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The electronic structure of cobalt nanocrystals suspended in liquid as a function of size has been investigated using in situ X-ray absorption and emission spectroscopy. A sharp absorption peak associated with the ligand molecules is found that increases in intensity upon reducing the nanocrystal size. X-ray Raman features due to d−d and to charge-transfer excitations of ligand molecules are identified. The study reveals the local symmetry of the surface of -Co phase nanocrystals, which originates from a dynamic interaction between Co nanocrystals and surfactant solvent molecules. [ABSTRACT FROM AUTHOR]

Electrospinning provides a simple approach to fabricating nanofibers and assemblies with controllable hierarchical structures. In this communication, we demonstrate that electrospinning can be combined with calcination to further maneuver the morphology and phase structure of nanofibers. More specifically, single-crystal V2O5 nanorods could be grown on rutile nanofibers by carefully calcining composite nanofibers consisting of amorphous V2O5, amorphous TiO2, and poly(vinylpyrrolidone). The size of the resulting V2O5 nanorods could be conveniently controlled by varying the composition of the nanofibers and/or the calcination temperature. In addition to the nanorod-on-nanofiber hierarchical structure, we believe this approach can also be extended to fabricate other more complex architectures. [ABSTRACT FROM AUTHOR]

Photocatalytic reduction of CO2 holds great promises for addressing both the environmental and energy issues that are facing the modern society. The major challenge of CO2 photoreduction into fuels such as methane or methanol is the low yield and poor selectivity. Here, we report an effective strategy to enhance the reduction potential of photoexcited electrons by fluorination of mesoporous single crystals of reduced TiO2-x. Density functional theory calculations and photoelectricity tests indicate that the Ti3+ impurity level is upswept by fluorination, owing to the built-in electric field constructed by the substitutional F that replaces surface oxygen vacancies, which leads to the enhanced reduction potential of photoexcited electrons. As a result, the fluorination of the reduced TiO2-x dramatically increases the CH4 production yield by 13 times from 0.125 to 1.63 µmol/g·h under solar light illumination with the CH4 selectivity being improved from 25.7% to 85.8%. Our finding provides a metal-free strategy for the selective CH4 generation from CO2 photoreduction. [ABSTRACT FROM AUTHOR]

We report a novel CsX-stripping mechanism that enables the efficient chemical transformation of nonluminescent Cs4PbX6 (X = Cl, Br, I) nanocrystals (NCs) to highly luminescent CsPbX3 NCs. During the transformation, Cs4PbX6 NCs dispersed in a nonpolar solvent are converted into CsPbX3 NCs by stripping CsX through an interfacial reaction with water in a different phase. This process takes advantage of the high solubility of CsX in water as well as the ionic nature and high ion diffusion property of Cs4PbX6 NCs, and produces monodisperse and air-stable CsPbX3 NCs with controllable halide composition, tunable emission wavelength covering the full visible range, narrow emission width, and high photoluminescent quantum yield (up to 75%). An additional advantage is that this is a clean synthesis as Cs4PbX6 NCs are converted into CsPbX3 NCs in the nonpolar phase while the byproduct of CsX is formed in water that could be easily separated from the organic phase. The as-prepared CsPbX3 NCs show enhanced stability against moisture because of the passivated surface. Our finding not only provides a new pathway for the preparation of highly luminescent CsPbX3 NCs but also adds insights into the chemical transformation behavior and stabilization mechanism of these emerging perovskite nanocrystals. [ABSTRACT FROM AUTHOR]

When nanocrystals are made to undergo chemical transformations, there are often accompanying large mechanical deformations and changes to overall particle morphology. These effects can constrain development of multistep synthetic methods through loss of well-defined particle morphology and functionality. Here, we demonstrate a surface protection strategy for solution phase chemical conversion of colloidal nanostructures that allows for preservation of overall particle morphology despite large volume changes. Specifically, via stabilization with strong coordinating capping ligands, we demonstrate the effectiveness of this method by transforming β-FeOOH nanorods into magnetic Fe3O4 nanorods, which are known to be difficult to produce directly. The surface-protected conversion strategy is believed to represent a general self-templating method for nanocrystal synthesis, as confirmed by applying it to the chemical conversion of nanostructures of other morphologies (spheres, rods, cubes, and plates) and compositions (hydroxides, oxides, and metal organic frameworks). [ABSTRACT FROM AUTHOR]

Developing efficient photoreversible color switching systems for constructing rewritable paper is of significant practical interest owing to the potential environmental benefits including forest conservation, pollution reduction, and resource sustainability. Here we report that the color change associated with the redox chemistry of nanoparticles of Prussian blue and its analogues could be integrated with the photocatalytic activity of TiO2 nanoparticles to construct a class of new photoreversible color switching systems, which can be conveniently utilized for fabricating ink-free, light printable rewritable paper with various working colors. The current system also addresses the phase separation issue of the previous organic dye-based color switching system so that it can be conveniently applied to the surface of conventional paper to produce an ink-free light printable rewritable paper that has the same feel and appearance as the conventional paper. With its additional advantages such as excellent scalability and outstanding rewriting performance (reversibility >80 times, legible time >5 days, and resolution >5 μm), this novel system can serve as an eco-friendly alternative to regular paper in meeting the increasing global needs for environment protection and resource sustainability. [ABSTRACT FROM AUTHOR]

Structural defects have been proven to determine many of the materials' properties. Here, we demonstrate a unique approach to the creation of Ag nanowires with high-density defects through controllable nanoparticles coalescence in one-dimensional pores of mesoporous silica. The density of defects can be easily adjusted by tuning the annealing temperature during synthetic process. The high-density defects promote the adsorption and activation of more reactants on the surface of Ag nanowires during catalytic reactions. As a result, the as-prepared Ag nanowires exhibit enhanced activities in catalyzing dehydrogenative coupling reaction of silane in terms of apparent activation energy and turnover frequency (TOF). We show further that the silane conversion rate can be enhanced by maximizing the defect density and thus the number of active sites on the Ag nanowires, reaching a remarkable TOF of 8288 h-1, which represents the highest TOF that has been achieved by far on Ag catalysts. This work not only proves the important role of structural defects in catalysis but also provides a new and general strategy for constructing high-density defects in metal catalysts. [ABSTRACT FROM AUTHOR]

Colloidal plasmonic metal nanoparticles have enabled surface-enhanced Raman scattering (SERS) for a variety of analytical applications. While great efforts have been made to create hotspots for amplifying Raman signals, it remains a great challenge to ensure their high density and accessibility for improved sensitivity of the analysis. Here we report a dealloying process for the fabrication of porous Au-Ag alloy nanoparticles containing abundant inherent hotspots, which were encased in ultrathin hollow silica shells so that the need of conventional organic capping ligands for stabilization is eliminated, producing colloidal plasmonic nanoparticles with clean surface and thus high accessibility of the hotspots. As a result, these novel nanostructures show excellent SERS activity with an enhancement factor of ~1.3 x 107 on a single particle basis (off-resonant condition), promising high applicability in many SERS-based analytical and biomedical applications. [ABSTRACT FROM AUTHOR]

Silver nanoplates with an extremely high aspect ratio (up to over 400) and a widely tunable surface plasmon resonance (SPR) band have been successfully synthesized by combining the concepts of selective ligand adhesion and seeded growth. Citrate ligands are used as the sole surfactant to effectively block overgrowth on the basal {111} facets and only allow growth in the lateral direction. By slowing down the reaction rate using Ag−citrate complex as precursor, the thin nature of Ag nanoplates is maintained with the edge length grown up to 4 μm, which ensures the high aspect ratio and the widely tunable SPR band. We also observe a size distribution focusing effect that helps to produce uniform nanoplates as well as narrow SPR bands over a wide range, which is important in many practical applications. [ABSTRACT FROM AUTHOR]

The high carrier mobility of graphene makes it an attractive candidate for future electronic device applications.1In SiO2/Si-supported graphene devices, the mobility typically varies from 2000 to ∼20000 cm2V−1s−1.2By removing SiO2,3,4much higher mobility (2 × 105cm2V−1s−1in the latter) has been obtained, suggesting the importance of the Coulomb scattering in graphene transport. Although such elaborate device fabrication is clearly effective, the mobility of finished devices is fixed thereafter and can vary from device to device. In this work, we first demonstrate a significant enhancement in carrier mobility in SiO2-supported graphene decorated with a layer of ligand-bound nanoparticles (NPs) such as iron oxide, titanium dioxide, or cadmium selenide acting as a charge reservoir. By transferring charges between graphene and the NP reservoir through the molecules, we show a remarkable reversible tunability in mobility (4000−19000 cm2V−1s−1) in the samedevice, which unambiguously proves that the charged impurity scattering is the prevailing mechanism for graphene mobility. In addition, the charge neutral point or the Dirac point can also be independently tuned over a wide gate voltage range. The reversible tuning is useful for fabricating large-area graphene devices such as nonvolatile memory with enhanced sensitivity. [ABSTRACT FROM AUTHOR]

We report the rapid formation of photonic crystal structures by assembly of uniform nonmagnetic colloidal particles in ferrofluids using external magnetic fields. Magnetic manipulation of nonmagnetic particles with size down to a few hundred nanometers, suitable building blocks for producing photonic crystals with band gaps located in the visible regime, has been difficult due to their weak magnetic dipole moment. Increasing the dipole moment of magnetic holes has been limited by the instability of ferrofluids toward aggregation at high concentration or under strong magnetic field. By taking advantage of the superior stability of highly surface-charged magnetite nanocrystal-based ferrofluids, in this paper we have been able to successfully assemble 185 nm nonmagnetic polymer beads into photonic crystal structures, from 1D chains to 3D assemblies as determined by the interplay of magnetic dipole force and packing force. In a strong magnetic field with large field gradient, 3D photonic crystals with high reflectance (83%) in the visible range can be rapidly produced within several minutes, making this general strategy promising for fast creation of large-area photonic crystals using nonmagnetic particles as building blocks. [ABSTRACT FROM AUTHOR]

We describe a “surface-protected etching” strategy that allows convenient conversion of sol−gel derived silica into porous structures. Poly(vinyl pyrrolidone) is used to protect the near surface layer, and NaOH is used to selectively etch the interior of the silica spheres. Etching initially yields porous structures and eventually removes the core to leave behind hollow silica spheres with porous shells. This strategy is useful for constructing core−shell systems where active nanomaterials are embedded in silica shell for enhanced stability against aggregation. We experimentally demonstrate use of the surface-protected etching approach to create openings on silica shells; these openings allow dissolved chemical species to reach embedded catalytic particles to be chemically transformed while the porous shells continue to act as effective barriers against aggregation and loss of activity of the core particles. We also show that, by controlling the extent of etching, it is possible to control the permeation rate of the chemical species through the shells. [ABSTRACT FROM AUTHOR]

Hydrophobic inorganic nanocrystals have been transferred from organic solvent to aqueous solution through a robust and general ligand exchange procedure. Polyelectrolytes such as poly(acrylic acid) and poly(allylamine) are used to replace the original hydrophobic ligands on the surface of nanocrystals at an elevated temperature in a glycol solvent and eventually render the nanocrystals highly water soluble. The physical properties of the nanocrystals, such as superparamagnetism, photocatalytic activity, and photoluminescence, are maintained or improved after ligand exchange. [ABSTRACT FROM AUTHOR]

A self-templated approach has been developed for the synthesis of TiO2microcapsules with tunable size and wall thickness by heating sol−gel derived TiO2microspheres with poly(acrylic acid) (PAA) in a diethylene glycol (DEG) solution. PAA plays a crucial role in the formation of microcapsules by crosslinking the surface TiO2nanoparticles and preventing them from dissolution by DEG. Hollow microcapsules form when DEG molecules penetrate the outer layer and remove the core materials by forming soluble titanium glycolate. [ABSTRACT FROM AUTHOR]

By slowing the rate of atomic addition to singly twinned seeds, we have grown silver nanobeams with lengths of 3−30 m, widths ranging from 17 to 70 nm, and a width to thickness ratio of 1.4. The well-defined dimensions, smooth surface, and crystallinity of nanobeams make them promising candidates for studying the effects of size on electron transport. With a simple method that allows rapid characterization of single nanobeams, we find that even the thinnest nanobeams largely retain the low resistivity of bulk silver. Nanobeams can support remarkably high current densities of up to 2.6 × 108A cm-2before the conduction path is broken by the formation of a nanogap. [ABSTRACT FROM AUTHOR]