Fabrication of (Y0.95Eu0.05)2O3 phosphors with enhanced properties by co-precipitation of layered rare-earth hydroxide

Article abstract of DOI:10.1016/j.jallcom.2019.07.046

This work is devoted to a simple method for synthesizing phosphors based on layered rare earth hydroxonitrates (LRH). Layered precursors of (Y_0.95Eu_0.05)₂(OH)₅(NO₃)·xH₂O with different morphologies were obtained via co-precipitation at a constant pH and hydrothermal methods. It was established that LRHs co-precipitated at pH 7 are highly ordered close-packed micron-sized plates with low porosity. With an increase in the pH of co-precipitation to 9.5, the phase composition is retained, but the isotropy and porosity of the particles increases sharply: the morphology changes from micron flat plates to curved nanosheets and nanoflakes. Co-precipitated LRHs crystallize more intensively into cubic (Y_0.95Eu_0.05)₂O₃ during heat treatment compared to precursors obtained under mild hydrothermal conditions (120 °C, 24 h). The structural characteristics of the material laid at the stage of synthesis directly determine its optical properties and the fundamental parameters of photoluminescence. The phosphors that co-precipitate at pH 7 demonstrate the highest yield of luminescence due to a more ordered structure, larger crystallite size, lower asymmetry factor and surface to volume ratio. The proposed co-precipitation method with a constant pH value of 7 allows one to obtain (Y_0.95Eu_0.05)₂O₃ phosphor with enhanced structural and optical properties compared to materials synthesized in hydrothermal conditions. The LRHs co-precipitated in this work are a promising precursor for the preparation of crystal-oriented and highly-efficient luminescent coatings and nanofilms.

Full text of DOI:10.1016/j.jallcom.2019.07.046

Accepted Manuscript
Fabrication of (Y
precipitation of layered rare-earth hydroxide
Eu ) O phosphors with enhanced properties by co-
0.95 0.05 2 3
Danil K. Aleshin, Maxim A. Mashkovtsev, Yulia A. Kuznetsova, Vladimir N. Rychkov,
Anatoly F. Zatsepin, Egor V. Gordeev
PII:
S0925-8388(19)32538-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.07.046
JALCOM 51334
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 24 April 2019
Revised Date: 22 June 2019
Accepted Date: 4 July 2019
Please cite this article as: D.K. Aleshin, M.A. Mashkovtsev, Y.A. Kuznetsova, V.N. Rychkov, A.F.
Zatsepin, E.V. Gordeev, Fabrication of (Y
precipitation of layered rare-earth hydroxide, Journal of Alloys and Compounds (2019), doi: https://
doi.org/10.1016/j.jallcom.2019.07.046.
Eu ) O phosphors with enhanced properties by co-
0.95 0.05 2 3
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Fabrication of (Y0.95Eu0.05)2O3 phosphors with enhanced properties by
co-precipitation of layered rare-earth hydroxide
Danil K. Aleshin,a Maxim A. Mashkovtsev,a* Yulia A. Kuznetsova,a Vladimir N. Rychkov,a Anatoly
F. Zatsepina and Egor V. Gordeeva
a. Institute of Physics and Technology, Ural Federal University, 19 Mira Str., 620002 Yekaterinburg,
Russia
Abstract
This work is devoted to a simple method for synthesizing phosphors based on layered rare earth
hydroxonitrates (LRH). Layered precursors of (Y0.95Eu0.05)2(OH)5(NO3)·xH2O with different
morphologies were obtained via co-precipitation at a constant pH and hydrothermal methods. It was
established that LRHs co-precipitated at pH 7 are highly ordered close-packed micron-sized plates
with low porosity. With an increase in the pH of co-precipitation to 9.5, the phase composition is
retained, but the isotropy and porosity of the particles increases sharply: the morphology changes from
micron flat plates to curved nanosheets and nanoflakes. Co-precipitated LRHs crystallize more
intensively into cubic (Y0.95Eu0.05)2O3 during heat treatment compared to precursors obtained under
mild hydrothermal conditions (120 °C, 24 h). The structural characteristics of the material laid at the
stage of synthesis directly determine its optical properties and the fundamental parameters of
photoluminescence. The phosphors that co-precipitate at pH 7 demonstrate the highest yield of
luminescence due to a more ordered structure, larger crystallite size, lower asymmetry factor and
surface to volume ratio. The proposed co-precipitation method with a constant pH value of 7 allows
one to obtain (Y0.95Eu0.05)2O3 phosphor with enhanced structural and optical properties compared to
materials synthesized in hydrothermal conditions. The LRHs co-precipitated in this work are a
promising precursor for the preparation of crystal-oriented and highly-efficient luminescent coatings
and nanofilms.
Keywords: layered rare earth hydroxides, co-precipitation, oriented growth, photoluminescence.
1. Introduction
At present, a large number of studies are aimed at the development of highly efficient
thermo- and photostable inorganic phosphors based on layered rare earth hydroxides (LRH)
[1-7]. New sensors [8-10], MRI contrasts [11, 12], nano-drug delivery systems [13, 14],
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catalysts for biofuel production [15], cryogenic magnetic coolers [16] and other promising
LRH-based materials are being actively developed.
Layered rare earth hydroxides are a new class of inorganic multilayer materials with the
general formula: Me2(OH)6-m[An-n]m/n·xH2O, where: Ме – rare earth cation (III+), An – anion
(I-, II-) and 1≤m≤2, 0≤x≤2 [17]. LRH are structural analogues of layered double hydroxides, as
they are assemblies of two-dimensional “infinite” layers of positively charged Ме (III+)
hydroxide complexes arranged parallel to each other. The interlayer space (galleries) contains
anions that compensate for the positive charge of the host layers and coordinated water
molecules [18]. The variation of the cationic and anionic compositions at different stages of the
synthesis of the LRH allows one to adjust the lattice structure, charge the density of layers and
the interlayer distance, thereby allowing for the synthesis of layered products with the desired
physical and chemical properties. In particular, it is important for the fabrication of phosphors
with enhanced properties, since a change in the morphology, crystallite size, surface features,
and other parameters of particles affects the local environment of the emission centers and is
accompanied by a change in the luminescent properties. In a number of studies, it has been
shown that highly ordered structures [19], large crystallites [20] and low surface area [21] of
phosphors lead to an increased luminescence intensity. Thus, the unique electrical, optical,
magnetic and catalytic properties of rare-earth elements reveal a wide range of promising
functional materials based on LRHs.
Among the various LRH synthesis methods, the most used are: hydrothermal treatment of
a mixture of aqueous solutions of salts of rare-earth elements (III+) with alkali or ammonium
solutions [1-3, 5, 7] and homogeneous precipitation in which a precipitant is formed during the
decomposition reaction [4, 13, 22, 23]. The precipitates obtained are highly ordered plate
particles with lateral sizes up to tens of micrometers and with a thickness up to a few
nanometers. Such structures have strongly anisotropic properties; when deposited on the
substrate, they form highly oriented crystalline nanofilms, and as a result are very suitable for
designing high-performance phosphors [5, 24]. However, the methods can only be
implemented on special equipment of periodic action. Varying the conditions of synthesis does
not allow for the fine adjustment of particle size during their growth; equally, the wide
application of these methods is limited by the use of highly diluted stock solutions, as well as
the 70-80% yield of synthesis products [23]. On the other hand, LRHs obtained by
conventional precipitation methods (direct or reverse) are gel-like, highly disordered
precipitates prone to the agglomeration of primary particles with the formation of irregularly
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shaped hard aggregates [25]. To suppress agglomeration in precipitates, different methods are
utilized such as washing in ethanol or acetone, introducing surfactants or using ultrasonic
dispersion [26]. For the reasons outlined above, the precipitation methods are considered
unfavorable for obtaining LRHs, as evidenced by a few publications devoted to this topic.
Despite this, the co-precipitation method is widely used to obtain highly crystalline, ultrafine
layered double hydroxides of various compositions [27, 28]. The isomorphic substitution of
metal cations in the structure of hydroxide complexes and the homogeneous distribution of
components are achieved by simultaneously introducing solutions of metal salts (II+)/(III+)
and the precipitant into a reactor, as well as keeping the precipitation conditions at a constant
level (in particular, the ratio of reactants, pH and temperature values). The co-precipitation
method is often used when it is necessary to organize a one-reactor synthesis and to obtain
large batches of the product [29]. Despite the advantages listed above, little research is devoted
to the properties of LRHs synthesized at a constant pH value [30]. In this work, the co-
precipitation method was used to synthesize highly ordered LRHs with controlled properties.
The advantages of the method are ease of implementation, the use of conventional chemical
equipment, the absence of surfactants and the possibility of scaling up the production to
industrial volumes.
In the present work, layered hydroxonitrates (Y0.95Eu0.05)2(OH)5(NO3)·xH2O (LRH) were
obtained by co-precipitation at a constant pH value, from which cubic (Y0.95Eu0.05)2O3 powders
were synthesized. Y2O3 doped with 5 mol % Eu2O3 is one of the most widely used phosphors
in the long-wavelength region of the visible spectrum [1, 5]. A comparison of the
physicochemical properties of the LRHs synthesized by co-precipitation at a constant pH and
under hydrothermal conditions was carried out. It is shown that the pH value during co-
precipitation has a significant effect on the properties of layered hydroxides. The conditions for
the co-precipitation of LRHs with improved properties compared to the products of
hydrothermal synthesis have been established. The effect of heat treatment on the phase
composition and morphology of the LRHs and the surface and luminescent properties of the
synthesized (Y0.95Eu0.05)2O3 powders were also investigated.
2. Experimental Section
2.1 Synthesis by co-precipitation
The initial solution of Y(NO3)3 was prepared by dissolving Y2O3 (99.98%) in HNO3
(Chem. grade) diluted in water (1:1) while heating the solution. Then, the Y3+ concentration
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was determined by the gravimetric method and the solution was adjusted to the stoichiometric
ratio of Y(NO3)3. After that, a solution of Eu(NO3)3 from Eu2O3 (99.99%) was prepared in a
similar way, mixed with a solution of Y(NO3)3 in a molar ratio of Y/Eu=19 and diluted with
water to 0.50 mol/L in Me3+. A solution of NH4OH (Analytical grade, 25%) diluted to 5.3
mol/L was used as a precipitant. An aqueous solution of NH4NO3 (Chem. grade, 99%) with a
concentration of 1.26 mol/L was used as a starting solution, which maintained the constant
concentration of by-products at the beginning of the precipitation. The required concentration
was calculated according to reaction:
0.5ꢀꢁ.ꢂꢃꢄꢅꢁ.ꢁꢃ(ꢆꢇ)ꢈ + 1.25ꢆꢉꢊꢇꢉ + ꢋꢉꢌꢇ = 0.25(ꢀꢁ.ꢂꢃꢄꢅꢁ.ꢁꢃ)ꢌ(ꢇꢉ)ꢃꢆꢇꢈ ∗ ꢋꢉꢌꢇ + 1.25ꢆꢉꢊꢆꢇꢈ (1)
The initial solutions were filtered and aged for at least 24 hours before co-precipitation.
Co-precipitation was carried out in a mixing reactor at room temperature with a stirring
speed of 600 rpm. The solutions were introduced dropwise simultaneously via peristaltic
pumps at a constant rate. During the co-precipitation, the introduction of the NH4OH solution
was automatically adjusted to maintain a constant pH value. Before the start of each
experiment, 0.15 L of the starting solution was poured into the reactor. Co-precipitation was
carried out at constant pH values from 7 to 9.5 (samples are designated below as YEHN - pH
“pH”): for each experiment, 2 L of (Y0.95Eu0.05)(NO3)3 was used. Fresh precipitates were
washed twice in distilled deionized water (18 MΩ) and once in absolute isopropyl alcohol,
dried at room temperature for 24 hours and then at 50 °C for 24 hours and finally calcined at
specified temperatures for 2 hours.
2.2 Hydrothermal Synthesis
The sample obtained via the hydrothermal method (indicated below as YEHN-H) was
synthesized using the initial solutions described above. The rare earth precursor solution was
diluted with water to 0.03 mol/L in Me3+, adjusted to a pH~7.3 with an NH4OH solution while
stirring and aged in an autoclave for 24 hours at 120 °С. After natural cooling, the sample was
centrifuged, washed and dried in the same way [1].
2.3 Characterization Methods
The X-ray diffraction patterns of the samples were obtained on an XPertProMPD
diffractometer (PANanalytical) in СuКα=1.5405 Å radiation using the XPertHighScorePlus
software. The crystallite size (coherent scattering region, CSR) was determined by the Scherrer
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method. The IR spectra of tablets pressed from samples mixed with KBr were obtained using a
Vertex 70 (Brucker) spectrometer in the range of 370 - 4000 cm-1 with a resolution of 1 cm-1.
Thermogravimetric and differential thermal analyzes were performed for washed and air-dried
samples using TGA / SDTA 851 (MettlerToledo) at a temperature range from 50 to 1000 °C,
with a heating rate of 10 °C/min. The morphology and sizes of individual particles were
examined using microphotographs obtained with an AurigaCrossBeam scanning electron
microscope (CarlZeiss). The surface properties and measurement of the specific surface area of
the samples were investigated using the BET method for isotherms of low-temperature
nitrogen sorption on a Nova 1200e analyzer (Quantachrome Instruments). The elemental
analysis and the molar ratio of Y3+ and Eu3+ in the samples was determined using an Elan 9000
mass spectrometer (PerkinElmer).
The excitation and photoluminescence spectra were recorded on a LS-55 spectrometer
(PerkinElmer). The slits on the monochromators were set 2.5 nm on the luminescence and
excitation pathways. The second orders of diffraction of the grating monochromator was
eliminated by means of a G-shaped filter with a transmission of 2.5 eV and below.
3. Results and Discussion
3.1 Phase Composition and Interlayer Distance of Co-precipitated LRHs
According to the X-ray phase analysis presented in Figure 1, the synthesized samples are
identified as: Y2(OH)5.14(NO3)0.86·H2O (ICDD 00-032-1435) [31]. Since this compound
-
contains weakly bound groups (NO3 ) and different amounts of coordinated water, in general
terms they are denoted as: Me2(OH)5(NO3)·xH2O [19, 23, 30, 32, 33].
A series of intense diffractions {00l}: (002) and (004) (Figure 1) clearly indicate that the
synthesized samples have a layered type of structure. A series of diffractions {hk0}
characterizes the lattice parameters of the {[Me2(OH)5·xH2O]+,}∞ hydroxocomplex host layer,
and their high intensity indicates the crystalline state of the sample after co-precipitation [1, 23,
33]. The interlayer distance (d{00l}) depending on the pH of co-precipitation of the LRH was
calculated through the interplanar distance of the lattice parameter (с/2) according to the Wulff-
Bragg condition [18]. It was found that during co-precipitation at pH>7 there is a slight
decrease in d{00l} by 0.015 nm (Table 1). In addition, as the pH increases, an increase in the
half-width of all of the series of diffractions is observed, which is equivalent to a decrease in
the density of stacking layers and disordering of the layer structure. Similar position of the
{hk0} series of diffractions on the X-ray diffraction patterns indicates a uniform distribution of
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Eu3+ cations in the layered structures regardless of the co-precipitation’s pH. The elemental
analysis of the samples confirm the co-precipitation of Y3+ and Eu3+ and obtaining precipitates
with practically the initial molar ratio of components: [Eu3+]/([Y3++Eu3+]) ≈0.05 (Table 1). The
hydrothermal samples obtained in this work also have highly crystalline properties, but the
orderliness of the layered structures is somewhat inferior to the sample obtained by co-
precipitation at pH7.
Figure 1. XRD patterns of the LRH co-precipitated at: pH7 (a); pH8 (b); pH9 (c); pH9.5 (d); hydrothermal synthesis (e)
Table 1. Interlayer distance (d), the size of the CSR (D) for reflections (002), (220) and the molar ratio
Eu/(Y+Eu) of LRHs after drying at 50 °C.
YEHN- d(002), D(002), d(220), D(220), [Eu3+]/[Y3++Eu3+],
"pH"
7
8
9
9.5
H
nm
nm
31
13
9
11
23
nm
nm
37
29
20
23
30
mol %
4.8
4.7
4.7
4.7
0.911
0.896
0.901
0.901
0.911
0.311
0.311
0.312
0.312
0.311
5.1
Layered precipitates with similar properties were obtained by other authors by conducting
hydrothermal and homogeneous synthesis of various LRHs depending on humidity,
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temperature and cation composition [4, 23, 34]. According to other research, the high value of
the interlayer distance is due to the effective retention of water molecules in the interlayer
space. Thus, it is shown that co-precipitation in the pH range from 7 to 9.5 allows one to
synthesize highly ordered single-phase crystalline LRHs.
3.2 Thermal Stability of Co-precipitated LRH and the Hydration Number x
The TG curves on Figure 2 clearly show a three-stage mass loss characteristic of LRHs
[30-33]. It is known that at the first stage (up to 210 °C) mainly the interlayer water is
removed. The intermediate stage (up to 350 °C) is associated with the transformation of (OH-)
bridges into (O2-) in the structure of the layer and the formation of the Me2O2(OH)(NO3),
which is equivalent to removing two water moleculesper formula unit. During the final stage
(up to 600 °C), complete removal of hydroxyl and interlayer groups in the form of H2O and
NO2 takes place, after which a final degradation of the layered structures with the formation of
oxides is observed. The established values of mass loss at each stage of the thermolysis
confirm the X-ray phase analysis data on the formation of layered LRHs and allow us to
estimate the amount of interlayer water or the hydration number
(Y0.95Eu0.05)2(OH)5(NO3)· H2O (Table 2).
х in the formula
х
As a result, it was found that with an increase in the pH of LRH co-precipitation, the
amount of interlayer water increases. In addition, samples obtained at pH 9 and 9.5 are
characterized by a smoother TG-curve, in which the steps of the thermolysis stages are less
distinct. It is worth noting that the TG-curve for hydrothermal samples has distinct well-
distinguishable transitions between decomposition stages, which indicates that its properties are
close to LRHs co-precipitated at low pH. For all samples, the difference in the total mass loss
between the observed and theoretically calculated values did not exceed 0.8 mass. %.
As noted above, LRHs co-precipitated at high pH values are characterized by a low
structural order. Similar results for precipitates of yttrium hydroxonitrates are found in [30]. A
wide variation in the properties of layered particles leads to an inhomogeneous multistage
decomposition of the layers during the separation of (ОН-) groups from the Y3+ coordination
-
sphere and the removal of interlayer (NO3 ) groups. The processes of dehydration, oxolation
and denitrationgradually replace each other and include intermediate stages of decomposition,
which leads to smoothing of TG curves and the appearance of additional steps.
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Figure 2. TG curves of the LRH co-precipitated at: pH7 (a); pH8 (b); pH9 (c); pH9.5 (d); hydrothermal
synthesis (e)
Table 2. Observed (∆) and calculated (∆t) values of mass loss at different stages of LRH thermolysis according to the TG
curves in Figure 2.
I
II
III
Sum
(50-210 °C)
(210-340 °C)
(340-600 °C)
YEHN-"pH"
Hydration
number, х
1.67
∆,%
∆,%
∆t ,%
∆,%
∆t ,%
∆,%
∆t ,%
7
8
9
9.5
H
8.33
7.76
8.99
9.46
8.64
10.03
10.02
9.42
10.28
10.07
9.97
10.02
9.89
9.84
9.93
16.78
16.78
17.25
15.99
17.12
17.43
17.54
17.30
17.22
17.37
35.14
34.56
35.66
35.73
35.83
35.72
35.33
36.19
36.51
35.94
1.55
1.82
1.92
1.74
3.3 Interlayer Anionic Groups and Carbonatization of the LRH Precipitates
The IR absorption spectra for co-precipitated and hydrothermal LRHs are shown in
Figure 3. A wide absorption band with a center in the frequency range of 3500 cm-1 and a
shoulder in the range of 3340 cm-1 correspond to the valent vibrations of the ν1 and ν3 (ОН-)
group that make up the layers of the hydroxocomplexesand interlayer H2O molecules,
respectively [5, 33, 34]. The deformation vibrations of water molecules ν2 (H-O-H) are
represented by peaks in the region of 1634 сm-1 [35, 36].
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Figure 3. IR spectra of LRH co-precipitated at: pH7 (a); pH8 (b); pH9 (c); pH9.5 (d); hydrothermal synthesis (e)
Narrow bands of 1384 сm-1 are associated with the valentvibrations of ν3, which are
-
characteristic of the uncoordinated interlayer (NO3 ) groups connected with the layers through
electrostatic interaction [1, 5, 23, 35]. Weak peaks: 826 сm-1, 1050 сm-1 and 1763 сm-1,
-
respectively, are related to deformation ν2, valent ν1 and a combination of ν1+ν4 (NO3 ) group
vibrations, the latter of which allows us to reliably distinguish the absorption process of nitrate
groups from carbonate groups [22, 35, 36]. In our work, strong carbonate bands were found for
samples co-precipitated at pH>8. According to the spectra shown in Figure 3, with an increase
in co-precipitation pH an increase in absorption is observed in the frequency ranges of 1088
cm-1 and 1520 cm-1, which is due to deformation ν2, symmetrical ν1 and antisymmetric valent
2-
vibrations ν3 (CO3 ), respectively [22, 36, 37]. The latter indicates the coordinated positioning
of the carbonate groups relative to Y3+ [22, 23, 37]. A wide absorption band with a center in the
frequency range of 640 cm-1 is associated with valent vibrations ν (Y(Eu)-OH) [22, 36]. The
infrared spectrum of the sample obtained under hydrothermal conditions is described by the
same absorption frequency bands; in addition, it contains an insignificant amount of carbonate
groups similar to samples co-precipitated at pH 7 and 8.
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It is known that with an increase in the pH of the solution, the content of dissolved
2-
carbonate groups (CO3 ) increases due to an increase in the solubility of CO2 from
atmospheric air. Despite the efforts being made, the presence of carbonate groups is one of the
main drawbacks that accompanies conventional and homogeneous methods of the precipitation
of layered materials [23, 35, 36]. Following other authors, we believe that during co-
precipitation these carbonate groups embed into the Y3+ coordination sphere and partially
replace the coordination (ОН-) groups [23]. This probably leads to an additional compensation
of the hydroxocomplexes charge, a decrease in the charge density of the layers and, as a
consequence, a decrease in the orderliness of the layered structures. As a result, it was found
that co-precipitation at a constant pH of 7 allows one to synthesize LRHs with uncoordinated
-
2-
interlayer (NO3 ) groups and a low content of coordinated (CO3 ) groups typical for materials
obtained by the hydrothermal method. The increase of the pH of co-precipitation in the
presence of CO2 from atmospheric air leads to a significant carbonization of the sample, which
may contribute to the disordering of the layers and the formation of heterogeneous synthesis
products with irreproducible compositions. The conclusions put forward are in good agreement
with the data of the TG and X-ray phase analysis.
3.4 LRH Particles Morphology. Particle Shape and Phase Composition of Calcined Powders
In Figure 4-a, the SEM image of the surface of YEHN-pH7 particles indicates the
presence of a strictly oriented, ordered edge growth of straight plates with a thickness of about
50 nm. Some “deficiency” in the side (lateral) surface most likely leads to the oriented growth
of new plates on the lateral surface of already formed plates. The increase in the size of the
lateral plates leads to a mutual growth or mutual “intergrowth” of the particles with the
formation of spherical agglomerates and morphology, such as “desert rose” or “house of cards”
[26, 30]. According to the image,shown in Figure 4-c, an increase in pH to 8 leads to a large
decrease in the lateral dimensions and thickness of the LRH sheets, which is caused by the
more intensive and erratic growth of new sheets on the lateral surface of the particles. As a
result, the surface of the LRH sheets is curved and takes the form of flakes, which together
form huge (~ 50 µm) near spherical agglomerations. The surface images of the YEHN-pH9 and
YEHN-pH9.5 particles, shown in Figure 4-e and 4-g, clearly indicate a change in the nature
and degree of aggregation of the layered particles as the co-precipitation pH increases. Near-
spherical agglomerates are replaced by large formations of irregular shape, consisting of a set
of randomly connected nanoscale plates. The isotropy of the plates, as well as their
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agglomeration increases. LRHs obtained by the hydrothermal method, shown in Figure 4-i
,
have a similar form of agglomerates. Despite this, the agglomerates consist of direct submicron
plate particles with a thickness of several tens of nanometers.
According to the micrographs in Figure 4-b, the calcined YEOx-pH7 particles have a
plate-like form close to that of the original precursor particles (Figure 4-a). The lateral
dimensions of the straight plates in the aggregates, as well as the spherical aggregates
themselves, did not undergo significant changes during heat treatment. The calcined flakes in
the YEOx-pH8, shown in Figure 4-d, still have their initial lateral dimensions but have
increased several times in thickness and lost their continuity, which led to the formation of
weakly bound isotropic nanoparticles withsize of about 50 nm. Large aggregates of irregular
shapes YEOx-pH9 and YEOx-pH9.5, shown in Figure 4-f and
h, respectively, are formed
from the precipitates obtained at pH 9 and 9.5. According to the SEM images, the surface of
the submillimeter aggregates is represented by compacted coral clusters of rounded
nanoparticles with sizeof about 40 nm. The plate form of the particles has completely
disappeared. A similar transformations for layered particles was observed by the authors of
other works in which LRHs were obtained by direct and inverse precipitation methods [26, 33].
The plate-like particles of LRHs obtained by the hydrothermal method partially lost their
continuity and shape after heat treatment at 1000 °C, similar to the YEOx-pH8 sample. At the
same time, the calcined YEOx-H powders show a decrease in the proportion of large
aggregates, which is probably due to the weakening of the bonds between the plates, leading to
partial disaggregation of the powders.
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Figure 4. SEM images of LRH precipitates dried at 50 °С (left) and (Y0.95Eu0.05)2O3 calcined at 1000 °С (right):
pH7 a) and b); pH8 c) and d); pH9 e) and f); pH9.5 g) and h); hydrothermal synthesis i) and j), respectively (on the
insets are images of the agglomerates).
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According to the diffractograms presented in Figure 5, all synthesized samples after
calcination at 1000 °С correspond to cubic Y2O3 (ICDD 01-086-1107). The dependences of the
size of the CSR for (Y0.95Eu0.05)2O3 powders on the calcination temperature shown in
Figure 6 indicate a higher crystallization intensity of the particles obtainedby co-precipitation
at pH 7 compared with the other samples.
Thus, the regulation of the pH value of co-precipitation in the range from 7 to 9.5 can
significantly affect the morphology, orderliness and size of layered LRHs. Variation of the pH
value most likely leads to different ratios between the nucleation and crystal growth of new
layers and edge growth on the formed surface. This, in turn, affects the shape and packing
density of layered structures at the micro level, as well as the size and type of agglomerates at
the macro level. This work demonstrates well the close relationship between the properties of
the co-precipitated layered LRHs and the properties of the materials after heat treatment. It is
shown that the final properties of the calcined powders (Y0.95Eu0.05)2O3 are determined directly
at the stage of synthesizing the precursor. It should be noted that the highly ordered structure
and dense packing of the layers of YEHN-pH7 plate particles causes an increased
crystallization intensity and leads to the formation of larger crystalline cubic (Y0.95Eu0.05)2O3
crystallites during heat treatment. In this regard, we believe that the method of co-precipitation
at a constant value of pH7 has great potential for creating crystal-oriented luminescent
materials and nanofilms.
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Figure 5. XRD patterns of (Y0.95Eu0.05)2O3 powders calcined at 1000 °С co-precipitated at: pH7 (a); pH8
(b); pH9 (c); pH9.5 (d); hydrothermal synthesis (e)
Figure 6. The effect of annealing temperature on the size of the CSR of (Y0.95Eu0.05)2O3 powders co-precipitated at: pH7
(a); pH8 (b); pH9 (c); pH9.5 (d); hydrothermal synthesis (e)
3.5 The Porosity of LRHs and Calcined Powders
The surface and structural transformations of layered particles occurring during
calcination were evaluated through studying porosity at different stages of the synthesis using
low-temperature nitrogen sorption methods. The isotherms for LRHs in Figure 7 are typical IV
adsorption-desorption curves characteristic of mesoporous materials with a predominantly H1-
type of pores (cylindrical) [38]. Determination of the specific surface area (SSA) of the
samples using the BET method revealed a proportional dependence of the SSA values on the
pH of co-precipitation (Table 3), which is in good agreement with other research data [30].
Similarly, proportional growth is observed for the total pore volume (Vp). Both factors indicate
an increasing contribution of the surface and texture of particles to the overall properties of the
final materials as the pH of the co-precipitation of the LRHs increases. YEHN-pH7 has the
lowest SSA values of 7.3 m2/g and Vp 0.025 cm3/g, which is consistent with the above
assumptions about the high ordered and strictly oriented growth of large multilayer plates.
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Figure 7. Low temperature nitrogen sorption isotherms and pore size distribution by the Barrett-Joyner-
Halenda method BJH (on the inset) for LRH samples co-precipitated at: pH7 (a); pH8 (b); pH9 (c); pH9.5
(d); hydrothermal synthesis (e)
The isotherms for the (Y0.95Eu0.05)2O3 samples calcined at 1000 °С presented in Figure 8
belong to the same type of curves and have the same type of pore as the precursors. It is
established that the precursors obtained at pH>7 intensively lose porosity during decomposition
and crystallization; additionally, after heat treatment the values of SSA for different pH are
quite close. Vp values decreased by about an order of magnitude for these samples. In contrast,
YEOx-pH7, after calcination at 1000 °С, undergoes a slight increase in the SSA value
compared to the precursor, but nevertheless has the lowest value of 11.6 m2/g compared to the
other oxide powders. Note that the pore volume for this sample is about four times larger than
the precursor and becomes higher than the other annealed powders. In addition, YEOx-pH7 has
a significantly different pore size distribution calculated by the Barrett-Joyner-Halenda (BJH)
method, as shown in the Figure 8
.
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Figure 8. Low-temperature nitrogen sorption isotherms and pore size distribution according to the Barrett-
Joyner-Handenda BJH method (on the insert) for (Y0.95Eu0.05)2O3 powders calcined at 1000 °С co-
precipitated at: pH7 (a); pH8 (b); pH9 (c); pH9.5 (d); hydrothermal synthesis (e)
The main contribution to the porosity of YEOx-pH7 is made by relatively large
mesopores with an average diameter of 37.9 nm (Table 3), while the pore diameter in other
samples is much smaller and have a wide size distribution. Comparing the surface data with the
SEM images, it is assumed that the registered pores in YEOx-pH7 can be clearly distinguished
in Figure 4-b. In this image, uniformly distributed holes are clearly visible on the side surface
of the plates, the shape of which with high probability is close to cylindrical. Note also that in
the image of the YEHN-pH7 plate surface, shown in Figure 4-a, there are no holes and the
porosity according to the tab in Figure 7 is represented by a small number of pores with a wide
size distribution not visible in the SEM images. It has been established that the surface
properties of samples obtained by the hydrothermal method have similar characteristics with
YEHN-pH8 and have pores with a wide size distribution in both cases.
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Table 3. The size of the CSR (D), specific surface area (Ssa), pore volume (Vp) and average pore diameter (dp) of
LRH samples dried at 50 °С and (Y0.95Eu0.05)2O3 powders calcined at 1000 °С.
50°С
Vp, cm3/g
1000°С
YEHN-
"pH"
D(220),
nm
37
D(220),
nm
45
Ssa, m2/g
dp, nm
Ssa, m2/g
Vp, cm3/g
dp, nm
7
8
9
9.5
H
7.3
53.0
95.1
137.9
53.8
0.025
0.552
0.853
1.122
0.227
13.9
41.6
35.9
32.6
16.9
11.6
30.1
26.6
26.4
26.9
0.110
0.066
0.082
0.081
0.049
37.9
8.8
12.4
12.2
7.3
29
20
23
30
35
38
35
35
Thus, the porosity of the LRH obtained at pH>7 significantly decreases after heat
treatment at 1000 °С with the formation of (Y0.95Eu0.05)2O3 powders. Despite this, the pores
have a wide size distributiondue to irregular morphology and low orderliness of nanoscale
particles. In contrast, the sample obtained at pH 7 undergoes minor change and has the lowest
SSA value. The porosity of the oxide particles is formed by regularly holes of a cylindrical
shape on the lateral surface of the plate particlesin the absence of significant destruction.
Probably, due to thehighly ordered structure, high amount and dense packing of the layers,
effective mass transfer of substances and more intensive and uniform recrystallization of the
particles are provided, which in turn leads to an increased crystallization during heat treatment.
In this regard, a study and comparison of the main luminescent properties of oxide powders
obtained at 1000 °С was conducted.
3.6 Optical Properties of Calcined Powders
The excitation and photoluminescence (PL) spectra of (Y0.95Eu0.05)2O3 powders obtained
at a temperature of 1000 °С are shown in Figure 9 (left panel). The most intense band of 248
nm in the excitation spectrum of PL corresponds to charge transfer from the 2p-orbital of the
O2- ion to the 4f-orbital of the Eu3+ ion [39]. Significantly less intense bands in the longer
wavelength region are associated with 4f-intra configuration optical transitions in Eu3+ ion
7
from the ground F0 state to several excited states. At intacenter excitation there are no any
qualitative changes in the emission spectra, but the luminescence yield is significantly lower as
compared to the emission intensity at excitation of 248 nm. For this reason, we focus on the
emission spectra excited at 248 nm to obtain the highest luminescence yield and, as a result, the
smallest error when comparing the emission characteristics of different samples.
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PL spectra of (Y0.95Eu0.05)2O3 powders at excitation by O2-→Eu3+ charge transfer consist
of lines corresponding to the 5D0→7FJ transitions (Figure 9, right panel). It should be noted that
the sample obtained at pH 7 is characterized by a higher PL intensity compared to all powders
under study. It is known that the morphology, crystall size and porosity of particles, which are
determined by the conditions of synthesis, have a significant impact on their optical and
luminescent properties [19-21]. In general, an increase in the size of particles or crystallites
leads to a decrease in the surface area of the sample, which is accompanied by a decrease in the
surface to volume ratio and, accordingly, in the number of atoms with high surface energy. The
YEOx-pH7 has a higher crystallization intensity during heat treatment and, as a consequence, a
larger CSR value (Figure 6). In addition, the SSA value is the smallest for this phosphor
(
Table 3), which provides fewer surface defects, usually playing the role of luminescence
“killers”. In total, all these features lead to minimization of non-radiative losses and, as a result,
to an increase in the luminescence intensity.
According to the SEM images (Figure 4), the YEOx-pH8is characterized by a high
degree of agglomeration, a large number of weakly bound particles andsmaller CSR value
(
Figure 6), which indicates more disordered structure as compared to the powders obtained at
other pH values. In addition, the surface area has a highest value for this powder, pointing to a
large number of surface channels for non-radiative relaxations (Table 3). These peculiarities
explain the lower emission intensity of YEOx-pH8.
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Figure 9. Photoluminescence excitation (λem=607 nm) and emission (λex=248 nm) spectra of (Y0.95Eu0.05)2O3
powders calcined at 1000 °С co-precipitated at: pH7 (a); pH8 (b); pH9 (c); pH9.5 (d);
hydrothermal synthesis (e)
A europium ion is known as a marker for the determination of the local symmetry in a
host lattice within the nearest atomic environment [40, 41]. The electric-dipole transition
5D0→7F2 of Eu3+ corresponding to the emission bands at 607 and 620 nm is generally parity-
forbidden by the Laporte selection rule. This transition becomes partially allowed when the ion
is situated at the low symmetry site, so the integrated intensity of PL bands at 607 and 620 nm
is hypersensitive to the local environment of the Eu3+ ion. In contrast, the magnetic-dipole
5D0→7F1 transition (emission at 583-596 nm) is allowed and is insensitive to the site symmetry.
5
5
The ratio of integrated intensities of D0→7F2 and D0→7F1 transitions is called the asymmetry
factor and characterizes the local symmetry in a host around the Eu3+ centre. The higher the
asymmetry factor, the less symmetry in the local environment of the Eu3+ ion in a host.
In a cubic Y2O3 lattice, there are two nonequivalent cationic positions: a C2-site without
inversion symmetry and a S6-site with an inversion centre [42]. The ratio of the number of
these sites in the host is 3:1. As such, even in ideal Y2O3 host, the largest contribution to the PL
5
spectra of Eu3+ ions is due to the D0→7F2 electric-dipole transition; the asymmetry factor is
higher than 1. Nonetheless, a change in the value of the asymmetry factor allows a qualitative
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estimate of the host distortions in the local environment of Eu3+. In Table 4, the calculated
values of the integrated intensity ratio ʃI(5D0→7F2)/ʃI(5D0→7F1), generally denoted as R21, are
listed for all studied samples. It can be seen that the sample synthesized at pH 7 is
characterized by the lowest R21 value, which indicates the highest symmetry in the local
environment of the emission centre. Indeed, YEOx-pH7 has the most ordered and closely
packed structure (Figure 4) and the smallest SSA (Table 3), which indicates a low
defectiveness, low surface to volume ratio and, accordingly, a relatively low number of Eu3+
emission centers located on the near-surface regions. The sample obtained by the hydrothermal
method is characterized by the highest R21 value (Table 4), as well as higher surface area and
smaller crystallite size compared to YEOx-pH7.
Table 4. Asymmetry factor around the Eu3+ ions in (Y0.95Eu0.05)2O3 powders synthesized in different conditions.
YEHN-"pH"
Asymmetry factor
7
8
9
9.5
H
9.19
9.23
9.31
9.38
9.39
The PL spectra of (Y0.95Eu0.05)2O3 powders obtained at pH 7 and calcined at different
temperatures are shown in Figure 10. It is shown that the PL intensity increases with the
calcination temperature of the samples. Additionally the temperature dependence of the
intensity correlates well with the dependence of crystallite size on temperature (Figure 10
,
upper panel). Similar correlations were observed by the authors of [43], however, the
crystallization of the samples in our case leads to a linear decrease of R21 from 9.42 to 8.89,
which can be explained by an increase in the symmetry of the emission centres during the
recrystallization process.
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Figure 10. Photoluminescence emission spectra(λex=248 nm) of (Y0.95Eu0.05)2O3 powders obtained at a pH 7
and calcined at different temperatures.
4. Conclusions
In our work, layered hydroxonitrates (Y0.95Eu0.05)2(OH)5(NO3)·
x
H2O were obtained via
co-precipitation at a constant pH and hydrothermal methods, from which (Y0.95Eu0.05)2O3
luminescent powders with different properties were synthesized. It has been shown that single-
phase layered materials are formed in a wide range of pH values from 7 to 9.5. Co-precipitation
-
at constant pH 7 leads to the formation of LRHs with uncoordinated interlayer (NO3 ) groups
and the lowest content of carbonate groups originating from atmospheric air. Under low pH
values of LRH co-precipitation, the formation of micron close-packed straight plates is
observed. As the pH increases, the lateral size and the plate thickness drop sharply, and the
straight particles bend to form strongly agglomerated nanoscale flakes. It has been established
that highly ordered close-packed plate LRHs obtained at pH 7 have the most intensive growth
of crystallites during heat treatment, which leads to the highest crystalline powders of cubic
(Y0.95Eu0.05)2O3. Oxide powders calcined at 1000 °С sharply lose their porosity, which is
accompanied by significant surface and morphological transformations. However, the heat-
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treated particles obtained at pH 7 retain the morphology of the precursor and have the lowest
porosity, which is represented mainly by large regular-shaped mesopores uniformly distributed
on the lateral plane of the lamellar particles.
A complex of materials characteristics, including morphology, structural disordering,
defectiveness and surface to volume ration, is determined by the synthesis conditions and
directly affects the practically important functional parameters of (Y0.95Eu0.05)2O3 phosphors.
The sample obtained at pH7 and calcined at 1000 °C has the highest luminescence yield
compared to the other investigated phosphors. The improvement of optical properties is due to
the formation of a more perfect crystalline structure with less defectiveness and a lower surface
to volume ratio.
As a result of comparing the properties of LRHs synthesized by various methods, it was
concluded that the co-precipitation method with a constant pH value of 7 allows one to obtain
more ordered close-packed layered structures and produce more efficient phosphors
(Y0.95Eu0.05)2O3 compared to materials obtained at mild hydrothermal conditions (120 °C, 24
h). Thus, the layered rare earth hydroxides synthesized in this work are suitable for applying to
the methods of intercalation chemistry and therefore have great prospects for the preparation of
highly efficient crystal-oriented luminescent coatings and nanofilms.
Acknowledgements
The study was financially supported by the Russian Science Foundation (agreement №
18-79-00188). The study was supported by the Ministry of Education and Science of the
Russian Federation (Government Task No. 3.1485.2017/4.6). The authors acknowledge the
UCCP “Modern Nanotechnology” INSaM UrFU for providing the measurement equipment.
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Highlights
• Co-precipitation at low pH causes oriented growth of layered rare-earth hydroxides
• Oriented growth form ordered plates with larger crystallite size and low surface
• The calcined phosphors inherit the morphology of the precursors
• Oxide phosphors from the plate-like particles show the highest luminescence yield