MetalâOrganic Framework-Derived Hollow Hierarchical Co3O4

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00733 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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ACS Applied Materials & Interfaces

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 *Author to whom correspondence should be addressed Email: [email protected]; Fax: +82-2-928-3584; Tel: +82-2-3290-3282 Keywords: Gas sensor, hollow hierarchical nanocages, zeolitic imidazolate framework, methylbenzene, Co3O4

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 ZIF67 (Zeolitic imidazolate framework-67) rhombic dodecahedra, followed by solvothermal synthesis of Co3O4 nanocages using ZIF-67 self-sacrificial templates, and subsequent heat treatment for developing high-performance methylbenzene sensors. The sensor based on

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hollow hierarchical Co3O4 nanocages with the size of ~1.0 µm not only exhibited 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.

1. INTRODUCTION 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 the reliable monitoring of the quality of indoor air. Oxide semiconductor-based gas sensors exhibit many unparalleled advantages such as a 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


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materials should be newly designed or optimized further. In previous contributions, our research group reported that p-type Co3O4 oxide semiconductor is a good candidate for detecting 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 nano-porous 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 nano-porosity, 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 sensors.24-29 However, a systematic design of highperformance gas sensors based on MOF-derived hollow hierarchical nanostructures has been rarely reported. Zeolitic imidazolate frameworks (ZIFs) are representative MOFs30 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 nano-architectures that show 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/meso-porosity, and shell thickness of hollow hierarchical nanostructures by a one-step hydrothermal/solvothermal



self-assembly reaction.9 Accordingly, we aimed to prepare hollow hierarchical Co3O4 nanostructures with independent control of the size, morphology, and shell thickness for designing high-performance gas sensors. In this contribution, 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 sufficient for monitoring indoor air quality could be accomplished 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 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 as 03-, 10-, 20-, and 40-ZIF-67, respectively) were precipitated by pouring 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


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precipitation, rhombic dodecahedral ZIF-67 particles were washed 3 times with methanol via centrifugation at 11000 rpm for 8 min and dried at room temperature for 24 h. Synthesis of hollow hierarchical Co-LDH particles. As-prepared 0.02 g of 03-, 10-, 20and 40-ZIF-67 particles were dispersed in 60 mL of methanol containing 0.175 g of cobalt nitrate hexahydrate. 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 solvothermally react at 120 °C for 1 h. This produced hollow hierarchical Co-layered double hydroxide (Co-LDH) particles with different sizes (referred 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.

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 Co-LDH 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 micro-heater on its bottom surface. The TG analysis (e.g., see the result for 20-Co-LDH in Figure S1) showed that the dehydration and decomposition of Co-layered double hydroxide are complete at < 247.4 °C. Thus, the sensors were annealed at 400 °C for 2 h in air to remove the organic components and convert the Co-LDH particles into hollow hierarchical Co3O4 nanocages (referred as 03-, 10-, 20-, and 40-Co3O4). The sensors based on hollow Co3O4 dodecahedra (referred as H-Co3O4) were prepared by heating the 10-ZIF-67 particles at 400 °C for 2 h to investigate the effect of the hierarchical



morphology on the gas sensing characteristics. For comparison, the sensors based on commercial Co3O4 powders (referred as C-Co3O4) were also prepared by coating the powder and subsequent heat treatment at 400 °C for 2 h. The average sensing film thickness of C-, 03-, 10-, 20-, and 40-Co3O4 sensors were measured to be 13.2 ± 2.6, 13.2 ± 0.8, 13.7 ± 2.4, 12.3 ± 2.3, and 14.5 ± 1.1 µm, respectively.

Characterization. The phase and crystallinity of the materials were characterized by Xray diffraction (XRD, Rigaku D/MAX-2500V/PC) using Cu Kα radiation (λ = 1.5418 Å), Microstructures of the materials were observed by field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi Co. Ltd., Japan) and transmission electron microscopy (HR-TEM, Titan, FEI). The specific surface areas of the powders were estimated by the Brunauer-Emmett-Teller (BET) analysis of nitrogen adsorption isotherms (TriStar 3000).

Gas-sensing properties. The fabricated sensors were heat-treated again at 350 °C for 8 h in air to remove any residual organic materials and to stabilize the Co3O4 phase. The sensors were placed in a specially designed quartz tube (1.5 cm3) and the atmosphere was controlled using a four-way valve to ensure a constant flow rate of 200 cm3 min-1 of dry air and the analyte gas. Two-probe direct current (DC) resistance of the sensor was measured using an electrometer interfaced with a computer. The gas responses (S = Rg/Ra; Ra: resistance in air and Rg: resistance in the presence of an analyte gas) to 5 ppm ethanol, p-xylene, toluene, benzene, formaldehyde, and carbon monoxide were analyzed in the temperature range of 200–300 °C.


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3. RESULTS AND DISCUSSION Nearly monodisperse rhombic dodecahedral ZIF-67 particles with different sizes were prepared by changing the concentration of 2-methylimidazole used for precipitation (Figure 1a–d). X-ray diffraction analysis revealed that all the particles show ZIF-67 structures (Figure S2a–d).20 The monodispersity of the particle size can be attributed to the short burst of nucleation upon pouring the solution of 2-methylimidazole in methanol to the methanol solution of cobalt nitrate at the initial stage of precipitation and subsequent slow diffusive particle growth.31 The average sizes (diameters of circumscribed spheres) of ~400 ZIF-67 rhombic dodecahedra prepared from methanol solutions containing 2.624, 1.312, 0.984, or 0.656 g of 2-methylimidazole are 0.28 ± 0.05, 0.95 ± 0.12, 2.21 ± 0.32, and 3.75 ± 0.43 µm, respectively. The decrease in the particle size with increasing 2-methylimidazole concentration in the present study is consistent with the previously reported size control of Zn-based ZIF-8 particles,32 which can be rationalized by the enhancement of the nucleation rate. Moreover, the particle sizes are linearly related to the 2-methylimidazole concentration on a log-log scale (Figure S3), indicating the possibility of a precise control of the particle size. These results clearly demonstrate that monodisperse, size-tunable, and well-dispersed Co-based ZIF-67 particles could be prepared by the facile chemical reaction. The methanol slurry solution containing different sizes of ZIF-67 particles and cobaltnitrate were solvothermally reacted at 120 °C for 1 h. After the reaction, whereas the overall sizes of the particles remained similar, the surface morphology changed significantly (Figure 1e–h). The particles obtained after the solvothermal reaction were identified as Co-layered double hydroxide (Co-LDH)22 from X-ray diffraction analysis (e.g., see XRD pattern for 10Co-LDH in Figure S2e). This indicates the growth of Co-LDH with nanosheet morphology on the surface of ZIF-67 rhombic dodecahedra. The absence of the ZIF peak in the XRD



pattern after the solvothermal reaction suggests that the ZIF-67 structures cannot be maintained during the reaction and Co-based ZIF-67 particles essentially acted as selfsacrificial templates. The formation mechanism of Co-LDH and Co3O4 hollow hierarchical nanocages is shown in Scheme 1. When the cobalt nitrate methanol solution was added to the methanol slurry containing Co-based ZIF-67 particles, the pH value of the mixture decreased to 5.3 with an increase in the concentration of cobalt nitrate (Figure S4). Under the acidic condition, Co2+ ions in ZIF-67 dissolved in the solution and a part of the dissolved Co2+ ions oxidized into Co3+ ions. Co-LDH is known to form under the co-existence of Co2+ and Co3+ ions.22,33 Accordingly, Co-LDH starts to form on the facets of ZIF-67 dodecahedra, which then grow into nanosheets at the expense of Co components in the ZIF-67 particles and slurry solution (Scheme 1b). The dissolution of the inner Co component and vertical growth of Co-LDH on the self-sacrificial templates of ZIF-67 rhombic dodecahedra lead to the formation of hollow hierarchical Co-LDH nanocages with different cage sizes (03-, 10-, 20-, and 40-Co-LDH) (Scheme 1c). This formation mechanism is supported by the observation of yolk-shell morphology for the samples prepared by the solvothermal reaction of a slurry containing 20ZIF-67 rhombic dodecahedra for a shorter reaction time (15 min) (Figure S5). The 03-, 10-, 20-, and 40-Co-LDH particles were converted into 03-, 10-, 20-, and 40Co3O4 hollow hierarchical structures by heat treatment at 400 °C for 2 h (Scheme 1d, Figure 1i-l and 2). The overall size and hollow hierarchical morphology are maintained in 10-, 20-, and 40-Co3O4 specimens (Figure 1j-l, Figure 2b–d, Figure 2f–h) even after the heat treatment whereas the hollow morphology of 03-Co3O4 particles became less distinct (Figure 1i, 2a, and 2e) owing to the very thin configuration of the shells. The bright contours at the center of the nanocages in the TEM images (Figure 2b–d) indicate the hollow space. High-resolution


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TEM analysis further revealed that the 10-, 20-, and 40-Co3O4 hollow hierarchical nanocages are assembled from thin Co3O4 nanosheets (Figure 2f–h). The average thickness of the nanosheets in 10-, 20-, and 40-Co3O4 nanocages are 12.9 ± 2.3, 13.6 ± 2.2, and 13.7 ± 2.3 nm, respectively (Figure 2f–h). The average shell thickness of 10-, 20- and 40-Co3O4 nanocages were estimated to be 89 ± 15, 292 ± 41, and 849 ± 129 nm (Figure S6), respectively. The larger the nanocages, the thicker the shells formed. When the same weight of the selfsacrificial templates of Co-based ZIF-67 rhombic dodecahedra are solvothermally reacted in the Co-nitrate solution, the shell thickness is determined by the size of ZIF-67 templates (i.e., the Co amount within ZIF-67 templates) and the amount of Co in the solution. Accordingly, the thinner shells of the smaller nanocages can be attributed to the increase in the surface area per unit volume. This indicates that both the size and shell thickness of the nanocages can be controlled independently by tuning the precipitation of ZIF-67 and subsequent solvothermal reaction. All the commercial Co3O4 nanopowders and Co3O4 hollow hierarchical nanocages were identified to be crystalline face-centered cubic Co3O4 (JCPDS no. 42-1467) from X-ray diffraction analysis (Figure 3). The crystallite sizes of commercial Co3O4 nanopowders and 03-, 10-, 20-, and 40-Co3O4 hollow hierarchical nanocages are calculated with Scherrer equation to be 48.3 ± 1.8, 36.9 ± 5.9, 33.6 ± 3.0, 36.6 ± 3.1, and 39.7 ± 2.6 nm, respectively, under assuming isotropic crystallite shape factor of 0.9. The Co3O4 nanosheets within hollow hierarchical nanocages are ~ 13 nm thick (Figure 2f-h) and ~ 80 nm wide (not shown here). Accordingly, the crystallite sizes determined by XRD (33.6 – 39.7 nm) larger than those observed in TEM analysis (~ 13 nm) emanated from anisotropic nature of crystallites.34 Note that the 03-, 10-, 20-, and 40-Co3O4 hollow hierarchical particles derived from Co-LDH show similar crystallite sizes.



The dynamic sensing transients of the C-, 03-, 10-, 20-, and 40-Co3O4 sensors showed representative p-type gas sensing characteristics: An increase in the resistance upon exposure to reducing gas and the recovery of the sensor resistance upon exposure to air, respectively (Figure 4a–e). The responses of all the sensors to 5 ppm p-xylene, toluene, benzene, ethanol, HCHO, and CO were measured at 200–300 °C. Owing to the very slow recovery rate, the gas sensing characteristics below 200 °C were not evaluated. The C-Co3O4 sensor showed relatively low responses to p-xylene (S = 8.5) and toluene (S = 5.4) at 200 °C (Figure 4f). The responses to all the gases became negligibly low at ≥ 250 °C. Although the 03-Co3O4 sensor exhibited slightly higher responses to ethanol (S = 23.0), p-xylene (S = 14.2), and toluene (S = 7.1) (Figure 4g) at 200 °C compared to the C-Co3O4 sensor, all the gas responses decreased to a low level upon increasing the sensor temperature above 250 °C. Note that remarkably high gas responses are achieved for 10-, 20-, and 40-Co3O4 sensors (Figure 4h–j). In particular, the 10-Co3O4 sensor showed unprecedentedly high responses to p-xylene (S = 120.0), ethanol (S = 99.2), and toluene (S = 76.6) at 200 °C (Figure 4h). Further, the 20- and 40-Co3O4 sensors also exhibited comparably high responses to p-xylene, toluene, and ethanol (Figure 4i,j). Notably, the responses of 10-, 20-, and 40-Co3O4 sensors to ethanol decreased rapidly with an increase in the sensor temperature from 200 to 225 °C, whereas those to pxylene and toluene decrease gradually. Thus, highly selective detection of xylene and toluene without significant sacrifice of the gas responses could be achieved at the sensing temperature of 225 °C. For quantification, the gas selectivity values (SX/SE and ST/SE) toward p-xylene and toluene over ethanol interference are calculated as a function of sensor temperature (Figure 4k–o). Note that the SX/SE and ST/SE values of 10-, 20-, and 40-Co3O4 sensors (Figure 4m–o) are significantly higher than those of C- and 03-Co3O4 sensors (Figure 4k,l). For example, the maximum SX/SE values of 10-, 20-, and 40-Co3O4 sensors are 7.5, 7.9,


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and 10.5, respectively, whereas those of C- and 03-Co3O4 sensors are 3.6 and 1.6, respectively. This clearly demonstrates that the hollow hierarchical Co3O4 nanocages are promising platforms for highly selective and sensitive detection of methylbenzenes. The pore size distribution and specific surface areas were measured from N2 adsorption/desorption (Figure 5) isotherms. Abundant mesopores with sizes in the range of 2 to 30 nm are found in 10-, 20-, and 40-Co3O4 hollow hierarchical structures (blue region in Figure 5d–f), whereas the 03-Co3O4 and C-Co3O4 particles show a relatively low volume of the mesopores (Figure 5b,c). These suggest that most of mesopores originated from the hollow hierarchical morphologies. The specific surface areas of the C-, 03-, 10-, 20- and 40Co3O4 particles are 9.6, 21.9, 53.7, 54.9, and 37.7 m2·g-1 (Figure 5g–k), respectively, which show a similar tendency as the pore volumes. Accordingly, the high gas responses of 10-, 20-, and 40-Co3O4 sensors can be explained by the highly gas-accessible hollow hierarchical nanostructures consisting of abundant mesopores and nanosheets with a high surface area. The 90% response and 90% recovery times (τres and τrecov) of the sensors, that is, the times required to reach 90% of the sensor resistance upon exposure to 5 ppm p-xylene and air, respectively, were calculated from the sensing transient at 225 °C. The τres values of 10-, 20and 40-Co3O4 sensors (63, 93, and 128 s) are significantly shorter than those of C- and 03Co3O4 sensors (314 and 324 s) (Figure 6a) and the τrecov values of 10-, 20- and 40-Co3O4 sensors (86, 74, and 77 s) are also remarkably shorter than those of C- and 03-Co3O4 sensors (390 and 328 s) (Figure 6b). The fast sensing and recovery kinetics in 10-, 20- and 40-Co3O4 sensors can be rationalized by the rapid diffusion of the analyte gases through the highly gasaccessible hollow hierarchical structures with abundant mesopores, whereas the sluggish response and recovery in C- and 03-Co3O4 sensors are attributed to the agglomerated configuration of sensing materials which decreases the gas accessibility.



The differences of gas responses for 03-, 10-, 20, and 40-Co3O4 sensors related with gas diffusion through sensing films and following gas sensing reaction. In the present study, similar thicknesses (~13 µm) of sensing films are coated on the substrates with two electrodes. In this electrode configuration, the conduction will occur mainly through the lower part of sensing film close to the electrodes. Thus, it is essential to transfer the analyte gases effectively to the sensing surfaces at the lower part of sensing film. Note that the 03Co3O4 particles show relatively agglomerated configuration (Figure 1i) and smaller amount of mesopores (size: 3 – 10 nm) (Figure 5c) compared to 10-, 20-, and 40-Co3O4 particles (Figure 1j-l, Figure 5d-f). Thus, the low gas responses of 03-Co3O4 sensors are attributed to the blocking of gas transfer to the lower part of sensing film because of agglomerated configuration of sensing film. This is in line with the prolonged τres and τrecov values in 03Co3O4 sensors (Figure 6). It should be noted that the 10-, 20-, and 40-Co3O4 sensors showed high gas responses because the sensing films consist of porously packed Co3O4 hollow hierarchical nanocages. However, their gas responses are different from each other. For example, the overall gas responses of the 40-Co3O4 sensor are lower than those of the 10Co3O4 sensor. We reported that In2O3 hollow spheres with thin shells (thickness: ~150 nm) show 1.89 times higher response to 100 ppm ethanol than those with thicker shells (thickness: ~300 nm).35 Note that the shells of 40-Co3O4 nanocages (thickness: 849 ± 129 nm thick) are ~10 times thicker than those of 10-Co3O4 nanocages (thickness: 89 ± 15 nm). This suggests that the lower gas response of the 40-Co3O4 sensor emanated from the decreased gasaccessibility owing to the thick shell layer. Highly selective detection of methylbenzenes is explained by the catalytic activity of sensing materials and the oxidation of interface gases during the transport through catalytic sensing materials. The Co3O4 is known as an excellent catalyst to oxidize aromatic volatile


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organic compounds such as benzene, methylbenzene, alcohol, and aldehyde.7,36 There have been many studies to report highly sensitive and selective detection of xylene and/or toluene using pure, Cr-loaded, and Pd-loaded Co3O4 nanostructures.5,6,37,38 In contrast, most of n-type oxide semiconductor gas sensors such as SnO2, ZnO, and In2O3 sensors often show high responses to highly reactive ethanol and low responses to less reactive xylene and toluene.3941

Accordingly, high catalytic activity of Co3O4 for oxidation of volatile organic compounds

explains the high responses of 10-, 20-, and 40- sensors to p-xylene, toluene, and ethanol at 200 °C (Figure 4h-j). The tuning of gas selectivity can be achieved further by the control of sensing temperature. For instance, in 10-, 20-, and 40-Co3O4 sensors, the ethanol responses decreased to negligible level with an increase in the sensor temperature from 200 to 225 °C, whereas those to p-xylene and toluene decrease gradually (Figure 4h-j). It means that the less reactive methylbenzenes can diffuse to the lower part of sensing film without significant oxidation whereas most of highly reactive ethanol oxidizes into non-reactive or less reactive species such as CO2 and H2O at the upper part of sensing film. In addition, the hollow hierarchical nanostructures will be more advantageous to oxidize highly reactive ethanol at the upper part of sensing materials. Accordingly, high methylbenzene selectivity of 10-, 20-, and 40-Co3O4 sensors in the present study can be attributed to the high catalytic activity of Co3O4 sensing materials and efficient catalytic oxidation of highly reactive interference gas such as ethanol during the gas transport to the lower part of sensing film. In order to investigate the effect of the hierarchical morphology on the gas sensing characteristics, hollow Co3O4 nanocages (H-Co3O4) were prepared by heat treatment of 10ZIF-67 particles at 400 °C for 2 h (Figure S7a). Hollow dodecahedra without hierarchical surface morphology are observed (Figure S7b,c). The maintenance of the dodecahedral morphology and overall size (average size: 0.68 ± 0.09 µm) during the heat treatment



suggests that hollow Co3O4 nanocages are formed by the oxidation of Co-components from the surface of 10-ZIF-67 rhombic dodecahedra. Note that the shell thickness of the H-Co3O4 nanocages (71 ± 10 nm) is similar to that of 10-Co3O4 nanocages (89 ± 15 nm). Further, the amount of mesopores (size: 2–30 nm) in H-Co3O4 nanocages (Figure S7f) was substantially lower than that in 10-Co3O4 nanocages (Figure 5d), once again confirming that the origin of mesopores in 10-Co3O4 nanocages is the hierarchical morphology. The overall gas responses of H-Co3O4 sensor were substantially lower than those of 10Co3O4 sensor despite the similar size and shell thickness of the nanocages. For instance, the responses of the H-Co3O4 sensor to 5 ppm p-xylene and toluene (28.2 and 15.4) at 225 °C are approximately 1/3 those of the 10-Co3O4 sensor (78.6 and 43.8) (Figure S8). Lü et al.25 also prepared porous Co3O4 concave nanocubes by thermal annealing of Co-MOF (ZIF-67) concave nanocubes at 300 – 400 °C. The responses to 10 ppm ethanol and toluene at 300 °C were ~ 1.80 and 1.15, respectively, which were significantly lower than those of the present sensor. This strongly suggests that the hierarchical morphology plays a key role in the enhanced gas responses, which is consistent with previous studies that demonstrate high responses of oxide chemiresistors with gas-accessible hierarchical nanostructures.5,42–44 The sensing transients of the 10-Co3O4 sensor to 0.25–5 ppm p-xylene are shown in Figure 7a. The sensor showed stable sensing and recovery. The response to 0.25 ppm p-xylene is 3.8, indicating that the detection of xylene at the ppb-level is possible (Figure 7a). Note that the response of 10-Co3O4 sensor to xylene is among the highest gas responses reported for sensors based on pure Co3O4 (Figure 7b).5,6,45–55 Further, the 10-times repetitive sensing transients toward 5 ppm of p-xylene indicate the reproducibility and stability of the 10-Co3O4 sensor (Figure 7c). Toluene and xylene are aromatic hydrocarbons with one and two methyl groups,


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respectively, on the benzene ring. These methylbenzenes are mainly used as solvents in gasoline, paint thinners, printing inks, and in the leather tanning process. Despite being widely used in everyday life, the exposure to methylbenzenes can cause irritation of the skin, eyes, stomach, and respiratory system, and furthermore, they can affect the memory and nervous system. Accordingly, the Agency for Toxic Substances and Disease Registry has set the minimum risk levels for exposure to xylene and toluene in acute-duration (14 days or less) inhalation to 2 ppm each, and in the chronic-duration (365 days or more) inhalation, it is set to 0.06 and 1 ppm,56,57 respectively. Note that, for a long time exposure, even a low concentration of methylbenzenes can have a harmful effect on human health. Because people spend over 90% of their time in enclosed buildings and vehicles,58 it is very important to monitor the indoor air pollution using highly sensitive sensors. Furthermore, xylene and toluene, two representative indoor pollutants, should be measured without interference from one of the most common VOCs in real life, ethanol, which is very reactive toward most of the oxide semiconductor gas sensors.59 This suggest that ultrasensitive and highly selective xylene sensor using hollow hierarchical Co3O4 nanocages in the present study can be applied for monitoring indoor air quality. The main accomplishment of this study is the size- and morphology-tunable synthesis of hollow hierarchical Co3O4 nanocages for high-performance gas sensors. The size of the Cobased ZIF-67 rhombic dodecahedra could be controlled at the stage of precipitation. The four different-sized monodisperse Co-based ZIF-67 dodecahedra served as excellent selfsacrificial templates for the solvothermal growth of hollow hierarchical nanostructures consisting of Co-LDH nanosheets layers. The size and hierarchical morphology were maintained after the conversion of the Co-LDH nanocages into Co3O4 nanocages by heat treatment. The main parameters to determine the gas response of hollow/hierarchical oxide nanostructures are the size, shell thickness, the crystallite size and surface area to volume ACS Paragon Plus Environment


ratio. Note that the most key parameters can be separately and precisely controlled by manipulating the self-sacrificial templates, the Co concentration, and reaction temperature during the solvothermal reaction. Thus, the synthetic route developed in this study is promising for the design of high-performance gas sensors for monitoring the quality of indoor air and can be used as a general strategy to enhance the gas sensing characteristics of nanostructures.


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4. CONCLUSIONS Ultrasensitive and highly selective methylbenzene sensors for monitoring indoor air were designed by tuning the size, hierarchical morphology, shell thickness, and mesopores of catalytic Co3O4 nanocages. The overall sizes of Co-based ZIF-67 rhombic dodecahedra could be controlled at the stage of their precipitation. The solvothermal growth of Co-layered double hydroxide on four different-sized (~0.3, 1.0, 2.0, and 4.0 µm) self-sacrificial ZIF-67 dodecahedra and subsequent heat treatment enabled the size- and morphology-tunable synthesis of nearly-monodisperse hollow hierarchical Co3O4 nanocages. The gas sensing characteristics were found to be critically dependent upon the size, shell thickness, mesopores, and hollow/hierarchical morphology. Extremely high gas responses of the hollow hierarchical Co3O4 nanocages with a size of ~1 µm toward methylbenzenes were attributed to the highly gas-accessible hollow hierarchical morphology, thin shells, and abundant mesopores, whereas the high gas selectivity originated from the high catalytic activity of Co3O4 toward aromatic VOCs. The MOF-derived hollow hierarchical Co3O4 nanocages with tunable size and morphology could detect sub-ppm-level methylbenzenes with ultrahigh response and high selectivity, and can be used in indoor air quality monitoring systems.

ASSOCIATED CONTENT Supporting Information. TG curve of 20-Co-LDH hollow hierarchical nanocages; diffraction patterns of rhombic dodecahedral ZIF-67 particles; size difference of ZIF-67 at various 2-methylimidazole/Co molar ratios; pH values according to the concentration of cobalt nitrate in the methanol slurry; SEM images showing conversion from ZIF-67 dodecahedra to Co-LDH hollow hierarchical nanocages during solvothermal reaction; low magnification TEM images of 10-Co3O4; X-ray diffraction pattern, SEM image, TEM image,



N2 adsorption/desorption isotherm, surface area, and pore size distribution for H-Co3O4 nanocages; gas response and selectivity of H-Co3O4 and 10-Co3O4.

Notes The authors declare no competing financial interest. Acknowledgements This work was supported by a grant from the National Research Foundation of Korea (NRF), which was funded by the Korean government (Ministry of Education, Science, and Technology (MEST), Grant No. 2016R1A2A1A05005331).


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SCHEME and FIGURES

Scheme 1. Schematic of the preparation of the hollow hierarchical Co3O4 nanocages: a) formation of the ZIF-67 dodecadedra; b) growth of Co-LDH nanosheets on the facets of ZIF67 particles; schematic illustrations of hollow hierarchical c) Co-LDH and d) Co3O4 nanocages.



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 cobalt-layered double hydroxide 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) 20Co3O4, and l) 40-Co3O4.


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Figure 2. Transmission electron microscopy (TEM) images of a, e) 03-Co3O4, b, f) 10-Co3O4, c, g) 20-Co3O4, and d, h) 40-Co3O4 hollow hierarchical particles.



Figure 3. X-ray diffraction (XRD) patterns of a) commercial Co3O4 nanopowders, and hollow hierarchical Co3O4 particles: b) 03-Co3O4, c) 10-Co3O4, d) 20-Co3O4, and e) 40Co3O4.


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Figure 4. Dynamic-sensing transients of a) C-Co3O4, b) 03-Co3O4, c) 10-Co3O4, d) 20-Co3O4, and e) 40-Co3O4 hollow hierarchical particles to 5 ppm of p-xylene at 225 °C; gas responses (Rg/Ra) of f) C-Co3O4, g) 03-Co3O4, h) 10-Co3O4, i) 20-Co3O4, and j) 40-Co3O4 sensors to 5 ppm of ethanol, p-xylene, toluene, benzene, formaldehyde, and carbon monoxide at 200 – 300 °C; selectivity toward p-xylene and toluene over the interfering ethanol gas (SX/SE and ST/SE) of k) C-Co3O4, l) 03-Co3O4, m) 10-Co3O4, n) 20-Co3O4, and o) 40-Co3O4 sensors.



Figure 5. (a) N2 adsorption/desorption isotherms and (b–f) pore size distributions and (g–k) BET surface areas of (b,g) C-Co3O4, (c,h) 03-Co3O4, (d,i) 10-Co3O4, (e,j) 20-Co3O4, and (f,k) 40-Co3O4.


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Figure 6. (a) 90% response time (τres) and (b) 90% recovery time (τrecov) of C-, 03-, 10-, 20-, and 40-Co3O4 sensors upon exposure to 5 ppm p-xylene and air, respectively, at 225 °C.



Figure 7. a) Dynamic sensing transients of 10-Co3O4 sensor to 0.25 – 5 ppm p-xylene at 225 °C; b) responses of 10-Co3O4 sensor to 5 ppm p-xylene at 200 °C (△), 0.25 – 5 ppm p-xylene at 225 °C (☆), and xylene responses in the literature.5,6,45–55; c) ten repetitive sensing transients of 10-Co3O4 sensor to 5 ppm p-xylene at 225 °C.


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