Recycling of carbon dioxide to fuels or splitting of water for the generation of hydrogen are highly desirable solutions aiming to establish a sustainable energy supply and to reduce carbon emissions. Ideally, only the energy of the sun would be required for such a process. This is already possible using photocatalysts, in which the energy of light is converted into chemical energy by photogeneration of charge carriers. However, in spite of decades of research, no system has yet been developed that would function efficiently on the industrial scale. One challenge is the development of materials that can also absorb the large visible portion of the sun light, and not only the few percent of UV light that can be used, for example, by the widely established material TiO₂. This article highlights a new approach for the tailor-made development of photoactive oxide nanoparticles able to function under irradiation with visible light. Since the energy needed to photoexcite a certain material increases with a decrease in particle size, the excitation energy can be tuned by a variation of the size of the photoactive species. In the project described here, the whole size range, starting from large particles with typical properties of extended semiconductors over various sizes of clusters and agglomerates down to isolated photoactive species is studied for vanadium oxide. The synthesis of those different structures is possible using suitable inert supports with acidic or basic surface properties, in which a higher basicity of the surface will favor formation of more and more isolated vanadyl species. While the crystalline oxide V₂O₅ in the form of nanoparticles absorbs in the visible range, the excitation energy shifts more and more to the UV region when going to small oligomers and finally to isolated vanadyl species. It can clearly be shown that the photocatalytic activity of the different vanadium oxide species is a function of their size, in which only the isolated species show activity in organic dye degradation as photocatalytic model reaction. Our study highlighted here is a first showcase to generally establish the use of the quantum size effect for the development of photocatalysts with carefully designed light absorption properties and suitable positions of the energy levels.