The formation kinetics of metal nanoparticles are generally described via mass transport and thermodynamics-based models, such as diffusion limited growth and classical nucleation theory (CNT). However, metal monomers are commonly assumed as precursors, leaving the identity of molecular intermediates and their contribution to nanoparticle formation unclear. Here we utilize liquid phase transmission electron microscopy (LPTEM) and reaction kinetic modeling to establish the nucleation and growth mechanisms and discover molecular intermediates during silver nanoparticle formation. Quantitative LPTEM measurements showed that their nucleation rate decreased while growth rate was nearly invariant with electron dose rate. Reaction kinetic simulations showed that Ag4 and Ag- followed a statistically similar dose rate dependence as the experimentally determined growth rate. We demonstrate that experimental growth rates are consistent with diffusion limited growth via attachment of these species to nanoparticles. Dose rate dependence of nucleation rate was inconsistent with CNT. We propose a reaction limited nucleation mechanism and demonstrate that experimental nucleation kinetics are consistent with Ag42+ aggregation rates at millisecond time scales. Reaction throughput analysis of the kinetic simulations uncovered formation and decay pathways mediating intermediate concentrations. The work demonstrates the power of quantitative LPTEM combined with kinetic modeling for establishing nanoparticle formation mechanisms and the principal intermediates.

A key challenge encountered by printed electronics is that the conductivity of sintered metal nanoparticle (NP) traces is always several times smaller than the bulk metal conductivity. Identifying the relative roles of the voids and the residual polymers on NP surfaces in sintered NP traces, in determining such reduced conductivity, is essential. In this paper, we employ a combination of electron microscopy imaging and detailed simulations to quantify the relative roles of such voids and residual polymers in the conductivity of sintered traces of a commercial (Novacentrix) silver nanoparticle-based ink. High resolution transmission electron microscopy imaging revealed details of the morphology of the inks before and after being sintered at 150 °C. Prior to sintering, NPs were randomly close packed into aggregates with nanometer thick polymer layers in the interstices. The 2D porosity in the aggregates prior to sintering was near 20%. After heating at 150 °C, NPs sintered together into dense aggregates (nanoaggregates or NAgs) with sizes ranging from 100 to 500 nm and the 2D porosity decreased to near 10%. Within the NAgs, the NPs were mostly connected via sintered metal bridges, while the outer surfaces of the NAgs were coated with a nanometer thick layer of polymer. Motivated by these experimental results, we developed a computational model for calculating the effective conductivity of the ink deposit represented by a prototypical NAg consisting of NPs connected by metallic bonds and having a polymer layer on its outer surface placed in a surrounding medium. The calculations reveal that a NAg that is 35%-40% covered by a nanometer thick polymeric layer has a similar conductivity compared to prior experimental measurements. The findings also demonstrate that the conductivity is less influenced by the polymer layer thickness or the absolute value of the NAg dimensions. Most importantly, we are able to infer that the reduced value of the conductivity of the sintered traces is less dependent on the void fraction and is primarily attributed to the incomplete removal of the polymeric material even after sintering.

Journal Article Probing Reaction Intermediates, Kinetics, and Surface Chemistry during Nanoparticle Synthesis and Assembly with Liquid Phase TEM Get access Taylor Woehl, Taylor Woehl Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, College Park, MD 20742, USA Search for other works by this author on: Oxford Academic Google Scholar Mei Wang, Mei Wang Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, College Park, MD 20742, USA Search for other works by this author on: Oxford Academic Google Scholar Umesha Dissanayake, Umesha Dissanayake Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, College Park, MD 20742, USA Search for other works by this author on: Oxford Academic Google Scholar Jiayue Sun, Jiayue Sun Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA Search for other works by this author on: Oxford Academic Google Scholar Asher Leff Asher Leff Sensors and Electron Devices Directorate, Combat Capabilities Development Command, United States Army Research Laboratory, Adelphi, MD 20783, United StatesGeneral Technical Services, LLC, Wall Township, New Jersey 07727, United States Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 1822–1823, https://doi.org/10.1017/S1431927622007188 Published: 01 August 2022

An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

Colloidal synthesis of alloyed multimetallic nanocrystals with precise composition control remains a challenge and a critical missing link in theory-driven rational design of functional nanomaterials. Liquid phase transmission electron microscopy (LP-TEM) enables directly visualizing nanocrystal formation mechanisms that can inform discovery of design rules for colloidal multimetallic nanocrystal synthesis, but it remains unclear whether the salient chemistry of the flask synthesis is preserved in the extreme electron beam radiation environment during LPTEM. Here we demonstrate controlled in situ LP-TEM synthesis of alloyed AuCu nanoparticles while maintaining the molecular structure of electron beam sensitive metal thiolate precursor complexes. Ex situ flask synthesis experiments showed that nearly equimolar AuCu alloys formed from heteronuclear metal thiolate complexes, while gold-rich alloys formed in their absence. Systematic dose rate-controlled in situ LP-TEM synthesis experiments established a range of electron beam synthesis conditions that formed alloyed AuCu nanoparticles with similar alloy composition, random alloy structure, and particle size distribution shape as those from ex situ flask synthesis, indicating metal thiolate complexes were preserved under these conditions. Reaction kinetic simulations of radical-ligand reactions revealed that polymer capping ligands acted as effective hydroxyl radical scavengers during LP-TEM synthesis and prevented metal thiolate oxidation at low dose rates. In situ synthesis experiments and ex situ atomic scale imaging revealed that a key role of metal thiolate complexes was to prevent copper atom oxidation and facilitate formation of prenucleation cluster intermediates. This work demonstrates that complex ion precursor chemistry can be maintained during LP-TEM imaging, enabling probing nanocrystal formation mechanisms with LP-TEM under reaction conditions representative of ex situ flask synthesis.

Observations of nanoparticle superlattice formation over minutes during colloidal nanoparticle synthesis elude description by conventional understanding of self-assembly, which theorizes superlattices require extended formation times to allow for diffusively driven annealing of packing defects. It remains unclear how nanoparticle position annealing occurs on such short time scales despite the rapid superlattice growth kinetics. Here we utilize liquid phase transmission electron microscopy to directly image the self-assembly of platinum nanoparticles into close packed supraparticles over tens of seconds during nanoparticle synthesis. Electron-beam induced reduction of an aqueous platinum precursor formed monodisperse 2-3 nm platinum nanoparticles that simultaneously self-assembled over tens of seconds into 3D supraparticles, some of which showed crystalline ordered domains. Experimentally varying the interparticle interactions (e.g., electrostatic, steric interactions) by changing precursor chemistry revealed that supraparticle formation was driven by weak attractive van der Waals forces balanced by short ranged repulsive steric interactions. Growth kinetic measurements and an interparticle interaction model demonstrated that nanoparticle surface diffusion rates on the supraparticles were orders of magnitude faster than nanoparticle attachment, enabling nanoparticles to find high coordination binding sites unimpeded by incoming particles. These results reconcile rapid self-assembly of supraparticles with the conventional self-assembly paradigm in which nanocrystal position annealing by surface diffusion occurs on a significantly shorter time scale than nanocrystal attachment.

Colloids in low-frequency (<1 kHz) oscillatory electric fields near planar electrodes aggregate in neutral pH electrolytes due to electrohydrodynamic (EHD) flow but separate in alkaline pH electrolytes. Colloid ζ-potential and electrolyte ion mobilities are thought to play roles in the underlying mechanism for this phenomenon, but a unifying theory for why particles aggregate in some electrolytes and separate in others remains to be established. Here, we show that increasing local pH near the electrode with an electrochemical reaction causes a colloidal aggregation-to-separation transition in oscillatory electric fields that induce strong attractive EHD flows. An electroactive molecule, para-benzoquinone, was electrochemically reduced at the electrode to locally increase the solution pH near the colloids. Superimposing a sufficiently large steady electrochemical potential onto an oscillatory potential caused a reversible aggregation-to-separation transition. Counterintuitively, decreasing frequency, which increases attractive EHD drag forces, caused a similar aggregation-to-separation transition. Even more interesting, multiple transitions were observed while varying the oscillatory potential. Taken together, these results suggested that the oscillatory potential induced a repulsive hydrodynamic drag force. Scaling arguments for the recently discovered asymmetric rectified electric field (AREF) showed that a repulsive AREF-induced electroosmotic (EO) flow competed with attractive EHD flow. A pairwise colloidal force balance including these competing flows exhibited flow inversions qualitatively consistent with experimentally observed aggregation-to-separation transitions. Broadly, these results emphasize the importance of AREF-induced EO flows in colloid aggregation and separation in low-frequency oscillatory electric fields.

An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

Liquid-phase transmission electron microscopy (LP-TEM) enables real-time imaging of nanoparticle self-assembly, formation, and etching with single nanometer resolution. Despite the importance of organic nanoparticle capping ligands in these processes, the effect of electron beam irradiation on surface-bound and soluble capping ligands during LP-TEM imaging has not been investigated. Here, we use correlative LP-TEM and fluorescence microscopy (FM) to demonstrate that polymeric nanoparticle ligands undergo competing crosslinking and chain scission reactions that nonmonotonically modify ligand coverage over time. Branched polyethylenimine (BPEI)-coated silver nanoparticles were imaged with dose-controlled LP-TEM followed by labeling their primary amine groups with fluorophores to visualize the local thickness of adsorbed capping ligands. FM images showed that free ligands crosslinked in the LP-TEM image area over imaging times of tens of seconds, enhancing local capping ligand coverage on nanoparticles and silicon nitride membranes. Nanoparticle surface ligands underwent chain scission over irradiation times of minutes to tens of minutes, which depleted surface ligands from the nanoparticle and silicon nitride surface. Conversely, solutions of only soluble capping ligand underwent successive crosslinking reactions with no chain scission, suggesting that nanoparticles enhanced the chain scission reactions by acting as radiolysis hotspots. The addition of a hydroxyl radical scavenger, tert-butanol, eliminated chain scission reactions and slowed the progression of crosslinking reactions. These experiments have important implications for performing controlled and reproducible LP-TEM nanoparticle imaging as they demonstrate that the electron beam can significantly alter ligand coverage on nanoparticles in a nonintuitive manner. They emphasize the need to understand and control the electron beam radiation chemistry of a given sample to avoid significant perturbations to the nanoparticle capping ligand chemistry, which are invisible in electron micrographs.

Liquid phase transmission electron microscopy (LP-TEM) enables real-time imaging of nanoparticle self-assembly, formation, and etching with single nanometer resolution. Despite the importance of organic nanoparticle capping ligands in these processes, the effect of electron beam irradiation on surface bound and soluble capping ligands during LP-TEM imaging has not been investigated. Here we use correlative LP-TEM and fluorescence microscopy (FM) to demonstrate that polymeric nanoparticle ligands undergo competing crosslinking and chain scission reactions that non-monotonically modify ligand coverage over time. Branched polyethylenimine (BPEI) coated silver nanoparticles were imaged with dose-controlled LP-TEM followed by labeling their primary amine groups with fluorophores to visualize the local thickness of adsorbed capping ligands. FM images showed that free ligands crosslinked in the LP-TEM image area over imaging times of tens of seconds, enhancing local capping ligand coverage on nanoparticles and silicon nitride membranes. Nanoparticle surface ligands underwent chain scission over irradiation times of minutes to tens of minutes, which depleted surface ligands from the nanoparticle and silicon nitride surface. Conversely, solutions of only soluble capping ligand underwent successive crosslinking reactions with no chain scission, suggesting nanoparticles enhanced the chain scission reactions by acting as radiolysis hotspots. The addition of a hydroxyl radical scavenger, tert-butanol, eliminated chain scission reactions and slowed the progression of crosslinking reactions. These experiments have important implications for performing controlled and reproducible LP-TEM nanoparticle imaging as they demonstrate the electron beam can significantly alter ligand coverage on nanoparticles in a non-intuitive manner. They emphasize the need to understand and control the electron beam radiation chemistry of a given sample to avoid significant perturbations to the nanoparticle capping ligand chemistry, which are invisible in electron micrographs.

An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

Nucleation is ubiquitous during liquid phase crystallization processes, both in natural and synthetic systems [1], and controls phenomena such as polymorph selection during crystallization of drug molecules and proteins, biomineral nucleation, in vivo aggregation of pathogenic proteins, and synthesis of nanomaterials. Except for very clean systems, nucleation is almost always heterogeneous because the presence of a solid interface significantly decreases the free energy barrier to nucleation [2]. The free energy barrier for a given heterogeneous nucleation site depends sensitively on the local surface topography and surface chemistry (e.g. functional groups). The identity and kinetics of nucleation at heterogeneous nucleation sites on a liquid-solid interface remains elusive because it is challenging to determine the free energy barrier for a given nucleation site with high spatial resolution.

Nucleation underlies the formation of many liquid-phasesynthetic and natural materials with applications in materials chemistry, geochemistry,biophysics, and structural biology. Most liquid-phase nucleation processes areheterogeneous, occurring at specific nucleation sites at a solid-liquidinterface; however, the chemical and topographical identity of these nucleationsites and how nucleation kinetics vary from site-to-site remains mysterious. Herewe utilize in situ liquid cell electronmicroscopy to unveil counterintuitive nanoscale non-uniformities inheterogeneous nucleation kinetics on a macroscopically uniform solid-liquidinterface. Time-resolved in situ electronmicroscopy imaging of silver nanoparticle nucleation at a water-silicon nitrideinterface showed apparently randomly-located nucleation events at the interface.However, nanometric maps of local nucleation kinetics uncovered nanoscale interfacialdomains with either slow or rapid nucleation. Interestingly, the interfacialdomains vanished at high supersaturation ratio, giving way to rapid spatiallyuniform nucleation kinetics. Atomic force microscopy and nanoparticle labelingexperiments revealed a topographically flat, chemically heterogeneous interfacewith nanoscale interfacial domains of functional groups similar in size to thoseobserved in the nanometric nucleation maps. These results, along with asemi-quantitative nucleation model, indicate that a chemically non-uniforminterface presenting different free energy barriers to heterogeneous nucleationunderlies our observations of non-uniform nucleation kinetics. Overall, ourresults introduce a new imaging modality, nanometric nucleation mapping, andprovide important new insights into the impact of surface chemistry on microscopicspatial variations in heterogeneous nucleation kinetics that have not beenpreviously observed.

Journal Article A Fluorescence Microscopy Assay for Assessing Beam Damage to Nanoparticle Capping Ligands During Liquid Cell Electron Microscopy Get access Thilini U Dissanayake, Thilini U Dissanayake Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, MD, USA Search for other works by this author on: Oxford Academic Google Scholar Mei Wang, Mei Wang Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, MD, USA Search for other works by this author on: Oxford Academic Google Scholar Taylor J Woehl Taylor J Woehl Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, MD, USA Corresponding author: tjwoehl@umd.edu Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 25, Issue S2, 1 August 2019, Pages 1672–1673, https://doi.org/10.1017/S1431927619009097 Published: 01 August 2019

The pillared MWW (PMWW or MCM-36) and pillared MFI (PMFI) are two prominent representatives of meso-/microporous lamellar zeolites, which are made by pillaring of layered MCM-22(P) and multilamellar MFI precursors, respectively. They both consist of a 10-membered ring sinusoidal micropore channel within each zeolitic layer and mesopore voids created by pillars between two adjacent zeolitic layers. Although catalytic activities of PMWW and PMFI have been compared, identification of mesopore differences between these two pillared lamellar zeolites has not been attempted. In this work, we report the differences in mesopore characteristics between PMWW and PMFI zeolites probed by atomic layer deposition (ALD) of titania (ALD-TiO2) species and a combination of structural, textural, composition and catalytic property analyses of the samples before and after ALD treatment, respectively. The results suggest PMFI has cylindrical mesopores with uniform sizes from pore entrance to main body, while PMWW has ink-bottle mesopores with the neck size much smaller than pore body. The communication between micropore and mesopore in PMFI is direct, but no communication exists in PMWW zeolites. The ALD-TiO2 process considerably modified external surface composition of PMFI and PMWW zeolites, which consequently led to a new application of the pillared lamellar zeolite materials, i.e., photo-catalytically active meso-/microporous zeolites for environmental remedy reactions.

Journal Article Toward Quantitative Liquid Cell Electron Microscopy through Kinetic Control of Solution Chemistry Get access Mei Wang, Mei Wang Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, College Park, USA Search for other works by this author on: Oxford Academic Google Scholar Chiwoo Park, Chiwoo Park Department of Industrial and Manufacturing Engineering, Florida State University, Tallahassee, USA Search for other works by this author on: Oxford Academic Google Scholar Taylor Woehl Taylor Woehl Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, College Park, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 25, Issue S1, 1 February 2019, Pages 23–24, https://doi.org/10.1017/S1431927618015854 Published: 01 February 2019

Platinum nanomaterials have been widely used as heterogeneous catalysts for oxidative dehydrogenation of organic alkanes and catalytic reforming [1-2]. Catalytic reactions are sensitive to the size, shape, and surface structure of catalyst nanoparticles, which control selectivity and activity of catalytic reaction processes [3]. Further, mesoscopic effects, like spatial vicinity of catalyst particles and confinement, are known to greatly affect activity and selectivity [4-5]. However, the reaction kinetics and nanocrystal formation processes that control the atomic scale and mesoscale properties of catalyst nanoparticles during synthesis are not well-known due to the lack of in situ characterization methods. Liquid cell scanning transmission electron microcopy (LC-STEM) provides a way to visualize and quantify the kinetics of these processes on the single nanoparticle scale, which can provide us with new insights into synthesis that will guide developing catalysts with desired structures and enhanced activity and selectivity.

Despite the vast number of carbon-supported catalysts in industrial processes, the relationship between the active catalyst and the carbon framework is not well understood. In this study, we report the importance of the carbon porosity on Cu-catalyzed carbon dioxide reduction (CO2RR) using carbon aerogels, a class of high surface area carbon foams with tunable structural properties. Systematically adjusting the porosity of the carbon aerogels leads to significant differences in catalytic selectivity and activity. We demonstrate that the pore widths of the carbon aerogels are correlated with CO2RR product selectivity, with an optimal mesopore size of 30-50 nm for CO and C2H4 formation. In this size region, the CO2RR activity experiences a 2-fold enhancement, while HER activity remains similar across all tested samples. Given the ubiquity of carbon materials, our findings will inform the design of carbon-supported catalysts for CO2RR and other industrially relevant chemical transformations. (C) 2018 Elsevier Ltd. All rights reserved.