Article pubs.acs.org/crystal
Controllable Synthesis of Bismuth Chalcogenide Core−shell Nanorods Zhen-Hua Ge and George S. Nolas* Department of Physics, University of South Florida, Tampa, Florida 33620, United States S Supporting Information *
ABSTRACT: Bi2S3/Bi and Bi2Te3/Bi core−shell nanorods were successfully synthesized by a hydrothermal method using Bi2S3 nanorods as the template. The thickness of the Bi shell was controlled by varying the reaction time or the Bi2S3-to-Te ratio. Phase-pure Bi and Bi2Te3 nanorods were also obtained by this synthetic approach. The formation of Bi2S3/Bi and Bi2Te3/Bi core−shell nanorods can be separated into two different processes. One is the reduction of Bi2S3 nanorods by hydrazine. The other is the formation of Bi2Te3 through the exchange of Te2− and S2− and the subsequent reaction of Bi with Te. The formation mechanisms for the core−shell nanorods are discussed in detail.
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INTRODUCTION
Nanostructured metal chalcogenides exhibit interesting physical properties and, therefore, are of interest for energy-related applications including fuel cells, solar cells, light-emitting diodes, sensors, and thermoelectrics.1−4 This is due, in part, to progress in the development of synthetic techniques that produce new nanoscale materials of different sizes and morphologies. Size control is one important aspect in allowing for the tunability of the optical and electrical properties of nanomaterials by changing the band gap.5,6 Semiconductor heterostructures and core−shell nanoparticles represent another interesting direction toward nanostructured materials synthesis.7,8 Nanoscale core−shell materials have previously been synthesized in various compositions.9 Core−shell materials such as SiO2/Au,10 CdSe/CdS,11 and PbTe/PbS12 have potential applications in photoluminescence, photocatalysis, and thermoelectrics. Bismuth chalcogenides are known to be good thermoelectric materials13,14 and are also of interest for photocatalysis and as topological insulators.15,16 Herein, we demonstrate the formation of bismuth chalcogenide core−shell nanorods by hydrothermal synthesis at moderate temperatures. Two types of core−shell nanorods, Bi2Te3/Bi and Bi2S3/Bi, were prepared using Bi2S3 nanorods as the template material. Bi2S3 has a chainlike structure and can be synthesized as nanorods for use as templates by hydrothermal processing.17,18 The thickness of the shell was controlled by varying the reaction time or the starting material ratio.
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Figure 1. Powder XRD patterns of the products prepared at 180 °C for holding times of (a) 0, (b) 3, (c) 6, and (d) 12 h. Shown at the bottom is the XRD powder pattern of Bi2S3 from JCPDS 17-0320.
shown in the figure, with increasing reaction time, an increasing amount of Bi was formed, with pure Bi obtained at 180 °C for 12 h. To confirm the formation of Bi2S3/Bi core−shell nanorods, high-resolution transmission electron microscopy (HRTEM) images were obtained, and selected-area electron
RESULTS AND DISCUSSION
Received: September 3, 2013 Revised: December 6, 2013 Published: December 17, 2013
Figure 1 shows the X-ray diffraction (XRD) patterns of the products prepared at 180 °C for different reaction times. As © 2013 American Chemical Society
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diffraction (SAED) was performed. Figure 2a shows a TEM image of the product prepared at 180 °C for 3 h without
Figure 3. XRD patterns of the products prepared at 180 °C for 12 h with Bi2S3-to-Te ratios of (a) 1:0, (b) 1:0.3, (c) 1:0.7, (d) 1:1.5, (e) 1:2.5, and (f) 1:3. Figure 2. (a) TEM image of the product prepared at 180 °C for 3 h without Te, indicating the formation of a core and shell. The rod depicted is relatively large to show the shell-and-core structure clearly and collect the SAED pattern of the shell without overlap with the core. (b) HRTEM image and (c,d) SAED patterns of the (c) shell and (d) core from the rod shown in panel a.
addition of the Te source and indicates that the product was rodlike with typical dimensions of 100 nm in diameter and 1 μm in length. The contrast shown in Figure 2a is an indication of the different compositions, core and shell, that constitute the rod. From HRTEM analyses (Figure 2b), the formation of grain boundaries is visible. The SAED pattern (Figure 2c) collected from the shell shown in Figure 2b indicates oriented polycrystalline Bi. The SAED pattern (Figure 2d) collected from the core is very similar to that of the shell because of electron diffraction from both shell and core; however, the diffraction pattern of single-crystal Bi2S3 (marked by circles) is also visible, as shown in the Figure 2d. This is due to the reduction of Bi2S3 to Bi from the outside, whereas the residual single-crystal Bi2S3 remains in the middle, as the reaction time is not long enough to completely reduce Bi2S3 to Bi. Figure 3 shows the XRD patterns of the products prepared at 180 °C for 12 h with different starting material (Bi2S3-to-Te) ratios. As shown in Figure 3a, the XRD pattern can be indexed to pure Bi. With increasing Te content in the starting material the Bi2Te3 XRD lines become more prominent, suggesting an increasing amount of Bi2Te3 with a decreasing amount of Bi in the final products. Finally, the Bi lines disappear, and all XRD lines match those of Bi2Te3 (Figure 3f), indicating that pure Bi2Te3 was obtained at a Bi2S3-to-Te ratio of 1:3. These results illustrate how Bi2Te3/Bi core−shell nanorods with controllable Bi-to-Bi2Te3 content ratios were obtained by varying the starting material (Bi2S3-to-Te) ratios. A possible reaction process for the formation of Bi2Te3/Bi nanorods is as follows:
(Te3 + 1)2 − → 3Te + Te 2 −
(2)
Bi 2S3 + NH 2NH 2 → Bi + N2 + H 2S
(3)
Bi 2S3 + 3Te2 − → Bi 2Te3 + 3S2 −
(4)
2Bi + 3Te → Bi 2Te3
(5)
Bi 2Te3 + NH 2NH 2 → Bi + N2 + H 2Te
(6)
In eqs 1 and 2 Te is of two types, Te2+ ions and highly reactive Te formed by the reaction with NaOH and hydrazine.19 This highly reactive Te can react with Bi directly. This is different than the case of Te powder used as the Te source. (Henceforth. we will indicate highly reactive Te with italics.) In eq 3, Bi2S3 was reduced to Bi by hydrazine. Bismuth ions (Bi3+) can be reduced to metallic Bi, and Te can readily be reduced to Te2− by hydrazine in alkaline conditions,20 as indicated by the standard redox potentials (E0Bi3+/Bi = 0.168 V, E0OH−/N2H4 = −1.15 V, and E0Te/Te2− = −0.845 V).20,21 There are two routes for the formation of Bi2Te3: One is ion exchange between S2− and Te2− (eq 4), and the other is direct reaction of Bi with Te (eq 5). However, as is the case for Bi2S3, Bi2Te3 can also be reduced to Bi by hydrazine. Apparently, there is dynamic equilibrium between the formation and dissolution of Bi2Te3 on the surface of the nanorods. Figure 4 shows the XRD patterns of the products with a Bi2S3-to-Te ratio of 1:1.5 prepared at 180 °C for 12 h with different amounts of NaOH added. In the reaction process NaOH acts as a reactant, as shown in eqs 1 and 2. With a NaOH content of 0.1 g, Te was detected in the final product (Figure 4a). Tellurium was not observed in the products as the NaOH content was increased to more than 0.5 g (Figure 4b). The Bi2Te3 content increased with increasing NaOH content as a result of an increase of Te2+ and Te. The amount of Bi2Te3 was reduced sharply as the NaOH content increased to 2 g (Figure 4d). This might be due to an excess of NaOH leading to an enhancement of the reducing ability of hydrazine. Mi et al.22 reported that excess NaOH will lead to the emergence of a
Te + NH 2NH 2 + OH− → (Te3 + 1)2 − + N2 + 2H 2O (1) 534
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polycrystalline Bi and the core was polycrystalline Bi2Te3. The SAED patterns (Figure 5c,d) confirmed the HRTEM results. To understand the reaction and growth mechanism of Bi2Te3/Bi polycrystalline core−shell nanorods, different reaction temperatures and holding times were investigated. Figure S1 of the Supporting Information shows the XRD patterns of the products prepared with a fixed Bi2S3-to-Te ratio of 1:1.5 but at different temperatures for 12-h reactions. When the temperature was 120 °C (Figure S1a, Supporting Information), four phases (Te, Bi2S3, Bi, and Bi2Te3) were present in the final product, indicating that the reaction (eqs 1−6) did not progress to completion at this temperature. As the temperature was increased from 120 to 170 °C (Figure S1a−c, Supporting Information), the XRD lines for Te disappeared, whereas those for Bi2S3 decreased in intensity and those for Bi and Bi2Te3 increased in intensity. This suggests that temperature is the determining factor in this reaction. In the final product, only Bi and Bi2Te3 were detected at a reaction temperature of 180 °C (Figure S1d, Supporting Information). The XRD patterns of the products prepared at 180 °C with a Bi2S3-to-Te ratio of 1:1.5 and with different reaction times are shown in Figure S2 of the Supporting Information and indicate that an increasing amount of Bi2Te3 and a decreasing amount of Bi was obtained with increasing reaction times. Based on these results, we believe that the formation of Bi2S3/Bi and Bi2Te3/Bi core−shell nanorods can be separated into two different processes. One is the reduction of Bi2S3 nanorods by hydrazine. The other is the formation of Bi2Te3 through the exchange of Te2− and S2− and the subsequent reaction of Bi and Te, as shown in Figure 6. When the Te
Figure 4. XRD patterns of the products prepared at 180 °C for 12 h with a Bi2S3-to-Te ratio of 1:1.5 and NaOH contents of (a) 0.1, (b) 0.5, (c) 1, and (d) 2 g. Shown at the bottom is the XRD powder pattern of Bi2S3 from JCPDS 17-0320.
Bi impurity during the synthesis of Bi2Te3 as a result of the enhanced role of NaOH as the reducing agent. Figure 5a shows a typical TEM image of the products prepared at 180 °C for 12 h with a Bi2S3-to-Te ratio of 1:1.5 and again indicates a rodlike morphology with typical dimensions of 50−100 nm in diameter and 500 nm in length. The HRTEM image (Figure 5b) indicates that the shell was
Figure 6. Illustration of the reaction and growth processes. The formation of core−shell nanorods can be separated into two processes. One is the reduction of Bi2S3 nanorods by hydrazine. The other is the formation of Bi2Te3. When the Te source was absent, Bi2S3/Bi core− shell nanorods were obtained. When Te was added, Bi2Te3/Bi core− shell nanorods were obtained. With a sufficient amount of Te introduced into the process, Bi2Te3 nanorods were obtained.
source was absent, Bi2S3/Bi core−shell nanorods were obtained. Bi2Te3 nanorods were obtained when the Te source was sufficient for completion of the reaction. For a specific amount of Te, Bi2Te3/Bi core−shell nanorods were obtained. Apparently, the shell is always Bi, for both Bi2S3/Bi and Bi2Te3/ Bi core−shell nanorods, because of the fact that both Bi2S3 and Bi2Te3 react with hydrazine to form Bi. In the synthesis of Bi2Te3/Bi core−shell nanorods, the core and shell were polycrystalline because of the relatively more complex formation mechanism of both the Bi2Te3 core and Bi shell. The Bi shell thickness for Bi2S3/Bi can be controlled by varying the reaction time, whereas the shell thickness for
Figure 5. (a) Typical TEM image of a product prepared at 180 °C for 12 h with a Bi2S3-to-Te ratio of 1:1.5. (b) HRTEM image and (c,d) SAED patterns of the (c) shell and (d) core from the rod shown in panel a. From the HRTEM image, the fringe spacing of the shell is in agreement with that of the distance of the Bi(012) crystal plane. The SAED patterns indicate that both the core and shell are polycrystalline. 535
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation and Department of Energy Partnership on Thermoelectric Devices for Vehicle Applications (Grant 1048796).
Bi2Te3/Bi can be controlled by varying the Te source content. Bi2S3/Bi and Bi2Te3/Bi with different shell thicknesses can result in different physical properties, because of a change in band gap, for example. Controlling the band gap in other core− shell materials by varying the shell thickness has been achieved.6,9
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
REFERENCES
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CONCLUSIONS Bi and Bi2Te3 nanorods and Bi2S3/Bi and Bi2Te3/Bi core−shell nanorods were prepared from Bi2S3 nanorods through a facile hydrothermal process. The core−shell nanorods can form from two processes: the reduction of Bi2S3 nanorods by hydrazine and the formation of Bi2Te3 by anion exchange between S2− and Te2− and the subsequent reaction between Bi and Te. Our work therefore allows for a method to prepare core−shell materials with a controllable shell thickness and is another illustration of the versatility of utilizing hydrothermal processing for the preparation of core−shell materials. This approach can also be used to investigate new or improved properties for a variety of applications, including optical electronics, photocatalysis, and thermoelectrics.
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Article
All chemical reagents used in this study were of analytical grade. The core−shell nanostructure materials were synthesized by a two-step process. As template materials, [001]-oriented single-crystal Bi2S3 nanorods of 50−100 nm in diameter and from 500 nm to 1 μm in length were prepared as described previously.23 Smaller-diameter core−shell nanorods will result from template materials having smaller diameters. For the preparation of Bi2S3/Bi core−shell nanorods, in a typical experiment, 1 g of NaOH was first added to 20 mL of deionized (DI) water, and the mixture was stirred for 10 min. Then, 10 mL of hydrazine was added to the solution, and the mixture was again stirred for 20 min. Following this step, 0.5 mmol of Bi2S3 nanorod powder was added to the solution, which was again stirred for 30 min. These stirring steps throughout the process ensured thorough mixing of the solution. The resulting solution was then transferred into a Teflon-lined stainless steel autoclave (100 mL capacity) together with an additional 30 mL of DI water. For the preparation of Bi2Te3/Bi core−shell nanorods, the process was the same with the exception that Te powder was added together with the resulting solution in the Teflon-lined stainless steel autoclave. The sealed autoclave was heated to different temperatures and held for different times to investigate the effects of the hydrothermal parameters on the composition and morphology of the products. The final black solid product was filtered, washed with DI water and ethanol three times, and then dried under a vacuum at room temperature. The as-synthesized powders were characterized by XRD (Bruker AXS D8) and transmission electron microscopy (TEM, JEOL 2100 and HRTEM FEI Tecnai F20 S-Twin TEM).
S Supporting Information *
Additional figures as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 536
dx.doi.org/10.1021/cg401323t | Cryst. Growth Des. 2014, 14, 533−536
