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Photoinduced “Stick-Slip” on Superhydrophilic Semiconductor Surfaces Kieth R. Denison†,‡ and Colin Boxall*,† Centre for Materials Science, UniVersity of Central Lancashire, Preston PR1 2HE, United Kingdom, and Oxley DeVelopments Company Ltd, Priory Park, UlVerston, Cumbria LA12 9QG, United Kingdom ReceiVed NoVember 16, 2006. In Final Form: January 29, 2007 Transparent mesoporous TiO2 (M-TiO2) thin films were prepared on quartz via a reverse micelle, sol-gel, spincoating technique. Films were characterized by atomic force microscopy (AFM) and Raman and UV-vis spectroscopies and were found to be mostly anatase with low surface roughness (Rt ≈ 5 nm). The time dependence of film photoinduced superhydrophilicity (PISH) was measured by observation of the spreading of a sessile water drop using a new, continuous measurement technique wherein the drop was first applied to the semiconductor surface and then was filmed while it and the underlying substrate were illuminated by 315 nm ultraband gap light. Results obtained at 100% relative humidity (RH) at 293 K showed that drops on M-TiO2 surfaces exhibited a photoinduced “stick-slip” behavior, the first time such an effect has been observed. The thermodynamic driving force for this photoinduced stick-slip was the departure of the system from capillary equilibrium as, with increasing illumination time, the concentration of surface Ti-OH groups increased and the equilibrium contact angle of the drop, θ0, decreased. A simple theoretical description of photoinduced stick-slip is derived and is used to calculate a value of the potential energy barrier associated with surface inhomogeneities that oppose onset of movement of the triple line, U ) 6.63 × 10-6 J m-1. This is the first time that U has been quantified for a surface with photoinduced superhydrophilicity. Triple line retreat measurements on an evaporating drop on M-TiO2 in the dark, RH ) 60%, T ) 293 K, gave a value of U ) 9.4 × 10-6 J m-1, indicating that U decreases upon UV illumination and that U in the light is primarily associated with inhomogeneities that are unaffected by an increase in the surface Ti-OH population, such as the physical roughness of the surface. In the dark evaporation experiment, the drop was found to retreat with an areal velocity of 1.48 × 10-8 m2 s-1. However, under UV illumination, the drop was found to spread at a substantially faster velocity of 2.33 × 10-5 m2 s-1, the latter being of the order of the velocities of 10-4 m2 s-1 observed in (dark) drop-spreading experiments conducted in the presence of trisiloxane surfactant superspreaders. This suggests that, once slip has started, the triple line processes over a thin precursor film of condensed water whose formation has been promoted by the photoinduced increase in the Ti-OH population at the semiconductor surface.
1. Introduction Wettability is one of the most significant properties of solid surfaces and is influenced by both surface energy and the geometrical microstructure of the surface.1-4 Control of wettability is an important technology in areas such as printing, adhesion, and coating. A range of methods for achieving control have been reported, including the use of temperature gradients,5 the application of electric fields,6,7 chemical surface modification,8 and the control of surface microsctructure.9 This report is concerned with the recently discovered phenomenon of photoinduced superhydophilicity, which is currently attracting significant attention as a new method for control of surface wettability. * To whom correspondence should be addressed. Tel.: +44 1772 893530; fax: +44 01772 892996; e-mail:
[email protected]. † University of Central Lancashire. ‡ Oxley Developments Company Ltd. (1) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (2) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395. (3) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Science 1999, 283, 46. (4) Lenz, P. AdV. Mater. 1999, 11, 1531. Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (5) Cazabat, A. M.; Heslot, F.; Troian, S. M.; Charles, P. Nature 1990, 346, 824. (6) Schneemilch, M.; Welters, W. J. J.; Hayes, R. A.; Ralston, J. Langmuir 2000, 16, 2924. (7) Blake, T. D.; Clarke, A.; Stattersfield, E. H. Langmuir 2000, 16, 2928. (8) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 256, 1539. (9) Abdelsalam, M. E.; Bartlett, P. N.; Kelf, T.; Baumberg, J. Langmuir 2005, 21, 1753.
In addition to their photocatalytic properties,10-17 various oxide semiconductors, including metal titanates,18 WO3,19 SnO2, ZnO, V2O3,20 and both porous and nonporous TiO2 surfaces,21-23 also (10) Mills, A.; LeHunte, S. J. Photochem. Photobiol., A 1997, 108, 1. (11) Litter, M. I. Appl. Catal., B 1999, 23, 89. (12) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (13) Tryk, D. A.; Fujishima, A.; Honda, K. Electrochim. Acta 2000, 45, 2363. (14) Fujishima, A.; Rao, T. N.; Tryk, D. A. Electrochim. Acta 2000, 45, 4683. (15) LeGurun, G.; Boxall, C.; Taylor, R. J. The Application of Photocatalysis in Transition Metal and Actinide Redox Chemistry. In Recent Research DeVelopments in Photochemistry Photobiology; Halder, D., Pandalai, S. G., Eds.; Transworld Research Network: Trivandum, India, 2004; Vol 7, pp 39-79. (16) Robertson, P. K. J.; Bahnemann, D. W.; Robertson, J. M. C.; Wood, F. Photocatalytic Detoxification of Water and Air. In Handbook of EnVironmental Chemistry Vol. 2 EnVironmental Photochemistry; Robertson, P. K. J., Bahnemann, D. W., Boule, P., Eds.; Springer-Verlag: Heidleberg, Germany, 2005; pp 367423. (17) Boxall, C.; Le Gurun, G.; Taylor, R. J.; Xiao, S. The Applications of Photocatalytic Waste Minimisation in Nuclear Fuel Processing. In Handbook of EnVironmental Chemistry Vol. 2 EnVironmental Photochemistry; Robertson, P. K. J., Bahnemann, D. W., Boule, P., Eds.; Springer-Verlag: Heidleberg, Germany, 2005; pp 451-481. (18) Irie, H.; Hashimoto, K. Photocatalytic Active Surfaces and Photo-Induced High Hydrophilicity/High Hydrophobicity. In Handbook of EnVironmental Chemistry Vol. 2 EnVironmental Photochemistry; Robertson, P. K. J., Bahnemann, D. W., Boule, P., Eds.; Springer-Verlag: Heidleberg, Germany, 2005; pp 425450. (19) Miyauchi, M.; Nakajima, A.; Hashimoto, K.; Watanabe, T. AdV. Mater. 2000, 12, 1923. (20) Miyauchi, M.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Chem. Mater. 2002, 14, 2812. (21) Miyauchi, M.; Nobuo, K.; Hishita, S.; Mitsuhashi, T.; Nakajima, A.; Watanabe, T., Hashimoto, K. Surf. Sci. 2002, 511, 401. (22) Yu, J. C.; Yu, Y.; Ho, W.; Zhao, J. J. Photochem. Photobiol., A 2002, 148, 331. (23) Yu, J. C.; Yu, J.; Zhao, J. Appl. Catal. B 2002, 36, 31.
10.1021/la063347+ CCC: $37.00 © 2007 American Chemical Society Published on Web 03/17/2007
Photoinduced “Stick-Slip”
Figure 1. Mechanism of photoinduced superhydrophilicity.
Figure 2. Contact angle of a sessile drop as defined by Young’s equation, (a). Also shown: stick-slip behavior of a constant volume drop during illumination. From the initial state in the dark (a), surface energy of the solid-vapor and solid-liquid interfaces, and so their surface tensions, γSV, γSL, change upon illumination while drop dimensions remain constant (b). When an upper limit to the tension on the triple line is obtained, the line “jumps” (c) from radius r to r + δr with concomitant decreases in θ and h.
exhibit photoinduced aqueous wettability or light-induced superhydrophilicity12-14,18 after UV illumination. The accepted mechanism18,24 on TiO2 is shown in Figure 1. Absorption of ultraband gap photons by the TiO2 results in the generation of conduction band electrons and valence band holes. These migrate to the TiO2 surface where electrons reduce Ti(IV) cations to Ti(III) (not shown) and holes oxidize bridging O2- anions. The latter results in the expulsion of an O atom followed by adsorption of water molecules at the resultant vacancy site, producing new OH groups, so increasing the hydrophilicity of the surface. That the generation of superhydrophilic character is hole driven is supported by the work of Sakai et al.24 wherein anodic polarization of illuminated TiO2 surfaces leads to higher rates of hydrophilic conversion. Technological application of photoinduced superhydrophilicity requires an understanding of the thermodynamics, kinetics, and mechanism of the process in the context of the factors influencing surface wettability, for example, surface energy and microstructure. Surface wettability is conveniently evaluated by the measurement of the contact angle, θ, of a sessile drop of the liquid in question25 (Figure 2a). Thus, in an attempt to describe the kinetics or thermodynamics of light-induced superhydro(24) Sakai, N.; Fujishima, A.; Watananbe, T.; Hashimoto, K. J. Phys. Chem. B 2001, 105, 3023. (25) Mittal, K. L. Contact Angle, Wettability and Adhesion; VSP: Utrecht, The Netherlands, 1993.
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philicity, this paper concerns the study of photoinduced changes in θ as a function of time at illuminated TiO2 surfaces. As superhydrophilic character is generated at illuminated oxide semiconductor surfaces, θ decreases. In the case of oxides such as TiO2, θf0 after only 30 min of moderate UV irradiation. All previous studies of the time dependence of this phenomenon have been conducted through discrete, static measurements, that is, the surface is illuminated first, followed by application of the drop and measurement of θ, this cycle being repeated over the period of illumination or until θ ≈ 0.19-22,26-28 We refer to this as the discrete measurement experiment. To avoid any variation in the wetting hysteresis effects (vide infra) that may arise during the multiple drop applications that such experiments involve, we elected to first apply the drop and then to illuminate the surface. θ of that single drop is then continuously measured as a function of time. We refer to this as the continuous measurement experiment. Conventional (i.e., non-photochemically induced) drop dynamics and surface thermodynamics can be interrogated by studying the spreading or receding of a drop. The former involves observation of the spreading of a nonevaporating, sessile drop directly after application onto a flat, rigid substrate as θ relaxes from 180° at contact to its equilibrium contact angle θ0. In this partial wetting regime (i.e., the drop retains the shape of a spherical cap and does not spread to completely cover the substrate), the dynamics of postapplication spreading are usually described through the energy balance between the capillary driving force (Figure 2a) and the resistance to spreading, the latter being derived from (1) viscous flow within the core of the drop and (2) frictional processes in the vicinity of the advancing contact line which stem from the attachment of fluid particles to the substrate surface.29 In the case of the latter, attachment may be promoted by the presence of surface inhomogeneities derived from chemical (including surface-phase segregation, dissolution, and liquidinduced swelling of the solid) or physical sources (including roughness, presence of pores and asperities, molecular orientation, and surface strain). Quantitative modeling of spread generally assumes that one dissipation route dominates, leading to two classes of model. The first takes a macroscopic approach to describing the energy balance between the capillary driving force and the hydrodynamic (viscous) resistance to spreading, giving30-34
V≈
γLVθ(cos θ0 - cos θ) 3η ln(2θ2ra-1)
(1)
where V is the triple line velocity during spreading, θ is the measured instantaneous contact angle, η is the dynamic viscosity of the liquid, and a is the radius of some cylindrical region centered at the symmetry axis of the drop where the radial velocity of the liquid is 0. Typically, a is of molecular dimensions but is rarely determined analytically; more commonly, a is an adjustable parameter whose value is extracted from the fit to experimental data. The second model uses molecular kinetics theory to describe motion of the three-phase boundary, or triple line, as an activated (26) Mills, A.; Elliott, N.; Parkin, I. P.; O’Neill, S. A.; Clark, R. J. J. Photochem. Photobiol., A 2002, 151, 171. (27) Mills, A.; Lepre, A.; Elliott, N.; Bhopal, S.; Parkin, I. P.; O’Neill, S. A. J. Photochem. Photobiol., A 2003, 160, 213. (28) Karuppuchamy, S.; Jeong, J. M. Mater. Chem. Phys. 2005, 93, 251. (29) de Ruijter, M. J.; De Coninck, J.; Oshanin, G. Langmuir 1999, 15, 2209. (30) Huh, C.; Scriven, L. E. J. Colloid Interface Sci. 1971, 35, 85. (31) de Gennes, P. G. ReV. Mod. Phys. 1985, 57, 827. (32) de Gennes, P. G.; Hua, X.; Levinson, P. J. Fluid Mech. 1990, 212, 55. (33) Carre´, A.; Shanahan, M. E. R. Langmuir 1995, 11, 24. (34) Shanahan, M. E. R. Langmuir 2001, 17, 3997.
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process involving molecular “jumping” between adsorption sites35-38 giving
V ) 2K0λ sinh
[
]
γLV(cos θ0 - cos θ) 2nkBT
(2)
where n is the number of adsorption sites per unit area, K0 is the equilibrium frequency of molecular displacements, and λ is the average length of those displacements. Some attempts have been made to combine the two approaches in the same model39,40 with recent studies by de Ruijter and co-workers suggesting that the dominant route of dissipation changes with time29,41 moving, in chronological order, from frictional processes in the vicinity of the advancing line to hydrodynamic behavior. In effect, the former, activated molecular process applies at large θ, while the latter dominates at smaller θ.29,31,33,34,39 Drop-receding experiments usually involve the study of the triple line after the sessile drop has been applied, has obtained its equilibrium contact angle, and then has started to evaporate.42-44 Occasionally observed during drop receding and advancing experiments is stick-slip where the line is static for most of the time and then moves abruptly. In keeping with the observations that friction forces in the vicinity of the line are the main source of resistance to movement toward the beginning of drop spread,29,34 analysis of stick-slip during receding experiments can provide information about the energetics of the surface inhomogeneities (vide supra) from which the stick-slip phenomenon is thought to be derived.42 As will be described below, we have observed stick-slip behavior during continuous measurement-based, drop-spreading studies of photoinduced wettability of mesoporous TiO2 surfaces wherein evaporation has been suppressed. The origins of photoinduced stick-slip within such a scenario may be understood from Figure 2, where γLV, γSL, and γSV are the liquid/vapor (LV), solid/liquid (SL), and solid/vapor (SV) interfacial free energies, θ0 is the equilibrium contact angle of the system, and θ0,ph is the equilibrium contact angle under illumination. Initially, dark, capillary equilibrium is obtained, and Young’s equation applies, Figure 2a. On illumination, γSL and γSV change. Capillary equilibrium is no longer in effect, with a net force being exerted on the triple line. However, as in the dark experiment, onset of movement of the line is opposed by surface inhomogeneities with the measured angle being unaltered for a period after the commencement of illumination, Figure 2b. γSV and γSL continue to change with illumination time, moving the drop further from equilibrium, until the force on the triple line is sufficient to overcome the opposition offered by the surface inhomogeneities and slip occurs, Figure 2c. In many ways, this is analogous to the conventional dark drop-spreading experiment, suggesting that the dynamics of photoinduced spreading postonset of triple line movement may be described in terms of the energy balance between capillary equilibrium restitution and dissipation (35) Galsstone, S.; Laidler, K. J.; Eyring, H. The Theory of Rate Processes; McGraw Hill: New York, 1941. (36) Blake, T. D.; Haynes, J. M. J. Colloid Interface Sci. 1969, 30, 421. (37) Blake, T. D.; Clarke, A.; De Coninck J.; de Ruijter, M. J. Langmuir 1997, 13, 2164. (38) Ruckenstein, E.; Dunn, C. S. J. Colloid Interface Sci. 1977, 59, 135. (39) Brochard-Wyart, F.; de Gennes, P. G. AdV. Colloid Interface Sci. 1992, 39, 1. (40) Petrov, P. G.; Petrov, J. G. Langmuir 1992, 8, 1762. (41) de Ruijter, M. J.; Charlot, M.; Voue´, M.; De Coninck, J. Langmuir 2000, 16, 2363. (42) Shanahan, M. E. R. Langmuir 1995, 11, 1041. (43) Bourges-Monnier, C.; Shanahan, M. E. R. Langmuir 1995, 11, 2820. (44) Joyce, M. J.; Todaro, P.; Penfold, R.; Port, S. N.; May, J. A. W.; Barnes, C.; Peyton, A. J. Langmuir 2000, 16, 4024.
forces. Additionally, if stick-slip behavior is observed during photoinduced spreading, then parallels may be drawn with stickslip in the dark and the energetics of the onset of photoinduced spreading described by the balance between capillary forces and the anchoring effects of inhomogeneties at the substrate surface. In the present study, we present the first report on the time dependence of the generation of photoinduced superhydrophilicity as determined by continuous measurements. In so doing, we use a mesoporous TiO2 (M-TiO2) substrate preparation that is known to give rise to surfaces that exhibit high levels of photoinduced superhydrophilicity22 and find that these surfaces exhibit stickslip behavior during photoinduced drop spreading. Thus, this paper also presents the first report of photoinduced stick-slip and the first theoretical model of the same. The latter allows for the quantitative analysis of the surface energy barrier opposing onset of photoinduced triple line movement and comparison with the barrier opposing movement during an evaporatively driven receding drop experiment in the dark. 2. Experimental Section 2.1. Reagents. All chemicals were of analytical grade or better and were used as received. Distilled water from a homemade still was further purified by a deionization system (E pure model 04642, Barnstead/Thermodyne, Dubuque, Iowa) to a resistivity of 1.8 × 105 Ωm. 2.2. Mesoporous Thin Film Fabrication. Mesoporous TiO2 thin films were fabricated using the hydrothermal reverse micelle route of Yu et al.22 First, Triton X-100 (26.0 g) and cyclohexane (150 cm3) were mixed with vigorous stirring. After 30 min, 1.08 g of distilled doubly deionized water was added to the solution at 293 K forming a reverse micellar turbid solution. The turbidity cleared upon dropwise addition of (23.0 g) titanium isopropoxide. The solution was then stirred for 60 min at 293 K to allow hydrolysis of the titanium alkoxide to go to completion, so forming a colloidal suspension of TiO2 nanoparticles. Finally, 10 cm3 of acetylacetone was added to the solution, resulting in a stable colloid of TiO2. Quartz glass slides measuring 60 × 25 × 2 mm (Multi-Lab Ltd, Newcastle-upon-Tyne, United Kingdom) were used as substrates for the M-TiO2 thin films. Films were deposited by spin coating the colloidal solution onto the slides, rotated at 2900 rpm for 30 s using an inverted model 636 modified rotating disk electrode system (Princeton Applied Research, TN). The coated slides were then heat-treated in air at a ramp rate of 3 °C/min up to 500 °C at which they were held for 60 min before being allowed to cool at the same rate. The slides were then stored in air in the dark for >48 h with no subsequent cleaning prior to use. 2.3. Film Characterization. UV-vis spectra of films were obtained using UV-vis spectrophotometry (Hewlett-Packard 8452A, Bristol, United Kingdom). Phase identification was achieved through Raman spectroscopy using a Renishaw Ramascope 1000 (Renishaw, Gloucestershire, United Kingdom) with backscattering geometry and a 17 mW He-Ne (632.8 nm) laser excitation source. Surface topography, relative roughness, and so forth were assessed using atomic force microscopy (AFM; Q-Scope 250, Quesant, CA). 2.4. Contact Angle Measurements. Contact angles of sessile water drops on test substrates (glass, M-TiO2, etc.) were measured using the assembly shown in Figure 3. A sealed enclosure allowed for control of the relative humidity (RH) during measurement. An air pump (Hi-Tech 1500, Weiko UK, Berkshire, United Kingdom) was used to transfer moist air from a Drechsel bottle fitted with a no. 1 filter into the enclosure. The pump, with an adjustable valve to regulate air flow, was fitted in-line between the enclosure and the bottle, the bottle being immersed in a temperature-controlled water bath (Clifton NE4-22D, Bennett Scientific, Devon, United Kingdom). Control of RH was provided by adjustment of the in-line valve and the bath temperature, the initial value of which, for the target RH, was determined from the Van’t Hoff isochore. A thermohygrometer (Rotronic A1, Rotronic, West Sussex, United Kingdom) was used to measure chamber RH and temperature. Pump speed and bath
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Figure 3. Schematic of homemade apparatus for measurement of the contact angle of a known volume sessile drop under controlled humidity in the presence and absence of ultraband gap illumination. temperature were fine adjusted until the desired RH and temperature were reached. In all experiments, a sessile drop of distilled, doubly deionized water with a volume of ∼1 µL was vertically deposited onto the test substrate/slide using a 5.0-µL microsyringe. The edge on profile of the drop was then filmed as a function of time. Image capture was performed using a Sony XC-ST50 CCD camera and Studio DC10plus v8.0 image editing software (Pinnacle Systems, Middlesex, United Kingdom). Images were typically captured in 10-s bursts every 10 min at 25 frames s-1. To aid visualization, drops were back lit with a 100 W quartz iodine lamp (Rank Bros, Cambridge, United Kingdom) fitted with a borosilicate glass UV cutoff filter (L G Optical Ltd, St Leonards-on-Sea, Sussex, United Kingdom) to prevent its light inducing any superhydrophilic effect. Images were then transferred to AutoCAD 2000 (Autodesk, CA) for analysis of drop instantaneous contact angle θ, height h, contact diameter or chord d, and volume V. Two types of contact angle measurement were made on as-prepared M-TiO2 thin films: those on evaporating sessile drops in the dark and those on nonevaporating sessile drops under ultraband gap illumination. In the case of the former, the RH and chamber temperature were preset to 60% and 293 K. The deposited drops were then filmed as they were allowed to evaporate. In the case of the latter, the RH and chamber temperature were preset to 100% and 293 K. The deposited drops were then illuminated with ultraband gap light and were filmed as they spread as a result of photoinduced superhydrophilicity. Monochromated light was directed perpendicularly “top down” onto the test slide via a quartz light guide coupled to a 900 W xenon arc lamp (Applied photophysics, Surrey, United Kingdom) fitted with an f/3.4 monochromator. The illumination wavelength was 315 nm, and the incident intensity was 143 mW cm-2.
3. Results and Discussion 3.1. Characterization of M-TiO2 Films. Raman spectroscopy of our films confirms that they are predominantly anatase (supporting information (SI) 1). UV-vis spectra (SI 2) indicate a band gap of 3.54 eV, blue-shifted from the bulk value of 3.2 eV45 as the films are comprised of particles small enough to exhibit the quantum size effect. By comparison with the similarly blue-shifted M-TiO2 film absorbance reported by Yu et al.,22 this allows us to attribute a maximum size of 13.1 nm to the crystallites within our layers, maximum layer 170 nm. Convolution of the UV-vis spectra with the spectral output of the lamp indicates that the maximum in number of photons s-1 absorbed by the films occurs at λ ) 315 nm. This wavelength was used in all photoinduced superhydrophilicity measurements, the light (45) Hurum D. C.; Agrios, A. G.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. J. Phys. Chem. B 2003, 107, 4545.
Figure 4. (a) Two-dimensional and (b) three-dimensional intermittent contact mode AFM images of the surface of M-TiO2 deposited on quartz by one coating cycle at 500 °C.
intensity being 143 mW cm-2 as measured using ferrioxalate actinometry.46,47 Figure 4 shows AFM images of our M-TiO2 films. Constituent particle diameter and highest peak-to-deepest valley total roughness, Rt, are smaller for our layers,
