Showing papers in "Advanced Functional Materials in 2006"

Charge transport and photocurrent generation in poly (3-hexylthiophene): Methanofullerene bulk-heterojunction solar cells

Abstract: The effect of controlled thermal annealing on charge transport and photogeneration in bulk-heterojunction solar cells made from blend films of regioregular poly(3-hexylthiophene) (P3HT) and methanofullerene (PCBM) has been studied. With respect to the charge transport, it is demonstrated that the electron mobility dominates the transport of the cell, varying from 10 –8 m 2 V –1 s –1 in as-cast devices to ≈ 3× 10 –7 m 2 V –1 s –1 after thermal annealing. The hole mobility in the P3HT phase of the blend is dramatically affected by thermal annealing. It increases by more than three orders of magnitude, to reach a value of up to ≈ 2× 10 –8 m 2 V –1 s –1 after the annealing process, as a result of an improved crystallinity of the film. Moreover, upon annealing the absorption spectrum of P3HT:PCBM blends undergo a strong red-shift, improving the spectral overlap with solar emission, which results in an increase of more than 60 % in the rate of charge-carrier generation. Subsequently, the experimental electron and hole mobilities are used to study the photocurrent generation in P3HT:PCBM devices as a function of annealing temperature. The results indicate that the most important factor leading to a strong enhancement of the efficiency, compared with non-annealed devices, is the increase of the hole mobility in the P3HT phase of the blend. Furthermore, numerical simulations indicate that under short-circuit conditions the dissociation efficiency of bound electron–hole pairs at the donor/acceptor interface is close to 90 %, which explains the large quantum efficiencies measured in P3HT:PCBM blends.

Direct ink writing of 3D functional materials

Abstract: The ability to pattern materials in three dimensions is critical for several technological applications, including composites, microfluidics, photonics, and tissue engineering. Direct-write assembly allows one to design and rapidly fabricate materials in complex 3D shapes without the need for expensive tooling, dies, or lithographic masks. Here, recent advances in direct ink writing are reviewed with an emphasis on the push towards finer feature sizes. Opportunities and challenges associated with direct ink writing are also highlighted.

Effects of Structural Variation on the Photocatalytic Performance of Hydrothermally Synthesized BiVO4

Abstract: Highly crystalline monoclinic scheelite BiVO4 powders are synthesized from aqueous Bi(NO3)3 and NH4VO3 solutions over a wide range of pH by a hydrothermal process BiVO4 powders with various morphologies, surface textures, and grain shapes are selectively synthesized by adjusting the pH The dependence of the Raman peak position and intensity on the synthesis conditions indicates that the symmetry distortions in the local structure of the synthesized BiVO4 are affected by the preparation conditions These variations in the local structure result in the modification of the electronic structure of BiVO4, which results in a blue-shift in the UV-vis absorption spectrum of hydrothermally synthesized BiVO4 in comparison with a well-crystallized sample prepared by homogeneous coprecipitation The photocatalytic activities for O2 evolution from an aqueous AgNO3 solution under visible-light irradiation are strongly dependent on the pH used in the synthesis The differences in the photocatalytic activities between BiVO4 samples prepared under various conditions is attributed to the degree of structural distortion, leading to differences in the mobility of photogenerated holes formed in the valence band, which consists of Bi 6s and O 2p orbitals

Synthesis of Hexagonal-Phase NaYF4:Yb,Er and NaYF4:Yb,Tm Nanocrystals with Efficient Up-Conversion Fluorescence

Abstract: IR-to-visible up-conversion fluorescent nanocrystals of hexagonal-phase NaYF4:20 %Yb,2 %Er and NaYF4:20 %Yb,2 %Tm have been synthesized by decomposition of multiprecursors of CF3COONa, (CF3COO)3Y, (CF3COO)3Yb, and (CF3COO)3Er/(CF3COO)3Tm in oleylamine at 330 °C. The average particle size is 10.5 ± 0.7 nm (from random measurements of 200 particles from five transmission electron microscopy images) and 11.1 ± 1.3 nm (from dynamic-light-scattering measurements). The up-conversion fluorescence intensity of the hexagonal nanocrystals in this work is much higher than that of other cubic-phase NaYF4:Yb,Er nanocrystals, including the ones in this work (by a factor of 7.5). Mechanisms for nucleation and growth of the hexagonal-phase nanoparticles are proposed. These nanocrystals are easily dispersed in organic solvents, producing a transparent colloidal solution. The hydrophobic surfaces of the particles are made hydrophilic using a bipolar surfactant. These nanoparticles and their dispersions in various media have potential applications in optical nanodevices and bioprobes.

Ion‐Current Rectification in Nanopores and Nanotubes with Broken Symmetry

Abstract: This article focuses on ion transport through nanoporous systems with special emphasis on rectification phenomena. The effect of ion-current rectification is observed as asymmetric current–voltage (I–V) curves, with the current recorded for one voltage polarity higher than the current recorded for the same absolute value of voltage of opposite polarity. This diode-like I–V curve indicates that there is a preferential direction for ion flow. Experimental evidence that ion-current rectification is inherent to asymmetric, e.g., tapered, nanoporous systems with excess surface charge is provided and discussed. The fabrication and operation of asymmetric polymer nanopores, gold nanotubes, glass nanocapillaries, and silicon nanopores are presented. The possibility of tuning the direction and extent of rectification is discussed in detail. Theoretical models that have been developed to explain the ion-current rectification effect are also presented.

Fabrication of Single-Crystalline Silicon Nanowires by Scratching a Silicon Surface with Catalytic Metal Particles†

Abstract: A novel strategy for preparing large-area, oriented silicon nanowire (SiNW) arrays on silicon substrates at near room temperature by localized chemical etching is presented. The strategy is based on metal-induced (either by Ag or Au) excessive local oxidation and dissolution of a silicon substrate in an aqueous fluoride solution. The density and size of the as-prepared SiNWs depend on the distribution of the patterned metal particles on the silicon surface. High-density metal particles facilitate the formation of silicon nanowires. Well-separated, straight nanoholes are dug along the Si block when metal particles are well dispersed with a large space between them. The etching technique is weakly dependent on the orientation and doping type of the silicon wafer. Therefore, SiNWs with desired axial crystallographic orientations and doping characteristics are readily obtained. Detailed scanning electron microscopy observations reveal the formation process of the silicon nanowires, and a reasonable mechanism is proposed on the basis of the electrochemistry of silicon and the experimental results.

Hydrogen adsorption in metal-organic frameworks: Cu-MOFs and Zn-MOFs compared

Abstract: Hydrogen adsorption in two different metal–organic frameworks (MOFs), MOF-5 and Cu-BTC (BTC: benzene-1,3,5-tricarboxylate), with Zn2+ and Cu2+ as central metal ions, respectively, is investigated at temperatures ranging from 77 K to room temperature. The process responsible for hydrogen storage in these MOFs is pure physical adsorption with a heat of adsorption of approximately –4 kJ mol–1. With a saturation value of 5.1 wt.-% for the hydrogen uptake at high pressures and 77 K, MOF-5 shows the highest storage capacity ever reported for crystalline microporous materials. However, at low pressures Cu-BTC shows a higher hydrogen uptake than MOF-5, making Cu-based MOFs more promising candidates for potential storage materials. Furthermore, the hydrogen uptake is correlated with the specific surface area for crystalline microporous materials, as shown for MOFs and zeolites.

A Hybrid Supercapacitor Fabricated with a Carbon Nanotube Cathode and a TiO2–B Nanowire Anode

Abstract: Recently, a new hybrid supercapacitor, integrating both the advantages of supercapacitors and lithium-ion batteries, was proposed and rapidly turned into state-of-the-art energy-storage devices with a high energy density, fast power capability, and a long cycle life. In this paper, a new hybrid supercapacitor is fabricated by making use of the benefits of 1D nanomaterials consisting of a carbon nanotube (CNT) cathode and a TiO2–B nanowire (TNW) anode, and the preliminary results for such an energy-storage device operating over a wide voltage range (0–2.8 V) are presented. The CNT–TNW supercapacitor is compared to a CNT–CNT supercapacitor, and discussed with regards to available energy densities, power capabilities, voltage profiles, and cycle life. On the basis of the total weight of both active materials, the CNT–TNW supercapacitor delivers an energy density of 12.5 W h kg–1 at a rate of 10 C, double the value of the CNT–CNT supercapacitor, while maintaining desirable cycling stability. The combination of a CNT cathode and a TNW anode in a non-aqueous electrolyte is proven to be suitable for high-performance hybrid supercapacitor applications; this can reasonably be assigned to the interesting synergistic effects of the two nanomaterials. It is hoped that the results presented in this study might renew interest in the design of nanomaterials that are applicable not only to hybrid supercapacitors, but also to energy conversion and storage applications of the future.

Fabrication of electrospinning-derived carbon nanofiber webs for the anode material of lithium-ion secondary batteries

Abstract: A carbon nanofiber-based electrode, exhibiting a large accessible surface area (derived from the nanometer-sized fiber diameter), high carbon purity (without binder), relatively high electrical conductivity, structural integrity, thin web macromorphology, a large reversible capacity (ca. 450 mA h g–1), and a relatively linearly inclined voltage profile, is fabricated by nanofiber formation via electrospinning of a polymer solution and its subsequent thermal treatment. It is envisaged that these characteristics of this novel carbon material will make it an ideal candidate for the anode material of high-power lithium-ion batteries (where a high current is critically needed), owing to the highly reduced lithium-ion diffusion path within the active material.

Hybrid Solar Cells from Regioregular Polythiophene and ZnO Nanoparticles

Abstract: Blends of nanocrystalline zinc oxide nanoparticles (nc-ZnO) and regioregular poly(3-hexylthiophene) (P3HT) processed from solution have been used to construct hybrid polymer–metal oxide bulk-heterojunction solar cells. Thermal annealing of the spin-cast films significantly improves the solar-energy conversion efficiency of these hybrid solar cells to ∼ 0.9 %. Photoluminescence and photoinduced absorption spectroscopy demonstrate that charge-carrier generation is not quantitative, because a fraction of P3HT appears not to be in contact with or in close proximity to ZnO. The coarse morphology of the films, also identified by tapping-mode atomic force microscopy, likely limits the device performance.

Accurate Measurement and Characterization of Organic Solar Cells

Abstract: Methods to accurately measure the current–voltage characteristics of organic solar cells under standard reporting conditions are presented. Four types of organic test cells and two types of silicon reference cells (unfiltered and with a KG5 color filter) are selected to calculate spectral-mismatch factors for different test-cell/reference-cell combinations. The test devices include both polymer/fullerene-based bulk-heterojunction solar cells and small-molecule-based heterojunction solar cells. The spectral responsivities of test cells are measured as per American Society for Testing and Materials Standard E1021, and their dependence on light-bias intensity is reported. The current–voltage curves are measured under 100 mW cm–2 standard AM 1.5 G (AM: air mass) spectrum (International Electrotechnical Commission 69094-1) generated from a source set with a reference cell and corrected for spectral error.

Electrical Switching and Bistability in Organic/Polymeric Thin Films and Memory Devices

Abstract: Recently, films created by incorporating metallic nanoparticles into organic or polymeric materials have demonstrated electrical bistability, as well as the memory effect, when subjected to an electrical bias. Organic and polymeric digital memory devices based on this bistable electronic behavior have emerged as a viable technology in the field of organic electronics. These devices exhibit fast response speeds and can form multiple-layer stacking structures, demonstrating that organic memory devices possess a high potential to become flexible, ultrafast, and ultrahigh-density memory devices. This behavior is believed to be related to charge storage in the organic or polymer film, where devices are able to exhibit two different states of conductivity often separated by several orders of magnitude. By defining the two states as “1” and “0”, it is now possible to create digital memory devices with this technology. This article reviews electrically bistable devices developed in our laboratory. Our research has stimulated strong interest in this area worldwide. The research by other laboratories is reviewed as well.

Influence of solvent mixing on the morphology and performance of solar cells based on polyfluorene copolymer/fullerene blends

Abstract: The influence of the solvent on the morphol. and performance of polymer solar cells was studied in devices based on blends of the polyfluorene copolymer, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2', 1',3'-benzothiadiazole)), and [6,6]-phenyl-C61-butyric acid Me ester. The blends are spin-coated from CHCl3 or from CHCl3 mixed with small amts. of xylene, toluene, or chlorobenzene. The devices are characterized under monochromatic light and AM1.5 solar illumination. An enhancement of the photocurrent d. was obsd. in diodes made from CHCl3 mixed with chlorobenzene, and a decreased photocurrent d. is obsd. in diodes made from CHCl3 mixed with xylene or toluene, compared to diodes made from neat CHCl3. The open-circuit voltages are almost the same for all diodes. The surfaces of the active layers were imaged with AFM. Height images indicate that a finer and more uniform distribution of domains corresponds to the diodes with enhanced photocurrent that are made from CHCl3 mixed with chlorobenzene, while a structure with larger domains is assocd. with the lower photocurrents in the diodes made from CHCl3 mixed with xylene or toluene. The influence of the morphol. on the excited-state dynamics and charge generation was studied using time-resolved spectroscopy. Fast formation of bound charge pairs followed by their conversion into free charge carriers was resolved, and excitation-intensity-dependent non-geminate recombination of free charges was obsd. A significant enhancement in free-charge-carrier generation was obsd. on introducing chlorobenzene into CHCl3. Imaging photocurrent generation from the solar cells with a light-pulse technique shows an inhomogeneous photocurrent distribution, which is related to the undulations in the thickness of the active layer. Thicker parts of the diodes yield higher photocurrent values. [on SciFinder (R)]

Nanostructured Vanadium Oxide Electrodes for Enhanced Lithium-Ion Intercalation†

Abstract: This article summarizes our most recent studies on improved Li + -intercalation properties in vanadium oxides by engineering the nanostructure and interlayer structure. The intercalation capacity and rate are enhanced by almost two orders of magnitude with appropriately fabricated nanostructures. Processing methods for single-crystal V 2 O 5 nanorod arrays, V 2 O 5 ·n H 2 O nanotube arrays, and Ni/V 2 O 5 ·nH 2 O core/shell nanocable arrays are presented; the morphologies, structures, and growth mechanisms of these nanostructures are discussed. Electrochemical analysis demonstrates that the intercalation properties of all three types of nanostructure exhibit significantly enhanced storage capacity and rate performance compared to the film electrode of vanadium pentoxide. Addition of TiO 2 to orthorhombic V 2 O 5 is found to affect the crystallinity, microstructure, and possible interaction force between adjacent layers in V 2 O 5 , and subsequently leads to enhanced Li + -intercalation properties in V 2 O 5 . The amount of water intercalated in V 2 O 5 is found to have a significant influence on the interlayer spacing and electrochemical performance of V 2 O 5 ·n H 2 O. A systematic electrochemical study has demonstrated that the V 2 O 5 ·0.3 H 2 O film has the optimal water content and exhibits the best Li + -intercalation performance.

Fabrication of Hollow Inorganic Microspheres by Chemically Induced Self‐Transformation

Abstract: Two contrasting approaches, involving either polymer-mediated or fluoride-mediated self-transformation of amorphous solid particles, are described as general routes to the fabrication of hollow inorganic microspheres. Firstly, calcium carbonate and strontium tungstate hollow microspheres are fabricated in high yield using sodium poly(4-styrenesulfonate) as a stabilizing agent for the formation and subsequent transformation of amorphous primary particles. Transformation occurs with retention of the bulk morphology by localized Ostwald ripening, in which preferential dissolution of the particle interior is coupled to the deposition of a porous external shell of loosely packed nanocrystals. Secondly, the fabrication process is extended to relatively stable amorphous microspheres, such as TiO2 and SnO2, by increasing the surface reactivity of the solid precursor particles. For this, fluoride ions, in the form of NH4F and SnF2, are used to produce well-defined hollow spheroids of nanocrystalline TiO2 and SnO2, respectively. Our results suggest that the chemical self-transformation of precursor objects under morphologically invariant conditions could be of general applicability in the preparation of a wide range of nanoparticle-based hollow architectures for technological and biomedical applications.

CO2 Capture by As-Prepared SBA-15 with an Occluded Organic Template

Abstract: A novel CO 2 capture phenomenon is observed by modifying as-prepared mesoporous silica SBA-15 (SBA(P)) with tetraethylenepentamine (TEPA), not only conserving the energy and time required for removing the template, but also opening the way to utilizing the micelle for dispersing guest species. The TEPA species dispersed within the channels of SBA(P) are highly accessible to CO 2 molecules; moreover, the hydroxyl group of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (Pluronic P123) template is able to modify the interactions between CO 2 and the amine to enhance the adsorptive capacity of this system. The remarkably high adsorption capacity (173 mg g -1 ) of this mesoporous silica-amine composite suggests potential CO 2 trapping applications, especially at low CO 2 concentrations during prolonged cyclic operations.

Learning from Diatoms: Nature's Tools for the Production of Nanostructured Silica

Abstract: Diatoms are eukaryotic, unicellular algae that are ubiquitously present in almost any water habitat on earth Diatoms dominate phytoplankton populations and algal blooms in the oceans They are responsible for about 25 % of the world's net primary production Apart from this ecological significance, diatoms are mainly known for the intricate geometries and spectacular patterns of their silica-based cell walls These patterns are species specific They are precisely reproduced in each generation documenting a genetic control of this biomineralization process Biogenesis of the diatom cell wall is considered to be a paradigm for the controlled production of nanostructured silica Biochemical studies demonstrated that diatom biosilica is a composite material containing zwitterionic proteins (silaffins) and long-chain polyamines in addition to silica Functional studies indicate a crucial role of these organic components in guiding silica precipitation as well as in the formation of species-specific nanopatterns These activities can be explained by molecular self-assembly and phase-separation processes Moreover, diatom cell walls also exhibit very exciting properties from the physical point of view: they are extremely stable and they may act as photonic crystals

Composite Silica Spheres with Magnetic and Luminescent Functionalities

Abstract: A new class of highly fluorescent, photostable, and magnetic core/shell nanoparticles in the submicrometer size range has been synthesized from a modified Stober method combined with the layer-by-layer (LbL) assembly technique. Luminescent magnetic nanoparticles are prepared via two main steps. The first step involves controlled addition of tetraethoxysilane to a dispersion of Fe3O4/γ-Fe2O3 nanoparticles, which are thereby homogeneously incorporated as cores into monodisperse silica spheres. The second step involves the LbL assembly of polyelectrolytes and luminescent CdTe quantum dots onto the surfaces of the silica-coated magnetite/maghemite particles, which are finally covered with an outer shell of silica. These spherical particles have a typical diameter of 220 ± 10 nm and a saturation magnetization of 1.34 emu g–1 at room temperature, and exhibit strong excitonic photoluminescence. Nanoparticles with such a core/shell architecture have the added benefit of providing a robust platform (the outer silica shell) for incorporating diverse functionalities into a single nanoparticle.

Dispersions of Surface‐Modified Carbon Nanotubes in Water‐Soluble and Water‐Insoluble Polymers

Abstract: Microscale aggregate formation, resulting from high intrinsic filler attractions, is one of the major issues in nanocomposite preparation and processing. Herein, the dispersive effects achieved by a wide range of surface-active agents, as well as surface oxidation and functionalization, are investigated. The aim of our research is to form a uniform, multiwalled carbon nanotube (MWNT) distribution in water-soluble (poly(ethylene glycol)) and water-insoluble (polypropylene) polymers. In order to understand the surface-charge-related stability of the treated nanotubes solutions, zeta-potential measurements are applied. Quantification of the state of the MWNT dispersion is derived from particle-size analysis, while visual characterization is based on optical and electron microscopy. To estimate the nucleating ability of the surface-modified carbon nanotubes, the temperature of crystallization and the degree of crystallinity are calculated from differential scanning thermograms. Finally, we suggest general guidelines to produce uniform MWNT dispersions using a dispersive agent and/or surface treatment in water-soluble and water-insoluble polymers.

Carbide-Derived Carbons: Effect of Pore Size on Hydrogen Uptake and Heat of Adsorption**

Abstract: Cryoadsorption is a promising method of enhancing gravimetric and volumetric onboard H2 storage capacity for future trans- portation needs. Inexpensive carbide-derived carbons (CDCs), produced by chlorination of metal carbides, have up to 80% open-pore volume with tunable pore size and specific surface area (SSA). Tuning the carbon structure and pore size with high sensitivity by using different starting carbides and chlorination temperatures allows rational design of carbon materials with en- hanced C-H2 interaction and thus increased H2 storage capacity. A systematic experimental investigation of a large number of CDCs with controlled pore size distributions and SSAs shows how smaller pores increase both the heat of adsorption and the total volume of adsorbed H2. It has been demonstrated that increasing the average heat of H2 adsorption above 6.6 kJmol -1 substantially enhances H2 uptake at 1 atm (1 atm=101325 Pa) and -196°C. The heats of adsorption up to 11 kJmol -1 exceed values reported for metal-organic framework compounds and carbon nanotubes.

Colloidal Synthesis of Hollow Cobalt Sulfide Nanocrystals

Abstract: Formation of cobalt sulfide hollow nanocrystals through amechanism similar to the Kirkendall Effect has been investigated indetail. It was found that performing the reaction at>120oC leads tofast formation of a single void ins ide each shell, whereas at roomtemperature multiple voids are formed within each shell, which can beattributed to strongly temperature-dependent diffusivities for vacancies.The void formation process is dominated by outward diffusion of cobaltcations; still, significant inward transport of sulfur anions can beinferred to occur as the final voids are smaller in diameter than theoriginal cobalt nanocrystals. Comparison of volume distributions forinitial and final nanostructures indicates excess apparent volume inshells implying significant porosity and/or a defective structure.Indirect evidence for shells to fracture during growth at lowertemperatures was observed in shell size statisticsand TEM of as-grownshells. An idealized model of the diffusional process imposes two minimalrequirements on material parameters for shell growth to be obtainablewithin a specific synthetic system.

Pharmacokinetics of Nanoscale Quantum Dots: In Vivo Distribution, Sequestration, and Clearance in the Rat†

Abstract: Advances in nanotechnology research on quantum dots (QDs)—water soluble ZnS-capped, CdSe fluorescent semiconductor nanocrystals—for in vivo biomedical applications have prompted a close scrutiny of the behavior of nanostructures in vivo. Data pertaining to pharmacokinetics and toxicity will undoubtedly assist in designing better in vivo nanostructure contrast agents or therapies. In vivo kinetics, clearance, and metabolism of semiconductor QDs are characterized following their intravenous dosing in Sprague–Dawley rats. The QDs coated with the organic molecule mercaptoundecanoic acid and crosslinked with lysine (denoted as QD-LM) are cleared from plasma with a clearance of 0.59 ± 0.16 mL min–1 kg–1. A higher clearance (1.23 ± 0.22 mL min–1 kg–1) exists when the QDs are conjugated to bovine serum albumin (denoted as QD-BSA, P < .05 (P = statistical significance). The biodistribution between these two QDs is also different. The liver takes up 40 % of the QD-LM dose and 99 % of QD-BSA dose after 90 min. Small amounts of both QDs appear in the spleen, kidney, and bone marrow. However, QDs are not detected in feces or urine for up to ten days after intravenous dosing.

CdIn2S4 Nanotubes and “Marigold” Nanostructures: A Visible‐Light Photocatalyst

Abstract: Nanostructured photocatalysts with high activity are sought for solar production of hydrogen. Spinel semiconductors with different nanostructures and morphologies have immense importance for photocatalytic and other potential applications. Here, a chemically stable cubic spinel nanostructured CdIn2S4 prepared by a facile hydrothermal method is reported as a visible-light driven photocatalyst. A pretty, marigold-like morphology is observed in aqueous-mediated CdIn2S4, whereas nanotubes of good crystallinity, 25 nm in diameter, are obtained in methanol-mediated CdIn2S4. The aqueous- and methanol-mediated CdIn2S4 products show excellent photocatalytic activity compared to other organic mediated samples, and this is attributed to their high degree of crystallinity. The CdIn2S4 photocatalyst gives quantum yields of 16.8 % (marigold-like morphology) and 17.1 % (nanotubes) at 500 nm, respectively, for the H2 evolution reaction. The details of the characteristics of the photocatalyst, such as crystal and band structure, are reported. Considering the importance of hydrogen energy, CdIn2S4 will be an excellent candidate as a catalyst for “photohydrogen” production under visible light. Being a nanostructured chalcogenide semiconductor, CdIn2S4 will have other potential prospective applications, such as in solar cells, light-emitting diodes, and optoelectronic devices.

Synthesis of Nearly Monodisperse Iron Oxide and Oxyhydroxide Nanocrystals

Abstract: Uniform magnetite, hematite, and goethite nanocrystals were prepared through an attractive method based on an oleic acid/alcohol/water system. By adjusting the synthetic parameters (base concentration, alcohol content, categories of alcohols, etc.), the controlled synthesis of uniform magnetite, hematite, and goethite nanocrystals can be easily achieved. Detailed investigations on the effect of the experimental parameters on the morphology of the final products and the phase transitions among the magnetite, hematite, and goethite phases were carried out. Finally, a method of doping other metal ions into magnetite was developed and the magnetic properties of magnetite doped with different metal elements were studied.

Stable, Superhydrophobic, and Conductive Polyaniline/Polystyrene Films for Corrosive Environments

Abstract: A polyaniline/polystyrene composite film with a lotus-leaf-like structure is prepared via a simple electrospinning method. The film shows stable superhydrophobicity and conductivity, even in many corrosive solutions, such as acidic or basic solutions over a wide pH range, and also in oxidizing solutions. The special surface composition and morphology are the two important aspects that induce such unusual properties. The polystyrene content can strongly influence the morphology of the composite films, which thus display different superhydrophobicities and conductivities.

Highly Efficient Green‐Emitting Phosphorescent Iridium Dendrimers Based on Carbazole Dendrons

Abstract: Green-emitting iridium dendrimers with rigid hole-transporting carbazole dendrons are designed, synthesized, and investigated. With second-generation dendrons, the photoluminescence quantum yield of the dendrimers is up to 87% in solution and 45% in a film. High-quality films of the dendrimers are fabricated by spin-coating, producing highly efficient. non-doped electrophosphorescent organic light-ernitting diodes (OLEDs). With a device structure of indium tin oxide/poly(3,4-ethylenedioxythiopheiie):poly(styrene sulfonic acid)/neat dendrimer/1,3,5-tris(2-N-phenylbenzimidazolyl)benzene/LiF/Al, a maximum external quantum efficiency (EQE) of 10.3% and a maximum luminous efficiency of 34.7 cd A(-1) are realized. By doping the dendrimers into a carbazole-based host, the maximum EQE can be further increased to 16.6%. The integration of rigid hole-transporting dendrons and phosphorescent complexes provides a new route to design highly efficient solution-processable dendrimers for OLED applications.

Uniform hexagonal plates of vaterite CaCO3 mesocrystals formed by biomimetic mineralization

Abstract: Vaterite mesocrystals with hexagonal morphology and uniform size have been successfully synthesized in the presence of a N-trimethylammonium derivative of hydroxyethyl cellulose via aggregation-mediated crystallization using a simple gas-diffusion method. The uniform hexagonal plates display sharp facets and edges, even though they are formed by the aggregation of nanocrystals. The results demonstrate that each vaterite plate can be explained as consisting of aggregates of nanoparticles that share the same three-dimensional orientation. A mechanism for the formation of hexagonal vaterite mesocrystals made of primary nanoparticles and hexagonal units is also presented. An understanding of the mesoscale transformation process will be helpful in controlling the aggregation-driven formation of complex higher-order structured materials and will provide new insights into biomineralization mechanisms. For example, the spines of sea urchins can be discussed within the framework of the mesocrystal concept. This study could provide an additional tool for designing advanced materials and could be used for the synthesis of more complex crystalline three-dimensional structures.

Nanocrystalline TiO2 Photocatalytic Membranes with a Hierarchical Mesoporous Multilayer Structure: Synthesis, Characterization, and Multifunction†

Abstract: A novel sol–gel dip-coating process to fabricate nanocrystalline TiO2 photocatalytic membranes with a robust hierarchical mesoporous multilayer and improved performance has been studied. Various titania sols containing poly(oxyethylenesorbitan monooleate) (Tween 80) surfactant as a pore-directing agent to tailor-design the porous structure of TiO2 materials at different molar ratios of Tween 80/isopropyl alcohol/acetic acid/titanium tetraisopropoxide = R:45:6:1 have been synthesized. The sols are dip-coated on top of a homemade porous alumina substrate to fabricate TiO2/Al2O3 composite membranes, dried, and calcined, and this procedure is repeated with varying sols in succession. The resulting asymmetric mesoporous TiO2 membrane with a thickness of 0.9 μm exhibits a hierarchical change in pore diameter from 2–6, through 3–8, to 5–11 nm from the top to the bottom layer. Moreover, the corresponding porosity is incremented from 46.2, through 56.7, to 69.3 %. Compared to a repeated-coating process using a single sol, the hierarchical multilayer process improves water permeability significantly without sacrificing the organic retention and photocatalytic activity of the TiO2 membranes. The prepared TiO2 photocatalytic membrane has great potential in developing highly efficient water treatment and reuse systems, for example, decomposition of organic pollutants, inactivation of pathogenic microorganisms, physical separation of contaminants, and self-antifouling action because of its multifunctional capability.

Controlling the Edge Length of Gold Nanoprisms via a Seed‐Mediated Approach

Abstract: A straightforward method is investigated for controlling and reinitiating the growth of single-crystalline Au nanoprisms. This work is based on seeding methodology, and depends on the slow reduction of metal ions onto the surface of a growing nanoprism. In this manner, we can tailor the edge length of Au nanoprisms between 100 and 300 nm without changing their thickness or crystallinity. Each nanoprism size has been characterized by UV-vis-NIR (NIR: near-IR) spectroscopy, transmission electron microscopy (TEM) techniques, and statistical analysis. Based on this work and existing silver halide crystal-growth theories, a preliminary mechanism is proposed which comments on the interplay between crystal growth and surface chemistry that ultimately dictates the morphology of the resulting nanostructure.

Superparamagnetic Polymer Nanocomposites with Uniform Fe3O4 Nanoparticle Dispersions

Abstract: Magnetic nanoparticles embedded in polymer matrices are good examples of functional nanostructures with excellent potential for applications such as electromagnetic interference shielding, magneto-optical storage, biomedical sensing, flexible electronics, etc. Control over the dispersion of the nanoparticle phase embedded in a polymer matrix is critical and often challenging. To achieve excellent dispersion, competition between polymer–polymer and polymer–particle interactions have to be balanced to avoid clustering of particles in polymer nanocomposites. We report the first deposition of magnetic nanocomposite poly(methyl methacrylate)/polypyrrole bilayers from solution using spin-coating. Fe3O4 nanoparticles have been synthesized using a chemical co-precipitation route. Using a combination of dissolving the polymer and mixing fatty acid surfactant coated Fe3O4 nanoparticles, we have demonstrated the formation of nanocomposites with uniform nanoparticle dispersion. Cross-sectional scanning electron microscopy, transmission electron microscopy, and magnetic measurements confirm the excellent dispersion and superparamagnetic response. Low-frequency impedance measurements on these bilayers are also presented and analyzed.