Cu₂O is a promising material for photocatalysis because of its absorption ability in the ultraviolet (UV)-visible light range. Cu₂O deposited on conductive Ti and fluorine-doped tin oxide (FTO) substrates behaves as a photocathode. Cu₂O deposited on an n-type semiconductor such as TiO₂ nanotube arrays (TNA)/Ti behaves as a photoanode and has demonstrated better photocatalytic activity than that of TNA/Ti. The substrate-dependent photocatalytic properties of Cu₂O heterojunctions are not well studied. In this work, the photocatalytic properties of a Cu₂O/TNA/Ti junction as a photoanode and of Cu₂O/Ti and Cu₂O/FTO junctions as photocathodes without bias were systematically studied to understand their performance. The Cu₂O/TNA/Ti photoanode exhibited higher photocurrent spectral responses than those of Cu₂O/Ti and Cu₂O/FTO photocathodes. The photoanodic/photocathodic properties of those junctions were depicted in their energy band diagrams. Time-resolved photoluminescence indicated that Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO junctions did not enhance the separation of photogenerated charges. The improved photocatalytic properties of Cu₂O/TNA/Ti compared with TNA/Ti were mainly attributed to the UV-visible light absorption of Cu₂O.