Journal of Sol-Gel Science and Technology 19, 365–369, 2000 c 2000 Kluwer Academic Publishers. Manufactured in The Netherlands. °

Preparation of Titania from Tetrakis(diethylamino)titanium via Hydrolysis YOSHIZUMI ISHIKAWA, HIDEYASU HONDA AND YOSHIYUKI SUGAHARA∗ Department of Applied Chemistry, School of Science and Engineering, Waseda University, Ohkubo-3, Shinjuku-ku, Tokyo 169-8555, Japan [email protected]

Abstract. The conversion of tetrakis(diethylamino)titanium (Ti(NEt2 )4 ) into titania via either a combination of hydrolysis (Ti(NEt2 )4 : THF : H2 O = 1 : 10 : x, x = 2, 4, 10) at ambient conditions and calcination (method A) or hydrolysis in a water-tetrahydrofuran (THF) mixture (Ti(NEt2 )4 : THF : H2 O = 1 : 10 : 100) at reflux (method B) was investigated. Titanium tertiary butoxide (Ti(Ot Bu)4 ) was also used as a substitute for Ti(NEt2 )4 . The hydrolysis via method A resulted in the formation of amorphous solids containing organics. Thermal analyses showed that the hydrolysis products showed mass losses up to 500◦ C probably due to the presence of diethylamine (Et2 NH) formed via the hydrolysis of Ti(NEt2 )4 in the hydrolysis products, while a mass loss of the hydrolysis product from Ti(Ot Bu)4 was completed up to about 200◦ C. After calcination at ≥600◦ C, anatase or a mixture of anatase and rutile was obtained. The crystallization behavior of the hydrolysis products from Ti(NEt2 )4 was different from that of the hydrolysis product from Ti(Ot Bu)4 . The hydrolysis via method B gave only an amorphous material from Ti(NEt2 )4 , while a crystalline titania (anatase and brookite) formed from Ti(Ot Bu)4 . Keywords:

1.

titania, tetrakis(diethylamino)titanium, new precursor, hydrolysis

Introduction

Sol-gel processes have been developed for decades as chemical routes for glass and oxide ceramics. As starting materials, metal alkoxides are generally utilized [1], but other types of compounds, such as metal halides, metal acetates [2], and metal diketonates [3] have also been utilized. Since all of these compounds are hydrolyzed in the general sol-gel processes, the hydrolysis behavior of M X bonds (X = halides) and M O C bonds are one of the important factors. Thus, it is expected to tune hydrolysis behavior by utilizing appropriate starting compounds. Metal dialkylamides have been known since 1856, and a variety of compounds have been prepared with various metals and metalloids [4]. In terms of chemical properties, metal dialkylamides are considered to be located between metal alkyls and metal alkoxides [5]. Since metal-nitrogen bonds are unstable against ∗ To

whom all correspondence should be addressed.

hydrolysis, metal dialkylamides are readily hydrolyzed to form metal oxides or hydroxides. As far as we know, however, there have been no studies on utilizing metal dialkylamides as precursors to oxides. Here we report our attempt to prepare metal oxide from metal dialkylamide. Since titanium dialkylamide is one of the most common compounds, we prepared titania, which is also one of the most popular and attractive oxides, from tetrakis(diethylamino)titanium (Ti(NEt2 )4 ) via hydrolysis. The preparation of titania was attempted by hydrolysis and subsequent calcination and by reflux without further calcination. Preparation of titania from titanium alkoxide was also conducted using (Ti(Ot Bu)4 ) for comparison. 2.

Experimental

Tetrakis(diethylamino)titanium (Ti(NEt2 )4 ) was prepared according to the previous report [6], and purified by distillation. The formation of Ti(NEt2 )4 was confirmed spectroscopically; 1 H NMR(270 MHz, CDCl3 ):

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δ1.0 (t, CH2 CH3 , 3H), 3.5 (q, CH2 CH3 , 2H); IR (cm−1 ) 608 (νTi-N ). Titanium tertiary butoxide (Ti(Ot Bu)4 ) was purified by distillation before use. Tetrahydrofuran (THF) was distilled over sodium. Ti(NEt2 )4 or Ti(Ot Bu)4 was hydrolyzed via the following two different methods. Method A. The reaction was conducted with Ti(NEt2 )4 (Ti(Ot Bu)4 ) : THF : H2 O = 1 : 10 : x (x = 2, 4, 10). Ti(NEt2 )4 (or Ti(Ot Bu)4 ) was dissolved in a half amount of THF, and water was also dissolved in the rest of THF. After cooling the Ti(NEt2 )4 solution down to 0◦ C, water (dissolved in THF) was added to the solution dropwise. The resultant solution was stirred at room temperature for 24 h, and the solvent was removed under reduced pressure. The hydrolysis product was then calcined under dry air flow at various temperatures for 1 h. The hydrolysis of a mixture of Ti(Ot Bu)4 and Et2 NH was conducted in a similar way (Ti(Ot Bu)4 : Et2 NH : THF : H2 O = 1 : 4 : 10 : 2). Method B. The reaction was conducted in the water-rich system (Ti(NEt2 )4 (Ti(Ot Bu)4 ) : THF : H2 O = 1 : 10 : 100). Ti(NEt2 )4 (or Ti(Ot Bu)4 ) was dissolved in a half amount of THF, and was heated at reflux. Then, a THF-water mixture (using the rest of THF) was added dropwise during refluxing. The solution was further refluxed for 24 h to complete hydrolysis. The precipitate was separated by centrifugation, and dried under a reduced pressure. Calcination of the precipitate was not attempted in method B. The amounts of titanium in the hydrolysis products were determined by Inductively-coupled plasma emission spectroscopy (ICP; Nippon Jarrell Ash ICAP-575 II) after dissolving the sample with aqua regia. The amounts of carbon, nitrogen and hydrogen were determined by elemental analysis. Thermal properties of the hydrolysis products were determined with a thermogravimetry-differential thermal analysis (TGDTA) instrument (Mac Science TG-DTA 2000S) operated in an air atmosphere. The heating rate was 10◦ C/min and α-Al2 O3 was used as a standard. XRD patterns were obtained by using a Mac Science MXP3 diffractometer (monochromated Cu Kα radiation). Transmission electron microscopic characterization was performed using a Hitachi H-8100A operated at 200 kV.

3. 3.1.

Results and Discussion Preparation of Titania via Hydrolysis (H2 O/Ti = 2, 4, 10) and Calcination (Method A)

When water was added to the THF solution of Ti(NEt2 )4 , precipitates were immediately formed. No gelation occurred under the present conditions. The precipitate was light yellow (H2 O/Ti(NEt2 )4 = 2) or white (H2 O/Ti(NEt2 )4 = 4, 10). XRD results revealed that the hydrolysis products from Ti(NEt2 )4 and (Ti(Ot Bu)4 (H2 O/Ti(Ot Bu)4 2) were amorphous. The compositions of the hydrolysis products with different compositions are listed in Table 1. Even with H2 O/Ti(NEt2 )4 = 10, the C/Ti and N/Ti ratios were comparable with those for H2 O/Ti(NEt2 )4 = 2. It is known that Ti N bonds are easily hydrolyzed [4]. Hence, the presence of a certain amount of unreacted diethylamino groups ( NEt2 ) is unlikely, since a stoichiometric (H2 O/Ti(NEt2 )4 = 2) or an excess (H2 O/Ti(NEt2 )4 = 4, 10) amount of water for hydrolysis was present in the system. Thus, this observation should be ascribed to the incorporation of organics in the products (vide infra). The thermal properties of hydrolysis products were studied by comparing them with those of hydrolysis products from Ti(Ot Bu)4 . Figure 1 shows the TGDTA curves for the hydrolysis product from Ti(NEt2 )4 (DTA, Fig. 1(a); TG, Fig. 1(b)) with a TG curve for that from Ti(Ot Bu)4 (Fig. 1(c)) (H2 O/Ti = 2). The ceramic yields up to 900◦ C were 69.2% (for Ti(NEt2 )4 ) and 66.2% (for Ti(Ot Bu)4 ). The mass loss for the product obtained by hydrolysis of Ti(NEt2 )4 was observed up to ∼500◦ C with a steep mass loss (5.7%) starting at ∼480◦ C accompanying a sharp exothermic peak (∼480–510◦ C). The TG curves of the hydrolysis

Table 1. The variation in the compositions of the hydrolysis products with the H2 O/Ti(NEt2 )4 ratio. The molar ratio of each elements to titaniuma H2 O/Ti(NEt2 )4

C

N

H

2

1.5

0.26

5.5

4

2.0

0.35

7.4

10

1.8

0.31

7.0

a The

molar ratios were calculated by the results of ICP and elemental analysis.

Preparation of Titania from Tetrakis(diethylamino)titanium via Hydrolysis

Figure 1. TG and DTA curves of hydrolysis products from different precursors. (a) DTA curve of the product from Ti(NEt2 )4 . TG curves of the products from (b) Ti(NEt2 )4 , (c) Ti(Ot Bu)4 , and (d) the mixture of Ti(Ot Bu)4 and Et2 NH.

products with H2 O/Ti(NEt2 )4 = 4 and 10 were similar to that with H2 O/Ti(NEt2 )4 = 2, and the mass losses above ∼480◦ C were also around 5%. On the contrary, the mass loss was essentially completed up to ∼200◦ C for the product obtained by hydrolysis of Ti(Ot Bu)4 . In order to study the difference in thermal behavior, the mixture of Ti(Ot Bu)4 and Et2 NH was hydrolyzed in a similar manner Et2 NH/Ti(+ OBu = 4, H2 O/Ti = 2). The TG curve (Fig. 1(d)) showed a very similar mass loss starting at ∼470◦ C. Thus, it is likely that part of Et2 NH formed by hydrolysis of Ti(NEt2 )4 remained in the products at high temperature, and Et2 NH or its pyrolyzed products caused exothermic reaction above 480◦ C. Since C/N ratios of the hydrolysis products were higher than that of Et2 NH (C/N = 4) (as shown in Table 1), THF was expected to be present in the products. The surface of titania powders showed Lewis-acidic and protonic characters, and bases (ammonia [7, 8], pyridine [7, 9], trimethylamine [7, 9]) were adsorbed via hydrogen bonding or acid-base reactions. These bases were resistive to the evacuation under vacuum even above 200◦ C [7, 9], indicating strong interactions. Thus, Et2 NH formed in the present study via hydrolysis can be strongly adsorbed on the hydrolysis products by interacting with the surface hydroxyls. It is likely

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that Et2 NH can be trapped in the pores of the products, implying better thermal stability against desorption than molecules adsorbed on the surface. Therefore, these previous reports further suggest that Et2 NH or its pyrolyzed products trapped in the hydrolysis products could cause exothermic reaction starting at ∼480◦ C. This assignment appears to be consistent with the TGMS study of the butylamine-modified titania, showing the desorption and/or decomposition of butylamine at 200–500◦ C [10]. Figure 2 shows the XRD patterns of the ceramic residues obtained by calcination of the hydrolysis products. When the hydrolysis products were heated at 400◦ C, a crystalline phase of anatase was obtained only from Ti(Ot Bu)4 . The delay of crystallization of the hydrolysis products obtained from Ti(NEt2 )4 seems to be related to the presence of organics at higher temperature, as shown by the thermal analyses. Upon heating at 600◦ C, only anatase was obtained from both the starting compounds. The difference was evident, when both the hydrolysis products were calcined at 800◦ C; the main phase was anatase for the product from Ti(NEt2 )4 , while rutile was the main phase in the product from Ti(Ot Bu)4 . The XRD patterns of both the residues calcined at 1000◦ C were similar; they showed the formation of rutile as the main phase with small amounts of anatase.

Figure 2. XRD patterns of the ceramic residues calcined under dry air flow. The products from (a) Ti(NEt2 )4 and (b) Ti(Ot Bu)4 .

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Figure 3. Transmission electron micrographs of the ceramic residues obtained from (a) Ti(NEt2 )4 and (b) Ti(Ot Bu)4 . (Calcined at 600◦ C.)

Titania exhibits at least seven types of crystal structures, and anatase, rutile and brookite are commonly observed in nature [11]. Although the preparative temperature is the dominant factor for determining the crystalline phases formed, other factors cannot be ignored. The relative amounts of these three phases in titania powders prepared from titanium alkoxides (Ti(OR)4 ) via hydrolysis and calcination depend on the R group in Ti(OR)4 [12], the type of a catalyst [13, 14], and the type of an additive [12, 15, 16–18], in addition to calcination temperature [13–15, 19]. Thus, the observed difference in crystalline phases obtained from two starting compounds via hydrolysis and calcination could be ascribed to the difference in released molecules via hydrolysis (t BuOH and Et2 NH) and in the structures of the oxide networks formed after hydrolysis. Figure 3 shows the transmission electron micrographs of the ceramic residues obtained from Ti(NEt2 )4 and Ti(Ot Bu)4 (calcined at 600◦ C). Both the residues consist of crystallites with diameters of ∼20–30 nm, which are consistent with the diameters calculated from XRD patterns (∼15 nm). 3.2.

Preparation of Titania via Hydrolysis (H2 O/Ti = 100) and Refluxing (Method B)

Figure 4 shows the XRD patterns of the hydrolysis products from Ti(NEt2 )4 and Ti(Ot Bu)4 (H2 O/Ti = 100). The product obtained from Ti(Ot Bu)4 (Fig. 4(b)) showed broad XRD peaks that are assginable to anatase and brookite. We cannot exclude the presence of an amorphous phase in this hydrolysis product. Preparation of crystalline titania from titanium

Figure 4. XRD patterns of the products obtained from (a) Ti(NEt2 )4 and (b) Ti(Ot Bu)4 via refluxing.

alkoxides obtained solely by refluxing (or similar operation) has already been reported [13–16, 19]. The presence of both anatase and brookite in the uncalcined products is consistent with these reports. On the contrary, the product from Ti(NEt2 )4 was amorphous (Fig. 4(a)). Bokhimi et al. [14, 15] prepared titania by drying (70◦ C) gels obtained via refluxing with the sufficient amount of water (H2 O/Ti = 16) and various catalysts. When oxalic acid was used as the catalyst, no crystalline titania was obtained, while crystalline titania was obtained with other catalysts (HCl, NH3 , AcOH). Thus, we assume that Et2 NH formed by the hydrolysis of Ti(NEt2 )4 play a certain role for inhibiting crystallization of titania by refluxing.

4.

Conclusions

We have demonstrated that Ti(NEt2 )4 can be converted into crystalline titania via hydrolysis (Ti(NEt2 )4 : THF : H2 O = 1 : 10 : x, x = 2, 4, 10) and subsequent calcination above 600◦ C. The hydrolysis of Ti(NEt2 )4 with a large amount of water (Ti(NEt2 )4 : THF : H2 O = 1 : 10 : 100) at reflux, however, did not give crystalline titania. The comparison synthesis using Ti(Ot Bu)4 revealed that Ti(Ot Bu)4 was more favorable for the crystallization of titania in both of the methods, which is likely to be ascribed to the effects of Et2 NH formed via hydrolysis of Ti(NEt2 )4 .

Acknowledgment The authors are deeply grateful to Prof. Kazuyuki Kuroda (Waseda University) for invaluable discussion.

Preparation of Titania from Tetrakis(diethylamino)titanium via Hydrolysis

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