Synthesis and Thermoelectric Properties of Bi2Se3 Nanostructures
© Kadel et al. 2010
Received: 16 July 2010
Accepted: 9 September 2010
Published: 1 October 2010
Bismuth selenide (Bi2Se3) nanostructures were synthesized via solvothermal method. The crystallinity of the as-synthesized sample has been analyzed by X-ray diffraction, which shows the formation of rhombohedral Bi2Se3. Electron microscopy examination indicates that the Bi2Se3 nanoparticles have hexagonal flake-like shape. The effect of the synthesis temperature on the morphology of the Bi2Se3 nanostructures has also been investigated. It is found that the particle size increases with the synthesis temperature. Thermoelectric properties of the Bi2Se3 nanostructures were also measured, and the maximum value of dimensionless figure of merit (ZT) of 0.096 was obtained at 523 K.
where S is Seebeck coefficient, σ is electrical conductivity, Κ is thermal conductivity, and T is absolute temperature at which figure of merit is measured. The quantity S2σ is most commonly referred as power factor. Increase in power factor and decrease in thermal conductivity are required for the enhancement of ZT value. Theoretical predictions and experimental results show that a nanostructured low-dimensional TE material can exhibit high thermoelectric efficiency [3–5]. Nanostructures can induce the reduction in thermal conductivity by the interface or boundary scattering of phonons and the increment in power factor by quantum confinement of electrons [3, 6]. In the recent years, researchers have shown increased interest to investigate thermoelectric properties of various solid-state TE materials in their nanostructured form. Recently, a high ZT of about 2.5 for Bi2Te3/Sb2Te3 superlattices has been reported at 300 K .
According to Slack , semiconductors having narrow band gap and high mobility carriers are best suited as thermoelectric materials. Bismuth selenide (Bi2Se3) is a V-VI semiconductor with a narrow band gap of about 0.3 eV [9, 10], which has potential application in optical recording system , photoelectrochemical devices , and thermoelectric devices [9, 10]. In recent years, bismuth chalcogenides gained much research interest due to their good thermoelectric properties and high ZT values at room temperature [7, 13].
Over the years, a wide variety of synthesis techniques have been developed to synthesize various nanostructures of Bi2Se3. Wang et al.  reported low-temperature solvothermal method to obtain Bi2Se3 nanostructures in ethylenediamine (EN), Giani et al.  used chemical vapor deposition method to synthesize Bi2Se3 thin film, and Jiang et al.  synthesized Bi2Se3 nanosheets by microwave heating in the presence of ionic liquid. Among the various synthesis techniques employed for the formation of Bi2Se3 nanostructures, the solvothermal/hydrothermal process is attracting much interest due to the advantages of high yield, low synthesizing temperature, high purity, and high crystallinity. Xie et al.  and Yu et al.  synthesized Bi2Se3 nanostructures using ethylenediamine (EN) as solvent, and Batabyal et al.  reported synthesis of Bi2Se3 nanorods using dimethyl formamide (DMF) as solvent. Solvothermal/hydrothermal process has been successfully employed to synthesize different nanostructures of Bi2Se3 [20, 21], and . To the best of authors' knowledge, measurement of thermoelectric properties of Bi2Se3 is seldom reported. Recently, Lin et al.  reported the thermoelectric measurement of nanostructured Bi2Se3 obtained from decomposition of the single-source precursor. In this communication, we report the synthesis of flake-like Bi2Se3 nanostructures via solvothermal route in DMF at various synthesis temperatures for different durations. The effect of the synthesis temperatures on the structure and morphology of the Bi2Se3 nanostructures has been investigated. TE properties of the Bi2Se3 nanostructures have also been measured and found superior to their bulk counterpart.
Analytically pure bismuth nitrate pentahydrate (Bi(NO3)3.5H2O, Fisher Scientific) and selenium (Se, Acros) powder were used as precursor materials for the synthesis of Bi2Se3, and 1 mmol of Bi(NO3)3.5H2O and 1.5 mmol of Se powder (in molar ratio of 2:3) were measured and added into a Teflon-liner. Then, 4 mmol of sodium hydroxide (NaOH, Acros) as a pH-controlling and pH-reducing agent, and 2 mmol of ethylenediaminetetraacetic acid (EDTA, Acros) as a shape-directing additive were added. Later, the Teflon-liner was filled up to 80% of its total volume with DMF and was placed in an ultrasonicator for 30 min to obtain a uniform reaction mixture. After the sonication, the Teflon-liner was placed in an autoclave and sealed tightly. Then, the autoclave was heated in the furnace at 140 and 200°C for 24 h. After the synthesis, the autoclave was allowed to cool down to room temperature naturally. The black precipitate resulted from the reaction was vacuum filtered, rinsed with ethanol and distilled water several times, and dried at 100°C in vacuum for 4 h to get the sample in powder form. Samples prepared in DMF at 140 and 200°C for 24 h are termed as BiSe-1 and BiSe-2, respectively. In order to measure the TE properties of the material, a large amount of as-prepared BiSe-2 powder sample was annealed in the presence of hydrogen and argon for 4 h before TE properties were measured.
X-ray diffraction (XRD) measurements were taken using Siemens D5000 diffractometer equipped with a Cu anode operated at 40 kV and 40 mA. The XRD patterns were collected with a step size of 0.01° and a scan rate of 1 step/s. Surface morphology analysis was performed by a field-emission scanning electron microscope (SEM, JEOL JSM-6330F, 15 kV). Transmission electron microscopy (TEM) images, selected-area electron diffraction (SAED) patterns, and energy dispersive X-ray spectroscopy (EDS) spectrum were obtained from FEI Tecnai F30 apparatus operated at an accelerating voltage of 300 kV with a point-to-point resolution of 2Å.
For TE properties measurement, the powder sample BiSe-2 was pressed at 500°C in graphite dies with a 12.7 mm central cylindrical opening diameter using a dc hot-press method to obtain cylindrical bulk discs. Since the pressure applied to the sample is very high (~80 MPa), these bulk samples are highly dense. The measured density of the sample by using Archimedes's principle is 6.59 gcm-3, which is at around 97% of the material's theoretical density (6.798 gcm-3). These bulk samples were then cut into 2 mm × 2 mm × 12 mm bars for four-probe electrical conductivity and Seebeck coefficient measurements and also into 12.7-mm-diameter discs with appropriate thickness for the thermal conductivity measurement. The electrical conductivity and Seebeck coefficient were measured by using commercial equipment (Ulvac, ZEM-3) from room temperature to 523 K, and the thermal conductivity was measured by using a laser flash system (Netzsch LFA 457) from room temperature to 523 K.
Result and Discussion
Structure Characterization of Bi2Se3 Nanoparticles
Thermoelectric Property of the Bi2Se3 Nanoparticles
Bi2Se3 nanoflakes were synthesized via solvothermal route at different synthesis conditions using DMF as solvent. The surface morphology and crystal structure of the nanoflakes were analyzed, and the results show that the as-prepared samples are rhombohedral phase of Bi2Se3. The size of the Bi2Se3 nanoflakes increases with the synthesis temperature. From the thermoelectric property measurement, the maximum ZT value of 0.096 was obtained at 523 K, and a ZT value of 0.011 was obtained at room temperature. The as-prepared Bi2Se3 nanoflakes exhibit a higher Seebeck coefficient and a low thermal conductivity compared with the bulk counterpart at room temperature, which can be attributed to their nanoscale size. The improvement on the thermoelectric property indicates the promising aspect of the as-prepared Bi2Se3 nanoflakes as a good thermoelectric material at room temperature.
We would like to thank Dr. Wei Chen for the XRD measurements. This work is supported by the National Science Foundation under grant DMR-0548061. The material sintering and thermoelectric measurement were done at Boston College. TEM work was performed at the user facility at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
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