Synthesis of hexagonal NaY(Gd)F4 ultrafine nanocrystals using sodium acetate as ionic mediator

Article abstract of DOI:10.1016/j.jfluchem.2017.07.004

Hexagonal phase ultrafine nanoparticles of NaY(Gd)F₄ were synthesized using chloride precursors and sodium acetate to provide liquid environment for ionic transportand as a source of sodium ions. Prepared ultrafine nanocrystals were analysed by XRD, laser Doppler electrophoresis and TEM measurements. Absence of fluoroacetic acid was confirmed by NMR of wash water obtained by dispersion centrifugation. Synthesized nanocrystals were 8.9?nm in diameter and had a hexagonal crystal structure. The method is appropriate for further experimental development, e.g. the synthesis of luminescent fluorides based on NaY(RE)F₄ system, and for possible industrial application because of its ability to prepare large amounts of nanocrystals and due to the fact that the process is affordable and environmental friendly.

Full text of DOI:10.1016/j.jfluchem.2017.07.004

JournalofFluorineChemistry200(2017)142–145
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Short Communication
Synthesis of hexagonal NaY(Gd)F4 ultrafine nanocrystals using sodium
acetate as ionic mediator
Vilém Bartůněka,⁎, Anna Poryvaib, Pavel Ulbrichc
a
Department of Inorganic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic
Department of Organic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic
Department of Biochemistry and Microbiology, Faculty of Food and Biochemical Technology, University of Chemistry and Technology Prague, Technická 3, 166 28
b
c
Prague 6, Czech Republic
A R T I C L E I N F O
A B S T R A C T
Keywords:
NaYF4
Hexagonal phase ultrafine nanoparticles of NaY(Gd)F4 were synthesized using chloride precursors and sodium
acetate to provide liquid environment for ionic transportand as a source of sodium ions. Prepared ultrafine
nanocrystals were analysed by XRD, laser Doppler electrophoresis and TEM measurements. Absence of fluor-
oacetic acid was confirmed by NMR of wash water obtained by dispersion centrifugation. Synthesized nano-
crystals were 8.9 nm in diameter and had a hexagonal crystal structure. The method is appropriate for further
experimental development, e.g. the synthesis of luminescent fluorides based on NaY(RE)F4 system, and for
possible industrial application because of its ability to prepare large amounts of nanocrystals and due to the fact
that the process is affordable and environmental friendly.
RE fluorides
Sodium acetate
Nanocrystals
Gadolinium
Ionic transport
1. Introduction
agent [6–10]. Microwave syntheses [13–16] or chemical etching [17]
are another methods of choice. Most broadly studied and applied up-
Rare earth fluorides have unique optical properties such as scintil-
lation, down-conversion or up-conversion. These properties determine
their considerable potential for both biological and technical applica-
tions. Due to the low systemic toxicity and cytotoxicity ionic insoluble
rare earth fluorides are very suitable for medical applications. These
materials seem to be a possible way to create complex drug delivery
systems for anticancer treatment [1]. Also a possibility of use of their
luminescent properties in infrared photodynamic therapy of cancer [2]
is intensively studied. Similarly, luminescent rare earth nanomaterials
are interesting for bioprobe applications [3]. NaYF4 doped by various
rare earth ions, e.g Yb3+ and Er3+ for up-converting nanocrystals, are
commonly used for optical applications. From all available crystal-
lographic modifications of these compounds, hexagonal modification
exhibited the best quantum yields [23].
converting mixed rare earth fluorides are commonly prepared using
hydrothermal synthesis in oleic acid [18], mostly in the form of sodium
salt [19–21] or with various other cations such as barium [22].
Sodium acetate is a common affordable and non-toxic chemical
substance, which is widely used in industry for example as a food ad-
ditive or as a base for the reaction mixtures neutralisation. Its trihydrate
form has low melting temperature of 58 °C and simultaneously it is the
source of sodium ions.
Thus, in the presented work we use sodium acetate trihydrate as a
solvent and as a sodium source for affordable and green low tempera-
ture synthesis of Na(Gd)F4 hexagonal nanocrystals in the boundary
between solvothermal and solid state reaction.
2. Results and discussion
Large range of methods can be utilised for rare earths nanofluorides
synthesis. Standard method is synthesis using oleic acid as surfactant
and 1-octadecene as a solvent [4,5]. Another methods are based on
high-temperature co-precipitation [11], on thermal decomposition of
lanthanide organic precursors [12] or on solvothermal synthesis em-
ploying ionic liquids (1-butyl-3-methylimidazolium chloride, 1-butyl-3-
methylimidazolium hexafluorophosphate (bmimPF6), or 1-butyl-3-me-
thylimidazolium tetrafluoroborate) as a solvent or as a fluorination
Synthesized ultrafine NaY(Gd)F4 nanoparticles were characterized
by X-Ray powder diffraction analysis to obtain the information about
both phase composition and average sizes of nanocrystals from peak
broadening. A mixture of cubic and hexagonal modifications was ob-
tained in experiments with only yttrium contribution. Addition of one
molar equivalent of gadolinium shifted phase composition to hexagonal
as has been previously reported in literature [23]. Therefore, hexagonal
⁎
Corresponding author.
E-mail address: vilem.bartunek@vscht.cz (V. Bartůněk).
http://dx.doi.org/10.1016/j.jfluchem.2017.07.004
Received 11 October 2016; Received in revised form 7 July 2017; Accepted 7 July 2017
Availableonline10July2017
0022-1139/©2017ElsevierB.V.Allrightsreserved.

V. Bartůněk et al.
JournalofFluorineChemistry200(2017)142–145
Fig. 1. XRD patterns of prepared hexagonal NaY(Gd)F4 nanocrystals: a) sample synthe-
sized at 90 °C, b) sample synthesized at 120 °C. PDF cards from the top down: hexagonal
phase, cubical NaF and cubical phase.
Fig. 3. Size distribution of the particles in methanol dispersion measured by laser Doppler
electrophoresis: a) aggregates before ultrasonic treatment, b) nanocrystals after ultrasonic
treatment.
phase of NaY(Gd)F4 was prepared (reference pattern 00-027-0699) only
with negligible impurity in the form of cubic phase (Fig. 1). According
to the Scherrer formula, average diameters of prepared nanocrystals
were estimated to be 8.9 nm.
treatment we used laser Doppler electrophoresis (ZetaSizer). As can be
seen in Fig. 3, initially prepared dispersion in methanol contained
larger particles corresponding by size with those observed by TEM.
After ultrasonic treatment we obtained the dispersion containing ul-
trafine nanocrystals. However, their stability was dependent on sub-
sequent conditions and processing, e.g surface stabilisation.
Sodium acetate proved to be very suitable solvent for ionic trans-
port, serving as a source of sodium ions. Its great advantage is also its
solubility in water, thus the purification process can be very effective,
affordable and environmental friendly, as well as the synthetic process.
Fluoroacetic acid is dangerous compound, so we decided to analyse
their presence in the remaining solution. However we did not expect
their occurrence since the reaction process is based on ionic exchange
and not on radical reaction. Remaining solution obtained by cen-
trifugation (15 ml) was acidified with aq. HCl to pH = 1, extracted with
3 ml of CDCl3 and analysed by 1H NMR and 13C NMR. As expected, we
found no signs of fluoroacetic acid. NMR spectra (Fig. 4) contain
characteristic resonances of acetic acid: 1H NMR (CDCl3): 2.10 (s, CH3)
and 13C NMR (CDCl3): 20.88 (CH3), 177.15 (COOH).
Temperature and time dependence of the process were investigated
in order to figure out whether the lower temperature could be used for
successfull preparation of nanocrystals. We found that lower tempera-
ture (90 °C) during the synthesis is not suitable, as can be seen in
Fig. 1a, where the diffraction pattern of the sample treated 15 h at 90 °C
is shown (for the comparison, the sample treated at 120 °C has the re-
action time only 1 h). Reaction mechanism involves the formation of
both NaF and cubical NaY(Gd)F4 phase as shown in Fig. 1a. Both of
these phases subsequently dissolve in hexagonal NaY(Gd)F4. Final
product prepared at 120 °C contains no NaF and a negligible amount of
cubical NaY(Gd)F4 phase.
TEM measurements of prepared nanocrystals (Fig. 2) showed that
these crystals are ultrafine in sizes and tend to aggregate into larger
clusters. However, these clusters can be separated by using of ultra-
sound (as may be seen on bottom panels of Fig. 2) and fabricated as
needed, for e.g. stabilized in selected solution.
For the measurements of the dispersion before and after ultrasonic
Obtained nanocrystals were quite small in diameter (in ultrafine
Fig. 2. TEM photographs of the material synthesized at
120 °C.
143

V. Bartůněk et al.
JournalofFluorineChemistry200(2017)142–145
Fig. 4. a) 1H NMR and b) 13C NMR spectra of remaining solution after the nanocrystals synthesis.
range) and thus the described synthetic procedure may be adapted for
the luminescent nanoparticles preparation by doping of various rare
earths ions, based on the requirements. The thermodynamic stability of
synthesized material was very good and XRD pattern measured after
more than a half of a year yielded the same results as in the case of
freshly prepared samples. Also the possibility to obtain entities with
larger sizes than is the threshold for nanoparticles (obtained aggregates
were larger than 100 nm in diameter), and on the other hand ultrafine
nanocrystals below 10 nm, could be very useful. This offers a potential
application of prepared nanocrystals in e.g. biomedical or cosmetic
research. Our method possesses more than 200 mg of the product in
higher than 80% yield and is apparently suitable to gain even much
bigger amounts of the final product. Besides, it is cost effective and
environmental friendly due to the using of very affordable and water
soluble precursor, sodium acetate. Therefore, the method can be
adapted also to an industrial use.
using Scherrer formula.
The samples for transmission electron microscopy were prepared by
the deposition of a 6 μl drop of studied solution onto carbon coated
copper grid, excess od solution was removed and grids were dried by
Whatman filtration paper. The samples were observed by transmission
electron microscope JEOL JEM-1010 at accelerating voltage of 80 kV.
Pictures were taken by SIS MegaView III digital camera (Soft Imaging
Systems) and analysed by AnalySIS v.3.2 software.
NMR spectra were recorded with an Agilent 400-MR DDR2 (1H:
400 MHz, 13C: 100 MHz). Chemical shifts (δ) are expressed in ppm and
are referred to the residual peak of the solvent.
BANDELIN SONOPULS 3200 ultrasonic homogenizer with probe
PH163 was used for disintegration of aggregates in order to obtain
dispersion of the nanocrystals. Small amount of the material was dis-
persed in methanol using ultrasonic bath. Subsequently, 0.25 kJ pulses
from homogenizer were applied with 10 s active/10 s delay regime.
Overall time of ultrasound treatment was 5 min.
Zetasizer Nano series instrument from Malvern Instruments Ltd. was
used for the measurements of particle’s sizes using laser Doppler elec-
trophoresis and Zetasizer Software 6.32 Malvern Instruments Ltd. was
used for the data processing.
3. Conclusions
NaY(Gd)F4 ultrafine nanoparticles were synthesized using chloride
precursors and sodium acetate as both ionic transport medium and
basic source of sodium ions. Obtained product was characterized by
XRD, laser Doppler electrophoresis and by TEM measurements. The
presence of fluoroacetic acid in remaining solution after the cen-
trifugation of dispersion was determined by NMR. Synthesized nano-
crystals were 8.9 nm in diameter and possessed hexagonal crystal
structure. Presented method is suitable for further development for
preparation of luminescent fluorides and for industrial use, since large
volumes of nanocrystals can be easily prepared. Moreover, this method
is affordable and environmental friendly.
Acknowledgement
This work was supported by specific university research (MSMT No.
20-SVV/2016).
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