One-step electrochemical approach to the synthesis of Graphene/MnO2 nanowall hybrids

We have demonstrated a one-step and effective electrochemical method to synthesize graphene/MnO₂ nanowall hybrids (GMHs). Graphene oxide (GO) was electrochemically reduced to graphene (GN), accompanied by the simultaneous formation of MnO₂ with a nanowall morphology via cathodic electrochemical deposition. The morphology and structure of the GMHs were systematically characterized by scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman spectroscopy. The resulting GMHs combine the advantages of GN and the nanowall array morphology of MnO₂ in providing a conductive network of amorphous nanocomposite, which shows good electrochemical capacitive behavior. This simple approach should find practical applications in the large-scale production of GMHs.      

Strain-induced semiconductor to metal transition in the two-dimensional honeycomb structure of MoS2

The electronic properties of two-dimensional honeycomb structures of molybdenum disulfide (MoS₂) subjected to biaxial strain have been investigated using first-principles calculations based on density functional theory. On applying compressive or tensile bi-axial strain on bi-layer and mono-layer MoS₂, the electronic properties are predicted to change from semiconducting to metallic. These changes present very interesting possibilities for engineering the electronic properties of two-dimensional structures of MoS₂.      

Monodisperse upconversion NaYF4 nanocrystals: Syntheses and bioapplications

A facile strategy using cheap and readily available precursors has been successfully developed for the synthesis of rare-earth doped hexagonal phase NaYF₄ nanocrystals with uniform shape and small particle size as well as strong photoluminescence. Due to their optical properties and good biocompatibility, these multicolor nanocrystals were successfully used as a bio-tag for cancer cell imaging. This novel synthetic method should also be capable of extension to the synthesis of other fluoride nanocrystals such as YF₃ and LaF₃.      

A high performance cobalt-doped ZnO visible light photocatalyst and its photogenerated charge transfer properties

Highly photocatalytically active cobalt-doped ZnO (ZnO:Co) nanorods have been prepared by a facile hydrothermal process. X-ray diffraction, X-ray photoelectron spectroscopy, Raman scattering and UV-vis diffuse reflectance spectroscopy confirmed that the dopant ions substitute for some of the lattice zinc ions, and furthermore, that Co²⁺ and Co³⁺ ions coexist. The as-prepared ZnO:Co samples have an extended light absorption range compared with pure ZnO and showed highly efficient photocatalytic activity, only requiring 60 min to decompose ∼93% of alizarin red dye under visible light irradiation (λ > 420 nm). The photophysical mechanism of the visible photocatalytic activity was investigated with the help of surface photovoltage spectroscopy. The results indicated that a strong electronic interaction between the Co and ZnO was present, and that the incorporation of Co promoted the charge separation and enhanced the charge transfer ability and, at the same time, effectively inhibited the recombination of photogenerated charge carriers in ZnO, resulting in high visible light photocatalytic activity.      

Ionic liquid-assisted one-step hydrothermal synthesis of TiO2-reduced graphene oxide composites

We have demonstrated a facile and efficient strategy for the fabrication of soluble reduced graphene oxide sheets (RGO) and the preparation of titanium oxide (TiO₂) nanoparticle-RGO composites using a modified one-step hydrothermal method. It was found that graphene oxide could be easily reduced under solvothermal conditions with ascorbic acid as reductant, with concomitant growth of TiO₂ particles on the RGO surface. The TiO₂-RGO composite has been thoroughly characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and thermogravimetric analysis. Microscopy techniques (scanning electron microscopy, atomic force microscopy, and transmission electron microscopy) have been employed to probe the morphological characteristics as well as to investigate the exfoliation of RGO sheets. The TiO₂-RGO composite exhibited excellent photocatalysis of hydrogen evolution.      

One-step synthesis of magnetically recyclable Au/Co/Fe triple-layered core-shell nanoparticles as highly efficient catalysts for the hydrolytic dehydrogenation of ammonia borane

Magnetically recyclable Au/Co/Fe core-shell nanoparticles (NPs) have been successfully synthesized via a one-step in situ procedure. Transmission electron microscope (TEM), energy dispersive X-ray spectroscopic (EDS), and electron energy-loss spectroscopic (EELS) measurements revealed that the trimetallic Au/Co/Fe NPs have a triple-layered core-shell structure composed of a Au core, a Co-rich inter-layer, and a Fe-rich shell. The Au/Co/Fe core-shell NPs exhibit much higher catalytic activities for hydrolytic dehydrogenation of ammonia borane (NH₃BH₃, AB) than the monometallic (Au, Co, Fe) or bimetallic (AuCo, AuFe, CoFe) counterparts.      

Synthesis of monodisperse CoPt3 nanocrystals and their catalytic behavior for growth of boron nanowires

Monodisperse CoPt₃ nanocrystals (NCs) have been synthesized in oleylamine solution by an organic solvothermal method. The NCs were ellipsoidal particles with a diameter around 6.6 nm and length around 10 nm with a good single crystal structure. Using CoPt₃ NCs as catalysts, large-area boron nanowires with diameters ranging from 30 to 50 nm were successfully prepared by chemical vapor deposition using a C/B/B₂O₃ mixture as the precursor. Structural analysis indicated that these nanowires were single crystalline with a β-rhombohedral structure. Measurement of the field emission properties of boron nanowire films showed that the boron nanowires have good field emission characteristics.      

One-dimensional ZnO nanostructures: Solution growth and functional properties

One-dimensional (1D) ZnO nanostructures have been studied intensively and extensively over the last decade not only for their remarkable chemical and physical properties, but also for their current and future diverse technological applications. This article gives a comprehensive overview of the progress that has been made within the context of 1D ZnO nanostructures synthesized via wet chemical methods. We will cover the synthetic methodologies and corresponding growth mechanisms, different structures, doping and alloying, position-controlled growth on substrates, and finally, their functional properties as catalysts, hydrophobic surfaces, sensors, and in nanoelectronic, optical, optoelectronic, and energy harvesting devices.      

Low-cost synthesis of robust anatase polyhedral structures with a preponderance of exposed {001} facets for enhanced photoactivities

Anatase polyhedral materials with a preponderance of exposed {001} facets have been produced using (NH₄)₂TiF₆ and water as raw materials. The crystallographic structure and the growth mechanism of the anatase TiO₂ product were investigated systematically by XRD (X-ray diffraction), scanning electron microscopy (SEM), TEM (transmission electron microscope), and ultraviolet (UV) visible and photoluminescence spectroscopy. The products exhibited significantly higher activities than commercial P25 titania nanoparticles in the photocatalytic degradation of methylene blue dye. Moreover, the materials have large particle sizes and are very robust, making them suited for practical uses.      

Wafer-level site-controlled growth of silicon nanowires by Cu pattern dewetting

An approach for the wafer-level synthesis of size- and site-controlled amorphous silicon nanowires (α-SiNWs) is presented in this paper. Microscale Cu pattern arrays are precisely defined on SiO₂ films with the help of photolithography and wet etching. Due to dewetting, Cu atoms shrink to the center of patterns during the annealing process, and react with the SiO₂ film to open a diffusion channel for Si atoms to the substrate. α-SiNWs finally grow at the center of Cu patterns, and can be tuned by varying critical factors such as Cu pattern volume, SiO₂ thickness, and annealing time. This offers a simple way to synthesize and accurately position a SiNW array on a large area.

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Spatially resolved photoelectric performance of axial GaAs nanowire pn-diodes

The spatially resolved photoelectric response of a single axial GaAs nanowire pn-diode has been investigated with scanning photocurrent and Kelvin probe force microscopy. Optical generation of carriers at the pn-junction has been shown to dominate the photoresponse. A photocurrent of 88 pA, an open circuit voltage of 0.56 V and a fill factor of 69% were obtained under AM 1.5 G conditions. The photocurrent followed the increasing photoexcitation with 0.24 A/W up to an illumination density of at least 90 W/cm², which is important for potential applications in concentrator solar cells.     

Facile synthesis of ZnO nanocrystals via a solid state reaction for high performance plastic dye-sensitized solar cells

We report the facile synthesis of ZnO nanocrystals via a one-step solid state reaction at room temperature and their application as the photoanode in plastic dye-sensitized solar cells (DSCs). ZnO nanoparticles were prepared utilizing zinc acetate dihydrate and sodium hydroxide with a short grinding time and without a sintering process. The as-prepared samples with the polycrystalline hexagonal wurtzite structure were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The obtained ZnO nanoparticles exhibited high crystallinity even without a high temperature sintering treatment during the preparation process. The effects of compression post-treatment on the photovoltaic performance of DSCs were also investigated using intensity modulated photocurrent spectroscopy (IMPS), incident photo-to-current conversion efficiency (IPCE), and electrochemical impedance spectroscopy (EIS). The results indicate that the improvement of power conversion efficiency after compression post-treatment of ZnO photoelectrode can be attributed to its high photoelectron collection efficiency and effective electron transport. Under the optimized conditions, a full plastic D149-sensitized ZnO solar cell measured under illumination of 100 mW·cm⁻² (AM 1.5G) presents an energy conversion efficiency of 3.76% with open-circuit voltage of 0.688 V, short-circuit current density of 8.55 mA·cm⁻², and fill factor of 0.64. These results demonstrate that the one-step solid state reaction is a convenient and effective method for the synthesis of ZnO nanocrystals for use in plastic DSCs.      

Temperature-dependent photoconductance of heavily doped ZnO nanowires

Ga-doped ZnO nanowires have been synthesized by a pulsed laser chemical vapor deposition method. The crystal structure and photoluminescence spectra indicate that the dopant atoms are well integrated into the ZnO wurtzite lattice. The photocurrent properties at different temperatures have been systematically investigated for nanowires configured as a three-terminal device. Among the experimental highlights, a pronounced semiconductor-to-metal transition occurs upon UV band-to-band excitation. This is a consequence of the reduction in electron mobility arising from the drastically enhanced Coulomb interactions and surface scattering. Another feature is the reproducible presence of two resistance valleys at 220 and 320 K upon light irradiation. This phenomenon originates from the trapping and detrapping processes in the impurity band arising from the native defects as well as the extrinsic Ga dopants. This work demonstrates that due to the dimensional confinement in quasi-one-dimensional structures, enhanced Coulomb interaction, surface scattering, and impurity states can significantly influence charge transport.      

Electrochemical Characterization of a Porous Pt Nanoparticle ＂Nanocube-Mosaic＂ Prepared by a Modified Polyol Method with HCI Addition

Porous and single crystalline platinum (Pt) nanoparticles (NPs) have been successfully synthesized by reduction of H₂PtCl₆·6H₂O and then investigated by optical spectroscopy and transmission electron microscopy. H₂PtCl₆·6H₂O was reduced using ethylene glycol in the presence of polyvinylpyrrolidone under highly acidic conditions (pH < 1) to form single crystalline Pt particles about 5 nm in size. These particles were then stacked via {100} facets, forming 50-nm length porous nanocubes with a mosaic structure. The porous Pt NPs exhibited excellent catalytic properties for methanol oxidation. In particular, the electrochemical surface area was ∼63 m²/g, five times higher than that for non-porous Pt NPs prepared using a conventional method. We suggest that the high catalytic activity of porous Pt NPs is due to a combination of the crystalline structure having exposed {100} facets and a porous morphology.      

Polarized light microscopy and spectroscopy of individual single-walled carbon nanotubes

Polarized light microscopy (PLM) is used to image individual single-walled carbon nanotubes (SWNTs) suspended in air across a slit opening. The imaging contrast relies on the strong optical anisotropy typical of SWNTs. We combine PLM with a tunable light source to enable hyperspectral excitation spectroscopy and nanotube chirality assignment. Comparison with fluorescence microscopy and spectroscopy confirms the assignment made with PLM. This represents a versatile new approach to imaging SWNTs and related structures.      

Aqueous-phase synthesis of Ag-TiO2-reduced graphene oxide and Pt-TiO2-reduced graphene oxide hybrid nanostructures and their catalytic properties

We demonstrate an aqueous solution method for the synthesis of a Ag-TiO₂-reduced graphene oxide (rGO) hybrid nanostructure (NS) in which the Ag and TiO₂ particles are well dispersed on the rGO sheet. The Ag-TiO₂-rGO NS was then used as a template to synthesize Pt-TiO₂-rGO NS. The resulting hybrid NSs were characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, UV-vis spectroscopy, Raman spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), and catalytic studies. It was found that TiO₂-rGO, Ag-TiO₂-rGO and Pt-TiO₂-rGO NSs all show catalytic activity for the reduction of p-nitrophenol to p-aminophenol by NaBH₄, and that Pt-TiO₂-rGO NS exhibits the highest catalytic activity as well as excellent stability and easy recyclability.      

Semiconducting carbon nanotube/fullerene blended heterojunctions for photovoltaic near-infrared photon harvesting

We demonstrate that the near-infrared (NIR) absorptivity of semiconducting single-walled carbon nanotubes (s-SWCNTs) can be harnessed in blended heterojunctions with the fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). Photogenerated charge separation is efficiently driven by the ultrahigh interfacial area of the blends and the favorable energy offsets between the two materials. NIR-sensitive photovoltaic and photodetector devices utilizing the stack (indium tin oxide/ca. 10 nm s-SWCNT:PCBM/100 nm C₆₀/10 nm 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Ag) were fabricated with NIR power conversion efficiencies >1.3% and peak, zero bias external quantum efficiency of 18% at λ = 1205 nm.      

Sub-2.0-nm Ru and composition-tunable RuPt nanowire networks

Recently, the synthesis of ultrathin nanostructures has attracted increasing interest because of their unique structure and properties. In this work, we report the synthesis of sub-2.0-nm Ru and composition-tunable RuPt nanowire networks using an environmentally friendly aqueous method. The structures were characterized using transmission electron microscopy (TEM), high-resolution TEM, X-ray diffraction (XRD), and energy-dispersive X-ray (EDX) spectroscopy. Moreover, the combined utilization of sodium n-dodecyl sulfate and potassium fluoride was determined to play a key role in the formation of these ultrathin nanostructures. The electrocatalytic properties of the sub-2.0-nm RuPt nanowire networks were investigated for methanol oxidation in an acidic medium. The nanostructures displayed composition-dependent properties, and compared with commercial Ru₅₀Pt₅₀/C, the as-synthesized Ru₅₆Pt₄₄ ultrathin nanowire network exhibited enhanced stability.

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Surface dislocation nucleation mediated deformation and ultrahigh strength in sub-10-nm gold nanowires

The plastic deformation and the ultrahigh strength of metals at the nanoscale have been predicted to be controlled by surface dislocation nucleation. In situ quantitative tensile tests on individual 〈111〉 single crystalline ultrathin gold nanowires have been performed and significant load drops observed in stress-strain curves suggest the occurrence of such dislocation nucleation. High-resolution transmission electron microscopy (HRTEM) imaging and molecular dynamics simulations demonstrated that plastic deformation was indeed initiated and dominated by surface dislocation nucleation, mediating ultrahigh yield and fracture strength in sub-10-nm gold nanowires.      

The oxidation characteristics of silicon nanowires grown with an au catalyst

In this study, we investigated the thermal oxidation of silicon nanowires (SiNWs) grown via the vapor-liquid-solid (VLS) method with an Au catalyst. We systematically analyzed the oxidation mechanism of the SiNWs in both the radial and axial directions and mapped the behavior of the Au atoms on the sidewall and at top of the wire as a function of oxidation time. After thermal oxidation at a temperature of 900 °C, two kinds of oxidation behavior in SiNWs were observed: one was conventional radial oxidation and the other was axial oxidation. In particular, the axial oxidation rate at the Si/Au interface increased dramatically compared with the radial oxidation rate, which can be explained by the reaction between the Si atoms precipitated from the Au tip and the O₂ gas injected in the area surrounding the Au tip. Additionally, we observed that the oxidation rate in the axial direction was inversely proportional to the wire diameter, which is related to the SiO₂ surrounding the Si wire. Moreover, the Au shape changed with respect to the wire diameter, suggesting that both the stress in the Au-Si alloy and the SiO₂ shell thickness of the wire critically affect the growth of SiO₂ on Au.