Hydrothermal Synthesis and Thermoelectric Transport Properties of

The growth process of the pearl-necklace-shaped PbTe nanowires can be reasonably explained by an oriented attachment mechanism. Thermoelectric ...
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Hydrothermal Synthesis and Thermoelectric Transport Properties of Uniform Single-Crystalline Pearl-Necklace-Shaped PbTe Nanowires Guoan Tai, Wanlin Guo,* and Zhuhua Zhang Institute of Nanoscience, Nanjing UniVersity of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, P.R. China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2906–2911

ReceiVed December 21, 2007; ReVised Manuscript ReceiVed April 16, 2008

ABSTRACT: Uniform single-crystalline pearl-necklace-shaped PbTe nanowires with an average diameter of about 30 nm, smaller than its average excitonic Bohr radius of 46 nm, were successfully synthesized by a hydrothermal process using tellurium nanowires as templates and Pb(NO3)2 as a precursor at a molar ratio of 1:1. It is shown that the reaction temperature, duration, and concentration of Pb(NO3)2 play important roles in the formation of the PbTe nanowires. The growth process of the pearl-necklace-shaped PbTe nanowires can be reasonably explained by an oriented attachment mechanism. Thermoelectric transport measurement indicates that the film composed of the obtained PbTe nanowires has a Seebeck coefficient of about 307 µV/K, up to about 16% higher than that of the state-of-the-art bulk PbTe at room temperature. The synthetic route can be applied to obtain other low-dimensional semiconducting telluride nanostructures and optimization of the thermoelectric properties through nanowire alignment and doping may lead to practical applications. 1. Introduction Thermoelectric materials have important applications in power energy generation, cooling, and thermal sensing.1,2 The efficiency of a thermoelectric device is related to the figure-ofmerit ZT of its constituent thermoelectric material3

ZT )

TS2σ ke + kL

(1)

where S is the Seebeck coefficient, σ is the electrical conductivity, ke and kL are the electronic and lattice thermal conductivity of the thermoelectric material, and T is the absolute temperature. The total thermal conductivity k ) ke + kL. Effective thermoelectric materials should have a low thermal conductivity but high electrical conductivity and high Seebeck coefficient. To be competitive with conventional refrigerators and generators, thermoelectric materials with ZT > 3 should be developed. In bulk material, the Weidmann-Franz law limits the ratio σ/k, which makes optimization of ZT very difficult.1 However, if the dimensionality is decreased, the new variable of size becomes available. In recent years, both theoretical predications4,5 and experimental results6 have strongly suggested that large improvements in ZT could be achieved in nanostructured systems because of both a high electronic density of states near the Fermi level and an increased phonon scattering or reduced lattice thermal conductivity. Much larger enhancements in thermoelectric conversion efficiency are predicted in true quantum-confined systems.4,5 Recent reports of ZT values at room temperature up to 2.4 with the use of quantum well7 and quantum dot superlattices8 have stimulated more interests. One-dimensional (1D) nanomaterials are of great interest for the construction of high-performance thermoelectric devices.9 The calculations4,5 showed that the maximum ZT of 1D quantum wire is greater than that of both the bulk material and quantum well, which stimulates researchers to synthesize various 1D thermoelectric nanomaterials. To date, the most successful synthesis of 1D thermoelectric materials has been achieved by electrodeposition within alumina templates. A series of Bi2Te3,10 * Corresponding author. Telephone: 86-25-84895827. Fax: 86-25-84895827. E-mail: [email protected].

Bi2-xSbxTex,11 Bi2Te3-ySey,12 and Bi1-xSbx13 nanowires were prepared using the template-based method. Although the size and uniformity of nanowires can be well-controlled by the hard template, the nanowires can only be prepared in low yields in each synthesis process and may be destroyed when they are separated from the template. Importantly, no significant improvement in ZT value was reported and the template materials were suspected to cause complications for transport measurements.14 It is also possible that the as-prepared nanowires are usually polycrystalline.12,15 It is therefore essential to develop an alternative approach to prepare a large quantity of singlecrystalline thermoelectric nanowires to meet the demand of highperformance thermoelectric applications. Lead telluride (PbTe) is a very promising thermoelectric materials due to its narrow band gap (0.31 eV at 300 K), facecentered cubic (rock salt) structure and large average excitonic Bohr radius (about 46 nm) allowing for strong quantum confinement within a large range of size.8,16 It is reported that PbTe quantum dot superlattices8 and bulk compounds with nanostructures16 have largely improved energy conversion efficiency compared with their bulk samples. Recently, Heremans, et al.17 also showed experimentally and theoretically that an enhancement in the Seebeck coefficient of PbTe nanoparticles with grain sizes on the order of 30-50 nm, which is in magnitude similar to that reported in the literature for PbTe/ PbSexTe1-x quantum dot superlattices.8 Therefore, the synthesis of PbTe nanomaterials has attracted intensive attentions. Monodispersed PbTe nanocrystals,18 nanorods,19 nanoboxes,20 hollow spheres and nanotubes,21 and nanowires22 have been synthesized. However, to date, the average diameter of the obtained PbTe nanowires is over 60 nm, larger than its average excitonic Bohr radius below which strong quantum confinement effect will occur.22 Moreover, few study for the thermoelectric properties of 1D PbTe nanostructures has been reported. To improve the thermoelectric properties of PbTe remarkably, nanowires with diameter smaller than its average excitonic Bohr radius are highly desirable. More interestingly, the performance of periodic or superlattice nanowires may outperform completely smooth nanowires because of the multiple scattering of acoustic phonons in periodic structured nanowires and increasing wire surface area.4,5 Consequently, it is very important to study the

10.1021/cg701262x CCC: $40.75  2008 American Chemical Society Published on Web 06/27/2008

Pearl-Necklace-Shaped PbTe nanowires

synthesis and thermoelectric transport properties of the uniform and periodic small-size nanowires to understand the underlying physics. In this article, an alternative hydrothermal synthesis method has been developed to synthesize single-crystalline pearlnecklace-shaped PbTe nanowires with the average external diameter of about 30 nm in large quantities by controlling the reaction conditions such as the reaction temperature, duration, and concentration of Pb(NO3)2. We found that the periodic nanowires are formed by an oriented attachment mechanism. Even measurements on the thin film sample of the obtained nanowires can achieve a 16% enhancement in the Seebeck coefficient over that of the state-of-the-art bulk PbTe material at room temperature.

Crystal Growth & Design, Vol. 8, No. 8, 2008 2907

Figure 1. Typical FESEM images of the pearl-necklace-shaped PbTe nanowires synthesized at 453 K for 12 h. (a) Low magnification; (b) high magnification. The inset of (a) is the diameter distribution of the PbTe nanowires.

2. Experimental Section All chemicals are analytical grade products purchased from Shanghai Chemical Reagents Company and were used as received without further purification. 2.1. Synthetic Procedure for Tellurium Nanowires. Tellurium nanowires with diameter of 3-20 nm are prepared using a modified method according to that reported previously by Qian, et al.23 In a typical experiment, 0.30 g of polyvinyl pyrrolidone (PVP, K30, MW ) 40000) and 0.1808 g of Na2TeO3 (0.8 mmol, 98%) were put into a Teflon-lined stainless steel autoclave with a volume capacity of 50 mL and dissolved in 25 mL of double distilled water under vigorous magnetic stirring to form a homogeneous solution at room temperature for a while. Then, 1.5 mL of hydrazine hydrate (85%) and 3 mL of aqueous ammonia (25%) solutions were added into the mixed solution, respectively. The final solution was clear, and 10.5 mL of double distilled water was added again under vigorous magnetic stirring up to 80% capacity of the total volume of the autoclave. The autoclave was then sealed and maintained at 453 K for 24 h. After that, the container was cooled to room temperature naturally. 2.2. Synthetic Procedure of Pearl-Necklace-Shaped PbTe Nanowires. Pb(NO3)2 (99%) (0.268 g, 0.8 mmol) was added into the above solution under magnetic stirring at 278 K for 12 h. Subsequently, the autoclave was sealed and maintained at 453 K for 12 h. After the reaction, the black flocculating product was collected from the solution by centrifugation. The precipitates were washed many times with double distilled water and absolute ethanol and dried in a vacuum at 353 K for 12 h. 2.3. Structural Characterization. The morphology and structural properties of the as-prepared products were analyzed by field emission scanning electron microscopy (FESEM, Sirion 200, 10 kV) and powder X-ray diffractometer with Cu radiation (XRD, Bruker D8 Advance), respectively. Transmission electron microscopy (TEM), high-resolution morphologies, the typical selected area electron diffraction (SAED) patterns and energy-dispersive X-ray spectroscopy (EDX) of the nanostructures were performed on JEM-2010 (200 KV). 2.4. Thermoelectric Transport Measurement. To measure the electrical conductivity and Seebeck coefficient of the product, thin film sample was prepared by drop-casting the nanowires from dispersed ethanol solutions on a glass substrate. The film with a length of 25 mm, width of 15 mm, and thickness of 8 µm was obtained by being pressed using a glass slide after drying naturally in order to make it more dense and even, and then dried at 353 K for 6 h in a vacuum. The four-probe method was adopted for the electric conductivity measurement. Silver pastes were used as electrical contacts of the electrodes to the film. The Seebeck coefficient S was measured on a patented thermoelectric measurement instrument (HGTE-II model, national invention 200710051933 of China). The test set up is illustrated in Figure 5c. The absolute temperature and temperature differences were determined by using calibrated nickel chromium-nickel silicon thermocouples. An internal thermal screen with monitored temperature distribution was used to minimize the parasitic heat losses around the sample. A temperature difference of about 6-15 K between cool and hot ends of the film was applied for the Seebeck coefficient measurements. A temperature gradient was established in the nanowire film when the electrical power was applied by a thermoelectric pile. The two thermocouples were contacted with the film surface to determine

Figure 2. X-ray diffraction patterns of Te nanowires (a), and PbTe samples synthesized at different time intervals: (b) 1, (c) 3, (d) 6, (e) 9, and (f) 12 h. The Miller indices correspond to cubic PbTe crystals. The peaks marked by * are from Pb3O4. The peaks positions of the bulk PbTe XRD profile are indicated by short dashed lines for comparison. the temperature drop. Electrodes pressed on the film sample were used to monitor the Seebeck voltage drop between the two thermocouples.

3. Results and Discussion 3.1. Phase and Morphology Characterization of the Products. Figure 1 shows the FESEM images of the obtained pearl-necklace-shaped PbTe nanowires. The nanowires have external diameters of 20-40 nm (inset of Figure 1a), smaller than the 46 nm Bohr radius, and lengths of several micrometers. Such miniaturization and periodic chainlike structures hold more promise because of size-reduction and novel structure induced thermal conductivity decrease. The present nanowire diameters are smaller than its excitonic Bohr radius (46 nm), below which an enhancement of ZT over the corresponding bulk value should occur because of strong quantum effects. Elemental analysis by EDX spectrum reveals an atomic ratio of Pb:Te ) 50.81: 49.19 (Figure 3d), which is consistent with the stoichiometric PbTe within experimental errors. The signals for Cu, Cr peaks originate from the substrate. XRD analyses of the as-synthesized samples show that the transformation from tellurium nanowires to pearl-necklace-shaped PbTe nanowires occurs with increasing reaction duration. The diffraction peaks in Figure 2f can be indexed to the rocksalt face-centered cubic (fcc) phase of PbTe (space group: Fm3jm, No. 225) with lattice constants of a ) b ) c ) 0.645 nm (JCPDS card: 78-1905). The peaks on the XRD profile of the obtained PbTe nanowires by 12 h hydrothermal growth are in excellent agreement with that of the

2908 Crystal Growth & Design, Vol. 8, No. 8, 2008

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Figure 3. (a) TEM image of the PbTe nanowires synthesized for 12 h at 453 K. Inset: Unit cell of PbTe (pink dots are Pb atoms; blue dots are Te atoms). (b) TEM image of a single pearl-necklace-shaped nanowire with diameters of about 23 nm in the junction section and about 37 nm in the bead section. Inset: SAED pattern indexed for cubic PbTe. (c) High-resolution TEM image taken from (b), which corresponds to the position referred by the downward-right arrow. (d) The EDX spectrum of the nanowires.

corresponding bulk sample (indicated in Figure 2 with short dashed lines), which means there is no obvious lattice distortion in the nanowires. The peaks marked with “*” is from Pb3O4, which resulted from a small quantity of nanoplates produced in our product (not shown). A little oxygen dissolved in the solution can oxidize Pb into PbO2 in the reaction system, and then the PbO2 can transform into Pb3O4 in the hydrothermal process.24 TEM and high-resolution TEM (HRTEM) provide further insight into the details of the pearl-necklace-shaped PbTe nanowires (Figure 3). The SAED (inset of Figure 3b) pattern obtained from the individual nanowires shows that the obtained nanowires are single crystalline. And the diffraction spots can be indexed as a cubic, room temperature phase of PbTe. Additionally, the HRTEM study (Figure 3c) shows that the PbTe nanowires are highly crystalline with d spacing of 0.3225 nm, which is in good agreement with the d(200) planes of the fcc PbTe lattice. This value corresponds exactly to the strongest peak observed in the XRD pattern (Figure 2f). Therefore, the nanowires grow along the 〈100〉 direction. Defects such as stacking faults and dislocations are not seen inside the nanowires (e.g., Figure 3c), indicating that the single crystals are of high quality. 3.2. Investigation of the Growth Mechanism. To understand the growth mechanism of the obtained pearl-necklace-shaped PbTe nanowires, the effects of the reaction temperature, duration, and concentration of Pb(NO3)2 on the resulting products were systematically investigated. When aged for more than 12 h at room temperature, the mixture of nanoplates and nanowires can be formed (Figure 4b). Increasing the reaction temperature to 413 K leads to a mixture of the nanoparticles and nanoplates (not shown). When the rest conditions remain the same, the obtained products are short nanowires within 3 h (images c and d in Figure 4). A large amount of nanoparticles

Figure 4. TEM images of the tellurium nanowires (a) and PbTe nanowires (b-f). PbTe nanowires synthesized at room temperature (b) and at 453 K at different time intervals: (c) 1, (d) 3, (e) 6, and (f) 9 h. The insets of (a) and (e) are the diameter distribution of the tellurium nanowires and the PbTe nanoparticles, respectively.

can be produced when prolonging the reaction duration up to 6 h (Figure 4e). When the reaction duration is increased to 9 h, the as-prepared products are short pearl-necklace-shaped nanowires (Figure 4f). Further prolonging the reaction duration to 12 h, well-defined and straight pearl-necklace-shaped nanowires can be obtained (Figures 2 and 3). In addition, when the molar ratio of Pb(NO3)2:Te is up to 3, the obtained product is composed of microbelts and nanoparticles (not shown). On the basis of the above experimental results, the transformation mechanism from t-Te nanowires to well-defined and single-crystalline pearl-necklace-shaped PbTe nanowires can be reasonably proposed. When all source materials were added to the autoclave, lead ions (Pb2+) could be reduced to metal lead atoms readily by the residual hydrazine hydrate at high temperature according to their redox potential.

E0Pb2+/Pb ) 0.126 V

(2)

E0N2H4/N2 ) -1.15 V

(3)

Then Pb atoms diffused into the lattice of t-Te to generate the cubic PbTe. In our system, PVP was introduced as an adsorption agent and architecture soft template, and plays an important role in the transformation process from the t-Te nanowires to the pearl-necklace-shaped PbTe nanowires. The PVP protecting the t-Te nanowires in the first step can also efficiently protect the PbTe nanowires produced in the second step. It has been shown that there is a strong interaction between the surfaces of nanoparticles and PVP through coordination bonding with the O and N atoms of the pyrrollidone ring.25 Very importantly,

Pearl-Necklace-Shaped PbTe nanowires

Crystal Growth & Design, Vol. 8, No. 8, 2008 2909 Scheme 1. Overall Synthetic Procedure for the Pearl-Necklace-Shaped PbTe Nanowires: Tellurium Nanowires (a) Are Transformed into Short PbTe Nanowires (b) and Then into Single Nanoparticles (c); (d) Pearl-Necklace-Shaped Pbte Nanowires Produced by Oriented Attachment of Spherical Nanoparticles and Partially Accompanied by Ostwald Ripening to Form Partially Smooth Surface Nanowires (e)

Figure 5. (a) I-V characteristics obtained from a PbTe nanowire thin film. (b) Dependence of the Seebeck voltage as a function of temperature differences along the thermoelectric film. (c) Set up of the measurement configuration for Seebeck coefficient.

because PVP can strongly bind metal cations,26 it can trap the Pb2+ during the dissolving process and leads to the formation of micelles with Pb2+-polychelate cores.27 As capping molecules, the PVP could selectively stabilize the {111} faces because it can strongly interacts with the charged {111} faces containing Pb or Te rather than the uncharged {100} faces containing mixed Pb/Te through coordination bonding. Decreasing surface energy on the {111} faces can raise the growth rate on the {100} faces in relative to the {111} faces. Therefore, the products prefer to grow along the 〈100〉 direction, which is in excellent agreement with the above XRD result. Furthermore, PVP is necessary in synthesizing the PbTe nanowire. The formation of the PbTe nanowires in this hydrothermal process is believed to be a direct combination pathway.

N2H4 · H2O + 4OH- h N2+5H2O + 4e-

(4)

2Pb2+ + N2H4 + 4OH- f 2Pb + N2 + 4H2O

(5)

Pb + Te f PbTe

(6)

Very interestingly, pearl-necklace-shaped PbTe nanowires can be produced in the transformation process from t-Te nanowires to PbTe nanowires. With increasing reaction duration the long tellurium nanowires will break and rupture into short sections and further into a large quantity of nanoparticles (Figure 4c-e). The transformation process may be attributed to two reasons: (1) The diffusion coefficient and atomic radius of Pb are greater than those of Te, which will lead to the formation of vacancies (namely, Frenkel disorder28) in the nanowire shell. (2) The instability of the micelles at high temperature will result in failure of the template PVP.29 As observed in this experiment, PbTe nanoparticles transformed into short pearl-necklace-shaped nanowires (Figure 4f) and finally into well-defined, long and straight pearl-necklace-shaped PbTe nanowires (Figure 1). The

transformation process from nanoparticles to pearl-necklaceshaped nanowires should be due to an orientation attachment mechanism. Because the diameters of the formed nanowires are close to the sizes of the nanoparticles as seen from the histogram for particle size (inset of Figure 4e) and wire-diameter distribution (inset of Figure 1a). In addition, the defects seen in the growth process of the nanowires are characteristic of the oriented-attachment process (inset of Figure 4f). Oriented attachment refers to the phenomenon that generates nanowires by attaching existing dot-shaped nanocrystals along a given crystal orientation. Since it was first reported a few years ago,30 this mechanism has been observed for several systems, such as CdTe,31 CdSe32 and PbSe nanowires,33 ZnO,34 ZnSe,35 and ZnS36 nanorods. In the case of anisotropic crystal structures, the mechanism can explain the formation of long wires. In the case of structures with higher symmetry, however, a symmetrybreaking mechanism needs to be operative for the formation of long wires.37 A dipole mechanism was invoked to explain the symmetry-breaking involved in the formation of the wire structures for semiconductors.31,33,36 In a stoichiometric nanoparticle of any of these compound semiconductor materials, some faces will be terminated with the cation and others with the anion. This creates a net dipole moment on each nanoparticle that tends to orient the particles in a common direction.33 Cho, et al.33 documented impressive evidence of the importance of the dipole moment of PbSe nanodots for the oriented attachment. They reported small PbSe nanocrystals are generally terminated by six {100} and eight {111} facets, and the largest permanent dipole moment was predicted along the 〈100〉 direction. Shim et al.38 also reported ZnSe nanocrystals with centrosymmetric zinc blend lattice have a largest permanent dipole moment along the 〈100〉 direction. We believe the formation of the pearlnecklace-shaped PbTe nanowires is also according to the alignment of preformed nanoparticles by dipole-dipole interactions as the mechanism responsible for the anisotropy associated with the attachment. This is consistent with recent observations of PbSe wires and ZnSe nanocrystals prepared by other methods,33,38 in which the wire axis was always aligned with the 〈100〉 direction of the crystal lattice. An interesting feature is the subsequent coarsening of the final nanowires by the Ostwald ripening mechanism, partially “smoothing” the nanowires surface (Figure 1, reaction duration for 12 h), which is similar to the synthesis of ZnS nanorods.36 The overall synthetic process is described in Scheme 1. These results are very interesting because the PbTe nanowires with cubic structure can

2910 Crystal Growth & Design, Vol. 8, No. 8, 2008

be formed from the oriented attachment growth of spherical cubic PbTe nanoparticles. 3.3. Thermoelectric Transport Properties of the Obtained PbTe Nanowires. To assess the thermoelectric transport properties of the pearl-necklace-shaped PbTe nanowires, the electrical conductivity and Seebeck coefficient were measured using the thin film of the PbTe nanowires at room temperature. SEM images (not shown here) reveal that the drop-cast films are web structure, with nonuniform thickness. The film exhibits a linear current-voltage (I-V) curve (see Figure 5a) that is symmetric about the origin, indicating that the contacts are ohmic. The slope yields a resistance of ∼764 Ω. By using an average film thickness estimated based on cross-sectional SEM images in a one-dimensional (1D) electrical transport model, an electrical conductivity σ ≈ 273 S/m can be obtained, which is two orders in magnitude lower than that of the corresponding bulk sample. The Seebeck coefficient of the nanowires is about 307 µV/K (Figure 5b), higher than 265 µV/K of the bulk sample up to about 16%. The scattered data are due to the temperature control precision using thermocouples. Heremans et al.17 attributed the enhancement of S to the separation of higher energy electrons from lower energy electrons and the selective scattering of electrons or the so-called electron energy filtering if the feature size of the nanostructures is less than the electron mean free path. The nanostructured films should efficiently reduce the thermal conductivity κ of the corresponding thermoelectric materials.7,8 It is shown that single thermoelectric nanowires have lower thermal conductivity compared with the corresponding thin film.5a Fardy et al.22a revealed that the thermal conductivity of the single PbTe nanowire with the diameter of 180 nm is 2-3 orders in magnitude lower than that of the corresponding stateof-the-art bulk sample (2.0 W m-1 K-1), and also 1-2 orders of magnitude lower than that of superlattice quantum dots.8 As the thermal conductivity of nanowires generally can be reduced by decreasing diameter, our nanowires should have much lower thermal conductivity due to the smaller diameters.5a 4. Conclusions In summary, a simple hydrothermal process has been developed to synthesize uniform pearl-necklace-shaped PbTe nanowires in large quantities by a hydrothermal process using tellurium nanowires as templates and Pb(NO3)2 as a precursor. First, tellurium nanowires with diameters of 3-20 nm were prepared using polyvinyl pyrrolidone and Na2TeO3. The pearlnecklace-shaped PbTe nanowires were then successfully synthesized by the hydrothermal process using the tellurium nanowires as templates and Pb(NO3)2 as a precursor at a molar ratio of 1:1 at 373 K for 12 h. HRTEM, SAED, and SEM investigations show that the obtained PbTe nanowires are singlecrystalline and have external diameters of 20-40 nm, smaller than the 46 nm Bohr radius, and lengths of several micrometers. The growth process of the pearl-necklace-shaped PbTe nanowires can be reasonably explained by an oriented attachment mechanism, and the reaction temperature, duration, and concentration of Pb(NO3)2 play important roles in the formation of the PbTe nanowires. Thermoelectric measurements show that the Seebeck coefficient has a 16% enhancement, whereas the electronic conductivity is lowered by two orders in the PbTe nanowire thin film when compared with that of the bulk PbTe. As it has been proven that the thermal conductivity of the single PbTe nanowire with the diameter of 180 nm can be at least 2 orders of magnitude lower than that of the corresponding bulk sample, and the thermal conductivity of nanowires decreases

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with reduced diameter, the obtained nanowires should have much lower thermal conductivity because of the smaller diameters. Furthermore, as periodic or superlattice nanowires may outperform completely smooth nanowires because of the multiple scattering of acoustic phonons in periodic structured nanowires and increasing wire surface area, the pearl-necklaceshaped PbTe nanowires should have more potential thermoelectric performance enhancement. Acknowledgment. The work is supported by the 973 program (2007CB936204), NSFC (10732040), and the Ministry of Education (No.705021, IRT0534) of China, Jiangsu Province NSF and the doctor Innovation Funds of NUAA (No. BCXJ0604).

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