Enhanced Charge Separation of TiO₂ Nanotubes Photoelectrode for Efficient Conversion of CO₂

For the production of TiO₂ nanotubes (TNTs) efficient photoelectrodes they must have efficient charge separation by trapping holes and transfer of electrons. In this study, MnO_x and Pd codecorated TNTs photoelectrodes were successfully constructed using a simple impregnation method, followed by an electrochemical deposition process. The photocatalytic activities for CO₂ conversion by the optimized TNTs photoelectrode (10Pd/0.8Mn/TNTs) were increased by 2.8 times to produce 40.3 ± 2.5 mg L^-1 acetic acid, and by 2.5 times to generate 24.6 ± 1.9 mg L^-1 formic acid compared to a bare TNTs photoelectrode. The optimized photoelectrode also showed the highest transient photocurrent of 1.15 mA cm^-2. The improved performance was due to the elevated charge separation through bidirectional modulation of photogenerated holes and electrons, on the basis of the steady-state surface photovoltage and analysis with the formed •OH concentrations, electrochemical reduction tests with N₂ or CO₂ atmospheres, and electrochemical impedance spectra. The decorated MnO_x effectively trapped the photogenerated holes, and the decorated Pd facilitated photogenerated electron transfer and promoted visible light absorption. The decorated MnO_x and Pd also played catalytic roles in the redox reactions involved with the photogenerated charge carriers.