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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: hu.junqing@dhu.edu.cn

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

CrystEngComm, 2012, 14, 2172–2179 | 2173

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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.

CrystEngComm, 2012, 14, 2172–2179 | 2175

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,

This journal is ª The Royal Society of Chemistry 2012

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).

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