Rational design of a g-C3N4 quantum dots-based S-scheme heterojunction with built-in electric field and π-π* transitions for PMS-activated photo-Fenton-like tetracycline hydrochloride removal and piezophotocatalytic CO2 reduction.

Pharmaceutical contamination of water and rising atmospheric CO2 are defining environmental burdens, yet are almost always addressed separately, even though semiconductor photoexcitation generates oxidizing holes and reducing electrons capable of addressing both. This work aimed to develop a single catalyst coupling peroxymonosulfate (PMS)-activated antibiotic degradation with piezo-photocatalytic CO2 reduction. A quaternary-doped Ni0.94Fe0.02Zr0.02Nd0.02O1-ᵨ (NFZNO)/g-C3N4 QDs/NiMoO4 (NGN) heterojunction was prepared by ultrasonic-assisted wet impregnation and characterized by XRD, FTIR, HR-TEM, XPS and EIS, supported by DFT calculations. Under visible light with PMS activation, NGN removed 98.73% of tetracycline hydrochloride within 40 min and achieved 63.2% mineralization after 300 min; scavenging experiments identified •OH and SO4•- as the principal oxidants, while LC-MS and ECOSAR analyses showed the degradation intermediates to become progressively less toxic. Response surface optimization predicted 97.96% removal. The same material reduced CO2 to CH4 at 472.08 μmol g-1 h-1 under coupled ultrasonic and visible-light excitation, a 2.72-fold gain over photocatalysis alone, with CH4 accounting for more than 98.5% of the electrons consumed under all conditions examined; 13CO2 labelling confirmed the carbon source. That selectivity remained unchanged across a 2.7-fold change in turnover, indicating that the piezoelectric field enhances carrier flux rather than altering product branching. These results show that oxidative water treatment and reductive CO2 valorization can both be driven by a single bifunctional catalyst.