Showing papers by "Christopher B. Murray published in 2018"

Photocatalytic Hydrogen Evolution from Substoichiometric Colloidal WO3- x Nanowires

Abstract: We report direct photocatalytic hydrogen evolution from substoichiometric highly reduced tungsten oxide (WOx) nanowires (NWs) using sacrificial alcohol. WOx NWs are synthesized via nonaqueous colloidal synthesis with a diameter of about 4 nm and an average length of about 250 nm. As-synthesized WOx NWs exhibit a broad absorption across the visible to infrared regions attributed to the presence of oxygen vacancies. The optical band gap is increased in these WOx NWs compared to stoichiometric bulk tungsten oxide (WO3) powders as a result of the Burstein–Moss shift. As a consequence of this increase, we demonstrate direct photocatalytic hydrogen production from WOx NWs through alcohol photoreforming. The stable H2 evolution on platinized WOx NWs is observed under conditions in which platinized bulk WO3 and bulk WO2.9 powders either do not show activity or show very low rates, suggesting that increased surface area and specific exposed facets are key for the improved performance of WOx NWs. This work demonstr...

Interplay between spherical confinement and particle shape on the self-assembly of rounded cubes.

Abstract: Self-assembly of nanoparticles (NPs) inside drying emulsion droplets provides a general strategy for hierarchical structuring of matter at different length scales. The local orientation of neighboring crystalline NPs can be crucial to optimize for instance the optical and electronic properties of the self-assembled superstructures. By integrating experiments and computer simulations, we demonstrate that the orientational correlations of cubic NPs inside drying emulsion droplets are significantly determined by their flat faces. We analyze the rich interplay of positional and orientational order as the particle shape changes from a sharp cube to a rounded cube. Sharp cubes strongly align to form simple-cubic superstructures whereas rounded cubes assemble into icosahedral clusters with additionally strong local orientational correlations. This demonstrates that the interplay between packing, confinement and shape can be utilized to develop new materials with novel properties.

Favorable Core/Shell Interface within Co2P/Pt Nanorods for Oxygen Reduction Electrocatalysis.

Abstract: Nanostructures with nonprecious metal cores and Pt ultrathin shells are recognized as promising catalysts for oxygen reduction reaction (ORR) to enhance Pt efficiency through core/shell interfacial strain and ligand effects. However, core/shell interaction within a real catalyst is complex and due to the presence of various interfaces in all three dimensions is often oversimply interpreted. Using Co2P/Pt core/shell structure as a model catalyst, we demonstrate, through density functional theory (DFT) calculations that forming Co2P(001)/Pt(111) interface can greatly improve Pt energetics for ORR, while Co2P(010)/Pt(111) is highly detrimental to ORR catalysis. We develop a seed-mediated approach to core/shell Co2P/Pt nanorods (NRs) within which Co2P(001)/Pt(111) interface is selectively expressed over the side facets and the undesired Co2P(010)/Pt(111) interface is minimized. The resultant Co2P/Pt NRs are highly efficient in catalyzing ORR in acid, superior to benchmark CoPt alloy and Pt nanoparticle cataly...

A Study of Tetrahydrofurfuryl Alcohol to 1,5-Pentanediol Over Pt–WOx/C

Abstract: The hydrogenolysis of tetrahydrofurfural alcohol to 1,5-pentanediol was studied over a series of carbon-supported metal/metal-oxide pairs. In agreement with previous reports, specific pairs, especially Pt and Ir paired with WOx, MoOx and ReOx, exhibited high activity and selectivity, even though the individual components were not active or selective. Only reducible oxides produced selective catalysts. A more comprehensive study of the Pt–WOx system indicated that the active form of the catalyst exists as a thin, submonolayer film of the oxide on the Pt surface. This film could be formed by atomic layer deposition (ALD) of W(CO)6 onto the Pt nanocrystals and STEM–EDS mapping demonstrated that ALD deposition occurred selectively on the Pt. When the catalyst was formed by impregnation of Pt and W salts, the WOx was mobile and able to move onto the Pt. The implications of these result for preparing selective catalysts are discussed.

Improved Models for Metallic Nanoparticle Cores from Atomic Pair Distribution Function (PDF) Analysis

Abstract: X-ray atomic pair distribution functions (PDFs) were collected from a range of canonical metallic nanomaterials, both elemental and alloyed, prepared using different synthesis methods and exhibiting drastically different morphological properties. Widely applied shape-tuned attenuated crystal (AC) fcc models proved inadequate, yielding structured, coherent, and correlated fit residuals. However, equally simple discrete cluster models could account for the largest amplitude features in these difference signals. A hypothesis testing based approach to nanoparticle structure modeling systematically ruled out effects from crystallite size, composition, shape, and surface faceting as primary factors contributing to the AC misfit. On the other hand, decahedrally twinned cluster cores were found to be the origin of the AC structure misfits for a majority of the nanomaterials reported here. It is further motivated that the PDF can readily differentiate between the arrangement of domains in these multiply twinned mo...

Nanoimprinted Chiral Plasmonic Substrates with Three-Dimensional Nanostructures.

Abstract: We report a large-area fabrication method to prepare chiral substrates patterned with arrays of multilayer, three-dimensional nanostructures using a combination of nanoimprint lithography and glancing angle deposition. Several structures are successfully fabricated using this method, including L-shaped, twisted arc and trilayer twisted Au nanorod structures, demonstrating its generality. As one typical example, arrays of L-shaped nanostructures, consisting of two layers of orthogonally oriented Au nanorods separated by a Ge dielectric layer in the thickness direction, exhibit giant optical chirality in the infrared region with an experimentally achieved g-factor as high as 0.38. Electromagnetic simulations show that the optical chirality results from plasmon hybridization between the two orthogonal Au segments. To demonstrate scalability, a 1 cm2 chiral substrate is fabricated with uniform chiral optical property. This method combines both high throughput and precise geometrical control and is therefore promising for applications of chiral metamaterials.

Charge Transport Modulation in PbSe Nanocrystal Solids by AuxAg1–x Nanoparticle Doping

Abstract: Nanocrystal (NC) solids are an exciting class of materials, whose physical properties are tunable by choice of the NCs as well as the strength of the interparticle coupling. One can consider these NCs as “artificial atoms” in analogy to the formation of condensed matter from atoms. Akin to atomic doping, the doping of a semiconducting NC solid with impurity NCs can drastically alter its electronic properties. A high degree of complexity is possible in these artificial structures by adjusting the size, shape, and composition of the building blocks, which enables “designer” materials with targeted properties. Here, we present the doping of the PbSe NC solids with a series of AuxAg1–x alloy nanoparticles (NPs). A combination of temperature-dependent electrical conductance and Seebeck coefficient measurements and room-temperature Hall effect measurements demonstrates that the incorporation of metal NPs both modifies the charge carrier density of the NC solids and introduces energy barriers for charge transpor...

Alignment of Nanoplates in Lamellar Diblock Copolymer Domains and the Effect of Particle Volume Fraction on Phase Behavior

Abstract: Polymer nanocomposites (PNCs) that employ diblock copolymers (BCPs) to organize and align anisotropic nanoparticles (NPs) have the potential to facilitate self-assembling hierarchical structures. However, limited studies have been completed to understand the parameters that guide the assembly of nonspherical NPs in BCPs. In this work, we establish a well-defined nanoplate system to investigate the alignment of two-dimensional materials in a lamellar-forming poly(styrene-b-methyl methacrylate) (PS-b-PMMA) BCP with domains oriented parallel to the substrate. Monodisperse gadolinium trifluoride rhombic nanoplates doped with ytterbium and erbium [GdF3:Yb/Er (20/2 mol %)] are synthesized and grafted with phosphoric acid functionalized polyethylene glycol (PEG-PO3H2). Designed with chemical specificity to one block, the nanoplates align in the PMMA domain at low volume fractions (ϕ = 0.0083 and ϕ = 0.017). At these low NP loadings, the BCP lamellae are ordered and induce preferential alignment of the GdF3:Yb/Er...

Thermal and Photocatalytic Reactions of Methanol and Acetaldehyde on Pt-Modified Brookite TiO2 Nanorods

Abstract: The influence of adding Pt on the catalytic and photocatalytic activity of monodispersed brookite phase TiO2 (B-TiO2) nanorods (NRs) was investigated. Pt was deposited on the NRs by photodeposition in solution, and the Pt-modified NRs were characterized using XPS, STEM, and LEIS. The thermal and photocatalytic activity of the Pt-modified NRs were then evaluated using temperature-programmed desorption (TPD) in ultrahigh vacuum (UHV). It was found that while Pt primarily acted as a site blocker for thermal reactions, Pt also acted as a recombination center for photogenerated electrons and holes, resulting in suppressed photocatalytic activity. Upon pretreatment with O2, however, the Pt-modified NRs exhibited enhanced photoactivity, indicating that adsorbed oxygen prevents electron–hole recombination by reacting with photogenerated conduction band electrons from the B-TiO2 to produce stable superoxide species on the Pt surface deposits. These results clearly demonstrate how the dynamics of charge carriers at...

A Characterization Study of Reactive Sites in ALD-Synthesized WOx/ZrO2 Catalysts

Abstract: A series of ZrO2-supported WOx catalysts were prepared using atomic layer deposition (ALD) with W(CO)6, and were then compared to a WOx/ZrO2 catalyst prepared via conventional impregnation. The types of sites present in these samples were characterized using temperature-programmed desorption/thermogravimetric analysis (TPD-TGA) measurements with 2-propanol and 2-propanamine. Weight changes showed that the WOx catalysts grew at a rate of 8.8 × 1017 W atoms/m2 per cycle. Scanning transmission electron microscopy/energy-dispersive spectroscopy (STEM-EDS) indicated that WOx was deposited uniformly, as did the 2-propanol TPD-TGA results, which showed that ZrO2 was completely covered after five ALD cycles. Furthermore, 2-propanamine TPD-TGA demonstrated the presence of three types of catalytic sites, the concentrations of which changed with the number of ALD cycles: dehydrogenation sites associated with ZrO2, Bronsted-acid sites associated with monolayer WOx clusters, and oxidation sites associated with higher WOx coverages. The Bronsted sites were not formed via ALD of WOx on SiO2. The reaction rates for 2-propanol dehydration were correlated with the concentration of Bronsted sites. While TPD-TGA of 2-propanamine did not differentiate the strength of Bronsted-acid sites, H–D exchange between D2O and either toluene or chlorobenzene indicated that the Bronsted sites in tungstated zirconia were much weaker than those in H-ZSM-5 zeolites.

Improved Chemical and Colloidal Stability of Gold Nanoparticles through Dendron Capping.

Abstract: Nanoparticle (NP) stability is imperative for commercialization of nanotechnology. In this study, we compare the stability of Au NPs with surfaces functionalized with oleylamine, dodecanethiol, and two dendritic ligands of different generations. Dendrimer ligands provide a significant increase in the chemical stability of Au NPs when analyzed by cyanide-induced NP decomposition as well as an investigation into their colloidal stability at ambient conditions. These results were supported by absorption measurements, transmission electron microscopy, thermogravimetric analysis, nuclear magnetic resonance, and small-angle transmission X-ray scattering and show that dendrimers play a key role in improving the chemical and colloidal stability of NPs.

3D Nanofabrication via Chemo-Mechanical Transformation of Nanocrystal/Bulk Heterostructures

Abstract: Planar nanocrystal/bulk heterostructures are transformed into 3D architectures by taking advantage of the different chemical and mechanical properties of nanocrystal and bulk thin films. Nanocrystal/bulk heterostructures are fabricated via bottom-up assembly and top-down fabrication. The nanocrystals are capped by long ligands introduced in their synthesis, and therefore their surfaces are chemically addressable, and their assemblies are mechanically "soft," in contrast to the bulk films. Chemical modification of the nanocrystal surface, exchanging the long ligands for more compact chemistries, triggers large volume shrinkage of the nanocrystal layer and drives bending of the nanocrystal/bulk heterostructures. Exploiting the differential chemo-mechanical properties of nanocrystal and bulk materials, the scalable fabrication of designed 3D, cell-sized nanocrystal/bulk superstructures is demonstrated, which possess unique functions derived from nanocrystal building blocks.

Morphological Dependence of the Thermal and Photochemical Reactions of Acetaldehyde on Anatase TiO2 Nanocrystals

Abstract: Size and shape selected anatase TiO2 (A-TiO2) nanocrystals that had either a truncated bipyramidal morphology exposing predominantly (101) facets, or a platelet morphology exposing a high fraction of (001) facets and ranging in size from 10 to 25 nm were used to study the influence of crystallite shape and size on the thermal and photocatalytic reactions of acetaldehyde on TiO2 using temperature-programmed desorption in ultra-high vacuum. The primary thermal reaction pathways on the bipyramidal and platelet nanocrystals were aldol condensation to produce crotonaldehyde and reductive coupling to butene. The platelet morphology, however, exhibited higher thermal activity, which was attributed to the higher fraction of exposed (001) facets. For both morphologies crystallite size was found to be important with smaller nanocrystals favoring butene and larger ones favoring crotonaldehyde. The dependence of the selectivity on crystallite size can be attributed to the populations of planar and edge sites exposed by the nanocrystals. The photocatalytic activity of the nanocrystals was also found to vary with size and shape with the platelets having higher activity for acetaldehyde photo-oxidation than the bipyramidal morphology. For both morphologies, photoactivity also increased as the size of the nanocrystals increased.

Preparation of silica coated and 90Y-radiolabeled β-NaYF4 upconverting nanophosphors for multimodal tracing

Abstract: Rare-earth (RE) compounds have been actively pursued for therapeutic and diagnostic applications due to their ability to upconvert near infrared light into the UV–vis range. Through nanoengineering and bottom-up synthesis, additional functionality can be added to these upconverting systems. Herein, we report the synthesis of 90Y-doped β-NaYF4:Er, Yb upconverting nanophosphors (UCNPs) to enable β-particle emission and upconversion by the same UCNP. To homogenously incorporate the radionuclides, we employ a hydroxide metathesis method to produce the RE precursor required for the solvothermal synthesis of monodisperse UCNPs. Once incorporated, we find that the β-emitting 90Y dopants do not influence the energy pathways required for upconversion, enabling simultaneous radio- and optical-tracing. The resulting large (>100 nm in height and width), anisotropic, 90Y-radiolabeled β-NaYF4 UCNPs are then coated with silica using a modified, micelle-driven Stober process to enable their dispersion in polar solvents. Doing so highlights the importance of surfactant (Igepal CO-520) and silica source (tetraethyl orthosilicate) interactions to the continuity of the silica shell and makes the vast library of silica surface chemistry and functionality accessible to upconverting radiotracers.