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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: TixOy, NbxOy and TaxOy. 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 TixOy samples are composed of 2 nm-sized particles with anatase-like core and amorphous shell structure. In case of NbxOy and TaxOy, 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 TaxOy best. For TixOy and NbxOy 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 TixOy 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 TaxOy compared to the other systems, a flow cell approach was used to track TaxOy 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.
