Stability and structural phase transitions of cobalt porphyrin adlayers on Au(100) surfaces

The stability and phase transitions of adlayers of two cobalt(II) porphyrins, 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II) (CoTPP) and 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine cobalt(II) (CoOEP), formed on Au(100) were investigated under electrochemical conditions. In situ scanning tunneling microscopy (STM) was employed to investigate the structure of CoTPP and CoOEP adlayers in 0.1 M HClO4. The CoTPP and CoOEP adlayer structures were varied with the modification time and the concentration. The in situ STM observations showed that the underlying reconstructed atomic structure was lifted to a (1 × 1) atomic arrangement by either the adsorption of CoTPP/CoOEP during modification in a benzene solution or positive potential manipulation in 0.1 M HClO4. Ordered CoTPP arrays with two different hexagonal and square packing arrangements were found on an Au(100)-(1 × 1) surface, along with characteristic Au islands. The CoOEP molecules also formed a close-packed hexagonal structure on an Au(100)-(hex) surface; CoOEP molecules were arranged in a semi-square structure on the Au(100)-(1 × 1) surface by the lifting of reconstruction. The results of this study showed that the interaction between the cobalt porphyrins and the Au(100) substrate depended on the modification conditions and the electrochemical potential.