Facile Synthesis of Titanate Nanoflowers by a Hydrothermal Route Ji-quan Huang, Zhi Huang, Wang Guo, Mei-li Wang, Yong-ge Cao,* and Mao-chun Hong Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2444–2446
ReceiVed January 9, 2008; ReVised Manuscript ReceiVed April 3, 2008
ABSTRACT: The novel sodium titanate nanoflowers were synthesized through a facile hydrothermal method. The as-grown nanoflowers were self-assembled by folded broad and thin nanosheets, which were formed by the assistance of ZnO nanostructure templates. The crystal structure was identified to be layered NaxH2-xTi2O5 · H2O, and the formation process of the titanate nanoflowers was proposed as follows: ZnO flower-like nanorods in the alkali solution formed at the initial stage, and then the formed titanate nanosheets deposited on the surface of the frameworks of the ZnO nanorods, and finally the titanate nanoflowers were formed after the removal of ZnO framework.
1. Introduction Titanium dioxide is one of the most important transition metal oxides which has been used in paints, ointments, sunscreens, self-cleaning devices, gas sensors, photocatalysts, catalyst supports, Li-ion battery materials, and solar cells, etc.1–5 It is found that the performance of TiO2-based devices depends on the crystal shape and phase dimension.1,2,6,7 For example, the electrochromism effect of the titanate nanotube is stronger than that of titania film,6 and the UV-vis absorption intensity of titanate nanowires is enhanced in the whole absorption region compared to monodisperse titania spheres,7 and so on. Moreover, the nanostructures of titania/titanate exhibit unusual properties that cannot be determined by extrapolation of bulk characteristics.1,2 Therefore, increasing interest in the controlled synthesis of titania/titanate nanostructures with different shapes and sizes has developed in recent years. So far, most synthetic efforts have been directed toward monodisperse nanoparticles,8 nanotubes,9–11 nanowires,5,7,10 and nanoribbons,12 etc. The synthesis of 0-dimensional (0D) and one-dimensional (1D) nanostructures has been widely investigated and well developed. However, reports on the synthesis of complex three-dimensional (3D) titanate nanostructures remain uncommon. On the other hand, recent research on the synthesis of 3D nanomaterials show that this kind of nanostructure may have potentially advanced applications in electronics and optoelectronics.13,14 Design and controllable synthesis of 3D nanostructures is highly desirable in advanced materials. Herein, we report a facile one-step hydrothermal synthesis of titanate 3D nanoflowers, which are composed of nanoribbons, in concentrated NaOH solution, and the relative formation mechanism is proposed.
2. Experimental Procedures Analytical reagent (AR) grade NaOH pellets, Zn(NO3)2 · 6H2O, and anatase powders with a particle size of about 7 nm, were used as source raw materials for fabrication. Titanate nanoflowers were fabricated through a hydrothermal reaction between NaOH solution and the mixture of TiO2 and Zn(NO3)2 · 6H2O. In a typical synthesis, 2 g of Zn(NO3)2 · 6H2O and 0.3 g of TiO2 powders were added to 16 mL of 5 M NaOH aqueous solution and then heated at 120 °C for 10 h in an autoclave of volume 20 mL. The precipitate was washed with HCl solution (0.1 M), distilled water, and absolute ethanol in order, and * To whom correspondence should be addressed. Fax & Tel: +86-59183721039. E-mail:
[email protected].
Figure 1. SEM images of the fabricated titanate nanoflowers (T1). then dried in air. This sample was denoted as T1. For a comparison, a mixture of 0.3 g of TiO2 powders and 16 mL of 5 M NaOH aqueous solution was hydrothermally treated by the same procedure, and the relative product was denoted as T2. The morphology and microstructure of the resulted samples were characterized by X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM).
3. Results and Discussion Titanate nanoflowers were synthesized through the hydrothermal route from the mixture of TiO2 and Zn(NO3)2 in NaOH solution. Figure 1a,b shows the SEM images of the titanate nanoflowers (sample T1). It is observed that the titanate product T1 contains numerous flowerlike aggregates. These flowers were composed of broad and thin nanoribbons, as shown in the SEM image in Figure 1b. The formation of the nanoflowers was further confirmed by TEM observation (Figure 2). Figure 2, panels a,b show the TEM images of T2 and T1, respectively; while the plain view and side view in the HRTEM images for the nanosheet of T1 are displayed in Figure 2, panels c, d, and e, respectively. It is obviously that the “petals” of the nanoflowers (i.e., the titanate nanoribbons) were ultra thin, containing only several titanate atomic layers. Two sets of lattice fringes with separations of 0.23 and 0.34 nm were observed in Figure 2c. Figure 2d revealed another set of fringes with a larger spacing of 0.75 nm, which could correspond to the interplanar distances of the titanate, and was in agreement with the results reported by Du et al. for titanate nanotubes15 and Yuan et al. for titania nanoribbons/nanorods.16 Furthermore, a TEM image (Figure 2a) revealed that the products were nanosheets for sample T2, which was hydrothermally synthesized through reaction between NaOH solution and TiO2, without Zn(NO3)2 addition. The different morphologies between T1 and T2 imply
10.1021/cg800030y CCC: $40.75 2008 American Chemical Society Published on Web 06/07/2008
Facile Synthesis of Titanate Nanoflowers
Crystal Growth & Design, Vol. 8, No. 7, 2008 2445
Figure 3. The XRD patterns of the samples T1, T2 and of the nanorods. The titanate nanorods, used as a reference, were obtained by the hydrothermal treatment of anatase powders in the 10 M NaOH solutions held for 24 h at 180 °C.
Figure 2. TEM images of (a) T2 and (b) T1. And HRTEM images of the nanosheet of T1 in (c) plain view and (d, e) side views.
that the introduction of Zn(NO3)2 should play an important role in the formation of titanate nanoflowers. What is the final product of the hydrothermal reaction between NaOH and titania is still a question that has been intensively argued in the literature.17–20 Several possibilities were proposed, such as TiO2,9 Na2Ti3O7, H2Ti3O7, NaxH2-xTi3O7, Na2Ti6O13, Na2Ti3O7 · nH2O, Na2Ti2O4(OH)2, HxTi2-x/40x/4O4 (0 ) vacancy), H2Ti2O5 · · H2O, etc.1,9,17–23 For example, the lattice space of 0.23 nm was ascribed to the interplanar distance of (202) plane in TiO2 with brookite structure24 or (112) plane of anatase phase (JCPDS Card No. 84-1286) or (501) plane of H2Ti2O5.H2O (JCPDS Card No. 47-0124), the lattice space of 0.75 nm was ascribed to the interplanar distances of H2Ti3O7,15 and 0.34 nm was attributed to the lattice space of (1j11) plane in Na2Ti6O1318 or (012) plane in Na2Ti3O718 or (101) plane in anatase25 (JCPDS Card No. 84-1286). However, it is strange that all of the three lattice distances of 0.23, 0.34, and 0.75 nm were detected in our fabricated nanoflowers. None of the compositions referred above provide a satisfactory fit to these interplanar distances obtained by TEM observation as shown in Figure 2, which implies that we should make more efforts to identify the exact compositions of the resulting titanate. It is reasonable and comprehensible that the interplanar distances determined by TEM is sometimes misleading because the electron beam would cause dehydration of the titanate under high vacuum.19 So X-ray diffraction (XRD) analysis is performed to further determine the crystal structure of the samples. Figure 3 shows the XRD patterns of the samples T1 and T2.
Here, the XRD patterns of titanate nanorods are shown to be a reference. The titanate nanorods were obtained by the hydrothermal reaction between anatase and 10 M NaOH solution at 180 °C for 24 h. It is shown in Figure 3 that the positions of all XRD peaks for nanoflowers (T1) are consonant with that of the nanorods, as well as nanosheets (T2), and no additional peaks were observed for the nanoflowers. It is confirmed that the phase of the nanoflowers, nanosheets, and nanorods are the same, and the nanoflower is one of the existing forms of the sodium titanate nanostructure. The broad peaks at 2θ ) 9.4°, 24.5°, 28.5°, 33.5°, 38.8°, 48.5°, and 62° correspond basically with (200), (110), (310), (301), (501), (020), and (002) planes of orthorhombic H2Ti2O5 · H2O (JCPDS Card No. 47-0124). Thus, the structure of the nanoflowers should be assigned to layered NaxH2-xTi2O5 · H2O, which is the same with the titanate nanotube structure proposed by Teng et al.23 The lattice distances of 0.23, 0.34 nm determined by TEM are consistent with that obtained from XRD measurements (peaks at 2θ ) 24.5° and 38.8° corresponding to d values of 0.36 and 0.23 nm, respectively). However, the interplanar d-spacing value of 0.75 nm determined by TEM is far smaller than that obtained from XRD measurement (peak at 2θ ) 9.4° corresponding to a d-spacing of 0.91 nm). This deviation may be caused by the dehydration of the titanate during the TEM observation. Because there was no addition of Zn(NO3)2 · 6H2O in the source materials, no nanoflowers could be observed experimentally; we believe that the introduction of zinc nitrate should play a key role in the formation of titanate nanoflowers. It is also reported that ZnO nanoflowers composed of nanorods could be hydrothermally fabricated easily in the NaOH solution.26–28 In addition, zinc nitrate is much more reactive in the NaOH solution than TiO2, which makes it easier to form ZnO nanostructures preferentially than to form titanate. As a consequence, the formation mechanism of the nanoflowers is proposed as follows. At the initial stage during the hydrothermal procedure, Zn2+ may react with OH- to form the precipitation of Zn(OH)2 and subsequently solution of [Zn(OH)42-],26,28 in which the ZnO can nucleate on the surface of Zn(OH)2 precipitates to form multinuclei aggregates. The multinuclei aggregates may further provide the preferential sites for the growth of ZnO nanoflowers26 self-assembled by nanorods (first stage as shown in Figure 4). The synthesized ZnO nanoflowers will served as templates for the subsequent deposition of titanate nanosheets formed by reaction of anatase with NaOH solution (second stage and third stage in Figure 4). Several nanosheets could heap together to further form the nanoribbons. Finally, ZnO can be removed by chemical etching in the dilute HCl solution during the washing procedure, and thus the titanate nanoflowers are obtained (fourth stage in Figure 4). The possible
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References
Figure 4. A suggested four-step schematic formation process of the titanate nanoflowers.
query against this mechanism is the formation of ZnO nanoflowers. The formed ZnO precipitation may redissolve in concentrated NaOH due to the high reactivity of ZnO in NaOH solution. However, it was reported that ZnO nanoflowers could be synthesized by the hydrothermal reaction of zinc salt with concentrated NaOH solution.26–28 Moreover, the existence of ZnO nanorods could be confirmed indirectly by titanate nanoribbons. The existence of ZnO nanorods would lead to the rumpling of titanate nanoribbons, and these rumples might also exist after the elimination of ZnO, as shown in Figure 2e. And we had examined nearly 100 TEM images of titanate nanoribbons obtained from the hydrothermal treatment of TiO2 with NaOH solution and without zinc salt under different conditions, and no rumpling of titanate nanoribbons could be observed. Thus, the presence of rumples confirmed indirectly the presence of ZnO nanorods.
4. Conclusion Titanate nanoflowers were fabricated by hydrothermal reaction between anatase and NaOH solution with the assistance of zinc oxide nanostructure which may serve as the template. The synthesized novel nanoflowers are composed of numerous nanoribbon aggregates which are extreme thin with several nanometers’ thickness. The formation mechanism is proposed based on XRD patterns and SEM/TEM observations. Acknowledgment. This work was supported by the National Basic Research Program (No. 2007CB936703), Fund of National Engineering Research Center for Optoelectronic Crystalline Materials (No. 2005DC105003: 2007K01), and 100 talent program in CAS.
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CG800030Y
