Interpretation of Ni2p XPS spectra of Ni conductors and Ni insulators |
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| Authors: | H W Nesbitt D Legrand G M Bancroft |
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| Institution: | (1) Department of Earth Sciences, Univeristy of Western Ontario, London, Ontario, Canada, N6A 5B7 e-mail: hwn@julian.uwo.ca, CA;(2) Department of Chemistry, University of Western Ontario, London, Ontario, Canada, N6A 5B7, CA |
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| Abstract: | Ni2p3/2 X-ray photoelectron spectral peak binding energies of Ni metal, NiS, and NiAs (all conductors) span a range of about 0.5 eV
and are, consequently, insensitive to formal Ni oxidation state and to the nature of the ligand to which Ni is bonded, relative
to other metals (e.g., Fe). Ni2p3/2 peak structures and binding energies reflect two energetic contributions. The major contribution is that associated with
the electrostatic field produced by ejection of the Ni(2p) photoelectron, the minor contribution is the relaxation energy
associated with filling unoccupied, conduction band 3d9 and 4s Ni metal orbitals. These conduction band orbitals become localized on the Ni photoion (and sometimes filled) in response
to the field created by the photoemission event. Because only the core Ni2p electron and nonbonding orbitals of predominantly
metallic character are affected, the main peak of all three conductors are affected similarly, leading to similar Ni2p3/2 main peak binding energies.
NiO, Ni(OH)2, and NiSO4 are insulators in which Ni is divalent and is bonded to oxygen. Although Ni is bonded to oxide in these phases, Ni2p binding
energies differ substantially, and reflect primarily the nature of the ligand (O2−, OH−, SO4
2−) to which Ni is bonded. The influence of the ligand is the result of charge (electron) transfer from valence band bonding
orbitals of dominantly ligand character, to unoccupied conduction band orbitals localized on Ni photoions. Relaxation energy
resulting from charge transfer is acquired by the emitted photoelectron, thus Ni2p3/2 photopeak binding energies of these insulators reflect the nature of the ligand to which Ni is bonded.
The Ni2p main peak binding energy of these conductors and insulators is a poor guide to Ni oxidation states. The Ni2p3/2 binding energies of insulators reflect, however, the nature of the ligand in the first coordination sphere of Ni.
The intensity of the Doniach–Sunjic contribution to Ni2p XPS spectra of NiS and NiAs is dependent on the nature of the ligand.
The Doniach–Sunjic contribution to ligand XPS core-level photopeaks (e.g., S2p of NiS and As3d of NiAs) has not been explained
and is poorly understood.
Received 24 May 1999 / Revised, accepted: 30 June 1999 |
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| Keywords: | Ni-conductors Ni-insulators XPS binding energies |
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