ZnCr2O4@ZnO/g-C3N4: A Triple-Junction Nanostructured Material for Effective Hydrogen and Oxygen Evolution under Visible Light

A triple-junction nanostructured material consisting of porous exfoliated graphitic carbon nitride (g-C3N4) nanosheets, ZnO, and ZnCr2O4 is prepared by a one-pot synthesis method through the calcination of a mixture of urea, thiourea, and Zn-Cr layered double hydroxide (LDH) at 450 degrees C. The structural, morphological, and optical properties of the prepared nanocomposites are characterized by various physicochemical techniques. This synthesis process simultaneously makes the material porous, produces exfoliated sheets of g-C3N4, and disperses mixed metal oxides on the surface of g-C3N4 owing to the slow evolution of significant amount of gases such as H2O, CO2, NH3, and H2S. The dispersion of ZnO and ZnCr2O4 on the surface of the g-C3N4 exfoliated nanosheets results in a preferable resolution for visible-light-induced photocatalytic H-2 and O-2 evolution. An optimal g-C3N4 content (60%) in the ZnCr2O4@ZnO/g-C3N4 nanostructured composite results in maximum H-2 (847 mu mol in 2h) and O-2 (455 mu mol in 2h) production in the presence of CH3OH and AgNO3, respectively, as sacrificial reagents. The apparent conversion efficiencies for H-2 and O-2 evolution are 28.01 and 15.0%, respectively. The increased photocatalytic activity is attributed to the proper alignment of the band structure, synergistic effects owing to the good coordination between g-C3N4 (Lewis base) and Zn-II ions (Lewis acid), and the suppression of electron-hole recombination owing to the formation of g-C3N4 nanosheets.