Porphyrin Functionalized Graphene Sheets in Aqueous Suspensions: From the Preparation of Graphene Sheets to Highly Conductive Graphene Films

Graphene has a unique structure and potential applications in nanoelectronics, nanocomposites, conductive and transparent films, etc. Synthesis of graphene is one of the major research efforts in order to make rapid developments in graphene research. In this study, a new method, that makes use of the π−π interactions between porphyrin and graphene to stabilize the chemically converted graphene (CCG), has been developed for preparation of CCG via chemical reduction of exfoliated graphene oxide (GO). Optical absorption spectroscopy measurement shows that π−π interactions take place between GO and porphyrins, 5,10,15,20-tetraphenyl-21H, 23H-porphine-p,p′,p′′,p′′′-tetrasulfonic acid tetrasodium hydrate (TPP-SO3Na) and 5,10,15,20-tetrakis(4-trimethylammoniophenyl) porphyrin tetra(p-toluenesulfonate). TPP-SO3Na functionalized CCG can form a stable aqueous suspension due to the electrostatic repulsion between the CCG sheets, which results from the aggregation of TPP-SO3Na molecules on the surfaces of the CCG sheets. Atomic force microscope observation shows that the TPP-SO3Na functionalized CCG sheets are single-layer entities, which are sandwiched by TPP-SO3Na molecules. Conductive graphene films with various thicknesses have been prepared by using the TPP-SO3Na functionalized CCG suspension via a vacuum filtration method. It is found that the change of the sheet resistance of the CCG films follows the percolation mechanism. A sheet resistance as low as ca. 5 KΩ·◻−1 of the CCG films with 80% transparency at 550 nm has been obtained. Such low sheet resistance is contributed to improved sp2 networks of the CCG sheets, low contact resistance between the CCG sheets, and the healing of the defective vacancies on CCG sheets by porphyrin molecules, which are achieved by combination of chemical reduction of GO and thermal annealing of the resultant CCG films.