Air- and Light-Stable Superhydrophobic Colored Surfaces Based on

Dec 23, 2009 - Juan R. Sanchez-Valencia, Angel Barranco, Juan P. Espinos and Agustin R. Gonzalez-Elipe. Instituto de Materiales de Sevilla (CSIC-US), ...
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Air- and Light-Stable Superhydrophobic Colored Surfaces Based on Supported Organic Nanowires Ana Borras* and Pierangelo Gr€oning Empa, Swiss Federal Laboratories for Materials Testing and Research (nanotech@surfaces Laboratory), Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland

Juan R. Sanchez-Valencia, Angel Barranco, Juan P. Espinos, and Agustin R. Gonzalez-Elipe Instituto de Materiales de Sevilla (CSIC-US), Avd. Americo Vespucio 49, 41092 Sevilla, Spain Received September 30, 2009. Revised Manuscript Received December 11, 2009 In this work, we report on a new type of superhydrophobic material consisting of supported organic nanowires prepared by vacuum deposition. Different intensely colored surfaces with water contact angles as high as 180° can be fabricated depending on the composition, morphology, and density of the nanowires. These surfaces are stable in air and under intense light irradiation. The wettability properties of coatings made of metalloporphyrins and metallophthalocyanines nanowires as well as other heterostructured binary and open core@shell nanowires are studied.

Introduction Functional 1D organic nanostructures have deserved great attention during the past few years for their potential use in nanoscale optical and electronic devices. Among the organic molecules utilized as building blocks for organic nanostructures, those forming π-stacking crystal structures are especially relevant because of their outstanding conductive, magnetic, and optical properties.1 In general, the methodologies used for the growth of organic nanofibers and nanowires can be included into three different approaches: self-assembly via solution deposition, template methods, and vapor transport process.1,2 Nowadays, crystal 1D organic nanostructures formed by π-conjugated molecules are being successfully challenged in an ample collection of applications including active components for photonic devices, organic field effect transistors (OFETs), phototransistors, vapor (gas) nanosensors, and solar cells.1,2 In this work, we address the use of supported organic nanowires from a different point of view. In fact, to the best of our knowledge this is the first time that layers formed by a high density of porphyrins and phthalocyanine nanowires are employed as superhydrophobic surfaces (i.e., surfaces with a water contact angle higher than 150°). So far, two strategies have been utilized for the fabrication of superhydrophobic surfaces, namely, the chemical modification of the surface with the incorporation of hydrophobic chemical groups, for instance, in the form of self-assembled monolayers (SAMs), *Corresponding author. Tel: þ41332282706. Fax: þ41332284490. E-mail: [email protected]. (1) (a) Briseno, A. L.; Mannsfeld, S. C. B.; Jenekhe1, S. A.; Bao, Z.; Xia, Y. Mater. Today 2008, 11, 38. (b) Zhao, Y. S.; Peng, A.; Fu, H.; Ma, Y.; Yao, J. Adv. Mater. 2008, 20, 1661. (2) (a) Yan, P.; Chowdhury, A.; Holman, M. W.; Adams, D. M. J. Phys. Chem. B 2005, 109, 724. (b) Su, W.; Zhang, Y.; Zhao, C.; Li, X.; Jiang, J. ChemPhysChem 2007, 8, 1857. (c) Kriha, O.; Goring, P.; Milbradt, M.; Agarwal, S.; Steinhart, M.; Wehrspohn, R.; Wendorff, J. H.; Greiner, A. Chem. Mater. 2008, 20, 1076. (d) Tang, Q.; Li, H.; Song, Y.; Hu, W.; Jiang, L.; Liu, Y.; Wang, X.; Zhu, D. Adv. Mater. 2006, 18, 3010. (e) Tong, W. Y.; Djurisic, A. B.; Xie, M. H.; Ng, A. M. C.; Cheung, K. Y.; Chan, W. K.; Leung, Y. H.; Lin, H. W.; Gwo, S. J. Phys. Chem. B 2006, 110, 17406. (f) Xiao, K.; Tao, J.; Pan, Z.; Puretzky, A. A.; Ivanov, I. N.; Pennycook, S. J.; Geohegan, D. B. Angew. Chem. 2007, 119, 2704. (3) (a) Blossey, R. Nature 2003, 2, 301. (b) Li, X. -M.; Reinhoudt, D.; CregoCalama, M. Chem. Soc. Rev. 2007, 36, 1350. (c) Marmur, A. Langmuir 2008, 24, 7573.

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and the modification of the surface topography.3 Hybrid approaches involving both chemical modifications and the control of topography have been developed as well.4 Besides, some developments of superhydrophobic materials based on nano- and microfibrous structures have been published recently.5 Superhydrophobic materials are of the utmost interest in both academia and industry because of their wide applications as water-repellent and stainless coatings, self-cleaning and antifogging surface designs, and laboratory-on-a-chip devices. As a matter of fact, Blossey wrote in 20033a that key advances in the understanding and fabrication of surfaces with controlled wetting properties are about to make true the dream of a contamination-free surface. Microfluidics, a currently expanding topic, would also benefit from advances in this subject.3 The optical properties of superhydrophobic surfaces such as transparency, reflectance, and fluorescence can be a determinant in the control of their performance for many of the applications listed above. In this context, the use of organic and hybrid colored layers has been suggested to be an environmentally friendly candidate for the decoration of industrial and domestic glassware.6 The first advantage of these coatings is that they provide access to a wide variety of colors. In addition, colored glassware based on organic and hybrid dyes would be straightforwardly recyclable when used on uncolored glass substrates, thus avoiding coloration by transition metals or other glass pigments or impurities that are very difficult to remove or remelt.6 Moreover, the use of porphyrins and phthalocyanines for this application has important advantages such as their low cost and facile preparation on large scales, their chemical and thermal stability, and the fact that many of them are nontoxic and biocompatible. (4) Coffinier, Y.; Janel, S.; Addad, A.; Blossey, R.; Gengembre, L.; Payen, E.; Boukherroub, R. Langmuir 2007, 23, 1608. (5) (a) Borras, A.; Barranco, A.; Gonzalez-Elipe, A. R. Langmuir 2008, 24, 8021. (b) Yuan, J.; Liu, X. G.; Akbulut, O.; Hu, J.; Suib, S. L.; Kong, J.; Stellacci, F. Nat. Nanotechnol. 2008, 3, 332. (c) Li, J.; Sambandam, S.; Lu, W. J.; Lukehart, C. M. Adv. Mater. 2008, 20, 420. (d) Ma, M.; Hill, R. M.; Rutledge, G. C. J. Adhes. Sci. Technol. 2008, 22, 1799. (e) Zhu, Y.; Li, J.; Wan, M.; Jiang, L. Macromol. Rapid Commun. 2008, 29, 239. (f) Han, D.; Steckl, A. J. Langmuir 2009, 25, 9454. (g) Gao, L. C.; Fadeev, A. Y.; McCarthy, T. J. MRS Bull. 2008, 33, 747. (6) Sanchez, C.; Lebeau, B.; Chaput, F.; Boilot, J. P. Adv. Mater. 2003, 15, 1969.

Published on Web 12/23/2009

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In this work, we present a new approach to the fabrication of superhydrophobic colored surfaces based on the formation of a high-density organic nanowire (ONW) film. First we show the dependence of the water contact angle of a surface formed by a high density of organic nanowires on the composition of such nanowires: “simple” organic nanowire films formed by metalloporphyrins (yellow, orange, red, and pink) and metallophthalocyanines (blue and green), “binary” organic nanowires films (red), and “open core@shell” organic nanowire films (blue). Secondly, we study the dependence of the water contact angle (WCA) on the density of the organic nanowires. Finally, we characterize how these coatings would behave under ambient conditions and the effect on the wettability properties of the UV and visible irradiation.

Scheme 1. Chemical Structure of the Organic Molecules Used as Building Blocks of Organic Nanowires

Experimental Section Nanowire Deposition. The Ag substrates were prepared by dc sputtering and underwent an oxygen plasma treatment before nanowire deposition.7,8a Columnar TiO2 thin films were deposited by plasma-enhanced chemical vapor deposition in a microwave electron cyclotron resonance (MW-ECR) plasma reactor at room temperature.8b-8d The fabrication of the nanowires is based on the sublimation of the dye molecules in an Ar atmosphere. The dyes molecules were placed in a 10 mL Oled Knudsen cell (Lesker) in a high-vacuum chamber pumped up to 5  10-6 mbar before the deposition. The distance between the substrate and the sublimation source was 8 cm. The Ar pressure in the chamber during the deposition was fixed at 0.020 mbar using a calibrated mass flow controller (MKS). The growth rate was controlled by a quartz crystal microbalance located in the same plane as the substrates. The deposition rates ranged between 0.30 and 0.50 A˚ s-1 considering a density in the balance of 0.50 g cm-3. These experimental parameters were common to all of the porphyrins (Frontier Sci. and Aldrich) and phthalocyanines (Aldrich) studied in this work. These materials were used as purchased. The temperatures of the substrates during deposition ranged between 130 and 150 °C for the porphyrins and between 215 and 260 °C for the phthalocyanines. The optimal thickness of the silver layer depends on the composition of the nanowires. In this work, three different thicknesses have been used in the deposition of the nanowires: Ag(1) < 15 nm and Ag(2) ∼30 nm for porphyrins and Ag(3) ∼60 nm for phthalocyanines. These values refer to the nominal thickness of the layer deposited by dc sputtering and measured by SEM before the plasma treatment.7 Sample Characterization. SEM images were obtained in a Hitachi S4800. The UV-vis transmission spectra of samples deposited on glass slides were recorded in a Cary 50 spectrophotometer in the range from 200 to 1100 nm. The XPS study of the samples was carried out with a VG Escalab 210 XPS spectrometer operated at a constant pass energy of 20 eV. Nonmonochromatized Mg Kr radiation was used as the excitation source. Each reported static water contact angle is the mean value of at least five measurements by the Young method of droplets with volumes of ∼5 μL. The contact angle hysteresis (CAH, difference between the advancing and receding angles) and sliding angles were measure with droplets with volumes of ∼8 to 9 μL. Water droplets were placed on the surface of the samples, and their contact angles were measured with a CAM100 instrument (KSV (7) (a) Borras, A.; Aguirre, M.; Groening, O.; Lopez-Cartes, C.; Groening, P. Chem. Mater. 2008, 20, 7371. (b) Borras, A.; Groening, O.; Koeble, J.; Groening, P. Adv. Mater., in press, 2009, DOI: 10.1002/adma.200901724. (c) Borras, A.; Groening, O.; Aguirre, M.; Gramm, F.; Groening, P., Langmuir, submitted for publication. (8) (a) Borras, A.; Barranco, A.; Espinos, J. P.; Cotrino, J.; Holgado, J. P.; Gonzalez-Elipe, A. R. Plasma Process. Polym. 2007, 4, 515. (b) Borras, A.; YanguasGil, A.; Barranco, A.; Cotrino, J.; Gonzalez-Elipe, A. R. Phys. Rev. B 2007, 76, 235303. (c) Sanchez-Valencia, J. R.; Borras, A.; Barranco, A.; Rico, V. J.; Espinos, J. P.; Gonzalez-Elipe, A. R. Langmuir 2008, 24, 9460. (d) Borras, A.; Sanchez-Valencia, J. R.; Garrido, J.; Barranco, A.; Gonzalez-Elipe, A. R. Microporous Mesoporous Mater. 2009, 118, 314.

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Instruments Ltd., Finland). The angle of the sample for the sliding-angle measurements was controlled by using an ad hoc sample holder. The illumination of the sample PtOEP nanowire film on TiO2 was carried out with a Xe lamp. The photon intensity at the position of the samples was 2 W cm-2 for the complete spectrum of the lamp (i.e., UV, visible, and IR photons) and 1.6 W cm-2 with the UV filter. The lamp provides UV photons with wavelengths higher than 200 nm and a constant transmission between ∼400 and 800 nm when actuated with the UV filter.

Results and Discussion Very recently, we have developed a vacuum process for the growth of supported organic squared nanowires and nanobelts formed by π-conjugated molecules.7 This methodology is a general process in two different respects: (i) it is applicable to the growth of organic nanowires based on different π-conjugated molecules and (ii) the fabrication of organic nanowires is independent of the substrate’s chemical nature. We have already demonstrated the growth of porphyrins and metalloporphyrins (M-OEP with M: Pt and Pd), phthalocyanines and metallophthalocyanines (M-Pc with M: Co, Fe and Cu), and perylene (Me-PTCDI) nanowires on silver-decorated silicon and glass substrates (Ag/Si) and on TiO2, SiO2, ZnO optical coatings, and other oxide thin films. In the present work, we have also included the formation of F16CuPc nanowires. (The chemical structure of the molecules is presented in Scheme 1.) In Figure 1, the growth of PtOEP nanowires at 140 °C on different substrates is shown. In this Figure, the formation of a high density of 1D nanostructures (∼109 NWs cm-1) on both Ag/Si and TiO2 thin films (Figure 1a, c-f)), is apparent. The silver-decorated silicon substrates were prepared by dc sputtering of silver and subsequent treatment with Ar/O2 plasma. In the case of PtOEP, a silver layer with a thickness of ∼30 nm was required to achieve a high density of nanowires (i.e., silver substrate named Ag(2); see the Experimental Section).7,8a The Langmuir 2010, 26(3), 1487–1492

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Figure 1. (a) Planar view SEM micrograph of the surface containing a high density of PtOEP nanowires on Ag/Si substrates; the inset shows a water droplet on this surface. (b) Cross-sectional SEM micrograph of a PtOEP thin film on Si(100). (c, d) Crosssectional SEM micrographs of PtOEP nanowires on Ag/Si. (e, f) Idem on columnar TiO2 substrates.

columnar TiO2 thin film was deposited by plasma-enhanced chemical vapor deposition (PECVD) at room temperature.8b-8d As we have previously demonstrated, the organic nanowires are single-crystalline.7 Squared nanowires and nanobelts are formed at random angles with respect to the substrate surface (Figure 1c-f). However, a PtOEP thin film is obtained under the same experimental conditions on Si(100) where no nanowires can be found (Figure 1b). The water contact angle measured in air on the layer formed by a high density of PtOEP nanowires is virtually 180° (Figure 1a inset)). Because of the high hydrophobicity of the surface, the droplets are bounced off of the sample. (See the movie in the Supporting Information.) To capture a droplet on the surface of the PtOEP nanowire film, we increased its volume; consequently, the inset of Figure 1a shows a droplet deformed by gravity effects. A much lower value of ∼104° for the CA was measured in the equivalent thin film of PtOEP, as summarized in Table S1. High-density ONW films formed by other molecules showing π stacking were fabricated following the same protocol. Table S1 in the Supporting Information lists the different samples under study with a short description of the experimental conditions used for the growth and measurement of contact angles. The requirements to comply to a superhydrophobic surface are a water contact angle higher than 150°; small contact angle hysteresis (i.e., a small difference between the advancing and receding contact angle), by convention less than 10°; and finally, a low threshold sliding angle (the angle at which droplets of determined sizes start sliding as the surface is tilted).3-5 Contact angle hysteresis and sliding angle measurements are included in the characterization of a surface contact angle by dynamic methods. Because the main aim of this preliminary work is to obtain superhydrophobic surfaces based on organic nanowires, we just include here the analysis of the dynamic contact angle of Langmuir 2010, 26(3), 1487–1492

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pure water in the case of superhydrophobic surfaces (Table S1). A full characterization of wettability including an analysis of the surface tension for different compositions of organic nanowires is outside the proposed scope of this letter and will be the subject of forthcoming work. The first rows in Table S1 account for the ONW films formed by simple nanowires (i.e., nanowires produced by the repetitive π stacking of the same type of molecules). The WCA values corresponding to the thin film counterpart are included for comparison in those cases where the measurement was reliable. (The low adhesion of the crystals forming the OEP thin films (cf. Figure 1b) hindered the measurement of the CA in this case.) In principle, hydrophobic behavior is expected for a flat, homogeneous surface formed by porphyrins and phthalocyanines molecules because of their nonpolar character. This behavior is proven by the WCAs measured for the flat films of single molecules, which are always between 93 and 110°. Taking these reference values into account, a very important result in Table S1 is that the ONW films, with WCAs between 131 and 180°, have much higher values than the flat thin films prepared under the same experimental conditions on flat substrates. This result can be explained within the framework of the Wenzel model,3,5a,9 as discussed in detail below. In general, the WCA of a rough surface is expected to increase with respect to the same material forming a flat surface when the material in question is hydrophobic. Table S1 also indicates that in our case there is also a relationship between the CA and the chemical composition of the nanostructures. Thus, the surfaces formed by ONWs of OEP, M-OEP, and F16CuPc are superhydrophobic (i.e., water CAs higher than 150°). The virtual 180° shown by the PtOEP and F16CuPc ONW films is particularly interesting. For these samples, the micrometric water droplets bounce off of the surfaces, making the WCA measurement extremely difficult. (Please note that in the movie in the Supporting Information the droplets are macroscopic.) From now on, we write 180° in italics to mark this behavior. In the case of F16CuPc, both the high roughness due to the nanowires and the presence of highly hydrophobic elements such as F seem to be the main factors contributing to the high superhydrophobicity of these surfaces. However, the difference in the CA between the PtOEP ONW film and the rest of the layers may be related to the superior stability of PtOEP under room conditions. The water CA measurements were carried out at room temperature on the samples handled in air and not stored in the dark. Very likely, under such conditions the organic nanowires undergo a superficial partial oxidation process. Figure S1 gathers the photoemission peaks corresponding to the cation in the center of the molecule for the PtOEP, PdOEP, and CuPc ONW films analyzed by X-ray photoelectron spectroscopy (XPS) in samples stored in air for more than 1 month. The binding energy (BE) values in eV were calibrated in all of these samples positioning the corresponding C 1s peaks at 284.5 eV. The chemical state of Pt, Pd, and Cu can be established by a comparison of the binding energies of the main peaks in Figure S1 with those in the literature for different states of oxidation of such metal elements.11 In Table S2 of the Supporting Information section, the different binding energy values for Pt, Pd, and Cu and the different oxides are summarized. The results indicate that the nanowires formed by PtOEP molecules are more inert to oxidation in air after relatively long storage times. Figure 2 shows pictures of the colored samples corresponding to different nanowire compositions. The UV-vis transmission (9) (a) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (b) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754.

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Figure 2. (a) Color picture of the ONW film grown on a silver film deposited on glass slices to show its high homogeneity over the covered area. (b) Colored NWs films corresponding to different molecules and different heterostructures as labeled. (c) UV-vis transmission curves for organic nanowires films on TiO2 thin films as labeled.

spectrum of selected samples is presented in Figure S1. One of the main advantages of our method of fabrication is that it provides high homogeneity over large deposition areas (Figure 2 top). As shown in Figure 2 bottom, different colors can be obtained by this method. The growth methodology also enables the fabrication of new families of 1D heterostructures in the form of binary and open core@shell nanowires, among others.7 The binary nanowires are composed of either two porphyrins or two phthalocyanines deposited simultaneously to form a single-crystalline nanowire with similar characteristics to those presented in Figure 1. (See Figure S2 reporting a high-density film of binary nanowires of PtOEP and PdOEP.) The water CA of the binary PtOEP and PdOEP ONW film was 180°. Therefore, the global wetting behavior of its surface is like that of the ONW films of PtOEP. Such a result implies that superhydrophobic surfaces can be fabricated by the addition of PtOEP molecules to ONWs formed by other metal porphyrins in such a way that their optical properties and color are preserved with very few changes (cf. Figure S2 and Figure 2b). One of the requirements for the growth of binary nanowires is that it involves miscible molecules. However, the fabrication of the open core@shell nanowires is not constricted in this regard because it is based on the neighboring development of a squared nanowire and a nanobelt.7a-7c We use the term open core@shell with reference to the morphology of the nanowires, an inner wire (core) partially wrapped with a belt 1490 DOI: 10.1021/la903701j

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(open shell) (refs 7aand 7 c), and SEM micrographs in Figure S3. A high density of these nanostructures can be fabricated by properly selecting the experimental conditions and the substrates. In the case of inner nanowires formed by PtOEP, the water CA of the samples formed by open core@shell reaches a virtual value of 180°. Moreover, the color of the samples is mostly due to the belts because the number of PtOEP molecules is relatively very low (Figure 2b). Finally, a color such as purple can be achieved by the growth of a similar number of phthalocyanine and porphyrins nanowires. This is the case for the sample labeled M1-PtOEPCoPc (Figures 2 and S4), where PtOEP squared nanowires and nanobelts can be found simultaneously with equivalent structures of CoPc. Besides the nanowire compositions, obtaining superhydrophobic surfaces also depends on the roughness of the films. The growth of ONWs by our methodology is a thermally activated process determined, among other factors, by the morphology of the substrate.7 In the case of the ONW films deposited on heterostructured silver/Si substrates, the amount of silver is one of the most critical factors controlling the density of the nanowires.7a,7c The explanation of this phenomenological finding relies on the growth mechanism proposed in ref 7c. The formation of the nanowires is basically a crystallization process where the heterogeneities of the substrate act as nucleation sites for the organic nanowires. In this case, different amounts of silver deposited on the Si(100) substrates are related to different distributions of the nucleation sites of the organic nanowires. Moreover, the optimal experimental conditions (temperature of the substrates, distribution of the nucleation sites, growth rates, etc.) depend strongly on the molecular chemical composition and structure.7a,7c Figure 3 shows planar SEM views of three PtOEP samples fabricated during the same experiment on different substrates and their corresponding water CA. On the silver-free Si(100) substrate (Figure 3a), the deposition of PtOEP results in a rather homogeneous film where very few nanowires have grown. Meanwhile, for higher amounts of silver Ag(1) (Figure 3b, sample M2-PtOEP in Table S1), film deposition is accompanied by the formation of squared nanowires and nanobelts that render a rougher surface and a higher CA value. Finally, for the substrate labeled Ag(2) (i.e., Ag thickness ∼30 nm), a very high density of nanowires is achieved, no flat thin film deposition remains, and the CA of this sample is virtually 180°. Although the thin film in Figure 3a is not completely smooth, in the following discussion we will adopt this film as the flat, homogeneous counterpart of the nanowire film surfaces. According to the Wenzel model3,5a,9 the hydrophobicity can be related to the development of water/air and water/material local interfaces in rough surfaces. The direct correlation between the water contact angle and the number of nanowires per square micrometer agrees with the predictions of this model applied to an idealized surface accounting for the type of roughness of our material. Such an idealized surface would consist of homogeneous and cylindrical nanowires standing perpendicular to the substrate. The main assumption of the Wenzel model is that the actual contact angle on a real surface (θ0 ) can be related to the contact angle on a flat surface of the same material (θ) through a roughness parameter rW according to the following expression: cosðθ0 Þ ¼ rW cosðθÞ

ð1Þ

The rW parameter is defined as the ratio between the total area of the surface and the geometric projected area. On our idealized surface, rW depends upon the thickness, length, and number of nanowires. A straightforward estimation of the CA in our Langmuir 2010, 26(3), 1487–1492

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Figure 3. (a-c) Normal view micrographs and water contact angle of PtOEP NW films deposited at 140 °C on different substrates as labeled. (d-f) Parametric curves showing the dependency of the rW parameter on the density of the nanowires. (The horizontal line corresponds to an rW value of ∼4, at which the contact angle of the surface reaches ∼180°).

ONW-film system can be made by assuming nanowires perpendicular to a flat substrate with a constant diameter D and length L.5a On the basis of such considerations, rW can be expressed as rW ¼ 1 þ nπLD

ð2Þ

In this equation, n is the surface concentration of nanowires per square micrometer. The dependence of eq 2 on n, L, and D is presented in Figure 3d-f. This Figure shows that the contact angle on the surface of the idealized system tends to increase with the surface density of the nanowires as well as their width and length. In eq 1, by substitution of the values corresponding to θ0 ∼180° (we adopt 179° to avoid incongruence) and θ ∼104° (Table S1) an rW value of ∼4 is obtained. It is important to remark that the only experimental WCA measurement taken into account in this calculation is that corresponding to the equivalent flat surface; therefore, the error in the value given for rW ∼4 is independently of the precision of the characterization of the WCA of the superhydrophobic surfaces. In the case of M1PtOEP, we can assume average values of L = 3.5 μm (Figure 1) and D = 0.055 μm (estimated from the thickness distribution histogram reported in Figure S5). For these values, a minimum density of 5 NW μm-2 is required to reach the virtual value of 180° in the CA. After Figure 3c, such a threshold value for the density of nanowires is clearly reached in sample M1-PtOEP where n > 15. By contrast, the geometrical characteristics of the nanowires in sample M2-PtOEP (L = 0.9 μm and D = 0.085 μm) would require higher density values (Figure 3f) to reach the superhydrophobic state, and experimentally, we have found a value of 133° for the CA of this film. The good agreement between our experimental findings with respect to the CA values and the predictions of the Wenzel model clearly proves that the density of nanowires is a critical magnitude for the control of hydrophobicity. From a practical point of view, it also opens the way for the tailored synthesis of superhydrophobic surfaces with a Langmuir 2010, 26(3), 1487–1492

predetermined value of CA by tuning the density and morphology of surface nanowires. As is well known, the water contact angle of TiO2 can be controlled through the illumination of its surface with UV and visible light. Numerous papers in the literature have reported the conversion of TiO2 from partially hydrophobic to hydrophilic under UV light treatment under ambient conditions and its recovery by storage in the dark, heating in air, or illumination with visible light.5a,10 Such behavior is also shown by other oxide thin films as reported elsewhere.12 Figure 1e,f has clearly shown that a high density of crystalline organic nanowires can be obtained on different oxide thin films of controlled microstructure. With the aim of testing the stability of the superhydrophobic behavior of the Pt-OEP ONW films under ambient conditions, we have carried out different UV and visible light irradiation experiments in air with Pt-OEP ONW films on TiO2 thin film substrates. The evolution of the CAs of these films as a function of the illumination time is presented in Figure 4. The results in Figure 4 reveal that the CA of the PtOEP nanowire surfaces remains constant at ∼180°, even though the corresponding CA of the TiO2 substrate undergoes a drastic decrease during the same experiments. The constancy of CA was also found after prolonged irradiation of the other porphyrin and phthalocyanine nanowire surfaces. These results indicate that the light dose converting semiconducting inorganic substrates into hydrophilic substrates does not affect the semiconducting organic NW film. Further measurements of the CA carried out at periods (10) (a) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Adv. Mater. 1998, 10, 135. (b) Stevens, N.; Priest, C. I.; Sedev, R.; Ralston, J. Langmuir 2003, 19, 3272. (11) Practical Surface Analysis; Briggs, D., Seah, M. P., Eds.; Wiley: New York, 1990. (12) (a) Rico, V.; Lopez, C.; Borras, A.; Espinos, J. P.; Gonzalez-Elipe, A. R. Sol. Energy Mater. Sol. Cells 2006, 90, 2944. (b) Rico, V.; Borras, A.; Yubero, F.; Espinos, J. P.; Frutos, F.; Gonzalez-Elipe, A. R. J. Phys. Chem. C 2009, 113, 3775.

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nanowires of porphyrins and phthalocyanines grown by physical vapor deposition. Superhydrophobic surfaces with contact angles as high as 180° have been obtained with PtOEP nanowires, adding PtEOP molecules during the growth of other porphyrin and phthalocyanine nanowires, and with fluorinated phthalocyanines. As a result, an ample variability of colored surfaces displaying water contact angles higher than 150° have been fabricated. A model to account for the values of the water contact angles has also been proposed. The surfaces are air-stable, chemically inert, and very robust to visible and UV light irradiation. Moreover, the methodology permits us to grow superhydrophobic ONW films on hydrophilic substrates such as SiO2. The combination of superhydrophobic films with hydrophobic, hydrophilic, or reversible substrates can find straightforward applications in microfluidics applications and as decorative or optical coatings. Figure 4. Evolution of the water contact angle under UV and visible illumination measured on amorphous TiO2 thin films with and without PtOEP NWs. The changes in the CA of the TiO2 thin films agree with the hydrophobic/hydrophilic reversibility of TiO2 under UV/vis irradiation. Note that CA corresponding to the PtOEP NWs remains unalterable under both UV and visible irradiation.

Acknowledgment. We thank the EU for financial support (PHODYE STREP project contract no. 033793) and acknowledge Spanish national grants (MAT2007-65764, Consolider-Ing. 2010-CSD2008-00023, and TEP2275) and the Domingo Martinez Foundation.

of longer than a year after the preparation of the samples have also shown that the hydrophobic and/or superhydrophobic character of the samples containing porphyrin and phthalocyanine nanowires is not affected by the aging time.

Supporting Information Available: Summary of the WCA values (static and dynamic measurements) for different nanowire thin film compositions. Chemical structure, UV-vis transmission spectra, and additional characterizations. A movie showing macroscopic droplets bouncing off of the surface of the PtOEP nanowire film. This material is available free of charge via the Internet at http://pubs.acs. org.

Conclusions We have studied the wettability properties of the surfaces formed by a high density of single-crystalline organic

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