Solvothermal synthesis and thermoelectric properties of indium telluride nanostring-cluster hierarchical structures
© Tai et al; licensee Springer. 2011
Received: 7 October 2010
Accepted: 13 April 2011
Published: 13 April 2011
A simple solvothermal approach has been developed to successfully synthesize n-type α-In2Te3 thermoelectric nanomaterials. The nanostring-cluster hierarchical structures were prepared using In(NO3)3 and Na2TeO3 as the reactants in a mixed solvent of ethylenediamine and ethylene glycol at 200°C for 24 h. A diffusion-limited reaction mechanism was proposed to explain the formation of the hierarchical structures. The Seebeck coefficient of the bulk pellet pressed by the obtained samples exhibits 43% enhancement over that of the corresponding thin film at room temperature. The electrical conductivity of the bulk pellet is one to four orders of magnitude higher than that of the corresponding thin film or p-type bulk sample. The synthetic route can be applied to obtain other low-dimensional semiconducting telluride nanostructures.
PACS: 65.80.-g, 68.35.bg, 68.35.bt
where σ, S, k, and T are the electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature, respectively. The thermal conductivity comprises the combination of heat carried by phonons or lattice vibrations (k l), and electrical carriers (k e). A good thermoelectric material should be perfective combination of high power factor (S 2σ) with low thermal conductivity. In a bulk material, the Weidmann-Franz law limits the ratio σ/k, which makes optimization of ZT very difficult [5, 6]. Recently, nanostructures lowering k and the distortion of electronic density of states enhancing S are effective approaches to enlarge ZT[7–10]. The ZT value of silicon nanowires is over 100-fold improvement up to ZT = 0.6 to 1 than that of bulk Si at near room temperature [11, 12]. A maximum ZT value of approx. 2.4 was observed in a p-type Bi2Te3/Sb2Te3 superlattice thin film  and a ZT value of 1.6 was also reported in PbSeTe/PbTe quantum dots . Particularly, layered semiconductors such as bismuth telluride (Bi2Te3), indium selenide (In4Se3) with nanostructures are very promising for thermoelectric applications [7, 13, 15].
In addition, hierarchically ordered multiscale architectures have attracted great interest because of their emergent properties [16–19]. Specially, hierarchical ordered structures have great potential in developing high-efficient thermoelectric materials and devices because of very low thermal conductivity and high Seebeck coefficient [18, 20]. Up to now, most hierarchical nanostructures were prepared by the surfactants or biomoleculars, which can control the shape and size of semiconductor nanomaterials but they make post-treatment for the materials very difficult and influence the optical and electrical, thermal, magnetic properties of the products . Therefore, it is necessary to develop a facile, surfactant-free, and high-efficient approach to produce hierarchically structural thermoelectric materials at mild temperature and pressure.
Layered binary chalcogenide alloys A2 IIIB3 VI (A = Al, Ga, In and B = S, Se, Te) with semiconducting properties have important applications in energy conversion and information devices [15, 22–24]. Among these compounds, indium telluride (In2Te3) possessing disordered structure with respect to metal atom is a promising candidate for thermoelectric, optoelectronic, switching and memory devices [25–28]. It exhibits two crystalline phases labeled as α and β corresponding to low and high temperature formation, respectively. α-In2Te3 has a face-centered cubic (fcc) lattice with a = 1.850 nm, which is approximately two times more than the lattice parameter of β-In2Te3 (a = 0.616 nm). The transition temperature between the two phases is about 600°C . To date, α- and β-In2Te3 thin films, three-dimensional open-framework In2Te3 and it's supertetrahedral T2 clusters have been prepared by electrochemical atomic layer deposition, thermal evaporation, electron beam evaporation, etc. [30–33]. However, synthesis and application of hierarchically one-dimensional (1D) In2Te3 nanostructures have not yet been reported, and it is necessary to understand In2Te3 material properties in low dimensionality.
In this article, a facile, surfactant-free, and high-efficient solvothermal approach has been developed to successfully synthesize α-In2Te3 hierarchical structures using ethylenediamine (EDA) as the reducing and complexing agent. The typically well-oriented nanoplatelet in the hierarchical structures possesses an edge length of approx. 700 nm and a thickness of approx. 150 nm. The Seebeck coefficient of the bulk pellet pressed by the obtained In2Te3 samples exhibits a remarkable enhancement about 43% over that of the reported corresponding thin film at room temperature.
All chemicals are analytical grade products purchased from Shanghai Chemical Reagent Company and were used as received without further purification.
In a typical synthesis process, 0.3071 g (0.8 mmol) of In(NO3)3 and 0.2712 g (1.2 mmol) of Na2TeO3 were put into a Teflon-lined stainless steel autoclave of 50 mL capacity and dissolved in ethylene glycol (EG) (35.56 mL) under vigorous magnetic stirring to form a clear solution at room temperature for 1 h. Then, EDA (4.44 mL) was added into the mixed solution. The solution was stirred for 30 min again. Then the autoclave was closed and maintained at 200°C for 3, 6, 12, 18, and 24 h. After the treatment, the autoclave was cooled to room temperature naturally. The black flocculating product was collected from the solution by centrifugation, washed several times with absolute ethanol and ultrapure water with resistivity of 18 MΩ-cm, and then dried at 60°C in vacuum for 10 h; as a result, the black powders were obtained.
The structural properties of the as-prepared products were analyzed by power X-ray diffractometer, which were obtained using Bruker D8 Advance diffractometer operating at 40 kV and 40 mA (Cu Kα radiation, λ = 0.154178 nm). The morphology of the as-prepared products was analyzed by field-emission scanning electron microscopy (FE-SEM, Sirion 200, 10 kV). Transmission electron microscope (TEM) images and energy dispersive X-ray spectroscopy (EDX) were obtained at 200 kV using a JEM-2010 microscope by dropping a dilute ethanol solution of the powders onto the ultrathin carbon-coated copper grids.
Results and discussion
Phase and morphology characterization of the In2Te3 hierarchical structures
Growth mechanism of the In2Te3 hierarchical structures
Thermoelectric transport properties of the In2Te3 hierarchical structures
To evaluate thermoelectric properties of the In2Te3 hierarchical structures, the room-temperature electrical conductivity and Seebeck coefficient of the bulk pellets were measured, which were pressed using the as-prepared powders at 460 MPa for 30 min. The Seebeck voltage measured between cool and hot ends of the bulk pellet varies linearly with the temperature difference. The slope yields a Seebeck coefficient of -300 μV/K (Figure 1c). The negative sign indicates that the bulk pellet behaves as a n-type semiconductor, which is consistent with the EDX result (Figure 4b). The Seebeck coefficient of the bulk pellet is about 43% enhancement over 210 μV/K of the reported corresponding best thin film sample (seen in ref. ). Additionally, the bulk pellet exhibits a linear current-voltage (I-V) curve (see Figure 1a) that is symmetric about the origin, indicating that the contacts are ohmic. The slope yields a resistance of 436 Ω. An electrical conductivity σ of 6.42 Ω-1 m-1 can be obtained by using an average bulk pellet thickness in a 1D electrical transport model, which is one to two orders of magnitude higher than that of the corresponding thin film sample (0.66 Ω-1 m-1) [26, 36], and four orders of magnitude higher than that of the corresponding p-type bulk sample . In addition, bulk In2Te3 has very small thermal conductivity about 1.4 W (m K)-1. The thermal conductivity will further decrease by nanostructuring corresponding thermoelectric materials [2, 38].
In summary, a simple, reproducible, surfactant-free, and high-efficient solvothermal approach has been for the first time developed to successfully synthesize n-type α-In2Te3 thermoelectric nanomaterials. The nanostring-cluster hierarchical structure were prepared using In(NO3)3 and Na2TeO3 as the reactants, EDA as the reducing and complexing agent, and EG as the reductant and solvent at 200°C for 24 h. The typically well-oriented platelet in the hierarchical structures possesses an edge length of approx. 700 nm and a thickness of approx. 150 nm. A diffusion-limited reaction mechanism based on the XRD patterns and FE-SEM images with different durations was proposed to explain the formation of the In2Te3 hierarchical structures. t-Te nanowires are formed initially using EG as the reductant. Then In3+ is reduced into indium by EDA at high temperature and high pressure. Finally, the hierarchically structural In2Te3 can be obtained by reacting indium and t-Te. The room temperature Seebeck coefficient of the bulk pellet pressed by the obtained samples exhibits a 43% enhancement over that of the reported corresponding thin film. The electrical conductivity of the bulk pellet is one to four orders of magnitude higher than that of the corresponding thin film or p-type bulk sample. This is a promising approach to grow semiconducting telluride nanostructures through a solution-based chemical route under controlled conditions without the presence of any catalysts or templates.
energy dispersive X-ray spectroscopy
field-emission scanning electron microscopy
transmission electron microscope
X-ray powder diffraction.
This work is supported by the 973 Program (no. 2007CB936204), National NSF (no. 10732040), Jiangsu Province NSF (BK2008042), GT is supported by National and Jiangsu Province Postdoctoral Science Foundation (201003582, 20090451207, 0901073C), Jiangsu Province NSF (BK2010501), RFDP Funding (20103218120035), and NUAA Research Funding (4015-909322, NS2010220).
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