Synthesis of monolithic zirconia aerogel via a nitric acid assisted epoxide addition method

Article abstract of DOI:10.1039/c4ra04601c

Monolithic zirconia aerogels were prepared using zirconium oxychloride (ZrOCl₂) as the precursor in the presence of the epoxide (PO) in both ethanol and mixed ethanol-water solutions, respectively. The effects of nitric acid and water contents on the gel formation behavior and the properties of the resulting aerogel were investigated. The results show that at a molar ratio of HNO₃/ZrOCl₂ of 1.0 and H₂O/ZrOCl₂ of 10, the translucent monolithic ZrO₂ aerogels were obtained and they exhibited a well-developed mesoporous structure and a high specific surface area of 454 m² g^-1. The as-prepared aerogels were composed of mainly amorphous phase and a spot of ZrO₂ crystal. After calcination at 750°C for 2 h, the ZrO₂ transformed into tetragonal phase. These results suggested that this HNO₃-assisted epoxide addition method was a simple and controllable way to get translucent monolithic zirconia aerogels.

Full text of DOI:10.1039/c4ra04601c

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Synthesis of monolithic zirconia aerogel via a nitric
acid assisted epoxide addition method
Cite this: RSC Adv., 2014, 4, 31666
a
a
a
a
b
Liang Zhong, Xiaohong Chen,* Huaihe Song, Kang Guo and Zijun Hu*
2
Monolithic zirconia aerogels were prepared using zirconium oxychloride (ZrOCl ) as the precursor in the
presence of the epoxide (PO) in both ethanol and mixed ethanol–water solutions, respectively. The effects
of nitric acid and water contents on the gel formation behavior and the properties of the resulting aerogel
were investigated. The results show that at a molar ratio of HNO
translucent monolithic ZrO aerogels were obtained and they exhibited a well-developed mesoporous
structure and a high specific surface area of 454 m
mainly amorphous phase and a spot of ZrO
transformed into tetragonal phase. These results suggested that this HNO
method was a simple and controllable way to get translucent monolithic zirconia aerogels.
3 2 2 2
/ZrOCl of 1.0 and H O/ZrOCl of 10, the
2
Received 16th May 2014
Accepted 15th July 2014
2
ꢀ1
g . The as-prepared aerogels were composed of
ꢁ
2
crystal. After calcination at 750 C for 2 h, the ZrO
2
DOI: 10.1039/c4ra04601c
3
-assisted epoxide addition
www.rsc.org/advances
epoxides undergoes irreversible ring opening. The metal oxide
1
. Introduction
ꢀ
gels prepared with bases like OH and NH are precipitation
3
Mesoporous zirconia (ZrO
2
) has attracted a lot of attention in due to the rapid reaction rate of the base with hydrated metal
15,16
recent years because of its unique structures and wide potential species.
applications in catalysts, catalyst supports and solid oxide induce the formation of precipitation immediately. With time
However, the epoxide is not a strong enough base to
1–3
4,5
6
fuel cells (SOFCs). Thus, extensive effort has been made to goes, the epoxides consume smoothly the counterion and the
synthesize mesoporous zirconia. According to the types of proton hydrate in the metal salt solvent. This allows the
precursors, it is convenient here to divide these preparation uniform formation of oligomers, which link together to be sol
7
–9
approaches for ZrO into two broad groups: alkoxide and non- particles and then lager clusters that subsequently cross-link to
2
6,17
alkoxide routes. In alkoxide process, not only are the alkoxides form a homogeneous gel. Christopher et al.
synthesized
difficult to prepare and handle, but also the sol–gel process of yttria-stabilized zirconia aerogel using the “epoxide addition”
alkoxides are hard to control. Therefore, researchers explored method. Using the epoxide (PO) as the gel initiator is more
lots of methods using non-alkoxides as precursor, such as convenient to get a higher specic area aerogel than using the
1
0
11
alcohol-thermal method, alcohol–water-thermal method,
oxetane. However, the gelation occurred rapidly. It only took
12
and the electrolysis method. The non-alkoxides used as several minutes in pure water system while just a few seconds in
precursors are cheaper and more manageable than alkoxides. alcohol–water system. The gel time is too short to provide a
However, their resultant products are usually in forms of chance for preparing aerogel composite materials via this
powders or highly cracked monoliths. In 2001, Gash et al. approach. In addition, under high temperature supercritical
reported a new sol–gel route to synthesize several transition and drying the as-prepared aerogel is coarse and dense. Therefore, it
main-group metal oxide aerogels using epoxides as proton is a great challenge to control the process of gel–sol and inte-
13
scavenger from inorganic salts in polar protic solvents. The grate the mesoporous structures into a ZrO
mechanism of proton-scavenging was reported to occur in two epoxide addition method. Much less work has been devoted to
steps: (1) protonation of the epoxide oxygen by the hydrolysis of synthesize the monolithic ZrO aerogel using this process.
hydrated metal species and (2) nucleophilic attack of the
Herein, we present a controllable strategy for the preparation
2
monolith using
2
14
protonated ring by the nucleophilic anionic. As a result, the of translucent and monolithic zirconia aerogel without any
stabilizer via the epoxide addition method by adjusting the water
and acid contents. The process of gel–sol can be easily controlled
a
State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of
by the molar ratio of HNO /ZrOCl and H O/ZrOCl . The gel time
3
2
2
2
Electrochemical Process and Technology for Materials, Beijing University of
Chemical Technology, Beijing, 100029, P. R. China. E-mail: chenxh@mail.buct.edu.
cn; Fax: +86-010-64434916; Tel: +86-010-64434916
can be largely prolonged, which helps to form a more inter-
connected network and also provides more chance of preparing
aerogel composites. Additionally, the as-prepared zirconia aero-
b
National Key Laboratory of Advanced Functional Composite Materials, Aerospace
Research Institute of Materials & Processing Technology, Beijing 100076, P. R. gel displayed a well-developed mesoporous structure and a high
China. E-mail: huzijun@hotmail.com; Fax: +86-010-68755517; Tel: +86-010-
6
8755517
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2
ꢀ1
specic surface area of 454 m
g
. Besides, the metastable varied from translucent to opaque depending on the reaction
ꢁ
tetragonal ZrO can be obtained aer calcinated at 750 C for 2 h. conditions. Assisted by the proton-scavenging properties of
2
epoxide, the hydrated metal species are induced to undergo
hydrolysis and condensation reactions to from small oligomers.
Then, these oligomers grow to be sol particles and lager clusters
in sequence. Finally, the gels with interconnected network are
produced. Table 1 shows the effects of the molar ratio of acid to
zirconium (N) and water to zirconium (H) on the gel time and
2
. Experimental sections
2
.1 Synthesis of ZrO
ZrO -precursor gels were prepared by the addition epoxide
method followed by supercritical drying in ethanol. Zirconium
oxychloride octahydrate (ZrOCl $8H O), propylene oxide (PO),
and HNO (65% w/w) were used as received. Two beakers were
prepared: one with ZrOCl $8H O, ethanol, H O and HNO ; the
2
Aerogel
2
2
the characteristic of ZrO gels. The gel time (tgel) is the interval
2
2
between addition of the epoxide and the point at which the sols
no longer ow when the reaction containers are tilted.
With the N value increasing from 0 to 1.2, the tgel raise form 5
3
2
2
2
3
other with ethanol and PO. The total molar ratio of ZrOCl , H O,
HNO and PO was kept constant at 1 : 10 : 1 : 6. The two solutions
2
2
2
s to 45 min as well as the ZrO gels vary from opaque to trans-
3
lucent. Basically, the formation of gels lies on the hydrolysis and
successive condensation of oligomers. In the acidic condition,
the rate of the sequential condensation can be effectively
inuenced by the acid concentration, because these reactions
are positively charged and stabilized by the repulsive electro-
static interaction in a fashion similar to that observed for
were mixed and vigorously stirred until the vortex created by the
stirring disappeared owing to gelation. Furthermore, a water bath
ꢁ
(
20 C) was needed in the process of hybrid. Aer 24 h aging, the
alcogels were washed with ethanol every 24 h and renewed 2–3
times in order to make sure the solvents in the gel network were
fully replaced by ethanol. The resultant alcogels were then
supercritically dried by ethanol in an autoclave. The autoclave was
pressurized to 2.5 MPa with purity nitrogen. Then, the tempera-
ture was heated to 538 K, while the pressure was controlled at 7.2
MPa. Aer keeping this supercritical state for 1 h, the uid was
slowly released to atmospheric pressure. When the autoclave was
cooled to room temperature, the resulting zirconia aerogel was
obtained. To determine the role of the acid in gel formation for
this system, zirconia aerogels were prepared from varying acid
content using a similar way described above. The aerogels with
18,19
colloidal particles-dispersed system.
Therefore, with the use
of higher acid concentration, the rate of condensation can be
obviously slowed down. What is more, the relative hydrolysis
and condensation rates play an important role on the different
20
wet-chemical characteristics of the sol–gel products. When N
is 0.6, fast relative rates of hydrolysis and condensation gener-
ally lead to white-opaque appearance, indicating the presence of
localized condensation. If the N value increased, i.e., for N ¼ 1,
fast hydrolysis with low condensation yield translucent poly-
21–23
meric alcogels with
a
well cross-linked network.
varied acid contents are noted as N and X (¼0, 0.6, 1.0, 1.2) is the
x
The obtained gels are translucent and bouncy. However, when
N ¼ 1.2, the resulting gels are easy to shrink and crack, illus-
trating that excessive consumption of epoxy by nitric acid
results in that clusters do not grow large enough to connect to
become an extended network. Moreover, at much higher value
of N, gelation will be completely suppressed. These gelation
molar ratio of acid to zirconium. Similarly, the aerogels with
varied water contents are called H
x
and X (¼2, 10, 20) is the molar
ratio of water to zirconium. The resulting aerogels were calcined
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
at 350 C, 450 C, 550 C, 750 C, 850 C, and 1000 C in air for 2 h
ꢁ
ꢀ1
with a heating rate 5.0 C$min , respectively.
24
behaviors are similar with the result of Sul et al. who prepared
the alkoxide-drived gels under the different acid concentration.
Likewise, the effects of the molar ratio of water to zirconium
2.2 Physical Characterization
The morphologies and structures of the samples were investi-
gated via transmission electron microscope (TEM, Hitachi H-
(H) on the gel time and the appearance of gels are also inves-
800) and high-resolution transmission electron microscope
tigated by varying the value of H from 2 to 20. To avoid the effect
of acid on the gelation behavior, the value of N is kept constant
at 1.0 in this process. As shown in Table 1, the tgel raises from 16
to 40 min with the value of H increasing from 2 to 20. The
(HRTEM, JEOL JEM-2100). Nitrogen sorption isotherms were
measured with ASAP 2020 (Micromeritics, USA), and the
ꢁ
sample was degassed at 150 C overnight. The specic surface
area and pore size distribution were calculated by the Bru-
nauer–Emmett–Teller (BET) method and Barrett–Joyner–
Halenda (BJH) method, respectively. The products were char-
acterized by X-ray powder diffraction (XRD) on a Rigaku D/max-
2
Table 1 Summary of synthetic conditions for synthesis of ZrO Gels
(
C ¼ 0.39 mol$Lꢀ , Repox ¼ 6)
1
a
2
500B2+/PCX system operating at 40 kV and 20 mA with Cu Ka
ꢁ
ꢁ
radiation (l ¼ 1.5406 A , 2q ¼ 10–90 ). Raman spectroscopy was
b
c
Sample
N
H
tgel (min)
Morphology
carried out using a 785 nm laser (Aramis, Jobin Yvon).
H
H
N0
2
1
1
0
0.6
1.0
1.2
0
20
10
10
10
10
16
40
ꢂ5s
2.5
25
Opaque white
Translucent white
Opaque white
Opaque white
Translucent white
Translucent white
20
3. Results and discussion
N
N
N
0.6
3
.1 Effects of reaction parameters on the formation of ZrO
gels
Addition of epoxide (PO) to ethanol or aqueous ethanol solu-
tions of ZrOCl
2
1.0 (H10
1.2
)
45
a
b
The molar ratio of epoxide to zirconium. The molar ratio of acid to
c
2
resulted in the formation of monolithic gels that zirconium. The molar ratio of H O to zirconium.
2
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transparency of gel is also varied from opaque to translucent
The TEM images of aerogel N0.6, N1.0 and H
2
are shown in
with the increase of the value of H. The increase of gel time may Fig. 2a–c. The degree of network and size of the cluster varied
ꢀ
be a result of the reduced nucleophilicity of the counterion (Cl ) depending on the contents of nitric acid and water. The opaque
in water versus ethanol, leading to a slower condensation reac- aerogel N0.6 prepared with fast reaction conditions (Fig. 2a)
25
tion. Baumann et al. also attributed this reasoning to similar shows less network and larger clusters than the translucent
changes for the epoxide-initiated gelation of AlCl $6H O in aerogel N1.0 prepared with relatively slow reaction conditions
3
2
ethanol versus water. Besides the nucleophilicity of the coun- (Fig. 2b). This microscopy results support the proposal, set
terion, the dielectric constants of reaction media should also be earlier, that nitric acid suppresses the presence of localized
considered. It has been recognized that the dielectric constants condensation resulting in small clusters and ne skeleton
ꢁ
26
of water and ethanol are 80.4 and 25.0 at 20 C, respectively.
formation, which also can been conrmed by SEM image shown
The dielectric constants of mixed water–ethanol solution are in Fig. 2f. Aerogel H10 has similar cluster sizes with N0.6. This
higher with more water. A higher dielectric constant corre- shows that high nucleophilicity of the counterion and low
sponds to a higher solubility, which predicts less clusters solubility of sol particles in pure ethanol result in the greater
numbers and lower growing rates of clusters. This is similar clusters growth. Fig. 2d and e provides a ne representation of
with the “nucleation” and “growth” of crystal in crystallography. the size, shape, connectivity of the clusters. It appears that these
It seems to provide another reasonable explanation for why the particles are relative spheres with a diameter in the 5–10 nm
1
3
2
gel time of H is shorter than that of H20. Gash et al. used this range. They seem to connect to each other to form the lager
hypothesis to explain the effect of different solvent on the gel clusters, in further, to form the network of aerogels. Moreover, it
time, when they prepared Cr O aerogel by epoxide addition is revealed crystallization has occurred in the skeleton structure
2
3
method. Above all, the gel time and the appearance of gels can of aerogel N1.0. The crystallization size was measured to be
be controlled by acid concentration and solvent employed.
about 5 nm.
2
3.2 Morphologies and structures of ZrO aerogels
Fig. 1 shows the photographs of typical zirconia aerogels.
Although the obtained aerogel N0.6 is opaque monolith, it is
easy to grind to ne powders. The aerogel N1.0 is translucent
and relatively rigid monolith, which is not easily crushed by
hand. The aerogel N1.2 is cracked and broke apart, indicating
the networks of gel are destroyed during the aerogel drying
2
process. Beside all that, aerogel H are cracked opaque
monoliths.
2
Fig. 2 TEM images of zirconia aerogels: (a) N0.6, (b) N1.0, (c) H , (d)
Fig. 1 The photographs of zirconia aerogels: (a) N0.6, (b) N1.0, (c) N1.2 HRTEM of N1.0 and (e) enlarged HRTEM image of the selected area
and (d) H marked by a box in (d); (f) the SEM image of N1.0
2
.
.
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.3 Surface Area and pore structure of ZrO
Fig. 3a shows nitrogen adsorption/desorption isotherms of the
aerogels N , N0.6, N1.0, and N1.2, respectively. All samples exhibit
hysteresis at relatively high P/P , which can be classied as Type
2
aerogels
prepared aerogels. The large specic surface area and large
pore volume are attributed to the absence of localized
condensation, resulting in more branched networks. This is
supported by the TEM data, discussed earlier. Furthermore, the
mesopore fraction of N1.0 has the maximum value of 97.1%, and
that of N0.6 is 66.2%, which suggests more mesopores are
formed with higher acid concentration. However, continuing to
increase N to 1.2, the specic surface area and pore volume are
0
o
IV according to the IUPAC nomenclature. The presence of hys-
terisis loop, caused by the capillary condensation in the meso-
pores (2–50 nm), suggests the resulting aerogels are
27
mesoporous materials.
2
ꢀ1
ꢀ1
406 cm
g
and 1.5 ml g , respectively. In addition, the
The BJH pore size distributions of N , N0.6, N1.0, and N1.2 are
0
mesopore fraction of N1.2 decreases to 54.7%. These decreases
may result from the formation of a weakly cross-linked structure
by clusters, which is easy to collapse and agglomerate.
The water content was also an important factor affecting the
pore structures. As shown in Fig. 3b, the as-prepared aerogels
also display a Type IV isotherm with a closed desorption branch
hysteresis loop, which is the typical characteristic of mesopore.
The trend of most probable pore size for varying acid concen-
tration is similar with that for varying water content. The
translucent aerogel H10 have a considerably higher surface area
present in the insert of Fig. 3a. The most probable pore size is
nearly identical for the N and N0.6. However, the most probable
0
pore size is found to shi towards larger pore size with higher
acid concentration. The aerogels N0.6, N1.0 and N1.2 have pore
size distribution centered at 14.8 nm, 18.3 nm and 31.6 nm,
respectively. With higher acid concentration, highly branched
network is easier to be formed by clusters. Presumably the more
branched the gel network is, the larger interconnected clusters
may form larger pore. The surface area, pore volume and mes-
opore fraction for the zirconia aerogels are summary in Table 2.
The aerogel N1.0 exhibits the maximum specic surface area
than opaque aerogel H , which suggests water, improving the
2
2
ꢀ1
ꢀ1
solubility of reaction media and reducing the nucleophilicity of
the counterion, slowing down the growth of clusters. Thus, the
pore structures of as-prepared aerogels are tailored via
addressing the nitric acid and water contents.
(454 m g ) and pore volume (1.8 ml g ) among all the as-
3.4 Effects of heat treatment on the structures of ZrO2
aerogel
As well known, calcination can result in the growth and trans-
formation of zirconia crystallite. To investigate the effect of
temperature on the crystallographic evolutions for as-prepared
zirconia, XRD patterns of zirconia N1.0 calcinated at 350–
ꢁ
ꢁ
ꢀ1
1
000 C in air for 2 h with a heating rate 10 C min , respec-
tively, and the results are given in Fig. 4a.
It can be seen that aerogel N1.0 without calcination has broad
diffraction pattern which is characteristic of amorphous
zirconia. However, there is a weak diffraction peak at approxi-
ꢁ
mately 50 , indicating that a spot of crystallization has occurred
21
during supercritical drying. This is consistent with the
HRTEM results discussed earlier. With the increase of the
calcination temperature, the broad diffraction peak become
sharpened gradually, reecting an evidently crystallite growth of
Table 2
2
N Adsorption–desorption results as a function of reaction
condition for As-prepared aerogels
SA BET
(m g
Pore volume
Mesopore
fraction (%)
2
ꢀ1
ꢀ1
a
Sample
)
(ml g
)
N
N
N
N
H
H
0
113
226
454
407
185
406
0.5
0.6
1.8
1.5
0.5
2.5
84.1
66.2
97.1
54.7
66.7
59.5
0.6
1.0 (H10
)
1.2
Fig. 3 Nitrogen adsorption–desorption isotherms for ZrO
a) N , N0.6, N1.0, and N1.2. The insert in (a) shows the pore size distri-
butions of N , N0.6, N1.0, and N1.2; (b) H , H10 and H20.The insert in (b)
shows the pore size distributions of H , H10 and H20
2
aerogels:
2
(
0
20
0
2
a
2
.
The ratio of the pore volume of pores mesoporous (2–50 nm) to totally
pore volume of pores between 1.7 nm and 300.0 nm width determined
by BJH method.
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values of tetragonal (t) phase (PDF #50-1089). Thus, it can be
deduced that the t-phase zirconia is obtained. However, as
shown in Fig. 4b, the XRD pattern for standard cubic (c) and
tetragonal (t) phase ZrO are similar and no splitting is observed
2
at the aforementioned 2q. So the assignment of cubic and
tetragonal phases based solely on the XRD is very difficult. It has
been well documented that Raman spectroscopy, sensitive to
polarizability of the oxygen ions, is recognized as a powerful tool
for identifying different polymorphs of metal oxides. According
to group theory, monoclinic (m-ZrO ), tetragonal (t-ZrO ) and
2
2
cubic (c-ZrO ) phase of zirconia are expected to have 18(9A +
2
g
9
Bg), 6(1A1g + 2B1g + 3E ) and 1T Raman active modes,
g 2g
28–32
respectively.
Therefore, Raman spectroscopy can easily
distinguish between cubic phase and tetragonal. As shown in
Fig. 5, the Raman spectrum have bands at 181, 265, 317, 470,
ꢀ
1
ꢁ
and 641 cm for zirconia N1.0 calcinated at 750 C, which are
assigned to the Raman-active modes for the tetragonal phase of
ZrO2.
28,30,33
ꢀ1
However, the single Raman peak at 490 cm for
cubic zirconia (ref. 33) does not appear. Thus, the metastable
t-phase below the m-t transition temperature (approximately
ꢁ
1
150 C) is obtained when the as-prepared zirconia aerogel is
heated. Fig. 4c shows the XRD pattern of as-prepared products
ꢁ
at 850 C. This gure indicates that a mixture of monoclinic (m)
and tetragonal (t) of zirconia is obtained. The Raman spectrum
(
5
8
Fig. 5) has bands at 177, 189, 220, 305, 331, 345, 378, 475, 497,
ꢀ
1
34, 561, 615, and 634 cm for zirconia N1.0 calcinated at
ꢁ
50 C, which are approximately the same as reported by the
3
4–36
Fig. 4 XRD patterns of aerogel N1.0 calcinated at different tempera- previous works.
ture: (a) calcinated at 350–1000 C, respectively; (b) aerogel N1.0 modes of m-ZrO
They can be identied to Raman active
ꢁ
ꢀ1
2
. In addition, the band at 256 cm can also be
ꢁ
calcinated at 750 C. The XRD pattern for standard cubic and tetrag-
are shown as lines and points with different colors in (b); (c)
aerogel N1.0 calcinated at 850 C. The XRD pattern for standard
ꢁ
observed for zirconia N1.0 calcinated at 850 C, illuminating the
presence of t-ZrO
onal ZrO
2
. This agrees with the XRD result. When
ꢁ
2
ꢁ
calcination temperature was increased to 1000 C (Fig. 4a), the
amount of m-phase is considerably increased. In addition, as
observed the selected area marked by a box in Fig. 4a, the
intensity of peak corresponding to t-phase increases propor-
tionally with that of m-phase, increasing the calcined temper-
ature from 850 C to 1000 C, which indicates that the m-phase
and t-phase ZrO may be formed simultaneously at a relatively
high temperature.
2
monoclinic ZrO is also shown as lines and points.
ꢁ
ꢁ
2
4. Conclusions
Zirconia monolithic aerogels had been successfully prepared
via the epoxide addition method by adjusting the solvent and
acid content. The opaque gels are always obtained at a lower
molar ratio of acid to zirconium and water to zirconium, while
with the increase of the acid and water contents, the obtained
ꢁ
ꢁ
Fig. 5 Raman spectra of aerogel N1.0 calcined at 650 C, 750 C, and
3 2
gels are translucent. When the HNO /ZrOCl molar ratio is 1.0,
ꢁ
8
50 C, respectively.
the gel time is prolonged to 25 min, which is not only helpful for
forming a stable gel with an extended network but also allows
for preparation of aerogel composite materials. However, over-
much acid and water contents decrease the strength of gels
structure leading to the occurrence of cracks and shrinkage
during supercritical drying process. Additionally, the acid and
water contents were found to be critical factors in the
morphology and microporous structure of the resulting aero-
the zirconia samples. Moreover, several new weak diffraction
peaks are also observed, and these peaks are sharpened
constantly with the increase of calcination temperature. It can
be seen in Fig. 4b that the major peaks are at 30 , 35 , 50 , and
0 with weak peaks at 62 , 74 , and 82 for the zirconia sample
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
6
ꢁ
calcined at 750 C. These diffraction peaks are very close to the
3 2 2 2
gels. At a molar ratio of HNO /ZrOCl of 1.0 and H O/ZrOCl of
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0, the resultant ZrO
RSC Advances
1
2
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developed mesoporous structure and a high specic surface
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the molar ratio of HNO /ZrOCl was 1.2), the as-prepared aer- 18738–18743.
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ꢀ1
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (51202009 and 51272016), New Teachers'
Fund for Doctor Stations, Ministry of Education of China
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