Enhanced charge separation in g-C3N4–BiOI heterostructures for visible light driven photoelectrochemical water splitting

Abstract

Heterojunctions of the low bandgap semiconductor bismuth oxyiodide (BiOI) with bulk multilayered graphitic carbon nitride (g-C₃N₄) and few layered graphitic carbon nitride sheets (g-C₃N₄-S) are synthesized and investigated as an active photoanode material for sunlight driven water splitting. HR-TEM and elemental mapping reveals formation of a unique heterostructure between BiOI platelets and the carbon nitride (g-C₃N₄ and g-C₃N₄-S) network that consisted of dendritic BiOI nanoplates surrounded by g-C₃N₄ sheets. The presence of BiOI in g-C₃N₄-S/BiOI and g-C₃N₄-S/BiOI nanocomposites extends the visible light absorption profile from 500 nm up to 650 nm. Due to excellent charge separation in g-C₃N₄/BiOI and g-C₃N₄-S/BiOI, evident from quenching of the carbon nitride photoluminescence (PL) and a decrease in the PL lifetime, a significant increase in photoelectrochemical performance is observed for both types of g-C₃N₄–BiOI heterojunctions. In comparison to heterojunctions of bulk g-C₃N₄ with BiOI, the nanocomposite consisting of few layered sheets of g-C₃N₄ and BiOI exhibits higher photocurrent density due to lower recombination in few layered sheets. A synergistic trap passivation and charge separation is found to occur in the g-C₃N₄-S/BiOI nanocomposite heterostructure which results in a higher photocurrent and a lower charge transfer resistance.