This study employed atomic layer deposition (ALD) to fabricate an Al₂O₃ passivation layer to optimize the performance of ultraviolet (UV) photodetectors with a TiO₂-nanorod-(NR)-containing active layer and a solid–liquid heterojunction (SLHJ). To reduce the processing time and enhance light absorption, a hydrothermal method was used to grow a relatively thick TiO₂-NR-containng working electrode. Subsequently, a 5-nm-thick Al₂O₃ passivation layer was deposited on the TiO₂ NRs through ALD, which has excellent step coverage, to reduce the surface defects in the TiO₂ NRs and improve the carrier transport efficiency. X-ray photoelectron spectroscopy revealed that the aforementioned layer reduced the defects in the TiO₂ NRs. Moreover, high-resolution transmission electron microscopy indicated that following the annealing treatment, Al, Ti, and O atoms diffused across the interface between the Al₂O₃ passivation layer and TiO₂ NRs, resulting in the binding of these atoms to form Al–Ti–O bonds. This process effectively filled the oxygen vacancies in TiO₂. Examination of the photodetector device revealed that the photocurrent-to-dark current ratio exhibited a difference of four orders of magnitude (10⁻⁴ to 10⁻⁸ A), with the switch-on and switch-off times being 0.46 and 3.84 s, respectively. These results indicate that the Al₂O₃ passivation layer deposited through ALD can enhance the photodetection performance of SLHJ UV photodetectors with a TiO₂ active layer.