J. Phys. Chem. C 2007, 111, 14521-14529
14521
Superhydrophobic TiO2 Surfaces: Preparation, Photocatalytic Wettability Conversion, and Superhydrophobic-Superhydrophilic Patterning Xintong Zhang, Min Jin, Zhaoyue Liu, Donald A. Tryk, Shunsuke Nishimoto, Taketoshi Murakami, and Akira Fujishima* Kanagawa Academy of Science and Technology, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan ReceiVed: June 8, 2007; In Final Form: July 23, 2007
We report herein the preparation and UV-stimulated wettability conversion of superhydrophobic TiO2 surfaces, as well as the preparation of superhydrophilic-superhydrophobic patterns by use of UV irradiation through a photomask. A CF4 plasma was used to roughen smooth TiO2 sol-gel films to produce a nanocolumnar morphology, and subsequent hydrophobic modification with octadecylphosphonic acid (ODP) rendered the roughened surfaces superhydrophobic. The superhydrophobic properties of these surfaces were evaluated by both static and dynamic water contact angle (CA) measurements. It was found that the surface morphology of the TiO2 film, which was dependent on the etching time, has a great influence on the observed superhydrophobic properties. The nanocolumnar surface morphology exhibited large water CA and small contact angle hysteresis (CAH); this is discussed in terms of the Wenzel equation and the Cassie-Baxter equation. Under low-intensity UV illumination (1 mW cm-2), the superhydrophobic TiO2 surface underwent a gradual decrease of water CA and finally became superhydrophilic, due to photocatalytic decomposition of the ODP monolayer. Readsorption of ODP molecules led to the recovery of the superhydrophobic state. This UV-stimulated wettability conversion was employed to prepare superhydrophilic stripes (50 and 500 µm wide) on a superhydrophobic TiO2 surface. The pattern was able to guide water condensation, as well as the evaporation of a polystyrene microsphere suspension, due to the extremely large wettability contrast between superhydrophobic and superhydrophilic areas.
Introduction The control of surface wettability is important for many biological processes1 and industrial applications.2 Recently, superhydrophobicity (water contact angle (CA) >150°) and superhydrophilicity (water CA 150°) after ODP modification. The receding CA, however, showed a decrease in the initial stage of ∼20 s and afterward showed a rapid increase with extended etching. As a result, the CAH curve rose initially and then decreased rapidly (Figure 2, inset). The hysteresis decreased to only 6° for the 135 s etched sample and only 2° for the 150 s etched sample. The very low hysteresis, together with the large CA, allowed water droplets to roll off these surfaces spontaneously rather than adhering (see Supporting Information). Thus, θs values for the surfaces etched over 135 s were not measurable and are not provided in Figure 2. The evolution of water CA depicted in Figure 2, especially the receding CA and CAH, should be directly related to the changes in surface morphology as a result of plasma etching.3a In the first 20 s or so, the CF4 plasma attacks the TiO2 surface, producing large numbers of pinholes around the grain boundaries. Water droplets can intrude into the ODP-modified surface through these pinholes. This, on the one hand, enlarges the contact area between the drop and the surface.3a According to the Wenzel equation (eq 4), the apparent water CA, either static or advancing (θs* and θa*), should increase as compared to the corresponding values for the smooth surface (θs and θa), depending on the roughness factor r, defined as the ratio between the actual contact area and the projected area.24 On the other hand, the intrusion of water into the surface easily results in the pinning of the contact line; the surface thus exhibits a receding angle smaller than that for the smooth surface or even exhibits hydrophilic behavior, in spite of the hydrophobic nature of the surface.3a As a result, the CAH becomes larger than that for the smooth surface.
TiO2 + 2CF2 f TiF4 + 2CO
(3)
cos θ* ) r cos θ
Figure 2. Water contact angles of an ODP-modified TiO2 surface as a function of etching time in the CF4 plasma: (4) static contact angle; (b) advancing contact angle; and (9) receding contact angle. Inset: CAH of the ODP-modified TiO2 surface as a function of CF4 plasma etching time.
A 15 °C increase in substrate temperature nearly doubled the thinning rate. Conversely, lowering the substrate temperature slowed the thinning rate. Referring to the etching reactions of SiO2, the reactions occurring in the etching of the TiO2 film may be written as follows:21
The removal of TiF4 from the film surface may account for this temperature-dependent etching, considering the quite high boiling point of this material (284 °C at 1 atm). The formation of a columnar microtexture is quite interesting since the plasma etching is of an isotropic nature.21 One possible reason is the higher etching rate at grain boundaries as compared to the micrograins themselves. The material at or close to the grain boundaries is etched more rapidly, and thus, micrograins at the topmost surface are separated from each other during the subsequent plasma etching. Similarly, etching polymer materials with an O2 plasma often gives rise to needle-like surface morphologies with a mechanism that has been suggested to be based on the faster etching of the noncrystalline phase of the polymer as compared to the crystalline phase.11,22 Superhydrophobic TiO2 Surfaces. Clean, smooth TiO2 surfaces show a water CA of ∼5°. The plasma etched TiO2 films show even smaller CA, approaching 0°. Thus, hydrophobic modification is necessary to make these surfaces hydrophobic or even superhydrophobic. We chose ODP to modify the TiO2 surfaces since this molecule can form hydrophobic selfassembled monolayers on the TiO2 surface.20,23 After ODP modification, the water CA of the smooth, nonetched TiO2 film increased to 108°; this value increased further, even exceeding 160°, for the plasma etched surfaces. Figure 2 depicts water CA, both static and dynamic (advancing/receding) CA, of ODPmodified TiO2 surfaces as a function of etching time. The contact angle hysteresis (CAH) (i.e., the difference between advancing and receding contact angles) is also shown in the inset. Both static (θs) and advancing (θa) CA showed a rapid increase in the figure with increasing the etching time up to 1 min; beyond 1 min, the CA increased slowly with extended etching. The surfaces etched over 45 s were all superhydro-
(4)
Further etching (over 45 s) leads to the formation of nanocolumnar TiO2 surfaces. These nanocolumnar surfaces, after hydrophobic modification, exhibit an increasing trend for all three kinds of water CA (θs, θa, and θr) and decreasing CAH as a function of etching time. The CAH can be as small as 2° for a sample etched over 150 s, much smaller than that for the smooth surface (31°). These phenomena, in particular, the small CAH, strongly suggest that the nanocolumnar surfaces under the water droplet are composite ones that consist of two phases (i.e., TiO2 columns and air pockets).3a,d As described by the Cassie equation (eq 5), the apparent CA (θs* and θa*) for a composite surface are influenced greatly by the surface fraction of solid (f1) versus air pockets (f2)25
cos θ* ) f1 cos θ - f2
(5)
According to this equation, a large surface fraction of air pockets, or a small fraction of TiO2 columns at the surface, would favor the hydrophobic properties of the surface. This explains the gradual increase of water CA, both static and advancing, with extended etching since plasma etching leads to a decrease in the fraction of TiO2 columns at the surface, as shown in Figure 1. Moreover, the existence of air pockets reduces the contact area of the drop with the solid surface, which weakens the interaction.3a,f The contact line moves on the surface instead of pinning, when the surface is tilted or when the drop is enlarged or diminished in size; this results in the small CAH values. Photocatalytic Wettability Conversion. Under UV illumination, the TiO2 surface can decompose the ODP monolayer as a result of photocatalytic action. The decomposition process was studied with IR spectroscopy, shown in Figure 3a. The frequen-
Preparing and Patterning a Superhydrophobic TiO2 Surface
J. Phys. Chem. C, Vol. 111, No. 39, 2007 14525
Figure 3. (a) Infrared difference spectra of an ODP-modified 90 s etched TiO2 film measured after UV illumination for various times, obtained by subtraction of a spectrum measured after UV illumination for 300 min. (b) Decay of water contact angles (2) and methyl and methylene vibration signals (4) of the ODP-modified 90 s etched TiO2 film measured during UV illumination (1 mW cm-2).
cies of the ODP monolayer were similar for all of the samples, with values of 2918 and 2850 cm-1, respectively, for the νa(CH2) and νs(CH2) stretching modes. The half-width was about 18 cm-1 for νa(CH2). Such values, close to those of a crystalline alkane, are typically taken as evidence of ordered packing of long-chain alkyls in self-assembled monolayers.23 For a highly disordered film, the frequency of the CH2 stretching is close to that of a liquid alkane (νa ) ∼2924 cm-1).23 Under the lowintensity UV illumination (1 mW cm-2), the vibrational bands of the methylene group lost intensity gradually; the band position shifted to a higher wavenumber with UV illumination, and the half-width showed a tendency to increase. These changes indicate the decomposition of hydrocarbon chains and the loss of orderly packing of the ODP molecules.23 After UV illumination for 180 min, the signal for the hydrocarbon chain vibration decreased to the noise level. The decomposition of the ODP monolayer leads to an interesting UV-stimulated wettability change of the superhydrophobic TiO2 surface. Figure 3b depicts the change of the water CA of an ODP-modified 90 s etched TiO2 surface during UV illumination. A series of water CA was obtained between 166 and 100°, dependent on the UV illumination time. A sharp transition from hydrophobic to hydrophilic occurred at a CA of approximately 90°, at which the hydrocarbon vibrational signal remained at only 10% of the initial value. After a 240 min illumination, the surface became superhydrophilic, indicating that all of the long-chain alkyls were decomposed and removed from the surface. The rate of the wettability change was approximately dependent on the square root of the light intensity under the present light intensity conditions (1-10 mW cm-2), which is similar to the case with other photocatalytic reactions;4f,16a the rate decreased further with the extension of plasma etching time since the thinning of the TiO2 film weakened the absorption of UV light. We also measured the θa and θr values of the ODP-modified 90 s etched TiO2 surface during UV illumination (Supporting Information). While θa remained rather stable during the first 40 min of UV illumination, the θr value decreased even faster than the static value, shown in Figure 3b; as a result, the CAH increased during the course of UV illumination. After a 40 min illumination, the CAH became 62°, much greater than the initial value (23°). This suggests that the wetting type of the nanocolumnar surface had changed from the Cassie type to the Wenzel type (i.e., the water droplet tended to intrude into the surface instead of standing over it) as a result of decomposition and disordering of the ODP monolayer.
Figure 4. Reversible superhydrophobic-superhydrophilic conversion of the 90 s etched TiO2 film (9) by alternating ODP modification and UV illumination. For comparison, the data for a smooth TiO2 film (O) are also shown in the figure.
An interesting phenomenon was observed: dipping the UVilluminated TiO2 surface, which was superhydrophilic, again into the ODP solution served to recover the superhydrophobic state as a result of readsorption of the ODP monolayer. This UV treatment-readsorption cycle was repeated 4 times with only a slight decrease in the water CA, as shown in Figure 4. The decrease was believed to be a result of the interference from residual phosphate ions on the packing density of the ODP monolayer; the IR signal of the methylene group for the fourth readsorbed ODP monolayer was only ca. 60% of the initial value, with a shift of νa(CH2) to 2921 cm-1. In some experiments, we washed the UV-illuminated samples with sodium carbonate solution and pure water in sequence. This washing was found to be effective in improving the quality of the readsorbed ODP monolayer as a result of the removal of the contaminating phosphate ions. Superhydrophobic-Superhydrophilic Patterns. The UVstimulated wettability conversion described previously can be applied to the preparation of interesting superhydrophilic patterns on superhydrophobic TiO2 surfaces. The wettability contrast for the pattern, defined as the difference between the receding CA of the superhydrophobic area and the advancing CA of the superhydrophilic area, should be greater than 140° or even close to 160°. Such a large wettability contrast is not achievable with conventional hydrophobic-hydrophilic patterning.
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Figure 5. (a) Image of a water bulge in a 500 µm wide superhydrophilic stripe taken in the direction normal to the stripe. (b) Image of the same water bulge taken in the direction parallel to the stripe. (c) Image taken in the same direction as panel b when more water was added to the bulge.
Figure 6. SEM image of a superhydrophobic-superhydrophilic pattern prepared using a photomask (line/space ratio: 1:1). Bright stripes in the image are superhydrophobic, and dark stripes are superhydrophilic. The scale bar corresponds to 100 µm.
We placed a water droplet (∼1 µL) in a 500 µm wide stripe with a microsyringe. The droplet spread entirely in the stripe, with a bulge forming in the middle of the stripe, as shown in Figure 5a. Such behavior indicates the superhydrophilic property of the stripe. The bulge, when imaged along the stripe, showed a nearly spherical cross-section, with a contact angle of 138°, as shown in Figure 5b. We added more water to the bulge and observed an increase of the contact angle to 159° (Figure 5c). As discussed by Lipowsky et al., for a fluid in a hydrophilic channel surrounded by hydrophobic domains, the CA of the fluid at the boundaries could be any value between that of the hydrophilic channel and that of the hydrophobic domains.15,26 Therefore, the large CA of the water bulge observed at the boundaries of the superhydrophilic stripe indicate that the superhydrophilic stripe was surrounded by a superhydrophobic area. The bulge moved in the superhydrophilic stripe when the surface was tilted slightly in the direction parallel to the stripe but remained pinned in the stripe when the surface was tilted in the direction normal to the stripe. The bulge can become over 6 times wider than the stripe due to the confinement of the superhydrophobic stripes (see Supporting Information). For a stripe pattern prepared on a smooth TiO2 surface, however, water tended to spread outside the stripe rather than forming a stable bulge since the moderate wettability contrast (150°) even after it had lost ca. 36% of the methylene and methyl groups, as shown in Figure 3b. These attributes allowed us to prepare superhydrophilic features 50 µm in size, or even smaller, surrounded by superhydrophobic areas through careful control of the UV illumination time. We cooled the 50 µm stripe pattern to 5 °C at ∼60% relative humidity. During cooling, water condensed selectively in the superhydrophilic stripes to form first a water droplet and then a water column in sequence, as shown in Figure 7. We also observed the formation of a water bulge in the 50 µm wide superhydrophilic stripes, just as in the 500 µm wide stripes, when a large amount of water was condensed in the stripes. Very few, if any, tiny water droplets were observed in the superhydrophobic area as a result of the presence of the superhydrophilic stripes. This is entirely different from the superhydrophobic surface, on which water condensed randomly into tiny water droplets. This suggests that the superhydrophilic stripes can affect the water condensation on the superhydrophobic surface. Any water droplet that is formed on the
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Figure 7. (a-c) Optical micrographs of a superhydrophobic-superhydrophilic pattern cooled below the dew point with a cooling/heating stage for 10 s (a), 3 min (b), and 14 min (c). (d) Micrograph of a superhydrophobic surface cooled below the dew point for 3 min. The scale bars in the micrographs correspond to 100 µm.
superhydrophobic area can spontaneously move to the superhydrophilic area and be confined there if it touches the border of the two areas. The driving force for the movement is the unbalanced Young force, F, that results from the wettability contrast between the superhydrophobic area (1) and the superhydrophilic area (2) for the droplet28
F ) γl(cos θ2a - cos θ1r)
(6)
Note that a rather large wettability contrast is required to observe this guided water condensation. Water condensed on a striped smooth TiO2 surface with a moderate wettability contrast (
