Showing papers by "Sara E. Skrabalak published in 2017"

Facet-Dependent Deposition of Highly Strained Alloyed Shells on Intermetallic Nanoparticles for Enhanced Electrocatalysis

Abstract: Surface strains can enhance the performance of platinum-based core@shell electrocatalysts for the oxygen reduction reaction (ORR). Bimetallic core@shell nanoparticles (NPs) are widely studied nanocatalysts but often have limited lattice mismatch and surface compositions; investigations of core@shell NPs with greater compositional complexity and lattice misfit are in their infancy. Here, a new class of multimetallic NPs composed of intermetallic cores and random alloy shells is reported. Specifically, face-centered cubic Pt–Cu random alloy shells were deposited on PdCu B2 intermetallic seeds in a facet-dependent manner, giving rise to faceted core@shell NPs with highly strained surfaces. High-resolution transmission electron microscopy revealed orientation-dependent surface strains, where the compressive strains were greater on Pt–Cu {200} than {111} facets. These core@shell NPs provide higher specific area and mass activities for the ORR when compared to conventional Pt–Cu NPs. Moreover, these intermetall...

Metal nanomaterials for optical anti-counterfeit labels

Abstract: The global economic, security, and health challenges presented by counterfeit goods require new approaches toward anti-counterfeit labels. This review describes recent advances in the use of metal nanomaterials for optical anti-counterfeit labels that may offer a multiplexed approach to security tags that can be easily fabricated, offer large coding capacity, and be interrogated throughout the supply chain and by the end user. This review also critically discusses the current approach to developing continuously more complex labels and offers awareness toward the need for simple, yet unclonable, taggants.

Enhanced Photoactivity from Single-Crystalline SrTaO 2 N Nanoplates Synthesized by Topotactic Nitridation.

Abstract: There are few methods yielding oxynitride crystals with defined shape, yet shape-controlled crystals often give enhanced photoactivity. Herein, single-crystalline SrTaO2N nanoplates and polyhedra are achieved selectively. Central to these synthetic advances is the crystallization pathways used, in which single-crystalline SrTaO2N nanoplates form by topotactic nitridation of aerosol-prepared Sr2Ta2O7 nanoplates and SrTaO2N polyhedra form by flux-assisted nitridation of the nanoplates. Evaluation of these materials for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) showed improved performance for the SrTaO2N nanoplates, with a record apparent quantum efficiency (AQE) of 6.1 % for OER compared to the polyhedra (AQE: 1.6 %) and SrTaO2N polycrystals (AQE: 0.6 %). The enhanced performance from the nanoplates arises from their morphology and lower defect density. These results highlight the importance of developing new synthetic routes to high quality oxynitrides.

Seed-mediated co-reduction in a large lattice mismatch system: synthesis of Pd-Cu nanostructures.

Abstract: Metal nanoparticles (NPs) are of interest for applications in catalysis, electronics, chemical sensing, and more. Their utility is dictated by their composition and physical parameters such as particle size, particle shape, and overall architecture (e.g., hollow vs. solid). Interestingly, the addition of a second metal to create bimetallic NPs adds multifunctionality, with new emergent properties common. However, synthesizing structurally defined bimetallic NPs remains a great challenge. One synthetic pathway to architecturally controlled bimetallic NPs is seed-mediated co-reduction (SMCR) in which two metal precursors are simultaneously co-reduced to deposit metal onto shape-controlled metal seeds, which direct the overgrowth. Previously demonstrated in a Au–Pd system, here SMCR is applied to a system with a larger lattice mismatch between the depositing metals: Pd and Cu (7% mismatch for Pd–Cu vs. 4% for Au–Pd). Through manipulation of precursor reduction kinetics, the morphology and bimetallic distribution of the resultant NPs can be tuned to achieve eight-branched Pd–Cu heterostructures with Cu localized at the tips of the Pd nanocubes as well as branched Pd–Cu alloyed nanostructures and polyhedra. Significantly, the symmetry of the seeds can be transferred to the final nanostructures. This study expands our understanding of SMCR as a route to structurally defined bimetallic nanostructures and the synthesis of multicomponent nanomaterials more generally.

Surface Passivation and Supersaturation: Strategies for Regioselective Deposition in Seeded Syntheses

Abstract: Crystal growth theory predicts that heterogeneous nucleation will occur preferentially at defect sites, such as the vertices rather than the faces of shape-controlled seeds. Platonic metal solids are generally assumed to have vertices with nearly identical chemical potentials, and also nearly identical faces, leading to the useful generality that heterogeneous nucleation preserves the symmetry of the original seeds in the final product. Herein, we test the limits of this generality in the extreme of low supersaturation, in an effort to expand the methods available for inducing anisotropic overgrowth. We formulate a strategy for favoring localized deposition that differentiates between both different vertices and different edges or faces, i.e., regioselective deposition. Deposition followed a simple kinetic model for nucleation rate, depending on wetting, supersaturation, and temperature. We demonstrate our ability to independently study the effects of varying supersaturation and surface passivation. Regio...

Designing Efficient Catalysts through Bimetallic Architecture: Rh@Pt Nanocubes as a Case Study

Abstract: Bimetallic nanocatalysts often have increased activities and stabilities over their monometallic counterparts due to surface strain effects and electron transfer between the two metals. Here, we demonstrate that the performance of a nanocatalyst can be precisely manipulated in shape-controlled nanocrystals through a bimetallic core@shell architecture. This ability is achieved in a model core@shell Rh@Pt nanocube system through control of shell thickness. The enhanced performance with thin-shelled nanocrystals is correlated with the weakening of surface–adsorbate interactions. In these thin-shelled Rh@Pt nanocubes, the maximum current density achieved during formic acid oxidation was over 2 times greater than that achieved with similarly sized Pt nanocubes, with a decreased CO poisoning ratio as well. The strategy employed here should also enhance the performance of many other bimetallic nanomaterials composed of more cost-effective metals too.

Role of Short-Range Chemical Ordering in (GaN)1–x(ZnO)x for Photodriven Oxygen Evolution

Abstract: (GaN)1–x(ZnO)x (GZNO) is capable of visible-light driven water splitting, but its bandgap at x ≤ 0.15 (>2.7 eV) results in poor visible-light absorption. Unfortunately, methods to narrow its bandgap by incorporating higher ZnO concentrations are accompanied by extensive Urbach tailing near the absorption-edge, which is indicative of structural disorder or chemical inhomogeneities. We evaluated whether this disorder is intrinsic to the bond-length distribution in GZNO or is a result of defects introduced from the loss of Zn during nitridation. Here, the synthesis of GZNO derived from layered double hydroxide (LDH) precursors is described which minimizes Zn loss and chemical inhomogeneities and enhances visible-light absorption. The average and local atomic structures of LDH-derived GZNO were investigated using X-ray and neutron scattering and are correlated with their oxygen evolution rates. An isotope-contrasted neutron-scattering experiment was conducted in conjunction with reverse Monte Carlo (RMC) simu...

Simple Reactor for Ultrasonic Spray Synthesis of Nanostructured Materials

Abstract: Developing a facile and general synthetic strategy toward particles with size, shape, and compositional control is of importance to nanotechnology applications. Ultrasonic spray synthesis (USS) is a continuous route to micro- and nanoscale particles with structural control, which are often difficult to obtain for inorganic solids with complex compositions or of metastable phases. This protocol describes the design and assembling of components for a simple reactor for USS. Components include a nebulizer, a nebulization chamber, a furnace, a furnace tube and the corresponding adapters, and a product collection apparatus. Details of our house-made components are provided as well as insights on material selection based on different synthetic requirements. We exemplify USS with a step-by-step procedure to single-crystalline NaSbO3 nanoplates, and this procedure can be easily modified to accommodate other chemistries. The integration of USS and molten salt synthesis for single-crystalline NaSbO3 nanoplates demo...

Synthesis of Core@Shell Nanostructures in a Continuous Flow Droplet Reactor: Controlling Structure through Relative Flow Rates

Abstract: Bimetallic nanostructures are primarily synthesized in small volume batches. However, droplet-based reactors are receiving attention due to their ability to maintain thermal and compositional equilibrium within and between droplets, enabling flow operations for inline analyses and the scale-up of nanomaterial syntheses. Here, the syntheses of shape-controlled core@shell Au@Pd nanostructures with variable shell thicknesses are reported through control of the relative flow rates of reagents within the microreactor. Specifically, Pd shells were grown on cubic or octahedral Au seeds, selected as a model system. In batch reactions, shell thickness is determined by precursor concentration; however, as shown here, precursor feedstock concentration can be held constant, with the precursor concentration within the droplets being controlled through relative flow rates. This approach allows process conditions to be modified inline rather than from batch to batch to achieve particles with different shell thicknesses,...

Facile synthesis of porous La–Ti–O and LaTiO2N microspheres

Abstract: Photocatalysts offer an excellent opportunity to shift the global energy landscape from a fossil fuel-dependent paradigm to sustainable and carbon-neutral solar fuels. Oxynitride materials such as LaTiO2N are potential photocatalysts for the water splitting reaction due to their high oxidative stability and their narrow band gaps, which are suitable for visible light absorption. However, facile synthetic routes to metal oxynitrides with controlled morphologies are rare. Ultrasonic spray synthesis (USS) offers a facile method toward complex metal oxides which can potentially be converted to oxynitrides with preservation of the microsphere structures that typify the products from such aerosol routes. Here, La–Ti–O microspheres were facilely produced by USS and converted by ammonolysis to LaTiO2N microspheres with porous shells and hollow interiors. This particle architecture is accounted for by coupling suitable combustion chemistry with the aerosol technique, producing precursor particles where the La3+ and Ti4+ are well-mixed at small length scales; this feature enables preservation of the microsphere morphology during nitridation despite the crystallographic changes that occur. The LaTiO2N microspheres are comparable oxygen evolving photocatalysts to samples produced by conventional solid state methods. These results demonstrate the utility of USS as a facile, potentially scalable route to complex photocatalytic materials and their precursors with distinct morphologies.

Oriented Attachment Is a Major Control Mechanism To Form Nail-like Mn-Doped ZnO Nanocrystals.

Abstract: Here, we present a controlled synthesis of Mn-doped ZnO nanoparticles (NPs) with predominantly nail-like shapes, whose formation occurs via tip-to-base-oriented attachment of initially formed nanopyramids, followed by leveling of sharp edges that lead to smooth single-crystalline “nails”. This shape is prevalent in noncoordinating solvents such as octadecene and octadecane. Yet, the double bond in the former promotes oriented attachment. By contrast, Mn-doped ZnO NP synthesis in a weakly coordinating solvent, benzyl ether, results in dendritic structures because of random attachment of initial NPs. Mn-doped ZnO NPs possess a hexagonal wurtzite structure, and in the majority of cases, the NP surface is enriched with Mn, indicating a migration of Mn2+ ions to the NP surface during the NP formation. When the NP formation is carried out without the addition of octadecyl alcohol, which serves as a surfactant and a reaction initiator, large, concave pyramid dimers are formed whose attachment takes place via bas...