Remarkable changes in the optical properties of CeO2 nanocrystals induced by lanthanide ions doping

Article abstract of DOI:10.1021/ic0701256

Highly uniform and well-dispersed CeO₂ and CeO ₂:Eu³⁺ (Sm³⁺, Tb³⁺) nanocrystals were prepared by a nonhydrolytic solution route and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), UV/vis absorption, and photoluminescence (PL) spectra, respectively. The result of XRD indicates that the CeO₂ nanocrystals are well crystallized with a cubic structure. The TEM images illustrate that the average size of CeO₂ nanocrystals is about 3.5 nm in diameter. The absorption spectrum of CeO₂:Eu³⁺ nanocrystals exhibits red-shifting with respect to that of the undoped CeO₂ nanocrystals. Under the excitation of 440 nm (or 426 nm) light, the colloidal solution of the undoped CeO₂ nanocrystals shows a very weak emission band with a maximum at 501 nm, which is remarkably enhanced by doping additional lanthanide ions (Eu³⁺, Tb³⁺, Sm³⁺) in the CeO₂ nanocrystals. The emission band is not due to the characteristic emission of the lanthanide ions but might arise from the oxygen vacancy which is introduced in the fluorite lattice of the CeO₂ nanocrystals to compensate the effective negative charge associated with the trivalent ions.

Full text of DOI:10.1021/ic0701256

Inorg. Chem. 2007, 46, 5237−5242
Remarkable Changes in the Optical Properties of CeO Nanocrystals
2
Induced by Lanthanide Ions Doping
Zhenling Wang,†,‡ Zewei Quan,† and Jun Lin*,†
Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, P. R. China, and Department of Chemistry,
Zhoukou Normal UniVersity, Zhoukou 466000, P. R. China
Received January 25, 2007
:Eu3 (Sm3+, Tb3+) nanocrystals were prepared by a nonhydrolytic
+
Highly uniform and well-dispersed CeO
2
and CeO
2
solution route and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray
photoelectron spectra (XPS), UV/vis absorption, and photoluminescence (PL) spectra, respectively. The result of
XRD indicates that the CeO
that the average size of CeO
nanocrystals exhibits red-shifting with respect to that of the undoped CeO
40 nm (or 426 nm) light, the colloidal solution of the undoped CeO nanocrystals shows a very weak emission
band with a maximum at 501 nm, which is remarkably enhanced by doping additional lanthanide ions (Eu3+, Tb3+,
Sm3+) in the CeO
nanocrystals. The emission band is not due to the characteristic emission of the lanthanide ions
2
nanocrystals are well crystallized with a cubic structure. The TEM images illustrate
nanocrystals is about 3.5 nm in diameter. The absorption spectrum of CeO
2
:Eu3
+
2
2
nanocrystals. Under the excitation of
4
2
2
but might arise from the oxygen vacancy which is introduced in the fluorite lattice of the CeO
compensate the effective negative charge associated with the trivalent ions.
2
nanocrystals to
Introduction
three-way catalysts (TWC) for exhaust gas treatment from
automobiles, oxygen ion conductors in solid oxide fuel cells,
polishing agents for the chemical mechanical planarization
Rare-earth oxides with different nanostructures are of great
importance because of their applications in high-performance
luminescent devices, biological labeling, magnets, and
catalysts based on the electronic, optical, and chemical
characteristics arising from their 4f electrons.1-8 Compared
(CMP) process, gate oxides in metal oxide semiconductor
devices, and ultraviolet (UV) blocking materials in UV
6
-10
shielding, respectively.
2
Nanostructured CeO has been
11
prepared by various methods such as forced hydrolysis,
the microemulsion method, solvothermal synthesis, the
two-stage precipitation process, nonhydrolytic sol-gel
synthesis, solid-state reactions, mechanochemical reac-
tions, sonochemical synthesis,
and so on.
with the other nanostructured lanthanide oxides, CeO
nanocrystals have been widely investigated as important
nanoparticles can be used as
2
1
2
6a,8
1
3
functional materials.6-17 CeO
2
6
b,7
14
15
16,17
18
microwave irradiation,
*
To whom correspondence should be addressed. E-mail: jlin@
ciac.jl.cn.
†
Chinese Academy of Sciences.
Zhoukou Normal University.
‡
(
1) (a) Wu, G. S.; Zhang, L. D.; Cheng, B. C.; Xie, T.; Yuan, X. Y. J.
Am. Chem. Soc. 2004, 126, 5976. (b) Yu, M.; Lin, J.; Fu, J.; Zhang,
H. J.; Han, Y. C. J. Mater. Chem. 2003, 13, 1413.
(9) Miki, T.; Ogawa, T.; Haneda, M.; Kakuta, N.; Ueno, A.; Tateishi, S.;
Matsuura, S.; Sato, M. J. Phys. Chem. 1990, 94, 6464.
(10) Tsunekawa, S.; Fukuda, T.; Kasuya, A. J. Appl. Phys. 2000, 87, 1318.
(11) Hsu, W. P.; R o¨ nnquist, L.; Matijevi c´ , E. Langmuir 1988, 4, 31.
(12) Masui, T.; Fujiwara, K.; Machida, K.; Adachi, G.; Sakata, T.; Mori,
H. Chem. Mater. 1997, 9, 2197.
(13) Djuri e` iæ, B.; Pickering, S. J. Eur. Ceram. Soc. 1999, 19, 1925.
(14) Li, F.; Yu, X. H.; Pan, H. J.; Wang, M. L.; Xin, X. Q. Solid State Sci.
2000, 2, 767.
(15) Li, Y. X.; Zhou, X. Z.; Wang, Y.; You, X. Z. Mater. Lett. 2003, 58,
245.
(16) Xia, B.; Lenggoro, I. W.; Okuyama, K. J. Mater. Chem. 2001, 11,
2925.
(2) Wakefield, G.; Keron, H. A.; Dobson, P. J.; Hutchison, J. L. J. Colloid
Interface Sci. 1999, 215, 179.
(
3) Wang, H. Z.; Uehara, M.; Nakamura, H.; Miyazaki, M.; Maeda, H.
AdV. Mater. 2005, 17, 2506.
(
(
(
4) Cao, Y. C. J. Am. Chem. Soc. 2004, 126, 7456.
5) Feldmann, C. AdV. Funct. Mater. 2003, 13, 101.
6) (a) Zhang, Y. W.; Si, R.; Liao, C. S.; Yan, C. H.; Xiao, C. X.; Kou,
Y. J. Phys. Chem. B 2003, 107, 10159. (b) Si, R.; Zhang, Y. W.;
You, L. P.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44, 3256.
7) Yu, T.; Joo, J.; Park, Y. I.; Hyeon, T. Angew. Chem., Int. Ed. 2005,
(
(
44, 7411.
(17) Yin, L. X.; Wang, Y. Q.; Pang, G. S.; Koltypin, Y.; Gedanken, A. J.
Colloid Interface Sci. 2002, 246, 78.
8) Wang, Z. L.; Feng, X. D. J. Phys. Chem. B 2003, 107, 13563.
1
0.1021/ic0701256 CCC: $37.00
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 13, 2007 5237
Published on Web 05/23/2007

Wang et al.
However, it is difficult to synthesize highly uniform and
well-dispersed CeO nanocrystals on the basis of the fol-
2
lowing reasons. First, it is not easy to choose the appropriate
precursor complexes and the crystalline temperatures for rare-
earth oxides are relatively high. Second, the agglomeration
of nanopcrystals is very common because the nanocrystals
tend to decrease the exposed surface to lower the surface
energy. Hence the literature on the synthesis of highly
6b,7,12,19a
uniformandwell-dispersedCeO
On the other hand, most of the investigations on CeO
nanocrystals with particle size less than 10 nm were focused
2
nanocrystalsislimited.
2
19
on the lattice relaxation effect, ultraviolet absorption
properties, and catalytic activity.6
a,10,12,19a,j
So far only a few
systems
researchers have reported lanthanide ion-doped CeO
together with their luminescent properties.20 To the best of
2
our knowledge, the synthesis of uniform and well-dispersed
3
+
3+
3+
3+
2 2
Figure 1. XRD patterns of CeO (a), CeO : Eu (b) nanocrystals, and
lanthanide ion-doped (Eu , Tb , Sm ) CeO
and their luminescent properties have not been reported. In
this paper, we synthesized the CeO nanocrystals doped and
2
nanocrystals
the standard data for bulk CeO2 (c; JCPDS Card No. 75-0076).
2
off by heating. Typically, 0.4 mmol of cerium(III) acetate hydrate
or the stoichiometric amounts of the rare earth acetates (the doping
concentrations of Eu , Tb , and Sm are 5 mol % of that of
3+
3+
3+
undoped with other lanthanide ions (Eu , Tb , Sm ) via
a high-boiling-point solvent process and reported their optical
3+
3+
3+
properties. It is interesting to note that the undoped CeO
nanocrystals show a very weak emission band peaking at
01 nm, which is remarkably enhanced by doping additional
2
3+
Ce ), 0.55 mL of oleyamine, and 0.60 mL of oleic acid were mixed
with 7.0 mL of hexadecane in a three-neck flask equipped with a
condenser. The system was pumped under vacuum at room
temperature for 15 min and then heated at 90 °C for 1 h, to form
a clear light yellow solution. At 90 °C, 0.06 g of NaOH was added
to the clear solution and the system was pumped again under
vacuum to remove the minimum water. Then, the solution was
heated to 280 °C and remained for 3 h under agitation and argon
protection. The solution changed gradually to a brown turbid slurry
with the increase of reaction time. These turbid slurries were cooled
to room temperature and then isolated and washed by adding a
sufficient amount of ethanol and separating by centrifugation. The
yielded precipitate can be well dispersed in hexane to form a
transparent colloidal solution.
5
3+
3+
3+
lanthanide ions (Eu , Tb , Sm ) in the CeO
2
nanocrystals.
3
+
3+
The greatly increased emission band of CeO
2
:Eu (Tb ,
Sm ) nanocrystals (located at 500 nm) is not attributed to
3
+
3
+
3+
the characteristic emission of the lanthanide ions (Eu , Tb ,
3+
Sm ) but may arise from the presence of an oxygen vacancy
in the lattices of CeO nanocrystals.
2
Experimental Section
Synthesis of CeO Nanocrystals Undoped and Doped with
2
Lanthanide Ions. The CeO -based nanocrystals were prepared by
2
the nonhydrolytic solvent method in the hexadecane-oleylamine-
oleic acid system.21 Cerium(III) acetate hydrate (99.999%, Aldrich),
hexadecane (g98%, Fluka), oleyamine (70%, Aldrich), oleic acid
Characterization. X-ray diffraction (XRD) was carried out on
a Rigaku-Dmax 2500 diffractometer with Cu KR radiation (λ )
0.154 05 nm). TEM images were obtained using a JEOL 2010
transmission electron microscope operating at 200 kV. Samples for
TEM were prepared by depositing a drop of the colloidal solution
onto a carbon-coated copper grid. The excess liquid was wiped
out with filter paper, and the grid was dried in air. The particle
size distribution histograms were measured with dynamic light
(
analytical reagent, AR, Beijing Chemical Reagent Co.), and sodium
hydroxide (g96.0%, AR, Beijing Chemical Reagent Co.) were used
as received for starting materials. Eu(CH COO) , Sm(CH COO)
and Tb(CH COO) , were prepared by dissolving the corresponding
oxides in acetic acid, and the water in the solutions was distilled
3
3
3
3
,
3
3
A
scattering (DLS) using a ZETASIZER 1000 HS instrument. The
(
18) Liao, X. H.; Zhu, J. M.; Zhu, J. J.; Xu, J. Z.; Chen, H. Y. Chem.
Commun. 2001, 937.
UV/vis absorption spectra were measured on a TU-1901 spectro-
photometer. The X-ray photoelectron spectra (XPS) were taken on
a VG ESCALAB MK II electron spectrometer using Mg KR
(1253.6 eV) as the exciting source. The excitation and emission
spectra were taken on an F-4500 spectrophotometer equipped with
a 150 W xenon lamp as the excitation source. All the measurements
were performed at room temperature.
(
19) (a) Tsunekawa, S.; Sivamohan, R.; Ohsuna, T.; Kasuya, A.; Takahashi,
H.; Tohji, K. Mater. Sci. Forum 1999, 315-317, 439. (b) Tsunekawa,
S.; Sahara, R.; Kawazoe, Y.; Ishikawa, K. Appl. Surf. Sci. 1999, 152,
53. (c) Tsunekawa, S.; Ishikawa, K.; Li, Z. Q.; Kawazoe, Y.; Kasuya,
A. Phys. ReV. Lett. 2000, 85, 3440. (d) Tsunekawa, S.; Ito, S.;
Kawazoe, Y. Appl. Phys. Lett. 2004, 85, 3845. (e) Tsunekawa, S.;
Wang, J.-T.; Kawazoe, Y. J. Alloys Compd. 2006, 408-412, 1145.
(f) Zhou, X.-D.; Huebner, W. Appl. Phys. Lett. 2001, 79, 3512. (g)
Zhang, F.; Chan, S.-W.; Spanier, J. E.; Apak, E.; Jin, Q.; Robinson,
R. D.; Herman, I. P. Appl. Phys. Lett. 2002, 80, 127. (h) Wu, L. J.;
Wiesmann, H. J.; Moodenbaugh, A. R.; Klie, R. F.; Zhu, Y.; Welch,
D. O.; Suenaga, M. Phys. ReV. B 2004, 69, 125415. (i) Zhang, F.; Jin,
Q.; Chan, S.-W. J. Appl. Phys. 2004, 95, 4319. (j) Tsunekawa, S.;
Wang, J.-T.; Kawazoe, Y.; Kasuya, A. J. Appl. Phys. 2003, 94, 3654.
20) (a) Linares, R. C. J. Opt. Soc. Am. 1966, 56, 1700. (b) Yugami, H.;
Nakajima, A.; Ishigame, M.; Suemoto, T. Phys. ReV. B 1991, 44, 4862.
Results and Discussion
Phase Structure and Morphology of the Nanocrystals.
XRD. Figure 1 shows the XRD patterns of CeO
2
(a), CeO
2
:
3+
Eu (b) nanocrystals, and the standard data for bulk CeO
2
(
(
2
(c). The results of XRD indicate that the CeO nanocrystals
(
c) Fujihara, S.; Oikawa, M. J. Appl. Phys. 2004, 95, 8002.
are well-crystallized and the patterns are in good agreement
with a cubic structure (space group, Fm 3h m (No. 225); cell,
0.5389 × 0.5389 × 0.5389 nm and R ) â ) γ ) 90°)
21) (a) Liu, Q.; Lu, W.; Ma, A.; Tang, J.; Lin, J.; Fang, J. J. Am. Chem.
Soc. 2005, 127, 5276. (b) Gu, H.; Soucek, M. D. Chem. Mater. 2007,
3
19, 1103.
5238 Inorganic Chemistry, Vol. 46, No. 13, 2007

2
Optical Properties of CeO Nanocrystals
Figure 3. UV/vis absorption spectra of CeO2 (a) and CeO2:Eu3 (b)
nanocrystals solution. Insets are the photographs of these two samples (in
hexane solution).
+
is larger than that (2.0 nm) calculated from the Scherrer
equation. This demonstrates that the CeO
of polycrystalline nature. The high-resolution TEM image
left inset) displays lattice fringes for nanocrystals, indicating
that these CeO nanocrystals possess high crystallinity. The
interplanar spacing of the CeO nanocrystals is about 0.31
nm, which is identical with the (111) facet distance of bulk
CeO phase. The selected area electron diffraction (SAED)
patterns (right inset) are consistent with a cubic structure of
CeO with strong ring patterns due to (111), (200), (220),
and (311) planes, in good agreement with the XRD result.
The particle size distribution histogram of CeO nanocrystals
2
nanocrystals are
(
2
2
2
2
2
measured with the dynamic light scattering technique is
shown in Figure S-1 (Supporting Information). It can be
2
seen that the size distribution of CeO nanocrystals is
narrow with an average particle size of 3.8 nm, in agreement
Figure 2. TEM micrograph of CeO2 nanocrystals (a; insets are the high-
resolution TEM image and a selected area electron diffraction pattern,
with the result observed by TEM image. As for CeO
2
:Eu3
+
3
+
SAED) and the HRTEM image of CeO2: Eu nanocrystals (b).
3
+
nanocrystals, the doping of Eu ions resulted in the
increase of crystallinity, the presence of an irregular mor-
phology, and a little larger particle size compared with the
undoped nanocrystals, as shown in the typical high-resolution
known from bulk CeO
cording to the Scherrer equation, the strongest peak (111) at
θ ) 28.7° and the peak (220) at 2θ ) 47.1° were used to
calculate the average crystallite size of CeO nanocrystals,
2
(JCPDS Card No. 75-0076). Ac-
2
3+
2
2
TEM image of CeO :Eu nanocrystals (Figure 2b, bottom).
determined to be around 2.0 nm. The calculated cell
parameter (a) is equal to 0.5426 nm, a little larger than that
This result is basically consistent with that observed by XRD
data.
of bulk CeO
2
(0.5389 nm, JCPDS Card No. 75-0076). This
Optical Properties. UV/Vis Absorption Spectra. As
is due to the lattice expansion effect resulting from increased
oxygen vacancies and Ce ions with decreasing particle
2
ultraviolet blocking materials, CeO nanocrystals have strong
absorption properties in the ultraviolet range. Figure 3a shows
the UV/vis absorption spectrum for the hexane colloidal
3+
1
9b,c,f-h
3+
size.
ions, all diffraction peaks can still be indexed to the cubic
structure of CeO phase, as shown in Figure 1b. Compared
with the undoped CeO nanocrystals, the peak (200) at 2θ
33.0° was present in the XRD patterns of the doped
sample, which may be due to the anisotropic growth induced
2
When CeO nanocrystals were doped with Eu
2
solution of CeO nanocrystals (deep yellow, inset a in Figure
2
3). There is a strong absorption band below 400 nm in the
spectrum, which is due to the charge-transfer transition from
2
2
-
4+
6a
)
O
(2p) to Ce (4f) orbitals in CeO
2
.
When CeO
2
3
+
nanocrystals were doped with Eu ions, we could obtain
the CeO :Eu nanocrystals using the same experimental
2
3
+
3+
by the doping ions. The average crystal size of CeO
2
:Eu
nanocrystals estimated by the Scherrer equation increased
to 2.6 nm, and the calculated cell parameter values decrease
slightly (0.5394 nm).
procedure. Similar to the undoped CeO
2
nanocrystals, the
3
+
Eu -doped CeO
2
nanocrystals can also be dispersed in
hexane to form a transparent and light yellow colloidal
solution, as shown in inset b of Figure 3. The absorption
TEM. Figure 2a shows a typical TEM image of CeO
nanocrystals. From the TEM image it can be seen that the
mean diameter of CeO nanocrystals is about 3.5 nm, which
2
3
+
spectrum of CeO
2
:Eu nanocrystals (Figure 3b) exhibited
nanocrystals. For
2
a red shift compared with that of CeO
2
Inorganic Chemistry, Vol. 46, No. 13, 2007 5239

Wang et al.
:Eu3
+
red-shifting occurs in the absorption spectrum of CeO
2
4+
nanocrystals because of the decrease of the change from Ce
3
+
3+
to Ce . Meanwhile, the peaks of Eu 3d are present in the
XPS spectrum, as shown in Figure 4 (inset), and the presence
3
+
of Eu element in the CeO
2
:Eu nanocrystals can also be
confirmed by the energy-dispersive X-ray spectrum (Figure
3
+
3+
S-2). The absorption spectra of CeO
solutions (Figure S-3) also exhibited a red shift compared
with that of CeO nanocrystals; however, the red-shift is not
as evident as that of CeO
According to the data for the absorption spectra, the optical
2
:Sm /Tb nanocrystals
2
3
+
2
:Eu nanocrystals.
3
+
g 2 2
band gaps (E ) of CeO and CeO :Eu nanocrystals can be
estimated by using the following equation:
n
Rhν ) C(hν - Eg)
(1)
Here R is the absorption coefficient, hν is photon energy, C
is the constant, n ) 1/2 for a directly allowed transition,
1
7
and n ) 2 for an indirectly allowed transition. The optical
absorption coefficient R was calculated according to the
3
equation R ) (2.303 × 10 AF)/lc, where A is the absorbance
-
3
2
of a sample, F is the real density of CeO (7.28 g cm ), l is
the path length, and c is the concentration of the ceria
1
2
2
suspensions. For direct transitions, the plots of (Rhν) versus
3
+
photon energy for CeO
2 2
(a) and CeO :Eu (b) nanocrystals
are shown in Figure S-4. The intersection of the extrapolated
linear portions gives the direct band gap energy (E
d
), and
:Eu3
+
the values are 3.66 for CeO and 3.33 eV for CeO
2
2
nanocrystals, respectively. For indirect transitions, the plots
1
/2
of (Rhν) versus photon energy for the above two samples
are shown in Figure S-5, and the intersection of the
3+
Figure 4. XPS spectra of Ce in CeO2 (a) and CeO2:Eu (b) nanocrystals.
extrapolated linear portions gives the indirect band gap
3
+
(Inset is the Eu 3d peaks in the latter.)
3+
i i 2 2
energy (E ). The E values of CeO and CeO :Eu nanoc-
rystals are 3.13 and 2.69 eV, respectively. The values of E
and E for CeO
reported in the literatures.
present CeO nanocrystals (E
d
CeO
2
nanocrystals with a particle size of several nanometers,
i
2
nanocrystals are consistent with the results
3+
it is well-known that there coexisted a small amount of Ce
1
7,23
The absorption edge of the
) 3.66 eV) is blue-shifted
3+
at the surface of CeO
with decreasing particle size.1
in the CeO nanocrystals can be confirmed by XPS analysis,
as shown in Figure 4 (top).22 In the XPS spectrum of Ce in
CeO
2
nanocrystals, the peaks of Ce4 3d3/2 and Ce 3d5/2 at
binding energies of 917.2 and 899.0 eV, respectively, are
very weak, while those of Ce3 3d3/2 and Ce 3d5/2 located
2
and the fraction of Ce ions increased
2
d
9h,22
The presence of Ce3+ ions
evidently compared with that of the bulk CeO
3.15 eV), which is mainly resulted from quantum
confinement effect. The estimated Bohr exciton radius for
2 d
powders (E
2
)
+
4+
1
9e
CeO
2
is about 7-8 nm, and the average size of our CeO
2
nanocrystals is only around 3.5 nm, which should be small
enough for the quantum confinement effect to take place.
+
3+
at 901.8 and 882.9 eV are relatively strong. This indicates
that the Ce3 ions are mainly located on the surface of CeO
This effect on the direct band gap (E
the following equation:
d
) can be calculated by
+
2
1
9i
4+
3+
nanocrystals. Due to the change from Ce to Ce ions in
CeO nanocrystals, the charge-transfer gap between O 2p
and Ce 4f bonds increased, which led to the blue-shift of
absorption spectrum for CeO
CeO
compared with the undoped CeO
seen from its XPS spectrum (Figure 4, bottom) that the peaks
2
2
2
2
p π
1
1
1.8e
ꢀR
E ) E +
+
-
(2)
d
g
2
[
]
me mh
2
R
2
nanocrystals compared with
bulk powders.10 As for CeO
3+
2
2
:Eu nanocrystals,
Here E
is the band gap (for CeO
powders, E
) 3.15 eV),
g
2
g
2
nanocrystals, it can be
R is the radius of the nanoparticles, m
effective masses of the electron and hole, respectively, and
is the relative dielectric constant of CeO (24.5). Using m
) 0.4m (m is the mass of a free electron) and R )
.5 nm, the calculated value of E is about 3.30 eV, which
is less than the E value (3.66 eV) of CeO nanocrystals
h
and m are the
e
4
+
4+
of Ce 3d3/2 and Ce 3d5/2 become strong, while those of
ꢀ
2
e
3+
3+
Ce 3d3/2 and Ce 3d5/2 are relatively weak. This indicates
19i
)
m
h
3+
that the doping of Eu ions in the CeO
in the decrease of the Ce fraction on the surface due to
the replacement of Ce3 by trivalent Eu ions. Hence, the
2
nanocrystals results
3
g
3+
d
2
+
3+
calculated from the absorption spectrum. It is assumed that
(22) Tsunekawa, S.; Fukuda, T.; Kasuya, A. Surf. Sci. 2000, 457, L437.
(23) Yu, S. H.; C o¨ lfen, H.; Fischer, A. Colloids Surf., A 2004, 243, 49.
5240 Inorganic Chemistry, Vol. 46, No. 13, 2007

2
Optical Properties of CeO Nanocrystals
different 5d components split by the crystal fields, and the
emissions generally show a double band with an energy
-1
difference around 2000 cm due to the ground state splitting
3
+
2
2
1b
of Ce ( F5/2, F7/2). Obviously, these general characteristics
3
+
for the luminescence of Ce have not been observed in the
excitation and emission spectra of CeO
nanocrystals. Furthermore, if the emission is from Ce in
3+
2 2
and CeO :Eu
3+
3+
CeO
2 2
and CeO :Eu nanocrystals, the emission intensity of
the former might be stronger than that of the latter because
3
+
more Ce ions may exist in the former than the latter (on
the basis of XPS results, Figure 4). But the experimental
results do not support this assumption. So we conclude that
3
+
the luminescence is not from Ce .
To explain the above luminescent phenomena, it is
necessary to propose a possible luminescence mechanism.
3
+
As described above, there coexists Ce ions on the surface
3
+
of CeO
2
nanocrystals and the fraction of Ce ions increases
Figure 5. Excitation (a) and emission (b) spectra of CeO2 (red dash line)
1
9h,22
3
+
with decrease of the particle size.
Due to the presence
nanocrystals, oxygen vacancies are
introduced in the crystal to compensate the effective negative
and CeO2:Eu (black solid line) nanocrystals in hexane solution.
3
+
of Ce ions in CeO
2
the valence change from Ce4+ to Ce3+ on the surface of
nanocrystals might have some additional contributions to the
2
0b,24-26
charge associated with the trivalent ions.
The forma-
blue-shifting of CeO
2
nanocrystals because the change from
tion of intrinsic oxygen vacancies can be expressed by the
following equation:
+
4 to +3 increases the charge-transfer gap between O 2p
10
and Ce 4f bands, as reported previously. The presence of
•
•
3
+
Ce in CeO
2
nanocrystals has been confirmed by the XPS
CeO f Ce (IV)‚Ce (III)‚O
‚(x/2)Vo + (x/4)O
2
1-x
x
2-x/2 2
3+
study mentioned before. When Eu ions were doped into
nanocrystals, the change from Ce4+ to Ce ions
3+
••
CeO
2
2
Here Vo is oxygen vacancy. When CeO nanocrystals are
3
+
••
decreased evidently (by XPS, Figure 4) and the contribution
of blue-shifting arising from this valence change became very
doped with Eu ions, extrinsic oxygen vacancies (Vo ) will
be generated in the fluorite lattice of the nanocrystals and
:Eu3 nanocrystals
+
3+
small. Hence, the E
d
(3.33 eV) of CeO
2
the fraction of trivalent Ce ions in the doped nanocrystals
must be decreased due to the replacement of Ce by trivalent
Eu ions. The decreasing of Ce ions and the presence of
2
Eu ions on the surface of CeO :Eu nanocrystals have
3+
obtained by UV/vis absorption spectra is very close to the
calculated value of E (3.30 eV).
Photoluminescence Properties. The photoluminescent
3
+
3+
g
3
+
3+
:Eu3 nanocrystals were
+
(PL) properties of CeO
2
and CeO
2
been confirmed by XPS, EDS analysis, and absorption
spectra data. The reaction equation can be expressed as
examined under the identical instrumental conditions. It is
shown that CeO nanocrystals exhibit only a very weak broad
2
•
•
band emission, which is remarkably enhanced (more than
CeO f Ce (IV)‚Ln (III)‚O
‚(y/2)Vo + (y/4)O
2
1-y
y
2-y/2 2
+
2
0 times) by Eu3 doping. Figure 5 gives the excitation (a)
3
+
••
and emission (b) spectra for the colloidal solutions of CeO
2
where Ln is a trivalent lanthanide ion (e.g., Eu ). The Vo
:Eu3 (black solid lines) nanocrys-
+
3+
(red dash lines)and CeO
2
2
concentration in CeO :Eu nanocrystals must be higher than
tals. From Figure 5, it can be seen that the excitation and
emission spectra (enlarged by 10 times in the figure) for
that in the undoped CeO
2
nanocrystals because the doping
3
+
concentration of Eu is much larger than the fraction of
3
+
colloidal solution of CeO
2
nanocrystals consist of a broad
Ce in undoped nanocrystals. Considering that the PL
3
+
band with maxima at 445 and 501 nm, respectively, whereas
2
intensity of the CeO :Eu nanocrystals is so strong while
+
those of CeO
2
:Eu3 nanocrystals show much stronger excita-
that of CeO
2
nanocrystals is very weak (Figure 5), we can
safely deduce that the strong emission of CeO
2
:Eu3 nanoc-
+
tion and emission bands with maxima at 426 and 498 nm,
3
+
respectively. The characteristic f-f emission lines of Eu
rystals excited at 440 nm may arise from the presence of a
:Eu3 nanocrystals. Further-
+
••
have not been detected in CeO
2
large amount of Vo in the doped nanocrystals.
more, it is surprising that the excitation spectra of CeO
2
and
The assumed photoluminescence mechanism can be il-
lustrated by the schematic diagram of the energy band of
3
+
CeO
2
:Eu nanocrystals are greatly different from their
absorption spectra (Figure 3). This indicates that the emis-
:Eu3 nanocrystals are not caused
+
(24) McBride, J. R.; Hass, K. C.; Poindexter, B. D.; Weber, W. H. J. Appl.
Phys. 1994, 76, 2435.
(25) Li, L. P.; Li, G. S.; Che, Y. L.; Su, W. H. Chem. Mater. 2000, 12,
sions from CeO
2
and CeO
2
2-
4+
by the charge-transfer transition from O to Ce but from
other emission centers. Basically, we can exclude the
possibility that the luminescence is from Ce3 according to
2567.
(
26) (a) Dutta, P.; Pal, S.; Seehra, M. S.; Shi, Y.; Eyring, E. M.; Ernst, R.
D. Chem. Mater. 2006, 18, 5144. (b) Verma, A.; Karar, N.; Bakhshi,
A. K.; Chander, H.; Shivaprasad, S. M.; Agnihotry, S. A. J. Nanopart.
Res. 2007, 9, 317. (c) Ou, D. R.; Mori, T.; Ye, F.; Kobayashi, T.;
Zou, J.; Auchterlonie, G.; Drennan, J. Appl. Phys. Lett. 2006, 89,
171911.
+
3
+
the following facts. First, if the emission is from Ce due
to its 5d-4f transition, the excitation spectrum may be
structured due to the transition from 4f ground state to
Inorganic Chemistry, Vol. 46, No. 13, 2007 5241

Wang et al.
that may bring about the absence of Eu3 emission in CeO
+
2
3+
nanocrystals. One possible reason is that the Eu ions are
4
+
located at O
h 2
centrosymmetric sites (Ce in CeO ) in which
3
+
the radiative transition probability of Eu is very small.
Other reasons include the formation of Eu -Ce ion pairs,
which are not luminescent. Furthermore, the absorption of
2+
4+
2
8
UV energy occurred in CeO
Eu -O charge transfer and f-f transitions of Eu , but
the absorbed energy is unable to transfer to Eu . All these
factors can quench the emission of Eu in CeO
2
very easily compared to the
3
+
2-
3+
3
+
3
+
2
nanocrys-
tals.
Conclusions
Figure 6. Schematic diagram for the luminescence process in CeO2:Eu3+
nanocrystals. The conduction band (CB), valence band (VB), oxygen
vacancy (VO ), electron (Q), and hole (x) are shown inside the figure.
We have synthesized the CeO
undoped with trivalent lanthanide ions (Eu , Sm , Tb )
and discussed their optical properties. The as-prepared CeO
2
nanocrystals doped and
•
•
3+
3+
3+
2
+
CeO
2
:Eu3 nanocrystals, as shown in Figure 6.27 The
3+
3+
3+
2
and CeO :Eu (Sm , Tb ) nanocrystals are highly uniform
excitation band peaking at 426 nm (2.93 eV) can be basically
attributed to the indirectly allowed transition (the calculated
and can be dispersed in hexane to form a transparent colloidal
3+
solution. The absorption spectrum of CeO
2
:Eu nanocrystals
value of E
of 426 nm, electrons were excited from the valence band
VB) to the conduction band (CB), producing holes in VB.
The electrons in CB move freely and are trapped by the
i
is about 2.70 eV). Upon excitation with the light
exhibits a red shift with respect to that of undoped CeO
2
nanocrystals; i.e., the blocking range for UV is enhanced by
(
2 2
doping lanthanide ions in CeO nanocrystals. The CeO
:Eu3
+
nanocrystals show a remarkable enhanced PL intensity with
respect to the CeO nanocrystals due to the increase of
oxygen vacancy in the CeO nanocrystals.
•
•
O
oxygen vacancy (V ) to form the donor; meanwhile the
2
holes may populate above the VB to form the acceptor. The
recombination of the electrons and holes thus produces the
2
••
Acknowledgment. This project is financially supported
by the Foundation of “Bairen Jihua” of the Chinese Academy
of Sciences, the MOST of China (Grant 2003CB314707),
and the National Natural Science Foundation of China
visible emission around 500 nm. If the oxygen vacancy (Vo )
3
+
in CeO
visible emission band, then CeO
other trivalent lanthanide ions should produce a similar
2
:Eu nanocrystals is the key factor to lead to the
2
nanocrystals doped with
3+
3+
(Grants NSFC 50572103, 20431030, 00610227).
emission band. So we synthesized Sm - and Tb -doped
CeO nanocrystals and measured their PL spectra. Figure
S-6 shows the PL spectra of CeO
nanocrystal solutions, respectively. It can be seen that the
2
Supporting Information Available: Figures S1 (particle size
:Sm3 and CeO
+
3+
2
2
:Tb
distribution histogram for CeO
Eu nanocrystals), S3 (UV/vis absorption spectra), S4 and S5 (plots
nanocrystals), S2 (EDS of CeO
2
:
2
3+
3
+
3+
2
1/2
excitation and emission spectra of Sm - and Tb -doped
2 2
of (Rhν) and (Rhν) versus photon energy for CeO and CeO :
:Eu3 nanoc-
+
3+
CeO
2
nanocrystals are similar to that of CeO
2
Eu nanocrystals), and S6 (excitation and emission spectra of
:Sm3 and CeO
CeO :Tb nanocrystals). This material is available
2 2
+
3+
rystals. This further confirms that the emission band (∼500
free of charge via the Internet at http://pubs.acs.org.
nm) is not the characteristic emission of doping lanthanide
ions and must be relative to the oxygen vacancy in CeO
nanocrystals.
2
IC0701256
(
27) Yang, J.; Lin, C. K.; Wang, Z. L.; Lin, J. Inorg. Chem. 2006, 45,
973.
(28) Blasse, G.; Bril, A. J. Chem. Phys. 1967, 47, 1920.
Finally, we explain the absence of Eu3 emission in CeO
+
2
8
nanocrystals briefly. We think there are several possibilities
5242 Inorganic Chemistry, Vol. 46, No. 13, 2007