Fluorine-doped SnO2 (FTO) substrates (TEC 7 from Pilkington) were cleaned with soap and water. The substrates were then immersed for 20 min in a concentrated 2.2 M NaOH in ethanol/water (10:1 v/v %) and rinsed with deionized water in an ultrasonic bath for 15 min. The substrates were subsequently heated at 500°C for 15 min.