Howard J. Walls

Facile Conversion of Hydroxy Double Salts to Metal–Organic Frameworks Using Metal Oxide Particles and Atomic Layer Deposition Thin-Film Templates

Rapid room-temperature synthesis of metal-organic frameworks (MOFs) is highly desired for industrial implementation and commercialization. Here we find that a (Zn,Cu) hydroxy double salt (HDS) intermediate formed in situ from ZnO particles or thin films enables rapid growth (<1 min) of HKUST-1 (Cu3(BTC)2) at room temperature. The space-time-yield reaches >3 × 10(4) kg·m(-3)·d(-1), at least 1 order of magnitude greater than any prior report. The high anion exchange rate of (Zn,Cu) hydroxy nitrate HDS drives the ultrafast MOF formation. Similarly, we obtained Cu-BDC, ZIF-8, and IRMOF-3 structures from HDSs, demonstrating synthetic generality. Using ZnO thin films deposited via atomic layer deposition, MOF patterns are obtained on pre-patterned surfaces, and dense HKUST-1 coatings are grown onto various form factors, including polymer spheres, silicon wafers, and fibers. Breakthrough tests show that the MOF-functionalized fibers have high adsorption capacity for toxic gases. This rapid synthesis route is also promising for new MOF-based composite materials and applications.

Ultra-Fast Degradation of Chemical Warfare Agents Using MOF–Nanofiber Kebabs

The threat associated with chemical warfare agents (CWAs) motivates the development of new materials to provide enhanced protection with a reduced burden. Metal-organic frame-works (MOFs) have recently been shown as highly effective catalysts for detoxifying CWAs, but challenges still remain for integrating MOFs into functional filter media and/or protective garments. Herein, we report a series of MOF-nanofiber kebab structures for fast degradation of CWAs. We found TiO2 coatings deposited via atomic layer deposition (ALD) onto polyamide-6 nanofibers enable the formation of conformal Zr-based MOF thin films including UiO-66, UiO-66-NH2 , and UiO-67. Cross-sectional TEM images show that these MOF crystals nucleate and grow directly on and around the nanofibers, with strong attachment to the substrates. These MOF-functionalized nanofibers exhibit excellent reactivity for detoxifying CWAs. The half-lives of a CWA simulant compound and nerve agent soman (GD) are as short as 7.3 min and 2.3 min, respectively. These results therefore provide the earliest report of MOF-nanofiber textile composites capable of ultra-fast degradation of CWAs.

Conformal and highly adsorptive metal–organic framework thin films via layer-by-layer growth on ALD-coated fiber mats

Integration of metal–organic frameworks (MOFs) on textiles shows promise for enabling facile deployment and expanding MOF applications. While MOFs deposited on flat substrates can show relatively smooth surface texture, most previous reports of MOFs integrated on fibers show poor conformality with many individual crystal domains. Here we report a new low-temperature (<70 °C) method to deposit uniform and smooth MOF thin films on fiber surfaces using an energy enhanced layer-by-layer (LbL) method with an ALD Al2O3 nucleation layer. Cross-sectional TEM images show a well-defined core@shell structure of the MOF-functionalized fiber, and SEM shows a flat MOF surface texture. We analyze the thickness and mass increase data of LbL HKUST-1 MOF thin films on ALD-coated polypropylene fibers and find the growth rate to be 288–290 ng cm−2 per LbL cycle. Unlike planar LbL MOF embodiments where adsorption capacities are difficult to quantify, the large volume quantity on a typical fiber mat enables accurate surface area measurement of these unique MOF morphologies. After 40 LbL cycles the MOFs on fibers exhibit N2 adsorption BET surface areas of up to 93.6 m2 gMOF+fiber−1 (∼535 m2 gMOF−1) and breakthrough test results reveal high dynamic loadings for NH3 (1.37 molNH3 kgMOF+fiber−1) and H2S (1.49 molH2S kgMOF+fiber−1). This synthesis route is applicable to many polymer fibers, and the fiber@ALD@MOF structure is promising for gas filtration, membrane separation, catalysis, chemical sensing and other applications.

Copper Benzenetricarboxylate Metal-Organic Framework Nucleation Mechanisms on Metal Oxide Powders and Thin Films formed by Atomic Layer Deposition

Chemically functional microporous metal-organic framework (MOF) crystals are attractive for filtration and gas storage applications, and recent results show that they can be immobilized on high surface area substrates, such as fiber mats. However, fundamental knowledge is still lacking regarding initial key reaction steps in thin film MOF nucleation and growth. We find that thin inorganic nucleation layers formed by atomic layer deposition (ALD) can promote solvothermal growth of copper benzenetricarboxylate MOF (Cu-BTC) on various substrate surfaces. The nature of the ALD material affects the MOF nucleation time, crystal size and morphology, and the resulting MOF surface area per unit mass. To understand MOF nucleation mechanisms, we investigate detailed Cu-BTC MOF nucleation behavior on metal oxide powders and Al2O3, ZnO, and TiO2 layers formed by ALD on polypropylene substrates. Studying both combined and sequential MOF reactant exposure conditions, we find that during solvothermal synthesis ALD metal oxides can react with the MOF metal precursor to form double hydroxy salts that can further convert to Cu-BTC MOF. The acidic organic linker can also etch or react with the surface to form MOF from an oxide metal source, which can also function as a nucleation agent for Cu-BTC in the mixed solvothermal solution. We discuss the implications of these results for better controlled thin film MOF nucleation and growth.

Large effect of titanium precursor on surface reactivity and mechanical strength of electrospun nanofibers coated with TiO2 by atomic layer deposition

Encapsulating and functionalizing polymer nanofibers can improve the polymers chemical resistance and surface reactivity, enabling new applications including biosensing, flexible electronics, gas filtration, and chemical separations. Polymer fiber functionalization typically involves energy intensive wet chemical treatments and/or plasma exposure. Recent results show low temperature atomic layer deposition (ALD) to be a viable means to coat nanofibers with uniform and conformal inorganic and hybrid organic–inorganic layers. For this article, the authors describe how the mechanical properties of nylon-6 nanofibers are affected by ALD coatings of TiO2 and other metal oxides. They find that the stress–strain behavior of nylon-6 nanofibers depends strongly on the specific precursor chemistry used in the coating process. For ALD TiO2 coatings, titanium tetrachloride tended to embrittle the fibers, whereas titanium isopropoxide had a more subtle effect. Physical characterization shows that the TiCl4 diffused in...

Generation and sampling of nanoscale infectious viral aerosols

ABSTRACT Airborne viruses represent a potentially significant health threat. However, only recently have researchers begun to characterize the size and infectivity of viral bioaerosols in the nanoscale size range. There are limitations in the generation of test viral aerosols and the ability to sample with acceptable efficiency. Reported here is use of a laminar-flow water condensation method to efficiently sample nanoscale bioaerosols to sizes well below 100 nm. We used MS2 bacteriophage in water to provide an aerosol with particles sizes from 300 nm down to 45 nm for sampling by both an all-glass impinger (4 mm; AGI-4) and the water condensation bioaerosol sampler. We demonstrated the existence of infectious viral particles below 100 nm and a higher collection efficiency by the water condensation sampler compared to the AGI-4 at nanoscale sizes. For example, the water condensation bioaerosol sampler that collected particles at 45 nm in diameter had 20 times more infective virions per collected particle compared to the AGI-4. However, when we corrected the AGI-4 data for particle size–dependent collection efficiency, the results were similar. We also used quantitative reverse transcription polymerase chain reaction, along with culturing for infectivity to determine the percent infectivity of the aerosol by particle size. Finally, we used a simple calculation to determine that a large fraction of sub-100 nm particles did not contain infectious virus because of the low titer concentration of virus in the Collison fluid.

Photoluminescent Nanofibers for Solid-State Lighting Applications

Photoluminescent nanofibers (PLN) can be formed by combining electrospun polymeric nanofibers and luminescent particles such as quantum dots (QD). The physical properties of PLNs are dependent upon many different nanoscale parameters associated with the nanofiber, the luminescent particles, and their interactions. By understanding and manipulating these properties, the performance of the resulting optical structure can be tailored for desired end-use applications. For example, the quantum efficiency of quantum dots in the PLN structure depends upon multiple parameters including quantum dot chemistry, the method of forming the PLN nanocomposites, and preventing agglomeration of the quantum dot particles. This is especially important in solution-based electrospinning environments where some common solvents may have a detrimental effect on the performance of the PLN. With the proper control of these parameters, high quantum efficiencies can be readily obtained for PLNs. Achieving high quantum efficiencies is critical in applications such as solid-state lighting where PLNs can be an effective secondary conversion material for producing white light. Methods of optimizing the performance of PLNs through nanoscale manipulation of the nanofiber are discussed along with guidelines for tailoring the performance of nanofibers and quantum dots for application-specific requirements.

Multi-sample Couette viscometer for polymer formulations

We describe a prototype scalable viscometer aimed at facilitating rapid measurements of multiple samples. The device simultaneously measures the viscosity of four samples in an array of Couette cells. Each cell contains a rotor with an embedded permanent magnet that is driven by an oscillating magnetic field generated by a two-axis Helmholtz coil. The angular displacement of the rotors is recorded by a charge-coupled device camera. Image analysis determines the amplitude and phase of the rotors with respect to the coils sinusoidal current. The viscosity of each sample influences the amplitude and phase of the rotor, which can be predicted with a viscously damped oscillator model. We present data for Newtonian fluids with viscosities from 50 to 2000 mPa s.

Long-term viable bioaerosol sampling using a temperature- and humidity-controlled filtration apparatus, a laboratory investigation using culturable E. coli.

ABSTRACT Sampling for culturable (e.g., viable) aerosolized microbes (bioaerosols) is a useful means to provide information for public health monitoring and studies. However, it is challenging to maintain microbe culturability when sampling at high flow rates (>12 L/min) and extended periods of time (≥4 h). We developed a first-generation, viable bioaerosol collection system (VBCS) utilizing temperature (T) and relative humidity (RH)–conditioned filtration at a flow rate of 25 L/min. A two-stage system of tube-in-shell Nafion™ exchange units provides cooling to ≤10°C and RH conditioning to 80–95%. Aerosol particles are collected on a polyurethane nanofiber filter providing a physical collection efficiency of >95% for sizes 0.06–10 µm. The T and RH conditions at the collection filter are maintained, despite changes to ambient conditions. The initial testing of the VBCS was done under indoor, laboratory conditions with aerosolized, vegetative E. coli. A scenario of a 30-min challenge of bioaerosol followed by continued sampling of clean air for various times was used to judge culturability maintenance under extended-term sampling. An initial loss of culturability upon collection onto the filter was observed; 23 ± 13% relative to 4-mm all-glass impinger. However once collected, 98% of culturability was maintained for an additional 4.5 h of sampling. An exponential decay in culturability was observed from 8 h to 15 h of sampling. Also, 24-h cold storage of the filters collected was studied. The VBCS is based on the use of dry filter cassettes, needs minimal maintenance, and preserves culturability of vegetative bacteria for >4 h.

Use of nanofibers in high-efficiency solid-state lighting

Nanofibers made from non-absorbing polymers such as poly(methyl methacrylate) are solid structures that have one dimension (diameter) in the 10-1,000 nanometer (nm) range, while the other dimension (length) can be quite long. These nanofibers can be formed in either an oriented or random packing structure, and the surface morphology of the fiber can range from smooth to nanoporous. Quantum dots (QD) or other luminescent nanoparticles (diameter 1-10 nm) can be added to the nanofiber to create the photoluminescent nanofiber (PLN). Because PLNs are nanocomposites of fluorescent nanoparticles and polymer nanofibers, the optical properties of the nanocomposite, including absorption, emission, and light scattering, can be tailored for application-specific requirements. Nanofibers may have several applications in solid-state lighting, including serving as a light diffuser, providing optical filtering of low photopic sensitive wavelengths (i.e., blue) to increase conversion to higher luminosity wavelengths, and providing a convenient vehicle for handling and blending QDs to achieve a high color-rendering index.