Insight into the role of the oxidized graphite precursor on the properties of copper-based MOF/graphite oxide composites

Abstract

Graphites of different origin were oxidized using Brodie method and then used as composite components with copper-based metal–organic frameworks, MOF. The degree of oxidation, crystallite size and chemistry of oxygen-containing groups on the surface of graphite oxides were investigated. The parent materials, MOF and graphite oxides, and the composites were characterized by X-ray diffraction, adsorption of nitrogen, X-ray photoelectron spectroscopy, elemental analysis, scanning electron microscopy in combination with energy dispersive X-ray spectroscopy, transmission electron microscopy, thermal analysis and Fourier transform infrared spectroscopy. The results show that not only epoxy groups but also carboxylic groups at the edges of graphene layers, when present in a predominant quantity, are engaged in the formation of metal–organic framework units. The contribution of the latter mechanism of the composite formation results in a more pronounced synergistic effect, demonstrated in the formation of new pores, and the distorted MOF units at the edges of the graphene layers. When carboxylic groups are absent or present in a small quantity, epoxy, phenol and carbonyl groups are the main reactive sites for the composite formation. The disruption of the MOF crystallites, governed by the geometry and chemistry of graphite oxides, also leads to the formation of copper salts at the edges of the graphene crystals. The morphology of the composite materials and their apparent chemistry depends on the chemistry of graphite oxide, its crystallite size and its amount in the composite.