Ammonia is an important chemical raw material and non-carbon-based fuel. Photocatalytic ammonia production technology as a mild alternative to the traditional Harbor–Bosch route is carried out at the air, liquid, and solid three-phase interface. Promoting the activation of N₂, depressing hydrogen evolution reaction (HER), and increasing the local N₂ concentration around the catalyst surface are critical factors in achieving high conversion efficiency. In this paper, we proposed that defective TiO₂ is surface-modified by alkyl acids with different carbon chain lengths (C₂, C₅, C₈, C₁₁, and C₁₄) to tune the catalyst surface properties. The defect sites greatly promote N₂ adsorption and activation. The wettability of the catalyst can be regulated from hydrophilic to hydrophobic by the length of the alkyl chain. The hydrophobic surface enhances the N₂ adsorption and increases the local N₂ concentration due to its aerophile. Meanwhile, it depresses the proton adsorption and HER. Overall, the nitrogen reduction reaction (NRR) is greatly promoted. Among the series of samples, they present a systematic change and have a maximal NRR performance for n-octanoic acid-defective TiO₂ (C₈-Vo-TiO₂; Vo = oxygen vacancy). The rate of ammonia production can be as high as 392 μmol·g⁻¹·h⁻¹. This work provides a new strategy for efficient ammonia synthesis at the three-phase interface using photocatalyst technology.