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Structural Characteristics of Oriented Mesostructured Silica Thin Films Fang-Fang Xu,*,†,‡ Fang-Ming Cui,† Mei-Ling Ruan,‡ Lin-Lin Zhang,‡ and Jian-Lin Shi† †
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Shanghai 200050, People’s Republic of China, and ‡Test and Analysis Center of Inorganic Materials, Shanghai Institute of Ceramics, Shanghai 200050, People’s Republic of China Received November 16, 2009. Revised Manuscript Received January 18, 2010 Mesoporous thin films synthesized via an electrochemical strategy (ref 1) generally show granular domains, each of which is composed of hexagonally packed one-dimensional channels oriented uniquely perpendicular to the film surface. Grain boundaries either parallel or normal to the channel direction might affect the properties and subsequent application of the film. In this study, the structural details of oriented mesostructured silica thin films have been examined by transmission electron microscope. The pore structures are characterized using the traditional crystallographic concepts but show different structural properties from that of polycrystalline materials. The boundary structures vary much depending on the residual internal stress and the orientation relationship between the bounded grains. A variety of structural features, typically near the large-angle tilt boundaries, have been observed including coincidence site lattices, lattice distortion, lattice displacement, and dislocations. According to the present structural analysis, microstructure evolution and potential applications have been discussed with respect to the oriented mesoporous films.
Introduction Supported mesostructured thin films are of great importance and interest for their applications in areas such as adsorption, chromatography, catalysis, sensor technology, and gas storage.2-4 Films of cubic phases possess transport pathways across the film owing to their three-dimensional (3D) network of interconnected pores,5 while 2D hexagonal mesoporous silica films were usually characterized by mesoporous channels oriented parallel to the film surface.3,6,7 Efforts have been made attempting to control nucleation and orientation through chemical modification of solid-liquid interfaces and/or dimensional confinement of the self-assembly process.8-10 However, alignment of the entire mesoporous channels in a direction perpendicular to the film surface has proved to be very difficult. Recently, Walcarius et al. developed an electrochemical strategy which successfully induced self-assembly of surfactant-templated (organo)silica thin films on various conducting supports, with mesopore channels projected normal to the solid surface over wide areas.1 The synthetic approach involves film growth starting from the support surface by potential-controlled sol-gel deposition and surfactant (cetyltrimethylammonium bromide (CTAB)) assemblies.
The mesoporous silica thin films grown by the electrochemicalassisted strategy exhibit hexagonally packed 1D channels oriented normal to the electrode surface. However, the films typically show granular microstructures with the domain size of 50-100 nm. Each of grains has rotated around the channel axis by certain angles with respect to the others. Though some of the grain boundaries are narrow and clean, many of them are quite thick and even show large area of randomly patterned lattice especially at triple junctions. Therefore, the boundary region weights around 10-15% of the whole film area, which must have considerable effects on the properties and subsequent application of the mesoporous films. In this paper, we report the transmission electron microscopy (TEM) analysis of the microstructures of the oriented mesostructured silica thin films prepared using the similar electrochemical approach. The complicated pore structures at the grain boundaries have been explicitly determined, which were related to the formation of coincidence site lattices (CSLs), dislocations, and lattice distortion induced by the internal stress during preparation. The present result could be helpful for the microstructure evolution in the electrochemical kinetics and hence the structure tailoring of mesoporous films for different applications.
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[email protected]; Tel þ86 21 5241 2574; Fax þ86 21 5241 5615.
Experimental Section
(1) Walcarius, A.; Sibottier, E.; Etienne, M.; Ghanbaja, J. Nat. Mater. 2007, 6, 602–608. (2) Ogawa, M. Curr. Top. Colloid Interface Sci. 2001, 4, 209–217. (3) Nicole, L.; Boissiere, C.; Grooso, D.; Quach, A.; Sanchez, C. J. Mater. Chem. 2005, 15, 3598–3627. (4) Chao, K.-J.; Liu, P.-H.; Huang, K.-Y. C. R. Chim. 2005, 8, 727–739. (5) Lu, Y.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature 1997, 389, 364– 368. (6) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Nature 1996, 379, 703–705. (7) Yang, H.; Coombs, N.; Ozin, G. A. J. Mater. Chem. 1998, 8, 1205– 1411. (8) Lu, Q.; Gao, F.; Komarneni, S.; Mallouk, T. E. J. Am. Chem. Soc. 2004, 126, 8650–8651. (9) Koganti, V. R.; Rankin, S. E. J. Phys. Chem. B 2005, 109, 3279–3283. (10) Fukumoto, H.; Nagano, S.; Kawatsuki, N.; Seki, T. Chem. Mater. 2006, 18, 1226–1234.
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Materials. The oriented mesostructured silica thin films were prepared by an electrochemical assisted deposition method reported by Walcarius.1 The clean indium tin oxides (ITO) glasses with the square resistance of 80-90 Ω/0 and the size of 20 50 2 mm were used as the substrate. Typically, the precursor solution was prepared by mixing 20 mL of aqueous solution of 10-3 M HCl, 20 mL of ethanol, 0.62 g of TEOS, 0.35 g of CTAB, and 0.17 g of NaNO3 in a beaker, followed by a stirring process for 2.5 h. The substrate as a working electrode, an Ag/AgCl reference, and a Pt wire auxiliary electrode were immersed in the precursor solution, and the electrodepsition was carried out under a quiescent condition by applying a cathodic potential of -1.3 V on the ITO substrate with a defined period of time 10-20 s. The substrate was drawn out from the solution and immediately rinsed with
Published on Web 01/28/2010
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deionized water as soon as the electrodeposition process was finished. The as-prepared films were then dried and aged at 100 °C in an oven for 12 h. Characterization. TEM observations were carried out on a JEM-2100F field emission transmission electron microscope with an accelerating voltage of 200 kV. Image processing and structural modeling were performed using DigitalMicrograph and CrystalMaker software packages. The TEM samples were prepared by mechanically scraping the mesostructured thin film from the ITO substrate. The scratched film species were then ultrasonically treated in ethanol and transferred on the copper TEM grid.
Results and Discussion Well-defined perpendicular hexagonal 1D channels have generally been observed in the inner regions of the aligned domains in the as-prepared mesostructured silica thin film, while grain boundaries and their vicinity show plenty of different structural features as indicated in Figure 1a. These include dislocations, CSLs, lattice displacement and/or distortion, and inclined channels. A widely observed structural feature is the Moire fringes (marked by a triangle in Figure 1a) in the vicinity of grain boundaries, which extend several to tens of nanometers. This suggests that there exist two separate compartments in the direction perpendicular to the film surface, which rotate by a small angle with respect to each other.11 Figure 1b illustrates the schematic drawing of a Moire pattern formed by a rotation angle of ∼10° between the top and bottom hexagonal lattices. The smaller the rotation angle is, the larger the coherence length of the Moire pattern would be. Therefore, the Moire pattern shown in Figure 1a refers to a rotation angle of ∼12°. The tilting angle between grains G1 and G2 and G1 and G3 are measured to be 9° and 17°, respectively, none of which matches the observed Moire configuration. This means additional rotation was introduced at the boundary region when two grains contacted. The appearance of Moire pattern indicates partial superimposition of two grains at the boundary area, which is characterized by an interface either normal or incline to the channel direction. This is proved by the cross-section view of the mesoporous film (see regions marked by solid arrows in Figure 1c). Apart from the twist of channel lattice near the boundary as derived from the above analysis of Moire pattern, lattice displacement is also observed as shown in Figure 1d, i.e. the magnified image of the framed region (marked by a square) in Figure 1a. An edge dislocation (marked by “T”) was formed in the close vicinity of the grain boundary. The lattice strain near the dislocation core causes the channels break and shift, leading to appearance of splitting of the bright dots (indicated by pairs of dashed lines) on the image. At the triple junction, the channel lattice is generally more disordered as typically represented by the asterisk-marked region in Figure 1a. Though the bright-dot contrast remains, suggesting channels still align normal to the film surface, the loss of hexagonal symmetry for the CSLs (the ring contrast) indicates lattice distortion occurred. Such deformed structure will be characterized in detail later. The above observed structural features near the boundaries, either CSLs or lattice displacement or lattice distortion, suggest possible discontinuity of channels across the film, as schematically illustrated in Figure 1e. It has been reported that bending of long channels is fairly common in “soft” mesoporous materials (also see the marked region by a blank arrow in Figure 1c).12,13 Such defects (11) Williams, D. B.; Carter, C. B. Transmission Electron Microscopy; Plenum Press: New York, 1996; Chapter 27. (12) Yang, S. M.; Yang, H.; Coombs, N.; Sokolov, I.; Kresge, C. T.; Ozin, G. A. Adv. Mater. 1999, 11, 52–55. (13) Yang, H.; Coombs, N.; Ozin, G. A. J. Mater. Chem. 1998, 8, 1205–1211.
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Figure 1. (A) TEM plan view of the present oriented mesoporous silica film. (B) Structural model of a Moire pattern. (C) TEM cross-section image of the film. (D) Enlarged image of the area framed by a square in (A) showing lattice displacement. (E) Schematic drawing of the channel configuration at the superimposed region.
may similarly form Moire or CSL patterns in a plan-view TEM image but still keep the channel continuous. However, in the present case, the bright-dot contrast near those defected regions as cited above rather than blurred-bar contrast typically for a bending configuration (for examples, see the circled regions in Figure 1a) imply that a shear really took place at the boundaries. This is further evidenced in Figure 1c in which the regions marked by solid arrows clearly show sudden lattice shifts leading to discontinuity of the channels. Thus, a gate with the size smaller than the pore diameter might have been created at these boundaries when the rotation angle or displacement vector is not too large. It seems that internal stresses are widely and commonly generated in the electrochemically synthesized mesoporous films, probably during the electro-assisted deposition process. The boundaries are usually the sites concentrating the stress,14 leading to the deviation from the perfect ordered structure. The microstructure of oriented mesostructured film is quite similar to that of metal and semiconductor polycrystalline films which grow in a specific crystallographic direction. In the context, we use the terminology of “polycrystalline” to describe the microstructure of the present mesostructured film but keep in mind that it refers to the arrangement of channels rather than atoms. In this specific oriented “polycrystalline” film, the grain boundary should be mainly the tilt boundary which defines the two bounded crystallites having interfaces parallel to the common projected axis (in this case, the channel axis) of the two grains while they tilt a certain angle around the axis with respect to each other. The boundaries could be classified as small-angle and largeangle tilt boundaries. Dislocations are usually generated at the small-angle tilt boundaries, while CLS relationship is established (14) Sutton, A. P.; Balluffi, R. W. In Interfaces in Crystalline Materials; Brook, R. J., Ed.; Clarendon Press: Oxford, UK, 1995; p 161.
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Figure 3. Structural models illustrating modification of Σ7 CSL structures via lattice displacements.
Figure 2. TEM micrographs of grain boundaries in the oriented “polycrystalline” mesoporous film. (A) A small-angle tilt boundary. (B) A triple junction where the arrows refer to the displacement vectors as will be described in Figure 3. The inset shows the diffractogram of the area in (B), in which arrows represent the internal stresses.
at the large-angle tilt boundaries so as to decrease the interfacial energy.15-17 In the present mesostructured “polycrystalline” thin film, the configuration of tilt boundaries somewhat differs from the conventional structures in the real polycrystalline solids because the lattices we will examine are the channels rather than atoms. Figure 2a shows the TEM micrograph of a small-angle tilt boundary which consists of two different configurations. An array of edge dislocations, as generally observed in the ceramic materials, formed at the lower part of the boundary. At the upper part, the boundary shows zigzag morphology, which terminates at the {1010} faces of either grains. The small-angle tilt boundary is generally very narrow, showing blurred contrast of only one channel layer, while the lattice maintains perfect in its vicinity. The zigzag configuration for small-angle tilt boundary is quite special that can hardly be observed in polycrystalline materials owing to that such boundary configuration generally refers to a nonrelaxed interface which involves high interfacial energy.15 However, this is reasonable to understand here because structural relaxation at the boundaries generally occurs within atomicscaled regions15,18 while the lattice of channels in the present porous film is in mesoscale. A thorough zigzag small-angle boundary can be found in Figure 2b, which shows perfect hexagonal channel lattice without any nanometer-scale relaxation across the boundary. However, the thin blurred contrast at the boundary implies that the grain boundary mismatch could be (15) Marcinkowski, M. J. Phys. Status Solidi A 1980, 60, 109–116. (16) Kronberg, M. L.; Wilson, F. H. Trans. Metall. Soc. AIME 1949, 185, 501– 514. (17) Ranganathan, S. Acta Crystallogr. 1966, 21, 197–199. (18) Bollmann, W. Crystal Defects and Crystalline Interfaces; Springer-Verlag: Berlin, 1970; p 221.
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effectively accommodated via slight distortion of a single layer of channels in such a soft mesostructured material. The large-angle tilt boundaries are usually thick and show diverse structural features. The generally observed one is the CSL whose size, configuration, and even symmetry may differ. The circled region in Figure 2b refers to CSLs formed at a large-angle tilt boundary. The measured intersect angle of 22° suggests a Σ7 [0001] tilt grain boundary.16,17,19 The dimensions of the CSL lattice at the intercross region match the Σ7 configuration, while the pattern shows triangle rather than ring shapes as often the case in polycrystalline materials. This is attributed to a slight lattice shift occurred between the top-and-bottom counterparts, which could be explained via structural modeling presented in Figure 3. The left-handed model represents a standard Σ7 CSL generally observed in, for example, a layered hexagonal crystal structure19 (in such a case, the circle refers to an atom). When a slight lattice displacement occurs with the magnitude equal to the width of splitting for the channel indicated by a solid triangle in Figure 3, the CSL pattern changes though its dimensions are preserved. Apparently, the displacement vector determines the final CSL configurations. In Figure 2b, it can be seen that there exist two orientation of triangular CSL pattern. Figure 3 explicitly resolves that this is due to the change of displacement direction. In the present case, the [1100]- and [1010]-oriented lattice shifts inside the hexagonal CSL lattice result in the observed discrepancy of CSL configurations. Therefore, the stress distribution at the large angle tilt boundaries must be quite heterogeneous and complicated. The inset of Figure 2b shows the diffractgram (the Fourier transformed pattern) of a broader region including that illustrated in this figure. The oval rather than the circular diffraction pattern suggests inhomogeneous distortion of the channel lattice. The measurement of the diffractogram gives the short axis of the oval ring referring to 3.3 nm interplanar spacing while the long axis referring to 3.16 nm spacing. The repeat spacing measured from an individual “single crystalline” grain showing perfect hexagonal symmetry is 3.3 nm, which agrees well with the reported value.1 Therefore, a compressive stress exists in this area with the direction parallel to the long axis of the oval ring, as indicated (19) Xu, F. F.; Bando, Y. Acta Crystallogr. A 2003, 59, 168–171.
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Figure 4. Disordered area at a triple junction showing a deformed CSL (framed region). The inset is the diffractogram of this area.
in the inset of Figure 2b. Though the small-angle and large-angle tilt boundaries bear the similar compressive stress, the former has been marginally affected while the latter shows broadened interface where lattice shift involves (also see Figure 1d). In polycrystalline materials, the majority of the residual stresses are concentrated at the triple junctions.14,20 This could be more directly and clearly viewed in the present soft “polycrystalline” mesoporous material. As has already been shown in Figure 1a, the channels are usually disordered at the multijunctions. The lattice deviates from the rigid 2D hexagonal symmetry via shift, distortion, or inclination. Figure 4 shows channel lattices at a multijunction area. Apart from the randomly distributed channels in which the defected structural features cited above are widely involved, the CSL with the symmetry other than hexagonal geometry is quite unusual (as also similarly shown in Figure 1a). Lattice distortion must have taken place. The structural models in Figure 5 illustrate the formation kinetics of the observed CSL configuration. The unusual CSL shows an approximate rectangular shape in which the coherence length of the long border corresponds to a Σ13 CSL (for a tilting angle of 27.8°) while that of the short border corresponds to a Σ7 CSL (for a tilting angle of 21.8°). None of the CSLs match the measured tilting angle (i.e., 16.4°) of the bounded grains. An additional lattice twist must be involved in the boundary area. No matter what the parent CSL (either Σ7 or Σ13) was, the final deformed CSL could be obtained via certain lattice distortion within one of the superimposed components. Parts a and b of Figure 5 show the perfect Σ13 and Σ7 CSL, respectively. In Figure 5a, when a horizontal stress is applied in a Σ13 CSL, the channel lattice represented by blank circles is supposed to undergo regular shears similar to the formation of twins.21 Here, the shear makes the C site coincide with C0 site, D with D0 , E with E0 , and F with F0 . The sites labeled with primed letters finally constitute a new CSL, i.e., the monoclinic rather than hexagonal coincidence site lattice (see Figure 5c). Figure 5d illustrates the magnified image of the deformed CSL superimposed with the proposed structural model, which shows perfect agreement. In Figure 5b, a vertical stress results in the same monoclinic CSL starting from a Σ7 CSL. It could be seen in Figure 5a,b that though both parent CSL of Σ7 and Σ13 could eventually transform into the same monoclinic CSL via a certain shear, the direction of the strain differs. The inset of Figure 4 shows the diffractogram which again indicates (20) Herring, C. In The Physics of Powder Metallurgy; Kingston, W. E., Ed.; McGraw-Hill Book Co.: New York, 1951; p 143. (21) Cahn, R. W. Adv. Phys. 1954, 3, 363–445.
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Figure 5. Formation mechanism for the deformed CSL shown in Figure 4. (A) The parent Σ13 CSL. (B) The parent Σ7 CSL. (C) The deformed CSL after certain lattice distortion described in (A) and (B). (D) Superimposition of observed image and structural model, showing good agreement. The solid and blank circles in the structural models refer to either of the superimposed layers.
internal compressive stress within this area. Since the stress direction is fairly parallel to the short border of the CSL, it is reasonable to consider that the parent CSL should be Σ7 because the stress direction in the model (an arrow in Figure 5b) is consistent with that of the experimental observation. Therefore, when the grains meet at the multijunction, the internal compressive stress makes the boundary area disordered through lattice twist, lattice shear, and dislocations. Plenary TEM examination found that the channels usually did not corrupt near the boundaries but rotated, displaced, and inclined, demonstrating certain mechanical stability of the mesostructured films. The cross-section view of the mesostructured thin film (Figure 1c) exhibits that the short channels generally maintain straight and rarely bend or curve even in the defected area which bears internal stresses. This implies a certain mechanical strength of the present mesostructured thin film. Small-angle tilt boundaries are well-defined in the form of narrow borders while large angle boundaries and majority of the triple junctions show disordered channel lattices in which lattice offset between the up and bottom film compartment (i.e., the formation of grain boundaries other than parallel to the channels) made the channels discontinuous across the film. In the present mesoporous film synthesized via electrochemical kinetics, disordered boundary regions occupy up to 15% of the film area. For some applications of oriented mesostructured films in, for example, drug delivery or role as a template for the synthesis of 1D quantum structure, the long and continuous channels are required. Therefore, single crystalline films or polycrystalline films with large grain size and high percentage of small angle tilt boundaries are the goals for the microstructure evolution. However, the presence of disordered region is not the least useless, especially if its volume percent can increase a great deal. As can be seen in Figure 1e, CSLs and lattice displacement result in the generation of gates, with smaller size than the pore diameter of ∼2 nm, at the interfaces of the overlapped parallel channels. Such structural characteristics may suggest that only atoms or very small molecules are allowed to pass through the mesoporous film, an implication of special applications, e.g., as membrane filters with the size of the straightforward pores down to 1 nm. To reach this Langmuir 2010, 26(10), 7535–7539
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end, several strategies can be taken into account. These include synthesizing “polycrystalline” films with smaller grain size and large amount of large-angle tilt boundaries, imposing a suitable stress to induce the lattice deformation, or manufacturing double- or multilayered mesoporous films. Since the TEM observation reveals that the channels keep orienting perpendicular or approximately perpendicular to the film surface albeit the “polycrystalline” nature of the present mesoporous film, which suggests its mechanical stability, microstructure tailoring, and evolution should be possible.
Conclusion Structural characteristics of oriented mesostructured silica thin film have been examined using transmission electron microscope. The film consists of nanosized domains which exhibit hexagonal patterned channels uniquely projecting normal to the film surface but rotate by a certain angle with respect to each other. Such microstructure is analogous to that of polycrystalline materials but differs in the boundary configurations. The small-angle tilt boundaries are narrow and show zigzag interfaces on {1010} planes of hexagonal channel lattice in addition to the conventional array of edge dislocations. The channel lattices intersect and are disordered in the lateral directions at the large-angle tilt
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boundaries or multijunctions which are usually very thick. Such boundary regions exhibit a variety of structural features including lattice distortion, lattice displacement, dislocations, and CSLs. Electron diffraction revealed the existence of residual internal stresses in the as-synthesized thin film, which induced disordering of channel lattices near those boundaries of high energy. The disordered region occupies up to 15% of the film area, implying that microstructure evolution is necessary for the purpose of different applications. One prospect concerns that the lateral disordering of channel lattice generates gates across the mesoporous film with the gate size smaller than the channel diameter, suggesting that only atoms or very small molecules are allowed to pass through the mesoporous film. Therefore, the oriented mesoporous film could be used as membrane filters with pore size down to 1 nm if corresponding microstructure evolution is realized. Acknowledgment. The authors thank the financial support by the National Natural Science Foundation of China (Grant No. 50452002, 20703055), the Talents Program of Chinese Academy of Sciences, the 973 programs (Grant No. 2007CB936704, 2009CB939904), and the SICCAS Innovation Program.
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