Coordination Polymers Derived Multi-shelled Mixed Ni-Co Oxides

Mixed Ni-Co Oxides Microspheres for Robust and Selective Detection of Xylene ... Multi-shelled metal oxide is one of the most intensively investigated...
0 downloads 9 Views 4MB Size
Subscriber access provided by Kaohsiung Medical University

Surfaces, Interfaces, and Applications

Coordination Polymers Derived Multi-shelled Mixed Ni-Co Oxides Microspheres for Robust and Selective Detection of Xylene Fengdong Qu, Wenan Shang, Dongting Wang, Shiyu Du, Tiju Thomas, Shengping Ruan, and Minghui Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03487 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 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

ACS Applied Materials & Interfaces

Coordination Polymers Derived Multi-shelled Mixed Ni-Co Oxides Microspheres for Robust and Selective Detection of Xylene Fengdong Qu†,‡, Wenan Shang‡, Dongting Wang‡, Shiyu Du‡, Tiju Thomas±, Shengping Ruan†,* , Minghui Yang‡,* †

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and

Engineering, Jilin University, Changchun 130012, PR China ‡

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences,

Ningbo 315201, PR China ±

Department of Metallurgical and Materials Engineering, Indian Institute of Technology

Madras, Chennai, Tamil Nadu, India KEYWORDS: coordination polymers; multishell; Ni-Co oxide; gas sensor; xylene

ABSTRACT: Multi-shell, stable, porous metal-oxide microspheres (Ni-Co oxides, Co3O4 and NiO) have been synthesized through amorphous-coordination-polymers based self-templated method. Both oxides of Ni and Co show poor selectivity to xylene, but the composite phase has substantial selectivity (eg. Sxylene/Sethanol = 2.69), and remarkable sensitivity (11.5 to 5 ppm xylene at 255 oC). The short response and recovery times (6 and 9 seconds), excellent humidity-

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 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

Page 2 of 30

resistance performance (with coefficient of variation = 11.4 %), good cyclability and long-term stability (sensitivity attenuation of ~9.5% after 30 days; stable sensitivity thereafter) all show that this composite is a competitive solution to the problem of xylene sensing. The sensing performances are evidently due to the high specific surface area and the nano-heterostructure in the composite phase.

ACS Paragon Plus Environment

2

Page 3 of 30 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

ACS Applied Materials & Interfaces

1. INTRODUCTION Metal oxides with complex interior structures are a unique class of functional materials that have attracted widespread interest due to its high specific surface area and porosity that enable tuning for a wide range of applications.1-4 In particular, the prospect of the material for gas sensing is substantial.5-6 Multi-shelled metal oxide is one of the most intensively investigated systems; their appeal is owing to effective gas diffusion and mass transportation across shells and reduced aggregation of nanosized subunits.7-9 For example, Wang et al. reported hollow, multi-shelled ZnSnO3 cubes for high performance formaldehyde sensors.10 Zhu et al. utilized one-pot synthesis strategy to prepare Cu2O microspheres and applied them as gas sensing materials to improve the performance of ethanol detection.7 Despite acceptable sensitivities, poor selectivity as well as humidity-resistance needs further improvement in metal oxides. Designing composites is widely regarded as one of the most effective ways to improve selectivity.11-12 Efforts have been devoted to forming composites to enhance gas sensing performance and to achieve satisfactory results.1317

Decorating or doping catalyst with selective catalytic materials is another accepted method to

improve selectivity.18-19 Hence preparation of composites with mixed oxide component and multi-shelled structure is desirable for fabricating high performance gas sensors, for specific purposes. Various strategies have been developed to synthesize multi-shelled mixed oxide composites. These strategies include hard or soft template-assisted and etching-assisted methods.20-24 For example, Wang et al. reported the synthesis of hollow, multi-shelled Cu2O using a cationic surfactant, cetyltrimethylam-monium bromide as soft template.25 However, these methods are often multistep, time-consuming and costly. Besides, the interior structure obtained is less complex, and limited by the degree of cations diffusing into the template. In particular, self-

ACS Paragon Plus Environment

3

ACS Applied Materials & Interfaces 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

Page 4 of 30

sacrificing template methods show potential in achieving multi-shelled nanostructured materials.26 However, developing an effective and facile self-templated strategy to prepare multishelled mixed oxide composites continues to be an issue that needs to be addressed in a case/composition specific manner. Herein, multishelled mixed Ni-Co oxide microspheres have been synthesized successfully through coordination polymers (CPs); commonly known as metal-organic frameworks (MOFs). The CPs used in this synthesis approach paves way for self-templating. The general strategy includes synthesis of Ni-Co CPs and subsequent transformation into mixed Ni-Co oxides by oxidizing the CPs using a thermal treatment in air. The strategy utilized in this work can be readily adapted to obtain multi-shelled microspheres with tunable diameter, composition and morphology. The multishell mixed Ni-Co oxide microspheres show properties that make it 2. EXPERIMENTAL SECTION 2.1. Chemical reagent All of the chemicals are purchased from Aladdin Industrial Corporation (Shanghai, China) and are of analytical grade. 2.2. Synthesis process 2.2.1. Synthesis of multishelled mixed Ni-Co oxides. Typically, 0.04 mmol Co(NO3)2·6H2O, 0.04 mmol of Ni(NO3)2·6H2O, and 0.08 mmol of isophthalic acid (H2IPA) are dissolved in a mixture of dimethyl formamide (DMF; 5 mL in volume) and acetone (5 mL) to form a clear solution by stirring for 6 h. The solution is then

ACS Paragon Plus Environment

4

Page 5 of 30 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

ACS Applied Materials & Interfaces

transferred to a Teflon-lined stainless steel autoclave and kept at 160 oC for 4 h. After cooling to room temperature, the as-obtained Ni-Co coordination polymer spheres (CPSs) are separated by centrifugation. The multi-shelled Ni-Co oxide microspheres are generated through a thermal treatment of Ni-Co CPSs in air at a temperature of 500 oC for 10 min, with a heating rate of 5 oC min-1. 2.2.2. Synthesis of NiO and Co3O4 muti-shell microspheres. The synthesis procedures of multi-shell NiO and Co3O4 are similar to those of multi-shelled Ni-Co oxide microspheres, except for using nickel nitrate, cobalt nitrate or the mixture of them with the molar ratios of 1:0 and 0:1 as reactants to prepare the CP precursors. The multi-shelled particles are generated through a thermal treatment of the corresponding CP precursors. 2.2.3. Synthesis of NiO/Co3O4 mixture. NiO and Co3O4 were weighed by a molar ratio of 1:1 (Ni:Co) and mixed together. The wellmixed powder was charged in a 50 ml tungsten carbide (WC) crucible containing three WC balls of 11 mm in diameter. The crucible was then closed and sealed with a Viton O-ring under air and fixed on a laboratory SPEX mill. The high energy milling was performed at 1000 rpm for 2 h. 2.3. Characterization Powder X-ray diffraction (XRD) patterns are collected on a Rigaku MiniFlex 600 powder Xray diffractometer with Cu Kα radiation (λ=1.5418 Å). Scanning electron microscopy (SEM) images are performed on a JSM-7800F (Japan) instrument. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observations are conducted on a JEM-2100 (Japan) instrument. N2 adsorption-desorption isotherms are measured

ACS Paragon Plus Environment

5

ACS Applied Materials & Interfaces 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

Page 6 of 30

at 77 K on a Micromeritics ASAP 2420 system. Surface area and pore size distribution are evaluated using Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. The chemical component is characterized by X‒ray photoelectron spectroscopy (XPS) is measured on a VG ESCALAB MKII spectrometer (VG, Britain) with Mg Kα excitation (1253.6 eV). Magnetic analysis are performed on Quantum Design 9T Physical Properties Measurement System (PPMS). 2.4. Fabrication and measurement of gas sensor Typically, multishelled mixed Ni-Co oxides powders are dispersed in water and ground to obtain a slurry. Then the slurry is drop-coated on an aluminium oxide ceramic substrate where two Au electrodes are printed on, and a microheater is printed on the other side. Gas-sensing measurements are conducted on an instrument of intelligent gas sensing analysis system from Beijing Elite Tech Co., Ltd., China under laboratory conditions. A gas mixing line equipped with mass flow controllers is designed to prepare target gases at specific concentrations (Fig. S1). Gas response (S = Rg/Ra) to reducing gases is defined as the ratio of the resistance of the sensor in gas (Rg) and air (Ra). The response and recovery time is defined as the time which the sensor takes to obtain 90 % total resistance change. Xylene has been widely used as solvents in printing, rubber and leather industries, as well as in household products, which can impair the human respiratory system, the central nervous system, eyes, liver and skin.27 CO, H2S and acetone are other common indoor pollutants, which may have intense interference when detecting xylene.28 3. RESULTS AND DISCUSSION 3.1. Structural and morphological characteristics

ACS Paragon Plus Environment

6

Page 7 of 30 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

ACS Applied Materials & Interfaces

Ni-Co coordination polymer spheres (CPs) as precursors is prepared firstly through coprecipitation of Ni2+ and Co2+ with organic ligand of H2IPA under solvothermal conditions. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis are conducted to investigate the structures and typical morphologies of the products. These Ni-Co CPs show smooth surfaces with uniform size of 1~1.2 um (Fig. S2). Besides, the Ni-Co CPs has a solid structure observed from the TEM images. Powder x-ray diffraction of Ni-Co CPs, shown in Fig. S2, indicates that the Ni-Co CPs are amorphous with no crystal structure. After thermal treatment in air, the Ni-Co CPs precursor transform into mixed Ni-Co oxides with more rough surfaces, as shown in Fig. 1a. The surface shrinks after calcination treatment, resulting in a smaller size of about ~1 µm. Besides many pores can be observed at the surface, which consists of small oxide nanograins, offering excellent gas accessibility. TEM (Fig. 1b) shows that the mixed Ni-Co oxides have crumpled multi-shell structure with more than five shells. Besides, many parts can be observed between two neighboring shells throughout the whole mixed Ni-Co oxides microspheres; this would exhibit resistance to aggregation of nanosized subunits and improve the structural stability. The EDX elemental mappings in Fig. 1d-f present the consistent and uniform distribution of nickel, cobalt and oxide. EDS analysis (Fig. 1g) of mixed Ni-Co oxides reveals the Ni/Co atom ratio is about 1:1; which is consistent with the amount of Ni and Co precursors used in the synthesis process. In order to confirm the composition of mixed Ni-Co oxides, M-H hysteresis loops are measured, shown in Fig. S3. The magnetization of Co3O4 increases almost liner with the increasing of magnetic field, which indicates Co3O4 is an antiferromagnetic material. The NiO also exhibits antiferromagnetic characteristics, which are well consistent with others works.29 The presence of NiCo2O4 spinel phase will increase hysteresis loops, which is caused by its ferromagnetic properties. It can be observed from the

ACS Paragon Plus Environment

7

ACS Applied Materials & Interfaces 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

Page 8 of 30

hysteresis loops of Ni-Co oxides that the magnetization has not come into saturation when the applied field reaches 30 kOe, which is caused by the co-contribution of antiferromagnetic and ferromagnetic phase.30 Thus the composites should contain NiO, NiCo2O4 and Co3O4.

Fig. 1 (a) SEM image and (b) TEM image of mixed Ni-Co oxides. The EDX elemental mapping of mixed Ni-Co oxides, (c) position, (d) nickel, (e) cobalt and (f) oxygen. (g) EDS spectrum of mixed Ni-Co oxides. NiO and Co3O4 are also prepared through a similar method except for different amount of reactants. SEM and TEM observations (Fig. S4) are conducted to evaluate the morphology and structure. SEM characterizations reveals NiO has a relative smooth surface; Co3O4 exhibits similar morphology with that of mixed Ni-Co oxides microspheres with ratio of 1:1. From TEM observations, both microspheres show multishell structures but with different numbers of shells. XRD analysis confirms the crystal structure of these samples, shown in Fig. S5.

ACS Paragon Plus Environment

8

Page 9 of 30 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

ACS Applied Materials & Interfaces

Fig. 2a exhibits schematic illustration of the formation process of mixed Ni-Co oxides. The morphologies and structures of solid CPs precursor changed from york-single-shelled composites to york-multi-shelled composites and multi-shelled oxides eventually under thermal treatment to 360, 390 and 500 oC, respectively, which was confirmed from TEM observations (shown in Fig. 2b-e). Besides, the surface becomes wrinkly after calcination, which means partly porous of shells can be expected. The formation mechanism of multishell structure can be explained by the co-interaction of two opposite forces of contraction (Fc) from decomposition of organic species and adhesion (Fa) from the dense shell.31 Appropriate calcination temperature is one of key parameters to synthesize desired complex interior structures.

Fig. 2. (a) Schematic illustration of the formation process of mixed Ni-Co oxides. (b-e) TEM images of solid CPs precursor and composites obtained through thermal treatment of solid CPs precursor to 360, 390 and 500 oC, respectively. The scale bar is 100 nm. XRD measurement of as-synthesized mixed Ni-Co oxides is conducted to check the phase composition and purity, shown in Fig. 3a. As can be seen from the pattern, the diffraction peaks can be indexed to cubic spinel structured NiCo2O4 or Co3O4 (JCPDS no. 20-0781 or 42-1467) and cubic NiO (JCPDS no. 71-1179). No other clear sharp peaks appear throughout the whole pattern, which implies that the product had a high purity without other crystal impurity. The specific surface area and porosity are checked through nitrogen adsorption/desorption isotherms

ACS Paragon Plus Environment

9

ACS Applied Materials & Interfaces 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

Page 10 of 30

and BJH pore size distribution analysis, shown in Fig. 3b. The N2-BET surface area of the mixed Ni-Co oxides is calculated to be 86.3 m2 g−1, which is larger than that of NiO (67.7 m2 g−1) and Co3O4 (58.5 m2 g−1). This is due to more shells and complex inner structures. The nitrogen adsorption/desorption isotherm of Ni-Co oxides, NiO and Co3O4 are classical type IV systems; they exhibit H3 hysteresis loop at a pressure range of 0.6–0.8; this indicates the presence of mesopores and macropores.32-33 The pore size distribution calculated from the adsorption branch using the Barrett-Joyner-Halenda (BJH) method shows that the average pore size of Ni-Co oxides, NiO and Co3O4 microspheres centered at 3.8, 9.7 and 16.7 nm, respectively. The high specific surface area of mixed Ni-Co oxide microspheres means an amplified target-receptor interface, which makes this material relevant for several surface-dependent applications (including gas sensors and catalysis).

ACS Paragon Plus Environment

10

Page 11 of 30 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

ACS Applied Materials & Interfaces

Fig. 3. (a) XRD patterns of mixed Ni-Co oxides. (b) Pore size distributions of mixed Ni-Co oxides, Co3O4 and NiO. XPS spectrum of mixed Ni-Co oxides, (c) all, (d) Ni 2p, (e) Co 2p, and (f) O 1s. The inset image of (b) and (c) is the N2 adsorption/desorption isotherms and the atomic concentrations of Ni, Co and O in Ni-Co oxides, respectively. X-ray photoelectron spectroscopy (XPS) data is collected to investigate the surface chemical composition for mixed Ni-Co oxides. The peaks observed in the survey spectrum (Fig. 3c) indicate the presence of Ni, Co, and O on the surface of mixed Ni-Co oxides. The binding energy for the C 1s peaks (284.8 eV) is used as an internal reference. Thus the peaks indexed to C evidently come from the reference. No other peaks can be observed from the survey spectrum, indicating the absence of impurities; which is consistent with the result of XRD and EDS. The Ni 2p XPS spectrum (Fig. 3d) shows two edges of Ni 2p1/2 and Ni 2p3/2 confirming the presence of NiO 34. The Ni 2p1/2 main peak and its satellites (at 872.6 eV and 879.8 eV), and Ni 2p3/2 main peak and its satellite (853.9 eV and 861.1 eV) are evidence of presence of Ni2+ (as in NiO). The energy difference between Ni 2p3/2 and Ni 2p1/2 splitting is 15.3 eV, which is indicated by the well-defined symmetry of Ni2+ in oxide form. The Co 2p XPS spectrum (Fig. 3e) shows two major peaks centered at 780.3 and 795.3 eV, which are assigned to the Co 2p1/2 and Co 2p3/2, respectively and two weak shake-up satellite peaks centered at 790 and 805 eV. The energy gap between the Co 2p main peak is about 9.7 eV, which indicates that the Co cation valence could be assigned to 3+.35-37. Besides, the appearance of two peaks at ~781.9 and 797.1 eV demonstrates the existence of Co (II), which indicates the presence of NiCo2O4 as well as Co3O4. O 1s curves of Ni-Co oxides (Fig. 3f) is deconvoluted into three fitted peaks, which implies the different states of oxygen species on the surface of the samples, OL (lattice oxygen species) component, OV (oxygen-vacancy) component and the OC (surface chemisorbed oxygen species) component. The XPS spectrum of NiO and Co3O4 is shown in Fig. S6. The O 1s peak(s) related information obtained from mixed Ni-Co oxides, Co3O4 and NiO, is listed in Table 1. For

ACS Paragon Plus Environment

11

ACS Applied Materials & Interfaces 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

Page 12 of 30

chemoresistive gas sensors based on metal oxide semiconductors, the ability to absorb oxygen species is one of the most vital factors to determine the performance of sensors. The increase of OC component of mixed Ni-Co oxides means that the surface chemisorbed oxygen species could aid in the gas sensing related reactions occurring on the surface of the sensing materials. Table 1 Results of curve fitting of O 1s XPS Spectra of Ni-Co oxides, Co3O4 and NiO samples. OL (Ni-O/Co-O)

OV (vacancy)

OC (chemisorbed)

529.7

531.3

532.8

Relative percentage (%) 56.8

24.6

18.6

Binding energy (eV)

530.9

532.7

Relative percentage (%) 68.0

15.6

16.4

Binding energy (eV)

530.4

531.3

22.3

10.9

Samples Ni-Co oxides

Binding energy (eV)

529.4

Co3O4 529.9

NiO Relative percentage (%) 66.8

3.2. Gas sensing properties The gas sensing performances of mixed Ni-Co oxides, Co3O4 and NiO microspheres are evaluated. Fig. 4a shows the response and response time of mixed Ni-Co oxides, Co3O4 and NiO microspheres to 5 ppm xylene as a function of working temperature. It can be observed that the response of mixed Ni-Co oxides, Co3O4 and NiO microspheres-based sensor exhibits a “volcano” shape with a maximum value of 11.5, 2.1and 1.6 at 255, 221, and 302 oC, respectively. The following two factors accounts for the phenomenon. Firstly, when the operating temperature increases from low temperature, the xylene molecules will achieve more energy to overcome the activation energy barrier to react with the adsorbed oxygen species. In addition, the adsorbed oxygen species will transfer from O2- to O- or O2-, which will capture more electrons from the sensing material

38

. When the temperature is too high, the gas adsorption ability will decrease,

ACS Paragon Plus Environment

12

Page 13 of 30 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

ACS Applied Materials & Interfaces

which results in the decrease of response.39 As also shown in Fig. 4a, with the increasing of working temperature, the response time decreases and comes into saturation with a further increase of temperature. Thus the optimum operating temperature of the Ni-Co oxides, Co3O4 and NiO microspheres sensor is suggested to be 255, 221, and 302 oC, respectively.

Fig. 4. (a) Gas response (Rg/Ra) and response time (s) of Ni-Co oxides, Co3O4 and NiO sensors to 5 ppm xylene at 138–344 °C. (b) Cross-responses of Ni-Co oxides, Co3O4 and NiO sensors to 5 ppm various gases (the concentration of CO is 100 ppm) at 255, 221 and 302 oC, respectively. (c) Dynamic gas sensing transients of Ni-Co oxides sensor to 1-5-1 ppm of xylene, and (d) responses to xylene as a function of concentration at 255 oC. (e) Humidity dependence of response of Ni-Co oxides, Co3O4 and NiO sensors to 5 ppm acetone at 255, 221 and 302 oC, respectively. (f) Coefficient of variation of the Ni-Co oxides, Co3O4 and NiO sensors by varying RH from 11% to 95 %.

Fig. 4b shows the cross-responses of Ni-Co oxides, Co3O4 and NiO sensors to ~5 ppm gases (the concentration of CO is ~100 ppm) at 255, 221 and 302 oC, respectively. Mixed Ni-Co oxides sensor exhibits a high response to xylene and a relatively low cross-response to ethanol, acetone, toluene, benzene, CO and H2S, whereas. pure Co3O4 and NiO sensors show none

ACS Paragon Plus Environment

13

ACS Applied Materials & Interfaces 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

Page 14 of 30

obvious selectivity toward one particular gas (xylene). The selectivity of mixed Ni-Co oxides sensor may be attributed to the heterostructure between nickel oxides and cobalt oxides.11, 40 Besides, to evaluate the interferential effects of other analytes, the response of mixed Ni-Co oxides sensor toward mixed gases (5 ppm xylene + 5 ppm ethanol or/and acetone) are measured and the results are shown in Fig. S7. It can be observed that the response of mixed Ni-Co oxides sensor shows increase but not cumulative rise when the target gases changing from xylene to xylene-containing mixed gases, which means mixed Ni-Co oxides shows well selectivity to xylene.27, 41-42 The dynamic sensing transients of Ni-Co oxides sensor to 1-5-1 ppm xylene operated at 255 o

C is shown in Fig. 4c. The sensor exhibited excellent response and recovery characteristics as

well as reproducibility. The response and recovery times obtained from sensing transients (Fig. S8) are 6 and 9 s, respectively. Fig. 4d shows the responses of Ni-Co oxide sensor to 1-5 ppm xylene at 255 oC. An almost linear relationship between response and xylene concentration can be observed, which indicates that the sensor exhibits potential in quantitative gas analysis when applied to practical situations. Besides, according to the standard defined by IUPAC (signal-tonoise ratio of 3), the theoretical limit of detection to xylene is determined to be 0.097 ppm with the slope of 1.98 ppm-1 and the root-mean-square noise of 0.064.33, 43 In addition, the gas sensing performance of NiO/Co3O4 mixture are further checked to clarity the enhanced sensing characteristics of mixed Ni-Co oxides, shown in Fig. S9. It can be observed that the response of NiO/Co3O4 mixture to 5 ppm xylene is 4.9 that is much lower than that of mixed Ni-Co oxides (~11.5) at their optimum operating temperature, indicating the improved response of mixed NiCo oxides.

ACS Paragon Plus Environment

14

Page 15 of 30 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

ACS Applied Materials & Interfaces

Long-term stability plays key role in actual application, which endows the sensor high reliability. Fig. S10 exhibited the response of mixed Ni-Co oxide microspheres-based sensor to 5 ppm xylene at 255 oC during long-term measurement over a period of ~30 days. The sensor showed a fast decrease in the first 14 days and then stabilized. The total attenuation value is about 9.45 %, which implies that the sensor does have a well long-term stability. The humidity dependence of Ni-Co oxides, Co3O4 and NiO sensors responses to 5 ppm xylene are shown in Fig. 4e. It can be observed that the humidity has remarkable influence on the response. With the humidity increasing, the responses of Ni-Co oxides, Co3O4 and NiO sensors decreases; this may be caused by the reaction between water molecules (H2O) and adsorbed oxygen species on the surface, resulting in formation of reactive hydroxyl groups (OH-) and hence deteriorating the gas response.44-46 To check the effect of humidity on the response, the coefficient of variation (CV) is introduced, which is defined as follows: CV = SSD/Saverage × 100%

[1]

Here SSD and Saverage are the standard deviation and average value of responses at different humidity values, respectively. The lower the CV value, the better humidity-resistance performance. The Ni-Co oxide sensors exhibits a low CV value (lower than 20%), indicating a humidity resistant performance when changing relative humidity (RH) from 11% to 95%, shown in Fig. 4f. However, Co3O4 and NiO sensor shows poor humidity resistance performance with a CV value of 29.13 % and 26.19 %, respectively. Thus the multishelled mixed Ni-Co oxide microsphere sensor exhibits excellent humidity independent characteristics. Better humidity resistance performance of Ni-Co oxides sensors may be attributed to the high affinity of water molecules to NiO, which acts as hydroxyl absorbers. 47

ACS Paragon Plus Environment

15

ACS Applied Materials & Interfaces 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

Page 16 of 30

The xylene sensing properties reported in literature are summarized in Table S1.18, 33, 39, 48-54 The mixed Ni-Co oxides reported here exhibits a higher response of 11.5 to 5 ppm xylene, and show excellent humidity-resistance properties with a CV value of 7.32 %. The evidently desirable sensing performance with regards to relative high response at moderate operating temperature makes mixed Ni-Co oxides sensor competitive when compared with most other high-performance xylene gas sensors. For example, H-W. Jang et al. reported that a sensor based on NiO-decorated Co3O4 nanorods exhibits response of 2.0 to 5 ppm xylene at 355 oC.49 Y. Li et al. reported Cr-doped Co3O4 hierarchical nanostructures can detect 5 ppm xylene with a response of 6.38 at 139 oC.50 Although the operating temperatures are relative low, the responses are in clear need of improvement. J. Lee and his co-operators reported that the Cr-doped NiO nanowires sensor has higher response (24.5) to xylene (5 ppm), but the operating temperature of the sensor is as high as 425 oC, restricting its application. 18 Typically, the sensing performances of metal-oxide-based chemiresistive gas sensors depends on the redox reaction between adsorbed oxygen species and target analytes on the surface of the sensing materials.55-56 The gas response is determined by the amount of active reaction sites which has a positive correlation with specific surface areas. Therefore the enhanced response of mixed Ni-Co oxide microspheres sensor could be explained by the high specific surface when compared with that of pure multshelled NiO and Co3O4 microspheres.57 Besides, the formation of heterostructures between NiO and NiCo2O4 or Co3O4 is another key element in improving the response through the modulation by adsorbing and desorbing oxygen species at the interfaces.11 Ni-Co oxide microspheres sensor exhibited enhanced selectivity properties, which may be accounted by the following two factors. The first one is the selective catalytic methyl group oxidation of Co3O4 and NiO catalyst at high temperature (200-400 oC).58-60 Besides, some

ACS Paragon Plus Environment

16

Page 17 of 30 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

ACS Applied Materials & Interfaces

synergistic effect between Co3O4 (NiCo2O4) and NiO seems to exist, for the pure Co3O4 and NiO reported here showed no obvious selectivity toward xylene. The heterostructure is evidently playing an important role.14, 61-63 Carriers can be modulated through heterostucture-formation, resulting in depletion layer (Co3O4 or NiCo2O4) and accumulation layer (NiO), which likes as an oxygen pump, enhancing the response and selectivity, as shown in Fig. 5 or Fig. S11.64 However, the precise reasons need further investigation.

Fig. 5. (a) The energy band structure of Co3O4/NiO heterojunctions - before and after contact. (b) Schematic representation of the mechanism influencing the gas sensing behavior.

4. CONCLUSIONS Complex, multi-shell, highly porous metal-oxide microspheres (Ni-Co oxides, Co3O4 and NiO) have been synthesized through a coordination polymers-based self-template method. The method uses amorphous coordination polymers, which are complex and calcined to obtain mixed Ni-Co oxides. Interestingly both oxides of Ni and Co show no selectivity to xylene; however they are robust to interferents. The composite however showed substantial selectivity, and remarkable sensitivity (11.5 to 5 ppm xylene at 255 oC). The selectivity (eg. Sxylene/Sethanol = 2.69), short

ACS Paragon Plus Environment

17

ACS Applied Materials & Interfaces 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

Page 18 of 30

response and recovery times (~6 and 9 seconds), excellent humidity-resistance performance (coefficient of variation = 11.4 %), good cyclability and long-term stability all show that this composite is a competitive solution to the problem of xylene sensing. Continuous run-time testing over a 30 day period resulted in an attenuation of sensitivity by ~9.5%; however the sensor performance stabilized thereafter and showed no further deterioration. The excellent sensing performances may be attributed to the combination of high specific surface area as well as nanoheterostructure between Co3O4 and NiO. Our findings will likely spur further work on selective and sensitive xylene sensors that operate at even lower temperatures. The composite reported here may also find applications in other surface science related applications. ASSOCIATED CONTENT Supporting Information. The schematic circuit diagram of the testing system and gas mixing line equipment, XRD pattern, TEM images, SEM images, XPS spectrum of NiO and Co3O4, response/recovery time graphs, long-term stability of the mixed Ni-Co oxide microspheres-based gas sensor and xylene sensing performances of reported in this and previous works. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Shengping Ruan, Tel/Fax.: +86-431-85168241, E-mail: [email protected]; * Minghui Yang, Tel/Fax.: +86-574-87608176, E-mail: [email protected]. Author Contributions

ACS Paragon Plus Environment

18

Page 19 of 30 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

ACS Applied Materials & Interfaces

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ORCID Fengdong Qu: 0000-0001-8681-3288 Minghui Yang: 0000-0003-1071-1327 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by NSF China through Grant 21471147, Program for JLU Science and Technology Innovative Research Team, Project of Science and Technology Plan of Jilin Province and Project of special fund for industrial innovation of Jilin Province. M. Yang would like to thank for the National “Thousand Youth Talents” program of China. We thank the Department of Science and Technology, Government of India for support through project nos. DST FILE NO. YSS/2015/001712 and DST 11-IFA-PH-07. REFERENCES (1) Yao, D.; Xin, W.; Liu, Z.; Wang, Z.; Feng, J.; Dong, C.; Liu, Y.; Yang, B.; Zhang, H. Phosphine-Free Synthesis of Metal Chalcogenide Quantum Dots by Directly Dissolving Chalcogen Dioxides in Alkylthiol as the Precursor. ACS Applied Materials & Interfaces 2017, 9 (11), 9840-9848, DOI: 10.1021/acsami.6b16407. (2) Liu, H.; Wu, Z.; Gao, H.; Shao, J.; Zou, H.; Yao, D.; Liu, Y.; Zhang, H.; Yang, B. One-Step Preparation of Cesium Lead Halide CsPbX3 (X = Cl, Br, and I) Perovskite Nanocrystals by Microwave Irradiation. ACS Applied Materials & Interfaces 2017, 9 (49), 42919-42927, DOI: 10.1021/acsami.7b14677. (3) Zhao, X.; Yu, R.; Tang, H.; Mao, D.; Qi, J.; Wang, B.; Zhang, Y.; Zhao, H.; Hu, W.; Wang, D. Formation of Septuple-Shelled (Co2/3Mn1/3)(Co5/6Mn1/6)2O4 Hollow Spheres as Electrode Material for Alkaline Rechargeable Battery. Advanced Materials 2017, 29 (34), 1700550, DOI: 10.1002/adma.201700550.

ACS Paragon Plus Environment

19

ACS Applied Materials & Interfaces 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

Page 20 of 30

(4) Zong, L.; Xu, J.; Jiang, S.; Zhao, K.; Wang, Z.; Liu, P.; Zhao, H.; Chen, J.; Xing, X.; Yu, R. Composite Yttrium-Carbonaceous Spheres Templated Multi-Shell YVO4 Hollow Spheres with Superior Upconversion Photoluminescence. Advanced Materials 2017, 29 (9), 1604377, DOI: 10.1002/adma.201604377. (5) Wang, L.; Lou, Z.; Deng, J.; Zhang, R.; Zhang, T. Ethanol Gas Detection Using a Yolk-Shell (Core-Shell) α-Fe2O3 Nanospheres as Sensing Material. ACS Applied Materials & Interfaces 2015, 7 (23), 13098-13104, DOI: 10.1021/acsami.5b03978. (6) Song, X.-Z.; Meng, Y.-L.; Tan, Z.; Qiao, L.; Huang, T.; Wang, X.-F. Concave ZnFe2O4 Hollow Octahedral Nanocages Derived from Fe-Doped MOF-5 for High-Performance Acetone Sensing at Low-Energy Consumption. Inorganic Chemistry 2017, 56 (22), 13646-13650, DOI: 10.1021/acs.inorgchem.7b02425. (7) Zhang, H.; Zhu, Q.; Zhang, Y.; Wang, Y.; Zhao, L.; Yu, B. One-Pot Synthesis and Hierarchical Assembly of Hollow Cu2O Microspheres with Nanocrystals-Composed Porous Multishell and Their Gas-Sensing Properties. Advanced Functional Materials 2007, 17 (15), 2766-2771, DOI: 10.1002/adfm.200601146. (8) Lee, J.-H. Gas sensors using hierarchical and hollow oxide nanostructures: Overview. Sensors and Actuators B: Chemical 2009, 140 (1), 319-336, DOI: https://doi.org/10.1016/j.snb.2009.04.026. (9) Yang, H. M.; Ma, S. Y.; Jiao, H. Y.; Chen, Q.; Lu, Y.; Jin, W. X.; Li, W. Q.; Wang, T. T.; Jiang, X. H.; Qiang, Z.; Chen, H. Synthesis of Zn2SnO4 hollow spheres by a template route for highperformance acetone gas sensor. Sensors and Actuators B: Chemical 2017, 245, 493-506, DOI: https://doi.org/10.1016/j.snb.2017.01.205. (10) Zhou, T.; Zhang, T.; Zhang, R.; Lou, Z.; Deng, J.; Wang, L. Hollow ZnSnO3 Cubes with Controllable Shells Enabling Highly Efficient Chemical Sensing Detection of Formaldehyde Vapors. ACS Applied Materials & Interfaces 2017, 9 (16), 14525-14533, DOI: 10.1021/acsami.7b03112. (11) Miller, D. R.; Akbar, S. A.; Morris, P. A. Nanoscale metal oxide-based heterojunctions for gas sensing: A review. Sensors and Actuators B: Chemical 2014, 204, 250-272, DOI: https://doi.org/10.1016/j.snb.2014.07.074. (12) Xu, S.; Gao, J.; Wang, L.; Kan, K.; Xie, Y.; Shen, P.; Li, L.; Shi, K. Role of the heterojunctions in In2O3-composite SnO2 nanorod sensors and their remarkable gas-sensing performance for NOx at room temperature. Nanoscale 2015, 7 (35), 14643-14651, DOI: 10.1039/C5NR03796D. (13) Kim, J.-H.; Lee, J.-H.; Mirzaei, A.; Kim, H. W.; Kim, S. S. Optimization and gas sensing mechanism of n-SnO2-p-Co3O4 composite nanofibers. Sensors and Actuators B: Chemical 2017, 248, 500-511, DOI: https://doi.org/10.1016/j.snb.2017.04.029. (14) Kim, T.-H.; Kwak, C.-H.; Lee, J.-H. NiO/NiWO4 Composite Yolk–Shell Spheres with Nanoscale NiO Outer Layer for Ultrasensitive and Selective Detection of Subppm-level pXylene. ACS Applied Materials & Interfaces 2017, 9 (37), 32034-32043, DOI: 10.1021/acsami.7b10294. (15) Ahmad, M. Z.; Sadek, A. Z.; Latham, K.; Kita, J.; Moos, R.; Wlodarski, W. Chemically synthesized one-dimensional zinc oxide nanorods for ethanol sensing. Sensors and Actuators B: Chemical 2013, 187, 295-300, DOI: https://doi.org/10.1016/j.snb.2012.11.042. (16) Li, L.; Zhang, C.; Chen, W. Fabrication of SnO2-SnO nanocomposites with p-n heterojunctions for the low-temperature sensing of NO2 gas. Nanoscale 2015, 7 (28), 1213312142, DOI: 10.1039/C5NR02334C.

ACS Paragon Plus Environment

20

Page 21 of 30 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

ACS Applied Materials & Interfaces

(17) Wang, X.; Cao, R.; Zhang, S.; Hou, P.; Han, R.; Shao, M.; Xu, X. Hierarchical flowerlike metal/metal oxide nanostructures derived from layered double hydroxides for catalysis and gas sensing. Journal of Materials Chemistry A 2017, 5 (45), 23999-24010, DOI: 10.1039/C7TA06809C. (18) Kim, H.-J.; Yoon, J.-W.; Choi, K.-I.; Jang, H. W.; Umar, A.; Lee, J.-H. Ultraselective and sensitive detection of xylene and toluene for monitoring indoor air pollution using Cr-doped NiO hierarchical nanostructures. Nanoscale 2013, 5 (15), 7066-7073, DOI: 10.1039/C3NR01281F. (19) Woo, H.-S.; Kwak, C.-H.; Chung, J.-H.; Lee, J.-H. Co-Doped Branched ZnO Nanowires for Ultraselective and Sensitive Detection of Xylene. ACS Applied Materials & Interfaces 2014, 6 (24), 22553-22560, DOI: 10.1021/am506674u. (20) Wang, J.; Tang, H.; Zhang, L.; Ren, H.; Yu, R.; Jin, Q.; Qi, J.; Mao, D.; Yang, M.; Wang, Y.; Liu, P.; Zhang, Y.; Wen, Y.; Gu, L.; Ma, G.; Su, Z.; Tang, Z.; Zhao, H.; Wang, D. Multi-shelled metal oxides prepared via an anion-adsorption mechanism for lithium-ion batteries. Nature Energy 2016, 1, 16050, DOI: 10.1038/nenergy.2016.50. (21) Wang, J.; Tang, H.; Ren, H.; Yu, R.; Qi, J.; Mao, D.; Zhao, H.; Wang, D. pH-Regulated Synthesis of Multi-Shelled Manganese Oxide Hollow Microspheres as Supercapacitor Electrodes Using Carbonaceous Microspheres as Templates. Advanced Science 2014, 1 (1), 1400011, DOI: 10.1002/advs.201400011. (22) Xu, S.; Hessel, C. M.; Ren, H.; Yu, R.; Jin, Q.; Yang, M.; Zhao, H.; Wang, D. [small alpha]Fe2O3 multi-shelled hollow microspheres for lithium ion battery anodes with superior capacity and charge retention. Energy & Environmental Science 2014, 7 (2), 632-637, DOI: 10.1039/C3EE43319F. (23) Sun, J.; Lv, C.; Lv, F.; Chen, S.; Li, D.; Guo, Z.; Han, W.; Yang, D.; Guo, S. Tuning the Shell Number of Multishelled Metal Oxide Hollow Fibers for Optimized Lithium-Ion Storage. ACS Nano 2017, 11 (6), 6186-6193, DOI: 10.1021/acsnano.7b02275. (24) Shen, L.; Yu, L.; Yu, X.-Y.; Zhang, X.; Lou, X. W. Self-Templated Formation of Uniform NiCo2O4 Hollow Spheres with Complex Interior Structures for Lithium-Ion Batteries and Supercapacitors. Angewandte Chemie International Edition 2015, 54 (6), 1868-1872, DOI: 10.1002/anie.201409776. (25) Xu, H.; Wang, W. Template Synthesis of Multishelled Cu2O Hollow Spheres with a SingleCrystalline Shell Wall. Angewandte Chemie International Edition 2007, 46 (9), 1489-1492, DOI: 10.1002/anie.200603895. (26) Yu, L.; Hu, H.; Wu, H. B.; Lou, X. W. Complex Hollow Nanostructures: Synthesis and Energy-Related Applications. Advanced Materials 2017, 29 (15), 1604563, DOI: 10.1002/adma.201604563. (27) Wang, B.; Huynh, T. P.; Wu, W.; Hayek, N.; Do, T. T.; Cancilla, J. C.; Torrecilla, J. S.; Nahid, M. M.; Colwell, J. M.; Gazit, O. M.; Puniredd, S. R.; McNeill, C. R.; Sonar, P.; Haick, H. A Highly Sensitive Diketopyrrolopyrrole‐Based Ambipolar Transistor for Selective Detection and Discrimination of Xylene Isomers. Advanced Materials 2016, 28 (21), 4012-4018, DOI: doi:10.1002/adma.201505641. (28) Jin, H.; Haick, H. UV regulation of non-equilibrated electrochemical reaction for detecting aromatic volatile organic compounds. Sensors and Actuators B: Chemical 2016, 237, 30-40, DOI: https://doi.org/10.1016/j.snb.2016.05.135. (29) Kodama, R. H.; Makhlouf, S. A.; Berkowitz, A. E. Finite Size Effects in Antiferromagnetic NiO Nanoparticles. Physical Review Letters 1997, 79 (7), 1393-1396, DOI: 10.1103/PhysRevLett.79.1393.

ACS Paragon Plus Environment

21

ACS Applied Materials & Interfaces 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

Page 22 of 30

(30) Cabo, M.; Pellicer, E.; Rossinyol, E.; Estrader, M.; Lopez-Ortega, A.; Nogues, J.; Castell, O.; Surinach, S.; Baro, M. D. Synthesis of compositionally graded nanocast NiO/NiCo2O4/Co3O4 mesoporous composites with tunable magnetic properties. Journal of Materials Chemistry 2010, 20 (33), 7021-7028, DOI: 10.1039/C0JM00406E. (31) Liu, B.; Li, X.; Zhao, Q.; Hou, Y.; Chen, G. Self-templated formation of ZnFe2O4 doubleshelled hollow microspheres for photocatalytic degradation of gaseous o-dichlorobenzene. Journal of Materials Chemistry A 2017, 5 (19), 8909-8915, DOI: 10.1039/C7TA02048A. (32) Kruk, M.; Jaroniec, M. Gas Adsorption Characterization of Ordered Organic−Inorganic Nanocomposite Materials. Chemistry of Materials 2001, 13 (10), 3169-3183, DOI: 10.1021/cm0101069. (33) Qu, F.; Jiang, H.; Yang, M. Designed formation through a metal organic framework route of ZnO/ZnCo2O4 hollow core-shell nanocages with enhanced gas sensing properties. Nanoscale 2016, 8 (36), 16349-16356, DOI: 10.1039/C6NR05187A. (34) Peck, M. A.; Langell, M. A. Comparison of Nanoscaled and Bulk NiO Structural and Environmental Characteristics by XRD, XAFS, and XPS. Chem. Mater. 2012, 24 (23), 44834490, DOI: 10.1021/cm300739y. (35) Barreca, D.; Massignan, C.; Daolio, S.; Fabrizio, M.; Piccirillo, C.; Armelao, L.; Tondello, E. Composition and Microstructure of Cobalt Oxide Thin Films Obtained from a Novel Cobalt(II) Precursor by Chemical Vapor Deposition. Chemistry of Materials 2001, 13 (2), 588593, DOI: 10.1021/cm001041x. (36) Gulino, A.; Fiorito, G.; Fragala, I. Deposition of thin films of cobalt oxides by MOCVD. Journal of Materials Chemistry 2003, 13 (4), 861-865, DOI: 10.1039/B211861K. (37) Wei, W.; Chen, W.; Ivey, D. G. Rock Salt−Spinel Structural Transformation in Anodically Electrodeposited Mn−Co−O Nanocrystals. Chemistry of Materials 2008, 20 (5), 1941-1947, DOI: 10.1021/cm703464p. (38) Lee, A. P.; Reedy, B. J. Temperature modulation in semiconductor gas sensing. Sensors and Actuators B: Chemical 1999, 60 (1), 35-42, DOI: https://doi.org/10.1016/S0925-4005(99)002415. (39) Jeong, H.-M.; Kim, J.-H.; Jeong, S.-Y.; Kwak, C.-H.; Lee, J.-H. Co3O4–SnO2 Hollow Heteronanostructures: Facile Control of Gas Selectivity by Compositional Tuning of Sensing Materials via Galvanic Replacement. ACS Applied Materials & Interfaces 2016, 8 (12), 78777883, DOI: 10.1021/acsami.6b00216. (40) Staerz, A.; Kim, T.-H.; Lee, J.-H.; Weimar, U.; Barsan, N. Nanolevel Control of Gas Sensing Characteristics via p–n Heterojunction between Rh2O3 Clusters and WO3 Crystallites. The Journal of Physical Chemistry C 2017, 121 (44), 24701-24706, DOI: 10.1021/acs.jpcc.7b09316. (41) Wang, B.; Huynh, T.-P.; Wu, W.; Hayek, N.; Do, T. T.; Cancilla, J. C.; Torrecilla, J. S.; Nahid, M. M.; Colwell, J. M.; Gazit, O. M.; Puniredd, S. R.; McNeill, C. R.; Sonar, P.; Haick, H. A Highly Sensitive Diketopyrrolopyrrole-Based Ambipolar Transistor for Selective Detection and Discrimination of Xylene Isomers. Advanced Materials 2016, 28 (21), 4012-4018, DOI: 10.1002/adma.201505641. (42) Jin, H.; Huynh, T.-P.; Haick, H. Self-Healable Sensors Based Nanoparticles for Detecting Physiological Markers via Skin and Breath: Toward Disease Prevention via Wearable Devices. Nano Letters 2016, 16 (7), 4194-4202, DOI: 10.1021/acs.nanolett.6b01066. (43) Li, J.; Lu, Y.; Ye, Q.; Cinke, M.; Han, J.; Meyyappan, M. Carbon Nanotube Sensors for Gas and Organic Vapor Detection. Nano Letters 2003, 3 (7), 929-933, DOI: 10.1021/nl034220x.

ACS Paragon Plus Environment

22

Page 23 of 30 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

ACS Applied Materials & Interfaces

(44) Yoon, J.-W.; Kim, J.-S.; Kim, T.-H.; Hong, Y. J.; Kang, Y. C.; Lee, J.-H. A New Strategy for Humidity Independent Oxide Chemiresistors: Dynamic Self-Refreshing of In2O3 Sensing Surface Assisted by Layer-by-Layer Coated CeO2 Nanoclusters. Small 2016, 12 (31), 42294240, DOI: 10.1002/smll.201601507. (45) Santarossa, G.; Hahn, K.; Baiker, A. Free Energy and Electronic Properties of Water Adsorption on the SnO2(110) Surface. Langmuir 2013, 29 (18), 5487-5499, DOI: 10.1021/la400313a. (46) Egashira, M.; Nakashima, M.; Kawasumi, S.; Selyama, T. Temperature programmed desorption study of water adsorbed on metal oxides. 2. Tin oxide surfaces. The Journal of Physical Chemistry 1981, 85 (26), 4125-4130, DOI: 10.1021/j150626a034. (47) Kim, H.-R.; Haensch, A.; Kim, I.-D.; Barsan, N.; Weimar, U.; Lee, J.-H. The Role of NiO Doping in Reducing the Impact of Humidity on the Performance of SnO2-Based Gas Sensors: Synthesis Strategies, and Phenomenological and Spectroscopic Studies. Advanced Functional Materials 2011, 21 (23), 4456-4463, DOI: 10.1002/adfm.201101154. (48) Bai, S.; Tian, K.; Tian, Y.; Guo, J.; Feng, Y.; Luo, R.; Li, D.; Chen, A.; Liu, C. C. Synthesis of Co3O4/TiO2 composite by pyrolyzing ZIF-67 for detection of xylene. Applied Surface Science 2018, 435, 384-392, DOI: https://doi.org/10.1016/j.apsusc.2017.10.080. (49) Suh, J. M.; Sohn, W.; Shim, Y.-S.; Choi, J.-S.; Song, Y. G.; Kim, T. L.; Jeon, J.-M.; Kwon, K. C.; Choi, K. S.; Kang, C.-Y.; Byun, H.-G.; Jang, H. W. p–p Heterojunction of Nickel OxideDecorated Cobalt Oxide Nanorods for Enhanced Sensitivity and Selectivity toward Volatile Organic Compounds. ACS Applied Materials & Interfaces 2018, 10 (1), 1050-1058, DOI: 10.1021/acsami.7b14545. (50) Li, Y.; Ma, X.; Guo, S.; Wang, B.; Sun, D.; Zhang, X.; Ruan, S. Hydrothermal synthesis and enhanced xylene-sensing properties of pompon-like Cr-doped Co3O4 hierarchical nanostructures. RSC Advances 2016, 6 (27), 22889-22895, DOI: 10.1039/C5RA26466A. (51) Sun, C.; Su, X.; Xiao, F.; Niu, C.; Wang, J. Synthesis of nearly monodisperse Co3O4 nanocubes via a microwave-assisted solvothermal process and their gas sensing properties. Sensors and Actuators B: Chemical 2011, 157 (2), 681-685, DOI: https://doi.org/10.1016/j.snb.2011.05.039. (52) Liu, L.; Zhong, Z.; Wang, Z.; Wang, L.; Li, S.; Liu, Z.; Han, Y.; Tian, Y.; Wu, P.; Meng, X. Synthesis, Characterization, and m-Xylene Sensing Properties of Co–ZnO Composite Nanofibers. Journal of the American Ceramic Society 2011, 94 (10), 3437-3441, DOI: 10.1111/j.1551-2916.2011.04528.x. (53) Sui, L.; Zhang, X.; Cheng, X.; Wang, P.; Xu, Y.; Gao, S.; Zhao, H.; Huo, L. Au-Loaded Hierarchical MoO3 Hollow Spheres with Enhanced Gas-Sensing Performance for the Detection of BTX (Benzene, Toluene, And Xylene) And the Sensing Mechanism. ACS Applied Materials & Interfaces 2017, 9 (2), 1661-1670, DOI: 10.1021/acsami.6b11754. (54) Qu, F.; Feng, C.; Li, C.; Li, W.; Wen, S.; Ruan, S.; Zhang, H. Preparation and XyleneSensing Properties of Co3O4 Nanofibers. International Journal of Applied Ceramic Technology 2014, 11 (4), 619-625, DOI: 10.1111/ijac.12160. (55) Sun, Y.-F.; Liu, S.-B.; Meng, F.-L.; Liu, J.-Y.; Jin, Z.; Kong, L.-T.; Liu, J.-H. Metal Oxide Nanostructures and Their Gas Sensing Properties: A Review. Sensors 2012, 12 (3), 2610, DOI:10.3390/s120302610 (56) Qu, F.; Yuan, Y.; Guarecuco, R.; Yang, M. Low Working-Temperature Acetone Vapor Sensor Based on Zinc Nitride and Oxide Hybrid Composites. Small 2016, 12 (23), 3128-3133, DOI: 10.1002/smll.201600422.

ACS Paragon Plus Environment

23

ACS Applied Materials & Interfaces 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

Page 24 of 30

(57) Lü, Y.; Zhan, W.; He, Y.; Wang, Y.; Kong, X.; Kuang, Q.; Xie, Z.; Zheng, L. MOFTemplated Synthesis of Porous Co3O4 Concave Nanocubes with High Specific Surface Area and Their Gas Sensing Properties. ACS Applied Materials & Interfaces 2014, 6 (6), 4186-4195, DOI: 10.1021/am405858v. (58) Hronec, M.; Majling, J. Liquid-phase oxidation of p-xylene catalyzed by supported metal oxides. Applied catalysis 1987, 29 (1), 67-71, DOI: 10.1016/S0166-9834(00)82607-X. (59) Hronec, M.; Ilavský, J. Kinetics of p-xylene oxidation catalyzed by cobalt oxide. Reaction Kinetics and Catalysis Letters 1987, 33 (2), 299-303, DOI: 10.1007/BF02128079. (60) Mizsei, J. How can sensitive and selective semiconductor gas sensors be made? Sensors and Actuators B: Chemical 1995, 23 (2), 173-176, DOI: 10.1016/0925-4005(94)01269-N. (61) Liu, C.; Wang, B.; Wang, T.; Liu, J.; Sun, P.; Chuai, X.; Lu, G. Enhanced gas sensing characteristics of the flower-like ZnFe2O4/ZnO microstructures. Sensors and Actuators B: Chemical 2017, 248, 902-909, DOI: https://doi.org/10.1016/j.snb.2017.01.133. (62) Ren, F.; Gao, L.; Yuan, Y.; Zhang, Y.; Alqrni, A.; Al-Dossary, O. M.; Xu, J. Enhanced BTEX gas-sensing performance of CuO/SnO2 composite. Sensors and Actuators B: Chemical 2016, 223, 914-920, DOI: https://doi.org/10.1016/j.snb.2015.09.140. (63) Zhang, J.; Liu, X.; Neri, G.; Pinna, N. Nanostructured Materials for Room-Temperature Gas Sensors. Advanced Materials 2016, 28 (5), 795-831, DOI: 10.1002/adma.201503825. (64) Wang, C.; Cheng, X.; Zhou, X.; Sun, P.; Hu, X.; Shimanoe, K.; Lu, G.; Yamazoe, N. Hierarchical α-Fe2O3/NiO Composites with a Hollow Structure for a Gas Sensor. ACS Applied Materials & Interfaces 2014, 6 (15), 12031-12037, DOI: 10.1021/am501063z.

ACS Paragon Plus Environment

24

Page 25 of 30 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

ACS Applied Materials & Interfaces

Insert Table of Contents Graphic and Synopsis Here

ACS Paragon Plus Environment

25

ACS Applied Materials & Interfaces 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

Fig. 1 (a) SEM image and (b) TEM image of mixed Ni-Co oxides. The EDX elemental mapping of mixed Ni-Co oxides, (c) position, (d) nickel, (e) cobalt and (f) oxygen. (g) EDS spectrum of mixed Ni-Co oxides. 158x141mm (300 x 300 DPI)

ACS Paragon Plus Environment

Page 26 of 30

Page 27 of 30 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

ACS Applied Materials & Interfaces

Fig. 2. (a) Schematic illustration of the formation process of mixed Ni-Co oxides. (b-e) TEM images of solid CPs precursor and composites obtained through thermal treatment of solid CPs precursor to 360, 390 and 500 oC, respectively. The scale bar is 100 nm. 237x108mm (300 x 300 DPI)

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

Fig. 3. (a) XRD patterns of mixed Ni-Co oxides. (b) Pore size distributions of mixed Ni-Co oxides, Co3O4 and NiO. XPS spectrum of mixed Ni-Co oxides, (c) all, (d) Ni 2p, (e) Co 2p, and (f) O 1s. The inset image of (b) and (c) is the N2 adsorption/desorption isotherms and the atomic concentrations of Ni, Co and O in Ni-Co oxides, respectively. 210x221mm (300 x 300 DPI)

ACS Paragon Plus Environment

Page 28 of 30

Page 29 of 30 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

ACS Applied Materials & Interfaces

Fig. 4. (a) Gas response (Rg/Ra) and response time (s) of Ni-Co oxides, Co3O4 and NiO sensors to 5 ppm xylene at 138–344 °C. (b) Cross-responses of Ni-Co oxides, Co3O4 and NiO sensors to 5 ppm various gases (the concentration of CO is 100 ppm) at 255, 221 and 302 oC, respectively. (c) Dynamic gas sensing transients of Ni-Co oxides sensor to 1-5-1 ppm of xylene, and (d) responses to xylene as a function of concentration at 255 oC. (e) Humidity dependence of response of Ni-Co oxides, Co3O4 and NiO sensors to 5 ppm acetone at 255, 221 and 302 oC, respectively. (f) Coefficient of variation of the Ni-Co oxides, Co3O4 and NiO sensors by varying RH from 11% to 95 %. 179x160mm (300 x 300 DPI)

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

Fig. 5. (a) The energy band structure of Co3O4/NiO heterojunctions - before and after contact. (b) Schematic representation of the mechanism influencing the gas sensing behavior. 166x139mm (300 x 300 DPI)

ACS Paragon Plus Environment

Page 30 of 30