Synthesis and characterization of Gd2Zr2O7 defectfluorite oxide nanoparticles via a homogeneous precipitation-solvothermal method

Article abstract of DOI:10.1039/c7ra11019g

Defect-fluorite structured Gd₂Zr₂O₇ nanoparticles were successfully synthesized via a homogeneous precipitation-solvothermal method using urea as a precipitant. The obtained nanoparticles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) analysis and transmission electron microscopy (TEM). Compared to the traditional solvothermal method, this homogeneous precipitationsolvothermal method has the advantage of producing nanoparticles with small grain sizes, a narrow sizedistribution, high surface areas and little agglomeration. Particularly, the mean crystallite size of Gd₂Zr₂O₇ obtained by this method is 20-30 nm, providing a great opportunity of using these nanoparticles as starting nano-sized building blocks for low temperature preparation of homogeneous and dense ceramics.

Full text of DOI:10.1039/c7ra11019g

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Synthesis and characterization of Gd Zr O defect-
2
2 7
fluorite oxide nanoparticles via a homogeneous
Cite this: RSC Adv., 2017, 7, 54980
precipitation-solvothermal method
Zhe Tang, Zhangyi Huang, Jianqi Qi, *acaXbicaofeng Guo,
ab
ac
de
ab
Wei Han,
ab
a
Mao Zhou, Shuting Peng and Tiecheng Lu*
2 2 7
Defect-fluorite structured Gd Zr O nanoparticles were successfully synthesized via a homogeneous
precipitation-solvothermal method using urea as a precipitant. The obtained nanoparticles were
characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning
electron microscopy (SEM), Brunauer–Emmett–Teller (BET) analysis and transmission electron
microscopy (TEM). Compared to the traditional solvothermal method, this homogeneous precipitation-
solvothermal method has the advantage of producing nanoparticles with small grain sizes, a narrow size-
Received 6th October 2017
Accepted 27th November 2017
2 2 7
distribution, high surface areas and little agglomeration. Particularly, the mean crystallite size of Gd Zr O
DOI: 10.1039/c7ra11019g
obtained by this method is 20–30 nm, providing a great opportunity of using these nanoparticles as
rsc.li/rsc-advances
starting nano-sized building blocks for low temperature preparation of homogeneous and dense ceramics.
small grain sizes, nanostructured materials have been reported
to display enhanced or discrepant properties comparing to
Introduction
12–14
Gd
2
Zr
2
O
7
is one of the most important rare-earth-stabilized coarse-grained counterparts.
Previous studies on RESZ show
zirconia (RESZ) materials due to its potential application as that compared to the conventionally synthesized microcrystal-
a ceramic waste matrix for immobilizing actinides and ssion line structures, nanosized ceramics show enhanced radiation
1,2
products. This crystalline system has a general formula resistance, decreased thermal conductivity and increased
1
5–19
A
2
B
2
O
7
, with one eight-coordinated A site (16c) and one six- conductivity.
So far, there are reports on using wet chemistry
5
coordinated B site (16d) that are capable of incorporating methods, the sol–gel processing, the gel combustion process-
a large amount of radionuclides (e.g. Pu, U, Np, Hf, Th, Am and ing, the co-precipitation method, the hydrothermal method
Cm). Previous studies show that Gd Zr O has a high thermal and others, to prepare Gd
20
21
22
3
6
2
Zr
2
O
7
nanoparticles with different
2
2 7
4–7
stability, high chemical resistance and durability,
which chemical homogeneity and phase assemblage. However, those
suggests that it can serve as a stable, robust and long-lived waste former methods cannot produce nanoparticles with well-
host. In addition, due to the good mechanical properties controlled grain sizes due to the agglomeration as a result of
(
strength and hardness), thermodynamic stability and leaching pH variation.
durability, Gd Zr also nds versatile applications in many
Thus, in this paper, we introduced a facile homogeneous-
elds, such as thermal barrier coatings, solid electrolytes in solvothermal precipitation method to synthesize well-
solid oxide fuel cells, transparent ceramics, photoactive mate- crystallized and dispersed Gd Zr nanoparticles with grain
sizes of 20–30 nm. We demonstrated that this method has
2
2 7
O

2
2 7
O
8–11
rials, etc.
Controls of sizes and morphologies of grains can greatly a better control of the grain size distribution through pH vari-
affect the physical and chemicals properties of material in ation by adjusting the molar ratio of urea. In addition, it
related to its applications. Due to the signicant advantages in provides a more efficient way to synthesis well-dispersed
nanoparticles than other wet chemistry routes.
a
College of Physical Science and Technology, Sichuan University, Chengdu 610064, P.
R. China. E-mail: qijianqi@scu.edu.cn; lutiecheng@scu.edu.cn
Experimental section
b
Key Laboratory of High Energy Density Physics of Ministry of Education, Sichuan
University, Chengdu 610064, Sichuan, P. R. China
In the synthesis experiment, Gd(NO
3 3 2
) $6H O (>99.99%, Ruike
c
Key Laboratory of Radiation Physics and Technology of Ministry of Education,
RE, China), ZrOCl $8H O (>99.99%, Aladdin) were mixed with
2
2
Sichuan University, Chengdu 610064, Sichuan, P. R. China
a stoichiometric ratio of 1 : 1 and dissolved in deionized water.
Ethanol and different amounts of urea were added to the mixed
solution while it was constantly stirring. Aer a fully mixing, the
d
Department of Chemistry, Washington State University, Pullman, Washington 99163,
USA
e
The Alexandra Navrotsky Institute for Experimental Thermodynamics, Washington
ꢀ
State University, Pullman, Washington 99163, USA
solution was heated at 200 C for 24 h in a Teon beaker and
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Fig. 2 XRD patterns of the calcined powders with different concen-
trations of urea.
Fig. 1 The scheme of the synthesis route.
inuence on the formation of the nal products (Fig. 2). At
a precipitate was yielded. The precipitate was then ltered and a lower concentrated level of urea, we observed a weak peak
washed with deionized water and ethanol in turns for several attributed to the (440) reection of Gd O , which was not shown
2
3
ꢀ
times. The reactants diluted by ethanol were dried at 60 C. in the product when the urea molar ratio is 30. Though at a very
Finally, the dry precursors were calcined at 800 C for 2 h. The high urea concentration (molar ratio of 60), the XRD pattern
ꢀ
synthesis scheme is shown in Fig. 1. We compared our synthetic shows no formation of Gd Zr O but Gd O and ZrO instead
2
2
7
2
3
2
samples by the homogeneous precipitation-solvothermal (Fig. 3).
method with those obtained by the co-precipitation method.
The relationship between the urea concentration and the
In the co-precipitation method, the dilute ammonium nal product can be explained by the pH variation. When the
hydroxide was used as the precipitant, which was mixed with temperature of mixed solution increases, the hydrolysis reac-
3
+
4+
the solution containing Gd and Zr . Aer stirring the solution tion takes place and provides hydroxide ions described in eqn
for 60 min and aging for 24 h, the formed gel-like precipitate (1) and (2),
was obtained from the solution through the centrifugal sepa-
ration. Then the precipitate was washed with deionized water
CO(NH
2
)
2
+ H
2
O ¼ CO
2
+ 2NH
3
(1)
(2)
ꢀ
and ethanol for several times and dried at 60 C. Gd
2
ꢀ
Zr
2
O
7
4
3
+
ꢃ
NH + H
3
2
O ¼ NH
+ OH
powders were nally synthesized aer a calcination at 800 C for
h.
The synthetic products were characterized by X-ray diffrac-
2
The hydrolysis reaction in the solution can constantly
provide hydroxide ions with which Gd and Zr are combined,
so that the mixing of Gd and Zr happens in a molecular level
3
+
4+
tion (XRD), Scanning electron microscope (SEM), transmission
electron microscopy (TEM), Infrared spectroscopy (IR), and
Brunauer–Emmett–Teller (BET) surface area measurements.
The quantitative phase analysis was derived from the rene-
ment of the XRD patterns using MAUD Rietveld program.
2 2 7
which is the basis for synthesizing homogeneous Gd Zr O
nanocrystals. Thus, the concentration of hydroxide ions is
critical as it can determine the phases formed in the nal
products. If the concentration of urea is low, the concentration
ꢃ
of hydroxide ions is also low and not enough for OH to be
Results and discussion
2 2 7
The XRD patterns (Fig. 2) of Gd Zr O synthesized by the
homogeneous-solvothermal precipitation method suggest
a good crystallinity. Particularly, when the molar ratio of ure-
3
+
4+
a : Gd : Zr is 30 : 1 : 1, the phase is completely crystallized in
the defect-uorite structure, evidenced by the strong and sharp
peaks resulting from the (111), (200), (220), (311) and (222)
reections. By using the Scherrer equation implemented by
JADE 6, we performed a pattern prole tting that determined
the average crystallite size to be 7.0 ꢁ 0.3 nm which is similar to
2
3,24
that reported by Popov et al. (ꢂ10 nm).
Because of our
powders were well crystallized in small sizes, we can control the
particle size by applying different calcination temperatures to
the preliminary particles. Furthermore, the XRD patterns
Fig. 3 Schematic diagram illustrating the formation and structural
3
+
4+
2 2 7
suggest that the molar ratio of urea : Gd : Zr has an evolution of Gd Zr O nanocrystalline powders.
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3
+
4+
reacted with Gd and Zr . In addition, though theoretically solvothermal process. The mixed solution resulted from the
urea could provide enough hydroxide ions for reactions with homogeneous precipitation was heated in a thermostatted oil
3
+
4+
ꢀ
Gd and Zr , the hydrolysis reactions described in eqn (1) and bath that maintained at 120 C for 2 h with a constant stirring.
2) are in dynamic equilibrium in a hermetic reactor, which Then the suspension was cooled and the white precipitate was
(
could result in an insufficient concentration of hydroxide ions. washed multiple times with deionized water and ethanol. Then
ꢀ
For instance, no precipitate is formed when the molar ratio of the precursors were dried in vacuum at 60 C and then calcined
3
+
4+
ꢀ
urea : Gd : Zr is lower than 2 : 1 : 1. As the concentration of at 800 C for 2 h. The XRD patterns of the products are given in
urea increased, the content of hydroxide ions is also increased Fig. 4. It's clear that without a solvothermal process, the
as seen from eqn (1) and (2). Aer the molar ratio of urea rea- synthesized powder is pure ZrO
2
. Numbers of studies show both
ches 3.5, the solution contains enough hydroxide ions to be A B O oxides and rare earth oxides can be synthesized by
2
2 7
3
+
4+
combined with Gd and Zr (Fig. 2) that yielded phases consist homogeneous precipitation method using urea as precipi-
25,26
of a defect-uorite phase Gd Zr O as the main phase with tant.
Nevertheless, due to the low-temperature calcination,
as a secondary phase. We found out that using only homogeneous precipitation method could lead to
the amount of the impurity (Gd ) decreases as the mole ratio oxides mixed with BO crystalline phases and perhaps
of urea increases up to certain values; and at a proper mole rate poor crystallized AO
30 : 1 : 1), we produced pure Gd Zr
2
2 7
2 3
a tiny amount of Gd O
2
O
3
A
2
B
2
O
7
2
2
or A-containing polymers, partly due to
ꢀ
(
2
2
O
7
in defect-uorite struc- the closed formation enthalpies of these phases at 800 C.
ture (Fig. 2). However, on the other hand, if the concentration of Similar phenomena were observed in other complex oxide
2
7,28
urea is too high, such as 60 : 1 : 1, the hydroxide ions would systems, such as garnet.
During the solvothermal process,
3
+
4+
ꢃ
segregate Gd and Zr , and react with them separately to form Gd Zr O was formed because of more OH was produced by
2
2 7
Gd(OH) and Zr(OH) instead of (Gd, Zr) hydroxide that even- the hydrolysis of urea with the aids of high temperature and
3
4
3
+
4+
tually leads to products as Gd
2
O
3
and ZrO
2
(Fig. 2).
high pressure. Compared with Gd , Zr can be precipitated at
The hydrolysis reactions described by eqn (1) and (2) are lower pH value. With the absence of solvothermal process, only
4
+
3+
known as homogeneous precipitations that produce a homoge- Zr , rather than Gd , was precipitated because of the relatively
neous precipitation of (Gd, Zr) hydroxide as described in eqn low pH value provided by the insufficient hydrolysis of urea,
22
(
2
3). Then the next step occurred under heating is the sol- giving rise to the formation of the precursor of ZrO . Thus, our
vothermal process, during which the added ethanol can effec- homogeneous precipitation-solvothermal method was proved
ꢀ
tively break up the aggregates in the (Gd, Zr)-oxides at 200 C, to be a new feasible route for synthesizing pure phase Gd Zr O
2
2
7
and the alkoxyl group can substitute hydroxyl group as as a base for various functional nanomaterials.
described in eqn (4),
ꢃ
ZrOCl
2
+ Gd(NO
3
)
3
+ OH / (Gd, Zr)-hydroxide
(3)
(
Gd, Zr)-hydroxide + ROH / Gd Zr O nanocrystal + mH O
2 2 7 2
(4)
In this homogeneous precipitation-solvothermal method,
the solvothermal process is the key step for synthesizing pure
defect-uorite phase Gd Zr O nanocrystals. As a comparison,
2
2 7
we performed
a co-precipitation synthesis without the
Fig. 5 IR absorption spectra of calcined powders with different
ꢃ1
ꢃ1
Fig. 4 XRD patterns of calcined powders without the solvothermal concentrations of urea (a) wavenumber between 400 cm
process.
and
ꢃ1
ꢃ1
4000 cm ; (b) wavenumber between 400 cm and 600 cm .
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powders synthesized by these two methods (Fig. 6). Fig. 6(b)
shows large particles of pure defect-uorite nanocrystalline
phase Gd Zr O powders prepared by the homogeneous
2
2 7
precipitation-solvothermal method with urea mole ratio as 30.
The morphology suggests well-dispersed and homogeneous
features. While in Fig. 6(a), particles prepared by the co-
precipitation method show elevated agglomerations that
grains are interconnected with each other to form larger micro-
structures that have irregular morphologies and porous
networks. Compared to co-precipitation method, our new
method produces Gd Zr O powders with a better dispersity
Fig.
6 SEM of powders synthesized by (a) the co-precipitation
method; (b) the homogeneous precipitation-solvothermal method.
2
2 7
and a low level of aggregation. Furthermore, the particle sizes of
samples synthesized by our method estimated by Nano
Measurer 1.2 are in a relatively-narrow particle-size distribution
within the range of 20–30 nm with a small mean particle size as
IR absorption spectra are used to determine the site prefer-
ences of Gd and Zr in Gd Zr O as it is sensitivity to the change
2 2 7
of the local structure resulting from distortion of polyhedra and
element substitutions. The IR spectra of samples prepared by
the homogeneous precipitation-solvothermal method with
different concentrations of urea are presented in Fig. 5.
According to former studies, the infrared spectra of A B O
2 7
2
8.7 ꢁ 0.8 nm (shown in Fig. 7).
TEM micrographs of our synthetic samples suggest the
nature of nanocrystalline as evidenced in Fig. 8. It is concluded
by the morphological image (Fig. 8(a)) that Gd Zr O powders
2
2 7
2
are well-dispersed nanoparticles without tight aggregations.
The crystal structure is conrmed by selected area diffraction
29,30
compounds contain seven IR-active optic modes.
Fig. 5(a)
shows the IR absorption spectra of powders with different
(SAED) to be defect-uorite shown in Fig. 8(b). The crystallite
ꢃ1
concentrations of urea in the wave range of 400 cm
to
size determined by HRTEM (Fig. 8(c)) is ꢂ7 nm in agreement
ꢃ1
4
000 cm . There are several characteristic absorption bands at
ꢃ1
ꢃ1
ꢃ1
ꢃ1
about 436 cm , 540 cm , 1380–1630 cm , and 3500 cm . It
has been known that the appearances of the band centered at
ꢃ1
ꢃ1
approximately the 1380–1630 cm and 3500 cm bands are
evidence of the existence of water molecules attached to the
powders. In Fig. 5(b), the spectrum of the sample with the mole
ꢃ1
ratio of urea as 60 shows peaks in the range of 400 cm to
00 cm , from which the peak at 540 cm is attributed from
Zr–O stretching modes, and that at 436 cm is caused by the O–
Gd–O bending modes. These are another evidence besides XRD
ꢃ1
ꢃ1
6
ꢃ1
2 3 2
that show the presence of Gd O and ZrO when the urea is high
concentrated during the homogeneous precipitation-
solvothermal synthesis.
In order to understand the micro-morphologies and micro-
structures of synthetic products made by the homogeneous
precipitation-solvothermal method as compared to co-
2 2 7
precipitation method, we performed SEM on Gd Zr O
2 2 7
Fig. 7 The particle size distribution of Gd Zr O nanocrystal powders Fig. 8 (a) TEM image of morphology, (b) SAED pattern, (c) and (d)
synthesized by the homogeneous precipitation-solvothermal method. HRTEM images of Gd Zr O powders.
2 2 7
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Gd
2
Zr
2
O
7
-based functional nanomaterials and other uorite-
oxide nanocrystals (like La Zr O ).
2
2 7
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China under Grant No. 11505122 and Grant No.
91326103, the ITER Program (No. 2014GB125002).
Fig.
2 2 7
9 Nitrogen adsorption–desorption isotherms of Gd Zr O
nanocrystalline powders prepared by the homogeneous precipitation- References
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3
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6
7
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2
2 7
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