Self-Assembly, Optical, and Mechanical Properties ... - ACS Publications

Dec 16, 2008 - M. Abdul Wahab* and Chaobin He* ..... The BET surface area, pore size (PS), and pore volume (PV) are shown in Table 1 and Figure 5...
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Langmuir 2009, 25, 832-838

Self-Assembly, Optical, and Mechanical Properties of Surfactant-Directed Biphenyl-Bridged Periodic Mesostructured Organosilica Films with Molecular-Scale Periodicity in the Pore Walls M. Abdul Wahab* and Chaobin He* Department of Materials Synthesis and Integration, Institute of Materials Research and Engineering (IMRE), The Agency for Science, Technology, and Research (A*STAR), 3 Research Link, Singapore 117602, Republic of Singapore ReceiVed September 29, 2008. ReVised Manuscript ReceiVed NoVember 11, 2008 Self-assembly, optical, and mechanical properties of surfactant-directed biphenyl-bridged periodic mesoporous organosilica thin films (PMOF-Bp’s) with molecular-scale periodicity in the pore walls were successfully demonstrated for the first time. The biphenyl-bridged organosilica precursor, 4,4-bis(triethoxysilyl)biphenyl (Bp-TES) has been used as the sole precursor (100%) for preparing PMOF-Bp films with molecular-scale periodicity in the pore walls via the surfactant-mediated one-step mild acidic self-assembly process. High-resolution X-ray diffraction (HRXRD) patterns and transmission electron microscope (TEM) images of PMOF-Bp materials confirmed the formation of a biphenylbridged periodic mesophase with molecular-scale periodicity in the organosilica framework. Fourier transform infrared (FT-IR) and NMR spectroscopic data also strongly suggested that the biphenyl organic segment is covalently bonded with silicon atoms in the acidic ethanol-washed biphenyl-bridged mesoporous framework. The emission behavior is sensitive to synthesis and thermal treatment temperatures. The biphenyl-bridged PMO films show absorption and emission due to the presence of biphenyl segment in pore walls. Nanoindentation hardness of the PMOF-Bp films could be controlled by temperature, degree of pore ordering and molecular periodicity, and even thickness of films. For example, well-organized PMOF-Bp film with molecular-scale periodicity in the pore walls showed a higher hardness value (0.23 GPa) than that of less mesoordered PMOF-Bp film (0.13 GPa). For all solvent-extracted PMO samples, N2 gas sorption experiments showed the surface area (from 714 to 688 m2/g), the pore volume (from 0.76 to 0.68 cm3/g), and pore size (2.81 to 3.1 nm). The solid-state NMR and FT-IR spectroscopic data were used to propose plausible interpretations of the formation of hydrogen-bonded molecular periodicity in the pore walls. The experimental periodicity value 1.40 nm was strongly supported by the periodicity obtained by the structural model (1.389 nm).

1. Introduction Surfactant-directed functional periodic mesoporous organosilica films (PMOs) constitute a very interesting class of periodic mesostructured materials because they can be developed into organic-inorganic periodic nanostructured materials with unique sensor, optical, catalytic, electrical properties, whereby the organic functional groups of mesoporous host can play an important role.1 For example, functionalized nanochannels in mesostructured films with specific functions have attracted considerable interest for a variety of promising applications such as pH sensor,2 hard coating,3 energy transfer and luminescent probes,4,5 catalysis,6 patterning,7 optical and fluorescent materials,4,5,8,9 * Corresponding author. E-mail: [email protected] (M.A.W.); [email protected] (C.H.). Fax: 65 6872 7528. Tel: 65 6874 1972. (1) (a) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611–9614. (b) Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A. Chem. Mater. 1999, 11, 3302–3308. (c) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867–871. (2) Wirnsberger, G.; Scott, B. J.; Stucky, G. D. Chem. Commun. 2001, 119– 120. (3) Sellinger, A.; Weiss, P. M.; Nguyen, A.; Lu, Y.; Assink, R. A.; Gong, W.; Brinker, C. J. Nature 1998, 394, 256–261. (4) Minoofar, P. N.; Hernandez, R.; Chia, S.; Dunn, B.; Zink, J. I. J. Am. Chem. Soc. 2001, 123, 1248–1249. (5) Minoofar, P. N.; Hernandez, R.; Chia, S.; Dunn, B.; Zink, J. I.; Franville, A. C. J. Am. Chem. Soc. 2002, 124, 14388–14396. (6) Yang, Q.; Liu, J.; Yang, J.; Kapoor, M. P.; Inagaki, S.; Li, C. J. Catal. 2004, 228, 265–272. (7) Doshi, D. A.; Huesing, N. K.; Lu, M.; Fan, H.; Lu, Y.; Simmons-Potter, K.; Potter, B. G.; Hurd, A. J.; Brinker, C. J. Science 2000, 290, 107–111. (8) Goto, Y.; Mizoshita, N.; Ohtani, O.; Okada, T.; Shimada, T.; Tani, T.; Inagaki, S. Chem. Mater. 2008, 20, 4495–4498. (9) Wong, E. M.; Markowitz, M. A.; Qadri, S. B.; Golledge, S.; Castner, D. G.; Gaber, B. P. J. Phys. Chem. B 2002, 106, 6652–6658.

adsorbent,10 low dielectric film,11,12 drug delivery,13 and chemical and biochemical reactions.14 Previously, such periodic nanostructures have often been prepared in powder form by the direct synthesis method, which allows the control of pore function and distribution of organic functionally in the pore walls compared to the postsynthetic grafting method.15-21 Previous reports usually focused on powder materials which have limited applications, (10) (a) Rebbin, V.; Schmidt, R.; Froba, M. Angew. Chem., Int. Ed. 2006, 45, 5210–5214. (b) Hoffmann, F.; Cornelius, M.; Morell, J.; Froba, M. Angew. Chem. Int. Ed. 2006, 45, 3216–3251. (c) Wong, E. M.; Markowitz, M. A.; Qadri, S. B.; Golledge, S.; Castner, D. G.; Gaber, B. P. Langmuir 2002, 18, 972–974. (11) (a) Lu, Y. F.; Fan, H. Y.; Doke, N.; Loy, D. A.; Assink, R. A.; LaVan, D. A.; Brinker, C. J. J. Am. Chem. Soc. 2000, 122, 5258–5261. (b) Yu, K.; Smarsly, B.; Brinker, C. J. AdV. Func. Mater. 2003, 13, 47–52. (12) (a) Dag, O.; Yoshina-Ishii, C.; Asefa, T.; MacLachlan, M. J.; Grondey, H.; Coombs, N.; Ozin, G. A. AdV. Funct. Mater. 2001, 11, 213–217. (b) Hatton, B. D.; Landskron, K.; Whitnall, W.; Perovic, D. D.; Ozin, G. A. AdV. Func. Mater. 2005, 15, 823–829. (13) Ji, Q.; Miyahara, M.; Hill, J. P.; Acharya, A.; Vinu, A.; Yoon, S. K.; Yu, J. S.; Sakamoto, K.; Ariga, K. J. Am. Chem. Soc. 2008, 130, 2376–2377. (14) Angelome, P. C.; Soller-Ilia, G. J. A. A. Chem. Mater. 2005, 17, 322– 331. (15) (a) Kapoor, M. P.; Inagaki, S. Bull. Chem. Soc. Jpn. 2006, 79, 1463–1475. (b) Grosso, D.; Cagnol, F.; Soler-Ilia, G. J. A. A.; Crepaldi, E. L.; Amenitsch, H.; Brunet, A. B.; Bourgeos, A.; Sanchez, C. AdV. Funct. Mater. 2004, 14, 309– 322. (c) Soler-Ilia, G. J. A. A.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. ReV. 2002, 102, 4093–4138. (d) Corma, A. Chem. ReV. 1997, 97, 2373–2420. (e) Wight, A. P.; Davis, M. E. Chem. ReV. 2002, 102, 3589–3614. (f) Ying, J. Y; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 57–77. (16) Zulkalova, M.; Zukul, A.; Kavan, L.; Nazeeruddin, M. K.; Liska, P.; Gratzel, M. Nano Lett. 2005, 5, 1789–1792. (17) (a) Stein, A.; Melde, B.; Schroden, R. C. AdV. Mater. 2000, 12, 1403– 1419. (b) Burkett, S. L.; Sims, S. D.; Mann, S. Chem. Commun. 1996, 1367–1368. (c) Mercier, L.; Pinnavaia, T. J. Chem. Mater. 2000, 12, 188–196. (d) Sayari, A.; Hamoudi, S. Chem. Mater. 2001, 13, 3151–3168. (18) Aida, T.; Tajima, K. Angew. Chem., Int. Ed. 2001, 40, 3803–3806. (19) Alvaro, M.; Aprile, C.; Benitez, M.; Bourdelande, J. L.; Carcia, H.; Herance, J. R. Chem. Phys. Lett. 2005, 414, 66–70.

10.1021/la803192z CCC: $40.75  2009 American Chemical Society Published on Web 12/16/2008

PMO with Molecular-Scale Periodicity

whereas the thin film-based PMOs show more advantages compared with powder-based mesostructures in terms of sensitivity, connection to the substrate such as electrode, organic-inorganic functions, patterning, uniformity, and transduction characteristics that are required for various applications such as sensing.2-9,11-15,22-27 Importantly, the performance of these nanostructured films could be enhanced by incorporating organic functions in the pore walls via functionalization process. Thus, to increase the utility of thin films, recent investigations have focused on the incorporation of a number of functional groups within the pore walls of mesoporous frameworks.22-30 The functional group in the pore framework allows control of the final properties of the mesostructures. It would therefore appear that the PMOs, synthesized from 100% or less organicbridged silsesquioxanes [(C2H5O)3-R-(OC2H5)3, where R is a bridging organic function], have extremely extended the potentiality of mesoporous thin films and have attracted many researchers as various organic active functions can directly be added into the pore walls by selection of the R framework organic functions.15,28-36 Particularly, 100% bridged-silesquioxanederived PMO materials are promising because the densely packed organic function in the mesoporous framework can enhance its functionalities relative to that of organically modified inorganic mesostructures.8-12,24-26,28-36 Meanwhile, various types of PMOs containing different organic functions have been reported. Among the studied PMOs, ethane-bridged PMOs with different functions have been reported extensively,28-30 whereas phenyl (20) (a) Wang, Y. Q.; Yang, C. M.; Zibrowins, B.; Spiethoff, B.; Linden, M.; Fchuth, F. Chem. Mater. 2003, 15, 5029–5035. (b) Choi, M.; Cho, H. S.; Srivastava, R.; Venkatesan, C.; Choi, D. H.; Ryoo, R. Nat. Mater. 2006, 5, 718–723. (c) Kruk, M.; Jaroniec, M.; Sayari, A. Langmuir 1999, 15, 5683–5688. (d) Zhu, H.; Jone, D. J.; Zajac, J.; Dutarte, R.; Rhomari, M.; Roziere, J. Chem. Mater. 2002, 14, 4886–4894. (e) James, E. M.; Mark, T. A.; Judy, O.; Paula, N. Langmuir 1997, 13, 4133–4141. (21) (a) Wahab, M. A.; Imae, I.; Kawakami, Y.; Ha, C. S. Chem. Mater. 2005, 17, 2165–2174. (b) Wahab, M. A.; Ha, C. S. J. Mater. Chem. 2001, 15, 508–516. (c) Wahab, M. A.; Kim, I.; Ha, C. S. Microporous Mesoporous Mater. 2004, 69, 19–27. (d) Wahab, M. A.; Kim, I.; Ha, C. S. J. Solid State Chem. 2004, 177, 3439–3447. (e) Wahab, W. A.; Imai, I.; Kawakami, Y.; Kim, I.; Ha, C. S. Microporous Mesoporous Mater. 2006, 92, 201–211. (f) Wahab, M. A.; Kim, I.; Ha, C. S. J. Nanosci. Nanotechnol. 2007, 8, 3532–3539. (22) (a) Ogawa, M. J. Am. Chem. Soc. 1994, 116, 7941–7942. (b) Ogawa, M.; Kurodda, K.; Mori, J. Langmuir 2002, 18, 744–749. (c) Yui, T.; Tsuchino, T.; Itoh, T.; Ogawa, M.; Fukushima, Y.; Takagi, K. Langmuir 2005, 21, 2644–2646. (d) Ogawa, M. Langmuir 1995, 11, 4639–4641. (23) Yang, H.; Kupermann, A.; Coombs, N.; Afra, S. M.; Ozin, G. A. Nature 1996, 379, 703–705. (24) (a) Eckert, H.; Ward, M. Chem. Mater. 2001, 12, 1–231. (b) Loy, D. A. MRS Bull. 2001, 26, 364–365. (c) Sol-gel products news. J. Sol-Gel Sci. Technol. 2000, 18, 287-292. (25) Miyata, H.; Suzuki, T.; Fukuoka, A.; Sawada, T.; Watanabe, M.; Noma, T.; Takada, K.; Mukaide, T.; Kuroda, K. Nat. Mater. 2004, 3, 651–656. (26) (a) Wahab, M. A.; Sudhakar, S.; Yeo, E.; Sellinger, A. Chem. Mater. 2007, 20, 1855–1261. (b) Wahab, A.; Sellinger, A. Chem. Lett. 2006, 10, 1240– 1241. (27) Kirmayer, S.; Dovgolevsky, E.; Kalina, M.; Lakin, E.; Cadars, E.; Epping, J. D.; Arteaga, A. F.; Abreu, C. R.; Chmelka, B. F.; Frey, G. L. Chem. Mater. 2008, 20, 3745–3756. (28) Hatton, B. D.; Landskron, K.; Whitnall, W.; Perovic, D.; Ozin, G. A. Acc. Chem. Res. 2005, 38, 305–312. (29) Hoffmann, H.; Cornelius, M.; Morell, J.; Froba, M. J. Nanosci. Nanotechnol. 2006, 6, 265–288. (30) Fujita, S.; Inagaki, S. Chem. Mater. 2008, 20, 891–908, and all references therein. (31) (a) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. Nature 2002, 416, 304–307. (b) Yang, Q.; Kapoor, K. P.; Inagaki, S. J. Am. Chem. Soc. 2002, 124, 9694–9695. (c) Kapoor, M. P.; Yang, Q.; Inagaki, S. J. Am. Chem. Soc. 2002, 124, 15176–15177. (d) Kapoor, M. P.; Inagaki, S.; Ikeda, S.; Kikiuchi, K.; Suda, S.; Shimada, T. J. Am. Chem. Soc. 2005, 127, 8174–8178. (32) Bion, N.; Ferreira, P.; Valente, A.; Goncalves, I. S.; Rocha, J. J. Mater. Chem. 2003, 13, 1910–1913. (33) (a) Sayari, A.; Wang, W. J. Am. Chem. Soc. 2005, 127, 12194–12195. (b) Cornelius, M.; Hoffmann, F.; Froba, M. Chem. Mater. 2005, 17, 6674–6678. (c) Yong, Y.; Sayari, A. Chem. Mater. 2007, 19, 4117–4119. (34) Kuroki, M.; Asefa, T.; Whitnal, W.; Kruk, M.; Yoshina-Ishii, C.; Jaroniec, M.; Ozin, G. A. J. Am. Chem. Soc. 2002, 124, 13886–13895. (35) Sheng, Y.; Han, S. H.; Huang, M. H. Chem. Mater. 2007, 19, 5986–5990. (36) Suzuki, T.; Miyata, H.; Kuroda, K. J. Mater. Chem. 2008, 18, 1239–1244.

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and biphenyl-bridged PMOs are the less studied mesostructures.30-36 Phenyl and biphenyl-bridged PMO powders with crystalline pore walls have been reported.31-33 Huang et al. also reported phenyl-bridged PMO films with structural properties only.35 The structural and optical properties of aromatic-bridged organosilica mesoporous films without molecular periodicity in the pore walls have been reported.8,11,12,36 However, mechanical properties of these hybrid PMO films have yet to be studied in detail, and, so far, literature shows that the dependence of the experimental conditions such as degree of pore ordering, thickness, and so forth on the optical and mechanical properties of biphenyl-bridged PMO thin films with molecular periodicity in the pore walls has not been reported anywhere in detail. In this paper, we successfully report the mechanical and optical properties of surfactant-mediated self-assembled biphenyl-bridged PMO films with molecular periodicity of 1.40 nm in the pore walls for the first time. In addition, the dependence of degree of pore ordering and molecular periodicity, thickness, and degree of cross-linking on the nanoindendation mechanical hardness of biphenyl-PMO films is systematically described. The mesophase formation with molecular periodicity obtained by high-resolution X-ray diffraction (HRXRD) is very consistent with transmission electron microscopy (TEM) images. The emission behavior of biphenyl-bridged PMO films is also found to be influenced by thermal treatment and synthesis conditions. The 13C and 29Si cross-polarization/magic-angle spinning (CP MAS) NMR results have confirmed the integrity of the Si-C bonds in biphenylbridged mesoporous frameworks, supported by Fourier transform infrared (FT-IR) spectroscopy. The structural model also confirms the hydrogen-bonded periodicity obtained by HRXRD.

2. Experimental Section 2.1. Sols and Film Preparation via the One-Step Self-Assembly Process. Initially, transparent bis-silanetriols sols (4,4′-bis(trihydroxysilyl)biphenyl) and films from 4,4′-bis(triethoxysilyl)biphenyl (Bp-TES) were prepared according to Figure 1. The sol solution was prepared by mixing Bp-TES (Aldrich), ethanol, water, and HCl (0.07M) with molar ratios of 1.00:52.2:7.2:2.2 × 10-3, respectively. In order to get rid of moisture, a tech-grade ethanol-containing 50 mL bottle cap was sealed, then Bp-TES was added into bottle. Water and finally HCl (0.07 M HCl) were injected into the solution without opening the bottle cap. This work uses the aforementioned adding sequence, which is very important for maintaining the transparency of the sols and also for fabricating films. The mixture was obtained as a transparent solution of bis-silanetriols sol of (4,4′-bis(trihydroxysilyl)biphenyl) after stirring for about 10 min at room temperature and was held at room temperature for 15 min. Then 3.25 wt % of cetyltrimethylammonium bromide (CTAB) was added into the solution. Then the surfactant-containing solution was cast onto precleaned glass substrate to make PMO films. A rotation speed of 1000 rpm for 30 s was used for preparing biphenyl-bridged PMO films. The PMO films containing substrates were dried at room temperature for several hours. Films were washed with acidic ethanol (50 mL ethanol and 1 mL HCl) for 15 min for removing surfactant moleculesfromtheporechannelsofPMOs.ForBrunauer-Emmett-Teller (BET) and other measurements, the solvent-extraction process was used to remove surfactant molecules by using same acidic ethanol for 12 h. 2.2. Cleaning Glass Substrates. All glass substrates were cleaned and stored in isopropyl alcohol (IPA). Before film casting, previously cut (1 in. × 1 in. pieces) glass slides were immersed in 1 M H2SO4 and allowed to undergo ultrasonication for 1 h. The glass slides were again ultrasonicated for 30 min each in deionized (DI) water, acetone, and IPA. All glass slides were stored in IPA solution. Prior to use, all slides were just rinsed with IPA solution several times and dried at room temperature, then used for film casting.

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Figure 1. (a) Sol solution containing 4,4′-bis(trihydroxysilyl)biphenyl, water, HCl and ethanol. (b) Self-assembled biphenyl-bridged PMO film formed via the one-step self-assembly process. (c) Structural model of layer arrangement of partially condensed Bp-TES units in pore walls. The biphenyl segment and molecular periodicity by structural model are also shown in panel c.

2.3. Measurement and Characterization Techniques. The periodic mesostructure of PMO film was evaluated by HRXRD (PANalytical X’Pert PRO High-Resolution XRD) and TEM (Philips CM300 FEG TEM microscope; operated at 300 kV). The scanning 2θ angle for HRXRD was in the range of 0-40°. For the TEM image, the biphenyl-bridged PMO films were scratched off from the glass substrates. The collected powder particles were dispersed into methanol by ultrasonication for 10 min, and then the solution was poured onto a holey carbon grid. FT-IR spectroscopy, solid-state 29 Si MAS NMR, and 13C CP MAS NMR were also used for structural characterization of the as-synthesized and acidic ethanol-washed biphenyl-bridged PMOs. For verifying the experimental hydrogenbonded periodicity, a structural model was built using MS Modeling (Accelrys Software, Inc.) and optimized using molecular mechanics methods (with a force field COMPASS). Surface areas and pore structures of powder samples were measured by a N2 gas sorption experiment at 77 K using a Quantachrome BET surface area analyzer (model NOVA 1200) apparatus and applying the BET and Barrett-Joyner-Halenda (BJH) methods. The pore-size distribution curves were obtained from the desorption branch by use of the BJH method. For this experiment, all powder samples were dehydrated at 150 °C for 18 h. The nanoindentation tests were carried out on an MTS Nano XP with a Berkovich (3-side pyramid) indenter. A constant strain rate of 5% s-1 was applied during the loading segment. A 60 s holding time was used after the indenter reached maximum depth. The same rate was applied for the unloading segment. The reported hardness values are averaged over a depth range of 25-70 nm where the total film thicknesses were about 1450-2000 nm. Hardness values were obtained from the following equation: H ) P/Amax, where H is the hardness, P is the maximum indentation load, and Amax is the maximum contact area. A field emission scanning electron microscope (SEM; JEOL FESEM JSM6700F) was used in order to observe the thickness of the PMO film. A diamond knife was used to cut it into a few pieces. Then a piece of fractured film was mounted on a carrier prepared by using double-sided carbon

tape. The cross-section of the film was coated with a film of gold. This coated film was used for observing the cross-section of PMO films. The UV-vis photospectrometer (UV-3101PC) was also used for this study.

3. Results and Discussion Preparation of sols and optimization of sol-gel conditions. For this study, at first we made the sols, which contain hydrophobic biphenyl chromophore as a bridging ligand and two hydrophilic triol groups at both ends of the biphenyl hydrophobic segment, which undergo further bonding with neighboring silanols for producing biphenyl-bridged PMOs with molecular periodicity in the pore walls. The structural and film formation properties of the films strongly depend on the hydrolysis and condensation of the Bp-TES precursor and the used sol-gel reaction conditions such as the amount of ethanol and HCl, nature of solvent, temperature, and water. The use of a higher amount of ethanol and HCl led to an opaque solution within a very short time, and the final materials were simple mesostructures with reduced molecular periodicity. The sample PMOF-Bp-W shows good transmittance (88-90%) in the visible region, whereas the PMO film made using a little longer spin coating time (such as 10 min) shows transmittance 85-86% in the aforementioned visible region. The sol-gel harsh conditions also did not form good films, and hydrothermal synthesis did not allow fabricating good films because sols show some particles as a result of the prompt condensation of precursors. Nanoindentation hardness also depends on the thickness, degree of pore ordering, and used reaction conditions. It is concluded therefore that a precise control over the sol-gel reaction conditions is very important for the preparation of Bp-TES-based PMO films. The optimum sol-gel conditions were adopted in this study.

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Figure 2. (a) HRXRD of PMOF series (as-prepared and washed samples). (b) TEM image of washed PMOF-Bp-W sample; scale bar is 10 nm. Inset in panel b shows the lattice fringes and their direction. (c) Structural model of layer structure of the less condensed Bp-TES units in which the measured periodicity is about 1.389 nm (see white line in panel c).

Periodic Mesostructure with Molecular-Scale Periodicity in the Pore Walls. Figure 2 shows the HRXRD patterns, TEM image, and structural model. Intense peaks were observed at 2θ ) 2.35° and 2.40°, corresponding to d-spacings of 3.76 and 3.66 nm for the as-prepared and washed biphenyl-bridged PMO films, respectively, indicating the formation of periodic mesostructured films via the mild acidic sol-gel self-assembly process,3-5,7-12,22,23,26 also supported by the TEM image in Figure 2b. The washed PMO film shows the small contraction (2.66%) of the d-spacing and the increased peak intensity in Figure 2a due to the removal of the CTAB molecules from the pore channels and the accompanying co-condensation of silanol groups of pore walls.3,22,23,26 The XRD diffraction patterns are very consistent with the previously reported PMO mesostructured films.8-12,26 Importantly, higher angle peaks were also clearly observed at d-spacings of 1.40, 0.68, 0.45, 0.35, 0.28, and 0.24 nm, which directly corresponds to the molecular-scale periodic structure present in the biphenyl-derived PMO films.31-37 The discernible higher angle peaks of the acidic ethanol-washed PMO film suggest the retention of the molecularly ordered structures in the final PMO film. However, the calculated molecular periodicity from HRXRD is about 1.40 nm in the pore walls as a result of the organization of the biphenyl-organosilica, and the obtained periodicity value is well-supported with the periodicity (1.389) obtained by the structural model in Figure 2c.35,37 Previously, higher angles of several XRD peaks at d -spacings of 1.16, 0.59, 0.39, 0.29, and 0.24 nm (2θ ) 6-40°) were also reported for covalently bonded biphenyl-bridged PMO powder materials formed via the base-catalyzed self-assembly process;31-33 the investigated biphenyl-bridged PMO film in this study also possesses similar molecular periodicity in the mesoporous organosilica framework. The very small peaks in the XRD patterns next to the (002) peak for both cases are very similar with previously reported works,37 but at present it is difficult to attribute these peaks. The TEM image in Figure 2b shows the formation of a periodic mesoporous structure along with a periodicity of 1.40 nm on the pore walls of the PMO material.31-36 The calculated spacing between adjacent channels (Figure 2b) is ∼4.2 nm, which matches with the obtained unit-cell parameter (a) in Figure 2a. The inset image in Figure 2b clearly shows the lattice fringes and channel direction. In this image, the lattice fringes are not running perfectly perpendicular to the pore channel axis,35,38 in sharp contrast to (37) (a) Cerveau, G.; Corriu, R. J. P.; Dabiens, B.; Bideau, J. L. Angew. Chem, Int. Ed. 2000, 39, 4533–4537. (b) Ben, F.; Boury, B.; Corriu, R. J. P.; Strat, V. Organometalics 2000, 12, 3249–3252. (38) Brandhuber, D.; Peterlik, H.; Huesing, N. Small 2006, 2, 503–506.

Figure 3. A cross-sectional SEM image of the acidic ethanol-washed biphenyl-bridged PMO film. The film is about 1.47 µm thick, and dense, continuous, and homogeneous on the glass substrate.

the previously reported perpendicular layer arrangement to the pore channels.31 The lattice fringes in Figure 2b (inset) make an angle of roughly 52.1° (to be horizontal) with respect to the direction of pore channels. Such results can be observed when the mesoporous structures have organized periodic mesostructures with periodicity in the pore walls. Importantly, the obtained periodicity value of 1.40 nm by HRXRD is also very close to the value of 1.389 nm obtained by the structural model in Figure 2c, which is larger than that of the previously reported covalently bonded biphenyl-bridged PMO powders (∼1.16 nm).31c The present study shows a periodicity of 1.40 nm, which is larger than the previously reported periodicity of 1.16 nm for the same precursor, due to the formation of the hydrogen-bonded network with neighboring silanols of Bp-TES precursor, which may allow the organization of hydrophobic biphenyl-bridged silsesquioxane segments.37 Previous studies have confirmed that the replacement of covalent bonds with hydrogen bonds allowed the formation of elongated periodic interlayer spacings.31-33,35,37 So, it could be clearly demonstrated that the hydrogen bonding from partially condensed structures may result in the formation of the relatively elongated periodic interlayer spacing of 1.40 nm for this study.31,35,37 Figure 3 shows an SEM image of a cross-section of the acidic ethanol-washed biphenyl-bridged PMO film. The 1.47 µm thick film is dense, continuous, and homogeneous on the glass substrate. Solid-State NMR and FT-IR Studies. For this study, the solid-state NMR and FT-IR results in Figure 4 suggest that low condensation of Bp-TES may lead to the formation of a hydrogenbonded network with neighboring silanols, and eventually the hydrogen bonding elongates the periodic interlayer spacing.31-37

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Figure 4. (left) Solid-state 29Si CP MAS NMR spectrum of (a) the washed PMOF-Bp-W and (b) PMOF-Bp-WSM; (right) FT-IR spectra of (a) as-prepared PMOF-Bp-A, (b) acidic ethanol-washed PMOF-BpW, and (c) PMOF-Bp-WSM. Table 1. Physical Properties of Biphenyl-Bridged PMO Materials sample code

d100a (nm)

SBETb (m2/g)

PMOF-Bp-Ae PMOF-Bp-Wg PMOF-Bp-WHh PMOF-Bp-WSMi.

3.76 3.66 3.64 3.66 (broad)

41 714 721 688

PVc (cm3/g)

PSd (nm)

NOf 0.76 0.68 0.73

NO 3.10 2.88 2.81

a nλ ) 2d100 sin θ. b BET surface area from BET measurements. c PV ) pore volume. d PS ) pore size. e NO ) not obtained. f A ) as-prepared. g W ) washed with acidic ethanol. h H ) heat treated sample at 75 ( 2 °C. i. SM ) simple mesostructure with reduced periodicity.

Figure 4a shows three intense resonances at -63, -73, and -86 ppm, which directly correspond to the T1 [CSi(OSi)(OH)2], T2 [CSi(OSi)2(OH)], and T3 [CSi(OSi)3] sites, respectively.8,11,12,31-35,37 The absence of Qn sites in the range of -100 to -125 ppm confirms that the stability of Si-C bonds in the O1.5Si-(C6H4)2-SiO1.5 framework and the O1.5Si-(C6H4)2SiO1.5 units are completely retained in the final mesoporous framework under used conditions.8,11,12,31-37 The T1 and T2 intensities are higher than the T3 intensity, suggesting the lower degree of organosilicate condensation. Therefore, such a high concentration of silanols may easily form the hydrogen-bonded network. According to previous studies, the base-catalyzed biphenyl-bridged PMO powders with periodicity of 1.16 nm showed that the peak intensities of Tn were in the order of T3 > T2 > T1 sites.31a,c,d,35,37 Thus it could be demonstrated that the periodicities also depend on the organic spacer, the nature of bonding, and the used sol-gel reaction conditions.31-39 Other studies also confirmed that the covalently bonded periodic lattices are always smaller than hydrogen-bonded periodic lattices for same compound such as phenyl/biphenyl-bridged sol-gel materials.31,35,37 The FT-IR spectra in Figure 4 were also used to investigate the presence of biphenyl-bridged organosilica segment, Si-O-Si, Si-OH, and the removal of CTAB surfactant molecules from the pore channels of PMO materials.14-25 Surfactant moleculerelated peaks at 2924 and 2847 cm-1 have been significantly reduced and broadened after washing with acidic ethanol, indicating the removal of surfactant molecules from the pore channels of PMO materials, which is also consistent with the BET results in Table 1. The Si-O vibrations of Si-OH and Si-O-Si groups are observed at 896 and 1038 cm-1 respectively, whereas peaks at 1156 and 1411 cm-1 are attributed to the C-H vibrations of the biphenyl group. The responsible peaks for Si-OH and H2O may overlap in the range of 3200-3500 cm-1. A similar broad peak was obtained when the Si-OH groups participate in hydrogen bonding with neighboring hydrolyzed silanols of organosilica materials.14-26 (39) (a) Shea, S. K.; Loy, D. A.; Webster, O. J. Am. Chem. Soc. 1992, 114, 6700–6710. (b) Loy, D. A.; Shea, K. J. Chem. ReV. 1995, 95, 1431–1442.

Figure 5. Nitrogen isotherms (a) and PSD (b) of PMOF-Bp-W and PMOF-Bp-WH.

Structural Model Study. Figure 2c shows a structural model made for the layer arrangement of the partially condensed BpTES units in the pore walls of mesostructured biphenylorganosilica. Previously, the value of experimental molecular periodicity was justified by molecular structural model value. As shown in Figure 2c, minimizing the framework energy of the hydrogen-bonded molecular lattice using a molecular structural model resulted in an interlayer spacing of 1.389 nm, which is very consistent with the 1.40 nm periodicity observed by HRXRD, substantiating the formation of a layer structure in the pore walls via hydrogen bonding. These consistent results occur because the final biphenyl-bridged PMO structures are stabilized via hydrogen bonding.37 A periodicity of 1.40 nm of biphenyl-bridged organosilica sol-gel powders is also reported in previous work on the structural analysis of 4,4-bis(trihydroxysilyl)biphenyl, where the stabilization of molecular periodic structure was the result of the formation of a strongly hydrogen-bonded network, which induces the organization of a hydrophobic biphenyl moeity.37 It should be mentioned here that the trihydroxysilyl molecules, a triol analogue of the biphenyl-bridged organosilica precursors, are arranged in a head-to-tail manner to form a layerordered structure.31a,35-37 Thus it is very reasonable that the employed Bp-TES precursor could be arranged in a layer structure in the pore walls of mesoporous PMOs since we have Bp-TES molecules with alternatively hydrophilic ends (triols) of hydrophobic biphenyl pillars. Hydrophobic and hydrophilic interactions may also significantly favor the self-assembly of Bp-TES-based PMOs with molecular periodicity in the pore walls.31a-c,35-37 Physical Properties. The BET surface area, pore size (PS), and pore volume (PV) are shown in Table 1 and Figure 5. Asprepared PMOF-Bp-A shows very small surface area (47 m2/g) without any pore volume and isotherms, indicating the pores are blocked with surfactant molecules.19-21,26a,31 This is more obvious in the solvent-washed sample. High surface area, pore volume, and pore size for the solvent extracted PMOF-W series were 688-721 m2/g, 0.68-0.76 cm3/g, and 2.81-3.1 nm, respectively, as a result of the removal of surfactant molecules from the pore

PMO with Molecular-Scale Periodicity

Figure 6. Absorption spectrum (a), and emission spectra of (b) PMOFBp-W and (c) PMO-Bp-WH.

channels of PMOs. These results are supported by the FT-IR spectra in Figure 4 (see region 2800-3000 cm-1).21,26a Two representative samples (isotherms and pore size distribution (PSD) curves) are shown in Figure 5. All samples show almost the same kinds of isotherms (type IV) with relative pressures at 0.3 to 0.4 to 0.6 to 0.7, under studied conditions. The PSD curves for two samples are also compared. Expectedly, both PSD curves also show the same shape without any difference. These results are in good agreement with those of the previously reported PMOs and even with phenyl/biphenyl-bridged PMO materials.31 Optical and Mechanical Properties. The optical characteristics of the Bp-TES-based PMO films were evaluated. Figure 6a shows the normalized absorption spectrum of a Bp-TESbased PMOF film. The present film shows a strong absorption due to the densely packed biphenyl groups in the pore walls.8,36 The absorption is at around 264 nm. Figure 6 shows a normalized emission spectrum of Bp-TES PMO film. The emission spectra in Figure 6b,c indicate that biphenyl groups in pore walls are also responsible for this emission.8,36 The observed optical results closely resemble that of the previously reported biphenyl-bridged PMO film. In order to see how thermal treatment affects emission behavior, the film was heated at 75 ( 2 °C for 5 h. Then the observed emission spectrum (Figure 6c) was slightly different from that of the PMO film without thermal treatment (Figure 6b), despite the slightly different degree of cross-linking of the biphenyl-bridged organosilica framework. It is obvious that heating the film increases the full wide at half-maximum, and the spectrum in Figure 6c also seems to be slightly shifted to a higher wavelength. The obtained results indicate that emission behavior could be changed by the synthesis and thermal treatment process.36 Very recently, Suzuki et al. have discussed the influence of the synthesis and thermal treatment temperatures on the emission behavior of phenyl-bridged PMO films.36 They have found that the emission spectrum became broader when the films were heated from 80 to 100 °C for several hours. The increased motion of the phenylene groups upon thermal treatment may induce Π interactions between the groups, which result in the preferred formation of excimers, and even the shrinkage of the PMO film increases the density of phenylene functions in the final framework. These synthesis factors of PMO films have also influenced the emission behavior. The results are also consistent with other recently reported aromatic silica films without molecular periodicity by Inagaki et al.8 The nanoindentation hardness of the biphenyl-bridged PMO mesoporous films was systematically investigated using a nanoindentation test. To date, only a few reports on the mechanical properties of mesoporous silica films have been discussed, although the mechanical response of thin coatings is very important for the evaluation of materials performance for various applications such hybrid coatings and microelectronics.3,11,12,26a The systematic results of nanoindentation experiments on various biphenyl-bridged PMO films are shown in Figure 7. In Figure

Langmuir, Vol. 25, No. 2, 2009 837

Figure 7. Nanoindentation hardness of (a) as-prepared PMOF-Bp-A film, (b) acidic ethanol-washed PMOF-Bp-W film, (c) acidic ethanolwashed PMOF-Bp-WH film (heated 75 ( 2 °C for 5 h), (d) acidic ethanol-washed PMOF-Bp-WSM film with simple mesostructure and reduced periodicity, (e) acidic ethanol-washed PMOF-Bp-WHT film with higher thickness (1.8 µm), and (f) the PMOF-Bp-WLT washed for longer time (30 min).

7, acidic ethanol-washed PMOF-Bp-W film shows a higher hardness value (0.23 GPa) compared to that of the as-prepared PMOF-Bp-A film, indicating that the removal of soft organic surfactant molecules from pore channels increases the hardness of the film as does the more condensed mesostructure, as evidence by the HRXRD in Figure 2a, which shows a shifting of peak position from 2.35° to 2.40° (2θ). Previous studies also show that the removal of the soft organic surfactant molecules from the pore channels increases the hardness.26a,40 Importantly, this study considers a number of variables such as temperature, thickness, and the degree of pore ordering to see how variables affect the hardness values of the final PMO films. For example, the washed PMOF-Bp-WH film is treated at 75 ( 2 °C for 5 h. This PMOF-Bp-WH film shows an increased hardness value of 0.27 GPa after heat treatment, perhaps due to the increased degree of cross-linking in the final PMO film.40 For sample PMOFBp-WSM, when the concentration of the ethanol and HCl initial mixture increases and stirring time also increases to 30-40 min, the resulting solution becomes opaque within a short time and leads to a mesostructure PMO film with a reduced degree of periodicity. The hardness value for this PMO film decreases from 0.23 to 0.13 GPa, as shown in Figure 7d (sample PMOBp-WSM), indicating that the degree of periodicity enables one to control the hardness of the PMO film.26a The PMO-Bp-WHT film with a thickness of 1.8 µm in Figure 7e also reduces the hardness value (0.18 GPa), indicating that the thickness of the PMO film also decreases the hardness value (0.18 GPa). For the same sample, increasing the washing time from 15 to 30 min (the repeatedly washed film (PMO-Bp-WLT)) increases the hardness value to 0.20 GPa. In this case, we would like to suggest that the trapped soft organic surfactant molecules inside the pore channels of the PMOF-Bp-WHT film cause a reduction in the hardness value since the repeatedly washed film shows higher hardness values. The degree of surfactant removal also depends on the thickness and employed washing time.

4. Conclusions In this study, we have succeeded in the preparation of biphenylbridged PMO films (from 100% bridged-silsesquioxane) with long-range molecular periodicity of 1.40 nm in the pore walls for the first time. The final PMO film is derived from 100% biphenyl-bridged organosilica precursor. The final framework is composed of alternatively arranged hydrophilic silicate layer (40) (a) Chemin, A.; Klotz, M.; Rouessac, V.; Ayral, A.; Barthel, E. Thin Solid Films 2006, 495, 210–213. (b) James, E. M.; Mark, T. A.; Judy, O.; Paula, N. Langmuir 1997, 13, 4133–4141.

838 Langmuir, Vol. 25, No. 2, 2009

and hydrophobic biphenyl layers. The mild acidic self-assembly process allows the production of a low degree of organosilicate sites that are responsible for the formation of strong hydrogenbonded network with neighboring triols. The hydrogen-bonded molecular periodicity 1.40 nm, which is larger by 0.21 nm than that of even same molecules used for preparing covalently bonded PMOs powders with a high extent of polycondensation.1-12 This one-step self-assembly route could be extended to other functional aromatic-based PMO films with molecular periodicity in the pore walls. The PMO films show absorption and emission due to the biphenyl function in the pore walls. The dependence of synthesis parameters such as degree of pore ordering and periodicity, thickness, heating time, and surfactant removal on nanoindentation hardness of biphenyl-bridged PMO films has been extensively described for the first time. The hardness of the well-organized PMO with molecular periodicity in the pore walls

Wahab and He

is higher (0.23 GPa) than that of PMO with simple mesotructures (0.13 GPa). The degree of cross-linking has also significantly increased the hardness value from 0.23 to 0.27 GPa. This material might be very useful in the areas of film-based application such as coatings, optical, and host membranes for the preparation of functional films, sensors, and dielectric interlayers. The favorable combination of mechanical, emission and hydrophobic pore (100% biphenyl-organosilica) with molecular periodicity could be used especially for highly hydrophobic functional nanostructured guest molecules. Acknowledgment. Authors thank the IMRE and the Agency for Science, Technology and Research (A*STAR) for financial support. The authors thank T. T. Lin for molecular modelling analysis. LA803192Z