In situ EFM experiments were performed on a commercial atomic force microscope (AFM, Dimension Icon, Bruker) using a conductive AFM probe (SCM-PIT-V2, Pt/Ir-coated, Bruker). Before the EFM measurements, to induce local charges at different points on the TiO2 film, the AFM probe was biased with different voltages and brought into contact with the sample at discrete points. The EFM was operated under a dual-pass tapping mode. On the first pass, the topographical image was obtained without applying any voltage between the probe and the sample. On the second pass, an external bias was applied to the conductive probe as it was lifted by a constant height of 20 nm above the sample surface. The phase shift signal of the cantilever was fed into two lock-in amplifiers to demodulate the 1ω and 2ω components.