efficient photoluminescence of isotropic rare-earth
TRANSCRIPT
doi.org/10.26434/chemrxiv.11494584.v1
Efficient Photoluminescence of Isotropic Rare-Earth OxychlorideNanocrystals from a Solvothermal RouteGuillaume Gouget, Morgane Pellerin, Lauriane Pautrot-d’Alencon, Thierry Le Mercier, Christopher B. Murray
Submitted date: 02/01/2020 • Posted date: 07/01/2020Licence: CC BY-NC-ND 4.0Citation information: Gouget, Guillaume; Pellerin, Morgane; Pautrot-d’Alencon, Lauriane; Mercier, Thierry Le;Murray, Christopher B. (2020): Efficient Photoluminescence of Isotropic Rare-Earth Oxychloride Nanocrystalsfrom a Solvothermal Route. ChemRxiv. Preprint. https://doi.org/10.26434/chemrxiv.11494584.v1
Eu3+-doped sub-10 nm LaOCl nanocrystals with 43 % photoluminescence quantum yield were prepared bysolvothermal synthesis from hydrated rare-earth chlorides. As-obtained nanocrystals are nearly spherical,monodisperse and stable as colloidal dispersions. These combined features should intensify the interest fornanocrystalline rare-earth oxyhalides and their optical properties.
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a. Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States.
b. Solvay, Research and Innovation Center Paris, F-93308, Aubervilliers, France.
c. Address here.
Electronic Supplementary Information (ESI) available: experimental section and additional data, Fig. S1-S9.
Efficient photoluminescence of isotropic rare-earth oxychloride nanocrystals from a solvothermal route
Guillaume Gouget,a Morgane Pellerin,
b Lauriane Pautrot‐D'Alençon,
b Thierry Le Mercier,
b and
Christopher B. Murray*a
Eu3+
-doped sub-10 nm LaOCl nanocrystals with 43 %
photoluminescence quantum yield were prepared by
solvothermal synthesis from hydrated rare-earth chlorides. As-
obtained nanocrystals are nearly spherical, monodisperse and
stable as colloidal dispersions. These combined features should
intensify the interest for nanocrystalline rare-earth oxyhalides and
their optical properties.
For several decades, rare-earth (RE) oxyhalides (REOX) have
been investigated for a variety of applications including bio-
imaging,1 small molecule sensing,
2 high-energy photon
detection,3,4
heterogeneous catalysis in the gas or liquid
phase,5–9
halide ion conduction,10
and light-emitting displays.11
REOXs are attractive for their luminescence properties: (i)
several REs can be readily combined in a single REOX structure
with various proportions, yielding optimized luminescence
features,1,4,12-16
and (ii) REOXs are low phonon energy hosts
(e.g. ca. 440 cm-1
for LaOCl),6,17
which prevents the excited RE-
emitting centers from luminescence quenching via multi-
phonon relaxation. PL excitation may occur in various energies
ranges, from infrared (IR) all the way to X-rays. LaOCl bandgap
is 4.17 eV according to density functional theory (DFT)
calculation,18
and it displays several absorption features in the
UV range.19,20
IR and visible light directly excite REs emitters,
whereas UV or X-Ray light excitation occur via the absorption
from LaOCl followed by energy transfer to the RE emitter.
LaOCl then sensitizes RE emission in the UV or X-Ray range.
The production of stable colloidal dispersions of REOX
nanocrystals (NCs) in the sub-20 nm regime is beneficial for
their processability (e.g. formation of thin close-packed films)
and for bio-compatibility.21
The search for routes to REOX NCs
has become more intense in the past ten years. They were
recently reviewed by Sarbajit Banerjee and co-workers.22
The
LaOCl NCs systematically display platelet morphologies, in
relation with their tetragonal structure (c axis orthogonal to
the most expressed facets). LaOCl structure (space group
P4/nmm, No. 129, Fig. 1) comprises layers of Cl anions
intercalated between slabs of LaO units (formally +1
charge·unit-1
) along the ab plane. Each La atom is coordinated
to 4 O and 5 Cl in a C4v symmetry environment. The separation
of cation sites along c limits cross-relaxation mechanisms
between emitting RE3+
centers.4,13,23
The lamellar structure is
also attractive for Cl ion conduction.10
Structural anisotropy is
thus beneficial for optical and transport properties. In 2009,
Du et al. reported ultra-thin Eu3+
-doped LaOCl nanoplates
(aspect ratio from 3.2 to 4.4). Hydrated RE trichloroacetates
single-source precursors were decomposed in the presence of
various amines.24
The relatively low PL quantum yield (PLQY)
of 5 % Eu3+
-doped LaOCl nanoplates (PLQY = 2.8 %) was
attributed to the predominant non-radiative pathways due to
high-frequency vibrations of long-chain amine ligands on the
surface of the nanoplates. Anisotropic morphologies can have
negative consequences on the photoluminescence (PL). More
isotropic (ideally cubic or spherical) morphologies would
provide a lower surface-to-volume ratio, i.e. a higher volume
for Eu3+
centers to be spatially separated from the interface,
and consequently higher PL efficiency.
Herein we report a synthetic route towards sub-10 nm
spherical LaOCl colloidal NCs, relying on the reaction of
hydrated chloride salts and oleylamine (OLA) at 220 °C.
Monodisperse samples were obtained, leading to self-
assembling of the dispersed crystals into close-packed films.
Fig. 1 Structure of LaOCl (P4/nmm, No. 129). The grey
polyhedron highlights La 9-fold coordinated environnement.
Using Eu3+
-doped (5 %) LaOCl NCs as a case study, PL efficiency
was improved, as detailed later. The correlations between
synthetic conditions, morphology and luminescence properties
of the NCs were discussed.
Details of the syntheses are reported in the Experimental
Section of Supporting Information. Briefly, hydrated RE
chlorides (starting either from LaCl3.7H2O only or a mix of
LaCl3.7H2O and EuCl3.6H2O (La:Eu molar ratio of 19:1)) were
dissolved in methanol (MeOH) prior to the addition of (OLA) in
air, at RT. The MeOH was evaporated and the mixture was
then degassed for 1 h at 120°C and 1 Torr. The reaction was
held at 220 °C during 1 h under N2, then oleic acid (OA) was
injected to the mixture and the reactant medium was cooled
to RT by blowing cool air over the vessel. Purification was
performed by two successive (i) additions of toluene and
MeOH followed by (ii) centrifugation to yield a precipitate and
(iii) removal of the supernatant. The final white pellet was
redispersed in chloroform, leading to a clear colorless solution.
X-ray diffraction (XRD) patterns in Fig. 2a were performed on
films obtained by dropcasting both dispersions on glass slides.
In both cases, LaOCl was unambiguously identified as a pure
phase (see also Supporting Information, Fig. S1). Significant
broadening of the peaks were attributed to small crystalline
domains. Using the Scherrer formula, apparent crystal sizes
from XRD diagrams were 6.6±0.3 and 7.1±0.6 nm for the Eu3+
-
doped and undoped samples, respectively.25
Energy dispersive
spectroscopy (EDS) (Fig. S2) indicated a molar ratio
RE:Cl = 1.00, in agreement with the expected value for REOCl.
Focusing on Eu3+
-doped LaOCl, the effective Eu insertion rate
(Eu:(La+Eu) molar ratio) was 4.8±0.2 %, according to elemental
analysis by inductive coupled plasma-optical emission
spectrometry (ICP-OES) measurements. The insertion rate of
Eu corresponds to the initial 19:1 (La:Eu) molar ratio.
Transmission electron microscopy (TEM, Fig. 2b and c) showed
ordered spherical NCs (see also Fig. S3). Selected area electron
diffraction (Fig. 2b, inset) confirmed that the NCs have the
oxychloride structure, as observed by XRD. Their average size
was 8.2±1.1 nm, according to TEM. This value was 24 % higher
than the apparent crystal size calculated from XRD, indicating
that the nanoparticles were monocrystalline. The deviation
was attributed to underestimation of apparent crystal size,
due to crystals strains contributing to XRD peak broadening.
The average aspect ratio was 1.1 according to TEM. The NCs
self-assembled upon dropcasting the dispersion on a TEM grid.
Monolayers (and bilayers, Fig. S3) with hexagonal packing
occured, as observed on the fast Fourier transform (FFT,
Fig. 2c, inset). The average center-to-center distance between
two neighboring NCs was 10.0 nm, so the shortest edge-to-
edge distance was 1.8 nm. This separation length is typically
observed for NCs capped with OLA or OA.26
Fourier transform
infra-red spectroscopy indicated that OLA binds to the surface
of the NCs after the purification steps (Fig. S4). OA was added
at the end of the heating step at 220 °C to avoid irreversible
precipitation in the early stages of the purification. It was
washed away in the final product, as confirmed by FTIR.
Hydrated lanthanum chloride was used for the first time as a
precursor to LaOCl NCs. The synthesis was performed at
220 °C. Alternative routes based on the solvothermal
decomposition of RE alkoxides or haloalkoxide intermediates
are reported between 300 and 330 °C.24,30
Other routes require
annealing steps above 500 °C.6,27–29
LaCl3.7H2O partially
dehydrates between 50 and 200 °C in argon atmosphere,31
and
it yields LaOCl and LaCl3 as competitive products.32
The
precursor contains La-Cl and La-O bonds already formed in the
initial precursor, as opposed to previously reported routes.22
We degassed the mixture of LaCl3.7H2O and OLA at 120 °C
prior to the reaction at 220 °C. At the end of the degassing
step, La probably maintained a Cl- and O-rich first coordination
sphere. A lower temperature of synthesis is then explained by
a reaction path involving successive dehydrations of the
intermediate, instead of the decomposition of an alkoxide.
Near-spherical LaOCl NCs (aspect ratio of 1.1) were
synthetized, while nanoplates were obtained in previous
reports.22
The anisotropic morphology was related to the
layered structure of LaOCl (tetragonal crystal cell) along the c
axis. Prefered growth along <110> directions was observed
when LaOCl ultrathin nanoplates (aspect ratio 4.0) were
synthetized from La(CCl3COO)3 with OLA.24
In contrast, our
study provided an example of isotropic morphology, which we
attributed to equal growth rates along the inequivalent
directions. We propose that the reactivity of intermediate
species dictates the relative growth rates, then resulting in the
difference in shapes between previous work24
and ours. The
nature of the intermediate species remains unclear, however
LaCl3.7H2O probably reacted into a Cl- and O-rich monomer,
which was responsible for lower temperatures of synthesis
and near-spherical morphologies. Future nvestigations on
intermediate species should be performed to better
understand and control shapes of LaOCl NCs. GdOCl and LaOBr
NCs were also synthetized by reacting gadolinium chloride and
lanthanum bromide (Fig. S5 and S6, respectively) in OLA.
Fig. 2 Structural characterization of LaOCl NCs. a) XRD patterns
of undoped and 5 % Eu3+
-doped samples. TEM pictures of b)
(inset: SAED) and c) (inset: FFT) 5 % Eu3+
-doped sample.
Fig. 3 Optical characterization of LaOCl NCs dispersed in
chloroform. a) mass exctinction coefficient of undoped and 5
% Eu3+
-doped samples, picture of a dispersion in inset. b) PL
(inset: dispersion irradiated at 300 nm) and c) PL excitation
spectra of 5 % Eu3+
-doped sample.
A controlled amount of Eu3+
doping was achieved by
introducing the desired ratio of RE precursors at the beginning
of the synthesis. The mass exctinction coefficient of the Eu3+
-
doped and undoped sample dispersions were compared on
Fig. 3a. Neither showed significant light absorption above 400
nm, yielding transparent solutions, as observed in inset. Both
doped and undoped samples displayed a peak at 286 nm,
before the mass exctinction coefficient dramatically increases
around 270 nm. These features were attributed to LaOCl, as
observed in previous work.18-20
The slope of absorption front
edge was sharper for the undoped sample than for the doped
one. An additional broad contribution was detected for the
Eu3+
-doped sample between 280 and 330 nm. This feature was
also observed in other Eu3+
-doped LaOCl materials.18,19
The
optical bandgap of undoped LaOCl sample determined by the
Tauc method (Fig. S7) was 4.11 eV.33
The value was close to
the one calculated for LaOCl structure from DFT (4.17 eV).18
The PL spectrum of Eu3+
-doped LaOCl spherical NCs excited at
λexc = 300 nm (Fig. 3b) showed narrow emission peaks in the
visible region, with a maximized intensity at λem = 618 nm. It
was responsible for the red color of the dispersion under UV
irradiation. Each peak was attributed to Eu3+
4f-4f inner
transitions (energy diagram in Fig. S8), and labeled accordingly.
Focusing on 5D0→
7F2 transition, two main peaks are detected
at 615 and 618 nm due to Stark splitting.34,35
In the case of
LaOCl nanoplates,24
inversion of relative intensity occured in
favour of the lower wavelength, which was attributed to a
lower symmetry of Eu3+
in a distorted environment compared
to bulk crystalline sample.36,37
In contrast, here the LaOCl NCs
display luminescence features similar to the bulk phase,
suggesting less distortion of Eu3+
environment from C4v
symmetry (Fig. 1). The PL excitation spectrum was performed
at λem = 618 nm (Fig. 3c). The main broad contribution
centered on 300 nm was related to charge-transfer band (CTB)
from O 2p to Eu 4f orbitals.29
The CTB was responsible for the
absorption feature between 280 and 330 nm in Eu3+
-doped
LaOCl (Fig. 3a).18
The narrower and weaker peaks above
350 nm were due to Eu3+
4f-4f excitations. The emission
spectrum at λexc = 395 nm (corresponding to Eu3+
7F0→
5L6
transition) in Fig. S9 showed a set of emission peaks similar to
the peaks observed at λexc = 300 nm (Fig. 3b).
The efficiency of radiative recombination pathways for Eu3+
in
LaOCl NCs dispersed in chloroform was probed by PL decay
and absolute PLQY measurements (Fig. 4). The PL decay at 618
nm was fit with a single exponential component (χ2 = 1.07).
The lifetime of excited states was 1.0 ms, which was 33 %
longer than for Eu3+
-doped LaOCl nanoplates (0.75 ms).24
The
lifetime of Eu3+
excited states increased, i.e. the rate of non-
radiative relaxation decreased. The absolute PLQY was
calculated from Equation (1), using an integrating sphere:
𝑃𝐿𝑄𝑌𝜆𝑒𝑥𝑐=
𝑁𝑒𝑚
𝑁𝑎𝑏𝑠 (1),
where Nabs is the number of photons absorbed by the sample,
Nem the number of photons emitted and λexc the wavelength of
excitation. These quantities were calculated as the intensity
difference between the Eu3+
-doped sample and the solvent as
reference, for the scattered light (Nabs) and the emitted light
(Nem). Intensities of absorption and emission were plotted as a
function of wavelength in Fig. 4b at λexc = 300 nm,
corresponding to the excitation in the O 2p→Eu 4f CTB. The
associated PLQY300 is 43 %. A similar experiment at
λexc = 395 nm (direct Eu3+
7F0→
5L6 excitation) leads to
PLQY395 = 14 %. The PLQY395 was 5 times higher than reported
perviously for LaOCl nanoplates.24
Higher PL efficiency is
observed for near-spherical NCs (aspect ratio of 1.1), as
evidenced by lifetime and PLQY measurements. PL quenching
is due to non-radiative recombination taking place at the
surface of the nanoparticles. The spherical morphology
minimizes the surface-to-volume ratio of the NCs. The increase
in lifetime and PLQY were then attributed to emissive centers
(Eu3+
) being, in average, further away from the surface.
Fig. 4 PL efficiency of Eu3+
emission in LaOCl NCs. a) PL decay
diagram fitted with a single exponential function. b) light
intensity absorbed (black) and emitted (red) by the sample
exposed at λexc = 300 nm in an integrating sphere.
In summary, monodisperse sub-10 nm LaOCl spherical NCs
have been obtained for the first time, starting from hydrated
RE chlorides and using lower temperatures than previously
reported. PL efficiency was improved by accessing isotropic
morphologies, as confirmed by longer PL lifetime and higher
PLQY. This work stresses the importance of exploring new
synthetic methods to diversify morphologies of LaOCl NCs,
which yields improved optical properties. In addition to their
processability, exemplified by the formation of close-packed
NCs films, isotropic LaOCl NCs doped with RE emitters should
be highly advantageous in various applications relying on their
optical properties. They can be further improved by different
strategies: (i) delineating conditions favoring isotropic
morphologies for REOX NCs (ii) improving the crystallinity of
the nanoparticles, and (iii) forming a protective crystalline shell
around the NCs. These strategies all highlight the need of more
experimental work on this new route, together with a better
understanding of the intermediate species involved during the
synthesis.
Conflicts of interest
There are no conflicts to declare.
Notes and references
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1
SUPPORTING INFORMATION
Efficient photoluminescence of isotropic rare-earth oxychloride nanocrystals
from a solvothermal route
Guillaume Gouget,a Morgane Pellerin,
b Lauriane Pautrot‐D'Alençon,
b Thierry Le Mercier,
b and Christopher B.
Murray*,a
aDepartment of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
bSolvay, Research and Innovation Center Paris, F-93308, Aubervilliers, France
EXPERIMENTAL SECTION 2
Chemicals 2
Synthesis of LaOCl nanocrystals 2
Structural characterization 2
Optical characterization 3
ADDITIONAL DATA 3
Figure S1. Le Bail refinements of undoped and 5 % Eu3+-doped LaOCl nanocrystals. 3
Figure S2. SEM-EDS spectrum of a region of aggregated Eu3+-doped LaOCl NCs. 4
Figure S3. TEM of 5 % Eu3+-doped LaOCl. 4
Figure S4. FTIR spectrum of Eu3+-doped LaOCl nanocrystals. 4
Figure S5. Structural characterization of GdOCl nanocrystals. 5
Figure S6. Structural characterization of LaOBr nanocrystals. 5
Figure S7. Tauc plot of dispersed LaOCl NCs. 6
Figure S8. Partial energy diagram of Eu3+ (4f6). 6
Figure S9. Emission spectrum of 5 % Eu3+-doped LaOCl NCs excitated at λexc = 395 nm. 7
REFERENCES 7
2
EXPERIMENTAL SECTION
Chemicals. Heptahydrated lanthanum trichloride (GFS Chemicals, 99.9 %), hexahydrated europium
trichloride (GFS Chemicals, 99.9 %), oleylamine (OLA, Sigma-Aldrich, 70 %), oleic acid (OA, Aldrich,
90 %), methanol (MeOH), toluene and chloroform (Fisher, ACS grade) were stored in air prior to use.
Synthesis of LaOCl nanocrystals. Hydrated RE chlorides in various proportions (0.5 mmol total RE
molar content) were dissolved in MeOH (10 mL) prior to addition of OLA (20 mL) in air, at RT. MeOH
was evaporated and the mixture was then degassed 1 h at 120°C and 1 Torr. The temperature was
increased to 220 °C in 13 min under N2. After 1 h at 220 °C, OA (5 mL) was injected to the mixture,
the temperature suddenly dropped to 200 °C, and then reactant medium was further cooled down to
RT by blowing cool air on the vessel. 10 mL of toluene was added, then 10 mL of MeOH. The
precipitation of a white solid was observed. The precipitate was centrifuged at 8900 g during 3 min,
the supernatant was discarded and a white pellet was recovered. The pellet was redispersed in 10 mL
of toluene, and then the dispersion was reprecipitated with 15 mL of MeOH. After purification, the
pellet was redispersed in chloroform.
Structural characterization. Powder X-ray diffraction (XRD) diagrams were acquired on a Rigaku
Smartlab diffractometer with Cu Kα radiation (λ =1.5416 Å, 40 mA and 30 kV) and parallel beam
setting in the θ-2θ geometry. 2θ range was screened between 10 and 80 ° with 0.05° (2θ) steps and
during 35 s per step. Peak identification was performed using the cif files from the crystallographic
open database (COD), collection code 9009170 (LaOCl) and the inorganic crystal structure database
(ICSD), collection codes 77820 (GdOCl) and 84336 (LaOBr)). Apparent crystal size εhkl were calculated
using the Scherrer formula:
β is the integral width of the peak, λ is the X-ray source wavelength and θ is the incident beam angle.
β was calculated for each separate experimental peak (hkl) by fitting it with a pseudo-Voigt function,
using the WinPLOTR function (FullProfSuite).1 ε was then averaged from all values calculated on a
single diagram.
Energy dispersive spectroscopy (EDS) was performed on a JEOL 7500F high resolution scanning
electron microscope equipped with an EDS detector. Acceleration tension of the source was 10 kV.
RE/Cl atomic ratio was calculated from the average of EDS spectra measured on three distinct
100 x 50 µm2 areas of aggregated particles. Data were analyzed using the INCA software (Oxford).
The sample was dropcasted on a Si wafer, followed by drying, before a carbon layer was sputtered.
Inductive coupled plasma-optical emission spectrometry (ICP-OES) was performed on a Spectro
Genesis spectrometer with a concentric nebulizer. Calibrated solution of La and Eu were purchased
from Inorganic Ventures. Solid samples were dissolved in HNO3 (70 %mas.) at 60 °C.
Transmission electron microscopy (TEM) images of NCs were collected on a JEOL JEM-1400
microscope operating at 120 keV. Samples were prepared by depositing a drop of NCs in chloroform
(3.10-2 g.L-1) on a carbon-coated copper grid. Selected area electron diffraction (SAED) was performed
with a camera distance of 20 cm. Interreticular planes were deduced from distances calibrated with a
known sample of silicon particles. Size distribution data were extracted from at least 100 particles on
pictures taken on at least 3 distinct regions of a TEM grid.
3
Optical characterization. Absorption spectra of the dispersions were measured in a 10 mm quartz
cuvette, on a Cary 5000 UV−vis−NIR spectrophotometer (Agilent Technologies) from 800 to 200 nm
with a 1 nm step. The same cuvette filled with solvent was used as a reference. Photoluminescence
(PL) experiments were performed on a FLS1000 fluorimeter (Edinburgh Instruments) equipped with a
Xe lamp. PL spectrum was acquired from 310 to 800 nm with 1 nm steps and 0.1 s per step. Slit
bandwidths on both incident and emission sides were 1.1 nm. PL excitation spectrum was acquired
from 270 to 610 nm, 1 nm steps and 0.1 s per step. Slit bandwidths on both incident and emission
sides were 2.0 nm. PL decay measurements were performed with a pulsed laser source (MCS) at
378 nm, 125 Hz pulse frequency. For absolute PL quantum yield measurement, an integrating sphere
was adapted on the fluorimeter. Emission spectra were acquired in the same condition for the
reference (cuvette with solvent) and sample: λexc = 300 nm (or 395 nm), 10 nm slit bandwidth on the
incident side, 1.0 nm slit bandwidth on the emission side, 270 (or 370) to 800 nm, 1 nm steps and
0.5 s per step.
ADDITIONAL DATA
Figure S1. Le Bail refinements of a) undoped and b) 5 % Eu3+-doped LaOCl nanocrystals. The peak at 2θ = 44.5 ° is due to the aluminum plate. It is taken into account in the fit (Al, Fm-3m, ICSD .cif file code 64700) in the refinements. In both cases, refined cell parameters of LaOCl (P4/nmm) are similar to values reported (COD .cif file 9009170): a = 4.119 Å and c = 6.883 Å. Refinements were performed on JANA 2006.2
4
Figure S2. SEM-EDS spectrum of a region of aggregated Eu3+-doped LaOCl NCs. Si detection is attributed to a contamination from sample preparation or SEM chamber. It is not considered for quantification, nor O and C.
Figure S3. a) TEM picture of 5 % Eu3+-doped LaOCl nanocrystals forming close-packed monolayers and bilayers. b) SAED picture with diffraction rings indexed along with LaOCl structure. Lower left inset is a scheme reproducing (full lines) major and *(dashed) minor rings. c) Size distribution diagram of 5 % Eu3+-doped LaOCl nanocrystals (black) and gaussian function centered at 8.2 nm with a 1.1 nm standard deviation.
Figure S4. FTIR spectrum of Eu3+-doped LaOCl nanocrystals showing bond vibrations typical of OLA, as
opposed to OA.3,4
5
Figure S5. Structural characterization of GdOCl nanocrystals. The synthesis was adapted from LaOCl experiment, using hexahydrate gadolinium chloride as RE precursor, and without addition of OA at the end of the heating step. a) XRD diagram showed that GdOCl (P4/nmm, No. 129) is the only phase obtained. The various peak widths were due to strong shape anisotropy, in relation with crystalline structure. Especially, the narrowest peaks corresponded to (hk0) interreticular planes (blue), i.e. orthogonal to c axis. b-d) TEM pictures showed stacks of thin plates with rough edges, forming 200-500 nm spherical aggregates. e) Peak position and apparent crystal size (from Scherrer formula) of some interreticular planes (hkl). εhk0 = 16.6±1.1 nm and ε00l = 4.2±0.4 nm confirmed that GdOCl NCs were anisotropic with a smaller dimension in c direction, whereas facets orthogonal to c were the biggest. This is in agreement with the plate shape observed by TEM (b)).
Figure S6. Structural characterization of LaOBr nanocrystals. The synthesis was adapted from LaOCl experiment, using lanthanum bromide instead of the chloride, heating at 330 °C instead of 220 °C, and without addition of OA at the end of the heating step. a) XRD diagram showed that LaOBr (P4/nmm, No. 129, ICSD code 84336) was the only phase obtained. b-d) TEM pictures of nanoplates 15-20 nm thick and 30-100 nm long. Particles displayed an elongated hexagon-like shape when oriented along the edge. This shape was similar than LaOCl obtained by Depner et al..5 Rod-like arrays (single- and multi-linear) formed via face-to-face alignement of the nanoplates. e) Peak position and apparent crystal size (from Scherrer formula) of some interreticular planes (hkl). εhk0 = 20.6±0.1 nm ε00l = 15.1±0.5 nm confirmed that LaOBrl NCs are anisotropic with a smaller dimension in c direction, whereas the biggest facets were perpendicular to c axis. This was in agreement with the shape observed by TEM (b)). εhk0 was smaller than plates length observed by TEM. The size difference could be due to tha nanoplates being polycrystalline.
6
Figure S7. Tauc plot of dispersed LaOCl NCs.
For direct bandgap semiconductors such as LaOCl,6 optical bandgap measurement was based on the Tauc relationship (2) between the absorption coefficient α, the photon energy hν and the bandgap energy Eg:
where B was a constant for allowed transitions. For relatively low absorption A (A < 1), the absorption coefficient of a perfectly non-scattering dispersion followed the linear Beer-Lambert law:
with l the optical path length (1 cm).
Figure S8. Partial energy diagram of Eu3+ (4f6) taking into account interelectronic repulsion and spin-orbit coupling.7
7
Figure S9. PL spectrum of Eu3+-doped LaOCl NCs dispersed in chloroform, excitation at λexc = 395 nm. REFERENCES 1 T. Roisnel and J. Rodríquez-Carvajal, Mater. Sci. Forum, 2001, 378–381, 118–123. 2 V. Petrícek, M. Dušek and L. Palatinus, Zeitschrift fur Krist., 2014, 229, 345–352. 3 S. Mourdikoudis and L. M. Liz-Marzán, Chem. Mater., 2013, 25, 1465–1476. 4 I. O. Perez De Berti, M. V Cagnoli, G. Pecchi, J. L. Alessandrini, S. J. Stewart, J. F. Bengoa and S.
G. Marchetti, Nanotechnology, 2013, 24, 175601. 5 S. W. Depner, K. R. Kort, C. Jaye, D. A. Fischer and S. Banerjee, J. Phys. Chem. C, 2009, 113,
14126–14134. 6 L. Lv, T. Wang, S. Li, Y. Su and X. Wang, CrystEngComm, 2016, 18, 907–916. 7 K. Binnemans, Coord. Chem. Rev., 2015, 295, 1–45.
download fileview on ChemRxivLaOCl_ChemRxiv.pdf (2.18 MiB)
Efficient photoluminescence of isotropic rare-earth oxychloride nanocrystals from a solvothermal route
Guillaume Gouget,a Morgane Pellerin,b Lauriane Pautrot‐D'Alençon,b Thierry Le Mercier,b and Christopher B. Murray*a
a. Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States.
b. Solvay, Research and Innovation Center Paris, F-93308, Aubervilliers, France.c. Address here.Electronic Supplementary Information (ESI) available: experimental section andadditional data, Fig. S1‐S9.
Eu3+-doped sub-10 nm LaOCl nanocrystals with 43 %
photoluminescence quantum yield were prepared by
solvothermal synthesis from hydrated rare-earth chlorides. As-
obtained nanocrystals are nearly spherical, monodisperse and
stable as colloidal dispersions. These combined features
should intensify the interest for nanocrystalline rare-earth
oxyhalides and their optical properties.
For several decades, rare‐earth (RE) oxyhalides (REOX) have
been investigated for a variety of applications including bio‐
imaging,1 small molecule sensing,2 high‐energy photon
detection,3,4 heterogeneous catalysis in the gas or liquid
phase,5–9 halide ion conduction,10 and light‐emitting
displays.11 REOXs are attractive for their luminescence
properties: (i) several REs can be readily combined in a
single REOX structure with various proportions, yielding
optimized luminescence features,1,4,12‐16 and (ii) REOXs are
low phonon energy hosts (e.g. ca. 440 cm‐1 for LaOCl),6,17
which prevents the excited RE‐emitting centers from
luminescence quenching via multi‐phonon relaxation. PL
excitation may occur in various energies ranges, from
infrared (IR) all the way to X‐rays. LaOCl bandgap is 4.17 eV
according to density functional theory (DFT) calculation,18
and it displays several absorption features in the UV
range.19,20 IR and visible light directly excite REs emitters,
whereas UV or X‐Ray light excitation occur via the
absorption from LaOCl followed by energy transfer to the
RE emitter. LaOCl then sensitizes RE emission in the UV or
X‐Ray range.
The production of stable colloidal dispersions of REOX
nanocrystals (NCs) in the sub‐20 nm regime is beneficial for
their processability (e.g. formation of thin close‐packed
films) and for bio‐compatibility.21 The search for routes to
REOX NCs has become more intense in the past ten years.
They were recently reviewed by Sarbajit Banerjee and co‐
workers.22 The LaOCl NCs systematically display platelet
morphologies, in relation with their tetragonal structure (c
axis orthogonal to the most expressed facets). LaOCl
structure (space group P4/nmm, No. 129, Fig. 1) comprises
layers of Cl anions intercalated between slabs of LaO units
(formally +1 charge·unit‐1) along the ab plane. Each La atom
is coordinated to 4 O and 5 Cl in a C4v symmetry
environment. The separation of cation sites along c limits
cross‐relaxation mechanisms between emitting RE3+
centers.4,13,23 The lamellar structure is also attractive for Cl
ion conduction.10 Structural anisotropy is thus beneficial for
optical and transport properties. In 2009, Du et al. reported
ultra‐thin Eu3+‐doped LaOCl nanoplates (aspect ratio from
3.2 to 4.4). Hydrated RE trichloroacetates single‐source
precursors were decomposed in the presence of various
amines.24 The relatively low PL quantum yield (PLQY) of
5 % Eu3+‐doped LaOCl nanoplates (PLQY = 2.8 %) was
attributed to the predominant non‐radiative pathways due
to high‐frequency vibrations of long‐chain amine ligands on
the surface of the nanoplates. Anisotropic morphologies
can have negative consequences on the
photoluminescence (PL). More isotropic (ideally cubic or
spherical) morphologies would provide a lower surface‐to‐
volume ratio, i.e. a higher volume for Eu3+ centers to be
spatially separated from the interface, and consequently
higher PL efficiency.
Herein we report a synthetic route towards sub‐10 nm
spherical LaOCl colloidal NCs, relying on the reaction of
hydrated chloride salts and oleylamine (OLA) at 220 °C.
Monodisperse samples were obtained, leading to self‐
assembling of the dispersed crystals into close‐packed
films.
Fig. 1 Structure of LaOCl (P4/nmm, No. 129). The grey
polyhedron highlights La 9‐fold coordinated
environnement.
Using Eu3+‐doped (5 %) LaOCl NCs as a case study, PL
efficiency was improved, as detailed later. The correlations
between synthetic conditions, morphology and
luminescence properties of the NCs were discussed.
Details of the syntheses are reported in the Experimental
Section of Supporting Information. Briefly, hydrated RE
chlorides (starting either from LaCl3.7H2O only or a mix of
LaCl3.7H2O and EuCl3.6H2O (La:Eu molar ratio of 19:1)) were
dissolved in methanol (MeOH) prior to the addition of
(OLA) in air, at RT. The MeOH was evaporated and the
mixture was then degassed for 1 h at 120°C and 1 Torr. The
reaction was held at 220 °C during 1 h under N2, then oleic
acid (OA) was injected to the mixture and the reactant
medium was cooled to RT by blowing cool air over the
vessel. Purification was performed by two successive (i)
additions of toluene and MeOH followed by (ii)
centrifugation to yield a precipitate and (iii) removal of the
supernatant. The final white pellet was redispersed in
chloroform, leading to a clear colorless solution. X‐ray
diffraction (XRD) patterns in Fig. 2a were performed on
films obtained by dropcasting both dispersions on glass
slides. In both cases, LaOCl was unambiguously identified
as a pure phase (see also Supporting Information, Fig. S1).
Significant broadening of the peaks were attributed to
small crystalline domains. Using the Scherrer formula,
apparent crystal sizes from XRD diagrams were 6.6±0.3 and
7.1±0.6 nm for the Eu3+‐doped and undoped samples,
respectively.25 Energy dispersive spectroscopy (EDS) (Fig.
S2) indicated a molar ratio RE:Cl = 1.00, in agreement with
the expected value for REOCl. Focusing on Eu3+‐doped
LaOCl, the effective Eu insertion rate (Eu:(La+Eu) molar
ratio) was 4.8±0.2 %, according to elemental analysis by
inductive coupled plasma‐optical emission spectrometry
(ICP‐OES) measurements. The insertion rate of Eu
corresponds to the initial 19:1 (La:Eu) molar ratio.
Transmission electron microscopy (TEM, Fig. 2b and c)
showed ordered spherical NCs (see also Fig. S3). Selected
area electron diffraction (Fig. 2b, inset) confirmed that the
NCs have the oxychloride structure, as observed by XRD.
Their average size was 8.2±1.1 nm, according to TEM. This
value was 24 % higher than the apparent crystal size
calculated from XRD, indicating that the nanoparticles were
monocrystalline. The deviation was attributed to
underestimation of apparent crystal size, due to crystals
strains contributing to XRD peak broadening. The average
aspect ratio was 1.1 according to TEM. The NCs self‐
assembled upon dropcasting the dispersion on a TEM grid.
Monolayers (and bilayers, Fig. S3) with hexagonal packing
occured, as observed on the fast Fourier transform (FFT,
Fig. 2c, inset). The average center‐to‐center distance
between two neighboring NCs was 10.0 nm, so the shortest
edge‐to‐edge distance was 1.8 nm. This separation length is
typically observed for NCs capped with OLA or OA.26 Fourier
transform infra‐red spectroscopy indicated that OLA binds
to the surface of the NCs after the purification steps (Fig.
S4). OA was added at the end of the heating step at 220 °C
to avoid irreversible precipitation in the early stages of the
purification. It was washed away in the final product, as
confirmed by FTIR.
Hydrated lanthanum chloride was used for the first time as
a precursor to LaOCl NCs. The synthesis was performed at
220 °C. Alternative routes based on the solvothermal
decomposition of RE alkoxides or haloalkoxide
intermediates are reported between 300 and 330 °C.24,30
Other routes require annealing steps above 500 °C.6,27–29
LaCl3.7H2O partially dehydrates between 50 and 200 °C in
argon atmosphere,31 and it yields LaOCl and LaCl3 as
competitive products.32 The precursor contains La‐Cl and
La‐O bonds already formed in the initial precursor, as
opposed to previously reported routes.22 We degassed the
mixture of LaCl3.7H2O and OLA at 120 °C prior to the
reaction at 220 °C. At the end of the degassing step, La
probably maintained a Cl‐ and O‐rich first coordination
sphere. A lower temperature of synthesis is then explained
by a reaction path involving successive dehydrations of the
intermediate, instead of the decomposition of an alkoxide.
Near‐spherical LaOCl NCs (aspect ratio of 1.1) were
synthetized, while nanoplates were obtained in previous
reports.22 The anisotropic morphology was related to the
layered structure of LaOCl (tetragonal crystal cell) along the
c axis. Prefered growth along <110> directions was
observed when LaOCl ultrathin nanoplates (aspect ratio
4.0) were synthetized from La(CCl3COO)3 with OLA.24 In
contrast, our study provided an example of isotropic
morphology, which we attributed to equal growth rates
along the inequivalent directions. We propose that the
reactivity of intermediate species dictates the relative
growth rates, then resulting in the difference in shapes
between previous work24 and ours. The nature of the
intermediate species remains unclear, however LaCl3.7H2O
probably reacted into a Cl‐ and O‐rich monomer, which was
responsible for lower temperatures of synthesis and near‐
spherical morphologies. Future nvestigations on
intermediate species should be performed to better
understand and control shapes of LaOCl NCs. GdOCl and
LaOBr NCs were also synthetized by reacting gadolinium
chloride and lanthanum bromide (Fig. S5 and S6,
respectively) in OLA.
Fig. 2 Structural characterization of LaOCl NCs. a) XRD
patterns of undoped and 5 % Eu3+‐doped samples. TEM
pictures of b) (inset: SAED) and c) (inset: FFT) 5 % Eu3+‐
doped sample.
Fig. 3 Optical characterization of LaOCl NCs dispersed in
chloroform. a) mass exctinction coefficient of undoped and
5 % Eu3+‐doped samples, picture of a dispersion in inset. b)
PL (inset: dispersion irradiated at 300 nm) and c) PL
excitation spectra of 5 % Eu3+‐doped sample.
A controlled amount of Eu3+ doping was achieved by
introducing the desired ratio of RE precursors at the
beginning of the synthesis. The mass exctinction coefficient
of the Eu3+‐doped and undoped sample dispersions were
compared on Fig. 3a. Neither showed significant light
absorption above 400 nm, yielding transparent solutions, as
observed in inset. Both doped and undoped samples
displayed a peak at 286 nm, before the mass exctinction
coefficient dramatically increases around 270 nm. These
features were attributed to LaOCl, as observed in previous
work.18‐20 The slope of absorption front edge was sharper
for the undoped sample than for the doped one. An
additional broad contribution was detected for the Eu3+‐
doped sample between 280 and 330 nm. This feature was
also observed in other Eu3+‐doped LaOCl materials.18,19 The
optical bandgap of undoped LaOCl sample determined by
the Tauc method (Fig. S7) was 4.11 eV.33 The value was
close to the one calculated for LaOCl structure from DFT
(4.17 eV).18 The PL spectrum of Eu3+‐doped LaOCl spherical
NCs excited at λexc = 300 nm (Fig. 3b) showed narrow
emission peaks in the visible region, with a maximized
intensity at λem = 618 nm. It was responsible for the red
color of the dispersion under UV irradiation. Each peak was
attributed to Eu3+ 4f‐4f inner transitions (energy diagram in
Fig. S8), and labeled accordingly. Focusing on 5D0→7F2
transition, two main peaks are detected at 615 and 618 nm
due to Stark splitting.34,35 In the case of LaOCl nanoplates,24
inversion of relative intensity occured in favour of the lower
wavelength, which was attributed to a lower symmetry of
Eu3+ in a distorted environment compared to bulk
crystalline sample.36,37 In contrast, here the LaOCl NCs
display luminescence features similar to the bulk phase,
suggesting less distortion of Eu3+ environment from C4v
symmetry (Fig. 1). The PL excitation spectrum was
performed at λem = 618 nm (Fig. 3c). The main broad
contribution centered on 300 nm was related to charge‐
transfer band (CTB) from O 2p to Eu 4f orbitals.29 The CTB
was responsible for the absorption feature between 280
and 330 nm in Eu3+‐doped LaOCl (Fig. 3a).18 The narrower
and weaker peaks above 350 nm were due to Eu3+ 4f‐4f
excitations. The emission spectrum at λexc = 395 nm
(corresponding to Eu3+ 7F0→5L6 transition) in Fig. S9 showed
a set of emission peaks similar to the peaks observed at
λexc = 300 nm (Fig. 3b).
The efficiency of radiative recombination pathways for Eu3+
in LaOCl NCs dispersed in chloroform was probed by PL
decay and absolute PLQY measurements (Fig. 4). The PL
decay at 618 nm was fit with a single exponential
component (χ2 = 1.07). The lifetime of excited states was
1.0 ms, which was 33 % longer than for Eu3+‐doped LaOCl
nanoplates (0.75 ms).24 The lifetime of Eu3+ excited states
increased, i.e. the rate of non‐radiative relaxation
decreased. The absolute PLQY was calculated from
Equation (1), using an integrating sphere:
N|¿|
PLQY λexc=
N em
¿ (1),
where Nabs is the number of photons absorbed by the
sample, Nem the number of photons emitted and λexc the
wavelength of excitation. These quantities were calculated
as the intensity difference between the Eu3+‐doped sample
and the solvent as reference, for the scattered light (Nabs)
and the emitted light (Nem). Intensities of absorption and
emission were plotted as a function of wavelength in Fig.
4b at λexc = 300 nm, corresponding to the excitation in the
O 2p→Eu 4f CTB. The associated PLQY300 is 43 %. A similar
experiment at λexc = 395 nm (direct Eu3+ 7F0→5L6 excitation)
leads to PLQY395 = 14 %. The PLQY395 was 5 times higher
than reported perviously for LaOCl nanoplates.24 Higher PL
efficiency is observed for near‐spherical NCs (aspect ratio of
1.1), as evidenced by lifetime and PLQY measurements. PL
quenching is due to non‐radiative recombination taking
place at the surface of the nanoparticles. The spherical
morphology minimizes the surface‐to‐volume ratio of the
NCs. The increase in lifetime and PLQY were then attributed
to emissive centers (Eu3+) being, in average, further away
from the surface.
Fig. 4 PL efficiency of Eu3+ emission in LaOCl NCs. a) PL
decay diagram fitted with a single exponential function. b)
light intensity absorbed (black) and emitted (red) by the
sample exposed at λexc = 300 nm in an integrating sphere.
In summary, monodisperse sub‐10 nm LaOCl spherical NCs
have been obtained for the first time, starting from
hydrated RE chlorides and using lower temperatures than
previously reported. PL efficiency was improved by
accessing isotropic morphologies, as confirmed by longer
PL lifetime and higher PLQY. This work stresses the
importance of exploring new synthetic methods to diversify
morphologies of LaOCl NCs, which yields improved optical
properties. In addition to their processability, exemplified
by the formation of close‐packed NCs films, isotropic LaOCl
NCs doped with RE emitters should be highly advantageous
in various applications relying on their optical properties.
They can be further improved by different strategies: (i)
delineating conditions favoring isotropic morphologies for
REOX NCs (ii) improving the crystallinity of the
nanoparticles, and (iii) forming a protective crystalline shell
around the NCs. These strategies all highlight the need of
more experimental work on this new route, together with a
better understanding of the intermediate species involved
during the synthesis.
Conflicts of interest
There are no conflicts to declare.
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download fileview on ChemRxivGG_LaOCl_ChemRxiv.docx (5.25 MiB)
SUPPORTING INFORMATION
Efficient photoluminescence of isotropic rare-earth oxychloride nanocrystals
from a solvothermal route
Guillaume Gouget,a Morgane Pellerin,b Lauriane Pautrot D'Alençon,‐ b Thierry Le Mercier,b and Christopher B.
Murray*,a
aDepartment of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United StatesbSolvay, Research and Innovation Center Paris, F-93308, Aubervilliers, France
EXPERIMENTAL SECTION 2
Chemicals 2
Synthesis of LaOCl nanocrystals 2
Structural characterization 2
Optical characterization 3
ADDITIONAL DATA 3
Figure S1. Le Bail refinements of undoped and 5 % Eu3+-doped LaOCl nanocrystals. 3
Figure S2. SEM-EDS spectrum of a region of aggregated Eu3+-doped LaOCl NCs. 4
Figure S3. TEM of 5 % Eu3+-doped LaOCl. 4
Figure S4. FTIR spectrum of Eu3+-doped LaOCl nanocrystals. 4
Figure S5. Structural characterization of GdOCl nanocrystals. 5
Figure S6. Structural characterization of LaOBr nanocrystals. 5
Figure S7. Tauc plot of dispersed LaOCl NCs. 6
Figure S8. Partial energy diagram of Eu3+ (4f6). 6
Figure S9. Emission spectrum of 5 % Eu3+-doped LaOCl NCs excitated at λexc = 395 nm. 7
REFERENCES 7
1
EXPERIMENTAL SECTION
Chemicals. Heptahydrated lanthanum trichloride (GFS Chemicals, 99.9 %), hexahydrated europium
trichloride (GFS Chemicals, 99.9 %), oleylamine (OLA, Sigma-Aldrich, 70 %), oleic acid (OA, Aldrich,
90 %), methanol (MeOH), toluene and chloroform (Fisher, ACS grade) were stored in air prior to use.
Synthesis of LaOCl nanocrystals. Hydrated RE chlorides in various proportions (0.5 mmol total RE
molar content) were dissolved in MeOH (10 mL) prior to addition of OLA (20 mL) in air, at RT. MeOH
was evaporated and the mixture was then degassed 1 h at 120°C and 1 Torr. The temperature was
increased to 220 °C in 13 min under N2. After 1 h at 220 °C, OA (5 mL) was injected to the mixture, the
temperature suddenly dropped to 200 °C, and then reactant medium was further cooled down to RT
by blowing cool air on the vessel. 10 mL of toluene was added, then 10 mL of MeOH. The
precipitation of a white solid was observed. The precipitate was centrifuged at 8900 g during 3 min,
the supernatant was discarded and a white pellet was recovered. The pellet was redispersed in 10 mL
of toluene, and then the dispersion was reprecipitated with 15 mL of MeOH. After purification, the
pellet was redispersed in chloroform.
Structural characterization. Powder X-ray diffraction (XRD) diagrams were acquired on a Rigaku
Smartlab diffractometer with Cu Kα radiation (λ =1.5416 Å, 40 mA and 30 kV) and parallel beam
setting in the θ-2θ geometry. 2θ range was screened between 10 and 80 ° with 0.05° (2θ) steps and
during 35 s per step. Peak identification was performed using the cif files from the crystallographic
open database (COD), collection code 9009170 (LaOCl) and the inorganic crystal structure database
(ICSD), collection codes 77820 (GdOCl) and 84336 (LaOBr)). Apparent crystal size εhkl were calculated
using the Scherrer formula:
β is the integral width of the peak, λ is the X-ray source wavelength and θ is the incident beam angle.
β was calculated for each separate experimental peak (hkl) by fitting it with a pseudo-Voigt function,
using the WinPLOTR function (FullProfSuite).1 ε was then averaged from all values calculated on a
single diagram.
Energy dispersive spectroscopy (EDS) was performed on a JEOL 7500F high resolution scanning
electron microscope equipped with an EDS detector. Acceleration tension of the source was 10 kV.
RE/Cl atomic ratio was calculated from the average of EDS spectra measured on three distinct
100 x 50 µm2 areas of aggregated particles. Data were analyzed using the INCA software (Oxford). The
sample was dropcasted on a Si wafer, followed by drying, before a carbon layer was sputtered.
Inductive coupled plasma-optical emission spectrometry (ICP-OES) was performed on a Spectro
Genesis spectrometer with a concentric nebulizer. Calibrated solution of La and Eu were purchased
from Inorganic Ventures. Solid samples were dissolved in HNO3 (70 %mas.) at 60 °C.
Transmission electron microscopy (TEM) images of NCs were collected on a JEOL JEM-1400
microscope operating at 120 keV. Samples were prepared by depositing a drop of NCs in chloroform
(3.10-2 g.L-1) on a carbon-coated copper grid. Selected area electron diffraction (SAED) was performed
with a camera distance of 20 cm. Interreticular planes were deduced from distances calibrated with a
known sample of silicon particles. Size distribution data were extracted from at least 100 particles on
pictures taken on at least 3 distinct regions of a TEM grid.
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Optical characterization. Absorption spectra of the dispersions were measured in a 10 mm quartz
cuvette, on a Cary 5000 UV−vis−NIR spectrophotometer (Agilent Technologies) from 800 to 200 nm
with a 1 nm step. The same cuvette filled with solvent was used as a reference. Photoluminescence
(PL) experiments were performed on a FLS1000 fluorimeter (Edinburgh Instruments) equipped with a
Xe lamp. PL spectrum was acquired from 310 to 800 nm with 1 nm steps and 0.1 s per step. Slit
bandwidths on both incident and emission sides were 1.1 nm. PL excitation spectrum was acquired
from 270 to 610 nm, 1 nm steps and 0.1 s per step. Slit bandwidths on both incident and emission
sides were 2.0 nm. PL decay measurements were performed with a pulsed laser source (MCS) at
378 nm, 125 Hz pulse frequency. For absolute PL quantum yield measurement, an integrating sphere
was adapted on the fluorimeter. Emission spectra were acquired in the same condition for the
reference (cuvette with solvent) and sample: λexc = 300 nm (or 395 nm), 10 nm slit bandwidth on the
incident side, 1.0 nm slit bandwidth on the emission side, 270 (or 370) to 800 nm, 1 nm steps and
0.5 s per step.
ADDITIONAL DATA
Figure S1. Le Bail refinements of a) undoped and b) 5 % Eu3+-doped LaOCl nanocrystals. The peak at2θ = 44.5 ° is due to the aluminum plate. It is taken into account in the fit (Al, Fm-3m, ICSD .cif filecode 64700) in the refinements. In both cases, refined cell parameters of LaOCl (P4/nmm) are similarto values reported (COD .cif file 9009170): a = 4.119 Å and c = 6.883 Å. Refinements were performedon JANA 2006.2
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Figure S2. SEM-EDS spectrum of a region of aggregated Eu3+-doped LaOCl NCs. Si detection isattributed to a contamination from sample preparation or SEM chamber. It is not considered forquantification, nor O and C.
Figure S3. a) TEM picture of 5 % Eu3+-doped LaOCl nanocrystals forming close-packed monolayers andbilayers. b) SAED picture with diffraction rings indexed along with LaOCl structure. Lower left inset is ascheme reproducing (full lines) major and *(dashed) minor rings. c) Size distribution diagram of 5 %Eu3+-doped LaOCl nanocrystals (black) and gaussian function centered at 8.2 nm with a 1.1 nmstandard deviation.
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Figure S4. FTIR spectrum of Eu3+-doped LaOCl nanocrystals showing bond vibrations typical of OLA, as
opposed to OA.3,4
Figure S5. Structural characterization of GdOCl nanocrystals. The synthesis was adapted from LaOClexperiment, using hexahydrate gadolinium chloride as RE precursor, and without addition of OA atthe end of the heating step. a) XRD diagram showed that GdOCl (P4/nmm, No. 129) is the only phaseobtained. The various peak widths were due to strong shape anisotropy, in relation with crystallinestructure. Especially, the narrowest peaks corresponded to (hk0) interreticular planes (blue), i.e.orthogonal to c axis. b-d) TEM pictures showed stacks of thin plates with rough edges, forming 200-500 nm spherical aggregates. e) Peak position and apparent crystal size (from Scherrer formula) ofsome interreticular planes (hkl). εhk0 = 16.6±1.1 nm and ε00l = 4.2±0.4 nm confirmed that GdOCl NCswere anisotropic with a smaller dimension in c direction, whereas facets orthogonal to c were thebiggest. This is in agreement with the plate shape observed by TEM (b)).
Figure S6. Structural characterization of LaOBr nanocrystals. The synthesis was adapted from LaOClexperiment, using lanthanum bromide instead of the chloride, heating at 330 °C instead of 220 °C,and without addition of OA at the end of the heating step. a) XRD diagram showed that LaOBr(P4/nmm, No. 129, ICSD code 84336) was the only phase obtained. b-d) TEM pictures of nanoplates15-20 nm thick and 30-100 nm long. Particles displayed an elongated hexagon-like shape whenoriented along the edge. This shape was similar than LaOCl obtained by Depner et al..5 Rod-like arrays(single- and multi-linear) formed via face-to-face alignement of the nanoplates. e) Peak position andapparent crystal size (from Scherrer formula) of some interreticular planes (hkl). εhk0 = 20.6±0.1 nmε00l = 15.1±0.5 nm confirmed that LaOBrl NCs are anisotropic with a smaller dimension in c direction,
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whereas the biggest facets were perpendicular to c axis. This was in agreement with the shapeobserved by TEM (b)). εhk0 was smaller than plates length observed by TEM. The size difference couldbe due to tha nanoplates being polycrystalline.
Figure S7. Tauc plot of dispersed LaOCl NCs.
For direct bandgap semiconductors such as LaOCl,6 optical bandgap measurement was based on theTauc relationship (2) between the absorption coefficient α, the photon energy hν and the bandgapenergy Eg:
where B was a constant for allowed transitions. For relatively low absorption A (A < 1), the absorptioncoefficient of a perfectly non-scattering dispersion followed the linear Beer-Lambert law:
with l the optical path length (1 cm).
Figure S8. Partial energy diagram of Eu3+ (4f6) taking into account interelectronic repulsion and spin-orbit coupling.7
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Figure S9. PL spectrum of Eu3+-doped LaOCl NCs dispersed in chloroform, excitation at λexc = 395 nm.
REFERENCES1 T. Roisnel and J. Rodríquez-Carvajal, Mater. Sci. Forum, 2001, 378–381, 118–123.2 V. Petrícek, M. Dušek and L. Palatinus, Zeitschrift fur Krist., 2014, 229, 345–352.3 S. Mourdikoudis and L. M. Liz-Marzán, Chem. Mater., 2013, 25, 1465–1476.4 I. O. Perez De Berti, M. V Cagnoli, G. Pecchi, J. L. Alessandrini, S. J. Stewart, J. F. Bengoa and S.
G. Marchetti, Nanotechnology, 2013, 24, 175601.5 S. W. Depner, K. R. Kort, C. Jaye, D. A. Fischer and S. Banerjee, J. Phys. Chem. C, 2009, 113,
14126–14134.6 L. Lv, T. Wang, S. Li, Y. Su and X. Wang, CrystEngComm, 2016, 18, 907–916.7 K. Binnemans, Coord. Chem. Rev., 2015, 295, 1–45.
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