(Invited) Improvement of Electron Transfer Rate at Metal/Organic Interfaces Using Pd Atomic Layers

Chemical modification of a metal electrode is one of promising methods to realize various electrochemical functions.  For example, photocurrent generation may be achieved by surface modification of a metal electrode with chromophores.  Electrocatalytic activity may be enhanced by immobilization of catalyst molecules on a metal surface.  However, these molecule-induced functionalities are strongly affected by metal/organic interface properties.  In particular, the electronic conductance at metal/organic interfaces may be one of essential issues to limit the efficiency of the electrochemical functions.  In this work, the electron transfer kinetics at metal/organic interfaces was experimentally investigated using Cobalt porphyrin (CoTPP) modified Au electrodes.  To control the connection between Au and CoTPPs, bridge monolayers were formed on the electrode using various thiol- or isocyanide-molecules, and CoTPPs were then immobilized on top of the bridge layers.  The electron transfer rate for oxygen reduction reaction (ORR) on CoTPPs was thoroughly investigated for various modified electrodes using the rotating disk electrode (RDE) method. 

The interface structure between the Au electrode and the molecular layers was controlled with and without forming Pd monoatomic layers (Fig. (a)).  The Pd overlayer on Au surface (Au/Pd_ML) was prepared by the underpotential deposition (UPD) method.  The Au and Au/Pd_ML electrodes were self-assembly covered with various diisocyanide molecules such as TPDI (4,4’-terphenyl diisocyanide).  CoTPPs were attached on these molecular layers.  Figure (b) shows Tafel plots for ORR on CoTPPs on SAM-covered Au or Au/Pd_ML electrode.  Electron transfer rate on the Au/Pd_ML was much faster than that on the Au while the Tafel slop was not affected by the presence of Pd overlayer.  This result indicates that the performance of chemically functionalized electrodes can be improved by the metal/organic interface control.