g 1 = 1.994, g 2 = 1.986, and g 3 = 1.975; moreover, it was seen the narrow line of E
0
Si centers at g = 2.001 and the resonances of non-bridging oxygen hole
centers Si-O
Á and peroxy radicals Si-O-O
Á at g 1 = 2.009, g 2 = 2.008, and
g 3 = 2.002. In the glass with high Sn content, color like dark yellow, it appears also
the resonance of the singly ionized oxygen vacancies of crystalline SnO 2 . The
electron spin resonance spectra revealed thus a fantastic method to distinguish
well-diluted Sn-doped silica glasses, from those which contain segregated
nanoparticles of crystalline SnO 2 . These nanoparticles can be recognized by an
absorption at 340 nm, corresponding to the energy gap of SnO 2 , and also show the
ionized oxygen vacancies of the SnO 2 lattice at g = 1.89.
5.41 Chemical Message
By using the solgel procedure, essentially starting from solutions of the element
precursors, elements different from the oxygen, silicon can be substituted by Sn and
this leads to obtain homogeneous glasses with different amounts of dopant. Then, it
results in the possibility of designing glasses with various and controlled refractive
indices to be used in projecting optical fibers. The success of the synthesis is
controlled by the physicochemical characterization of the defects induced by the
dopant element.
5.42 The Cases of Photocatalytic TiO 2
Titanium dioxide is widely used in different applications, where the most relevant is
the photocatalysis (induced by UV irradiation) [22]. TiO 2 exists in different crystalline forms: anatase, rutile, and brookite. These are expected and have been
verified to have a different catalytic activity; nevertheless, the reasons of the differences are still object of investigation. In fact, while some authors claimed to the
higher surface area or porosity, as origin of higher efficacy, others suggested that
the difference in surface defects definitely decides the activity.
It only remains to select specific crystalline TiO 2 phases and investigate the
location of the defects. Defects in TiO 2 , generated by UV irradiation, consist of
holes (electron vacancies in the valence band of TiO 2 ) (h
+ ) and free electrons (e
− )
in the oxide conduction band. They have in charge the photomineralization process,
following the reactions here reported.
5.40 Discussion of the Case
123
0
Si centers at g = 2.001 and the resonances of non-bridging oxygen hole
centers Si-O
Á and peroxy radicals Si-O-O
Á at g 1 = 2.009, g 2 = 2.008, and
g 3 = 2.002. In the glass with high Sn content, color like dark yellow, it appears also
the resonance of the singly ionized oxygen vacancies of crystalline SnO 2 . The
electron spin resonance spectra revealed thus a fantastic method to distinguish
well-diluted Sn-doped silica glasses, from those which contain segregated
nanoparticles of crystalline SnO 2 . These nanoparticles can be recognized by an
absorption at 340 nm, corresponding to the energy gap of SnO 2 , and also show the
ionized oxygen vacancies of the SnO 2 lattice at g = 1.89.
5.41 Chemical Message
By using the solgel procedure, essentially starting from solutions of the element
precursors, elements different from the oxygen, silicon can be substituted by Sn and
this leads to obtain homogeneous glasses with different amounts of dopant. Then, it
results in the possibility of designing glasses with various and controlled refractive
indices to be used in projecting optical fibers. The success of the synthesis is
controlled by the physicochemical characterization of the defects induced by the
dopant element.
5.42 The Cases of Photocatalytic TiO 2
Titanium dioxide is widely used in different applications, where the most relevant is
the photocatalysis (induced by UV irradiation) [22]. TiO 2 exists in different crystalline forms: anatase, rutile, and brookite. These are expected and have been
verified to have a different catalytic activity; nevertheless, the reasons of the differences are still object of investigation. In fact, while some authors claimed to the
higher surface area or porosity, as origin of higher efficacy, others suggested that
the difference in surface defects definitely decides the activity.
It only remains to select specific crystalline TiO 2 phases and investigate the
location of the defects. Defects in TiO 2 , generated by UV irradiation, consist of
holes (electron vacancies in the valence band of TiO 2 ) (h
+ ) and free electrons (e
− )
in the oxide conduction band. They have in charge the photomineralization process,
following the reactions here reported.
5.40 Discussion of the Case
123