Sharpening behaviour of W-wire electrodes in 10–25wt.% NaOH solutions

A W-wire was etched in aqueous NaOH solutions to form a sharp-tipped probe for scanning tunnelling microscopy. Experimental results show that a high anodic dissolution rate of the W-wire occurred in the electrode beneath the electrolyte surface, forming a tip shape. The tip sharpness depended strongly on the etching potential in the limiting-current plateau and the NaOH concentration. Three different tip morphologies of the etched W-wires were observed: a sharp tip with a negative curvature, a stepwise tip, and a long tip with a positive curvature. The anodic dissolution behaviour of the above-mentioned tips is discussed.