Porous graphene oxide frameworks: Synthesis and gas sorption properties

We report detailed synthesis of a range of porous graphene oxide frameworks (GOFs) by expansion of graphene oxide (GO) sheets with various linear boronic acid pillaring units in a solvothermal reaction. The GOF structures develop through boronate-ester formation as a result of B-O bonding between boronic acids and oxygen functional groups on the GO layers. Synthesized GOFs exhibit periodic layered structures with largely expanded interlayer spacing as characterized by X-ray powder diffraction (XRD). The boronate-ester link formation is further evidenced by Fourier transform infrared (FTIR) and Raman spectroscopy. Furthermore, the strong boronate-ester bonds between GO layers results in improved thermal stability over the precursor GO. The solvent-free, evacuated frameworks provide highly increased accessible surface area for nitrogen adsorption compared to GO alone, which depends on the type and length of the boronic acid, indicating the importance of pillaring unit. Both isosteric heat of adsorption (Q(st)) and the adsorbed hydrogen capacity per surface area are twice as large as typical porous carbon materials and comparable to metal-organic frameworks (MOFs) with open metal centers. This enhanced Q(st) and adsorption capacity is attributed to optimum interlayer spacing between graphene planes such that hydrogen molecules interact with both surfaces. Finally, our systematic study reveals the profound effect of both synthesis and activation temperatures to obtain porous framework structures.