Facile synthesis of silica-coated Bi 2 S 3 nanorods and hollow silica nanotubes

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1 Journal of Crystal Growth 282 (2005) Facile synthesis of silica-coated Bi 2 S 3 nanorods and hollow silica nanotubes Shu Fen Wang, Feng Gu, Zhong Sen Yang, Meng Kai Lu, Guang Jun Zhou, Wen Guo Zou State Key Laboratory of Crystal Materials, Shandong University, Jinan , China Received 27 January 2005; accepted 25 April 2005 Available online 21 June 2005 Communicated by M. Schieber Abstract Silica coated bismuth sulfide (Bi 2 S 3 ) nanorods have been prepared by a simple coating process. The coating resulted in the improved stability and luminescent intensity of the Bi 2 S 3 nanorods. Hollow silica nanotubes were also obtained by using Bi 2 S 3 nanorods as a removal template. r 2005 Elsevier B.V. All rights reserved. PACS: Ys; n Keywords: A1. Characterization; A1. Nanostructures; B1. Bi 2 S 3 ; B1. Nanorods 1. Introduction Up to date, hollow tubes of a variety of inorganic materials including oxides [1], nonoxides [2] and metals [3] have been widely investigated, which can find potential applications in nanoscale electronics, optoelectronics and biochemical-sensing fields. Among them, silica nanotubes are of special interest because of their hydrophilic nature, easy formation of colloidal suspension and surface functional accessibility for Corresponding author. Tel: address: mengkailu@icm.sdu.edu.cn (M.K. Lu ). both inner and outer walls [4]. With respect to preparation, traditional routes such as hightemperature reaction [5] and sol gel method [6] have been used. The drawback of these methods lies in that the purity and the structural controllability of the products are poor. Very recently, a template-directed method was put forward, which offers various advantages in synthesis well-structured silica nanotubes. Templates such as carbon nanotubes [7a], gold nanorods [7b] and some organic complex [7c] have been applied. Here, we select Bi 2 S 3 as a removing template, which is the first attempt on the synthesis of hollow silica nanotubes. Bismuth sulfide (Bi 2 S 3 ), a direct band /$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi: /j.jcrysgro

2 80 S.F. Wang et al. / Journal of Crystal Growth 282 (2005) gap semiconductor, is useful for photodiode and photovaltoic converters [8]. It can also find applications in thermoelectric cooling technologies based on the Peltier effect [9]. One-dimensional (1D) Bi 2 S 3 nanostructures especially nanorods have been successfully prepared and novel properties were exploded [10]. Due to their unstable property in nature, it is critical to employ a protective sheath of thermally and chemically stable materials around them to enhance their performance for their promising applications. In this communication, silica acts as the coating layer and the coating process is speculated to extend the application range of Bi 2 S 3 nanorods greatly. 2. Experimental section Preparation of Bi 2 S 3 nanorods: Bi 2 S 3 nanorods were prepared following the process described earlier as rapid polyol method [10c]. In brief, Bi(NO 3 ) 3 5H 2 O (analytical grade, 1.14 g) and excessive thiourea (analytical grade, 0.5 g) were dissolved in 100 ml ethylene glycol. Then the homogeneous solution was placed in a 250 ml round-bottomed flask, heated and refluxed at 197 1C for 30 min. The products were filtered and washed several times with absolute alcohol and water and then vacuum dried at 60 1C for 3 h. In order to investigate the affect of the coating on the PL properties of the Bi 2 S 3 nanorods, 2% Dy 3+ - doped sample was also prepared. Synthesis of silica coated nanorods and hollow silica nanotubes: The formation of silica coating involved hydrolysis of TEOS (analytical grade) to generate silica sols using ammonia as a catalyst. Typically, 0.1 g of the as-obtained Bi 2 S 3 nanorods was added to the mixed solution of 30 ml distilled water and 20 ml absolute ethanol. Subsequently, 7ml NH 3 H 2 O (26%) and certain amount of TEOS were added dropwise. The solution was stirred for about 12 h at room temperature. After standing for a day, another 4 h stirring was conducted. The resulting silica/bi 2 S 3 core-shell nanorods were obtained by centrifugation, washing repeatedly with ethanol followed by drying in a vacuum at 60 1C for 5 h. The heat treatment at 400 1C for 1 h was also conducted. By immersing the products in HCl (1:1) or through focusing the electron beam on the heat-treated sample, hollow silica nanotubes can be obtained. The whole process is illustrated in Scheme 1. Characterization: The products were characterized by the X-ray diffraction (Japan Rigaku D/ max-ga X-ray diffractometer), the Fourier transform infrared spectra (NEXUS FT-IR 670 infrared spectrometer) and transition electron microscope (Japan JEM-100CXII). The optical properties of the samples were characterized by a Edinburgh FLS920 fluorescence lifetime and steady-state spectrometer. 3. Results and discussion The transmission electron microscopy (TEM) images of the plain and silica-coated Bi 2 S 3 nanorods are shown in Fig. 1. As shown in Fig. Scheme 1. Schematic illustration of the formation of silica/bi 2 S 3 nanorods and silica hollow nanotubes.

3 S.F. Wang et al. / Journal of Crystal Growth 282 (2005) Fig. 1. TEM images of (a) as-obtained Bi 2 S 3 nanorods, (b) and (c) silica-coated Bi 2 S 3 nanorods (60 1C) with different enlarge scales and inset of (c) electron diffraction of a single-coated nanorod. 1a, well crystalline Bi 2 S 3 nanorods can be obtained by the rapid polyol method. Most of them have diameters of about nm, and a small amount of them have diameters of about nm. Fig. 1b and c show the images of the silica/bi 2 S 3 nanorods with different magnifications. Fig. 1b displays that the coating is complete and nearly no bare zone exists. The enlarged image in Fig. 1c reveals that silica is smoothly coated on the surface of Bi 2 S 3 nanorods as a thin layer of about nm. The wall thickness is dependent on the concentration of the TEOS for a certain amount of NH 3 H 2 O and Bi 2 S 3 nanorods. From Fig. 1c, we can also see that most of the coated rods have one end open, which is favorable for the template removal process. The electron diffraction (ED) pattern of a single-coated rod (inset of Fig. 1c) further indicates the single crystal nature of Bi 2 S 3 and the presence of silica layers. The diffractive spots can be indexed to the reflections of orthorhombic Bi 2 S 3 and the weak diffused rings reveal that the formed silica layers are amorphous in nature. The existence of the silica layer was further demonstrated by the X-ray diffraction (XRD) patterns and the Fourier transform infrared (FT- IR) spectra as shown in Fig. 2. Fig. 2(1) shows XRD patterns of plain and silica-coated Bi 2 S 3 nanorods, the sharp peaks index well to the bismuthinite structure (JCPDS no ) indicating well crystallinity of the products. The presence of the amorphous silica sheath has made attribution in Fig. 2(1)(b) and (c) as increasing background at lower diffraction angles (2y: ). After heat-treated at 400 1C for 1 h (Fig. 2(1)(c)), the pattern related to Bi 2 S 3 is nearly unchangeable, indicating the shielding function of silica. For the FT-IR spectrum (Fig. 2(2)(c)) of silica/bi 2 S 3, a pronounced change was noticed in the region of cm 1, which clearly indicates the formation of the silica layers. The peak at 806 cm 1 is due to the symmetric stretching of the Si O Si group [11]. The sharp band at 1096 cm 1 corresponds to the characteristic oxygen asymmetric stretching mode [12]. As it is reported [13], the splitting of the asymmetric stretching mode (broad doublet band at cm 1 ) is probably due to the presence of strained siloxane links and surface silanols, which favors the coating of silica on the surface of the Bi 2 S 3. In semiconductors, band gaps have been found to be particle size dependent. The band gap increases with decreasing particle size. As Bi 2 S 3 is a narrow band gap semiconductor (Eg is 1.3 ev for bulk), with the decrease of the diameter into nanoscale, novel optical properties may be observed. Fig. 3 displays the emission spectrum of the silica/bi 2 S 3 :Dy 3+ nanorods. For comparison, the emission spectrum of Bi 2 S 3 : Dy 3+ nanorod was also included. As shown in Fig. 3b, under excited at 295 nm, two emission bands can be observed from the coated samples. The band at 395 nm can be considered to originate from the Bi 2 S 3 nanorods. Though the origin of the sharp bands is still under consideration, the shape of the peak suggests that the emission does not arise from the defects but from the intrinsic luminescence of Bi 2 S 3 nanorod. Maybe it ascribed to the interaction of excimers [14]. Another band at 480 nm arises from the characteristic 4 F 9/2 6 H 15/2 transition of Dy 3+. After coating, the properties of Bi 2 S 3 :Dy 3+ nanorods are improved including the stability and the luminescent efficiency. We interpret

4 82 S.F. Wang et al. / Journal of Crystal Growth 282 (2005) Fig. 2. XRD patterns and FT-IR spectra of silica/bi 2 S 3 nanorods. For XRD patterns (a) as-prepared Bi 2 S 3 nanorods, (b) silica/bi 2 S 3 nanorods (60 C), and (c) silica/bi 2 S 3 nanorods (400 1C) and the FT-IR patterns (a) Bi 2 S 3 nanorods, (b) silica/bi 2 S 3 nanorods. the enhancement of the luminescence intensity as removal of electronic capture centers on the surface of Bi 2 S 3 nanorods or removal of nonradiative decay channels because of the silica shell. Furthermore, Bi 2 S 3 is unstable especially when meeting acid or suffering thermal treatment, which motivated us to synthesize hollow silica nanotubes by removing the present Bi 2 S 3 nanorods using

5 S.F. Wang et al. / Journal of Crystal Growth 282 (2005) such methods. Fig. 4a displays the TEM image of the hollow silica nanotubes after dissolving the silica/bi 2 S 3 samples into HCl aqueous solution for about 5 h. Typically, the silica-coated Bi 2 S 3 sample (0.1 g) was added to 10 ml HCl solution (1:1). In this stage, the solution color changed from black to colorless gradually leaving some white products. These nanotubes have lengths of mm and diameters of nm. The wall thickness is about 40 nm. The residual indside the hollow tubes may be Bi 2 S 3 nanorods, which are removed incompletely due to the shortage of dissolving time. Meanwhile, it is interesting to find that by focusing the electron beam on the heat-treated sample (400 1C for 1 h) for about 5 s, hollow silica nanotubes can also be obtained. Fig. 4b and c shows the products obtained by this method. It is considered that upon irradiating the sample, a rapid increase of temperature, together with the low pressure in the TEM chamber favors the movement of the metastable Bi 2 S 3 nanorods. Similar phenomena have also been observed in the exploring of Sn-filled ZnS nanotubes [15]. B y these two means, hollow silica nanotubes with one open end were obtained. The wall thickness can be adjusted by changing the concentration of the TEOS. The interior diameter can also be changed by choosing Bi 2 S 3 nanorods with different diameters. This adjustable property makes silica hollow tubes have potential applications more widely. 4. Conclusions In summary, we have demonstrated a simple method for coating Bi 2 S 3 nanorods with a uniform silica layer. The stability and the luminescent properties of these rods were improved. Furthermore, well-defined silica hollow tubes can also be obtained by removing Bi 2 S 3 core. This facile procedure can be easily adjusted for the largescale synthesis of silica hollow tubes. Acknowledgment Fig. 3. Emission spectra of (a) Bi 2 S 3 :Dy 3+ nanorods and (b) silica/bi 2 S 3 :Dy 3+ nanorods. This work was supported by the awarded funds to excellent State Key Laboratory by Chinese Ministry of Education (No ). Fig. 4. TEM images of silica hollow tubes obtained by (a) immersing in HCl and (b), (c) focusing electron beam on the thermal treated samples.

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