(41e) Synthesis of Rhodium Nanoclusters on Graphene/Cu(111) from Organometallic Precursors: A Combined DFT and STM Study

This paper aims to achieve fundamental understanding of surface reaction chemistry and catalysis by formation of rhodium metal nanoclusters from organometallic compounds (Rh(CO)₂(acac) and Rh₆(CO)₁₆) on an epitaxially grown graphene layer over Cu(111). This model catalyst was studied with ultrahigh vacuum scanning tunneling microscopy (UHV-STM) and Density Functional Theory (DFT) calculations. Several moiré patterns with different periodicities and different rotational alignments were observed in STM images, which indicate the weak interaction between graphene monolayer and Cu(111) substrate. DFT calculations show that Rh₉ cluster has the strongest bond to the graphene/Cu(111) surface with an adsorption energy of -4.16 eV and Rh-C bond of 2.11 Å. DFT calculations also shows that organometallic compounds that are introduced to the sample should first be dissociated on the surface so that rhodium nanoclusters can be formed on graphene hollow sites as oxygen weakly interacts with the surface (adsorption energies of -0.19 and -0.37 eV for RhCO and Rh(CO)2, respectively). These results explain the weak interaction of Rh₆(CO)₁₆ on the surface, which is attributable to the repulsive effect by oxygen atoms. Both STM and DFT results confirm that rhodium nanoclusters have preferential nucleation sites on graphene moiré pattern on Cu(111).