Synthesis and photophysical properties of EuS nanoparticles from the thermal reduction of novel Eu(III) complex

Article abstract of DOI:10.1021/jp0567826

EuS nanoparticles were synthesized by the thermal reduction of single source precursor (SSP), (PPh₄)[Eu(S₂-CNEt ₂)₄]·2H₂O, under microwave irradiation. The average size of the EuS nanoparticles was found to be 8 nm (3-16 nm in size). The organic products on the EuS surface were observed by using FT-IR, NMR, and MS analyses. We have found that these are resulted from the chemical reactions of SSP and cover the nanocrystal surface. A thermal reaction of SSP gave EuS nanoparticles and the organic product (SCN(Et) ₂). The organic product would make a dimmer, (Et)₂NC(S)- (S)CN(Et)₂, by the couping of the radicals formed in the thermal reaction and/or thiopolymer in the solution through the polymerization of the radicals. The effective surface modification by the organic products led to protection of the EuS surface, resulting in the formation of the strongly luminescent EuS nanoparticles at room temperature (emission peak = 350 nm, fwhm = 58 nm, emission quantum yield = 27 ± 5%).

Full text of DOI:10.1021/jp0567826

9008
J. Phys. Chem. B 2006, 110, 9008-9011
Synthesis and Photophysical Properties of EuS Nanoparticles from the Thermal Reduction
of Novel Eu(III) Complex
Yasuchika Hasegawa,*,† Yoshiko Okada,‡ Tomoharu Kataoka,‡ Takao Sakata,§
Hirotaro Mori,§ and Yuji Wada‡
Graduate School of Material Science, Nara Institute of Science and Technology, 8916-5 Takayama-cho,
Ikoma, Nara 630-0192, and Material and Life Science, Graduate School of Engineering, Osaka UniVersity
2-1 Yamadaoka, Suita, Osaka 565-0871 Japan
ReceiVed: NoVember 23, 2005; In Final Form: February 2, 2006
EuS nanoparticles were synthesized by the thermal reduction of single source precursor (SSP), (PPh4)[Eu(S2-
CNEt2)4]‚2H2O, under microwave irradiation. The average size of the EuS nanoparticles was found to be 8
nm (3-16 nm in size). The organic products on the EuS surface were observed by using FT-IR, NMR, and
MS analyses. We have found that these are resulted from the chemical reactions of SSP and cover the
nanocrystal surface. A thermal reaction of SSP gave EuS nanoparticles and the organic product (•SCN(Et)2).
The organic product would make a dimmer, (Et)2NC(S)-(S)CN(Et)2, by the couping of the radicals formed
in the thermal reaction and/or thiopolymer in the solution through the polymerization of the radicals. The
effective surface modification by the organic products led to protection of the EuS surface, resulting in the
formation of the strongly luminescent EuS nanoparticles at room temperature (emission peak ) 350 nm,
fwhm ) 58 nm, emission quantum yield ) 27 ( 5%).
Introduction
nanoparticles (emission quantum yield ) 0.05%). However,
photochemical reaction also gives some side reactions and
complicated byproducts.8 The complicated byproducts should
be obstacles to synthesis of EuS nanoparticles with photophysi-
cal properties.
Europium(II) sulfide (EuS) is a characteristic semiconductor
having the degenerated 4f orbitals between the conduction band
(5d orbitals of Eu(II)) and the valence band (3p orbitals of S2-).1
The 4f-5d electronic transition and spin configuration of EuS
lead to unique photophysical and magnetooptical properties.2
These properties are strongly dependent on their sizes and the
environments surrounding the particle.3 Especially, the photo-
physical properties of quantum-dot nanoparticles are affected
by size and their surface conditions.4 A key point for creation
of EuS nanoparticles having the characteristic photophysical
properties is to stabilize the surface structure by surface
modification. According to the protection of the nanoparticles
surface, single-source precursors (SSP) have been utilized for
the synthesis of nanoparticles in the presence of a suitable
capping agent.5 Recently, Scholes and co-workers have reported
the synthesis of EuS nanocrystals using SSP ([Eu(S2CNEt2)3]-
(2,2′-bipyridine) or (1,10-phenanthroline) and capping agents
(trioctylphosphine and oleylamine) by heating at 300 °C.6
Thermal decomposition of SSP with suitable capping agents
gives monodispersed nanocrystals.
Here, we have successfully synthesized EuS nanoparticles
by the thermal reaction of a new SSP, (PPh4)[Eu(S2CNEt2)4]‚
2H2O (Figure 1). The thermal reaction of the SSP was carried
out under 80 °C by using the microwave irradiation to avoid
uncontrollable decomposition of the ligand, giving complicated
byproducts. Previous SSP, Na[Eu(S2CNEt2)4], causes incorpora-
tion of impurity of Na+ cation into EuS, damaging the EuS
photophysical properties. Therefore, the SSP has been improved
by exchanging the cation of Na+ with a characteristic organic
cation (PPh4+: tetraphenylphosphine). From the TEM analyses,
the average size of the EuS nanoparticles has been found to be
8 nm. The surface modification of the EuS nanoparticles was
confirmed by using of FT-IR, NMR, and MS analyses. The
emission quantum yields of the EuS nanoparticles prepared in
the present work have been found to be 27 ( 5% at room
temperature.
Recently, we have reported that the EuS nanoparticles can
be synthesized from SSP (Na[Eu(S2CNEt2)4]‚1.5H2O) without
capping agents under white LED irradiation.7 This photochemi-
cal reaction gives EuS nanoparticles and organic species such
as SCN(Et)2 and/or (Et)2CNS-SCN(Et)2 at room temperature.
The organic species function as a capping agent on the EuS
surface during the nanoparticle growth. The effective surface
modification by organic species leads to protection of the EuS
surface, resulting in the formation of the luminescent EuS
Experimental Section
Materials. Europium(III) chloride hexahydrate (EuCl3‚6H2O)
was purchased from Kanto Chemical Co. Sodium N,N-dieth-
yldithiocarbamate trihydrate (Na(S2CNEt2)‚3H2O) and aceto-
nitrile (CH3CN) were purchased from Nakalai Tesque. Tet-
raphenylphosphonium bromide (BrPPh4) and ethanol were
purchased from Wako Pure Chemical Industries Ltd. Acetoni-
trile-d3 (CD3CN) was purchased from Aldrich.
* To whom correspondence should be addressed.
† Nara Institute of Science and Technology.
‡ Graduate School of Engineering, Osaka University.
§ Research Center for Ultrahigh Voltage Electron Microscopy, Osaka
University.
Preparation of Tetraphenylphosphonium Tetrakis(dieth-
yldithiocarbamate)europium(III) dihydrates ((PPh4) [Eu-
(Et2dtc)4]‚2H2O. A solution of Na(S2CNEt2)‚3H2O (1.13 g) in
4 mL ethanol was added to EuCl3‚6H2O (0.35 g) dissolved in
10.1021/jp0567826 CCC: $33.50 © 2006 American Chemical Society
Published on Web 04/19/2006

EuS Nanoparticles from a Novel Eu(III) Complex
J. Phys. Chem. B, Vol. 110, No. 18, 2006 9009
Figure 1. Schematic process for converting precursor into EuS
nanoparticles.
4 mL ethanol with stirring.9 After the reaction mixture was
filtered, a solution of BrPPh4 in 2 mL ethanol was added to the
filtrated solution. The resulting precipitate was separated by
filtration and washed several times with ethanol. Yield: 40%.
FT-IR spectra (KBr): 1485-1482 (C-N) cm-1, 1442 (P-
Figure 2. TEM image of EuS nanoparticles. (a) HRTEM image of
nanoparticles. (b) Size distribution histogram. (c) Electron diffraction
pattern image.
1
Phenyl) cm-1, 1007 (C-S) cm-1. H NMR spectra (CD3CN):
7.91 (4H, t, J ) 6.3 Hz, [Eu(S2CNEt2)](P(C6H5)4(p)), δ7.70
(16H, m, [Eu(S2CNEt2)](P(C6H5)4(o, m)), δ3.17 (8H, q, J )
7 Hz, [Eu(S2CNCH2CH3)2](PPh4). δ1.61 (12H, q, [Eu(S2-
CNCH2CH3)2](PPh4). Found: C: 47.14, H: 5.37, N: 4.99%.
Calcd for C44H64EuN4O2PS8: C: 47.17, H: 5.76, N: 5.00%.
Preparation of EuS Nanoparticles. The precursor, (PPh4)-
[Eu(S2CNEt2)4]‚2H2O, was dissolved in acetonitrile. After the
solution was refluxed under microwave irradiation (by CEM
Discover) for 6 h, yellowish-white powder was obtained. The
reaction was monitored as a function of time by recording the
optical absorption spectrum of an aliquot. The resulting yel-
lowish-white powder was cooled to room temperature. The color
might be based on the nonreacted (PPh4)[Eu(S2CNEt2)4]‚2H2O.
This power was separated by centrifugation and washed with
acetonitrile. After washing, white powder was obtained.
Apparatus. FT-IR measurements were performed at room
temperature by a Perkin-Elmer system 2000 FT-IR spectrometer.
1H NMR data were obtained with a JEOL EX-270 spectrometer.
1H NMR chemical shifts were determined using tetramethyl-
silane (TMS) as internal standard. 31P NMR data were obtained
with a JEOL ECP-400 spectrometer. 31P NMR chemical shifts
were determined using phosphoric acid (85.0%) as internal
standard. Elemental analyses were performed with a Perkin-
Elmer 240C. Fast atom bombardment mass spectra (FAB-MS)
were recorded in the positive ion mode with a 3-nitrobenzyl
alcohol matrix on a JEOL JMS-700 mass spectrometer. FAB
mass spectroscopy was measured using a dispersion in aceto-
nitrile under positive ion mode: FAB+ (spectrum type: normal
ion). The matrix of the FAB-MS measurement is 3-nitrobenzyl
alcohol. High-resolution images of the EuS nanocrystals were
obtained with a Hitachi H-9000 TEM (high-resolution transmis-
sion electron microscopy) equipped with a tilting device ((10
degrees) and operating at 300 kV (Cs ) 0.9 mm). UV-vis
absorption spectra were measured on a JASCO V-570 spectro-
photometer at room temperature. The emission spectra were
measured using a SPEX fluorolog fluorescence spectrophotom-
eter. The emission quantum yields were obtained by using a
conventional route, integrating the photoluminescence band of
the sample (absorbance at 290 nm: 0.101, excitation at 290
nm) in acetonitrile and comparing the intensity to that of
terphenyl10 (absorbance at 290 nm: 0.092, excitation at 290
nm, the emission quantum yield ) 95%) in acetonitrile.11
persed EuS nanoparticles with clear lattice fringes. The crystal-
linity was clear and well-resolved and lattice planes of EuS (0.29
nm (200)) were observed. The average diameter of the nano-
particles was found to be 8 nm (Figure 2b). The electron
diffraction pattern of the nanocrystals revealed spacings of 2.98,
2.11, 1.80, and 1.34 dÅ, corresponding to (200), (220), (311),
and (420) planes of NaCl-type EuS nanocrystal, in agreement
with those reported previously for EuS nanoparticles (Figure
2c).3,7 The results indicated that thermal reaction of SSP under
microwave irradiation gave the EuS nanoparticles at 80 °C.
To observe the reacting solution, the absorption spectrum of
the sample including SSP (5.0 mM) in acetonitrile was recorded
at time intervals. The absorption spectra are shown in Figure
3. The peak of the absorption band of SSP was located at 420
nm and assigned to the S f Eu LMCT transition of the SSP,
(PPh4)[Eu(S2CNEt2)4], due to the comparison with Eu(III)
dithiocarbamate complexes reported previously.9 Strong absorp-
tion below 400 nm should be due to the π-π* transition of
[S2CNEt2]- ligands. After microwave irradiation, we could
follow the reaction by observing the decrease of the LMCT
band. The LMCT band was gradually decreased during the first
1 h. We propose the thermal reaction of SSP as follows (Figure
•
3). First, this reaction gives Eu(II) ion and S2CNEt2 through
•
the electron transfer from the ligand to Eu(III). The S2CNEt2
•
moiety gives S2- and SCNEt2. The EuS nanoparticles might
be formed by the reaction of Eu(II) with S2-, followed by
particle growth. Figure 3 shows appearance of reactive solvent
under microwave irradiation. The reactive solution includes EuS
nanoparticles and nonreacted yellow reagents, (PPh4)[Eu(S2-
CNEt2)4]‚2H2O. After washing with acetonitrile, white powder
without absorption at 400 nm was obtained.
It is known that the microwave irradiation is a useful tool
for nanocrystal formation with narrow size distributtion.12 This
advantage was also realized in the present work.
Surface Analyses of EuS Nanoparticles. The organic
products on the EuS surface were observed by using FT-IR,
NMR, and MS analyses. An FT-IR spectrum of the sample is
shown in Figure 4a. The vibration band at 1075 cm-1 was
assigned to C-S stretching of the organic products. The
wavenumber of this broad band was shifted to longer than that
of corresponding SSP (Eu dithiocarbamate complex: 1007
cm-1). This result suggests that the C-S bonds transform from
single bond character to double bond. We also observed the
vibration bands between 1600 and 1400 cm-1 (C-N stretching).
Results and Discussion
Characterization of EuS Nanocrystals and Reaction
Mechanism. A TEM image of the nanoparticles is shown in
Figure 2a. In the high-resolution image, we found monodis-
In the FAB-MS spectrum, we found the signal at 116.1 for
an acetonitrile solution containing EuS nanoparticles. This signal

9010 J. Phys. Chem. B, Vol. 110, No. 18, 2006
Hasegawa et al.
Figure 5. Emission and excitation spectra of EuS nanoparticles in
acetonitrile at room temperature. Excitation wavelength for measure-
ment of emission spectrum is a 290 nm. Monitor wavelength for
measurement of excitation spectrum is a 360 nm.
signals of the organic cation, tetraphenylphosphine, for a EuS
nanoparticle solution (7.10, 6.58 and 3.97 ppm) in comparison
with those for an SSP solution (7.90 (triplet, 1H) and 7.70
(multiplet 4H) ppm). 31P NMR spectra of EuS nanoparticles
and SSP in acetonitrile-d3 are also shown in Figure 4c. The
signal of tetraphenylphosphine for a EuS nanoparticle solution
(-12.22 ppm) was also shifted to higher magnetic field than
that for SSP (24.47 ppm). In general, NMR signals of organic
compounds on surface of nanoparticles are shifted to higher
magnetic field.13 We propose that EuS nanoparticles are
coordinated by organic products, such as the Eu complex. This
supposition is caused by an upper-field shift of NMR data of
tetraphenylphosphonium. Suppositional compounds, •SCN(Et)2
or their derivatives, would also coordinate on the EuS nano-
particle surface. However, discussion about higher magnetic
field signals of the suppositional compounds is not clear at the
present time because the chemical structures of suppositional
compounds are different from that of corresponding SSP,
(PPh4)[Eu(S2CNEt2)4]‚2H2O. Furthermore, we have measured
elemental analysis of our surface-modified EuS nanoparticles
(Found: C: 11.91, H: 2.17, N: 0.56%). These data suggest
the presence of organic products on the EuS surface. A small
percentage of nitrogen might be caused by small amount of
•SCN(Et)2 or derivatives. We propose that EuS nanoparticles
have a sulfide-rich surface, assuming that the nanoparticles plus
ligands, Ph4P+, is overall neutral.
Figure 3. (a) UV-vis absorption spectra of the experimental solution
in each reaction time. The absorption at 420 nm shows S f Eu LMCT.
(b) Variation of the absorption at 420 nm taken at different time
intervals. (c) Decomposition mechanism of (PPh4) [Eu(S2CNEt2)4]
precursor.
In this work, a thermal reaction of SSP gave EuS nanopar-
ticles and the organic product (•SCN(Et)2). The organic product
may make a dimmer, (Et)2NC(S)-(S)CN(Et)2, by the couping
of the radicals formed in the thermal reaction and/or thiopolymer
in the solution through the polymerization of the radicals. We
could not observed the dimmer or polymer, however, Wei and
co-workers have suggested dithiocarbamate assembly on nano-
particles, recently.14 Then we propose that the EuS nanoparticle
surface is covered with the dimer or the polymer.
Optical Properties. The emission and the excitation spectra
of EuS nanoparticles in acetonitrile are shown in Figure 5. When
the sample was irradiated at 290 nm, the emission spectrum
exhibited a peak at λ ) 350 nm. This emission band is due to
a 4f-5d transition. The energy gap in nanoparticles of 8 nm
was estimated by a zero-zero band between the emission and
excitation spectra, giving the energy gap of 3.7 eV. This shows
a blue shift in comparison with previous EuS nanoparticles (of
ca. 10 nm: 3.1 eV).15 The level of the conduction band
constructed from 5d orbitals should be affected by the particle
1
Figure 4. (a) IR spectra of nanoparticles, (b) H NMR spectra, and
(c) 31P NMR spectra of spectra of EuS nanoparticles and SSP.
was assigned to C3H10NS. These results indicate that the
organic product has the structure SCN(Et)2. Therefore, we
propose that the organic product, SCN(Et)2, is formed by a
thermal reaction of Eu (III) dithiocarbamate complex.
•
•
1H NMR spectra of EuS nanoparticles and SSP (Eu dithio-
carbamate complex) in acetonitrile-d3 are shown in Figure 4b.
The two resonance signals at 3.10 and 1.19 ppm for an
acetonitrile solution containing EuS nanoparticles were assigned
to ethyl groups of the organic product (•SCN(Et)2). These signals
were shifted to higher magnetic field than those of the
corresponding peaks at 3.16 ppm (triplet, 3H) and 1.59 ppm
(quartet, 2H) for an SSP solution. We observed the shift of the

EuS Nanoparticles from a Novel Eu(III) Complex
J. Phys. Chem. B, Vol. 110, No. 18, 2006 9011
size of EuS (a quantum size effect). It seems that the 3p orbitals
of sulfides are also affected by the quantum size effect.
However, the emission bands are dependent on the transition
between 5d orbitals of Eu(II) and degenerated 4f orbitals, f-d
transition. The emission spectrum with higher energy gap (3.7
eV) might be based on the 5d orbitals. The effective mass of
EuS is estimated at 0.3-1.1 by previous reports.16 Therefore,
the electron Bohr radius is calculated to be 0.7-2.3 nm. EuS
nanoparticles with smaller size (diameter ∼5 nm) would be
affected by the quantum effect, strongly. We propose that larger
energy gap of prepared EuS nanoparticles (3-16 nm in size)
might be due to the quantum size effect. Particle size from single
source precursor depends on the condition of thermal reduction.
Smaller EuS nanoparticles (average diameter < 5 nm) would
be prepared by improvement of solvent, temperature, concentra-
tion, and structure of SSP. Concerning the emission process of
EuS nanoparticles, we are now trying to analyze the emission
lifetime. The detailed analysis of the emission lifetimes would
lead to demonstration of the emission process of EuS nanopar-
ticles soon. The emission quantum yield of EuS nanoparticles
was found to be 27 ( 5% at room temperature. This was higher
than that of corresponding EuS prepared by white LED
irradiation (0.05%).7 We also measured photophysical properties
of EuS nanoparticles from Na[Eu(S2CNEt2)4]‚1.5H2O. The
emission spectrum exhibited a peak at λ ) 340 nm. We propose
that the average size of EuS nanoparticles from Na[Eu(S2CNEt2)
4]‚1.5H2O is similar to that of corresponding EuS nanoparticles
from (PPh4)[Eu(S2CNEt2) 4]‚2H2O. However, The quantum
yield of EuS from Na[Eu(S2CNEt2)4]‚1.5 H2O was found to be
0.53%. These results indicate that tetraphenylphosphonium
cations play an important role of enhanced luminescence. We
suggest that this strong emission comes from the following two
reasons: (1) absence of impurities cation, Na+ ions, (2) perfect
protection on the EuS nanoparticle surface by organic products,
(3) less amount of the organic byproducts. As a result, EuS
nanoparticles synthesized from a thermal reaction of SSP should
show strong luminescent properties with high emission quantum
yield.
Organization) and by 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.
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Luminescent EuS nanoparticles were successfully synthesized
by the thermal reaction of a new single-source precursor (SSP).
We identified the organic product on the EuS surface using
NMR, FT-IR, and mass spectra. The emission quantum ef-
ficiency of the EuS nanoparticles with the organic product was
the highest one among reported EuS crystals at room temper-
ature. We suggest that luminescent EuS nanoparticles are
obtained by the perfect protection of the surface. The surface-
protected condition using the organic product can be manipu-
lated by molecular design of SSP, especially ligands. The
manipulation of the surface-protected condition of EuS nano-
particles would be linked to advanced applications such as
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Acknowledgment. This work was supported partly by
NEDO (New Energy Industrial Technology Development