CRYSTAL GROWTH & DESIGN
High-Temperature Formation of Titanate Nanotubes and the Transformation Mechanism of Nanotubes into Nanowires
2009 VOL. 9, NO. 8 3632–3637
Jiquan Huang, Yongge Cao,* Qiufeng Huang, Hong He, Yuan Liu, Wang Guo, and Maochun Hong Key Lab of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China ReceiVed April 6, 2009; ReVised Manuscript ReceiVed May 25, 2009
ABSTRACT: Large-scale pure titanate nanotubes were synthesized through the hydrothermal reaction between TiO2 powders and concentrated NaOH under an unexpected high temperature of 240 °C, while it was generally claimed that it is impossible to form nanotubes at temperatures higher than 180 °C. The titanate nanotube was found to be an inevitable intermediate product, which finally transformed into a nanowire upon increasing the hydrothermal treatment duration. It was proven that the successive appearance of nanosheets, nanotubes, and nanowires are three unavoidable kinetic products of the reaction. Increasing the temperature could only accelerate the nanotube-nanowire transformation process but could not affect the sequence of the reaction events. The transformation kinetics from nanotubes to nanowires under different reaction temperatures was studied. Detailed studies indicate that this transformation process was accompanied by a coarsening process induced by both oriented attachment (OA) and Ostwald ripening (OR) mechanisms simultaneously; thereafter, the OA-OR cooperative mechanism was proposed. Introduction Since the first report of titanate nanotubes derived from hydrothermal treatment of TiO2 powder with concentrated NaOH aqueous solution by Kasuga et al.1 in 1998, great attention has been paid to the structures2-8 and formation mechanisms7-18 of the high-aspect-ratio one-dimensional (1D) titanate nanostructures, such as nanotubes and nanowires. These kinds of materials are attracting more and more attention because of their potential applications in various fields, such as catalysts,19 proton conduction,20 gas sensors,21 hydrogen storage,22 Li-ion batteries,23 solar batteries,24 electrochromism,25 and absorption materials for radioactive ions.26 How to tailor the desired 1D nanostructure morphologies is crucial for their effective utilization, which makes it essential to clarify the growth kinetics of nanotubes and nanowires. In the past decade, great efforts6-18 were focused on the effects of hydrothermal parameters on the compositions and morphologies and clarification of the sequential events in the formation process of titanate nanotubes and nanowires. However, the formation mechanisms of 1D titanate nanostructures are still disputed. It is accepted by contemporary reports that nanotubes are formed at mild hydrothermal temperatures in the range of 100-180 °C, while only nanowires (or nanoribbons) can be synthesized at high temperature (e.g., T > 200 °C).11,15-17 Additionally, titanate nanotubes can further transform into nanowires with prolonged hydrothermal duration.15,17,18 Controversially, Zhu17 and Elsanousi18 explained this transformation by Ostwald ripening (OR), while Kukovecz15 ascribed it to a new crystal growth mechanism named “oriented attachment” (OA). However, no direct and detailed experimental evidence has been presented for either of the above two mechanisms. In this paper, we report the successful synthesis of titanate nanotubes at a relatively “ultrahigh temperature” of 240 °C, and the nanotubes could transform into nanowires abruptly with increasing hydrothermal reaction duration. This transformation was confirmed by our experimental observations to be induced by mixed OA and OR mechanisms. It is hoped that these * Corresponding author. E-mail:
[email protected]. Fax: +86-59183721039. Tel: +86-591-83721039.
investigations would help to further the understanding of titanate nanomaterials and accelerate the application process. Experimental Section Analytical reagent (AR) grade NaOH pellets and anatase TiO2 powder with particle size of about 10 nm were used as raw materials for synthesis. Anatase powder (0.2 g) was put into a 20 mL aqueous solution of 8 M NaOH in a Teflon-lined autoclave. The autoclave was then directly put into a 240, 180, or 160 °C oven for 1-240 h. After the hydrothermal treatment, the white precipitate was washed several times with distilled water and with absolute ethanol and then dried in air. The morphology and microstructure of the resulting samples were characterized by X-ray diffraction pattern (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The surface area and Barret-Joiner-Halenda (BJH) pore distribution were obtained on an ASAP 2020 Micromeritics instrument.
Results and Discussion Figure 1 shows the SEM (a-d) and TEM (e-h) images of the samples obtained by treating anatase with concentrated NaOH aqueous solution at 240 °C for different durations. SEM and TEM images (Figure 1b,f, respectively) clearly show that titanate nanotubes with outer diameters of about 9 nm and lengths of several hundred nanometers were synthesized under high hydrothermal temperature of 240 °C. Ordinarily, it is considered that such a high temperature is not favorable for formation of tubular titanate.15,16 It has also been stated that the nanotubes can only form at 90-180 °C while higher temperatures favor the formation of nanowires by virtue of enhanced thermal stability compared with nanotubes.15-17 Bavykin et al.11,27 further claimed that the morphology of the titanate nanostructure depends on the concentration of Ti(IV) in alkaline solution during hydrothermal treatment. They suggested that formation of nanotubes occurs in just a certain range of Ti(IV) concentration, which depends only on the temperature presented by the following equation: ln(cTi(IV) NaOH) ) 0.355 - 2623/ (T + 273).27 Consequently, nanotubes can only be obtained in a lower temperature range, while the formation of nanowires is favorable at higher temperature. Yuan et al.16 also suggested that the morphologies of the products are temperature-dependent
10.1021/cg900381h CCC: $40.75 2009 American Chemical Society Published on Web 06/15/2009
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Figure 1. SEM (a-d) and TEM (e-h) images of the hydrothermal products synthesized at 240 °C for different durations: (a, e) 1 h; (b, f) 2 h; (c, g) 3 h; (d, h) 240 h. Panel I shows XRD patterns of the hydrothermal products synthesized at 240 °C for different durations.
and be controlled by thermodynamics when the temperature is above 180 °C, and thus the product should be nanowires at temperatures higher than 180 °C. However, our observation indicates that the titanate nanotube is the consequent product of the reaction of TiO2 with concentrated NaOH with certain reaction durations, and the treating temperature only affects the kinetic reaction rate. The morphology evolution of hydrothermal products at 240 °C is displayed in Figure 1. Similar to previously reported experiments carried out at lower temperature (e.g., 110-150 °C),4,7,10 the dissolving of TiO2 in NaOH solution results in the precipitation of ultrathin titanate nanosheets (Figure 1a,e), which roll up to form one-dimensional nanotubes (Figure 1b,f). As the treatment duration increased to 3 h, the product was nanowires (Figure 1c,g). The nanowires have widths ranging from 50 to 300 nm and lengths of several tens of micrometers. No further morphology evolution could be observed by increasing the treatment duration, which means that the nanowires have reached the equilibrium state. The product is still nanowires even when the duration is as long as 240 h. Recently, Wu et al. proposed a “universal” formation mechanism.7 They claimed that the thickening of thin nanosheets is accelerated by increasing the temperature and thus thick layers were generated. Finally, the split of the thick layers results in the formation of nanowires. However, in our present study, no thick layers were observed. Contrarily, we found that the thin
nanosheets evolved toward nanotubes and thereafter these nanotubes transformed into nanowires, as shown in Figure 1. This observation demonstrates that the successive nanosheets, nanotubes, and nanowires are three unavoidable kinetic products of the reaction of TiO2 with concentrated NaOH. A schematic drawing depicting the formation process of these titanate products is shown in Figure 2. Figure 1i shows the XRD patterns of the hydrothermal products synthesized at 240 °C for different durations. It is clear that samples synthesized at 240 °C for 1 h and longer times show the typical XRD pattern of titanate without any trace of TiO2. The XRD patterns of both titanate nanosheets and nanotubes are similar to those of titanate nanotubes obtained under mild temperature conditions.4,5,11,17 The crystal structure of the nanotubes (and nanosheets) is similar to that of H2Ti2O5 · H2O (JCPDS Card No. 47-0124). However, XRD patterns of the titanate nanowires are different. The crystal structure of the nanowires obtained at 240 °C for 3 h is similar to that of nanotubes. With increasing reaction duration, the diffraction peaks at about 2θ 9.7° and 28.5° shift to higher angle and more peaks appear. At the same time, the intensity of the diffraction peaks increase, which suggests that the crystallinity of the nanowires is improved by continued hydrothermal treatment.
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Figure 2. Schematic of the formation process of titanate nanosheets, nanotubes, and nanowires.
Figure 3 shows the dependence of the transition process from tinanate nanotubes to nanowires on the hydrothermal duration at different temperatures. Figure 3a shows the plots of the diameter and length of the product as a function of the hydrothermal duration at 240 °C. For 1 < t < 2.3 h, the product is nanotubes with outer diameter of about 9 nm and average length of about 350 nm, and no obvious crystal growth with increasing duration was observed. The final product consists of a mixture of nanotubes and nanowires in the time range of 2.3-2.9 h. At t > 2.9 h, nanowires with average diameter of about 90 nm and average length of about 70 µm are the final products. As a comparison, the dependence of the transition processes from nanotube to nanowire on the hydrothermal temperature and duration at lowered temperature (180 and 160 °C) was also studied, as shown in Figure 3b. The activation energy for the transformation process was calculated to be 103.8 kJ/mol (Figure 3c), which was high enough to hinder the product changing from nanotube to nanowire below 180 °C. However, as shown in Figure 3, it is obvious that the transition rate increases significantly with the reaction temperature. Thus, the transformation would be accelerated by increasing the temperature, and the system undergoes a fast formation of nanotubes. Similarly, the nanotube-nanowire transformation at 240 °C also becomes extremely rapid. Consequently, the existence of nanotubes can only be observed in short time experimental design as our current study. The fact that previous researchers did not produce nanotubes at temperatures higher than 180 °C can be due to their long reaction duration. With longer sampling intervals, the nanotubes may be missed and only nanowires can be observed, which led to the misunderstanding of “only nanowires can be formed at high temperature”. The effect of the treatment duration on the morphology of the titanate nanostructure had been studied recently by several
Huang et al.
groups.15,17,18 Zhu17 and Elsanousi18 explained the morphological evolution of nanotubes into nanowires by Ostwald ripening in which nanowires grow at the expense of smaller crystals. However, this explanation is suspect. In our present work, the product undergoes a nanotube-nanowire transition if the hydrothermal duration is increased, where the crystal size changes by 1 or 2 orders of magnitude in a short time (from t ) 2 h to t ) 3 h), as shown in Figure 1 and Figure 3a. Obviously, this abrupt particle coarsening could not be explained by the OR mechanism. In order to develop a further understanding of the transformation mechanism, more detailed observations were conducted by means of TEM and HRTEM. Figure 4 shows the HRTEM images of the nanotubes and nanowires synthesized at 240 °C for 2.25 to 23 h. After 2.25 h of reaction, parallel nanotubes were observed everywhere (Figure 4a). Upon increase of the reaction duration to 2.75 h, the adjacent nanotubes were found to be attached together. Figure 4b shows that two parallel nanotubes “bunch” to be attached with each other, which displays the particular characteristic of the OA mechanism.28,29 Simultaneously, the OR process also accompanied the OA, which is confirmed by our HRTEM observations as shown in Figure 4b,c, in which titanate nanopaticles were found to precipitate at both the inner and outer surfaces of the nanotubes. Based on these observations, it is proposed preliminarily that the morphological evolution from nanotubes into nanowires is induced by cooperation of both the OA and OR mechanisms: the violent coarsening of the crystal is achieved by the coalescence of smaller nanotubes, while both the inner channels of tubes and the interspaces (i.e., boundary pores) among tubes are covered by precipitation of Ti(IV). In other words, the rapid crystal coarsening via tube bunching is through an OA process, while the formation of wire-like titanate by filling of inner channels follows the OR route. Figure 4d shows the HRTEM image of a nanowire obtained after hydrothermal treatment for 3 h. It is clear that the nanowire contains defects with high concentration. The tailing of the diffraction spots in the inset of Figure 4d clarifies the typical short-range disorder within long-range order in the crystal lattice, which gives us a clue that large amounts of defects were generated during the tubebunching OA process. These generated defects can be removed through elongating the hydrothermal reaction duration. As shown in Figure 4e, as the treatment duration increased to 23 h, most of the nanowire products show nearly defect-free lattice characteristics with clear spot diffraction pattern (inset in Figure 4e), implying that the crystallinity of the bunched nanowires via OA route was significantly increased after continuous hydrothermal treating. The nanowires synthesized at 240 °C for 3 h present characteristics of mesopores. Figure 5 shows the pore volume
Figure 3. Dependence of the transition process from tinanate nanotubes to nanowires on the hydrothermal duration at (a) 240 and (b) 180 and 160 °C. Panel c presents the corresponding calculated activation energy, Ea. V is the mean variation rate of diameters from nanotubes to nanowires.
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Figure 4. HRTEM images of the nanotubes and nanowires synthesized at 240 °C for different durations: (a) 2.25 h, (b, c) 2.75 h, (d) 3 h, and (e) 23 h. Insets in panels d and e show the corresponding selected area electron diffraction (SAED) patterns.
Figure 5. Pore volume distribution (BJH desorption) (a) and nitrogen adsorption-desorption isotherms (b) of the nanotubes/nanowires synthesized at 240 °C for different durations: (9) 2 h (nanotubes), the BET surface area is 156 m2/g; (b) 3 h (nanowires), the BET surface area is 108 m2/g; (2) 78 h (nanowires), the BET surface area is 26 m2/g.
distribution curves and nitrogen adsorption-desorption isotherms for titanate nanotubes and nanowires synthesized at 240 °C for different durations. The titanate nanotubes (t ) 2 h) exhibit a narrow pore size distribution with a maximum peak of 3.9 nm and a broad peak around 10 nm, as shown in Figure 5a. Simultaneously, the isotherm for the nanotubes is type IV with type H3 hysteresis loop at relative pressures higher than 0.4, as shown in Figure 5b, indicating the presence of mesopores (2-50 nm).30 It is known from Figure 4a,c that the inner diameter of the nanotubes is about 4 nm, thus the smaller pores (∼3.9 nm) can be attributed to the pores inside the nanotubes; the larger pores (broadening peak from 6 nm to more than 20 nm) may be attributed to the pores formed by the aggregation of the nanotubes.11 The nanowires synthesized at 240 °C for 78 h only show a wide pore size distribution (13-40 nm) with a suppressed peak at 19 nm (Figure 5a). The disappearance of the smaller pores (
