ZIF-X (8, 67) based nanostructures for gas-sensing applications
-
Tengfei Shi
,
Chuanxin Ge
Abstract
ZIF-8 and ZIF-67 are the most investigated zeolitic imidazolate frameworks (ZIFs) materials that have aroused enormous scientific interests in numerous areas of application including electrochemistry, gas storage, separation, and sensors by reason of their fascinating structural properties. Recently, there is a rapidly growing demand for chemical gas sensors for the detection of various analytes in widespread applications including environmental pollution monitoring, clinical analysis, wastewater analysis, industrial applications, food quality, consumer products, and automobiles. In general, the key to the development of superior gas sensors is exploring innovative sensing materials. ZIF-X (8, 67) based nanostructures have demonstrated great potential as ideal sensing materials for high-performance sensing applications. In this review, the general properties and applications of ZIF-X (8, 67) including gas storage and gas adsorption are first summarized, and then the recent progress of ZIF-X (8, 67) based nanostructures for gas-sensing applications and the structure-property correlations are summarized and analyzed.
1 Introduction
Over the previous decades, the development and practical application of porous materials have been one of the critical tasks of modern technologies (e.g., separation technologies). The family of porous materials has been dramatically expanded by numerous new types of porous materials, such as zeolite-type, carbon nanotubes, activated carbon, and porous metal–organic materials. Metal organic frameworks (MOFs) are one of the hot materials in the field of porous nanomaterials, which are constructed by linking single-metal cations or metal clusters with organic linkers (Furukawa et al. 2013; Li et al. 2012; Wang et al. 2018c; Yuan et al. 2018). To date, numerous facile synthesis routes have been developed to prepare MOFs, such as solvothermal/hydrothermal (Soltanolkottabi et al. 2020; Zhang et al. 2020b), microwave-assisted (Lucero et al. 2020; The Ky et al. 2019; Xu et al. 2020b), mechanochemical (Brekalo et al. 2020; Chen et al. 2020; Li et al. 2014b), electrochemical (Martinez Joaristi et al. 2012; Stock and Biswas 2012), sonochemical (Othong et al. 2019; Soleimani et al. 2020) and microfluidic methods (Faustini et al. 2013; Hu et al. 2020; Wu et al. 2019a). Owing to the features of their crystalline structure, MOFs are also considered ideal templates for the derivation of various porous materials such as carbonaceous materials, metal compounds, and their composites (Kaneti et al. 2017; Salunkhe et al. 2017; Shen et al. 2016; Wang et al. 2018a; Yan et al. 2018; Zhang et al. 2019f). MOFs based nanomaterials have been widely applied in numerous fields, including gas storage, gas separation, biomedical imaging, energy storage, catalysis, drug delivery, chemical sensors, and others (Adatoz et al. 2015; Dong et al. 2022; Kong et al. 2012; Koo et al. 2019; Kreno et al. 2011; Li et al. 2012, 2018b, 2019, 2020a, 2021; Mason et al. 2014; Pham et al. 2020; Taheri et al. 2021; Wu et al. 2020; Yang and Yang 2020; Zhang et al. 2021).
Zeolitic imidazole frameworks (ZIFs) are an exceptional subfamily of MOFs composed of inorganic metal cations (M2+) and imidazolate-type ligands (Park et al. 2006). The representative ZIFs framework structures in previous reports are displayed in Figure 1. Recently, ZIFs have been widely studied for several applications due to their promising features including ultra-large surface area, exceptionally high hydrothermal robustness, and excellent chemical resistance (Guo et al. 2010; Han et al. 2018; Pan et al. 2012; Song et al. 2020; Sun et al. 2012; Xu et al. 2017). To date, more than 150 novel ZIFs structures have been synthesized, some of which exhibit structures topology similar to zeolites, and others also can be found in materials different from traditional zeolites (Anh Phan et al. 2009). Figure 2 shows the number of research publications about “ZIF” and “gas sensing” from the Web of Science. As can be seen from Figure 2, ZIF-8 and ZIF-67 are the most studied materials among numerous ZIFs. Both ZIF-8 and ZIF-67 with a sodalite topology are isostructural with zeolites, and they have similar lattice parameters and the same 2-methylimidazole organic ligands, but they use different metal centers (Zn2+ or Co2+) to build their structures.
The framework structures of ZIFs (Anh Phan et al. 2009; Ying et al. 2015; Zakzeski et al. 2011). Copyright permission obtained to reproduce from RSC Publishing, Elsevier and American Chemical Society.
Trends in ZIF publication.
As the most popular materials among numerous ZIFs materials, ZIF-8 and ZIF-67 possess extremely stable structures, giant surface areas, controllable pore apertures, tunable morphologies, and easily synthesis. Inspired by these charming characteristics, ZIF-X (8, 67) based nanostructures have been further employed as ideal sensing elements for gas-sensing applications. Further, compared with other common sensing materials (e.g., semiconductor metal oxide, conducting polymer, and graphene), ZIF-X (8, 67) based nanostructures exhibit high sensitivity, excellent gas selectivity, and good stability, while pure ZIFs tend to exhibit low sensitivity due to their poor electrical conductivity. However, the derivatives from ZIF-X (8, 67) and semiconductor metal oxide gas sensors tend to have a high operating temperature, while graphene and conducting polymer gas sensors can operate at room temperature, but their sensitivity and selectivity are poor.
So far, a number of excellent reviews on synthesis, properties, and sensing applications (e.g., optical chemical sensing, electrochemical sensing (Liu et al. 2020), metal ion sensing and biosensing (Kukkar et al. 2021) of ZIFs have been published). However, there is no dedicated review on ZIF-X (8, 67) based nanostructures (e.g., pure ZIFs, ZIFs composites, and derivatives) for gas sensing applications. In this review, we systematically summarize the ongoing progresses of ZIF-X (8, 67) based nanostructures for gas-sensing applications. First, we discuss the structural characteristics of the ZIF-X (8, 67) materials, such as surface area, tunable structures, stability. Then, the general properties (chemical and physical properties) and applications (gas storage and gas separations) of ZIF-X (8, 67) have been examined. In the third part, we detail the preparation process, sensing performances, and the sensing mechanism of the ZIF-X (8, 67) based nanostructures, especially focusing on the structure-property relationship. All gas sensors utilizing ZIF-X (8, 67) based nanostructures as the gas-sensing materials can be divided into two groups: volatile organic compounds (VOCs) sensor and inorganic gas sensor. Finally, the prevailing challenges and future research regarding ZIF-X (8, 67) based nanostructures for gas-sensing applications are also summarized.
2 The structure of ZIF-X (8, 67)
In recent years, numerous different strategies have been studied to design and tune the particle sizes and morphologies of ZIFs, and some classic morphologies of ZIFs are summarized in Table 1. The manipulation of particle sizes and morphologies of ZIFs is significant for optimizing the properties of the ZIFs and their derivatives. Therefore, Pan et al. (2011) have developed a facile method to change the morphology of ZIF-8 nanocrystals from the typical rhombic dodecahedron to truncated cubic and truncated rhombic dodecahedron, and various morphologies of ZIF-8 nanocrystals were shown in Figure 3A–D. This approach is based on precisely controlling the concentration of surfactant cetyltrimethylammonium bromide to achieve morphological control. Hollow nanostructures have been extensively studied for their quite important practical applications in recent years. Li et al. (2015) reported that the unique hierarchical porous ZnO hollow cubes structure could be constructed by directly decomposing ZIF-8 with cube-like morphology. The as-prepared ZnO nanostructure possesses excellent structural properties (e.g., high surface area) owing to maintaining the original structure of the ZIF-8 precursors. Besides, Xu et al. (2016) have demonstrated that the hollow ZIF-8 spheres with uniform shell thickness can be obtained by the MOF-to-MOF conversion method in the 2-methylimidazole solution. The fabrication of the hollow ZIF-8 spheres is based on the structural features of MOF-5 (e.g., high porosity) and the stronger coordination bond between Zn2+ and 2-methylimidazole. Furthermore, the ZIF-8 with the cage-like morphology have also been synthesized as precursors to prepare the hierarchical porous ZnO nanocages for their excellent gas-sensing performance (Zhang et al. 2019d).
The morphologies of representative ZIFs in previous reports.
| ZIF-n | Composition | Net | Morphology | References |
|---|---|---|---|---|
| 8 | Zn(MeIM)2 | Sod | Rhombic dodecahedra | Torad et al. (2013) |
| 8 | Zn(MeIM)2 | Sod | Truncated rhombic dodecahedral | Pan et al. (2011) |
| 8 | Zn(MeIM)2 | Sod | Cube-like | Li et al. (2015) |
| 8 | Zn(MeIM)2 | Sod | Hollow spheres | Lee et al. (2012) and Xu et al. (2016) |
| 8 | Zn(MeIM)2 | Sod | Cage-like | Zhang et al. (2019d) |
| 67 | Co(MeIM)2 | Sod | Rhombic dodecahedra | Guo et al. (2016) |
| 67 | Co(MeIM)2 | Sod | Truncated rhombic dodecahedral | Guo et al. (2016) |
| 67 | Co(MeIM)2 | Sod | Hollow prisms | Yu et al. (2016) |
| 67 | Co(MeIM)2 | Sod | Nanotube | Yu et al. (2015) |
| 67 | Co(MeIM)2 | Sod | Hollow spheres | Zhou et al. (2020a) and Tan and Zeng (2016) |
| 67 | Co(MeIM)2 | Sod | Flower-like | Wang et al. (2019) |
Images of ZIF-8 crystals. SEM images of (A) pure ZIF-8 crystal, (B) ZIF-8 crystal with 0.0025% CTAB, (C) ZIF-8 crystal with 0.01% CTAB, and (D) ZIF-8 crystal with 0.025% CTAB (Pan et al. 2011). (E) The schematic diagrams of preparation of ZIF-67 hollow prisms (Yu et al. 2016). (F) The synthesis and corresponding morphologies of ZIF-67 precursors (Wang et al. 2019). Copyright permission obtained to reproduce from RSC Publishing, STM Signatory Publisher and Elsevier.
Lou et al. (2016) have synthesized the uniform ZIF-67 hollow prisms structure though the ion-exchange method directly converting cobalt acetate hydroxide into ZIF-67 in ethanol. Furthermore, the CoS2 bubble-like hollow nanoparticles can be further constructed by ZIF-67 hollow prisms via a sulfidation reaction, then the CoS2 nanobubble hollow prisms can be obtained from the conversion of CoS2 bubble-like hollow nanoparticles under the annealing treatment (Figure 3E). Yu et al. (2015) reported that four novel kinds of ZIF-67 had been prepared through a simple and green method at room temperature by converting crystalline cobalt carbonate hydroxide. Importantly, the aqueous solution involved in this method without any toxic organic solvents. By particularly controlling some experimental conditions including reaction time and temperature, and the concentration of the solution, diverse morphologies of ZIF-67 can be obtained, such as core-shell nanowires, bead-on-string structure, nanotube, and nanoflake. Moreover, a series of ZIF-67 precursors with various shapes and sizes have also been synthesized for preparing their derivative porous metal oxides (Figure 3F) (Wang et al. 2019). These four morphologies of ZIF-67 were specially designed via the suitable ratios of Co2+ ions and 2-methylimidazole, which showed excellent shape-controllable features.
The pore size of ZIFs is also essential to optimize the performance of ZIFs and their derivatives. ZIFs inherit the structural characteristics of zeolites and MOFs with controllable pore size configuration and large surface areas, and are therefore promising candidates for gas storage and gas separation. Benefit from the unique pore structure characteristics, they exhibit significant sieving advantages in that gas with sizes larger than the pore size would be intercepted. For instance, the ZIF-8 membranes with a pore size of 0.34 nm are often recommended as a promising candidate for hydrogen (0.29 nm) separation from CO2 (0.33 nm), N2 (0.36 nm), and CH4 (0.38 nm). Therefore, these ZIFs with controllable pore size and large surface area are often employed in gas sensing applications to separate the interfering gases from the target gases and thus can greatly improve the selectivity and sensitivity of the gas sensors.
3 General properties and applications of ZIF-X (8, 67)
To this day, numerous porous materials have been explored as porous adsorbents or membranes for gas adsorption and separation, such as carbon nanotubes, activated carbon, zeolites, aluminophosphates, and silica gels. As an important class of crystalline porous materials, ZIFs are considered ideal candidates for gas storage and separation (Aceituno Melgar et al. 2015; Gong et al. 2017; Guan et al. 2020). Among various ZIFs materials, Zn-ZIF-8 and Co-ZIF-67 are the most representative materials, have been widely applied in gas adsorption applications by reason of their distinct structural features. Compared with other ZIF materials, ZIF-8 and ZIF-67 not only exhibit unexpected chemical stability but also show outstanding thermal stability, which is of great significance to their practical applications. For example, ZIF-67 can basically maintain the original crystal morphology at a high-temperature of 350 °C, and ZIF-8 can stable up to 500 °C (Park et al. 2006; Qian et al. 2012). In addition, ZIF-8 and ZIF-67 possess comparable chemical stability, and they can stable in poor conditions such as boiling water and some of the organic solvents (e.g., boiling methanol). Moreover, ZIF-8 and ZIF-67 share the same sod topology with sodalite zeolites. Both materials exhibit excellent structural features among ZIFs materials, and they possess a high surface area (e.g., 1821 m2/g for ZIF-8,1888 m2/g for ZIF-67) and high pore volumes (Zhou et al. 2017).
Over the years, various ZIFs materials have been applied as novel adsorbent materials for gas storage to meet the global energy and environmental challenges. Numerous studies have already demonstrated that the gas adsorption capacities of porous ZIFs have a strong correlation with their pore volumes or surface areas (Houndonougbo et al. 2013; Li et al. 2014a; Meng et al. 2020). Therefore, owing to the excellent structural characteristics including huge surface area and ordered porous structures, both ZIF-8 and ZIF-67 exhibit superior gas adsorption capacity (e.g., hydrogen storage, carbon dioxide capture, and methane storage). In 2017, Zhou et al. reported a comparative study on ZIF-X (8, 67) for the gas (CH4, N2, and CO2) adsorption properties. In this work, compared with CH4 and N2 adsorbed gases, both ZIF-8 and ZIF-67 exhibit the largest adsorption capacity for CO2. Therefore, ZIF-8 and ZIF-67 have also been regarded as promising materials for CO2 adsorption. In addition, to obtain better CO2 adsorption properties of ZIF-X (8, 67) based nanomaterials, various functionalization strategies have been developed. For instance, Tsai et al. (2018) reported that the organic linkers of ZIF-8 were successfully exchanged with several known imidazoles by solvent assisted ligand exchange method. The modified ZIF-8 has various functional groups, particles with SH functionalities exhibit a 2-fold improvement in CO2 adsorption capacity compared with unexchanged ZIF-8. Moreover, Song et al. (2020) synthesized two series of ion-exchanged ZIF-67 (i.e., Li-ZIF67 and Na-ZIF-67) by a novel one-step ion-exchange strategy to improve CO2 uptake performance. By comparison, the ion-exchanged ZIF-67 exhibits a higher CO2 adsorption capacity, resulting from the enhanced electrostatic interaction caused by the ion-exchange method. Similarly, Akbari Beni and Niknam Shahrak (2020) have successfully designed three alkali-functionalized ZIF-8 nanostructures (i.e., Li, K, and Na) using density functional theory calculations in a molecular simulation study. The simulated CO2 adsorption isotherms of functionalized ZIF-8 at 298 k from 0 to 150 kPa. The uptake capacity of pristine ZIF-8 was lower than that of alkali-functionalized ZIF-8. Furthermore, among the functionalized ZIF-8 structures, Li-ZIF-8 shows the highest CO2 adsorption of 6 mmol/g. The higher storage performance in Li-ZIF-8 could be attributed to the enhanced interaction between Li-functionalized ZIF-8 and CO2 molecules.
In 2014, Cheng and Hu (2014) prepared a stable H2O-functionalized ZIF-8 via a simple and effective water treatment approach, which exhibits a significantly enhanced H2 adsorption capacity at ambient temperature compared to pristine ZIF-8. Furthermore, Pt@ZIF-8/graphene oxide composites were synthesized through the simple liquid impregnation route, in which Pt nanoparticles act as the metal catalyst (Zhou et al. 2015). In this work, the hydrogen storage capacities of pure ZIF-8 and the as-synthesized composites were systematically investigated. Pt@ZIF-8/GO composite show the remarkably enhanced H2 capacity at ambient temperature compared to ZIF-8 and ZIF-8/GO. Such improved H2 capacity could be ascribed to the catalytic spillover mechanism. As mentioned above, ZIF-8 and ZIF-67 show great potential for gas storage applications and various functionalisation techniques (e.g., ion exchange, ligand functionalization, and inorganic substance-ZIF composites) have been developed to further improve gas adsorption capacity.
4 Gas-sensing applications
In recent years, due to the poor air quality caused by excessive exhaust emissions from factories, people are increasingly concerned about health problems. Therefore, the detection of toxic and harmful gases is urgently needed and thereby facilitate the solution of air pollution problems. More importantly, apart from the environmental analysis, high-performance gas sensors have also been applied in versatile fields, such as clinical analysis, wastewater analysis, industrial applications, food quality, consumer products, automobile, and domestic appliances (Figure 4) (Bhargav et al. 2014; Blanco-Rodriguez et al. 2018; Tai et al. 2020; Wales et al. 2015; Wojnowski et al. 2017). It is known that the key factor for the development of superior performance gas sensors is the high-performance sensing materials. With the rapid development of materials science, numerous novel types and structures of gas-sensing materials are continually emerging, which provides a promising opportunity for the development and application of gas sensors.
The important parameters and applications of gas sensors.
As a burgeoning novel class of porous material, ZIFs materials have attracted tremendous attention as gas-sensing materials, and extensive strategies have been developed to design various nanostructures of ZIFs materials to optimize their functionality and applications. In fact, various ZIF-X (8, 67) based nanostructures for gas-sensing applications have been intensively investigated due to the following attractive properties:(1) They possess distinct structural characteristics (such as tunable pore aperture enables them to serve as molecular sieves, which can significantly improve the selectivity of sensitive materials, high surface areas and ultra-porous nature ensure that they can provide sufficient adsorption sites for the target gas and oxygen molecules, which is of great benefit to the detection sensitivity). (2) Extremely high stability as traditional zeolites, and can maintain stability under some harsh environmental conditions, which can ensure a great long-term stability for sensing materials. (3) ZIF-8 and ZIF-67 are an emerging platform that can be easily functionalized (e.g., loaded metal nanoparticles), and the functionalized composites can then be used as templates for the preparation of their derivatives to enable the achievement of high-performance sensitive materials. As of today, numerous advanced nanostructures of ZIF-X (8, 67) and their derivatives have been designed to detect distinct toxic and harmful gases. In general, the same sensitive materials with different structures and morphologies would exhibit significantly different structural features such as different specific surface area, pore size, and surface active sites. All those structural parameters are the key factors in optimizing the gas-sensing performance. Therefore, the control of the morphology and nanostructure of ZIF-X (8, 67) is vital for optimizing the parameters of ZIF-X (8, 67) based sensors. Meanwhile, it is urgent to further explore the relationship between gas-sensing performance and structural features for superior gas-sensing performances. Various ZIF-X (8, 67) based gas sensors have been developed in previous reports, which can be divided into two categories: VOCs sensor (Table 2) and inorganic gas sensor (Table 3). (Wang et al. 2018b; Zhang et al. 2020a, 2020c, 2020e)
Summary of the sensing performances of ZIF-X (8, 67) based nanostructures gas-sensing applications for the detection of VOCs.
| ZIF | Derivatives | Nanostructure | Analyte | Working temperature (°C) | Response [SG2] | Concentration (ppm) | tRes/tRec (s) | LOD (ppm) | References |
|---|---|---|---|---|---|---|---|---|---|
| ZIF-8/ZIF-67 | ZnO/Co3O4 | Porous | Acetone | 275 | 15.17 | 1 | 317/390 | – | Xiao et al. (2018) |
| Au/ZIF-8 | Au/ZnO | Porous | Acetone | 275 | 17.1 | 1 | 231/215 | 0.1 | Xia et al. (2017) |
| ZIF-67 | Au/Co3O4 | Porous cubic | Acetone | 220 | 22.19 | 100 | 27/70 | 10 | Cui et al. (2019) |
| ZIF-67 | Co3O4/FGH | Porous | Acetone | 250 | 81.2 | 50 | 20 | 1 | Ding et al. (2018) |
| ZIF-8 | ZnO/S,N: GQDs/PANI | Porous | Acetone | RT | 2 | 0.5 | 15/27 | 0.1 | Zhang et al. (2019a) |
| ZIF-8/ZIF-67 | ZnO/Co3O4 | Hollow | Acetone | 300 | 30.1 | 100 | 8/2 | 1 | Zhang et al. (2019) |
| ZIF-67/Co-Zn LDH | Co3O4/ZnCo2O4 | Hollow | Acetone | 255 | 16.3 | 100 | 41/47 | 0.51 | Qu et al. (2018) |
| ZIF-67 | Co3O4/Fe2O3 | Hollow cubes | Acetone | 250 | 3.27 | 100 | – | – | Xu et al. (2020a) |
| Pd-ZIF-67 | Co3O4/PdO/n-SnO2 | Hollow nanocubes | Acetone | 450 | 22.8 | 5 | 90.8/108.4 | 0.005 | Jang et al. (2017) |
| Pd@ZIF-8 | PdO@ZnO–SnO2 | Hollow nanotubes | Acetone | 400 | 5.06 | 1 | 20/64 | 0.01 | Koo et al. (2017a) |
| Pd-ZIF-67 | PdO-Co3O4 | Hollow nanocages | Acetone | 350 | 2.51 | 5 | – | 0.1 | Koo et al. (2017b) |
| ZIF-67/Ni–Co LDH | Co3O4/NiCo2O4 | Double-shelled nanocages | Acetone | 238.9 | 3.09 | 100 | 8/20 | – | Qu et al. (2017a) |
| ZIF-67 | Co3O4 | Core–shell | Acetone | 190 | 13 | 200 | 4/8 | – | Zhang et al. (2018a) |
| ZIF-8 | ZnO@MoS2 | Nanosheets core/shell | Acetone | 350 | 150 | 0.5 | 9/17 | 0.005 | Chang et al. (2020) |
| Au-ZnO@ZIF-8 | – | Nanowire array | Acetone | 275 | 231 | 100 | ∼180/∼60 | 0.0034 | Yao et al. (2019) |
| ZnO@ZIF-8 | – | Hollow nanofibers | Acetone | 263 | 60 | 100 | – | 2 | Tian et al. (2020) |
| ZIF-8/ZIF-67 | – | Hexagonal shapes | Ethanol | 180 | – | 90 | 390/348 | 10 | Matatagui et al. (2018) |
| ZIF-67 | Co3O4 | Porous concave nanocubes | Ethanol | 300 | – | 200 | <10 | 10 | Lu et al. (2014) |
| ZIF-67 | Co3O4 | Porous hollow | Ethanol | 200 | 221.99 | 100 | 129/216 | 1 | Zhang et al. (2020b) |
| ZIF-67 | ZnO–Co3O4 | Nanofibers | Ethanol | 200 | 106 | 1000 | 7/236 | 1 | Xiong et al. (2017) |
| ZIF-67 | TiO2/Co3O4 | Porous polyhedral | Ethanol | 150 | 16.7 | 150 | – | 5 | Zhang et al. (2018b) |
| ZIF-8 | ZnO | Hollow nanocages | Ethanol | 325 | 139.41 | 100 | 2.8/56.4 | 0.025 | Zhang et al. (2019d) |
| Ag–ZIF-8 | Ag–ZnO | Hollow nanocages | Ethanol | 250 | 84.6 | 100 | 5/10 | 0.0231 | Zhang et al. (2019) |
| ZIF-8 | Fe2O3/ZnO/Au | Hexagonal-like | Ethanol | 280 | 170 | 100 | 4/5 | 10 | Chen et al. (2018) |
| ZIF-8 | CNT@ZnSnO3 | Hollow boxes | Ethanol | 240 | 166 | 100 | 6 | 1 | Guo et al. (2020) |
| ZIF-8@ZnO | ZnO | Neck-connected ZnO films | Ethanol | 375 | 124 (I g /I a ) | 50 | 120/70 | 0.5 | Qi et al. (2019) |
| ZnO@ZIF-8 | – | Core–shell | Ethanol | 160 | 35.9 | 100 | 23/31 | – | Ren et al. (2019) |
| ZIF-67 | – | Porous | Formaldehyde | 150 | 13.9 | 100 | – | 5 | Chen et al. (2014) |
| ZIF-8 | – | Porous | Formaldehyde | RT | 6.25 | 100 | 94/79 | – | Reddy et al. (2020) |
| ZnO@ZIF-8 | – | Core–shell | Formaldehyde | 300 | – | 100 | 16/9 | 5.6 | Tian et al. (2015) |
| POM@ZIF-8@ZnO | – | Core–shell | Formaldehyde | 25 | – | 100 | 15.1/16.2 | 0.387 | Wang et al. (2018a) |
| ZIF-67/Co–Fe | Co3O4/CoFe2O4 | Double-shelled nanocubes | Formaldehyde | 139 | 12.7 | 10 | 4/9 | 0.3 | Zhang et al. (2019b) |
| ZIF-8/Co-Zn hydroxide | ZnO/ZnCo2O4 | Hollow core–shell nanocages | Xylene | 320 | 34.26 | 100 | – | 0.126 | Qu et al. (2016) |
| ZIF-8/Ni-Zn LDH | ZnO/Ni0.9Zn0.1O | Double-shelled nanocages | Xylene | 240 | 54.7 | 100 | – | 0.12 | Qu et al. (2017b) |
| Ti loaded ZIF-67 | Co3O4/TiO2 | Rhombic dodecahedron | Xylene | 120 | 6.17 | 50 | <23 | 8.6 | Bai et al. (2018) |
| ZIF-67 | Co3O4 | Hollow nanocages | Xylene | 225 | 78.6 | 5 | 63/86 | 0.25 | Jo et al. (2018) |
| Pd@ZIF-8 | Pd@ZnO–WO3 | Nanofibers | Toluene | 350 | 22.22 | 1 | 20 | 0.1 | Koo et al. (2016) |
| ZIF-8/ZIF-67 | – | Hexagonal shapes | Toluene | 180 | – | 90 | 150/186 | 10 | Matatagui et al. (2018) |
| ZIF-67 | Co3O4 | Porous | n-Butanol | 100 | 21 | 100 | 146/90 | 5 | Wang, et al. (2019) |
| ZIF-8 | ZnO | Clusters | n-Butanol | 300 | 37.8 | 10 | 10 | 0.1 | Zhang et al. (2019c) |
| SnS2/ZIF-8 | SnO2/ZnO | Porous flower-like | Triethylamine | 200 | 17.7 | 50 | 9/10 | – | Ma et al. (2019) |
| ZIF-8@ZIF-67 | Co3O4/ZnO | Co3O4 quantum dots/ZnO nanocages | Trimethylamine | 190 | 232 | 50 | 2.1/11.8 | 0.013 | Li et al. (2020b) |
| ZIF-8 | ZnO | Hierarchical hollow cubes | Benzene | 400 | (1.21) | 0.1 | 76 ± 16/82 ± 17 | 0.1 | Li et al. (2015) |
Summary of the sensing performances of ZIF-X (8, 67) based nanostructures gas-sensing applications for the detection of inorganic gas.
| ZIF | Derivatives | Nanostructure | Analyte | Working temperature (°C) | Response [SG3] | Concentration (ppm) | tRes/tRec (s) | LOD (ppm) | References |
|---|---|---|---|---|---|---|---|---|---|
| ZIF-8/ZIF-67 | – | Hexagonal shapes | H2 | 180 | – | 90 | 132/186 | 10 | Matatagui et al. (2018) |
| ZnO@ZIF-8 | – | Core-shell nanorods | H2 | 250 | 3.28 | 50 | – | – | Wu et al. (2017) |
| ZnO@ZIF-8 | – | Core-shell nanorods | H2 | 275 | – | 50 | 50/130 | – | Khudiar et al. (2020) |
| ZnO@ZIF-8 | – | Core-shell microrods | H2 | 125 | 8.61 | 50 | 100/20 | 0.5 | Cui et al. (2018) |
| ZnO@ZIF-8 | – | Core-shell nanowires | H2 | 300 | 1.44 | 50 | – | – | Drobek et al. (2016) |
| ZIF-8/Pd/ZnO | – | Core-shell nanowires | H2 | 200 | 8.5 ± 0.4 | 50 | – | – | Weber et al. (2018) |
| ZIF-8@ZnO | – | Core-shell nanowires | H2 | 250 | 72.3 | 100 | – | – | Jeon et al. (2020) |
| ZIF-67 | PdO–Co3O4 | Nanocubes | NO2 | 100 | 44.11 | 20 | – | 1 | Choi et al. (2017) |
| ZIF-67 | MWCNTs | Nanocubes | NO2 | RT | 1 | 5 | – | 0.1 | Rui et al. (2018) |
| WS4_ZIF-67 | WS2_Co–N–HCNC | Hollow nanocages | NO2 | RT | 48.2 | 5 | – | 0.1 | Koo et al. (2018) |
| ZnCo-ZIF | – | Nanosheets | NO2 | RT | 54.6 | 100 | 1.01/12 | 0.01 | He et al. (2021) |
| ZIF-8/ZIF-67 | – | Hexagonal shapes | NO2 | 180 | – | 90 | 348/330 | 10 | Matatagui et al. (2018) |
| In2O3/ZIF-8 | – | Core-shell In2O3 nanofibers | NO2 | 140 | 16.4 | 1 | 80/133 | 0.01 | Liu et al. (2019) |
| ZIF-8@ZnO | – | Nanowires | NO2 | 250 | 60.6 | 20 | – | – | Jeon et al. (2020) |
| ZIF-8/ZnO | – | Nanorod | H2S | 25 | 52.1 | 10 | 420/- | 0.05 | Wu et al. (2019b) |
| Pt@ZIF-8 | Pt/ZnO | Three-dimensional macroporous | H2S | 320 | 11.2 | 1 | 8.7/19.4 | 0.025 | Zhou et al. (2020b) |
| ZIF-67/Ni−Co | Co3O4/NiCo2O4 | Double-shelled nanocages | H2S | 250 | 57 | 50 | 153/40 | – | Tan et al. (2020) |
| ZIF-8/ZIF-67 | – | Hexagonal shapes | CO | 180 | – | 90 | 456/264 | 10 | Matatagui et al. (2018) |
| ZIF-67 | Co3O4 | Porous | CO | 150 | 220 | 100 | – | 0.5 | Qin et al. (2020) |
| SnO2@ZIF-67 | – | Core–shell | CO2 | 205 | 16.5 | 5000 | 96/25 | 5000 | Me et al. (2018) |
| ZIF-8 | – | Porous | NH3 | RT | 1.33 | 10 | 63/45 | – | Reddy et al. (2020) |
| ZIF-8@ZnO | – | Nanowires | NH3 | 250 | 77.7 | 10 | – | 0.5 | Jeon et al. (2020) |
| Zinc doped ZIF-67 | CoZn-NCNTs | Porous polyhedral | SO2 | RT | 28.9 | 30 | – | 30 | Li et al. (2018a) |
4.1 VOCs sensors
4.1.1 Acetone sensors
Acetone sensor as a critical class of VOCs sensors, various ZIF-X (8, 67) based nanostructures have been intensively investigated as gas sensing materials to enhance its sensitivity and selectivity. As a family of metal oxide semiconductors (MOSs), ZnO and Co3O4 have gained tremendous attention for gas sensing due to their fast response and relatively low cost. However, the traditional metal oxide based gas sensors commonly exhibit low selectivity and high operating temperature, which are great obstacles for their further application. To enhance the poor gas-sensing properties, numerous MOSs with unique nanostructures derived from ZIF-X (8, 67) materials have been explored for their ultra-high surface areas and abundant chemical active sites. It is believed that porous structures are one of the most popular nanostructures owing to their unique features. Therefore, numerous ZIF-X (8, 67) based gas-sensing materials with porous structures have been designed to enable excellent sensing performance. For example, the porous ZnO/Co3O4 nanomaterials derived from ZIF-X (8, 67) can detect ppb to ppm level of acetone at 275 °C, and exhibited a notable response of 15.17 towards 1000 ppb of concentration (Xiao et al. 2018). Moreover, to further improve gas-sensing performance, the introduction of noble metals (e.g., Au) into porous nanostructures is considered as a promising method. Xia et al. (2017) synthesized porous Au/ZnO nanocomposites by direct pyrolysis of Au/ZIF-8 nanoparticles. The as-synthesized porous Au/ZnO nanocomposites at 275 °C performed obviously enhanced response and acetone selectivity at low concentration. For example, the gas-sensing sensitivity of Au/ZnO (17.1) nanocomposites is higher than the porous ZnO nanoparticles (7.9) towards 1000 ppb of acetone. Meanwhile, they also constructed porous Au/Co3O4 microstructures using the same strategy described above (Cui et al. 2019). The as-synthesized porous Au/Co3O4 samples retained a similar morphology to the ZIF-67 template and greatly improved its pore structure. Therefore, combining the structural characteristics of Co3O4 and the catalysis of Au, the porous Au/Co3O4 composites as acetone-sensing materials exhibits higher gas-sensing performance than pure Co3O4. The porous Au/Co3O4 composites performed obviously improved acetone response (R g /R a = 21.95) compared to singular Co3O4 nanoparticles (R g /R a = 11.35) toward 100 ppm acetone. In addition, the porous Au/Co3O4 composites exhibited a shorter response-recovery time toward acetone gas. The two works above demonstrated that the Au/ZnO and Au/Co3O4 exhibited enhanced sensing performance can be attributed to the unique porous structure, the spillover effect of Au nanoparticles, and its catalytic and electronic activities (Figure 5A–D). Meanwhile, these results might provide a novel way for the design of porous noble metal/semiconductor metal oxide composites for development of high performance gas sensors.
Sensing mechanism of Au/Co3O4 nanocomposites. (A–D) The sensing mechanism of Au/Co3O4 nanocomposites (Cui et al. 2019). (E) The preparation process of Co3O4/ZnCo2O4 nanostructure using ZIF-67 as precursor, and f the acetone sensing mechanism (Qu et al. 2018). Copyright permission obtained to reproduce from Elsevier.
Ding et al. (2018) fabricated porous Co3O4/FGH composites from the precursor ZIF-67/functionalized graphene oxide hydrogels (FGH) composites. Take advantage of unique porous nanostructures and optimization of electrical properties of composite materials with the chemically functionalized FGH, the porous Co3O4/FGH composites showed a markedly high acetone response (Rgas/R0 = 81.2@ 50 ppm), which was ∼20-fold higher compared with pure Co3O4 film. Furthermore, the porous Co3O4/FGH composites exhibited rapid response time (within 20 s) and excellent selectivity to acetone gas. In addition, Zhang et al. (2019a) reported the ZnO/S, N: GQDs/PANI (GQDs = graphene quantum dots, PANI = polyaniline) nanocomposites synthesized through in-situ polymerization strategy. The sensor based on ZnO/S, N: GQDs/PANI nanocomposites exhibited ppb-level sensitivity, remarkable acetone selectivity, excellent long-term stability, and rapid response/recovery speed towards acetone at room temperature. Meanwhile, the effect of humidity on the gas sensing performances of the gas sensor is negligible, making it appropriate for the detection of exhaled acetone gas.
Recently, hollow nanomaterials have been extensively designed as high-performance gas sensing materials because of their remarkable structural characteristics (e.g., high specific surface area and low densities). For example, using the ZIF-8/ZIF-67 mixture as the self-sacrificing template, Zhang et al. (2019b) fabricated hollow ZnO/Co3O4 heterostructures nanocomposite. The use of as-synthesized ZnO/Co3O4 hollow nanostructure samples as acetone sensing materials shows a significantly enhanced sensing performance (e.g., superior response, remarkable selectivity, outstanding repeatability, and excellent linearity) compared with pristine ZnO. Similarly, hollow nanostructure Co3O4/ZnCo2O4 composites have also been developed via a self-templating strategy (Figure 5E) (Qu et al. 2018). This strategy includes three steps, the ZIF-67 template was first synthesized and then transformed into Co/Zn-ZIF@Co-Zn layered double hydroxide (LDH) precursor. Finally, hollow Co3O4/ZnCo2O4 composites were prepared through calcination of as-prepared Co/Zn-ZIF@Co-Zn LDH precursor. The as-prepared hollow nanostructure Co3O4/ZnCo2O4 composites showed enhanced gas sensing performance compared with hollow ZnCo2O4 and Co3O4. The acetone sensing properties of Co3O4/ZnCo2O4 heterojunctions nanostructures are strongly affected by the surface adsorbed oxygen species, and the acetone sensing mechanism shown in Figure 5F. Once acetone gas is injected, the acetone gas would react with the adsorbed oxygen ions, and then release the trapped electrons back to Co3O4 and ZnCo2O4, and the reaction can be described by Eq. (1): CH3COCH3ads + 8O−ads→3CO2 + 3H2O + 8e−). And the constructed heterojunctions nanostructures can provide more active sites for adsorption of oxygen and acetone molecules; therefore, the construction of heterojunctions is one of the important factors the enhancement of sensing properties. Furthermore, the excellent sensing properties are also attributable to the porous structure and the formation of thin shells.
Xu et al. (2020) synthesized hollow Co3O4/Fe2O3 nanocubes through the MOF hybrid-assisted method. The as-synthesized hollow Co3O4/Fe2O3 nanocubes exhibited 3.06-fold higher response value (R g /R a = 3.27) to acetone gas than that of Co3O4 nanocubes, which could be attributed to unique hollow porous structures and the formation of p–n heterojunctions.
In 2017, The group of Kim (Jang et al. 2017; Koo et al. 2017a; Koo et al. 2017b) reported three catalyst functionalized acetone-sensing materials with hollow structures. The first one, Co3O4−PdO loaded n–SnO2 hollow nanocubes (HNCs) were prepared via galvanic replacement reaction (GRR), in which ultra small PdO nanoparticles and Co3O4 island as cocatalysts to functionalize n–SnO2 (Figure 6A) (Jang et al. 2017). The GRR-treated SnO2 showed significantly enhanced gas sensing performance compared with hollow p-type Co3O4 derived from ZIF-67. For example, the n–SnO2–Co3O4–PdO HNCs showed 21.9 times higher response (Rair/Rgas = 22.8–5 ppm) toward acetone compared with hollow p-type Co3O4. Meanwhile, this group also prepared the PdO@ZnO–SnO2 nanotubes through the electrospinning and followed calcination (Koo et al. 2017a). The nanoscale PdO catalysts (3–4 nm) and heterojunction structures (ZnO–SnO2 and PdO–ZnO) were effectively loaded on the surface of Co3O4 HNCs. Benefiting from the advantages of the unique hollow nanotube structures, PdO@ZnO−SnO2 NTs can exhibit high specific surface areas and considerable open porosity. Therefore, the PdO@ZnO−SnO2 NTs as acetone gas sensing materials showed a superior response (Rair/Rgas = 5.06 to 1 ppm at 400 °C), excellent acetone selectivity, and rapid response/recovery speed (20/64 s) at substantially humid conditions (95% RH). Kim et al. (2017) also reported that ultrasmall Pd catalyst well-dispersed Co3O4 hollow nanocages (HNCs) derived from ZIF-67 templates were good candidates as acetone gas sensor. With the unique structure of Co3O4 HNCs and functionalization of PdO NPs, the PdO–Co3O4 HNCs showed a markedly enhanced response (R g /R a = 2.51@5 ppm acetone at 350 °C) compared to Co3O4 NPs (1.45), Co3O4 HNCs (1.96), and PdO−Co3O4 NPs (1.98). Moreover, the PdO−Co3O4 HNCs exhibited outstanding selectivity toward acetone gas.
PdO loaded Co3O4–SnO2 for acetone sensing. (A) The formation of PdO loaded Co3O4–SnO2 from Pd loaded ZIF-67 precursor (Jang et al. 2017). (B) The acetone-sensing performances of ZnO@MoS2 core/shell nanostructures at 350 °C (Chang et al. 2020). (C) Response of ZnO@MoS2 core/shell nanostructures toward 100 ppb acetone under different RH. The charge transfer and interaction energy between acetone (D), CO2 (E), CH4 (F), NH3 (G), H2S (H), H2 (I), H2O (J), O2 (K) and MoS2 (Chang et al. 2020). Copyright permission obtained to reproduce from American Chemical Society and Elsevier.
Qu et al. (2017a) reported Co3O4/NiCo2O4 double-shelled nanocages (DSNCs) obtained by thermal treatment of ZIF-67/Ni-Co LDH precursor in air. When used as a gas sensing material, the obtained Co3O4/NiCo2O4 DSNCs give an improved sensitivity, outstanding reversibility, and excellent selectivity toward acetone gas compared to Co3O4 NCs. Zhang et al. (2018a) have successfully prepared four novel porous Co3O4 hierarchical structures by controlling the calcination environment (i.e., atmosphere, temperature, and calcination rate) from ZIF-67 templates. Take the advantages of core−shell hierarchical structure, both core−shell Co3O4 sample and porous core−shell Co3O4 sample performed a higher response value (i.e., 13 and 11, respectively) toward acetone gas at 190 °C than the response values of porous popcorn and nanoparticle structures (i.e., 7.9 and 2.6, respectively). In addition, both core−shell and porous core−shell Co3O4 samples exhibited short response/recovery time (share the same values of 4/8 s) toward acetone gas, which is far shorter than the values of porous popcorn and nanoparticle structures.
More recently, Chang et al. (2020) reported the highest response value toward acetone gas among ZIF-X (8, 67)-based sensing materials. In their report, ZIF-8 was used as a precursor to obtaining ZnO, and then the core/shell ZnO@MoS2 nanosheets heterojunctions were prepared via a facile hydrothermal method. The ZnO@MoS2 heterogeneous nanostructures exhibited ∼80-fold improvement of response value than that of singular ZnO toward 100 ppb of acetone at 350 °C (Figure 6B). Notably, the as-prepared ZnO@MoS2 gave a super-rapid response time (60 s)/recovery time (40 s) toward super-low acetone gas concentration of 5 ppb, excellent acetone selectivity, and a negligible influence of humidity on the gas sensing performance (Figure 6C). To provide an in-depth understanding of the sensing mechanism of MoS2, they calculated the interaction energy and charge transfer between various gas molecules and the MoS2 surface by density functional theory (DFT) calculations, as shown in Figure 6(D–K). It is clear from the results of the DFT calculations that there is a stronger interaction between acetone and MoS2 compared to other gases, which can be used to explain the excellent sensitivity and selectivity of ZnO@MoS2 sensor towards acetone.
As discussed above, ZIF-based membranes have been extensively applied in gas storage and separation due to their outstanding structural features for gas adsorption and filtration. Thus, combining MOSs with the excellent selectivity of ZIFs together is also considered as a promising design strategy to improve the sensing performance of MOSs. For example, to further improve the MOSs selectivity to the analytic gases with similar sensing activity, Yao et al. (2019) developed a novel Au-ZnO@ZIF-8-Dimethylbenzimidazole (DMBIM) core-shell nanowires array with different thick ZIF-8 thin film (5, 30 and 50 nm). The as-prepared MO@MOF 5 nm-DMBIM exhibited the best acetone selectivity over benzene compared to all reported materials. Meanwhile, the as-prepared nanomaterials performed excellent sensitivity, remarkable long-term stability and rapid response/recovery speed. Similarly, Tian et al. (2020) developed unique ZnO@ZIF-8 hollow nanofibers to pursue superior sensing properties of ZnO-based chemoresistive sensor. Benefits from porous ZIF-8 thin layer and ZnO hollow nanofibers, the as-synthesized hollow ZnO@ZIF-8 showed remarkably enhanced sensitivity and selectivity to acetone gas compared with pure ZnO. The obtained material can also detect acetone gas at 263 °C and maintain relatively stable under humid atmosphere (15–60% RH). Overall, the above described novel ZIF-X (8, 67) based nanostructures acetone gas sensors including their derivatives (e.g., metal oxide, noble metal-metal oxide, metal oxide composites) and ZIF/metal oxide composites exhibit superior acetone gas sensing properties, which not only develops several excellent acetone sensors for detection of exhaled breath, but also presents various novel and efficient ways for preparing high performance sensing materials.
4.1.2 Ethanol sensors
The ethanol sensing performance of porous Co3O4 concave nanocubes derived from ZIF-67 was reported by Lu et al. (2014). In this work, porous Co3O4 nanomaterial was prepared by a two-step strategy, the ZIF-67 concave nanocubes were first synthesized as self-sacrificial templates and then calcining the as-synthesized ZIF-67 templates at 300 °C (Figure 7A). They also systematically investigated the influence of calcination condition (i.e., calcination temperature) on the formation mechanism of specific morphologies and ethanol gas sensing properties. Benefiting from the significantly high surface area of 120.9 m2/g that can absorb more oxygen species, the as-prepared Co3O4 concave nanocubes performed excellent selectivity, good sensing reproducibility, and high sensitivity with rapid response/recovery speed (less than 10 s). In addition, Zhang et al. (2020c) synthesized porous Co3O4 polyhedral nanostructures derived from the ZIF-67 sacrificial template, which showed the highest sensitivity to ethanol vapor compared to other reported Co3O4 materials. Apart from the high response, the porous Co3O4 sensors possess remarkable selectivity, excellent repeatability, and low operating temperature of 200 °C.
Hollow structures and gas sensing properties. (A) The fabrication of Co3O4 concave nanocubes by pyrolysis of ZIF-67 precursor Lu et al. (2014). (B) The synthesis procedure of Ag@ZnO hollow nanocages. (C) Responses of various ZnO nanostructures versus ethanol concentrations (10–1000 ppm) at 250 °C. (D) Sensing characteristics of Ag–ZnO hollow nanocages to various analytes at 250 °C. (E) The sensing mechanism to ethanol for CNT@ZnSnO3 hollow structures (Guo et al. 2020). Copyright permission obtained to reproduce from American Chemical Society and Elsevier.
Xiong et al. (2017) synthesized porous ZnO–Co3O4 hollow polyhedron with p–n heterojunction nanostructure via a morphology-inherited thermal treatment of ZIF-67 precursor at 300 °C. The as-synthesized ZnO–Co3O4 heterostructure exhibited excellent performances to 1000 ppm of ethanol gas included high response value (106), applausive selectivity, and fast response/recovery speed (7/236 s) in comparison with pure porous Co3O4 nanomaterials, which can be explained by the formation of heterostructure and the porous hollow polyhedron structure. Additionally, the necklace-like TiO2/Co3O4 nanofibers heterostructures were also successfully prepared through an electrospinning method and subsequent calcination treatment. Similarly, taking the advantages of the excellent catalytic activity of Co3O4, the formation of heterojunctions, and the unique hierarchical structures, the TiO2/Co3O4 heterostructures sensor presented a high ethanol response value of 16.7–150 ppm ethanol at 150 °C. Zhang et al. (2019g) constructed hierarchical ZnO-QD (quantum dots) hollow nanocages (HNCs) by direct pyrolysis of ZIF-8 templates at 390 °C for 50 min. Applied as sensing material, the obtained ZnO-QD (HNCs) exhibited ultra-high ethanol response (R a /R g = 139.41–100 ppm) at the working temperature of 325 °C and the response/recovery speed (2.8/56.4 s). Moreover, the as-prepared material can present a notable response value of 5.1 upon an ultra-low concentration of ethanol (<25 ppb). Such remarkable sensing performance benefits from the high specific surface area caused by quantum dots and the hierarchical hollow nanocages structure.
Furthermore, the Ag@ZnO HNCs were prepared via a MOF-based route, in which the nanoscale Ag with the size of 10 nm as catalysts to functionalize ZnO hollow nanocages (Figure 7B). The functionalized ZnO HNCs exhibited significantly enhanced response (84.6@100 ppm) toward ethanol and decreased working temperature of 250 °C compared to pure ZnO NCs and ZnO NPs (Figure 7C). Besides, the Ag@ZnO HNCs showed short response/recovery (5/10 s), remarkable response reproducibility, and excellent selectivity (Figure 7D). Such enhanced sensing performances could come from the catalytic effect of Ag NPs, porous hollow nanostructure, and larger surface area. Work by Chen et al. (2018) compared the sensing performance of α-Fe2O3/ZnO/Au nanocomposites, α-Fe2O3/ZnO heterostructures and α-Fe2O3 nanoplates to ethanol. Correspondingly, the α-Fe2O3/ZnO/Au nanocomposites display the largest BET surface area among the three materials (79.08 m2/g for α-Fe2O3/ZnO/Au, 61.27 m2/g for α-Fe2O3/ZnO, and 37.94 m2/g for α-Fe2O3). Used as sensing materials, the as-prepared α-Fe2O3/ZnO/Au heterostructures showed outstanding ethanol sensing performances owing to the unique heterostructure, high surface area, the well-dispersed Ag NPs, and the synergistic effects of phase compositions. For instance, the α-Fe2O3/ZnO/Au nanocomposites exhibited a high response (170@100 ppm ethanol) at 280 °C, outstanding recycling stability, and fast response/recovery (4/5 s). More recently, Guo et al. (2020) reported a chemical sensor based on CNTs based perovskite type ZnSnO3 fabricated by a facile hydrothermal reaction, where ZIF-8 used as Zn2+ source. The obtained CNT@ZnSnO3 nanostructures featured a unique hierarchical structure with an exceptionally high surface area of 45.73 m2/g, which is dramatically higher compared with solid ZnSnO3 cube and hollow boxes ZnSnO3 (6.53 and 28.41 m2/g respectively). Thus, the chemical sensor was highly sensitive to ethanol (Rair/Rgas = 166–100 ppm) at 240 °C. The schematic illustration of ethanol sensing mechanism for CNT@ZnSnO3 hollow structures is illustrated in Figure 7E. When the CNT@ZnSnO3 hollow structure is exposed to ambient atmosphere, the O− and O2− will be produced through the reaction between O2 molecules and electrons of ZnSnO3, thus forming a thick electron-depleted layer. Upon exposure to ethanol gas, the formed O− and O2− can react with target gas molecules and release electrons back to CNT@ZnSnO3 hollow structures, resulting in a low resistance value of CNT@ZnSnO3 sensor.
In addition, it exhibited excellent ethanol selectivity, fast response time (6 s), and long-term stability. In addition, sensors based on neck-connected ZnO films exhibited dramatically enhanced sensitivity to ethanol (Qi et al. 2019). In this work, core-shell ZIF-8@ZnO films were used as precursors, and then the neck-connected ZnO films obtained by annealing treatment of the as-synthesized core-shell ZIF-8@ZnO films. The neck-connected ZnO films achieved markedly enhanced ethanol response value (I g /I a = 124–50 ppm), which increased by ∼six times compared with ZnO film. Meanwhile, Ren et al. (2019) fabricated core-shell ZnO@ZIF-8 microspheres via a self-template method. In contrast with pristine hollow ZnO hollow microspheres, the core-shell ZnO@ZIF-8 microspheres exhibited significantly improved sensing properties towards ethanol, such as high response (35.90@100 ppm ethanol) and excellent selectivity to ethanol. The improved ethanol sensing properties were attributed to the enrichment effect of the uniform ZIF-8 shell.
4.1.3 Formaldehyde sensors
Chen et al. (2014) reported the gas sensing performance of pure MOFs ZIF-67 for the first time. Benefiting from the large surface area (1832.2 m2/g) and remarkable stability, ZIF-67 exhibited high sensitivity and excellent selectivity at 150 °C to formaldehyde as gas sensor. It is worth noting that the ZIF-67 sensor can sense a low concentration of 5 ppm formaldehyde with a particular response value of 1.8. Additionally, the highly humid atmosphere (less than 70% RH) has a negligible effect on the formaldehyde sensing properties. These results demonstrated that porous pure MOFs ZIF-67 could be a promising gas-sensing material. Another pure MOF (ZIF-8) has also been investigated as sensing material for the formaldehyde gas sensor (Reddy et al. 2020). The ZIF-8 exhibited a low response value of 6.25–100 ppm formaldehyde. In 2015, Tian et al. fabricated core−shell ZnO@ZIF-8 nanorods via a self-template strategy, in which ZnO nanorods serve as the self-template and the source of Zn2+ ions for synthesis of ZIF-8. The as-fabricated ZnO@ZIF-8 nanorods heterostructures showed significantly improved formaldehyde selectivity compared to the ZnO nanorods sensor, which was mainly attributed to the role of the ZIF−8 shell. Meanwhile, the response/recovery speeds were increased owing to the barriers of ZIF-8 shell. Likewise, Wang et al. (2018) reported the core-shell polyoxometalate@ZIF-8@ZnO nanocomposite for enhancing the formaldehyde sensing performances. Take advantage of the structural features of ZIF-8 shell and photocurrent enhancement of polyoxometalate, the polyoxometalate@ZIF-8@ZnO sensors exhibited excellent selectivity for formaldehyde and remarkable sensitivity.
More recently, Zhang et al. (2019d) constructed Co3O4/CoFe2O4 double-shelled nanocages (DSNCs) via a MOF-assisted strategy (Figure 8A). In brief, ZIF-67/PBA core-shell nanocubes were prepared via the ion-exchange method as templates, then the final products obtained by pyrolyzing as-prepared templates at 550 °C in air. Interestingly, Co3O4/CoFe2O4 DSNCs exhibited superior formaldehyde sensing performances included the excellent detection capability of sub-ppm-level formaldehyde, large response (R g /R a = 12.7–10 ppm at 139 °C) (Figure 8B), with good high selectivity (Figure 8C) and fast response speed (∼4 s). The formaldehyde sensing mechanism of CCFO DSNCs composites is based on the change in the amount of electrons on the surface of the CoFe2O4 and Co3O4 during the ionization of oxygen molecules and the oxidation/reduction reaction of the formaldehyde gas with the O2−and O−. And the enhanced formaldehyde sensing properties of CCFO DSNCs composites can be mainly ascribed to the unique double-shelled nanostructures, large specific surface area, porous structure and p–p heterojunctions (Co3O4/ZnCo2O4).
Co3O4/CoFe2O4 double-shell nanocubes for formaldehyde sensing. (A) The fabrication of Co3O4/CoFe2O4 double-shell nanocubes from ZIF-67 nanocubes (Zhang et al. 2019d). (B) Formaldehyde-sensing properties of Co3O4/CoFe2O4 double-shell nanocubes at different temperature (Zhang et al. 2019d). (C) The selectivity of Co3O4/CoFe2O4 double-shell nanocubes at room temperature (Zhang et al. 2019d). Copyright permission obtained to reproduce from Elsevier.
4.1.4 Xylene sensors
The group of Qu et al. (2016) constructed hollow core-shell ZnO/ZnCo2O4 nanocages (HCSNCs) via a two-step treatment of ZIF-8 templates, in which ZIF-8/Co–Zn double hydroxides sacrificial templates were first made, and then subsequent thermal treatment of as-made sacrificial templates in air (Figure 9A). The obtained ZnO/ZnCo2O4 HCSNCs displayed a surface area of 113.2 m2/g, which is significantly larger than that of ZnO NCs (45.6 m2/g) and ZnCo2O4 shells (83.8 m2/g). Benefits from their unique hollow nanostructure and the formation of ZnO/ZnCo2O4 heterojunction, the ZnO/ZnCo2O4 HCSNCs showed good reversibility, high response, and excellent selectivity toward xylene. Another work reported by this group, the ZnO/Ni0.9Zn0.1O double-shelled nanocages (DSNCs) as sensing material, were prepared via the same strategy as above. Similarly, the sensor based on ZnO/Ni0.9Zn0.1O DSNCs showed superior selectivity and high sensitivity to xylene (Figure 9B, C). Moreover, it exhibited obviously enhanced sensing performances compared with ZnO NCs.
ZnO/ZnCo2O4 HCSNCs for Xylene-sensing performances. (A) Schematic representation of the synthesis of ZnO/ZnCo2O4 HCSNCs via a two-step process (Qu et al. 2016). (B, C) Xylene-sensing performances of ZnO/Ni0.9Zn0.1O double-shelled nanocages. Copyright permission obtained to reproduce from Royal Society of Chemistry and Elsevier.
Bai et al. (2018) synthesized Co3O4/TiO2 p–n heterojunction through calcination of Ti ion loaded ZIF-67 at 450 °C in air. The Co3O4/TiO2 composites exhibited a response value of 6.17 toward 50 ppm of xylene, which was 5-fold higher compared with singular Co3O4 nanoparticles. In addition, this sensor also exhibited fast response/recovery speed, remarkable reversibility, and excellent selectivity due to porous structure and the formation of p–n heterojunction. The xylene sensing mechanism of Co3O4/TiO2 composites heterojunction can be interpreted with the adsorption-reaction mechanism. When Co3O4/TiO2 composites are exposed to air atmosphere, the surface of the Co3O4/TiO2 sensor is susceptible to be occupied by adsorbed oxygen species. Once xylene gas is introduced, O2−(ads) react with xylene molecules, and will form free electrons, CO2 and H2O. Eventually, the depletion layer formed in the ethanol gas will be even wider than the depletion layer in air. Jo et al. (2018) reported hollow hierarchical Co3O4 nanocages by using rhombic dodecahedral ZIF-67 as precursors showed the highest response toward p-xylene (R g /R a = 78.6@5 ppm) at 225 °C among all reported pure Co3O4 gas sensing materials. Moreover, the sensor exhibited a high response to toluene (43.8) and remarkable methylbenzene selectivity in the interference of ethanol. Such exceptional sensing performances of the hollow hierarchical Co3O4 nanocages were attributed to its unique hierarchical morphology and the excellent catalytic activity of Co3O4. Notably, several key parameters such as the morphology, shell thickness, and pore sizes of Co3O4 nanocages that significantly determine the gas-sensing properties of the sensor, could be easily controlled via changing the precipitation reaction of the ZIF-67 templates.
4.1.5 n-butanol sensors
Wang et al. (2019) synthesized porous Co3O4 assemblies with different morphologies and sizes for sensing n-butanol by direct pyrolysis of Co-MOF (ZIF-67). To investigate the influence of structural features of porous Co3O4 assemblies on the gas-sensing performance, four kinds of ZIF-67 precursors were specially designed with different structures. Then, the products acquired by calcining obtained ZIF-67 precursors at 300 °C for 4 h, displayed well-controlled morphologies and sizes. As shown in Figure 3F and Figure 10A–D, the as-synthesized samples Co3O4-1, Co3O4-3, and Co3O4-4 share similar shapes with corresponding precursors ComIM-1, ComIM-3, and ComIM-4, respectively. However, the shape of the Co3O4-2 sample was quite different from that of its precursor, ComIM-2. Furthermore, the nanoparticles of Co3O4-2 samples exhibited slightly bigger sizes (>10 nm) compared with Co3O4-1, Co3O4-3, and Co3O4-4 with similar sizes (4–7 nm). Meanwhile, the Co3O4-1 and Co3O4-2 had much lower BET surface areas (17.3 and 11.0 m2/g, respectively) than Co3O4-3 and Co3O4-4 (54.2 and 50.9 m2/g, respectively). Among them, Co3O4-4 exhibited the highest response (R g /R a = 21 @100 ppm n-butanol) and the best n-butanol selectivity. In addition, the porous Co3O4 sensor showed a relatively low detection limit toward n-butanol (<5 ppm) and high stability. The n-butanol sensing mechanism of Co3O4 semiconducting materials is based on surface-resistance controlled mechanism, and the detail n-butanol sensing procedure of porous Co3O4 assemblies is depicted in Figure 10E. Clearly, under different atmospheres, various reactions will occur on the surface of the porous Co3O4 assembly sensor, including the adsorption of oxygen molecules and the redox reaction of the target gas, while the reaction will be accompanied by changes in both the hole-accumulation layer and the resistance of the surface of the porous Co3O4 assembly sensor. This work demonstrated that the structural features of the Co3O4 assemblies have a close correlation with their gas-sensing properties. Therefore, this finding can provide a promising strategy to design high-performance gas-sensing materials.
Co3O4 nanoparticles and gas sensing. (A–D) SEM images of the obtained Co3O4 nanoparticles. (E) The n-butanol sensing mechanism (Wang et al. 2019). Copyright permission obtained to reproduce from Elsevier.
Zhang et al. (2019e) demonstrated that ZnO assemblies exhibited the highest sensing response toward n-butanol. They prepared Zn-MOF (ZIF-8) templates with varied crystallinity and particle size through ligand exchange during the nucleation process. Then, the ZnO assemblies with different pore sizes were obtained by annealing ZIF-8 templates. The as-prepared ZnO-2 with the micropore size of 1 nm showed a high response (37.8@10 ppm) to n-butanol, which was 30 and 20 times gas responses of ammonia and formaldehyde, respectively. In addition, the ZnO-2 sensor exhibited excellent recyclable properties and a relatively low detection limit (<0.1 ppm).
4.1.6 Other VOCs sensors
Koo et al. (2016) reported toluene sensor properties of MOF-derived Pd@ZnO complex catalyst-loaded WO3 synthesized via electrospinning followed by calcination (Figure 11A). Interestingly, the Pd@ZnO complex catalyst was effectively incorporated into WO3 nanofibers to produce Pd−ZnO and ZnO−WO3 heterogeneous structures. Due to the functionalization of the Pd NPs catalyst and the introduction of multi-heterojunction (Figure 11B), the Pd@ZnO-WO3 sensor showed superior toluene sensitivity (R a /R g = 4.37@100 ppb toluene at 350 °C), with rapid gas response time (<20 s). In addition, the sensor exhibited remarkable sensitivity to toluene (R a /R g = 22.22@1 ppm) and a negligible response toward the other interfering gases such as ethanol, hydrogen sulfide, nitric monoxide, and ammonia.
Sensing mechanism of Pd@ZnO-WO3 nanofibers. (A) The schematic diagram showing the fabrication of Pd@ZnO-WO3 nanofibers. (B) Schematic diagrams of the sensing mechanism for Pd@ZnO-WO3 nanofibers (Koo et al. 2016). Copyright permission obtained to reproduce from American Chemical Society.
Li et al. (2015) synthesized hierarchical ZnO hollow cubes by direct calcination of ZIF-8 templates. The as-synthesized ZnO hollow cubes well maintained the original cube-like morphologies of ZIF-8 and displayed a high BET specific surface area of 45 m2/g. The as-fabricated chemical sensor based on hierarchical ZnO hollow cubes is able to detect benzene of low concentration (0.1–5.0 ppm) with higher response and the drastically faster response speed compared to the pristine ZnO particles (Figure 12A, B). The enhanced benzene sensing mechanism of ZnO hollow cubes is primarily originated from the unique hierarchical porous nanostructures, and the unique nanostructures of ZnO hollow cubes can provide more exposed oxygen vacancies and faster surface reaction kinetics of benzene molecules, thus realizing the improved sensing performance of hierarchical ZnO hollow cubes. Therefore, this sensor based on hierarchical ZnO hollow cubes has promising application prospects in benzene of low concentrations detection. Ma et al. (2019) have successfully synthesized the porous SnO2/ZnO flower-like nanostructures by using SnS2/ZIF-8 as sacrificial template. The as-synthesized SnO2/ZnO heterogeneous composites displayed a large BET surface area of 108.88 m2/g, which contributes to enhancing the sensing performances of SnO2/ZnO heterostructures. Thus, the SnO2/ZnO showed a high response value of 17.7 toward 50 ppm triethylamine at 200 °C, which increased by about 2.2 times compared to that of pure SnO2. Moreover, this sensor exhibited rapid response time (∼9 s) and excellent selectivity to triethylamine.
Benzene-sensing properties. (A, B) Benzene-sensing properties (0.1–5 ppm) of ZnO hollow cubes and pristine ZnO particles at 400 °C (Li et al. 2015). Copyright permission obtained to reproduce from Royal Society of Chemistry.
Li et al. (2020b) prepared Co3O4/ZnO p–n heterojunctions (Co3O4/ZnO NHs) derived from heterogeneous MOFs (ZIF-8@ZIF-67), used for trimethylamine-sensing. The Co3O4/ZnO NHs based sensor exhibited superior sensing properties toward trimethylamine included high response (Rair/R g = 232@50 ppm at 190 °C), rapid response time (2.1s@10 ppm), low detection limit (∼13 ppb), high trimethylamine selectivity, and excellent stability, compared to pristine ZnO nanocages. In particular, they combined the quasi-in-situ XPS studies and DFT calculations to explain the outstanding sensing performances of the sensor. Specifically, through their DFT calculations and Bader charge analysis, it is evident that a significant charge transfer behavior occurred during the sensing of ZnO to trimethylamine gas, which demonstrates that the enhanced sensing mechanism of Co3O4/ZnO NHs is attributable to the production of metallic zinc. Meanwhile, the formation of p–n heterojunctions in Co3O4/ZnO NHs results in a considerably higher resistance in the air as compared to pure ZnO, which also contributes to the significant enhancement of the trimethylamine gas sensing response. Finally, they attribute the excellent trimethylamine sensing performance to the combined effect of nano-heterojunction formation and the surface metallization.
4.2 Inorganic gas sensors
4.2.1 H2 sensors
Up to date, MOF(ZIF-8)-based membranes have been extensively studied as an auxiliary filter layer due to its unique properties (e.g., the molecular sieving properties), to improve the poor selectivity of MOSs (e.g., ZnO) gas sensors. For instance, Wu et al. (2017) developed a hydrogen sensor using core-shell ZnO@ZIF-8 nanorod film as sensing material. As shown in Figure 13A, for pristine ZnO film and the as-synthesized ZnO@ZIF-8 film, the H2 response increases with increasing working temperature and the highest response value at 250 °C. In contrast with the pristine ZnO film, the as-synthesized ZnO@ZIF-8 film showed a significantly enhanced sensitivity toward H2. Notably, owing to the strengthened molecular sieving properties of the ZIF-8 thin shell, this ZnO@ZIF-8 film exhibited a negligible CO sensitivity (Figure 13B, C). Similarly, Cui et al. (2018) developed a ZnO@ZIF-8 microrod sensor for the detection of H2. This sensor exhibited a higher response (8.61@50 ppm), lower optimum working temperature of 125 °C, better selectivity to H2 over CH4, C2H2, C2H4, C2H6, and CO compared with pure ZnO microrod sensor. Recently, Khudiar et al. (2020) also synthesized the ZnO@ZIF-8 hybrid materials for selective hydrogen gas detection. The as-fabricated ZnO@ZIF-8 gas sensor displayed an outstanding selective response to H2 over benzene vapor compared with a raw ZnO sensor. This result is realized by the molecular sieving effect of ZIF-8 coating, hindering the diffusion of larger benzene vapor molecules pass through the pore aperture.
Responses of ZnO@ZIF-8 nanorod. (A) Responses of ZnO@ZIF-8 nanorod and singular ZnO nanorod film toward 50 ppm H2 at different temperature (150–300 °C) (Wu et al. 2017). (B) Responses of ZnO@ZIF-8 nanorod and singular ZnO nanorod film toward 50 ppm CO (Wu et al. 2017). (C) Selectivity property of ZnO@ZIF-8 film for H2 over CO at 200 °C (Wu et al. 2017). Response curves of ZnO@ZIF-8 nanowires and ZnO nanowires toward 10, 30, and 50 ppm of (D) H2, (E) C7H8, and (F) C6H6 (Drobek et al. 2016). Copyright permission obtained to reproduce from STM Signatory Publisher and American Chemical Society.
To enhance the selectivity of ZnO nanowires (NWs) sensors, Drobek’s group reported ZnO@ZIF-8 nanocomposite NWs for hydrogen detection (Drobek et al. 2016). The obtained nanocomposites exhibited significantly enhanced selectivity to H2 over C6H6 and C7H8 as compared to the pure ZnO NWs (Figure 13D–F). In addition, Weber and co-workers reported ZIF-8 nanomembrane coated Pd/ZnO NWs as a highly selective hydrogen detector (Weber et al. 2018). The ZIF-8 coated Pd/ZnO NWs showed a dramatically high response (R a /R g = 6.7@50 ppm) to H2 gas at 200 °C, while negligible responses toward other analytes including C6H6, C7H8, CH3COCH3, and C2H5OH. However, the Pd/ZnO NWs showed noticeable responses to all tested gases, which can clearly confirm the poor selectivity of the gas sensor without ZIF-8 nanomembrane. Such high-performance hydrogen sensors were attributed to the addition of Pd nanoparticles (NPs) and the ZIF-8 nanomembrane overcoat. Furthermore, Jeon et al. (2020) synthesized ZIF-8@ZnO NWs core-shell hybrid nanomaterials and studied the influence of the ZIF-8 membrane on the gas sensing performances of ZnO NWs. In this hybrid system, owing to its unique properties such as excellent gas adsorption capability, exceptional stability, and the molecular sieving effect caused by porous nanostructures, the ZIF-8 membrane can act as dual roles (i.e., the pre-concentrator and the gas separator). Therefore, the ZIF-8@ZnO NWs exhibited a high H2 response (74.0%@100 ppm of H2 at 250 °C) with excellent selectivity and retaining a stable response value of 99.1% toward H2 after 20 days. In general, benefiting from the unique structural characteristics of ZIF-8, it can be combined as a molecular sieve with metal oxides to form metal oxide@ZIF-8 composites, which can significantly improve the sensitivity and selectivity of metal oxide gas sensors, which provides a promising strategy for the development of highly selective sensors, as well as paving the way for other promising ZIF materials.
4.2.2 NO2 sensors
In 2107, Choi et al. (2017) reported that Pd@ZIF-67 derived PdO-Co3O4 HNCs functionalized by single-walled carbon nanotubes (SWCNTs) then integrated on the Ni/Au-colorless polyimide (cPI) heater for detection of NO2. This sensor exhibited excellent sensing performances toward NO2, such as high sensitivity (S = (Rair−Rgas)/Rair = 44.11%@20 ppm NO2 at 100 °C), excellent detection capability that could detect as low as 1 ppm NO2 (S = 3.09%), and high mechanical stability. Similarly, Rui et al. (2018) developed the room-temperature NO2 sensors based on ZIF-67/multiwalled carbon nanotubes (MWCNTs) and as-derived Co3O4/MWCNT hybrid fibers. Those two sensors can detect an ultralow NO2 gas concentration of 100 ppb with high sensitivity at room temperature. Additionally, they can maintain excellent gas-sensing performance even if they are bent into different angles.
Koo et al. (2018) developed a facile strategy that can effectively confine the few-layered WS2 nanoplates in hollow carbon nanocages composites derived from ZIF-67 (Figure 14A). Benefiting from the numerous edge sites of WS2 nanoplates, WS2/carbon nanocages composites showed dramatically enhanced sensing performances toward NO 2 at room temperature than that of as-derived carbon nanocages composites. The NO2 sensor based on WS 2 /carbon nanocages composites displayed high sensitivity toward NO2 (ΔR/R g = 48.2%@5 ppm) with a low detection limit of 100 ppb at room temperature (Figure 14B), good NO 2 selectivity, excellent NO 2 sensing stability. In addition, the WS2/carbon nanocages composites showed a comparatively high response value of 21.8% toward 5 ppm NO2 even in a high humidity (90% RH), as shown in Figure 14C. Liu et al. (2019) first reported that ZIF-8 nanocrystals coated In2O3 nanofibers (NFs) were highly sensitive to NO2 at an optimal testing temperature of 140 °C. As displayed in Figure 14D, E, the In2O3/ZIF-8 NFs heterostructures displayed a much higher response compared with pristine In2O3 NFs. And more notably, the In2O3/ZIF-8 (4:1) NFs heterostructures sensor could detect NO2 at an extremely low concentration of 10 ppb, showing a relatively high response value (R g /R a = 5.6). Furthermore, the In2O3/ZIF-8 (4:1) NFs showed a far faster response/recovery speed (80/133 s) and better humidity resistance than that of the pristine In2O3 NFs. He et al. (2021) prepared ZnCo-ZIF intercalated GO nanosheets (ZnCo-ZIF/GN) by a facile solvothermal method for NO2 detection at room temperature. The ZnCo-ZIF/GN composite showed a high sensing response (R a /R g = 54.61) under a RH of 30% towards 100 ppm NO2. In addition, the ZnCo-ZIF/GN based NO2 sensor displayed excellent gas selectivity, good stability and a remarkably low detection limit of 10 ppb performance acetone gas sensor. These excellent gas sensing results provide a facile strategy to construct poor conductivity of ZIF with highly conductive graphene nanosheet for constructing advanced high-performance gas-sensing application.
Formation process of WS2/carbon nanocages composites and Gas sensing. (A) Formation process of WS2/carbon nanocages composites (Koo et al. 2018). (B) Calculated responses of the various sensors toward NO2 at room temperature (Koo et al. 2018). (C) Response characteristics of the WS2/carbon nanocages composites under 95% RH (Koo et al. 2018). (D) Responses to 1 ppm NO2 versus operating temperature (80–180 °C) (Koo et al. 2018). (E) Response of In2O3/ZIF-8 nanofibers toward different concentrations of NO2 at 140 °C (Liu et al. 2019). Copyright permission obtained to reproduce from STM Signatory Publisher and Elsevier.
4.2.3 H2S sensors
Wu et al. (2019b) developed an advanced H2S sensor based on MOF(ZIF-8)-loaded ZnO nanorods prepared via a self-template method. The sensor showed excellent room-temperature H2S-sensing performance. In particular, the as-made sensor not only has the unique structural features of porous ZIF-8 but also possess attractive gas sensing performances of semiconductor-metal-oxide. Thus, the sensor exhibited excellent sensing performances toward target gas H2S. The ZIF-8/ZnO showed a significantly enhanced sensing response to H2S ((ΔR)/R a = 52.1@10 ppm) at room temperature, which increased by 15 times compared with pure ZnO nanorods (Figure 15A). It is noteworthy that the response values of ZIF-8/ZnO nanorods to H2S gas only slightly decreased even when the humidity increased to 70%, as shown in Figure 15B. In addition, the sensor exhibited remarkable H2S selectivity, ppb-level detection limit (∼50 ppb), and superior stability (i.e., excellent hydrophobic property). Zhou et al. (2020) prepared 3D inverse opal (3DIO) Pt/ZnO nanocomposites via the thermolysis of 3DIO Pt NPs@ZIF-8 template for the sensing of trace-level H2S gas. The obtained 3DIO Pt/ZnO nanocomposites featured an exceptionally high surface area of 171.6 m2/g. Further, taking advantage of the size effect of noble metal Pt NPs, 3DIO Pt/ZnO nanocomposites exhibited a high response (11.2) toward 1 ppm of H2S at working temperature of 320 °C, an ultralow detection limit (∼25 ppb), excellent H2S sensitivity, and long-term stability.
Responses to H2S gas. (A) The response of raw ZnO and ZIF-8-loaded ZnO nanorod toward H2S at room temperature. (B) Response properties of ZIF-8-loaded ZnO nanorod under different humidity from 0 to 90% (Wu et al. 2019). (C) The response of Co3O4/NiCo2O4 DSNCs from ZIF-67 rhombododecahedron at different temperatures (Tan et al. 2020). (D) Selectivity properties of Co3O4/NiCo2O4 DSNCs toward various analytes at 50 ppm (Tan et al. 2020). Copyright permission obtained to reproduce from Elsevier.
Using ZIF-67/Ni-Co yolk-shell nanostructure as the templates, our group reported the template-assisted synthesis of Co3O4/NiCo2O4 DSNCs as gas sensing materials for detection of H2S (Tan et al. 2020). The as-synthesized Co3O4/NiCo2O4 DSNCs displayed a large BET surface area of 103 m2/g. As shown in Figure 15C, for the synthesized Co3O4/NiCo2O4 DSNCs H2S gas sensors, the response values at different gas concentrations (30, 50, 70 and 100 ppm) all exhibit an increase first with increasing operating temperature from 50 to 250 °C, and then a decrease in response at higher operating temperatures. The Co3O4/NiCo2O4 DSNCs exhibited the best sensitivity and selectivity to H2S, with negligible responses to other analytes. In particular, this sensor showed superior response and selectivity toward 100 ppm of H2S at 250 °C (Figure 15D). Such excellent sensing performances demonstrated that hollow double-shelled architectures have the remarkable potential in the fabrication of superior gas sensors.
4.2.4 CO/CO2 sensors
Matatagui et al. (2018) constructed a chemoresistive sensor based on ZIF-8/ZIF-67 nanostructures for the detection of CO, H2, NO2, toluene and ethanol. This chemoresistive sensor could detect these target gases with significant resistance change at relatively low concentrations (down to 10 ppm). Notably, the ZIF-8/ZIF-67 combination-based sensor exhibited significantly enhanced responses to toluene and hydrogen of 10 ppm compared with ZIF-67 nanocrystals. In addition, it showed rapid response, excellent repeatability, and reversibility. Qin et al. (2020) prepared Co3O4 nanostructures with numerous octahedral active Co3+ sites, derived from ZIF-67 precursors by controlling the calcination conditions (i.e., calcination temperature and heating rate). Owing to the high activity of Co3+ for CO oxidation, the as-prepared Co3O4 nanostructures with abundant active Co3+ sites showed significantly improved CO gas sensing properties, compared to Co3O4 nanoparticles. For example, the sample obtained through calcination of ZIF-67 precursor at 300 with heating rate of 1 °C min−1 exhibited an ultrahigh response (∼20 times) toward 100 ppm of CO at a significantly lower working temperature compared with Co3O4 nanoparticles. Moreover, this sensor showed fast response/recovery speed (10/2 s), low detection limit (at least 500 ppb), and excellent CO selectivity and superior stability. To further enhance the interaction between CO2 gas and SnO2 surface, Me et al. (2018) synthesized core-shell SnO2 @ZIF-67 hybrid nanostructure to utilize excellent CO2 capturing properties of ZIF-67 and the synergistic effects of the hybrid nanostructure. The response of SnO2 @ZIF-67 hybrid nanostructure was 12-fold in enhancement for 50% CO2 sensing than the pristine SnO2 sensor. This sensor exhibited the highest gas response (16.5 ± 2.1% toward 0.5% CO2) at 205 °C, among the reported SnO2-based sensors. In addition, the as-prepared hybrid nanostructure showed remarkable recovery time (22.04 ± 4.33 s for 50% CO2, 25.5 ± 4.5 s for 5000 ppm CO2) and a relatively low operating temperature of 205 °C. These gas sensing results show that the assembly of ZIF-X (8, 67) on metal oxide nanoparticles is an attractive prospect for achieving high performance gas sensing properties.
4.2.5 Other inorganic gas sensors
Reddy et al. (2020) synthesized pure MOF (ZIF-8) via a facile chemical route method as gas-sensing material for detection of NH3 at room temperature. However, the NH3 sensing performances of as-prepared ZIF-8 were unsatisfactory. For instance, the ZIF-8 sensor showed a relatively low response value (9) toward 100 ppm concentration of NH3.
Furthermore, the semiconductor gas sensing material (ZnO nanowires) hybridized with ZIF-8 nanocrystals prepared by Jeon et al. (2020) also displayed sensitivity to NO2 and NH3. Take advantage of this unique hybrid nanostructure, ZIF-8@ZnO nanowires showed excellent responses (62.3% for 20 ppm NO2, and 77.2% for 10 ppm NH3) at 250 °C, excellent long-term stability and low gas detection limit (1 ppm for NO2, and 500 ppb for NH3). Additionally, the ZIF-8@ZnO hybrid nanostructure sensor exhibited significantly enhanced selectivity at 250 °C for NO2 and NH3 over CH4 and C3H8 compared with the sensor based on pure ZnO nanowires (Figure 16A, B).
The selectivity (A) and response (B) of pure ZnO nanowires and ZIF-8@ZnO hybrid nanostructure to various target gases (Jeon et al. 2020). (C) Schematic illustration of the nanostructure of CoZn-NCNTs. (D) SO2-sensing properties of the CoZn-NCNTs sensor toward various toxic gases (Li et al. 2018). Copyright permission obtained to reproduce from Elsevier and Royal Society of Chemistry.
Li et al. (2018a) prepared a CoZn-NCNTs material through the direct pyrolysis of zinc doped ZIF-67 (i.e., bimetallic MOFs ZnZIF-67) under Ar atmosphere. The synthesized CoZn-NCNTs retain the nano-polyhedral morphology of the ZnZIF-67 with a high surface area of 446.35 m2/g, and have abundant interconnected CNTs on the external faces (Figure 16C). As shown in Figure 16D, the CoZn-NCNTs exhibited remarkable performances for the sulfur dioxide (SO2) detection at room temperature. For instance, it showed a high response value ((ΔR/R0) = 28.9%) toward 30 ppm of SO2 with fast response/recovery speed (78/32 s). In addition, the CoZn-NCNTs exhibited excellent SO2 selectivity, superior chemical stability and response repeatability. To deeply understand the SO2 sensing mechanism of CoZn-NCNTs, they explored the effect of the doped Zn on the band structure of nanotube using first-principles calculations, and the calculations showed that the doped Zn would alter the band structure of the CNTs, which led to a significant enhancement of the SO2 adsorption capacity, ultimately enabling the CoZn-NCNTs to exhibit excellent SO2 sensing performance. This work presents a novel zeolitic imidazolate frameworks based strategy for the preparation of the excellent sensing materials for the detection of SO2 gas at room temperature.
5 Summary and perspective
In summary, we have discussed the recent development of ZIF-X (8, 67) based nanostructures for gas-sensing applications, including pristine ZIF-X (8, 67), ZIF-X (8, 67) based membranes, and derivatives. To obtain high-performance ZIF-X (8, 67) based gas sensors, several promising strategies have been explored in recent years. Through the rational heteroatom-doping, nanostructure design, and construction of heterojunction, ZIF-X (8, 67) based sensing materials could provide superior gas-sensing performances. In particular, we summarized which nanostructures are best to choose for detecting a specific target gas based on the crucial parameters of gas-sensing applications including sensitivity, cross-selectivity, response time, and long-term stability (Table 4). As an excellent sub-family of the crystalline materials MOFs, ZIFs have ultrahigh surface areas, diverse structures, high hydrothermal robustness, and excellent chemical resistance. Featuring these desirable properties, ZIFs have gained extensive interest in widespread fields, including electrochemistry, gas storage and separation, and sensing. In particular, ZIF-8 and ZIF-67 are ZIF materials of the most representative, which exhibit superior thermal and chemical stabilities, and unique structural properties.
The summary of the optimal ZIF-X (8, 67) based nanostructures for the detection of a specific target gas up to now.
| ZIF | Metal oxide | Response | Target gas | References |
|---|---|---|---|---|
| ZIF-8 | ZnO@MoS2 | 150 at 0.5 ppm | Acetone | Chang et al. (2020) |
| ZIF-67 | Co3O4 | 222 at 100 ppm | Ethanol | Zhang et al. (2020b) |
| ZIF-67/Co–Fe | Co3O4/CoFe2O4 | 12.7 at 10 ppm | Formaldehyde | Zhang et al. (2019b) |
| ZIF-67 | Co3O4 | 78.6 at 5 ppm | Xylene | Jo et al. (2018) |
| Pd@ ZIF-8/PVP/AMH | Pd@ZnO–WO3 | 22.2 at 1 ppm | Toluene | Koo et al. (2016) |
| ZIF-8 | ZnO | 37.8 at 10 ppm | n-Butanol | Zhang et al. (2019c) |
| SnS2/ZIF-8 | SnO2/ZnO | 17.7 at 50 ppm | Triethylamine | Ma et al. (2019) |
| ZIF-8@ZIF-67 | Co3O4/ZnO | 232 at 50 ppm | Trimethylamine | Li et al. (2020b) |
| ZIF-8 | ZnO | 1.21 at 0.1 ppm | Benzene | Li et al. (2015) |
| ZnO@ZIF-8 | / | 72.3 at 100 ppm | H2 | Jeon et al. (2020) |
| WS4_ZIF-67 | WS2_Co–N–HCNC | 48.2 at 5 ppm | NO2 | Koo et al. (2018) |
| Pt@ZIF-8 | Pt/ZnO | 11.2 at 1 ppm | H2S | Zhou et al. (2020b) |
| ZIF-67 | Co3O4 | 220 at 100 ppm | CO | Qin et al. (2020) |
| SnO2@ZIF-67 | / | 16.5 at 5000 ppm | CO2 | Me et al. (2018) |
| ZnO@ZIF-8 | / | 77.7 at 10 ppm | NH3 | Jeon et al. (2020) |
| Zinc doped ZIF-67 | CoZn-NCNTs | 28.9 at 30 ppm | SO2 | Li et al. (2018a) |
In this review, we summarized the current advances of ZIF-X (8, 67) based nanostructures, especially for gas-sensing applications. Firstly, due to their porous structure with tunable pore aperture and exceptional stabilities, ZIF-X (8, 67) materials are especially suitable for gas storage including CO2, H2, and CH4, and ZIF-X (8, 67) based mixed matrix membranes have also shown promising potential in the excellent gas separation applications such as CO2/CH4, H2/CH4, C3H6/C3H8 and so on. Furthermore, more novel strategies are needed to develop ZIF-X (8, 67) based materials to further expand those gas storage and gas separation applications in the future.
Lastly, we specifically emphasize the nanostructure-sensing properties relationship in gas-sensing applications. In particular, we focused on the key parameters of the gas sensors (sensitivity, response rate, selectivity, stability, and sensing mechanisms) and detailed the sensing performances of ZIF-X (8, 67) based nanostructures toward inorganic analytes and VOCs. To obtain a high-performance gas sensor (i.e., high sensitivity and selectivity, fast response, low cost, and remarkable long-term stability), numerous strategies have been suggested for developing the unique ZIF-X (8, 67) based nanostructures (pure ZIF, ZIF-X (8, 67) based nanocomposite and derivatives), and the obtained nanostructures possess numerous extraordinary advantages. Firstly, these unique nanostructures possess huge surface area and a great deal of active sites, which can significantly promote the target gases adsorptions or surface reactions and can dramatically enhance the sensitivity of gas sensors. Secondly, the easily adjustable pore sizes permit them to serve as molecular sieves, which can dramatically enhance the selectivity of gas sensors. In addition, the sensing materials derived from ZIF-X (8, 67) based nanostructures would exhibit high porosity during pyrolysis, which is crucial for sensing performance due to their excellent gas accessibility, ultra-high surface area, and innumerable exposure active sites. It is noteworthy that mesoporous structures are the most common amongst sensing materials with porous nanostructure and are most beneficial for enhancing the sensing properties. Lastly, the ZIF-X (8, 67) based nanostructures possess superior thermal and chemical stability and can be stable in some harsh environments. Therefore, ZIF-X (8, 67)-based nanostructures exhibit excellent stability including good repeatability and long-term stability during gas-sensitive reactions while being utilized as sensitive materials. Therefore, these novel ZIF-X (8, 67) based nanostructures have great potential to be efficient sensing materials for wide applications in environmental protection (e.g., environmental harmful gas sensors), medical diagnosis (e.g., exhaled breath analyzers), indoor air quality monitoring, and food technology.
Despite some innovative advancements achieved in ZIF-X (8, 67) based nanostructures for gas sensors, several worthwhile directions are still pursued. (1) Metal oxides derived from ZIF-X (8, 67) nanostructures, tend to require high operating temperatures in gas sensing reactions, for which graphene or conductive polymers can be combined with ZIF-X (8, 67) based nanostructures to achieve lower operating temperatures as well as room temperature operation. (2) Low detection limits and high response are also important in practical applications, which may be further enhanced by the construction of novel heterostructures and the introduction of nanoscale metal NPs catalysts (e.g. Pd, Pt, Au, Ag, etc.) to obtain higher response values and lower detection limits. (3) Excellent selectivity is a key factor in achieving accuracy in gas sensors, and this can be achieved by developing simple and promising methods to design homogeneous ZIF membranes with specific pore sizes to improve their ability to act as molecular sieves against interfering gases, thus enabling highly selective sensors. All in all, further development of ZIF-X (8, 67) based nanostructure (pure ZIF, ZIF membrane, and their derivatives) is still needed and can further expand their possible applications in the future.
Funding source: National Natural Science Foundation of China
Award Identifier / Grant number: 52172069, 51950410596
Funding source: Key Research and Development Plan
Award Identifier / Grant number: BE2019094
Funding source: Six Talent Peaks Project
Award Identifier / Grant number: TD-XCL-004
Funding source: Qing Lan Project
Award Identifier / Grant number: [2016]15
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Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
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Research funding: This work was supported by the National Natural Science Foundation of China (52172069, 51950410596), the Key Research and Development Plan (BE2019094), the Six Talent Peaks Project (TD-XCL-004) and the Qing Lan Project ([2016]15) of Jiangsu Province.
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Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
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© 2022 Walter de Gruyter GmbH, Berlin/Boston
Artikel in diesem Heft
- Frontmatter
- ZIF-X (8, 67) based nanostructures for gas-sensing applications
- From passive to emerging smart silicones
- Corrosion inhibition relevance of semicarbazides: electronic structure, reactivity and coordination chemistry
- A review on the application of machine learning for combustion in power generation applications
- Methacrylic acid based microgels and hybrid microgels
- Ultrasonic viscosity-reduction vacuum residue oil
Artikel in diesem Heft
- Frontmatter
- ZIF-X (8, 67) based nanostructures for gas-sensing applications
- From passive to emerging smart silicones
- Corrosion inhibition relevance of semicarbazides: electronic structure, reactivity and coordination chemistry
- A review on the application of machine learning for combustion in power generation applications
- Methacrylic acid based microgels and hybrid microgels
- Ultrasonic viscosity-reduction vacuum residue oil
