Unravelling the photo-excited chlorophyll-a assisted deoxygenation of graphene oxide: formation of a nanohybrid for oxygen reduction

Abstract

We attempted to expand the horizons of graphene oxide (GO) reduction methods using chlorophyll-a (CHL-a), which is inclined to react favourably with GO by virtue of photo-excited electron transfer from the singlet excited CHL-a LUMO (−0.7 V) to GO (−0.4 V) in aqueous media assisted by the interactive affinity between CHL-a and reduced graphene oxide (RGO), which is ensured through π–π interaction between the CHL-a macro-cycle and the GO surface. This results in the formation of a CHL-a⁺ radical cation which might favor the oxidation of water with oxygen evolution. The formation of RGO after photo-exposure can also be confirmed via TEM and Raman spectroscopy. Gradual restoration of sp² hybridisation in the GO framework with increasing CHL-a concentration can be correlated with the enhanced contribution of the conformation in which electron transfer is efficient from CHL-a to GO (also supported by XPS and XRD data). This fact corroborates the faster component (a₁) augmentation with increasing CHL-a concentration towards overall excited state lifetime. The applicability of this RGO/CHL-a nano-hybrid as a possible electro-catalyst, to be used for oxygen reduction in energy conversion systems such as fuel cells, has also been explored through cyclic voltammetry. All these results cumulatively highlight the effective, environmentally friendly mechanism of the photo-excited CHL-a assisted deoxygenation of GO in aqueous media, which eventually gives rise to a RGO/CHL-a nano-hybrid as a potential electro-catalyst in next generation bio-fuel cells.