Photocatalytic Degradation Of Dyes Over MoO₃, SnO_2-x And Zn₂SnO₄ Modified G-C₃N₄ Composites

Photocatalysis has attracted more and more attention as an ideal technology for the treatment of environmental pollution and clean energy production in recent years. Therefore, it is highly desirable to develop photocatalysts with high activities under visible-light irradiation. Great efforts have been devoted on the developing of visible-light driven photocatalysts. Polymeric graphitic carbon nitride (g-C3N4) is a novel metal free visble-light-driven semiconductor with narrow band gap energy of 2.7 eV. It has the merit of low cost, and can be easily prepared via numerous facile methods. Furthermore, g-C3N4 has the properties of high thermal and chemical stability. These advantages indicate that the metal-free g-C3N4 has promising potential in the photocatalysis fields. However, the photocatalytic activity of g-C3N4 is still low. The modification of g-C3N4 is needed to. improve its photocatalytic activity. Therefore, in this paper, several studies were made to improve the visible-light photocatalytic performance of g-C3N4 photocatalysts.First, we have prepared a novel MoO3/g-C3N4 composite photocatalyst by the milling and heating method. Only MoO3 and g-C3N4 are detected via XRD and FT-IR analyses. N2 adsorption and DRS results suggest that the addition of MoO3 slightly affects the specific surface area and the photoabsorption performance. The high photocatalytic activity of MoO3/g-C3N4 is mainly attributed to the synergetic effect of MoO3 and g-C3N4 in electron-hole pair separation via the charge migration between the two semiconductors. The charge transfer follows direct Z-scheme mechanism, which is proven by the reactive species trapping experiment and the ·OH-trapping photoluminescence spectra.Second, high-efficient SnO2-x/g-C3N4 composite photocatalysts were synthesized using a simple calcination of g-C3N4 and Sn6O4(OH)4. The synthesized composite exhibited excellent photocatalytic performance for Rhodamine B (RhB) degradation under visible light irradiation. In addition, the SnO2-x/g-C3N4 composite also showed high activity for photodegradation of other organics. The thorough investigation indicated that the introduction of SnO2-x on g-C3N4 promoted its surface area and light absorption performance. However, more important was the formation of a hetero-junction structure between SnO2-x and g-C3N4, which efficiently promoted the separation of electron-hole pairs by a direct Z-scheme mechanism to enhance the photocatalytic activity.Finally, we prepared a novel photocatalyst, Zn2SnO4/g-C3N4 composite via simple calcination of Zn2SnO4-melamine mixture. The synthesized composite was characterized by various techniques including BET, XRD, SEM,TEM, XPS, DRS, and PL. Characterization results indicated that the decoration of Zn2SnO4 on g-C3N4 showed nearly no effect on its light absorption performance, but promoted the surface area of the composite, which benefited the adsorption of dyes and the subsequent photocatalytic process. However, more significant was the formation of hetero-junction structure between Zn2SnO4 and g-C3N4, which efficiently promoted the separation of electron-hole pairs and enhanced the photocatalytic performance of Zn2SnO4/g-C3N4.