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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

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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-

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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.


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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-


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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


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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


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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


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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


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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.


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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


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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).


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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


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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

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


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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


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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.


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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


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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


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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


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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


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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.


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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)


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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)



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)


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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)



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)


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