Phase-controlled synthesis and gas-sensing properties of zinc stannate (ZnSnO3 and Zn2SnO4) faceted...
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Phase-controlled synthesis and gas-sensing properties of zinc stannate(ZnSnO3 and Zn2SnO4) faceted solid and hollow microcrystals
Guanxiang Ma, Rujia Zou, Lin Jiang, Zhenyu Zhang, Yafang Xue, Li Yu, Guosheng Song, Wenyao Liand Junqing Hu*
Received 26th September 2011, Accepted 25th November 2011
DOI: 10.1039/c2ce06272k
Well-defined faceted zinc stannate, including cubic ZnSnO3 and octahedral Zn2SnO4, microcrystals
were synthesized in a large scale by a one-step chemical solution route, in which the phase control was
simply accomplished by only changing stannic precursors. These faceted cubic ZnSnO3 and octahedral
Zn2SnO4 microcrystals are easily converted to faceted hollow structures with a shape preserved through
an acid etching process. Possible growth and etching mechanisms of these faceted microcrystals have
been proposed. The hollow structures of zinc stannate were exploited as gas sensors and exhibit
improved sensing performances to a series of gases (especially with regard to H2S and C2H5OH);
moreover, the sensitivity and recovery time of Zn2SnO4 hollow octahedral structures to H2S and
C2H5OH are both higher than those of the cubic structures, which may find potential industrial
applications in detecting gases.
1. Introduction
Complex metal oxides are an important class of functional
materials that exhibit a wide range of properties, such as
magnetism, superconductivity, catalysis, and lithium intercala-
tion. However, phase control of these oxides with a desired
composition is still a challenge owing to their various stoichi-
ometries (e.g. typical formulas of ABO3, A2BO4 and AB2O4) and
the associated complex structures.1,2 In recent years, considerable
efforts have been devoted to the synthesis of these complex metal
oxides. But the phase or stoichiometry of those complex metal
oxides is not well controlled. Also, more effort is needed to reveal
their phase evolution and formation mechanism, which is much
important for addressing their properties and technological
potentials with a specific phase.3,4 Zinc stannate, a multifunc-
tional material, exists as two typed oxides with a different Zn/Sn/
O ratio and crystallographic structure: the orthorhombic
ZnSnO3 and spinel-type cubic Zn2SnO4,5,6 and has potential
applications in gas sensing,7–10 photocatalysis,11,12 photo-
conductors,13 lithium ion batteries14 and dye-sensitized solar
cells.15,16 So far, considerable interest has been focused to
synthesize zinc stannate material, including ZnSnO3 and
Zn2SnO4. Some zinc stannate micro-/nanostructures, including
nanowires/nanorods,17 nanocubes,18 micro-spheres,19,20 faceted
crystals,8 and so on, have been synthesized. However, the two
phases of ZnSnO3 and Zn2SnO4 materials have been synthesized
by different and separate routes. For example, Fang11 et al. have
State Key Laboratory for Modification of Chemical Fibers and PolymerMaterials, College of Materials Science and Engineering, DonghuaUniversity, Shanghai, 201620, China. E-mail: [email protected]
2172 | CrystEngComm, 2012, 14, 2172–2179
prepared ZnSnO3 nanowire architectures by using fructose as
a molecule template; Zhu17 et al. have prepared Zn2SnO4 nano-
rods by a hydrothermal process using hydrazine hydrate as an
alkaline mineralizer; Wang9 et al. and Ji21 et al. have prepared
ZnSnO3 cubic crystals and Zn2SnO4 octahedron structures
assembled with some intercrossed hexagon nanoplates via
hydrothermal reactions, respectively. However, in the above
routes, the synthetic conditions for ZnSnO3 and Zn2SnO4
materials are independent, i.e., the phase-controlled synthesis of
ZnSnO3 and Zn2SnO4 materials including well-defined faceted
crystals through a simple synthetic route by only changing
reactions conditions, e.g., stannic precursor, has not yet been
simultaneously accomplished. As we know, different preparative
methods have important effects on the phase, structure, and
properties of materials. A facile solution chemical synthetic route
has been often utilized to allow the phase, composition and
morphology to be controlled.
Compared with the solid counterparts, the fantastic hollow-
structured micro-/nanomaterials possess characteristics such as
low density, high surface-to-volume ratio, and low coefficients of
thermal expansion that enable them broad applications in
sensors,22 catalysis,23 Li-ion batteries,24 biomedicines,25 and
many others.26,27 Various strategies have been employed to
synthesize different hollow micro-/nanostructures, such as hard
templating synthesis,28 soft templating synthesis,29 and template-
free methods.30,31 However, mostly of these approaches
mentioned here described the synthesis of hollow spherical
structures, and methods to synthesise non-spherical or faceted
hollow structures are relatively few. Recently, Jiang32 et al. have
prepared Pt/ZnSnO3 polyhedral hollow structures by a simulta-
neous reduction-etching route; Zeng33 et al. have fabricated
This journal is ª The Royal Society of Chemistry 2012
Fig. 1 (a) XRD patterns (an upper curve: the resulted product; a bottom
curve: the standard ZnSnO3 powder from JCPDS card, no.11-0274), (b)
SEM image of the as-synthesized ZnSnO3 sub-micrometre cubic crystals
from a hydrothermal route, the inset shows such a well-defined cube.
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ZnSnO3 hierarchical nanocages via a solution synthetic method.
To the best of our knowledge, the synthesis of ZnSnO3 and
Zn2SnO4 materials with hollow cubic and octahedral architec-
tures has not been realized.
In the present study, we report the phase-controlled synthesis
of well-defined faceted cubic ZnSnO3 and octahedral Zn2SnO4
microcrystals in a large scale by a one-step facile solution
chemical route, in which the phase control of them was accom-
plished by only changing stannic precursors. As-synthesized zinc
stannate faceted microcrystals are easily converted to faceted
hollow structures with a shape preserving through an acid
etching process. Gas sensors based on these hollow zinc stannate
structures show a high sensitivity, fast response, and short
recovery time to H2S and C2H5OH.
2. Experimental section
Synthesis of ZnSnO3 and Zn2SnO4 microcrystals
All of the chemicals were analytically pure, and purchased from
Shanghai Chemical Industrial Co. Ltd. and used without further
purification. In a typical synthesis, starting materials, including
tin tetrachloride (SnCl4$5H2O), stannous chloride (SnCl2$7H2O),
zinc acetate (ZnAc2$2H2O), and sodium hydroxide (NaOH) were
dissolved in distilled water to form four transparent solutions,
respectively. For the synthesis of cubic ZnSnO3 microcrystals,
a ZnAc2 solution (0.02 M, 15 mL) was added to a SnCl4 solution
(0.02M, 15 mL) at room temperature with vigorous agitation for
10 min, forming a mixture of solutions. Then, a NaOH solution
(0.2M, 15mL) was added to themixture, with further continuous
stirring for 10 min. (A molar ratio of Zn2+/Sn4+/Na+ was
1 : 1 : 10.). The final mixture was put into a Teflon-lined stainless
steel autoclave with 60 mL capacity, and a hydrothermal reaction
proceeded at 130 �C for 6 h. The white products were collected by
centrifugation and washed repeatedly with anhydrous ethanol
and distilled water. Zn2SnO4 octahedral microcrystals were
synthesized by the similar hydrothermal procedures to those used
for the preparation of ZnSnO3 cubic microcrystals, except that
SnCl2$7H2O was substituted for SnCl4$5H2O.
Synthesis of ZnSnO3 and Zn2SnO4 hollow structures
0.05 g of as-synthesized ZnSnO3 cubic or Zn2SnO4 octahedral
crystals were added to a HNO3 solution (1 M, 5 mL), which was
kept at 25 �C for 2 h, and then ultrasonically dispersed for 3–5
min. The products were washed with distilled water and absolute
alcohol for several times, and dried at 60 �C for 6 h in air.
Characterizations
The phase of the ZnSnO3 and Zn2SnO4 microcrystals were
determined by powder X-ray diffraction (XRD; Rigaku D/Max
2550, Cu KR radiation). The morphology and microstructure of
the as-synthesized products were investigated by a field-emission
scanning electron microscope (SEM; Hitachi S-4800) and
transmission electron microscope (TEM; JEM-2100F). The
surface area, pore size, and pore-size distribution of the products
were determined by Brunauer–Emmett–Teller (BET) nitrogen
adsorption–desorption and Barett–Joyner–Halenda (BJH)
methods (Quantachrome, Auto-sorb-1MP).
This journal is ª The Royal Society of Chemistry 2012
Sensing tests
The gas sensing tests were operated in a system of HW-30A
(Hanwei Electronics Co. Ltd., P. R. China). The products were
mixed with terpineol forming a paste and then coated onto an
alumina tube-like substrate (7 mm in length and 1.5 mm in
diameter) with a pair of Au electrodes on each end. The unit was
then calcined at 400 �C for 2 h. A small Ni–Cr alloy coil was
placed through the tube as a heater to increase a working
temperature. In order to improve the long-term stability, the
sensors withstood the working temperature for several days. A
stationary state gas distribution method was carried out for gas
response testing. Detected gases, such as H2S, were injected into
a closed test chamber and mixed with air (air humidity: 37%).
After each measurement, the sensor was exposed to the atmo-
spheric air by opening the chamber. In the measuring electric
circuit, a 4.7 MU load resistor was connected in the series with
the gas sensors. The circuit voltage was 5.0 V, and the output
voltage (Vout) was the terminal voltage of the load resistor. The
working temperature (circa 270 �C, optimally) of the sensors was
adjusted by varying the heating voltage. The resistance of the
sensor in air or test gas was measured by monitoring theVout The
gas response of the sensor was defined as Sr ¼ Ra/Rg, where Ra
and Rg were the resistance in air and in the test gas, respectively.
The response or recovery time was estimated as the time taken
from the sensor output to reach 90% of its saturation after
applying or switching off the gas in a step function.
3. Results and discussion
3.1 Structure and morphology characterizations
The phase and the purity of as-obtained zinc stannate materials
were examined by the XRD pattern. Fig. 1(a) shows the XRD
pattern of a ZnSnO3 sample. All of the diffraction peaks (upper
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curve) can be indexed to the standard ZnSnO3 material with
a face-centered-cubic perovskite structure (bottom curve: the
XRD pattern from the standard ZnSnO3 powder on JCPDS
card, No.11-0274); no peaks from other phases can be detected,
indicating that the as-synthesized product has high purity.
Fig. 1(b) shows a low-magnification SEM image of the synthe-
sized ZnSnO3 product. It can be clearly seen that the ZnSnO3
material is composed of large-scaled, uniform, monodisperse
cubes. An inset of this figure shows a high magnification SEM
image of a ZnSnO3 cube, clearly demonstrating that the cube
have smooth faces and sharp edges with a mean size of �500 �500 � 500 nm3. These results suggest that the present facile
hydrothermal route results in large scaled synthesis of uniform,
monodispersed, well-defined, and pure ZnSnO3 sub-micrometre
sized cubes.
More interestingly, these ZnSnO3 cubic solid crystals can be
converted into hollow structures (or boxes) with an original
shape preserved via a simple acid-etching route, which was per-
formed in a solution of HNO3 upon an ultrasonic dispersion.
Fig. 2 shows the SEM and TEM images of the ZnSnO3 hollow
structures. We can see that the ZnSnO3 cubic hollow structures
retained their original size, shape and dispersity through the
etching process, Fig. 2(a). However, compared with the smooth
faces of the ZnSnO3 original solid crystals, the boxes have very
rough faces with many nanoparticles on them, shown by an inset
of this figure. Most of these hollow structures are unbroken and
kept without damage; a few of them are broken and collapsed,
Fig. 2(b). Their transparency to the electron beam confirms that
Fig. 2 (a, b) SEM images of the ZnSnO3 hollow cubic boxes by etching
with a solution of HNO3, inset in (b) of an enlarged image clearly
demonstrating the rough surface of such a cubic box. (c, d) TEM images
of these ZnSnO3 cubic hollow boxes with a thin wall thickness and
a spacious internal hollow space. (e) A high-magnification TEM image
and the corresponding ED pattern of this area. (f) XRD patterns of the
ZnSnO3 hollow cubic boxes formed by acid etching.
2174 | CrystEngComm, 2012, 14, 2172–2179
the hollow boxes’ thickness is as thin as several nm, thus forming
a spacious internal hollow space, Fig. 2(c) and 2(d). A high-
magnification TEM image of a hollow box reveals that each box
is composed of numerous nanocrystals with a diameter of less
than 30 nm (Fig. 2(e)). The selected area electron diffraction
(ED) of a box exhibits discontinued ring patterns of the ZnSnO3
phase, in which the four intensively bright ED rings (from the
one with the smallest diameter) are in good agreement with the
(200), (220), (042), and (422), indicating that the box shows
a polycrystalline nature consisting of numerous ZnSnO3 nano-
crystallites, rather than a single crystal. Fig. 2(f) shows the XRD
pattern of a ZnSnO3 sample after the acid etching process. There
are no different peaks compared with XRD patterns of the
original ZnSnO3 product in Fig. 1(a), indicating that the phase of
ZnSnO3 cubic microcrystals has not been changed after the acid
etching process. The single-step acid-etching process for the
ZnSnO3 boxes was simple and highly reproducible, and the size
and the shape of the original ZnSnO3 cubes were unchanged
through the etching process. These ZnSnO3 boxes may be suit-
able for gas-sensing applications due to these large specific
surface areas, compared to that of the solid structures.
Fig. 3(a) shows the SEM image of the Zn2SnO4 octahedral
crystals with an average size of �2.0 mm, which were clearly
enclosed by 8 well-defined and faceted triangles. Also, a small
number of spherical particles (as indicated by a square in
Fig. 3(a)) were observed in the product. Fig. 3(b) shows the XRD
pattern of the product, in which the main strong diffraction
peaks were assigned to the cubic spinel-type structure of
Zn2SnO4 (JCPDS No.24-1470) material, except the weak
diffraction peaks due to a small amount of SnO2 phase (as in
indicated by stars in Fig. 3(b)). Fig. 3(c)–(e) shows SEM and
TEM images for the Zn2SnO4 octahedral crystals viewed from
different directions. The Zn2SnO4 crystal shown in Fig. 3(c)
exactly stands on its octahedral pinnacle, demonstrating 4 equal
triangles commonly sharing this pinnacle; Fig. 3(d) shows the
Fig. 3 (a) A SEM image of the Zn2SnO4 octahedral crystals. (b) The
XRD pattern of the Zn2SnO4 product, the star-label indicates SnO2
diffraction peaks. (c–f) SEM and TEM images showing two Zn2SnO4
crystals viewed from different directions. (g) A HRTEM image of the
Zn2SnO4 crystal, the inset showing the corresponding FFT pattern along
the [114] zone axis.
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TEM image of this octahedral crystal with such an orientation to
the carbon film support, displaying a square shape. The Zn2SnO4
crystal shown in Fig. 3(e) is oriented with a triangle plane parallel
to the SEM holder surface, revealing a regular triangle and three
banked triangles sharing a wedge with the regular triangle,
respectively. Fig. 3(f) shows the TEM image of this octahedral
crystal with such an orientation to the carbon film support,
displaying a regular hexagon. In fact, further analyzing its crystal
structure, the electron beam angles in (c, d) and (e, f) were
parallel to the [001] and [110] directions of the Zn2SnO4 crystal,
respectively. All the exposed faces of the Zn2SnO4 octahedral
crystals were composed of eight equivalent (111) planes. Clear
and continuous lattice-fringe images can be resolved in Fig. 3(g),
and the distance between neighboring fringes was measured to be
0.262 nm, close to the (311) lattice spacing (0.261 nm) in the
Zn2SnO4 crystal. The inset in Fig. 3(g) shows the corresponding
fast-Fourier-transform (FFT) pattern, which can be indexed to
the [114] zone axis of the Zn2SnO4 crystal.
More interestingly, these octahedral Zn2SnO4 solid crystals
can also be converted into hollow structures with an original
shape preserved via a simple acid-etching route, as has been
described earlier about the ZnSnO3 hollow structures. Fig. 4
shows the SEM and TEM images of the hollow octahedral
Zn2SnO4 structures. The SEM image in Fig. 4(a) shows that
there are several holes formed on the surfaces of these hollow
structures. TEM images show a thin wall thickness and some
materials remaining due to partial etching. Clearly, these hollow
structures basically kept their original shape and size. An ED
pattern taken from a hollow structure shows discontinued ring
patterns of the Zn2SnO4 phase, indicating a polycrystalline
nature of this Zn2SnO4 structure consisting of numerous nano-
crystallites, rather than a single crystal. Fig. 4(d) shows the XRD
patterns of the Zn2SnO4 hollow octahedral structures formed by
acid etching. The diffraction peaks are assigned to the cubic
spinel structure (JCPDS No.24-1470), indicating the octahedral
Zn2SnO4 phase were stable under acidic conditions. Also, these
Fig. 4 (a) A SEM image of the Zn2SnO4 hollow octahedral structures by
etching with a solution of HNO3. (b) A TEM image of the Zn2SnO4
hollow octahedral structures. (c) A TEM image and a corresponding ED
pattern from the edge of a hollow octahedral structure. (d) The XRD
pattern of the Zn2SnO4 hollow octahedral structures formed by acid
etching.
This journal is ª The Royal Society of Chemistry 2012
Zn2SnO4 hollow octahedral structures may find gas-sensing
applications due to these large specific surface areas, compared
to that of the solid structures.
3.2 Possible growth and etching mechanisms
To examine the formation mechanism of these zinc stannate
faceted crystals, time-dependent experiments were carried out.
Fig. 5 shows the SEM and XRD images of the ZnSnO3 material
at a reaction time of 1 h, 2 h, 6 h, and 18 h, with other synthetic
parameters unchanged. Fig. 5(a–d) correspond to the reaction
time of 1 h, 2 h, 6 h, 18 h. From Fig. 5(a) we can clearly see that
only a small quantity of cubes formed at a shorter reaction time
and most products cannot be defined as a shape. Fig. 5(b) reveals
that the products formed after a reaction of 2 h are mostly
uniform, regular cubes with an edge length of about 400–500 nm
and accumulate into a whole. With a reaction time prolonged
to 6 h, Fig. 5(c), all the as-prepared ZnSnO3 products are
regular and monodisperse cubes with a larger edge length of
about 600–800 nm. With a reaction time further extending to 18
h, Fig. 5(d), the crystals further grow and thus their length are up
to about 2–3 mm. Fig. 5(e) shows the XRD patterns of the
ZnSnO3 samples prepared at different reaction time of 1 h, 2 h, 6
h, and 18 h, respectively. All of the diffraction peaks can be
readily indexed to those of the standard ZnSnO3 powders with
the perovskite structure (JCPDS No.11-0274), also confirming
that no crystal structure changes occurred within these different
reaction periods. Clearly, no other crystalline phases were
detected from the XRD patterns, indicating that the ZnSnO3
material with a high purity could be obtained under current
synthetic conditions via the above different reaction times. Also,
Fig. 5 SEM images showing the as-synthesized ZnSnO3 material at
different reaction times: (a) 1 h, (b) 2 h, (c) 6 h, and (d) 18 h. (e) The
corresponding XRD patterns of the as-prepared ZnSnO3 cubic.
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Fig. 6 Possible mechanisms show the growth and etching processes of
the ZnSnO3 cubic microcrystals, respectively. Typical TEM image shows
the corresponding etching stages from a solid single crystal to hollow
polycrystals.
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as shown in Fig. 5(e), the intensity of the diffraction peaks
increases with the extended reaction time, indicating that the
crystalline degree of the ZnSnO3 samples becomes higher and
higher. On the basis of the results above, it could be concluded
that the shorter reaction time was contributive to the smaller
sized ZnSnO3 cubic crystals, and a longer reaction time was
beneficial to the formation of ZnSnO3 cubic crystals and their
growth into large sized cubes.
In our experiments, it is found that the raw materials and
reaction time are important parameters for the crystal growth of
the zinc stannate, and from a chemical reaction point of view
a mechanism for these microcrystals can be proposed. When the
stannic precursors were SnCl4$5H2O and ZnAc2$2H2O (under
alkaline conditions), with a molar ratio of Zn2+/Sn4+/Na+ of
1 : 1 : 10), ZnSnO3 was formed. The involved reactions resulting
in the formation of ZnSnO3 can be described as follows in eqn
(1)–(2):
Zn2+ + Sn4+ + 6OH� / ZnSn(OH)6 (1)
ZnSn(OH)6 / ZnSnO3 + 3H2O (2)
ZnAc2 + SnCl4 + 6NaOH ¼ ZnSnO3 + 4NaCl
+ 2NaAc + 3H2O (3)
Here, the combination of Zn2+, Sn4+ and OH� in the precursor
solution led to the formation of the unstable intermediate
ZnSn(OH)6 in the solution (eqn (1)).33 With the hydrothermal
conditions, the as-formed ZnSn(OH)6 unstable intermediate is
transformed into ZnSnO3 (eqn (2)). As a whole, these two
reactions can be merged as eqn (3). In contrast, when a stannic
precursor was SnCl2$7H2O and other reaction conditions are
kept, Zn2SnO4 was formed, and the chemical reactions for the
formation of the Zn2SnO4 can be expressed as the following eqn
(4)–(6):
Sn2+ + ½O2 / Sn4+ + O2� (4)
2Zn2+ + Sn4+ + 8OH� / Zn2Sn(OH)8 (5)
Zn2Sn(OH)8 / Zn2SnO4 + 4H2O (6)
2ZnAc2 + SnCl2 + 6NaOH + ½O2 ¼ Zn2SnO4
+ 2NaCl + 4NaAc + 3H2O (7)
In this case, SnCl2$7H2O can dissolve in the water with diffi-
culty, and under the present hydrothermal conditions, Sn2+ can
gradually be oxidized into Sn4+ in the solution by oxygen in the
autoclave (eqn (4)), where the ratio of Zn2+/Sn4+ is different from
that of the formation of ZnSnO3, resulting in another unstable
intermediate Zn2Sn(OH)8 (eqn (5)). As the reaction was on-
going, intermediate Zn2Sn(OH)8 decomposed into Zn2SnO4
(eqn (6)). On the whole, the entire reactions can be summarized
as eqn (7).
From a crystal growth point of view, surface energies associ-
ated with different crystallographic planes are usually different,
and a general sequence may follow an order of g{111} < g{100}
< g{110} for face centered cubic (fcc) spinel-type crystals.34 In an
ideal growth habit, these crystals usually exist and are enclosed
with {111} lattice planes as the basal surfaces, and the {100} or
2176 | CrystEngComm, 2012, 14, 2172–2179
{110} lattice planes with high surface energies disappear during
the growth of the crystals.8 However, the crystal morphology
depends not only on the intrinsic crystal structure but also on the
synthetic conditions, especially for the case of spinel-type fcc
crystals.36 The different solubility of the raw materials
(SnCl4$5H2O and SnCl2$7H2O) or other parameters (such as
temperature and pressure) may change the order of free energies
on these facets. In fact, this crystal growth phenomenon has been
widely observed in other cases of inorganic crystals.35 For
example, Cu2O crystals enclosed by six equivalent {100} facets36
have been successfully synthesized via reducing the copper-citrate
complex solution with glucose, single crystals of the spinel-type
LiMn2O4 with equivalent eight (111) planes have also been
successfully grown by a solvent evaporation flux method at 1173
K,37 respectively. Here, taking ZnSnO3 cubic microcrystals for
an example, a growth process is schematically illustrated in
Fig. 6. A formation and then decomposition of unstable phase
ZnSn(OH)6 results in a nucleation and growth of ZnSnO3
nanocrystals under the hydrothermal conditions, involving the
above chemical reactions (step 1). Upon the introduction of
additional reactants into the reaction mixture, more ZnSnO3
nanocrystallites were produced and further assembled and
packed into slightly larger sized nanocrystallites. Obviously, to
minimize high-energy surfaces, the as-formed ZnSnO3 nano-
crystals undergo a ‘‘dissolution–recrystallization’’ process and
then aggregate into a larger particle (step 2), which may follow
the rule of Ostwald ripening.38 By modifying the ideal growth
habit, these crystals develop into a cubic morphology several
microns in size and are enclosed with {100} lattice planes as the
basal surfaces with the extending reaction time (step 3). Finally,
these ZnSnO3 cubic crystals continually adsorb the nanocrystals
within the reaction system onto their surfaces, resulting in the
formation of large cubes. Because the acid-etching process does
not involve the interchange of species, the etching process is not
due to the Kirkendall effect.39 Here, taking ZnSnO3 cubic hollow
boxes as an example, it is only caused by the dissolution of the
ZnSnO3 material in the acid solution of HNO3. Due to the fact
that ZnSnO3 microcubes are homogeneous in chemical elements,
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Fig. 7 The XRD pattern of the as-prepared Zn2SnO4 hollow cubic
structures, which was obtained from the ZnSnO3 hollow cubic structures
after being annealed at 600 �C.
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as for why the surface material of the ZnSnO3 microcubes was
significantly maintained through the acid-etching, resulting in
the formation of the ZnSnO3 hollow microcubes, it is believed
that most area on the surface of a given ZnSnO3 microcube is
smooth and compact, as suggested by the SEM imaging, but
a few partial areas on the cubes have some crystal growth defects
at a micro- and nanometre sized scale, such as steps, cracks, and
holes, while the interior matter of the ZnSnO3 microcubes is
loose and even porous, as demonstrated in the previous crystal
growth of the material.32,33 These surface growth defects provide
an energetically favored site for the absorption of H+ from the
acid solution of HNO3 and helpful for the dissolution of the
ZnSnO3 material, and then the acid solution of HNO3 enters into
the interior of the ZnSnO3 microcubes and further etches the
loose and even porous matter of the ZnSnO3. By comparison, the
smooth and compact surfaces of these ZnSnO3 microcubes will
not be good for the dissolution of the ZnSnO3 material and thus
the surface matter of this material will be significantly kept,
except the areas with some growth defects, resulting in the
formation of the ZnSnO3 hollow structures at a given etching
time. Certainly, if the etching time is long enough, these ZnSnO3
hollow boxes will be dissolved completely by the acid. In the
present case, it seems that there are two diffusion processes
during the formation of the ZnSnO3 boxes, i.e., the outward
diffusion and inward diffusion of reactive species (step 1). An
etching on the surface of the ZnSnO3 microcubes drives the
outward diffusion of cations and further increases the surface
defects of the cubic crystals, enhancing the inward dissolution of
the ZnSnO3 of the cubes. Then, the continuous outward diffusion
of the reactive species and the accumulation of pores inside the
crystal lead to the formation of the large void under the surface
matter (or shell) (step 2). Finally, the crystal shell is maintained
by the balance of the inward diffusion and the outward diffusion
of the ZnSnO3 material (step 3) at a given etching time.
3.3 Sensing properties
Recently, zinc stannate was found to be a sensor material for
H2S, C2H5OH and HCHO; the sensor made of the Zn2SnO4
calcined flowerlike hierarchical nanostructures exhibited higher
sensitivity than that of the Zn2SnO4 uncalcined material, and the
response of the annealed sensor was superior to that of the
unannealed sensor.7 In the present work, as-prepared ZnSnO3
cubic and Zn2SnO4 octahedral hollow boxes were both calcined
in air at 600 �C. It is found that the Zn2SnO4 product was stable
during heat treatment, but the ZnSnO3 product decomposed to
SnO2 and Zn2SnO4 mixed phases after being annealed, as
confirmed by the XRD pattern in Fig. 7. So, it indicated that the
Zn2SnO4 product was more thermally stable than the ZnSnO3
product; in fact, in previous reports,7 Zn2SnO4 had been
demonstrated to be the most thermodynamically stable, while
ZnSnO3 has been found to be a thermodynamically metastable
crystal phase.6 Although the ZnSnO3 hollow structures decom-
posed into the mixture of SnO2 and Zn2SnO4, the size and the
shape of the original material were unchanged through the heat
treatment. Considering the characteristic internal cavity of
a large area and thermal stability, the sensing material we
examined was the Zn2SnO4 hollow cubic and octahedral struc-
tures, respectively.
This journal is ª The Royal Society of Chemistry 2012
To demonstrate the performance of the Zn2SnO4 hollow
structures as a sensing material, the responses to a series of gases
are investigated at an operating temperature of 260 �C. As shown
in Fig. 8, the responses of the Zn2SnO4 hollow material based gas
sensors to H2S, C2H5OH, HCHO, C3H6O, NH3, CO, H2 and
NO2 are examined, and are found to be excellent to H2S,
C2H5OH, and HCHO among them. Clearly, the responses of the
Zn2SnO4 hollow octahedral structures to all of the gases tested
are higher than those of the Zn2SnO4 hollow cubic structures,
especially with regard to H2S and C2H5OH. In order to further
confirm the relationship between the hollow structures and gas
sensing performances, nitrogen adsorption and desorption
measurements of the above two hollow products were carried out
to estimate the properties. As shown from the nitrogen adsorp-
tion and desorption cyclic curves in Fig. 8(b), the adsorbed
quantity of the hollow octahedral structures and hollow cubic
structures are marked by black and red curves, respectively. In
fact, the BET surface area of the two hollow structures was
calculated to be 43.768 and 17.895 m2 g�1, respectively, indicating
a downtrend of the active surface among them. So it can be
concluded that the hollow octahedral structures contribute to
a large surface area, and hence lead to high sensitivity. Pore size
distribution curves of the two hollow products were shown in
inset of Fig. 8(b). The size of the pores based on desorption data
mainly centered at 1.972 nm and 1.967 nm with a relatively
narrow distribution for the Zn2SnO4 hollow octahedral struc-
tures and cubic structures, respectively (inset in Fig. 8(b)), which
is in the mesoporous range.
Fig. 9 shows the typical dynamical response curves of the
Zn2SnO4 hollow cubic (a, c) and octahedral (b, d) materials
based gas sensors to H2S (from 1 ppm to 5 ppm, 10 ppm, and 50
ppm) and C2H5OH (from 1 ppm to 5 ppm, 10 ppm, and 50 ppm,
and then to 1ppm again) with their different concentrations at
260 �C. It reveals that with an increase (such as H2S concentra-
tion increasing from 1 ppm to 5 ppm, 10 ppm, and 50 ppm) of the
gas concentration, the sensitivities increase, but the sensitivity of
the octahedral Zn2SnO4 hollow structures increases faster than
that of the cubic form. Also, the Zn2SnO4-based gas sensor
presents sensitive and reversible responses to both H2S and
C2H5OH. Specifically, the resistance of the sensor decreases upon
its exposure to C2H5OH for less than 2 s, and the resistance
recovers to its initial value after being in air for 20 s; the response
and recovery times of the Zn2SnO4-based sensors to H2S are
within 10 and 25 s, respectively. So, it can be concluded that the
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Fig. 8 (a) The plots of the sensitivity of the sensors based on the Zn2SnO4 hollow octahedral (black) and cubic (red) structures, respectively, to different
gases. (b) Typical N2 gas adsorption–desorption isotherm cyclic curves of the Zn2SnO4 hollow octahedral (black) and cubic (red) structures, inset
showing BJH pore size distribution of these structures, respectively.
Fig. 9 Typical dynamical response curves of the Zn2SnO4 hollow cubic
(a) and octahedral (b) structures for gas sensors to H2S with the
concentration increasing. The response curves of the Zn2SnO4 hollow
cubic (c) and octahedral (d) structures based gas sensors to C2H5OHwith
different concentrations.
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Zn2SnO4-based sensors exhibit high response and rapid
recovery time to H2S and C2H5OH. Moreover, the sensitivity of
the Zn2SnO4 hollow structures to H2S and C2H5OH is higher
than that of the Zn2SnO4 solid crystals. Obviously, the
improvements of sensitivity and recovery time are ascribed to
the higher surface area associated with the Zn2SnO4 hollow
structures.
The gas sensing mechanism of our Zn2SnO4-based sensors
should follow the surface charge model, and can be explained by
the change in resistance of the sensor upon exposure to different
gas atmospheres. Therefore, ‘‘surface accessibility’’ is crucial to
maintain the high sensitivity of the structures. A Zn2SnO4
hollow octahedral box has a larger active surface area on the
{111} facets than that of a Zn2SnO4 hollow cubic box, which can
provide more active space for the interaction between Zn2SnO4
material and the detected gases, and thus shows a higher sensi-
tivity. When the gas sensor is exposed to the test gas atmosphere,
the resistance of the material decreases owing to the electrons
produced from the reaction, which results in an increase of the
output voltage. When a Zn2SnO4 hollow structure is exposed to
2178 | CrystEngComm, 2012, 14, 2172–2179
air, oxygen molecules can be adsorbed onto the surface to
form chemisorbed oxygen species by capturing free electrons
from the conduction band. After sufficient adsorption processes,
arriving at a certain equilibrium state, the decrease of the elec-
tron concentration in the conduction band results in a stabiliza-
tion of high surface resistance. When the Zn2SnO4 material is
exposed to C2H5OH, HCHO, H2S or other reductive gas
atmosphere, these gas molecules can react with adsorbed oxygen
species on its surface. This process releases the trapped electrons
back to the conduction band and finally leads to an increase
of electron concentration, which results in a decrease in the
resistances.
4. Conclusions
We have developed a facile chemical solution route to the phase-
controlled synthesis of well-defined faceted cubic ZnSnO3 and
octahedral Zn2SnO4 microcrystals in a large scale. The as-
synthesized zinc stannate faceted microcrystals are easily con-
verted to hollow structures with a shape preserved through an
acid etching process. The effects of reaction conditions, such as
time and temperature, on the formation of products were care-
fully examined, and the size of the crystallites can be easily tuned
by varying the reaction time. Possible growth and etching
mechanisms for the faceted crystals have been proposed. The
hollow structures of zinc stannate were exploited as gas sensors
and exhibited improved sensing performances to H2S, C2H5OH
and HCHO; moreover, the sensitivity and recovery time of
Zn2SnO4 hollow octahedral structures to H2S and C2H5OH are
both higher than those of the cubic structures, which may find
potential industrial applications in detecting gases.
Acknowledgements
This work was supported from the National Natural Science
Foundation of China (Grant No. 21171035 and 50872020), the
Program for New Century Excellent Talents of the University in
China, the ‘‘Pujiang’’ Program of Shanghai Education
Commission (Grant No. 09PJ1400500), the ‘‘Dawn’’ Program of
the Shanghai Education Co mmission (Grant No. 08SG32), and
the Science and Technology Commission of Shanghai-based
‘‘Innovation Action Plan’’ Project (Grant No. 10JC1400100).
This journal is ª The Royal Society of Chemistry 2012
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