Anastasios P. Angelopoulos

Microscopic observations of condensation of water on lotus leaves

We report an in situ observation of water condensation and evaporation on lotus leaf surfaces inside an environmental scanning electron microscope. The real-time observation shows, at the micrometer length scale, how water drops grow to large contact angles during water condensation, and decrease in size and contact angle during the evaporation phase of the experiment. To rationalize the observations, we propose a geometric model for liquid drops on rough surfaces when the size of the drop and surface roughness scale are comparable. This model suggests that when drop size and surface roughness are of the same magnitude, such as micrometer size water drops on lotus leaves, well-known equations for wetting on rough surfaces may not be applicable.

Silica nanoparticle layer-by-layer assembly on gold.

Layer-by-layer (LBL) assembly of silica nanoparticles is investigated as a means of controlling the surface wetting properties of gold electroplated onto 316 L stainless-steel substrates while maintaining a low electrical surface contact resistance. The strong polyelectrolyte acrylamide/beta-methacryl-oxyethyl-trimethyl-ammonium copolymer is used as the cationic binder. The impact of silica nanoparticle zeta (zeta) potential for a range of -37.1 to 5.9 mV in the thickness, wettability, and contact resistance of the final LBL-assembled coatings is presented. The zeta potential is varied by altering both the pH and alcohol (ethanol) content of the silica suspensions and polymer suspension, consistent with the predictions of the Debye-Huckel equation. Nanoparticle adsorption is found to occur rapidly, with surface coverage equilibration obtained after only 1 min and uptake that is nearly linear with respect to the number of bilayers deposited. An increase in the absolute value of the (negative) zeta potential in the silica suspension is found to increase the bilayer thickness to an average value as high as 82% of the individual nanoparticle diameter for the smaller nanoparticles investigated, suggesting that nearly complete surface coverage may be achieved after the application of only a single nanoparticle-polymer bilayer (a coating thickness as low as 15.6 nm) and that nanoparticle adsorption is enhanced by electrostatic attraction between substrate and adsorbate. Counterintuitively, a more porous bilayer structure is observed if the zeta potential of the previously deposited nanoparticles is increased while the substrate is immersed in the cationic copolymer suspension, suggesting that copolymer adsorption in inhibited by substrate-solvent interactions. Wetting measurements demonstrate that silica LBL assembly results in a substantial reduction in contact angle from 84 degrees on the bare substrate surface to as low as 15 degrees after the application of a single bilayer and 7 degrees after the application of eight bilayers. A monotonic increase in coating contact resistance is observed with an increase in the thickness with a characteristic volumetric electrical through-plane resistivity of as low as 1.63 kOmega.cm obtained from contact resistance measurement.

Portable method of measuring gaseous acetone concentrations.

Measurement of acetone in human breath samples has been previously shown to provide significant non-invasive diagnostic insight into the control of a patients diabetic condition. In patients with diabetes mellitus, the body produces excess amounts of ketones such as acetone, which are then exhaled during respiration. Using various breath analysis methods has allowed for the accurate determination of acetone concentrations in exhaled breath. However, many of these methods require instrumentation and pre-concentration steps not suitable for point-of-care use. We have found that by immobilizing resorcinol reagent into a perfluorosulfonic acid polymer membrane, a controlled organic synthesis reaction occurs with acetone in a dry carrier gas. The immobilized, highly selective product of this reaction (a flavan) is found to produce a visible spectrum color change which could measure acetone concentrations to less than ppm. We here demonstrate how this approach can be used to produce a portable optical sensing device for real-time, non-invasive acetone analysis.

Enhanced Electrocatalytic Oxygen Reduction through Electrostatic Assembly of Pt Nanoparticles onto Porous Carbon Supports from SnCl2-Stabilized Suspensions

Monodisperse Pt nanoparticles with atomic structures that span the cluster to crystal transition have recently been synthesized in electrostatically stabilized, aqueous-based suspensions. In the present study, the anionic charge from the stabilizing SnCl(2) sheath adsorbed on the surface of these particles is used for the first time to assemble Pt directly onto porous carbon supports via electrostatic assembly. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) reveals that these assemblies have substantially higher Pt-C dispersions than obtained from precipitation methods commonly used for commercial electrocatalyst systems. Energy dispersive spectroscopy (EDS) and inductively coupled plasma-mass spectrometry (ICP-MS) are used to determine that loadings of 10-30% by weight Pt (particle packing fractions from 0.05 to 0.25) are obtained through a single electrostatic application of these particles on Vulcan carbon, depending on particle size. The highest average oxygen reduction reaction (ORR) mass activity obtained using this approach is 90.4 A/g(Pt) at 0.9 V vs RHE in 0.1 M perchloric acid is with 1-2 nm particles that exhibit a transitional atomic structure. This activity compares to an average value of 74.0 A/g(Pt) obtained from densely packed electrostatic layer-by-layer (LbL) assemblies of unsupported particles and 36.7 A/g(Pt) commercial Vulcan electrocatalyst from Tanaka Kikinzoku Kogyo (TKK). Enhanced activity is observed with electrostatic assembly of any particle size on Vulcan relative to unsupported or commercial electrocatalyst with comparable durability. Such enhanced activity is attributed to improved reactant accessibility to the catalyst surface due to the increase in particle dispersion. An extinction coefficient of 7.41 m(2)/g at 352 nm is obtained across the entire cluster to crystal transition from 20 atom clusters to 2.9 nm single crystal nanoparticles, indicating that observed variation in ORR activity with particle size may be associated primarily with changes in atomic surface structure as opposed to the metallic character of the nanoparticles as assessed by UV-vis spectroscopy.

Improved electrocatalytic ethanol oxidation activity in acidic and alkaline electrolytes using size-controlled Pt-Sn nanoparticles.

The promotion of the electrocatalytic ethanol oxidation reaction (EOR) on extended single-crystal Pt surfaces and dispersed Pt nanoparticles by Sn under acidic conditions is well known. However, the correlation of Sn coverage on Pt nanoparticle electrocatalysts to their size has proven difficult. The reason is that previous investigations have typically relied on commercially difficult to reproduce electrochemical treatments of prepared macroscopic electrodes to adsorb Sn onto exposed Pt surfaces. We demonstrate here how independent control over both Sn coverage and particle size can yield a significant enhancement in EOR activity in an acidic electrolyte relative to previously reported electrocatalysts. Our novel approach uses electroless nanoparticle synthesis where surface-adsorbed Sn is intrinsic to Pt particle formation. Sn serves as both a reducing agent and stabilizing ligand, producing particles with a narrow particle size distribution in a size range where the mass-specific electrocatalytic activity can be maximized (ca. 1-4 nm) as a result of the formation of a fully developed Sn shell. The extent of fractional Sn surface coverage on carbon-supported Pt nanoparticles can be systematically varied through wet-chemical treatment subsequent to nanoparticle formation but prior to incorporation into macroscopic electrodes. EOR activity for Pt nanoparticles is found to be optimum at a fractional Sn surface coverage of ca. 0.6. Furthermore, the EOR activity is shown to increase with Pt particle size and correlate with the active area of available Pt (110) surface sites for the corresponding Sn-free nanoparticles. The maximum area- and mass-specific EOR activities for the most active catalyst investigated were 17.9 μA/cm(2)Pt and 12.5 A/gPt, respectively, after 1 h of use at 0.42 V versus RHE in an acidic electrolyte. Such activity is a substantial improvement over that of commercially available Pt, Pt-Sn, and Pt-Ru alloy catalysts under either acidic or alkaline conditions.

A nanoscale-modified LaMer model for particle synthesis from inorganic tin–platinum complexes

The size-tunable structure and properties of Pt nanoparticles at the atomic length scale have attracted significant attention across a wide variety of fields including magnetics, electrocatalysis, optics, and gas-phase synthesis. Mechanisms responsible for the formation Pt nanoparticles remain unclear because of the difficulty generating in situ data for the time-evolution of size, shape, distribution, volume fraction, particle number density, and oxidation state from the starting complexes. We here demonstrate the use of simultaneous small- and wide-angle X-ray scattering combined with UV-vis spectroscopy to measure these key synthesis metrics for the reduction of Pt(IV) by Sn(II) in aqueous solution. This synthesis approach has been previously shown to permit continuous control over Pt nanoparticle size from 0.9 to 2.6 nm to within 10% standard deviation. Such fine control led to the discovery of densely packed amorphous structures at ca. 1.7 nm with substantially enhanced electrocatalytic oxygen reduction relative to nanocrystals and commercial electrocatalysts. Ex situ UV-vis and in situ X-ray scattering are here shown to reveal four distinct stages during synthesis: (1) autoreduction of a ligand/noble metal complex with a unique structure that depends on the Sn(II)/Pt(II) ratio, (2) generation of Pt primary particles and the formation of Pt nuclei at a rate that depends on the structure of the initial complex, (3) nanoparticle growth via LaMers diffusion of these primary particles to the nuclei, and (4) growth termination due to capping from a stabilizing, two-layer ligand shell. We derive a set of consecutive rate equations and associated kinetic parameters that describe each step. The kinetics of ligand rearrangement has been previously found to limit the rate of nanoparticle growth. We incorporate this phenomenon into LaMers classic diffusion-limited growth scheme to extend it to the nanoscale regime. This new model provides detailed understanding of how metal ligands serve as both reducing and stabilizing agents and allow for unprecedented, continuous control over both size and distribution. Systematic variation of temperature permits detailed time resolution at the very onset of Pt primary particle formation, as well as a means to determine temperature sensitivity of nanoparticle growth.

Acceleration of Electrocatalytic Activity on Pt–Sn Nanoparticles

A SnCl 2 shell on Pt metal core nanoparticle synthesis technique has recently been demonstrated to permit electrostatic layer-by-layer (LbL) assembly of well-ordered electrocatalysts without precipitation onto porous carbon supports. In this paper, the electrocatalytic activity of the LbL-assembled Pt nanoparticles is shown to depend critically upon removal of surface-adsorbed Sn (Sn ads ). By subjecting the synthesized Pt nanoparticle electrodes to potential sweeps greater than 1.0 V vs reversible hydrogen electrode, Sn ads are removed and a nearly threefold enhancement in oxygen reduction reaction (ORR) specific activity over commercial catalysts is obtained. In contrast to this electrochemical acceleration approach, we also investigate electroless, wet-acceleration methods for Sn ads removal. Energy-dispersive spectroscopy and inductively coupled plasma-mass spectrometry are used to quantify the Pt/Sn ratio in the electrode assemblies as a function of immersion time in solution (both alkaline and acidic) and during electrochemical acceleration, respectively. Charging current for the underpotential deposition of protons on the Pt nanoparticle surface is used to monitor the removal of Sn ads during electrochemical acceleration, followed by ORR activity measurement in saturated perchloric acid (HClO 4 ). Wet-chemical acceleration in NaOH solution is found to remove similar amounts of Sn as compared to the electrochemical technique.

Perfluorosulfonic Acid Membrane Catalysts for Optical Sensing of Anhydrides in the Gas Phase

Continuous, on-site monitoring of personal exposure levels to occupational chemical hazards in ambient air is a long-standing analytical challenge. Such monitoring is required to institute appropriate health measures but is often limited by the time delays associated with batch air sampling and the need for off-site instrumental analyses. In this work, we report on the first attempt to use the catalytic properties of perfluorosulfonic acid (PSA) membranes to obtain a rapid, selective, and highly sensitive optical response to trimellitic anhydride (TMA) in the gas phase for portable sensor device application. TMA is used as starting material for various organic products and is recognized to be an extremely toxic agent by the National Institute for Occupational Safety and Health (NIOSH). Resorcinol dye is shown to become immobilized in PSA membranes and diffusionally constrain an orange brown product that results from acid-catalyzed reaction with more rapidly diffusing TMA molecules. FTIR, UV/vis, reaction selectivity to TMA versus trimellitic acid (TMLA), and homogeneous synthesis are used to infer 5,7- dihydroxyanthraquinone-2-carboxylic acid as the acylation product of the reaction. The color response has a sensitivity to at least 3 parts per billion (ppb) TMA exposure and, in addition to TMLA, excludes maleic anhydride (MA) and phthalic anhydride (PA). Solvent extraction at long times is used to determine that the resorcinol extinction coefficient in 1100 EW PSA membrane has a value of 1210 m(2)/g at 271.01 nm versus a value of 2010 m(2)/g at 275.22 nm in 50 vol% ethanol/water solution. The hypsochromic wavelength shift and reduced extinction coefficient suggest that the polar perfluorosulfonic acid groups in the membrane provide the thermodynamic driving force for diffusion and immobilization. At a resorcinol concentration of 0.376 g/L in the membrane, a partition coefficient of nearly unity is obtained between the membrane and solution concentrations and a maximum conversion rate of one ambient TMA molecule for every two membrane-immobilized resorcinol molecules is observed in 15 min.

Competing effects of silanol surface concentration and solvent dielectric constant on electrostatic layer-by-layer assembly of silica nanoparticles on gold

Two types of silica nanoparticles having differing concentrations of ionizable surface groups are used to investigate the interplay between nanoparticle surface charge and solvent dielectric constant in nanostructure development during layer-by-layer assembly with a cationic polyacrylamide. Zeta (zeta) potential measurements are used to determine the extent of silanol dissociation with pH. For 19-nm-diameter X-Tec 3408 silica nanoparticles from Nano-X GmbH (NanoX), complete dissociation yields a zeta-potential value of about -44mV and occurs between pH 5 and 6 in 50% ethanol-in-water mixture by volume. By contrast, 65-nm-diameter polishing silica from Electron Microscopy Supply (EMS) has a zeta potential that does not equilibrate even up to pH 7 with a value of -59mV under otherwise similar solution conditions. The more negative zeta potential at a given pH is found to substantially reduce nanoparticle adsorption. This behavior is opposite that observed when the dielectric constant of the suspension is decreased, independent of particle size. Nanoparticle surface chemical heterogeneity is discussed as a plausible explanation for such seriously discrepant behavior and the effects on multilayer electrical contact resistance for proton-exchange membrane (PEM) fuel-cell coating applications are presented.

Gold Surface Adsorption Properties of the Enzymatically Polymerized Amphiphilic Decyl Ester of L‐Tyrosine

In aqueous solution, both amphiphilic decyl esters of the amino acids D & L‐Tyrosine have been shown to self‐assemble at identical low mM range c.m.c. values to form similar rod or plate‐like fibers a few microns wide and with lengths ranging from tens to hundreds of microns [Mat. Sci. Eng. C11 2000 155–163]. These monomers have only slightly different rates of polymerization with horseradish peroxidase below their c.m.c. and have been shown to possess pH dependent self‐assembly properties, as well as significantly increased binding to the gold surface of a quartz crystal microbalance with increasing pH [Biotech. Progress 15 1999 522–528]. In the present study, we quantitate the pH dependence and gold surface adsorption kinetics of the decyl ester of L‐tyrosine using X‐ray photoelectron spectroscopy (XPS) and then correlate these data with monomer solution titration studies. These data support the notion of increasing gold surface adsorption with increasing pH in the range from 5–8. We present evidence that this involves the lowered solubility of the neutral charged species resulting from the deprotonation of the α‐NH3 + in each monomer unit. At 24 h adsorption at pH 7.0, we observed XPS evidence for complete coverage of the Au surface. From these data for the Au 4f photoelectron signal attenuation, we estimate a film thickness of 5.89 nm from the attenuation of the Au4f photoelectron intensity by the growing film.