J. Phys. Chem. C 2010, 114, 14849–14853
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Synthesis and Thermoelectric Property of Cu2-xSe Nanowires Yan Zhang, Chenguo Hu,* Chunhua Zheng, Yi Xi, and Buyong Wan Department of Applied Physics, Chongqing UniVersity, Chongqing 400044, P.R. China ReceiVed: June 17, 2010; ReVised Manuscript ReceiVed: July 27, 2010
Single crystalline Cu2-xSe nanowires with lengths up to 50 µm are synthesized via a modified composite hydroxide mediated method. Experiments reveal that both quantity and species of hydroxides or amount of water used in the preparation would affect final morphology of Cu2-xSe crystals, from nanoplates to nanorods to nanowires. The UV-visible-near-infrared reflection spectrum demonstrates the absorption edges of the Cu2-xSe nanowires in the ultraviolet and near-infrared region, which could be interpreted in terms of direct transitions from a band gap of 2.38 eV and indirect transitions from a band gap of 1.12 eV. The band-to-band transition model has been proposed. In addition, Seebeck effect based on the Cu2-xSe nanowire film is investigated, demonstrating the Seebeck coefficient of 180 µV/K. 1. Introduction
2. Experimental Method
In recent years, the semiconductor copper selenide has been drawing extensive attention, due to its unique photoelectrical properties and wide applications in electronic and optoelectronic devices. Copper selenide is also widely used in solar cells as an optical filter and superionic material1-4 and as sensor and laser materials.5 A variety of techniques have been used to prepare copper selenide nanocrystals. Copper selenide nanocrystals have been successfully prepared by various methods, including solvothermal6 and gamma-radiation,7 sonochemical,8,9 electrochemical,10 microwave assisted,11,12 photochemical,13 template-based,14 chemical vapor deposition,15 chemical bath deposition,16 and so on. However, there have been only a few reports on the synthesis of copper selenide nanowires, especially of single crystalline structure. The usual length of the copper selenide nanowires is no longer than 20 µm,14,15 although Yang et al. have reported the polycrystalline Cu2-xSe nanowire bundles with diameters of 100-300 nm and lengths up to hundreds of micrometers via water evaporation induced self-assembly.17 Due to fundamental importance and potential applications of copper selenide nanowires in building its functional nanoscale devices, it is vital that a rational synthesis method be found to obtain single crystalline copper selenide nanowires with uniform diameters and long lengths. Herein, we report a novel and convenient route to synthesize Cu2-xSe nanowires by the modified composite hydroxide mediated (M-CHM) approach without any surfactants. The CHM approach is a new strategy that provides a one-step, convenient, low-cost, environment friendly and possibly massproduction route for synthesizing nanostructures of functional materials.18-21 The single crystalline Cu2-xSe nanowires with diameters ∼200 nm and lengths larger than 50 µm can be obtained by the M-CHM method at a temperature of 200 °C when a proper amount of water is added in the reaction mixture. In addition, the band-to-band transitions are studied by UV-visible-near-infrared reflection spectrum, and the conductivity and Seebeck effect of the Cu2-xSe nanowires is also investigated.
All of the reagents are of analytical grade and are used without further purification. In a typical synthesis of Cu2-xSe nanowires, first, 3 g of a mixture of NaOH and KOH with Na/K ratio of 51.5:48.5 as well as 10 mL of deionized water are put in a Teflon-lined autoclave of 25 mL capacity and stirred until a uniform mixture solution is formed. Second, 1 mmol of CuCl2 · 2H2O and 0.5 mmol of Se powder, 2 mL of hydrazine hydrate (as reducing agent) are added into the vessel. After ultrasonic agitation for about 30 min, the vessel is sealed and moved into a furnace preheated to 200 °C. The autoclave remains in the furnace for 24 h, and then it is taken out and allowed to cool to room temperature naturally. The black-colored precipitate is collected by centrifugation and then washed with deionized water and ethanol several times, and then dried in air at room temperature. The synthesis of the Cu2-xSe nanorods is similar to that of nanowires except that 9 g of a mixture of composite hydroxide (NaOH and KOH with Na/K ratio of 51.5: 48.5) or 3 g of KOH is used instead of 3 g of a mixture of composite hydroxide. When 3 g composite hydroxide is replaced by 3 g NaOH, the nanoplates are obtained. The chemical composition and crystal structure of the samples are characterized by X-ray diffraction (XRD) with Ni-filtered CuKa radiation (λ ) 1.5406 Å) and energy dispersive X-ray spectroscopy (EDS). The morphology and the size of the assynthesized samples are characterized by scanning electron microscope (SEM, Nova 400 Nano), field emission scanning electron microscope (FE-SEM), and transmission electron microscope (TEM, TECNAI20, Philips). The reflection spectra are carried out in the wavelength range 240-2600 nm using a computer aided double-beam spectrophotometer (UV-vis-NIR, Hitachi 4100). In order to investigate the electrical conductivity and Seebeck coefficient of the samples, we prepared the Cu2-xSe nanowire film by casting the dispersed Cu2-xSe nanowires in ethanol on a dielectric substrate. We used silver paste to connect the film and copper wires to make an electrode. The dimension of the film is 20 mm in length, 5 mm in width, and ∼100 µm in thickness. The measurement of electrical properties is performed by a computer-controlled multifunctional measuring system (Keithley 2400 source meter), and the thermoelectromotive force (thermo emf) is measured by changing the temperature at the
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[email protected].
10.1021/jp105592d 2010 American Chemical Society Published on Web 08/16/2010
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Figure 1. XRD patterns of the Cu2-xSe samples obtained with 3 g of NaOH (a), 3 g of KOH (b), 9 g of composite hydroxides (c), and 3 g of composite hydroxides at 200 °C for 24 h.
Figure 3. SEM and TEM images of the Cu2-xSe samples obtained at 200 °C for 24 h, with 3 g of NaOH (a and b) and 3 g of KOH (c). A total of 9 g of composite hydroxides (d).
Figure 2. Low (a) high (b) magnification SEM image of the Cu2-xSe nanowires obtained with 3 g of composite hydroxides at 200 °C for 24 h. EDS spectrum (c) showing the composition of the nanowires; the Si signal is from the substrate. TEM image (d) and SAED pattern (inset d) from the corresponding nanowire.
hot end, while keeping the cold end at room temperature. The temperatures at the hot and cold ends are measured by two thermocouples. 3. Results and Discussion The as-prepared products are first characterized by X-ray powder diffraction (XRD) spectra, and the results are shown in Figure 1. Comparing with the standard JCPDS cards, we have found that all peaks of the samples readily match those of a pure face-centered-cubic Cu2-xSe structure (JCPDS 06-0680) with lattice constant a ) 0.5739 nm. No impurity peaks are observed, indicating that the products are of single phase and high purity. In addition, the strong and sharp diffraction peaks suggest that the as-synthesized products are well crystallized. The morphology of the Cu2-xSe products is characterized in detail by a scanning electron microscope (SEM). Figure 2a,b shows that the product contains a large quantity of uniform nanowires with lengths up to 50 µm. The inset of the Figure 2b
shows a high magnification SEM image, indicating that the width of the nanowires is about 200 nm. EDS spectrum in Figure 2c shows only Cu and Se peaks (the Si signal is from the substrate), suggesting that these nanowires are composed of Cu and Se with atom ratio of Cu/Se close to 1.85:1, a nonstoichiometric Cu2-xSe. The TEM image in Figure 2d further demonstrates the nanowire morphology and single crystalline structure (inset d). The nanowire grows along 〈100〉 direction according to the diffraction spots. Figure 3a shows the SEM image of the uniform hexagonal nanoplates obtained by 3 g of NaOH. The high magnification SEM image (inset a) suggests that the thickness of single nanoplates is about 100 nm. The TEM image in Figure 3b further demonstrates the plate-like morphology. Figure 3c displays many rods with different lengths, which are obtained by 3 g of KOH. However, we find that there is little difference in morphology when we restrictively limit the same concentration of OH- ions as 3 g of NaOH contained in these experiments. Figure 3d shows the SEM image of the nanorods obtained by 9 g of the mixture of NaOH and KOH. The average length of the rods is about 8 µm. The inset TEM image further demonstrates the nanorod morphology. From the above experimental results, we can conclude that both quantity and species of hydroxides used in the preparation would induce different morphologies of Cu2-xSe crystals. In order to compare the M-CHM method with the hydrothermal method, we carried out detailed studies by varying the amount of deionized water without changing other experimental conditions. Figure 4a shows the XRD patterns of the products by the varied amount of deionized water (5, 10, and 18 mL) with 3 g of composite hydroxides at 200 °C for 24 h. The diffraction peak at 36.5° corresponding to main peak (111) of Cu2O, begins emerging when the amount of deionized water is up to 18 mL (80% of the vessel volume, a standard hydrothermal method). Figure 4b-d give the SEM images of the samples corresponding to the XRD spectra given in Figure 4a, indicating that 10 mL of deionized water is the most suitable for the growth
Properties of Cu2-xSe Nanowires
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(2Na+ + Se2- f Na2Se
Figure 4. XRD patterns (a) of the Cu2-xSe samples obtained with 5, 10, and 18 mL water and 3 g of composite hydroxides at 200 °C for 24 h. The arrow marked peak belongs to Cu2O. SEM images of the products prepared with 5 (b), 10 (c), and 18 mL (d) of deionized water, respectively. Each inset presents a diameter distribution histogram of corresponding sample in the image.
of uniform Cu2-xSe nanowires. The insets are the diameter distribution histograms obtained from the SEM images. However, broad diameter distribution is found of the sample synthesized by the hydrothermal method. Since the NaOH/KOH composite melts possess high viscidity, the viscidity would be decreased by increasing the content of water in the melts, which would enhance the transportation speed of crystal seeds and result in the formation of longer nanowires.21 On the other hand, the hydrazine hydrate (N2H4 · H2O) as a reducing agent plays an important role in controlling the growth of crystals.22,23 Factors affecting the crystal growth involve kinetics and crystallography. It has been concluded by Murphy24 that selective absorption of molecules and ions in solution on different crystal faces directs the growth of seeds into various shapes by controlling the growth rates along different crystal axes. The competitive absorption of N2H4 molecules on different crystal faces in the reaction mixture affects the shape of Cu2-xSe crystals. Selective adsorption of N2H4 molecules on different faces induces anisotropic growth. However, the change the content of water19 in the mixture also affects the adsorption of N2H4 molecules.25 Therefore, the condition of 3 g of NaOH/ KOH mixture with 10 mL of water is a suitable ambience either for the growth of Cu2-xSe nanowires or for an effective restraint of Cu2O formation. The obtained Cu2-xSe nanocrystals result from redox reactions. The possible reaction steps are suggested as follows:
N2H4 · H2O
Se
f
Se2-
(1)
or
2K+ + Se2- f K2Se) (2)
Cu2+ + Se2- f CuSeV
(3)
(2 - x)CuSe f Cu2-xSe V + (1 - x)Se
(4)
Selenium powder can be reduced to Se2- ion by reducing agent N2H4 · H2O (reaction 1).26 Meanwhile, dissociative Se2might partially combine with Na or K ion to form Na2Se or K2Se in alkaline solution (reaction 2).17 CuSe is easier to precipitate than Cu2Se due to the solubility product constant Ksp (CuSe) ) 7.94 × 10-49 and Ksp(Cu2Se) ) 1.58 × 10-61 (reaction 3).17 The transformation of Cu2-xSe occurs spontaneously according to the calculated electrode potential (reaction 4).17,27 The freshly produced selenium is active and can dissolve in the excessive alkaline solution at once.27 However, as Na2Se is more active than K2Se, which induces the higher reaction rate in the solution containing NaOH than that in the solution containeing KOH when they dissolve into Na+/K+ and OH-, different reaction rates would affect the action of surface adsorption of N2H4 molecules in reaction dynamics. Therefore, a different morphology of Cu2-xSe crystals is formed. Band gap of the nanowires can be estimated by optical spectrum analysis. The reflectance R(%) spectrum of the Cu2-xSe nanowires is given in Figure 5a. We note that there exist two absorption edges; one is in the ultraviolet region and the other in the near-infrared region. The absorption coefficient R can be calculated using the Kubelka-Munk function:28 R ) const × (1 - R)2/2R. As the thickness of the sample is much larger than the size of individual crystals, an ideal diffuse reflection can be assumed.29 Copper selenide is well-known as p-type semiconductor possessing a direct as well as an indirect optical gap.30 The curves of the absorption coefficient R derived from the diffuse reflectance data are shown in Figure 5b. The absorption spectrum presents a stronger near-infrared absorption peak at 883 nm and a weaker ultraviolet absorption peak at 420 nm, indicating the absorption edges of 1104 and 520 nm, respectively. The band gap of the Cu2-xSe sample can be estimated according to the absorption edges, and the values of direct and indirect band gap are 2.38 and 1.12 eV, respectively, which are close to those reported in refs 31 and 32. The nanowires produced by our method have potential applications in solar cells as well as photoelectron devices in the near-infrared region according to the estimated band gaps. Basically, the band to band transitions of the nanowires can be interpreted in terms of the diagram as is shown in Figure 5c. Based on calculations, the conduction band (CB) bottom is composed of Se 4p states, and the valence band (VB) top is composed of the Cu 4s states mixed somewhat with the Cu 3d states. There exist two transitions; one is the transition from Cu 4s states to the conduction band (CB) Se 4p states, and the other is the transition from Cu 3d states to the conduction band (CB) Se 4p states when the radiation energy increases.33 Cu2Se is a kind of thermoelectric material. To explore the thermoelectric property of the Cu2-xSe nanowires, we have measured the electrical conductivity and Seebeck coefficient of the thin film (Sf) electrode made from the nanowires. The linear current-voltage (I-V) curve in Figure 6a indicates that the nanowires on the substrate have a good Ohmic contact with the Cu electrodes. The Seebeck voltage measured between two Cu electrodes at the ends of film varies linearly with the temperature difference. Seebeck coefficient (SCu - Sf, SCu can
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Figure 5. Reflectance R (%) spectrum of Cu2-xSe nanowires obtained with 3 g of composite hydroxides at 200 °C for 24 h (a) and the optical absorption spectra derived from diffuse reflectance data (b) and the schematic sketch of the band to band transitions (c).
Figure 6. I-V curves (a) and temperature difference dependence of Seebeck voltage (b) of Cu2-xSe nanowire films.
be neglected) can be obtained from the slope of the curve.34 The Seebeck coefficient of the Cu2-xSe film is about 180 µV/ K, as is shown in Figure 6b. The positive value indicates its p-type semiconductor characteristic, which agrees with the previous reports.17,35,36 The Seebeck coefficient of the Cu2-xSe nanowire film is much lager than the reported β-Cu2Se at room temperature35 and also larger than that of the thermoelectric material Ag2Se.37 4. Conclusions High quality Cu2-xSe nanowires with lengths up to 50 µm have been synthesized by the M-CHM method using hydrazine hydrate as a reducing agent. The method is simple and costeffective and dispenses with the use of a large amount of organic solvent, and the method may be extended to synthesize other chalcogenide nanowires as well. The nanowires are of single crystals growing along direction. Both quantity and species of hydroxides or amount of water used in the preparation would affect final morphology of Cu2-xSe crystals. The optical absorption of the Cu2-xSe nanowires in the ultraviolet and near-
infrared region demonstrates the direct band gap of 2.38 eV and indirect band gap of 1.12 eV. The Seebeck coefficient of the Cu2-xSe nanowire film is up to 180 µV/K, which is much larger than the β-Cu2Se and Ag2Se at room temperature reported previously and indicates its p-type semiconductor characteristic. Acknowledgment. This work is supported by the NSFC (60976055), the Fundamental Research Funds for the Central Universitis (CDJXS10102208), Postgraduates’ Innovative Training Project (S-09109) of the 3rd-211 Project, and the largescale equipment sharing fund of Chongqing University. References and Notes (1) Lakshmikumar, S. T.; Rastogi, A. C. Sol. Energy. Mater. Sol. Cells. 1994, 32, 7–19. (2) Toyoji, H.; Hiroshi, Y. Jpn. Kokai Tokkyo Koho 1990, 173–622. (3) Chen, W. S.; Stewart, J. M.; Mickelson, R. A. Appl. Phys. Lett. 1985, 46, 1095. (4) Levy-Clement, C.; Neumann-Spallart, M.; Haram, S. S.; Santhanam, K. S. V. Thin. Solid. Films. 1997, 302, 12. (5) Heske, C.; et al. Appl. Phys. Lett. 1997, 70, 1022.
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