Mapping Valence Band and Interface Electronic Structure Changes during the Oxidation of Mo to MoO₃ via MoO₂ and MoO₃ Reduction to MoO₂: A NAPPES Study

Abstract

Tuning the surface energetics, especially work function (ϕ) of the materials, is of a great deal of interest for a wide range of surface- and interface-based devices and applications. How the ϕ of a solid surface changes under the reaction conditions is of paramount interest to the chemists, particularly in the areas of surface-dependent phenomena such as, catalysis and electrochemistry. In the present study, by using the valence band and core-level photoelectron spectroscopy, surface electronic changes from Mo to MoO3 via MoO2 was studied under relevant near-ambient pressure (NAP) and high temperature conditions. A very significant change in ϕ from Mo to MoO3 was observed and it is well corroborated with the changes in gas-phase vibrational features of O2 in both near-ambient pressure ultraviolet photoelectron spectra (NAPUPS) as well in NAP X-ray photoelectron spectroscopy. Reversible changes in the electronic structure is observed when MoO3 was reduced in H2 to MoO2. On the basis of the extent of oxidation/reduction of MoO x , NAPUPS has shown, one or two additional peaks in the band gap at 0.6 and 1.6 eV below the Fermi level. Mo5+ features are identified in the VB and in the Mo 3d core levels with distinct features. Mo5+ features are also stable and essential to bridge MoO2 and MoO3 layers, and their co-existence. In addition, characteristic changes in Mo 4d and O 2p features observed from Mo to MoO3 and well corelated to the band gap of MoO3. Oxidation and reduction propagate from the surface to bulk; indeed, this has significant implications in surface-dependent phenomena. The present study demonstrates (a) the uniqueness of NAPUPS in identifying the subtle to large changes in the electronic structure on solid surfaces under common oxidation and reduction (in general, under reaction) conditions, and (b) relevance of NAPUPS to all surface-dependent phenomena, such as catalysis and electrochemistry.

Hot Carriers and Photothermal Effects of Monolayer MoOx for Promoting Sulfite Oxidase Mimetic Activity

“…The microscopic morphology and structure of the aforementioned samples were analyzed by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). As shown in Figure b–d, the as-prepared MoO x appears to be ultrathin rose-shaped flowers with an average size of 500 nm which are different from the bulk MoO 3 (labeled as b-MoO 3 ) with traditional rectangle-like nanobelt morphology − in Figure S1. To further explore the detail structure characteristics, the high-resolution TEM (HRTEM) and select-area electronic diffraction analysis were conducted to investigate the local crystalline structure of ML-MoO x (Figure e).…”

Oxygen Vacancy‐Engineered PEGylated MoO3−x Nanoparticles with Superior Sulfite Oxidase Mimetic Activity for Vitamin B1 Detection

“…With the occurrence of high‐temperature repairing for defect sites (removal the oxygen vacancies), the intensity of monoclinic MoO 2 phase was generally decreased (Figure S15a, Supporting Information). The decline in the intensity of monoclinic MoO 2 could effectively inhibit the electron transfer trough the core of the P‐MoO 3 −x , which has also been proved by Gopinath and co‐workers, Pettes co‐workers, and Dunn and co‐workers . Furthermore, after most of the Mo 4+ and Mo 5+ were transformed into Mo 6+ , bringing about the lattice ordering and blockage of the catalytic sites.…”

NO2 Interactions with MoO3 and CuO at Atmospherically Relevant Pressures

“…Mane and co-workers predict that NO 2 does bind and oxidize O vac sites on MoO 3 while forming NO that immediately desorbs. , The decrease in Mo 5+ species with a lack of N-containing surface species (Figure c) supports this mechanism. The removal of electrons on the Mo 5+ atoms at O vac sites results in fewer Mo 5+ electronic states in the MoO 3 energy gap at higher NO 2 pressures (Figure d); and others have attributed a higher resistivity to this decrease …”

“…The removal of electrons on the Mo 5+ atoms at O vac sites results in fewer Mo 5+ electronic states in the MoO 3 energy gap at higher NO 2 pressures (Figure 3d); and others have attributed a higher resistivity to this decrease. 60 In contrast to the lack of N-containing species on the MoO 3 , NO 2 does adsorb on the CuO surface. The Cu 2p 3/2 and Cu Ledge absorption spectra (Figure 6a,b, respectively) show that the Cu remains in the Cu 2+ oxidation state throughout the entire experiment.…”