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.      

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.      

Production of graphene nanospheres by annealing of graphene oxide in solution

We report a simple method to produce graphene nanospheres (GNSs) by annealing graphene oxide (GO) solution at high-temperature with the assistance of sparks induced by the microwave absorption of graphite flakes dispersed in the solution. The GNSs were formed by rolling up of the annealed GO, and the diameters were mostly in the range 300–700 nm. The GNS exhibited a hollow sphere structure surrounded by graphene walls with a basal spacing of 0.34 nm. Raman spectroscopy and X-ray photoelectron spectroscopy of the GNSs confirmed that the GO was efficiently reduced during the fabrication process. The resulting GNSs may open up new opportunities both for fundamental research and applications, and this method may be extended to the synthesis of other nanomaterials and the fabrication of related nanostructures.      

Nanoseparation-inspired manipulation of the synthesis of CdS nanorods

CdS nanorods have been sorted by length using a density gradient ultracentrifuge rate separation method. The fractions containing longer rods showed relatively stronger oxygen-related surface trap emission, while the shorter ones had dominant band-edge emission. These results suggest that the final length distribution of CdS nanorods is not a result of random nucleation and growth, but is related to the local synthesis conditions. Inspired by these findings, different synthesis environments (N₂, air, and O₂) have been employed in order to tailor the length distribution. In addition to the rod length, the photoluminescence properties of CdS nanorods can also be manipulated. Increasing the oxygen partial pressure significantly changed the growth behavior of CdS nanorods by improving the anisotropic growth.      

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.      

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.      

Scalable synthesis of sub-100 nm hollow carbon nanospheres for energy storage applications

Sub-100 nm hollow carbon nanospheres with thin shells are highly desirable anode materials for energy storage applications. However, their synthesis remains a great challenge with conventional strategies. In this work, we demonstrate that hollow carbon nanospheres of unprecedentedly small sizes (down to ～32.5 nm and with thickness of ～3.9 nm) can be produced on a large scale by a templating process in a unique reverse micelle system. Reverse micelles enable a spatially confined Stöber process that produces uniform silica nanospheres with significantly reduced sizes compared with those from a conventional Stöber process, and a subsequent well-controlled sol–gel coating process with a resorcinol–formaldehyde resin on these silica nanospheres as a precursor of the hollow carbon nanospheres. Owing to the short diffusion length resulting from their hollow structure, as well as their small size and microporosity, these hollow carbon nanospheres show excellent capacity and cycling stability when used as anode materials for lithium/sodium-ion batteries.

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Optical properties of SiO2 and ZnO nanostructured replicas of butterfly wing scales

SiO₂ and ZnO inverse structure replicas have been synthesized using butterfly wings as templates. The laser diffraction performance of the SiO₂ inverse structure replica was investigated and it was found that the zero-order light spot split into a matrix pattern when the distance between the screen and the sample was increased. This unique diffraction phenomenon is closely related to the structure of the SiO₂ inverse structure replica. On the other hand, by analyzing the photoluminescence spectrum of the ZnO replica, optical anisotropy in the ultraviolet band was demonstrated for this material.      

Controlled synthesis and multifunctional properties of FePt-Au heterostructures

In this work, FePt-Au heterostructured nanocrystals (HNCs) such as tadpole-, dumbbell-, bead-, and necklace-like nanostructures were synthesized by a facile heteroepitaxial growth of Au NCs onto FePt nanorods (NRs). A study of the growth mechanism revealed that the morphology control of the final products can be correlated with the adsorption sites of hydrogen onto the FePt NRs, which can be manipulated by the amount of the forming gas (Ar/7% H₂) added. Not only the optical characteristic and magnetic properties of the intrinsic materials were retained in the products, but also the FePt-Au HNCs showed the tunable multifunctional properties resulted from the interactions between Au and FePt. Moreover, for methanol oxidation, the FePt-Au HNCs exhibited enhanced catalytic activity and CO tolerance on the catalyst surface compared to commercial Pt catalysts. It is worth noting that as multifunctional units, the FePt-Au HNCs also possess a heterogeneous surface, which could potentially enable their site-specific functionalization for targeting or imaging purposes in biomedical applications. More interestingly, the catalytic properties of the FePt-Au HNCs also endow this material with application potentials in nanocatalysis.      

Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> nanostructures: A new photocatalyst

Uniform colloidal Bi₂S₃ nanodots and nanorods with different sizes have been prepared in a controllable manner via a hot injection method. X-ray diffraction (XRD) results show that the resulting nanocrystals have an orthorhombic structure. Both the diameter and length of the nanorods increase with increasing concentration of the precursors. All of the prepared Bi₂S₃ nanostructures show high efficiency in the photodegradation of rhodamine B, especially in the case of small sized nanodots—which is possibly due to their high surface area. The dynamics of the photocatalysis is also discussed.      

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.      

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.      

Controlled synthesis of porous spinel cobalt manganese oxides as efficient oxygen reduction reaction electrocatalysts

In this article, we report a facile precursor pyrolysis method to prepare porous spinel-type cobalt manganese oxides (Co _x Mn_3-x O₄) with controllable morphologies and crystalline structures. The capping agent in the reaction was found to be crucial on the formation of the porous spinel cobalt manganese oxides from cubic Co₂MnO₄ nanorods to tetragonal Co₂Mn₄ microspheres and tetragonal Co₂Mn₄ cubes, respectively. All of the prepared spinel materials exhibit brilliant oxygen reduction reaction (ORR) electrocatalysis along with high stability. In particular, the cubic Co₂MnO₄ nanorods show the best performance with an onset potential of 0.9 V and a half-wave potential of 0.72 V which are very close to the commercial Pt/C. Meanwhile, the cubic Co₂MnO₄ nanorods present superior stability with negligible degradation of their electrocatalytic activity after a continuous operation time of 10,000 seconds, which is much better than the commercial Pt/C electrocatalyst.

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Densely aligned graphene nanoribbons at ∼35 nm pitch

We demonstrate the fabrication of high-density aligned graphene nanoribbon (GNR) arrays by plasma etching of graphene sheets through a nanomask derived from self-assembled poly (styrene-block-dimethylsiloxane) (PS-PDMS) diblock copolymer films. This approach produces parallel GNR (～12 nm wide) arrays at ～35 nm pitch. Microscopy and polarized Raman spectroscopy are used to reveal the high-degree of alignment of GNRs. Electrical measurements show that parallel GNRs in a 1 μm wide region can deliver ～0.38 mA current at a source-drain bias of 1 V. This novel patterning approach allows for the fabrication of densely aligned GNR arrays on various substrates and could provide a route to large scale integration of GNRs into nanoelectronics, optoelectronics and biosensors.

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Convective assembly of linear gold nanoparticle arrays at the micron scale for surface enhanced Raman scattering

A convective assembly technique at the micron scale analogous to the writing action of a “pipette pen” has been developed for the linear assembly of gold nanoparticle strips with micron scale width and millimeter scale length for surface enhanced Raman scattering (SERS). The arrays with interparticle gaps smaller than 3 nm are hexagonally stacked in the vicinity of the pipette tip. Variable numbers of stacked layers and clean surfaces of the assembled nanoparticles are obtained by optimizing the velocity of the pipette tip. The SERS properties of the assembled nanoparticle arrays rely on their stacking number and surface cleanliness.      

Polyol synthesis and chemical conversion of Cu<Subscript>2</Subscript>O nanospheres

Polyol synthesis route, which is a popular and effective way of synthesizing noble metal nanocrystals, has been employed for the fabrication of Cu₂O nanospheres. With this method, the particle size of the product can be readily tailored by tuning the concentration of Cu(NO₃)₂ and/or poly(vinyl pyrrolidone). It has been demonstrated that the main driving force of this reaction is the difference in redox potentials between ethylene glycol (EG) and NO₃⁻, and not that between those of EG and Cu²⁺. The resulting Cu₂O nanospheres were used as a solid precursor for generating hollow nanospheres of copper sulfide with different sulfiding degrees, as well as CuO, via suitable chemical conversions. The Kirkendall effect determined the final hollow structure. The results in this paper provide a good example of the broadening of the scope of application of polyol synthesis route and may supply a thinking clue for the synthesis of other oxide materials.      

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.      

Novel photodetectors based on double-walled carbon nanotube film/TiO2 nanotube array heterodimensional contacts

A new kind of photodetector based on a double-walled carbon nanotube (DWCNT) film and a TiO₂ nanotube array with hetrodimensional non-ohmic contacts has been fabricated. Due to the dimensionality difference effect, the DWCNT film/TiO₂ nanotube array photodetector exhibits a much higher photocurrent-to-dark current ratio and photoresponse relative to an Au film/TiO₂ nanotube array device, even at small bias voltage. The photocurrent-to-dark current ratio reached four orders of magnitude and a high photoresponse of 2467 A/W was found upon irradiation at 340 nm. Furthermore, the photosensitive regions could be extended into the visible range. The photocurrent-to-dark current ratio reached approximately three orders of magnitude upon irradiation at 532 nm, where the photon energy is much lower than the band gap of TiO₂.      

Ultra-thin double-walled carbon nanotubes: A novel nanocontainer for preparing atomic wires

Double-walled carbon nanotubes (DWCNTs) with high graphitization have been synthesized by hydrogen arc discharge. The obtained DWCNTs have a narrow distribution of diameters of both the inner and outer tubes, and more than half of the DWCNTs have inner diameters in the range 0.6–1.0 nm. Field electron emission from a DWCNT cathode to an anode has been measured, and the emission current density of DWCNTs reached 1 A/cm² at an applied field of about 4.3 V/μm. After high-temperature treatment of DWCNTs, long linear carbon chains (C-chains) can be grown inside the ultra-thin DWCNTs to form a novel C-chain@DWCNT nanostructure, showing that these ultra-thin DWCNTs are an appropriate nanocontainer for preparing truly one-dimensional nanostructures with one-atom-diameter.      

Ammonium bicarbonate reduction route to uniform gold nanoparticles and their applications in catalysis and surface-enhanced Raman scattering

A new protocol for the synthesis of nearly monodisperse gold nanoparticles with controllable size is described. The pathway is based on the reduction of AuCl₄⁻ by ammonium bicarbonate in the presence of sodium stearate under hydrothermal conditions. The particle sizes could be easily tuned by regulating the reaction conditions including precursor concentration, reaction temperature and growth time. A tentative explanation for the reduction and growth mechanism of uniform gold nanoparticles has been proposed. The as-prepared gold particles showed good catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol by excess NaBH₄, and a surface-enhanced Raman scattering (SERS) study suggested that the gold nanoparticles exhibited a high SERS effect on the probe molecule Rhodamine 6G.