Effects of aging on the kinetics of nanocrystalline anatase crystallite growth

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1 Materials Chemistry and Physics 95 (2006) Effects of aging on the kinetics of nanocrystalline anatase crystallite growth Hsing-I Hsiang, Shih-Chung Lin Department of Resources Engineering, National Cheng Kung University, Tainan, Taiwan, ROC Received 1 December 2004; received in revised form 2 June 2005; accepted 7 June 2005 Abstract In this study, the effects of aging on crystalline phases, crystallite sizes and agglomeration of TiO 2 gels were investigated using thermogravimetry (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), BET specific surface area analyzer and transmission electron microscopy (TEM). The activation energies of anatase crystallite growth for aged and unaged gels were calculated to determine and explain the growth mechanism. TiO 2 aging treatment decreased the number for hydroxyl ions existing in the anatase, resulting in lowering the rate of surface area reduction and decreasing the amount of agglomeration after calcination. The activation energy values for crystallite growth in samples with aging treatment were greater than that found in unaged samples. In nanocrystalline anatase samples without aging treatment, the oriented attachment controlled the crystallite growth. However, grain boundary diffusion controlled the crystallite growth for nanocrystalline anatase samples with aging treatment Elsevier B.V. All rights reserved. Keywords: Crystallite growth; Titania oxide; Kinetics; Oriented attachment; Anatase 1. Introduction TiO 2 is widely used as a catalyst, pigment and in electronic materials [1 3]. However, these applications demand high purity titania with controlled crystallite size, definite phase composition and surface properties. Therefore, it is important to control the particle size, morphology and texture of the TiO 2 system. Recently, many different methods have been used to prepare nanosized TiO 2 powders [4 7]. Among these various methods, the thermal treatment of precursors obtained using sol gel [4,5], TiCl 4 precipitation [6] and Ti(SO 4 ) 2 precipitation [7] have been widely investigated because of the easily controlled process. There are three wellknown TiO 2 crystalline types: anatase, rutile and brookite. The phase transition of a-tio 2 r-tio 2 takes place during calcination [8]. The phase transformation, which occurs during calcination, gives rise to a transformed Corresponding author. Tel.: x62821; fax: address: hsingi@mail.ncku.edu.tw (H.-I. Hsiang). rutile powder that has undergone considerable aggregation and grain growth [9]. The above characteristics are detrimental to the formation of nanosized TiO 2 powders. The kinetics of anatase crystallite growth has been studied extensively. The activation energy values reported in the literature for anatase crystallite growth are very diverse, ranging from 5 to 230 kj mol 1 [10 15]. However, to the best of our knowledge, the disagreement between the previous reports has not been discussed. Aging is a process in which the gel physical properties can be changed as the result of polymerization, coarsening and phase transformation [16]. The changes in the gel structure and properties that occur during aging have a profound effect on the subsequent sintering process [10]. Various studies have shown that the amorphous TiO 2 gels crystallize into anatase through the aging process [17,18]. These reports focused mainly on the aging effects the crystallization, specific surface area and crystallite size of the TiO 2 powders. However, the aging treatment mechanism effects on the crystallite growth of TiO 2 powders during calcination have not been well understood /$ see front matter 2005 Elsevier B.V. All rights reserved. doi: /j.matchemphys

2 276 H.-I. Hsiang, S.-C. Lin / Materials Chemistry and Physics 95 (2006) The aim of this study was to investigate the aging effects on the crystalline growth phases and kinetics for TiO 2 gels obtained from TiCl 4 using thermogravimetry (TG), differential thermal analysis (DTA), X-ray diffraction (XRD) and dilatometer. 2. Experimental 2.1. Sample preparation Titanium chloride (reagent grade) was dissolved in distilled water to prepare Ti 4+ solution. The TiO 2 gel was prepared by adding Ti 4+ solution, drop by drop to NH 4 OH solution at ph 9.5 under vigorous stirring. The ph was maintained at 9.5 by adding 10% NH 4 OH solution. After precipitation, the gels were aged in a boiling solution, kept at ph 9.5 for various periods (0, 12, 24 and 48 h). Distilled water was used for filtering and washing three times. The gel was then washed with ethanol and dried at 80 C. The powders were calcined at 600 C. This powder acted as the precursor for anatase crystallite growth. The powders were annealed in air at temperatures between 600 and 650 C at different amounts of time Characterization The crystalline phases of the samples were identified using the XRD powder method using Ni-filtered Cu K radiation (Siemens D5000). The crystallite sizes of the TiO 2 in these specimens were determined using the Scherrer equation. DTA/TG (Netzsch STA 409C) analyses were performed using alumina as the reference material. The specific surface areas of the TiO 2 powders were measured using the conventional nitrogen absorption (BET) technique (Gemini 2360, Micromeritics). The morphology of the calcined powder was observed by transmission electron microscopy (TEM) (HF- 2000, Hitachi, Tokyo, Japan). Fig. 1. The XRD patterns of TiO 2 gel aged for various durations. may correspond to the decomposition of organic species. The third exotherm (440 C) is attributed to anatase phase crystallization. With increasing aging time, the amount of anatase phase and crystallite sizes increased and correspondingly, the exotherm intensity due to organic decomposition and anatase crystallization decreased. The variation in the thermal gravimetric analysis with various aging times is shown in Fig. 3. The as-precipitated gel exhibited a total loss of 15 wt%, while the aged gels had a total weight loss of only about 2 5 wt%. This can be attributed to the loss of hydroxyl ions and structural water from the titanium hydrous oxide, which increased with increased aging time [16] Aging effects on agglomeration and crystallite growth The crystallite size is characterized from XRD line broadening, while the BET method determines the agglomerate size. The XRD crystallite size ratio to the BET particle size gives an estimate of the average number of primary particles in each agglomerate. Assuming that the particle shape is 3. Results and discussion 3.1. Effects of aging on the thermal behavior of TiO 2 gel Fig. 1 shows the XRD patterns of TiO 2 gel aged at various time periods. The as-precipitated gel was amorphous. Anatase peaks were observed for TiO 2 gels aged for 12, 24 and 48 h. Aging promoted anatase phase crystallization. The anatase phase crystallinity increased with increasing aging time on the basis of the full width at half maximum of X-ray diffraction peak. Fig. 2 shows the changes in the DTA profiles of TiO 2 gels at various aging times. Three peaks were observed in the as-precipitated gel at around 100, 300 and 440 C. The first endothermic peak (100 C) is attributed to the desorption of water molecules. The second exothermic peak (300 C) Fig. 2. The changes in the DTA profiles of TiO 2 gels with various aging times.

3 H.-I. Hsiang, S.-C. Lin / Materials Chemistry and Physics 95 (2006) Fig. 3. The variation in the thermal gravimetric analysis with various aging times. spherical, the agglomeration number can be derived from the following relationship: AN (50) = D BET D XRD 3 where D BET is the BET particle surface area and D XRD is the crystallite size measured by XRD line broadening. Fig. 4 shows the agglomeration numbers for TiO 2 gels calcined at various temperatures. Apparently, the agglomeration numbers for TiO 2 gels with and without aging treatment increased rapidly as the calcination temperature increased above 600 and 800 C, respectively. In addition, the agglomeration numbers for TiO 2 gels with aging treatment were much lower than those for TiO 2 gels without aging treatment as the calcination temperature was raised above 600 C. Hebrard et al. [19] investigated the surface area reduction inatio 2 anatase powder under various partial water and oxy- Fig. 4. The agglomeration numbers for TiO 2 gels calcined at various temperatures. gen pressures. The surface diffusion of hydroxyl ions was reported to be the rate-limiting step in surface area reduction. During the surface diffusion process, material from the particle surface region with positive curvature diffused along the particle surface and then precipitated at the region with negative curvature in the neck between the particles. This process lead to neck development between particles, which is characteristic of hard agglomerates. With increasing aging time, the hydroxyl ions existing in the anatase lattice decreased, resulting in a lowered surface area reduction rate and decreased agglomeration numbers. The crystallite growth kinetics generally follow the empirical equation: D D 0 = kt 1/m, where D 0 is the mean initial crystallite size, D the mean crystallite size at time t, k a temperature-dependent constant and m is the crystallite growth exponent. The exponent m and rate constant k can Fig. 5. Plots of ln(d D 0 ) vs. ln t for samples aged for various times.

4 278 H.-I. Hsiang, S.-C. Lin / Materials Chemistry and Physics 95 (2006) Table 1 The calculated exponent m and rate constant k Aging time (h) Temperature ( C) m ln k Generally speaking, crystallite growth in nanocrystallites can be regarded as the coalescence of small neighboring crystallites due to atomic diffusion or discrete orientation attachment, crystallographically oriented crystallites [15]. The nanocrystallites, with their low mass and very large surface area, may cluster to decrease their surface, attracted by adjacent exposed surfaces and rolling and sliding past one another in the oriented attachment process. Fig. 6. Arrhenius plot for samples aged for various times. be obtained from the slope and intercept on the y-axis of the plot ln(d D 0 ) versus ln t. Fig. 5 shows a plot of ln(d D 0 ) versus ln t with a regression line fitted. The calculated exponent m and rate constant k are shown in Table 1. Fig. 6 shows the rate constant reported in Table 1 on an Arrhenius plot. The activation energy values for anatase crystallite growth, calculated from the slope of the lines and n, are summarized in Table 2. It can be concluded that: (i) the activation energy calculated for samples with aging treatment (mean value 230 kj mol 1 ) are greater than that found for samples without aging treatment (mean value 53 kj mol 1 ) and (ii) the activation energy values calculated for samples with aging treatment are similar, regardless of the different aging times. This result shows that the crystallite growth mechanism depends on the TiO 2 gel aging treatment. Table 2 The activation energy values for anatase crystallite growth Aging time (h) Activation energy (kj mol 1 ) Fig. 7. HRTEM image of the anatase obtained by calcining unaged gel at 600 C for 4 min.

5 H.-I. Hsiang, S.-C. Lin / Materials Chemistry and Physics 95 (2006) In oriented attachment based coarsening, two small nanocrystallites combine to form a larger crystallite without the need for atomic diffusion. Thus, crystallite growth based on the oriented attachment mechanism is dependent upon the probability of contact between nanocrystallites in appropriate orientations. The higher the agglomeration number for the powder, the smaller the distance between the nanocrystallites and the greater the chance for the nanocrystallites to contact each other and coarsen via oriented attachment. The activation energy calculated for samples without aging treatment agreed with the previous observation reported by Zhang and Banfield who analyzed the anatase crystallite growth kinetics based on the oriented attachment (activation energy = 68.6 kj mol 1 ) [15]. Fig. 7 shows a HRTEM image of the anatase obtained by calcining unaged gel at 600 C for 4 min. The schematic outlines show one possible attachment process that could develop the observed crystal morphology. Therefore, it is believed that the growth mechanism of oriented attachment may operate during the early anatase coarsening obtained by calcining TiO 2 gel without aging due to the larger agglomeration numbers. The activation energy for crystallite growth in samples with aging treatment was about 230 kj mol 1, which is very high compared with that for samples without aging treatment. However, this is close to the activation energy for grain boundary diffusion in nanocrystalline TiO 2 [13]. For samples with aging treatment, the agglomeration number was low, meaning fewer anatase crystallites and crystallite crystallite contacts in one cluster. Because anatase crystallite growth based on oriented attachment involves only crystallite crystallite anatase contact, oriented attachment becomes less important and diffusion along the grain boundaries should become increasingly important for samples with aging treatment. Based on the above discussions, pure and nanosized size of anatase powder can be obtained by aging treatment due to the lesser aggregation and higher activation energy for anatase crystallite growth. 4. Conclusion 1. TiO 2 aging treatment decreased the number of hydroxyl ions existing in the anatase and resulted in lowering the surface area reduction rate and decreasing the agglomeration after calcination. 2. The activation energy for crystallite growth in samples with aging treatment was greater than that found for samples without aging treatment. 3. For nanocrystalline anatase samples without aging treatment, oriented attachment controlled the crystallite growth. However, grain boundary diffusion controlled the crystallite growth for nanocrystalline anatase samples with aging treatment. Acknowledgement The authors would like to express their thanks to the National Science Council of the Republic of China for financial support of this Project (NSC E ). References [1] S. Matsuda, A. Kato, Appl. Catal. 8 (1983) 149. [2] H.F. Rayne, Organic Coating Technology, vol. II, Wiley, NY, 1961, p [3] C.P. Li, J.F. Wang, W.B. Su, H.C. Chen, Y.J. Wang, D.X. Zhuang, Mater. Lett. 57 (2003) [4] F. Cot, A. Larbot, G. Nabias, L. Cot, J. Eur. Ceram. Soc. 18 (1998) [5] J. Yang, S. Mei, J.M.F. Ferreira, J. Am. Ceram. Soc. 84 (2001) [6] Y. Zheng, E. Shi, Z. Chen, W. Li, X. Hu, J. Mater. Chem. 11 (2001) [7] H.K. Park, Y.T. Moon, D.K. Kim, J. Am. Ceram. Soc. 79 (1996) [8] K.J.D. MacKenzie, Trans. J. Br. Ceram. Soc. 74 (1975) 29. [9] A. Gribb, J.F. Banfield, Am. Mineral. 82 (1997) 717. [10] H. Hu, B.B. Rath, On the time exponent in isothermal grain growth, Met. Trans. 1 (1970) [11] K.J.D. Mackenzie, P.J. Melling, Trans. J. Brit. Ceram. Soc. 73 (1974) 179. [12] J.A. Eastman, J. Appl. Phys. 75 (1994) 770. [13] H.J. Hofler, R.S. Averback, Scripta Met. 24 (1990) [14] H. Zhang, J.F. Banfield, Am. Mineral. 84 (1999) 528. [15] H. Zhang, J.F. Banfield, Chem. Mater. 14 (2002) [16] C.J. Brinker, G.W. Scherer, Sol Gel Science: The Physics and Chemistry of Sol Gel Processing, Academic Press Inc., San Diego, 1990, pp [17] D.S. Seo, J.K. Lee, H. Kim, J. Cryst. Growth 233 (2001) 298. [18] D.S. Seo, J.K. Lee, E.G. Lee, H. Kim, Mater. Lett. 51 (2001) 115. [19] J.L. Hebrard, P. Nortier, M. Pijolat, M. Soustelle, J. Am. Ceram. Soc. 73 (1990) 79.