Chemistry, Department of

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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.

Document type: 
Article
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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.

Document type: 
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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Article
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Improved Adhesion and Compliancy of Hierarchical Fibrillar Adhesives

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2015-07-13
Abstract: 

The gecko relies on van der Waals forces to cling onto surfaces with a variety of topography and composition. The hierarchical fibrillar structures on their climbing feet, ranging from mesoscale to nanoscale, are hypothesized to be key elements for the animal to conquer both smooth and rough surfaces. An epoxy-based artificial hierarchical fibrillar adhesive was prepared to study the influence of the hierarchical structures on the properties of a dry adhesive. The presented experiments highlight the advantages of a hierarchical structure despite a reduction of overall density and aspect ratio of nanofibrils. In contrast to an adhesive containing only nanometer-size fibrils, the hierarchical fibrillar adhesives exhibited a higher adhesion force and better compliancy when tested on an identical substrate.

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Article
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Material Versatility Using Replica Molding for Large-Scale Fabrication of High Aspect-Ratio, High Density Arrays of Nano-Pillars

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2014-06-27
Abstract: 

Arrays of high aspect-ratio (AR) nano-pillars have attracted a lot of interest for various applications, such as for use in solar cells, surface acoustic sensors, tissue engineering, bio-inspired adhesives and anti-reflective surfaces. Each application may require a different structural material, which can vary in the required chemical composition and mechanical properties. In this paper, a low cost fabrication procedure is proposed for large scale, high AR and high density arrays of nano-pillars. The proposed method enables the replication of a master with high fidelity, using the subsequent replica molds multiple times, and preparing arrays of nano-pillars in a variety of different materials. As an example applied to bio-inspired dry adhesion, polymeric arrays of nano-pillars are prepared in this work. Thermoset and thermoplastic nano-pillar arrays are examined using an atomic force microscope to assess their adhesion strength and its uniformity. Results indicate the proposed method is robust and can be used to reliably prepare nano-structures with a high AR.

Document type: 
Article
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Harnessing Tunable Scanning Probe Techniques to Measure Shear Enhanced Adhesion of Gecko-Inspired Fibrillar Arrays

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2014-12-30
Abstract: 

The hierarchical arrays of mesoscale to nanoscale fibrillar structures on a gecko’s foot enable the animal to climb surfaces of varying roughness. Adhesion force between the fibrillar structures and various surfaces is maximized after the gecko drags its foot in one direction, which has also been demonstrated to improve the adhesion forces of artificial fibrillar arrays. Essential conditions that influence the magnitude of these interactions include the lateral distance traveled and velocity between the contacting surfaces, as well as the velocity at which the two surfaces are subsequently separated. These parameters have, however, not been systematically investigated to assess the adhesion properties of artificial adhesives. We introduce a systematic study that investigates these conditions using a scanning probe microscope to measure the adhesion forces of artificial adhesives through a process that mimics the mechanism by which a gecko climbs. The measured adhesion response was different for arrays of shorter and longer fibrils. These results from 9000 independent measurements also provide further insight into the dynamics of the interactions between fibrillar arrays and contacting surfaces. These studies establish scanning probe microscopy techniques as a versatile approach for measuring a variety of adhesion properties of artificial fibrillar adhesives.

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Article
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Self-Assembly of Nanoparticles onto the Surfaces of Polystyrene Spheres with a Tunable Composition and Loading

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2015-01-09
Abstract: 

Functional colloidal materials were prepared by design through the self-assembly of nanoparticles (NPs) on the surfaces of polystyrene (PS) spheres with control over NP surface coverage, NP-to-NP spacing, and NP composition. The ability to control and fine tune the coating was extended to the first demonstration of the co-assembly of NPs of dissimilar composition onto the same PS sphere, forming a multi-component coating. A broad range of NP decorated PS (PS@NPs) spheres were prepared with uniform coatings attributed to electrostatic and hydrogen bonding interactions between stabilizing groups on the NPs and the functionalized surfaces of the PS spheres. This versatile two-step method provides more fine control than methods previously demonstrated in the literature. These decorated PS spheres are of interest for a number of applications, such as catalytic reactions where the PS spheres provide a support for the dispersion, stabilization, and recovery of NP catalysts. The catalytic properties of these PS@NPs spheres were assessed by studying the catalytic degradation of azo dyes, an environmental contaminant detrimental to eye health. The PS@NPs spheres were used in multiple, sequential catalytic reactions while largely retaining the NP coating.

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Article
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Microwave Assisted Formation of Monoreactive Perfluoroalkylsilane-based Self-Assembled Monolayers

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2014-12-15
Abstract: 

We demonstrate the use of microwave radiation as a tool to accelerate the formation of perfluoroalkylsilane based self-assembled monolayers (SAMs) on silicon oxide surfaces. Surface coverage of these SAMs of monoreactive perfluoroalkylsilanes increased in proportion to the duration over which the solutions were heated by microwave radiation. 

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Article
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