Multiple Growth Stages and Their Kinetic Models of Anatase

Aug 11, 2010 - In this work, we present a unique three-stage hydrothermal growth of ... each growth stage, a kinetic model by using mathematic equatio...
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J. Phys. Chem. C 2010, 114, 14461–14466

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Multiple Growth Stages and Their Kinetic Models of Anatase Nanoparticles under Hydrothermal Conditions Hongquan Zhan, Xianfeng Yang, Chaomin Wang, Chaolun Liang, and Mingmei Wu* MOE Key Laboratory of Bioinorganic and Synthetic Chemistry/State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, and Instrumental Analysis and Research Center, Sun Yat-Sen UniVersity, Guangzhou 510275, P. R. China ReceiVed: July 6, 2010; ReVised Manuscript ReceiVed: July 29, 2010

Hydrothermal growth, structural analyses, and growth kinetics of anatase nanoparticles have been paid extensive attention. The growth was generally related to either Ostwald ripening (OR) or orientation attachment (OA). However, very few works about hydrothermal crystallization of anatase nanoparticles have covered both OR and OA. In this work, we present a unique three-stage hydrothermal growth of anatase nanoparticles in succinic acid: OA, shrinkage, and OR. The growth procedures are experimentally monitored by powder X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) in detail. The calculated crystalline sizes by the Scherrer equation unambiguously suggest the three-stage growth mechanism. This is further evidenced by vivid high resolution TEM (HRTEM) images and their related fast Fourier transform (FFT) patterns. For each growth stage, a kinetic model by using mathematic equations is proposed and addressed in detail. 1. Introduction Nanocrystal growth mechanism and microstructure development have received more and more attention in current nanochemistry, especially in view of the significance of controlling crystal sizes and morphologies.1-3 Early studies on the kinetics model for coarsening of bulk materials were generally based on Ostwald ripening (OR),4-6 which involves the growth of larger particles at the expense of smaller ones. However, an important nonclassical growth mechanism, defined as orientation attachment (OA), has been highlighted as a common step during the growth of some nanosized crystals. Orientation attachment is an alternative growth pathway in which larger crystals formed by crystallographically oriented assembly of smaller nanocrystals.7,8 To express crystal growth behavior Via such a mechanism, kinetic models were often described as similar to that of Smoluchowski’s,9,10 which was quite complex. Huang and Banfield et al. proposed a much simpler OA kinetics model to account for hydrothermal coarsening of thiol-capped ZnS nanoparticles.11 In addition, Ribeiro et al. suggested an easyaccepted theoretical model according to the collision of early yielded nanoparticles and successfully elucidated the OA growth of SnO2 nanoparticles.12,13 These models can fit well for OA growth Via the aggregation of a few nanoparticles, but obvious deviations occurred when many nanoparticles aggregated through OA growth. Research on solution growth and kinetic models of titanium dioxide (TiO2) has attracted increasing interest in the last decades because of the excellent capability of TiO2 in photocatalysis, solar cells, sensors, and energy storage.14-20 Recent studies indicate that the photoactivity of TiO2 nanoparticles is significantly microstructure-dependent.20-25 Therefore, obtaining TiO2 nanopaticles with precise microstructure control and theoretical prediction during crystal growth is a key step to improving their performance. After the work by Penn and Banfield, in which the concept of OA growth of TiO2 nanoparticles was first * To whom correspondence should be addressed. E-mail: ceswmm@ mail.sysu.edu.cn.

proposed,7,8 many in-depth investigations about their growth mechanism have been carried out. It has been reported that TiO2 nanocrystals can self-assemble together and produce anisotropic nanostructures with the assistant of organic ligands.24,26,27 The size and aggregation of TiO2 nanoparticles can be modified by adding organic acids into the reaction solution.28-32 A one-step OR growth kinetics was proposed by Li et al.,33 and Smoluchowski’s OA model was simulated by Zhang et al.10 to address crystal coarsening of TiO2 nanocrystals, respectively, but they did not observe the entire evolution of TiO2 microstructure under microscopy. In the present work, the growth kinetics of TiO2 nanoparticles in succinic acid solution under hydrothermal conditions was investigated by monitoring crystal size change and microstructure evolution and by analyzing growth kinetics. A new multistep growth mechanism of single crystalline nanoparticles of TiO2 has been developed, which included formation of single-crystal-like TiO2 nanoaggregates Via OA, their size-shrinking, and their Ostwald ripening to actual single crystals. A new modified kinetics model has been built for interpreting the formation of TiO2 nanoaggregates through OA. Another simplified model has been proposed to express the novel size-shrinking of TiO2 nanoaggregates (nanoparticles). Finally, the traditional OR mechanism has been adopted to address the ripening of TiO2 single crystals. 2. Experiment Titanium n-butoxide [Ti(OC4H9)4, briefly denoted as TNB] in A.R. grade was used as the titanium source without further purification. Initially, 2.362 g of succinic acid [HOOC(CH2)2COOH] was added into deionized water to form an aqueous solution with a molar concentration of 2.0 M. Subsequently, TNB was dropwise added while stirring to form a mixture as feedstock with a molar ratio of [succinic acid]:[Ti] at 4:1. The feedstock (ca. 10 mL) was then transferred into a Teflon-lined autoclave (23 mL) and immediately tightly closed. The hydrothermal reaction was conducted at 220 °C for a period of time in a range of 0.5 h to 30 days in an oven. After reaction, the autoclave was taken out and cooled down naturally to ambient

10.1021/jp1062308  2010 American Chemical Society Published on Web 08/11/2010

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Zhan et al. determined by using Barrett-Joyner-Halenda (BJH) algorithm according to the desorption data of the N2 isotherm.

Figure 1. Powder XRD patterns of the products synthesized at 220 °C with different reaction times from 30 min to 720 h: (a) 30 min; (b) 2 h; (c) 12 h; (d) 72 h; (e) 720 h.

temperature. The powdery products were isolated Via filtration, washed with ethanol and deionized water, respectively, and then placed into a desiccator for drying. Powder X-ray diffraction (XRD) characterization was performed on a Rigaku D/MAX 2200 VPC diffractometer using Cu KR radiation (λ ) 0.15045 nm) and a graphite monochromator. The instrument was operated at 40 kV and 30 mA. Samples for transmission electron microscopic (TEM) examination were prepared by dispersing powders on a carbon film supported copper grid. TEM observations were performed on a JEOL JEM-2010HR electron microscope equipped with a Gatan GIF Tridiem system. The Brunauer-Emmet-Teller (BET) nitrogen physisorption experiments were carried out on a Micromeritics ASAP 2010 system. The BET surface area was estimated using adsorption data in a relative pressure range from 0.05 to 0.2, and the pore-size distributions of the materials were

3. Results and Discussion 3.1. The Experimental Results. The powder X-ray diffraction (XRD) patterns of hydrothermal products synthesized from the above-mentioned succinic acid solution are presented in Figure 1. The diffraction patterns reveal that all the products are crystallized in the anatase phase (JCPDS card no. 21-1272). As shown in Figure 1, the crystallization of the products becomes more significant with the reaction making progress. The crystalline sizes in the products were calculated by the Scherrer equation from the full width at half-maximum (fwhm) of 101 diffraction peaks. The calculated crystalline sizes as a function of reaction times are plotted in Figure 2a. The curve in Figure 2a indicates that the coarsening of anatase nanoparticles may have different crystallization behavior at different growth stages, which is further illustrated by electron microscopy (Figure 3). Figure 3 shows the TEM and high resolution TEM (HRTEM) images of a variety of typical samples grown after a variety of periods of reaction times at 220 °C. Both TEM and HRTEM observations propose that there are different dominant growth behaviors during each stage. Each of the nanoparticles in Figure 3a which are yielded at an early stage is generally composed of several subnanocrystals attaching together along one specific crystallographic orientation. This is clearly illustrated in the HRTEM image (Figure 3a2). The coalescence of lattice fringes at the boundary of two subnanocrystals inside the nanoparticle is quite obvious. Each nanoparticle thus assembled by the coalescent attachment of smaller nanocrystals appears with irregular shapes and unclear edges. The FFT pattern in the upper right corner from the nanocrystal-assembled nanoparticle at the center of Figure 3a2 indicates it has a single-crystalline-like

Figure 2. Calculated crystalline sizes vs reaction time at (a) 220 °C, (b) 180 °C, and (c) 140 °C. The inset is an enlarged plot for the first stage.

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Figure 4. Nitrogen adsorption/desorption isotherm and BarrettJoyner-Halenda (BJH) pore-size distribution plot (inset) of the sample growth at 220 °C for 2 h.

Figure 3. TEM and HRTEM images of the specimens prepared at 220 °C with different reaction times: (a) 30 min; (b) 2 h; (c) 12 h; (d) 72 h; (e) 720 h. The insets at the top right corners are typical FFT patterns from the whole crystal in the HRTEM image.

nature. This experimental evidence consequently suggests a particular orientation attachment (OA) character of these smaller subnanocrystals to build a larger nanoparticle. Due to such an OA, the nanoparticles in Figure 3a1 are in most cases in the form of elongated geometry.

After continuing the reaction in succinic acid to 2 h, the elongated nanoparticles tend to be granular (Figure 3b) and more nanocrystals as building blocks in each nanoparticle are observed (Figure 3b2). The sizes of these nanoparticles tend to be much larger. These nanocrystals aggregate by OA, which is confirmed by both HRTEM image and FFT pattern (Figure 3b2). In the HRTEM image, the lattice fringes at the boundaries among nanocrystals are highly coherent and are confused with each other. In the FFT pattern, the distinguished spots are regularly distributed. All the spots are indexed to be anatase with single crystalline nature. Because of the attachment of nanocrystals, there are some interspaces inside each of the nanoparticles and the voids can be identified obviously in either the TEM image or HRTEM image (Figure 3b). The presence of porous structure can be assessed by the isothermal curve and pore-size distribution (Figure 4). Both the isothermal curve and pore-size distribution reveal the mesoporous characteristic of the nanoparticles. During this early growth stage, the crystalline nanoparticles grow remarkably quickly (Figure 2a). The quick growth of the nanoparticles is actually attributed to OA at this moment, and particle sizes can be regarded as the aggregation of the subnanocrystals (Figure 3a and b). The OA plays a key role in the early nanoparticle growth. With continuing hydrothermal OA, more nanoparticles tend to attach together and the resultant nanoparticles tend to be greater and greater (Figure 2a, inset). The X-ray diffraction intensities tend to be stronger, and the diffraction widths tend to be narrower (Figure 1a-c). However, after hydrothermal reaction for 12 h, both the diffraction intensities and the diffraction widths almost remain unchangeable (Figure 1d). The crystalline size of these nanoparticles calculated from the Scherrer equation seems to reach a limited maximum around 20 nm (Figure 2a, inset). Under microscopy, the tracks of mesopores on nanoparticles can be observed at this moment but the boundaries between subnanocrystals nearly disappear (Figure 3c). This is further suggested by both isothermal curves and pore-size distribution curves (Figure S1, Supporting Information). This is expected to result from coherent fusing of subnanocrystals inside the hydrothermal system. During the OA of nanocrystals, hydrothermal crystallization (HC) is making progress and the geometrical outlines of these nanoparticles become more and more obvious. That the calculated nanoparticle sizes around 20 nm remain almost unchangeable can be up to 24 h, indicating the growth based on OA has weakened during this period.

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Interestingly, with a further hydrothermal crystallization to 72 h, the calculated crystalline sizes tend to be decreased to some extent (Figure 2a). The dispersed nanoparticles in Figure 3d appear slightly smaller than those in Figure 3c. The original pores inside each nanoparticle are not so obvious (Figure 3d) as those in Figure 3c, which was further demonstrated by adsorption-desorption measurement (Figure S1, Supporting Information). The nanoparticles become more compacted. The geometrical outline becomes clearer, and the spots in the FFT pattern (Figure 3d, inset) become stronger and sharper. This surely indicates the nanocrystal-attached nanoparticles in Figure 3a have developed into real single crystals (Figure 3d). According to the TEM image in Figure 3d, it can be observed that the surface of these nanoparticles as single crystals becomes smoother but the sizes become smaller than those in Figure 3c. The lattice fringes of {101} are always observed, indicative of their preserving. This unambiguously indicates a slight shrinking of these anatase nanoparticles during crystallization at this stage, as illustrated in Figure 2a with a reaction time ranging from 24 to 72 h. This might be the first example of shrinking growth behavior of anatase nanoparticles during hydrothermal aging, after OA growth. A similar phenomenon is discovered in Pt nanocrystal aggregation growth.34 If the hydrothermal growth was taken at 180 and 140 °C, negligible shrinking phenomena were detected (Figure 2b,c). If the reaction was carried out in an aqueous solution of butanediol and PEG-200 (polyethylene glycol 200), respectively, such a shrinkage behavior was not identified (Figures S2 and S3, Supporting Information). Therefore, such a shrinkage growth for anatase nanoparticles is unique and will be of great interest. After a remarkably long hydrothermal growth, such as at 220 °C for 720 h, the nanoparticles develop into more regular single crystals with much smoother and clearer edges (Figure 3e). Both HRTEM image and FFT patterns show unambiguous evidence that the nanoparticles originally from the OA of subnanocrystals have become actual and complete single crystals. The crystal sizes are slightly greater than those in Figure 3d. The crystal growth is significantly slower than early stages up to 10 h (Figures 2 and 3). These results show a common Ostwald ripening (OR) mechanism which takes a key role in this stage, i.e., since point C (at 72 h) in Figure 2. These developed nanocrystals are mainly enclosed by common {101} lattice planes. In fact, even at much early growth stages, {101} lattice fringes emerge. They preserve and develop during the entire three stages: OA, shrinkage, and OR. 3.2. Growth Kinetic Models. As described above, the whole crystal growth can be divided into three stages. The first stage is from starting to point B in Figure 2a. During this stage, the crystals grow rapidly up to a limited calculated size of 20.3 nm while the reaction time is at 10 h. During this period, OA plays a dominant role in the crystal growth. During the following reaction, i.e., up to 24 h, the crystal growth speed tends to be much slower. The maximum nanocrystal sizes during this stage can only approach a calculated size of 20.8 nm. After reaching such a maximum size, the further hydrothermal crystallization leads to a slight decrease of crystal size. This stage is defined as the second one which is from the highest point, i.e., point B, to the lowest one, i.e., point C, in Figure 2a. The slight decreasing of crystal sizes results from a volume shrinkage of nanoparticles. The last stage is from point C to the end, i.e., point D on the curve of Figure 2a. In the third stage, the crystal growth is reinitiated, but the growth is much slower than that at the initial period in the first stage.

Zhan et al. TABLE 1: Simulated Data for the Multiple Stages of Growth of Anatase Nanoparticles stage

T (°C)

220

180

140

Eaa (kJ/mol)

the 1st stage

m k1 (h-1) R2 Dh (nm) k2 (h-1) R2 Dm (nm) k3 (h-1/2) n R2

6.81 1.46 0.93 20.58 0.068 0.99 17.33 0.30 2.0 0.93

4.89 1.27 0.99

4.25 0.36 0.99

30.40

15.12 0.23 2.0 0.92

12.90 0.15 2.0 0.92

the 2nd stage the 3rd stage

14.83

Note: Ea is deduced from the Arrhenius equation, ln k ) -(Ea/RT) + A0. a

Now it is clear there are three growth stages in the present crystallization of anatase nanoparticles. The TEM images in Figure 3a-c evidently indicate that an OA mechanism is dominant in the first stage. In order to understand the growth mechanisms quantitatively, some mathematic equations are adopted and kinetic models are built. Using Huang’s equation of describing OA growth of ZnS nanoparticles,11 it is found that fitting data cannot match well with our above experiment results. Thus, a modified kinetic model is proposed to elucidate the experiment results on the basis of early work by Huang et al.11 The derived equation can be described as follows:

d ) d0

1 + mk1t 1 + k1t

(1)

The detailed derivation of eq 1 is mentioned in the Supporting Information, where t is time, d is the average particle size at time t, d0 is the initial average particle size at the starting point, m is defined as the aggregation factor which represents the degree of particle aggregation, and k1 represents the rate constant. The crystal size is calculated to be about 3 nm at point A, i.e., d0 ) 3 nm. The first stage of crystal growth can be fitted well by using eq 1. The resultant fitting data are listed in Table 1. According to these data in Table 1, a fitting curve is obtained (the red curve in Figure 2), which agrees well with the experiment results. The good agreement indicates eq 1 can express the actual crystal growth based on OA. With the increase of reaction temperature (T), both the aggregation factor (m) and rate constant (k) tend to be greater (Table 1). This implies more subnanocrytals aggregate to build a larger single crystalline nanoparticle at higher temperature, and the aggregation tends to be facilitated because of the increase of collision probabilities of colloid nanocrystals. In the second stage, the particles shrink. The phenomena can be described by the viscous flow model which has been used to describe liquid phase sintering of particles.35-37 During this stage, the volume of each nanoparticle generated from several coherently assembled subnanocrystals reduces a little because these nanoparticles inside connect and fuse together more tightly with each other. Such a shrinking phenomenon can be expressed by Frenkel’s equation36 as the viscous flow. A simplified equation is illustrated as follows:

d ) dh - k2(t - th)

(2)

The derivation of eq 2 is expressed in the Supporting Information, where t is time, th is the beginning time of the

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SCHEME 1: Schematic Illustrating the Three-Stage Growth Mechanism of TiO2 Nanoparticles in the Presence of Succinic Acid

second stage (herein, th is equal to 24 h), and dh is the initial particle size at the beginning of the second stage. The obtained parameters are listed in the middle row of Table 1. The green curve in Figure 2a deduced from eq 2 with these parameters agrees well with the experimental data. The fitted value of dh is 20.6 nm at 220 °C, consentaneous with calculated data from the Scherrer equation (Figure 2a). The datum of k2 is obtained to be 0.0068. Such a small value of k2 suggests a slight shrinkage behavior. For the third stage, existing models have been used to assess coarsening Via Ostwald ripening (OR). As normal, the growth model can be mathematically expressed by the following equation, as suggested in previous work:6,11

alkyl chain ends may serve as a chelating agent to bridge two or more tiny nanocrystals along some specific orientation.17,18,32 This is similar to the evolution of mesocrystals assisted by organic surfactants.38 Instead of succinic acid, if either butanediol or PEG-200 was adopted as a part of cosolvent in the distilled water with identical molar ratio, such a three-stage growth could not be identified. The growth mechanism is significantly different. The key growth behavior is attributed to classical OR (either Figure S2 or S3 in the Supporting Information). Therefore, the three-stage growth is remarkably unique in our present work. To the best of our knowledge, this might be the first example for the novel growth of anatase nanoparticles. 4. Conclusion

d ) k3(t - tm)

1/n

+ dm

(3)

in which t is time, k3 is a temperature-dependent rate constant, n is an exponent, and dm is the crystal size at the starting time point of tm (herein, tm ) 72 h). When the exponent n is equal to 2, it is inferred that the crystal growth is controlled by ion diffusion along the matrix-particle boundary; when n ) 3, the growth is controlled by volume diffusion of ions in the matrix; and when n ) 4, the growth is controlled by dissolution kinetics at the particle-matrix interface.11 The resultant parameters in eq 3 to fit the experimental data are listed in Table 1, and the simulated curve is illustrated in Figure 2 with a blue one. The fitted results for the starting particle diameter (dm ≈ 17.3 nm) at 220 °C at the beginning of this stage match perfectly with the shrinking end of the second stage (17.5 nm). Other mathematic equations such as those in the work by Li and his coauthors33 cannot be well matched with our present experimental results. Therefore, our present growth mechanisms are considerably unique as compared with previous work. With raising reaction temperature, the values of k3 increase significantly, indicative of the highly improved growth Via OR. The exponent value of n is fitted to be 2, suggesting that the growth kinetics of the third stage is significantly controlled by species diffusion along the solution-particle boundary. Due to a redissolution and mass transport among surfaces of nanoparticles as a main growth behavior, the present OR results in a much slower crystal growth than that Via OA. According to the Arrhenius equation and the above data of rate constant in Table 1 for OA and OR, respectively, the deduced activation energies (Ea) are derived to be 30.4 and 14.83 kJ mol-1. The much greater Ea value for OA than that for OR reveals that a higher barrier gap is present during OA of subnanocrystals than that during OR. In order to overcome such a higher Ea, some specific chemical species may be necessary in the OA growth. The presence of succinic acid may play a crucial role in the threestage growth of our present anatase nanocrystals. Because of the strong affinity of the carboxylate group to titanium atom, it is possible that succinic acid with double carboxylate groups on both

A novel three-stage hydrothermal growth of anatase nanoparticles in the presence of succinic acid at 220 °C has been observed by using XRD patterns and TEM images. Both experimental data and theoretical models indicate the growth does not follow a simple classic behavior. HRTEM images and their related FFT patterns reveal several anatase nanocolloids aggregate with lattice fringes coherently attaching at boundaries and evolve into a compact nanoparticle with the 011 lattice fringe preserved. At such an early stage, the quick increase of crystalline sizes is crucially dependent on OA (Scheme 1, OA). The further hydrothermal crystallization results in a volume shrinkage of single-crystal-like as-aggregated nanoparticles (Scheme 1, SH). After this, a slow crystal growth related to classical OR is simulated and the nanoparticles gradually develop into actual single crystals with well-defined outer geometries (Scheme 1, OR). Three simulated curves based on mathematic equations have been illustrated to address the threestage growth, which match perfectly with the experimental data. The calculated activation energies for both OA and OR indicate OA needs a much higher barrier energy than OR. This confirms that OA rarely happens during crystal growth as compared to general OR. Thus, for most crystal growths, OR is very common. If the hydrothermal reaction was carried out in the presence of either butanediol or PEG-200, the evidence about such a three-stage growth was not be observed. They only obeyed OR models. Therefore, such a three-stage growth of anatase nanoparticles in the presence of succinic acid should be significantly unique and will be of great interest in crystal growth and nanochemistry. Acknowledgment. This work is financially supported by National Natural Science Foundation (NNSF) of China, the Government of Guangdong Province and Guangzhou City for NSF and industry program (Nos. U0734002, 50872158, 20090171110025, 8251027501000010, and 10C22051347). Supporting Information Available: Additional isothermal curves and pore-size distributions and the dependence of crystal sizes of reaction time in the reaction solutions in the presence

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