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

Abstract

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.