One-step synthesis of conductive copper-based metal-organic framework/graphene-like composites

Metal-organic frameworks (MOF) are hybrid structures obtained by the assembly of metallic centres and organic linkers through strong covalent bonds. MOF recently attracted a great attention due to their high surface area (up to 4500 m2/g [1]) and the possibility to change their metallic centres and organic functionalities, adapting their properties to potential applications towards adsorption and separation of specific gases. In order to expand the field of application of MOF, the preparation of hybrid materials combining MOF and carbonaceous materials (carbon nanotubes [2], graphite oxide [3]) has been very recently proposed. This paper focuses on 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, a water-stable MOF containing Cu2+ dimers as the metallic units linked to oxygen atoms from benzene tricarboxylate (BTC) has been selected. Graphene-like layers used to intercalate the MOF structure were obtained by a two-step oxidation/reduction wet treatment of a high surface carbon black (CB). The oxidation with nitric acid destroys the CB backbone and provides functionalization with oxygenated groups (mainly carboxylic) at the edge of the graphitic layers leaving untouched the basal plane. The oxidized CB was treated with hydrazine hydrate to reduce the hydrophilic character and to promote self-assembling phenomena as a consequence of the hydrophobic interaction between the graphitic planes. 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 and solvent mixture using 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 carefully characterized by elemental analysis, X-ray diffraction, FT-IR and UV-Visible spectroscopy, thermal analysis, scanning electron microscopy, BET. Electrical characterization were performed through resistive measurements in four contact geometry by using the Van der Pauw method. The evaluated sheet resistance and bulk resistivity reveal a substantially insulating character as concerns the pristine MOF while the conducibility rapidly increases with the added graphene-like percentage, ranging on several orders of magnitudes for the investigated samples.