Grain Boundary Engineering in Bottom-Up Synthesized Thermoelectric Nanocomposites

Grain boundaries play a critical role in determining the thermoelectric performance of materials by simultaneously influencing electrical and thermal transport. Compared to conventional composites synthesized via melting-annealing methods, nanocomposites prepared through wet-chemical routes are more susceptible to grain boundary effects due to their high specific surface area. However, most previous studies have primarily focused on tuning the composition of nanoparticles, while grain boundary engineering has been relatively underexplored. In this perspective, we first review the general mechanisms of energy filtering and low-frequency phonon scattering at grain boundaries and their contributions to thermoelectric enhancement. We then highlight three promising bottom-up strategies for grain boundary engineering via nanoparticle surface modification: nanoparticle blending, colloidal exchange, and small-molecule recovery. Finally, we outline several key questions and challenges that future research must address to further advance this field.