Article pubs.acs.org/JPCC
Multiscale Rough Titania Films with Patterned Hydrophobic/ Oleophobic Features G. Soliveri, R. Annunziata, S. Ardizzone, G. Cappelletti, and D. Meroni* Department of Chemistry, Università degli Studi di Milano, Via Golgi 19, 20133 Milan, Italy S Supporting Information *
ABSTRACT: Oxide-based hybrids are promising systems to modulate the surface properties and impart different functionalities. Here, the wettability features of rough titanium dioxide (TiO2) layers derivatized by siloxanes are tailored with respect to both water and nonaqueous solvents. The adopted synthetic procedure is very simple and may be extended to different substrates, as it is based on the direct functionalization of homemade, tailored, TiO2 nanoparticles by different siloxanes, both fluorinated and nonfluorinated. Nanotitania provides a multiscale roughness able to impart superhydrophobicity and its photocatalytic activity can be exploited to obtain surfaces with patterned wettability by photocatalytic lithography. The behavior of the different siloxanes (oleophobicity degree, self-cleaning properties, and kinetics of ultraviolet degradation) is related to the surface energy components of the bare siloxane films, evaluated by Owens−Wendt classical model, and to the structure of the siloxane monolayer at the TiO2 surface, as determined by 13C and 29Si solid-state nuclear magnetic resonance. Finally, patterned structures with tunable hydrophobic and oleophobic patches are obtained by using the photocatalytic activity of the oxide. The siloxane photodegradation process is analyzed by Fourier transform infrared spectroscopy. The resulting wetting contrast is exploited to obtain a site selective adsorption of a dye molecule. The presented procedure can be applied to obtain the site selective deposition or growth of a large variety of materials, such as semiconductor quantum dots, polymers, or biological molecules.
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INTRODUCTION The tailoring of materials wettability has been the topic of numerous experimental and theoretical researches1 mainly devoted to the preparation of superhydrophobic surfaces.2−4 Superhydrophobic properties are observed when water contact angles (CAs) on a surface are larger than 150° and the CA hysteresis is very low. The superhydrophobicity is the result of a specially textured topography of the surface and of the presence of a low energy coating material.5 Superhydrophobic surfaces bear relevance to numerous applications like selfcleaning, protection of outdoor cultural heritage, anticorrosion coatings, and biomaterials.6−8 In several instances, however, the wettability of a material should be tuned to some predetermined level without reaching superhydrophobic conditions, which might imply the total lack of contact of water with the substrate, as the droplets roll off the surface. For example, the maximum adsorption of proteins, as well as cell adhesion and growth, is observed on a moderate hydrophilic surface with a water CA around 60°.9 Further, the tuned localization of hydrophobic/hydrophilic patches on a surface may control numerous physicochemical properties of a system, e.g., the condensation of water from the gas phase,10 the controlled adsorption of metal ions,11 or the transport of electrolytes and gas bubbles into interfaces such as porous environments or complex flow networks.12−14 Photo© 2012 American Chemical Society
lithography is one of the most advanced patterning techniques employed to obtain controlled localized hydrophobic/hydrophilic patches. Photochemical patterning without the use of the expensive photoresist can be performed by irradiating a selfassembled monolayer with ultraviolet (UV) light with a wavelength shorter than 185 nm; however, this approach is limited to specific functional groups and under constrained environments.15 In contrast, photoactive semiconductors can be directly patterned by the selective oxidation, supported by the photocatalyst itself, of chemisorbed hydrophobic monolayers in the presence of a patterning mask. This surface modification technique, besides bearing straightforward applications for offset printing and printed-circuit boards, may offer the starting point for additional functionalization of the underlying material.11 One of the aims of the present work is to test the photocatalytic lithography efficiency of the synthesized films. The control of the wettability by water has been extensively studied in the literature, while the preparation of oleophobic surfaces is by far less investigated, despite the numerous, economically relevant, potential applications in fields such as Received: September 21, 2012 Revised: November 26, 2012 Published: November 30, 2012 26405
dx.doi.org/10.1021/jp309397c | J. Phys. Chem. C 2012, 116, 26405−26413
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Table 1. List of Adopted Siloxanes
crude oil transfer, fluid power systems, antifouling materials, and also in several energy storage systems.16 Notwithstanding the very relevant applicative impact of nano-TiO2 films with tunable wettability for both water and nonaqueous solvents, to the authors best knowledge, only very few recent works deal with this topic.17−21 Most of these works18,20,21 rely on complex multistep synthetic procedures, often limited to a special kind of substrate: Wang et al. obtained engineered oleophobic TiO2 surfaces by a complex procedure implying a combination of anodization of a Ti foil and laser technology;18 also, Kim et al. modified metal Ti foils by electrochemical etching and hydrothermal processes;21 Zhang and coauthors obtain titania/single-walled carbon nanotube composites by complex processes.20 In the present work, the wettability features of rough functionalized TiO2 layers are tailored with respect to both water and nonaqueous solvents. The adopted synthetic procedure is very simple, and it may be extended to different substrates, as it is based on the direct functionalization of homemade, tailored, sol−gel nano-TiO2 particles by different siloxanes in isopropanol. The derivatizing molecules are different kinds of siloxanes, both fluorinated and nonfluorinated. The surface activity of the functionalizing molecules is studied by surface free energy (SFE) determinations obtained for the given molecule deposited onto smooth glass slides. The structure of the functionalizing layer and the attachment modes of Si atoms at the TiO2 surface are analyzed by combining data of solid-state nuclear magnetic resonance (NMR) analyses with the molecule surface energy components and CA measurements in several solvents. Finally, patterned structures with tunable hydrophobic and oleophobic patches are obtained by photocatalytic lithography. The resulting wetting contrast is exploited to obtain a site selective adsorption of a dye molecule.
ment and crystallite size determinations were performed as previously reported.22 Specific surface areas were determined by the classical Brunauer−Emmett−Teller (BET) procedure using a Coulter SA 3100 apparatus. Desorption isotherms were used to determine the pore size distribution using the Barrett, Joyner, and Halenda (BJH) method.23 Particle size distribution was evaluated by dynamic light scattering (DLS) by using a Beckman Coulter Nanosizer N5. High-resolution transmission electron microscopy (HRTEM) images of the as-synthesized TiO2 particles were acquired using a JEOL 3010 TEM microscope, 300 kV acceleration, and LaB6 single crystal filament. All samples were dry deposited on Cu holey carbon grids (200 mesh). For the evaluation of the roughness factor and surface topography of the composite samples, an atomic force microscopy (AFM) microscope (NTMDT Solver PRO-M) working in tapping mode was used; the roughness factor was obtained on areas of 5 × 5 μm2. The adopted siloxanes were commercial products by Sigma Aldrich (Table 1): trimethoxy(octadecyl)silane (Si-18C), triethoxy(octyl)silane (Si-8C), and 1H,1H,2H,2H-perfluorooctyl-triethoxysilane (Si-8C(F)). Nano-TiO2 was functionalized with the siloxane compounds using the following procedures. The titania powders (0.2 g) were dispersed in 2-propanol (10 mL). The selected amount of siloxane was then added to the TiO2 dispersion under vigorous stirring. The amount of added siloxane was calculated on a weight basis (33%). This value was selected on the grounds of previous results, relative to other siloxane molecules, which indicated an invariance of the hybrid features (CA, NMR, and infrared spectroscopy) for siloxane amounts around 30%.8 The solvent removal was achieved by means of a vacuum oven (at 400 mbar, 40 °C). The siloxane−TiO2 composite powders (0.1 g) were suspended in 2-propanol (2 mL), and the resulting dispersion was spin coated (2000 rpm, 20 s) onto previously cleaned glass slides to obtain thin films.24 CA measurements were performed by a Krüss Easy Drop instrument. Static CA determinations on the siloxane bare films deposited onto smooth glass slides were used to determine the SFE of the different organic molecules, according to the procedure reported by Lee et al.25 Dynamic CA measurements (advancing (θa) and receding (θr)) of hybrid siloxane−TiO2 films were obtained by the following procedure: a drop of 3 μL was placed on the surface (static CA), then the drop size was changed with a speed of 18 μL min−1 (dynamic CA). Movies with 100 images were recorded. The reported CAs values are the average of at least five independent determinations taken at different sample locations.
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EXPERIMENTAL SECTION All used chemicals were of reagent grade purity and were employed without further purification; doubly distilled water passed through a Milli-Q apparatus was used to prepare solutions and suspensions. Nanometric TiO2 powders were prepared by a sol−gel synthesis starting from Ti propoxide in 2-propanol by adding water fast under stirring at 65 °C at spontaneous pH. The resulting suspension was dried at room pressure at 90 °C and subsequently calcined at 300 °C for 5 h under O2 stream. Room-temperature X-ray powder diffraction (XRPD) patterns were collected between 10 and 80 °C with a Siemens D500 diffractometer, using Cu Kα radiation. Rietveld refine26406
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Figure 1. (a) HRTEM image of TiO2 particles; (b) AFM 3D image (5 × 5 μm2) of a TiO2 film.
photocatalytic activity. The final product shows a nanocrystalline structure with a controlled enrichment in anatase and brookite (75% anatase and 25% brookite) and a crystallite size of 6 nm (Figure S1, Supporting Information). The concomitant presence of different polymorphs is thought to enhance the photocatalytic activity of the oxide by reducing the recombination rate of the photogenerated electrons and holes.24 Moreover, this composite structure may provide random exposure of facets affecting the in-homogeneity of the texture and thus its wetting properties. The morphology of the titania particles was evaluated by HRTEM images (Figure 1a): the particles are clearly well separated, made up of medium closely packed spherical nanoparticles, exhibiting defined contours and characterized by a highly crystalline aspect. The average mean diameter of the pristine particles was estimated to be in the range 5−8 nm by statistical evaluation on several independent images. Inspection of the spacing distances most frequently observed indicates 0.352 to 0.355 nm: this distance is ascribable to the relevant TiO2 anatase polymorph29 and, namely, the (101) crystal planes, in both cases. Film morphology was studied by means of AFM. The AFM 3D (Figure 1b) image of the bare TiO2 film shows the complete coverage of the glass substrate by the titania film. The surface presents a multiscale roughness, due to the presence of both nanometric particles and micrometric aggregates. While the DLS analysis showed a monomodal distribution in the nanometer range, during the drying step, the particle selfassembled in aggregates of variable sizes resulting in multiscale roughness. The average roughness and the root-mean-square (RMS) are 120 and 150 nm, respectively. No activation treatment is needed prior to the siloxane deposition to improve the surface reactivity of the oxide nanoparticles, as the adopted synthetic conditions and mild calcination temperatures promote a high degree of surface hydroxylation. No effects are introduced in the structural features of TiO2 by siloxane addition, with respect to both the polymorphs enrichment and the crystallite size as it can be seen comparing XRPD spectra before and after functionalization (Figure S1, Supporting Information). The functionalization by the organic molecules affects instead the specific surface area of the particles and their porosity. The oxides modified by the two siloxanes bearing a C8 chain show a decrease in the surface area by about 80% with respect to the bare oxide, this decrease being mainly the result of the loss in pore volume (Figure 2). In the case of functionalization by T_Si-18C, the surface area drops severely, and it is reduced to a few square meters per gram. The
The structure and the attachment modes to the TiO2 surface of the siloxane molecules were investigated by solid-state NMR spectroscopy. Solid-state NMR spectra were obtained at 125.62 (13C) and 99.36 (29Si) MHz on a Bruker Avance 500 spectrometer, equipped with a 4 mm magic angle spinning (MAS) broadband probe (spinning rate νR up to 3 kHz). The MAS spectra were recorded on solid samples, typically 0.15 g. Each sample was packed into a 4 mm MAS rotor (50 μL sample volume) spinning at 3 kHz and at a temperature of 300 K; no resolution improvement was found at higher rate spinning and/or temperature. Variable amplitude cross-polarization (CP) method was used for recording the 29Si spectra, with 1 ms of CP contact time, 20.0 s of delay and 10 000 scans, while direct polarization (DP) method and proton decoupled mode was employed for 13C spectra with 2.0 s of delay and 15 000 scans. All chemical shifts were externally referenced to TMS. Self-cleaning tests, by methyl orange (dissolved in water) and methylene blue (dissolved in CH2I2), were performed on the titania films, prepared as reported above. The siloxane−TiO2 composite films were photocatalytically26 litographed by irradiating with an iron halogenide lamp (Jelosil HG 500), emitting in the UV-A (wavelength range, 315−400 nm), in the presence of paper mask onto the TiO2 film. The time of irradiation depended on the nature of the siloxane molecules. The irradiated areas could be wetted by water or CH2I2, and so colored by water or CH2I2 soluble dyes. The kinetics of the photodegradation of the siloxane on the TiO2 was studied. During 2 h, the loss of hydrophobicity and oleophobicity was investigated by the CA variation of water and diiodomethane, respectively. In addition, the functionalized TiO2 before and after UV irradiation was analyzed by Fourier transform infrared spectroscopy (FTIR) using a Jasco 4200 spectrometer, accessorized by attenuated total reflectance (ATR) module.
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RESULTS AND DISCUSSION Nano-TiO2 Composites: Synthesis and Characterization. The features of the particles were carefully tailored due to the key role played by the layer morphology on the wettability of the film. High water/alkoxide ratio and the solvent/water ratios (100 and 20, respectively) were used to provoke an extensive hydrolysis of the alkoxide, favoring nucleation vs particle growth, in order to produce small crystallites and, consequently, large particle surface area.27,28 The ensuing thermal treatment was performed to promote the crystallinity of the oxide, which is fundamental to obtain a good 26407
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These data are in agreement with AFM determinations, which show that the presence of the siloxane molecules does not significantly modify the roughness features. Wettability Features. Before considering the wettability of the nano-TiO2−-siloxane composites, the surface features of the bare siloxanes deposited onto smooth glass slides were investigated. According to Owens and Wendt’s approach,30 the measurement of CAs of test liquids, with known surface tension and relative components, enables the determination of SFE, as the sum of polar and disperse parts. Table 2 reports the total SFE and relative polar and disperse components obtained for the present siloxanes by using the Table 2. SFE Values for the Siloxane Films and Dipole and Polar Components by OWRK Model γs of siloxane films (mN m−1) siloxane
SFE
polar
disperse
Si-8C Si-8C(F) Si-18C
44 22 46
11 5 6
33 17 40
method reported by Lee et al.25 Nonfluorinated molecules (Si8C and Si-18C) show similar SFE values but a different polar/ disperse component ratio, the weight of the disperse component increasing with increasing the length of the alkyl chain. The fluorinated compound (Si-8C(F)), instead, exhibits the lowest total SFE and an almost halved disperse component with respect to the corresponding unfluorinated compound (Si8C) in agreement with the expected weak dispersive interactions of fluorocarbons.31,32 The different component partition appears directly by the comparison of the water and CH2I2 CAs (Figure S4, Supporting Information); the CA increases in passing from the nonfluorinated to the fluorinated molecules, especially for the nonaqueous solvent. For rough surfaces, the simple picture represented by Young’s relation33 fails. For very rough hydrophobic surfaces, air pockets remain trapped in the network created by the hydrophobic posts giving rise to a composite surface consisting of both solid and air, as proposed in the Cassie−Baxter model.34 The resulting material shows superhydrophobic behavior. For all the present TiO2−siloxane composites, the measurement of the water CA was not straightforward; in fact, because of the enhanced water repellency, water drops bounced and rolled off the layers. Even performing the measurements by rising, very slowly, the support up to the contact with the pendant drop, the simple detachment of the drop from the tip produced, almost invariably, rolling off of the drop. In Table 3, the values of water CAs are, in any case, reported as larger than 150°, with very low measured hystereses (150 >150 >150
