From the methods section. Interesting."Fe@CNO was purified using...

From the methods section. Interesting.

"Fe@CNO was purified using two methods [59,60]. In the high-temperature method, Fe@CNO was heated at 3000 °C for 0.5 h under Ar to remove encapsulated Fe nanoparticles, resulting in purified CNOs, denoted as HT-CNO. In the electrochemical purification method, a DC of 10 V was applied to a working electrode made of Fe@CNO and a Pt counter electrode in 0.1 M (NH₄)₂SO₄ aqueous solution. Charge ions intercalated into graphitic layers, and Fe slowly leached over 20 h. The resulting purified CNOs were denoted as EC-CNO."

Fe@CNO is our graphite nano-onions with the speck of iron catalyst at the center (just to be clear).