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Capillary rise between two TiO2 thin-films: evaluating photo-activated wetting
Thin Solid Films 446 (2004) 232–237
Capillary rise between two TiO2 thin-films: evaluating photo-activated wetting Sho Kataoka1, Marc A. Anderson* Environmental Chemistry and Technology Program, University of Wisconsin – Madison, 660 N. Park Street, Madison, WI 53706, USA Received 25 February 2003; received in revised form 10 September 2003; accepted 1 October 2003
Abstract The measurement of capillary rise was used for evaluating the photo-enhanced wetting ability of TiO2 . Two glass plates, which were coated with TiO2 thin-films, were vertically oriented having a small gap and immersed in the several solutions. In the channel between two TiO2 thin-films, the capillary rise of liquids is determined by the surface tension of liquids and the physical– chemical nature of the contacting surfaces. The capillary rise of water was measured to be 28 mm for a TiO2 samples equilibrated in the dark for 2 h (immediately after the calcination), 29 mm for the surface pre-illuminated with UV light and 11 mm for aged samples (i.e. kept in a dark box for several weeks). A companion FT-IR study supported the fact that the capillary rise has a strong relation with the amount of adsorbed water on the surface of the TiO2 thin-films. While contact angles have been routinely measured for photoactive TiO2 systems, capillary rise is an alternative criterion for the wetting ability without expensive equipment. We also show that capillary rise can be expanded to photo-responsive motion of liquids in the TiO2 channel. The capillary rise of water tested between the two TiO2 thin-film surfaces increased by a factor of three under UV light exposure. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Capillary rise; Wetting ability; TiO2; UV illumination
1. Introduction TiO2, known to be a photocatalyst, changes its wetting ability after being illuminated with UV light w1–5x. Both hydrocarbons (liquids) and water completely wet the surface of TiO2 after UV exposure w6–16x. This phenomenon is commonly referred to as ‘photo-induced wetting ability.’ Many attempts to elucidate the mechanisms of the wetting property from a mechanistic and microscopic point of view have been reported w16–20x. The current premise attributes this phenomenon to oxygen (bridging oxygen) defects (holes), which are produced via the photocatalytic process. This event is considered to be structural change and is supported by recent experiments showing that the surface of TiO2 sputtered with Arq should behave as if it were illuminated with UV light w9x. *Corresponding author. Tel.: q1-608-262-2674; fax: q1-608-2620454. E-mail address: [email protected] (M.A. Anderson). 1 Present address: Department of Chemistry, Texas A&M University, Dr. Cremer Group, 3255 TAMU College Station, TX 77843, USA.
While photo-enhanced wetting has now been studied for several years, we feel that the thermodynamic and macroscopic evaluation of this phenomenon has not been adequately addressed. This is partially because the surface tension of solids is not directly measurable. Contact angles are commonly measured to obtain the extent of the surface tension of solids based on Young’s equation w21x: gLV cos usgSVygSL
(1)
where g is a surface tension, u is a contact angle and subscripts L, V and S are represent liquid, vapor and solid, respectively. Although contact angle measurements are easy and useful, problems still remain. Since it has not been possible to directly test the Young’s equation experimentally w21x, many correction factors have been proposed, e.g. a roughness factor for surfaces, the shape of droplets and others w11,15,21–24x. Furthermore, it is particularly difficult to evaluate this photoactivated phenomenon on the surface of solids when contact angles are small (;08). With respect to the surface of TiO2 after UV illumination, the contact angle
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.10.001
S. Kataoka, M.A. Anderson / Thin Solid Films 446 (2004) 232–237
of water (gLV is f73 mNym) approaches zero degrees, which is characterized as nearly complete ‘spreading’ w21x. Since water has one of the highest surface tension among common liquids—except for mercury, most other liquids also are likely to spontaneously spread on the surface of irradiated TiO2. To illustrate this last point, we attempted to measure the contact angle of mercury droplets (gLV is f485 mNym) on TiO2 surfaces before and after UV exposure. The contact angle was 122– 1408 before and 105–1168 after the illumination. However, since the mercury droplet deformed in shape due to the weight of mercury, it was difficult to determine exact contact angles. To the best of our knowledge, only contact angle measurements have been applied to the characterization of the wetting ability of TiO2 w6–16x. Capillary rise is a well-known interfacial phenomenon caused by the surface tension of liquids and their wetting ability. When a meniscus is ideally ‘hemi-cylindrical’ concave in shape, the capillary rise between two flat plates is expressed as follows w21x: hs
2gLV cos u , rgd
(2)
where gSV is the surface tension of a liquid, u is a contact angle between a liquid and a substrate, r is the density of a liquid, g is the gravitational acceleration (i.e. 9.8 mys2) and d is a distance between two plates. If capillary rise is applied to TiO2 surface wetting ability instead of contact angle measurements, small contact angles may be able to be projected to capillary rise with larger scale by changing the distance of plates. The idea of alternatively using the capillary rise method is that this method could evaluate wetting ability with higher precision in an inexpensive way. Some recent publications have appeared concerning the light-mediated dynamic motion of liquids on surfaces w24–31x. This phenomenon is based on the well-known Marangoni effect. Liquids move on surface-responsive materials (especially the trans–cis photo-trans isomerisable azobenzene groups) that are coated on substrates w26x. In a similar fashion we can also cause fluids to rise in capillaries between TiO2 thin-films simply by illuminating the surface with UV light. 2. Experimental details 2.1. Capillary rise test A TiO2 sol was prepared via a sol–gel processing procedure w3x and was dip-coated onto 1=12-inch glass plates (Cardinal CG, Spring Green, WI) at a removal velocity of 0.95 mmys. The supported catalyst was calcined at 350 8C for 3 h after drying at room temperature in a 100 8C oven. Plastic strips (0.5-mm
233
Fig. 1. Schematic diagram of the system used for pre-illumination test and Marangoni test.
thickness) were positioned between two coated glass plates forming a small capillary. This assembly was firmly fastened with clips and vertically immersed in test liquids. Capillary rise was measured from the top surface of liquids. Two types of tests were conducted in the capillary rise tests. For the first test, TiO2-coated glass plates were illuminated with UV light (F4T8BL, Viko, Japan) for 2 h prior to immersing into liquids (pre-illumination test). The results were compared with those of samples not illuminated. The tests were performed with both samples immediately after calcination and samples kept in a dark box (60% RH and 23 8C) for several weeks after the calcination. For the convenience, the former is called a fresh surface, and the latter is an aged surface. Contact angles of samples were also measured using a contact angle goniometer (NRL contact angle goniom´ eter, Rame-Hart Inc., Mountain Lakes, NJ). The second test was aimed at changing the capillary rise during the illumination (Marangoni test). The diagram of the testing scheme is illustrated in Fig. 1. The samples were immersed into liquids and afterwards were simultaneously illuminated with UV light for 2 h or longer. In the second test, only the aged samples were employed. The test liquids in the study constitute water (MilliQ, Millipore, Bedford, MA), ethanol (Aldrich, Milwaukee, WI), ethanolywater mixtures (4, 8 and 20 wt.%), 0.025 M tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt, Aldrich) aqueous solutions and an aqueous solution of Rhodamine-B (0.0025 M, Aldrich). The surface tension of aqueous ethanol solution changes with concentration and was estimated from Ref. w21x. Aqueous ethanol solutions were also expected to scavenge holes on the surface of TiO2. Likewise, tiron, also known as a superoxide scavenger, was added for the purpose of scavenging superoxides produced on the surface of TiO2 under UV illumination. 2.2. Fourier transform spectroscopy In order to better define the character of the respective surfaces of TiO2 (aged, fresh and UV-illuminated), the
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Fig. 2. Photograph of capillary rise of colored water in a channel between TiO2-coated glasses: no UV illumination (left), UV pre-illumination (right).
amount of water adsorbed on those surfaces of TiO2 was measured with infrared (IR) spectroscopy using a TiO2 thin-film transmission IR cell. The experimental details about the thin-film transmission cell were thoroughly described in the previous publication w32x. The previous study showed that this cell allows one to analyze the surface of TiO2 both in the presence and absence of UV illumination in a closed environment. The same TiO2 sol as was used for glass plates in the capillary rise test was also sprayed on a Si wafer and was calcined in the same way. For aging the surface, this supported catalyst was stored at a dark box (60% RH, 23 8C) for several weeks after the calcination. For the fresh surface, the supported catalyst used in the test with the aged surface was calcined again at 350 8C for an hour and was afterwards exposed to humidified air at the dark box for 12 h (60% RH, 23 8C) in order to obtain the same experimental condition with respect to surface chemistry of the TiO2 thin-film. The transmission IR cell containing the supported catalyst was flushed with dry air (-0.7% RH) for 12 h prior to taking a background spectrum under the dry air condition. Subsequently, humidified air (60% RH) was supplied through the cell, and IR spectra were collected with an IR spectrometer (Nexus 670, Nicolet Instrument, Madison, WI), which contained an MCT-A (HgCdTe) detector and KBr beam splitter. During the course of the UV-illuminated experiment, the catalyst was simultaneously illuminated with an UV lamp (a 200-W medium pressure Hg vapor lamp, Model 7825-32, Ace Glass, Vineland, NJ) covered with a cooling jacket.
and their respective capillary rise between the two TiO2 plates both with and without UV pre-illumination are presented in Table 1. As a general trend among the test liquids, the capillary rise becomes smaller as the surface tension of liquids becomes smaller. These results are supported by the general theory of capillarity presented previously in Eq. (2). When the tests were compared with and without UV pre-illumination, the difference in the capillary rise was small. In addition, the capillary rise of the tiron solution showed a similar trend to that of water, and the additional effect on capillary rise was not obvious for the sample that was pre-illuminated with UV irradiation. When pure ethanol was employed, the capillary rise did not differ between the samples with and without UV pre-illumination. When the aged samples were employed, the capillary rise of water was 11 mm for the sample not illuminated and 29 mm for the illuminated sample. Thus, it would appear that sample age plays a substantial role with respect to capillary rise. 3.2. Marangoni tests From the results of the pre-illumination testing, it appeared that aged surfaces of thin-film TiO2 samples displayed the largest change upon UV irradiation and were subsequently used in the following studies. After immersing the sample plates in a bath of water, the capillary rise reached 11 mm. During a 2-h period of UV illumination upon these TiO2-coated plates, the capillary rise of water increased to 29 mm. The change was not rapid, taking approximately 2 h for the top of the meniscus to reach its maximum. For comparison, an 8 wt.% ethanol aqueous solution and a tiron solution were tested in the same manner. The results of these tests are listed in Table 2. The maximum heights of the capillary rise of all liquids in all of these Marangoni tests were equivalent to those obtained in previous preillumination testing. Table 1 Capillary rise and contact angle of liquids in parallel test
3. Results 3.1. Pre-illumination tests Pictures of the experimental apparatus with the aged samples are shown in Fig. 2. The solution was colored with Rhodamine-B for this picture alone in order to exhibit the height of the liquid. The capillary rise of the aqueous solution of Rhodamine-B was 15 mm for the sample without UV pre-illumination and 25 mm for the sample with UV pre-illumination. A list of liquids tested
Water 4 wt.% EtOH aq. 8 wt.% EtOH aq. 20 wt.% EtOH aq. EtOH Tiron aq. Rhodamine-B aq.b Waterb
No UV
UV
g (mNym) h (mm) u (8)
h (mm) u (8)
73 58a 49a 38a 22 – – 73
29 22 21 18 10 28 25 29
–, not available. Estimated based on Ref. w15x. b Aged surface. a
28 21 22 15 10 25 15 11
18 15 15 15 1 10 – 35–47
7 5 5 8 1 1 – 6
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235
Table 2 The motion of liquids during UV illumination Before UV
Water 8 wt.% EtOH aq. Tiron aq.
After UV
g (mNym)
h (mm)
u (8)
h (mm)
u (8)
73 49
11 8 10
34–42 33–39 55
29 21 29
6 5 1
–
–, not available.
3.3. FT-IR spectra In a previous study, we developed a unique IR transmission technique that allowed us to directly monitor the behavior of water molecules adsorbed on the TiO2 thin-film w32x. The size of bands in the 3600– 2800ycm range that are assigned to O–H stretching bonds (nOH mode) corresponds to the amount of water adsorbed on the thin-film. For purposes of finding a common condition before testing, all of the thin-film TiO2 samples used in these studies were previously kept in a dark box with 60% RH before being set in the cell and subsequently flushed for 12 h with dry air prior to taking a background spectrum. The spectra of a thin-film TiO2 with aged, fresh and UV-illuminated surfaces were collected under humid condition (60% RH at 295 K) and are displayed in Fig. 3. The size of the bands for the aged surface was significantly smaller than both the fresh and UVilluminated surfaces. This result indicates that these aged surfaces were not capable of adsorbing as much water as the others. Slightly larger amounts of water were adsorbed on UV-illuminated than fresh surfaces (i.e. not illuminated). However, this amount of water was almost equivalent to the case of the fresh surface. This fact suggests that the UV-illuminated surface did not adsorb a significantly larger quantity of water over that of a fresh surface. 4. Discussion
Fig. 3. FT-IR spectra of TiO2 surfaces under 60% RH at 23 8C: aged, fresh and fresh with UV illumination from the bottom.
tension of TiO2 upon UV illumination. When ethanol and tiron, which are known hole and superoxide scavengers, respectively, were added to water, the capillary rise maintained good agreement with the theory and were directly related to the surface tension of the liquids and its contact angle with the TiO2 surface. Thus, the effect of holes andyor superoxides on the capillary rise seemed to be minimal for our conditions. However, when the contact angle of water was large (35–478) as is the case of the aged sample, the measured capillary rise (11 mm) was considerably different from the calculated value (f23 mm). We believe that this disagreement is caused both by errors in Young’s equation and Eq. (2), neither of which lack account for the weight of these liquids. Droplets are compressed due to their weight particularly when their contact angles become large. In addition, the shape of meniscus of the capillary rise is not always spherical which is also due to the weight of liquids. Correction factors for Young’s equation w21,23,24x or contact angles measured with Wilhelmy plate w21x may provide better agreement between measured contact angles and the capillary rise.
4.1. Capillary rise and wetting properties From the results of the pre-illumination testing, we can conclude that the capillary rise between TiO2-coated glass plates changed with the surface tension of liquids and the TiO2 surface condition. Eq. (2) relates capillary rise to the surface tension of a liquid and its contact angle on a given surface. This relation is illustrated in Fig. 4, where the measured values of capillary rise were also plotted against independently measured contact angles. For cases of small contact angles (lower than 208), the obtained capillary rises is in reasonable agreement with theory, and would suggest that this capillary rise method is a suitable alternative to measuring contact angles with respect to determining the change in surface
Fig. 4. Relation between contact angles and capillary rises (points are measured data and lines are calculated data).
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Capillary rise changed with the condition of TiO2 surface, i.e. freshly prepared, aged or UV-illuminated. FT-IR results also demonstrated that the aged surfaces were not capable of adsorbing the water as much as the fresh or UV-illuminated surfaces after all of the samples were exposed to dry air in dark control for 12 h prior to taking background spectra. The simple comparison of Table 1 and Fig. 3 suggests that the analogy between the height of the bands of IR spectra and the capillary rise is obvious. Capillary rise and the wetting ability are induced by the adsorption of water on the surface, and the adsorption of water is enhanced by UV illumination and is diminished by aging. In fact, the aging effects were also reported by the previous publications w8,14x, where contact angles increased from 10 to 728 when TiO2 surfaces were stored in dark boxes. The present investigation was not designed to elucidate the mechanism of aging the surface of TiO2. However, considering the premise on the wetting ability that oxygen vacancies on the TiO2 surface induces high wetting ability (i.e. high surface energy) w16x, we speculate that oxygen vacancies were diminished in a dark box (i.e. via the aging process). Water molecules and oxygen molecules selectively adsorb on oxygen vacancies in reversible way in a dark control at ambient temperature w18,19x. Therefore, oxygen vacancies gradually decrease under the condition, and TiO2 surfaces get low wetting ability. In order to prove the hypothesis, the aged surface needs to be carefully investigated with long term experiments. 4.2. Photo-responsive (Marangoni type) motion Based on the idea that the capillary rise changes with the surface conditions from the pre-illumination testing, liquid motion can be induced simply by exposure to a UV source. Marangoni tests revealed that the capillary rise of water significantly increased by a factor of two during the course of UV illumination. However, we should like to point out that the highest point of the capillary rise did not change from that obtained in the pre-illumination test. Therefore, capillary rise is simply determined by the surface tension of liquids and their interaction between this liquid and the surface of TiO2, but not by the process. Likewise, adding ethanol (8 wt.% EtOH aq.) and tiron (tiron aq.) did not show any specific effect on the capillary rise, similarly to the preillumination test. Again, a hole or a superoxide scavenger did not change the interaction between the surface and the liquid and did not diminish the capillary rise. From these results, we believe that the surface wetting ability causing the capillary rise was not changed by reacting fluids between TiO2 thin-films via photocatalytic reaction and that the interface between TiO2 and air changed its surface wetting ability via photocatalytic events.
Currently, this process was neither instantaneous nor irreversible. Moreover, only aged surfaces induced a significant liquid motion during UV illumination while freshly prepared surfaces showed only a small change in capillary rise. From an analysis of Fig. 4, it appears that the capillary rise does not change to a large extent when contact angles were small (e.g. 0–158). Some recent publications w7,15x mentioned that contact angles of TiO2 increased by providing ultrasonic waves or visible lights even after UV illumination. If one could reliably utilize some of these manners to instantaneously decrease the surface energy of TiO2, this fluid movement might become reversible process. It is important to note that in recent publications w9x concerning the motion of liquids, particularly on the surface of glass coated with azobenzene groups, one could not observe the motion of liquids that had high surface tensions, e.g. water and ethylene glycol due to the low surface energy of the coatings. In contrast here, by using irradiated TiO2 thinfilms on glass plates we can amplify the capillary rise of water, a high surface tension liquid, by simply employing UV light. 5. Conclusions In this study, we have suggested that the capillary rise be considered an alternative method to be employed for evaluating the wetting ability of TiO2 thin-films that change the surface wetting ability upon UV irradiation. While contact angle measurements have been the preferred measuring technique, capillary rise methods can easily measured under irradiation using simple systems. The capillary rise is not only determined by liquids but also by the surface condition: the fresh, the aged, or the UV-illuminated. FT-IR investigations illustrated the relationship between the adsorption of water and capillary rise. Since adding a hole and a superoxide scavengers to solutions between TiO2 films showed no effects on capillary rise, fluids between TiO2 films do not contribute significantly to change of the wetting ability in these systems. Capillary rise in the channels of TiO2 thin-films has been shown to be a photo-responsive liquid motion induced by UV exposure. Although the effect is currently limited only to an aged surface, it is worth highlighting that this means that we can move water, a high surface tension material simply by illuminating UV light. Acknowledgments The authors thank Prof. George Zografi in School of Pharmacy at University of Wisconsin – Madison for informative comments on this paper. We would also like to acknowledge the helpful comments and valuable
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experimental assistance of Mr Jeffrey R.S. Brownson and Mr Kenneth A. Walz during these studies. References w1x A. Fujishima, K. Honda, Nature 238 (1972) 37. w2x M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. w3x X.Z. Fu, L.A. Clark, W.A. Zeltner, M.A. Anderson, J. Photochem. Photobiol. A—Chem. 97 (1996) 181. w4x M.E. Zorn, D.T. Tompkins, W.A. Zeltner, M.A. Anderson, Appl. Catal. B—Environ. 23 (1999) 1. w5x S. Kataoka, D.T. Tompkins, W.A. Zeltner, M.A. Anderson, J. Photochem. Photobiol. A—Chem. 148 (2002) 323. w6x R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431. w7x N. Sakai, R. Wang, A. Fujishima, T. Watanabe, K. Hashimoto, Langmuir 14 (1998) 5918. w8x R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Adv. Mater. 10 (1998) 135. w9x T. Watanabe, A. Nakajima, R. Wang, M. Minabe, S. Koizumi, A. Fujishima, K. Hashimoto, Thin Solid Films 351 (1999) 260. w10x R. Wang, N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B 103 (1999) 2188. w11x M. Miwa, A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Langmuir 16 (2000) 5754. w12x M. Miyauchi, A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Chem. Mater. 12 (2000) 3.
237
w13x A. Nakajima, K. Hashimoto, T. Watanabe, K. Takai, G. Yamauchi, A. Fujishima, Langmuir 16 (2000) 7044. w14x R.D. Sun, A. Nakajima, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B 105 (2001) 1984. w15x M. Miyauchi, N. Kieda, S. Hishita, T. Mitsuhashi, A. Nakajima, T. Watanabe, K. Hashimoto, Surf. Sci. 511 (2002) 401. w16x N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B 105 (2001) 3023. w17x R. Nakamura, K. Ueda, S. Sato, Langmuir 17 (2001) 2298. w18x M.A. Henderson, Langmuir 12 (1996) 5093. w19x M.A. Henderson, Surf. Sci. Rep. 46 (2002) 5. w20x S.H. Szczepankiewicz, A.J. Colussi, M.R. Hoffmann, J. Phys. Chem. B 104 (2000) 9842. w21x A.W. Adamson, A.P. Gast, Physical Chemistry of Surfaces, Wiley, New York, 1997. w22x G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994. w23x A.C. Pierre, Introduction to Sol–gel Processing, Kluwer Academic, Boston, 1998. w24x N.W.F. Kossen, P.M. Heertjes, Chem. Eng. Sci. 20 (1965) 593. w25x J.Y. Shin, N.L. Abbott, Langmuir 15 (1999) 4404. w26x S. Abbott, J. Ralston, G. Reynolds, R. Hayes, Langmuir 15 (1999) 8923. w27x K. Ichimura, S.K. Oh, M. Nakagawa, Science 288 (2000) 1624. w28x C.L. Feng, Y.J. Zhang, J. Jin, Y.L. Song, L.Y. Xie, G.R. Qu, L. Jiang, D.B. Zhu, Langmuir 17 (2001) 4593. w29x C.D. Bain, Chem. Phys. Chem. 2 (2001) 580. w30x S. Daniel, M.K. Chaudhury, J.C. Chen, Science 291 (2001) 633. w31x T.P. Russell, Science 297 (2002) 964. w32x S. Kataoka, Ph.D. Thesis, Environmental Chemistry and Technology, University of Wisconsin – Madison, USA, 2003.
Capillary rise between two TiO2 thin-films: evaluating photo-activated wetting Sho Kataoka1, Marc A. Anderson* Environmental Chemistry and Technology Program, University of Wisconsin – Madison, 660 N. Park Street, Madison, WI 53706, USA Received 25 February 2003; received in revised form 10 September 2003; accepted 1 October 2003
Abstract The measurement of capillary rise was used for evaluating the photo-enhanced wetting ability of TiO2 . Two glass plates, which were coated with TiO2 thin-films, were vertically oriented having a small gap and immersed in the several solutions. In the channel between two TiO2 thin-films, the capillary rise of liquids is determined by the surface tension of liquids and the physical– chemical nature of the contacting surfaces. The capillary rise of water was measured to be 28 mm for a TiO2 samples equilibrated in the dark for 2 h (immediately after the calcination), 29 mm for the surface pre-illuminated with UV light and 11 mm for aged samples (i.e. kept in a dark box for several weeks). A companion FT-IR study supported the fact that the capillary rise has a strong relation with the amount of adsorbed water on the surface of the TiO2 thin-films. While contact angles have been routinely measured for photoactive TiO2 systems, capillary rise is an alternative criterion for the wetting ability without expensive equipment. We also show that capillary rise can be expanded to photo-responsive motion of liquids in the TiO2 channel. The capillary rise of water tested between the two TiO2 thin-film surfaces increased by a factor of three under UV light exposure. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Capillary rise; Wetting ability; TiO2; UV illumination
1. Introduction TiO2, known to be a photocatalyst, changes its wetting ability after being illuminated with UV light w1–5x. Both hydrocarbons (liquids) and water completely wet the surface of TiO2 after UV exposure w6–16x. This phenomenon is commonly referred to as ‘photo-induced wetting ability.’ Many attempts to elucidate the mechanisms of the wetting property from a mechanistic and microscopic point of view have been reported w16–20x. The current premise attributes this phenomenon to oxygen (bridging oxygen) defects (holes), which are produced via the photocatalytic process. This event is considered to be structural change and is supported by recent experiments showing that the surface of TiO2 sputtered with Arq should behave as if it were illuminated with UV light w9x. *Corresponding author. Tel.: q1-608-262-2674; fax: q1-608-2620454. E-mail address: [email protected] (M.A. Anderson). 1 Present address: Department of Chemistry, Texas A&M University, Dr. Cremer Group, 3255 TAMU College Station, TX 77843, USA.
While photo-enhanced wetting has now been studied for several years, we feel that the thermodynamic and macroscopic evaluation of this phenomenon has not been adequately addressed. This is partially because the surface tension of solids is not directly measurable. Contact angles are commonly measured to obtain the extent of the surface tension of solids based on Young’s equation w21x: gLV cos usgSVygSL
(1)
where g is a surface tension, u is a contact angle and subscripts L, V and S are represent liquid, vapor and solid, respectively. Although contact angle measurements are easy and useful, problems still remain. Since it has not been possible to directly test the Young’s equation experimentally w21x, many correction factors have been proposed, e.g. a roughness factor for surfaces, the shape of droplets and others w11,15,21–24x. Furthermore, it is particularly difficult to evaluate this photoactivated phenomenon on the surface of solids when contact angles are small (;08). With respect to the surface of TiO2 after UV illumination, the contact angle
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.10.001
S. Kataoka, M.A. Anderson / Thin Solid Films 446 (2004) 232–237
of water (gLV is f73 mNym) approaches zero degrees, which is characterized as nearly complete ‘spreading’ w21x. Since water has one of the highest surface tension among common liquids—except for mercury, most other liquids also are likely to spontaneously spread on the surface of irradiated TiO2. To illustrate this last point, we attempted to measure the contact angle of mercury droplets (gLV is f485 mNym) on TiO2 surfaces before and after UV exposure. The contact angle was 122– 1408 before and 105–1168 after the illumination. However, since the mercury droplet deformed in shape due to the weight of mercury, it was difficult to determine exact contact angles. To the best of our knowledge, only contact angle measurements have been applied to the characterization of the wetting ability of TiO2 w6–16x. Capillary rise is a well-known interfacial phenomenon caused by the surface tension of liquids and their wetting ability. When a meniscus is ideally ‘hemi-cylindrical’ concave in shape, the capillary rise between two flat plates is expressed as follows w21x: hs
2gLV cos u , rgd
(2)
where gSV is the surface tension of a liquid, u is a contact angle between a liquid and a substrate, r is the density of a liquid, g is the gravitational acceleration (i.e. 9.8 mys2) and d is a distance between two plates. If capillary rise is applied to TiO2 surface wetting ability instead of contact angle measurements, small contact angles may be able to be projected to capillary rise with larger scale by changing the distance of plates. The idea of alternatively using the capillary rise method is that this method could evaluate wetting ability with higher precision in an inexpensive way. Some recent publications have appeared concerning the light-mediated dynamic motion of liquids on surfaces w24–31x. This phenomenon is based on the well-known Marangoni effect. Liquids move on surface-responsive materials (especially the trans–cis photo-trans isomerisable azobenzene groups) that are coated on substrates w26x. In a similar fashion we can also cause fluids to rise in capillaries between TiO2 thin-films simply by illuminating the surface with UV light. 2. Experimental details 2.1. Capillary rise test A TiO2 sol was prepared via a sol–gel processing procedure w3x and was dip-coated onto 1=12-inch glass plates (Cardinal CG, Spring Green, WI) at a removal velocity of 0.95 mmys. The supported catalyst was calcined at 350 8C for 3 h after drying at room temperature in a 100 8C oven. Plastic strips (0.5-mm
233
Fig. 1. Schematic diagram of the system used for pre-illumination test and Marangoni test.
thickness) were positioned between two coated glass plates forming a small capillary. This assembly was firmly fastened with clips and vertically immersed in test liquids. Capillary rise was measured from the top surface of liquids. Two types of tests were conducted in the capillary rise tests. For the first test, TiO2-coated glass plates were illuminated with UV light (F4T8BL, Viko, Japan) for 2 h prior to immersing into liquids (pre-illumination test). The results were compared with those of samples not illuminated. The tests were performed with both samples immediately after calcination and samples kept in a dark box (60% RH and 23 8C) for several weeks after the calcination. For the convenience, the former is called a fresh surface, and the latter is an aged surface. Contact angles of samples were also measured using a contact angle goniometer (NRL contact angle goniom´ eter, Rame-Hart Inc., Mountain Lakes, NJ). The second test was aimed at changing the capillary rise during the illumination (Marangoni test). The diagram of the testing scheme is illustrated in Fig. 1. The samples were immersed into liquids and afterwards were simultaneously illuminated with UV light for 2 h or longer. In the second test, only the aged samples were employed. The test liquids in the study constitute water (MilliQ, Millipore, Bedford, MA), ethanol (Aldrich, Milwaukee, WI), ethanolywater mixtures (4, 8 and 20 wt.%), 0.025 M tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt, Aldrich) aqueous solutions and an aqueous solution of Rhodamine-B (0.0025 M, Aldrich). The surface tension of aqueous ethanol solution changes with concentration and was estimated from Ref. w21x. Aqueous ethanol solutions were also expected to scavenge holes on the surface of TiO2. Likewise, tiron, also known as a superoxide scavenger, was added for the purpose of scavenging superoxides produced on the surface of TiO2 under UV illumination. 2.2. Fourier transform spectroscopy In order to better define the character of the respective surfaces of TiO2 (aged, fresh and UV-illuminated), the
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Fig. 2. Photograph of capillary rise of colored water in a channel between TiO2-coated glasses: no UV illumination (left), UV pre-illumination (right).
amount of water adsorbed on those surfaces of TiO2 was measured with infrared (IR) spectroscopy using a TiO2 thin-film transmission IR cell. The experimental details about the thin-film transmission cell were thoroughly described in the previous publication w32x. The previous study showed that this cell allows one to analyze the surface of TiO2 both in the presence and absence of UV illumination in a closed environment. The same TiO2 sol as was used for glass plates in the capillary rise test was also sprayed on a Si wafer and was calcined in the same way. For aging the surface, this supported catalyst was stored at a dark box (60% RH, 23 8C) for several weeks after the calcination. For the fresh surface, the supported catalyst used in the test with the aged surface was calcined again at 350 8C for an hour and was afterwards exposed to humidified air at the dark box for 12 h (60% RH, 23 8C) in order to obtain the same experimental condition with respect to surface chemistry of the TiO2 thin-film. The transmission IR cell containing the supported catalyst was flushed with dry air (-0.7% RH) for 12 h prior to taking a background spectrum under the dry air condition. Subsequently, humidified air (60% RH) was supplied through the cell, and IR spectra were collected with an IR spectrometer (Nexus 670, Nicolet Instrument, Madison, WI), which contained an MCT-A (HgCdTe) detector and KBr beam splitter. During the course of the UV-illuminated experiment, the catalyst was simultaneously illuminated with an UV lamp (a 200-W medium pressure Hg vapor lamp, Model 7825-32, Ace Glass, Vineland, NJ) covered with a cooling jacket.
and their respective capillary rise between the two TiO2 plates both with and without UV pre-illumination are presented in Table 1. As a general trend among the test liquids, the capillary rise becomes smaller as the surface tension of liquids becomes smaller. These results are supported by the general theory of capillarity presented previously in Eq. (2). When the tests were compared with and without UV pre-illumination, the difference in the capillary rise was small. In addition, the capillary rise of the tiron solution showed a similar trend to that of water, and the additional effect on capillary rise was not obvious for the sample that was pre-illuminated with UV irradiation. When pure ethanol was employed, the capillary rise did not differ between the samples with and without UV pre-illumination. When the aged samples were employed, the capillary rise of water was 11 mm for the sample not illuminated and 29 mm for the illuminated sample. Thus, it would appear that sample age plays a substantial role with respect to capillary rise. 3.2. Marangoni tests From the results of the pre-illumination testing, it appeared that aged surfaces of thin-film TiO2 samples displayed the largest change upon UV irradiation and were subsequently used in the following studies. After immersing the sample plates in a bath of water, the capillary rise reached 11 mm. During a 2-h period of UV illumination upon these TiO2-coated plates, the capillary rise of water increased to 29 mm. The change was not rapid, taking approximately 2 h for the top of the meniscus to reach its maximum. For comparison, an 8 wt.% ethanol aqueous solution and a tiron solution were tested in the same manner. The results of these tests are listed in Table 2. The maximum heights of the capillary rise of all liquids in all of these Marangoni tests were equivalent to those obtained in previous preillumination testing. Table 1 Capillary rise and contact angle of liquids in parallel test
3. Results 3.1. Pre-illumination tests Pictures of the experimental apparatus with the aged samples are shown in Fig. 2. The solution was colored with Rhodamine-B for this picture alone in order to exhibit the height of the liquid. The capillary rise of the aqueous solution of Rhodamine-B was 15 mm for the sample without UV pre-illumination and 25 mm for the sample with UV pre-illumination. A list of liquids tested
Water 4 wt.% EtOH aq. 8 wt.% EtOH aq. 20 wt.% EtOH aq. EtOH Tiron aq. Rhodamine-B aq.b Waterb
No UV
UV
g (mNym) h (mm) u (8)
h (mm) u (8)
73 58a 49a 38a 22 – – 73
29 22 21 18 10 28 25 29
–, not available. Estimated based on Ref. w15x. b Aged surface. a
28 21 22 15 10 25 15 11
18 15 15 15 1 10 – 35–47
7 5 5 8 1 1 – 6
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Table 2 The motion of liquids during UV illumination Before UV
Water 8 wt.% EtOH aq. Tiron aq.
After UV
g (mNym)
h (mm)
u (8)
h (mm)
u (8)
73 49
11 8 10
34–42 33–39 55
29 21 29
6 5 1
–
–, not available.
3.3. FT-IR spectra In a previous study, we developed a unique IR transmission technique that allowed us to directly monitor the behavior of water molecules adsorbed on the TiO2 thin-film w32x. The size of bands in the 3600– 2800ycm range that are assigned to O–H stretching bonds (nOH mode) corresponds to the amount of water adsorbed on the thin-film. For purposes of finding a common condition before testing, all of the thin-film TiO2 samples used in these studies were previously kept in a dark box with 60% RH before being set in the cell and subsequently flushed for 12 h with dry air prior to taking a background spectrum. The spectra of a thin-film TiO2 with aged, fresh and UV-illuminated surfaces were collected under humid condition (60% RH at 295 K) and are displayed in Fig. 3. The size of the bands for the aged surface was significantly smaller than both the fresh and UVilluminated surfaces. This result indicates that these aged surfaces were not capable of adsorbing as much water as the others. Slightly larger amounts of water were adsorbed on UV-illuminated than fresh surfaces (i.e. not illuminated). However, this amount of water was almost equivalent to the case of the fresh surface. This fact suggests that the UV-illuminated surface did not adsorb a significantly larger quantity of water over that of a fresh surface. 4. Discussion
Fig. 3. FT-IR spectra of TiO2 surfaces under 60% RH at 23 8C: aged, fresh and fresh with UV illumination from the bottom.
tension of TiO2 upon UV illumination. When ethanol and tiron, which are known hole and superoxide scavengers, respectively, were added to water, the capillary rise maintained good agreement with the theory and were directly related to the surface tension of the liquids and its contact angle with the TiO2 surface. Thus, the effect of holes andyor superoxides on the capillary rise seemed to be minimal for our conditions. However, when the contact angle of water was large (35–478) as is the case of the aged sample, the measured capillary rise (11 mm) was considerably different from the calculated value (f23 mm). We believe that this disagreement is caused both by errors in Young’s equation and Eq. (2), neither of which lack account for the weight of these liquids. Droplets are compressed due to their weight particularly when their contact angles become large. In addition, the shape of meniscus of the capillary rise is not always spherical which is also due to the weight of liquids. Correction factors for Young’s equation w21,23,24x or contact angles measured with Wilhelmy plate w21x may provide better agreement between measured contact angles and the capillary rise.
4.1. Capillary rise and wetting properties From the results of the pre-illumination testing, we can conclude that the capillary rise between TiO2-coated glass plates changed with the surface tension of liquids and the TiO2 surface condition. Eq. (2) relates capillary rise to the surface tension of a liquid and its contact angle on a given surface. This relation is illustrated in Fig. 4, where the measured values of capillary rise were also plotted against independently measured contact angles. For cases of small contact angles (lower than 208), the obtained capillary rises is in reasonable agreement with theory, and would suggest that this capillary rise method is a suitable alternative to measuring contact angles with respect to determining the change in surface
Fig. 4. Relation between contact angles and capillary rises (points are measured data and lines are calculated data).
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Capillary rise changed with the condition of TiO2 surface, i.e. freshly prepared, aged or UV-illuminated. FT-IR results also demonstrated that the aged surfaces were not capable of adsorbing the water as much as the fresh or UV-illuminated surfaces after all of the samples were exposed to dry air in dark control for 12 h prior to taking background spectra. The simple comparison of Table 1 and Fig. 3 suggests that the analogy between the height of the bands of IR spectra and the capillary rise is obvious. Capillary rise and the wetting ability are induced by the adsorption of water on the surface, and the adsorption of water is enhanced by UV illumination and is diminished by aging. In fact, the aging effects were also reported by the previous publications w8,14x, where contact angles increased from 10 to 728 when TiO2 surfaces were stored in dark boxes. The present investigation was not designed to elucidate the mechanism of aging the surface of TiO2. However, considering the premise on the wetting ability that oxygen vacancies on the TiO2 surface induces high wetting ability (i.e. high surface energy) w16x, we speculate that oxygen vacancies were diminished in a dark box (i.e. via the aging process). Water molecules and oxygen molecules selectively adsorb on oxygen vacancies in reversible way in a dark control at ambient temperature w18,19x. Therefore, oxygen vacancies gradually decrease under the condition, and TiO2 surfaces get low wetting ability. In order to prove the hypothesis, the aged surface needs to be carefully investigated with long term experiments. 4.2. Photo-responsive (Marangoni type) motion Based on the idea that the capillary rise changes with the surface conditions from the pre-illumination testing, liquid motion can be induced simply by exposure to a UV source. Marangoni tests revealed that the capillary rise of water significantly increased by a factor of two during the course of UV illumination. However, we should like to point out that the highest point of the capillary rise did not change from that obtained in the pre-illumination test. Therefore, capillary rise is simply determined by the surface tension of liquids and their interaction between this liquid and the surface of TiO2, but not by the process. Likewise, adding ethanol (8 wt.% EtOH aq.) and tiron (tiron aq.) did not show any specific effect on the capillary rise, similarly to the preillumination test. Again, a hole or a superoxide scavenger did not change the interaction between the surface and the liquid and did not diminish the capillary rise. From these results, we believe that the surface wetting ability causing the capillary rise was not changed by reacting fluids between TiO2 thin-films via photocatalytic reaction and that the interface between TiO2 and air changed its surface wetting ability via photocatalytic events.
Currently, this process was neither instantaneous nor irreversible. Moreover, only aged surfaces induced a significant liquid motion during UV illumination while freshly prepared surfaces showed only a small change in capillary rise. From an analysis of Fig. 4, it appears that the capillary rise does not change to a large extent when contact angles were small (e.g. 0–158). Some recent publications w7,15x mentioned that contact angles of TiO2 increased by providing ultrasonic waves or visible lights even after UV illumination. If one could reliably utilize some of these manners to instantaneously decrease the surface energy of TiO2, this fluid movement might become reversible process. It is important to note that in recent publications w9x concerning the motion of liquids, particularly on the surface of glass coated with azobenzene groups, one could not observe the motion of liquids that had high surface tensions, e.g. water and ethylene glycol due to the low surface energy of the coatings. In contrast here, by using irradiated TiO2 thinfilms on glass plates we can amplify the capillary rise of water, a high surface tension liquid, by simply employing UV light. 5. Conclusions In this study, we have suggested that the capillary rise be considered an alternative method to be employed for evaluating the wetting ability of TiO2 thin-films that change the surface wetting ability upon UV irradiation. While contact angle measurements have been the preferred measuring technique, capillary rise methods can easily measured under irradiation using simple systems. The capillary rise is not only determined by liquids but also by the surface condition: the fresh, the aged, or the UV-illuminated. FT-IR investigations illustrated the relationship between the adsorption of water and capillary rise. Since adding a hole and a superoxide scavengers to solutions between TiO2 films showed no effects on capillary rise, fluids between TiO2 films do not contribute significantly to change of the wetting ability in these systems. Capillary rise in the channels of TiO2 thin-films has been shown to be a photo-responsive liquid motion induced by UV exposure. Although the effect is currently limited only to an aged surface, it is worth highlighting that this means that we can move water, a high surface tension material simply by illuminating UV light. Acknowledgments The authors thank Prof. George Zografi in School of Pharmacy at University of Wisconsin – Madison for informative comments on this paper. We would also like to acknowledge the helpful comments and valuable
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experimental assistance of Mr Jeffrey R.S. Brownson and Mr Kenneth A. Walz during these studies. References w1x A. Fujishima, K. Honda, Nature 238 (1972) 37. w2x M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. w3x X.Z. Fu, L.A. Clark, W.A. Zeltner, M.A. Anderson, J. Photochem. Photobiol. A—Chem. 97 (1996) 181. w4x M.E. Zorn, D.T. Tompkins, W.A. Zeltner, M.A. Anderson, Appl. Catal. B—Environ. 23 (1999) 1. w5x S. Kataoka, D.T. Tompkins, W.A. Zeltner, M.A. Anderson, J. Photochem. Photobiol. A—Chem. 148 (2002) 323. w6x R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431. w7x N. Sakai, R. Wang, A. Fujishima, T. Watanabe, K. Hashimoto, Langmuir 14 (1998) 5918. w8x R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Adv. Mater. 10 (1998) 135. w9x T. Watanabe, A. Nakajima, R. Wang, M. Minabe, S. Koizumi, A. Fujishima, K. Hashimoto, Thin Solid Films 351 (1999) 260. w10x R. Wang, N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B 103 (1999) 2188. w11x M. Miwa, A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Langmuir 16 (2000) 5754. w12x M. Miyauchi, A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Chem. Mater. 12 (2000) 3.
237
w13x A. Nakajima, K. Hashimoto, T. Watanabe, K. Takai, G. Yamauchi, A. Fujishima, Langmuir 16 (2000) 7044. w14x R.D. Sun, A. Nakajima, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B 105 (2001) 1984. w15x M. Miyauchi, N. Kieda, S. Hishita, T. Mitsuhashi, A. Nakajima, T. Watanabe, K. Hashimoto, Surf. Sci. 511 (2002) 401. w16x N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B 105 (2001) 3023. w17x R. Nakamura, K. Ueda, S. Sato, Langmuir 17 (2001) 2298. w18x M.A. Henderson, Langmuir 12 (1996) 5093. w19x M.A. Henderson, Surf. Sci. Rep. 46 (2002) 5. w20x S.H. Szczepankiewicz, A.J. Colussi, M.R. Hoffmann, J. Phys. Chem. B 104 (2000) 9842. w21x A.W. Adamson, A.P. Gast, Physical Chemistry of Surfaces, Wiley, New York, 1997. w22x G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994. w23x A.C. Pierre, Introduction to Sol–gel Processing, Kluwer Academic, Boston, 1998. w24x N.W.F. Kossen, P.M. Heertjes, Chem. Eng. Sci. 20 (1965) 593. w25x J.Y. Shin, N.L. Abbott, Langmuir 15 (1999) 4404. w26x S. Abbott, J. Ralston, G. Reynolds, R. Hayes, Langmuir 15 (1999) 8923. w27x K. Ichimura, S.K. Oh, M. Nakagawa, Science 288 (2000) 1624. w28x C.L. Feng, Y.J. Zhang, J. Jin, Y.L. Song, L.Y. Xie, G.R. Qu, L. Jiang, D.B. Zhu, Langmuir 17 (2001) 4593. w29x C.D. Bain, Chem. Phys. Chem. 2 (2001) 580. w30x S. Daniel, M.K. Chaudhury, J.C. Chen, Science 291 (2001) 633. w31x T.P. Russell, Science 297 (2002) 964. w32x S. Kataoka, Ph.D. Thesis, Environmental Chemistry and Technology, University of Wisconsin – Madison, USA, 2003.