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.

Modulation of Charge Transport at Grain Boundaries in SrTiO3: Toward a High Thermoelectric Power Factor at Room Temperature

“…This challenging target might be achievable via the advancement of ceramic processing technologies and/or development of composites in which the secondary components (e.g., graphene) restrain grain growth, while facilitating the creation of oxygen vacancies at the GB. 6 , 7 , 22 , 40 , 43 …”

“… 9 Recently, interest has grown in the optimization and enhancement of thermoelectric properties of inorganic compounds via grain boundary (GB) engineering. 1 , 3 , 6 , 7 …”

“… 7 Generally, this single-crystal-like electrical conductivity at low temperature is attributed to the decrease in GB resistance with the reduction of the built-in potential barrier at the grain boundaries. 3 , 6 , 7 , 18 Localized changes in GB chemistry and structure (elementary composition, chemical state, etc.) are thought to be the primary reasons for the reduction of the potential barrier at the grain boundaries.…”

Applications of Ceramic/Graphene Composites and Hybrids

“…The maximum ZT value measured at 1100 K was 0.29 and electron mobility was improved three times compared to the plain ceramic. Rahman et al [ 118 ] proposed that, in addition to doping, increasing the carrier mobility and electrical conductivity of STO controlled the formation of double Shottky barriers at grain boundaries, due to strontium and oxygen vacancies, which acted as scattering centers for electron mobility. The use of 0.7% rGO as a high conductivity phase at grain boundaries resulted in the reduction of Shottky barrier height, releasing the trapped electrons and enhancing mobility.…”

Challenges in Improving Performance of Oxide Thermoelectrics Using Defect Engineering

“…SrTiO 3 have been studied widely and is considered as one of the favorite n-type TE materials due to its high absolute Seebeck coefficient. [32,33]. Virgin SrTiO 3 is considered to be an insulator and with a bandgap of 3.25 eV.…”