Interpreting the enhanced photoactivities of 0D/1D heterojunctions of CdS quantum dots/TiO2 nanotube arrays using femtosecond transient absorption spectroscopy

Heterojunctions of CdS quantum dots (QDs)/TiO2 nanotube arrays (NTAs) were synthesized via a simple vapor transport approach. The 0D/1D QDs/NTAs electrodes exhibited 5-fold improvement efficiency for photocatalytic dye degradation and hydrogen evolution compared to pure TiO2 NTAs in a photoelectrochemistry cell, which was attributed to the faster transportation and retarded recombination of electron-hole pairs in the 0D/1D heterostructures. Femtosecond transient absorption offers deep insights into the dynamics of the charge carriers, showing that surface-trapped electrons were generated within 1.2 ps. The lifetimes for recombination of electrons at shallow trap sites with holes and deep trapped electrons with holes are prolonged to 73.2 ps and 622.6 ps, respectively. The reduced charge transfer resistance is verified by electrochemical impedance spectroscopy. Mott-Schottky plots indicated downward shifts for both the conduction band and valence band in the heterostructure. Finally, a Z-scheme photoreactive mechanism is proposed to explain the enhanced catalytic activities.