Efficient Bias-Free Degradation of Sulfamethazine by TiO₂ Nanoneedle Arrays Photoanode and Co₃O₄ Photocathode System under LED-Light Irradiation

Solving high electrical-energy input for pollutants degradation is one of the core requirements for the practical application of photoelectrocatalytic (PEC) technology. Herein, we developed a self-driven dual-photoelectrode PEC system (TiO₂ NNs-Co₃O₄) composed of a TiO₂ nanoneedle arrays (TiO₂ NNs) photoanode and Co₃O₄ photocathode for the first time. Under light-emitting-diode (LED) illumination, the bias-free TiO₂ NNs-Co₃O₄ PEC system exhibited excellent PEC performance, with an internal bias as high as 0.19 V, achieving near complete degradation (99.62%) of sulfamethazine (SMT) with a pseudo-first-order rate constant of 0.042 min⁻¹. The influences of solution pH, typical inorganic anions, natural organic matter, and initial SMT concentration on the PEC performance were investigated. Moreover, the main reactive oxygen species (h⁺, ^•OH, ^•O₂⁻) in the dual-photoelectrode PEC system for SMT decomposition were elaborated. The practical application feasibility for efficient water purification of this unbiased PEC system was evaluated. It was proved that the TiO₂ NNs photoanode provided a negative bias while the Co₃O₄ photocathode provided a positive bias for the photoanode, which made this system operate without external bias. This work elucidated the cooperative mechanism of photoelectrodes, providing guidance to develop a sustainable, efficient, and energy-saving PEC system for wastewater treatment.