Synthesis of lanthanum fluoride nanocrystals and modification of their surface

Article abstract of DOI:10.1134/S1070363211020010

The LaF₃ nanoparticles were synthesized in the presence of citric acid and glycine. The products were characterized by X-ray phase analysis, transmission electron microscopy, dynamic light scattering, and infrared spectroscopy. In the presence of organic acids the synthesis was shown to result in a decrease in size of the formed particles. The IR spectroscopic studies revealed that citric acid and glycine acted as modifiers of the surface of LaF₃ particles forming a chemical bond with the surface ions La³⁺. A suggestion was advanced on the structure of the grafted surface layer. The features of the colloidal behavior of the systems were investigated.

Full text of DOI:10.1134/S1070363211020010

ISSN 1070-3632, Russian Journal of General Chemistry, 2011, Vol. 81, No. 2, pp. 277–281. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © A.V. Safronikhin, G.V. Ehrlich, G.V. Lisichkin, 2011, published in Zhurnal Obshchei Khimii, 2011, Vol. 81, No. 2, pp. 177–181.
Synthesis of Lanthanum Fluoride Nanocrystals
and Modification of Their Surface
A. V. Safronikhin, G. V. Ehrlich, and G. V. Lisichkin
Lomonosov Moscow State University, Leninskie Gory 1, Moscow, 119991 Russia
e-mail: safronikhin@yandex.ru
Received April 1, 2010
3
Abstract—The LaF nanoparticles were synthesized in the presence of citric acid and glycine. The products
were characterized by X-ray phase analysis, transmission electron microscopy, dynamic light scattering, and
infrared spectroscopy. In the presence of organic acids the synthesis was shown to result in a decrease in size of
the formed particles. The IR spectroscopic studies revealed that citric acid and glycine acted as modifiers of the
3+
3
surface of LaF particles forming a chemical bond with the surface ions La . A suggestion was advanced on
the structure of the grafted surface layer. The features of the colloidal behavior of the systems were
investigated.
DOI: 10.1134/S1070363211020010
Nano-sized crystals of ionic inorganic compounds
belong to the least studied class of inorganic
nanomaterials. The only exceptions are semicon-
ductors of A B and A B structure, although in this
respect also there are many unsolved problems.
NaF (method 1, used, in particular in [1,2]) and the so-
called double-drop method [3] that consists in the
simultaneous slow addition of solutions of reagents to
a large volume of water (method 2). In the second case
one can expect to obtain smaller particles and their
more narrow distribution due to the strong dilution of
the initial reagents and maintaining unchanged reaction
conditions in the course of time. In the synthesis of
II VI
III
V
The rare-earth element fluorides which are in-
soluble in water and biological fluids are of particular
interest for the purposes of medical diagnostics. In
particular, in [1] the synthesis was described of GdF3
nanoparticles for the application in magnetic imaging.
On the basis of europium fluoride luminescent
materials for diagnostics can be created. In extension
of our studies on the europium fluoride [2], in this
work we studied the methods of synthesis of the LaF3
nanoparticles, the modification of their surface, and the
colloid-chemical behavior of the objects obtained.
LaF we varied the technique and the temperature of
3
the reaction mixture. Comparative characteristics of
the products are listed in Table 1.
The X-ray diffraction analysis confirmed the
formation of LaF phase with a hexagonal structure
3
(JCPDS no. 32-0483, space group P3c1) in all
syntheses. The most intense reflection corresponds to
the interplanar distance d = 3.23 Å (face 111) [4]. No
reflections of other phases or impurities were detected
by the XRD. The relatively low intensity of reflections
and their broadening indicate a polycrystalline nature
of samples and a small size of the crystals. The data on
the size of the primary particles obtained from the
X-ray photographs with the Scherrer equation, the
transmission electron microscopy (TEM), and the
dynamic light scattering (DS) presented in Table 1
indicate that simultaneous adding of reagents to the
aqueous solution results in the formation of the
particles with a narrow size distribution. Increasing the
reaction temperature from 20 to 75°C leads to a
The most common method of synthesis of
nanocrystals of water-insoluble salts is the exchange
reactions in solution. The size and morphology of the
resulting nanoparticles depend on several parameters,
primarily on the concentration of reacting solutions,
temperature, and order of the addition of the reagents.
These particles, which can be conventionally termed as
primary, are capable of association with each other to
form larger aggregates up to precipitation.
In this study, we used two methods of synthesis: the
3
+
slow addition of solution of La salt to a solution of
2
77

2
78
SAFRONIKHIN et al.
Table 1. Comparative characteristics of the synthesized samples of LaF
Size of primary particles, nm
3
Average particle size, nm
(dynamic light scattering)
Method of synthesis Temperature, °C
Transmission
electron microscopy
X-ray phase
analysis
Method 1
Method 2
20
18
20
13
16
5−20
5−30
250
150
180
100
7
5
20
5−15
7
5
10−30
relatively small increase in the size of primary
particles, reduces the extent of their aggregation, and
results in the formation of more ordered crystals.
particle size distribution became narrower. For the
precipitation of these particles we used ultra-
centrifugation or addition of alcohol or acid solution
(in the case of modification with citric acid).
Primary LaF particles synthesized at 75°C have a
3
regular hexagonal structure, in contrast to the particles
obtained at 20°C (see the figure).
The isolated Citr@LaF and Gly@LaF powders
3
3
were characterized by X-ray diffraction, transmission
electron microscopy, and IR spectroscopy. Calcula-
tions of the Debye patterns showed the same phase
composition in all cases that corresponded, as in the
case of the synthesis in the absence of modifier, to the
In all cases, the colloidal system formed was
unstable: there was precipitation of lanthanum
fluoride. The traditional method of stabilizing the
nanopowder particles is a chemical modification of
their surface. For the ionic crystals it is logical to use
as modifiers complexing organic compounds capable
to form coordination bonds with the metal ions on the
surface.
hexagonal LaF . The distribution of point reflections
3
along the circles indicates the polycrystalline nature of
the modified samples and their lower ordering
compared with the unmodified ones.
At the same time, the infrared spectroscopy showed
the presence of molecules of the ligand in the products.
Table 2 shows the values of the principal absorption
In the present work we used citric acid and glycine
as the modifiers. Proceeding from the above data, the
synthesis of modified LaF particles was carried out by
3
bands in the infrared spectra of LaF , citric acid
3
method 2 at 75°C in situ, that is, by adding dropwise
CitrH , and the modified sample Citr@LaF , and their
3
3
NaF and LaCl solutions to the solution of a modifier.
3
assignment. In the spectra of unmodified LaF the
3
The obtained colloidal systems of modified
lanthanum fluoride are significantly more stabile than
the unmodified samples. Their study by the DC
method showed that in the presence of the modifier the
size of aggregates was reduced to 60–80 nm, and the
bands of stretching and bending vibrations occur of
water molecules forming a crystal hydrate with
lanthanide fluoride and adsorbed on the particle
surface, and the vibrations of the La–F bond. After
modification, in the spectra appear the bands
(
a)
(b)
(c)
1
00 nm
100 nm
100 nm
3
The images of LaF nanoparticles synthesized by (a) method 2 at 20°C and by method 1 at (b) 20°C and (c) 75°C obtained by
transmission electron microscopy.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 2 2011

SYNTHESIS OF LANTHANUM FLUORIDE NANOCRYSTALS
279
–
1
3 3 3
Table 2. Assignment of vibration frequencies (cm ) in the IR spectra of LaF , citric acid CitrH , and Citr@LaF
LaF
3
CitrH
3
Citr@LaF
3
Assignment [5, 6]
O)
C–OH(α-hydroxy group)
OH(COOH dimers)
as(COOH),
δ(H O)
as(COO )
3
600−3200
3600−3200
ν
OH(H
2
3
2
496
665
ν
ν
1
1
755, 1728, 1703
ν
1
637
2
−
1
591, 1583, 1571
ν
429, 1390, 1360
ν
s
(COOH)
−
1
458, 1438, 1420, 1411
1303, 1255, 1188
1078
ν
s
(COO )
1
292, 1173
082
ρ
r
2 τ 2 ω 2
(CH ), ρ (CH ), ρ (CH )
1
νC–OH(α-hydroxy group),
ν
C–O(carbonyl), νС–C
−
5
61
δ(COO ) + δ(LaOC)
−
5
05
ρ
r
(COO )
4
19, 411
408
δ(LaF)
characteristic of citric acid: vibrations of carboxy,
hydroxy, and C–H groups. The decrease in the
vibration frequencies of the carboxy groups in the
spectra of Citr@LaF3 compared with citric acid
indicates that they are in the deprotonated, ionized
state. The difference between the frequencies of
asymmetric and symmetric stretching vibrations of the
and the appearance of the bands of the La–O bending
–
1
vibrations at 585 cm . From comparison of the
frequencies of asymmetric and symmetric stretching
vibrations of carboxy groups Δ = 223 follows that the
bond of the carboxy group to the La(III) ion is
monodentate. This leads apparently to the formation on
the surface of chelate complexes of LaF nanoparticles
3
–
–
coordinated carboxy group, Δ = ν (COO ) – ν (COO ),
as
s
can be used as a criterion showing how the carboxy
–
1
group is coordinated. In the case of Citr@LaF this
Table 3. The assignment of vibration frequencies (cm ) in
3
–
1
difference is equal to 163 cm . This value is smaller
than the corresponding value for the sodium citrate ion
the IR spectra of glycine (Gly) and the lanthanum fluoride
(Gly@LaF
3
) modified with glycine
–
1
(
172 cm ), and, according to [7,8], we can conclude
Gly
Gly@LaF
3
Assignment [9−13]
that the carboxy groups of citric acid are coordinated
with the La(III) ions in the nanoparticle surface with
the formation, mainly, of bridging or bidentate structures.
3
600−3300
ν
OH(H
2
O)
+
3
167
ν
s
(NH
3
)
1
636
δ(H
2
O)
+
La
O
1618
593
525, 1508
1619
1630
δ
as(NH
3
)
−
O
O
1
ν
as(COO )
R
La
R
+
3
1
1
1523, 1508
1455
1408
1340
1127
1036
908
δ
s
(NH
)
La
O
1
1
1
444
414
335
δ(CH
2
)
−
s
ν (COO )
Table 3 shows the assignment of absorption bands
ρ(CH
ρ(NH
2
+
3
)
in the IR spectra of glycine and Gly@LaF . In the
3
133, 1114
034
12
)
spectra of the modified samples the vibration bands
were detected of amino, carboxy, and methylene
groups, indicating the presence of glycine in the
1
ν
as(CCN)
9
ρ(CH
2
)
–
product. The COO bands are shifted relative to the
894
699
608
867
s
ν CCN
−
relevant bands of glycine zwitterions. This indicates
the formation of bonds between carboxy group and the
surface La(III) ions. The most striking evidence of this
bonding is the disappearance in the IR spectrum of
669
ρ
w
(COO )
−
δ(COO )
−
585
532
δ(COO ) + δ(LaOC)
δ(CNH) + δ(COH) + δ(NCH)
–
1
−
Gly@LaF of the band at 506 cm , which belongs to
506
ρ
(COO )
3
r
the rotational vibrations of unbound carboxy group,
419, 408
δ(LaF)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 2 2011

2
80
SAFRONIKHIN et al.
with simultaneous coordination of amino and carboxy
groups:
ions occurs only at the processing of the powder with
the concentrated acid.
H
Thus, as a result of the study we demonstrated the
2
N
possibility of obtaining the LaF nanoparticles with
3
modified surface. At the interaction of organic acids
with the surface La(III) ions complexes are formed
with the composition and structure depending on the
nature of the modifier: in the case of glycine they are
chelates involving one N atom of the amino group and
one O atom of the carboxy group; in the case of citric
acid bridging or bidentate complexes are formed
involving two O atoms of the carboxy group.
La
O
O
At such coordination, there are no functional
groups on the surface capable of entering into
protolytic interactions with the components of the
solution with changing the surface charge. As a
consequence, the stability of such a system is almost
independent of pH.
Polycarboxyic acids can be used to stabilize the
resulting colloids because the carboxy groups unbound
to the surface carry a negative charge. The particles
obtained in the presence of citric acid can be
repeatedly exposed to the processes of coagulation–
dispersion by varying the pH of the system in the range
from 3 to 8. These processes occur without destroying
the grafted layer. This feature can be used for isolation
Fundamentally another situation is observed in the
case of Citr@LaF . As we have already shown earlier
3
[
2], the citric acid is attached to the EuF surface by
3
one carboxy group, and one of the two remaining
groups is deprotonated. Similar fixation of citrate ions
occurs in the case of the surface of gold nanoparticles
[
14]. At the contact with the surface lanthanide ions
and purification of nano-sized LaF particles. The
3
the citrate ions form a charged layer on the surface of
the particle. Due to electrostatic repulsion, the
similarly charged particles are less aggregating and do
not precipitate. Lowering the medium pH by adding
acid leads to complete deposition of particles at the pH
investigated method of modification can be used for
the synthesis of colloids of other ionic compounds, in
particular, of the lanthanide salts.
EXPERIMENTAL
4
.5–3. This is obviously a result of protonation of the
ionized carboxy group, which removes the surface
charge:
The X-ray phase analysis data were obtained on a
DRON-3M instrument (CoK radiation, λ = 0.179021 nm).
α
O
O
The morphology of the synthesized samples was
studied using a LEO912 AB OMEGA transmission
electron microscope. This instrument was also used to
obtain the X-ray photographs of modified samples by
the Debye–Scherrer method. The particle size
distribution was determined by dynamic light scat-
tering on a Nanosizer Malven device. The IR spectra
of the samples in KBr pellets were recorded on a
Thermonicolet Fourier transform IR200 infrared
HO
HO
O
O
+
H+
H+
−
O
OH
−
HO
HO
O
O
O
O
LaF3
LaF3
La
La
La
La
–
1
Repeated changes in pH of the system have no
effect on the behavior of the system: at pH > 4.5 the
particles become dispersed in solution, at pH < 4.5
they coagulate. This shows that the grafted citrate layer
is rather stable. This feature can be used for the
isolation of the resulting nanoparticles and their
separation from impurities. In addition, the carboxy
groups unbound with the nanoparticle surface can
coordinate metal ions from solutions. At the interaction
spectrometer (resolution 4 cm , the number of scans
64).
Synthesis of LaF nanoparticles. Method 1. 2 ml
3
of LaCl (0.0017 mol) was uniformly added dropwise
3
at either 20 or 75°C to 25 ml of NaF (0.0051 mol) at
continuous stirring. The formed LaF particles were
3
isolated in a centrifuge (8000 rpm), washed several
times with water, and dried in air.
2
+
of a solution containing Cu ions with Citr@LaF the
Method 2. To 100 ml of water at either 20 or 75°C
3
particles are colored blue. The resulting bimetallic
complexes are very strong: desorption of the copper
at continuous stirring were added dropwise simulta-
neously 15 ml of solution of LaCl (0.0017 mol) and
3
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 2 2011

SYNTHESIS OF LANTHANUM FLUORIDE NANOCRYSTALS
281
1
5 ml of solution of NaF (0.0051 mol). The formed
4. Staritzky, E. and Asprey, L.B., Anal. Chem., vol. 29,
no. 5, p. 856.
LaF particles were isolated in a centrifuge (8000 rpm),
3
washed several times with water, and dried in air.
5. Vanhoyland, G., Pagnae, J., D’Haen, J., Mullens, S.,
and Mullens, J., J. Solid State Chem., 2005, vol. 178,
p. 166.
Synthesis of Citr@LaF and Gly@LaF nano-
3
3
particles. Synthesis was carried out by method 2 at
6
. da Silva, M.F.P., Matos, J.R., and Isolani, P.C.,
7
1
5°C, the solutions of reagents were added dropwise to
00 ml of a neutral (pH 7–8) solution of citric acid or
J. Therm. Anal. Cal., 2008, vol. 94, no. 1, p. 305.
7
. Nakamoto, K., Infrared and Raman Spectra of Inorganic
and Coordination Compounds, Moscow: Mir, 1991,
p. 256.
aminoacetic acid (0.0017 mol).
ACKNOWLEDGMENTS
8. Deacon, G.B. and Phillips, R.J., Coord. Chem. Rev.,
1
980, vol. 33, p. 227.
The authors express their gratitude to Associate
Professor F.M. Spiridonov for help with the X-ray
analysis of products. This study was financially
supported by the Russian Foundation for Basic
Research (grant no. 09-03-00875-a).
9
. Kumar, S., Rai, A.K., Singh, V.B., and Rai, S.B.,
Spectrochim. Acta, Part A: Mol. Biomol. Spectrosc.,
2005, vol. 61, nos. 11–12, p. 2741.
0. Baran, J. and Ratajczak, H., Spectrochim. Acta, Part A:
Mol. Biomol. Spectrosc., 2005, vol. 61, no. 7, p. 1611.
1
1
1. Nakamoto, K, Infrared and Raman Spectra of Inorganic
and Coordination Compounds, Moscow: Mir, 1991,
p. 259.
REFERENCES
1
2
3
. Evanies, F., Diamente, P.R., van Veggel, F.C.J.M.,
Stanasz, G.J., and Prosser, R.S., Chem. Mater., 2006,
vol. 18, p. 2499.
1
1
2. Panyushkin, V.T., Bukov, N.N., and Abramov, D.E.,
Polyhedron, 2003, vol. 22, no. 2, p. 271.
3. Panyushkin, V.T., Spektrokhimiya koordinatsionnykh
soedinenii RZE (Spectrochemistry of the Rare Earth
Elements Coordination Compounds), Rostov-on-Don:
Rostov. Gos. Univ., 1984.
. Safronikhin, A., Shcherba, T., Ehrlich, H., and
Lisichkin, G., Appl. Surf. Sci., 2009, vol. 255, no. 18,
p. 7990.
1
4. Wulandari, P., Li, X., Tamada, K., and Hara, M.,
J. Nonlin. Optic. Phys. Mater., 2008, vol. 17, no. 2, p. 185.
. Zhilenko, M.P., Erlikh, G.V., and Lisichkin, G.V.,
Rossiiskie Nanotekhnologii, 2009, vol. 4, nos. 5–6, p. 33.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 2 2011