Investigation on nanobubbles on graphite substrate produced by the water-NaCl solution replacement

► On HOPG surfaces, water-NaCl solution exchange can repeatedly produce nanobubbles. ► Nanobubble density increases with salt concentrations ranging from 0.15 M to 4.00 M. ► Their diameter decreases with the salt concentrations. ► For 2.00 and 4.00 M of NaCl, the density of nanobubbles has little difference. ► The dissolved gas played an essential role in the process of nanobubbles formation.