Showing papers in "Journal of Physical Chemistry B in 1999"
TL;DR: In this paper, the surface plasmon absorption of noble metal nanoparticles was studied and the effects of size, shape, and composition on the plasman absorption maximum and its bandwidth were discussed.
Abstract: The field of nanoparticle research has drawn much attention in the past decade as a result of the search for new materials. Size confinement results in new electronic and optical properties, possibly suitable for many electronic and optoelectronic applications. A characteristic feature of noble metal nanoparticles is the strong color of their colloidal solutions, which is caused by the surface plasmon absorption. This article describes our studies of the properties of the surface plasmon absorption in metal nanoparticles that range in size between 10 and 100 nm. The effects of size, shape, and composition on the plasmon absorption maximum and its bandwidth are discussed. Furthermore, the optical response of the surface plasmon absorption due to excitation with femtosecond laser pulses allowed us to follow the electron dynamics (electron−electron and electron−phonon scattering) in these metal nanoparticles. It is found that the electron−phonon relaxation processes in nanoparticles, which are smaller than t...
3,635 citations
TL;DR: In this paper, the size and temperature dependence of the plasmon absorption of 9, 15, 22, 48, and 99 nm gold nanoparticles in aqueous solution were studied.
Abstract: The size and temperature dependence of the plasmon absorption is studied for 9, 15, 22, 48, and 99 nm gold nanoparticles in aqueous solution. The plasmon bandwidth is found to follow the predicted behavior as it increases with decreasing size in the intrinsic size region (mean diameter smaller than 25 nm), and also increases with increasing size in the extrinsic size region (mean diameter larger than 25 nm). Because of this pronounced size effect a homogeneous size distribution and therefore a homogeneous broadening of the plasmon band is concluded for all the prepared gold nanoparticle samples. By applying a simple two-level model the dephasing time of the coherent plasmon oscillation is calculated and found to be less than 5 fs. Furthermore, the temperature dependence of the plasmon absorption is examined. A small temperature effect is observed. This is consistent with the fact that the dominant electronic dephasing mechanism involves electron−electron interactions rather than electron−phonon coupling.
2,480 citations
TL;DR: Ordered carbon molecular sieves exhibiting Bragg diffraction of X-ray lines have been synthesized for the first time, using mesoporous silica sieves as the template.
Abstract: Ordered carbon molecular sieves exhibiting Bragg diffraction of X-ray lines have been synthesized for the first time, using mesoporous silica molecular sieves as the template. Sucrose was converted to carbon inside the mesopores of the silica molecular sieves through a mild carbonization process using a sulfuric acid catalyst. The carbon molecular sieves were obtained after the removal of the silica framework using an aqueous solution of sodium hydroxide. The X-ray diffraction, transmission electron microscopy, and pore size analysis showed that the structure of the carbon molecular sieves consisted of a three-dimensional regular array of uniform mesopores 3 nm in diameter. The structure was not simply a negative replica of the used silica template, but the synthesis mechanism involved the unique transformation into a new ordered array that was triggered by the removal of the silica frameworks. The highly ordered mesoporous texture suggested its scientific and technological importance as a new shape-selec...
2,298 citations
TL;DR: In this paper, a linear relationship was found between the absorption maximum of the longitudinal plasmon resonance and the mean aspect ratio as determined from TEM and it was shown that such a linear dependence is also predicted theoretically.
Abstract: Gold nanorods with different aspect ratios are prepared in micelles by the electrochemical method and their absorption spectra are modeled by theory. Experimentally, a linear relationship is found between the absorption maximum of the longitudinal plasmon resonance and the mean aspect ratio as determined from TEM. It is shown here that such a linear dependence is also predicted theoretically. However, calculations also show that the absorption maximum of the longitudinal plasmon resonance depends on the medium dielectric constant in a linear fashion for a fixed aspect ratio. Attempts to fit the calculations to the experimental values indicate that the medium dielectric constant has to vary with the aspect ratio in a nonlinear way. Chemically, this suggests that the structure of the micelle capping the gold nanorods is size dependent. Furthermore, comparison with the results obtained for rods of different aspect ratios made by systematic thermal decomposition of the long rods further suggests that the medi...
1,539 citations
TL;DR: In this paper, the authors developed a unified and generally applicable theory of solvation of small and large apolar species in water, where hydrogen bonding of water is hindered yet persists near the solutes, leading to drying of extended apolar surfaces, large forces of attraction and hysteresis on mesoscopic length scales.
Abstract: We develop a unified and generally applicable theory of solvation of small and large apolar species in water. In the former, hydrogen bonding of water is hindered yet persists near the solutes. In the latter, hydrogen bonding is depleted, leading to drying of extended apolar surfaces, large forces of attraction, and hysteresis on mesoscopic length scales. The crossover occurs on nanometer length scales, when the local concentration of apolar units is sufficiently high, or when an apolar surface is sufficiently large. Our theory for the crossover has implications concerning the stability of protein assemblies and protein folding.
1,535 citations
TL;DR: In this article, a co-reduction of chlorauric acid HAuCl4 and silver nitrate AgNO3 with sodium citrate is used to form homogeneous gold−silver alloy nanoparticles.
Abstract: Gold−silver alloy nanoparticles with varying mole fractions are prepared in aqueous solution by the co-reduction of chlorauric acid HAuCl4 and silver nitrate AgNO3 with sodium citrate. As the optical absorption spectra of their solutions show only one plasmon absorption it is concluded that mixing of gold and silver leads to a homogeneous formation of alloy nanoparticles. The maximum of the plasmon band blue-shifts linearly with increasing silver content. This fact cannot be explained by a simple linear combination of the dielectric constants of gold and silver within the Mie theory. On the other hand, the extinction coefficient is found to decrease exponentially rather than linearly with increasing gold mole fraction xAu. Furthermore, the size distribution of the alloy nanoparticles is examined using transmission electron microscopy (TEM). High-resolution TEM (HRTEM) also confirms the formation of homogeneous gold−silver alloy nanocrystals.
1,327 citations
TL;DR: In this article, the synthesis of high quality single-walled carbon nanotubes (SWNTs) is accomplished by optimizing the chemical compositions and textural properties of the catalyst material used in the chemical vapor deposition (CVD) of methane.
Abstract: The synthesis of bulk amounts of high quality single-walled carbon nanotubes (SWNTs) is accomplished by optimizing the chemical compositions and textural properties of the catalyst material used in the chemical vapor deposition (CVD) of methane A series of catalysts are derived by systematically varying the catalytic metal compounds and support materials The optimized catalysts consist of Fe/Mo bimetallic species supported on a novel silica−alumina multicomponent material The high SWNT yielding catalyst exhibits high surface-area and large mesopore volume at elevated temperatures Gram quantities of SWNT materials have been synthesized in ∼05 h using the optimized catalyst material The nanotube material consists of individual and bundled SWNTs that are free of defects and amorphous carbon coating This work represents a step forward toward obtaining kilogram scale perfect SWNT materials via simple CVD routes
1,046 citations
TL;DR: In this article, a new family of molten salts is reported, based on the Nalkyl, N-alkyl pyrrolidinium cation and the bis(trifluoromethane sulfonyl)imide anion.
Abstract: A new family of molten salts is reported, based on the N-alkyl, N-alkyl pyrrolidinium cation and the bis(trifluoromethane sulfonyl)imide anion. Some of the members of the family are molten at room temperature, while the smaller and more symmetrical members have melting points around 100 °C. Of the room-temperature molten salt examples, the methyl butyl derivative exhibits the highest conductivity; at 2 × 10-3 S/cm this is the highest molten salt conductivity observed to date at room temperature among the ammonium salts. This highly conductive behavior is rationalized in terms of the role of cation planarity. The salts also exhibit multiple crystalline phase behavior below their melting points and exhibit significant conductivity in at least their higher temperature crystal phase. For example, the methyl propyl derivative (mp = 12 °C) shows ion conductivity of 1 × 10-6 S/cm at 0 °C in its higher temperature crystalline phase.
1,016 citations
TL;DR: In this paper, the work function and density of states (DOS) of multiwall carbon nanotubes (MWNTs) using ultraviolet photoelectron spectroscopy (UPS) were studied.
Abstract: We have studied the work function and density of states (DOS) of multiwall carbon nanotubes (MWNTs) using ultraviolet photoelectron spectroscopy (UPS) Raw MWNTs were purified by successive sonication, centrifugation, sedimentation, and filtration processes with the aid of a nonionic surfactant The purified MWNTs showed a slightly lower work function (43 eV) than that of highly oriented pyrolytic graphite (44 eV) Effects of three different oxidative treatments, air-, oxygen plasma-, and acid-oxidation, have also been studied It was found that oxidative treatments affect the DOS of valence bands and increase the work function X-ray photoelectron spectroscopy (XPS) measurements have suggested that gas-phase treatment preferentially forms hydroxyl and carbonyl groups, while liquid-phase treatment forms carboxylic acid groups on the surface of MWNTs These surface chemical groups disrupt the π-conjugation and introduce surface dipole moments, leading to higher work functions up to 51 eV We expect the
919 citations
TL;DR: Poncela et al. as discussed by the authors extended the TraPPE-UA force field to include Lennard-Jones interaction parameters for methine and quaternary carbon groups by fitting to critical temperatures and saturated liquid densities of branched alkanes.
Abstract: A new generalization of the configurational-bias Monte Carlo method is presented which avoids the problems inherent in a Boltzmann rejection scheme for sequentially generating bond bending and torsional angles. The TraPPE-UA (transferable potentials for phase equilibria united-atom) force field is extended to include Lennard-Jones interaction parameters for methine and quaternary carbon groups by fitting to critical temperatures and saturated liquid densities of branched alkanes. Configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to determine the vapor−liquid coexistence curves (VLCC) for six alkane isomers with four to eight carbons. Results are presented for two united-atom alkane force fields: PRF [Poncela, et al. Mol. Phys. 1997, 91, 189] and TraPPE-UA. Standard-state specific densities for the TraPPE-UA model were studied by simulations in the isobaric−isothermal ensemble. It is found that a single set of methyl, methylene, methine, and quaternary carbon parameters g...
803 citations
TL;DR: In this paper, the structure and dynamics of photosynthetic light-harvesting pigment−protein complexes have been studied and detailed understanding of the light harvesting and light harvesting has been achieved.
Abstract: Great progress in the study of structure and dynamics of photosynthetic light-harvesting pigment−protein complexes has recently resulted in detailed understanding of the light-harvesting and light-...
TL;DR: In this article, the size-tunable silver nanoparticle arrays by nanosphere lithography and their structural characterization by atomic force microscopy (AFM) were used to determine the in-plane diameter and out-of-plane height of Ag nanoparticles.
Abstract: Nanosphere lithography (NSL) is an inexpensive, inherently parallel, high-throughput, and materials-general nanofabrication technique capable of producing well-ordered 2D periodic particle arrays of nanoparticles. This paper focuses on the synthesis of size-tunable silver nanoparticle arrays by nanosphere lithography and their structural characterization by atomic force microscopy (AFM). The in-plane diameter, a, of Ag nanoparticles was tuned from 21 to 126 nm by systematic variation of the nanosphere diameter, D. Similarly, the out-of-plane height, b, was tuned from 4 to 47 nm by varying the mass thickness, dm, of the Ag overlayer. Experimental measurements of a, b, and interparticle spacing dip of many individual nanoparticles as a function of D and dm were carried out using AFM. These studies show (i) b = dm, (ii) dip accurately corresponds to predictions based on the nanosphere mask geometry, (iii) a, after correction for AFM tip convolution, is governed only by the mask geometry and the standard devi...
TL;DR: The fusion and spreading of phospholipid bilayers on glass surfaces was investigated as a function of pH and ionic strength and this result suggests that van der Waals and electrostatic interactions govern the fusion process.
Abstract: The fusion and spreading of phospholipid bilayers on glass surfaces was investigated as a function of pH and ionic strength. Membrane fusion to the support was favorable at high ionic strength and low pH for vesicles containing a net negative charge; however, neutral and positively charged vesicles fused under all conditions attempted. This result suggests that van der Waals and electrostatic interactions govern the fusion process. Membrane spreading over a planar surface was favorable at low pH regardless of the net charge on the bilayer, and the process is driven by van der Waals forces. On the other hand membrane propagation is impeded at high pH or on highly curved surfaces. In this case a combination of hydration and bending interactions is primarily responsible for arresting the spreading process. These results provide a framework for understanding many of the factors that influence the effectiveness of scratches on planar supported bilayers as barriers to lateral diffusion and lead to a simple meth...
TL;DR: In this article, the reduction of Ag+ in these solutions is brought about by the 1-hydroxyalkyl radical generated in the radiolysis of 2-propanol; citrate does not act as a reductant but solely as a stabilizer of the colloidal particles formed.
Abstract: Colloidal silver sols of long-time stability are formed in the γ-irradiation of 1.0 × 10-4 M AgClO4 solutions, which also contain 0.3 M 2-propanol, 2.5 × 10-2 M N2O, and sodium citrate in various concentrations. The reduction of Ag+ in these solutions is brought about by the 1-hydroxyalkyl radical generated in the radiolysis of 2-propanol; citrate does not act as a reductant but solely as a stabilizer of the colloidal particles formed. Its concentration is varied in the range from 5.0 × 10-5 to 1.5 × 10-3 M, and the size and size distribution of the silver particles are studied by electron microscopy. At low citrate concentration, partly agglomerated large particles are formed that have many imperfections. In an intermediate range (a few 10-4 M), well-separated particles with a rather narrow size distribution and little imperfections are formed, the size slightly decreasing with increasing citrate concentration. At high citrate concentrations, large lumps of coalesced silver particles are present, due to ...
TL;DR: In this article, a simple method to control the size of monodispersed Pt nanoparticles with the use of an alcohol reduction is described. But this method is not suitable for nanoparticles of arbitrary diameter.
Abstract: We describe a simple method to control the size of Pt nanoparticles with the use of an alcohol reduction. They then are assembled on the electrode by an electrophoretic deposition. The mean diameter of monodispersed Pt nanoparticles can be controlled from 19 to 33 A in one-step reaction by changing the kind and/or the concentration of alcohol in water and the amount of protective polymer, poly(N-vinyl-2-pyrrolidone) (PVP). The monodispersed Pt nanoparticles of smaller diameter are obtained in the order of methanol > ethanol > 1-propanol, indicating that a faster reduction rate of [PtCl6]2- ions is an important factor to produce the smaller particles. The particle diameter decreases linearly with concentration of alcohol in water. Furthermore, increasing the amount of PVP makes the size of Pt nanoparticles smaller, the size distribution remaining quite narrow. By the combination of the one-step reaction with the stepwise growth reaction, Pt nanoparticles of arbitrary diameter in the range 19−50 A can be ob...
TL;DR: In this article, a UV light illuminated TiO2 single-crystal surface exhibited a 0° contact angle for both water and oil, indicative of a highly amphiphilic surface against its native hydrophobic surface.
Abstract: Reversible surface wettability conversion on titanium dioxide (TiO2) single crystals has been achieved, and its mechanism has been examined by means of contact angle measurement and X-ray photoelectron spectroscopy (XPS). A UV light illuminated TiO2 single-crystal surface exhibited a 0° contact angle for both water and oil, indicative of a highly amphiphilic surface against its native hydrophobic surface. This was ascribed to photoreduction of surface Ti4+ to Ti3+ at definite sites, leading to preferential adsorption of dissociative water on top. A long-term storage of the highly amphiphilic surface resulted in reconversion of the surface wettability. It was found that the amphiphilic-to-hydrophobic reconversion is due to the replacement of the adsorbed hydroxyl groups by oxygen, which returns the surface geometric and electronic structures similar to the native TiO2 surface. The result of angle-resolved XPS measurement revealed that the surface reactions occurred at the uppermost layers of the single cry...
TL;DR: In this article, the electronic structure and domain size of tungsten oxide species in crystalline isopolytungstates, monoclinic WO3, and dispersed WOx species on ZrO2 surfaces were investigated.
Abstract: UV−visible diffuse reflectance spectroscopy was used to probe the electronic structure and domain size of tungsten oxide species in crystalline isopolytungstates, monoclinic WO3, and dispersed WOx species on ZrO2 surfaces. UV−visible absorption edge analysis, CO2 chemisorption, and Raman spectroscopic results show that three distinct regions of WOx coverage on ZrO2 supports appear with increasing WOx surface density: submonolayer region (0−4 W nm-2), polytungstate growth region (4−8 W nm-2), and polytungstate/crystalline WO3 coexistence region (>8 W nm-2). The structure and catalytic activity of WOx species on ZrO2 is controlled only by WOx surface density (W nm-2), irrespective of the WOx concentration, oxidation temperature, and ZrO2 surface area used to obtain a particular density. The submonolayer region is characterized by distorted octahedral WOx species that are well dispersed on the ZrO2 surface. These species show a constant absorption edge energy, they are difficult to reduce, and contain few a...
TL;DR: In this paper, a series of thiol-capped crystalline CdSe nanoparticles has been synthesized in aqueous solution using mercapto- alcohols (2-mercaptoethanol, 1-thioglycerol), and mercaptos acids (thioglobal acid, thiolactic acid) as stabilizers.
Abstract: As an expansion to the wet chemical route for the preparation of quantum-sized II−VI semiconductor materials, a series of thiol-capped crystalline CdSe nanoparticles has been synthesized in aqueous solution using mercapto- alcohols (2-mercaptoethanol, 1-thioglycerol), and mercapto acids (thioglycolic acid, thiolactic acid) as stabilizers The smaller (app 14−22 nm diameter) CdSe particles were obtained using thioalcohols as capping agents; the use of thioacids as stabilizers produced larger (21−32 nm diameter) CdSe particles CdSe nanoparticles were separated from the crude solutions as redissolvable powder samples with narrow size distributions using a size-selective fractionation and have been characterized by UV−vis absorption and photoluminescence spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy, and energy-dispersive X-ray analysis A calculation of the HOMO−LUMO gap of CdSe particles as a function of their size has been done using an extended effective mass appr
TL;DR: In this article, the solvent dependence of the formal redox potentials of the ferrocenium/ferrocene (Fc+/0) and 1,2,3,4,5-pentamethylferrocenium (Me5Fc+,/0), compared with the decamethy-lferricenium/decamethyelferrocenes (Me10Fc-,/
Abstract: The solvent dependence of the formal redox potentials of the ferrocenium/ferrocene (Fc+/0) and 1,2,3,4,5-pentamethylferrocenium/1,2,3,4,5-pentamethylferrocene (Me5Fc+/0) couples versus the decamethylferrocenium/decamethylferrocene (Me10Fc+/0) couple indicates that the latter is a superior redox standard for studying solvent effects on the thermodynamics of electron transfer. The couples were studied in 29 solvents and the differences in formal redox potentials between the MenFc+/0 (n = 5, 10) and Fc+/0 couples are surprisingly solvent dependent. In the case of the Fc+/0 couple versus the Me10Fc+/0 couple, the potential difference ranges from +583 mV in 2,2,2-trifluoroethanol to +293 mV in water. The positive shifts for the Me5Fc+/0 couple versus the Me10Fc+/0 couple were about half of these values. The Me10Fc+/0 redox couple can also be used in easily oxidized solvents, such as N-methylaniline and N,N-dimethylaniline, or in conjunction with a Hg working electrode. Statistical multiparameter analysis of th...
TL;DR: The sum-frequency generation (SFG) technique has been shown to be the only technique available that can provide detailed information about a liquid interface at the molecular level as mentioned in this paper, which has been proven to be a most powerful analytical tool for liquid interfaces.
Abstract: Liquid interfaces play a fundamental role in science and technology. The development of surface-sensitive probes capable of yielding molecular information about these interfaces is of great importance. This paper gives an overview of our and some others' recent work on vibrational spectroscopy of liquid interfaces by the sum-frequency generation (SFG) technique. This technique, being highly surface specific and applicable to all interfaces accessible by light, has been proven to be a most powerful analytical tool for liquid interfaces. A wide range of systems have been studied including neat liquid surfaces, solid/liquid and liquid/liquid interfaces, surfactants at liquid interfaces, and electrochemical interfaces. In many cases, SFG is shown to be the only technique available that can provide detailed information about a liquid interface at the molecular level.
TL;DR: In this paper, the authors examined the effect of solvent on the optical extinction spectrum of periodic arrays of surface-confined silver nanoparticles fabricated by nanosphere lithography (NSL) and showed that the defect sites that occur as a byproduct of the NSL fabrication process play a negligible role in the macroscale extinction spectrum.
Abstract: In this paper we examine the effect of solvent on the optical extinction spectrum of periodic arrays of surface-confined silver nanoparticles fabricated by nanosphere lithography (NSL). By use of NSL, it is possible to systematically vary the out-of-plane height of the nanoparticles, and by thermal annealing, we can control the nanoparticle shape. We have studied four separate samples of nanoparticle arrays; three samples have nanoparticles that are truncated tetrahedral in shape but that differ in out-of-plane height and one sample has nanoparticles that are oblate ellipsoidal in shape. By performing UV−vis extinction spectroscopy measurements at 12 μm spatial resolution, we show that the defect sites that occur as a byproduct of the NSL fabrication process play a negligible role in the macroscale extinction spectrum. We find that the extinction spectrum of the nanoparticles that are oblate ellipsoidal in shape is least sensitive to the surrounding dielectric medium, and the extinction spectrum of the na...
TL;DR: In this paper, the shape change and size reduction of gold particles were investigated in a pulsed Nd:YAG laser at 532 nm to gold particles of less than 50 nm in aqueous solution.
Abstract: Irradiation of a pulsed Nd:YAG laser at 532 nm to gold particles of less than 50 nm in aqueous solution was found to cause the shape change and size reduction of the particles. Typically, the nonspherical gold particles between 20 and 50 nm in diameter disappeared, whereas the number of gold particles of spherical shape less than 10 nm increased. The size reduction ceased after 5 min irradiation. The maximum diameter in the size distribution decreased to ca. 10 nm when the laser fluence was increased up to nearly 800 mJ cm-2. The temperature of the gold particles was estimated from the absorbed laser energy by the particles and was found to rise as high as the boiling point of gold; these results were supported by the measurements of the blackbody radiation from the particles. The shape change and size reduction are considered to occur through melting and vaporization of the gold particles. The high temperature, which causes melting and vaporization, is a result of the strong absorption of the laser energ...
TL;DR: In this paper, the photocurrent−voltage (IV) characteristics of the plain NiO electrode recorded potentiostatically in a standard three-electrode setup upon UV illumination demonstrate p-type behavior, while the IV characteristics of a dye-sensitized nanostructured NiO electrodes coated with erythrosin B show cathodic photocurrent under visible light illumination.
Abstract: Nanostructured NiO film was prepared by depositing nickel hydroxide slurry on conducting glass and sintering at 500 °C to a thickness of about 1 μm. The photocurrent−voltage (IV) characteristics of the plain nanostructured NiO electrode recorded potentiostatically in a standard three-electrode setup upon UV illumination demonstrate p-type behavior, while the IV characteristics of a dye-sensitized nanostructured NiO electrode coated with erythrosin B show cathodic photocurrent under visible light illumination. The highest incident photon-to-current conversion efficiencies of tetrakis(4-carboxyphenyl)porphyrin (TPPC) and erythrosin B-coated NiO films were 0.24% and 3.44%, respectively. In sandwich solar cells with a platinized conducting glass as counter electrode exposed to light from a sun simulator (light intensity: 68 mW/cm2), a short-circuit cathodic photocurrent density (ISC) of 0.079 mA/cm2 and an open-circuit voltage (VOC) of 98.5 mV for TPPC-coated NiO electrode were achieved. Similarly, ISC = 0.2...
TL;DR: The physical and chemical properties of a new class of lithium conducting polymer electrolytes formed by dispersing ceramic powders at the nanoscale particle size into a poly(ethylenoxide) (PEO)− lithium salt, LiX complexes, are reported and discussed in this paper.
Abstract: The physical and chemical properties of a new class of lithium conducting polymer electrolytes formed by dispersing ceramic powders at the nanoscale particle size into a poly(ethylenoxide) (PEO)− lithium salt, LiX complexes, are reported and discussed. These true solid-state PEO−LiX nanocomposite polymer electrolytes have in the 30−80 °C range an excellent mechanical stability (due to the network of the ceramic fillers into the polymer bulk) and high ionic conductivity (promoted by the high surface area of the dispersed fillers). These important and unique properties are accompanied by a wide electrochemical stability and by a good compatibility with the lithium electrode (assured by the absence of any liquids and by the interfacial stabilizing action of the dispersed filler), all this making these nanocomposite electrolytes of definite interest for the development of advanced rechargeable lithium batteries.
TL;DR: In this paper, the crystallization of anatase from amorphous titania has been controlled using the hydrothermal technique Crystallite size and surface area can be controlled by careful alterations of the hydroth.
Abstract: The crystallization of anatase from amorphous titania has been controlled using the hydrothermal technique Crystallite size and surface area can be controlled by careful alterations of the hydroth
TL;DR: In this paper, the solvation of individual carbon nanotubes was verified by dispersing the tubes on a mica substrate and examining them with atomic force microscopy (AFM), which revealed that light sonication in alcohol solvents does not remove significant amounts of the fluorine.
Abstract: Highly purified single-wall carbon nanotubes (SWNTs) were fluorinated to form “fluorotubes”, which were then solvated as individual tubes in various alcohol solvents via ultrasonication. The solvation of individual fluorotubes was verified by dispersing the tubes on a mica substrate and examining them with atomic force microscopy (AFM). Elemental analysis of the tubes reveals that light sonication in alcohol solvents does not remove significant amounts of the fluorine. While these solutions are metastable, they will persist long enough (over a week) to permit solution-phase chemistry to be carried out on the fluorotubes. For example, the solvated fluorotubes can be precipitated out of solution with hydrazine to yield normal, unfluorinated SWNTs, or they can be reacted with sodium methoxide to yield what are apparently methoxylated SWNTs. These reaction products have been examined with elemental analysis and a variety of spectroscopies and microscopies.
TL;DR: The configurational-bias Monte Carlo (CBMC) technique has been used for computing the adsorption isotherms for linear and branched 2-methylalkanes on silicalite.
Abstract: The configurational-bias Monte Carlo (CBMC) technique has been used for computing the adsorption isotherms for linear and branched 2-methylalkanes on silicalite. The carbon numbers of the alkanes ranged from four to nine. For branched alkanes inflection behavior was observed for all carbon numbers studied. The inflection was found to occur at a loading of four molecules per unit cell. Below this loading the branched alkanes are seen to be located predominantly at the intersections of the straight and zigzag channels. To obtain loadings higher than four, the branched alkane must seek residence in the channel interiors which is energetically more demanding and therefore requires disproportionately higher pressures; this leads to the inflection behavior. Linear alkanes with six and more carbon atoms also were found to exhibit inflection behavior. Hexane and heptane show inflection due to commensurate “freezing”; the length of these molecules is commensurate with the length of the zigzag channels. This leads ...
TL;DR: In this paper, a spin-polarized DFT calculation was used to estimate the probability of an adsorbed O2 molecule following the molecular and dissociative channels in a 1:1 ratio with each vacancy site.
Abstract: vacancy population), presumably due to dissociative filling of the vacancy sites in a 1:1 ratio. Above a coverage of 4 10 13 molecules/cm 2 , a first-order O2 TPD peak appears at 410 K. Oxygen molecules in this peak do not scramble oxygen atoms with either the surface or with other coadsorbed oxygen molecules. Sequential exposures of 16 O2 and 18 O2 at 120 K indicate that each adsorbed O2 molecule, irrespective of its adsorption sequence, has equivalent probabilities with respect to its neighbors to follow the two channels (molecular and dissociative), suggesting that O 2 adsorption is not only precursor-mediated, as the sticking probability measurements indicate, but that all O2 molecules reside in this precursor state at 120 K. This precursor state may be associated with a weak 145 K O2 TPD state observed at high O2 exposures. ELS measurements suggest charge transfer from the surface to the O2 molecule based on disappearance of the vacancy loss feature at 0.8 eV, and the appearance of a 2.8 eV loss that can be assigned to an adsorbed O2 - species based on comparisons with Ti-O2 inorganic complexes in the literature. Utilizing results from recent spin-polarized DFT calculations in the literature, we propose a model where three O2 molecules are bound in the vicinity of each vacancy site at 120 K. For adsorption temperatures above 150 K, the dissociation channel completely dominates and the surface adsorbs oxygen in a 1:1 ratio with each vacancy site. ELS measurements indicate that the vacancies are filled, and the remaining oxygen adatom, which is apparent in TPD, is transparent in ELS. On the basis of the variety of oxygen adsorption states observed in this study, further work is needed in order to determine which oxygen-related species play important roles in chemical and photochemical oxidation processes on TiO2 surfaces.
TL;DR: In this article, the authors report cooperative enhancement of two-photon absorption in multi-branched structures which may lead to new design criteria for development of highly efficient twophoton materials.
Abstract: Recent reports of molecular structures with considerably enhanced two-photon absorption cross-section have generated considerable interest in this phenomenon from both fundamental and applications perspectives. In this letter, we report cooperative enhancement of two-photon absorption in multi-branched structures which may lead to new design criteria for development of highly efficient two-photon materials. The multi-branched structures were synthesized using coupling of two and three two-photon active asymmetric donor−acceptor chromophores linked together by a common amine group. The two-photon cross-sections measured both with nanosecond and femtosecond pulses show that the value for the trimer is more than six times larger than that for the monomer, and not just three times larger as expected from the number density increase.
TL;DR: The generalized Born model proposed by Still and co-workers is parametrized specifically for proteins, peptides, and nucleic acids within the CHARMM all hydrogen and polar hydrogen force fields and the performance of these models in reproducing overall molecular solvation trends is found to be quite good.
Abstract: The generalized Born model proposed by Still and co-workers (Qui, D.; Shenkin, P. S.; Hollinger, F. P.; Still, W. C. J. Phys. Chem. A 1997, 101, 3005−3014) is parametrized specifically for proteins, peptides, and nucleic acids within the CHARMM all hydrogen and polar hydrogen force fields. A database of atomic electrostatic environments and molecular electrostatic solvation free energies comprising amino acid residues, nucleic acid bases, dipeptides, dinucleotides, proteins, and DNA strands is established, and numerical finite difference Poisson calculations are solved using atomic radii and charges from the corresponding force field parameters. These data provide the necessary input to parametrize generalized Born models for a particular force field. The performance of these models in reproducing overall molecular solvation trends is examined and found to be quite good. Furthermore, calculations of electrostatic solvation free energy differences suggest that conformational free energy changes are well re...