Grain Boundary Interfaces Controlled by Reduced Graphene Oxide in Nonstoichiometric SrTiO3-δ Thermoelectrics

Abstract

Point defect or doping in Strontium titanium oxide (STO) largely determines the thermoelectric (TE) properties. So far, insufficient knowledge exists on the impact of double Schottky barrier on the TE performance. Herein, we report a drastic effect of double Schottky barrier on the TE performance in undoped STO. It demonstrates that incorporation of Reduced Graphene Oxide (RGO) into undoped STO weakens the double Schottky barrier and thereby results in a simultaneous increase in both carrier concentration and mobility of undoped STO. The enhanced mobility exhibits single crystal-like behavior. This increase in the carrier concentration and mobility boosts the electrical conductivity and power factor of undoped STO, which is attributed to the reduction of the double Schottky barrier height and/or the band alignment of STO and RGO that allow the charge transfer through the interface at grain boundaries. Furthermore, this STO/RGO interface also enhances the phonon scattering, which results in low thermal conductivity. This strategy significantly increases the ratio of σ/κ, resulting in an enhancement in ZT as compared with pure undoped STO. This study opens a new window to optimize the TE properties of many candidate materials.

Figure 1

Schematic of (a) double Schottky barrier and (b) double Schottky barrier height at a grain boundary.

Figure 2

Room temperature (a) XRD patterns of SrTiO₃-RGO (0–4 wt% RGO) composites and (b) Raman spectra of STO-0.7 wt% RGO composite.

Figure 3

Microstructural characterization of the fractured SrTiO₃-RGO (0–4 wt% RGO) composites. Yellow arrows in (d–f) represent the agglomerated RGOs.

Figure 4

Microstructural characterization of the SrTiO₃-RGO composites; (a) HRTEM micrograph of the SrTiO₃-RGO hybrid powder, (b) diffraction pattern, (c,d) low and high magnification micrograph of the SrTiO₃–0.7 wt% RGO composite, and (e,f) low and high magnification micrograph of the SrTiO₃-4 wt% RGO composite. Yellow arrows represent the RGO at the grain boundaries.

Figure 5

Room temperature calculated mobilities for reduced single crystal STO as a function of carrier concentration and its comparison with STO-RGO composites.

Figure 6

Temperature dependent (a) carrier concentrations, (b) mobilities, (c) electrical conductivities, and (d) Seebeck coefficients of the STO-RGO composites.

Figure 7

(a) UPS spectra of pristine STO and RGO (b) schematic of band alignment, and (c) schematic of double Schottky barrier height of the STO-RGO composites.

Figure 8

Schematic diagram of RGO coated STO grains, blocking Sr sublimation and pumping out oxygen.

Figure 9

Temperature dependent (a) total thermal conductivity, and (b) lattice thermal conductivity of the STO-RGO composites. The inset in (b) shows the plot of κ_L vs. T⁻¹ of the STO-RGO composites and (c–e) is the schematic representation of various RGO content in STO matrix.

Figure 10

Temperature dependent (a) powder factors and (b) ZT values of the STO-RGO composites.