Registered growth of mesoporous silica films on graphite Hong Yang,a Neil Coombs,b Igor Sokolova and Georey A. Ozin*a aMaterials Chemistry Research Group, L ash Miller Chemical L aboratories, University of T oronto, 80 St. George Street, T oronto, Ontario, Canada M5S 3H6 bImagetek Analytical Imaging, 32 Manning Avenue, T oronto, Ontario, Canada M6J 2K4 It has recently been demonstrated that hexagonal mesoporous silica films can be grown at mica/water interfaces. In this report it is established that the films can also be synthesized on cleaved pyrolytic graphite.Diraction and microscopy studies of the growth, structure and form of the films indicate that the channels are aligned along the hexagonal symmetry axes of the graphite surface. The registry of the mesoporous silica film with the underlying substrate may be facilitated by an organized hemicylindrical surfactant monolayer assembly that is observed at the boundary between the graphite and water.Growth of the mesoporous silica film most likely begins within a liquid-crystal surfactant/silicate film that is aligned with the hemimicelle–graphite overstructure. The recently reported surfactant-based syntheses of hexagonal a horizontal position.The film forming process commenced within minutes under static conditions in an oven at 80 °C. mesoporous silica films at air/water, oil/water and solid/water interfaces1–5 raises a number of issues. These include the Film growth was allowed to proceed for a period of one day to a week. The films so formed were transferred from the vessel structure of the interfacial surfactant, details of the nucleation and growth processes, the factors that determine the alignment using tweezers and washed with deionized water.The calcination of graphite-supported films was conducted in air and of the channels, and the domain and defect architecture of the films. in a furnace attached to an Omega CN-2010 programmable temperature controller. The temperature ramp was Here, we present experimental details relating to the growth, structure and form of a hexagonal mesoporous silica film <1°C min-1 and typically the sample was held at 450 or 540°C for 4 h.deposited onto the surface of freshly cleaved pyrolytic graphite. A combination of powder X-ray diraction (PXRD), highresolution scanning electron microscopy (HRSEM), atomic Characterization force microscopy (AFM) and transmission electron microscopy Powder X-ray diraction.PXRD data was obtained on a (TEM), provide converging evidence that the channels of the Siemens D 5000 diractometer using Ni-filtered Cu-Ka radi- film appear to be aligned with the hexagonal symmetry axes ation with l=1.54178 A° . Samples of the graphite-supported of the graphite surface.A well ordered hemimicellar surfactant films were mounted on top of a home-made low-background assembly is shown, by pre-contact electrical double layer quartz cell for recording PXRD data. (EDL) AFM, to pre-exist at the graphite/water interface. The polymerization of a pre-organised liquid-crystal surfactant/ Scanning electron microscopy.SEM images were obtained silicate assembly located at the surface of graphite is believed on a Hitachi S-4500 field emission microscope using a low to direct the nucleation and growth of the mesoporous silica acceleration voltage of 2–5 kV to minimize the charging of film. film surfaces. Samples were generally imaged directly except on one occasion when the sample was coated with a very thin layer of gold before imaging. Experimental Transmission electron microscopy.TEM images of the films Synthesis were recorded on a Philips 430 microscope operating at an The graphite used in this work is taken from a 12 mm×12 mm×2 mm pyrolytic graphite monochromator (Grade ZYB). This was a generous gift donated by the Advanced Ceramics Corporation. Thin sheets of graphite substrate were obtained by peeling a uniform section from the graphite block with Scotch tape and then cutting it into the desired size.Tetraethylorthosilicate (TEOS, 99+%, Aldrich), cetyltrimethylammonium chloride (CTACl, 29 mass% aqueous solution, Pflatz & Bauer) and hydrochloric acid (36.5–38 mass% aqueous solution, BDH) were used as received. The synthesis of mesoporous silica films at the graphite/water interface was conducted under quiescent acidic conditions.1,2,6 The reactant mole ratios used were 100H2O57HCl50.11 CTACl50.1TEOS The CTACl surfactant solution was mixed with the TEOS source of silica and stirred for ca. 2 min at room temperature and then transferred into a polypropylene bottle and allowed to achieve a stable air/water interface.A ca. 10 mm×10 mm freshly cleaved pyrolytic graphite substrate was then positioned Fig. 1 A representative PXRD pattern for an as-synthesized mesoporous silica film grown on freshly cleaved pyrolytic graphite at the surface of the synthesis solution and allowed to float in J. Mater. Chem., 1997, 7(7), 1285–1290 1285accelerating voltage of 100 kV. In order to get ultrathin sections the tapping mode was engaged, the tip was elevated manually by the motor for the vertical displacements.Normally the (100–300 A° ) of the mesoporous silica film grown on graphite, a gold-coated film was embedded in epoxy, heated to form a elevation needed was about 100–200 nm. The jump into precontact mode was clearly evident every time. block and cut at dierent angles using a diamond knife following the standard ultramicrotome procedure.The gold coating was used both to identify the film position in the Results epoxy block and the sides of the film, since the adhesion forces between the film and the graphite substrate are not suciently The PXRD pattern of the film grown on graphite shows only two low-angle peaks assigned to the (100) and (200) reflections strong to withstand the microtome cutting force.Consequently the film is easily cleaved from the graphite upon sectioning. of a hexagonal symmetrymesoporous silica, Fig. 1. The absence of the (110) reflection for the film, compared to that of a randomly oriented powdered sample,7 confirms that the chan- Atomic force microscopy. AFM experiments were conducted on a NanoScope III microscope (Digital Instruments, CA) nel axis is aligned parallel to the graphite surface.Calcination of the film was performed at 450 °C in air for 4 h. The using silicon integrated tip cantilevers (Park Scientific Instruments, CA) for height mode scanning of the films, and high-temperature treatment causes the PXRD d100-spacing to contract by 1–3 A° with concomitant changes in the intensity special silicon cantilevers (Digital Instruments, CA, FESP type, 227 mm cantilever, resonance frequency 70–100 KHz) for tap- of the PXRD pattern.These eects are associated with the polymerization of residual SiOH groups. The integrity of the ping mode scanning in fluids. The cantilevers were used as received. Images of mesoporous silica film surfaces were film and its mesostructure are well maintained throughout this thermal process.obtained by using direct contact (dc) scanning mode. Images of CTACl assemblies on graphite were obtained by using two SEM images of the films grown on the graphite substrate are shown in Fig. 2. The low-magnification images reveal that dierent methods. The first employed a pre-contact EDL height imaging mode.The second utilized a combined EDL the films are essentially continuous, Fig. 2(a). Intriguing equilateral triangle-shaped pits are observed in the films, pre-contact and tapping imaging mode. This method is a new way to image soft structures with minimal disturbance of their Fig. 2(b)–(d). Also, the growth fronts of the films typically have parallel filamentous extensions, terminated with edges inclined integrity. It was applied successfully in this work to image surfactant assemblies adsorbed at the graphite/water interface.at 60° or 120°, and which sometimes display sharp 60° or 120° bends, Fig. 2(a), (b). The triangle-shaped pits display both CTACl aqueous solution with concentrations of 4–18 mM was used. The frequency of the cantilever tapping in the surfactant straight and convex edges, Fig. 2(b)–(d). It is noteworthy that the edges of the triangles are exclusively aligned with respect solution was about 33 KHz. To achieve pre-contact mode during tapping, the following procedure was adopted. After to each other as well as in alignment with the edges of the Fig. 2 SEM images of an as-synthesized mesoporous silica film grown on freshly cleaved pyrolytic graphite: (a) a large area and a growth front of the film; (b) a growth front showing the filaments and equilateral triangular pits (note that this sample has been coated with gold before imaging); (c), (d) close-up views of the triangular pits 1286 J.Mater. Chem., 1997, 7(7), 1285–1290filaments at the growing front of the film, Fig. 2(b).A closer AFM soft imaging technique.5,8–10 We have found in this study that the perturbation of the surfactantassembly by the scanning examination of the inner walls of the triangular pits byHRSEM reveals a multi-layer topology with step features aligned with tip can be further reduced and the quality of the AFM images enhanced by using the tapping mode in conjunction with the the sides of the triangles, Fig. 2(c), (d). TEM images of the mesoporous silica films grown on pre-contact (EDL) technique. This can be understood by recalling that the EDL consists graphite show that the channels are hexagonally close-packed with a centre-to-centre distance of about 40–45 A° and run of a bilayer of oppositely charged surfactant cations and counter anions adsorbed on the AFM tip.The result is an parallel to the graphite/water growth boundary, Fig. 3(a). The TEM estimated unit-cell dimensions agree well with the PXRD additional EDL force of repulsion between the tip and the surfactant assembly, adsorbed on graphite, being imaged. Thus, d100-spacings of 37–39 A° . A TEM image of a cross-section of the film cut along the channel axis is shown in Fig. 3(b). The the apex of the tip floats over the fragile surfactant assembly with a gentler type of interaction. This type of pre-contact observed parallel lines have a repeat distance of 40–45 A° which corresponds well with the separation of the walls of the scanning mode, in conjunction with the tapping action of the probe, will cause less deformation of the assembly being imaged mesopores and confirms that the channels grow parallel to the graphite surface. The smooth bottom surface of the films because of a lack of friction force. According to previous experiments, and our own work, the shown in Fig. 3 implies that it corresponds to the side of the mesoporous silica film that has grown from the graphite direct pre-contact mode scanning along the boundary of the EDL provides an image of the adsorbed hemicylindrical surfac- surface.The mesoporous silica films can be grown on graphite to a thickness of around 0.5–0.6 mm. tant monolayer assembly on the graphite surface. The evidence for this being a hemimicellar monolayer comes from the AFM is an eective probe of the topology of the films grown on graphite, Fig. 4. AFM direct contact (dc) mode images of thickness measurement of the surfactant assembly.This is achieved by increasing the scanning force, whereby the EDL the outer surface of the film reveal a mottled structure at the 50–200 nm length scale. A representative example of this repulsion is overcome and the tip jumps through the film.5,8–10 Pre-contact (EDL) tapping mode AFM images of an aque- texture on the surface of the film surrounding a triangular pit is shown in Fig. 4(a). Overall, these surface images portray the ous solution of CTACl adsorbed on graphite have been obtained over the concentration range of 4–18 mM. One can occurrence of a multi-layer step structure in the central regions of the films, Fig. 4(b), as well as on the walls of the triangular discern well defined parallel stripes with a periodicity of ca. 53 A° , Fig. 5(a), and a thickness up to 500 A° , Fig. 5(b). The film pits, Fig. 4(a), and on the filamentary growth fronts of the films, Fig. 4(c). thickness was estimated by measuring the height dierence when the tip jumps from the pre-contact tapping to the contact The organization of adsorbed surfactants at the boundary between water and silica, mica and graphite substrates has tapping mode.The collapse of the surfactant assembly was either spontaneous or induced by the change in the setpoint recently been observed using the pre-contact mode (EDL) of the AFM scanning. This implies the existence of up to 10 layers of 50 A° diameter surfactantcylindrical micelles organized as a liquid-crystal film at the boundary between water and graphite.The AFM images also reveal the presence of domains within which parallel arrays of cylindrical micelles meet at boundaries with either 60° or 120° angles, Fig. 5(c). This suggests that the director of the liquid-crystal overlayer is aligned with the hexagonal unit-cell axes of the graphite surface as well as the axis of the adsorbed hemicylindrical micellar monolayer.The ability to observe, for the first time, such surfactant multilayer assemblies without their destruction apparently stems from the lack of friction force in the AFM tapping mode compared to that of the direct contact mode. Discussion The above results provide a basis for beginning to understand the origin of the preferred orientation and apparent registry of the channels of the mesoporous silica film with the hexagonal symmetry axes of the graphite surface.The assembly process in the absence of TEOS probably begins with adsorption of CTACl on the graphite surface. The hydrophobic interaction between the alkane chain and the graphite causes the surfactants to lie flat on the surface.8,9 Geometrical matching of the methylene groups in the all-trans alkane chain with the aromatic carbon six-rings in the planar graphite surface, favours a head-to-head and tail-to-tail packing arrangement of the surfactant along the hexagonal symmetry axes of graphite.This geometryis driven by hydrophobic, electrostatic ion-pair and image dipole forces between the CTACl and the electrically conducting graphite surface.8,9 The result is 50 A° parallel stripes of surfactants running orthogonally to the graphite hexagonal symmetry axes.This Fig. 3 TEM images of an as-synthesized mesoporous silica film show- organized surfactant monolayer assembly then serves as a ing (a) the hexagonally closed-packed channel structure with a centre- template for the further adsorption of CTACl from solution to-centre spacing of ca. 45 A° ; (b) the channels running parallel to the and the formation of a monolayer of 50 A° diameter hemi- graphite surface.Bottom (smooth side), grown at the graphite/water cylindrical micelles with a thickness of ca. 25 A° .8,9 Using interface; top (rough side), grown in solution. (Note, imaging the entire film compromises visualization of the channels.) pre-contact AFM imaging mode we have reproduced this J.Mater. Chem., 1997, 7(7), 1285–1290 1287nm nm nm nm nm nm 400.00 500.00 600.00 ( a ) (b ) ( c ) Fig. 4 AFM dc mode images of the outer surface of an as-synthesized mesoporous silica film on graphite: (a) mottled texture on the film surrounding a triangular pit; (b) steps on the edges of the triangular pits; (c) terraces on the filaments at a growth front result over the 4–18 mM surfactant concentration range investi- selves in alignment with the edges of the filaments that comprise the growth fronts of the film.These structural features likely gated in this study. However, for a surfactant concentration above roughly 9 mM and when the AFM imaging is changed arise from the ability of the surfactant/silicate/graphite assembly to control the alignment of the channels in the to the pre-contact tapping mode, we still observe 50 A° parallel stripes but now with a film thickness of about 500 A° , Fig. 5(b). growth of the mesoporous silica film, along the three symmetry equivalent hexagonal axes of graphite. Growth fronts are This implies that continued accretion of CTACl from solution leads to the development of a multilayer of close-packed 50 A° expected to meet at 60° in the body of the mesoporous silica film to form triangular features and to terminate at the diameter cylindrical micelles.These are presumably organized in the form of a liquid-crystal film registered with the extremities of the film to create filaments displaying 60° and 120° angular features, Fig. 2. graphite surface. In the presence of TEOS, nucleation of the mesoporous The stepped textures observed for the films, the triangular pits and the filaments, Fig. 4, most likely originate from silica film is initiated by polymerization of charge-balancing silicate anions in the headgroup region of a liquid-crystal film polymerization and growth of cylindrical surfactant/silicate seeds11 in the liquid-crystal film adsorbed on the graphite that has its director axis registered with the hexagonal symmetry axes of the underlying graphite substrate, Plate 1.surface. Further studies will be required to elucidate the details of the nucleation and growth processes that give rise to these Growth of the mesoporous silica film is likely to be determined by the charge and structure encoded in the surfactant/silicate/ mesoporous silica films.graphite film. This process could produce a mesoporous silica film in which the channels are aligned with the hexagonal Conclusion symmetry axes of the graphite surface. Subsequent deposition and polymerization of surfactant/silicate micellar assemblies In this paper, we have demonstrated that mesoporous silica films can be synthesized on the hydrophobic surface of freshly results in the continuous growth and thickening of the mesoporous silica film up to the observed value of about half a cleaved pyrolitic graphite.Our combined PXRD, HRSEM, TEM and AFM data provide converging evidence that the micrometre. This templating model for the polymerization of a surfactant/ channels of the film are probably registered with the hexagonal symmetry surface structure of the underlying graphite.This silicate liquid-crystal film on graphite is consistent with the observation of mutually aligned triangular-shaped pits in the arrangement can be understood in terms of the growth of the mesoporous silica film within a liquid-crystal surfactant/silicate body of the resulting mesoporous silica film, which are them- 1288 J.Mater. Chem., 1997, 7(7), 1285–1290Fig. 5 Pre-contact (EDL) tapping mode AFM images (using phase detection) of CTACl assemblies adsorbed on graphite showing (a) parallel stripes with a periodicity of ca. 53A° , and (b) a thickness of ca. 500 A° ; (c) domains of surfactants with parallel arrays of cylindrical micelles that meet at sharp boundaries with an angle of either 60° or 120°.The surfactant concentration was in the range of 9–18 mM. film that is aligned with the symmetry axes of graphite. Overall, the synthesis of mesoporous silica films on hydrophilic mica and hydrophobic graphite surfaces, as well as free-standing ones formed at the boundary between air and water, augurs well for their use in a range of applications, such as large-molecule catalysis, membrane separations and chemical sensing.Financial support from Mobil Technology Company is deeply appreciated. H.Y. is grateful for an Ontario Graduate Scholarship and a University of Toronto Open Scholarship. H.Y. also thanks Mr S. Boccia and Mr F. Neud for the helpful technical tutorial on using HRSEM. We would also like to express our deepest gratitude to Dr Grant Henderson for the use of his AFM equipment and for valuable technical discussions.References 1 H. Yang, A. Kuperman, N. Coombs, S. Mamiche-Afara and G. A. Ozin, Nature (L ondon), 1996, 379, 703. 2 H. Yang, N. Coombs, I. Sokolov and G. A. Ozin, Nature (L ondon), 1996, 381, 589. 3 G. A. Ozin, D. Khushalani and H. Yang, Proceedings of the NAT O Plate 1 Graphical illustration of proposed model for the formation of Advanced Research Workshop on Self Assembly, ed. J. Wuest, Val Morin, May 1996. a mesoporous silica film on graphite. Red, surfactant tail; yellow, surfactant headgroup; light blue, silicate building-block; dark blue, 4 S. Schacht, Q. Huo, I. G. Voigt-Martin, G. D. Stucky and F. Schu�th, Science, 1996, 273, 768. silica; black, graphite. J. Mater. Chem., 1997, 7(7), 1285–1290 12895 I. A. Aksay, M. Trau, S. Manne, I. Honma, N. Yao, L. Zhou, 8 S. 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