The in situ XRR measurements were performed at the ID10 beamline of the ESRF synchrotron (Grenoble, France). To ensure the reactor transparency for the X-ray radiation with the beam energy of 22 keV, an aluminum reactor wall was replaced with a copy made of beryllium. The vertical beam size was 15 µm. A two-dimensional MAXIPIX detector was used. The XRR data from the curved liquid Cu surface were treated according to the method described recently in ref. to eliminate the curvature's effect and extract the normalized reflectivity profiles. The resulting XRR curves were fitted using Refl1D software to a slab model consisting of one (in case of bare Cu) or three (in case of Cu covered with Gr) layers: a Cu substrate, a void, and a C monolayer. The density of the liquid Cu, the C layer, and the thickness of the C layer were fixed to 7.99 g/cm 3 , [53] 5.36 g/cm 3 , and 1.42 Å, respectively. The fitting parameters were the roughness of the Cu and Gr layers together with the width of the void layer. The roughness of the bare liquid Cu, of 1.1±0.1 Å, was obtained from fitting the reflectivity measured on it prior to Gr growth. This value was applied as a starting parameter for the fit of the Gr on the Cu curve. As a result, the roughness values of the Cu and Gr layer were 1.0±0.1 and 1.1±0.1 Å, respectively, and the void value was 1.5±0.1 Å. Note that both parameters (the roughness and the void width) are independent of each other.