Reversible Superhydrophobic to Superhydrophilic Conversion of Ag

Jun 25, 2008 - Reversible Superhydrophobic to Superhydrophilic Conversion of Ag@TiO2 Composite Nanofiber Surfaces. Ana Borras*, Angel Barranco and ...
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Langmuir 2008, 24, 8021-8026

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Reversible Superhydrophobic to Superhydrophilic Conversion of Ag@TiO2 Composite Nanofiber Surfaces Ana Borras,*,†,‡ Angel Barranco,† and Agustı´n R. Gonza´lez-Elipe† Instituto de Ciencia de Materiales de SeVilla, CSIC-UniVersidad de SeVilla, AVda. Ame´rico Vespucio 49, 41092 SeVilla, Spain, and Nanotech@surfaces Laboratory, EMPA Materials Science and Technology, Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland ReceiVed January 13, 2008. ReVised Manuscript ReceiVed March 26, 2008 A new type of superhydrophobic material consisting of a surface with supported Ag@TiO2 core-shell nanofibers has been prepared at low temperature by plasma-enhanced chemical vapor deposition (PECVD). The fibers are formed by an inner nanocrystalline silver thread which is covered by a TiO2 overlayer. Water contact angles depend on the width of the fibers and on their surface concentration, reaching a maximum wetting angle close to 180° for a surface concentration of ∼15 fibers µm-2 and a thickness of 200 nm. When irradiated with UV light, these surfaces become superhydrophilic (i.e., 0° contact angle). The decrease rate of the contact angle depends on both the crystalline state of the titania and on the size of the individual TiO2 domains covering the fibers. To the best of our knowledge, this is one of the few examples existing in the literature where a superhydrophobic surface transforms reversibly into a superhydrophilic one as an effect of light irradiation.

Introduction Superhydrophobic materials, characterized by water contact angles higher than 150°,1 are of the utmost interest from both fundamental and applied points of view.1–7 Owing to their ample use as super water-repellent surfaces, as antistain surfaces, for tribological applications, etc., much effort has been dedicated during the recent years to the artificial synthesis of superhydrophobic materials.7–11 Two strategies have been utilized with this purpose, namely, the chemical modification of the surface with the incorporation of hydrophobic chemical groups and the modification of the surface topography.7–14 Hybrid approaches implying both chemical modifications and control of topography have also been developed.7 Particularly in connection with the control of topography, the development of bioinspired procedures for surface modification has been a very successful approach with still many open possibilities.15,16 Accounting for the reasons * Corresponding author. E-mail: [email protected]. Tel: +41332282706. Fax: +41332284490. † CSIC-Universidad de Sevilla. ‡ EMPA Materials Science and Technology. 31.

(1) Blossey, R. Nat. Mater. 2003, 2, 301. (2) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132,

(3) Yaminsky, V.; Ohnishi, S.; Ninham, B. In Long-range hydrophobic forces due to capillary bridging. Handbook of Surf. and Interf. of Mater.; Nalwa, H. S., Eds.; Academic Press: San Diego, 2001; Vol. 4, p 131. (4) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (5) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. AdV. Mater. 2004, 16, 1929. (6) Gao, L.; McCarthy, T. J. J. Am. Chem. Soc. 2006, 128, 9052. (7) Coffinier, Y.; Janel, S.; Addad, A.; Blossey, R.; Gengembre, L.; Payen, E.; Boukherroub, R. Langmuir 2007, 23, 1608. (8) Woodward, I.; Schofield, W. C. E.; Roucoules, V.; Badyal, J. P. S. Langmuir 2003, 19, 3432. (9) Coulson, S. R.; Woodward, I. S.; Badyal, J. P. S.; Brewer, S. A.; Willis, C. Langmuir 2000, 16, 6287. (10) Lee, W.; Jin, M.-K.; Yoo, W.-Ch.; Jang, E-S.; Choy, J.-H.; Kim, J.-H.; Char, K.; Lee, J.-K Langmuir 2004, 20, 287. (11) Wu, Y.; Sugimura, H.; Inoue, Y.; Takai, O. Chem. Vap. Deposition 2002, 8, 47. (12) Tae, G.; Lammertink, R. G. H.; Kornfield, J. A.; Hubbell, J. A. AdV. Mater. 2003, 15, 66. (13) Greene, G.; Yao, G.; Tannenbaum, R. Langmuir 2003, 19, 5869. (14) Yu, K.; Zhiqiang, W.; Jiang, Y.; Zhang, X. Langmuir 2006, 22, 4483. (15) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (16) Gao, X.; Jiang, L. Nature 2004, 432, 36.

of the superhydrophobicity of modified surfaces is also a very active field of research where theoretical and experimental contributions are required.7–17 The quest for materials that may change their contact angle in a reversible and controllable way has also been a challenge in many of the late investigations. Electrical fields or light irradiation have been used for the transformation of surfaces from hydrophobic to hydrophilic and vice versa.18,19 In this context surfaces containing nanotubes and nanopillars of TiO2 or ZnO have been reported to present a superhydrophobic response (water contact angles around 165°)20–23 and to transform reversibly into superhydrophilic (water contact angle smaller than 10°) when subjected to UV light irradiation. Preparation techniques of these materials include electrochemical anodization of Ti20,21 and sol-gel.22,23 Recently, we reported the low-temperature synthesis by plasma-enhanced chemical vapor deposition (PECVD) of Ag@TiO2 nanofibers supported on a silver substrate (hereafter, we will use the notation Ag@TiO2 for core-shell nanofibers and Ag/TiO2 for composite materials consisting of TiO2 deposited on silver substrates which, in most cases, have nanofibers on their surface). Depending on the conditions, a high concentration of nanofibers, ranging between 2 and 15 fibers µm-2, formed by a central core of silver and a TiO2 overlayer, have been obtained.24,25 Since its external surface consists of TiO2, it is expected that it presents the hydrophobic-hydrophilic transformation typical of this oxide when it is subjected to UV ¨ ner, D.; Youngblood, J.; McCarthy, (17) Chen, W.; Fadeev, Y.; Hsieh, M. C.; O T. J. Langmuir 1999, 15, 3395. (18) Lahann, J.; Mitragotri, S.; Tran, T.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. A. Science 2003, 299, 371. (19) Crevoisier, D.; Fabre, P.; Corpart, J.; Leibler, L. Science 1999, 285, 1246. (20) Balaur, E.; Macak, J. M.; Tsuchiya, H.; Schmuki, P. J. Mater. Chem. 2005, 15, 4488. (21) Balaur, E.; Macak, J. M.; Taveira, L.; Schmuki, P. Electrochem. Commun. 2005, 7, 1066. (22) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2004, 126, 62. (23) Liu, H.; Feng, L.; Zhai, J.; Jinag, L.; Zhu, D. Langmuir 2004, 20, 5659. (24) Borras, A.; Barranco, A.; Yubero, F.; Gonzalez-Elipe, A. R. Nanotechnology 2006, 17, 3518. (25) Borras, A.; Barranco, A.; Espinos, J. P.; Cotrino, J.; Holgado, J. P.; Gonza´lez-Elipe, A. R. Plasma Process. Polym. 2007, 4, 515.

10.1021/la800113n CCC: $40.75  2008 American Chemical Society Published on Web 06/25/2008

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irradiation.26–30 In the present work we study the evolution of the water contact angle on the surface of this Ag/TiO2 composite material and correlate it with the thickness and surface concentration of fibers present on its surface. The increase in contact angle up to values close to 180° (i.e., superhydrophobicity) is explained by means of a simple model based on the considerations of Wenzel1,2,31 about the effect of roughness on the wetting angles. Another characteristic feature of these composite surfaces is that they transform from superhydrophobic into superhydrophilic upon UV light irradiation. This constitutes one of the few examples in the literature where such a reversible superhydrophobic-superhydrophilic transformation has been reported.20–23

Experimental Methods Details about the synthesis by PECVD and characterization of Ag/TiO2 composite materials have been reported in two recently published works by our group.24,25 For the objectives of the present work it is important to comment that surfaces with supported nanofibers develop on a rough silver substrate subjected to preoxidation with a plasma of oxygen. TiO2 is then plasma-deposited on these surfaces by using a Ti isopropoxide precursor (TTIP) and plasmas of oxygen or mixtures of Ar + O2 as plasma gas. A high concentration of Ag@TiO2 core-shell nanofibers develop on this surface when the deposition is carried out with the substrate at a temperature in the range between 403 and 523 K. At the latter temperature the deposited titania is crystalline (anatase). The central core of the fibers consists of a silver thread of about 20-30 nm width and a length of 1-3 µm. The TiO2 overlayer has a variable thickness depending on the deposition time.25 Surfaces containing fibers with a width between 30 and 400 nm depending on the sample have been used for water contact angle experiments. As materials for reference, TiO2 was simultaneously deposited on flat Si(100) substrates. These samples were studied by cross-sectional SEM to estimate the “equivalent” thickness of the TiO2 layer deposited on flat surfaces. The obtained value was proportional to the thickness of the TiO2 shell in the Ag@TiO2 nanofibers. Cross-sectional and planar view SEM micrographs were obtained in an Hitachi S-5200 field emission microscope. Water contact angles have been determined by the Young equation1,2 on samples that were stored in the dark for a considerable period of time. This storage period was necessary because the “asprepared” samples where superhydrophilic as a result of the illumination with UV photons during their preparation in a plasma environment.32 Water droplets of 5 µL were placed on the surface of the samples and their contact angle measured with a CAM100 instrument (KSV Instruments Ltd., Finland). Illumination of the samples during defined periods of time was carried out with a Xe lamp providing a photon intensity at the position of the samples of 2 W cm-2 for the complete spectrum of the lamp (i.e., UV, visible, and IR regions) and 1.6 W cm-2 when a UV filter was used. For simplicity we will write UV illumination when the complete spectrum is used and VIS illumination when a UV filter is placed between the lamp and the sample. The surface state of the Ag/TiO2 material was investigated, before and after UV illumination “in situ”, by X-ray photoemission spectroscopy (XPS) in a VG Escalab 210 electron spectrometer (26) Wang, R.; Hahimoto, K.; Fujishima, A.; Chinkuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 432. (27) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188. (28) Borras, A.; Lopez, C.; Rico, V.; Gracia, F.; Gonzalez-Elipe, A. R.; Richter, E.; Battiston, G.; Gerbasi, R.; McSporran, N.; Sauthier, G.; Gyo¨rgy, E.; Figueras, A. J. Phys. Chem. C 2007, 111, 1801. (29) Stevens, N.; Priest, C. I.; Sedev, R.; Ralston, J. Langmuir 2003, 19, 3272. (30) Miyauchi, M.; Kieda, N.; Hishita, S.; Mitsuhashi, T.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Surf. Sci. 2002, 511, 401. (31) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (32) Han, J. B.; Wang, X.; Wang, N.; Wei, Z. H.; Yu, G. P.; Zhou, Z. G.; Wang, Q. Q. Surf. Coat. Technol. 2006, 200, 4876.

Borras et al. supplied with a prechamber where the samples were exposed to the UV light in the presence of 133 Pa of oxygen.

Results Water Contact Angle on the Ag/TiO2 Composite Substrates. The preparation by PECVD of surfaces containing Ag@TiO2 supported nanofibers has been the subject of previously published works by our group,24,25 showing that the surface concentration of fibers could be controlled by a proper adjustment of the deposition conditions. For the studies on the wetting behavior of these surfaces, we have selected a series of samples which exemplified the different surface topographies achievable with our procedure. For comparison, essays have also been carried out with thin films of TiO2 prepared simultaneously on flat substrates (i.e., silicon (100)) under the same conditions. Figure 1 shows a series of planar view SEM micrographs corresponding to different Ag/TiO2 surfaces and to the TiO2 thin films grown simultaneously on flat substrates. For the samples in this figure, deposition is carried out by keeping the substrate at 298 K (images (e) and (f), note that no fibers are formed), 403 K (images (a) and (b), note that fibers are now formed), and 523 K (images (c) and (d)). In the latter case the TiO2 is crystalline in the form of anatase as determined by X-ray diffraction and deduced from the small crystallites observable in the image. Typical surface concentrations of Ag@TiO2 composite fibers were 15 and 3 fibers µm-2 for the samples prepared at 403 and 523 K, respectively. It is important to remark here that the fibers are formed by a silver core and a TiO2 overlayer. Therefore, the material brought in contact with the liquid is always TiO2, although the topography of the surface can be different depending on the deposition conditions. The insets in this figure show images of the water droplets formed on each surface during contact angle measurement. From the analysis of the shape of these droplets according to the Young equation different values of the wetting contact angles were obtained. As a general tendency, it appears that the contact angles are always higher on the Ag/TiO2 surfaces than on TiO2. However, most remarkable is that very high contact angles (i.e., 150° and 155°) are obtained on the Ag/TiO2 composite surfaces (c) and (e) and that a completely superhydrophobic surface (i.e., virtually 180° of contact angle) is obtained for sample (a). On the surface of this latter material there is a high concentration (i.e., around 15 fibers µm-2) of Ag@TiO2 nanofibers. Note that taking the picture of the water droplet in this case was only possible after folding slightly the silver substrate; otherwise, the drops runoff from the surface or remain attached to the tip of the needle used to put them on the surface for contact angle measurements. A video showing further details about this experiment can be found in the Supporting Information. The water contact angle on the pristine silver substrate without TiO2 was 60°. The previous images and experiments were carried out on samples where a TiO2 layer with an equivalent thickness higher than 200 nm had been deposited. According to our previous studies about the formation of these fibers, their surface concentration and their length remain practically constant since the beginning of the deposition experiments.24,25 This means that the silver core of the fibers develop at the initial stages of the deposition and are covered by a TiO2 outer layer whose thickness grows with the deposition time. Figure 2 shows a series of SEM micrographs taken for Ag/TiO2 composite surfaces prepared at 403 K for increasing deposition times. This figure clearly shows that thicker fibers are obtained as the deposition time increases. A similar behavior was obtained for the crystalline fibers formed at 523 K (i.e., under conditions similar to those of Figure 1 b).

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Figure 1. SEM micrographs (planar view) of the Ag/TiO2 composite (left) and TiO2 (right) surfaces prepared by PECVD on respectively a silver membrane and a flat Si substrate. Preparation conditions were, from top to bottom, as follows: 100% O2-403 K (Ag/TiO2-403 K), 10% O2-523 K (Ag/TiO2-523 K), and 100% O2-298 K (Ag/TiO2-298 K). The inset shows the images of the water droplets that formed on the surface of the different samples during water contact angle measurements and the value measured for this parameter.

Meanwhile, irrespective of the TiO2 thickness, the surface topography was rather similar for all samples prepared at 298 K (i.e., under deposition conditions similar to those of Figure 1c). In this case only small changes in the water contact angles were found as a function of the TiO2 deposition time and no further analysis of these data will be done here. For the Ag/TiO2 samples prepared at 403 and 523 K the water contact angles measured as a function of the equivalent thickness of the deposited TiO2 layer are reported in Figure 3. The equivalent thickness of TiO2 was measured on a flat Si substrate used as a reference during the same experiment. Although the actual thickness of the fibers is smaller than the equivalent thickness of the TiO2 layer, it can be considered that the width of the fibers follows the same tendency as the nominal thickness measured on Si substrates. The data in Figure 3 reveal that the water contact angle increases with the thickness of the fibers and reaches a maximum value for an equivalent thickness of TiO2 of ca. 200 and 420 nm for the 403 and 523 K samples, respectively. It is interesting that, for a TiO2 thickness of 200 nm, the sample with the maximum concentration of fibers (i.e., the one prepared at 403 K) depicted superhydrophobic behavior (i.e., water contact angle around 180°). Superhydrophobic to Superhydrophilic Conversion. TiO2 is a photoactive material that becomes hydrophilic when it is irradiated with UV light.26–30 This transformation is reversible and these surfaces recover their initial contact angle after storing them in the dark for a prolonged period of time or, in an accelerated way, by heating the samples in air. The heating treatment can be carried out by placing the sample on a hot plate at temperatures higher than 373 K or by the illumination of the sample surface with visible (VIS) light. As the TiO2 absorbs only UV light, it

is commonly admitted that VIS illumination (including the IR region of the spectrum) produces a local heating of the oxide.30 The possibility to control the water contact angle on our composite surfaces upon UV light irradiation is of the outmost interest since variations from 180° to virtually 0° could be expected. Therefore, water contact angle experiments were carried out with Ag/TiO2 composite surfaces that were subjected to UV irradiation. Figure 4(top) shows a comparison of the evolution of the water contact angles of a series of TiO2 and Ag/TiO2 surfaces subjected to UV irradiation for increasing periods of time. A first assessment of the data in this figure reveals that all the samples become superhydrophilic (i.e., water contact angles smaller than 10°) after UV irradiation for prolonged periods of time. According to the typical behavior of TiO2,28–30 the samples recovered their initial contact angle after illumination with VIS light for ∼18 h (c.f. Figure 4(bottom)). The reversibility from the superhydrophilic state is also achieved by prolonged storage in the dark (∼96 h) or by heating the samples in air at 383 K for ∼7 h. This behavior is particularly interesting for sample Ag/TiO2-403 K, where a reversible variation from 180° to 0° is found (see Supporting Information). The surface state of the samples, before and after UV illumination, was investigated by XPS “in situ”. However, this analysis did not show any significant change in the surface characteristics of the sample apart from a slight decrease in the surface concentration of adventitious carbon and a slight change in the shape of the O 1s signal. Figure 4(top) also shows that the kinetics of the evolution of the water contact angle is dependent on the characteristics of the sample. Kinetics studies based on the time dependence of the water contact angles for irradiated TiO2 surfaces have been

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Figure 3. Variation of the water contact angle measured on the Ag/TiO2 surfaces prepared at 403 and 523 K as a function of the equivalent thickness of the deposited TiO2 layer.

Figure 2. SEM micrographs (planar view) of Ag/TiO2 surfaces prepared at 403 K for increasing periods of time. Values of the equivalent thickness of the TiO2 are from top to bottom: 90, 200, and 250 nm.

reported by Seki and Tachiya.33 Our data for the flat TiO2 surfaces, included in Figure 4 for comparison, indicate that the decrease in contact angle is very fast for the flat samples of TiO2 prepared at 523 K. A key characteristic of this material is that it is crystalline.24,25 A slower kinetic is found for amorphous TiO2 samples grown at 403 K on Si(100). This trend is reproduced by the Ag/TiO2 surfaces, where the surface with crystalline TiO2 (i.e., sample Ag/TiO2-523 K) presents the fastest rate of contact angle decrease. Meanwhile, the fact that upon illumination sample Ag/TiO2-403 K presents the slowest decrease of contact angle confirms that UV irradiation is less efficient in inducing wetting (33) Seki, K.; Tachiya, M. J. Phys. Chem. B 2004, 108, 4806. (34) Miyauchi, M.; Shimai, A.; Tsuru, Y. J. Phys. Chem. B 2005, 27, 13307. (35) Rico, V.; Lopez, C.; Borras, A.; Espinos, J. P.; Gonzalez-Elipe, A. R. Sol. Ener. Mater. Solar Cells 2006, 90, 2944.

Figure 4. (Top) Evolution of the water contact angle measured on TiO2 and Ag/TiO2 surfaces subjected to UV irradiation for increasing periods of time. (Bottom) Recovered angles after irradiation with VIS light. The lines are plotted to guide the eyes.

angle changes on amorphous TiO2 layers. However, the recovery of the original contact angles show the opposite trend as is shown in Figure 4(bottom). Under VIS illumination, the amorphous samples (i.e., Ag/TiO2-403 K and TiO2-403 K) recovered the partially hydrophobic state after a few minutes of irradiation and a superhydrophobic value for the contact angle on the Ag/TiO2403 K after 10 h of treatment. Meanwhile, crystalline samples follow a slower kinetics. This behavior agrees with previous results in the literature about the influence of the crystalline phase and crystal size on the TiO2 photoactivity.36–38 Thus, the Ag/TiO2-523 K surface presents a faster recovery than the TiO2523 K due to the fact that the anatase grains forming the fibers (36) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. AdV. Mater. 1998, 10, 135. (37) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33. (38) Calza, P.; Pelizzetti, E.; Mogyorsi, K.; Kun, R.; Dekany, I. Appl. Catal., B 2007, 72, 314.

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are smaller than those found in the TiO2 flat layers grown at 523 K on Si substrates (see Figure 1 and refs 24 and 25).

Discussion The previous results have shown that surfaces containing Ag@TiO2 fibers may present superhydrophobic behavior that depends on the surface concentration of fibers and on their width. Very high contact angles of water have been reported for a variety of materials with a very strict control of their surface roughness and structure.6–9 On surfaces with TiO2 and ZnO nanotubes or nanorods a superhydrophobic behavior characterized by water contact angles between 160 and 180° have also been reported.20–23,34 Our system consisting of Ag@TiO2 deposited nanofibers is characterized by very high contact angles. A maximum value of virtually 180° is found for samples with a fiber concentration of about 15 fibers µm-2. These nanofibers are prepared by PECVD at relatively low temperatures. For this material we have found that good control of the contact angle is possible by adjusting the thickness of the deposited TiO2 layer and, therefore, the width of the Ag@TiO2 nanofibers. This possibility is a unique feature of our preparation procedure by which the width and surface concentration of the nanofibers is directly related to respectively the deposition time of TiO2 and the temperature of the substrate during the deposition. The fact that the water contact angle varies with the width of the fibers can be explained on the basis of an idealized surface model for our material. According to the simple model of Wenzel31 to account for the effect of surface roughness on contact angle, hydrophobicity can be related to the development of water/ air and water/material local interfaces in rough surfaces. As an overall result, the actual contact angle on a real surface (θ′) can be related to the contact angle on a flat surface of the same material (θ) according to eq 1:

cos(θ′) ) rW cos(θ)

(1)

where rW is a roughness related parameter, which is defined as the ratio between the total area of the surface and the geometric projected area.31 Thus, this parameter depends on both the lateral area of the surface structures and the number of such structures. A rough estimation of water contact angles in our fiber system can be done by assuming an idealized surface structure consisting of vertical fibers of constant width and a length of 1.5 µm (i.e., similar to the mean value measured for the Ag@TiO2 nanofibers) placed on a flat surface. This structure is represented in Figure 5(top). Based on simple geometrical considerations, rW can be expressed as shown in eq 2:

rW ) Areal/Ageom. ) 1 + nπ1.5D

(2)

In this parametrized formula n is the surface concentration of fibers and D its diameter expressed in nanometers. The factor 1.5 has been taken for fibers of 1.5 µm in length. A similar model was proposed by Chen et al.17 to account for superhydrophobicity behavior. Figure 5 shows the evolution of the rW parameter as a function of the density of fibers for fibers with different widths varying between 35 and 200 nm. This figure shows that the contact angle on the surface of this idealized system formed by nanofibers tends to increase as the surface density of the fibers and their width increase. As represented in Figure 3, this result agrees with the behavior found experimentally for our Ag/TiO2 surfaces. According to this figure, for a concentration of fibers above a certain value and a given width, the contact angle on these surfaces reaches a virtual value of 180°. The horizontal dotted line in Figure 5(bottom) indicates the minimum value of rW necessary to achieve the virtual contact angle (θ′) of 180° by

Figure 5. (Top) Idealized representation of the surface of the Ag/TiO2 composite material where a certain number of nanometric fibers (n) of diameter D and length L are formed. (Bottom) Representation of the roughness factor calculated with eq2 as a function of fiber concentration onto the surface for different fiber widths. The dotted line indicates the rw values at which superhydrophobicity is found.

adding fibers to a flat surface with a contact angle (θ) of 100° (see eq1). This latter value of contact angle is similar to that found for the thin films of this material deposited on a flat substrate (c.f. Figure 1). Note that different values ranging from 30° to 110° have been reported for flat TiO2 surfaces.26–30 In our case, the chosen value of 100° is a good average for flat thin films of this material prepared by PECVD under the same conditions as those for the Ag/TiO2 surfaces. A very interesting characteristic of TiO2 or ZnO surfaces is that they become completely hydrophilic when they are irradiated with UV light.26–30,32–38 This property is also depicted by nanotubes, nanorods, or similar structures of these materials, where superhydrophobic into superhydrophilic transformations have been reported.20–23 This transformation from hydrophobic into hydrophilic upon illumination with UV light was interpreted by Wang et al.26,27,36 as due to the light-induced dissociation of water from the atmosphere on the surface of the TiO2. A similar behavior is depicted by our Ag/TiO2 surfaces (Figure 4), indicating that they become superhydrophilic under UV illumination. The control of the density of fibers and their width attainable by our plasma method provide an easy way of controlling the initial contact angle (i.e., its superhydrophobic state) and therefore provides a straightforward way of controlling the angles among which this transformation occurs. In addition, our results in Figure 4 show that the decrease rate of contact angle is dependent on the crystallinity of the surface. Thus, flat surfaces of anatase depict the fastest transformation into the superhydrophilic state. Similarly, the crystalline Ag/TiO2 surfaces depict the fastest transformation from all the composite surfaces. It is known that crystalline TiO2, particularly in the form of anatase, presents the highest photoactivity of this material owing to a relatively low recombination rate of photogenerated electrons and holes.37,38 This feature is in agreement with the observed behavior of our TiO2 thin films and composite Ag/TiO2 surfaces, although our results also show that significant differences exist when comparing their respective responses. Thus, for the Ag/TiO2 surfaces the hydrophilic state is reached after much longer irradiation times

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than those for TiO2 flat surfaces. According to the amphiphilic model of Wang et al.,26,27,36 UV irradiation of TiO2 in air produces the dissociation of water molecules and an additional hydroxylation of certain regions of its surface. As an overall effect, the existence of surface patches with an extra hydroxylation degree converts the surface into hydrophilic. Assuming that a similar transformation of the surface state also occurs along the Ag@TiO2 nanofibers, the lower transformation rate found in our case suggests that this extra hydroxylation of surface patches is only effective if they have a critical size and/or connectivity. The existence of size constraints would be in agreement with the morphological analysis of the Ag@TiO2 nanofibers,24,25 showing that the size of the TiO2 particles/crystals forming their external layer is smaller than that of the crystal/particles formed on flat TiO2 thin films prepared under similar conditions. We can therefore conclude that the Ag/TiO2 composite material behaves as superhydrophobic because of the presence on its surface of Ag@TiO2 nanofibers. We have shown that in this superhydrophobic state the water contact angle depends on the

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surface concentration of fibers and on their thickness and can reach a value or virtually 180°. The surface can be converted into a superhydrophilic state when it is illuminated with UV light and back again to the superhydrophobic state when the samples are heated or illuminated with VIS light for a sufficient period of time. This change is due to the known transformation of the surface of irradiated TiO2 as previously reported in the literature.26,27,30,36,37 Acknowledgment. We are thankful for the financial support of the Ministry of Science and Education of Spain (ref. MAT 2007-65764) and of the European Union (NATAMA Project NMP3-CT-2006-032583). Supporting Information Available: Video showing the evolution under UV light irradiation of a water droplet on a composite surface with Ag@TiO2 nanofibers. This material is available free of charge via the Internet at http://pubs.acs.org. LA800113N