High quality carbon nanodots (C-dots) with high purity were synthesized through a mild, one-step electrochemical approach, without the assistance of any chemicals but only pure water. This high productivity method makes the synthetic process of C-dots synthesis both economical as well as environment-friendly. The as prepared C-dots are predominantly multi-layer graphene oxide, with luminescence and high up-conversion photoluminescence (emission of light at shorter wavelengths than the excitation wavelength). Meanwhile, C-dots showed peroxidise mimetic function and visible-light-sensitive photocatalytic activity for methyl orange degradation. In addition, a novel photocatalyst (TiO₂/C-dots) was obtained by combining C-dots with TiO₂ through an easy hydrothermal method. Remarkably, TiO₂/C-dots exhibited an excellent visible-light photocatalytic activity.