Abstract
Oxide nanoparticles in the size range of a few nanometers are typically synthesized in solution or via laser ablation techniques, which open numerous channels for structural change via chemical reactions or fragmentation processes. In this work, neutral vanadium oxide nanoparticles are instead synthesized by sublimation from bulk in combination with a pickup by superfluid helium droplets. Mass spectroscopy measurements clearly demonstrate the preservation of the bulk stoichiometric ratio of vanadium to oxygen in He-grown nanoparticles, indicating a tendency towards tetrahedral coordination of the vanadium centers in finite geometries. This unexpected finding opens up new possibilities for a combined on-the-fly synthesis of nanoparticles consisting of metal and metal-oxide layers. In comparison to mass spectra obtained via direct ionization of vanadium oxide in an effusive beam, where strong fragmentation occurred, we observe a clear preference for (V2O5) n oligomers with even n inside the He nanodroplets, which is further investigated and explained using the electronic structure theory.
Highlights
Vanadium oxides, a prominent class of the early 3d transition metal compounds, have been objects of great interest in the past few decades due to their very broad range of oxidation states
The correct stoichiometry for all oligomers (V2O5)n for n up to 25 is con rmed via time-of- ight (TOF) mass spectra, which is a important result for the ongoing research on the synthesis of mixed-metallic core–shell nanoparticles with the same technique,[37,43,44] as it opens up the possibility of an additional coating with metal-oxide catalysts in future experiments
Two groups of peaks appear, which can be assigned to the structures (V2O5)[2], (V2O5)[3], and to fragments of both, with one or two oxygen atoms removed
Summary
A prominent class of the early 3d transition metal compounds, have been objects of great interest in the past few decades due to their very broad range of oxidation states This feature, together with the comparatively high abundance of vanadium, makes this group of oxides highly valuable for catalysis applications such as NOx reduction by NH3 We will show that a pickup of pure, neutral vanadium(V) oxide particles from vapour over the heated bulk material is possible, and demonstrate that a follow-up ionization of these particles inside the He droplets is less destructive than direct ionization in an effusive beam This is highly advantageous, as it allows us to distinguish fragments, undesired byproducts of the ionization process, from the actual particle distribution of vanadium oxide vapour over heated bulk, and helps to clarify previous discussions on stability and abundance of certain oligomers. The correct stoichiometry for all oligomers (V2O5)n for n up to 25 is con rmed via time-of- ight (TOF) mass spectra, which is a important result for the ongoing research on the synthesis of mixed-metallic core–shell nanoparticles with the same technique,[37,43,44] as it opens up the possibility of an additional coating with metal-oxide catalysts in future experiments
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