Psidium guajava mediated green synthesized cobalt oxide nanoparticles dispersed on reduced graphene oxide for electrocatalytic water splitting†

In this research, we synthesized (Co₃O₄)_aq and (Co₃O₄)_et nanoparticles (NPs) utilizing aqueous and ethanolic extracts, respectively, of Psidium guajava leaves. The biosynthesized NPs were sonicated with reduced graphene oxide (rGO) to produce rGO@(CO₃O₄)_aq and rGO@(Co₃O₄)_et nanocomposites (NCs) and their respective calcined (700 °C) products rGO@(CO₃O₄)_aqc and rGO@(Co₃O₄)_etc. The nanomaterials (NMs) were characterized through XRD, FTIR, UV-visible spectroscopy, SEM, TGA, and DSC analyses. They exhibited crystallite sizes of 10–15.4 nm and band gaps of 5.1–5.9 mV. Their surfaces were coated with organic moieties from plant extracts. TGA and DSC analyses showed the endothermic loss of moisture and exothermic evolution of organic contents. SEM images revealed the rough and porous surfaces of NPs, making them efficient catalysts for water splitting. Linear swap voltammetry (LSV) measurements for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), Tafel slopes and double layer capacitance (C_dl) values reflected a decrease in electrocatalytic water splitting efficiency in the following order: rGO@(Co₃O₄)_aq > (Co₃O₄)_aq > rGO@(Co₃O₄)_aqc and rGO@(Co₃O₄)_et > (Co₃O₄)_et > rGO@(Co₃O₄)_etc. Each aqueous extract-derived nanomaterial was electrocatalytically more active than its respective ethanolic extract-derived counterpart. Moreover, the non-calcined rGO decorated Co₃O₄ products showed superior electrocatalytic performance compared with their calcined counterparts and therefore, can be recommended as the best choices for electrocatalytic water splitting applications.