Real-Time Deduction of Mechanisms and Kinetics Underlying Photocatalytic Water Disinfection: Cell Motility and Particle Tracking.

The current methods used to study photocatalysis-assisted water disinfection at a laboratory scale may not lead to process scale-up for large-scale implementation. These methods do not capture the process complexity and address all the factors underlying disinfection kinetics, including the physical characteristics (e.g., shape and size) of the photocatalyst, the light intensity, the form of the catalyst (e.g., free-floating and immobilized), and the photocatalyst-microorganism interaction mode (e.g., collision mode and constant contact mode). This drawback can be overcome using in situ methods to track the interaction between the photocatalysts and the microorganisms (e.g., Escherichia coli) and thereby engineering the resulting disinfection kinetics. Contextually, this study employed microscopy and particle-tracking algorithms to quantify in situ cell motility of E. coli undergoing titanium dioxide (TiO2) nanowire-assisted photocatalysis, which was observed to correlate with cell viability closely. This experimentation also informed that the E. coli bacterium interacted with the photocatalysts through collisions (without sustained contact), which allowed for phenomenological modeling of the observed first-order kinetics of E. coli inactivation. Addition of fluorescent-tagging assays to microscopy revealed that cell membrane integrity loss is the primary mode of bacterial inactivation. This methodology is independent of the microorganism or the photocatalyst type and hence is expected to be beneficial for engineering disinfection kinetics.

Shape memory polymer composites (SMPCs) using interconnected nanowire network foams as reinforcements.

Shape memory polymers (SMPs), although offer a suite of advantages such as ease of processability and lower density, lag behind their shape memory alloy counterparts, in terms of mechanical properties such as recovery stress and cyclability. Reinforcing SMPs with inorganic nanowires and carbon nanotubes (CNTs) is a sought-after pathway for tailoring their mechanical properties. Here, inorganic nanowires also offer the added advantage of covalently binding the fillers to the surrounding polymer matrices via organic molecules. The SMP composites (SMPCs) thus obtained have well-engineered nanowire-polymer interfaces, which could be used to tune their mechanical properties. A well-known method of fabricating SMPCs involving casting dispersions of nanowires (or CNTs) in mixtures of monomers and crosslinkers typically results in marginal improvements in the mechanical properties of the fabricated SMPCs. This is owed to the constraints imposed by the rule-of-mixture principles. To circumvent this limitation, a new method for SMPC fabrication is designed and presented. This involves infiltrating polymers into pre-fabricated nanowire foams. The pre-fabricated foams were fabricated by consolidating measured quantities of nanowires and a sacrificial material, such as (NH4)2CO3, followed by heating the consolidated mixtures for subliming the sacrificial material. Similar to the case of traditional composites, use of silanes to functionalize the nanowire surfaces allowed for the formation of bonds between both the nanowire-nanowire and the nanowire-polymer interfaces. SMPCs fabricated using TiO2nanowires and SMP composed of neopentyl glycol diglycidyl ether and poly(propylene glycol) bis(2-aminopropyl ether) (Jeffamine D230) in a 2:1 molar ratio exhibited a 300% improvement in the elastic modulus relative to that of the SMP. This increase was significantly higher than SMPC made using the traditional fabrication route. Well-known powder metallurgy techniques employed for the fabrication of these SMPCs make this strategy applicable for obtaining other SMPCs of any desired shape and chemical composition.

Nanowire-based thermoelectrics.

Research on thermoelectrics has seen a huge resurgence since the early 1990s. The ability of tuning a material's electrical and thermal transport behavior upon nanostructuring has led to this revival. Nevertheless, thermoelectric performances of nanowires and related materials lag far behind those achieved with thin-film superlattices and quantum dot-based materials. This is despite the fact that nanowires offer many distinct advantages in enhancing the thermoelectric performances of materials. The simplicity of the strategy is the first and foremost advantage. For example, control of the nanowire diameters and their surface roughnesses will aid in enhancing their thermoelectric performances. Another major advantage is the possibility of obtaining high thermoelectric performances using simpler nanowire chemistries (e.g., elemental and binary compound semiconductors), paving the way for the fabrication of thermoelectric modules inexpensively from non-toxic elements. In this context, the topical review provides an overview of the current state of nanowire-based thermoelectrics. It concludes with a discussion of the future vision of nanowire-based thermoelectrics, including the need for developing strategies aimed at the mass production of nanowires and their interface-engineered assembly into devices. This eliminates the need for trial-and-error strategies and complex chemistries for enhancing the thermoelectric performances of materials.

Engineering efficient thermoelectrics from large-scale assemblies of doped ZnO nanowires: nanoscale effects and resonant-level scattering.

Recent studies focusing on enhancing the thermoelectric performance of metal oxides were primarily motivated by their low cost, large availability of the component elements in the earth's crust, and their high stability. So far, these studies indicate that n-type materials, such as ZnO, have much lower thermoelectric performance than their p-type counterparts. Overcoming this limitation requires precisely tuning the thermal and electrical transport through n-type metal oxides. One way to accomplish this is through the use of optimally doped bulk assemblies of ZnO nanowires. In this study, the thermoelectric properties of n-type aluminum and gallium dually doped bulk assembles of ZnO nanowires were determined. The results indicated that a high zT of 0.6 at 1000 °C, the highest experimentally observed for any n-type oxide, is possible. The high performance is attributed to the tailoring of the ZnO phase composition, nanostructuring of the material, and Zn-III band hybridization-based resonant scattering.

Lithiation-induced shuffling of atomic stacks.

In rechargeable lithium-ion batteries, understanding the atomic-scale mechanism of Li-induced structural evolution occurring at the host electrode materials provides essential knowledge for design of new high performance electrodes. Here, we report a new crystalline-crystalline phase transition mechanism in single-crystal Zn-Sb intermetallic nanowires upon lithiation. Using in situ transmission electron microscopy, we observed that stacks of atomic planes in an intermediate hexagonal (h-)LiZnSb phase are "shuffled" to accommodate the geometrical confinement stress arising from lamellar nanodomains intercalated by lithium ions. Such atomic rearrangement arises from the anisotropic lithium diffusion and is accompanied by appearance of partial dislocations. This transient structure mediates further phase transition from h-LiZnSb to cubic (c-)Li2ZnSb, which is associated with a nearly "zero-strain" coherent interface viewed along the [001]h/[111]c directions. This study provides new mechanistic insights into complex electrochemically driven crystalline-crystalline phase transitions in lithium-ion battery electrodes and represents a noble example of atomic-level structural and interfacial rearrangements.

Non-conformal decoration of semiconductor nanowire surfaces with boron nitride (BN) molecules for stability enhancement: degradation-resistant Zn3P2, ZnO and Mg2Si nanowires.

A simple and reliable strategy for stabilizing the surfaces of compound semiconductors was presented. The strategy involved decorating the surfaces of compound semiconductor nanowires non-conformally with small molecules of boron nitride (BN). More specifically, Zn3P2, ZnO and Mg2Si nanowires, highly useful in energy conversion device fabrication (e.g., photovoltaics and thermoelectrics), have been stabilized against air- and acid-assisted degradation by decorating their surfaces with small molecules of BN. It is believed that the decoration of the nanowire surfaces with BN molecules made the nanowire surfaces non-wettable to water and aqueous acid solutions, and thereby imparted them enhanced resistance against water- and acid-assisted degradation. This procedure did not alter the bandgap of the nanowires. Moreover, this procedure aided in retaining the electrical conduction between the nanowire interfaces when the nanowires are assembled into mats or pellets. This strategy solves one of the primary bottlenecks in the widespread use of nanowires in energy conversion device fabrication, namely their stability. It is believed that this strategy is applicable for stabilizing other compound semiconductor nanowires, including nitrides, sulfides, silicides and antimonides.

Thermoelectric properties of large-scale Zn3P2 nanowire assemblies.

Gram quantities of both unfunctionalized and 1,4-benzenedithiol (BDT) functionalized zinc phosphide (Zn3P2) nanowire powders, synthesized using direct reaction of zinc and phosphorus, were hot-pressed into highly dense pellets (≥98% of the theoretical density) for the determination of their thermoelectric performance. It was deduced that mechanical flexibility of the nanowires is essential for consolidating them in randomly oriented fashion into dense pellets, without making any major changes to their morphologies. Electrical and thermal transport measurements indicated that the enhanced thermoelectric performance expected of individual Zn3P2 nanowires is still retained within large-scale nanowire assemblies. A maximum reduction of 28% in the thermal conductivity of Zn3P2 resulted from nanostructuring. Use of nanowire morphology also led to enhanced electrical conductivity in Zn3P2. Interface engineering of the nanowires in the pellets, accomplished by hot-pressing BDT functionalized nanowires, resulted in an increase on both the Seebeck coefficient and the electrical conductivity of the nanowire pellets. It is believed that filtering of low energy carriers resulting from the variation of the chemical compositions at the nanowire interfaces is responsible for this phenomenon. Overall, this study indicated that mechanical properties of the nanowires along with the chemical compositions of their surfaces play a hitherto unknown, but vital, role in realizing highly efficient bulk thermoelectric modules based on nanowires.

Compositional disorder and its effect on the thermoelectric performance of Zn3P2 nanowire-copper nanoparticle composites.

Recent studies indicated that nanowire format of materials is ideal for enhancing the thermoelectric performance of materials. Most of these studies were performed using individual nanowires as the test elements. It is not currently clear whether bulk assemblies of nanowires replicate this enhanced thermoelectric performance of individual nanowires. Therefore, it is imperative to understand whether enhanced thermoelectric performance exhibited by individual nanowires can be extended to bulk assemblies of nanowires. It is also imperative to know whether the addition of metal nanoparticle to semiconductor nanowires can be employed for enhancing their thermoelectric performance further. Specifically, it is important to understand the effect of microstructure and composition on the thermoelectric performance on bulk compound semiconductor nanowire-metal nanoparticle composites. In this study, bulk composites composed of mixtures of copper nanoparticles with either unfunctionalized or 1,4-benzenedithiol (BDT) functionalized Zn3P2 nanowires were fabricated and analyzed for their thermoelectric performance. The results indicated that use of BDT functionalized nanowires for the fabrication of composites leads to interface-engineered composites that have uniform composition all across their cross-section. The interface engineering allows for increasing their Seebeck coefficients and electrical conductivities, relative to the Zn3P2 nanowire pellets. In contrast, the use of unfunctionalized Zn3P2 nanowires for the fabrication of composite leads to the formation of composites that are non-uniform in composition across their cross-section. Ultimately, the composites were found to have Zn3P2 nanowires interspersed with metal alloy nanoparticles. Such non-uniform composites exhibited very high electrical conductivities, but slightly lower Seebeck coefficients, relative to Zn3P2 nanowire pellets. These composites were found to show a very high zT of 0.23 at 770 K, orders of magnitude higher than either interface-engineered composites or Zn3P2 nanowire pellets. The results indicate that microstructural composition of semiconductor nanowire-metal nanoparticle composites plays a major role in determining their thermoelectric performance, and such composites exhibit enhanced thermoelectric performance.

Large-scale synthesis and in situ functionalization of Zn3P2 and Zn4Sb3 nanowire powders.

A simple method for the large-scale synthesis of gram quantities of compound semiconductor nanowires without the need for any external catalysts or templates is presented. This method is demonstrated using zinc phosphide (Zn3P2) and zinc antimonide (ß-Zn4Sb3) nanowires as example systems. Large-scale synthesis of Zn3P2 and Zn4Sb3 nanowire powders was accomplished using a hot-walled chemical vapor deposition chamber by transporting phosphorus and antimony, respectively, via the vapor phase onto heated zinc foils. The zinc foils were rolled concentrically into coils to maximize the substrate surface area, and consequently, the nanowire yield. Using this method, 250 mg of Zn3P2 nanowires were obtained on 480 cm(2) of zinc foil in a span of 45 minutes. Furthermore, a process of exposing the synthesized nanowires to a vapor of organic functional molecules immediately after their synthesis and before their removal from the vacuum chamber was developed to obtain large quantities of surface functionalized nanowire powders. This in situ vapor-phase functionalization procedure passivated the nanowire surfaces without adversely affecting their morphology or dimensions. Our studies revealed that both 4-aminothiophenol and 3-propanedithiol functionalized Zn3P2 nanowires were stable over a 120 day duration without any agglomeration or degradation. This method of mass producing nanowires can also be extended to other binary semiconductors.

Chemical vapor deposition of conformal, functional, and responsive polymer films.

Chemical vapor deposition (CVD) polymerization utilizes the delivery of vapor-phase monomers to form chemically well-defined polymeric films directly on the surface of a substrate. CVD polymers are desirable as conformal surface modification layers exhibiting strong retention of organic functional groups, and, in some cases, are responsive to external stimuli. Traditional wet-chemical chain- and step-growth mechanisms guide the development of new heterogeneous CVD polymerization techniques. Commonality with inorganic CVD methods facilitates the fabrication of hybrid devices. CVD polymers bridge microfabrication technology with chemical, biological, and nanoparticle systems and assembly. Robust interfaces can be achieved through covalent grafting enabling high-resolution (60 nm) patterning, even on flexible substrates. Utilizing only low-energy input to drive selective chemistry, modest vacuum, and room-temperature substrates, CVD polymerization is compatible with thermally sensitive substrates, such as paper, textiles, and plastics. CVD methods are particularly valuable for insoluble and infusible films, including fluoropolymers, electrically conductive polymers, and controllably crosslinked networks and for the potential to reduce environmental, health, and safety impacts associated with solvents. Quantitative models aid the development of large-area and roll-to-roll CVD polymer reactors. Relevant background, fundamental principles, and selected applications are reviewed.

Selective sensing of volatile organic compounds using novel conducting polymer-metal nanoparticle hybrids.

Conducting polymer-metal nanoparticle hybrids, fabricated by assembling metal nanoparticles on top of functionalized conducting polymer film surfaces using conjugated linker molecules, enable the selective sensing of volatile organic compounds (VOCs). In these conducting polymer-metal nanoparticle hybrids, selectivity is achieved by assembling different metals on the same conducting polymer film. This eliminates the need to develop either different polymers chemistries or device configurations for each specific analyte. In the hybrids, chemisorption of the analyte vapor induces charge redistribution in the metal nanoparticles and changes their work function. The conjugated linker molecule causes this change in the work function of the tethered nanoparticles to affect the electronic states in the underlying conducting polymer film. The result is an easily measurable change in the resistance of the hybrid structure. The fabrication of these sensing elements involved the covalent assembly of nickel (Ni) and palladium (Pd) metal nanoparticles on top of poly(3,4-ethylenedioxythiophene-co-thiophene-3-acetic acid), poly(EDOT-co-TAA), films using 4-aminothiophenol linker molecules. The change in resistance of hybrid Pd/poly(EDOT-co-TAA) and Ni/poly(EDOT-co-TAA) hybrid films to acetone and toluene, respectively, is observed to be in proportion to their concentrations. The projected detection limits are 2 and 10 ppm for toluene and acetone, respectively. A negligible response (resistance change) of the Pd/poly(EDOT-co-TAA) films to toluene exposure confirmed its selectivity for detecting acetone. Similarly, lack of response to acetone confirmed the selectivity of the Ni/poly(EDOT-co-TAA) stacks for detecting toluene. It is anticipated that the assembly of other metals such as Ag, Au and Cu on top of poly(EDOT-co-TAA) would provide selectivity for detecting and discriminating other VOCs.

Hierarchical multifunctional composites by conformally coating aligned carbon nanotube arrays with conducting polymer.

A novel method for the fabrication of carbon nanotube (CNT)-conducting polymer composites is demonstrated by conformally coating extremely high aspect ratio vertically aligned-CNT (A-CNT) arrays with conducting polymer via oxidative chemical vapor deposition (oCVD). A mechanical densification technique is employed that allows the spacing of the A-CNTs to be controlled, yielding a range of inter-CNT distances between 20 and 70 nm. Using this morphology control, oCVD is shown to conformally coat 8-nm-diameter CNTs having array heights up to 1 mm (an aspect ratio of 10(5)) at all inter-CNT spacings. Three phase CNT-conducting polymer nanocomposites are then fabricated by introducing an insulating epoxy via capillary-driven wetting. CNT morphology is maintained during processing, allowing quantification of direction-dependent (nonisotropic) composite properties. Electrical conductivity occurs primarily along the CNT axial direction, such that the conformal conducting polymer has little effect on the activation energy required for charge conduction. In contrast, the conducting polymer coating enhanced the conductivity in the radial direction by lowering the activation energy required for the creation of mobile charge carriers, in agreement with variable-range-hopping models. The fabrication strategy introduced here can be used to create many multifunctional materials and devices (e.g., direction-tailorable hydrophobic and highly conducting materials), including a new four-phase advanced fiber composite architecture.

Large-scale, hot-filament-assisted synthesis of tungsten oxide and related transition metal oxide nanowires.

A scalable and versatile method for the large-scale synthesis of tungsten trioxide nanowires and their arrays on a variety of substrates, including amorphous quartz and fluorinated tin oxide, is reported. The synthesis involves the chemical-vapor transport of metal oxide vapor-phase species using air or oxygen flow over hot filaments onto substrates kept at a distance. The results show that the density of the nanowires can be varied from 10(6)-10(10) cm(-2) by varying the substrate temperature. The diameter of the nanowires ranges from 100-20 nm. The results also show that variations in oxygen flow and substrate temperature affect the nanowire morphology from straight to bundled to branched nanowires. A thermodynamic model is proposed to show that the condensation of WO(2) species primarily accounts for the nucleation and subsequent growth of the nanowires, which supports the hypothesis that the nucleation of nanowires occurs through condensation of suboxide WO(2) vapor-phase species. This is in contrast to the expected WO(3) vapor-phase species condensation into WO(3) solid phase for nanoparticle formation. The as-synthesized nanowires are shown to form stable dispersions compared to nanoparticles in various organic and inorganic solvents.

Photoluminescence of GaN nanowires of different crystallographic orientations.

We utilized time-integrated and time-resolved photoluminescence of a-axis and c-axis gallium nitride nanowires to elucidate the origin of the blue-shifted ultraviolet photoluminescence in a-axis GaN nanowires relative to c-axis GaN nanowires. We attribute this blue-shifted ultraviolet photoluminescence to emission from surface trap states as opposed to previously proposed causes such as strain effects or built-in polarization. These results demonstrate the importance of accounting for surface effects when considering ultraviolet optoelectronic devices based on GaN nanowires.

Mechanisms of 1D crystal growth in reactive vapor transport: indium nitride nanowires.

Indium nitride (InN) nanowire synthesis using indium (In) vapor transport in a dissociated ammonia environment (reactive vapor transport) is studied in detail to understand the nucleation and growth mechanisms involved with the so-called "self-catalysis" schemes. The results show that the nucleation of InN crystal occurs first on the substrate. Later, In droplets are formed on top of the InN crystals because of selective wetting of In onto InN crystals. Further growth via liquid-phase epitaxy through In droplets leads the growth in one dimension (1D), resulting in the formation of InN nanowires. The details about the nucleation and growth aspects within these self-catalysis schemes are rationalized further by demonstrating the growth of heteroepitaxially oriented nanowire arrays on single-crystal substrates and "tree-like" morphologies on a variety of substrates. However, the direct nitridation of In droplets using dissociated ammonia results in the spontaneous nucleation and basal growth of nanowires directly from the In melt surface, which is quite different from the above-mentioned nucleation mechanism with the reactive vapor transport case. The InN nanowires exhibit a band gap of 0.8 eV, whereas the mixed phase of InN and In(2)O(3) nanowires exhibit a peak at approximately 1.9 eV in addition to that at 0.8 eV.

Vapor phase synthesis of tungsten nanowires.

A concept is presented for synthesizing metal nanowires directly from the vapor phase using chemical vapor transport to temperatures higher than the corresponding metal oxide decomposition temperature. Specifically, this concept is demonstrated with the synthesis of tungsten metal nanowires with sizes ranging from 70 to 40 nm by increasing the condensation temperature. The simultaneous condensation and decomposition of the tungsten oxide species during nucleation and growth is suggested for 1-D growth of metallic tungsten nanowires. This synthesis concept could potentially be extended to the vapor phase synthesis of metal nanowires of several other nonvolatile and refractory metals. The tungsten nanowires could find potential applications in gas sensors and as electron sources in electron microscopes.