The use of natural sunlight or indoor light for heterogeneous semiconductor photocatalysis is a promising alternative technology that could help address increasing environmental threats including water pollution and the energy crisis. A novel ternary hetero-junction of rGO/ZnIn₂S₄/g-C₃N₄ photo-electro-catalyst was prepared by a simple hydrothermal method.

By SEM or XRD, the morphology and the composition of the rGO/ZnIn₂S₄/g-C₃N₄, ZnIn₂S₄/g-C₃N₄, ZnIn₂S₄ and g-C₃N₄ were studied. Anode coated with rGO/ZnIn₂S₄/g-C₃N₄ has more smaller nanoparticles than others. This means that the positive effect of the layered GO and g-C₃N₄ increased the surface areas and made the catalyst more uniformly. XRD of the rGO/ZnIn₂S₄/g-C₃N₄ showed the typically peaks of ZnIn₂S₄. GO and g-C₃N₄ induce the formation and growth of ZnIn₂S₄ on the surface. The UV-vis diffuse reflection spectrums are compared in Fig. 2. Both the pure g-C₃N₄ and ZnIn₂S₄ have an absorption in the visible light region. After the combination of GO, ZnIn₂S₄ and g-C₃N₄, the enhanced absorption in the visible region is clearly observed compared to pure ZnIn₂S₄ or g-C₃N₄.

The anode was the rGO/ZnIn₂S₄/g-C₃N₄ catalyst coated on carbon fiber and carbon fiber with Pt nanoparticles was used as the cathode in a self-sustained catalytic fuel cell for triclosan degradation. Electron transferred spontaneously without additional applied voltage and this save the energy consumption. Triclosan (98.8%) was degraded after 30 minutes in the self-biased system (1000 Ω). Potential degradation mechanisms for pollutants were proposed. ZnIn₂S₄ was the P-type semiconductor and g-C₃N₄ was a N-type semiconductor. The conduction band of g-C₃N₄ is more negative than ZnIn₂S₄ and ZnIn₂S₄ is more negative than rGO. An interior bias can be produced to transfer the localized e^- of g-C₃N₄ to ZnIn₂S₄ and and combination with the h⁺ of ZnIn₂S₄. Aeration and activation of molecular oxygen by the e^- of ZnIn₂S₄ in the fuel cell, lead to the formation of reactive oxidizing species. The holes of g-C₃N₄ oxidized H2O and generated •OH for triclosan removed.

This photoelectrochemical system was conducive to the separation of photo-generated e^-/hole pairs and e^- transfer to the cathode irrespective of light. Compare to the traditional method, it can remove pollutants without light irradiation or any externally applied voltage. This new catalytic pollution control route can lower the energy consumption and may degrade many kinds of pollutants.

Figure captions:Mechanism of the self-biased photoelectrochemical system.