Pristine and oxidised Ag-nanoparticles on free-standing graphene as explored by X-ray photoelectron and Auger spectroscopy

Abstract

Silver nanoparticles with a mean diameter of 15 nm have been grown on free-standing graphene and explored by soft X-ray synchrotron radiation using the ESCAmicroscopy beamline at Elettra. In particular, the 3d core orbitals and valence band (4d/5 s) of pristine and plasma-oxidised Ag-nanoparticles have been investigated by X-ray photoelectron spectroscopy (XPS), X-ray induced valence band photoemission, and MNN Auger spectroscopy. The X-ray spectra of the pristine Ag-nanoparticles, i.e. XPS, Auger and UPS spectra, are very similar to these of the metallic Ag-bulk film while the spectra of the oxidised Ag-nanoparticles exhibit distinct differences with respect to the oxidised Ag-bulk film. This is inferred from a larger valence band width, an extended valence band photoemission intensity nearly up to the Fermi level of the bulk metal, a smaller two-hole Coulomb correlation energy and a rather bulk-like Auger spectrum. We interpret this as to result from a shell-like composition of the Ag-nanoparticles with a thin passivating oxide layer and an inner metallic core. Moreover, the 3d photoemission peak of the oxidised nanoparticles is shifted to higher binding energy with respect to the bulk oxide peak. This is attributed not only to an XPS chemical shift but also to an additional final state Coulomb charging according to the electrostatic liquid drop model. Comparing the core binding energy shift of Ag-nanoparticles with the electrostatic potential of a spherical compact particle and a flat nanoislands a morphology transformation of the oxidised nanoparticles from 3D to 2D is obvious in the size range 5–15 nm.