Lanthanide-activated scheelite nanocrystal phosphors prepared by the low-temperature vapor diffusion sol-gel method

Article abstract of DOI:10.1039/C6DT03382B

A series of Eu³⁺-, Tb³⁺-, and Tm³⁺-doped CaWO₄ phosphor nanocrystals have been synthesized under benign conditions using the vapor diffusion sol-gel method. The high degree of synthetic flexibility inherent to this approach has enabled the synthesis of a CaWO₄:(Eu,Tb) dual-sensitized white light emitting nanocrystal phosphor upon commercial UV excitation at 366 nm with a long lifetime exceeding 1 ms.

Full text of DOI:10.1039/C6DT03382B

Dalton
Transactions
View Article Online
View Journal
COMMUNICATION
Lanthanide-activated scheelite nanocrystal
phosphors prepared by the low-temperature
Cite this: DOI: 10.1039/c6dt03382b
vapor diffusion sol–gel method†
Received 29th August 2016,
Accepted 25th October 2016
Sean P. Culver and Richard L. Brutchey*
DOI: 10.1039/c6dt03382b
www.rsc.org/dalton
A series of Eu3+-, Tb3+-, and Tm3+-doped CaWO4 phosphor nano- and Tb3+-doped CaWO4 via the Pechini method, in order to
crystals have been synthesized under benign conditions using the make use of their characteristic red and green emission,
vapor diffusion sol–gel method. The high degree of synthetic flexi- respectively.12 Additionally, Liao and co-workers hydrother-
bility inherent to this approach has enabled the synthesis of a mally generated CaWO4 microspheres with a range of Tb3+
CaWO4:(Eu,Tb) dual-sensitized white light emitting nanocrystal concentrations to yield green emitting phosphors.13 In yet
phosphor upon commercial UV excitation at 366 nm with a long another study, Zhang et al. incorporated various concen-
lifetime exceeding 1 ms.
trations of Dy3+ into CaWO4 using a traditional solid state syn-
thesis to ultimately produce a white light emitting phosphor
through host sensitization coupled with subsequent emissive
energy transfer to Dy3+.14 Unfortunately, all of the aforemen-
tioned techniques, including previously used methods for pre-
paring Ln3+-activated scheelite nanocrystals, use high tempera-
ture (≥130 °C), high pressure, and/or non-neutral pH to
induce crystallization and incorporate Ln3+ ions,15–17 which
often inhibits morphological control and leads to compo-
sitional inhomogeneities that negatively affect the targeted
emission properties.
Alternatively, the vapor diffusion sol–gel (VDSG) method
allows for the synthesis of compositionally controlled, crystal-
line metal oxide nanoparticles under ultrabenign conditions
(low temperature, ambient pressure, and neutral pH) by pro-
viding kinetic control over the delivery of water vapor to the
gas–liquid interface of an alcohol solution containing essential
metal alkoxides.18–20 Diffusion of water vapor into the compo-
sitionally tailored precursor solution initiates hydrolysis and
polycondensation, resulting in the slow nucleation and growth
of sub-30 nm metal oxide nanocrystals. This method has pre-
viously been applied to the synthesis of many complex multi-
nary metal oxides, including a series of red emitting Eu3+-doped
BaZrO3 and BaTiO3 nanocrystals.21,22 Of note, the method has
also previously been adapted for the large-scale production of
BaTiO3 nanocrystals.23 Herein, three Ln3+ ions with intrinsic
primary color emission (i.e., Eu3+, Tb3+, and Tm3+) have been
doped into sub-30 nm CaWO4 nanocrystals under benign con-
ditions by employing the VDSG method. Structural and mor-
phological characterizations are provided, along with steady-
state photoluminescence (PL) spectra, chromaticity coordi-
nates, and PL lifetimes to elucidate the resultant emission pro-
The development of inorganic luminescent materials has gar-
nered substantial interest in recent years due to their diverse
applicability (e.g., bioimaging, display technologies, solid state
lighting).1–3 The success of these materials has in a large part
been dependent on a subclass of materials reliant upon the
tunable emission harnessed from rare earth lanthanide (Ln)
elements. More specifically, the incorporation of small concen-
trations of Ln dopants into inorganic oxide hosts (e.g., phos-
phates, vanadates, tungstates) has led to a variety of phosphors
exhibiting emission across the visible spectrum.4–7 Among the
available oxides, scheelite-structured tungstates have emerged
as attractive hosts owing to their excellent chemical and
thermal stability, in addition to their unique self-activated
luminescence properties.8,9
In particular, the CaWO4 scheelite has proven to be an
effective host matrix for a multitude of Ln ions.10–14 For this
reason, numerous studies have exploited CaWO4 hosts towards
achieving down conversion solid-state phosphors. Recently,
Wang and co-workers fabricated luminescent films of Eu3+-
Department of Chemistry, University of Southern California, Los Angeles, CA 90089,
USA. E-mail: brutchey@usc.edu
†Electronic supplementary information (ESI) available: All experimental infor-
mation; calculated structural parameters and the corresponding Rietveld fits;
TEM images of the as-synthesized and calcined nominal CaWO4:1%Ln nanocrys-
tals with the corresponding size distribution histograms; room temperature
emission spectra for the CaWO4 nanocrystals; room temperature emission
spectra for the CaWO4:1%Eu and CaWO4:1%Tb nanocrystals excited at 366 nm;
CaWO4:(1%Eu,1%Tb) PL lifetime curves; room temperature emission spectra for
the CaWO4:(1%Eu,1%Tb) nanocrystals excited at 393 and 485 nm. See DOI:
10.1039/c6dt03382b
This journal is © The Royal Society of Chemistry 2016
Dalton Trans.

View Article Online
Communication
Dalton Transactions
○
Fig. 1 Rietveld analysis of powder XRD patterns of (a) as-synthesized and (b) calcined CaWO4 nanocrystals. Experimental ( ) and calculated (
)
patterns are shown along with the difference curve ( ). Tickmarks () corresponding to the phase refined are given in (a) and (b). (c) XRD patterns of
as-synthesized and calcined CaWO4:1%Ln nanocrystals.
perties. Furthermore, a white light emitting CaWO4 nanocrys- and rationalized as follows: (1) Ln3+ is known to substitute
talline phosphor with a long lifetime was synthesized through onto Ca2+ lattice positions in this system,27 (2) Ln3+ ions have
dual activator ion incorporation for the first time.
appreciably smaller ionic radii relative to Ca2+ (rLn3+ = 1.07,
Powder X-ray diffraction (XRD) was used to probe the crys- 1.04, and 0.99 Å for 8-coordinate Eu, Tb, and Tm, respectively
tallinity and phase purity of the CaWO4:xLn (x = 0–1 mol%) and rCa2+ = 1.12 Å),28 and (3) substitution of Ca2+ by Ln3+
nanocrystals (Fig. 1). All diffraction maxima can be indexed to creates a charge imbalance that is compensated through
the tetragonal scheelite structure (PDF no. 07-0210), belonging calcium vacancy formation (V″Ca).29
to the I41/a space group. Moreover, the absence of any second-
The morphology of the as-synthesized and calcined
ary phases (e.g., Ln2W2O9) indicates that the nanocrystals are CaWO4:1%Ln nanocrystals was investigated by transmission
indeed phase pure. Incorporation of the Ln3+ ions into the electron microscopy (TEM). Representative TEM images depict-
host lattice via the VDSG method was verified by performing ing the as-synthesized and calcined CaWO4:1%Eu nanocrystals
inductively coupled plasma – atomic emission spectroscopy are shown in Fig. 2. The nanocrystals exhibit a quasispherical
(ICP-AES) and spectrofluorometry (vide infra). According to shape with no compositional size or shape dependence over
ICP-AES, the dopant concentrations of Eu3+, Tb3+, and Tm3+ the studied concentration range. TEM images for all other
were 0.54, 0.70, and 0.65 mol% for nominally CaWO4:1%Eu, CaWO4:1%Ln nanocrystals are provided in the ESI.† Mean dia-
CaWO4:1%Tb, and CaWO4:1%Tm nanocrystals, respectively. It meters were found to be 12.5 3.2, 10.9 2.9, 10.2 2.7, and
should also be noted that the dopant concentration could be 8.8
2.6 nm for the as-synthesized CaWO4, CaWO4:1%Eu,
nominally tuned from 0–5 mol% with a doping efficiency of 52 CaWO4:1%Tb, and CaWO4:1%Tm nanocrystals, respectively.
6% (ESI†). Though post-synthetic thermal treatment (i.e., cal-
cination) was not required to induce crystallization and incor-
porate the Ln3+ ions, the as-synthesized nanocrystals were cal-
cined at 600 °C for 15 min in air to remove organic moieties
from the surface (∼10 wt% by thermogravimetric analysis) that
otherwise quench the desired luminescence (ESI†).24,25
Rietveld analysis of the powder XRD patterns was con-
ducted to further probe the structure of the as-synthesized and
calcined CaWO4:1%Ln nanocrystals. As shown in Fig. 1, the
calcined nanocrystals exhibit an increase in crystallinity, but
remain phase pure; no segregation of secondary phases such
as CaCO3, WO3, Ln2O3, or Ln2W2O9 is observed. All calculated
structural parameters and the corresponding Rietveld fits are
provided in the ESI.† Visual assessment of the fits to the
experimental patterns, as well as low values of the goodness-
of-fit indicators (Rwp and χ2) suggest that the structure is ade-
quately described by the tetragonal scheelite framework. The
lattice parameters and unit cell volumes extracted from the
structural analysis correspond well with the literature values
for bulk CaWO4 for all dopant concentrations.26 Notably,
slight contractions in the unit cell volumes are observed for all
Fig. 2 TEM images of (a) as-synthesized and (b) calcined CaWO4:1%Eu
CaWO4:Ln nanocrystals, further confirming Ln3+ incorporation nanocrystals.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2016

View Article Online
Dalton Transactions
Communication
Fig. 3 Assigned room temperature excitation and emission spectra of (a) CaWO4:1%Eu, (b) CaWO4:1%Tb, and (c) CaWO4:1%Tm nanocrystals.
Upon calcination at 600 °C for 15 min in air, the nanocrystals Meanwhile, the corresponding emission spectrum exhibits a
show an increase in both the mean diameter and the degree of peak pertaining to the relaxation of the 1D2 excited state to the
agglomeration; however, the nanoscale morphology is retained 3F4 (454 nm) state. The CIE coordinates for this composition
with all particle sizes remaining sub-30 nm. Mean diameters are (0.183, 0.168), which corresponds to a blue color. It should
of 21.8 5.8, 22.3 5.5, 22.1 5.5, and 16.8 4.1 nm were also be noted that self-activated blue emission from the host
found for the calcined CaWO4, CaWO4:1%Eu, CaWO4:1%Tb, was negligible at all of the aforementioned excitation wave-
and CaWO4:1%Tm nanocrystals, respectively. The nanocrystal lengths (ESI†).
growth exhibited upon calcination can be attributed to
thermally driven Ostwald ripening.
Photoluminescence lifetime measurements corresponding
to the nominal CaWO4:1%Eu, CaWO4:1%Tb, and CaWO4:1%
The optical properties of the calcined CaWO4:xLn powders Tm nanocrystals are given in Fig. 4. Lifetimes of 1.01, 1.12,
were probed using steady-state and time-resolved spectro- and 0.26 ms were obtained for the nominal CaWO4:1%Eu
5
7
fluorometry. Room temperature excitation and emission (λexc = 393 nm, λem = 614 nm, D0 → F2), CaWO4:1%Tb (λexc
=
=
5
7
spectra displaying the characteristic f–f transitions of the 485 nm, λem = 544 nm, D4 → F5), and CaWO4:1%Tm (λexc
Ln3+ ions are given in Fig. 3. With regards to the nominal 358 nm, λem = 454 nm, D2 → F4) nanocrystals, respectively.
CaWO4:1%Eu nanocrystals, the excitation spectrum monitored These results compare favorably against previous studies on
at 615 nm exhibits peaks corresponding to excitation of the 7F0 lanthanide-doped CaWO4 (e.g., τ = 0.5 ms for hydrothermally
1
3
5
5
5
ground state to the D4 (359 nm), L7 (380 nm), L6 (393 nm),
5D3 (414 nm), 5D2 (462 nm), and 5D1 (471 nm) states
(Fig. 3a).30 Upon excitation at 393 nm, emission peaks stem-
ming from the relaxation of the 5D0 excited state to the 7F1
7
7
(590 nm), F2 (615 nm), and F3 (653 nm) states are observed.
The calculated Commission Internationale de l’Élairage (CIE)
coordinates for this composition are (0.595, 0.399), which
corresponds to a reddish orange color. Importantly, the most
dominant emission intensity belongs to the 5D0 → 7F2 (electric
dipole) transition, indicating that the Eu3+ ions occupy sites
that lack inversion symmetry (i.e., Ca2+ sites; S4 point sym-
metry without an inversion center), further corroborating that
the Ln3+ ions are being incorporated into the host lattice at the
Ca2+ lattice position.30 Turning to the excitation spectrum of
the nominal CaWO4:1%Tb nanocrystals monitored at 544 nm,
7
peaks stemming from the excitation of the F6 ground state to
5
5
5
the D2 (357 nm), L10 (375 nm), and D4 (485 nm) states are
observed (Fig. 3b).13 The associated emission spectrum reveals
5
peaks corresponding to the relaxation of the D4 excited state
to the 7F5 (544 nm), 7F4 (588 nm), and 7F3 (620 nm) states
when excited at 485 nm. The CIE coordinates for this compo-
sition are (0.442, 0.548), which corresponds to a yellowish
green color. Finally, the excitation spectrum of the nominal
CaWO4:1%Tm nanocrystals was obtained by monitoring at
454 nm, where a peak arising from the excitation of the 3H6
Fig. 4 Room temperature luminescence lifetime curves of (a)
CaWO4:1%Eu, (b) CaWO4:1%Tb, and (c) CaWO4:1%Tm nanocrystals. The
associated excitation and emission wavelengths, and lifetimes (τ) are
ground state to the 1D2 (358 nm) state is present (Fig. 3c).31 also provided.
This journal is © The Royal Society of Chemistry 2016
Dalton Trans.

View Article Online
Communication
Dalton Transactions
the VDSG method. The ability of this approach to realize
highly crystalline luminescent phosphors under benign con-
ditions with doping flexibility has been demonstrated.
Moreover, as a result of facile compositional tailoring, a phos-
phor exhibiting white light emission has been achieved using
an excitation wavelength equivalent to commercial UV exci-
tation. Given the rich diversity of Ln ions (i.e., emission pro-
files) and scheelite host compositions available, further
luminescence tuning may be easily accomplished with the
VDSG method. Importantly, this work highlights the synthetic
potential of this method towards the fabrication of nanocrys-
tals for white light emitting applications.
Fig. 5 Room temperature emission spectrum of CaWO4:1%Eu,1%Tb
nanocrystals. Inset depicts CIE chromaticity coordinates for single
doped Eu and Tb nanocrystals (black stars) and co-doped nanocrystals
(open black circle) excited at the optimal wavelengths reported in the
text.
Acknowledgements
This material is based on work supported by the Department
of Energy Office of Basic Energy Sciences under Grant No.
DE-FG02-11ER46826. The authors thank Dr P. Djurovich for
helpful discussions and Dr P. Cottingham with help with
prepared CaWO4:Eu submicron phosphors32 and τ = 0.3 ms thermal analysis.
for sonochemically prepared CaWO4:Eu submicron phos-
phors33), and demonstrate the efficacy of radiative lanthanide
emission associated with these VDSG-prepared, sub-30 nm
Notes and references
nanocrystals.13,31
In order to attain a white light emitting phosphor using the
VDSG method, Eu3+ and Tb3+ were co-doped within a single
CaWO4 host. Elemental analysis by ICP-AES revealed Eu3+ and
Tb3+ concentrations to be 0.62 and 0.75%, respectively, in the
nominal CaWO4:1%Eu,1%Tb nanocrystals. The increased
overall Ln3+ concentration had no observable effect on nanocrys-
tal morphology (ESI†). The emission spectrum of the nominal
CaWO4:1%Eu,1%Tb nanocrystals exhibits the characteristic f–f
transitions of Eu3+ and Tb3+ upon excitation at 366 nm (i.e.,
commercial UV excitation; Fig. 5). In addition to the previously
assigned radiative transitions from Eu3+ and Tb3+, higher energy
emission peaks arising from the 5D4 → 7F6 (488 nm), 5D3 → 7F4
1 M.-K. Tsang, C.-F. Chan, K.-L. Wong and J. Hao, J. Lumin.,
2015, 157, 172.
2 R. Saraf, C. Shivakumara, N. Dhananjaya, S. Behera and
H. Nagabhushana, J. Mater. Sci., 2015, 50, 287.
3 A. J. Peter and I. B. S. Banu, J. Mater. Sci: Mater. Electron.,
2014, 25, 2771.
4 J. W. Stouwdam, G. A. Hebbink, J. Huskens and F. C. J.
M. van Veggel, Chem. Mater., 2003, 15, 4606.
5 J. W. Stouwdam, M. Raudsepp and F. C. J. M. van Veggel,
Langmuir, 2005, 21, 7003.
6 G.-H. Lee, T.-H. Kim, C. Yoon and S. Kang, J. Lumin., 2008,
128, 1922.
(437 nm), and 5D3
→
7F5 (413 nm) Tb3+ relaxations are also
7 F. Wang and X. Liu, Acc. Chem. Res., 2014, 47, 1378.
8 X. Yang, J. Liu, H. Yang, X. Yu, Y. Guo, Y. Zhou and J. Liu,
J. Mater. Chem., 2009, 19, 3771.
observed.34 Addtionally, excitation wavelengths of 393 and
485 nm can be applied to the CaWO4:(Eu,Tb) nanocrystals in
order to isolate the characteristic individual emission profiles of
9 W. van Loo, Phys. Status Solidi A, 1975, 28, 227.
Eu3+ and Tb3+, respectively (ESI†). The emission spectrum of 10 M. V. Nazarov, D. Y. Jeon, J. H. Kang, E.-J. Popvici,
CaWO4:(Eu,Tb) nanocrystals excited at 485 nm does however
show enhanced intensity at 615 nm (i.e., the 5D0 7F2 tran-
L.-E. Muresan, M. V. Zamoryanskaya and B. S. Tsukerblat,
→
Solid State Commun., 2004, 131, 307.
sition from Eu3+), owing to the established energy transfer from 11 Y. Zhang, W. Gong, J. Yu, Z. Cheng and G. Ning, RSC Adv.,
Tb3+ to Eu3+ that occurs when both ions are present within the
same host matrix.35,36 Photoluminescence lifetimes related to 12 W. Wang, P. Yang, Z. Cheng, Z. Hou, C. Li and J. Lin, ACS
the relaxation of Eu3+ (λem = 615 nm) and Tb3+ (λem = 544 nm)
Appl. Mater. Interfaces, 2011, 3, 3921.
excited states were 1.09 and 1.18 ms with an excitation wave- 13 J. Liao, B. Qiu, H. Wen and W. You, Opt. Mater., 2009, 31,
length of 366 nm, respectively (ESI†), commensurate with the
1513.
lifetimes of the singly activated CaWO4:1%Eu and CaWO4:1%Tb 14 Y. Zhang, W. Gong, J. Yu, H. Pang, Q. Song and G. Ning,
nanocrystal phosphors. The CIE coordinates for this compo-
RSC Adv., 2015, 5, 62527.
sition under 366 nm excitation are (0.349, 0.356), which corres- 15 Maheshwary, B. P. Singh and R. A. Singh, New J. Chem.,
ponds to white light with a color temperature of 4850 K.
2015, 39, 4494.
In conclusion, a series of three prototypical Ln3+ ions have 16 K. G. Sharma and N. R. Singh, New J. Chem., 2013, 37,
2016, 6, 30886.
been incorporated into sub-30 nm CaWO4 nanocrystals using
2784.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2016

View Article Online
Dalton Transactions
Communication
17 K. G. Sharma, Th. P. Singh and N. R. Singh, J. Alloys
Compd., 2014, 602, 275.
A. Muñoz and G. Aquilanti, Phys. Rev. B: Condens. Matter,
2005, 72, 174106.
18 F. A. Rabuffetti and R. L. Brutchey, Chem. Mater., 2011, 23, 27 R. F. Gonçalves, L. S. Cavalcante, I. C. Nogueira, E. Longo,
4063.
M. J. Godinho, J. C. Sczancoski, V. R. Mastelaro, I. M. Pinatti,
I. L. V. Rosa and A. P. A. Marques, CrystEngComm, 2015, 17,
1654.
19 F. A. Rabuffetti, J. S. Lee and R. L. Brutchey, Chem. Mater.,
2012, 24, 3114.
20 S. P. Culver, M. J. Greaney, A. Tinoco and R. L. Brutchey, 28 R. D. Shannon, Acta Crystallogr., Sect. A: Fundam.
Dalton Trans., 2015, 44, 15042. Crystallogr., 1976, 32, 751.
21 F. A. Rabuffetti, S. P. Culver, J. S. Lee and R. L. Brutchey, 29 S. Shi, J. Gao and J. Zhou, Opt. Mater., 2008, 30, 1616.
Nanoscale, 2014, 6, 2909.
22 F. A. Rabuffetti, J. S. Lee and R. L. Brutchey, Adv. Mater.,
2012, 24, 1434.
30 W. Wang, P. Yang, S. Gai, N. Niu, F. He and J. Lin,
J. Nanopart. Res., 2010, 12, 2295.
31 J. Liao, B. Qiu, H. Wen, J. Chen, W. You and L. Liu, J. Alloys
Compd., 2009, 487, 758.
23 T. Ould-Ely, M. Luger, L. Kaplan-Reinig, K. Niesz,
M. Doherty and D. E. Morse, Nat. Protoc., 2011, 32 F. Lei and B. Yan, J. Solid State Chem., 2008, 181, 855.
6, 97.
33 C. Li, C. Lin, X. Liu and J. Lin, J. Nanosci. Nanotechnol.,
2008, 8, 1183.
34 E. Cavalli, P. Boutinaud, R. Mahiou, M. Bettinelli and
P. Dorenbos, Inorg. Chem., 2010, 49, 4916.
24 T. A. Shakhverdov, Opt. Spectrosc., 2003, 95, 571.
25 J. Georges, Analyst, 1993, 118, 1481.
26 D. Errandonea, J. Pellicer-Porres, F. J. Manjón, A. Segura,
Ch. Ferrer-Roca, R. S. Kumar, O. Tschauner, P. Rodríguez- 35 L. G. van Uitert, J. Electrochem. Soc., 1967, 114, 1048.
Hernández, J. López-Solano, S. Radescu, A. Mujica, 36 M. Nazarov and D. Y. Noh, J. Rare Earths, 2010, 28, 1.
This journal is © The Royal Society of Chemistry 2016
Dalton Trans.