Enhanced photocatalytic CO₂ reduction and 2,4-dichlorophenol degradation of TiO₂ nanotubes via bi-directionally controlling electrons and holes

In this study, TiO₂ nanotubes (TiNTs) co-modified with MnO_x (Mn) and reduced-graphene-oxide (RGO) were prepared by a simple electrochemical deposition process. The photocatalytic activities for reducing CO₂ and degradating 2,4-dichlorophenol of TiNTs were enhanced after co-modification with proper amounts of MnO_x and RGO. The yields of acetic acid and formic acid of the amount-optimized TiNTs (6RGO/4Mn/TiNTs) were 36.3 ± 0.9 and 21.6 ± 0.7 mg L⁻¹ h⁻¹, which were ∼2.5 times and ∼2.2 times of bare TiNTs and the 2,4-dichlorophenol degradation efficiency of 6RGO/4Mn/TiNTs was 36.9% for 1 h, which was 21.3% higher than bare TiNTs. According to the results of photophysical and photochemical experiments, the enhanced photocatalytic activity was attributed to the greatly-elevated photogenerated charge separation via the bi-directional control mechanism of electrons and holes. The photogenerated holes could be captured by MnO_x and the transfer of photogenerated electrons could be accelerated through the RGO. In addition, the formed [rad]OH was found to act as major active species in the 2,4-dichlorophenol degradation process. The degradation paths by [rad]OH attacking was proposed by analyzing the main intermediates.