Douglas A. Blom

Tunable Plasmonic Nanoparticles with Catalytically Active High-Index Facets

Noble metal nanoparticles have been of tremendous interest due to their intriguing size- and shape-dependent plasmonic and catalytic properties. Combining tunable plasmon resonances with superior catalytic activities on the same metallic nanoparticle, however, has long been challenging because nanoplasmonics and nanocatalysis typically require nanoparticles in two drastically different size regimes. Here, we demonstrate that creation of high-index facets on subwavelength metallic nanoparticles provides a unique approach to the integration of desired plasmonic and catalytic properties on the same nanoparticle. Through site-selective surface etching of metallic nanocuboids whose surfaces are dominated by low-index facets, we have controllably fabricated nanorice and nanodumbbell particles, which exhibit drastically enhanced catalytic activities arising from the catalytically active high-index facets abundant on the particle surfaces. The nanorice and nanodumbbell particles also possess appealing tunable plasmonic properties that allow us to gain quantitative insights into nanoparticle-catalyzed reactions with unprecedented sensitivity and detail through time-resolved plasmon-enhanced spectroscopic measurements.

Atomic structure of three-layer Au/Pd nanoparticles revealed by aberration-corrected scanning transmission electron microscopy

The study of nanomaterials can be greatly improved with the use of aberration-corrected transmission electron microscopy (TEM), which provides image resolutions at the level of 1 A and lower. Sub-Angstrom image resolution can yield a new level of understanding of the behavior of matter at the nanoscale. For example, bimetallic nanoparticles are extremely important in catalysis applications; the addition of a second metal in many cases produces much-improved catalysts. In this paper, we study the structure and morphology of Au/Pd bimetallic particles using primarily the high-angle annular dark-field (HAADF) imaging mode in an aberration-corrected STEM/TEM. It is well established that, when recorded under appropriate illumination and collection geometries, incoherent HAADF-STEM images are compositionally sensitive and provide direct information on atomic positions. We matched the experimental intensities of atomic columns with theoretical models of three-layer Au/Pd nanoparticles, in different orientations. Our findings indicate that the surface layer of the nanoparticle contains kinks, terraces and steps at the nanoscale. The effect of adding a second metal induces the formation of such defects, which might very likely promote the well-known improved catalytic activity of this system.

Preparation of Cross-Sectional Samples of Proton Exchange Membrane Fuel Cells by Ultramicrotomy for TEM

Transmission electron microscopy (TEM) microstructural characterization and microchemical analysis are two tools which can help relate proton exchange membrane (PEM) processing and resulting structure to fuel cell performance as well as structural changes to performance loss. A necessary first step is the preparation of specimens suitable for TEM characterization that maintain the spatial relationships among the various macro- and microcomponents of an entire PEM fuel cell. By carefully keeping the structure intact during the sample preparation, valuable information regarding the relationships among the many components of the fuel cell can he preserved. Previous TEM studies of PEM fuel cells have only been conducted on parts of the cell. No single preparation technique has resulted in keeping the entire fuel cell intact to image the entire cross section. In the present work, diamond knife ultramicrotomy has successfully been used to create thin sections suitable for TEM of entire PEM membrane electrode assemblies both before and after use in a fuel cell test stand.

Surface microstructuring and long-range ordering of silicon nanoparticles

Pulsed-laser irradiation was used to induce the formation of linear arrays of nanoparticles that can extend over millimeter distances. On flat surfaces, the irradiation induces the fragmentation and clustering of a thin silicon film pulsed-laser deposited on silicon into nanoparticles that grow to 30–40 nm in diameter. The nanoparticles aggregate into clusters that migrate, forming short curvilinear groups that exhibit a short-range ordering. If a region containing a microscopic roughness is introduced, the nanoparticles are forced to align into long and remarkably straight lines with line spacing very close to the laser wavelength. A close connection is established between the nanoparticle alignment and the evolution of laser-induced periodic surface structures. The microscopic roughness solely serves as a trigger to produce the alignment.

New catalytic liquid-phase ammoxidation approach to the preparation of niacin (vitamin B3).

New highly dispersed bimetallic nanoscale catalysts based on rhenium combined with antimony or bismuth have been shown to be highly effective for the ammoxidation of 3-picoline to nicotinonitrile (precursor for vitamin B3) under mild conditions in the liquid phase.

Condensed phase growth of single-wall carbon nanotubes from laser annealed nanoparticulates

Single-wall carbon nanotubes (SWNT) were grown to micron lengths by laser-annealing nanoparticulate soot containing short (∼50 nm long) nanotube “seeds.” The “seeded” nanoparticulate soot was produced by restricting the time spent by an ablation plume inside an 800 °C oven following laser vaporization of a C–Ni–Co target. The soot collected from the laser vaporization apparatus was placed inside graphite crucibles under argon, and heated by a CO2 laser. In situ pyrometry was used to estimate the sample temperature. Length distributions of SWNT bundles in the unannealed and annealed samples were measured by transmission electron microscopy and field emission scanning electron microscopy. Annealing treatments exceeding 1600 °C produced no increase in nanotube length, while lower temperatures in the 1000–1300 °C range were optimal for growth. These experiments indicate that SWNT grow by the conversion of condensed phase nanomaterial during annealing, a similar mechanism to that proposed for growth during nor...

Facet Control of Gold Nanorods

While great success has been achieved in fine-tuning the aspect ratios and thereby the plasmon resonances of cylindrical Au nanorods, facet control with atomic level precision on the highly curved nanorod surfaces has long been a significantly more challenging task. The intrinsic structural complexity and lack of precise facet control of the nanorod surfaces remain the major obstacles for the atomic-level elucidation of the structure-property relationships that underpin the intriguing catalytic performance of Au nanorods. Here we demonstrate that the facets of single-crystalline Au nanorods can be precisely tailored using cuprous ions and cetyltrimethylammonium bromide as a unique pair of surface capping competitors to guide the particle geometry evolution during nanorod overgrowth. By deliberately maneuvering the competition between cuprous ions and cetyltrimethylammonium bromide, we have been able to create, in a highly controllable and selective manner, an entire family of nanorod-derived anisotropic multifaceted geometries whose surfaces are enclosed by specific types of well-defined high-index and low-index facets. This facet-controlled nanorod overgrowth approach also allows us to fine-tune the particle aspect ratios while well-preserving all the characteristic facets and geometric features of the faceted Au nanorods. Taking full advantage of the combined structural and plasmonic tunability, we have further studied the facet-dependent heterogeneous catalysis on well-faceted Au nanorods using surface-enhanced Raman spectroscopy as an ultrasensitive spectroscopic tool with unique time-resolving and molecular finger-printing capabilities.

Optimized imaging using non-rigid registration.

The extraordinary improvements of modern imaging devices offer access to data with unprecedented information content. However, widely used image processing methodologies fall far short of exploiting the full breadth of information offered by numerous types of scanning probe, optical, and electron microscopies. In many applications, it is necessary to keep measurement intensities below a desired threshold. We propose a methodology for extracting an increased level of information by processing a series of data sets suffering, in particular, from high degree of spatial uncertainty caused by complex multiscale motion during the acquisition process. An important role is played by a non-rigid pixel-wise registration method that can cope with low signal-to-noise ratios. This is accompanied by formulating objective quality measures which replace human intervention and visual inspection in the processing chain. Scanning transmission electron microscopy of siliceous zeolite material exhibits the above-mentioned obstructions and therefore serves as orientation and a test of our procedures.

Atomic-level imaging of Mo-V-O complex oxide phase intergrowth, grain boundaries, and defects using HAADF-STEM

In this work, we structurally characterize defects, grain boundaries, and intergrowth phases observed in various Mo-V-O materials using aberration-corrected high-angle annular dark-field (HAADF) imaging within a scanning transmission electron microscope (STEM). Atomic-level imaging of these preparations clearly shows domains of the orthorhombic M1-type phase intergrown with the trigonal phase. Idealized models based on HAADF imaging indicate that atomic-scale registry at the domain boundaries can be seamless with several possible trigonal and M1-type unit cell orientation relationships. The alignment of two trigonal domains separated by an M1-type domain or vice versa can be predicted by identifying the number of rows/columns of parallel symmetry operators. Intergrowths of the M1 catalyst with the M2 phase or with the Mo5O14-type phase have not been observed. The resolution enhancements provided by aberration-correction have provided new insights to the understanding of phase equilibria of complex Mo-V-O materials. This study exemplifies the utility of STEM for the characterization of local structure at crystalline phase boundaries.

Irreversible xenon insertion into a small-pore zeolite at moderate pressures and temperatures

Pressure drastically alters the chemical and physical properties of materials and allows structural phase transitions and chemical reactions to occur that defy much of our understanding gained under ambient conditions. Particularly exciting is the high-pressure chemistry of xenon, which is known to react with hydrogen and ice at high pressures and form stable compounds. Here, we show that Ag16Al16Si24O8·16H2O (Ag-natrolite) irreversibly inserts xenon into its micropores at 1.7 GPa and 250 °C, while Ag(+) is reduced to metallic Ag and possibly oxidized to Ag(2+). In contrast to krypton, xenon is retained within the pores of this zeolite after pressure release and requires heat to desorb. This irreversible insertion and trapping of xenon in Ag-natrolite under moderate conditions sheds new light on chemical reactions that could account for the xenon deficiency relative to argon observed in terrestrial and Martian atmospheres.