Decoupling ROS generation and photocatalytic activity in Au-Ag-Pt/g-C3N4 systems under visible light.

The incorporation of noble and plasmonic metals into graphitic carbon nitride (g-C₃N₄) is widely used to enhance visible-light photocatalytic performance; however, the relationship between reactive oxygen species (ROS) generation and pollutant degradation efficiency remains insufficiently understood. In this work, a composition-controlled Au-Ag-Pt/g-C₃N₄ photocatalyst platform was designed with a fixed total metal loading of 1 wt.% to distinguish genuine interfacial effects from simple loading contributions. Monometallic, bimetallic, and trimetallic systems were systematically investigated under visible-light irradiation.

Powder X-ray diffraction and nitrogen physisorption confirmed preservation of the g-C₃N₄ framework and comparable surface areas across the catalyst series, indicating that photocatalytic differences primarily originate from electronic effects. Excited-state dynamics studied by time-correlated single photon counting revealed distinct metal-dependent charge-transfer behaviour. Au prolonged the average photoluminescence lifetime from 4.37 ns for pristine CN to 7.87 ns, whereas Pt and Ag shortened the lifetime to 3.61 and 2.88 ns, respectively, indicating efficient interfacial electron extraction.

Hydroxyl radical generation was quantified using the coumarin fluorescence method. Au/g-C₃N₄ exhibited the highest absolute •OH formation, while Ag/Pt displayed the strongest positive radical synergy. However, photocatalytic degradation of bisphenol A (BPA) revealed that Pt/g-C₃N₄ achieved the highest degradation efficiency despite comparatively lower radical intensity. Furthermore, systems exhibiting strong •OH synergy did not necessarily show enhanced BPA conversion.

The results demonstrate a clear decoupling between hydroxyl radical generation, charge-carrier lifetime, and photocatalytic degradation efficiency. Rather than maximising radical production, the metal identity primarily governs oxidative pathway selectivity and interfacial charge-transfer processes. Pt-containing systems likely promote kinetically favourable oxidation pathways involving efficient charge utilisation, while Au primarily enhances •OH formation without maximising pollutant degradation. These findings highlight the importance of pathway engineering, interfacial charge-transfer control, and mechanistic understanding for the rational design of multimetal photocatalysts under visible-light irradiation.