H-terminated Si quantum dots (Si QDs) with 3 nm diameter are converted to water-soluble uniform-sized Si QDs after controlled oxidation in an EtOH/H2O2 solution. These dots present Si/SiOxHy core/shell nanostructures and can be finely tuned to emit light in seven different colors due to the quantum size effect in the Si cores, exhibiting excellent photocatalytic activity in the visible range.