Mechanistic Insights into the Photocatalytic Indigo Carmine Dye Decolorization by CoO/TiO.

TiO is widely studied as an efficient UV-light photocatalyst for organic compound degradation through reactive oxygen species (ROS) generation. TiO can be modified to show photocatalytic activity under visible light illumination by combining with visible-light absorbing metal oxides. Here, we investigated CoO/TiO composite materials as visible-light absorbing photocatalysts, with various weight loadings of CoO, for the decolorization of wastewater pollutant indigo carmine. Under green LED light, 1.4 wt% CoO/TiO showed the highest decolorization rate compared to other weight loadings and bare TiO. While UV-Vis spectroscopy indicated that CoO/TiO composite materials and bare TiO cause similar dye decolorization behavior, NMR spectroscopy showed that after 24 h, reaction products were present in the reaction mixture for 1.4 wt% CoO/TiO, while TiO showed no reaction products. The lack of photocatalytic activity of CoO/zeolite and other CoO/oxide composite materials suggests a synergistic effect between CoO and TiO, where a small amount of CoO enables TiO 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.