A water-resistant precursor in a wet process for Ti02 thin film formation Mitsunobu Sate,*" Hiroki Hara,' Toshikazu Nishideb and Yutaka Sawadac "Research Institute for Science and Technology, Kogakuin University, Nakano, Hachioji City, Tokyo 192, Japan bResearch Center, Nissan Motor Co., Ltd., Natsushima, Yokosuka City, Kanagawa 237, Japan 'Department of Industrial Chemistry, Faculty of Engineering, Tokyo Institute of Polytechnics, Iiyama, Atsugi City, Kanagawa 243-02, Japan Deposition of anatase TiO, thin films on soda-lime glass has been achieved by firing an adhered precursor titanium@) complex of ethylenediamine-N,N,N',"-tetraacetic acid (H,edta) between 450 and 550 "C in air. The coating solution was prepared by the reaction of a neutral [Ti( H,O)(edta)] complex, obtained from TiCl, and H4edta in an air-oxidation process, with dipropylamine in ethanol.The crystal structures of the oxide films on glass substrates were examined by XRD, and some of their optical properties and electronic structure were investigated by XPS. It was shown that TiOz film formation was attainable employing a water-resistant precursor derived from a stable metal complex. The thermal properties of the initial [Ti(H,O)(edta)] complex and the facile preparation of the precursor ethanol solution are also reported. Titanium(1v) dioxide thin films are important materials for a wide variety of applications based on their optical, electrical and photoelectrochemical properties, etc.'Y2 The deposition of anatase thin films, one of three crystalline forms of titania, has been studied recently by the spray inductively coupled plasma techniq~e,~low-pressure CVD4 and atomic layer dep~sition.~ Anatase film formation by anodic oxidative hydrolysis6 of TiC13 and by an organic self-assembled monolayer method7 from aqueous solution has also been performed.The conventional sol-gel process employing alkoxides as precursors is still of importance in the formation of anatase thin film~.~-'~ In this process, the rigorous exclusion of water from the system is essential for the synthesis and conservation of the precursor alkoxides and their solutions, since the process is based on partial or complete hydrolysis of such metal a1k0xides.l~ From this point of view, a novel wet process, in which films can be formed by facile coating procedures of precursor solutions of a water-resistant precursor, may afford practical advantages.In recent work, we found that an ethanol solution of a monomeric tributylammonium cobalt (m)-edta complex was a useful precursor in a wet process for the formation of Co,O, thin films on glass s~bstrates.'~ We report here the formation of anatase thin films on soda-lime glass by firing an adhered film consisting of a titanium(1v) complex salt. The precursor solution was prepared by reaction of a neutral [Ti( H,O)(edta)] complex with dipropylamine in ethanol. The starting complex and the TiO, thin films formed by firing in air at various temperatures up to 550°C were studied by several spectroscopic methods.The influence of added water to the precursor solution was also examined. From these results, it was elucidated that such ethanol solutions are not only suitable for the formation of precursor films adherable to substrates, but also robust to humid atmospheric conditions. Experimental Materials Ethylenediamine-N,N,N',N'-tetraacetic acid and a hydro-chloric acid solution of titanium(II1) chloride (min. 20 mass%) of the highest commercially available grade were purchased from Kanto Chemicals Co., Inc. Ethanol was purchased from Amakasu Chemical Industries Co., Ltd., and was of GR grade (99.5%) and dried upon molecular sieves 4A (Wako Pure Chemicals Co., Ltd.), before use. Other materials were used without further purification.Preparation of [Ti( H,O) (edta)] To an aqueous solution (250 ml) of 29.26 g (0.1 mol) of H4edta, was added 54.0 g of a 20% HC1 solution of TiC1, (67 mmol) with stirring at 90°C in an open beaker. The violet solution obtained was stirred at 90 "C for 30 min. During the reaction, white crystals gradually formed in the violet solution. The solution was then continuously stirred at 50°C for 3 h. The white crystals which formed were collected, washed with a small amount of water and ethanol, and air-dried. Yield: 23.5 g (81%). Elemental analysis indicated high purity (calc. for [Ti(H20)(CloH12N208)] (C10H14N209Ti,M, =354.1 1): C, 33.92; H, 3.98; N, 7.91. Found: C, 33.81; H, 4.03; N, 7.95%). Preparation of an ethanol solution for coating An ethanol solution containing 12 mass% precursor was prepared by reaction of [Ti( H,O)(edta)] with dipropylamine in ethanol.[Ti(H,O)(edta)] [1.00 g, 2.83 mmol] was sus-pended in 15 ml of dried ethanol and 0.62 g (6.16 mmol) of dipropylamine was added slowly to the suspended solution in a flask, and the mixed solution was heated at reflux for 10 min. The yellow transparent solution obtained was used to adhere the precursor onto soda-lime glass. An ethanol solution of 20 mass% was also prepared by the same procedure. Each log aliquot of the solution was then adjusted to 20 g by the addition of measured amounts of water and ethanol. Thus 5 and 10 mass% water solutions, with precursor contents of lo%, were prepared, and applied onto soda-lime glass after storing at ambient temperature for 10 days.Note, the transparency of these solutions was retained even after several months and neither the colour nor the viscosity of the solutions changed. A conventional flow-coat procedure was employed for the coating of these precursor solutions onto the substrate in order to prepare the samples for firing. A spinsoating method was also practicable. In both cases, transparent yellow films were obtained after drying. The samples were fired in air at various temperatures for 30min during which the films became colourless, and showed strong adherence to the glass. The film thickness after firing was controlled to be ca. 100nm for X-ray photoelectron J. Mater. Chew., 1996,6(ll), 1767-1770 1767 spectroscopic (XPS) and ellipsometric measurements or ca.200-300 nm for X-ray diffraction (XRD) measurements. Measurements IR spectra were measured on a Perkin Elmer FTIR 1600 spectrophotometer for sample tablets diluted with KBr. Thermogravimetry (TG) and differential thermal analyses (DTA) were performed using a Rigaku TAS-200 instrument. The crystal structures of the oxide films were determined by XRD performed with a Rigaku RINT-2500V model (50 kV, 300 mA) using graphite-monochromated Cu-Ka radiation and a parallel beam optic system (incident angle: 0.1"). The XP spectrum was recorded on a Perkin Elmer PHI model 5600 spectrometer, with Mg-Ka radiation (1253.6 eV) operated at 10 kV and 200 W used as the X-ray excitation source.The C 1s binding energy (284.6 eV) of the trace amount of hydro-carbon originally present in the air was used to calibrate the binding energy. Under these conditions the Ag 3d,,, peak (367.9 eV) had a FWHM value of 0.96 eV. The operating pressure was within (2-8) x Pa. Transmittance spectra were measured with a Hitachi U4000 spectrophotometer. The refractive indices were measured with a Mizojiri DHA-OLX ellipsometer employing a He-Ne laser source of 632.8 nm. The film thicknesses were measured using a Dektak 3030 stylus profilometer. Results and Discussion The complex [Ti( H,O)(edta)] was isolated and the pentagonal-bipyramidal structure around the Ti" ion was crystallograph-ically determined by Fackler, Jr.et a1.16 They isolated the complex by a similar procedure to that used by Weighardt et al., by employing H,edta in place of nitrilotriacetic acid (H,nta) or iminodiacetic acid (H,ida) as the polyaminopolycar-boxylic acid ligand.17 In their method, Ti"' was first oxidized to TiIV by the addition of nitric acid to the starting TiCl, solution prior to reaction with H,edta. In contrast, we synthe-sized the same compound in good yield by a milder air-oxidation process of a solution of Ti"' and H,edta. The IR spectrum of the complex (Fig. 1) is almost identical to that reported by Sawyer and McKinnie." TG-DTA measurements were carried out in order to study the decomposition of [Ti(H,O)(edta)]. Several exothermic peaks were observed by DTA, shown along with the TG curve in Fig.2. The small peak at 240°C in DTA is attributable to the elimination of the coordinated water molecule, as verified by the corresponding decrease of mass in the TG curve. The broad exothermic peak and the accompanying decrease of mass between ca. 300 and 500°C may be due to the sequential decomposition of the complex and oxidation of the organic ligand. The TG-DTA \F cn -0 cn3E 100 200 300 500 T/"C Fig. 2 TG-DTA curves of the complex [Ti(H,O)(edta)] results show that complete decomposition of the complex and organic substrates occurs at 515 "C. Since [Ti(H,O)(edta)] alone shows low solubility in organic solvents we examined the reaction of [Ti(H,O)(edta)] with dipropylamine in ethanol, guided by the good solubility of the alkylammonium salt of an anionic cobalt (m)-edta complex which was prepared as the precursor for Co,O, thin films on glass substrates.15 The reaction of two mole equivalents of dipropylamine with colourless [Ti( H,O)(edta)] gave a trans-parent yellow solution indicating that the precursor complex formed in solution is different from the starting aqua complex, although its structure is as yet unknown. Adhered films obtained from the precursor complex were fired between 400 and 550°C and in all cases transparent films were obtained.In order to identify the transparent films, XRD spectra of the thin films after firing were examined (Fig. 3). In each spectrum, the broad band observed in the range 28= 15-35" may be attributed to the film and the substrate.It is seen that firing at 400 "C leads to an amorphous film, since no peak due to a crystalline compound was observed [Fig. 3(a)]. In contrast, characteristic peaks of anatase were found in the spectra of the samples obtained above 450°C, as shown in Fig. 3(b)-(d). The peaks at 25.4, 37.9, 48.2, 54.2, 55.1, 62.9 and 75.2" are assigned to the (101), (004), (200),(105), (211),(204) and (215) reflections of anatase,19 respectively. The strongest, at 28 =25.4", is characteristic of anatase ( 101), as previously rep~rted.'~?~~Since no peaks due to other compounds were observed, it is evident that the fired films are anatase, although that obtained at 400°C is amorphous. Thus, it is clear that the reaction of [Ti(H,O)(edta)] with dipropylamine in ethanol leads to an excellent precursor solution for the preparation of anatase thin films.Use of the aqua complex alone was not feasible owing to its low solubility in a variety of solvents. 20 40 3000 ' 2000 1500 I000 500 28/degrees wavenumber/cm-' Fig.3 YRD patterns of the thin films formed on soda-lime glass at Fig. 1 IR spectrum of the complex [Ti(H,O)(edta)] various temperatures: (a) 400, (b) 450, (c) 520 and (d) 550 "C 1768 J. Muter. Chern., 1996, 6(ll), 1767-1770 The relationship between the refractive indices and the firing temperatures of the thin films is shown in Fig. 4. The absolute values of the refractive indices and their tendency to increase with increasing firing temperature mirror the results for films prepared by a sol-gel method reported by Nishide and Mizukami,13 as shown in Fig.4. The transmittance spectra of the thin films prepared by either method are also very similar. An XPS measurement was carried out on the thin film fired at 500"C, in order to examine the electronic structure of the TiO, thin film on the glass substrate. The binding energies measured for Ti 2p3,, and Ti 2pl,, are 458.7 and 464.2eV, respectively. These are comparable with the values, 458.8 eV for Ti 2p3,, and 464.3 eV for Ti 2p1,,, reported for TiO, in the literature.21 Significantly, the difference, 5.5 eV, between the binding energies for Ti 2p3,, and Ti 2p1,, is almost identical to the reported value of 5.54 eV.Thus, the observations on the electronic structure of the thin film support the formation of TiO, involving the Ti4+ ion. In order to examine the optical properties of the trans- parent thin films obtained by firing the adhered films which were coated from the precursor solutions, ellipsometric measurements were made; results for precursors B and C, containing 5 and 10% water as well as for the water-free precursor A are given in Fig. 4. The refractive indices at each firing temperature for samples B and C differed slightly from that obtained from the water-free precursor (precursor A) mentioned above. These results suggest that the optical properties of the anatase thin films can be controlled by the addition of water to the precursor solution.On the basis of these results, the present procedure is useful for the prep- aration of TiO, thin films on glass substrates. Note that optical properties, which can be affected by the co-existence of organic substrates, i.e. the ligand and the amine, of the films are comparable with those obtained from the conven- tional sol-gel procedure, where a water-free precursor solution was employed. Furthermore, samples containing up to 10% water can be used as precursor solutions, the presence of water only slightly affecting the refractive indices of the thin films formed. A schematic representation of the novel pro- cedure for the preparation of the thin films is shown in Fig. 5. Diethylamine was used instead of dipropylamine, in order to investigate the formation reaction of the adherable precur- sor.We found that the aqua complex also reacts with this amine in ethanol to form a similar yellow solution which can also adhere to a glass substrate. This result suggests that the chemical species involving TiIV ions in both solutions are comparable. From the viewpoint of the coordination chemistry 2*20 r .2.10 Xa m .-C ' .-92.00 CI .c3 p! 1.90 A I I I1.80 I I 350 400 450 500 550 600 firing temperature/% Fig. 4 Relationship between the reflactive index and the firing tempera- ture for the TiO, thin films. 0,water-free precursor (A); 0,5% water content (B); A 10% water content (C). The corresponding relationship in the sol-gel method, indicated by U, is from ref. 13.Ti /HCI Solution1 edLinH20 [Ti(HzO)(edta)] Precursory Solution Coating FiringI Ti02 Thin Film Fig. 5 Schematic representation of the novel wet process of the TiIv-edta complex, it is important to clarify the structure of the ethanol-soluble form obtained in the presence of amine. Investigations of the effects of the N-alkyl groups of the amines on the properties and the isolation of the adhered precursor complexes obtained from the reaction of the neutral [Ti(H,O)(edta)] complex are now in progress, along with the application of the novel procedure to form mixed-metal oxide systems involving Ti'V. Conclusion It has been shown that an ethanol solution obtained by the reaction of neutral [Ti(H,O)(edta)] and 2 mole equivalents of dipropylamine is useful as a novel precursor for TiO, thin films.By firing the adhered precursor in the temperature range 450-550°C, anatase thin films were obtained, while firing at 400°C gave an amorphous film. The optical properties of the thin films are comparable with those prepared by the conven- tional sol-gel process. It was furthermore elucidated that the adherent property of the precursor ethanol solution is unaffec- ted by the addition of water. This, to our knowledge, is the first report on TiO, film formation employing a facile coating method with a water-resisting precursor derived from a stable metal complex. This work was partially supported by a grant-in-aid for scientific research from the Ministry of Education, Science and Culture of Japan (No.03650687) and a special grant from our institute. References C. J. Brinker and G. W. 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