MetalâOrganic Frameworks-Derived Hierarchical Co3O4 Structures as

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Metal-Organic Frameworks (MOFs) Derived Hierarchical Co3O4 Structures as Efficient Sensing Materials for Acetone Detection Rui Zhang, Tingting Zhou, Lili Wang, and Tong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17669 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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ACS Applied Materials & Interfaces

Metal−Organic Frameworks (MOFs) Derived Hierarchical Co3O4 Structures as Efficient Sensing Materials for Acetone Detection Rui Zhang, Tingting Zhou, Lili Wang,* Tong Zhang* State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China

E-mail address: [email protected] and [email protected] *Corresponding author: E-mail address: [email protected] and [email protected]

KEYWORDS Metal-Organic Framework, ZIF-67, Core shell, Gas sensor, Pore-rich, High Performance

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ABSTRACT:

High sensitive and stable gas sensors have attracted much attention because they are the key to innovations in the fields of environment, health, energy savings and security, etc. Sensing materials, which influence the practical sensing performance, are the crucial parts for gas sensors. Metal-organic frameworks (MOFs) are considered as alluring sensing materials for gas sensors due to the possession of high specific surface area, unique morphology, abundant metal sites and functional linkers. Herein, four kinds of porous hierarchical Co3O4 structures have been selectively controlled by optimizing the thermal decomposition (temperature, rate and atmosphere) using ZIF-67 as precursor that was obtained from coprecipitation method with the co-assistant of cobalt salt and 2-methylimidazole in the solution of methanol. These hierarchical Co3O4 structures, with controllable cross-linked channels, meso-/micropores, and adjustable surface area, are efficient catalytic materials for gas sensing. Benefits from structural advantages, core shell and porous core shell Co3O4 exhibit enhanced sensing performance than those of porous popcorn and nanoparticle Co3O4 to acetone gas. These novel MOF templated Co3O4 hierarchical structures are so fantastic that can be expected as efficient sensing materials for development of low temperature operating gas sensor.


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1.

INTRODUCTION

Cobalt oxides, as one of the most common and applied non-noble compound, are designed and applied for gas sensing device through fine controlling morphology and composition. In particular, tricobalt tetraoxygen (Co3O4) stands out because of its excellent catalytic activity as a robust electrocatalyst to offer significant potential in various fields, especially in the field of gas sensors.1-4 However, the poor response of p-type metal oxide semiconductor (MOS)based gas sensors are regarded as big obstacles for the purpose of actual commercialization compared with that of n-type MOS. The synthesis of high-sensitive sensing materials is urgently demanded and have become an active research area in order to further develop high performance sensing devices.5-8 Gas sensors operate based on the mechanism of reaction and adsorption/desorption of gas molecular happening on the surface of sensing layer. Therefore, the suitable sensing materials for gas sensor should have a large surface area, proper pore size and excellent electrical conductivity. Metal-organic frameworks (MOFs) are constructed by metal ions and organic linkers. And because of the enriched organic ligands in the MOFs structure, it has been claimed to be promising meso/micro-porous materials, which possess low density, ultra-high porosities and high specific surface areas.9-10 MOFs can be customized in accordance with their final application, for example, tailoring their pore sizes, choosing specific metal sites, controlling surface area, etc.11,

12

However, microporous regime, which restrains their interactions with

active sites of MOFs and the diffusion of chemical species, limits its wide applications in spite of the unique structural features.13 They could be easily converted to a series of derivatives by heating treatment, including porous carbons, metal/metal oxide, carbon/metal oxide hybrids.14, 15

Usually, MOF templated derived porous or hollow MOSs own well-defined morphology and

inherit the porosity from MOFs with innumerable reaction site and gas accessibility, leading to


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high pore volumes, periodic alternation and extraordinary tenability. Attributed to these advantages, MOSs derived from MOFs are investigated in excellent applications, such as lithium-ion batteries, electrochemical sensing, supercapacitors, solar cells and so on.11, 13,

16-19

Recently, Li and co-workers synthesized ZIF-67-derived Co3O4 honeycomb-like structures only through a simple one-step calcination process for electrochemical water splitting.20 Moreover, MOF templated MOS-based chemical gas sensors have been reported in some articles to utilize its high surface area. Li et al. prepared MOF-5-derived ZnO hollow nanocages, which performed sub-ppm level sensitivity towards benzene and acetone.21 Zhou et al. fabricated 3D hierarchical Co3O4 nano-architectures by calcining Co5-MOF and investigated sensing performance in detecting formaldehyde. 22 Herein, this work reported that using MOF (ZIF-67) as precursor template could directly synthesize porous hierarchical Co3O4 structures. By controlling calcination environment, the morphology, specific surface area and pore size of Co3O4 materials can be tuned, enabling efficient gas sensing response. The as-synthesized Co3O4 products present the core shell (CS), porous core shell (PCS), porous popcorn (PPC) and nanoparticle (NP) morphologies. 2.

EXPERIMENTAL SECTION

2.1. Materials. Cobaltous nitrate hexahydrate (Co(NO3)2•6H2O, 98.5%), 2-methylimidazole (C4H6N2, 98%) and methanol (CH3OH, 99.5%) were the starting chemicals. All of them were of analytical grade in this study and used without further purification. 2.2. Synthesis Process. Preparation of ZIF-67 templates: A simple and reproducible coprecipitation method was carried in order to obtain uniform ZIF-67 templates,20 2 mmol of Co(NO3)2•6H2O was dissolved in 50 mL of methanol and 8 mmol of 2-methylimidazole were dissolved in 50 mL of methanol


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under continuous stirring in the meanwhile. Then the solution was mixed. After three days, purple precipitate appeared. The solid product was collected and washed using methanol. And the final product was ZIF-67 templates. The four Co3O4 products with different morphologies were shaped via one-step pyrolysis of ZIF-67 templates. The thermal decomposition temperature was on the basis of the thermogravimetric analysis (TGA) results. In a typical procedure, a little of ZIF-67 was placed in a muffle furnace under controlling a certain temperature, calcining rate and gas condition. Synthesis of Core shell Co3O4 (CS-Co3O4) structures derived from ZIF-67: The furnace was set to heat to the stated temperature of 350 °C under air flow for 3 h at a rate of 1 °C min-1. Synthesis of Porous core shell Co3O4 (PCS-Co3O4) structures derived from ZIF-67: The furnace was set to heat to the stated temperature of 350 °C under air flow for 3 h at a rate of 10 °C min-1. Synthesis of Porous popcorn Co3O4 (PPC-Co3O4) structures derived from ZIF-67: The furnace was set to heat to the stated temperature of 500 °C under N2 flow for 1 h and then under air flow for 3 h at a rate of 1 °C min-1. Synthesis of Nanoparticle Co3O4 (NP-Co3O4) structures derived from ZIF-67: The furnace was set to heat to the stated temperature of 600 °C under air flow for 3 h at a rate of 10 °C min-1. 2.3. Characterization. The samples were characterized by X-ray diffraction patterns (XRD) via a Rigake D/Max2550 diffractometer with Cu Kα radiation (0.15418 nm) at a scanning rate of 7° min-1, JEOL JSM-7500F (FESEM) and Tecnai G2 20S-Twin microscope (HRTEM). The composition analysis was obtained by a PREVAC X-ray photoelectron spectrometer (XPS). The specific


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surface area of Brunauer-Emmett-Teller (BET) was conducted by N2 gas adsorption and desorption using a JW-BK132F analyzer.

Scheme 1. Schematic illustration of the formation of different Co3O4 structures. Detail experimental conditions for the Co3O4 products are shown in Scheme 1. Through controlling the pyrolysis temperature, rate and air condition, four kinds of Co3O4 with different morphologies were obtained, and the collected products were all black powder. 2.4. Fabrication and measurement of the gas sensor. The as-synthesized samples were mixed with deionized water (at a weight ratio of 1:4) and ground to form black paste with good consistency subsequently. Next, the paste was dropped on a hollow Al2O3 tube (outer diameter= 1.2 mm, inner diameter= 0.8 mm), which is printed with two parallel Au electrodes (distance=1 mm) and inserted with a Ni-Cr microheater in the hollow section (Figure S1). The electrical properties of the as-fabricated gas sensors were conducted to diverse target gases (acetone, xylene, ammonia, benzene, methanal and trimethylamine) at 160−230 °C. The data collection was realized on an acquisition of CGS-8


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(Beijing Alite). The sensing response (S=Rg/Ra) was defined as the ratio of resistance in gas (Rg) to that in air (Ra) in this study. Moreover, the response/recovery times were measured by the times cost to achieve 90 % of the total resistance changes when analytes were injected and withdrew, respectively. 3.

RESULTS AND DISCUSSIONS 3.1. Structural and Morphological Characteristics. ZIF-67 templates were conducted to transform to Co3O4 powders after the calcination

above 350 °C. During this process, metal ions (cobalt ions) were oxidized to cobalt oxide and simultaneously the organic linkers were decomposed, resulting in formation of porous structures. The formation mechanism of the products with different morphologies derived from ZIF-67 were demonstrated as followed: the ball-in-dodecahedron hollow Co3O4 obtained from one step calcination of ZIF-67 dodecahedrons is supposed to be similar to the formation process of yolk-shell structures derived from solid precursors through thermal decomposition. Non-equilibrium heat treatment may cause a heterogeneous contraction process, which could be utilized to illustrate this phenomenon.23 During the non-equilibrium and heterogeneous heating process (1 °C min-1), two kinds of forces (cohesive and adhesive forces) formed in the opposite direction at the interface of the Co3O4 shell and the core because of the gradual decomposition of ZIF-67. The adhesive force may prevent the inward contraction of the Co3O4 resulting in the formation of outer shell. In the opposite, the cohesive force produces the inward contraction of the inner Co3O4 core. Finally, large cavities appeared between the shell and the core of Co3O4. Compared with core-shell (CS and PCS) structure, PPC-structure was obtained by addition of procedure of presintering in nitrogen. It could be proved by DTG that ZIF-67 only lost a small amount of weight (about 6 %) at about 500 °C in N2. Under the protection of nitrogen, the organic ligands may cross link with each other to form partial


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network as supported materials instead of direct decomposition firstly. Then the calcination process in air removed the organic fraction to output Co3O4 with the PPC-structure. It should be mentioned that regulation of calcination gradient could not change the core shell structure in this study. Partly fragmentary PCS-Co3O4 was observed at a larger calcination gradient. However, when the calcination temperature increased, the inner vacancy owned high thermal energy and diffused to the outer surface, leading to the collapse of hollow structure, and NPstructure was observed.24 In particular, when used as sensing materials in acetone detection, the CS-structure showed the best performance among these four structures because the multilevel structure could not only maintain a stable structure but also offer a high contact area between the material surface and gas molecules.

Figure 1. (a) The photograph of ZIF-67 dispersed in solvent; (b) SEM and (c) TEM image of ZIF-67 rhombic dodecahedron; (d) N2 adsorption-desorption isotherm (Inset is the pore size distribution); (e) XRD pattern and (f) corresponding ideal geometrical model of ZIF-67.


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Through mixing cobalt acetate and 2-methylimidazole in the solution of methanol under no interruption of 3 days, monodispersed ZIF-67 rhombic dodecahedrons were obtained (Figure 1a). The template is composed of rough porous polyhedron with an average size of about 2.5 µm and exhibits solid structure (Figure 1b-c and Figure S2). In addition, a welldefined morphology of rhombic dodecahedral with straight edges and facets presented. Moreover, the porous structure of ZIF-67 was analyzed using by the test curves of the nitrogen adsorption−desorption isotherm (Figure 1d). In particular, N2 uptake appeared under relatively low pressure, indicating that the ZIF-67 is regarded as a microporous material (type-I behavior)25 with a high BET surface area of 1460 m2g−1.

Figure 2. SEM images of the as-synthesized Co3O4 products derived from ZIF-67: (a) CSCo3O4, (b) PCS-Co3O4, (c) PPC-Co3O4, and (d) NP-Co3O4; (a1−d1) TEM image of various Co3O4 structures; (a2−d2) Corresponding ideal geometrical models of individual Co3O4 structures (a3−d3) HRTEM images pointed out by red boxes in (a1−d1); (a4−d4) SAED pattern of single Co3O4 structure; (a5-d5) Size distributions of as-synthesized Co3O4 product.


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X-ray diffraction (XRD) pattern of ZIF-67 was presented in Figure 1e. All the observed diffraction peaks of the formed ZIF-67 in the 2θ range of 8-50° match well with the simulated published results in the literature.20, 26, 27 Figure 1f shows the ideal dodecahedral structure of ZIF-67. It could be seen that the vertex shares three or four edges and all the faces are rhombic. And the observed SEM image was coincident with the simulated structure. It is worthwhile to note that the morphologies of Co3O4 were well-designed by changing the atmosphere and temperature of heating treatment. The microstructure and morphology of the as-synthesized Co3O4 samples were characterized by SEM, TEM, HRTEM and SAED in Figure 2. It demonstrated that the effective maintain of morphological profile of the ZIF-67 could be obtained through controlling thermal decomposition condition of precursor to different degrees (Figure 2a-c). When observed in detail, the SEM (TEM) images of the CS-Co3O4, PCS-Co3O4, PPC-Co3O4 and NP-Co3O4 were exhibited in panels a (a1), b (b1), c (c1) and d (d1) of Figure 2, respectively. Moreover, significant shrinkage in size was observed in Co3O4 products after thermal decomposition. The average apex to apex lengths of the four final products were around 1.7 µm, 1.5 µm, 1.6 µm and 150 nm (Figure 2a5-d5), respectively. For core-shell (CS and PCS) structures, the void spaces and shells could be clearly distinguished by the dark edges and bright regions (Figure 2a1 and b1), and it was apparent that the interior contain near-spherical cores along with a vast space between the core and shell. However, PCS-Co3O4 exhibited a morphology of interconnecting nanoparticles with each other, forming enormous opened cracks, holes and interstitial structures on the exterior surfaces. In addition, PPC-Co3O4 presented a homogeneous dodecahedral morphology with concave porous surfaces (Figure 2c and c1). The obtained nanoparticles are in serious aggregation with an average size of 150 nm approximately (Figure 2d). The HRTEM images in Figure 2a3-d3 are taken from the single nanoparticles pointed out by red cycles in Figure a1-d1. 0.16 nm, 0.29


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nm and 0.47 nm interval of the lattice fringes agree well with the spacing of (511), (220) and (111) planes of Co3O4 (JCPDS no. 42-1467), respectively. The selected area electronic diffraction (SAED) patterns of as-synthesized Co3O4 were given and shown in Figure 2a4-d4. For CS-Co3O4 and PPC-Co3O4, a series of discontinuous concentric rings presented, which indicated that the primary nanocrystals are attached with each other randomly. For PCSCo3O4 and NP-Co3O4, many scattered points were observed, revealing the quasimonocrystalline structure. Based on the above observation, it is rationally deduced that thermal treatment could induce and realize the release of a large number of small molecules from the interior of ZIF-67, leading to a remarkable porosity left. Different from ZIF-67, the four kinds of MOF-derived Co3O4 structures displayed a mesoporous structure (type IV behavior)25 with a BET surface area of 44.5, 42.4, 42.8 and 12.2 for CS-Co3O4, PCS-Co3O4, PPC-Co3O4, and NP-Co3O4, respectively (Figure 3a). The peaks of the most pore size are centered at about 3.0 nm (Figure 3b). It is believed that the obtained mesoporous structures are in favour of access and departure of gas molecules on the surface regions of Co3O4 samples, which could facilitate a fast mass transfer.28 The thermogravimetric analysis (TGA) curves of the ZIF-67 templates were obtained through thermal treatment in nitrogen and in air at a heating rate of 10 °C min-1 (Figure 3c). As a results, the template showed different thermal stability in nitrogen and in air. ZIF-67 presents thermally stable till a rather high temperature of about 470 °C in nitrogen. Both morphology and composition of ZIF-67 was changeless after calcination in N2 for 2 h at 300 °C as shown in Figure 3S. However, the templates underwent fast weight loss at around 320 °C in air, which may be due to the oxidation and decomposition of 2-methylimidazole. The weight loss of ZIF67 templates dropped sharply and finally flattens at about 350 °C till 42.2 %. Such a total weight change in the decomposition procedure well agrees well with the weight loss of


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transformation from ZIF-67 to Co3O4.26,

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Thus, it were believed to decompose into Co3O4

products at a chosen temperature of 350 °C in air. The above results indicated that ZIF-67 framework is sensitive to the existence of oxygen at higher temperature based on the phenomenon of lower thermal stability in air than that in nitrogen.

Figure 3. (a) N2 adsorption-desorption isotherms and (b) pore size distribution of MOF-derived Co3O4 structures; (c) TG curves of ZIF-67 under different atmosphere; (d) XRD patterns of the as-synthesized Co3O4 samples and XPS spectra of CS-Co3O4: (e) Co 2p, and (f) O 1s. In order to further confirm the conversion of ZIF-67 after calcination, X-ray powder diffraction (XRD) peaks of the MOF-derived as-synthesized samples were shown in Figure 3d.


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It could be seen that all peaks of the as-synthesized samples could be ascribed to the cubic Co3O4 (JCPDS no. 42-1467), indicating that ZIF-67 has been transformed into the Co3O4 successfully. In addition, X-ray photoelectron spectroscopy (XPS) analysis gives out the chemical and electronic states of the CS-Co3O4 architectures. The binding energies were standardized through referencing them to the C 1s peak (284.5 eV) in the XPS analysis. The full XPS survey scan spectra were shown in Figure S4: the prominent peaks of C, O and Co elements were contained. Figure 3e presents the high-resolution spectrum of Co 2p. Two prominent peaks, with a spin-orbit splitting of around 15.0 eV, were shown at about 796 and 781 eV, which were assigned to the Co 2p1/2 and Co 2p3/2 peaks, respectively. Co 2p3/2, Co3+ (780.52 eV) and Co2+ (782.67 eV) included, was observed at the binding energy of around 781 eV. While, Co 2p1/2, Co3+ (795.58 eV) and Co2+ (797.33 eV) included, was observed at the binding energy of about 796 eV as a shoulder peak.29-31 Figure 3f presents the high-resolution spectrum of O 1s. Two fitted peaks of binding energy at about 532.35 eV and 530.39 eV were attributed to the chemisorbed oxygen and lattice oxygen species in the CS-Co3O4 architectures, respectively.27, 32 3.2. Gas Sensing Properties. It is known that the gas sensing properties are mightily dependent on operating temperature. Thus, responses to 200 ppm acetone of the four sensors were investigated under different temperatures and shown in Figure 4a. For all sensors, the responses increase to reach a maximum at 190 °C and then decrease during 160 °C -230 °C. It can be explained as follow: Lower operating temperature bring out slow diffusion of gas molecules and low chemical activation of Co3O4 sensing surface, resulting in a poor response to acetone. However, higher operating temperature may endow adsorbed gas molecules with high activation, and they may escape from sensing surface easily, leading to a decreasing


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response as well. In addition, the maximal values of response for CS-, PCS-, PPC- and NPCo3O4 are 13, 11, 7.9 and 2.6, respectively. Thus, further gas sensing measurements were all conducted at the optimum operating temperature of 190 °C. Table 1. Comparison of acetone-sensing properties of Co3O4-based sensors. Materials

Structure

BET/ [m2g-1]

R0/[kΩ]

Res.

Tr1/[s]

Tr2/[s]

Co3O4

core shell

44.5

10.6

13

4

8

Co3O4

porous core shell

42.4

11.7

11

4

8

Co3O4

porous popcorn

42.8

8.6

7.9

47

33

Co3O4

nanoparticle

12.2

6.0

2.6

106

252

♦R0: Resistance in air; ♦Res.: Response; ♦Tr1: Response time; ♦Tr2: Recovery time Selectivity of CS-Co3O4-based sensor to 200 ppm of various analytes at 190 °C was shown in Figure 4b. Compared with other gases, the sensor exhibits the highest response to acetone. In addition, the resistances of CS-, PCS-, PPC- and NP-Co3O4 increase quickly when they exposed to reducing acetone, and then decrease with the separation of acetone gas (Figure 4c-d and Figure S5). This result indicated the typical p-type property of MOS.33 CSand PCS-Co3O4 showed fast response/recovery speed to acetone with the same values (4 s/8 s). However, PPC- and NP-Co3O4 exhibited longer response/recovery times of 47 s/33 s and 106 s/252 s, respectively. The related sensing performances of the four sensors towards acetone in this work are summarized in Table 1. The slower response and recovery speed of PPC- and NP-Co3O4-based sensors could be ascribed to the retention of gas molecules. That is, the different morphology features of the as-synthesized Co3O4 samples will bring out an irregular response/recovery character.34


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Figure 4. (a) Responses to 200 ppm acetone vs. operating temperature and (b) polar graphs to different target gases of CS-, PCS-, PPC- and NP-Co3O4 based sensors; Dynamic resistance curves of the (c) CS-Co3O4 and (d) PCS-Co3O4 based sensors towards 200 ppm acetone at 190 °C; (e) Response/recovery graphs and (f) relationship of responses vs. the acetone concentration of CS-Co3O4 and PCS-Co3O4 based sensors at 190 °C. Dynamic response and recovery curves of the CS- and PCS-Co3O4-based sensors to 10500 ppm acetone were shown in Figure 4e. It can be seen that when acetone was injected into the tested chamber, the responses of the sensors rapidly increased, and the response values


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varied with the concentration changing. Thus a stepwise distribution of the curves appeared, which indicated that the core-shell (CS and PCS) structures are sensitive to acetone. The calculated CS- and PCS-Co3O4-based sensor responses are plotted in Figure 4f for quantitative and clear descriptions. It could be clearly seen that the gas responses of CSCo3O4- and PCS-Co3O4-based sensors increase rapidly when exposed to 10-100 ppm of acetone, but the responses increased slowly with the increasing acetone concentration to reach saturate at 200-2000 ppm. This is possibly due to the lack of surface adsorbed oxygen species when the surface was covered with sufficient target gas molecules, leading to a slower growth of response. The core-shell (CS and PCS) structures-based sensors showed enhanced acetone sensing performance compared to those of reported in literature, as shown in Table 2,

1, 34-36

which gives out the promising application of the MOS-based sensors in trace

acetone monitoring. Table 2 Sensing properties of various Co3O4-based sensors to acetone in the present study and literature. Materials

Structure

Tem./ [ºC]

Con./[ppm]

Res.

Ref.

Co3O4

nanorod

240

1000

13.5

[34]

Co3O4

nanosphere

100

1000

7.5

[35]

Co3O4

nanocube

200

100

3.2

[36]

Co3O4

nanosphere

220

500

2.9

[1]

Co3O4

nanocube

240

500

4.88

[1]

Co3O4

core shell

190

200

13

[This work]

Co3O4

porous core shell

190

200

11

[This work]

♦Tem: Working temperature; ♦Con.: Concentration; ♦Res.: Response; ♦Ref.: Reference


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Figure 5. (a) Eight response cycles and (b) long-term stability of the sensor based on CS- and PCS-Co3O4 towards 200 ppm acetone at 190 °C; (c) Schematic of sensing mechanism of Co3O4 samples. The fluctuation of the responses is a big obstacle for gas sensor in practical applications, thus the continuous measurements of the sensors were conducted within eight cycles (Figure 5a). The results distinctly indicated that the responses of the sensors could be retained well, which confirmed the superior stability of the sensors. In order to evaluating the long term stability of CS- and PCS-Co3O4-based sensors, continuous tests of repeated every two days were carried out in a month when exposed to 200 ppm acetone. As shown in Figure


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5b, it could be observed that both sensors exhibit excellent reproducibility with small deviations. In order to research the structural stability of the as-synthesized Co3O4 materials, SEM and TEM images of the four kinds of Co3O4 structures were provided after repeated sensing measurements to acetone (Figure S6). Most of Co3O4 materials retain their original morphologies and little Co3O4 were observed disintegrated and cracked in view. The above results suggested a good stability of the Co3O4 materials. 3.3. Gas Sensing Mechanism. The reasonable devotions of such excellent performances of as-synthesized porous Co3O4 derived from ZIF-67 to acetone are attributed to two factors. Firstly, it is known that sensing properties of MOS-based gas sensors are seriously affected by the surface reaction between sensing materials and target gases taking place on the sensing layers. That is to say, the sensing performance largely depends on the structure, size, surface state and morphology of the sensing materials.2 In this work, CS-, PCS- and PPC-Co3O4 have porous structures with the specific surface areas of 44.5, 42.8, 42.4 m2g-1, respectively, which are larger than that of NP-Co3O4 (12.2 m2g1

). The larger surface allows to adsorb more gas molecules, leading to higher response at the

same testing condition. Thus, the responses of CS- (13), PCS- (11) and PPC- (7.9) Co3O4based sensors are higher than that of NP-Co3O4 (2.6). However, although PPC-Co3O4 shows a porous hierarchical structure, lack of big cavity leads to difficulty diffusing throughout the particles, resulting in a decrease of the response/recovery speed (47 s/33 s) than that of CSCo3O4 (4 s/ 8 s) and PCS- Co3O4 (4 s/ 8 s). Moreover, the interior structure of NP-Co3O4 is so compact compared with the other three porous samples according to the TEM analysis (Figure 2a1-d1). Thus, the sensing speed of the NP-Co3O4 (106 s/ 252 s) was so long due to the


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detained gas diffusion. Because of the difference in morphological characteristics, the assynthesized Co3O4 samples presented different sensing properties. Secondly, hole is the charge carrier of p-type Co3O4, the sensing mechanism of which is mainly attributed to surface conductivity modulation through adsorption and desorption process.34, 37 When oxygen molecules chemisorbed on Co3O4, hole accumulation layers are created on the surface of Co3O4. Thus, Co3O4-based sensors often show a highly conductive state in air because of the rich existence of holes. Next, when reductive acetone molecules introduced, reaction happened between oxygen species and sensing material surface. And in this process, the trapped electrons were released and the conductivity of the Co3O4-based sensor decreased obviously. Thus a high resistance state was observed. The related reaction was described as follows.38-40 O2+e-→O2O2-+e-→2OC3H6O+8O-(ads) → 3CO2↑+3H2O+8e4. CONCLUSIONS In summary, it is found that the proper manipulation of heating temperature, heating rates and gas condition during the ramping process may disturb the balance to build some general principles during the hollowing process for fabricating complex Co3O4 hollow structures. The hierarchical MOF-derived Co3O4 samples provide more exposed surface and active locations, which could be the reactive sites for the pre-adsorbed oxygen molecules and reducing analytes. Four kinds of Co3O4 samples were obtained and described as CS-, PCS-, PPC- and NP-Co3O4 with a diameter of about 1.7 µm, 1. 5 µm, 1. 6 µm and 150 nm, respectively. The sensors based on the CS- and PCS-Co3O4 with core-shell structures presented excellent sensing performance to acetone at 190 °C with response of 13/ 11, and the same response/


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recovery times of 4/ 8 s, respectively. It could be concluded from the above results that the core-shell material with a loose interior structure could be derived from MOF and exhibited superior sensing performance. The above work clearly emphasized the architecturedependent performance in gas sensing, providing a facile step to synthesize high performance Co3O4-based sensors.


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ASSOCIATED CONTENT SUPPORTING INFORMATION The FESEM and TEM images, XRD analysis of the full range spectra and detailed sensing performance of sensing materials are listed. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected]

NOTES The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Postdoctoral Science Foundation of China (No. 2015M571361 and 2016T90251), the Natural Science Foundation Committee (NSFC, Grant No. 51502110 and 61673191), and Central Universities Basic Scientific Research.


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REFERENCES (1) Zhou, T. T.; Zhang, T.; Deng, J. N.; Zhang, R.; Lou, Z.; Wang, L. L. P-type Co3O4 Nanomaterials-based Gas Sensor: Preparation and Acetone Sensing Performance. Sens. Actuators B 2017, 242, 369-377. (2) Liu, Y. J.; Zhu, G. X.; Ge, B. L.; Zhou, H.; Yuan, A. H.; Shen, X. P. Concave Co3O4 Octahedral

Mesocrystal:

Polymer-Mediated

Synthesis

and

Sensing

Properties.

CrystEngComm 2012, 14, 6264-6270. (3) Yoon, J. W.; Hong, H. J.; Park, G. D.; Hwang, S. J.; Hady, F. A.; Wazzan, A. A.; Kang, Y. C.; Lee, J. H. Kilogram-scale Synthesis of Pd-loaded Quintuple-shelled Co3O4 Microreactors and Their Application to Ultrasensitive and Ultraselective Detection of Methylbenzenes. ACS Appl. Mater. Interfaces 2015, 7, 7717-7723. (4) Wang, L. L.; Deng, J. N.; Lou, Z.; Zhang, T. Nanoparticles-assembled Co3O4 Nanorods ptype Nanomaterials: One-pot Synthesis and Toluene-Sensing Properties. Sens. Actuators B 2014, 201, 1-6. (5) Wang, L. L.; Chen, D.; Jiang, K.; Shen, G. Z. New Insights and Perspectives into Biological Materials for Flexible Electronics. Chem. Soc. Rev. 2017, 46, 6764-6815. (6) Wang, L. L.; Jackman, J. A.; Tan, E. L.; Park, J. H.; Potroz, M. G.; Hwang, E. T.; Cho, N. J. High-Performance, Flexible Electronic Skin Sensor Incorporating Natural Microcapsule Actuators. Nano Energy 2017, 26, 38−45.


22

Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7) Wang, L. L.; Jackman, J. A.; Ng, W. B.; Cho, N. J. Flexible, Graphene-Coated Biocomposite for Highly Sensitive, Real-Time Molecular Detection. Adv. Funt. Mater. 2016, 26, 8623−8630. (8) Wang, L. L.; Ng, W. B.; Jackman, J. A.; Cho, N. J. Graphene-Functionalized Natural Microcapsules: Modular Building Blocks for Ultrahigh Sensitivity Bioelectronic Platforms. Adv. Funt. Mater. 2016, 26, 2097−2103. (9) Kitao, T.; Zhang, Y. Y.; Kitagawa, S.; Wang, B.; Uemura, T. Hybridization of MOFs and Polymers. Chem. Soc. Rev. 2017, 46, 3180-3133. (10) Lian, X. Z.; Fang, Y.; Joseph, E.; Wang, Q.; Li, J. L.; Banerjee, S.; Lollar, C.; Wang, X.; Zhou, H. C. Enzyme–MOF (metal–organic framework) composites. Chem. Soc. Rev. 2017, 46, 3386-3401. (11) Shieh, F. K.; Wang, S. C.; Yen, C. I.; Wu, C. C.; Dutta, S.; Chou, L. Y.; Morabito, J. V.; Hu, P.; Hsu, M. H.; Wu, K. C. W.; Tsung, C. K. Imparting Functionality to Biocatalysts via Embedding Enzymes into Nanoporous Materials by a de Novo Approach: Size-Selective Sheltering of Catalase in Metal−Organic Framework Microcrystals. J. Am. Chem. Soc. 2015, 137, 4276−4279. (12) Shieh, F. K.; Wang, S. C.; Leo, S. Y.; Wu, K, C, W. Water-Based Synthesis of Zeolitic Imidazolate Framework-90 (ZIF-90) with a Controllable Particle Size. Chem. Eur. J. 2013, 19, 11139−11142 (13) Shieh, F. K.; Wang, S. C.; Yen, C. I.; Wu. C. C.; Dutta, S.; Chou, L. Y.; Morabito, J. V.; Hu, P.; Hsu, M. H.; Wu, K. C. W.; Tsung, C. K. Strategies for Improving the Functionality of


23


Page 24 of 28

Zeolitic Imidazolate Frameworks: Tailoring Nanoarchitectures for Functional Applications. Adv. Mater. 2017, 29, 1700213-(31). (14) Salunkhe, R. R.; Tang, J.; Kamachi, Y.; Nakato, T.; Kim, J. H.; Yamauchi, Y. Asymmetric Supercapacitors Using 3D Nanoporous Carbon and Cobalt Oxide Electrodes Synthesized from a Single Metal Organic Framework. ACS Nano 2015, 6, 6288-6296. (15) Zhu, Q. L.; Xu, Q. Metal–organic Framework Composites. Chem. Soc. Rev. 2014, 43, 5468-5512. (16) Wang, Y. Z.; Kong, M. G.; Liu, Z. W.; Lin, C. C.; Zeng, Y. Morella-Rubra-Like Metal– Organic-Frameworkderived Multilayered Co3O4/NiO/C Hybrids as Highperformance Anodes for Lithium Storage. J. Mater. Chem. A 2017, 5, 24269-24274. (17) Yang, J.; Ye, H. L.; Zhao, F. Q.; Zeng, B. Z. A Novel CuxO Nanoparticles@ZIF‑8 Composite Derived from Core−Shell Metal−Organic Frameworks for Highly Selective Electrochemical Sensing of Hydrogen Peroxide. ACS Appl. Mater. Interfaces 2016, 8, 20407−20414. (18) Maiti, S.; Pramanik, A.; Mahanty, S. Extraordinarily High Pseudocapacitance of Metal Organic Framework Derived Nano-Structured Cerium Oxide. Chem. Commun. 2014, 50, 11717-11720. (19) Liu, Y. R.; Sun, Y. X.; Zhang, J. B. Metal–Organic-Framework Derived Porous Conducting Frameworks for Highly Efficient Quantum Dot-Sensitized Solar Cells. J. Mater. Chem. C 2017, 5, 4286-4292.


24

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(20) Li, L. L.; Tian, T.; Jiang, J.; Ai, L. H. Hierarchically Porous Co3O4 Architectures with Honeycomb-like Structures for Efficient Oxygen Generation from Electrochemical Water Splitting. J. Power Sources 2015, 294, 103-111. (21) Li, W. H.; Wu, X. F.; Han, N.; Chen, J. Y.; Qian, X. H.; Deng, Y. Z.; Tang, W. X.; Chen, Y. F. MOF-Derived Hierarchical Hollow ZnO Nanocages with Enhanced Low-Concentration VOCs Gas-Sensing Performance. Sens. Actuators B 2016, 225, 158-166. (22) Zhou, W.; Wu, Y. P.; Zhao, J.; Dong, W. W.; Qiao, X. Q.; Hou, D. F.; Bu, X. H.; Li, D. S. Efficient Gas-Sensing for Formaldehyde with 3D Hierarchical Co3O4 Derived from Co5‑Based MOF Microcrystals. Inorg. Chem. 2017, 56, 14111-14117. (23) Shao, J.; Wan, Z. M; Liu, H. M.; Zheng, H. Y.; Gao, T.; Shen, M.; Qu, Q. T.; Zheng, H. H. Metal Organic Frameworks-Derived Co3O4 Hollow Dodecahedrons with Controllable Interiors as Outstanding Anodes for Li Storage. J. Mater. Chem. A 2014, 2, 12194-12200. (24) Koo, W. T.; Yu, S.; Choi, S. J.; Jang, J. S.; Cheong, J. Y.; Kim, II. 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. (25) Ji, W. L.; Xu, Z. L.; Liu, P. F.; Zhang, S. Y.; Zhou, W. Q.; Li, H. F.; Zhang, T.; Li, L. J.; Lu, X. H.; Wu, J. S.; Zhang, W. N.; Huo, F. W. Metal−Organic Framework Derivatives for Improving the Catalytic Activity of the CO Oxidation Reaction. ACS Appl. Mater. Interfaces 2017, 9, 15394-15398.


25


Page 26 of 28

(26) Wu, R. B.; Qian, X. K.; Rui, X. H.; Liu, H.; Yadian, B. L.; Zhou, K.; Wei, J.; Ya, Q. Y.; Feng, X. Q.; Long, Y.; Wang, L. Y.; Huang, Y. Z. Zeolitic Imidazolate Framework 67-Derived High Symmetric Porous Co3O4 Hollow Dodecahedra with Highly Enhanced Lithium Storage Capability. Small 2014, 10, 1932-1938. (27) Lv, 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, 41864195. (28) Zhou, T. T.; Zhang, T.; Zhang, R.; Lou, Z.; Deng, J. N.; Wang, L. L. Hollow ZnSnO3 Cubes with Controllable Shells Enabling Highly Efficient Chemical Sensing Detection of Formaldehyde Vapors. ACS Appl. Mater. Interfaces 2017, 9, 14525-14533. (29) Xu, W.; Chen, J. H.; Yu, M. H.; Zeng, Y. X.; Long, Y. B.; Lu, X. H.; Tong, Y. X. SulphurDoped Co3O4 Nanowires as an Advanced Negative Electrode for High-Energy Asymmetric Supercapacitors. J. Mater. Chem. A 2016, 4, 10779-10785. (30) Wang, K.; Cao, Y. L.; Hu, J. D.; Li, Y. Z.; Xie, J.; Jia, D. Z. Solvent-Free Chemical Approach to Synthesize Various Morphological Co3O4 for CO Oxidation. ACS Appl. Mater. Interfaces 2017, 9, 16128−16137. (31) Wang, Y.; Wang, C. Y.; Wang, Y. J.; Liu, H. K.; Huang, Z. G. Superior Sodium-Ion Storage Performance of Co3O4@Nitrogen-Doped Carbon: Derived from a Metal–Organic Framework. J. Mater. Chem. A 2016, 4, 5428-5435. (32) Wei. R. J.; Zhou, X. L.; Zhou, T. F.; Hu, J. C.; Hu, J. C.; Ho, J. C. Co3O4 Nanosheets with In-Plane Pores and Highly Active {112}


26

Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Exposed Facets for High Performance Lithium Storage. J. Phys. Chem. C 2017, 121, 1900219009. (33) Kim, H. J.; Lee, J. H. Highly Sensitive and Selective Gas Sensors Using P-type Oxide Semiconductors: Overview. Sens. Actuators B 2014, 192, 607-627. (34) Xu, J. M.; Zhang, J.; Wang, B. B.; Liu, F. Shape-regulated Synthesis of Cobalt Oxide and Its Gas-sensing Property. J. Alloys Compd. 2015, 619, 361-367. (35) Park, J.; Shen, X. P.; Wang, G. X. Solvothermal Synthesis and Gas-sensing Performance of Co3O4 Hollow Nanospheres. Sens. Actuators B 2009, 136, 494-498. (36) Sun, C.; Su, X. T.; Xiao, F.; Niu, C. G.; Wang, J. D. Synthesis of Nearly Monodisperse Co3O4 Nanocubes via A Microwave-assisted Solvothermal Process and Their Gas Sensing Properties. Sens. Actuators B 2011, 157, 681-685. (37) Wang, L. L.; Deng, J. N.; Lou, Z.; Zhang, T. Cross-linked p-type Co3O4 octahedral nanoparticles in 1D n-type TiO2 nanofibers for high-performance sensing devices. J. Mater. Chem. A 2014, 2, 10022-10028. (38) Zhang, R.; Zhang, T.; Zhou, T. T.; Deng, J. N.; Lou, Z.; Wang, L. L. Fast and Real-time Acetone Gas Sensor Using Hybrid ZnFe2O4/ZnO Hollow Spheres. RSC Adv. 2016, 6, 6673866744. (39) Wang, L. L.; Jackman, J. A.; Park, J. H.; Tan, E. L.; Cho, N. J. A Flexible, UltraSensitive Chemical Sensor with 3D Biomimetic Templating for Diabetes-Related Acetone Detection. J. Mater. Chem. B 2017, 5, 4019-4024.


27


Page 28 of 28

(40) Zhang, J.; Song, J. M.; Niu, H. L.; Mao, C. J.; Zhang, S. Y.; Shen, Y. H. ZnFe2O4 Nanoparticles: Synthesis, Characterization, and Enhanced Gas Sensing Property for Acetone. Sens. Actuators B 2015, 221, 56-62.

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