Alkylamine Integrated Metal-Organic Framework-Based Waveguide

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Surfaces, Interfaces, and Applications

Alkylamine Integrated Metal-Organic Framework-Based Waveguide Sensors for Efficient Detection of Carbon Dioxide from Humid Gas Streams Ki-Joong Kim, Jeffrey T. Culp, Paul R Ohodnicki, Patricia Cvetic, Sean Sanguinito, Angela Lea Goodman, and Hyuk Taek Kwon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12052 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Applied Materials & Interfaces

Alkylamine Integrated Metal-Organic Framework-Based Waveguide Sensors for Efficient Detection of Carbon Dioxide from Humid Gas Streams Ki-Joong Kim,†,‡,* Jeffrey T. Culp,†,‡ Paul R. Ohodnicki,† Patricia C. Cvetic,†,‡ Sean Sanguinito,†,‡ Angela L. Goodman,† and Hyuk Taek Kwon§ †National ‡Leidos

Energy Technology Laboratory, 626 Cochrans Mill Rd., Pittsburgh, Pennsylvania 15236, United States

Research Support Team, 626 Cochrans Mill Rd., Pittsburgh, Pennsylvania 15236, United States

Department of Chemical Engineering, Pukyong National University, 45 Yongso-ro, Nam-gu, Busan 48513, South Korea §

KEYWORDS. carbon dioxide sensing, metal organic framework thin film, water mitigation, post-synthetic modification, optical fiber sensor.

ABSTRACT: Metal-organic framework (MOF) based chemical sensors have recently been demonstrated to be highly selective, sensitive, and reversible for CO2 sensing across a range of platforms including optical fiber and surface acoustic wave-based sensors. However, interference of water molecules is a primary issue in CO2 sensing systems based upon MOF layers due to cross-sensitivity, stability of MOF-based materials in humid conditions, and associated baseline drift over the lifetime of sensors. Herein, we develop a simple approach of alleviating the negative effect of water vapor to the optical fiber sensor by using alkylamine (i.e. oleylamine) to form a protective hydrophobic layer on the surface of MOFs for improving water stability. This strategy leads to an enhanced CO2 sensitivity and retention of the intrinsic CO2 sorption capacities even under humid conditions.

excellent sorption kinetics, reversibility, and guestinduced changes in the MOF structure and/or properties.5-11 Therefore, many efforts have been devoted in developing MOF-based sensors in the last decade.

INTRODUCTION Development of carbon dioxide (CO2) gas sensors became a subject of great importance due to their potential implementation in areas such as environment monitoring, indoor air quality supervision, and industrial production. The sensor devices developed for such applications include the use of nondispersive infrared absorption sensors and chemi-resistive sensors based on semiconducting metal oxides.1-4 These sensing platforms, however, present several drawbacks including high energy consumption and expensive optical systems in the case of infrared sensors, and cross-sensitivity and high working temperatures in semiconducting sensors. As such, there is a need for new types of cost-effective sensing and measurement systems, allowing highly sensitive and selective CO2 detection with stable response even at ambient temperature and pressure.

Very recently, optical fibers integrated with MOF materials such as ZIF-812-14 and HKUST-115-19 and UiO6620,21 have demonstrated a rapid increase in the detection of chemicals such as CO2, methane, volatile organic compounds, nitrobenzene, and 4-aminopyridine. The sensing mechanism for MOF-integrated waveguidebased optical fiber sensor was demonstrated by our prior work.12 Namely, refractive index increases within the MOF layer induced by physisorption of gas lead to a more light propagates from the optical fiber into the MOF layer. Through the application as physi-sorbents, these MOF-based sensors also benefit from the inherent adsorption reversibility and selectivity for CO2 over other atmospheric gases.

Nanoporous materials such as metal-organic frameworks (MOFs) have attracted great attention for potential application as gas sensing materials due to the

Water vapor is a typical interference of MOF-based CO2 sensors in the air due to the highly competitive

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nature of the adsorption of these two components.22-24 In some cases, the presence of high humidity levels may even promote the decomposition of MOF based sensing layers.25-27 For this reason, it is typically desired that an incoming gas stream is completely dehydrated to minimize adverse impacts of humidity on sensing layer performance and stability. However, the requirement for dehydration severely limits the ability to deploy low-cost sensors in optimal locations with minimal size and optimized performance characteristics.

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RESULTS AND DISCUSSION We first investigated the crystal morphology, structure, and porosity of pristine bulk Co/ZIF8 powder obtained from the growth solution (See experimental details in SI) and OLA modified Co/ZIF8 (hereafter denoted as OLA-Co/ZIF8) by scanning electron microscopy (SEM), X-ray diffraction (XRD) and N2 adsorption studies. XRD results confirm that the framework of Co/ZIF8 remain unchanged after OLA modification (Figure 1a). SEM images show that crystal morphology of OLA-Co/ZIF8 is identical to that of pristine Co/ZIF8 with rhombic dodecahedron morphology with an average crystallite size of ~ 70 nm.

Post-synthetic modification (PSM) has been widely applied to the surface of MOFs with different functionalities to improve water stability, thereby preserving crystallinity and ensuring that porosity required for desired performance remains intact.28-30 For example, covalently modifying aldehyde groups with alkyl or fluoroalkyl groups in MOFs through imine condensation reactions have resulted in significantly enhanced water stability.31-34 Most reported PSM processes, however, require more stringent environmental conditions such as elevated temperatures and/or catalysts for the PSM reaction to proceed. These are not ideal conditions for large scale commercial production of MOF thin film coated sensing devices such as optical fiber sensors. Thus, it would be very beneficial to develop a simple approach for mitigating water vapor effects on MOF sensing layers to allow for more effective sensing of CO2 gases under realistic condition. We herein introduce an uncomplicated PSM strategy, which prominently enhanced the stability and CO2 sensitivity of MOF coated optical fibers in the presence of water vapor by modifying the surface of the MOF layer with a hydrophobic alkylamine with a long hydrocarbon chain such as oleylamine (OLA). OLA is a long-chain primary alkylamine which exhibits a strong affinity for metal complexation through the NH2 functional group.35 Binding of OLA to the surface metal sites of MOF particles results in a dense aliphatic layer on the surface which significantly increases the hydrophobic character of the MOFs. In addition, the cis-C=C bond in the middle of the OLA molecule results in a kinetic diameter of 0.68 nm which is much larger than the 0.34 nm pore aperture for ZIF8. This restricts migration of the OLA into the interior of the ZIF8 particles and ensures that modification with OLA can only occur at the surface while preserving the critical underlying framework pore structure. In addition, the optical sensor performance was investigated in the visible wavelength range with cobalt-doped ZIF8 (Co/ZIF8) by replacing d10 Zn2+ with d7 Co2+ in the ZIF8. The use of Co2+ provides an additional strong absorption band in the visible range due to the dd band transition of the cobalt ion without degrading the porosity of ZIF8.36 The introduced visible range absorption significantly enhances the optical sensing response associated with CO2 absorption in the MOF layer.12

Figure 1. (a) XRD patterns (Inset shows SEM images for each sample), (b) XPS spectra of N 1s, and (c) Raman spectra of Co/ZIF8 and OLA-Co/ZIF8. Nitrogen isotherms of OLA-Co/ZIF8 show a decrease in BET surface area from 1557 to 1188 m2 g-1 (23.7%) and in total pore volume from 0.70 to 0.56 cm3 g-1 (20.0%), respectively (Figure S1). The decrease in surface area could be attributed to a blockage of some surface pores due to inclusion of the OLA molecule. This inclusion can occur in the crystallize surface pores even though the OLA cross-section is larger than the pore aperture through a known process involving a reversible dissociation of the 2-methyl-imidazole (2-mIm) ligands located along the pore aperture.37 This phenomenon is confined to the outer layers of the particles and would only partially restrict access of nitrogen to the bulk of the material. This explanation is further confirmed by the pore size distribution results which imply that bulk pore properties of OLA-Co/ZIF8 are not significantly affected


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metal-bound amine does not have a significant effect on CO2 adsorption in OLA-Co/ZIF8 under dry condition.

by the surface functionalization. Analysis of the OLACo/ZIF8 material using thermal gravimetric analysis indicated the onset of decomposition through the loss of OLA occurs at 300 °C (Figure S2). An OLA loading of approximately 1.5 wt% was determined from the mass of residual ZnO/Co3O4, thus confirming the confinement of OLA to the surface region of the particles. X-ray photoelectron spectroscopy (XPS) and Raman were used to probe the local structural bonding environment on the surface of OLA-Co/ZIF8 (Figure 1b). The peak shift of about 0.2 eV to higher binding energy (397.2 eV to 397.4 eV) in a pyridinic environment is observed when OLA is bound to metal, which is associated to the increased N-coordination to the metal.38 The Raman modes (Figure 1c) observed at 685, 1146, and 1460 cm-1 are attributed to methyl group and imidazole ring vibrations39 and the nitrogen-metal-nitrogen (N-ZnN or N-Co-N) vibration at 176 cm-1.40 Upon OLA modification, these bands are all in the same position and intensity, indicating structural equality. However, the Raman modes of observed at 305 and 422 cm-1 assigned to Zn-N (or Co-N) bending and stretching blue shifts by 4 cm-1 after OLA modification, implying that the N atom of the OLA is weakly connected to the metal ions.41 These results suggest further evidence of the amine to the metal ions on the surface of Co/ZIF8, presumely via van der Waals interactions.

Figure 2. FT-IR spectra of Co/ZIF8 and OLA-Co/ZIF8 at 25 °C under vacuum and loaded with dry 32 psi CO2. Arrows indicate the positions of the new peaks after OLA modification. Inset shows the zoomed spectral region for the C-H aliphatic chain of the OLA-Co/ZIF8. Uniform and tight MOF sensing layer with a thickness of ~200 nm on the optical fiber sensor were fabricated through a simple solution method at room temperature12 (Figure 3a) and then modified with OLA (for details see the Experimental section). CO2 sensing tests were performed at atmosphere pressure and room temperature in dry and wet conditions (80% relative humidity). The gas chamber was first purged with dry N2 gas and then dry or wet N2 was used as the reference and/or carrier gas, followed by recording reference spectrum (100 %T). After this, the CO2 gases of different concentrations were fed into the chamber and the transmitted spectrum was recorded.

The functionalization reaction between OLA and Co/ZIF8 was confirmed by in-situ FT-IR (Figure 2). The FT-IR spectra were recorded on samples evacuated overnight at 25 °C. Structural integrity after modification with OLA is evidenced by the similar FT-IR spectra for both the original Co/ZIF8 and OLA-Co/ZIF8 in the region below 1600 cm-1 which is associated with vibrations of the 2-mIm ligands. The ring stretching are still characteristic of the 2-mIm, indicating that the underlying framework has not been compromised after OLA modification. The inclusion of OLA in the modified material is verified by several new peaks appearing in the spectrum. These peaks include those at 1320 and 1200 cm-1 which correspond to the C-N stretch35,42 and C=CH bending of free OLA40, respectively. The presence of OLA is also supported by the appearance of peaks at 2923 and 2852 cm-1 assigned to the C-H stretching modes from the OLA aliphatic chain.43 No peak shift of C-H stretching was observed in the OLA-Co/ZIF8 (Figure 4b), indicating that OLA does not strongly intereact with the sbu sites in the MOF.44,45 The samples were exposed in-situ to gas phase CO2 in order to determine if the incorporated OLA showed any reactivity with the CO2 gas. The FT-IR results showed only spectral features consistent with physisorption of CO2 at around 2330 and 660 cm-1 with no evidence of a chemical reaction between the surface amine groups of OLA-Co/ZIF8 and CO2 molecules such as via the formation of carbamates.46 These results indicate that the

Gas sensing performance of MOF coated optical fiber sensor was evaluated at room temperature in a sealed quartz chamber of tube. We first examined the MOF coated optical fiber sensors in dry conditions. The chamber was purged with N2 gas for a few hours and then the optical spectrum as a reference (100 %T) was collected with a tungsten halogen lamp. After this the CO2 gas was inserted in the chamber and the optical spectrum was recorded (Figure S3). These two spectra were used to obtain the transmitted spectrum (%T) of the gas for a given concentration of the gas. Absorption of transmitted light intensity in a wavelength range of 540 ~ 600 nm is predominant due to a strong absorption band caused by d-d band transition of the cobalt ions within the Co/ZIF8 as demonstrated by the similarity with the absorption peak wavelength observed for the Co/ZIF8 nanoparticles in solution. Overall, the transmittances of

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compositions. Supprisingly, the OLA-Co/ZIF8 exhibited an enhanced sensitivity for wet CO2 as compared the dry CO2 conditions, albeit with a slightly longer response time. The increased response time for OLA-Co/ZIF8 sensing layer is likely due to the hindered diffusion of CO2 through the dense OLA surface layer. Figure 3e presents an optical response of the OLA-Co/ZIF8 on optical fiber sensor showing the linear relationship between %T and the CO2 concentration even under high humidity.

both Co/ZIF8 and OLA-Co/ZIF8 sensors decrease as the CO2 concentration increases (Figure 3b) in dry conditions. This observation indicates a guest moleculeinduced optical response due to a modification of the refractive index in the MOF thin film, as demonstrated by our prior work.12

Since the sensing response of the MOF-coated optical fiber depends on the amount of gases adsorbed, we investigated the dynamic CO2 breakthrough curves of Co/ZIF8 and OLA-Co/ZIF8 samples at 25 °C to see the adsorption properties of the samples in the dry and wet conditions (Figure 4a). The column packed with sample was first pretreated at 120 °C for 1 hour under He flow, and 10%-CO2/He gas was fed at 25 °C. The OLA coating has a clear influence on the CO2 uptake as well as the CO2 and H2O diffusion properties of the OLA-Co/ZIF8 particles. The breakthrough time of CO2 for OLACo/ZIF8 increased in wet condition (Figure 4a) and the sorption kinetics for H2O is slower than Co/ZIF-8 (Figure S4). Integrating the breakthrough curves for OLACo/ZIF8 gave CO2 uptakes of 0.195 mmol g-1, which is 40% more adsorb CO2 than Co/ZIF8 in wet condition (0.139 mmol g-1) (Figure 4b) even though water vapor adsorption between the samples was similar (Figure S5), indicating that the reaction between amine group and CO2 is highly specific, such that H2O cannot interfere with the reaction.47 A similar phenomenon was observed for various amine-functionalized MOFs, showing an enhanced the CO2 capacity and CO2/N2 selectivity in comparison with those under dry conditions.46,48,49 It has been shown that the presence of water vapor in the stream resulted in the formation of bicarbonate, and thus promoted more CO2 to be adsorbed on amine-modified MOFs.

Figure 3. (a) Cross-sectional FE-SEM image of ~200 nm Co/ZIF8 coated optical fiber modified with OLA. (b) Transmission spectra of Co/ZIF8 and OLA-Co/ZIF8 coated optical fibers after exposure to CO2 in dry and wet conditions. Absorption spectrum of Co/ZIF8 nanoparticles was obtained from dilution of powders in methanol solution. Dynamic sensing response at 590 nm of (c) Co/ZIF8 and (d) OLA-Co/ZIF8 to different CO2 concentration in dry and wet conditions. (e) %T of OLACo/ZIF8 as a function of CO2 concentration in dry and wet conditions.

The increased CO2 adsorption in wet conditions was also observed in the gravimetric adsorption measurement results in Figure S6, which show a higher mass gain for OLA-Co/ZIF8 in wet CO2 compared to pristine Co/ZIF8 even though both materials showed similar amounts of H2O adsorption in humidified N2 and equal amounts of CO2 uptake under dry conditions. These findings are directly related to the CO2 sensing performance of the MOF sensor in wet conditions (Figure 3). The Co/ZIF8 coated sensor fails since the sorbent has a higher affinity for H2O than CO2. The OLA-Co/ZIF8 coated sensor, in contrast, shows the expected sensing response due to the increased CO2 affinity in wet condition, which leads to increased in total gas adsorbed when CO2 is introduced into the gas stream. The decreased %T of OLA-Co/ZIF8 with CO2 is due to adsorb more CO2 in wet condition. For example, when N2 replaces CO2 in wet condition of Co/ZIF8, the increased

However, the sensing performance of the Co/ZIF8 sensor with exposure to 80% relative humidity (RH) (Figure 3c) shows a deteriorated signal upon exposure to CO2 along with a significant baseline drift over the time. In addition, the transmitted spectrum never recovers to 100 %T when the CO2 is replaced by N2. These results indicate that the water vapor is significantly impacting the integrity of the MOF sensing layer on optical fiber during the CO2 sensing test. In contrast, the OLACo/ZIF8 sensor under the same humidity conditions showed reversible sensing responses to a wide range of CO2 concentrations without showing significant variations in %T (Figure 3d) upon returning to baseline


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Moreover, the highly hydrophobic behavior of OLACo/ZIF8 remains unchanged over a period of time as required for realistic sensing applications in humid environments. For example, we have demonstrated that the as-prepared bulk OLA-Co/ZIF8 particles are able to float on the surface of water (Figure S8a) and is retained the underlying framework pore structure (Figure S8b) for 1 year or even longer. Lee et al. reported the decoration of long hydrocarbon such as dodecylamine on the surface of MOF membrane.54 The resuling membranes showed an improved water resistance with intact morphology and porosity. Water uptake of the Co/ZIF8 and OLA-Co/ZIF8 was also observed by FT-IR after long air exposures more than 1 year (Figure S9). Subtle absorption band associated with OH stretching bands at 3300 cm-1 can be clearly seen on OLA-Co/ZIF8, indicating that OLA-Co/ZIF8 mitigate the water molecules compared to Co/ZIF8.

%T is observed due to the desorption of preadsorbed H2O by inserting CO2, therefore, the total adsorbed amount of gases is decreased. While the decreased %T of OLACo/ZIF8 with CO2 is due to more adsorbed CO2 in wet conditions. Moreover, a slower response times of OLACo/ZIF8 sensor could be explained by slower sorption kinetics of H2O molecules through the OLA layer. Little to no difference in CO2 adsorption capacity under dry and wet conditions is observed in the fresh Co/ZIF8 (Figure 4b), indicating that initial exposure to H2O does not have a significant effect on CO2 adsorption. This is associated to the existence of -CH3 group in the Co/ZIF8 limiting the penetration of H2O molecules into the pores, therefore H2O resided predominantly at the MOF surfaces.50,51 However, the CO2 adsorption ability of Co/ZIF8 decayed considerably after extended contact with wet CO2 (Figure 4c), revealing the destruction of the initial framework. This can be directly correlated with the significant baseline drift over the time of the Co/ZIF8 sensors in wet condition. Further characterization by CO2 sorption for the OLA-Co/ZIF8 has been evaluated after testing in wet conditions (Figure 4c). CO2 uptake capacities of OLA-Co/ZIF8 show almost constant values from 0.706 mmol g-1 to 0.705 (3 days) and 0.650 mmol g-1 (5 days), respectively. However, Co/ZIF8 loses its CO2 capacity after extended exposure to wet CO2 condition. To evaluate the reversibility of the OLA-Co/ZIF8 MOF, a temperature programmed desorption experiment was conducted (Figure S7). Nearly all of the adsorbed CO2 and H2O are physi-sorbed and released at room temperature under dry He flow. Trace amounts of CO2 (1.0 %) and H2O (0.15 %) are desorbed at around 70 °C, indicative of a very small amount of chemisorbed CO2 and H2O, possibly due to interaction with uncoordinated amine sites in the OLA. This reversible physisorption of CO2 and H2O is of critical importance for remote sensor applications since it does not require heat input for regeneration. These properties may also be advantageous for other applications such as CO2 separation in contrast to current technologies which use amine-functionalized porous materials to absorb CO2, followed by high temperature regeneration.46,52 Structural stabilities of Co/ZIF8 and OLA-Co/ZIF8 samples were explored by CO2 flow with 80% RH for 5 days (Figure 4d and 4e). XRD pattern of Co/ZIF8 exposure to humid gas flues shows the weakening of the main peak at 2θ = 7.3° and characteristic peaks at 2θ = 11.1° associated to zinc/cobalt carbonates (Figure 4d)53, suggesting structural transformation. In contrast, the framework and crystallinity of OLA-Co/ZIF8 remain unaltered after the same treatment (Figure 4e). These results are in good agreement with the above CO2 isotherm investigations as well as the reversible and stable CO2 sensing responses.

Figure 4. (a) Breakthrough curves for CO2 (C0 and C are the inlet and outlet concentrations, respectively) and (b) CO2 uptake (calculated from breakthrough curves) of Co/ZIF8 and OLA-Co/ZIF8 in dry and wet conditions. (c) CO2 adsorption capacities (obtained from gravimetric adsorption measurement) of Co/ZIF8 and OLA-Co/ZIF8



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samples exposure to humid gas flues for different times. XRD patterns of (d) Co/ZIF8 and (e) OLA-Co/ZIF8 samples exposure to humid gas flues for different times. We measured in-situ FT-IR spectra of Co/ZIF8 and OLA-Co/ZIF8 samples at 25 °C under 32 psi while flowing wet CO2 onto dried samples placed in an environmental chamber (Figure 5). For Co/ZIF8 under wet CO2 condition (Figure 5a), the absorption peak assigned to the v3 band of adsorbed CO2 at ~2337 cm-1 rapidly decreased in intensity with increasing RH in the chamber, indicating that H2O adsorption is highly competitive with CO2. In addition to this, a very broad adsorption peak due to the adsorbed H2O appeared and increased from 2800 to 3700 cm-1, consistent with the fact that large amount of H2O is adsorbed into Co/ZIF8 in wet CO2 condition (Figure S10). Conversely, the area of the adsorbed CO2 peak for the OLA-Co/ZIF8 showed a moderate increase with higher RH levels indicating that H2O vapor does not adversely impact the total CO2 uptake in the sample. Intensities of the adsorbed H2O peaks, with the increase of the RH, were identical (Figure S10). The FT-IR results further support the gravimetric adsorption and breakthrough studies verify that OLA modification of Co/ZIF8 greatly enhances the CO2/H2O selectivity of the sample.

Figure 5. (a) In-situ FT-IR spectra showing the v3 band of adsorbed CO2 in Co/ZIF8 (left) and OLA-Co/ZIF8 (right) measured at 25 °C under 32 psi CO2 with increasing RH. Note that CO2 uptake decreases drastically with increasing RH in the pristine Co/ZIF8, but moderately increases in the OLA-Co/ZIF8. 0% RH* means the spectrum measured upon returning to dry CO2 conditions following the 80% RH measurement. (b) FTIR spectra of OLA-Co/ZIF8 in dry and wet CO2 (80% RH) conditions.

The mechanism for the additional CO2 uptake under wet conditions remains elusive. One possible explanation would involve reaction of CO2, H2O and OLA to form a carbamate or carbamic acid. However, no evidence for this reaction was observed in the in-situ FT-IR data under wet CO2 as shown in Figure 5b. From the observation, bands associated with the OLA (~1650, 1320, and 1200 cm1) changed significantly under wet CO condition. This 2 could indicate a re-arrangement of the OLA molecules on the surface in response to the increased polarity of the high humidity environment. The re-arrangement could provide a new favorable CO2 adsorption site on the surface of the MOF resulting in the observed increase in CO2 uptake relative to dry conditions.

CONCLUSION In summary, we report, for the first time, the use of alkylamine-modified MOF thin film on optical fiber sensors for the detection of CO2 under wet conditions. The above results suggest that the alkylamine (such as OLA) modification occurs on the surface of Co/ZIF8 MOF without affecting the bulk pore structure of the material. Furthermore, it has been demonstrated that a simple strategy of appending a long chain aliphatic amine to metal sites at the surface of MOFs is a viable strategy for creating moisture stable MOF-based chemical sensors. We anticipate that this approach could be useful for some MOFs with intermediate aperture sizes (0.6 ~ 0.7 nm) such as UiO-66 or HKUST-1. Compared with other PSM methods, the current technique, through the use of solution-based methods under mild synthetic conditions is also well-suited for thin film applications involving porous MOFs. The deposition of a hydrophobic layer of OLA to the Co/ZIF8 sensing layer greatly enhanced the stability of the material in the presence of wet CO2 while retaining the inherent porosity of the material. More surprisingly, the OLA-modified MOFs also showed an enhanced CO2 sensitivity in humid gas streams, which is an indispensable ability for practical applications such as CO2 separation.


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effect of RH on the sample, the same procedure was followed with the exception that CO2 and H2O was flowed into the sample cell by using a relative humidity apparatus. The relative humidity was measured by using a relative humidity probe that was installed upstream of the sample cell.

EXPERIMENTAL SECTION Materials. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), and methanol of analytical grade were purchased from Sigma-Aldrich. 2-methyl-imidazole (2-mIm) was obtained from Alfa Aesar. Fiber optic cables were purchased from ThorLabs (FG105LCA, Multimode fiber, 0.22 NA, low-OH, 105 um).

Preparation of Cobalt-doped ZIF-8 (Co/ZIF-8) Thin Films on Optical Fiber. Polymer jacket of fiber optic was first removed and then etched (~5 cm long) in a buffered oxide hydrofluoric acid (HF) etchant solution for 60 minutes. Then it is washed thoroughly by DI water and dried naturally. The etched fiber optic was also cleaned in Piranha solution (H2SO4/H2O2, 70/30 v/v%) at 70 °C for 30 minutes. Then it is washed thoroughly by DI water and dried under N2 flow. (Caution: HF and Piranha solutions are an acutely toxic chemical and are dangerous to handle without personal protective equipment; splash goggles or a face shield with safety glasses, lab coat, 18 mil neoprene gloves, chemically-resistant sleeves and apron, and closed toed shoes. Working alone with this chemical is not recommended). To grow MOF thin film, the etched/cleaned fiber optic was immersed in a freshly mixed methanolic solution of 2-mIm (5 mmol, 50 mL) and mixture solution of Zn(NO3)2·9H2O (1.25 mmol, 25 mL) and Co(NO3)·6H2O (1.25 mmol, 25 mL) for 60 min at room temperature, followed by washing using methanol and drying in air. This process was repeated 10 times and a MOF thickness of ~200 nm was obtained (Figure 3a). Co/ZIF-8 nanoparticles were obtained from the growth mixture solution by centrifuge.

Characterization. X-ray diffraction (XRD) pattern was obtained using a Rigaku Ultima IV Diffractometer, operating at 40 kV and 40 mA with Cu kα radiation (0.154 nm) in the range from 5 to 50 degrees with a step size of 0.01. Field emission-scanning electron microscope (FESEM) analysis was conducted with a FEI Quanta 600 using 20 kV accelerating voltage. Nitrogen adsorption isotherm were measured at -196 °C using a Micromeritics ASAP 2020 analyzer. Before measurement, the sample (~100 mg) was degassed in vacuum at 150 °C for 2 hours. X-ray photoelectron spectroscopy (XPS) was carried out with a PHI 5600ci instrument using a monochromatized Al Kα X-ray source (1486.6eV). The pass energy of the analyzer was 23.5 eV and binding energies were calibrated using the C 1s signal for adventitious carbon, which was assigned a binding energy of 284.8 eV. Raman spectroscopy was carried out on a Horiba LabRam HREvolution spectrometer with 532 nm laser as excitation source. Thermal gravimetric analysis was performed on a Mettler Toledo, TGA-DSC 3+ Star system. The sample (~10 mg) was placed into an alumina crucible and heated under air flow (75 mL min-1) at a rate of 3 °C up to 600 °C. UV-Vis-NIR spectra were measured by using a Lambda 1050 spectrophotometer equipped with an integrating sphere. Gravimetric gas adsorption measurements were obtained using a Hiden IGA microbalance at room temperature using gas densities calculated from REFPROP for buoyance corrections. The sample was dried under vacuum at 90 °C until the mass loss stabilized prior to measurement.

MOF thin films (or MOF nanoparticles) were modified by immersing into 5 mmol OLA methanolic solution (25 mL) at room temperature for 3 hours, followed by washing with methanol several times. Breakthrough experiments of CO2 in dry and wet conditions. The column breakthrough tests were carried out in a dynamic breakthrough measurements system under atmospheric pressure and room temperature. Before the experiment, 0.33 g of samples were loaded in a fixed-bed column, and the sample was pretreated with helium for 1 hr at 120 °C. The 10%-CO2/He gas with 10 mL min-1 of total flow rate fed into column at room temperature and the concentration of the gases were monitored by mass spectroscopy. The desorption of gases was carried out using a temperature programmed desorption technique, where the temperature was raised from room temperature to 150 °C at a rate of 5 °C min-1.

Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) was performed using a Thermo Electron Nexus 4700 FTIR ESP. This singlebeam FT-IR spectrometer took measurements from 4000 to 600 cm-1 with a scanning resolution of 4 cm-1 and averaged 250 scans. Samples were coated on a ZnSe crystal and placed in custom-designed Spectra Tech ATR cells. Sample coating occurred by suspending the OLACo/ZIF8 and Co/ZIF8 in dichloromethane and then painting a thin coating onto the ZnSe crystal. The sample was placed under vacuum overnight prior to the start of the experiment. A temperature of 25 °C was kept constant using a recirculation bath. A leak test was performed using 60 psi of N2. Once the cell was vented slowly, a scan was taken at 0 psi of N2 prior to injecting 32 psi of CO2. A scan was then taken after 20 minutes which allowed for the system to approach equilibrium. For investigating the

Sensing Measurement. In-situ optical transmission spectroscopy was recorded by connecting the one end of the fiber with a spectrometer (JAZ, Ocean Optics) operating over a wavelength range of ∼400-900 nm. The other end of the fiber was connected to a broad-band deuterium tungsten halogen light source (DH-2000-BAL, Ocean Optics). Gas sensing performance of MOF coated optical fiber sensor was evaluated at room temperature



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any agency thereof, nor any of their employees, nor LRTS, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

in a sealed quartz chamber of tube coupled with an automated gas delivery system and monitored the transmittance (%T) of the sensor in various gases. The total flow rate was maintained at 100 mL min-1. Different gas concentrations were obtained by controlling the flow rates of pure N2 and CO2 gas. The chamber was first purged with N2 gas for a few hours and then the reference spectrum was recorded (100 %T). After this the CO2 gas was inserted in the chamber and the transmitted spectrum was recorded. These two spectra were used to obtain the spectrum of the gas for a given concentration of the gas. All sensing tests were performed at room temperature and ambient conditions without pretreatments such as heating and/or evacuating under vacuum, and pure N2 was used as the reference and/or carrier gas.

REFERENCES (1) Wang, Y.; Nakayama, M.; Watanabe, K.; Yagi, M.; Nishikawa, M.; Fukunaga, M. The NDIR CO2 Monitor with Smart Interface for Global Networking. IEEE Trans. Instrum. Meas. 2005, 54, 1634-1639. (2) Eranna, G.; Joshi, B. C.; Runthala, D. P.; Gupta, R. P. Oxide Materials for Development of Integrated Gas Sensors - A Comprehensive Review. Crit. Rev. Solid State Mater. Sci. 2004, 29, 111-188. (3) Capone, S.; Forleo, A.; Francioso, L.; Rella, R.; Siciliano, P.; Spadavecchia, J.; Presicce, D. S.; Taurino, A. M. Solid State Gas Sensors: State of the Art and Future Activities. J. Optoelectron. Adv. Mater. 2003, 5, 1335-1348. (4) Rohwedder, J. J. R.; Pasquini, C.; Fortes, P. R.; Raimundo, I. M.; Wilk, A.; Mizaikoff, B. iHWG-µNIR: A Miniaturised NearInfrared Gas Sensor Based on Substrate-Integrated Hollow Waveguides Coupled to a Micro-NIR-Spectrophotometer. Analyst 2014, 139, 3572-3576. (5) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Metal-Organic Framework Materials as Chemical Sensors. Chem. Rev. 2012, 112, 1105-1125. (6) Stassen, I.; Burtch, N.; Talin, A.; Falcaro, P.; Allendorf, M.; Ameloot, R. An Updated Roadmap for the Integration of MetalOrganic Frameworks with Electronic Devices and Chemical Sensors. Chem. Soc. Rev. 2017, 46, 3185-3241. (7) Liu, B. Metal-Organic Framework-Based Devices: Separation and Sensors. J. Mater. Chem. 2012, 22, 10094-10101. (8) Allendorf, M. D.; Houk, R. J. T.; Andruszkiewicz, L.; Talin, A. A.; Pikarsky, J.; Choudhury, A.; Gall, K. A.; Hesketh, P. J. Stress-Induced Chemical Detection using Flexible MetalOrganic Frameworks. J. Am. Chem. Soc. 2008, 130, 14404-14405. (9) Strauss, I.; Mundstock, A.; Treger, M.; Lange, K.; Hwang, S.; Chmelik, C.; Rusch, P.; Bigall, N. C.; Pichler, T.; Shiozawa, H.; Caro, J. Metal-Organic Framework Co-MOF-74-Based Host– Guest Composites for Resistive Gas Sensing. ACS Appl. Mater. Interfaces 2019, 11, 14175-14181. (10) Drobek, M.; Kim, J.-H.; Bechelany, M.; Vallicari, C.; Julbe, A.; Kim, S. S. MOF-Based Membrane Encapsulated ZnO Nanowires for Enhanced Gas Sensor Selectivity. ACS Appl. Mater. Interfaces 2016, 8, 13, 8323-8328. (11) Kim, J.-O.; Kim, J. Y.; Lee, J.-C.; Park, S.; Moon, H. R.; Kim, D.-P. Versatile Processing of Metal-Organic FrameworkFluoroPolymer Composite Inks with Chemical Resistance and Sensor Applications. ACS Appl. Mater. Interfaces 2019, 11, 4, 4385-4392. (12) Kim, K.-J.; Lu, P.; Culp, J. T.; Ohodnicki, P. R. MetalOrganic Framework Thin Film Coated Optical Fiber Sensors: A

ASSOCIATED CONTENT Supporting Information. N2 adsorption isotherms, pore size distribution, TGA, and CO2 adsorption isotherms of Co/ZIF8 and OLA-Co/ZIF8; breakthrough curves and adsorbed/desorbed amount for H2O and CO2 from Co/ZIF8 and OLA-Co/ZIF8; Photographs and XRD patterns of as-synthesized Co/ZIF8 and OLACo/ZIF8 in the mixture of water and hexane. Optical spectra of OLA-Co/ZIF8 under N2 and CO2 flow; FT-IR spectra of Co/ZIF8 and OLA-Co/ZIF8 after long air exposures and under humid gas stream. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge Sonia Hammache for her assistance in breakthrough analyzing system, Bret Howard for XRD analysis, Thuy-Duong Nguyen-Phan for Raman analysis, and John Baltrus for XPS work and his useful discussion. This work was performed in support of the US Department of Energy’s Fossil Energy Crosscutting Technology Research Program, Briggs White, NETL Technology Manager. The Research was executed through the NETL Research and Innovation Center’s Carbon Storage Field Work Proposal. Research performed by Leidos Research Support Team staff was conducted under the RSS contract 89243318CFE000003. This work was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with Leidos Research Support Team (LRST). Neither the United States Government nor


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SYNOPSIS TOC. “for TOC only”


Alkylamine Integrated Metal-Organic Framework-Based Waveguide

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