Co-doped Y2O3 optical functional nanoparticles and novel self-assembly squama-like aggregates

Article abstract of DOI:10.1016/j.jallcom.2007.03.042

Co-doped Y₂O₃ optical functional nanoparticles with size of 4-10 nm have been synthesized by the plasma arc-discharge method. Meanwhile, novel three-dimensional squama-like macro-aggregates were self-assembled by disordered nanoparticles synthesized simultaneously in the arc-discharge process. The formation mechanism of the aggregates is ascribed to the periodic coagulation of Y nanoparticles. The valence states of Co in Y₂O₃ lattice can be determined as +2 from XPS spectrum with an etching depth of 6 nm. For Co-doped Y₂O₃ nanoparticles, upon excitation with 426 nm, a red emission (620 nm) has been observed which originates from ⁴A₂(⁴F) → ⁴T₁(⁴P) d-d transitions of Co²⁺ ions in tetrahedral co-ordination.

Full text of DOI:10.1016/j.jallcom.2007.03.042

Journal of Alloys and Compounds 457 (2008) 517–521
Co-doped Y O optical functional nanoparticles
2
3
and novel self-assembly squama-like aggregates
∗
X.G. Liu , J. Du, D.Y. Geng, S. Ma, J.M. Liang, M. Tong, Z.D. Zhang
Shenyang National Laboratory for Material Science, Institute of Metal Research and International Centre for Material Physics,
Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China
Received 23 January 2007; received in revised form 8 March 2007; accepted 8 March 2007
Available online 12 March 2007
Abstract
Co-doped Y
2
3
O optical functional nanoparticles with size of 4–10 nm have been synthesized by the plasma arc-discharge method. Meanwhile,
novel three-dimensional squama-like macro-aggregates were self-assembled by disordered nanoparticles synthesized simultaneously in the arc-
discharge process. The formation mechanism of the aggregates is ascribed to the periodic coagulation of Y nanoparticles. The valence states of Co
2 3 2 3
in Y O lattice can be determined as +2 from XPS spectrum with an etching depth of 6 nm. For Co-doped Y O nanoparticles, upon excitation
4
4
4
4
2+
with 426 nm, a red emission (620 nm) has been observed which originates from A
co-ordination.
2
( F) → T
1
( P) d–d transitions of Co ions in tetrahedral
©
2007 Elsevier B.V. All rights reserved.
Keywords: Nanostructures; X-ray diffraction; Luminescence; Magnetic measurements
2
+
1
. Introduction
in Co -doped B2O3–ZnO–PbO glasses, upon excitation with
80 nm, a red emission (625 nm) has been observed which
5
2
2
4
4
2+
Yttria has received great attentions in various fields of
originates from E ( G) → A2(F ) transition of Co ions in
tetrahedral co-ordination [5]. However, little work has been
reported about the emission spectrum of Y2O3 nanoparticles
doped by Co2 ions until now.
advanced functional materials, due to its excellent properties
such as a high dielectric constant, a low absorption in a broad
range (from near-UV to IR), a superior electrical break-down
+
(
[
>3 MV/cm) and a low leakage current onto silicon substrates
1]. Yttria is one of the best hosts for lanthanide ions, because
Nanoparticles can be prepared by various methods with
different mechanisms, including physical, chemical, physico-
chemical and mechanical ones. The structural, physical and
biomedical properties of nanoparticles depend sensitively on
the sample-preparation processes [6]. A wide variety of syn-
thesis techniques have been developed for the production of
pure and doped nanopowders, including wet chemical methods
its ionic radius and crystal structure are very similar to those of
many lanthanide oxides [2]. Doping with a variety of lanthanide
ions can yield materials with different fluorescent spectra, such
as Eu for red, Tb for green, Dy for yellow and Tm for blue
[
3]. Regarding optical materials doped by transition-metal ions,
2+
only a little work has so far been reported. Co is a kind of
[7], laser ablation [8], sol–gel [4] and combustion [9]. Differ-
laser active ion. Tetrahedrally coordinated Co2 in many bulk
+
ent sets of parameters for each synthesis method determine
the structural and optical properties of the final products. As
a well-known method for producing the nanoparticles, the arc-
discharge method has been widely used for the formation of
fullerenes and related materials [6]. For instance, the iron group
metals (Fe, Co, Ni) were encapsulated within hollow graphitic
cage [10]. To the best of our knowledge, there has been no report
on the studies of optical functional nanoparticles prepared by
arc-discharge method.
crystals, such as Co:Y3Al O12 and Co:MgAl2O4, can exhibit
intense absorption at 1.54 ␮m, and these materials can be used as
the Q-switched material [4]. Sun et al. reported that Co -doped
Y3Al O12 nanocrystallites exhibit a red emission (630 nm) upon
5
2+
5
excitation with 420 nm [4]. Lakshminarayana et al. found that
∗
Corresponding author.
E-mail address: liuxg@imr.ac.cn (X.G. Liu).
0
925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.03.042

5
18
X.G. Liu et al. / Journal of Alloys and Compounds 457 (2008) 517–521
During preparation of the nanoproducts, the nanounits, such
as nanoparticles, nanoclusters, nanowires and nanorods, can
be also self-assembled into the novel structural aggregates by
several routes, including the chemical vapor deposition [11],
the laser vaporization–condensation [12], the charge transfer-
ring [13], the electron irradiation deposition [14], the organic
reagent-assisted method [15], the solution–liquid–solid method
[
16] and the catalytic vapor–liquid–solid growth [17]. With
these routes, various nanoscale or microscale aggregates can
demonstrate novel architectures, including web-like, spherical,
nanowire-like, network, tree-like and fishbone-like aggregates.
In a recent work, we successfully prepared the novel coral-
like aggregates self-assembled by GdAl2 nanocapsules by the
arc-discharge process, which showed the feasibility of the arc-
discharge technique [18].
Fig. 2. Macro-morphology of the aggregates on the segmental baffle. The arrows
(a) and (b) mark squama-like and film-like architectures, respectively.
In this paper, we synthesized Co-doped Y2O3 nanoparticles
by the arc-discharge method and investigated the optical prop-
erties. At the same time, regularly aligned three-dimensional
macro-aggregates self-assembled by the nanoparticles were
simultaneously synthesized in the arc-discharge process.
nanoparticles was observed by a transmission electron microscope (TEM JEOL-
2010) with an emission voltage of 200 kV. The emission spectrum of the
nanoparticles was measured using an FL4500 fluorescence spectrophotome-
ter. X-ray photoelectron spectroscopy (XPS) measurements were performed on
ESCALAB-250 with a monochromatic X-ray source (an aluminum K␣ line of
1486.6 eV energy and 150 W) to characterize the valence of Co in the Y2O3 at
2
. Experimental procedures
a depth of 6 nm. The magnetic properties of the nanoparticles were measured
on a Quantum Design MPMS-7 superconducting quantum interference device
(
SQUID).
The Co-doped Y2O3 nanoparticles, which self-assembled into the squama-
like aggregates, were synthesized by the arc-discharge method with modified
strategies. In purified argon atmosphere, an Y67Co33 (at%) alloy button was
prepared by arc-melting Y and Co flakes of 99.9 wt% purity at least four times
so that the components were homogeneous. The experimental procedure of the
arc-discharge was similar to that in the previous reports [18,19], but with some
changes on the arc-discharge furnace. Fig. 1 represents the schematic diagram
of the arc-discharge furnace with a segmental baffle in front of the target. The
cathode was a tungsten needle with a diameter of 3 mm. The anode target (i.e., the
Y67Co33 alloy button) was placed into one pit of a water-cooled copper crucible.
The distance and the angle between the anode and the cathode were about 3 mm
3
. Results and discussion
The macro-morphology of the aggregates, deposited on the
segmental baffle during the arc-discharge, is shown in Fig. 2.
It is clearly seen that there are the squama-like architectures
(marked by the arrow (a)) and the film-like structures (marked by
the arrow (b)) with thickness of 2 mm. The macro-morphology
of the aggregates deposited on the segmental baffle is greatly
different from that of the aggregates deposited on the anode
(copper crucible) [18,19]. Unfortunately, the aggregates with the
squama-like architectures are toosoft to be observed in detailsby
SEM. Nevertheless, the morphology of the film-like structures
was examined by SEM, as shown in Fig. 3, which might pro-
vide with information on the initial stage of the growth of the
squama-like architectures. The SEM image indicates that the
film-like structures with some isolated particles are not dense,
which distinctly differ from the previous reports [18,19]. The
◦
and 60 , respectively. A mixture gas of Ar (20000 Pa) and H2 (4000 Pa) was
−3
introduced into an evacuated chamber (6.6 × 10 Pa), before a potential was
applied between the cathode and the anode. During the arc-discharging process,
the current was maintained at 100 A for 5 h, while the voltage was maintained at
1
6 V. After being passivated in argon atmosphere (0.02 MPa) for 24 h, squama-
like macro-aggregates (Fig. 2) in a large-area were collected from the surface of
the segmental baffle in front of the target.
X-ray powder diffraction (XRD) pattern was recorded with a D/max-2000
diffractometer at a voltage of 50 kV and a current of 250 mA with Cu K␣
(
λ = 1.54056 A˚ ). The architecture of the aggregates self-assembled by nanopar-
ticles was characterized by a Philips SSX-550 scanning electron microscope
SEM), with an emission voltage of 20 kV. The detailed morphology of the
(
Fig. 1. Schematic diagram of the arc-discharge furnace employed to synthesize
the nanoparticles.
Fig. 3. SEM image of the aggregates with film-like structures marked by the
arrow (b) in Fig. 2.

X.G. Liu et al. / Journal of Alloys and Compounds 457 (2008) 517–521
519
plasma, the rapid decrease of the temperature lets the clusters be
in the over saturation state in dynamic equilibrium; thus they are
dissociated. The rapid cooling leads to the nucleation of crys-
tallites and the formation of nanoparticles. At the same time,
nanoparticles with high speed are diffused around the region of
the plasma. Most of the Y nanoparticles approach the surface of
the segmental baffle with a certain angle incidence, due to the
low density of Y (Fig. 1). Meanwhile, the coarse surface of the
segmental baffle also provides more chances to the nanoparti-
cles at initial times of the growth to capture incident particles
and form the nucleation of the aggregates [19]. The source mate-
rials for the growth of the aggregates can be directly supplied
by the process of oblique angle incident. At the initial stage of
the growth of the squama-like architectures, Y nanoparticles are
rapidly coagulated to form film-like structure, due to the low
temperature of segmental baffle. Film-like structures elevate the
surface temperature of segmental baffle. Afterwards, Y nanopar-
ticles are slowly coagulated in a certain period, which induces
the formation of the squama-like aggregates. Yttrium nanopar-
ticles, due to higher surface energy, are completely oxidized to
form Y2O3 nanoparticles when they are exposed to air.
Fig. 4. X-ray diffraction (XRD) spectrum of the Co-doped Y2O3 nanoparticles
constructing aggregates.
significant difference in the micro-morphology can be related to
theformationandcoagulationbehaviorofthenanoparticles[20].
The formation mechanism of nanoparticles in plasma is that
matter clusters with a high activity exist in the plasma, which
could exchange rapidly the energy with the reacted matter clus-
ters, beneficial to the reaction between them. When the reacted
matter clusters leave the high temperature flam tail region of the
The XRD pattern in Fig. 4 shows the phase components of
the as-prepared products prepared by arc-discharging Y67Co33
Fig. 5. (a) TEM morphology of the Co-doped Y2O3 nanoparticles. (b) The corresponding electron diffraction patterns of (a). (c) XPS spectrum and corresponding
fitting curve of the Co in Y2O3 at a depth of 6 nm.

5
20
X.G. Liu et al. / Journal of Alloys and Compounds 457 (2008) 517–521
Fig. 6. Excitation (a) and emission (b) spectra of Co-doped Y2O3 nanoparticles.
alloy. All XRD peaks could be indexed to the single-phase body-
centered cubic Y2O3. The result is different from the previous
reports that fcc (or hcp) cobalt exists in nanoparticles prepared
by arc-discharging cobalt-based alloy [19,21–23]. According to
the (6 2 2) diffraction peak, the calculated lattice parameter of
the excited wavelength is 426 nm in Fig. 6(a). The intense emis-
sion bands appeared at the wavelengths of 566 and 620 nm,
respectively. The position of the emission peak of Co-doped
Y2O3 nanoparticles is similar to that of Y3Al O12 nanocrystals
5
2+
doped with tetrahedral Co ions in the visible region [4]. This
indicates that Co2 ions locate in the tetrahedral sites of Y2O3
+
˚
the as-prepared Y2O3 nanoparticles is 10.65 A, which is bigger
2+
˚
than 10.59 A of the pure Y2O3 phase. Y2O3 is the structure of c-
nanoparticles. In its neutral charge state, the Co ions have
7
4
RE (rare earths) style, the six tetrahedral gaps are occupied by O
ionsandtheothertetrahedralgapswithoutOions [24]. Andionic
an [Ar] 3d electron configuration. The atomic F ground state
splits under the influence of the tetrahedral component of the
radius of O2 (0.132 nm) is bigger than that of Co (0.072 nm).
The lattice expansion suggests that some cobalt ions may enter
into the remanent tetrahedral gaps without oxygen ions in the
body-centered cubic Y2O3 lattice. In addition, the size of the
particles is estimated to be 4–6 nm, according to the half-width
of XRD peaks of Y2O3.
−
2+
crystal field into a A2 ground state and T2 + T1 excited states.
The smaller trigonal distortion and spin–orbit interaction split
4
4
4
4
the ground A2 state into E1/2 + E3/2 [28]. The emission peaks
around 620 and 566 nm in the visible range are derived from sep-
4
4
4
4
4
4
4
4
arately A2( F) → T1( P) and A2( F) → A1( G) transitions
of tetrahedral Co2 ions [29,30]. Because the dopant ions affect
the band gap structure of the host materials [31], the emission
spectra of the Co-doped Y2O3 present a red shift, compared with
that reported early in Ref. [27]. Measurements at room temper-
ature generally exhibit some inhomogeneous broadening of the
spectral lines. In addition, small particles are more susceptible
to surface defects and disorder than larger particles. Disordered
states contribute to the broadening of the spectrum [2].
+
Fig. 5(a) shows TEM morphologies of the Y2O3 nanoparti-
cles. The Y2O3 nanoparticles are of irregular spherical shape,
and the diameter distribution is from 4 to 10 nm, as measured
from Fig. 5(a), which is well consistent with the sizes estimated
above. Besides the irregular shapes, the nanoparticles show also
coarse surfaces. The irregular shapes and the coarse surfaces of
the nanoparticles are ascribed to the rapid quenching and the
coexistence of different phases during the arc-discharge process
Magnetization measurement shows a distinct ferromagnetic
behavior. Fig. 7 represents magnetic hysteresis loop at 300 K
of the Co-doped Y2O3 nanoparticles. The curve in Fig. 7 shows
[
25]. A common feature of the samples is that they have some
agglomerates. Compared withthe previous report [26], the Y2O3
nanoparticles show higher dispersivity in Fig. 5(a). Different
synthetic conditions caused some differences in the dispersivity
of nanoparticles [26]. From the selected area electron diffrac-
tion (SAED) pattern (Fig. 5(b)), the formation of Y2O3 can be
determined from the characteristic diffraction rings, indicating
the characteristic lattice planes (2 2 2), (4 0 0), (4 2 0), (4 4 0),
(
6 2 2) and (8 3 1) of Y2O3, which is consistent with the XRD
results. The valence states of Co in Y2O3 lattice can be deter-
mined as +2 from the binding energy peak of 781.2 eV [27],
which is shown in the XPS spectrum with an etching energy
depth of 6 nm (Fig. 5(c)). The existence of Co2 in Y2O3 can be
explained that a small amount of dissociated Co evaporate into
bcc phase as doped ions in the arc-discharging [21].
+
The emission spectrum at room temperature of Co-doped
Y2O3 nanoparticles in visible region is shown in Fig. 6(b), and
Fig. 7. Magnetic hysteresis loop at 300 K of Co-doped Y2O3 nanoparticles. The
inset shows magnified portion of the curves.

X.G. Liu et al. / Journal of Alloys and Compounds 457 (2008) 517–521
521
obvious hysteresis with the coercive field Hc of 135 Oe at 300 K,
which is higher than that of Co-doped ZnO films, but the satura-
tionmagnetizationMs issmallerthanthatofCo2 inatetrahedral
crystal field (3.0 μB/Co) [32]. It is reasonable to assume that the
observed optical and magnetic properties are intrinsic nature of
Co element in Y2O3 nanoparticles.
[7] R. Bazzi, M.A. Flores-Gonzalez, C. Louis, K. Lebbou, C. Dujardin, A.
Brenier, W. Zhang, O. Tillement, E. Bernstein, P. Perriat, J. Lumin. 102–103
(2003) 445.
+
[
[
8] H. Eilvers, B.M. Tissue, Chem. Phys. Lett. 251 (1996) 74.
9] J. Hao, S.A. Studenikin, M. Cocivera, J. Lumin. 93 (2001) 313.
[
10] Z.D. Zhang, J. Mater. Sci. Technol. 23 (2007) 1.
[11] K.A. Dick, K. Deppert, M.W. Larsson, T. Martensson, W. Seifert, L.R.
Wallenberg, L. Samuelson, Nat. Mater. 3 (2004) 380.
[
12] M.S. Elshall, S.T. Li, D. Graiver, U. Pernisz, ACS Symposium Series, vol.
22, 1996, pp. 79–99.
13] K. Naka, H. Roh, Y. Chujio, Langmuir 19 (2003) 5496.
4
. Conclusion
6
[
In conclusion, Co-doped Y2O3 nanoparticles have been syn-
[14] S.O. Cho, E.J. Lee, H.M. Lee, J.G. Kim, Y.J. Kim, Adv. Mater. 18 (2006)
0.
6
thesized using the arc-discharge method. At the same time,
novel three-dimensional squama-like macro-aggregates were
self-assembled by Y2O3 nanoparticles synthesized simultane-
ously in the arc-discharge process. The nanoparticles’ size is
in the range of 4–10 nm. The valence states of Co in Y2O3
lattice can be determined as +2 from XPS spectrum with an
etching energy depth of 6 nm. For Co-doped Y2O3 nanoparti-
cles, upon excitation with 426 nm, a red emission (620 nm) has
[
[
15] Q.Y. Lu, F. Gao, D.Y. Zhao, Nanotechnology 13 (2002) 741.
16] A.K. Boal, F. Ilhan, J.E. Derouchey, T. Thurn-Albrecht, T.P. Russell, V.M.
Rotello, Nature 404 (2000) 6779.
[
[
[
[
[
[
[
[
[
[
17] Y.Q. Zhu, W.K. Hsu, W.Z. Zhou, M. Terrones, H.W. Kroto, D.R.M. Walton,
Chem. Phys. Lett. 347 (2001) 337.
18] S. Ma, D.Y. Geng, W.S. Zhang, W. Liu, X.L. Ma, Z.D. Zhang, Nanotech-
nology 17 (2006) 5406.
19] X.G. Liu, D.Y. Geng, S. Ma, J.M. Liang, Z.D. Zhang, Nanotechnology, in
preparation.
20] Y.J. Leng, S.H. Chan, K.A. Khor, S.P. Jiang, P. Cheang, J. Power Sources.
117 (2003) 26–34.
4
4
4
4
been observed which originates from A2( F) → T1( P) d–d
transitions of Co2 ions in tetrahedral co-ordination. The arc-
discharge method opens up a new way of synthesizing optical
functional materials.
+
21] S. Ma, Y.B. Wang, D.Y. Geng, J. Li, Z.D. Zhang, J. Appl. Phys. 98 (2005)
094304.
22] D.Y. Geng, Z.D. Zhang, W.S. Zhang, P.Z. Si, X.G. Zhao, W. Liu, K.Y. Hu,
Z.X. Jin, X.P. Song, Scripta Mater. 48 (2003) 593.
23] P.Z. Si, R.S. Turtelli, R. Grossinger, A. Reissner, M. Kuepferling, Z.D.
Zhang, J. Alloys Compd. 379 (2004) 82–86.
24] Z.T. Zhang, J.Y. Zhang, Inorganic Photoluminescence Materials and their
Application, Chemical Industry Press, Beijing, 2005, pp. 19–21.
25] P.Z. Si, E. Br u¨ ck, Z.D. Zhang, I. Skorvanek, J. Kovac, M. Zhang, Mater.
Res. Bull. 39 (2004) 1005.
Acknowledgment
This work was supported by the National Natural Science
Foundation of China under grant number 50331030.
References
26] L. Wen, X.D. Sun, Q. Lu, G.X. Xu, X.Z. Hu, Opt. Mater. 29 (2006)
239.
[
[
[
1] Y.C. Wu, S. Parola, O. Marty, J. Mugnier, Opt. Mater. 27 (2004) 21–27.
2] D. Dosev, B. Guo, I.M. Kennedy, J. Aerosol Sci. 37 (2006) 402.
3] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, Nan-
otechnology 15 (2004) 75.
4] Z.H. Sun, D.R. Yuan, X.L. Duan, X.C. Wei, H.Q. Sun, C.N. Luan, Z.M.
Wang, X.Z. Shi, D. Xu, M. Lv, J. Cryst. Growth 260 (2004) 171.
5] G. Lakshminarayana, S. Buddhudu, Spect. Acta: Part A. 63 (2006) 295.
6] Z.D. Zhang, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nan-
otechnology, vol. 6, American Scientific, California, 2004, pp. 77–160.
[27] B.J. Tan, K.J. Klabunde, P.M.A. Sherwood, J. Am. Chem. Soc. 113 (1991)
855.
[28] C. Liu, F. Yun, H. Morkoc, J. Mater. Sci. 16 (2005) 555.
[29] S. Ramachandran, A. Tiwari, J. Narayan, Appl. Phys. Lett. 84 (2004) 5255.
[30] K.J. Kim, Y.R. Park, Appl. Phys. Lett. 81 (2002) 1420.
[31] P. Yang, M.K. Lu, C.F. Song, D. Xu, D.R. Yuan, Phys. Stat. Sol. (b) 231
(2002) 106.
[
[
[
[32] X.C. Liu, E.W. Shi, Z.Z. Chen, H.W. Zhang, L.X. Song, H. Wang, S.D.
Yao, J. Cryst. Growth. 296 (2006) 135.