Nucleation at three-phase junction lines: in situ atomic force microscopy of the electrochemical reduction of sub-micrometer size silver and mercury(I) halide crystals immobilized on solid electrodes

The electrochemical reduction of sub-micrometer size silver halide crystals immobilized on the surface of gold and platinum electrodes starts at the three-phase junction line where the three phases "metal", "silver halide" and "electrolyte solution" meet. Following nucleation at this line the reaction advances within seconds on the surface of the silver halide crystals until the entire surface is covered with about 20 atomic layers of silver and the reduction is terminated. The silver layer can be oxidized anodically and the next layer of the silver halide crystals becomes accessible for further reduction. This sequence of reductions and oxidations can be repeated. The nucleation of silver at the three-phase junction line can be detected by atomic force microscopy (AFM) measurements when, after a short reduction pulse and dissolution of the remaining silver halide, a thin ring of silver is observed at the place where the three-phase junction line was situated. The entire scenario of electrochemical reduction of immobilized silver halide crystals depends on the crystal size. Large crystals (about 100 mum edge-length) immobilized on the surface of optically transparent indium tin oxide electrodes show the growing of silver whiskers on the crystal surface, similar to what is known for the reduction of silver halides with photographic developers. However, also in the case of the large crystals, the reduction starts at the three-phase junction line. The electrochemical reduction of immobilized sub-micrometer size crystals of Hg2Cl2, and Hg2Br2 starts also at the three-phase junction. In the case of gold electrodes the formation of liquid mercury is followed by the formation of a solid crystalline gold amalgam. In the case of platinum electrodes the liquid mercury wets the platinum surface but does not destroy it.