Graphene boosts thermoelectric performance of a Zintl phase compound

The concept of nanocomposites derived by incorporating a second minor phase in bulk thermoelectric materials has established itself as an effective paradigm for optimizing high thermoelectric performance. In this work, this paradigm is for the first time extended to bulk Zintl phase Mg3Sb2 and its isoelectronically Bi-doped derivative Mg3Sb1.8Bi0.2 system. Herein, we report the synthesis, microstructural details, electronic structure and thermoelectric properties of (Mg3Sb2, Mg3Sb1.8Bi0.2)/ graphene nanosheet (GNS) nanocomposites with different mass ratios. Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) investigation reveals that Mg3Sb2 nanoparticles are homogenously anchored on the surface of GNS. We demonstrate that Mg3Sb2-based materials incorporated with a small content of graphene outperform optimally, resulting in potential p-type thermoelectric materials. The present nanocomposite additive of GNS deriving such a novel nanocomposite of (Mg3Sb2, Mg3Sb1.8Bi0.2)/GNS, enhances the electrical conductivity significantly, thereby resulting in a substantially large increase in the power factor. The enhanced electrical conductivity of these nanocomposites is attributed to the increase in the carrier concentration and high carrier mobility owing to the ultra high mobility of graphene. X-ray photoelectron spectroscopy (XPS) core level spectra confirm weak bonding between GNS and Mg3Sb2. Increase in carrier concentration is reflected in XPS valence band spectra and change in spectral weight near valence band maxima is indicative of increased electrical conductivity in the nanocomposite material. The thermal conductivity of these nanocomposites is noted to be reduced at high temperature. These favorable conditions lead to enhanced thermoelectric figure-of-merit (ZT) = 0.71 at 773 K for Mg3Sb2/GNS and a ZT = 1.35 at 773 K for Mg3Sb1.8Bi0.2/GNS nanocomposites with the mass ratio of 80 : 1 which are similar to 170% and similar to 125% higher values compared to bare Mg3Sb2 and bare Mg3Sb1.8Bi0.2 respectively. We strongly believe that the present novel strategy of fabricating such a nanocomposite of a Zintl compound by utilizing GNS as a nanocomposite additive, may provide an emerging path for improving thermoelectric properties of various Zintl phase compounds.