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Dec 22, 2015 - Sieving to Improve Selectivity of Formaldehyde Gas Sensor. Hailin Tian .... The response/recovery times were defined as the time to rea...
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Zeolitic Imidazolate Framework Coated ZnO Nanorods as Molecular Sieving to Improve Selectivity of Formaldehyde Gas Sensor Hailin Tian, Huiqing Fan,* Mengmeng Li, and Longtao Ma State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, P. R. China

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S Supporting Information *

ABSTRACT: Zinc oxide (ZnO) and zeolitic imidazolate framework-8 (ZIF−8) core−shell heterostructures were obtained by using the selftemplate strategy where ZnO nanorods not only act as the template, but also provide Zn2+ ions for the formation of ZIF−8 shell. The ZIF−8 shell was uniformly deposited to form ZnO@ZIF−8 nanorods with core−shell heterostructures at 70 °C for 24 h as the optimum reaction time by the hydrothermal synthesis. Transmission electron microscopy (TEM) images revealed that the ZnO@ZIF−8 heterostructures are composed of ZnO as core and ZIF−8 as shell. Nitrogen (N2) sorption isotherms demonstrated that the as-prepared ZnO@ZIF−8 nanorods are a typical microporous material. Additionally, the ZnO@ZIF−8 nanorods sensor exhibited distinct gas response for reducing gases with different molecule sizes. The selectivity of the ZnO@ZIF−8 nanorods sensor was obviously improved for the detection of formaldehyde owing to the limitation effect of the aperture of ZIF−8 shell. This study demonstrated that semiconductor@MOF core−shell heterostructures may be a novel way to enhance the selectivity of the gas sensing materials. KEYWORDS: ZnO, ZIF−8, core−shell heterostructures, formaldehyde, gas sensor

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material with large cavities (11.6 Å) and small pore apertures (3.4 Å) has been extensively reported as the sensing material.22,26,27 In addition, zinc oxide (ZnO), a typical ntype transparent oxide semiconductor with a wide band gap of 3.37 eV, has been considered an excellent material for the gas sensors, due to its high mobility of electrons, low cost, and environmental friendliness.28−30 Compared with pure MOFs, the heterostructures with the functional materials as core and MOFs as shell are attractive as the sensing material because we can rationally utilize the synergistic effect of them. Zhan and coworkers first reported that the well-defined ZnO@ZIF−8 nanorod with core−shell heterostructure was prepared by the synthetic strategy with ZnO nanorods as self-sacrificial temples for the ZIF−8 shell. ZnO@ZIF−8 nanorods exhibited distinct response toward hydrogen peroxide or ascorbic acid in photoelectrochemical sensors due to the limitation of the aperture of ZIF−8.31 Inspired by this idea, it is possible that ZnO@ZIF−8 core−shell heterostructures possess potential application in gas sensors with improving selectivity toward gas molecules of different sizes. The self-template method is believed to be a feasible strategy for fabricating the MOFs core−shell heterostructures. The metal oxide template can provide metal ions through dissolving

etal oxide gas sensors (MOGS) are solid-state resistivetype devices which are widely used in applications ranging from health and safety (e.g., medical diagnostics, air quality monitoring, food processing and detection of toxic, flammable, and explosive gases) to energy efficiency and emission control in combustion processes.1−3 As we know, one of the major challenges for metal oxide-based gas sensors is to improve selectivity. Many gas species may cause changes in the resistance, making it impossible for a single sensor to properly identify the gas, especially for diverse volatile organic compounds (VOCs). Many studies have been reported to improve the selectivity and other important sensing parameters of resistive-type MOGS.4−7 However, novel enhancement ways in the selectivity of the materials are necessary to simplify the device design and eliminate false-positive or interfering signals. As a new class of crystalline porous materials, metal−organic frameworks (MOFs) has attracted tremendous attention in recent years.8−12 Generally speaking, the pores within MOFs can be functionalized by tuning the pore sizes to make use of size-exclusive effects for the separation and purification of small molecules.13,14 Consequently, porous MOFs have been widely developed as multifunctional materials and potential applications on gas storage,15,16 separation,17,18 heterogeneous catalysis,19,20 and sensing.21−24 Zeolitic imidazolate frameworks−8 (ZIF−8) has the formula Zn(mim)2 (mim = 2methylimidazole) with a sodalite (SOD)-related zeolite type structure.25 This chemically robust and thermally stable © 2015 American Chemical Society

Received: November 22, 2015 Accepted: December 22, 2015 Published: December 22, 2015 243

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Figure 1. (a) Schematic diagram of the ZnO@ZIF−8 nanorods synthesized with ZnO nanorods as a template in the mixed solvent at 70 °C for 24 h. (b) Low-magnification TEM image of the ZnO@ZIF−8 nanorods with core−shell heterostructures. (c) Two aperture sizes of ZIF−8 structure. (d) Schematic illustration of the ZnO@ZIF−8 nanorod sensor with selectivity for different VOCs molecule. solvent of DMF and H2O. Finally, ZnO@ZIF−8 nanorods with core− shell heterostructures can be obtained in this strategy. Synthesis of ZnO Nanorods. ZnO nanorods were obtained via a hydrothermal synthesis reported in our previously published procedure.32 In a typical experiment, Zn(NO3)2·6H2O (2.26 g, 7.6 mmol) and NaOH (2.43 g, 60.8 mmol) were dissolved into 40 mL deionized water. The white suspension was continuously stirred for 30 min under magnetic stirring. Then, the resultant solution was heated in a sealed autoclave with a stainless steel shell at 140 °C for 12 h inside an oven. Subsequently, the white precipitate was collected by centrifugation after cooling naturally to room temperature, washed with deionized water and ethanol several times, and then dried at 60 °C for 8 h. Synthesis of ZnO@ZIF−8 Nanorods with Core−Shell Heterostructures. The growth of ZIF−8 on the ZnO nanorods template proceeded via a hydrothermal route in the reported study.31 The asprepared ZnO nanorods (0.0407 g, 0.5 mmol) and 2-methylimidazole (0.324 g, 4 mmol) were successively added to a beaker (100 mL) containing a mixed solvent of DMF/H2O (40 mL, 3:1 of v/v). After sonication for 5 min, the mixture was transferred to a sealed autoclave with a stainless steel shell and heated to 70 °C for 24 h inside an oven. Subsequently, the white product was collected by centrifugation after cooling naturally to room temperature, washed by fresh DMF and ethanol for several times, and then dried at 60 °C for 8 h. Characterization. Phase compositions of the as-synthesized materials were examined by using X-ray diffraction (XRD; X’pert PRO MPD, Philips, Eindhoven, The Netherlands) with Cu Kα radiation (λ = 1.5406 Å) in the range of 5−50°. Product morphology and microstructure were observed by using scanning electron microscopy (SEM; JSM−6701F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM; Tecnai F30G,2 FEI, Hillsboro, OR, USA). Brunauer−Emmett−Teller (BET) surface area of the products were performed by a full analysis of nitrogen adsorption− desorption tests (3H−2000PS4, Beishide Ltd., Beijing, China). Gas Sensing Measurement. Gas-sensing measurements were performed by using a gas response instrument (HW−30A, Hanwei

in solvents and then construct MOFs with organic molecules on the surface of metal oxide. The size and morphology of the MOFs core−shell heterostructures can be straightforwardly controlled by the template. In this regard, ZnO nanorods not only can be easily synthesized at nanoscale, but also are the most suitable template to fabricate the MOFs core−shell heterostructures. In this paper, we successfully synthesized ZnO@ZIF−8 nanorods with core−shell heterostructures, ZnO nanorods were the self−template that provided Zn2+ ions for the formation of ZIF−8, and ZIF−8 as shell was coated on the outside of ZnO nanorods. The uniform and continuous ZIF−8 shell was formed on the surface of ZnO nanorods when the reaction time was 24 h by the hydrothermal synthesis. In the gas sensing measurement, the as-prepared ZnO@ZIF−8 nanorod gas sensors exhibited good selectivity toward formaldehyde gas at 300 °C in comparison with other VOCs, such as ammonia, acetone, toluene, methanol, and ethanol. The limitation of the aperture of ZIF−8 was the main reason that ZnO@ZIF−8 nanorods with core−shell heterostructures displayed distinct gas response to organic vapors with different molecular sizes.

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EXPERIMENTAL SECTION

Chemicals. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%) and 2-methylimidazole (97%) were purchased from Alfa Aesar Chemical Regent Co., Ltd. Sodium hydroxide (NaOH, 96%) and N,Ndimethylformamide (DMF, 99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. Synthetic Procedure. ZnO nanorods were synthesized by a hydrothermal method. Then, ZnO nanorods acting as the template as well as the Zn2+ source were coated by 2-methylimidazole in the mixed 244

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ACS Sensors Ltd., Zhengzhou, China) (Figure S1a). The measuring electric circuit of this instrument can be seen in Figure S1b. A load resistor is connected in series with a gas sensor. The circuit voltage Vc is 5 V, and the output voltage (Vout) is the terminal voltage of the load resistor RL. The heating voltage Vh is adjusted for the working temperature of the sensors. The volume of the gas-sensing test chamber is 15 L. When an amount of tested gas is injected into the chamber, the sensor’s resistance is changed. As a result, the output voltage is changed. The gas response (S) was defined as the ratio of Rair/Rgas, where Rair and Rgas are the resistances measured in air and reductive gas, respectively. The response/recovery times were defined as the time to reach 90% of the final equilibrium value. Gas sensor devices were fabricated according to our previous study.28 The ZnO@ZIF−8 nanorods with core−shell heterostructures were mixed and ground with adhesive in an agate mortar to form a paste. An alumina tube with Au electrodes and platinum wires was used as the substrate. The sensors of the ZnO@ZIF−8 nanorods were prepared on alumina tube by the slurry spin coating. A Ni−Cr alloy crossed the center of the alumina tube, which was applied as a heating resistor for both substrate heating and temperature controlling (Figure S1c). Finally, the gas sensors were dried under infrared radiation light for several minutes and calcined at 300 °C for 2 h in air. For each composition, three sensors were made and tested three times at the same conditions to obtain an average value of sensitivity.

Morphology and phase composition of the ZnO@ZIF−8 nanorods with core−shell heterostructures were primarily confirmed by using SEM, XRD, and TEM. The typical SEM image of the ZnO@ZIF−8 nanorods with core−shell heterostructures can be seen in Figure 2a. The pristine ZnO

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RESULTS AND DISCUSSION The synthetic procedure of the ZnO@ZIF−8 nanorods with core−shell heterostructures by using the self-template strategy can be seen in Figure 1a. The ZnO nanorods act as the template as well as the Zn2+ source. Coated as shell on the surface of ZnO nanorods, ZIF−8 is synthesized with Zn2+ and 2-methylimidazole in the mixed solvent of DMF and H2O. Figure 1b shows a TEM image of the ZnO@ZIF−8 nanorods with core−shell heterostructures. The diameter of ZnO core is about 300 ± 50 nm, while the surface of ZnO nanorods appears rough and is deposited with ZIF−8 shell of 100 ± 50 nm. For the ZIF−8 structure, the Zn atom center is solely coordinated by the N atoms in the 1,3-positions of the five-membered ring. The neutral frameworks are based on nets of linked ZnN4 tetrahedra.25 The structure of this porous material with small apertures (3.4 Å) and large cavities (11.6 Å) is illustrated in Figure 1c. As shown in Figure 1d, the ZnO@ZIF−8 nanorods reveal distinct gas response to VOCs with different molecular sizes. By combining with ZIF−8, the ZnO nanorods surface was coated with the porous ZIF−8 shell which has two apertures. Oxygen passes through the aperture of the ZIF−8 shell, forming four kinds of oxygen species on the surface of ZnO nanorods at different working temperatures, which are O2 (80 °C), O2− (150 °C), O− (300−400 °C), and O2− (550 °C).33,34 On contact with reducing gases, the oxygen species are consumed in the redox reaction and electrons are released back to ZnO grains, leading to a decrease in electrical resistance.35 Among VOCs, formaldehyde molecules can easily pass through the ZIF−8 pores and are oxidized by the oxygen species (O−) at working temperature of 300 °C (HCOH + 2O− → CO2 + H2O + 2e−). The electrons are formed and transported to the electrode substrate along the axial direction of ZnO nanorods. Compared with formaldehyde, other VOC molecules may be blocked at the ZIF−8 layer as a result of their molecule sizes being bigger than formaldehyde molecules, leading to only a few VOC molecules passing through ZIF−8 shell and reacting with the oxygen species on the surface of the ZnO nanorods. Accordingly, it is reasonable to believe that the ZnO@ZIF−8 nanorods will exhibit a good selectivity for formaldehyde even in the presence of other VOCs.

Figure 2. (a) SEM image of the ZnO@ZIF−8 nanorods (Inset: SEM image of ZnO nanorods). (b) TEM image of a single ZnO@ZIF−8 nanorod with core−shell heterostructure. (c) XRD pattern of the ZnO@ZIF−8 nanorods and the simulated XRD pattern of ZIF−8 and ZnO. (d,e) EDS elemental scan of area−1 and area−2 in panel b.

nanorods are presented in the inset of Figure 2a. The surface of ZnO nanorods is coated with ZIF−8 giving the shell a rough appearance. The morphology of the ZnO@ZIF−8 nanorods is similar to that of ZnO, suggesting that the self-template method is an effective way to fabricate the core−shell heterostructures. Figure 2b shows a low-magnification TEM image of the ZnO@ ZIF−8 nanorods with core−shell heterostructures. The outer shell with light contrast is the ZIF−8 layer, which is uniformly coated on the surface of the inner ZnO nanorods with dark contrast. Energy dispersive X-ray spectra (EDS) of area−1 and area−2 with the white dotted lines are shown in Figure 2d and e. The weight percent of C, O, and Zn elements in the core area (Area−1) is 47.8%, 11.06%, and 41.12%, respectively, while the atomic percent of Zn and O in the core area is 11.86% and 13.04%, respectively (Figure 2d). The atomic ratio of Zn and O in area−1 is about 1:1, suggesting that ZnO is the main phase composition in the core area of the ZnO@ZIF−8 nanorods, while the weight percent of C, O, and Zn elements in the shell area (Area−2) is 72.12%, 2.8%, and 25.06%, respectively (Figure 2e). It is concluded that ZIF−8 is the main phase composition in the shell area because C and Zn are two main elements of ZIF−8 and ZnO barely exists in the outer layer as a result of the small weight percent of O element. As shown in Figure 2c, the XRD pattern characterization reveals that the ZnO@ZIF−8 nanorods are composed of two kinds of materials 245

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Both the ZnO nanorods sensor and the ZnO@ZIF−8 nanorods sensor showed that the proper working temperature was 350 °C (Figure S3), but the framework of ZIF−8 began to decompose at 350 °C according to the reports. 26,27 Furthermore, we have measured TEM and XRD experiments for ZnO@ZIF−8 nanorods after calcination at 300 °C for 2 h in air. The result of the TEM image and XRD pattern demonstrated that the ZIF−8 shell was stable and ZnO@ZIF− 8 core−shell nanorods were not destroyed at 300 °C for 2 h in air (Figure S4). Therefore, we chose 300 °C as the proper working temperature to operate the subsequent experiments in order to keep the good gas sensing property of ZnO and avoid the breakdown of ZIF−8. On the other side, water molecules can easily across the aperture of the ZIF−8 shell due to the smaller diameter. So, humidity interference is an important parameter in formaldehyde detection. We measured the gas sensing performance for formaldehyde in 100 ppm at different humidity conditions. The controlled humidity environments were achieved by 100 mL saturated aqueous solutions of different salts of LiCl, MgCl2, Mg(NO3)2, NaCl, KCl, and K2SO4 in a beaker at room temperature, which yielded 11%, 33%, 54%, 75%, 85%, and 97% RH, respectively.39,40 We put the prepared solution in the gas sensing test chamber and spent 30 min to ensure the air in the chamber reached equilibrate state. Figure 4a illustrates the gas response transient curves of the ZnO@ZIF−8 nanorods sensor at working temperature of 300 °C for formaldehyde in 100 ppm at different humidity conditions. The gas response and response time of the ZnO@ ZIF−8 nanorods sensor is shown in Figure 4b. The

with different crystal structures. The (1 0 0), (0 0 2), (1 0 1), and (1 0 2) peaks located at 30−50° can be indexed as the hexagonal structure of ZnO (JCPDS card: 36−1451); the residual diffraction peaks agree well with the simulated XRD pattern of ZIF−8 according to the published crystal structure date.36 Additionally, Figure S2 illustrates low-magnification TEM images of the ZnO@ZIF−8 nanorod obtained at the hydrothermal time of 12, 24, and 36 h, respectively. As shown in Figure S2a, the surface of ZnO nanorods is partly covered with the ZIF−8 shell at 12 h by the hydrothermal synthesis, indicating that the reaction time is too short and the surface of ZnO nanorods is still exposed. The ZIF−8 shell on the surface of ZnO nanorods is uniform and continuous when the reaction time is prolonged to 24 h (Figure S2b). As the reaction time is added to 36 h, the ZIF−8 shell will become thicker and thicker, while the ZnO core is thinner and thinner (Figure S2c). It is demonstrated that the thickness of the ZIF−8 shell can be controlled by the hydrothermal time. The ZnO@ZIF−8 nanorods with core−shell heterostructures cannot be perfectly formed at a short reaction time. Meanwhile, the diffusion of formaldehyde can be blocked and the response time of the sensor is increased at the reaction time of 36 h (Figure S2d−f). Therefore, the ZIF−8 shell was uniformly deposited to form ZnO@ZIF−8 nanorods with core−shell heterostructures at 24 h as the optimum hydrothermal time. On the basis of the above results, it is demonstrated that the well-defined ZnO@ZIF−8 nanorods have been successfully synthesized in our work. As the gas sensing material, the permanent porosity and absorptive capacity of ZnO@ZIF−8 nanorods were confirmed by using N2 adsorption measurement at 77 K. The nitrogen sorption isotherms of ZnO nanorods are type IV with an H1 loop according to the IUPAC classification in Figure 3a.37

Figure 3. (a) Nitrogen sorption isotherms for the ZnO nanorod sample and the ZnO@ZIF−8 nanorod sample measured by 77 K. Solid and open circles represent adsorption and desorption branches, respectively. (b) BET surface area of two samples.

Meanwhile, the nitrogen sorption isotherms for the ZnO@ ZIF−8 nanorods exhibit classic type I characterized by a sharp uptake under low relative pressure (Figure 3a), a signature feature of the microporous materials. The Brunauer−Emmett− Teller (BET) surface area of the ZnO@ZIF−8 nanorods is 307.4 m2·g−1 which is much larger than that of ZnO nanorods (Figure 3b). The lack of hysteresis indicates that the adsorption and desorption mechanisms are similar and that adsorption is reversible.38 It means that ZnO@ZIF−8 nanorods are suitable for the gas sensing material.

Figure 4. (a) Gas response transient curves of the ZnO@ZIF−8 nanorods sensor at 300 °C for formaldehyde in 100 ppm at different humidity conditions. (b) Gas response and response time for formaldehyde in 100 ppm as a function of the relative humidity. 246

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Figure 5. (a,b) Gas response transient curves and the selectivity of the ZnO@ZIF−8 nanorods sensor to 100 ppm of different VOCs at working temperature of 300 °C. (c,d) Gas sensing transient curves and the selectivity of the ZnO nanorods sensor to 100 ppm of different VOCs at 300 °C.

curve of the ZnO@ZIF−8 nanorods sensor for formaldehyde displays an apparent difference from other VOCs as shown in Figure 5a, which means the diffusion of VOCs onto the surface of ZnO nanorods is greatly affected due to the presence of ZIF−8. The kinetic diameter of all detected VOCs increases in the order: formaldehyde (2.43 Å)41 < ammonia (2.90 Å) < methanol (3.63 Å) < ethanol (4.53 Å) < acetone (4.60 Å) < toluene (5.25 Å).42 Formaldehyde and ammonia can easily pass the small pore aperture (3.40 Å) of ZIF−8 shell and other VOCs molecules cannot go through those apertures and are blocked by the ZIF−8 shell. Additionally, the ZnO-based gas sensors exhibit a low response for ammonia at this experimental condition, which means ammonia is not an interferential gas. The interfering signals are decreased or the selectivity is promoted, which is the role of the ZIF−8 shell in the gas sensing detection. The gas response of the ZnO@ZIF−8 nanorods sensor for formaldehyde is more than four times the gas response of other VOCs (Figure 5b). In contrast, the response curves of the ZnO nanorods sensor exhibit high response for formaldehyde, ethanol, acetone, and methanol, indicating that the ZnO nanorods sensor has low selectivity (Figure 5c,d). Meanwhile, the formaldehyde selectivity of different materials at optimum working temperature is represented in Table 1. This suggests that the anti-interference capacity of the ZnO@ZIF−8 nanorods sensor has been considerably enhanced and it has a good selectivity for formaldehyde, which is in accordance with an idea in Figure 1d.

performance of the sensor has been reduced when the relative humidity is added, particularly above 80%. The response time is increased from 14 to 21 s with enhancement of the relative humidity. It is likely that water molecules can occupy the active sites on the surface of ZnO and interfere with the adsorption of formaldehyde at a high relative humidity. Additionally, water molecules can be adsorbed in the aperture of ZIF−8 with hydrogen bonding interaction, which is a probable reason to prolong the response time. Therefore, the gas sensing of the sensor was measured and the data was collected at 50−60%RH in our work. The relative humidity in the gas sensing test chamber was monitored by a standard humidity sensor and adjusted by the desiccant. Improvement of the selectivity is the main purpose in designing the ZnO@ZIF−8 nanorods with core−shell heterostructures. To demonstrate this purpose, we tested the gas sensing experiment of ZnO nanorods before and after coating the ZIF−8 shell in the presence of different VOCs, such as formaldehyde, ethanol, ammonia, acetone, toluene, and methanol. We take formaldehyde as a detectable gas, because the formaldehyde concentration is an important parameter to evaluate our living environment. Meanwhile, other VOCs also need to be detected, such as ethanol; acetone and methanol are the interferential gases which usually lead to false positive or interfering signals for the ZnO-based gas sensors. Figure 5 shows the selectivity of the ZnO@ZIF−8 nanorods sensor and the ZnO nanorods sensor in different VOCs of 100 ppm at 300 °C. After ZnO nanorods are coated with ZIF−8, the response 247

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fit line that the detection limit of the ZnO@ZIF−8 nanorods sensor for formaldehyde is about 5.6 ppm. In Figure 6c, the response and recovery times of the ZnO@ZIF−8 nanorods sensor are 16 and 9 s in 100 ppm formaldehyde at 300 °C, respectively. Compared with other ZnO-based sensors,32,47 the response and recovery times of the ZnO@ZIF−8 nanorods sensor are increased, as the gas molecules need to move successfully through the shell of ZIF−8 and react with the oxygen species on the surface of the ZnO nanorods. It is concluded that the ZnO@ZIF−8 nanorods sensor reasonably utilized the molecule-size-selective ability of the ZIF−8 shell and gas sensing properties of ZnO nanorods. The stability of the ZnO@ZIF−8 nanorods sensor for 100 ppm formaldehyde at 300 °C is represented in Figure 6d. The result indicated that the ZnO@ZIF−8 nanorods sensor had a good stability in a natural environment after 4 weeks. Based on the above results, it is reasonable to believe that the ZnO@ZIF−8 nanorods sensor is potentially applicable to detecting the formaldehyde concentration in our living environment.

Table 1. Selectivity of Different Materials for 100 ppm Formaldehyde at Optimum Working Temperature materials ZnO@ZIF−8 core−shell nanorods ZIF−67a CuO−TiO2 nanofibers SnO2−graphene nanocomposite Hierarchical α-Fe2O3/NiO hollow structure

methods

working temperature (°C)

selectivity ratiob (SF/ S E)

ref

hydrothermal synthesis by selftemplate strategy solution-based synthesis electrospinning and hydrothermal synthesis solid-state synthesis

300

4

this work

150

2

43

200

5

44

260

2

45

hydrothermal synthesis

200

1.5

46

a

ZIF−67 represents Co-based zeolitic imidazolate framework (Co(mim)2; mim = 2−methylimidazolate). bSelectivity ratio (SF/SE) represents the selectivity of the gas sensor between formaldehyde and ethanol.

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CONCLUSIONS In summary, ZnO@ZIF−8 nanorods with core−shell heterostructures were successfully synthesized in a hydrothermal condition at 70 °C for 24 h by using a self-template strategy. The morphology and phase composition of the ZnO@ZIF−8 nanorods with core−shell heterostructures were confirmed by the results of SEM, TEM, XRD, and BET. In the gas sensing measurement, the as-prepared ZnO@ZIF−8 nanorods sensor revealed distinct gas response toward different VOC molecules

Figure 6a illustrates the typical gas response transient curves of the ZnO@ZIF−8 nanorods sensor, when it is exposed to formaldehyde with concentrations of 10, 20, 50, 80, 100, and 200 ppm at 300 °C. The di-logarithm of gas response (S) and formaldehyde concentration (C) is shown in Figure 6b. The data can be fitted linearly, with slope of 0.793 and intercept of −0.5914, respectively. It indicated that the ZnO@ZIF−8 nanorods sensor revealed concentration dependence for formaldehyde. Besides, it can be calculated from the extended

Figure 6. (a) Gas response transient curves of the ZnO@ZIF−8 nanorods sensor in formaldehyde at concentration range from 10 to 200 ppm at 300 °C. (b) Di-logarithm of gas response (S) and formaldehyde concentration (C) for the ZnO@ZIF−8 nanorods sensor. (c) Response and recovery times of the ZnO@ZIF−8 nanorods sensor to 100 ppm formaldehyde at 300 °C. (d) Stability of the ZnO@ZIF−8 nanorods sensor for 100 ppm formaldehyde at 300 °C in 4 weeks. 248

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Article

ACS Sensors

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due to the size-selective effects of the ZIF−8 shell. Although the response and recovery times of the ZnO@ZIF−8 nanorods sensor were increased, the ZnO@ZIF−8 nanorods sensor exhibited a better selectivity for formaldehyde compared with the ZnO nanorods sensor. We demonstrated that the ZnO@ ZIF−8 nanorods sensor can eliminate false positive or interfering signals for the detection of formaldehyde. Therefore, semiconductor@MOF core−shell heterostructures may be a novel way to enhance the selectivity of the gas sensing materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.5b00236. Photograph of the gas-sensing system, measuring electric circuit of the gas-sensing instrument, schematic diagram of the compositions of a gas sensor, low-magnification TEM images and gas response transient curves of the ZnO@ZIF−8 nanorod, gas response and response time at different hydrothermal times, dynamic response and recovery curves for ZnO nanorods and the ZnO@ZIF−8 nanorods, and the gas response of two samples at different working temperatures, TEM image and XRD pattern of the ZnO@ZIF−8 core−shell nanorods at 300 °C for 2 h in air, ac impedance spectra and dc I−V curves of ZnO@ZIF−8 nanorods, ZIF−8 and ZnO (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-29-88494463. Fax: +8629-88492642. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51172187), the SPDRF (20116102130002, 20116102120016) and 111 Program (B08040) of MOE, the Xi’an Sci&Tec Foundation (CXY1510-2), and the Fundamental Research Funds for the Central Universities (3102014JGY01004) of China.

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REFERENCES

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DOI: 10.1021/acssensors.5b00236 ACS Sens. 2016, 1, 243−250