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Metachromatic Effects and Photodegradation of Basic Blue on Nanocrystalline Titania Films Elias Stathatos,† Panagiotis Lianos,*,† and Christos Tsakiroglou‡ University of Patras, Engineering Science Department, 26500 Patras, Greece, and FORTH-ICE/HT, Stadiou Street, Platani, P.O. Box 1414, 26504 Patras, Greece Received May 16, 2004. In Final Form: July 14, 2004 Titania nanocrystalline films have been deposited on solid substrates by a sol-gel procedure carried out in Triton X-100 reverse micelles. When the dye Basic Blue is adsorbed on these films, it demonstrates a strong metachromatic effect; that is, it aggregates, resulting in a blue shift of its absorption spectrum. Metachromasy in this system is related to the hydrophilicity of the film surface and to the humidity of the film environment. Films composed of 67% titania and 33% silica gave an intense and reversible metachromatic effect that can be exploited to make a handy humidity sensor. Photodegradation of Basic Blue on titania films is faster in humid environments than in dry environments, and this goes in parallel with metachromatic effects.
Introduction Titanium dioxide is the most popular catalyst for the photodegradation of organic pollutants, both in solution and in air.1-4 The greatest mass of the related research is devoted to the study of the synthesis procedures and doping of titanium dioxide, as related to photodegradation efficiency5-10 and to the hydrophilic properties of this catalyst,11-13 while effort is concentrated on increasing its efficiency and on engineering means for its recovery.14,15 In order for a substance to be photocatalytically degraded in air, its molecules must be adsorbed on titania particles. For this reason, it is necessary to know the physical state of the adsorbed molecules. This last matter has received relatively little attention.16,17 In the course of studying the photodegradation of dyes adsorbed on nanocrystalline titania films, we were intrigued by the metachromatic † ‡
University of Patras. FORTH-ICE/HT.
(1) Subramanian, V.; Kamat, P. V.; Wolf, E. Ind. Eng. Chem. Res. 2003, 42, 2131. (2) Stylidi, M.; Kondarides, D.; Verykios, X. Appl. Catal., B 2003, 40, 271. (3) Wu, T.; Liu, G.; Zhao, J.; Hidaka, H.; Serpone, N. J. Phys. Chem. B 1998, 102, 5845. (4) Sopyan, I.; Watanabe, M.; Murasawa, S.; Hashimoto, K.; Fujishima, A. J. Photochem. Photobiol., A 1996, 98, 79. (5) Stathatos, E.; Petrova, T.; Lianos, P. Langmuir 2001, 17, 5025. (6) Yu, J.; Zhang, L.; Yu, J. Chem. Mater. 2002, 14, 4647. (7) Yu, J.; Yu, J.; Ho, W.; Jiang, Z.; Zhang, L. Chem. Mater. 2002, 14, 3808. (8) Di Paola, A.; Marci, G.; Palmisano, L.; Schiavello, M.; Uosaki, K.; Ikeda, S.; Ohtani, B. J. Phys. Chem. B 2002, 106, 637. (9) Guillard, C.; Beaugiraud, B.; Dutriez, C.; Herrmann, J.-M.; Jaffrezic, H.; Jaffrezic-Renault, N.; Lacroix, M. Appl. Catal., B 2002, 39, 331. (10) Mills, A.; Hill, G.; Bhopal, S.; Parkin, I.; O’Neill, S. J. Photochem. Photobiol., A 2003, 160, 185. (11) Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (12) Miyauchi, M.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Chem. Mater. 2002, 14, 2812. (13) Yu, J.; Yu, J.; Ho, W.; Zhao, J. J. Photochem. Photobiol., A 2002, 148, 331. (14) Mehrvar, M.; Anderson, W.; Moo-Young, M. Adv. Environ. Res. 2002, 6, 411. (15) Peiro, A.; Peral, J.; Domingo, C.; Domenech, X.; Ayllon, J. Chem. Mater. 2001, 13, 2567. (16) Barazzouk, S.; Lee, H.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. B 2000, 104, 3616. (17) Coon, S. R.; Zakharian, T. Y.; Littlefield, N. L.; Loheide, S. P.; Puchkova, E. J.; Freeney, R. M.; Pak, V. N. Langmuir 2000, 16, 9690.
behavior of the adsorbed dye. We then realized that the same effects responsible for metachromasy may also be responsible for the efficiency and, generally, the evolution of the photodegradation procedure and decided to further study these effects. Metachromasy is the change of color of a dye when it is adsorbed on film or on colloidal particles in solution, and it is due to dimerization or higher order aggregation.17 For this reason, it is also observed in concentrated solutions of the dye. The metachromatic effect was originally studied as a means to observe stained tissue for histological purposes.18,19 Most of the studied cases report metachromasy as a hypsochromic shift17-19 (i.e., blue shift) in the absorption spectrum. Hypsochromic aggregates are due to repulsive interactions, which induce shifts to higher energies.16,20 Metachromasy is explained in early works19 as the consequence of blocking outer π-electrons for bonding purposes upon dimerization, thus inducing transitions by inner π-electrons, therefore, by higher transition energies. Quantitative examples of metachromasy are shown in the present work. We can generally say that metachromasy induces a change of color, easily observable by the bare eye. As will be explained below, it is a reversible effect,17 and since it is sensitive to humidity, it can be employed as a humidity sensor. In the present work, we study the metachromasy of the cationic dye Basic Blue when adsorbed on pure titania nanocrystalline films or on films made of TiO2/SiO2 mixtures. The chemical structure of Basic Blue is shown in the inset of Figure 1. Experimental Section All reagents used in the present work were purchased from Aldrich, except Triton X-100 [polyoxyethylene-(10) isooctylphenyl ether, Serva], and were used as received. Synthesis of Titania Nanocrystalline Films. Titania nanocrystalline films were deposited on glass slides (microscope slides) by a sol-gel procedure carried out inside reverse micelles, as in previous publications.5,21 Briefly, to 6.7 mL of cyclohexane, we added 1.29 g of Triton X-100, then 0.072 mL of H2O, and, (18) Michaelis, L.; Granick, S. J. Am. Chem. Soc. 1945, 67 (7), 1212. (19) Schubert, M.; Levine, A. J. Am. Chem. Soc. 1955, 77 (16), 4197. (20) McRae, E. G.; Kasha, M. J. Phys. Chem. 1958, 28, 721. (21) Stathatos, E.; Lianos, P.; Del Monte, F.; Levy, D.; Tsiourvas, D. Langmuir 1997, 13, 4295.
10.1021/la048788c CCC: $27.50 © 2004 American Chemical Society Published on Web 09/08/2004
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Figure 1. Absorption spectrum of the titania film (lower curve) without and (upper curve) with adsorbed BB. Inset: chemical structure of BB. finally, 0.57 mL of titanium isopropoxide, under vigorous stirring. The final molar ratio was 0.2:0.4:0.2 M for Triton X-100/water/ titanium isopropoxide, respectively. The stirring continued for ∼10 min under ambient conditions. Then, a slide, previously cleaned in sulfochromic acid solution, washed, and dried in hot air, was dipped in the above sol and withdrawn by hand. The obtained organic-inorganic nanocomposite film was calcinated at 500 °C to produce the titania film. Synthesis of Silica Films. We used the same procedure for the synthesis of silica films as that used for the synthesis of titania films. To 6.7 mL of cyclohexane, we added 1.29 g of Triton X-100, then 0.072 mL of acidified water (pH 3), and, finally, 0.6 mL of tetramethoxysilane (TMOS), under vigorous stirring. The final molar ratio was 0.2:0.4:0.5 M for Triton X-100/water/TMOS, respectively. This sol was continuously stirred under ambient conditions for 2 days, since the sol-gel procedure with TMOS in reverse micelles is very slow. A film was made by dipping a slide in the above sol and by calcinating it at 500 °C, as was done for titania. Synthesis of Mixed Titania/Silica Films. Films were made by mixing alkoxides at different molar proportions. First, TMOS was introduced in the reverse micellar solution, and after 12 h of stirring, titanium isopropoxide was finally added. After 10 min, a film was made by dipping. In this way, we provided enough time for the TMOS to be hydrolyzed and polymerized before adding the fast reacting titanium isopropoxide. Apparatus and Measuring Methods. Ultraviolet-visible (UV-vis) absorption measurements were made with a Cary 1E spectrophotometer and IR measurements with a Perkin-Elmer Spectrum RX1 Fourier transform infrared (FTIR) spectrometer. The equilibrium contact angle was measured by using the sessile drop method (i.e., by depositing a drop of a premeasured volume on the surface of the film) and recording the image by using a stereoscope and computer image analysis. Illumination of the samples was done by an Oriel 450 W xenon lamp. The illumination intensity was 100 mW/cm2, that is, equivalent to ∼1 sun at AM1.5. Control of the intensity of the incident light was obtained with superposed multiple wire grids. The intensity of the incident radiation was measured at the position of the sample using an Oriel radiant power meter.
Results and Discussion The titania films made by the sol-gel procedure given in the Experimental Section are composed of anatase nanocrystals of a particle size of ∼16 nm. The sizes were calculated by X-ray diffraction (XRD) analysis and verified by atomic force microscopy (AFM) image analysis. The absorption spectrum of such a titania film is given in Figure 1 (lower curve). The oscillating part of the spectrum is due to interference fringes. By using an index of
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Figure 2. Absorption spectra of BB adsorbed on titania film (1) before and (2) after 20 min of irradiation. Curve 3 is the same spectrum as that of curve 2, but it is normalized to the height of curve 1, for comparison. The spectra are corrected for titania absorption. Inset: variation of (a) the absorbance at maximum and (b) the absorbance at 525 nm of BB adsorbed on titania films vs irradiation time.
refraction of 2.0, a value previously found for similar films, the thickness of the films was calculated to be around 600 nm. When these films were dipped in an aqueous 10 mM solution of the dye Basic Blue (BB), they were immediately colored, since they adsorbed an important quantity of the dye. However, as a general rule, films were dipped and left in BB solution for 20 min. The pH of these solutions was 7.9. The absorption spectrum of the adsorbed dye is also shown in Figure 1 (upper curve), together with its chemical structure. Illumination of the film with adsorbed dye by simulated solar radiation leads to photodegradation of the dye, which is rapid, as is seen in the inset of Figure 2. More details about the photodegradation of BB by titania films have been presented in previous works.5,22,23 The photodegradation procedure was monitored by recording the absorbance of the film after every 2 min of irradiation time. The absorption maximum continuously decreased, but at the same time, it was continuously blue shifted so that, after ∼20 min of illumination, it was shifted by ∼25 nm. The absorption spectrum, corrected for the absorption of titania itself, is presented in Figure 2, showing absorption before and after 20 min of irradiation. The shape of the absorption curve did not change, but it was shifted to shorter wavelengths. We believe that this shift is not due to chemical modifications of the dye, which of course take place anyway, but is due to aggregation, in connection with the modification of the hydrophilicity of the titania film during irradiation and dye photodegradation. In other words, what one sees by recording the absorption spectra is not the intermediate products of photodegradation but the intact dye at different aggregation phases. This conclusion is supported by the following data. As already said, the shift of the absorption spectrum is not accompanied by changes in the structure of the spectrum. When an aqueous solution of BB was photodegraded in the presence of titania films immersed in the solution, we observed a decrease of absorbance, again without any structural change in the absorption spectrum. (22) Stathatos, E.; Tsiourvas, D.; Lianos, P. Colloids Surf., A 1999, 149, 49. (23) Bouras, P.; Stathatos, E.; Lianos, P. Appl. Catal., B 2004, 51, 275.
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Figure 3. FTIR spectra of titania films with adsorbed BB at different irradiation times. The films were supported on doublesided, polished silicon wafers. Figure 5. Absorption spectra of BB adsorbed on titania film (1) before and (2) after drying. The spectra are corrected for titania absorption and normalized to the same height, for comparison. Table 1. Values of the Contact Angle of Water Drops on the Surface of Titania Film with Adsorbed Basic Blue vs Irradiation Time
Figure 4. Absorption spectra of aqueous solutions of BB: (1) 10-5 M; (2) 10-2 M. The spectra are normalized to the same height, for comparison.
In addition, the IR spectra of the titania films with adsorbed dye deposited on silicon wafers revealed that the height of the IR absorption peaks decreased during photodegradation, but as seen in Figure 3, no indications of IR-detectable intermediates were recorded. Even though UV-vis and IR spectroscopies are not the safest methods for detecting intermediates, which are difficult to detect on films anyway, they are a strong indication that what is spectroscopically monitored is the nondegraded dye, rather than intermediates. Additional data further support this argument. When the sample with the half-degraded dye giving the spectrum of Figure 2 (curves 2 and 3) was put in a dry atmosphere, it suffered a red shift in absorption and recovered the same position as curve 1 of Figure 2. This last result is a sound indication that the shift is not due to intermediate species of photodegradation but is due to dye aggregation. The top spectrum of Figure 3 is the result of 30 min of irradiation, and it is identical to the corresponding IR spectrum of pure titania film deposited on silicon wafers. The procedure then leads to complete degradation of the dye, and it results in a film with practically no organic residues. The absorption maximum of the adsorbed on the titania film dye is around 525 nm. This is not the position where monomers absorb. This is seen in the spectrum of Figure 4, showing the absorption by aqueous solutions of BB. Low concentrated solutions (10-5 M) reveal an absorption maximum around 605 nm. Highly concentrated solutions give a main
irradiation time (min)
contact angle (deg)
0 10 20 30
27.7 22.3 18.4 7.3
absorption peaking around 550 nm, while higher order aggregates give a new absorption building up around 500 nm. This is a typical metachromatic effect in solution. Obviously, when BB is adsorbed on titania films, it is already in an aggregated form, and it aggregates even further when subjected to irradiation by simulated solar radiation. This aggregation tendency of BB when adsorbed on titania is due to the hydrophilicity and modifications of the hydrophilicity of the titania film as well as to the amphiphilic properties of BB, as will be analyzed in the next paragraph. Figure 5 shows the absorption bands of BB adsorbed on titania films under normal ambient conditions and after drying the sample by blowing hot dry air on it for several minutes. The drying resulted in a shift of the absorption maximum to longer wavelengths by ∼40 nm, while a shoulder grew at the position where the monomer absorption was expected (see Figure 4). This shift is reversible. When exposed to humidity, the original spectrum was recovered. This result indicates that water, which has a high affinity for titania, is adsorbed on the substrate and that the dye aggregates in the presence of water. This is justified by the fact that BB, as can be seen by its chemical structure, which is shown in Figure 1, contains a sizable hydrophobic group, giving it amphiphilic behavior. Such a structure facilitates aggregation of the dye in the presence of water, as happens with all amphiphilic substances and as happens with the present dye when dissolved in water at sufficiently high concentrations. When BB is adsorbed on titania, the hydrophobic part of the dye is exposed, and this makes the titania/dye surface hydrophobic. Indeed, contact angle measurements made with water drops gave the values shown in Table 1. The original (nonirradiated) film gave a relatively high contact angle, but it continuously decreased with the destruction of the dye. It is possible that, as the dye is photodegraded and the surface of the film becomes more
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Figure 6. Photodegradation of BB on titania films under wet and dry conditions at different illumination times. The values on the vertical axis correspond to the absorbance at maximum.
hydrophilic, more water is adsorbed on the film due to the ambient humidity, which induces further aggregation of the adsorbed dye, hence, the blue shift in the absorption spectrum of Figure 2. In addition to the increase of hydrophilicity due to dye destruction, it is known that the illumination of titania results in photoinduced hydrophilic conversion.24,25 Another possibility for the cause of the spectral shift during illumination might be some kind of acid-base interactions between BB and titania, the extent of which varies due to OH variations. This matter necessitates further studies which are in progress in our laboratories. In this respect, it must be noted that Coon et al.17 have reported a metachromatic effect with crystal violet adsorbed on titania powder, where the dye is aggregated in the absence and not in the presence of water. Obviously, aggregation conditions may change from one system to the other. We believe that it is the amphiphilic nature of BB in combination with the nature of the titania nanocrystalline film surface that defines its aggregation conditions in the present case. The physical state of the dye is then affected by humidity. The obvious question that subsequently arises is whether these effects affect the rates of photodegradation. The answer is straightforward, and it is given in Figure 6. The photodegradation data of Figure 6 were recorded in a closed chamber with controlled humidity. The chamber was a Pyrex cylinder where a stream of dry air could be supplied by side tubes. The photodegradation rates in Figure 6 are slower than those in Figure 2, since the intensity of the light entering the reaction chamber was 80 mW/cm2, that is, lower than that in open air (100 mW/cm2). In one case, the film communicated with ambient conditions being in a rather humid environment. In the second case, a dried-air stream passed through the reaction chamber, providing an environment of low humidity. The photodegradation rate was higher in the presence of humidity. This is consistent with efficient photodegradation observed in aqueous solutions.23 There are a couple of reasons that make the presence of water beneficial. The presence of water means there is an abundance of hydroxyl groups which are necessary for forming radicals and assist in photodegradation. However, (24) Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 1028. (25) Stevens, N.; Priest, C. I.; Sedev, R.; Ralston, J. Langmuir 2003, 19, 3272.
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Figure 7. Absorption spectra of BB adsorbed on 2:1 molar ratio titania/silica film before and after drying. The spectra are corrected for substrate absorption and normalized to the same height, for comparison. Inset: the color of the film surface under wet (violet) and dry (blue) conditions.
it should not be excluded that dye aggregation in humid environments may facilitate photodegradation by enhanced photosensitization. Such enhancement is due to the change of the excited state of the aggregated species that results in better matching with the conduction band of the semiconductor and easy excited electron transfer, a fact that has been previously observed with other systems.16 Since humidity and dye aggregation are directly related in a reversible manner and since dye aggregation is immediately observable by metachromasy, we can exploit this phenomenon to make a handy humidity sensor. For this purpose, it has been found that the most intense and fully reversible metachromatic effects are observed not by using pure titania but by using a TiO2/SiO2 mixture at a 2:1 molar ratio. Figure 7 shows the absorption spectra of BB adsorbed on such a film under “dry” or “wet” conditions. The long wavelength absorption, that is, low aggregation conditions in a dry environment, is strongly blue colored. The short wavelength absorption, that is, high aggregation conditions in a wet environment, is strongly violet colored. BB adsorbed on pure silica was always blue, that is, nonaggregated, and was not affected by humidity. It was previously reported26 that the TiO2/ SiO2 mixture at about the above 2:1 proportion leads to superhydrophilic films with a high water adsorption capacity. The variation of the quantity of adsorbed water results in variation of the surface electric resistance, and this has been exploited to make a humidity sensor.27 When the amount of SiO2 is higher than ∼30%, then, the active surface decreases26 due to sintering, so that the amount of water carried by the film accordingly decreases. Increased hydrophilicity of the TiO2/SiO2 2:1 molar ratio mixture was also detected in the present study, as can be seen in Table 2, where contact angle measurements were performed using paraffin oil drops. The above mixture possesses the less lipophilic and therefore the more hydroscopic surface. These results are in agreement with the rest of the above data. No aggregation of BB was observed on pure SiO2, apparently because the dispersion of this dye on silica is extensive and is not affected by (26) Machida, M.; Norimoto, K.; Watanabe, T.; Hashimoto, K.; Fujishima, A. J. Mater. Sci. 1999, 34, 2569. (27) Tai, W.-P.; Oh, J.-H. Sens. Actuators, B 2002, 85, 154.
Metachromatic Effects and Photodegradation of BB Table 2. Values of the Contact Angle of Paraffin Oil Drops on the Surface of Mixed Titania/Silica Films at Various Compositions TiO2 molar percent
contact angle (deg)
0 33 66 100
11.4 12.5 13.8 13.0
humidity. On the contrary, pure titania or titania/silica mixtures induce humidity-sensitive BB aggregation. However, comparison of Figure 5 with Figure 7 reveals that the absorption maximum appears at a lower wavelength when pure titania is used than when TiO2/SiO2 is used, even though the opposite is expected, since the mixture is more hydrophilic. Apparently, additional
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unknown parameters may play an important role which is yet to be determined. Conclusions Basic Blue was adsorbed on nanocrystalline titania or titania/silica films, where it was always found in an aggregated form, with the exception of pure silica films, where the dye is in a nonaggregated form. The state of aggregation is demonstrated by a net change of color (metachromatic effect). Thus, the dye appears blue when it is a monomer but it looks violet when it is aggregated, as can be seen in Figure 7. Wet films are more efficient photocatalysts than dry films, and this is beneficial for practical applications. LA048788C
