Structures self-assembled from anionic graphene and cationic manganese porphyrin: Characterization and application in artificial photosynthesis

Carboxylic-rich graphene oxide (CGO) and Mn^III tetrakis(N-methyl-4-pyridinio)porphyrin (MnTMPyP) form self-assembled leaf-like structures. These were obtained through electrostatic and π-π stacking interactions, as indicated by the redshift of the metalloporphyrin Soret band throughout the pH range of 6-12. Voltammetry and XPS measurements confirmed that CGO does not significantly affect the chemical environment of the metal ion in MnTMPyP. However, the effect of CGO is apparent when determining by rotating ring disk electrode (RRDE) voltammetry the ability of CGO-Mn^IVTMPyP films to catalyze water oxidation and evolve oxygen. The amount of O₂ evolved at pH 10 and 1.6 V versus Ag/AgCl is 50 % higher than that of MnTMPyP films. When used for the photoelectrochemical oxidation of water, CGO-MnTMPyP films on iron/hematite foams exhibited substantial activity, as evidenced by the current density (0.54 mA/cm² at 1.23 V vs. NHE) and incident photons to current efficiency (IPCE; 9.0 % at 1.43 V vs. NHE) measured at pH 10.