Charge Redistribution in Pd-Ag Alloys from a Local Perspective

Abstract

Using Pd and Ag L 3,2 -edge x-ray-absorption near edge structures (XANES) and x-ray photoemission spectroscopy, we have investigated the charge distribution in a series of Pd-Ag alloys (Pd 3 Ag, PdAg, and PdAg 3 ) from a local perspective. It is found that, relative to the pure element, both Pd and Ag gain d and lose non-d (s and p) charge upon alloying. The results are discussed in terms of band filling, rehybridization, electronegativity, and electroneutrality considerations. The possibility of using the L 3,2 -edge XANES white line to circumvent the need of an appropriate volume for charge transfer discussions and for other applications are noted. [S0031-9007(96)01801-7] PACS numbers: 78.70.Dm, 71.55.AkThe electronic properties of 4d noble metals, Pd and Ag, are determined largely by their band structure, particularly the energy distribution of the d electrons and the interplay between the d and the non-d (s and p) conduction electrons [1]. Thus d-charge redistribution upon alloy formation has a profound effect on the electronic behavior of the alloy. It is generally recognized that the Pd d band extends above the Fermi level, e F , and exhibits high unoccupied densities of d states at the Fermi level while the Ag d band is entirely below e F [2]. Experimentally, the occupied portion of the d band can be studied with photoemission, and, to a lesser extent, x-ray emission techniques, while the unoccupied densities of states of d character can be investigated with the L 3,2 -edge x-ray-absorption near edge structure (XANES). This is because L 3,2 XANES probes the d holes via dipole transitions. It has been shown that, owing to the presence of localized densities of states (DOS) of the unoccupied d band at the Fermi level, the Pd L 3,2 -edge XANES exhibits an intense spike (commonly known as a white line) at the onset of the L 3,2 -edge XANES, while the XANES of Ag, of which the d band is nominally full, does not [3]. In the latter, however, close examination reveals that there exist some unoccupied states of 5d character above the Fermi level in Ag as the result of sp-d rehybridization [4].The Pd-Ag alloy system, which has complete miscibility, fcc structure, and negligible volume effects, is of particular interest in that the center of gravity of the d band of the pure elements, which can be determined by an atomiclike one-electron energy ͑e d ͒, is well separated and that Pd-Ag alloy is known to have two separate dband components which closely correspond to that of the pure constituents [5,6]. The energy position of e d changes only slightly in the alloys, although the individual d component narrows as the constituent becomes more dilute. In this paper, we examine the nature and the direction of charge transfer in Pd-Ag alloys in connection with the behavior of the alloy d band from a local perspective. Pd and Ag L 3,2 -edge XANES [3,7], which is ideally suited as a local probe for the study of unoccupied d states above the Fermi level in Pd-Ag alloys, was used together with photoemission in this study. A chan...

Size and alloying induced shift in core and valence bands of Pd-Ag and Pd-Cu nanoparticles

“…Although this argument works well for compounds of non-transition elements, it is generally not valid for metallic alloys with full or nearly full d-bands. 40,41 In particular, it is observed that the 3d core level shifts of both Ag and Pd have the same sign. 42 Abrikosov et al have explained this behaviour in terms of intra-atomic charge re-distribution due to valence electron hybridization.…”

XRay Studies of PdAg Membranes for Hydrogen Separation

“…The pre-edges and backgrounds were subtracted and the measurements were normalised at 50 eV beyond the edge. Some saturation was visible when comparing the results to the literature [8]. A numerical compensation was determined for Pd and Pd/Ag under vacuum, which was also applied to the hydrogen measurements.…”

Pd L3edge XANES investigation of the electronic and geometric structure of Pd/Ag–H membranes

“…The value of m B was determined by steering the relative intensity of the white-line (compared to the baseline) to the values for pure Pd respectively Pd 0.8 Ag 0.2 as determined by Coulthard and Sham,16 while the baseline was kept at 1 with the aid of a normalization factor. The energy scales of the spectra were calibrated on the point of inflection of the Pd L 3 edge, 3173.3 eV, 22 of the pure Pd sample in vacuum.…”

“…Some saturation effects due to self-absorption were visible when comparing our fluorescence results to transmission results from Coulthard and Sham. 16 A numerical correction was determined based on the vacuum measurements of the two samples and applied to the hydrogen measurements. The correction was based on work by Schroeder; 21 for a 901 scattering angle the intensity of the fluorescence yield (I F ) is given as a function of the absorption cross section (m x ) and the background absorption (m B ):…”

“…2(a) shows that the addition of Ag to Pd gives a lower intensity of the white-line, which is in agreement with results in the literature. 16 This can be attributed to a decrease of the number of 4d holes in Pd, i.e., to a shift of electron density from the Ag to the Pd metal. The peak at approximately 3204 eV (indicated with arrows) is shifted to lower energy for Pd/Ag.…”