Efficient Gas-Sensing for Formaldehyde with 3D Hierarchical Co3O4

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Cite This: Inorg. Chem. 2017, 56, 14111-14117

Efficient Gas-Sensing for Formaldehyde with 3D Hierarchical Co3O4 Derived from Co5‑Based MOF Microcrystals Wei Zhou,† Ya-Pan Wu,*,† Jun Zhao,† Wen-Wen Dong,† Xiu-Qing Qiao,† Dong-Fang Hou,† Xianhui Bu,‡ and Dong-Sheng Li*,† †

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College of Materials and Chemical Engineering, Hubei Provincial Collaborative Innovation Center for New Energy Microgrid, Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang, 443002, China ‡ Department of Chemistry and Biochemistry, California State University, Long Beach, California 90840, United States S Supporting Information *

ABSTRACT: Detecting formaldehyde at low operating temperature and maintaining long-term stability are of great significance. In this work, a hierarchical Co3O4 nanostructure has been fabricated by calcining Co5-based metal−organic framework (MOF) microcrystals. Co3O4-350 particles were used for efficient gas-sensing for the detecting of formaldehyde vapor at lower working temperature (170 °C), low detection limit of 10 ppm, and long-term stability (30 days), which not only is the optimal value among all reported pure Co3O4 sensing materials for the detection of formaldehyde but also is superior to that of majority of Co3O4-based composites. Such extraordinarily efficient properties might be resulted from hierarchically structures, larger surface area and unique pore structure. This strategy is further confirmed that MOFs, especially Co-clusters MOFs, could be used as precursor to synthesize 3D nanostructure metal oxide materials with high-performance, which possess high porosity and more active sites and shorter ionic diffusion lengths.

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on.11−14 In this context, much effort has been made to explore metal oxides deriving from different MOFs and extend their performance. Zheng et al. have fabricated high-purity porous Co3O4 concave nanocubes by calcining ZIF-67 and investigated sensing capacity in detecting volatile organic compounds, especially to ethanol.15 Similarly, Kim’s group has reported PdO nanoparticles (NPs) functionalized Co3O 4 hollow nanocages (HNCs) deriving from ZIF-67 and improved its acetone sensing performances.16 However, as one of the obvious characteristics of MOFs, secondary building units, such as the metal oxide clusters, seemed to be ignored especially to be treated as precursor of fabricating nanoarchitectural metal oxides. Our group has also reported 3D hierarchical copper oxide by calcining Cu-cluster-based MOF exhibiting excellent TEAsensing performances.17

INTRODUCTION Formaldehyde sensing is widely applied in household environmental monitoring, pollutant gas leakage in chemical plant.1 In previous reports, a variety of formaldehyde sensors was skillfully fabricated by metal oxides, such as In2O3, SnO2, NiO, ZnO, and ZnCo2O4.2−5 However, some metal oxides not only often exhibit an optimum performance at a relative higher operating temperature but many of them also show a long response time. Given this, to find an ideal gas sensor with low operating temperature, high selectivity, and time-saving is of great significance. An efficient synthetic strategy for solving this problem is to construct 3D nanoarchitectural nanoparticles, which dramatically promote gas diffusion and electron transport.6,7 Metal oxides prepared from MOF templates have shown extraordinary tunability, periodic alternation, and highly pore volumes.8−10 Due to those advantages, metal oxides derived from MOFs exhibit excellent applications in fuel cells, lithium-ion batteries, supercapacitors, oxygen-evolution reaction, and so © 2017 American Chemical Society

Received: September 1, 2017 Published: November 7, 2017 14111

DOI: 10.1021/acs.inorgchem.7b02254 Inorg. Chem. 2017, 56, 14111−14117

Article

Inorganic Chemistry

We investigated the transformation of Co-MOF through thermal gravimetric analysis (TGA). The TGA curve of the precursor is shown in Figure S2. As the temperature rises, water or ethanol guest molecule are eliminated. At around 350 °C, the second weight loss occurs, leading to conversion of Co-MOF to Co3O4, and the framework begins to collapse. Co-MOFs were initially decomposed, and spherical Co3O4 was formed (Figure 2b). To further understand the formation mechanism of spherical Co3O4 and the effect of different calcination temperature on the final gas-sensing performance, different calcination temperatures from 400 and 450 °C were also assessed. After calcination under 400 and 450 °C, spherical Co3O4 starts to collapse and agglomerate (Figure 2c,d). To explore the morphology of Co3O4-350 in more detail, the TEM images are shown in Figure 3. Figure 3a shows a lowmagnification TEM image, exhibiting a loose and rough surface. About 22.5% of the particles are distributed at ∼28 nm; 80% of the particles are distributed at 20−40 nm (Figure 3b). The selected-area electron diffraction (SAED) pattern reveals that the entire Co3O4 sphere is single-crystalline, which shows the diffraction spots representing (220), (311), (511), (110), and (400) lattice planes(Figure 3c). Moreover, Figure 3d displays a HRTEM lattice micrograph of the Co3O4 sphere, and the spacing between the lattice planes is nearly 0.205, 0.247, and 0.321 nm corresponding to the d311, d400, and d511 spacing of spinel-type Co3O4, respectively. To understand the influence of specific surface area on gas sensitivity, the nitrogen isothermal adsorption experiments were run at 77 K (Figure 4). The N2-BET surface area and gas-sensing parameters of Co3O4 samples are calculated in Table 1. By contrast, the Co3O4-350 provides a larger surface for formaldehyde in contact with the Co3O4 samples. The gas-sensing parameters seems to be linked with N2-BET surface area. The bigger the specific surface area of the Co3O4 samples, the more contact bewteen the Co3O4 and formaldehyde, and it will increase the probability of electron exchange to further improve sensitivity. Moreover, the pore size distribution was carried out by the BJH method (Figure 4, inset). The pore diameter of the Co3O4-350 was 0−20 nm, mainly 5 nm, which favors formaldehyde transportation. After the analysis above, Co3O4350 has the largest surfaces area, appropriate microporous and mesoporous structure, and loose surfaces. It is precisely these factors which finally determine the distinctive gas-sensing performance. The surface chemical compositions of sensing materials are of great significance to the final gas-sensing performances. For this purpose, XPS analysis was also conducted (Figure 5, the data for the other two samples are shown in Figures S3 and S4). We show Co2p, O1s, and C1s spectra to further verify the results of XRD analysis in Figure 5a. The results show the existence of Co, O, C, and N elements. In Figure 5b, two strong peaks, at 780.8 eV for Co 2p3/2 and 796.4 eV for Co 2p1/2, are found; meanwhile, the existence of two satellite peaks located (786.8 and 801.9 eV), further confirm the Co3O4 phase. The N1s spectrum shows one peak at ca.400 eV, which shows the existence of N elements. The C1s spectra shows three peaks at 284.5, 285.5, and 288.2 eV in the Co3O4 sample, which are associated with CC, C−O, and C O,19 respectively (Figure 5d). On the basis of the data above, we are convinced that the as-synthesized Co3O4 products are Ndoped Co3O4 compounds. Gas-Sensing Performance Analysis of Co3O4 Sensors. As is well-known, Co3O4 is a kind of p-type semiconductor materials and has been widely used in supercapacitors, lithium-

Inspired by the pioneering work, hierarchical Co3O4 nanoarchitectures have successfully been obtained at appropriate calcinating temperature by utilizing Co5-MOF (Co5(μ3OH)2(1,4-ndc)4(bix)2]n) bar-shaped microcrystal (Scheme 1). Scheme 1. Forming Process of Hierarchical Co3O4-350 Nanoparticles

Hereinto, the spheral Co3O4-350 exhibits excellent formaldehyde vapor sensing performance with lower detecting limits (10 ppm) as well as better long-term stability (at least 30 days), the optimal value among all reported pure Co3O4 sensing materials for the detection of formaldehyde (see the Table S1).

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RESULTS AND DISCUSSION Characterizations of Co3O4 Samples. The structure of the precursor and Co3O4 was determined by powder X-ray diffraction (XRD). As is shown in Figure S1, all peaks of the XRD pattern match well with the simulated one. In Figure 1, the

Figure 1. XRD patterns of the synthesized Co3O4 particles.

diffraction peaks can be indexed to pure spinel-type phase Co3O4 (Powder Diffraction File No. 43−1003, Joint Committee on Powder Diffraction Standards, 1991). The morphologies of the precursor and as-synthesized Co3O4 were presented by optical microscopy and SEM images (Figure 2). It can be seen that stripshaped purple crystals were obtained by experimental steps in advance and most of them were regular morphology (Figure 2a). 14112

DOI: 10.1021/acs.inorgchem.7b02254 Inorg. Chem. 2017, 56, 14111−14117

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Figure 2. (a) Optical microscopy of Co-MOF; (b−d) typical SEM images of the Co3O4-350 sphere and Co3O4-400, Co3O4-450, respectively.

Figure 3. (a) Low-magnification TEM image of the Co3O4-350 sphere; (b) size distribution, based on >100 observed Co3O4-350 species counted from low-magnifications images. (c) SAED pattern recorded from the red circle of panel (a). (d) HRTEM image of the Co3O4-350.

ion batteries, and gas sensor.20−22 Considering that using MOFs as a template of preparating metal oxide nanoparticles has more advantages than traditional inorganic materials synthesis methods, we choose the Co5-cluster MOF as precursor to investigate the sensing performance. Figure 6a depicts the responses of the Co3O4-350 to 200 ppm formaldehyde vapor at different operating temperatures, as well as other benzene, ether, acetone. At any operating temperature we tested, Co3O4-350 always exhibits the highest response to formaldehyde than other vapors. The response value to formaldehyde is about 14, which is 4 times higher than that to benzene vapor and 2 times higher than that to acetone vapor. It should be pointed out that the response of the formaldehyde

varies with the operating temperature notably. Meanwhile, the response to formaldehyde reach a maximum value at about 14; after that, the response starts to decline. This indicates the optimal operating temperature for detecting formaldehyde to Co3O4 samples is 170 °C. In addition, we found that the optimal operating temperature vary with different vapors for the same gas-sensing materials. As is shown in Figure 6b, the Co3O4-350 sensor shows better sensitivity than that of other calcination temperature. The corresponding sensitivity of different concentrations also showed a certain linear relationship. These results illustrated that the special spherical morphology of Co3O4, as well as larger specific surface area, have great influence on the sensitivity and dynamic sensing performances. 14113

DOI: 10.1021/acs.inorgchem.7b02254 Inorg. Chem. 2017, 56, 14111−14117

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stabilities of the Co3O4-350 sensor (Figure 9) exposed to 100 ppm formaldehyde for 30 days. It is obvious that the response values collected every 2 days are almost stable, designating that the microsphere Co3O4-350 sensor displays excellent long-term stability. This may depend on the high thermal stability of mesoporous materials calcinated by Co5-cluster MOF and low operating temperature of the Co3O4-350 sensor. All the data analysis above strongly demonstrated that the special Co3O4 sphere morphology synthesized by calcinating Co5-cluster MOF, the loose and larger surface areas, and the special surface chemical compositions, have jointly decided the better gassensing performance. Gas-Sensing Mechanism of Co3O4 Sensors. As a typical p-type semiconductor, the surface reaction and chemical interaction on the sensing layers has a decisive effect on the sensing characteristics.23 The whole process involves gas adsorption, surface reaction, and desorption processes. When the Co3O4 sensor is exposed to air, oxygen molecules adsorb on the surface forming ionized oxygen species (O−) and holes by capturing electrons from Co3O4 particles (Figure 10, eq i). When the Co3O4-350 sensor was exposed to formaldehyde, the absorbed formaldehyde vapor reacts with the ionized oxygen species and releases the trapped electrons (eq ii), which leads to an increase in the resistance of the Co3O4-350 sensor. As the concentration of 200 ppm formaldehyde was injected into the chamber, the response increase slowly and tends to saturation. The surface adsorbed oxygen species are not ample for the reaction i, subsequently, the surface active sites are nearly fully covered with formaldehyde vapor, which leads to reaction ii moving slowly. Given this, the morphology, pore structure, and specific surface area of sensing materials are of great importance to the sensing response as well as response and recovery time. Co3O4-350 owns special morphology, larger surface area, and unique pore structure, which offer enough chemical reaction sites and gas molecules diffusion path. Thus, Co3O4-350 shows better formaldehyde-sensing performance than not only those materials deriving from the same precursor but also other typical formaldehyde sensing materials.

Figure 4. N2 adsorption−desorption isotherm of Co3O4 samples and BJH pore size distribution plot (inset).

To seek for an excellent sensor at lower and wide detection concentration is of great importance. Herein, we investigated the dynamic response and recovery of the three Co3O4 sensors. As is displayed in Figure 7a, the resistances increases after exposure to formaldehyde vapor; it is obvious that the Co3O4-350 sensor displays an excellent response to formaldehyde vapor (the corresponding resistance change curve of Co3O4-350 is shown in Figures S5−S7, as well as those of other two sensors). When the Co3O4-350 sensor is exposed to air, the response returns nearly to the baseline level. While exposed to formaldehyde vapor, the response value achieves a maximum and maintains a plateau, which can be explained by the saturation of reaction sites changing little and not cause effective conductivity variation. For low formaldehyde concentration sensing characteristics, the response values of the Co3O4-350 sphere that correspond to 2.6, 3.5, 4.9, 7, and 9 for 10, 20, 30, 40, and 50 ppm of formaldehyde vapor, respectively. Meanwhile, for high formaldehyde concentration sensing characteristics, the response values of the Co3O4 sphere that correspond to 11.7, 14.1, 15.1, 16, and 17 for 100, 200, 300, 400, and 500 ppm of formaldehyde vapor, respectively. It is worthwhile mentioning that there is a good linearity between concentration and sensitivity in wide concentration range (Figure 8b, inset). The response and recovery times are also important parameters for a gas sensor. As shown in Figure 8a, the resistance of the Co3O4-350 sensor increases quickly. The response and recovery times were 46 and 98 s. By contrast, the response and recovery times of other sensors are summarized in Table 1. It indicates that the special pore structure and lager surface area is vital to good response capability to formaldehyde vapor. Finally, reproducibility and stability is also an important indicator for gas-sensing devices. In Figure S8, three reversible cycles of the response curve indicated that both sensors have good reproducibility and temporary stability. To explore the stability of the Co3O4-350 sensor in detail, we tested the

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EXPERIMENTAL SECTION

Materials and Preparation. All reagents and solvents were commercially available and used as received. The precursor of CoMOF was prepared according to the literature.18 A mixture of naphthalene-1,4-dicarboxylicacid (0.1 mmol, 21.6 mg), 1,4-bis(imidazol-1-ylmethyl)benze (0.1 mmol, 23.8 mg), Co(ClO4)2·6H2O (0.2 mmol, 96.5 mg), NaOH (0.2 mmol, 8.0 mg), and H2O/EtOH (10 mL, v/v 1:1) were placed in a 25 mL Teflon-lined stainless-steel vessel, heated to 140 °C for 3 days, and then cooled to room temperature over 24 h. After that, the as-prepared Co-MOFs were heated to the desired temperature (350, 400, 450 °C) for 3 h under N2 with a heating rate of 5 °C·min−1 to obtain different Co3O4-x samples (x represents the carbonization temperature), respectively. Physical Measurements. Thermogravimetric analysis was performed on a Netzsch Model STA 449C microanalyzer heated from 25 to 900 °C with a heating rate of 10 °C min−1 in air atmosphere. Powder Xray diffraction (XRD) was studied on a Rigaku Ultima IV diffractometer (Cu Kα radiation, λ = 1.5406 Å). The N2 adsorption−desorption

Table 1. Comparison of N2-BET Surface Area and Gas Sensitivity of Different Co3O4 Samples samples

N2-BET surface (m2·g−1)

mean pore diameter (nm)

sensitivity (100 ppm)

response time (s)

recovery time (s)

Co3O4-350 Co3O4-400 Co3O4-450

30.78 25.52 8.97

25.01 29.98 31.72

12 10 3.2

46 91 100

98 94 111

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DOI: 10.1021/acs.inorgchem.7b02254 Inorg. Chem. 2017, 56, 14111−14117

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Figure 5. (a) XPS spectra of as-obtained Co3O4-350 microspheres: (a) survey, (b) Co 2p spectrum, (c) N 1s spectrum, and (d) C 1s spectrum.

Figure 6. (a) Gas-sensing responses of the Co3O4-350 samples to 200 ppm of ether, formaldehyde, benzene, and acetone at different temperatures. (b) Sensitivity of the Co3O4 samples to different formaldehyde concentration at 170 °C.

Figure 7. Typical response and recovery curves to different formaldehyde low (a) and high (b) concentrations of Co3O4-350 samples at the operating temperature of 170 °C. isotherms were measured on an automatic surface analyzer (SSA-7300, China). Before the measurement, the samples were outgassed at 150 °C

for 5 h. The morphologies and microstructure were investigated using a field-emission scanning electron microscope (FE-SEM, JEOLJSM14115

DOI: 10.1021/acs.inorgchem.7b02254 Inorg. Chem. 2017, 56, 14111−14117

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Inorganic Chemistry

Figure 8. (a) Response and recovery times of the Co3O4-350 sensor to 100 ppm at 170 °C. (b) Sensitivity of the Co3O4 sensor to different formaldehyde vapor concentrations at 170 °C. paste was then dispersed on the commercial ceramic substrates with Ag−Pd interdigitated eletrodes. The prepared sensors were dried at 80 °C in air for 24 h, the gas-sensing performances were experimented by a chemical gas sensor-4 temperature pressure (CGS 4-TPs, Beijing Elite Tech Co., Ltd., China) intelligent gas-sensing analysis system. The test temperature was adjusted from room temperature to 220 °C. Herein, the responses (Rg/Ra) of the sensors were defined as the ratio of the resistance of the Co3O4 materials in air and in target gas atmosphere. The responce time and recovery times are defined as the resistances of sensors changes 90% of the whole resisitances during the target gas occurrence and disappearance.

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CONCLUSION We have designed and prepared Co3O4 samples by calcinating Co5-MOF at different temperature. Among them, the spherical Co3O4-350 exhibits excellent formaldehyde sensing performance at low operating temperature, lower detecting limits (10 ppm), and better long-term stability (at least 30 days), which exceed those of Co3O4 particles reported in previous literature. This strategy is further confirmed that MOFs, especially Co-clusters MOFs, could be used as precursor to synthesize highperformance 3D nanostructure metal oxide materials. This facile method can be expanded to obtain metal oxides to explore multifunctional applications, such as fuel cells, lithium-ion batteries, and oxygen-evolution reaction and so on.

Figure 9. Stabilities of the Co3O4-350 sensors injected 100 ppm formaldehyde vapor at 170 °C for 30 days.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02254.

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Table, SEM images of the Co3O4, response-recovery curve, linear dependence of response curves (PDF)

AUTHOR INFORMATION

Corresponding Authors

Figure 10. Schematic illustration of formaldehyde-sensing mechanism for Co3O4-350.

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

6700F, Japan) operating at 15 kV and a transmission electron microscopy (TEM, JEOLJEM-2010F) with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopic (XPS) was performed using an ESCALAB 250 with a monochromatic Al Kα X-ray source. Fabrication and Measurement of Gas Sensors. Co3O4 samples were dispersed in the absolute ethanol to form a paste, and the prepared

Xianhui Bu: 0000-0002-2994-4051 Dong-Sheng Li: 0000-0003-1283-6334 Notes

The authors declare no competing financial interest. 14116

DOI: 10.1021/acs.inorgchem.7b02254 Inorg. Chem. 2017, 56, 14111−14117

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Solutions: Transformation of Co/Ni-MOF-74 to Co x Ni 1−x @ CoyNi1−yO@C for the Oxygen Evolution Reaction. Inorg. Chem. 2017, 56, 5203−5209. (12) Liu, J.; Zhao, Q. L.; Wu, C.; Wang, Y.; Wei, W.; Wang, X. Y.; Yi, L. H. Performance improvement of activated nanoporous carbon supported gold catalyst as an anode for direct borohydride-hydrogen peroxide fuel cells. RSC Adv. 2014, 4, 17129−17135. (13) Cao, K. Z.; Jiao, L. F.; Xu, H.; Liu, H. Q.; Kang, H. Y.; Zhao, Y.; Liu, Y. C.; Wang, Y. J.; Yuan, H. T. Reconstruction of Mini-Hollow Polyhedron Mn2O3 Derived from MOFs as a High-Performance Lithium Anode Material. Adv. Sci. 2016, 3, 1500185. (14) Maiti, S.; Pramanik, A.; Mahanty, S. Extraordinarily High Pseudocapacitance of Metal Organic Framework Derived Nanostructured Cerium Oxide. Chem. Commun. 2014, 50, 11717−11720. (15) Lü, Y. Y.; Zhan, W. W.; He, Y.; Wang, Y. T.; Kong, X. J.; Kuang, Q.; Xie, Z. X.; Zheng, L. S. MOF-Templated Synthesis of Porous Co3O4 Concave Nanocubes with High Specific Surface Area and Their Gas Sensing Properties. ACS Appl. Mater. Interfaces 2014, 6, 4186−4195. (16) Koo, W. T.; Yu, S.; Choi, S. J.; Jang, J. S.; Cheong, J. Y.; Kim, I. D. Nanoscale PdO Catalyst Functionalized Co3O4 Hollow Nanocages using MOF Templates for Selective Detection of Acetone Molecules in Exhaled Breath. ACS Appl. Mater. Interfaces 2017, 9, 8201−8210. (17) Wu, Y. P.; Zhou, W.; Dong, W. W.; Zhao, J.; Qiao, X. Q.; Hou, D. F.; Li, D. S.; Zhang, Q. C.; Feng, P. Y. Temperature-Controlled Synthesis of Porous CuO Particles with Different Morphologies for Highly Sensitive Detection of Triethylamine. Cryst. Growth Des. 2017, 17, 2158−2165. (18) Li, D. S.; Zhao, J.; Wu, Y. P.; Liu, B.; Bai, L.; Zou, K.; Du, M. Co5/ Co8-Cluster-Based Coordination Polymers Showing High Connected Self-Penetrating Networks: Syntheses, Crystal Structures and Magnetic Properties. Inorg. Chem. 2013, 52, 8091−8098. (19) Nie, R. F.; Shi, J. J.; Du, W. C.; Ning, W. S.; Hou, Z. Y.; Xiao, F. S. A sandwich N-doped graphene/Co3O4 hybrid: an efficient catalyst for selective oxidation of olefins and alcohols. J. Mater. Chem. A 2013, 1, 9037−9041. (20) Salunkhe, R. R.; Tang, J.; Kamachi, Y.; Nakato, T.; Kim, J. H.; Yamauchi, Y. A Concept for Asymmetric Supercapacitors Using 3D Nanoporous Carbon and Cobalt Oxide Electrodes Synthesized from Single Metal Organic Framework. ACS Nano 2015, 9, 6288−6296. (21) Liu, B.; Zhang, X. B.; Shioyama, H.; Mukai, T.; Sakai, T.; Xu, Q. Converting cobalt oxide subunits in cobalt metal-organic framework into agglomerated Co3O4 nanoparticles as an electrode material for lithium ion battery. J. Power Sources 2010, 195, 857−861. (22) Wang, C. X.; Yin, L. W.; Zhang, L. Y.; Xiang, D.; Gao, R. Metal Oxide Gas Sensors: Sensitivity and Influencing Factors. Sensors 2010, 10, 2088−2106. (23) Wang, C.; Cheng, X. Y.; Zhou, X.; Sun, P.; Hu, X. L.; Shimanoe, K.; Lu, G. Y.; Yamazoe, N. Hierarchical α-Fe2O3/NiO Composites with a Hollow Structure for a Gas Sensor. ACS Appl. Mater. Interfaces 2014, 6, 12031−12037.

ACKNOWLEDGMENTS This work was financially supported by the NSF of China (Nos. 21373122, 21673127, 51572152, 51502155, and 21671119).

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REFERENCES

(1) Vellingiri, K.; Deep, A.; Kim, K. H.; Boukhvalov, D. W.; Kumar, P.; Yao, Q. The sensitive detection of formaldehyde in aqueous media using zirconium-based metal organic frameworks. Sens. Actuators, B 2017, 241, 938−948. (2) Wang, S. M.; Cao, J.; Cui, W.; Li, X. F.; Li, D. J. Facile synthesis and excellent formaldehyde gas sensing properties of novel spindle-like In2O3 porous polyhedral. Sens. Actuators, B 2016, 237, 944−952. (3) Li, Y. X.; Chen, N.; Deng, D. Y.; Xing, X. X.; Xiao, X. C.; Wang, Y. D. Formaldehyde detection: SnO2 microspheres for formaldehyde gas sensor with high sensitivity, fast response/recovery and good selectivity. Sens. Actuators, B 2017, 238, 264−273. (4) Lai, X. Y.; Shen, G. X.; Xue, P.; Yan, B. Q.; Wang, H.; Li, P.; Xia, W. T.; Fang, J. Z. Ordered Mesoporous NiO with Thin Pore Walls and Their Enhanced Sensing Performance for Formaldehyde. Nanoscale 2015, 7, 4005−4012. (5) (a) Tian, H. L.; Fan, H. Q.; Li, M. M.; Ma, L. T. Zeolitic Imidazolate Framework Coated ZnO Nanorods as Molecular Sieving to Improve Selectivity of Formaldehyde Gas Sensor. ACS Sens 2016, 1, 243−250. (b) Long, H.; Harley-Trochimczyk, A. H.; Cheng, S. Y.; Hu, H.; Chi, S. W.; Rao, A.; Carraro, C.; Shi, T. L.; Tang, Z. R.; Maboudian, R. Nanowire-Assembled Hierarchical ZnCo2O4 Microstructure Integrated with Low Power Microheater for Highly Sensitive Formaldehyde Detection. ACS Appl. Mater. Interfaces 2016, 8, 31764−31771. (6) Zhang, Y. M.; Zhang, J.; Zhao, J. H.; Zhu, Z. Q.; Liu, Q. J. AgLaFeO3 fibers, spheres, and cages for ultrasensitive detection of formaldehyde at low operating temperatures. Phys. Chem. Chem. Phys. 2017, 19, 6973−6977. (7) Wang, Y.; Liu, J. F.; Wang, M.; Pei, C. J.; Liu, B.; Yuan, Y. K.; Liu, S. Z.; Yang, H. Q. Enhancing the Sensing Properties of TiO2 Nanosheets with Exposed {001} Facets by a Hydrogenation and Sensing Mechanism. Inorg. Chem. 2017, 56, 1504−1510. (8) (a) Xia, W.; Mahmood, A.; Zou, R. Q.; Xu, Q. Metal-Organic frameworks and their derived nanostructures for electrochemical energy storage and conversion. Energy Environ. Sci. 2015, 8, 1837−1866. (b) Chen, E. X.; Fu, H. R.; Lin, R.; Tan, Y. X.; Zhang, J. Highly selective and sensitive trimethylamine gas sensor based on cobalt imidazolate framework material. ACS Appl. Mater. Interfaces 2014, 6, 22871−22875. (c) Chen, E. X.; Yang, H.; Zhang, J. Zeolitic imidazolate framework as formaldehyde gas sensor. Inorg. Chem. 2014, 53, 5411−5413. (d) Cao, X. H.; Tan, C. L.; Sindoro, M.; Zhang, H. Hybrid micro-/nanostructures derived from metal-organic frameworks: preparation and applications in energy storage and conversion. Chem. Soc. Rev. 2017, 46, 2660−2677. (e) Salunkhe, R. R.; Kaneti, Y. V.; Yamauchi, Y. MetalOrganic Framework-Derived Nanoporous Metal Oxides toward Supercapacitor Applications: Progress and Prospects. ACS Nano 2017, 11, 5293−5308. (9) (a) Wang, L.; Han, Y. Z.; Feng, X.; Zhou, J. W.; Qi, P. F.; Wang, B. Metal-organic frameworks for energy storage: Batteries and supercapacitors. Coord. Chem. Rev. 2016, 307, 361−381. (b) Wu, Y. P.; Zhou, W.; Zhao, J.; Dong, W. W.; Lan, Y. Q.; Li, D. S.; Sun, C.; Bu, X. Surfactant-assisted phase-selective synthesis of new cobalt MOFs and their efficient electrocatalytic hydrogen evolution reaction. Angew. Chem., Int. Ed. 2017, 56, 13001−13005. (10) Kaneti, Y. V.; Tang, J.; Salunkhe, R. R.; Jiang, X. C.; Yu, A. B.; Wu, K. C. W.; Yamauchi, Y. Nanoarchitectured Design of Porous Materials and Nanocomposites from Metal-Organic Frameworks. Adv. Mater. 2017, 29, 1604898. (11) (a) Tang, H. L.; Cai, S. C.; Xie, S. L.; Wang, Z. B.; Tong, Y. X.; Pan, M.; Lu, X. H. Metal-Organic-Framework-Derived Dual Metal- and Nitrogen-Doped Carbon as Efficient and Robust Oxygen Reduction Reaction Catalysts for Microbial Fuel Cells. Adv. Sci. 2016, 3, 1500265. (b) Sun, D. R.; Ye, L.; Sun, F. X.; García, H.; Li, Z. H. From Mixed-Metal MOFs to Carbon-Coated Core-Shell Metal Alloy@Metal Oxide Solid 14117

DOI: 10.1021/acs.inorgchem.7b02254 Inorg. Chem. 2017, 56, 14111−14117