Supporting Information
Particle
Size
and
Crystal
Phase
Dependent
Photoluminescence
of
La2Zr2O7:Eu3+ Nanoparticles
Madhab Pokhrel1, Mikhail G. Brik2, 3, 4 and Yuanbing Mao1*
1
Department of Chemistry, University of Texas-Pan American, 1201 West University Drive,
Edinburg, Texas 78539, USA
2
College of Sciences, Chongqing University of Posts and Telecommunications, Chongqing
400065, China
3
Institute of Physics, University of Tartu, Ravila 14C, Tartu 50411, Estonia
4
Institute of Physics, Jan Dlugosz University, Armii Krajowej 13/15, PL-42200 Czestochowa,
Poland
*To whom correspondence should be addressed: Tel: +1 956 665 2417; Fax: +1 956 665 5006;
E-mail: maoy@utpa.edu
ESI-1 Synthesis of 5 % Eu3+ doped La(OH)3·ZrO(OH)2·nH2O precursor:
The starting materials including lanthanum nitrate hexahydrate (La(NO3)3•6H2O, 99.0%),
zirconium dinitrate oxide hydrate (ZrO(NO3)2•xH2O, 99.9%), europium (III) nitrate hexahydrate
(Eu(NO3)3•6H2O, 99.9%), potassium nitrate (KNO3, 99.9%), sodium nitrate (NaNO3, 98%) and
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ammonia (NH4OH, 28.0−30.0%) were purchased from Sigma Aldrich. All chemicals are of
analytical grade reagents and used directly without further purification.
In the first step, single-source complex precursors of La(OH)3·ZrO(OH)2:Eu(OH)3·nH2O were
prepared via a co-precipitation route. In a typical synthesis process, 4.75 mmol lanthanum nitrate
hexahydrate, 5 mmol zirconium dinitrate oxide hydrate, and 0.25 mmol europium (III) nitrate
hexahydrate were dissolved in 200 mL of deionized water to form a clear solution. Then a 200 mL
of dilute ammonia solution with a specific concentration was slowly dropped into the above metal
nitrate solution under magnetical stirring. After further agitation and aging for another 2 hours, the
formed white precipitation was collected by centrifuged and washing with deionized water for
several times. Finally, the single-source complex precursor of La(OH)3·ZrO(OH)2:Eu(OH)3·nH2O
was air dried overnight. Herein, in order to adjust the size of final La2Zr2O7:5%Eu3+ NPs, ammonia
solutions with concentrations of 10, 5 and 2.5(v/v) % were used.
Synthesis of La2Zr2O7:5%Eu3+ NPs
In the second step, La2Zr2O7:5%Eu3+ NPs were synthesized size-controllably through a facile
molten
salt
synthetic
process
using
the
single-source
complex
precursors
of
La(OH)3·ZrO(OH)2:Eu(OH)3·nH2O prepared above. Typically, 0.35 g of the as-prepared
precursor was first ground together with 60 mmol of nitrate mixture (NaNO3:KNO3 = 1:1, molar
ratio). After being ground homogenously, the mixture was transferred into a covered ceramic
crucible and heated up to 650 ℃ with a rate of 10 ℃/min in a box furnace. After isothermally
treated for 6 hours, the sample was cooled down to room temperature at a rate of 10 °C/min. Then
the resulting product was washed with copious amount of deionized water and centrifuged for
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collection. After dried in an oven at 120 ℃ overnight, the La2Zr2O7:5%Eu3+ NPs were obtained.
Corresponding to the concentrations of the added ammonia solutions, i.e. 10, 5 and 2.5 (v/v) %,
for preparing the precursor of La (OH)3·ZrO(OH)2:Eu(OH)3·nH2O, the as-prepared
La2Zr2O7:5%Eu3+ NPs were labeled as S1, S2 and S3, respectively. The as-prepared sample S3
was further calcinated at 800 and 1000 ℃ in air for 6 h and noted as S4 and S5, respectively. The
total synthetic procedure is summarized schematically as shown in Fig. 1.
ESI-2. QY Setup and measurements for the La2Zr2O7:5%Eu3+ NPs
All the excitation and emission spectra were corrected for the spectral sensitivity of the system and
detector, as well as intensity variation in the Xenon lamp light source using a reference diode. In
addition, the temporal evolutions (photoluminescence decay) of the emission intensity as a
function of time were measured utilizing a pulsed xenon flash-lamp excitation source. The source
has a pulse width of approximately 2 µs and the wavelength was controlled by the excitation
monochromater. The collected decay curve was analyzed using Exponential Fit Analysis software
F980 provided by Edinburg Instruments. All the emission and excitation spectra measurements
were performed at room temperature.
For absolute QY measurements, a 150 mm BenFlect coated integrating sphere was employed in
the Edinburgh Instruments FLS980 fluorescence spectrometer. Spectral sensitivity for the
fluorescence spectrometer and sphere was corrected using a calibrated lamp for spectral light
throughput. Powder samples of the as-prepared La2Zr2O7:5%Eu3+ NPs were held in a specially
designed sample holder mounted at the middle of the integrating sphere, engineered with baffles
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to ensure light is homogenized before exiting the sphere. Samples were excited at 258 nm using
the Xenon lamp light source.
To be consistent with all experiments performed, the same sample weight (90 mg) and
monochromater slit size (3 nm for both excitation and emission monochromaters) were kept
identical for all experiments, i.e. same setup was used to compare fluorescence intensities between
samples. Fluorescence spectra in the range of 500-750 nm and excitation spectra of 250-272 nm
not absorbed by the sample and reference were collected after diffuse reflectance from the samples
relative to a non-absorbing standard at the excitation wavelength and emission spectra under the
same condition. To avoid reabsorption, very thin layer of samples was used in all experiments.
The fluorescence QY was measured by finding the ratio of the area under the corrected emission
spectra to the difference in corrected area under the diffuse reflectance spectra for the samples and
the reference.1
The spectra were collected after diffuse reflectance from the samples relative to a non-absorbing
standard (BenFlect) at the excitation wavelength and emission spectra under the same condition.
The emission spectra of 500-750 nm and excitation spectra of 240-272 nm not absorbed by the
samples and reference was measured using TE-cooled photo-multiplier tube (Hamamatsu, Model
R928P) as shown in Figure S6. The QY was measured by finding the ratio of the area under the
emission spectra to the difference in corrected area under the diffuse reflectance of the excitation
spectra for the sample and the reference as shown in equation below.
QY 
Asample
NPemitted
 Nem d 

,
NPabsorbed  N absd Areference  Asample
Including all the possible errors, such as reflectivity of the reference (< 3%), particle size effects
(< 2%), and diffuse reflectance from the sample holder (< 3%), we estimated the error in calculated
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QYs is about 10 %. Similarly, we estimated the fluctuation for excitation power is about 2%. The
excitation power was measured to be 6 mW.
Figure S2. Excitation and emission spectra collected after the diffuse reflectance from the sample
and the reference. Emission spectra were multiplied 10 times for better visibility.
Table S2. PL QY for sample S5 at RT under 258 nm and 244 nm excitation after integrating the
wavelength range from 500 to 750 nm.
Samples
S5
S5
Excitation Wavelength
258
244
QY (%)
6.01±0.60
5.96±0.59
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ESI-3. Evaluation of the particle size of the La2Zr2O7:5%Eu3+ NPs using Scherrer Equation
Profile fitting is the most precise way to determine diffraction peak position, intensity, and width
for calculating lattice parameters and crystallite size. Figure S2 shows the Gaussian fitting for
sample S1 and S2, and an estimation of FWHM for the XRD peak corresponding to
La2Zr2O7:5%Eu3+ centered at 2θ = 28.53 °. The corrected FWHM and the calculation of particle
size for all other three samples have also been listed in Table S1.
Figure S3. Profile fitting to determine FWHM for calculating crystallite size of the
La2Zr2O7:5%Eu3+ NPs: sample S1 (left panel) and sample S2 (right panel).
Table S3. The evaluated crystalline size of the La2Zr2O7:5%Eu3+ NPs using Scherrer equation.
Samples
cos(θ)→θrad
FWHM (Rad)
D = Kλ/βcosθ, λ = 0.154 nm
S1
Θ = (28.53/2)°
0.016
9 nm
S2
0.0091
17 nm
S3
0.0055
28 nm
S4
0.0040
38 nm
S5
0.0022
70 nm
ESI-4. Comparisions of Raman spectra between samples of S3, S4 and S5
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Figure S4. Raman spectra for La2Zr2O7:5% Eu3+ NP samples of S3, S4 and S5.
From the Raman spectra, the strongest band located at ~302 cm−1 can be assigned to the internal
La-O stretching mode, the band of 390 cm−1 is attributed to the O-Zr-O stretching. The other
bands at 508, 621,730, and 842 are attributed to T2g mode of fluorite structure.2 The La-O
stretching mode located at ~302 cm−1 can be used to identify the ordering degree of oxygen
vacancies and cations. For samples S3 and S4, the corresponding Raman spectra are boarder that
that of sample S5, consistent with the fact of disordered fluorite structure vs pyrochlore structure
of the La2Zr2O7:5% Eu3+ NPs by XRD. At the meantime, sample S4 became less disorder after
thermal annealing at 800 ℃, as confirmed with sharper Raman peak at ~302 cm−1 compared to
that from sample S3.
ESI-5. BET Method for Determining Surface Areas for samples of S1, S3, S4 and S5
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Figure S5. (a) Adsorption isotherms for nitrogen in La2Zr2O7:5% Eu3+ NP especially samples of
S1, S3, S4 and S5 at 77 K, (b) evaluation of BET surface areas calculated from the standard BET
pressure range.
ESI-6. Structure and EDX Analysis of the La2Zr2O7:5%Eu3+ NPs
In addition, the compositional distributions of each element in the La2Zr2O7:5%Eu3+ NPs were
confirmed by Energy Dispersive X-ray (EDX) spectral mapping analysis as shown in Figures S3a
and S3b. Spectral mapping was performed in a cluster of particles as shown Figure S3a. EDX
signals obtained from K, L, and M shell electrons were clearly traced across the region of the
La2Zr2O7:5%Eu3+ NPs as shown in Figure S3a which guarantees the doping of Eu3+ in the
La2Zr2O7:5%Eu3+ NPs.
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Figure S6. As-prepared La2Zr2O7:5%Eu3+ sample S3: (a) SEM image, (b) corresponding EDX
spectrum showing the chemical composition. Extra peaks in the spectrum, such as aluminum and
carbon peaks, correspond to the SEM stub and carbon tape to hold the La2Zr2O7:5%Eu3+ NP
sample.
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ESI-7 Purposed energy transfer diagram of the La2Zr2O7:5%Eu3+ NPs under 258 nm
excitation
Figure S7. A simplified energy level diagram and the proposed energy transfer mechanisms under
258 nm excitation and Eu3+ ions emissions for the La2Zr2O7:5%Eu3+ NPs.
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ESI-8. Shift in charge transfer (CT) band of the La2Zr2O7:5%Eu3+ NPs
CT bands corresponding to Eu3+-O2- was located at shorter wavelengths for sample S1 compared
to those of S2 and S3. Maximum shift of 1 nm was observed between sample S1 and S3 (Figure
S4a, panel a). However CT band corresponding to Eu3+-O2- was located at 244 nm for sample S5
compared to that 258 nm for S3 (Figure S4a, panel b). The reasons for the above differences may
be related to crystal structure (fluorite vs. pyrochlore) and order of Eu3+ symmetry in these two
crystal phases of the La2Zr2O7 host.
Figure S8. (a) Excitation spectra for samples S1 and S3 showing the shift in CTB between the
samples prepared with different concentrations of the added ammonia, (b) excitation spectra for
sample S3 before and after annealing at 800 and 1000 °C for 6 hrs showing the shift in CTB among
the samples S3, S4 and S5.
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ESI-9. Multiexponential fitting for decay of the La2Zr2O7:5%Eu3+ NPs
The PL decay of the emission intensity at 612 nm under 258 nm excitation as a function of time
was measured utilizing a pulsed xenon flash-lamp excitation source. The entire system was
controlled though Edinburgh Instruments F980 data acquisition software. The collected decay
curve was analyzed using F980 software provided by Edinburg Instruments. Figure S7 shows the
decay monitored for sample S3 at 612 nm emission and its fitting. In addition, fitting parameter,
i.e chi-square for each fitting was in the range of 0.95 to 1.2. To quantify the luminescent dynamics,
we estimate the effective lifetime (τeff) by using the following equation.
 eff 
A1 1  A2 2
A1 1  A2 2
2
2
where A1 and A2 are the fitting parameters and 1 and 2 are the fluorescence decay times
Figure S9. Luminescence decay monitored for the La2Zr2O7:Eu3+ NPs at 612 nm under 258 nm
excitation. Two exponential functions were used for fitting.
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Table S9. Effective lifetime (τeff) evaluation by fitting the decay curves with a multi exponential
function.
sample
S1
S2
S3
S4
S5
τ1
0.708
0.728
0.764
0.806
0.851
τ2
1.932
1.952
1.973
2.139
2.459
τ1^2
0.501
0.529
0.583
0.649
0.724
τ2^2
3.73
3.81
3.89
4.54
6.00
 eff 
A1 1  A2 2
A1 1  A2 2
2
2
1.79
1.78
1.82
1.93
2.24
ESI-10: Site symmetry Analysis of the La2Zr2O7:5%Eu3+ NPs
The spectrum of Eu3+(4f6) in La2Zr2O7 was obtained between 500 nm and 750 nm at room
temperature (RT) at different excitation wavelengths as shown in Fig. 7a and 7b for both crystal
phase of La2Zr2O7: Eu3+ nanoparticles. The relative similarity both in the spectra and the Stark
splitting’s of emission bands was observed both under band (Eu3+ band) and above band (258 nm)
excitation as shown Fig. 7a and 7b. This indicates that all the emission in La2Zr2O7: Eu3+ is from
Eu3+ single site. The observed number of Stark splittings for the 5D0→7F2 transition at under band
(Eu3+ band) excitation is consistent with Eu3+ ions entering a substitutional La site with D3d
symmetry as reported in the previous literature. 3 In general, Eu3+ is assumed to replace the site of
La3+ in the D3d crystalline field because of the same valence and similar ion radius. In addition, the
I612/I592 ratios was always larger than 1 and the 612 nm emission is the most dominating one in
both fluorite (Fig. S7 (a) and pyrochlore crystal phases as shown in Fig. S7 (b). Overall similar
emission profiles observed in this study ensure that the original cation (La3+) site symmetry (D3d)
is preserved.
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Figure S10. PL spectra of (a) sample S3 and (b) sample S5, showing the dependence of the
5
D0→7F1, 5D0→7F2-4 emissions intensity on the excitation wavelengths. Overall similar spectral
profile was observed in this study for all Eu3+ band excitation, however PL intensity was found
maximum for 258 nm excitation.
References
1. M. Pokhrel, G. A. Kumar, and D. K. Sardar, "Highly efficient NIR to NIR and VIS upconversion in Er3+ and
Yb3+ doped in M2O2S (M = Gd, La, Y)," Journal of Materials Chemistry A, 1[38] 11595-606 (2013).
2. Y. Tong, S. Zhao, W. Feng, and L. Ma, "A study of Eu-doped La2Zr2O7 nanocrystals prepared by saltassistant combustion synthesis," Journal of Alloys and Compounds, 550[0] 268-72 (2013).
3. K. Holliday, S. Finkeldei, S. Neumeier, C. Walther, D. Bosbach, and T. Stumpf, "TRLFS of Eu3+ and Cm3+
doped La2Zr2O7: A comparison of defect fluorite to pyrochlore structures," Journal of Nuclear
Materials, 433[1–3] 479-85 (2013).
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