Preparation of FTO/TiO2 substrates: TiO2 layers on FTO were prepared using an established doctorblading protocol. Briefly, 0.25 g TiO2 powder (Hombikat UV-100, pure anatase) was added to 1.25 mL anhydrous ethanol. The mixture was treated in ultrasonic bath for 10 min to produce a welldispersed suspension. The FTO glass substrates with a size of 1.5 × 2.5 cm were first cleaned by acetone for removing residual organic contaminants by ultrasonication for 20 min. The cleaned FTO glass was then etched in 0.1 M NaOH and rinsed with deionized water. Two FTO glass pieces were placed between microscope glasses and fixed using a 3M scotch tape as frame and spacer, leaving an exposed area of ~ 1.5 cm × 1.5 cm. Then, 200 μL TiO2 suspension was dropped on the microscope glass and gently swept by a glass stick onto the FTO glass pieces. After drying at 70 °C for 20 min, the TiO2 films were pressed at 10 4 N to improve the mechanical stability. All samples were calcined at 450 °C for 30 min in air before any tests or further treatments. The FTO/TiO2 substrates are abbreviated as TiO2 in this report.