A highly efficient g-C3N4/SiO2 heterojunction: the role of SiO2 in the enhancement of visible light photocatalytic activity

Abstract

SiO₂, an insulator, hardly has any photocatalytic acitivity due to its intrinsic property, and it is generally used as a hard template to increase the surface area of catalysts. However, in this work, we found that the surface state of the insulator SiO₂ can promote the migration of photogenerated charge carriers, leading to the enhancement of the photooxidation ability of graphitic carbon nitride (g-C₃N₄). A one-pot calcination method was employed to prepare g-C₃N₄/SiO₂ composites using melamine and SiO₂ as precursors. The composites present considerably high photocatalytic degradation activities for 2,4-dichlorophenol (2,4-DCP) and rhodamine B (RhB) under visible light (λ > 420 nm) irradiation, which are about 1.53 and 4.18 times as high as those of bulk g-C₃N₄, respectively. The enhancement of the photocatalytic activity is due to the fact that the introduction of the insulator SiO₂ in g-C₃N₄/SiO₂ composites can greatly improve the specific surface area of the composites; more importantly, the impurity energy level of SiO₂ can help accelerate the separation and transfer of electron–hole pairs of g-C₃N₄. Electron paramagnetic resonance (EPR) spectroscopy and trapping experiments with different radical scavengers show that the main active species of g-C₃N₄ are superoxide radicals, while holes also play a role in photodegradation. For g-C₃N₄/SiO₂-5, besides superoxide radicals and holes, the effect of hydroxyl radicals was greatly improved. Finally, a possible mechanism for the photogenerated charge carrier migration of the g-C₃N₄/SiO₂ photocatalyst was proposed.