Cu 2 O rhombic dodecahedra, octahedra, and cubes were densely modified with conjugated 4-ethynylaniline (4-EA) for facet-dependent photocatalytic activity examination. Infrared spectroscopy affirms bonding of the acetylenic group of 4-EA onto the surface copper atoms. The photocatalytically inactive Cu 2 O cubes showed surprisingly high activity toward methyl orange photodegradation after 4-EA modification, while the already active Cu 2 O rhombic dodecahedra and octahedra exhibited a photocatalytic activity enhancement. Electron, hole, and radical scavenger experiments prove that the photocatalytic charge transport processes have occurred in the functionalized Cu 2 O cubes. Electrochemical impedance spectroscopy also indicates reduced charge transfer resistance of the functionalized Cu 2 O crystals. A band diagram constructed from UV−vis spectral and Mott−Schottky measurements reveals significant band energy shifts in all Cu 2 O samples after decorating with 4-EA. From density functional theory (DFT) calculations, a new band has emerged slightly above the valence band maximum within the band gap of Cu 2 O, which has been found to originate from 4-EA through banddecomposed charge density analysis. The increased charge density localized on the 4-EA molecule and the smallest electron transition energy to reach the 4-EA-generated band are factors making {100}-bound Cu 2 O cubes photocatalytically active. Proper molecular decoration represents a powerful approach to improving the photocatalytic efficiency of semiconductors.