Strong photoluminescence and sensing performance of nanosized Ca0.8Ln0.1Na0.1WO4 (Ln = Sm,Eu) compounds obtained by the dry top-down grinding method

Article abstract of DOI:10.1039/c9dt02109d

Two lanthanide doped nanosystems Ca_0.8Ln_0.1Na_0.1WO₄ (Ln = Eu, Sm), denoted as Eu?CWO and Sm?CWO, were prepared by a top-down approach in three simple steps: Activation, miniaturization by high-energy milling, and further calcination. The solids were thoroughly characterized by X-ray powder diffraction (XRPD) and Scanning-electron microscopy (SEM). Also, analyses of the structure of the compounds and the impact of milling on the crystallite shape and size were carried out through Rietveld refinements. Solid-state photoluminescence was studied in terms of excitation, emission, lifetimes (τ_obs) and europium-quantum yields. Finally, the Eu?CWO sample was employed as a potential water-stable chemical sensor towards toxic cations, showing a quenching effect in the presence of iron ions.

Full text of DOI:10.1039/c9dt02109d

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View Article Online
DOI: 10.1039/C9DT02109D
ARTICLE
Strong photoluminescence and sensing performance of nanosized
Ca Ln Na WO (Ln=Sm, Eu) compounds obtained by dry “top-
down” grinding method
0
.8 0.1
0.1
4
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
b
DOI: 10.1039/x0xx00000x
Germán E. Gomez,* Carlos A. López,* R. Lee Ayscue III , Karah E. Knope , María del R. Torres
c
a
Deluigi and Griselda E. Narda
4
Two lanthanide doped nanosystems Ca0.8Ln0.1Na0.1WO (Ln=Eu, Sm), denoted as Eu@CWO and Sm@CWO, were prepared
by a “top-down” approach in three simple steps which included activation, miniaturization by high-energy milling, and
further calcination. The solids were thoroughly characterized by X-ray powder diffraction (XRPD), Raman spectroscopy and
scanning-electron microscopy (SEM). Also, analyses of the compounds’ structure and the impact of the milling on
crystallite shape and size were carried out through Rietveld refinements. The solid-state photoluminescence was studied
in terms of excitation, emission, lifetimes (obs) and europium-quantum yields. Finally, the Eu@CWO sample was employed
as potential water-stable chemical sensor towards toxic cations, finding a quenching effect in the presence of iron ions.
4
The ABO structure (A=Ca, Sr, Ba; B=W, Mo) has been used
Introduction
as traditional host platforms for lanthanide ions. Using this
material Gd , Er , Yb @CaWO or carbon-dots@CaWO (or
4 4
3
+
3+
3+
7
Studies focused on the optical properties of the lanthanide
ions in diverse host materials provide fundamental data that
includes energetic transitions, radiative and non-radiative
decay processes, intrinsic and overall quantum yields, and
lifetimes. Such information is critical for assessing and/or
designing optical devices such as lasers, upconverters, color
displays, and amplifiers. Towards this end, many recent efforts
have aimed to develop new or improved optical devices via the
appropriate pairing of host materials (chemical compositions)
3
+
3+
BaWO
4
) doped with Er ,Yb as upconverting (UC) micro/nano
8
materials have been reported. Additionally, CaWO
4
has been
3
+
employed as a matrix for Eu and a variety of hierarchical
architectures that show red emission under UV excitation
were obtained by controlling doping through different
Zhou et al. studied and compared the
luminescence properties of CaMoO
9
–15
synthetic routes.
3
+
3+
+
4
:Eu and CaMoO
4
:Eu -A ,
+
3+
where Aꢀ=ꢀLi, Na, K, and deduced that Li -doped CaMoO
4
:Eu
produced the optimal red emission upon excitation at
1
–4
with doped lanthanide ions. Trivalent lanthanide ions are
1
6
3+
4
4
64ꢀnm. Despite many investigations on CaBO :Eu (B= Mo,
0
characterized by a gradual filling of the 4f orbitals, from 4f
W) to the best of our knowledge, extensive research regarding
important photophysical parameters such as luminescence
3
+
14
3+
N
(
La ) to 4f (Lu ). These electronic [Xe]f configurations (N =
5
0
-14) generate a variety of electronic energy levels, resulting
lifetime (obs) or even quantum yield parameters (QYtot or QYEu
)
6
in intricate optical properties. Consequently, each lanthanide
ion exhibits narrow and characteristic 4f-4f transitions. All Ln
ions (except La3+ and Lu ) exhibit f-f emissions that range in
has not been reported. Yet the obs value is an important
property to be evaluated for designing new luminescent
materials. Furthermore, the potential sensing applications of
3
+
3+
wavelength from ultraviolet (UV) to visible and near-infrared
3
+
17
CaBO
a set of selected lanthanide-doped ABO
Commonly, ABO phases are synthesized through
4
:Ln phases are scarcely investigated. Table 1 displays
3
+
3+
3+
3+
(NIR) spectral regions. For example, Sm , Eu , Tb , and Tm
4
luminescent phases.
ions can emit orange, red, green, and blue light, respectively,
3
+
3+
3+
4
while Nd , Yb and Er ions display the well-known near-
infrared luminescence.
conventional methods at high temperatures for long periods of
times. Additionally, hydrothermal or microwave assisted
syntheses have served as an activation process for further
calcinations at high temperatures.18,19 By comparison, the use
a. Instituto de Investigaciones en Tecnología Química (INTEQUI). Universidad
Nacional de San Luis, CONICET, Almirante Brown 1455, 5700 San Luis,
of high-energy milling for these types of inorganic compounds
Argentina. E-mails: gegomez@unsl.edu.ar, calopez@unsl.edu.ar
Georgetown University, Washington, D.C., USA.
Laboratorio de Microscopia Electrónica y Microanálisis (LABMEB), Facultad de
2
0
b.
is scarcely reported. Additionally, sonochemical methods
have been employed to obtain luminescent ABO phases as a
c.
4
Química, Bioquímica y Farmacia, Universidad Nacional de San Luis, Ejército de
los Andes 950, 5700 San Luis.
2
1
fast and solvent-free route of synthesis. In this work, we
report a novel three-step “top-down” synthesis that follows a
†
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: tables of Rietveld analysis, f-f
transitions assignments, decay profiles, emission spectra and extra micrographies.
See DOI: 10.1039/x0xx00000x
“green” methodology through a solvent-free approach with a
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Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
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Journal Name
final calcination at lower temperatures than those previously Eu@CWO sample was evaluated as potential sensor towards
View Article Online
DOI: 10.1039/C9DT02109D
reported in the literature.
toxic cations.
Table 1: Select ABO
4
lanthanide-doped phases (A=Ca, Sr, Ba; B=W, Mo).
Experimental
Synthesis
Size-
Refere
nce
Compound
carbon-dots
Synthesis method
hydrothermal
Property
UC
particles
The title materials were obtained through a novel three step-
synthesis. Unlike traditional synthetic routes, this methodology
does not use solvent and has a relatively low calcination
temperature; thus, achieving a “green” method to obtain
3
+
@CaWO
4
:Yb ,
4 m
8
7
9
Er3+
3
-4 m
3
+
3+
Gd , Er ,
hydrothermal and
further calcination
and 100
nm
UC
3
+
Yb @CaWO
4
x x 4
optical materials. Ca1-2xLn Na WO samples, with x = 0 and 0.1
co-deposition in the
water/oil phase and
further calcination
and Ln = Eu and Sm, were obtained as white powders by
employing a mechanothermal method, where the solid-state
reaction was mechanically activated by high-energy milling in a
0
.12µm-
.79µm
CaMoO
4
:Eu3+
red-emitter
1
Ca0.8Ln0.1Na0.1
red and orange
emitters/chemica
l sensing
planetary ball mill. The starting reagents were CaCO
3 2 3
, Eu O (or
High energy milling and
further calcination
50-150
nm
this
WO
4
(Ln=Sm,
Eu)
Sm ) Na CO and WO . The ball milling processes were
O
2 3
2
3
3
work
carried out using a Fritsch Pulverissette 6 planetary ball mill
equipped with a cylindrical tungsten carbide vial (80 cm )
together with 15 mm diameter WC balls. The ball mass–
powder mass ratio was 29:1, the rotation speed was 500 rpm,
and the milling time was 1 h in intervals of 15 minutes. From
nitrate–citrate gel
combustion
500 nm-
1m
: Eu3+
red-emitter
red-emitter
10
22
12
3
CaMoO
4
3
+
CaMoO
4
: Bi ,
solid-state reaction and
calcination
not
Eu3+
reported
3
+
CaMoO : Eu ,
4
red and green
emitter
flux crystal growth
1-5 mm
3
+
3+
these conditions, the weight-normalized cumulative kinetic
Dy , Tb
: Na+,
energy (Ecum) was calculated to be 0.2 kJ∙g-1 according to Rojac
CaMoO
4
solid-state reaction and
calcination
not
red emitter
red emitter
13
14
Eu3+
reported
23
et al.
After this mechanical activation, the samples were
: Eu3+
calcined for 1 h at 700 °C which produced pure and well-
crystallized phases with the scheelite structure type. To obtain
miniaturized powders, an aliquot of both samples was re-
milled under the same conditions described previously, but
with a ball mass–powder mass ratio of 40:1 over 8 h, yielding
an Ecum of 2.2 kJ∙g . These reduced size samples were used for
luminescent characterizations. A schematic view of the
synthesis procedure is illustrated in scheme 1.
CaMoO
4
sonochemical
6 m
3
+
CaWO
4
: Eu , sol-gel methodology and
not
red, green and
blue emitters
1
5
3
+
3+
Dy , Tb
CaMoO
nanocasting
reported
: A+,
4
3
+
Eu (A=Li, K,
Na)
microwave sintering
3,82 m
red emitter
16
21
-1
cathodo- and
photo-
3
+
CaWO
4
:Eu ,
120-160
nm
sonochemical
Tb3+
luminescence
3
+
4
CaWO :Er ,Yb solid-state reaction and
~
1-4 m
thermal sensing
red-emitter
17
18
20
3
+
calcination
CaWO
4
4
: Eu3+
: Eu3+
hydrothermal
4 m
high energy milling and
further calcination
not
CaWO
red-emitter
reported
not
CaWO
4
: Eu3+
microwave
red-emitter
16
reported
Scheme 1: Synthesis of Ln@CWO compounds.
X-ray powder diffraction (XRPD)
4
In this work, we synthesized Ca0.8Ln0.1Na0.1WO (Ln=Eu, Sm)
(
namely as Eu@CWO and Sm@CWO) samples via a simple The identification and characterization of the solid phases
three step method that included two mechanochemical were carried out by laboratory XRPD using a Rigaku Ultima IV
treatments followed by a soft calcination process. Double diffractometer with CuKα (λ = 1.5418 Å) radiation. The
3
+
+
2+
doping (e.g. 0.5Ln 0.5Na for Ca ) was required to maintain patterns were refined by the Rietveld method using the
electroneutrality in the scheelite structure and to obtain a FullProf program.24,25 The profile shape was modelled using
2
6
defect-free compound. The nanosized solids were the Thompson-Cox-Hastings pseudo-Voigt function , and the
characterized by X-ray powder diffraction and scanning- instrumental resolution parameters were considered in the
electron microscopy. Structural analysis was carried out by refinements in order to obtain the microstructural parameters.
Rietveld refinements which provided information about Scanning Electron Microscopy (SEM)
crystallite size. The solid-state photoluminescence (SSPL) was
Images were obtained on a Zeiss LEO1450VP instrument;
analysed by recording the excitation and emission spectra and
samples were placed on adhesive carbon tape coated with
calculating the obs from the decay profiles. Also, the
gold. SEM images were also collected with a Zeiss SUPRA 55-
VP scanning electron microscope at an acceleration voltage of
europium-quantum yields were measured. Finally, the
2
| J. Name., 2012, 00, 1-3
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3
0 kV with an in-lens detector. The samples were mounted occupancies are in accordance with the expected
View Article Online
stoichiometry: Ca0.8Ln0.1Na0.1WO
4
. The m
D
ic
O
r
I
o
: 1
s
0
tr.1
u
0
c
3
t
9
u
/Cra
9
l
D
a
T
n
02
a
1
l
0
9
D
directly onto conductive carbon "Lift-N-Press" adhesive tabs.
Dispersion was completed by introducing 2 mg of sample into
of these samples does not suggest a peak broadening being
indicative of a large size of crystallite.
2
mL of n-hexanes and then sonicated for 45 minutes.
Photoluminescence spectra and lifetime measurements
Photoluminescence measurements were obtained on solid
samples at room temperature using a Horiba PTI QM-400
system. Excitation and emission spectra as well as lifetimes
were collected on ground solid powders crushed between two
glass slides. Time-resolved lifetime measurements were
acquired using a Xenon flash lamp as the average of 10,000
shots. Reported lifetimes and standard deviation are the
average of three collections.
Quantum yields determinations
Quantum yields were collected for Eu@CWO using an 8.9 cm
integrating sphere coated in Spectralon fluoropolymer.
Samples were ground to homogenous powders and loaded
into a Teflon sample holder placed within the sphere. Slit
widths were maintained at 1 nm, and the spectra recorded
were background-corrected using a blank sample holder and
corrected for the wavelength dependence of the
spectrofluorometer, sampling optics, and integrating sphere.
Reported quantum yields are averages of three independent
determinations.
Figure 1: Effect on the XRPD pattern of grinding method of Eu@CWO nanoparticles.
The milled samples were further refined with special attention
given to line broadening. As mentioned previously, the line
shape was modelled using the Thompson-Cox-Hastings
pseudo-Voigt function. The U, V, and W parameters are fixed
and correspond to the instrumental broadening; hence, only
the Lorentzian isotropic strain (X) and size (Y) were refined.
From these parameters, the apparent size and strain were
calculated from the Scherrer and Stokes-Wilson formula,
respectively. Figure 2 shows the best fits for the Ln doped
phases after milling treatments. Table S2 summarizes the cell
and microstructural parameters of these samples. No
appreciable change in the observed cell parameters after
milling suggests that the size reduction process did not modify
the crystallographic features of lanthanide-doped scheelite
structure type. Furthermore, the apparent size and strain of
crystallite is similar in both samples. Obtained strain
parameters are high and in accordance with a milling process
that generates grains with a distorted surface. The crystallite
size of ~18 nm is greatly reduced with respect to well
crystallized samples.
Chemical sensor studies
The sensing activity of the Eu-based compound was
investigated by monitoring the emission at 616 nm, when
exciting the samples at 393 nm. A quartz cuvette with an
optical path of 1 cm was employed. Stock aqueous solutions of
2
+
2+
2+
2+
2+
2+
2
2
00 ppm of each metal (Cu , Fe , Zn , Co , Mn , UO ,
4
+
2+
2+
Th , Pb and Hg ) were prepared. Suspensions were
prepared by introducing 1.5 mg of powdered sample into 4 mL
0.37 mg∙mL ) of each metal stock solution. The samples were
-1
(
previously ultrasonicated for 15 minutes. The excitation and
emission slits were set at 1.5 nm.
Results and discussion
Crystal structure
The structural identification and characterization of all samples
were performed from XRPD data for well-crystallized, milled
samples. All samples were indexed to the scheelite structure
4
(CaWO ), and no impurities were observed. Moreover, as
illustrated in Figure 1, milling resulted in a strong line
broadening associated with microstructural effects for the Eu-
doped sample.
All XRPD patterns were refined in the tetragonal space
group I4
center is located in a tetrahedral site; Ca is coordinated by
eight oxygen atoms. Each CaO polyhedron edge-shares with
four other polyhedra; WO tetrahedra are distributed between
/a(88).27 In the scheelite structure the W metal
1
8
4
Figure 2: Refinement for Eu@CWO and Sm@CWO (left). Observed (red points),
these sites. The results of the Rietveld refinement of well calculated (black full line) and difference (bottom) Rietveld profiles for both
crystallized samples is summarized in Table S1. For doped compounds at 298 K. Eu@CWO refined structure (right), with Ca (blue), Eu/Na (pink),
phases, the Ca/(Ln0.5Na0.5) ratio was refined, and the site W(grey), and O (red)
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Morphology analysis
(Figure 4, bottom). The remaining peaks are observed at 564,
View Article Online
DOI: 10.1039/C9DT02109D
6
08, 706 nm for J=5/2, 7/2, 11/2 respectively. The excitation
The morphology characterization was carried out by SEM and
Rietveld analysis in order to study the impact of the synthesis
approach on the obtained particle sizes. From SEM analysis,
the averaged particle sizes were found into the 300-400 nm
and 50-150 nm ranges for the samples before and after
miniaturization “top-down” treatment (Figures 3 and S1-3).
However, according to Figure 3, a high grade of crystalline
aggregation was achieved. . Basing on the Rietveld refinement
analysis of Ln@CWO nanosystems (Table S2), it was possible
to state that each particle is composed by crystallites of ~18
nm.
spectrum shows a complex combination of transitions with
maxima at 405 and 482 nm. 1931 Comission Internationale de
l’Éclairage (CIE) coordinates of Sm@CWO emission are 0.62,
0
.37 (x,y) (see Figure 6).
Sample Eu@CWO exhibits an emission spectrum with peaks
3
+
corresponding to the transitions within the Eu ion’s 4f shell,
5
7
7
0 J 2
D → F (J=0-4), with a maximum emission at 616 nm ( F ). The
remaining peaks are positioned at 580, 592, 655 and 702 nm
for J=0, 1, 3, and 4 transitions, respectively (Figure 5 bottom).
The excitation spectrum shows a complex combination of
transitions with maxima at 395, 465 and 535 nm. Notably, the
two lower energy transitions are exceptionally deep into the
visible region (Figure S5 and S6). 1931 CIE Coordinates of
Eu@CWO are 0.66, 0.33 (x,y) (Figure 6). The corresponding
assignment of the 4f–4f excitation transitions for Eu@CWO
and Sm@CWO are presented in Table S3.
Figure 3: SEM micrographies of Eu@CWO before (top) and after (bottom) milling
miniaturization.
Solid state photoluminescence (SSPL)
Illustrated in Figure 4, the excitation and emission spectra of
4
host matrix CaWO (CWO) were studied. The excitation band
located at 330 nm is ascribed to a charge transfer band (CTB)
2
-
6+
associated with an electron transfer from O ligand to W .
The corresponding blue emission is observed as a broad band
with a maximum located at 400 nm (see Figure S4).
Figure 4: Excitation (top) and emission (bottom) spectra of Sm@CWO.
The excitation spectra of Sm@CWO and Eu@CWO exhibit
broad bands in the 260-350 nm region which are attributed to
a CTB (Figures 4 and 5, top). In general, CTBs can arise through
three different mechanisms: (i) host absorption which involves
2
-
6+
electron transfer from O ligand to W (ii) inter-valence
charge transfer due to electronic transition from 4f state of
3
+
3+
6+
Eu (or Sm ) to W and (iii) electron transfer from the filled
p orbitals of O2− anions to the vacant 4f orbitals of Ln .
3+ 28
2
Such broadness in CTB can potentially be arised by the
contribution from all the three types of electronic transition.
4
6
J
Sm@CWO sample shows the G5/2 → H transitions (J=5/2,
4
6
7
/2, 9/2, 11/2) with a max emission at 646 nm ( G5/2→ H9/2
)
4
| J. Name., 2012, 00, 1-3
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―푡
―푡
휏
휏
View Article Online
(1)
DOI: 10.1039/C9DT02109D
1
2
퐼 = 퐴 + 퐼 .푒
+ 퐼 .푒
1
2
For the case of Sm@CWO, lifetimes were collected at an
excitation wavelength of 405 nm monitoring the emission at
4
6
6
46 nm ( G5/2→ H9/2). Exponential fitting was modelled
between 112-1200 µs to avoid the influence of initial
excitation and account for estimated zero intensity
2
luminescence. The biexponential fitting (R =0.9984) resulted in
average lifetimes of 12 µs and 106 µs.
In both cases, the single exponential fitting showed
disagreement with the decay profiles, demonstrated by higher
2
R values than those employing the bifunctional model. This
model would suggest the presence of two emitting centres in
the structure. This behaviour can be related with the
crystallographic features. As it was discussed previously, each
3
+
Ln (in the centre of polyhedron LnO
8
) is edge-connected to
four AO polyhedra. Considering these features, a statistical
8
analysis revealed that 66% of the lanthanides are isolated,
while the other 34% are connected to another emitting centre.
3
+
Assuming that the lifetime of each Ln is affected by the
vicinity of another one, the presence of two emitting centres
can be assigned to differences in the lanthanide
2
9
environments.
To qualify the ability of the matrix to sensitize the emission of
Eu(III) centres, and to get information of the relationship
between the structure and photoluminescence properties, the
photophysical parameters of Eu@CWO were analysed. These
parameters included: the overall quantum yield (QYtot),
intrinsic quantum yield (QYLn), and the efficiency of the matrix-
to-metal energy transfer (ηsens). The relationship between
these terms is given in equation 2.
Figure 5: Excitation (top) and emission (bottom) spectra of Eu@CWO.
QYtot = QYLn• sens
(2)
The total quantum yields (QYtot) were experimentally collected
using an integrating sphere and calculated according to
equation 3:
퐸푠 ― 퐸푏
QYtot = 퐿푏 ― 퐿푠
(3)
where E
is the integrated emission spectrum of the blank (integrating
sphere and empty sample holder), L the blank absorption, and
the sample absorption at the excitation wavelength.
s b
is the integrated emission spectrum of the sample, E
b
L
s
Figure 6: CIE 1931 chromaticity diagram showing the (x,y) color coordinates for For a correct evaluation of the SSPL efficiency of the Eu-doped
Eu@CWO and Sm@CWO nanoparticles after being excited with the corresponding
wavelengths. Pictures show the samples under direct excitation in the fluorometer.
sample, the QYLn was calculated. This parameter expresses
how well the radiative processes compete with the non-
radiative pathways and provides optimized quantum
efficiencies for these materials. Assuming that non radiative
For lifetime measurements, time based decay spectra were
collected in triplicate (Figure S7). For Eu@CWO, lifetimes were
(
k
nr) and radiative (k
r
) processes are essentially involved in the
collected at excitation wavelengths of 395, 465, and 535 nm
5
depopulation of D
0
state, the QYLn can be expressed as:
5
7
0 2
monitoring the emission at 616 nm ( D → F ) in which all
values were consistent. Exponential fitting was modelled QYLn = k
r
/(k
r
+knr
)
(4)
between 160-4000 µs to avoid influence of initial excitation
In general, contributions of the knr include back-energy
transfer to the sensitizer, electron transfer quenching, and
quenching by matrix vibrations. Moreover, C-H, O-H, and N-H
and account for estimated zero intensity luminescence. The
2
biexponential fitting (R = 0.9999) (equation 1) revealed
average lifetimes (obs) of 126 µs and 445 µs.
This journal is © The Royal Society of Chemistry 20xx
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vibrations which are commonly found in organic moieties have hypersensitive transition in the presence of the metals could
View Article Online
3
0
DOI: 10.1039/C9DT02109D
an important contribution to knr
.
The radiative contribution k
r
be calculated as:
can be calculated from the equation:
QE% = (I - I)/I •100
0
0
(9)
k
r
= 1/
r
(5)
0
where I and I represent the emission intensity values in the
The so-called radiative lifetime 
Eu(III) by the equation :
r
can be approximated for absence and presence of the quencher, respectively. Based on
3
1
2+
these results, Fe represents an efficient quencher for the
luminescence of Eu@CWO, exhibiting a QE of 65%, being the
highest value for the set of metal solutions.
3
k
r
= (1/
r
) = AMD,0 • n •(Itot/IMD
)
(6)
The plausible mechanism for luminescence quenching by iron
In equation 6, AMD,0 is the spontaneous emission probability of
the D → F magnetic dipole transition equal to 14.65 s , n is
0 1
5
7
-1
ions has been further investigated. In this context, many
reasons for such a quenching effect have been explored: (1)
the collapse of the framework, (2) ion exchange between the
targeted ions and central metal ions of the sensors or (3) the
the refractive index, Itot is the total integrated emission of the
5
7
D
0
→ F
J
(J= 0-6) transitions and IMD is the integrated emission
5
7
of the D
0
→ F
1
transition.
3
7
resonance energy transfer, among the most relevant.
r
If the radiative lifetime,  , is known, QYLn can be calculated
using the observed luminescence lifetime. Based on eqn. (1)
and (4), the QYLn can be calculated as:
2
+
38
According to absorption spectrum of Fe solution, it has a
large overlap with the excitation spectrum of Eu@CWO.
2
+
Therefore, this overlap of the Fe absorption band and the
excitation of Eu@CWO (395 nm) may be responsible for the
quenching effect through the electron-transfer mechanism.
QYLn = obs/
r
(7)
At the same time, knowledge of both obs and 
r
enables
determination of the overall rate of non-radiative deactivation.
Hence, the radiative lifetime is an important parameter in the
photophysical description of lanthanide luminescence. The
SSPL parameters of Eu@CWO are summarized in Table 2.
Sensing studies
In the context of chemical sensor design, Tb3+ and Eu3+
inorganic compounds have received significant interest due to
5
7
5
7
their hypersensitive transitions ( D
4
→ F
5
and
0 2
D → F ,
respectively). These materials possess optical properties which
are readily affected by their chemical and physical
3
2
environment. During the last fifteen years, Eu and Tb hybrid
materials have been extensively used as efficient platforms for
3
3
34
35
sensing ions, water, VOCs (volatile organic compounds),
3
6
36
explosives and agrotoxics.
Regarding its water stability and robust emission signal, the
chemosensing performance of Eu@CWO was investigated by
measuring the redPL in presence of cationic species (see Figure
7
).
In this study the emission intensities were used as an optical
handle for sensing. The emission band centered at 616 nm
decreases when Eu@CWO is suspended in 200 ppm of metallic
solutions (see Figure 7). The quenching efficiency (QE) of the
Table 2: Photophysical parameters, measured in triplicated, of the Eu@CWO.
QYtot
QYEu
(%)
η
sens
k
(s )
obs
k
(s )
r
k
nr
-1
τ
r
τ
nr
Sample
I
tot
1
I )
MD (7F
-1
-1
(%)
(%)
8.22
7.13
(s )
1619
1647
(µs)
1591
1666
(µs)
618
607
1
78338
75862
6162
6247
6232
2.30
1.91
27.97
26.72
2247
2247
2247
629
600
2
3
75774
76658 ±
(1456)
2.03
2.08±
(20)
26.75
27.15±
(72)
7.59
7.65±
(55)
601
610±
(16)
1646
1637±
(16)
1664
1640±
(43)
608
611±
(6)
Average ±
6214±
(46)
2
247
**
*
Assuming average (obs)av of 445 µs the following data table is calculated. ** Standard deviation.
6
| J. Name., 2012, 00, 1-3
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Page 7 of 8
Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Journal Name
ARTICLE
Figure 7: Emission of Eu@CWO (top) under different metallic suspensions of 200 ppm.
Quenching Efficiency of each cation species to the europium hypersensitive transition
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Two lanthanide nano-systems based on Ca0.8Ln0.1Na0.1WO
4
8
9
(Ln=Eu, Sm) were successfully prepared by a novel and simple
three step methodology including: activation, calcinations, and
a “top-down” miniaturization by high-energy milling. The
novelty of the synthesis lies in the fact that it is solvent free, 10
implying shorter calcinations times than previously reported
methods to obtain analogous phases of nanoparticles with 11
sizes around 50-150 nm. All the compounds were fully
characterized by X-ray powder diffraction () and Scanning- 12
electron microscopy (SEM). Moreover, the impact of milling on
the shape and size was carried out through the structural 13
analysis of Rietveld’s refinements. The photoluminescence of
the two Ln-doped samples were studied in terms of excitation, 14
emission, CIE (x,y) chromaticities, lifetimes, and europium-
quantum yields. The higher values of intrinsic quantum yield, 15
strong red emission, and characteristic lifetimes at lower
europium concentration were shown by Eu@CWO. Finally, the 16
Eu@CWO was tested as chemical sensor towards toxic cations,
based on the quenching effect of iron ions through an 17
electron-transfer mechanism. The results verified the
outstanding potential of Eu@CWO as highly stable optical 18
material without previous activation and promising for the
development of specific chemical sensors by a fast synthesis 19
and “green” methodology.
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2
0
Conflicts of interest
There are no conflicts to declare.
21
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This work was supported by the Consejo Nacional de
2
006, 26, 3711–3716.
Investigaciones Científicas
Universidad Nacional de San Luis (Projects: PIP_CONICET
20CO and PROICO 02-2016). G.E.G. thanks the Fulbright
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Técnicas (CONICET) and
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C.A.L. acknowledges ANPCyT (Project PICT 2014-3576). R.L.A.
thanks the DOD-SMART fellowship, and K.E.K. recognizes the
Clare Boothe Luce Foundation for support.
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For Table of Contents Only
Two new nanosized lanthanide-doped materials (Sm and
Eu) obtained by high-energy milling showed a suitable
platform with solid-state photoluminescence properties for
sensing applications.
8
| J. Name., 2012, 00, 1-3
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