Easy preparation of recyclable thermally stable visible-light-active graphitic-C3N4/TiO2 nanocomposite photocatalyst for efficient decomposition of hazardous organic industrial pollutants in aqueous medium

Abstract

Graphitic-C3N4/TiO2 nanocomposite was prepared as a photocatalyst (PC) active under visible light (λ ≥ 420 nm) by preparation of graphitic carbon nitride (g-C3N4) from melamine followed by an effective easy impregnation method. Several g-C3N4/TiO2 composites containing 1 to 12 wt% g-C3N4 were synthesized and characterized using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential thermal analysis (DTA), photoluminescence (PL) spectroscopy, diffusion reflectance spectroscopy (DRS), and Brunauer–Emmett–Teller (BET) measurements. A photocatalytic mechanism is proposed based on the relative positions of the energy bands of the two constituents. Compared with its individual components, g-C3N4/TiO2 demonstrated unusually high photocatalytic activity for phenol decomposition in aqueous phase under visible-light irradiation. The heterojunction was optimized in the 5 wt% g-C3N4/TiO2 nanocomposite due to the well-matched bandgap structure (optimum loading) and excellent electron–hole pair separation in the conduction and valence band of TiO2 and g-C3N4, respectively. After 2 h of visible-light irradiation, 68 % degradation was observed when using this optimum composition. The performance was slightly decreased (to 66 %) after recycling of the catalyst four times (used a total of five times), but remained reliable for industrial applications considering other factors. In this system, TiO2 (Degussa P25) seems to play the principal PC role, while g-C3N4 acts as a sensitizer for absorption of visible light. Due to the enhanced visible-light absorption ability enabled by g-C3N4 in the composite, stable electron–hole (e−–h+) pairs produced at the interface of the heterojunction lead to generation of highly reactive free radicals (·O2, ·OH, etc.) which together initiate degradation of phenol but individually suffer from some limitation that must be overcome. The thermal stability and recycling efficiency of this PC will enable its use in industrial applications as a cost-effective sustainable cleanup candidate. The prepared g-C3N4/TiO2 exhibits stable electron–hole (e−–h+) pair separation at the heterojunction under visible light for enhanced degradation of organic pollutants via a redox mechanism. The g-C3N4 loading affects the photocatalytic activity, with the 5 wt% g-C3N4/TiO2 composite exhibiting the highest degradation, with recycling.