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Chemistry Letters Vol.32, No.8 (2003)
Size Selective Synthesis of Surface-modified EuS Nanocrystals
Using Pyridine and Their Physical Properties
Supitcha Thongchant, Yasuchika Hasegawa, Yuji Wada, and Shozo Yanagidaꢀ
Material and Life Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Osaka 565-0871
(Received March 4, 2003; CL-030190)
The particle size of EuS nanocrystals was gradually in-
Table 1. Amount of pyridine added in the reaction, average
particle sizes and absorption peaks of the EuS nanocrystals
creased by addition of pyridine to the reaction mixture of euro-
pium metal and hydrogen sulfide in liquid ammonia. The size
effect of EuS nanocrystals was found in the absorption spectra
for the first time. The size effect on the magnetic properties of
the nanocrystals was also observed.
Samples
Pyridine
/mL
Particle
sizes/nm
Absorption
peaks/nm
EuS-0
0
20
21
30
36
400, 665
295, 560
385, 600
365, 630
EuS-0.5-p
EuS-2.5-p
EuS-10-p
0.5
2.5
10.0
Photophysical and optomagnetic properties of europium
chalcogenides are important in applying them as luminescent
materials and optical isolator.1–3 These properties are strongly
dependent on their sizes and the environments surrounding
them. For example, an enhancement of photoluminescence
was found for EuS nanoclusters in zeolite Y.4 A large Faraday
effect was observed in EuS microcrystals-embedded TiO2 thin
films.2 The size and the environment were considered to change
the specific physical properties of the low-dimension EuS crys-
tals. However, there had been no report on the clear relation be-
tween particle sizes and physical properties of nanoscaled EuS
crystals.
We reported the preparation of the first EuS and EuO nano-
crystals by using liquid ammonia as a reaction medium.5,6 The
liquid ammonia reaction is advantageous in preparing EuS
nanocrystals (20 nm); however, their crystal sizes had not been
controllable yet.
We have found pyridine, which was soluble in liquid am-
monia, as a suitable surface modification-reagent of the nano-
crystals and succeeded in controlling the particle sizes of the
EuS nanocrystals. The size-controlled EuS nanocrystals pre-
pared by addition of pyridine were characterized by XRD,
TEM, UV–vis spectra and magnetic measurements.
A liquid ammonia solution including europium metal and
H2S was prepared in the same way as the previous report.5
To this liquid ammonia solution (50 mL), distilled pyridine
(0.5 mL, 2.5 mL, 10 mL) was added. Liquid ammonia was re-
moved by evaporation at room temperature in 1.5 h after the re-
action was completed. The resulting products were purple pow-
ders.
average sizes of 30 nm and 36 nm were obtained by the addition
of pyridine 2.5 mL and 10 mL, respectively. An increase of the
amount of pyridine enlarged the particle sizes of EuS nanocrys-
tals. A TEM image of the surface-modified EuS nanocrystals
(EuS-0.5-p) is shown in Figure 1. The observed particle sizes
determined in the TEM images agreed well with those calculat-
ed from the Scherer equation. The clear lattice fringes indicated
high crystalinity of the EuS nanocrystals. We observed the
change of the absorption spectra of the dispersion solutions of
the nanocrystals in pyridine and found that EuS-0.5-p was more
stably dispersed than EuS-0. We prepared pyridine dispersion
solutions of surface-modified EuS-0.5-p and bare EuS-0
(10À4 wt%), and stood them for 8 h. As the result, the absorption
of bare EuS-0 decreased rapidly because of sedimentation of the
nanocrystals (Figure 2). In contrast, surface-modified EuS-0.5-p
showed high dispersion ability even after 8 h. The result sug-
gests that pyridine molecules prevent aggregation of EuS nano-
crystals because of their surface-modification by the pyridine
added in the synthesis.
The absorption spectra of the EuS nanocrystals in KBr are
shown in Figure 3. The absorption of the surface-modified EuS
nanocrystals ((b) EuS-0.5-p, 21 nm) shifted to shorter wave-
length than that of similar size of pristine EuS nanocrystals
((a) EuS-0, 20 nm). Note that absorptions of europium chalco-
Each powder was rinsed with methanol for 5 times and
dried in vacuum. The resulting compounds were identified by
the X-ray diffraction patterns. The diffraction peaks observed
at 2ꢀ ¼ 25:9ꢁ, 30.0ꢁ, 43.0ꢁ, 50.8ꢁ, 53.4ꢁ, 62.3ꢁ, 68.7ꢁ, 70.9ꢁ,
79.0ꢁ were assigned to the (111), (200), (220), (311), (222),
(400), (331), (420), (422) planes of NaCl-type EuS. The inten-
sity ratios of the diffraction patterns agreed with those of bulk
EuS. The average sizes of the prepared EuS samples calculated
by the Scherer equation from the XRD spectra are shown in
Table 1. By addition of 0.5 mL of pyridine, we obtained EuS
nanocrystals with average size of 21 nm, almost the same as that
prepared without pyridine addition.5 EuS nanocrystals with
Figure 1. A TEM image of EuS-0.5-p (21 nm).
Copyright ꢀ 2003The Chemical Society of Japan