726
Ind. Eng. Chem. Res. 2002, 41, 726-731
Development of Photocatalytic Coating Agents with Indicator Dyes J. F. Zhi,†,‡ H. B. Wang,§ and A. Fujishima*,§ Open Lab, Special Research Laboratory for Optical Science, Kanagawa Academy of Science and Technology, KSP Building East 412, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan, and Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan
A new dye-doped photocatalytic (TiO2) coating agent, together with a precoating agent, was developed in order to detect debonded regions or gaps in the coating layers during or soon after the coating process. The dyes present in the coating layers act as visible indicators of the coating gaps due to their colors. Various commercial dyes were examined for their detection performance and the bleaching time after the gap detection. The results indicated the suitability of several selective fluorescent dyes for precoating agents and vividly colored dyes for photocatalytic agents. After the detection of the coating gaps, these dyes were bleached by photocatalytic activity of TiO2 over a period of 3 to 7 days under sunlight. Field tests on alumina plates coated with this new photocatalytic coating were found to be successful. Introduction Titanium dioxide (TiO2) exhibits unique photofunctionalities that make it an excellent choice for photocatalysis applications.1-6 The ability of titanium dioxide to work efficiently under low UV light intensities has enabled it to be useful for self-cleaning,3 self-sterilizing,7 and air-cleaning8 applications in indoor and outdoor environments. It is one of the unique aspects of TiO2 that there are actually two distinct photoinduced phenomena: the first is the well-known original photocatalytic phenomenon,1,3 which leads to the breakdown of organics, and the second involves high wettability, which is known as superhydrophilicity.3,9-10 Even though they are intrinsically different processes, they must take place on the same TiO2 surface. TiO2, being a wide band gap (3.2 eV) semiconductor, is transparent to visible light. This property, together with the self-cleaning function, make TiO2 a potential transparent photocatalytic coating material on any kind of exterior or interior building surface to maintain it clean under ambient conditions. Earlier, we have demonstrated the preparation of highly transparent TiO2 thin-film coatings on various substrates such as glass11,12 and ceramic tiles.2 Recently, a transparent photocatalytic coating agent of TiO2 that can be used on the outdoor walls of buildings has been developed by several Japanese companies (NIPPON SODA, etc.). A transparent precoat agent, which was developed by the same company, has been found to be necessary before applying the photocatalytic coating in order to avoid damaging the normal coating surface because of the strong oxidation power of TiO2. As described in Figure 1, this photocatalytic coating agent applied on the precoated surfaces has been found to be able to photodegrade various organic and inorganic contaminants under sunlight. Depending on the climate, the products may be washed off by rainfall.3 * To whom correspondence should be addressed. Phone: +81-3-5841-7245. Fax: +81-3-3812-6227. E-mail: akira-fu@ fchem.chem.t.u-tokyo.ac.jp. † Kanagawa Academy of Science and Technology. ‡ Japan Science and Technology Corporation (JST) Domestic Research Fellow. § University of Tokyo.
Figure 1. Schematic diagram of the photocatalytic coating agent used in this study.
Figure 2. Optical images of artificial gap regions (red regions indicated by arrows) made on alumina substrates by use of a mask followed by coating a photocatalytic agent. After the removal of the mask (right-side images), the gaps are not visible because of the transparency of the coatings.
However, one barrier to put this coating agent into practical application is that there can easily be accidental gaps, namely, uncoated or debonded regions which exist in the precoat and photocatalytic coating layer during painting because of the transparency of the coating agents. Moreover, these gaps cannot be detected during or even soon after painting. Figure 2 shows the artificial gap regions (red regions indicated by arrows in the left-side images) prepared on an alumina sub-
10.1021/ie0104722 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/17/2002
Ind. Eng. Chem. Res., Vol. 41, No. 4, 2002 727
Figure 3. Schematic models of the two types of coating gaps that can occur on (A) a precoat layer and (B) a photocatalytic coating layer.
strate by application of a mask, followed by coating with a photocatalytic agent. As shown in the right-side images, the gaps could not be seen after removing the mask because of the transparency of the coatings. These images demonstrate the difficulty in locating the gap areas after performing the coating in real situations. Furthermore, all of these gaps usually lead to two obvious problems, as described in Figure 3. In the absence of the precoat layer (i.e., the debonded regions appearing in the precoating layer), the strongly oxidizing properties of the TiO2 contained in the photocatalytic coating layer will destroy the normal substrate surface, especially organic substrates (Figure 3A). In the other case, it is clear that the debonded regions appearing in the photocatalytic coating layer will lead to the surface becoming fouled locally with time (Figure 3B). Therefore, this problem severely restricts the practical application of TiO2-based coating agents. To apply TiO2 coating agents for buildings and other substrate surfaces practically, the problem of coating gaps must be solved. From a practical application point of view, the most appropriate method to solve this problem is to find a visible indictor that enables the location of coating gaps or debonded regions. Here, we report the development of TiO2 photocatalytic coating agents that can overcome this drawback by the addition dye of indicators with which the gaps or debonded regions in the coating layer can be detected conveniently during and after painting. Therefore, such gaps can be avoided easily. The key point in the selection of the visible indicator is that it should be bleached after the detection of the gaps. Therefore, the objective of the present study is to search for an indictor that satisfies the aforementioned requirements. Generally, to the best of our knowledge, the use of dyes as indicators appears to be an attractive alternative in view of practical application. There are two important advantages in the use of the dye as an indicator of coating gaps. First, the dyes must be bright in color; thus, they make the transparent coating agents visible. Second, the dyes must be bleached photochemically or photocatalytically by TiO2 under sunlight. This possibility has been recognized by numerous studies.13-16 Therefore, the most promising way is to dope the coating
Table 1. Characteristic of Dyes Studied
dyes Rhodamine 123 Rhodamine 6G Coumarin-6 Thioflavin-T Lumogallion Fluorscein Sodium Methylene Blue Procion Red MX-5B Inchigo Carmine Acridine Orange Tartrazine Phthalocyanine Blue Malachite Green NK-719a NK-3a NK-5a NK-382a NK-138a NK-1590a
classification
abbreviation
basic Rh123 basic Rh6G coumarin Cm-6 TFT azo Lg acid FS basic MB reaction MX-5B acid IC basic AO acid TT pigment PB basic MG cyanine NK-719 cyanine NK-3 cyanine NK-5 cyanine NK-382 cyanine NK-138 cyanine NK-1590
solubility in coating agent
λmax (nm)
soluble soluble soluble soluble soluble soluble soluble soluble soluble soluble soluble soluble soluble soluble soluble insoluble soluble soluble soluble
575 520 630 417 490 475 655 538 605 465 435 620 621 491 604, 560 706, 649 595 653 662
a These dyes are commercial products of Hayashibara Biochemical Laboratories, Inc., Kankoh-Shikiso Institute.
agent with a suitable dye as an indictor to locate the gaps in the coating layers, to cover the gaps with the coating agent, and then to allow the exposure of the coating layers to sunlight to enable the bleaching of the dyes. Materials and Painting Methods The coating agents BISTOREITA NRC-300A (precoating agent) and BISTOREITA NRC-300C (photocatalytic coating agent) were obtained from the NIPPON SODA Corp. (Tokyo, Japan). Table 1 lists the dyes used in our studies. The tested dyes were commercial products and were used without further purification. The dye concentration was controlled in the range of 0.01 ∼ 0.05 wt %. The sol-gel dip-coating process and brush painting method were used for the fabrication of the coating films. Substrates such as soda-lime glass, alumina, and aluminum plate coated by white paint were used. The coating agents were coated on the soda-lime glass and
728
Ind. Eng. Chem. Res., Vol. 41, No. 4, 2002
Figure 4. Optical images of precoat layers coated on the alumina substrates after doping with fluorescent dye indicator Rh123 under visible (left-side) and black light (right-side) illumination.
alumina substrate surface uniformly by dipping the substrates into the coating agents and then lifting them at a controlled speed (1.5 mm/s). The resulting samples were dried in an electric furnace at 100 °C for 15 min to form the coating layers, and the organic substances were also burned out during drying. In the case of aluminum plate coated with white paint, the coating was carried out by a brush-painting method, followed by drying in air at room temperature. The thickness of the precoating and photocatalytic-coating layers was in the range of 0.5 ∼ 1 µm. Absorbance measurements of dyes were performed using a Shimadzu Model UV-160 double-beam spectrophotometer. The decolorization effect of dyes on the alumina and aluminum plate by photocatalytic oxidation reaction was measured by use of a reflectance spectrophotometer, “Handy-Spec” (GY-Gardner). A 10-W black light lamp and an artificial sunlight lamp (SOLAX, XC-100, Baiotto Corp., Japan) were used for illumination of the coated layers. In addition, the outdoor exposure test of the resulting layers was carried out on the rooftop of a building in Tokyo, (Hongo, Bunkyo-Ku), for one week in December. The angle of exposure was ca. 45°, and the UV power of sunlight was about 1.75 mW/cm2. Results and Discussion 3.1. Evaluation of Dyes as Dopants for Precoat Agents. As mentioned previously, the photocatalytic coating agent used includes two components. One is a precoating agent that is used as a barrier layer to prevent the substrate from reacting with TiO2. The other one is a photocatalytic agent, which is composed of TiO2 We focus first on the precoating agent. The requirements for the selection of the dye indicator are its vivid color, as well as the easy and complete decomposition (bleaching) after the detection. In an effort to use several kinds of vivid dyes (shown in Table 1) as indicators, we noted that, for example, MB, MG, PB, and so forth were not well-suited for this application because, after locating the gaps, the photodecomposition of these dyes in the precoating layer was poor under the irradiation of artificial sunlight or black light (UV). It is pertinent to note that the precoat does not contain any photocatalyst and, hence, the direct photodecomposition of the dye is necessary after locating the gaps. However, we found that the addition of certain types of pale-colored fluorescent dyes leads to the ability to act as indicator of
gaps when the resulting coating layers were illuminated with only a black light lamp. Rh123, Cm-6, Rh6G, and TFT are widely used as fluorescent dyes. These dyes were doped into the precoat layer and then coated on the alumina substrate by use of the dip-coating technique or brush-painting method. Figure 4 shows the optical images of the precoat layers on alumina after dip-coating with the Rh123 fluorescent dye. Although artificial gaps (letters A and F, indicated by the arrows) introduced in the layer are not easily detectable under visible light, they are detectable under the black light (UV) illumination. Similar results were observed for the Cm-6, Rh6G, and TFT. These results indicate that these fluorescent dyes are very useful for the location of gaps in the precoat layer under UV light at night. Some of the fluorescent dyes may not exhibit fluorescence when they are doped into the precoat, as observed in the case of FS. Next, as noted previously, the focus shifted to solving another important problem: how to decolorize these fluorescent dyes after detecting the gaps to restore the original transparency of the coating layers. In other words, these fluorescent dyes must be decomposed after the detection of the coating gaps in the precoating layer, even though the fluorescent dyes used are not vivid. In an attempt to bleach the fluorescent dyes, they were irradiated with black light (UV) irradiation. The UV irradiation effect on TFT photobleaching was evaluated by comparing the changes in the absorption (λmax, 417 nm) as a function of irradiation. As a result, as illustrated in Figure 5A (curve a), no significant change in the absorption was observed for TFT-doped precoat layer upon black light (UV) irradiation. This indicates that TFT could not be bleached simply with UV or sunlight irradiation. Similar behavior was observed for Rh123 (Figure 5B, curve a) as well as for Rh6G and Cm-6. Because photobleaching of the aforementioned dyes was not successful, we have attempted to examine the photocatalytic bleaching of these dyes by TiO2. The photocatalytic bleaching of several dyes on TiO2 surfaces has been verified by a number of researchers.13-16 If this is effective for the fluorescent dyes present in the precoat layer, we postulate that the photocatalytic agent coated over the precoat layer can decolorized these fluorescent dyes because of the photocatalytic reaction through the interface. To examine such a possibility, we first dipped a glass slide coated with dye-doped precoat layers into a solution of the photocatalytic coating agent,
Ind. Eng. Chem. Res., Vol. 41, No. 4, 2002 729 Table 2. Appropriate Dyes for the Precoat and Photocatalytic Coating Agents
Figure 5. Observed variation in absorbance versus irradiation time (t) of black light for (A) TFT and (B) Rh-123 dye-doped precoat films (a) before and (b) after depositing a TiO2 coating over them. The λmax for TFT and Rh-123 are 417 and 575 nm, respectively.
and the resulting samples were dried in an electric furnace at 100 °C for 15 min to form the photocatalytic coating layers, which were then to subjected black light illumination. The observed variation in the absorption (λmax, 417 nm) of TFT as a function of UV irradiation time is illustrated in Figure 5A (curve b). As expected, the photocatalytic process promotes a rapid bleaching of TFT, as evident from the decrease in the intensity of the characteristic absorption band (λmax, 417 nm) of TFT. Similar results were observed for Rh123 (Figure 5B, curve b), Cm-6, and Rh6G. Similar sets of experiments were carried out with the same fluorescent dyes under illumination with an artificial sunlight lamp. The results indicate that the bleaching rate under artificial sunlight is slower than that under the black light; however, the bleaching time was only about 3 (Rh123) to 7 days (Cm-6). Thus, the
dye indicators appropriate for precoating layer
dye indicators appropriate for photocatalytic coating layer
Rhodamine 123 (Rh123) Rhodamine 6G (Rh6G) Coumarin-6 (Cm-6) Thioflavin T (TFT)
Methylene Blue (MB) Cyanine dyes (list in Table 1) Procion Red MX-5B (MX-5B) Indigo Carmine (IC) Acridine Organge (AO)
photocatalytic oxidation effect of TiO2 is effective for the bleaching of the fluorescent dyes (Table 2) doped in the precoating layer under sunlight. The UV light available in sunlight is sufficient to activate TiO2 to induce the bleaching reaction, although the reaction is slower in comparison to that under black light illumination. It is pertinent to note that this slow reaction is advantageous for the present application, as it enables us to have sufficient time to detect the coating gaps in real situations. These results also indicate the suitability of certain fluorescent dyes (Table 2) as dopants for the precoat layer. 3.2. Evaluation of Dyes as Dopants for Precoat Agents. Similar sets of experiments as in the case of the precoat agent were also carried out to select suitable dyes for the photocatalytic coating agent. Experiments carried out with fluorescent dyes were not successful as the fluorescent effect of the dye diminished after doping into the photocatalytic coating layer because of the quenching effect of TiO2. However, the mixing of vividly colored dyes (Table 1), such as MB, NK-719, NK3, and MX-5B, to the photocatalytic coating agent enabled the observation of color in the coating layers during or soon after coating. This phenomenon is shown pictorially in Figure 6 of the NK-719 (precoat dopant is Rh123) dye. Furthermore, the vivid colors could be decolorized by photocatalytic activity of TiO2 itself under illumination with an of artificial sunlight lamp for about 20 h (maximum) to obtain the transparent coating layers. Similar results were observed for the MB (precoat dopant is Rh6G), NK-3 (precoating is Cm-6), and MX-5B (precoating dopant is Rh123) system. The disappearance rates of the vivid dyes used were measured using absorption and reflectance spectra. The UV-visible spectral changes taking place during the photooxidation of dyes in the glass/precoat (Cm-6)/TiO2 (MB) system is illustrated in Figure 7. A rapid decrease in the absorption band of MB in the film is clearly seen as a function of time. The complete bleaching of the dye
Figure 6. Optical images of alumina/(dye) precoat layer/(dye) photocatalytic layer films before and after irradiation by artificial sunlight. The artificial gaps are indicated as broad lines and letters A and F. The dopant dye indicator is NK-719. The dye indicated in parentheses is the dopant in the precoat layer. A reference is also shown which is coated without any coating agents.
730
Ind. Eng. Chem. Res., Vol. 41, No. 4, 2002
Figure 7. Variation of UV-visible absorption spectrum of a precoat (Cm-6)/photocatalytic coating (MB) system under illumination with artificial sunlight. The irradiation times (t/min) are indicated on the respective spectra. (Inset: change relative absorbance of MB as a function of irradiation time of artificial sunlight; A0 is the absorbance at 0 time, and the A is the absorbance at t time.)
effect. A similar effect is shown in Figure 8 for another system containing glass/precoat (Rh 123)/TiO2 (NK-719). These experiments were successful for various combinations of dyes listed in Table 2. Except for these dyes, the other dyes listed in Table 1 failed as indicators of photocatalytic coating, for example, MG, PB, and so forth because of their poor decolorization and FS, TT, and so forth because of their poor colors. 3.3. Outdoor Exposure Test. Because the dye-doped photocatalytic agents are mainly applicable for coating on large area surfaces such as building walls, it is necessary to carry out field tests to examine their practicability. Furthermore, the field test allows us to know if the low intensity of UV light in sunlight is sufficient to bleach the dyes in the photocatalytic films, because UV light is necessary to activate TiO2 photocatalyst. We combined the different dye indicators studied previously into the two kinds of coating agents (precoat and photocatalytic coating), and then the resulting layers were exposed on the rooftop of a building in the heart of Tokyo for one week in December. Figure 9 shows the results of our exposure tests using the glass/ precoat (Rh123)/TiO2 (MB/NK-719) system. It can be seen that the dye indicators tested showed a good ability to detect debonded regions and satisfactory decolorizing after exposure to the outdoor environment for 7 days. Similar results were observed for the other systems listed in Table. 2 This also represents a final confirmation of the feasibility of doping dyes as indicators into transparent photocatalytic coating layers that are capable of detecting debonded regions and coating gaps. Conclusion
Figure 8. Variation of UV-visible absorption spectrum of precoat (Rh-123)/photocatalytic coating (NK-719) system under illumination with artificial sunlight. The irradiation times (t/h) are indicated on the respective spectra. (Inset: change relative absorbance of NK-719 as a function of irradiation time of artificial sunlight; A0 is the absorbance at 0 time, and the A is the absorbance at t time.
is evident after a 300-min exposure. This is indicative of the significant role of the photocatalytic decolorization
In this work, a new photocatalytic coating agent along with a precoat agent was developed for the detection of coating gaps during or soon after the coating process. For both kinds of coating agents (precoat and photocatalytic coating), the addition of the fluorescent and the vivid dyes, respectively, enables the convenient detection of coating gaps. After the detection of the gaps, the dyes can automatically undergo photocatalytic bleaching under sunlight because of the photocatalytic activity of TiO2, thus retaining the transparency of the coating. This development provides a great possibility
Figure 9. Outdoor test results: optical images of a precoating (RH123)/photocatalytic coating (MB and NK-719) system before and after exposure to the outdoor environment for 7 days (urban Tokyo rooftop, December, ca. 45° incidence, ca. 1.75 mW/cm2). A reference is also shown which is coated without any coating agents.
Ind. Eng. Chem. Res., Vol. 41, No. 4, 2002 731
for the practical application of these photocatalytic coating agents to enable gap-free coatings. Such agents are especially useful for the preparation of coatings on large object such as building walls. Acknowledgment The authors gratefully thank Dr. Tata N. Rao for fruitful discussions. Literature Cited (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Watanabe, T.; Kitamura, A.; Kojima, E.; Nakayama, C.; Hashimoto, K.; Fujishima, A. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. E., Al-Ekabi, H., Eds.; Elsevier: New York, 1993; pp 767. (3) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalysis: Fundamentals and Applications; BKC, Inc.: Tokyo, Japan, 1999. (4) Fujishima, A.; Tata N., Rao.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (5) Frank, S. N.; Bard, A. J. J. Phys Chem. 1977, 81, 1484. (6) Heller, A. Acc. Chem. Res. 1995, 28, 503-508. (7) Kikuchi, K.; Sunada, K.; Iyoda, T.; Hashimoto, K.; Fujishima, A. J. Photochem. Photobiol., A 1997, 106, 51.
(8) Sopyan, I.; Murusawa, S.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1977, 723. (9) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (10) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Adv. Mater. 1998, 10, 135. (11) Negishi, N.; Iyoda, T.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1995, 841. (12) Sopyan, I.; Watanabe, M.; Murasawa, S.; Hashimoto, K.; Fujishima, A. J. Photochem. Photobiol., A 1996, 98, 79. (13) Tanaka, K.; Padermpole, K.; Hisanaga, T. Water Res. 2000, 34, 327. (14) Mills, A.; Wang, J. S.; J. Photochem. Photobiol., A 1999, 127, 123. (15) Wu, T. X.; Liu, G. M.; Zhao, J. C.; Hidaka, H.; Serpone, N. New J. Chem. 2000, 2, 93. (16) Lakshmi, S.; Renganathan, R.; Fujita, S. J. Photochem. Photobiol., A 1995, 88, 163.
Received for review May 29, 2001 Revised manuscript received November 4, 2001 Accepted November 8, 2001 IE0104722
