FTO conductive glass sheets (Xingyan Tech. Ltd, Hong
Kong) were used as substrates. TiO2 paste was prepared by
adding 50 mg of TiO2 nanopowder to 100 mg of
polyethylene glycol then grinding the mixture for half an
hour until a homogeneous paste was obtained. The paste was
deposited on the FTO substrates using doctor-blade method
to obtain the TiO2 film. The area of the TiO2 layer was
restricted to 0.25 cm2. The TiO2 layer was sintered in an
oven at 450 °C for 40 min. The films were left in the oven to
cool down slowly to 60 °C. The thickness of the TiO2 layer
was measured using Olypus Polarizing Microscope BX53-P
equipped with DP73 camera, and found to be 21±1 μm. The
TiO2 films were then immersed in the natural extracts for one
day