Low Power Mixed Gas Sensor

The invention is directed to a chemically robust, highly-selective, low power sensor that can be used for the direct electrical detection of mixed gases. In particular, metal-organic frameworks (MOFs) offer exceptional chemical and structural tunability as mixed-gas capture materials. As an example of the invention, the influence of interfering gases on trace NO2 detection in a simulated flue gas stream was investigated. The unique interaction of NO2 with the MOF's metal center leads to orders of magnitude decrease in MOF resistance. More broadly, the coadsorption of specific gases (e.g., H2O, SO2) can be beneficial to the electrical detection of the target gas (e.g., NO2), and careful electrical measurements can discern their presence independent of the target gas.

FIELD OF THE INVENTION

The present invention relates to gas sensing and, in particular, to a low power mixed gas sensor that can detect individual gases in mixed gas streams.

BACKGROUND OF THE INVENTION

Selective gas sensors with long lifetimes, low power consumption, and high chemical durability are needed for myriad of applications, ranging from environmental monitoring to removing toxic gases from industrial waste streams. See M. A. A. Mamun and M. R. Yuce,IEEE Sensors J.19, 7771 (2019); and A. Kaushik et al.,Chem. Rev.115, 4571 (2015). Toxic acid gases present in flue gas streams are of particular importance, where multiple toxic gases such as SOxand NOxare present. See F. Rezaei et al.,Energy Fuels29, 5467 (2015); and H. Wang et al.,Chem. Eng. J.378, 122155 (2019).

SUMMARY OF THE INVENTION

The invention is directed to a low power mixed gas sensor, comprising an electrically insulating substrate; a pair of interdigitated electrodes disposed on the substrate; a mixed-gas-capture film disposed on the pair of interdigitated electrodes and the substrate; and a frequency response analyzer for measuring the impedance response of the mixed-gas-capture film when a mixed gas steam is absorbed in the mixed-gas-capture film and an alternating voltage is applied to the pair of interdigitated electrodes. The mixed-gas-capture film can comprise a metal organic-framework (MOF) material, such as M-MOF-74, where M is cobalt, magnesium, zinc, or nickel, or RE-DOBDC, where RE is a rare-earth element and DOBDC is dihydroxyterephthalic acid. The frequency can correspond to a RC transition frequency that leverages the capacitive component of the MOF to increase the signal strength while still enabling a larger signal change associated with the DC resistance to be calculated. A high impedance interface can be connected in series with the frequency response analyzer.

The invention is further directed to a method for detecting individual gases in a mixed gas stream, comprising providing a low power mixed gas sensor, exposing the mixed gas sensor to a mixed gas stream; and measuring the impedance response of the mixed-gas-capture film.

As an example of the invention, the impedance response of Ni-MOF-74 was systematically evaluated at 50° C. under 16 unique combinations of N2, NO2, SO2, and CO2under both dry and humid (0.8% H2O) conditions. Through this analysis it is seen that NO2drives the large (up to 6060×) change in RMOF. This change in RMOFoccurs by two electrically distinct processes, one fast and one slower. CO2inhibits NO2adsorption, specifically the faster adsorption process, though this effect can be reversed by adding SO2. The adsorption of H2O by Ni-MOF-74 is rapid (<120 s), much faster than NO2. Moreover, co-adsorption of competing gases under humid conditions enhances the change in RMOF. Together, these results suggest that coadsorption of specific gases (e.g., H2O, SO2) can be beneficial to the electrical detection of the target gases (NO2), and that careful electrical measurements can even discern their presence independent of the target gas. Thus, to enhance the overall electrical response, it is not necessary for the MOF to be 100% selective for a given gas species; intentionally designing the MOF to accept complementary gases can be beneficial.

DETAILED DESCRIPTION OF THE INVENTION

Key attributes for long-lived, near-zero power chemical sensors include: (1) extremely high resistance (>109Ω) in the activated state, (2) strong active material-analyte binding to prevent escape of captured analytes, and (3) large per-unit-cell adsorption capacity coupled with (4) a favorable redox potential of the analyte with respect to the active capture material. These characteristics enable a low power sensor with irreversible analyte capture and a corresponding large change in sensor electrical response.

As shown inFIGS.1A and1B, the low power mixed gas sensor10comprises a mixed-gas-capture film11disposed on interdigitated electrodes (IDEs)12. The mixed-gas-capture material is preferably a metal-organic framework (MOF) or a microporous aluminosilicate (zeolite). The IDEs12comprise a pair of interlocking comb-shaped arrays of metallic electrodes deposited on the surface of an electrically insulating substrate13. The substrate is preferably more electrically insulating (i.e., has a higher resistivity) than the mixed-gas-capture material. For example, the substrate can be a high resistance silica glass.

A variety of analyzers can be used to detect changes in electrical properties of the mixed-gas-capture material when exposed to a mixed-gas stream. For example, impedance spectroscopy can be used to measure the electrical impedance of the coated IDEs over a range of frequencies. Therefore, the sensor10can further comprise a frequency response analyzer14for measuring the impedance response of the mixed-gas capture film when an AC voltage15is applied to the IDEs12. In materials where the dielectric loss is very small and the permittivity is large, a high impedance interface16can be connected in series with the frequency response analyzer14to provide a more accurate impedance measurement.

A well-known family of MOFs, MOF-74, has been extensively studied for the interaction of the metal center with different acid gases, such as NOx, SOx, CO2, and H2O, and the competitive binding of each investigated by both computational and experimental methods. See K. Tan et al.,Chem. Mater.29, 4227 (2017). M-MOF-74 are a series of isostructures with a variety of metals (M=Mg, Ni, Co, Zn) and the same organic ligand, 2,5-dihydroxyterephthalic acid (DHTP). Current literature has highlighted the uniqueness of electronic structure in M-MOF-74 as a function of metal choice and response to various adsorbed gases. See A. de Oliveira et al.,Chem. Phys. Lett.691, 283 (2018); K. Tan et al.,Chem. Mater.27, 2203 (2015); and K. Tan et al.,Chem. Mater.29, 4227 (2017). Each adsorbed gas has been calculated to modify M-MOF-74 electronic structures to a varying degree, indicating the possibility for sensing of unique chemical species. For application in electrical sensing, the change in electronic structure, due to gas adsorption, modifies the effective masses of electrons and holes, therefore changing the conductivity of the MOF material.

In particular, Ni-MOF-74 is a promising candidate for these applications, having demonstrated large changes in electrical properties upon adsorption of dry NO2. See L. J. Small et al.,Adv. Funct. Mater.30, 2006598 (2020). The performance under more industrially-relevant conditions, however, is not well known, where multiple competing gases (H2O, SO2, CO2, etc.) may be present and coadsorb on the MOF. See L. Ding and A. O. Yazaydin,J. Phys. Chem. C116, 22987 (2012). Detailed FT-IR studies by Tan et al. have shown complex competition between these gases in MOF-74, where kinetic limitations strongly influence exchange of adsorbed gases. See K. Tan et al.,Chem. Mater.27, 2203 (2015); and K. Tan et al.,Chem. Mater.29, 4227 (2017). It is worth noting that unlike gas detection via mass changes, using electrical properties to detect gas adsorption may result in conditions where gas is adsorbed, but not detected, if the adsorption of the gas does not influence the electronic structure of the MOF, or otherwise alter the electrical properties of the MOF film (e.g., surface conduction via adsorbed water along the MOF crystallite exterior). See L. Sun et al.,Chem. Sci.8, 4450 (2017).

Another MOF that can be used with the invention includes RE-DOBDCs (where RE is a rare earth element and DOBDC is dihydroxyterephthalic acid). These RE-DOBDC MOFs have recently been shown to have strong binding of N2O and H2O and have been used for the photoluminescence-based detection of acid gases, including NOx. See Patent Appl. No. US 2021/0231628, which is incorporated herein by reference.

A number of microporous aluminosilicates (zeolites) are also suitable as mixed-gas-capture materials.

Frequency Response

Direct electrical detection of gaseous analytes by MOFs can be performed through either a change in the capacitance or resistance of the MOF-containing sensor. Changes in capacitance are typically measured by an alternating voltage at relatively high frequency (e.g., 1 MHz), such as the SO2sensor Chernikova et al. built to successfully detect ppb levels of SO2. See V. Chernikova et al.,J. Mater. Chem. A6, 5550 (2018). Changes in capacitance for MOF-based sensors are typically small, however, as the change is based on the real permittivity of the MOF having adsorbed one gas (e.g., N2) versus another (e.g., SO2). On the other hand, changes in MOF resistance in response to a gaseous analyte are typically recorded as DC measurements, e.g., a chemiresistor. See I. Stassen et al.,ACS Cent. Sci.5, 1425 (2019); M. L. Aubrey et al.,J. Amer. Chem. Soc.141, 5005 (2019); and M.-S. Yao et al.,Angew. Chem. Int. Ed.58, 14915 (2019). While this approach offers potentially large changes in signal for the right MOF-analyte combinations, it can be technically challenging, as many MOFs possess resistivities approaching those of common insulators, such as alumina. See L. J. Small and T. M. Nenoff,ACS Appl. Mater. Interfaces9, 44649 (2017); L. J. Small et al.,ACS Appl. Mater. Interfaces11, 27982 (2019); A. A. Talin et al.,Science343, 66 (2014); and L. Sun et al.,J. Amer. Chem. Soc.137, 6164 (2015). Therefore, many groups have worked towards identifying lower resistivity MOFs. See I. Stassen et al.,Chem. Soc. Rev.46, 3185 (2017); and S. K. Bhardwaj et al.,J. Mater. Chem. A6, 14992 (2018). A downside to decreasing the MOF resistivity, however, is an increase in the sensor's power consumption. While resistive components dissipate power, purely capacitive (e.g., reactive, or imaginary impedance) components do not. See E. Barsoukov and J. R. Macdonald,Impedance Spectroscopy: Theory, Experiment, and Applications,2nd Ed., Wiley, Hoboken, N.J., USA (2005).

The impedance can be related to the capacitance and conductivity of the MOF material. When an alternating voltage is applied to the IDE, some energy is stored by the capacitance, and some is dissipated by the resistance effects. Therefore, the resulting current will exhibit a phase lag. The capacitance effect is known as the permittivity (or dielectric constant), and the resistive effect as dielectric loss. The sensor can be operated at an AC frequency corresponding to a RC transition frequency that leverages the capacitive component of the MOF material to increase the signal strength while still enabling the larger signal change associated with the DC resistance to be calculated. The high impedance interface enables a reference measurement to be obtained on precision internal reference capacitors which are automatically substituted for the sample; a second measurement is made, this time on the sample itself. The two results can be used to derive an accurate measurement of the permittivity of the MOF material—in effect, the first measurement is used to eliminate the effects of extraneous capacitance.

The present invention marries the high signal strength and low power consumption of a capacitive MOF sensor with the large signal change of a resistive MOF sensor. The invention uses a hybrid approach whereby impedance spectroscopy is first applied in the lab to understand the AC frequency response across a wide range (1 mHz-1 MHz), and then used to extrapolate the DC resistance of the mixed gas sensor. From this data, a single RC transition frequency (e.g., 100 mHz) is selected, leveraging the capacitive component of the MOF material to increase the signal strength while still receiving information about the MOF's DC resistance. With this hybrid approach, both the high resistivity and mixed gas selectivity of an exemplary Ni-MOF-74 was leveraged to create an active material for a near-zero power sensor which detects the presence of individual gases in a mixed gas stream through changes in the electrical properties of MOF-74.

As described in U.S. Pat. No. 11,573,217, a low power nitrogen oxide sensor has been developed that provides direct electrical detection of trace (0.5-5 ppm) NO2at relatively low temperatures (50° C.) via changes in the electrical properties of nitrogen-oxide-capture active materials. The high impedance of MOF-74 enables applications requiring a low power sensor, with 0.8 mg MOF-74 active material drawing <15 pW for a macroscale sensor 35 mm2area. As will be described below, this same sensor can be used to detect individual gases in mixed gas streams, for example comprising two or more of NOx, H2O, SOx, and/or COx.

MOF-Based Mixed Gas Sensor Fabrication

A series of sensors were fabricated by dropcasting Ni-MOF-74 powders on IDEs on glass substrates. In order to evaluate the influence of the MOF-74 metal center on mixed gas sensing ability, a series of IDEs were coated with Ni-MOF-74, activated at 200° C. under vacuum, and interrogated with impedance spectroscopy at varying mixed gas compositions.

Ni-MOF-74 was synthesized using a literature procedure, with minor alterations. See S. M. Vornholt et al.,Dalton Trans.46, 8298 (2017); S. E. Henkelis et al.,Cryst. Eng. Commun.21, 1857 (2019); and L. J. Small et al.,Adv. Funct. Mater.2006598, 1 (2020). To synthesize Ni-MOF-74, nickel acetate tetrahydrate (1.24 g, 5.00 mmol) was dissolved in water (14 mL) with stirring. 2,5-dihydroxyterephthalic acid (0.5 g, 2.50 mmol) was dissolved in sodium hydroxide (1 M, 10 mL) and added dropwise to the salt solution in 1 mL aliquots over 5 mins. The reaction solution was heated to reflux for 16 hr and then allowed to cool. The powder was collected by filtration, washed with methanol (2×100 mL) and water (2×100 mL) and allowed to dry overnight in air. For use in the dropcast powder sensor, the Ni-MOF-74 were ground into a fine powder using a mortar and pestle.

Platinum IDEs on glass substrates were obtained from DropSens (product G-IDEPT10). These IDEs contain 125 pairs of platinum lines 250 nm thick and 10 μm wide with a spacing of 10 μm between lines. The IDEs were cleaned under N2flow, and their impedance magnitude at 100 mHz was verified to be greater than 3×1010Ω. In a 10 mL glass vial, 25 mg of the MOF-74 powder and 1 mL acetone were mixed. The mixture was sealed and stirred vigorously for 30 minutes, after which 12.5 μL was pipetted onto the active area of the IDE. The IDE was allowed to dry at room temperature for 5 mins, followed by deposition of another 12.5 μL of the MOF suspension. This resulted in 0.8 mg of MOF-74 being deposited on the active area of the IDE (˜35 mm2). Film thickness of the dropcast powder coatings was determined to be on the order of 10 μm.

Mixed Gas Exposure and In-Situ Electrical Testing

Ni-MOF-74-coated IDEs were loaded into a custom-built gas exposure chamber that enabled MOF activation and subsequent in-situ electrical testing under varying gaseous environments without exposure to lab atmosphere. Impedance spectra were simultaneously recorded using either (1) a Solartron 1260 Frequency Response Analyzer connected in series with a Solartron 1296 Dielectric Interface or (2) a Solartron Modulab with both femtoammeter and reference measurement cards. Regardless of instrument, all measurements utilized the internal reference capacitors to increase measurement accuracy.

During the initial measurements when 1.5 h of pure N2was flowed, impedance spectra were recorded at 0 V DC and 100 mV (RMS) AC over 1 MHz to 10 mHz (10 points/decade) for IDEs in three separate test slots. Afterwards, continuous measurements were recorded at 100 mHz for IDEs in one or two of the test slots. Measurements at 100 mHz continued until the gas mixture of interest had been flowed for 5 h. Impedance spectra over 1 MHz-10 mHz were then recorded for all IDEs. The chamber was opened, all three IDEs were immediately removed, and they were either (1) examined in Fourier transform infrared (FT-IR) spectroscopy, (2) interrogated in powder X-ray diffraction (PXRD), or (3) sealed in a glass vial for subsequent analysis at a synchrotron. All electrical measurements and gas exposures occurred at 50° C.

A study was conducted to determine the relation between competing gas composition, gas coadsorption in Ni-MOF-74, and resulting changes in electrical properties for Ni-MOF-74 when detecting trace NO2. To understand how the competitive adsorption of N2, NO2, SO2, CO2, and H2O influences the ability of Ni-MOF-74-based gas sensors to electrically detect trace NO2, a large batch of Ni-MOF-74 powder was synthesized, dropcast onto interdigitated electrodes, activated at 200° C. under vacuum, and exposed to different gas combinations at 50° C. Mimicking an industrial acid-gas stream, N2carrier gas was used to introduce NO2or SO2at 1 ppm, CO2at 10% (100,000 ppm), or H2O at 0.8% (8,000 ppm); all gas combinations are outlined in Table 1. MOF phase stability was verified via PXRD, while gas adsorption was confirmed via FT-IR. The sensor electrical response before, during, and after gas exposure was correlated to specific gas adsorption sites identified by differential electron density analysis.

Impedance Response to Mixed Gas Exposure

The impedance response of the Ni-MOF-74 coated IDEs were recorded before and after gas exposure; example spectra are provided inFIGS.2A and2Bfor exposure to humid NO2+SO2+CO2. The bare IDE shows a highly capacitive response, with impedance magnitude (|Z|) displaying a straight line, and the phase angle residing near −90° except for very low frequencies <0.1 Hz. Addition of the activated Ni-MOF-74 retains most of this capacitive response, with only a slight decrease in |Z| and increase in phase angle, implying that the Ni-MOF-74 is less resistive than the bare IDE. Upon exposure to humid NO2+SO2+CO2, a transition to a resistive response is observed: |Z| plateaus to near 0.8 GΩ for frequencies <100 Hz, while the phase angle shifts to 0°. In order to extract the DC resistance, RMOF, of Ni-MOF-74, the data were fit with an equivalent circuit shown inFIG.3. This equivalent circuit is similar to that described in U.S. Pat. No. 11,573,217, except that it contains another constant phase element (CPE) to account for changes in the background capacitance of the measurement setup when humidity is introduced. See L. J. Small et al.,Adv. Funct. Mater.30, 2006598 (2020); L. J. Small and T. M. Nenoff,ACS Appl. Mater. Interfaces9, 44649 (2017); L. J. Small et al.,ACS Appl. Mater. Interfaces11, 27982 (2019). The resulting fits are overlaid for the example spectra inFIGS.2A and2B. Across all fits, the goodness of fit, as judged by χ2, was better than 0.0059. Uncertainty of fitting individual circuit elements was small (<5%), less than sample-to-sample variability.

To verify that these changes in impedance were due to the Ni-MOF-74, several control experiments were performed. First, an uncoated IDE was activated and tested under each of the dry gases; no single gas or combination gave any response from the uncoated IDE. Thus, it is concluded that any observed electrical responses under dry conditions are due to changes in the electrical properties of the Ni-MOF-74, and not the underlying IDE. Under humid conditions, however, 1.56×, 2.60×, and 2.39×changes were observed in response to “humid N2,” “humid NO2,” and “humid NO2+SO2+CO2,” respectively. This result is not surprising; as water vapor condenses on the IDE's glass surface and gas species dissolve into it, the surface resistance across the glass IDE is expected to decrease. It is not possible to fit these differences to Rgiass, which is fixed from measurements of the dry IDE before MOF deposition, to spectra which also have Ni-MOF-74. From this control experiment, it is concluded that small changes in RMOFunder humid conditions may be due to changes in the underlying IDE. The large, three orders of magnitude change in impedance inFIG.2A, however, cannot be solely attributed to changes in the IDE; it must be due to the Ni-MOF-74.

To verify that the Ni-MOF-74 structure was maintained after gas exposure, PXRD was performed on all samples and select data are plotted inFIG.4. Here the as-synthesized Ni-MOF-74 powder (plot A) displays all major peaks expected from the reference pattern. Upon dropcasting Ni-MOF-74 onto the IDE (plot C), a broad peak at 22° associated with the glass substrate is seen, in addition to a sharp Pt(111) peak near 40° attributed to the Pt lines on the IDE (plot B). The Ni-MOF-74 peak intensity is suppressed due to low loading (1.21 mg), but major peaks near 6.9° and 12° are still readily identified. After exposure to humid NO2+SO2+CO2, no appreciable change in PXRD is observed (plot D). Across all gas combinations, no significant changes in PXRD pattern were observed, implying structural stability of Ni-MOF-74 across all gas combinations and consistent with other literature reports. See L. J. Small et al.,Adv. Funct. Mater.30, 2006598 (2020).

FT-IR corroborated adsorption of each of the tested gases (except N2) by Ni-MOF-74. From this complementary set of characterizations, it is inferred that the adsorption of the gas molecules by the stable Ni-MOF-74 structure altered the electrical properties of the MOF, creating the observed changes in electrical response.

Competing Gases Alter the Ni-MOF-74 Electrical Response

To compare the influence of the competitive gas environment on the electrical response of Ni-MOF-74, impedance spectra from all Ni-MOF-74 sensors (N=59) under all 16 gas conditions were fitted before and after gas exposure, and RMOFand CMOFwere extracted for each. The ratio of RMOFbefore gas exposure to that after gas exposure is plotted inFIG.5for each gas combination.

FromFIG.5, it is readily seen that the presence of NO2is necessary to achieve multiple orders of magnitude decrease in RMOF, in excess of 6,000× in some cases. Gas combinations which do not contain NO2display significantly smaller decreases, all less than 4×. No statistically significant change was observed for dry N2or dry CO2. A small change (1.07±0.03) was seen from dry SO2, but this response was quenched for SO2+CO2(0.94±0.08). Upon adding 0.8% humidity, the baseline N2response increased to 1.68±0.19, though much of this change can be attributed to the IDE alone (1.56) and may not represent any appreciable contribution from the MOF film. The humid SO2, CO2, and SO2+CO2responses (3.58, 3.20, 3.94) are slightly higher than those observed for a blank IDE exposed to humid NO2(2.60) or humid NO2+SO2+CO2(2.39). It is hypothesized that the slightly increased response is due to the Ni-MOF-74 powder providing more exterior surface area for electrically conductive gas-laden H2O to be adsorbed and decrease the resistance across the bulk Ni-MOF-74 powder, as compared to the bare IDE.

Once NO2is added to the gas mixture, a large change in RMOFis observed. Under dry conditions this change is 238±63, increasing to 1020±220 under 0.8% humidity. Adding SO2to dry NO2has no appreciable effect on RMOF, while adding CO2hinders the ability of NO2to change RMOF. Remarkably, when added together, SO2and CO2enhance the dry NO2response, and under humid conditions enhance the response individually or combined, leading to the largest observed changes in RMOF. Moreover, all competing gases tested, except dry CO2, enhance the NO2response of the Ni-MOF-74 compared to dry N2.

Evaluating the capacitance of the MOF thin film, CMOF, provides even more insight, and the ratio of CMOFbefore gas exposure to that after gas exposure is plotted inFIG.6. These changes in capacitance provide a convenient method by which to classify the gas combinations as either (1) nonresponsive gases (N2, SO2, CO2) or dry NO2(Cbefore/Cafter=1.0×), (2) humid, no NO2(Cbefore/Cafter=0.5-0.6×), or (3) NO2+other gases (Cbefore/Cafter=0.7-0.9×), where humidity (H2O) is included as “another gas.” These results suggest that there is a significant interaction between the different adsorbed gas molecules in the MOF, increasing the capacitance after adsorption (Cbefore/Cafter<1.0×). The nonresponsive gases (SO2, CO2), creating no change themselves, alter the capacitive response when added with NO2. NO2appears to moderate the changes in capacitance created by H2O. Interestingly, the change created by dry NO2+SO2appears similar to that of humid NO2+SO2, while dry NO2+CO2has a lesser effect.

Together, the resistance and capacitance values can be used to differentiate the effects of NO2and H2O. If there is a large change in RMOF(Rafter/Rbefore>4×), then NO2must be present. If there is no additional change in CMOF, then H2O is absent. However, if the change in RMOFis accompanied by an moderate change in CMOF(0.7<Cbefore/Cafter<0.9), then H2O is also present.

Real-Time Sensor Response Reveals Gas Competition

The changes in electrical properties of Ni-MOF-74 discussed above are after 5 h of gas exposure, sufficient time for the Ni-MOF-74 to equilibrate with the gas environment. To better understand how competitive gas adsorption influences the transient electrical response, impedance measurements at 100 mHz were continuously recorded during these gas exposures and the resulting data is plotted inFIG.7A, comparing the NO2-containing gas combinations to the N2control. Here the data has been normalized by the initial impedance magnitude (|Z|0) to enable more meaningful comparison between samples. For all dry gas combinations, a constant impedance is initially recorded under dry N2flow. Once the NO2-containing dry gas combination is introduced, an exponential decay in impedance magnitude is observed, consistent with previous results for a zeolite-based NO2sensor. See S. J. Percival et al.,Ind. Eng. Chem. Res.60, 14371 (2021). In this previous work, the exponential decay was related to the consumption of adsorption sites via chemical reaction kinetics of the first order. A single exponential decay from an initial (|Z|0) to final (|Z|f) can be approximated as follows:

Here |Z(t)| is the impedance magnitude as a function of time (t), and τ is the time constant of the decay. The data inFIG.7Awas normalized by |Z|0, and can be linearized as follows, enabling extraction of τ.

A plot of ln

versus time is shown inFIG.7B. For dry NO2, dry NO2+SO2, and dry NO2+SO2+CO2, two distinct slopes are seen: a steeper slope (smaller τ) during the first hour, followed by a shallower slope (larger τ) over hours 1-4. The final slope (t>4 h) is an artifact of the analysis; an ideal exponential decay will approach, but never obtain its final value, whereas here |Z|fwas approximated as the value after 5 h. From this data, it is inferred that Ni-MOF-74 exposed to dry NO2, dry NO2+SO2, or dry NO2+SO2+CO2displays two distinct processes that contribute to the decrease in impedance: a fast initial process, and a slower second process. Similar results have been seen for the impedance change of Ni-SSZ-13 zeolite exposed to trace NO2, where the different processes were attributed to NO2adsorbed on the crystallite exterior surfaces (faster) and interior surfaces (slower). See S. J. Percival et al.,Ind. Eng. Chem. Res.60, 14371 (2021).

In contrast, Ni-MOF-74 exposed to dry NO2+CO2exhibits a constant slope over the first 4 h of exposure, indicating a single time constant and one process. Moreover, this single process is similar in slope (time constant) to the slower process of the other NO2-containing gas combinations. This suggests that CO2interferes with the fast NO2adsorption process. This effect is not simply due to differences in concentration of NO2(1 ppm) and CO2(100,000 ppm), as the addition of SO2(1 ppm) reenables the fast NO2adsorption process. Under dry conditions, the displacement of CO2by SO2is likely more favorable than CO2by NO2. This hypothesis is supported by the fact that NO2+CO2displayed a smaller change in RMOFcompared to NO2; even after 5 h all of the electrically active adsorption sites occupied by CO2could not be displaced by NO2.

Addition of humidity (H2O) to the gas combination creates a significant difference in the transient impedance response and can be related to the gases present and competing for the Ni-MOF-74 adsorption sites. This difference is most pronounced during the first hour and is plotted inFIG.7C. During the initial exposure to dry N2all sensors display a constant |Z|. Upon exposure to the humid gas streams, a sharp decrease in |Z| is immediately observed (<120 s), followed by a gradual increase in |Z|. This relatively fast diffusion and adsorption of H2O into MOF-74 films was observed via FT-IR by Tan et al. See K. Tan et al.,Chem. Mater.27, 2203 (2015). For humid N2, this increase stabilizes to within 2.6% of the final value within the first 0.25 h. Clearly, H2O is rapidly adsorbed by Ni-MOF-74. For all NO2-containing gas combinations, this increase is short-lived and |Z| gradually decreases again. Gas combinations containing CO2required a longer time to finally decrease in |Z|, consistent with the dry case, where CO2interfered with NO2adsorption. Likewise, humid NO2+SO2+CO2was faster to recover than humid NO2+CO2was, consistent with the observation in the dry case the SO2moderates CO2's deleterious effects on NO2adsorption.

At times >1 h, a plot of ln

versus time yields a constant slope for humid NO2, suggesting a single adsorption process analogous to the dry cases. This process is likely the displacement of the quickly adsorbed H2O by much slower NO2. Addition of other gases, however, creates curves which are not simply described by a linear fit. This suggests a more complex interplay between the competing gases under humid conditions as an equilibrium adsorption configuration is gradually achieved across all MOF adsorption sites.

In sum, changes in the transient impedance response may be attributed to the adsorption and exchange of the competing gases. The adsorption of NO2is the driver for the large decrease in impedance observed. Co-adsorption of CO2hinders the kinetics of this process, though it can be moderated by the addition of SO2. The adsorption of H2O is much faster than that of NO2, and the adsorption of NO2in the H2O saturated Ni-MOF-74 is further hindered by CO2.

It is important to understand how the various gases might interact with Ni-MOF-74 during H2O detection. H2O, N2O, and CO2are known to be adsorbed by M-MOF-74. These gases and SO2, also adsorbed by M-MOF-74, are often present in environments containing industrial flue gases.

The source of this selective electrical response is believed to be related to the relative electronic structures of the Ni-MOF-74 and the competing gas species. While many different gas molecules will readily adsorb to the unsaturated metal sites in Ni-MOF-74, a large change in RMOFis only expected if there is a significant amount of MOF-adsorbate electron transfer, creating new unoccupied electron states that facilitate charge transport in Ni-MOF-74. Thus, adsorption of triply bound N2, with its tightly held electrons, is not expected to influence RMOF.

On the other hand, NO2is a radical molecule that can serve as both an electron acceptor and donor. The NO2highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) sits below the Ni-MOF-74's HOMO, which contains significant contributions from Ni electrons. See K. Tan et al.,Chem. Mater.29, 4227 (2017); L. Sun et al.,Chem Sci.8, 4450 (2017); and J. A. Rodriquez et al.,J. Mol. Catal. A: Chem.167, 47 (2001). Electrons from the Ni-MOF-74 HOMO may be transferred to the NO2LUMO, creating newly reorganized unoccupied states in Ni-MOF-74, facilitating electronic transport, and decreasing RMOF.

Compared to NO2, SO2displays a LUMO at energy levels much closer to vacuum level. See K. Tan et al.,Chem. Mater.29, 4227 (2017); and J. A. Rodriquez et al.,J. Mol. Catal. A: Chem.167, 47 (2001). This significantly impedes electron transfer; reports have suggested that SO2adsorption in M-MOF-74 is more akin to physisorption than chemisorption. See K. Tan et al.,Chem. Mater.29, 4227 (2017). Therefore, no significant change in RMOFis expected upon adsorption of SO2.

Likewise, significant charge transfer is not expected for CO2due to relative band locations. Moreover, studies on the competitive adsorption of CO2and H2O suggest that CO2is preferentially exchanged for H2O, making significant adsorption of CO2from air unlikely.

Interactions with H2O are more complex. The LUMO of water sits above Ni-MOF-74's HOMO; significant electron transfer is not expected, consistent with reports of molecularly adsorbed H2O, and no dissociation. Nevertheless, high binding energy of H2O, only slightly less than that of NO2, indicates a strong interaction with the metal center. See K. Tan et al.,Chem. Mater.27, 2203 (2015). Calculations have predicted that the decreased electron density on a Zn-MOF-74 metal center leads to an decrease in effective mass for Zn-MOF-74 electrons. See P. Canepa et al.,J. Mater. Chem. A3, 986 (2015). As effective mass is theoretically proportional to resistivity, a similar decrease in RMOFis expected and observed.

The present invention has been described as low power mixed gas sensor. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.