Cluster Core-Level Binding-Energy Shifts: The Role of Lattice Strain

Abstract

Our combined experimental and theoretical analysis of the shifts, with particle size, of core-level binding energies (BE's) of metal nanoparticles on insulating supports, shows that these shifts have an important initial state contribution arising, in large part, because of lattice strain. This contribution of BE shifts has not been recognized previously. Lattice strain changes the chemical bonding between the metal atoms and this change induces BE shifts. DOI: 10.1103/PhysRevLett.93.026805 PACS numbers: 73.22.-f, 79.60.-i Photoelectron spectroscopy (PES) is often used to deduce information on the electronic structure of molecules, solids, and at surfaces [1]. There is increasing interest in nanostructured materials, especially in clusters grown on inert surfaces since they may have high catalytic activity [2]. Thus, the question of how the PES of deposited nanoparticles reflects their electronic and geometric structures is quite important. It is known that the corelevel BE's of metal clusters on insulating supports [3][4][5][6][7][8][9][10][11][12] shift to lower BE with increasing size by BE 1 eV from small clusters to bulk metal. However, there is substantial disagreement over the assignment of these shifts to initial or final state effects; see Ref.[13] for a rigorous definition of these two contributions. Mason [7] argued that changes in the electron configuration of the atoms in smaller clusters, not relaxation energies, were primarily responsible for the shift. On the other hand, Wertheim and collaborators [4,5] assigned the shift as due to final state screening effects and related the magnitude of BE to an effective cluster radius. For Cu clusters grown on thin Al 2 O 3 films, Wu et al. [11] concluded from an Auger parameter analysis that initial state contributions are small and may be significant only for very small clusters. This separation is quite important since the initial state BE reflect changes in the electronic structure before ionization of a core level and, hence, are of direct interest for materials properties. In the present work, we show that the initial and final state contributions to the BE are of comparable magnitude. Further, we relate the initial state BE to a lattice strain that exists because the average bond distances in small clusters are shorter than in the bulk [14,15]. The chemical bonding is different at the shorter bond distances and this, in turn, leads to changes in the core-level BE's. The chemical changes important for the BE involve an increased d to sp promotion, or hybridization, for shorter bond distances.Our work is based on a combination of experimental and theoretical methods that allow us to decompose the BE into initial and final state contributions. Our theoretical approach involves the calculation of electronic wave functions (WF's) for clusters and the determination of the BE's where relaxation in response to ionization is excluded, a pure initial state BE, and where this relaxation is allowed [13]. Our experimental approach involves use of an Auger para...

Size dependence of core and valence binding energies in Pd nanoparticles: Interplay of quantum confinement and coordination reduction

“…The quantum confinement effect as well as the lattice strain, and hence the coordination number reduction, is reported to induce the binding energy shifts. 1,9 It is interesting to note that the effects due to quantum confinement have larger dependence on particle size ͑ϳR −2 ͒ than those due to coordination reduction ͑ϳR −1 ͒. 17,18 Indicating that the inner core electrons are more strongly affected by the quantum confinement, the valence electrons are strongly affected by the coordination reduction.…”

“…The common observation in the XPS studies of metal nanoparticles reported in the literature ͑Au, Ag, Pd, Pt, Ni, Cu, and Co͒ is the positive core level shift with respect to the bulk values. [1][2][3][4][5][6][7][8][9] In a few studies, shifts in the binding energy of valence band with respect to Fermi level and narrowing of valence band spectra have also been reported. 3,7,8 Although considerable efforts have been focused on elucidating the physical origin of the observed size dependent changes, the interpretation of the observed behavior is still unclear.…”

“…Zhang and Sham 8 studied the electronic properties of the thiol-capped Au nanoparticles ͑1 Ͻ R Ͻ 5 nm͒ using XPS and argued that the modifications in the binding energy and the full width at half maximum ͑FWHM͒ of the Au 4f core levels and the valence 5d band are mainly due the initial state effects. Whereas, Richter et al 9 studied the Co clusters ͑R Ͻ 1 nm͒ deposited on Al 2 O 3 grown on NiAl ͑110͒ by Auger parameter analysis along with theoretical modeling and argued that the initial and final state contributions to the core level shifts obtained were of comparable magnitude. Thus there is a substantial disagreement over the assignment of these shifts with initial or final state effects.…”

“…It may be mentioned here that in most of the reported studies on size dependence of the electronic properties of metal ͑Au, Ag, Pd, Pt, Ni, Cu, and Co͒ at nanodimensions, the samples were prepared by evaporating a small quantity of metal onto the substrate. [1][2][3][4][5][6]9 The isolated metal islands normally have a rather broad and often not well characterized size distribution. The electronic properties were studied as a function of coverage, which is approximately mentioned as the particle size.…”

Tuning ultrafine manganese oxide nanowire synthesis seeded by Si particles and its superior Li storage behaviors

“…35,36 A small positive shift of the absorption edge compared with pristine MnO@C could be related to the pulverization of the MnO during the charge/discharge process. 37 The radial-structure functions of the Mn K-edge (Figure 8c) Tuning MnO nanowire synthesis seeded by Si particles H Wei et al demonstrate the obvious increase in the peak height at 1.5 angstroms, implying an increase in the ordering of the O 2 À ions in the MnO structure, which induces the fast diffusion of Li ions and a low internal resistance. 38 As for the change in the radial distribution function of the second shell of the MnO@C nanowire, it may be consistent with the increase in the disorder degree of the Mn-Mn shell corresponding to the coordination of the second shell, which was supported by the increases of Debye-Waller factor of Mn-Mn.…”