Selective conversion of CO₂ to CO using blue TiO₂ with an r-GO shell and quantitative measurement of excited electrons with four-wave mixing microspectroscopy

The conversion of CO₂ into hydrocarbon or syngas (CO) using sunlight can address numerous current environmental issues and future challenges in the energy domain. Although numerous visible light-responsive photocatalysts have been reported, the quantum yield remains limited. Furthermore, analytical tools are yet to be established for quantitatively monitoring the amount of excited electrons, which play a critical role in catalytic reactions. The results of this study revealed that the presence of a reduced graphene oxide (r-GO) shell on blue-TiO₂ (b-TiO₂) considerably improves the photocatalytic performance in selectively converting CO₂ into CO under visible light. The formation of the r-GO shell on b-TiO₂ narrowed the b-TiO₂ bandgap. Moreover, the r-GO shell increased the absorption of visible light and facilitated electron transfer, resulting in approximately eight times the CO yield from b-TiO₂@r-GO compared to that of TiO₂. The presence of the r-GO shell improved the photocatalytic stability of b-TiO₂. Furthermore, four-wave mixing microspectroscopy was performed to analyze the amount of excited electrons. The results of microscopy revealed the amount of excited electrons in b-TiO₂@r-GO was approximately 35 times that of TiO₂. These results not only proposed a strategy for increasing the stability and efficiency of TiO₂-based photocatalysts but also are useful for evaluating the improvement of photocatalyst materials.