Improvement of surface light absorption of ZnO photoanode using a double heterojunction with α-Fe2O3/g-C3N4 composite to enhance photoelectrochemical water splitting

This study investigated a double-heterojunction of a ZnO/alpha-Fe2O3/g-C3N4 photoanode to overcome each of their intrinsic drawbacks for use as photocatalysts or photoelectrocatalysts ZnO thin film having a wide bandgap and low reduction potential, alpha-Fe2O3 having insufficient reduction potential, and g-C3N4 having low oxidation potential as photocatalysts. The elemental and chemical mapping of the materials species and free metal ions were investigated for the first-time using time-of-flight-secondary ion mass spectrometry (TOF-SIMS), which is a surface-sensitive analytical method. The depth profile of TOF-SIMS indicated that the alpha-Fe2O3 and g-C3N4 species were homogeneously covered and chemically connected on the surface of ZnO nanorods. The photo-electrocatalytic (PEC) performance of the ZnO/alpha-Fe2O3/g-C3N4 photoanode was obtained, and it showed a photocurrent density of 0.97 mA/cm2 at 1.23 VRHE and under 100 mW/cm2 illumination, which was (1.6 and 4.8) fold greater than those of the ZnO/alpha-Fe2O3 and ZnO photoanode, respectively. The ZnO/alpha-Fe2O3/g-C3N4 photoanode generated 29.28 mu mol/cm2 of H2 and 14.15 mu mol/cm2 of O2 at 2 h illumination of 100 mW/cm2 light along with 1.23 VRHE. The inclusion of the dual heterojunction in the ZnO/alpha-Fe2O3/g-C3N4 photoanode pro-vided lower charge transfer resistance (Rct) and higher electrochemical active surface area (ECSA), which led to simultaneous enhancements in the electron-hole separation and surface light absorption at higher wavelengths, ultimately resulting in substantially improved PEC performance.