Metal-organic framework and graphene-like layers: new perspectives for conductive microporous composite materials

Metal-organic framework (MOF) intercalated with conductive graphene-like (GL) layers were synthesized and characterized. The selected MOF, the copper-based HKUST-1, combines high surface area, water stability, simple preparation and low costs. GL layers used to intercalate the MOF structure were obtained by a two-step, high-controlled, oxidation/reduction wet treatment of a high surface carbon black [1,2]. MOF composites were produced at different carbonaceous layers load. Elemental analysis demonstrated a complete incorporation of the carbonaceous layers into the hybrid structure. X-ray diffraction measurements revealed patterns resembling the ones reported for pure HKUST-1, indicating that the presence of intercalated GL layers does not prevent the formation of linkages between Cu ions and the BTC molecule, leading to the formation of HKUST-1 crystals (with negligible presence of the competing CuO2 phase). The Scanning Electron Microscope observations are in agreement with such structural results. Ar adsorption/desorption isotherms showed the preservation of the microporous character of the samples with the GL intercalation, revealing however a decrease in the average size of the pores for higher concentrations [3]. Spectroscopic and electrical conductivity characterizations were performed on GL, MOF and hybrid samples. The analysis of Infrared spectra and x-ray absorption spectra suggests that the carboxylic functionalities of the graphenic layers have interacted with the Cu dimers, and chemical interactions are involved in the formation of the composites. Likely, the presence of residuals of carboxylic groups, located at the edges of the GL basal planes, helps the interaction with the MOF crystals, similarly with those observed for MOF/Graphite-Oxide composites. Once again, this seems to not interfere with the MOF crystal formation [3]. The electrical conductivity rapidly increases with the added GL percentage, ranging over 5 orders of magnitude when the GL amounts changes between 5% and 40% wt. The increase does not follow a linear behavior. The observed strongly superlinear trend vs. doping and the observation of a threshold doping can be interpreted in terms of percolation theory. The relatively large threshold and saturation values for the GL concentration suggest a weak interaction between the filler particles inside the matrices, probably due to a good dispersion triggered by the high porosity of the host material [3].