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A ciprofloxacin based 1D Cd(II) coordination polymer with highly efficient humidity sensing performance
Inorganic Chemistry Communications 108 (2019) 107541
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Short communication
A ciprofloxacin based 1D Cd(II) coordination polymer with highly efficient humidity sensing performance
T
Ting Lib,c, Le-Xi Zhanga,c, , Yue Xinga, Heng Xua, Yu-Qing Yueb, Qi Lia, Hao Donga, ⁎⁎ Hai-Yang Wanga, Yan-Yan Yinb,c, ⁎
a
School of Materials Science and Engineering, Tianjin Key Lab for Photoelectric Materials and Devices, Key Laboratory of Display Materials and Photoelectric Devices (Ministry of Education), Tianjin University of Technology, Tianjin 300384, China b Department of Environmental Science and Engineering, Nankai University Binhai College, Tianjin 300270, China c Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China
GRAPHICAL ABSTRACT
A new 1D coordination polymer {[Cd0.5(cfH)(4,4′-bpy)0.5](NO3)·H2O}n (named as CdL, cfH = ciprofloxacin, 4,4′-bpy = 4,4′-bipyridine) further connected by hydrogen bonds forming a 3D supramolecular network was synthesized and assembled into an impedimetric humidity sensor. The CdL humidity sensor exhibits high response, small hysteresis, and short response–recovery times.
ARTICLE INFO
ABSTRACT
Keywords: Cadmium Ciprofloxacin Impedance Humidity sensor
Coordination polymers (CPs) have recently be considered as promising candidates for electrical sensors, especially impedimetric ones, yet still limited by low sensitivity, slow response, and large humidity hysteresis. In this contribution, {[Cd0.5(cfH)(4, 4′-bpy)0.5](NO3)·H2O}n (named as CdL, cfH = ciprofloxacin, 4,4′-bpy = 4,4′-bipyridine) has been synthesized via a facile solvent evaporation method at room temperature. The CdL CP possesses a one dimensional (1D) structure that is further connected by hydrogen bonds forming a three dimensional (3D) supramolecular network. Taking into account of rich hydrogen bonds and uncoordinated atoms (N, O, and F), the CdL CP was assembled into an impedimetric sensor and thus the humidity sensing performance was investigated systemically. The CdL based sensor exhibited good humidity sensing performance in a wide humidity range, including high sensitivity, small hysteresis, and relatively rapid response–recovery. Moreover, the sensing mechanism was discussed by complex impedance analysis in detail, so as to confirm the charge
Correspondence to: L.-X. Zhang, School of Materials Science and Engineering, Tianjin Key Lab for Photoelectric Materials and Devices, Key Laboratory of Display Materials and Photoelectric Devices (Ministry of Education), Tianjin University of Technology, Tianjin 300384, China. ⁎⁎ Correspondence to: Y.-Y. Yin, Department of Environmental Science and Engineering, Nankai University Binhai College, Tianjin 300270, China. E-mail addresses: [email protected] (L.-X. Zhang), [email protected] (Y.-Y. Yin). ⁎
https://doi.org/10.1016/j.inoche.2019.107541 Received 19 July 2019; Received in revised form 14 August 2019; Accepted 15 August 2019 Available online 21 August 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 108 (2019) 107541
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carrier contribution at different relative humidity (RH) range. These results indicate the potential application of CdL in humidity sensors, and further reveal the promising prospects for construction of CP-based electrical sensory devices.
Humidity sensors are widely applied in industry, meteorology, agriculture, medicine, aviation, electronic engineering and so on. Sensitive material is the core component of a humidity sensor, thus the research and development of humidity sensitive material has drawn more and more attention [1]. Since R.W. Dunmore reported the pioneering work using electrolyte LiCl in 1938 [2], great progress has been made on the research and application of inorganic ceramic oxides, organic polymers, and their composites [3–13]. However, these materials are usually suffered from some disadvantages such as complicated preparation, high working temperature, unsatisfactory stability, etc. [14]. Therefore, considerable efforts in this aspect have been made to search for new materials with superior humidity sensing performance. As a kind of star materials, coordination polymers (CPs) have drawn much attention not only because of their good structural tunability and easy processability [15], but also due to their tremendous potential applications in fields of gas storage [16], catalysis [17] and chemical sensors [18,19]. Taking one focus application of chemical sensors as an example, CPs have been recently assembled as active materials towards moisture, the methods of which are mainly based on optical and mechanical signals [19]. As is known to all, the transduction of electrical signals can be well detected much more accurately and conveniently, compared with the above-mentioned optical and mechanical manners. However, due in part to their large band gap and low electrical conductivity [20] that usually result in poor performance, the electrical sensory devices are still limited [19,21], especially the impedimetric sensors, which hold advantages of high sensitivity, cheap equipment, and real-time monitoring. Previously, it was considered that porous coordination polymers (PCPs) that are well known as metal-organic frameworks (MOFs) are necessary for constructing electrical humidity sensors due to their abundant pores, high specific surface area, as well as modified functional groups [21]. Following this way, interesting progresses have been made on the utility of pristine MOFs as humidity active materials [22–24], as well as modified MOFs [25,26] and decorated MOFs [27]. Very recently, Y.-Y. Yin and co-workers demonstrated the possibility of assembling CPs not confined to MOFs as impedimetric humidity sensing components [28], even the CP with very small specific surface area and certainly quite low water vapor adsorption capacity [29]. These contributions have open a new way for searching CPs not only limited to MOFs as effective humidity sensing materials, however the rational design and synthesis of electrically active CPs is still at its early stage. Ciprofloxacin (cfH = 1-cyclopropyl6-flfluoro-1,4-dihydro-4-oxo-7-(1-pip- erazinyl)-3-quinoline carboxylic acid), of which the structure is shown in Fig. S1 in the Supplementary data, is one of the second generation fluoroquinolone compounds currently used in clinical practice for treatment of various bacterial infections [30]. Due to its rich coordination modes, cfH has been used to link various metal ions building novel CPs [31,32], however, the sensing property of cfH based CPs has not been reported yet. In this work, a novel single crystal of {[Cd0.5(cfH)(4, 4′-bpy)0.5] (NO3)·H2O}n (named as CdL, 4,4′-bpy = 4,4′-bipyridine) has been synthesized successfully via a facile solvent evaporation method at room temperature. The structure of CdL was determined by singlecrystal X-ray diffraction and further confirmed by elemental analysis and FT-IR spectra. Subsequently, the humidity sensing performance of CdL was examined systemically. In addition, the humidity sensing mechanism of CdL has been discussed on the basis of complex impedance analysis in detail. All reagents and solvents were purchased from J&K Scientific Ltd. (China) and used as received without further purification. The CdL CP was prepared based on a solvent evaporation method at room
Fig. 1. ORTEP drawing an asymmetric unit of CdL with the atomic labeling scheme of all atoms (thermal ellipsoids at 30% probability).
temperature. In a typical synthesis, 0.5 mmol Cd(NO3)2·4H2O (0.15 g), 0.5 mmol ciprofloxacin hydrochloride (0.19 g), and 0.5 mmol 4,4′-bpy (0.078 g) were completely dissolved in 10 mL water, 1 mL water, and 2 mL ethanol under magnetic stirring for 10 min, respectively. Then the above solutions were well mixed together for another 10 min stirring. After precipitate was filtered out form the mixture, the clear solution was kept stand still without any disturbance at room temperature. Colourless transparent plate-shaped crystals were finally obtained through slow evaporation of the filtered solution in air for 3 days. The synthesis procedure can be repeated easily with an average yield of about 50% based on Cd2+. Elemental analysis calculated for CdL with a formula of Cd0.5C22H24N5O7F (%): C = 48.42, H = 4.43, and N = 12.83. Found for CdL: C = 48.44, H = 4.42, and N = 12.80. The Fourier transform infrared (FT-IR) spectrum of CdL is shown in Fig. S2 (cm−1): 3440(s), 3030(s), 1630(vs), 1610(vs), 1560(vs), 1480(vs), 1390(vs), 1330(vs), 1300(vs), 1270(vs), 1160(s), 1040(s), 942(vs), 819(vs). One CdL crystal with suitable size (0.40 mm × 0.40 mm × 0.20 mm) was selected and mounted on a glass fiber tip of a goniometer head in air at room temperature. And singlecrystal X-ray diffraction (SXRD) measurement was performed on a XtaLAB PRO diffractometer (Rigaku, Japan/Netherlands) with graphite-monochromated Cu-Kα radiation (λ = 1.54056 Å). Diffraction intensities were collected with a CCD area detector image plate diffractometer using the ω/φ scan technique. The absorption correction was carried out with semi-empirical calculations from multi-scans. The structure was solved by direct methods with SHELXS-97 [33] and refined by least-squares procedures on Fo2 using SHELXL-97 [34] by minimizing the function Σω(Fo2-Fc2)2, where Fo and Fc are the observed and calculated structure factors, respectively. All non‑hydrogen atoms were refined anisotropically. Powder X-ray diffraction (PXRD) pattern were collected from 5° to 40°, with a step of 0.02° and data collection time of 0.2 s, on a D/max-2500 advance diffractometer (Rigaku, Japan) with Cu-Kα radiation (λ = 1.54056 Å). Elemental analysis was carried on a Vario EL Cube elemental analyzer (Elementar, German). FT-IR spectra were recorded on a Nicolet iS10 spectrometer in KBr pellets in the range of 4000–700 cm−1 (Thermo Fisher Scientific, USA). The assembling of a CdL sensing device is presented as follows [28,29]: CdL paste was obtained by grinding appropriate amount of CdL in deionized water, which was then coated on a alumina substrate 2
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Fig. 2. The coordination environment (a), 1D coordination chain (b), 2D supramolecular layer (c), and 3D supramolecular network (d) of CdL. Atomic labeling scheme of nonhydrogen atoms: Cd (cyan), C (gray), N (blue), O (red), F (green); All of the hydrogen atoms have been omitted for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(8 mm × 4 mm × 0.5 mm) with five pairs of AgePd interdigitated electrodes (IDE, electrode width and gap: 0.15 mm). The experiments were performed at an ambient temperature of 20 °C for sensor aging and test. The sensor was dried at 80 °C for 5 h and aged under 97% relative humidity (RH) at 1 V (AC, 100 Hz) for 24 h, so as to improve the stability and durability. The humidity sensing properties were tested in different RH, which is achieved by several saturated salt solutions. The humidity gradient was controlled by six jars (1 L) with rubber stoppers filled with different saturated salt solutions. The sensor in a shielded socket is sealed in a rubber stopper which is the same as that used for 6 jars. Saturated solutions of LiCl, MgCl2, Mg(NO3)2, NaCl, KCl, and K2SO4 in closed vessels yield 11%, 33%, 54%, 75%, 85%, and 97% RH levels, respectively [35]. In order to ensure the accuracy of RH, 8 h stabilization of the jars was necessary to get fully liquid-gas equilibrium before humidity sensing test. In a test circle, the sensor was placed in one jar until the impedance value achieves balance then it was switched to another jar. After reaching steady, the sensor was handled back to the first jar waiting the next impedance stability, finally ending a cyclic test. The experiments were performed at room temperature on a humidity sensing system (CHS-1 Humidity Sensing Analysis System, Beijing Elite Tech. Co. Ltd., China) at 1 V AC with frequency varying from 50 Hz to 100 kHz. The sensor response is defined by: SRH = Z11%/ ZRH, where Z11% and ZRH represent the impedance values of the CdL sensor taken at 11% RH and a certain RH. The response time (τres) or recovery time (τrec) is defined as the time duration needed for the sensor to achieve 90% of total impedance change in the case of adsorption and desorption, respectively. According to the crystal structure refinement based on SXRD results, CdL crystallizes in the triclinic space group P-1. Details of crystallographic data and refinement results for CdL are listed in Table S1. Selected bond lengths and angles of CdL are given in Tables S2 and S3,
respectively. As shown in Fig. 1, the asymmetric unit of CdL with the formula of {[Cd0.5(cfH)(4,4′-bpy)0.5](NO3)·H2O}n contains one Cd2+ cation (half-occupied), one cfH ligand and half a 4,4′-bpy, as well as one NO3− anion and two lattice H2O (half-occupied) molecules. The Cd2+ ion is coordinated by two nitrogen atoms from two 4,4′-bpy ligands and four oxygen atoms from two cfH ligands forming a [CdN2O4] distorted octahedron configuration, as shown in Fig. 2(a). In the cfH ligand, the piperazine ring holds a chair configuration and the quinoline unit is essentially planar. As to two coordinated oxygen atoms in one cfH molecule, one is the carbonyl oxygen (O3), another is the carboxyl oxygen (O1) that is homogenized with another carboxyl oxygen (O2). Subsequently, the proton dissociated from the carboxyl group is further neutralized by the imino group forming a quantenary ammonium group. The coordination mode of cfH is similar to previous reported metal-cfH complexes, and the CdeO bond distances are in the range of 2.227(6)–2.288(4) Å, which all fall into the normal range of reported CdeO bonds in Cd-cfH CPs [31,32]. As shown in Fig. 2(b), the neighboring Cd2+ are linked by 4,4′-bpy molecules to generate a 1D chain structure. Adjacent 1D chains can be further connected via hydrogen bonds (N3⋯H⋯O2) to generate a 2D supramolecular layer, as can be seen in Fig. 2(c). It is worth noting that rich hydrogen bonds exist between supramolecular layers due to plenty of oxygen atoms from cfH ligands as well as counterions NO3− and lattice water molecules. The data of selected hydrogen bond lengths and angles of CdL are shown in Table S4. As a result of this connection, the supramolecular layer is finally linked into a 3D porous structure, as can be seen in Fig. 2(d). PXRD pattern of as-synthesized CdL sample is exhibited in Fig. S3, so is the simulated pattern using SXRD data from CdL single crystal. Clearly, all PXRD peaks are consistent well with those simulated data. Characteristic peaks of impurities arising from reactants or intermediate products are not observed, implying high crystalline purity of 3
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[22,24,25,28,29]. The CdL sensor indicates much larger responses in the low frequency region (50–100 Hz) than that in the high one (1–100 kHz). In the later frequency region, the impedance decreases not so evident as that in the former region that is almost independent of RH, indicating the disappearance of dielectric phenomenon, because the polarization of adsorbed water molecules cannot be able to keep pace with the change of AC electric fields. It should be pointed out that impedance values cannot be recorded at far lower frequency (50 Hz) and RH range (11%–54%) limited by the test scope of CHS-1 (0.5 Ω–1 GΩ). Therefore, the operation AC voltage and frequency were set at 1 V and 100 Hz in the following experiments for gaining higher responses. At 100 Hz, the impedance changes higher than three orders of magnitude (S97% = 3.15 × 103), which is much larger than that of reported CP-based humidity sensors, such as Fe-BTC (1, 3, 5-benzenetricarboxylate) [14], [Cd(TMA)(DPP)0.5∙H2O]n (H2TMA = 3-thiophenemalonic acid, DPP = 1,3-di(4-pyridyl)propane) [24], NH2-MIL-125(Ti) [25], FeCl3-NH2-MIL-125(Ti) [27], and [Pb(TMA)]n [29], and slightly better than that of MIL-101(Cr) [22], [Eu(H2O)2(mpca)2Eu(H2O)6M (CN)8]·nH2O (mpca = 2-pyrazine-5-methyl-carboxylate, M = Mo, W) [23], however still smaller than that of [Pb0.5(TAA)]n (HTAA = 3thiopheneacetic acid) [28] and (NBu4)2Cu2(dhbq)3 [37], as listed in Table 1. Compared with other non-CPs humidity sensing materials, the sensitivity of CdL is a lot higher than that of PDDA/RGO [3], MoS2/ SnO2 [5], and Ba5−xSrxNb4O15 [13], although not as good as that of Ag@mpg-CN [9], Ag/SnO2 [10], In-SnO2/meso CN [11], and CeO2 aero-gel [12]. The dynamic response behavior of the CdL sensor, as exhibited in Fig. 3(b), is switched between 11% RH and 97% RH at 100 Hz. In the switching test cycle, the impedance at 11% RH decreases rapidly to an equilibrium value at 97% RH and then increases rapidly to the baseline at 11% RH with response-recovery times of 12 s and 53 s, respectively. As can be seen in Table 1, the response/recovery time of present work is comparable with that of most CPs sensors [22,24,25,27–29,37], but far shorter than that of Fe-BTC [14] and EuM [23]. Except that of PDDA/RGO [3], the response/recovery time of our work is still not as short as that of conventional oxide-based humidity sensors [5,9–13]. It is considered that the relatively rapid response behavior is originated from numerous active sites and hydrogen bonds network that facilitate the absorption of water molecules and charge carriers transfer. These advantages all result in significant enhancement of large sensing signal and short response time. Additionally, the humidity hysteresis characteristic of the CdL sensor was examined by increasing the RH from 11% to 97% for moisture absorption and then decreasing back to 11% for moisture absorption at different RH. As can be seen in Fig. 3(c), the max humidity hysteresis (ΔHmax) is 3.7% RH occurred at 85% RH in the high RH range (70%–100%), indicating the dominant physisorption of water rather than chemisorption. To further understand the conduction mechanism of CdL towards moisture, complex impedance spectra (CIS) have been analyzed at different RH in the frequency range of 50 Hz–100 kHz, since CIS have been proved to be a powerful tool for exploring the conduction process of humidity sensors [14,22–25,27–29]. As shown in Fig. 4, complex impedance diagrams were constructed in Nyquist plots that can be resolved into real (ReZ) and imaginary (ImZ) parts. Because of strong electronic interactions, water molecules chemisorb onto active sites of CdL surface at low RH range (11%–54%). Only a few water molecules are captured by CdL surface forming a discontinuous water coverage, as sketched in Fig. 5(a). Water molecules chemisorb on active sites of CdL and then dissociate into H+ and OH– ions, forming hydroxyl groups on CdL surface. Large impedance values (hundreds of MΩ) are recorded in Fig. 3(a), because of the electrolytic conduction (H2O → H+ + OH−) [36] is difficult to happen in this stage. The impedance is arc of a semicircle, as shown in Fig. 4(a), indicating a kind of polarization, which is modeled by an equivalent circuit illustrated as an inset of Fig. 4(a). There are two series parts in the equivalent circuit [22,27,29]: one is the electrode represented by a resistor (Rs), the portion of which
Fig. 3. (a) Impedance-RH relationships of the CdL sensor at different frequencies. (b) The characteristic response-recovery behavior of the CdL sensor switching between 11% RH and 97% RH. (c) The humidity hysteresis of the CdL sensor.
the CdL sample. The CdL crystal enjoys uncoordinated fluorine and oxygen atoms in cfH ligands, which are high density active sites for capturing moisture from the environment and thus forming physisorption of water layer on CdL surface. More importantly, abundant oxygen atoms from interchain NO3− anions and lattice H2O molecules generate hydrogen bond networks throughout the whole crystal, which are perfect proton transport channels [36] for humidity sensing. Taking into account of numerous active sites and hydrogen bonds, CdL was assembled into an impedimetric humidity sensor, and the corresponding sensing performance was investigated systematically. Totally speaking, impedance values of the CdL sensor decrease gradually with the increase of RH, as shown in Fig. 3(a). Simultaneously, the CdL sensor exhibits typical humidity sensing behavior that is impedance-dependent at the frequency ranging from 50 Hz to 100 kHz, the phenomenon of which is agreed well with other impedimetric humidity sensors based on CPs 4
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Table 1 Summarized humidity sensing performance based on various materials. Material
Method
CdL Fe-BTC MIL-101(Cr) EuM [Cd(TMA)(DPP)0.5∙H2O]n NH2-MIL-125(Ti) FeCl3-NH2-MIL-125(Ti) (NBu4)2Cu2(dhbq)3 [Pb0.5(TAA)]n [Pb(TMA)]n PDDA/RGO MoS2/SnO2 Ag@mpg-CN Ag/SnO2 In-SnO2/meso CN CeO2 aero-gel Ba5−xSrxNb4O15
S = Z11%
RH/Z97% RH
AC, AC, AC, AC, AC, AC, AC, DC AC, AC, DC AC, AC, AC, AC, AC, DC
or S = Z97%
RH range (%)
100 Hz 1 Hz 100 Hz 500 Hz 100 Hz 100 Hz 100 Hz 100 Hz 100 Hz 100 Hz 100 Hz 100 Hz 100 Hz 50 Hz
RH/Z11% RH
S 3
11–97 0–1.2 33–95 53–100 11–97 11–95 11–95 30–90 11–97 11–97 11–97 0–97 11–98 11–98 11–98 11–98 33–95
3.15 × 10 1.35 1.93 × 103 ~103 3.52 × 102 27.5 3.67 × 102 ~2.1 × 104 1.46 × 104 3.1 × 102 3.96 ~17 3.16 × 104 6.7 × 104 1.48 × 105 3.16 × 104 52.9
τres (s)
τrec (s)
Ref.
12 ~mins 17 380 11 45 11 54 13 9 108–147 5 3 4 3.5 4.6 2
53 ~mins 90 390 56 50 86 6 30 38 94–133 13 1.4 6.5 1.5 2.7 17
This work [14] [22] [23] [24] [25] [27] [37] [28] [29] [3] [5] [9] [10] [11] [12] [13]
[3].
Fig. 4. Complex impedance property of the CdL sensor at various RH ranges: (a) 11%–54%, and (b) 75%–97%. Inset: the equivalent circuits of complex impedance plots at (a) low RH (11%–54%) and (b) high RH (75%–97%).
Fig. 5. The proposed humidity sensing mechanism of the CdL based sensor under different RH ranges.
continuous liquid-like layer, as is diagrammed in Fig. 5(b). In the liquidlike layer, water molecules are ionized with the help of electrostatic field to generating moveable proton (H+) and hydronium ions (H3O+) [38]. These charge carriers significantly reduce the impedance based on the ion transfer mechanism of Grotthuss reaction: H2O + H3O+ → H3O+ + H2O [39], since this reaction ensures the easy transfer of H3O+ in the continuous water layer. With the increase of RH, the semicircle becomes smaller, which can be described as typical Warburg
is irrespective to RH; The other is the CdL sensing film expressed as parallel connection of a resistor (R) and a constant phase element (CPE1). With the increase of RH (54%–85%), the arc grows to be a semicircle, the radius of which becomes small gradually, and then a straight line emerges attached to the semicircle in the low frequency range (right half part in a Nyquist plot) at 97% RH, as shown in Fig. 4(b). Upon the first chemisorpted water layer, more and more water molecules physisorpted onto the CdL surface, forming a 5
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impedance [40]. According to the protons conductivity model [41], the H3O+ from a bare proton and a water molecule is more stable than a bare proton. Hence the H3O+ will be the dominant charge carrier in this stage. Under alternating electric fields, these H3O+ ions can form directional movement then produce leakage current, and finally decrease the impedance obviously. The existence of leakage current makes the complex impedance curve become a completely semicircle. As shown in the inset of Fig. 4(b), the equivalent circuit at high RH (54%–97%) is similar to that at low RH (11%–54%), except that the resistance (R) in Fig. 4(a) is expressed by a series part including a resistance (R) and a constant phase element (CPE2). The Grotthuss reaction usually occurs in low frequency range, because the the mass of H3O+ is relatively larger than that of H+. Therefore, the impedance value decreases with the increasing frequency until the H3O+ movement is unable to keep pace with the changing of AC electric field [27]. So in CIS, the part of semicircle curve disappears slowly at high frequency, and an upturned line appears at low frequency. At 97% RH, the straight line at low frequencies of CIS can also be signified by CPE2, which indicates the impact of H3O+ in the physisorbed water layer through H+ jumping between adjacent water molecules, as illustrated in Fig. 5(c). Based on CIS analysis, the sensing mechanism of this CdL sensor indicates significant roles contributed by distinct charge carriers at different RH ranges. In this work, a novel 1D CP CdL with the formula of {[Cd0.5(cfH) (4,4′-bpy)0.5](NO3)·H2O}n was synthesized through a facile solvent evaporation method at room temperature. Then CdL was assembled to be an impedimetric humidity sensor, which exhibited good sensing properties, including highly response, small hysteresis, and short response-recovery time in a wide humidity range. This contribution reported a highly efficient candidate material for practical humidity sensors. More importantly, this work highlights the potential application of CPs as promising sensing materials for next-generation electrical devices.
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Stock, M. Tiemann, T. Wagner, Surface-modified CAU-10 MOF materials as humidity sensors: impedance spectroscopic study on water uptake, Phys. Chem. Chem. Phys. 17 (2015) 21634–21642. [27] Y. Zhang, B. Fu, K. Liu, Y. Zhang, X. Li, S. Wen, Y. Chen, S. Ruan, Humidity sensing properties of FeCl3-NH2-MIL-125(Ti) composites, Sensors Actuators B Chem. 201 (2014) 281–285. [28] Y.-Y. Yin, Y. Xing, Y.-N. Li, J.-N. Wang, T. Li, L.-X. Zhang, A novel impedimetric humidity sensor based on a one dimensional [Pb0.5(TAA)]n (HTAA = 3-thiopheneacetic acid) coordination polymer, Inorg. Chem. Commun. 97 (2018) 103–108. [29] Y.-Y. Yin, L.-X. Zhang, X. Wang, B.-L. Zhang, X.-Q. Gong, L. Liu, R.-H. Sun, A honeycomb-layered 2D coordination polymer of [Pb(TMA)]n (H2TMA = 3-thiophenemalonic acid) for impedimetric humidity sensing with high performance, Inorg. Chem. Commun. 100 (2019) 38–43. [30] G.-F. Zhang, X. Liu, S. Zhang, B. Pan, M.-L. Liu, Ciprofloxacin derivatives and their antibacterial activities, Eur. J. Med. Chem. 146 (2018) 599–612. [31] D.-R. Xiao, E.-B. Wang, H.-Y. An, Z.-M. Su, Y.-G. Li, L. Gao, C.-Y. Sun, L. Xu, Rationally designed, polymeric, extended metal–ciprofloxacin complexes, Chem. Eur. J. 11 (2005) 6673–6686. [32] J.-H. He, D.-Z. Sun, D.-R. Xiao, S.-W. Yan, H.-Y. Chen, X. Wang, J. Yang, E.-B. Wang, Syntheses and structures of five 1D coordination polymers based on quinolone antibacterial agents and aromatic polycarboxylate ligands, Polyhedron 42 (2012) 24–29. [33] G.M. Sheldrick, SHELXS-97, a Program for Automatic Solution of Crystal Structure, University of Göttingen, Germany, 1997. [34] G.M. Sheldrick, SHELXL-97, a Program for Crystal Structure Refinement, University of Göttingen, Germany, 1997. [35] L. Greenspan, Humidity fixed points of binary saturated aqueous solutions, J. Res. Natl. Bur. Stand. 81 A (1977) 89–96.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC Grant No. 21601094 and No. 21401139); Tianjin Education Commission Scientific Research Plan Project (Grant No. 2018KJ271); Tianjin Natural Science Council (Grant No. 15JCQNJC02900); State Environmental Protection Key Laboratory of Odor Pollution Control, P. R. China (Grant No. 201903202), Student's Platform for Innovation and Entrepreneurship Training Program (Grant No. 201813663002 and No. 201810060025), and 111 Project (B12015). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.107541. References [1] H. Farahani, R. Wagiran, M.N. Hamidon, Humidity sensors principle, mechanism, and fabrication technologies: a comprehensive review, Sensors 14 (2014) 7881–7939. [2] R.W. Dunmore, An electrometer and its application to radio meteorography, J. Res. Nat. Bur. Std. 20 (1938) 723–725. [3] D. Zhang, J. Tong, B. Xia, Humidity-sensing properties of chemically reduced graphene oxide/polymer nanocomposite film sensor based on layer-by-layer nano selfassembly, Sensors Actuators B Chem. 197 (2014) 66–72. [4] D. Zhang, J. Tong, B. Xia, Q. Xue, Ultrahigh performance humidity sensor based on layer-by-layer self-assembly of graphene oxide/polyelectrolyte nanocomposite film, Sensors Actuators B Chem. 203 (2014) 263–270. [5] D. Zhang, Y. Sun, P. Li, Y. Zhang, Facile fabrication of MoS2-modified SnO2 hybrid nanocomposite for ultrasensitive humidity sensing, ACS Appl. Mater. Interfaces 8 (2016) 14142–14149.
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T. Li, et al. [36] K. Tan, S. Zuluaga, Q. Gong, P. Canepa, H. Wang, J. Li, Y.J. Chabal, T. Thonhauser, Water reaction mechanism in metal organic frameworks with coordinatively unsaturated metal ions: MOF-74, Chem. Mater. 26 (2014) 6886–6895. [37] X.J. Lv, M.S. Yao, G.E. Wang, Y.Z. Li, G. Xu, A new 3D cupric coordination polymer as chemiresistor humidity sensor: narrow hysteresis, high sensitivity, fast response and recovery, Sci. China Chem. 60 (2017) 1197–1204. [38] P.G.M. Mileo, T. Kundu, R. Semino, V. Benoit, N. Steunou, P.L. Llewellyn, C. Serre,
G. Maurin, S.D. Vinot, Highly efficient proton conduction in a three-dimensional titanium hydrogen phosphate, Chem. Mater. 29 (2017) 7263–7271. [39] B.M. Kulwicki, Humidity sensors, J. Am. Ceram. Soc. 74 (1991) 697–708. [40] L. Wang, X. Duan, W. Xie, Q. Li, T. Wang, Highly chemoresistive humidity sensing using poly(ionic liquid)s, Chem. Commun. 52 (2016) 8417–8419. [41] J.H. Anderson, G.A. Parks, Electrical conductivity of silica gel in the presence of adsorbed water, J. Phys. Chem. 72 (1968) 3662–3668.
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Short communication
A ciprofloxacin based 1D Cd(II) coordination polymer with highly efficient humidity sensing performance
T
Ting Lib,c, Le-Xi Zhanga,c, , Yue Xinga, Heng Xua, Yu-Qing Yueb, Qi Lia, Hao Donga, ⁎⁎ Hai-Yang Wanga, Yan-Yan Yinb,c, ⁎
a
School of Materials Science and Engineering, Tianjin Key Lab for Photoelectric Materials and Devices, Key Laboratory of Display Materials and Photoelectric Devices (Ministry of Education), Tianjin University of Technology, Tianjin 300384, China b Department of Environmental Science and Engineering, Nankai University Binhai College, Tianjin 300270, China c Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China
GRAPHICAL ABSTRACT
A new 1D coordination polymer {[Cd0.5(cfH)(4,4′-bpy)0.5](NO3)·H2O}n (named as CdL, cfH = ciprofloxacin, 4,4′-bpy = 4,4′-bipyridine) further connected by hydrogen bonds forming a 3D supramolecular network was synthesized and assembled into an impedimetric humidity sensor. The CdL humidity sensor exhibits high response, small hysteresis, and short response–recovery times.
ARTICLE INFO
ABSTRACT
Keywords: Cadmium Ciprofloxacin Impedance Humidity sensor
Coordination polymers (CPs) have recently be considered as promising candidates for electrical sensors, especially impedimetric ones, yet still limited by low sensitivity, slow response, and large humidity hysteresis. In this contribution, {[Cd0.5(cfH)(4, 4′-bpy)0.5](NO3)·H2O}n (named as CdL, cfH = ciprofloxacin, 4,4′-bpy = 4,4′-bipyridine) has been synthesized via a facile solvent evaporation method at room temperature. The CdL CP possesses a one dimensional (1D) structure that is further connected by hydrogen bonds forming a three dimensional (3D) supramolecular network. Taking into account of rich hydrogen bonds and uncoordinated atoms (N, O, and F), the CdL CP was assembled into an impedimetric sensor and thus the humidity sensing performance was investigated systemically. The CdL based sensor exhibited good humidity sensing performance in a wide humidity range, including high sensitivity, small hysteresis, and relatively rapid response–recovery. Moreover, the sensing mechanism was discussed by complex impedance analysis in detail, so as to confirm the charge
Correspondence to: L.-X. Zhang, School of Materials Science and Engineering, Tianjin Key Lab for Photoelectric Materials and Devices, Key Laboratory of Display Materials and Photoelectric Devices (Ministry of Education), Tianjin University of Technology, Tianjin 300384, China. ⁎⁎ Correspondence to: Y.-Y. Yin, Department of Environmental Science and Engineering, Nankai University Binhai College, Tianjin 300270, China. E-mail addresses: [email protected] (L.-X. Zhang), [email protected] (Y.-Y. Yin). ⁎
https://doi.org/10.1016/j.inoche.2019.107541 Received 19 July 2019; Received in revised form 14 August 2019; Accepted 15 August 2019 Available online 21 August 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
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carrier contribution at different relative humidity (RH) range. These results indicate the potential application of CdL in humidity sensors, and further reveal the promising prospects for construction of CP-based electrical sensory devices.
Humidity sensors are widely applied in industry, meteorology, agriculture, medicine, aviation, electronic engineering and so on. Sensitive material is the core component of a humidity sensor, thus the research and development of humidity sensitive material has drawn more and more attention [1]. Since R.W. Dunmore reported the pioneering work using electrolyte LiCl in 1938 [2], great progress has been made on the research and application of inorganic ceramic oxides, organic polymers, and their composites [3–13]. However, these materials are usually suffered from some disadvantages such as complicated preparation, high working temperature, unsatisfactory stability, etc. [14]. Therefore, considerable efforts in this aspect have been made to search for new materials with superior humidity sensing performance. As a kind of star materials, coordination polymers (CPs) have drawn much attention not only because of their good structural tunability and easy processability [15], but also due to their tremendous potential applications in fields of gas storage [16], catalysis [17] and chemical sensors [18,19]. Taking one focus application of chemical sensors as an example, CPs have been recently assembled as active materials towards moisture, the methods of which are mainly based on optical and mechanical signals [19]. As is known to all, the transduction of electrical signals can be well detected much more accurately and conveniently, compared with the above-mentioned optical and mechanical manners. However, due in part to their large band gap and low electrical conductivity [20] that usually result in poor performance, the electrical sensory devices are still limited [19,21], especially the impedimetric sensors, which hold advantages of high sensitivity, cheap equipment, and real-time monitoring. Previously, it was considered that porous coordination polymers (PCPs) that are well known as metal-organic frameworks (MOFs) are necessary for constructing electrical humidity sensors due to their abundant pores, high specific surface area, as well as modified functional groups [21]. Following this way, interesting progresses have been made on the utility of pristine MOFs as humidity active materials [22–24], as well as modified MOFs [25,26] and decorated MOFs [27]. Very recently, Y.-Y. Yin and co-workers demonstrated the possibility of assembling CPs not confined to MOFs as impedimetric humidity sensing components [28], even the CP with very small specific surface area and certainly quite low water vapor adsorption capacity [29]. These contributions have open a new way for searching CPs not only limited to MOFs as effective humidity sensing materials, however the rational design and synthesis of electrically active CPs is still at its early stage. Ciprofloxacin (cfH = 1-cyclopropyl6-flfluoro-1,4-dihydro-4-oxo-7-(1-pip- erazinyl)-3-quinoline carboxylic acid), of which the structure is shown in Fig. S1 in the Supplementary data, is one of the second generation fluoroquinolone compounds currently used in clinical practice for treatment of various bacterial infections [30]. Due to its rich coordination modes, cfH has been used to link various metal ions building novel CPs [31,32], however, the sensing property of cfH based CPs has not been reported yet. In this work, a novel single crystal of {[Cd0.5(cfH)(4, 4′-bpy)0.5] (NO3)·H2O}n (named as CdL, 4,4′-bpy = 4,4′-bipyridine) has been synthesized successfully via a facile solvent evaporation method at room temperature. The structure of CdL was determined by singlecrystal X-ray diffraction and further confirmed by elemental analysis and FT-IR spectra. Subsequently, the humidity sensing performance of CdL was examined systemically. In addition, the humidity sensing mechanism of CdL has been discussed on the basis of complex impedance analysis in detail. All reagents and solvents were purchased from J&K Scientific Ltd. (China) and used as received without further purification. The CdL CP was prepared based on a solvent evaporation method at room
Fig. 1. ORTEP drawing an asymmetric unit of CdL with the atomic labeling scheme of all atoms (thermal ellipsoids at 30% probability).
temperature. In a typical synthesis, 0.5 mmol Cd(NO3)2·4H2O (0.15 g), 0.5 mmol ciprofloxacin hydrochloride (0.19 g), and 0.5 mmol 4,4′-bpy (0.078 g) were completely dissolved in 10 mL water, 1 mL water, and 2 mL ethanol under magnetic stirring for 10 min, respectively. Then the above solutions were well mixed together for another 10 min stirring. After precipitate was filtered out form the mixture, the clear solution was kept stand still without any disturbance at room temperature. Colourless transparent plate-shaped crystals were finally obtained through slow evaporation of the filtered solution in air for 3 days. The synthesis procedure can be repeated easily with an average yield of about 50% based on Cd2+. Elemental analysis calculated for CdL with a formula of Cd0.5C22H24N5O7F (%): C = 48.42, H = 4.43, and N = 12.83. Found for CdL: C = 48.44, H = 4.42, and N = 12.80. The Fourier transform infrared (FT-IR) spectrum of CdL is shown in Fig. S2 (cm−1): 3440(s), 3030(s), 1630(vs), 1610(vs), 1560(vs), 1480(vs), 1390(vs), 1330(vs), 1300(vs), 1270(vs), 1160(s), 1040(s), 942(vs), 819(vs). One CdL crystal with suitable size (0.40 mm × 0.40 mm × 0.20 mm) was selected and mounted on a glass fiber tip of a goniometer head in air at room temperature. And singlecrystal X-ray diffraction (SXRD) measurement was performed on a XtaLAB PRO diffractometer (Rigaku, Japan/Netherlands) with graphite-monochromated Cu-Kα radiation (λ = 1.54056 Å). Diffraction intensities were collected with a CCD area detector image plate diffractometer using the ω/φ scan technique. The absorption correction was carried out with semi-empirical calculations from multi-scans. The structure was solved by direct methods with SHELXS-97 [33] and refined by least-squares procedures on Fo2 using SHELXL-97 [34] by minimizing the function Σω(Fo2-Fc2)2, where Fo and Fc are the observed and calculated structure factors, respectively. All non‑hydrogen atoms were refined anisotropically. Powder X-ray diffraction (PXRD) pattern were collected from 5° to 40°, with a step of 0.02° and data collection time of 0.2 s, on a D/max-2500 advance diffractometer (Rigaku, Japan) with Cu-Kα radiation (λ = 1.54056 Å). Elemental analysis was carried on a Vario EL Cube elemental analyzer (Elementar, German). FT-IR spectra were recorded on a Nicolet iS10 spectrometer in KBr pellets in the range of 4000–700 cm−1 (Thermo Fisher Scientific, USA). The assembling of a CdL sensing device is presented as follows [28,29]: CdL paste was obtained by grinding appropriate amount of CdL in deionized water, which was then coated on a alumina substrate 2
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Fig. 2. The coordination environment (a), 1D coordination chain (b), 2D supramolecular layer (c), and 3D supramolecular network (d) of CdL. Atomic labeling scheme of nonhydrogen atoms: Cd (cyan), C (gray), N (blue), O (red), F (green); All of the hydrogen atoms have been omitted for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(8 mm × 4 mm × 0.5 mm) with five pairs of AgePd interdigitated electrodes (IDE, electrode width and gap: 0.15 mm). The experiments were performed at an ambient temperature of 20 °C for sensor aging and test. The sensor was dried at 80 °C for 5 h and aged under 97% relative humidity (RH) at 1 V (AC, 100 Hz) for 24 h, so as to improve the stability and durability. The humidity sensing properties were tested in different RH, which is achieved by several saturated salt solutions. The humidity gradient was controlled by six jars (1 L) with rubber stoppers filled with different saturated salt solutions. The sensor in a shielded socket is sealed in a rubber stopper which is the same as that used for 6 jars. Saturated solutions of LiCl, MgCl2, Mg(NO3)2, NaCl, KCl, and K2SO4 in closed vessels yield 11%, 33%, 54%, 75%, 85%, and 97% RH levels, respectively [35]. In order to ensure the accuracy of RH, 8 h stabilization of the jars was necessary to get fully liquid-gas equilibrium before humidity sensing test. In a test circle, the sensor was placed in one jar until the impedance value achieves balance then it was switched to another jar. After reaching steady, the sensor was handled back to the first jar waiting the next impedance stability, finally ending a cyclic test. The experiments were performed at room temperature on a humidity sensing system (CHS-1 Humidity Sensing Analysis System, Beijing Elite Tech. Co. Ltd., China) at 1 V AC with frequency varying from 50 Hz to 100 kHz. The sensor response is defined by: SRH = Z11%/ ZRH, where Z11% and ZRH represent the impedance values of the CdL sensor taken at 11% RH and a certain RH. The response time (τres) or recovery time (τrec) is defined as the time duration needed for the sensor to achieve 90% of total impedance change in the case of adsorption and desorption, respectively. According to the crystal structure refinement based on SXRD results, CdL crystallizes in the triclinic space group P-1. Details of crystallographic data and refinement results for CdL are listed in Table S1. Selected bond lengths and angles of CdL are given in Tables S2 and S3,
respectively. As shown in Fig. 1, the asymmetric unit of CdL with the formula of {[Cd0.5(cfH)(4,4′-bpy)0.5](NO3)·H2O}n contains one Cd2+ cation (half-occupied), one cfH ligand and half a 4,4′-bpy, as well as one NO3− anion and two lattice H2O (half-occupied) molecules. The Cd2+ ion is coordinated by two nitrogen atoms from two 4,4′-bpy ligands and four oxygen atoms from two cfH ligands forming a [CdN2O4] distorted octahedron configuration, as shown in Fig. 2(a). In the cfH ligand, the piperazine ring holds a chair configuration and the quinoline unit is essentially planar. As to two coordinated oxygen atoms in one cfH molecule, one is the carbonyl oxygen (O3), another is the carboxyl oxygen (O1) that is homogenized with another carboxyl oxygen (O2). Subsequently, the proton dissociated from the carboxyl group is further neutralized by the imino group forming a quantenary ammonium group. The coordination mode of cfH is similar to previous reported metal-cfH complexes, and the CdeO bond distances are in the range of 2.227(6)–2.288(4) Å, which all fall into the normal range of reported CdeO bonds in Cd-cfH CPs [31,32]. As shown in Fig. 2(b), the neighboring Cd2+ are linked by 4,4′-bpy molecules to generate a 1D chain structure. Adjacent 1D chains can be further connected via hydrogen bonds (N3⋯H⋯O2) to generate a 2D supramolecular layer, as can be seen in Fig. 2(c). It is worth noting that rich hydrogen bonds exist between supramolecular layers due to plenty of oxygen atoms from cfH ligands as well as counterions NO3− and lattice water molecules. The data of selected hydrogen bond lengths and angles of CdL are shown in Table S4. As a result of this connection, the supramolecular layer is finally linked into a 3D porous structure, as can be seen in Fig. 2(d). PXRD pattern of as-synthesized CdL sample is exhibited in Fig. S3, so is the simulated pattern using SXRD data from CdL single crystal. Clearly, all PXRD peaks are consistent well with those simulated data. Characteristic peaks of impurities arising from reactants or intermediate products are not observed, implying high crystalline purity of 3
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[22,24,25,28,29]. The CdL sensor indicates much larger responses in the low frequency region (50–100 Hz) than that in the high one (1–100 kHz). In the later frequency region, the impedance decreases not so evident as that in the former region that is almost independent of RH, indicating the disappearance of dielectric phenomenon, because the polarization of adsorbed water molecules cannot be able to keep pace with the change of AC electric fields. It should be pointed out that impedance values cannot be recorded at far lower frequency (50 Hz) and RH range (11%–54%) limited by the test scope of CHS-1 (0.5 Ω–1 GΩ). Therefore, the operation AC voltage and frequency were set at 1 V and 100 Hz in the following experiments for gaining higher responses. At 100 Hz, the impedance changes higher than three orders of magnitude (S97% = 3.15 × 103), which is much larger than that of reported CP-based humidity sensors, such as Fe-BTC (1, 3, 5-benzenetricarboxylate) [14], [Cd(TMA)(DPP)0.5∙H2O]n (H2TMA = 3-thiophenemalonic acid, DPP = 1,3-di(4-pyridyl)propane) [24], NH2-MIL-125(Ti) [25], FeCl3-NH2-MIL-125(Ti) [27], and [Pb(TMA)]n [29], and slightly better than that of MIL-101(Cr) [22], [Eu(H2O)2(mpca)2Eu(H2O)6M (CN)8]·nH2O (mpca = 2-pyrazine-5-methyl-carboxylate, M = Mo, W) [23], however still smaller than that of [Pb0.5(TAA)]n (HTAA = 3thiopheneacetic acid) [28] and (NBu4)2Cu2(dhbq)3 [37], as listed in Table 1. Compared with other non-CPs humidity sensing materials, the sensitivity of CdL is a lot higher than that of PDDA/RGO [3], MoS2/ SnO2 [5], and Ba5−xSrxNb4O15 [13], although not as good as that of Ag@mpg-CN [9], Ag/SnO2 [10], In-SnO2/meso CN [11], and CeO2 aero-gel [12]. The dynamic response behavior of the CdL sensor, as exhibited in Fig. 3(b), is switched between 11% RH and 97% RH at 100 Hz. In the switching test cycle, the impedance at 11% RH decreases rapidly to an equilibrium value at 97% RH and then increases rapidly to the baseline at 11% RH with response-recovery times of 12 s and 53 s, respectively. As can be seen in Table 1, the response/recovery time of present work is comparable with that of most CPs sensors [22,24,25,27–29,37], but far shorter than that of Fe-BTC [14] and EuM [23]. Except that of PDDA/RGO [3], the response/recovery time of our work is still not as short as that of conventional oxide-based humidity sensors [5,9–13]. It is considered that the relatively rapid response behavior is originated from numerous active sites and hydrogen bonds network that facilitate the absorption of water molecules and charge carriers transfer. These advantages all result in significant enhancement of large sensing signal and short response time. Additionally, the humidity hysteresis characteristic of the CdL sensor was examined by increasing the RH from 11% to 97% for moisture absorption and then decreasing back to 11% for moisture absorption at different RH. As can be seen in Fig. 3(c), the max humidity hysteresis (ΔHmax) is 3.7% RH occurred at 85% RH in the high RH range (70%–100%), indicating the dominant physisorption of water rather than chemisorption. To further understand the conduction mechanism of CdL towards moisture, complex impedance spectra (CIS) have been analyzed at different RH in the frequency range of 50 Hz–100 kHz, since CIS have been proved to be a powerful tool for exploring the conduction process of humidity sensors [14,22–25,27–29]. As shown in Fig. 4, complex impedance diagrams were constructed in Nyquist plots that can be resolved into real (ReZ) and imaginary (ImZ) parts. Because of strong electronic interactions, water molecules chemisorb onto active sites of CdL surface at low RH range (11%–54%). Only a few water molecules are captured by CdL surface forming a discontinuous water coverage, as sketched in Fig. 5(a). Water molecules chemisorb on active sites of CdL and then dissociate into H+ and OH– ions, forming hydroxyl groups on CdL surface. Large impedance values (hundreds of MΩ) are recorded in Fig. 3(a), because of the electrolytic conduction (H2O → H+ + OH−) [36] is difficult to happen in this stage. The impedance is arc of a semicircle, as shown in Fig. 4(a), indicating a kind of polarization, which is modeled by an equivalent circuit illustrated as an inset of Fig. 4(a). There are two series parts in the equivalent circuit [22,27,29]: one is the electrode represented by a resistor (Rs), the portion of which
Fig. 3. (a) Impedance-RH relationships of the CdL sensor at different frequencies. (b) The characteristic response-recovery behavior of the CdL sensor switching between 11% RH and 97% RH. (c) The humidity hysteresis of the CdL sensor.
the CdL sample. The CdL crystal enjoys uncoordinated fluorine and oxygen atoms in cfH ligands, which are high density active sites for capturing moisture from the environment and thus forming physisorption of water layer on CdL surface. More importantly, abundant oxygen atoms from interchain NO3− anions and lattice H2O molecules generate hydrogen bond networks throughout the whole crystal, which are perfect proton transport channels [36] for humidity sensing. Taking into account of numerous active sites and hydrogen bonds, CdL was assembled into an impedimetric humidity sensor, and the corresponding sensing performance was investigated systematically. Totally speaking, impedance values of the CdL sensor decrease gradually with the increase of RH, as shown in Fig. 3(a). Simultaneously, the CdL sensor exhibits typical humidity sensing behavior that is impedance-dependent at the frequency ranging from 50 Hz to 100 kHz, the phenomenon of which is agreed well with other impedimetric humidity sensors based on CPs 4
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Table 1 Summarized humidity sensing performance based on various materials. Material
Method
CdL Fe-BTC MIL-101(Cr) EuM [Cd(TMA)(DPP)0.5∙H2O]n NH2-MIL-125(Ti) FeCl3-NH2-MIL-125(Ti) (NBu4)2Cu2(dhbq)3 [Pb0.5(TAA)]n [Pb(TMA)]n PDDA/RGO MoS2/SnO2 Ag@mpg-CN Ag/SnO2 In-SnO2/meso CN CeO2 aero-gel Ba5−xSrxNb4O15
S = Z11%
RH/Z97% RH
AC, AC, AC, AC, AC, AC, AC, DC AC, AC, DC AC, AC, AC, AC, AC, DC
or S = Z97%
RH range (%)
100 Hz 1 Hz 100 Hz 500 Hz 100 Hz 100 Hz 100 Hz 100 Hz 100 Hz 100 Hz 100 Hz 100 Hz 100 Hz 50 Hz
RH/Z11% RH
S 3
11–97 0–1.2 33–95 53–100 11–97 11–95 11–95 30–90 11–97 11–97 11–97 0–97 11–98 11–98 11–98 11–98 33–95
3.15 × 10 1.35 1.93 × 103 ~103 3.52 × 102 27.5 3.67 × 102 ~2.1 × 104 1.46 × 104 3.1 × 102 3.96 ~17 3.16 × 104 6.7 × 104 1.48 × 105 3.16 × 104 52.9
τres (s)
τrec (s)
Ref.
12 ~mins 17 380 11 45 11 54 13 9 108–147 5 3 4 3.5 4.6 2
53 ~mins 90 390 56 50 86 6 30 38 94–133 13 1.4 6.5 1.5 2.7 17
This work [14] [22] [23] [24] [25] [27] [37] [28] [29] [3] [5] [9] [10] [11] [12] [13]
[3].
Fig. 4. Complex impedance property of the CdL sensor at various RH ranges: (a) 11%–54%, and (b) 75%–97%. Inset: the equivalent circuits of complex impedance plots at (a) low RH (11%–54%) and (b) high RH (75%–97%).
Fig. 5. The proposed humidity sensing mechanism of the CdL based sensor under different RH ranges.
continuous liquid-like layer, as is diagrammed in Fig. 5(b). In the liquidlike layer, water molecules are ionized with the help of electrostatic field to generating moveable proton (H+) and hydronium ions (H3O+) [38]. These charge carriers significantly reduce the impedance based on the ion transfer mechanism of Grotthuss reaction: H2O + H3O+ → H3O+ + H2O [39], since this reaction ensures the easy transfer of H3O+ in the continuous water layer. With the increase of RH, the semicircle becomes smaller, which can be described as typical Warburg
is irrespective to RH; The other is the CdL sensing film expressed as parallel connection of a resistor (R) and a constant phase element (CPE1). With the increase of RH (54%–85%), the arc grows to be a semicircle, the radius of which becomes small gradually, and then a straight line emerges attached to the semicircle in the low frequency range (right half part in a Nyquist plot) at 97% RH, as shown in Fig. 4(b). Upon the first chemisorpted water layer, more and more water molecules physisorpted onto the CdL surface, forming a 5
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impedance [40]. According to the protons conductivity model [41], the H3O+ from a bare proton and a water molecule is more stable than a bare proton. Hence the H3O+ will be the dominant charge carrier in this stage. Under alternating electric fields, these H3O+ ions can form directional movement then produce leakage current, and finally decrease the impedance obviously. The existence of leakage current makes the complex impedance curve become a completely semicircle. As shown in the inset of Fig. 4(b), the equivalent circuit at high RH (54%–97%) is similar to that at low RH (11%–54%), except that the resistance (R) in Fig. 4(a) is expressed by a series part including a resistance (R) and a constant phase element (CPE2). The Grotthuss reaction usually occurs in low frequency range, because the the mass of H3O+ is relatively larger than that of H+. Therefore, the impedance value decreases with the increasing frequency until the H3O+ movement is unable to keep pace with the changing of AC electric field [27]. So in CIS, the part of semicircle curve disappears slowly at high frequency, and an upturned line appears at low frequency. At 97% RH, the straight line at low frequencies of CIS can also be signified by CPE2, which indicates the impact of H3O+ in the physisorbed water layer through H+ jumping between adjacent water molecules, as illustrated in Fig. 5(c). Based on CIS analysis, the sensing mechanism of this CdL sensor indicates significant roles contributed by distinct charge carriers at different RH ranges. In this work, a novel 1D CP CdL with the formula of {[Cd0.5(cfH) (4,4′-bpy)0.5](NO3)·H2O}n was synthesized through a facile solvent evaporation method at room temperature. Then CdL was assembled to be an impedimetric humidity sensor, which exhibited good sensing properties, including highly response, small hysteresis, and short response-recovery time in a wide humidity range. This contribution reported a highly efficient candidate material for practical humidity sensors. More importantly, this work highlights the potential application of CPs as promising sensing materials for next-generation electrical devices.
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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC Grant No. 21601094 and No. 21401139); Tianjin Education Commission Scientific Research Plan Project (Grant No. 2018KJ271); Tianjin Natural Science Council (Grant No. 15JCQNJC02900); State Environmental Protection Key Laboratory of Odor Pollution Control, P. R. China (Grant No. 201903202), Student's Platform for Innovation and Entrepreneurship Training Program (Grant No. 201813663002 and No. 201810060025), and 111 Project (B12015). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.107541. References [1] H. Farahani, R. Wagiran, M.N. Hamidon, Humidity sensors principle, mechanism, and fabrication technologies: a comprehensive review, Sensors 14 (2014) 7881–7939. [2] R.W. Dunmore, An electrometer and its application to radio meteorography, J. Res. Nat. Bur. Std. 20 (1938) 723–725. [3] D. Zhang, J. Tong, B. Xia, Humidity-sensing properties of chemically reduced graphene oxide/polymer nanocomposite film sensor based on layer-by-layer nano selfassembly, Sensors Actuators B Chem. 197 (2014) 66–72. [4] D. Zhang, J. Tong, B. Xia, Q. Xue, Ultrahigh performance humidity sensor based on layer-by-layer self-assembly of graphene oxide/polyelectrolyte nanocomposite film, Sensors Actuators B Chem. 203 (2014) 263–270. [5] D. Zhang, Y. Sun, P. Li, Y. Zhang, Facile fabrication of MoS2-modified SnO2 hybrid nanocomposite for ultrasensitive humidity sensing, ACS Appl. Mater. Interfaces 8 (2016) 14142–14149.
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