A Highly Sensitive and Fast-Responding Ethanol Sensor Based on

Aug 21, 2008 - A Highly Sensitive and Fast-Responding Ethanol Sensor Based on CdIn2O4 Nanocrystals Synthesized by a Nonaqueous Sol−Gel Route...
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Chem. Mater. 2008, 20, 5781–5786

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A Highly Sensitive and Fast-Responding Ethanol Sensor Based on CdIn2O4 Nanocrystals Synthesized by a Nonaqueous Sol-Gel Route Minhua Cao,*,†,‡ Yude Wang,*,†,§ Ting Chen,§ Markus Antonietti,† and Markus Niederberger| Max Planck Institute of Colloids and Interfaces, Colloid Department, Research Campus Golm, 14424 Potsdam, Germany, Department of Chemistry, Northeast Normal UniVersity, 13324, Changchun, China, Department of Materials Science and Engineering, Yunnan UniVersity, 650091, Kunming, China, and Swiss Federal Institute of Technology (ETH) Zu¨rich, Department of Materials, Wolfgang-Pauli-Strasse 10, 8093 Zu¨rich, Switzerland ReceiVed March 19, 2008. ReVised Manuscript ReceiVed July 8, 2008

CdIn2O4 nanoparticles with crystallite sizes of about 10 nm were obtained by a nonaqueous sol-gel route involving the reaction of cadmium acetate and indium isopropoxide in benzyl alcohol. Without the use of any additional stabilizing agents, the crystal growth is restricted by the solvent and/or by the organic reaction products. The as-fabricated sensor showed high response, fast response, and recovery times toward ethanol gas. The performance of a home-built CdIn2O4 nanocrystal sensor was even better than the competing SnO2 and In2O3 sensors, making this material not only interesting for sensor devices, but also for a number of related electrochemical applications.

Introduction The demand for better environmental control and safety has stirred up much interest in the research of gas sensors. Rapid and accurate detection and quantification of ethanol vapor are especially important, because it is closely connected to public health and safety. Developing portable ethanol sensing devices with high response and selectivity are of obvious use. Semiconducting metal oxides have been extensively investigated and used for gas detection, because of their sensitivities for different gaseous species.1,2 So far, commercially available ethanol sensing devices are basically restricted to the binary metal oxide SnO2.3-6 Although SnO2based sensors have good response, there are still limiting factors such as selectivity and low thermodynamic stability. Therefore, other sensor materials for the detection of ethanol are still to be developed. Just recently, Liu et al.7 reported an improved response, when vanadium pentoxide nanobelts are used as the active sensing medium. The unique electron transport through nanobelts makes this material a particularly attractive ethanol gas sensor with enhanced response and * To whom correspondence should be addressed. E-mail: minhua.cao@ mpikg.mpg.de (M.C.). † Max Planck Institute of Colloids and Interfaces. ‡ Northeast Normal University. § Yunnan University. | Swiss Federal Institute of Technology (ETH) Zu¨rich.

(1) Franke, M. E.; Koplin, T. J.; Simon, U. Small 2006, 2, 36. (2) Eranna, G.; Joshi, B. C.; Runthala, D. P.; Gupta, R. P. Crit. ReV. Solid State Mater. Sci. 2004, 29, 111. (3) Law, M.; Kind, H.; Messer, B.; Kim, F.; Yang, P. D. Angew. Chem. 2002, 114, 2511; Angew. Chem., Int. Ed. 2002, 41, 2405. (4) Comini, E.; Faglia, G.; Sberveglieri, G.; Pan, Z.; Wang, Z. L. Appl. Phys. Lett. 2002, 81, 1869. (5) Li, C.; Zhang, D.; Liu, X.; Han, S.; Tang, T.; Han, J.; Zhou, C. Appl. Phys. Lett. 2003, 82, 1613. (6) Wang, Y. L.; Jiang, X. C.; Xia, Y. N. J. Am. Chem. Soc. 2003, 125, 16176. (7) Liu, J. F.; Wang, X.; Peng, Q.; Li, Y. D. AdV. Mater. 2005, 17, 764.

selectivity.7 Ternary metal oxides are promising candidates for the sensor devices, because they exhibit better reconcilability of chemical and physical properties by changing the compositions than binary metal oxides. Nevertheless, only little research has been focused on the sensing properties of these materials, probably because of the lack of simple synthetic procedures for these materials with both high crystallinity and purity. 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. In view of the sensing mechanism, the particle size, defects, surface, interface properties, and stoichiometry directly affect the state and the amount of oxygen species on the surface of the sensors, and consequently the performance of metal oxide-based sensors. Among these factors, control over the particle size is particularly advantageous for drastically enhancing the sensing performance with increasing surface area.8-10 The preparation method for the sensing material therefore plays an important role in tailoring the morphological characteristics of the sensor. Classical aqueous sol-gel techniques have been widely applied for the synthesis of ternary metal oxide nanoparticles, because they offered several advantages over the high temperature solid state methods, such as high purity and homogeneity, and low processing temperatures.11,12 However, the aqueous sol-gel chemistry still suffers from some drawbacks resulting from the high reactivity of the precursors toward hydrolysis. On (8) Yamazoe, N. Sens. Actuators, B 1991, 5, 7. (9) Pinna, N.; Neri, G.; Antonietti, M.; Niederberger, M. Angew. Chem., Int. Ed. 2004, 43, 4345. (10) Sahm, T.; Ma¨dler, L.; Gurlo, A.; Barsan, N.; Pratsinis, S. E.; Weimar, U. Sens. Actuators B 2004, 98, 148. (11) Kurihara, L. K.; Suib, S. L. Chem. Mater. 1993, 5, 609. (12) Mao, Y. B.; Park, T. J.; Wong, S. S. Chem. Commun. 2005, 5721.

10.1021/cm800794y CCC: $40.75  2008 American Chemical Society Published on Web 08/21/2008

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the one hand, the synthesis of ternary and multimetal oxides due to the different reactivities of the individual precursors is particularly difficult in water, and on the other hand, the resulting precipitates are often amorphous, which means that subsequent heat treatment is necessary to induce crystallization. This annealing step undoubtedly leads to particle growth. It is, therefore, rather difficult to obtain nanocrystals of just a few nanometers in size by this method. In view of these disadvantages, nonaqueous sol-gel procedures in organic solvents to crystalline ternary metal oxide nanoparticles13-15 are beneficial, because they offer better control over particle size without the use of surfactants, high crystallinity at relatively low temperature, and simplification of synthesis parameters.15,16 In this paper, we report promising sensing properties of CdIn2O4 nanoparticles with crystallite sizes of about 10 nm obtained by a nonaqueous sol-gel route. CdIn2O4 is wellknown as an n-type semiconductor, characterized by its high free carrier concentration. As a transparent conducting oxide (TCO) material, CdIn2O4 with a carrier concentration of about 1 × 1020 cm-3 can be achieved without additional doping.17 Its conductivity is determined mainly by the concentration of stoichiometric defects, in which oxygen vacancies provide the donor states.18a Gas sensing properties of CdIn2O4 thin films have been studied.18b In view of the gas sensing mechanism involving gas adsorption, a good sensing material must offer a high surface area, which is a direct consequence of the appropriate preparation method. CdIn2O4 powders reported previously in literature were prepared either by a high temperature solid state route or by chemical coprecipitation,19 generally leading to particles with a much larger and less uniform crystallite size. Here, CdIn2O4 nanocrystals with a high surface area were obtained by a nonaqueous sol-gel route. The reaction between cadmium acetate and indium isopropoxide in benzyl alchohol results in the formation of CdIn2O4 nanocrystals under solvothermal conditions. Without the use of any additional stabilizing agents, the crystal growth is restricted by the solvent and/or by the organic reaction products. Furthermore, the procedure can easily be scaled-up, which is particularly attractive for industrial applications. Experimental Section Synthesis. All procedures were started in a glovebox (O2 and H2O < 0.1 ppm). In a typical synthesis of nanoparticles, 0.5 mmol of cadmium acetate (98.0%; Aldrich) and 0.5 mmol of indium (13) Niederberger, M.; Pinna, N.; Polleux, J.; Antonietti, M. Angew. Chem., Int. Ed. 2004, 43, 2270. (14) Garnweitner, G.; Hentschel, J.; Antonietti, M.; Niederberger, M. Chem. Mater. 2005, 17, 4594. (15) Garnweitner, G.; Niederberger, M. J. Am. Ceram. Soc. 2006, 89, 1801. (16) Niederberger, M.; Garnweitner, G. Chem.sEur. J. 2006, 12, 7282. (17) Li, B.; Zeng, L.; Zhang, F. S. Phys. Status Solidi, A 2004, 201, 960. (18) (a) Zakrzewskta, K.; Pisarkiewicz, T.; Czapla, A. Phys. Status Solidi, A 1987, 99, 141. (b) Peng, D. L.; Jiang, S. R.; Xie, L. Phys. Status Solidi, A 1993, 136, 441. (19) (a) Shannon, R. D.; Gillson, J. L.; Bouchard, R. J. J. Phys. Chem. Solids 1977, 38, 877. (b) Chu, X. F.; Liu, X. Q.; Song, Y. C.; Meng, G. Y. Sens. Actuators, B 1999, 61, 19. (c) Chu, X. F.; Liu, X. Q.; Meng, G. Y. Mater. Res. Bull. 1999, 34, 693. (20) (a) Wang, S. X.; Wang, W. L.; Liao, K. J. Rare Met. Mater. Eng. 2002, 31, 73. (b) Pinna, N.; Garnweitner, G.; Beato, P.; Niederberger, M.; Antonietti, M. Small 2005, 1, 112.

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Figure 1. XRD powder pattern of as-prepared CdIn2O4 nanocrystals.

isopropoxide (99.9%; Aldrich) were added to 20 mL of anhydrous benzyl alcohol (99.8%; Aldrich). The reaction mixture was transferred into an autoclave (Parr acid digestion bombs, Teflon cups with 45 mL inner volume). The autoclave was taken out of the glovebox and heated at 200 °C for about 2 days. The resulting cloudy suspension was centrifuged, and the precipitate was thoroughly washed with ethanol and dried at room temperature. Characterization. The X-ray powder diffraction (XRD) diagrams were measured in the reflection mode (Cu KR radiation) on a Bruker D8 diffractometer equipped with a scintillation counter. TEM investigations were performed either on a Zeiss EM 912 Ω microscope, operating at 100 kV, or on a JEM-3000F microscope, operating at 300 kV (HRTEM). Thermogravimetric analysis (TGA) was performed on a Netzsch TG 209 F1 at a scanning rate of 10 K/min in air. Fourier transform infrared spectra (FTIR) were obtained with a rapid FTIR spectrometer (Varian 1000). Sensing Tests. The sensor was prepared by coating assynthesized CdIn2O4 nanocrystals dispersed in water as a sensing layer with a thickness of about 0.6-0.8 mm on a prefabricated alumina tube (7 mm in length and 1.5 mm in diameter) with gold electrodes and platinum wires. The element was calcined at 400 °C for 2 h. A Ni-Cr alloy crossing the alumina tube was used as a resistor, which ensured substrate heating and temperature control. Before measuring the gas sensing properties, we first aged the gas sensors at 250 °C for 150 h in air. The gas-sensing properties were tested in a chamber through which a controlled atmosphere was allowed to flow. The resistance of the sensor was measured by using a conventional circuit in which the element was connected with an external resistor in series at a circuit voltage of 10 V. The electrical response of the sensor was measured with an automatic test system, controlled by a personal computer.

Results and Discussion The X-ray powder diffraction (XRD) pattern of the assynthesized nanoparticles is shown in Figure 1. All diffraction peaks can be assigned to the cubic CdIn2O4 phase (JCPDS 31-174) without any indication of crystalline byproducts such as CdO or In2O3. This result proves that it is possible to obtain the pure ternary metal oxide by this soft-chemistry approach. By the Debye-Scherrer analysis, the crystal size of the as-synthesized CdIn2O4 nanocrystal is about 9.8 nm. Representative transmission electron microscopy (TEM) images of the CdIn2O4 sample are shown in Figure 2. The

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Figure 2. (A) TEM overview image of as-synthesized CdIn2O4 nanocrystals; (B) SAED pattern of an ensemble of CdIn2O4 nanocrystals; (C) HRTEM image of one nanocrystal.

overview image in Figure 2a at low magnification proves that the sample entirely consists of individual CdIn2O4 particles with sizes of about 10 nm, larger particles or agglomerates being mostly absent. The selected area electron diffraction (SAED) pattern displays diffraction rings that are characteristic for crystalline nanoparticles and that match with the cubic CdIn2O4 structure (Figure 2b). The high-resolution TEM (HRTEM) image shows the spherelike shape in more detail and the well-developed lattice planes additionally demonstrate the high crystallinity (Figure 2c). The Brunauer-Emmett-Teller (BET) surface area of the as-synthesized CdIn2O4 sample amounts to 56 m2/g, which provides the high surface area desired for gas adsorption applications. The surface properties of the as-synthesized CdIn2O4 nanocrystals were investigated by Fourier transform infrared spectroscopy (FTIR). In addition to the bands below 1000 cm-1, which can be assigned to the Cd-O-In bond,20a the FTIR spectrum in Figure S1 of the Supporting Information displays two strong absorption bands at 1538 and 1391 cm-1, and two weak bands at 2950 and 714 cm-1. These bands can be assigned to the asymmetric and symmetric vibrations of carboxylate groups,20b and C-H stretching and out-ofplane C-H vibrations of phenyl groups, respectively, suggesting that a small quantity of benzoic acid molecules, formed in situ during the reaction, are adsorbed on the surface of the as-synthesized nanocrystals.20b Thermogravimetric analysis (TGA) reveals that the weight fraction of the adsorbed organic molecules is around 5 wt % (Figure S2 in the Supporting Information). The molar ratio of cadmium acetate to indium isopropoxide has a large effect on the purity of final product and therefore has been investigated in detail. From the molecular formula of CdIn2O4, we know the stoichiometric ratio of Cd to In is 0.5. However, pure CdIn2O4 phase discussed above is formed with a cadmium acetate to indium isopropoxide ratio of 1. To highlight the influence of the molar ratio of above two precursors on the purity of final product, we carried out the reference experiments, where the ratio of cadmium acetate to indium isopropoxide was varied from 0.5 to 0.85 while keeping a constant reaction temperature of 200 °C. The composition of final product was monitored by corresponding XRD pattern. As shown in Figure 3a, the XRD pattern of the sample obtained with a ratio of cadmium acetate to indium isopropoxide of 0.5 indicates that all diffraction peaks can be indexed as a mixture of CdIn2O4

Figure 3. XRD patterns of samples obtained at different ratios of cadmium acetate to indium isopropoxide: (a) 0.5, (b) 0.7, and (c) 0.85.

and In2O3. With increasing the ratio to 0.7 and 0.85, the relative intensities of the diffraction peaks for In2O3 become gradually weaker (patterns b and c in Figure 3). Pure CdIn2O4 phase was finally obtained with the ratio of 1 (Figure 1). The reason may be that cadmium acetate and indium isopropoxide have different activity in benzyl alcohol, thus leading to their different conversion to corresponding oxides. In addition, the reaction temperature also has an important effect on the purity of final product. If the reaction temperature was below 200 °C, pure CdIn2O4 phase could not be obtained. Figure 4A shows a sketch of the sensor device used here, together with a photograph of the as-fabricated gas sensor using the CdIn2O4 nanocrystals (Figure 4B). This particular setup for the sensor device exhibits several advantages such as high response, fast response and recovery time, and good stability. The sensor signal was defined as the ratio of the electrical resistance of the sensor in air (Ra, at a relative humidity of 56%) to that in the test gas (Rg). Before the gas sensing properties were measured, the sensors were calcined at 400 °C for 2 h, followed by aging at 250 °C for 150 h in air to improve the stability and repeatability. The XRD and TEM results of the calcined sample are shown in Figures S3 and S4 in the Supporting Information. No large agglomerates were observed from the TEM image. The size of the nanocrystals was calculated to be 10 nm by the Debye-Scherrer analysis according to the XRD image, which is almost same as that of the CdIn2O4 nanocrystals prior to heat treatment, indicating their good thermal stability. The electrical conductivity of a sensor not only depends on the gas atmosphere but also on the operating temperature.2 Figure 5A shows the relation between the resistance and the operating temperature of the CdIn2O4 nanocrystal sensor. The resistance increases with increase of temperature in the range of 150-260 °C (line a in Figure 5A). This result is probably due to the fact that the physical adsorption predominant at low temperatures may gradually transforms to chemical adsorption, and O-2(ads), O-(ads), OH-(ads), and O2-(ads), were formed on the surface by capturing electrons from the sensing layer, thus leading to the increase of resistance. If the temperature continues to

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Figure 4. (A) Sketch of the structure of a typical CdIn2O4 nanocrystal gas sensor; (B) photograph of the gas sensor.

Figure 5. (A) Relation between the resistance and the operating temperature; (B) effects of the operating temperature on the response of the sensor at different concentrations of C2H5OH.

increase, a dynamic equilibrium occurs between the adsorption and desorption of oxygen on the surface of the sensor material, leading to a constant resistance.21 According to Figure 5A (line b), the resistance is nearly independent of the operating temperature in the range of 260-320 °C, which means that a sensor based on these CdIn2O4 nanoparticles is expected to exhibit excellent thermal stability over a relatively broad operating temperature range. This feature is of great significance especially for applications in areas with temperature fluctuations. The temperature-dependent resistivity profile as observed in Figure 5A, line a and b, is characteristic of a typical surface-controlled model.22 However, above 320 °C, the resistance decreases (line c in Figure (21) Mcaleer, J. F.; Moseley, P. T.; Norris, J. O. W.; Williams, D. E. J. Chem. Soc., Faraday Trans. I 1987, 85, 1323. (22) Takata, M.; Tsubone, D.; Yanagida, H. J. Am. Ceram. Soc. 1976, 59, 4.

5A), probably because of intrinsic defects that become responsible for the conductance of the sensor at higher temperature,23 and the increased mobility of the charge carriers. Figure 5B shows the sensing properties of the CdIn2O4 nanocrystals toward ethanol gas. The gas response is considerably better than the one recently reported for classical CdIn2O4- and In2O3-based ethanol sensors.20a,24-29 According to Figure 5B, the sensor response of the CdIn2O4 nanocrystals has an optimal operating temperature of about 255-275 °C. The sensor response first increases gradually and then decreases with increasing operating temperature. Furthermore, ethanol gases of different concentrations ranging from 100 to 1000 ppm (Figure 5B) result in an increase of the sensor response with larger ethanol contents, whereas the temperature with the highest response is almost constant. This behavior enables a pronounced discrimination of ethanol gas of different concentrations in this temperature range. A quick response/recovery time was observed with this sensor at an operating temperature of 260 °C. The response time is defined as the time needed to reach 90% of the equilibrium value after the injection of the test gas. The recovery time is defined as the time needed to return to 10% above the original response in air after stopping the flow of the test gas.7 Figure 6A shows the typical dynamic response curve of the CdIn2O4 sensor toward different ethanol concentrations. Response and recovery times calculated from the case of 1000 ppm are 6 and 30 s, respectively. We have to point out that our CdIn2O4 nancrystal-based ethanol sensor even outperforms previous ethanol sensors based on SnO2 and V2O5 nanostructures.7,27-29 Furthermore, the selectivity of the CdIn2O4 nanocrystal sensor was also investigated. As shown in Figure 6B, we tried to detect gasoline and butane gases under the same conditions at an operating temperature of 260 °C. From the curve of the sensor response vs gas concentration, we find quite a low response to gasoline and butane. This result indicates that CdIn2O4 nanocrystals are (23) Wang, Y. D.; Chen, Z. X.; Li, Y. F.; Zhou, Z. L.; Wu, X. H. SolidState Electron. 2001, 45, 639. (24) Dong, Y. F.; Wang, W. L.; Liao, K. J. Sens. Actuators, B 2000, 67, 254. (25) Li, B. X.; Xie, Y.; Jing, M.; Rong, G. X.; Tang, Y. C.; Zhang, G. Z. Langmuir 2006, 22, 9380. (26) Chu, X. F.; Wang, C. H.; Jiang, D. L.; Zheng, C M. Chem. Phys. Lett. 2004, 399, 461. (27) Chiu, H. C.; Yeh, C. S. J. Phys. Chem. C 2007, 111, 7256. (28) Sun, F. Q.; Cai, W. P.; Li, Y.; Jia, L. H.; Lu, F. AdV. Mater. 2005, 17, 2872. (29) Liu, Y.; Koep, E.; Liu, M. L. Chem. Mater. 2005, 17, 3997.

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Figure 6. (A) Typical dynamic response curve of CdIn2O4 nanoparticle sensor toward ethanol with increasing concentrations at 260 °C. (B) Variation in responses of the sensor toward ethanol, gasoline, or butane at 260 °C.

a good new gas-sensing material for detecting ethanol, which can be applied for monitoring alcohol in the environment and food or the drinking status of drivers. It should also be pointed out that a highly efficient alcohol sensing material is the first step toward the material base of an improved direct ethanol fuel cell, which relies on the same elemental steps (oxygen binding and dissociation, alcohol binding and oxidation), however in a spatially separated anode/cathode geometry with a separation layer of high O2- mobility. All that can potentially be provided by the current material. All the sensing experiments were carried out at a relative humidity (RH) of 56%. Additional tests were performed at RHs of 40, 80, and 90% with the result that the resistances were almost constant for all RHs, suggesting that this sensor can be applied over a wide range of humidity. CdIn2O4 is well-known as an n-type semiconductor, characterized by its high free carrier concentration. Its conductivity is determined mainly by the concentration of stoichiometric defects, in which oxygen vacancies provide the donor states.30 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 operationg temperature of the sensing material exposed to the test gas, thus resulting in the space-charge layer changes and band modulation.2 According to Wolkenstein’s model for semiconductors,31 We propose an analogous model for the CdIn2O4 nanoparticles, as schematically shown in Figure 7. First, oxygen species were adsorbed on the surface of particles in the air, and then were ionized into O-(ads) or O2-(ads) by capturing free electrons from the particles, thus leading to the formation of thick spacecharge layer and increasing of potential barrier (Figure 7, upper part). Here, the resistance of the sensor was high. When the sensor was put into a tested gas, for example ethanol in our case, C2H5OH would react with the O-(ads) or O2-(ads) to form CO2, H2O, and C2H5OC2H5 accompanied by the (30) Zakrzewskta, K.; Pisarkiewicz, T.; Czapla, A. Phys. Status Solidi, A 1987, 99, 141. (31) Wolkenstein, T. Electronic Processes on Semiconductor Surfaces during Chemisorption; Plenum: New York, 199135182.

Figure 7. Schematic diagram of the proposed reaction mechanism of CdIn2O4 sensors to ethanol on the particle surface.

release of electrons32 (Figure 7, lower part). 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 change of the resistance in air and in tested gas showed the sensing properties of the sensor for the tested gas. The chemical processes can be summarized as follows32 (32) (a) Wu, X. H.; Wang, Y. D.; Liu, H. L.; Li, Y. F.; Zhou, Z. L. Mater. Lett. 2002, 56, 732. (b) Wang, Y. D.; Chen, J. B.; Wu, X. H. Mater. Lett. 2001, 49, 361.

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O2(gas) a O2(ads) O2(ads) + e- a O2 (ads) O2 (ads) + e a 2O (ads)

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fast response and recovery times toward ethanol gas. The performance of a home-built CdIn2O4 nanocrystal sensor was even better than the competing SnO2 and In2O3 sensors, making this material not only interesting for sensor devices but also for a number of related electrochemical applications.

O-(ads) + e- a O2-(ads) C2H5OH(gas)+O2-(ads) f C2H5O-(ads)+OH-(ads) C2H5O-(ads) f (C2H5)2O(ads)+O-(ads) + eC2H5OH(gas) + O2-(ads) + h f CO2 + H2O + V¨o In conclusion, the successful synthesis of CdIn2O4 nanocrystals by using a nonaqueous approach involving the reaction of cadmium acetate and indium isopropoxide with benzyl alcohol is presented. The high crystallinity and high surface-to-volume ratio of the as-synthesized CdIn2O4 nanocrystals make them ideal candidates for ethanol gas-sensing devices. The as-fabricated sensor showed high response and

Acknowledgment. Financial support by the Max-PlanckSociety is gratefully acknowledged. M. C. thanks the Alexander von Humboldt Foundation for supporting her research stay in Germany and the Natural Science Foundation Council of China (NSFC) (20401005 and 20771022). Supporting Information Available: FTIR spectrum and TGA curve of the as-synthesized CdIn2O4 nanocrystals; XRD pattern and TEM image of the CdIn2O4 nanocrystals after annealing at 400 °C for 2 h, followed by aging at 250 °C for 150 h in air (PDF). This material is available free of charge via the Internet at http://pubs. acs.org. CM800794Y