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Cite this: DOI: 10.1039/c5dt00897b

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The effect of temperature on thermoelectric properties of n-type Bi2Te3 nanowire/graphene layer-by-layer hybrid composites† Hyun Ju and Jooheon Kim* The thermoelectric properties of Bi2Te3 nanowire/graphene composites prepared at different sintering temperatures have been investigated. The as-synthesized ultrathin Bi2Te3 nanowires are uniformly distributed between the graphene layers, leading to the formation of Bi2Te3 nanowire/graphene layer-by-layer hybrid structures. The electrical conductivity of the as-sintered composites increases dramatically with

Received 5th March 2015, Accepted 18th May 2015

the sintering temperature, as the relative density and grain size increase and the interface density

DOI: 10.1039/c5dt00897b

decreases. This in turn lowers the Seebeck coefficient due to the reduction of the potential barrier for carriers and their scattering at the interface. The fabricated n-type Bi2Te3 nanowire/graphene composites

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exhibit an enhanced figure of merit of 0.25 at an optimal sintering temperature of 623 K.

1.

Introduction

In the last few decades, great efforts have been made to explore and develop alternative technologies to meet increasing energy demands. Thermoelectric devices, which can convert thermal energy from a temperature gradient into electrical energy and vice versa, show great potential for energy harvesting applications. They are also expected to play an increasingly important role in meeting future energy requirements. Previous studies have focused on inorganic bulk thermoelectric materials, which have been utilized commercially for the past 50 years. The figure of merit of these materials (ZT, dimensionless) has reached values around 1.1,2 The figure of merit is calculated as ZT = (S2σT )/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. Graphene, on the other hand, has a theoretical ZT value of ∼4 at room temperature,3 which along with its outstanding flexibility makes it an outstanding candidate for thermoelectric applications. In theory, a thermoelectric material must possess a high electrical conductivity, a high Seebeck coefficient, and a low thermal conductivity for its ZT to be high. However, previous experimental studies on graphene have highlighted its poor thermoelectric efficiency that stems from a low Seebeck coefficient (∼50 μV K−1) and a high thermal con-

School of Chemical Engineering & Materials Science, Chung-Ang University, Seoul 156-756, Republic of Korea. E-mail: [email protected]; Fax: +82-2-812-3495; Tel: +82-2-820-5763 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c5dt00897b

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ductivity (up to ∼5000 W m−1 K−1).4–6 To date, therefore, graphene has seldom been studied as a thermoelectric material. The high thermal conductivity of graphene reflects its inplane thermal conductivity, which is four orders of magnitude larger than through the plane.7 Moreover, heterointerfaces with neighboring (non-graphene) layers can be introduced to further reduce interfacial thermal transport. The scattering of phonons between the adjacent layers is markedly enhanced in bilayers such as these. Seol et al. found that the thermal conductivity of SiO2-supported graphene was lower than that of suspended graphene,6 and an even lower thermal conductivity is predicted for graphene sandwiched between non-graphene layers.8 Furthermore, the introduction of a nanostructured material as a neighboring layer substantially reduces the thermal conductivity of graphene compared to its bulk layers because phonon scattering is enhanced at the interfaces with the intercalated nanomaterials.9 Indeed, Poudel et al. reported that the reduced thermal conductivity of nanostructured bismuth–antimony–telluride bulk alloys was due to increased phonon scattering.10 A large Seebeck coefficient is also required for achieving high thermoelectric efficiency. The addition of telluriumbased nanostructured materials with a high Seebeck coefficient, such as Bi2Te3, PbTe, and Sb2Te3,11–16 can increase the Seebeck coefficient of pristine graphene,8 enhancing its thermoelectric performance. Nanostructured materials can also increase the Seebeck coefficient via potential barrier scattering17–19 or quantum confinement effects.20 Among nanostructured materials, Bi2Te3 is the most efficient n-type thermoelectric material, because of its high Seebeck coefficient and low thermal conductivity at room temperature.1 The

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study of layer-by-layer heterostructures of graphene intercalated with Bi2Te3 nanostructured materials is therefore promising in view of enhancing the ZT of n-type thermoelectric materials. This paper presents a promising strategy for enhancing the thermoelectric properties of layer-by-layer hybrid composites. Thermoelectric Bi2Te3 nanowire/graphene layer-by-layer composites were prepared by facile wet chemical synthesis and subsequent sintering. Most prior studies of thermoelectric materials have focused on material fabrication. However, the processing conditions must also be optimized in order to achieve further ZT enhancements and guarantee the scalability of the process. Indeed, Son et al.21 and Keawprak et al.22 have highlighted the effects of the sintering temperature on the thermoelectric properties of the resulting materials, which these authors exploited to achieve dramatically improved ZT values. These effects were also investigated here in view of optimizing the processing conditions. Specifically, the electrical conductivity, Seebeck coefficient, thermal conductivity, power factor, and ZT of Bi2Te3 nanowire/graphene composites sintered at various temperatures were measured and compared.

2. Experimental methods 2.1

Materials

Graphite nanopowder (mean particle size

graphene layer-by-layer hybrid composites.

The thermoelectric properties of Bi2Te3 nanowire/graphene composites prepared at different sintering temperatures have been investigated. The as-synth...
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