Tetragonal faceted-nanorods of anatase TiO₂ single crystals with a large percentage of active {100} facets

Abstract

Tetragonal faceted-nanorods of single-crystalline anatase TiO₂ with a large percentage of higher-energy {100} facets have been synthesized by hydrothermal transformation of alkali titanate nanotubes in basic solution.

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In recent years, intensive studies have been reported to prepare TiO₂ because it has been widely used in photocatalysts, photosplitting water, dye-sensitized solar cells, photochromic devices, and gas sensing.^1–4 In particular, anatase had been undoubtedly proved to show the most excellent applications among all the TiO₂ crystallographic phases.^5,6 It has been demonstrated that the intrinsic properties of anatase TiO₂ depended on shapes, sizes and especially the exposed facets.^7–9 According to the Wulff construction,^10,11 the equilibrium crystal shape of a basic anatase crystal closely resembles the shape of the naturally occurring anatase mineral, which is a truncated bipyramidal structure enclosed by eight {101} facets and two {001} facets, and the exposed {101} facets amounted to more than 94% (Scheme 1(a)). Recently, micron-sized anatase crystals which exposed a large percentage of {001} facets have been synthesized by using fluorine to stabilize the higher surface energy facets of {001} (Scheme 1(b)).¹² Since then, tremendous efforts have been conducted on the degree of truncation,^13,14 the particle sizes¹⁵ and the synthesis methods^16,17 for truncated octahedral anatase (Scheme 1, path 2). However, anatase TiO₂ crystals with four well-defined lateral active {100} facets (Scheme 1(c)) have never been synthesized. Theoretical studies demonstrated that anatase {100} facets are more active and accordingly exhibit higher catalytic activity than {001} and {101} facets.^17–19 Barnard et al.^20,21 have theoretically demonstrated that hydroxyl can lower the surface free energy of {100} facets. However, this process has never been verified in experiments, because most available titanium precursors are very reactive under basic conditions. Synthesis of anatase TiO₂ single crystals which predominantly expose higher-energy {100} facets is still a big challenge.

Scheme 1 Schematic drawings of anatase shapes: (a) equilibrium crystal shape with large percentage of {101} facets; (b) truncated octahedral shape with large percentage of {001} facets; (c) tetragonal faceted-nanorod with four well lateral {100} facets.

Herein, we first report a facile hydrothermal route for the synthesis of tetragonal faceted-nanorods (TFNRs) of anatase TiO₂ which predominantly exposed higher-energy {100} facets. This process involves the formation of Na-titanate nanotubes by hydrothermal reaction of P25 in NaOH solution²² first and hydrothermal transformation of the precursors into anatase phase of TiO₂ in basic solution. Traditionally, transformation of Na-titanate to pure anatase TiO₂ needed exchanging alkali ions with protons to form the H-titanates in advance.^23–25 This is also the first report on one-step transformation of Na-titanate precursor into anatase TiO₂.

Typically, the Na-titanate nanotubes are 5–20 nm in diameter and several micrometres in length. Energy dispersive X-ray spectroscopy reveals that the atomic ratio of sodium to titanium was ca. 18% (Fig. S1 in ESI† ). Fig. 1 shows the scanning electron microscopy (SEM) image of the products obtained by hydrothermal reaction of Na-titanate nanotubes, indicating a tetragonal nanorod shape. All the particles had well-defined lateral facets with sharp edges and the adjacent facets were perpendicular as well as with the same width (inset of Fig. 1(b)). Fig. 2 shows X-ray diffraction patterns of the Na-titanate precursors and the TFNR particles. The diffraction peaks in Fig. 2(b) match well with the crystal structure of the anatase TiO₂ phase (tetragonal, I4₁/amd, JCPDS 21-1272), indicating all of the Na-titanate precursors have been transferred into an anatase TiO₂ phase.

Fig. 1 (a) Lower and (b) higher SEM images of tetragonal faceted-nanorods. The inset of (b) is the top view of a single TFNR.

Fig. 2 X-Ray diffraction patterns of: (a) Na-titanate precursors and (b) the TFNR products.

More detailed structural information of the TFNRs was revealed by transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). By measuring the SAED patterns in Fig. 3(a), the axis of the rod was parallel to the 〈001〉 direction, indicating that the rod grows along the 〈001〉 direction. The angle between 〈100〉 and 〈001〉 directions is 90°, in good agreement with the model of TFRN anatase TiO₂ enclosed by {100} facets projected along the [010] direction (Fig. 3(b)). Fig. 3(c) shows the high-resolution TEM image taken from Fig. 3(a) indicated by the rectangle. Three sets of lattice fringes with spacings of 0.35, 0.35 and 0.48 nm can be attributed corresponding to (101), (101̄) and (002) of the anatase phase, respectively. The exposed facet (under electron-beam irradiation) was flat and the thickness of the rod was found to be constant by measuring the titanium content across the plane (Fig. S2(a) and 2(b) in ESI† ). To further confirm the exposed facets of the TFNR, the same rod was rotated to the [11̄0] zone axis from the [010] zone axis. As shown in Fig. 3(d), the diameter of the rod was 1.4 times larger than that in Fig. 3(a). The rod exhibited a sharp edge and a symmetric sharp peak of the titanium content was observed (Fig. S2(c) and 2(d) in ESI† ). Both the SEAD pattern (inset of Fig. 3(d)) and HRTEM image (Fig. 3(f)) also confirm that the TFNR has a single-crystalline structure of anatase phase with a growth direction along 〈001〉, which matches well with the TFNR model enclosed by {100} facets, projected along [11̄0] direction (Fig. 3(e)). On the basis of the above observations and structural analysis, we conclude that the exposed lateral facets of the as-prepared TFNRs are mainly the {100} facets.

Fig. 3 (a) Typical TEM image of an individual TFNR viewed along the [010] direction; inset: the corresponding SEAD pattern. (b) Schematic model of an ideal TFNR with four lateral {100} facets, projected along the [010] direction. (c) HRTEM image taken from (a) indicated by the rectangle. (d) TEM image of the same TFNR viewed along the [11̄0] direction; inset: the corresponding SAED pattern. (e) Schematic model of an ideal TFNR with four lateral {100} facets, projected along the [11̄0] direction. (f) HRTEM image taken from (c) indicated by the rectangle.

To investigate the transformation process from Na-titanate nanotubes to the TFNRs of anatase TiO₂, a detailed time-dependent morphology evolution study was conducted at 200 °C (Fig. S3 in ESI† ). It was observed that more particles appeared and the particles gradually grew up while the diameter and length of Na-titanate nanotubes decreased. When the hydrothermal reaction time was longer than 6 h, the Na-titanate completely transformed to TFNRs of anatase TiO₂. Furthermore, we measured the pH value of the reaction solution. Before hydrothermal reaction, the solution pH was 10.1. When reaction was stopped at 1 h, the solution pH was 10.9 and the obtained products contained both Na-titanate and anatase phases (Fig. S4(a) in ESI† ). It was found that the pH would gradually increase with the reaction time and the final pH reached 11.8. In addition, no anatase was found if the Na-titanate precursors were treated in an ethanol bath at 200 °C for 24 h (Fig. S4(b) in ESI† ).

Upon hydrothermal treatment of the Na-titanate nanotubes, the transformation would proceed with dissolution and nucleation. At the beginning of the transformation, the surface of Na-titanate nanotubes was gradually decomposed and produced Ti(OH)₄ fragments, hydroxyl and sodium ions under hydrothermal condition, according to eqn (1).

Then, the Ti(OH)₄ fragments were rearranged through dehydration reaction between Ti–OH and HO–Ti so as to share edges, resulting in the formation of anatase crystal nuclei, according to eqn (2). Subsequently, the TiO₂ nuclei gradually grew up to form nanocrystalline TiO₂ when Ti(OH)₄ fragments diffused to their surface. Due to anisotropy in adsorption stability of the capping reagents, the adsorbates adsorbed onto certain crystallographic planes more strongly than others, which lowers the surface energy of the bound plane and hinders the growth of crystals or some crystal planes. Barnard et al.^20,21 had calculated the surface structure and energies of selected low-index surfaces of both rutile and anatase and found that in basic conditions, O-terminated (100) facets had lower surface free energy than O-terminated (101) facets and (001) facets. In the present system, we also believe that the hydroxyl ions gradually released from Na-titanate nanotubes would preferentially adsorb onto anatase (100) facets, which lower the surface energy of the (100) facets and thus limited the crystal growth along the a- and b-axis. Our results were different from other reports,^26,27 because they used different titanium precursors and at a fixed basic condition, in which the concentration of hydroxyl would gradually decrease with the reaction, resulting in spindle or round-rod shapes. It is also of note that the present transformation process was also different from our previous reported²³ topochemical reaction process under neutral or acidic conditions.

In general, the catalytic activity of inorganicnanocrystals depends greatly on the exposed crystal facets.^28,29 These TFNR anatase TiO₂ single crystals are expected to have higher reactivity due to the large percentage of {100} facets compared with crystals having normal majority {101} facets. Here, we have investigated the photocatalytic activity of the TFNRs by measuring the formation of active hydroxyl radicals (˙OH), the most important oxidative species in photocatalysis reactions.³⁰Terephthalic acid (TA) was used as a fluorescence probe. Fig. 4 displays the fluorescence spectra of the UV light irradiated tetragonal nanorod anatase TiO₂ in 3 mM terephthalic acid and 0.01 M NaOH solution at different irradiation times. For comparison, the capability of forming ˙OH of commercially available anatase powder (Sinopharm Chemical Reagent Co., Ltd, purity >99%) was also studied due to their similar BET surface area. The unique fluorescence peak at 426 nm is originated from 2-hydroxyterephthalic (TAOH) acid produced by the reaction of TA with ˙OH in basic solution.³¹ The linear relationships between fluorescence intensity and irradiation time are found for both the TFNRs and commercial anatase powders, as shown in the inset of Fig. 4. We found that the TFNRs exhibit much higher activities (2.2 times) than that of the commercial anatase powders (inset of Fig. 4). This result suggests that the reactive {100} facets may play an important role in the photocatalytic reaction.

Fig. 4 Fluorescence spectra of UV light (maximum emission was 254 nm) irradiation of the TFNRs in 3 mM terephthalic acid and 0.01 M NaOH solution at different irradiation times. Inset is the time dependences of fluorescence intensity at 426 nm, where squares and circles correspond to the TFNR sample and the commercial TiO₂ powder, respectively.

In summary, we have reported a novel and facile route for the preparation of tetragonal faceted-nanorods of single-crystalline anatase TiO₂ with predominately exposed higher-energy {100} facets by hydrothermal transformation of Na-titanate in alkaline solution. This is the first report that alkali titanate (instead of the usual H-titanate) can be completely transferred to pure anatase TiO₂ in basic conditions viahydrothermal reaction. A “dissolution–nucleation” process is proposed to explain the transformation from alkali titanate to TFNR anatase TiO₂. Furthermore, an enhanced photocatalytic activity for the TFNR sample is observed.

This work was supported by the NSFC (Grant Nos. 20525309, 20673008, 50821061) and MSTC (MSBRDP, Grant Nos. 2006CB806102, 2007CB936201).

Notes and references

1. A. Fujishima and K. Honda, Nature, 1972, 238, 37.
2. M. Grätzel, Nature, 2001, 414, 338.
3. X. Chen and S. S. Mao, Chem. Rev., 2007, 107, 2891.
4. B. O'Regan and M. Grätzel, Nature, 1991, 353, 737.
5. M. A. Fox and M. T. Dulay, Chem. Rev., 1993, 93, 341.
6. A. L. Linsebigler, G. Q. Lu and J. T. Yates, Chem. Rev., 1995, 95, 735.
7. R. Hengerer, L. Kavan, P. Krtil and M. Gratzel, J. Electrochem. Soc., 2000, 147, 1467.
8. L. Kavan, M. Gratzel, S. E. Gilbert, C. Klemenz and H. J. Scheel, J. Am. Chem. Soc., 1996, 118, 6716.
9. H. H. Deng, H. Zhang and Z. H. Lu, Chem. Phys. Lett., 2002, 363, 509.
10. M. Lazzeri, A. Vittadini and A. Selloni, Phys. Rev. B: Condens. Matter Mater. Phys., 2001, 63, 155409.
11. U. Diebold, N. Ruzycki, G. Herman and A. Selloni, Catal. Today, 2003, 85, 93.
12. H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H. M. Cheng and G. Q. Lu, Nature, 2008, 453, 638.
13. H. G. Yang, G. Liu, S. Z. Qiao, C. H. Sun, Y. G. Jin, S. C. Smith, J. Zou, H. M. Cheng and G. Q. Lu, J. Am. Chem. Soc., 2009, 131, 4078.
14. X. Han, Q. Kuang, M. Jin, Z. Xie and L. Zheng, J. Am. Chem. Soc., 2009, 131, 3152.
15. Y. Dai, C. M. Cobley, J. Zeng, Y. Sun and Y. Xia, Nano Lett., 2009, 9, 2455.
16. D. Zhang, G. Li, X. Yang and J. C. Yu, Chem. Commun., 2009, 4381.
17. B. Wu, C. Guo, N. Zheng, Z. Xie and G. D. Stucky, J. Am. Chem. Soc., 2008, 130, 17563.
18. A. Beltrán, J. R. Sambrano, M. Calatayud, F. R. Sensato and J. Andrés, Surf. Sci., 2001, 490, 116.
19. M. Calatayud and C. Minot, Surf. Sci., 2004, 552, 169.
20. A. S. Barnard, P. Zapol and L. A. Curtiss, Surf. Sci., 2005, 582, 173.
21. A. S. Barnard and L. A. Curtiss, Nano Lett., 2005, 5, 1261.
22. T. Kasuga, M. Hiramatsu, A. Hoson, T. Sckino and K. Niihara, Langmuir, 1998, 14, 3160.
23. Y. Yu and D. Xu, Appl. Catal., B, 2007, 73, 166.
24. Y. Mao and S. S. Wong, J. Am. Chem. Soc., 2006, 128, 8217.
25. H. Zhu, Y. Lan, X. Gao, S. Ringer, Z. Zheng, D. Song and J. Zhao, J. Am. Chem. Soc., 2005, 127, 6730.
26. A. Chemseddine and T. Mortiz, Eur. J. Inorg. Chem., 1999, 235.
27. Y. Gao and S. A. Elder, Mater. Lett., 2000, 44, 228.
28. N. Tian, Z. Y. Zhou, S. G. Sun, Y. Ding and Z. L. Wang, Science, 2007, 316, 732.
29. L. Hu, Q. Peng and Y. Li, J. Am. Chem. Soc., 2008, 130, 16136.
30. T. Hirakawa and Y. Nosaka, Langmuir, 2002, 18, 3247.
31. J. C. Yu, J. Yu, W. Ho, Z. Jiang and L. Zhang, Chem. Mater., 2002, 14, 3808.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and figures. See DOI: 10.1039/b923755k

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