J. Phys. Chem. C 2008, 112, 14825–14829
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Rapid Synthesis and Electrochemical Property of Ag2Te Nanorods Pengfei Zuo, Shengyi Zhang,* Baokang Jin, Yupeng Tian, and Jiaxiang Yang Department of Chemistry, Anhui UniVersity, Hefei 230039, China ReceiVed: May 12, 2008; ReVised Manuscript ReceiVed: July 8, 2008
A rapid chemical method has been developed for the synthesis of the Ag2Te nanorods with monoclinic crystalline phase. The method is based on the template-engaged synthesis in which the Te nanorods were used as template reagents. On the basis of a series of experiments and characterizations, the effect factors and the formation mechanism of the Ag2Te nanorods were discussed. Furthermore, the electrochemical property of the Ag2Te nanorods was determined by the voltammetric technique. Introduction
Experimental Section
Recently, the silver chalcogenides have attracted much attention due to their interesting properties and great potential applications. As a kind of chalcogenides, silver telluride (Ag2Te) has much more technological prospects.1-3 As reported,4 Ag2Te can transfer its structural phase from the low-temperature monoclinic structure (β-Ag2Te) to the high-temperature facecentered cubic structure (R-Ag2Te) at about 145 °C. The β-Ag2Te is a narrow band gap semiconductor with high electron mobility and low lattice thermal conductivity, whereas the R-Ag2Te shows superionic conductivity because Ag1+ cations can easily move through the cubic sublattice formed with tellurium anions.5,6 Generally, Ag2Te bulk material was prepared by the solid reaction between elemental Ag and Te at elevated temperature in evacuated tubes, or by the aqueous reaction of metal-salt solutions with toxic gaseous H2Te. Going with the increasing interest in the nanostructured materials and nanodevices, a number of methods for the synthesis of the Ag2Te nanomaterials with different morphologies have been developed. For example, nanocrystalline Ag2Te was synthesized in an ethylenediamine system by high-intensity ultrasonic irradiation at room temperature.7 Ag2Te film was prepared by the reaction of a Ag film with vacuum-deposited Te8 and by cathodic deposition from dimethyl sulfoxide solution containing AgNO3 and TeCl4.9 Although it is a challenge to fabricate Ag2Te into onedimensional (1D) nanostructured materials that could be applied to design various nanodevices, some strategies for the synthesis of 1D Ag2Te nanostructures have been devised. For example, Ag2Te nanorods were prepared by the reaction between AgCl and elemental Te in the mixed solvent of ethylenediamine and hydrazine hydrate.10 Ag2Te nanowires were obtained by cathodic electrolysis from dimethyl sulfoxide solution containing AgNO3 and TeCl4 in porous anodic alumina membranes.11 Ag2Te nanotubes were synthesized by a hydrothermal method in which AgNO3 and Na2TeO4 were used as precursors.3 In this paper, Ag2Te nanorods were prepared by a rapid chemical reaction route, in which the Te nanorods as-obtained were used as template reagents. In addition, for their optoelectronic and magnetoelectric applications, the electrochemical property of the Ag2Te nanorods was studied by the voltammetric technique.
The reagents are of analytical grade and were used in experiments without further purification. The Te nanorods that would be used as the template reagents were prepared as described in the literature.12 With the Te nanorods as-obtained, Ag2Te nanorods were typically synthesized as follows. First, under magnetic stirring, 1 mL of 1.0 M hydrazine (N2H4 · H2O) was added dropwise into 20 mL of 0.01 M AgNO3 aqueous solution containing 0.1 M Na2EDTA. Then, when the reaction solution turned light yellow, 0.1 mmol of Te nanorods were added. After aging at room temperature for 20 min, the reaction solution was kept still in a water bath at 60 °C for 30 min. Final product was obtained by centrifuge-separating and waterwashing the black precipitate formed in the solution. The characterization instruments and electrochemistry experiment are described in detail in the Supporting Information.
* Corresponding author. Tel/fax: +86 551 5107342; e-mail: syzhangi@ 126.com.
Results and Discussion Characterization. The SEM image (Figure 1a) for the asobtained Te product discloses its rod-like morphology. The XRD pattern (Figure 1b) indicates that the Te product is composed of a hexagonal crystalline phase whose unit cell constants are a ) 0.4450 nm and c ) 0.5996 nm, corresponding well to those in the literature (JCPDS Card, No.36-1452). Figure 1c shows the rod-like morphology of the typical Ag2Te product, whose crystalline phase was determined by XRD (Figure 1d). Clearly, all of the diffraction peaks on the XRD pattern can be indexed to the monoclinic Ag2Te phase, and the unit cell constants calculated from the diffraction peaks are a ) 0.8084 nm, b ) 0.9013 nm, c ) 0.7915 nm, and β ) 113.5°, which are consistent with those in the literature (JCPDS Card, No.34-0142). Notably, in the conversion of the Te template to Ag2Te product, the shape transformation is from straight to curve, and the crystalline phase is from hexagonal to monoclinic. To understand the chemical situation of elements in the product, the XPS test was performed, and the results are shown in Figure 2 (the XPS patterns for Ag3d and O1s are shown in Supporting Information Figure S1). Comparing the XPS spectra of the Ag2Te samples before (Figure 2a and 2b) and after (Figure 2c and 2d) surface Ar1+ etching, it can be concluded that there is little oxides on the surface of the product. The oxides may result from the surface oxidation reaction of the Ag2Te nanorods with O2: Ag2Te + O2 ) 2 Ag + TeO2. The binding energies obtained from the XPS spectra are 368.1 and 572.8 eV for
10.1021/jp804164h CCC: $40.75 2008 American Chemical Society Published on Web 08/29/2008
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Figure 1. SEM images and XRD patterns of the Te template (a and b) and Ag2Te product (c and d).
Ag3d5/2 and Te3d5/2, respectively, which means that the product is composed of Ag (I+) and Te (II-). In addition, according to the quantification of XPS peaks, the molar ratio of Ag to Te is 1.997:1.000, close to the stoichiometry of Ag2Te. The thermal analysis results for the Ag2Te product are shown in Supporting Information Figure S2. From the TG curve, it can be seen that there is no significant weight loss between 25 and 250 °C, which indicates that the chemical composition of the product did not change. The endothermic peak on the DSC curve indicates that the Ag2Te nanorods could undergo a structural phase transition from the low-temperature monoclinic structure (β-Ag2Te) to the high-temperature face-centered cubic structure (R-Ag2Te) at about 150 °C.3,4 Reaction Mechanism. In synthesis, the Ag2Te nanorods were produced by the reaction of Te nanorods with neonatal Ag particles. The Te nanorods were prepared in advance and the Ag particles were formed in solution by the follow reactions:
Ag(EDTA)1-)Ag1++EDTA21+
4Ag
(1)
+ N2H4 · H2O + 4OH )4Ag + N2 + 5H2O (2) 1-
0
The experimental results show that the freshly formed light yellow Ag particles had a high reactivity and could readily start the synthesis reaction with Te nanorods at room temperature. In synthesis, the stable Ag(EDTA)1- complex was first formed,
since the molar ratio of Na2EDTA and Ag1+ was 10:1 in the reaction solution. During the aging time, the Ag1+ could be slowly released from the Ag(EDTA)1- and gradually reduced to elemental Ag by hydrazine (N2H4 · H2O). As a result, the neonatal Ag particles could be continually supplied to react with Te nanorods. Through the topotactic transformation of Te templates, the Ag2Te nanorods were formed by the diffusion of Ag atom into the Te nanorods. Similarly, the copper and silver chalcogenides have been prepared by this template transformation mechanism.13-16 Due to the change of atom radius and bond length in the conversion of Te to Ag2Te, the shape of the product transformed from straight to curve nanorods, and the crystalline phase from hexagonal to monoclinic phase. In the interest of probing into the effect of the atom ratio in precursors on the product, a series of experiments were performed, and the results are shown in Supporting Information Figure S3. As above, in typical synthesis, pure Ag2Te nanorods were obtained with the 2.0:1.0 atom ratio of Ag and Te in the precursors. When this ratio is decreased or increased, the Te/ Ag2Te or Ag/Ag2Te composites were obtained, which had been confirmed by the XRD patterns (as shown in Figure S3). This phenomenon indicates that pure Ag2Te nanorods could be obtained by the complete reaction of the Ag and Te atoms in the precursors, as performed in typical synthesis.
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Figure 2. XPS spectra of the Ag2Te samples: the survey spectrum (a) and narrow spectrum for Te3d (b) obtained before sample surface Ar1+ etching; the survey spectrum (c) and narrow spectrum for Te3d (d) obtained after sample surface Ar1+ etching.
Figure 3. The voltammograms of Ag2Te nanorods: (a) the voltammogram (curve I) obtained by positive-going scan and the voltammogram (curve II) obtained by negative-going scan; (b) voltammograms obtained by three successive cyclic scan. (in 0.10 mol/L KNO3, at 50 mV/s).
Electrochemistry. Figure 3a shows the experimental results of linear scan voltammetry by carbon paste electrode (CPE) modified with the Ag2Te nanorods. Consulting the results reported about the electrochemistry of Ag2Te and CdTe particles,17,18 we attribute three anodic peaks on curve I, namely, peaks 1 (at 0.11 V), 2 (at 0.37 V), and 3 (at 0.48 V), to the oxidation of the Ag2Te nanorods (Ag2Te f Te0 + 2e +2Ag1+), the oxidation of Te0 (Te0 f Te4+ + 4e), and the oxidation of Te4+ (Te4+ f Te6+ + 2e), respectively. Then, the cathodic peaks 4 (at 0.18 V) and 5 (at -0.1 V) on curve II could be assigned
to the reduction of Te4+ (and Te6+) and Ag1+, respectively. Figure 3b shows the successive cyclic voltammograms of the Ag2Te nanorods. From the first cyclic voltammogram, it can be seen that the peaks 1-5 are similar to that in Figure 3a and can be designated as above. Obviously, anodic peak 1 (at 0.10 V) in the first cycle voltammogram is low and then heightened up (denoted as peak 6) in the second and third cyclic voltammograms. As designated for Ag2Se nanotubes in literature,16 peak 1 is assigned to the oxidation of Ag2Te nanorods, and peak 6 formed in subsequent cycles is attributed the oxidation of Ag0
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Figure 4. The voltammograms obtained at different potential regions: (a) with blank CPE (without Ag2Te) at -0.6 to 0.8 V; (b) at -0.25 to 0.25 V; (c) at -0.20 to 0.40 V; (d) at 0 to 0.80 V. (in 0.10 mol/L KNO3, at 50 mV/s).
that had been produced by the reduction of Ag1+ (behaving as peak 5 in Figure 3b). To verify the above designations, several experiments were carried out, and the results are shown in Figure 4. From Figure 4a, it can be seen that there was no redox peak on the voltammogram obtained with the blank CPE. The large current at about -0.5 V came from the electrolyte solution itself, which explains the origin of the large current (at about -0.5 V, Figure 3) on the voltammogram obtained with Ag2Te nanorods. For Ag2Te nanorods, it has been found that there was no cathodic peak on the voltammogram obtained with the negative-going scan from 0 V to -0.6 V. However, there was a cathodic peak (at -0.13 V) on the voltammogram if this negative-going scan was performed after a positive-going scan from 0 to 0.3 V, in which the Ag2Te could be oxidized to Te0 and Ag1+. Clearly, this cathodic peak came from the reduction of Ag1+ to Ag0, which supports the designation for peak 5 in Figure 3, panels a and b. When the scan potential scope was narrowed from -0.60 to 0.80 V to -0.25 to 0.25 V, there only are two peaks on the cyclic voltammogram (Figure 4b). Here, since the Te0 could not be further oxidized in this scope, the anodic peak (peak 1) and the cathodic peak (peak 2) could be readily assigned to the oxidation of the Ag2Te to Te0 + Ag1+ and the reduction of
Ag1+ to Ag, respectively. If the positive-going scan verge was extended from 0.25 to 0.40 V, an additional pair of redox peaks (peak 2 at 0.35 V and peak 3 at 0.20V, as shown in Figure 4c) appears, which may result from the oxidation of Te0 to Te4+ and the reduction of Te4+ to Te0, respectively. When the positive-going scan verge was further extended from 0.40 to 0.80 V, another anodic peak (peak 2 in Figure 4d) presents. This anodic peak may come from the oxidation of Te4+ to Te6+. The electrochemical property of the Ag2Te nanorods was further studied by changing the rate of potential scan and the acidity of the electrolyte solution, and the results are shown in Figure 5. From Figure 5a, it can be seen that the scan rate affected the intensity and position of the redox peaks. However, peak 1, produced by the oxidation of Ag2Te nanorods, was little affected by the scan rate. This voltammetric characteristic is the inherence of solid electroactive particles.16 Contrarily, the intensity of peaks 2 and 5, which originated from the oxidation of Te0 and the reduction of Ag1+, respectively, are largely affected by the scan rate, which is the voltammetric characteristic of the electroactive particles with hydroponic diffusibility. On the basis of this voltammetric characteristic, it can be assumed that the Ag1+ existed as free Ag(H2O)1+ before being deoxi-
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Figure 5. The cyclic voltammograms obtained: (a) at a different scan rate in 0.10 M KNO3, and (b) in different electrolyte solution (at 50 mV/s).
dized, and the Te0 existed as unagglomerate particles before being oxidized on the surface of the electrode. From Figure 5b, it can be seen that the electroactivity of Ag2Te nanorods is relevant with the acidity of electrolyte solution. Comparing with that in neutral KNO3 solution, the oxidation peaks of Ag2Te to Te0, Te0 to Te4+, and Te4+ to Te6+ are all positive-shifted from peaks 1-3 to peaks 1′-3′ (Figure 5b) in acidic (HNO3) solution, due to their difficult oxidation.17 Because soluble Ag1+ can be easily deoxidized in acidic solution, the reduction peak for Ag1+ is also positive-shifted from peak 5 to peak 5′. However, in NaOH solution, there is little current peaks on the cyclic voltammogram, which indicates that the Ag2Te nanorods are electrochemically stable in alkaline medium. Conclusion In summary, the Ag2Te nanorods with monoclinic crystalline phase were rapidly prepared by the template-engaged synthesis in which the Te nanorods were used as template reagents. It was found that the neonatal Ag particles could readily react with Te nanorods at room temperature, and the pure Ag2Te nanorods could be obtained with a 2.0:1.0 atom ratio of Ag and Te in the precursors. In addition, the electrochemical property of the Ag2Te nanorods was studied by the voltammetric technique in different conditions. The results show that the electrode reaction of Ag2Te nanorods is irreversible and the Ag2Te nanorods are electrochemically stable in alkaline medium. Importantly, the as-prepared Ag2Te nanorods might be useful in various nanodevices, and the present strategy for the synthesis of Ag2Te nanorods is potentially practicable to other 1D nanostructures. Acknowledgment. Support for this work from the National Natural Science Foundation of China (Nos. 20775001, 50532030,
20771001) and the Anhui Research Project (Nos. 050440702, 2006KJ007TD) is gratefully acknowledged. Supporting Information Available: The detailed characterization and electrochemistry processes, TG and DSC curves, XRD patterns, and XPS spectra are provided. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Liang, B. Q.; Chen, X.; Wang, Y. J.; Tang, Y. J. Phys. ReV. B 2000, 61, 3239. (2) Hussmann, A.; Betts, J. B.; Boebinger, G. S.; Migliori, A.; Rosenbaum, T. F.; Saboungi, T. F. Nature. 2002, 417, 421. (3) Qin, A. M.; Fang, Y. P.; Tao, P. F.; Zhang, J. Y.; Su, C. Y. Inorg. Chem. 2007, 46, 7403. (4) Martin, C. R. Science. 1994, 266, 1961. (5) Dalven, R.; Gill, R. J. Appl. Phys. 1967, 38, 753. (6) Chen, R.; Xu, D.; Guo, G.; Gui, L. J. Mater. Chem. 2002, 12, 2435. (7) Li, B.; Xie, Y.; Liu, Y.; Huang, J. X.; Qian, Y. T. J. Solid State Chem. 2001, 158, 260. (8) Sa´fra´n, G.; Geszti, O.; Radno´czi, G. Vacuum 2003, 71, 299. (9) Chen, R. Z.; Xu, D. S.; Guo, G. L.; Gui, L. L. Electrochim. Acta 2004, 49, 2243. (10) Jiang, Y.; Wu, Y.; Yang, Z. P.; Xie, Y.; Qian, Y. T. J. Cryst. Growth 2001, 224, 1. (11) Chen, R. Z.; Xu, D. S.; Guo, G. L.; Gui, L. L. J. Mater. Chem. 2002, 12, 2435. (12) Xi, G. C.; Peng, Y. Y.; Yu, W. C.; Qian, Y. T. Cryst. Growth Des. 2005, 5, 325. (13) Kaito, C.; Nonaka, A.; Kimura, S.; Suzuki, N.; Saito, Y. J. Cryst. Growth. 1998, 186, 386. (14) Gates, B.; Mayers, B.; Wu, Y. Y.; Sun, Y. G.; Cattle, B.; Yang, P. D.; Xia, Y. N. AdV. Funct. Mater. 2002, 12, 679. (15) Zhang, S. Y.; Fang, C. X.; Tian, Y. P.; Zhu, K. R.; Jin, B. K.; Shen, Y. H.; Yang, J. X. Cryst. Growth Des. 2006, 6, 2809. (16) Zhang, S. Y.; Fang, C. X.; Wei, W.; Jin, B. K.; Tian, Y. P.; Shen, Y. H.; Yang, J. X.; Gao, H. W. J. Phys. Chem. C 2007, 111, 4168. (17) Perdicakis, M.; Grosselin, N.; Bessie`re, J. Electrochim. Acta 1997, 42, 3351. (18) Kentaro, A.; Kuniaki, M.; Tetsuji, H.; Yasuhiro, A. Electrochim. Acta 2006, 51, 4987.
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