Structural and Electrochemical Characterization of Nanographenes Adlayers on Au(111)
Authors: Obara Yui , Yoshimoto Soichiro 
Published: December 2024

The development of efficient hydrogen production technology by water splitting electrolysis with low overvoltage is underway to realize a hydrogen society, and the development of a technology to precisely design and sheet the chemical structure along with catalyst development is very important to understand the electrocatalytic reaction mechanism such as hydrogen evolution and oxygen reduction reaction and the effect of gap spacing on the active site at the nanoscale. This is very important for visualizing and understanding the mechanisms of electrocatalytic reactions, such as hydrogen evolution and oxygen reduction reactions (HER and ORR), and the effect of gap spacing on the active sites at the nanoscale. For example, it has been reported that ORR activity is enhanced on surfaces with adsorbed hydrophobic organic molecules on Pt compared to that on clean surfaces (Nat. Commun. 2018, 9, 4378). The effect of nanopores (vacancy sites) on the HER is unclear, although it is attributed to the limited reaction sites of Pt atoms owing to the adsorption of organic molecules. Furthermore, it is difficult to control the size of the nanopores and their dispersion (distance between nanopores) at the nano-level using a top-down approach, in which graphene sheets are mechanically and chemically processed, and a bottom-up 2D sheet fabrication method should be established. The development of a technique to precisely design chemical structures and fabricate sheets is important to understand the electrocatalytic reaction mechanism at the nanoscale and to visualize the effect of nanopore size and spacing on the active sites at the nanoscale.

In this study, we investigated the formation of nanostructures by the intermolecular condensation of nanographene, such as ovalene and dicoronylene (Fig. 1), and investigated the thermal condensation conditions by adsorption on a Au(111) substrate under various conditions. The pore size of the nanostructures formed by ovalene and dicoronylene was characterized by using a scanning tunneling microscope (STM).

A clean Au (111) substrate was immersed in an ovalene- and/or dicoronylene-saturated benzene solution for 10 – 60 s, and the coated Au(111) electrode was investigated using cyclic voltammetry (CV) measurements. The current density was significantly reduced before and after the vacuum heat treatment, suggesting that the ovalene molecules were rearranged on Au(111) and condensed between molecules. In addition, as a result of observation of the surface state in the atmosphere with STM, it was found before heating (Fig. 2a), confirming that the ovalene molecule was adsorbed over a wide area, and after heating (Fig. 2b), nanostructures with voids of 2.5 ~ 3 nm in diameter were observed. These results suggest that condensation may have occurred between the ovalene molecules due to heating, resulting in the formation of nanostructures with many nanopores. In contrast, dicoronylene forms a diamond-shaped lattice structure with sides of approximately 1.5 to 2.5 nm. Considering the molecular size of dicoronylene, it is difficult to judge whether intermolecular condensation among dicoronylene molecules occurred; however, the adlayer structure was different from that observed at room temperature. These results strongly suggest that intermolecular condensation reaction among ovalene and dicoronylene occurred on Au(111) upon elevating the temperature in vacuum. From the CV measurements, no significant change in the electric double-layer region was observed before and after the vacuum heating treatment; however, we found small change in the overpotential on the hydrogen evolution reaction.

Figure 1