David A. Boyd

Heterogenous Catalysis Mediated by Plasmon Heating

We introduce a new method for performing and miniaturizing many types of heterogeneous catalysis involving nanoparticles. The method makes use of the plasmon resonance present in nanoscale metal catalysts to provide the necessary heat of reaction when illuminated with a low-power laser. We demonstrate our approach by reforming a flowing, liquid mixture of ethanol and water over gold nanoparticle catalysts in a microfluidic channel. Plasmon heating of the nanoparticles provides not only the heat of reaction but the means to generate both water and ethanol vapor locally over the catalysts, which in turn allows the chip and the fluid lines to remain at room temperature. The measured products of the reaction, CO(2), CO, and H(2), are consistent with catalytic steam reforming of ethanol. The approach, which we refer to as plasmon-assisted catalysis, is general and can be used with a variety of endothermic catalytic processes involving nanoparticles.

Single-step deposition of high-mobility graphene at reduced temperatures

Current methods of chemical vapour deposition (CVD) of graphene on copper are complicated by multiple processing steps and by high temperatures required in both preparing the copper and inducing subsequent film growth. Here we demonstrate a plasma-enhanced CVD chemistry that enables the entire process to take place in a single step, at reduced temperatures (<420 °C), and in a matter of minutes. Growth on copper foils is found to nucleate from arrays of well-aligned domains, and the ensuing films possess sub-nanometre smoothness, excellent crystalline quality, low strain, few defects and room-temperature electrical mobility up to (6.0±1.0) × 10(4) cm(2) V(-1) s(-1), better than that of large, single-crystalline graphene derived from thermal CVD growth. These results indicate that elevated temperatures and crystalline substrates are not necessary for synthesizing high-quality graphene.

Chemical Separations by Bubble-Assisted Interphase Mass-Transfer

We show that when a small amount of heat is added close to a liquid-vapor interface of a captive gas bubble in a microchannel, interphase mass-transfer through the bubble can occur in a controlled manner with only a slight change in the temperature of the fluid. We demonstrate that this method, which we refer to as bubble-assisted interphase mass-transfer (BAIM), can be applied to interphase chemical separations, e.g., simple distillation, without the need for high temperatures, vacuum, or active cooling. Although any source of localized heating could be used, we illustrate BAIM with an all-optical technique that makes use of the plasmon resonance in an array of nanoscale metal structures that are incorporated into the channel to produce localized heating of the fluid when illuminated by a stationary low-power laser.

Strain-induced pseudo-magnetic fields and charging effects on CVD-grown graphene

Atomically resolved imaging and spectroscopic characteristics of graphene grown by chemical vapor deposition (CVD) on copper are investigated by means of scanning tunneling microscopy and spectroscopy (STM/STS). For CVD-grown graphene remaining on the copper substrate, the monolayer carbon structures exhibit ripples and appear strongly strained, with different regions exhibiting different lattice structures and electronic density of states (DOS). In particular, ridges appear along the boundaries of different lattice structures, which exhibit excess charging effects. Additionally, the large and non-uniform strain induces pseudo-magnetic field up to ~ 50 T, as manifested by the DOS peaks at quantized energies that correspond to pseudo-magnetic field-induced integer and fractional Landau levels. In contrast, for graphene transferred from copper to SiO_2 substrates after the CVD growth, the average strain on the whole diminishes, so do the corresponding charging effects and pseudo-magnetic fields except for sample areas near topological defects. These findings suggest feasible nano-scale “strain engineering” of the electronic states of graphene by proper design of the substrates and growth conditions.

Polymer sphere lithography for solid oxide fuel cells: a route to functional, well-defined electrode structures

As a first step towards mechanistic studies of fuel cell electrodes with both well-defined and functionally representative structural features, two-dimensional anti-dot metal films with tunable features are prepared. The fabrication employs a facile, sacrificial templating method, known as polymer sphere lithography, and the resulting metal films are fully connected, yet fully porous. Using initial bead sizes in the range of 500 nm to 3.2 μm and oxygen plasma etching to remove from ¼ to ¾ of the original bead diameter, computed triple phase boundary densities in the porous films of 2,000 to 43,500 cm cm−2 are achieved. Image analysis shows the computed (theoretical) and experimental structural features to be in good agreement, demonstrating sufficient perfection in the films for electrochemical studies. Furthermore, thermal stability under hydrogen of thermally evaporated Ni films is excellent, with negligible change in triple phase boundary length as required for quantitative electrochemical measurements. Ultimately, these two-dimensional metallic networks may also serve as the platform for future fabrication of three-dimensional electrodes with truly optimized structural features.

Growth of biaxially textured BaxPb1–xTiO3 ferroelectric thin films on amorphous Si3N4

We prepared highly aligned, biaxially textured BaxPb1–xTiO3 (PBT) on amorphous Si3N4 by using an ion-beam-assisted deposited MgO as a template layer. PBT was deposited on a biaxially textured MgO using sol-gel synthesis, metal-organic chemical-vapor deposition, and molecular beam epitaxy. The biaxial texture of the PBT was inherited from the MgO template. The reflection high-energy electron diffraction (RHEED) and cross-section transmission electron microscopy (TEM) experiments suggest that exposure of the MgO template to atmospheric moisture before PBT heteroepitaxy resulted in a significant narrowing of the PBT in-plane orientation distribution. The microstructures of the biaxially textured PBT films were analyzed by x-ray diffraction, RHEED, and TEM. The dynamic contact mode electrostatic force microscopy polarization hysteresis loops confirmed that these films are ferroelectric.

Infrared absorption study of physisorbed carbon monoxide on graphite

Abstract We have applied infrared reflection absorption spectroscopy (IRRAS) to the study of carbon monoxide monolayers adsorbed on a single surface of graphite (HOPG). Concurrent monitoring by laser ellipsometry determines the time of appearance of the first and second layer and of bulk CO. CO is one of the few adsorbates previously studied on graphite by IRRAS. Our spectra agree with the previously published results at 35 K and extend the temperature range to 20–40 K, including all four known solid monolayer phases in this range, and unambiguously establish which spectrum corresponds to the bilayer. Integrated absorption strength provides estimates of the average tilt of the molecular axes with respect to the surface in the nominally flat phases. Frequency shifts due to dynamic dipole coupling may contain information on the short-range correlations of tilt azimuths. We find evidence for specific tilt correlations in the commensurate phases.

Proton conductivity of columnar ceria thin-films grown by chemical vapor deposition

Columnar thin films of undoped ceria were grown by metal-organic chemical vapor deposition. The films, deposited on Pt-coated MgO(100) substrates, display a columnar microstructure with nanometer scale grain size and ~30% overall porosity. Through-plane (thickness mode) electrical conductivity was measured by AC impedance spectroscopy. Proton conduction is observed below 350-400 °C, with a magnitude that depends on gas-phase water vapor pressure. The overall behavior suggests proton transport that occurs along exposed grain surfaces and parallel grain boundaries. No impedance due to grain boundaries normal to the direction of transport is observed. The proton conductivity in the temperature range of 200-400 °C is approximately four times greater than that of nanograined bulk ceria, consistent with enhanced transport along aligned grain surfaces in the CVD films.

Study of SF6 adsorption on graphite using infrared spectroscopy

We report an experimental study of adsorbed monolayers of SF(6) on graphite using infrared reflection absorption spectroscopy supplemented by ellipsometry. The asymmetric S-F stretch mode nu(3) near 948 cm(-1) in the gas is strongly blueshifted in the film by dynamic dipole coupling. This blueshift is very sensitive to the intermolecular spacing in the SF(6) layer. We convert the measured frequency nu(3) to a lattice spacing a, using a self-consistent field calculation, calibrated by the frequency in the commensurate phase. The resolution in lattice spacing is 0.002 A, although there is a larger systematic uncertainty associated with nondynamic-dipole contributions to the frequency shift. We map the commensurate-incommensurate transition, a transition between two incommensurate phases, and the melting transition. These results are compared to previous x-ray data. We provide a new determination of the layer critical point (156 K), the layer condensation line down to 110 K, and the spreading pressure at saturation in this temperature range.

Solar fuel photoanodes prepared by inkjet printing of copper vanadates

Widespread deployment of solar fuel generators requires the development of efficient and scalable functional materials, especially for photoelectrocatalysis of the oxygen evolution reaction. Metal oxides comprise the most promising class of photoanode materials, but no known material meets the demanding photoelectrochemical requirements. Copper vanadates have recently been identified as a promising class of photoanode materials with several phases exhibiting an indirect band gap near 2 eV and stable photoelectrocatalysis of the oxygen evolution reaction in a pH 9.2 electrolyte. By employing combinatorial inkjet printing of metal precursors and applying both calcination and rapid thermal processing, we characterize the phase behaviour of the entire CuO–V2O5 composition space for different thermal treatments via automated analysis of approximately 100 000 Raman spectra acquired using a novel Raman imaging technique. These results enable the establishment of structure–property relationships for optical absorption and photoelectrochemical properties, revealing that highly active photoelectrocatalysts containing α-Cu2V2O7 or α-CuV2O6 can be prepared using scalable solution processing techniques. An additional discovery results from the formation of an off-stoichiometric β-Cu2V2O7 material that exhibits high photoelectroactivity in the presence of a ferri/ferrocyanide redox couple with excellent stability in a pH 13 electrolyte, demonstrating that copper vanadates may be viable photoanodes in strong alkaline electrolytes.