Synthesis of copper-based MOF/graphene-like composites and probing of their conductive properties

Metal-organic frameworks (MOF) are hybrid structures obtained by the assembly of metallic centres and organic linkers through strong covalent bonds. The great interest toward MOF is driven by their high surface area and the possibility to tune their properties by changing their metallic centres and organic linkers. The electrical properties of MOF were very rarely studied due to their insulating nature. Recently the preparation of hybrid materials combining MOF and carbonaceous materials (carbon nanotubes, graphite oxide) has been reported. This paper proposes the synthesis and description of the structure of a copper-based MOF composite formed with conductive graphene-like layers. For the preparation of the composite the HKUST-1 metal-organic framework was selected. HKUST-1 is a water-stable MOF containing Cu2+ dimers linked to oxygen atoms from benzene tricarboxylate. Graphene-like layers used to intercalate the MOF structure were produced by a two-step oxidation/reduction in water solution of a high surface carbon black. The water suspension of graphene-like material was readily used to prepare the composite. The preparation of the composites was done by adding the water suspension of graphene-like layers to MOF precursors in solution following the same procedure as in the preparation of the pristine MOF. The added graphene-like layers ranges from 5 to 45 wt.% of the final material weight. These compounds, as well as the parent materials, present a high specific surface area and were characterized by elemental analysis, X-ray diffraction, FT-IR and UV-Visible spectroscopy, thermal analysis, scanning electron microscopy, BET. Electrical characterization was performed through resistive measurements in a four contact geometry via the Van der Pauw method. The evaluated sheet resistance and bulk resistivity reveal a substantially insulating character as concerns the pristine MOF while the conductivity rapidly increases with the added graphene-like percentage, ranging on several orders of magnitudes for the investigated samples.