Amorphous Cu x O nanoflakes with a thickness of 10-50 nm were synthesized through dye-assisted transformation of rhombic dodecahedral Cu2O nanocrystals using a facile solution process. The morphology evolution observed by electron microscopy is highly dependent on the reaction between the surface and the dye. The crystal grain shrinks during the process until the formation of a purely amorphous nanoflake. The amorphous Cu x O nanoflake consists of a combination of Cu(I) and Cu(II) with a ratio close to 1:1. It shows enhanced photocatalytic reactivity toward the degradation of methyl orange compared to that of rhombic dodecahedral Cu2O nanocrystals with all active (110):Cu facets. The chemical composition and architecture remain the same after repeating degradation tests. The high surface-to-volume ratio contributes to its superior photocatalytic performance, whereas its low surface energy, confirmed by density functional theory simulations, explains its improved stability. The nanoflakes also show the ability of degrading nitrobenzene effectively, thus demonstrating great promise as a highly stable and active photocatalyst for environmental applications.