Solvothermal Synthesis of MIL-96 and UiO-66-NH<sub>2</sub> on Atomic Layer Deposited Metal Oxide Coatings on Fiber Mats

Heather F. Barton; Alexandra K. Davis; Dennis T. Lee; Gregory N. Parsons

doi:10.3791/57734

JoVE Journal > Chemistry

Chemie

Solvothermal Synthesis of MIL-96 and UiO-66-NH₂ on Atomic Layer Deposited Metal Oxide Coatings on Fiber Mats

Published: June 13, 2018

doi:

10.3791/57734

Heather F. Barton¹, Alexandra K. Davis¹, Dennis T. Lee¹, Gregory N. Parsons¹

¹Department of Chemical and Biomolecular Engineering,North Carolina State University

Summary

Metal-organic frameworks are effective in gas storage and heterogeneous catalysis, but typical synthesis methods result in loose powders that are difficult to incorporate into smart materials. We demonstrate a method of first coating fabrics with ALD metal oxides, resulting in conformal films of MOF on the fabrics during solvothermal synthesis.

Abstract

Metal-organic frameworks (MOFs), which contain reactive metal clusters and organic ligands allowing for large porosities and surface areas, have proven effective in gas adsorption, separations, and catalysis. MOFs are most commonly synthesized as bulk powder, requiring additional processes to adhere them to functional devices and fabrics that risk decreasing the powder porosity and adsorption capacity. Here, we demonstrate a method of first coating fabrics with metal oxide films using atomic layer deposition (ALD). This process creates conformal films of controllable thickness on each fiber, while providing a more reactive surface for MOF nucleation. By submerging the ALD coated fabric in solution during solvothermal MOF synthesis, the MOFs create a conformal, well-adhered coating on the fibers, resulting in a MOF-functionalized fabric, without additional adhesion materials that may block MOF pores and functional sites. Here we demonstrate two solvothermal synthesis methods. First, we form a MIL-96(Al) layer on polypropylene fibers using synthetic conditions that convert the metal oxide to MOF. Using initial inorganic films of varying thicknesses, diffusion of the organic linker into the inorganic allows us to control the extent of MOF loading on the fabric. Second, we perform a solvothermal synthesis of UiO-66-NH₂ in which the MOF nucleates on the conformal metal oxide coating on polyamide-6 (PA-6) fibers, thereby producing a uniform and conformal thin film of MOF on the fabric. The resulting materials can be directly incorporated into filter devices or protective clothing and eliminate the maladroit qualities of loose powder.

Introduction

Metal-organic frameworks are crystalline structures consisting of reactive metal cluster centers bridged by organic molecule linkers to provide large porosities and surface areas. Their structure, porosity, and functionality can be designed by choosing appropriate clusters and linkers, leading to surface areas as high as 7,000 m²/g_MOF¹^,². Their high porosity and surface area have made MOFs diversely applicable in adsorption, separation, and heterogeneous catalysis in fields ranging from energy production to environmental concerns to biological processes¹^,³^,⁴^,⁵^,⁶.

Numerous MOFs have proven successful in selectively adsorbing volatile organic compounds and greenhouse gases or to catalytically degrade chemicals that may prove harmful to human health or the environment. In particular, MIL-96 (Al) has shown to selectively adsorb nitrogenous volatile organic compounds (VOCs) due to the availability of lone pair electrons in the nitrogen groups to coordinate with the weak Lewis acid Al present in the metal clusters⁷. MIL-96 has also been shown to adsorb gases such as CO₂, p-xylene, and m-xylene⁸^,⁹. MOF adsorption selectivity is dependent on both the Lewis acid of the metal cluster, as well as pore size. The pore size of MIL-96 increases with temperature, resulting in increased adsorption capacity of trimethylbenzene with increased temperature, and presents the opportunity of tuning selectivity with adsorption temperature⁹.

The second MOF of focus here, UiO-66-NH₂ has been shown to catalytically degrade chemical warfare agents (CWAs) and simulants. The amine group on the linker provides a synergistic effect in degrading nerve agents, while preventing agent degradation products from binding irreversibly to the zirconium clusters and poisoning the MOF¹⁰. UiO-66-NH₂ has catalytically hydrolyzed dimethyl p-nitrophenylphosphate (DMNP) with a half-life as short as 0.7 minutes in buffered conditions, nearly 20 times faster than its base MOF UiO-66¹¹^,¹².

While these adsorption and catalytic properties are promising, the physical form of the MOFs, primarily bulk powder, can be difficult to incorporate into platforms for gas capture and filtration without adding significant bulk, clogging pores, or reducing MOF flexibility. An alternative is to create MOF functionalized fabrics. MOFs have been incorporated into fabrics in myriad ways, including electrospinning MOF powder/polymer slurries, adhesive mixes, spray coating, solvothermal growth, microwave syntheses, and a layer-by-layer growth method¹³^,¹⁴^,¹⁵^,¹⁶^,¹⁷^,¹⁸. Of these, electrospinning and polymer adhesives can result in blocked functional sites on the MOF as they are encapsulated in the polymer, significantly decreasing adsorption capacity and reactivity. Additionally, many of these techniques fail to create conformal coatings on the fibers due to line of sight difficulties or poor adhesion/nucleation and the reliance on purely electrostatic interactions. An alternative method is to first coat the fabric with a metal oxide to allow for stronger surface interactions with the MOF¹⁸^,¹⁹.

One method of metal oxide deposition is atomic layer deposition (ALD). ALD is a technique for depositing conformal thin films, controllable to the atomic scale. The process utilizes two half reactions that occur only at the surface of the substrate to be coated. The first step is to dose a metal containing precursor, which reacts with hydroxyls on the surface, leaving a metallated surface while excess reactant is purged from the system. The second reactant is an oxygen-containing reactant, typically water, which reacts with the metal sites to form a metal oxide. Again, excess water and any reaction products are purged from the system. These alternating doses and purges can be repeated until the desired film thickness is achieved (Figure 1). Atomic layer deposition is particularly useful because the small-scale vapor phase precursors allow for conformal films on every surface of substrates with complex topology, such as fiber mats. Additionally, for polymers such as polypropylene, the ALD conditions can allow the coating to diffuse into the fiber surface, providing a strong anchor for future MOF growth²⁰.

The metal oxide coating allows for increased nucleation sites on the fibers during traditional solvothermal synthesis by increasing functional groups and roughness¹⁸^,²⁰. Our group has previously shown the ALD metal oxide base layer is effective for UiO-6X, HKUST-1, and other syntheses through various routes of solvothermal, layer-by-layer, and hydroxy-double salt conversion methods¹³^,¹⁷^,¹⁸^,²¹^,²²^,²³. Here we demonstrate two synthesis types. The MIL materials are formed by converting the Al₂O₃ ALD coating directly to MOF by diffusion of the organic linker. By submerging an Al₂O₃ ALD coated fiber mat in trimesic acid solution and heating, the organic linker diffuses into the metal oxide coating to form MIL-96. This results in a strongly adhered, conformal MOF coating on every fiber surface. The second synthesis approach calls for typical UiO-66-NH₂ hydrothermal synthesis using metal and organic precursors, but adds a metal oxide coated fiber mat on which the MOF nucleates. For both synthesis approaches, the resulting products consist of conformal thin films of MOF crystals strongly adhered to the supporting fabric. In the case of MIL-96, these can be incorporated into filters for adsorption of VOCs or greenhouse gases. For UiO-66-NH₂ these fabrics can be easily incorporated into lightweight protective clothing for military personnel, first responders, and civilians for continuous defense against CWA attacks.

Protocol

1. Atomic Layer Deposition (ALD) of Al₂O₃ on Fiber Mats

Place a 2.54 x 2.54 cm² polypropylene fabric sample in the reactor boat (a thin, rigid, metal mesh holder). A schematic of the reactor is presented in Figure 2.
Open the pressure gauge. Remove the clasp from the reactor cap. Turn on manual control in the LabView system. Close the carrier nitrogen and gate valve on the ALD reactor. Open the vent nitrogen.
After removing the reactor cap, load the fabric sample into the ALD reactor. Replace the reactor cap and open the gate valve. Close the vent and open the carrier nitrogen. Turn off manual control.
Load the recipe for Al₂O₃ on fabrics. The recipe will alternately dose trimethylaluminum (TMA) for 1.2 s, followed by a 30 s dry nitrogen purge or dose water for 1 s followed by a 60 s dry nitrogen purge. Set the recipe to run 1000 cycles.
Set the mass flow controller to 20 cfm and the furnace temperature to 90 °C (84 °C on the furnace interface).
Open the manual valve to the TMA and water. Close the pressure gauge. Replace the clasp on the reactor cap. Press Start on the interface.
Upon completion of the recipe, open the pressure gauge. Remove the clasp from the reactor cap. Turn on manual control in the system. Close the carrier nitrogen and gate valve on the ALD reactor. Open the vent nitrogen.
Remove the reactor cap and sample boat. Re-seal the reactor.
NOTE: The procedure may be paused at this point.

2. Atomic Layer Deposition (ALD) of TiO₂ on Polyamide-6 (PA-6) Fiber Mats

Place a 2.54 x 2.54 cm² PA-6 fabric sample in the reactor boat (a thin, rigid, metal mesh holder).
Open the pressure gauge. Remove the clasp from the reactor cap. Turn on manual control in the LabView system. Close the carrier nitrogen and gate valve on the ALD reactor. Open the vent nitrogen.
After removing the reactor cap, load the fabric sample into the ALD reactor. Replace the reactor cap and open the gate valve. Close the vent and open the carrier nitrogen. Turn off manual control.
Load the recipe for TiO₂ on fabrics. The recipe will alternately dose TiCl₄ for 1 s, followed by a 40 s dry nitrogen purge or dose water for 1 s followed by a 60 s dry nitrogen purge. Set the recipe to run 300 cycles.
Set the mass flow controller to 20 cfm and the furnace temperature to 90 °C (84 °C on the furnace interface).
Open the manual valve to the TiCl₄ and water. Close the pressure gauge. Replace the clasp on the reactor cap. Press Start on the interface.
Upon completion of the recipe, open the pressure gauge. Remove the clasp from the reactor cap. Turn on manual control in the system. Close the carrier nitrogen and gate valve on the ALD reactor. Open the vent nitrogen.
Remove the reactor cap and sample boat. Re-seal the reactor.
NOTE: The procedure may be paused at this point.

3. Solvothermal Synthesis of MIL-96

Add 0.0878 g of H₃BTC to an 80 mL glass beaker.
Add 12 mL of H₂O and 12 mL of ethanol to the beaker.
Stir magnetically for 10 min or until the H₃BTC is fully dissolved.
Place the solution in a Teflon lined pressure vessel.
Add the Al₂O₃ coated polypropylene to the solution and prop the fabric on a mesh support so it does not lie flat against the bottom of the vessel.
Seal the pressure vessel and place it in the furnace at 110 °C for 24 h.
After allowing the sample to cool, place the fabric sample in a mesh basket in a beaker. Wash twice with ethanol, each for 12 h.
Sample activation requires heating at 85 °C for 6 h under vacuum, followed by heating at 110 °C for 12 h under vacuum.
NOTE: The procedure can be stopped here. All samples should be stored in a desiccator to maintain sample activation.

4. Solvothermal Synthesis of UiO-66-NH₂

Add 0.08 g of ZrCl₄ to a 20 mL glass scintillation vial.
Add 20 mL of N,N-dimethylformamide (DMF) to the ZrCl₄ in 5 mL increments. Cap the vial between increments and allow fumes to dissipate.
Sonicate the solution for 1 min.
Add 0.062 g of 2-aminoterephthalic acid to the vial, and magnetically stir the solution for 5 min.
Add 25 µL of deionized water to the vial.
Add 1.33 mL of concentrated HCl to the vial.
Submerge the TiO₂ ALD coated fabric swatch in the solution and cap the vial.
Place the sample in the furnace at 85 °C for 24 h.
After allowing the sample to cool, place the fabric sample in a mesh basket in a beaker. Wash twice with 80 mL of DMF, each for 12 h. Wash 3 times with 80 mL of ethanol, each for 12 h.
After removing the fabric swatch, filter the residual MOF powder. Wash twice with 80 mL of DMF, each for 12 h. Wash 3 times with 80 mL of ethanol, each for 12 h.
Sample activation requires heating at 85 °C for 6 h under vacuum, followed by heating at 110 °C for 12 h under vacuum.
NOTE: The procedure can be stopped here. All samples should be stored in a desiccator to maintain sample activation.

Representative Results

To describe the MOF/fabric materials, we delineate two terms related to measured surface area. First, projected surface area, cm²_projected, refers to the macroscopic size of the fabric swatch as measured with a ruler, i.e., the area of the sample's projected shadow. The second surface area of interest is the BET surface area, calculated from a nitrogen isotherm obtained at 77 K. These values are given in units of m²/g_Fabric, m²/g_MOF, or m²/g_MOF+Fabric, corresponding respectively to the measured or estimated total surface area per gram of sample for the fabric before MOF loading, the MOF itself, or the fabric after loading with MOF. For ALD coated fabrics and MIL-96 coated fabrics, the surface areas were calculated from a partial pressure range of 0.05 to 0.3. For samples containing UiO-66-NH₂, surface areas were calculated using a partial pressure range of 0.02 to 0.08, due to the presence of microporosity. All samples had correlation coefficients of 0.995 or higher. Fit parameters are listed for each sample in Table 1. The specific surface area of a MOF on fabric, m²/g_MOF, is calculated using measured mass and surface area of MOF on fabric:

After coating fabrics with 1000 cycles of Al₂O₃ ALD, the polypropylene fabric appeared visually unchanged, although some additional stiffness could be felt by hand. Ellipsometry of monitor silicon wafers revealed 1100 ±15 Å of Al₂O₃ growth using a Cauchy model. The ALD coating resulted in a mass gain of 1.16 mg_Al2O3/cm²_projected. This process was repeated with 500 and 2000 cycles of Al₂O₃, resulting in 600 ±15 and 2010 ±40 Å on the monitor silicon wafers. The mass increase was 0.65 mg_Al2O3/cm²_projected and 2.26 mg_Al2O3/cm²_projected on the 500 and 2000 cycle samples respectively. The BET surface area of the Al₂O₃(1000) coated polypropylene was 4.7 m²/g_Fabric.

Following the MOF synthesis, the resulting solution was clear and free of loose MOF powder, indicating strong MOF and ALD adhesion on the fiber. After washing and drying, the sample mass increase on the 500, 1000, and 2000 cycle samples was 40, 73, and 77% of the mass of the initial samples, respectively. Parallel exposure of Al₂O₃ coated fabric samples to synthesis conditions in the absence of MOF linker or metal-cluster precursors revealed an inherent mass gain of 10-20%, suggesting the mass measurements exaggerate the MOF loading. Examination with scanning electron microscopy (SEM) showed conformal MOF crystal thin films on all fibers, resembling a cobblestone pattern (Figure 3b–3c). When the Al₂O₃ was reduced to 500 cycles, the film began to break apart as the MOF formed, resulting in a patchy coating (Figure 3a). A bare polypropylene sample with no Al₂O₃ coating was also exposed to MIL-96 synthesis conditions (Figure 3d), but XRD showed no detectable MOF present on the fibers. Cross-sectional images of these samples revealed the 500 cycle Al₂O₃ base layer completely reacted, while a fraction of the Al₂O₃ base layer remained for the 1000 and 2000 cycle samples (Figure 4d–4f). Cross-sections of the original Al₂O₃ ALD coated polypropylene are shown in Figure 4a-4c. In the 24 hour reaction time, approximately 80±20 nm of Al₂O₃ reacted or was potentially etched away in the acidic synthesis conditions. Electron dispersion spectroscopy images of the cross-section revealed the carbon based polypropylene core and predominantly Al₂O₃ shell (Figure 5). X-ray diffraction patterns of the MOF coated fabric, matching the simulated PXRD pattern of MIL-96, are shown in Figure 6. The measured surface area after MOF growth was 6.0 m²/g_MOF+Fabric, 6.7 m²/g_MOF+Fabric, and 19.9 m²/g_MOF+Fabric, for the 500, 1000, and 2000 cycle samples respectively. Adsorption and desorption isotherms are shown in Figure 7.

The PA-6 fiber mats appeared slightly yellowed after deposition of 300 cycles of TiO₂, but the mat felt nearly unchanged in stiffness. Ellipsometry revealed 175 ±15 Å of TiO₂ for ALD at 50, 90, or 200 °C on the monitor silicon. The ALD mass loading was 0.17, 0.20, and 0.25 mg_TiO2/cm²_{projected area PA-6} for the 50, 90, and 200 °C samples. The BET surface area of the PA-6 fabric coated with 300 cycles of TiO₂ at 90 °C was 8.2 m²/g_Fabric.

Following solvothermal MOF synthesis, XRD patterns revealed UiO-66-NH₂ was present on the fibers (Figure 8). The MOF mass gain on the 50, 90, and 200 °C samples was 2.4, 78, and 0%. A parallel exposure of TiO₂ coated nylon to synthesis conditions in the absence of MOF metal-cluster or linker precursors revealed a mass gain of 10-20%. Additionally, fabric was easily torn during the MOF synthesis and the acidic conditions may etch the TiO₂ film, leading to uncertainties in the MOF loading. SEM images showed the MOF coatings on each sample, with flaky coatings on the 50 °C samples, dense coatings on the 90 °C samples, and sparse coatings on the 200 °C samples (Figure 9a–9c). An uncoated PA-6 sample was also exposed to UiO-66-NH₂ synthesis conditions, resulting in a relatively sparse coating of MOF crystals (Figure 9d). The measured BET surface areas after MOF synthesis were 16.0 m²/g_MOF+Fabric, 19.8 m²/g_MOF+Fabric, and 4.67 m²/g_MOF+Fabric, for the 50, 90, and 200 °C samples respectively. Adsorption and desorption isotherms are shown in Figure 10.

Figure 1. Schematic of Al₂O₃ ALD process: In the first step, precursor dosing, trimethyl aluminum precursor reacts with the hydroxyl terminated surface. The excess precursor is then purged from the system, resulting in a uniform aluminum-dimethyl terminated surface. During the water dose step the water reacts to replace the methyl groups, resulting in a newly hydroxyl terminated surface. In the last step of the cycle, the excess water is purged from the system. Please click here to view a larger version of this figure.

Figure 2. ALD reactor schematic: The system is a home-built, hot-walled viscous flow reactor with a dry nitrogen carrier gas. The precursor lines are wrapped with heat tape, while the actual deposition zone holding the mesh sample boat is located within a furnace. The system is operated under vacuum at ~1.8 Torr. Please click here to view a larger version of this figure.

Figure 3. SEM images of PP with (a) Al₂O₃(500)/MIL-96, (b) Al₂O₃(1000)/MIL-96, (c) Al₂O₃(2000)/MIL-96, and (d) no ALD coating after exposure to the MIL-96 synthesis conditions. Please click here to view a larger version of this figure.

Figure 4. SEM images of the cross-section of PP with (a) Al₂O₃ (500), (b) Al₂O₃(1000), (c) Al₂O₃(2000), (d) Al₂O₃ (500)/MIL-96, (e) Al₂O₃(1000)/MIL-96, (f) Al₂O₃(2000)/MIL-96. Please click here to view a larger version of this figure.

Figure 5. EDS images of the cross-section of PP/Al₂O₃ (500)/MIL-96 reveals the carbon based polypropylene core with the predominantly Al₂O₃ shell. Please click here to view a larger version of this figure.

Figure 6. (black) Simulated PXRD pattern of MIL-96, (red) XRD pattern of Al₂O₃ coated polypropylene, (green) MIL-96 on Al₃O₃(500) coated polypropylene, (bue) MIL-96 on Al₃O₃ (1000) coated polypropylene, (purple) MIL-96 on Al₃O₃(2000) coated polypropylene and (gray) bare PP after exposure to MIL-96 synthesis conditions. Please click here to view a larger version of this figure.

Figure 7. (grey) N₂ adsorption and desorption isotherms for MIL-96 on 500 cycles of Al₂O₃ on polypropylene (blue) adsorption and desorption isotherms for MIL-96 on 1000 cycles of Al₂O₃ on polypropylene (black) adsorption and desorption isotherms for MIL-96 on 2000 cycles of Al₂O₃ on polypropylene. Please click here to view a larger version of this figure.

Figure 8. (black) Simulated PXRD pattern of UiO-66-NH_2, (red) XRD pattern of TiO₂ coated PA-6, (green) UiO-66-NH₂ on TiO₂(50 °C), coated PA-6 (blue) UiO-66-NH₂ on TiO₂(90 °C) coated PA-6, (purple) UiO-66-NH₂ on TiO₂(200 °C) coated PA-6 and (gray) UiO-66-NH₂ on bare PA-6. Please click here to view a larger version of this figure.

Figure 9. SEM images of PA-6/TiO₂/UiO-66-NH₂ with ALD deposition at (a) 50 °C, (b) 90 °C and (c) 200 °C and (d) UiO-66-NH₂ on PA-6 with no ALD base coat, demonstrating the higher ALD temperature results in greater diffusion of the ALD precursors into the fiber, altering the MOF adhesion. Please click here to view a larger version of this figure.

Figure 10. N₂ Adsorption and desorption isotherms for PA-6/TiO₂/UiO-66-NH₂ with ALD deposition at (grey) 50 °C (blue) 90 °C, and (black) 200 °C. Please click here to view a larger version of this figure.

Sample	C	Y (g/mmol)	Slope (g/mmol)	Qm (mmol/G)
PP/Al₂O₃ (1000)	6.61	3.13	17.59	0.048
PP/Al₂O₃ (500)/MIL-96	7.01	2.31	13.588	0.062
PP/Al₂O₃ (1000)/MIL-96	9.24	1.58	13.01	0.069
PP/Al₂O₃ (2000)/MIL-96	4.06	1.21	3.69	0.2
Nylon/TiO₂ (90 °C)	2.99	3.97	10.57	0.072
Nylon/TiO₂ (50 °C)/UiO-66-NH₂	63.09	0.096	5.99	0.16
Nylon/TiO₂ (90 °C)/UiO-66-NH₂	599	0.0082	4.92	0.2
Nylon/TiO₂ (200 °C)/UiO-66-NH₂	32.43	0.644	20.24	0.048

Table 1. List of BET fit parameters for each sample.

Discussion

The ALD coating strongly influences the adhesion and loading of the MOF. First, depending on the type of substrate and ALD precursor, the ALD layer can either form a distinct outer shell around the fiber, or diffuse into the fiber to create a gradual transition to the metal oxide coating²⁰. The hard shell has been observed on cotton and nylon substrates, while diffusive layers can be observed in polypropylene under proper conditions. Second, the diffusion into the fiber can also be controlled by varying the deposition temperature²⁰^,²⁴. Higher temperatures increase the diffusion of the ALD precursors into the fiber. Lastly, the metal oxide coating must be thick enough after diffusion to form a conformal outer coating and provide functional groups and increased surface roughness for the MOF to nucleate¹⁸^,²⁰. While silicon wafers were used to monitor the ALD growth and estimate film thickness, polymer spin-coated on QCM crystals can serve as a more accurate means to track mass uptake, coupled with FTIR intensities to estimate film thicknesses²⁴. These methods would require more time and materials, but could account for delayed or accelerated nucleation on polymer films, rather than estimating based on the ALD growth on silicon. Alternatively, TEM cross-sectional imaging could be used, but this may result in breaking or compressing the fiber coatings.

Unlike conventional MOF synthesis, the MIL synthesis relies on a metal source anchored on a fiber. Under proper conditions, the Al₂O₃ coating on polypropylene can diffuse into the fiber, helping to anchor the MOF after synthesis. However, if the metal oxide is fully reacted or if the ALD diffusion is limited, the adhesive forces may be slightly diminished. An example of this is present for the MIL-96 grown using 500 ALD cycles of Al₂O₃, as shown in the SEM image in Figure 3a. The patchy MOF coverage and loose fragments are evidence of the MOF layer peeling away from the fiber after the metal oxide has been fully reacted, confirmed by the cross-sectional images in Figure 4. For the thicker metal oxide layers, this peeling is not observed. The MOF loading of the MIL is limited by the metal source on the fiber. The MOF loading on the 500 cycle sample was likely low because the Al was fully consumed. The uniform MOF adhesion on the 1000 cycle and 2000 cycle samples, and their cross-sectional images, suggest the Al₂O₃ was not fully consumed. The loading was limited by the diffusion rate of the trimesic acid organic linker into the Al₂O₃ and longer synthesis time may reveal a higher MOF loading on the thicker Al₂O₃ coatings.

Separately from the MOF syntheses on fabrics, Al₂O₃ powder was used in place of the Al₂O₃ ALD coating during a MIL-96 synthesis. The powder did not react. To understand the difference in reactivity between the powder and film, the dielectric constants were compared. Using ellipsometry measurements on the film, the refractive index was found to be 1.63, giving a dielectric constant of 2.66, while the literature value of Al₂O₃ is 10²⁵. This suggests the ALD film is much more likely to form a dipole, making it more reactive. Given the low ALD temperature, this is likely due to hydroxyls remaining in the film creating defects.

The 2000 cycle samples had the highest BET surface area, consistent with a larger mass loading than on the 500 cycle samples. The smaller BET surface area of the MIL-96 on the fibers coated with 500 ALD cycles reflects the small mass loading. The literature value for the BET surface area of synthesized MIL-96 is approximately 600 m²/g_MOF⁷^,⁸_.Using the mass measurements and surface areas, the calculated specific surface area of MIL on fabrics was only one tenth of the literature values, though this is improved with thicker ALD base layers. This estimate may be artificially low due to exaggerated mass measurements and insufficient material in the BET.

For the UiO-66-NH₂ synthesis, the TiO₂ on PA-6 fibers interacts with the backbone of the fiber to change the structural properties while also forming a hard outer shell on the microfibers²⁰^,²⁶. The coatings deposited at 50 °C resulted in pealing and poor adhesion after MOF synthesis because the low temperature limited the diffusion of the precursor into the fiber. For metal oxides deposited at 90 °C, this peeling was largely eliminated due to the increased temperature of deposition, though some cracks can still be observed in the film. At 200 °C, the diffusion into the fiber eliminated peeling and cracking, but at the expense of thinning the available TiO₂ at the surface of the fiber. The thick outer shells deposited at 50 and 90 °C still resulted in MOF growth, but the MOF growth was very limited on TiO₂ deposited at 200 °C, likely because the outermost shell is so thin. The BET surface area of these samples reflects the growth on the TiO₂ layers. The UiO-66-NH₂ powder surface area was 1325 m²/g_MOF, in agreement with literature reported values. Back calculating the MOF surface area from the mass measurements and sample surface areas reveals the MOF powders on fabrics had at best half the surface area per gram MOF. In all cases, while the mass loadings could be misleading, the thicker outer ALD layers correlated to larger BET surface areas post MOF growth, possibly resulting in better MOF crystallinity as the MOF precursors interacted less with the fibers.

Future studies may examine atomic layer deposition for a variety of metal oxides, including ZnO, ZrO₂, and HfO₂, which may be applicable for alternative MOF syntheses²⁷. However, some of these processes require much higher deposition temperatures, limiting feasible fabrics for deposition. Additionally, MOFs with much more complex metal centers, such as Zr₆ clusters, may be much more difficult to achieve due to the limited mobility of the film. However, in choosing appropriate ALD precursors and temperatures, fluidity of the film may be achieved at higher MOF synthesis temperatures²⁸.

Offenlegungen

The authors have nothing to disclose.

Acknowledgements

The authors thank their collaborators at RTI International, US Army Natick Soldier RD&E Center, and Edgewood Chemical and Biological Center. They also thank their funding source, the Defense Threat Reduction Agency.

Materials

trimethylaluminum	Strem Chemicals	93-1360
home-built ALD reactor	N/A
nitrogen cylinder	Arc3	UN1066
trimesic acid	Sigma-Aldrich	482749-500G
ethanol	Koptec	V1001
teflon lined autoclave	PARR Instrument Company	4760-1211
isotemp furnace	Fisher Scientific	F47925
Zirconium (IV) chloride	Alfa Aesar	12104
2-aminoterephthalic acid	Acros Organics	278031000
N,N-dimethylformamide	Fisher Scientific	D119-4
Hydrochloric Acid	Fisher Scientific	A481-212
Polypropylene fiber mats	N/A
Polyamide fiber mats	N/A

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Barton, H. F., Davis, A. K., Lee, D. T., Parsons, G. N. Solvothermal Synthesis of MIL-96 and UiO-66-NH₂ on Atomic Layer Deposited Metal Oxide Coatings on Fiber Mats. J. Vis. Exp. (136), e57734, doi:10.3791/57734 (2018).