Synthesis and Characterization of Hexagonal CuSe Nanotubes by

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Synthesis and Characterization of Hexagonal CuSe Nanotubes by Templating against Trigonal Se Nanotubes Zhang,*,†

Fang,†

Sheng-Yi Chun-Xia Yu-Peng Yu-Hua Shen,† and Jia-Xiang Yang†

Tian,†

Ke-Rong

Zhu,‡

Bao-Kang

Jin,†

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 12 2809-2813

Department of Chemistry and School of Physics & Material Science and Center of Modern Experimental Technology, Anhui UniVersity, Hefei, 230039 China ReceiVed July 10, 2006; ReVised Manuscript ReceiVed September 20, 2006

ABSTRACT: A novel method has been developed for the preparation of crystalline nanotubes of copper selenide (CuSe). The method was based on template-directed synthesis in which the trigonal Se nanotubes were used as a template-directed reagent. Apart from CuSe nanotubes, one-direction (1D) nanocrystallites of Cu3Se2, Cu2-xSe, and Cu2Se were also obtained by changing the atom ratio of Cu and Se in the precursors. The products were characterized by scanning electron microscopy (SEM), X-ray diffractometry (XRD), laser Raman spectrography (LRS), and X-ray photoelectron spectroscopy (XPS). On the basis of a series of experiments and characterizations, the formation mechanism of the CuSe nanotubes is discussed. 1. Introduction In the past few decades, the semiconductor copper selenide (CuSe) has received great attention due to its particular photoelectrical properties and wide applications in electronic and optoelectronic devices, such as the solar cell,1-3 optical filter,4 super ionic conductors,5,6 thermo electric converters,7 etc. Interestingly, as a kind of binary compound, copper selenide can exist in a wide range of stoichiometric compositions (CuSe, Cu2Se, CuSe2, Cu3Se2, Cu5Se4, Cu7Se4, etc.) and non-stoichiometric compositions (Cu2-xSe),8-12 and can be constructed into several crystallographic forms (monoclinic, cubic, tetragonal, hexagonal, etc.).13 The special constitutions and properties of these compositions make copper selenide an ideal candidate for scientific research. Therefore, considerable progress on the study of copper selenides has been made in recent years. A number of methods for the synthesis of copper selenides have been explored, such as the mechanical alloying method,14 gammairradiation,15 microwave-assistedheating,16 sonochemicalmethod,17,18 hydrothermal method,19 vacuum evaporation,20 electrodeposition,21 solid-state chemical reaction,22 and solution-phase chemical reaction.23-25 However, the objects of synthesis are mostly films or nanoparticles of copper selenides, and reports on the copper selenides with one-direction (1D) nanostructures are very few.26 Of the various synthesis techniques for copper selenides, the chemical reaction technique has many advantages, such as simplicity, no requirement for sophisticated instruments, nontoxic operation, low-cost, and the potential for large-scale production. In this paper, we report a novel chemical reaction route for the preparation of crystalline nanotubes of CuSe, in which single-crystalline nanotubes of trigonal Se (t-Se) asprepared were used as a template-directed reagent. As known, template-directed synthesis in which the template served as a physical scaffold against which other nanomaterials were assembled has been extensively explored to generate various nanostructures.27-32 In some cases, the template as one of the reactants was actively engaged in the synthesis process.33,34 In * To whom correspondence should be addressed. Tel/Fax: +86 551 5107342. E-mail: [email protected]. † Department of Chemistry. ‡ School of Physics & Material Science and Center of Modern Experimental Technology.

our synthesis process, the t-Se nanotubes acting as templates and reactants were converted into crystalline nanotubes of CuSe by reacting with Cu nanoparticles freshly produced in aqueous CuSO4 solution. Apart from nanotubes of CuSe, several copper selenides with 1D nanostructure, such as Cu3Se2, Cu2-xSe, and Cu2Se, were also obtained by changing the atom ratio of Cu and Se in the precursors. 2. Experimental Section In the experiments, all the reagents are of analytical grade and were used without further purification. The t-Se nanotubes, being used as a template-directed reagent for the synthesis of CuSe nanotubes, were synthesized as our report.35 Typically, the synthesis experiment was carried out at room temperature as follows. First, 1.0 mL of 0.1 M hydrazine (N2H4‚H2O) and 5.0 mL of aqueous ammonia solution (25 wt%) with an appropriate amount of Na2SO3 were mixed together. Then, 1.0 mL of 0.1 M CuSO4 was dropwise added into the above mixture solution under stirring. Finally, 0.1 mmol of t-Se nanotubes as-prepared was added into the mixture solution when the solution turned light yellow. After the reaction solution aged for 30 min, the black flocculent product was separated by centrifugation. The final product was obtained by washing with distilled water and baking at 503 K for 10 min. The products before and after baking were characterized by scanning electron microscopy (SEM) with energy-dispersive X-ray analysis (EDX, 1530 VP Ger. LEO with OXFORD INCA-300), X-ray diffraction (XRD, Japan Rigaku D/max-RA X-ray diffractometer, with graphite monochromatized Cu KR1 radiation, λ ) 0.15406 nm), laser Raman spectrography (LRS, JY HR800, equipped with a 488 nm, 10 mw argon ion laser) and X-ray photoelectron spectroscopy (XPS, ESCA 3 Mk II, VG Scientific, UK, equipped with a Mg KR 1253.6 eV X-ray source).

3. Results and Discussion Characterization Results. The SEM images and XRD patterns of the products are shown in Figure 1. Figure 1a shows the SEM image of the Se nanotubes asprepared to be used as the templates for the synthesis of copper selenides. The EDX analysis confirmed the high purity of the Se nanotubes since the Se peak is only observed in the EDX pattern. The crystallization of the Se nanotubes was studied by XRD (as shown in Figure 1b), and all the diffraction peaks in the XRD pattern can readily be indexed to the trigonal phase Se (t-Se). Figure 1c shows the SEM image of the product before baking. The image (as the inset in Figure 1c) indicates that tubular

10.1021/cg0604430 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/26/2006

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Figure 1. SEM images and XRD patterns of the products obtained in the typical experiment: (a, b) t-Se; (c, d) the product before baking; (e, f) the product after baking.

nanostructures were obtained. The element constitution of the tubular nanostructures was studied by EDX, and the result indicates that the atom ratio of Cu and Se is 52.14:47.86. The crystallization of the tubular nanostructures was further characterized by XRD, and the result is shown in Figure 1d. The diffraction peaks at 23.5°and 29.7° in the XRD pattern can be indexed to the (100) and (101) planes of t-Se, respectively, and the diffraction peaks at 25.0°, 27.8°, 28.6°, 39.7°, and 49.7° can be indexed to the (101), (200), (111), (220), and (311) planes

of tetragonal phase Cu3Se2, respectively. Obviously, the product before baking is made of tetragonal Cu3Se2 with little t-Se. Figure 1e shows the SEM image of the final product after baking. From the image (as the inset in Figure 1e), it is observed that the product also possesses the tubular nanostructures. The element constitution of the tubular nanostructures was studied by EDX to give the atom ratio of Cu and Se as 52.04:47.96. The crystallization of the tubular nanostructures was further characterized by XRD, and the result (as shown in Figure 1f)

Synthesis of Hexagonal CuSe Nanotubes

Crystal Growth & Design, Vol. 6, No. 12, 2006 2811

Figure 4. The XRD patterns of the products with different atom ratios of Cu and Se in the precursors: (a) 0.5:1, (b) 1.0:1, (c) 1.5:1, (d) 2.0: 1, (e) 2.5:1. (* Se, # Cu3Se2, O Cu2-xSe, ∆ Cu2Se). Table 1. XRD and EDX Results of the As-Prepared Products

Figure 2. The LRS patterns of the products as-prepared: (a) t-Se; (b) the product before baking; (c) the product after baking.

suggests that the tubular nanostructures are made of hexagonal CuSe since all diffraction peaks in the XRD pattern can be indexed to the hexagonal phase CuSe. The crystalline change of the products was also studied by LRS, and the result is shown in Figure 2. From the LRS pattern, it can be seen that the three peaks in the curve for t-Se nanotubes do not appear in the curves for the products before and after baking, which means that there is little or no t-Se in the products. Noticeably, the intensive longitudinal optical (LO) phonon mode peak of t-Se36 at 235.0 cm-1 blue-shifts to 263.0 cm-1 for the Cu3Se2 and CuSe. Consulting the Raman result (262.0 cm-1) for Cu2Se as reported,37 the peak at 263.0 cm-1 probably originates from the LO phonon mode of the Cu‚‚‚Se bond. Here, the frequency ratio of 263.0 and 235.0 cm-1 is 1.119. Supposing the bond force constants of Cu‚‚‚Se and Se‚‚‚Se to be equal, this frequency ratio should be 1.115 according to the relation of mass with frequency.37 Obviously, the hypothesis above is acceptable; therefore, the blue-shift of the LO phonon mode peak results mainly from the mass change of the bonding atoms (Cu and Se). Similarly, the transverse optical (TO) phonon mode peak at 140.9 cm-1 38 and the first LO overtone peak at 436.9 cm-1 39 of t-Se blue-shift to 191.2 and 510.9 cm-1, respectively, for Cu3Se2. But these two peaks disappear for CuSe, perhaps due to CuSe high symmetry in structure. Here, it should be pointed out that the discussion about LRS has ignored the size effect of the products. As a confirmatory analysis for the chemical situation of elements in the product, the XPS test was performed, and the

Figure 3. The XPS patterns of the final product: (a) Cu-2p; (b) Se-3d.

no. (as Figure 4)

atom ratio of Cu and Se in the precursors

atom ratio of Cu and Se determined by EDX

a

0.5:1

58.67:41.33

b c

1.0:1 1.5:1

52.14 :47.86 65.36:34.64

d e

2.0:1 2.5:1

65.86:34.14 67.21:32.79

phase of products (based on the XRD results and consulting literature) t-Se and tetragonal Cu3Se2 (JCPDS72-1421) t-Se and Cu3Se2 Cu3Se2 and cubic Cu2-xSe (JCPDS06-0680) Cu2-xSe Cu2-xSe and orthorhombic Cu2Se (JCPDS37-1187)

result is shown in Figure 3. The XPS pattern indicates that the binding energy values of Cu2p3/2, Cu2p1/2, and Se3d are 932.0, 952.0, and 53.80 eV, respectively. On the basis of this result and referencing the Xie’s result,18 apart from the Cu(II) in CuSe, little Cu(I) may exist in the product. As shown in Figure 3b, the binding energy of Se3d is 53.80 eV, which corresponds to that of the Se(-II),18 since the binding energies of Se3d for Se(0), Se(+IV), and Se(+VI) are ca. 55 eV,35 59 eV,40 and 61 eV,41 respectively. In addition, the shoulder-peak (as arrowed in Figure 3b) may result from the oxidation of little Se in the product.40 Reaction Mechanism. In the literature, copper selenide crystals have been synthesized by the reaction of Cu particles with Se film42 or Se particles with Cu film.43 In our synthesis process, the copper selenide nanotubes were synthesized by the

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Figure 5. The SEM images of the products obtained with different atom ratio of Cu and Se in the precursors: (a) 0.5:1, (b) 1.5:1, (c) 2.0:1, (d) 2.5:1.

reaction of Cu particles with the t-Se nanotubes. The t-Se nanotubes were prepared in advance, and the Cu particles with light yellow were freshly produced by the reduction of Cu2+ with hydrazine in the experiment. It is found that the Cu particles first formed could swiftly start the synthesis reaction. During the aging time, this reaction between elemental Cu and Se could steadily progress due to the following reasons. On the one hand, the smaller Cu particles with high reactivity could be continually supplied by the reduction of Cu2+ that was slowly released from [Cu(NH3)4]2+.44 On the other hand, the oxidation reaction of the Cu particles with O2 could be prevented owing to the presence of Na2SO3. As above, the Cu3Se2 phase was first produced in the synthesis process at room temperature. This phenomenon may result from the fact that the Cu3Se2 phase is more stable than other phases at room temperature. Lakshmi and co-workers have proved that the Cu2-xSe phase could convert to the Cu3Se2 phase by aging at room temperature.12 Ohtani and co-workers found that Cu2Se and CuSe could react with each other to form Cu3Se2 at room temperature.45 The Cu3Se2 phase that is unstable at temperatures above 413 K12 should dissolve into Cu2Se and CuSe at 503 K, according to the Cu-Se phase diagram. Then, the dissolved Cu2Se reacted with elemental Se existing in the product to form CuSe. Therefore, the hexagonal phase CuSe that resulted from both the dissolved CuSe and the CuSe formed from Cu2Se reacting

with Se was produced by baking at 503 K. Jiang and his coworkers46 removed the remainder of the t-Se in the product by baking at 503 K, which is over the melting point (490 K) of t-Se. In our experiment, the remainder Se in the product took part in the formation of CuSe but was not lost at 503 K. This reaction mechanism is supported by the EDX results (given above) in which the atom ratios of Cu and Se before (Cu/Se ) 52.14:47.86) and after (Cu/Se ) 52.04:47.96) baking were almost unchanged in the product. The CuSe with tubular nanostructure may result from the template formation mechanism on which the product was formed by the diffusion of Cu atoms into the t-Se nanotubes (tubular templates) like into the Se film as reported.42,43 Also based on this template formation mechanism, the wall thickness and diameter of the CuSe nanotubes are ca. 80 and 300 nm, respectively, which correspond well to that of the t-Se nanotubes. Perhaps the shape transformation from the straight t-Se nanotubes to the bent CuSe nanotubes and the length change from 10-20 µm for t-Se nanotubes to several micrometers for copper selenides are all the result of the change in atom bond length and energy in the reaction process between Cu and Se. The detailed formation mechanism for CuSe nanotubes is being studied. Effect of the Atom Ratio of Cu and Se in the Precursors on the Product. The atom ratio of Cu and Se in the precursors could affect the phase and constitution of the product.47 In our typical synthesis, the atom ratio of Cu and Se in the precursors

Synthesis of Hexagonal CuSe Nanotubes

is 1:1. In the interest of probing the effect of the atom ratio on the product, a series of experiments were performed at room temperature by changing the molar ratio of CuSO4 and t-Se nanotubes. The XRD patterns, as shown in Figure 4, indicate that the copper selenides with various phases were produced with different ratios of the precursors. XRD results compared with the EDX results of the products are summarized in Table 1. From the table, it can be found that the atom ratio of Cu and Se in the precursors played an important role in the phase transformation of copper selenides. When the atom ratio of Cu and Se is low, such as 0.5:1 or 1:1, tetragonal Cu3Se2 was formed from the remainder t-Se. With the atom ratio of 1.5:1, apart from Cu3Se2, some cubic Cu2-xSe was also produced due to the increase of Cu. When the atom ratio increased up to 2.0:1, the product was Cu2-xSe, and then a little orthorhombic Cu2Se would be produced if the reaction lasted longer (2 h) since some cavities in the Cu2-xSe phase were filled by Cu. According to this fill mechanism, the mixture of Cu2-xSe and Cu2Se was obtained in a typical reaction time (30 min) by making use of a higher atom ratio (2.5:1). Figure 5 shows the morphologies of the products obtained with different atom ratios of Cu and Se in the precursors. From the images, it can be seen that all products possess 1D nanostructures but with a bent shape. Notably, 1D nanostructures with a straight shape in Figure 5a are made of unreacted Se, which was confirmed by EDX results. 4. Conclusion In summary, we have successfully synthesized CuSe nanotubes by using t-Se nanotubes as a template-directed reagent. The XRD pattern indicates that the CuSe nanotubes possess the structure of a hexagonal phase. LRS patterns disclosed the phase transformation of the products, and the force constants of Cu‚‚‚Se and Se‚‚‚Se almost are equal by reckoning with the shift of the LRS peaks. The effect of the atom ratio of Cu and Se in the precursors on the product was studied, and Cu3Se2, Cu2-xSe and Cu2Se were obtained with the increase of the atom ratio of Cu and Se. Importantly, the present method is rapid and convenient and is being used for the synthesis of other 1D selenides in our group. Acknowledgment. Support for this work from the National Natural Science Foundation of China (No. 20475001, 50532030, 20471001) and Anhui Scientific Project (No. 050440702, 2005KJ011ZD, 2006KJ007TD) is gratefully acknowledged. References (1) Hivoto, U. Jpn. Kokai Tokkyo Koho JP 01,298,010, 1989, (2) Haram, S. K.; Santhanam, K. S. V.; Numann-Spallar, M.; LevyClement, C. Mater. Res. Soc. Bull. 1992, 27, 1185. (3) Lakshmikvmar, S. T. Sol. Energy Mater. Sol. Cells 1994, 32, 7. (4) Toyoji, H.; Hiroshi, Y. Jpn. Kokai Tokkyo Koho, JP 02,173,622, 1990. (5) Chen, W. S.; Stewart, J. M.; Mickelson, R. A. Appl. Phys. Lett. 1985, 46, 1095. (6) Levy-Clement, C.; Neumann-Spallart, M.; Haram, S. S.; Santhanam, K. S. V. Thin Solid Films 1997, 302, 12. (7) Bhuse, V. M.; Hankare, P. P.; Garadkar, K. M.; Khomane, A. S. Mater. Chem. Phys. 2003, 80, 82. (8) Shafizade, K. B.; Ivanova, I. V.; Kaizinets, M. M. Thin Solid Films. 1978, 55, 211. (9) Haram, S. K.; Santhanam, K. S. V.; Numann-Spallar, M.; LevyClement, C. Mater. Res. Soc. Bull. 1992, 27, 1185.

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