ZIF-8 derived hierarchical hollow ZnO nanocages with quantum dots for sensitive ethanol gas detection

Sensors & Actuators: B. Chemical 289 (2019) 144–152

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

ZIF-8 derived hierarchical hollow ZnO nanocages with quantum dots for sensitive ethanol gas detection

T ⁎

Xin Zhang, Weiying Lan, Junlan Xu, Yantao Luo, Jiang Pan, Cunyi Liao, Linyu Yang, Wenhu Tan , Xintang Huang Department of Physical Science and Technology, Central China Normal University, 430079, Wuhan, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Zinc oxide Gas sensor ZIF-8 Hierarchical hollow nanocube Quantum dot

As an important semiconductor metal oxide, zinc oxide has been intensely investigated for the application of gassensing. Herein, we developed a metal–organic framework (MOF)-based strategy to synthesize novel hierarchical hollow ZnO nanocages with quantum dots (denoted as HHQD-ZnO nanocages). For comparison, solid ZnO and hollow ZnO nanocages derived from the rhombic dodecahedral (RD) ZIF-8 and 470-ZIF-8, respectively have also been synthesized. Gas sensing measurement indicated that the sensor based on the HHQD-ZnO nanocages exhibits super high response of 139.41 upon 100 ppm ethanol at 325 °C. In addition, the HHQD-ZnO nanocagesbased sensor can detect ethanol gas at a detection limit of well below 25 ppb with the response of 5.1. Such excellent performance, especially the ultrahigh response to ethanol is explained with respect to particle-interpenetrating framework with good connectivity, hierarchical porous hollow nanostructure and quantum dots.

1. Introduction With the progress of society and the development of science and technology, gas sensors are widely used in environmental monitoring, domestic safety, public medical security and industrial production [1,2] to detect the inflammable, explosive, or toxic gases. In the past few decades, many researchers worldwide have focused on the oxide semiconductor gas sensors based on SnO2 [3,4], TiO2 [5], ZnO [6], In2O3 [7,8], WO3 [9,10], Fe2O3 [11,12], CuO, Co3O4, NiO and Cr2O3. As a nontoxic and low-cost n-type semiconductor with a wide band gap (3.4 eV) and a large exciton binding energy (60 meV), ZnO is attractive and has been considered as a promising semiconductor material for solar cell [13,14], photocatalysts [15,16], light-emitting diodes [17], piezoelectric devices, acousto-optical devices [18,19] and gas sensors [20,21]. Many approaches, such as chemical vapor deposition (CVD), selfassembly [22], template techniques [23–25] and hydrothermal method [26], have been developed to synthesize ZnO nanomaterials with different morphologies. To date, ZnO nanotubes [27], nanowires [28], nanorods [29–31] nano-tetrapods [32], nanobelts [33], nanorings [34], nanosaws [34], and dendritic patterns [35] have been prepared by various physical and chemical methods. Multilayered ZnO Nanosheets with 3D Porous Architectures were synthesized and highly promising for gas sensor applications [36]. The metal-doped ZnO-based gas senor

⁎

could be reliably used for detection of inflammable gases [37,38]. The ZnO-related heterostructures are expected superior gas-sensing materials for gas sensing applications [39,40]. ZnO@ZIF−8 nanorod gas sensors exhibited good selectivity toward formaldehyde gas in comparison with ammonia, acetone, toluene, methanol, and ethanol [41]. Due to the excellent electrochemical performance [42–47], different types of MOF derived nanomaterial with various morphologies have been synthesized by many researchers. As one of the MOF series, Znrelated ZIFs have been set out to be applied to sensing device field [48–50] because of the thermal and chemical properties. ZnO [51] and ZnO-related nanomaterials were prepared from Zn-based ZIF-8 precursors and used in gas sensors [52,53]. Hollow structures have attracted tremendous interest owing to their unique structural features. Furthermore, hollow nanostructures could provide more active sites for electrochemical reactions compared with the solid nanostructures [54]. Therefore, enhanced gas sensing activities can be expected for hollow nanostructured zinc oxide. Quantum dots (QDs) are Low-dimensional semiconductor nanostructured materials and have attracted a great deal of attention due to their outstanding properties. It is should be noticed that quantum dots have exhibited extremely important potential applications in gas sensor, due to the low-cost, solution processibility, extremely small size (a few nanometers in diameter), and high activity surface [55]. For example, ZnO quantum dots exhibited potential applications in H2S gas sensors

Corresponding author. E-mail address: [email protected] (W. Tan).

https://doi.org/10.1016/j.snb.2019.03.090 Received 26 September 2018; Received in revised form 18 March 2019; Accepted 20 March 2019 Available online 20 March 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.

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slowly added into 2-methylimidazole and 1-methylimidazole solution under stirring at room temperature and maintained 2 h. Then, a resultant milk-like suspension was obtained, and the ZIF-8 products with the size of about 470 nm (470-ZIF-8 nanocrystals) were collected by centrifugation and washing with ethanol five times. The obtained white powders were dried at 80 ℃ for 12 h. 20 mg of the as prepared 470-ZIF-8 nanocrystals were incubated in 10 ml of a tannic acid solution (5 g /L) and aged for 6 min. After collected by centrifugation and washed with water and methanol, the obtained products were placed in a tube furnace and heated to 420 °C with a ramping rate of 5 °C min−1 and then maintained at 420 °C for 90 min in air atmosphere. Finally, the Hollow ZnO nanocages were obtained.

[56]. Up to now, some MOF-derived ZnO nanostructures have been reported. Hierarchical ZnO parallelepipeds were synthesized from Znbased MOF precursors and used as an efficient scattering layer in dyesensitized solar cells [57]. Hierarchical-structured hollow ZnO cubes by converting the ZIF-8 MOF into ZnO were applied for benzene gas-sensing [52]. Nitrogen-doped porous carbon-ZnO nanopolyhedras derived from ZIF-8 were used for photoelectrochemical biosensors [53]. It was reported that the C, N-doped ZnO derived from ZIF-8 was applied for dye degradation and water oxidation [58]. However, until now the hierarchical hollow ZnO nanocages with quantum dots (HHQD-ZnO) have been rarely reported. In this work, we synthesized hierarchical hollow ZnO nanocages with quantum dots (HHQD-ZnO) based on ZIF-8 precursors. The as-prepared HHQD-ZnO nanocages-based gas sensor exhibited excellent sensitivity, good selectivity and fast response toward ethanol gas.

2.4. Characterization and gas-sensing measurements The as-prepared products were characterized by X-ray diffraction (XRD, PAN-alytical Empyrean, Cu-K α radiation with λ = 1.5418 Å). Scanning electron microscopy (SEM, JSM-6700 F; 10 kV) was used to investigate the morphology and microstructure. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) are obtained via a JEM-2010FEF microscope operated at 200 kV. The specific surface area was measured by the Brunauer–Emmett–Teller (BET, Belsorp Mini) equation based on the N2 adsorption–desorption analyses. The gas sensing properties of HHQDZnO nanocages-based sensor were detected by a Navigation 4000NMDOG gas sensing measurement instrument. Our previous work has described the processes of gas sensor fabrication and gas-sensing measurement [59]. The concentration of target gas was calculated by the following formula [52,60,61]:

2. Experimental The reagents were analytic grade and used without further purification. 2.1. Synthesis of HHQD-ZnO nanocages For details, 0.3 g Zn(OAc)2·2H2O and 0.25 ml triethanolamine were dissolved in 5 ml isopropanol to form a clear solution after 70 ℃ heated with vigorous stirring for several minutes. Meanwhile, 1.1 g 2-methylimidazole was dissolved in 5 ml isopropanol with vigorous stirring for 30 min. After that, the zinc acetate solution was slowly added into 2methylimidazole solution under stirring at room temperature (˜27 ℃) and maintained 2 h. Then, a resultant milk-like suspension was obtained, and the ZIF-8 products with the size of about 170 nm (170-ZIF-8 nanocrystals) were collected by centrifugation and washing with ethanol five times. The obtained white powders were dried at 80 ℃ for 12 h. The obtained 170-ZIF-8 precursors were placed in a tube furnace and heated to 390 °C with a ramping rate of 5 °C min−1 and then maintained at 390 °C for 50 min in air atmosphere. Finally, the HHQDZnO nanocages were obtained.

C=

22.4 × ϕ × ρ× V1 × 1000 M×V2

(1)

Where C (ppm) is the target gas concentration, φ is the required gas volume fraction,ρ(g/mL) is the density of the liquid, V1 (μL) is the volume of liquid, V2 (L) is the volume of the chamber, and M (g/mol) the molecular weight of the liquid. The gas response of the sensor was defined as S= R a R g S= R a/R g (the resistance of the gas sensor in clean air (Ra) and target gas (Rg)). The response/recovery time is defined as the time needed to reach 90% of total signal change.

2.2. Synthesis of solid ZnO nanocages derived from rhombic dodecahedral (RD) ZIF-8

3. Result and discussion

The solid ZnO nanocages derived from rhombic dodecahedral (RD) ZIF-8 were synthesized and used for the performance comparison with HHQD-ZnO nanocages. Typically, a 5 ml volume of an aqueous solution of Zn(OAc)2·2H2O (300 mg) was added to 6.4 ml of an aqueous solution of 2.72 M 2-methylimidazole with gentle stirring. After few seconds the transparent mixture turned white and was then left undisturbed at room temperature for 2 h. The resulting RD-ZIF-8 particles were washed three times with DI water with centrifugation [47]. The obtained white powders were dried at 80 ℃ for 12 h. The obtained RD-ZIF-8 precursors were placed in a tube furnace and heated to 400 °C with a ramping rate of 5 °C min−1 and then maintained at 400 °C for 1 h in air atmosphere. Finally, the solid ZnO nanocages were obtained.

3.1. Structural and morphological characteristic The microstructures and surface morphology of the 170-ZIF-8 precursors and HHQD-ZnO nanocages were investigated by SEM (scanning electron micrographs). Fig. 1a and b show that the as-synthesized 170ZIF-8 precursors have uniform cage-like morphologies and the size of the 170-ZIF-8 particles is about 170 nm. What’s more, good connectivity between 170-ZIF-8 particles can be observed. Fig. 1c and d show the typical SEM image of HHQD-ZnO nanocages derived from 170-ZIF-8 shown in Fig. 1a and b. It is obvious that the cage-like morphology of precursors were retained and there were no apparent collapsed structures in the final sample. The as-obtained HHQD-ZnO nanocages were further investigated to obtain deeper insight into the structural features by TEM and HRTEM. From the large-area morphology (Fig. 2a) and high resolution image (Fig. 2b) of HHQD-ZnO nanocages, it is clear that the nanocage-like morphology was not destroyed. As Fig. 2c shown, the hollow structure of single HHQD-ZnO nanocage can be observed. The high-resolution transmission electron microscopy (HRTEM) images show the morphology of as-synthesized HHQD-ZnO with an average diameter of about 4.5 nm (Fig. 2d and e). N2 adsorption–desorption isotherms were measured to get the specific surface area and pore distribution of HHQD-ZnO nanocages (Fig. 2f). The specific surface area calculated

2.3. Synthesis of hollow ZnO nanocages For details, 0.3 g Zn(OAc)2·2H2O, 0.01 g CTAB (Hexadecyl trimethyl ammonium Bromide) and 0.25 ml triethanolamine were dissolved in 5 ml isopropanol to form a clear solution after 70 ℃ heated with vigorous stirring for several minutes. Meanwhile, 1.1 g 2-methylimidazole and 0.3 g 1-methylimidazole were dissolved in 5 ml isopropanol with vigorous stirring for 30 min. After that, the zinc acetate solution was 145

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Fig. 1. SEM images of 170-ZIF-8 precursors (a) and (b); HHQD-ZnO nanocages (c) and (d) derived from 170-ZIF-8.

from the N2 isotherm adsorption/desorption is 59.4 m2 g−1. As Fig. 2g shown, pore sizes of HHQD-ZnO nanocages were distributed in the range between 1.83 and 34.6 nm with central size of 18.05 nm. The X-ray diffraction (XRD) patterns of the as-synthesized 170-ZIF8 shown in Fig. 3a match well with previous reports [62–64]. The X-ray diffraction patterns of HHQD-ZnO nanocages are shown in Fig. 3b. The diffraction peaks of Bragg angles (2 θ) are 31.77°, 34.42°, 36.25°, 47.54°, 56.60°, 62.86° and 67.96°, corresponding to the(100), (002), (101), (102), (110), (103) and (112) planes of the wurtzite ZnO

structure (Joint Committee on Powder Diffraction Standards (JCPDS) File Card No. 36–1451). From the XRD results, the corresponding full width at the half maximum intensity (FWHM) of these diffraction peaks are 0.02782, 0.02953, 0.0285, 0.03186, 0.03146, 0.03207 and 0.03658 rad. According to the Scherer equation (Eq. (2)) [65], the crystallite size was calculated to be 5.12, 4.86, 5.06, 4.70, 4.95, 5.01 and 4.52 nm of corresponding diffraction peaks based on the FWHM of these diffraction peaks, respectively. The mean of these calculated crystallite sizes are 4.89 nm

Fig. 2. TEM images (a)-(e); BJH pore size distribution (f) and Nitrogen gas adsorption–desorption isotherms (g) of HHQD-ZnO nanocages. 146

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Fig. 3. X-ray diffraction patterns of (a) 170-ZIF-8 precursors; (b) HHQD-ZnO nanocages derived from 170-ZIF-8.

ZnO nanostructures. Due to the good dispersity, each RD-ZIF-8 particle is relative independence. After calcination at high temperature, the RDZIF-8 particles decomposed and shrank into solid ZnO nanocages without the influence of the surrounding particles (Fig. S5a and b). In the heating process, the decomposition of 170-ZIF-8 can result in the shrink. On the opposite, the interaction force of surrounding 170-ZIF-8 particles will resist the shrink due to the good connectivity. Under certain conditions, such as suitable temperature and annealing time, the interaction of shrink and drawing force can achieve a balance and lead to the formation of HHQD-ZnO nanostructure. With the increase of temperature and annealing time, this balance is destroyed and the 170ZIF-8 precursors break even into nanoparticles instead of shrink (Fig. S5c and d). Fig. 4. Responses of the sensors based on HHQD-ZnO, hollow ZnO and solid ZnO nanocages to 100 ppm ethanol gas at different operating temperatures, respectively.

L=

Kλ βcosθ

3.2. Gas sensing properties Typically, the response of the semiconductor oxide gas sensor is highly influenced by the operating temperature greatly [62]. Fig. 4 shows the response of the sensor to 100 ppm ethanol under operating temperature range from 270 to 350 °C. The results of the tested sensor show the volcano-shaped patterns of temperature dependence of the response, which have been observed in many reports [71–78]. A diffusion-controlled response model has been proposed and successfully used to describe these frequently observed temperature-dependent sensing responses [79]. The results of the temperature-dependent response display clearly that the maximum responses of HHQD-ZnO, hollow ZnO and solid ZnO nanocages-based senosrs are 139.41, 15.86 and 10.31 to 100 ppm ethanol gas at the operating temperature of 325 °C, respectively. It is clear that 325 °C is the optimal operating temperature and could be useful for gas sensing measurements. Fig. 5 displays the dynamic responses of gas sensors based on

(2)

where L is the mean crystallite size; K is the Scherrer constant; λ is the X-ray wavelength; β is the full width at half the maximum intensity (FWHM) of the diffraction peak; θ is the Bragg angle. Fig. S1 shows the weight loss curve of 170-ZIF-8 measured by thermogravimetric analysis (TGA) under air atmosphere. Between 100 and 250 °C about 1% weight reduction is incurred, corresponding to the removal of guest molecules occluded within and adsorbed on the 170ZIF-8 surface. The structural degradation of 170-ZIF-8 began from 330 °C [66]. After 350 ℃, the TGA curve indicated a steep weight loss of thermal decomposition of 170-ZIF-8, which leads to the formation of zinc oxide [67–70]. SEM images of RD-ZIF-8 (Fig. S2a and b) and solid ZnO nanocages (Fig. S2c and d) derived from RD-ZIF-8 are shown in Fig. S2. From the TEM image of Fig. S2e, the size of solid ZnO nanocages are observed about 470 nm. Fig. S2f shows that the lattice fringe spacing of 0.28 nm, corresponding to the (100) plane of the hexagonal phase of ZnO. The XRD patterns of RD-ZIF-8 and solid ZnO nanocages are displayed in Fig. S3a and b. The SEM image of hollow ZnO nanocages (Fig. S4b) derived from 470-ZIF-8 (Fig. S4a) shows that the size is about 470 nm and the broken nanocages confirm the hollow nanostructure. The TEM image (Fig. S4c) further strengthens the evidence of hollow nanocages. The high-resolution TEM (HRTEM) image (Fig. S4d) reveals that the distinct lattice fringes of 0.26 nm corresponding to (002) plane of ZnO. The XRD patterns of hollow ZnO nanocages shown in Fig. S4e confirm the formation of ZnO. Connectivity (or dispersity) of ZIF-8 precursors can play an important role in the formation of final structures of HHQD-ZnO and solid

Fig. 5. Sensing transients of the sensors based on solid ZnO, hollow ZnO and HHQD-ZnO nanocages to 100–1000 ppm ethanol gas, respectively. 147

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Fig. 6. (a) Gas responses of solid ZnO, hollow ZnO and HHQD-ZnO nanocage-based sensors and to 20–1000 ppm ethanol, respectively; (b) Dilogarithm linear fitting of the gas response to the concentration of ethanol gas.

HHQD-ZnO nanocages, hollow ZnO and solid ZnO nanocages to different concentrations of ethanol at 325 °C, respectively. The responses of the HHQD-ZnO nanocage-based sensor are 139.41, 257.29, 425.99, 535.12, 610.63 and 679.54–100, 200, 400, 600, 800 and 1000 ppm of ethanol, respectively. The solid ZnO nanocage-based sensor get the responses of 10.31, 17.55, 26.68, 33.46, 39.13 and 43.81, while hollow ZnO-based gas sensor acquires the responses of 15.86, 28.02, 42.27, 57.53, 70.26 and 78.95 under the same ethanol gas concentration condition. The responses of gas sensors as functions of the concentration of ethanol at 325 °C are shown by Fig. 6a. From the curves it can be observed that the responses increase with the increase of the ethanol gas concentration. The responses are almost linear with gas concentration at the range of 20–100 ppm. When the target gas concentrations reach higher levels, the response tends to saturation slightly. The response of the HHQD-ZnO nanocage-based sensor is over 13 times than that of the solid ZnO nanocages-based sensor. The response of semiconducting oxide gas sensor as a function of the gas concentration can be empirically represented as:

S= 1 + Ag ·(Pg )α

Fig. 7. Response/recovery time of HHQD-ZnO nanocage-based sensor to 100 ppm ethanol at 325 °C.

and 32.58 to 25 ppb, 250 ppb, 2.5 ppm, 10 ppm, 15 ppm and 20 ppm ethanol gas at 325 °C (Fig. 8b). It means that the detection limit of the HHQD-ZnO nanocages-based sensor is below 25 ppb to ethanol gas. Fig. 9 shows the selectivity of the HHQD-ZnO nanocage-based ethanol gas sensor compared with the responses of ammonia, isopropanol, toluene, acetone, DMF (N N-Dimethyl formamide), formaldehyde, methanol, propanol and butanol. Obviously, the HHQDZnO nanocages-based sensor has excellent selectivity to ethanol at the temperature of 325 °C. Table 1 lists a performance comparison between the HHQD-ZnO nanocages-based sensor and the previous reports of ZnO nanostructures. It is obvious that the HHQD-ZnO nanocages exhibit excellent performance, especially the much better response and faster response speed than those ZnO nanomaterials reported in the previous research [52,82–86]. The long-term stability of the HHQD-ZnO nanocages-based sensor towards 100 ppm ethanol gas at 325 °C is displayed by Fig. S6. It is found that the sensor showed excellent stability for more than 60 days.

(3)

where Pg is the target gas partial pressure, which is directly proportional to the gas concentration. The response S is characterized by the prefactor Ag and exponent α directly derived from the surface interaction between chemisorbed oxygen and reductive gas to n-type semiconductor directly [39,80]. The logarithm of gas response can be linear with that of gas concentration [80,81]. From Fig. 6b, the logarithm of the response S can be fitted linearly with the logarithm of ethanol concentration perfectly and the equations are as follows: Y=−0.10238 + 0.58189X

(4)

Y=−0.14448 + 0.68204X

(5)

Y = 0.52659 + 0.79010X

(6)

3.3. Gas sensing mechanism

The value α toward to ethanol gas is 0.58189, 0.68204 and 0.79010 and the correlation coefficient (R) of fitting linear is 0.99505, 0.99515 and 0.99013 for solid ZnO, hollow ZnO and HHQD-ZnO nanocages, respectively. Hence, the sensitivity of the sensor base on HHQD-ZnO nanocages is significantly higher than that of solid ZnO and hollow ZnO nanocages. As shown in Fig. 7, one circle of dynamic response to 100 ppm ethanol gas at 325 °C was tested to further investigate the performance of HHQD-ZnO nanocage-based sensor. It is observed that the response/ recovery time is 2.8/56.4 s with the high response of 139.41. Fig. 8a shows sensing behaviors of HHQD-ZnO nanocage-based sensor to 25 ppb ethanol gas. The responses are 5.10, 5.72, 10.13, 20.15, 25.03

The change of resistance is mainly caused by the adsorption and desorption of gas molecules on the surface of the sensing structure for ZnO-based sensors. The mechanism of sensing of HHQD-ZnO sensors can be explained by the modulation model of the depletion layer (Fig. S7). The adsorption of oxygen species on the surface of HHQD-ZnO results in extracting free electrons from the conduction band and the consequent formation of a thick space charge layer (SCL), which increases the potential barrier (PB) and thus results in a higher resistance (Fig. S7a) [36]. Once exposed to reductive atmosphere, the reaction of adsorbed oxygen species with the reductive gas molecules releases the trapped electrons back to the conduction band. This leads to increasing 148

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Fig. 8. (a) Sensing transients of the sensor based on HHQD-ZnO nanocages to 25 ppb ethanol gas; (b) Gas responses of HHQD-ZnO nanocage-based sensor to 25 ppb20 ppm ethanol gas, respectively.

Fig. 9. (a) Dynamic sensing curves and (b) response of gas sensor based on HHQD-ZnO nanocages toward 100 ppm different target gases (ammonia, formaldehyde, DMF (dimethylformamide), acetone, toluene, isopropanol, methanol, propanol, butanol and ethanol, respectively).

γ O2 + δe−1 → Oδγ− 2

carrier concentration and decreasing the thickness of SCL and resistances of sensors (Fig. S7b). From above performance analysis we notice that the response of the HHQD-ZnO nanocages-based sensor is much higher than that of the hollow ZnO and solid ZnO nanocage-based sensor although they are both ZnO nanomaterials. The enhanced properties of the HHQD-ZnO nanocages-based sensor to ethanol gas are possibly attributed to the follow factors. Physisorption and chemisorption are the two well-accepted types of adsorption. Physisorption is associated to a neutral state for adsorption and there is no true chemical bonds made between physisorbed species and the ZnO surface. The chemisorption of oxygen on ZnO results in the formation of an electron-depleted surface layer, causing a decrease in the electronic conductivity. In air, oxygen molecules are adsorbed on the surface of ZnO and could interact with the surface defects and electrons as follow [55,87,88]:

(7)

where δ and ν show the ionized degree of surface adsorbed oxygen. The adsorption of oxygen on ZnO results in the decrease in the electronic conductivity for n-type semiconductor. The adsorbed oxygen is mainly in the form of O− at temperatures ranging from 200 to 350 ℃ [89]. The reaction of gas molecules (such as C2H5OH, CH3OH, NH3, HCHO and C7H8) with chemisorbed oxygen on HHQD-ZnO results in the removal of these oxygen molecules, which causes a decrease in the electric resistance of the sensor devices. When the sensor was exposed to the target gas, such as ethanol, the target gas will react with adsorbed oxygen species on the ZnO surface to form CO2 and H2O, which leads to an increasing carrier concentration of the sample and decreasing resistances of sensors (Fig. S8a). The chemical reaction between the adsorbed ionized oxygens and the target gas molecules at temperature ranging from 200 to 350 °C can be depicted by the followed reaction

Table 1 Performance Comparison of gas sensors designed by other approaches in previous reports (Tres and Trecov are defined as response/recovery time). Sensing materials

Temperature (°C)

Concentration

Tres (s)

Trecov (s)

Response

ZnO spheres [82] ZnO cubes [52] ZnO nanowires [83] Porous ZnO nanoplates [84] ZnO nanorods [85] ZnO nanorods [86] HHQD-ZnO nanocubes

350 400 300 380 332 320 325

100 ppm/ethanol 5 ppm/toluene 100 ppm/ethanol 100 ppm/ethanol 100 ppm/ethanol 100 ppm/ethanol 100 ppm/ethanol

> 50 49 ± 16 >3 320 – 47 2.8

> 300 121 ± 19 ˜60 17 – 50 56.4

˜24 ˜1.9 ˜35 8.9 13.5 11 139.41

149

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not larger than twice the excitonic Bohr radius of ZnO [98]. It is possible that the formation of ZnO quantum dots can introduce some structural defects and result in stronger surface effect. [99,100], which improve the electrochemical reaction of quantum dots. The possible defects (such as the lattice mismatch in the fingerprint area with red dotted line of Fig. 2c) in gas sensor could promote the charge separation capability, retard the recombination of hole-electron pairs and consequently enhance gas sensitivity [101–103]. All the above-mentioned factors improve the oxidative reactive capacity towards ethanol molecules, which results in the significantly enhanced performance of HHQD-ZnO nanocage-based sensor.

equations [90–92]:

C2 H5 OH + 6O− → 2CO2 + 3H2 O+ 6e−

(8)

CH3 OH + 3O− → CO2 + 3H2 O+ 3e−

(9)

NH3 + 1.5O− → 0.5N2 + 1.5H2 O+ 1.5e−

(10)

HCHO + 2O− → CO2 + H2 O+ 2e−

(11)

C7 H8 + 2O− → C7 H6 O− + H2 O+ e−

(12)

That is to say, the reaction of target gas molecules with adsorbed oxygen on ZnO results in the removal of adsorbed oxygen molecules, which causes a decrease in the electric resistance of the sensor devices. Based on Eqs. (8)–(12) we can see that the quantitative proportional relationship of released electrons is 6:3:1.5:2:1 to the same number of C2H5OH, CH3OH, NH3, HCHO and C7H8 molecules by consuming O−. Much more electrons are released in the reactions of ethanol compared with that of CH3OH, NH3, HCHO and C7H8. Consequently, the higher sensitivities were obtained by ethanol, which results in the good selectivity of ethanol. It was reported that the catalytic activity of Auloaded TiO2 and Au-ZnO for ethanol is higher than that for toluene [93,94]. The possible higher catalytic activity of HHQD-ZnO results in higher sensitivity for ethanol. It is known that ethanol, propanol and butanol are homologues. Due to higher chemical activity of oxidation, ethanol is easier to be oxidized, which results in higher sensitivity of ethanol in contrast to that of and butanol. From the SEM images of Figs. 1 and 2, it can be observed that there is excellent connectivity between 170-ZIF-8 (Fig. 1a and b) particles, meanwhile, the RD-ZIF-8 particles have good dispersity (Fig. S2a and b). After annealing the 170-ZIF-8 and RD-ZIF-8 precursors, the obtained products HHQD-ZnO nanocages and solid ZnO nanocages basically maintain the original connection characteristics of their precursors, respectively. It means that with the particle-interpenetrating framework, the HHQD-ZnO nanocages (Fig. 1c and d; Fig. 2a and b) have good connectivity compared with the solid ZnO nanocages (Fig S2 c–e). Connectivity between primary particles is an important parameter for achieving high sensitivity [52,91,92]. The good connectivity of HHQDZnO nanocages can be a factor to ensure the higher performance, especially the excellent sensitivity compared with the solid ZnO nanocages. From Fig. 2, Figs. S2 and 7 it can be observed that the size of HHQDZnO nanocages, hollow ZnO nanocages and solid ZnO nanocages are about 170 nm, 470 nm and 470 nm, respectively. It is means that the size of solid ZnO nanocages is about 3 times than that of hollow ZnO and HHQD-ZnO nanocages. The reduction in size of HHQD-ZnO nanocages can result in the increase of surface activity of ZnO [95–97]. In addition, it is worth noticed that with the similar particle shape and size, the response of hollow ZnO nanocage-based sensor is much higher than that of solid ZnO nanocages. It is obvious that the hollow cages provide more surface exposure of active reactive sites because of integrated hollow and interpenetrated in-sheath porosity, producing higher specific surface area. Then it is obvious that the unusual hierarchical hollow nanostrucure of HHQD-ZnO provides a large surface-tovolume ration and surface exposure of active reactive sites. Both the target gas and the background gas can access all of the surfaces of particles. Fig. S8b shows the ethanol gas diffusion in HHQD-ZnO nanocages. The ethanol molecules not only spread and be adsorbed on the external surface, they can spread inward or through the hollow particleinterpenetrating framework (as yellow and brown green arrow shown in Fig. S8b, respectively). Then there are more adsorption sites for gas adsorption, leading to enhancing ethanol-sensing performance of HHQD-ZnO nanocage-based sensor. The smaller size of HHQD-ZnO nanocages with hierarchical porous hollow nanostructure can obtains high surface area and pore volume compared with the relative large size of solid ZnO nanocages with the similar particle shapes, which results in the super higher response. Moreover, the average crystallite size of HHQD-ZnO nanocages is

4. Conclusions In summary, we have developed easy metal–organic framework (MOF)-based strategies to synthesize hollow ZnO, solid ZnO and hierarchical hollow ZnO nanocages with quantum dots (HHQD-ZnO nanocages) for gas sensors from Zn-based metal organic frameworks (ZIF-8) nanocages precursors. The HHQD-ZnO nanocage-based gas sensor shows the enhanced response of 139.41 with the response and recovery time being 2.8 s and 56.4 s compared with the solid ZnO nanocagesbased gas sensor towards 100 ppm ethanol gas at 325 °C. The excellent gas sensing performance of HHQD-ZnO nanocages is likely attributed to the unique hollow interpenetrated nanostructures, relative small size and quantum dots with large specific surface area. Author contributions Xin Zhang and Weiying Lan contributed equally to this work. Acknowledgments This work is financially supported by self-determined research funds of CCNU from the colleges’ basic research and operation of MOE. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.03.090. References [1] G. Zhu, H. Xu, Y. Xiao, Y. Liu, A. Yuan, X. Shen, Facile fabrication and enhanced sensing properties of hierarchically porous CuO architectures, Appl. Mater. Interfaces 4 (2012) 744–751. [2] C. Wang, X. Cheng, X. Zhou, P. Sun, X. Hu, K. Shimanoe, G. Lu, N. Yamazoe, Hierarchical α-Fe2O3/NiO composites with a hollow structure for a gas sensor, Appl. Mater. Interfaces 6 (2014) 12031–12037. [3] F. Wei, H. Zhang, M. Nguyen, M. Ying, R. Gao, Z. Jiao, Template-free synthesis of flower-like SnO2 hierarchical nanostructures with improved gas sensing performance, Sens. Actuators B Chem. 215 (2015) 15–23. [4] P. Sun, Y. Cao, J. Liu, Y. Sun, J. Ma, G. Lu, Dispersive SnO2 nanosheets: hydrothermal synthesis and gas-sensing properties, Sens. Actuators B Chem. 156 (2011) 779–783. [5] I. Kim, A. Rothschild, B. Lee, D. Kim, S. Jo, H. Tuller, Ultrasensitive chemiresistors based on electrospun TiO2 nanofibers, Nano Lett. 6 (2006) 2009–2013. [6] J. Zhang, S. Wang, Y. Wang, M. Xu, H. Xia, S. Zhang, W. Huang, X. Guo, S. Wu, ZnO hollow spheres: preparation characterization, and gas sensing properties, Sens. Actuators B Chem. 139 (2009) 411–417. [7] X. Li, S. Yao, J. Liu, P. Sun, Y. Sun, Y. Gao, G. Lu, Vitamin C-assisted synthesis and gas sensing properties of coaxial In2O3 nanorod bundles, Sens. Actuators B Chem. 220 (2015) 68–74. [8] S. Elouali, L.G. Bloor, R. Binions, I. Parkin, C. Carmalt, J. Darr, Gas sensing with nanoindium oxides (In2O3) prepared via continuous hydrothermal flow synthesis, Langmuir 28 (2012) 1879–1885. [9] Z. Cai, H. Li, X. Yang, X. Guo, NO sensing by single crystalline WO3 nanowires, Sens. Actuators B Chem. 219 (2015) 346–353. [10] C. Rout, K. Ganesh, A. Govindaraj, C. Rao, Sensors for the nitrogen oxides, NO2, NO and N2O, based on In2O3 and WO3 nanowires, Appl. Phys. A 85 (2006) 241–246. [11] W. Yan, H. Fan, Y. Zhai, C. Yang, P. Ren, L. Huang, Low temperature solutionbased synthesis of porous flower-like α-Fe2O3 superstructures and their excellent gas-sensing properties, Sens. Actuators B Chem. 160 (2011) 1372–1379.

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Xin Zhang is currently a MS student of physical science and technology department, Central China Normal University, Wuhan, PR China. Weiying Lan is currently a MS student of physical science and technology department, Central China Normal University, Wuhan, PR China. Junlan Xu received her BS degree from the Department of Physical Science and Technology in Central China Normal University. She is majoring in preparation ofnanomaterial and its application. Yantao Luo is currently a MS student of physical science and technology department, Central China Normal University, Wuhan, PR China. Jiang Pan is currently a MS student of physical science and technology department, Central China Normal University, Wuhan, PR China. Cunyi Liao is currently a MS student of physical science and technology department, Central China Normal University, Wuhan, PR China. Linyu Yang is currently a MS student of physical science and technology department, Central China Normal University, Wuhan, PR China. Wenhu Tan received his Ph.D. degree from the Electronic Information School, Wuhan University. He is mainly engaged in the research of gas sensors, electronic design automatic and embedded system. Xintang Huang is a professor in Institute of Nanoscience and Nanotechnology, Central China Normal University, Wuhan, PR China. He is mainly engaged in the research of gas sensors, electrochemical biosensors, lithium-ion battery, dye sensitized solar cells and supercapacitors.

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