Ionic liquids combined with Pt-modified ordered mesoporous carbons as electrolytes for the oxygen sensing

Sensors and Actuators B 254 (2018) 490–501

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Ionic liquids combined with Pt-modified ordered mesoporous carbons as electrolytes for the oxygen sensing Ying Liu a , Jingyuan Liu a,∗ , Qi Liu a , Hongsen Zhang a , Zhanshuang Li a , Xiaoyan Jing a , Yi Yuan a , Hongquan Zhang a,c , Peili Liu b , Jun Wang a,b,∗ a

Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, PR China Institute of Advanced Marine Materials, Harbin Engineering University, 150001, PR China c School of Automation, Harbin Engineering University, 150001, PR China b

a r t i c l e

i n f o

Article history: Received 23 February 2017 Received in revised form 11 July 2017 Accepted 12 July 2017 Available online 15 July 2017 Keywords: Cyclic voltammetry Room temperature ionic liquids Ordered mesoporous carbons Platinum nanoparticles Oxygen sensor

a b s t r a c t In recent years, room temperature ionic liquids (RTILs) have been highly utilized in electrochemical fields. Specifically, ionic liquids (ILs) as electrolytes augur promising potential for the next generation of advanced and miniaturized electrochemical gas sensors. In this paper, 1-ethyl-3-methylimidazolium hexafluorophosphate ([EMIM][PF6 ]), 1-propyl-3-methylimidazolium hexafluorophosphate ([PMIM][PF6 ]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6 ]) were synthesized through a facile one-pot method. We found that [EMIM][PF6 ] and [PMIM][PF6 ] exist in the solid state at room temperature. Hence, these ILs can effectively avoid the leakage of electrolytes. However, [BMIM][PF6 ] can be present in liquid state at room temperature as the melting point decreases, due to the increasing of alkyl chain (Cn , n < 6) for the ILs. Compared to solid electrolyte gas sensors, sensors based on liquid electrolytes offer a reliable response and achieve sufficient ion mobility to sense oxygen. Our work provides an incentive to enhance the properties of a liquid electrolyte by mixing it with Pt-modified ordered mesoporous carbons (Pt-OMCs), which not only prevents the aggregation of OMCs but also provides a large number of active sites for oxygen reduction. The limit of detection (LOD) in oxygen sensors based on Pt/CMK-3COOH-[BMIM][PF6 ] is relatively low (1.10% O2 ). Our results indicate that the synergistic combination of ILs and Pt-OMCs markedly promotes the performance of electrochemical oxygen sensors. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Electrochemical oxygen sensors have been discussed widely in recent years [1]. The electrolyte is one of the significant components, which contacts all electrodes effectively and solubilizes the reactants and products for efficient mass transport. Therefore, electrolyte should be chemically and physically stable under all conditions of oxygen sensors operation [2]. One kind of wellknown electrochemical gas sensors is based on a solid electrolyte, such as Ag2 SO4 [3], ZrO2 [4] and ZnCr2 O4 [5]. However, the solid electrolytes show a low conductivity due to the combination of insufficient electrical carriers at room temperature, so oxygen sensors based on solid-state depend strongly on temperature [6,7]. Normally, these sensors are used at high temperature [8,9], result-

∗ Corresponding authors at: Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, PR China. E-mail addresses: [email protected] (J. Liu), [email protected] (J. Wang). http://dx.doi.org/10.1016/j.snb.2017.07.069 0925-4005/© 2017 Elsevier B.V. All rights reserved.

ing in large power consumption which is inconsistent with the contemporary needs [7,10]. However, using liquid electrolytes can result in the drying out of solutions and the inconvenience in fabrication and transportation of sensors. For example, acetonitrile, dimethylformamide and dimethylsulfoxide [11], which are traditional liquid electrolyte solutions, have been applied for various researches. In contrast to conventional electrolytes, ionic liquids (ILs) are preferable for manufacturing oxygen sensors, because of their wide electrochemical potential window, remarkable physical and chemical stability as well as excellent solubility for a range of materials. Moreover, one unique advantage is that the low vapor pressure of ILs can eliminate the need for a membrane and thus simplify sensor design [12–18]. Hence, the electrochemical gas sensors based on ILs tend to have a longer lifetime. However, the majority of ILs-based electrolytes exist in the liquid state at room temperature. They have some disadvantages such as having a degree of fluidity, which limits their usage. As reported by Junqiao Lee et al. [19], 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([EMIM][NTf2 ]) was incorporated with poly(methyl methacrylate) (PMMA), which improved

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the mechanical stability of [EMIM][NTf2 ] and also reduced the impact from atmospheric impurities and moisture. However, the peak current and sensitivity of the oxygen sensor systematically decreased with the increasing content of PMMA. Thus we expect to improve the mechanical robustness of the electrochemical oxygen sensors by investigating ILs, which facilitates solid-liquid conversion by heating or cooling to meet the demand in certain situations, e.g. moving, shaking or tilting of the electrode, where loss of the ILs might occur [20]. Furthermore, the relatively low conductivity manifested by the inherently high viscosity of ILs and the comparatively small diffusion coefficients of gas molecules in ILs usually lead to slow responses and small limiting currents [21]. To solve these problems, great attempts are made to facilitate the diffusion of gas analytes in IL electrolytes. The most efficient strategy involves the formation of thin IL layers on microfabricated electrode arrays [22,23], which is expensive and could not be widely employed in the gas sensor field. This strategy produces IL layers with thicknesses up to several micrometers at the sensing interfaces and therefore effectively improves the performance of the IL-based sensors. However, it does not meet the rules of low cost and easy-to-use techniques of the contemporary society. So many researchers apply carbon nanotubes (CNTs)-modified electrolytes. Ming Zhou et al. [24] compared the responses of using OMCs and CNTs to modify glassy carbon electrodes for electrochemistry of eight kinds of materials. The study discovered that the OMCs-modified glassy carbon electrode has superior electron transfer kinetics in contrast to the CNTs-modified glassy carbon electrode. The OMCs exhibit a high specific surface area, a periodic mesoporous structure and chemical inertness, which make OMCs suitable for various of many applications including sensors [24–30]. The CMK-3, which is a type of OMC, possesses a marked ability to support ILs. To reduce the cost of sensors, the aggregation of Pt nanoparticles (NPs) should be avoided by being loaded on carbon supports. Therefore, Pt NPs were uniformly loaded onto the surface of CMK-3-COOH by the reduction of hydrogen hexachloroplatinate hexahydrate (H2 PtCl6 ·6H2 O). Herein, we found that [EMIM][PF6 ] and [PMIM][PF6 ] can be present in solid state at room temperature but they easily melt after heating owing to their low melting points. Therefore, under the same high-temperature conditions, the ILs can be present in liquid state, which has higher electrical conductivity compared to solid electrolytes. The spill-less electrolytes are very robust for instability caused by moving, thus they possess portable application, avoiding a number of problems such as the leakage of liquid electrolytes. Hence, to a great extent, the lifetime of a gas sensor is prolonged and the field of application is broadened. Moreover, Pt/CMK-3-COOH is firstly introduced into imidazole-based ILs for detecting oxygen. An improved outcome is observed with a linear current response for oxygen reduction from 0 to 100% O2 . A sensor assembled with Pt/CMK-3-COOH and imidazole-based ILs exhibits outstanding gas sensing performance towards oxygen. This is ascribed to the large pore volume and specific surface area of CMK-3, which support Pt NPs for forming a porous composite. More importantly, the presence of Pt/CMK-3-COOH in the electrolyte accelerates the direct charge transportation in ILs [31–34].

491

Fig. 1. Scheme of preparation for Pt/CMK-3-COOH (a); Procedure of mixing Pt/CMK3-COOH with [BMIM][PF6 ] (b); Illustration of the structure of O2 sensor (c).

2. Experimental section 2.1. Chemical reagents ILs, 1-methylimidazole (Xiya Reagent), bromoethane (Tianjin Guangfu), 1-bromopropane (Aladdin Industrial) and 1bromobutane (Xiya Reagent) were used directly without further purification. H2 PtCl6 ·6H2 O and potassium hexafluorophosphate (KPF6 ) were obtained from Xiya Reagent. The CMK-3 was supplied

by XFNANO, INC, and sodium boronitride (NaBH4 ) and silver nitrate (AgNO3 ) was purchased from Sinopharm Chemical Reagent Co., Ltd. Acetonitrile (MeCN, Sigma-Aldrich) solvent was used for washing the electrode before and after using with ILs. High purity N2 , 20% O2 , 40% O2 , 60% O2 , 80% O2 and 100% O2 , which were purchased from Qinghua gases (Harbin, China) and used for electrochemical experiments.

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Fig. 2. FT-IR spectra of [EMIM][PF6 ] (a), [PMIM][PF6 ] (b), [BMIM][PF6 ] (c) as well as CMK-3 and CMK-3-COOH (d).

2.2. Preparation of 1-alkyl-3-methylimidazolium hexafluorophosphate ([AMIM][PF6 ]) 0.050 mol 1-methylimidazole, 0.055 mol bromoalkane and 0.060 mol KPF6 were added into a 100 mL flask, which was connected with a condenser pipe. The mixture was stirred continuously for 3 h at 353 K, then it was rinsed many times with ultrapure water until no yellowish precipitate remained in the water layer, as detected by 0.5 mol L−1 AgNO3 solution. The product was dried in a vacuum oven at 333 K for 24 h to remove the moisture and avoid the influence of trace impurities before the electrochemical measurements. The reaction equation is as follows:

(1)

following step, 0.052 g H2 PtCl6 ·6H2 O was added into the previous solution. The mixture was vigorously stirred for 30 min and a relative excess amount of 0.1 mol L−1 NaBH4 solution was dropped into the mixture. After stirring for 3 h, the solid part of the solution was collected following centrifugation, then the product was fully washed with ultrapure water and dried in air at 333 K overnight. The sample was named as Pt/CMK-3-COOH. 2.4. Preparation of Pt/CMK-3-COOH-[BMIM][PF6 ] 20 mg of Pt/CMK-3-COOH and 0.1 mL hydrophobic [BMIM][PF6 ] were mixed by stirring in a crucible to form a black gel as displayed in Fig. 1b. So the Pt/CMK-3-COOH-[BMIM][PF6 ] mixture could be pipetted onto the electrode surface. The fabrication procedure is illustrated in Fig. 1c and Fig. S1b. 2.5. Instrumentation

2.3. Preparation of Pt/CMK-3-COOH To enhance the active spots at the surface of CMK-3, functionalization of the CMK-3 with carboxylic groups was employed via acid treatment[35]. Specifically, the CMK-3 was treated with 0.5 mol L−1 HNO3 for 3 h at 353 K, then the product was centrifuged several times and washed with ultrapure water. The CMK-3-COOH was obtained after drying in air at 333 K overnight. The uniform loading of Pt NPs onto the CMK-3-COOH sample was completed by wet chemical reduction method[36], as it shown in Fig. 1a. In brief, 0.100 g CMK-3-COOH was added to 10 mL H2 O, then the solution was put into an ultrasonic bath for 30 min. In the

The as-prepared samples were characterized by X-ray diffraction (XRD, Rigaku TTR-III) with high-intensity Cu-K␣ radiation ( = 0.15406 nm) operating at 40 kV and 150 mA to investigate the crystal structure thoroughly. The morphology of samples was examined by field-emission scanning electron microscopy (FESEM, HitachiSU8000) operating at an accelerating potential of 2.0 kV. Fourier transform infrared spectroscopy (FTIR) measurements were carried out on a Perkin-Elmer Spectrum 100 Fourier transform infrared spectrometer to investigate the molecular structure of ILs in the 400–4000 cm−1 region by using the KBr-disk method. 1 H NMR (500 MHz) measurements were taken on a Bruker Advance

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Fig. 3.

1

H NMR spectra of [EMIM] [PF6 ] (a), [PMIM][PF6 ] (b) and [BMIM][PF6 ] (c).

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Fig. 4. Small-angle XRD pattern (a) and large-angle XRD pattern (b) of as-prepared samples.

III-500 spectrometer with deuterated dimethylsulfoxide (DMSO) as the solvent at room temperature. Thermogravimetric (TG) analysis and different scanning calorimetry (DSC) were carried out with an SDTQ-600 (TA, U.S.) thermogravimetric analyzer at a heating rate of 2 K/min from 300 K to 350 K in a nitrogen atmosphere. The specific surface areas of CMK-3, CMK-3-COOH and Pt/CMK-3-COOH were calculated by Brunauer-Emmett-Teller (BET) equation from a Micromeritics ASAP 2020 instrument (U.S.) at liquid nitrogen temperature. Pore size distribution and pore volume were estimated by using Barrett-Joyner-Halenda (BJH) method from the desorption branch of the isotherm. We conducted a series of electrochemical tests including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry and linear sweep voltammetry (LSV) by using a computer-controlled ␮-Autolab (PGSTAT302N FRA32 M). The CV measurements were conducted in [BMIM][PF6 ] and Pt/CMK-3COOH-[BMIM][PF6 ] at different scan rates within a potential range of −5 to 5 V (vs Pt) in 20% O2 . Under different concentrations of oxygen, the CV curves were measured with a constant sweep rate of 100 mV s−1 . The impedance spectra obtained within 20% O2 atmosphere were carried out at the open circuit potential. The other parameters of impedance spectra, the AC amplitude, initial frequency and final frequency, were set at ±5 mV, 105 Hz and 0.1 Hz, respectively. As parameters of chronoamperometry, a constant potential was set at −2 V (vs Pt), and a run time of 800 s in 20% O2 . LSV was carried out in the N2 atmosphere, measured at a potential of −2 V (vs Pt), and the range of potential was −5–5 V (vs Pt). All electrochemical experiments were conducted with an interdigitated electrode and the sensor was deposited in a temperature-controlled box to reduce ambient electromagnetic interferences. The working electrode (WE) and reference electrode (RE) of the interdigitated electrodes were made of Pt (0.1 mm diameter). The detailed structures of interdigitated electrode and sensor are shown in Fig. S1. Prior to the addition of gases, ILs and Pt/CMK-3-COOH[BMIM][PF6 ] were deposited under vacuum to remove moisture and avoid the influence of trace impurities. About 20 ␮L sample was delivered by micropipette onto the surface of the integrated electrodes. The ILs were left in N2 atmosphere overnight to purge any oxygen, other absorbed gases and volatilize solvents before being employed. The O2 volume percentages of this experiment were 20%, 40%, 60%, 80% and 100%, with N2 making up the remainder of the gas mixture. All electrochemical measurements were performed at

10-min intervals to ensure that the gases were saturated and the CVs were stable totally. 3. Results and discussion 3.1. Structural characteristics of ILs and CMK-3 The chemical structure of 1-alkyl-3-methylimidazolium hexafluorophosphate [AMIM][PF6 ] is shown in Fig. 2a–c. The FTIR spectra of [EMIM][PF6 ], [PMIM][PF6 ] and [BMIM][PF6 ] are in agreement with the expected results of ILs possessing the same characteristic peaks. The origin of [AMIM][PF6 ] vibrational bands are as follows: the peak at 3300–2780 cm−1 is ascribed to the C H stretching vibration. The peaks at 3171.4 and 3125.1 cm−1 are clearly visible and attributed to the C H ring stretching vibration of imidazolium. The peaks at 2966.4 and 2878.6 cm−1 support the presence of the stretching vibration of the saturated C H bond. The bands at 1574.9 and 1468.4 cm−1 are attributed to skeletal vibration of the imidazole ring. The peak at 1339.4 cm−1 is assigned to the C H deformation vibration of methyl. The band at 1169.6 cm−1 commences the stretching mode of vibration for imidazole ring. The peak at 839.0 cm−1 is characteristic of the P-F mode formed of [AMIM][PF6 ]. The low wavenumber region exhibits a series of vibration peaks, which correspond to the plane bending vibration of C H mode. The results demonstrate that [EMIM][PF6 ], [PMIM][PF6 ] and [BMIM][PF6 ] are successfully synthesized. Fig. 2d shows FTIR spectra of the CMK-3 and CMK-3-COOH samples. The bands at 1579.28 and 1258.62 cm−1 are attributed to C C and C O C stretching vibrations [37]. A new band appears at 1738.13 cm−1 in the spectra of CMK-3-COOH sample, which is assigned to the stretching vibration of C O for −COOH [38]. Therefore, the FTIR patterns confirm the existence of −COOH in CMK-3-COOH. The 1 H NMR spectrum of [BMIM][PF6 ] (Fig. 3c) portrays the following characteristic signals: (a) ı = 9.11, 7.77 and 7.71 ppm, attributed to hydrogen bonding with carbon atoms 1, 2 and 3 of the imidazole ring; (b) ı = 3.87 ppm due to hydrogen bonding with carbon atom 4 of the methyl group; (c) ı = 4.18, 1.79, 1.27 and 0.93 ppm corresponding to hydrogen bonding to carbon atoms 5, 6, 7 and 8 of the butyl group. Compared with [PMIM][PF6 ] (Fig. 3b), the 1 H NMR spectrum of [BMIM][PF6 ] has another peak with ı = 1.27 ppm, which confirms the existence of a propyl group in [PMIM][PF6 ]. No peak is found with ı = 1.79 and 1.27 ppm, which indicates that [EMIM][PF6 ] (Fig. 3a) is synthesized successfully. The above-

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Fig. 5. SEM images of CMK-3 (a and b), CMK-3-COOH (c and d) and Pt/CMK-3-COOH (e and f).

mentioned conclusions are consistent with the results determined by FTIR. Various spectroscopic and analytical methods were used to examine the physicochemical properties of pristine CMK3, CMK-3-COOH and Pt/CMK-3-COOH samples. From Fig. 4a, the small-angle XRD pattern of CMK-3 sample shows a main (100) diffraction peak at 2␪ = 1.03◦ , which indicates an ordered mesoporous structure with a two-dimensional (2-D) hexagonal symmetry. The (100) diffraction peak is also consistent with the pattern of CMK-3-COOH, suggesting that the 2-D hexagonal symmetry is not destroyed [39]. As shown in Fig. 4b, the large-angle XRD patterns of CMK-3, CMK-3-COOH and Pt/CMK-3-COOH samples show a broad diffraction peak at 24.60◦ , which ensures the presence of the C(002) [36]. As for CMK-3-COOH modified with Pt NPs, the large-angle XRD pattern indicates individual (111), (200), (220), (311) and (222) diffraction peaks at 2␪ = 39.75◦ , 46.23◦ , 67.45◦ , 81.24◦ and 85.69◦ (JCPDS.No.65-2868), respectively, demonstrating that Pt NPs can be compounded at the surface of CMK-3, which

is functionalized by using HNO3 [36,39]. The average particle size of Pt can be calculated using the Scherer equation [40]: d = 0.9␭/B2␪ cos␪

(2)

where d is the average size of Pt particles,  is the X-ray wavelength ( = 0.15406 nm),  is the angle of the peak, and B2 is the halfpeak width. Pt nanoparticle size was calculated about 2.0 nm for Pt/CMK-3-COOH. The morphologies of the as-prepared samples were observed by SEM (Fig. 5). As shown in a series of figures below, Fig. 5a and b exhibit a panoramic image of CMK-3, from which we observe the aggregated rod-like nanostructures. Fig. 5c and d are the SEM images of CMK-3, which are functionalized with HNO3 . The CMK3-COOH sample appears as a well-dispersed rod-like morphology with a relatively uniform shape, and the average particle size (Fig. 5c) becomes smaller as compared to that in Fig. 5b. The SEM images of Pt/CMK-3-COOH (Fig. 5e and f) show that the CMK-3COOH sample is ideally decorated with Pt NPs. The results reveal

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Fig. 6. TG and DSC curves of the [EMIM][PF6] (a) and [PMIM][PF6] (b) in a nitrogen atmosphere. N2 adsorption-desorption isotherm (c) and the BJH pore size distribution plot (d) of CMK-3, CMK-3-COOH and Pt/CMK-3-COOH.

that Pt NPs are successfully loaded onto the surface of CMK3-COOH. In addition, the immobilized Pt NPs exhibit excellent adhesive characteristics for the CMK-3-COOH samples, which is vital for their utilization in sensors. Fig. 6a and b present TG and DSC curves of the [EMIM][PF6 ] and [PMIM][PF6 ] in a nitrogen atmosphere, respectively. For two ILs, the TG plots show a platform in the range of 300–350 K, demonstrating that the weight and composition of as-prepared samples remain unchanged. To accurately ascertain the melting point of [EMIM][PF6 ] and [PMIM][PF6 ], DSC measurement was conducted. As displayed in Fig. 6a, a sharp endothermic peak appears from 328 K to 338 K, and the peak point at 332 K represents the melting point of [EMIM][PF6 ]. Similarly, the range of endothermic peak for [PMIM][PF6 ] is 310–321 K, and the melting point is 313 K (Fig. 6b). As for [BMIM][PF6 ], it exists in liquid state around 298 K and below. This result is in accord with the finding that ILs (Cn < 6)[41] with longer alkyl chains have lower melting temperatures. The textural properties of as-obtained products were investigated by N2 adsorption-desorption isothermal technique (Fig. 6c and d). From Fig. 6c, due to the hierarchical mesoporous and macroporous structures, the shape of the isotherms are classified as type IV, as indicated by strong adsorbent-adsorbate interaction [42,43]. The isotherms show that desorption starts straightway after completion of adsorption. The isotherms of CMK-3 and CMK-3-COOH show a well-defined hysteresis loop at 0.4–0.9 relative pressures (P/P0 ), indicating the existence of a significant amount of mesopores. As for Pt/CMK-3-COOH, a narrow hysteresis loop occurs at 0.4–0.8 P/P0 . Moreover, the amount of adsorbed volume of nitrogen rose rapidly at 0.95–1.0 P/P0 , which is attributed to a capillary condensation phenomenon in the mesopores. Fig. 6d exhibits the pore size distributions calculated by Barrett-Joyner-Halenda (BJH)

Table 1 Physical properties of CMK-3, CMK-3-COOH and Pt/CMK-3-COOH. Sample

SBET a [m2 g−1 ]

Smicro b [m2 g−1 ]

Vtotal c [cm3 g−1 ]

CMK-3 CMK-3-COOH Pt/CMK-3-COOH

811.72 752.13 539.70

315.96 265.65 174.91

0.067 0.036 0.019

a

Specific surface area is calculated by Brunauer-Emmett-Teller (BET) equation. Specific surface area of micropores is calculated by the t-plot method. The total pore volume is determined from the amount of nitrogen adsorbed at a relative pressure of 0.99. b

c

method from the desorption branch of the isotherm. The pore size of as-prepared samples is 2–265 nm. This finding is consistent with the results obtained by N2 adsorption-desorption isotherm. More importantly, the pore diameters of CMK-3, CMK-3-COOH and Pt/CMK-3-COOH specimens are mainly ∼4.5 nm. The porous textural details of the three materials are summarized in Table 1. By comparison, CMK-3 has the largest BET surface area (CMK–3 > CMK-3-COOH > Pt/CMK-3-COOH), and the specific surface area of micropore and total pore volume show the same order. In particular, after loading of Pt NPs onto CMK-3-COOH, the BET surface area remains at 539.70 m2 g−1 , which contributes to a faster process of O2 reduction. 3.2. Electrochemical characteristics of ILs and Pt/CMK-3-COOH-[BMIM][PF6 ] Chronoamperometry is an electrochemical technique in which the potential of the working electrode is stepped-up, and the resulting current from faradaic processes occurring at the electrode (caused by the potential step) is monitored as a function of time.

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Fig. 7. (a) Chronoamperometric current at a Pt electrode modified by [EMIM][PF6 ], [PMIM][PF6 ] and [BMIM][PF6 ] in 20% O2 . The electrode potential is kept at −2.0 V vs Pt; (b) LSV at a Pt electrode modified by [EMIM][PF6 ], [PMIM][PF6 ] and [BMIM][PF6 ] in N2 atmosphere. Scan range is −5–5 V vs Pt. Scan rate: 100 mV s−1 ; (c) CV curves in [EMIM][PF6 ], [PMIM][PF6 ] and [BMIM][PF6 ] within 20% O2 at 100 mV s−1 ; (d) The impedance spectra in [EMIM][PF6 ], [PMIM][PF6 ] and [BMIM][PF6 ] within 20% O2 . Temperature: 340 K.

Fig. 7a shows oxygen reduction in different electrolytes with a constant potential of −2 V vs Pt in 20% O2 . The response of the current tends to decrease at the beginning, and the current of three ILs is steady at −0.67 ␮A. [BMIM][PF6 ] at the first tends to be stable, whereas [EMIM][PF6 ] takes the longest time. Linear sweep voltammetry (LSV) is a voltammetric method, which is used to calculate the electrochemical potential window of ILs. The electrochemical potential window of an electrolyte is the potential range between, which the electrolyte is neither oxidized nor reduced. And the electrochemical potential window is one of the most important characteristics to be identified for solvents and electrolytes used in electrochemical applications. Fig. 7b suggests that electrochemical potential window of [BMIM][PF6 ] is 6.09 V, which shows the widest range of the three ILs. For [PMIM][PF6 ] and [EMIM][PF6 ], the electrochemical potential windows are 4.75 V and 3.66 V, respectively. Cyclic voltammetry (CV) is a type of potentiodynamic electrochemical measurement, which is used widely. From Fig. 7c, the cathodic and anodic peaks in [EMIM][PF6 ], [PMIM][PF6 ] and [BMIM][PF6 ] are close, but the current of the cathodic peak in [BMIM][PF6 ] is largest. EIS was employed for further investigating the electrochemical performance of as-prepared samples (Fig. 4d). It is clear that the resistances (Rs ) between the working and reference electrodes were evaluated to be 2977, 2972 and 2912  for [EMIM][PF6 ], [PMIM][PF6 ] and [BMIM][PF6 ] from the intercepts of the plots at the Z’ axis, respectively. [BMIM][PF6 ] also exhibits a higher productivity than other ILs in the course of the experiment. And the FTIR spectrograms show that the [BMIM][PF6 ] is the purest,

which is attributed to the low melting point of [BMIM][PF6 ]. Therefore, we chose [BMIM][PF6 ] to conduct the follow-up experiments. We investigate the influence of scan rate on the redox peak response with the results in Fig. 8a and b, showing that a pair of symmetrical redox peaks appears. The presence of the oxidation peak, which corresponds to the oxidation of superoxide back to oxygen, testifies that the generated superoxide is stable and the reduction of oxygen is electrochemically reversible in this system. The reaction mechanism is as follows [14,44]: O2 + e−  O2 −

•

(3)

Fig. 8a shows that the overlay of cyclic voltammograms in [BMIM][PF6 ] and Pt/CMK-3-COOH-[BMIM][PF6 ] electrolytes at a scan rate of 100 mV s−1 within 20% O2 . It can be seen that the cathodic and anodic peaks in [BMIM][PF6 ] are very close in comparison to those in Pt/CMK-3-COOH-[BMIM][PF6 ]. Moreover, for the [BMIM][PF6 ] system, the redox current is relatively weak. But in the Pt/CMK-3-COOH-[BMIM][PF6 ] system, two obvious redox peaks appeared, confirming that CMK-3-COOH modified with Pt NPs has high electrochemical activity. The reason for this enhanced electrolyte property is the high porosity and high specific surface area of CMK-3 and the excellent conductivity of Pt NPs. As a result, Pt/CMK-3-COOH-[BMIM][PF6 ] exhibits an excellent improved performance to promote electron transfer and the sensor based on this electrolyte has been developed for detecting oxygen. For Fig. 8b, the peak currents increase gradually with increasing scan rates and reduction peaks occurred with a negative shift. Meanwhile, oxidation peaks moved to a positive direction. The inset

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Fig. 8. (a) Overlayed cyclic voltammograms in [BMIM][PF6 ] and Pt/CMK-3-COOH-[BMIM][PF6 ] electrode at scan rate 100 mV s−1 in 20% O2 ; (b) Overlayed cyclic voltammograms at a Pt electrode with different scan rates (100 ∼ 1000 mV s−1 ) in Pt/CMK-3-COOH-[BMIM][PF6 ] within 20% O2 . Inset: calibration curves of peak current for oxygen reduction with proportion of scan rates; (c) Overlayed cyclic voltammograms at a Pt electrode with different concentrations of oxygen (0 ∼ 100% O2 ) in Pt/CMK-3-COOH[BMIM][PF6 ], scan rate: 100 mV s−1 ; (d) The corresponding plots of peak current for oxygen reduction vs. oxygen concentration. Temperature: 298 K.

of Fig. 8b shows that the corresponding plots of peak current vs. the square root of scan rate are linear, indicating that the reduction of oxygen is diffusion controlled. The equation for the calibration curves is I = −42.501/2 –45.54, where I/␮A is the measured current, 1/2 /(V s−1 )1/2 is the square root of scan rate, and linear regression coefficient (R2 ) is 0.9997. The influence of oxygen concentration on redox peak response was investigated in Fig. 8c. The background current of Pt/CMK3-COOH-[BMIM][PF6 ] was measured prior to the introduction of oxygen, which is 0.94 ␮A. The relatively small background currents suggest that there is a low-level impurity in the samples. The peak current increased with oxygen concentration and rose to a level of ca. 0.26 mA for 100% O2 . It can be observed that an excellent linear relationship is achieved in Fig. 8d, which describes the relationship between the reduction peak current and the oxygen concentrations from 0 to 100% O2 . The equation for the calibration curves is I = −2.5647[O2 ]−0.2543, where I/␮A is the measured current and [O2 ]/vol% is the oxygen concentration. The corresponding R2 value is 0.9998, and the limit of detection (LOD), based on three times the standard deviation of the intercept, is 1.10%. It demonstrates that Pt/CMK-3-COOH-[BMIM][PF6 ] is a promising material for oxygen sensors due to its wide detection range. Since the sensor’s repeatability test is performed by alternatingly purging different concentrations of oxygen and pure nitrogen, the long-term chronoamperometry (LTCA) gives a better S/N ratio in comparison to other amperometric techniques, making it a

simple and sensitive method for developing real-world sensor applications. The LTCA was performed to investigate the ability of the device to continuously monitor oxygen concentrations. This can be considered as quite a harsh technique due to the accumulation of superoxide at the working electrode. Fig. 9a shows LTCA for ca. 3000 s in the Pt/CMK-3-COOH-[BMIM][PF6 ], and the stable values at each step are collected to form the plots of current vs. O2 concentration, as shown in Fig. 9b. Both the first ascending and descending plots from 0–100 vol% O2 have similar currents. However, for the second ascending concentration set, the currents are slightly smaller. The sensor turns out to respond quite quickly to the changes in oxygen concentration, in that currents reach stationary values past both their increase and decrease in about 5 s. Long-term stability is one of the most important properties of sensors. Working stability of the electrolyte was verified by monitoring the electrochemical behavior of a reversible couple after successive sweeps of cyclic voltammograms. In Fig. 9c, it is noticed that the current can retain about 95% of its original value for consecutive 50 cycles CV test from −5 V to 5 V at 100 mV s−1 in Pt/CMK-3-COOH-[BMIM][PF6 ] within 20% O2 . Moreover, in order to further evaluate the long-term stability of the response to O2 for our sensor based on IL electrolytes, the sensor was stored dry at room temperature and the current is integrated over relatively long time intervals by the CV peak current drift, which is relatively defined to the original peak current at an interval of 5 days for a total period of 30 days. The sensor retained 99.95%, 99.89%, 98.34%, 96.03%, 93.47% and 90.69% of its initial ampero-

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Fig. 9. (a) Long-term chronoamperometry (LTCA) at different O2 gas concentrations in oxygen sensor based on Pt/CMK-3-COOH-[BMIM][PF6 ]. The gas flow was alternated between 100 vol% N2 and varying O2 concentrations: 20, 40, 60, 80, 100 vol%. (b) Plots of stable current vs. concentration for ascending (1), descending (2), subsequent ascending (3) order of concentrations. (c) Cyclic voltammograms of the Pt/CMK-3-COOH-[BMIM][PF6 ] for 50 cycles, volume fraction of O2 : 20%, scan rate: 100 mV s−1 . (d) The amperometric responses of the Pt/CMK-3-COOH-[BMIM][PF6 ] in 30 days within 20% O2 . (e) The corresponding plots of current vs. time in 30 days. Temperature: 298 K.

metric responses after 5, 10, 15, 20, 25 and 30 days, as shown in Fig. 9d and e. The long-term stability of the sensor turned out to be satisfactory, in that only a ca. 9.31% decline of the current response was observed after one month of continuous use, which indicated that the sensor has relatively high long-term storage stability of Pt/CMK-3-COOH-[BMIM][PF6 ] for the electrochemical application. 4. Conclusion In this study, [EMIM][PF6 ], [PMIM][PF6 ] and [BMIM][PF6 ] were synthesized through a facile one-pot method. [EMIM][PF6 ] and [PMIM][PF6 ] have the property of solid-liquid conversion with

the increasing of temperature, which effectively avoids the leakage of electrolytes in the course of utilization and the instability in transportation. To further enhance the performance of electrochemical oxygen sensors, we introduced Pt-modified CMK-3 into ILs. As we anticipated, when Pt/CMK-3-COOH-[BMIM][PF6 ] was utilized as the electrolytes for oxygen sensors, plots of reduction peak current vs. oxygen concentration showed an excellent linear relationship, which suggests that the electrode overlaid with the Pt/CMK-3-COOH-[BMIM][PF6 ] exhibited a marked improvement in gas response behavior for oxygen. Overall, it is worthwhile to point out that this paper provides a method to prolong the lifetime and

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broaden the application field of oxygen sensors. This strategy can also be used to manufacture other amperometric gas sensors. Acknowledgments The authors would like to express their thanks to the support from the National Natural Science Foundation of China (51402065 & 61473095), Heilongjiang Province Natural Science Funds for Distinguished Young Scholar (JC201404), Special Innovation Talents of Harbin Science and Technology for Distinguished Young Scholar (2014RFYXJ005), Natural Science Foundation of Heilongjiang Province (B201404), Fundamental Research Funds of the Central University (HEUCFZ) and Program of International S&T Cooperation special project (2013DFR50060). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2017.07.069. References [1] J. Wecker, A. Bauch, S. Kurth, G. Mangalgiri, M. Gaitzsch, M. Meinig, et al., Oxygen detection system consisting of a millimeter wave Fabry-Pérot resonator and an integrated SiGe front-end, Proc. of SPIE 9747 (2016) 97470C. [2] H. Li, X. Mu, Y. Yang, A.J. Mason, Low power multimode electrochemical gas sensor array system for wearable health and safety monitoring, IEEE Sens. J. 14 (2014) 3391–33919. [3] S.S. Bhoga, K. Singh, Electrochemical solid state gas sensors: an overview, Ionics 13 (2007) 417–427. [4] J.W. Yoon, M.L. Jrilli, E.D. Bartolomeo, R. Polini, E. Travirsa, The NO2 response of solid electrolyte sensors made using nano-sized LaFeO3 electrodes, Sens. Actuators B 76 (2001) 483–488. [5] N. Miura, M. Nakatou, S. Zhuiykov, Impedance-based total-NOx sensor using stabilized zirconia and ZnCr2O4 sensing electrode operating at high temperature, Electrochem. Commun. 4 (2002) 284–287. [6] R. Ramamoorthy, P.K. Dutta, S.A. Akbar, Oxygen sensors: materials, methods, designs and applications, J. Mater. Sci. 38 (2003) 4271–4282. [7] P. Li, R.G. Compton, Electrochemical high concentration oxygen sensing using a phosphonium cation based room temperature ionic liquid: analytical studies, Electroanal 27 (2015) 1550–1555. [8] R. Wang, T. Okajima, F. Kitamura, T. Ohsaka, A novel amperometric O2 gas sensor based on supported room-temperature ionic liquid porous polyethylene membrane-coated electrodes, Electroanalysis 16 (2004) 66–72. [9] W. Gopel, G. Reinhardt, M. Rosch, Trends in the development of solid state amperometric and potentiometric high temperature sensors, Solid State Ionics 136–137 (2000) 519–531. [10] Z. Wang, P. Lin, G.A. Baker, J. Stetter, X. Zeng, Ionic liquids as electrolytes for the development of a robust amperometric oxygen sensor, Anal. Chem. 83 (2011) 7066–7073. [11] M.C. Buzzeo, O.V. Klymenko, J.D. Wadhawan, C. Hardacre, K.R. Seddon, R.G. Compton, Voltammetry of oxygen in the room-temperature ionic liquids 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide and hexyltriethylammonium bis((trifluoromethyl)sulfonyl)imide: one-electron reduction to form superoxide. Steady-state and transient behavior in the same cyclic voltammogram resulting from widely different diffusion coefficients of oxygen and superoxide, J. Phys. Chem. A 107 (2003) 8872–8878. [12] A. Pahlavan, V.K. Gupta, A.L. Sanati, F. Karimi, M. Yoosefian, M. Ghadami, ZnO/CNTs nanocomposite/ionic liquid carbon paste electrode for determination of noradrenaline in human samples, Electrochim. Acta 123 (2014) 456–462. [13] Y. Pan, L. Shang, F. Zhao, B. Zeng, A novel electrochemical 4-nonyl-phenol sensor based on molecularly imprinted poly (o-phenylenediamine-co-o-toluidine)-nitrogen-doped graphene nanoribbons-ionic liquid composite film, Electrochim. Acta 151 (2015) 423–428. [14] A. Rehman, X. Zeng, Methods and approaches of utilizing ionic liquids as gas sensing materials, RSC Adv. 5 (2015) 58371–58392. [15] S. Zhang, X. Chen, G. Liu, X. Hou, Y. Huang, J. Chen, et al., A novel sensing platform based on ionic liquid integrated carboxylic-functionalized graphene oxide nanosheets for honokiol determination, Electrochim. Acta 155 (2015) 45–53. [16] L. Zhao, B. Zeng, F. Zhao, Electrochemical determination of tartrazine using a molecularly imprinted polymer −multiwalled carbon nanotubes—ionic liquid supported Pt nanoparticles composite film coated electrode, Electrochim. Acta 146 (2014) 611–617. [17] F. Li, D. Pan, M. Lin, H. Han, X. Hu, Q. Kang, Electrochemical determination of iron in coastal waters based on ionic liquid-reduced graphene oxide supported gold nanodendrites, Electrochim. Acta 176 (2015) 548–554.

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Biographies Ying Liu is currently a graduate student of the Key Laboratory of Superlight Material and Surface Technology, Harbin Engineering University in China, under the supervision of Professor Zhanshuang Li. Her current research is focused on design and preparation of electrochemical oxygen sensors. Jingyuan Liu received her Ph.D. from the College of Material Science and Chemical Engineering, Harbin Engineering University, China. Her current research is focused on the synthesis of semiconductor nanostructures and their gas-sensing applications. Qi Liu is a postgraduate, focusing on gas sensors for doctor degree at the College of Material Science and Chemical Engineering, Harbin Engineering University, China. Hongsen Zhang is a Ph.D. student of the College of Material Science and Chemical Engineering, Harbin Engineering University, China. His current research is focused on the design and synthesis of synthesis of new nanostructured materials and nanocomposites.

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Zhanshuang Li is a professor of the College of Material Science and Chemical Engineering, Harbin Engineering University, China. His research interests focus on design and synthesis of nano-materials, as well as their applications for gas sensor and environmental disciplines. Xiaoyan Jing is a professor of the College of Material Science and Chemical Engineering, Harbin Engineering University, China. Her research interests focus on design and synthesis of nano-materials, as well as their applications for gas sensor and lithium batteries applications. Yi Yuan is a professor of the College of Material Science and Chemical Engineering, Harbin Engineering University, China. Her research interests focus on polymer and composite materials. Hongquan Zhang is a professor of School of Automation, Harbin Engineering University, China. His research interests focus on design and synthesis of nanomaterials and semiconductor materials and their application and gas sensors and electron field. Peili Liu is a professor of Institute of Advanced Marine Materials, Harbin Engineering University. His research interests focus on polymer and composite materials. Nanomaterials and Applied in the marine application and shipbuilding field. Jun Wang received his Ph.D. from the College of Material Science and Chemical Engineering, Harbin Engineering University under the supervision of Prof. Milin Zhang. He was appointed as a full professor at the College of Material Science and Chemical Engineering in 2007. His research interests include synthesis of new nanostructured materials and nanocomposites, and their application in energy storage and environment areas, such as super-capacitors and gas sensors.