Photoemission study of the electronic structure of Mo and Mo oxides

Abstract

The electronic structure of Mo and Mo oxides has been studied using ultraviolet and X-ray photoelectron spectroscopy. The valence band spectrum of Mo(100) is compared with calculations of the density of states (DOS). A fairly good agreement of the characteristic peak positions with the DOS maxima is obtained. The Mo 3d core lines indicate well-defined MoO2 and MoO3 oxidation states with binding energies respectively 0.9 to 5.0 eV higher than that of the Mo metal. By comparing the atomic photoionisation cross sections and electron state occupation numbers with the experimental band intensities the highly ionic character of MoO2 has been estimated. The XPS valence band of MoO2 can be qualitatively described using the molecular orbital model based on a distorted rutile structure. The MoO3 valence band is composed of the O 2p derived band with a characteristic structure around 5 eV reflecting the existence of cationic d interactions. A small peak at about 1 eV below EF is observed in the photoemission of blue substoichiometric MoO3-x (in contrast to transparent MoO3). This is assigned to a small occupied defect band of trapped electrons in oxygen vacancies. Ion bombardment of transparent MoO3 results in a drastic change of the valence band indicating the MoO3 to MoO2 surface modification due to oxygen depletion.