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Nanotube Formation from a Sodium Titanate Powder via Low-Temperature Acid Treatment Chien-Cheng Tsai and Hsisheng Teng* Department of Chemical Engineering and Center for Micro/Nano Science and Technology, National Cheng Kung UniVersity, Tainan 70101, Taiwan ReceiVed September 12, 2007. In Final Form: NoVember 6, 2007 Through structure-monitoring of nanotube formation from a lamellar sodium titanate, the present work explicitly elucidated the structure of the titanate nanotubes obtained from hydrothermal treatment of TiO2 with NaOH. A new compound of an orthorhombic lepidocrocite-type sodium titanate was synthesized from calcination of a solid-state mixture of TiO2 anatase and Na2CO3 powders followed by hydrothermal treatment with NaOH. By treating with acid at 25 °C for Na+ exchange with H3O+, the titanate compound exfoliated and then proceeded with sheet-scrolling to form nanotubes, which had a structure and morphology very close to those of the nanotubes derived from NaOH treatment on TiO2. During the low-temperature acid treatment, the lepidocrocite-type titanate is transformed from the orthorhombic C-base-centered symmetry to the body-centered symmetry. This transformation, accompanied by a size-contraction of TiO6-octahedron units, was critical for the formation of nanotubes. The present work provides direct evidence, for the first time, that the widely reported TiO2-derived titanate nanotubes can be obtained at low temperatures by scrolling the sheets exfoliated from the orthorhombic lepidocrocite-type titanate.
Introduction Hydrothermal recrystallization of TiO2 in concentrated aqueous NaOH solution to produce high-purity titanate nanotubes has drawn great attention because of the unique structural features of the nanotubes for numerous applications.1-9 However, the titanate structure that constitutes the nanotubes is still a topic under debate. It has been generally recognized that the nanotubes are formed by scrolling sheets exfoliated from a lamellar titanate that was suggested to be A2Ti3O7,10-20 A2Ti2O4(OH)2 (or A2Ti2O5‚H2O),21-24 or lepidocrocite-type AxTi2-x/40x/4O4 (A: * Corresponding author. E-mail:
[email protected]. Fax: +8866-2344496. Tel: +886-6-2385371. (1) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14, 3160-3163. (2) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. AdV. Mater. 1999, 11, 1307-1311. (3) Zhang, Q.; Gao, L.; Sun, J.; Zheng, S. Chem. Lett. 2002, 226-227. (4) Wang, Y. Q.; Hu, G. Q.; Duan, X. F.; Sun, H. L.; Xue, Q. K. Chem. Phys. Lett. 2002, 365, 427-431. (5) Yao, B. D.; Chan, Y. F.; Zhang, X. Y.; Zhang, W. F.; Yan, Z. T.; Wang, N. Appl. Phys. Lett. 2003, 82, 281-283. (6) Tsai, C. C.; Teng, H. S. Chem. Mater. 2004, 16, 4352-4358. (7) Wang, W.; Varghese, O. K.; Paulose, M.; Grimes, C. A. J. Mater. Res. 2004, 19, 417-422. (8) Bavykin, D. V.; Parmon, V. N.; Lapkin, A. A.; Walsh, F. C. J. Mater. Chem. 2004, 14, 3370-3377. (9) Bavykin, D. V.; Friedrich, J. M.; Walsh, F. C. AdV. Mater. 2006, 18, 2807-2824. (10) Du, G. H.; Chen, Q.; Che, R. C.; Yuan, Z. Y.; Peng, L. M. Appl. Phys. Lett. 2001, 79, 3702-3704. (11) Chen, Q.; Du, G. H.; Zhang, S.; Peng, L. M. Acta Crystallogr., Sect. B 2002, 58, 587-593. (12) Chen, Q.; Zhou, W.; Du, G.; Peng, L. M. AdV. Mater. 2002, 14, 12081211. (13) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Xu, H. J. Am. Chem. Soc. 2003, 125, 12384-12385. (14) Zhang, S.; Peng, L.-M.; Chen, Q.; Du, G. H.; Dawson, G.; Zhou, W. Z. Phys. ReV. Lett. 2003, 91, 256103. (15) Sun, X.; Li, Y. Chem.sEur. J. 2003, 9, 2229-2238. (16) Yuan, Z. Y.; Su, B. L. Colloids Surf., A 2004, 241, 173-183. (17) Thorne, A.; Kruth, A.; Tunstall, D.; Irvine, T. S. J.; Zhou, W. J. Phys. Chem. B 2005, 109, 5439-5444. (18) Zhang, S.; Chen, Q.; Peng, L. M. Phys. ReV. B 2005, 71, 014104. (19) Bavykin, D. V.; Friedrich, J. M.; Alexei, A.; Lapkin, A. A.; Walsh, F. C. Chem. Mater. 2006, 18, 1124-1129. (20) Morgado, E., Jr.; de Abreu, M. A. S.; Praia, O. R. C.; Marinkovic, B. A.; Jardim, P. M.; Rizzo, F. C.; Araujo, A. S. Solid State Sci. 2006, 8, 888-900.
Na and/or H; 0: vacancy).25,26 These suggested structures can principally be divided into two crystalline systems: (1) the monoclinic A2Ti3O7 system in which the trebled TiO6 octahedra are joined via corner-sharing to form puckered sheets as the host layers; (2) the orthorhombic system in which TiO6 octahedra are combined via edge-sharing to form two-dimensional lepidocrocite-type sheets as the host layers (see Supporting Information). A2Ti2O4(OH)2 (or A2Ti2O5‚H2O), which also has lepidocrocitetype host layers, belongs to the orthorhombic system. The present work would systematically offer a complete disclosure of the nanotube structure through monitoring the formation of nanotubes directly from a well-characterized sodium titanate powder. To identify the structure and formation mechanism of the nanotubes, a previous study conducted hydrothermal treatment on the trititanate Na2Ti3O7 powder in concentrated NaOH solution.27 This treatment did not transform the monoclinicphase trititanate into any tubular species. On the basis of the result, the study concluded that the trititanate is a stable compound, and a soft-chemical treatment cannot exfoliate the layered structure for nanotube formation. In a preliminary experiment we conducted the hydrothermal treatment (130 °C for 24 h) on Na2Ti3O7 in NaOH with subsequent acid treatment to assist nanotube formation.2,6,15 Still, no tubular structure was found. Under this circumstance, the present work alternatively focused the sheet-scrolling examination on the titanates belonging to the orthorhombic system. In order to examine the orthorhombic-phase titanates, we synthesized here a new sodium titanate compound via a solidstate method, in which a TiO2 anatase/Na2CO3 mixture was calcined to obtain the desired compound. We subjected this newly (21) Yang, J.; Jin, Z.; Wang, X.; Li, W.; Zhang, J.; Zhang, S.; Guo, X.; Zhang, Z. Dalton Trans. 2003, 3898-3901. (22) Zhang, M.; Jin, Z.; Zhang, J.; Guo, X.; Yang, J.; Li, W.; Wang, X.; Zhang, Z. J. Mol. Catal. A: Chem. 2004, 217, 203-210. (23) Tsai, C. C.; Teng, H. S. Chem. Mater. 2006, 18, 367-373. (24) Nian, J N.; Teng, H. S. J. Phys. Chem. B 2006, 110, 4193-4198. (25) Ma, R.; Bando, Y.; Sasaki, T. Chem. Phys. Lett. 2003, 380, 577-582. (26) Ma, R.; Fukuda, K.; Sasaki, T.; Osada, M.; Bando, Y. J. Phys. Chem. B 2005, 109, 6210-6214. (27) Kukovecz, A.; Hodos, M.; Horvath, E.; Radnoczi, G.; Konya, Z.; Kiricsi, I. J. Phys. Chem. B 2005, 109, 17781-17783.
10.1021/la702839u CCC: $40.75 © 2008 American Chemical Society Published on Web 01/30/2008
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synthesized titanate to hydrothermal treatment in NaOH, which was followed by acid treatment at low temperatures for nanotube formation. The present work analyzed and monitored the structure of the compound from each treatment stage, attempting to shed light on the structure and formation mechanism of the titanate nanotubes. Experimental To synthesize the sodium titanate compound via a solid-state route, a ball-milled mixture of commercial TiO2 anatase (Acros; 100% anatase with a crystal size of 30 nm) and Na2CO3 (Fullin), with a Ti/Na molar ratio of 0.9/1, was calcined in air at 600 °C for 2 h. Hydrothermal treatment on this solid-state synthesized specimen was conducted by treating 1 g of this specimen with 70 mL of 10 N NaOH solution in a sealed Teflon-lined autoclave at 130 °C for 24 h. The influence of pressure would not be an important issue because the saturation pressure of 10 N NaOH was reported to be ca. 1.2 bar at 120 °C.28 The product from filtration was washed with ethanol to obtain a hydrothermally treated specimen, or washed with 0.1 N HNO3 solution at 25 °C to reach pH values of 6 and 1.7 to obtain other specimens. The phase identification of the specimens was conducted with powder X-ray diffraction (XRD) at a scanning rate of 4°/min by using a Rigaku RINT2000 diffractometer equipped with CuKR radiation. The XRD patterns for Rietveld analysis were collected with a step size of 0.01° and a count time of 6 s per step. The Generalized Structure and Analysis Software (GSAS) package was used for Rietveld structural refinement.29 The proposed crystal structure of titanates were constructed with the Ca.R.Ine version 3.1 crystallography program package.30 The microstructures were explored with a high-resolution transmission electron microscope (HRTEM, Hitachi FE-2000). The Na/Ti ratio of the specimens was analyzed by using an inductively coupled plasma-mass spectrometer (ICP-Mass, Hewlett-Packard 4500). The composition analysis was carried out after dissolving the speciomens in HCl solution. The specific surface area was measured by N2 adsorption at -196 °C using an adsorption apparatus (Micromeritics, ASAP 2010), with the adsorption data fitted to the Brunauer-Emmett-Teller (BET) equation. The specific pore volume was obtained from the total amount adsorbed at relative pressures near unity. The pore size distribution was analyzed by using the Barrett-Joyner-Halenda (BJH) method.
Figure 1. Powder XRD patterns of the titanate specimens obtained from the solid-state synthesis (designated as Sodium-Titanate), from the hydrothermal treatment on Sodium-Titanate (designated as Sodium-Titanate-H), and from the acid treatment on SodiumTitanate-H to pH values of 6 and 1.7 (designated as Sodium-TitanateHA and NT, respectively). The diffraction peaks are indexed according to the orthorhombic-phase structure shown in Figure 4a,b for Sodium-Titanate and Sodium-Titanate-HA, respectively.
Results and Discussion In the solid-state synthesis of the new sodium titanate compound, kinetic control, including temperature and reaction time adjustment, was a critical issue for obtaining a phase-pure titanate here. Above 600 °C or longer calcination time (>2h), there showed the presence of Na2Ti3O7 impurity. The content of the impurity increased with the synthesis temperature and time. The XRD pattern of the 600 °C-synthesized sodium titanate, which is designated as Sodium-Titanate, is shown in Figure 1. The transmission electron microscopic (TEM) image of the Sodium-Titanate specimen is shown in Figure 2a. This titanate specimen consisted of plate-like particles with well-defined crystalline structure, as reflected by the selected area electron diffraction patterns shown in Figure 2b,c. These figures also provide the corresponding HRTEM images showing the lattice fringes directing along different zone axes. The lattice fringe in Figure 2c clearly reflects the lamellar feature of this newly synthesized Sodium-Titanate. (28) Perry, R. H.; Chilton, C. H. Perry’s Chemical Engineers’ Handbook, 7th ed.; McGraw-Hill Book Co.: New York, 1997. (29) Toby, B. H. J. Appl. Cryst. 2001, 34, 210-213. (30) Ca.R.Ine Crystallography, version 3.1;International Union of Crystallography: Senlis, France, 1994.
Figure 2. TEM images of the Sodium-Titanate specimen obtained from the solid-state synthesis (a) and the selected area electron diffraction patterns with their corresponding HRTEM images showing the lattice fringes directing along the zone axis [21h1 ] (b) and the axis [11h1] (c).
To identify the crystalline structure of Sodium-Titanate, the GSAS program was used for Rietveld analysis on the diffraction pattern.31 All peaks on the diffraction pattern of Sodium-Titanate can be indexed as belonging to the orthorhombic unit cell. Thus,
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Figure 4. Proposed crystal structures of NaxTi2-x/40x/4O4 for the Sodium-Titanate specimen in orthorhombic C-base-centered symmetry (a) and Nax+y-zHzTi2-x/40x/4O4(OH)y‚nH2O for the SodiumTitanate-HA specimen in orthorhombic body-centered symmetry (b). In both models, the interlayer sites are shown in a fully occupied situation. Because of the small scattering factor for H atoms, OH and H2O are not distinguished in the structural simulation. The change from the C-base-centered symmetry to the body-centered symmetry can be caused by a slip of the middle layer by c0/2 along the c-axis against the top and bottom layers. Table 1. Refined Structure Parameters of NaxTi2-x/40x/4O4 for the Sodium-Titanate Specimen in the Orthorhombic System with C-base-centered Symmetry and Cmc21 Space Group
Figure 3. Observed (dot) and Rietveld-refinement simulated (line) XRD profiles for the Sodium-Titanate and Sodium-Titanate-HA specimens. The difference between the observed and simulated data is shown at the bottom of the figures. The reliability factors are Rwp ) 0.129 and Rp ) 0.085 for Sodium-Titanate, and Rwp ) 0.123 and Rp ) 0.087 for Sodium-Titanate-HA.
the atomic coordinates for lepidocrocite-type titanates were considered the initial parameter-trial for the Rietveld structural refinement on Sodium-Titanate.29 The result of Rietveld refinement (the simulated pattern and its difference from the observed one) is shown in Figure 3a, which accounts for satisfactory reliability factors of Rwp ) 0.129 and Rp ) 0.085. The parameters Rp and Rwp stand for the regression sum of relative errors and the regression sum of weighted squared errors, respectively. It gave the refined values of orthorhombic phase lattice parameters of a0 ) 0.4346 nm, b0 ) 2.0513 nm, and c0 ) 0.4065 nm with C-base-centered symmetry and Cmc21 space group (No. 36). The proposed crystal structure of Sodium-Titanate is displaced in Figure 4a, showing host layers of two-dimensional lepidocrocite-type sheets that comprise edge-shared TiO6 octahedra. Table 1 shows the atomic fractional coordinates of the refined Sodium-Titanate structure, in which the site occupancies are 0.93 and 0.27 for Ti and Na, respectively. The atomic coordinates are in good agreement with the TEM diffraction patterns shown in Figure 2b,c. The Na/Ti atomic ratio of Sodium-Titanate was found to be 0.29 by using an ICP-Mass spectrometer. This gave Sodium-Titanate a lepidocrocite-type formula of NaxTi2-x/40x/4O4, where x ) 0.54. On the basis of the crystal structure shown in Figure 4a, this x value was in excellent agreement with the Ti and Na occupancies (Table 1) determined from the Rietveld analysis. (31) Groult, D.; Mercey, C.; Raveau, B. J. Solid State Chem. 1980, 32, 289296.
atom
characteristics of positiona
x
y
z
occupancy
UISOb
Ti O O Na
4a 4a 4a 4a
0 0 0 0
0.3136 0.3835 0.2178 0.0746
0.6390 0.0960 0.1480 0.6465
0.93 1 1 0.27
0.4061 0.0091 0.1268 0.0189
a The number represents the multiplicity of the position, and the letter represents the Wyckoff notation, which specifies a position and runs in the sequence of a, b, c, .... b The isotropic thermal parameter.
Hydrothermal treatment in 10 N NaOH at 130 °C for 24 h was carried out for the lamellar Sodium-Titanate, in an attempt to lessen the interlayer attraction via intercalation with ions or molecules for structure exfoliation or even sheet-scrolling into tubes. This hydrothermally treated titanate (with subsequent washing with ethanol), designated as Sodium-Titanate-H, has a XRD pattern similar to that of Sodium-Titanate (see Figure 1). However, the Na/Ti atomic ratio was increased to 0.46 because of this hydrothermal treatment. There should be intercalation of Na+ ions, together with OH- ions for charge balance, into the interstices of this lamellar structure. This gave Sodium-Titanate-H a chemical formula of Nax+yTi2-x/40x/4O4(OH)y‚nH2O, where x ) 0.54 and y ) 0.32. The formula also reflects that there might be H2O molecules, composed of exchangeable H+ and OHions, situated between the host layers of the titanate lattice. The hydrothermal treatment reinforced the crystallinity of the titanate, as reflected by the better-defined diffraction peaks for SodiumTitanate-H relative to those for Sodium-Titanate (Figure 1). Essentially, the lattice parameters of Sodium-Titanate-H were similar to those of Sodium-Titanate. In preparing titanate nanotubes from NaOH treatment on TiO2, some previous studies considered neutralization washing a critical step for scrolling the titanate sheets into nanotubes.2,6,15,23 We washed Sodium-Titanate-H with 0.1 N HNO3 at 25 °C to reach a equilibrated pH value of 6 before filtration. The XRD pattern
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Table 2. Na/Ti Atomic Ratios and Pore Structures of the Titanate Specimen Obtained from the Solid-State Synthesis (Sodium-Titanate) and those Obtained after the Consecutive Post-treatments: The Hydrothermal Treatment in NaOH (Sodium-Titanate-H), the Acid Treatment to pH ) 6 (Sodium-Titanate-HA), and the Acid Treatment to pH ) 1.7 (NT)
titanate specimen
Na/Ti atomic ratio
surface area (m2/g)
pore volume (cm3/g)
Sodium-Titanate Sodium-Titanate-H Sodium-Titanate-HA NT
0.29 0.46 0.28 0.11
4.0 46 98 310
0.037 0.24 0.38 0.71
of this acid washed titanate, designated as Sodium-Titanate-HA, is also shown in Figure 1. There were significant changes in the diffraction pattern observed due to this acid-washing, although an auxiliary TEM analysis showed that Sodium-Titanate-HA was still in the plate-like morphology. The result from the Rietveld structural refinement (Figure 3b) revealed that this SodiumTitanate-HA compound belongs to an orthorhombic system with body-centered symmetry and Immm space group (No. 71). It gave the orthorhombic phase lattice parameters of a0 ) 0.3801 nm, b0 ) 1.8880 nm, and c0 ) 0.3076 nm and satisfactory reliability factors of Rwp ) 0.123 and Rp ) 0.087. These cell parameters were validated by TEM diffraction analysis. The proposed crystal structure of Sodium-Titanate-HA is shown in Figure 4b, in which the interlayer sites are occupied by Na and OH (or H2O). The Na/Ti atomic ratio of Sodium-Titanate-HA was found to be 0.28 by the ICP-Mass analysis. This ratio reflects that the Na+ contained in Sodium-Titanate-H could be exchanged with H3O+ during the washing with the HNO3 solution, thus maintaining the overall charge neutrality (except for residual microscopic charges). The Na/Ti atomic ratios of the different titanate specimens are summarized in Table 2. The interlayer spacing, corresponding to the parameter b0 of Sodium-Titanate-H or Sodium-Titanate-HA, is seen to decrease because of the ion exchange during acid washing. It is of interest to observe the smaller values of the lattice parameters a0 and c0 for Sodium-Titanate-HA relative to those for Sodium-TitanateH. The host layers of the titanates are situated on the a×c planes, as shown in Figure 4. The decrease in the values of a0 and c0 reflects that the ion exchange must have led to a size contraction of the TiO6 octahedra constituting the two-dimensional host layers of the titanates. This contraction in TiO6 was accompanied by a slip of every other layer by c0/2 along the c-axis against the stationary layers, as reflected by the difference between the Sodium-Titanate (or Sodium-Titanate-H) and Sodium-TitanateHA crystal structures shown in Figure 4. As a result, SodiumTitanate-H with the C-base-centered symmetry changed, as a result of the exchange of Na+ by H3O+, to Sodium-Titanate-HA with body-centered symmetry. This layer slip and TiO6 contraction imply the loosening of the lamellar titanate framework in which the TiO6 layers are coordinated by the Na+ cations. The removal of Na+ leads to weakened interaction between neighboring TiO6 layers.32 The loosening promotes the possibility of sheetexfoliation. Previous studies also reported that the titanate nanotubes obtained from hydrothermal treatment of TiO2 with NaOH had an orthorhombic structure with body-centered symmetry.23,25 With further acid-washing Sodium-Titanate-H (also at 25 °C) to reach a pH value of 1.7, we found the formation of nanotubes,
as shown by the TEM image in Figure 5. These nanotubes had open ends and were 10-15 nm wide and 100-200 nm long with 4-6 wall layers. The Na/Ti atomic ratio of the nanotubes was reduced to 0.11. Figure 2 shows the XRD pattern of the nanotube specimen, which is designated as NT. By comparing the diffraction pattern with those extensively reported in literature,10-26 we found that the peak positions for NT derived from acidwashing the lepidocrocite-type Sodium-Titanate-H was very close to those for the nanotubes derived from NaOH treatment on TiO2. This similarity in the structure of the nanotubes evidently reflects the fact that the TiO2-derived titanate nanotubes are obtained by scrolling titanates belonging to the orthorhombic system, in which TiO6 octahedra are combined via edge-sharing to form two-dimensional lepidocrocite-type sheets as the host layers. A study also reported, on the basis of molecular dynamics calculations, that the lepidocrocite-type titanate is the final product obtained from the hydrothermal treatment of TiO2 with NaOH.33 Both the Sodium-Titanate-H synthesized in the present work and the A2Ti2O4(OH)2 (or A2Ti2O5‚H2O) reported by previous studies belong to this orthorhombic system.21,23 For the treatments following the solid-state synthesis, including both the hydrothermal and acid-washing, there are mechanisms of charge balance and intercalation or exchange of ions involved in the treatments. The kinetics of these charge or ion introduction processes should affect structure exfoliation and sheet-scrolling into tubes. We found that a high yield of NT formation required a gradual acid-washing process. Apart from the crystalline structure, we also monitored the surface morphology of the titanate specimens by using N2 adsorption. The adsorption-desorption isotherms (see Supporting Information) exhibit hysteresis behavior, indicating that the specimens are mainly mesoporous. The BJH method was employed to analyze the pore size distribution, and the results are shown in Figure 6. The distribution clearly shows that the porosity of the titanate specimens is contributed by mesopores (2-100 nm in pore size). The interstice between the crystal grains of the specimens should be the main contributor to the porosity. The surface area and pore volume calculated according to the adsorption data are summarized in Table 2. The porosity of Sodium-Titanate from the solid-state synthesis was very small. With the progress of the treatments, including the hydrothermal treatment and acid-washing, the porosity of the titanate specimen increased significantly. Obviously, these treatments not only
(32) Wei, M. D.; Konishi, Y.; Zhou, H. S.; Sugihara, H.; Arakawa, H. Solid State Commun. 2005, 133, 493-497.
(33) Alvarez-Ramirez, F.; Ruiz-Morales, Y. Chem. Mater. 2007, 19, 29472959.
Figure 5. TEM images of the NT specimen obtained from acidwashing the Sodium-Titanate-H specimen to a pH value of 1.7. The inset shows the multilayer feature of the tube wall.
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Summary and Conclusions
Figure 6. Pore size distribution of the titanate specimen obtained from the solid-state synthesis (Sodium-Titanate) and those obtained after the consecutive post-treatments: the hydrothermal treatment in NaOH (Sodium-Titanate-H), the acid treatment to pH ) 6 (SodiumTitanate-HA), and the acid treatment to pH ) 1.7 (NT).
loosened the crystalline lattice but also segregated the crystal grains for larger interstitial space (likely through the introduction of residual microscopic charges). After exfoliation of the lamellar structure and subsequent sheet-scrolling into nanotubes, the NT specimen obtained reached a specific surface area as large as 310 m2/g (Table 2). A large surface area was generally observed for titanate nanotubes derived from the hydrothermal treatment of TiO2 with NaOH,6,24 and was considered a beneficial characteristic for a great variety of applications.34-38 (34) Fang, X. S.; Ye, C. H.; Zhang, L. D.; Zhang, J. X.; Zhao, J. W.; Yan, P. Small 2005, 1, 422-428. (35) Fang, X. S.; Ye, C. H.; Zhang, L. D.; Xie, T. AdV. Mater. 2005, 17, 1661-1665.
We monitored the structure change during titanate nanotube formation from low-temperature acid treatment on a lepidocrocitetype sodium titanate compound. This new titanate compound was obtained via calcination on a solid-state NaCO3/TiO2 mixture followed by hydrothermal treatment of the resulting product in NaOH. The acid treatment led to exchange of Na+ ions in the titanate, to structure transformation from the orthorhombic C-basecentered symmetry to the body-centered symmetry (caused by a slip of every other layer along the c-axis), and eventually to scrolling the nanosheets exfoliated from the lamellar titanate into nanotubes. This structure transformation was accompanied by size contraction of the TiO6 octahedra constituting the host layers of the titanate. The contraction as well as the layer slip was an indication for the loosening of the interlayer coordination. Knowledge obtained from this structure-monitoring will be useful to elucidate the structural feature and formation mechanism of the TiO2-derived titanate nanotubes, which have been found to be useful in a great variety of applications. Acknowledgment. This research is supported by the National Science Council of Taiwan (NSC 95-2221-E-006-408-MY3 and NSC 95-2218-E-006-019) and the Center for Micro/Nano Science and Technology of National Cheng Kung University. Supporting Information Available: Structure models of the monoclinic and orthorhombic titanate systems; N2 adsorption-desorption isotherms for the titanate specimen obtained from the solid-state synthesis and those obtained after the consecutive posttreatments. This material is available free of charge via the Internet at http://pubs.acs.org. LA702839U (36) (a) Bavykin, D. V.; Lapkin, A. A.; Plucinski, P. K.; Friedrich, J. M.; Walsh, F. C. J. Catal. 2005, 235, 10-17. (b) Nian, J. N.; Chen, S. A.; Tsai, C. C.; Teng, H. S. J. Phys. Chem. B 2006, 110, 25817-25824. (37) Riss, A.; Berger, T.; Grothe, H.; Bernardi, J.; Diwald, O.; Kno¨zinger, E. Nano Lett. 2007, 7, 433-438. (38) Miyauchi, M.; Tokudome, H. J. Mater. Chem. 2007, 17, 2095-2100.
