, respectively. In each case, the size of the grains is considerably larger than that of the grains in MCM-41, which was reported to be 50-120 nm.14 On the basis of the results of structural characterization, it is shown that the samples have cylindrical pores with regular sizes, and the pore channels are significantly longer than those of MCM-41. This indicates that these samples are appropriate for the experimental clarification of the correlation between the positron lifetime and the cylindrical pore size. Positron Annihilation Characteristics. The five-component fitting results are tabulated in Table 2. The obtained lifetime profiles could not be satisfactorily fitted using four components. The first and second components are probably combinations of the annihilation of p-Ps and that of a free or a trapped positron in the silica framework and will not be discussed in detail in this paper. The fourth and fifth components, which are longer than 15 ns, are due to the annihilation of o-Ps in some of the open spaces. In this case, two kinds of open spaces have to be considered: the primary cylindrical pore and the intergrain space. The lifetime of the longest component exceeds 100 ns and is

0.5 ( 0.07 0.8 ( 0.07 3.6 ( 0.13 7.6 ( 0.50 4.1 ( 0.19 2.8 ( 0.15 2.3 ( 0.27 1.7 ( 0.48

16.0 ( 0.23 9.8 ( 0.09 10.7 ( 0.12 3.2 ( 0.32

1.010 0.997 1.044 1.365

Koshimizu et al.

Figure 5. Correlation between the pore radius and the positron lifetime. The experimental data are compared with the several curves derived from the theoretical models: the Tao,1–3 INU,6 extended Tao,4 and RTE models.7 For the RTE model, the correlation for the channel-like pore with an infinite length is depicted in the figure, and the diameter is defined as the width of the channel. In the case of the extended Tao and RTE models, the temperature is set to 300 K. The thickness of the overlap region of the electron cloud and the Ps wave function, ∆R, is set to 0.166 nm.

χ2 is the variance of the fit. a

1.0 ( 0.05 1.1 ( 0.04 1.0 ( 0.05 1.7 ( 0.04 SBA-3-12 SBA-3-14 SBA-3-16 SBA-3-18

0.38 ( 0.007 0.32 ( 0.004 0.34 ( 0.005 0.43 ( 0.002

5.2 ( 0.43 4.6 ( 0.36 3.7 ( 0.34 13.8 ( 2.4

17.8 ( 0.24 22.2 ( 0.33 31.4 ( 0.44 40.9 ( 7.9

106.4 ( 19.1 120.2 ( 13.2 125.5 ( 4.4 126.3 ( 5.1

62.3 ( 0.49 73.6 ( 0.84 70.1 ( 0.92 80.4 ( 0.50

17.2 ( 1.17 12.9 ( 0.58 13.3 ( 0.69 7.0 ( 0.15

I5 (%) I3 (%) τ5 (ns) τ4 (ns) τ3 (ns) τ2 (ns) τ1 (ns)

TABLE 2: Five-Component Fitting Results for the Positron Lifetime Profiles of SBA-3a

I1 (%)

I2 (%)

I4 (%)

χ2

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almost independent of the length of the surfactant molecule of the template. Therefore, this component is attributed to the annihilation in the intergrain space, similarly to a previous report on MCM-41.14 In contrast, the lifetime of the fourth component increases with the length of the surfactant molecule; this trend is the same as that observed for the pore size. Therefore, the fourth component can be attributed to the annihilation of o-Ps in the primary cylindrical pore. This is in contrast to the case of MCM-41 where the components corresponding to the annihilation in the primary pore have not been clearly detected. This difference between SBA-3 and MCM-41 is attributed to the difference in the size of the powder particles. In the case of MCM-41, the size of the powder particle is smaller than the diffusion length of o-Ps; therefore, almost all the o-Ps’s in the primary pore diffuse out to the intergrain space, which is larger than the primary pore. In contrast, in the case of SBA-3, as is observed in the SEM micrographs, the sizes of the powder particles are comparable to or larger than the reported value of the diffusion length of several µm for the o-Ps with a lifetime of approximately 100 ns.7 Hence, the component corresponding to annihilation in the primary pore can be observed. This consideration is further supported by the intensities of the fourth and the fifth components (I4 and I5). For SBA-3-12 and SBA3-14, the fourth component dominates the fifth component. The contribution of the fourth component decreases with a further increase in the pore size. This behavior of the intensity of the long lifetime components is explained by the following factors: (i) As the pore size increases, the lifetime in the primary pore becomes longer, and the diffusion length of o-Ps increases. Therefore, the escape probability of o-Ps increases with the pore size. (ii) As shown in Figure. 4, the size of the grain decreases for long surfactant molecules; this leads to a larger escape probability for samples with larger pore sizes. The correlation between the pore radius and the lifetime of the fourth component is shown in Figure 5. The experimental data are compared with the results of several theoretical models. The Tao model assumes spherical pores, and Ps inside the pore is expressed using a wave function spreading over the pore.1–3 The INU model also assumes Ps in spherical pores and treats Ps as a particle colliding with the pore wall.6 In contrast, the RTE model considers rectangular pores,7 and the correlation curve corresponding to the channel with infinite length is shown in Figure 5. The curves for Tao, extended Tao, and RTE model

Positron Annihilation in SBA-3 almost coincide for the pore diameter smaller than 2 nm, because the population of the excited states can be ignored for smaller pores. Among the above-mentioned models, the curves for these three models appear to be appropriate for the experimental results in this study; this is because these models assume a cylindrical or channel pore, which is best suited for the samples used in this study. The lifetime of SBA-3-18 obtained in this study is slightly shorter than that predicted by the cylindrical or channel models. This shortening is possibly due to the escape of o-Ps from the primary pore to the intergrain space. During this escape, the “apparent” lifetime of the fourth component decreases because the escape acts as an additional decay channel for Ps related to this component. This consideration is in line with the above discussion on the intensities of the fourth and fifth components. For SBA-3-12, a longer lifetime is obtained as compared to that predicted by the model, even when ambiguities in the determination of the pore size are considered. This may be due to possible structural irregularities manifested in the broad XRD pattern. As for the third component, at present, a clear attribution is not possible. The third component is weak and shows significant variations in the lifetime for different samples. However, this component is very interesting from the viewpoint of possible annihilation in the additional micropores in the silica framework. The presence of micropores in the silica wall in SBA-3-16 was detected by XRD measurement26 and adsorption measurement.27 It has also been shown that some of the micropores penetrate the silica wall and connect the primary cylindrical pores.27 Therefore, by taking advantage of such micropores, ordered carbon cubes can be prepared by using SBA-3 as the template.27 In our study, the lifetime of the third component corresponds to a pore diameter of approximately 0.8-1.5 nm, according to the Tao model assuming that the micropore is a spherical cavity.1–3 By use of the cylindrical model,4 the pore radius is estimated to be 0.3-0.6 nm. Particularly, the lifetime of the third component for SBA-3-18 is significantly longer than those for other samples, suggesting that this component carries some information on the pore structure of the sample. Although no conclusive information is obtained from the third component at present, this component requires further detailed investigation such as comparison with TEOS-treated SBA-3 samples for selectively blocking the micropore.23,27 Conclusions Positron lifetime measurements were performed for ordered porous silica (SBA-3). The lifetime of the fourth component increased for large pores, and this component is attributed to the annihilation of o-Ps in the primary cylindrical pores. The lifetime component corresponding to the primary pores was obtained because the sizes of the grains were larger than or comparable to the diffusion length of o-Ps; this is in contrast to the case of MCM-41 where the grain size is very small. The

J. Phys. Chem. C, Vol. 112, No. 24, 2008 8783 relation between the positron lifetime and the pore size was compared to the relations obtained from several theoretical models; the cylindrical or channel models agreed rather well with the experimental results, although significant deviations were found for the SBA-3-12 and SBA-3-18 samples. These deviations can be explained by possible irregularities in the sample and the escape of o-Ps from the primary pores. References and Notes (1) Tao, S. J. J. Chem. Phys. 1972, 56, 5499–5510. (2) Eldrup, M.; Lightbody, D.; Sherwood, J. N. Chem. Phys. 1981, 63, 51–58. (3) Nakanishi, H.; Jean, Y. C.; Positron and Positronium Chemistry; Schrader, D. M., Jean,Y. C., Eds.; Elsevier: Amsterdam, 1988; Chapter 5. (4) Goworek, T.; Ciesielski, K.; Jası´nska, B.; Wawryszczuk, J. Chem. Phys. 1998, 230, 305–315. (5) Ciesielski, K.; Dawidowicz, A. L.; Goworek, T.; Jası´nska, B.; Wawryszczuk, J. Chem. Phys. Lett. 1998, 289, 41–45. (6) Ito, K.; Nakanishi, H.; Ujihira, Y. J. Phys. Chem. B 1999, 103, 4555–4558. (7) Gidley, D. W.; Frieze, W. E.; Dull, T. L.; Yee, A. F.; Ryan, E. T.; Ho, H.-M. Phys. ReV. B 1999, 60, R5157-R5160. (8) Goworek, T.; Jası´nska, B.; Wawryszczuk, J.; Zaleski, R.; Suzuki, T. Chem. Phys. 2002, 280, 295–307. (9) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710–712. (10) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C.T-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834–10843. (11) Ito, K.; Yagi, Y.; Hirano, S.; Miyayama, M.; Kudo, T.; Kishimoto, A.; Ujihira, Y. J. Ceram. Soc. Jpn. 1999, 107, 123–127. (12) He, Y. J.; Zhang, H. Y.; Chen, Y. B.; Wang, H. Y.; Horiuchi, T. J. Phys.: Condens. Matter 2001, 13, 2467–2472. (13) Goworek, J.; Wawryszczuk, J.; Zaleski, R. J. Colloid Interface Sci. 2001, 243, 427–432. (14) Wawryszczuk, J.; Goworek, J.; Zaleski, R.; Goworek, T. Langmuir 2003, 19, 2599–2605. (15) Xu, J.; Moxom, J.; Yang, S.; Suzuki, R.; Ohdaira, T. Chem. Phys. Lett. 2002, 364, 309–313. (16) Zaleski, R.; Wawryszczuk, J.; Goworek, T. Chem. Phys. Lett. 2003, 372, 800–804. (17) Zaleski, R.; Wawryszczuk, J.; Goworek, J.; Boro´wka, A.; Goworek, T. J. Colloid Interface Sci. 2003, 262, 466–473. (18) Zaleski, R.; Wawryuszczuk, J.; Boro´wka, A.; Goworek, J.; Goworek, T. Microporous Mesoporous Mater. 2003, 62, 47–60. (19) Goworek, J.; Boro´wka, A.; Zaleski, R.; Kusak, R. J. Therm. Anal. Cal. 2005, 79, 555–560. (20) Zaleski, R.; Boro´wka, A.; Wawryszczuk, J.; Goworek, J.; Goworek, T. T. Chem. Phys. Lett. 2003, 372, 794–799. (21) Zaleski, R.; Wawryszczuk, J.; Goworek, T. Radiat. Phys. Chem. 2007, 76, 243–247. (22) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147–1160. (23) Chen, F.; Shen, S.; Xu, X.-J.; Xu, R.; Kooli, F. Microporous Mesoporous Mater. 2005, 79, 85–91. (24) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373–380. (25) Kirkegaard, P.; Eldrup, M.; Mogensen, O. E.; Pedersen, N. J. Comput. Phys. Commun. 1981, 23, 307–335. (26) Albouy, P.-A.; Ayral, A. Chem. Mater. 2002, 14, 3391–3397. (27) Chen, F.; Xu, X.-J.; Shen, S.; Kawi, S.; Hidajat, K. Microporous Mesoporous Mater. 2004, 75, 231–235.

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