A Strategy for Architectural Growth of Tubular and Nontubular CaTi

May 24, 2013 - State Key Laboratory of Pulsed Power Laser Technology, Electronic Engineering Institute, 460 Huangshan Road, Hefei, Anhui. 230037, Peop...
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High-Symmetry Epitaxial Growth under Solvothermal Conditions: A Strategy for Architectural Growth of Tubular and Nontubular CaTiO3 Microstructures with Regular Geometrical Morphologies and Tunable Dimensions Dabin Yu,* Jinhua Zhang, Feng Wang, Minghui Zhao, Kai Du, Shiwen Shu, Jiwei Zou, and Yan Wang State Key Laboratory of Pulsed Power Laser Technology, Electronic Engineering Institute, 460 Huangshan Road, Hefei, Anhui 230037, People’s Republic of China S Supporting Information *

ABSTRACT: This paper reports on the architectural growth of CaTiO3 microstructures with regular geometrical morphologies and tunable dimensions under simple solvothermal conditions by using a mixture of ethanol and water as solvent, calcium nitrate and tetrabutyltitanate as starting materials, and NaOH as mineralizer. The microstructures can be divided into tubular and nontubular types, both of which can be further divided into one-, two-, and three-dimensional microstructures. The tubular/ nontubular structures and the dimensions of the microstructures can be achieved by adjusting the initial NaOH concentration and the volume ratio of ethanol to water, respectively. The growth mechanism responsible for the formation of the CaTiO3 microstructures was investigated. A high-symmetry epitaxial growth process confers regular geometrical morphologies on the CaTiO3 microstructures: the fast stacking interplay of the {111} planes results in a rectangular structure, and the epitaxial growth limited in the three perpendicular directions forms a perpendicular structure, thus generating the regular geometrical microstructures. The results suggest that highsymmetry epitaxial growth under solvothermal conditions should be a convenient and effective approach for the growth of regular geometrical CaTiO3 microstructures, which may find importance in many fields. and nanostructures,14 and they can be divided into one-step routes and two-step routes. For example, nanolamellate structures and butterfly-like dendrites were fabricated by Liu’s group and Zhao’s group,15 respectively, through a corresponding one-step route. On the basis of the high reactivity of titanate (TiO2) and metal ions, microtubular structures with rectangular cross-section,16 dendrites and rectangular prisms,14 and polyhedra17 were successively obtained by different groups through a corresponding two-step route, i.e., the preparation of titanate (TiO2) nanoparticles used as precursors, followed by the fabrication of the CaTiO3 MSs or nanostructures. As an exception, CaTiO3 hollow crystals were fabricated in water-free solution by Zhou et al.9 Despite these, there are still very limited approaches to produce CaTiO3 nanostructures and MSs. Herein, we report a strategy for growth of high-quality CaTiO3 MSs with regular geometrical morphologies and tunable dimensions via a high-symmetry epitaxial growth process under simple solvothermal conditions. The MSs can be divided into tubular and nontubular types, both of which can be further divided into one-, two-, three-dimensional (1D, 2D, and 3D) MSs. To the best of our knowledge, the growth of

1. INTRODUCTION Architectural growth of crystalline microstructures (MSs) of inorganic functional materials has been a fundamental issue in materials science and technology.1 The MSs with well-defined shapes, especially those with regular geometrical morphologies, have been gaining research attention because of the requirements to uncover and map their shape-dependent properties and to achieve their practical applications.2 Calcium titanium (CaTiO3), the representative compound in the perovskite materials family, is of both fundamental interest and practical importance in many disciplines such as mineralogy,3 chemistry,4 materials science,5 electronic engineering,6 catalyst control techniques,7 and biotechnology,8 due to its unique orthorhombic structure and excellent chemical and physical properties. Stimulated by fundamental study and technological application, research interest has been focused on the design of rational methods for synthesizing CaTiO3 MSs with desired morphologies.9 Although many chemical methods, such as solid-state reactions,10 sol−gel method,11 and mechanochemical synthesis,12 have been used to synthesize CaTiO3 powders, most of them suffer from high-processing temperatures or produce powders with impurities, wide size distribution, and poorly defined morphologies.13 In recent years, hydrothermal methods have been developed for the synthesis of CaTiO3 MSs © 2013 American Chemical Society

Received: April 8, 2013 Revised: May 24, 2013 Published: May 24, 2013 3138

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such high-quality CaTiO3 MSs with regular geometrical morphologies and tunable dimensions has not been reported so far.

2. EXPERIMENTAL SECTION Analytical grade Ca(NO3)2, Ti(OC4H9)4, and NaOH were purchased from Shanghai Chemical Industrial Company and were used without further purification. In a typical synthesis procedure of 1D tubes, 2.0 mmol of Ti(OC4H9)4 and 64.0 mmol of NaOH were dissolved in 25 mL of ethanol and 20 mL of deionized water, respectively. The NaOH solution was then added dropwise into the Ti(OC4H9)4 solution under vigorous stirring to form a white flocculent mixture. After the mixture was stirred for 10 min, 5 mL of Ca(NO3)2 (0.4 M) was added under continuous stirring. Subsequently, the mixture was transferred into a Teflon-lined stainless steel autoclave for solvothermal treatment at 150 °C for 12 h. The vessel was then cooled to ambient temperature. The precipitate was successively washed with distilled water and ethanol, and then dried in a desiccator at ambient temperature. The synthesis of the other MSs was carried out by the same procedure with adjusting the amount of the added starting materials and the NaOH concentration (details see Supporting Information). The products were characterized by X-ray diffraction, recorded on a MAC Science Co. Ltd. MXP 18 AHF X-ray diffractometer with monochromatized Cu Kα radiation (λ = 1.54056 Å). Field emission scanning electron microscopy (FESEM) images were taken on a LEO1530 scanning electron microscope. Transmission electron microscopy (TEM, including high-resolution TEM) images and SAED patterns were obtained on a JEM-ARM200F atomic resolution transmission electron microscope.

3. RESULTS AND DISCUSSION 3.1. Characterization of the CaTiO3 MSs. FESEM clearly shows that there are two kinds of products, i.e., tubular (Figure 1a−d) and nontubular (Figure 1e−h) MSs. When the NaOH concentration was between 0.5 M and 2 M, the products were dominated by tubes. As shown in Figure 1a−d, with the NaOH concentration fixed at 1 M, increasing EtOH/H2O led to the evolution of products from 1D to 2D to 3D tubes. When EtOH/H2O was 1:1, the product was a large number of 1D rectangular tubes with smooth side surfaces and average size of about 3 × 3 × 6 μm (Figure 1a), accompanied by a small number of 2D (cross-like) tubes (top-right inset in Figure 1a) and 3D tubes with six branches symmetrically arranged in three perpendicular directions (bottom-right inset in Figure 1a). When EtOH/H2O was increased to 2:1 and 3:1, the products were the mixture of 2D and 3D tubes (Figure 1b) and uniform 3D tubes (Figure 1c), respectively. However, the branches with a relatively large cross section of the 2D and 3D tubes (Figure 1b,c) were split into four symmetrical parts with different sizes and lengths, which is attributed to the special growth mechanism. When the concentrations of starting materials was incresed to 0.02 M, with EtOH/H2O fixed at 3:1, the product was a large number of tube clusters (Figure 1d), which can be divided into two types: 1D side-by-side type (top-right inset in Figure 1d) and 3D perpendicular type with four parallel tubular branches in each of the three perpendicular directions (bottom-right inset in Figure 1d). Compared to the synthesis of the tubular MSs, the synthesis of nontubular MSs was carried out at elevated NaOH concentration (>2.5 M). As shown in Figure 1e−h, when the NaOH concentration was 3.5 M, with the concentrations of the starting materials fixed at 0.01 M, increasing EtOH/H2O led to the evolution of the products from one dimension to three dimensions. When EtOH/H2O ranged from 0.5:1 to 1:1, 1D rectangular prisms with smooth side surfaces were obtained

Figure 1. FESEM images of the tubular (a−d) and nontubular (e−h) CaTiO3 MSs with various dimensions: (a) 1D rectangular tubes; (b) 2D and 3D tubes; (c) 3D tubes; (d) tube clusters; (e, f) 1D rectangular prisms with different length diameter ratios; (g) 1D rectangular prisms and 2D MSs; (h) 3D nontubular MSs.

(Figure 1e,f), but their length diameter ratio increased with increasing EtOH/H2O. When EtOH/H2O was 1:1.5, the MSs tend to form perpendicular branches, i.e., cross-like and T-like structures as indicated by the white arrows in Figure 1g. When EtOH/H2O was further increased to 2:1, the product was a mixture of the 2D T-like and cross-like MSs and 3D MSs (Figure 1h). The tubular/nontubular MSs gave very similar X-ray diffraction (XRD) patterns (Figure 2). All the diffraction peaks in the XRD patterns can be indexed to the orthorhombic CaTiO3 with unit cell a = 5.430 Å, b = 7.656 Å, and c = 5.398 Å, close to those in the literature (JCPDS card No. 22-0153, space group Pnma). The (040) diffraction peak shows the strongest intensity in the XRD patterns, and its intensity is abnormally stronger than that of the commonly observed strongest (121) peak. Thus, the crystal growth exhibits preferential [010] orientation. Figure 3a shows a transmission electron microscopy (TEM) image of an individual tube with perpendicular branches. Because of the thick sample, the contrast of the TEM image cannot fully display its tubular structure apart from at the ends of the branches. The selected area electron diffraction (SAED) 3139

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and 0.272 nm, respectively, in agreement with the SAED analyses. The magnified HRTEM image (bottom-right inset in Figure 3c) indicates that the (040) plane with the lattice fringe of ca. 0.192 nm is along the axis of the branch, further confirming the preferential [010] growth orientation as exhibited by the XRD analysis. To clarify the relationship between the intrinsic crystal structure and the perpendicular morphologies of CaTiO3 MSs, the structural characteristics of the crystal were further investigated by TEM and SAED techniques. Figure 4a,b

Figure 2. Typical powder XRD patterns of tubular (a) and nontubular (b) CaTiO3 MSs.

Figure 3. (a) TEM image of an individual tube with perpendicular branches. (b) The associated SAED pattern obtained from the marked area (a black pane) in (a) with the [001̅] zone axis of the orthorhombic CaTiO3. (c) The corresponding HRTEM image with the fringe spacing of 0.385 and 0.272 nm, consistent with the d-spacing of the (020) and (200) planes, respectively. The bottom-right inset in (c) shows a magnified image obtained from the area marked with a black pane.

Figure 4. (a, b) SAED patterns of the two perpendicular branches of an individual tube (the bottom-left inset in a) taken from the marked areas A and B, respectively, with the [101]̅ zone axis of the orthorhombic CaTiO3. (c−e) Schematic illustrations of the crystal structures of an individual 1D, 2D, and 3D tubes, respectively. (f) Schematic illustrations of the unit cell of the orthorhombic CaTiO3 and the corresponding pseudocubic perovskite subcell (marked with bold lines). (g) The identical crystal lattices in three perpendicular directions according to the pseudocubic perovskite subcell.

pattern (Figure 3b) and the corresponding high-resolution TEM (HRTEM) image (Figure 3c) of the tube were obtained from the marked area of one branch (the black pane in Figure 3a) with the [001̅] zone axis of the orthorhombic CaTiO3. As shown in Figure 3b, the diffraction spots are assigned to the dspacing of 0.770 and 0.385 nm, consistent with the lattice spacing of the (010) and (020) planes of the orthorhombic CaTiO3, respectively. However, due to systemic extinction, the (010) and (020) diffraction peaks cannot be exhibited in the XRD patterns. As for the SAED pattern, the (010) and (020) diffraction spots result from the secondary diffraction because g010 = ⇀ g210 + they meet the conditions of reciprocal vectors: ⇀ ⇀ ⇀ ⇀ ⇀ g g 200 and 020 = g220 − g200 , respectively. The corresponding ̅ HRTEM image (Figure 3c) shows a single crystalline feature of the sample, and the 2D lattice fringes are examined to be 0.385

shows the SAED patterns of two perpendicular branches of an individual tube (bottom-left inset in Figure 4a), and they were obtained from the areas marked with panes A and B, respectively, by directing the incident electron beam perpendicular to the plane of the two perpendicular branches. The two SAED patterns have the same spot array, but the directions of the spot array are also in a perfect perpendicular manner, matching well with the axes of the two perpendicular branches. The rectangle spot array in the SAED patterns is indexed to (010) and (101) of the orthorhombic CaTiO3. The directions of (010) and (101) are parallel to and perpendicular to the axis of the branches, respectively, indicating that the two perpendicular branches have identical [010] growth orientation and the four side surfaces of a rectangular branch are bound by 3140

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{101} lattice planes. Because of the stacking of alternating neutral CaO and TiO2 planes,16 the rectangular branches exposing the {010}, {101}, and {101̅} planes is a stable structure for CaTiO3 crystals, which is regarded as the characteristics of the perovskite materials.20 Therefore, the side surfaces of the rectangular structures in the 1D, 2D, and 3D MSs are schematically illustrated in Figure 4c−e. Consequentially, as for the two perpendicular branches of an individual MSs, the {101} lattice planes of one branch must meet the {010} lattice planes of the other branch at the intersection. With regard to the orthorhombic CaTiO3 with space group Pnma, the real orthorhombic unit cell can be schematically shown as Figure 4f, in which the area depicted with bold lines is known as the pseudocubic perovskite subcell, the size of which is calculated to be 3.828 Å according to XRD data (Supporting Information). If the orthorhombic unit cell is regarded as a √2 × √2 × 2 superunit cell based on the cubic subcell, there is a cubic-to-orthorhombic transformation of the Miller indices: (100)c to (101)o, (110)c to (200, 121)o, (111)c to (022, 220)o, (200)c to (040)o, etc., where the subscripts c and o denote the cubic perovskite subcell and the orthorhombic unit cell, respectively. That is, the {101} lattice planes of the orthorhombic CaTiO3 are, in fact, corresponding to the {100}c lattice planes of the cubic CaTiO3. Therefore, the six side surfaces of a rectangular branch can be regarded as the {100} lattice planes of the cubic CaTiO3 so that there are identical lattice planes in the three perpendicular directions (Figure 4g). Therefore, the identical crystal lattices of the cubic CaTiO3 can be shared by the perpendicular branches at the intersection. Besides these, it should be noted that, different from the HRTEM experiment as shown in Figure 3, we failed to obtain clear HRTEM images to see the detailed crystal structure by directing the incident electron beam along this direction ([101̅] zone axis), because the tube wall observed from this direction is much thicker than that from the [001]̅ direction (Supporting Information), which, on the contrary, further confirm the structures as schematically illustrated in Figure 4c−e. 3.2. Growth Mechanism of the CaTiO3 MSs. In order to investigate the growth mechanism of the CaTiO3 MSs, the growth process was monitored using XRD and FESEM. According to the XRD analysis, the chemical reactions were almost completed within 8 h (Supporting Information), but the crystal growth was found to continue until the synthesis time reached 12 h. Upon adjusting some synthesis conditions (Supporting Information), a kind of interesting quatrefoil-like MSs, consisting of four edges with 4-fold symmetry, was successfully observed as the synthesis had proceeded for 8 h (marked with red circles in Figure 5a). When the synthesis had proceeded for 12 h, a number of 1D chapped tubes (Figure 5b) together with a few of the tubes with smooth side surfaces (inset in Figure 5b) were obtained. This indicates that the quatrefoil-like MSs grew into the 1D chapped tubes with an increase of the synthesis time. To understand the crystal growth of the 1D chapped tubes, we return to the structure of the tube clusters as mentioned in Figure 1d. Compared to the 1D chapped tubes, the 1D side-byside type tube clusters have an open intersection (Figure 5e), clearly showing that the four parallel tubes are symmetrically attached to a center particle (as indicated by white arrows in Figure 5e). Similarly, the 3D perpendicular type tube cluster also has a center particle (as indicated by white arrows in Figure 5f), on which there are four parallel tubes symmetrically

Figure 5. (a, b) FESEM images of the samples obtained after the synthesis had proceeded for 6 and 10 h, respectively. (c, d) The magnified FESM images of the 1D chapped tubes viewed from the side and the top, respectively. (e, f) The magnified FESEM images of the 1D side-by-side type and the 3D perpendicular type tube clusters, respectively.

attached in each of the 3D perpendicular directions. The 1D tubes (smooth surfaces), chapped tubes, and side-by-side type tube clusters had experienced the same growth mechanism because they were obtained by the same route. On the basis of these observations and all the analyses described above, the growth mechanism of CaTiO3 MSs is schematically illustrated in Figure 6. As shown in Figure 6a, the crystal growth process of the MSs can be divided into two stages, i.e., the growth of the center particles (Stage 1), followed by the epitaxial growth on the center particles (Stage 2). At the initial stage, the hydrolysis of TNB led to the formation of Ti(OH)4 and TiO2 (Supporting Information). Meanwhile, Ti(OH)4 and TiO2 reacted with OH− to produce TiO2(OH)22−, HTiO3−, and TiO32−, which further reacted with Ca2+ and OH− to produce CaTiO3. Thus, the formation of CaTiO3 can be simply expressed by an overall equation: Ca 2 + + Ti(OH)4 + 2OH− = CaTiO3 + 3H 2O

(1)

With the nucleation and growth of the crystal, CaTiO3 nanoparticles were formed. Under sovlothermal conditions, the CaTiO3 nanoparticles underwent an Ostwald ripening process, during which some nanoparticles further aggregated into large particles by oriented attachment (Stage 1).9 The center particle has the feature of a single crystal as depicted by the red cube in Figure 6a, which is important for the growth of the regular geometrical MSs. However, its shape is still irregular; that is, its side surfaces are not bound by the stable {010} and {101} planes, thus remaining reactive areas (sites) for further crystal growth. 3141

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The growth of the tubular and nontubular MSs is attributed to the interaction between the growth rate and the mass transport rate.20 When the growth rate of crystals greatly exceeds the mass transport rate of the ions that feeds the growing crystals, the anisotropic growth feature of the crystals will be fully exhibited. The fast growth of the four edges will greatly reduce the concentrations of the species (TiO2(OH)22−, HTiO3−, etc.) in the bulk solution, and mass transportation to the growing edges leads to the unsaturation in the center of the four edges, thus resulting in a hollow structure (tubular structure), which can be clearly seen from the quatrefoil-like particles and the 1D chapped tubes (Figure 5a,d). On the other hand, under the thermal treatment, the crystal growth process is accompanied by an Ostwald ripening process, which will result in an extension of recrystallization from the inner surface to the tube wall, thus enhancing the formation of the tubular structure. The formation of various tubes can be simply summarized in Figure 6b,c: if the four edges together with the four side surfaces grow synchronously, rectangular tubes with smooth surfaces are obtained (Figure 6I); if the four edges grow independently, thus resulting in a fissure along the axis in each side surface, the chapped tubes (Figure 6II), including the branches with relatively large cross section of the 2D and 3D tubes as shown in Figure 1b,c, are obtained; if the four edges in one direction are completely separated, each of them further evolves into a tubular structure, thus resulting in the formation of the tube clusters (Figure 6III, VI). In addition, controlled by the mass transportation, the growth of the inside edges of the tube clusters was limited, thus resulting in the defective edges as indicated by the white arrows in Figure 6III and VI. Compared to the tubular CaTiO3 MSs, the nontubular MSs were synthesized at elevated NaOH concentration (about several times that for the synthesis of the tubular MSs). NaOH is, in fact, one of the reactants. Increasing its concentration will greatly increase the formation rate of TiO2(OH)22−, HTiO3−, etc. That is, the concentrations of these species for the synthesis of the nontubular MSs are much higher than those for the synthesis of the tubular CaTiO3 MSs. In this case, both the crystal growth rate and the mass transport rate are not governed by the concentrations of these species any longer. The mass transport rate of these species is high enough to feed the growing crystals until the stable surfaces are achieved (bound by the {010}, {101}, and {101̅} planes).20 In addition to NaOH, EtOH/H2O is another main factor that influences the formation of the CaTiO3 MSs by affecting the dimensions of the products. To obtain the regular geometrical CaTiO3 MSs, the optimal EtOH/H2O is between 1:4 and 4:1. Adjusting EtOH/H2O can change the polarity of the solvent, the solubility of the starting materials, and even the mass transport rate, the interplays of which result in the evolution of the products from one dimension to three dimensions with increasing EtOH/H2O. Besides these, other synthesis conditions such as reaction temperature, reaction time, and the concentrations of the starting materials are also important for the formation of these CaTiO3 MSs under the solvothermal conditions. However, the exact effect of these synthesis parameters on the formation of the regular geometrical CaTiO3 MSs is worthy of further investigation.

Figure 6. Schematic illustrations of the growth mechanism of various CaTiO3 tubes: (a) the growth process of the tubular structure; (b) the formation of various 1D tubes (I−III); (c) the formation of 2D or 3D tubes (IV−VI). The blue double arrows in (a) indicate the formation of sharp edges resulting from the fast stacking interplay of the {111} planes.

At the second stage, the MSs are formed by a high-symmetry epitaxial growth process on the basis of the center particles. From a kinetics point of view, the different stacking rate on the surfaces of a crystal will determine its anisotropic growth. As a member in the titanate-based perovskite (A2+B4+O3), the {111} surfaces of CaTiO3 are polar and reactive because of the alternating charged planes of CaO3 and Ti,18 thus leading to fast stacking in this direction. As calculated by Lee’s group,19 the {111} surfaces of CaTiO3 are particularly reactive because of the larger charge density of Ca2+. The fast stacking interplay of the (111) and (111̅) planes (as indicated by the black arrows in Figure 6a) consequentially results in the fast formation of a sharp edge in [010] orientation (the blue double solid arrow in Figure 6a), because the edge is on the junction of the two planes and its growth is simultaneously enhanced by the two planes. Because of the high symmetry of the {111} planes in the center particle, the other three edges are symmetrically formed by each two of other {111} planes in the same direction (the three blue double dotted arrows in Figure 6a). As a result, a regular rectangular structure, i.e., the four parallel edges together with the four side surfaces bounded by {101} lattice planes, is conferred on all the MSs. On the other hand, from the crystal structure point of view, the identical [010] growth orientation of the branches of the 2D and 3D MSs corresponds to the [100] orientation of the cubic CaTiO3 according to the transformation of the Miller indices. Therefore, the growth of the MSs can be regarded as the epitaxial growth along the {100} planes of the cubic CaTiO3 on the center particles. Because of the high-symmetry {100} planes of the cubic CaTiO3, the epitaxial growth is strictly limited in the three perpendicular directions, thus further conferring a perfect perpendicular structure on the 2D and 3D MSs. Interestingly, the preferential [010] growth orientation of the crystal as mentioned in the XRD and HRTEM analyses is attributed to the fast stacking of the {111} planes rather than the preferential growth of the (010) plane itself.

4. CONCLUSION In summary, high-quality CaTiO3 MSs with regular geometrical morphologies and tunable dimensions have been, for the first time, achieved on a large scale under simple solovthermal 3142

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(17) Yang, J.; Geng, B. Y.; Ye, Y. X.; Yu, X. CrystEngComm. 2012, 14, 2959−2965. (18) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, UK, 1994; pp 38−44. (19) Chen, P. C.; Tsai, M. C.; Huang, Y. J.; Chiub, H. T.; Lee, C. Y. CrystEngComm 2012, 14, 1990−1993. (20) López, C. M.; Choi, K. S. Langmuir 2006, 22, 10625−10629.

conditions. The intrinsic crystal structure of orthorhombic CaTiO3 leads to a high-symmetry epitaxial growth of the crystals. The fast stacking interplay of the {111} planes results in a rectangular structure (cross-section), and the epitaxial growth limited in the three perpendicular directions forms a perpendicular structure, thus generating the MSs with regular geometrical morphologies. Although more detailed investigations concerning the formation process of these MSs are still needed, this strategy will offer great opportunities to design perovskite crystals with desired morphologies concerning fundamental scientific study, to explore the dependence of the properties of the materials on the morphologies, and even to manufacture potential nano- and microdevices.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details for the MSs synthesis, related characterization, and supplementary results and discussion. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Nature Science Foundation of China (NSFC, Grant No. 51072227). REFERENCES

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