Showing papers in "Physical Chemistry Chemical Physics in 2009"

Density functional theory for transition metals and transition metal chemistry

Abstract: We introduce density functional theory and review recent progress in its application to transition metal chemistry. Topics covered include local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and catalysis, including molecules, clusters, nanoparticles, surfaces, and solids.

Reevaluation of absolute luminescence quantum yields of standard solutions using a spectrometer with an integrating sphere and a back-thinned CCD detector.

Abstract: We reevaluate the absolute fluorescence and phosphorescence quantum yields of standard solutions by using a novel instrument developed for measuring the absolute emission quantum yields of solutions. The instrument consists of an integrating sphere equipped with a monochromatized Xe arc lamp as the light source and a multichannel spectrometer. By using a back-thinned CCD (BT-CCD) as the detector, the sensitivity for spectral detection in both the short and long wavelength regions is greatly improved compared with that of an optical detection system that uses a conventional photodetector. Using this instrument, we reevaluate the absolute fluorescence quantum yields (Φf) of some commonly used fluorescence standard solutions by taking into account the effect of reabsorption/reemission. The value of Φf for 5 × 10−3 M quinine bisulfate in 1 N H2SO4 is measured to be 0.52, which is in good agreement with the value (0.508) obtained by Melhuish by using a modified Vavilov method. In contrast, the value of Φf for 1.0 × 10−5 M quinine bisulfate in 1 N H2SO4, which is one of the most commonly used standards in quantum yield measurements based on the relative method, is measured to be 0.60. This value is significantly larger than Melhuish’s value (0.546), which was estimated by extrapolating the value of Φf for 5 × 10−3 M quinine bisulfate solution to infinite dilution using the self-quenching constant. The fluorescence quantum yield of 9,10-diphenylanthracene in cyclohexane is measured to be 0.97. This system can also be used to determine the phosphorescence quantum yields (Φp) of metal complexes that emit phosphorescence in the near-infrared region: the values of Φp for [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) are estimated to be 0.063 in water and 0.095 in acetonitrile under deaerated conditions at 298 K, while that in aerated water, which is frequently used as a luminescent reference in biological studies, is reevaluated to be 0.040.

Lanthanide ions as spectral converters for solar cells

Abstract: The use of lanthanide ions to convert photons to different, more useful, wavelengths is well-known from a wide range of applications (e.g. fluorescent tubes, lasers, white light LEDs). Recently, a new potential application has emerged: the use of lanthanide ions for spectral conversion in solar cells. The main energy loss in the conversion of solar energy to electricity is related to the so-called spectral mismatch: low energy photons are not absorbed by a solar cell while high energy photons are not used efficiently. To reduce the spectral mismatch losses both upconversion and downconversion are viable options. In the case of upconversion two low energy infrared photons that cannot be absorbed by the solar cell, are added up to give one high energy photon that can be absorbed. In the case of downconversion one high energy photon is split into two lower energy photons that can both be absorbed by the solar cell. The rich and unique energy level structure arising from the 4fn inner shell configuration of the trivalent lanthanide ions gives a variety of options for efficient up- and downconversion. In this perspective an overview will be given of recent work on photon management for solar cells. Three topics can be distinguished: (1) modelling of the potential impact of spectral conversion on the efficiency of solar cells; (2) research on up- and downconversion materials based on lanthanides; and (3) proof-of-principle experiments. Finally, an outlook will be given, including issues that need to be resolved before wide scale application of up- and downconversion materials can be anticipated.

Laser ablation synthesis in solution and size manipulation of noble metal nanoparticles

Abstract: In the past years, laser ablation synthesis in solution (LASiS) emerged as a reliable alternative to traditional chemical reduction methods for obtaining noble metal nanoparticles (NMNp). LASiS is a “green” technique for the synthesis of stable NMNp in water or in organic solvents, which does not need stabilizing molecules or other chemicals. The so obtained NMNp are highly available for further functionalization or can be used wherever unprotected metal nanoparticles are desired. Surface functionalization of NMNp can be monitored in real time by UV-visible spectroscopy of the plasmon resonance. However LASiS has some limitations in the size control of NMNp, which can be overcome by “chemical free” laser treatments of NMNp. In this paper we provide an overview of LASiS, size manipulation by laser irradiation and functionalization of NMNp, with special care in pointing out some of the main issues about this research area.

On the concept of ionicity in ionic liquids

Abstract: Ionic liquids are liquids comprised totally of ions However, not all of the ions present appear to be available to participate in conduction processes, to a degree that is dependent on the nature of the ionic liquid and its structure There is much interest in quantifying and understanding this ‘degree of ionicity’ phenomenon In this paper we present transport data for a range of ionic liquids and evaluate the data firstly in terms of the Walden plot as an approximate and readily accessible approach to estimating ionicity An adjusted Walden plot that makes explicit allowance for differences in ion sizes is shown to be an improvement to this approach for the series of ionic liquids described In some cases, where diffusion measurements are possible, it is feasible to directly quantify ionicity via the Nernst–Einstein equation, confirming the validity of the adjusted Walden plot approach Some of the ionic liquids studied exhibit ionicity values very close to ideal; this is discussed in terms of a model of a highly associated liquid in which the ion correlations have similar impact on both the diffusive and conductive motions Ionicity, as defined, is thus a useful measure of adherence to the Nernst–Einstein equation, but is not necessarily a measure of ion availability in the chemical sense

Quantum dot-based resonance energy transfer and its growing application in biology.

Abstract: We provide an overview of the progress made in the past few years in investigating fluorescence resonance energy transfer (FRET) using semiconductor quantum dots (QDs) and the application of QD-based FRET to probe specific biological processes. We start by providing some of the pertinent conceptual elements involved in resonance energy transfer, and then discuss why the Forster dipole-dipole mechanism applies to QD fluorophores. We then describe the unique QD photophysical properties of direct relevance to FRET and summarize the main advantages offered, along with some of the limitations encountered by QDs as exciton donors and/or acceptors. Next we describe the overall progress made and discuss a few representative examples where QD-based FRET sensing of specific biological processes has been demonstrated. We also detail some of the advances of single molecule FRET using QD-conjugates and highlight the unique information that can be extracted. We conclude by providing an assessment of where QD-based FRET investigations may be evolving in the near future.

HOx radical regeneration in the oxidation of isoprene

Abstract: We propose, and quantify from first principles, two novel HOx-regenerating unimolecular reactions in isoprene oxidation, which are estimated to yield in pristine tropical forest conditions about 0.7 HO2 and 0.03 OH radicals per isoprene oxidized; it is further argued that the photolabile coproduct of HO2 can be a major source of OH, with a yield of the order of 1. The newly proposed chemistry could provide a rationalization for the unexpectedly high OH concentrations often observed in forested environments, such as over the Amazon forest in the recent Gabriel campaign.

Screened hybrid density functionals for solid-state chemistry and physics.

Abstract: Density functional theory incorporating hybrid exchange–correlation functionals has been extraordinarily successful in providing accurate, computationally tractable treatments of molecular properties. However, conventional hybrid functionals can be problematic for solids. Their nonlocal, Hartree–Fock-like exchange term decays slowly and incorporates unphysical features in metals and narrow-bandgap semiconductors. This article provides an overview of our group’s work on designing hybrid functionals for solids. We focus on the Heyd–Scuseria–Ernzerhof screened hybrid functional [J. Chem. Phys. 2003, 118, 8207], its applications to the chemistry and physics of solids and surfaces, and our efforts to build upon its successes.

How to turn your pump-probe instrument into a multidimensional spectrometer: 2D IR and Vis spectroscopies via pulse shaping.

Abstract: We have recently developed a new and simple way of collecting 2D infrared and visible spectra that utilizes a pulse shaper and a partly collinear beam geometry. 2D IR and Vis spectroscopies are powerful tools for studying molecular structures and their dynamics. They can be used to correlate vibrational or electronic eigenstates, measure energy transfer rates, and quantify the dynamics of lineshapes, for instance, all with femtosecond time-resolution. As a result, they are finding use in systems that exhibit fast dynamics, such as sub-millisecond chemical and biological dynamics, and in hard-to-study environments, such as in membranes. While powerful, these techniques have been difficult to implement because they require a series of femtosecond pulses to be spatially and temporally overlapped with precise time-resolution and interferometric phase stability. However, many of the difficulties associated with implementing 2D spectroscopies are eliminated by using a pulse shaper and a simple beam geometry, which substantially lowers the technical barriers required for researchers to enter this exciting field while simultaneously providing many new capabilities. The aim of this paper is to provide an overview of the methods for collecting 2D spectra so that an outsider considering using 2D spectroscopy in their own research can judge which approach would be most suitable for their research aims. This paper focuses primarily on 2D IR spectroscopy, but also includes our recent work on adapting this technology to collecting 2D Vis spectra. We review work that has already been published as well as cover several topics that we have not reported previously, including phase cycling methods to remove background signals, eliminate unwanted scatter, and shift data collection into the rotating frame.

(14)N HYSCORE investigation of the H-cluster of [FeFe] hydrogenase: evidence for a nitrogen in the dithiol bridge.

Abstract: Hydrogenases are enzymes catalyzing the reversible heterolytic splitting of molecular hydrogen. Despite extensive investigations of this class of enzymes its catalytic mechanism is not yet well understood. In this paper spectroscopic investigations of the active site of [FeFe] hydrogenase are presented. The so-called H-cluster consists of a bi-nuclear catalytically active subcluster connected to a [4Fe4S] ferredoxin-like unit via a Cys–thiol bridge. An important feature of the H-cluster is that both irons in the bi-nuclear subcluster are coordinated by CN and CO ligands. The bi-nuclear site also carries a dithiol bridge, whose central atom has not yet been identified. Nitrogen and oxygen are the most probable candidates from a mechanistic point of view. Here we present a study of the 14N nuclear quadrupole and hyperfine interactions of the active oxidized state of the H-cluster using advanced EPR methods. In total three 14N nuclei with quadrupole couplings of 0.95 MHz, 0.35 MHz and 1.23 MHz were detected using hyperfine sublevel correlation spectroscopy (HYSCORE). The assignment of the signals is based on their 14N quadrupole couplings in combination with DFT calculations. One signal is assigned to the CN ligand of the distal iron, one to a Lys side chain nitrogen and one to the putative nitrogen of the dithiol bridge. Hence, these results provide the first experimental evidence for a di-(thiomethyl)amine ligand (–S–CH2–NH–CH2–S–) in the bi-nuclear subcluster. This finding is important for understanding the mechanism of [FeFe] hydrogenases, since the nitrogen is likely to act as an internal base facilitating the heterolytic splitting/formation of H2.

Lighting the way ahead with boron dipyrromethene (Bodipy) dyes

Abstract: This review covers some recent advances made using boron dipyrromethene (Bodipy) compounds, highlighting aspects such as new sensing applications for reactive oxygen species and solvent rheology. The light-harvesting capabilities of the dye especially in the crystalline state are also discussed emphasising Bodipy derivatives as potential candidates for solid-state solar concentrators.

Measurement of fragmentation and functionalization pathways in the heterogeneous oxidation of oxidized organic aerosol

Abstract: The competition between the addition of polar, oxygen-containing functional groups (functionalization) and the cleavage of C–C bonds (fragmentation) has a governing influence on the change in volatility of organic species upon atmospheric oxidation, and hence on the loading of tropospheric organic aerosol. However the relative importance of these two channels is generally poorly constrained for oxidized organics. Here we determine fragmentation–functionalization branching ratios for organics spanning a range of oxidation levels, using the heterogeneous oxidation of squalane (C30H62) as a model system. Squalane particles are exposed to high concentrations of OH in a flow reactor, and measurements of particle mass and elemental ratios enable the determination of absolute elemental composition (number of oxygen, carbon, and hydrogen atoms) of the oxidized particles. At low OH exposure, the oxygen content of the organics increases, indicating that functionalization dominates, whereas for more oxidized organics the amount of carbon in the particles decreases, indicating the increasing importance of fragmentation processes. Once the organics are moderately oxidized (O/C ≈ 0.4), fragmentation completely dominates, and the increase in O/C ratio upon further oxidation is due to the loss of carbon rather than the addition of oxygen. These results suggest that fragmentation reactions may be key steps in the formation and evolution of oxygenated organic aerosol (OOA).

Electrodeposition of copper composites from deep eutectic solvents based on choline chloride.

Abstract: Here we describe for the first time the electrolytic deposition of copper and copper composites from a solution of the metal chloride salt in either urea–choline chloride, or ethylene glycol–choline chloride based eutectics. We show that the deposition kinetics and thermodynamics are quite unlike those in aqueous solution under comparable conditions and that the copper ion complexation is also different. The mechanism of copper nucleation is studied using chronoamperometry and it is shown that progressive nucleation leads to a bright nano-structured deposit. In contrast, instantaneous nucleation, at lower concentrations of copper ions, leads to a dull deposit. This work also pioneers the use of the electrochemical quartz crystal microbalance (EQCM) to monitor both current efficiency and the inclusion of inert particulates into the copper coatings. This technique allows the first in situ quantification or particulate inclusion. It was found that the composition of composite material was strongly dependent on the amount of species suspended in solution. It was also shown that the majority of material was dragged onto the surface rather than settling on to it. The distribution of the composite material was found to be even throughout the coating. This technology is important because it facilitates deposition of bright copper coatings without co-ligands such as cyanide. The incorporation of micron-sized particulates into ionic liquids has resulted, in one case, in a decrease in viscosity. This observation is both unusual and surprising; we explain this here in terms of an increase in the free volume of the liquid and local solvent perturbation.

Multiscale modeling of emergent materials: biological and soft matter

Abstract: In this review, we focus on four current related issues in multiscale modeling of soft and biological matter. First, we discuss how to use structural information from detailed models (or experiments) to construct coarse-grained ones in a hierarchical and systematic way. This is discussed in the context of the so-called Henderson theorem and the inverse Monte Carlo method of Lyubartsev and Laaksonen. In the second part, we take a different look at coarse graining by analyzing conformations of molecules. This is done by the application of self-organizing maps, i.e., a neural network type approach. Such an approach can be used to guide the selection of the relevant degrees of freedom. Then, we discuss technical issues related to the popular dissipative particle dynamics (DPD) method. Importantly, the potentials derived using the inverse Monte Carlo method can be used together with the DPD thermostat. In the final part we focus on solvent-free modeling which offers a different route to coarse graining by integrating out the degrees of freedom associated with solvent.

Interactions and dynamics in electrolyte solutions by dielectric spectroscopy

Abstract: Despite its immense abilities to quantify many aspects of ion-ion and ion-solvent interactions, dielectric relaxation spectroscopy (DRS) has long been neglected as a tool for the investigation of the structure and dynamics of electrolyte solutions. The reasons for this are briefly discussed and it is shown that many of the difficulties associated with this technique have been overcome in recent years by technological developments. Representative applications of DRS to the investigation of ion solvation and ion association in electrolyte solutions of chemical, industrial, geochemical and biological interest, including room temperature ionic liquids and polyelectrolyte systems, are discussed. The advantages of linking DRS measurements to information obtained from other experimental techniques and from computer simulations are highlighted.

Cross-diffusion and pattern formation in reaction-diffusion systems.

Abstract: Cross-diffusion, the phenomenon in which a gradient in the concentration of one species induces a flux of another chemical species, has generally been neglected in the study of reaction–diffusion systems. We summarize experiments that demonstrate that cross-diffusion coefficients can be quite significant, even exceeding “normal,” diagonal diffusion coefficients in magnitude in systems that involve ions, micelles, complex formation, excluded volume effects (e.g., surface or polymer reactions) and other phenomena commonly encountered in situations of interest to chemists. We then demonstrate with a series of model calculations that cross-diffusion can lead to spatial and spatiotemporal pattern formation, even in relatively simple systems. We also show that, in the absence of cross-diffusion among the reacting species, introduction of a nonreactive species that induces appropriate cross-diffusive fluxes with reactive species can lead to pattern formation.

Computation of accurate excitation energies for large organic molecules with double-hybrid density functionals

Abstract: Time-dependent double-hybrid density functional methods are evaluated for the calculation of vertical singlet–singlet valence excitation energies of a wide variety of organic molecules. Beside the already published TD-B2-PLYP method, an analogous approach based on the recently published ground state B2GP-PLYP functional is presented for the first time. Double-hybrid functionals contain a hybrid-GGA-like part for which a conventional TDDFT linear response treatment is carried out. The thus obtained excitation energies are afterwards corrected by adding a non-local correlation portion, which is based on an CIS(D) type excited state perturbative correction. Both, TD-B2-PLYP and TD-B2GP-PLYP, are first applied to the 142 vertical singlet excitation energies in a benchmark set by Schreiber et al., that contains small and medium sized organic molecules. In a second part, a new benchmark set composed of five large organic dyes is proposed. Accurate reference values are derived from experimental 0–0 excitation energies in solution. A back-correction scheme based on TDDFT computations is presented by which solvent, relaxation and vibrational effects are removed, yielding experimental vertical gas phase excitation energies with an estimated accuracy of about ±0.1 eV. The TD-B2-PLYP, TD-B2GP-PLYP and a variety of conventional TDDFT methods are then applied to this new benchmark set. The results for both considered test sets show that the new double-hybrid approaches yield the smallest mean absolute deviations of 0.22 eV for the first benchmark set and 0.19 eV (TD-B2-PLYP) and 0.16 eV (TD-B2GP-PLYP) for the new organic dye test set. Apart from a break-down of the perturbative correction for very high-lying transitions (larger than 8 eV), it is generally found that the double-hybrid functionals show high robustness and accuracy that cannot be obtained with conventional density functionals (e.g. B3-LYP).

Molecular dynamics simulation of the electrochemical interface between a graphite surface and the ionic liquid [BMIM][PF6]

Abstract: The structure of the electrical double layer in the ionic liquid l-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) near a basal plane of graphite was investigated by molecular dynamics simulation. The calculations were performed both for an uncharged graphite surface and for positively and negatively charged ones. It is found that near an uncharged surface the ionic liquid structure differs from its bulk structure and represents a well-ordered region, extending over ∼20 A from the surface. Three dense layers of ca 5 A thick are clearly observed at the interface, composed of negative ions and positively charged rings. It is established that in the first adsorption layer the imidazolium ring in the [BMIM]+ cation tends to be arranged in parallel to the graphite surface at a distance of 3.5 A. The [PF6]− anion is oriented in such a way that the phosphorus atom is at a distance of 4.1 A from the surface and triplets of fluorine atoms form two planes parallel to the graphite surface. Ions adsorbed at the uncharged surface are arranged in a highly defective 2D hexagonal lattice and the corresponding lattice spacing is approximately four times larger than that of the graphene substrate. The influence of the electrode potential on the distribution of electrolyte ions and their orientation has also been investigated. Increase in the electrode potential induces broadening of the angle distribution of adsorbed rings and a shift of the most probable tilt angle towards bigger values. It was shown that there are no adsorbed anions on the negatively charged surface (σ = −8.2 μC cm−2), but the surface concentration of adsorbed cations on the positively charged surface (σ = +8.2 μC cm−2) has a nonzero value. In addition, the influence of the surface charge (±σ) on the volume charge density and electric potential profiles in an electrolyte was studied. The differences in the cation and anion structure result in the fact that the integral capacitance of the electrical double layer depends on the electrode polarity and equals C = 4.6 μF cm−2 at σ = −8.2 μC cm−2 and C = 3.7 μF cm−2 at σ = +8.2 μC cm−2.

Reactions at surfaces in the atmosphere: integration of experiments and theory as necessary (but not necessarily sufficient) for predicting the physical chemistry of aerosols

Abstract: While particles have significant deleterious impacts on human health, visibility and climate, quantitative understanding of their formation, composition and fates remains problematic. Indeed, in many cases, even qualitative understanding is lacking. One area of particular uncertainty is the nature of particle surfaces and how this determines interactions with gases in the atmosphere, including water, which is important for cloud formation and properties. The focus in this Perspective article is on some chemistry relevant to airborne particles and especially to reactions occurring on their surfaces. The intent is not to provide a comprehensive review, but rather to highlight a few selected examples of interface chemistry involving inorganic and organic species that may be important in the lower atmosphere. This includes sea salt chemistry, nitrate and nitrite ion photochemistry, organics on surfaces and heterogeneous reactions of oxides of nitrogen on proxies for airborne mineral dust and boundary layer surfaces. Emphasis is on the molecular level understanding that can only be gained by fully integrating experiment and theory to elucidate these complex systems.

Water adsorption on the stoichiometric and reduced CeO2(111) surface: a first-principles investigation

Abstract: We present a density functional theory investigation of the interaction between water and cerium oxide surfaces, considering both the stoichiometric and the reduced surfaces. We study the atomic structure and energetics of various configurations of water adsorption (for a water coverage of 0.25 ML) and account for the effect of temperature and pressure of the environment, containing both oxygen and water vapor, employing the ab initio atomistic thermodynamics approach. Through our investigation, we obtain the phase diagram of the water–ceria system, which enables us to discuss the stability of various surface structures as a function of the ambient conditions. For the stoichiometric surface, we find that the most stable configuration for water is when it is bonded at the cerium site, involving two O–H bonds of hydrogen and oxygen atoms at the surface. If oxygen vacancies are introduced at the surface, which is predicted under more reducing conditions, the binding energy of water is stronger, indicating an effective attractive interaction between water molecules and oxygen vacancies. Water dissociation, and the associated activation energies, are studied, and the role of oxygen vacancies is found to be crucial to stabilize the dissociated fragments. We present a detailed analysis of the stability of the water–ceria system as a function of the ambient conditions, and focus on two important surface processes: water adsorption/desorption on the stoichiometric surface and oxygen vacancy formation in the presence of water vapor. A study of the vibrational contribution to the free energy allows us to estimate the effect of this term on the stability range of adsorbed water.

Intrinsic quantum yields and radiative lifetimes of lanthanide tris(dipicolinates)

Abstract: The efficiency with which the surroundings of trivalent lanthanide ions sensitize their luminescence (ηsens) is a key parameter in the design of highly emitting molecular edifices and materials. Evaluation of ηsens requires the measurement of the overall and intrinsic quantum yields obtained upon ligand and metal excitation, respectively. We describe a modified integration sphere enabling absolute determination of these quantities on small amounts of solid samples or solutions (60 μL). The sphere is tested for linear response of emitted versus absorbed light intensities with increasing concentration of Cs3[Ln(dpa)3] solutions (Ln = Eu, Tb). The overall (QEuL = 29 ± 2%) and intrinsic (QEuEu = 41 ± 2%) quantum yields obtained for Eu allow the direct calculation of ηsens (71 ± 6%) while the radiative lifetime (τrad = 4.1 ± 0.3 ms) is calculated from QEuEu and the observed lifetime. The intrinsic quantum yield matches the value extracted from emission parameters using the simplified equation proposed by Werts et al. but, on the other hand, the theoretical estimate using spontaneous transition probabilities calculated from Judd–Ofelt (JO) parameters is off by −25% (3.15 ms). In the case of Cs3[Tb(dpa)3], the molar absorption coefficient of the 5D4←7F6 transition is too small to measure QTbTb for the solution but this quantity could be determined for the microcrystalline sample (72 ± 5%, τrad = 1.9 ± 0.1 ms). In this case, the JO theoretical estimate leads to a much too short τrad value. The large difference in ηsens for microcrystalline samples of Eu (85%) and Tb (42%) tris(dipicolinates) is attributed to back energy transfer in the latter compound consecutive to a sizeable overlap between the 5D4→7F6 emission and the absorption spectrum of the dipicolinate triplet, this overlap being smaller in the case of the solution. The overall quantum yield of Na3[Yb(dpa)3] in aqueous solution is very low (0.015 ± 0.002%) due to both poor sensitization efficiency (8%) and small intrinsic quantum yield (QYbYb = 0.178 ± 0.003%; τrad = 1.31 ± 0.02 ms). For evaluating intrinsic quantum yields of Yb in aqueous solutions of coordination compounds from lifetimes, a value of 1.2–1.3 ms is recommended.

Silver nanoparticles self assembly as SERS substrates with near single molecule detection limit.

Abstract: Highly sensitive SERS substrates with a limit of detection in the zeptomole (for Nile blue A and oxazine 720) range were fabricated through a bottom-up strategy. Ag nanoparticles (Ag NPs) were self-assembled onto glass slides by using 3-mercaptopropyltrimethoxysilane (MPTMS) sol–gel as linker. The substrates were characterized by UV-Vis and AFM after each deposition of Ag NPs. It was found that the glass slide presented just a few Ag NPs aggregates scattered throughout the surface after just one deposition. The glass surface was gradually covered by a homogeneous distribution of Ag NPs aggregates as the deposition number increased. Surface-enhanced Raman scattering (SERS) of the substrates was examined at different numbers of Ag NPs deposition using nile blue A and oxazine 720 as probe molecules and two laser excitations (632.8 nm and 785 nm). Optimum SERS was observed after six depositions of Ag NPs. SERS mapping indicated that at lower deposition numbers (less than 3 Ag NPs depositions) the substrates presented a few SERS “hot–spots” randomly distributed at the surface. After 7 Ag NPs depositions, spatial distribution of the SERS signal followed a Gaussian statistics, with a percent relative standard deviation (RSD%) of ∼19%. In addition, the sample-to-sample reproducibility of the SERS intensities under both laser excitations was lower than 20%. It was also found that these substrates can provide giant Raman signal enhancement. At optimum conditions and with a 632.8 nm laser, the signal from an estimated of only ∼44 probe molecules (100× objective) can still be detected.

XPS study of nitrogen dioxide adsorption on metal oxide particle surfaces under different environmental conditions

Abstract: The adsorption of nitrogen dioxide on gamma aluminium oxide (gamma-Al(2)O(3)) and alpha iron oxide (alpha-Fe(2)O(3)) particle surfaces under various conditions of relative humidity, presence of molecular oxygen and UV light has been investigated. X-Ray photoelectron spectroscopy (XPS) is used to monitor the different surface species that form under these environmental conditions. Adsorption of NO(2) on aluminum oxide particle surfaces results primarily in the formation of surface nitrate, NO(3)(-) with an oxidation state of +5, as indicated by a peak with binding energy of 407.3 eV in the N1s region. An additional minority species, sensitive to the presence of relative humidity and molecular oxygen, is also observed in the N1s region with lower binding energy of 405.9 eV. This peak is assigned to a surface species in the +4 oxidation state. When irradiated with UV light, other species form on the surface. These surface-bound photochemical products all have lower binding energy, between 400 and 402 eV, indicating reduced nitrogen species in the range of N oxidations states spanning +1 to -1. Co-adsorbed water decreases the amount of these reduced surface-bound products while the presence of molecular oxygen completely suppresses the formation of all reduced nitrogen species on aluminum oxide particle surfaces. For NO(2) on iron oxide particle surfaces, photoreduction is enhanced relative to gamma-Al(2)O(3) and surface bound photoreduced species are observed under all environmental conditions. Complementing the experimental data, N1s core electron binding energies (CEBEs) were calculated using DFT for a number of nitrogen-containing species in the gas phase and adsorbed on an Al(8)O(12) cluster. A range of CEBEs is calculated for various nitrogen species in different adsorption modes and oxidation states. These calculated values are discussed in light of the peaks observed in the XPS N1s region and the possible species that form following NO(2) adsorption and photoreaction on metal oxide particle surfaces under different conditions of relative humidity, presence of molecular oxygen and UV light.

First-principles study on defect chemistry and migration of oxide ions in ceria doped with rare-earth cations

Abstract: Oxygen transport in rare-earth oxide (RE2O3) doped CeO2 with fluorite structure has attracted considerable attention owing to both the range of practical usage (e.g., fuel cells, sensors, etc.) and the fundamental fascination of fast oxide ion transport in crystalline solids. Using density-functional theory, we have calculated the formation energies of point defects and their migration properties in RE2O3 doped CeO2(RE = Sc, Y, La, Nd, Sm, Gd, Dy, and Lu). The calculated results show that oxygen vacancies are the dominant defect species obtained by RE3+ doping. They form associates with the RE3+ ions, and the corresponding defect association energy is a strong function of the ionic radii of the RE3+ dopants. The migration of an oxygen vacancy was investigated using the nudged elastic band method. The lowest activation energy for oxygen vacancy hopping is obtained for a straightforward migration path between two adjacent oxygen sites. The migration energy of an oxygen vacancy also strongly depends on the ionic radii of the neighbouring dopant cations. Accordingly, we have identified two factors that affect the oxygen vacancy migration; (1) trapping (or repelling) of an oxygen vacancy at the NN site of the RE3+ dopant, and (2) reduction (or enlargement) of the migration barrier by RE3+ doping. These findings provide insight for atomistic level understanding of ionic conductivity in doped ceria and would be beneficial for optimizing ionic conductivity.

Elastic strain at interfaces and its influence on ionic conductivity in nanoscaled solid electrolyte thin films--theoretical considerations and experimental studies.

Abstract: Ionic transport in solids parallel to grain or phase boundaries is usually strongly enhanced compared to the bulk. Transport perpendicular to an interface (across an interface) is often much slower. Therefore in modern micro- and nanoscaled devices, a severe influence on the ionic/atomic transport properties can be expected due to the high density of interfaces. Transport processes in boundaries of ionic materials are still not understood on an atomic scale. In most of the studies on ionic materials the interfacial transport properties are explained by the influence of space charge regions. Here we discuss the influence of interfacial strain at semicoherent or coherent heterophase boundaries on ionic transport along these interfaces in ionic materials. A qualitative model is introduced for (untilted and untwisted) hetero phase boundaries. For experimental verification, the interfacial oxygen ionic conductivity of different multilayer systems consisting of cubic ZrO2 stabilised by aliovalent dopands (YSZ, CSZ) and an insulating oxide is investigated as a function of structural mismatch. Recent results on extremely fast ionic conduction in YSZ/SrTiO3 thin film systems (“colossal ionic concuctivity at interfaces”) is discussed from the viewpoint of strain effects.

Piezoelectric properties of poly(vinylidene fluoride) and carbon nanotube blends: β-phase development

Abstract: In order to get poly(vinylidene fluoride) (PVDF) films containing high β-phase content, multiwalled carbon nanotubes (MWCNTs) were blended with PVDF. For drawn samples, the content of piezoelectric β-form crystal was increased with MWCNT addition due to the rapid crystallization rate offered by the nucleating action of MWCNT, but soon reached a plateau. Poling on the drawn samples helps additional β-phase formation when the added MWCNT content was less than 0.2 wt%; at this MWCNT amount, almost pure β-phase crystal was obtained. More MWCNT addition induced depolarization to reduce the β-phase content. Undrawn samples show monotonous increase of β-phase content with MWCNT amount when subjected to poling.

The potential role of hydrogen bonding in aprotic and protic ionic liquids.

Abstract: Cohesion energies determine the phase behavior of materials. The understanding of interaction energies is in particular interesting for ionic liquids. Here we show experimentally that, in accord with theoretical work, the intermolecular cation-anion interactions in ionic liquids can be detected by far FTIR spectroscopy. The measured vibrational bands of aprotic and protic ionic liquids in the low-frequency range can be referred to the interaction strength between cations and anions in various combinations. It can be shown by DFT B3LYP calculations that these interactions are described by characteristic ratios between Coulomb forces and hydrogen bonds. These ratios can be tuned towards increasing hydrogen bond contributions which is reflected in important macroscopic properties of ionic liquids such as enthalpies of vaporization and viscosities. This opens a new path for tuning the desired properties of this new class of material.

Reconsideration of the excited-state double proton transfer (ESDPT) in 2-aminopyridine/acid systems: role of the intermolecular hydrogen bonding in excited states.

Abstract: In the present work, the excited-state double proton transfer (ESDPT) in 2-aminopyridine (2AP)/acid systems has been reconsidered using the combined experimental and theoretical methods. The steady-state absorption and fluorescence spectra of 2AP in different acids, such as formic acid, acetic acid, propionic acid, etc. have been measured. We demonstrated for the first time that the ESDPT reaction can take place between 2AP and all of these acids due to the formation of the intermolecular double hydrogen bonds. Furthermore, the vitally important role of the intermolecular double hydrogen bonds between 2AP and acids for ESDPT reaction has also been confirmed by the disappearance of ESDPT when we add the polar acetonitrile to the 2AP/acids systems. This may be due to that the respective polar solvation of 2AP and acids by the acetonitrile solvent disrupts the formation of intermolecular double hydrogen bonds between 2AP and acids. Moreover, the intermolecular double hydrogen bonds are demonstrated to be significantly strengthened in the electronic excited state of 2AP/acid systems using the time-dependent density functional theory (TDDFT) method. The ESDPT reaction is facilitated by the electronic excited-state hydrogen bond strengthening. In addition, potential energy curves of the electronic excited state along the proton transfer coordinate are also calculated by the TDDFT method. The stepwise mechanism of the ESDPT reaction in the 2AP/acid systems is theoretically reconfirmed, and the concerted mechanism is theoretically excluded. At the same time, the sequence of the double proton transfers is theoretically clarified for the first time using the potential energy curves calculated by TDDFT method.

Restricted active space spin-flip configuration interaction approach: theory, implementation and examples.

Abstract: A new formulation of the spin-flip (SF) method is presented. The electronic wave function is specified by the definition of an active space and through α-to-β excitations from a Hartree–Fock reference. The method belongs to the restricted active space (RAS) family, where the CI expansion is restricted by classifying the molecular orbitals in three subspaces. Properties such as spin completeness, variationality, size consistency, size intensivity, and orbital invariance are discussed. The implementation and applications use a particular truncation of the wave function, with the inclusion of hole and particle contributions such that for fixed active space size, the number of amplitudes is linear in molecular size. This approach is used to investigate single and double bond-breaking, the singlet–triplet gap of linear acenes, electronic transitions in three Ni(II) octahedral complexes, the low-lying states of the 2,5-didehydrometaxylylene (DDMX) tetraradical and the ground state multiplicity of 28 non-Kekule structures. The results suggest that this approach can provide a quite well balanced description of nearly degenerate electronic states at moderate computational cost.

Aluminium siting in the ZSM-5 framework by combination of high resolution 27Al NMR and DFT/MM calculations

Abstract: The Al siting in the ZSM-5 zeolite was investigated by 27Al 3Q MAS NMR spectroscopy and QM/MM calculations. It was found that the occupation of the framework T-sites by Al and the concentration of Al in these T-sites are neither random nor controlled by a simple rule. They both depend on the conditions of the zeolite synthesis. At least 12 out of the 24 distinguishable framework T-sites of ZSM-5 are occupied by Al in the set of the investigated zeolite samples. A partial identification of the Al sites is possible. The calculated 27Al NMR shielding values were converted to 27Al isotropic chemical shifts using the experimental isotropic chemical shift of 60.0 ppm referenced to the aqueous solution of Al(NO3)3 and the corresponding calculated NMR shielding of 490.0 ppm of a silicon rich (Si/Al 38) chabazite structure zeolite as a secondary internal standard. The observed 27Al isotropic chemical shifts of 50.0 and 54.7 ppm correspond to Al atoms in the T20 and T6 sites, respectively. The pair of measured isotropic chemical shifts of 52.9 and 53.7 ppm can be assigned to the T4, T8 pair. At the low-shielding end, two assignments are plausible. The smallest deviations between the calculated and observed isotropic chemical shifts are reached for the assignment as follows: T24 (64.8 ppm) is not occupied in the samples and that the observed isotropic chemical shifts 63.6, 62.8, and 60.0 ppm belong to T1, T17, and T7, respectively. It follows then that T-sites T12 (60.8 ppm), T3 (61.7 ppm), and T18 (62.0 ppm) are most likely not occupied by Al in our ZSM-5 samples. If we assume that the calculated isotropic chemical shifts are systematically larger than the observed ones then we can assign the largest observed isotropic chemical shifts of 63.6 and 62.8 ppm to the least shielded T24 and T1 sites, respectively, and 60.0 ppm to T12. Then the sites T3 (61.7 ppm), T18 (62.0 ppm), and T17 (62.5 ppm) would be unoccupied by Al in our ZSM-5 samples. It was further shown that there is no simple linear relationship between the observed 27Al isotropic chemical shifts and the average Al–O–Si angles.