Showing papers in "Journal of Physical Chemistry B in 2000"
TL;DR: In this paper, the fundamental physics behind the scarcity of ferromagnetic ferroelectric coexistence was explored and the properties of known magnetically ordered ferro-electric materials were examined.
Abstract: Multiferroic magnetoelectrics are materials that are both ferromagnetic and ferroelectric in the same phase. As a result, they have a spontaneous magnetization that can be switched by an applied magnetic field, a spontaneous polarization that can be switched by an applied electric field, and often some coupling between the two. Very few exist in nature or have been synthesized in the laboratory. In this paper, we explore the fundamental physics behind the scarcity of ferromagnetic ferroelectric coexistence. In addition, we examine the properties of some known magnetically ordered ferroelectric materials. We find that, in general, the transition metal d electrons, which are essential for magnetism, reduce the tendency for off-center ferroelectric distortion. Consequently, an additional electronic or structural driving force must be present for ferromagnetism and ferroelectricity to occur simultaneously.
3,146 citations
TL;DR: In this paper, the authors introduce the fundamentals of TEM and its applications in structural determination of shape-controlled nanocrystals and their assemblies, and demonstrate in situ TEM for characterizing and measuring the thermodynamic, electric, and mechanical properties of individual nanostructures, from which the structure−property relationship can be registered with a specific nanoparticle/structure.
Abstract: The physical and chemical properties of nanophase materials rely on their crystal and surface structures. Transmission electron microscopy (TEM) is a powerful and unique technique for structure characterization. The most important application of TEM is the atomic-resolution real-space imaging of nanoparticles. This article introduces the fundamentals of TEM and its applications in structural determination of shape-controlled nanocrystals and their assemblies. By forming a nanometer size electron probe, TEM is unique in identifying and quantifying the chemical and electronic structure of individual nanocrystals. Electron energy-loss spectroscopy analysis of the solid-state effects and mapping the valence states are even more attractive. In situ TEM is demonstrated for characterizing and measuring the thermodynamic, electric, and mechanical properties of individual nanostructures, from which the structure−property relationship can be registered with a specific nanoparticle/structure.
1,980 citations
TL;DR: In this paper, the impact of particle size on phase stability and phase transformation during growth of nanocrystalline aggregates was investigated using titania (TiO2) samples consisting of anatase and brookite.
Abstract: To understand the impact of particle size on phase stability and phase transformation during growth of nanocrystalline aggregates we conducted experiments using titania (TiO2) samples consisting of nanocrystalline anatase (46.7 wt %, 5.1 nm) and brookite (53.3 wt %, 8.1 nm). Reactions were studied isochronally at reaction times of 2 h in the temperature range 598−1023 K and isothermally at 723, 853, and 973 K by X-ray diffraction (XRD). A numerical deconvolution method was developed to separate overlapping XRD peaks, and an analytical method for determining phase contents of anatase, brookite, and rutile from XRD data was established. Results show that, in contrast to previous studies, anatase in our samples transforms to brookite and/or rutile before brookite transforms to rutile. Thermodynamic and kinetic analyses further support this conclusion. For general titania samples, the transformation sequence among anatase and brookite depends on the initial particle sizes of anatase and brookite, since partic...
1,408 citations
TL;DR: The wavelength corresponding to the extinction maximum, λmax, of localized surface plasmon resonance (LSPR) of silver nanoparticle arrays fabricated by nanosphere lithography (NSL) can be systematically tuned from ∼400 nm to 6000 nm as discussed by the authors.
Abstract: The wavelength corresponding to the extinction maximum, λmax, of the localized surface plasmon resonance (LSPR) of silver nanoparticle arrays fabricated by nanosphere lithography (NSL) can be systematically tuned from ∼400 nm to 6000 nm. Such spectral manipulation was achieved by using (1) precise lithographic control of nanoparticle size, height, and shape, and (2) dielectric encapsulation of the nanoparticles in SiOx. These results demonstrate an unprecedented level of wavelength agility in nanoparticle optical response throughout the visible, near-infrared, and mid-infrared regions of the electromagnetic spectrum. It will also be shown that this level of wavelength tunability is accompanied with the preservation of narrow LSPR bandwidths (fwhm), Γ. Additionally, two other surprising LSPR optical properties were discovered: (1) the extinction maximum shifts by 2−6 nm per 1 nm variation in nanoparticle width or height, and (2) the LSPR oscillator strength is equivalent to that of atomic silver in gas or...
1,207 citations
TL;DR: In this paper, a dye-sensitized rutile-based solar cell was developed and optimized to achieve a PV response close to that of the anatase-based cell.
Abstract: The objective of this work is to develop and optimize the new dye-sensitized solar cell technology. In view of the infancy of rutile material development for solar cells, the PV response of the dye-sensitized rutile-based solar cell is remarkably close to that of the anatase-based cell.
1,060 citations
TL;DR: In this paper, gate-dependent measurements of n-type and p-type silicon nanowires have been made and characterized by electrical transport measurements, and it is shown that the carrier mobility of these materials is consistent with diffusive transport, and that it is possible to heavily dope SiNWs and approach a metallic regime.
Abstract: Single-crystal n-type and p-type silicon nanowires (SiNWs) have been prepared and characterized by electrical transport measurements. Laser catalytic growth was used to introduce controllably either boron or phosphorus dopants during the vapor phase growth of SiNWs. Two-terminal, gate-dependent measurements made on individual boron-doped and phosphorus-doped SiNWs show that these materials behave as p-type and n-type materials, respectively. Estimates of the carrier mobility made from gate-dependent transport measurements are consistent with diffusive transport. In addition, these studies show it is possible to heavily dope SiNWs and approach a metallic regime. Temperature-dependent measurements made on heavily doped SiNWs show no evidence for Coulomb blockade at temperatures down to 4.2 K, and thus testify to the structural and electronic uniformity of the SiNWs. Potential applications of the doped SiNWs are discussed.
1,048 citations
TL;DR: In this article, the surface-enhanced Raman scattering (SERS) of single molecules of Rhodamine (R6G) is investigated and it is shown that the SERS signal is significantly higher when the excitation wavelength is resonant with the absorption band of R6G.
Abstract: Atomic force microscopy (AFM) measurements show that the Ag nanoparticles that yield surface-enhanced Raman scattering (SERS) of single molecules of Rhodamine (R6G) are all compact aggregates consisting of a minimum of two individual particles. Comparison of 514.5 and 632.8 nm excitation shows that the single molecule R6G signal is significantly higher when the excitation wavelength is resonant with the absorption band of R6G and suggests that the Raman excitation spectrum follows the absorption profile for R6G. We have also observed an interesting superlinear power dependence of the SERS signal. On average, by increasing the incident power by 2 orders of magnitude and decreasing the integration time by the same factor to maintain constant fluence, increases of 4 to 6 times were observed in the SERS intensity. We discuss these results in terms of model where the R6G molecule that yields single molecule SERS signals is located at the junction of two Ag nanoparticles. We have also modeled the system using m...
1,046 citations
TL;DR: The analytic generalized Born approximation is modified to permit a more accurate description of large macromolecules, while its established performance on small compounds is nearly unaffected, and is adapted to describe molecules with an interior dielectric constant not equal to unity.
Abstract: The analytic generalized Born approximation is an efficient electrostatic model that describes molecules in solution. Here it is modified to permit a more accurate description of large macromolecul...
982 citations
TL;DR: In this article, a nonphonon energy relaxation mechanism involving electron−hole (e−h) spatial separation strongly affects electron intraband dynamics in CdSe nanocrystals with a variety of surface passivations.
Abstract: Femtosecond transient absorption in the visible and infrared spectral ranges has been applied to study carrier dynamics and mechanisms for resonant optical nonlinearities in CdSe nanocrystals (NCs) with a variety of surface passivations. Sequential filling of the 1S, 1P, and 1D atomic-like electron orbitals, governed by Fermi statistics, is clearly observed in the NC bleaching spectra recorded at progressively higher pump intensities. We observe that electron−hole (e−h) spatial separation strongly affects electron intraband dynamics. Such dependence indicates a nonphonon energy relaxation mechanism involving e−h interactions. A strong difference in electron and hole relaxation behavior in the stage following initial intraband relaxation is observed. In contrast to electron relaxation, which is sensitive to the quality of surface passivation (i.e., is affected by trapping at surface defects), depopulation dynamics of the initially-excited hole states are identical in NCs with different surface properties, ...
904 citations
TL;DR: In this article, the results of steady-state and time-resolved luminescence measurements performed on suspensions of nanocrystalline ZnO particles of different sizes and at different temperatures are presented.
Abstract: This report presents the results of steady-state and time-resolved luminescence measurements performed on suspensions of nanocrystalline ZnO particles of different sizes and at different temperatures. In all cases two emission bands are observed. One is an exciton emission band and the second an intense and broad visible emission band, shifted by approximately 1.5 eV with respect to the absorption onset. As the size of the particles increases, the intensity of the visible emission decreases while that of the exciton emission increases. As the temperature decreases, the relative intensity of the exciton emission increases. In accordance with the results presented in a previous paper, we assume that the visible emission is due to a transition of an electron from a level close to the conduction band edge to a deeply trapped hole in the bulk ( ) of the ZnO particle. The temperature dependence and size dependence of the ratio of the visible to exciton luminescence and the kinetics are explained by a model in w...
827 citations
TL;DR: The presence of interchain species in the photophysics of conjugated polymer films has been the subject of a great deal of controversy as discussed by the authors, and much of the controversy can be resolved by noting that the polymer samples in different studies had different side groups or were prepared in different ways and thus have different degrees of inter-chain interaction.
Abstract: The presence of interchain species in the photophysics of conjugated polymer films has been the subject of a great deal of controversy. In this paper, we present strong evidence that interchain species do form in conjugated polymer films, and that the degree of interchain interactions can be controlled by varying the solvent and polymer concentration of the solution from which the films are cast. Thus, much of the controversy in the literature can be resolved by noting that the polymer samples in different studies had different side groups or were prepared in different ways and thus have different degrees of interchain interaction. The photoluminescence (PL) of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene), MEH-PPV, changes both its spectral shape and quantum yield when the films are prepared from different solutions or when the morphology is varied by annealing. Increasing the amount of interchain interactions enhances the red portion of the film’s PL, a result assigned to a combination of changes in the vibronic structure of the PL of the exciton and increased numbers of weakly emissive interchain species. Photoluminescence excitation spectroscopy shows that excitation to the red edge of the absorption band preferentially enhances the red emission, suggesting that the interchain species are aggregates with a distinct ground state absorption. Scanning force microscopy shows topographic features that correlate with the degree of interchain interactions, verifying that the morphology of conjugated polymer films changes with polymer concentration, choice of solvent, and spin-casting speed. Even at low excitation intensities, photooxidative damage occurs quickly in MEH-PPV films excited in air, and the rate at which damage occurs is sensitive to the packing of the polymer chains. For samples under vacuum at low excitation intensity, a long-lived emissive tail, in combination with excitedstate absorption dynamics that do not match those of the emissive species, provide direct evidence for the production of interchain aggregates. Annealing an MEH-PPV film produces a photophysical signature similar to photooxidation, implying that defects in conjugated polymer films are intrinsic and depend on the details of how the chains are packed. At higher excitation intensities, we find that exciton -exciton annihilation occurs, and that the probability for annihilation can vary by an order of magnitude depending on the degree of interchain contact in the film. Finally, we show that changing the film morphology has a direct effect on the performance of MEH-PPV-based light-emitting diodes. Higher degrees of interchain interaction enhance the mobility of carriers at the expense of lower quantum efficiencies for electroluminescence. Taken together, the results reconcile much of the contradictory literature and provide a prescription for the optimization of conjugated polymer films for particular device applications.
TL;DR: The role of electrical potential, charge transport, and recombination in determining the photopotential and photocurrent conversion efficiency of dye-sensitized nanocrystalline solar cells was studied in this paper.
Abstract: The role of electrical potential, charge transport, and recombination in determining the photopotential and photocurrent conversion efficiency (IPCE) of dye-sensitized nanocrystalline solar cells was studied. Electrostatic arguments and electrical impedance spectroscopy (EIS) are used to obtain information on the electrical and electrochemical potential distribution in the cell. It is shown that on the macroscopic level, no significant electrical potential drop exists within the porous TiO2 when it contacts the electrolyte and that the electrical potential drop at the transparent conducting oxide substrate (TCO)/TiO2 interface occurs over a narrow region, one or two layers of TiO2. Analyses of EIS and other data indicate that both the photopotential of the cell and the decrease of the electrical potential drop across the TCO/TiO2 interface are caused by the buildup of photoinjected electrons in the TiO2 film. The time constants for the recombination and collection of the photoinjected electrons are measur...
TL;DR: In this paper, a laser ablation of a metal silver plate in an aqueous solution of sodium dodecyl sulfate, C12H25OSO3Na, was shown to be essentially the same as that of silver nanoparticles chemically prepared in a solution.
Abstract: Silver nanoparticles were produced by laser ablation of a metal silver plate in an aqueous solution of sodium dodecyl sulfate, C12H25OSO3Na. The absorption spectrum of the silver nanoparticles is found to be essentially the same as that of silver nanoparticles chemically prepared in a solution. The size distribution of the nanoparticles measured by an electron microscope shifts to a smaller size with increase in the concentration of sodium dodecyl sulfate and with a decrease in the irradiation laser power. These findings are explained by a scheme that the nanoparticles are formed via rapid formation of an embryonic silver particle and a consecutive slow particle growth in competition with termination of the growth due to SDS coating on the particle.
TL;DR: In this article, the shape transformations of gold nanorods were investigated by using visible absorption spectroscopy and transmission electron microscopy (TEM) in order to analyze the final shape and size distribution.
Abstract: Gold nanorods have been found to change their shape after excitation with intense pulsed laser irradiation. The final irradiation products strongly depend on the energy of the laser pulse as well as on its width. We performed a series of measurements in which the excitation power was varied over the range of the output power of an amplified femtosecond laser system producing pulses of 100 fs duration and a nanosecond optical parametric oscillator (OPO) laser system having a pulse width of 7 ns. The shape transformations of the gold nanorods are followed by two techniques: (1) visible absorption spectroscopy by monitoring the changes in the plasmon absorption bands characteristic for gold nanoparticles; (2) transmission electron microscopy (TEM) in order to analyze the final shape and size distribution. While at high laser fluences (∼1 J cm-2) the gold nanoparticles fragment, a melting of the nanorods into spherical nanoparticles (nanodots) is observed when the laser energy is lowered. Upon decreasing the...
TL;DR: In this paper, the CHARMM all-atom force field for molecular simulations of lipids was improved by a combination of unexpected simulation results and recent high-level ab initio quantum mechanical calculations.
Abstract: Improvements in the CHARMM all-atom force field for atomic-level molecular simulations of lipids are reported. Substantial adjustments have been made to the Lennard-Jones (LJ) hydrocarbon and torsional parameters and to the partial atomic charges and torsional parameters of the phosphate moiety. These changes were motivated by a combination of unexpected simulation results and recent high-level ab initio quantum mechanical calculations. The parameter optimization procedure is described, and the resulting energy function validated by an 11 ns molecular dynamics simulation of a hydrated phospholipid bilayer. Of note is the influence of the hydrocarbon LJ parameters on the conformational properties of the aliphatic tails, emphasizing the importance of obtaining the proper balance between the bonded and nonbonded portions of the force field. Compatibility with the CHARMM all-atom parameter sets for proteins and nucleic acids has been maintained such that high quality simulations of biologically interesting me...
TL;DR: In this paper, the authors explain the cause for photovoltage in nanocrystalline, mesoporous dye-sensitized solar cells, in terms of the separation, recombination, and transport of electronic charge as well as electron energetics.
Abstract: We explain the cause for the photocurrent and photovoltage in nanocrystalline, mesoporous dye-sensitized solar cells, in terms of the separation, recombination, and transport of electronic charge as well as in terms of electron energetics. On the basis of available experimental data, we confirm that the basic cause for the photovoltage is the change in the electron concentration in the nanocrystalline electron conductor that results from photoinduced charge injection from the dye. The maximum photovoltage is given by the difference in electron energies between the redox level and the bottom of the electron conductor's conduction band, rather than by any difference in electrical potential in the cell, in the dark. Charge separation occurs because of the energetic and entropic driving forces that exist at the dye/electron conductor interface, with charge transport aided by such driving forces at the electron conductor/contact interface. The mesoporosity and nanocrystallinity of the semiconductor are importa...
TL;DR: A series of giant metal-cluster compounds, each composed of a gold core and a glutathione (GSH) adsorbate layer, have been prepared from Au(I)SG polymers and separated by gel electrophoresis, using methods reported in a recent Letter as discussed by the authors.
Abstract: A series of giant metal-cluster compounds, each composed of a gold core and a glutathione (GSH) adsorbate layer, have been prepared from Au(I)SG polymers and separated by gel electrophoresis, using methods reported in a recent Letter [J. Phys. Chem. B 1998, 102, 10643−6]. Identification of the separated compounds by core mass is accomplished through laser desorption mass spectrometry of matrix-diluted films. Three principal compounds have core masses of ca. 4.3, 5.6, and 8.2 kDa (in the range of ∼20−40 Au atoms), and show structured optical absorption spectra with clear optical absorption onsets near 1.7, 1.3, and 1.0 eV, respectively. Each of these shows unusually strong chiroptical activity in the metal-based electronic transitions across the near-infrared, visible, and near-ultraviolet regions, whereas neither the crude (unseparated) mixture nor its higher molecular weight components possess such strong optical activity. The location and strength of the optical activity suggest a metal electronic struc...
TL;DR: In this paper, a class of copolymers based on poly(l-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) was found to spontaneously adsorb from aqueous solutions onto several metal oxide surfaces, such as TiO2, Si0.4Ti0.6O2 and Nb2O5, as measured by the in situ optical waveguide lightmode spectroscopy technique and by ex situ X-ray photoelectron spectrograph.
Abstract: The generation of surfaces and interfaces that are able to withstand protein adsorption is a major challenge in the design of blood-contacting materials for both medical implants and bioaffinity sensors. Poly(ethylene glycol)-derived materials are generally considered to be particularly effective candidates for the fabrication of protein-resistant materials. Most metallic biomaterials are covered by a protective, stable oxide film; converting such oxide surfaces, which are known to strongly interact with proteins, into noninteractive surfaces requires a specific design of the surface/interface architecture. A class of copolymers based on poly(l-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) was found to spontaneously adsorb from aqueous solutions onto several metal oxide surfaces, such as TiO2, Si0.4Ti0.6O2, and Nb2O5, as measured by the in situ optical waveguide lightmode spectroscopy technique and by ex situ X-ray photoelectron spectroscopy. The resulting adsorbed layers are highly effective in reducing th...
TL;DR: In this article, a systematic study has been performed in order to find an appropriate medium for solubilization/dispersion of pristine single-walled carbon nanotubes (SWCNTs).
Abstract: A systematic study has been performed in order to find an appropriate medium for solubilization/dispersion of pristine single-walled carbon nanotubes (SWCNTs). Five solvents, all featuring high electron pair donicity (β) and low hydrogen bond parameter (α) have demonstrated the ability to readily form stable dispersions. The best dispersions have been characterized by UV/visible-NIR spectra, ESR spectra, and atomic force microscopy (AFM).
TL;DR: In this article, a more detailed picture of the SWNT electronic band structure is developed and compared with experimental tunneling spectroscopy measurements, and experimental and theoretical investigations of localized structures, such as bends and ends in nanotubes, are presented.
Abstract: Scanning tunneling microscopy, spectroscopy, and tight-binding calculations have been used to elucidate the unique structural and electronic properties of single-walled carbon nanotubes (SWNTs). First, the unique relationship between SWNT atomic structure and electronic properties, and the richness of structures observed in both purified and chemically etched nanotube samples are discussed. Second, a more detailed picture of SWNT electronic band structure is developed and compared with experimental tunneling spectroscopy measurements. Third, experimental and theoretical investigations of localized structures, such as bends and ends in nanotubes, are presented. Last, quantum size effects in nanotubes with lengths approaching large molecules are discussed. The implications of these studies and important future directions are considered.
TL;DR: In this paper, the pore size, pore volume and specific surface area of ordered mesopores of SBA-15 silica were studied using nitrogen adsorption and novel methods based on selective pore blocking via organosilane modification.
Abstract: Microporosity and connectivity of ordered mesopores of SBA-15 silica were studied using nitrogen adsorption and novel methods based on selective pore blocking via organosilane modification, and on the imaging of inverse platinum replica of ordered mesoporous structure. It was found that SBA-15 exhibits a relation between the pore size, pore volume and specific surface area which is significantly different from that for cylindrical or hexagonal pores, which suggests that the SBA-15 structure is more complex than an array of hexagonally ordered channels, even if they are corrugated. Nitrogen and argon adsorption measurements provided evidence that large mesopores are accompanied by a certain amount of significantly smaller pores (of the size below about 3.4 nm) with a broad distribution primarily in the micropore/small-mesopore range. The modification of SBA-15 via chemical bonding of small trimethylsilyl ligands partially blocked the complementary pores, and the bonding of larger octyldimethylsilyl groups ...
TL;DR: In this article, the authors address the kineti cation of RuII(2,2‘-bipyridyl-4,4‘dicarboxylate)2(NCS)2-sensitized nanocrystalline TiO2 films results in injection of an electron into the semiconductor.
Abstract: Optical excitation of RuII(2,2‘-bipyridyl-4,4‘dicarboxylate)2(NCS)2-sensitized nanocrystalline TiO2 films results in injection of an electron into the semiconductor. This paper addresses the kineti...
TL;DR: In this article, the authors used NiO as a cocatalyst for water splitting into H2 and O2 in pure water without any additives under UV irradiation, and found that NiO increased the photocatalytic activity of the NiO(0.15 wt %)/Sr2Ta2O7 photocatalyst.
Abstract: Sr2Ta2O7 and Sr2Nb2O7 with similar layered perovskite structure showed activities for water splitting into H2 and O2 in pure water without any additives under UV irradiation. The band gaps of Sr2Ta2O7 and Sr2Nb2O7 were 4.6 and 3.9 eV, respectively. Sr2Ta2O7 gave H2 and O2 from pure water under UV irradiation even in the absence of a cocatalyst. The activity of Sr2Ta2O7 was much increased by loading NiO as a cocatalyst even without pretreatment. The quantum yield of the NiO(0.15 wt %)/Sr2Ta2O7 photocatalyst was 12% at 270 nm. On the other hand, native Sr2Nb2O7 did not possess the activity. The high activity was obtained for the Sr2Nb2O7 photocatalyst when NiO was loaded and pretreated. Factors affecting the photocatalytic activities were discussed by using the Sr2Ta2O7 and Sr2Nb2O7 powder with similar layered perovskite structure. A predominant factor affecting the photocatalytic behavior of Sr2Ta2O7 and Sr2Nb2O7 is the conduction band levels formed by Ta5d and Nb4d.
TL;DR: In this article, a series of TiO2 samples with different anatase-to-rutile ratios were prepared by calcination, and the roles of the two crystallite phases of titanium(IV) oxide (TiO2) on the photocatalytic activity in oxidation of phenol in aqueous solution were studied.
Abstract: A series of TiO2 samples with different anatase-to-rutile ratios was prepared by calcination, and the roles of the two crystallite phases of titanium(IV) oxide (TiO2) on the photocatalytic activity in oxidation of phenol in aqueous solution were studied. High dispersion of nanometer-sized anatase in the silica matrix and the possible bonding of Si−O−Ti in SiO2/TiO2 interface were found to stabilize the crystallite transformation from anatase to rutile. The temperature for this transformation was 1200 °C for the silica−titania (ST) sample, much higher than 700 °C for Degussa P25, a benchmarking photocatalyst. It is shown that samples with higher anatase-to-rutile ratios have higher activities for phenol degradation. However, the activity did not totally disappear after a complete crystallite transformation for P25 samples, indicating some activity of the rutile phase. Furthermore, the activity for the ST samples after calcination decreased significantly, even though the amount of anatase did not change muc...
TL;DR: The lifetime τn and diffusion coefficient Dn of photoinjected electrons have been measured in a dye-sensitized nanocrystalline TiO2 solar cell over 5 orders of magnitude of illumination intensity using intensity-modulated photovoltage and photocurrent spectroscopies τn was found to be inversely proportional to the square root of the steady state light intensity, I0, whereas Dn varied with I0068.
Abstract: The lifetime τn and diffusion coefficient Dn of photoinjected electrons have been measured in a dye-sensitized nanocrystalline TiO2 solar cell over 5 orders of magnitude of illumination intensity using intensity-modulated photovoltage and photocurrent spectroscopies τn was found to be inversely proportional to the square root of the steady-state light intensity, I0, whereas Dn varied with I0068 The intensity dependence of τn is interpreted as evidence that the back reaction of electrons with I3- may be second order in electron density The intensity dependence of Dn is attributed to an exponential trap density distribution of the form Nt(E) ∝ exp[−β(E − Ec)/(kBT)] with β ≈ 06 Since τn and Dn vary with intensity in opposite senses, the calculated electron diffusion length Ln = (Dnτn)1/2 falls by less than a factor of 5 when the intensity is reduced by 5 orders of magnitude The incident photon to current efficiency (IPCE) is predicted to decrease by less than 10% over the same range of illumination in
TL;DR: In this paper, an electrochemical procedure based on the dissolution of a metallic anode in an aprotic solvent has been used to obtain silver nanoparticles ranging from 2 to 7 nm.
Abstract: An electrochemical procedure, based on the dissolution of a metallic anode in an aprotic solvent, has been used to obtain silver nanoparticles ranging from 2 to 7 nm. By changing the current density, it is possible to obtain different silver particle sizes. The influence of the different electrochemical parameters on the final size was studied by using different kinds of counter electrodes. The effect of oxygen presence in the reaction medium as well as the type of particle stabilizer employed have also been investigated. In some conditions an oscillatory behavior is observed. Characterization of particles was carried out by TEM and UV−vis spectroscopy. The maximum and the bandwidth of the plasmon band are both strongly dependent on the size and interactions with the surrounding medium. The presence of different silver clusters was detected by UV−vis spectroscopy. By using this technique, the existence of an autocatalytic step in the synthesis mechanism is proposed.
TL;DR: The Transferable Potentials for Phase Equilibria-United Atom (TraPPE-UA) force field for hydrocarbons is extended to alkenes and alkylbenzenes by introducing the following pseudo-atoms as mentioned in this paper.
Abstract: The Transferable Potentials for Phase Equilibria-United Atom (TraPPE-UA) force field for hydrocarbons is extended to alkenes and alkylbenzenes by introducing the following pseudo-atoms: CH2(sp2), CH(sp2), C(sp2), CH(aro), R−C(aro) for the link to aliphatic side chains and C(aro) for the link of two benzene rings. In this united-atom force field, the nonbonded interactions of the hydrocarbon pseudo-atoms are solely governed by Lennard-Jones 12−6 potentials, and the Lennard-Jones well depth and size parameters for the new pseudo-atoms were determined by fitting to the single-component vapor−liquid-phase equilibria of a few selected model compounds. Configurational-bias Monte Carlo simulations in the NVT version of the Gibbs ensemble were carried out to calculate the single-component vapor−liquid coexistence curves for ethene, propene, 1-butene, trans- and cis-2-butene, 2-methylpropene, 1,5-hexadiene, 1-octene, benzene, toluene, ethylbenzene, propylbenzene, isopropylbenzene, o-, m-, and p-xylene, and naphth...
TL;DR: In this article, the effect of solvent refractive index and core charge on the surface plasmon absorbance of alkanethiolate monolayer-protected gold clusters (MPCs) was investigated.
Abstract: This report describes the effect of solvent refractive index and core charge on the surface plasmon absorbance of alkanethiolate monolayer-protected gold clusters (MPCs). Solution spectra of dodecanethiolate MPCs (5.2 nm average diameter) reveal an 8-nm shift in plasmon band position as the solvent refractive index is varied from nd20 = 1.33 to 1.55. The spectral shift agrees with predictions of Mie theory when the alkanethiolate monolayer is accounted for in the calculations. Electronic charging of the MPC gold core by electrolysis, from its rest potential (−0.16 V) to +0.82 V vs a Ag quasi-reference electrode, causes a 9-nm (516 to 525 nm) shift in the surface plasmon band position. The shift in surface plasmon band position with core charge is compared to the predictions of the concentric sphere model for MPC capacitance and Mie theory, as a first step toward probing effects of quantized core charging on this important optical property.
TL;DR: In this article, the atomic structures of nanocrystalline powders of ceria, CeO2, and ceria-zirconia solid solution, (Ce,Zr)O2 were studied by the pulsed neutron diffraction technique.
Abstract: The atomic structures of nanocrystalline powders of ceria, CeO2, and ceria-zirconia solid solution, (Ce,Zr)O2, were studied by the pulsed neutron diffraction technique. Ceria is used as an oxygen storage component in automotive exhaust emission control systems, but the degradation of its oxygen storage capacity (OSC) after extended use at high temperatures has been a problem. Our results for the first time establish a direct correlation between the concentration of vacancy-interstitial oxygen defects and OSC. The surface area, on the other hand, exhibits much less correlation with OSC. The results also show that zirconia, which is known to retard the degradation when incorporated into ceria, reduces ceria and preserves oxygen defects. It is suggested that oxygen defects are the source of OSC in ceria-based catalyst supports, and the preservation of oxygen defects is critical for the stability of OSC against thermal aging.
TL;DR: In this paper, the photoactivity of the prepared coated particles was lower than that of single-phase TiO2 and was found to decrease with an increase in the heat treatment.
Abstract: Magnetic photocatalysts were synthesized by coating titanium dioxide particles onto colloidal magnetite and nano-magnetite particles. The photoactivity of the prepared coated particles was lower than that of single-phase TiO2 and was found to decrease with an increase in the heat treatment. These observations were explained in terms of an unfavorable heterojunction between the titanium dioxide and the iron oxide core, leading to an increase in electron−hole recombination. Interactions between the iron oxide core and the titanium dioxide matrix upon heat treatment were also seen as a possible cause of the observed low activities of these samples. Other issues considered include the physical and chemical characteristics of the samples, such as surface area and the presence of surface hydroxyl groups. Depending on the calcination conditions, these photocatalysts were found to suffer from varying degrees of photodissolution. Photodissolution tests revealed the greater the extent of the heat treatment, the low...