Oxygen Adsorption on Graphene-Encapsulated Palladium Nanoparticles Imaged by Kelvin Probe Force Microscopy

Abstract

Graphene encapsulated metal nanoparticles (G@NPs) offer a possibility to observe confined reactions in the nanocontainer formed by the NP's facets and graphene. However, direct experimental detection of adsorbed atomic and molecular species under the graphene cover is still challenging and the mechanisms of intercalation and adsorption are not well understood. Here we show that Kelvin probe force microscopy (KPFM) can largely contribute to the understanding of adsorption and desorption at the single NP level, which we exemplify by comparing oxygen adsorption experiments obtained at asprepared PdNPs and G@PdNPs, both supported on highly oriented pyrolytic graphite (HOPG) and studied under ultra-high vacuum (UHV) conditions. We show that oxygen adsorption at room temperature occurs at a much higher partial oxygen pressure on G@PdNPs compared to as-prepared PdNPs. Similarly, the removal of oxygen via a reaction with the residual gas of the UHV is slower on the G@PdNPs compared to as-prepared PdNPs. The differences can be explained by a limited facility for reactant and product molecules to enter and desorb from the nanocontainer via the defects of the graphene. Observed WF changes are supported by assisting density functional theory (DFT) calculations. ACKNOWLEDGMENTS Support from the Agence Nationale de la Recherche and the Deutsche Forschungsgemeinschaft (ANR, DFG) through project REACTIVITY (grant ANR-17-CE09-0045) is gratefully acknowledged. H.G. acknowledges support from the Swedish Reserach Council (2016-05234). The calculations were performed at C3SE (Göteborg) via an SNIC grant. C.B. acknowledges J. Murgia and P. Bindzi for maintainance work at the UHV chamber. ASSOCIATED CONTENT Supporting Information. Details about the sample preparation, UHV techniques (STM, nc-AFM and KPFM), image analysis, density functional theory (DFT) calculations, choice of WF values from literature, and supporting experiments and calculations (PDF)

Characterizing the Water-Forming Reaction on Graphite- and Ceria-Supported Palladium Nanoparticles and Nanoislands by the Work Function

“…As can be also seen in the corresponding profile 1 (red) below the WF image, the WF of the NP increases by +0.3 eV upon the O 2 pulse. The increase in the WF is in agreement with previous observations: 35 during the pulse, oxygen dissociatively adsorbs on the PdNPs, increasing the WF of the palladium. The increase here is smaller than the previously observed one (+0.6 eV 35 ), which is due to the strong tip−surface convolution effect mentioned above.…”

“…Note that because the AFM tips have been exposed to O 2 and H 2 prior to all experiments shown here, the tip's WF does not change when dosing O 2 and H 2 . For more details, see the Supporting Information for this article and that for ref 35.…”

Oxygen-Adsorption-Induced Charge State Change of a Pd Nanocluster on the Al2O3/NiAl(110) Surface

“…O 2 dissociation on the Pd surface was extensively studied at different temperatures, by temperature-programmed desorption, scanning tunneling microscopy, and density functional theory as well as KPFM. ,− Oxygen adsorption accompanies significant electron transfer to O 2 , creating a surface dipole toward the surface or the formation of superoxo species. , However, in our results, we observed a positively charged nanocluster, which does not fit either of the above case. The oxygen molecule bound to a Pd nanocluster may exist, which occurs with a positive charge of the metal nanocluster or at low temperature, but we measured a negatively charged Pd nanocluster and performed the experiment at RT.…”

“…Kelvin probe force microscopy (KPFM) is a complementary technique of noncontact atomic force microscopy (NC-AFM), in which the electrostatic interaction caused by an AC bias voltage is compensated during topographic imaging. − This compensation voltage corresponds to the local work function difference between the sample and the tip, whereas for insulating films, it gives information about the local surface charge . Therefore, KPFM is a powerful tool for investigating catalyst activity and selectivity. − Compared with the averaging of chemical shifts over the catalyst by X-ray photoelectron spectroscopy, KPFM allows us to follow the individual size and shape of metal catalysts that are adsorbed at different sites of the support, for example, terraces, steps, or kinks, and to distinguish the charge state and composition of adsorbed species . When the charge transfer between Pd nanoclusters and the substrate can be determined on each nanocatalyst in a gas environment, much progress can be made in catalyst characterization.…”

High-temperature oxidation and reduction of the inverse ceria/Cu(111) catalyst characterized by LEED, STM, nc-AFM and KPFM

“…Interestingly, the WF difference decreases on a time scale of days when the sample is left in the UHV. As KPFM has shown to be very sensitive on adsorbed species on surfaces [71], we strongly anticipate that the sample gets contaminated by species from the residual gas of the UHV. Such contamination was observed also on PdNPs [72] and we suspect that it is in particular the reactive Cu(111) surface that reacts with the residual gas.…”