Properties of oxygen sites at the MoO3(0 1 0) surface: density functional theory cluster studies and photoemission experiments

Ab initio density functional theory cluster studies on the MoO3(0 1 0) surface as well as ultraviolet photoemission (UPS) experiments on well-crystallized single phase Mo oxides are carried out to examine the electronic structure of the oxide systems. In addition, electronic details of different surface oxygen vacancies are studied by appropriate vacancy clusters. Calculations on embedded clusters as large as Mo15O56H22 confirm the mixed covalent/ionic character of the oxide. The computed width of the O 2sp dominated valence band region of MoO3, about 7 eV, agrees well with the present photoemission data for MoO3(0 1 0) samples. The overall shape of the computed densities of states (DOS) in the O 2sp region of MoO3 is rather similar to the measured UPS intensity curves indicating weak energy dependence of corresponding transition matrix elements. Calculated vacancy energies for the different surface oxygen sites at MoO3(0 1 0) yield rather large values, 6.8–7.6 eV, which shows that oxygen is bound quite strongly to the substrate. Vacancy formation leads to reduction of neighboring molybdenum centers which expresses itself by increased metal d electron occupation and corresponding DOS contributions above the O 2sp region. This is consistent with the experimental UPS data for MoO3(0 1 0) where oxygen vacancies have been introduced by mild ion bombardment. It is further supported by the present UPS data for well-crystallized intermediate molybdenum oxides, such as Mo18O52, Mo8O23, or Mo4O11. These oxides show, depending on the degree of reduction, one or two additional peaks above the valence band. Characteristic changes in the intensity ratios of the O 2sp peaks can be interpreted on the basis of the theoretical DOS results as a preferential loss of bridging oxygen from the MoO3 lattice. Mild ion bombardment, a technique which is often used to clean surfaces in UHV experiments, results in the case of MoO3(0 1 0) in considerable surface reduction. The reduced Mo species is comparable to that in MoO2 as indicated by the UP spectra. Therefore, mild ion bombardment cannot be considered a suitable tool for the preparation of molybdenum oxide surfaces.