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Abstract

Especially in catalysis, understanding the structural relationships is of high importance
to lay the bridge between structure and catalytic performance. In case of amorphous
materials, local structures are not accessible by conventional X-ray diffraction. Instead,
X-ray total scattering and subsequent pair distribution function (PDF) analysis provide
valuable structural information on these types of materials. This method was used in
this study to shed light on the local structural arrangements of amorphous metal
oxides synthesised from metal ethoxide precursors. The systems in focus are three
transition metal oxides which are photocatalytically active for light-induced water splitting: Ti_xO_y, Nb_xO_y and Ta_xO_y. The analysis starts with the local structures of the pure metal ethoxide precursor materials. The structural entities in all three systems
form aggregates of several hundreds of nanometres and show coherently scattering
domain sizes of less than 1 nm. To identify a common motif of the amorphous and the
crystalline polymorphs, amorphous metal oxides were crystallised by in situ
temperature-dependent total scattering experiments. The analyses reveal that Ti_xO_y
samples are composed of 2 nm-sized particles with anatase-like core and amorphous shell structure. In case of Nb_xO_y and Ta_xO_y, PDFs obtained from the amorphous
materials show many similarities. The structural motifs composed of two layers of
pentagonally arranged Ta-O polyhedra represents the PDF obtained from the
amorphous Ta_xO_y best. For Ti_xO_y and Nb_xO_y systems, PDF analyses were performed on
the suspensions darkened upon ultraviolet (UV) light exposure with the formation of
Ti³⁺ and Nb⁴⁺
species. Apart from some structural modifications, PDFs did not display
much difference compared to those obtained from non-UV treated counterparts.
Scanning transmission electron microscopy (STEM) examinations and electron energy
loss spectroscopy (EELS) performed on Ti_xO_y pointed to the reduction of Ti⁴⁺ by UV
illumination even after the colour has changed back. Spatially resolved EEL spectra showed indications for reduction at regions close to the surfaces. Finally, due to the high electron density in Ta_xO_y compared to the other systems, a flow cell approach was used to track Ta_xO_y entities in a dilute suspension (0.2 wt%) that flow through an in situ analysis cell. The flow cell approach was shown to work successfully. The studies show that with PDF analysis, obtaining insights into the motifs in non-crystalline metal oxides, monitoring the formation of nanoparticles in dilute solution and examination of effects under UV illumination can be carried out successfully.