Photocatalytic CO₂ Reduction Using Ti₃C₂X (X = Oxo, OH, F, or Cl) MXene-ZrO₂: Structure, Electron Transmission, and...

CO₂ photoreduction using a semiconductor-based photocatalyst is a promising option for completing a new carbon-neutral cycle. The short lifetime of charges generated owing to light energy is one of the most critical problems in further improving the performance of semiconductor-based photocatalysts. This study shows the structure, electron transmission, and stability of Ti₃C₂X (X = oxo, OH, F, or Cl) MXene combined with a ZrO₂ photocatalyst. Using H₂ as a reductant, the photocatalytic CO formation rate increased by 6.6 times to 4.6 μmol h^-1 g_cat^-1 using MXene (3.0 wt %)-ZrO₂ compared to that using ZrO₂, and the catalytic route was confirmed using ¹³CO₂ to form ¹³CO. In clear contrast, using H₂O (gas) as a reductant, CH₄ was formed as the major product using Ti₃C₂X MXene (5.0 wt %)-ZrO₂ at the rate of 3.9 μmol h^-1 g_cat^-1. Using ¹³CO₂ and H₂O, ¹²CH₄, ¹²C₂H₆, and ¹²C₃H₈ were formed besides H₂¹²CO, demonstrating that the C source was the partial decomposition and hydrogenation of Ti₃C₂X. Using the atomic force and high-resolution electron microscopies, 1.6 nm thick Ti₃C₂X MXene sheets were observed, suggesting ∼3 stacked layers that are consistent with the Ti-C and Ti···Ti interatomic distances of 0.218 and 0.301 nm, respectively, forming a [Ti₆C] octahedral coordination, and the major component as the X ligand was suggested to be F and OH/oxo, with the temperature increasing by 116 K or higher owing to the absorbed light energy, all based on the extended X-ray absorption fine structure analysis.