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

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. HRTEM 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 gC3N4 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.