High-Quality Brookite TiO2 Flowers: Synthesis, Characterization, and

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High-Quality Brookite TiO2 Flowers: Synthesis, Characterization, and Dielectric Performance Wanbiao Hu, Liping Li, Guangshe Li,* Changlin Tang, and Lang Sun State Key Lab of Structural Chemistry, Fujian Institute of Research on the Structure of Matter and Graduate School of Chinese Academy of Sciences, Fuzhou 350002, P. R. China

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 8 3676–3682

ReceiVed April 9, 2009; ReVised Manuscript ReceiVed May 31, 2009

ABSTRACT: High-quality brookite flowers were fabricated via a facile solution chemistry technique. The synthetic conditions to the flower-like brookite were monitored by a series of time-resolved experiments and further optimized by adjusting the concentrations of the Na+ and OH- species involved in the reaction system. Careful sample characterizations by the combined techniques of X-ray diffraction, Raman, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectra indicate the formation of highly phase-pure and well-crystallized brookite with an extremely low defect concentration. Different from the natural brookite mineral with an indirect transition (Zallen, R.; Moret, M. P. Solid State Commun. 2006, 137, 154), the present high-quality brookite flowers showed a direct transition with a bandgap energy of 3.4 ( 0.1 eV, which is larger than those of its two other polymorphs, that is, a direct band gap of 3.0 ( 0.1 eV for rutile and indirect band gap of 3.2 ( 0.1 eV for anatase. Room-temperature alternative current impedance measurements indicate that the permittivity for the brookite flowers is 93 at 40 Hz, which is much higher than that for anatase but slightly lower than rutile as opposed to what is theoretically predicted in the literature. Strikingly, the flower shape also enables high quality brookite TiO2 with a high structural stability up to 900 °C in air, impossibly accessible when using other preparation methods. These observations pave the way for high-quality brookite flowers to find a broad class of technological uses.

1. Introduction Exploring the dielectric properties associated with a stable structure and controlled morphology has drawn great attention because they are necessary for novel electronic devices and dielectric material manufacture such as gigabit level dynamic random access memory, imbedded-capacitor, microelectro-mechanical systems, and advanced metal-oxide-semiconductortransistors.1 There are various lattice symmetrical factors that are involved in the dielectric properties. As a consequence, a majority of the dielectric applications are strongly influenced by discrepant crystalline polymorphs and morphologies.2 Systematic investigations of these issues will be essential and significant for both the cognitive level and application fields, which may exemplify the property tailorings through material synthesis and morphological control. TiO2 is a model oxide compound in this regard since it has three polymorphs of different symmetries: rutile, anatase, and brookite, all of which can be described in terms of distorted TiO6 octahedra with different symmetries or arrangements.3 Therefore, investigating the dielectric properties of TiO2 of different polymorphs is fundamentally important, which may help to understand the roles that microstructure plays in the dielectric response. Plenty of literature reports have concluded that rutile TiO2 is an excellent dielectric material as it features a dielectric constant of 180 along the c-axis and 90 along the a-axis, while anatase TiO2 exhibits much smaller dielectric constant of 2.48.1a,2,4 Comparatively, brookite TiO2 is theoretically predicted to be a promising dielectric with a static dielectric constant much higher than anatase and rutile;5 nevertheless, its dielectric constant remains unclear experimentally, most likely because (1) brookite is a metastable phase and shows very complicated and lowly symmetric structure, and (2) brookite formation is always accompanied by secondary phases such as * To whom correspondence should be addressed. Tel: +86-591-83702122. Fax: +86-591-83714946. E-mail: [email protected].

anatase or/and rutile,6 since anatase shows a relatively lower surface energy in comparison with brookite.7 Therefore, exploring the dielectric performance of brookite is very interesting and practical, while the prerequisites can be high-quality brookite synthesis. Numerous synthetic strategies have been developed for the fabrication of brookite phase,6a,c,8 in which several preparation conditions including inorganic salts, organic substances, pH value, reaction time, and temperature were investigated. However, the mechanism pertinent to the formation of brookite phase is still controversial.9 As a result, the majority of theoretical work7a-c,10 on the structure and property prediction of brookite is not confirmed by experimental evidence, which challenges the uses of brookite for future technologies of electronic devices and dielectric material manufacture. Since the dielectric properties are strongly dependent on the polarization nature and thus are very sensitive to the interfaces, grain scale, and morphologies,11 in this work, we first designed a facile solution chemistry procedure to fabricate high-quality brookite. On the basis of this facile synthesis and the polarization induced by distorted TiO6 octahedra of brookite flowers, we also performed a systematic investigation of the structure, spectral characteristics, and dielectric properties of this unique polymorph.

2. Experimental Section 2.1. Synthesis. Sample synthesis was performed by a solution chemistry using the following procedure: 50 mL of 0.21 M NaOH (99.9%) solution was slowly added into 100 mL of 0.31 M TiOSO4 (98%) solution with stirring, which yielded a white suspension. This suspension was then filtered thoroughly and dispersed into water as the precursors. The precursors were sealed in Teflon-lined stainless steel autoclaves and reacted at 220 °C for 48 h. The synthetic conditions were optimized by adjusting the pH values using different concentrations of HNO3 or NaOH. 2.2. Characterization. The phase purity of the final products was examined by X-ray diffraction (XRD) (Rigaku Dmax2500, Cu KR

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Figure 2. (a-d) SEM and TEM of the samples obtained under the conditions: 220 °C, 48 h, and pH ) 12.5. Figure 1. XRD patterns of the samples prepared at 220 °C for 48 h with varied pH values: (a) pH ) 0.8; (b) pH ) 6; (c) pH ) 7; (d) pH ) 10.5; and (e) pH ) 12.5. Vertical bars in bottom layers denote the standard data for brookite (JCPDS, No. 29-1360), anatase (JCPDS, No. 71-1167), and titanate (JCPDS, No 83-0703), respectively. Symbol “+” denotes the internal standard Ni. radiation, λ ) 0.15418 nm). Ni powders were chosen as the internal standard for peak positions determination. Morphologies of the final products were observed by field emission scanning electron microscopy (FE-SEM) (JEOL JSM-6700) and transmission electron microscopy (TEM) (JEM-2010). Raman spectra of the final products were obtained on a Renishaw, UV-vis Raman System 1000 with an excitation line of 532 nm. Infrared spectra of the samples were measured on a PerkinElmer IR spectrophotometer using a KBr pellet technique. Optical diffuse reflectance spectra of the samples were measured using a Lambda 900 UV-vis spectrometer at room temperature. The valence and defect states were studied by X-ray photoelectron spectroscopy (XPS) employing ESCA-LAB MKII spectrometer from VG Co., with Al KR radiation and by electron paramagnetic resonance (EPR) operating on a Bruker BioSpin EPR with the microwave frequency of 9.86 GHz. The alternative current (AC) impedance measurements were carried out in a frequency range from 40 to 30 MHz and at an oscillation voltage of 0.5 V using a precision LCR meter (Agilent 4294A) with an assistant clamp of Agilent 16451B.

3. Results and Discussion 3.1. Structure, Morphology, and Formation Mechanism. Figure 1 shows XRD patterns of the samples prepared with given pH values at 220 °C for 48 h. When the pH value of reaction system was as low as 0.8, the product was a pure anatase phase. The average crystallite size was calculated to be 8.5 nm using the Scherrer equation. After the pH value was adjusted to 6 by decreasing the HNO3 amount, the products were still pure phase anatase, while the crystallite size grew up to 24 nm. In neutral conditions of pH ) 7 in the absence of HNO3 or NaOH, the product was layered titanates12 (Figure 1c). When the pH value was adjusted to 10.5, the brookite component appears as characterized by a diffraction peak at about 2θ ) 30.8° (JCPDS No. 29-1360). The content of the brookite component was about 50% as calculated by XRD data refinements.6b Because most of the diffraction peaks of the brookite are superposed with those of anatase (JCPDS, No 71-1167), several previous literature reports have used the intensity ratio of diffraction peaks, I121brookite/(I120brookite + I101anatase) to determine the phase composition of brookite,6a,c which seems however questionable, in particular when considering the peak broadening and strain effects of nanophases as well as the diffraction intensity dependence of morphology. It is noted that

Figure 3. XRD patterns of the samples prepared at 220 °C for (a) 2 h, (b) 3 h, and (c) 6 h at pH ) 12.5. Vertical bars in bottom layers denote the standard data of brookite and titanate, respectively. Symbol “+” denotes the internal standard Ni.

one characteristic peak of anatase at 2θ ) 62.57°, which is often ignored previously, does not overlap with any diffraction peaks of brookite. In this work, we used this peak as well as the ratio of I121brookite/(I120brookite + I101anatase) to identify the existence of anatase in the as-prepared samples. For the sample obtained at pH ) 12.5 (Figure 1e), the ratio of I121brookite/(I120brookite + I101anatase) equals to 1. After careful examination, as shown in the inset of Figure 1e, the diffraction peak at 2θ ) 62.57° for anatase disappeared completely. Therefore, it can be concluded that the sample prepared at pH ) 12.5 is pure-phase brookite. The corresponding morphology is flower-shaped with a dimension of about 2 µm, as indicated by SEM and TEM images (Figure 2). The crystallinity is excellent, and the distinct lattice fringe with d ) 0.247 nm matched well with the (012) plane of brookite TiO2 (Figure 2d). Even though anatase TiO2 nanocrystals were obtained at low pH values, the brookite component seems to not come from the conversion of anatase. To confirm this, a series of timeresolved experiments with fixed pH ()12.5) and reaction temperature (220 °C) were carried out. It is found that when the reaction time was as short as 2 h, the product was layered titanate (Figure 3a), which is almost the same as that of the precursor (Figure S1, Supporting Information). This layered titanate consisted of fine particles with dimensions of several

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Figure 4. Morphological evolutions as monitored by SEM images for the samples obtained with pH ) 12.5 at 220 °C for (a) 2, (b) 3, (c) 6, (d) 12, (e) 24, and (f) 48 h.

tens of nanometers (Figure 4a). Prolonging the reaction time to 3 h, the product was a mixed phase of titanate and brookite (Figure 3b) with no apparent changes in particle morphology. Pure-phase brookite was formed after 6 h reactions (Figure 3c), showing particles of a spindle-like shape (Figure 4c). Further prolonging the reaction time, these spindle-like particles aggregated together and yielded flower-like brookite TiO2 (Figure 4e,f). These observations strongly indicated that the formation of brookite flowers should be originated from the direct transformation of layered titanate rather than the anatase. Brookite TiO2 with many kinds of morphologies has been reported in the literature. For example, Cozzoli et al.13 prepared anisotropically shaped brookite TiO2 nanocrystals using a surfactant-assisted nonaqueous strategy, and announced that brookite is formed via a direct solid-state phase transformation of the initially formed c-axis-elongated anatase, in which the homogeneous nucleation and heterogeneous nucleation accompanying with growth processes were supposed to explain the switch from anatase to brookite. Wang et al.14 concluded that the assembled spheres of bicrystalline (brookite and rutile) titania nanoparticles were prepared in the presence of methylcellulose (MC) and NaCl, which was considered to be related to the hydrophilicity of MC that has promoted the olation and oxolation reactions and depressed the formation of other TiO2 phases. Obviously, the formation of the present flower-like brookite TiO2 could be different from these mechanisms. On the basis of our time-resolved experiments (Figures 3 and 4), we proposed that three steps may exist toward the formation of flower-like brookite TiO2: (1) the transformation of layer titanate into brookite nanoparticles, and (2) the growth of brookite particles up to the spindle-like shape, and (3) the assemble of these spindle-like particles into flower morphology. For the first step, the layered titanate shows TiO6 octahedral layers that are usually held by the strong static interaction between Na+ cations and TiO6 units, and therefore can act as the key precursor to brookite TiO2 nanophases such as nanotubes8h and brookite TiO2.15 In the present reaction system, Na+ or H+ ions located in the interlayer spaces of the layered titanate could be gradually released, which disturbs the static interaction and introduces

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the internal structural tension. With an increase of this tension, the layered structure would become unstable and finally transform into anatase in the acidic condition (Figure 1a,b) or brookite in the basic condition (Figures 1d,e and 3). In the subsequent processes, spindle- and flower-like brookite TiO2 was formed, which may undergo a preferential coarsening of the nanoparticles as is governed by the surface charger and surface energy. The equilibrium morphology of nanophases could be spherical when considering the liquid model. Namely, if the surface energy is isotropic, the obtained crystal should be nearly spherical, while if the surface energy is anisotropic, the energy-minimizing shape could be formed by the limiting planes of the possible lowest surface energy. The observed spindle-like shape should be related to the surface energy difference of the lattice planes, since as indicated by a theoretical study, among (100), (010), (001), (110), (011), and (111) planes of brookite TiO2, (001) plane holds the smallest surface formation energy of 0.62 eV, while (100) exhibits the larger value 0.88 eV.7c This calculation suggests that brookite crystal prefer growth along the [001] direction to give the nonspherical morphology. On the other hand, coarsening of brookite TiO2 nanoparticles strongly depends on the surface charges. Once the brookite TiO2 nanocrystals were formed, Na+ and OH- ions in the reaction solution would be adhered on their surfaces which may destroy the local equilibrium concentrations to give rise to gradient distributions of charges.16 Thus, the bigger particles grew up to show a spindle-like shape (Figure 4d) at the expense of the shrinkage of the smaller particles, typical for the Ostwald ripening process.16 With a prolonged aging time, these spindlelike brookite TiO2 particles would weld together to reduce the surface energy and finally produce brookite flowers (Figure 4e). In the synthesis, we also found that pH values and Na+ are two key factors for the formation of brookite flowers. When the pH value was reduced slightly to 12 and the reaction time kept the same at 48 h, the product was nearly pure brookite TiO2 but with obviously weakened assembly of particles (Figure 5). As discussed above, the assembling of small particles affects the surface chargers of the crystallites. Decreasing the pH value would reduce the concentration of Na+ and OH- ions and further weaken the interactions of small spindle-like particles. Therefore, the building rate of brookite flowers was slower compared to that at pH ) 12.5. When NaOH was replaced by LiOH or KOH, the products did not contain any brookite phase but a mixture of anatase and rutile with small amount unknown phases (Figure 6) even though the pH value was kept at 12.5. Brookite TiO2 is thus transformed from layered titanate. When LiOH or KOH was introduced into the reaction system, the ionic exchange process between Na+ ions in the precursors and Li+ (or K+) ions in the solution would occur. These newly formed layered titanates containing Li+ (or K+) would transform into anatase or rutile under hydrothermal conditions. From these formation reactions, it is likely that alkali ions play an important role in nucleation or nucleus growth of given phases of TiO2. It is well documented5,17 that sodium and potassium ions could lead to the formation of brookite and anatase, respectively. In combination with these literature works, it can be concluded that sodium ions may promote the brookite nucleation, while other alkali ions may prefer the nucleation of anatase and rutile, which explains why pure phase brookite TiO2 is difficult to obtain without having sodium involved. 3.2. Spectroscopic and Valence States Characteristic of Flower-Like Brookite TiO2. Raman and infrared spectra are effective to investigate the vibration behaviors of the chemical bonds and also to identify the phase structures, especially for

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No band related to anatase at 513 cm-1 was observed, which further confirms the formation of pure phase brookite. Consequently, the present preparation method is superior over other methodologies reported in the literature for pure phase brookite,8c,d since for the latter cases, the final products are often coexistent with traces of anatase. The flower-like brookite TiO2 also has clean surfaces, since as indicated by FT-IR spectrum (Figure 7b), four distinct vibrations were observed at 420, 488, 564, 710 cm-1, characteristic of the bending vibrations of Ti-O and O-Ti-O bonds in TiO6 octahedron, while no other vibrations at 3400 and 1635 cm-1 for physisorbed water19 or at 1395 cm-1 for CO32- species were detected. UV-visible diffusion reflectance spectrum of the as-prepared flower-like brookite TiO2 is shown in Figure 8. The relevant data for anatase and rutile TiO2 are also given for comparison. A strong absorption due to the interband transition20 was observed at about 400 nm, which is interestingly intermediated between those of anatase and rutile. The band gap, Eg, is usually estimated from the absorption edge wavelength of the interband transition according to the following equation:

K(hυ - Eg)1/n R) hυ Figure 5. (a) XRD and (b) SEM images of the samples obtained with pH ) 12 at 220 °C for 48 h. Vertical bars in the bottom layer denote the standard data of brookite (JCPDS, No. 29-1360), and the symbol “+” denotes the internal Ni standard, respectively.

Figure 6. XRD patterns of the sample obtained at 220 °C using (a) LiOH and (b) KOH for tuning the pH to 12.5. Vertical bars in the bottom layers denote the standard data of rutile (JCPDS, No. 88-1172) and anatase (JCPDS, No. 71-1167), respectively.

distinguishing the brookite from anatase.6a,8b,18 It is known that the lattice of brookite TiO2 has a D2h15 symmetry (space group: Pbca). According to the space group theory, 69 optical modes may exist that can be expressed by the following irreducible representation:18a

9A1g + 9B1g + 9B2g + 9B3g + 9A1u + 8B1u + 8B2u + 8B3u (1) where A1g, B1g, B2g, and B3g modes are Raman active, Bu, B2u, and B3u modes are infrared active, and A1u mode is inactive in both Raman and infrared. As indicated by Figure 7a, the Raman spectrum of the as-prepared flower-like brookite TiO2 shows 15 vibration bands in the range of 100 to 700 cm-1, which can be well assigned to the modes of A1g (155, 194, 247,412, 636 cm-1), B1g (213, 322, 501 cm-1), B2g (366, 395, 460, 583 cm-1), and B3g (172, 287, 545 cm-1) of brookite TiO2, respectively.18a

(2)

where R is the absorbance, K is a constant, and n equals 2 for direct transition and 1/2 for indirect transition.21 Recent optical absorption studies on the pale brown brookite mineral from the vicinities of Tremadoc, Wales and Bourg d’Oisans, France, suggested that brookite is an indirect semiconductor22 based on the broad absorption that extends throughout the visible spectral range. Nevertheless, as for almost all minerals, plenty of impurity ions can be expected for the mineral brookite, which may account for the visible absorptions. These considerations along with some literature work5,23 and our latest theoretical calculation (Figure S2, Supporting Information) lead us to consider the as-prepared high-quality brookite flowers as a direct-transition semiconductor, somewhat like rutile TiO2, but quite different from the indirect transition semiconductor anatase. According to the plots of (F(R)*hν)n versus energy (hν) in Figure 8b, the flower-like brookite TiO2 showed a direct band gap of 3.4 ( 0.1 eV, which is larger than that of 3.0 ( 0.1 eV for rutile and the indirect band gap of 3.2 ( 0.1 eV for anatase. The band gap obtained for the present brookite flower is almost the same as that reported by Koelsch et al.8a The valence states of Ti ions of the as-prepared flower-like brookite TiO2 were determined by XPS. The core level spectra for the flower-like brookite TiO2 are illustrated in Figure 9. The O 1s spectrum consists of a strong photoelectron signal around 529.8 eV and a shoulder around 531.5 eV, which are, respectively, attributed to the bulk oxygen (O2-) and surface adsorbed oxygen.24 Ti 2p spectra consist of the distinct Ti 2p1/2 and Ti2p3/2 photoelectron signals that are located at 464.3 and 458.5 eV, respectively. The spin-orbital splitting between these peaks is 5.8 eV, which is comparable with that of 5.74 eV reported previously.25 Both Ti 2p signals are highly symmetric, and no shoulders were observed on the lower energy sides of Ti 2p3/2 signal. Moreover, the bonding energy difference, ∆E, between O1s and Ti 2p3/2 is 71.3 eV, which is close to that of 71.5 eV for TiO2, but is smaller than that of 73.4 eV for Ti2O3 and 75.0 eV for TiO,26 indicating the extremely low defect concentration from Ti3+.19 This conclusion is also confirmed by EPR (Figure S3, Supporting Information). It is well-known that Ti3+ has a 3d1 configuration which expects to give an EPR signal at g )

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Figure 7. (a) Raman and (b) FT-IR spectra of the flower-like brookite TiO2.

Figure 8. (a) UV-visible diffusion reflectance spectra and (b) energy dependence of (F(R)*hν)n for the as-prepared brookite TiO2 flowers. The corresponding data for the rutile and anatase TiO2 are also given for comparison.

Figure 9. High-resolution XPS spectra of the as-prepared brookite TiO2 flowers.

1.96. The EPR spectrum of the as-prepared brookite TiO2 was characterized by a flat baseline without any paramagnetic signal, which indicates the absence of Ti3+ species. 3.3. Conductivity and Dielectric Properties. The electric properties of the flower-like brookite TiO2 were investigated by complex AC impedance measurements, which were performed on the brookite pellet obtained by pressing the mixture of as-prepared brookite powders with 4% PVB (poly (vinyl butyral)) and heating between 200 and 240 °C, and then calcining at 500 °C to sufficiently eliminate the rudimental PVB. It is noted that the pellet maintained the pure brookite phase without microstructural collapse or apparent morphological and scale changes (Figure S4, Supporting Information). The imped-

Figure 10. (a) Impedance plot, and frequency dependences of (b) ε′ and (c) ε′′ for the flower-like brookite TiO2. Red arc denotes the fitting result.

ance spectrum measured at room temperature is shown in Figure 10a, which can be fitted by equivalent circuit (RbQ1)(RgQ2), where Rb and Rg represent the bulk and grain boundary resistances, and Qi is the phase element. The fitting result, as given in the inset of Figure 10a, showed that the bulk resistance Rb is 1.06 × 106 Ω with the corresponding conductivity of σ ) 3.08 × 10-7 S cm-1, which is much smaller than the grain boundary resistance Rg > 107 Ω. The complex permittivity is determined by the formalism,27

ε* ) 1/iwC0Z*(w)

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Figure 11. Frequency dependence of loss tangent for the flower-like brookite TiO2. The peak frequencies noted by arrows are about 1700 and 45500 Hz, respectively.

where ω)2πf is the angular frequency and C0 is the vacuum capacitance. Figure 10b,c present the frequency dependence of the real (ε′) and imaginary (ε′′) components of the as-prepared flower-like brookite TiO2. Low frequency permittivity ε′ is as high as 93 at 40 Hz, which is far higher than that of anatase, but slightly lower than that of rutile. ε′ decreased slightly with increasing the frequency. Comparatively, the variation of ε′′as a function of the frequency exhibited two humps at about 850 and 18000 Hz. Similar to the previous reports,4b,c,28 the variation of permittivity can mainly be ascribed to the deformation of electron shells and polarization of both Ti4+ and O2- ions promoted by strong local internal electric field induced by the external applied electric field. Continuous fall of ε′with increasing frequency occurs because dipoles are not able to faithfully follow the impressed oscillating field.29 In the high frequency region >5 × 105 Hz, the permittivity ε′ trended to be a constant at 6.5 with increasing frequency. For the loss tangent, two loss peaks at about 1700 and 45500 Hz are clearly seen (Figure 11), which are supposed to be originated from the interface dipole polarization and ionic relaxation polarization, respectively.4a,28 Generally, the dielectric relaxation can be expressed by a Debye relaxation relation:30

ε* - ε∞ )

εs - ε∞ 1 + (ωτ)

2

-j

ωτ(εs - ε∞) 1 + (ωτ)2

(3)

where εs is the static field dielectric constant, ε∞ is the high frequency dielectric constant of the material, ω is the applied angular frequency, and τ is the relaxation time. Then, we get,

tan δ )

(εs - ε∞) εs + ε∞(ωτ)

2

) 2 tan δm

ωτ1 1 + (ωτ1)2

(4)

where tan δm is the maximum loss. From eq 4, it is obvious that tan δ has a maximum at wτ1 ) 1.0. On the basis of the data in Figure 11, the relaxation times for two peaks were calculated to be about 90 and 3 µs. These observations demonstrate that the limited interfaces for the present flowershaped brookite may show merits of space charge restraint and ionic transfer, which may decrease the dielectric loss while increase the electric conductivity. As a good candidate for photovoltaic device material of brookite itself, these advantages could be inherited to other systems by controlling the interfaces or various post-treatment techniques.1a,c,31 Our latest preliminary results (Figure S5, Supporting Information) showed that the high-quality brookite TiO2 flowers can stand stably up to a high temperature of 900 °C in air with no apparent changes in the phase and morphology, while upon

calcination in air at 1000 °C, these brookite TiO2 flowers completely transformed into rutile as is accompanied by a morphological change into a sphere-like shape. Comparatively, brookite prepared by other methodologies8h,33 usually becomes destabilized and transforms into rutile at temperatures below 600 °C. Such an abnormally high structural stability is closely related to the flower-shape, since as indicated by some theoretical simulation,32 the existence of the strong interactions among the fine particles for the assembly of the brookite flowers may decrease the surface energy which could enhance the structural stability. The methodology reported in this work may pave the way for new brookite TiO2 morphologies to find a broad class of technological uses.

4. Conclusions High-quality brookite TiO2 in a flower-like shape was synthesized by optimizing the hydrothermal conditions. A series of time-resolved experiments indicates that the formation of brookite phase underwent three dominant processes: the transformation of layered titanate into brookite nanoparticles, the growth of brookite nanoparticles into the spindle-like shape, and ultimately the assembly of spindle-like particles into flower morphology. These processes were governed by the concentrations of OH- and Na+, and surface charges as well as the surface energy of the fine particles. The flowerlike brookite TiO2 crystallized in a single phase structure with an extremely low defect concentration. The direct band gap for flower-like brookite TiO2 was 3.4 ( 0.1 eV, which is larger than those of its other two polymorphs, that is, 3.0 ( 0.1 eV for rutile and indirect band gap of 3.2 ( 0.1 eV for anatase. The electric properties investigated by complex AC impedance indicate that the flower-like brookite TiO2 had a bulk conductivity of about 3.08 × 10-7 S cm-1. After increasing the frequency, real component of complex permittivity decreased slightly, while the imaginary one exhibited two humps at about 850 and 18000 Hz. The loss peaks associated with the interface dipole polarization and ionic relaxation polarization were observed in the loss tangent curve at about 1700 and 45500 Hz. According to the Debye relaxation relation, the relaxation times for two peaks were calculated to be about 90 and 3 µs. Acknowledgment. This work was financially supported by NSFC under the contract (No. 20671092, 20831004, 20773132, 20771101), National Basic Research Program of China (No. 2007CB613301, 2009CB939801), Directional program (KJCXZYW-MO5), and Knowledge Innovation Program of the Chinese Academy of Sciences, and FJIRSM key program (No. SZD07004-3). Supporting Information Available: XRD pattern of the precursor; electronic structure of brookite, rutile, and anatase calculated by Materials Studio software; EPR spectrum of the as-prepared flowerlike brookite TiO2; XRD patterns and SEM images of brookite after annealing at 500, 900, and 1000 °C. This information is available free of charge via the Internet at http://pubs.acs.org.

References (1) (a) Lee, Y. H.; Chan, K. K.; Brady, M. J. J. Vac. Sci. Technol. A 1995, 13, 596. (b) Wilk, G. D.; Wallace, R. M.; Anthony, J. M. J. Appl. Phys. 2000, 87, 484. (c) Qi, L.; Lee, B. I.; Chen, S. H. AdV. Mater. 2005, 17, 1777. (d) Sarkar, A.; Krupanidhi, S. B. Solid State Commun. 2007, 143, 510. (2) Lee, C. Phys. ReV. B 1994, 50, 13379. (3) (a) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (b) Gateshki, M.; Yin, S.; Ren, Y.; Petkov, V. Chem. Mater. 2007, 19, 2512.

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(4) (a) Parker, R. A.; Wasilik, J. H. Phys. ReV. 1960, 120, 1631. (b) Parker, R. A. Phys. ReV. 1961, 124, 1719. (c) Samara, G. A.; Peercy, P. S. Phys. ReV. B 1973, 7, 1131. (5) Mo, S. D.; Ching, W. Y. Phys. ReV. B 1995, 51, 13023. (6) (a) Kominami, H.; Kohno, M.; Kera, Y. J. Mater. Chem. 2000, 10, 1151. (b) Isley, S. L.; Penn, R. L. J. Phys. Chem. B 2006, 110, 15134. (c) Li, J. G.; Ishigaki, T.; Sun, X. D. J. Phys. Chem. C 2007, 111, 4969. (7) (a) Gong, X. Q.; Selloni, A.; Batzill, M.; Diebold, U. Nat. Mater. 2006, 5, 665. (b) Beltran, A.; Gracia, L.; Andres, J. J. Phys. Chem. B 2006, 110, 23417. (c) Gong, X. Q.; Selloni, A. Phys. ReV. B 2007, 235307 (1), 76. (d) Ranade, M. R.; Navrotsky, A.; Zhang, H. Z.; Banfield, J. F.; Elder, S. H.; Zaban, A.; Borse, P. H.; Kulkarni, S. K.; Doran, G. S.; Whitfield, H. J. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6476. (8) (a) Koelsch, M.; Cassaignon, S.; Guillemoles, J. F.; Jolivet, J. P. Thin Solid Films 2002, 403, 312. (b) Li, J. G.; Tang, C. C.; Li, D.; Haneda, H.; Ishigaki, T. J. Am. Ceram. Soc. 2004, 87, 1358. (c) Addamo, M.; Bellardita, M.; Paola, A. D.; Palmisano, L. Chem. Commun. 2006, 4943;(d) Bakardjieva, S.; Stengl, V.; Szatmary, L.; Subrt, J.; Lukac, J.; Murafa, N.; Niznansky, M.; Cizek, K.; Jirkovskyc, J.; Petrova, N. J. Mater. Chem. 2006, 16, 1709. (e) Hosono, E.; Fujihara, S.; Imai, H.; Honma, I.; Masaki, I.; Zhou, H. S. ACS Nano 2007, 1, 273. (f) Iskandar, F.; Nandiyanto, A. B.; Yun, K. M.; Hogan, C. J.; Okuyama, J. K.; Biswas, P. AdV. Mater. 2007, 19, 1408. (g) Ardizzone, S.; Bianchi, C. L.; Cappelletti, G.; Gialanella, S.; Pirola, C.; Ragaini, V. J. Phys. Chem. C 2007, 111, 13222. (h) Deng, Q. X.; Wei, M. D.; Ding, X. K.; Jiang, L. L.; Ye, B. H.; Wei, K. M. Chem. Commun. 2008, 3657. (9) (a) Pottier, A.; Chaneac, C.; Tronc, E.; Mazerolles, L.; Jolivet, J. P. J. Mater. Chem. 2001, 11, 1116. (b) Lee, J. H.; Yang, Y. S. J. Eur. Ceram. Soc. 2005, 25, 3573. (c) Li, Y. Z.; Lee, N. H.; Hwang, D. S.; Song, J. S.; Lee, E. G.; Kim, S. J. Langmuir 2004, 20, 10838. (10) Posternak, M.; Baldereschi, A. Phys. ReV. B 2006, 125113 (1), 74. (11) (a) Kinoshita, K.; Yamaji, A. J. Appl. Phys. 1976, 47, 371. (b) Arlt, G.; Hennings, D.; With, G. J. Appl. Phys. 1985, 58, 1619. (c) Rothschilda, A.; Komem, Y. Appl. Phys. Lett. 2003, 82, 574. (d) Buscaglia, M. T.; Viviani, M.; Buscaglia, V. Phys. ReV. B 2006, 064114 (1), 73. (12) Zhu, H. Y.; Lan, Y.; Gao, X. P.; Ringer, S. P.; Zheng, Z. F.; Song, D. Y.; Zhao, J. C. J. Am. Chem. Soc. 2005, 127, 6730. (13) Buonsanti, R.; Grillo, V.; Carlino, E.; Giannini, C.; Kipp, T.; Cingolani, R.; Cozzoli, P. D. J. Am. Chem. Soc. 2008, 130, 11223. (14) Wei, J. P.; Yao, J. F.; Zhang, X. Y.; Zhu, W.; Wang, H. T.; Rhodes, M. J. Mater. Lett. 2007, 61, 4610. (15) Nagase, T.; Ebina, T.; Iwasaki, T.; Hayashi, H.; Onodera, Y.; Chatterjee, M. Chem. Lett. 1999, 911.

Hu et al. (16) Wong, E. M.; Bonevich, J. E.; Searson, P. C. J. Phys. Chem. B 1998, 102, 7770. (17) Kiyama, M.; Akita, T.; Tsutsumi, Y.; Takada, T. Chem. Lett. 1972, 21. (18) (a) Tompsett, G. A.; Bowmaker, G. A.; Cooney, R. P.; Metson, J. B.; Rodgers, K. A.; Seakins, J. M. J. Raman Spectrosc. 1995, 26, 57. (b) Zhang, Y. H.; Chan, C. K.; Porter, J. F.; Guo, W. J. Mater. Res. 1998, 13, 2602. (c) Su, W. G.; Zhang, J.; Feng, Z. C.; Chen, T.; Ying, P. L.; Li, C. J. Phys. Chem. C 2008, 112, 7710. (19) Li, G. S.; Li, L. P.; Boerio-Goates, J.; Woodfield, B. F. J. Am. Chem. Soc. 2005, 127, 8659. (20) Zhang, W. Z.; Froba, M.; Wang, J. L.; Tanev, P. T.; Wong, J.; Pinnavaia, T. J. J. Am. Chem. Soc. 1996, 118, 9164. (21) Kortuem, G. Reflectance Spectroscopy: Principles, Methods, Applications; Springer-Verlag: Berlin-Heidelberg, 1969. (22) Zallen, R.; Moret, M. P. Solid State Commun. 2006, 137, 154. (23) Beltran, A.; Gracia, L.; Andres, J. J. Phys. Chem. B 2006, 110, 23417. (24) Kim, K. S.; Winograd, N. Surf. Sci. 1974, 43, 625. (25) Sdergren, S.; Siegbahn, H.; Rensmo, H.; Lindstrm, H.; Hagfeldt, A.; Lindquist, S. E. J. Phys. Chem. B 1997, 101, 3087. (26) Sayers, C. N.; Armstrong, N. R. Surf. Sci. 1978, 77, 301. (27) (a) Jawad, S. A.; Alnajjar, A.; Abdallah, M. H. Appl. Phys. A: Mater. Sci. Process. 1997, 64, 199. (b) Barsoukov, E.; Macdonald, J. R. Impedance Spectroscopy; Wiley: New York, 2005; p 18. (c) Chen, L. J.; Li, L. P.; Li, G. S. J. Solid State Chem. 2008, 18, 2073. (28) Zhang, L. D.; Zhang, H. F.; Wang, G. Z.; Mo, C. M.; Zhang, Y. Phys. Stat. Sol. A 1996, 157, 483. (29) Gupta, V.; Bamzai, K. K.; Kotru, P. N.; Wanklyn, B. M. Mater. Sci. Eng. B 2006, 130, 163. (30) El-Desoky, M. M.; Salem, S. M.; Kashif, I. J. Mater. Sci.-Mater. Electron. 1999, 10, 279. (31) (a) Gilmer, D. C.; Colombo, D. G. Chem. Vap. Deposition 1998, 4, 9. (b) Wilk, G. D.; Wallace, R. M.; Anthony, J. M. J. Appl. Phys. 2000, 87, 484. (c) Paily, R.; DasGupta, A.; DasGupta, N.; Bhattacharya, P.; Misra, P.; Ganguli, T.; Kukreja, L. M.; Balamurugan, A. K.; Rajagopalan, S.; Tyagi, A. K. Appl. Surf. Sci. 2002, 187, 297. (d) Manjunath, S.; Anilkumar, K. R.; Revanasiddappa, M.; Prasad, M. V. Ferroelectr. Lett, 2008, 35, 36. (32) (a) Horsch, M. A.; Zhang, Z. L.; Glotzer, S. C. Phys. ReV. Lett. 2005, 95, 056105. (b) Chen, J. Z.; Zhang, C. X.; Sun, Z. Y.; Zheng, Y. S.; An, L. J. J. Chem. Phys. 2006, 124, 104907. (c) Chen, J. Z.; Zhang, C. X.; Sun, Z. Y.; An, L. J.; Tong, Z. J. Chem. Phys. 2007, 127, 024105. (33) Li, J. G.; Ishigaki, T. Acta Mater. 2004, 52, 5143.

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