The spectra of the as-prepared OCT and DOD samples show the presence of
resonances at g = 1.98, attributable to electrons trapped in singly ionized oxygen
vacancies V 0
þ . No signal was detected in NBA nanoparticles. Air does not affect
the spectra, while under CO/Ar atmosphere the V 0
þ amount increases only for
OCT and DOD nanocrystals. This suggests that new oxygen vacancies are created
according to the following reactions.
O 0 þ CO ! CO 2 þ V 0
V 0 ! V 0
þ
þ e
À
The amount of paramagnetic oxygen vacancies is expected to be relatable to the
electrons entering the conduction band and responsible for the electrical properties.
In order to get deeper insight into the role that the different crystal surfaces play in
determining the electrical response, the amount of V 0
þ centers detected after CO/
Ar treatment of OCT, DOD, and NBA SnO 2 nanoparticles was related to the
structure of the different exposed surfaces and plotted versus the area of exposed
{110} crystal face. The very sensitive OCT nanoparticles do not possess any {110}
crystal face, while entirely expose the {221} surfaces; OCT shows the highest
amount of V 0
þ . A lower amount of paramagnetic defects was observed in DOD
nanoparticles which have a certain amount of {110} crystal faces beside the {111}
ones. NBA nanoparticles contain high amount of {110} crystal face, while no
paramagnetic defects (Fig. 5.43).
(a)
(b)
(c)
Fig. 5.42 a As-prepared sample, b after treatment under air stream, c after treatment with CO/
argon. Insets in b show the corresponding shapes [26]
5.52 Discussion of the Case
137