Gas adsorption at metal sites for enhancing gas sensing performance

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Gas adsorption at metal sites for enhancing gas sensing performance of ZnO@ZIF-71 nanorod arrays Tingting Zhou, Yutong Sang, Yanling Sun, Congyi Wu, Xiaoxia Wang, Xing Tang, Tian Zhang, Hao Wang, Changsheng Xie, and Dawen Zeng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02642 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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Gas adsorption at metal sites for enhancing gas sensing performance of ZnO@ZIF-71 nanorod arrays Tingting Zhou,†,§ Yutong Sang,† Yanling Sun,† Congyi Wu,† Xiaoxia Wang,*† Xing Tang,† Tian Zhang,† Hao Wang,† Changsheng Xie,† Dawen Zeng*†,§ †

State Key Laboratory of Materials and Processing Die & Mould Technology, Nanomaterials and Smart Sensors Research Laboratory, Department of Materials Science and Engineering, Huazhong University of Science and Technology, No. 1037, Luoyu Road, Wuhan 430074, China.

§

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei University, Wuhan 430062, China.

KEYWORDS: Zeolitic imidazolate framework (ZIF); gas sensor; adsorption; metal site; DFT

ACS Paragon Plus Environment

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ABSTRACT: The detection of trace amount of volatile organic compounds (VOCs) has been covered by tons of researches, which are dedicated to improve the detection limit and insensitivity to humidity. In this work, we have synthesized ZnO@ZIF-71 nanorod arrays (NRAs) equipped with the adsorption effect at metal site that promoted the detection limit of ethanol and acetone, to which also have great selectivity. The gas sensor not only exhibits shorter response/recovery time (53/55% for ethanol, 48/31% for acetone), but also excellent insensitivity to humidity and improved detection limit (10 times improved at 21 ppb for ethanol, 4 times at 3 ppb for acetone) at low working temperature (150 °C). By the analysis of in-situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and calculation of density functional theory (DFT), the mechanism of enhanced gas sensing performance from ZnO@ZIF-71 NRAs is proved. It shows ethanol and acetone gas molecules can be adsorbed at the metal sites of ZIF-71. This work provides a new idea to improve the detection limit and humidity-insensitivity of gas sensor towards specific gas molecules.


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Introduction Due to the low cost, long lifetime, real-time monitor and portability, metal oxide semiconductor (MOS) gas sensor, such as ZnO, SnO2, In2O3 and NiO have been developed for toxic, combustible gases and volatile organic compounds (VOCs) detection over the past decades in industrial processes, environmental and indoor gas supervision, and health care aspects.1-2 Compared with other materials, ZnO nanorod arrays (NRAs) have advantages like lower temperature fabrication and higher surface-to-volume ratios.3-4 It is extensively used for detecting many kinds of gases, such as H2, H2S, VOCs and applied in several areas.5-7 As known, ethanol is an important biomarker for identifying the qualification of food,8 while acetone can anaesthetize the central nervous system of human and cause damage to kidney, pancreas, and liver.9 Moreover, 1 ppm acetone in human breath is believed to be the threshold value for diagnosing of type II diabetes.10 Therefore, it is necessary to develop methods which can enhance gas sensing performance in the accurate detection of ppb/ppm level ethanol and acetone for clinic application. To date, ZnO NRAs are often processed by several paths like doping, p-n junction, and noble metal decoration to pursue higher sensitivity, shorter response and recovery time, and humidity-insensitivity.11-12 However, it still needs a method to achieve overall improved ZnO gas sensing performance simultaneously. Recently, one kind of typical porous materials, metal-organic frameworks (MOFs) were widely used for gas separation and adsorption.13-14 MOFs are crystalline materials composed of metal cations bridged by organic electron donors. In fact, several MOFs have open metal sites


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in pore surface. The considerable interest in these materials is a result of their potential uses in several applications, including gas storage and separations, electronics, catalysis, biomedicine, chemical sensing, and radiation detection. This inspired some researches about improving the selectivity and sensitivity of gas sensors and it promoted a single MOS-based gas sensor in accurate detection of low concentration VOCs.15-18 In our pervious work, the selectivity of ZnO NRA gas sensor was successfully regulated by MOFs materials decoration, which is determined by whether the gas molecules can penetrate the MOF shell to ZnO or not.19 In the meantime, some researchers make use of MOF to form the resistive gas sensors and use them to improve the detection limit of the gas sensing. Koo and his co-workers used Pd nanowires@ Zeolitic imidazolate framework (ZIF)-8 to enhance gas sensing performance for H2.20 The detection limit of sensors is down to 0.06% for Pd nanowires@ZIF-8 compared to that 0.1% of Pd NWs. In addition, Yao et al. prepared ZnO@5 nm ZIF-CoZn device to detect acetone which is down to trace level with the value of 19 ppb.9 Interestingly, in previous work, we noticed the ZnO NRAs demonstrated enhanced response to ethanol and acetone after ZIF71 decoration, which has important significance for promoting the detection limit towards ethanol and acetone. This endows the ZnO@ZIF-71 NRA gas sensor the potential to accurately detect ppb levels of ethanol and acetone. As discussed above, it has great significance to explore the mechanism of the enhanced gas sensing performance of ZnO@ZIF-71 gas sensor. One particular aspect of these materials is that they are often permanently microporous and exhibit extremely high surface areas. While MOFs have been intensively investigated for their adsorptive properties. Wales concluded that


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MOFs provide high surface areas and active sites for adsorption. 21 Thus, gas sensors comprised of MOFs can also be used for direct adsorption of the analyte or as a support to improve sensitivity of MOS. As manifested by many researches, the open metal sites in pore surface play an important role in gas adsorption. For example: Lv et al. determined that aniline molecules were adsorbed in MOF-5 so it made MOF-5 employed for aniline detection.22 And ZIF-71 shows adsorption capacity to ethanol according to other reported results.23 Moreover, as manifested by many researches, open metal sites in pore surface of MOFs play an important role in gas adsorption,24 such as H2;25 CO226 and acetylene27 gases. Based on these, we reasonably deduced that the enhanced response and the improved detection limit were due to the adsorption of ethanol and acetone on the ZIF-71 layer. And the Zn2+ cations as metal sites are potential adsorption sites during this process. Therefore, this mechanism can be proved by in-situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and density functional theory (DFT) calculation as well. The type of adsorption and adsorption sites are further explored at the same time. In this work, ZnO@ZIF-71 shows overall improvement of low concentration ethanol and acetone gas sensing performance in various aspects such as selectivity, detection limit, and humidity-insensitivity. In addition, a detailed discussion about the mechanism of the enhanced gas sensing performance of ZnO@ZIF-71 gas sensor is carried out, which is based on both in-situ DRIFT spectroscopy result and DFT calculation as well. ZIF-71 can physically adsorb ethanol and acetone at Zn2+ metal cluster site. Furthermore, ZnO@ZIF-71 also shows excellent insensitivity to humidity due to the waterproof effect by ZIF-71.28-29 This work


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develops a novel approach to enhance the performance of low concentration and explores the relationship between gas adsorption effect of MOF and MOS@MOF gas sensing performance.

Materials and methods Chemicals Zinc

nitrate

hexahydrate

(Zn(NO3)2·6H2O),

sodium

hydroxide

(NaOH),

hexamethylenetetramine (HMTA), and N, N-dimethylformamide (DMF) were bought from Sinopharm Chemical Reagent Co. Ltd. 4,5-dichloroimidazole was purchased from Alfa Aesar Chemical Regent Co., Ltd. All the chemicals used in this study were of analytical grade and used without further purification. Distilled water was used in all the experiments. Preparation of ZnO@ZIF-71 nanorod arrays ZnO NRAs were synthesized by a two-step process. The alumina substrates coated with interdigital electrode were ultrasonically cleaned and followed by drying at 80 °C. In the first step, ZnO nanocrystals were spin-coated on the substrate to form a seed layer. In the second step, ZnO NRAs were synthesized by a hydrothermal method on ZnO seed layer. After that, ZnO NRAs were immersed in 16 mL mixture of 12 mL DMF and 3 mL distilled water containing 4, 5-dichloroimidazole (0.04 g, 0.5 mmol) at 70 °C for 8 h to grow the ZIF-71 shell in a Telfon-lined stainless-steel autoclave (50 mL). Characterization


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The phases of the materials were analyzed by X-ray diffraction (XRD). The XRD patterns were recorded by a Philips X’Pert diffractometer from 2θ = 5 ° to 40 ° using Cu-Kα radiation (λ = 1.5406 Å). The morphology and microstructure of the products were observed on a fieldemission scanning electron microscope (FESEM, FEI Sirion 200), which was operated under an acceleration voltage of 10 kV. Transmission electron microscopy (TEM) analyses were carried out under a working voltage of 300 kV on the instrument FEI Tecnai G2 S-TWIM. TGA has been analyzed by TA Instruments SDT 2960, under air atmosphere in the temperature range 35-800°C (heating rate 5°C/min). Brunauer-Emmett-Teller (BET) surface areas of the products were calculated from nitrogen adsorption-desorption curves (TriStar II 3020, Micromeritics Instrument Corporation). The DRIFT was tested by Fourier transform infrared spectrometer (FT-IR) on the instrument Bruker vertex 70. Gas sensing measurements Gas sensing measurements were made using a semiconductor device analyzer (B1500A, Agilent Technologies Co., LTD). The substrate of the sensor is 3 mm×3 mm alumina ceramic plate. It is covered with interdigitated electrode and the reverse side of the plate is covered with ruthenium oxide heating electrode. The ZnO NRAs and ZnO@ZIF-71 NRAs are synthesized based on the ZnO seed layer spin-coated on alumina substrate. All the gas sensing devices were pre-matured. During the measurement, dry air was pumped into the testing chamber and the sensor was heated at 150 °C. Then, Ethanol and acetone were pumped into the chamber for about 300 s and the total flow rate of the mixed gas was set to 200 mL/min,


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respectively. The concentration was controlled by two parallel mass flow controllers (Beijing Sevenstar Electronics Co., Ltd.) that mix dry air and target gas (all 200 ppm, Shanghai Weichuang Standard Gas Analytical Technology Co., Ltd.) according to required proportions. The gas response value is defined as: Response = ΔI/Ia = (Ig–Ia)/Ia × 100%

(1)

where Ig and Ia were the average current within a cycle in target gas and air, respectively. Computational Details DFT calculations by the Project NWchem30-31 were performed to obtain the several site configurations and the corresponding adsorption energies (Eads) between adsorbates and ZIF71. The DFT approach has been successfully employed to estimate the interaction energies between organic molecules. Eads was calculated as:

Eads = Ecomplex − (EZIF-71 + Eadsorbate)

(2)

The calculation method is B3LYP. For each adsorbate-framework complex. The 6-31+g (d, p) basis sets are used for the non-metal elements carbon, hydrogen, nitrogen, and chlorine. Metal elements zinc are used for Lanl2DZ basis sets treatment.32 Optimization was performed without any additional conditions.

Results and discussion The ZnO@ZIF-71 core-shell structure has been characterized in our pervious work. Based on XRD, SEM and TEM characterization results, it can be concluded that the composite material is successfully synthesized in this work. The XRD patterns of synthesized ZnO NRAs


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and ZnO@ZIF-71 NRAs shown in Figure 1 ZnO NRAs and ZnO@ZIF-71 NRAs both have (100), (002) and (101) three peaks of ZnO crystal and with a (002) crystal plane orientation. According to the simulated XRD pattern of ZIF-71 from the reported crystal structure,33 both crystalline phases of ZIF-71 and ZnO can be identified in the XRD patterns of ZnO@ZIF-71 NRAs.

Figure 1. XRD patterns of the ZnO NRAs and ZnO@ZIF-71 NRAs. The peaks (♦) of Al2O3 are from the ceramic substrate.

Figure 2. (a) SEM image of ZnO@ZIF-71 NRAs surface. (b) The cross-section SEM image of ZnO@ZIF-71 NRAs.


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The microstructure of ZnO@ZIF-71 NRAs was analyzed using SEM and TEM, and the SEM images of ZnO NRAs are shown (Figure S1,Supporting Information). The ZnO NRAs are well vertically-aligned and the diameter of ZnO nanorods is in the range of 200-250 nm. The SEM images of ZnO@ZIF-71 NRAs’ surface and cross section are shown in Figure 2, the dense coating of ZIF-71 on the surface of ZnO nanorods is visibly identifiable and the length of nanorods is about 1.5 μm. Moreover, Figure S1(c) and S1(d) reveal the different element composition between ZnO NRAs and ZnO@ZIF-71 NRAs by the energy dispersive X-ray spectrometer (EDS) point scan of these two samples. The TEM image depicts the core-shell structure intuitively (Figure 3(a)). The corresponding cross-sectional composition line profiles (Figure 3(b)) demonstrate that N and C elements mostly locate in the outer layer of the nanorod, while Zn element distributes relatively higher concentration in the core area. Note that N and C are two main elements of 4,5-dichloroimidazole which only exists in the ZIF-71 shell and O element only exists in ZnO core area. Judging by the XRD, SEM, and TEM results, the ZnO@ZIF-71composite nanorods with a core−shell structure have been successfully prepared by self-template method.


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Figure 3. TEM characterization of ZnO@ZIF-71 nanorod: (a) TEM image of ZnO@ZIF-71 nanorod core-shell structure, and (b) the EDS line scan result reveals the chemical composition along the red line in Figure 3a. To investigate the gas sensing performance of the ZnO@ZIF-71 NRAs gas sensor compared with that of the ZnO NRAs gas sensor more intuitively, we used a lower operating temperature (150 °C) to complete all the experiments owing to the difficulty to obtain low-concentration test gases. The thermal stability of ZnO@ZIF-71material has been evaluated by TGA analysis (Fig. S2). And the TG&DTG results indicate that the ZnO@ZIF-71 NRAs can be stable at 150 °C. Then, it can be deduced from Figure 4 that the response of the ZnO @ ZIF-71 NRAs gas sensor is higher than the pure ZnO NRAs at the same test conditions. Figure 4(a) and 4(c) show that


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ZnO NRA sensor has no response for low concentrations of ethanol and acetone at 1 ppm and 500 ppb, respectively, while the composite NRA device has obvious response to the corresponding concentrations of ethanol and acetone. In addition, Figure 4(b) and 4(d) show the response-concentration plots from the repeatability test of two sensors at different gas concentration (Figure S3-S4). For comparison, Table S2-S3 summarize the sensing response to ethanol and acetone for various sensors. Although ZnO@ZIF-71 dose not exhibit high response, it shows lower operating temperature and lower detection limits. And It shows good linearity and the details of the linear fitting are shown in Table S1. Therefore, it is evidenced that the coating of the ZIF-71 layer greatly improves the detection limit of the sensor. Based on the above results, the lowest detectable concentration was limited by the present experimental setup. We can derive the theoretical detection limit of each sensor based on the signal-to-noise ratio (see the Supporting Information ).34-35 It is obvious that ZnO@ZIF-71 NRAs has lower detection limit than ZnO NRAs, which is down to trace level with the value of 21 ppb for ethanol and 3 ppb for acetone at 150 °C. Compared with ZnO NRAs sensor, ZnO@ZIF-71 NRAs gas sensor exhibits almost 4 times and 1 time improved gas sensing response for 10 ppm ethanol and 5 ppm acetone, respectively. Furthermore, the theoretical detection limit is also promoted by almost 10 times for ethanol and 4 times for acetone (Table 1). This indicates that the ZnO @ ZIF-71 gas sensor can reduce the detection limit of ethanol and acetone to ppb level.


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Figure 4. Gas sensing performance comparison of ZnO and ZnO@ZIF-71 NRAs: (a) Responserecovery curves and (b) response-concentration plots of ZnO and ZnO@ZIF-71 NRAs to ethanol of different concentration. (c) Response-recovery curves and (d) responseconcentration plots of ZnO and ZnO@ZIF-71 NRAs to acetone of different concentration. The MOF structure obviously enhances the sensitivity of bare ZnO NRAs to ethanol and acetone.


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Figure 5. (a-b) Response-recovery curves to 10 ppm ethanol and 5 ppm acetone with different relative humidity. (c-d) Normalized current curves to 10 ppm ethanol and 5 ppm acetone for better comparison of response and recovery time.


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Table 1. Summarized gas sensing properties to ethanol and acetone of ZnO NRAs and ZnO@ZIF-71 NRAs sensors. Gas Ethanol 10 ppm

Acetone 5 ppm

Temp.

150 °C

150 °C

Sample

Response

tres (s)

trec (s)

detection limit

ZnO

2.59%

419.46

988.28

207 ppb

ZnO@ZIF-71

13.40%

194.37

442.17

21 ppb

Enhanced

~4 times

53%

55%

~10 times

ZnO

20.62%

373.34

772.45

12 ppb

ZnO@ZIF-71

38.90%

195.9

535.5

3 ppb

Enhanced

~1 time

48%

31%

~4 times

As we know, humidity is one of important interference parameters in VOC detection. What’s more, humidity is an unneglectable factor during the process of detecting trace amount of ethanol and acetone. However, the traditional MOS such as ZnO sensor is very sensitive to humidity.36-37 Fortunately, the hydrophobic ZIF materials may be the solution to inhibit humidity interference according to several reports.38 At the same time, ZIF-71 has also been previously reported to have hydrophobic properties second only to ZIF-8,39 so ZIF-71 is expected to be used to synthesize a gas sensor with humidity-insensitivity like ZIF-8.40 Owing to the reported hydrophobicity and solvent separation from ethanol-water of ZIF-71,28, 41 we consider it necessary to verify the sensitivity of the ZnO@ZIF-71 NRA gas sensor to water vapor. We measured the gas sensing performance for ethanol and acetone at different humidity conditions. The controlled humidity environments were achieved by saturated vapor pressure


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method.40The coefficient of variation (CV) is used to represent the effect of humidity on responses, which is defined as CV = RSD/Raverage ×100%

(3)

where RSD and Raverage are the standard deviation (SD) and average value of responses with different humidity, respectively.9 The lower CV value means the better anti-interference performance. Figure 5(a) and 5(b) show results of ZnO@ZIF-71 NRAs sensor exposed in 10 ppm ethanol and 5 ppm acetone with 20%, 40%, 65%, 80% relative humidity. CV values of ethanol and acetone gas with different relative humidity are 4.49% and 5.02%. Both CV values are below 10% that mean ZnO@ZIF-71 is a gas sensor insensitive to water vapor. Whereas ZnO as a kind of traditional MOS gas sensor is easy to be disturbed by humidity during VOC detection. In addition, the change of the response of ZnO sensor in different relative humidity is shown in Figure S5. Comparatively, ZnO NRAs shows a variation as high as 59.4% and 37.3% at the same conditions. In summary, the combination of ZIF-71 and ZnO can significantly improve the humidity anti-interference of ZnO sensor. The response time of the sensor is defined as the time required increasing the current to 90% of the saturation value and the recovery time is the time required decreasing the saturated current to its 10%. Figure 5 illustrates the normalized response and recovery characteristic curves of the sensors based on of 10 ppm ethanol and 5 ppm acetone in 1000 s gas-in time. ZnO@ZIF-71 gas sensor shows the shorter response and recovery times. Compared with ZnO NRA sensor, the response time of ZnO@ZIF-71 NRA sensor is reduced by 48% for 10 ppm


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ethanol and 40% for 5 ppm acetone, and 55% and 31% reduction of recovery time for the same conditions. (Table 1) As ZIFs are a class of porous materials, ZnO@ZIF-71 NRAs have permanent porosity and adsorption capacity. These can be verified by the nitrogen adsorption and desorption isotherms. The BET surface area testing results show clearly that ZnO@ZIF-71 nanorods have higher specific surface area and gas adsorption capacity than ZnO (Figure S6).15, 42 Thus, larger surface area permits ZnO@ZIF-71 composite nanorods to adsorb more ethanol and acetone gas molecules. It is one of the reasons that why ZIF-71 layer shows improved sensitivity of ZnO gas sensors.

Figure 6. In-situ DRIFT spectrum of ZnO@ZIF-71 NRAs in different atmosphere. The vibrational peak of O-H at 3407 cm-1 derives from ethanol while the peak at 1644 cm-1 derives from acetone. In addition, ZIF-71 is an attractive candidate for gas adsorption and molecular separation. For example, a molecular simulation study has shown that ZIF-71 can adsorb ethanol. To


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investigate the ethanol and acetone adsorption behavior of ZnO@ZIF-71 NRAs, the in-situ DRFIT measurement is proceeded. The ZnO NRAs and ZnO@ZIF-71 NRAs samples were introduced into dry air and 200 ppm of ethanol and acetone gases at 150 ℃, respectively. Then the scanning spectra (Figure 6) were obtained.


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Figure 7. DFT-optimized interaction configurations of ethanol (a-c) and acetone (d-f) on different site of ZIF-71. (a, d) is Zn site and (d-c, e-f) is Cl site specifically. From the in-situ DRFIT results, ZnO@ZIF-71 and ZnO devices show differences in the adsorption of the two gases. As can be seen in Figure 6, after the introduction of dry air and ethanol into ZIF-71 respectively, the vibrational peak of O-H appeared in the position of 3407 cm-1, whereas the ZnO sample did not show any corresponding O-H peak (Figure S7).43-45 Therefore, it is reasonable to assume that O-H is derived from the adsorption and enrichment of ethanol on the ZIF-71 layer. Similarly, after the two samples were respectively introduced into acetone and dry air, the vibrational peak of C=O was only formed at 1644 cm-1 after acetone was introduced into ZnO@ZIF-71. And the carbonyl was derived from acetone adsorbed by ZIF-71. Next, we performed a temperature programmed desorption (TPD) measurement on the ZnO@ZIF-71 sample. The details of experimental operation and TPD results can be seen in Supporting Information. Figure S8 shows that ZnO@ZIF-71 has a


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desorption peak for ethanol and acetone at about 150 ℃ (Peak α). It indicates that ZnO@ZIF71 composite has better adsorption and desorption properties for ethanol and acetone than pure ZnO. It is further proved that ZnO@ZIF-71 can adsorbs ethanol and acetone gases.

Table 2. Summarized DFT results simulation of possible adsorption site configurations between ZIF-71 and two kinds of adsorbed gases. Ethanol-1

Ethanol-2

Ethanol-3

Acetone-1 Acetone-2

Acetone-3

Adsorption Site

C-O…Zn

O-H…Cl

O-H…Cl

C=O…Zn

C=O…Cl

C=O…Cl

Eads (KJ/mol)

-12.50

5.39

93.48

-6.82

3.06

-2.81

Adsorption

√

×

×

√

×

√

Determined whether it can be adsorbed according to the value of adsorption energy (√ suggest the gas molecule can be adsorbed at corresponding adsorption site, and reversely for ×).

Figure 8. Schematic illustration of the enhanced gas sensing performance of ZnO@ZIF-71 NRAs gas sensor, dependent on the adsorption of ethanol and acetone gas molecule at metal site by ZIF-71 layer.


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As we mentioned before, the Zn2+ as metal sites of ZIF-71 are potential adsorption sites in adsorption process. Therefore, the adsorption sites of gas molecules in the ZIF-71 structure have been discussed below. Figure 7 shows the structure of ZIF-71 and possible active adsorption sites to ethanol and acetone, respectively. The results based on DFT simulation is showed in Table 2 manifest the Eads of Ethanol-1, Acetone-1 and Acetone-3 structures is negative, which indicates that ethanol and acetone can be adsorbed onto these structures of ZIF-71. Moreover, these Eads values higher than -13 KJ/mol reveal that the adsorption process is physical adsorption.46-47 And the more negative values of Eads indicate that metal cluster site Zn2+ are preferable adsorption sites for gas adsorption. In other words, adsorption energies of different site configurations between gas molecules and frameworks shows ZIF-71 can physically adsorb ethanol and acetone at Zn2+ metal cluster site, which leads to the improvement of gas sensing performance. It has been widely accepted that Zn2+ site is thermodynamically favored by small organic molecules, allowing adsorption uptakes due to its strong interaction affinity with adsorbates.48-49 In our follow-up work, the use of other metal cluster site, which has a higher adsorption of VOC gas, will be considered to further increase the gas sensing performance. According to DFT calculation results, ZIF-71 has larger adsorption energy for ethanol than acetone, which explains why ZnO@ZIF-71 NRAs has a stronger enhanced effect on ethanol response than acetone in Table 1. The mechanism of the enhanced gas sensing performance of ZnO@ZIF-71 NRAs is shown in Figure 8, which is dependent on the physical adsorption of ethanol and acetone gas molecules at metal sites by ZIF-71 layer. This gives explanation for the overall improved gas sensing performance of


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ZnO@ZIF-71 than pure ZnO, such as higher response, improved detection limit, quicker response/recovery behavior. It develops a new approach for applying metal−organic frameworks in enhancing gas sensor performance for accurate detection of low concentration VOC.

Conclusions ZnO@ZIF-71 NRAs with core-shell structure have been successfully synthesized by templating method. By coating ZIF-71 layer onto the ZnO gas sensor, the gas sensing performance for ethanol and acetone is significantly enhanced. The ZnO@ZIF-71 NRA gas sensor shows enhanced response, shorter response and recovery time and improved detection limit at low working temperature (150 °C). Furthermore, ZnO@ZIF-71 also shows excellent insensitivity to humidity due to the hydrophobic effect by ZIF-71. In addition, we proved that the ZIF-71 could adsorb and enrich ethanol and acetone gases according to in-situ DRIFT. And the DFT results further show that ethanol and acetone physically adsorb at Zn2+ site of ZIF71. This work can develop a new meaningful approach for applying metal−organic frameworks in enhancing gas sensor performance. Moreover, it further promotes low cost and portable MOS gas sensors for accurate detection of low concentration VOC. ASSOCIATED CONTENT Supporting Information. The SEM image of ZnO nanorod arrays, EDS point scan spectra, BET surface area results, detailed computing process of theoretical detection limit, in-situ drift IR


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spectrum of ZnO NRAs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (D. Zeng). *E-mail: [email protected] (X. Wang) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant number 51572075) and Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials. Also, the FESEM, TEM, FT-IR analyses were supported by the Analytical Testing Center of HUST, and the XRD measurements were supported by the State Key Laboratory of Materials and Processing Die & Mould Technology.


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Table of Contents


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Gas adsorption at metal sites for enhancing gas sensing performance

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