174
A.A. Fedorova et al. / Journal of Fluorine Chemistry 178 (2015) 173–177
160
a solubility in water due to the hydrophilic alcohol groups gives the
cyclodextrin molecules an ability to form a noncovalent inclusion
complexes with other organic molecules in aqueous solutions. In
such complexes, the cyclodextrin molecules play a role of the ‘‘host
molecules’’.
Important property of cyclodextrins is the ability to form
supramolecular structures in aqueous solutions. It is assumed that
beta-cyclodextrin molecules form structures in a form of columns
[27]. The molecules of beta-cyclodextrin are arranged by alterna-
tion of the head-to-head and tail-to-tail stacking, thus forming a
system of extended channels.
a)
140
120
100
80
60
40
20
2. Experimental
0
0
100
200
300
400
Temperature (0C)
2.1. Sample preparation
Fig. 1. Thermogravimetric analysis of metal trifluoroacetates hydrates: (a)
Er(CF3COO)3Á4.5H2O; (b) Yb(CF3COO)3Á4.0H2O; (c) Y(CF3COO)3Á4.9H2O; (d)
CF3COONa. The graphs are displaced on the ordinate axis relatively to each other.
In our synthesis the following compounds were used as
precursors: Na2CO3 (>98%), Y2(CO3)3ÁxH2O (>98%), Er2(CO3)3ÁyH2O
H2O (>99%), Yb (>99%), CF3COOH (99.9%), beta-cyclodextrin
hydrate (>98%), distilled water.
Yttrium and erbium trifluoracetates hydrates were obtained
from the respective metal carbonates hydrates. The precursors
were dissolved in nearly two-fold excess of 50% trifluoracetic acid
prepared by diluting the initial acid with distilled water. Sodium
trifluoracetate was obtained from sodium carbonate that was
dissolved in 50% trifluoracetic acid taken in a 1.5 molar excess.
Ytterbium trifluoracetate hydrate was prepared from ytterbium
and a two-fold excess of 50% trifluoracetic acid. Composition of
trifluoroacetates hydrates of Y, Yb and Er was determined by a
complexometric titration with 0.02 M EDTA aqueous solution and
xylenol orange as indicator and by thermogravimetric analysis.
To study the influence of beta-cyclodextrin on the decomposi-
tion of metal trifluoroacetates hydrates and on the phase
composition of fluorides formed, two reaction mixtures for
synthesis NaYF4 were prepared. The first mixture contained
stoichiometric quantities of CF3COONa and Y(CF3COO)3Á4.9H2O
and the second one has the same composition but with the
addition of beta-cyclodextrin. The weight ratio of the mixture of
the metal trifluoroacetates hydrates to beta-cyclodextrin was 2:1.
The resulting mixtures were put in alundum crucibles and distilled
water was added into each crucible (2 ml of H2O for 2.5 g of the
mixture). Then the crucibles were placed in a muffle furnace and
heated from 20 to 150 8C for 30 min. At a temperature close to
100 8C the mixtures were completely dissolved in water forming
transparent solutions. Crucibles were then thermally treated for
2 h at 150 8C to remove water. Then mixtures were heated from
150 to 420 8C for 1 h and annealed for 2 h at 420 8C. The annealing
temperature was selected from the analysis of thermogravimetric
curves (Fig. 1).
2001, JCPDS-ICDD) was used to identify phases in the solid
materials.
The thermal analysis of metal trifluoroacetates hydrates was
performed on a NETZSCH STA 409 PC/PG instrument in the
temperature range of 20–500 8C at a heating rate of 10 K min–1 in
an air flow.
Scanning electron microscopy (SEM) was performed on a Leo
Supra 50 VP instrument in a low vacuum mode. Energy dispersive
X-ray analysis (EDX) was carried out at the same instrument using
spectrometer Inca Energy +.
Transmission electron microscope (TEM) analysis was carried
out using a JEM 2100 F transmission electron microscope under a
working voltage of 200 kV.
3. Results and discussion
Thermogravimetric analysis of Y(CF3COO)3Á4.9H2O, Er(CF3-
COO)3Á4.5H2O, Yb(CF3COO)3Á4.0H2O shows that their decomposi-
tion proceeds in two main stages (Fig. 1). The first stage corresponds
to a loss of water (100–150 8C) and is observed as several small steps
of a weight loss on the thermogravimetric curves. The second stage
(280–320 8C) relates to the decomposition of metal trifluoroacetate
to the corresponding metal fluoride. The thermal decomposition of
sodium trifluoroacetate proceeds in one stage at 225–280 8C. So, we
can conclude that the decomposition of trifluoroacetates of Na, Y, Yb
and Er occurs in close temperature ranges. It is probably helpful for
the formation of complex fluorides by the decomposition of
mixtures of these metal trifluoroacetates hydrates.
In our work we propose to use beta-cyclodextrin as a reaction
agent which allows to suppress pyrohydrolysis and to obtain
porous metal fluorides. The influence of the addition of beta-
cyclodextrin on the composition of the resulting product was
studied by X-ray diffraction analysis (XRD). The XRD patterns
(Fig. 2) show that the presence of beta-cyclodextrin in a reaction
mixture of sodium and yttrium trifluoroacetates hydrates prevents
pyrohydrolysis and leads to the formation of the single-phase
sample (beta-NaYF4). The products obtained by the decomposition
of a mixture of metal trifluoroacetates hydrates without beta-
cyclodextrin contain impurities of oxifluorides phases, indicating
the pyrohydrolysis process.
Doped samples NaYF4: Yb3+ (20 at.% Yb) were obtained from
mixtures of the corresponding metal trifluoroacetates hydrates
with and without using beta-cyclodextrin. Sample NaYF4: Yb3+
,
Er3+ (17 at.% Yb and 3 at.% Er) was synthesized only from the
mixture of the corresponding metal trifluoroacetates hydrates
with using beta-cyclodextrin. Heat treatment mode was similar to
the described above. For the NaYF4: Yb3+ the weight ratio of metal
trifluoroacetates hydrates to beta-cyclodextrin was 2:0.66. Data on
the sample synthesis and their composition are given in Table 1.
2.2. Sample characterization
The X-ray diffraction (XRD) patterns of metal fluorides were
collected on Rigaku D/MAX 2500 X-ray diffractometer with
CuKalpha radiation. The data were processed using the STOE
software. The powder data base (PCPDFWIN, Version 2.2, June
In the case of a sample NaYF4: Yb3+ (CD) (sample names are
given in Table 1) smaller content of beta-cyclodextrin was used
as compared with the synthesis of a sample NaYF4 (CD) (25 wt.%
instead of 33 wt.%). This led to the appearance of weak reflexes