Selective Enhancement in Phonon Scattering Leads to a High Thermoelectric Figure-of-Merit in Graphene Oxide-Encapsulated ZnO Nanocomposites

Abstract

ZnO is a promising candidate as an environment friendly thermoelectric (TE) material. However, the poor TE figure of merit (zT) needs to be addressed to achieve significant TE efficiency for commercial applications. Here we demonstrate that selective enhancement in phonon scattering leads to increase in zT of RGO encapsulated Al-doped ZnO core shell nanohybrids, synthesized via a facile and scalable method. The incorporation of 1 at% Al with 1.5 wt% RGO into ZnO (AGZO) has been found to show significant enhancement in zT (=0.52 at 1100 K) which is an order of magnitude larger compared to that of bare undoped ZnO. Photoluminescence and X-ray photoelectron spectroscopy measurements confirm that RGO encapsulation significantly quenches surface oxygen vacancies in ZnO along with nucleation of new interstitial Zn donor states. Tunneling spectroscopy reveals that the band gap of ~ 3.4 eV for bare ZnO reduces effectively to ~ 0.5 eV upon RGO encapsulation, facilitating charge transport. The electrical conductivity enhancement also benefits from the more than 95% densification achieved, using the spark plasma sintering method, which aids reduction of GO into RGO. The same Al doping and RGO capping synergistically brings about drastic reduction of thermal conductivity, through enhanced phonon-phonon and point defect-phonon scatterings. These opposing effects on electrical and thermal conductivities enhances the power factors as well as the zT value. Overall, a practically viable route for synthesis of oxide -RGO TE material which could find its practical applications for the hightemperature TE power generation.

Ba1/3CoO2: A Thermoelectric Oxide Showing a Reliable ZT of ∼0.55 at 600 °C in Air

“…The specific heat capacity used for the TDTR simulation was measured using differential scanning calorimetry (DSC, DSC8500, PerkinElmer) (Supporting Information, Figure S3). The in-plane κ of the films was extracted according to the following equation as previously reported. , where φ, κ obsd , κ // and κ ⊥ are the inclination angle of the layers relative to the perpendicular of the film surface, observed thermal conductivity, in-plane κ, and cross-plane κ, respectively (Supporting Information, Figure S4). In this study, φ = 35°, which was proven by scanning transmission electron microscopy (STEM) observations in our previous study …”

“…Since most metal oxides show good chemical and thermal stability in air as well as a harmless nature compared to chalcogenide-based state-of-the-art thermoelectric materials, oxide-based thermoelectric materials that show a high ZT are promising. , Several high- ZT thermoelectric oxides have been reported thus far; Fujita et al reported that the Na x CoO 2 single crystal exhibited a ZT of ∼1.2 at 800 K. Acharya et al . reported that Nb-doped SrTiO 3 with natural graphite exhibited a ZT of ∼1.42 at 1050 K. Biswas et al reported that graphene oxide-encapsulated ZnO nanocomposites exhibited a ZT of ∼0.52 at 1100 K. However, the reliability is considerably low due to a lack of careful observation of their stability at elevated temperatures.…”

Effects of Heavy Ion Irradiation on the Thermoelectric Properties of In2(Te1−xSex)3 Thin Films

“…Then, the number of scattering centers leads to a rapid increase in electrical resistivity. Biswas et al reported that the thermal conductivity of Al-doped ZnO quantum dots is reduced due to the selective phonon scattering by point defects and interfaces, and thereby an enhanced PF value [ 41 ]. Further, the lattice thermal conductivity of this composite could be dominated by grain boundaries and hence phonon–phonon scatterings.…”

Effects of Co-doping and Microstructure on Charge Carrier Energy Filtering in Thermoelectric Titanium-Doped Zinc Aluminum Oxide

“…For TE applications, ZnO doped with cations, such as Al 3+ , ,− Cd 2+ , Ni 2+ , − and Ga 3+ , and those co-doped with other cations have been studied. ,,,− However, due to the low solubility of solute ions in ZnO and its relatively high κ values (e.g., κ ∼ 8 W/(m·K) at 900 °C, its TE performance is rather limited. The low solubility limit of dopants in ZnO leads to the formation of second phases, which restricts its TE performance.…”