Journal of Nanomaterials

Volume 2014 (2014), Article ID 835450, 5 pages

http://dx.doi.org/10.1155/2014/835450

Research Article

Influence of Sn Doping on Phase Transformation and Crystallite Growth of Nanocrystals

Guozhu Fu,¹ Yanqiu Yang,² Gang Wei,¹ Xin Shu,¹ Ning Qiao,¹ and Li Deng2,3

¹State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

²Beijing Bioprocess Key Laboratory, Beijing University of Chemical Technology, Beijing 100029, China

³Amoy-BUCT Industrial of Biotechnovation Institute, Amoy 361026, China

Received 16 January 2014; Accepted 2 February 2014; Published 10 March 2014

Academic Editor: Shao-Wen Cao

Copyright © 2014 Guozhu Fu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Sn doped TiO₂ nanocrystals were synthesized via a single-step hydrothermal method and the influences of Sn doping on TiO₂ have been investigated. It is found that Sn doping not only facilitates the crystal transfer from anatase to rutile but also facilitates the morphology change from sphere to rod. The states of Sn were studied by XPS and the creation of oxygen vacancies by Sn doping is confirmed. Moreover, the HRTEM results suggest that Sn facilitates preferential growth of resulting nanocrystals along (110) axis, which results in the formation of rod-like rutile nanocrystals.

1. Introduction

In recent years, the applications of semiconductors in photocatalysis and other fields have attracted much interest [1, 2], and many semiconductor nanomaterials and their heterogeneous structures have been developed for their application in energy and environmental applications [3–5], for example, the investigations of TiO₂ on various organic pollutants photodegradation [6] due to its excellent photocatalytic activity, physical and chemical stability, and nontoxicity [7]. Because most of the solar energy is focused on the visible light, it is important to develop the visible-light-driven photocatalysts. However, TiO₂ is only sensitive to UV light because of its large band gap (3.2 eV). In order to efficiently use solar energy, many methods have been studied. An effective way is to introduce foreign ions into TiO₂, including rare earth element doping [8], metals doping [9], and nonmetals doping [10, 11].

The property of TiO₂ can also be affected by foreign metal ions doping. It has been shown that the photocatalytic activity of the modified TiO₂ improves to different extents depending on different ion doping, such as Mn²⁺ [12], Zr⁴⁺ [13], and Fe³⁺ [14]. Moreover, the phase transformation behavior and structure of TiO₂ are also affected by foreign metal ions. For example, Ag⁺ [15], Mn²⁺ [16], and Cr³⁺ [17] are proved to promote phase transformation from anatase to rutile, while silicon ion doping strongly restrains the phase transformation [18] and lowers the phase transition temperature [19]. But, so far, no detailed study has been reported on the influence of Sn⁴⁺ doping on the phase transformation and structure of hydrothermal synthesis TiO₂.

In our work, Sn doped TiO₂ nanocrystals were prepared by hydrothermal method. The existing states of Sn and its role in phase transformation as well as the morphology evolution were investigated. Sn facilitates the phase conversion from anatase to rutile and prefers the morphology evolution from spherical shape to nanorods.

2. Experimental Section

Sn doped TiO₂ nanoparticles were prepared by hydrothermal method. 2.9 mL acetic acid was added to 17 mL tetrabutyl titanate and stirred for 15 min. The mixture was then poured into 73 mL of water and vigorously stirred for 1 hour. After adding 1 mL concentrated nitric acid, the mixture was heated to 80°C and peptized for 75 min. Then the volume was adjusted with water to 80 mL. The mixture was kept in a 100 mL autoclave and heated at 200°C for 12 h. For Sn doped samples, appropriate volume of tin tetrachloride was added to distilled water in advance (the feed molar ratios of Sn/Ti were modulated as 0.25/100, 0.5/100, 0.75/100, and 1.0/100, resp.).

The powder XRD experiments were performed on Bruker D8 Advance X-ray diffractometer using monochromic Cu Kα radiation ( nm). The scanning electron microscopy (SEM) images were recorded using S4700 Hitachi Ltd. The transmission electron microscopy (TEM) was performed with a Tecnai G2 20 transmission electron microscope of HongKong Co., Ltd. The X-ray photoelectron spectroscopy (XPS) experiments were carried out on Thermo ESCALAB 250 and the binding energies are calibrated by C1 photoelectron peak (284.6 eV).

3. Results and Discussion

The X-ray diffraction patterns of TiO₂ samples with different Sn doping ratios are shown in Figure 1. It can be seen that the undoped TiO₂ is mainly composed of anatase and weak diffraction peaks of rutile TiO₂ can also be found in the XRD pattern. With the addition of Sn, the samples undergo “reutilization” to give rutile as the predominant polymorph, and the conversion from anatase to rutile is completed on 0.75% and 1.0% Sn doped samples, which suggests that the doped Sn ions can promote the formation of rutile.

Figure 1: XRD patterns of TiO₂ samples prepared with different Sn doping ratios: (a) 0%, (b) 0.25%, (c) 0.50%, (d) 0.75%, and (e) 1.00%.

The TEM results in Figure 2 show that the particle shape changed from sphere (diameter of 10–20 nm) to rod (width of ~20 nm and length of 100–200 nm) by Sn doping, which are consistent with SEM results (not shown here). The space between the lattice planes of Sn-free TiO₂ (Figure 2(a)) is 0.35 nm, which corresponds well to the value of (101) plane for anatase. The lattice space of 0.33 nm displayed in Sn–TiO₂ samples (Figures 2(b)–2(e)) is equal to the value of (110) plane for rutile. Therefore, it is reasonable to conclude that the Sn-free TiO₂ spherical nanocrystals are mainly anatase structure and the Sn doped TiO₂ rod-like particles are rutile. As for our samples, with an increasing of Sn doping amount in TiO₂, the content of the spherical anatase decreases and the rod-like rutile increases, illustrating that Sn doping facilitates the phase conversion from anatase to rutile, which is consistent with XRD. Furthermore, the HRTEM images reveal rod-like building units and nanocrystal growth along the [110] axis, indicating that the formation of the rod-like TiO₂ is the result of the preferential growth (PG) in crystallographic orientation favored by Sn incorporation. Since Ti (IV) and Sn (IV) ions have the similar ionic radii, it is reasonable to deduce that Sn ions substitute lattice Ti, which is confirmed by our subsequent experiment. The EDX spectrum in Figure 2(e) also confirms the existence of Sn in TiO₂ nanocrystals.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2: TEM images of TiO₂ samples prepared with different Sn doping ratios: (a) 0%, (b) 0.25%, (c) 0.50%, (d) 0.75%, (e) 1.00%, and (f) EDX spectrum of 1% Sn doped TiO₂ nanocrystals.

Figure 3 shows the XPS spectra of Sn doped TiO₂ sample with Sn/Ti atomic ratio of 1%. As for Ti 2 spectrum in Figure 3(a), the two peaks at 458.1 eV and 458.7 eV could be assigned to O–Ti–O and Ti–O–Sn, respectively. Due to the electronegativity of Sn (1.96) which is larger than Ti (1.54) [20], the substitution of Sn for Ti in the lattice leads to the shift of binding energy to a higher value.

(a)

(b)

Figure 3: XPS spectra of the 1% Sn doped TiO₂ nanocrystals.

As for O1 spectrum in Figure 3(b), the peak at 532.2 eV is attributed to surface hydroxyl oxygen atoms. The peak at 529.3 eV is the binding energy of O1 in Ti–O–Ti, while the occurrence of the peak at 530 eV is the result of Sn substitute for Ti leading to the positive shift. These observations all confirm the formation of Ti−O−Sn structure in the Sn doped TiO₂, owing to the substitution of Ti by Sn.

As shown in Figure 4, both anatase and rutile phase coexist at short treatment time, and the ratio of rutile/anatase increases with treatment time prolonging. A similar finding was reported by Zhang and Gao [21] who stated that phase transformation occurred simultaneously with particle growth.

Figure 4: XRD patterns of the 1% Sn doped samples with different times: (a) 3 h, (b) 6 h, (c) 9 h, (d) 12 h, and (e) 15 h.

It has been shown that the oxygen vacancies of TiO₂ increase if foreign cations replace Ti⁴⁺ ions [22]. Vemury and Pratsinis [23] found that the formation of rutile phase was enhanced either by introducing dopant oxides with the same crystal structure as rutile or by creating oxygen vacancies by doping cations. It has been reported that the rutile fraction increases at a higher Eu³⁺ addition owing to the creation of oxygen vacancies by replacing the Ti⁴⁺ sites with subvalent Eu³⁺ ions in the TiO₂ [24]. In our samples, the creation of oxygen vacancies is by replacing the Ti⁴⁺ sites with Sn⁴⁺ ions in TiO₂ and therefore the rutile formation can be enhanced. Moreover, it was supposed that there is a relationship between the phase transformation and the nanocrystal growth process [25] and the preferential growth process aids phase transformation. The minimization of the area of high-energy surface faces promoted by the preferential growth process may be an extra driving force for the phase transformation from anatase to rutile phase.

4. Conclusions

In summary, the promoting roles of Sn⁴⁺ in both TiO₂ phase transformation and morphology change have been confirmed in our study. The result demonstrated that morphology transition was related to the preferential growth process and the phase transformation was related to the creation of oxygen vacancies caused by Sn. Our observations of preferential growth coupled with phase transformation process led us to understand the preferential growth process aided phase transformation.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was supported by Beijing Key Laboratory of Bioprocess, State Key Laboratory of Chemical Resource Engineering, China 863 Project (nos. 2009AA03Z802 and 2009AA03Z803), and Amoy Major Scientific and Technological Innovation Project (2012S0305).

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