Flame synthesis of calcium-, strontium-, barium fluoride nanoparticles and sodium chloride

Article abstract of DOI:10.1039/b419099h

Non-oxidic salts such as NaCl, CaF₂, SrF₂ and BaF₂ were synthesised using a flame spray method; optional doping of such fluorides with rare earth elements suggests possible applications in optics. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b419099h

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Flame synthesis of calcium-, strontium-, barium fluoride nanoparticles
and sodium chloride
Robert N. Grass and Wendelin J. Stark*
Received (in Cambridge, UK) 21st December 2004, Accepted 21st January 2005
First published as an Advance Article on the web 3rd February 2005
DOI: 10.1039/b419099h
2-ethylhexanoate and chlorobenzene resulted in ordinary stone
salt (NaCl) particles of high purity, as demonstrated by dissolving
the product in water and measuring the resulting pH that remained
neutral. To illustrate the feasibility of rare earth incorporation into
fluorides a sample of BaF2 doped with 1at% Ho was prepared
using a precursor containing barium 2-ethylhexanoate with 1at%
holmium 2-ethylhexanoate and a stochiometric amount of
hexafluorobenzene.
Non-oxidic salts such as NaCl, CaF2, SrF2 and BaF2 were
synthesised using a flame spray method; optional doping of
such fluorides with rare earth elements suggests possible
applications in optics.
The current need for computer power has pushed chip
manufacturing to UV-applications using 193 and 154 nm
lithography. A major hurdle towards such large scale applications
is the cost effective production of high purity fluoride crystals and
glasses.1a Current fibre optics rely on doped silica that is
manufactured in a flame process1b for its inherently low
entrainment of impurities. Such impurities result in scattering
centers and signal loss upon transmission. The present work
describes a method to prepare metal salts, especially halides by a
similar flame process and extends the similar, yet existing
technology to this novel material class. Among the different halide
salts, mainly fluorides have wide applications in optics as windows,
lenses, scintillation crystals1c and as host crystals for rare earth ions
(Ho, Er, Eu, Nd, Ce) exhibiting interesting properties in
optoelectronics such as lasing,1d light amplification and upconver-
sion.1e,2a,b Fluoride-based fibre optics may be obtained from doped
ZrF4 and BaF2 based glasses.2c,d
The produced powders consist of nanoparticles with diameters
of 20 nm for CaF2 to well above 100 nm for NaCl, which is
consistent with nitrogen adsorption measurements (BET, see
Table 1) and peak width from X-ray diffraction patterns (Fig. 1)
calculated using the Scherrer formula. Transmission electron
microscopy (TEM) of the fluorides (Fig. 2) shows regularly shaped
cubic crystallites for BaF2 and slightly irregular spheroid
nanoparticles for SrF2 and CaF2. When switching from CaF2 to
SrF2 or BaF2 an increase of the primary particle size can be
observed together with a decrease of the degree of agglomeration.
Scanning electron microscopy (SEM) of sodium chloride (Fig. 3)
Table 1 Products synthesised by flame spray pyrolysis
Iksue et al.3a have shown that polycrystalline structures with
crystal sizes in the nanometer regime show very high transparency
and optical properties similar to single crystals for laser
applications. Currently nano-crystalline salts can be produced by
liquid phase methods using microemulsions.3b–d They are limited
to very low concentrations and often suffer from the uptake of
impurities or remaining surfactants. Flame synthesis of inorganic
materials has rapidly evolved during the past years:4a Starting from
the preparation of mixed oxides for catalysis,4b–d more complex
materials such as doped apatites,4e carbonates4f or yttrium–
aluminium–garnet4g have become accessible by a continuous,
dry, gas-phase process. Here we want to extend the present
products from flame synthesis to metal fluorides and chlorides.
Alkaline earth fluorides and sodium chloride were produced by
mixing corresponding metal 2-ethylhexanoate liquid precursors4i–k
with stochiometric amounts of a fluoro- or chlorobenzene and
spraying the resulting liquid into a methane–oxygen-flame.4h The
product powder was collected on a preheated filter plate to avoid
the condensation of water. Neither the off-gas nor the product
contained any measurable amounts of acids (HF or HCl).
Experiments were performed using either calcium, strontium
or barium 2-ethylhexanoate together with hexafluorobenzene,
resulting in CaF2, SrF2 and BaF2, respectively. Sodium
M in metal
2- ethylhexanoate
Halide
Source
Surface
areaa/m2g21
Product
dpb/nm
Ca
Sr
Ba
Na
C6F6
C6F6
C6F6
C6H5Cl
C6F6
CaF2
SrF2
BaF2
NaCl
Ho–BaF2
139
84
36
30
14
17
34
92
26
Ba + 1at% Ho
48
a
b
Measured by nitrogen adsorption; errors ¡3%.
primary particle diameter assuming spherical particles.
Calculated
Fig. 1 X-ray diffraction patterns of the as-synthesised salts. Only the
CaF2 pattern contains peaks from calcium oxide impurities.
*wendelin.stark@chem.ethz.ch
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Fig. 4 Cumulative and differential particle size distribution of the BaF2
dispersed in ethylene glycol, measured by X-ray disc centrifuge. The dotted
line shows the best log normal fit to the data with a geometric standard
deviation of 1.33 consistent with theoretical calculations.5b,c
Fig. 2 Transmission electron micrograph of flame-made alkaline earth
fluorides. The powders show high crystallinity and low agglomeration in
the case of BaF2.
Fig. 5 UV-VIS transmission spectra of BaF2 and Ho–BaF2. RDG
represents the best fit according to the Rayleigh–Debye–Gans model. The
bottom line represents the subtraction of the measured data from the
model giving the holmium spectrum. Vertical lines show the expected
absorption peaks according to Travis et al..5e Inset: Photograph of a BaF2
pill (1.3 cm diameter).
Fig. 3 Scanning electron micrograph of flame-made NaCl. Irregular
crystallites of a wide size range are visible.
shows a much broader particle size distribution as well as larger
particles. The latter may result from grain growth due to moisture
as observed by Sata.5a
diameter of 10 nm. The optical properties of the pill as well as a pill
pressed from the holmium doped BaF2 powder were analysed
using transmission UV-VIS spectroscopy. Fig. 5 shows the
corresponding transmission spectra of the two pills. The transmis-
sion decay with the reciprocal wavelength can be accredited to
light scattering. The loss in transmission can be explained
according to the Rayleigh–Debye–Gans (RDG) theory5d as
X-ray diffraction (XRD) analysis (Fig. 1) shows high crystal-
linity for all products. While the CaF2 diffraction pattern shows
small contributions of CaO, the other diffraction patterns indicate
the absence of oxidic species. For holmium doped BaF2 no
additional signals arising from inhomogenities could be observed
and corroborate a good holmium incorporation.
Particle size analysis by X-ray disc centrifuge (XDC) of BaF2
powder dispersed in ethylene glycol after short sonication is shown
in Fig. 4. The particle size distribution can be described as log
normal with a geometric standard deviation of 1.33, which is
consistent with free molecular regime, diffusion-limited coagu-
lation of non-fractal particles.5b,c
ꢀ
ꢁ
T~(1{Rs) exp { lC2
,
where Rs accounts for the reflection losses at the two sample
surfaces, l is the wavelength and C is a constant containing the
sample thickness, the difference in refractive index and the particle
radius. The constants C and Rs could be fitted to the Ho–BaF2 line
from 400 to 780 nm and then subtracted from the measured data
resulting in the holmium absorbance spectrum with main peaks at
640, 536, 485, 452 and 416 nm.5e
The BaF2 powder could be formed into an optically transparent
pill (Fig. 5) when pressed uniaxially for 5 min at 370 MPa at room
temperature. During the compaction the specific surface area
decreased to 28 m2 g21, equivalent to an increase in mean particle
1768 | Chem. Commun., 2005, 1767–1769
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a Shimadzu UV-1650PC spectrometer in transmission mode.
Brookhaven Instruments X-ray disc centrifuge was used for particle size
distribution analysis.
A
The present work shows that beyond metal oxides, phosphates
and carbonates even oxygen-free metal salts such as fluorides and
chlorides can be formed by flame synthesis using carboxylate
precursors and a suitable halide anion source. The high affinity of
fluorine and chlorine towards alkali and alkaline earth metals
promotes the rapid formation of the corresponding salts and
suppresses oxide formation. With optional rare earth metal doping
such products may find applications in optical fibre manufacture
for UV-VIS applications and lasing materials.{
We thank Dr F. Krumeich for electron microscopy imaging and
U. Krebs, C. Furrer and H.-P. Schla¨pfer for reactor construction.
This work has been supported by the CTI grant Nr. 7103.1
NMS-NM.
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Robert N. Grass and Wendelin J. Stark*
Institute for Chemical and Bioengineering, ETH Ho¨nggerberg HCI E
107, CH-8093 Zu¨rich, Switzerland.
E-mail: wendelin.stark@chem.ethz.ch; Fax: +41 44 633 10 83;
Tel: +41 44 632 09 80
Notes and references
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{ The precursors were prepared by dissolving calcium hydroxide (Riedel-
de-Haen, p.a.), strontium acetate (ABCR), barium acetate (Strem, 99%),
sodium hydrogencarbonate (Merck, p.a.) and holmium oxide (Fluka,
99.9%) in 2-ethylhexanoic acid (Soctec, tech). Hexafluorobenzene
(Lancaster, 99%) or chlorobenzene (Fluka, 98%) was added in a
stoichiometric amount and mixed with 50 vol% xylene (Fluka, 97%).
The liquid mixtures were fed through a capillary (diameter 0.4 mm) into a
methane (1.13 l min21, Pan Gas, tech)/oxygen (2.4 l min21 Pan Gas,
tech) flame using a gear-ring pump (HNP Mikrosysteme) at a rate of
5 ml min21. Oxygen (5 l min21, Pan Gas, tech, constant pressure drop at
the capillary tip 1.5 bar) was used to disperse the liquid leaving the
capillary. A stable combustion was achieved by applying a sheath gas
(oxygen, 230 l h21, Pan Gas, tech) through a concentric sinter metal ring.4h
Calibrated mass flow controllers (Brooks) were used to monitor gas flows.
The as-formed particles were collected on a glass fibre filter (Whatmann
GF/A, 25.7 cm in diameter), placed on a cylinder mounted above the flame,
by the aid of a vacuum pump (Busch). For analysis, BET measurements
were performed on a Micromeritics Tristar. X-ray diffraction was
conducted on a Siemens powder X-ray diffractometer using Ni-filtered
CuKa radiation in step mode with a step size of 0.3u. Transmission electron
microscopy images were recorded on a CM30 ST (Philips, LaB6 cathode,
˚
operated at 300 kV, point resolution y4 A). Particles were deposited onto
a carbon foil supported grid. UV-Vis measurements were performed using
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 1767–1769 | 1769