Kelvin Probe Force Microscopy in Surface Chemistry: Reactivity of Pd Nanoparticles on Highly Oriented Pirolytic Graphite

The reactivity of 10–50 nm large Pd nanoparticles (PdNPs) supported on the highly oriented pirolytic graphite (HOPG) surface has been studied under ultra-high-vacuum (UHV) conditions by monitoring the work function (WF) during Kelvin probe force microscopy. The WF of as-prepared PdNPs can vary by more than 600 meV under similar deposition conditions. Because of a chemical reaction between the PdNPs and the residual gas of the UHV at room temperature, the WF always continuously decreases until an equilibrium value is reached. We suspect carbon contamination resulting from the dissociation of CO and hydrocarbons for both phenomena. Smaller PdNPs exhibit a higher reactivity (contamination) than larger ones, and site effects can be observed. We show that annealing at high temperatures in an oxygen atmosphere is sufficient to obtain clean PdNPs, which exhibit the expected WF difference between HOPG and Pd.