Ammonia (NH₃) is one of the important energy sources for sustainable chemical products and carbon-free energy carriers. Artificial N₂ photofixation is one of the promising approaches for the clean production of NH₃ using photocatalysts in N₂-dissolved water as a hydrogen source. We prepared defect-engineered TiO₂/CuO heterojunction photocatalysts by the simple evaporation-induced self-assembly (EISA) method followed by post-thermal annealing under inert gas flow. The formation of a type-Ⅱ using TiO₂ and CuO facilitated light absorption in near IR to UV light and the separation of photoexcited electron-hole pairs. In addition, the further post-thermal annealing under N2 gas flow resulted not only in an increase in crystallinity for the photoactive Anatase TiO₂ and Tenorite CuO in the bulk but also in the in-situ formation of Ti³⁺ defect sites on the TiO₂ surface. The increased crystallinity enhanced the photoexcited charge transport, while the defect sites improved N₂ adsorption and activation, promoting the photocatalytic conversion of N₂ to NH₃. The defect-engineered TiO₂/CuO photocatalysts exhibited a high NH₃ production rate (1.575 μmol g⁻¹h⁻¹) under visible light irradiation (λ≧ 420 nm) without any sacrificial agent, this value is 9.4 times and 2.5 times higher than those obtained with pristine TiO₂ and TiO₂/CuO photocatalysts, respectively. This work clearly demonstrates the synergistic effect of heterojunction and defect-engineering and provides insights into how each strategy can affect solar-driven NH₃ production.