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Effect of Withdrawal Speed on Film Thickness and Hexagonal Pore–Array Dimensions of SBA-15 Mesoporous Silica Thin Film Junho Hwang, Naoko Shoji, Akira Endo, and Hirofumi Daiguji Langmuir, Just Accepted Manuscript • DOI: 10.1021/la5037713 • Publication Date (Web): 04 Dec 2014 Downloaded from http://pubs.acs.org on December 14, 2014

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Effect of Withdrawal Speed on Film Thickness and Hexagonal Pore–Array Dimensions of SBA-15 Mesoporous Silica Thin Film

Junho Hwang†, Naoko Shoji‡, Akira Endo§, and Hirofumi Daiguji*†‡

†

Department of Mechanical Engineering, Graduate School of Engineering, The University of Tokyo

7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡

Division of Environmental Studies, Graduate School of Frontier Sciences, The University of Tokyo

5-1-5 Kashiwanoha, Kashiwa 277-8563, Japan §

National Institute of Advanced Industrial Science and Technology (AIST)

AIST Central 5-2, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

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ABSTRACT In this study, two-dimensional hexagonal mesoporous silica thin films of SBA-15 were synthesized on Si substrates via dip-coating using an evaporation-induced self-assembly process. The effect of the withdrawal speed on the thicknesses, one-dimensional pore alignments, and two-dimensional hexagonal pore arrays of the films was elucidated. Detailed analyses of FE-SEM and TEM images and XRD and XRR patterns of the synthesized thin films clarified that the pore sizes, interplanar spacings, and film thicknesses depend on the withdrawal speed. Furthermore, the same films were synthesized on Si substrates with microtrenches. The local flow of coating solutions around microtrenches affects the pore direction as well as the film thickness. In order to form well-ordered mesoporous silica thin films with large surface areas, it is important to control the synthetic conditions such as the local flow of the coating solutions as well as the physicochemical properties of the silica precursor solutions or template molecules.

KEYWORDS: Dip-coating, Self-assembly, Pore alignment, X-Ray diffraction (XRD) and reflectivity (XRR), Field-emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM).

1. INTRODUCTION Micelle-templated mesoporous silica materials have attracted considerable interest in a variety of practical fields, including catalysis, optics, electronics, biotechnology, and separation, owing to their highly ordered and uniform pore sizes, high surface area–to-volume ratios, and simple syntheses (i.e., the liquid crystal templating mechanism).1–3 Precise control of the properties of mesoporous silica, such as pore size and structure, is important for their practical application and is attained by adjusting the surfactant that acts as an organic template.4–9 In particular, mesoporous silica materials with two-dimensional hexagonal arrays (P6mm), i.e., SBA-15, have simple geometries and easily analyzed pore structures. In general, SBA-15 thin films are fabricated using the dip-coating method because of the increased ability to align the pores parallel to the substrate in the desired direction as compared to spin-coating.10 2 ACS Paragon Plus Environment

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Using the dip-coating process, a well-ordered mesostructured thin film can be deposited onto the substrate by evaporation-induced self-assembly.11–13 The thickness of thin films has been controlled by the concentration of coating solutions. Recently, it was found that the thickness can also be controlled by the withdrawal speed of the substrate within the range of 10−2 and 10 mm s−1. Le Berre et al.14 reported that the thicknesses of multilayered phospholipid films deposited on horizontally moving solid substrates depends on the moving speed, v. At high speeds ( v > ~1 mm s−1), viscous drag forces interacting with the substrates have a predominant role in deposition of the thin films and the liquid film dragged onto the substrate dries quickly (i.e., the Landau-Levich regime). The thicknesses of thin films formed in this regime increased with increasing withdrawal speed at a power of 2/3 based on the classical Landau-Levich model.15 At speeds that are sufficiently low ( v < ~10−1 mm s−1) to neglect viscous drag forces, solutes accumulate near the contact line of the solution and substrate because of evaporation-induced flow, depositing the dried film above the meniscus (i.e., the evaporation regime). The thicknesses of thin films formed in this regime increased with decreasing withdrawal speed at a power of −1. At a crossover point in the intermediate regime between both regimes, the thickness of thin films was minimized at a critical speed, vcrit. Faustini et al.16 showed the same tendency to form a minimum of the thickness at some critical velocity in the sol-gel film formation using the dip-coating method. Brewer et al.17 also revealed this tendency for the layer deposition of particulate coatings from aqueous 20-nm-diameter silica dispersions via a forced-convection-assisted drag-out operation. In the evaporation regime, Dimitrov and Nagayama18,19 studied the vertical-driven colloidal particle deposition system and concluded that the particle deposition near the meniscus resulted from the convective particle transfer caused by the solvent evaporation at a low withdrawal speed and the deposition rate was proportional to the inverse of the withdrawal speed. The similar results could be obtained in different systems theoretically (dip-coating20) and experimentally (a Hele–shaw cell consisting of two plates21,22). Berteloot et al. proposed a simple model of the hydrodynamics near a contact line for determining the thickness of deposited layers of colloidal particles. They reported that the film thickness is proportional to the withdrawal speed at the power of -2,23 but later, they corrected that the thickness is proportional to the withdrawal speed at the power of 3 ACS Paragon Plus Environment

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-1 by considering the effect of particle concentration gradients near the contact line.24 There could be several different mechanisms for colloidal particle transfer in the meniscus region such as liquid flow due to capillary pressure gradient and particle diffusion due to concentration gradient. However, their final equation to estimate the film thickness, in which the film thickness is proportional to the withdrawal speed at the power of -1, does not include the parameters in the meniscus region such as the location of the stagnation point and the particle concentration at the stagnation point but includes the parameters in the initial and final states, i.e. the particle concentrations of the bulk solution and the dried film and the meniscus length.24 It is fair to say that their model is basically the same to the mass-balance model proposed by Le Berre et al.14 The deposition process in the upper portion of the meniscus on a moving substrate is similar to the phenomenon known as the “coffee-ring effect”, which is caused by non-uniform evaporation flux at the surface of drying droplets.25,26 Both in the deposition process on a moving substrate and coffee-ring effect, the fast solvent evaporation near the contact line with the substrate surface causes the evaporation-induced outward flow. However, in the deposition process on a moving substrate, the contact line is moving at a constant rate, and thus the receding contact angle is fixed regardless of solvent evaporation. On the other hand, in the “coffee ring effect,” the contact line is fixed and the radius of the droplet does not shrink with drying, and thus the contact angle of the droplet decreases gradually with drying. As a result, films of uniform thickness can be obtained in the deposition process on a moving substrate, whereas in the “coffee-ring effect,” the film around the contact line is much thicker than that inside the droplet. In the Landau-Levich regime, fluid dynamics of Newtonian and non-evaporating liquids have been studied for several decades theoretically and experimentally. While the studies of non-Newtonian and evaporating liquids have been conducted recently as mentioned above.14,16,17,24 For mesoporous silica thin films, the uniaxially oriented alignment, interplanar spacing (in-plane and normal lattice constant with respect to the substrate), and pore size are important to improve their performance in various application devices with mesoporous structures. In particular, the uniaxially oriented alignment of pores is vital for improving the mass transport rate through pores. Until now, a variety of techniques have been 4 ACS Paragon Plus Environment

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reported for the production of two-dimensional hexagonal mesoporous thin films with the desired mesopores. The methods can be categorized into four groups: i) synthesis under external forces such as electric field,27 magnetic field,28 and air flow,29 ii) surface treatment by photoinduced anisotropic polymer film30 and cubic mesoporous film,31 and rubbing treatment,32 iii) geometrical surface modification such as lithographically designed confined space,33,34 and iv) addition of alignment-controlling agents.35 However, in most of these methods, the evaporation-induced self-assembly process was not considered significantly. The process plays an important role in determining the structure of mesostructured silica films, and the relevant fundamental research of the process should contribute to the advancement of the methods described above. Controlling the synthetic conditions to affect the film-formation process is as important as adjusting the initial sol composition in determining the structures of highly ordered mesostructured thin films. Especially, the withdrawal speed in dipcoating affects the film-formation process. In this study, we focused on the effect of withdrawal speeds ranging from 0.02 to 3.0 mm s−1 on the thicknesses and pore structures of SBA-15 mesoporous silica thin films with two-dimensional hexagonal pore structures. We used the mass-balance model14 and the Landau-Levich model15 to predict the film thickness in the capillarity and the draining regimes, respectively. In addition, we proposed a model to predict the distance from the top of the meniscus of the coating solution to the drying line and the time required to dry the thin film for the investigation of the effect of the withdrawal speed on the uniaxially oriented pore alignment and twodimensional hexagonal pore structure. The film thicknesses, pore sizes, and pore structures were investigated via field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and Xray diffraction (XRD) and X-ray reflectivity (XRR) analysis of the synthesized thin films. Furthermore, to investigate the effect of the local flows on the alignment of the mesopores, we synthesized the SBA-15 thin film on Si substrates with microtrenches in the capillarity (0.02 mm s−1) and draining (3.0 mm s−1) regimes, respectively.

2. METHODS 5 ACS Paragon Plus Environment

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2.1. Preparation of SBA-15 mesoporous silica thin films The (100)-oriented Si substrates (Kyodo International Inc., Japan) used underwent sequential 10 min cleanings with n-hexane (96.0%, Wako pure chemistry industries, Ltd, Japan), acetone (99.5%, Wako), and deionized water (18.3MΩ, Millipore, Germany) in an ultrasonic bath. The substrates were then soaked in piranha solution (H2SO4 (95.0%, Wako)/ H2O2 (30%, Wako) = 2 : 1, vol%) for 30 min and rinsed with deionized water to remove contaminants and make them wettable with a water/ethanol solution. They were then cleaned with 2-propanol (99.7%, Wako) for 1 h and dried in air. The precursor solution was prepared as follows: Tetraethylorthosilicate (TEOS (~96%, Tokyo Chemical Industry Co., Ltd., Japan); 1.50 g) as the silica source, Ethanol (EtOH (95.8%, Wako); 15.0 g), deionized water (0.78 g), and 0.1 M Hydrogen chloride (HCl (99.7%, Wako); 0.15 g) were stirred at room temperature for 1 h in a capped vial. Triblock copolymer based on poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol), (Pluronic P123 (BASF, Germany); 0.30 g), as the structure-directing agent, and EtOH (14.25 g) were stirred at room temperature for 1 h in a separate capped vial. After stirring, the two different solutions were mixed and then stirred at room temperature for 1 h. The final TEOS/P123/EtOH/H2O/0.1 M HCl molar ratio of the precursor solution was 1:0.0072:88.2:7.17:0.0021. The prepared precursor solution was then deposited on a Si substrate using the dip-coating method, as follows: The cleaned Si substrate with or without microtrenches was vertically immersed in the precursor solution and withdrawn at a rate of 0.02, 0.05, 0.1, 0.2, 0.5, 1.0, or 3.0 mm s−1 then aged for 24 h. The coating processes were performed at 298 K at a relative humidity (RH) of 60% inside a glove box. Subsequently, the as-prepared thin films were dried at 343 K for 1 h and calcined to remove the block copolymer surfactant at 773 K for 5 h in air at a very slow ramping rate (< 0.5 K min−1) to minimize deformation caused by thermal stress.

2.2. Characterization of SBA-15 mesoporous silica thin films The synthesized transparent mesoporous silica thin films were observed using a field-emission scanning electron microscope (S-4800, Hitachi High-Technologies, Japan) at an accelerating voltage of 1.0–1.2 kV 6 ACS Paragon Plus Environment

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without any metal coating. Furthermore, TEM images of cross-sections of the film were obtained using a transmission electron microscope (H-9000UHR, Hitachi High-Technologies, Japan) at an acceleration voltage of 300 kV. Prior to the TEM analysis, the specimen was prepared using the ion-beam milling method with a GATAN 691 PIPS at an acceleration voltage of 6 kV; an Ar ion beam was used and the milling angle was 5°. The average film thicknesses and hexagonal pore–array dimensions of the SBA-15 thin films were analyzed using an X-ray reflectivity and X-ray diffraction system (SmartLab 9 kW system, RIGAKU, Japan) with Cu Kα radiation (wavelength = 1.54 Å) at 45 kV/200 mA. GlobalFit software was used to analyze the XRR data.

3. RESULTS 3.1. Synthesis of mesoporous silica thin films using the dip-coating process Figure 1(a) shows a schematic diagram of the dip-coating process. The cleaned Si substrate was vertically immersed in the precursor solution and withdrawn at a constant rate, v, and then the liquid film was coated on the Si substrate. During a drying process of the coating solution, rod-like micelles of Pluronic P123 were aligned parallel to the Si substrate by evaporation-induced self-assembly and silica was solidified around the micelles as shown in Figure 1(b). By removing the micelles, a well-ordered mesoporous silica thin film was formed. This mesoporous silica thin film has a two-dimensional hexagonal pore structure as shown in Figure 1(c). Assuming the hexagon is regular, the interplanar spacings normal and tangential to the Si substrate can be defined as d(100) and d(110).

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Figure 1. (a) Schematic diagram of the dip-coating process, (b) enlarged cross-sectional view of a deposited SBA-15 thin film onto the Si substrate, and (c) schematic diagram of hexagonal mesopore array corresponding to the cross-sectional plane of SBA-15 thin films. d(100) and d(110) denote the interplanar spacings normal and tangential to the Si substrate, respectively.

3.2. TEM measurements Figure 2 shows cross-sectional TEM images of SBA-15 thin films synthesized on Si substrates at four different withdrawal speeds, i.e., 0.02, 0.5, 1.0, and 3.0 mm s−1. The white part in TEM images corresponds to the mesopores aligned parallel to the Si substrate. SEM images only show the surface of a cross-section; therefore, they cannot be used to determine if the pore structures along the dip-coating direction behind the cross-sectional image are the same as those on the surface. In contrast, TEM images show a projection of the interior of a specimen; thus, the alignment of the pores in the thin film along the dip-coating direction can be verified. In this study, TEM analyzed the SBA-15 thin film specimens to a depth of a few hundred nanometers. As the withdrawal speed increased from 0.5 to 3.0 mm s−1, the film thickness increased; similarly, as the withdrawal speed decreased to 0.02 mm s−1, the film thickness also increased. The local thicknesses of the 8 ACS Paragon Plus Environment

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thickest layer measured by TEM images of the thin films produced at withdrawal speeds of 0.02, 0.5, 1.0, and 3.0 mm s−1 were 81.6, 23.7, 28.9, and 51.3 nm, respectively. The TEM images revealed that the pore arrays contained the most regularly arrayed pores with twodimensional hexagonal structures when the samples were withdrawn at 3.0 mm s−1 in comparison with the samples withdrawn at the other rates. As shown in the TEM images, thin films at 0.02, 1.0, and 1.0 mm s−1 had well-controlled two-dimensional hexagonal pore structures but lacked perfect pore arrays. Furthermore, some pores were not independent but rather were connected to adjacent pores; this supports that the directions of the pore array varied in some planes within a few hundred nanometers deep in the specimens and the pores were not perfectly aligned in one direction over the entire film, although the local two-dimensional hexagonal pore arrays were well controlled (see Supporting Information).

Figure 2. Cross-sectional TEM images of SBA-15 thin films synthesized on Si substrates at four withdrawal speeds: (a) 0.02, (b) 0.5, (c) 1.0, and (d) 3.0 mm s−1. 9 ACS Paragon Plus Environment

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In the TEM images, the interplanar spacing of the (100) planes [d(100)] as shown in Figure 1 (c) decreased with increasing film thickness. The local d(100) values were 6.3, 7.9, 7.2, and 6.4 nm for withdrawal speeds of 0.02, 0.5, 1.0, and 3.0 mm s−1, respectively. Furthermore, the sizes of the pores also decreased with increasing film thickness. The reasons for this are explained later in this article. Image analyses showed that, as the film thickness increased, both the interplanar spacing and pore size decreased, yielding more compact structures.

3.3. XRR measurements To determine the average thicknesses of the SBA-15 thin films, we performed XRR measurements. The XRR patterns were analyzed using GlobalFit software (RIGAKU, Japan), and the average thickness of a 5 mm × 5 mm area of the SBA-15 thin films was calculated by comparing the periodic vibration, ∆θ, of the XRR spectrum with simulated patterns within 2θ < 2°. Figure 3(a) shows simulated XRR patterns of SBA-15 thin films synthesized on Si substrates at seven different withdrawal speeds, i.e., 0.02, 0.05, 0.1, 0.2, 0.5, 1.0, and 3.0 mm s−1. Higher frequency oscillations correspond to a thicker film of mesoporous silica.

Figure 3. (a) XRR patterns of SBA-15 thin films synthesized on Si substrates at withdrawal speeds of 0.02, 0.05, 0.1, 0.2, 0.5, 1.0, and 3.0 mm s−1. The vertical axis has a logarithmic scale. (b) Plots of the film thickness versus withdrawal speed. 10 ACS Paragon Plus Environment

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The measured XRR patterns show that the minimum frequency occurred at a withdrawal speed of 0.2 mm s−1 (i.e., the critical withdrawal speed, vcrit) and increased with either increasing or decreasing withdrawal speed. Figure 3(b) shows plots of the film thicknesses versus withdrawal speeds for seven different withdrawal speeds. The calculated thicknesses were 88.9, 44.4, 30.6, 24.0, 26.6, 33.6, and 55.7 nm for films produced at withdrawal speeds of 0.02, 0.05, 0.1, 0.2, 0.5, 1.0, and 3.0 mm s−1, respectively. Figure 3(b) also shows the prediction curve based on semi-empirical models (further details are provided in Section 4.1). This curve was composed of two different models describing the deposition process by capillarity-induced flow (capillarity regime) and viscous drag flow (draining regime) that corresponded to ultraslow and fast withdrawal speeds, respectively. The prediction curves from these two models were determined using experimental data for the thicknesses of films produced at withdrawal speeds of 0.02 and 3.0 mm s−1, respectively. We confirmed that the linear combination of these two models approximated the film thicknesses at intermediate withdrawal speeds of 0.05–1.0 mm s−1.

3.4. XRD measurements We also performed XRD measurements to confirm the formation of SBA-15 films with hexagonally arrayed pores. Figure 4(a) shows the first intense d(100) peak of out-of-plane XRD patterns of SBA-15 thin films synthesized at seven different withdrawal speeds, i.e., 0.02, 0.05, 0.1, 0.2, 0.5, 1.0, and 3.0 mm s−1. These XRD patterns were in good agreement with previously reported patterns for SBA-15 mesoporous silica materials.5,36−38 The diffraction patterns of the SBA-15 films produced at a withdrawal speed of 3.0 mm s−1 featured intense peaks with d values of 6.33 nm (100) and 3.24 nm (200), which indicated the formation of a 2D hexagonal mesostructure parallel to the substrate. The 2θ of the first peak was minimized at 0.2 mm s−1 and increased with either increasing or decreasing withdrawal speed. Figure 4(b) shows the interplanar spacings of the (100) planes that were obtained from the first peak in the corresponding XRD pattern versus withdrawal speed for the seven different withdrawal speeds.

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Figure 4. (a) Out-of-plane XRD patterns of SBA-15 thin films synthesized on Si substrates at seven different withdrawal speeds, i.e., 0.02, 0.05, 0.1, 0.2, 0.5, 1.0, and 3.0 mm s−1. The vertical axis has a logarithmic scale. (b) Plots of the interplanar distance [d(100)] versus withdrawal speed.

The measured d(100) values for withdrawal speeds of 0.02, 0.05, 0.1, 0.2, 0.5, 1.0, and 3.0 mm s−1 were 5.84, 6.75, 7.08, 7.68, 7.24, 7.03, and 6.33 nm, respectively. The results showed that the interplanar spacing of the (100) planes decreased with increasing film thickness. The d(100) values measured from the TEM images were slightly larger than those of the XRD patterns. However, all the results featured similar trends. The second and third peaks at 3.0 mm s−1 appeared more clearly than those at 0.02 mm s−1, although the intensity of the peaks generally increased as the thickness of thin film increased under the same conditions. This also supported that the pore array parallel to the substrate of thin films produced at 3.0 mm s−1 was better than that at 0.02 mm s−1. We also performed in-plane XRD measurements to confirm the formation of SBA-15 films with hexagonally arrayed pores.39,40 Figure 5 shows the in-plane XRD patterns of SBA-15 thin films synthesized at three different withdrawal speeds, i.e., 0.02, 0.5, and 3.0 mm s−1. The thin films were set as the dip-coating direction was parallel to the incident X-ray direction. The incident angle was 0.2°. All of the in-plane XRD patterns showed the peak for the d(110) plane was around 1.24–1.32 degree, indicating that the SBA-15 films 12 ACS Paragon Plus Environment

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had two-dimensional hexagonal pore structures. The measured d(110) values for withdrawal speeds of 0.02, 1.0, and 3.0 mm s−1 were 6.69, 6.80, and 7.12 nm, respectively. If the hexagon is regular, d(110) should be 0.58 times smaller than d(100). However, the measured d(110) value was close to d(100), suggesting that the hexagon was wider in the tangential direction to the Si substrate. The cross-sectional TEM images of SBA-15 thin films shown in Figure 2 also support this result. The measured d(110) value slightly increased with increasing the withdrawal speed, but further investigation is needed to clarify the relationship between the d(110) interplanar spacing and the withdrawal speed.

Figure 5. (a) In-plane XRD patterns of SBA-15 thin films synthesized on Si substrates at three different withdrawal speeds, i.e., 0.02, 0.5, and 3.0 mm s−1. The vertical axis has a logarithmic scale.

4. DISCUSSION 4.1. One-dimensional alignments and two-dimensional hexagonal arrays of pores in mesoporous silica thin films During the dip-coating of sol-gel solutions, two opposite film-formation regimes exist that are dependent on the withdrawal speeds. At low withdrawal speeds (i.e., the capillarity regime), the film thickness is governed by evaporation-induced flow (the capillarity rises to replenish those deposited onto the substrate) because of 13 ACS Paragon Plus Environment

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continuous evaporation of the solvent near the upper part of the meniscus; in contrast, at high withdrawal speeds (i.e., the draining regime), the film thickness is mainly dependent on viscous drag flow. At intermediate speeds, both phenomena overlap and there is a critical speed at which the film thickness is minimized.14 However, the dimensions of the pore arrays inside the thin films and their dependence on the withdrawal speeds have not been fully elucidated even though the mesoporous silica SBA-15 thin films produced in each regime undergo completely different formation processes. Experimentally, as shown in Figure 2, the SBA-15 thin films formed at various withdrawal speeds (i.e., the capillarity, intermediate, and draining regimes) featured two-dimensional hexagonal pore structures; however, the pores of the film produced in the three different regimes showed obvious differences in the one-dimensional alignments and two-dimensional hexagonal arrays. For these differences, we noted the distance, ∆z, from the top of the meniscus of the solution to the drying line (vapor-liquid-solid, three-phase boundary) and the time, ∆t, required to dry the film (see Figure 1(a)). We considered the mass-balance model.14 The rate of solvent evaporation, E (m3 s−1), is given by E = m& 0 v0 ∆zL ,

(1)

where m& 0 (kg m−2 s−1) is the flux of solvent evaporation, v0 (m3 kg−1) is the specific volume of the solvent, ∆z (m) is the distance from the top of the meniscus of the solution to the drying line, and L (m) is the width of the substrate; ∆zL indicates the area of solvent evaporation. The rate of film formation, F (m3 s−1), is given by F = vxs L ,

(2)

where v (m s−1) is the withdrawal speed and xs is the thickness of the solid film. The following massconservation equation should be satisfied between the rate of solvent evaporation, E, and the rate of film formation, F:

αρ F = cME ,

(3)

where α is the volume fraction of inorganic solid material, ρ (g cm−3) is the density of the thermally stabilized inorganic solid material, c (mol L−1) is the inorganic molar concentration of a solution, and M (g mol−1) is the molar weight. 14 ACS Paragon Plus Environment

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In the capillarity regime, the rate of solvent evaporation, E, restricts the rate of solution feeding to the upper zone of the meniscus, which affects the rate of film formation, F. From Eqs. 1–3, assuming that the rate of solvent evaporation, E, is constant (E = E0), the thickness of the solid film, xs, is given by  cME 0 xs =   αρ L

1  = C1v −1 , v

(4)

where C1 is a constant. The solvent is so dilute that m& 0 and v0 can be assumed to be constant in the z direction during the drying process. From Eq. 1, if E is constant (E = E0), ∆z is also constant as follows: ∆z =

E0 . m& 0 v 0 L

(5)

On the other hand, in the draining regime, at faster withdrawal speeds, the formation of a film is usually described by the Landau-Levich equation.15 In this equation, the thickness of the liquid film, xl, is given by 1

x l = 0.94l c Ca 2 3

2

 γ  2  ηv  3    , = 0.94  ρg   γ 

(6)

where lc [(γ/ρg)1/2] is the capillary length and Ca [ηv/γ] is the capillary number. Here, η, γ, and ρ are the viscosity, surface tension, and density of the fluid, respectively, while g is gravitational acceleration. Assuming that the thickness of the solid film, xs, is proportional to that of the liquid film, xl, xs is given by xs = C 2 v 2 3 ,

(7)

where C2 is a constant. From Eqs. 1–3 and 7, ∆z is given by ∆z =

αρC 2 cMm& 0 v 0

v5 3 .

(8)

Furthermore, the time required to dry the film, ∆t (s), is defined as ∆t = ∆z v . From Eqs. 1–3, ∆t is given by  αρ ∆t =   cMm& 0 v0

  xs . 

(9)

Because ∆t is proportional to xs, ∆t and xs show the same trends with respect to withdrawal speed, v, in both the capillarity and draining regimes.

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For the film thickness, C1 was calculated to be 1.778 × 10−12 m2 s−1 by substituting the measured values of xs = 89.9 nm and v = 0.02 mm s−1 into Eq. 4. On the other hand, by substituting xs = 55.7 nm and v = 3.0 mm s−1 into Eq. 7, C2 was calculated to be 2.677 × 10−6 m1/3 s2/3. From these results, the thickness of the solid film, xs, can be expressed as xs = C1v −1 + C 2 v 2 3 .

(

(10)

) (

) + (2.677 × 10

Specifically, xs [m ] = 1.778 × 10 -12 [m 2 s -1 ] × v [m s -1 ]

-1

-6

) (

)

[m 1 3 s 2 3 ] × v [m s -1 ]

23

. In Figure 3(b),

the capillarity and draining model lines were plotted using Eq. 10. From the proposed model for investigating a direct correlation between the pore structures and withdrawal speed, v, the distance from the top of the meniscus of the solution to the drying line, ∆z, was constant regardless of v in the capillarity regime and proportional to ~v5/3 in the draining regime. In the dip-coating experiments, the measured ∆z in the capillarity regime was ~1 mm, and, in the draining regime, ∆z increased as v increased and was ~35 mm at 3.0 mm s−1. ∆z is related to the one-dimensional alignment of pores. As ∆z increased, the gradient of c in the z direction decreased. This suggested that the mesoporous silica film dried from a more uniform liquid film of silica precursor solutions. Thus, the one-dimensional alignment should be improved by increasing the withdrawal speed (see Supporting Information). The time required to dry the thin film, ∆t, is related to the two-dimensional hexagonal arrays of pores. As ∆t increased, the liquid film dried more slowly. This suggested that the template molecules were sufficiently selfassembled and well-ordered pore structures formed in the silica thin films. If the liquid film dried too fast, it could lead to higher viscosity in the liquid film and disrupt complete self-assembly of the silica precursors and surfactant mesophase because of the shorter period of time available for organization.41 Higher viscosity in the thin liquid film prevented the poorly condensed inorganic network surrounding the surfactant mesophase from facile rearrangement, resulting in a disordered structure. Furthermore, from Eq. 9, thicker films had better pore structures because ∆t was proportional to xs. Because the films produced at withdrawal speeds of 0.5 and 1.0 mm s−1 (i.e., in the intermediate regime) were too thin, i.e., ∆t was too short, it was difficult to form a wellordered mesostructure in the film. Indeed, the extent of the silica condensation reaction played a critical role in 16 ACS Paragon Plus Environment

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the formation of self-assembled mesostructures; environmental conditions such as temperature, relative humidity, and evaporation rate during dip-coating also affected the formation. When the relative humidity was reduced from 60 to 40%, the thicknesses of the thin films did not change at both 0.02 and 3.0 mm s−1; however, the structures of the pores were disordered at 0.02 mm s−1 but identical at 3.0 mm s−1 (see Supporting Information). From Eqs. 4 and 9, the increase in m& 0 with reduced relative humidity led to decreased ∆z and ∆t values, resulting in disordered pore structures. However, the influence of the reduction of relative humidity at 3.0 mm s−1 was smaller than that at 0.02 mm s−1 because ∆z at 3.0 mm s−1 was much larger than that at 0.02 mm s−1. The simultaneous control of ∆z and ∆t was essential for producing more complete mesoporous silica thin films with two-dimensional hexagonal pore structures.

4.2. Interplanar spacing and pore sizes in mesoporous silica thin films The drying process was likely dependent on the liquid film thickness: As the liquid film thickness increased, it should have dried more slowly and remained in a tunable steady state for a longer time period. As a result, the solid films could have more compact structures. In contrast, ultrathin films dried too quickly to sufficiently age the inorganic species and complete micelle self-assembly;12,13,41 as a result, the interplanar spacing increased and well-defined pore structures were not formed over the entire surface of the Si substrate. The TEM images and XRD patterns of the synthesized solid films reflected this trend. Furthermore, to clarify the effect of calcination on the compact structure, the interplanar spacing of d(100) was measured by XRD before calcination. The measured d(100) values for withdrawal speeds of 0.02, 0.05, 0.1, 0.2, 0.5, 1.0, and 3.0 mm s−1 were 8.70, 10.19, 10.37, 10.23, 10.17, 9.89, and 9.00 nm, respectively. The reduction ratio of interplanar spacing of d(100) before and after calcination was about 30% for all withdrawal speeds (see Supporting Information). Thus, we concluded that the correlation between the interplanar spacing of d(100) and withdrawal speed was determined by the dip-coating process and not contraction due to calcination. In general, the pore-size distribution of mesoporous silica is determined from N2 or Ar adsorption data using Barrett-Joyner-Halenda (BJH), Dollimore-Heal (DH), or nonlocal density functional theory (NLDFT) methods. 17 ACS Paragon Plus Environment

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However, N2 adsorption–desorption isotherm measurements could not be applied to the characterization of pore-size distribution inside the thin films because it was difficult to acquire a sufficient amount of sample for the measurement. For the exact measurement, at least several milligrams of samples are required. If a 1-cm square Si substrate is used and the film thickness is 50 nm, hundreds of the thin films are required to determine the pore size. In addition, the ellipsometric porosimetry has been recently developed to evaluate the pore size. This technique has ability to measure porosity of 10-nm thick thin films but the exact value of the anisotropy parameter of elliptic cylinder pores is crucial for the pore size determination.42 Alternatively, we evaluated the out-of-plane pore radius from the cross-sectional TEM images. Figure 6 shows a magnified cross-sectional TEM image of a SBA-15 mesoporous silica thin film synthesized on a Si substrate at a withdrawal speed of 0.5 mm s−1. The direct magnification was 500 000× and silicon crystal structures can be even observed.

Figure 6. Cross-sectional TEM image of a SBA-15 mesoporous silica thin film synthesized on a (100)-oriented Si substrate at a withdrawal speed of 0.5 mm s−1. The direct magnification was 500 000×.

In the TEM images, the pore geometries appeared to be elliptical. The in-plane pore radius was not evaluated in this work, as it was likely to be overestimated due to the distortion of one-dimensional alignment of 18 ACS Paragon Plus Environment

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mesochannels from in-plane XRD measurement. The out-of-plane radius was evaluated; the measured values were 0.90, 1.62, 1.37, and 1.05 nm for withdrawal speeds of 0.02, 0.5, 1.0, and 3.0 mm s−1, respectively. The out-of-plane radius and d(100) interplanar spacing showed similar trends with respect to the withdrawal speed: As the film thickness increased, both the out-of-plane radius and interplanar spacing decreased, yielding more compact structures.

4.3. The effect of local flows on pore alignment To clarify the effect of local flows on copolymer micelle alignment, we synthesized SBA-15 thin films on Si substrates with microtrenches in the capillarity (0.02 mm s−1) and draining regimes (3.0 mm s−1). The microtrenches were prepared by deep reactive ion etching (DRIE). The width, depth, and length of the trenches were ~3 µm, ~0.8 µm, and ~2 mm, respectively. Figures 7(a) and (b) show a schematic diagram of a Si substrate with microtrenches and a SEM image of the top surface, respectively. When the width of the trenches was less than ~1 µm, the mesopores were well-aligned along the trench direction, suggesting that the micelle assembly was controlled by the solid–liquid interface.27 In this study, we intentionally used wider microtrenches to investigate the effect of local flows on pore alignment. Figures 7(c-I) and (c-II) show cross-sectional SEM images of SBA-15 films synthesized at two different withdrawal speeds, i.e., 0.02 and 3.0 mm s−1, respectively. Figures 7(d-I) and (d-II) show enlarged views of areas close to the central part of the films shown in Figures 7(c-I) and (c-II), respectively. The central part of the film produced in the trench was much thicker than that produced on a planar substrate both in the capillarity and draining regimes. The thicknesses of the films in the capillarity and draining regimes were ~320 and ~150 nm, respectively, which are 3.6 and 2.7 times larger, respectively, than those produced on a planar substrate. In the capillarity regime, the increase in the liquid film thickness was attributed to enhanced capillary rise due to overlapping menisci in contact with three different surfaces. The film produced by capillarity rise from bulk sol solution showed well-ordered structures. However, the film formed at 0.02 mm s−1 was so thick that a crack occurred near the side walls of the trench, as shown in Figure 7(c-I). On the other hand, in the draining regime, the increase in the liquid film thickness was attributed 19 ACS Paragon Plus Environment

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to liquid flow into the trench from the top surface of the substrate. The mesopores near the gas-film interface were obliquely arranged along the local flow direction, as shown in Figure 7(e), while the mesopores near the side wall–film interface were well-ordered along the trench direction because there was little influence from the local flows. Hillhouse et al. demonstrated that the flow field could induce a preferred orientation of hexagonal mesoporous silica films.43 More recently, Shan et al. exhibited that mesoporous silica films deposited onto the planar substrate could be aligned along the incident angle of hot air flow.44 These results indicated that the synthetic conditions should be determined based on the shape of the substrate in order to produce uniaxially oriented pores. For wider trenches, films including uniaxially oriented mesopores could be produced when the withdrawal speed was within the capillarity regime. It should be noted that the capillary-driven flow and the flow at the vapor–liquid interface could affect the alignment of the mesopores. Active control of the local flow of coating solutions could enable the formation of well-ordered mesoporous silica thin films with large surface areas as well as control of the physicochemical properties of the silica precursor solutions or template molecules.

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Figure 7. (a) Schematic diagram of a Si substrate with microtrenches. (b) SEM image of the top surface of the Si substrate with microtrenches. (c-I, c-II) Cross-sectional SEM images of a SBA-15 mesoporous silica thin film synthesized in the microtrench at withdrawal speeds of 0.02 and 3.0 mm s−1, respectively. (d-I, d-II) Enlarged views of Figs. 7(c-I) and 7(c-II) near the central part of the microtrench. (e) Enlarged view of Figs. 7(c-II) near the side-wall of the microtrench.

5. CONCLUSIONS In this study, we synthesized two-dimensional hexagonal mesoporous silica SBA-15 thin films on Si substrates via dip-coating and investigated the influences of withdrawal speeds in the range of 0.02–3.0 mm s−1 on the film thicknesses and pore structures using FE-SEM, TEM, XRD, and XRR analyses. Furthermore, to

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investigate the effect of the local flows on pore alignment, we synthesized the same films on Si substrates with microtrenches. The following conclusions were drawn from this study: 1. During dip-coating, the distance from the top of the solution meniscus to the drying line was constant in the capillarity regime regardless of withdrawal speed. In contrast, the distance increased with increasing withdrawal speed in the draining regime. As the distance increased, the concentration gradient of a solution decreased, resulting in well-aligned mesopores along the dip-coating direction. 2. The time required to dry the thin film was proportional to the film thickness. As the time increased, the liquid film dried more slowly resulting in sufficient self-assembly of the template molecules and wellordered two-dimensional hexagonal pore arrays. 3. The interplanar spacing of the d(100) planes and pore sizes increased with increasing withdrawal speed in the capillarity regime and decreased with increasing withdrawal speed in the draining regime. The interplanar spacing and pore size were maximized at a critical withdrawal speed in the intermediate regime. The effect of the withdrawal speed on the interplanar spacing and pore size showed the opposite trend to that on the film thickness. As the liquid film thickness increased, it dried more slowly and thus remained in a tunable steady state for a longer period, leading to more compact structures in the thicker films. 4. The local flow in the microtrenches affected the alignment of the mesopores. Active control of the local flow of coating solutions could enable the formation of well-ordered mesoporous silica thin films.

ASSOCIATED CONTENT

Supporting Information. FE-SEM images of the top surface of SBA-15 thin films synthesized at different withdrawal speeds, effect of withdrawal speed on the density of thin films, SBA-15 thin films synthesized at 40% relative humidity (RH) and an interplanar spacing of d(100) before and after calcination. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION 22 ACS Paragon Plus Environment

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Corresponding Author *E-mail: [email protected], Phone/Fax: +81 3 5841 6971.

ACKNOWLEDGMENT This research was supported by a Grant-in-Aid for Scientific Research (B) (No. 23360092) from the Japan Society for the Promotion of Science (JSPS) and JST-CREST. We thank the Center for Applied Technology at Rigaku Industrial Corporation for their assistance with the XRD and XRR measurements.

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