A Highly Proton-Conductive 3D Ionic Cadmium–Organic Framework

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 1713−1722

A Highly Proton-Conductive 3D Ionic Cadmium−Organic Framework for Ammonia and Amines Impedance Sensing Ruilan Liu,† Yaru Liu,‡ Shihang Yu,† Chenglin Yang,† Zifeng Li,† and Gang Li*,† †

College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, Henan 450001, P. R. China School of Science, North University of China, Taiyuan, Shanxi 030051, P. R. China

‡

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S Supporting Information *

ABSTRACT: Lately, the progressive study of metal−organic frameworks (MOFs) for the detection of ammonia and amines has made infusive achievements. Nevertheless, the investigation of proton-conductive MOFs used to detect the low concentrations of ammonia and amine gases at different relative humidities (RHs) at room temperature is relatively restricted. Herein, by solvothermal reaction of Cd(NO3)2 with 2-methyl1H-imidazole-4,5-dicarboxylic acid (H3MIDC), a three-dimensional ionic MOF {Na[Cd(MIDC)]}n (1) bearing ordered one-dimensional channels was successfully synthesized. Our research indicates that the uncoordination carboxylate sites are beneficial to proton transfer and the recognition of ammonia and amine compounds. The optimized proton conductivity of 1 reaches a high value of 1.04 × 10−3 S·cm−1 (100 °C, 98% RH). The room temperature sensing properties of ammonia and amine gases were explored under 68, 85, and 98% RHs, respectively. Satisfactorily, the detection limits of MOF 1 toward ammonia, methylamine, dimethylamine, trimethylamine, and ethylamine are 0.05, 0.1, 0.5, 1, and 4 ppm, respectively, which is one of the best room-temperature sensors for ammonia among previous sensors based on protonconductive MOFs. The proton conducting and sensing mechanisms were highlighted as well. KEYWORDS: ionic MOF, crystal structure, proton conductivity, amine recognition, mild temperature inorganic/organic thin films,14 conducting polymers,15 and nanocomposites.16 Representatively, Campbell et al. developed an electrically conductive 2D Cu-based MOF that exhibited excellent NH3 gas sensitivity.11 Xu et al. prepared one kind of MOF-based nanofilm for chemoresistive NH3 sensing at room temperature.10 Note that the chemoresistive sensors based on the above sensing materials of ammonia and amine gases are studied by means of electronically conductive methods. But the research on proton- conducting MOF-related chemical sensors is still relatively limited. Recently, we have studied the proton conduction of several imidazole dicarboxylate-based MOFs at NH 3 ·H2 O vapors. 17−20 It is found that their proton conductivities could be greatly affected by the concentrations of NH3·H2O vapors. Inspired by this, our group has preliminarily studied the ammonia impedance-sensing properties of two neutral proton-conducting MOFs.21,22 On the other hand, although the research on ionic MOFs has appealed considerable attention,23 the relevant reports on chemical sensors are still rare. Accordingly, the utilization of protonconductive ionic MOFs needs more explorations in the chemoresistive gas sensor area. We herein designed a novel ionic MOF, {Na[Cd(MIDC)]}n (1) (H3MIDC = 2-methyl-

1. INTRODUCTION As one kind of high-potential gas-sensitive materials, metal− organic frameworks (MOFs) have been widely explored. Because of the diversity of metal cations or clusters and multifunctional bridging ligands, the structural features of MOFs (pore size, specific surface area, etc.) can be modulated by the selection of different structural units.1 MOFs have become extremely promising in the development of crystalline porous materials because of the advantages of their unusual cavity shape, mild synthetic conditions, and the controllability of cavities. By functionalization of a ligand or introduction of functional metal ions, the target MOFs can possess great potential in catalysis,2 capture of gases,3 drug delivery,4 chemical sensors,5−7 and proton conduction.8 In particular, MOFs of specific spatial structures have high selectivity to the target molecules with different sizes and structures, which is a hotspot in the field of chemical sensor at present. More recently, to further improve the functionality of MOFderived materials, chemoresistive gas sensors have been exploited. Chemoresistive gas sensors have lately received considerable interest owing to the increasing demand for monitoring harmful gases including ammonia, sulfur dioxide, carbon monoxide, NOx, and so on.9 MOFs are used as a new intriguing class of templates, which can be designed into diverse types of chemoresistive gas sensors.10−12 Ammonia and amine sensors gradually developed from semiconductors,13 © 2018 American Chemical Society

Received: October 28, 2018 Accepted: December 9, 2018 Published: December 10, 2018 1713

DOI: 10.1021/acsami.8b18891 ACS Appl. Mater. Interfaces 2019, 11, 1713−1722

Research Article

ACS Applied Materials & Interfaces

The total resistance (R) of the sample was acquired by arc extrapolation to the Z′ axis from the Nyquist plots. The thickness (L) and surface area (S) of the pellet were considered when calculating the conductivity: σ = L/(RS). Activation energy (Ea) values were obtained from the Arrhenius equation: Tσ = σ0exp(−Ea/kT). 2.6. Gas-Sensing Characterization. The sensing measurements were performed on a homemade device reported in our previous work (as displayed in Figure S1).22 The sample was hung in the testing box and bonded with a couple of Pt electrodes. The precise control of humidity and temperature and the determination of the accurate analyte gas concentration in the measuring device are carried out according to the literature method.22 The experimental details of impedance gas-sensing experiments are described in the Supporting Information. 2.7. Analysis on Impedance Plots. In the impedance diagram, at high frequencies, a semicircle can be observed, and at the low −frequencies, a spur can be observed. The linear part of the Nyquist diagram shows an ion-barrier effect on electrodes and eliminates the probability of electron transfer.28 An equivalent circuit R(C(R(Q(R(C(RW)))))) tried to fit the Nyquist plot by the ZSimpWin program. The selected fitting results are displayed in Figures S2−S5.

1H-imidazole-4,5-dicarboxylic acid), which delivered highly mobile Na+ ions as charge-balancing cations. In this work, we demonstrate that the ionic MOF 1 can be used to detect low concentrations of ammonia and amine gases under different RHs at room temperature. In comparison to our previous findings,21,22 the sensing properties of the ionic MOF 1 have been greatly improved.

2. EXPERIMENTAL SECTION 2.1. Reagents and Apparatus. The reagents used in the experiment were obtained from commercial channels. According to the previous literature,24 the compound H3MIDC was synthesized. The water used here was deionized water. The elemental analysis was performed on a FlashSmart analyzer. The IR spectra (KBr pellet; 400−4000 cm−1) were determined using a BRUKER TENSOR 27 spectrophotometer. Thermogravimetric (TG) measurement was conducted on a Netzsch STA 409PC differential thermal analyzer with a heating rate of 10 °C min−1 in air. X-ray powder diffraction (PXRD) was measured on a Panalytical X’pert PRO X-ray diffractometer (λ = 1.5418 Å). N2, H2, O2, CO, CO2, benzene, and methanol vapor adsorption−desorption isotherms were determined at 25 °C on an ASAP 2420 adsorptiometer. H2O and NH3 vapors adsorption−desorption isotherms were collected on a 3H-2000PW multistation weight method analyzer at 25 °C (BeiShiDe Instrument Technology (Beijing) Co. Ltd.). 2.2. Preparation of {Na[Cd(MIDC)]}n. A mixture of Cd(NO3)2· 6H2O (30.8 mg, 0.1 mmol), NaOH (0.02 mmol), and H3MIDC (17.0 mg, 0.1 mmol) in 7 mL deionized water was stirred for 0.5 h. Then, the mixture was transferred into a 25 mL autoclave, sealed, and heated at 155 °C for 84 h. Subsequently, the mixture was gradually cooled to 25 °C. Colorless transparent cubic crystals of 1 were obtained after rinsing with deionized water for three times (74% yield based on Cd). Calcd for C6H3CdN2NaO4: C, 23.81; H, 0.99; N, 9.26%. Found: C, 23.55; H, 1.04; N, 9.48%. IR (cm−1, KBr): 3374 (m), 2940 (w), 2404 (s), 1565 (s), 1479 (s), 1344 (m), 1258 (w), 1179 (m), 853 (w), 772 (m), 494 (w). 2.3. Crystal Structure Determinations. A Bruker smart APEXII CCD diffractometer was used for MOF 1 measurement with Mo Kα radiation (λ = 0.710 73 Å). The single crystal was picked out and stuck to the top of the fine glass filament. The crystal data were collected by the ω−2θ scan technique at a low temperature (100 K), and the Lorentz polarization effects were corrected. Modification was used for secondary extinction. Direct method was used to solve the structure, which was expanded by the Fourier technique. H atoms on C were located geometrically and used as a ride model to refine. H atoms on O were found at reasonable positions in the differential Fourier map located there. The SHELXL program25,26 was adopted to all calculations. The CCDC no. of 1 is 1849575. 2.4. Water Treatment and Activation. The crystalline solids of 1 (25 mg) were soaked in H2O for 1 month or refluxed in H2O for 24 h. Subsequently, the solids were collected from the water and then airdried for the PXRD test. The crystals of MOF 1 (25 mg) were soaked in an aqueous solution (15 mL) including hydrochloric acid or sodium hydroxide with pH = 1−11 for 24 h, and then the solids were isolated and dried for PXRD testing. The samples were soaked in ethanol for 3 days to be activated, and the ethanol was replaced 3 times a day. The filtered samples were dried at 70 °C in a vacuum for 8 h. Then, the corresponding adsorption−desorption tests were carried out. 2.5. Proton Conductivity Measurement. The impedance data were collected using a PARSTAT 2273 impedance analyzer bearing a traditional quasi-four-probe electrochemical cell (AC voltage of 100 mV; 0.1−1 MHz). AC impedance spectroscopy was carried out on cylindrical pellets with a couple of Pt electrodes. The cylindrical pellet was made from about 30 mg of sample under 2.5 MPa of pressure for 3 min. Its diameter and thickness were determined using a Vernier caliper. The pellets were equilibrated under different humidities for 18 h to ensure constant water content and stable proton conductivity.

3. RESULTS AND DISCUSSION 3.1. Structural Descriptions of MOF 1. MOF 1 crystallizes in the tetragonal space group P42/nmc (Table S1). The asymmetric unit of MOF 1 contains one Cd(II) cation, one MDIC3− ligand, and one sodion. The Cd2+ center is located in a slightly distorted [CdN2O4] octahedral environment, in which the Cd2+ center is coordinated by four O atoms (O1, O1#, O1#2, and O1#3) and two N atoms (N1 and N1#) from three MDIC3− anions (Figure S6). As denoted in Table S2, the Cd−O and Cd−N bond lengths are all within the normal ranges.27 Each MIDC3− ligand adopts the same coordination mode as μ4-k N,O: kO: kN′, O′: kO′ (Figure S7). The adjacent two Cd(II) sites are linked by the MIDC3− ligand to construct a dual-core secondary structure unit with the Cd···Cd distance being 3.846 Å. The adjacent secondary structural units form an infinitely extended onedimensional chain through linkages of carboxyl oxygens and N atoms. The alternating chains are bridged into a 2D layer by the MIDC23− ligand. Furthermore, the adjacent layers are joined by the N and O atoms of the ligand to form a threedimensional (3D) structure (Figure 1). Note that half of the 1D channels of the framework are occupied by the uncoordinated carboxylate oxygen atoms and free sodium ions, which offers a remarkable proton transfer pathway. Thus, this will enhance the impedance sensing performance of 1 toward ammonia and amines. Before the proton conduction tests of MOF 1, TG analysis was performed to evaluate its thermal stability in air (Figure S8), which demonstrates that MOF 1 remains thermally stable up to 280 °C. It firstly loses a weight of 27.99% (calculated, 28.00%) between 280 and 440 °C, which corresponds to the partial loss of MDIC3− ligands. Upon further heating, it goes through a major weight loss step from 440 to 540 °C (observed, 19.74%; calculated, 19.44%) corresponding to the release of the remaining organic ligands. Eventually, a white amorphous residue, which is CdO and 0.5Na2O, is obtained (observed, 52.27%; calculated, 52.56%). In short, the excellent thermal stability of 1 makes it an ideal candidate for protonconducting and proton-sensing materials. 3.2. Water and Chemical Stability. High water stability and wide pH tolerances are very important for the application of proton conductors or identification applications. To check for the water stability of 1, the samples were soaked in H2O for 1714

DOI: 10.1021/acsami.8b18891 ACS Appl. Mater. Interfaces 2019, 11, 1713−1722

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foundation for proton conduction and proton-sensing applications. 3.3. Proton-Conductive Properties. As exhibited in Figure 3 and Figures S9−S13, all the Nyquist plots reveal one semicircle with a tail at low frequencies suggesting the blockage of proton ions at the Pt electrodes.28 Note that the compound does not present any proton-conductive behavior under anhydrous condition at room temperature. Meanwhile, it starts showing proton conductivity after the pellet was equilibrated for 18 h under a certain RH value, which incisively demonstrates the role of the water molecules as efficient carriers in the proton transfer process. At a fixed temperature, the proton conductivities for MOF 1 were found to be the low values of 8.32 × 10−8 to 1.13 × 10−5 S·cm−1 at room temperature and 68−98% RHs (Figure S9). The cause for the low conductivities of MOF 1 may be due to the restricted absorption of proton carriers in the 1D channels under low-humidity conditions, and merely a handful of water molecules can be ionized into H+ or H3O+ under the lowtemperature conditions. At 100 °C, the conductivity increases from 7.16 × 10−5 S·cm−1 (under 68% RH) to 1.04 × 10−3 S· cm−1 (under 98% RH) as RH increases (Figure 3a). With the increase of RH, more water molecules will be absorbed into the channels and interact with carboxylate O atoms to build up rich H-bonding nets, which will be conducive to proton conduction. Meanwhile, in the case of low humidity, less water molecules are adsorbed in the channels The above phenomena indicate that its proton transfer is dominated by watermediated proton conduction. The temperature-dependent proton conductivities for MOF 1 were measured at 25−100 °C and fixed RHs. Under fixed RH, the proton conductivity could be increased with an increase in temperature. As shown in Figure 3b, under 98% RH, the conductivity is 1.13 × 10−5 S· cm−1 at 25 °C, subsequently increases gradually with the rise in temperature, and attains 1.04 × 10−3 S·cm−1 at 100 °C. A similar phenomenon also appears under 93, 85, 75, and 68% RHs (Table 1, Figures S10−S13), which is a typical proton conduction behavior. As denoted in Table S3, under similar testing conditions, although the optimized proton conductivity (1.04 × 10−3 S·cm−1, at 100 °C, and 98% RH) of 1 is not as high as the conductivity values of several MOFs recently

Figure 1. 3D framework of MOF 1 showing the 1D channels.

1 month and refluxed in H2O for 1 day, the PXRD profiles for the simulated ones and water-treatment solids matched perfectly (Figure 2a). To determine the chemical stability of 1, the crystalline powders were soaked in acidic or alkali aqueous solutions for 24 h. Consequently, the PXRD patterns of the simulated ones, as-synthesized, and acid-base solutiontreatment solids were almost consistent, which indicates that the compound can maintain its structural rigidity in the pH range of 1−11 (Figure 2b). Note that the PXRD patterns for the simulated and as-synthesized MOF 1 did not match perfectly at about 15°. This is probably because of the variation in the preferred orientation of the sample during the PXRD data collection process. Evidently, MOF 1 has good chemical and excellent water stabilities, which provides the good

Figure 2. (a) PXRD patterns of 1 for the simulated ones and of the water-treated solids, and (b) PXRD patterns of 1 after immersion in different pH solutions and the simulated ones. 1715

DOI: 10.1021/acsami.8b18891 ACS Appl. Mater. Interfaces 2019, 11, 1713−1722

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Figure 3. (a) Impedance spectra of 1 at 100 °C and at 68−98% RHs, and (b) impedance spectra of 1 at 25−100 °C and at 98% RH.

Table 1. Proton Conductivities (S·cm−1) of 1 at Various RHs and Temperatures σ (S·cm−1) temperature 25 °C 35 °C 40 °C 50 °C 60 °C 70 °C 80 °C 85 °C 90 °C 95 °C 100 °C

68% RH 8.32 1.04 1.43 2.24 3.21 7.25 4.91 9.36 1.92 5.60 7.16

× × × × × × × × × × ×

10−8 10−7 10−7 10−7 10−7 10−7 10−6 10−6 10−5 10−5 10−5

75% RH 1.35 2.45 5.04 9.53 1.36 1.75 2.62 3.63 5.25 8.16 1.12

× × × × × × × × × × ×

85% RH

10−6 10−6 10−6 10−6 10−5 10−5 10−5 10−5 10−5 10−5 10−4

3.87 6.14 9.15 1.45 4.06 5.28 6.47 8.98 1.74 2.04 3.02

reported,29,30 the conductivity can be compared to that of previous MOFs,22,31 which is much higher than that of some other MOFs.32−34 It should be pointed out that MOF 1 presents high water and chemical stabilities, especially good proton conductivity, which lays a good material foundation for the future exploration of practical application in fuel cells. Although the contribution of sodium ions to the conductivity of MOF 1 at room temperature can be ignored, with the increase of temperature, the thermal motion of sodium ions may have a certain degree of influence on the conductivity value of 1.35 To explore the proton conduction mechanism, we determined the H2O vapor adsorption properties of MOF 1. As displayed in Figure S14, the water vapor absorption value is ca. 74 mg/g as P/P0 is 0.05. As P/P0 reaches 0.95, the water absorption value reaches ca. 156 mg/g. In other words, with the increase of P/P0 value, the water absorption value of the MOF will increase. On the basis of this, to further probe and figure out the proton conduction mechanism, it prompted us to calculate the activation energy (Ea) from the least-squares fits of the slopes. The Ea values are 1.32 eV under 68% RH and 0.35 eV under 98% RH (Figure 4). Apparently, the mixed mechanisms containing both vehicle (>0.4 eV) and Grotthuss

× × × × × × × × × × ×

10−6 10−6 10−6 10−5 10−5 10−5 10−5 10−5 10−4 10−4 10−4

93% RH 5.21 1.15 1.52 2.55 2.99 5.99 8.11 1.24 3.00 3.92 5.67

× 10−6 × 10−5 × 10−5 × 10−5 × 10−5 × 10−5 ×1 0−5 × 10−4 × 10−4 × 10−4 × 10−4

98% RH 1.13 2.17 3.75 5.48 6.25 7.68 1.09 1.46 3.19 6.32 1.04

× × × × × × × × × × ×

10−5 10−5 10−5 10−5 10−5 10−5 10−4 10−4 10−4 10−4 10−3

Figure 4. Arrhenius plots of the proton conductivities of 1 at 68 and 98% RHs.

(hopping) ( DMA (548%) > TMA (381%) > EA (231%). Then, we studied the response to the analytes under 85% RH. The detection limits of ammonia, MA, DMA, TMA, and EA were determined to be 1, 2, 4, 6, and 9 ppm, respectively (Table S4). Figure 5b shows the good linear relationship from detection limits of 30 ppm and is consistent with 98% RH. Considering the practicability of low humidity, we further study the response of MOF 1 to the analytes at 68% humidity (Table S5). The detection limits of the analytes are 3 ppm (ammonia), 6 ppm (MA), 9 ppm (DMA), 12 ppm (TMA), and 15 ppm (EA). Meanwhile, linear relationship and response are consistent with those under 98 and 85% RHs (Figure 5c). The above statement indicates that they are typical protonconducting behaviors.37 Note that the response values increase with the increase of gas concentrations and show a good linear relationship 1718

DOI: 10.1021/acsami.8b18891 ACS Appl. Mater. Interfaces 2019, 11, 1713−1722

Research Article

ACS Applied Materials & Interfaces between response and concentrations below 30 ppm under different RHs. The gas response value increases with increasing RH, and the sensitivity of the MOF under high humidity is obviously superior to that under low humidity. For instance, the room temperature gas response (1379%) to 30 ppm NH3 at 98% RH is 2.1 times and 3.8 times those at 85% (662%) and 68% RH (363%), respectively. The detection limits toward ammonia gas decrease successively from high humidity to low humidity. It is worth mentioning that 98% RH is the optimal condition for the sensor, which has the highest gas response and the lowest detection limit at room temperature. At 98% RH, the detection limit of the ammonia gas sensor is 0.05 ppm, with a good gas response of 21% (Figure 5a). It should be pointed out that because of the importance and urgency of the identification of ammonia gas, a wide variety of ammonia gas sensors based on various materials, such as oxides, organic polymers, and MOFs, have been developed in recent years. However, it is found in the literature that a considerable number of ammonia sensors require a high operating temperature.38−40 There are only a few reports on the identification of ammonia gas at mild temperatures.10,11,21,22,41−60 Considering the characteristics of the room-temperature sensor reported in this paper, we deliberately listed the identification performance of some ammonia sensors under mild conditions in Table 3 and conducted comparative studies. As listed in Table 3, MOF 1 is significantly superior to other previous NH3 gas sensors using different sensing materials in high response and low detection limits at moderate temperatures.10,11,21,22,41−60 It should be emphasized that most of the previously reported ammonia sensors can show good recognition performance under dry conditions, such as dry N2 or dry air.10,44−52,54−57,60 There are very few reports on the identification of ammonia under certain humidity,41,43,59 especially at high humidity.11,21,22,58 Only, Dinca et al.,11 Kim et al.,58 and our group21,22 have recently described several NH3 sensors under high RHs (60−98% RHs). This lays a good foundation for the application of room-temperature ammonia recognition under high humidity of these ammonia sensors. Also, the response values of volatile amines illustrate the same trend that the response value increases with the increase of RH (Figure 5, Tables 2, and Tables S4 and S5). According to the literature search, in the past, people used traditional detection methods, such as UV−vis spectroscopy or chromatography, to identify these amines (MA, DMA, TMA, and EA): The identification of MA was mostly in aqueous solution,61,62 the identification of TMA was mostly carried out at high temperatures,63−65 and the identification of DMA and EA was very limited.66,67 As far as we know, the study of gas identification using protonconductive MOFs for these substances is firstly reported herein. A pivotal issue for the future development of sensory devices based on the MOF is whether it has selectivity in chemoreceptive responses observed here. Beyond the above research, the chemoreceptive sensing for other gases (N2, H2, O2, CO, CO2, benzene, MeOH, n-hexane, and toluene) was further studied at 30 ppm at 25 °C and 68−98% RHs (Figure 6 and Figure S16). For these gases, the maximum response value of gas sensors is not more than 9%, much lower than that of ammonia gas and volatile amine at 25 °C and 68−98% RHs. To better highlight the selectivity to the analytes, the values of the selectivity to analytes compared to other gases (s = response (analytes)/response (gas)) are shown in Tables S6−

Figure 6. Column chart of the responses of MOF 1 to NH3 compared to the different gases (30 ppm) under 68−98% RHs and at 25 °C.

S11. The results indicate that MOF 1 has good sensitive identification of NH3, MA, DMA, TMA, or EA gases in the presence of other disturbing gases (N2, H2, O2, CO, CO2, benzene, MeOH, n-hexane, and toluene). To confirm the reproducibility of the sensor, we conducted the following cycling experiment: First, the detected gas was injected into the device, the sensor’s resistance was found to fall to a constant value after 8 min, and then the detected gas was blown out of the system. After 5 min, the sensor’s resistance was found to rise to a constant value. After 30 cycles, the cycling characteristics show that the response decreases very little (Figures S17−S19). The PXRD patterns of the recycled samples were very similar to those of the newly synthetized samples, showing that they maintained their structural stability after the cycling test (Figure S20). In addition, the sensor can be supported for 2 months under the experimental conditions and maintain the integrity of its structure (Figure S21). Hence, the MOF-based sensors show good stability and reproducibility in the process of detection, which provides sufficient evidence for the development of sensors in the future. Ea values of MOF 1 toward 30 ppm ammonia and amines at 98 and 68% RHs and 25−100°C are calculated to further gain insight into the sensing mechanism (Figures S22 and S23). Under 98% RH, the activation energies for ammonia, MA, DMA, TMA, and EA are in the range of 0.12−0.30 eV (Figure 7a). This suggests that the proton conduction pertains to a Grotthuss mechanism, which involves proton transfer between extraneous guest species and inherent structure. As the proton conductivities were controlled by the carrier concentration and the proton transferring efficiency, the carriers and pathways of the transmission process are discussed. For efficient proton conduction, the rich H-bonding networks are extremely crucial. The water, ammonia, and amine molecules adsorbed by MOF 1 can easily form hydrogen-bonded networks with the uncoordination carboxylate oxygens in the channels, which gives a sequential channel for proton transfer. Under 68% RH, the Ea values for ammonia, MA, DMA, TMA, and EA are 0.46, 0.52, 0.59, 0.63, and 0.68 eV, respectively (Figure 7b). The differences in Ea values are accounted to the difference in the concentrations and mobility of the proton carriers in ammonia and amine gases. As mentioned already, typical Ea values are 0.1−0.4 eV and 0.5−0.9 eV for Grotthuss and vehicle 1719

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Figure 7. Arrhenius plots of the proton conductivities of 1 at (a) 98% and (b) 68% RHs toward different amine gases.

mechanisms, respectively.68−70 So, the Grotthuss mechanism is likely to occur under low RH toward different amine gases. As the adsorption capacity of MOF 1 to ammonia and amine vapors may strongly influence the proton conduction behavior, it is necessary to find out this performance to further interpret the mechanism. The absorption and desorption isotherms of NH3 or amine vapors were determined (Figure S24) at 25 °C. The NH3, MA, DMA, TMA, and EA gas absorptions are ca. 42, 38, 34, 31, and 27 mg/g at P/P0 = 0.05, respectively. As P/P0 is 0.9, the maximum adsorption capacities of these gases are ca. 225, 202, 190, 176, and 164 mg/g, respectively. From these data, we can get two points: One is that MOF 1 shows a certain adsorption capacity to these compounds; the other is that the adsorption amount is related to the size of the molecules. The larger the molecules are, the smaller the adsorption amount of the MOF is. When these amine compounds enter the channels, it is easy to form a rich hydrogen-bond network with the carboxylate oxygen atoms inside the e channels, which is conducive to proton transfer. Additionally, these amine compounds can also interact with the water adsorbed by the MOF forming ammonium units, which is also conducive to proton conduction. For ammonia gas, its small size makes it easy to be adsorbed and react with water molecules to form ionized protons, so its identifying performance is the best. The amino groups of methylamine or ethylamine are all on the side of the molecule; the polarity of methylamine is less than that of ethylamine, so the performance of methylamine is better. The amino groups of both dimethylamine and trimethylamine are in the middle of the molecule, and the ability of dimethylamine to combine with water molecules to ionize protons is stronger; thus, the performance is relatively better.

cm−1. On the basis of this study, we explored the sensing properties of the MOF 1-based sensor to ammonia and amine gases. The room-temperature sensitivity of the MOF-based sensor toward ammonia and amine gases at different RHs (68−98% RHs) was fully studied. The detection limits and response values of the MOF were also investigated. The detection performance of these amine gases was the best under 98% RH. Moreover, reversible and reusable behaviors were observed during the exploring process. We have proven that this MOF-based sensor has a remarkable sensitivity to ammonia and amine gases, which has high potential in the field of electrochemical sensing.

4. CONCLUSIONS In this research, a novel proton-conducting 3D ionic MOF (1) with 1D channels has been successfully solvothermally synthesized and structurally characterized by multiple determination methods. First, the investigation of proton conductivity at 68−98% RHs verifies that humidity and temperature have influence on its proton conduction. The conductivity can reach a maximum value of 1.04 × 10−3 S·

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21571156 and J1210060).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b18891.

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Details of crystal data, impedance analysis, PXRD patterns, gas adsorption/desorption (PDF) Crystallographic information of MOF 1 (CCDC no. 1849575) (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gang Li: 0000-0001-9049-4208 Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acsami.8b18891 ACS Appl. Mater. Interfaces 2019, 11, 1713−1722