One-Pot Synthesis and Gas-Sensing Properties of Hierarchical ZnSnO3 Nanocages

Yi Zeng,† Tong Zhang,*,† Huitao Fan,† Wuyou Fu,‡ Geyu Lu,† Yongming Sui,‡ andHaibin Yang‡

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, JilinUniVersity, Changchun 130012, People’s Republic of China, and State Key Laboratory of Superhard Materials,Jilin UniVersity, Changchun 130012, P.R. China

ReceiVed: June 4, 2009; ReVised Manuscript ReceiVed: September 20, 2009

We have successfully fabricated hierarchical ZnSnO3 nanocages via a facile one-pot solution synthesis method.Field emission scanning electron microscopic and transmission electron microscopic results reveal that thecubic ZnSnO3 samples with hollow interior and porous shells are cage-like structure with the side length of200-400 nm, where the subunits are irregular-shaped nanoparticles. The time-dependent morphology of theZnSnO3 samples has been investigated, and a possible formation mechanism of these hierarchical structuresis proposed. Moreover, gas sensor based on hierarchical ZnSnO3 nanocages exhibits better sensing propertiescompared with the solid ZnSnO3 nanocubes. The facile preparation method may provide an easy path to theextendable synthesis of other functional nanomaterials with hollow structure and further exploitation of thepotential applications.

1. Introduction

Over the past few years, the controlled arrangements ofnanosized building blocks into hierarchical hollow structureshave become a hot topic in recent self-assembly research fields.1

Functional materials with this structure can provide higherspecific area and peculiar characteristics than their solidstructures, which can be further used to exploit new potentialapplications. To fabricate the complex architectures, variousstrategies have been employed successfully to assemble buildingblocks into different hollow micro- and nanostructures in theform of hollow spheres,2 nanocages,3 nanoskeletons,1c,4 andhollow octahedra.5 However, these approaches mentioned aboveusually involve costly templates or raw materials and complexoperating steps, which possibly result in the increased cost andfurther limit the potential applications. Therefore, there stillremains a great challenge to develop a facile, mild, and low-cost method for the fabrication of hollow micro- and nanostruc-tures.

Zinc stannate (ZnSnO3), as a famous multifunctional material,has attracted extensive attention due to its potential applicationsin various fields.6 ZnSnO3 is a structure of perovskite oxidesand ilmenite structure, forming face-centered-cubic (fcc) closedpacking.7 Recently, several groups have synthesized ZnSnO3

micro- and nanostructures by various synthesis routes, includingthermal evaporation, coprecipitation method, and low temper-ature ion exchange.6-8 Significantly, Wang et al. have reportedthe preparation of ZnSnO3 hollow spheres and hierarchicalnanosheets by the cetyltrimethyl ammonium bromide (CTAB)-assisted hydrothermal method.9 Geng et al. have reported thesynthesis of polyhedral ZnSnO3 microcrystals by differentsurfactant-assisted wet-chemical methods at low temperature (85°C).7 In that regard, the use of the organic functional groups tomodify the relatively growth rate of {100} and {111} planes

represents an important route to explore the formation ofcomplex ZnSnO3 nanostructures. Until now, one-pot solutionsynthesis for the cubic ZnSnO3 nanocages with hierarchicalarchitecture had not been reported.

In this paper, we report the synthesis of cubic ZnSnO3

nanocages with hierarchical architecture via a hexamethylene-tetramine (HMT)-assisted hydrothermal process. The as-prepared products are illuminated in terms of their crystallinity,morphology, and structure. Moreover, the formation process hasbeen investigated through the morphology evolution withdifferent reaction times, and a possible formation mechanismis proposed. To demonstrate the potential applications, we havefurther investigated the gas-sensing properties of the sensorsbased on these unique ZnSnO3 nanostructures.

2. Experimental Procedures

All the reagents in the experiment were analytical grade(Beijing Chemicals Co., Ltd.) and used as received withoutfurther purification. A typical procedure is as follows: Zn-(CH3COO)2 ·2H2O (20 mM), SnCl4 ·5H2O (20 mM), NaOH(0.25 g), and (CH2)6N4 (HMT, 0.08 g) were mixed withdeionized water under magnetic stirring vigorously until a 40mL uniform suspension was formed. Then the suspension wastransferred to a Teflon-lined stainless steel autoclave, sealedtightly, and maintained at 160 °C for 12 h. After the hydro-thermal procedure, the autoclave cooled naturally down to roomtemperature. The precipitates on the bottom of the autoclavewere collected by centrifugation, washed with absolute ethylalcohol, deionized water several times, and dried in air at 90°C to get the final products.

An X-ray diffraction (XRD) pattern was recorded using aRigaku D/max-2500 X-ray diffractometer with Cu KR radiation.Field-emission scanning electron microscopic (FESEM) imageswere obtained using a JEOL JSM-7500F microscope operatedat an acceleration voltage of 15 kV. Transmission electronmicroscopic (TEM), selected-area electron diffractive (SAED),and high-resolution transmission electron microscopic (HRTEM)images were obtained on a Hitachi H-8100 microscope and a

* To whom correspondence should be addressed. Tel.: +86-431-85168385. Fax: +86-431-85168417. E-mail: zhangtong@jlu.edu.cn.

† State Key Laboratory on Integrated Optoelectronics, College ofElectronic Science and Engineering.

‡ State Key Laboratory of Superhard Materials.

J. Phys. Chem. C 2009, 113, 19000–1900419000

10.1021/jp905230h CCC: $40.75 2009 American Chemical SocietyPublished on Web 10/12/2009

JEOL JEM-2100F microscope, respectively, both with anaccelerating voltage of 200 kV. The gas-sensing properties ofZnSnO3 were measured using a RQ-2 gas-sensing characteriza-tion system (Figure S1, Supporting Information). In the gasresponse measurement, a given amount of testing gas wasinjected into a closed chamber, and the sensor was put into thechamber for the measurement of the sensitive performance. Aftereach measurement, the sensor was exposed to the atmosphericair by opening the chamber. The RQ-2 test meter can measurethe dynamic electrical signal, including resistance, current, andvoltage of the sensing materials. The signals were collected bya computer automatically through a data acquisition card(ADLINK PCI-9111DG). Special application software wasdeveloped using the LabVIEW environment (Version 7.1) togovern the work of the whole system. The sensitivity of sensorwas defined as the ratio (S ) Ra/Rg) of the resistance of thesensor in dry air (Ra) to that in the testing gas (Rg). The responseand recovery time were defined as the time taken by the sensorto achieve 90% of the total resistance change in the case ofadsorption and desorption, respectively.

3. Results and Discussion

Structure and Morphology. Figure 1 shows a typical XRDpattern of the as-prepared products. All of the diffraction peakscan be indexed to the standard ZnSnO3 with the perovskitestructure (JCPDS No. 11-0274). No diffraction peaks from anyother impurities are observed. The morphologies and micro-structures of the ZnSnO3 products were illuminated by FESEMand TEM observations. A panoramic FESEM image of the as-prepared ZnSnO3 obtained on a large scale after hydrothermaltreatment at 160 °C for 12 h is shown in Figure 2a. It can beobserved from the enlarged FESEM image of Figure 2b thatthe products are composed of cubic-shaped ZnSnO3 nanostruc-tures with similar side length of about 200-400 nm, whichexhibit a hierarchical structure composed of compactly ag-gregated ZnSnO3 subunits of irregular-shaped polyhedra. Thehigh-magnification FESEM image shows the detailed morphol-ogies of the hierarchical ZnSnO3 nanocubes. Apparently, therough and porous surfaces of the cubic nanostructures areexhibited (Figure 2c). An individual ZnSnO3 cube with a brokensurface is shown in Figure 2d, and its apparent cavity confirmsthat the hierarchical ZnSnO3 cubes are actually hollow struc-tures. It can also be observed that the inner surfaces of thehollow structure of the broken cube are very rough. Thishierarchical cage-like ZnSnO3 may be suitable to gas-sensing

applications due to these large specific surface areas, includingthe outer and inner regions of hollow cubes, compared to thatof the solid nanostructures.

In addition, the hollow ZnSnO3 nanocages are furthercharacterized by TEM and HRTEM. It is noteworthy that thehierarchical cage-like structures are sufficiently stable, whichcannot be destroyed even after ultrasonication for a long time.The typical TEM image in Figure 3a shows that the size andshape of ZnSnO3 are similar to those of the FESEM observa-tions. Moreover, from the TEM image, we can see an obviousdifference between the dark edge and the relatively bright centerof the individual ZnSnO3 cube, further proving that the

Figure 1. XRD pattern of the as-prepared ZnSnO3 products andstandard XRD pattern of ZnSnO3 (JCPDS No. 11-0274). Figure 2. Morphological characterization of the as-prepared ZnSnO3

products: (a) panoramic, (b) enlarged, and (c,d) high-magnificationFESEM images. The inset of panel d shows a broken part of anindividual ZnSnO3 cage.

Figure 3. (a) TEM and (b) the corresponding HRTEM images of anindividual ZnSnO3 nanocage. The inset of panel a shows the SAEDpattern of the nanocage.

One-Pot Synthesis of Hierarchical ZnSnO3 Nanocages J. Phys. Chem. C, Vol. 113, No. 44, 2009 19001

hierarchical ZnSnO3 nanostructures have cage-like hollowstructures. The corresponding SAED pattern (inset of Figure3a) and the HRTEM image confirm that the hierarchical ZnSnO3

nanocages are polycrystalline structures in nature (Figure 3b).Growth Process and Mechanism. To understand the forma-

tion process of hierarchical ZnSnO3 nanocages and the possiblegrowth mechanism, the morphology evolution of ZnSnO3

nanocages with different reaction time have been investigatedin detail, which is shown in Figure 4. When the hydrothermaltime is 10 min, it can be observed that the sample consists ofsome agglomerates, as shown in a panoramic image (FigureS2, Supporting Information). The low- and high-magnificationFESEM observation indicates that the products are actuallycomposed of a large number of nanoparticles (Figure 4a andthe inset). As the hydrothermal process is prolonged to 40 min,FESEM image shows that these aggregations transform into thequasi-cubic particles with coarse surfaces and the size of about200-400 nm (Figure 4b). The enlarged FESEM image shownin the inset of Figure 4b indicates that the surface of nanostruc-tures exhibit a hierarchical structure, where the subunits are somesmaller nanoparticles enchased on larger nanoparticles. Withthe reaction time increasing to 4 h, the quasi-cubic structure ofZnSnO3 evolves into regular cubic shape, and the craterlets inthe surfaces of cubes can be observed (Figure 4c). With thereaction time prolonged to 8 h, more craterlets in the cubicsurfaces can be observed in Figure 4d, and these craterlets may

evolve into pores and further serve as active sites for theformation of hollow cage-like structure. When the reaction timeis 12 h, the hierarchical ZnSnO3 cages in bulk quantity areobtained, and the detailed characteristics of them are describedin the above part. Extending the reaction time to 16 h, theFESEM image indicates that the pore size is enlarged, and thepores gradually merge to form one hole at the cubic surfaces(Figure 4e). When the reaction time is further up to 24 h, thecubic nanostructures are gradually etched into nanoparticles andfragments, which consist of a large scale of irregular nanopar-ticles with the diameter of about 80 nm (Figure 4f).

On the basis of the experimental results and the investigationsdescribed above, the formation of hierarchical ZnSnO3 nano-cages probably involves four key steps, which are schematicallyillustrated in Figure 5. In the first step, the presence of Zn2+

and Sn4+ is easy to form ZnSn(OH)6 nanoparticles by thecomplexation with OH- in alkaline solution. With the reactiontime being prolonged, the self-aggregated process between theseZnSn(OH)6 nanoparticles to form incompact cubic aggregatesis structurally and energetically favorable (step 2). Then, theZnSn(OH)6 intermediate phase is transformed into ZnSnO3

phase, and the sample exhibits a hierarchical structure with acubic shape, where the subunits are irregular and regular ZnSnO3

polyhedra with the size of several tens of nanometers. Thestructural defects in the surface of a nanocube act as active pointsto form the interior cavity by a selective dissolution-recrystalli-zation process with increasing hydrothermal time.10 In the fourthstep, it is difficult to maintain the hierarchical cage-like structuresin the growth and restructure of crystallite process when thehydrothermal time is further prolonged. The obtained sampleis mainly composed of cubic fragments and nanoparticles.

To investigate the role of HMT on the formation of cubicstructure, the controlled experiments of the hydrothermal processwithout HMT and with the replaced CTAB have been carriedout, respectively, keeping other experimental conditions con-stant. The TEM observation confirms that the absence of HMTleads to the distinctly morphological change of ZnSnO3 products,which are composed of nanoparticles with a diameter of about20 nm (Figure S3, Supporting Information). When we add adifferent surfactant of CTAB instead of HMT, products withdifferent size and shape are obtained. It can be observed thatthe as-prepared product consists of a large scale of ruggedmicrospheres with the sizes of 0.5-1.1 µm and many brokenmicrospheres, which confirms that these spheres are indeedhollow structures. The enlarged inset indicates that the shellsof hollow microspheres consist of numerous compact-aggregatednanoparticles (Figure S4, Supporting Information). Therefore,on the basis of the morphological study, it can be concludedthat HMT plays an important role in forming ZnSnO3 nanocages.

Gas-Sensing Properties. For gas-sensing measurement, gassensors were fabricated by spin-coating an aqueous paste ofZnSnO3 nanostructures onto a ceramic tube, which was equippedwith a pair of Au electrodes and a Ni-Cr heating coil in thetube to control the operating temperature. The gas-sensingproperties of hierarchical ZnSnO3 nanocages to ethanol andHCHO were measured. Figure 6 shows the response curves of

Figure 4. FESEM images of morphology evolution of ZnSnO3

nanostructures prepared with different reaction time: (a) 10 min, (b)40 min, (c) 4 h, (d) 8 h, (e) 16 h, and (f) 24 h. The insets of a, b, ande show the enlarged images, and the scale bar is 100 nm.

Figure 5. Schematic illustration of the formation process of hierarchical ZnSnO3 nanocages.

19002 J. Phys. Chem. C, Vol. 113, No. 44, 2009 Zeng et al.

the ZnSnO3 gas sensor to ethanol and HCHO with increasingconcentration at their optimal operating temperatures of 270 and210 °C, respectively. The difference of the optimal temperatureseems to correlate with the different activation energy barriersrelated to different testing gases.11 It can be seen that the ZnSnO3

gas sensor presents sensitive and reversible responses to bothethanol and HCHO. Specifically, the resistance of the sensordecreases upon exposure to ethanol for less than 2 s, and theresistance recovers to its initial value after being in air for 30 s.The response and recovery times of the ZnSnO3 sensor toHCHO are within 25 and 90 s, respectively. It can be concludedfrom Figure 6 that the ZnSnO3 sensor exhibits high responseand rapid response and recovery time to ethanol. Moreover, thesensitivity of hierarchical ZnSnO3 nanocages to 10 ppm ethanolis about 3.1, which is higher than that of the solid ZnSnO3

nanocubes (Figure S5, Supporting Information). The improve-ment of sensitivity and recovery time may be ascribed to thehigher surface area, and further gas-sensing performance hasto be studied in detail.

For the gas sensing mechanism of our ZnSnO3 sensor, itshould follow the surface charge model, which may be explainedby the change in resistance of the sensor upon exposure todifferent gas atmospheres.12 When a ZnSnO3 nanostructure isexposed to air, oxygen molecules can be adsorbed onto theZnSnO3 surface to form chemisorbed oxygen species (O2

-, O2-,O-) by capturing free electrons from the conduction band.13

After sufficient adsorption process to arrive at a certainequilibrium state, the decrease of the electron concentration inthe conduction band results in stabilization of high surfaceresistance. When ZnSnO3 is exposed to ethanol, HCHO, or otherreductive gas atmosphere, these gas molecules can react withadsorbed oxygen species on its surface.14 This process releasesthe trapped electrons back to the conduction band and finallyleads to an increase of electron concentration, which results ina decrease in the resistances. ZnSnO3 is an n-type semiconduc-tor, and the sensing mechanism also shows this kind ofcharacteristic. The high sensitivity of the ZnSnO3 gas sensor isattributed to the small grain size and high surface-to-volumeratio of the hollow structure. As an n-type semiconductor,ZnSnO3 has potential application in gas sensors. More in-depthinvestigation is necessary to further exploit the synthesis routeand improve gas sensing properties.

4. Conclusions

In summary, a simple one-pot solution route has been reportedfor the synthesis of peculiar ZnSnO3 nanostructure withhierarchical hollow-cubic shapes. The hierarchical ZnSnO3

nanocages with the side length of 200-400 nm are composedof many nanoparticles. The time-dependent morphology ofZnSnO3 samples is studied in detail, and a possible formationmechanism is discussed from the viewpoint of nucleation andself-assembly of building blocks. The capping reagent of HMTexerts crucial influences on these unique ZnSnO3 architectures.In addition, the gas-sensing properties of our ZnSnO3-basedsensor exhibit high sensitivity, fast response, and good repeat-ability to ethanol. On the basis of this hydrothermal route andthe gained unique nanostructures, it is significant for exploitingthe synthesis of other semiconductors with novel shapes andvarious potential applications.

Acknowledgment. This work was financially supported byNational Natural Science Foundation of China (Grant No.10672139), the National Science Fund for DistinguishedYoung Scholars of China (Grant No. 60625301), the Scienceand Technology Office, Jilin Province, China (Grant No.2006528), and the Open Project of the Key Laboratory ofLow Dimensional Materials & Application Technology(Xiangtan University), Ministry of Education, China (GrantNo. KF0706).

Supporting Information Available: Figure S1 shows aphotograph of the RQ-2 gas-sensing characterization system.Figure S2 shows a panoramic FESEM image of samplesobtained after the hydrothermal process of 10 min. FigureS3 shows the TEM image of samples obtained without HMT.Figure S4 and its inset exhibit the morphology of samplesobtained with CTAB instead of HMT. Figure S5 shows thedynamical resistances of hierarchical ZnSnO3 nanocages andsolid cubes to 10 ppm ethanol. The inset is the FESEM imageof solid ZnSnO3 nanocubes. This material is available freeof charge via the Internet at http://pubs.acs.org.

References and Notes

(1) (a) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai,G. A.; Alivisatos, A. P. Science 2004, 304, 711. (b) Park, S.; Lim, J.; Chung,S.; Mirkin, C. A. Science 2004, 303, 348. (c) Lu, X. M.; Au, L.; McLellan,J.; Li, Z. Y.; Marquez, M.; Xia, Y. N. Nano Lett. 2007, 7, 1764.

(2) (a) Fowler, C. E.; Khushalani, D.; Mann, S. Chem. Commun. 2001,19, 2028. (b) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (c)Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400. (d) Kim,S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124,7642.

(3) (a) Sun, Y.; Mayers, B. T.; Xia, Y. Nano Lett. 2002, 2, 481. (b)Skrabalak, S. E.; Chen, J.; Au, L.; Lu, X.; Li, X.; Xia, Y. AdV. Mater.2007, 19, 3177. (c) Xiong, Y.; Wiley, B.; Chen, J.; Li, Z.-Y.; Yin, Y.; Xia,Y. Angew. Chem., Int. Ed. 2005, 44, 7913. (d) Cobley, C. M.; Campbell,D. J.; Xia, Y. AdV. Mater. 2008, 20, 748.

(4) (a) Kim, D.; Park, J.; An, K.; Yang, N.-K.; Park, J.-G.; Hyeon, T.J. Am. Chem. Soc. 2007, 129, 5812. (b) Wang, X.; Fu, H. B.; Peng, A. D.;Zhai, T. Y.; Ma, Y.; Yuan, F. L.; Yao, J. N. AdV. Mater. 2009, 21, 1.

(5) Yin, Y. D.; Erdonmez, C.; Aloni, S.; Alivisatos, A. P. J. Am. Chem.Soc. 2006, 128, 12671.

(6) (a) Zhang, T. S.; Shen, Y. S. Mater. Lett. 1995, 23, 69. (b) Xu,J. Q.; Jia, X. H.; Lou, X. D.; Xi, G. X.; Han, J. J.; Gao, Q. H. Sens. ActuatorsB 2007, 120, 694. (c) Shen, Y. S.; Zhang, T. S. Sens. Actuators B 1993,12, 5. (d) Wu, X. H.; Wang, Y. D.; Li, Y. F.; Zhang, Z. L. Mater. Chem.Phys. 2002, 77, 588.

(7) Geng, B. Y.; Fang, C. H.; Zhan, F. M.; Yu, N. Small 2008, 4,1337.

(8) Xue, X. Y.; Chen, Y. J.; Li, Q. H.; Wang, C.; Wang, Y. G.; Wang,T. H. Appl. Phys. Lett. 2006, 88, 182102.

(9) Wang, W. W.; Zhu, Y. J.; Yang, L. X. AdV. Funct. Mater. 2007,17, 59.

Figure 6. Dynamical response curves of the ZnSnO3 gas sensorexposed to different concentrations of (a) HCHO and (b) ethanol attemperatures of 210 and 270 °C, respectively.

One-Pot Synthesis of Hierarchical ZnSnO3 Nanocages J. Phys. Chem. C, Vol. 113, No. 44, 2009 19003

(10) (a) Xi, G.; Peng, Y.; Yu, W.; Qian, Y. Cryst. Growth Des. 2005,5, 325. (b) Yu, S.; Colfen, H.; Antonietti, M. J. Phys. Chem. B 2003, 107,7396. (c) Yu, J. C.; Xu, A.; Zhang, L.; Song, R.; Wu, L. J. Phys. Chem. B2004, 108, 64.

(11) Kim, Y. S.; Ha, S. C.; Kim, K.; Yang, H.; Park, J. T.; Lee, C. H.;Choi, J.; Paek, J.; Lee, K. Appl. Phys. Lett. 2005, 86, 213105.

(12) (a) Gergintschew, Z.; Forster, H.; Kositza, J.; Schipanski, D. Sens.Actuators B 1995, 26, 170. (b) Williams, D. E. Sens. Actuators B 1999, 57,1. (c) Gong, H.; Wang, Y. J.; Teo, S. C.; Huang, L. Sens. Actuators B1999, 54, 232. (d) Sberveglieri, G.; Baratto, C.; Comini, E.; Faglia, G.;

Ferroni, M.; Ponzoni, A.; Vomiero, A. Sens. Actuators B 2007, 121, 208.(e) Coppa, B. J.; Fulton, C. C.; Kiesel, S. M.; Davis, R. F.; Pandarinath,C.; Burnette, J. E.; Nemanich, R. J.; Smith, D. J. J. Appl. Phys. 2005, 97,103517.

(13) Morrison, S. R. Sens. Actuators B 1982, 2, 329.(14) (a) Gao, T.; Wang, T. H. Appl. Phys. A: Mater. Sci. Process. 2005,

80, 1451. (b) Kolmakov, A.; Zhang, Y.; Chen, G.; Moskovits, M. AdV.Mater. 2003, 12, 997.

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