Synthesis of nano-sized lanthanum oxyfluoride powders by mechanochemical processing

Article abstract of DOI:10.1016/S0925-8388(02)00837-X

Lanthanum oxide (La₂O₃) was subjected to grinding with polytetrafluoroethylene (PTFE, (CF₂CF₂)_n) in air using a planetary mill followed by thermal treatment to synthesize lanthanum oxyfluoride ultra-fine particles. The grinding operation causes mechanochemical (MC) reaction between the two materials. The MC reaction is almost finished by 240 min, and the products ground for 240 min or more are composed of LaOF, amorphous La(CO₃)F and amorphous carbon (C). Heating this MC reaction product at 600°C enables us to eliminate amorphous C and decompose La(CO₃)F into LaOF, so that pure LaOF material can be obtained as the final product. The average particle size of the final product (purified LaOF) is around 10 nm.

Full text of DOI:10.1016/S0925-8388(02)00837-X

Journal of Alloys and Compounds 348 (2003) 214–219
L
www.elsevier.com/locate/jallcom
Synthesis of nano-sized lanthanum oxyfluoride powders by
mechanochemical processing
*
Jaeryeong Lee, Qiwu Zhang , Fumio Saito
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
Received 25 December 2001; received in revised form 27 March 2002; accepted 22 April 2002
Abstract
Lanthanum oxide (La O ) was subjected to grinding with polytetrafluoroethylene (PTFE, (CF CF ) ) in air using a planetary mill
2
3
2
2 n
followed by thermal treatment to synthesize lanthanum oxyfluoride ultra-fine particles. The grinding operation causes mechanochemical
MC) reaction between the two materials. The MC reaction is almost finished by 240 min, and the products ground for 240 min or more
(
are composed of LaOF, amorphous La(CO )F and amorphous carbon (C). Heating this MC reaction product at 600 8C enables us to
3
eliminate amorphous C and decompose La(CO )F into LaOF, so that pure LaOF material can be obtained as the final product. The
3
average particle size of the final product (purified LaOF) is around 10 nm.

2002 Elsevier Science B.V. All rights reserved.
Keywords: PTFE; Mechanochemical reaction; Lanthanum oxyfluoride with nano-structure; Purification
1
. Introduction
related gases [16–19]. In addition, this method has the
potential to utilize wastes such as a fluorine resin waste
instead of lanthanum fluoride or ammonium fluoride. The
product synthesized by MC method would contain a
greater quantity of impurities than that synthesized by
solid-state high temperature reaction. The impurities result
from wearing of pot and balls due to intensive grinding. In
this research, the authors used a zirconia pot and balls,
which are very stable against wearing, in order to limit the
contamination considerably.
Lanthanum oxyfluoride, LaOF, is known to be an
activator, a host material of phosphors, a catalyst for
oxidative coupling of methane and/or oxidative dehydro-
genation of ethane [1–5]. This material has been syn-
thesized by mainly two processes: one is a solid-state
synthesis process under high temperature and pressure,
causing direct reaction among starting materials [6–10],
the other is an electro-winning process, which needs
pretreatment for preparing an aqueous solution or a molten
salt [11]. In these synthesis methods, lanthanum fluoride or
ammonium fluoride has been used as a fluoride source, so
that the cost is very high. A more economical method for
synthesizing LaOF ultra-fine particles would be strongly
expected in the allied industries. The authors have pro-
posed a novel and alternative method for synthesizing
particulate functional materials from their constituent
mixture without heating [12–15]. This method uses only
an intensive grinding-machine such as a planetary mill, so
that it is an environmentally friendly method due to no
emission of any harmful substances such as fluorine-
The main purpose of this paper is to provide information
on synthesis of LaOF ultra-fine particles from La O and
2
3
PTFE by mechanochemical (MC) reaction and subsequent-
ly thermal treatment.
2. Experimental
Analytical grade lanthanum oxide (La O , Wako Pure
2
3
Chemical Industries, Japan) and polytetrafluoroethylene
with guaranteed purity (PTFE, [–CF CF –] , molecular
2
2
n
weight (M )55000–20 000; n, polymerization degree)
w
were chosen as starting materials. Two starting materials
were mixed at equivalent molar ratio of F to La, and the
mixture was kept in a desiccator. A planetary mill
*
Corresponding author. Tel./fax: 181-22-217-5137.
E-mail addresses: jrlee@iamp.tohoku.ac.jp
zhangqw@tagen.tohoku.ac.jp (Q. Zhang), saito@tagen.tohoku.ac.jp (F.
Saito).
(J.
Lee),
(
Pulverisette-7, Fritsch, Germany) was used for grinding
the mixture. The mill is composed of a pair of pots made
0
925-8388/02/$ – see front matter
 2002 Elsevier Science B.V. All rights reserved.
PII: S0925-8388(02)00837-X

J. Lee et al. / Journal of Alloys and Compounds 348 (2003) 214–219
215
of zirconia installed on a rotating disc being driven by an
electric motor. Four grams of the mixture were put in the
zirconia pot (45 cm inner volume) with seven zirconia
the following methods: an X-ray diffraction (XRD) analy-
sis (RAD-B, Rigaku, Japan) using Cu Ka radiation was
conducted to identify the phases formed in the product. An
X-ray photoelectron spectroscopic (XPS) analysis (PHI
5600 ESCA system, Ulvac-Phi., Japan) was performed to
obtain information on the chemical bonds of materials in
the ground product. Infrared spectra of the samples were
also measured using an FT-IR spectrometer (Bio-RAD,
FTS-40A) with the KBr disc method. Thermogravimetric
(TG) analysis was carried out for the ground product using
a TG/DTA analyzer (TAS-200, Rigaku) at 5 K/min in air.
Morphology and microstructure of the ground and heated
products were observed using a high-resolution transmis-
sion electron microscope with selected area diffraction
(SAD) (JEM-ARM1250, JEOL, Japan).
3
balls of 15 mm diameter. The mixture was subjected to
grinding in air at 700 rev./min for various periods of time.
Thermal treatment of the ground samples was conducted at
about 600 8C for 2 h with two objects: one is to analyze
quantitatively the free PTFE content (W) in the ground
sample, and using this analysis data we calculated the MC
reactivity (Y 5 1 2W). The other is to purify LaOF fine
particles in the ground sample. The details of the quantita-
tive analysis of W are as follows: during thermal treatment
(
600 8C, 2 h), F gas was generated from the free PTFE in
2
the ground sample, subsequently it was collected com-
pletely by absorbing with sodium hydroxide (NaOH, 0.2
N) solution, forming NaF and H O. During these opera-
2
tions, the reactions as a monomer unit reaction for the
polymer are
3. Results and discussion
6
00 8C in air
[
CF CF ](s)
→
2F (g) 1 2CO (g)
(1)
(2)
(3)
2
2
2
2
Fig. 1 shows XRD patterns of the PTFE–La O mix-
2
3
tures ground for different periods of time. Peaks of the two
materials (La O and La(OH) ) are observed in the XRD
2
4
F (g) 1 2H O(l) → 4HF(l) 1 O (g)
2 2 2
2
3
3
pattern of the mixture ground for 30 min. The formation of
La(OH) is due to hydration of La O by its adsorption of
HF(l) 1 4NaOH(l) → 4NaF(l) 1 4H O(l)
2
3
2
3
moisture from the environment, as given by La O 1
2
3
2
F
ion concentration in the NaOH solution was mea-
3H O→2La(OH) . This implies that the mechanochemical
2 3
sured by an ion chromatograph (IC) (LC10 series,
Shimadzu, Japan) to calculate the content of free PTFE in
the ground product.
(MC) reaction has not been achieved yet until 30 min.
However, these peaks of La O and La(OH)3 decrease
gradually and disappear by 120 min. Instead of this
change, new peaks appear in the patterns of the mixtures
2
3
The ground and heated products were characterized by
Fig. 1. XRD patterns of the La O –PTFE mixture ground for different periods of time.
2
3

2
16
J. Lee et al. / Journal of Alloys and Compounds 348 (2003) 214–219
ground for 120 min or more, and their intensities increase
as the grinding progresses. They are consistent with the
peaks of LaOF (tetragonal, JCPDS no. 5-0470), and only
peaks of LaOF are observed in the patterns of the products
ground for 240 min or more, entailing that LaOF can be
synthesized by grinding for 240 min or more. Of course,
the ground products are not only LaOF and they contain
other materials at this stage. In addition, the color of the
ground products had changed from white to black pro-
gressively with an increase in grinding time, suggesting the
formation of carbon. This will be discussed in the follow-
ing XPS analysis.
Fig. 2 shows TG curves of the mixtures ground for
different periods of time. These curves can be categorized
into two groups; one is a group of short grinding period
and the other is a prolonged ground group. As for the
former group, significant weight loss in the mixtures can
be seen in the temperature range from 300 to 450 8C,
reaching about 10 wt.%, irrespective of grinding time. The
weight loss is due to the thermal decomposition of mainly
PTFE in the mixture, forming F2 and CO2 gases. In
addition, the initiation temperature in the weight loss is
slightly shifted towards the lower side with increase of the
grinding time. This means that the polymerization degree
of the PTFE decreases due to cutting of the chain as the
grinding proceeds. On the other hand, the weight loss of
the mixtures in the latter group is observed in the tempera-
ture range from about 400 to 500 8C, but the final values
are about 8 wt.%, which is smaller than that (10 wt.%) of
the former group. Moreover, the initiation temperature in
the weight loss is marginally higher by about 100 8C than
that for the former group. This suggests that the products
and thermally decomposed materials of the mixtures
ground for 240 min or more are different from those of the
mixtures ground for 120 min or less. Generally, the
compounds synthesized mechanochemically have structur-
al defects more or less, which would influence the XRD
analysis of them. In these cases, in order to detect even the
minor compounds using an XRD analyzer, it is necessary
to improve the crystal quality, and annealing of the ground
mixture at relatively low temperature is conducted. In this
research, annealing was conducted at 300 8C for 2 h. The
XRD patterns of the annealed samples are shown in Fig. 3.
Only the peaks of LaOF exist in the XRD pattern of the
mixture ground for 120 min while new peaks, besides the
peaks of LaOF, were observed weakly in the XRD patterns
of the mixtures ground for 240 min or more, although they
cannot be found in Fig. 1. These new peaks are consistent
with those of La(CO )F (Hexagonal, JCPDS no. 41-595).
3
It is unexpected that La(CO )F can be synthesized at such
3
a low temperature as 300 8C. Therefore, this result indi-
cates that the structural defects of La(CO )F were made
3
better by thermal treatment at 300 8C, and the XRD peaks
can be observed. That is, the mixtures ground for 240 min
or more have included LaOF and La(CO )F synthesized
3
by grinding, not by thermal treatment. To confirm these
results, FT-IR analysis has been conducted on the follow-
ing three samples; 480-min-ground product and the prod-
ucts heated at 300 and 600 8C, respectively for 2 h, and the
results are shown in Fig. 4. The color of samples has
changed gradually from black to white with the increase of
temperature. Although it is difficult to identify the peaks in
the ground mixture, as for the pattern of the ground
mixture heated at 300 8C, the three main peaks are clearly
2
1
observed: the peak at around 1490 cm corresponds to
Fig. 2. TG curves of the La O –PTFE mixture ground for different
Fig. 3. XRD patterns of the heated mixtures after grinding for different
2
3
periods of time.
periods of time (T5300 8C for 2 h).

J. Lee et al. / Journal of Alloys and Compounds 348 (2003) 214–219
217
carbonate can be decomposed by heating at 600 8C.
Therefore, the weight loss of the latter group in Fig. 2 is
due to the decarbonation of La(CO )F and the combustion
3
of C.
Fig. 5A and B show XPS spectra on F1s and C1s of the
products ground for different periods of time, respectively.
Peak intensity at about 689 eV in the spectra shown in Fig.
5
A due to the existence of C–F bond decreases with an
increase in grinding time, and disappears at 240 min. This
implies that defluorination of PTFE has been achieved as
the grinding proceeds. Instead of this change, a new peak
appears at about 685 eV in the patterns of the mixtures
ground for 120 min or more, and its intensity increases
gradually as the grinding progresses. About 3.5 eV in
energy difference between the two peaks are seen in F1s,
suggesting that the fluoride-binding state in the product has
been transformed from covalent to ionic bonds [20]. The
ionic bond formed is attributed to the formation of LaOF
in the ground product, and this is consistent with the
results shown in Fig. 1. As for the C1s spectra, two peaks
are observed due to the presence of C–F binding at
Fig. 4. FT-IR spectra of the 480-min-ground mixture heated at 300 and
00 8C for 2 h.
2
9160.5 eV and C–C binding at 28560.5 eV, respectively.
6
The peak positioned at 29160.5 eV tends to decrease
gradually as the grinding progresses and ultimately dis-
appear by about 240 min of grinding. On the contrary, the
new peak at 29060.5 eV caused by the presence of C–O
binding, is observed in the pattern of 120-min ground
mixture and this peak increases gradually as grinding
proceeds. The peak at 28560.5 eV increases continuously
with an increase of grinding time. In other words, two
2
1
21
the C–O stretch, the peaks at 880 cm and 740 cm
correspond to C–O out-of-plane bend and C–O in-plane-
bend, respectively. This result indicates the existence of
inorganic carbonate and agrees well with the result shown
in Fig. 3. There are no significant peaks in the pattern of
the product heated at 600 8C, indicating that the inorganic
Fig. 5. XPS spectra of the mixture ground for different periods of time.

2
18
J. Lee et al. / Journal of Alloys and Compounds 348 (2003) 214–219
peaks at 29060.5 eV and 28560.5 eV are observed in the
patterns of the mixtures ground for 240 min or more and
they increase gradually with an increase in grinding time,
implying that C–O and C–C bonds have been formed in
the product, respectively [21–24]. These bonds are due to
the existence of carbonate and carbon, as described earlier.
Thus, the mechanochemical reaction between PTFE and
La O occurred to form LaOF, La(CO )F and C. There-
2
3
3
fore, the MC reaction between La O and PTFE during the
2
3
grinding can be expressed by Eq. (4), as a monomer unit
reaction for the polymer, and the decomposition by
thermal treatment at 600 8C in the mixture ground for 240
min or more can be expressed by Eqs. (5) and (6).
Grinding for 240 min or more:
2
La O (s) 1 [CF CF ](s) → 3LaOF(s) 1 La(CO )F(s) 1
2 3 2 2 3
C(s)
(4)
Thermal treatment at 600 8C for 2 h:
La(CO )F(s) → LaOF(s) 1 CO (g)
(5)
(6)
3
2
Fig. 7. XRD patterns of the 480-min-ground mixture, and that heated at
3
00 and 600 8C, respectively.
C(s) 1 O (g) → CO (g)
2
2
Fig. 6 shows the MC reactivity as a function of grinding
time. The reactivity increases slightly in the early stage of
grinding, and reaches about 0.4 at 120 min, then it
increases rapidly, reaching about 0.99 at 240 min. Sub-
sequently, the yield tends to increase with leveling off and
approaching 1.0 at 360 min. This implies that MC reaction
to synthesize LaOF would be accomplished nearly by 240
min and it is completely finished by prolonged grinding for
Fig. 7 shows XRD patterns of the 480-min-ground
sample, and that heated at 300 and 600 8C, respectively,
for 2 h. The heating at 300 8C makes it possible to improve
the crystallinity of La(CO )F formed in the product, and
3
the heating at 600 8C enables us to purify LaOF, with
decomposition from La(CO )F to LaOF and from C to
3
CO in the mixture ground for 240 min or more, as given
2
by Eqs. (5) and (6), as well as with improvement of
crystallinity in LaOF.
3
60 min or over.
Fig. 8A–C shows a SAD pattern of the mixture ground
for 480 min, its TEM dark field image, and a TEM
photograph of the product heated at 600 8C for 2 h. The
interplanar spacing calculated by ring pattern in SAD is
consistent with that of LaOF (Tetragonal, JCPDS no.
5
-0470). The TEM dark field image of the grinding
product indicates crystalline fine particles of LaOF, dis-
persing in amorphous phase, and their first order particle
sizes are confirmed to be a few nanometres. The amor-
phous phase in the TEM image is regarded as amorphous
carbon and amorphous La(CO )F. As shown in the TEM
3
photo of the 480-min-ground mixture heated at 600 8C for
2
h, Fig. 8C, LaOF fine particles with good crystallinity are
obtained and their size is about 10 nm.
4
. Conclusions
Grinding the mixture of PTFE and La O in air by
2
3
planetary mill was conducted to synthesize LaOF and
subsequently thermal treatment at 600 8C was carried out
to purify LaOF in the mixture ground for 240 min or more.
Fig. 6. Reactivity of mechanochemical reaction in PTFE to form LaOF as
a function of grinding periods of time.

J. Lee et al. / Journal of Alloys and Compounds 348 (2003) 214–219
219
Fig. 8. SAD pattern and typical TEM dark field images. (A) SAD pattern of the 480-min-ground mixture, (B) TEM dark field image of the
4
80-min-ground mixture, (C) TEM image of the 480-min-ground mixture heated at 600 8C for 2 h.
[
5] H. Jorma, S. Eija, Y. Pia, A.F. Elisabeth, L.B. Michele, P. Pierre, J.
The following conclusions can be made based on the
experimental results.
Chem. Soc. Faraday Trans. 94 (4) (1998) 481.
6] W. Klemm, Z. Anorg. Allg. Chem. 248 (1941) 167.
7] W.H. Zachariasen, Acta Cryst. 4 (1951) 231.
[
[
[
1
2
3
. LaOF can be synthesized mechanochemically by grind-
ing of the mixture, and the reaction proceeds with an
increase in grinding time. The reaction is almost
completed by grinding for about 240 min.
8] J.H. M u¨ ller, T. Petzel, J. Alloys Comp. 224 (1995) 18.
[9] J.W. Fergus, J. Mater. Sci. Lett. 16 (4) (1997) 267.
10] S.N. Achary, B.R. Ambekar, M.D. Mathews, A.K. Tyagi, P.N.
[
Moorthy, Thermochim. Acta 320 (1998) 239.
[
[
11] Y. Zhao, L. Wang, W. Liu, Zhongguo Xitu Xuebao 5 (1987) 25.
12] F. Saito, G. Mi, M. Hanada, Solid State Ionics 101–103 (1997) 37.
[13] G. Mi, F. Saito, S. Suzuki, Y. Waseda, Powder Technol. 97 (1998)
178.
14] Q. Zhang, T. Nakagawa, F. Saito, J. Alloys Comp. 308 (2000) 121.
15] J. Lee, Q. Zhang, F. Saito, J. Am. Ceram. Soc. 84 (4) (2001) 863.
16] S .V . Gangal, in: 2nd Edition, Encyclopedia of Polymer Science and
Engineering, Wiley, New York, 1989.
. The prolonged ground product consists of LaOF, amor-
phous La(CO )F and amorphous carbon and LaOF
3
synthesized mechanochemically is dispersed in the
amorphous phases.
[
[
[
. Thermal treatment at about 600 8C for the mixture
ground for 240 min or more enables us to purify the
fine particles of LaOF with good crystallinity, without
emission of fluorine-related gases. The particle sizes of
LaOF purified are about 10 nm.
[
17] J. Johnson, Nature 367 (1994) 223.
[18] M.C. Gupta, S.G. Viswanath, Ind. Eng. Chem. Res. 37 (1998) 2707.
19] T. Yoshioka, K. Fukukawa, T. Sato, A. Okuwaki, J. Appl. Polym.
[
[
[
[
[
[
Sci. 70 (1998) 129.
20] D. Briggs, M.P. Seah, in: 2nd Edition, Practical Surface Analysis,
Wiley, New York, 1990.
21] J.L. He, W.Z. Li, L.D. Wang, J. Wang, H.D. Li, Nucl. Instrum.
Methods B. 135 (1998) 512.
22] B. Belzung, K. Brace, C. Combellas, M. Delamar, F. Kanoufi,
M.E.R. Shanahan, A. Thi e´ bault, Polymer 39 (20) (1998) 4867.
23] V. Boittiaux, F. Boucetta, C. Combellas, F. Kanoufi, A. Thi e´ bault,
M. Delamar, P. Bertrand, Polymer 40 (1999) 2011.
24] T.Q. Li, M.Q. Zhang, L. Song, H.M. Zeng, Polymer 40 (1999)
References
[
[
[
[
1] C.D. Mello, G.J. Dirksen, H.F. Folkerts, A. Meijerink, G. Blasse, J.
Phys. Chem. Solids 56 (2) (1995) 267.
2] Z.S. Chao, X.P. Zhou, H.L. Wan, K.R. Tsai, Appl. Catal. A 130
(
1995) 127.
3] H. Wan, Z. Chao, W. Weng, X. Zhou, J. Cai, K. Tsai, Catal. Today
0 (1996) 67.
4] C.T. Au, Y.Q. Zhang, H. He, S.Y. Lai, C.F. Ng, J. Catal. 167 (1997)
54.
4
451.
3
3