Synthesis and thermoelectric properties of PbTe nanorods and microcubes

Materials Science and Engineering B 163 (2009) 57–61

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Synthesis and thermoelectric properties of PbTe nanorods and microcubes Buyong Wan a,b , Chenguo Hu a,∗ , Bin Feng a , Yi Xi a , Xiaoshan He a a b

Department of Applied Physics, Chongqing University, Chongqing 400044, PR China College of Physics and Information Technology, Chongqing Normal University, Chongqing 400047, PR China

a r t i c l e

i n f o

Article history: Received 26 January 2009 Received in revised form 5 April 2009 Accepted 2 May 2009 Keywords: Chemical synthesis Lead telluride Nanostructures Seebeck coefficient

a b s t r a c t The pure face-centered cubic lead telluride nanorods and microcubes have been synthesized by the composite-hydroxide-mediated (CHM) approach using hydrazine and NaBH4 as a reducing agent, respectively. The method is based on reactants in hydroxide melts at a eutectic temperature of 200 ◦ C and normal atmosphere without using an organic dispersant or surface capping agent. Scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy are used to characterize the structure, morphology, and composition of the samples. The results show that the diameter of nanorods is 40–70 nm, while the size of microcubes is up to several micrometers. The measurement of thermoelectricity shows that the room temperature Seebeck coefficient value of the nanorods is up to 679.8 ␮V/K, about 2.56 times larger than that of the lead telluride bulk material, while the Seebeck coefficient value of the microcubes is only 195.5 ␮V/K. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Thermoelectric (TE) materials have potential applications in electrical-energy generation, cooling, and thermal sensing. Lead telluride (PbTe) is particularly promising for high-ZT thermoelectric devices, owing to its narrow band gap (0.31 eV at 300 K), facecentered cubic structure and large average exciton Bohr radius (∼46 nm) possessing strong quantum confinement within a large range of size. Theoretical calculations [1,2] and practical measurements [3,4] indicate that significant improvement in TE efficiency could be achieved in nanostructured systems owing to both a high electronic density of states near the Fermi level and an increased phonon scattering or reduced lattice thermal conductivity. Especially, one-dimensional (1D) structures of thermoelectric materials are of particular interest because they represent the smallest dimension structure that can efficiently transport thermal and electrical carriers, and are suitable for building device, which allows the study of thermoelectric property of an individual nanostructure. Recently, it has been reported that Seebeck coefficient of PbTe nanowires is greatly improved [5–7] in comparison with their bulk counterparts, similar to that of PbTe-based compound quantum dot superlattices [3]. Therefore, the preparation of PbTe nanostructures has attracted intensive attention. Up to now, many methods have been developed to synthesize crystalline PbTe, including magnetron sputtering [8], ultrasonic synthesis [9], sonoelectrochemistry [10], hydrothermal crystallization [11], microemulsion [12], solvothermal synthesis [13], hot wall

technique [14] etc. These methods usually depend on high vacuum, or high pressure, or salt–solvent mediated high temperature, or surface capping agent. Though, PbTe-based 1D structures can be synthesized by an electrochemical deposition with hard templates [15,16], the 1D structures are usually polycrystalline and not well dispersed after separating from the template. Limited success has been reported in PbTe 1D structure growth by solution-based softtemplating approaches. Therefore, seeking a simple approach for low-cost, lower-temperature, large-scale, controlled growth of PbTe low dimension structure at atmospheric pressure is of essential importance. The composite-hydroxide-mediated (CHM) approach is a newly developed method for synthesis of nanomaterials [17–20]. Here we synthesize PbTe nanorods and PbTe microcubes via the CHM approach using hydrazine and NaBH4 as reducing agent, respectively. The advantages of this method are of a simple process, easy to scale-up, low-cost, at normal atmosphere and without using any organic dispersant or surface capping agent. The measurement of thermoelectricity has been carried out on the film made from the PbTe nanorods and microcubes. The results demonstrate that a very large Seebeck coefficient of 679.8 ␮V/K can be obtained from the nanorods at room temperature, about 2.56 times larger than that of the PbTe bulk material [5]. But the Seebeck coefficient of the microcubes is a little less than that of the PbTe bulk material. 2. Experimental 2.1. Synthesis of lead telluride

∗ Corresponding author. E-mail address: [email protected] (C.G. Hu). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.05.004

PbTe crystals were synthesized by the CHM approach. All chemicals are of analytical grade. Typically, an amount of 9 g of mixed

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hydroxides (NaOH:KOH = 51.5:48.5) was placed in a 25 mL Teflon vessel. A mixture of anhydrous Pb(NO3 )2 and Te powder at 0.5 mmol each was used as the raw material and was placed on the top of the hydroxides in the vessel and, 2 mL hydrazine (N2 H4 ·H2 O) (>80%) was added in as reducing agent. The vessel was then put into a furnace preheated to 200 ◦ C. The vessel remained in the furnace for 24 h and then was taken out and led cool naturally down to room temperature. The black solid product obtained was washed several times using deionized water and filtered, and then it was washed with diluted HCl (pH 3.0) to remove other hydroxides on the surface of the nanorods. The clean PbTe nanorods were thus obtained after being washed twice with deionized water again. For the synthesis of the PbTe microcubes, all the synthesis procedures were the same as those stated above except that 2 mL hydrazine was replaced with 2 mmol NaBH4 powders as reducing agent.

2.3. Thermoelectric transport measurement To assess the electrical conductivity and Seebeck coefficient of the products, a film of the PbTe nanorods/microcubes was prepared by casting the dispersed PbTe crystals in ethanol solution on a dielectric substrate; after being dried naturally, it was pressed under 20 MPa pressure at room temperature using a press moulding machine for 30 min to make it more dense and even. Silver pastes were used to get Ohm contact between the film and Cu wires, and then the film was dried at 120 ◦ C for 1 h. The measurement of electrical properties was performed by a computer-controlled multifunctional measuring system (Keithley 2400 source meter), and the thermoelectromotive force (thermo emf) was measured by changing the temperature at the hot end, while keeping the cold end at room temperature. The temperatures of the hot and cold ends of the films were measured by the thermocouples contacted on the surfaces.

2.2. Structural characterization 3. Results and discussion The morphology and the size of the synthesized samples are determined at 20 kV by a field emission scanning electron microscopy (Nova 400 Nano SEM) and at 400 kV by a JEOL 4000EX high resolution electron microscope (HREM). An energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometer (XRD) with Cu K␣ radiation ( = 1.5418 Å), in step of 4◦ per min were used to investigate the crystal phase and chemical composition.

Fig. 1 shows the morphology of the PbTe products. Fig. 1a gives the SEM image of the PbTe nanorods synthesized using N2 H4 ·H2 O as reducing agent, from which we can see diameters of the nanorods range from 40 nm to 70 nm and typical lengths are of 300–500 nm. EDS in the inset of Fig. 1a shows that the elements in PbTe nanorods are lead and Te only (Si represents the peak of substrate). The electronic diffraction in Fig. 1b demonstrates the single crystalline

Fig. 1. SEM image (a) and TEM image (b) with the electron diffraction pattern (inset b) of the PbTe nanorods. SEM image (c) and magnified image (d) of PbTe microcubes. EDS spectra in the inset of (a) and (c) showing the presence of Pb and Te, the Si and C signals are from the substrate.

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Fig. 2. XRD spectrum of the PbTe before pressing and after pressing: (a) nanorods and (b) microcubes. The enlarged XRD spectrum of PbTe nanorods and microcubes from 26.5◦ to 28.5◦ in the inset of (b).

structure and the diffraction spots can be indexed as the facecentered cubic phase. The SEM images of PbTe prepared using NaBOH4 as the reducing agent were shown in Fig. 1c and d. The morphology of PbTe is of cubes with their sizes up to several micrometers. EDS in the inset of Fig. 1c shows that the elements in PbTe cubes are lead and Te only (Si and C are from the substrate). XRD diffraction peaks index that both nanorods and microcubes are rocksalt face-centered cubic (fcc) phases of PbTe, as is shown in Fig. 2. The lattice constants of PbTe nanorods and microcubes are 6.468 Å and 6.449 Å, respectively, which are consistent with the standard value (a = 6.454 Å) of bulk face-centered cubic PbTe (Fm-3m (225), JCPDS: 78-1905). From the inset of Fig. 2b, it can be found that the size of products with NaBH4 as the reducing agent is much larger than that with N2 H4 ·H2 O as the reducing agent and the crystallization of PbTe microcubes is better, due to the full-widthhalf-maximum (FWHM) peaks of PbTe microcube is smaller than that of PbTe nanorods and the intensity of peaks of PbTe microcube is stronger than that of PbTe nanorods. The formation of both PbTe nanorods and microcubes results from redox reactions. In the formation process of PbTe nanorods, the hydrazine hydrate plays a key role in controlling the nucleation and growth of PbTe nanorods. According to electrochemical theory, element Te cannot be efficiently reduced to Te2− ions by hydrazine hydrate due to close redox potentials (E 0 2− = Te/Te

0 −1.143 V and EN

ily reduced (E 0

2 /N2 H4

Pb2+ /Pb

= −1.15 V), while Pb2+ ions can be eas-

= −0.126 V). However, our experiments show

that Te powder could quickly dissolve into strong alkaline hydrazine hydrate accompanied with the production of N2 , and the color of the solution quickly turns red and finally deep red-brown. Fig. 3 shows there are two peaks centered around 510 nm and ranged from 250 nm to 360 nm, which might correspond to the absorption , Te2− , Te2− , respectively [21]. peak of polymer tellurium ions Te2− 2 3 4

reducibility of NaBH4 , the reactions can be expressed as below: NaBH4 + 4Te + 3NaOH → 4NaHTe + H3 BO3

(2)

−

(3)

NaHTe + Pb

2+

+ 2OH → PbTe + NaOH + H2 O

Factors affecting the crystal growth involve kinetics and crystallography. It has been concluded by Murphy [23] that the selective absorption of molecules and ions in solution on different crystal faces directs the growth of nanoparticles into various shapes by controlling the growth rates along different crystal axes. Wang [24] suggested that the shape of an fcc nanocrystal is to be determined mainly by the ratio of growth rate in <1 0 0> to that in <1 1 1>. Then, with respect to the fcc structure of PbTe, we speculate that the nanorods of PbTe form in an analogous process. Due to the existence of N2 H4 molecule in the hydroxide melts, different rates of adsorption and de-adsorption of hydrazine molecules on the different planes of PbTe nuclei would kinetically affect the growth rate of these planes. So the PbTe nuclei preferentially grow along a specific direction to form the rods. Moreover, because the existence of the polymerized tellurium anions and metal (II) amides decreases the free Te2− or Pb2+ concentration, the speed of whole reaction reduces, which prevents crystals from growing large. However, as the growth rate in <1 0 0> direction is almost equal to that in <1 1 1> direction with NaBH4 as a reducing agent in hydroxide melts, the speed of the whole reaction is faster. Therefore, microcube structures are obtained. To investigate the thermoelectric transport properties of asprepared PbTe products, the electrical conductivity and Seebeck

Polytelluride anions in form of Te2− x (x = 2–6) with rings or chains should exist in the above solution, which is similar to S2− x in liquid ammonia system [22]. The possible reaction was listed below: xTe + N2 H4 + 4OH− → N2 + 4H2 O + Te2− x

(1)

Why does not element Pb exist in the final products? In the hydroxide melts, the hydrazine not only plays a role as reducing agent but also as the complexing reagent to form a complex, metal(II) amides [22]: Pb(NH2 )2 or Pb(NH3 )2+ . With the increase of temperature and reaction time, metal(II) amides rupture and release the cation Pb2+ , which could react with active tellurium source Te2− released from Te2− x under attack of nucleophilic hydrazine in the hydroxide melts to gradually nucleate and grow to form PbTe nanocrystals. In the formation process of PbTe microcubes, due to good selective

Fig. 3. UV–vis absorption spectra of Te dissolving in the strong alkaline hydrazine hydrate.

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coefficient were measured using thin films of the PbTe nanorods and microcubes at room temperature. The thermoelectric properties of PbTe nanorod film (sample 1) and microcube film (sample 2) are shown in Figs. 4 and 5, respectively. The SEM images in Figs. 4a and 5a reveal that the average thickness of the sample 1 and the sample 2 are around 74.4 ␮m and 69.6 ␮m, respectively. The cracks in cross-section of the film in Fig. 4a result from separation of the film from the substrate to make a sample for SEM. The SEM images of PbTe nanorod and microcube films show some gaps on the surfaces, as is shown in Figs. 4b and 5b. The remarkable diffraction peak shift (about 1◦ to big angle) is observed in the film after pressing (Fig. 2a and b), which reveals that the crystalloid structures of PbTe materials would not have changed under pressure, except a little bit decrease of the interplanar spacing “d” and lattice constant “a”. The almost symmetric linear I–V curve in Figs. 4c and 5c indicates a good Ohmic contact between the films and Cu electrodes. The resistance of the films, as calculated from the slope of the I–V curve, is about 3.10 k and 90.9 , respectively. Based on the resistance and the size (including length, width, and thickness of the film), the electrical conductivities of sample 1 and sample 2 are estimated to be at 9.73 S/m and 355.6 S/m, respectively, which are two to three orders of magnitude less than that of the PbTe bulk sample. The lower conductivity is probably due to insufficient contact among nanorods and microcubes in the thermoelectric films. The Seebeck voltage measured between both Cu electrodes

Fig. 5. Thermoelectric properties of PbTe microcubes films: (a) SEM image of the film in side view showing the thickness of 69.6 ␮m; (b) superficial morphology of film consisted of PbTe microcubes; (c) I–V characteristics; (d) Seebeck voltage–temperature difference relation of the two ends of device with the TE film.

at the ends of films varies linearly with the temperature difference (Figs. 4d and 5d). A Seebeck coefficient can be obtained from the slope. The room temperature values of sample 1 and sample 2 are approximately equal to 679.8 ␮V/K and 195.5 ␮V/K, respectively, and the positive numbers indicate p-type semiconductor behavior. It can be found that the Seebeck coefficient of PbTe nanorod films is about 2.56 times larger than that of the PbTe bulk material (265 ␮V/K [5]), while the Seebeck coefficient of PbTe microcubes films is slightly less than that of the bulk material. The increased Seebeck coefficient of PbTe nanorods might origin from a quantum confinement effect by the small diameter (40–70 nm) according to an induced modification of the density of state (DOS) [25], or from an increased scattering at the interfaces of the nanoscale inclusions [26]. Yan also owed it to the phonon drag caused by the increased scattering between phonon and charge carrier [7]. In addition, the sizeable reduction in thermal conductivity is largely the result of the scattering of mid- and long-wavelength phonons at the interfaces of the nanoscale inclusions [27,28]. 4. Conclusions

Fig. 4. Thermoelectric properties of PbTe nanorods films: (a) SEM image of the film in side view showing the thickness of 74.4 ␮m; (b) superficial morphology of film consisted of PbTe nanorods; (c) I–V characteristics; (d) Seebeck voltage–temperature difference relation of the two ends of device with the TE film.

We have achieved the synthesis of highly pure PbTe nanorods and PbTe microcubes via a novel composite-hydroxide-mediated approach at temperature of 200 ◦ C. The hydrazine used as a reducing agent is the key factor to induce 1D growth of PbTe, as

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the existence of Te2− and metal (II) amides (Pb(NH2 )2 ) in the x hydrazine hydroxide melts can decrease the active ion concentration, which allows a PbTe nuclei to grow selectively along a specific direction to form the nanorods. While, since the NaBH4 as a reducing agent causes the isotropic and fast growth, microcubes are obtained. The film made from the as-synthesized PbTe nanorods has very high Seebeck coefficient (679.8 ␮V/K) at room temperature, which is much higher than that of the PbTe bulk material. But the room temperature Seebeck coefficient of the PbTe microcubes is only 195.5 ␮V/K. However, the conductivities measured in the as-prepared PbTe films are still low owing to the unoptimized interconnections. Possibly, anneal or assembly treatment could be an effective strategy for enhancing the electrical conductivity of the nanostructured film. Acknowledgments This work has been funded by the NSFC (20741006) and the Science and Technology Research Project of Chongqing Municipal Education Commission of China (KJ080819). The authors thank Professor Xia Lin of Chongqing University for the English editing. References [1] [2] [3] [4]

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