Electronic structure of a Pd monolayer on Nb(110)

Abstract

We present angle-resolved photoemission data and calculations for Pd monolayers adsorbed on Nb(110). We find good overall agreement between theory and experiment and demonstrate directly that the Pd states have a noble-metal configuration on the Nb surface but not as a free monolayer. We discuss the implications for hydrogen uptake in this system. There has recently been considerable interest in thin layers of palladium on niobium, ' particularly in connection with their influence on hydrogen absorption by the underlying metal. The experiments were performed on recrystallized foils of Nb which produced sharp low-energy-electron diffraction patterns indicating a (110) surface. At one monolayer coverage the Pd is commensurate with this structure. It is interesting that the near-neighbor distance for Nb(110) planes is 2.86 A, only about 4% larger than that for Pd(111) planes which is 2.75 A.. So it is perhaps not so surprising that the Pd monolayer can distort and become commensurate with the underlying Nb. This structure also has an interesting interaction with hydrogen, At one monolayer Pd the system does not adsorb hydrogen, i.e. , the sticking coefficient is zero. This is unusual because the sticking coefficient for clean Nb and clean Pd are both large in the range of 0.2-0.6 as determined by photoemission and work-function change. 'In this paper we present angle-resolved photoemission data for the Nb+ Pd system and electronic structure calculations for Pd monolayers and Nb films with adsorbed palladium. This represents the first detailed comparison of angle-resolved photoemission data and theory for a metal-on-metal system.The two-dimensional Brillouin zone is shown in Fig. 1. The (110) face of a bcc crystal has C2"symmetry so at the zone center (and at N ) the states can be classifed as XI where i = 1 -4. To fix ideas we note that s, d 2, and d 2 2 belong to Xi, d~b elongs to X2, d~, belongs to X3 4 Along 1"-N the only symmetry is a vertical reflection plane, so Xi and X4 can mix, as can X2 and X3. Similarly, along I -H, Xi and X3 mix, as do X2 and X4. States along these lines can be classifed by even or odd symmetry under reflection. In the isolated monolayer there is an additional symmetry, reflection in the plane (z -z), which prevents these mixings. This fact goes a long way toward explaining the difference between the single layer and the multilayer systems.The experiment was performed on 0.005-in. Nb single crystals. Pd films were evaporated onto the

Core-level spectroscopy of the Pd/W(110) interface: Evidence of long-range Pd-island–W interactions at submonolayer Pd coverages

“…For example, the electronic structure of ultrathin Pd films on transition metal substrates varies systematically with the substrate across the periodic table [2][3][4][5][6][7]. In fact, for Pd grown on bcc(110) metal surfaces (Nb, Ta, Mo, and W) the electronic structure of monolayer, pseudomorphic Pd most closely resembles that of a noble metal [2,[8][9][10][11][12], leading to surface chemical properties that are very different than those at the surface of bulk Pd [2,4,[13][14][15][16][17][18][19][20][21][22][23][24]. Clearly, a surface atom's properties can be substantially influenced by its bonding with the underlying substrate.…”

Observation of a low-energy adsorbate core-level satellite for CO bonded to palladium: Coordination-dependent effects

“…57,58 A way to get a more quantitative measure of the valenceband effects is to look at the position of the 4d band with valence photoemission spectroscopy. For 1 ML Pd on Nb, W, and Ta, which like Mo are refractory metals, the Pdderived 4d states typically appear at binding energies between 2 and 4 eV, [59][60][61][62] whereas the 4d states for 1 ML Pd on Au͑111͒ are characterized by a peak at 1.5 eV with a tail stretching down to E F . 57 Since the molecular CO 2 level is situated between E F and the vacuum level E V , a shift of the Pd 4d states to higher binding energies reduces the overlap and weakens the 2-d hybridization.…”

Alloying of Pd thin films with Nb(0 0 1)

“…For novel chemical reactivity properties it is not volume intermixing that is of interest, but rather intermixing in the near surface region and especially at the very surface itself. Many studies of metal-metal bonding have thus focused on Pd deposited on Nb(1 1 0) [1][2][3][4][5][6][7][8][9][10] and Ta(1 1 0) [1,[10][11][12][13][14][15][16][17][18][19][20]. The adsorption of Pd on Nb or Ta was reported to lead to a reactivity similar to that of Ag [1,[15][16][17][18][19][20].…”