Hybrid-structured ZnO thermoelectric materials with high carrier mobility and reduced thermal conductivity

Nanostructure engineering has been extensively applied to ZnO in an effort to improve its performance in thermoelectric material, solar cell, and nanogenerator applications. Nano-structured ZnO bulks are limited by their inherently low mobility caused by the high density of grain boundaries and interfaces. In this study, a hybrid micro/nano structure composed of nearly coherent grain boundaries with a low misorientation degree among the nanograins was successfully fabricated in Zn_1−xAl_xO (x = 0, 0.01, 0.02, 0.03, 0.04 mol) bulks via hydrothermal synthesis and spark plasma sintering. Despite the large amount of nanograin boundaries and interfaces in the resulting material, a high carrier mobility value (50.7 cm² V⁻¹ s⁻¹) was obtained in the x = 0.2 sample-close to the level shown by ZnO single crystals and far higher than that of its ordinary nano-structured counterparts (<15 cm² V⁻¹ s⁻¹). A reduced thermal conductivity value of 2.1 W m⁻¹ K⁻¹ at 1073 K was also obtained in the micro/nano-structured x = 0.02 bulk due to extremely effective scattering at boundaries and interfaces also present in the nano-structured counterparts. After the simultaneous optimization of both electrical and thermal transport properties, the micro/nano-structured x = 0.02 sample showed a high ZT value (up to 0.36) at 1073 K. The proposed micro/nano-structure may also be applicable to other thermoelectric materials for further ZT enhancement.