Tuning the Anchoring and ORR Activity of Sub-Nano Pt Clusters on HOPG Via Ion Implantation and Oxygen Functionalization

Sub-nanometer supported clusters are of interest in catalysis due to nearly complete precious metal atom utilization, generally high catalytic activity, and attractive tunability via size and clustersupport interactions. The stability of such clusters remains a challenge, however. We demonstrate anchoring and stabilization of size-selected Ptn, n = 2 - 10 clusters soft-landed on HOPG, achieved 2 by modifying the HOPG with 100 eV kinetic energy N(𝑛 1, 2) and Ar+ ions, with and without O2 oxidation of the ion-modified surfaces, prior to Ptn soft-landing. Ion modification is found to significantly improve the sticking probability for soft-landed Ptn, implying increased adsorption energies, and O2 exposure prior to cluster deposition increases the sticking probability to ~unity. DFT finds that Ptn bind preferentially to under-coordinated carbon sites created by the ion impacts, rather than to implanted N atoms, and X-ray photoelectron spectroscopy and scanning tunneling microscopy both show that the defect-anchored clusters are stable against sintering. There is net electron transfer from the defect-bound clusters to the support, and the local defect structure may undergo significant restructuring due to Pt-C bond formation. To probe the effects of ion-modified HOPG on the chemical properties of the adsorbed Ptn clusters, we tested electrodes prepared by Ptn deposition on ion-modified HOPG for the oxygen reduction reaction (ORR). The ion-modified electrodes showed ORR activity consistent with cluster size being maintained, and O2 modification enhanced both cluster anchoring and ORR activity. Thus, ion modification provides means to stabilize sub-nano cluster electrodes, while oxygen functionalization of the anchoring sites tunes the electrocatalytic activity.