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Low-temperature chemical route to bismuth-doped tellurium single-crystalline nanorods
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Materials Letters 62 (2008) 1983 – 1985 www.elsevier.com/locate/matlet
Low-temperature chemical route to bismuth-doped tellurium single-crystalline nanorods Hongmei Liu, Songxiu Liu, Kaixun Huang ⁎ Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, China Received 13 September 2007; accepted 29 October 2007 Available online 4 November 2007
Abstract One-dimensional bismuth-doped tellurium nanorods have been synthesized via a simple chemical route by the reduction of tellurium oxide and bismuth nitrate in neutral solution at low temperature of 50 °C. Scanning electron microscopy (SEM) images revealed that the size of the nanorods was uniform and ∼ 25 nm in diameter and ∼ 500 nm in length. And the nanorods were characterized to be single crystalline by high-resolution transmission electron microscopy (HRTEM) analysis. The optical properties of the nanorods were investigated and the results showed that the nanorods exhibited strong luminescence emission at room temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: Tellurium; Dope; Nanorod; Single-crystalline; Photoluminescence
1. Introduction
2. Experimental
The motivation for the fabrication of nano-scaled materials lies in that the material in the size of nanometer could probably possess different properties (e.g. conductivity, optics, magnetism) from that in bulk size. As a semiconductor material with a band gap energy of 0.35 eV, tellurium is of special interest for its versatile properties such as photoconductivity; catalytic activity, piezoelectricity, and thermoelectricity [1–3]. So far, many chemical methods have been developed for the synthesis of one-dimensional (1D) tellurium nanostructures including nanowires, nanorods, nanotubes, and nanobelts [4–6]. Among these methods, 1D tellurium nanostructures were always synthesized by chemical reduction of tellurium compounds through hydrothermal process [7–9]. Still, it is a challenge to develop low cost and convenient synthetic routes. Here in this paper, we presented a facile and highly reproductive route to synthesize uniform bismuth-doped tellurium nanorods at low temperature. And the optical properties of the nanorods are investigated. To our best knowledge, this fabrication process of the bismuth-doped tellurium nanorods has not been reported so far.
All chemicals are of analytical grade and purchased from Shanghai Chemical Reagent Co. Ltd.
⁎ Corresponding author. Tel.: +86 27 8754 3532; fax: +86 27 8754 3632. E-mail address: [email protected] (K. Huang). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.10.058
2.1. Synthesis In one glass beaker, 4 mL NH3·H2O (30 wt.%) and 0.29 g ethylene diamine tetraacetic acid (EDTA) were dissolved in 50 mL distilled water. Then 0.32 g Bi(NO3)3·5H2O was added under stirring to form a homogenous solution, which is labelled as solution A. In another glass beaker, 0.1 g NaOH, 0.16 g TeO2, and 5 g poly(vinyl pyrrolidone) (PVP, Mw = 40,000) were dissolved in 40 mL distilled water subsequently. And this solution was labelled as solution B. Under vigorous stirring, solution B was added slowly into solution A to form a homogeneous mixture. The final volume of mixture was 100 mL by adding distilled water, and the pH value was adjusted to 7.0 by concentric nitric acid. After adding 4 mL N2H4·H2O (85 wt.%) into the neutral mixture, the solution was immediately transferred to a glass flask and sealed by polyethylene plastic film, and maintained at 50 °C for 48 h. The product was collected by centrifugation and washed by water and ethanol in sequence for three times before drying in vacuum at 40 °C for 5 h. In the contrast experiment for the
1984
H. Liu et al. / Materials Letters 62 (2008) 1983–1985
preparation of pure Te, the whole process was the same except the addition of Bi(NO3)3·5H2O to solution A. 2.2. Characterization X-ray diffraction (XRD) patterns of the samples were recorded on a X'Pert PRO diffraction meter (Cu Kα radiation, λ = 1.5418 Å, 20 kV, 150 mA). Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectrometer (EDS) measurements were performed by using FEI SIRION 200 and Oxford INCA. The TEM images were taken on a JEM2010FEF operated at a voltage of 200 kV. UV–Vis absorption spectrum of sample was measured by using Hitachi U-3010 spectrophotometer. Composition of product was also analyzed by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) with a PerkinElmer Optima 4300 DV. The photoluminescence (PL) spectra were recorded with a Jasco FP-6500 fluorescence spectrophotometer at room temperature. 3. Results and discussion 3.1. The structure and composition of products XRD patterns of the products are illustrated in Fig. 1. These diffraction peaks match well with that reported in the standard card (PDF No. 36-1452), which corresponds to that of hexagonal structure tellurium with a cell constant of a = 4.456 Å and c = 5.922 Å. From the XRD pattern of Fig. 1a, it can be seen that the sample is of high purity since no other peaks from impurities could be found in the result. However, the sample prepared from the solution containing Bi3+ ions also exhibits only Te signals in XRD pattern (Fig. 1b). This could be explained that the XRD pattern of product was not affected by the ultra low concentration of Bi. The sample prepared from the solution containing Bi3+ ions was analyzed by EDS in Fig. 2. The results showed that only Te and Bi signals were observed, indicating Bi and Te not in the form of oxide or other salt. The accurate element analysis of the sample was performed by ICP, which gave a composition of 99.21:0.79 in molar ratio of Bi:Te. This indicates that a small amount of bismuth ions were also reduced
Fig. 2. EDS spectrum of the sample prepared in the presence of Bi3+ ions.
during the reduction process of tellurium ions, and Bi-doped tellurium was formed as a result. The low concentration of Bi in the product is probably due to the difference in the reactivity of bismuth and tellurium ions in this system. It should be pointed that EDTA played an important role to keep Bi3+ ions from hydrolyzing to Bi(OH)3 precipitate in this neutral solution during reduction process. 3.2. Morphologies of the product The morphologies of the prepared sample were investigated by SEM and TEM (Fig. 2). In the SEM image, it is found that the sample consists of one-dimensional nanorods with diameter of 20–30 nm and length of 500–600 nm (Fig. 3a). The formation of the nanorod structure is attributed to the surfactant effect of PVP since nanobelts with 2 μm length and 500 nm width were obtained in the absence of PVP. The TEM image of an individual nanorod is represented in the inset picture of Fig. 3a. The nanorod has a uniform diameter of 25 nm and length of 500 nm. The EDS analysis of a single nanorod demonstrated that about 0.64 at.% Bi was uniformly distributed in the nanorod. Corresponding to the square region labeled in the inset picture of Fig. 3a, a HRTEM image was recorded and a regular crystal lattice was shown in Fig. 3b. Analysis on the lattice fringes gives lattice spacings of 5.90 and 3.89 Å, which match well with (001) and (100) planes of bulk tellurium, respectively. The (001) plane is parallel to the edge of the nanorod, indicating that the nanorods have grown along the (001) direction. In addition, a fast Fourier transform (FFT) was conducted on the image (Fig. 3b inset). The sharp spots in the FFT image confirm that the nanorod is single crystalline in nature. And the spots could be well indexed to the (100), (001), and (101) planes, respectively. Although element Bi atoms existed in the sample, the lattice fringe was same as that of pure tellurium. It was probably due to that the concentration of Bi was too low and Bi atoms only occupied the vacancy sites of tellurium, which did not change the lattice structure. 3.3. Optical properties of the product
Fig. 1. X-ray diffraction patterns of samples prepared (a) in the absence of Bi3+ ions and (b) in the presence of Bi3+ ions.
Although the dried as-prepared Bi-doped Te nanorods are black in color, the solution exhibits deep blue color when the nanorods are dispersed in the water. This phenomenon is quite different from that of solution containing pure Te sample, which is in grey color. The special color suggests that the Bi-doped Te nanorods may possess special optical property different from that of bulk tellurium. Fig. 4 shows the
H. Liu et al. / Materials Letters 62 (2008) 1983–1985
1985
Fig. 3. (a) SEM image of Bi-doped Te sample. Inset picture is the TEM image of one nanorod; (b) Corresponding HRTEM image of the square area labeled in the inset picture of (a). Inset picture is the FFT of the image.
UV–vis absorption spectrum and photoluminescence spectrum of the nanorods. It is clearly in the UV–vis spectrum that there are two absorption peaks at 284 and 652 nm, which are close to the work reported by Qian et al. (278 and 586 nm) [4]. The first absorption peak at 284 nm may result from the allowed direct transition from the valence band (p-bonding triplet) to the conduction band (p-antibonding triplet), and the second peak at 652 nm was probably ascribed to a forbidden direction transition [10,11]. Compare this UV–vis result with that of Qian et al., the absorption peaks are sharper and there is a red-shift in peak value. This indicates that the bismuth doping has an effect on the optical property of tellurium nanorod. The existence of bismuth dopant may change the structure of valence and conduction bands of tellurium, which is favorable for the light absorption. It is scarcely reported on the photoluminescence property of nanostructured tellurium. In this work, the photoluminescence spectrum of the sample at excitation wavelength of 372 nm was studied. The result in Fig. 4 shows three strong emission peaks located at 415, 440, and 470 nm. We also conducted the photoluminescence spectrum of the pure Te sample and there was no emission peaks observed. The special optical property of the Bi-doped Te nanorods may open a new
application for the Te nanomaterials. Furthermore, the synthesis of other element doped tellurium is also in study.
4. Conclusion Uniform single-crystalline nanorods of Bi-doped tellurium were successfully synthesized through a convenient chemical reduction route. Different from other normally used hydrothermal methods, the reaction process could be completed at temperature as low as 50 °C. SEM and ICP results showed that the nanorods with diameter of 25 nm and length of 500 nm consisted of 99.21 at.% tellurium and 0.79 at.% bismuth. In addition, the product displayed strong photoluminescence property at room temperature and three strong emission peaks located at 415, 440, and 470 nm were observed at excitation wavelength of 372 nm. Further investigation may focus on the extension of this process to the production of tellurium nanomaterials doped with other elements. References
Fig. 4. UV–vis spectrum (a: right Y scale) and photoluminescence spectrum (excitation wavelength: 372 nm) of the Bi-doped Te (b: left Y scale) at room temperature.
[1] P. Tangney, S. Fahy, Phys. Rev. B 65 (2002) 054302-14. [2] A.A. Kudryavstev, The Chemistry and Technology of Selenium and Tellurium, Collet's Ltd, London, 1974. [3] B. Mayers, Y.N. Xia, J Mater Chem 12 (2002) 1875–1881. [4] H.S. Qian, S.H. Yu, J.Y. Gong, L.B. Luo, L.F. Fei, Langmuir 22 (2006) 3830–3835. [5] W. Zhu, W. Wang, H. Xu, L. Zhou, L. Zhang, J. Shi, Cryst. Growth Des. 6 (2006) 2804–2808. [6] Y.J. Zhu, W.W. Wang, R.J. Qi, X.L. Hu, Angew. Chem. Int. Ed. 43 (2004) 1410–1414. [7] M. Mo, J. Zeng, X. Liu, W. Yu, S. Zhang, Y. Qian, Adv Mater 14 (2002) 1658–1662. [8] H. Yin, Z. Xu, J. Bai, H. Bao, Y. Zheng, Mater Lett 59 (2005) 3779–3782. [9] A.M. Qin, Y.P. Fang, C.Y. Su, Inorg. Chem. Commun. 7 (2004) 1014–1016. [10] H.M. Isomaski, J.V. Boehm, Phys. Scr. 25 (1982) 801–803. [11] R. Swan, A.K. Ray, C.A. Hogarth, Phys. Status Solidi, A 127 (1991) 555–560.
Materials Letters 62 (2008) 1983 – 1985 www.elsevier.com/locate/matlet
Low-temperature chemical route to bismuth-doped tellurium single-crystalline nanorods Hongmei Liu, Songxiu Liu, Kaixun Huang ⁎ Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, China Received 13 September 2007; accepted 29 October 2007 Available online 4 November 2007
Abstract One-dimensional bismuth-doped tellurium nanorods have been synthesized via a simple chemical route by the reduction of tellurium oxide and bismuth nitrate in neutral solution at low temperature of 50 °C. Scanning electron microscopy (SEM) images revealed that the size of the nanorods was uniform and ∼ 25 nm in diameter and ∼ 500 nm in length. And the nanorods were characterized to be single crystalline by high-resolution transmission electron microscopy (HRTEM) analysis. The optical properties of the nanorods were investigated and the results showed that the nanorods exhibited strong luminescence emission at room temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: Tellurium; Dope; Nanorod; Single-crystalline; Photoluminescence
1. Introduction
2. Experimental
The motivation for the fabrication of nano-scaled materials lies in that the material in the size of nanometer could probably possess different properties (e.g. conductivity, optics, magnetism) from that in bulk size. As a semiconductor material with a band gap energy of 0.35 eV, tellurium is of special interest for its versatile properties such as photoconductivity; catalytic activity, piezoelectricity, and thermoelectricity [1–3]. So far, many chemical methods have been developed for the synthesis of one-dimensional (1D) tellurium nanostructures including nanowires, nanorods, nanotubes, and nanobelts [4–6]. Among these methods, 1D tellurium nanostructures were always synthesized by chemical reduction of tellurium compounds through hydrothermal process [7–9]. Still, it is a challenge to develop low cost and convenient synthetic routes. Here in this paper, we presented a facile and highly reproductive route to synthesize uniform bismuth-doped tellurium nanorods at low temperature. And the optical properties of the nanorods are investigated. To our best knowledge, this fabrication process of the bismuth-doped tellurium nanorods has not been reported so far.
All chemicals are of analytical grade and purchased from Shanghai Chemical Reagent Co. Ltd.
⁎ Corresponding author. Tel.: +86 27 8754 3532; fax: +86 27 8754 3632. E-mail address: [email protected] (K. Huang). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.10.058
2.1. Synthesis In one glass beaker, 4 mL NH3·H2O (30 wt.%) and 0.29 g ethylene diamine tetraacetic acid (EDTA) were dissolved in 50 mL distilled water. Then 0.32 g Bi(NO3)3·5H2O was added under stirring to form a homogenous solution, which is labelled as solution A. In another glass beaker, 0.1 g NaOH, 0.16 g TeO2, and 5 g poly(vinyl pyrrolidone) (PVP, Mw = 40,000) were dissolved in 40 mL distilled water subsequently. And this solution was labelled as solution B. Under vigorous stirring, solution B was added slowly into solution A to form a homogeneous mixture. The final volume of mixture was 100 mL by adding distilled water, and the pH value was adjusted to 7.0 by concentric nitric acid. After adding 4 mL N2H4·H2O (85 wt.%) into the neutral mixture, the solution was immediately transferred to a glass flask and sealed by polyethylene plastic film, and maintained at 50 °C for 48 h. The product was collected by centrifugation and washed by water and ethanol in sequence for three times before drying in vacuum at 40 °C for 5 h. In the contrast experiment for the
1984
H. Liu et al. / Materials Letters 62 (2008) 1983–1985
preparation of pure Te, the whole process was the same except the addition of Bi(NO3)3·5H2O to solution A. 2.2. Characterization X-ray diffraction (XRD) patterns of the samples were recorded on a X'Pert PRO diffraction meter (Cu Kα radiation, λ = 1.5418 Å, 20 kV, 150 mA). Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectrometer (EDS) measurements were performed by using FEI SIRION 200 and Oxford INCA. The TEM images were taken on a JEM2010FEF operated at a voltage of 200 kV. UV–Vis absorption spectrum of sample was measured by using Hitachi U-3010 spectrophotometer. Composition of product was also analyzed by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) with a PerkinElmer Optima 4300 DV. The photoluminescence (PL) spectra were recorded with a Jasco FP-6500 fluorescence spectrophotometer at room temperature. 3. Results and discussion 3.1. The structure and composition of products XRD patterns of the products are illustrated in Fig. 1. These diffraction peaks match well with that reported in the standard card (PDF No. 36-1452), which corresponds to that of hexagonal structure tellurium with a cell constant of a = 4.456 Å and c = 5.922 Å. From the XRD pattern of Fig. 1a, it can be seen that the sample is of high purity since no other peaks from impurities could be found in the result. However, the sample prepared from the solution containing Bi3+ ions also exhibits only Te signals in XRD pattern (Fig. 1b). This could be explained that the XRD pattern of product was not affected by the ultra low concentration of Bi. The sample prepared from the solution containing Bi3+ ions was analyzed by EDS in Fig. 2. The results showed that only Te and Bi signals were observed, indicating Bi and Te not in the form of oxide or other salt. The accurate element analysis of the sample was performed by ICP, which gave a composition of 99.21:0.79 in molar ratio of Bi:Te. This indicates that a small amount of bismuth ions were also reduced
Fig. 2. EDS spectrum of the sample prepared in the presence of Bi3+ ions.
during the reduction process of tellurium ions, and Bi-doped tellurium was formed as a result. The low concentration of Bi in the product is probably due to the difference in the reactivity of bismuth and tellurium ions in this system. It should be pointed that EDTA played an important role to keep Bi3+ ions from hydrolyzing to Bi(OH)3 precipitate in this neutral solution during reduction process. 3.2. Morphologies of the product The morphologies of the prepared sample were investigated by SEM and TEM (Fig. 2). In the SEM image, it is found that the sample consists of one-dimensional nanorods with diameter of 20–30 nm and length of 500–600 nm (Fig. 3a). The formation of the nanorod structure is attributed to the surfactant effect of PVP since nanobelts with 2 μm length and 500 nm width were obtained in the absence of PVP. The TEM image of an individual nanorod is represented in the inset picture of Fig. 3a. The nanorod has a uniform diameter of 25 nm and length of 500 nm. The EDS analysis of a single nanorod demonstrated that about 0.64 at.% Bi was uniformly distributed in the nanorod. Corresponding to the square region labeled in the inset picture of Fig. 3a, a HRTEM image was recorded and a regular crystal lattice was shown in Fig. 3b. Analysis on the lattice fringes gives lattice spacings of 5.90 and 3.89 Å, which match well with (001) and (100) planes of bulk tellurium, respectively. The (001) plane is parallel to the edge of the nanorod, indicating that the nanorods have grown along the (001) direction. In addition, a fast Fourier transform (FFT) was conducted on the image (Fig. 3b inset). The sharp spots in the FFT image confirm that the nanorod is single crystalline in nature. And the spots could be well indexed to the (100), (001), and (101) planes, respectively. Although element Bi atoms existed in the sample, the lattice fringe was same as that of pure tellurium. It was probably due to that the concentration of Bi was too low and Bi atoms only occupied the vacancy sites of tellurium, which did not change the lattice structure. 3.3. Optical properties of the product
Fig. 1. X-ray diffraction patterns of samples prepared (a) in the absence of Bi3+ ions and (b) in the presence of Bi3+ ions.
Although the dried as-prepared Bi-doped Te nanorods are black in color, the solution exhibits deep blue color when the nanorods are dispersed in the water. This phenomenon is quite different from that of solution containing pure Te sample, which is in grey color. The special color suggests that the Bi-doped Te nanorods may possess special optical property different from that of bulk tellurium. Fig. 4 shows the
H. Liu et al. / Materials Letters 62 (2008) 1983–1985
1985
Fig. 3. (a) SEM image of Bi-doped Te sample. Inset picture is the TEM image of one nanorod; (b) Corresponding HRTEM image of the square area labeled in the inset picture of (a). Inset picture is the FFT of the image.
UV–vis absorption spectrum and photoluminescence spectrum of the nanorods. It is clearly in the UV–vis spectrum that there are two absorption peaks at 284 and 652 nm, which are close to the work reported by Qian et al. (278 and 586 nm) [4]. The first absorption peak at 284 nm may result from the allowed direct transition from the valence band (p-bonding triplet) to the conduction band (p-antibonding triplet), and the second peak at 652 nm was probably ascribed to a forbidden direction transition [10,11]. Compare this UV–vis result with that of Qian et al., the absorption peaks are sharper and there is a red-shift in peak value. This indicates that the bismuth doping has an effect on the optical property of tellurium nanorod. The existence of bismuth dopant may change the structure of valence and conduction bands of tellurium, which is favorable for the light absorption. It is scarcely reported on the photoluminescence property of nanostructured tellurium. In this work, the photoluminescence spectrum of the sample at excitation wavelength of 372 nm was studied. The result in Fig. 4 shows three strong emission peaks located at 415, 440, and 470 nm. We also conducted the photoluminescence spectrum of the pure Te sample and there was no emission peaks observed. The special optical property of the Bi-doped Te nanorods may open a new
application for the Te nanomaterials. Furthermore, the synthesis of other element doped tellurium is also in study.
4. Conclusion Uniform single-crystalline nanorods of Bi-doped tellurium were successfully synthesized through a convenient chemical reduction route. Different from other normally used hydrothermal methods, the reaction process could be completed at temperature as low as 50 °C. SEM and ICP results showed that the nanorods with diameter of 25 nm and length of 500 nm consisted of 99.21 at.% tellurium and 0.79 at.% bismuth. In addition, the product displayed strong photoluminescence property at room temperature and three strong emission peaks located at 415, 440, and 470 nm were observed at excitation wavelength of 372 nm. Further investigation may focus on the extension of this process to the production of tellurium nanomaterials doped with other elements. References
Fig. 4. UV–vis spectrum (a: right Y scale) and photoluminescence spectrum (excitation wavelength: 372 nm) of the Bi-doped Te (b: left Y scale) at room temperature.
[1] P. Tangney, S. Fahy, Phys. Rev. B 65 (2002) 054302-14. [2] A.A. Kudryavstev, The Chemistry and Technology of Selenium and Tellurium, Collet's Ltd, London, 1974. [3] B. Mayers, Y.N. Xia, J Mater Chem 12 (2002) 1875–1881. [4] H.S. Qian, S.H. Yu, J.Y. Gong, L.B. Luo, L.F. Fei, Langmuir 22 (2006) 3830–3835. [5] W. Zhu, W. Wang, H. Xu, L. Zhou, L. Zhang, J. Shi, Cryst. Growth Des. 6 (2006) 2804–2808. [6] Y.J. Zhu, W.W. Wang, R.J. Qi, X.L. Hu, Angew. Chem. Int. Ed. 43 (2004) 1410–1414. [7] M. Mo, J. Zeng, X. Liu, W. Yu, S. Zhang, Y. Qian, Adv Mater 14 (2002) 1658–1662. [8] H. Yin, Z. Xu, J. Bai, H. Bao, Y. Zheng, Mater Lett 59 (2005) 3779–3782. [9] A.M. Qin, Y.P. Fang, C.Y. Su, Inorg. Chem. Commun. 7 (2004) 1014–1016. [10] H.M. Isomaski, J.V. Boehm, Phys. Scr. 25 (1982) 801–803. [11] R. Swan, A.K. Ray, C.A. Hogarth, Phys. Status Solidi, A 127 (1991) 555–560.