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Langmuir 1999, 15, 2130-2133
Preparation and Characterization of Novel Nanoscopic Titanium Dioxide/Phthalocyanine Complex Films Weijun Liu, Yuan Wang,* Linlin Gui, and Youqi Tang Institute of Physical Chemistry, Peking University, Beijing 100871, P. R. China Received August 23, 1998. In Final Form: December 15, 1998 Complex films composed of nanoscopic particles of phthalocyanine and titanium dioxide have been successfully prepared for the first time by coating the mixed colloids of titanium dioxide and phthalocyanine on glass substrates. The obtained transparent blue complex films decolored obviously when illuminated with UV light due to the UV-light induced catalysis of the titanium dioxide particles for the oxidizing decomposition of phthalocyanine. The complex films and their decoloration were characterized by transmission electron microscope, X-ray diffraction patterns, UV-visible spectra, and FTIR spectra.
Introduction Intensive studies on nanoscopic materials have provided great potentials and effective methods for fabricating various novel functional systems and materials.1-9 Much attention has been paid to the new high-density optical recording media with the increasing use of computers by our information society. The area density of optical recording is principally determined by the diffraction limit of light, so the shorter is the wavelength, the denser is the optical recording. Therefore, recording materials using UV lasers are becoming more and more important for scientists to study.10 Phthalocyanine is a very important class of organic dye, with many applications due to its photochemical, photophysical, and electrochemical properties, biological functions, high stability and innocuity, and mainly has two characteristic absorption bands: Soret band (300-400 nm) and Q-band (600-800 nm).11,12 The molecular structures of 29H, 31H-phthalocyanine and oxovanadium phthalocyanine are shown in Chart 1. Phthalocyanine embedded in some organic polymers has been applied in optical recording materials which records with the IR-laser hole burning technique.10,13-17 When a UV laser is used instead of an IR laser, however, phthalocyanine cannot be decomposed efficiently because (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Tennakone, K.; Kumara, G. R. R. A.; Kumarasinghe, A. R.; Sirimanne, P. M.; Wijayantha, K. G. U. J. Photochem. Photobiol., A: Chem. 1996, 94, 217. (3) Wang, R.; Hashimoto, K.; Fujishima, A. Nature 1997, 388, 431. (4) Lakshmi, B. B.; Dorhout, P. K.; Martin C. R. Chem. Mater. 1997, 9, 857. (5) Schmid, G., Ed. Clusters and Colloids; VCH: Weinheim, Germany, 1994. (6) Toshima, N.; Wang, Y. Langmuir 1994, 10, 4574. (7) Wang, Y.; Liu, H.; Toshima, N. J. Phys. Chem. 1996, 100, 19533. (8) Klabunde, K. J.; Tan, B. J. Chem. Mater. 1991, 3, 30. (9) Kim, D. H.; Anderson M. A. J. Photochem. Photobiol. A: Chem. 1996, 94, 221. (10) Seto, J.; Tamura, S.; Asai, N.; Kishii, N.; Kijima, Y.; Matsuzawa, N. Pure Appl. Chem. 1996, 68, 1429. (11) Law, K.-Y. Chem. Rev. 1993, 93, 449. (12) Stillman, M. J.; Nyokomg, T. Phthalcyanines, Properties and Applications; Leznoff, C. C., Lever, A. B. P., Eds., VCH: New York, 1989; Chapter 3. (13) Fabian, J.; Nakazumi, H.; Matsuoka, M. Chem. Rev. 1992, 92, 1197. (14) Tooru, Y. JP 09 58, 129 [97, 58, 129]. (15) Yoshuki, N.; Hideki, N.; Hisamitsu, K. JP 08, 287, 514 [96, 287, 514], 1996. (16) Shuichi, K.; Tetsuya, K.; Shinichiro, M.; Mare, S. JP 09, 202, 047 [97, 202, 047], 1997. (17) Noboru, S.; Tatsuya, T.; Yasunobu U.; Tsutomu, S. JP 09, 226, 249 [97, 226, 249], 1997.
Chart 1
the heat effect of the UV light is much lower than that of IR light. To solve this problem, a proper kind of UV-lightinduced catalyst can be selected to mix with phthalocyanine and prepare a complex film in which the catalyst can increase the rate of phthalocyanine decomposition under the illumination of UV light. It has been known that titanium dioxide (TiO2) is a kind of UV-light-induced catalyst which has been widely studied and applied to decompose organic pollutants in the environment.18-26 When illuminated with light of wavelength shorter than 380 nm, titanium dioxide produces electrons and holes that subsequently initiate reduction and oxidation reactions. The energy states for the dye-sensitization process of a n-TiO2/TiOPc system have been studied.27 According to the reported data, it can be expected that titanium dioxide can catalyze the oxidizing decomposition of phthalocyanine species under the irradiation of UV light if it is mixed with phthalocyanine to form a thin film. In this paper, we report the preparation and UV-lightinduced decoloration of a new kind of UV-light recording material that is a complex film composed of nanoscopic particles of titanium dioxide and phthalocyanine. (18) Fox, M. A.; Abdel-Wahab, A. A. J. Catal. 1990, 126, 693. (19) Abdullah, M.; Low, G. K.-C.; Matthews, R. W. J. Phys. Chem. 1990, 94, 6820. (20) Navio, J. A.; Mota, J. F.; Adrian M. A. P.; Gomez, M. G. J. Photochem. Photobiol., A: Chem. 1990, 52, 91. (21) Anderson, M. A.; Gieselmann, M. J.; Xu, Q. J. Membr. Sci. 1988, 39, 243. (22) Ohko, Y.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. B 1998, 102, 2699. (23) Murakata, T.; Yamamoto, R.; Yoshida, Y.; Hinohara, M.; Ogata, T.; Sato, S. J. Chem. Eng. Jpn. 1998, 31, 21. (24) Zhao, J.; Wu, K.; Wu, T.; Hidaka, H.; Serpone, N. J. Chem. Soc., Faraday Trans. 1998, 94, 673. (25) Bahnemann, D. W.; Hilgendorff, M.; Memming, R. J. Phys. Chem. B 1997, 101, 4265. (26) Inel, Y.; Okte, A. N. Toxicol. Environ. Chem. 1996, 55, 115. (27) Norskar, S.; Pillay, S. A.; Chanda, M. J. Photochem. Photobiol., A 1998, 113, 257.
10.1021/la981122w CCC: $18.00 © 1999 American Chemical Society Published on Web 02/19/1999
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Experimental Section Materials and Equipment. 29H,31H-phthalocyanine (H2Pc) and oxovanadium phthalocyanine (VOPc) were purchased from Aldrich Chemical Co. Tetraisopropyl orthotitanate (Ti(iPrO)4) was purchased from Fluka Chemie Co. Other reagents used in this work had at least a level of GR and were used without further purification. Transmission electron microscopy (TEM) photographs were taken by using a JEM-200CX transmission electron microscope. Particle sizes were measured from enlarged TEM photographs by using a magnifier with a magnifying power of 10 times. X-ray diffraction (XRD) patterns were recorded by a Dmax-2000 diffractometer (Rigaku Co.). UV-vis spectra were measured by using a TU-1221 UV-vis spectrometer (Beijing General Instrument Co.). FT-IR spectra were measured by using a Nicolet FT-IR 550 spectrophotometer. Preparation of Nanoscopic Titanium Dioxide Colloid. Nanoscopic colloids of TiO2 were prepared by a modified method similar to that described in the literature.28 Titanium isopropoxide (5 mL, 17 mmol) was diluted by 5 mL of ethanol and then hydrolyzed at room temperature by adding it to 30 mL of 0.1 M hydrochloric acid solution dropwise under vigorously stirring; a white precipitate was formed instantaneously. Right after the hydrolysis, the obtained slurry was heated to 80 °C and stirred for 8 h. Then the colloidal solution was treated by ultrasonic methods for 2 h and filtered on a glass frit to remove nonpepetized agglomerate. Water was then evaporated until the final concentration of TiO2 became 20 mg/mL. Preparation of Nanoscopic Phthalocyanine Colloids. A known quantity (2.5 g, 6.7 mmol) of surfactant, hexadecyltrimethylammonium bromide (C16H33N(CH3)3Br), was added to 400 mL water and heated at 50 °C under stirring to get a transparent solution. H2Pc (0.3 g, 0.5 mmol) dissolved in 15 mL of concentrated H2SO4 was added dropwise to the surfactant solution under vigorously stirring at 0-5 °C to give a transparent blue colloidal solution. The obtained colloidal solution was then washed to neutral pH with water by using an ultrafilter. A trace amount of H2SO4 was removed by anion-exchange resins. The neutral colloidal solution was then concentrated to 20 mg/mL by using the ultrafilter. A nanoscopic VOPc colloidal solution was prepared by the same method. Preparation and Decoloration of Titanium Dioxide/ Phthalocyanine Complex Films. The TiO2 colloid (2 mL, 20 g/L) was mixed with the H2Pc colloid (1 mL, 20 g/L) to form a transparent mixed colloidal solution. A transparent blue TiO2/ H2Pc complex film on a glass substrate was prepared by the spin coating technique (1.5 kr/min) and dried at 100 °C for 1 h. For testing the decoloration properties of the complex film, the films were illuminated by a 250 W high-pressure mercury vapor lamp. An obvious decoloration process from blue to colorless can be observed by the naked eye after several minutes. The decoloration process was followed by using a TU-1221 UV-vis spectrometer.
Results and Discussion Preparation and Characterization of the Titanium Dioxide/Phthalocyanine Complex Film. To prepare the titanium dioxide/phthalocyanine complex film, phthalocyanine species must be mixed with the titanium dioxide colloid homogeneously. However, phthalocyanine is a type of organic compound that is very difficult to be dissolved in almost all kinds of solvents. Two methods can be adopted to solve this problem. One is to use dissolvable derivatives of phthalocyanine, in which some polar groups are introduced to phthalocyanine molecules by chemical synthesis to make them dissolvable in titanium dioxide colloidal solutions. Another way is using nanoscopic colloidal particles of phthalocyanine, i.e., the phthalocyanine colloids with particle size at nanometer scope are prepared at first and then they are mixed with the titanium dioxide colloid. In this work, the second (28) Shklover, V.; Nazeeruddin, M.-K.; Zakeeruddin, S. M.; Barbe`, C.; Kay, A.; Haibach, T.; Steurer, W.; Hermann, R.; Nissen H.-U.; Gra¨tzel, M. Chem. Mater. 1997, 9, 430.
Figure 1. TEM micrographs of colloids: (a) TiO2; (b) H2Pc; (c) VOPc.
method is proposed because it not only can solve the problem of dissolvability of phthalocyanine but also can increase the colority of the complex films due to the use of nanoscopic particles.29 This is especially important when the film is somewhat very thin. Meanwhile we believe that a complex film composed of nanoscopic particles of an inorganic semiconductor and an organic semiconductor has great potential for developing new functional materials. Colloids of titanium dioxide and phthalocyanine were prepared through the methods mentioned in the Experimental Section. Figure 1 shows the TEM photographs (29) Nakazumi, H.; Makita K.; Nagashiro, R. J. Sol-Gel Sci. Technol. 1997, 8, 901.
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Figure 2. XRD pattern of TiO2 dried at 100 °C for 1 h.
and particle size distributions of the TiO2, H2Pc and VOPc colloids. The particle sizes of TiO2 (3.4-5.4 nm; dav ) 4.1 nm) are a little larger than those of VOPc (2.0-3.5 nm; dav ) 2.3 nm) and similar to those of H2Pc (4.2-7.2 nm; dav ) 5.1 nm). The particles in the TiO2 colloid and the VOPc colloid are well separated from each other without heavy aggregation. The first particles of H2Pc can be clearly observed although there are some aggregates that may be formed during the desiccation process of TEM samples. This suggests that the dispersability of the H2Pc particles is poorer than that of metal phthalocyanine (such as VOPc) particles. When the titanium dioxide colloid is mixed with the phthalocyanine colloid, particles of titanium dioxide will quickly deposit if SO42- ions in the phthalocyanine colloid are not removed completely. So the phthalocyanine colloids have to be washed to neutral pH by using an ultrafilter with deioned water, and then the remaining trace amounts of SO42- ions are eliminated from the colloidal solution by anion-exchange resin. The phthalocyanine colloid without SO42- ions can be mixed with the titanium dioxide colloid homogeneously, and the mixed colloidal solution keeps stable for several months at least. A blue transparent complex film can be easily prepared from the mixed colloidal solution by the spin coating technique. During the process of ultrafiltering, large quantities of excessive surfactants are removed from the phthalocyanine colloidal solution. Phthalocyanine in the obtained complex film keeps stable when heated in the air at 150 °C for 1 h but decomposes to some extent when heated at 200 °C for 1 h. The VOPc/TiO2 complex film is more stable than the H2Pc/TiO2 complex film. This is mainly determined by the different stability of different phthalocyanine itself. It has been known that titanium dioxide has several kinds of crystal phases (such as anatase and rutile) and the anatase phase is generally thought to be the most efficient catalyst in the UV-light-induced catalytic oxidizing process. Figure 2 is the XRD pattern of the TiO2 xerogel used in the complex films (dried at 100 °C for 1 h). Peaks in the pattern (25.46, 37.52, 47.64, 54.36, 62.88°) show that the crystal phase of the TiO2 xerogel is anatase. The crystal grain size (d101) calculated by the Scherrer formula is 3.7 nm, which accords well with the particle size measured from the TEM photographs of the TiO2 colloid (Figure 1a). This fact indicates that anatase is one of the main components of the TiO2 xerogel. UV-Light-Induced Decoloration of the Titanium Dioxide/Phthalocyanine Complex Films. When illuminated by the UV light, the titanium dioxide/phthalocyanine complex films decolor significantly. For comparison, a poly(vinyl alcohol) (PVA) film containing the H2Pc colloidal particles was prepared and illuminated. Figure 3a-c shows the UV-vis spectra of the VOPc/TiO2, H2Pc/TiO2, and H2Pc/PVA complex films, respectively,
Figure 3. UV-vis spectra of TiO2/phthalocyanine complex films. (A) TiO2/H2Pc: a, before illumination; b-e, each after additional 10 min of illumination; f-h, each after additional 20 min of illumination. (B) TiO2/VOPc: a, before illumination; b-f, each after additional 10 min of illumination; g, after additional 120 min of illumination. (C) PVA/H2Pc, measured under the same conditions of (A).
which are illuminated and measured under the same conditions. The complex films of VOPc/TiO2 and H2Pc/ TiO2 obviously decolored after being illuminated by the UV light; however, the spectra of the H2Pc/PVA film did not change significantly. This clearly shows the lightinduced catalytic activity of the TiO2 particles in the complex films. Another evidence for the light-induced catalytic decomposition process is that the complex film decolored very slowly when covered by a piece of quartz glass having a coating film of TiO2 on it, while the complex film covered by quartz glass itself decolored just as the quartz glass did not exist. These experiments prove that it is the light in the UV region (about 280 nm) absorbed by TiO2 that works in the light-induced decoloration process. Because different phthalocyanines have different stabilities, the decoloration rates of the complex films differ
Titanium Dioxide/Phthalocyanine Complex Films
Figure 4. Decoloration rate curves of different complex films: 9, TiO2/H2Pc; 2, TiO2/VOPc; b, PVA/H2Pc. The ordinate represents the decay of absorbance (ABS).
from each other when different kinds of phthalocyanines are used in the films. Figure 4 shows the effect of different kinds of phthalocyanines on the decoloration rate of the complex films. The ordinate represents the decay of absorbance (ABS) that equals ABSi/ABS0 100% (where the ABS0 is the absorbance of a complex film before illumination and ABSi is the absorbance of the film after UV-light illumination). The VOPc/TiO2 complex films (Figure 4a) decolored more slowly than the H2Pc/TiO2 films (Figure 4b). Figure 5 shows the FT-IR spectra of the H2Pc/TiO2 films before and after the UV-light illumination. Before the illumination, the spectrum (Figure 5b) of the complex film has peeks (1500, 1437, 1327, 1116, 1006 cm-1) of H2Pc (Figure 5a), while, after the illumination, all these peaks disappeared (Figure 5c) and a new peak (1385 cm-1) appeared at the same time, indicating that the H2Pc in the complex film has been decomposed during the decoloration process. The new peak can be attributed to intermediate products of the UV-light-induced decomposition process.
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Figure 5. FTIR spectrum of the TiO2/H2Pc complex film: (a) H2Pc; (b) TiO2/H2Pc before UV-light illumination; (c) TiO2/H2Pc after UV-light illumination.
Conclusions This paper demonstrates a convenient approach to prepare titanium dioxide/phthalocyanine complex films constructed by nanoscopic particles, which we believe to be of universal significance for preparing other kinds of inorganic semiconductor/organic semiconductor complex films with nanostructures. When illuminated with UV light, the complex films decolor from blue to colorless due to the UV-light-induced catalytic properties of the titanium dioxide particles in the films as designed. The decoloration rate of the complex films depends on the kind of phthalocyanine in the films. FT-IR measurements indicate that phthalocyanine in the films has been decomposed after illuminated under the UV light. Acknowledgment. The authors express their sincere thanks to the TEM lab of Peking University for their help in the TEM measurements. LA981122W
