Microwave-assisted synthesis and characterization of ultrafine neodymium oxide particles

Article abstract of DOI:10.1016/j.jallcom.2007.04.060

Nanocrystalline neodymium oxide precursor particles have been prepared for the first time by a microwave-assisted hydrothermal route from a solution containing Nd(CH₃COO)₃·H₂O. Further thermal treatment of the as-prepared precursors resulted in the formation of the well-crystallized Nd₂O₃ (cubic or trigonal) nanoparticles with fibrous or rod-like morphology. It was found that neodymium oxide is mesoporous material owning the specific surface area up to 130 m²/g. The proposed synthesis, in contrast to conventional hydrothermal method, offers very rapid heating with better yield and high reproducibility.

Full text of DOI:10.1016/j.jallcom.2007.04.060

Journal of Alloys and Compounds 451 (2008) 297–300
Microwave-assisted synthesis and characterization of
ultrafine neodymium oxide particles
Mirosław Zawadzki∗
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, PO Box 1410 Wrocław 2, Poland
Available online 13 April 2007
Abstract
Nanocrystalline neodymium oxide precursor particles have been prepared for the first time by a microwave-assisted hydrothermal route from a
solution containing Nd(CH3COO)3·H2O. Further thermal treatment of the as-prepared precursors resulted in the formation of the well-crystallized
Nd2O3 (cubic or trigonal) nanoparticles with fibrous or rod-like morphology. It was found that neodymium oxide is mesoporous material owning
the specific surface area up to 130 m2/g. The proposed synthesis, in contrast to conventional hydrothermal method, offers very rapid heating with
better yield and high reproducibility.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Microwave-assisted hydrothermal synthesis; Neodymium oxide; Nanoparticles
1. Introduction
in various fields as luminescent materials [6], catalysts [7], pro-
tective coatings or components for advanced ceramic materials
[8].
Hydrothermal methods, which involve low temperatures and
wet chemical technique, offer the possibility for the synthesis of
high-purity, homogeneous, and ultrafine materials. In particular,
the use of microwaves as the heating source may offer benefits
in terms of cost savings through the reduction in processing time
and energy input and may result in improved product yields [1].
Due to the properties of internal and volumetric heating, thermal
gradients during microwave processing are avoided, providing a
uniform environmental for reaction and the consequent dramatic
increaseinreactionrates. Recently, theapplicationofmicrowave
irradiation to materials science has shown rapid growth due
to its usefulness in the preparation of a variety of nanosized
inorganic materials [2,3]. Compared with conventional heat-
ing, microwave heating has an advantage of high-efficiency and
rapid formation of nanoparticles with a narrow size distribution
and no serious agglomeration. In particular, microwave-assisted
hydrothermal (or more generally solvothermal) method may
be very useful in production of finely dispersed, nanocrys-
talline materials of great technological importance [4,5]. To
such group of materials belong nanocrystalline rare earth oxides.
Among them, neodymium oxide has been attracting a great
deal of interest due to its unique properties and applications
Rare earth oxides are usually produced by oxalic acid pre-
cipitation in industry. By that procedure, the products are pure
and easily filtered but the size of the powders is in the range
of micrometers [9]. In order to get ultrafine rare earth oxide
particles, such routs as chemical vapor deposition, laser abla-
tion, sol–gel processes, polyol method, etc. can be used. At
the nanometer scale Nd2O3 is reported to have been elaborated
only in the form of agglomerated powders by microemulsion
technique [10], sol gel auto-combustion [11], hydrogen plasma-
metal reaction [12]. Recently, hydrothermal methods have been
successfullyappliedforthepreparationofdispersedneodymium
oxide nanoparticles with various morphology [13]. In this paper
the preparation of nanocrystalline neodymium oxide precursors
for the first time by a microwave-assisted hydrothermal route is
reported.
2. Experimental
2.1. Samples
Neodymium acetate hydrate, prepared in our laboratory, was used as starting
material. XRD pattern of the acetate (not shown) corresponded well with the
pattern calculated from the single crystal data of Nd(ac)3·H2O as reported by
Junk et al. [14]. Other reagents were of analytical grade and used without further
purification. Neodymium acetate was prepared by slow dissolution of the oxide
(Aldrich) in a hot acetic acid (glacial) followed by a vacuum evaporation of
∗
Tel.: +48 71 3435021; fax: +48 71 3441029.
E-mail address: m.zawadzki@int.pan.wroc.pl.
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.04.060

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M. Zawadzki / Journal of Alloys and Compounds 451 (2008) 297–300
the excess acid at room temperature. 1.96 g acetate was added to 50 ml distilled
water (previously degassed at boiling temperature in order to remove CO2) with
continuous stirring until it was completely dissolved. The microwave-assisted
hydrothermal synthesis were performed in a microwave accelerated reaction
system MW Reactor ERTEC Model 02-02, described elsewhere [15], which
operates at 2.45 GHz frequency with 0–100% of full power (1000 W). A
microwave accelerated reaction system equipped with a teflon vessel of 70 ml
capacity was filled with the prepared solution up to 70% of the total volume.
The reactor was sealed and then the vessel was microwave heated to the desired
temperature (220 and 290 ◦C, respectively, for samples A and B) at a rate of
15 ◦C/min. The pressure of the reactor gradually increased, about 20 and 40 bar,
respectively, and maintained during the holding period (1 and 4 h, respectively,
for samples A and B). The light violet product was obtained which was washed
at least five times by repeated cycles of centrifugation and re-dispersion in
deionized water. Finally, the nanoparticles in the product were concentrated and
separated from the solvent by centrifugation and subsequent vacuum drying
at room temperature for several hours. Conventional thermal treatment of the
as-prepared powder was performed at temperatures up to 800 ◦C for 4 h to
obtain the well-crystallized neodymium oxide.
Transmission electron micrographs (TEM) and corresponding electron
diffraction patterns (SAED) of the samples were obtained by using Tesla BS
500 microscope at 90 kV. Samples for the TEM observations were prepared
by ultrasonically dispersing the powders in water and putting a droplet of the
suspension on a copper microscope grid covered with perforated carbon.
The textural properties of samples (specific surface area and porosity) were
determined by nitrogen adsorption–desorption isotherms at liquid nitrogen tem-
perature by using an automatic volumetric apparatus (FISONS Sorptomatic
1900). Prior to measuring, all samples were degassed at 250 ◦C for some
hours and 10−3 Torr. Specific surface areas, SBET, were measured by the
Brunauer–Emmett–Teller (BET) method. The pore distribution was analyzed
following the Dollimore–Heal (D–H) method, which was applied to the desorp-
tion branch of each isotherm.
3. Results and discussion
Phase composition and morphology of the samples pre-
pared under microwave-assisted hydrothermal conditions from
Nd(ac)3·H2O precursor depend on the reaction parameters.
Fig. 1 shows XRD patterns of product A (220 ◦C, 1 h), as-
prepared and heated at various temperatures. The analysis of
the pattern for the as-prepared sample (Fig. 1a) indicates that it
could not be assigned to any known neodymium compound [17].
It resembles to some extent the pattern of neodymium hydroxide
but the presence of a strong reflection at low 2θ angle suggest that
the structure may be similar to that observed in layered hydrox-
2.2. Methods of characterization
The structure and crystallinity of the samples were characterized by X-ray
powder diffraction (XRD) method. The XRD data were collected using DRON-3
diffractometer with Cu K␣ radiation. The ICDD database was utilized for phase
identification. The average crystallite size D of Nd2O3 was calculated using the
Schererr equation [16] from the broadening of the X-ray line (2 2 2) and (1 0 1),
for C and A type oxide, respectively, which was calibrated from high purity
silicon.
Fig. 1. XRD patterns of product A: (a) as-prepared; (b) heated at 600 ◦C; (c)
heated at 800 ◦C.
Fig. 2. XRD patterns of product B: (a) as-prepared; (b) heated at 600 ◦C; (c)
heated at 800 ◦C.

M. Zawadzki / Journal of Alloys and Compounds 451 (2008) 297–300
299
ide metal acetates [18]. After heat treatment the strongest low
angle reflection disappears and complete reordering of the sam-
ple with crystallization of neodymium oxide phase was noticed.
In particular, when product A is heated at 600 ◦C (Fig. 1b) crys-
talline reflections appear in the pattern, which could be assigned,
to two forms of Nd2O3: cubic [19] and trigonal [20], respec-
tively as major and minor phase. Further treatment at 800 ◦C
(Fig. 1c) leads to crystallization of trigonal phase (A-Nd2O3)
but some amounts of cubic phase (C-Nd2O3) is still detected.
The average crystallite size of neodymium oxide particles, cal-
culated from the half-width of the diffraction peaks, increases
from about 9 nm (cubic phase) to 17 nm (trigonal phase) as the
heating temperature rises from 600 to 800 ◦C.
The XRD results of the product B (290 ◦C, 4 h) thermal evolu-
tion are presented in Fig. 2. It is seen that the as-prepared sample
(Fig. 2a) matched well with the pattern of Nd(OH)3 calculated
from single crystal data [21]. After further heat treatment the
hydroxide transformed completely into cubic oxide. The phase
identification reveals the characteristic patterns of C-Nd2O3
and no other phases are detected after heating up to 800 ◦C.
According to literature cubic neodymium oxide is observed
during decomposition of Nd(OH)3 at temperature range from
Fig. 3. TEM micrographs of product A: (a) as-prepared; (b) heated at 600 ◦C;
(c) heated at 800 ◦C; with corresponding SAED patterns.
Fig. 4. TEM micrographs of product B: (a) as-prepared; (b) heated at 600 ◦C;
(c) heated at 800 ◦C; with corresponding SAED patterns.

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M. Zawadzki / Journal of Alloys and Compounds 451 (2008) 297–300
400 to 600 ◦C, and then progressive transformation into trigonal
phase occurs [22]. Our results indicate that neodymium hydrox-
ide obtained by microwave-assisted hydrothermal method (at
severe conditions) is suitable precursor when preparing Nd2O3
with preserved cubic structure is necessary. It should be also
noticed that the average crystallite size of C-Nd2O3 particles was
changedslightlyfrom10to14 nmwhenheatingtemperaturewas
raised from 600 to 800 ◦C. It also suggests good thermal stability
of neodymium oxide obtained from sample B as precursor.
Fig. 3 shows microstucture and morphology evolution of
the product A after heat treatment up to 800 ◦C. From TEM
image (Fig. 3a) for the as-prepared sample, the arrays of fibrous
nanoparticles randomly oriented are observed with the particle
size of the length in micrometer range and the thickness from
6 nm, and some of them are slightly aggregated into bundles
due to the higher surface energy of the nanoparticles. SAED
pattern from the sample contains broad rings typical for amor-
phous materials. After heat treatment (Fig. 3b and c) the fibers
preserve their shape, though become shorter and often stick
together to form belts up to tens nanometers. In SAED pat-
terns well-developed rings of cubic and trigonal Nd2O3 are seen,
respectively, for the sample heated at 600 and 800 ◦C.
TEM micrographs of the product B, as-prepared and after
furtherheatingareshowninFig. 4. SAEDandTEMrevealedthat
the as-prepared sample (Fig. 4a) consists of Nd(OH)3 rod-like
crystals with diameter from 7 nm and length up to 300 nm. When
sample B is heated at 600 ◦C (Fig. 4b) or even at 800 ◦C (Fig. 4c),
the shape of the particles is preserved however some signs of
aggregation occur and an increase in the particle diameter is
also noticeable. Both SAED patterns (at 600 and 800 ◦C) exhibit
C-Nd2O3 structure of particles.
B (according to IUPAC classification [23]). This means, that
Nd2O3 is mesoporous material with a very low contribution
of micropores, responsible for the adsorption observed at low
pressure P/P0 < 0.1. Inset on Fig. 5 indicates that both samples
have broad monomodal pore size distribution, centered at 4.3
and 6.1 nm for the samples A and B, respectively. It should be
noticed, however, that the most of pore diameters include in the
range of 2–10 nm. It was found that specific surface area is 130
and 90 m2/g for the samples A and B, respectively.
4. Conclusions
The nanocrystalline neodymium oxide precursors of differ-
ent morphology were successfully obtained under hydrothermal
conditions using microwave heating for a relatively short time.
It was found that the reaction parameters such as tempera-
ture/pressure and hold time determined morphology, as well as
crystal structure. At mild conditions very long fibrous nanopar-
ticles are formed while shorter rod-like particles are obtained
at serve conditions (higher temperature/pressure and prolonged
time). After heating both products reveals Nd2O3 structure
(cubic or trigonal) with interesting textural properties includ-
ing high surface area, mesoporosity with pore size distribution
below 10 nm.
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Fig. 5. Nitrogen adsorption–desorption isotherms and pore size distribution of
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