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

Abstract

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 blueTiO2 (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.