Structure of thiocyanate adlayers on Rh(111): an in situ STM study

In situ scanning tunneling microscopy (STM) was used to examine the structure of thiocyanate adlayers specifically adsorbed on Rh(111) in solutions of potassium hydroxide and perchloric acid, both containing potassium thiocyanate (KSCN). An atomically flat terrace-step structure was consistently observed on Rh(111) surfaces prepared by the flame-annealing-quenching method. The Rh(111)-(1 x 1) atomic structure was discerned on the atomically flat terrace even in the alkaline solution. High-resolution STM images disclosed two different structures of the SCN- adlayers, (2 x root 3) and (2 x 2), in the alkaline and the acidic media, respectively. In each structure, an individual adsorbed SCN- ion appeared as a single spot with a constant corrugation height in STM images, suggesting that SCN- ions adsorbed predominantly with their S-ends at particular bonding sites on Rh(111). The difference in the adlayer structure in the two solutions can be attributed to the interaction between adsorbed SCN- and coadsorbed K+ in the alkaline solution, and is different from that between adsorbed SCN- and H+ in the acidic solution.