Publications by authors named "Michael R Zachariah"

88 Publications

Magnetic-Field Directed Vapor-Phase Assembly of Low Fractal Dimension Metal Nanostructures: Experiment and Theory.

J Phys Chem Lett 2021 Apr 22;12(16):4085-4091. Epub 2021 Apr 22.

Department of Chemical and Environmental Engineering, University of California, Riverside, California 92521, United States.

While gas-phase synthesis techniques offer a scalable approach to production of metal nanoparticles, directed assembly is challenging due to fast particle diffusion rates that lead to random Brownian aggregation. This work explores an electromagnetic-levitation technique to generate metal nanoparticle aggregates with fractal dimension () below that of diffusion limited assembly. We demonstrate that in addition to levitation and induction heating, the external magnetic field is sufficient to compete with random Brownian forces, which enables the formation of altered fractals. Ferromagnetic metals (Fe, Ni) form chain-like aggregates, while paramagnetic Cu forms compact nanoparticle aggregates with higher values. We have also employed a Monte Carlo simulation to evaluate the necessary field strength to form linear chains in the gas phase.
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http://dx.doi.org/10.1021/acs.jpclett.0c03463DOI Listing
April 2021

Modelling and simulation of field directed linear assembly of aerosol particles.

J Colloid Interface Sci 2021 Jun 19;592:195-204. Epub 2021 Feb 19.

University of California, Riverside, CA 92521, United States. Electronic address:

Unlike liquid phase colloidal assembly, significantly changing the structure of fractal aggregates in the aerosol phase, is considered impractical. In this study, we discuss the possibility of applying external magnetic and electric fields, to tune the structure and fractal dimension (D) of aggregates grown in the aerosol phase. We show that external fields can be used to induce dipole moments in primary nanoparticles. We found that an ensemble of particles with induced dipole moments will interact through directional attractive and repulsive forces, leading to the formation of linear, chain-like aggregates with D ~ 1. The aggregate structure transition is dependent on the primary particle sizes, temperature and applied field strength which was evaluated by performing a hybrid ensemble/cluster-cluster aggregation Monte Carlo simulation. We demonstrate that the threshold magnetic field strength required to linearly assemble 10-500 nm particle sizes are practically achievable whereas the electric field required to assemble sub-100 nm particles are beyond the breakdown strength of most gases. To theoretically account for the enhanced coagulation rates due to attractive interactions, we have also derived a correction factor to both free molecular and transition regime coagulation kernel, based on magnetic dipolar interactions. A comparison has been made between the coagulation time-scales estimated by theory and simulation, with the estimated magnetization time-scales of the primary particles along with oscillation time period of the magnetic field, to demonstrate that sub-50 nm superparamagnetic primary particles can be magnetized and assembled at any temperature, while below the Curie temperature ferromagnetic particles of all sizes can be magnetized and assembled, given the applied field is higher than the threshold.
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http://dx.doi.org/10.1016/j.jcis.2021.02.050DOI Listing
June 2021

Revealing High-Temperature Reduction Dynamics of High-Entropy Alloy Nanoparticles Transmission Electron Microscopy.

Nano Lett 2021 Feb 11;21(4):1742-1748. Epub 2021 Feb 11.

Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, Illinois 60607, United States.

Understanding the behavior of high-entropy alloy (HEA) materials under hydrogen (H) environment is of utmost importance for their promising applications in structural materials, catalysis, and energy-related reactions. Herein, the reduction behavior of oxidized FeCoNiCuPt HEA nanoparticles (NPs) in atmospheric pressure H environment was investigated by gas-cell transmission electron microscopy (TEM). The reduction reaction front was maintained at the external surface of the oxide. During reduction, the oxide layer expanded and transformed into porous structures where oxidized Cu was fully reduced to Cu NPs while Fe, Co, and Ni remained in the oxidized form. chemical analysis showed that the expansion of the oxide layer resulted from the outward diffusion flux of all transition metals (Fe, Co, Ni, Cu). Revealing the H reduction behavior of HEA NPs facilitates the development of advanced multicomponent alloys for applications targeting H formation and storage, catalytic hydrogenation, and corrosion removal.
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http://dx.doi.org/10.1021/acs.nanolett.0c04572DOI Listing
February 2021

Silicon Nanoparticles for the Reactivity and Energetic Density Enhancement of Energetic-Biocidal Mesoparticle Composites.

ACS Appl Mater Interfaces 2021 Jan 29;13(1):458-467. Epub 2020 Dec 29.

University of California, Riverside, California 92521, United States.

Biocidal nanothermite composites show great potential in combating biological warfare threats because of their high-energy-release rates and rapid biocidal agent release. Despite their high reactivity and combustion performance, these composites suffer from low-energy density because of the voids formed due to inefficient packing of fuel and oxidizer particles. In this study, we explore the potential of plasma-synthesized ultrafine Si nanoparticles (nSi, ∼5 nm) as an energetic filler fuel to increase the energy density of Al/Ca(IO) energetic-biocidal composites by filling in the voids in the microstructure. Microscopic and elemental analyses show the partial filling of mesoparticle voids by nSi, resulting in an estimated energy density enhancement of ∼21%. In addition, constant-volume combustion cell results show that nSi addition leads to a ∼2-3-fold increase in reactivity and combustion performance, as compared to Al/Ca(IO) mesoparticles. Oxidation timescale analyses suggest that nSi addition can promote initiation due to faster oxygen transport through the oxide shell of Si nanoparticles. At nSi loadings higher than ∼8%, however, slower burning characteristics of nSi and sintering effects lead to an overall degradation of combustion behavior of the composites.
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http://dx.doi.org/10.1021/acsami.0c17159DOI Listing
January 2021

Oxidation Studies of High-Entropy Alloy Nanoparticles.

ACS Nano 2020 Nov 20;14(11):15131-15143. Epub 2020 Oct 20.

Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, Illinois 60607, United States.

Although high-entropy alloys (HEAs) have shown tremendous potential for elevated temperature, anticorrosion, and catalysis applications, little is known on how HEA materials behave under complex service environments. Herein, we studied the high-temperature oxidation behavior of FeCoNiCuPtHEA nanoparticles (NPs) in an atmospheric pressure dry air environment by gas-cell transmission electron microscopy. It is found that the oxidation of HEA NPs is governed by Kirkendall effects with logarithmic oxidation rates rather than parabolic as predicted by Wagner's theory. Further, the HEA NPs are found to oxidize at a significantly slower rate compared to monometallic NPs. The outward diffusion of transition metals and formation of disordered oxide layer are observed in real time and confirmed through analytical energy dispersive spectroscopy, and electron energy loss spectroscopy characterizations. Localized ordered lattices are identified in the oxide, suggesting the formation of FeO, CoO, NiO, and CuO crystallites in an overall disordered matrix. Hybrid Monte Carlo and molecular dynamics simulations based on first-principles energies and forces support these findings and show that the oxidation drives surface segregation of Fe, Co, Ni, and Cu, while Pt stays in the core region. The present work offers key insights into how HEA NPs behave under high-temperature oxidizing environment and sheds light on future design of highly stable alloys under complex service conditions.
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http://dx.doi.org/10.1021/acsnano.0c05250DOI Listing
November 2020

High-Temperature Pulse Method for Nanoparticle Redispersion.

J Am Chem Soc 2020 Oct 21;142(41):17364-17371. Epub 2020 Sep 21.

Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States.

Nanoparticles suffer from aggregation and poisoning issues (e.g., oxidation) that severely hinder their long-term applications. However, current redispersion approaches, such as continuous heating in oxidizing and reducing environments, face challenges including grain growth effects induced by long heating times as well as complex procedures. Herein, we report a facile and efficient redispersion process that enables us to directly transform large aggregated particles into nanoscale materials. In this method, a piece of carbon nanofiber film was used as a heater and high treatment temperature (∼1500-2000 K) is rapidly elevated and maintained for a very short period of time (100 ms), followed by fast quenching back to room temperature at a cooling rate of 10 K/s to inhibit sintering. With these conditions we demonstrate the redispersion of large aggregated metal oxide particles into metallic nanoparticles just ∼10 nm in size, uniformly distributed on the substrate. Furthermore, the metallic states of the nanoparticles are renewed during the heat treatment through reduction. The redispersion process removes impurities and poisoning elements, yet is able to maintain the integrity of the substrate because of the ultrashort heating pulse time. This method is also significantly faster (ca. milliseconds) compared to conventional redispersion treatments (ca. hours), providing a pragmatic strategy to redisperse degraded particles for a variety of applications.
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http://dx.doi.org/10.1021/jacs.0c04887DOI Listing
October 2020

Aerosol Synthesis of High Entropy Alloy Nanoparticles.

Langmuir 2020 Mar 20;36(8):1985-1992. Epub 2020 Feb 20.

University of California Riverside, Riverside, California 92521, United States.

Homogeneously mixing multiple metal elements within a single particle may offer new material property functionalities. High entropy alloys (HEAs), nominally defined as structures containing five or more well-mixed metal elements, are being explored at the nanoscale, but the scale-up to enable their industrial application is an extremely challenging problem. Here, we report an aerosol droplet-mediated technique toward scalable synthesis of HEA nanoparticles with atomic-level mixing of immiscible metal elements. An aqueous solution of metal salts is nebulized to generate ∼1 μm aerosol droplets, which when subjected to fast heating/quenching result in decomposition of the precursors and freezing-in of the zero-valent metal atoms. Atomic-level resolution scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy analysis reveals that all metal elements in the nanoparticles are homogeneously mixed at the atomic level. We believe that this approach offers a facile and flexible aerosol droplet-mediated synthesis technique that will ultimately enable bulk processing starting from a particulate HEA.
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http://dx.doi.org/10.1021/acs.langmuir.9b03392DOI Listing
March 2020

Synergistically Chemical and Thermal Coupling between Graphene Oxide and Graphene Fluoride for Enhancing Aluminum Combustion.

ACS Appl Mater Interfaces 2020 Feb 28;12(6):7451-7458. Epub 2020 Jan 28.

Department of Mechanical Engineering , Stanford University , Stanford , California 94305 , United States.

Metal combustion reaction is highly exothermic and is used in energetic applications, such as propulsion, pyrotechnics, powering micro- and nano-devices, and nanomaterials synthesis. Aluminum (Al) is attracting great interest in those applications because of its high energy density, earth abundance, and low toxicity. Nevertheless, Al combustion is hard to initiate and progresses slowly and incompletely. On the other hand, ultrathin carbon nanomaterials, such as graphene, graphene oxide (GO), and graphene fluoride (GF), can also undergo exothermic reactions. Herein, we demonstrate that the mixture of GO and GF significantly improves the performance of Al combustion as interactions between GO and GF provide heat and radicals to accelerate Al oxidation. Our experiments and reactive molecular dynamics simulation reveal that GO and GF have strong chemical and thermal couplings through radical reactions and heat released from their oxidation reactions. GO facilitates the dissociation of GF, and GF accelerates the disproportionation and oxidation of GO. When the mixture of GO and GF is added to micron-sized Al particles, their synergistic couplings generate reactive oxidative species, such as CF and CFO, and heat, which greatly accelerates Al combustion. This work demonstrates a new area of using synergistic couplings between ultrathin carbon nanomaterials to accelerate metal combustion and potentially oxidation reactions of other materials.
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http://dx.doi.org/10.1021/acsami.9b20397DOI Listing
February 2020

Quantifying protein aggregation kinetics using electrospray differential mobility analysis.

J Pharm Biomed Anal 2020 Jan 30;177:112845. Epub 2019 Aug 30.

National Institute of Standards and Technology, 100 Bureau Dr., MS 8520, Gaithersburg, MD, 20899-8520, United States; University of Maryland, College Park, United States; University of California, 900 University Ave., Riverside, CA, 92521, United States. Electronic address:

Protein aggregation is a critical concern in bioprocessing, where its presence can result in serious adverse interactions in clinical end-use applications. In this study, an aerosol-based technique, electrospray differential mobility analysis (ES-DMA), was used to quantify thermally-induced protein aggregation kinetics for bovine serum albumin (BSA) and α-chymotrypsinogen A (α-chymo), employing a new methodology to modify the solution for compatibility with the electrospray process. Results are compared orthogonally with asymmetrical-flow field-flow fractionation (AF4), a hydrodynamic separation technique with UV detection. Measurements were conducted over a range of protein concentrations and temperatures. Both techniques successfully resolved the protein monomer and dimer populations, allowing quantification of monomer loss. BSA and α-chymo exhibited second and first order kinetics, respectively, confirming different limiting steps for the two species. The Arrhenius equation yielded activation energies for BSA of (240 ± 20) kJ mol and (190 ± 10) kJ mol by ES-DMA and AF4, respectively. The rates determined by ES-DMA were equal to or slightly faster than those measured by AF4, so instrumental differences were analyzed to identify potential sources of bias. An important factor may be the applicable concentration range for each method; notably, AF4 operates at the mg mL level, while ES-DMA is sensitive at μg mL and therefore requires much smaller samples for analysis (typically several μL are injected). The limitations of each method are detailed in the discussion and demonstrate the importance of orthogonal measurement strategies for the analysis of protein kinetics. ES-DMA provides a potentially useful alternative to size exclusion chromatography to screen the stability of formulation conditions for protein therapeutics; neither ES-DMA nor AF4 rely on column interactions for separation.
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http://dx.doi.org/10.1016/j.jpba.2019.112845DOI Listing
January 2020

High temperature shockwave stabilized single atoms.

Nat Nanotechnol 2019 Sep 12;14(9):851-857. Epub 2019 Aug 12.

Department of Materials Science and Engineering, University of Maryland, College Park, MD, USA.

The stability of single-atom catalysts is critical for their practical applications. Although a high temperature can promote the bond formation between metal atoms and the substrate with an enhanced stability, it often causes atom agglomeration and is incompatible with many temperature-sensitive substrates. Here, we report using controllable high-temperature shockwaves to synthesize and stabilize single atoms at very high temperatures (1,500-2,000 K), achieved by a periodic on-off heating that features a short on state (55 ms) and a ten-times longer off state. The high temperature provides the activation energy for atom dispersion by forming thermodynamically favourable metal-defect bonds and the off-state critically ensures the overall stability, especially for the substrate. The resultant high-temperature single atoms exhibit a superior thermal stability as durable catalysts. The reported shockwave method is facile, ultrafast and universal (for example, Pt, Ru and Co single atoms, and carbon, CN and TiO substrates), which opens a general route for single-atom manufacturing that is conventionally challenging.
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http://dx.doi.org/10.1038/s41565-019-0518-7DOI Listing
September 2019

Ultrafast, Controllable Synthesis of Sub-Nano Metallic Clusters through Defect Engineering.

ACS Appl Mater Interfaces 2019 Aug 8;11(33):29773-29779. Epub 2019 Aug 8.

Department of Materials Science and Engineering , University of Maryland , College Park , Maryland 20742 , United States.

Supported metallic nanoclusters (NCs, < 2 nm) are of great interests in various catalytic reactions with enhanced activities and selectivities, yet it is still challenging to efficiently and controllably synthesize ultrasmall NCs with a high-dispersal density. Here we report the in situ synthesis of surfactant-free, ultrasmall, and uniform NCs via a rapid thermal shock on defective substrates. This is achieved by using high-temperature synthesis with extremely fast kinetics while limiting the synthesis time down to milliseconds (e.g., ∼1800 K for 55 ms) to avoid aggregation. Through defect engineering and optimized loading, the particle size can be robustly tuned from >50 nm nanoparticles to <1 nm uniform NCs with a high-dispersal density. We demonstrate that the ultrasmall NCs exhibit drastically improved activities for catalytic CO oxidation as compared to their nanoparticulated counterparts. In addition, the reported method shows generality in synthesizing most metallic NCs (e.g., Pt, Ru, Ir, Ni) in an extremely facile and efficient manner. The ultrafast and controllable synthesis of uniform, high-density, and size-controllable NCs paves the way for the utilization and nanomanufacturing of NCs for a range of catalytic reactions.
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http://dx.doi.org/10.1021/acsami.9b07198DOI Listing
August 2019

In-operando high-speed microscopy and thermometry of reaction propagation and sintering in a nanocomposite.

Nat Commun 2019 Jul 10;10(1):3032. Epub 2019 Jul 10.

Department of Chemical and Environmental Engineering, University of California, Riverside, CA, 92521, USA.

An important proposed mechanism in nanothermites reactions - reactive sintering - plays a significant role on the combustion performance of nanothermites by rapidly melting and coalescing aggregated metal nanoparticles, which increases the initial size of the reacting composite powders before burning. Here, we demonstrate a high-speed microscopy/thermometry capability that enables ~ µs time and ~ µm spatial resolution as applied to highly exothermic reaction propagation to directly observe reactive sintering and the reaction front at high spatial and temporal resolution. Experiments on the Al+CuO nanocomposite system reveal a reaction front thickness of ~30 μm and temperatures in excess of 3000 K, resulting in a thermal gradient in excess of 10 K m. The local microscopic reactive sintering velocity is found to be an order of magnitude higher than macroscale flame velocity. In this observed mechanism, propagation is very similar to the general concept of laminar gas reaction theory in which reaction front velocity ~ (thermal diffusivity x reaction rate).
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http://dx.doi.org/10.1038/s41467-019-10843-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6620330PMC
July 2019

Ultrafast, scalable laser photothermal synthesis and writing of uniformly dispersed metal nanoclusters in polymer films.

Nanoscale 2019 Jul 4;11(28):13354-13365. Epub 2019 Jul 4.

Department of Chemical and Environmental Engineering, University of California, Riverside, CA 92507, USA.

This paper presents a fast CO laser synthesis and writing technique - laser photothermal synthesis and writing (LPSW) - to generate and write a high concentration of unaggregated, spherical sub-10 nm metal nanoparticles (sMNPs). The method is generic, and we demonstrate the fabrication of Ni, Cu, and Ag directly in polymer thin films. A partly IR-absorbing thin polymer film can be heated by the laser to high temperatures in a short time, triggering metal-reduction, nucleation, and growth. Rapid quenching of polymer films suppresses particle diffusion and traps the generated sMNPs in the polymer film. As a result, these particles are immobilized in the laser illuminated spot ("written" by the laser) on quenching. Here, Ag-polymer films are used as a model to demonstrate how laser parameters - pulse duration, laser energy flux, and number of pulses (pulsed thermal load) - can be varied to tune particle size distributions of metal sMNPs. Using this approach, we have been able to generate 4-12 nm Ag sMNPs with thermal pulses as short as 35 ms. Fast heating timescales employed in this approach allow for the scalable manufacturing of high yields of metal sMNPs, which we estimate to be around 1 g min. This rapid, general synthesis and writing technique may have potentially important applications in fast, large-scale additive manufacturing and patterning of metal-loaded polymer multilayers, flexible electronics, and sensors.
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http://dx.doi.org/10.1039/c9nr02839kDOI Listing
July 2019

Vapor-Phase Strategy to Pillaring of Two-Dimensional Zeolite.

J Am Chem Soc 2019 06 28;141(22):8712-8716. Epub 2019 May 28.

Department of Chemical and Biomolecular Engineering , University of Maryland , College Park , Maryland 20742 , United States.

Two-dimensional (2D) layered zeolites are new forms of 3D zeolite frameworks. They can be pillared to form more open porous structures with increased access for reactants that are too big for the micropores of zeolites. The current pillaring procedure, however, requires intercalation of pillaring precursors by dispersing 2D zeolite in an alkoxide liquid and hydrolizing entrapped alkoxide to form inorganic oxide pillars in an aqueous alkaline solution. Both steps use excess solvents, generate significant waste, and require multiple synthesis and separation steps. Here we report a vapor-phase pillarization (VPP) process to produce pillared zeolites from 2D layered zeolite structures. The VPP process has ∼100% efficiency of alkoxide usage in the intercalation step, requires less (and, in some cases, zero) water addition in the hydrolysis step, does not require separation for product recovery, and generates no liquid waste. Furthermore, synthesis of pillared zeolites via the VPP process can be accomplished within a single apparatus with one-time operation. The pillared zeolite prepared by the VPP method preserved the zeolitic layered structure as well as acidity and showed enhancement in catalytic alkylation of mesitylene with benzyl alcohol compared to 2D layered zeolite without pillarization treatment.
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http://dx.doi.org/10.1021/jacs.9b03479DOI Listing
June 2019

Direct Writing of a 90 wt% Particle Loading Nanothermite.

Adv Mater 2019 Jun 16;31(23):e1806575. Epub 2019 Apr 16.

Department of Chemical and Environmental Engineering, The University of California, Riverside, CA, 92521, USA.

The additive manufacturing of energetic materials has received worldwide attention. Here, an ink formulation is developed with only 10 wt% of polymers, which can bind a 90 wt% nanothermite using a simple direct-writing approach. The key additive in the ink is a hybrid polymer of poly(vinylidene fluoride) (PVDF) and hydroxy propyl methyl cellulose (HPMC) in which the former serves as an energetic initiator and a binder, and the latter is a thickening agent and the other binder, which can form a gel. The rheological shear-thinning properties of the ink are critical to making the formulation at such high loadings printable. The Young's modulus of the printed stick is found to compare favorably with that of poly(tetrafluoroethylene) (PTFE), with a particle packing density at the theoretical maximum. The linear burn rate, mass burn rate, flame temperature, and heat flux are found to be easily adjusted by varying the fuel/oxidizer ratio. The average flame temperatures are as high as ≈2800 K with near-complete combustion being evident upon examination of the postcombustion products.
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http://dx.doi.org/10.1002/adma.201806575DOI Listing
June 2019

Fast quantification of nanorod geometry by DMA-spICP-MS.

Analyst 2019 Mar;144(7):2275-2283

Materials Measurement Science Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.

A fast, quantitative method for determining the dimensions of nanorods (i.e., length and diameter) is described, based on hyphenation of differential mobility analysis (DMA) with single particle inductively coupled plasma mass spectrometry (spICP-MS). Seven gold nanorod samples with different dimensions (diameters 11.8 nm to 38.2 nm, aspect ratios 1.8 to 6.9) were used to validate the method. We demonstrate that DMA-spICP-MS can (1) achieve quantification of both length and diameter comparable with TEM analysis, (2) make statistically meaningful measurements in minutes at low concentrations (<108 mL-1) and (3) separate nanorods from spheres and quantify the geometry of each population. A robustness analysis of this method was performed to evaluate potential biases in this approach.
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http://dx.doi.org/10.1039/c8an02250jDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692075PMC
March 2019

Thermal Shock Synthesis of Metal Nanoclusters within On-the-Fly Graphene Particles.

Langmuir 2019 Mar 21;35(9):3413-3420. Epub 2019 Feb 21.

Department of Chemical and Environmental Engineering , University of California, Riverside , Riverside , California 92521 , United States.

Metal nanoclusters (1-10 nm) have drawn great attention because of their potential applications including energy storage, catalysis, nanomedicine, and electronic devices. However, manufacturing ultrasmall metal nanoparticles at high concentrations in an unaggregated state is not a solved problem. Here, we report an aerosol-based thermal shock technique for in situ synthesis of well-dispersed metal nanoclusters in on-the-fly graphene aerosols. A rapid thermal shock to the graphene aerosol has been used to nucleate and grow the metal nanoclusters with subsequent quenching to freeze the newly formed nanoclusters in the graphene aerosol matrix. A characteristic time analysis comparison with the experiment shows that the nanocluster formation is governed by nucleation and subsequent surface growth and that the graphene retards coagulation, enabling unaggregated metal nanoclusters. The method is generic, and we show the formation of sub-10 nm Ni, Co, and Sn nanoclusters. This continuous aerosol-based thermal shock technique offers considerable potential for the scalable synthesis of well-dispersed and uniform metal nanoclusters stabilized within a host matrix. As an example of potential application, we demonstrate very favorable catalytic properties.
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http://dx.doi.org/10.1021/acs.langmuir.8b03532DOI Listing
March 2019

What atomic properties of metal oxide control the reaction threshold of solid elemental fuels?

Phys Chem Chem Phys 2018 Oct;20(42):26885-26891

Department of Chemistry and Biochemistry and Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20740, USA.

The redox reaction between fuel (metal, metalloid, etc.) and metal oxide is ubiquitous. On the other hand simple thermodynamic considerations do not seem to yield much insight into what makes for a vigorous oxidizer. In this study, two different systematically doped metal oxide systems: perovskites (9 compounds) and δ-Bi2O3 (12 compounds) were synthesized in a manner such that for each system the crystal structure and morphology were maintained. Four fuels (Al, B, Ta, C) with different physical properties, covering almost all fuel types, were included in this study. The initiation temperature and oxygen release temperature was measured by fast heating (>105 K s-1) temperature-jump/time-of-flight mass spectrometry coupled with high-speed imaging. These results were then correlated with the average metal-oxygen bond energy in the oxidizer, and overall metal-oxygen electronegativity. In general, within each systematic metal oxide, we found linear relationships between average bond energy and electronegativity of the metal oxides with initiation temperature for all four fuels, despite their very different physical/chemical properties. These results indicate that intrinsic atomic properties of metal oxide control fuel-metal oxide reaction initiation.
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http://dx.doi.org/10.1039/c8cp01671bDOI Listing
October 2018

Mechanistic Studies of [AlCp*] Combustion.

Inorg Chem 2018 Jul 5;57(14):8181-8188. Epub 2018 Jul 5.

Department of Chemistry and Biochemistry , University of Maryland-College Park , College Park , Maryland 20742 , United States.

The combustion mechanism of [AlCp*] (Cp* = pentamethylcyclopentadienyl), a ligated aluminum(I) cluster, was studied by a combination of experimental and theoretical methods. Two complementary experimental methods, temperature-programmed reaction and T-jump time-of-flight mass spectrometry, were used to investigate the decomposition behaviors of [AlCp*] in both anaerobic and oxidative environments, revealing AlCp* and AlOCp* to be the major decomposition products. The observed product distribution and reaction pathways are consistent with the prediction from molecular dynamics simulations and static density functional theory calculations. These studies demonstrated that experiment and theory can indeed serve as complementary and predictive means to study the combustion behaviors of ligated aluminum clusters and may help in engineering stable compounds as candidates for rocket propellants.
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http://dx.doi.org/10.1021/acs.inorgchem.8b00589DOI Listing
July 2018

Carbothermal shock synthesis of high-entropy-alloy nanoparticles.

Science 2018 03;359(6383):1489-1494

Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA.

The controllable incorporation of multiple immiscible elements into a single nanoparticle merits untold scientific and technological potential, yet remains a challenge using conventional synthetic techniques. We present a general route for alloying up to eight dissimilar elements into single-phase solid-solution nanoparticles, referred to as high-entropy-alloy nanoparticles (HEA-NPs), by thermally shocking precursor metal salt mixtures loaded onto carbon supports [temperature ~2000 kelvin (K), 55-millisecond duration, rate of ~10 K per second]. We synthesized a wide range of multicomponent nanoparticles with a desired chemistry (composition), size, and phase (solid solution, phase-separated) by controlling the carbothermal shock (CTS) parameters (substrate, temperature, shock duration, and heating/cooling rate). To prove utility, we synthesized quinary HEA-NPs as ammonia oxidation catalysts with ~100% conversion and >99% nitrogen oxide selectivity over prolonged operations.
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http://dx.doi.org/10.1126/science.aan5412DOI Listing
March 2018

Calculating the rotational friction coefficient of fractal aerosol particles in the transition regime using extended Kirkwood-Riseman theory.

Phys Rev E 2017 Jul 18;96(1-1):013110. Epub 2017 Jul 18.

Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, USA.

We apply our extended Kirkwood-Riseman theory to compute the translation, rotation, and coupling friction tensors and the scalar rotational friction coefficient for an aerosol fractal aggregate in the transition flow regime. The method can be used for particles consisting of spheres in contact. Our approach considers only the linear velocity of the primary spheres in a rotating aggregate and ignores rotational and coupling interactions between spheres. We show that this simplified approach is within approximately 40% of the true value for any particle for Knudsen numbers between 0.01 and 100. The method is especially accurate (i.e., within about 5%) near the free-molecule regime, where there is little interaction between the particle and the flow field, and for particles with low fractal dimension (≲2) consisting of many spheres, where the average distance between spheres is large and translational interaction effects dominate. Our results suggest that there is a universal relationship between the rotational friction coefficient and an aggregate Knudsen number, defined as the ratio of continuum to free-molecule rotational friction coefficients.
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http://dx.doi.org/10.1103/PhysRevE.96.013110DOI Listing
July 2017

Growth of Sub-5 nm Metal Nanoclusters in Polymer Melt Aerosol Droplets.

Langmuir 2018 01 5;34(2):585-594. Epub 2018 Jan 5.

University of Maryland , College Park, Maryland 20742, United States.

Ultrasmall metal nanoparticles are inherently unstable because of their high specific surface area. This work investigates how growth and aggregation of these nanostructures can be circumvented by incorporating them into a polymer matrix in an on-the-fly growth process. We demonstrate the formation of sub-5 nm particles of Ni, Co, and Cu nanoparticles in a polymer matrix using an aerosol single-drop reactor approach. The rapid thermal pulse given to the aerosol particles enables the formation of nuclei and growth, with subsequent rapid quenching to freeze in the structure. The role of the temperature as well as the precursor concentration of the resulting size and morphology is discussed. A characteristic time analysis and an analysis of the particle size distributions lead to the conclusion that growth is governed by nucleation and surface growth, with little coagulation or Ostwald ripening. Finally, we note that this aerosol route is amenable to scale-up for large-scale production of nanoclusters that can either be used as is within the polymer or released by solvent extraction, depending on the application.
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http://dx.doi.org/10.1021/acs.langmuir.7b02900DOI Listing
January 2018

Surface Modification of Cisplatin-Complexed Gold Nanoparticles and Its Influence on Colloidal Stability, Drug Loading, and Drug Release.

Langmuir 2018 Jan 18;34(1):154-163. Epub 2017 Dec 18.

University of Maryland, College Park, Maryland 20742, United States.

Cisplatin-complexed gold nanoparticles (Pt-AuNP) provide a promising strategy for chemo-radiation-based anticancer drugs. Effective design of such platforms necessitates reliable assessment of surface engineering on a quantitative basis and its influence on drug payload, stability, and release. In this paper, poly(ethylene glycol) (PEG)-stabilized Pt-AuNP was synthesized as a model antitumor drug platform, where Pt is attached via a carboxyl-terminated dendron ligand. Surface modification by PEG and its influence on drug loading, colloidal stability, and drug release were assessed. Complexation with Pt significantly degrades colloidal stability of the conjugate; however, PEGylation provides substantial improvement of stability in conjunction with an insignificant trade-off in drug loading capacity compared with the non-PEGylated control (<20% decrease in loading capacity). In this context, the effect of varying PEG concentration and molar mass was investigated. On a quantitative basis, the extent of PEGylation was characterized and its influence on dispersion stability and drug load was examined using electrospray differential mobility analysis (ES-DMA) hyphenated with inductively coupled plasma mass spectrometry (ICP-MS) and compared with attenuated total reflectance-FTIR. Using ES-DMA-ICP-MS, AuNP conjugates were size-classified based on their electrical mobility, while Pt loading was simultaneously quantified by determination of Pt mass. Colloidal stability was quantitatively evaluated in biologically relevant media. Finally, the pH-dependent Pt release performance was evaluated. We observed 9% and 16% Pt release at drug loadings of 0.5 and 1.9 Pt/nm, respectively. The relative molar mass of PEG had no significant influence on Pt uptake or release performance, while PEGylation substantially improved the colloidal stability of the conjugate. Notably, the Pt release over 10 days (examined at 0.5 Pt/nm drug loading) remained constant for non-PEGylated, 1K-PEGylated, and 5K-PEGylated conjugates.
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http://dx.doi.org/10.1021/acs.langmuir.7b02354DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6057618PMC
January 2018

Dimethyl Methylphosphonate Adsorption Capacities and Desorption Energies on Ordered Mesoporous Carbons.

ACS Appl Mater Interfaces 2017 Nov 8;9(46):40638-40644. Epub 2017 Nov 8.

Department of Chemistry and Biochemistry, University of Maryland , College Park, Maryland 20742, United States.

In this study, we determine effective adsorption capacities and desorption energies for DMMP with highly ordered mesoporous carbons (OMCs), 1D cylindrical FDU-15, 3D hexagonal CMK-3, 3D bicontinuous CMK-8, and as a reference, microporous BPL carbon. After exposure to DMMP vapor at room temperature for approximately 70 and 800 h, the adsorption capacity of DMMP for each OMC was generally proportional to the total surface area and pore volume, respectively. Desorption energies of DMMP were determined using a model-free isoconversional method applied to thermogravimetric analysis (TGA) data. Our experiments determined that DMMP saturated carbon will desorb any weakly bound DMMP from pores >2.4 nm at room temperature, and no DMMP will adsorb into pores smaller than 0.5 nm. The calculated desorption energies for high surface coverages, 25% DMMP desorbed from pores ≤2.4 nm, are 68-74 kJ mol, which is similar to the DMMP heat of vaporization (52 kJ mol). At lower surface coverages, 80% DMMP desorbed, the DMMP desorption energies from the OMCs are 95-103 kJ mol. This is overall 20-30 kJ mol higher in comparison to that of BPL carbon, due to the pore size and diffusion through different porous networks.
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http://dx.doi.org/10.1021/acsami.7b12033DOI Listing
November 2017

Doped δ-bismuth oxides to investigate oxygen ion transport as a metric for condensed phase thermite ignition.

Phys Chem Chem Phys 2017 May;19(20):12749-12758

Department of Chemical and Biomolecular Engineering and Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA.

Nanothermites offer high energy density and high burn rates, but are mechanistically only now being understood. One question of interest is how initiation occurs and how the ignition temperature is related to microscopic controlling parameters. In this study, we explored the potential role of oxygen ion transport in BiO as a controlling mechanism for condensed phase ignition reaction. Seven different doped δ-BiO were synthesized by aerosol spray pyrolysis. The ignition temperatures of Al/doped BiO, C/doped BiO and Ta/doped BiO were measured by temperature-jump/time-of-flight mass spectrometer coupled with a high-speed camera respectively. These results were then correlated to the corresponding oxygen ion conductivity (directly proportional to ion diffusivity) for these doped BiO measured by impedance spectroscopy. We find that ignition of thermite with doped BiO as oxidizer occurs at a critical oxygen ion conductivity (∼0.06 S cm) of doped BiO in the condensed-phase so long as the aluminum is in a molten state. These results suggest that oxygen ion transport limits the condensed state BiO oxidized thermite ignition. We also find that the larger oxygen vacancy concentration and the smaller metal-oxide bond energy in doped BiO, the lower the ignition temperature. The latter suggests that we can consider the possibility of manipulating microscopic properties within a crystal, to tune the resultant energetic properties.
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http://dx.doi.org/10.1039/c6cp08532fDOI Listing
May 2017

High Temperature Synthesis of Single-Component Metallic Nanoparticles.

ACS Cent Sci 2017 Apr 13;3(4):294-301. Epub 2017 Apr 13.

Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States.

Nanoparticles (NPs) dispersed within a conductive host are essential for a range of applications including electrochemical energy storage, catalysis, and energetic devices. However, manufacturing high quality NPs in an efficient manner remains a challenge, especially due to agglomeration during assembly processes. Here we report a rapid thermal shock method to synthesize well-dispersed NPs on a conductive fiber matrix using metal precursor salts. The temperature of the carbon nanofibers (CNFs) coated with metal salts was ramped from room temperature to ∼2000 K in 5 ms, which corresponds to a rate of 400,000 K/s. Metal salts decompose rapidly at such high temperatures and nucleate into metallic nanoparticles during the rapid cooling step (cooling rate of ∼100,000 K/s). The high temperature duration plays a critical role in the size and distribution of the nanoparticles: the faster the process is, the smaller the nanoparticles are, and the narrower the size distribution is. We also demonstrated that the peak temperature of thermal shock can reach ∼3000 K, much higher than the decomposition temperature of many salts, which ensures the possibility of synthesizing various types of nanoparticles. This universal, , high temperature thermal shock method offers considerable potential for the bulk synthesis of unagglomerated nanoparticles stabilized within a matrix.
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http://dx.doi.org/10.1021/acscentsci.6b00374DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5408342PMC
April 2017

Direct In Situ Mass Specific Absorption Spectra of Biomass Burning Particles Generated from Smoldering Hard and Softwoods.

Environ Sci Technol 2017 May 4;51(10):5622-5629. Epub 2017 May 4.

Material Measurement Laboratory, National Institute of Standards and Technology , Gaithersburg, Maryland 20899, United States.

Particles from smoldering biomass burning (BB) represent a major source of carbonaceous aerosol in the terrestrial atmosphere. In this study, mass specific absorption spectra of laboratory-generated smoldering wood particles (SWP) from 3 hardwood and 3 softwood species were measured in situ. Absorption data spanning from λ = 500 to 840 nm were collected using a photoacoustic spectrometer coupled to a supercontinuum laser with a tunable wavelength and bandwidth filter. SWP were size- (electrical mobility) and mass-selected prior to optical characterization allowing data to be reported as mass-specific absorption cross sections (MAC). The median measured MAC at λ = 660 nm for smoldering oak particles was 1.1 (0.57/1.8) × 10 m g spanning from 83 femtograms (fg) to 517 fg (500 nm ≤ mobility diameter ≤950 nm), MAC values in parentheses are the 16 and 84 percentiles of the measured data (i.e., 1σ). The collection of all six wood species (Oak, Hickory, Mesquite, Western redcedar, Baldcypress, and Blue spruce) had median MAC values ranging from 1.4 × 10 m g to 7.9 × 10 m g at λ = 550 nm with absorption Ångström exponents (AAE) between 3.5 and 6.2. Oak, Western redcedar, and Blue spruce possessed statistically similar (p > 0.05) spectra while the spectra of Hickory, Mesquite, and Baldcypress were distinct (p < 0.01) as calculated from a point-by-point analysis using the Wilcox rank-sum test.
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http://dx.doi.org/10.1021/acs.est.7b00810DOI Listing
May 2017

Friction factor for aerosol fractal aggregates over the entire Knudsen range.

Phys Rev E 2017 Jan 4;95(1-1):013103. Epub 2017 Jan 4.

Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, USA.

We develop an approach for computing the hydrodynamic friction tensor and scalar friction coefficient for an aerosol fractal aggregate in the transition regime. Our approach involves solving the Bhatnagar-Gross-Krook equation for the velocity field around a sphere and using the velocity field to calculate the force on each primary sphere in the aggregate due to the presence of the other spheres. It is essentially an extension of Kirkwood-Riseman theory from the continuum flow regime to the entire Knudsen range (Knudsen number from 0.01 to 100 based on the primary sphere radius). Our results compare well to published direct simulation Monte Carlo results, and they converge to the correct continuum and free molecule limits. Our calculations for clusters with up to 100 spheres support the theory that aggregate slip correction factors collapse to a single curve when plotted as a function of an appropriate aggregate Knudsen number. This self-consistent-field approach calculates the friction coefficient very quickly, so the approach is well-suited for testing existing scaling laws in the field of aerosol science and technology, as we demonstrate for the adjusted sphere scaling method.
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http://dx.doi.org/10.1103/PhysRevE.95.013103DOI Listing
January 2017

Ultra-fast self-assembly and stabilization of reactive nanoparticles in reduced graphene oxide films.

Nat Commun 2016 08 12;7:12332. Epub 2016 Aug 12.

Department of Materials Science and Engineering, University of Maryland College Park, 1208 Engineering Lab Building, College Park, Maryland 20742, USA.

Nanoparticles hosted in conductive matrices are ubiquitous in electrochemical energy storage, catalysis and energetic devices. However, agglomeration and surface oxidation remain as two major challenges towards their ultimate utility, especially for highly reactive materials. Here we report uniformly distributed nanoparticles with diameters around 10 nm can be self-assembled within a reduced graphene oxide matrix in 10 ms. Microsized particles in reduced graphene oxide are Joule heated to high temperature (∼1,700 K) and rapidly quenched to preserve the resultant nano-architecture. A possible formation mechanism is that microsized particles melt under high temperature, are separated by defects in reduced graphene oxide and self-assemble into nanoparticles on cooling. The ultra-fast manufacturing approach can be applied to a wide range of materials, including aluminium, silicon, tin and so on. One unique application of this technique is the stabilization of aluminium nanoparticles in reduced graphene oxide film, which we demonstrate to have excellent performance as a switchable energetic material.
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http://dx.doi.org/10.1038/ncomms12332DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4990634PMC
August 2016

Electrospray-Differential Mobility Hyphenated with Single Particle Inductively Coupled Plasma Mass Spectrometry for Characterization of Nanoparticles and Their Aggregates.

Anal Chem 2016 09 15;88(17):8548-55. Epub 2016 Aug 15.

University of Maryland , College Park, Maryland 20742, United States.

The novel hyphenation of electrospray-differential mobility analysis with single particle inductively coupled plasma mass spectrometry (ES-DMA-spICPMS) was demonstrated with the capacity for real-time size, mass, and concentration measurement of nanoparticles (NPs) on a particle-to-particle basis. In this proof-of-concept study, the feasibility of this technique was validated through both concentration and mass calibration using NIST gold NP reference materials. A detection limit of 10(5) NPs mL(-1) was determined under current experimental conditions, which is about 4 orders of magnitude lower in comparison to that of a traditional ES-DMA setup using a condensation particle counter as detector. Furthermore, independent and simultaneous quantification of both size and mass of NPs provides information regarding NP aggregation states. Two demonstrative applications include gold NP mixtures with a broad size range (30-100 nm), and aggregated gold NPs with a primary size of 40 nm. Finally, this technique was shown to be potentially useful for real-world samples with high ionic background due to its ability to remove dissolved ions yielding a cleaner background. Overall, we demonstrate the capacity of this new hyphenated technique for (1) clearly resolving NP populations from a mixture containing a broad size range; (2) accurately measuring a linear relationship, which should inherently exist between mobility size and one-third power of ICPMS mass for spherical NPs; (3) quantifying the early stage propagation of NP aggregation with well-characterized oligomers; and (4) differentiating aggregated NPs and nonaggregated states based on the "apparent density" derived from both DMA size and spICPMS mass.
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http://dx.doi.org/10.1021/acs.analchem.6b01544DOI Listing
September 2016