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

Abstract

This study investigated a double-heterojunction of a ZnO/α–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, α–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 α–Fe2O3 and g–C3N4 species were homogeneously covered and chemically connected on the surface of ZnO nanorods. The photoelectrocatalytic (PEC) performance of the ZnO/α–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/α–Fe2O3 and ZnO photoanode, respectively. The ZnO/α–Fe2O3/g–C3N4 photoanode generated 29.28 μmol/cm2 of H2 and 14.15 μmol/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/α–Fe2O3/g–C3N4 photoanode provided 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.