Boosting photocatalytic oxidation of organic pollutants on Z-scheme carbon nitride/Ag3PO4/Ag composites by coupling with carbon nanotubes

The present study reports the development of ternary and quaternary carbon nitride (CN) based nanocomposites through the coupling of CN with single-walled carbon nanotubes (SWCNTs), Ag3PO4 (AP) and Ag nanoparticles (NP). A facile wet-chemistry process was developed for the synthesis of the composites. The prepared materials were characterised using XRD, ATR, DR-UV-Vis, photoluminescence (PL) spectroscopy and EPR. Photocatalytic dye degradation (rhodamine b, RhB) is greatly improved in composite materials. The most active photocatalyst was the quaternary composite bearing SWCNTs, Ag3PO4 and Ag NP, that presented a 2-and a 3-fold increase in photoactivity compared with the CN functionalised only with SWCNTs and the pure CN, respectively. The enhanced activity is associated with improvements in several important mate-rials properties. Adsorption in the visible light is improved by the presence of Ag3PO4 and Ag NP. The latter provided plasmon resonance effects, absorbing light up to 700 nm. The presence of SWCNTs reduced charge recombination phenomena reducing by half the PL intensity. In addition, charge availability was further increased due to the formation of heterojunctions through the coupling of CN with Ag3PO4 and Ag NP. The PL intensity of the ternary composite was reduced to 1% of the pure CN PL intensity. In-situ EPR spectroscopy using spin-traps revealed the formation of superoxide anion radicals (O-2(center dot)) that provide evidence for charge transfer via a Z-scheme mechanism. O-2(center dot) production was enhanced 8.5 times in the ternary composite compared with the pure CN. The importance of the stepwise functionalization of CN through the formation of ternary and quaternary composites towards the development of advanced photoactive materials is highlighted herein.