Monodisperse NaxY(OH)yF3+x-y Mesocrystals with tunable morphology and chemical composition: PH-Mediated ion-exchange

Article abstract of DOI:10.1021/acs.cgd.6b01569

This work reports self-Assembled uniform monodisperse Na_xY(OH)_yF_3+x-y (0 < x < 1, 0 < y < 3) mesocrystals with tunable morphology and chemical composition obtained from a hydrothermal ionexchange reaction. Detailed crystal structure analysis illustrates that the special crystal structure of NaxY(OH)_yF_3+x-y (0 < x < 1, 0 < y < 3) facilitates the pH-mediated ion-exchange process. A possible topochemical-based mechanism has been proposed to this ion-exchange process. By carefully adjusting the pH value of the solution, the tunable three-dimensional (3D) Na_xY(OH)_yF_3+x-y (0 < x < 1, 0 < y < 3) mesocrystal architectures are obtained through an oriented self-Assembly. The photoluminescence properties of the rare earth ions (such as Eu³⁺, Tb³⁺, and Ce³⁺) doped as-prepared mesocrystal samples are investigated, and multicolor emissions are realized.

Full text of DOI:10.1021/acs.cgd.6b01569

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Article
x
y
3+x-y
Monodisperse NaY(OH)F mesocrystals with tunable morphology
and chemical composition: pH-mediated ion-exchange
Jiao Wang, Bi-Qiu Liu, Gaoshan Huang, Zhi-Jun Zhang, and Jing-Tai Zhao
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01569 • Publication Date (Web): 21 Dec 2016
Downloaded from http://pubs.acs.org on December 23, 2016
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Monodisperse Na Y(OH) F mesocrystals with
x
y 3+xꢀy
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tunable morphology and chemical composition: pHꢀ
mediated ionꢀexchange
a
b
c, d
e
e
Jiao Wang ‡*, BiꢀQiu Liu ‡, Gaoshan Huang , ZhiꢀJun Zhang *, JingꢀTai Zhao
a
School of Information Science & Engineering, Fudan University, Shanghai 200433, P. R.
China.
b
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, P. R. China.
c
Department of Materials Science, Fudan University, Shanghai 200433, P. R. China.
d
National Key Laboratory of Fundamental Science of Micro/NanoꢀDevice and System
Technology, Chongqing University, Chongqing 400030, P. R. China.
e
School of Materials Science and Engineering, Shanghai University, Shanghai, 200072, P. R.
China.
ABSTRACT This work reports selfꢀassembled uniform monodisperse Na Y(OH) F (0<x<1,
y 3+xꢀy
x
0<y<3) mesocrystals with tunable morphology and chemical composition obtained from a
hydrothermal ionꢀexchange reaction. Detailed crystal structure analysis illustrates that the
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special crystal structure of Na Y(OH) F (0<x<1, 0<y<3) facilitates the pHꢀmediated ionꢀ
y 3+xꢀy
x
exchange process. A possible topochemicalꢀbased mechanism has been proposed to this ionꢀ
exchange process. By carefully adjusting the pH value of the solution, the tunable threeꢀ
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dimensional (3D) Na Y(OH) F (0<x<1, 0<y<3) mesocrystal architectures are obtained
y 3+xꢀy
x
through an oriented selfꢀassembly. The photoluminescence properties of the rare earth ions (such
3
+
3+
3+
as Eu , Tb and Ce ) doped asꢀprepared mesocrystal samples are investigated and multicolor
emissions are realized.
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INTRODUCTION
Generally, the chemical composition, crystal structure, particle size, morphology, and
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, 2
dimensionality show profound impacts on the application of nanoꢀ/microꢀmaterials.
Controlled synthesis of inorganic nanoꢀ/microꢀcrystals with specific architectures and research
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ꢀ6
on their architecturalꢀrelated properties are interesting research subjects. Among various nanoꢀ
microꢀarchitectures, threeꢀdimensional (3D) singleꢀcrystallineꢀlike architecture termed
mesocrystals’ have received extensive attentions during the recent decade due to their
interesting intrinsic feature such as singleꢀcrystalline structure, complex 3D morphology and
/
‘
7
, 8
easy processing ability.
Different from single crystals or superlattices, novel mesocrystals
offer a new possibility for design and building multifunctional materials with exciting properties
9
, 10
and wide field of applications.
For those novel microꢀ/nanoꢀarchitectures which are difficult to obtain by traditional synthetic
strategies, the topochemicalꢀbased ionꢀexchange synthetic route facilitate the synthesis of novel
2
, 11
materials,
due to the final products can inherit the crystal structure, particle size, and
1, 2, 12ꢀ18
morphology of the parents expeditiously.
Thus, topochemical ionꢀexchange reaction also
was proposed to be used for obtaining high quality luminescent materials, such as βꢀNaYF4
1
4, 17ꢀ19
20, 21
22,
microꢀ/nanoꢀcrystals,
oriented Gd O monolayer film,
LnOF microꢀ/nanoꢀcrystals,
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3
23
and etc. The topotactic transformation strategy can be used to directly synthesize high quality
βꢀNaYF nanoꢀ/microꢀ/mesoꢀcrystals from lanthanide hydroxides [Ln(OH) ] and lanthanide
4
3
hydroxide fluoride [Ln(OH) F ] precursors by avoiding the intermediate routine cubicꢀ
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3ꢀx
2
, 12, 13, 15, 17ꢀ19, 24ꢀ26
hexagonal (αꢀβ) phase transfer.
The reversal transformation process from βꢀ
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7
NaYF to Ln(OH) F also was reported. Very recently, lanthanide oxyfluorides with special
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3 3ꢀx
nanoꢀ/microꢀstructures also can be prepared by topotactic transformation from different
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precursors.
Experimental variables have a profound influence on morphology, size,
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and composition of the final samples.
morphological evolution and chemical composition changing of the Y(OH)xF3ꢀx and
When other experimental parameters remain, the
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NaY(OH)xF4ꢀx exhibit a dependence on the changing of the OH amount or pH values.
Thus, it is eagerly demanded to give detail discussion on the topochemical ionꢀexchange
processes during the hydrothermal processes.
Herein, a series the uniform monodisperse 3D Na Y(OH) F (0<x<1, 0<y<3) hierarchical
y 3+xꢀy
x
mesocrystals with tunable morphology and chemical composition was synthesized by a facial
hydrothermal method. Of particular interest to this research is that the result provides solid
evidence on the topochemical ionꢀexchange progress from Ln(OH) to βꢀNaYF . Besides, the
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4
morphology and chemical composition of the final monodisperse 3D mesocrystal products can
be tailored in a wide range by carefully adjusting the pH values of the solution. One possible
topochemicalꢀbased mechanism for the formation of such 3D Na Y(OH) F (0<x<1, 0<y<3)
y 3+xꢀy
x
hierarchical mesocrystals are proposed, and the variable luminescence properties of rare earth
RE) ions doped samples are studied.
(
EXPERIMENTAL SECTION
Materials. In a typical synthesis procedure, rare earth oxides (Y O , Eu O , Tb O and CeO ,
2
3
2
3
4
7
2
9
9.99%), ammonium hydrogen fluoride (NH HF , analytical purity), sodium citric acid (Na CA,
4 2 3
analytical purity), sodium hydroxide (NaOH, analytical purity) and dilute nitric acid (HNO3,
analytical purity) were used as starting materials without further purification.
Synthesis procedure. The samples with controllable morphologies were synthesized via a
hydrothermal process. Firstly, rare earth oxides were dissolved in diluted nitric acid to prepare
RE(NO ) aqueous solution. The pH values of above solutions were adjusted to 3 by NaOH
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3
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aqueous solution (5 M). Then, 10 ml of RE(NO ) (0.2 M) aqueous solution was added into 10
3
3
3
−
ml of aqueous solution containing 7 mmol of sodium citric acid to form the CA –metal
complex under continuous stirring for 1 h. Excess amount of NH HF aqueous solution (0.9 M,
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2
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1
0 ml) was added into the mixed solution gradually, under continuous stirring for 1 h. The pH
values (12.0 – 13.8) of the solutions were adjusted to be desired target by NaOH aqueous
solutions (5 M). Subsequently, the mixture was transferred into a 50 ml Teflonꢀlined stainless
steel autoclave, which heated at 180 °C for 12 h. The asꢀprepared samples were washed several
times by deionized water and anhydrous ethanol, respectively. Finally, these asꢀprepared
samples were dried at 80 °C under vacuum overnight. Different samples obtained under different
pH condition are denoted as S1 – S13 (pH = 12.0 – 13.8, Table S1 and Figure 1).
Characterization. Xꢀray diffraction (XRD) patterns were collected at room temperature on a
HUBER G670 (Cu Kα1 radiation, λ = 1.54056 Å, Ge monochromator) for phase identification.
The 2θ range was from 5 to 100 ° with the step of 0.005 °. Scanning electron microscopy (SEM,
Sꢀ4800) and transmission electron microscopy (TEM, JEOL200CX) were used for the
observation of the morphology and particle size. Fourier Transform infrared (FTIR)
spectrophotometer (Nicolet Avatar 370) was used to detect infrared spectra by the KBr pellet
method. The chemical composition was measured by inductively coupled plasmaꢀoptical
emission spectroscopy (ICPꢀOES) on a BarianVista RL spectrometer with radial plasma
3
+
3+
observation. The ultravioletꢀvisible (UVꢀvis) spectra of Eu and Ce ꢀdoped samples were
detected on a fluorescence spectrometer FLS920. The vacuum ultravioletꢀvisible (VUVꢀvis)
3
+
spectroscopic properties of Tb ꢀdoped samples were detected at the VUV spectroscopy
beamline (4B8ꢀVUV) of the Beijing Synchrotron Radiation Facility, Institute of High Energy
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Physics, Chinese Academy of Sciences. All luminescence spectra were detected at room
temperature.
RESULTS AND DISCUSSION
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Figure 1. (a) Comparison of XRD patterns of the asꢀobtained samples obtained at different pH
values (pH = 12.0 ꢀ 13.8, S1 ꢀ S13) with the JCPDS for βꢀNaYF (No. 16ꢀ0334), Y(OH)1.57F1.43
4
(
No. 80ꢀ2008) and Y(OH) (No. 24ꢀ1422). (b) The corresponding enlargement XRD patterns in
3
o
o
the region from 28 to 31.2 .
Effect on crystal structure and chemical composition. Figure 1 shows the XRD patterns of
the samples obtained from hydrothermal processes under different pH values as described in
Experimental Section. These XRD profiles with peak shift and relative intensity changing
indicate that the similarity of crystal structures of these samples is high. These XRD profiles can
be well indexed as hexagonal structure and exhibit high similarity to the crystal structure of
hexagonal βꢀNaYF (JCPDS No. 16ꢀ0334), Y(OH)1.57F1.43 (JCPDS No. 80ꢀ2008) and Y(OH)3
4
(JCPDS No. 24ꢀ1422), indicating that their crystal structures are identical. The ionꢀexchange
reaction can easily occurs among such isostructural compounds with close lattice parameters
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matching. The pH value exhibits a great influence on crystal structure and chemical
composition while other experimental parameters are fixed. With the increasing of pH value, the
diffraction peak positions of (110) and (101) planes shift to low angle obviously (Figure 1b).
When the pH value was adjusted from 12.0 to 13.8, the diffraction peak position of (110) planes
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o
o
shifted fast from 29.7 to 28.7 and the diffraction peak position of (101) planes shifted slightly
o
o
from 30.6 to 30.1 , while the space between (110) and (101) diffraction peaks increased from
o
o
0
.86 to 1.44 (Figure 1b). This phenomenon indicates that the pH value of initial solution also
exhibits a great influence on the lattice parameters of asꢀobtained samples. The detail discussions
are given in the following parts (Figure 2 and Figure 3).
Based on the XRD data in Figure 1, the lattice constants and cell volumes for finial samples
are plotted in Figure 2. Figure 2a indicates that the lattice constant a increases in pH < 12.6 and
pH > 13.2, and remains almost constant in 12.6 < pH < 13.2. On the other hand, the lattice
parameter c nearly remain constant, independent of pH value especially in 12.4 < pH < 13.4. The
lattice constant c first slightly increases in pH < 12.4 then slightly decreases in pH > 13.4. It is
worth noting that, the lattice constant c/a ratio linear decreases with the increasing of pH value
(Figure 2a). Despite the evolution of lattice parameters and unit cell volumes, the composition of
the finial samples also shows a dependence on the variation of pH value (Figure 2b and Table
S1). As shown in Figure 2b, the increasing of pH value causes a decreasing of the F/Y (and
Na/Y) ratio and an increasing of the O/Y (H/Y) ratio in finial samples. The ionꢀexchange process
ꢀ
ꢀ
+
2, 12ꢀ15, 17ꢀ19,
leads to the substitution of F by OH and releasing of Na ions from crystal structure.
2
4ꢀ29
ꢀ
ꢀ
ꢀ
In basic solution, the excessive OH keeps the substitution of F by OH is kinetically
favorable. In contradiction to the variation trend of O/Y, the variation trends of F/Y and Na/Y
ꢀ
are similar. This phenomenon indicates that the variation of F ions in this crystal structure is
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related to the variation of Na ions. The decreasing rate of F/Y (Na/Y) ratio and the increasing
rate of O/Y (H/Y) ratio in pH < 12.4 seems to be higher than that in pH > 12.4 (Figure 2b). With
ꢀ
the increasing of pH value, the increasing of OH content leads to the decrease of local
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coordination space of Na ions, which is responsible for the decreasing of Na content. Thus,
this topotactic ionꢀexchange transformation can be described as follows: with the substitution of
ꢀ
ꢀ
+
2, 12ꢀ15,
F ions for OH ions, the inserted Na ions release from the tunnels in the crystal structure.
17ꢀ19, 24ꢀ29
This ionꢀexchange leads to the formation of finial samples with tunable chemical
2
, 12ꢀ15, 17ꢀ19, 24ꢀ29
composition at different pH values.
The formula of the asꢀprepared samples may be
defined as Na Y(OH) F3+xꢀy. This topotactic transformation is one reversible process: βꢀNaYF4
x
y
2
7
↔
Na Y(OH) F
↔ Y(OH) . The formula βꢀNaYF (x=1, y=0) and Y(OH) (x=0, y=3) are
3+xꢀy 3 4 3
x
y
specific conditions of the intermediate product Na Y(OH) F3+xꢀy. Then, reasonable limits of x
x
y
and y should be given as Na Y(OH) F (0<x<1, 0<y<3).
y 3+xꢀy
x
The lattice constant evolution also shows a dependence on the chemical composition variation
ꢀ
ꢀ
+
(
Figure 2c). With the substitution of F ions by OH ions, the inserted Na ions release from the
tunnels in the crystal structure. These changes of lattice parameters can be attributed to the ionic
ꢀ
ꢀ
+
size differences between F (r = 1.285 Å, CN = 2) and OH (r = 1.32 Å, CN = 2), Na (r = 1.24 Å,
3
+
30
CN = 9) and Y (r = 1.075 Å, CN = 9), respectively. Meanwhile, the increment of the unit cell
ꢀ
ꢀ
30
volume can be mainly attributed to the substitution of F by OH ions. However, the different
evolution between lattice constant c and lattice constant a is noticeable (Figure 2c). The lattice
ꢀ
parameter a increases with decreasing of F concentration in finial produces. Meanwhile, the
lattice constant c first slightly increases with decreasing F/Y ration in Na Y(OH) F (0<x<1,
y 3+xꢀy
x
0
<y<3) hosts (F/Y > 1.36) then slightly decreases in F/Y < 1.36 (Figure 2c). This phenomenon is
31, 32
consistent with the results of Y(OH) F and Yb(OH) F .
The thermal stability of this type
3
ꢀx
x
3ꢀx x
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ꢀ
crystal structure containing OH group is not good. Based on the previous report on the thermal
gravity (TG) curves of this type compound, the typical thermal decomposition temperature is
o
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around 350 ꢀ 500 C, and this decomposition releases H O molecular.
Novel
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micro/nano REꢀdoped LnOF luminescent materials can be effectively obtained via this further
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thermal decomposition.
Figure 2. (a) The dependence of the lattice lengths (a and c) and the rate of c/a of the asꢀ
prepared samples on the pH values. (b) The dependence of chemical composition on the pH
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values. (c) The dependence of the lattice lengths (a and c) and the rate of c/a of the asꢀprepared
samples on the contents of fluorine elements (F/Y) in host matrix for different samples.
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Figure 3. The crystal structure models of (a) βꢀNaYF (No. 16ꢀ0334), (b) sample S2 (see SI), (c)
4
sample S9 (see SI) and (d) Y(OH) (No. 24ꢀ1422) viewed along [001] direction.
3
As a controllable and reversible reaction, this topochemical ionꢀexchange route is complex.
The XRD patterns indicate that the structures of βꢀNaYF4 (JCPDS No. 16ꢀ0334),
Na Y(OH) F
(0<x<1, 0<y<3) and Y(OH) (JCPDS No. 24ꢀ1422) are very similarly
3
x
y
3+xꢀy
hexagonal phases in space group P6 /m. The Y(OH) crystals can be transferred into βꢀNaYF
3
3
4
1
3, 17
crystals by keeping their frameworks and morphologies nearly unchanged.
Figure 3, the crystallographic structures of βꢀNaYF , Na Y(OH) F
As shown in
(0<x<1, 0<y<3) (S2 and
y 3+xꢀy
4
x
S9) and Y(OH) viewed along [001] direction further identify that these compounds are
3
3
+
ꢀ
ꢀ
15
isostructural. The Y is coordinated by nine F /OH ions and form a tricapped trigonal prism.
A crystal framework with hexagonal tunnels along c axis consists of these Y(O/F) tricapped
9
15
trigonal prism. In this framework, the Y(O/F) polyhedrons connect with each other in an edgeꢀ
9
sharing manner along the a and b axes, and in a faceꢀsharing manner along [001] direction,
1
5
ꢀ
ꢀ
+
respectively. The F ions in these structures can be easily substituted by OH ions and the Na
ions can be easily released from these hexagonal tunnels to form another stable crystal structure
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with the increasing of pH value, resulting the crystal transform from βꢀNaYF to Y(OH) step by
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3
ꢀ
step. During the ionꢀexchange reaction, with decreasing of F concentration, differ from
ꢀ
increasing of the a parameter, the lattice constant c remains almost constant especially in F < 1.8
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(
pH > 12.6, Figure 2a and c). The typical Y(O/F) tricapped trigonal prism consists of one
9
ꢀ
ꢀ
ꢀ
ꢀ
trigonal prismatic arrangement of six F /OH ions with the capping F /OH ions (three) located on
ꢀ
ꢀ
each of the rectangular faces. With a substitution of OH (r = 1.32 Å, CN = 2) for F (r = 1.285 Å,
+
ꢀ
ꢀ
CN = 2) and releasing of Na , three capping anions (F /OH ) rotate around the c axis in (0001)
plane and are located near the centre of each rectangular face. Consequently, the a parameters
are increased and c parameters remains almost constant. The hydrogen bond (OꢀH F) was
·
··
31, 32
assumed to play critical roles on these separations of lattice constant evolution.
Normally,
3
4ꢀ36
Y(OH) and βꢀNaYF contain little or no hydrogen bonds.
On the other hand, the FTIR
3
4
ꢀ
spectra of Na Y(OH) F (0<x<1, 0<y<3) confirm that the asꢀprepared sample contains OH
y 3+xꢀy
x
1
2, 31
and hydrogen bonding between the OH and F (OꢀH F) (Figure S1).
Based on the Y(O/F)9
···
tricapped trigonal prismatic geometry, the hydrogenꢀbonded layer in Na Y(OH) F (0<x<1,
y 3+xꢀy
x
31
0
<y<3) is perpendicular to the c direction. Thus, the c parameter is greatly limited by the
3
1
hydrogen bonds.
Effect on Morphology. The pH value also has a dramatically influence on the mescocrystal
size and morphology via hydrothermal route. As the pH value increased (F/Y decreased, Figure
2), the products evolved from a hexagonal prism to cantaloupeꢀlike bundle structure and finally
to microspindle bundle structure. The 3D architectures of all samples are mesocrystals as shown
in Figure 4 and Figure S3. The nanorods with diameters around 10 nm can be clearly observed
on the surface and in the interior of the 3D architecture (Figure 4 and S3). The selectedꢀarea
electron diffraction (SAED) patterns of the 3D architectures (Figure 4 and S3) exhibit sharp and
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highly symmetrical spots and reveal their singleꢀcrystalꢀlike mesocrystal nature. These loose
structure of the bundles with rough surface indicate that these mesocrystals are formed by the
oriented selfꢀassemble of nanorods stacking parallel along their c axes in a sideꢀbyꢀside manner.
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In Na Y(OH) F
(0<x<1, 0<y<3) crystal structure, the Na(F/O) polyhedra and vacancies
6
x
y
3+xꢀy
may retard the skeleton stacking of Y(O/F) tricapped trigonal prism along the a and b direction
9
1
4
in ed. On the other hand, the skeleton stacking of Y(O/F) tricapped trigonal prism leads to a
9
1
4, 15
preferential growth along the [001] direction.
Thus, for these hexagonal phases, it is
kinetically favorable to form oneꢀdimensional (1D) nanorods/nanowires firstly. Meanwhile, to
further decrease the total energy, these 1D nanorods/nanowires tend to selfꢀassemble into 3D
2
2
bundle structures by sharing the side surface.
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Figure 4. SEM images and SAED patterns (insets) of the asꢀprepared samples obtained in the
initial solutions with different pH values. The scale bar is 500 nm. The scale bar in inset is 5
1/nm.
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Figure 5. High magnification (aꢀd) SEM and (eꢀh) TEM micrographs of the asꢀprepared samples
obtained in the initial solutions with different pH values. The scale bar is 100 nm. HRTEM
image of the sample (pH = 12.3) is shown as inset (The scale bar is 2 nm).
Figure 5 shows close observation of the Na Y(OH) F (0<x<1, 0<y<3) mesocrystals, which
y 3+xꢀy
x
further confirms that all these samples consist of nanosized subunits (~ 10 nm). With the
increasing of pH values, the aspect ratio of the subunits increases. At low pH value condition,
Na Y(OH) F (0<x<1, 0<y<3) subunits are nanoparticles (Figure 5a and 5e, pH = 12.3). With
y 3+xꢀy
x
the increasing of pH value (pH = 12.3, Figure 5a), the Na Y(OH) F (0<x<1, 0<y<3) primary
y 3+xꢀy
x
crystals are encouraged growth along the [001] direction, and the morphologies Na Y(OH) F
y 3+xꢀy
x
(0<x<1, 0<y<3) primary crystals change from nanoparticle (Figure 5b and f, pH =12.5) to
nanorods (Figure 5c and g, pH =12.7), then to nanowires (Figure 5d and h, pH =13.8). The high
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resolution transmission electron microscopy (HRTEM) demonstrates the high crystallinity and
single crystal nature of the subunit particles (Figure 5 inset). In contradiction to the classic
crystal growth, these 3D hierarchical mesocrystals are built from the selfꢀassembling of nonꢀ
spherical nanoparticle subunits. The attachment of primary nanorods/nanowires is selfꢀassembly
on the (100) planes along c direction. On the other hand, at pH = 12.0, the single crystalꢀlike
mesocrystal hexagonal prism with smooth surfaces formed by Ostwald ripening (Figure 4a).
These morphology images (Figure 5) also indicate that higher pH value encourages superior
0
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growth of Na Y(OH) F (0<x<1, 0<y<3) subunits along [001] direction and inhibits the
y 3+xꢀy
x
growth along [100] direction. With the increasing of pH values, the rapidly growth along [001]
direction leads to the morphology variation of Na Y(OH) F (0<x<1, 0<y<3) subunits from
x
y 3+xꢀy
spherical nanoparticle to nanowire (Figure 5). In the relatively low pH value range (pH < 12.5),
smaller Na Y(OH) F (0<x<1, 0<y<3) subunits have a relatively spherical shape. These
x
y 3+xꢀy
subunits with spherical geometry show lower selectivity of selfꢀassembly. It is believed that the
selfꢀassemble of equiaxial nanocrystals facilitate the Ostwald ripening (Figure 4a). With the
increasing of pH value, the selectivity of selfꢀassembly increases. In the relatively intermediate
pH value range, Na Y(OH) F (0<x<1, 0<y<3) subunits show an extended aspect ratio in
y 3+xꢀy
x
[
001] direction, and assemble on 3D with higher selectivity of selfꢀassembly. In the relatively
high pH value range, the much further extended 1D Na Y(OH) F (0<x<1, 0<y<3) subunits
x
y 3+xꢀy
selfꢀassemble preferentially on (100) plane, leading to the 3D mesocrystal morphology. In
addition, the subunits selfꢀassembly process is concurrent with the preferentially growth of the
subunits along the fastest growing [001] direction. The subunits assembled in the middle have
more time to grow and are promoted to grow more longer along [001] than the subunits attached
37
on the outer. This leads to the finial 3D cantaloupeꢀlike bundle structure and finally to
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microspindle bundle structure. This morphology evolution is consistent with the results of selfꢀ
3
7
assembled structures of CuO. These imply the near perfect alignment of the Na Y(OH) F
y 3+xꢀy
x
(0<x<1, 0<y<3) primary crystals in the assembly. This evidence provides a strong support to the
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assembly mechanism presented above.
Effect on luminescence properties. Hydrothermal method facilitates the doping of RE ions in
host matrix. And, the samples with specified emission color can be obtained and adjusted by the
3
+
3+
3+
3+
3+
3+
(
coꢀ)doping of RE ions (RE = Ce , Tb , and Eu ) easily (Figure 6a). To visualize the RE
3
+
3+
3+
(RE = Ce , Tb , and Eu ) dopingꢀinduced variation of emission color, the CIE (Commission
International del’Eclairage) chromaticity coordinates (calculated from the relevant emission
3
+
3+
3+
3+
spectra) of RE ꢀdoped (RE = Ce , Tb , and Eu ) samples are provided in Figure 6b.
3
+
3+
3+
Obviously, as the efficient host matrix for luminescent materials, Ce , Tb , and Eu ꢀdoped
samples exhibit typical blue, green, and orange emission, respectively (Figure 6a and 6b). In this
3
+
type host matrix, Ce ion gives a strong broad emission due to the allowed 4f–5d electric dipole
transition. The four main sharp peaks (at 489, 542, 584, and 620 nm) in the emission spectrum of
3+
5
7
3+
Tb ꢀdoped sample S1 are attributed to the D ꢀ F (J = 6, 5, 4, 3) transitions of Tb ion,
4
J
5
7
giving bright green light emission. Meanwhile, the D ꢀ F (at 542 nm) transfer is dominant.
4
5
3
+
Figure 6a also shows the excitation and emission spectra of Eu ꢀdoped sample S1 (5 mol%)
o
3+
obtained under pH = 12.0 at 180 C for 12h. For S1:Eu mesocrystals (pH = 12.0), when excited
7
5
ion to the transition of F ꢀ L at 393 nm, the sharp emission peaks in emission spectra are
0
6
5
7
attributed to the transitions from the excited state D to the ground state F . The electricꢀdipole
J
J
5
7
3+
D ꢀ F transition at 617 nm indicates the doping Eu ions are located mainly at
0
2
noncentrosymmetric sites. Compared with the oxide compounds, the charge transfer band is
3+
absence in the excitation spectrum of Eu ꢀdoped sample S1 in ultraviolet region (200 – 400 nm,
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Figure 6a). The charge transfer band will shift to the vacuum ultraviolet region (< 200 nm), due
ꢀ
15
to the electronegativity of F ions.
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+
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Figure 6. (a) Excitation and emission spectra of RE ꢀdoped (RE = Ce , Tb , and Eu ) sample
3+
3+
3+
3+
S1 (pH = 12.0). (b) CIE chromaticity diagram of RE ꢀdoped (RE = Ce , Tb , and Eu )
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samples. (c) The dependence of the intensities of the emissions (617 nm ( D ꢀ F ) and 592 nm
0
2
5
7
3+
(
D ꢀ F )) of Eu in samples on the pH values of the initial solutions.
0 1
The luminescence properties of REꢀdoped Na Y(OH) F (0<x<1, 0<y<3) also show a
y 3+xꢀy
x
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dependence on the chemical composition variables. As shown in Figure 6c, the emission
5
7
5
7
3+
intensity of D ꢀ F and D ꢀ F transitions of Eu ꢀdoped samples decrease gradually with
0
1
0
2
the decreasing of F/Y ration in Na Y(OH) F
(0<x<1, 0<y<3) crystal structure (the increasing
x
y
3+xꢀy
3+
3+
of pH value). The Tb (Figure S4) and Ce (Figure S5) ions doped samples exhibit similarly
3
+
luminescence intensity evolution to the Eu ꢀdoped counterpart. This phenomenon is mainly
3
+
attributed to the variation of coordination environment. As mentioned above, RE ions locate in
ꢀ
the centre of the Y(O/F) tricapped trigonal prism, and connect with nine OH groups with high
9
ꢀ
energy vibrational modes. With the increasing of pH value, OH coordinator increases, leading to
the changing of the coordination environment. The luminescent intensity of REꢀdoped materials
20, 38ꢀ42
can be dramatically reduced with this revolution of coordination environment.
5
7
In addition, the luminescent intensity of D ꢀ F (electric dipole transition) is higher than
0
2
5
7
the luminescent intensity of the D ꢀ F (magnetic dipole transition) (Figure 6a and 6c).
0
1
5
7
5
7
Normally, as the asymmetric ratio, the intensity ratio R (R = I( D ꢀ F )/I( D ꢀ F )) exhibits a
0
2
0
1
3
+
dependence on the specific crystallographic or spectroscopic site symmetry of Eu ions. In
Na Y(OH) F (0<x<1, 0<y<3) crystal structure, the R value evaluation exhibits a dependence
x
y 3+xꢀy
on the crystal structure variation and the pH value changing. The R values increases with
decreasing F/Y ration in Na Y(OH) F (0<x<1, 0<y<3) hosts from 3.5 to 1.36, but the R
x
y 3+xꢀy
values are decreased when the F/Y ration further decreases lower than 1.36 (Figure 6c). The
3
+
chromaticity coordinates in CIE chromaticity diagram of Eu ꢀdoped Na Y(OH) F (0<x<1,
y 3+xꢀy
x
0<y<3) also show a dependence on the variation of R (Figure 6b). Generally, all this phenomena
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3+
are attributed to the distortions of the Eu site in the centre of Y(O/F) tricapped trigonal
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3+
prism. In βꢀNaYF crystal structure, the Eu ions locate at the 1a and 1f sites (C symmetry)
4
3h
2
with an inversion centre and 2h sites (C symmetry) without inversion centre at the same time.
s
0
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0
3+
On the other hand, the Eu ions occupy the 2c sites (C symmetry) with an inversion centre in
3
h
3
5, 36
ꢀ
ꢀ
Y(OH) crystal structure.
When F ions are substituted by OH , the revolution of
3
coordination environment of RE ions will cause a possible changing of coordination symmetry,
leading to the variation of R.
CONCLUSIONS
By adjusting the pH of the initial solution, the lattice constants, chemical compositions and 3D
morphologies of monodisperse Na Y(OH) F (0<x<1, 0<y<3) mesocrystals can be controlled
y 3+xꢀy
x
under facial hydrothermal approaches. In this study, the isostructural feature of βꢀNaYF4,
ꢀ
ꢀ
Na Y(OH) F
(0<x<1, 0<y<3) and Y(OH) allow F ions to be substituted by OH ions and
3
x
y
3+xꢀy
+
Na ions to be easily released from the hexagonal tunnels of crystal structure through pHꢀ
mediated topochemical ionꢀexchange. A broad miscibility range for Na Y(OH) F (0<x<1,
y 3+xꢀy
x
0
<y<3) exists by the random isomorphous substitution of ions. The asꢀobtained mesocrystals
with 3D hierarchical architectures are formed by oriented selfꢀassembly of nanoꢀsubunits in a
sideꢀbyꢀside manner. This result provides solid evidence on the topochemical ionꢀexchange
3
+
progress from Ln(OH) to βꢀNaYF . The asꢀprepared RE ꢀdoped Na Y(OH) F (0<x<1,
y 3+xꢀy
3
4
x
0<y<3) mesocrystals exhibit characteristic multicolor emissions. This interesting topochmical
ionꢀexchange provides one valuable synthesis strategy. These Na Y(OH) F (0<x<1, 0<y<3)
x
y 3+xꢀy
mesocrystals based luminescent materials may provide potential applications in other research
fields.
ASSOCIATED CONTENT
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Supporting Information. XRD patterns, TEM images, SAED patterns, and FTꢀIR spectra of the
product, as well as luminescent spectra of the synthesized mesocrystals. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
* Dr. J. Wang: email, wangjiao@fudan.edu.cn
*
Prof. Dr. Z.ꢀJ. Zhang: email, zhangzhijun@shu.edu.cn.
Author Contributions
‡ These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the National Natural Science Foundation of China (No.
51302039), and the Visiting Scholar Foundation of National Key Laboratory of Fundamental
Science of Micro/NanoꢀDevices and System Technology in Chongqing University (No.
2014MS03). All the authors gratefully acknowledge the valuable support of Prof. Ye Tao
and Dr. Yan Huang (Beijing Synchrotron Radiation Facility, Institute of High Energy
Physics, Chinese Academy of Sciences) in experiment.
REFERENCES
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(
1)
Gai, S.; Li, C.; Yang, P.; Lin, J., Recent Progress in Rare Earth Micro/Nanocrystals: Soft
Chemical Synthesis, Luminescent Properties, and Biomedical Applications. Chem. Rev. 2014,
14, (4), 2343ꢀ2389.
2) Xu, Z.; Li, C.; Yang, P.; Zhang, C.; Huang, S.; Lin, J., Rare Earth Fluorides
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(
Nanowires/Nanorods Derived from Hydroxides: Hydrothermal Synthesis and Luminescence
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Table of Contents Use Only
Monodisperse NaxY(OH)yF3+x-y mesocrystals with tunable morphology and chemical
composition: pH-mediated ion-exchange
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Jiao Wang, Bi-Qiu Liu, Gaoshan Huang, Zhi-Jun Zhang, Jing-Tai Zhao
Synopsis TOC
Novel multiꢀcolor luminescent uniform Na Y(OH) F mesocrystals with tunable morphology
y 3+xꢀy
x
and chemical composition are successfully prepared by hydrothermal method through a pHꢀ
mediated ionꢀexchange reaction. Each of the threeꢀdimensional (3D) Na Y(OH) F
y 3+xꢀy
x
mesocrystal architectures is an selfꢀassembly of aligned nanorod subunits in a sideꢀbyꢀside
manner.
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Figure 1. (a) Comparison of XRD patterns of the as-obtained samples obtained under different pH values
(
pH = 12.0 - 13.8, S1 - S13) with the JCPDS for β-NaYF
Y(OH) (No. 24-1422). (b) The corresponding enlargement XRD patterns in the region from 28 to 31.2 .
Figure 1
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(No. 16-0334), Y(OH)1.57F1.43 (No. 80-2008) and
o o
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Figure 2. (a) The dependence of the lattice lengths (a and c) and the rate of c/a of the as-prepared samples
on the pH values. (b) The dependence of chemical composition on the pH values. (c) The dependence of the
lattice lengths (a and c) and the rate of c/a of the as-prepared samples on the contents of fluorine elements
(
F/Y) in host matrix for different samples.
Figure 2
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Figure 3. The crystal structure models of (a) β-NaYF
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(No. 16-0334), (b) sample S2 (see SI), (c) sample S9
(see SI) and (d) Y(OH)
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Figure 3
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