neither its chemical shift nor its breadth. The broad component is connected with Sc
ions located closer to the surface and part of them are on nanocrystals surface. This
testifies to the effect that electron density on nuclei of these ions is dependent upon
removed pore water and especially upon removed surface and structural OH groups.
OH
-
groups removal is followed by generation of anion vacancies and decreasing
of local symmetry close to additive ions and thus leading to broadening of NMR line.
It is significant that the intensity of the narrow component when heated remains
practically unchanged while the broad component appears and grows in intensity.
And this component is distinctly indentified in the sample heated to 400°С i.e. at
the temperature when water molecules connected by strong hydrogen bond with
mezopores surface are being removed. This means that in the sites of location most of
scandium ions, molecules of water generate strong and inhomogeneous electric-field
gradients which due to quadrupole expansion lead to disappearance of the component
in NMR
45
Се spectrum. Removal of water molecules causes partial decrease in
quadrupole expansion, appearance of the broad component and chemical shift of the
narrow component. In this case no significant change in electronic environment in both
groups of scandium ions occurs.
Removal of surface and structural OH groups does not lead to significant
quadrupole expansion (breadth and intensity of the second component has been
increasing up to 700°C) but it significantly increases electronic shielding near most of
Sc ions, and causes removal of this component into high frequencies area. The breadth
of a NMR Sc spectrum component localized close to the surface and at the surface of
ZrO
2
nanocrystals, results from significant scattering of their chemical shifts. This
a)
b)
Figure 5. NMR
45
Sc spectrum: а) – sample 1 calcined at 1200°С,
and b) – scandium oxide at 20°С.
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