Size-controlled synthesis of quantum-sized EuS nanoparticles and tuning of their Faraday rotation peak

Article abstract of DOI:10.1039/b512594d

Size-controlled EuS nanoparticles were synthesized by the reaction of europium metal with thiourea as a sulfur source in liq. NH₃, whose surface was confirmed to be modified with thiourea by FT-IR measurement, and the opto-magnetic properties (Faraday effect) of the EuS nanoparticles were investigated by using PMMA films containing the nanoparticles, showing that their Faraday rotation peaks were adjustable by control of their particle size. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b512594d

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COMMUNICATION
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Size-controlled synthesis of quantum-sized EuS nanoparticles and
tuning of their Faraday rotation peak
Tomoharu Kataoka,a Yasunori Tsukahara,a Yasuchika Hasegawab and Yuji Wada*a
Received (in Cambridge, UK) 8th September 2005, Accepted 14th October 2005
First published as an Advance Article on the web 16th November 2005
DOI: 10.1039/b512594d
liquid containing Eu2+ and solvated electrons. When thiourea was
Size-controlled EuS nanoparticles were synthesized by the
introduced into the liquid, the liquid turned yellow, indicating that
the reaction of Eu2+ with thiourea proceeded. After reaction, the
liquid ammonia was removed by evaporation at room tempera-
ture, and the resulting powder was washed several times by
centrifugation with methanol. Prepared purplish powder was
identified by X-ray diffraction (XRD) measurements. The
diffraction peaks of the powders were observed at 2h = 26.0u,
30.1u, 43.1u, 50.9u, 53.4u, 62.5u, 70.9u and 78.7u, and they were
assigned to the (111), (200), (220), (311), (222), (400), (420) and
(422) planes of NaCl-type EuS (Fig. 1). Some other peaks were
also observed at 2h = 16.1u, 28.0u, 29.2u and 40.9u, assigned to the
(100), (110), (101) and (201) planes of hexagonal Eu(OH)3.
Because Eu(OH)3 is stable and not dissolved in general solvents, it
was difficult to isolate pure EuS nanoparticles. These peaks were
so weak compared with those of EuS, indicating a very small
amount of byproduct. The average sizes of EuS were calculated by
the Sherrer equation from the peaks corresponding to the (111),
(200) and (220) planes. It is noteworthy that the peaks broadened
as the amount of thiourea increased, indicating that the average size
of EuS nanoparticles depended on the amount of sulfur source, i.e.
thiourea. Excessive introduction of thiourea (thiourea/Eu mole
ratio = 2 or 10) gave smaller EuS nanoparticles (average size = 8 or
7 nm), although the stoichiometric reaction gave the larger size
(average size = 14 nm). This size control should be due to the effect
of thiourea as a surface-modification reagent, suppressing the
particle growth. From the above results, we have found that EuS
reaction of europium metal with thiourea as a sulfur source in
liq. NH3, whose surface was confirmed to be modified with
thiourea by FT-IR measurement, and the opto-magnetic
properties (Faraday effect) of the EuS nanoparticles were
investigated by using PMMA films containing the nanopar-
ticles, showing that their Faraday rotation peaks were
adjustable by control of their particle size.
Europium sulfide (EuS) has degenerate 4f orbitals located between
the conduction band (5d orbitals of Eu2+) and the valence band
(3p orbitals of S22).1,2 The 4f–5d electron transition and spin
configuration of EuS lead to unique magneto-optical properties,
i.e., Faraday effect.3,4 Note, that the energy gap between the 4f
orbitals and the conduction band can be manipulated by
controlling the size of the EuS particle (quantum size effect). The
manipulation of the energy gap is directly linked to a novel
wavelength-tunable Faraday device (isolator) for fiber-optical
telecommunication systems. The size control of EuS nanoparticles,
however, is extremely difficult, as long as the conventional
reactions are employed such as the reaction between Eu2+ and
H2S5 and the photochemical reaction of the Eu3+ complex.6,7 In
order to control the particle size, two criteria should be taken into
account for the synthesis of EuS nanoparticles; (1) selection of an
appropriate sulfur source for quantitative introduction of sulfur to
the reaction system, leading to an accurate control of the synthetic
conditions, and (2) introduction of surface modification reagents
to prevent particle aggregation.8 To satisfy these two criteria, we
have selected thiourea as an ideal sulfur source, which is expected
to perform not only as a quantitative sulfur source, but also as a
surface modification reagent.
In this communication, we report a novel method for the size-
controlled synthesis of EuS nanoparticles (7–14 nm) by reaction of
Eu metal with thiourea in ammonia, and we discuss the quantum
size effect on their energy gaps and Faraday effect.
EuS nanoparticles were prepared by reaction of europium metal
(0.2 g) with thiourea (0.1 g, 0.2 g, 1.0 g, thiourea/Eu mole ratio was
1, 2, 10, respectively) in liquid ammonia under a N2 atmosphere.
Eu metal was dissolved in liquid ammonia at 195 K, giving a blue
aDepartment of Material and Life Science, Division of Advanced Science
and Biotechnology, Graduate School of Engineering, Osaka University,
2-1, Yamadaoka.Suita, Osaka, 565-0871, Japan.
E-mail: ywada@mls.eng.osaka-u.ac.jp; Fax: +81 (0)6 6879 7875;
Tel: +81 (0)6 6879 7926
bResearch and Education Center for Materials Science, Nara Institute of
Science and Technology, Keihanna Science City, 8916-5, Takayama-
cho, Ikoma, Nara, 630-0192, Japan. E-mail: hasegawa@ms.naist.jp;
Fax: +81 743 72 6170; Tel: +81 743 72 6171
Fig. 1 XRD spectra of EuS nanoparticles prepared by reaction of Eu
metal and thiourea, (a: Thiourea/Eu ratio = 1, b: 2, c: 10).
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nanoparticles of controlled size can be prepared by changing the
amount of thiourea.
The absorption bands attributed to H–N–H deformation vibration
and SLC stretching vibration were observed at 1590 and
1000 cm21, respectively. The wavenumbers of these peaks were
shifted to low values compared to free thiourea molecules (1620
and 1090 cm21). These results indicate that the surface of the EuS
nanoparticles is capped by thiourea molecules, resulting in
stabilization of the nanoparticles.9,10
The transmission electron microscopy (TEM) images of EuS
nanoparticles are shown in Fig. 2.{ From the TEM image
observed at low magnification (Fig. 2 (a)), the EuS nanoparticles
(size: 14 nm) were found to be aggregated. This aggregated
situation was also observed in other samples. Some EuS
nanoparticles were barely observed with enough clear fringe to
determine the particle size in the high-resolution TEM (HRTEM)
images (Fig. 2 (b)–(d)). The particle sizes determined by the
HRTEM images (Fig. 2 (b); 13 nm, (c); 9 nm, and (d); 7 nm)
agreed with those calculated by the Sherrer equation based on
XRD data. However, there was not enough EuS nanoparticles
available for determination of their size distributions.
P. C. Rieke and S. B. Bentjen have proposed a reaction
mechanism for the preparation of CdS film from Cd(NO3)3 and
thiourea.7 Based on their mechanism, we would suppose the
reaction mechanism of the present synthesis of EuS nanoparticles
to be as follows:
The UV-Vis absorption spectra of EuS nanoparticles are shown
in Fig. 4.{ Broad absorption bands were observed at the region of
400–759 nm, which was assigned to the electronic transition
between 4f7 and 4f6(7FJ)5d(t2g). The absorption peak wavelength
Eu + NH3(solvent) A Eu2+ + 2e2
CS(NH2)2 A H2S + NH2CN
H2S + 2e2 A S22 + H2
Eu2+ + S22 A EuS
When europium metal is dissolved in liquid ammonia, divalent
europium ions (Eu2+) are formed with the solvated electrons.4
Thiourea molecules produce sulfur anions (S22) in the liquid
ammonia solution by combining with solvated electrons. Thus,
EuS nanoparticles are gradually formed by the crystal growth of
Eu2+ and S22 in the liquid ammonia. Thiourea molecules also
perform as a surface stabilizer suppressing the EuS crystal growth.
In order to prove the presence of thiourea on the surface, we
measured the FT-IR spectra of the EuS nanoparticles (Fig. 3).{
Fig. 3 IR spectra of the EuS nanoparticles, whose sizes were 14 nm
(black solid line), 8 nm (pale gray line), and 7 nm (dark gray line), and that
of a thiourea (dashed line). The spectra are shown in (a) the hole region,
(b) the region corresponding to the H–N–H deformation vibration of
thiourea, (c) the region corresponding to the SLC stretching vibration of
thiourea.
Fig. 2 (a) The TEM image of the EuS nanoparticles prepared by the
reaction of europium metal and thiourea (a: Thiourea/Eu ratio = 1). (b)–
(d) The high-resolution TEM image of the EuS nanoparticles prepared by
reaction of Eu metal and thiourea, (b: Thiourea/Eu ratio = 1, c: 2, d: 10).
Fig. 4 Absorption spectra of EuS nanoparticles.
Chem. Commun., 2005, 6038–6040 | 6039
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nanoparticles having a quantum size region.{ The size control of
EuS by using thiourea should open the gates for the creation
of new opto-magnetic devices such as wavelength-adjustable
optical-isolators.
We would like to express our grateful acknowledgement to Prof.
Hirotaro Mori and Dr Takao Sakata, Research Center for Ultra-
High Voltage Electron Microscopy, Osaka University for the
TEM measurements. We would also like to express our grateful
acknowledgement to Prof. Kazuyuki Hirao and Prof. Koji Fujita,
Department of Material Chemistry, Graduate school of
Engineering, Kyoto University for the Faraday measurements.
This research was partly supported by NEDO, and a Grant-in Aid
for Scientific Research No. 17042015 from the ministry of
Education, Science, Sports and Culture. This work was also
supported by a Grant-in-Aid for Scientific Research on Priority
Areas (417, No. 17029038) from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT) of the Japanese
Government and NEDO (New Energy Industrial Technology
Development Organization). This work was supported partly by a
Grant-in-Aid for Scientific Research (No. 15750155) and a Grant-
in-Aid for Scientific Research on Priority Area A of ‘‘Panoscopic
Assembling and High Ordered Functions for Rare Earth
Materials’’ from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan.
Fig. 5 (a) Absorption spectrum (up) and Faraday rotation spectrum
(down) of EuS nanoparticles (size: 14 nm). (b) The particle-size
dependence of the peak top wavelength on Faraday rotation.
of a (14 nm), b (8 nm), and c (7 nm) were found to be 515, 520,
and 540 nm, respectively. The blue shift of the absorption bands
accompanied by the decrease of the particle size should be due to
the quantum size effect. This is the first observation of the
quantum size effect on the energy gap of EuS induced by the size
control. With these photophysical findings, we carried out Faraday
effect measurements of the EuS nanoparticles.{ Polymeric thin
films containing dispersed EuS nanoparticles were prepared for the
Faraday measurements according to the procedure described in
the literature.4 The UV-Vis absorption (Abs.) spectrum and the
Faraday rotation (FR) spectrum of the EuS sample (size = 14 nm)
are shown in Fig. 5(a). The peak of the Faraday rotation spectrum
of the EuS sample appeared at a little higher wavelength than that
of the absorption peak. The Verdet constants (V) of the EuS
samples calculated from the Faraday rotation angles4 were
calculated to be 1.4 6 1023 deg. cm21 Oe21 (a), 6.3 6 1024 (b),
and 1.6 6 1023 (c) respectively. V values of EuS nanoparticles
already reported in the literature4 were 8.9 6 1023 deg. cm21 Oe21
(the EuS size = 23 nm), 2.361023 (the EuS size = 28 nm), and
4.261023 (the EuS size = 31 nm). These values were of the same
order as those of the EuS nanoparticles prepared in the present
work. The particle-size dependence of the Faraday rotation peak
wavelength is also shown in Fig. 5(b). In order to compare the
wavelength values of the FR peak of the EuS nanoparticles
prepared in this work with that of large EuS particles, the FR peak
wavelength of the EuS nanoparticles (35 nm in diameter) was
added in Fig. 5(b). The particles were prepared by reaction of
europium metal and H2S according to the literature.5 The size was
determined by the Sherrer equation as well as the case of EuS
nanoparticles prepared in this work. The EuS sample of smaller
particle size showed a blue shift in the Faraday rotation because of
the increased energy gap. This physical relation between the
particle size and the peak wavelength of the Faraday rotation
should be understood from the quantum size effect. We have
successfully manipulated the peak wavelength of the Faraday
rotation by using size-controlled EuS nanoparticles.
Notes and references
{ Transmission Electron Microscopy (TEM) images were obtained with a
Hitachi H-9000 TEM equipment operating at 300 kV. The samples for
TEM measurements were prepared by dropping a dilute dispersion of EuS
sample in ethanol on a copper grid and drying at room temperature. FT-IR
measurements were performed at room temperature on a Perkin-Elmer
system 2000 FT-IR spectrometer. KBr pellets, which were used as a
measurement sample, were prepared by compressing a mixture of EuS
powder (2 mg) and KBr matrix (0.2 g). UV-Vis absorption spectra were
measured on a Hitachi U-3300 spectrophotometer at room temperature.
The measurement samples were the same as those for FT-IR measure-
ments. Faraday effect measurements were carried out using a measurement
system for Faraday and Kerr effects (JASCO, Model K-250). The external
magnetic field was fixed at 1.5 T. EuS nanoparticles-doped PMMA films
were prepared as below. EuS nanoparticles (0.06 g) were added to a
2-propanone solution of PMMA (1.5 g, 10 wt%) and dispersed under
ultrasonic treatment, to give a colloidal suspension. The glass substrate was
covered on two parallel edges with adhesive tape to control the thickness of
the film. The colloidal suspension was applied to one of the free edges of
the glass and distributed with a glass roll sliding over the tape-covered edges
to give PMMA films containing EuS.
{ Whereas EuS single crystal and Eu2+ doped glass have been reported,
control of the Faraday spectrum has never reported.
1 P. Wachter, Handbook on the Physics and Chemistry of Rare Earths, 2nd
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Chem. B, 2003, 107, 2193.
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In conclusion, the particle size of EuS has been successfully
controlled by using thiourea, resulting in the control of the
Faraday spectra. To the best of our knowledge, this is the first
report on manipulation of the Faraday spectra for EuS
6040 | Chem. Commun., 2005, 6038–6040
This journal is ß The Royal Society of Chemistry 2005