Direct Formation of Zn-Al Layered Double Hydroxide Films with High Transparency on Glass Substrate by the Sol-Gel Process with Hot Water Treatment Naoko Yamaguchi,*,† Tomohiko Nakamura,† Kiyoharu Tadanaga,† Atsunori Matsuda,‡ Tsutomu Minami,† and Masahiro Tatsumisago†
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 7 1726-1729
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture UniVersity, Sakai, Osaka 599-8531, Japan, and Department of Materials Science, Toyohashi UniVersity of Technology, Toyohashi, Aichi 441-8580, Japan ReceiVed April 14, 2006; ReVised Manuscript ReceiVed May 21, 2006
ABSTRACT: Amorphous Al2O3-ZnO thin films with various Zn/Al ratios were prepared on glass substrates by the sol-gel method with a heat treatment at 400 °C for 30 min followed by immersion of the films in distilled water at 100 °C to form nanocrystalline Zn-Al layered double hydroxide (LDH) with hexagonal structure. Zn-Al LDH nanocrystals were precipitated on the amorphous Al2O3-ZnO films with various Zn/Al ratios, and the maximum amount of nanocrystals was formed on the film of the Zn/Al atomic ratio of unity. Zn-Al LDH crystals with the size of 200-500 nm have been precipitated on glass substrates through hot water treatment at temperatures lower than 100 °C. Introduction Many studies have been conducted on layered double hydroxides (LDHs), which are also known as “hydrotalcite-like clay”, over the past several decades due to their possible applications as anion exchangers, catalysts, and electrochemical sensors and for the immobilization hosts of biological materials by intercalation of various anions into the layers.1-14 In these studies, powder samples have been used since LDH is usually prepared through the coprecipitation process.1-14 To evaluate the intercalation reaction of LDH or observe the formation process of LDH crystals, immobilization of LDH nanocrystals on substrates is favorable. Only a limited number of reports on immobilization of the LDH crystals are present in the literature.5-9 We have investigated the formation of AlOOH or anatase nanocrystals on substrates through immersion of amorphous precursor thin film in hot water.15-21 Very recently, we have succeeded in the direct immobilization of Zn-Al LDH on substrates by treating Al2O3-ZnO amorphous thin films with the Zn/Al atomic ratio of unity with hot water.22 However, the size of LDH crystals formed on the Al2O3-ZnO thin films is large enough to cause scattering in the visible light region, and the Zn-Al LDH films become opaque after being immersed in boiling water for only 1 min. Control of the precipitation process should give transparent LDH films, which is favorable to evaluate optical properties of anion-intercalated LDH. In the present paper, the effects of Zn/Al atomic ratio in Al2O3-ZnO films on the surface morphology of LDH thin films are examined. To obtain the Zn-Al LDH films with high transparency on soda-lime silica substrate, immersion in hot water at a lower temperature is also reported. Experimental Section LDH thin films were prepared from aluminum tri-sec-butoxide (Al(O-sec-Bu)3) and zinc acetate dihydrate (Zn(OAc)2‚2H2O) according to the following procedure. Al(O-sec-Bu)3, ethylacetoacetate (EAcAc), and isopropyl alcohol (i-PrOH) were mixed and stirred at room * Corresponding author. Tel: +81-72-254-9334. Fax: +81-72-254-9913. E-mail:
[email protected]. † Osaka Prefecture University. ‡ Toyohashi University of Technology.
temperature for 3 h. A mixture of water and i-PrOH was then added dropwise to the solution for hydrolysis. Separately, Zn(OAc)2‚2H2O, diethanolamine (DEA), and i-PrOH were mixed and stirred at room temperature for 3 h. A mixture of water and i-PrOH was added dropwise to the solution. Then the two solutions were mixed together and stirred at room temperature for 1 h; the obtained solution was used for coating. The molar ratios of i-PrOH, EAcAc, H2O, and DEA to all the metal salts were 20, 1, 1, and 36, respectively. Molar ratios of Zn versus Al were chosen to be 20:80, 50:50, 67:33, 80:20, 95:5, and 100:0. Coating was carried out on soda-lime silica glass plates by dipping with a withdrawing speed of 3 mm/s. The coated films obtained were heattreated at 400 °C for 30 min to obtain amorphous Al2O3-ZnO films. The Al2O3-ZnO films were then immersed in hot water at 65 or 100 °C for 5 s to 4 h and dried overnight at 50 °C. The surface and the cross section of the coated films were examined by field emission type scanning electron microscopy (FE-SEM, Hitachi S-4500). X-ray diffraction (XRD) patterns were recorded with a Rigaku RINT 1100 X-ray diffractomater operating at 40 kV and 30 mA with Cu KR radiation. The XRD patterns were measured in the 2θ/θ mode within a 2θ range of 3-50°. Optical transmission spectra were measured using a UV-visible spectrophotometer (JASCO V-570).
Results and Discussion Figure 1 shows the surface of the Al2O3-ZnO films with various Zn/Al ratios, heat-treated at 400 °C and then immersed in hot distilled water at 100 °C for 15 min. The surface morphology due to the hot water treatment is different for each composition of the film. The surface morphology of the film with Zn/Al ratio of 20:80 (a) is similar to that of Al2O3 film with pseudoboehmite nanocrystals.15-18 In the film with Zn/Al atomic ratio of unity, 50:50 (b), nanocrystals with hexagonal structure were observed, and the crystal size is larger than 500 nm. The number of hexagonal nanocrystals formed on the film with Zn/Al ratio of 67:33 (c) is less than that formed on the film with the Zn/Al atomic ratio of unity, and the crystal size is larger than 1 µm. Small amounts of nanocrystals were only formed on the films with Zn/Al ratio of 80:20 (d) and 95:5 (e). Thus, the maximum amount of nanocrystals with hexagonal structure was formed on the film with the Zn/Al atomic ratio of unity, and the largest crystals were precipitated on the film with the Zn/Al atomic ratio of 2. The XRD patterns of the film with the Zn/Al atomic ratio of unity are shown in Figure 2.
10.1021/cg060220+ CCC: $33.50 © 2006 American Chemical Society Published on Web 06/20/2006
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Figure 1. FE-SEM micrographs of the surface of Al2O3-ZnO films coated on soda-lime glass substrates heated at 400 °C and immersed in boiling water for 15 min of various atomic ratios of Al/Zn: (a) 80:20, (b) 50:50, (c) 67:33, (d) 20:80, (e) 5:95, and (f) 0:100 (pure ZnO).
Figure 3. The optical transmission spectra of soda-lime glass substrate coated with Al2O3-ZnO immersed in boiling water for (a) 0 s, (b) 5 s, (c) 15 s, (d) 1 min, (e) 15 min, and (f) 3 h and (g) soda-lime glass substrate without coating. Figure 2. XRD patterns of Al2O3-ZnO film (Zn/Al ) 1) without the hot water treatment (a) and with the hot water treatment at 100 °C (b) and 65 °C (c).
Before the hot water treatment, the film was amorphous (a), but two distinct peaks were observed after immersion in hot water at 100 °C for 15 min (b). The interplanar spacing of the film after hot water treatment was 0.761 nm, which agrees well with the published value for Zn-Al layered double hydroxide (LDH) with carbonate anion.8 The interplanar spacing of the film with the Zn/Al atomic ratio of 2 was almost similar to the film with unity atomic ratio. Consequently, the nanocrystals with hexagonal structure formed on Al2O3-ZnO films are Zn-Al LDH crystals. As observed, the nanocrystals with hexagonal structure are formed directly on the substrate. In the pure Al2O3 system, the flowerlike structure consisting of pseudoboehmite was formed with the dissolution-reprecipitation process of solgel derived porous Al2O3 films in water. The formation process of LDH nanocrystals in this study is also proposed to be a dissolution-reprecipitation process, because the LDH crystals were newly precipitated on the sol-gel derived amorphous film. The Zn/Al atomic ratio of the LDH nanocrystals prepared in this study should also be 2. However, the largest amount of hexagonal crystals is precipitated with the Zn/Al atomic ratio of unity. In the calcination-rehydration process, which is known
as an effective procedure to intercalate various anions into layers of LDH powder, it is known that LDH with the Zn/Al atomic ratio of 2 is preferentially reconstructed independent of the composition of precursor Zn-Al LDH powder.1,4 In the film with the Zn/Al atomic ratio of unity, excess Al2O3 might enhance the nucleation, and therefore the largest amount of the LDH nanocrystals was observed on the film with the Zn/Al atomic ratio of unity. Figure 3 shows the optical transmission spectra of soda-lime glass substrate coated with Al2O3-ZnO (Zn/Al ) 1) immersed in boiling water for (a) 0 s, (b) 5 s, (c) 15 s, (d) 1 min, (e) 15 min, and (f) 3 h. The transmission spectrum of soda-lime glass substrate without coating (g) is also shown for comparison. The transmittance of the coated substrate decreases with an increase in hot water treatment time because hexagonal nanocrystals become large enough to cause the scattering of the visible light as the hot water treatment becomes long, and the film turned opaque with immersion in hot water. To control the precipitation process, hot water treatment at a lower temperature was examined. Figure 4 shows FE-SEM photographs of the 20° slanted cross section of the film after the hot water treatment at 65 °C. The FE-SEM observation in Figure 4 shows that LDH nanocrystals were formed after immersion in hot water at 65 °C, and the size of crystals was smaller compared to hot water
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Figure 4. FE-SEM micrographs of 20° slanted cross section of Al2O3-ZnO film (Zn/Al ) 1) coated on soda-lime glass substrates after heating at 400 °C (a) and immersed in hot water at 65 °C for (b) 1 min, (c) 15 min, (d) 3 h, and (e) 4 h.
ratios after hot water treatment. The surface morphology of the film was different for each composition of the film, and the maximum amount of LDH nanocrystals with hexagonal structure was obtained on the film with Zn/Al atomic ratio of unity. The Zn-Al LDH nanocrystals were formed through a dissolutionreprecipitation process. We have also succeeded in direct formation of transparent Zn-Al LDH films through hot water treatment at 65 °C. Direct formation of LDH films on glass surface by this simple process will expand the application fields of LDH. Figure 5. The optical transmission spectra of soda-lime glass substrate coated with Al2O3-ZnO immersed in hot water at for (a) 0 min, (b) 1 min, (c) 15 min, (d) 3 h, and (e) 4 h and (f) soda-lime glass substrate without coating.
treatment at 100 °C.22 Figure 5 shows the optical transmission spectra of soda-lime glass substrate coated with Al2O3-ZnO immersed in hot water at 65 °C for (a) 0 min, (b) 1 min, (c) 15 min, (d) 3 h, and (e) 4 h and (f) soda-lime glass substrate without coating. Even with immersion at 65 °C for 3 h, the transmittance was higher than 50% in the visible light region, probably due to the smaller crystal size. Thus, almost transparent LDH precipitated thin films were achieved. Immobilization of LDH nanocrystals on substrates should also be useful to evaluate the intercalation reaction of LDH or investigate the formation process of LDH crystals. The formation procedure for transparent LDH films must be effective for applications of LDH in, for example, optics. The preparation procedure of LDH thin film on glass substrate as developed in the present study is simple and will open new application fields of LDH. Conclusion Zn-Al LDH nanocrystals have been precipitated on solgel derived Al2O3-ZnO amorphous films with various Zn/Al
Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan and JSPS. References (1) Kooli, F.; Depege, C.; Ennaqadi, A.; Roy, A. D.; Besse, J. P. Clay Clay Miner. 1997, 45, 1997. (2) Crepaldi, E. L.; Pavan, P. C.; Valim, J. B. J. Mater. Chem. 2000, 10, 1337. (3) Leroux, F.; Taviot-Gueho, C. J. Mater. Chem. 2005, 15, 3628. (4) Prikhod’ko, R. V.; Sychev, M. V.; Astrelin, I. M.; Erdmann, K.; Mangel’, A.; van Santen, R. A. Russ. J. Appl. Chem. 2001, 74, 1573. (5) Lei, X.; Yang, L.; Zhang, F.; Evans, D. G.; Duan, X. Chem. Lett. 2005, 34 (12), 1610. (6) Lee, J. H.; Rhee, S. W.; Jung, D. Y. Chem. Commun. 2003, 2740. (7) Gardener, E.; Huntoon, K. M.; Pinnavaia, T. J. AdV. Mater. 2001, 13, 1263. (8) Gao, Y. F.; Nagai, M.; Masuda, Y.; Sato, F., Seo, W. S.; Koumoto, K. Langmuir 2006, 22, 3521. (9) Yamada, H.; Watanabe, Y.; Hashimoto, T.; Tamura, K.; Ikoma, T.; Yokoyama, S.; Tanaka, J.; Moriyoshi, Y. J. Eur. Ceram. Soc. 2006, 26, 463. (10) Newman, S. P.; Jones, W. New J. Chem. 1998, 105. (11) Khan, A. I.; O’Hare, D. J. Mater. Chem. 2002, 12, 3191. (12) Therese, G. H. A.; Kamath P. V. Chem. Mater. 2000, 12, 1195. (13) Leroux, F.; Besse, J. P. Chem. Mater. 2001, 13, 3507. (14) Sato, T.; Wakabayashi, T.; Shimada, M. Ind. Eng. Chem. Prod. Res. DeV. 1986, 25, 89. (15) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 1040.
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Crystal Growth & Design, Vol. 6, No. 7, 2006 1729 (20) Yamaguchi, N.; Tadanaga, K.; Matsuda, A.; Minami, T.; Tatsumisago, M. J. Sol-Gel Sci. Technol. 2005, 33 (1), 117. (21) Matsuda, A.; Matoda, T.; Kogure, T.; Tadanaga, K., Minami, T.; Tatsumisago, M. J. Sol-Gel Sci. Technol. 2003, 27, 61. (22) Yamaguchi, N.; Nakamura, T.; Tadanaga, K.; Matsuda, A.; Minami, T.; Tatsumisago, M. Chem. Lett. 2006, 35 (2), 174.
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