The main purpose of this thesis is the increase of the properties of the (001) and (101) surfaces of titanium dioxide (TiO2) such as band gap by placing auxiliary compounds on the surface which is important in dye-sensitized solar cells. Two compounds including 2-aminothiazole and amphetamine were selected for this study. 2-aminothiazole has five tautomers including T, C5HT, T3HT, C3HT and T5HT. The interaction of these molecules with the (001) and (101) surfaces of TiO2 were studied using density functional theory (DFT) and considering the periodic boundary conditions. To investigate the effect of interaction on the TiO2 surfaces, the physical properties including interaction energy, energy gap, density of states (DOS), electron density and molecular orbitals were calculated. To investigate the effect of the surface on the molecules the absorption spectra of the molecules interacting with the surface were calculated using ONIOM method at the (QM:QM) level of theory and compared with the absorption spectra of the molecules in the gas phase. The results showed that these compounds interact with the proper orientations with the surface and in some cases chemical bonds between the surface and molecule are formed which reduce the energy gap of TiO2 and increase the photocatalytic property of the surface. Similar studies were performed in the solvent (acetonitrile) to investigat the effect of the electrostatic field of the solvent and the explicit effect of the solvent molecules on the mentioned physical properties.