and Photoinduced Changes in the Water ... - ACS Publications

Feb 15, 2005 - Ian Wark Research Institute, University of South Australia, Mawson Lakes, ...... condensation of vicinal silanols/titanols to siloxane/...
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Thermally- and Photoinduced Changes in the Water Wettability of Low-Surface-Area Silica and Titania A. Kanta, R. Sedev, and J. Ralston* Ian Wark Research Institute, University of South Australia, Mawson Lakes, Adelaide, SA 5095, Australia Received September 12, 2004. In Final Form: January 12, 2005 The surface properties of silica and titania are mainly determined by the presence, density, and type of terminal hydroxyl groups (Si-OH “silanol” and Ti-OH “titanol”). Thermal treatment at elevated temperatures causes dehydroxylation on both surfaces, confirmed by streaming potential and ToF-SIMS measurements. The magnitude of the zeta potential markedly decreases after heat treatment, but the IEP is not affected. The intensity ratio MOH+/M+ (M ) Si or Ti), which reflects the surface density of OH groups, also decreases noticeably after high-temperature treatment. The mechanism is condensation of adjacent silanol/titanol groups into siloxane/titanoxane bonds. Ultraviolet light (λ ) 254 nm) has little effect on silica but rapidly induces hydrophilicity on titania surfaces. There is a strong correlation between the amount of hydrocarbons adsorbed on the surface and the density of titanol groups (thence the water contact angle). The effect of UV radiation can be entirely attributed to photolytic decomposition of organic contaminants. Dehydroxylated titania and silica (at 1050 °C) show very different wetting behavior: silica is moderately hydrophobic (water contact angle of about 40°), while titania is hydrophilic (0°). This dissimilarity can be explained with a simple model estimating the van der Waals and acid-base interfacial interactions.

Introduction The wettability of materials is an important factor in numerous industrial and natural processes. Metal oxides (e.g., SiO2, TiO2, etc.) are widely popular materials, and their wettability is of significant practical interest. The contact angle is the simplest characteristic of the physical interaction between a liquid and a solid surface and is determined by the nature of the three contacting phases (solid, liquid, and vapor). The wettability can be considerably modified by varying the composition of the system. However, it is often advantageous to affect the intrinsic wettability by applying an external influence, e.g., light irradiation,1-3 electric field,4-6 thermal treatment, etc. The surface of a metal oxide is a high-energy surface (i.e., the surface of a hard solid7) and therefore should be completely wetted by most liquids.7,8 This, however, applies for a clean surface only, as any adsorption will drastically modify the surface energetics.8 Obtaining and keeping a high-energy surface clean under the circumstances of contact angle measurement proved to be a major difficulty in the wetting studies of such surfaces.9 A contemporary review of the wettability of metals, glass, * Author to whom correspondence should be addressed. Phone: +(61 8) 8302-3225. Fax:+(61 8) 8302-3683. E-mail: [email protected]. (1) Abbott, S.; Ralston, J.; Reynolds, G.; Hayes, R. Langmuir 1999, 15, 8923. (2) Stevens, N.; Priest, C. I.; Sedev, R.; Ralston, J. Langmuir 2003, 19, 3272. (3) Richards, N.; Ralston, J.; Reynolds, G. Contact Angle, Wettability Adhes. 2003, 3, 361. (4) Quinn, A.; Sedev, R.; Ralston, J. J. Phys. Chem. B 2003, 107, 1163. (5) Peykov, V.; Quinn, A.; Ralston, J. Colloid Polym. Sci. 2000, 278, 789. (6) Schneemilch, M.; Welters, W. J. J.; Hayes, R. A.; Ralston, J. Langmuir 2000, 16, 2924. (7) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827. (8) Zisman, W. A. Adv. Chem. Ser. 1964, 43, 1. (9) White, M. L. In Clean Surfaces: Their Preparation and Characterization for Interfacial Studies; Goldfinger, G., Ed.; Marcel Dekker: New York, 1970; p 361.

and graphite was published by Schrader.10 The development of the controversy surrounding the wettability of gold is particularly instructivesit was not until contact angle measurements were performed with carefully purified components and in ultrahigh vacuum that the intrinsic hydrophilicity of gold was firmly established. A clean silica surface is hydrophilic (water contact angle is 0°), but when left exposed to the open atmosphere, it gradually turns into mildly hydrophobic (contact angle ∼40° 11,12) due to organic contamination.13 Such behavior is typical for high-energy surfaces, e.g., mica, metals, metal alloys,9 and metal oxides.13 The advancing water contact angle on metals kept under industrial conditions is typically in the range from 40° to 80°.14 Titanium dioxide is no exception to this rule2. The surface of silica is probably one of the most extensively studied surfaces.15-20 Its properties are chiefly determined by the density and type of the hydroxyl groups (Si-OH “silanol”) present on the surface. On the average, about five silanols per square nanometer are found on a fully hydroxylated silica surface. Thermal dehydroxylation (10) Schrader, M. E. In Modern Approaches to Wettability: Theory and Applications; Plenum Press: New York, 1992; p 53. (11) Lamb, R. N.; Furlong, D. N. J. Chem. Soc., Faraday Trans. 1 1982, 78, 61. (12) Yang, J.; Duan, J.; Fornasiero, D.; Ralston, J. J. Phys. Chem. B 2003, 107, 6139. (13) Takeda, S.; Fukawa, M.; Hayashi, Y.; Matsumoto, K. Thin Solid Films 1999, 339, 220. (14) Horsthemke, A.; Schroeder, J. J. Chem. Eng. Fundam. 1983, 2, 66. (15) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Marcel Dekker: New York, 1967. (16) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry; Wiley: New York, 1979. (17) Legrand, A. P. The Surface Properties of Silicas; John Wiley: New York, 1998. (18) Vansant, E. F.; van der Voort, P.; Vrancken, K. C. Characterization and Chemical Modification of the Silica Surface; Elsevier: Amsterdam, 1995. (19) Parfitt, G. D. Pure Appl. Chem. 1976, 48, 415. (20) Zhuravlev, L. T. Colloids Surf. A 2000, 173, 1.

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occurs at any temperature higher than about 150-200 °C according to the condensation reaction:

Dehydroxylation becomes irreversible above 400 °C, and there are less than 1 OH/nm2 after heat treatment at 800 °C. The surface with many siloxane bridges is much less effective in hydrogen bonding water, and the contact angle is about 40°.11 Rehydroxylation of silica is generally slow. Hydroxyl groups (“titanol”, Ti-OH) are also present on the surface of titania and are responsible for its adsorption and catalytic properties.15,19,21,22 On the average, about eight titanols per square nanometer are found on the surface of titania (theoretical estimates are even higher: 9-11 nm-2 for rutile and 12-14 nm-2 for anatase21). The chemistry of surface titanols must be analogous to that of surface silanols. In particular, thermal treatment should induce dehydroxylation as follows

After outgassing at 200 °C, only three hydroxyls are left per square nanometer, and virtually no OH groups are present after heat treatment at about 500 °C. While high-temperature treatment has a similar effect on silica and titania, the impact of UV irradiation is very different. This is hardly surprising as titania is a semiconductor with a band gap energy of about 3.2 eV, and therefore, wavelengths shorter than about 390 nm are capable of exciting electrons. In silica (band gap energy ) 9 eV 23), a much higher photon energy would be required. Indeed, the photocatalytic properties of titanium dioxide are well documented.24 UV light has little influence on silicasultraviolet radiation in the presence of oxygen is used for dry cleaning of silicon wafers from organic contamination.25 Halfpenny et al.26 have found that highintensity UV laser radiation can cause local dehydroxylation on the surface of silica (according to eq 1), but some damage was also detected. In 1997, Wang et al.27 announced that UV light generates hydrophilic TiO2 surfaces and strongly argued that the phenomenon is not directly related to the photocatalytic activity of titania but should rather be attributed to the reversible formation of hydrophilic nanodomains seen with scanning probe microscopy. Contamination was dismissed as a possible explanation of the experimental observations28 since (i) washing a hydrophobic TiO2 sample with acetone and then water did not change the water contact angle significantly (51° before and 48° after the cleaning29); (ii) when a hydrophilic (21) Parfitt, G. D. In Progress in Surface and Membrane Science; Cadenhead, D. A., Danielli, J. F., Eds., 1976; Vol. 11, p 181. (22) Textor, M.; Sitting, C.; Frauchiger, V.; Tosatti, S.; Brunette, D. M. In Titanium in Medicine; Brunette, D. M., Tengvall, P., Textor, M., Thomsen, P., Eds.; Springer: Berlin, 2001; p 171. (23) Sze, S. M. Physics of Semiconductor Devices, 2nd ed.; Wiley: New York, 1981. (24) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C 2000, 1, 1-21. (25) El-Kareh, B. Fundamentals of Semiconductor Processing Technologies; Kluwer Academic Publishers: Boston, 1995. (26) Halfpenny, D. R.; Kane, D. M.; Lamb, R. N.; Gong, B. Appl. Phys. A 2000, 71, 147. (27) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature (London) 1997, 388, 431. (28) Sun, R.-D.; Nakajima, A.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 2001, 105, 1984. (29) Sakai, N.; Wang, R.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Langmuir 1998, 14, 5918.

Figure 1. Simplified scheme of surface dehydroxylation (M ) Si or Ti): silanol/titanol condensation into siloxane/titanoxane bonds.

(UV-irradiated) sample was sonicated in pure water the contact angle of water increased from 0° to 11°;29 (iii) in an oxygen atmosphere, as compared with air, the decrease of the water contact angle after UV illumination was slower, and the increase after dark storage was faster.30 Miyauchi et al.31 investigated the water contact angles and the photodecomposition of Methylene Blue on a dozen different oxides and concluded that photocatalysis and photoinduced hydrophilicity are uncorrelated. We have recently shown2 that titania is easily converted into a hydrophilic state after exposure to UV light and then rapidly reverts to hydrophobic when left in open air. The contact angle change is quite significant (∼80°) and reversibly repeatable. The mechanism of the hydrophobichydrophilic conversion, however, has not been fully elucidated. In this investigation, we report the wettability changes on silica and titania flat plates after thermal treatment at elevated temperature and irradiation with ultraviolet light. We test the hypothesis that silica and titania surfaces have analogous behavior when heated (Figure 1). The impact of ultraviolet light is also investigated. The behavior of the terminal hydroxyl groups is probed with contact angle and streaming potential measurements, ToF-SIMS measurements, and AFM imaging. Materials and Methods Silicon wafers, P(100), were kindly provided by Dr. M. Bjelopavlic (MEMC Electronic Materials). The RMS roughness of the wafers was 0.24 nm, and the peak-to-valley height was 0.88 nm, as obtained with atomic force microscopy (AFM) imaging. Silicon wafers covered with a thin layer of titanium dioxide were obtained from Philips Research Laboratories (Eindhoven, The Netherlands). Magnetron sputtering was used to deposit 25 nm thick layers of pure and stoichiometric TiO2 (XPS detection limit was 0.1 at. wt%) in the amorphous state (details can be found elsewhere32). The root-mean-square (RMS) roughness of the layers was 0.3 nm, and the peak-to-valley height was 1.5 nm over an area of 1 µm2. Both titania and silica were stored in polypropylene boxes (Fluoroware H22-60) in a laboratory for several months. Samples were cut to size (10 × 10 mm2) and subsequently blown with a stream of filtered nitrogen to remove any debris left from the cutting process. These samples are further referred to as “untreated”. Silica and titania samples were subjected to two types of treatment: (i) thermal treatment at elevated temperatures (up to 1050 °C) and (ii) irradiation with ultraviolet light. Thermal treatment was carried out for 15 h in air in an oven (Jetflow model K2F, Australia). Samples were placed in a custom(30) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188. (31) Miyauchi, M.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Chem. Mater. 2002, 14, 2812. (32) Feiler, A.; Jenkins, P.; Ralston, J. Phys. Chem. Chem. Phys. 2000, 2, 5678.

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built quartz cell to minimize contamination. After heat treatment, samples were steadily cooled and subsequently loaded into the ToF-SIMS vacuum chamber or the cell of the streaming potential apparatus within 5 min of actual removal from the oven. Silica and titania samples were irradiated in ambient atmosphere with ultraviolet light of fixed wavelength (λ ) 254 nm) using two different sources: a spectral irradiator (MM3, BunkohKeiki Co., Japan, with a 2 kW Xenon arc lamp giving a flux of 13.6 mW/cm2) or, alternatively, a UV eraser (Lawtronics ME15). A constant radiant exposure (23.8 J/cm2) was used throughout the investigation. Advancing water contact angles were measured with the sessile drop method. Digital images of the droplet silhouette (624 pixels × 580 pixels, 256 gray levels) were captured with a progressive scan CCD camera (JAI CV-M10BX, Japan). The contact angle was determined by numerically drawing a tangent close to the edge of the droplet. All measurements were performed with highpurity water at pH ) 5.8 (resistivity > 18 MΩ‚cm and surface tension of 72.8 mN/m at 20 °C). Streaming potential measurements were performed using an apparatus based on the design of Scales et al.33 Electrolyte solution (10-4 and 10-3 M analytical grade KCl with a pH adjusted with analytical grade KOH or HCl) was circulated through the cell under constant hydrostatic pressure, P, and the streaming potential, E, was measured. The zeta potential, ζ, was calculated through the Smoluchowski equation:34

ζ)

ηλ ∆E 0 ∆P

(3)

where λ is the conductivity of the capillary, 0 is the permittivity of free space, η is the viscosity of the liquid, and  its dielectric constant. Two identical plates (75 × 25 × 1 mm3) were always used (silica plates were purchased from Herbert Groiss, Melbourne, Australia). The plates were rinsed with water, sonicated in ethanol for 20 min, rinsed again with high-purity water, dried in a stream of high-purity nitrogen, and finally plasma-treated for 60 s. The water contact angle was zero on both silica and titania surfaces after this procedure. Thermally treated silica and titania samples were only rinsed with high-purity water. All measurements were carried out at room temperature (∼22 °C) and in triplicate. Static SIMS measurements were performed on a PHI TRIFT 2100 time-of-flight secondary ion mass spectrometer (ToF-SIMS) equipped with a pulsed 15 keV Ga liquid metal ion gun (LMIG). The primary ion-beam current was 600 pA, the pulse rate was 10 kHz, and the acquisition time was 60 s. To ensure that only a small fraction of surface material is removed, the primary ion current density on the sample was reduced to less than 5 × 1012 ions/cm2 during acquisition. On each sample, 10 spots of size 100 × 100 µm2 were analyzed. Each intensity ratio is expressed as an average value with a 95% confidence interval. Prior to the statistical analysis, the data were normalized by dividing the intensity of the ion of interest by that of the ion yield for the sum of all peaks. The following set of fragments was used for normalization in all experiments with silica: Si+, SiOH+, C2H3+, C3H3+, C3H5+, C4H7+, and C4H9+. In experiments with titania the set was C2H3+, C2H5+, C3H5+, C3H7+, Ti+, TiH+, TiO+, and TiOH+. The intensity of 48TiOH+ was always used (i.e., the total intensity at 65 amu corrected for the contribution of 49TiO+ by using the 49Ti/48Ti abundance ratio35). Surface imaging was performed with an atomic force microscope (NanoScope III, Digital Instruments) in tapping mode (TMAFM). Silicon SPM cantilevers (Pointprobe NCH, Nanosensors) were used to collect detailed topographic information. RMS roughness and peak-to-valley height were obtained with the AFM running software. Raman spectra were acquired on a commercial Raman microscope (Renishaw Ramascope System 1000). The system (33) Scales, P. J.; Grieser, F.; Healy, T. W.; White, L. R.; Chan, D. Y. C. Langmuir 1992, 8, 965. (34) Hunter, R. J. Zeta Potential in Colloid Science: Principles and Applications; Academic Press: London, 1981. (35) Matolin, V.; Johanek, V. Vacuum 2002, 67, 177.

Figure 2. Advancing contact angle of water on titania plates: before (O) and after (b) heat treatment at various temperatures. The contact angle is measured immediately after cooling the sample down. consists of a single spectrograph fitted with holographic notch filters and a Peltier-cooled CCD detector upon which the Raman spectrum is dispersed. The spectrograph is coupled to a Leica DMLM microscope, rigidly fixed to the spectrograph baseplate. The spectrometer has a maximum lateral resolution of 2 µm and depth resolution of 2 µm. A solid-state diode laser (Renishaw, 532 nm, 20 mW) was used as the incident light source.

Results The influence of heat treatment on the wettability of titania is presented in Figure 2, which shows the advancing contact angle of water on titania plates before and after baking the sample at various temperatures. The contact angle on the plates kept in storage (clean polypropylene box in a dark drawer) is 81.0 ( 1.6°, and this remarkable reproducibility is probably one reason for attributing a fundamental significance to the hydrophobicity of titania. Heat-treating the samples reduces the contact angle (measured immediately after the sample has cooled to room temperature), and if the temperature is higher than 300 °C, the samples become totally hydrophilic. Baking the samples for 1 or 15 h gave the same result. It appears from these results that the surface was hydrophobic due to adsorbed contaminationsafter organic components have been pyrolyzed, at 300 °C or above, the surface is left clean and therefore hydrophilic. The titania surface remains hydrophilic even after being treated at 1050 °C (Figure 2) when dehydroxylation must be complete.21 This is in marked contrast with silica, which becomes moderately hydrophobic under similar conditions. The hydrophobicity of dehydroxylated silica is attributed to the significant reduction of the surface density of silanols due to their condensation into siloxane bonds. The ζ potential is very sensitive to the surface density of OH groups because their dissociation is the underlying mechanism for surface charge creation.34 The outcome of our streaming potential measurements on silica is given in Figure 3. The sign and magnitude of the ζ potential, as well as the isoelectric point (IEP ≈ 2), are in line with published data.33 Note the significant decrease in ζ potential after the silica samples have been heat-treated at 1050 °C. The widely accepted explanation is that almost full dehydroxylation of the surface has occurred.36 Streaming potential measurements were carried out with titania plates in exactly the same manner, and the results are shown in Figure 4. The values of the ζ potential are smaller, and the changes, induced by ionic strength and/or heat treatment, are also smaller when compared (36) Grieser, F.; Lamb, R. N.; Wiese, G. R.; Yates, D. E.; Cooper, R.; Healy, T. W. Radiat. Phys. Chem. 1984, 23, 43.

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Figure 3. Zeta potential on silica as a function of pH (O, b: 10-4 M KCl; 0, 9: 10-3 M KCl): before (O, 0) and after (b, 9) heat treatment at 1050 °C.

Figure 5. Raman spectra of a titania layer: (a) untreated; (b) heated at 600 °C (15 h); (c) heated at 800 °C (15 h); and (d) heated at 1050 °C (15 h).

Figure 4. Zeta potential on titania as a function of pH (O, b: 10-4 M KCl; 0, 9: 10-3 M KCl): before (O, 0) and after (b, 9) heat treatment at 1050 °C.

Figure 6. Advancing contact angle of water on silica (0) and titania (b) plates after exposure to UV light (λ ) 254 nm, 23.8 J/cm2) at room temperature. The contact angle is measured immediately after the illumination.

with silica. The overall behavior, however, is the same. The IEP ≈ 4.5 is very close to the value 4.3 reported previously for the same material.32 This value is also lower than the frequently reported values (≈6) for titania particles. It must be noted that the IEP for titania strongly depends on sample preparation and can vary widely.37 In full analogy with silica, the IEP of titania is not shifted after heat treatment (Figure 4) implying that the nature of the surface dissociation groups (titanols) has not been radically altered. The decreased magnitude of the surface charge is purely due to the decreased surface density of titanol groups. In other words, at high-temperature, dehydroxylation occurs on the surfaces of silica and titania in a similar manner (Figure 1). The thermal treatment at elevated temperature induces some morphological changes in the titania layer. AFM imaging showed no specific features on the surfaces before or after heat treatment. The untreated titania layer had a RMS roughness of 0.3 nm and a peak-to-valley height of 1.5 nm. After the sample is baked at 1050 °C for 15 h, these values increase to 3.5 and 9.5 nm, respectively. Such changes are not found on silica,12 and we speculate that the roughening of the titania surface is due to the mismatch of the thermal expansion coefficients of silica (0.5 × 10-6 1/K 23) and titania (7.1 × 10-6 1/K 38). This large difference means that the titania film is likely to be under residual stress39 which may roughen its surface. An alternative mechanismssurface reconstructions due to oxygen desorption at high temperature (titania is prone to reduction)shas been suggested by one of the reviewers. (37) Kosmulski, M. Adv. Colloid Interface Sci. 2002, 99, 255. (38) Sheppard, L. M. Am. Ceram. Soc. Bull. 1991, 70, 1467. (39) Madou, M. J. Fundamentals of Microfabrication: The Science of Miniaturization, 2nd ed.; CRC Press: Boca Raton, FL, 2002.

Another interesting aspect of the thermal treatment is revealed by Raman spectroscopysthe initially amorphous titania layer is gradually transformed into anatase (Figure 5). Although Raman spectroscopy is not a strict quantitative technique, it is certainly suited to detect different forms of TiO2. The characteristic peaks40 at 145 cm-1 (appearing after treatment at 600 °C) and also at 393 and 637 cm-1 (appearing after treatment at 800 °C) clearly show the gradual crystallization of anatase (the 520 cm-1 peak is due to the underlying silicon). It is generally accepted that rutile is the stable high-temperature phase41 but the transition temperature might be affected significantly by particle size because of the interplay between surface energy and polymorphism.42-44 There are reports of the transition been detected well above 1000 °C.42,45 The influence of UV illumination on the wettability of silica and titania is illustrated in Figure 6. Since titania is a semiconductor with a moderately wide band gap energy, excitation with UV light (λ e 390 nm) is possible, and therefore, catalytic oxidation of adsorbed organic species will be promoted. The contamination appears to be effectively removed and the intrinsic hydrophilicity of titanium dioxide is rapidly recovered (B, Figure 6). For all practical purposes, silica is an insulator and no (40) Chang, H.; Huang, P. J. J. Raman Spectrosc. 1998, 29, 97. (41) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6th ed.; Wiley: New York, 1999. (42) Zhang, H.; Banfield, J. F. Am. Mineral. 1999, 84, 528. (43) Ranade, M. R.; Navrotsky, A.; Zhang, H. Z.; Banfield, J. F.; Elder, S. H.; Zaban, A.; Borse, P. H.; Kulkarni, S. K.; Doran, G. S.; Whitfield, H. J. Proc. Natl. Acad. Sci. U.S.A. 2002, 99 Suppl. 2, 6476. (44) Navrotsky, A. Geochem. Trans. 2003, 34. (45) Sun, Y.; Egawa, T.; Zhang, L.; Yao, X. Jpn. J. Appl. Phys. 2 2002, 41, L945.

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Figure 7. Advancing contact angle of water on silica and titania plates exposed to laboratory atmosphere. Initially, the samples were rinsed with water, sonicated in ethanol, and then plasma cleaned.

wettability changes are registered under identical conditions (0, Figure 6). To provide further support for the above hypothesis, silica and titania samples were examined with ToF-SIMS. The ratio MOH+/M+ is taken as characteristic of the amount of hydroxyl groups on the surface. The SiOH+/Si+ ratio for the untreated surface is in acceptable agreement with values obtained by others (e.g., 0.04-0.25).26 It decreases when silica is baked at high temperature. This confirms the decrease in the density of hydroxyl groups on the surface, although the ratio cannot be quantitatively related to the area fraction of surface silanols. Even after a prolonged (15 h) heating at 1050 °C, some OH groups are still detected on the silica surface. Most probably this ensued from the fact that our samples were treated in air rather than in a vacuum. Indeed, when the hightemperature heat treatment was carried out in an evacuated (∼10-7 Torr) quartz furnace, dehydroxylation proceeded further and the SiOH+/Si+ ratio dropped down to about 0.01.46 The SIMS results for silica corroborate the conclusion drawn from the electrokinetic measurements. In the case of titania, however, the trend is more complicatedsthe surface density of titanols, as represented by the TiOH+/Ti+ ratio, decreases after thermal treatment but the value for the untreated surface is lower. This is an apparent contradiction as we demonstrate below. As explained in the Introduction, clean oxide surfaces are completely wetted by water. In a laboratory air environment, however, recontamination of high-energy surfaces is unavoidable. Silica and titania plates were precleaned and subsequently exposed to a laboratory (clean room) atmosphere. As organic molecules progressively adsorb, the initially hydrophilic surfaces gradually turn into hydrophobic ones. The changes can be easily quantified by monitoring the water contact angle as a function of time (Figure 7). The higher rate of contamination on titania correlates with the higher surface energy in comparison with silica.19,44 Recontamination is influenced by many factors (temperature and humidity, recent laboratory activities, mode of floor and bench cleaning, etc), and therefore, the above example should not be taken as a general rule. The main conclusion is that, under the same conditions, recontamination of titania is much faster (∼minutes) than that of silica (∼hours). In particular, standard procedures for keeping silica (or glass) at a (46) Wood, B. J.; Lamb, R. N.; Raston, C. L. Surf. Interface Anal. 1995, 23, 680.

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Figure 8. ToF-SIMS results for titania. Correlation between the intensity ratios TiOH+/Ti+ and C2H3+/Ti+. UV treatment: upper line. A, untreated sample; B, sample irradiated with UV light (23.8 J/cm2); C, two samples treated as in B after 1 h storage in a clean room (one in darkness and one exposed to visible light); D, sample irradiated with an increased UV dose (38.5 J/cm2); E, sample D after being kept for 19 h in the vacuum chamber of the SIMS instrument. Thermal treatment: lower line. The upper correlation is for hydroxylated titania, and the lower one is for dehydroxylated titania.

sufficient level of purity for interfacial studies might not be sufficiently stringent in the case of titania. Since hydrocarbon contamination plays a crucial role in the wettability of titania surfaces, it is more rigorous to report the TiOH+/Ti+ and C2H3+/Ti+ ratios simultaneously. The first one reflects the presence of surface hydroxyl groups, while the second one represents the adsorbed amount of hydrocarbons (similar results are derived if another organic fragment is used, e.g., C2H5+ or C3H5+; the C2H3+ signal was selected because it was the most intensive). The results for titania plates before and after UV irradiation are shown in Figure 8. Point A was obtained with an untreated sample (kept in storage for several weeks or longer) on which the contact angle of water was 80°. After exposure to UV light, the wettability of the sample improved radically (water contact angle, 0°) and the SIMS experiment gave point B in Figure 8, i.e., the surface contained more OH groups and less organic contamination. Two additional samples were then treated in parallel: one was intensively irradiated with visible light (λ ) 450 nm) for 1 h, while the other was simply kept in a dark box. Both samples remained exposed to one and the same laboratory atmosphere. Subsequent SIMS measurements yielded two statistically identical points labeled C in Figure 8. The contact angle of water was 62° on the sample exposed to visible light and 61° on the sample held in darkness. It therefore appears that visible light has no significant influence on the rate of recontamination or the final state of the titania surfaces. These results clearly demonstrate that the titania surface is very effectively cleaned by UV light (essentially due to catalytic photooxidation of the adsorbed hydrocarbons) but then is rapidly recontaminated (owing to the rather high surface energy of the clean TiO2 surface). The upper correlation in Figure 8 was further reinforced with the following experiment. An untreated titania sample was given a higher UV dose (38.5 J/cm2 as compared with 23.8 J/cm2), and the SIMS measurement showed less contamination and a higher TiOH+/Ti+ ratio (point D in Figure 8). The sample was left overnight (19 h) inside the chamber of the SIMS instrument (pressure ≈ 10-8-10-9 Torr), and the measurement was repeated. The result is labeled E in Figure 8. Even under vacuum conditions, some contamination has occurred (though much less, point E is closer to the standard UV-cleaned point B than to contaminated points A and C).

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Figure 9. Zeta potential on clean titania as a function of pH (O, b: 10-4 M KCl; 0, 9: 10-3 M KCl): before (O, 0) and after (b, 9) irradiation with UV light (λ ) 254 nm, 23.8 J/cm2) at room temperature.

Figure 10. Zeta potential on dehydroxylated (1050 °C, 15 h) titania as a function of pH (O, b: 10-4 M KCl; 0, 9: 10-3 M KCl): before (O, 0) and after (b, 9) irradiation with UV light (λ ) 254 nm, 23.8 J/cm2) at room temperature.

All these results lie on the same (upper) straight line in Figure 8. This strong correlation between hydrocarbon contamination and surface hydroxyl groups is a strong argument against any structural changes happening on the surface of titanium dioxide due to UV irradiation and allegedly independent of photocatalytic reactions. AFM imaging of titania samples before and after exposure to UV light showed no noticeable differences. Our findings imply that the hydrophobicity of titania is entirely due to contaminants present on the surface. The cleaner the surface, the closer the water contact angle to its intrinsic value on titania (0°). The upper correlation in Figure 8 also clarifies the apparent anomaly in the TiOH+/Ti+ ratiosthe value for the untreated surface is simply masked by the adsorption layer. The lower line in Figure 8 shows the correlation for SIMS measurements performed after heat-treating the titania samples. Thus, all SIMS data fall into two categories: hydroxylated (upper line in Figure 8) or dehydroxylated (lower line in Figure 8). The different slope is related to the different density of titanol groups on the surface (lower on the dehydroxylated one). On both types of surfaces, the quantity of detected OH groups is inversely proportional to the amount of hydrophobic contamination present. This corroborates the conclusion of other authors13 that surface OH groups act as adsorptive sites. Above a certain threshold (C2H3+/Ti+ ≈ 0.15), the adsorbed organic layer dominates the wettability of the surface, i.e., the surface becomes hydrophobic because the underlying hydrophilic titanium dioxide is masked as far as contact angle measurements are concerned.8 Finally, if ultraviolet light is simply oxidizing hydrocarbon contamination, it should have no influence on the surface of clean titania. Indeed, on clean titania, the same ζ potential is measured before and after UV exposure (Figure 9). The obvious conclusion is, since no contamination was present on the surface, one and the same population of titanols was sampled by the electrokinetic measurement before and after the UV illumination. The same is observed on dehydroxylated (and therefore clean) titania surfaces (Figure 10). The magnitude of the ζ potential is reduced due to the decreased density of surface OH groups. Subsequent irradiation with UV light has no effect whatsoever because the thermal treatment has also pyrolyzed any adsorbed hydrocarbons.

propensity for hydrocarbon adsorption.22 When left in a common laboratory atmosphere, titania is rapidly contaminated and often becomes quite hydrophobic (water contact angle ≈ 80°). The surface is most easily cleaned by irradiation with UV light. Recontamination proceeds swiftly (see Figure 7), and the wettability of the surface can be cycled.2 The supposed influence of visible lightspromoting the recontamination of the surfacesis not substantiated by the current results (points C in Figure 8). The visible light might have an apparent, i.e., indirect, effect on the hydrocarbon migration toward the clean surface, e.g., by inducing a local temperature change and/or a concentration gradient. Our results show a direct correlation between the amount of contamination on the surface and the contact angle of water. The strong influence of UV light can be attributed to the well-known photocatalytic properties of TiO2. This is supported by the fact that silica, under the same conditions, remains unchanged. No structural changes as a direct result of UV exposure were found, and there is no need to assume any such mechanism in order to explain our findings. The recent findings of Wang et al.47 lend strong support to our conclusion. Using a visible-infrared sum frequency generation technique, these authors have conclusively demonstrated that a thin organic layer is strongly bound to the titania surface. Furthermore, this layer is easily removed by UV irradiation and rapidly regenerated under ambient conditions. White et al.48 employed several ultrahigh vacuum techniques to investigate the behavior of water on clean and methylated titania surfaces. They have convincingly shown that clean titania surfaces are water wet, i.e., hydrophilic. Hydrophobicity, imparted by surface methylation, can be eliminated by exposure to UV light but is unrelated to the occurrence of oxygen vacancies, Ti3+, or water dissociation. These findings, together with our results, cast serious doubt on the validity of the mechanisms widely promoted by Hashimoto and co-workers.27-31,49 Dehydroxylation proceeds in a similar manner on both silica and titania. The decrease of the surface density of OH groups is clearly reflected in the ζ potential vs pH curves. Since the measurement takes several hours, it

Discussion Our findings confirm that the titanium dioxide surface is intrinsically hydrophilic, i.e., water will completely wet the surface provided that it is clean. Higher contact angles are easily observed on titania surfaces due to their

(47) Wang, C.-Y.; Groenzin, H.; Shultz, M. J. Langmuir 2003, 19, 7330. (48) White, J. M.; Szanyi, J.; Henderson, M. A. J. Phys. Chem. B 2003, 107, 9029. (49) Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 1028.

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Table 1. Input and Calculated Parameters for Dexydroxylated Silica and Titaniaa oxide

A × 1020 [J]

SiO2 TiO2

6.554

2 γLW S [mJ/m ]

2 WLW A [mJ/m ]

2 γ+ S [mJ/m ]

2 γS [mJ/m ]

2 WAB A [mJ/m ]

WA [mJ/m2]

35 97

55 92

0 0

52.7 52.7

73 73

129 165

18.156

a A, Hamaker constant; γLW, Lifshitz-van Der Waals component of the surface free energy; WLW, Lifshitz-van Der Waals component S A AB of the work of adhesion; γ+ S , acidic parameter of the solid surface; γS , basic parameter of the solid surface; WA , acid-base component of the work of adhesion; WA, (total) work of adhesion (eq 4)

also clearly indicates that rehydroxylation is not particularly fast (even on titania). The wetting behavior of silica and titania is different (the hydrophobicity of dehydroxylated silica is exceptional among other metal oxides50). In an attempt to clarify this difference, the work of adhesion of water to silica and titania is estimated below. Following van Oss, Good, and Chaudhury51,52 the work of adhesion, WA, is split into two components: AB WA ) WLW A + WA

(4)

The Lifshitz-van der Waals (LW) term accounts for van der Waals-London (dispersive), Keesom, and Debye types of interactions. The acid-base (AB) term includes the contribution of Lewis-type acid-base interactions across the interface, including hydrogen bonding. We consider fully dehydroxylated surfaces, i.e., assume that all surface OH groups have been removed. The LW component of the work of adhesion, WLW A , is then calculated as51,53 LW LW 1/2 WLW A ) 2(γS γL )

(5)

where γLW is the LW component of the solid surface S ) 21.8 mJ/m2 for water.53 The γLW energy and γLW is L S obtained from the Hamaker constant of the material, A, through51,54

γLW S )

A 24πl02

(6)

where l0 ) 1.57 ( 0.09 Å is an average (atomic) equilibrium distance.51 The figures for silica and titania are listed in Table 1 and shown with black bars in Figure 11. Both values are lower than the work of cohesion for water (WC ) 2γL ) 2 × 72.8 mJ/m2), shown with a dashed line in Figure 11, and therefore, both surfaces should be hydrophobic. The water contact angles can be estimated through the Young equation, written as

WA cos θ ) - 1 + 2 WC

(7)

and the values are unrealistically high (104° and 75°, respectively). As aptly described by Laskowski and Kitchener,55 “all solids would be hydrophobic, if they did not carry polar or ionic groups.” (50) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity, 2nd ed.; Academic Press: London, 1982. (51) Van Oss, C. J. Interfacial Forces in Aqueous Media; Marcel Dekker: New York, 1994. (52) Good, R. J.; van Oss, C. J. In Modern Approaches to Wettability; Schrader, M. E., Loeb, G. I., Eds.; Plenum Press: New York, 1992; p 1. (53) Fowkes, F. M. Adv. Chem. Ser. 1964, 43, 99. (54) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1985. (55) Laskowski, J.; Kitchener, J. A. J. Colloid Interface Sci. 1969, 29, 670.

Figure 11. Work of adhesion, WA, of water on silica and titania. The Lifshitz-van der Waals component, WLW A , is calculated through eq 5, and the acid-base component, WAB A , is calculated through eq 8; the values are listed in Table 1. The dashed line is the work of cohesion for water (WC ) 2γL ) 145.6 mJ/m2).

The acid-base component of the work of adhesion, 51,52 WAB A , is given by + - 1/2 + 1/2 WAB + 2(γA ) 2(γS γL ) S γL )

(8)

where γ+ i reflects the acidic (electron-pair acceptor) properties and γi the basic (electron-pair donor) prop2 erties, and for water, γ+ L ) γL ) 25.5 mJ/m (by definition). According to van Oss,51 the acidic parameter is rather small for most solids, and therefore, we have made the simplifying assumption that γ+ S ) 0 for both dexydroxylated oxides. The value of γS for silica was selected such that the contact angle calculated through eq 7 is equal to the experimentally observed value of 40°. The value of WAB A for titania was taken as equal to the value for silica (see Table 1 and gray bars in Figure 11). It can be seen clearly that the total work of adhesion (Table 1 and dark gray bars in Figure 11) is lower than the work of cohesion for water in the case of silica and higher in the case of titania. Thus, the different wettability of dehydroxylated silica and titania with respect to water is due to their considerably dissimilar Hamaker constants. This nicely illustrates the point that a high-energy surface is wetted by most liquids because of its higher polarizability rather than simply because of its high surface energy.7 The value of γLW S for silica (Table 1) is close to the lower end of the interval within which experimental values are scattered57 (from 40 to 105 mJ/m2), and therefore, eq 6 provides a reasonable estimate for our purpose. The values of γ+ S and γS for silica used here (Table 1) are similar to the values mentioned by Papirer and Ballard (0.13 and 62.4 mJ/m2, respectively).57 However, these authors did not specify the hydroxylation state of the silica surface. The acid-base parameters listed in Table 1 characterize a completely dehydroxylated oxide surface. Therefore, (56) Ackler, H. D.; French, R. H.; Chiang, Y.-M. J. Colloid Interface Sci. 1996, 179, 460. (57) Papirer, E.; Balard, H. In The Surface Properties of Silicas; Legrand, A. P., Ed.; Wiley: 1998; p 315.

Low-Surface-Area Silica and Titania

these acid-base properties are attributed to the siloxane bridges left behind the reacted hydroxyl groups. If the stoichiometry of eqs 1 and 2 is the same, then the number of siloxanes per unit area should be higher on titania (by virtue of the higher number of OH per unit area before dehydroxylation). For that reason, the value of WAB A for titania, given in Table 1, is probably underestimated (thus, reinforcing our conclusion). For partially or fully hydroxylated oxide surfaces, two more contributions to the interfacial free energy of adhesion must be considered: (i) hydrogen bonding and (ii) electrostatic interactions (if the surface is not at the point of zero charge). Under these conditions, even conservative estimates show that both silica and titania are hydrophilic.58 Conclusion The wettability of titania and silica surfaces is mainly governed by the density of hydroxyl groups (silanols and titanols, respectively). Intensive heat treatment reduces the number of OH groups per unit area. In both cases, the (58) Kaggwa, G.; Ralston, J. 2004, in preparation.

Langmuir, Vol. 21, No. 6, 2005 2407

condensation of vicinal silanols/titanols to siloxane/titanoxane bonds is plausible. Silica becomes moderately hydrophobic, while titania remains hydrophilic. UV irradiation has no effect on silica but is remarkably effective in removing organic contamination from titania surfaces. The effect of UV light can be entirely attributed to photocatalytic decomposition, and therefore removal, of organic contamination. The wettability difference between silica and titania can be rationalized with a simple model incorporating van der Waals and Lewis-type acid-base interfacial interactions. The wettability contrast is significant and can be used to produce micro-sized patterns on a flat surface.59 Acknowledgment. Financial support for this project from the Australian Research Council through the Special Research Centre Scheme is gratefully acknowledged. The authors thank one of the reviewers for his competent and stimulating comments. LA047721M (59) Kanta, A.; Sedev, R.; Ralston, J. 2005, submitted for publication.