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

Abstract

Abstract A triple‐junction nanostructured material consisting of porous exfoliated graphitic carbon nitride (g‐C 3 N 4 ) nanosheets, ZnO, and ZnCr 2 O 4 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 °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‐C 3 N 4 , and disperses mixed metal oxides on the surface of g‐C 3 N 4 owing to the slow evolution of significant amount of gases such as H 2 O, CO 2 , NH 3 , and H 2 S. The dispersion of ZnO and ZnCr 2 O 4 on the surface of the g‐C 3 N 4 exfoliated nanosheets results in a preferable resolution for visible‐light‐induced photocatalytic H 2 and O 2 evolution. An optimal g‐C 3 N 4 content (60 %) in the ZnCr 2 O 4 @ZnO/g‐C 3 N 4 nanostructured composite results in maximum H 2 (847 μmol in 2 h) and O 2 (455 μmol in 2 h) production in the presence of CH 3 OH and AgNO 3 , 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‐C 3 N 4 (Lewis base) and Zn II ions (Lewis acid), and the suppression of electron–hole recombination owing to the formation of g‐C 3 N 4 nanosheets.