(M = Ca, Sr, and Ba) Perovskite Oxides - ACS Publications - American

Feb 24, 2009 - Based on titanate nanofiber reactivity, ternary MTiO3 (M = Ca, Sr, and Ba) perovskite oxides with specific morphologies have been fabri...
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J. Phys. Chem. C 2009, 113, 4386–4394

Titanate Nanofiber Reactivity: Fabrication of MTiO3 (M ) Ca, Sr, and Ba) Perovskite Oxides Y. Li,† X. P. Gao,*,† G. R. Li,† G. L. Pan,† T. Y. Yan,† and H. Y. Zhu*,‡ Institute of New Energy Material Chemistry, Nankai UniVersity, Tianjin 300071, China, and School of Physical and Chemical Sciences, Queensland UniVersity of Technology, Brisbane, Qld 4001, Australia ReceiVed: December 8, 2008; ReVised Manuscript ReceiVed: January 23, 2009

Titanate nanofiber, as an important morphology of one-dimensional nanostructured titanate, not only can be regarded as the final product derived from alkaline-hydrothermal treatment of bulk TiO2 but also can serve as initial reactant for fabricating complex functional materials with their parent morphology, which may be difficult to achieve using their bulk material counterparts under mild hydrothermal conditions. Based on titanate nanofiber reactivity, ternary MTiO3 (M ) Ca, Sr, and Ba) perovskite oxides with specific morphologies have been fabricated in alkaline solution at low temperature via a soft chemical route. The resulting CaTiO3 products possessed a microtubular structure with rectangular cross-section, while SrTiO3 and BaTiO3 showed the assemblies consisting of aggregated particles in a compact fashion. On the basis of the experimental results, we have proposed two types of growth mechanisms to elucidate the formation processes of CaTiO3, SrTiO3, and BaTiO3 microstructures, respectively. The fabrication of microtubular CaTiO3 undergoes the initial dissolution of titanate nanofibers by Ostwald ripening process to convert into micrometer-sized fiber-bundles, while recrystallization occurs simultaneously until tubular microstrucures are obtained. The formation of SrTiO3 and BaTiO3 microstructures involves ion exchange reaction and in situ growth process at the self-sacrifice of titanate nanofibers framework based on the chemical reactivity. In addition, the photoelectrochemical properties of the as-obtained products are presented, and CaTiO3 microtubes exhibit better photoeletrochemical response relative to SrTiO3 and BaTiO3 microstructures. I. Introduction Exploiting diverse strategies for tunable construction of onedimensional building blocks into functional devices becomes a major research area in materials science and nanotechnology. In particular, the fabrication of nanostructured TiO2 has received extensive attention owing to its wide-ranging applications as photoelectrochemical devices,1,2 photocatalysts,3,4 lithium-ionbattery materials,5,6 and intelligent absorbents for radioactive ions,7 as well as the high activity involved in fabricating delicate composites. For example, titanate nanofibers can be readily converted into anatase and rutile TiO2 nanoparticle polymorphs via different mechanisms in a dilute acid solution.8 Nanocomposites of TiO2/H2Ti5O11 · H2O and ZnO/H2Ti5O11 · H2O were obtained in aqueous mediums, in which TiO2 and ZnO grew on the surface of the protonated pentatitanate nanobelts.9 Hierarchical nanostructures of V2O5 nanorods standing on rutile nanofibers were produced by electrospinning and an additional calcination process.10 As compared to TiO2 bulk materials, titanate nanostructures exhibit a inherently chemical reactivity, which is beneficial for designing complex titanate-based composites. Herein, we aimed at the synthesis of alkaline earth titanates MTiO3 (M ) Ca, Sr, and Ba) with adjustable morphology by utilizing the high activity of titanate nanofibers. Alkaline earth titanates MTiO3 (M ) Ca, Sr, and Ba) with a cubic perovskite structure have been investigated intensively due to their unique dielectric, piezoelectric, and ferroelectric properties, which are of great interest in the technological * To whom correspondence should be addressed. E-mail: xpgao@ nankai.edu.cn; [email protected]. † Nankai University. ‡ Queensland University of Technology.

applications such as capacitors, transducers, actuators, and nonvolatile random-access memory devices.11 Moreover, SrTiO3 is an important n-type semiconductor with band gap of about 3.2 eV.12 The stability, wavelength response, and current-voltage of SrTiO3 make it a promising candidate for efficient photocatalysts13 and photoelectrodes14,15 for splitting water into hydrogen and oxygen. Numerous studies have been focused on modifying SrTiO3 with transition metal ions16 and nitrogen17 for the visible light response, instead of generating the photocatalytic activity under ultraviolet light. Spatial charge separation of photochemical-induced oxidized and reduced products can occur on the surface of ferroelectric BaTiO3, which may decrease the probability of recombination of the photogenerated charge carriers.18 Due to the lack of interest in practical application, CaTiO3 has not been widely developed. It usually acts as a compositional substitute in other perovskite oxides for controlling the ferroeletctricity.19 Other properties, such as red luminescence of Pr3+-doped CaTiO320 and electro-mechanooptical conversion in BaTiO3-CaTiO3 multifunctional ceramics, have also been reported.21 Stimulated by fundamental scientific study and technological application, MTiO3 (M ) Ca, Sr, and Ba) have been synthesized through a variety of methods, including conventional solid-state reaction, hydrothermal synthesis,22 sol-gel method,23 inverse micelle microemulsion method,24 and molten salt synthesis.25 Alternatively, biological synthetic technique26 and nonaqueous route27 have been newly developed for the preparation of nanocrystalline BaTiO3 and SrTiO3. Recently, one-dimensional ferroelectric materials like nanowires and nanotubes have emerged as an important conformation. Single-crystalline BaTiO3 and SrTiO3 nanorods have been accomplished by a

10.1021/jp810805f CCC: $40.75  2009 American Chemical Society Published on Web 02/24/2009

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Figure 1. SEM image (a) and TEM image (b) of titanate nanofibers.

solution-based decomposition of bimetallic alkoxide precursors or by ion-exchange reaction.28 BaTiO3 and SrTiO3 nanotubes with an inner diameter of ∼4 nm an outer diameter of ∼8 nm have been generated from a hydrothermal reaction by using titanium oxide nanotubes as a precursor.29 Bundles of BaTiO3 nanotubes have been formed using alumina template membranes with sol-gel techniques.30 BaTiO3 nanowires and SrTiO3 nanocubes have been achieved by a one-step solid-state chemical reaction in the presence of NaCl and a nonionic surfactant.25 Hydrothermal treatment of the layered titanate in Ba(OH)2 aqueous solution resulted in a variety of nanostructured BaTiO3.31 With regard to CaTiO3, there were quite limited methods employed for the fabrication of nanostructured CaTiO3, except for the nanolamellate CaTiO3 in situ on the titanium substrate.32 Therefore, it would be highly desirable to develop a general approach to fabricate alkaline earth titanates with controllable size and morphology. In this work, we have successfully prepared the microtubes of CaTiO3 and the fiber-like assemblies of BaTiO3 and SrTiO3 via a facile hydrothermal treatment, mainly depending on the high reactivity of titanate nanofibers with different metal ions. Two possible formation mechanisms have been discussed. 2. Experimental Section Sample Preparation and Characterization. Titanate nanofibers were prepared from rutile TiO2 powder in concentrated NaOH solution. Typically, the rutile powder was mixed with 10 M NaOH solution under vigorous stirring for two hours. The mixture was then autoclaved at 180 °C for 48 h. The

Figure 2. SEM images of the as-obtained products at (a) 100 °C, (b) 110 °C, (c) 120 °C, and (d) 150 °C for 24 h in 1.0 M NaOH and (e) in 0.5 M NaOH, (f) in 2.0 M NaOH, and (g) in 5.0 M NaOH at 150 °C for 24 h.

precipitates recovered from the mixture were rinsed with distilled water and 0.1 M HCl solution until a pH value about 8 was reached. The sample was dried at 100 °C for 24 h and used as the reactant in the following step. Subsequently, taking the synthesis of CaTiO3 microtubes as example, the required amount of titanate nanofibers was dispersed into 50.0 mM CaCl2 aqueous solution (35 mL). The pH value of the mixture was adjusted to about 13 with 1.0 M NaOH solution (10 mL), and the white suspension was transferred into a Teflon-lined stainless steel autoclave and maintained at 150 °C for 24 h. The product was neutralized using 0.1 M HCl solution, washed with distilled water, and dried at 100 °C for 24 h. The microfiber-like assemblies of SrTiO3 and BaTiO3 were fabricated by analogy with the synthesis procedure of CaTiO3 microtubes. The morphology and phase transformation of the resultant samples were characterized by scanning electron microscopy (SEM, Hitachi S-3500N), transmission electron microscopy (TEM, FEI Tecnai 20) with an acceleration voltage of 200 kV, and X-ray diffraction (XRD, Rigaku D/max-2500) with Cu K radiation, respectively. The UV-visible spectra of the samples were recorded on a Varian Cary 5E UV-vis-NIR spectrophotometer. Photoelectrochemical Performance. To measure the photoelectrochemical properties, perovskite titanate slurry was prepared by slowly evaporating ethonal solution of MTiO3 (M ) Ca, Sr, and Ba). The different titanate thin films were coated onto the conducting glass substrates (FTO) using a doctore

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Figure 5. XRD patterns of the as-obtained CaTiO3 products in different alkaline concentrations: (a) 0.5 M NaOH, (b) 1.0 M NaOH, and (c) 2.0 M NaOH at 150 °C for 24 h.

Figure 3. TEM images of CaTiO3 microtubes obtained at different reaction temperatures for 24 h in 1.0 M NaOH: (a and b) 110 °C, (c and d) 120 °C, and (e and f) 150 °C. Figure 6. XRD patterns of (a) titanate nanofibers and of the as-prepared products with different barium concentrations: (b) [Ba2+] ) 2.5 mM, (c) [Ba2+] ) 5.0 mM, (d) [Ba2+] ) 10.0 mM, (e) [Ba2+] ) 20.0 mM, (f) [Ba2+] ) 30.0 mM, and (g) [Ba2+]) 50.0 mM.

substrate as working electrode, and a platinum electrode by sputtering on the conducting glass as counter electrode. The electrolyte solution consisted of 0.5 M LiI, 0.5 M 4-tertbutylpyridine, and 0.05 M I2. Photocurrent-voltage characteristics were measured using Zahner CIMPS-2 system by irradiating the sample with simulated solar light at the output intensity of AM 1.5 (A 500 W xenon lamp). The active cell area was 1 cm2. Electrochemical impedance spectroscopy (EIS) experiments were also conducted using a Zahner IM6ex electrochemical workstation at the open circuit potential with 5 mV amplitude of perturbation in the frequency range of 10 kHz-10 mHz under the illumination of AM 1.5 100 mW/cm2. Figure 4. XRD patterns of titanate nanofibers and the as-obtained CaTiO3 products at different reaction temperatures for 24 h in 1.0 M NaOH.

blading technique. The films were sintered at 450 °C for 30 min after drying in air at room temperature. Dye-sensitized titanate films were obtained by 12 h immersion in an ethanolic solution of cis-Di(thiocyanate)bis-(2,2-bipyridyl-4,4-dicarboxylate)-ruthenium(II) (N3). A sandwich-type solar cell was comprised of perovskite titanate films coated on conducting glass

3. Results and Discussion 3.1. Formation of CaTiO3 Microtubes. Figure 1 shows SEM and TEM images of the precursor titanate nanofibers prepared by the hydrothermal treatment of rutile powders. A large number of isolated straight nanofibers of about 100 nm in diameter and several micrometers in length were obtained. The surface of nanofibers is smooth. The X-ray diffraction pattern of titanate nanofibers (Figure 4, bottom) has the diffraction peaks similar to those of protonated titanates with a

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Figure 7. XRD patterns of (a) titanate nanofibers and of the as-prepared products with different strontium concentrations: (b) [Sr2+] ) 2.5 mM, (c) [Sr2+] ) 5.0 mM, (d) [Sr2+] ) 10.0 mM, (e) [Sr2+] ) 30.0 mM, and (f) [Sr2+] ) 50.0 mM.

monoclinic lattice (C2/m),33 which has a relatively poor crystallinity and high reactivity. Microtubular inorganic materials, such as SnO2,34 MgO,35 NaHoF4,36 BiVO4,37 andaluminum-richmulliteAl2(Al2+2xSi2-2x)O1038 δ, have been prepared by using various methods. The fabrication of CaTiO3 microtubes with rectangular cross-section under hydrothermal condition is interesting, based on the high reactivity of titanate nanofibers. SEM and TEM were employed to examine the typical morphology and structure of CaTiO3 microtubes obtained at 150 °C for 24 h in 1.0 M NaOH solution. The as-prepared products (Figure 2d) are composed of a large quantity of microtubes with widths in the range of 200∼500 nm and lengths of 0.5-2 µm, which is significantly larger than the precursor titanate nanofibers. The representative TEM images of CaTiO3 products are given in Figure 3, panels e and f, which clearly show the tubular structure. The individual CaTiO3 microtube has a rectangular cross-section with the wall thickness of about 80 nm. XRD pattern indicates that the asobtained microtubes are in good agreement with an orthorhombic lattice for CaTiO3 (JCPDS No. 81-561). To further understand the formation process of microtubular CaTiO3, we studied the effects of reaction temperature and alkaline concentration, as presented in Figure 2-5. By applying the reaction temperature ranging from 100 to 180 °C, we found that higher hydrothermal temperature was favorable to the morphology evolution from titanate nanofibers to the resultant microtubes. When the hydrothermal temperature was kept at 100 °C, the main phase was the starting titanate nanofibers. SEM image (Figure 2a) indicates that although the fiber-like structures are preserved, the as-prepared products become apparently shorter and thicker in contrast to titanate nanofibers. As the temperature increased to 110 °C, the trace of CaTiO3 with a poor crystallinity was observed in SEM images (Figure 2b) and XRD patterns (Figure 4). It is noticeable that the predominant products are nanofiber bundles up to several micrometers with a jagged tip, in which the tip of bundles consists of smaller needle-like nanofibers with diameter about 20-30 nm, as demonstrated in Figure 3a,b. Moreover, nanoparticles with diameter ranging from 20 to 100 nm appear near the tips. At 120 °C, titanate nanofibers have transformed into the tubular structure with a larger scale, whereas small amount of fiberbundles also existed in some regions. In this growth stage, the as-prepared CaTiO3 microtubes possessed various morphologies

Figure 8. SEM image (a) and TEM images of the as-prepared BaTiO3 products with different barium concentrations: (b) [Ba2+] ) 2.5 mM, (c) [Ba2+] ) 5.0 mM, (d) [Ba2+] ) 10.0 mM, (e) [Ba2+] ) 30.0 mM, and (f and g) [Ba2+] ) 50.0 mM. (pure BaTiO3).

(Figure 2c). It can be seen that some microtubes grow individually with regular square cross-section and others share the multisides to form the irregular polygon open-ends. In some cases, two or more CaTiO3 tubules stick together to construct two-dimensional arrangements. TEM images reveal the important information that some nanoparticles of about 20-80 nm occasionally attach inside and outside the microtubes, which is similar to those nanoparticles existing at the tip of fiber-bundles (Figure 3c,d). The phenomenon is consistent with Ostwald ripening process,39 in which larger particles are formed at expense of the smaller particles owing to their high reactivity. As temperature increasing to 150 °C, CaTiO3 microtubes became the prevalent products and exhibited a good crystallinity. There is no obvious change of the structure and composition of microtubes at 180 °C for 24 h. Therefore, the hydrothermal temperature is an essential factor for the formation of CaTiO3 tubular microstructure and for the increase of the crystallinity.

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Figure 10. Photocurrent-voltage curves of the cells comprised of dyesensitized MTiO3 (M ) Ca, Sr, and Ba) with various morphologies under Xe lamp irradation (100 mW/cm2).

Figure 9. SEM image of (a) and TEM images of (b-d) the as-prepared SrTiO3 microstructures.

In the hydrothermal synthesis, alkaline concentration can be used effectively for well-controlled growth of nanostructures with desired morphology and size.40 We have adjusted the alkaline concentration to examine its effect on the morphology evolution under the temperature of 150 °C. At the low alkaline concentration of 0.5 M NaOH, the as-obtained products are the mixtures of titanate nanofibers and CaTiO3 columns without open ends, indicating the occurrence of the morphology transformation. The increase of alkaline concentration to 2.0 M resulted in the mixture of microtubes and microcubes of CaTiO3, while the complete conversion of tubular structure to cubic CaTiO3 was achieved at the concentration of 5.0 M, as shown in Figure 2f,g. The surface area accordingly decreased from 9 m2/g to 3 m2/g as the morphological transformation from the hollow tube to the solid cube. The alkaline concentration plays an important role in utilizing the reactivity of starting nanofibers and controlling the morphology of the resulting CaTiO3. The optimal alkaline concentration for the formation of microtubes was 1.0 M at the temperature of 150 °C. 3.2. Formation of SrTiO3 and BaTiO3 Fiber-Like Microstructures. To reveal the reactivity and structure transition of the starting nanofibers to fabricate fiber-like BaTiO3, XRD patterns of the as-obtained products treated by different barium concentration were illustrated in Figure 6. When Ba2+ concentration was lower than 10.0 mM, the intensity of diffraction peaks of titanate nanofibers after hydrothermal treatment became weaker. By keeping the Ba2+ concentration at 10.0 mM, the pattern shows the coexistence of two phases of titanate nanofibers and BaTiO3. The peaks at 22.1, 31.5, and 38.8° can be indexed to (100), (110) and (111) planes of BaTiO3, suggesting the formation of BaTiO3 crystals. As the Ba2+ concentration reached 30.0 mM, BaTiO3 becomes the dominant phase with a trace of titanate nanofibers. The diffraction peaks of BaTiO3 became stronger and sharper, indicating the high crystallinity with increasing the concentration under hydrothermal treatment condition. With the concentration up to 50.0 mM, all of the diffraction peaks of the sample (Figure 6g) can be ascribed to the cubic structure of perovskite BaTiO3 (JCPDS Card No. 75-212). On the basis of XRD patterns in the Figure 7, the structure change of microstructured SrTiO3 shows the similarity to the growth patterns of BaTiO3, in which the relative intensity

Figure 11. Nyquist plots of the electrochemical impedance spectra (EIS) of different MTiO3 (M ) Ca, Sr, and Ba) DSSCs.

of SrTiO3 peaks increased gradually, accompanying with the disappearance of titanate nanofibers. All characteristic peaks can be indexed to the cubic structure of perovskite SrTiO3 (JCPDS Card No. 79-174). In conclusion, the precursor titanate nanofibers show a good chemical reactivity as the self-sacrifice templates to fabricate BaTiO3 and SrTiO3 microstructures by adjusting the concentration. The morphology evolution of products obtained at different reaction stages was examined by TEM images (Figure 8). At the low concentration of Ba2+ ([Ba2+] ) 2.5 mM), the samples essentially retained fiber-like nanostructures. Interestingly, it is shown in large magnification images (Figure 8b) that titanate nanofibers are embedded with some isolated stripes, which are quite different from the precursor with smooth surface. Such morphology is similar to Ag-doped TiO2 nanowires or Co-doped anatase nanorods.41 The HRTEM images (inset in Figure 8b) display that the interplanar spacing is about 0.36 nm, in accordance with the interplanar distance of the (101) plane of titanate nanorods.42 Thus, the phase and morphology of the precursor does not change after hydrothermal treatment at relatively low Ba2+ concentration. Ba2+ ions can be trapped irreversibly in the interlayer region of the nanofibers with the formation of heterogeneous stripes.7 With the increase of Ba2+ concentration to 5.0 mM, those titanate nanofibers with stripes

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Figure 12. SEM images of as-obtained products at different intervals of reaction time: (a) 2 h; (b) 4 h; (c) 6 h; (d) 12 h at 150 °C in 1.0 M NaOH.

Figure 14. XRD patterns of as-obtained CaTiO3 products at different intervals of reaction time: (a) 2 h; (b) 4 h; (c) 6 h; (d) 12 h at 150 °C in 1.0 M NaOH.

Figure 13. TEM images of as-obtained products at different intervals of reaction time: (a) 2 h; (b) 4 h; (c) 6 h; (d) 12 h at 150 °C in 1.0 M NaOH.

increased visibly. At the higher concentration ([Ba2+] ) 30.0 mM), the samples are the mixture of the stripes-decorated titanate nanofibers and the branch-like BaTiO3 microstructures (Figure 8e), which is responsible for the emergence of diffraction peaks of BaTiO3 in XRD patterns. As the reaction proceeded and Ba2+ concentration further increased to 50.0 mM, there are no starting precursors detected in the final product. SEM images in Figure 8a show the formation of bundles of BaTiO3 microstructures, which possessed fiber-like structure with a convex surface. Detailed structures reveal that the length of

BaTiO3 microstuctures can be as long as tens of micrometers and the width is about 200-500 nm in general. Actually, fiberlike BaTiO3 microstructures consisted of many irregular particles with the size about 100-200 nm. The nanoparticles connected tightly in growing process, and the interfaces between them can be distinguished clearly (inset in Figure 8f). In addition, the branch-like microstructures were formed due to the random orientations of nanoparticles (Figure 8g). Such similar morphology also exists in SrTiO3 products (Figure 9). Notably, there is a slight difference in the delicate structures of SrTiO3 microstructures, in which the connection of SrTiO3 particles are looser than that of BaTiO3 particles. The calculated lattice spacing in the HRTEM image (inset in Figure 8d) is about 0.278 nm in agreement with the interplanar distance of the (110) plane of cubic perovskite SrTiO3.

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Figure 15. Schematic illustration of the possible formation mechanism of MTiO3 (M ) Ca, Sr and Ba) perovskites with various morphologies.

3.3. Photoelectrochemical Properties. The UV-visible spectra of titanate nanofibers, as well as those of the CaTiO3 microtubes are presented in the Supporting Information (Figure S1). The microtubular CaTiO3 has similar absorption edges as titanate nanofibers with enhanced intensity to a certain extent. The UV-visible absorption spectra of BaTiO3 series and SrTiO3 series are shown in Figure S2 and S3, respectively. When the Ba2+ concentration is lower than 10.0 mM, the as-obtained products absorb UV light with wavelength below 350 nm, whereas the absorption edges of the products shift to a longer wavelength with increasing Ba2+ concentration to 50.0 mM, at which concentration BaTiO3 formed. Both barium titanate and strontium titanate are indirect semiconductor.43 The band gap of the resultant SrTiO3 and BaTiO3 microstructures can be estimated from the plot of transformed Kubelka-Munk function versus the photon energy.44 The tangent lines indicate the band gaps of 2.96 eV for BaTiO3 and 3.01 eV for SrTiO3 microstructures (Figure S4), which are nearly 0.24 and 0.19 eV smaller than those of bulk materials. In order to investigate the photoelectrochemical performance of MTiO3 (M ) Ca, Sr, and Ba) titanate cells, we measured the photocurrent-voltage characteristics and electrochemical impedance spectroscopy (EIS) under simulated sunlight AM 1.5 illumination conditions (100 mW/cm2). Figures 10 and 11 show the J-V curves and EIS plots of the cells composed of microtubular CaTiO3, fiber-like microstructrured SrTiO3, and BaTiO3, respectively. The dye-sensitized MTiO3 (M ) Ca, Sr, and Ba) showed much higher open-circuit voltage (Voc) and short-circuit current density (Jsc) than those of corresponding materials without dye-sensitization. The Voc and Jsc of the dyesensitized cells increased in the order with CaTiO3 > BaTiO3 > SrTiO3. Especially, the microtubule CaTiO3 DSSC exhibits relatively a good photoelectrochemical performance, with the open-circuit voltage of 0.7 V and the current density of 0.26 mA/cm2. The EIS is a powerful tool for investigating the surface state involving charge transfer and recombination of dyesensitized solar cells. Comparable with EIS spectra of TiO2 DSSCs,45 there are no the semicircle in the high frequency region, related to the impedance of Pt counter electrode, and

the semicircle in the low frequency region, associated with Warburg impedance of the diffusion of tri-iodide in electrolyte. The spectra of MTiO3 (M ) Ca, Sr, and Ba) DSSCs are similar with only one semicircle is presented in EIS spectra, indicating that the charge transfer process across the MTiO3/dye/electrolyte interfaces is the dominant reaction in the MTiO3 electrodes. Moreover, the diameter of the semicircle decreases gradually from SrTiO3 to CaTiO3, in good agreement with the photoelectrochemical activity of MTiO3 DSSCs as shown above. It is noted that the kinetics for the photoelectrochemical reaction is dependent on the configuration confinement of electrode materials, especially for charge carrier diffusion processes. Compared with zero-dimensional particles, the one-dimensional configuration can effectively reduce the contact resistance among grain boundaries to improve electron transport, leading to increasing photocurrent and conversion efficiency.46 It is believed that the photoelectrochemical performance of MTiO3 DSSCs may be further improved by optimizing the size and morphology of these perovskites. 3.4. Possible Formation Mechanism. The possible mechanisms were proposed to clarify the growth process of MTiO3 (M ) Ca, Sr, and Ba) with various microstructures (Figure 15). On the basis of the experimental results, we can infer that the formation of CaTiO3 microtubes and fiber-like MTiO3 (M ) Sr and Ba) assembly share different growth mechanisms. Unlike the case of rolling-up growth mechanism responsible for rectangular WO3 · H2O nanotubes formation,47 Ostwald ripening process would play a crucial role in the formation of microtubular CaTiO3 because the diameters of final microtubes are noticeably larger than those of nanofiber precursors. In order to explore the intrinsic growth mechanism of CaTiO3 microtubes, time-dependent experiments were carried out by keeping the reaction condition at 150 °C. SEM and TEM analyses (Figures 12 and 13) present the morphological evolution of intermediate products obtained after 2, 4, 6 and 12 h. Shortly after reaction duration for 2 h, it was observed that the precursor titanate nanofibers had a strong tendency to aggregate into fiberbundles with width obviously increasing to 0.2-0.5 µm, in good agreement with the fact that larger nanorods grew at the expense

Titanate Nanofiber Reactivity of individual nanotubes by Ostwald ripening process.42 Some fiber-bundles show regular ends, while others have V-shaped tips, likely due to the different dissolving rate in the alkaline solution. Prolonging the reaction time to 4 h, there was coexistence of partially converted microtubes with irregular open-ends and fiber-bundles with a coarse end. As the reaction proceeded to 6 h, the amount of incomplete microtubes with a concave cavity increased, accompanying with the microsized fiber-bundles. After the reaction time to 12 h, the partially transformed microtubes predominately existed with a trace of fiber-bundles. More interestingly, we observed that intermediate microtubes transformed into hollow cavity by eroding the fiberbundles at the open end of inside microtube (inset in Figure 13d). Based on the above observation, we infer that micrometersized fiber-bundles serving as the intermediate products, first grow larger at the consumption of smaller nanofibers, and recrystallization occurred simultaneously48 until the hollow microtubes were obtained. However, the exact growth mechanism of CaTiO3 microtubes with rectangular cross-section needs to be further explored. Another possible mechanism is proposed to understand the formation of the assembly of SrTiO3 and BaTiO3 microstructures. BaTiO3 (or SrTiO3)15 shares more common structure with titanate nanofibers, which contain the TiO6 octahedra by edge sharing. The layered titanates have the capability for ion exchange with good structure retention.49 In our experiment, Ba2+ (or Sr2+) ion, acting as substituted/trapped ions, can preferentially enter into the TiO6 lattice and undergo the exchange with H+ ion under the alkaline condition. Such ion exchange induces the generation of chemical reactive sites, corresponding to the strips embedded in titanate nanofibers. With increasing Ba2+ ion concentration, a large quantity of chemical reactive sites were formed and finally led to the occurrence of the phase transition to BaTiO3. Gradually, BaTiO3 particles grew larger in situ at the self-sacrifice of titanate nanofibers framework based on the chemical reactivity and aggregated with each other to constitute the fiber-like assembly. 4. Conclusion In summary, series of alkaline earth titanate with specific morphology on a large scale have been successfully prepared through hydrothermal treatment, based on the titanate nanofiber reactivity. Namely, CaTiO3 microtubes with a rectangular or polygen-shaped open ends and SrTiO3/BaTiO3 microstructures composed of nanoparticles were obtained in basic solution at 150 °C for 24 h. The experimental results indicate that the parent titanate nanofibers involved in the reaction exhibit high chemical reactivity as starting reactant. The ion concentration, reaction temperature and alkaline concentration play crucial roles in the phase transition and morphology evolution. The optical and photoelectrochemical properties of the as-obtained products were investigated by photocurrent-voltage measurement and electrochemical impedance spectroscopy. Two different growth mechanisms are proposed to clarify the conversion from titanate nanofibers into perovskite titanates with various morphologes. This simple and versatile strategy may provide a general way to use low-dimensional titanate nanostructures with the high reactivity as a precursor to fabricate ternary complex oxides, such as CoTiO3 and MnTiO3. Acknowledgment. This work is supported by the 973 Program (Grant 2009CB220100), China. Financial support from the Australian Research Council (ARC) is also gratefully acknowledged.

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