Synthesis of ZnOâAg Hybrids and Their Gas-Sensing Performance

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The synthesis of ZnO-Ag hybrids and their gas-sensing performance towards ethanol Jing Ding, Junwu Zhu, Pengcheng Yao, Jin Li, Huiping Bi, and Xin Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01711 • Publication Date (Web): 11 Aug 2015 Downloaded from http://pubs.acs.org on August 22, 2015

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Industrial & Engineering Chemistry Research

The synthesis of ZnO-Ag hybrids and their gassensing performance towards ethanol Jing Ding†, Junwu Zhu†*, Pengcheng Yao‡, Jin Li†, Huiping Bi†, and Xin Wang† †

Key Laboratory for Soft Chemistry and Functional Materials, Nanjing University of Science

and Technology, Ministry of Education, Nanjing 210094, China ‡

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].: +86 25 8431 5943; fax: +86 25 8431 5054.

ACS Paragon Plus Environment

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ABSTRACT: Herein, we describe a facile surfactant-free method to prepare the ZnO-Ag hybrids at room temperature for improving gas-sensing performance towards ethanol. Characterizations indicate that Ag nanoparticles are well deposited on the surface of ZnO nanorods. It is interesting to note that the introduction of only 1 wt% Ag in ZnO-Ag hybrids leads to the impressive enhancement in gas-sensing properties of ZnO. Notably, the ZnO-Ag hybrids can offer gas response value of 884.7 towards 1000 ppm of ethanol (approximately 12.6 times higher than pure ZnO nanorods). Meanwhile, the hybrids exhibit excellent stability with only ±5 % fluctuation of gas response value over a period of 480 cycles. Moreover, the abovementioned hybrids also exhibit well selectivity (about 4.7-52.9 times greater than that of other tested vapors). The high sensitivity, well selectivity and excellent stability presented by ZnO-Ag hybrids make it a potential material for developing an excellent gas-sensing sensor towards ethanol. 1. Introduction Volatile organic compounds (VOCs) such as alcohols and aromatic hydrocarbons are hazardous to human health due to their capabilities to stimulate the mucous membranes and upper respiratory tracts1. Nevertheless, the human olfactory system is limited to a qualitative detection of a few gases. Therefore, it is imperative to develop gas sensors not only for the detection of toxic and explosive gases that may come from spills and leaks, but also for the quality control of food and environmental monitoring. Recently, metal oxide semiconductors, such as SnO22-3, WO34 and Cu2O5, have been used as sensing materials to detect trace amounts of target gases in air. Apart from them, ZnO has also been extensively studied for detecting combustible and toxic gases, such as C2H5OH, CO and H2 owing to its high sensitivity, low cost and ease of availability6-8. However, in view


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of relatively low efficiency of ZnO sensors, there exists urgent need to improve its sensitivity and selectivity in order to meet the demand of practical application. It is known that, noble metals, such as Pt9, Pd10-11 and Au12-15, are frequently used in gas-sensing materials. Among them, as a comparatively cheap noble metal, Ag nanoparticles can act as catalysts to accelerate the chemisorption process of metal oxides and greatly improve their gas-sensing performance. But relatively few researches have been carried out on the Ag-modified ZnO with enhanced gas-sensing properties. Xiang et al.16 fabricated ZnO-Ag nanorods via a photochemical method, which can detect 10 ppm of ethanol. Zong et al.17 reported that ZnO-Ag nanorods can be fabricated by using Ag/C cables as the template, and the products exhibit high sensitivity towards 100 ppm of ethanol when operating at 332 oC. Moreover, Chen et al.18 presented the improved gassensing performance of ZnO films modified by Ag ion implantation. Nonetheless, suffering from difficult control of sizes and distribution of Ag nanoparticles on the surface of ZnO nanorods, the severe reaction conditions, the use of surfactant and high reaction temperature were needed in previous methods. Therefore, many researchers are still trying to find effective methods to prepare ZnO-Ag hybrids with high gas response value under mild condition. Herein, we report a facile approach to deposit Ag nanoparticles on the surface of ZnO nanorods at room temperature without using any surfactant and harmful reducing agents. According to our strategy, ZnO nanorods were firstly prepared, then a small amount of Ag nanoparticles were grown on the surface of ZnO nanorods through the reduction reaction between Ag ions and ethylene glycol (EG) at room temperature. It is important to note that, the incorporation of 1 wt% Ag nanoparticles can greatly improve the gas-


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sensing response of ZnO nanorods towards ethanol. This work shows that high efficient ZnO-Ag gas-sensing materials can be synthesized under mild condition, and it also opens a promising way to fabricate metal-metal oxides hybrids. 2. Experimental Section 2.1 Synthesis of ZnO nanorods and ZnO-Ag hybrids All of the chemicals were of analytical grade and used without further purification. The ZnO nanorods were synthesized using a similar procedure reported previously19. In a typical procedure, Zn(NO3)2·6H2O (3.7 g) was dissolved in ethanol (70 mL) and stirred for 20 min. Then NaOH solution (0.5 M) was dropped into the above solution until the pH reached 10. After that, the resulting ivory-white suspension was stirred for another 20 min, transferred into a 100 mL of Teflon-lined autoclave and heated at 180 oC for 12 h under autogenous pressure. After being cooled to room temperature, the obtained precipitate was centrifuged, washed with deionized water and ethanol. The ZnO nanorods were obtained after drying in a vacuum oven at 60 oC for 12 h. Subsequently, the ZnO-Ag hybrids with different contents of Ag nanoparticles (0.5, 1, 2, 3, 5 and 10 wt%, calculated by the mass ratio of Ag in ZnO-Ag hybrids) were synthesized. A typical experiment for the synthesis of ZnO-Ag hybrids with 1 wt% Ag can be described as follows: the obtained ZnO (1 g) and AgNO3 (1 mmol) were dissolved in EG (20 mL) at room temperature under ultrasonic treatment for over 30 min. Then Na2WO4·2H2O (0.35 mmol) dissolved in EG (20 mL) were added, and the reaction mixture was vigorously stirred for 30 min. Afterwards, the resultant precipitate was


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collected by centrifugation, washed and dried in air at 60 oC for 12 h, and the dried product was labeled as ZnO-Ag (1 %). Similarly, the hybrids with different contents of Ag nanoparticles were defined as ZnOAg (Ag content), such as ZnO-Ag (0.5 %), ZnO-Ag (2 %), ZnO-Ag (3 %), ZnO-Ag (5 %) and ZnO-Ag (10 %). 2.2 Characterization Powder X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ=1.5406 Å). The morphology of as-obtained products was characterized by transmission electron microscopy (TEM, JEOL JEM-2100) and field-emission scanning electron microscopy (FESEM, Semi-In-Lens SU8010). The elemental compositions and chemical states of samples were analyzed by a PHI QUANTERA II X-ray photoelectron spectroscopy (XPS), using monochromatic Al Kα X-ray as the excitation source. 2.3 Fabrication of gas sensors and response test The gas-sensing properties were performed on a WS-30A measuring system (Winsen Electronics Co. Ltd.) under static testing conditions. Fabrication of ZnO or ZnO-Ag hybrids gas sensors is similar to our previously reported process20-21. Typically, the ZnO nanorods or ZnO-Ag hybrids were mixed with a few drops of deionized water to form slurry through sufficient milling, which was coated onto a prefabricated ceramic tube (1 mm in diameter and 4 mm in length) positioned with two Au electrodes and four Pt wires on each end. A Ni-Cr heating wire was put through the ceramic tube to supply different operating temperatures, which could be controlled in the range of 100-400 oC. The


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ceramic tube was then welded onto a six-probe pedestal of test system. In order to keep the uniformity of the samples coating and the consistency of the samples quality on every ceramic tube, we adopted the method of brushing each ceramic tube with the same 4 times. To improve the stability and repeatability, the obtained sensors were aged at 300 o

C for 7 days in air prior to use. Typically, the obtained gas sensors were placed in a test

chamber. The electrical resistance of sensors was measured in air and in the mixture of target gas and air. Target gases were injected into the test chamber by a microinjector and were mixed with air by a little electric fan. The sensors were exposed to air again by removing the chamber. For reducing gases such as ethanol, acetone and formaldehyde, the gas response (sensitivity) is defined as S=Ra/Rg, where Ra is the resistance in ambient air and Rg is the resistance in target gas, respectively.


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3. Results and discussion 3.1 Structure and morphology of ZnO-Ag hybrids

Figure 1. (a) XRD patterns of pure ZnO, ZnO-Ag (1 %) and ZnO-Ag (10 %); XPS survey of (b) whole scan spectrum, (c) Ag 3d of ZnO-Ag (1 %) and (d) Zn 2p of ZnO-Ag (1 %). The typical XRD patterns of as-prepared pure ZnO, ZnO-Ag (1 %) and ZnO-Ag (10 %) are presented in Figure 1a. More XRD patterns of ZnO-Ag hybrids can be seen in Figure S1 (Supporting Information). The sharp diffraction peaks indicate high crystalline characteristic of ZnO-Ag hybrids. The peaks at 31.8o, 34.4o, 36.3o, 47.5o, 56.6o, 62.9o, 66.4o, 68.0o, 69.1o, 72.6o and 77.0o can be unambiguously indexed to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) planes of ZnO with hexagonal phase (JCPDS no. 05-0664), respectively. Obviously, no characteristic peaks of


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impurities such as Zn or Zn(OH)2 are observed. However, it is a remarkable fact that no typical diffraction peaks of Ag are observed in the XRD pattern of ZnO-Ag (1 %), probably because of the low content of Ag nanoparticles in the final hybrids and relatively strong diffraction intensity of ZnO. Accordingly, the ZnO-Ag (10 %) was synthesized using the same procedure for XRD investigation. As expected, the ZnO-Ag (10 %) shows obvious characteristic diffraction peaks (marked with asterisks shown in Figure 1a) of cubic Ag (JCPDS no. 03-0921), indicative of the presence of Ag in obtained hybrids. In addition, according to previous reports22, Ag ions can be incorporated in ZnO system either as a substituent for Zn2+ or occupy some interstitial sites. However, the absence of any peak shift for characteristic peaks of ZnO or Ag in ZnO-Ag hybrids excludes the possibility that Ag incorporates into the lattice of ZnO. Instead, the Ag nanoparticles might only deposit on the surface of ZnO nanorods. This conclusion is also supported by XPS results. As shown in Figure 1b, the spectrum of ZnO-Ag (1 %) shows that no peaks of other elements except Zn, O, Ag and C are observed, and the existence of carbon peak can be attributed to adventitious hydrocarbon from XPS instrument itself16. It is noteworthy that, compared with feature peaks of Zn and O, the peak intensity of Ag is obviously weaker. It is mainly because of the low content of Ag nanoparticles in ZnO-Ag (1 %)16, which matches very well with the XRD results. As shown in Figure 1c, the high-resolution scan of Ag 3d peaks of ZnO-Ag (1 %) presents a Ag 3d3/2 peak at 374.3 eV and a Ag 3d5/2 peak at 368.3 eV, with a spin-energy separation of 6 eV, indicating the metallic nature of Ag0 on the surface of ZnO nanorods, which is in consistent with the XRD results. Analogously, as displayed in Figure 1d, the Zn 2p3/2 locates at 1021.6 eV and the Zn 2p1/2 is at 1044.7 eV, with an interval of


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approximately 23.1 eV. The XPS spectra, in conjunction with XRD results, further confirm the deposition of metallic Ag0 on the surface of ZnO nanorods. Figure S2 represents the formation process of pure ZnO nanorods and Ag-ZnO hybrids. First, ZnO nuclei form from dehydration of Zn(OH)42− ions in an alkaline environment, which has a negatively charged surface. Then, Ag+ adsorbs on the surface of ZnO nanorods via the electrostatic attraction in AgNO3 solution. Particularly, in this method, sodium tungstate as a catalyst can dramatically speed up the reduction of silver ions, and EG acts as both a solvent and a reducing agent to reduce silver ions to Ag metal at room temperature, which has been reported in our previous result23. To investigate the influence of precursors, the control experiment was carried out with the other experimental conditions unchanged. When Na2WO4 were not involved, it was found that no Ag nanoparticles were formed. The dissolution of appropriate amounts of silver nitrate in ethylene glycol at room temperature could yield the colloidal silver dispersions, but the kinetics process is very slow, and the formation of silver nanoparticles takes several hours with low yield. Actually, the presence of Na2WO4 plays a key role in the formation of Ag nanoparticles, which determines whether or not silver ions would be chemically reduced by EG at room temperature 24-25.


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Figure 2. (a) FESEM of ZnO-Ag (1 %), the inset in (a) is EDS spectra of ZnO-Ag (1 %); TEM or HRTEM images of (b-d) ZnO-Ag (1 %), (e, f) ZnO-Ag (10 %). The morphologies of resulting products were characterized by TEM and FESEM. Figure 2a shows the FESEM image of synthesized ZnO-Ag (1 %), which verifies the rodliked morphology of ZnO with variable diameters

26-27

and lengths. Taking into account

the limit of resolution ratio of FESEM instrument and the small sizes (10-20 nm) of Ag


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nanoparticles, it is not easy to find the Ag nanoparticles on the surface of ZnO nanorods from SEM images. Even so, the Ag nanoparticles can still be observed in the FESEM images (marked by red circles in Figure 2a). To illustrate the existence of Ag nanoparticles, the composition of ZnO-Ag (1 %) is further verified by EDS result (inset of Figure 2a), which confirms the existence of Ag, Zn and O in ZnO-Ag (1 %). The morphology of ZnO-Ag (1 %) was further studied by TEM (Figure 2b-d). The relatively few Ag nanoparticles can also be observed from the TEM images (Figure 2b) of ZnO-Ag (1 %) originated from low content of Ag nanoparticles in hybrids. Moreover, the HRTEM image of ZnO-Ag (1 %) shown in Figure 2d (taken from the area encircled by a square in Figure 2c), indicates that two different lattice spacings can be found. The spacing of 0.193 nm can be assigned to the (102) plane of hexagonal wurtzite ZnO, while the spacing of 0.205 nm matches well with that of the (200) plane of Ag. The HRTEM image, combined with above discussion, implies the successful deposition of Ag nanoparticles onto the surface of ZnO nanorods tightly. To further confirm the aforementioned deduction, the ZnO-Ag (10 %) was investigated by TEM. It can be observed that the ZnO consist of nanorods with about 150-250 nm in lengths and diverse diameters, and some spherical ZnO particles are also observed (Figure 2e and 2f). Obviously, the Ag nanoparticles with diameters of 10-20 nm are deposited well on the surface of ZnO nanorods. Note that Ag nanoparticles are firmly anchored on ZnO nanorods even after ultrasonic dispersion for TEM characterization, indicating a strong linkage or contact between them, which might be helpful to the enhancement of gas-sensing properties.


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3.2 Gas-sensing performances

Figure 3. (a) The response of pure ZnO and ZnO-Ag (1 %) hybrid sensor towards 100 ppm of ethanol operated at different temperatures; (b) The response of pure ZnO and ZnO-Ag hybrids with different contents of Ag towards 100 ppm of ethanol; (c) The response of pure ZnO and ZnO-Ag (1 %) hybrid sensors towards ethanol with different concentrations; (d) The sensitivity tendency of pure ZnO and ZnO-Ag (1 %) vs. different gas concentrations, the inset shows corresponding calibration curves (5-100 ppm). To explore the potential application of as-prepared ZnO-Ag hybrids, the samples were fabricated into gas-sensing materials for investigating its gas-sensing performance towards ethanol. Generally, the gas-sensing response is strongly influenced by operating temperatures. The response of pure ZnO and ZnO-Ag (1 %) hybrid sensor towards 100 ppm of ethanol at different temperatures (160-400 oC) were investigated and shown in Figure 3a. For pure ZnO sensor, the sensitivity increases with increasing operating


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temperatures, and reaches the highest value operated at 400 oC in our experimental conditions. In contrast, it is found that the sensitivity of ZnO-Ag (1 %) hybrid sensor firstly increases as the operating temperature is increased to 370 oC, and then sharply decreases with further increasing temperatures. This result can be ascribable to the adsorption-desorption kinetics and thermodynamics of target gas on the surface of ZnOAg hybrids28. Target gas molecule would be adsorbed on the active sites on the surface of sensing material. The concentration of target gas on the surface of sensing material is dependent on the concentration of active sites. The response value is proportional to concentration of active sites and the O2- concentration in the sensing material. Given that the higher response value presented by ZnO-Ag hybrid sensor at 370 oC, we infer that an appropriate heating temperature may give more active sites and higher O2- concentration on the surface of sensing material

29

. Clearly, 370 oC is an optimal operating temperature

for ZnO-Ag (1 %) hybrid sensor, which is applied in all investigations hereinafter. It is worth mentioning that the Ag contents in hybrids have a profound influence on the responses of ZnO-Ag hybrids sensors. The gas response values of pure ZnO, ZnO-Ag (0.5 %), ZnO-Ag (1 %), ZnO-Ag (2 %), ZnO-Ag (3 %), ZnO-Ag (5 %) and ZnO-Ag (10 %) hybrids sensors towards 100 ppm of ethanol are 13.2, 43.7, 101.8, 64.4, 24.3, 21.2 and 8.0, respectively (Figure 3b). Obviously, the response value initially increases and then decreases with the elevated contents of Ag nanoparticles. Particularly, the ZnO-Ag (1 %) hybrid sensor reaches the highest response value in contrast to other products. For the sake of illustrating the key role of Ag in hybrids, the sensing properties of pure ZnO and ZnO-Ag hybrids towards ethanol were investigated. Figure 3c compares the gas response of pure ZnO and ZnO-Ag (1 %) hybrid sensors towards ethanol with different


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gas concentrations. It is clearly that the response amplitudes of two sensors increase sharply with increasing gas concentrations, and ZnO-Ag (1 %) hybrid sensor presents much higher response value than those of pure ZnO at all gas concentrations. Notably, the ZnO-Ag (1 %) can offer gas response value of 884.7 towards 1000 ppm of ethanol, which is approximately 12.6 times higher than pure ZnO nanorods, highlighting beneficial synergistic effects between Ag and ZnO. Furthermore, the gas response undergoes a drastic rise when the ethanol is injected, and recovers rapidly to its initial state after removing target gas in every cycle, indicating the good reversibility and repeatable performance. Figure 3d displays the sensitivity tendency of pure ZnO and ZnO-Ag (1 %) versus different gas concentrations of ethanol. In the range of 5-1000 ppm, both ZnO and ZnOAg (1 %) exhibit a near linear tendency of response. Similarly, the inset of Figure 3d shows the near linear curve with ethanol concentrations from 5 to 100 ppm. Generally, the sensors sensitivity (S=Ra/Rg) can be empirically represented by S=1+Ag(Pg)β30, where Pg is the partial pressure of target gas, which is directly proportional to gas concentration, Ag is the prefactor, and β is the exponent. Actually, β has a value of either 1 or 0.5, mostly depending on the surface interaction between chemisorbed oxygen and reducing gas to ntype metal oxide semiconductors31. When ethanol concentrations are in the range of 5−1000 ppm, β is found to be 1 for the two sensors. As shown in Figure 3d, the sensitivity of ZnO-Ag hybrids sensor is much larger than that of pure ZnO sensor at the same ethanol concentration, which is probably because that the ZnO-Ag hybrids sensor can provide more sites for oxygen adsorption and subsequent reaction with the target gas 32.


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Table 1. The comparative analysis of gas response of different materials. Materials ZnO-Ag nanorods ZnO-Ag nanorods ZnO-Ag film Ag-hierarchical ZnO microspheres ZnO nanorods array with an exposed (0001) facet Pd-ZnO nanoflowers ZnO-Ag hybrids ZnO-Ag hybrids

Ethanol (ppm)

Operating temperature(°C)

Gas response

100 10 100

332 280 300

38.5 6 14.6

17

100

350

66.9

33

100

370

69

34

100 10 100

300 370 370

22.6 10.2 101.8

35

Reference 16 18

this work this work

In addition, Table 1 presents the comparative analysis of gas response of different materials. It is evident that ZnO-Ag hybrids prepared by this procedure display higher gas response values towards ethanol compared with other materials reported, which further confirms that the ZnO-Ag hybrids are suitable for detecting ethanol with high sensitivity.


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Figure 4. (a) The response transients of ZnO-Ag (1 %) hybrid sensor towards 20 ppm of ethanol, the inset of (a) depicts the details of once adsorption and desorption; (b) Stability test of as-prepared ZnO-Ag (1 %) hybrid sensor towards 20 ppm of ethanol over a period of 480 cycles; (c) The response of ZnO-Ag (1 %) hybrid and ZnO-Ag (1 %) mechanical mixture sensors towards ethanol with different concentrations; (d) The response of ZnOAg (1 %) hybrid sensor towards 100 ppm of different target gases at 370 oC. For practical application, in addition to the required high sensitivity amplitudes, a sensor should have short response-recovery times. Generally, the response time is defined as the time to reach 90 % of the equilibrium value following a step increase in the target gas concentration, while the recovery time is defined as the time necessary for the sample to return to 10 % above the original electrical current in air after removing the target gas36. Figure 4a shows the response transients of ZnO-Ag(1 %) hybrid sensor towards 20 ppm of ethanol. Apparently, the initial volume of ethanol vapor is consumed to reach a nearly saturation state (as the light magenta region shown in Figure 4a). It takes about 15 s for saturation during the gas response value increases from 1 to about 19.9. Subsequently, rapid recovery (as the dark grey region shown in Figure 4a) happens by purging air. As displayed in the insert of Figure 4a, the response and recovery times of ZnO-Ag (1 %) hybrid sensor towards ethanol are about 15 s and 20 s, respectively, which are

attractive

for

possible

gas-sensing applications

like

food

analyses,

wine

identifications, electronic noses and breath analyzers. As another important feature of chemical sensors, good stability is the ability to successively respond to target gases without a visible decrease in sensor response. Therefore, we have also evaluated the stability of as-prepared ZnO-Ag (1 %) hybrid


16

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sensor towards 20 ppm of ethanol over a period of 480 cycles, as shown in Figure 4b. After 480 cycles between the target gas and fresh air, gas responses of the sensor could return to its initial value (only ±5 % fluctuation of gas response value), which indicates the outstanding stability, and further manifests that the adsorption of ethanol on the surface of ZnO-Ag hybrids is reversible. In order to verify the beneficial synergistic effects between ZnO and Ag nanoparticles, the control experiment was carried out. The Ag nanoparticles and ZnO nanorods were respectively prepared under the same condition as we mentioned in experimental section, and the ZnO-Ag (1 %) mechanical mixture was obtained by mixing ZnO nanorods with Ag nanoparticles. As shown in Figure 4c, the ZnO-Ag hybrids sensor displays a significant enhancement towards ethanol in comparison with as-obtained ZnO-Ag (1 %) mechanical mixture sensor. This result highlights the existence of synergistic effects between ZnO and Ag originated from the interaction. The selectivity is a key parameter of sensor for its practical application. Hence, gas-sensing responses of ZnO-Ag (1 %) hybrid sensor towards ethanol, acetone, methylbenzene, acetic acid, ethylene glycol, formaldehyde and acetaldehyde were examined. Figure 4d indicates that the ZnO-Ag (1 %) hybrid sensor presents greatly higher response towards ethanol than that towards other vapors, suggesting the outstanding selectivity towards ethanol vapor. It is noteworthy that the response value of ZnO-Ag (1 %) towards ethanol vapor is 4.7-52.9 times greater than those of other tested vapors. Furthermore, the sensing selectivity results of the obtained pure ZnO product were also studied and showed in Figure S4 (Supporting Information). Clearly, the ratio of gas response values of pure ZnO towards ethanol and methylbenzene is only 9.8, which is greatly lower than that of ZnO-Ag (1 %) (about 52.9). In addition, pure ZnO shows only 1.1-3.5


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times higher in selectivity towards ethanol than that of other tested vapors, suggesting the poor selective detection of ethanol. The outstanding performances displayed by ZnO-Ag hybrids gas sensor towards ethanol make it promising for gas-sensing applications.

Figure 5. Schematic illustrations for the gas-sensing mechanism of (a) pure ZnO nanorods; (b) ZnO-Ag hybrids. As an n-type semiconductor, the sensing mechanism of ZnO belongs to the surfacecontrolled type, where the gas sensitivity is generally interpreted by the modulation model of depletion layer

28, 30-32, 36-37

. For pure ZnO nanorods, the oxygen molecules from the

ambient can be adsorbed on the surface of ZnO nanorods when the sensors are exposed to air (Figure 5a, left). The oxygen molecules capture electrons from conduction band of ZnO to form adsorbed oxygen ions (O2-, O- and O2-)

38

, which can produce an electron

depletion layer on the surface region, and result in a decrease in the carrier concentration


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and electron mobility

39

. However, once the sensor is exposed towards ethanol, the

ethanol gas will be chemisorbed on the surface of ZnO, and the chemisorbed ethanol gas will be oxidized by the adsorbed oxygen ions (O2-) of ZnO. During this oxidation process, the depleted electrons can be fed back to the surface of ZnO to lower the number of trapped electrons, leading to a decreased resistance. In other words, the removed electrons are delivered to the conduction band. As a result, the depletion layer becomes thin and the electrical current increases (Figure 5a, right). In contrast, when a small quantity of Ag nanoparticles are deposited on the surface of ZnO nanorods, the produced ZnO-Ag hybrids make the electron depletion layer thicker, which can impressively enhance the gas-sensing responses of oxide semiconductors owing to the deeper extending electron depletion layer (Figure 5b, left; Figure S3). In the real applications of ZnO-Ag hybrids gas sensors, the decorated Ag nanoparticles can play the role of catalysts to accelerate oxygen ions conversion rate in air, and increase the amount of adsorbed oxygen, which result in faster electron depletion from the surface of ZnO-Ag hybrids. When the sensor is exposed to ethanol gas, Ag nanoparticles can also accelerate the reaction between ethanol and adsorbed oxygen ions via a spillover effect 40. As shown in the TEM images (Figure 2), the small Ag nanoparticles with average diameters of 10-20 nm are highly dispersed on the surface of ZnO nanorods, which are very beneficial for spillover effect, and play the role of catalysts for gas-sensing reactions. As a result, the deep electron depletion changes into a shallow layer. The effective shrinkage of the depletion region, in conjunction with the reduced resistance, endows the ZnO-Ag hybrids with an impressive enhancement in gas response (Figure 5b, right)

33

.

Consequently, for the ZnO-Ag hybrids gas sensors, the resistance variation of ZnO


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surface in air and target gas is much larger than that of pure ZnO sensor, leading to the significant enhancement of the sensor response. Actually, the Ag contents in hybrids have a profound influence on the responses of ZnO-Ag hybrids sensors. As a kind of excellent electric conductor, the Ag nanoparticles would connect with each other with the elevated contents of Ag nanoparticles, and the micro electric circuit would form both on surface and in body of ZnO-Ag hybrids coating when the content of Ag nanoparticles is too high. This makes the semiconductor’s resistance of ZnO-Ag hybrids coating decreased and further causes electrons to conduct along the metallic Ag owing to its superior conductivity, thus reducing the oxygen chemisorption ability and reaction activity on the surface of sensors

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. Consequently,

excessive Ag contents would further lower the gas response value to ethanol. Given that the more superior gas-sensing performances presented by ZnO-Ag (1 %) in comparison with pure ZnO, the as-prepared ZnO-Ag hybrids would be a promising sensor in detecting ethanol. 4. Conclusions In summary, ZnO-Ag hybrids were successfully fabricated by a surfactant-free and room temperature method without any harmful reducing agents. The Ag loading content can be tuned by varying the concentration of AgNO3. Compared with pure ZnO nanorods, the decoration of ZnO nanorods with an appropriate content of Ag nanoparticles significantly enhances the response and selectivity towards ethanol. The obtained hybrid exhibits quick response-recovery characteristics, good reproducibility and good stability, indicating their promising application as gas-sensing materials. This work opens a notable


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way to fabricate metal-metal oxides hybrids, and makes a significant contribution to gassensing application. ASSOCIATED CONTENT Supporting Information. Additional details about XRD, schematic diagrams of the formation process of pure ZnO nanorods and ZnO-Ag hybrids, the sensing selectivity of pure ZnO, and TGA patterns of pure ZnO and ZnO-Ag(1%). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This investigation was supported by the Jiangsu Funds for Distinguished Young Scientists (BK2012035), Program for New Century Excellent Talents in University (NCET-11-0834), the Natural Science Foundation of China (No. 51322212, 51472122), the Fundamental Research Funds for the Central Universities (No. 30920130111004) and PAPD of Jiangsu.


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