Showing papers by "Christopher B. Murray published in 2019"

Bimetallic synergy in cobalt–palladium nanocatalysts for CO oxidation

Abstract: Bimetallic and multi-component catalysts typically exhibit composition-dependent activity and selectivity, and when optimized often outperform single-component catalysts. Here we used ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and in situ and ex situ transmission electron microscopy (TEM) to elucidate the origin of composition dependence observed in the catalytic activities of monodisperse CoPd bimetallic nanocatalysts for CO oxidation. We found that the catalysis process induced a reconstruction of the catalysts, leaving CoOx on the nanoparticle surface. The synergy between Pd and CoOx coexisting on the surface promotes the catalytic activity of the bimetallic catalysts. This synergistic effect can be optimized by tuning the Co/Pd ratios in the nanoparticle synthesis, and it reaches a maximum at compositions near Co0.24Pd0.76, which achieves complete CO conversion at the lowest temperature. Our combined AP-XPS and TEM studies provide direct observation of the surface evolution of the bimetallic nanoparticles under catalytic conditions and show how this evolution correlates with catalytic properties.

Plasmonic Optical and Chiroptical Response of Self-Assembled Au Nanorod Equilateral Trimers.

Abstract: Assembling metamolecules from anisotropic, shape-engineered nanocrystals provides the opportunity to orchestrate distinct optical responses one nanocrystal at a time. The Au nanorod has long been a structural archetype in plasmonics, but nanorod assemblies have largely been limited to end-to-end or side-to-side arrangements, accessing only a subset of potential metamolecule structures. Here, we employ triangular templates to direct the assembly of Au nanorods along the edges of an equilateral triangle. Using spatially resolved, dark-field scattering spectroscopy in concert with numerical simulation of individual metamolecules, we map the evolution in surface plasmon resonances as we add one, two, and three nanorods to construct triangular nanorod assemblies. The assemblies exhibit rotation- and polarization-dependent hybridized plasmon modes, which are sensitive to variations in nanorod size, position, and orientation that lead to geometrical symmetry breaking. The triangular arrangement of nanorods supports magnetic plasmon modes where electric fields are directed along the perimeter of the triangle, and the magnetic field intensity within the triangle's open interior is enhanced. Circumferential displacements of the nanorods within the templates impart either a left- or right-handed sense of rotation to the structure, which generates a chiroptical response under unidirectional oblique illumination. Our results represent an important step in realizing and characterizing metamaterial assemblies with "open" structures utilizing anisotropic plasmonic building blocks, with implications for optical magnetic field enhancement and chiral plasmonics.

Generalized Synthetic Strategy for Transition-Metal-Doped Brookite-Phase TiO2 Nanorods.

Abstract: We report a generalized wet-chemical methodology for the synthesis of transition-metal (M)-doped brookite-phase TiO2 nanorods (NRs) with unprecedented wide-range tunability in dopant composition (M...

Tuning the Electrocatalytic Oxygen Reduction Reaction Activity of Pt-Co Nanocrystals by Cobalt Concentration with Atomic-Scale Understanding.

Abstract: The development of a suitable catalyst for the oxygen reduction reaction (ORR), the cathode reaction of proton exchange membrane fuel cells (PEMFC), is necessary to push this technology toward wide...

General Synthetic Route to High-Quality Colloidal III-V Semiconductor Quantum Dots Based on Pnictogen Chlorides.

Abstract: The synthesis of colloidal III–V quantum dots (QDs), particularly of the arsenides and antimonides, has been limited by the lack of stable and available group V precursors. In this work, we exploit...

Nanocrystal Core Size and Shape Substitutional Doping and Underlying Crystalline Order in Nanocrystal Superlattices

Abstract: Substitutional doping is a potentially powerful technique to control the properties of nanocrystal (NC) superlattices (SLs). However, not every NC can be substituted into any lattice, as the NCs have to be close in size and shape, limiting the application of substitutional doping. Here we show that this limitation can be overcome by employing ligands of various size. We show that small NCs with long ligands can be substituted into SLs of big NCs with short ligands. Furthermore, we show that shape differences can also be overcome and that cubes can substitute spheres when both are coated with long ligands. Finally, we use the NC effective ligand size, softness, and effective overall size ratio to explain observed doping behaviors.

Cluster-mining: an approach for determining core structures of metallic nanoparticles from atomic pair distribution function data

Abstract: A novel approach for finding and evaluating structural models of small metallic nanoparticles is presented. Rather than fitting a single model with many degrees of freedom, libraries of clusters from multiple structural motifs are built algorithmically and individually refined against experimental pair distribution functions. Each cluster fit is highly constrained. The approach, called cluster-mining, returns all candidate structure models that are consistent with the data as measured by a goodness of fit. It is highly automated, easy to use, and yields models that are more physically realistic and result in better agreement to the data than models based on cubic close-packed crystallographic cores, often reported in the literature for metallic nanoparticles.

Air-Stable CuInSe2 Nanocrystal Transistors and Circuits via Post-Deposition Cation Exchange.

Abstract: Colloidal semiconductor nanocrystals (NCs) are a promising materials class for solution-processable, next-generation electronic devices. However, most high-performance devices and circuits have bee...

Phase Behavior of Grafted Polymer Nanocomposites from Field-Based Simulations

Abstract: There are limited theoretically predicted phase diagrams for polymer nanocomposites (PNCs) because conventional modeling techniques are largely unable to predict the macroscale phase behavior of PN...

Dendrimer Ligand Directed Nanoplate Assembly

Abstract: Many studies on nanocrystal (NC) self-assembly into ordered superlattices have focused mainly on attractive forces between the NCs, whereas the role of organic ligands on anisotropic NCs is only in its infancy. Herein, we report the use of a series of dendrimer ligands to direct the assembly of nanoplates into 2D and 3D geometries. It was found that the dendrimer-nanoplates consistently form a directionally offset architecture in 3D films. We present a theory to predict ligand surface distribution and Monte Carlo simulation results that characterize the ligand shell around the nanoplates. Bulky dendrimer ligands create a nontrivial corona around the plates that changes with ligand architecture. When this organic-inorganic effective shape is used in conjunction with thermodynamic perturbation theory to predict both lattice morphology and equilibrium relative orientations between NCs, a lock-and-key type of mechanism is found for the 3D assembly. We observe excellent agreement between our experimental results and theoretical model for 2D and 3D geometries, including the percent of offset between the layers of NCs. Such level of theoretical understanding and modeling will help guide future design frameworks to achieve targeted assemblies of NCs.

Binary icosahedral quasicrystals of hard spheres in spherical confinement

Abstract: The influence of geometry on the local and global packing of particles is important to many fundamental and applied research themes such as the structure and stability of liquids, crystals and glasses. Here, we show by experiments and simulations that a binary mixture of hard-sphere-like particles crystallizing into the MgZn2 Laves phase in bulk, spontaneously forms 3D icosahedral quasicrystals in slowly drying droplets. Moreover, the local symmetry of 70-80% of the particles changes to that of the MgCu2 Laves phase. Both of these findings are significant for photonic applications. If the stoichiometry deviates from that of the Laves phase, our experiments show that the crystallization of MgZn2 is hardly affected by the spherical confinement. Our simulations show that the quasicrystals nucleate away from the spherical boundary and grow along five-fold symmetric structures. Our findings not only open the way for particle-level studies of nucleation and growth of 3D quasicrystals, but also of binary crystallization.

Designing Strong Optical Absorbers via Continuous Tuning of Interparticle Interaction in Colloidal Gold Nanocrystal Assemblies

Abstract: We program the optical properties of colloidal Au nanocrystal (NC) assemblies via an unconventional ligand hybridization (LH) strategy to precisely engineer interparticle interactions and design ma...

Experiments and Simulations Probing Local Domain Bulge and String Assembly of Aligned Nanoplates in a Lamellar Diblock Copolymer

Abstract: Within ordered poly(styrene-b-methyl methacrylate) (PS-b-PMMA) block copolymer (BCP) lamellae, oriented nanoplates [gadolinium trifluoride doped with ytterbium and erbium, GdF3:Yb/Er (20/2 mol %)] ...

The Influence of Surface Platinum Deposits on the Photocatalytic Activity of Anatase TiO2 Nanocrystals

Abstract: In this study, the impact of supported Pt particles on the thermal and photocatalytic activity of well-defined anatase TiO2 (A-TiO2) nanocrystals (NCs) was investigated. Pt-decorated NCs were characterized using scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The activity of the Pt-decorated NCs for the thermal and photocatalytic reactions of acetaldehyde was then studied using temperature-programmed desorption in ultrahigh vacuum. The bare TiO2 NCs demonstrated thermal activity primarily for aldol condensation, partial oxidation, and reductive coupling pathways. For the Pt-decorated NCs, the Pt deposits were found to act mainly as a site blocker for the thermal aldol condensation and partial oxidation pathways, as well as acting as recombination centers for photogenerated electrons and holes at the surface, suppressing the photocatalytic activity of the NCs. Upon pretreating with O2, however, the photocatalytic activity of the Pt-deco...

Author Correction: Bimetallic synergy in cobalt–palladium nanocatalysts for CO oxidation

Abstract: In the version of this Article originally published, the author Baran Eren was mistakenly affiliated with the Harbin Institute of Technology, China; it has now been corrected to Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.

Quantum dots and production method thereof

Abstract: A production method of a quantum dot comprising a Group IIIA-VA compound, the quantum dot as prepared, and an electronic device including the same, and the production method includes: supplying a Group VA element precursor including a halide of a Group VA element and a first ligand of a phosphine compound or a first amine compound; and performing a reaction between the Group VA element precursor and a Group IIIA metal precursor in the presence of a reducing agent in an organic reaction medium including a second amine compound.

Plasmonic elastic capsules as colorimetric reversible pH-microsensors

Abstract: There is a crucial need for effective and easily dispersible colloidal microsensors able to detect local pH changes before irreversible damages caused by demineralization, corrosion, or biofilms occur. One class of such microsensors is based on molecular dyes encapsulated or dispersed either in polymer matrices or in liquid systems exhibiting different colors upon pH variations. They are efficient but often rely on sophisticated and costly syntheses, and present significant risks of leakage and photobleaching damages, which is detrimental for mainstream applications. Another approach consists in exploiting the distance-dependent plasmonic properties of metallic nanoparticles. Still, assembling nanoparticles into dispersible colloidal pH-sensitive sensors remains a challenge. Here, we show how to combine optically active plasmonic gold nanoparticles and pH-responsive thin shells into "plasmocapsules". Upon pH change, plasmocapsules swell or shrink. Concomitantly, the distance between the gold nanoparticles embedded in the polymeric matrix varies, resulting in an unambiguous color change. Billions of micron-size sensors can thus be easily fabricated. They are non-intrusive, reusable, and sense local pH changes. Each plasmocapsule is an independent reversible microsensor over a large pH range. Finally, we demonstrate their potential use for the detection of bacterial growth, thus proving that plasmocapsules are a new class of sensing materials.