Controlled Synthesis of Tellurium Nanostructures from Nanotubes to

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Controlled Synthesis of Tellurium Nanostructures from Nanotubes to Nanorods and Nanowires and Their Template Applications Hangtian Zhu,†,‡ He Zhang,†,‡ Jingkui Liang,‡,§ Guanghui Rao,‡ Jingbo Li,‡ Guangyao Liu,‡ Zhenmin Du,† Haiming Fan,||,^,* and Jun Luo‡,* †

Department of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China § International Center for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, China Shanxi Key Laboratory of Degradable Biomedical Materials, School of Chemical Engineering, Northwest University, Xi’an, Shanxi 710069, China, ^ Department of Materials Science and Engineering, National University of Singapore, 119260, Singapore

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bS Supporting Information ABSTRACT: In the present study, we report a facile hydrothermal route developed to synthesize various 1D tellurium nanostructures including nanotubes, nanowires, and nanorods on a large scale. Te nanotubes with a tunable diameter from 200 nm to 2 μm are fabricated in NaOH solution. With poly(vinyl pyrrolidone) as the surfactant, ultrathin Te nanowires with a diameter of 5 8 nm are synthesized in NaOH solution. However, in both cases, only Te nanorods are obtained if NaOH is not added. The formation of Te nanotubes is attributed to the depletion of Te atoms at the surface of seeds. The influence of the reaction conditions including pH value, reaction temperature, reducing agent, and reactant concentration on the size and morphology of the Te nanostructures is investigated and the structural evolution with different growth rate is illustrated. A reduction reaction rate controlled growth mechanism has been proposed for the variable nanostructures. In addition, the obtained Te nanostructures are ideal templates to synthesize other Te-related nanocompounds, which is demonstrated by the synthesis of ultrathin Ag2Te nanowires through the direct reaction of ultrathin Te nanowires with AgNO3 at room temperature.

1. INTRODUCTION In the past 10 years, 1D nanostructures have been a subject of intense research for both fundamental interest and various applications in electrons, photonics, optoelectronics, biomedicine etc.1 4 Among these 1D nanostructured materials, trigonal tellurium (t-Te) is an interesting one which has a tendency to form 1D structures even without any template and surfactant. The anisotropic growth of t-Te is ascribed to its anisotropic crystal structure which is composed of insulating parallel helical chains held by van der Waals cohesion.5 As a valuable p-type narrow-bandgap semiconducting material, 1D t-Te nanostructures have demonstrated promising physical and chemical properties such as photoconductivity, photoelectricity, high piezoelectricity, thermoelectricity, nonlinear optical properties, and catalytic performance.6 More importantly, these t-Te nanostructures can serve as self-sacrificed templates to synthesize many novel nanostructures of tellurides by cation diffusion process.7 12 The size and shape of these Te nanostructures play a key role in the template reaction. Thus, it is strongly desired to develop a simple and inexpensive method for controllable synthesis of Te nanostructures as well as for understanding the correlations of the size, shape, and crystallinity r 2011 American Chemical Society

between Te nanostructures and the tellurides obtained by template synthesis. Significant progresses in the synthesis of various 1D t-Te nanostructures such as rods,6,13 tubes,14 17 wires,6,18,19 and belts15,20 have been made in the solution phase method. For the controlled synthesis of nanostructures in solution method, both thermodynamics and kinetic equilibrium should be considered. The basic principle for morphology control in the synthesis of 1D Te nanostructures is the separation of nucleation and growth process and the control of the growth rate of different crystal face. These modulations can be achieved by adjusting the reaction conditions like solvent selection, reaction temperature, reactant concentration and surfactant and so forth. Mayers and Xia14 have synthesized Te nanotubes by the reduction of orthotelluric acid in ethylene glycol. The size of the nanotubes can be tuned by controlling growth time and tellurium concentration. It is believed that a low decomposition rate is the key Received: January 11, 2011 Revised: February 21, 2011 Published: March 14, 2011 6375

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The Journal of Physical Chemistry C factor for the formation of tubular structure in such reaction system. Using poly(vinyl pyrrolidone) (PVP) as surfactant, Qian et al.20 have prepared Te nanobelts and nanowires by reducing NaTeO3 with hydrazine hydrate in aqueous ammonia, and nanowires with a diameter of 4 9 nm have been obtained in this method. The synthesis of Te nanobelts and nanotubes has also been reported by Mo15 based on a disproportionation reaction with a slow formation rate of tellurium atom. Gautam and Rao6 have reported the fabrication of 1D Te nanostructures with various morphologies by a self-seeding solution process. In addition, microwave assistant synthesis has also been applied to synthesize Te nanorods and nanowires in ionic liquid.13 Very recently, Te nanotubes with different cross-section have been synthesized by reducing TeO2 in ethylene glycol solution with cetyltrimethyl ammonium bromide and cellulose acetate.17 Although several growth mechanism have been proposed for various 1D Te nanostructures synthesized in different reaction media or with different fabrication procedures, the successful modulation of the morphology of Te nanostructures from nanotubes to nanorods and nanowires in a certain condition is rarely investigated. Moreover, the obtained Te nanostructures from different approaches have different surface properties as well as chemical activities, which in turn results in the different telluride products via template-synthesis process. Therefore, development of a novel fabrication method for the controlled growth of different 1D Te nanostructures is an important step toward future nanodevice applications of tellurium and telluride nanostructures. Meanwhile, it will also help to deepen our understanding of the underlying mechanism of the controllable growth of 1D Te nanostructures. In the present work, we report the successful synthesis of different 1D t-Te nanostructures including nanotubes, nanorods, and nanowires by a simple reduction of K2TeO3 in aqueous solution through an elaborated control of experimental conditions such as pH value and suitable surfactant. The influence of pH value, reaction temperature, precursor concentration, and surfactant on the formation of Te nanostructures is investigated. The mechanism of controllable growth of 1D Te nanostructures is discussed and compared with those of previous reports. Furthermore, ultrathin Te nanowires are used as template to successfully prepare Ag2Te nanowires, indicating the potential of Te nanostructures as the template to synthesize Te-based nanocompounds.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Te Nanotubes and Nanorods. All of the reagents used in the experiment were of analytical purity purchased and used without further purification. Large-scale synthesis of uniform Te nanotubes was conducted by a hydrothermal reduction of K2TeO3 without any surfactant. In a typical experiment, 305 mg (1.2 mmol) K2TeO3, and 5.3 g (50 mmol) NaH2PO2 3 H2O were put into a 60 mL Teflon-lined stainlesssteel autoclave and then dissolved in 40 mL of 0.1 mol/L NaOH solution under vigorous magnetic stirring to form a homogeneous solution at room temperature. The autoclave was then sealed and maintained at 120 C for 12 h. After that, the container was cooled to room temperature naturally. The precipitates were collected by centrifuging and washed several times by deionized water and absolute ethanol, and then dried at 80 C for 4 h. The above synthesis process for the preparation of Te nanotubes is defined as the standard synthesis condition in the context. However, if NaOH was not added, Te nanorods were obtained

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Figure 1. XRD pattern of Te nanotubes.

with the above synthesis procedure. It is worth noting that the Te precipitates must be dried in absolute ethanol, which protects the Te nanostructures from quick oxidization as observed in water. 2.2. Synthesis of Ultrathin Te Nanowires. The experimental procedure for the synthesis of Te nanowires is basically same to that for Te nanotubes. The only difference is that 160 mg PVP (K30, polymerization degree 360) was added to the reaction mixture prior to hydrothermal treatment. PVP served as the surfactant to induce the growth of Te nanowires. 2.3. Synthesis of Ultrathin Ag2Te Nanowires. The obtained ultrathin Te nanowires were ultrasonically redispersed in 200 mL deionized water. 50 mL of 30 mM AgNO3 solution was slowly added into the Te nanowires solution at room temperature. The mixture was stirred for 30 min and the precipitates were collected from the solution by centrifuging and washed several times by deionized water and absolute ethanol, and then dried at 80 C for 4 h. 2.4. Characterization. The obtained Te nanostructures and Ag2Te nanowires were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Phase identification and structure analysis of the sample were carried out by XRD using a Rigaku D/max 2500 diffractometer with Cu KR radiation (50 kV  250 mA) and a graphitic monochromator. XRD data were collected by the step-scan mode with a step width of 2θ = 0.02 and a sampling time of 1 s. The chemical composition of the sample was identified by energy-dispersive X-ray spectroscope (EDS). The overview morphologies and sizes of the samples were obtained by FESEM performed on a FEI-Sirion scanning electron microanalyzer at 10 kV. TEM images, high-resolution TEM (HRTEM) images, and selected area electron diffraction (SAED) patterns were recorded on a JEM-2010 transmission electron microscope using an accelerating voltage of 200 kV.

3. RESULTS AND DISCUSSION 3.1. Te Nanotubes and Nanorods. Figure 1 shows the XRD pattern of as-prepared Te nanotubes. All of the peaks can be perfectly indexed to the trigonal phase of tellurium (space group: P3121), which is consistent with the reported XRD data (JCPDS 36 1452). The SEM images of Te nanotubes are presented in Figure 2. As shown in parts a and b of Figure 2, the obtained Te nanotubes are of several micrometers long with the outer diameter of 200 400 nm and wall thickness of 20 50 nm. The hexagonal cross-section of the Te nanotube is clearly 6376

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Figure 4. SEM images of products obtained at the early stage of the reaction with different alkaline condition. Te nanotubes synthesized in 0.1 mol/L NaOH solution with a reaction time of (a) 1 h and (b) 2 h; (c) Te nanorods with a reaction time of 40 min (without NaOH); (d) schematic illustration of the growth mechanism for the hollow structure.

Figure 2. SEM images of 1D Te nanostructures. (a) Te nanotubes synthesized under the standard experimental condition (0.1 mol/L NaOH solution), and the hexagonal cross-section of the Te nanotube is clearly observed in (b); (c) and (d) Te nanorods are obtained in the absence of NaOH; (e) and (f) larger Te nanotubes with a diameter about 1 2 μm synthesized in 1 mol/L NaOH solution.

Figure 3. (a) TEM image of the Te nanotube (the hexagonal crosssection is highlighted by the white line); (b) HRTEM image of the Te nanotube, and the insert shows the SAED pattern; (c) TEM image and (d) HRTEM image of the Te nanorod, and the insert is the SAED pattern of the Te nanorod.

observed in part b of Figure 2, which should stem from the trigonal symmetry of Te crystal. According to our experiments, alkaline condition is crucial for the formation of tubular Te nanostructures. As shown in parts c and d of Figure 2, only Te nanorods are obtained if NaOH is not added. By simply increasing NaOH

concentration from 0.1 to 1 M, the Te nanotubes thus synthesized have the diameter 10 times larger as shown in parts e and f of Figure 2. Because of its anisotropic crystal structure, t-Te crystal usually grows preferentially along the [001] direction. The TEM image presented in part a of Figure 3 shows that the Te nanotube has a hexagonal cross-section. The inserts of parts b and d of Figure 3 show the SAED pattern of the Te nanotube and nanorod respectively confirming the trigonal phase of the Te nanotube and nanorod with the growth direction along [001]. As shown in part b of Figure 3, lattice fringes with a spacing about 0.39 nm, which corresponds to the (100) lattice planes of t-Te, are parallel to the Te nanotube. The lattice fringes with a lattice spacing of 0.59 nm, corresponding to the (001) plane of t-Te, are readily resolved along the Te nanorod (part d of Figure 3). In our synthetic strategy, varied Te nanostructures are mainly modulated by adjusting NaOH concentration in solution. Welldefined Te nanotubes are observed among corroded Te nanorods by hydrothermal treatment of the as-prepared Te nanorods in 40 mL of 0.1 M NaOH solution in the presence of 5.3 g NaH2PO2 3 H2O at 120 C for 12 h, as shown in Figure S1 of the Supporting Information, and the TeO32 ions with a certain concentration have been detected after the above hydrothermal treatment. This obviously implies that the as-prepared Te nanorods have been redissolved and reoxidized to TeO32 in the alkaline solution. And Te nanotubes are obtained when the content of TeO32 ions reaches the critical nucleation concentration. Details of the TeO32 ions detecting are presented in the Supporting Information. By rapid cooling with water, we have investigated the morphology evolution of Te nanotubes at the early stage of the reaction. For Te nanotubes, short nanorods with a diameter of 30 50 nm are observed after a reaction time of 1 h as shown in part a of Figure 4. According to our experiments, the small nanorods grow bigger in both lateral and longitudinal dimensions with prolonged reaction time (Figure S2 of the Supporting Information). As shown in part b of Figure 4, Te nanotubes with a diameter of 100 200 nm have been formed after a reaction time of 2 h. However, for the synthesis of Te nanorods, due to the rapid reduction reaction in the absence of NaOH, the seedlike product can only be found at the earlier stage with a reaction time of 40 min as shown in part c of Figure 4, and a large number of Te 6377

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Figure 5. SEM images of Te nanostructures obtained at different reaction conditions. (a) 160 C; (b) 180 C; (c) 0.25 mol/L NaH2PO2 3 H2O, 160 C; (d) 2.5 mol/L NaH2PO2 3 H2O, 160 C.

Figure 6. (a) SEM image, (b) TEM image, and (c) HRTEM image of the Te nanowires; (d) Enlarged image of the framed region of (c), and the insert of (d) shows selected-area FTT of (c).

nanorods are obtained after a reaction of 1 h (Figure S3 of the Supporting Information). According to the observation in parts a and b of Figure 4, the hollow structures are gradually generated from the seedlike nanorods. The formation process of Te nanotubes in our experiment is in good agreement with the seedinduced growth mechanism proposed by Mayers,14 which attributes the formation of Te nanotubes to the depletion of Te atoms at the surface of seeds. According to our experimental investigation mentioned above, the growth of Te nanotubes is very sensitive to the pH value of the reaction solution. As described in other synthesis of Te nanostructure by reduction of TeO32 in aqueous solution, pH value has dramatic influence on the reduction reaction rate of TeO32 , which is decreased with increasing pH value.18,21,22 The pH value influences the formation process of Te nanotubes mainly in two aspects. On the one hand, low reaction rate in alkaline condition (higher NaOH concentration) leads to low Te concentration in the bulk solution. As the free Te source is quickly consumed at the growing face of the seed, the (001) plane, the concentration of Te atoms near the central portion of this surface is undersaturated considering the limited mass transport rate in the reaction system.14 On the other hand, the dissolution reaction of the Te seed is promoted in alkaline solution.23 Because of the undersaturation of Te atoms in the central portion of the growing face, the dissolution reaction is most likely to take place at this area and eventually results in the tubular structure. A scheme of the formation process of Te nanotubes is illustrated in part d of Figure 4. The key factor of the successful formation of Te nanotubes is the low Te concentration in the bulk solution, which is realized in our synthesis through the low reaction rate in the alkaline condition. In order to further understand the influence of redox reaction rate on the formation of Te nanostructures, the synthesis has been carried out in various conditions to adjust the reaction rate, and the morphology evolution of Te nanostructures is observed. It has been proposed that the tubular structures have been destroyed at high temperature due to the high redox reaction rate and high mass transport rate.24 In this work, when the reaction temperature for Te nanotubes preparation is increased from 120 to 160 C, instead of hollow nanotubes, microrods with a diameter about 1 μm and a length of 2 5 μm are obtained (part a of Figure 5). Moreover, a featherlike structure is observed

when the reduction temperature is further increased to 180 C as shown in part b of Figure 5. Similar featherlike structures shown in Figure S4 of the Supporting Information are obtained by replacing NaH2PO2 3 H2O with NaBH4 (150 mg) and heated at 120 C. Because higher temperature and strong reducing agent leads to higher reaction rate, the formation of a featherlike structure is attributed to the rapid crystallization at large supersaturation.25,26 The evolution of the Te nanostructures form nanotubes to nanorods and finally to featherlike structures with increasing redox reaction rate has also been found by adjusting the concentration of the reducing agent. When the concentration of NaH2PO2 3 H2O is reduced to 0.25 mol/L (1/5 of the standard condition), instead of microrods mentioned above, tubular structures are observed again at 160 C due to the reduced reaction rate (part c of Figure 5). However, as shown in part d of Figure 5, the featherlike structures are obtained at 160 C owing to the accelerated reaction rate, when the concentration of NaH2PO2 3 H2O is doubled (2.5 mol/L). As mentioned above, the reaction rate of our synthesis is increased with decreasing pH value, and Te nanorods are obtained instead of nanotubes in the absence of NaOH at 120 C. When the reaction is carried out in 0.05 M HCl solution, featherlike structures are obtained as expected due to the higher reaction rate (Figure S5 of the Supporting Information). In the standard nanotubes synthesis condition, the increase of K2TeO3 from 1.2 to 4 mmol leads to the formation of the main product of nanorods and a very small amount of nanotubes (Figure S6 of the Supporting Information), which may be ascribed to the high reaction rate in the high reactant concentration condition. The evolution of Te nanostructures with different reaction rate is confirmed by adjusting NaOH concentration, reaction temperature, and reducing agent independently. Tubular structure can only be formed with a low reaction rate, which is associated with high pH value, low temperature, weak reductant, and low reactant concentration. With a higher reaction rate, the hollow structures are destroyed and solid rod structures emerge. If the reaction rate is further accelerated, featherlike structures are obtained. 3.2. Ultrathin Tellurium Nanowires. To further tuning the morphology of Te nanostructures, PVP is added into reaction mixture as the surfactant. Uniform Te nanowires of about 6 nm in diameter and 500 nm in length are synthesized on a large scale as shown in Figure 6. Figure 7 shows the XRD pattern of Te 6378

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Figure 7. XRD pattern of Te nanowires.

nanowires. All of the diffraction peaks are broadened as compared with those of Te nanotubes due to the small crystal size of nanowires. In comparison with the reported data (JCPDS 36 1452), the (100) and (110) peaks are obviously intensified. This indicates that the preferred growth direction of Te nanowires is along c axis since the 1D nanostructures prefer to lay on the substrate. The HRTEM images in parts c and d of Figure 6 and its fast Fourier transform (FFT) (insert of part d of Figure 6) exhibit the highly crystalline nature of the Te nanowire. Part d of Figure 6 shows three representative planes of the t-Te nanowire with lattice spacings of 0.59, 0.38, and 0.32 nm corresponding to (001), (100), and (101) respectively, which further confirms the [001] growth direction of the Te nanowire. In the presence of PVP, Te nanowires instead of nanotubes are synthesized in alkaline condition, indicating that PVP as a surfactant plays a key role in the formation of Te nanowires. PVP as a surfactant has been employed as a stabilizing agent and a structure-directing agent in the preparation of silver nanowires, which selectively absorbs on the surface of the growing particles.27 30 The strongly selectively absorbed PVP is an obstacle for Te diffusion, which highly suppresses the growth of Te particles.20,31 However, only PVP is not sufficient for the formation of highly anisotropic Te nanowires, the low reaction rate in the alkaline condition facilitates the anisotropic growth of Te nanowires in the presence of PVP. For example, short nanorods with a diameter about 70 nm are obtained if NaOH is not added (Figure S7 of the Supporting Information). This suggests that the alkaline condition is helpful for PVP to fully play its role as a surfactant for the synthesis of Te nanowires. More evidence have been found in shuttle-like nanorods obtained by increasing the reaction temperature to 180 C (Figure S8 of the Supporting Information), confirming the key role of suitable reaction rate in the growth of Te nanowires.32 Te nanostructures can react with many other elements and serve as templates to synthesize Te-based nanocompounds. Therefore, the Te nanowires are ideal templates to synthesize ultrathin telluride nanowires, which might have various applications. We demonstrate here that ultrathin Ag2Te nanowires can be prepared by the direct reaction of Te nanowires with AgNO3 at room temperature. The reaction takes place fast at room temperature, and the color of the solution changes from blue to dark immediately after the addition of AgNO3. Part a of Figure 8 shows the overview TEM image of the obtained Ag2Te nanowires. The nanowires have a diameter of

Figure 8. (a) Overview TEM image and (b) HRTEM image of Ag2Te nanowires, and insert of (b) shows the SAED pattern; (c) EDS spectrum and (d) XRD pattern of the Ag2Te nanowires.

5 10 nm, which is almost the same size as that of Te nanowires. The HRTEM image and diffraction pattern shown in part b of Figure 8 reveal the polycrystalline nature of Ag2Te nanowires. The lattice fringes with a spacing of 0.75 nm correspond to the (100) plane of Ag2Te. The chemical composition of the nanowires has been determined by EDS (part c of Figure 8), indicating the nanowires are composed of Ag and Te. The atomic ratio of Ag to Te is about 1.93, which is very close to 2. XRD pattern of the obtained nanowires are shown in part d of Figure 8, and the diffraction peaks are broadened due to the very small grain size of Ag2Te.33 35

4. CONCLUSIONS In summary, we demonstrate a simple route to synthesize different 1D Te nanostructures through a reduction reaction of K2TeO3 using NaH2PO2 in alkaline solution. By adjusting the alkalinity, size tunable Te nanotubes are obtained. In current synthetic strategy, the morphology of Te nanostructures can be successfully modulated from nanotube to nanorod, featurelike nanostructure through the control of the NaOH concentration, reaction temperature and reductant independently. With PVP as the surfactant, ultrathin Te nanowires with a diameter of 5 8 nm are synthesized in alkaline condition. The ultrathin Te nanowires are further demonstrated as templates to synthesize ultrathin Ag2Te nanowires. The systematic investigations on the effect of experimental condition on the morphology evolution of Te nanostructures suggest that the reduction reaction rate is a key factor for the controllable growth of Te nanostructures with different morphology and size. The principle observed in this work may apply to synthesize 1D nanostructures of other semiconductors via reduction reaction in solution. The various 1D Te nanostructures prepared in current method also provide the opportunity to synthesize other functional 1D telluride nanostructures for future nanodevice applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed description of the TeO32 ions detecting. SEM image of the nanotubes among corroded Te nanorods. SEM images of the Te nanorods which

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The Journal of Physical Chemistry C are synthesized in 0.1 mol/L NaOH solution with a reaction time of 1.5 h. SEM image of Te nanorods formed after a reaction of 1 h at 120 C (without NaOH). SEM images of Te nanostructures synthesized with strong reducing agent NaBH4, HCl solution and high K2TeO3 concentration at 120 C. SEM images of nanorods synthesized with the assistance of PVP but without NaOH or at high temperature 180 C. XRD pattern of Te nanorods prepared at 120 C for 12 h (without NaOH). SEM images of freshly prepared Te nanowires after being stored in ethanol for 36 h and 10 days. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (H.F.); [email protected] (J.L.).

’ ACKNOWLEDGMENT The work was financially supported by the National Basic Research Program of China (Grant No. 2007CB925003), the National Natural Science Foundation of China (Grant No. 21006079), and the Chinese Academy of Sciences.

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