Photoelectrocatalytic PNP removal using C3N4 nanosheets/α-Fe2O3 nanoarrays photoanode: Performance, mechanism and degradation pathways

Highlights: • Enhanced light absorption was obtained by integrating C₃N₄ with Fe₂O₃. • C₃N₄/Fe₂O₃ heterojunction facilitated the separation of photogenerated carriers. • The applied bias extracted e^- to external circuit and thus improved the quantum yield. • The synergy of photocatalysis and electrocatalysis was achieved using C₃N₄ NSs/Fe₂O₃ NRAs. • The photoelectrocatalysis using developed photoanode was caused by e^-, ·OH and h⁺. The massive discharge of p-nitrophenol (PNP) has adversely affected ecosystem and human health. In this work, the g-C₃N₄ nanosheets was integrated with α-Fe₂O₃ nanorod arrays (C₃N₄ NSs/Fe₂O₃ NRAs) through simple in-situ hydrothermal and electrochemical deposition, and applied for the photoelectrocatalytic degradation of PNP. At the optimum experiment conditions at PNP initial concentration of 10 mg/L, pH value of 6.1, electrolyte concentration of 0.2 mol/L, and bias voltage of 1.5 V, the optimized photoanode displayed the highest PNP removal rate. This improvement was attributed to the synergistic effect between photocatalysis and electrocatalysis, a synergy factor of 1.6 was achieved during this C₃N₄ NSs/Fe₂O₃ NRAs photoelectrocatalytic system. The C₃N₄/Fe₂O₃ heterojunction and bias voltage facilitated the generation, separation and transfer of electron/hole pairs. The electrolysis also provided rich active species, hydroxyl radicals, etc., which cooperated with the reactive oxygen species derived from enhanced photocatalysis to achieve rapid pollutants decomposition. Based on the intermediates detection and analysis, two PNP degradation pathways, namely direct oxidation and reduction–oxidation path, were proposed, that matched well the existence form and transformation rule of inorganic nitrogen. The radical capture experiments demonstrated that electrons, hydroxyl radicals and holes were the main species responsible for PNP photoelectrocatalytic removal.