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J. Phys. Chem. C 2008, 112, 1658-1662
Local Structure of TiO2-Derived Nanotubes Prepared by the Hydrothermal Process Takashi Kubo* and Atsushi Nakahira Department of Materials Science, Graduate School of Engineering, Osaka Prefecture UniVersity, 1-1, Gakuen-cho, Naka-ku, Sakai-shi, Osaka, 599-8531, Japan ReceiVed: August 21, 2007; In Final Form: NoVember 12, 2007
The structure of TiO2-derived nanotubes with about 10 nm outer diameter and 5 nm inner diameter and a few hundred nanometers in length prepared by the hydrothermal process in an aqueous NaOH solution was investigated. Especially, the local structure of TiO2-derived nanotubes was analyzed in detail on short-range order by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) measurements. It was found that TiO2-derived nanotubes were mainly composed of layered titanate and the anatase-like structure was partly present in the titanate-based nanotubes. The formation of the anatase-like structure was thought to be caused from the consolidation of nanotubes.
Introduction Since carbon nanotubes1 were discovered, the syntheses of micro- and nanotubes of TiO2 have been attempted by various methods such as template methods.2-4 On the contrary, Kasuga et al. treated TiO2 at 383 K in a 10 M NaOH aqueous solution without the replication or template, and nanotubes 8 nm in diameter and 100 nm in length were obtained by their experiments.5,6 This simple synthetic chemical method for nanotubes without templates was confirmed by many researchers. TiO2derived nanotubes synthesized by this chemical process are particularly interesting, because of their high specific surface area caused by nanotubular morphology, leading to the development of new photocatalytic activities.7-9 It is also reported that the TiO2-derived nanotubes synthesized by the hydrothermal treatment of TiO2 with NaOH aqueous solution, such as sodium or hydrogen titanate (NaxH1-xTi3O7 or H2Ti3O7), possess optical and magnetic properties.10 Furthermore, preparations of thin film and bulk for these nanotubes have been attempted vigorously.11-13 There is currently much discussion about the formation mechanism and phase of this nanotube, although many groups have investigated the structure, formation mechanism, and clarification of synthetic condition for these nanotubular products. These nanotubes are thought to be caused by scrolling of an exfoliated TiO2-derived nanosheet.10,14,17-20 Kasuga et al. and Yao et al. have concluded that the prepared nanotubes were composed of titania.5,6,15,16 Other researchers argued in the favor of titanate structures.10,13,14,17-21 For example, Du et al. have found that the prepared nanotubes were constructed of hydrogen titanate (H2Ti3O7) rather than titania,17 and subsequently Chen and co-workers have discussed the structure of this titanate nanotube by high-resolution transmission electron microscopy (HRTEM) and first-principle simulation.18,19 Ma et al. have reported that this product was composed of lepidocrocite-type H2Ti2-x0x/4O4 (0: vacancy).20 Thus, the detailed clarification of the structure of TiO2-derived nanotubes is now of specific interest and importance. However, since TiO2-derived nanotubes are low-crystalline materials as shown in X-ray diffraction (XRD) patterns (these may be like a noncrystalline or glass * Corresponding author. Phone: +81-72-254-8531. Fax: +81-72-2549912. E-mail:
[email protected].
material),13,14 it is difficult to investigate their structure with conventional XRD and TEM. Therefore, we focus on the X-ray absorption fine structure (XAFS) method. XAFS (X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)) has been regarded as a useful method to obtain local structural information of noncrystalline materials. Recently, the structural evaluation of the TiO2 nanosheet has been attempted by the fluorescent XAFS method.22,23 Moreover, Ma et al. have discussed the structure on the molecular scale of TiO2-derived nanotubes from Raman and XAFS measurements for as-prepared nanotube, lepidocrocite titanate, and various stepped titanates of Ti3O7, Ti4O9, and Ti5O11.24 In order to understand the structure of TiO2-derived nanotubes further, it is very important to investigate the difference of the structure between TiO2 particles on the molecular scale. In the present work, TiO2-derived nanotube was synthesized by the hydrothermal treatment for TiO2 in NaOH aqueous solution systems and characterized by various analytical methods, such as XRD, TEM, N2 adsorption, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectra (XPS), and XAFS, etc. In particular, the difference of the local structure between anatase-type TiO2 particles and TiO2-derived nanotubes prepared by the hydrothermal process was discussed from information on short-range order using Ti K edge XAFS measurements at SPring8. Consequently, our results revealed that TiO2-derived nanotubes were mainly composed of layered titanate and that the anatase-like structure was locally present in the titanate-based nanotubes. The formation of anatase-like structure is thought to be caused from the consolidation of nanotubes. Experimental Section Two grams of anatase-type TiO2 (3 m2/g, Kojundo Chem., Japan) were used as a starting material. They were added in 10 M NaOH aqueous solution (20 mL). Then the specimens were treated under a hydrothermal reaction at 383 K for 96 h. The obtained products after hydrothermal treatments were sufficiently washed with deionized water and dilute HCl aqueous solution (0.1 M) and were subsequently separated from the washing solution by filtration. This treatment was repeated until
10.1021/jp076699d CCC: $40.75 © 2008 American Chemical Society Published on Web 01/15/2008
Local Structure of TiO2-Derived Nanotubes the washing water showed pH < 7. The final pH value of the washing water was 6.8. After the washing treatment, they were filtered and subsequently dried at 323 K for more than 12 h in an oven. The phase identification was carried out by the XRD (Rint 2500, Rigaku Co., Ltd., Japan) method using Cu KR radiation at 40 kV and 50 mA. The XRD profiles were collected between 5° and 60° of 2θ angles with a step interval of 0.01° and scanning rate of 1°/min. Various microstructural analyses were performed by TEM (JEM2010/SP, JEOL, Japan) with an accelerating voltage of 200 kV. Sodium in prepared samples was analyzed by energy-dispersive X-ray (EDX) and XPS analysis. Nitrogen adsorption isotherms at 77 K were obtained by an automatic gas adsorption measurement apparatus (BELSORP 18PLUS-SPL, Japan-BEL, Japan). Some products were pretreated at 403 K for 10 h. The XPS analysis was performed in an ultrahigh vacuum (less than 1 × 10-8 Torr) using an ULVAC-PHI 5500MT system, with a Mg KR (hν ) 1253.6 eV) X-ray source operated at 15 kV. Binding energies were referenced to the C 1s level of residual graphitic carbon. FT-IR spectra of samples were obtained by the KBr pellet method using an FT-IR spectrophotometer (NICOLET AVATAR 370DTGS, Thermo Fisher Scientific K.K., Japan). The spectra were recorded at 4 cm-1 resolution. The Ti K edge XAFS (XANES and EXAFS) were recorded at room temperature at BL01B1 of SPring 8 in Japan. The Ti K edge XAFS data for this study was corrected with transmission mode using the Si(111) double-crystal monochromator (2d ) 0.627 nm). The data were collected with the ionization chambers filled with gas (I0 chamber: He/N2 ) 7/3, I chamber: N2). For the XAFS measurements, the samples were prepared as pellets with the thickness varied to obtain a 0.5-1 jump at the Ti K absorption edge. Ti metallic foil was used for the energy calibration. XANES were analyzed by subtracting a linear background computed by least-squares fitting from the preedge region and normalized. EXAFS were analyzed by using standard methods. The preedge region was subtracted, and then the EXAFS spectrum was extracted by fitting the absorption coefficient with a cubic spline method. Fourier transformation of the k3-weighted EXAFS oscillation from k (k is the photoelectron wavenumber) space to r space was performed over the range of 2.5-12 Å-1 to obtain the radial distribution function (FT-EXAFS). The analysis of EXAFS data was conducted using the commercial software “REX2000” (Rigaku Co. Ltd., Japan).
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Figure 1. (A and B) TEM images of the product prepared by the hydrothermal treatment of anatase at 383 K for 96 h and the subsequent washing treatment with H2O/HCl. (C) XRD patterns of anatase and the obtained nanotubular product.
Results and Discussion Figure 1 shows typical TEM images and the XRD pattern of the product prepared by a hydrothermal treatment of anatasetype TiO2 at 383 K at 96 h. As shown in Figure 1A, the obtained product possessed nanotubular structures with about 10 nm o.d. and 5 nm i.d. and a few hundred nanometers in length, and they were open-end with several wall layers on both sides. The measured interlayer spacing was about 0.90 nm. Moreover, it was found that the obtained nanotubular product had a scroll structure as shown in Figure 1B. In the XRD pattern of the nanotubular product as shown in Figure 1C, the broad reflection peaks were observed at 2θ of approximately 10°, 24°, 30°, 48°, and 62°. The XRD pattern of the synthesized nanotubular product in this study was consistent with nanotubular products prepared in other previous reports.9-11,17-20 In particular, the peak at 2θ ) ca. 10° (d ) 0.88 nm) was corresponding to the interlayer spacing value measured by TEM observation, indicating that this product might have a layered titanate structure.
Figure 2. FT-IR spectra of (a) anatase and (b) the nanotubular product. Arrows show O-H bonds.
Sodium hardly could be detected in the nanotubular product according to surface analytical methods such as EDX and XPS analysis. Figure 2 shows FT-IR spectra of anatase and the dried nanotubular product. For the nanotubular product, three absorption bands centered at 3400, 1630, and 950 cm-1 are assigned to O-H stretching mode for interlayer water, oxonium ions, and hydroxyl groups, H-O-H bending for water and oxonium ions, and O-H bending for hydroxyls, respectively.25,26 These results indicate the most replacement of Na+ in the nanotubular product by H+. Adsorption/desorption isotherms for the synthesized nanotubular product were measured. The typical isotherms for anatase-type TiO2 and the nanotubular product are shown in Figure 3. The standard isotherm for anatase-type TiO2 belonged to a metal oxide (type II according to IUPAC classification).
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Figure 3. Typical adsorption/desorption isotherms of anatase and the nanotubular product.
Figure 4. Ti 2p XPS of (a) anatase and (b) the nanotubular product.
In case of the nanotubular product, the adsorption hysteresis was observed in the region of a relative pressure P/P0 above 0.4, suggesting that the isotherm pattern showed type IV. Considering that the nanotubular product was formed by rollingup of nanosheet (Figure 1A), it is thought that platelike or sheetlike aggregates give rise to slit-shaped pores resulting in the hysteresis. The Brunauer-Emmett-Teller (BET) surface area value obtained by N2 adsorption measurement for the nanotubular product was over 250 m2/g. Figure 4 shows the Ti 2p XPS of the nanotubular product prepared by the hydrothermal process and anatase-type TiO2 as a starting material. The spin-orbit components (2p3/2 and 2p1/2) of each peak of anatase-type TiO2 were well deconvoluted by two curves (at approximately 458.9 and 464.6 eV, respectively) corresponding to Ti(IV) in a tetragonal structure.27 Through the hydrothermal process, the binding energy of Ti 2p peaks slightly shifted to lower binding energy (for the nanotubular product, the binding energies of Ti 2p3/2 and 2p1/2 were located at 458.5 and 464.2 eV, respectively). It has generally been reported that Ti-O-Ti linkages of TiO6 octahedra are broken in strong alkali solutions, leaving some oxygen vacancies in the formation of nanotubes.6,28,29 It is conjectured that the shift of Ti 2p peaks might be caused by the presence of some oxygen vacancies. Thus, these XPS results showed that the local structure around the Ti ion in the nanotubular product was different from the local structure around the Ti ion in anatase-type TiO2. The structure of nanotubular products prepared by the hydrothermal process was further investigated from information on local structure around Ti by Ti K edge XAFS (EXAFS and XANES) measurements. Figure 5A shows k3-weightd EXAFS oscillations for anatasetype TiO2 and the nanotubular product prepared by the hydrothermal treatment of anatase at 383 K for 96 h. As shown in Figure 5A, the EXAFS oscillation of the nanotubular product was obviously different from that of anatase in the amplitude. Figure 5B shows the results of FT-EXAFS for anatase-type TiO2 and the nanotubular product. They represent a radial distribution function plot around the Ti atom. From FT-EXAFS for anatasetype TiO2, the first and second peaks were indicated as the coordination number corresponding to the Ti-O and Ti-Ti bond distance, respectively. The magnitudes of the second peak
Kubo and Nakahira and the subsequent ones were further smaller in FT-EXAFS for the nanotubular sample in comparison to anatase-type TiO2, indicating that the nanotubular product has only periodic structures in short order. The decrease of magnitude of the second peak and subsequent ones is consistent with low crystallinity as shown in XRD patterns (Figure 1C). This may imply some vacancy of the neighboring atoms such as oxygen. To obtain the structural parameters, the curve fitting of the Ti-O shell within the range of 0.6-2 Å-1 was performed by inverse FT. The later each value was obtained from the simulation of the experimental spectrum using the theoretical curves calculated by Mckale et al.30 The inverse FT of first-shell signal and the best fit are show in Figure 6. In both samples, the agreements between the two curves are quite satisfactory. Notably, the nearest Ti-O peak for anatase-type TiO2 in FT-EXAFS was sharp, and the Ti-O distance and its average coordination number of anatase were approximately 1.96 Å and 6.0, respectively. On the other hand, the nearest Ti-O peak for the nanotubular product was broad, and its distance was 1.92 Å. The Ti-O average coordination number for the nanotubular product was 5.2. It is considered that the shift of the nearest Ti-O peak toward smaller distances and the difference of the peak geometry are due to highly distorted TiO6 octahedra. Thus, EXAFS results indicated that TiOx polyhedra in the nanotubular product were obviously different from ones in anatase-type TiO2 as a starting material. Figure 7A shows Ti K edge XANES spectra for the nanotubular product, anatase-type TiO2, rutile-type TiO2, Ti2O3, and TiO. The edge region in the absorption spectra provides much information on the environment geometry and the electronic structure of the absorption atom. Figure 7B shows the first-derivative regions of Ti K edge XANES spectra shown in Figure 7A. The edge energy is defined as the energy position corresponding to the peak maximum of the first-derivative function. The XANES spectrum of nanotubular product slightly revealed lower energy position for the peak maximum of the first-derivative function (edge energy), compared with the XANES spectrum of anatase. However, the peak position of the nanotubular product was very close to the peak position of TiO2. Furthermore, as shown in Figure 7A, the characteristic preedge peaks were also observed at 4960-4970 eV. The preedge can be assigned to forbidden transitions from the core 1s level to unoccupied 3d states of a Ti(IV). According to the position of edge energy and the presence of preedge, it was considered that Ti ions in the nanotubular product were mainly present as Ti(IV). These preedge features are widely used to derive information on the coordination environment of Ti(IV) in structurally complex oxide materials, such as titano-silicate glasses, Ti-containing zeolite, and Ti-containing mesoporous silica, etc.31-34 Figure 7C shows the preedge of Ti K edge XANES for anatase-type TiO2, nanotubular product, and K2Ti2O5 as a reference sample. K2Ti2O5 is a layered titanate material containing five-coordinate Ti.35 Here, the preedge spectrum of anatasetype TiO2 contains three major features in this region (peaks A, C, and D in Figure 7C). Although the origin of peak A is less clear, it is believed to be associated with Ti 3d-4p hybridized states.36 The C and D peaks are attributed to the 1s f t2g and eg electronic transitions, respectively.37 In the preedge of the Ti K edge XANES for K2Ti2O5, a strong peak is located at 4961 eV (peak B in Figure 7C). This peak B is assigned to the allowed 1s f t2g transition for tetrahedral symmetry. Such a peak B dominates the preedge XANES for titanate materials containing five-coordinate Ti.38 In the preedge of the Ti K edge
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Figure 5. (A) k3-weighted EXAFS oscillations and (B) FT-EXAFS for the nanotubular product and anatase.
Figure 6. Inverse Fourier transform (full line) and the best fit (dotted line). Panels A and B are the data for anatase and the nanotubular product, respectively.
Figure 7. (A) Ti K edge XANES for (a) the nanotubular product, (b) anatase, (c) rutile, (d) Ti2O3, and (e) TiO. (B) The first-derivative functions of the Ti K edge XANES spectra shown in (A). (C) The preedge of the Ti K edge XANES for (a) anatase, (b) K2Ti2O5, and (c) the nanotubular product.
XANES for the nanotubular product, the relative intensities of peaks B and C were higher than ones of anatase and the preedge features were similar to titanates such as K2Ti2O5, indicating the presence of five-coordinate Ti in the nanotubular product. It was considered that the TiO6 octahedron in anatase-type TiO2 has deformed with decrease of symmetry of the nearest oxygen
around the Ti ion on the formation of nanotubes, indicating that the local structure of the nanotubular product was similar to the one of titanate K2Ti2O5. Ma et al. have also discussed about the structure of the nanotubular product by the analytic methods such as XAFS measurements and concluded that the nanotubes might be composed of layered titanate structure.24 According to their report, the nanotubular products prepared by the hydrothermal process are still mainly composed of layered titanate. However, although peak C is confirmed for the nanotubular product in the preedge of the Ti K edge XANES, the peak is not consistent with ones for K2Ti2O5. Since this peak C is also observed in the preedge for anatase-type TiO2, it is thought that peak C was derived from the anatase in the nanotubular product. The XRD result showed the nanotubular products had no peaks from anatase-type TiO2 (as shown in Figure 1A), indicating that this peak around 4963.8 eV from the anatase structure was resulted from the nanotubular structure, not anatase-type TiO2 as a starting material. Tsai and Teng and Yang et al. have reported that the final pH value of the washing water after the washing process had much effect on the structure of the nanotubes, and the layered titanate transformed into a nanotube through Na+ f H+ substitution and eventually transformed into anatase-type TiO2.28,39,40 In their previous study, the nanotubes prepared by the washing treatments at pH value of 5-13 were composed of titanate compound, and the nanotubes prepared by acidic posttreatments washing at pH values of 0-2 had XRD patterns analogous to that of anatase-type TiO2 rather than that of the titanate compound. In fact, our XRD results also indicated that the product transformed to anatase-type TiO2 by excessive HCl washing process. Since the final pH value of the washing water was 6.8 in our synthesis process, it is considered that the layered titanate-like product transformed into a nanotube through Na+ f H+ substitution during the washing treatment. Therefore, this peak C is related with the formation of the nanotube. In other words, it is considered that the titanate nanotube partly had an anatase-like structure derived from the nanotubular structures. Consequently, it is found that the nanotubular products prepared by the hydrothermal process were mainly composed of layered titanate and partly contain anatase-like
1662 J. Phys. Chem. C, Vol. 112, No. 5, 2008 structure. The detailed study for the titanate-based nanotube is now under further investigation from information on intermediate-range order by high-energy X-ray diffraction (HE-XRD) and neutron diffraction (ND) at the Synchrotron Radiation Research Center.41 Thus, Ti K edge XAFS analysis revealed the anatase-like structure locally existing in titanate-based nanotubes prepared by the hydrothermal process in the NaOH system under our investigation. By examining different synthesized nanotubes, it will be possible to find an association between the titanate/ anatase ratio and the synthesis conditions such as final pH value in the washing process. Therefore, XAFS analysis may be a useful technique for the evaluation of nanomaterials with unique structures such as these titanate nanotubes. Conclusions In this study, TiO2-derived nanotube with about 10 nm o.d. and 5 nm i.d. and a few hundred nanometers in length was synthesized by the hydrothermal treatment of anatase-type TiO2 in 10 M NaOH aqueous solution and subsequent washing treatments with H2O/HCl. The structure of the synthesized nanotubular product was evaluated by various analytical methods. From TEM observation and N2 adsorption measurement, it was found that the nanotubular products had scroll structures of sheetlike product and possessed high BET surface area. The local structure of the TiO2-derived nanotube was analyzed in detail from information on short-range order by XAFS measurements of the Ti K edge. EXAFS results indicated that TiOx polyhedra in the nanotubular product were obviously different from ones in anatase-type TiO2 as a starting material. Furthermore, XRD, TEM, and XANES results showed that the nanotubular products were mainly composed of layered titanate. On the other hand, it was found that the anatase-like structure might be partly present in the as-synthesized titanate-based nanotubes by the preedge of the Ti K edge XANES. The formation of the anatase-like structure was thought to be resulting from the consolidation of nanotubes. Acknowledgment. XAFS measurements were carried out at BL01B1 in SPring8 (2004B0166). The authors are greatly thankful for the technical support and discussion from JASRI. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Ajayan, P. M.; Stephan, O.; Redlich, P.; Colliex, C. Nature 1995, 375, 564. (3) Hoyer, P. Langmuir 1996, 12, 1411. (4) Imai, H.; Takei, Y.; Shimizu, K.; Matsuda, M.; Hirashima, H. J. Mater. Chem. 1999, 9, 2971. (5) Kasuga, T.; Hirashima, M.; Hoson, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14, 3160. (6) Kasuga, T.; Hirashima, M.; Hoson, A.; Sekino, T.; Niihara, K. AdV. Mater. 1999, 11, 1307.
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