Fabrication of Y2Ti2O7:Yb 3+,Ho3+ nanoparticles by a gel-combustion approach and upconverting luminescent properties

Article abstract of DOI:10.1016/j.jallcom.2014.04.101

Yb³⁺, Ho³⁺ co-doped pyrochlore-structured (Y _0.99-xHo_0.01Yb_x) ₂Ti₂O₇ (x = 0, 2.5, 5.0, 7.5, 10.0 and 12.5 mol%) nanoparticles (NPs) were successfully fabricated via a gel-combustion approach. The products as-obtained were characterized by various techniques, i.e. X-ray diffraction, transmission electron microscope, Fourier transformed infrared spectra and upconverting spectra. The results indicate that the bright green (～540 nm) and red (～660 nm) emissions are observed in Y ₂Ti₂O₇:Ho³⁺,Yb³⁺ NPs under the 980 nm excitation, which is ascribed to the radiative transitions (⁵F₄,⁵S₂) → ⁵I ₈ and ⁵F₅ → ⁵I₈ of Ho³⁺ ions, respectively. It is also found that the calcining temperature and Yb³⁺ ion doping level have a great influence on the upconverting spectra of (Y_0.99-xHo _0.01Yb_x)₂Ti₂O₇ NPs. The emission intensities increase initially and then fall down from 800 to 1000 °C. The optimum doping level of Yb³⁺ ions is 7.5 mol%, and the intensity of upconverting emissions for (Y_0.915Ho _0.01Yb_0.075)₂Ti₂O₇ NPs is enhanced by the fold of 32 compared to the Yb³⁺-free samples. The dependence of upconverting intensity on the excitation power reveals the contribution of two photons to both the green and red upconverting process under lower excitation power, and the possible upconverting mechanisms have been proposed accordingly.

Full text of DOI:10.1016/j.jallcom.2014.04.101

Journal of Alloys and Compounds 608 (2014) 165–169
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
3
+
3+
Fabrication of Y Ti O :Yb ,Ho nanoparticles by a gel-combustion
2
2 7
approach and upconverting luminescent properties
a,
⇑
a
a
a
a
a
Zhongsheng Chen , Min Wang , Haiqing Wang , Zhanggao Le , Guolin Huang , Lixia Zou ,
a
b
b
c
Zhirong Liu , Dianyuan Wang , Qingkai Wang , Weiping Gong
a
State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China Institute of Technology, Nanchang, Jiangxi 330013, China
College of Science, Jiujiang University, Jiujiang, Jiangxi 332005, China
Electronic Science Department, Huizhou University, Huizhou, Guangdong 516001, China
b
c
a r t i c l e i n f o
a b s t r a c t
Yb3+
,
x 2 2 7
Ho3+ co-doped pyrochlore-structured (Y0.99ꢀxHo0.01Yb ) Ti O (x = 0, 2.5, 5.0, 7.5, 10.0 and
Article history:
Received 15 February 2014
Received in revised form 15 April 2014
Accepted 16 April 2014
12.5 mol%) nanoparticles (NPs) were successfully fabricated via a gel-combustion approach. The products
as-obtained were characterized by various techniques, i.e. X-ray diffraction, transmission electron micro-
scope, Fourier transformed infrared spectra and upconverting spectra. The results indicate that the bright
Available online 24 April 2014
3
+
3+
green (ꢁ540 nm) and red (ꢁ660 nm) emissions are observed in Y
2 2 7
Ti O :Ho ,Yb NPs under the 980 nm
5
5
5
and 5F
? 5I
3+
excitation, which is ascribed to the radiative transitions ( F
4
, S
2
) ? I
8
5
8
of Ho ions, respec-
Keywords:
3+
tively. It is also found that the calcining temperature and Yb ion doping level have a great influence on
Yttrium titanate
Gel-combustion
Upconverting luminescence
Nanophosphors
the upconverting spectra of (Y0.99ꢀxHo0.01Yb
then fall down from 800 to 1000 °C. The optimum doping level of Yb ions is 7.5 mol%, and the intensity
of upconverting emissions for (Y0.915Ho0.01Yb0.075 Ti NPs is enhanced by the fold of 32 compared to
the Yb -free samples. The dependence of upconverting intensity on the excitation power reveals the
contribution of two photons to both the green and red upconverting process under lower excitation
power, and the possible upconverting mechanisms have been proposed accordingly.
Ó 2014 Elsevier B.V. All rights reserved.
x 2 2 7
) Ti O NPs. The emission intensities increase initially and
3
+
)
2
2 7
O
3
+
ꢀ
1
1
. Introduction
(<712 cm ), high refractive index value (ꢁ2.2 at 550 nm), high
optical band gap (ꢁ3.7 eV), the ability to accept rare earth
elements in solid solutions, as well as excellent thermal and
In more recent years, research on rare-earth ions doped
3
+
upconverting nano-materials has received considerable attention
owing to their unique ability to convert the near-infrared (NIR)
excitation to visible (Vis) or even ultraviolet emissions, which
has opened up vast opportunities for creating novel biomedical
imaging, cancer therapy, temperature sensing, detection of heavy
metals, spectra solar converters for solar cells, and so on [1–5].
To investigate the upconverting properties of host matrices with
chemical stability. In these previous research, Er ions are used
as the activator, and much efficient upconverting process can be
achieved when co-doped with Yb as sensitizer for Yb ion has
3
+
3+
ꢀ21
2
a larger absorption cross section (ꢁ11.7 ꢂ 10
cm ) under the
980 nm excitation [15] and there occurs the large spectral overlap
3
+
4
4
3+
between Er
absorption ( I15/2 ? I11/2
)
and Yb
emission
4
4
3+
( F5/2 ? F7/2). Ho ion, another most popular emission center,
can give rise to the strong green and red emissions because of
energy levels capable of infrared pumping that closely match the
most attractive 980 nm. To the best of our knowledge, however,
4 3 2 3
low phonon energy, such as NaReF , ReF and Re O (Re = rare
earth), is of special interest [6–9] as the lower phonon energy
can help reduce the possibility of nonradiative relaxation, thus
resulting in the strong NIR-to-VIS upconverting emissions. It
3
+
3+
2 2 7
Yb , Ho co-doped Y Ti O nanophosphors and their upconver-
is noteworthy that pyrochlore-structured Re
2
Ti
2
O
7
recently has
ting luminescence have not yet been reported systematically so
far. Therefore, it is urgent to make a deeper research in the
been proposed as one of potential host material for excellent
upconverting luminescence [10–14] due to their unique structural
characteristics, such as the relatively low phonon energy
3
+
3+
2 2 7
upconverting properties of Ho , Yb co-doped Y Ti O NPs.
3
+
3+
Herein, in the present work, a series of Yb , Ho co-doped
Ti nanophosphors were fabricated via gel-combustion
approach using glycine as the fuel. The influence of calcining
Y
2
2 7
O
3
+
⇑
Corresponding author. Tel.: +86 79183897510; fax: +86 79183879031.
E-mail address: zhshcheng@ecit.cn (Z. Chen).
temperature and Yb ion doping concentration on the phase
evolution as well as upconverting properties have been discussed
http://dx.doi.org/10.1016/j.jallcom.2014.04.101
925-8388/Ó 2014 Elsevier B.V. All rights reserved.
0

1
66
Z. Chen et al. / Journal of Alloys and Compounds 608 (2014) 165–169
in detail. And the possible upconverting mechanisms have been
also proposed through the power dependence of upconverting
intensities.
crystallizes from the amorphous structure between 700 and
800 °C. The series of characteristic planes (222), (400), (331),
(440), (622), etc., is indexed based on face-centered cubic lattice
titanate (JCPDS No. 42-0413) as reported previously [18]. Addition-
ally, as the temperature increases, no other possible impure phases
are detected. And the increase in the diffraction intensity with the
rise of the calcining temperature indicates the crystallite growth of
2
. Experimental section
Ho3+, Yb3+ co-doped Y
2 2 7
Ti O nanocrystals were synthesized via a glycine–
nitrate gel-combustion method. The appropriate conditions (the pH value of the
precursor is 2.0, the fuel-to-oxidant ratio is 2.0) are chosen, and the details of the
2 2 7
the Y Ti O powders. According to the Scherrer’s formula [17], the
mean crystallite sizes of products calcined at 800, 900 and 1000 °C
are estimated at ꢁ24.8, 30.1, and 45.6 nm, respectively. On the
other hand, other as-prepared co-doped samples (Y0.99ꢀxHo0.01-
2 3
procedure have been given in our previous article [16]. Tetrabutyl titanate, Y O ,
Ho
of Y
Y(NO
2
O
3
and Yb
, Yb
, Yb(NO
of n(Y+Ho+Yb)/nTi 1:1 and varying Yb ion doping level, various Ho ,Yb co-doped
0.99ꢀxHo0.01Yb Ti (x = 0–12.5 mol%) NPs were fabricated.
The crystalline structure of as-prepared samples was determined by X-ray
diffraction (XRD, D/ruax2550PC, Tokyo, Japan) using Cu radiation
k = 0.154056 nm) over a 2h range from 10° to 80° at a scanning rate of 4°/min.
2
O
3
were employed as basic starting materials. Firstly, a given amount
and Ho was dissolved with concentrated HNO to convert into
and Ho(NO solutions, respectively. By fixing the molar ratio
2
O
3
2
O
3
2
O
3
3
3
)
3
3
)
3
3 3
)
x 2 2 7
Yb ) Ti O (x = 0, 2.5, 5.0, 10.0 and 12.5 mol%) also exhibit the same
3
+
3+
3+
effect of the calcining temperature on the phase evolution (not
shown here).
(
Y
x
)
2
2 7
O
Fig. 2 gives the typical TEM images of Ho3+, Yb3+ co-doped
Ka
(
Y
2
2 7
Ti O NPs calcined at 800 °C and 1000 °C for 1.0 h, respectively.
And the average crystallite size D (nm) of nanocrystals can be estimated from the
XRD spectra according to the Scherrer’s formula [17]:
Both the samples exhibit the same morphology characteristics:
Kk
D ¼
b cos h
where D (nm) is the crystallite size; K is a Scherrer constant and equals 0.9, k is the
X-ray wavelength 0.1540 nm, b (rad) is the full-width-at-half-maximum (FWHM)
of a characteristic diffraction peak and h (°) is the diffraction angle corresponding
to the XRD peak being considered, and here the most intensive XRD peaks (222)
are used. The morphologies and average size of samples were observed by transmis-
sion electron microscopy (TEM, JEOL-2010F, Tokyo, Japan) operated at 200 kV. TEM
samples were prepared by dispersing the powder in absolute alcohol by ultrasonic
treatment, dropping onto a porous carbon film supported on a copper grid, and then
dried in air. The adsorption of impurities on the host surface was conducted by
Fourier transform infrared spectra (FT-IR, Avatar 380, Thermo Nicolet Corporation,
ꢀ
1
USA) in the scan range of 4000–400 cm with KBr pellet technique. The measure-
ment of upconverting emission was carried out using a power-controllable 980 nm
diode laser (Newport Corporation, California, USA) as the excitation source. The laser
was focused on the sample (spot size of ꢁ1.0 mm) by use of a 10 cm focal length lens
and the fluorescence emission was collected in a reflection mode, in an angular
direction of 45° to normal incidence. The maximum pump power at the sample
surface was ꢁ2500 mW. The upconverting emission spectra were collected by a
spectrophotometer (Hitachi F-4500, Tokyo, Japan).
3
. Results and discussion
2 2 7
The XRD patterns of the products (Y0.915Ho0.01Yb0.075) Ti O cal-
cined in air at various temperatures for 1.0 h are presented in
Fig. 1. It is apparent that the powders calcined at 600 and 700 °C
are primarily amorphous, as evidenced by the broad continuum
diffraction peak centered at around 30° (2h) in Fig. 1b and c.
Nevertheless, all of the XRD patterns exhibit the single phase with
pyrochlore structure when calcined at 800 °C or higher tempera-
tures. Therefore, it can be safely concluded that the products
Fig. 1. XRD patterns of (Y0.915Ho0.01Yb0.075
atures for 1.0 h.
2
) Ti
2
O
7
NPs calcined at various temper-
Fig. 2. Typical TEM images of Ho3+, Yb3+ co-doped Y
and 1000 °C for 1.0 h, and the inserts show the particle size distribution.
2 2 7
Ti O NPs calcined at 800 °C

Z. Chen et al. / Journal of Alloys and Compounds 608 (2014) 165–169
167
most of the NPs are either spherical or spheroidal, and of good dis-
persity. On the basis of the size distribution, the average particle
size of the samples prepared at 800 and 1000 °C is estimated to
be ꢁ25.2 and 45.20 nm, respectively, which is fully consistent with
the aforementioned XRD results determined by the Scherrer’s
formula.
In order to demonstrate the adsorption of impurities on the host
calcined at various temperatures, the FT-IR spectra were examined,
as shown in Fig. 3. From Fig. 3(a), the two absorption bands at
ꢀ
1
ꢁ
1522 and 1080 cm can be observed, which are ascribed to the
carbonate. Generally, the carbonates are the unavoidable residues
after the thermal decomposition of precursors containing organic
ꢀ1
[
19,20]. The stretching vibration bands (ꢁ3450 cm ) and bending
ꢀ1
vibration bands (ꢁ1636 cm ) of hydroxyl groups (OAH), and the
ꢀ
1
ꢀ
3
characteristic absorption bands (1384 cm ) of nitrate ions (ANO )
still exists after combustion, implying H
2
O molecules in the air
ꢀ
and NO
3
are adsorbed on the products. When the powders were
calcined at 600 °C for 1.0 h, the adsorption of carbonate is reduced
dramatically, as the characteristic absorption bands (ꢁ1522 and
Fig. 3. FT-IR spectra of the products after combustion and calcined at various
temperatures, (a) after combustion, (b) 600 °C, (c) 700 °C, (d) 800 °C, (e) 900 °C and
ꢀ1
1
080 cm ) become much weaker, and almost disappeared when
(
f) 1000 °C.
calcining at 800 °C. The strength of the characteristic absorption
bands of H O and CO molecules are decreased gradually with
2
2
the increase of calcining temperatures, and then keep almost
unchanged at 800 °C or higher temperatures, which showed that
the adsorption amount of H O and CO molecules is reduced.
2 2
When the powders were calcined at 800 or high temperatures,
the FT-IR spectra of as-obtained products share the similar absorp-
tion bands except for the difference in the absorption strength,
which can be accounted for by the fact that the FT-IR spectra are
not very sensitive to the particle sizes of nanomaterials [21]. In
Fig. 3, the low wavenumber absorption bands appearing at 568,
ꢀ
1
4
68 and 410 cm are assigned to TiAO, YAO
1
and YAO stretching
2 7
Ti O , respec-
2
vibrations in the Y Ti (O O( ) polyhedron of Y
2
2
1
)
6
2
2
tively, which are the main features of the titanate pyrochlore [13].
Fig. 4 shows the effect of calcining temperatures on upconverting
spectra of (Y0.915Ho0.01Yb0.075) Ti O NPs under the 980 nm
2 2 7
excitation. In terms of all the samples excited by the weak pump
power of 610 mW, bright upconverting emissions could be
2 2 7
observed, suggesting that (Y0.915Ho0.01Yb0.075) Ti O is a high effi-
cient upconverting material. As shown in Fig. 4, the upconverting
spectra exhibit the green1 and red emission bands in the visible
region of 500–700 nm, which is similar to the upconverting
Fig. 4. The effect of the calcining temperatures on upconverting spectra of
Ti NPs (under the 980 nm excitation with the pump power
of 610 mW).
(
Y
0.915Ho0.01Yb0.075
)
2
2 7
O
3
+
3+
3+
3+
2 3 3 5
systems, Lu O :Ho ,Yb and Y Al O12:Ho ,Yb [22–24], and the
emission bands centered at ꢁ540 and ꢁ660 nm are originated from
5
5
5
5
5
3+
(
F
4
,
S
2
) ? I
8
, and
F
5
? I
8
electronic transitions of Ho ions,
carbonate ion, decrease with calcining temperatures from 600 to
800 °C, then keep constant essentially. Secondly, the surface
defects act also as nonradiative transition centers, and the upcon-
verting intensity may be influenced by the grain size (due to the
different surface-to-volume ratios) [26,27]. As aforementioned in
XRD and TEM results, the increase of the average particle size of
respectively.
And also, it is evidently seen from Fig. 4 that the upconverting
intensity is increased greatly with the temperature rise from 600
to 800 °C, approaches to the maximum, and then decreases from
8
00 to 1000 °C, this trend is not identical to our previous findings
10], in which upconverting intensity (Y0.91Er0.015Yb0.075 Ti
NPs is increased gradually with the temperature rise from 600 to
000 °C. The reason for this may be accounted for by the different
[
)
2
2
O
7
2 2 7
(Y0.915Ho0.01Yb0.075) Ti O nanophosphors prepared at 800, 900,
and 1000 °C implies that the surface defects are reduced with the
rise of calcining temperature, thereby leading to the increase of
luminescence intensity [26]. Thirdly but very importantly, how-
1
preparation method of phosphors. As a matter of fact, the influence
of the calcining temperature on the upconverting intensity seems
to be a little more complicated, which may be explained by the
three contradictory factors: the higher energy impurities adsorbed,
the surface defects and the local environment of activator ions.
3
+
ever, the local symmetry of Ho lattice sites can also be promoted
with the increase of calcining temperature, therein resulting in the
reduction of luminescence intensity [28]. Taking these factors
above into consideration, the first reason may be predominant to
account for the effect of calcining temperatures on the upconver-
ting intensity. It is proposed that, therefore, to reduce the hydroxyl
and carbonate groups absorbed on the host and to modify the local
symmetry of crystalline field of rare earth ions may improve the
upconverting efficiency of nanophosphors greatly.
ꢀ
2ꢀ
3
Firstly, the higher energy contaminants, i.e. OH and CO ,
could be readily absorbed onto the sample surface, consequently
reducing the upconverting intensity [25]. As demonstrated in the
FT-IR spectra above, the quenching centers, i.e. hydroxyl and
3
+
Fig. 5 shows the effect of Yb ion doping concentrations on the
upconverting properties of (Y0.99ꢀxHo0.01Yb Ti NPs calcined at
800 °C for 1.0 h. Apparently, the Yb ion doping level has a great
1
x
)
2 7
O
For interpretation of color in Figs. 4–6, the reader is referred to the web version of
2
3
+
this article.

1
68
Z. Chen et al. / Journal of Alloys and Compounds 608 (2014) 165–169
3
+
As it well known, Yb ion has a larger absorption cross section
ꢀ
21
2
(
ꢁ11.7 ꢂ 10
cm ) at 980 nm excitation [15]. Within a certain
3
+
range of Yb ion doping concentration, therefore, the increase of
3
+
Yb ions favors the absorption of pumping photons, and then
facilitates the efficient resonant energy transfer from Yb to Ho
ions, thus improving the efficiency of upconverting emissions. Too
3+
3+
3
+
higher Yb ions, however, may increase the odds of the back energy
3
+
3+
transfer Ho ? Yb [29,30], and on the other hand, it is possible
3
+
that the pairing or aggregation of Yb ions could come into being,
which resultantly may turn these ions into new quenching centers
31]. As a consequence, the Yb ions at increased doping level
3
+
[
may act as trapping centers and induce nonradiative relaxation,
herein reducing the upconverting efficiency. From Fig. 6, it is also
3
+
found that at Yb ions of 7.5 mol%, the intensities of green, red, total
upconverting emission is enhanced by 32 folds compared to
3
+
Yb -free samples, which evidences the important role of the
3
+
sensitizer Yb ions in the upconverting process. In addition, the
intensity ratios (Ired/Igreen) of the red to green emission keep almost
Fig. 5. The effect of the Yb3+ ion doping concentrations on upconverting spectra of
Ti NPs calcined at 800 °C for 1.0 h (under the 980 nm
3+
constant as the Yb
ion concentration increases from 0 to
(
Y
0.99ꢀxHo0.01Yb
)
x 2
2
O
7
3+
1
2.5 mol%, suggesting that Yb ions have a simultaneous influence
excitation with the pump power of 610 mW).
5
4 2 8 5 8
, S ) ? I and F radiative transitions of Ho ions.
? 5I
5
5
5
3+
on the ( F
To get a better understanding of the possible upconverting
3
+
3+
2 2 7
mechanism involved in Y Ti O :Ho ,Yb NPs, the pump power
dependence of the integral intensity of upconverting emissions is
plotted in Fig. 7. It has been widely recognized that, for nearly
any unsaturated upconverting process, the slope of the double-
logarithmic plots of the integral intensity versus pump power is
the number of NIR photons absorbed per visible photon emitted
[
32]. At relatively low pump powers, the slopes determined
through the linear fitting are 2.15 ± 0.02 and 2.18 ± 0.02 for the
5
5
5
5
5
green (( F
4 2 8 5 8
, S ) ? I ) and red ( F ? I ) transitions, respectively,
5
5
indicating the involvement of two photons excited to ( F
levels. This is consistent with other upconverting system, such
3 5 4
as Y Al O12:Ho ,Yb and NaYF :Ho ,Yb NPs [23,24,33]. When
4 2
, S ) and
5
F
5
3
+
3+
3+
3+
the pump power increases to a relatively high region, however,
the so-called ‘‘saturation effect’’ exists for both the red and green
emissions, i.e. the intensity decreases dramatically instead with
the increase of the pump power. This phenomena can be attributed
to thermal quenching caused by the absorption of the 980 nm light
when excited under too high pump power [34–36].
Fig. 6. Enhancement factors of the green, red, total upconverting emissions and
3
+
I
red/Igreen of (Y0.99ꢀxHo0.01Yb3
x
)
2
Ti
2
O
7
NPs as a function of Yb concentrations (the
Fig. 8 shows the energy level diagrams of Ho3+ and Yb3+ ions, as
well as the proposed upconverting processes under the laser
+
integrated intensity of Yb -free sample is scaled as unit, under the 980 nm
excitation). (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
3
+
excitation of 980 nm. Firstly, the electrons of Yb ions absorbs
4
one pump photon, and are excited from the ground state
the level
F7/2 to
4
F5/2 due to its lager cross sections, and then the energy
influence on the intensities of upconverting spectra. As for
3
+
Yb -free samples, the upconverting emissions are very weak. This
is due to the fact of the spectra mismatch between the energy level
of Ho ions and the 980 nm excitation although Ho3+ ions can be
3+
excited by the 980 nm diode laser directly. The upconverting
3+
2 2 7
efficiency of uni-doped Y Ti O :Ho nanophosphors is rather
low. By co-doping Yb3 ions, however, bright upconverting
+
emissions could be observed even by naked eyes, which confirms
that there exists the efficient energy transfer from Yb3+ to Ho3+
.
According to Fig. 5, the integral intensity of upconverting green,
red and overall emissions is normalized, respectively, and the
effect of the green, red, total upconverting emission and the
red/Igreen as a function of Yb3 ion doping concentrations can
+
I
be obtained, as shown in Fig. 6. Obviously, the green, red, total
3+
UC emission share the same trend with the Yb ion doping level:
3
+
the emission intensity increases drastically with increasing Yb
ions, up to a critical value of 7.5 mol%, and then decreases for more
3
+
Yb ions, i.e. the so-called concentration quenching effect takes
3
+
Fig. 7. The pump power dependence of the red and green emissions of (Y0.915-
Ti NPs calcined at 800 °C for 1.0 h (under the 980 nm excitation).
For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
place. This suggests that there exists an optimal Yb
concentration that gives maximum emission intensity in
Ti nanophosphors.
ion
Ho0.01Yb0.075
(
)
2
2 7
O
a
(
Y
0.99ꢀxHo0.01Yb
x
)
2
2 7
O

Z. Chen et al. / Journal of Alloys and Compounds 608 (2014) 165–169
169
optimum doping level of Yb3+ ions is 7.5 mol%, and the intensity
of upconverting emissions for (Y0.915Ho0.01Yb0.075 Ti NPs is
)
2
2 7
O
3
+
increased by 32 times compared to the Yb -free samples. The qua-
dratic pump power dependence of the green and red intensities
indicates two photons are responsible for the upconverting process
under low pump power, and the potential upconverting mecha-
nisms have been proposed.
Acknowledgments
This work is supported by Natural Science Foundation of China
(
Nos. 51362002, 11375043, 51171069 and 51062008), Science &
Technology Pillar Program from Jiangxi Provincial Department of
Science and Technology(No. 20122BBG70084-2), and the financial
supports from Key Laboratory of Radioactive Geology and Explora-
tion Technology Fundamental Science for National Defence (No.
2010RGET15) and State Key Laboratory Breeding Base of Nuclear
Resources and Environment (No. 060614 and NRE1319) are also
greatly appreciated.
Fig. 8. Energy level diagrams of Ho3+ and Yb3+ ion as well as the proposed
2 2 7
upconverting mechanisms of Y Ti O
:Ho3+,Yb NPs.
3+
of the excited electrons is transferred to the neighboring electrons of
3
+
5
Ho ions at the ground state I
about the population of the intermediate excited state I
excess energy consumed by the host lattice vibration. Then, most
8
(ETU1 in Fig. 8), thereby bringing
5
6
with the
References
Ho ions at the state 5
3
+
5
5
6 4 2
I are pumped to the levels ( F , S ) via
[
1] L. Cheng, C. Wang, Z. Liu, Nanoscale 5 (2013) 23–37.
excited state absorption (ESA) and energy transfer (ETU2). Lastly,
the green luminescence is observed via the radiative transitions
[2] K.E. Sapsford, W.R. Algar, L. Berti, K.B. Gemmill, B.J. Casey, E. Oh, M.H. Stewart,
I.L. Medintz, Chem. Rev. 113 (2013) 1904–2074.
[3] S. Zhou, S. Jiang, X. Wei, Y. Chen, C. Duan, M. Yin, J. Alloys Comp. 588 (2014)
5
5
5
from ( F
4
, S
2
) to I
8
, which is also depicted in detail as follows:
654–657.
[
4] X. Huang, S. Han, W. Huang, X. Liu, Chem. Soc. Rev. 42 (2013) 173–201.
4F7=2ðYb Þ þ h
3þ
m
980 ! F5=2ðYb3þ
4
Þ
ðGSAÞ
[5] Q. Liu, W. Feng, F. Li, Coord. Chem. Rev. (2014), http://dx.doi.org/10.1016/
j.ccr.2014.01.004.
5I
ðHo Þ þ F5=2ðYb Þ ! I
3þ
4
3þ
5
ðHo Þ þ F7=2ðYb3þÞ ðETU1Þ
3þ
4
8
6
6
6
[
6] D. Gao, X. Zhang, H. Zheng, W. Gao, E. He, J. Alloys Comp. 554 (2013) 395–399.
[7] C. Li, Y. Mei, J. Xie, W. Dai, G. Du, Z. Li, J. Alloys Comp. 560 (2013) 10–14.
8] Y. Li, S. Xu, J. Alloys Comp. 601 (2014) 195–200.
[9] L. Li, H. Lin, X. Zhao, Y. Wang, X. Zhou, C. Ma, X. Wei, J. Alloys Comp. 586 (2014)
55–560.
[10] Z.S. Chen, W.P. Gong, T.F. Chen, S.L. Li, D.Y. Wang, Q.K. Wang, Mater. Lett. 68
2012) 137–139.
5I
5I
5
3þ
4
3þ
5
5
3þ
4
3þ
ðHo Þ þ F5=2ðYb Þ ! ð F
4
; S
2
ÞðHo Þ þ F7=2ðYb Þ ðETU2Þ
[
3þ
5
5
ÞðHo3þ
ðHo Þ þ h
m
980 ! ð F
4
; S
2
Þ
ðESAÞ
5
5
3þ
5
ðHo3þÞ þ h
ð F
4
; S
2
ÞðHo Þ ! I
8
m
green ðRTÞ
(
[
[
11] X. Li, H. Cai, L. Ding, X. Dou, W. Zhang, J. Alloys Comp. 541 (2012) 36–40.
12] C.C. Ting, Y.C. Chien, W.F. Sung, ECS J. Solid State Sci. Technol. 2 (2013) R105–
R110.
In the case of the red emission, the two possible approaches are
3+
5
responsible for the population of Ho ions at the
F
5
level. On one
5
5
[13] B.P. Singh, A.K. Parchur, R.K. Singh, A.A. Ansari, P. Singh, S.B. Rai, Phys. Chem.
Chem. Phys. 15 (2013) 3480–3489.
[
hand, the excited electrons at the states ( F
4
, S
2
) decay nonradiative-
5
ly to the level of F
5
with the help of multi-phonon relaxation (MPR).
14] B.S. Cao, J.L. Wu, N.S. Yu, Z.Q. Feng, B. Dong, Thin Solid Films 550 (2014) 495–
498.
3+
On the other hand, a portion of electrons of Ho ions at the interme-
5
5
[15] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, J. Phys. Chem.
B 107 (2003) 1107–1112.
[
diate excited state I
6 7
decay to the level I via nonradiative transition
5
(
NRT), and then giving rise to the population of the state F through
5
16] Y. Zhang, M. Wang, Z. Le, G. Huang, L. Zou, Z. Chen, Ceram. Int. 40 (2014) 5223–
5230.
17] H.P. Klug, L.E. Alexander, X-Ray Powder Diffraction Procedures, John Wiley &
Sons, New York, 1959.
18] L. Brixner, Inorg. Chem. 3 (1964) 1065–1067.
19] P.A. Tanner, L.S. Fu, B.M. Cheng, J. Phys. Chem. C 113 (2009) 10773–10779.
the energy transfer (ETU3) or excited-state absorption (ESA). As a
result, the red emission around 660 nm is observed by the transition
[
5
5
from the level F
5
to I
8
, which can be described as follows.
[
[
4F7=2ðYb3þÞ þ h
m
980 ! F5=2ðYb3þÞ ðGSAÞ
4
[20] H. Palneedi, V. Mangam, S. Das, K. Das, J. Alloys Comp. 509 (2011) 9912–9918.
[
21] K. Koteswara Rao, T. Banu, M. Vithal, G.Y.S.K. Swamy, K. Ravi Kumar, Mater.
Lett. 54 (2002) 205–210.
5I
ðHo Þ þ F5=2ðYb Þ ! I
3þ
4
3þ
5
ðHo3þÞ þ F7=2ðYb3þ
ðMPRÞ
4
Þ
ðETU1Þ
ðETU3Þ
8
6
5
5
3þ
5
3þ
[22] L.Q. An, J. Zhang, M. Liu, S.W. Wang, J. Am. Ceram. Soc. 88 (2005) 1010–1012.
23] M. Liu, S.W. Wang, D.Y. Tang, L.D. Chen, J. Ma, J. Rare Earth 27 (2009) 66–70.
[24] X.P. Chen, W.J. Zhang, Q.Y. Zhang, Physica B 406 (2011) 1248–1252.
ð F
4
; S
2
ÞðHo Þ ! F
5
ðHo
Þ
[
5
3þ
5
3þ
I
5I
5I
6
7
7
ðHo Þ ! I
7
ðHo
Þ
ðMPRÞ
[
25] G.J. De, W.P. Qin, J.S. Zhang, Y. Wang, C.Y. Cao, Y. Cui, Solid State Commun. 137
(2006) 483–487.
26] M.L. Bailly, G. Costentin, H. Lauron-Pernot, J.M. Krafft, M. Che, J. Phys. Chem. B
3þ
4
3þ
5
ðHo3þÞ þ F7=2ðYb3þ
Þ
4
ðHo Þ þ F5=2ðYb Þ ! F
5
[
3þ
5
3þ
ðHo Þ þ h
ðHo Þ ! I
m
5
980 ! F
5
ðHo
Þ
ðESAÞ
109 (2005) 2404–2413.
5F
3þ
3þ
[27] T. Moon, S.T. Hwang, D.R. Jung, D. Son, C. Kim, J. Kim, M. Kang, B. Park, J. Phys.
Chem. C 111 (2007) 4164–4167.
[
5
8
ðHo Þ þ h
m
red ðRTÞ
28] C.C. Ting, Y.S. Chiu, C.W. Chang, L.C. Chuang, J. Solid State Chem. 184 (2011)
63–571.
5
[
[
29] L. Zhang, H. Hu, C. Qi, F. Lin, Opt. Mater. 17 (2001) 371–377.
30] F. Auzel, G. Baldacchini, L. Laversenne, G. Boulon, Opt. Mater. 24 (2003) 103–
4
. Conclusions
In summary, a variety of Yb3+, Ho3+ co-doped Y
1
09.
[31] J.L. Wagener, P.F. Wysocki, M.J.F. Digonnet, H.J. Shaw, D.J. Digiovanni, Opt. Lett.
8 (1993) 2014–2016.
32] M. Pollnau, D.R. Gamelin, S.R. Luthi, H.U. Gudel, M.P. Hehlen, Phys. Rev. B 61
2000) 3337–3346.
2
2 7
Ti O NPs were
1
prepared via the glycine-combustion approach. Under the 980 nm
[
excitation, the bright (ꢁ540 nm) and red (ꢁ660 nm) emissions are
(
5
5
5
5
5
observed, which is attributed to the ( F
4
, S
2
) ? I
8
and
F
5
? I
8
[33] Y. Bai, Y. Wang, G. Peng, K. Yang, X. Zhang, Y. Song, J. Alloys Comp. 478 (2009)
676–678.
[34] Y.Q. Lei, H.W. Song, L.M. Yang, L.X. Yu, Z.X. Liu, G.H. Pan, X. Bai, L.B. Fan, J.
Chem. Phys. 123 (2005) 174710.
3
+
transitions of Ho ions, respectively. The calcining temperatures
3
+
and Yb ion doping level have a great influence on the upconver-
ting spectra of (Y0.99ꢀxHo0.01Yb Ti NPs. The intensities of
emissions increase initially and then decrease after 800 °C. The
x
)
2
2
O
7
[35] A.H. Li, Q. Lü, EPL 96 (2011) 18001.
[36] J. Petit, B. Viana, P. Goldner, Opt. Express 19 (2011) 1138–1146.