pubs.acs.org/Langmuir © 2010 American Chemical Society
Wetting Properties of Polycrystalline TiO2 Surfaces: A Scaling Approach to the Roughness Factors Ana Borras* and Agustı´ n R. Gonzalez-Elipe Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla), Avd. Am erico Vespucio 49, 41092 Sevilla, Spain Received May 17, 2010. Revised Manuscript Received September 2, 2010 This work presents a thorough study on the wettability of polycrystalline anatase TiO2 thin films prepared at 250 °C in a microwave plasma enhanced chemical vapor deposition (MW-PECVD) reactor with Ar/O2 plasmas. Anatase polycrystalline thin films with different microstructures, textures, and surface roughness were obtained as a function of their thickness. The water contact angle of the samples was analyzed within the assumptions of the Wenzel, Cassie, and Miwa models to ascertain the effect of roughness and other surface heterogeneities on their characteristic parameters. The roughness factors defined in the different models were calculated from the atomic force microscopy (AFM) images of the films for two different observation scales within the premises of the dynamic scaling theories. The obtained results indicate that the wetting angle of an equivalent flat anatase surface with a value of 82° can only be properly estimated for observation scales of 5 5 μm2 and using the Miwa model. The analysis of the UV induced hydrophilization of the surface state of the anatase films and the posterior recovery of the partially hydrophobic character of these surfaces in the absence of UV photons suggest a clear dependence of the light induced wettability on their texture and size of crystalline domains.
Introduction Although the growth of crystalline and nanocrystalline TiO2 in the form of powders and thin films can be considered as a classical topic in photocatalysis, its interest has increased during the last years because of the potential applications of this material in other related fields such as photovoltaic solar cells, gas sensors, optical thin films and systems, self-cleaning surfaces, and so on.1 Control and reproducibility of the material properties is a requisite to improve its performance and reliability. This is certainly true when looking to the literature on the wetting contact angle (WCA) conversion of TiO2 thin films subjected to UV light illumination.2-6 In these works, the differences in light induced wetting behavior have been attributed to quite different factors without a widely accepted common picture of this phenomenon. This phenomenon was first refereed by Wang and co-workers who showed that under UV light irradiation the WCA of a polycrystalline TiO2 thin film changed from partially hydrophobic *To whom correspondence should be addressed. E-mail: anaisabel.
[email protected]. (1) (a) Gratzel, M. Prog. Photovoltaics 2000, 8, 171. (b) Lee, S. W.; Takahara, N.; Korposh, S.; Yang, D. H.; Toko, K.; Kunitake, T. Anal. Chem. 2010, 82, 2228. (c) Iftimie, N.; Crisan, M.; Braileanu, A.; Crisan, D. C.; Nastuta, A.; Rusu, G. B.; Popa, P. D.; Mardare, D. J. Optoelectron. Adv. Mater. 2008, 10, 2363. (d) Martinez-Ferrero, E.; Sakatani, Y.; Boissiere, C.; Grosso, D.; Fuertes, A.; Fraxedas, J.; Sanchez, C. Adv. Func. Mater. 2007, 17, 3348. (e) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Langmuir 2000, 16, 7044. (f) Fujishima, A.; Zhang, X. T.; Tryk, D. A. Surf. Sci. Rep. 2008, 63, 515. (2) (a) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (b) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Adv. Mater. 1998, 10, 135. (c) Miyauchi, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Chem. Mater. 2000, 12, 3. (d) Katsumata, K.; Nakajima, A.; Yoshikawa, H.; Shiota, T.; Yoshida, N.; Watanabe, T.; Kameshima, Y.; Okada, K. Surf. Sci. 2005, 579, 123. (3) (a) Zubkov, T.; Stahl, D.; Thompson, T. L.; Panayotov, D.; Diwald, O.; Yates, J. T. J. Phys. Chem. B 2005, 109, 15454. (b) Thompson, T. L.; Yates, J. T. Chem. Rev. 2006, 106, 4428. (c) Yates, J. T. Surf. Sci. 2009, 603, 1605. (4) White, J. M.; Szanyi, J.; Henderson, M. A. J. Phys. Chem. B 2003, 107, 9029. (5) Sanchez-Valencia, J. R.; Borras, A.; Barranco, A.; Rico, V. J.; Espinos, J. P.; Gonzalez-Elipe, A. R. Langmuir 2008, 24, 9460. (6) Stevens, N.; Priest, C. I.; Sedev, R.; Ralston, J. Langmuir 2003, 19, 3272.
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(WCA ∼80°) to superhydrophilic (WCA < 10°, i.e., the water droplet spreads completely onto the surface).2 This conversion was reversible, and the hydrophobic character of the surface could be restored by different manners, for instance, by irradiation of the surface with visible light. This behavior has promoted TiO2 as an important functional material for the fabrication of antifogging and self-cleaning surfaces already working in commercial glassware and mirrors.7 Although it has been more than a decade since this pioneer study by Wang and co-workers, the physicochemical processes responsible for the reversible conversion of the surface are still matters of debate within the scientific community. In a simplified way, two different approaches have been proposed: the amphiphilic model proposed by Wang et al. based on the hydroxylation of the surface due to the UV irradiation, and the explanation of the hydrophiliation conversion through the photocatalytic removal oxidation of carbon pollutants from the surface of the TiO2.3 Within this context, it is not yet clear what are the factors that may be contributing to efficiently control the superhydrophilic conversion of TiO2 surfaces illuminated with UV light and its recovery when they are left in the dark or illuminated with visible light.8 To contribute to this topic, we study here the hydrophobic to hydrophilic conversion and recuperation of polycrystalline anatase TiO2 thin films with different surface texture and crystal sizes. This investigation is a continuation of a recent work, about the wetting behavior under UV irradiation of amorphous and crystalline TiO2 thin films.9 Already in this previous work, as well (7) (a) Paz, Y.; Luo, Z.; Rabenberg, L.; Heller, A. J. Mater. Res. 1995, 10, 2842. (b) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalyst, Fundamentals and Applications; BKC: Tokyo, 1999. (c) Blossey, R. Nat. Mater. 2003, 2, 301. (d) Marmur, A. Langmuir 2008, 24, 7573. (e) Murugan, K.; Rao, T. N.; Gandhi, A. S.; Ashutosh, G. . S.; Murty, B. S. Catal. Commun. 2010, 11, 518. (8) (a) Nakajima, A.; Koizumi, S.; Watanabe, T.; Hashimoto, K. Langmuir 2000, 16, 7048. (b) Miyauchi, M.; Kieda, N.; Hishita, S.; Mitsuhashi, T.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Surf. Sci. 2002, 511, 401. (c) Rico, V.; Lopez, C.; Borras, A.; Espinos, J. P.; Gonzalez-Elipe, A. R. Sol. Energy Mater. Sol. Cells 2006, 90, 2944. (9) Rico, V.; Romero, P.; Hueso, J. L.; Espinos, J. P.; Gonzalez-Elipe, A. R. Catal. Today 2009, 143, 347.
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as in the present investigation, we have verified that many basic principles have been overlooked when trying to correlate wetting angles with surface characteristics. We think that this relatively confused situation concerning the photoactivity of TiO2 is likely part of the more ample and recent controversy on the basic principles of the wetting behavior of surfaces. This controversy has been generated along the reinterpretation made by McCarthy and co-workers10 of the Young equation based on a return to the origins of the wetting principles. Contrary to the models developed by Wenzel11 and Cassie and Baxter12 during the last century and since then widely utilized for most researchers interested in the relationships between wetting and surface roughness,6,7c,8b,12 McCarthy stresses that the Young equation relates forces and lengths at the perimeter of the sessile drops used to measure WCAs. According to this author, by decades most researchers have made an extensive and rather uncritical use of the surface tension concept instead of that of forces and have incorporated the surface roughness inside the drops as a basic parameter directly determining the WCA. The main objective of the present work is to correlate the surface properties of polycrystalline TiO2 thin films with their WCA. Since we follow an empirical approach to this problem and do not pretend any fundamental contribution to the principles of wetting, we will use the surface roughness or, in a more general way, the surface morphology as the main experimental running parameter of the studied surfaces. However, contrary to the majority of the works in literature making direct and uncritical use of surface roughness of the root mean square (RMS) value determined by atomic force microscopy (AFM), our approach relies on the concepts derived from the dynamic scaling theory of thin film growth.14 Thus, we make a critical appraisal of the different roughness values which can be determined by AFM for a single surface. In addition, the assessment of the WCA of different TiO2 surfaces by the Wenzel (WM),11 Cassie-Baxter (C-BM),12 and Miwa-Hashimoto (M-HM)15 models has permitted us to conclude that the last one describes the best the wetting behavior of polycrystalline anatase thin films. To study both the dependence of WCA on roughness and the wetting changes under UV light illumination, we have used polycrystalline anatase thin films deposited by plasma enhanced chemical vapor deposition (PECVD). In a previous work we studied the growth of this type of crystalline TiO2 thin film by providing a thorough characterization of their film microstructure, structure, and texture as a function of their thickness.16 Herein, the meaning of these terms is borrowed from that commonly used by the scientific community working in thin films. Thus, the term microstructure addresses the thin film (10) (a) Gao, L.; McCarthy, T. J. Langmuir 2008, 24, 9183. (b) Gao, L.; McCarthy, T. J. Langmuir 2009, 25, 7249. (c) Gao, L.; McCarthy, T. J.; Zhang, X. Langmuir 2009, 25, 14100. (d) Gao, L.; McCarthy, T. J. Langmuir 2009, 25, 14105. (11) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (12) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (13) (a) Hsieha, C. T.; Fan, W. S. App. Phys. Lett. 2006, 88, 243120. (b) Borras, A.; Barranco, A.; Gonzalez-Elipe, A. R. Langmuir 2008, 24, 8021. (c) Shieh, J.; Hou, F. J.; Chen, Y. C.; Chen, H. M.; Yang, S. P.; Cheng, C. C.; Chen, H. L. Adv. Mater. 2009, 21, 1. (d) Han, D.; Steckl, A. J. Langmuir 2009, 25, 9454. (e) Borras, A.; Groening, P.; Sanchez-Valencia, J. R.; Barranco, A.; Espinos, J. P.; Gonzalez-Elipe, A. R. Langmuir 2010, 26, 1487. (f) Ma, M.; Hill, R. M; Rutledge, G. C. J. Adhes. Sci. Technol. 2008, 22, 1799. (g) Tuteja, A.; Choi, W.; Ma, M. L.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Science 2007, 318, 1618. (14) (a) Barabasi, A. L.; Stanley, H. E. Fractal Concepts in Surface Growth; Cambridge University Press: Cambridge, UK, 1994. (b) Fang, S. J. J. Appl. Phys. 1997, 82, 5891. (c) Senthilkumar, M.; Sahoo, N. K.; Thakur, S.; Tokas, R. B. Appl. Surf. Sci. 2005, 252, 1608. (d) Borras, A.; Yanguas-Gil, A.; Barranco, A.; Cotrino, J.; Gonzalez-Elipe, A. R. Phys. Rev. B 2007, 76, 235303. (15) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754. (16) Borras, A.; Sanchez-Valencia, J. R.; Widmer, R.; Rico, V. J.; Justo, A.; Gonzalez-Elipe, A. R. Cryst. Growth Des. 2009, 9, 2868.
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morphology, that is, columnar microstructure, size and shape of the columns, crystal size, and so on; structure to differentiate between amorphous, rutile, and anatase phases and texture to indicate the preferential growth of certain crystal planes parallel to the sample surface. Making use of this knowledge about the film properties, here we try to determine the possible relationships existing between the roughness of the thin films with their water contact angles, as well as to show how they may be implied in the control of the wetting behavior under light irradiation.
Wetting Behavior and Surface Roughness The wetting angle of a drop on a flat surface is determined by the Young law10 and is the result of the balance among the cohesive forces acting in the contact line between the drop, the surface, and the air or environment (see eq 1). FSV ¼ FLV cosðθÞ þ FSL
ð1Þ
For many years, this equation has been reinterpreted by substituting forces by surface tension and turning the vectorial character of the equation into scalar according to cosðθÞ ¼
γSV - γSL γLV
ð2Þ
where γSL, γSV, and γLV refer to the interfacial energy in the solid-liquid, solid-vapor, and liquid-vapor interface, respectively. This equation implies that if the addition of γSV and γSL equals γLV, then the drop wets completely the surface, reaching what customarily is named as a superhydrophilic state.7c This is the case of surfaces with a high surface energy. When the solidvapor interface presents a low surface tension, the water contact angle increases. WCAs higher than 90° are usually referred as hydrophobic surfaces, while for WCA higher than 150° we reach a superhydrophobic state, typical of surfaces where a water droplet is expected to roll off.7c Equation 2 provides a very simple description of the wettability of a flat surface. However, since the earliest investigations on this topic, it was realized that the roughness and morphology of the surface are key properties in determining the WCAs.2,6-8,10,13 There exist several models attempting to relate the CA of a real surface. These models are characterized by a specific roughness and presenting morphological heterogeneities, with the CA of the smooth, compact, and homogeneous ideal surface of the same material. In this work, we will use three different models: a. Wenzel Model (WM)10. The WM relates the contact angle of a real surface (θ0 ) with the contact angle of the flat surface (θ) through the roughness factor rW: cosðθ0 Þ ¼ rW cosðθÞ
ð3Þ
In this equation, rW is defined as the relation between the real area of the surface and its geometric projected area. b. Cassie-Baxter Model (C-BM)11. This model includes the effect of the heterogeneity of the surface on the CA, that is, the role of the composition of the different materials forming the contact surface with the droplet. cosðθ0 Þ ¼ f1 cosðθ1 Þ þ f2 cosðθ2 Þ
ð4Þ
where f1 and f2 represent the solid fractions of the surface with contact angles θ1 and θ2, respectively. In the case of a porous or rough surface, the second material in contact would be the air, with a WCA of 180°. Then, eq 4 becomes cosðθ0 Þ ¼ fCB cosðθÞ þ fCB - 1
ð5Þ
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Scheme 1. Schematic of the Miwa-Hashimoto Model for a 1D Application
where fCB corresponds to the actual solid surface forming the liquid-solid interface. c. Miwa-Hashimoto (M-HM)15. Formally, this latter model combines the WM and the C-BM through the equation: cosðθ0 Þ ¼ rMH fMH cosðθÞ þ fMH - 1
ð6Þ
where rMH depends on the lateral contact area of the features on the surface and fMH on the fraction of surface area of the material in contact with the liquid. The factor rf represents the relation between the area of the liquid-solid interface and the projected surface area (lower than the actual area because of the effect of roughness). Miwa et al. proposed a linear model with triangular motives (see Scheme 1)15 and approached the value of these two parameters as a ð7Þ rMH ¼ b P fMH ¼ P
bþ
b P
c
ð8Þ
Herein, we have calculated the different coefficients used by the different models by analyzing the AFM images of the surface of the studied samples measured at room conditions with the WSxM software.17 The implementation made here of the MiwaHashimoto Model is based on these images, and it is a 2D modification of the 1D model originally proposed by these authors. Although more complex models have been recently developed in order to describe the changes in contact angles on real surfaces,13f,g in this work we utilize these three classical models that, besides their wide applications in the literature, are of facile implementation by looking via AFM to the surface state of the materials. Models such as those in refs 13f and 13g include in their formulation the shape of features not directly exposed to the surface (i.e., including 3D corrugations which are not accessible to commercial AFM). Since the scaling properties of surfaces are tightly related to the roughness observations by AFM, within the perspective of the present work only those model parameters easily determined by this technique would be considered and handled in our discussion. The roughness concept and, particularly, the way how to estimate it are not univocal. In the present work, to assess the roughness of the studied surfaces, we make use of the concepts of the so-called dynamic scaling theory (DST).14 Among other issues, the analysis of a growing surface through the DST permits one to relate its scaling behavior with the growing mechanisms of the thin films. These studies rely on the evolution of the surface roughness with the deposition time and the scale of measurement. Concretely, the DST predicts that for self-affine surfaces the roughness increases with the scale of measurement until a certain saturation value. Considering this principle within the perspective (17) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; GomezHerrero, J.; Baro, A. M. Rev. Sci. Instrum. 2007, 78, 013705.
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of the wetting behavior of surfaces, the existence of a scaling behavior implies that the roughness of the thin films and, therefore, the different coefficients defined in the models summarized above will depend on the scale used to characterize the surface roughness. As far as we know, a similar approach has not been used previously in the literature on wetting. For this analysis, we will assume that for a specific sample the characteristic roughness controlling the surface contact angle is the saturation roughness. The observation scale length at which the measured roughness reaches saturation depends on the type of material, deposition process, growth mechanism involved, and thickness of the thin films. In this work, we present a brief analysis of the surface roughness of polycrystalline TiO2 thin films to highlight the importance of a correct selection of the measurement scale for the calculation of the contact angle coefficients.
Experiment and Methodology Growth of Nanocrystalline TiO2 Thin Films by PECVD. Anatase TiO2 thin films were prepared by PECVD in an electron cyclotron resonance microwave (ECR-MW) plasma reactor working in downstream; that is, the substrates were located out of the glow discharge of the plasma during deposition.16 The plasma reactor was a SLAN type operated with a power of 400 W with a mixture Ar/O2 (90% Ar-10% O2) as plasma gas. During deposition, the substrates were heated at 523 K to induce the formation of the crystalline phase of TiO2. Titanium isopropoxide, Ti(OC3H7)4 (TTIP), was used as precursor. It was bubbled with oxygen and transported through a heated line up to the deposition chamber. Total pressure during deposition was ∼4 10-3 Torr. Further information about the experimental configuration can be found elsewhere.18 Films were deposited simultaneously on silicon and quartz substrates. After plasma deposition, the as-prepared films were hydrophilic as an effect of the plasma UV light irradiation.19 Since the surface state of the as prepared samples and their WCAs changed with time, the thin films were kept for 3 months in the dark under controlled ambient conditions before the analysis of their wetting properties. Thin Film Characterization. The structure of the thin films was determined by X-ray diffraction (XRD) in both BraggBrentano and glancing incident angle configurations (GAXRD). The analysis by normal XRD was carried out in a Siemens D5000 instrument. The experiments at glancing angle were carried out in a Siemens D500 instrument. Texture coefficients of crystal planes were determined by the typical formulas yielding information on preferential growth of crystalline planes parallel to the surface.20 The roughness and surface topography of the samples were examined by AFM working in tapping mode in a SPM-Veeco from Digital using high frequency levers of silicon. X-ray photoelectron spectroscopy (XPS) spectra of the films were recorded on a VG Escalab 210 spectrometer working under energy transmission constant conditions (data not shown). The Mg KR line was used for excitation of the spectra. They were calibrated in binding energy by referencing to the C 1s peak due to contamination at 284.6 eV. One of the most important factors in the chemical composition of the TiO2 thin films deposited by PECVD relies on the plasma composition.9,18 In our work, we study samples deposited under the same conditions of plasma composition (10% O2/90% Ar), precursor (TTIP), and temperature (523 K); therefore, no important changes in the surface chemistry can be (18) (a) Borras, A.; Cotrino, J.; Gonzalez-Elipe, A. R. J. Electrochem. Soc. 2007, 54, 152. (b) Romero-Gomez, P.; Rico, V.; Borras, A.; Barranco, A.; Espinos, J. P.; Cotrino, J.; Gonzalez-Elipe, A. R. J. Phys. Chem. C 2009, 113, 13341. (c) Gracia, F.; Holgado, J. P.; Gonzalez-Elipe, A. R. Langmuir 2004, 20, 1688. (19) (a) Rangel, E. C.; Gadioli, G. Z.; Cruz, N. C. Plasma Polym. 2004, 9, 35. (b) 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. (20) Pecharroman, C.; Gracia, F.; Holgado, J. P.; Oca~na, M.; Gonzalez-Elipe, A. R.; Bassas, J.; Santiso, J.; Figueras, A. J. Appl. Phys. 2003, 93, 1.
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expected.18 Indeed, the analysis showed that the films had a TiO2 composition and that the amount of spurious carbon incorporated in the films was similar in all the samples (∼15 atom % at the surface for samples handled in air). The surface of the as-grown samples is hydrophilic because the high amount of OH groups and other radicals inherent in the plasma deposition method. The 3 months of storage in dark allowed the surface of the samples to relax and eventually develop the hydrophobic character. In fact, the analysis of O 1s spectra corresponding to the different samples after their storage in dark demonstrates that the surface contribution of OH groups is negligible, that is, below the sensitivity of the XPS techinque. Furthermore, the O 1s spectra recorded for the three samples under study show no significant differences. Water droplets were placed on the surface of the samples, and their contact angle measured with a CAM100 instrument (KSV Instruments Ltd., Finland). The reported static water contact angles are the mean value from at least seven measurements by the Young method of droplets with volumes of ∼5 μL. The UV illumination experiments were 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 an almost constant emission between ∼400 and ∼800 nm when actuated with an UV filter.
Results and Discussion Texture, Morphology, and Surface Roughness of Polycrystalline Anatase Thin Films. The influence of the deposition rate and thickness on the structure, texture, and microstructure of TiO2 thin films prepared by PECVD has been studied in a previous work,16 where we showed that the growing process can be described according to the Kolomogorov model accounting for the growth of crystalline films from a homogeneous saturated medium. For the present study aiming at establishing a correlation between the WCA and the microstructure and, therefore, roughness of the surface, we have prepared three nanocrystalline TiO2 thin films: d30nmv1, d420nmv1, and d2μmv2. The first subscripts indicate the thickness and the second the growth rate (v1 ∼ 1.9 nm min-1 and v2 ∼ 5 nm min-1). These three samples are representative of growth stages dominated by nucleation (d30nmv1), columnar development according to several anatase habits (d420nmv1), and columnar development driven by stress and tension relaxation (d2μmv2).16 Figure 1 summarizes a series of experimental results concerning the morphology, structure, and texture of the selected thin films. The surface of sample d30nmv1 is formed by round grains of the anatase phase (c.f. Figure 1a and d). Since this sample is in the first stage of crystal development, it shows a low crystallinity and a small crystal size. No texture coefficients can be determined for this sample, since the diffractogram was recorded using the glancing angle configuration (Figure 1d). In samples d420nmv1 and d2μmv2, the morphology is markedly columnar.16 As a characteristic feature, the planar view scanning electron microscopy (SEM) image of sample d420nmv1 in Figure 1b shows the boundary zone between four different columnar domains (the dashed lines separating these domains have been added in the image to guide the eyes). Each domain is related with the development of the anatase crystals following different crystal habits. As a matter of fact, by the texture analysis of the samples, different planes appear with texture coefficients T(hkl) > 1 (i.e., planes (101) and (211), Figure 1e).20 By contrast, the triangular-shaped crystal facets of the surface of sample d2μmv2 is the termination of a columnar microstructure consisting of larger crystal sizes and a marked texturation according to the (112) planes. 15878 DOI: 10.1021/la101975e
The surface roughness is strongly related with the thin film microstructure. Figure 2 shows different results of the AFM analysis of the surfaces of the three studied samples. In the reported AFM micrographs and line profiles of samples d420nmv1 and d2μmv2 (Figure 2c,d and bottom), it is clear the development of microstructural domains and angular features. It is also important to remark that the mean heights of the sample surface features are also very different (Figure 2 bottom). As a consequence, there are large differences in RMS values (see Table 1), in good agreement with their quite distinct microstructures (Figure 2). Thus, for the thinner and smoother film, the RMS is ∼1 nm, while for the thicker film the RMS is ∼35 nm. These RMS values are statistically calculated from images taken with a 5 μm 5 μm measurement scale. For the analysis of the WCAs within the assumptions of Wenzel, Cassie, and Miwa-Hashimoto models, it is very important to realize that the surface roughness must be determined from images acquired over a 5 μm 5 μm area instead of 1 μm 1 μm, as it is common by WCAs of studies in thin films.13,15 The first and more obvious reason relies on the size of the surface domains and features of the studied samples. It is obvious that, for samples d420nmv1 and d2μmv2, roughness characterization with the 1 μm 1 μm images would certainly fail because the size of the surface features is in the order of or higher than the observation scale. The scaling dynamic approach to thin film growth establishes that the surface roughness of a thin film increases with the measurement scale until a threshold value. From this value on, the roughness reaches saturation, remaining constant for larger measurement scales.14 Such scaling behavior is obvious in the results summarized in Table 1, where the RMS values of the roughness calculated from AFM images acquired at various scales present different values. According with the surface morphology, the differences in the RMS roughness at different scales are more pronounced for the thicker samples. A way to analytically proceed to the surface analysis is by calculating the so-called potential spectral distribution (PSD), defined as the Fourier transform of the surface roughness in real space.14 Figure 3 shows a plot of the PSD curves as a function of k (where k is 1/L, the inverse of the length distances in the real space) determined for the three studied samples. According to the DST, the PSD of self-affine surfaces increases with L (i.e., for low k values) up to reach a saturation value that defines a threshold scale length for the system. For length scales lower than this saturation value, the roughness of the surface determined by AFM would be tightly dependent on the scale of measurement (i.e., it will present different values if determined for small or large scanned regions). Above this saturation length value, the roughness would be independent of the observation scale. The problem in many papers dealing with possible correlations between contact angles and roughness is that such dependences on observation scales are not considered explicitly producing as a consequence flaw results or misleading interpretations. This behavior is actually depicted by the series of RMS values reported in Table 1 that were obtained by using different observation scales. The roughness values gathered in Table 1 confirm that a critical view should be adopted when dealing with RMS values of thin films. Thus, according to the premises of the DST,14 the RMS roughness increases with the film thickness when the observation scale is of the order or above the threshold scale length of the system. This condition is clearly fulfilled by the RMS values in the first and second columns of the table. However, when the RMS values are determined on AFM images with a size smaller than the threshold scale length of the system (i.e., clearly the last column of the table), the RMS roughness present an odd behavior that does not comply with the DST premises (RMS of d420nmv1 at 0.5 μm 0.5 μm is bigger that the RMS of d2μmv2 at 0.5 μm 0.5 μm). Langmuir 2010, 26(20), 15875–15882
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Figure 1. AFM (a) and normal view SEM micrographs (b,c) of the anatase thin films as labeled. (d) GAXRD of d30nmv1 and XRD patterns of d420nmv1 and d2μmv2; the reference pattern corresponds to a randomly oriented sample. (e) Textural coefficients for samples d420nmv1 and d2μmv2 calculated from the XRD diagrams in (d). The dashed lines in panel (b) mark the boundaries between different microstructural domains.
Moreover, the threshold scale lengths estimated from the PSD curves in Figure 3 (i.e., 1000, 3000, and 4000 nm, signaled by vertical lines in the figure) are not equivalent for samples presenting different thickness and microstructure. The estimated threshold lengths determine the observation scales which can be used to extract RMS values for the different samples. Thus, for sample d30nmv1, the RMS values reported in Table 1 for a measurement scale above 1 μm are all equivalent and within the equivalent error. The same happens for the other two samples for the 10 and 5 μm observation scales. It is also worthy of note that the PSD of sample d420nmv1 is characterized by two slope regions (i.e., a first one from 3700 to 2000 nm and a second one L < 2000 nm). Tentatively, we attribute the first region to the development of big surface domains and the second to the columnar growth within these domains. Since the different parameters defined within the frame of the different contact angle models (c.f. equations eqs 3, 5, and 6) are ultimately related to the roughness and morphology of the surfaces, the PSD analysis of the roughness of samples suggests that these parameters should also depend on the length scale used for the analysis. We will demonstrate in the next section that this is actually the case. Langmuir 2010, 26(20), 15875–15882
Water Contact Angles and Roughness Parameters. Figure 4 shows the water contact angles of the samples represented against both their thickness and their RMS values deduced from 5 μm 5 μm AFM images. The three samples can be considered as hydrophobic with WCA values > 90° which are slightly higher than those reported in the literature for TiO2 surfaces.2a,8b,6,21 This difference can be a result of the distinct preparation methods and/or the sample characteristics (i.e., microstructure, roughness, residual content of carbonaceous residues, etc.). As expected, the WCA follows the same tendency as the surface roughness which, in turn, increases with the sample thickness. The maximum difference in WCAs (i.e., ∼22°) is observed between samples d30nmv1 and d2μmv2. If this increment was exclusively due to the different roughness of the two samples, according to the Wenzel model the equivalent flat surface WCA θ of the polycrystalline anatase would be g90°. Since this result would be in disagreement with the high surface energy measured for a single crystal TiO2,22 it seems unavoidable to consider other additional factors to properly account for the relationship between surface topography (21) Yang, T. S.; Shiu, C. B.; Wong, M. S. Surf. Sci. 2004, 548, 75. (22) Zisman, W. A. Adv. Chem. Ser. 1964, 43, 1.
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Figure 3. PSD curves as a function of the measurement scale for polycrystalline TiO2 surfaces deposited by PECVD. Vertical lines indicate the scale threshold for the roughness saturation.
Figure 2. (Top) 55 μm2 3D AFM images of the surface topography of the d30nmv1 (a), d420nmv1 (b), and d2μmv2 (c) samples and the corresponding RMS values. (d) Phase image derivative of (c) showing the angular features of d2μmv2. (Bottom) Line profiles extracted from the images in (a-c) (left) and height histograms and mean height (right). Table 1. Root Mean Square Roughness of the Three Samples Measured at Different Scales by AFM Imaging RMS (nm) 10 μm 10 μm 5 μm 5 μm 1 μm 1 μm 0.5 μm 0.5 μm d30nmv1 d420nmv1 d2μmv2
0.96 ( 0.03 23 ( 2 35 ( 5
a
0.97 ( 0.03 22 ( 2 35 ( 5
0.95 ( 0.01 11 ( 1.5 24 ( 2
0.90 ( 0.01 10.5 ( 1.5 8(1
a The error bars have been calculated by statistics of the RMS values in more than five images acquired in different areas of the surfaces.
and WCA of the thin films. Some of these factors can be related with the definition of roughness, and some others depend on samples characteristics such as their porosity and heterogeneity of their surfaces. To ascertain the influence of the observation scale on the calculated values of the r and f parameters in eqs 3, 5, and 6 we have preceded to their calculation on 1 1 μm2 and 5 5 μm2 AFM images. Table 2 summarizes the results of the analysis of the static water contact angle for the TiO2 polycrystalline thin films through the Wenzel and Cassie-Baxter approaches for the two selected observation scales. The difference in the calculated values of r and f for each sample confirms the need to choose a proper observation scale for their determination. Since the utilized scales are smaller (1 μm) and higher (5 μm) than the threshold lengths determined by the PSD curves (cf. Figure 3), we conclude that the second scale is the one to be used for the analysis of the wetting behavior of the studied samples. Table 2 also reports calculated values for θ, the wetting angle of an equivalent flat surface of 15880 DOI: 10.1021/la101975e
Figure 4. Water contact angle as a function of the thin film thickness (left) and roughness (right). The RMS values in the right panel were calculated from images of 5 μm 5 μm similar to those in Figure 2a-c. Table 2. Data Resulting from the Application of the Wenzel and Cassie-Baxter Models to the WCA Values Measured on Polycrystalline TiO2 Films of Different Roughness Measured on 1 μm 1 μm and 5 μm 5 μm AFM Images (see data in Table 1) Wenzel
θ0 (deg)
rW (1 μm)
θW (°)
rW (5 μm)
θW (°)
d30nmv1 d420nmv1 d2μmv2
95 110 117
∼1.017 ∼1.16 ∼1.17
∼95 ∼107 ∼113
∼1.004 ∼1.32 ∼1.25
∼95 ∼105 ∼111
Cassie-Baxter
θ0 (deg)
fCB (1 μm)
θCB (°)
fCB (5 μm)
θCB (°)
d30nmv1 d420nmv1 d2μmv2
95 110 117
∼0.53 ∼0.52 ∼0.54
∼43 ∼74 ∼89
∼0.645 ∼0.48 ∼0.47
∼65 ∼68 ∼85
TiO2. Since the calculated values vary depending not only on the observation scale but on the sample, we must admit that neither the Wenzel nor the Cassie-Baxter model describes properly the dependence between roughness and WCA in our samples. These inconsistencies clearly support the need of considering other surface features and roughness models to account for the wetting behavior of our films. The analysis of the AFM micrographs with the WSxM software17 permits highlighting the surface features which are above the mean height of the roughness variations. This selection of surface features can be used to calculate the equation parameters used within the MH model to account for the wetting behavior of surfaces (cf. eq 7). The images in Figure 5 indicate that the number of surface grains above the mean height increases with film thickness. According to the scheme used for the definition of Langmuir 2010, 26(20), 15875–15882
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Figure 5. 3D Miwa-Hashimoto model applied to the surface samples in AFM images. The number of the features above the mean height of the surface is indicated in the micrographs. The perimeter lines around the features are highlighted in the images. Table 3. Data Resulting from the Application of the Miwa-Hashimoto Model to the WCA Values Measured on Polycrystalline TiO2 Films with Different Roughness Measured on 1 μm 1 μm and 5 μm 5 μm AFM Images (see data in Table 1) Miwa-Hashimoto θ0 (deg) d30nmv1 d420nmv1 d2μmv2
95 110 117
area/perimeter (1 μm) ∼6.44 nm ∼36.6 nm ∼43 nm
θ0 (deg) area/perimeter (5 μm) d30nmv1 d420nmv1 d2μmv2
95 110 117
∼10.05 nm ∼36.6 nm ∼58 nm
rMH
fMH
∼1.007 ∼0.53 ∼1.13 ∼0.52 ∼1.04 ∼0.54 rMH
fMH
∼1.26 ∼0.645 ∼3.346 ∼0.48 ∼2.72 ∼0.47
θMH (°) ∼44 ∼76 ∼82 θMH (°) ∼81 ∼83 ∼82
eqs 7 and 8, the area to perimeter ratio of these features also determines rMH, although in an opposite way. Values of rMH and fMH calculated through the digital analysis of the AFM images for the three samples in Figure 5 and their equivalents but taken from images taken with a 1 1 μm2 observation scale are summarized in Table 3. Three main conclusions can be inferred from the results in Tables 2 and 3: (i) The values of the roughness factors defined in eqs 3, 5, 6, and 7 (as they do the measured RMS values reported in Table 1) depend on the scale of the images utilized for their calculation. This means that the scaling behavior of the roughness/surface morphology affects the roughness factors and through them to the calculated equivalent water contact angles θ. This is quite evident by comparing the calculated values of θCB (1 μm) with θCB (5 μm) and θMH (1 μm) with θMH (5 μm) that show quite different values for samples of different thickness. (ii) For the 5 5 μm observation scale, already above the saturation lengths for the three studied samples, the Miwa-Hashimoto model provides similar values of the equivalent contact angle independently of the particular sample used for the calculation. (iii) Taking this concordance as a proof of concept, the contact angle of the equivalent TiO2 surface, that is, homogeneous, smooth, and compact surface, is θ ∼ 82°. We must stress that this value of θ corresponds to polycrystalline samples that have been prepared by PECVD and kept stored for 3 months after their synthesis and that it, therefore, should not necessarily coincide with other theoretical or experimental values of θ determined for flat TiO2 surfaces.2a,8b,6,21 Certainly, it can be expected that samples prepared by using alternative procedures of synthesis will render distinct surface states with significant differences in their hydroxylation state, crystal planes, or hydrocarbon contamination (note that this is not the case for the samples used here; all of them are prepared and handled under the same conditions). It is also important to mention that the use of the MH model to describe the wetting behavior of the studied films proceeds through the calculation of a magnitude (i.e., area/ perimeter of surface features) with length (L) dimensions. This fact relates to McCarthy’s considerations on the Young equation Langmuir 2010, 26(20), 15875–15882
stressing that it relates forces at the perimeter of the sessile drops and that, therefore, roughness concepts based on surface dimensions (i.e., L2) should be avoided for a proper description of wetting. Although the objectives of the present work do not address specifically this controversial matter, the fact that the MH model describes properly the experimental findings incidentally supports the need for a critical revision of the main concepts explaining the wetting behavior of surfaces. Wetting Behavior under UV Irradiation. The comprehension of the photoactivation processes of TiO2 surfaces to transform them into superhydrophilic surfaces, a characteristic feature depicted by both crystalline and amorphous materials,2,3,7-9 has fostered several studies with single crystals to ascertain the influence of crystalline phase and crystal plane in determining this behavior. Thus, for example, the photoactivity of different single-crystalline anatase TiO2 surfaces, mainly focused on the (001) and (100) planes, has been the subject of different studies.2b,23 Similarly, the wetting behavior of polycrystalline TiO2 thin films seems to depend on the degree of crystallinity and crystal phases present at the surface.24 However, despite the ample literature on the subject, not many studies have addressed the effect of the microstructure, structure, and optical properties on the wetting behavior of polycrystalline TiO2 thin films.2,6,9,21 In this regard, we have demonstrated previously (cf. refs 9, 16, and 18 and Figure 1) that the PECVD methodology for the growth of polycrystalline anatase provides precise control of the texture and crystal size of the thin films. With the aim of evaluating the role played by these two latter characteristics of the thin films, we have studied here the evolution of the WCA of samples d30nmv1, d420nmv1, and d2μmv2 as a function of the UV irradiation time and their recovery under VIS light, heating in air, and storage in dark. The top panel of Figure 6 shows that the time evolution of the WCA under UV light clearly differs for the thinner sample (d30nmv1) with respect to the two other samples. For sample d30nmv1, the conversion from hydrophobic to hydrophilic is much slower, approaching the kinetics of an amorphous sample (inset in Figure 6 top). Such a result is in a good agreement with the poor crystallinity and small crystal size determined for this sample (Figure 1). The other two samples present a similar evolution, although the kinetics is slightly faster for sample d2μmv2 (Figure 6, top), precisely the thin film with the highest crystallinity. By contrast, this sample presents the slowest recovery of WCA when subjected to the different surface activation treatments after the UV irradiation (Figure 6, bottom). An inverse relationship between crystallinity and efficiency for recovery is clearly supported by the relatively fast increase of the WCA observed for sample d30nmv1. Thus, while this sample reaches a wetting angle of ∼70° (23) (a) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188. (b) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (24) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33.
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large facets (c.f. Figure 1 and ref 16.). We think that the inverse relationship found here between the texture and crystal size of the films and the recuperation kinetics of the hydrophobic character fits within the frame of the amphiphilic model originally proposed by Wang and co-workers2,8b by admitting that the wetting changes require a collective transformation of the crystalline planes exposed to the surface. Within this scheme, the recovery of the surface hydrophobicity at macroscopic level requires the total conversion of planes with localized hydrophilic sites (i.e., newly formed Ti-OH groups according to Wang and coworkers) into hydrophobic sites (i.e., Ti-O-Ti surface sites),2 a rearrangement that should require more activation energy on well crystallized surfaces or in surfaces with a preferential (112) texture. This hypothesis also sustains that, although sample d420nmv1 presents a full crystalline character, the smaller mean size of the surface crystals and its low texturation in comparison with d2μmv2 (cf. Figures 1 and 2) make it that the former sample presents a faster recovery of hydrophobicity than the latter. In this regard, the fastest recovery of wetting angle, quite similar to that of an amorphous sample (data not shown), is found for sample d30nmv1 because of its low crystallinity.
Conclusion
Figure 6. (Top) Evolution of the water contact angle of the surface of the polycrystalline thin films with the UV irradiation time. The inset shows the change in the WCA of an amorphous TiO2 thin film fabricated by PECVD at room temperature. (Bottom) Recuperation of the WCA of the thin films after UV illumination by irradiation with visible light, heating in air at 383 K, and storage in dark during a prolonged time. Images of the real water droplets have been included in both panels.
after 5 min of VIS irradiation, d2μmv2 starts a slow recovery of its pristine hydrophobic state after a much prolonged period in the dark. Meanwhile, sample d420nmv1 presents an intermediate behavior in both directions, transformation into superhydrophilic under UV illumination and recovery under VIS irradiation, heating, or storage in the dark (note that the effect of VIS light or heating does not bring about a very significant difference in the recuperation kinetics, in contrast with previously claimed effects of heat and light in this transformation).8b These differences among samples indicate that their texture and crystallinity are crucial factors to account for the photowetting behavior of TiO2 thin films. In particular, our results support that the hydrophilic to hydrophobic conversion should be related to the size and surface tension of the outer planes of the thin film acting as an interface at the contact line between the film and the water and the film and the air. Along with this view, the slowest recovery kinetics corresponds to the sample presenting a higher degree of texturation (d2μmv2) by the (112) planes and a characteristic surface morphology formed by a dense package of angular and
15882 DOI: 10.1021/la101975e
In this work, we have carried out a thorough study on the wetting behavior of polycrystalline TiO2 thin films deposited by PECVD. This investigation has focused on the analysis of the influence of roughness on the WCA in the dark and the on the influence of the UV irradiation into the WCA transformation. The precise AFM characterization of the surface topography of the samples has allowed us to demonstrate the scaling behavior of the roughness factors involved in the evaluation of the equivalent wetting angle for a flat and compact surface. It has been demonstrated that the scale of measurement is critical for a proper evaluation of this coefficients and that the empirical relationship between surface roughness and WCA is properly described by the Miwa-Hashimoto model. The optimum observation scale in our case has resulted to be 5 μm 5 μm, in any case higher than the typical correlation or threshold lengths found for each sample by the evaluation of their roughness within the premises of the DST. When the polycrystalline TiO2 thin films are subjected to UV irradiation, their surface transforms from hydrophobic into superhydrophilic. Some qualitative correlations have been observed for the time response of this transformation process and its reversal by VIS illumination, heating, or storage in the dark, and the surface texture and crystal size of the films. Acknowledgment. The authors thank the European Union (Project NATAMA STRP 032583, Contract No. 032583), Spanish Minister of Research and Innovation (Projects FUNCOAT (CONSOLIDER-INGENIO CDS2008-0023) and MAT200765764), and Junta de Andalucı´ a (P09-TEP-5283).
Langmuir 2010, 26(20), 15875–15882
