Chemistry Reference
In-Depth Information
the chemical composition. However, no photocatalytic and oxidation catalytic activity
measurements have been performed on these series.
Moreover, in order to overcome the drawbacks of titanium oxide, our approach has also
been to increase its 'anionic electronegativity', focussing our attention on the modification
of the nature and the number of anions in the vicinity of the cations and to decrease the
electronic polarizability. In this context, the Ti-O(OH)-F system was investigated.
29
Partial substitution of oxygen atoms by fluorine ones is associated with (i) the reduction
of the network electronic polarizability and of the dielectric constant; as a consequence,
compounds with lower refractive indices should be obtained with a higher transparency in
the visible light region and (ii) the occurrence of localized charges inducing the loss of
photocatalytic activity. The first-reported titanium oxyfluoride, TiOF
2
, was synthesized and
characterized in 1955 by Vorres and Donohue.
30
Focussing on this, in 2006, Reddy et al.
31
investigated the electrochemical behaviour of TiOF
2
with Li metal under discharge-charge
conditions. Topotactic Li reactions with other ReO
3
-type compounds have been investi-
gated.
32
More recently, Mel'nichenko et al.
33
have shown the occurrence of few other
titanium oxyfluorides; however, no information about the crystal structure of these
compounds was given. In addition other ammonium oxofluorotitanates were prepared
and the structures described.
34-35
Finally the optical and electrical properties of Ti
oxyfluorides adopting the rutile form were published.
36
During the synthesis investigation, the modification of the R
¼
[HF]/[Ti] ratio has
enabled the preparation of several compounds that adopt different structural types.
For increasing R values, compounds deriving from the anatase, the hexagonal tungsten
bronze and the ReO
3
types have thus been synthesized.
37
Among them, the structural
properties of the ReO
3
-derived compound obtained for R
¼
3, i.e. for the highest fluorine
content, are detailed.
29
The effect of the substitution of fluoride ions F
for O
2
ones
in the vicinity of Ti
4
þ
cations has been investigated in a first step on the basis of
electronic structure calculation, by determining the variation of dielectric function and
optical constants, refractive index and attenuation coefficient in UV-visible range.
38
These calculations showed that the light-scattering properties of fluorine-substituted
phases can be controlled by modifying the fluorine amount and consequently the cell
volume.
Chemical analysis, TGA and density measurements allow the accurate determination of
the chemical formula of Ti hydroxyfluorides. The structural features of the compounds
will correlated with their UV-absorption properties.
8.2
Synthesis of Tetravalent Ce and Ti-based Oxyfluorides
8.2.1
Preparation of Ce-Ca-based Oxyfluorides
The Ce-Ca based oxyfluorides were prepared by coprecipitation performed in a basic
medium at pH
>
12. The starting materials are Ce(III) nitrates and Ca chlorides.
Yamashita et al.
19
have already explored this route but only in the case of Ce-Ca based
oxides. In our case, the pH, which is a key parameter, was adjusted taking into account
