Prepare and cut a cylinder slab

Adjust the shape of the slab to "Cylinder", in the "Size" options adjust the cylinder length and radius to 80 Å and 15 Å respectively. You can display the slab using the "Show/Hide slab" interactor, also export the results of the cutting/passivating in a new project by activating the corresponding option.
Check on "Passivate surface" and, in the option dialog that immediately opens, and as illustrated select to passivate empty C-C bonds created when cutting the slab with water molecule.

Then just click on "Cut this slab now !" to create a new project, that will contain the new material: a C diamond cell where the atoms inside the cylinder are removed, creating a surface, and where non-bridging bonds have been passivated using H\(_2\)O molecules.