Mechanistic Insights into the Photocatalytic Indigo Carmine Dye Decolorization by Co₃O₄/TiO₂

TiO₂ is widely studied as an efficient UV-light photocatalyst for organic compound degradation through reactive oxygen species (ROS) generation. TiO₂ can be modified to show photocatalytic activity under visible light illumination by combining with visible-light absorbing metal oxides. Here, we investigated Co₃O₄/TiO₂ composite materials as visible-light absorbing photocatalysts, with various weight loadings of Co₃O₄, for the decolorization of wastewater pollutant indigo carmine. Under green LED light, 1.4 wt% Co₃O₄/TiO₂ showed the highest decolorization rate compared to other weight loadings and bare TiO₂. While UV-Vis spectroscopy indicated that Co₃O₄/TiO₂ composite materials and bare TiO₂ cause similar dye decolorization behavior, NMR spectroscopy showed that after 24 h, reaction products were present in the reaction mixture for 1.4 wt% Co₃O₄/TiO₂, while TiO₂ showed no reaction products. The lack of photocatalytic activity of Co₃O₄/zeolite and other Co₃O₄/oxide composite materials suggests a synergistic effect between Co₃O₄ and TiO₂, where a small amount of Co₃O₄ enables TiO₂ to utilize visible light without compromising the surface area available for ROS creation. Lastly, we emphasize the need to be cautious when drawing conclusions regarding the dye degradation, since we showed that decolorization does not necessarily equate to full degradation, using a unique combination of UV-Vis and nuclear magnetic resonance spectroscopy.