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Communication

Preparation and Enhanced Acetone-Sensing Properties of ZIF-8-Derived Co3O4@ZnO Microspheres

by
Xiangxiang Fan
1,2,*,
Susu Yang
1,
Chun Huang
1,
Yujie Lu
1 and
Pan Dai
1,2
1
School of Information Engineering, Huzhou University, Huzhou 313000, China
2
Zhejiang Province Key Laboratory of Smart Management & Application of Modern Agricultural Resources, Huzhou University, Huzhou 313000, China
*
Author to whom correspondence should be addressed.
Chemosensors 2023, 11(7), 376; https://doi.org/10.3390/chemosensors11070376
Submission received: 9 June 2023 / Revised: 30 June 2023 / Accepted: 3 July 2023 / Published: 5 July 2023
(This article belongs to the Special Issue Chemical and Biosensors Based on Metal-Organic Frames (MOFs))
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Abstract

:
In this work, ZIF-8-derived Co3O4@ZnO microspheres were prepared by a liquid-phase concentration-controlled nucleation strategy. The results of the material characterization showed that Co3O4@ZnO microspheres were obtained, and the surface structure could be controlled with the concentration of the ligand. Compared with pure Co3O4 microspheres, the operating temperature of optimized Co3O4@ZnO microspheres increased by 90 °C after the gas-sensing test. The response to 50 ppm acetone of Co3O4@ZnO microspheres was 4.5 times higher than that of pure Co3O4, and the detection limit reached 0.5 ppm. Meanwhile, Co3O4@ZnO microspheres showed a shorter response-recovery time and better selectivity. The enhanced-sensing mechanism of the ZIF-8-derived Co3O4@ZnO microspheres was also analyzed.

1. Introduction

Acetone is not only an important organic solvent but also a respiratory marker of diabetes. The concentration of acetone in the exhaled breath of a diabetic patient is 1.8 ppm, while the concentration for a healthy person is less than 0.9 ppm. In order to diagnose diabetes, it is of great significance to prepare an acetone sensor with a detection limit of the ppb level [1]. Due to the advantages of their small size, low cost, and easy preparation [2,3], metal-oxide semiconductor (MOS) gas sensors have been widely used for acetone detection, such as In2O3, SnO2, and Co3O4 [4,5,6]. Compared with the n-type MOS, previous studies have shown that Co3O4-based sensors exhibited the features of lower working temperature and higher selectivity [7,8]. Co3O4 is considered a promising MOS material for acetone-sensing with low power consumption. The response to 200 ppm acetone of Co3O4 nanoparticles prepared by Zhang et al. was 2.6 at 190 °C [9]. Park et al. synthesized Co3O4 hollow microspheres, and the response to 1000 ppm acetone was 7.5 at 100 °C [10]. However, the application of pure Co3O4-based sensors is still limited by their low sensitivities.
To obtain improved gas-sensing properties of Co3O4-based sensors, a general method is to prepare heterojunctions. The response to 50 ppm acetone of Co3O4/ZnO heterogeneous material prepared by Zhao’s team was approximately 5.1 times higher than the value of pure Co3O4 [11]. The response to 5 ppm TMA of Co3O4/SnO2 heterogeneous material prepared by Meng’s team was 7.2 times higher than the value of pure Co3O4 [12]. Therefore, enhanced sensing properties can be obtained by constructing a heterojunction. The structures of heterogeneous materials include a hybrid heterostructure, a decorated heterostructure, and a multilayer heterostructure [13,14,15,16]. Among them, the decorated heterostructure is beneficial for gas sensing due to its good synergistic effect between the core and shell components and larger heterointerface [17,18]. The response to 100 ppm formaldehyde of In2O3@SnO2 heterostructure Nanofibers prepared by Wan et al. was higher than that of pure In2O3 Nanofibers and pure SnO2 nanosheets [19]. A variety of decorated heterostructures have been reported as sensing materials, such as In2O3@NiO, CeO2@ZnSnO3, and Co3O4@WO3 [20,21,22]. All the reported decorated heterostructures have shown improved gas-sensing properties.
Metal-organic framework (MOF) materials, with their large specific surface area and high porosity, have shown great value for application in the fields of gas storage, energy storage, adsorption, and catalysis [23,24,25]. Since the porous MOS structure can be obtained via heating treatment, MOF materials have received extensive attention for the preparation of sensors [26,27,28]. More gas adsorption and reaction sites in porous MOS could significantly enhance the gas-sensing properties [29]. Therefore, more attention has been paid to the decorated heterostructures prepared by MOF. As a representative MOF material, ZIF-8 is bridged by 2-methylimidazole and Zn2+. ZnO could be obtained by annealing treatment of ZIF-8. The material of ZnO exhibits high charge-carrier mobility and high sensitivity [30,31], and ZIF-8-derived ZnO obtained extensive attention as a gas sensor. The response to 500 ppb acetone of ZIF-8-derived ZnO@MoS2 heterostructures prepared by Chang’s group reached 1.5 at 350 °C [32]. Zhan et al. prepared ZIF-8-derived SnO2@ZnO microspheres, which showed a response of 164 to 70 ppm NO2 at 300 °C [33]. The excellent gas-sensing performances benefit from the heterojunction and porous structure. However, these ZIF-8-derived n-type ZnO heterogeneous materials were always operated at a high temperature. As a sensing material with low power consumption, Co3O4 with ZIF-8-derived ZnO coating was rarely reported.
In this work, ZIF-8-derived Co3O4@ZnO microspheres were prepared using the liquid-phase concentration-controlled nucleation strategy. The surface structure was controlled with the concentration of the ligand. The acetone-sensing properties and mechanism of the heterostructure were analyzed.

2. Materials and Methods

2.1. Materials

2-methylimidazole (2-MeIM, AR), zinc acetate (Zn(COOH3)2·2H2O, AR), cobalt nitrate hexahydrate (Co(NO3)2·6H2O, AR), sodium hydroxide (NaOH, AR), ethanol (CH3CH2OH, AR), and methanol (CH3OH, AR) were all bought from Aladdin (Shanghai, China). All the reagents were utilized without pretreatment.

2.2. Synthesis of Co3O4 Microspheres

First, 58.21 g Co(NO3)2·6H2O was added to 40 mL deionized water to obtain a purple solution, then 2 g NaOH was added and stirred for 10 min. Second, the supernatant of the solution was transferred into a 50 mL Teflon-lined autoclave and kept at 180 °C for 5 h. After the reaction, the precipitate was obtained by centrifugation. The precipitate was washed with ethanol and water three times, and the Co3O4 precursor was obtained after being dried at 70 °C for 3 h. Finally, Co3O4 microspheres were obtained by calcining the precursor at 500 °C for 2 h.

2.3. Synthesis of Co3O4@ZnO Microspheres

Solution A was obtained by dissolving 0.0714 g Zn(COOH3)2·2H2O in 10 mL of methanol. Solution B was obtained by dissolving a certain amount of 2-methylimidazole and 50 mg of the Co3O4 precursor in 10 mL of methanol, followed by sonication for 1 h. Solutions A and B were mixed and stirred for 30 min. After standing for 24 h, the precipitate was obtained by centrifugation. The precipitate was washed with ethanol and water three times, and the precursor of Co3O4@ZnO microspheres was obtained after being dried at 60 °C for 12 h. After being annealed at 500 °C for 3 h, Co3O4@ZnO microspheres were obtained.
The amount of 2-MeIM added was set as 3.7 times, 37 times, and 110 times that of the molar amount of Zn2+, and the corresponding materials were named CZ-1, CZ-2, and CZ-3.

2.4. Material Characterization

Scanning electron microscopy (SEM, Apreo 2S, ThermoFischer, Waltham, MA, USA) and transmission electron microscopy (TEM, Talos F200S G2, ThermoFischer) were utilized to characterize the morphology and surface structure of the materials. The element compositions were analyzed by energy-dispersive X-ray spectroscopy (EDS, QUANTAX EDS, Bruker, Billerica, MA, USA). The crystal composition and crystallinity were analyzed by X-ray diffraction spectroscopy (XRD, Ultima IV, Rigaku, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS, ESCALAB Xi+, ThermoFischer) was utilized to characterize the chemical ingredients and states of the samples.

2.5. Preparation and Measurement of Sensors

The fabrication method of the sensors is consistent with our previous report [34]. Specifically, 10 mg of Co3O4@ZnO microspheres were mixed with 100 μL of ethanol to obtain the homogeneous paste, and it was evenly brushed onto the outer surface of a ceramic tube equipped with a Cr-Ni heating wire and a pair of gold electrodes. After being annealed at 300 °C for 12 h, the acetone sensor was obtained.
As shown in Figure 1, the test system includes a humidity generator, a test chamber, and a source meter. The humidity generator is used to generate different humidity environments. The air was filtered and passed through a saturated salt solution by an air pump. When the air was introduced into the test chamber, the environment with the required humidity was obtained and the relative humidity was determined by a humidity sensor. The measurement of our sensor was performed in an 18 L test chamber whose size was 300 mm × 300 mm × 200 mm. A source meter was used to record the data.
Gas-sensing properties were analyzed by injecting saturated acetone vapor into the test chamber or exhausting acetone from the chamber as per our previous report [35]. When acetone gas was injected, the gas was stirred evenly by the fans. The concentration of acetone is adjusted by changing the volume of saturated steam injected into the chamber [36]. The concentration could be calculated by the formula Cg = VgPg/P0Vc, where Vg is the volume of acetone injected into the chamber, Pg is the saturated vapor pressure of acetone, P0 is the ambient atmospheric pressure, and Vc is the volume of the test chamber. After the test, the acetone gas in the test chamber was replaced with fresh air through the air pump.
The ratios of (Rg − Ra)/Ra were used to describe the sensitivity of the sensors, where Rg and Ra are the resistances of the sensors in target gas and air. The times required to change the resistance by 90% after injecting or exhausting the target gas were used to define the response and recovery time.

3. Results

3.1. Materials Characterization

Figure 2a presents that the Co3O4 particle is a spherical structure with a smooth surface, and the size is approximately 300 nm. As shown in Figure 2b–d, all the samples of CZ-1, CZ-2, and CZ-3 still maintain spherical structures, but the surface characteristics have undergone dramatic changes. With the increase in the 2-methimidazole concentration, Co3O4 microspheres are gradually covered by nanoparticles, and the surfaces become rougher when the number of covered grains gradually increased. Therefore, the microspheres with decorated particles can be prepared by this method, and the surface structure can be adjusted by changing the ligand concentration.
The XRD spectra of Co3O4, CZ-1, CZ-2, and CZ-3 are displayed in Figure 3. The peaks of Co3O4 are located at 31.24°, 36.82°, 44.78°, 55.66°, and 59.22°, which correspond to (2 2 0), (3 1 1), (4 0 0), (4 2 2), and (5 1 1) crystal planes of spinel Co3O4 (JCPDS No.43-1003). The spectra of CZ-1, CZ-2, and CZ-3 also show identical diffraction peaks of Co3O4, indicating that Co3O4 crystal is formed in these samples. On the XRD spectrum of CZ-3, three different diffraction peaks are formed from other samples, which are located at 31.6°, 36.24°, and 56.42°. These peaks are indexed to the (1 0 0), (1 0 1), and (1 1 0) crystal planes of ZnO. The results indicate that the CZ-3 contains crystals of Co3O4 and ZnO. However, due to the low content of ZnO, the three peaks are not observed on the spectra of CZ-1 and CZ-2.
As shown in Figure 4, all the samples were further analyzed by TEM and EDS to confirm the morphologies and compositions. Figure 4a,b present that pure Co3O4 and CZ-1 are both smooth microspheres. Figure 4c,d exhibit that the Co3O4 microspheres are coated by ZnO nanoparticles and the amount of ZnO nanoparticles gradually increases with the increase in the 2-methimidazole concentration. In order to verify that CZ-1 is coated by ZnO, EDS was used to analyze the element on the surface. Figure 4e,f show that the element of Zn is evenly distributed on the surface of Co3O4 microspheres. The smooth surface of CZ-1 can contribute to the small size of ZnO nanoparticles. Figure 4g,h indicate that the main element in the nanoparticles coated on the microspheres is Zn. Figure 4i shows that the lattice width of the nanoparticles coated on the microspheres is 0.26 nm and is indexed to the (0 0 2) crystal plane of ZnO. The results indicate that Co3O4@ZnO microspheres are obtained by the liquid-phase concentration-controlled nucleation strategy.
Figure 5a shows that the elements of Zn, O, and Co are presented on the XPS spectrum of CZ-2, while only Co and O appear in Co3O4. Figure 5b shows the high-resolution XPS spectrum of Zn 2p in CZ-2, which includes two peaks corresponding to Zn2+ [37]. Therefore, it is further proved that the nanograins coated on Co3O4 microspheres are ZnO. Figure 5c presents the Co 2p spectra of Co3O4 and CZ-2. The fitting peaks located at 797 eV and 782.5 eV are associated with Co2+, while the fitting peaks located at 795 eV and 779.5 eV correspond to Co3+. This demonstrates the existence of Co3O4 particles in the samples [38]. Figure 5d presents the O 1 s spectra of Co3O4 and CZ-2. The three different fitting peaks are related to lattice oxygen (OL), oxygen vacancies (OV), and absorbed oxygen (OC) [39]. Among these, OV is the key component to affect the gas-sensing performance. The gas-sensing property is attributed to the redox reaction between the target gas with oxygen adsorbed on the surface of the sensing material. As the donor, the oxygen vacancies provide sites for oxygen adsorption and reaction [40]. The area ratio OV in Co3O4 is 29.8%, while that in CZ-2 is 39.1%. The OV proportion of CZ-2 is higher than that of pure Co3O4.

3.2. Gas-Sensing Properties

The sensing properties of the material are always affected by the operating temperature. Figure 6a shows the resistances of four samples in the air at a temperature ranging from 130 °C to 280 °C. The resistance of all the samples decreases significantly as the temperature increases. Because of the heterojunctions, the resistance of Co3O4@ZnO microspheres is higher than that of pure Co3O4. The resistance of CZ-2 and CZ-3 even exceeds the range of the source meter at temperatures lower than 190 °C, making it impossible to accurately measure the resistance values. Figure 6b shows the responses of four sensors to 50 ppm acetone, operated at a temperature ranging from 130 °C to 280 °C. Co3O4, CZ-1, CZ-2, and CZ-3 reach the maximum response at 160 °C, 190 °C, 250 °C, and 250 °C. The optimal operating temperatures of all the Co3O4@ZnO microspheres are higher than the value of pure Co3O4 because of the presence of n-type MOS. Among them, CZ-2 shows the highest response at 250 °C, and the resistance in the air is approximately 600 kΩ. The response value of CZ-2 is 4.5 times higher than the value of pure Co3O4. Therefore, the acetone sensitivity of Co3O4 is enhanced by the ZIF-8-derived ZnO.
The responses of pure Co3O4 and CZ-2 to 0.5–100 ppm acetone at the optimal operating temperature are displayed in Figure 7a. The response values of CZ-2 are larger than the values of Co3O4. Compared with pure Co3O4, the detection limit of CZ-2 reaches 500 ppb. As shown in Figure 7b, the response and recovery times of CZ-2 are 32 s and 98 s, which are shorter than that of pure Co3O4. Therefore, the sensitivity and response-recovery rates of Co3O4 are improved by the ZIF-8-derived ZnO.
As shown in Figure 8a, the responses of pure Co3O4 and CZ-2 to 50 ppm of different gases were tested at the optimized temperatures. The two samples show the highest response to acetone, which means that the two sensors have a selective response to acetone. To quantitatively investigate the selectivity of the two sensors, the ratios of Sothers/Sacetone are used and listed in Figure 8b, where Sacetone and Sothers are the responses of the sensor to acetone and other gas. Figure 8b presents that CZ-2 has lower ratios than pure Co3O4, which means that CZ-2 has better selectivity. According to the ratios, the selectivity of the sensor depends on the sensitivity of different gases, and the sensitivity of different gases is related to the reaction intensity of the gas on the surface of the sensing material. Due to the reaction intensity being influenced by the amount of adsorbed gas on the surface and the reaction energy, the material composition and working temperature are key factors to determine the selectivity, and the sensor exhibits the highest reaction intensity at the optimal operating temperature [13]. Compared with Co3O4, the acetone reaction intensity on the surface of CZ-2 is enhanced more than other gases, resulting in improved selectivity. The results show that the selectivity of Co3O4 is enhanced by ZIF-8-derived ZnO.
In order to evaluate the stability of the sensor, the multi-cycle tests of CZ-2 to 50 ppm acetone are shown in Figure 8c. The response curves of CZ-2 remain stable during the multi-cycle tests. The response curves of CZ-2 to 50 ppm acetone over 6 days are presented in Figure 8d. The responses of CZ-2 remain stable, and the relative standard deviation is 4.02%. The results exhibit that Co3O4@ZnO microspheres display good stability.
The response to acetone of CZ-2 was also investigated in an environment of relative humidity (RH) ranging from 30% to 80%. In order to quantitatively evaluate the anti-humidity property of CZ-2, the ratios of Sothers/S30 were used as a comparative quantity, where S30 and Sothers are the response in the environment of 30% RH and other RH. Figure 9 shows the ratios of CZ-2 as a function of relative humidity. As the humidity increased from 30% to 80%, the response of the CZ-2 decreased by 31% compared to the original. When the sensor is in a high-humidity environment, water molecules will adsorb on the surface of sensing materials and react with oxygen ions. A hydroxyl group with less activity is generated, and the sensitivity of the sensor decreases [41].
The acetone-sensing properties of different Co3O4-based sensors in this work and the previous reports are listed in Table 1 [42,43,44,45,46]. The ZIF-8-derived Co3O4@ZnO microspheres exhibit better sensitivity and a lower detection limit compared with the previous reports.

4. Discussion

The sensing mechanism of MOS is similar to the description of the previous report [47]. As shown in Figure 10a, when Co3O4 is in the air environment, the electrons in the material are captured by the absorbed oxygen molecules to form oxygen ions. A hole-accumulating layer is formed on the surface of the material [48]. The adsorption processes of oxygen are expressed by Formulas (1)–(3). As shown in Figure 10b, when Co3O4 is in acetone, the acetone molecules react with the oxygen ions and electrons are released into the sensing material. The reaction processes between oxygen ions with acetone molecules are expressed by Formulas (4) and (5). Due to the reaction, the hole-accumulating layer becomes narrow. For the sensing material of Co3O4, the accumulation layer is a conducting channel, and a change in the accumulation layer directly affects the response value.
O2(gas) → O2(ads)
O2(ads) + e− → O2−(ads)
O2−(ads) + e− → 2O−(ads)
CH3COCH3(gas) + 4O2−(ads) → 3H2O(gas) + 3CO2(gas) + 4e−
CH3COCH3(gas) + 8O−(ads) → 3CO2(gas) + 3H2O(gas) + 8e−
As the test results show, ZIF-8-derived Co3O4@ZnO microspheres present a p-type semiconductor behavior. The improved acetone-sensing properties of CZ-2 contribute to the formed p-n heterojunction between ZnO with Co3O4, the increased oxygen vacancy of material, and the porous structure of MOF-derived ZnO.
Firstly, the formed p-n heterojunction between ZnO and Co3O4 is the key reason for the enhanced gas sensitivity. As Figure 10c,d show, the band gaps of ZnO and Co3O4 are 3.37 eV and 1.6 eV. When they are in contact, the carrier flows between ZnO with Co3O4 to make the Fermi level equilibrium. The depletion layers are formed on the interface of two materials, and the resistance primarily depends on the width of the depletion layer. When the sensor is in the air, the reactions of Formulas (1)–(3) occur on the Co3O4@ZnO microspheres. The electrons flowing from ZnO to Co3O4 decrease, which causes a decrease in the width of the hole-depleting layer on the Co3O4 side [49]. When acetone gas is introduced, the reactions of Formulas (4) and (5) occur on the material, leading to the hole-depleting layer increasing. For the Co3O4@ZnO microspheres, the resistance change rate caused by the depletion layer is significant, leading to higher sensitivity. Secondly, oxygen vacancy is an active site, which provides a venue for oxygen adsorption and reaction [40]. From XPS characterization, it is clear that the OV component of CZ-2 is higher than that of pure Co3O4. This indicates that more sites of oxygen adsorption and reaction are present on the surface of Co3O4@ZnO microspheres. Finally, the porous structure derived by ZIF-8 is also a favorable channel for acetone gas transmission. With the porous structure, the gas reaches the interface of the heterojunction more easily. Therefore, the acetone-sensing properties of Co3O4@ZnO microspheres are improved.

5. Conclusions

ZIF-8-derived Co3O4@ZnO microspheres were prepared by a liquid-phase concentration-controlled nucleation strategy. This work indicated that the ZIF-8-derived surface structure could be controlled by changing the ligand concentration. Compared with pure Co3O4, the Co3O4@ZnO microspheres presented improved acetone-sensing properties. The sensor presents higher sensitivity, a faster response-recovery rate, and better selectivity and stability. This work shows that the preparation of an MOF-derived heterostructure is an effective way to enhance the gas-sensing performance of MOS materials.

Author Contributions

Conceptualization, X.F.; methodology, X.F. and P.D.; software, Y.L.; validation, X.F. and S.Y.; formal analysis, X.F. and C.H.; investigation, S.Y.; resources, Y.L.; data curation, S.Y. and C.H.; writing—original draft preparation, X.F. and S.Y.; writing—review and editing, X.F. and Y.L.; visualization, X.F. and S.Y.; supervision, X.F. and P.D.; project administration, X.F. and C.H.; funding acquisition, X.F. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Nature Science Foundation of China (No. 62201211).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Chemosensors 11 00376 g001 550
Figure 1. Schematic diagram of gas-sensing test system.
Figure 1. Schematic diagram of gas-sensing test system.
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Figure 2. SEM images of (a) Co3O4, (b) CZ-1, (c) CZ-2, and (d) CZ-3.
Figure 2. SEM images of (a) Co3O4, (b) CZ-1, (c) CZ-2, and (d) CZ-3.
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Figure 3. XRD spectra of (a) Co3O4, (b) CZ-1, (c) CZ-2, and (d) CZ-3.
Figure 3. XRD spectra of (a) Co3O4, (b) CZ-1, (c) CZ-2, and (d) CZ-3.
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Figure 4. TEM images of (a) Co3O4, (b) CZ-1, (c) CZ-2m and (d) CZ-3. EDS mapping images of (e,f) CZ-1 and (g,h) CZ-2. (i) HRTEM images of CZ-2.
Figure 4. TEM images of (a) Co3O4, (b) CZ-1, (c) CZ-2m and (d) CZ-3. EDS mapping images of (e,f) CZ-1 and (g,h) CZ-2. (i) HRTEM images of CZ-2.
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Figure 5. XPS spectra of (a) full survey for CZ-2 and Co3O4. High-resolution XPS spectra of (b) Zn 2p, (c) Co 2p, and (d) O 1 s.
Figure 5. XPS spectra of (a) full survey for CZ-2 and Co3O4. High-resolution XPS spectra of (b) Zn 2p, (c) Co 2p, and (d) O 1 s.
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Figure 6. (a) Resistances in air and (b) responses to 50 ppm acetone of four samples at different operating temperatures.
Figure 6. (a) Resistances in air and (b) responses to 50 ppm acetone of four samples at different operating temperatures.
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Figure 7. (a) Dynamic response curves of pure Co3O4 and CZ-2 to 0.5–100 ppm acetone. (b) Response-recovery curves of pure Co3O4 and CZ-2 to 50 ppm acetone.
Figure 7. (a) Dynamic response curves of pure Co3O4 and CZ-2 to 0.5–100 ppm acetone. (b) Response-recovery curves of pure Co3O4 and CZ-2 to 50 ppm acetone.
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Figure 8. (a) Response to 50 ppm different gases and (b) ratios (Sothers/Sacetone) of pure Co3O4 and CZ-2. (c) Cycle response curves and (d) response curves of CZ-2 during 6 days to 50 ppm acetone.
Figure 8. (a) Response to 50 ppm different gases and (b) ratios (Sothers/Sacetone) of pure Co3O4 and CZ-2. (c) Cycle response curves and (d) response curves of CZ-2 during 6 days to 50 ppm acetone.
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Figure 9. Response ratios (Sothers/S30) of CZ-2 as a function of relative humidity.
Figure 9. Response ratios (Sothers/S30) of CZ-2 as a function of relative humidity.
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Figure 10. Schematic diagram of energy band structure of (a) pure Co3O4 in air, (b) pure Co3O4 in acetone, (c) Co3O4@ZnO in air and (d) Co3O4@ZnO in acetone.
Figure 10. Schematic diagram of energy band structure of (a) pure Co3O4 in air, (b) pure Co3O4 in acetone, (c) Co3O4@ZnO in air and (d) Co3O4@ZnO in acetone.
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Table
Table 1. Comparison of the gas-sensing performances of Co3O4-based sensors to acetone.
Table 1. Comparison of the gas-sensing performances of Co3O4-based sensors to acetone.
MaterialOperating
Temperature		ResponseDetection
Limit		References
Co3O4 rectangular rods200 °C1.94 (50 ppm)5 ppm[42]
Co3O4/Fe2O3 hollow cubes250 °C2.27 (100 ppm)1 ppm[43]
Co3O4/NiCo2O4 nanocages238.9 °C2.09 (100 ppm)-[44]
PdO-Co3O4 hollow Nanocages350 °C1.51 (5 ppm)0.1 ppm[45]
Co3O4/ZnSnO3 nanorod array250 °C2.61 (100 ppm)-[46]
Co3O4@ZnO microspheres250 °C2.3 (50 ppm)0.5 ppmThis work
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MDPI and ACS Style

Fan, X.; Yang, S.; Huang, C.; Lu, Y.; Dai, P. Preparation and Enhanced Acetone-Sensing Properties of ZIF-8-Derived Co3O4@ZnO Microspheres. Chemosensors 2023, 11, 376. https://doi.org/10.3390/chemosensors11070376

AMA Style

Fan X, Yang S, Huang C, Lu Y, Dai P. Preparation and Enhanced Acetone-Sensing Properties of ZIF-8-Derived Co3O4@ZnO Microspheres. Chemosensors. 2023; 11(7):376. https://doi.org/10.3390/chemosensors11070376

Chicago/Turabian Style

Fan, Xiangxiang, Susu Yang, Chun Huang, Yujie Lu, and Pan Dai. 2023. "Preparation and Enhanced Acetone-Sensing Properties of ZIF-8-Derived Co3O4@ZnO Microspheres" Chemosensors 11, no. 7: 376. https://doi.org/10.3390/chemosensors11070376

APA Style

Fan, X., Yang, S., Huang, C., Lu, Y., & Dai, P. (2023). Preparation and Enhanced Acetone-Sensing Properties of ZIF-8-Derived Co3O4@ZnO Microspheres. Chemosensors, 11(7), 376. https://doi.org/10.3390/chemosensors11070376

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