Today’s biggest challenge in the field of renewable energy is undoubtedly the solar energy conversion and storage. Photoelectrolysis of water using solar light could be the most efficient and environmentally friendly way of producing sustainable H2 gas. In order to achieve it, a photo-semiconductor capable of absorbing solar energy efficiently is needed. Efficient photocatalysts should satisfy several conditions: suitable band gap (about 2-2.2 eV) for visible light absorption and appropriate band edge potentials for water splitting, capability to separate excited electrons and holes, minimal energy losses during charge transport, chemical stability to corrosion, suitable electron transfer properties from photocatalysts’ surface to water and low cost. Most of these properties of semiconductors are primarily determined by their electronic structure. Therefore, the density functional theory calculations which can provide insight into the electronic structure of the modified semiconductors can guide the selection of the proper dopants in semiconductors to meet the requirements needed for application. The effect expected from some dopant is mainly related to the location of its energy levels with respect to the band structure of the host. In particular, for efficient use of TiO2, which is one of the most promising photocatalysts for photocatalytic water splitting, it is necessary to find a dopant pair which narrows the band gap in order to shift the absorption edge to the visible light region, but at the same time does not lower the conduction band minimum (CBM), as the reduction potential level of water is just below the CBM of TiO2.