TEM Investigation and FBB Model Explanation to the Phase

Jun 14, 2010 - A generalized relationship of phase transitions between sodium/hydrogen titanates and titania by the wet-chemical process was proposed...
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TEM Investigation and FBB Model Explanation to the Phase Relationships between Titanates and Titanium Dioxides Hongwei Liu, Eric R. Waclawik, Zhanfeng Zheng, Dongjiang Yang, Xuebin Ke, Huaiyong Zhu, and Ray L. Frost* Discipline of Chemistry, Faculty of Science and Technology, Queensland UniVersity of Technology, Brisbane QLD 4001, Australia ReceiVed: April 23, 2010; ReVised Manuscript ReceiVed: May 31, 2010

Sodium and hydrogen titanates fibers and three TiO2 polymorphs (TiO2 (B), anatase, and rutile) were prepared from inorganic titanium compounds by hydrothermal reaction and calcination. The nature and morphologies of the nanofibers with layered structure were investigated by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was revealed that the phase transitions between titanates and titania (via proton titanate) could be interpreted systemically by the fundamental building blocks (FBB) model. The reaction also requires interconversion between some corner-linked and edge-linked pairs of TiO6 octahedrons. A generalized relationship of phase transitions between sodium/hydrogen titanates and titania by the wet-chemical process was proposed. A duplex pentagonal prism scheme was implied to summarize all of the phase relationships between titanates and titania. I. Introduction Much attention has been put into the design and fabrication of nanostructures based on metal oxides in the past decade because of their peculiar electronic and optic properties and their potential applications in technology.1-5 There is great interest in the development of titanates and TiO2-based solids with nanoscale dimensions and high morphological specificity6,7 such as nanofibers,8 nanosheets,9 and nanotubes10 because of their demonstratedpotentialinsolarenergyconversion,11 photocatalysis,12,13 photovoltaic devices,14,15 and as carrier for metallic nanoparticles.16 Hydrothermal treatment of different TiO2 precursors in a highly alkaline medium is a powerful way to prepare nanotubes and nanowires, but the information about the structure of the synthesized nanostructures is still unclear despite intensive investigations. In contradiction with previous investigations, Sun et al.17 report that as-synthesized nanotubes are titanates and can be described as NaxH2-xTi3O7. Thermal treatment of these materials leads to the formation of different titanates with the general formula Na2TinO2n+1. Recently, it was also mentioned that as-synthesized nanotubes or nanorods have an even more complex structure, NaxH2-xTinO2n+1 · yH2O.18-20 Treatment of these nanoparticles with a HCl solution produced nanotubes and nanorods of H2Ti3O7. Although the importance of continuing efforts to develop alternate approaches to the synthesis of the nanostructures has been realized,4 the potential for controlled reactions of these nanostructures has not drawn significant attention. In fact, phase transitions at moderate temperature are strongly preferred for constructing inorganic structures on nanometer scale (nanoscale)21 because the delicate nanostructures can easily be lost at high temperatures due to sintering. The present authors’ recent work has described a reversible scheme of transitions between nanoscale titanium dioxides to and from nanoscale titanates.22 We found for the first time that phase transitions from the titanate nanostructures to TiO2 polymorphs take place readily * Corresponding author. E-mail: [email protected].

in wet-chemical processes at temperatures close to ambient. Furthermore, the resultant TiO2 nanocrystals can react with concentrated NaOH solution, yielding hollow titanate nanotubes. In the present work, we report phase transitions from titanate fibers to hexatitanate, TiO2 (B), and anatase. The phase transitions are systematically explained by a fundamental building blocks (FBB) model, and a stereoschematic is implied on the basis of the above explanation. II. Experimental Section 1. Sample Preparation. NaOH pellets and HNO3 (both are AR grade from Aldrich) and TiOSO4 · xH2O (98%, from Fluka) were used in the synthesis. Titanate nanofibers in this study were prepared via a hydrothermal reaction between a concentrated NaOH solution and an inorganic titanium salt.15 Specifically, 10.7 g of TiOSO4 · H2O was dissolved into 80 mL of water and stirred until becoming clear. The resultant TiOSO4 solution was mixed with 100 mL of 15 M NaOH solution while stirring. The mixture (white suspension) was then transferred into a 200 mL Teflon-lined stainless steel autoclave and kept at a temperature between ambient and 200 °C for 48 h to yield titanate precipitates via a hydrothermal reaction. The white precipitate in the autoclaved mixture was recovered by centrifugation and washed with deionized water four times by dispersing the wet cake into 100 mL of water and recovering the solid by centrifugation. The sodium titanate product was labeled as T3. T3 was calcinated at 500 °C for 16 h under air flow to obtain a new titanate T6. The sodium titanates T3 and T6 were then neutralized using 0.1 M HCl solution and washed with water to remove most of the sodium ions. We dried the resultant hydrogen titanate (Htitanate; denoted as T3-H and T6-H, respectively) at 100 °C for 16 h and then dispersed this into a dilute (0.05 M) HNO3 solution, at temperatures selected to form anatase (TA). On the other hand, the resultant hydrogen titanate (H-titanate) was carried out at 500 °C in an atmosphere of air flow (20 cm3/ min) to obtain TiO2 (B) (denoted as “TB”). Standard laboratory safety precautions, including the use of appropriate hoods,

10.1021/jp103644x  2010 American Chemical Society Published on Web 06/14/2010

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SCHEME 1: Schematic Showing Phase Transformation Relationships between the Samples Used in This Studya

a T3(6) represents trititanate (hexatitanate). T3(6)-H represents proton trititanate (hexatitanate).

Figure 2. A generalized phase transition between sodium- and hydrogen-titanates, anatase, TiO2 (B), and rutile by the wet-chemical process.

Figure 1. Powder X-ray diffraction patterns of the titania-based nanofibers.

vessels, and safety gloves, were required when handling the strong acids and bases. 2. Sample Characterization. The microstructures and morphologies of products were investigated using transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. TEM images were recorded on a Philips CM20 TEM, employing an accelerating voltage of 200 kV and doubletilt holder. High-resolution transmission electron microscopy (HRTEM) was carried out on a Philips Tecnai F20 under an accelerating voltage of 200 kV. XRD patterns of the sample powders were recorded using a Shimadzu XRD-6000 diffractometer, equipped with a graphite monochromator. Cu KR radiation (λ ) 0.15418 nm) and a fixed power source (40 kV and 40 mA) were used. The samples were scanned at a rate of 1° (2θ)/min over a range of 2-80°, which covers the main characteristic diffraction peaks of the titanates, anatase, and TiO2(B). X-ray photoelectron spectroscopy (XPS) spectra were recorded in an ESCALAB 250 spectrometer, and Al KR radiation was used as the X-ray source. The C1s peak at 284.5 eV was used as a reference for the calibration of the binding energy (BE) scale. III. Results and Discussion 3.1. Phase Transformations from Titanate to Titania. The preparation procedure of the samples used here is shown in Scheme 1 summarized from the viewpoint of phase transformation. The phase transformations were investigated by X-ray diffraction (Figure 1). The directly obtained product (T3, Figure 1A) from hydrothermal reaction is consistent mainly with tritanate phase (Na2Ti3O7, monoclinic, S.G. P21/m, a ) 0.8566 nm, b ) 0.3804 nm, c ) 0.9133 nm, β ) 101.57°, ICSD #250000), although

there are some impurities, which is similar to the result given by Kolen’ko et al.23 X-ray photoelectron spectroscopy (XPS) of the surface indicated that this washed product possessed a sodium content of ∼10 wt % Na. The T3 was converted to T6 shown in Figure 1B (Na2Ti6O13, monoclinic, S.G. C2/m, a ) 1.513 nm, b ) 0.3745 nm, c ) 0.9159 nm, β ) 99.30°, ICSD #23877) after calcining at 500 °C. Proton titanate T6-H (H2Ti6O13, Figure 1C) was obtained by ion-exchange of the Na with H from the respective sodium titanate T6. It is noted that the basal structure of proton titanate was not changed, although the ion-exchange process results in the decrease of the d-spacing in the layered structure. Calcining the proton titanate T6-H yielded TB shown in Figure 1D (TiO2(B), monoclinic, S.G. C2/ m, a ) 0.6524 nm, b ) 0.3740 nm, c ) 01.218 nm, β ) 107.05°, ICSD #41056), TA shown in Figure 1E (anatase, tetragonal, S.G. I41/amd, a ) 0.3782 nm, c ) 0.9502 nm, ICSD #63711), depending on the calcination temperature. It is noteworthy that the fibril morphology was heritage from T3 for all of the above samples. All of the obtained titania phases can react with concentrated NaOH solution and yield trititanate. In addition, the phases involved here exhibit a complicated transition relationship, which has been summarized in Figure 2. All titanates on the green plane in Figure 2 can be converted into titania directly by an acid-assisted wet-chemical process as reported earlier.16 3.2. TEM Observation. The phase transition process was also confirmed by transmission electron microscopy (TEM) and corresponding electron diffraction (ED) patterns, as shown in Figure 3. The lattice parameters for the samples were calculated from ED patterns and listed in Table 1. Figure 3A shows a typical low-magnification TEM image of as-prepared nanofibers of T3 sample. The corresponding ED patterns (Figure 3B) indicate that it has a high crystallinity of the nanostructures, which are also confirmed by XRD. The ED patterns can be indexed as the monoclinic C2/m space group, using the Na2Ti3O7 parameters. Figure 3C shows a typical low-magnification TEM image of as-prepared nanofibers of T6 sample. The corresponding ED patterns (Figure 3D) indicate that this sample has also a high crystallinity of the nanostructures, which are confirmed by XRD. The ED patterns can be indexed as the monoclinic P21 space group, using the Na2Ti6O13 parameters. The zone axis is [001]. Figure 3E shows a low-magnification image of proton titanate T6-H (H2Ti6O13), a product of ion exchange of T6. The powder X-ray diffraction pattern of the sample reveals that the crystal structure is almost the same as that of T6 because the positions of peaks of the sample are almost coincident with those of T6. It could be deduced that the structure of the sample is the same

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Liu et al. TABLE 1: Phase Compositions and Crystallographic Parameters for Nanofibers crystal structure

phase reference composition Ti3 T6 Ti6-Ha TB TA a

Figure 3. TEM microstructure of the samples used in this study. Panels A, C, E, G, and I are T3, T6, T6-H, TB, and TA fibers, respectively. Panels B, D, F, H, and J are the electron diffraction patterns corresponding to the above samples in the same sequence.

as T6 based on the relationship of the crystal structure between T3 and T3-H. The ED taken down [001] is shown in Figure 2F. The lattice parameters determined by ED are about 1.46 nm for a, 0.382 nm for b, and 1.30 for c, which slightly depart from those of T6. Figure 3G and I show the TEM micrograph of TB (TiO2(B)) and TA (anatase) samples. The corresponding ED for TB sample (Figure 3H) down [001] can be indexed as the monoclinic C2/m space group, using the TiO2(B) parameters. The corresponding

Na2TinO2n+1, n ) 3,4,9 Na2Ti6O13 H2Ti6O13 TiO2(B) anatase

space group

parameters (nm) a

b

c

C2/m

1.513

0.375 0.916

P21 P21 C2/m I41/amd

1.56 0.378 1.22 1.46 0.38 1.30 1.216 0.374 0.651 0.3789 0.9537

β (deg) 99.3 105 105 107.29

Crystalline parameters for Ti6-H are measured in this study.

ED down [67j1] for TA is shown in Figure 3J, and can be indexed as the tetragonal I41/amd space group, using the anatase parameters. 3.3. FBB Model Explanation to Phase Transitions between Titanates and Titania. There are many ways developed to describe the structures and the relationships between inorganic crystals. Veblen has discussed the utility of the poly somaticseries approach.24 It cannot be simply applied to describe the TiO2 polymorphs, primarily because of the absence of slabs common to all polymorphs. In this work, the titanium oxide structures will be discussed on the approach based on the use of fundamental building blocks (FBB),25 emphasized by the similarities between the structures. Banfield26 has used this model to represent the relationships among the TiO2 minerals. All of the structures except rutile related in this study can be constructed from a unit composed of four edge-sharing octahedrons, which are shown in Figure 4. The difference between the corner-shared octahedrons and edge-shared octahedrons is marked with arrows. Thus, polyhedral representation of the structures of titanate and titania in this work can be described as Figure 5. It is shown in Figure 5 the polyhedral representation of the structures of titanate and titania. All of the projections are viewed down [010] except rutile (down [001]). In Figure 5a, every three TiO6 octahedrons form a slab by edge-sharing, and the slabs connect with each other to form a layer of TiO6 octahedrons in corner-linked mode. Thus, upperlayer and lower-layer TiO6 octahedrons form layered-structural trititanate. If atomic sites between TiO6 slab layers are occupied by sodium ions, it is sodium titanate (Figure 5a). If they are taken by hydrogen ions, it is hydrogen trititanate (Figure 5b). If TiO6 layers in trititanate are connected in corner-linked mode, it will change into microporous structure. Thus, sodium hexatitanate and hydrogen hexatitanate can be obtained as shown in Figure 5c and d. Given the absence of ions, if all TiO6 octahedrons are corner-linked, we can get rutile (Figure 5g). Similarly, if all octahedrons are edge-linked, we can get TiO2(B) (Figure 5e) or anatase (Figure 5f). By applying the FBB model, the present authors have successfully explained the structural evolution in several transition systems, such as the absorption of ammonium ions,27 bivalent Ba and Sr cations28,29 by trititanate, and conversion of trititanate to hexatitanate.30 A PBB schematic representing the transition of Na2Ti3O7 to TiO2(B) is implied in Figure 6. There are four steps that are needed for converting this structural transition. Figure 6A shows an undistorted representation of the Na2Ti3O7 structure viewed down [010]. The conversion from Na2Ti3O7 to TiO2(B) requires replacement of Na cations with hydrogen ions, which corre-

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Figure 4. A scheme of functional building blocks.

Figure 6. Diagram illustrating the stepwise conversion of Na2Ti3O7 (a) to TiO2(B) (d). Parts (b) and (c) are the proposed intermediate.

Figure 5. Polyhedral representation of the structures of titanate and titania. All of the projections are down [010] except rutile (down [001]).

sponds to step I. The reaction also requires interconversion between some corner-linked and edge-linked pairs of Ti-O6 octahedra, which have been illustrated in Figure 6b and c. The slab of TiO6 octahedron in trititanate is -3-3-3-; that is, every three edge-shared octahedrons connect mutually by cornersharing, which will become -2-2-2- after conversion in step II (Figure 6c) and step III (Figure 6d). The structure of TiO2 (B) (Figure 6e) thus becomes reality by relative slipping of upper layer and lower layer (step III) followed by dehydration (step IV), as shown in the enlarged inset of Figure 6. IV. Conclusion The phase transformations from sodium trititanate to hexatitanate and from proton titanate to TiO2(B) and anatase were investigated by XRD and TEM. It was revealed that the phase transitions and morphological derivation between titanates, anatase, and TiO2 (B) (via proton titanate) could be interpreted systemically by the fundamental building blocks model (FBB). A generalized relationship of phase transitions between sodium

and hydrogen titanates, anatase, TiO2(B), and rutile by the wetchemical process was then proposed. A duplex pentagonal prism scheme was then implied to summarize all of the relationships between titanates and titania including this work and previous work. It is the first time in which seven compounds of titanium have been made into one stereoscheme, showing the complex connections among them. Acknowledgment. The financial and infra-structure support of the Queensland University of Technology Inorganic Materials Research Program of the School of Physical and Chemical Sciences is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation. References and Notes (1) Li, D.; Xia, Y. AdV. Mater. 2004, 16, 1151–1170. (2) Limmer, S. J.; Cao, G. AdV. Mater. 2003, 15, 427–431. (3) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353–389. (4) Rao, C. N. R.; Deepak, F. L.; Gundiah, G.; Govindaraj, A. Prog. Solid State Chem. 2003, 31, 5–147. (5) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446–2461. (6) Hennings, D.; Klee, M.; Waser, R. AdV. Mater. 1991, 3, 334–340. (7) Newnham, R. E. MRS Bull. 1997, 22, 20–33. (8) Gao, X. P.; Zhu, H. Y.; Pan, G. L.; Ye, S. H.; Lan, Y.; Wu, F.; Song, D. Y. J. Phys. Chem. B 2004, 108, 2886–2872. (9) Sukpirom, N.; Lerner, M. M. Chem. Mater. 2001, 13, 2179–2185. (10) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14, 3160–3163. (11) Gra¨tzel, M. Nature 2001, 414, 338–344. (12) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalysis Fundamentals and Applications; BKC, Inc.: Tokyo, 1999.

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