Stanislaus S. Wong

Enhanced Electrocatalytic Performance of Processed, Ultrathin, Supported Pd–Pt Core–Shell Nanowire Catalysts for the Oxygen Reduction Reaction

We report on the synthesis, characterization, and electrochemical performance of novel, ultrathin Pt monolayer shell-Pd nanowire core catalysts. Initially, ultrathin Pd nanowires with diameters of 2.0 ± 0.5 nm were generated, and a method has been developed to achieve highly uniform distributions of these catalysts onto the Vulcan XC-72 carbon support. As-prepared wires are activated by the use of two distinctive treatment protocols followed by selective CO adsorption in order to selectively remove undesirable organic residues. Subsequently, the desired nanowire core-Pt monolayer shell motif was reliably achieved by Cu underpotential deposition followed by galvanic displacement of the Cu adatoms. The surface area and mass activity of the acid and ozone-treated nanowires were assessed, and the ozone-treated nanowires were found to maintain outstanding area and mass specific activities of 0.77 mA/cm(2) and 1.83 A/mg(Pt), respectively, which were significantly enhanced as compared with conventional commercial Pt nanoparticles, core-shell nanoparticles, and acid-treated nanowires. The ozone-treated nanowires also maintained excellent electrochemical durability under accelerated half-cell testing, and it was found that the area-specific activity increased by ~1.5 fold after a simulated catalyst lifetime.

One-dimensional noble metal electrocatalysts: a promising structural paradigm for direct methanol fuel cells

In this perspective, the catalytic shortfalls of contemporary DMFCs are discussed in the context of the materials that are currently being employed as electrocatalysts in both the anode and cathode. In light of these shortfalls, the inherent advantages of one-dimensional (1D) nanostructures are highlighted so as to demonstrate their potential as efficient, robust, and active replacements for contemporary nanoparticulate electrocatalysts. Finally, we review in detail the recent applications of 1D nanostructured electrocatalysts as both anodes and cathodes, and explore their potentially promising results towards improving DMFC efficiency and cost-effectiveness. In the case of cathode electrocatalysts, our group has recently prepared both 200 nm platinum nanotubes and ultrathin 2 nm platinum nanowires, which evinced two-fold and seven-fold enhancements in area specific ORR activity, respectively, as compared with contemporary commercial Pt nanoparticles. Similarly, the development of one-dimensional anodic electrocatalysts such as alloyed PtRu and PtCo nanowires, hierarchical Pt~Pd nanowires, and segmented PtRu systems have yielded promising enhancements towards methanol oxidation.

Carbon nanotube–nanocrystal heterostructures

The importance of generating carbon nanotube-nanoparticle heterostructures is that these composites ought to take advantage of and combine the unique physical and chemical properties of both carbon nanotubes and nanoparticles in one discrete structure. These materials have potential applicability in a range of diverse fields spanning heterogeneous catalysis to optoelectronic device development, of importance to chemists, physicists, materials scientists, and engineers. In this critical review, we present a host of diverse, complementary strategies for the reliable synthesis of carbon nanotube-nanoparticle heterostructures using both covalent as well as non-covalent protocols, incorporating not only single-walled and multi-walled carbon nanotubes but also diverse classes of metallic and semiconducting nanoparticles (221 references).

Size-Dependent Enhancement of Electrocatalytic Performance in Relatively Defect-Free, Processed Ultrathin Platinum Nanowires

We report on the synthesis, characterization, and electrocatalytic performance of ultrathin Pt nanowires with a diameter of less than 2 nm. An acid-wash protocol was employed in order to yield highly exfoliated, crystalline nanowires with a diameter of 1.3 +/- 0.4 nm. The electrocatalytic activity of these nanowires toward the oxygen reduction reaction was studied in relation to the activity of both supported and unsupported Pt nanoparticles as well as with previously synthesized Pt nanotubes. Our ultrathin, acid-treated, unsupported nanowires displayed an electrochemical surface area activity of 1.45 mA/cm(2), which was nearly 4 times greater than that of analogous, unsupported platinum nanotubes and 7 times greater than that of commercial supported platinum nanoparticles.

Purification strategies and purity visualization techniques for single-walled carbon nanotubes

Traditionally, SWNTs have been prepared by electric arc-discharge, laser ablation, or chemical vapor deposition (CVD) methods, yielding tubes of various diameter and length distributions. However, without exception, as-prepared SWNTs are contaminated with a number of impurities, decreasing the overall yield of usable material. Impurities include transition metal catalysts, such as Fe, Co, and Ni, which are necessary for the growth of SWNTs as well as carbonaceous species such as amorphous carbon, fullerenes, multishell carbon nanocapsules, and nanocrystalline graphite. In this Feature Article, we highlight the role of microscopy in designing and facilitating the interpretation of the success of strategies, including (a) oxidative methods including liquid and gas phase oxidation, (b) chemical functionalization protocols, (c) filtration and chromatography techniques, and (d) microwave heating methods, aimed at rationally purifying single-walled carbon nanotubes.

Mechanism-Based Tumor-Targeting Drug Delivery System. Validation of Efficient Vitamin Receptor-Mediated Endocytosis and Drug Release

An efficient mechanism-based tumor-targeting drug delivery system, based on tumor-specific vitamin-receptor mediated endocytosis, has been developed. The tumor-targeting drug delivery system is a conjugate of a tumor-targeting molecule (biotin: vitamin H or vitamin B-7), a mechanism-based self-immolative linker and a second-generation taxoid (SB-T-1214) as the cytotoxic agent. This conjugate (1) is designed to be (i) specific to the vitamin receptors overexpressed on tumor cell surface and (ii) internalized efficiently through receptor-mediated endocytosis, followed by smooth drug release via glutathione-triggered self-immolation of the linker. In order to monitor and validate the sequence of events hypothesized, i.e., receptor-mediated endocytosis of the conjugate, drug release, and drug-binding to the target protein (microtubules), three fluorescent/fluorogenic molecular probes (2, 3, and 4) were designed and synthesized. The actual occurrence of these processes was unambiguously confirmed by means of confocal fluorescence microscopy (CFM) and flow cytometry using L1210FR leukemia cells, overexpressing biotin receptors. The molecular probe 4, bearing the taxoid linked to fluorescein, was also used to examine the cell specificity (i.e., efficacy of receptor-based cell targeting) for three cell lines, L1210FR (biotin receptors overexpressed), L1210 (biotin receptors not overexpressed), and WI38 (normal human lung fibroblast, biotin receptor negative). As anticipated, the molecular probe 4 exhibited high specificity only to L1210FR. To confirm the direct correlation between the cell-specific drug delivery and anticancer activity of the probe 4, its cytotoxicity against these three cell lines was also examined. The results clearly showed a good correlation between the two methods. In the same manner, excellent cell-specific cytotoxicity of the conjugate 1 (without fluorescein attachment to the taxoid) against the same three cell lines was confirmed. This mechanism-based tumor-targeting drug delivery system will find a range of applications.

A Facile and Mild Synthesis of 1-D ZnO, CuO, and α-Fe2O3 Nanostructures and Nanostructured Arrays

ZnO nanowires, CuO nanowires, and alpha-Fe(2)O(3) nanotubes as well as their corresponding arrays have been successfully synthesized via a low cost, generalizable, and simplistic template method. Diameters of one-dimensional (1-D) metal oxide nanostructures ( approximately 60-260 nm), measuring micrometers in length, can be reliably and reproducibly controlled by the template pore channel dimensions. Associated vertically aligned arrays have been attached to the surfaces of a number of geometrically significant substrates, such as curved plastic and glass rod motifs. The methodology reported herein relies on the initial formation of an insoluble metal hydroxide precursor, initially resulting from the reaction of the corresponding metal solution and sodium hydroxide, and its subsequent transformation under mild conditions into the desired metal oxide nanostructures. Size- and shape-dependent optical, magnetic, and catalytic properties of as-prepared 1-D metal oxides were investigated and noted to be mainly comparable to or better than the associated properties of the corresponding bulk oxides. A plausible mechanism for as-observed wire and tube-like motifs is also discussed.

EFFECT OF MORPHOLOGY OF ZNO NANOSTRUCTURES ON THEIR TOXICITY TO MARINE ALGAE

The influence of ZnO nanoparticle morphology on its toxicity for marine diatoms was evaluated. Four ZnO nanoparticle motifs, possessing distinctive sizes and shapes, were synthesized without adding surfactants. Diameters of ZnO spheres ranged from 6.3 nm to 15.7 nm, and lengths of rod-shaped particles were 242 nm to 862 nm. Their effects on the growth of the marine diatoms, Thalassiosira pseudonana, Chaetoceros gracilis, and Phaeodactylum tricornutum, were determined in laboratory cultures. Between 4.1 and 4.9% of the Zn from all types of nanoparticles dissolved within 72 h and was neither concentration dependent nor morphology dependent. Addition of all nanoparticles at all concentrations tested stopped growth of T. pseudonana and C. gracilis, whereas P. tricornutum was the least sensitive, with its growth rate inversely proportional to nanoparticle concentration. Bioaccumulation of Zn released from nanoparticles in T. pseudonana was sufficient to kill this diatom. The toxicity of rod-shaped particles to P. triocornutum was noted to be greater than that of the spheres. The overall results suggest that toxicity studies assessing the effects of nanoparticles on aquatic organisms need to consider both the dissolution of these particles and the cellular interaction of nanoparticle aggregates.

Ambient Large-Scale Template-Mediated Synthesis of High-Aspect Ratio Single-Crystalline, Chemically Doped Rare-Earth Phosphate Nanowires for Bioimaging

A simple and effective template-mediated protocol has been developed for the large-scale, room-temperature preparation of high-aspect-ratio, single-crystalline Tb-doped CePO(4) nanowires, measuring approximately 12 nm in diameter and over 10 mum in length. Moreover, we also isolated sheaf-like bundles of nanostructures. The synthesis mechanism likely involved a crystal splitting step. The resulting nanowires demonstrated an intense redox-sensitive green photoluminescence, which was exploited, in addition to their inherently high biocompatibility and low toxicity, for potential applications in biological imaging and labeling of cells.

Tailoring the composition of ultrathin, ternary alloy PtRuFe nanowires for the methanol oxidation reaction and formic acid oxidation reaction

In the search for alternatives to conventional Pt electrocatalysts, we have synthesized ultrathin, ternary PtRuFe nanowires (NW), possessing different chemical compositions in order to probe their CO tolerance as well as electrochemical activity as a function of composition for both (i) the methanol oxidation reaction (MOR) and (ii) the formic acid oxidation reaction (FAOR). As-prepared ‘multifunctional’ ternary NW catalysts exhibited both higher MOR and FAOR activity as compared with mono-metallic Pt NWs, binary Pt7Ru3 and Pt7Fe3 NWs, and commercial catalyst control samples. In terms of synthetic novelty, we utilized a sustainably mild, ambient wet-synthesis method never previously applied to the fabrication of crystalline, pure ternary systems in order to fabricate ultrathin, homogeneous alloy PtRuFe NWs with a range of controlled compositions. These NWs were subsequently characterized using a suite of techniques including XRD, TEM, SAED, and EDAX in order to verify not only the incorporation of Ru and Fe into the Pt lattice but also their chemical homogeneity, morphology, as well as physical structure and integrity. Lastly, these NWs were electrochemically tested in order to deduce the appropriateness of conventional explanations such as (i) the bi-functional mechanism as well as (ii) the ligand effect to account for our MOR and FAOR reaction data. Specifically, methanol oxidation appears to be predominantly influenced by the Ru content, whereas formic acid oxidation is primarily impacted by the corresponding Fe content within the ternary metal alloy catalyst itself.