Under assistance of ionic liquid, the Cu2O crystal was successfully synthesized by a simple solution-phase method with several morphologies, including octahedrals, truncated octahedrals, facet-etched octahedrals, particle-coated octahedrals and aggregated spheres. The morphology of Cu2O crystals can easily be modified by tuning the adding amounts of ionic liquid 1-ethyl-3-methylimidazolium bromide. A possible growth mechanism for the crystal products can be suggested based on the DFT+D calculation and experiments. Under visible-light irradiation (lambda > 420 nm), the obtained Cu2O samples show an outstanding performance during photodegrading methylene blue (MB) dye. The higher photocatalytic activity for MB photodegradation by the Cu2O facet-etched octahedrals comparing to octahedrals can be attributed to the formation of surface heterojunction between (100) and (111) facets. Therefore, the ionic liquid-assisted solution-phase syntheses would be potentially useful in the fields of catalyst production for photodegrading hazardous pollutants. (C) 2017 Elsevier B.V. All rights reserved.