The potential-dependent polaron formation is also probed by electrostatic force microscopy (EFM) measurements, which are sensitive to the surface charges. The EFM topography image in Figure displays a uniform profile of the sample surface after being subjected to a range of tip biases from -1 V to -7 V, suggesting that no structural deterioration has taken place in the sample. The EFM 1ω signal mapping in Figure shows that when the TiO2 sample is exposed to negative tip biases, a noticeable surface charge appears. When the applied voltage is further decreased from -1 V to -2 V, the 1ω signal increases, illustrating the potential-dependent local charge accumulation, see Figure . We ascribe the observed EFM signals to the aggregation of Ti 3+ polarons on the surface. This assignment is in line with the absence of pronounced features in the 2ω signal, suggesting that the dielectric constant and thus the stoichiometry of the TiO2 sample remains unchanged under different electrode potential conditions see Figure .