Chemistry, Department of

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Determining Adhesion of Non-Uniform Arrays of Fibrils

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2012-08-10
Abstract: 

Dry adhesives containing nonuniform arrays of fibrils were tested for the uniformity of their adhesion strength. These arrays comprised fibrils with nanometer-scale dimensions and lengths tuned from 150 to 1500 nm. The surfaces of the fibrils were rendered hydrophobic through a vapor phase deposition of silane molecules to further tune the adhesion strength of the fibrillar structure. Adhesion force measurements over micrometer-length scales were obtained using a tipless cantilever controlled by a scanning probe microscope. Maps of the adhesion forces depicted diverse variations in adhesion strength with the nonuniform lateral changes in topography. Through an extensive data analysis, differences observed between samples were correlated to changes in processing conditions and surface chemistry modifications. The methods demonstrated in this paper are useful for identifying variations in the adhesion strength of dry adhesives made of nonuniform arrays of fibrils. These advancements are crucial for understanding the correlation between structure and function within nonuniform fibrillar adhesives.

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Article
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In situ X-ray Absorption Spectroscopic Studies of Magnetic Fe@FexOy/Pd Nanoparticle Catalysts for Hydrogenation Reactions

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2017-03-09
Abstract: 

Core@shell Fe@FexOy nano particles ( have attracted a great deal of interest as potential magnetic supports for catalytic metals via galvanic exchange reactions. In this study Fe@Fe x O y /Pd bimetallic NPs were synthesized through galvanic exchange reaction s using 50:1, 20:1 and 5:1 molar ratio s of Fe@Fe x O y NPs to Pd(NO 3 2 . The resulting Fe@Fe x O y /Pd NPs have Pd nanoparticles on the Fe ox ide surfaces, and the NPs are linked in chains as shown by TEM analysis and EDX mapping. After galvanic reactions with Pd , the particles still retain their response to external magnetic fields . The magnetic properties of the resulting materials led to the ir successful application as nanometer sized magnetic stir bars for hydrogenation reactions. The Fe@Fe x O y /Pd NPs derived f rom the 5:1 molar ratio of their respective salts (Fe:Pd) exhibit ed a higher catalytic activity than particles synthesized from 20:1 and 50:1 molar ratios for the hydrogenation of 2 methyl 3 buten 2 ol . T he highest turnover frequency reach ed 3600 h 1 using ethanol as a solvent. In situ X ray absorption near edge structure (XANES) spectra show that the Fe@Fe x O y core shell particles in the Fe@Fe x O y /Pd system are easily oxidized when dispersed in water, while they are very stable if ethanol is used as a solvent. This oxidative stability has important implications for the use of such particles in real world applications.

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Covalent Surface Modification of Silicon Oxides with Alcohols in Polar Aprotic Solvents

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2017-05-30
Abstract: 

Alcohol based monolayers were successfully formed on the surfaces of silicon oxides through reaction performed in polar aprotic solvents. Monolayers prepared from alcohol based reagents have been previously introduced as an alternative approach to covalently modify the surfaces of silicon oxides. These reagents are readily available, widely distributed, and are minimally susceptible to side reactions with ambient moisture. A limitation of using alcohol based compounds is that previous reactions required relatively high temperatures in neat solutions, which can degrade some alcohol compounds or could lead to other unwanted side reactions during the formation of the monolayers. To overcome these challenges, we investigate the condensation reaction for alcohols on silicon oxides carried out in polar aprotic solvents. In particular, propylene carbonate has been identified as a polar aprotic solvent that is relatively non-toxic, readily accessible, and can facilitate the formation of alcohol based monolayers. We have successfully demonstrated tuning the surface chemistry of silicon oxide surfaces with a variety of alcohol containing compounds. The strategy introduced in this research can be utilized to create silicon oxide surfaces with hydrophobic, oleophobic, or charged functionalities.

Document type: 
Article

Ordered Porous Electrodes by Design: Towards Enhancing the Effective Utilization of Platinum in Electrocatalysis

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2017-08-04
Abstract: 

Platinum‐nanoparticle‐functionalized, ordered, porous support electrodes are prepared and characterized as a potential new class of oxygen reduction reaction (ORR) electrocatalysts. This study aims to develop electrode materials that enhance the effective utilization of Pt in electrocatalytic reactions through improved mass transport properties, high Pt mass specific surface area, and increased Pt electrochemical stability. The electrodes are prepared using modular sacrificial templates, producing a uniform distribution of Pt nanoparticles inside ordered porous Au electrodes. This method can be further fine‐tuned to optimize the architecture for a range of characteristics, such as varying nanoparticle properties, pore size, or support material. The Pt‐coated Au, ordered, porous electrodes exhibit several improved characteristics, such as enhanced Pt effective utilization for ORR electrocatalysis. This includes a nearly twofold increase in Pt mass specific surface area over other ultrathin designs, superior mass transport properties in comparison to traditional catalyst layers of C black supported Pt nanoparticles mixed with ionomer, good methanol tolerance and exceptional stability toward Pt chemical and/or electrochemical dissolution through interfacial interactions with Au. The methods to prepare Pt‐coated ordered porous electrodes can be extended to other architectures for enhanced catalyst utilization and improved performance of Pt in electrochemical processes.

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Surfactant Controlled Growth of Niobium Oxide Nanorods

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2017-08-14
Abstract: 

his paper describes a solution-phase hydrothermal synthesis of crystalline niobium pentoxide (Nb2O5) nanorods. The methods reported herein yield uniform Nb2O5 nanorods with average diameters of 6 nm and lengths of 38 nm, which are directly synthesized from niobic acid by a hydrothermal process. The formation of Nb2O5 nanorods from niobic acid was studied in the presence of surfactants that stabilize the nanostructures. The crystalline Nb2O5 nanorods were relatively uniform in size and shape. The size of the Nb2O5 nanorods could be tuned through the choice of surfactant even in the absence of a worm-like micellar morphology. Amine, amide, ammonium, carboxylate, sulfonate, and sulfate containing surfactants were systematically evaluated for their influence on the ability to form uniform Nb2O5 nanorods. The surfactants in this study had hydrophobic tails that were either straight or branched, such as a polymer, and contained either a single or multiple head groups. The nanorods grew by a process of oriented attachment of nanoparticles that was regulated by the surfactants added into the reaction mixture. The results of these studies indicate that this synthetic approach serves as a tunable platform to prepare single crystalline niobium oxide based nanostructures with well-defined morphologies and dimensions. This surfactant assisted formation of crystalline Nb2O5 nanorods could also have important implications in the design of other transition metal oxide based nanomaterials.

Document type: 
Article

Block copolymer templated synthesis of PtIr bimetallic nanocatalysts for the formic acid oxidation reaction

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2017-09-27
Abstract: 

Arrays of PtIr alloy nanoparticle (NP) clusters are synthesized from a method using block copolymer templates, which allows for relatively narrow NP diameter distributions (~4–13 nm) and uniform intercluster spacing (~ 60 or ~100 nm). Polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer micelles were used to create thin film templates of NPs with periodic pyridinium-rich domains that are capable of electrostatically loading PtCl62- and IrCl62- anion precursors for the preparation of NP arrays. The composition of PtIr NPs was specified by the ratio of metal anions in a low-pH immersion bath. Formic acid oxidation, studied by cyclic voltammetry, shows that the arrays of clusters of PtIr alloy NPs are highly active catalysts, with mass activity values on par or exceeding current industrial standard catalysts. The uniformity in the NP population in a cluster and the small diameter range established by the block copolymer template permit an estimate of the optimal Pt:Ir ratio for the direct oxidation of formic acid, where, ~10 nm Pt16Ir84 alloy NPs were the most active with a mass activity of 37 A/g.  

Document type: 
Article

Hexagonal Arrays of Cylindrical Nickel Micro-structures for Improved Oxygen Evolution Reaction

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2017-02-06
Abstract: 

Fuel-cell systems are of interest for a wide range of applications, in part for their utility in power generation from nonfossil-fuel sources. However, the generation of these alternative fuels, through electrochemical means, is a relatively inefficient process due to gas passivation of the electrode surfaces. Uniform microstructured nickel surfaces were prepared by photolithographic techniques as a systematic approach to correlating surface morphologies to their performance in the electrochemically driven oxygen evolution reaction (OER) in alkaline media. Hexagonal arrays of microstructured Ni cylinders were prepared with features of proportional dimensions to the oxygen bubbles generated during the OER process. Recessed and pillared features were investigated relative to planar Ni electrodes for their influence on OER performance and, potentially, bubble release. The arrays of cylindrical recesses were found to exhibit an enhanced OER efficiency relative to planar nickel electrodes. These microstructured electrodes had twice the current density of the planar electrodes at an overpotential of 100 mV. The results of these studies have important implications to guide the preparation of more-efficient fuel generation by water electrolysis and related processes.

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Tuning Oleophobicity of Silicon Oxide Surfaces with Mixed Monolayers

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2016-11-21
Abstract: 

We demonstrate the formation of mixed monolayers derived from a microwave-assisted reaction of alcohols with silicon oxide surfaces in order to tune their surface oleophobicity. This simple, rapid method provides an opportunity to precisely tune the constituents of the monolayers. As a demonstration, we sought fluorinated alcohols and aliphatic alcohols as reagents to form monolayers from two distinct constituents for tuning the surface oleophobicity. The first aspect of this study sought to identify a fluorinated alcohol that formed monolayers with a relatively high surface coverage. It was determined that 1H,1H,2H,2H-perfluoro-1-octanol yielded high quality monolayers with a water contact angle (WCA) value of ∼110° and contact angle values of ∼80° with toluene and hexadecane exhibiting both an excellent hydrophobicity and oleophobicity. Tuning of the oleophobicity of the modified silicon oxide surfaces was achieved by controlling the molar ratio of 1H,1H,2H,2H-perfluoro-1-octanol within the reaction mixtures. Surface oleophobicity progressively decreased with a decrease in the fluorinated alcohol content while the monolayers maintained their hydrophobicity with WCA values of ∼110°. The simple and reliable approach to preparing monolayers of a tuned composition that is described in this article can be utilized to control the fluorocarbon content of the hydrophobic monolayers on silicon oxide surfaces.

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Article
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Rapid Covalent Modification of Silicon Oxide Surfaces through Microwave-Assisted Reactions with Alcohols

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2016-07-10
Abstract: 

We demonstrate the method of a rapid covalent modification of silicon oxide surfaces with alcohol-containing compounds with assistance by microwave reactions. Alcohol-containing compounds are prevalent reagents in the laboratory, which are also relatively easy to handle because of their stability against exposure to atmospheric moisture. The condensation of these alcohols with the surfaces of silicon oxides is often hindered by slow reaction kinetics. Microwave radiation effectively accelerates this condensation reaction by heating the substrates and/or solvents. A variety of substrates were modified in this demonstration, such as silicon oxide films of various thicknesses, glass substrates such as microscope slides (soda lime), and quartz. The monolayers prepared through this strategy demonstrated the successful formation of covalent surface modifications of silicon oxides with water contact angles of up to 110° and typical hysteresis values of 2° or less. An evaluation of the hydrolytic stability of these monolayers demonstrated their excellent stability under acidic conditions. The techniques introduced in this article were successfully applied to tune the surface chemistry of silicon oxides to achieve hydrophobic, oleophobic, and/or charged surfaces.

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Article
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Hierarchical Surface Coatings of Polystyrene Nanofibers and Silica Microparticles with Rose Petal Wetting Properties

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2016-06-05
Abstract: 

Surfaces with rose petal like properties, simultaneously exhibiting a high degree of hydrophobicity and a high adhesion to water, were prepared by spray coating of progressively smaller hydrophilic silica particles along with hydrophobic nanofibers onto surfaces of interest. Various polymer structures were achieved by tuning the spray coating flow rates during deposition of the polymer nanofibers and silica particles. At a reduced flow rate, polystyrene fibers were formed with diameters of less than 100 nm. Water contact angles (WCAs) of coatings prepared from the hierarchical assemblies of silica particles blended with polystyrene nanofibers were greater than 110°. Coatings prepared from the hierarchical assemblies either with or without incorporation of the polymer nanofibers pinned water droplets to their surfaces even after inverting the substrates, similar to the properties of a rose petal. Hierarchical coatings of silica particles without the polystyrene nanofibers also exhibited a high adhesion to water, pinning at least 30% more water on its surfaces. Conversely, hierarchical coatings containing the polystyrene nanofibers exhibited an increased water mobility across their surfaces. Further water retention experiments were performed to determine the ability of the different coatings to efficiently condense water vapor, as well as their efficiency to remove this condensed liquid from their surfaces. Both types of hierarchical coatings exhibited an excellent ability to retain water at a low humidity, while establishing a self-limiting condition for retaining water at a high humidity. These coatings could be prepared on a relatively large-scale and with a relatively low cost on the surfaces of a variety of materials to enhance their water resistance, water retention and/or ability to condense water vapor.

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