We succeeded in observing surface changes in p-type silicon in aqueous chloroplatinic acid solution under illumination or in the dark. The scanning tunneling microscopy (STM) image obtained after cathodic polarization in the dark implies that the crystal growth of deposited platinum predominates over nucleation. However, the STM image obtained after cathodic polarization under illumination implies that nucleation predominates in the initial deposition. The explanation for this behavior is that electron transfer occurs at a relatively high overpotential under illumination, i.e. the charge transfer potential of the electron under illumination is higher than that in dark.