**4. Conclusions**

The color of a thin copper oxide layer formed on a copper plate turned from reddish-brown to dark blue-purple by irradiation with 5 keV Ar<sup>+</sup> ions to a fluence of <sup>1</sup> <sup>×</sup> <sup>10</sup><sup>15</sup> Ar<sup>+</sup> cm−<sup>2</sup> . Nuclear reaction analysis revealed that a significant amount of oxygen (5 <sup>×</sup> <sup>10</sup><sup>15</sup> O atoms·cm−<sup>2</sup> ) was released from the irradiated layer. The reduction of cupric oxide (CuO) to cuprous oxide (Cu2O) occurred in the layer after the irradiation as confirmed by the decrease in intensity of a shake-up satellite line and the change in the shape of a Cu 2p3/2 line in photoemission spectra. The reduction led to the compositional change in the mixture of Cu/Cu2O/CuO, which would result in the chromatic change.

**Author Contributions:** Conceptualization, K.T. and T.K.; methodology, K.T.; RBS and NRA, F.N.; data curation, T.K. and K.T.; writing—original draft preparation, K.T.; writing—review and editing, K.T. and T.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
