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1.
J Am Chem Soc ; 135(46): 17246-9, 2013 Nov 20.
Artigo em Inglês | MEDLINE | ID: mdl-24125019

RESUMO

The behavior of graphene-coated n-type Si(111) photoanodes was compared to the behavior of H-terminated n-type Si(111) photoanodes in contact with aqueous K3[Fe(CN)6]/K4[Fe(CN)6] as well as in contact with a series of outer-sphere, one-electron redox couples in nonaqueous electrolytes. The n-Si/Graphene electrodes exhibited stable short-circuit photocurrent densities of over 10 mA cm(-2) for >1000 s of continuous operation in aqueous electrolytes, whereas n-Si-H electrodes yielded a nearly complete decay of the current density within ~100 s. The values of the open-circuit photovoltages and the flat-band potentials of the Si were a function of both the Fermi level of the graphene and the electrochemical potential of the electrolyte solution, indicating that the n-Si/Graphene did not form a buried junction with respect to the solution contact.

2.
Dalton Trans ; 40(44): 11809-14, 2011 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-21964596

RESUMO

We have resolved the enantiomers of a series of chiral modified metallophthalocyaninato complexes of nickel bearing alkoxy groups at the 14 and 28 positions on what would otherwise be a normal phthalocyaninato ligand and conforming to the general formula [14,28-(RO)(2)Pc]Ni(ii), where R = Me, Et, or n-Pr. The complex for which R = n-Pr is reported here for the first time. Resolution of the enantiomers of these complexes was accomplished via HPLC utilizing an immobilized carbohydrate-based stationary phase, resulting in baseline resolution of peaks corresponding to enantiomers of the complexes, with R(s) values in excess of five. Isolation of milligram quantities of the complexes bearing methoxy and n-propoxy groups in high enantiomeric excess has been achieved via semi-preparative-scale HPLC on the same stationary phase. Resolved samples of these compounds do not appear to racemize at an appreciable rate, nor do they readily exchange alkoxy groups with alcohols while stirring in alcoholic solution. The spectroscopic details and the crystallographically-determined solid-state structure for the complex where R = n-Pr are reported, and are highly similar to those that have been observed for the previously reported analogues. It has been shown by NMR that the chirality and C(2) molecular symmetry of the complex bearing n-propoxy groups is maintained in solution.

3.
Nano Lett ; 11(10): 4449-55, 2011 Oct 12.
Artigo em Inglês | MEDLINE | ID: mdl-21894947

RESUMO

We report the dislocation-driven growth of two-dimensional (2D) nanoplates. They are another type of dislocation-driven nanostructure and could find application in energy storage, catalysis, and nanoelectronics. We first focus on nanoplates of zinc hydroxy sulfate (3Zn(OH)(2)·ZnSO(4)·0.5H(2)O) synthesized from aqueous solutions. Both powder X-ray and electron diffraction confirm the zinc hydroxy sulfate (ZHS) crystal structure as well as their conversion to zinc oxide (ZnO). Scanning electron, atomic force, and transmission electron microscopy reveal the presence of screw dislocations in the ZHS nanoplates. We further demonstrate the generality of this mechanism through the growth of 2D nanoplates of α-Co(OH)(2), Ni(OH)(2), and gold that can also follow the dislocation-driven growth mechanism. Finally, we propose a unified scheme general to any crystalline material that explains the growth of nanoplates as well as different dislocation-driven nanomaterial morphologies previously observed through consideration of the relative crystal growth step velocities at the dislocation core versus the outer edges of the growth spiral under various supersaturations.

4.
Science ; 328(5977): 476-80, 2010 Apr 23.
Artigo em Inglês | MEDLINE | ID: mdl-20413496

RESUMO

Single-crystal nanotubes are commonly observed, but their formation is often not understood. We show that nanotube growth can be driven by axial screw dislocations: Self-perpetuating growth spirals enable anisotropic growth, and the dislocation strain energy overcomes the surface energy required for creating a new inner surface forming hollow tubes spontaneously. This was demonstrated through solution-grown zinc oxide nanotubes and nanowires by controlling supersaturation using a flow reactor and confirmed using microstructural characterization. The agreement between experimental growth kinetics and those predicted from fundamental crystal growth theories confirms that the growth of these nanotubes is driven by dislocations.

5.
Appl Phys Lett ; 95(20): 202501, 2009 Nov 16.
Artigo em Inglês | MEDLINE | ID: mdl-20011248

RESUMO

A soft x-ray absorption and x-ray magnetic circular dichroism (XMCD) study of the ferromagnetism in solution-grown EuS nanocrystals (NCs) is reported. The absorption edges of Eu M(5) and M(4), S K, O K, and P K were probed to determine elementally specific contributions to the magnetism of EuS NCs. Differential spin absorption was observed by XMCD at the Eu M(5,4) edges confirming the presence of a magnetic moment on the Eu(2+) 4f shell. No dichroic signal was observed for S, O, or P. Vibrating sample magnetometry of ensembles of NCs shows ferromagnetic properties consistent with the XMCD studies.

6.
J Am Chem Soc ; 131(45): 16461-71, 2009 Nov 18.
Artigo em Inglês | MEDLINE | ID: mdl-19845339

RESUMO

The basic characteristics of nanowire growth driven by screw dislocations were investigated by synthesizing hierarchical lead sulfide (PbS) nanowire "pine trees" using chemical vapor deposition of PbCl(2) and S precursors and systematically observing the effects of various growth parameters, such as hydrogen flow, temperature, pressure, and the growth substrates employed. Statistical surveys showed that the growth rate of the dislocation-driven trunk is about 6 mum/min and that of the vapor-liquid-solid (VLS) driven branch nanowire is about 1.2 mum/min under the typical reaction conditions at 600 degrees C, 900 Torr, and a hydrogen flow rate of 1.5 sccm. The onset of hydrogen flow plus the presence of fresh silicon have been identified as the critical ingredients for generating PbS nanowire trees reproducibly. To explain the experimental findings in the context of classical crystal growth theory, the former is suggested to create a spike in supersaturation of the actual sulfur precursor H(2)S and initiate dislocations with screw components that then propagate anisotropically to form the PbS nanowire trunks. Maintaining suitable hydrogen flow provides a favorable low supersaturation that promotes dislocation-driven trunk nanowire growth and enables the simultaneous VLS nanowire growth of branches. Furthermore, thermodynamic consideration and experiments showed that silicon fortuitously controls the supersaturation by reversibly reacting with H(2)S to form SiS(2) and that SiS(2) can also be a viable precursor for PbS nanowire growth. The key requirements of screw dislocation-driven nanowire growth are summarized. This study provides some general guidelines for further nanowire growth driven by screw dislocations.


Assuntos
Nanofios/química , Sulfetos/síntese química , Hidrogênio/química , Chumbo/química , Teste de Materiais , Tamanho da Partícula , Pressão , Silício/química , Sulfetos/química , Propriedades de Superfície , Temperatura
7.
Environ Sci Technol ; 43(5): 1598-604, 2009 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-19350941

RESUMO

Once released into the environment, engineered nanomaterials may be transformed by microbially mediated redox processes altering their toxicity and fate. Little information currently exists on engineered nanomaterial transformation under environmentally relevant conditions. Here, we report the development of an in vitro biomimetic assay for investigation of nanomaterial transformation under simulated oxidative environmental conditions. The assay is based on the extracellular hydroquinone-driven Fenton's reaction used by lignolytic fungi. We demonstrate the utility of the assay using CdSe(core)/ZnS(shell) quantum dots (QDs) functionalized with poly(ethylene glycol). QD transformation was assessed by UV-visible spectroscopy, inductively coupled plasma-optical emission spectroscopy, dynamic light scattering, transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). QDs were readily degraded under simulated oxidative environmental conditions: the ZnS shell eroded and cadmium was released from the QD core. TEM, electron diffraction analysis, and EDX of transformed QDs revealed formation of amorphous Se aggregates. The biomimetic hydroquinone-driven Fenton's reaction degraded QDs to a larger extent than did H202 and classical Fenton's reagent (H2O2 + Fe2+). This assay provides a new method to characterize transformations of nanoscale materials expected to occur under oxidative environmental conditions.


Assuntos
Materiais Biomiméticos/química , Meio Ambiente , Teste de Materiais/métodos , Nanoestruturas/química , Absorção , Arsênio/química , Cádmio/análise , Peróxido de Hidrogênio/química , Concentração de Íons de Hidrogênio , Hidroquinonas/química , Ferro/química , Oxirredução , Polietilenoglicóis/química , Pontos Quânticos , Espectrofotometria Ultravioleta , Zinco/análise
8.
Science ; 320(5879): 1060-3, 2008 May 23.
Artigo em Inglês | MEDLINE | ID: mdl-18451264

RESUMO

Hierarchical nanostructures of lead sulfide nanowires resembling pine trees were synthesized by chemical vapor deposition. Structural characterization revealed a screwlike dislocation in the nanowire trunks with helically rotating epitaxial branch nanowires. It is suggested that the screw component of an axial dislocation provides the self-perpetuating steps to enable one-dimensional crystal growth, in contrast to mechanisms that require metal catalysts. The rotating trunks and branches are the consequence of the Eshelby twist of screw dislocations with a dislocation Burgers vector along the 110 directions having an estimated magnitude of 6 +/- 2 angstroms for the screw component. The results confirm the Eshelby theory of dislocations, and the proposed nanowire growth mechanism could be general to many materials.

9.
Nano Lett ; 7(9): 2907-12, 2007 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-17672508

RESUMO

We report a chemical vapor deposition (CVD) synthesis of hyperbranched single-crystal nanowires of both PbS and PbSe using PbCl2 and S/Se as precursors under hydrogen flow. Multiple generations of nanowires grow perpendicularly from the previous generation of nanowires in an epitaxial fashion to produce dense clusters of a complex nanowire network structure. The flow rate and duration of the hydrogen co-flow in the argon carrier gas during the CVD reactions are found to have a significant effect on the morphology of the PbS/PbSe grown, from hyperbranched nanowires to micrometer-sized cubes. No intentional catalyst was employed for the nanowire synthesis, but it is suggested that elemental lead that has been reduced from the vapor by the hydrogen might serve as a vapor-liquid-solid (VLS) catalyst for the anisotropic growth of PbS/PbSe. The nanowires were also investigated with Raman spectroscopy. These PbS and PbSe nanostructures can have applications in photovoltaics because multiple exciton generation has been demonstrated in nanocrystals of both materials.


Assuntos
Cristalização/métodos , Hidrogênio/química , Chumbo/química , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Nanotecnologia/métodos , Compostos de Selênio/química , Sulfetos/química , Catálise , Gases , Substâncias Macromoleculares/química , Teste de Materiais , Conformação Molecular , Tamanho da Partícula , Propriedades de Superfície
10.
Chem Commun (Camb) ; (31): 3326-8, 2006 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-16883425

RESUMO

Solvothermal reactions of iron(II) acetate tetrahydrate and cobalt(II) acetate tetrahydrate with 1,2-dicyanobenzene in methanol solution result in the formation of racemic six-coordinate iron(III) and cobalt(III) complexes, respectively, with a new bicyclic pentadentate 14,28-[1,3-diiminoisoindolinato]phthalocyaninato ligand.

11.
Nano Lett ; 6(8): 1617-21, 2006 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-16895345

RESUMO

We report for the first time the chemical synthesis of free-standing single-crystal nanowires (NWs) of FeSi, the only transition-metal Kondo insulator and the host structure for ferromagnetic semiconductor Fe(x)Co(1-x)Si. Straight and smooth FeSi nanowires are produced on silicon substrates covered with a thin layer of silicon oxide through the decomposition of the single-source organometallic precursor trans-Fe(SiCl3)2(CO)4 in a simple chemical vapor deposition process. Unlike typical vapor-liquid-solid (VLS) NW growth, FeSi NWs form without the addition of metal catalysts, have no catalyst tips, and depend strongly on the surface employed. X-ray spectroscopy verifies the identity and the room-temperature metallic nature of FeSi NWs. Room-temperature electrical transport measurements using NW devices show an average resistivity of 210 micro Omega cm, similar to the value for bulk FeSi. Investigations into the low-temperature physical properties of the first one-dimensional Kondo insulator and the possible new NW growth mechanism are underway. This unique synthetic approach to FeSi NWs will be generally applicable to many other transition-metal silicides.


Assuntos
Cristalização/métodos , Instalação Elétrica , Ferro/química , Nanotecnologia/métodos , Nanotubos/química , Nanotubos/ultraestrutura , Silício/química , Condutividade Elétrica , Ferro/análise , Teste de Materiais , Conformação Molecular , Nanotecnologia/instrumentação , Tamanho da Partícula , Silício/análise
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