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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 8860−8868
Metal−Organic Framework-Derived Hollow Hierarchical Co3O4 Nanocages with Tunable Size and Morphology: Ultrasensitive and Highly Selective Detection of Methylbenzenes Young-Moo Jo,† Tae-Hyung Kim,† Chul-Soon Lee,† Kyeorei Lim,† Chan Woong Na,‡ Faissal Abdel-Hady,§ Abdulaziz A. Wazzan,§ and Jong-Heun Lee*,†,§ †
Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea Dongnam Regional Division, Korea Institute of Industrial Technology, Busan 46938, Republic of Korea § Department of Chemical and Materials Engineering, King Abdulaziz University, Jeddah 21589, Saudi Arabia ‡
S Supporting Information *
ABSTRACT: Nearly monodisperse hollow hierarchical Co3O4 nanocages of four different sizes (∼0.3, 1.0, 2.0, and 4.0 μm) consisting of nanosheets were prepared by controlled precipitation of zeolitic imidazolate framework-67 (ZIF-67) rhombic dodecahedra, followed by solvothermal synthesis of Co3O4 nanocages using ZIF-67 self-sacrificial templates, and subsequent heat treatment for the development of highperformance methylbenzene sensors. The sensor based on hollow hierarchical Co3O4 nanocages with the size of ∼1.0 μm exhibited not only ultrahigh responses (resistance ratios) to 5 ppm p-xylene (78.6) and toluene (43.8) but also a remarkably high selectivity to methylbenzene over the interference of ubiquitous ethanol at 225 °C. The unprecedented and high response and selectivity to methylbenzenes are attributed to the highly gas-accessible hollow hierarchical morphology with thin shells, abundant mesopores, and high surface area per unit volume as well as the high catalytic activity of Co3O4. Moreover, the size, shell thickness, mesopores, and hollow/hierarchical morphology of the nanocages, the key parameters determining the gas response and selectivity, could be well-controlled by tuning the precipitation of ZIF-67 rhombic dodecahedra and solvothermal reaction. This method can pave a new pathway for the design of high-performance methylbenzene sensors for monitoring the quality of indoor air. KEYWORDS: gas sensor, hollow hierarchical nanocages, zeolitic imidazolate framework, methylbenzene, Co3O4
1. INTRODUCTION
methylbenzenes because of its high catalytic activity toward aromatic compounds.5,6 To achieve an ultrahigh gas response, chemiresistive variation near the surface of the sensing materials should be enhanced by controlling the size of the nanostructures7,8 and the assembled configuration of the nano building blocks should be tailored to be highly porous and gas-accessible.9 Recently, metal−organic frameworks (MOFs), hybrid nanoporous materials, have received much attention in the applications of gas storage materials,10 gas membranes,11 catalysts,12 chemical sensors,13,14 and drug delivery systems15 because of their distinctive advantages of tunable nanoporosity, high surface area, and facile preparation. Further, MOFs have been used as sacrificial templates to prepare hollow and/or porous nanostructures via chemical reaction and/or thermal annealing,16 to enhance the performances of Li-ion batteries,17−20 supercapacitors,21,22 catalysts,23 and gas sen-
Indoor volatile organic compounds (VOCs) emitted from paint, adhesives, cleaning products, and furnishings induce not only asthma1 but also sick building syndrome2 with various symptoms such as headache, nausea, irritation of the eyes, and fatigue. In particular, methylbenzenes such as xylene and toluene are the key indoor air pollutants that should be monitored precisely. Considering the health impact on human beings, a high gas response is essential to detect sub-ppm-level methylbenzenes and a high selectivity toward methylbenzenes is also very essential for reliable monitoring of the quality of indoor air. Oxide semiconductor-based gas sensors exhibit many unparallel advantages such as high response, rapid sensing speed, excellent stability, and facile integration.3,4 However, ultrasensitive and highly selective detection of sub-ppm-level methylbenzenes using oxide semiconductor chemiresistors still remains challenging. This indicates that the sensing materials should be newly designed or optimized further. In previous contributions, our research group reported that the p-type Co3O4 oxide semiconductor is a good candidate for detecting © 2018 American Chemical Society
Received: January 14, 2018 Accepted: February 21, 2018 Published: February 21, 2018 8860
DOI: 10.1021/acsami.8b00733 ACS Appl. Mater. Interfaces 2018, 10, 8860−8868
Research Article
ACS Applied Materials & Interfaces
Figure 1. Scanning electron microscopy (SEM) images of rhombic dodecahedral ZIF-67 particles: (a) 03-ZIF-67, (b) 10-ZIF-67, (c) 20-ZIF-67, and (d) 40-ZIF-67; hollow hierarchical Co-LDH particles: (e) 03-Co-LDH, (f) 10-Co-LDH, (g) 20-Co-LDH, and (h) 40-Co-LDH; and hollow hierarchical Co3O4 particles: (i) 03-Co3O4, (j) 10-Co3O4, (k) 20-Co3O4, and (l) 40-Co3O4.
sors.24−29 However, a systematic design of high-performance gas sensors based on MOF-derived hollow hierarchical nanostructures has been rarely reported. Zeolitic imidazolate frameworks (ZIFs) are representative MOFs,30 and various structures of Zn- and Co-based ZIFs are considered as attractive self-sacrificial templates for the preparation of ZnO and Co3O4 nanostructures for gas sensors. Hollow and hierarchical oxide nanostructures are two of the most representative highly gas-accessible nanoarchitectures that show a high gas response as well as a rapid responding speed,9 and they can be synergistically combined into hollow hierarchical oxide nanostructures. However, in general, it is difficult to separately control the size, morphology, micro-/ mesoporosity, and shell thickness of hollow hierarchical nanostructures by a one-step hydrothermal/solvothermal selfassembly reaction.9 Accordingly, we aimed to prepare hollow hierarchical Co3O4 nanostructures with independent control of the size, morphology, and shell thickness for designing highperformance gas sensors. In this paper, Co-based ZIF-67 dodecahedra of four different sizes (∼0.3, 1.0, 2.0, and 4.0 μm) were prepared by controlling the precipitation reaction, and they were used as self-sacrificial templates to grow hollow hierarchical Co-layered double hydroxide (Co-LDH) nanosheets by a solvothermal reaction. The hollow hierarchical Co3O4 nanocages with different sizes were successfully prepared by heat-treating the Co-LDH nanocages, and their gas-sensing characteristics were investigated. The gas response and selectivity were closely dependent on the size and shell thickness of the nanocages as well as their hierarchical nanoarchitecture. The unprecedented high response and selectivity to xylene and toluene which are required for monitoring indoor air quality could be obtained by combining highly gas-accessible hollow and hierarchical morphology as well as tuning the shell thickness and sensing temperature. The sensing mechanism underlying the ultrasensitive and highly selective detection of methylbenzenes is also discussed.
2. EXPERIMENTAL SECTION 2.1. Preparation of Rhombic Dodecahedral ZIF-67. Rhombic dodecahedral ZIF-67 particles with average sizes (diameters of circumscribed spheres) of ∼0.3, 1.0, 2.0, and 4.0 μm (referred to as 03-, 10-, 20-, and 40-ZIF-67, respectively) were precipitated by adding 100 mL of a methanol solution containing 2.624, 1.312, 0.984, or 0.656 g of 2-methylimidazole (C4H6N2, 99%, Sigma-Aldrich, USA) into 100 mL of a methanol solution containing 1.17 g of cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 99.999%, Sigma-Aldrich, USA). The resulting solutions were stirred for 12 min and aged for 24 h at room temperature. After precipitation, rhombic dodecahedral ZIF-67 particles were washed three times with methanol via centrifugation at 11 000 rpm for 8 min and dried at room temperature for 24 h. 2.2. Synthesis of Hollow Hierarchical Co-LDH Particles. Asprepared 0.02 g of 03-, 10-, 20-, and 40-ZIF-67 particles were dispersed in 60 mL of methanol containing 0.175 g of Co(NO3)2·6H2O. After ultrasonication for 1 min and stirring for 5 min, this slurry was transferred to a Teflon-lined stainless-steel autoclave (volume: 100 mL), which was then sealed, and the contents were allowed to react solvothermally at 120 °C for 1 h. This produced hollow hierarchical Co-LDH particles with different sizes (referred to as 03-, 10-, 20-, and 40-Co-LDH), which were washed thrice with methanol via centrifugation. Subsequently, the 03-, 10-, 20-, and 40-Co-LDH particles (0.02 g) were dispersed in 1 mL of methanol. 2.3. Fabrication of a Gas-Sensing Film. An organic binder (0.2 mL, FCM, a terpineol-based ink vehicle, USA) was mixed with the aforementioned slurry (1 mL of the methanol slurry containing CoLDH particles) and aged for 10 min to evaporate methanol at room temperature. The resulting slurry was screen-printed on an alumina substrate (area: 1.5 mm × 1.5 mm; thickness: 0.25 mm) with two Au electrodes on its top surface (electrode widths: 1 mm; separation: 0.2 mm) and a microheater on its bottom surface. The thermogravimetric (TG) analysis (e.g., see the results for 20-Co-LDH in Figure S1) showed that the dehydration and decomposition of Co-LDH are complete at
