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PubMed2004

Catalyst-nanostructure interfacial lattice mismatch in determining the shape of VLS grown nanowires and nanobelts: a case of Sn/ZnO.

Vapor-liquid-solid (VLS) is a well-established process in catalyst-guided growth of nanowires. The catalyst particle is generally believed to be in liquid state during growth, and it is the site for adsorbing incoming molecules; the crystalline structure of the catalyst may not have any influence on the structure of the grown one-dimensional nanostructures. In this paper, using tin particle guided growth of ZnO nanostructures as a model system, we show that the interfacial region of the tin particle with the ZnO nanowire/nanobelt could be ordered (or partially crystalline) during the VLS growth, although the local growth temperature is much higher than the melting point of tin, and the crystallographic lattice structure at the interface is important in defining the structural characteristics of the grown nanowires and nanobelts. The interface prefers to take the least lattice mismatch; thus, the crystalline orientation of the tin particle may determine the growth direction and the side surfaces of the nanowires and nanobelts. This result may have important impact on the understanding of the physical chemical process in the VLS growth.

Journal Article↗
PubMed2005

DNA-templated three-branched nanostructures for nanoelectronic devices.

Three-branched DNA molecules have been designed and assembled from oligonucleotide components. These nucleic acid constructs contain double- and single-stranded regions that control the hybridization behavior of the assembly. Specific localization of a single streptavidin molecule at the center of the DNA complex has been investigated as a model system for the directed placement of nanostructures. Highly selective silver and copper metallization of the DNA template has also been characterized. Specific hybridization of these DNA complexes to oligonucleotide-coupled nanostructures followed by metallization should provide a bottom-up self-assembly route for the fabrication and characterization of discrete three-terminal nanodevices.

Copper↗
PubMed2004

Dispersion interactions enable the self-directed growth of linear alkane nanostructures covalently bound to silicon.

Current interest in methods for controllably adding organic molecules to silicon surfaces relates to proposed hybrid silicon-organic devices. It was recently shown that a "self-directed" growth process, requiring only limited scanned probe intervention, has the potential to permit rapid, parallel production of ordered molecular nanostructures on silicon with predefined absolute position, structure, composition, and extent of growth. The hybrid organic-silicon structures formed are bound by strong covalent interactions. In this work, we use scanning tunneling microscopy and density functional theory techniques to show that molecule-surface dispersion interactions enable the growth process and play a crucial role in the final configurations of the nanostructures.

Journal Article↗
PubMed2005

Mirror image nanostructures.

Chiral molecules that self-assemble to form chiral supramolecular structures exhibit interesting structural features reminiscent of tertiary and quaternary structures of proteins and have applications in catalysis and nonlinear optics. Often, these structures are hierarchical, with their chiral structure difficult to interpret on the molecular scale. In this communication, we observe chiral assembling molecules that form well-defined helices with a pitch of 28 nm. We observe the behavior in both R- and S-enantiomers of the molecule, forming mirror image nanostructures. The molecular chirality is determined by the dimethyloctyl alkyl coil of the molecule and is located more than 4 nm from the hydrogen-bonding segment. The nanostructures observed are not hierarchical, which could be a result of the significant separation between the stereocenter and hydrogen-bonding dendron. The subtle structural modification at the periphery of the molecule biases the supramolecular assembly, which is driven primarily by strong hydrogen-bonding and pi-pi stacking interactions.

Journal Article↗
PubMed2006

Ultralow-density nanostructured metal foams: combustion synthesis, morphology, and composition.

The synthesis of low-density, nanoporous materials has been an active area of study in chemistry and materials science dating back to the initial synthesis of aerogels. These materials, however, are most often limited to metal oxides, e.g., silica and alumina, and organic aerogels, e.g., resorcinol/formaldehyde, or carbon aerogels, produced from the pyrolysis of organic aerogels. The ability to form monolithic metallic nanocellular porous materials is difficult and sometimes elusive using conventional methodology. Here we report a relatively simple method to access unprecedented ultralow-density, nanostructured, monolithic, transition-metal foams, utilizing self-propagating combustion synthesis of novel transition-metal complexes containing high nitrogen energetic ligands. During the investigation of the decomposition behavior of the high-nitrogen transition metal complexes, it was discovered that nanostructured metal monolithic foams were formed in a post flame-front dynamic assembly having remarkably low densities down to 0.011 g cm(-3) and extremely high surface areas as high as 270 m(2) g(-1). We have produced monolithic nanoporous metal foams via this method of iron, cobalt, copper, and silver metals. We expect to be able to apply this to many other metals and to be able to tailor the resulting structure significantly.

Journal Article↗
PubMed2006

Manipulation of aqueous growth of CdTe nanocrystals to fabricate colloidally stable one-dimensional nanostructures.

The present article is devoted to systematically exploring the influence of various experimental variables, including the precursor concentration, the ligand nature, the counterion type, the Cd-to-Te molar ratio, pH, and temperature, on the aqueous growth of CdTe nanocrystals. The growth may be divided into two stages: the early fast growth stage and the later slow growth stage. The later stage is found to be dominated by Ostwald ripening (OR), being strongly dependent on all experimental conditions. In contrast, the early stage is dominated by adding monomers to nanocrystals, which may be dramatically accelerated by lowering precursor concentrations and using ligands with a molecular structure similar to that of thioglycolic acid (TGA). This fast growth stage is similar to that observed during organometallic growth of nanocrystals in hot organic media. On the basis of this finding, one-dimensional wurtzite CdTe nanostructures can be directly prepared in aqueous media by storing rather dilute precursor solution (2.4 mM with reference to ligand) in the presence of TGA at lower temperature (from room temperature to 80 degrees C). A low growth temperature is used to suppress OR during crystal growth. In addition, the simultaneous presence of both TGA-like ligand and 1-thioglycerol or 2-mercaptoethylamine leads to formation of colloidally stable 1D CdTe nanostructures with controlled aspect ratios.

Journal Article↗
PubMed2005

Synthesis and optical properties of S-doped ZnO nanostructures: nanonails and nanowires.

S-doped ZnO nanostructures such as nanonails and nanowires have been synthesized via a simple one-step catalyst-free thermal evaporation process on a large scale. The doping concentration of sulfur into ZnO nanonails and nanowire were 2 atm % and 7.5 atm %, respectively. Studies found that the S-doped ZnO nanonails and nanowires were single-crystalline wurtzite structure and grew along the (001) direction. The average diameters of the nanonails and nanowires were 70 and 50 nm, respectively. Low-temperature photoluminescence spectra of ZnO samples showed two luminescence peaks in the UV and green emission region, respectively. As the concentration of sulfur in the ZnO nanostructures increased, the intensity of the UV emission peak decreased dramatically, and it showed a little blue-shift while the intensity of the green emission increased greatly.

Journal Article↗
PubMed2005

Electronic and chemical properties of nanostructured cerium dioxide doped with praseodymium.

Nanostructured doped ceria is a prospective material for catalytic applications such as the construction of membranes with mixed electronic and ionic conductivity for effective syngas production. In this article, the surface properties of nanostructured ceria doped with praseodymium have been studied by X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and Fourier transform infrared spectroscopy of adsorbed carbon monoxide. The effects of supporting 1.4 wt % Pt as well as structural changes upon the reduction of the samples with methane have been investigated. While in samples without supported platinum, mainly praseodymium cations are reduced in a methane atmosphere; stronger reduction of cerium cations was found in the case of surface modification with Pt. The structural differences correlate with results from temperature-programmed reaction experiments with methane. Explanations are discussed in terms of different reaction mechanisms.

Journal Article↗
PubMed2005

Effects of nanostructure on catalytic degradation of ethanol on SrCO3 catalysts.

Degradation of ethanol over SrCO3 nanowires and nanoparticles was used as a model reaction to investigate the effect of nanostructure on chemical property. Differences in catalytic degradation activity with nanostructure are evaluated. The results indicated that catalytic activity of SrCO3 particles increases with decreasing of particle size due to high surface area. But this conclusion cannot be applicable to evaluating SrCO3 nanowires and nanoparticles. SrCO3 nanowires have lower ignition temperatures and wider working temperature ranges than SrCO3 nanoparticles, though nanowires had lower surface areas. Besides, ethanol degraded over nanowires in three ways, and the dominating reaction changes with reaction temperature. Consequently, the main degradation products of nanowires differed with temperature. But for nanoparticles, acetaldehyde is the only main product. Since transmission electron microscopy, X-ray diffraction, and bond equilibrium theory analysis demonstrated that nanowires and nanoparticles had similar crystal structure, surface area, and grain size, the differences in catalytic degradation activity between SrCO3 nanowires and nanoparticles can be attributed to different distributions of active sites, as proven by CO2 and ethanol temperature programmed desorption.

Journal Article↗
PubMed2005

Strong polarized enhanced raman scattering via optical tunneling through random parallel nanostructures in Au thin films.

Random parallel nanostructures (ridges and channels) were created by scratching gold thin films deposited on glass slides. Atomic force microscope (AFM) images showed that the width of the substructures within the scratches were of the order of a few hundred nanometers. These nanometric gold features can then support localized surface plasmon resonances in the direction perpendicular to the propagation of the scratches. This surface plasmon excitation led to a remarkable dependence of the intensity of the surface-enhanced resonance Raman scattering (SERRS) on the polarization direction of the incident light relative to the orientation of the scratch. The maximum SERRS intensities for oxazine 720 (a common laser dye) adsorbed on these nanostructures were obtained when the polarization of the light field was perpendicular to the direction of the substructures. The SERRS intensities followed a squared dependence on the polarization direction of the incident field.

Journal Article↗
PubMed2005

Illumination intensity dependence of the photovoltage in nanostructured TiO2 dye-sensitized solar cells.

The open-circuit voltage (V(oc)) dependence on the illumination intensity (phi0) under steady-state conditions in both bare and coated (blocked) nanostructured TiO2 dye-sensitized solar cells (DSSCs) is analyzed. This analysis is based on a recently reported model [Bisquert, J.; Zaban, A.; Salvador, P. J. Phys. Chem. B 2002, 106, 8774] which describes the rate of interfacial electron transfer from the conduction band of TiO2 to acceptor electrolyte levels (recombination). The model involves two possible mechanisms: (1) direct, isoenergetic electron injection from the conduction band and (2) a two-step process involving inelastic electron trapping by band-gap surface states and subsequent isoenergetic transfer of trapped electrons to electrolyte levels. By considering the variation of V(oc) over a wide range of illumination intensities (10(10) < phi0 < 10(16) cm(-2) s(-1)), three major regions with different values of dV(oc)/d phi0 can be distinguished and interpreted. At the lower illumination intensities, recombination mainly involves localized band-gap, deep traps at about 0.6 eV below the conduction band edge; at intermediate photon fluxes, recombination is apparently controlled by a tail of shallow traps, while, for high enough phi0 values, conduction band states control the recombination process. The high phi0 region is characterized by a slope of dV(oc)/d log phi0 congruent with 60 mV, which indicates a recombination of first order in the free electron concentration. The study, which was extended to different solar cells, shows that the energy of the deep traps seems to be an intrinsic property of the nanostructured TiO2 material, while their concentration and also the density ([symbol: see text]t approximately 10(18)-10(19) cm(-3)) and distribution of shallow traps, which strongly affects the shape of the V(oc) vs phi0 curves, change from sample to sample and are quite sensitive to the electrode preparation. The influence of the back-reaction of electrons from the fluorine-doped tin oxide (FTO) conducting glass substrate with electrolyte tri-iodide ions on the V(oc) vs phi0 dependence characteristic of the DSSC is analyzed. It is concluded that this back-reaction route can be neglected, even at low light intensities, when its rate (exchange current density, j0), which can vary over 4 orders of magnitude depending on the type of FTO used, is low enough (j0 < or = 10(-8)A cm(-2)). The comparison of V(oc) vs phi0 measurements corresponding to different DSSCs with and without blocking of the FTO-electrolyte contact supports this conclusion.

Journal Article↗
PubMed2005

Designed nanostructured pt film for electrocatalytic activities by underpotential deposition combined chemical replacement techniques.

Multiple-deposited Pt overlayer modified Pt nanoparticle (MD-Pt overlayer/PtNPs) films were deliberately constructed on glassy carbon electrodes through alternately multiple underpotential deposition (UPD) of Ag followed redox replacement reaction by Pt (II) cations. The linear and regular growth of the films characterized by cyclic voltammetry was observed. Atomic force spectroscopy (AFM) provides the surface morphology of the nanostructured Pt films. Rotating disk electrode (RDE) voltammetry and rotating ring-disk electrode (RRDE) voltammetry demonstrate that the MD-Pt overlayer/PtNPs films can catalyze an almost four-electron reduction of O(2) to H(2)O in air-saturated 0.1 M H(2)SO(4). Thus-prepared Pt films behave as novel nanostructured electrocatalysts for dioxygen reduction and hydrogen evolution reaction (HER) with enhanced electrocatalytic activities, in terms of both reduction peak potential and peak current, when compared to that of the bulk polycrystalline Pt electrode. Additionally, it is noted that after multiple replacement cycles, the electrocatalytic activities improved remarkably, although the increased amount of Pt is very low in comparison to that of pre-modified PtNPs due to the intrinsic feature of the UPD-redox replacement technique. In other words, the electrocatalytic activities could be improved markedly without using very much Pt by the technique of tailoring the catalytic surface. These features may provide an interesting way to produce Pt catalysts with a reliable catalytic performance as well as a reduction in cost.

Journal Article↗
PubMed2005

Electrochemical study of interfacial composite nanostructures: polyelectrolyte/gold nanoparticle multilayers assembled on phospholipid/dextran sulfate monolayers at a liquid-liquid interface.

The build up and electrochemical characterization of interfacial composite nanostructures containing a cationic polyelectrolyte and negatively charged mercaptosuccinic acid stabilized gold nanoparticles (AuNPs) is reported. The nanostructures were formed at the interface between two immiscible electrolyte solutions in which the organic phase is an immobilized 2-nitrophenyl octyl ether/PVC gel. The growth of the multilayer was verified with UV-vis spectra, and approximately a linear increase in UV-vis absorbance with increasing number of layers was observed. The interfacial capacitance of the multilayers was measured as a function of the potential and a theoretical model was developed to explain the results. The excellent agreement between theoretical and experimental capacitance curves allows us to conclude that nanocomposites behave similarly to polyelectrolyte multilayers, with the outmost layer determining the alternating sign of the outer surface charge density. Cyclic voltammograms were used to evaluate the transfer rate constant across the multilayers of a model drug, metoprolol, and the standard probe tetraethylammonium cation. The apparent rate constants were slightly larger than in other studies in the literature and decrease with the increasing number of layers.

Catalysis↗
PubMed2005

Electroless synthesis of Ag nanoparticles on deposited nanostructured Si films.

Two and three-dimensional Ag nanoparticle ensembles were synthesized on deposited nanostructured column-void Si films simply by film immersion into pure Ag(2)SO(4) or AgNO(3) solutions. In addition to functioning as a reducer, this nanostructured material provides immobilization and monodispersion of the Ag nanoparticles due to its systematic nanoscale topography. This is accomplished without the requirement of a surfactant, capping agent, or linker. Kinetics, as monitored from plasmon optical extinction, and infrared spectroscopy suggest accompanying oxide growth limits and finally inhibits synthesis enabling nanoparticle size control. Kinetics is also limited by Ag+ transport through the voids unless the Si film is ultrathin. Our synthesis approach offers significant advantages for surface-enhanced molecular detection, including the absence of any agents on the nanoparticle surfaces and the ability to obtain nanoparticle ensembles on any substrate.

Journal Article↗
PubMed2006

Photoluminescence investigation based on laser heating effect in ZnO-ordered nanostructures.

We synthesized ZnO-SiO2 composite opal and ZnO inverse opal by electrodeposition using SiO2-opal template and polystyrene (PS)-opal template, respectively. Compared with compact ZnO nanocrystal film also prepared by electrodeposition, ordered ZnO nanostructures exhibit more significant red-shift and broadening of the UV peak with increasing excitation power, which is due to a stronger local heating effect in ordered ZnO nanostructures. We developed a quantitative analytical method to investigate photoluminescence (PL) of ZnO based on laser heating effects. The experimental data agree well with fitting curves derived from the electron-phonon interaction model. Important parameters, such as electron-phonon coupling strength and thermal activation energy, can be obtained by fitting experimental data. The resonant Raman spectra provide further evidence that the analyses based on laser heating effects are feasible.

Lasers↗
PubMed2006

Nanostructural organization in ionic liquids.

Nanometer-scale structuring in room-temperature ionic liquids is observed using molecular simulation. The ionic liquids studied belong to the 1-alkyl-3-methylimidazolium family with hexafluorophosphate or with bis(trifluoromethanesulfonyl)amide as the anions, [C(n)mim][PF(6)] or [C(n)mim][(CF(3)SO(2))(2)N], respectively. They were represented, for the first time in a simulation study focusing on long-range structures, by an all-atom force field of the AMBER/OPLS_AA family containing parameters developed specifically for these compounds. For ionic liquids with alkyl side chains longer than or equal to C(4), aggregation of the alkyl chains in nonpolar domains is observed. These domains permeate a tridimensional network of ionic channels formed by anions and by the imidazolium rings of the cations. The nanostructures can be visualized in a conspicuous way simply by color coding the two types of domains (in this work, we chose red = polar and green = nonpolar). As the length of the alkyl chain increases, the nonpolar domains become larger and more connected and cause swelling of the ionic network, in a manner analogous to systems exhibiting microphase separation. The consequences of these nanostructural features on the properties of the ionic liquids are analyzed.

Journal Article↗
PubMed2006

One-dimensional ZnO nanostructure arrays: synthesis and characterization.

One-dimensional ZnO nanostructure arrays such as nanowires, nanonails, and nanotrees, have been synthesized by oxygen assisted thermal evaporation of metallic zinc on a quartz substrate over a large area. Morphological evolution of ZnO nanostructures at different time scales and different positions of the substrates have been studied by electron microscopy. A self-catalyzed vapor-liquid-solid (VLS) process is believed to be responsible for the nucleation and subsequently a vapor-solid process is operative for further longitudinal growth. The photoluminescence spectrum showed a weak UV and a broad green emission peak at 3.25 and 2.49 eV, respectively. The latter was attributed to the presence of zinc interstitial defects. Electrical resistivity as a function of temperature showed activated mechanisms to be present. The electrical response of the ZnO nanonail arrays to different gases (CO, NO2, and H2S) indicated that there could be possible application as gas sensors for this material.

Journal Article↗