(Invited) Reduced Thermal Conductivity of Tcnq and F4-TCNQ Infiltrated HKUST-1 Due to Suppression of Linker Flexural Phonon Modes

Abstract

MOFs are of great interest for applications in gas storage, separation, catalysis and more recently, thermoelectrics (TE) due to their extremely high porosity (surface area), large chemical and mechanical tailorability defined by the choice of the metal node, the organic ligand, and infiltrated guest molecules, and low thermal conductivities. In particular, new emergent properties, such as electronic conductivity and energy transfer, have been achieved by infiltrating MOF pores with specialized ‘guest’ molecules. The thermal transport in infiltrated MOFs is critically important for understanding the efficiency of both gas adsorption and thermoelectric applications. Understanding the impacts on thermal transport upon MOF infiltration with an electrically conductive guest molecule is critical for realizing an efficient TE material. This work experimentally investigates the impacts on the thermal properties of HKUST-1 MOFs infiltrated with charge accepting TCNQ and F4-TCNQ guest molecules. We provide experimental evidence paired with Molecular Dynamics (MD) results to describe the thermal transport mechanism as a loading-dependent structural change within the MOF. Since HKUST-1 exhibits negative thermal expansion (NTE), we note the unusual phenomena that the material softens as it becomes denser with increasing temperatures. We show this is achieved by which the ligands take on a contorted configuration as they absorb more energy at higher temperatures. Upon infiltration, the MOF softens, however the softening (and therefore reduction in phonon group velocity) cannot be completely responsible for the large reduction in thermal conductivity that is experimentally observed (~71% at room temperature). Spectral Energy Density (SED) and MD results indicate that localized phonon modes from the adsorbates act as additional scattering mechanisms to reduce the thermal conductivity. Further, experimental evidence of the existence of low frequency ZA-like flexural modes (usually found only in 2D materials) within the highly porous 3D MOF structure likely contributes to a large portion of the thermal conductivity, and is suppressed upon infiltration with a guest molecule, causing the large reduction in thermal conductivity.