TiO₂ is widely considered the star photocatalyst among various options due to its chemical stability, nontoxicity, and low cost. In recent years, many applications in environmental purification have been developed based on TiO₂ nanocrystals. TiO₂ nanocrystals are typically prepared by wet-chemical methods via the condensation reaction of [Ti(OH)_h(H₂O)_6-h]_4-h (where h is the hydrolysis ratio). We demonstrated that this ratio (h) is a key factor that affects the crystal structures of TiO₂. Based on the thermodynamic parameters of possible condensation reactions, the relationship between the hydrolysis ratio h and the crystal structure has been mapped and verified by experimental data. Based on this, we synthesized colloidal Cu-doped TiO₂ (Cu-TiO₂) nanocrystals using a facile sol-hydrothermal method. It has been found that the doping of Cu induced grain size reduction and bandgap narrowing of TiO₂ nanocrystals, as well as the formation of abundant oxygen vacancies in the TiO₂ nanocrystal. This resulted in improved performance for photocatalytic degradation of benzene and a high antibacterial rate of over 99.99% against both Escherichia coli and Staphylococcus aureus under visible light. Moreover, we successfully synthesized the Cu–TiO₂/CuS p-n heterojunction using an in-situ sulfidation method. This structure exhibited excellent photocatalytic performance for NO removal, with a maximum removal efficiency of 85%, due to enhanced optical absorption and highly efficient charge carrier separation.