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Cite This: Inorg. Chem. 2018, 57, 7104−7112
A Water-Stable Proton-Conductive Barium(II)-Organic Framework for Ammonia Sensing at High Humidity Kaimeng Guo, Lili Zhao, Shihang Yu, Wenyan Zhou, Zifeng Li, and Gang Li* College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, Henan, P. R. China
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S Supporting Information *
ABSTRACT: In view of environmental protection and the need for early prediction of major diseases, it is necessary to accurately monitor the change of trace ammonia concentration in air or in exhaled breath. However, the adoption of protonconductive metal−organic frameworks (MOFs) as smart sensors in this field is limited by a lack of ultrasensitive gas-detecting performance at high relative humidity (RH). Here, the pellet fabrication of a water-stable proton-conductive MOF, Ba(oCbPhH2IDC)(H2O)4]n (1) (o-CbPhH4IDC = 2-(2-carboxylphenyl)-1H-imidazole-4,5-dicarboxylic acid) is reported. The MOF 1 displays enhanced sensitivity and selectivity to NH3 gas at high RHs (>85%) and 30 °C, and the sensing mechanism is suggested. The electrochemical impedance gas sensor fabricated by MOF 1 is a promising sensor for ammonia at mild temperature and high RHs.
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INTRODUCTION Metal−organic frameworks (MOFs) are highly ordered crystalline solids that can be controllably constructed by judicious selection of metal ions/clusters and various bridging ligands. Accordingly, MOFs with unique structural diversity (such as topological structures, poles, and functions) could be obtained. Therefore, an opportune research platform is provided for obtaining multifunctional MOF materials.1−3 In this context, in the fields of energy-related applications, electrical- and proton-conductive MOFs have received great attention from researchers.4−15 In electrochemical sensing field, the distinctive electrochemical and physical advantages including the conductive sites introduced by activating metals and/or ligands make the conductive MOFs become excellent electrochemical sensors with high sensitivity and selectivity.16−20 At the same time, the outstanding structural features of the MOFs, such as types of the pores (shape and size) or the various interactions between the guest and host molecules (Hbonds and π−π and van der Waals interactions) can have a significant impact on the electrical signal, which will be very beneficial to the function of the sensor. Nevertheless, because the electrical and proton conductivities of most MOFs are relatively low, until now, the sensing applications based on the direct use of the conductive MOFs were less.11−20 Note that a © 2018 American Chemical Society
limited number of articles on electrochemical identification are mainly focused on electrically conductive MOFs.17−21 On the one hand, the reports of proton-conductive MOFs as electrochemical impedance sensors for ammonia are very scarce. On the other hand, the smart response of protonconductive MOFs upon external stimuli (e.g., temperature, humidity, and the vapor environment)22−26 has opened another door to develop new types of conductive MOF-based sensory devices. Ammonia, as a flammable and highly toxic gas, can be applied in a variety of industries and also is a major polluting gas in the environment released from decomposition of organic nitrogenous animal and vegetable matters and motor vehicles.27 It has also been found that the amount of ammonia in exhaled breath can be a prominent biomarker for stomach cancer or kidney disease.17,19,20,28−30 Recently, our group found that the proton conductivities for four Co(II)-based and one Sr(II)based imidazole dicarboxylate MOFs are markedly increased at aqua-ammonia vapor. What’s interesting is that the proton conductivity of these MOFs increases steadily with the increase of the concentration of ammonia vapor.24−26 Naturally, protonReceived: March 26, 2018 Published: June 7, 2018 7104
DOI: 10.1021/acs.inorgchem.8b00806 Inorg. Chem. 2018, 57, 7104−7112
Article
Inorganic Chemistry conductive MOFs-based electrochemical impedance sensors for ammonia catch our eyes. This motivates us to further explore this type of proton-conductive MOF as sensitively sensing device for ammonia recognition. Herein, we describe that a simple device can be employed for the reversible impedance sensing of NH3 gas at high relative humidity (RH) and 30 °C. It was made in this way: first, the crystalline powders of [Ba(o-CbPhH2IDC)(H2O)4]n (oCbPhH4IDC = 2-(2-carboxylphenyl)-1H-imidazole-4,5-dicarboxylic acid)31,32 (1) were pressed into a pellet, and then the pellet was put into a couple of Pt electrodes for sensing measurements. The MOF 1 shows excellent sensitivity and high selectivity recognition of ammonia gas under high RHs and mild temperature.
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Table 1. Crystallographic Data and Structure Refinement Information for Compound 1 MOF formula fw crystal system crystal size, mm space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Dc, Mg m−3 Z μ, mm−1 reflns collected/unique data/restraints/parameters final R indices [I > 2σ(I)] R indices (all data) GOF on F2 Δρmin and Δρmax, e Å−3
EXPERIMENTAL SECTION
Reagents and Apparatus. The chemicals were purchased from commercial channels and used as received. The o-CbPhH4IDC compound was synthesized in accordance with the methods described in early literature.31,32 Elemental analyses were conducted with a Vario El III analyzer. IR spectra were measured from KBr pellets with a Bruker Tensor II FTIR spectrophotometer. Thermogravimetric analyses (TGA) were performed on a Netzsch STA 449F3 differential thermal analyzer. The powder X-ray diffraction (PXRD) patterns were obtained on a Panalytical X’pert PRO X-ray diffractometer. H2O and NH3 vapors adsorption−desorption isotherms were determined on a 3H-2000P Multistation Weight method analyzer (BeiShiDe Instrument Technology Co. Ltd.). Preparation of [Ba(o-CbPhH2IDC)(H2O)4]n (1). A mixture of Ba(NO3)2 (0.0212 g, 0.1 mmol), o-CbPhH4IDC (0.0276 g, 0.1 mmol), and H2O (7 mL) was sealed in a 25 mL Teflon-lined bomb and heated at 150 °C for 4 d and then cooled to room temperature. Pale yellow rod crystals of 1 were obtained, washed with distilled water, and dried in air. Yield: 61% (based on Ba). Calcd for C12H14N2O10Ba: C, 29.80; H, 2.92; N, 6.35%. Found: C, 29.54; H, 3.05; N, 6.27%. IR (cm−1, KBr): 3454(w), 3059(w), 2817(w), 2541(w), 1964(w), 1685(m), 1573(s), 1509(s), 1423(s), 1282(m), 1135(s), 1018(m), 941(m), 881(m), 781(s), 733(s), 688(w), 526(m), 450(w). Crystal Structure Determinations. Single-crystal diffraction data of 1 were collected at 293(2) K on a Bruker smart APEXII CCD diffractometer using Mo Kα radiation (λ = 0.710 73 Å). The intensities were corrected for Lorenz-polarization effects. Structure was solved with direct methods and expanded using the Fourier technique with SHELXL program package.33,34 The H atoms were generated geometrically. Anisotropic thermal parameters were adapted to all non-H atoms. The crystallographic data, important bond distances and angles, and H-bonding data are listed in Tables 1, S1, and S2, respectively. CCDC No. 1537928. Stability Experiment and Activation. The crystalline samples of 1 were soaked in water for 7 d or refluxed in water for 24 h, then collected and dried in air to obtain samples for PXRD determinations. Crystalline solids of 1 (10 mg) were ground and soaked in 15 mL of aqueous solution with different pH values (pH = 1, 2, ..., 11) for 24 h, which as adjusted by HCl or NaOH. Then, the solids were collected and dried in air for PXRD measurements. The activation crystalline solids of 1 were prepared by immersing the solids in alcohol for 8 h to exchange the water molecules in the framework and then drying under vacuum at 65 °C for 14 h. Proton Conduction Measurement. The proton conductivity was recorded by a quasi-four-probe method using a Princeton Applied Research PARSTAT 2273 impedance analyzer with a frequency range from 0.1 Hz to 1 MHz and an applied alternating current voltage of 100 mV. The pressed pellet for electrochemical determination was prepared according to our recent description.25 Before the protonconductivity measurement, each pellet was equilibrated for at least 18 h under different relative humidity (RH). All electrochemical data were recorded through Power Suite program. Impedances (R) were
1 C12H14BaN2O10 483.59 triclinic 0.21 × 0.19 × 0.15 P1̅ 8.7117(13) 10.5772(17) 10.8169(17) 79.997(7) 67.061(6) 86.919(6) 903.8(2) 1.919 2 2.248 23 680/3181, R(int) = 0.0374R(int) = 0.0374 3181/276/226 R1 = 0.0276, wR2 = 0.0749 R1 = 0.0313, wR2 = 0.0761 1.129 1.048 and −0.768
calculated from the semicircles of the Nyquist plot. Proton conductivity was deduced from the equation σ = T/(RS), where T and S are the thickness (cm) and cross-sectional area (cm2) of the pellet, respectively. The activation energy (Ea) value was deduced from the slope of Arrhenius plots by least-squares fitting. Analysis on Impedance Plots. The equivalent circuit LR(C(R(Q(R(C(RW)))))) was used to fit the Nyquist plots of 1 at 30 and 100 °C under 98% RH through ZSimpWin program (Figures S1 and S2). Gas Sensor Characterizations. As shown in Figure 1, we adopt a homemade device to perform the sensing experiment. The pellet was
Figure 1. Diagram of gas-sensing experimental device for MOF 1.
placed into a couple of Pt mesh electrodes and then was hung inside the testing chamber. The two electrodes were linked to the electrochemical measuring instrument (Princeton Applied Research PARSTAT 2273 impedance analyzer). At the beginning of the test, the relative humidity in the testing chamber is kept constant by adjusting the proportion of saturated water steam and dry synthetic air. Subsequently, the target gas diluted by dry synthetic air was injected into the testing chamber by the precise control of the computer (CS200C, Beijing Seven Star Qualiflow Electronic Equipment Manufacturing Co., Ltd.). This testing chamber is equipped with a precise temperature-control device. The precise relative humidity and temperature in the testing chamber are measured by an instrument (Smart Sensor, AR837). The gas response of gas sensing was calculated from the following equation: 7105
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Figure 2. (a) 3D solid-state packing of MOF 1 supported by H-bonds. (b) Water chain inside the channel.
Figure 3. PXRD patterns of 1. (a) The simulated ones from the single-crystal data, as-synthesized, and after water-treatment samples. (b) The simulated ones from the single-crystal data and after soaked in various pH solutions solids.
were further linked by complicated intermolecular H-bonds to build from two-dimensional architecture (Figure S6) to threedimensional (3D) framework (see Figure 2a). Although compound 1 and 3D MOF [Sr(o-CbPhH2IDC)]n31 are prepared using the same imidazole dicarboxylate ligand, the coordination fashions of the organic ligand are different. Additionally, in MOF 1, the water participates in the coordination to meatal ion, which causes their structural difference. We emphasize that the coordination H2O molecules, imidazole, and carboxylate units all involve in the hydrogenbond networks (see Table S2). Therefore, the water chains with infinite eight-membered ring (O10−O8−O4−O5−O10− O8−O4−O5−O10) and six-membered ring (O9−O8−O10− O9−O8−O10−O9) supported by H-bonds are formed along the crystallographic b-axis (see Figure 2b). This will be very beneficial to the proton-transfer process of the MOF system.3,6,7 Thermal Gravimetric Analysis (TGA) of MOF 1. The plot (see Figure S7) indicated that the first step of weight loss arises from 77.5 to 197.5 °C (observed 14.98%, calculated 14.89%), which corresponds with the loss of four coordinated H2O molecules. Then, the organic ligands in 1 collapsed with the increase of the temperature. At last, a plateau region appeared in the temperature of 455−800 °C. The black residue
gas response% = (R h − R gas)/R gas × 100 where Rh and Rgas are the impedance of the sensor at fixed RH and target gas, respectively.
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RESULTS AND DISCUSSION Crystal Structure. Single-crystal X-ray diffraction determination (see Table 1) illustrates that the asymmetric unit of 1 includes one crystallographically independent Ba(II) ion, one independent o-CbPhH2IDC2− ligand, and four coordination H2O molecules. As shown in Figure S3a, Ba1 is 10-coordinated by four carboxylate O atoms (O6, O3#1, O2#1, and O3#3) of three o-CbPhH2IDC2− ligands, five O atoms (O7, O8, O9, O10, and O7#2) of five individual coordination H2O molecules, and one N atom (N1#1) from one o-CbPhH2IDC2− ligand, exhibiting a distorted pentadechedron with 14 trigons and 1 tetragon as faces (Figure S3b). As indicated in Figure S4, two neighboring Ba(II) atoms are linked by two water units and two μ2 imidazole carboxylate functionalities to form a binuclear [Ba2(o-CbPhH2IDC)4(H2O)8] unit. The Ba···Ba distance inside the binuclear unit is 4.1968(7) Å. Furthermore, these binuclear units are linked by two organic ligands as μ3-kO, N,O′:kO″:kO‴modes (see Figure S5) to build a one-dimensional (1D) Ba(II) chain (see Figure S4). These Ba(II) chains 7106
DOI: 10.1021/acs.inorgchem.8b00806 Inorg. Chem. 2018, 57, 7104−7112
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Figure 4. Impedance spectra of 1 at 75% RH and 30−100 °C.
Figure 5. Impedance spectra of 1 at 30 °C and 75%−98% RH.
Table 2. Proton Conductivities [S cm−1] for 1 at Different RHs and Temperatures 30 °Ca 68% 75% 85% 93% 98% a
40°Ca
50°Ca
b
RH RHb RHb RHb RHb
5.52 2.21 2.69 1.17
× × × ×
−7
10 10−6 10−6 10−5
6.01 3.30 1.32 1.82
× × × ×
−7
10 10−6 10−5 10−5
6.71 9.21 5.66 2.13 2.60
× × × × ×
60°Ca −8
10 10−7 10−6 10−5 10−5
1.91 1.01 1.13 2.95 5.09
× × × × ×
70°Ca −7
10 10−6 10−5 10−5 10−5
4.83 1.40 1.43 3.40 1.09
× × × × ×
80°Ca −7
10 10−6 10−5 10−5 10−4
8.83 1.96 1.73 4.54 2.56
× × × × ×
90°Ca −7
10 10−6 10−5 10−5 10−4
9.34 3.98 2.70 3.25 4.09
× × × × ×
100°Ca −7
10 10−6 10−5 10−4 10−4
2.05 5.69 5.09 6.68 1.36
× × × × ×
10−6 10−6 10−5 10−4 10−3
Temperature. bRelative humidity.
is estimated to be BaO (observed 31.38%, calculated 31.71%). We emphasize that below 100 °C the weight loss of 1 is only 2.59% equivalent to losing less than one coordination water molecule. Hence, three or more water molecules can be maintained less than 100 °C, which would benefit the protonconduction process in 1. Water and Chemical Stability of MOF 1. For MOFsbased proton conductors and electrochemical sensors, water and wide pH stability is an important factor for future applications. As exhibited in Figure 3a, all prime peaks of experimental PXRD of as-synthesized MOF 1 overlap well with the simulated ones, confirming its crystalline phase purity. The water stability of 1 was determined by refluxing the crystalline
powders in boiling water for 1 d or immersing the powders in water for 7 d. As displayed in Figure 3a, after water treatment, the PXRD of the samples remains unchanged, which proves the water stability of the framework. To further investigate its chemical stability, the crystalline solids of 1 were put into the aqueous solutions with pH values varying from 1 to 11 for 1 d. The accordant PXRD patterns before and after chemical treatment demonstrated that the compound still retains structural stability (see Figure 3b). Apparently, both above determinations illustrate its excellent water and chemical stability. To be noticed, although people have previously studied the thermal stability of several Ba(II) carboxylate MOFs,35−39 the research on the water stability is very rare. As 7107
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Inorganic Chemistry far as we know, 1 is first example of investigation on water stability of Ba(II) carboxylate MOFs. Proton Conduction of MOF 1. Impedance measurements were performed under controlled RHs and temperatures. The proton conductivity was obtained for the fitting of the Nyquist plot.40,41 Samples had fixed water content and stable conductivity values through pellets equilibrated under different RHs for 18 h. As shown in Figures 4, 5, and S8−S12, the Nyquist plots show one semicircle at high frequencies with a slope at low frequencies suggesting that the conduction species should be proton ions. We point out that at 30 and 40 °C under 68% RH or below this RH, we could not obtain useful impedance value due to the anomalous impedance spectra and promiscuous data. Thus, the electrochemical data were collected from 50 to 100 °C under 68% RH (Figure S8) and from 30 to 100 °C under 75−98% RHs (see Figures 4 and S9−S11). As observed from Table 2, the compound exhibits humidity- and temperature-dependent proton-conduction features. At fixed temperatures of 30 and 100 °C (see Figure 5 and S12), the conductivities augment from 5.52 × 10−7 (75% RH) to 1.17 × 10−5 S·cm−1 (98% RH) and from 2.05 × 10−6 (68% RH) to 1.36 × 10−3 S·cm−1 (98% RH), respectively. Under fixed RH, we can also observe that the conductivities of 1 increase as the temperature rises (see Figures 4 and S8−S11). As shown in Figure 6, Ea values of MOF 1 were 0.71 at 68% RH and 0.67 eV at 98% RH suggesting the proton-conduction
values of several previous compounds at similar condition.45−47 For example, Luo and coauthors reported an open-framework, (CH3NH3)2Ag4Sn3S8, having conductivity of 1.14 × 10−3 S· cm−1 at 67 °C and 99% RH.45 Sanda and co-workers prepared the 1D MOF (C10H2O8)0.5(C10S2N2H8)]·2H2O]}n showing conductivity of 0.44 × 10−3 S·cm−1 under 80 °C and 95% RH.46 Our group recently described a Co-based supramolecule [with conductivity of 0.31 × 10−3 S·cm−1 under 100 °C and 98% RH).47 Additionally, taking into account future applications in fuel cells, the time-dependent conductivity was measured at 100 °C and 98% RH. As displayed in Figure S14, after 8 h of continuous testing, the proton conductivity of MOF 1 remained basically unchanged. At the same time, the perfect matching PXRD data for 1 before and after electrochemical determinations reveal that the skeleton remains uniform, denoting its structural stability during the proton-conduction process (see Figure S15). The Impedance Ammonia Sensing Properties of MOF 1. As described in the previous section, this MOF exhibits good thermal stability, excellent water stability, and good stability toward different pH solutions (1−11). In addition, it is more important that the MOF has excellent proton conductivity under high humidity, which will be very beneficial to the development of sensing research. Therefore, considering the need of practical application, we investigate its electrochemical recognition performance of ammonia under mild temperature (30 °C) and high relative humidities (75%, 85%, 93%, and 98% RHs). We use the device shown in Figure 1 to perform the gassensing experiments of 1. In the test process, first of all, the relative humidity inside the test chamber is maintained at a constant value. Then, a certain target gas was injected into the chamber. As listed in Table 3, under fixed RH, the sensor indicates stable impedance value. With the introduction of ammonia and the extension of time, the resistance value of the sensor is slowly reduced and finally kept at a constant value. We measured the response time of the sensor toward 25 ppm of NH3 at different RHs. As revealed in Figure S16, the response times for the sensor toward NH3 gas are nearly 8, 10, 12, and 13 min under 98%, 93%, 85%, and 75% RHs, respectively. That is to say, the higher the humidity, the shorter the response time. The impedance spectra showing similar shapes with spikes bent to Z′ axis at ∼70° indicate that the conducting species must be proton ions (see Figure 7).23 When the ammonia vapor in the testing chamber is blown out, within 10 min, the sensor’s resistance value will gradually increase and then revert to its initial value under fixed RH. This indicates a good reversibility of the ammonia sensor. To be noticed, the reversible change of resistance toward 25 ppm of NH3 vapor at 30 °C and 98% RH could be observed over more than 10 successive cycles (see Figure S17). This reproducibility is a continuously dynamic test result. Detailed test conditions are as follows: First, at 30 °C and 98% RH, the starting
Figure 6. Arrhenius plots under 68% and 98% RH.
behaviors following the Vehicle mechanism.42−44 As shown in Figure S13, as P/P0 = 0.90, the maximum absorption value of H2O molecules is ca. 77.2 mg/g. This explains why the optimal conductivity of the MOF is relatively large, being 1.36 × 10−3 S· cm−1. The proton-conductivity value can be compared to the
Table 3. Impedance Values [Ω] for 1 upon Different Concentration of NH3 Gas at Different RHs and 30°C 75% 85% 93% 98% a
1 ppma
3 ppma
1.69 × 105 3.40 × 104
1.98 × 105 1.47 × 105 2.81 × 104
b
RH RHb RHb RHb
5 ppma 7.58 1.85 1.32 2.37
× × × ×
10 ppma
105 105 105 104
7.44 1.37 1.10 1.79
× × × ×
105 105 105 104
15 ppma 6.90 1.14 8.61 1.52
× × × ×
105 105 104 104
20 ppma 6.78 9.34 7.31 1.31
× × × ×
105 104 104 104
25 ppma 6.48 8.67 5.89 1.20
× × × ×
105 104 104 104
The concentration of NH3. bRelative humidity. 7108
DOI: 10.1021/acs.inorgchem.8b00806 Inorg. Chem. 2018, 57, 7104−7112
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Figure 7. Impedance spectra of 1-based sensor at 98% (a), 93% (b), 85% (c), and 75% (d) RHs and 30 °C upon 0−25 ppm of NH3 gas.
resistance value of MOF 1 was recorded. Then, 25 ppm of NH3 vapor was introduced into the box. A continuously decreasing impedance value can be observed. As a constant impedance value was obtained, the NH3 gas was pumped from the chamber through the continuous flow of 98% RH water vapor. After then, the impedance value increases steadily to the starting impedance value. This is a complete cycle test. Notably, at room temperature, when the ammonia sensor is put aside in air for 0.5 h or put in a vacuum device for 5 min, the adsorbed ammonia can be removed from the sensor. As exhibited in Figure S18, the excellent framework stability was confirmed by the PXRD determinations before and after NH3 sensing experiments, which will be very beneficial to the future application of ammonia gas identification. The gas response of MOF 1 toward diverse NH 3 concentrations under high RHs and 30 °C was explored. As displayed in Figure 8, the gas response increases linearly as the NH3 concentration (from 1 to 25 ppm at 98% and 93% RHs, from 3 to 25 ppm at 85% RH and from 5 to 25 ppm at 75% RH). This is the typical performance of a resistor gas sensor.48
Also, the gas response increases with the increasing RH. Apparently, the high proton-conductive ability of MOF 1 under high RH benefits its sensing for ammonia gas. For example, for sensing the 25 ppm of ammonia gas, the gas response under 98% RH is 243%, which is ∼8.1 times, ∼1.7 times, and ∼1.3 times for those under 75% (30%), 85% (143%), 93% (193%) RHs. Compound 1 indicates the highest gas response and the lowest detection limit toward ammonia under 98% RH. The sensor keeps a distinct gas response of 21.14% toward 1 ppm of ammonia gas. At 93% RH, the gas response reduces to 2.32% toward 1 ppm of NH3 gas. Under 85% RH, the MOF discerns 3 ppm of NH3 having gas response of 6.41%. Nevertheless, under 75% RH, the gas response varies from 11.21% (upon 5 ppm of NH3 gas) to 30.09% (upon 25 ppm of NH3 gas). The sensing properties of previously reported ammonia sensors at mild temperature were enumerated in Table S3. Obviously, the MOF 1-based ammonia sensor exhibits excellent performance showing high response and low detection limit (1 ppm) at 30 °C and high RHs, which is one of the best ammonia sensors 7109
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response values toward other gases, such as N2, O2, H2, benzene, acetone, and CO, their response values have a certain degree of increase. To study its ammonia recognition mechanism for 1, we determined the impedance spectra toward 25 ppm of NH3 vapor under 30−100 °C and 75% or 98% RHs (shown in Figures S19 and S20. Ea values were deduced from the Arrhenius plots to be 0.73 eV at 75% RH and 0.14 eV at 98% RH (see Figure 10). The higher and lower Ea values indicate
Figure 8. Gas response of the sensor toward different concentration of NH3 at different RHs and 30 °C.
previously described at high humidity (60%15 and 80% RHs49) and mild temperature. Figure 9 exhibits the response values of the sensor being exposed to various gases [N2, O2, H2, benzene, acetone, CO,
Figure 10. Arrhenius plots of 1 at 75% and 98% RHs toward 25 ppm of NH3.
the proton-conduction mechanisms are different under different RHs. The mechanisms are Vehicle mechanism and Grotthus mechanism under 75% and 98% RHs.31−33 Ea value being 0.73 eV illustrates that the proton conduction originates the movement of H2O and NH3 units inside the frameworks with NH4+ or H+(H2O)n ions. At 98% RH, the absorption H2O and NH3 or coordination H2O, imidazole, and carboxylic groups construct abundant hydrogen-bond nets for proton hopping, bringing about the smaller Ea value. As we recently discovered,24−26 H2O and NH3 units and protonation of NH3· H2O vapors reveal a synergistic effect for proton transfer, which explains exactly why the sensor based on MOF 1 exhibits wonderful NH3 gas-recognition performance at high RH. To further explain conduction mechanism, NH3 gas absorption and desorption isotherms are determined at room temperature employing activated crystalline solids. As indicated in Figure S21, the NH3 vapor absorption value is ca. 151 mg· g−1 as P/P0 equals 0.05. As P/P0 attains 0.95, the largest NH3 gas absorption is ca. 220.2 mg·g−1, which shows that the MOF has strong ammonia adsorption capacity. This shows that, in the process of NH3 gas identification, ammonia units can join with the coordination H2O, imidazole, and carboxylic groups forming a hydrogen-bond network and can also react with the adsorbed H2O units to produce NH4+ ions. Both the rich Hbonding nets and NH4+ ions may greatly improve the protonconductive properties of the MOFs. Again, this explains why the MOF 1 reveals excellent ammonia recognition performance under high humidity. To illustrate the correctness of this conclusion, we did a comparative experiment. The results showed that the MOF did not have any proton conduction and ammonia recognition in the absence of water or under dry ammonia at 30 °C.
Figure 9. Columnar chart about responses for 1 toward dissimilar gases.
CO2, H2S, EtOH, MeOH, and NH3] under concentration of 25 ppm at 30 °C and 98% RH. There are insignificant responses lower than 19% toward most of the above gases. By contrast, a high gas response of 243% upon NH3 vapor can be discovered. The values of selectivity (S = response (NH3)/ response (gas)) of NH3 toward the rest of the gases are 98.8 for nitrogen (N2), 100.8 for oxygen (O2), 96 for hydrogen (H2), 96.8 for benzene, 83.5 for acetone, 96.8 for carbon monoxide (CO), 19.74 for carbon dioxide (CO2), 13.32 for hydrogen sulfide (H2S), 14.24 for EtOH, and 12.74 for methanol, which indicates that the sensor still shows the sensitive identification of ammonia in the presence of other disturbing gases. We point out that, although acidic gases CO2 and H2S can react with water molecules like NH3 gas to produce more protons at high RHs, unlike NH3 molecule, they cannot form strong H-bonds with H2O molecules and the organic ligands of the MOF. It is not conducive to proton transmission. Thus, they display low response values in this case. However, compared with the 7110
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Inorganic Chemistry Table 4. Proton Conductivities [S cm−1] for 1 upon Different Concentration of NH3 Gas at Different RHs and 30°C 1 ppma 75% 85% 93% 98% a
3 ppma
5 ppma
b
RH RHb RHb RHb
−6
2.75 × 10−6 1.37 × 10−5
2.35 × 10 3.16 × 10−6 1.66 × 10−5
5.52 2.51 3.52 1.96
× × × ×
10 ppma −7
10 10−6 10−6 10−5
6.13 3.39 4.47 2.58
× × × ×
−7
10 10−6 10−6 10−5
15 ppma 6.25 4.08 5.40 3.06
× × × ×
−7
10 10−6 10−6 10−5
20 ppma 6.86 4.98 6.36 3.55
× × × ×
−7
10 10−6 10−6 10−5
25 ppma 7.18 5.36 7.89 3.88
× × × ×
10−7 10−6 10−6 10−5
The concentration of NH3. bRelative humidity.
Accession Codes
Finally, compared to solely water-assisted conductivities of 1 (see Table 2), the calculated conductivities of MOF 1 at different concentrations of NH3 gases (1−25 ppm) and RHs can increase in varying degrees (see Table 4). Obviously, the higher the ammonia concentration, the greater the amplitude of the proton conductivity that can be observed. This case is consistent with our previous findings.24−26 In our recent study about proton conduction of Sr-based MOF [Sr(μ2-H2PhIDC)2(H2O)4]·2H2O (H3PhIDC = 2phenyl-4,5-imidazole dicarboxylic acid),26 we found that its proton conductivity increases steadily with the increase of concentration of aqua-ammonia vapors (from 0.01 to 3.0 M NH3·H2O solution) at temperature range of 30−100 °C. The detection limits of the Sr-MOF are 0.1 M NH3·H2O at 30 and 40 °C, 0.05 M NH3·H2O at 50 °C, and 0.01 M NH3·H2O at 60−100 °C. Note that the RH was not being controlled precisely. Herein, the RH is controlled precisely. We mainly studied the performance of ammonia recognition of MOF 1 under 30 °C and 75−98% RHs. As mentioned earlier, at 30 °C, the NH3 gas detection limits of MOF 1 are 1, 3, and 5 ppm under 75%, 85%, and 98% RHs, respectively. Obviously, MOF 1 shows better ammonia recognition performance than the SrMOF.
CCDC 1537928 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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*E-mail:
[email protected]. ORCID
Gang Li: 0000-0001-9049-4208 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (21571156 and J1210060).
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REFERENCES
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CONCLUSION In summary, it is an ammonia sensor based on the pressed pellet from a proton-conductive MOF, Ba(o-CbPhH2IDC)(H2O)4]n (1) that could be prepared by a cost-effective method, and we proposed its sensing mechanism on the basis of structural determination, Ea calculations, H2O, and NH3 vapor absorptions and PXRD experiments. Our sensor displays fortified ammonia gas recognition performance under a high relative humility atmosphere and mild temperature (30 °C): the lowest NH3 gas detection limit of 1 ppm and highest response of 243% toward 25 ppm of NH3 under 98% RH; high selective detection of NH3 toward 10 kinds of interfering gases; an outstanding reversible change in resistance toward 25 ppm of NH3 vapor at 30 °C and 98% over 10 cycles. Considering above advantages for MOF 1-based sensor, especially high sensitivity and selectivity under high RH and 30 °C, MOF 1 can be expected as a wonderful sensing platform to monitor trace-level NH3 gas changes under these conditions. Although our research is still at a basic level, the fabrication of the MOFs into devices provided a good chance for development of new proton-conductive devices.
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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/acs.inorgchem.8b00806. Details of crystal data, impedance analysis, PXRD patterns, gas adsorption/desorption (PDF) 7111
DOI: 10.1021/acs.inorgchem.8b00806 Inorg. Chem. 2018, 57, 7104−7112
Article
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