pubs.acs.org/Langmuir © 2009 American Chemical Society
TiO2 Thin Films Self-Assembled with a Partly Fluorinated Surfactant Template Mark J. Henderson,† Kevin Zimny,‡ Jean-Luc Blin,‡ Nicolas Delorme,† Jean-Franc-ois Bardeau,† and Alain Gibaud*,† † Laboratoire de Physique de l’Etat Condens e, UMR CNRS 6087, Universit e du Maine, 72085 Le Mans Cedex 09, France, and ‡Equipe Physico-chimie des Colloides, UMR SRSMC 7565 Universit e Nancy-1/CNRS Facult e des Sciences, BP 23970, F-54506 Vandoeuvre-les-Nancy cedex, France
Received June 20, 2009. Revised Manuscript Received August 11, 2009 New TiO2 films have been self-assembled on solid substrate by dip-coating using TiCl4 as the titanium source and the partly fluorinated surfactant F(CF2)8C2H4(OC2H4)9OH as the liquid crystal template. By control over the dipwithdrawal speed, film thicknesses from a minimum of 43 nm were produced with rms roughnesses of 0.5-0.7 nm. The films were characterized by X-ray reflectivity, grazing incidence small-angle X-ray scattering, atomic force microscopy, contact angle measurements, and Raman spectroscopy. Their GI-SAXS patterns are characteristic of a 2-D hexagonal structure in which tubular rods of the fluorinated surfactant are packed hexagonally and aligned parallel to the substrate. Reflectivity and contact angle measurements of the as-prepared film indicate that a low-density hydrophilic TiO2 surface presents to the air.
1. Introduction Fluorocarbons are often added to silica gels to confer greater hydrophobicity to films deposited on glass substrate by dip or spray coating.1-3 Conversely, the specific solvent-molecular interactions between low dielectric fluids, such as CO2 and fluorocarbon surfactants,4 can be exploited for pore modification of mesostructured silica because the “CO2-philic” fluorinated tail is readily penetrated at high pressure.5 The interactions are expected to be further enhanced when semifluorinated surfactants are used, e.g., F(CF2)8C2H4(OC2H4)9OH (hereafter abbreviated as RF8(EO)9), because the local dipole created by the CF2 to CH2 transition induces CO2 quadrupole-solute interaction. Recently, semifluorinated surfactants were used to prepare mesoporous silica analogous to the M41S materials.6,7 These studies have shown that during the hydrothermal treatment better condensation of the silica is promoted, a result of the greater thermal stability of the fluorinated templates compared with their hydrogenated analogues.8 The phase behavior of one member of this class of surfactants, RF8(EO)9, was investigated in aqueous solution and subsequently applied to the preparation of mesoporous silicas.6 Applying the rule that one CF2 group is equivalent to 1.7 CH2 groups in lowering the critical micelle concentration,9 C16H33(OC2H4)10OH (known by the trade name Brij 56) can be *To whom correspondence may be addressed: Tel þ 00 33 2-43-83-32-01; Fax þ 00 33 2-43-83-35-18; e-mail
[email protected].
(1) Hong, B. S.; Han, J. H.; Kim, S. T.; Cho, Y. J.; Park, M. S.; Dolukhanyan, T.; Sung, C. Thin Solid Films 1999, 351, 274–278. (2) Monde, T.; Fukube, H.; Nemoto, F.; Yoko, T.; Konakahara, T. J. NonCryst. Solids 1999, 246, 54–64. (3) Brennan, T.; Dalgliesh, R. M.; Lovell, M. R.; Richardson, R. M.; Barnes, A. C.; Sergeant, S. A. Langmuir 2003, 19, 7761–7767. (4) Dardin, A.; DeSimone, J. M.; Samulski, E. T. J. Phys. Chem. B 1998, 102, 1775–1780. (5) Ghosh, K.; Bashadi, S.; Lehmler, H. J.; Rankin, S. E.; Knutson, B. L. Microporous Mesoporous Mater. 2008, 113, 106–113. (6) Blin, J. L.; Lesieur, P.; Stebe, M. J. Langmuir 2004, 20, 491–498. (7) Blin, J. L.; Stebe, M. J. Microporous Mesoporous Mater. 2005, 87, 67–76. (8) Michaux, F.; Carteret, C.; Stebe, M. J.; Blin, J. L. Microporous Mesoporous Mater. 2008, 116, 308–317. (9) Ravey, J. C.; Stebe, M. J. Colloids Surf., A 1994, 84, 11–31.
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considered as the hydrogenated analogue of RF8(EO)9. Brij 56 has also been used to prepare mesoporous silicas using both the micellar and liquid crystal templating routes.10,11 We have previously used a related hydrogenated surfactant from the same class of Brij surfactants, C16H33(OC2H4)20OH, Brij 58, to template lamellar and hexagonal mesostructured TiO2-based thin film by the self-assembly evaporation-induced (EISA) method.12 We used X-ray reflectivity (XR) to characterize the depth profile of the film and showed that these films, when prepared by dip coating or casting, were highly organized and comprised up to 17 bilayers of titanium oxide gel and surfactant but showed significant contraction of the mesostructure when the template was removed by a solvent extraction process. Larger pore sizes are obtained by infiltration of the occluded template by N,N-dimethyldecylamine13 or, as noted above, with CO2.5 As a virtue of RF8(EO)9 is the ability to fully extend its rigid hydrophobic chains,6 we produced organized TiO2-based films using the liquid crystal phase method from ethanolic solution. X-ray reflectometry marks the point of departure from our earlier study of TiO2 thin films prepared using Brij surfactants. In the present approach we have (i) unequivocally identified the film mesostructures using GI-SAXS, (ii) characterized the surface structures using AFM, (iii) demonstrated whether or not the hydrophobic surfactant tail is buried, and (iv) developed a new protocol for the removal of the fluorinated template to produce a porous oxide. Finally, a comparison between TiCl4 and aqueous TiOCl2 solution as the TiO2 source has allowed an insight into how the water content of the starting sol and the extent of precursor hydrolysis affect the dimensions of the 2-D unit cell without complications that can arise with the use of nonrigid hydrocarbon templates.
(10) Coleman, N. R. B.; Attard, G. S. Microporous Mesoporous Mater. 2001, 44, 73–80. (11) Blin, J. L.; Leonard, A.; Su, B. L. Chem. Mater. 2001, 13, 3542–3553. (12) Henderson, M. J.; Gibaud, A.; Bardeau, J. F.; White, J. W. J. Mater. Chem. 2006, 16, 2478–2484. (13) Sayari, A.; Hamoudi, S.; Yang, Y. Chem. Mater. 2005, 17, 212–216.
Published on Web 09/15/2009
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2. Materials and Methods 2.1. Film Synthesis. A film, targeted with a 55 wt % of RF8(EO)9 (expressed here as the weight percent of the surfactant [RF8(EO)9/(RF8(EO)9 þ TiO2) 100]), was prepared as follows. TiCl4 (Aldrich, 1.12 mL, 1.94 g, 10.2 mmol transferred in argon atmosphere) was added dropwise by syringe and needle to an icecold solution of RF8(EO)9 (1.0 g) in freshly distilled ethanol (37 g). The clear yellow solution was stirred for 15 min and then allowed to warm to room temperature. Water (18 MΩ quality, 0.6 g, 33.3 mmol) was then added slowly to the solution, and stirring continued at room temperature for a further 10 min. The concentrated sol was filtered through a 0.2 μm filter (Sartorius Minisart-plus). An aliquot (8.0 g) of the yellow stock solution was transferred to another container and diluted with ethanol (24.0 g). Where TiOCl2 solution (Fluka, 3.18 g, 10.2 mmol) was used the procedure was the same as that for TiCl4 with the exceptions that the solution was cooled to 0 °C and transferred in air and no water added to the sol. A custom-built dip-coating device equipped with a humidity chamber was used to coat microscope slide glass or silicon wafers at a relative humidity of 30% maintained by passing dry air through the chamber. The glass slide (Marienfeld, 75 mm 25 mm 1 mm) or silicon wafer (Siltronix; semiconductor quality silicon; 50 mm 25 mm 0.5 mm), precleaned with ethanol and then dried with pressurized, filtered air, was lowered into the dilute solution. After an immersion time of 1 min, the wafer was withdrawn at either 12.5 or 46 cm min-1 until the bottom edge of the wafer was just above the solution, whereupon the substrate was raised rapidly away from the solution. After 2-3 min the coated substrate was removed from the chamber into ambient conditions typically 21 °C and relative humidity 25-30%. A film was subsequently heated at 80 °C in air for 16 h to partly condense the inorganic phase. To remove the surfactant template, a film was either immersed in ethanol at 80 °C for 33 h and subsequently toluene at 80 °C for 24 h or directly fired in air at 350 °C for 1 h in a furnace using a temperature ramp of 2 °C/min and then rinsed in ethanol at 80 °C for 2 h. 2.2. Film Characterization. X-ray reflectometry (XR) experiments were performed in air with a wavelength of 1.54 A˚ on a Philips X’Pert reflectometer at the Laboratoire de Physique de l’Etat Condense (LPEC), Universite du Maine. The data were recorded as X-ray reflectivity, R, as a function of scalar momentum transfer qz 4π sin θ qz ¼ λ where λ is the X-ray wavelength (1.542 A˚) and θ the specular reflectance angle. The mesostructure of a thin film was further characterized by grazing incidence small-angle X-ray scattering (GI-SAXS). The measurements were also performed at LPEC with the sample in vacuum using the recently commissioned small-angle X-ray scattering instrument (high flux camera, Rigaku rotating Cu anode, 45 kV, 50 mA). An exposure time of 21 600 s and an incident angle of 0.25° were used to capture the patterns. The experimental X-ray reflectivity curves were calculated using the matrix technique14 and illustrated by Dourdain et al.15 Atomic force microscopy measurements were performed with an Agilent 5500 AFM. All the topography images were realized in tapping mode in air using the same “whisker type” NSC05-10 tip (NT-MDT) with a nominal 10 nm radius of curvature. Film thickness measurement was performed by measuring the step height of a scratch made with a scalpel. Image processing and roughness analysis were performed with the Gwyddion freeware. 2D-PSD spectra where (14) Gibaud, A. X-Ray and Neutron Reflectivity: Principles and Applications; Springer: Paris, 1999. (15) Dourdain, S.; Bardeau, J. F.; Colas, M.; Smarsly, B.; Mehdi, A.; Ocko, B. M.; Gibaud, A. Appl. Phys. Lett. 2005, 86, 113108.
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Figure 1. GI-SAXS patterns obtained for TiO2-based films deposited on (a) silicon substrate at a withdrawal speed of 12.5 cm min-1, (b) glass substrate at a withdrawal speed of 46 cm min-1, (c) glass substrate at a withdrawal speed of 46 cm min-1 and exposed to ethanol at 80 °C for 33 h film, and (d) glass substrate at a withdrawal speed of 46 cm min-1 after firing in air at 350 °C for 1 h and rinsing in ethanol at 80 °C for 2 h. All measurements were recorded in vacuum at θ = 0.25° and a collection time of 21 600 s.
obtained using the WSxM freeware. Contact angle measurements were recorded on a custom-built apparatus (LPEC) and images collected and analyzed using logAx(1.8.0) software. Raman experiments were performed at room temperature on a T64000 (Jobin-Yvon, Horiba) multichannel Raman spectrometer (LPEC). A 514.5 nm wavelength radiation from a coherent spectrum argon/krypton ion laser, operating at a power of 50 mW, was selected to provide a good signal-noise ratio. The laser light was focused onto the samples by using a microscope equipped with a 100 objective. The scattered light was analyzed by a spectrometer with a single monochromator (600 gratings mm-1), coupled to a nitrogen-cooled CCD detector.
3. Results and Discussion Figure 1 shows the GI-SAXS results for films prepared at withdrawal speeds of (a) 12.5 cm min-1 and (b) 46 cm min-1 from the same dilute solution of TiCl4, RF8(EO)9 ethanol, and water. From the angular relationship between the reflections, patterns typical 2-D hexagonal phase are observed in which tubular rods of RF8(EO)9 are packed hexagonally and aligned parallel to the substrate.16,17 Reflections in the GI-SAXS pattern of Figure 1a are indexed accordingly as the (11), (01), and(10) planes. The reflectivity patterns of Figure 2 display Kiessig fringes resulting from the interference of the X-rays reflected from the air/film and film/glass interfaces and with Bragg diffraction from constructive interference of reflected radiation from successive stacked bilayers. The parameters obtained from the fitted reflectivity curve for the film shown in Figure 2a are presented in Table 1. With the inclusion of a native SiO2 buffer layer of 1.4 nm thickness, a porous TiO2 cap layer of 2 nm, and eight repeating (16) Gibaud, A.; Bardeau, J. F.; Dutreilh-Colas, M.; Bellour, M.; Balasubramanian, V. V.; Robert, A.; Mehdi, A.; Reye, C.; Corriu, R. J. J. Mater. Chem. 2004, 14, 1854–1860. (17) Gibaud, A.; Grosso, D.; Smarsly, B.; Baptiste, A.; Bardeau, J. F.; Babonneau, F.; Doshi, D. A.; Chen, Z.; Brinker, C. J.; Sanchez, C. J. Phys. Chem. B 2003, 107, 6114–6118.
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Figure 3. (a) Atomic force micrograph of the as-prepared TiO2based film deposited on silicon substrate at a withdrawal speed of 12.5 cm min-1. (b) Height distribution profile showing the thickness of the film. Table 2. Contact Angles for Various Surfaces
Figure 2. R vs qz profiles obtained from TiO2-based films deposited on (a) silicon substrate at a withdrawal speed of 12.5 cm min-1 and (b) glass substrate at a withdrawal speed of 46 cm min-1. For (a) the calculated data (black line) is compared to the measured data (green circles). Table 1. Parameters Obtained from the Modeled X-ray Reflectivity Data of Figure 2a
surface
contact angle/deg
silicon (as received) glass (ethanol preclean) RF8(EO)9-TiO2 film (as prepared) pore-TiO2 film (toluene rinsed) pore-TiO2 film (fired and rinsed) fluorosurfactant monolayer20
20 ( 2 22 ( 2 50 ( 2 62 ( 1 67 ( 5 95 ( 3
bilayers of TiO2 and RF8(EO)9 of d-spacing = 4.9 nm, the total film thickness is ∼43 nm. Within the bilayers, the TiO2 layers are ∼2.0 nm thick and have an electron density of 0.57 e-/A˚3 (qc = 0.0284 A˚-1, as calculated using Fel = Qc2).18 This density is significantly less than that of crystalline anatase (1.10 e-/A˚3) or rutile (1.2 e-/A˚3) but is in agreement with the value of 0.52 e-/A˚3 obtained from related TiO2-based film where Brij 58 was used as the template.12 The result suggests that the titania walls in the asprepared films are porous, amorphous, or gel-like. The electron density of the fluorinated layer, 0.61 e-/A˚3 (0.0294 A˚-1), is in agreement with that reported for the density of solid phase of a monolayer of F(CF2)8(CH2)2OH self-assembled at a liquid-
liquid interface (1.85 that of the electron density of bulk water, 0.334 e-/A˚3).19 Its distance, 2.9 nm, is also in good agreement with the sum of the lengths of the hydrophobic and hydrophilic parts, 1.5 and 1.6 nm,6 respectively, indicating that the hydrophobic chains are fully extended. The similarity of the densities of the fluorinated surfactant and the TiO2 layers results in a low X-ray contrast; however, thicker films containing 18 bilayers produced by withdrawal speeds of 46 cm min-1 did display well-defined Bragg reflections (Figure 2b). However, the low contrast between the film and the underlying substrate produced a film where the air/film and film/substrate critical edges were difficult to discern compared with those obtained from the use of hydrogenated surfactant.12 The topography of the film surface as revealed by AFM (Figure 3) is homogeneous and flat. The root-mean-square roughness (rms), 0.5 nm, and the film thickness, 43.8 ( 2 nm, are in good agreement with the values extracted from the X-ray reflectometry analysis. The contact angles reported in Table 2 were obtained from the surface of the as-prepared RF8(EO)9-TiO2 film (∼50°) and from a nonionic Zonyl FSN fluorosurfactant monolayer
(18) Rousseau, J.-J.; Gibaud, A. Crystallographie Geometrique et Radiocristallographie, 3rd ed.; Dunod: Paris, 2007.
(19) Tikhonov, A. M.; Li, M.; Schlossman, M. L. J. Phys. Chem. B 2001, 105, 8065–8068.
qc/A˚ -1
roughness/A˚
0.0120 4.6 TiO2 cap layer 0.0285 5.5 TiO2 walls F 0.0284 b and following the formulas dkl = 2π/q and dkl = 1/(k2/b2 þ l2/c2)1/2, a unit cell ratio c/b of 1.1 was estimated from the positions of the (02) and (20) Bragg reflections along qz (c vector) and qy (b vector) The 2-D centered-rectangular phase was confirmed by the simulation using this unit cell. Figure 7b shows the X-ray reflectivity pattern obtained from the same film. Two well-defined Bragg peaks are observed with characteristic dspacing of 44.8 A˚ and correspond to the planes that are parallel to the film surface. The peaks are therefore indexed as (02) and (04) according to the spots on the longer axis of the reciprocal space lattice shown by the GI-SAXS pattern. As the ratio of the unit cell parameters is much less than that observed√ for true 2-D hexagonal (P6mm, No. 17) symmetry (where c = b 3), we conclude that significant shrinkage in the direction normal to the substrate has occurred during the formation of the mesophase. A low inor(24) Buchko, C. J.; Kozloff, K. M.; Martin, D. C. Biomaterials 2001, 22, 1289– 1300. (25) Dittert, B.; Stenzel, F.; Ziegler, G. J. Non-Cryst. Solids 2006, 352, 5437– 5443. (26) Cottineau, T.; Richard-Plouet, M.; Rouet, A.; Puzenat, E.; Sutrisno, H.; Piffard, Y.; Petit, P.-E.; Brohan, L. Chem. Mater. 2008, 20, 1421–1430.
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Figure 7. (a) Measured GI-SAXS pattern obtained from a TiO2based film produced from a TiOCl2 3 7H2O 3 1.4HCl precursor deposited on glass substrate at a withdrawal speed of 46 cm min-1, recorded in vacuo at θ = 0.2° and collection time 21 600 s showing Miller indices of the Bragg reflections of a 2-D simulated reciprocal space (black circles). (b) R vs qz profile obtained from the film.
ganic/organic volume ratio induced by a high water content and long evaporation times are reported to favor a thermally stable 2-D centered rectangular silicate phase.27 We suggest that the high water content and greater extent of hydrolysis of Ti precursor in the sol prepared using the TiOCl2 solution compared with that produced using TiCl4 influenced the structure observed here. (27) Zhou, X. F.; Yu, C. Z.; Tang, J. W.; Yan, X. X.; Zhao, D. Y. Microporous Mesoporous Mater. 2005, 79, 283–289.
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4. Conclusions Highly ordered mesostructured titania-based thin films have been produced by the evaporation-induced self-assembly route using F(CF2)8C2H4(OC2H4)9OH as the template and a TiCl4 precursor. X-ray diffraction analysis of the films by GI-SAXS reveals that the rigid chain, semifluorinated surfactant produced a 2-D structure in which tubular rods of the surfactant are packed hexagonally and aligned parallel to the silicon or glass substrates. The use of a commercially available and air-stable aqueous TiOCl2 solution resulted in a well-structured as-prepared film but one that exhibited significant shrinkage in the direction
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normal to the film surface. These results form a foundation for the synthesis conditions for the preparation of mesoporous TiO2 materials using the micellar template route for which dilute aqueous conditions are utilized. The use of F(CF2)8C2H4(OC2H4)9OH as the structure-directing agent and also opens the possibility of using supercritical CO2 processing of these mesostructured TiO2 films for pore size engineering. Acknowledgment. The authors thank Prof. M.-J. Stebe (Nancy Universite) for helpful discussions and DuPont de Nemours Belgium for providing a sample of F(CF2)8C2H4(OC2H4)9OH.
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