Simultaneous Nitrogen Doping and Pore Generation in Thermo-Insulating Graphene Films via Colloidal Templating

We report a simple method for preparing highly efficient thermoelectric materials through the fabrication of nitrogen-doped reduced graphene oxide (GO) with a porous structure. The samples were produced by thermal annealing of GO/nitrogen-rich polystyrene (N-PS) particle composite films using a colloidal templating method. N-PS particles served as a nitrogen dopant source for the nitrogen-doped thermally reduced graphene oxide (TrGO) as well as sacrificial particles for the porous structure. The S values of the porous TrGO films were negative, indicating that the samples were transformed into n-type materials. Their porous structures simultaneously resulted in materials with high σ values and low in-plane κ values by providing numerous air cavities for phonon scattering and destruction of the anisotropic structure, maintaining an interconnected structure for an electron transport path. Thus, the porous TrGO films exhibited enhanced power factors and low κ values. The highest ZT value of 1.39 × 10^-4 was attained for a porous TrGO film annealed at 1100 °C, which was 1200 times higher than that of a nonporous TrGO film. This study emphasizes that an isotropic orientation of two-dimensional materials has a significant effect on the suppression of in-plane κ, leading to their enhanced thermoelectric performance.