Size-Controlled Synthesis of ZnSnO3 Cubic ... - ACS Publications

Jul 27, 2010 - mechanism of cubic crystallites was attributed to a nucleation assembly ... And as the particle-size of ZnSnO3 cubic crystallites decre...
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J. Phys. Chem. C 2010, 114, 13577–13582

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Size-Controlled Synthesis of ZnSnO3 Cubic Crystallites at Low Temperatures and Their HCHO-Sensing Properties Zhengjun Wang, Jie Liu, Feijiu Wang, Siya Chen, Hui Luo, and Xibin Yu* Department of Chemistry, Shanghai Normal UniVersity, Shanghai 200234, People’s Republic of China ReceiVed: May 24, 2010; ReVised Manuscript ReceiVed: July 11, 2010

Uniform and monodisperse ZnSnO3 cubic crystallites were prepared via a solution process involving the reaction of zinc sulfate and sodium stannate at a reaction temperature as low as 0 °C without any surfactant. The size was readily controlled from 40 to 600 nm by varying the reaction temperature. The possible formation mechanism of cubic crystallites was attributed to a nucleation assembly process. The as-fabricated sensors based on ZnSnO3 cubic crystallites showed high sensitivity, fast response, and short recovery times toward HCHO gas. And as the particle-size of ZnSnO3 cubic crystallites decrease, the sensitivity of gas sensors based on them increases and the recovery time shorten rapidly. The detection limit of ZnSnO3 cubic crystallites sensor can reach as little as lower than one per million for HCHO. The performance of a home-built ZnSnO3 cubic crystallites sensor was even better than the competing SnO2 and In2O3 sensor, making this material interesting for sensor devices. Introduction Formaldehyde (HCHO) vapor is closely connected to public health and safety, and it is a colorless, strong-smelling, and wellknown carcinogen often used in the workplace and comes from the adhesive used in the manufacture of resins, plastics, coatings, and fabrics.1 HCHO injures the eyes, nose, and respiratory organs and causes allergies, which is called sick house syndrome at low levels1 and may cause death at concentrations higher than 15 ppm.2 There are some conventional methods to detect HCHO in the laboratory and industry, such as colorimetry,3 polarography,4 chromatography,5 spectroscopy,6,7 and fluorescence.8,9 However, they usually need large equipment and long detection time. Rapid, facile, and accurate detection of formaldehyde vapor is especially important from the practical point of view. Semiconducting metal oxides, for instance, SnO2,10-14 polyhedral zinc stannate (ZnSnO3),2 CdIn2O4 nanoparticles,15 vanadium pentoxide nanobelts,16 and so on, have been extensively investigated and used for gas detection because of their sensitivities for different gaseous species.17,18 The gas-sensing process of metal oxide sensors generally involves a catalytic reaction between the gas to be monitored and the adsorbed oxygen on the surface of the sensor. The particle size, defects, surface, and interface properties of metal oxide sensors directly affect the state and the amount of oxygen species on the surface of the sensors and consequently the performance of sensors. Among these factors, high surface area and lesser particle size of semiconducting metal oxide is particularly advantageous for evidently enhancing the sensing performance.19-21 The preparation method for sensing material therefore plays an important role in tailoring the morphological characteristics and control over the particle size and surface area of the sensor. Classical aqueous sol-gel techniques have been widely applied for the synthesis of metal oxide nanoparticles, because they offer several advantages over the high-temperature solidstate methods, such as high purity and homogeneity and low * To whom correspondence should be addressed. Phone: +86-2164324528. Fax: +86-21-64322511. E-mail: [email protected].

processing temperatures.22,23 However, the synthesis of composite metal oxides is particularly difficult in water, due to the different reactivities of the individual precursors. The resulting precipitates are often amorphous, which means that subsequent heat treatment is necessary to induce crystallization. This annealing step leads to particle growth. On the other hand, lowtemperature ion exchange24 and coprecipitation methods25-27 usually need complex operating procedures and expensive raw materials. The hydrothermal synthesis route28 also requires too much time and higher temperatures and wastes too much energy. In this paper, to our knowledge, is the first report of the synthesis of cubic ZnSnO3 nanocrystals with lesser particle size (∼40 nm) and high surface area via a solution process involving the reaction of zinc sulfate and sodium stannate at a reaction temperature of as low as 0 °C. In addition, we also found that cubic ZnSnO3 cubic crystallites based sensors shows high sensitivity, fast response, and short recovery times to HCHO gas and the detection limit of this sensor can reach as little as lower than one per million for HCHO. Furthermore, the procedure can easily be scaled-up, which is particularly attractive for industrial applications. Experimental Section Preparation of ZnSnO3 Cubic Crystallites. All reagents were of analytic grade from Aladdin reagent (China) Co., Ltd., and used without further purification. In a typical experiment for the synthesis of 40 nm cubic of ZnSnO3 cubic crystallites, 2.8754 g (10 mmol) of zinc sulfate heptahydrate (ZnSO4 · 7H2O) were added into 100 mL of deionized water, and the solution was stirred at room temperature until zinc sulfate heptahydrate was dissolved completely. Then, 20 mL of sodium stannate solution (Na2SnO3 · 3H2O) was poured into zinc sulfate heptahydrate solution, making the molar ration of zinc sulfate heptahydrate and sodium stannate to be 1:1. The mixed solution was vigorously stirred at 0 °C for 5 h. After the reaction, the precipitates were collected by centrifugation and washed with deionized water for several times to remove residual ions in the products. The final products were then dried in air at 100 °C before characterization.

10.1021/jp104733e  2010 American Chemical Society Published on Web 07/27/2010

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Wang et al.

Figure 1. XRD pattern of the ZnSnO3 products fabricated at different temperature and standard XRD pattern of ZnSnO3 (JCPDS No.11-0274).

General Characterization. X-ray powder diffraction (XRD) pattern was recorded using a Japan Regaku D/max c¸A X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.5418 Å) irradiated with a scanning rate of 4 degree/min. The Field-emission scanning electron microscopic (FESEM) images were obtained using a JEOL JSM-7500F microscope operated at an acceleration voltage of 15 kV. A JEOL JEM-200CX microscope operating at 160 kV in the bright-field mode was used for transmission electron microscopy (TEM). Selected area electron diffraction (SAED) patterns were performed on a JEOL JEM-2010 electron microscope operating at 200 kV. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface area by using adsorption data in the range of the relative pressures (p/p0) from 0.02 to 0.20. Sensing Tests. The gas-sensing properties were measured using a static test system of HW-30 made by Hanwei Electronics Co. Ltd., Henan Province, China. The sensors were fabricated by a modifying method described in another work.29 The obtained ZnSnO3 powders were dispersed into ethanol to form gas-sensing paste. Then the paste was applied on an alumina tube substrate. The ends of the tubes were coated by two Au electrodes and connected four Pt conducting wires. (Schematic drawing of a sensor element, the electric circuit, and photograph of the gas sensor are shown in the Supporting Information.) The tube was followed by drying at 80 °C for 3 h and subsequent calcination at 400 °C for 1 h in air. Before the sensitivity measurement, the samples were connected to the 5 V direct current source, and the heat voltage was maintained at 5 V until the stabilization of the baseline voltage. In our gas-sensing measurements, a given amount of test gas was injected into a closed chamber, and the sensor was put into the chamber for the measurement of the sensitive performance. After each measurement, the sensor was exposed to the atmospheric air by opening the chamber. Sensitivity was defined as the ratio (S) of the average resistance (in the range of 30 s) in air (Ra) to that in the test gas (Rg):S ) (Ra)/(Rg). The response and recovery time were defined as the time taken by the sensor to achieve 90% of the total resistance change in the case of adsorption and desorption, respectively. Results and Discussion The composition and phase purity of as-obtained products were first examined by X-ray powder diffraction (XRD) patterns. Figure 1 shows that the XRD pattern of a typical ZnSnO3 cubic

Figure 2. Typical FESEM images of the as-prepared ZnSnO3 products at (A) 0 °C; (B) 20 °C; (C) 40 °C; (D) 60 °C; (E) 80 °C.

crystallites sample fabricated at different temperature. All of the diffraction peaks can be indexed to the standard ZnSnO3 with the perovskite structure (JCPDS No.11-0274). No impurity phases were detected from the XRD pattern, indicating that ZnSnO3 cubic crystallites with high purity could be obtained under current synthetic conditions at the reaction temperature from 0 to 80 °C. FESEM images of typical sample of ZnSnO3 products fabricated at different temperatures are shown in Figure 2. It can be clearly seen that the ZnSnO3 products are composed of large-scale uniform and monodisperse cubic crystallites, suggesting the high yield achieved with this approach. For the study of the influence of the reaction temperature on the particle-size of ZnSnO3 cubic crystallite, the reaction temperature was increased from 0 to 80 °C by a step of 20 °C while other synthetic parameters were kept unchanged. Concomitant with the increase of reaction temperature, the particle-size of obtained ZnSnO3 cubic crystallites increased from 30-40 nm (reaction temperature 0 °C) to 550-600 nm (reaction temperature 80 °C), and the crystalline gradually increased (as shown in Figure 1). On the basis of the experimental data obtained above, it could be concluded that the lower reaction temperature was contributive to the smaller particle size of as-obtained ZnSnO3 cubic crystallites. Similarly controlled synthesis ZnSnO3 nano, microcrystallites were also reported by other science workers,22,23 but to our knowledge, this is the first time report of the synthesis of cubic ZnSnO3 nanocrystallites with so simple a procedure and at so low a reaction temperature. As the particle size of ZnSnO3 cubic crystallites produced at the lower temperature is similar to the size of amorphous particles, in order to facilitate observation of ZnSnO3 cubic crystallites formation process, we chose a series of ZnSnO3 cubic crystallites produced at 60 °C for different reaction times as the example. The XRD patterns and morphologies of the samples were carefully investigated by quenching the reaction time at different time intervals so as to gain a better understand-

Size-Controlled Synthesis of ZnSnO3 Crystallites

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13579 SCHEME 1: Schematic Illustration for the Possible Formation Mechanism of ZnSnO3 Cubic Crystallites

Figure 3. XRD pattern of the ZnSnO3 products fabricated at different reaction times.

Figure 4. Typical FESEM images of ZnSnO3 products at different reaction times (A) 20 min; (B) 1 h; (C) 3 h; (D) 5 h.

ing of the evolution mechanism of ZnSnO3 cubic crystallites. As shown in Figure 3, the intensity of diffraction peaks increases with the extended reaction time, indicating that the crystalline degree of the samples becomes higher and higher. Parts A-D of Figure 4 shows the FESEM images of the corresponding intermediates. When the reaction time is reduced to 20 min, the as-obtained products consist of amorphous nanoparticles with a mean diameter of about 10 nm and the precipitates are illcrystallized, as confirmed by the FESEM image (Figure 4A) and a few weak peaks in the XRD pattern (Figure 3). When the reaction time is prolonged to 1 h, amorphous nanoparticles begin to aggregate and to form random aggregation with approximate right angles as shown in Figure 4B. After 3 h, more cubes with smooth surface appear in the products, but there also are a lot of amorphous nanoparticles on the surface of cubes (Figure 4C). With the further extension of reaction time, the small particles gradually disappear. This can be explained by the Ostwald ripening law, because of surface tension (and surface to-volume ratios), small particles are much more soluble than large particles. A typical FESEM image of the as-prepared ZnSnO3 cubic crystallites at 60 °C for 5 h is shown in Figure 4D, where the amorphous nanoparticles disappear competently, the uniformed monodisperse ZnSnO3 cubic crystallites with side lengths of about 400 nm finally enlarge and separate from each other. In comparison with other template-based synthetic techniques, this adopted method has no catalyst to serve and has no template

to guide for the uniform cubic crystallites in the solution process. Thus the driving force for the anisotropic growth of the asprepared ZnSnO3 cubic crystallites may derive from the inherent crystal structure of the materials and their chemical potential in solution. Although the exact mechanism for the formation of these cubic crystallites is still unclear, it is believed that the growth of the cubic morphology is not assisted by a catalyst or directed by a template. In this case, it is likely that the crystal growth of as-formed cubic particles is governed by a solutionsolid (SS) process.30,31 In this process, first a rapid mixing of Zn2+ and SnO32- led to the formation of colloid particles which immediately appeared in solution. The as-formed colloid particles were partially dissolved in water to form a metastable supersaturated transparent solution. Then a large number of the amorphous precursor seeds formed through a homogeneous nucleation process. Subsequently they aggregated into nanoparticle clusters, and their sizes increased as the reaction temperature increased, which is the reason that the particle size can be controlled by changing the reaction temperature (step 1). During the subsequent crystal stage, the self-aggregated nanoparticle clusters grew into cubic crystallites. At the same time, a few amorphous nanoparticles attached on the surface of cubic crystals (step 2). According to the Ostwald ripening law, the attached small amorphous particles dwindled and the large cubic crystals grew (step 3). As the reaction time progressed, the suspended amorphous particles were completely consumed and the perfect and uniform cubic crystallites were formed eventually (step 4). This phenomenon is similar to previous reports of many other compounds.32,33 A possible schematic illustration of the formation mechanism of the ZnSnO3 cubic crystallites is shown in Scheme 1. Recently, ZnSnO3 was found to be a sensor material for HCHO. Xu et al have reported that the HCHO sensitivity of hexagonal ZnSnO3 microparticles can reach 4.5. Jia et al have reported that the HCHO sensitivity can be improved to 35 with doping Au into mixed oxides of ZnO/ZnSnO3. Geng et al. have reported that the detection limit of 14-faceted polyhedral ZnSnO3 can reach several parts per million. The sensitivity should improve to quickly detect lower concentration HCHO because of its large number of applications and highly toxicity. According to the sensing mechanism, control over particle size is particularly advantageous for drastically enhancing the sensing performance with increasing surface area.34-36 The growth of colloidal particles via solution routes can be believed to be a diffusion-controlled process, that is, each particle nucleates and grows in one limited diffusion-controlled domain, as originally described by Lamer et al.37 In general, according to the Stokes-Einstein equation

D)

kT 6πηr

(1)

where D is the diffusion coefficient, k is the Boltzmann constant, T is the reaction temperature, η is the viscosity of the solution,

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Figure 5. The UV-vis absorption spectra of ZnSnO3 cubic crystallites of different particle sizes (40 nm, 200 nm, 600 nm).

TABLE 1: Surface Area of ZnSnO3 Cubic Crystallites with Different Particle Sizes sample code

mean particle size (nm)

surface area (m2/g)

sample 1 sample 2 sample 3

40 200 600

109.5 68.0 36.4

and r is the effective hydrodynamic radius of the spherical particles or molecules. According to the equation above, the higher the reaction temperature is, the larger the diffusion coefficient of the reaction system is, the larger the particle-size of as-obtained ZnSnO3 cubic crystallites with other parameters being identical. Therefore, by varying the reaction temperature,

Wang et al. the particle size of as-obtained ZnSnO3 cubic crystallites could be deliberately controlled in the synthetic method. In the work described herein, different particle-size ZnSnO3 cubic crystallites have been prepared by varying the reaction temperature and their surface area are shown in Table 1. It has been proven that concomitant with the decrease of reaction temperature from 80 to 0 °C the particle size of obtained ZnSnO3 cubic crystallites decreased from 550 to 600 nm (reaction temperature 0 °C) to 30-40 nm (reaction temperature 80 °C) and the surface area increases quickly, which we consider may exhibit excellent gas-sensing properties. ZnSnO3 is an n-type semiconductor. The principle of gas detection of the resistance-type sensors is based on the conductance variation of the sensing element, which depends on the gas atmosphere and on the operating temperature of the sensing material exposed to the test gas, thus resulting in the space charge layer changes and band modulation. According to Wolkenstein’s model for semiconductors,38,39 oxygen species were adsorbed on the surface of ZnSnO3 sensors in the air and then were ionized into O- (ads) or O2- (ads) by capturing free electrons from the particles because of the strong electron negativity of the oxygen atom. Therefore, the concentration of electrons in the conduction band decreases, and the resistance of the material increases. When the sensor was put into a tested gas, for example, HCHO in our case, HCHO would react with the O- (ads) or O2-(ads) to form CO2, H2O and the reaction equation is following40

HCHO(gas) + O2-(ads) f H2O(g) + CO2(g) + 2e(1a)

Figure 6. (A) Response vs working voltage of ZnSnO3 cubic crystallites sensors of different size exposed to 100 ppm HCHO. (B) Sensitivity of ZnSnO3 cubic crystallites sensors of different size to different concentrations of HCHO. (C) Typical response curves of ZnSnO3 cubic crystallites sensors of different size to HCHO with increasing concentrations. (D) Sensitivity of the cubic ZnSnO3 (40 nm) sensor to different tested gases. In parts A-C, lines 1-3 correspond to the ZnSnO3 cubic crystallites of 40 nm, 200 nm, and 600 nm particle size, respectively.

Size-Controlled Synthesis of ZnSnO3 Crystallites This process results in thinning of space-charge layer and decreasing of potential barrier and thus current rises. In this case, the resistance of the sensor was low. The resistance of the material decreases owing to the electrons produced from the reaction, which results in an increase in the output voltage. The UV-vis absorption spectra of ZnSnO3 cubic crystallites with different particle-sizes are shown in Figure 5 and the Eg calculated on the basis of the corresponding absorption edges are 5.28 eV (600 nm), 5.23 eV (200 nm), and 5.00 eV (40 nm), respectively. The smaller the particle-size of ZnSnO3 cubic crystallites, the less Eg there is. According to the HCHO sensing mechanism of ZnSnO3 above, less band gap energy should help the O2 adsorption on the ZnSnO3 surface to trap electrons from the conduction band of ZnSnO3 and enhance the sensing performance. On the basis of the analysis above, ZnSnO3 cubic crystallites with smaller particle size have been prepared by varying the reaction temperature, which should exhibit higher sensitivity because of their larger surface area and less Eg. First, the different particle size ZnSnO3 cubic crystallites with the particle size of 40 nm (line 1), 200 nm (line 2), and 600 nm (line 3) based gas sensors are performed at different working voltages to 100 ppm HCHO, and the sensitivity as a function of the working voltage is shown in Figure 6A, which indicates that the sensitivity of the sensor increases and reaches maximum at 5.25 V and then decreases rapidly with further increase of the working voltage. So in the further test, the working voltage is fixed as 5.25 V. From Figure 6A, other important information is obtained: S(1) > S(2) > S(3). Figure 6B shows the sensitivity of ZnSnO3-based sensor gradually increases as the HCHO concentrations increase from 10 to 100 ppm, which demonstrates that ZnSnO3 based sensors have a wide detection limit for HCHO. It once again proven that S(1) > S(2) > S(3). Figure 6C shows the typical response curves of ZnSnO3-based gas sensors to HCHO with increasing concentration at room temperature. As can be seen, gas sensors can fast response to HCHO, and their sensitivity increases rapidly with HCHO concentrations. The results reveal that the sensitivities of the ZnSnO3-based gas sensors are excellent to HCHO. All the three sensors based on as-prepared ZnSnO3 exhibit much better sensitivity to HCHO than those of previous reports.41 It is also clear that with an increase in the HCHO concentration, the sensitivities increase, but the sensitivity of the ZnSnO3 cubic crystallites with the particle-size of 40 nm increases faster than that of the other two. Hence, the ZnSnO3 cubic crystallite with the particle size of 40 nm sensor is much more sensitive than the other two. The detection limit of the as-prepared ZnSnO3 sensor can reach as little as several parts per million for HCHO. At the same time, the recovery time of sensor based on 40 nm ZnSnO3 cubic crystallites is shorter than the other two is only 5-10 s. We have to point out that our ZnSnO3 cubic NCs-based HCHO sensor even outperforms previous HCHO sensors based on SnO2, In2O3, and V2O5 nanostructures.42-45 In our experiment, we found that as-prepared ZnSnO3 cubic crystallites also have good sensitivity to other test gases like hydrogen sulfide, ammonia, chloroform, toluene, carbon monoxide as shown in Figure 6D. Among them, toluene and chloroform have not been reported. Conclusions The successful synthesis of uniform and monodisperse ZnSnO3 cubic crystallites via a low temperature solution process is presented. The size of as-obtained ZnSnO3 cubic crystallites could be deliberately controlled in the range 40-600 nm by

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13581 varying the reaction temperature. The formation mechanism was discussed based on temperature dependent and time dependent experiments. The high crystallinity and high surface-to-volume ratio of the as-synthesized ZnSnO3 cubic crystallites make them ideal candidates for HCHO gas-sensing devices. The asfabricated sensors showed high sensitivity, fast response, and short recovery times toward HCHO gas. As the particle size of ZnSnO3 cubic crystallites decrease, the sensitivity of gas sensors based on them rapidly increases. The detection limit of ZnSnO3 cubic crystallites sensor can reach as little as lower than one per million for HCHO. The performance of a home-built ZnSnO3 cubic crystallites sensor was even better than the competing SnO2 and In2O3 sensor, making this material interesting for sensor devices. Acknowledgment. The authors would like to thank Innovation Program of Shanghai Municipal Education Commission (10YZ70, 09ZZ136), Science Foundation of Shanghai Normal University (SK201002), Shanghai Science and Technology Development Fund (no. 09520500500), and the Key Laboratory of Resource Chemistry of Ministry of Education of China for financial support. Supporting Information Available: Schematic drawing of a sensor element, the electric circuit, and photograph of the gas sensor. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Suzuki, Y.; Nakano, N.; Suzuki, K. EnViron. Sci. Technol. 2003, 37, 5695–5700. (2) Geng, B.; Fang, C.; Zhan, F.; Yu, N. Small 2008, 4, 1337–1343. (3) Thomas, C. L. P.; Meunier, F.; Veasey, C. A.; McGill, C. D. Anal. Commun. 1998, 35, 103–105. (4) Zhang, Z.-Q.; Zhang, H.; He, G.-F. Talanta 2002, 57, 317–322. (5) Tsai, S.-W. Appl. Occup. EnViron. Hyg. 1999, 14, 355–362. (6) Okachi, T.; Onaka, M. J. Am. Chem. Soc. 2004, 126, 2306–2307. (7) Hoq, M. F.; Indu, B.; Ernst, W. R.; Gelbaum, L. T. Ind. Eng. Chem. Res. 1992, 31, 1807–1810. (8) Wagner, B. K.; Carrinski, H. A.; Ahn, Y.-H.; Kim, Y. K.; Gilbert, T. J.; Fomina, D. A.; Schreiber, S. L.; Chang, Y.-T.; Clemons, P. A. J. Am. Chem. Soc. 2008, 130, 4208–4209. (9) Lai, J.-Y.; Li, Y.-T. Biomacromolecules 2010, 11, 1387–1397. (10) Law, M.; Kind, H.; Messer, B.; Kim, F.; Yang, P. Angew. Chem., Int. Ed. 2002, 41, 2405–2408. (11) Li, C. Z.; D.; Liu, X.; Han, S.; Tang, T.; Han, J.; Zhou, C. Appl. Phys. Lett. 2002, 81, 1869–1873. (12) Comini, E.; Faglia, G.; Sberveglieri, G.; Pan, Z.; Wang, Z. L. Appl. Phys. Lett. 2002, 81, 1869. (13) Wang, Y. L.; Jiang, X. C.; Xia, Y. N. J. Am. Chem. Soc. 2003, 125, 16176. (14) Law, M.; Kind, H.; Messer, B.; Kim, F.; Yang, P. D. Angew. Chem., Int. Ed. 2002, 114, 2511. (15) MinhuaCao;Wang, Y.; Chen, T.; Antonietti, M.; Niederberger, M. Chem. Mater. 2008, 20, 5781–5786. (16) Liu, J.; Wang, X.; Peng, Q.; Li, Y. AdV. Mater. 2005, 17, 764– 767. (17) Franke, M. E.; Koplin, T. J.; Simon, U. Small 2006, 2, 36–50. (18) Eranna, G.; Joshi, B. C.; Runthala, D. P.; Gupta, R. P. Crit. ReV. Solid State Mater. Sci. 2004, 29, 111–123. (19) Yamazoe, N. Sensors Actuators B 1991, 5, 7–16. (20) Pinna, N.; Neri, G.; Antonietti, M.; Niederberger, M. Angew. Chem., Int. Ed. 2004, 43, 4345–4349. (21) Sahm, T.; Ma¨dler, L.; Gurlo, A.; Barsan, N.; Pratsinis, S. E.; Weimar, U. Sensors Actuators B 2004, 98, 148–153. (22) Kurihara, L. K.; Suib, S. L. Chem. Mater. 1993, 5, 609–613. (23) Mao, Y.; Park, T.-J.; Wong, S. S. Chem. Commun. 2005, 5721– 5735. (24) Kovacheva, D.; Petrov, K. Solid State Ionics 1998, 109, 327–332. (25) Wrobel, G.; Piech, M.; Dardona, S.; Ding, Y.; Gao, P.-X. Cryst. Growth Des. 2009, 9, 4456–4461. (26) Xu, J.; Jia, X.; Lou, X.; Xi, G.; Han, J.; Gao, Q. Sens. Actuators B 2007, 120, 694–699. (27) Xue, X. Y.; Guo, T. L.; Lin, Z. X.; Wang, T. H. Mater. Lett. 2008, 62, 1356–1358.

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